Sirtuins (SIRTs) are nicotine adenine dinucleotide(+)-dependent histone deacetylases regulating critical signaling pathways in prokaryotes and eukaryotes, and are involved in numerous biological processes. Currently, seven mammalian homologs of yeast Sir2 named SIRT1 to SIRT7 have been identified. Increasing evidence has suggested the vital roles of seven members of the SIRT family in health and disease conditions. Notably, this protein family plays a variety of important roles in cellular biology such as inflammation, metabolism, oxidative stress, and apoptosis, etc., thus, it is considered a potential therapeutic target for different kinds of pathologies including cancer, cardiovascular disease, respiratory disease, and other conditions. Moreover, identification of SIRT modulators and exploring the functions of these different modulators have prompted increased efforts to discover new small molecules, which can modify SIRT activity. Furthermore, several randomized controlled trials have indicated that different interventions might affect the expression of SIRT protein in human samples, and supplementation of SIRT modulators might have diverse impact on physiological function in different participants. In this review, we introduce the history and structure of the SIRT protein family, discuss the molecular mechanisms and biological functions of seven members of the SIRT protein family, elaborate on the regulatory roles of SIRTs in human disease, summarize SIRT inhibitors and activators, and review related clinical studies.
The sirtuin (SIRT) protein family, which are conserved proteins belonging to class III histone deacetylases, comprises seven members.1 Notably, SIRTs share a nicotine adenine dinucleotide + (NAD) + -binding catalytic domain and may act specifically on different substrates depending on the biological processes in which they are involved.2 The sequence and length of SIRTs are different in both their N- and C-terminal domains, partially explaining their different localization and functions.2 Recently, more and more studies have shown their association with and involvement in different pathologies, such as (but not restricted to) cancer and cardiovascular diseases (CVDs).3,4,5,6 Additionally, increasing evidence supported the potential use of SIRT modulators for the treatment of different kinds of diseases,7,8,9,10,11 suggesting the critical roles of SIRTs in the diseases. Herein, to enhance our understanding of SIRTs, we provide a comprehensive summary of the roles of SIRTs in health and various diseases.
Historical review and structure of SIRT proteins
The history of SIRTs can be traced to founding member Sir2 nearly 40 years ago, which was first discovered in the budding Saccharomyces cerevisiae, and was originally known as mating-type regulator 1 protein.12 Subsequently, Sir2 has been found to function in transcriptional repression at ribosomal DNA loci,13 at silent mating-type loci14 and in telomeres,15 and this increasing knowledge has greatly improved exploration of its function. In the late 1990s, a study confirmed that Sir2 prolonged the lifespan of yeast by inhibiting genomic instability. Loss of Sir2 significantly shortened the lifespan of yeast, while an additional copy of Sir2 prolonged it by about 40%.16 Later evidence showed that Sir2 had NAD + -dependent HDAC enzymatic activity, which provided a molecular framework in which NAD-dependent histone deacetylation could be connected to genomic silencing and ageing in yeast, and possibly to higher eukaryotic metabolism as well, opening a new chapter of Sir2 enzymology.17 Sir2’s key role in the molecular mechanism of senescence in Caenorhabditis elegans was also later demonstrated.18 As Sir2 homologous genes have been successively isolated in bacteria, plants and mammals, the Sir2 homologous proteins in all species have been collectively referred to as SIRTs.19,20
Currently, seven mammalian homologs of yeast Sir2 named SIRT1 to SIRT7 have been identified, which are well-known as the β-NAD + or NAD + -dependent enzymes.21,22,23 Figure 1 shows a historical timeline summarizing studies on milestones in SIRT family members. Regarding to the molecular structures, SIRT1-7 share a chemically and structurally conserved catalytic core in general and there may be subtle differences in the infrastructure of active site.24 In detail, X-ray crystalline diffraction reveals that the catalytic core includes two bilobed globular domains consisting of approximately 275 amino acids residues, characterized by their necessity for NAD as a cofactor. The different N- and C-terminals of SIRT proteins are fairly variable in length, chemical composition, susceptibility to post-translational modifications (PTMs) (typically phosphorylation), and enable them to bind substrates.2,25,26 The large structural domain is composed of an inverted classical open α/β Rossmann-fold structure, which is a parallel β-sheet nucleotide-binding fold typical of many NAD-utilizing enzymes such as dehydrogenases; in addition, a smaller domain contains a zinc ribbon motif. These two domains form a pocket in the middle where NAD and acetylated peptides bind.2,27
Differences among members of the SIRT protein family were initially attributed to their discrete pattern of subcellular localization.28 As far as we know, SIRT1 is mainly localized in the nucleus and shuttles to the cytosol under specific circumstances.29,30 SIRT2 is predominantly cytosolic but also exists in the nucleus in the G2 to M phase transition of the cell cycle.31 SIRT3-5 localize primarily to mitochondria, and have a mitochondrial targeting sequence.32,33,34 Additionally, SIRT6 and SIRT7 are nuclear proteins. Of them, SIRT6 is principally located in the chromatin and SIRT7 is mostly found in the nucleolus.35,36 Additionally, the localization and subcellular shuttling of SIRTs depend on different kinds of cell types and cell cycle oscillation.37 For example, SIRT1 could be primarily located in the cytosol in some subsets of neurons, as well as expressed in both nucleus and cytosol in ependymal cells.30 Moreover, SIRT2 is in the cytosol during most phases of cell cycle, while SIRT2 is expressed in nucleus and associates with chromatin and deacetylates the histone H4K16 during G2/M transition and mitosis.31
The catalytic activity level of SIRT protein family members is thought to be their second most significant difference. Of note, the regulation of catalytic activity of SIRTs involves multiple steps: (a) NAD + and acetyl lysine substrates binding; (b) the glycosidic bond cleavage; (c) acetyl transfer; and (d) O-acetyl-ADPR, nicotinamide, and deacetylated lysine products formation. Concretely, the initial reaction of NAD + glycosidic bond cleavage is proceeded through either an SN1-like mechanism, as supported by the structure of Hst2 bound to carba-NAD + ,38 or an SN2-like mechanism, as supported by the structure of Sir2Tm bound to NAD+ and an acetyl lysine-containing peptide.39 Furthermore, available studies suggested a complex array of PTMs regulated by SIRTs. Initially, Sir2 was considered solely as a deacetylase enzyme.17 However, the functional range of enzymatic activities of SIRTs has been greatly expanded in mammals. SIRT1-3 sustain strong deacetylase activities. SIRT4 has ADP-ribose transferase activity and can down-regulate glutamate dehydrogenase activity in β cells, thereby reducing insulin secretion response.33 SIRT5 is involved in regulating protein post translational modifications such as lysine succinylation, malonylation, and glutarylation, etc.40,41 Moreover, SIRT6 can function as NAD + -dependent monoADP-ribosyl transferase and long-chain fatty acyl deacetylases.42,43 Meanwhile, SIRT7, the latest discovered SIRT family protein, has been relatively less studied, which was first found to be a β-NAD + -dependent deacetylase enzyme and is localized in nucleoli that govern the transcription of RNA polymerase I.44,45 Numerous target proteins, including histone and non-histone, have been shown to be modified by SIRTs, and participates in the regulation of multiple fundamental cellular functions including glucose, and lipid metabolism, mitochondrial biogenesis, DNA repair, oxidative stress, apoptosis, and inflammation.46 Hence, SIRTs are now recognized as a major regulator of cellular physiology. Nevertheless, the SIRT protein family still has multiple proven and unproven catalytic modification activities. Given our current limited understanding of the SIRT protein family, more investigation is warranted in this area.
The regulatory role of SIRTs in cellular biology
The role of SIRTs in inflammation
Inflammation is an essential immune response that enables survival during infection or injury and maintains tissue homeostasis under a variety of noxious conditions.47 It comes at the cost of a transient decline in tissue function, which can in turn contribute to the pathogenesis of diseases involving altered homeostasis and a variety of physiological and pathological processes.48 The molecular process of inflammation is varied and depends on the type of inflamed cells and organs. The inflammatory response is composed of several inseparable pathways involving inflammatory cells, inflammatory mediators induced by sensor cells, inflammatory pathway components, and the target tissues that are affected by the inflammatory mediators.47 Recently, with greater in-depth understanding of the process of inflammation, numerous studies have successfully illustrated how the SIRT protein family has a close association with inflammation. In this section, we summarize the role of the SIRT family in the inflammatory response and the major signaling pathways (Fig. 2).
The effect of SIRTs in inflammatory cells
The cells involved in the inflammatory response include inflammatory cells such as macrophages, mast cells and endothelial cells. SIRTs, especially SIRT1 and SIRT6, can affect the secretion of inflammatory mediators and play a central role in regulating the differentiation of dendritic cells (DCs) and the activation of macrophages.49,50 For example, SIRT1 participates in mediating inflammatory signaling in DCs, consequentially modulating the balance of proinflammatory T helper type 1 cells and anti-inflammatory Foxp3(+) regulatory T cells. SIRT1 knockout (KO) in DCs restrained the generation of regulatory T cells while driving T helper 1 cell development, resulting in enhanced T-cell-mediated inflammation against microbial responses.49 Moreover, SIRT6 deficiency in macrophages resulted in inflammation with increases in acetylation and greater stability of the forkhead box protein O1 (FoxO1). Conversely, the ectopic overexpression of SIRT6 in KO cells reduced the inflammatory response.50 Moreover, results from in vivo experiments demonstrated that SIRT3 overexpression in transfused macrophages not only induced M2 macrophage polarization, but also alleviated inflammation.51 Based on these current studies, the SIRT family may regulate the activation or differentiation of inflammatory cells, such as DCs and macrophages in the immune system.
The effect of SIRTs on inflammatory mediators
Inflammatory mediators are chemicals produced during inflammation that cause an inflammatory response. In response to the inflammatory process, inflammatory cells release specialized substances, including vasoactive amines and peptides, eicosanoids, proinflammatory cytokines and acute-phase proteins, which mediate the inflammatory process by preventing further tissue damage and ultimately resulting in healing and restoration of tissue function.52 Overexpressed or activated SIRTs, mainly SIRT1–3, can reduce the inflammatory response through anti-inflammatory effects, such as tumor necrosis factor-alpha (TNF-α), a multifunctional pro-inflammatory cytokine, which is produced by macrophages/monocytes during acute inflammation, and plays a critical role with orchestrating the cytokine cascade in various inflammatory diseases.53 For instance, increased SIRT1 protein expression can reduce acetylation of the nuclear factor kappa-B (NF-κB) p65 subunit, which results in the suppression of TNF-α-induced NF-κB transcriptional activation and reduction of TNF-α secretion in a SIRT1-dependent manner.54,55 In addition, SIRT1 knockdown increased, while SIRT1 activator treatment decreased TNF-α secretion from macrophages.55 One recent study verified that SIRT6 suppressed inflammatory responses and downregulated the expression of inflammatory factors interleukin (IL)-6 and TNF-α via the NF-κB pathway.56 For example, both SIRT1 and SIRT6 inhibited TNF-α-induced inflammation of vascular adventitial fibroblasts through reactive oxygen species (ROS) and the protein kinase B (Akt) signaling pathway.57 SIRT1 exerted anti-inflammatory effects against IL-1β-mediated pro-inflammatory stress through the Toll-like receptor 2 (TLR2)/SIRT1/NF-κB pathway.58 SIRT1 deficiency increased microvascular inflammation in obese septic mice, while resveratrol treatment decreased leukocyte/platelet adhesion and E-selectin/intercellular adhesion molecule (ICAM-1) expression accompanied by increased SIRT1 expression and improved survival.59 In addition, SIRT1 and SIRT6 inhibited inflammation by decreasing pro-inflammatory cytokines such as IL-6, IL-β, cytochrome oxidase subunit 2 and ICAM-1.60 Moreover, SIRT1 exerted anti-inflammatory effects against IL-1β-mediated pro-inflammatory stress through the TLR2/SIRT1/NF-κB pathway.58 SIRT1 deficiency increased microvascular inflammation in obese septic mice, while resveratrol treatment decreased leukocyte/platelet adhesion and E-selectin/ICAM-1 expression accompanied by increased SIRT1 expression and improved survival.59 Recently, SIRT2 as modulators have been shown to be effective in inhibiting lipopolysaccharide-stimulated production of TNF-α to suppress neuroinflammation.61,62 Moreover, Kurundkar et al. have determined that SIRT3 deficiency altered the proinflammatory responses of macrophages to lipopolysaccharides, with a greater increase in TNF-α production.63 Several studies have also shown an anti-inflammatory effect of SIRT3, which downregulates IL-1β and IL-18, inhibits inflammasomes and attenuates oxidative stress.64,65 SIRT3 KO mice have significantly increased inflammatory cell infiltration.66 These studies highlight the critical role of SIRT3 in the process of inflammation. In conclusion, then, as one of the most important pro-inflammatory cytokines, inflammatory mediators are closely regulated by the SIRT protein family and is widely involved in inflammation.
Currently, the SIRT family mainly exerts an anti-inflammatory effect in response to tissue stress or disease development, but there are exceptions. For example, early SIRT2 inhibition prevented neuroinflammation evidenced by reduced levels of glial fibrillary acidic protein, IL-1β, IL-6 and TNF-α and by increased levels of glutamate receptor subunits GluN2A, GluN2B and GluA1; however, SIRT2 inhibition was unable to reverse cognitive decline or neuroinflammation.67 In this case, SIRT2 exhibited a temporary proinflammatory effect. Furthermore, both pro- and anti-inflammatory effects have been attributed to SIRT2 and SIRT3.68 Single deficiency of SIRT2 or SIRT3 had minor or no impact on the antimicrobial innate immune responses, while SIRT2/3−/− macrophages secreted increased levels of both proinflammatory and anti-inflammatory cytokines.68 From these results, then, most SIRT proteins appear to play anti-inflammatory roles, but limited reports have found the opposite effect, as just described for SIRT2. These inconsistent results might be due to the specificity of SIRT2 mechanisms in the SIRT family, or may be temporary effects manifested at different stages of the disease process. Therefore, more research is needed to explore the reasons for these discrepancies.
Overall, SIRTs can act in concert or compensate each other for certain immune functions.68 It is also worth noting that the effects of various SIRTs may differ between diseases, or even have opposite effects. Therefore, research on SIRTs has left a number of gaps which require further exploration to pinpoint the role of the SIRT family in inflammatory responses and the underlying mechanisms of action, which may account for the different results.
The effect of SIRTs on inflammatory pathway components
The signaling pathway of inflammation is complex, but inflammatory pathway components have begun to be elucidated over the past several years. Currently, there are many studies on the mechanisms by which the SIRT family participates in inflammation, especially pathways involving NF-κB, TNF-α, and the NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome.
NF-κB is considered to be the central regulator of inflammation, which drives the expression of cytokines, chemokines, inflammasome components and adhesion molecules.69 It is mainly involved in immune and inflammatory responses and can induce the expression of downstream inflammatory cytokines.70,71 TNF-α is a pro-inflammatory cytokine mainly produced by macrophages and monocytes and is involved in normal inflammatory and immune responses.72 As an important component of innate immunity, the NLRP3 inflammasome plays an important role in the body’s immune response and inflammatory cell death (pyroptosis).73 In the following sections, we detail the role of the SIRT family as it affects three key inflammatory pathway components.
Majority of SIRTs exert anti-inflammatory effects by inhibiting the NF-κB pathway
NF-κB exists in multiple forms, with the heterodimer of p65 (RelA, Rel associated protein) and p50 subunits (p65/p50) being the most prevalent species.74 In the absence of stimulation, NF-κB is normally present in the cytoplasm in an inactive form. Upon stimulation by various pro-inflammatory cytokines (such as IL-1β, IL-6 and TNF-α), NF-κB rapidly translocates to the nucleus and regulates the transcription or expression of target genes.75,76 In addition, NF-κB activity can be modulated by PTMs of proteins, such as acetylation.77 Most members of the SIRT family are involved in regulation of the NF-κB pathway, primarily including SIRT1, SIRT2, SIRT6, and SIRT7.
Growing evidence suggests the significant role of SIRTs in the regulation of inflammation. SIRT1 has anti-inflammatory effects mediated by the deacetylation and inactivation of the p65 subunit of NF-κB.78 SIRT1 inhibits the transcriptional activity of NF-κB via deacetylation of the p65 (RelA) subunit at Ac-Lys310.78 Furthermore, the finding that lower SIRT1 activity levels may increase the expression of NF-κB, thus driving inflammation,79 also highlight the important role of SIRT1 during inflammation.
