Review

Oncogene (2008) 27, 225–233; doi:10.1038/sj.onc.1210907

Cancers take their Toll—the function and regulation of Toll-like receptors in cancer cells

R Chen1,2, A B Alvero2, D-A Silasi2, K D Steffensen3 and G Mor2

  1. 1Department of Molecular, Cellular and Developmental Biology, Yale University, New Haven, CT, USA
  2. 2Department of Obstetrics, Gynecology and Reproductive Sciences, Yale University School of Medicine, New Haven, CT, USA
  3. 3Department of Oncology, Vejle Hospital, Vejle, Denmark

Correspondence: Dr G Mor, Department of Obstetrics, Gynecology and Reproductive Sciences, Reproductive Immunology Unit, Yale University School of Medicine, 333 Cedar St. FMB 301 New Haven, CT 06520, USA. E-mail: gil.mor@yale.edu

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Abstract

Cancer could be deemed as an abnormal and uncontrolled tissue repair process. Therefore, it would not be surprising that factors that function in the tissue repair process, such as cytokines, chemokines, growth factors and Toll-like receptor (TLR) ligands, as well as growth signals for compensatory proliferation, would also be key factors in regulating and enhancing cancer progression. The TLR pathways, which play a critical role in tissue repair, are also key regulators in cancer progression as well as chemoresistance. TLRs serve as cell surface sensors that can initiate pathways leading to proliferation and chemoresistance; as well as mediators that are able to regulate the infiltrating immune cells to provide further support for cancer progression.

Keywords:

ovarian cancer, Toll-like receptors, MyD88, inflammation, tissue repair, chemoresistance

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Introduction

Inflammation is a relevant physiologic process necessary for immune protection, tissue repair and tissue remodeling. In tissue injury and the subsequent process of wound healing, the inflammatory process mediated by immune cells not only contains potential microorganism invasion, but also promotes, in the surrounding tissue, cell proliferation and neovascularization, which will ensure appropriate healing and repair of the wound. Similar process occurs in many organs of the body with high cellular turnover rates such as the gastrointestinal and reproductive tracts (Girling and Hedger, 2007).

In these tissues, the immune infiltrate and resulting inflammatory process secure the removal of dying cells and promotes the renewal of the tissue by enhancing cell proliferation and maintaining adequate blood supply (Mor et al., 2002). All these processes are mediated by the production of cytokines and chemokines, which function as the main regulators not only of immune cells, but the entire cellular component of the specific microenvironment (Lin and Karin, 2007).

In normal conditions, the inflammatory process is tightly controlled, to secure that cell proliferation only occurs until repair is completed or infection resolved. In contrast, a chronic inflammatory process represents a rich environment containing inflammatory cells and growth/survival factors that could enhance the growth of surrounding cells with already sustained DNA damage or mutations, which may lead to the formation of a tumor (Lin and Karin, 2007). Indeed, if we look at the tumor microenvironment, it contains significant amount of immune infiltrates and has all the characteristics of an ongoing inflammatory process (Chen et al., 2007c).

At present time, the relationship between inflammation and tumorigenesis is widely accepted; however, the cellular and molecular mechanism involved in this process is incompletely understood. Our studies suggest that cancer cells have acquired many properties characteristic of immune cells, allowing them to communicate and more importantly, regulate the immune system for its own survival and growth (Kelly et al., 2006; Chen et al., 2007b). The Toll-like receptors (TLRs), and their intracellular signaling components, constitute an important cellular pathway mediating this interaction and are associated not only with the inflammatory process but also with the development of chemoresistance. In this review we will discuss the role of TLRs in tumor cells and their role in the promotion of a proinflammatory/progrowth microenvironment and the acquisition of chemoresistance.

