Introduction

The modern sedentary lifestyle, unhealthy food with too much fat and salt, physical inactivity and psychological stress have led to a global epidemic of obesity in the last few decades.1, 2 In particular, obesity hypertension and metabolic syndrome have become major public health concerns.3, 4 According to the 2006 National Health and Nutrition Survey in Japan, one out of two men and one out of five women aged between 40 and 74 years are suffering from metabolic syndrome or are sufferers-to-be.

Recently, the nuclear receptor superfamily has been postulated as key molecules in metabolic syndrome.5, 6, 7 The nuclear receptors are ligand-activated transcription factors whose activity is regulated by small lipophilic molecules that include steroid hormones, fat-soluble vitamins, thyroid hormone, retinoids and dietary lipids, and control genes involved in glucose, lipid and energy metabolism.8 The family also includes orphan nuclear receptors, such as peroxisome proliferator-activated receptor (PPAR)-α, γ, δ, liver X receptor and farnesoid X receptor. Indeed, genetically engineered mice of estrogen receptor,9 androgen receptor,10 glucocorticoid activating enzyme 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1),11 PPAR-γ12 and PPAR-δ13 were reported to develop visceral obesity and metabolic syndrome. On the other hand, the renin–angiotensin–aldosterone system (RAAS) is also implicated in the pathogenesis of metabolic syndrome.14, 15 Aldosterone is a component of the RAAS and its receptor mineralocorticoid receptor (MR) belongs to the nuclear receptor superfamily. Recent studies have suggested an etiological role for aldosterone/MR in the development of metabolic syndrome. On the other hand, the aldosterone/MR system also has a critical role in the progression of target organ damage in metabolic syndrome. We have shown that renal and cardiac injuries in an experimental model of metabolic syndrome are strongly dependent on the activation of the aldosterone/MR system.16, 17, 18 In addition, MR activation causes target organ damage even in normal or low aldosterone states.19, 20 We identified a small guanosine triphosphate (GTP)-binding protein, Rac1, as a novel mediator of ligand-independent MR activation, and cross-talk between Rac1 and MR contributes to the nephropathy of several chronic kidney disease (CKD) models.21

The human MR gene has two alternative promoters, P1 and P2. The transgenic expression of SV40 large T antigen driven by the P1 promoter resulted in lethal hibernomas, unraveling a new functional link between aldosterone and energy homeostasis in brown adipose tissues.22, 23 Overexpression of human MR under the control of the P1 promoter resulted in abnormal urinary electrolyte excretion and dilated cardiomyopathy-like cardiac lesion, supporting the role in cardiovascular disease (CVD) and CKD.24

In this review, we first provide a general overview of obesity and hypertension and then focus on the link between aldosterone/MR and metabolic syndrome. Finally, we introduce our recent findings on the roles of aldosterone-dependent and aldosterone-independent MR activation in target organ complication associated with metabolic syndrome.

Obesity and hypertension

Both genetic and environmental factors contribute to the development of hypertension. Recent genome-wide association studies of BP and hypertension identified several loci associated with hypertension.25, 26 On the other hand, obesity is shown to be one of the major environmental factors to increase the risk of hypertension.

Obesity, especially visceral obesity, is closely related to hypertension.27, 28, 29, 30 Blood pressure (BP) increases ∼4.5 mm Hg for every 10 lb (4.5 kg) weight gain.31 During a 4-year follow-up, 5% weight gain was associated with 20–30% increased odds of hypertension.29 According to the Framingham Heart Study, 65–75% of the risk for hypertension is attributed to excess weight.28 Both obesity and hypertension convey increased risk for CVD. In addition, visceral obesity and hypertension often cluster with insulin resistance, dyslipidemia, inflammation and prothrombotic states.3, 32 This risk-factor clustering condition, known as metabolic syndrome, is a highly predisposing condition for target organ injury.33, 34 Furthermore, high salt intake increases BP and worsens cardiovascular and renal outcomes in patients with obesity and metabolic syndrome.35, 36, 37, 38