Repression of NF-κB activity is responsible for the anti-inflammatory effect of SIRT6.80 For instance, SIRT6 attenuated NF-κB expression by deacetylating histone H3K9 in the promoters of NF-κB target genes, hence decreasing inflammation.80 Additionally, SIRT6 overexpression suppressed NF-κB-mediated inflammatory responses in OA development.81 Since nuclear SIRT1 and SIRT6 deacetylate RelA/p65 and support its degradation by the proteasome, decreases in both SIRT1 or SIRT6 levels/activity increase NF-κB activity and amplify pro-inflammatory gene expression during chronic inflammation.82
Evidence concerning the role of SIRT7 in inflammatory processes has been somewhat inconsistent. In terms of mediating an anti-inflammatory response, knockdown of SIRT7 promoted the translocation of NF-κB p-p65 to the nucleus and subsequently increased the secretion of downstream inflammatory cytokines, while SIRT7 overexpression had the opposite effect.83,84 However, evidence also suggested that loss of SIRT7 promoted the translocation of NF-κB p65 to the cytoplasm.85 Thus, the roles of SIRT7 in p65 translocation is controversial. In addition, the decline of SIRT7 upregulated the levels of pro-inflammatory cytokines including IL-1β and IL-6 in human umbilical vein endothelial cells, while overexpression of SIRT7 effectively alleviated the inflammatory response.86 However, several studies have also revealed a pro-inflammatory role for SIRT7. For example, SIRT7-kidney-specific KO mice exhibited diminished inflammation with a reduction in the level of multiple inflammatory factors such as TNF-α, IL-1β and IL-6, and suppression of nuclear NF-κB p65 accumulation.87 These contradictory results imply that the regulatory effects of SIRT7 on the inflammatory process may be variable under specific pathologies, which will need further study.84
SIRT2 also participates in inflammatory responses. Inhibition of SIRT2 enhanced microglial activation and the release of pro-inflammatory cytokines via acetylation-dependent upregulation of NF-κB transcriptional activity.88 SIRT2 reduced the levels of pro-inflammatory cytokines and ameliorated the severity of arthritis by deacetylating the p65 subunit of NF-κB,89 further demonstrating the role of SIRT2 activation in suppression of the inflammatory response.
In summary, SIRTs are found to interfere with the NF-κB signaling pathway by preventing NF-κB translocation, influencing its expression and regulating its interactions, thereby having an anti-inflammatory function. Understanding the underlying molecular mechanisms of NF-κB pathway activation and its effects on inflammation may guide an approach to designing better pharmacological targets for alleviating inflammation and related therapies.
The activation of NLRP3 aggravates inflammation
NLRP3 is an important component of the NLRP3 inflammasome complex involved in inflammation.90,91 It is believed that activation of the NLRP3 inflammasome occurs in two sequential steps — first, it must be primed, and then it can be activated.71 When the body suffers from inflammatory disease, damage-associated molecules directly engage TLR4 and then quickly activate the NF-κB signaling pathway, resulting in augmented expression of NLRP3;92,93,94 this in turn generates inflammatory cytokines such as IL-1β, IL-18, TNF-α and transforming growth factor-beta (TGF-β) which aggravate inflammation.95 Some studies have found that SIRTs, especially SIRT1 and SIRT3, act on NLRP3 to exert anti-inflammatory functions. For example, SIRT1 plays an important protective role in the inflammation mediated by the attenuation of NLRP3 activity, which is the best characterized inflammasome.96,97 Mechanistic studies of acute liver injury98 demonstrated activation of a pathway involving SIRT1 and multipotent mesenchymal stromal/stem cell-mediated AMP-activated protein kinase (AMPK) α in macrophages, resulting in deacetylation of spliced X-box-binding protein 1 and subsequent inhibition of the NLRP3 inflammasome.
It was reported that mitophagy/autophagy blockade leads to the accumulation of damaged mitochondria generating ROS, and this in turn activates the NLRP3 inflammasome.99 For instance, a study carried out by Zhao et al. suggested that the mechanism of action by which SIRT3 protects against tissue damage involved the attenuation of ROS production and reduction of NLRP3 activity, resulting in the inhibition of oxidative stress and the downregulation of proinflammatory cytokines.64 However, little information is available on the relationship between SIRT3 and NLRP3; thus, further research is necessary to determine whether SIRT3 has a direct effect on the NLRP3 inflammasome.
The effect of SIRTs targeting noncoding RNAs on the inflammatory pathway
Current studies have mainly elucidated the role of the SIRT family in the inflammatory response. However, exploration of the molecular mechanism underlying how SIRTs affect inflammation is still limited, especially studies examining the interaction of SIRT1 with noncoding RNAs. For example, microRNAs (miRNAs) can negatively regulate inflammation by repressing SIRT1. Downregulation of miRNAs such as miR-217 and miR-543 mitigated the inflammatory response by regulating the SIRT1/AMPK/NF-κB signaling pathway.100 In the same way, miR-378 reduced SIRT1 activity and facilitated the inflammatory pathway involving NF-κB-TNFα by targeting 5'-AMPK subunit gamma-2.101 In addition, the RNase monocyte chemoattractant protein-induced protein 1 alleviated inflammatory responses by promoting the expression of SIRT1 mediated via miR-9.102 Furthermore, SIRT1 targets the p53/miR-22 axis to suppress inflammation, cyclooxygenase (COX)-2 and inducible nitric oxide synthase (iNOS) expression.103 These studies suggest that the regulation of SIRTs by noncoding RNAs may be a promising therapeutic strategy for inflammation-related diseases.
In summary, the SIRT family is involved in inflammation via various mechanisms. Although the details of SIRT-dependent regulation of inflammation are becoming clear, many unanswered questions remain. For example, further studies are needed to explore whether depletion of SIRTs is a common pathological change in the occurrence and development of inflammation-related diseases. Further attention is also needed to resolve some of the conflicting data and better understand the critical role of the SIRT family in the inflammatory response. The contradictory roles of the SIRT family in inflammation may result from their regulation of common signaling pathways under specific pathologic conditions. While determining what role the SIRT family plays in inflammation, researchers should also target its mechanism of action in order to lay the foundation for subsequent clinical translational studies. To summarize, we have focused on introducing relevant studies and the beneficial effects of the SIRT family through its regulation of inflammatory pathways, providing an important reference point for future studies.
The role of SIRTs in metabolism
Metabolism is the general term for a series of ordered chemical reactions that take place in the body to sustain life.104,105 These processes allow organisms to grow and reproduce, maintain their structure and respond to the external environment.106,107,108 Metabolism mainly includes glucose metabolism and lipid metabolism.104,109 Many metabolic processes occur in the mitochondria where SIRT3–5 proteins are located. In addition, SIRT proteins located in the nucleus may participate in regulating several metabolism-related genes.109,110 In this section, we focus on the SIRT proteins and their roles in maintaining metabolic homeostasis by participating in the regulation of glucose, glutamine, and lipid metabolism (Fig. 3).
The effect of SIRTs on glucose metabolism
Glucose metabolism refers to a series of complex chemical reactions after glucose, glycogen and other substances enter the body, including anaerobic glycolysis of glucose, aerobic oxidation, synthesis and decomposition of glycogen, and gluconeogenesis.111,112 Abnormal glucose metabolism and insulin resistance might cause metabolic diseases such as diabetes.113,114,115 The roles of SIRTs in glucose metabolism have been established. For example, SIRT1 is a key positive regulator of systemic insulin sensitivity and regulates pancreatic insulin secretion, thus contributing to increased systemic insulin sensitivity, which triggers glucose uptake and utilization.116,117,118 Mechanistically, SIRT1 participates in the regulation of glucose metabolism by upregulating AMPK, and activation of AMPK can ameliorate the glucose metabolic imbalance.116,119 Upregulated SIRT1 may reverse the development of diabetes by targeting the AMPK/acetyl CoA carboxylase signaling pathway.117 Similarly, decreased levels of SIRT1 may lead to AMPK deficiency, thereby impairing the improvement in glucose tolerance.119 Meanwhile, there are an interdependent relationship between AMPK and SIRT1,120,121 and activation of SIRT1 and its downstream signaling pathways could also be improperly triggered in AMPK-deficient states.121 Additionally, SIRT1 increases insulin sensitivity and lowers blood sugar by downregulating protein tyrosine phosphatase 1B, a key negative regulatory protein in the insulin signal transduction pathway.118 Thus, high expression of SIRT1 is benefit for maintaining blood sugar stability via the regulatory proteins of insulin signaling. However, the relationship between SIRT1 and other molecules (e.g., AMPK and protein tyrosine phosphatase 1B) that are closely associated with blood glucose regulation is still worth further exploration.
SIRT1, SIRT3, and SIRT6 also participate in glucose metabolism. The limited whole-body benefit of increasing hepatic SIRT3 during the development of diet-induced insulin resistance, which can be considered a pre-diabetic state, has also been demonstrated.122 Mechanistically, SIRT3 negatively regulates aerobic glycolysis by inhibiting hypoxia-inducible factor 1α (HIF-1α).123 SIRT6 takes part in the maintenance of glucose metabolic homeostasis in the whole body and in local tissues such as liver and skeletal muscle.124,125 For instance, SIRT6 in pancreatic β cells deacetylated FoxO1 and subsequently increased the expression of glucose-dependent transporter 2 to maintain the glucose-sensing ability of pancreatic β cells and systemic glucose tolerance.126 Improvement in SIRT6-mediated insulin signaling transduction has been reported in the liver of obese rats after exercise.127 Also, enhancement of insulin sensitivity in skeletal muscle and liver by physiological overexpression of SIRT6 has been described,128 suggesting potential functions of SIRT6 in glucose metabolism.
Finally, direct and indirect involvement of SIRTs in glucose metabolism may provide new insights into therapeutic targets for the treatment of abnormal glucose metabolism in the future. This may help reduce the human disease burden related to glucose metabolism, where SIRT proteins may play an important role in overcoming glucose metabolic diseases at an earlier time point.
The effect of SIRTs on lipid metabolism
Lipid metabolism means that most of the fat ingested by the human body is emulsified into small particles by bile, and the lipase secreted in the pancreas and small intestine hydrolyzes the fatty acids in the fat into free fatty acids, after which hydrolyzed small molecules are absorbed by the small intestine into the bloodstream.104,105,129 Notably, the SIRT protein family is involved in lipid metabolism.129,130 For SIRT1, Qiang et al. found that SIRT1-dependent cAMP Response Element Binding protein (Creb) deacetylation regulates lipid metabolism.131 Mechanistically, Lys136 is a substrate for SIRT1-dependent deacetylation that affects Creb activity by preventing cyclic adenosine monophosphate (cAMP)-dependent phosphorylation, leading to the promotion of hepatic lipid accumulation and secretion. Moreover, SIRT1 activates AMPK, which leads to lipid-lowering effects in vitro and in vivo.132 SIRT2 prevents liver steatosis and lipid metabolic disorders by deacetylation of hepatocyte nuclear factor 4α.133 Additionally, SIRT3 acts as a bridge in the lipid metabolism pathway. For example, pancreatic SIRT3 deficiency promoted hepatic steatosis by enhancing 5-hydroxytryptamine synthesis in mice with diet-induced obesity.134 In addition, roles for SIRT5 and SIRT6 were identified in lipid metabolism.135,136,137,138 For instance, SIRT5 inhibited preadipocyte differentiation and lipid deposition by activating AMPK and repressing mitogen-activated protein kinase (MAPK) signaling pathways, which has been verified in obese mice.135 Compared with control wild-type mice, SIRT6-KO mice had a significant increase in both body weight and fat mass and exhibited glucose intolerance and insulin resistance.138 Mechanistically, SIRT6-KO decreased expression of the adiponectin gene and Akt in white adipose tissue, while expression of the thermogenic gene UCP1 was diminished in brown adipose tissue.138
The effect of SIRTs on other metabolism
SIRT3 and SIRT4 have been found to play roles in regulating glutamine metabolism. In detail, Gonzalez-Herrera et al. reported that loss of SIRT3 promoted glutamine use in nucleotide biosynthesis.139 Conversely, SIRT4 inhibited glutamine metabolism in colorectal cancer cells, thereby acting as a tumor suppressor.140 In addition, SIRT3 affected mitochondrial metabolic reprogramming by activating the AMPK/peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) pathway, thereby maintaining the stability of mitochondrial membrane potential as well as mitochondrial structure.141 Moreover, silencing SIRT6 influenced collagen metabolism in human dermal fibroblasts by affecting the synthesis and degradation of collagen.142
As shown in the previous findings, SIRT1, SIRT3, and SIRT6 have been more frequently studied than other SIRTs in regulating human body metabolism, mainly through their effect on glucose and lipid metabolism. However, only a few studies have focused on the roles of other SIRT proteins, in particular SIRT2 and SIRT7. In the future, research should be focused on the role of these other SIRTs in regulating different metabolism subtypes. Overall, clarifying the various participating mechanisms of SIRTs in metabolism might provide future new ideas for research and novel therapeutic targets for the treatment of abnormal metabolism, thereby lessening the burden imposed on society by human lipid metabolism-related diseases.
The role of SIRTs in oxidative stress
Oxidative stress is considered to be an important factor in cell damage and is usually caused by the overproduction of ROS. Under physiological conditions, ROS are produced at low levels and are scavenged by the endogenous antioxidant system. When ROS exceed the scavenging capacity, however, cellular oxidative stress damage occurs.143 Oxidative stress plays an important role in the pathological process of various diseases.144 Recently, accumulating studies have shown that the SIRT protein family participates in the process of oxidative stress. Notably, SIRT proteins contribute to cellular tolerance to oxidative stress by regulating many genes and their related signaling pathways (as shown in Fig. 4). Herein, we review the regulation of different target genes or proteins by SIRTs, with the aim of understanding their mechanistic effects in the process of antioxidant stress damage.
The interaction between SIRT1, SIRT3, SIRT6 and AMPK
AMPK, is a major regulator of metabolic homeostasis and is often activated under oxidative stress conditions such as ischemia and hypoxia.145 SIRT1 participates in regulating AMPK and its related pathways. For example, AMPK can be activated by liver kinase B1 (LKB1), the upstream regulator of AMPK, while activated AMPK reduces oxidative stress injury by promoting insulin sensitivity, fatty acid oxidation and mitochondrial biosynthesis to generate ATP.146 SIRT1 overexpression leads to the deacetylation of LKB1, the translocation of LKB1 from the nucleus to the cytoplasm, and the activation of AMPK to alleviate oxidative stress.147 Additionally, SIRT1 lowers LKB1 activation in the liver, which subsequently abrogates Thr172-AMPKα phosphorylation, thereby increasing oxidative stress in severe acute hypoxia.148 It can be seen that SIRT1 may activate AMPK by regulating LKB1, thereby resisting oxidative stress damage and promoting cell survival.
In addition to the role of SIRT1 on AMPK, SIRT3 and SIRT6 can also interact with AMPK to exert an anti-oxidative effect on stress injury. Deficiency of AMPKα resulted in elevated expression of SIRT3, which modulated oxidative stress in heart tissue both in vitro and in vivo .149 It has also been shown that the AMPK activated SIRT3, limited oxidative stress and improved mitochondrial DNA integrity and function.150 In addition, SIRT3 reduced ROS and lipid peroxidation by improving mitochondrial function via deacetylation of LKB1 and activation of AMPK.151 As previously mentioned, a feedback loop may exist between AMPK and SIRT3. SIRT6 also promoted AMPK expression, thus upregulating antioxidant-encoding gene expression of manganese superoxide dismutase (MnSOD) and catalase (CAT), thereby suppressing oxidative stress.152 In brief, SIRT1, SIRT3 and SIRT6 act to counter oxidative stress by directly or indirectly interacting with AMPK. However, additional studies are required to clarify the relationship between other SIRT proteins and AMPK pathway under oxidative stress.
The effect of SIRT1, SIRT 2, and SIRT6 on Nuclear erythroid 2-related factor 2 (Nrf2)
Nrf2 is a leucine transcription factor that plays extremely important roles in antioxidant response element (ARE)-dependent transcriptional regulation of defense genes. When stimulated, Nrf2 dissociates from suppressor protein Keap1 in the nucleus and interacts with AREs to regulate the expression of antioxidant genes, suggesting a close association between Nrf2 and oxidative stress.153 Notably, SIRTs including SIRT1, SIRT2 and SIRT6 can activate Nrf2, regulate antioxidant gene expression, and thus fight oxidative stress damage. For example, SIRT1 activated Nrf2 by changing the structure of Keap1, leading to Nrf2 nuclear transfer and promoting the expression of antioxidant genes, such as glutathione S transferase and glucuronyl transferase.154,155 In addition, SIRT2 was downregulated in the spinal cord after peripheral nerve injury, which subsequently inhibited Nrf2 activity, leading to increased oxidative stress.156 The overexpression of SIRT6 in the brain through in vivo gene transfer enhanced Nrf2 signaling and reduced oxidative stress.157,158 SIRT6 protected human lens epithelial cells from oxidative damage via activation of Nrf2 signaling.159 Furthermore, SIRT6 protects cells against hydrogen peroxide-induced oxidative stress by promoting Nrf2/ARE signaling.160 Therefore, SIRTs can activate Nrf2, regulate antioxidant gene expression, and thus fight oxidative stress damage.
The effect of SIRT1 and SIRT3 on FoxOs
A family of SIRT targets are class O mammalian forkhead transcription factors (FoxO1, FoxO3, FoxO4 and FoxO6) which participate in regulating oxidative stress. FoxO1 can scavenge excessive ROS through the regulation of downstream target genes such as MnSOD and CAT, and thus reduce cellular oxidative stress damage. SIRT1 alleviates oxidative stress by controlling nuclear shuttling and transcriptional activity of FoxO1 and FoxO3a. For instance, SIRT1 induced the transfer of FoxO1 to the nucleus and increased the level of FoxO1 protein in adipocytes, reducing the production of ROS and oxidative stress.161 Moreover, SIRT1 promoted early-onset age-related hearing loss by suppressing FoxO3a-mediated oxidative stress resistance in vivo.162 Apart from SIRT1, SIRT3 has also been shown to participate in the regulation of oxidative stress via FoxO3.163,164 Mechanistically, SIRT3 activated FoxO3 gene expression, which increased transcription of MnSOD and CAT, enabling the elimination of ROS.165,166 The aforementioned studies show that SIRT1 and SIRT3 can interact with FoxOs to counteract oxidative stress.