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Inflammation and cancer

Numerous studies have provided convincing evidence supporting the notion that bacterial- and viral-induced inflammatory process can mediate tumorigenesis (Coussens and Werb, 2002). Surgical removal of a primary tumor is often followed by rapid growth of previously dormant metastases as observed in in vivo studies and lipopolysaccharide (LPS) has been suggested to be responsible for this effect (Pidgeon et al., 1999). Indeed, Balb/c mice receiving a tail vein injection of 4T1 mouse mammary carcinoma cells showed an increase in lung metastases following LPS injection (Harmey et al., 2002). In humans, chronic infection and inflammation are considered two of the most important epigenetic and environmental factors contributing to tumorigenesis and tumor progression (Balkwill and Coussens, 2004; Beachy et al., 2004).

In 1858, Rudolf Virchow noticed that the generation of cancer often happened at sites of chronic inflammation (Virchow, 1858, 1863). He also hypothesized that chronic inflammation could promote the proliferation of cells and thus, the development of cancer. In the past 15 years, numerous cancers have been shown to be associated with local chronic inflammation. Chronic inflammatory bowel diseases such as chronic ulcerative colitis and Crohn's disease have strong association with colon cancer (Balkwill and Coussens, 2004). Similarly, gastric cancer has a strong link with chronic Helicobacter pylori infection and the resulting inflammation (Ernst et al., 2001). Ovarian endometriosis is an established risk factor for certain types of epithelial ovarian cancers (Giudice and Kao, 2004; Riman et al., 2004; Sekizawa et al., 2004). Other examples include chronic bronchitis and lung cancer, schistosomiasis and bladder cancer, papillomavirus infection and cervical cancer, chronic pancreatitis and pancreatic cancer, chronic cholecystitis and gall bladder cancer, and hepatitis virus B and C infection and liver cancer (Balkwill and Mantovani, 2001; Coussens and Werb, 2002; Li et al., 2005). Moreover, epidemiological studies showed that regular intake of nonsteroidal antiinflammatory drugs lowered the risk of developing some types of cancers (Gupta and Dubois, 2001; Balkwill and Coussens, 2004).

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Toll-like receptor signaling pathways and their expression in cancer cells

TLRs play a key role in the innate immune system, particularly in inflammatory response against various invading exogenous pathogens, by recognizing receptor-specific pathogen-associated molecular patterns of highly conserved pathogenic components of bacteria, viruses, fungi and parasites (Takeda et al., 2003; Heil et al., 2004; Takeda and Akira, 2004; Zhang et al., 2004; Gorden et al., 2005; Yarovinsky et al., 2005). In addition, TLRs can also be activated by endogenous ligands, as shown in Table 1 (Leadbetter et al., 2002; Heil et al., 2004; Tsan, 2006). Most TLRs signal through a common pathway—the myeloid differentiation primary response gene 88 (MyD88)-dependent pathway (Figure 1), since they possess a common intracellular domain known as the Toll/interleukin (IL)-1 receptor (TIR) domain (Rock et al., 1998). Following TLR ligation, it recruits the adapter protein MyD88, which associates with the intracellular domain of the receptor through a TIR–TIR interaction (Muzio et al., 1997; Wesche et al., 1997; Burns et al., 1998; Sato et al., 2003) leading to subsequent downstream activation of the nuclear factor of kappa light polypeptide gene enhancer in B-cells (NF-κB) (early phase NF-κB activation) and mitogen-associated protein (MAP) kinase signaling pathways (such as the ERK-CREB pathway, the JNK-AP1 pathway, and the p38 pathways). This signaling cascade is responsible for the induction of a proinflammatory response characterized by the production of cytokines and chemokines, as well as cell proliferation (McDermott and O'Neill, 2002).

Figure 1.
Figure 1 - Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

Toll-like receptor (TLR) signaling pathways. Membranal TLRs (represented by TLR4) recognize external ligands (exogenous and endogenous), while cytoplasmic TLRs (TLR3) recognize intracellular signals. When activated, the majority of TLRs induce activation of NF-κB (early phase NF-κB activation) and cytokine production in a MyD88-dependent manner; while TLR4, like TLR3, can also signal in a MyD88-independent manner and induce the expression of type I interferons (IFN) and IFN-inducible proteins in addition to a late phase NF-κB activation.