Several candidate chromosomal loci or single nucleotide polymorphisms (SNPs) are postulated. For example, a whole-genome scan suggests a locus at 1p36 for obesity hypertension.39 Genetic studies in humans suggest the association of obesity hypertension with variants of several genes, including tumor necrosis factor-α (TNF-α),40 glucocorticoid receptor,41 CYP11B242 and serum and glucocorticoid-regulated kinase (Sgk)1.43 Diseases of civilization, including obesity and hypertension, may result from the mismatch between contemporary environment and ‘energy-thrifty genotype’ of genes, which helped our ancestors survive occasional famines.44, 45 According to the ‘fetal programming of adult disease’ hypothesis proposed by Barker et al.,46 obesity and metabolic syndrome in adulthood originate from malnutrition of the fetus during intrauterine life, which leads to functional and structural adaptive processes for its survival and a compensatory catch-up growth.

Mechanisms of obesity hypertension

Obese subjects have increased cardiac output and plasma volume as well as reduced peripheral vascular resistance. Enhanced renal tubular sodium reabsorption has a central role in the pathogenesis of obesity hypertension. According to Guyton's theory,47 sodium retention impairs pressure natriuresis; higher pressure is necessary for maintaining a sodium balance, resulting in hypertension (right shift in the pressure-natriuresis curve). Multiple factors are postulated to contribute to the enhanced sodium reabsorption, as described below (Figure 1).48, 49, 50

Figure 1
figure 1

Mechanisms of obesity hypertension. Activation of the sympathetic nervous system (SNS), renin–angiotensin system (RAS), aldosterone, hyperinsulinemia and mechanical compression of the kidney cause increased renal tubular sodium reabsorption and hypertension. Adipocyte-derived factors, such as leptin, angiotensinogen and aldosterone-releasing factors, are supposed to have important roles. TNF-α, tumor necrosis factor-α.

Full size image

Sympathetic nervous system

Renal sympathetic overactivity increases sodium reabsorption and vasoconstriction. Leptin, an adipokine secreted in proportion to adiposity, is believed to be an important mediator linking obesity, renal sympathetic activation and hypertension. Leptin acts on the ventromedial and dorsomedial hypothalamic nuclei and regulates energy homeostasis by reducing appetite and increasing energy expenditure. It also modulates renal sympathetic outflow through the melanocortin system. Obesity may cause ‘selective leptin resistance,’ whereby the sympathetic nervous system (SNS) responses to leptin are maintained, whereas its anorexic effect is blunted.51 Renal sympathetic activation is also caused by insulin, non-esterified fatty acids, angiotensin II and aldosterone.

Renin–angiotensin system

Despite marked sodium retention, obesity hypertension is associated with activation of the renin–angiotensin system (RAS). The increased renin secretion may be caused by the increased sympathetic stimulation. Alternatively, angiotensinogen produced by the adipose tissue may contribute to the high circulating angiotensinogen levels in obesity hypertension.15 Overexpression of 11β-HSD1 in the adipose tissue results in visceral obesity and metabolic syndrome. The mice had increased angiotensinogen in the plasma and in the adipose tissue, and hypertension was abolished by angiotensin II type 1 receptor antagonist (ARB).52

Aldosterone

Details are described in the next section.

Hyperinsulinemia

Obese subjects are characterized by hyperinsulinemia and insulin resistance. Hyperinsulinemia could increase sympathetic activity and sodium reabsorption, modify ion transport and stimulate proliferation of smooth muscle cells.

Similar to the case of leptin, insulin actions are blunted in the muscle and adipose tissues, whereas renal action is preserved and facilitated by hyperinsulinemia, resulting in enhanced sodium absorption. The former actions are mediated by insulin receptor substrates (IRS)-1, and the latter by IRS-2.53

Renal mechanical compression

Visceral fat mass may compress the kidney and increase tubular reabsorption. Changes in the renal medullary histology may increase interstitial hydrostatic pressure, compress the thin loops of Henle and vasa recta and enhance tubular reabsorption.