The effect of SIRT1 and SIRT3 on PGC-1α
PGC-1α is a coactivator of peroxisome proliferator-activated receptor-γ, which can act to block oxidative stress damage by scavenging excess ROS, inducing antioxidant enzyme expression and maintaining mitochondrial function.167 SIRT1 can activate PGC-1α through deacetylation, scavenge ROS caused by oxidative stress, and alleviate oxidative stress injury. Activation of the SIRT1-PGC-1α axis implies activation of antioxidant defense mechanisms, alleviating mitochondrial oxidative stress.168,169,170 Additionally, PGC-1α and SIRT3 can interact directly. PGC-1α increased respiratory capacity and reduced oxidative stress through SIRT3-mediated reduction of mitochondrial ROS.171,172 Furthermore, loss of SIRT3 resulted in the expression of PGC-1α, which produced a decrease in mitochondrial respiration. Inhibition of SIRT3 reduced PGC-1α expression and mitochondrial function, thereby lowering oxidative stress resistance.173,174 Thus, both SIRT1 and SIRT3 may interact with PGC-1α in order to resist oxidative stress damage.
The effect of SIRT1 and SIRT6 on p53
p53 is a stress response transcription factor and was the earliest discovered physiological substrate of SIRT1. p53 can promote oxidative stress injury by regulating different target proteins and further induce cellular responses.175 p53 exerted pro-oxidant activity and promoted oxidative damage by regulating its transcriptional targets, including p53-inducible gene 3, glutathione/NADH, p-FoxO3a and B-cell lymphoma -2-associated-X-protein (Bax).176 In contrast, p53 can act as an antioxidant factor to suppress oxidative stress by regulating several redox-related proteins, such as MnSOD, glutathione peroxidase 1, and Jun N-terminal kinase (JNK).176 When cells are under oxidative stress, multiple sites in the N-terminal of p53 are phosphorylated and multiple lysine sites in the C-terminal are acetylated.177 SIRT1 has a negative regulatory effect on p53; for example, depletion of SIRT1 abolished the increase in oxidative stress induced by p53 acetylation in THP-1 cells.178 SIRT1 activation also reversed p53 expression and accumulation brought on by H2O2-induced oxidative stress.179 The small molecule activator SRT2104 enhanced renal SIRT1 expression and activity and deacetylated p53, resulting in activation of antioxidant signaling.180 As for the role of SIRT6 in oxidative stress, relevant studies have been limited. For instance, SIRT6 protected cardiomyocytes by inhibiting p53/Fas-dependent cell death and augmenting endogenous antioxidant defense mechanisms.181 Hence, SIRT1 and SIRT6 can inhibit p53 activity through deacetylation and reduce oxidative factor expression, promoting resistance to oxidative stress injury.
The effect of SIRT1, SIRT3, and SIRT6 on NF-κB
NF-κB is a nuclear transcription factor. Activated NF-κB factors promote the production of ROS that damage tissues and organs.182 When oxidative stress occurs, enhanced ROS activity can stimulate the activation of NF-κB and induce the expression of ICAM-1 and monocyte chemotactic factor 1, which further activate NF-κB and lead to oxidative stress.183 SIRTs inhibited transcription by deacetylating the NF-κB subunit Rel/p65, reducing the production of oxygen radicals.79 SIRT1, SIRT3 and SIRT6 inhibited the transcriptional activity of NF-κB through deacetylation, thereby resisting oxidative stress injury. For example, downregulation of SIRT1 protein levels by NF-κB led to oxidative stress.184 In addition, SIRT3 regulated ROS generation, causing suppression of NF-κB activation and oxygen radicals.185 Moreover, loss of SIRT6 in cutaneous wounds aggravated the proinflammatory response by increasing NF-κB activation and promoting oxidative stress.186 Therefore, SIRT1, SIRT3, and SIRT6 can block oxidative stress damage by inhibiting NF-κB activity.
The effect of SIRTs on oxidative stress through other pathways
Many molecules are upstream regulators of SIRTs and have a regulatory effect on them under oxidative stress. For example, the expression of SIRT1 and SIRT6 was decreased by oxidative stress-dependent miR-34a activation in epithelial cells.187 SIRT5 was upregulated by Krüppel-like factor (KLF) 6 silencing, thereby reducing oxidative stress.188 Meanwhile, SIRTs target many downstream factors, such as HIF-1α and endothelial nitric oxide synthase (eNOS), and then participate in regulating oxidative stress. Activation of HIF-1α is associated with oxidative stress and can regulate ROS formation through direct or indirect effects.189 For example, SIRT4 reduced the accumulation of ROS by inhibiting HIF-1α, which is also an important mechanism underlying SIRT4 activity in oxidative stress.190,191 In addition, eNOS dysfunction in an oxidative stress environment led to increased generation of ROS. SIRTs play important roles in regulating the activity of eNOS as well. For instance, upregulation of SIRT1 reduced eNOS acetylation (inactive state) and enhanced eNOS phosphorylation (active state).192 Activation of the SIRT1/eNOS pathway has been found to reduce ROS production by inhibiting NF-κB expression.193 In brief, the mechanisms by which SIRTs regulate oxidative stress are diverse, and there are many more regulatory pathways that need to be verified.
Together, these aforementioned studies reflect the importance of the SIRT protein family in oxidative stress and can be expected to stimulate future research in order to decipher the SIRT protein mechanisms. As summarized in Fig. 4, SIRTs are involved in the regulation of redox homeostasis and oxidative stress involving many key genes and molecules. Indeed, SIRTs play important roles in maintaining intracellular homeostasis which keeps cells healthy, making them ideal for redox regulation studies. Additionally, SIRTs enhance intracellular homeostasis by acting synergistically through different mechanisms.
Further in-depth studies are needed to identify and elucidate the exact role of each SIRT and to determine whether different SIRTs have functional redundancy or overlapping roles in homeostasis, which may be important for regulating oxidative stress in cells and important pathological manifestations. SIRTs should be developed as modulators of redox-related diseases, and may also provide a mechanistic basis for the development and discovery of antioxidants. Given the interest in SIRTs as drug targets and their redox importance, studies addressing these questions may also provide therapeutic opportunities for the treatment of metabolic, age-related and other redox-related diseases.
The role of SIRTs in cell apoptosis
Cell apoptosis is an active form of cell death that involves programmed cellular machineries leading to progressive self-destruction of the cell.194 As a type of programmed cell death, apoptosis is a basic cellular mechanism and may occur in numerous diseases. Notably, one of the most extensive biological functions regarding the SIRT protein family is participation in the process of cell apoptosis. The SIRT protein family has functions in both physiological conditions and diseases by regulating the acetylation modification and/or influencing various apoptosis-related proteins by pathway crosstalk, and thus takes part in the pathogenesis of many diseases including cancer, CVDs and others (Fig. 5).195,196
The effect of SIRTs as histone deacetylases on apoptosis
Histones are the major protein components of chromatin, serve as spools around which DNA is wound, and play roles in gene regulation. The SIRT family-dependent epigenetic regulation of histone acetylation is an important link in the regulation of apoptosis.197 For example, SIRT1 can reduce the acetylation levels of histones in the promoters of genes, e.g., AR, BReast-CAncer susceptibility gene 1(BRCA1), ERS1, ERS2, EZH2 and EP300, which ultimately affected cancer cell apoptosis.197 Additionally, SIRT6 links histone H3K9 deacetylation to NF-κB-dependent gene expression and organismal life span.80 At the molecular level, SIRT6 binds to the promoters of extracellular signal-regulated kinase (ERK) 1 and ERK2 genes, and deacetylates histone H3K9, thereby inhibiting ERK1/2 expression.198 Moreover, SIRT6 induced the expression of GATA binding protein 5 (GATA5) through inhibition of Nkx3.2 transcription by deacetylating histone H3K9, thereby regulating GATA5-mediated signaling pathways to prevent endothelial injury.199 These studies have demonstrated the critical role of the SIRT protein family in regulating apoptosis. However, additional studies have found that the SIRT protein family regulates other novel modification types of histones, for example, sumoylation200 and ubiquitination.201 Whether these new types of histone modification participate in cell apoptosis remains largely unknown, which may be a new direction for further research.
The effect of SIRT1 on apoptosis by targeting apoptosis-related proteins and pathways
Among the SIRT protein family, SIRT1 is the most widely studied protein, especially in regulating cell apoptosis. A variety of transcription factors, including p53, NF-κB and FoxO, which act downstream of SIRT1, are closely related to cell apoptosis.103,202,203,204 Therefore, we focus here on how SIRT1 participates in regulating these three proteins and their related pathways.
SIRT1 mediates p53-dependent apoptosis by suppressing acetylated p53
As first discovered with non-histone targets of SIRT1, p53 plays a central role in the prevalence of diseases related to apoptosis.205,206,207 SIRT1 regulates p53 deacetylation, which is associated with the apoptosis-inhibiting signaling pathway, mainly including the p53-induced death domain protein Pidd,208 p21, Bax/Bad and caspases.209 For example, Zeng et al. reported that an extract of Anoectochilus roxburghii flavonoids reduced neuron apoptosis by positively regulating SIRT1 expression, thereby reducing expression of the apoptosis-related molecules p53, p21 and caspase-3, while increasing the ratio of B-cell lymphoma (Bcl)-2/Bax.210 SIRT1 also participated in the regulation of p53 protein through direct deacetylation. For example, SIRT1 deacetylating p53 at Lys379 inhibited p53-dependent apoptosis.211 In addition, SIRT1 can regulate the p53 signaling pathway by targeting proteins. The overexpression of SIRT1 resulted in markedly reduced mRNA and protein expression levels of p53 signaling pathway-related molecules (including p53 and Bax) in vitro, but increased Bcl mRNA and protein expression.212 p53 expression gradually decreased with increasing SIRT1 levels, thus indicating a gradual decrease in apoptosis.213 These findings thus show that SIRT1 inhibits apoptosis via inactivation of p53, suggesting a critical role for SIRT1 in regulating the p53 signaling pathway.
The SIRT1/NF-κB pathway is mainly involved in inflammation-induced apoptosis
Regarding the mechanism underlying SIRT1 involvement in apoptosis, NF-κB (p65) acetylation was significantly increased after inhibition/deletion of SIRT1.214 A large number of studies have shown that SIRT1 mediates NF-κB pathway modulation to mitigate inflammasome signaling and cellular apoptosis.203,214,215 For example, SIRT1 overexpression promoted mouse B lymphocytes cell proliferation, inhibited apoptosis, and upregulated pro-inflammatory cytokines by inhibiting the NF-κB pathway.216 Additionally, activating the NF-κB signaling pathway could ultimately induce apoptosis through regulation of the inflammatory process.217 Silencing interferon regulatory factor 9 curbed activity of the NF-κB signaling pathway by upregulating SIRT1, which further inhibited TNF-α induced changes in inflammatory cytokine secretion and promoted apoptosis.218 Therefore, it appears to be a double-edged sword that SIRT1 regulates NF-κB signaling to affect cellular inflammatory activation and apoptosis in different spatiotemporal dependencies.
SIRT1 regulates apoptosis by the regulation of FoxOs
FoxO transcription factors can control cell survival by regulating the expression of genes involved in cell-cycle progression and promoting apoptosis.219 SIRT1 is a key regulator of cell defenses and survival in response to stress, which deacetylates and represses FoxO-dependent apoptosis.219,220 SIRT1 mediates cell apoptosis through the deacetylation of FoxO proteins including FoxO1,221 and upregulation of SIRT1 can inhibit apoptosis via the FoxO1/β-catenin pathway.222 Moreover, SIRT1, FoxO1, and sterol regulatory element binding protein-1 (SREBP-1) may act as a pathway and play crucial roles in apoptosis. At both the protein and mRNA levels, SIRT1 and SREBP-1 were upregulated in progestin-resistant cells, while FoxO1 was downregulated.223 Interestingly, SIRT1 may be a potential target for cross-regulation of post-transcriptional modifications. For example, acetylation was required for FoxO3-induced apoptosis through phosphorylated-FoxO3 (p-FoxO3) formation, while expression or activation of SIRT1 blocked p-FoxO3 formation and apoptosis.224 Deacetylation of FoxO3 by SIRT1 resulted in S-phase kinase-associated protein 2-mediated FoxO3 ubiquitination and degradation.225 These fine-tuning mechanisms of FoxO3 regulation modulated by PTMs may be a new method to regulate apoptosis in a coordinated manner. In summary, then, SIRT1 can regulate the activity of FoxO, thereby modulating the balance between anti-apoptotic and apoptotic genes.226
miRNAs play important roles in the regulation of SIRT1
miRNAs, a subtype of non-coding RNAs, are small endogenous RNAs which can inhibit protein translation in apoptosis.227 Moreover, SIRT1 has been revealed to be targeted by miRNAs such as miR-34a, miR-181, miR-128, miR-449 and miR-30a-5p. For example, Yamakuchi et al. demonstrated a negative correlation between the expression of miR-34a and SIRT1, suggesting SIRT1 was a target of miR-34a.228 In addition, SIRT1 is a key player in the protection provided by miR-34a-5p inhibition during apoptosis.229 The overexpression of miR-181d-5p inhibited cell apoptosis and renal fibrosis in a mouse model by downregulating the SIRT1/p53 pathway.230 Furthermore, miR-181a increased FoxO1 acetylation and promoted granulosa cell apoptosis via SIRT1 downregulation.231 The previous study also suggested that miR-128 promoted apoptosis in human cancers via the p53/Bak axis.232 Upregulation of miR-128 promoted apoptosis in an epilepsy model in vivo and in vitro through the SIRT1/p53/Bax/cytochrome c/caspase signaling pathway.233 Other miRNAs, such as miR-449, have been investigated in a model of acute kidney injury model by detecting expression of its target SIRT1 and downstream factors p53/Bax.234 Inhibition of miR-449 elevated SIRT1 expression and inhibited acetylated p53 and Bax protein levels.234 Finally, miR-30a-5p targeted SIRT1 to activate the NF-κB/NLRP3 signaling pathway, resulting in cardiomyocyte apoptosis.227 These studies all demonstrate how miRNAs play important roles in the regulation of SIRT1, which should be further studied in various diseases in the future.
Other regulatory molecules or factors acting on SIRT1
Upstream of SIRT1, in addition to miRNAs, a novel fibroblast growth factor 1 variant could counteract adriamycin-induced apoptosis by decreasing p53 activity via upregulation of SIRT1-mediated p53 deacetylation.235 There have also been a series of studies on the anti-apoptotic effect of melatonin which regulates SIRT1 in various physiological processes.236,237,238,239 Additionally, some chemicals or drugs, like cambinol and ginsenoside Rc, have been shown to inhibit or activate SIRT1 to regulate the apoptotic process.240,241 Given that the above-mentioned molecules can regulate SIRT1-related signaling pathways, SIRT1 may be a potential therapeutic target in the apoptotic response.
The effect of SIRT2 on apoptosis
Several previous studies have suggested that SIRT2 has complex regulating mechanisms promoting or inhibiting apoptosis.242 In contrast to SIRT1, SIRT2 is predominantly a cytoplasmic protein and is able to deacetylate several cytoplasmic substrates,243 including p53,244 NF-κB,245 and FoxO3.246 In terms of its anti-apoptotic effects, SIRT2 downregulation alone is sufficient to cause apoptosis, and SIRT2 depletion leads to p53 accumulation causing activation of the p38 MAPK in cancer cell lines such as HeLa, but not in normal cells.247
On the other hand, SIRT2 can promote apoptosis mediated by the caspase, Bcl2/Bax and FoxO pathways. For example, She et al. demonstrated that the SIRT2 inhibitor AGK2 effectively reduced the levels of phospho-JNK and FoxO3a.248 As JNK is a well-known regulator of apoptosis, protein downregulation will lead to attenuation of the subsequent signaling cascade involving Bim, and eventually leads to suppression of the caspase cascade.248 In addition, SIRT2 overexpression induces cellular apoptosis via upregulating cleaved caspase 3 and Bax and downregulating anti-apoptotic protein Bcl-2,245 suggesting the important role of SIRT2 in apoptosis. As for the FoxO-related pathway, FoxO3a, which is the immediate downstream target for SIRT2-driven deacetylation, is a promoter of apoptotic pathways in many diseases.246,249 SIRT2 activates FoxO3a by deacetylating it, which promotes the activation of the pro-apoptotic pathways Akt/FoxO3a and JNK, and thus increases apoptosis. Additionally, the administration of specific inhibitors of SIRT2 attenuates neuronal cell death under ischemic conditions in vitro and in vivo.248 The confusing role of SIRT2 in the process of apoptosis might thus be attributed to regulation of different pathways affected by different conditions, but more studies verifying SIRT2 functions in apoptosis will be needed in the future.
The effect of SIRT3-7 on apoptosis
The critical roles of SIRT3-5 in regulating cell apoptosis
Three SIRT proteins, namely SIRT3–5, are localized to the mitochondrion, a dynamic organelle that functions as the primary site of endogenous apoptosis. Although mitochondrial SIRT proteins have not been as extensively studied as SIRT1, a growing body of studies have illustrated their importance in basic mitochondrial biology and apoptosis.