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Some TLRs (TLR3 and -4) can also induce NF-κB activation via a MyD88-independent manner through another TIR domain-containing adaptor molecule, the TIR domain-containing adaptor inducing IFN-β (TRIF) (Akira et al., 2003; Barton and Medzhitov, 2004; Takeda and Akira, 2004). The recruitment of different signaling molecules by TRIF upon TLR stimulation can lead to the activation of different downstream mediators and targets (Figure 1). The recruitment of TRAF6 to TRIF can activate NF-κB via a MyD88-dependent pathway-like manner (late phase NF-κB activation). The association of receptor-interacting protein 1 with TRIF also activates NF-κB through a yet unknown pathway (late phase NF-κB activation).

Most of the reports on TLRs have focused on their expression and function in cells of the immune system. Recently however, TLR expression and function in cancer cells and its association with tumorigenesis and tumor progression has become a very active field (Chen et al., 2007c). In a recent study, Huang et al. (2005) reported the expression of Tlr4 in murine tumor cell lines and showed that the activation of Tlr4 in these tumor cells by LPS-induced tumor evasion from immune surveillance. Recently, our group described the ubiquitous expression of TLR4 in human epithelial ovarian cancer (EOC) cells and showed that in a subgroup of EOC cells that express MyD88 (referred from now on as type I EOC cells), ligation of TLR4 by LPS-induced cell proliferation and enhanced cytokine/chemokine production. This response was however, not observed in the subgroup of EOC cells that do not express MyD88 (referred from now on as type II EOC cells) (Kelly et al., 2006).

Another recent study by Huang et al. (2007) showed that Listeria monocytogenes (Lm) promotes tumor growth through TLR2. Using an in vivo model, the authors found that vaccination against Lm inhibited the growth of H22 tumor cells while injection of Lm enhanced cell growth. They showed that this effect was mediated by direct interaction of the cancer cells with the bacteria, as demonstrated by increased NF-κB activity in cancer cells only in the animals expressing Tlr2 but not in the Tlr2 knockout mice, following injection of heat-killed Lm.

Similarly, He et al. (2007) described the expression of TLR4 in human lung cancer cells. TLR4 ligation on the cancer cells promoted the secretion of immunosuppressive cytokines and induced resistance to TNFα and TRAIL-induced apoptosis.

From these studies, it is reasonable to conclude that the activation of TLRs in cancer cells and the ensuing cytokine/chemokine production may have two major consequences: (1) the direct promotion of cancer cell survival, chemoresistance and therefore tumor progression; and (2) the regulation of immune response within the tumor microenvironment. Below, we will discuss each of these potential effects.

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TLRs promote tumor progression and chemoresistance

Inflammation-induced chemoresistance has been linked to the hyperactivation of NF-κB in cancer cells (reviewed in Nakanishi and Toi, 2005). The transcription factor NF-κB upregulates the expression of many proinflammatory cytokines, chemokines, growth factors, matrix metalloproteinases, adhesion molecules, and more importantly, antiapoptotic proteins (Shishodia and Aggarwal, 2002; Pikarsky et al., 2004).

It is well established that apoptosis or programmed cell death is the key mechanism of most antitumor therapies including chemotherapy, radiotherapy and immunotherapy (Alvero et al., 2006). Activation of NF-κB has been shown to induce upregulation of antiapoptotic proteins such as c-FLIP and XIAP, and to inhibit proapoptotic proteins such as Bax and Caspase-9 (Kreuz et al., 2004). Taken together, these cellular events may lead to chemo-resistance (Kamsteeg et al., 2003; Pommier et al., 2004).

Central questions in these observations have been to determine the factors inducing NF-κB activity in the cancer cells. TNFα, which has been postulated as a potential candidate, is a major inducer of NF-κB activity promoting the early phase of activation (Covert et al., 2005; Werner et al., 2005). Moreover, the activation of TLRs in cancer cells may represent another important pathway contributing to NF-κB activation. In ovarian cancer cells with a functional TLR-MyD88 pathway (type I EOC cells), treatment with ligands for TLR4 such as LPS or paclitaxel increased the expression of XIAP and phospho-AKT, two important regulators of cell survival (Kelly et al., 2006). Similarly, the ligation of TLR2 in lung cancer cells induced the activation of mitogen-activated protein kinases (MAPK) as well as NF-κB, which were shown to prolong cancer cell survival (Huang et al., 2007).