Aldosterone and obesity hypertension/metabolic syndrome

Aldosterone excess has been implicated in obesity-related disorders.

In 1981, Tuck et al.54 first suggested the involvement of aldosterone in the pathogenesis of obesity-associated hypertension. They indicated that weight reduction decreased plasma renin activity and aldosterone concentration, along with BP in obese patients. A recent study by Engeli et al.55 showed that in menopausal women, the obese group had higher plasma aldosterone compared with the lean group (62±25 vs. 38±17 ng l−1), and that weight reduction (−5%) by caloric restriction was accompanied by a reduction in BP (−7 mm Hg) and plasma aldosterone (−31%). Similarly, patients with visceral-type morbid obesity (body mass index 49.0±3.5 kg m−2) had increased plasma aldosterone concentration (1070±137 pM, normal: 190–932 pM), which was significantly reduced (699±90 pM) after the correction of body mass index (27.7±2.0 kg m−2) by gastric bypass surgery. Morbidly obese patients of subcutaneous type had lower plasma aldosterone levels (810±103 pM).56 Goodfriend et al.57, 58 showed a relationship between plasma aldosterone concentration and the amount of visceral fat, which was independent of renin. Accordingly, non-classical adrenal stimuli for aldosterone production had been reported, including oxidized products of linoleic acid59 and as-yet-unidentified potent mineralocorticoid-releasing factors secreted by adipocytes.60

Recent clinical evidence supports the intimate relationship between aldosterone and metabolic syndrome. Two cross-sectional clinical studies of African descent have shown that plasma aldosterone concentration is independently associated with metabolic syndrome.61, 62 The C allele of the −344C/T variant in the promoter of the aldosterone synthase (CYP11B2) gene, which is associated with hyperaldosteronemia, was shown to increase susceptibility to metabolic syndrome in European men.42 Fallo et al.63 observed that patients with primary aldosteronism had a higher incidence of metabolic syndrome than those with essential hypertension (41.1 vs. 29.6%; P<0.05). Furthermore, prospective studies of the Framingham Offspring Study participants indicated that individuals with higher circulating aldosterone levels have increased risk of developing hypertension and metabolic syndrome.64, 65

Epidemiological studies have shown that low birth weight (intrauterine growth retardation) is related to the occurrence of metabolic syndrome in later life (‘fetal programming’ hypothesis).46 The risk of developing metabolic syndrome of men whose birth weight was ≤2.95 kg was more than 10 times greater than that of men whose birth weight was >4.31 kg. Aldosterone as well as glucocorticoid is suggested to be involved in the mechanisms, possibly because of the activation of the hypothalamus–pituitary–adrenal axis.66

Aldosterone excess has also been reported in several animal models of obesity and metabolic syndrome. For example, obese, heart failure-prone SHHF/Mcc-fa cp is documented to have a higher plasma aldosterone level compared with +/+ control (209.4±14.3 vs. 107.0±17.0 ng l−1; P<0.05).67 de Paula et al.68 showed that aldosterone has a critical role in the pathogenesis of hypertension in diet-induced obesity by mediating glomerular hyperfiltration and sodium retention. High fat diet caused body weight gain (+53%) and BP elevation (+16±3 mm Hg) in dogs, which were associated with increased plasma aldosterone concentration (from 31±7 to 58±18 ng l−1), glomerular filtration rate (+38±6%) and cumulative Na balance (+472±110 mEq). These changes were efficiently attenuated by eplerenone treatment despite a similar body weight gain.