SIRT3 plays a pro-apoptotic role in that glycogen synthase kinase-3 β (GSK-3β)/Bax, Bax/Bcl-2 and bad/Bcl-x/L ratios regulate apoptosis.250,251 SIRT3 overexpression promoted apoptosis by enhancing caspase 9 cleavage in hepatocellular carcinoma (HCC) cells,252 and SIRT3 depletion downregulated cleaved caspase 3 levels in lung cancer (LC) cells.253 In contrast, several studies have found that SIRT3 has an anti-apoptotic effect. SIRT3 deficiency resulted in significantly increased apoptosis, increased Bax and caspase 3 mRNA levels, and decreased Bcl-2 mRNA levels in septic mice,254 and also significantly increased caspase 3 expression in SIRT3-KO mice. Thus, SIRT3 plays different roles in different diseases, both pro- and anti-apoptotic. A typical example is when SIRT3 expression inhibited the growth of cancer cells by promoting apoptosis and necroptosis. In a stress injury disease model, SIRT3 inhibited apoptosis and exerted a protective effect against various stressors. For example, SIRT3 deficiency produced more melanocyte apoptosis by inducing severe mitochondrial dysfunction and cytochrome c release into the cytoplasm.255 However, more research is needed in the future to determine whether SIRT3 promotes or inhibits apoptosis of the caspase 3 pathway in different types of diseases.
FoxO transcription factors are downstream targets of the serine/threonine protein kinase B/Akt, which promotes apoptosis signaling by affecting multiple mitochondria-targeting proteins.256 SIRT3 acetylation modulated FoxO1 and exerted apoptotic effects.51 In addition, SIRT3 post-translationally upregulated FoxO3a activity through deacetylation, dephosphorylation and deubiquitination to regulate apoptosis.257 Meanwhile, non-coding RNAs act as upstream regulators of SIRT3 to regulate apoptosis. For example, the miR-297 antagomir affected apoptosis by targeting SIRT3 to reduce the extent of IκBα and NF-κB phosphorylation and prevent activation of NLRP3.258 A similar study confirmed that SIRT3 was also a target of miR-421.259 Studies of the upstream and downstream regulatory mechanisms of SIRT3 regulating apoptosis are few and more research will be required in this area.
There are only limited studies on SIRT4 and cell apoptosis, but these few have indicated that SIRT4 prevents apoptosis by affecting the ratio of pro-caspase 9/caspase 9 or pro-caspase 3/caspase 3, and by altering Bax translocation.191,260 In addition, SIRT5 participates in the regulation of apoptosis as a deacetylated protein and may have an effect on apoptosis-related proteins. For example, SIRT5 deacetylated cytochrome c, a protein of the mitochondrial intermembrane space with a central function in oxidative metabolism as well as in apoptosis initiation.261 SIRT5 overexpression ameliorated cytochrome c leakage and activation of caspase 3 to alleviate apoptosis.262,263 Thus, these data implicate mitochondrial SIRTs as effective in protecting against pathological injury and apoptosis by inhibiting the cytochrome c/caspase 3 apoptosis pathway. Such research may form the basis for future treatment for apoptosis. However, the number of related studies on SIRT4 and SIRT5 is still limited and need to be expanded.
The role of SIRT6 and SIRT7 during apoptosis
At present, only a few studies have explored the role of SIRT6 and SIRT7, which could be a new research direction for the SIRT protein family. Both SIRT6 and SIRT7 mediate apoptosis by regulating p53.264,265 Furthermore, SIRT7 promoted cellular survival following genomic stress by attenuation of DNA damage and the p53 response.266 However, current studies on SIRT6 and SIRT7 are still in their infancy, and more research is needed in the future to explore their role in apoptosis.
In conclusion, one of the most extensive biological functions of the SIRT protein family is to participate in the process of apoptosis. As a family of bidirectional regulatory proteins, the function of SIRTs appears to be reversible depending on the cellular state. However, our current knowledge of SIRTs in apoptosis and its regulation is far from complete. More studies are needed in the future to explore the underlying molecular mechanisms of how the SIRT protein family is regulated in pathophysiological processes.
The role of SIRTs in autophagy
Autophagy is a cell self-digestion process via lysosomes that clears cellular waste, including aberrantly modified proteins or protein aggregates and damaged organelles.267 Recent studies have illustrated the important roles of the SIRT protein family in the autophagic process. Therefore, in this section, we aim to review recent research on the relationship between the SIRT protein family and autophagy, and discuss possible regulatory roles of SIRT proteins in autophagy, as well as the conditions under which they participate in autophagy in a positive or negative manner (Fig. 6).
The effect of SIRT1 on autophagy
SIRT1 regulates autophagy through deacetylation
SIRT proteins affect protein acetylation level, and this modification is closely involved in autophagy. There are complex roles for SIRT1-related deacetylation in the regulation of autophagy.268,269 For example, SIRT1 deacetylates autophagy-related proteins (such as Beclin-1 and microtubule-associated protein light chain 3 (LC3)) to promote autophagy. Deacetylation of Beclin-1 lysine residue by SIRT1 impairs autophagic flux; thus, autophagosome fusion with lysosomes is compromised.270,271 SIRT1 promotes autophagy of cancer cells by reducing acetylation of LC3.272 LC3 and autophagy related (Atg)7 deacetylation is disrupted in germ-cell-specific SIRT1 KO mice, which affects the redistribution of LC3 from the nucleus to the cytoplasm and activation of autophagy.273 Suppression of SIRT1 enhances acetylation level of unc-51 like kinase 1 (ULK1) and induces ROS-dependent autophagy.274 Therefore, SIRT1 could directly regulate autophagy through deacetylation of autophagic proteins.
SIRT1 regulates autophagy via deacetylation of autophagy-related proteins as well as through deacetylation of mitochondrial proteins.275 Mitochondrial proteins participate in the process of mitophagy; a selective autophagic process that is critical for cellular homeostasis and eliminates dysfunctional mitochondria.276 For example, induction of autophagy by SIRT1/HIF-1α activation is a novel therapeutic option for peripheral nerve injury.277 SIRT1 activity is involved in mitochondrial biogenesis through PGC-1α and participates in the balance of autophagy regulatory proteins.278 Mitofusins2 (MFN2) is a mitochondrial fusion factor and increasing evidence has shown that it is involved in the regulation of autophagy.279 For example, MFN2 is deacetylated by SIRT1, and loss of SIRT1 causes a sequential chain of defective autophagy in an MFN2-dependent manner.280 Mechanistically, SIRT1 deacetylates K655 and K662 residues at the C terminus of MFN2, leading to autophagy activation.281
In conclusion, SIRT1 acts on autophagy-related proteins and transcriptional factors mainly through modification of acetylation, and affects the occurrence or degradation of autophagosomes. However, there have been limited studies on other PTMs of SIRT1, and more research is needed to explore the regulatory mechanism of SIRT1 in the future.
Upstream and downstream signaling pathway of SIRT1 in autophagy
AMPK is an evolutionarily conserved serine/threonine-protein kinase. Under various physiological and pathological conditions, AMPK acts as an activator of SIRT1 and is involved in the regulation of autophagy. For example, inositol polyphosphate multi kinase enhances autophagy-related transcription by stimulating AMPK-dependent SIRT1 activation.282 AMPK can also be activated as a downstream molecule of SIRT1. SIRT1 promoted autophagy via AMPK activation.283 Autophagy impairment is mediated by downregulation of SIRT1/FoxO3a/AMPK/ peroxisome proliferators-activated receptors (PPAR)-α signaling.284 The SIRT1 activator resveratrol increases cAMP content, expression of protein kinase A, as well as the activity of AMPK. Besides, resveratrol pretreatment reduces tumor necrosis factor α-induced inflammation and increases LC3B expression and sequestosome 1(SQSTM1)/p62 degradation in a concentration-dependent manner.285 Activation of the AMPK/SIRT1 pathway alleviates cell damage and promotes autophagic flux via downregulation of p62.286 Therefore, SIRT1 recognizes resveratrol-induced autophagy in vitro and in vivo via the cAMP/phosphorylated protein kinase A (PRKA)/AMPK/SIRT1 signaling pathway.287,288 AMPK acts as an upstream molecule to regulate expression of SIRT1 active agent. SIRT1 affects autophagy by binding to molecules directly. SIRT1 forms a molecular complex with Atg5, Atg7 and Atg8, and transiently increased expression of SIRT1 is sufficient to stimulate basal rates of autophagy.289 SIRT1 interacts with the Cullin 4B-Ring E3 ligase complex, which promotes autophagy of cancer cells.290 In conclusion, these molecules play important roles as the upstream or downstream of SIRT1 in the process of autophagy, and affect the occurrence and development of diseases.
Noncoding RNAs in SIRT1-regulated autophagy
A variety of miRNAs have been found to affect autophagy by directly regulating expression of SIRT1. For example, miR-124 and miR-142 represses autophagy via targeting SIRT1 in cancer cells.291 Silencing of miR-150-5p increases autophagy by targeting the SIRT1/p53/AMPK pathway.292 miR-138-5p affects insulin resistance through inducing autophagy in HepG2 cells by regulating SIRT1, and overexpression of SIRT1 increases Beclin-1 and LC3 II/I levels, and the number of green fluorescent protein-LC3 dots, and decreases p62 level.293 miR-145 inhibition upregulates SIRT1 and attenuates autophagy via NF-κB-dependent Beclin-1.294
Both long noncoding RNAs (lncRNAs) and circular RNAs (circRNAs) modulate autophagy associated with SIRT1. For instance, lncRNA metastasis-associated lung adenocarcinoma transcript 1 enhances ox- low-density lipoprotein (LDL)-induced autophagy through the SIRT1/MAPK/NF-κB pathway.295 lncRNA growth arrest specific 5 inhibits macroautophagy and forms a negative feedback regulatory loop with the miR-34a/SIRT1/mammalian target of rapamycin (mTOR) pathway.296
In conclusion, SIRT1 is a key regulator of the autophagic process. Through its deacetylase activity, SIRT1 is involved in the regulation of different autophagic proteins from initiation to degradation. The level and function of SIRT1 are also regulated by many signaling pathways, such as AMPK. Some studies have shown the regulation of SIRT1 by ncRNAs. SIRT1-mediated autophagic dysregulation leads to progression of various diseases. In the future, we need more research evidence to improve and supplement the mechanism of SIRT1.
Effect of SIRT2 on autophagy
It has been indicated that SIRT2 controls the functional ability of the autophagic system through acetylation.297 Genetic manipulation of SIRT2 levels in vitro and in vivo modulates the levels of α-synuclein acetylation, its aggregation, and autophagy.298 SIRT2 loss of function either with AK1 (a specific SIRT2 inhibitor) or by SIRT2 KO recovers microtubule stabilization and improves autophagy.299 Additionally, SIRT2 directly binds to the 3'UTR of transcription factor EB and facilitates its mRNA stability. Transcription factor EB is a key transcription factor involved in the regulation of many lysosome-related genes and plays a critical role in the fusion of autophagosomes and lysosomes, indicating that SIRT2 modulates autophagic components.300 Although the precise mechanism is unresolved, SIRT2 plays a key role in regulating autophagy in certain diseases, and more research is needed.
Effect of SIRT3–5 on autophagy
As mitochondrial SIRTs (mtSIRTs) members, SIRT3–5 are all involved in regulating energy metabolism and metabolic homeostasis through regulation of mitophagy.301,302 SIRT3 regulates autophagy by activating different downstream signaling pathways. For example, overexpression of SIRT3 activates macroautophagy through activating the AMPK/ULK1 pathway.301 SIRT3 promotes expression of autophagic proteins Beclin-1 and LC3II via downregulation of the Notch-1/Hes-1 pathway.303 Functional studies showed that SIRT3 reversed Bnip3 expression and promoted Bnip3-required mitophagy activity via the ERK-CREB signaling pathway.304 SIRT3 is involved in the regulation of autophagy; however, its role as an autophagy regulator, particularly the molecular mechanism, remains poorly understood. One recent study found that SIRT3 was directly inhibited by miR-874-5p and promoted autophagy, while depletion of miR-874-5p inhibited autophagy.305 A related study indicated that SIRT3 regulated the LKB1/AMPK/mTOR autophagic signaling pathway through the lncRNA DYNLRB2-2/miR-298/SIRT3 axis.306 Compared with SIRT1, the studies related to autophagy in SIRT3 are still lacking.
Mitochondria represent a major source of ROS that affect mitochondrial function, resulting in autophagic clearance of damaged mitochondria.183 Localized in the mitochondria, SIRT4 regulates proteins involved in metabolic reactions, antioxidant pathways and autophagy, thus maintaining mitochondrial homeostasis.307 Overexpression of SIRT4 inhibits ROS production and autophagy by activating the Akt/mTOR signaling pathway.308 Furthermore, the SIRT4/optic atrophy 1 axis is causally linked to mitochondrial dysfunction and altered mitochondrial dynamics that translates into aging-associated decreased mitophagy.301 So far, there are few relevant studies on SIRT4 regulation of autophagy. Further studies need to explore the role of SIRT4 as an mtSIRT in mitochondrial processes, such as autophagy (mitophagy).
Unlike SIRT4, which inhibits autophagy, the role of SIRT5 in regulating autophagy is contradictory. In the case of inhibition of autophagy by SIRT5, mitochondrial size is increased and mitophagy decreased upon SIRT5 overexpression, whereas the opposite effect is observed in SIRT5-silenced cells or upon treatment with the SIRT5 inhibitor MC3482.302 However, SIRT5 could enhance autophagy in gastric cancer (GC) cells via the AMPK/mTOR pathway.309 Additionally, SIRT5-induced deacetylation of lactate dehydrogenase B triggers hyperactivation of autophagy; a key event in tumorigenesis.310 Succinyl-proteomics in brown adipose tissue of normal and SIRT5 KO mice. Overacylation due to SIRT5 deficiency leads to defective autophagy/mitophagy.311 Besides their functions in energy metabolism and mitochondrial respiratory chain complexes, all three mtSIRTs participate in the regulation of mitochondrial morphology/dynamics. They seem to promote mitochondrial fusion and/or inhibit fission, and thus might attenuate mitophagic clearance of dysfunctional mitochondria.302 At present, the mechanism of action of mtSIRTs on autophagy is still unclear.
Effect of SIRT6 on autophagy is mainly through inhibition of Akt-related pathway
SIRT6 is essential for the regulation of autophagy in cells. For example, overexpression of the SIRT6 gene could inhibit apoptosis and induce autophagy, which might be involved in repairing kidney damage caused by lipopolysaccharide (LPS).312 Autophagy controls cellular senescence by eliminating damaged cellular components and is negatively regulated by Akt signaling through mTOR. SIRT6 overexpression induces autophagy via attenuation of insulin-like growth factor (IGF)/Akt/mTOR signaling.313 Lu et al. revealed that SIRT6 positively regulates autophagy in cardiomyocytes. Mechanistically, SIRT6 promotes nuclear retention of FoxO3 transcription factor via attenuating Akt signaling, which is responsible for autophagic activation.314 SIRT6 can be inhibited by upstream miR-122, resulting in a significant reduction in the levels of elabela, thereby preventing angiotensin II (Ang II)-mediated loss of autophagy.315 However, the mechanism of SIRT6 promotion of autophagy needs further study.
Effect of SIRT7 on autophagy needs further investigation
There are few studies about the effects of SIRT7 in autophagy. For example, silencing forkhead box M1 promotes apoptosis and autophagy through the SIRT7/mTOR/IGF2 pathway in GC cells.316 SIRT7 protects against chondrocyte degeneration in OA via autophagic activation.317 SIRT7 depletion significantly inhibits androgen-induced autophagy in LNCap and 22Rv1 cells (in vitro). SIRT7 plays an important role in tumor growth and metastases and immunohistochemical analysis of 93 specimens and bioinformatic analysis revealed that SIRT7 expression was positively associated with androgen receptor (AR) (in vivo).318 SIRT7 promotes prostate cancer autophagy indirectly via the AR signaling pathway.318 These results suggest that SIRT7 plays a positive role in promoting apoptosis. However, the number of studies on SIRT7 is still limited and further research is needed.
Autophagy is a highly conserved catabolic process and a major cellular pathway for the degradation of long-lived proteins and cytoplasmic organelles. Growing evidence has suggested that the SIRT protein family plays an important role in pathophysiology by mediating autophagy, maintaining cellular homeostasis, integrating cellular energy metabolism, and clearing damaged and waste cells. Although there is still a lot of work to be done, based on the current research, it is confident that the SIRT family might become a target for future research on autophagy. Investigating the exact mechanism of SIRT-mediated autophagy in different diseases is a new field to be explored in the future. Further studies should focus on the biological mechanism of SIRT co-regulating autophagy with various molecular signals and its role in different subcellular localization. Moreover, autophagy modulators of SIRTs may also provide new pharmacological targets.