Cancer development shares many similarities with the tissue repair process. First, both processes are triggered by local tissue injury, and second, they are both aided with the activity of the innate immune system, by way of local inflammation (Coussens and Werb, 2002).

In both processes there is an increase in the stimulation of cell proliferation. Contrary to the normal tissue repair process, where cell proliferation is under strict control and the process ends when the wound is completely healed; in cancer, however, this process is out of control and as a result—tumor progression ensues.

Therefore, it is plausible that cancer development is an abnormal form of tissue repair, in which the control mechanism loses its function by either mutation or being aborted by abnormal proremodeling signals (such as proinflammatory cytokines and growth factors).

An important stimuli for TLRs and subsequent effect on cell proliferation and survival are molecules released during the process of tissue repair. The tissue repair process has been reported to depend on TLR4-MyD88 signaling (Fukata et al., 2005). LPS was shown to accelerate wound repair in vitro (Koff et al., 2006). TLR4-MyD88 signaling is important to maintain intestinal epithelial homeostasis in response to gut injury, and both TLR4 and MyD88 knockout mice displayed impaired compensatory proliferation and increased apoptosis (Rakoff-Nahoum et al., 2004; Fukata et al., 2005; Pull et al., 2005). Similarly, in a mouse model of acute lung injury, hyaluronan released from injured cells protected epithelial cells from apoptosis through the TLR2/4-MyD88-NF-κB pathway (Jiang et al., 2005).

Dying cells stimulate inflammation through TLRs and MyD88 present in the immune cells, inducing a process known as ‘sterile inflammatory response’ (Chen et al., 2007a). The expression of TLRs by cancer cells may facilitate a similar type of response to the presence of dying cells, resulting from either normal tissue turnover or in response to chemotherapy. Indeed, certain molecules released by necrotic or apoptotic cells have been described to be ligands for TLRs (Chen et al., 2007c). Therefore, in the context where tumor cells have a functional TLR-MyD88 pathway, as previously described for epithelial ovarian cancer cells, the presence of these molecules may initiate a signal leading to NF-κB activation, and therefore promotion of an antiapoptotic and progrowth microenvironment. There are multiple evidence demonstrating that use of continuous cycles of several chemotherapeutic drugs can induce initial tumor reduction (cell death of the sensitive cells), which, however, is followed immediately by recurrence of the tumor characterized by a more aggressive phenotype. We could argue that the partial induction of cell death in the tumor can trigger a sterile inflammatory response mediated by TLRs, present on cancer cells that survived therapy. This would then initiate a process of ‘tissue repair’, which in the context of neoplasia, involves tumor growth. As we will discuss below, this seems to be the case in chemoresistant ovarian cancer tumors.

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TLRs, cancer cells and the tumor immune infiltrate

Immune infiltration is a remarkable phenomenon in tumor development. The infiltrating host leukocytes, such as neutrophils, tumor-associated macrophages (TAMs), dendritic cells, eosinophils, mast cells and lymphocytes, are present in both the supporting stroma and the tumor sites, forming the inflammatory microenvironment (Ben-Hur et al., 2000a, 2000b; Balkwill and Mantovani, 2001; Coussens and Werb, 2002). Previously, the immune infiltrates were thought to help the host against the developing tumor, however, recent studies showed that instead of combating the tumor, the infiltrating immune cells contribute to cancer growth and metastasis, as well as immunosuppression (Balkwill and Mantovani, 2001; Coussens and Werb, 2002; Philip et al., 2004). By analysing 1919 cases, Menard et al. (1997) found that immune infiltration had no association with survival and prognosis of breast carcinoma patients more than 40 years of age. Furthermore, in previous studies we found that the infiltrating macrophages in breast carcinoma were a main source of estrogen, which is a major stimulus for cell proliferation and cancer progression (Mor et al., 1998). Similarly, Pollard reported that the infiltration of macrophages stimulated breast cancer progression and metastasis (Pollard, 2004), and knocking out colony-stimulating factor-1 (CSF-1, a cytokine that regulates macrophage differentiation and function) in a mouse strain inhibited breast cancer growth and metastasis. A study on oral epithelial squamous cell carcinoma also revealed that the level of immune cell infiltration was positively correlated with the level of morphological and pathological transformation from normal to malignant phenotypes (Gannot et al., 2002).