Urinary aldosterone excretion is reported to be elevated in db/db mice, an experimental model of obesity-associated type 2 diabetes mellitus that has a non-functional leptin receptor, compared with their control db/+ mice (0.24±0.02 vs. 0.09±0.02 ng mg−1 creatinine; P<0.05).69 In accordance with this, the expression of the aldosterone effector kinase, Sgk1, is increased in the kidney of this model.70 Hyperaldosteronism in db/db mice might be attributable, at least in part, to C1q TNF-related protein 1 (CTRP1). This 32-kDa protein is expressed at a high level in the adipose tissues of obese db/db mice and Zucker diabetic fatty (fa/fa) rats,71 and has the ability to stimulate aldosterone secretion in the adrenal cortical cell line, H295R.72 Aldosterone, in turn, may contribute to adipokine abnormalities in db/db mice, because the MR antagonist, eplerenone, corrected the obesity-related adipokine changes, such as increased TNF-α, monocyte chemoattractant protein (MCP)-1 and plasminogen activator inhibitor (PAI)-1, and decreased adiponectin and PPAR-γ.69 Supporting this, recent works have shown that MR is expressed in adipocytes and mediates mineralocorticoid and glucocorticoid effects on adipogenesis and white adipose tissue functions.69, 73, 74, 75

Aldosterone as a mediator of target organ damage

Aldosterone has long been considered as a hormone that regulates electrolyte, fluid volume and BP homeostasis.76 Recently, however, a paradigm shift has occurred in the field of aldosterone research.77, 78 Emerging evidence has shown aldosterone as an important mediator of cardiovascular and renal damage, in addition to its classic roles.78 Aldosterone acts on non-epithelial cells in the heart, vasculature, kidney and brain to cause tissue remodeling, inflammatory responses, fibrosis, induction of oxidative stress and endothelial dysfunction.79 These non-classical actions were highlighted by two large-scale randomized clinical trials, RALES and EPHESUS,80, 81 in which treatment with low-dose MR antagonists, spironolactone and eplerenone, dramatically improved the outcomes of patients with severe congestive heart failure or left ventricular dysfunction after myocardial infarction taking the standard therapy including angiotensin-converting enzyme (ACE) inhibitors, ARBs, diuretics, β-blockers and digitalis. In the kidney, aldosterone injures glomerular visceral epithelial cells (podocytes), which form the final filtration barrier to plasma macromolecules in the glomerulus, leading to proteinuria and ultimately to glomerulosclerosis.82 Aldosterone also triggers proinflammatory responses and perivascular and interstitial fibrosis in the kidney.83, 84

Historically, it was already shown in the 1940s (even before the isolation of aldosterone and MR!) that the stimulation of MR causes glomerulosclerosis and cardiac fibrosis. For example, Hans Selye is an advocator of ‘stress theory (general adaptation syndrome),’ in which he discovered the role of glucocorticoids in non-specific adaptation response to stressors. During the process of establishing this theory, he carried out a lot of experiments and found that the administration of desoxy (11-deoxy) corticosterone acetate (DOCA), the first synthetic steroid (which stimulates MR), to rats induces inflammatory and fibrotic changes in the heart and kidney. He described unilateral nephrectomy and salt loading as important conditioning factors. He considered that these phenotypes are due to mineralocorticoid actions of DOCA. Furthermore, he postulated a hypothesis that glucocorticoids have ‘anti-’inflammatory actions as adaptation to stress, whereas mineralocorticoids may have ‘pro-’inflammatory effects.85 Since then, the uninephrectomized DOCA/salt rat has been frequently used as a model of hypertensive renal damage. However, the amount of endogenous deoxycorticosterone, a precursor of aldosterone, is extremely small, except for special conditions, such as patients with congenital adrenocorticosteroid synthase deficiency. Half a century later, it was shown that aldosterone, the physiological mineralocorticoid in our body, provokes proteinuria, inflammatory and fibrotic lesions in the heart and kidney.83, 86 The pathogenetic role for aldosterone in CVD and CKD has been widely recognized after the publication of the epoch-making results of ‘RALES’ and ‘EPHESUS’.