Role of SIRTs in cell proliferation
Cell proliferation is the process by which a cell grows and divides to produce two daughter cells.319,320,321 Cell proliferation leads to an exponential increase in cell number and is, therefore, a rapid mechanism of tissue growth.321,322 Cell proliferation requires both cell growth and division to occur at the same time, which is the basis of organismal growth, development, reproduction and inheritance (Fig. 7).322,323,324
Effect of SIRT1 on cell proliferation
SIRT1 is involved in regulating cell proliferation in a bilateral way by regulating protein expression and acetylation.272,325 The opposite effects of SIRT1 on cell proliferation have been observed among different cell types or the regulation of different downstream molecules. For example, SIRT1 promotes cell proliferation by regulating LC3 and retinoblastoma (Rb) acetylation. At the molecular level, SIRT1 promotes the proliferation of endometrial cancer (EC) cells by reducing acetylation of LC3.272 SIRT1 deacetylates Rb protein in the Rb/ E2F transcription factor 1 (E2F1) complex, leading to dissociation of E2F1 and enhanced oligodendrocyte progenitor cell proliferation.326 SIRT1 directly regulates expression of transcription factor proteins, such as E2F1 and p53, subsequently promoting macrophage and HCC cell proliferation, respectively.327,328
However, SIRT1 can have an antiproliferative role via regulating expression of key proteins related to cell proliferation, such as AMPK and signal transducer and activator of transcription 3 (STAT3). For instance, SIRT1 exerts antiproliferative effects via the AMPK/mTOR pathway in the context of mutant p53 in HCC cells.329 SIRT1 overexpression inhibits the proliferation of renal cancer cells, while inhibition of SIRT1 expression has the opposite effects.325 SIRT1 might serve an anticancer role in cancer cells by upregulating expression of downstream AMPK.330 SIRT1 also inhibits GC cell proliferation via the STAT3/matrix metalloproteinase (MMP)-13 signaling pathway.331 SIRT1 has both promotive and inhibitory effects on proliferation in different cells. However, more studies are still needed to elucidate the mechanisms and establish under which conditions SIRT1 promotes or inhibits cell proliferation.
Effect of SIRT2 on cell proliferation
Participation of SIRT2 in cell proliferation was identified by a series of studies.332,333,334,335 At the molecular level, SIRT2 regulates Myc and results in promotion of cell proliferation. For example, SIRT2 enhances N-Myc and c-Myc protein stability and promotes cancer cell proliferation.332 On the contrary, SIRT2 functions as an HDAC and inhibits proliferation of neuroblastoma cells, renal podocytes, and neuroblastoma cells.336 SIRT2 upregulation reduces cell proliferation in renal podocytes under high-glucose conditions.337 The opposite effect of SIRT2 on cell proliferation might be due to the different cell types, which might be the direction for future studies.
Effect of SIRT3 on cell proliferation
SIRT3, the major deacetylase in mitochondria, also plays a bilateral role in regulating cell proliferation. For instance, SIRT3 is responsible for hydroxymethyl-transferase 2 (SHMT2) deacetylation, and the conversion of serine and glycine accomplished by SHMT2 deacetylation in mitochondria is significantly upregulated to support cell proliferation.338 Chen et al. found that increased activity of SIRT3 contributed to decreased ROS levels and increased cell proliferation.339 Conversely, the expression of SIRT3 is upregulated by Profilin-1, and subsequently negatively regulates HIF-1α protein levels and suppresses cell proliferation.340
Effect of SIRT4 on cell proliferation
SIRT4 inhibits proliferation of several types of cancer cells. For example, SIRT4 inhibits the proliferation of cancer cells by inhibiting glutamine metabolism.341,342 In addition, cell proliferation due to repression of SIRT4 by the mTORC1 pathway has been identified.343 Moreover, SIRT4 is the molecular switch mediating cellular proliferation through glutaminase (GLS)-mediated activation of the Akt/GSK3β/CyclinD1 pathway; mechanically, SIRT4 suppression activates glutaminase, thereby initiating Akt activation.344
Effect of SIRT5 on cell proliferation
SIRT5 promotes cell proliferation in most conditions by regulating activity of signaling proteins or protein PTM. For instance, SIRT5 promotes cell proliferation by increasing activity of the MAPK pathway through acetyl-CoA acetyltransferase 1.345,346 Moreover, citrate synthase desuccinylation by SIRT5 promotes cancer cell proliferation.347 Similarly, SHMT2 desuccinylation by SIRT5 drives cell proliferation.348 In addition, SIRT5 regulates cell proliferation directly or indirectly by influencing expression of transcription factors, such as E2F1 and pancreatic and duodenal homeobox 1 (PDX1).349 However, SIRT5 suppresses the proliferation of pancreatic β-cells in vitro by downregulating transcription of PDX1 by deacetylating H4K16.350 In conclusion, SIRT5 has dual functions in regulating proliferation of different cell types. However, the distinct mechanism for the bilateral roles of SIRT5 is worth further exploration.
Effect of SIRT6 on cell proliferation
SIRT6 is also reported to regulate cell proliferation in a bilateral manner via influencing downstream molecules, such as AMPK, ERK, Wnt signaling and the MAPK pathway. SIRT6 promotes expression of COX-2 by repressing AMPK signaling, thereby increasing cell proliferation.351 Moreover, overexpression of SIRT6 promotes cell proliferation via upregulating he phosphorylation of ERK.352 In addition, SIRT6 deletion promotes hematopoietic stem cell proliferation through aberrant activation of Wnt signaling.353 Using genetic and biochemical studies in vitro and in human multiple myeloma xenograft models, Cea et al. found that SIRT6 depletion enhanced cell proliferation via upregulating expression of MAPK.354 In conclusion, SIRT6 has both promotive and inhibitory effects on cell proliferation. The different results of SIRT6 in regulating cell proliferation need further study.
Effect of SIRT7 on cell proliferation
Previous studies have shown that SIRT7 has a positive role in regulating cell proliferation.355 Upregulation of SIRT7 protects against the proliferation of vascular smooth muscle cells (VSMCs) in atherosclerosis.355 Similarly, SIRT7 deficiency attenuates VSMC proliferation, thus attenuating neointimal formation following vascular injury.356 Moreover, SIRT7 depletion inhibits cancer cell proliferation by suppressing AR signaling and activating p38MAPK.318,357
The direct and indirect involvement of SIRTs in proliferation could provide new ideas and evidence in support of potential research and as therapeutic targets. This might be meaningful for the treatment of abnormal proliferation in the future, thereby reducing the human disease burden related to proliferation. However, at present, research is still focused on the effect of SIRTs on carcinoma, and other molecular mechanisms of proliferation is less researched. Therefore, research on other molecular mechanisms of proliferation should be increased in the future. More evidence from in vitro and in vivo models for different kinds of diseases to confirm undefined molecular mechanisms of proliferation as yet is awaited.
Roles of SIRTs in cell migration and invasion
Migration and invasion are vital phenotypes both in physiological and pathological status. They allow normal cells to change position within tissues during embryonic morphogenesis, wound healing, and immune-cell trafficking.358,359 Specifically, in human cancers, they allow neoplastic cells to enter lymphatic and blood vessels for undergoing metastatic growth in distant organs.360,361 An increasing number of studies have shown that SIRTs play important roles in the molecular mechanisms of cell migration and invasion, such as regulation of TGF-β signaling and epithelial-to-mesenchymal transition (EMT).362,363 Since these two phenotypes are hallmarks during tumor progression, we introduced the potential roles of SIRT protein family in cell migration and invasion, mainly depending on cancers (Fig. 8).
Effect of SIRT1 on cell migration and invasion
SIRT1 deacetylates many key proteins, which also contain transcription factors, mainly involved in EMT and integrin adhesion, thereby regulating cell migration and invasion.271,363 EMT is the most well-established example of changes in cell–cell adhesion, which refers to nonepithelial cells that are loosely embedded in an extracellular matrix (ECM).364 Integrin adhesion activates pathways including TGF-β, phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/Akt, and AMPK signaling pathways.
Several studies have shown that SIRT1 protein levels are lower in lesion tissues than in adjacent tumor tissues or normal tissues of patients with cancer.365,366,367 This phenomenon is also observed in autoimmune disorders, which indicates that SIRT1 plays important roles in regulating cell migration and invasion.368 SIRT1, as a deacetylase, influences the biological functions of proteins via regulating protein deacetylation, such as deacetylation of Beclin-1. In melanoma cells, SIRT1 deacetylates Beclin-1 and then accelerates autophagic degradation of the epithelial marker E-cadherin, finally promoting EMT.271 Additionally, SIRT1 could regulate the expression levels of several proteins that participate in cell migration and invasion, resulting in promotion of EMT. Both in vivo and in vitro studies have shown that expression of SIRT1 in chondrosarcoma cells could effectively take part in the metastatic plasticity of the cells by inducing EMT, via enhancing expression of Twist protein, which is a critical transcriptional factor of EMT.363 Zinc finger E-box binding homeobox 1 is an E-cadherin‐related transcription factor. Yu et al. have reported that there is positive feedback between SIRT1 and Zinc finger E-box binding homeobox 1, which enhances EMT of osteosarcoma.369 SIRT1 induces deacetylation of Beclin-1 and then accelerates autophagic degradation of the epithelial marker E-cadherin, further promoting EMT in melanoma cells.271 Epidermal SIRT1 plays a role in wound repair. SIRT1 knockdown inhibits EMT, cell migration, and TGF-β signaling in keratinocytes.370 Furthermore, SIRT1 activates downstream PI3K/Akt and Notch signaling pathways, which alleviates H9c2 cell injury induced by hypoxia, via promoting cell proliferation, migration and invasion, and by inhibiting apoptosis.371 In non-small cell LC (NSCLC), the SIRT1-mediated AMPK/mTOR signaling pathway could promote A549 and H1299 cell proliferation, invasion and apoptosis.372
Expression of SIRT1 can be regulated by ncRNAs, which further influence its effects in regulating cell migration and invasion. For instance, in colorectal cancer (CRC) cells, downregulation of SIRT1, by miR-34a transfection, increases the level of acetylated-p53 and inhibits cell migration and invasion.373 This situation is also found in HCC.374 Expression of SIRT1 can also be regulated by lncRNAs or circRNAs in a ceRNA-dependent manner. For example, SIRT1 promotes cell migration and invasion in HCC. Expression of SIRT1 is upregulated by lncRNA MALAT1 via sponging miR-204, which might have a pivotal role in treatment and prognosis.375 Furthermore, SIRT1 promotes the migration of fibroblast-like synoviocytes in rheumatoid arthritis, which providing new insight into SIRT1 during RA progression. Mechanistically, SIRT1 is positively regulated by circ0088036 via sponging miR-140−3p.368
Effect of SIRT2 on cell migration and invasion
SIRT2 participates in regulating cell migration and invasion through deacetylating target proteins. STAT3 is an important protein for regulating cell invasion and migration.376,377 STAT3 has been shown to affect EMT in several cancers.378 Previous studies have shown that SIRT2 can deacetylate Aldo-keto reductase family 1 member C1 (AKR1C1), which is a member of the human aldo-keto reductase protein family that catalyzes NADP + -dependent reduction. AKR1C1 deacetylation further inhibits the transactivation of STAT3 target genes, thus suppressing migration in NSCLC cells and xenograft models.379 It has been reported that isocitrate dehydrogenase 1 (IDH1) affects cell migration in malignant tumors, such as glioblastoma.380 In human CRC, SIRT2-dependent IDH1 deacetylation represses CRC cell migration and invasion both in vitro and in vivo.381
Effect of SIRT3-5 on cell migration and invasion
SIRT3-5 are three main deacetylases that are located in mitochondria, which appear to be suppressors of cell migration and invasion. Previous studies have demonstrated that SIRT3 and SIRT4 negatively regulate EMT. For instance, transplantation of sh-SIRT3 cells in nude mice resulted in rapid tumor growth and larger tumors. At the molecular level, SIRT3 depletion inhibits EMT by lower E-cadherin expression, leading to tumor suppression.382 Sun et al. suggested that SIRT4 suppressed EMT through promoting E-cadherin expression in GC cells.383 Li et al. reported that SIRT3 was involved in the inhibitory effect of nicotinic alpha7 subtype of nicotinic acetylcholine receptors on platelet-derived growth factor-BB, an angiogenic factor, induced VSMC migration. Activation of alpha7 subtype of nicotinic acetylcholine receptors attenuates migration in platelet-derived growth factor-BB-treated VSMCs via a mitochondrial SIRT3-dependent manner.384
SIRT5 regulates cell migration and invasion in several cancer cells. For example, Dang et al. found that SIRT5 promoted migration and invasion of HCC cells.385 The opposite findings were reported by Yao et al. in that the inhibition of SIRT5 increased migration and invasion of HCC in hypoxic microenvironments.386 This inconsistent phenomenon might be attributed to the hypoxic status of tumor microenvironments. However, further studies are required for illustrating its deeper regulatory mechanisms.
Effect of SIRT6 and SIRT7 on cell migration and invasion
SIRT6 and SIRT7 are the least studied of the seven SIRTs to date, which are both located in the nucleus. Both of them have been found to play a role in cell migration and invasion via regulating EMT and/or MMP expression. For example, in human HCC, SIRT6 promotes N-cadherin and vimentin expression via deacetylating FoxO3a in HCC cells.387 SIRT6 upregulates expression of MMP9 probably through the MAPK/ERK1/2 pathway, with increased migration and invasion of OS cells.388 Liu et al. found that forced expression of SIRT6 attenuated EMT by suppressing the TGF-β1/ small mothers against decapentaplegic protein (Smad)3 pathway and N-terminal kinase (c-Jun) in rat models of asthma.362 A recent study has shown that SIRT7 promotes CRC cell invasion through the inhibition of E‐cadherin, which is the most important protein in EMT.389 Furthermore, SIRT7 is overexpressed in EC cells compared with normal endometrial cells. SIRT7 downregulation inhibits the invasiveness of EC cells.390
Taken together, the above-discussed findings suggest that SIRT proteins are involved in regulating cell migration and invasion during physiological processes and the development of human cancers. However, current research mainly focuses on the function of SIRT1 in regulating cell migration and invasion. Much work is still needed to pinpoint the precise molecular mechanisms governing the functions of other SIRTs, especially SIRT6 and SIRT7, under those conditions. It is meaningful to continue to explore the role of SIRT proteins in other diseases, which might provide future beneficial alternatives against those devastating diseases.
Regulatory role of SIRTs in human diseases
SIRTs and cancer
Cancer is currently the second most common contributor to premature mortality worldwide.391 Since an early diagnosis and effective treatment for patients with cancer are critical, the identification and application of effective biomarkers and novel drug targets are urgently required. Recent evidence reveals that aberrant expression of SIRTs occurs in almost all cancer types with different mechanisms, including those involved in cancer metabolism, genome stability, and the tumor microenvironment.3 The functions of SIRTs in the tumor process are characterized as tumor suppressor and/or oncogene, depending on genetic context and tumor type and stage.392 Moreover, SIRTs could exert regulatory roles in the response of the tumor to chemotherapy.393 These unique features suggest that SIRTs serve as potentially targetable markers and play important roles in cancer therapy. In this section, we summarize the recent studies of SIRTs in diverse cancers, which is shown in Fig. 9.
Breast cancer (BC)
BC is the most common malignancy throughout the world and is the fifth leading cause of cancer-related deaths.394 SIRT2 and SIRT4 are downregulated,395,396 while SIRT1 and SIRT7 are upregulated in BC tissues compared to adjacent tissues or normal tissues.357,397 Besides, increased SIRT2 and SIRT4 expression is correlated with longer overall survival,395,396 whereas increased SIRT1 and SIRT7 expression predicts a poor prognosis in patients with BC.398,399 These disparities might be in respect to the different roles of them in BC progression.400
Regarding BC development, SIRTs are generally considered as tumor suppressors but might act as tumor promoters as well. SIRT1 has been extensively explored in comparison to other SIRTs for their roles in BC, and may influence BC progression by regulating many processes, especially EMT. SIRT1 plays a critical role in regulating EMT-associated programming and thus, consequently, eliciting BC invasion and metastasis in patients with triple-negative BC.401 SIRT1 expression suppresses BC metastasis by reducing EMT, and invasiveness in nude mice.402 The effect of SIRT1 modulation on EMT in breast cancer-related cancer stem cells has also been observed. This study indicates that loss of SIRT1 destabilizes EMT inducer paired related homeobox 1, disinhibits KLF4, and activates transcription of aldehyde dehydrogenase 1, which encourages cancer stem cells, resulting in metastatic reversion.403 In addition to its tumor suppressive roles in BC, SIRT1 overexpression, altered EMT programming, and a decrease in tumor-suppressive miR-200a may be consistently involved in BC development and subsequent distant metastasis.404 The plausible explanation of the contradictory functions of SIRT1-mediated BC regulation might be due to tumor grade, tumor stage of BC, and the use of animal or human samples with a different pathological subtype.