In ovarian cancer samples, staining for CD45, a marker for leukocytes, revealed intensive immune infiltration surrounding cancer cells and stroma (Figure 2a) (Silasi et al., 2006). Interestingly, necrotic centers represent as origins of the immune infiltrates and from these centers the cells migrate towards the rest of the tumor (Figure 2b). The presence of necrotic cells has been proposed to represent one of the stimuli for macrophage migration and differentiation (Lotze and Tracey, 2005). Indeed, contrary to what would be expected, the presence of necrosis in the tumor is associated with bad prognosis. Many of the cellular factors released by the dying cells may function as potent stimuli to the immune cells, primarily macrophages (Lotze and Tracey, 2005).

Figure 2.
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Immune infiltration in ovarian cancer samples. (a) CD45-positive cells (brown) infiltrate between cancer cells (blue) observed on a frozen section of primary ovarian cancer tumor. (b) Necrotic centers may serve as potential origins of the immune infiltrates and the cells may migrate into the rest of the tumor from these centers.

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Therefore, as discussed above, the inflammatory process is not only triggered by infection but also by dying cells, and as in infection, the products of dying cells need to be recognized by immune cells, or tumor cells, in order to elicit the inflammatory response. During both cancer development and tissue repair process, the immune infiltrate is characterized by the presence of a high number of macrophages. Further characterization of these cells had led to the conclusion of the existence of two subclasses of macrophages (M1 and M2 types of macrophages) based on their cytokine profile. M2 macrophages are the dominant subtype recognized during tissue repair and in the protumor inflammatory microenvironment. M2 macrophages promote tissue repair and remodeling and are present in established tumors and may promote tumor growth (Lewis and Pollard, 2006). Unlike M1 macrophages, which are known to mediate response against intracellular parasites and tumors by producing IL-12, IL-23, IFNγ, IL-18 and TNFα, M2 macrophages have high levels of scavenger, mannose and galactose-type receptors, produce VEGF, IL-6, IL-10, PG, iNOS and IDO, and have immunoregulatory and proliferation-stimulating functions.

What directs macrophage differentiation at the tumor microenvironment towards the M2 type or tumor-supportive phenotype? A potential explanation is the cancer cells themselves. We proposed three stages (Chen et al., 2007c): (1) Recruitment: via the production of chemokines (MCP-1, GROα and IL-8) cancer cells recruit immune cells to the tumor microenvironment; (2) Education: via the secretion of cytokines that regulate immune cell differentiation (IL-6, TNFα and MIF) tumor cells polarize immune cells towards tumor-supporting cells and (3) Response: the differentiated immune cells produce cytokines, hormones and growth factors at the tumor microenvironment that promote tumor growth and create immune tolerance (Figure 3).

Figure 3.
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Tumor-immune cell ‘Education’ model. Cancer cells secrete chemokines to recruit immune cells (Recruitment), induce their differentiation into protumor cells (Education) and receive support from the differentiated immune cells (Response). GROα, growth-regulated protein alpha; IL-8, interleukin-8; MCP-1, monocyte chemo-attractant protein-1; MIF, macrophage migration inhibitory factorTNFα, tumor necrosis factor-alpha.

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For all these three stages, the expression of TLRs by the cancer cells is a major factor. Through TLR2 or TLR4, cancer cells can recognize either microbial pathogens or cellular debris, and promote the expression and secretion of chemokines and cytokines (for example, MCP-1 and IL-8) which would work as mediators for the regulation of immune cell migration, differentiation and function (Figure 3).