Antiproteinuric effects of MR antagonists

Clinical and experimental studies have shown that MR antagonists effectively ameliorate proteinuria in patients or in animal models of hypertension, diabetes mellitus and CKD.78, 19, 87, 88, 89, 90, 91, 92, 93, 94

For example, White et al.87 compared the anti-albuminuric effect of eplerenone with the Ca blocker, amlodipine, in older patients with systolic hypertension. After 24 weeks of therapy, eplerenone reduced albuminuria to a greater extent than did amlodipine (−52 vs. −10%; P=0.04) in patients with microalbuminuria at baseline, although their hypotensive effects were comparable. Williams et al. compared the efficacy of eplerenone and enalapril in patients with stage 1 or stage 2 hypertension. After 12 months, the extent of BP reduction was similar between the two groups, whereas eplerenone was superior to enalapril in reducing albuminuria in patients who had an elevated value at baseline (−62 vs. −26%; P=0.01). These clinical studies implicate that proteinuria in these hypertensive patients is, at least in part, dependent on the aldosterone/MR system (beyond BP effect).

Mineralocorticoid receptor antagonists are supposed to be effective in patients who experience ‘aldosterone breakthrough.’95 When we administer ACE inhibitors or ARBs to patients, plasma aldosterone levels decrease at first. However, over the long term, the initially decreased aldosterone increases again in patients with ‘aldosterone breakthrough.’ The incidence reported ranges from 10% over 6 months to 53% over 1 year. Sato et al.90 reported that after 40 weeks of ACE inhibitor therapy in patients with diabetic nephropathy, urinary albumin excretion was markedly higher among those who experienced aldosterone breakthrough than among those who did not. Horita et al.96 found similar results in patients with IgA (immunoglobulin A) nephropathy. A prospective randomized open-label study by Bianchi et al.92 showed that in CKD patients already treated with ACE inhibitors and/or ARBs, the addition of spironolactone reduced proteinuria and retarded the decrease in estimated glomerular filtration rate after 1 year of therapy.

Metabolic syndrome and CKD: role of the aldosterone/MR system

Chronic kidney disease is becoming another urgent public health problem worldwide. Even minor renal dysfunction is shown to be a significant risk factor for CVD. Epidemiological studies have shown that metabolic syndrome is an important modifiable risk factor for CKD.34, 97, 98, 99 However, the mechanism linking metabolic syndrome to CKD has not been clearly elucidated. It cannot be solely attributable to the additive effects of individual components, such as hypertension and diabetes mellitus,97 and some unifying underlying mechanism has been suggested. We examined the role of aldosterone/MR signaling in this process, using spontaneously hypertensive rats (SHR)/NDmcr-cp (SHR/cp, obese SHR) as a rat model of metabolic syndrome.16 This rat is a derivative of SHR with cp mutation in the leptin receptor gene,100 and it manifested a clustering of obesity, hypertension, hyperinsulinemia and hypertriglyceridemia.

Proteinuria is a central symptom of CKD. Obese SHR/cp developed marked proteinuria in an age-dependent manner. By contrast, urinary protein excretion remained low in non-obese SHR despite a similar BP level. Proteinuria in SHR/cp was accompanied by podocyte injury, as indicated by the attenuation of the normal podocyte marker nephrin, induction of the injured podocyte marker, desmin, and foot process effacement under electron microscopic analysis. These findings suggest that podocyte injury underlies the etiology of proteinuria in SHR/cp.