Additionally, many other non-EMT factors that are known to function in other cellular processes in BC development could also be regulated by SIRT1. For example, the estrogen receptor (ER) and AR could mediate induction of estrogen- and androgen-responsive genes respectively and stimulate cell proliferation, and SIRT1 represses the transcriptional and proliferative response of BC cells to estrogens via an Erα-dependent mechanism.405 DNA polymerase delta1, the gene coding for DNA polymerase δ catalytic subunit p125, is upregulated by SIRT1, thus promoting proliferation and migration of BC cell line MCF-7.397 Metadherin, an oncogenic protein, has been implicated in promoting cancer progression, metastasis, and chemoresistance in BC.406 Activation of AMPK has been reported to reduce the expression of metadherin through enhanced SIRT1 activity along with GSK-3β in an independent manner in triple-negative BC.406
In addition to SIRT1, SIRT2 functions in a binary manner, as a tumor suppressor or promoter. SIRT2 acts as a tumor suppressor in BC by regulating mitosis and genome integrity. Evidence has shown that SIRT2 promotes BRCA1-BRCA1-associated RING domain protein 1 (BARD1) heterodimerization through deacetylation, thereby facilitating homologous recombination and tumor suppression.407 Additionally, in cancer biology, Slug, an EMT transcription factor, promotes tumor progression and metastasis.408 In basal-like BC, SIRT2 maintains Slug protein stability by deacetylation, which contributes to basal-like BC’s robust tumorigenic activity, along with enhanced invasive and metastatic capabilities.409 SIRT3, SIRT4 and SIRT7 illustrate different functions in BC progression. SIRT3 has been found to affect p53 by disruption of the ERα–p53 interaction, and decrease proliferation, colony formation, and migration in BC cells.410 Notably, SIRT4 could exert its tumor-suppressive activity in BC though negatively regulating SIRT1 expression via repressing glutamine metabolism, which suggests a novel crosstalk between mitochondrial and nuclear SIRT proteins in BC progression.411 SIRT7 depletion inhibits tumor growth via activating p38/MAPK signaling.357
Additionally, SIRT proteins can affect the sensitivity of BC cells to several drugs, including tamoxifen, paclitaxel and doxorubicin. For example, SIRT1 causes tamoxifen resistance in ER-α-positive BC cells through upregulation of multidrug resistance protein 2 by mediating deacetylation of FoxO1 protein.412 Subsequently, SIRT1 inhibition impairs nuclear FoxO1 and multidrug resistance protein 2 expression and augments the cytotoxic effect of paclitaxel and doxorubicin in tamoxifen-resistant BC cells.412 SIRT3 overexpression in BC cell line MTR-3 reduces the sensitivity of the resistant cells to tamoxifen.357,413 On the contrary, SIRT4 enhances the tamoxifen sensitivity of BC cells via inhibiting the STAT3 signaling pathway.
These findings indicate unique mechanisms of SIRT1 mediate BC regulation and its contribution to tumor development and resistance, which suggests that SIRTs are promising therapeutic targets in BC, and provides clinical strategy for overcoming drug resistance. However, the exact molecular mechanism is still uncertain and needs further investigation.
LC is the leading cause of cancer-related deaths and the second most diagnosed cancer worldwide, with NSCLC being the most common type.394 Significant differences in SIRT expression between NSCLC tissues and nontumor lung tissue or adjacent tissue have been observed, which indicates that SIRTs are promising biomarkers in the diagnosis of LC.414,415,416 Notably, serum SIRT3 distinguished LC patients from healthy individuals with an area under the curve of 0.918 and optimal cutoff value of 3.12, reaching sensitivity of 86.4% and specificity of 94%.416 SIRTs could be potential prognostic factors for NSCLC.414,415,417 For example, high SIRT1-3 expression is associated with poor survival in patients with NSCLC.414,415
Evidence has suggested that SIRTs are key factors involved in tumor development and treatment in LC.418,419 Regarding LC progression, the SIRTs play conflicting roles. SIRT1 upregulated by SNHG10 suppresses NSCLC cell proliferation, as a tumor suppressor.420 Overexpression of SIRT1 protects NSCLC cells against osteopontin-induced NF-κB p65 acetylation and EMT, thus attenuating OPN-induced cell proliferation, migration and invasion.421 However, SIRT1 upregulated by circ_0001946, could promote cell growth in lung adenocarcinoma by activating the Wnt/β-catenin signaling pathway.422 SIRT2 and SIRT6 have been shown to exert both pro- and anticarcinogenic effects in the process of LC. For example, SIRT2 suppressed the migration of NSCLC cells by deacetylating AKR1C1, and inhibiting transactivation of STAT3 target genes.379 In addition, SIRT2 deacetylates the K100 residue of glycolytic enzyme phosphoglycerate mutase and facilitates its activation, resulting in enhanced NADPH production and accelerated tumor growth in NSCLC cells.423 Similarly, SIRT6 illustrates opposite functions in the promotion of LC development, as tumor suppressor and promoter.424,425 For instance, SIRT6 can coordinate with chromatin remodeler chromodomain-helicase-DNA-binding 4 to promote chromatin relaxation and DNA repair, thereby exerting an anticarcinogenic role in LC.424 In contrast, SIRT6 can also drive EMT and metastasis in NSCLC via snail-dependent transrepression of KLF4.425 This dual action of these SIRTs might depend upon the cellular context, tumor types, cancer stage, and their involvement in various cellular pathways,392,418 and further studies are needed to explore the exact mechanisms underlying their dual roles in LC.
Expression of SIRTs could have an influence on the chemoresistance and radioresistance of LC. SIRT1 promotes cisplatin resistance of NSCLC cells by elevating vascular endothelial growth factor A expression.426 SIRT1 is upregulated in cisplatin-resistant NSCLC tissues and cells compared to cisplatin-sensitive groups.291 SIRT1 silencing enhances the cisplatin sensitivity of H1299/cisplatin cells via suppressing autophagy. Upregulation of SIRT2 in NSCLC cells increases the sensitivity to cisplatin treatment while SIRT3 promotion reduces cisplatin resistance in LC by modulating the FoxO3/Cdc10-dependent transcript 1 protein axis.253,427 In relation to LC radioresistance, SIRT3 promotes DNA damage repair and radioresistance through ataxia telangiectasia mutated–Chk2 in NSCLC cells.428 SIRTs can affect radioresistance in LC through the regulation of tumor metabolism. Overexpression of SIRT6 inhibits key-enzyme generation in A549 cells to inhibit glycolysis and enhance radiosensitivity.428
In LC, SIRTs are involved in tumor development, chemoresistance and radioresistance, and exert regulatory functions by targeting different target proteins. Thus, SIRTs represent promising therapeutic targets in the perspective of precision medicine and provide new insights into therapeutic strategies for LC.
HCC, the most common type of primary liver cancer with a relatively high mortality, is the sixth most common cancer and the third-leading cause of cancer-related mortality worldwide.394 Several bioinformatics studies have reinforced that nonmitochondrial and mitochondrial SIRTs are differentially expressed in HCC. For example, nonmitochondrial SIRT1, SIRT2 and SIRT6 are expressed at higher levels,429,430,431 while mitochondrial SIRT3-5 are expressed at lower levels in HCC tissues compared with normal liver or surrounding tumor tissue.432,433,434 SIRTs could be prognostic markers for patients with HCC. For instance, high expression of SIRT1 and SIRT7 is highly associated with poor survival, whereas low tumor levels of SIRT4 predicts a decreased survival time in HCC patients.434,435,436
Recent studies have suggested that SIRTs could play regulatory roles in HCC development by regulating the metabolic state of the cancer cells.437 Referring to mitochondrial SIRTs, SIRT4 exerts a tumor-suppressive function in HCC by inhibiting glutamine metabolism.434 SIRT5 prevents tumor immune evasion and suppresses HCC development by orchestrating bile acid metabolism.438 However, SIRT5 also exerts a tumor-promoting function as a metabolic regulator. The activation of mitochondrial SIRT5 contributes to the promotion of growth and metastasis of HCC cells via glucose metabolism reprogramming from oxidative phosphorylation to glycolysis.439 The possible explanation for the dual role of SIRT5 in HCC could be related to its involvement in different metabolic processes, including glucose and lipid metabolism, which might result in opposite effects on tumor progression.3 In addition to mitochondrial SIRTs, the nonmitochondrial SIRTs can influence HCC by regulating cancer-related metabolism, especially glucose metabolism. SIRT1 and SIRT6 deacetylate hnRNP A1 to suppress glycolysis and growth in HCC.440 SIRT6, stabilized by ubiquitin-specific peptidase 48, attenuates HCC glycolysis and impedes metabolic reprogramming, thereby hampering HCC malignancy.441 SIRTs can also play roles in modulation of the cell cycle in HCC, which are essential for tumor development. Evidence has shown that SIRT4 upregulates cell-cycle governing genes p16 and p21 expression, suppresses CyclinB1/Cdc2 and Cdc25c, which normally induce cell-cycle progression, and suppresses survival to induce apoptosis in HCC cells.442
Therapeutic advances targeting SIRTs are currently being explored as it is suggested that modulating SIRT3 abundance via cyclin-dependent kinase (CDK) 4/6 inhibition might enhance HCC therapy when combined with sorafenib.443 SIRT3 downregulates the mRNA and protein levels of glutathione S-transferase π1, a phase II detoxification enzyme involved in metabolism of chemotherapeutic agents, and SIRT3 overexpression promotes chemotherapeutic-agent-induced or sorafenib-induced apoptosis, thereby enhancing the drug sensitivity of HCC cells.252 Aside from mitochondrially directed deacetylase activity, SIRT6 depletion is reported to downregulate multidrug resistance protein 1 expression through the suppression of CCAAT/enhancer-binding protein, promoting enhanced HCC chemosensitivity.444
CRC ranks third in terms of cancer incidence worldwide and is the second most common cause of cancer deaths.394 Previous studies have shown that SIRT1 and SIRT7 are increased,389,445 whereas SIRT2, SIRT4 and SIRT6 are decreased in human CRC tissues compared to normal tissue, which suggests that SIRTs are potential diagnostic biomarkers for CRC.446,447,448 SIRTs are potential prognostic factors for CRC. For instance, overexpression of SIRT5 is correlated with poor prognosis in patients with CRC, while SIRT6 expression is related to improved survival.448,449 However, there still a need for further studies that make more clear analyses to verify the roles of SIRTs as biomarkers of CRC, such as receiver operating characteristic curve, sensitivity and specificity analyses.
The pleiotropic roles of SIRTs in the regulation of tumor cell metabolism and cell death are strongly linked to the progression of CRC. SIRT1 has been found to affect CRC in a dose-dependent manner by regulating glutamine metabolism and apoptotic pathways. Heterozygous deletion of SIRT1 induces c-Myc expression, enhancing glutamine metabolism and subsequent proliferation, autophagy and cancer formation. In contrast, homozygous deletion of SIRT1 triggers apoptotic pathways, increases cell death, diminishes autophagy, and reduces cancer formation.450 The dose-dependent regulation of cellular metabolism and apoptosis by SIRT1 mechanistically contributes to the observed dual roles of SIRT1 in tumorigenesis. SIRTs have an anticarcinogenic action via modulation of CRC-related metabolism. SIRT2-dependent IDH1 deacetylation regulates cellular metabolism and inhibits liver metastasis of CRC.381 SIRT4 upregulates E-cadherin expression and suppresses proliferation, migration and invasion through inhibition of glutamine metabolism in CRC cells.140 In addition to the anticarcinogenic effects of SIRTs, SIRT5 contributes to colorectal carcinogenesis by enhancing glutaminolysis in a deglutarylation-dependent manner.449 SIRTs exert their regulatory function in CRC development through the modulation of several autophagy-related pathways. In particular, SIRT5 can deacetylate lactate dehydrogenase B, thus promoting hyperactivation of autophagy and tumorigenesis in CRC.310
Recent evidence highlights that SIRTs are involved in various tumor processes related to chemoresistance and radioresistance in CRC. For example, overexpression of SIRT3 improves anticancer drug resistance of CRC cells through superoxide dismutase (SOD) 2 and PGC-1α regulation.451 In addition, SIRT4 increases the sensitivity of CRC cells to chemotherapeutic drug 5-fluorouracil by inhibiting the cell cycle.447 Regarding radioresistance of CRC, FoxQ1-mediated SIRT1 upregulation augments expression and nuclear translocation of β-catenin and benefits CRC-related intestinal pathological bacteria, thereby enhancing the radioresistance of CRC cells.452
SIRTs play a role in regulating CRC progression, which indicates that SIRT-small-molecule-activator/inhibitor-based therapy strategies is a rescue strategy for patients with CRC. However, there have been a limited number of studies of the molecular mechanisms of SIRTs in regulating CRC progression.
GC is the fifth most frequently diagnosed cancer with an incidence rate of 5.6%, and the fourth most common cause of cancer death with a mortality rate of 7.7% worldwide.394 During the past decades, SIRTs have been considered as potential druggable targets in the clinical treatment of GC. SIRT1 is upregulated in GC tissues and SIRT1 depletion promotes GC progression through activation of STAT3/MMP-13 signaling.331 SIRT4 and SIRT6 are downregulated in GC tissues, and their low expression is negatively correlated with tumor size and pathological grade, which predicts poor prognosis.383,453 Mechanistically, SIRT4 inhibits cell proliferation, migration, and invasion in GC via regulating EMT. SIRT6 inhibits the Janus kinase 2/STAT3 pathway, thereby suppressing the growth of GC. Regarding tumor resistance, SIRT6 silencing can overcome sorafenib resistance by promoting ferroptosis.454 Thus, SIRTs could act as novel biomarkers and therapeutic targets of GC.
Pancreatic cancer (PC)
PC has high mortality and ranks as the seventh leading cause of cancer-related deaths worldwide.394 PC is also affected by SIRT activity and expression. SIRT5 expression is directly correlated with favorable prognosis, as its loss promotes glutamic-oxaloacetic transaminase 1 acetylation, thus promoting cell proliferation by enhancing glutamine and glutathione metabolism.455 Upregulation of SIRT6 by tumor suppressor KLF10 activity influences glycolysis, EMT, and distant metastasis of PC.456 SIRTs are associated with drug resistance of PC. SIRT1 can facilitate chemoresistance of PC cells by regulating adaptive response to chemotherapy-induced stress.457
Collectively, SIRTs play important roles in tumor progression, chemoresistance and radioresistance in gastrointestinal cancer by regulating multiple cellular and physiological processes, including metabolism, cell cycle, cell death, and tumor microenvironment. Therefore, the potential selective modulation of SIRT protein family members represents a promising area in gastrointestinal cancer treatment. However, given the contribution of gastrointestinal cancer to worldwide morbidity and mortality, further research is needed to understand the exact mechanisms underlying the involvement of SIRTs in these cancers, especially for GC and PC.
Ovarian cancer (OC)
OC is one of the most aggressive female malignancies, with poor prognosis.394 SIRT1-3 and SIRT6 are significantly decreased, while SIRT5 is significantly increased in OC tissues compared to normal or adjacent tissues.458,459,460 High expression of SIRT2 and SIRT5-7 is correlated with favorable survival, while high expression of SIRT1 and SIRT4 is associated with poor survival,458 suggesting that SIRTs could serve as novel prognostic biomarkers. SIRTs are implicated in the development and treatment of OC. For example, SIRT1 expression suppresses high motility group box-1 protein expression and acetylation, thus inhibiting OC migration, invasion and angiogenesis.461 However, MHY2245, a new SIRT1 inhibitor, exert antitumor activity against OC cells by blocking the pyruvate kinase M2/mTOR pathway.462 In addition to SIRT1, overexpression of SIRT3 dramatically suppresses OC cell metastatic capability by inhibiting EMT via downregulation of Twist.463 Regarding OC treatment, cisplatin has been a pivotal drug, however, cisplatin resistance hinders the prognosis of patients.464 Overexpression of SIRT2 significantly enhances the sensitivity of cisplatin-resistant counterpart cells to cisplatin in OC.465 In addition, SIRT5 can promote cisplatin resistance in OC by suppressing DNA damage in a ROS-dependent manner via regulation of the Nrf2/Heme Oxgenase-1 pathway.460
EC is the most common gynecological cancer in high-income countries and its incidence is rising globally.466 Recent studies have shown that SIRTs participate in the development and progression of EC. For example, SIRT1 is elevated in EC cell lines and tissues and SIRT1 promotes autophagy and proliferation of EC cells by reducing acetylation of LC3.272 The expression of SIRT2 is increased in most human EC cell lines and SIRT2 overexpression promotes EC cell proliferation but inhibits apoptosis.467 In contrast to SIRT1 and SIRT2, SIRT6 might function as a tumor suppressor of EC cells. SIRT6 negatively affects the proliferation of AN3CA and KLE EC cells by repressing expression of the antiapoptotic protein surviving.468 Chemotherapy is crucial for postoperative adjuvant therapy of EC. SIRT1 promotes the growth and cisplatin resistance of EC cells.469 SIRT2 has been shown to promote cell stemness and activate the MEK/ERK signaling pathway while repressing chemosensitivity in EC.470
Cervical cancer (CC)
CC is one of the most severe and prevalent female malignancies and a global health issue.394 Abnormal expression of SIRTs in CC tissue may be related to disease progression. For instance, the expression of SIRT2 is decreased in CC tissue compared with paired adjacent tissue, and SIRT2 expression in tumor tissue is negatively correlated with tumor size, and lymph node metastasis, which predicts favorable survival.471 For mechanistic studies, SIRT1 has been found to be overexpressed in HPV-infected CC cells and SIRT1 expression is correlated with poor clinical outcomes in CC.472 SIRT1 enables HPV-infected CC cells to continue growing by nullifying absent in melanoma 2 inflammasome-mediated immunity. Moreover, SIRT3 contributes to the reprogramming of fatty acid synthesis by upregulating acetyl-coA carboxylase 1 to promote de novo lipogenesis by SIRT3 deacetylation, thereby promoting the invasion and metastasis of CC cells.473
SIRTs are implicated in tumor development and chemotherapy resistance in gynecological cancer including OC, EC, and CC, thus SIRTs might serve as indicators of prognosis and as promising therapeutic targets for gynecological cancer. However, evidence about the roles of SIRTs in gynecological cancer is still limited, so more studies are needed to further explore the underlying molecular mechanism by which SIRTs regulate tumor processes in these cancers.