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The TLR4-MyD88-NF-κB pathway in ovarian cancer

Epithelial ovarian cancer is the most common form of ovarian cancer and all patients with this diagnosis are treated with the same regimen that includes the combination of paclitaxel and carboplatin (Schwartz, 2002). However, the clinical response is not the same, suggesting that there are additional differences that are not associated with the histologic characteristics.

Our initial analysis for the expression of TLR4 in ovarian cancer revealed a ubiquitous expression of this receptor in those tumors; however, the response to TLR4 ligation was not the same. Further analysis of the intracellular components led us to the identification of type I and type II EOC cells based on their TLR4/TNFα response which is associated with characteristics of tumor adaptation and malignancy (Chen et al., 2007b). Type I cells are characterized by the following: (i) a functional TLR4 signaling pathway; (ii) constitutive cyclic activity of NF-κB; (iii) continuous production of cytokines, which is further enhanced by TLR4 ligation (with either LPS or paclitaxel) and TNFα stimulation; (iv) high expression of MyD88, low IκBα and a high IKKβ/IKKα ratio; and (v) chemoresistance. Type II cells, however, do not have a functional TLR-MyD88 pathway and do not constitutively produce cytokines, and are chemosensitive.

Interestingly, the functional TLR-MyD88 pathway in type I cells contributes to the maintenance of the proinflammatory microenvironment by secreting a substantially elevated level of cytokines and chemokines, all of which lead to further tumor growth. The characteristic proinflammatory environment generated by type I EOC cells was lost upon the knockdown of MyD88, suggesting that an active MyD88-dependent TLR4 signaling is responsible for the LPS-induced, NF-κB-mediated EOC cell proliferation and cytokine secretion (Kelly et al., 2006). In contrast, type II EOC cells do not constitutively secrete cytokines, and no changes in cell proliferation or cytokine production were observed following LPS treatment.

Type I EOC cells also use TLRs to communicate with and ‘educate’ the immune cells. Indeed, we observed that the upregulated chemokine expression in type I EOC cells upon TLR4 stimulation by LPS-induced in vitro monocyte migration and differentiation into a protumor phenotype (unpublished data).

Tumors are made of heterogeneous cell populations, which is also evidenced in ovarian cancer tumors in term of type I and II (Figure 4). The ratio of type I (potentially cancer stem cells, highly resistant to proapoptotic signals) versus type II (more differentiated and sensitive to apoptotic signals) may determine the growth of the tumor and the outcome in response to therapy. The type I cells may serve as resources of cytokines, chemokines and other signaling molecules that promote tumor progression, neovascularization and ‘education’ of the immune infiltrates; while the turnover and/or chemotherapy-induced apoptosis of the sensitive type II cells would further activate the tissue repair potential of the infiltrated immune cells (M2 macrophages in particular) and lead to even elevated tumor growth.

Figure 4.
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Heterogeneity of epithelial ovarian cancer (EOC) tumors. Type I and type II EOC cells could both be seen on a frozen section of a representative primary EOC tumor. This indicates that the ratio between type I and type II cells may determine the characteristics of the tumor in terms of growth, recurrence and chemoresistance.

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In summary, these findings suggest that the proinflammatory conditions observed at the tumor microenvironment may not only originate from the immune cells, but also from the cancer cells as well. However, this capacity is not intrinsic to all the cancer cells, but to a selected population which has acquired a functional TLR4-MyD88-NF-κB signaling pathway. We hypothesize that it is the elevated repair capacity of type I EOC cells after chemotherapeutic drug treatment that results in the subsequent development of chemoresistance and tumor recurrence. Finally, cancer cells can ‘educate’ the immune infiltrates to produce the type of cytokines that will facilitate tumor growth and metastasis as well as acquiring immune tolerance (Figure 5).

Figure 5.
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Inflammation—Toll-like receptor (TLR)- and tumor immune interaction. TLR-mediated NF-κB activation induces proinflammatory cytokine production, which could promote cancer cell proliferation and induce immune cells differentiation toward a tumor-supporting phenotype. Cytokines produced by the tumor induce the differentiation of macrophages towards a M2 phenotype, prevents maturation of iDC and therefore maintaining a tolerance stage increase the number of T reg cells with subsequent inhibition of T cells. Yellow arrows, promotion; red arrows, inhibition.