It is noted that the serum aldosterone level was higher in obese SHR/cp than in non-obese SHR, and that there was a positive correlation between circulating aldosterone concentration and proteinuria. Expression of Sgk1, a downstream effector of aldosterone, was upregulated in the whole kidney and in the glomerular fraction of SHR/cp, supporting the causative role of aldosterone/MR activation. Indeed, the selective MR antagonist, eplerenone, effectively reduced proteinuria in SHR/cp. In parallel, eplerenone improved podocyte injury in SHR/cp, as shown by the changes in nephrin and desmin expressions. These data suggest that the aldosterone-provoked podocyte injury has a pivotal role in the pathogenesis of proteinuria in SHR/cp. We propose that adipocyte-derived aldosterone-releasing factors (ARFs), although still unidentified, may account for aldosterone elevation and the resultant target organ complication in SHR/cp.16, 101

Salt accelerates target organ damage in SHR/cp through MR activation

Recent clinical studies have shown the increased salt sensitivity of BP and target organ injury in patients with obesity and metabolic syndrome.34, 35, 36, 38 For example, Verhave et al.36 showed that higher sodium intake increases urinary albumin excretion in overweight individuals but not in non-overweight people. However, the mechanisms have not been clearly elucidated. Therefore, we examined the effects of salt loading on the nephropathy of SHR/cp.17

High-salt diet markedly enhanced proteinuria in SHR/cp (Figure 2a). Interestingly, the MR antagonist, eplerenone, perfectly inhibited the salt-induced exacerbation, suggesting the involvement of the aldosterone/MR cascade. Similarly, salt worsened glomerular podocyte impairment and renal histopathological findings (glomerulosclerosis, tubulointerstitial injury and renal arteriolar lesions), which were completely mitigated by eplerenone. Although salt loading suppressed circulating renin and aldosterone, it paradoxically activated renal MR signaling, as shown by increased MR in the nuclear fraction, induction of aldosterone effector kinase Sgk1 and upregulation of putative mediators of aldosterone-evoked organ damage, such as transforming growth factor-β1, PAI-1 and MCP-1 in the kidney of salt-loaded SHR/cp. Eplerenone completely inhibited these MR-dependent cascades. The paradoxical MR activation might be attributable in part to adipocyte-derived ARFs. Although the RAS-regulated aldosterone generation is counterbalanced by salt, our preliminary data suggest that aldosterone production by ARFs lacks negative feedback regulation in response to high salt intake. As a result, the suppression of the circulating aldosterone level might be less than expected, causing inappropriately high aldosterone for the amount of salt intake.

Figure 2
figure 2

(a) Proteinuria of spontaneously hypertensive rats (SHR)/cp fed a normal diet (SHR/cp), SHR/cp fed a high-salt diet (SHR/cp+HS) and SHR/cp+HS treated with the mineralocorticoid receptor (MR) antagonist, eplerenone (SHR/cp+HS+Epl).17 (b) Left ventricular diastolic function was evaluated as −dp/dtmax by direct LV pressure monitoring through cardiac catheterization.18 (c) Schematic representation of our hypothesis that obesity and salt, two cardinal features of a modern, civilized society, cause inappropriate activation of the aldosterone/MR system, leading to renal and cardiac injuries. **P<0.01 vs. SHR/cp; #P<0.05 vs. SHR/cp+HS; ##P<0.01 vs. SHR/cp+HS.

Full size image

Cross-talk between the kidney and the cardiovascular system has recently become a major topic.102 We examined whether the same mechanism can be extrapolated to the pathogenesis of cardiac injury in our rats.18 Cardiac catheterization and Doppler echocardiographic analysis indicated that the left ventricular diastolic function was impaired in salt-loaded SHR/cp, which was fully recovered by eplerenone (Figure 2b).

These findings corroborate our hypothesis that obesity and salt, two cardinal features of a modern, civilized society, cause MR activation, leading ultimately to CVD and CKD (Figure 2c).

Novel mechanism of ligand-independent MR activation and its implication in CKD

So far, we have described MR activation and target organ injury in high aldosterone models. Recently, we and others19, 20, 103 reported that MR activation could also have a crucial role in target organ damage even in normal or low aldosterone states. Dahl salt-hypertensive rats develop podocyte injury, proteinuria, glomerulosclerosis and cardiac failure under a high-salt diet. Although the circulating aldosterone level was low, MR activation was noted in the target organ, and eplerenone dramatically retarded the progression of renal and cardiac diseases. These findings raise the possibility that molecules other than aldosterone may activate MR.