Glioma is the most common and malignant primary tumor of the central nervous system, with a poor prognosis, especially glioblastoma.474 SIRT1 and SIRT7 are upregulated,475,476 while SIRT3 and SIRT6 are downregulated in glioma tissues compared with normal or adjacent brain tissues.477,478 Glioma patients with higher SIRT1 or SIRT3 expression exhibit worse prognosis, whereas downregulation of SIRT5 is significantly correlated with shorter survival time in glioblastoma. These situations have suggested that SIRTs are promising prognostic biomarkers of glioma and might be involved in tumor progression.475,477,479
SIRT1 and SIRT6 exert a tumor suppressor effect in glioma. SIRT1-mediated p21-Activated kinase 1-deacetylation at K420 hinders autophagy and glioblastoma growth.480 Besides, SIRT6 suppresses glioma cell growth via induction of apoptosis, inhibition of oxidative stress, and inhibition of the activation of the Janus kinase 2/STAT3 signaling pathway.478 On the contrary, SIRT3 and SIRT7 are reported to play positive roles in the development of glioma. SIRT3 can stabilize Ku70–Bax interaction to enhance glioma cell viability.477 Moreover, SIRT7 affects the malignancy of glioma cells mainly by promoting glioma proliferation and invasion through ERK and STAT3 signaling.476 Evidence also suggests that SIRTs participate in the transformation of chemoresistance and radioresistance in glioma. For instance, SIRT1 inhibition increases the sensitivity of glioma cells for temozolomide via facilitation of intracellular ROS generation.475 In addition, CDK1-mediated SIRT3 activation could enhance mitochondrial function and contribute to adaptive radioresistance in glioma cells.481 Therefore, SIRTs are potential biomarkers for the prognosis and diagnosis of glioma and promising therapeutic targets.
Leukemia is a malignant clonal disease of hematopoietic stem cells, and most leukemias are sporadic and their specific etiology remains elusive.482 SIRTs participate in the development and therapeutic resistance of leukemia. SIRT1 promotes T-cell acute lymphoblastic leukemia progression by regulating the phosphorylation and degradation of p27 through deacetylating cyclin-dependent kinase 2.483 SIRT2 is overexpressed in primary acute myeloid leukemia blasts, and SIRT2 activation by nicotinamide phosphoribosyltransferase (NAMPT) reduces proliferation and induces apoptosis in human acute myeloid leukemia, possibly via the Akt/GSK-3β/β-catenin pathway.335 Inhibition of SIRT2 suppresses the in vitro growth and in vivo engraftment of T-cell acute lymphoblastic leukemia cells via diminished LIM domain only 2 (LMO2) deacetylation.484 This dual action in tumor development of SIRT2 might be due to different types of leukemia.
Regarding leukemia treatment, the combination of chemotherapeutics with SIRT modulators could provide a novel therapeutic strategy. For example, pharmacological targeting or RNAi-mediated knockdown of SIRT1 inhibits cell growth and sensitizes AML cells to tyrosine kinase inhibitor treatment.485 Moreover, shSIRT6-induced DNA repair deficiencies are potently synergistic with NAMPT targeting in acute myeloid leukemia treatment, which shows promising in vivo efficacy compared with monotherapy.486 SIRT7 expression increases with the positive response to treatment, but shows reduction when patients progress or relapse, which suggests that SIRT7 potentially serves as a general biomarker for monitoring treatment response in myeloid stem cell disorders.487 Accordingly, these results suggest that targeting SIRTs represents an attractive therapeutic strategy and provides a rationale for the novel combination-based treatments for leukemia.
We have reviewed the role of different SIRTs in diverse cancers, focusing on them as new anticancer therapeutic targets. Various investigations have indicated that different SIRTs show differential patterns of expression depending upon the pathological subtype, tumor grade and stage. It could be concluded that SIRTs serve as prognostic factors/biomarkers in patients with cancer. The discrepancy in the role of SIRTs in tumor progression and tumor chemoresistance or radioresistance might be attributed to various tumor types, stages, microenvironment, and their involvement in various tumor processes, such as cellular metabolism, cell death, cell cycle, and DNA damage/repair. Notably, several SIRTs exert a dual action in cancer. Thus, figuring out the underlying mechanisms and conditions that enable their opposing roles in cancer might be one of the main challenges and of great therapeutic significance. Collectively, SIRTs could be utilized as promising target molecules to be used as potential biomarkers for diagnosis and prognosis in patients with cancer. A variety of available SIRT modulators could be developed and further utilized to promote treatment efficacy of various cancers by themselves or, more likely, in combination with different anticancer drugs.
SIRTs and CVDs
Over the past decades, the incidence of CVDs, such as heart failure, atherosclerosis, and hypertension, has been increasing.488 CVDs are the major cause of mortality worldwide.489,490 According to the Global Burden of Disease Study 2019, prevalent cases of total CVDs have increased from 271 million to 523 million in 204 countries and territories between 1990 and 2019. The number of CVD deaths has also increased from 12.1 million to 18.6 million.491 Epigenetic modification plays a critical role in the occurrence and development of CVD488 and regulates the function and expression level of CVD-related genes through DNA methylation, histone modification, and non-coding RNA mechanism.492 Therefore, SIRT protein family has received much attention in CVD research due to its crucial role in regulating histone deacetylation.488 In addition to HDAC function, SIRTs also have multiple non-histone deacetylase and mono-ADP-ribosyl transferase activities.493 These functions also play an important role in CVDs (Fig. 10). SIRTs regulate crucial pathological processes, such as cell proliferation, cell senescence, DNA damage, oxidative stress, inflammation, and cell metabolism, thereby influencing the occurrence and development of CVDs.493,494
Cardiac hypertrophy and fibrosis
Cardiac hypertrophy is an adaptive and compensatory mechanism for maintaining cardiac output during physiological and pathological stimuli.495 However, some detrimental processes, such as pressure or volume overload, can lead to pathological cardiac hypertrophy.495 Cardiac fibrosis induces fibroblast proliferation and excessive deposition of extracellular proteins.496 Pathological cardiac hypertrophy and fibrosis are the main characteristics of cardiac remodeling.497 It is crucial to reveal the molecular mechanisms associated with cardiac hypertrophy and fibrosis, as there are currently no effective treatments for cardiac remodeling.497 Therefore, SIRT proteins, which have been reported to play important roles in the occurrence and development of cardiac hypertrophy and fibrosis, have received extensive attention, especially SIRT1, SIRT3, and SIRT6.
The impact of SIRT1 on cardiac hypertrophy is inconsistent, with both alleviating and exacerbating effects having been reported.4 Some in vivo studies have suggested that SIRT1 overexpression can alleviate Ang II-induced cardiac hypertrophy by reducing cardiomyocyte apoptosis and promoting autophagy.498,499 In addition, SIRT1 overexpression can ameliorate cardiac hypertrophy induced by phenylephrine by inhibiting protein kinase C (PKC)‐ζ activation.500 However, some studies have shown the opposite effect. For example, SIRT1 exacerbated cardiac hypertrophy by promoting membrane localization and activation of Akt and phosphoinositide-dependent protein kinase 1, while impaired Akt activation in the hearts of SIRT1-deficient mice was related to decreased cardiac hypertrophy in response to physical exercise and Ang II.501 These opposite effects might be dependent on the degree of SIRT1 expression.502 For instance, the low (2.5-fold) or moderate (7.5-fold) overexpression of SIRT1 in the hearts of transgenic mice attenuated cardiac hypertrophy. However, a high overexpression (12.5-fold) level of SIRT1 increased cardiac hypertrophy.502 These conflicting effects imply that SIRT1 has different effects on cardiac hypertrophy in different contexts and models.503 Therefore, more studies are needed to further explore the complex effects of SIRT1 on cardiac hypertrophy.503
SIRT3 has a protective role in cardiac hypertrophy.504 Its expression was reduced in the hearts of Ang II-induced cardiac hypertrophic mice and in Ang II-treated cardiomyocytes.505 In addition, SIRT3 overexpression protects myocytes from hypertrophy, whereas SIRT3 silencing exacerbates Ang II-induced cardiomyocyte hypertrophy.505 Resveratrol can be used to activate SIRT3 with protective effects on hypertrophy through activation of SIRT3 and subsequent autophagy.506 However, the protective effects of resveratrol have not been observed after the addition of siRNA-SIRT3.506 SIRT3 promotes autophagy in Ang II-induced myocardial hypertrophy via deacetylation of FoxO1,507 blocks the cardiac hypertrophic response by augmenting FoxO3a-dependent antioxidant defense mechanisms in mice,164 and exerts protective effects against cardiac hypertrophy by reducing the level of acetylation and activity of poly (ADP-ribose) polymerase-1.508
SIRT6 also protects against cardiac hypertrophy.509 Both in vivo and in vitro studies have revealed that SIRT6 inhibits isoproterenol-induced cardiac hypertrophy via activation of autophagy.314 Specifically, SIRT6 promotes nuclear retention of FoxO3 transcription factor, possibly by attenuating Akt signaling, which is responsible for autophagy activation.314 In addition, SIRT6 protects cardiomyocytes from hypertrophy by decreasing the protein level of p300 and subsequently the acetylation and transcriptional activity of NF-κB p65 subunit.510 It also blocks IGF-Akt signaling and cardiac hypertrophy development by targeting c-Jun.511 Moreover, STAT3 suppression has been reported to be involved in the protective effect of SIRT6 against cardiomyocyte hypertrophy.512
In addition, SIRT2, SIRT5, and SIRT7 have protective effects on cardiac hypertrophy, while SIRT4 appears to have the opposite effect. The protein level of SIRT2 is reduced in cardiac hypertrophy, and SIRT2 overexpression attenuates agonist-induced cardiac hypertrophy in a cell-autonomous manner.513 On a molecular level, SIRT2 binds to and deacetylates the nuclear factor of activated T-cell c2 transcription factor, thereby regulating nuclear factor of activated T-cell c2 transcription activity and exerting protective effects on cardiac hypertrophy.513 In contrast, loss of SIRT2 has been reported to reduce AMPK activation, thereby promoting aging-related and Ang II-induced cardiac hypertrophy and blunting metformin-mediated cardioprotective effects.514 These findings have suggested that SIRT2 might be a potential target for the treatment of cardiac hypertrophy. In addition, SIRT5 prevents age-related cardiac hypertrophy,515 while SIRT7 also ameliorates stress-induced cardiac hypertrophy by interacting with and deacetylating GATA4.516 Interestingly, SIRT4 seems to have an adverse effect on cardiac hypertrophy. For instance, an in vivo study has revealed that SIRT4 overexpression aggravates Ang II-induced cardiac hypertrophy by inhibiting MnSOD activity.517 However, further studies are needed to confirm this result.
In cardiac fibrosis, TGF-β is a key profibrotic cytokine that exerts profibrotic effects.518 TGF-β is involved in the protective effect of SIRT1, SIRT3, and SIRT6 on cardiac fibrosis by regulating the activity of Smad family transcription factors.519,520,521 For instance, activation of both SIRT1 and SIRT3 by resveratrol attenuates cardiac fibrosis in mice by inhibiting the TGF-β/Smad3 pathway519,520 and systematic SIRT6 KO induces cardiac fibrosis in mice by activating the TGF-β/Smad3 pathway.521 On a molecular level, the study has also shown that SIRT3 overexpression partially prevents the inflammatory and profibrotic effects by modulating the FOS/activator protein-1 pathway in human and rat cardiomyocytes.65 SIRT6 prevents Ang II-mediated cardiac fibrosis and injury by targeting AMPK-Angiotensin-converting enzyme 2 signaling.522
In addition, other SIRTs affect cardiac fibrosis. For example, SIRT2 overexpression protects against Ang II-induced cardiac fibrosis and rescues cardiac function.514 This protective effect of SIRT2 is associated with the promotion of AMPK activation by deacetylating the kinase LKB1.514 SIRT5 KO mice have shown increased fibrosis compared to age-matched wild-type mice,515 although relevant mechanisms need to be further explored. However, SIRT4 appears to contribute to cardiac fibrosis, and global SIRT4 KO in mice confers resistance to Ang II infusion by significantly suppressing fibrosis deposition.517 Similarly, enhanced expression and phosphorylation of SIRT7 plays a role in promoting cardiac fibrosis via activation of Smad2 and ERK signaling pathways.523 However, SIRT7 KO in mice has been reported to result in cardiac fibrosis.265
Overall, SIRTs play an important role in cardiac hypertrophy and fibrosis. SIRT1, SIRT3, and SIRT6 might protect against cardiac hypertrophy and fibrosis by affecting important biological processes and regulating downstream signaling pathways, such as autophagy and TGF-β/Smad3 pathways. Of note, SIRT1 might have bidirectional effects on cardiac hypertrophy, which might be dependent on the degree of SIRT1 expression. Furthermore, SIRT2 and SIRT5 might also have protective effects on cardiac hypertrophy and fibrosis. In contrast, SIRT4 might exacerbate cardiac hypertrophy and fibrosis. Evidence has suggested that SIRT7 has a protective effect on cardiac hypertrophy, but its effect on cardiac fibrosis is inconsistent. Considering that there are few studies on SIRT7 in cardiac hypertrophy and fibrosis, further research is needed in the future.
Heart failure is the most common endpoint of most CVDs,524 affecting an estimated 64.3 million people worldwide.391,525 It is a complex disease and involves various molecular and cellular alterations that affect the cardiac structure and impair the contractile function.526 However, the underlying mechanisms of heart failure remain not fully understood.527 Recently, growing evidence has suggested that SIRTs play key roles during the process of heart failure. The following section summarizes this evidence.
SIRT1 has beneficial effects on the development of heart failure. The expression of SIRT1 is decreased in the hearts of advanced heart failure patients and rat models.528,529 Heart failure is closely related to some biological processes, such as oxidative stress and cell apoptosis.530,531 SIRT1 might attenuate oxidative stress and protect cells from oxidative damage and apoptosis through several mechanisms.531 For example, levels of MnSOD, thioredoxin1, and Bcl-xL (an anti-apoptotic molecule) are significantly decreased in cardiomyocytes from individuals with advanced heart failure.528 The low expression of SIRT1 might downregulate antioxidants and upregulate pro-apoptotic molecules by increasing p53 acetylation and decreasing FoxO1 translocation in the nucleus.528 In addition, an in vivo study has suggested that SIRT1 overexpression reduces cardiomyocyte apoptosis through the NF-κB p65/miR-155/brain-derived neurotrophic factor (BDNF) signaling pathway, thereby alleviating heart failure in rats.532 Furthermore, reduced level and activity of sarco-endoplasmic reticulum Ca2+-ATPase (SERCA2a) are major features of heart failure, and SIRT1 KO elevated the acetylation of SERCA2a, which in turn leads to SERCA2a dysfunction and cardiac defects in a failing heart.533 In contrast, the pharmacological activation of SIRT1 restores SERCA2a activity via deacetylation at K492.533 Overall, the above evidence has indicated that SIRT1 is involved in the occurrence and development of heart failure and might be a promising therapeutic target for heart failure treatment.
In addition, mitochondrial energy metabolism disorder contributes to the progression of heart failure.534 Myocardial acetylproteomics demonstrates that there is extensive mitochondrial protein lysine hyperacetylation in mouse models of early-stage heart failure and in end-stage failing human hearts.534 As a mitochondrial deacetylase, SIRT3 plays an important role in maintaining the mitochondrial function,535 and provides a protective effect during heart failure.536 SIRT3 deficiency might impair cardiac mitochondrial function and aggravate heart failure during aging.537 In addition, SIRT3 is involved in the regulation of endothelial metabolism and angiogenesis, thereby affecting the occurrence and development of heart failure.538,539 For instance, an in vivo study has suggested that the endothelial‐specific SIRT3 KO disrupts glucose transport from endothelial cells to cardiomyocytes, decreases cardiomyocyte glucose utilization via apelin in a paracrine manner, and sensitizes pressure overload-induced heart failure.538
Similar protective effects during heart failure have also been observed in SIRT6.511 SIRT6 expression is significantly decreased in the hearts of patients with chronic heart failure as well as animal models of heart failure.527 SIRT6 overexpression increases the survival of transverse aortic constriction-induced heart failure mice, which might be associated with telomerase upregulation, such as telomerase reverse transcriptase and telomeric repeat binding factor 1.540
Compared to SIRT1, SIRT3, and SIRT6, studies on SIRT2,541 SIRT4, SIRT5,542 and SIRT7 in heart failure are limited. SIRTs might play important roles in the occurrence and development of heart failure and their further exploration is needed in the future. Studies on SIRT2, SIRT4, SIRT5, and SIRT7 might reveal promising research directions for the treatment of heart failure.