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Epigenetic regulation of TLR function—miRNAs, TLR signaling and cancer

microRNAs (miRNAs) are a class of small RNA molecules that regulate gene expression at post-transcriptional level (Ruvkun, 2001; Rana, 2007). They work by complementing sequence-specific targets in the 3′-UTR of mRNAs through incorporation to RNA-induced silencing complex (RISC), which leads to either the degradation of the target mRNA (if the miRNA and its target sequence is a perfect match), or the inhibition of the target mRNA translation (if the match between miRNA and its target sequence is imperfect) (Wiemer, 2007). Recent studies have demonstrated a link between miRNAs and TLR function, and therefore potential association with inflammation and cancer formation.

miR-155 is one of the most studied miRNAs related to inflammation and cancer. It has been found highly expressed in B-cell lymphoma, breast and lung cancers, and pancreatic adenocarcinomas (Tam et al., 1997; Eis et al., 2005; Calin and Croce, 2006; Esquela-Kerscher and Slack, 2006; Wiemer, 2007). Recent works by various groups gradually unveil the function of miR-155. O'Connell et al. (2007) observed that ligands for TLR2, TLR3, TLR4 and TLR9 could all induce the upregulation of miR-155 expression, through both MyD88- and TRIF-dependent pathways, and the expression of miR-155 is JNK-dependent. Our group also found that miR-155 was highly expressed in type I EOC cells, suggesting that miR-155 may be necessary for maintaining a normal proinflammatory protumor environment mediated by these cells (Chen et al., 2007b).

miR-146 has been reported to be highly expressed in breast, prostate, pancreatic, stomach and papillary thyroid carcinomas (He et al., 2005; Volinia et al., 2006; Wiemer, 2007). He et al. (2005) also found that the high expression of miR-146 distinguished papillary thyroid cancers from healthy controls. In ovarian cancer cells, we observed high level of miR-146a expression in type I but not type II EOC cells, indicating a role of miR-146 in regulating the proinflammatory characteristics of type I cells (Chen et al., 2007b). Indeed, both miR-146a and miR-146b are targets of NF-κB, and are upregulated upon TLR2, TLR4 or TLR5 ligation as well as TNFα or IL-1β stimulation (Taganov et al., 2006). Taganov et al. (2006) also identified IRAK1 and TRAF6 as direct targets of miR-146, indicating that miR-146 may be important negative regulators of the TLR signaling pathway.

In addition, we recently reported the role of miR-199a as regulator of IKKβ expression in ovarian cancer cells (Chen et al., 2007b). In this case the absence of miR-199a expression in type I cells results in a high IKKβ expression level, which is associated with a functional TLR/MyD88 pathway, cytokine production and more important, chemoresistance. The case is vice versa in type II cells. The loss of the inhibitory effect of miR-199a on IKKβ expression in type I cells may represent an evolutionary fine-tuning during the development of chemoresistant cells. These findings suggest that miRNAs may provide invaluable targets to treat chemoresistant cancers.

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Conclusion

Cancer could be deemed as an abnormal and uncontrolled tissue repair process. Therefore, it would not be surprising that factors that function in the tissue repair process, such as cytokines, chemokines, growth factors and TLR ligands, as well as growth signals for compensatory proliferation, would also be key factors in regulating and enhancing cancer progression. The TLR pathways, which play a critical role in tissue repair, are also key regulators in cancer progression as well as chemoresistance. On the one hand, they serve as cell surface sensors that can initiate pathways leading to proliferation and chemoresistance; on the other, they are also mediators that are able to regulate the infiltrating immune cells to provide further support for cancer progression. Moreover, microRNAs that regulate at different levels of the TLR pathways, may function as either oncogenes or tumor suppressors and their abnormal expression may further facilitate cancer growth. Better understanding of the function and regulation of the TLR signaling pathways in cancer may shed new lights on understanding the mechanisms of cancer formation and progression, as well as provide new targets for more effective regimens to treat cancer.

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