To date, little is known about the mechanisms of ligand-independent MR activation.104, 105 MR belongs to the nuclear receptor superfamily acting as a transcription factor; on ligand binding, the ligand–receptor complex translocates into the nucleus, where it dimerizes and interacts with the mineralocorticoid response element in the promoter region of the target genes to activate gene transcription. From this point of view, the activity of MR should be modulated by multiple factors other than the ligand level. As for the other steroid receptors, intracellular signaling molecules were shown to influence the receptor activity.106, 107, 108, 109 We identified a small G protein, Rac1, as a potent activator of MR.21

To address the possibility that the Rho family GTPases can modulate MR activity, we first carried out in vitro transfection assays in human embryonic kidney 293 cells. In luciferase reporter assays, MR-dependent transcriptional activity was upregulated in response to aldosterone. Overexpression of constitutively active (CA)-Rac1, not wild-type or dominant-negative Rac1, potentiated this response further. CA-Rac1 also increased the luciferase activity even without aldosterone. We further assessed the nuclear translocation of green fluorescence protein (GFP)-tagged MR. Without aldosterone, GFP fluorescence was distributed mainly in the cytoplasm; on activation by aldosterone, MR-GFP was promptly targeted to the nucleus. Quantitative analysis showed that CA-Rac1 substantially increased the amount of nuclear MR-GFP, both in the absence and presence of aldosterone. These results indicate that CA-Rac1 causes MR activation.

Thereafter, we investigated whether this ‘Rac1-evoked MR activation’ might contribute to the pathogenesis of renal injury in vivo, using the Rho guanosine diphosphate (GDP) dissociation inhibitor (RhoGDI)α knockout (KO) mice,110 a kidney-specific Rac1 activation model. At 12 weeks of age, the KO mice showed massive albuminuria, glomerular podocyte damage and focal and segmental glomerulosclerosis. Treatment with the Rac-specific inhibitor, NSC23766,111, 112 substantially ameliorated the renal impairment concomitantly with repression of Rac1 activity. We examined the aldosterone/MR status in this model. Although the serum aldosterone concentration and BP were not increased, the amount of MR in the nuclear fraction and the expression of Sgk1, a downstream target of MR, were upregulated in the kidneys of KO mice, suggesting an aldosterone-independent MR activation. Therefore, we treated the KO mice with MR antagonist. Eplerenone dramatically ameliorated albuminuria, renal histopathological changes, podocyte injury and induction of MR-related injury mediators, such as MCP-1 and PAI-1. Furthermore, NSC23766 also suppressed MR activation, as assessed by the expressions of Sgk1, PAI-1 and MCP-1. These results indicate that the Rac1-mediated MR activation has a central role in the renal phenotype of RhoGDIα KO mice (Figure 3). Traditionally, Rac1 GTPase is known to have diverse biological functions, such as actin cytoskeletal organization, cell migration and generation of oxidative stress, as components of NADPH oxidase.113 On the other hand, Rac1 was recently shown to be indispensable for the nuclear localization of β-catenin in canonical Wnt signaling114 and of STAT5 in cytokine signaling,115 in addition to our finding of nuclear MR translocation, highlighting novel roles for Rac1 in the nucleocytoplasmic shuttling of transcription factors.

Figure 3
figure 3

Mechanisms of renal injury in Rho guanosine diphosphate (GDP) dissociation inhibitor (RhoGDI)α knockout (KO) mice. Depletion of GDIα causes Rac1 activation in the kidney, which mediates aldosterone-independent mineralocorticoid receptor (MR) activation and the resultant podocyte injury, albuminuria and chronic kidney disease.