Atherosclerosis is a chronic inflammatory disease4 that results from a series of events, including increased levels of LDL cholesterol in the plasma, dysfunctional endothelial cells, inflammation with immune cell infiltration, and ultimately plaque formation.494,543 SIRTs have been reported to directly affect atherogenesis and plaque stability by preventing endothelial cell dysfunction, VSMC senescence, and macrophage foam cell formation via regulation of key biological processes, such as DNA damage repair and anti-apoptosis and anti-inflammatory pathways.493
SIRT1 has a protective effect on atherosclerosis.544 A prior in vivo study has shown that endothelial cell-specific overexpression of SIRT1 protects against atherosclerosis in apolipoprotein E KO mice,545 which was associated with inhibited endothelial cell apoptosis via eNOS expression activation.545 In addition, SIRT1 activation by SRT1720 in aging mice ameliorates endothelial dysfunction by increasing COX-2 signaling and reducing oxidative stress and inflammation.546 On VSMC level, SIRT1 protects against DNA damage and inhibits atherosclerosis partly by activating the repair protein Nijmegen breakage syndrome-1.544 Moreover, macrophage foam cell formation is a key initiation event in the pathogenesis of atherosclerosis.547 SIRT1 activation reduces Lox-1-mediated foam cell formation via suppression of the NF-κB signaling pathway.548 In contrast, suppression of the SIRT1 signaling pathway by mTOR signaling promotes foam cell formation and inhibits foam cell egress.549 Several miRNAs have been revealed to have a key role in atherosclerosis by regulating the expression of SIRT1.550,551 For example, miR-217 downregulation might alleviate atherosclerosis via inhibition of macrophage apoptosis and inflammatory response.550 SIRT1 is a direct target of miR-217. SIRT1 silencing can eliminate the effects of miR-217 downregulation.550 The above evidence has suggested that SIRT1 is associated with the occurrence and development of atherosclerosis and might be a promising therapeutic target for atherosclerosis treatment.
Compared to SIRT1, a relatively limited number of studies have explored the roles of other SIRTs in atherosclerosis. SIRT2 decreases atherosclerotic plaque formation in LDL receptor-deficient mice by regulating macrophage polarization.552 SIRT3 gene expression is associated with endothelial cell apoptosis in atherosclerosis rats,553 and SIRT3/SOD2 signaling can be activated by circ_0,003,423, thereby protecting human umbilical vein endothelial cells from oxLDL-induced dysfunction.554 SIRT4 suppresses the PI3K/Akt/NF-κB signaling pathway and relieves oxLDL-induced human umbilical vein endothelial cells injury.555 SIRT6 protects against endothelial dysfunction, VSMC senescence, and atherosclerosis in mice.201,556,557 In addition, SIRT6 overexpression reduces oxLDL uptake in RAW macrophages, and SIRT6 knockdown enhances it and increases the expression of macrophage scavenger receptor 1.558 Finally, SIRT7 has been reported to regulate the VSMC proliferation and migration via the Wnt/β-catenin signaling pathway, which provides a promising therapeutic strategy for anti-atherosclerosis.559
In conclusion, the role of SIRT1 in atherosclerosis has received extensive attention. SIRT1 deficiency in endothelial cells, VSMCs, and monocytes/macrophages promotes atherosclerosis.560 Therefore, SIRT1 might be a potential therapeutic target for the treatment of atherosclerosis. Other SIRTs might also have protective effects on atherosclerosis. However, due to a relatively low number of studies, the relevant mechanisms need to be further explored in the future.
Coronary artery disease (CAD)
CAD is the result of atherosclerotic plaque development in the walls of coronary arteries.561 It is one of the most common causes of death in the developed countries and is responsible for about one in every five deaths.562 Current studies on the role of SIRTs in CAD mainly focus on SIRT1, which has a protective effect on CAD by regulating some crucial biological processes, such as oxidative stress, inflammation, cell apoptosis, and cell proliferation.
Epidemiological studies have suggested that genetic SIRT1 polymorphisms are associated with the risk of CAD,563 while the expression level of SIRT1 is reduced in CAD patients.564 SIRT1 inhibition causes oxidative stress and inflammation in CAD patients.565 On a molecular level, expression of downregulated SIRT1 in human CAD monocytes is related to the enhanced acetylated p53 expression levels.565 In contrast, SIRT1 overexpression in human CAD monocytes mitigates pro-apoptotic events and attenuates some proinflammatory events, such as upregulating expression of NF-κB and iNOS and NO concentrations.565 SIRT1 has been reported to be involved in the regulation of CAD via noncoding RNAs.566,567 For example, promoted expression of SIRT1 by elevated expression of lncRNA C2dat1 and subsequent suppressed miR-34a expression increases VSMC proliferation and migration in CAD.567
Except for SIRT1, epidemiological studies also suggest that genetic polymorphisms of SIRT3 and SIRT6 are associated with the risk of CAD,563 but the related mechanism needs to be further explored. Given the protective role of SIRT1 in CAD, other SIRTs might also be potential therapeutic targets for CAD. Therefore, exploring the roles of other SIRTs in CAD might be a promising research direction in the future.
Myocardial ischemia/reperfusion (MI/R) injury
In recent years, the morbidity and mortality of ischemic cardiac diseases, such as myocardial infarction, have shown an upward trend.568 With the development of recanalization technology, the treatment of myocardial infarction has made remarkable progress.568 However, MI/R injury can be induced as the treatments progress.569 MI/R injury is closely related to oxidative stress and apoptosis,4 and SIRTs play crucial roles in MI/R by controlling the above biological processes.
SIRT1 has a protective effect on MI/R injury and reduces the infarct area of the heart.570,571 Cardiac-specific SIRT1 KO mice have shown a significantly increased myocardial infarction area size.572 In contrast, cardiac-specific SIRT1 overexpression was significantly reduced in the myocardial infarction area.572 As for its potential mechanism, overexpression of SIRT1 leads to upregulation of antioxidant pathways mediated by FoxO1 and MnSOD and downregulation of pro-apoptotic pathways mediated by caspase-3 and Bax, thereby protecting the heart from MI/R injury.572 In addition, SIRT1 overexpression has been shown to be involved in ameliorating miRNA inhibition associated with MI/R injury.573,574 For example, upregulated SIRT1 expression resulting from miR-132 inhibition might ameliorate MI/R injury by inhibiting oxidative stress and pyroptosis through activation of PGC-1α/Nrf2 signaling.573 The SIRT1/AMPK/PGC-1α pathway is involved in the process by which lncRNA Oip5-as1 attenuates MI/R injury by sponging miR-29a.574 Like SIRT1, nuclear deacetylase SIRT6 also has a protective effect on MI/R injury. On a molecular level, SIRT6 protects against MI/R injury by increasing FoxO3α-dependent antioxidant defense mechanisms575 and attenuating aging-related charged multivesicular body protein 2B accumulation.576
In addition, the protective effects of mitochondrial SIRT3-5 have been observed in MI/R injury. An in vivo study has revealed that SIRT3 deficiency exacerbates MI/R injury.577 Both in vitro and in vivo models have shown that SIRT4 is downregulated in cardiomyocytes after MI/R injury, and that SIRT4 overexpression decreases myocardial infarct size.578 This protective effect of SIRT4 against MI/R injury has been reported to be associated with preserved mitochondrial function and reduced myocardial apoptosis.578 Similarly, a prior in vivo study has demonstrated that SIRT5 loss increased myocardial infarct size and MI/R injury, which might be associated with the effect of SIRT5 on modulating protein succinylation in the heart.579
This evidence suggests that SIRT1-6 might play critical roles in alleviating myocardial infarction and M/IR by regulating some important biological processes, such as oxidative stress and apoptosis. However, relevant molecular mechanisms behind these processes need to be further explored. Moreover, few studies have focused on the roles of SIRT2 and SIRT7 in M/IR injury, and further research is needed in the future.
Hypertension, defined as systolic blood pressure of ≥ 140 mmHg and/or diastolic blood pressure of ≥ 90 mmHg, is the risk factor for other CVDs,580 affecting an estimated 1.39 billion people worldwide in 2010. Its prevalence is still rising globally.581 In recent years, increasing studies have focused on the protective effects of SIRT1 and SIRT3 on hypertension.582,583 In vivo studies have shown that SIRT1 overexpression in VSMCs attenuates Ang II-induced hypertension in mice.582 Similarly, SIRT3 overexpression attenuates Ang II and deoxycorticosterone acetate salt-induced hypertension in transgenic mice.583 Both SIRT1 and SIRT3 have been reported to be involved in the regulation of oxidative stress in hypertension.584,585,586 For example, SIRT1 activation attenuates Klotho deficiency-induced arterial stiffness and hypertension by increasing AMPKα and eNOS activity.584 SIRT1 overexpression mediated by NAMPT alleviates Ang II-mediated ROS production.585 In addition, diminished SIRT3 expression and redox inactivation of SIRT3 leads to SOD2 inactivation and contributes to the pathogenesis of hypertension.586
SIRTs also play important roles in the complications of hypertension. For example, decreased urinary levels of SIRT1 can be seen as a non-invasive biomarker of early renal damage in hypertension.587 SIRT3 alleviates the development of hypertensive renal injury by suppressing EMT.588 Endothelial-specific deletion of SIRT6 significantly enhances blood pressure and exacerbates endothelial dysfunction and cardiorenal injury in experimental hypertension by targeting Nkx3.2-GATA5 signaling.589
These findings indicate that SIRTs have protective effects on the occurrence and development of hypertension and might be valuable predictive biomarkers as well as promising therapeutic targets for hypertension complications. However, relevant mechanisms still need to be further explored, especially for SIRT2, SIRT4, SIRT5, and SIRT7, which have not been extensively investigated.
This section summarized the effects of SIRTs on CVDs. The effects of SIRT1, SIRT3, and SIRT6 have received extensive attention. Most studies have shown that they have a protective effect on CVDs, such as cardiac fibrosis, heart failure, atherosclerosis, and M/IR injury. Compared to SIRT1, SIRT3, and SIRT6, studies on SIRT2, SIRT4, SIRT5, and SIRT7 are relatively limited, even though they play important roles in CVDs. The protective effects of SIRT2, SIRT5, and SIRT7 in several CVDs (e.g., hypertrophy) have been observed. Of note, SIRT4 might aggravate cardiac hypertrophy and fibrosis. Overall, SIRTs are promising therapeutic targets, and the pharmacological modulation of SIRTs can be used in the prevention and treatment of CVDs.
SIRTs and respiratory system diseases
Respiratory diseases are one of the biggest threats to human health.590 Common respiratory diseases, including asthma, chronic obstructive pulmonary disease (COPD), lung fibrosis (LF), coronavirus disease 2019 (COVID-19), and other lung injures, seriously affect physical and mental health.590 SIRTs have received considerable attention due to their important effects on respiratory diseases.591 Herein, we summarize the related studies in several common respiratory system diseases (Fig. 11).
COPD is a common disease characterized by persistent respiratory symptoms and progressive airflow obstruction.592 Most chronic respiratory disease-attributable deaths are due to COPD, which is the fourth leading cause of death worldwide and considered to be a global public health challenge.593,594,595 Oxidative stress, inflammation, and apoptosis are the most important influencing factors for COPD occurrence596 and are closely related to SIRT family.593 Cigarette smoking (CS) is a causative factor for COPD. The level of SIRT1 is substantially decreased in lungs of patients with COPD/emphysema, as well as in lungs of rodents exposed to CS.597 Moreover, SIRT1 has been found to have anti-inflammatory, anti-apoptotic, and antioxidant roles in the pathogenesis of COPD.598 For example, SIRT1 plays a pivotal role in regulating NF-κB-dependent proinflammatory mediators in lungs of smokers and patients with COPD.599 Apart from NF-κB regulation, SIRT1 also mediates COPD via deacetylation of the FoxO3 transcription factor and tumor suppressor p53 involved in lung cell senescence and oxidative stress-induced cellular apoptosis.597,599 Moreover, the SIRT1 activator SRT1720 might be able to inhibit LPS-induced cytokine release from cultured peripheral blood mononuclear cells in patients with COPD. Thus, pharmacological activation of SIRT1 might have considerable potential as a novel form of chronopharmacology in COPD.600
SIRT6 plays an important role in the regulation of autophagy in COPD.591 For example, reduced SIRT6 expression level is associated with COPD development through enhancement of cellular senescence created by insufficient autophagy during CS exposure.601 SIRT6 overexpression weakens autophagy via IGF–Akt–mTOR signaling.601 Similar to SIRT1, reduced SIRT6 level is also implicated in COPD.602 Therefore, SIRT6 deficiency might contribute to the development of COPD.
SIRT3 is a mitochondrial deacetylase regulating mitochondrial function, and its role in the pathogenesis of COPD has also been mentioned. For instance, SIRT3 inhibits airway epithelial mitochondrial oxidative stress, thereby contributing to attenuating the progression of COPD.603 Therefore, activating the SIRT3 signaling pathway might present a novel therapeutic target to slow or prevent the pathogenesis of COPD.
With the understanding of the positive roles of SIRT1, SIRT3, and SIRT6 in COPD, their pharmacological activation by specific agents might be a promising strategy against COPD. However, other SIRT family members have not yet been studied in the respiratory system. SIRTs mainly mediate this disease via inflammation- or autophagy-related pathways.604,605 In addition, COPD is commonly thought to be associated with other chronic diseases, especially those where accelerated aging is involved. Therefore, the defection of anti-aging molecules, such as SIRTs, has been proposed as a mechanism for accelerated lung aging in COPD.606 Given the severity and complexity of COPD, further studies are necessary to validate the exact roles of SIRTs.
LF is a leading cause of death in the industrialized world, which significantly increases with age.607 An epidemiological study has shown that approximately 45% of global deaths have been attributed to fibrosis.607 The pathogenesis of LF is complex and involves environmental influences and microorganisms.608 Recent developments in the field of LF have pointed towards the pivotal role of SIRTs in regulating disease progression, thereby qualifying as potential anti-fibrotic drug targets.607 Four of the seven SIRTs (SIRT1, SIRT3, SIRT6, and SIRT7) have been investigated in LF, while the functional roles of the remaining SIRTs (SIRT2, SIRT4, and SIRT5) remain elusive.
SIRT1 loss might be involved in the pathogenesis of LF. Thus, its activation might be an effective treatment for LF. SIRT1 plays an important role in regulating alveolar epithelial cell 2 progenitor renewal and LF.609 Mechanistically, SIRT1 activation promotes self-renewal and differentiation of alveolar epithelial cell 2 in lung tissues of idiopathic pulmonary fibrosis (IPF) patients and aged mice.609 However, the opposite results have been reported for SIRT1 changes in LF. According to the study performed by Zeng et al., SIRT1 expression was significantly increased in lungs from patients with IPF, as well as in lungs from bleomycin-induced LF mouse models.610 Nevertheless, SIRT1 activation or overexpression attenuates LF through regulation of canonical TGF-β1/p300 signaling. In addition, SIRT1 activation has been used in aging-related LF prevention and therapy.611 As the expression of SIRT1 in LF is controversial, more studies are needed to explore this notion in the future.
Due to the preferential mitochondrial association with extended life span in humans, SIRT3 is a protein of particular interest in age-related diseases, including LF.612 For example, there is a SIRT3 deficiency within the murine aging lung, which promotes the fibrotic response mediated by TGF-β1.612 TGF-β1 is a major multifunctional cytokine that is known as a mediator implicated in LF pathogenesis.613 In addition, SIRT3 deficiency promotes LF by augmenting alveolar epithelial cell mitochondrial DNA damage and apoptosis.614 Cheresh et al. have suggested that SIRT3 overexpression can ameliorate asbestos-induced pulmonary fibrosis.615 Thus, improvement in SIRT3 expression might be a novel therapeutic focus for managing patients with IF.
It is possible that SIRT6 participates in the inhibition of cellular fibrosis by regulating the TGF-β1 signaling pathway. SIRT6 can also be an ambitious target molecule for understanding the pathogenesis of IPF through the inhibitory role in TGF-β-induced cellular senescence.616 Additionally, Chen et al. have shown that targeting SIRT6 is a potential novel therapeutic strategy for pulmonary fibrosis that involves inactivating the TGF-β1/Smad2 signaling pathway.617 Furthermore, SIRT6 prevents TGF-β1-induced lung myofibroblast differentiation by inhibiting the TGF-β1/Smad2 and NF-κB signaling pathways.618 SIRT6 also inhibits EMT during IPF by inactivating TGF-β1/Smad3 signaling,619 highlighting the critical role of SIRT6 in LF. Moreover, all SIRTs show a tendency to be expressed at lower levels in fibroblasts from patients compared to controls, but the greatest decrease is observed with SIRT7.620 Furthermore, the decline in SIRT7 in LF has a profibrotic effect, which is mediated by changes in Smad3 levels.620
The above evidence shows that SIRT1, SIRT3, SIRT6, and SIRT7 are beneficial for preventing and improving the pathogenesis of LF. However, the modulatory roles of other SIRT members remain unclear. Mazumder et al. have reviewed the regulatory roles of under-reported SIRTs (mainly SIRT2, SIRT4, and SIRT5, which lack direct reported associations with LF) in basic cellular and mitochondrial metabolic pathways critical to LF.607 Overall, they have suggested that SIRTs appear to exert a protective action in LF, except SIRT2, which might have a pro-fibrotic action given its proinflammatory effects observed in asthma.607 In summary, studies on the function of SIRTs in regulating LF have potential.
Asthma is a chronic inflammator