Full size image

As RhoGDIα KO mice constitute a rather artificial model, we evaluated the involvement of the Rac1–MR pathway in a more common CKD model. We administered NSC23766 to the above-mentioned Dahl salt-sensitive rats, which elicited MR activation in the kidney despite a low serum aldosterone level. Salt triggers BP elevation and glomerular damage in this model. Rac1 was activated in the kidneys of salt-loaded Dahl salt-sensitive rats, and the administration of NSC23766 significantly suppressed MR activation, proteinuria and glomerulosclerosis. These findings imply that Rac1 activation might contribute, at least in part, to the non-aldosterone-mediated MR activation in this model.

Altogether, we identified Rac1 as a ligand-independent activator of MR. This alternative pathway of MR activation actually has a significant role in the progression of renal injury in some CKD models, implicating Rac1 as a novel therapeutic target for CKD. We have preliminary data indicating that Rac1 is activated in response to various stimuli relevant to metabolic syndrome (data not shown). Thus, MR can be activated in the kidney of metabolic syndrome by several different pathways, both aldosterone dependently (through ARFs) and independently (through Rac1), and mediates renal injury.

Vertebrate evolution and the aldosterone/MR system

Finally, we describe evolutional perspectives on the aldosterone/MR system. Aldosterone, a principal regulator of sodium reabsorption in the kidney, is postulated to have had a crucial role in the phylogenetic transition from aquatic fishes to land-living tetrapods.116 As a means to sustaining life on land with little salinity, terrestrial animals used aldosterone/MR to recapitulate the sea water environment within the body. Indeed, aquatic fishes do not possess aldosterone, whereas tetrapods acquire it. On the other hand, MR, the receptor, had already been present long before aldosterone evolved. During aquatic life without aldosterone, MR served as a receptor for other ligands and conveyed its own function. The probability of simultaneous acquisition of both ligand and receptor during evolution would be very low. In the case of aldosterone, the hormone ‘exploited’ other ligands' receptor MR and compelled it to a new role in electrolyte homeostasis, which eventually became the main function of MR.

When humans first appeared, the amount of available salt was limited, and salt was one of the most precious commodities. During a long period of salt scarcity, people with stronger ‘salt retention’ genes had a significant survival advantage. In our modern, industrialized societies, we now have plenty of salt, and an energy imbalance causes a pandemic of obesity. The cultural changes have far outpaced any possible genetic adaptations, and obesity and salt synergistically cause inappropriate activation of the aldosterone/MR system, according to our animal data. Individuals who have genes for enhanced ‘salt-retentive’ activity are especially predisposed to diseases of civilization, such as salt-sensitive hypertension, CVD and CKD, when faced with the contemporary environment. Target organ damage by aldosterone might be a manifestation of the exaggerated original function of MR during the aquatic life.

Perspectives

We have summarized the literature on obesity hypertension/metabolic syndrome and aldosterone/MR. We have shown that SHR/cp, a metabolic syndrome model, is susceptible to renal and cardiac injuries, especially when the animal is fed a high-salt diet. Inappropriate activation of the aldosterone/MR system underlies target organ diseases. We have shown that MR can be activated by several different pathways, both aldosterone dependently (through ARFs) and independently (through Rac1, for example). These findings support our hypothesis that obesity and salt, two central features of the modern society, cause MR overactivation, leading to CVD and CKD.

Future studies are necessary to identify the clinical conditions in which Rac1 is overactivated and the Rac inhibitor is effective, and to determine the intrarenal localization of activated Rac1. For this purpose, clinical studies using a renal biopsy specimen of CKD patients are under investigation. We expect that ARFs and Rac1 can be novel targets of therapy for metabolic syndrome and CKD. We also have to establish handy diagnostic tools to evaluate the tissue MR activation state, because circulating aldosterone concentration does not necessarily reflect MR activity in the target organ. Finally, we have to accumulate clinical evidence to prove the efficacy of MR blockade in patients with obesity and metabolic syndrome by large-scale clinical trials.