Mineralocorticoid receptors and the heart, multiple cell types and multiple mechanisms: a focus on the cardiomyocyte

Renal effects and safety between Asian and non-Asian chronic kidney disease and type 2 diabetes treated with nonsteroidal mineralocorticoid antagonists

BACKGROUND

Asians bear a heavier burden of chronic kidney disease (CKD), a common comorbidity of type 2 diabetes mellitus (T2DM), than non-Asians. Nonsteroidal mineralocorticoid receptor antagonists (MRAs) have garnered attention for their potential advantages in renal outcomes. Nevertheless, the impact on diverse ethnic groups remains unknown.

METHODS

The PubMed, Embase, Cochrane Library, China National Knowledge Infrastructure (CNKI), Wanfang database, and clinical trial registries were searched through August 2023 with the following keywords: nonsteroidal MRAs (finerenone, apararenone, esaxerenone, AZD9977, KBP-5074), CKD, T2DM, and randomized controlled trial (RCT). A random effects model was used to calculate overall effect sizes.

RESULTS

Seven RCTs with 14 997 participants were enrolled. Nonsteroidal MRAs reduced urinary albumin to creatinine ratio (UACR) significantly more in Asians than non-Asians: (weighted mean difference [WMD], -0.59, 95% CI, -0.73 to -0.45, p < .01) vs (WMD, -0.29, 95% CI, -0.32 to -0.27, p < .01), respectively. The average decline of estimated glomerular filtration rate (eGFR) was similar in Asians and non-Asians (p > .05). Regarding systolic blood pressure (SBP), nonsteroidal MRAs had a better antihypertension performance in Asians (WMD, -5.12, 95% CI, -5.84 to -4.41, p < .01) compared to non-Asians (WMD, -3.64, 95% CI, -4.38 to -2.89, p < .01). A higher incidence of hyperkalemia and eGFR decrease ≥30% was found in Asians than non-Asians (p < .01).

CONCLUSIONS

Nonsteroidal MRAs exhibited significant renal benefits by decreasing UACR and lowering SBP in Asian than that of non-Asian patients with CKD and T2DM, without increase of adverse events except hyperkalemia and eGFR decrease ≥30%.

Mineralocorticoid Receptors in Vascular Smooth Muscle: Blood Pressure and Beyond

After half a century of evidence suggesting the existence of mineralocorticoid receptors (MR) in the vasculature, the advent of technology to specifically knockout the MR from smooth muscle cells (SMCs) in mice has elucidated contributions of SMC-MR to cardiovascular function and disease, independent of the kidney. This review summarizes the latest understanding of the molecular mechanisms by which SMC-MR contributes to (1) regulation of vasomotor function and blood pressure to contribute to systemic and pulmonary hypertension; (2) vascular remodeling in response to hypertension, vascular injury, obesity, and aging, and the impact on vascular calcification; and (3) cardiovascular pathologies including aortic aneurysm, heart valve dysfunction, and heart failure. Data are reviewed from in vitro studies using SMCs and in vivo findings from SMC-specific MR-knockout mice that implicate target genes and signaling pathways downstream of SMC-MR. By regulating expression of the L-type calcium channel subunit Cav1.2 and angiotensin II type-1 receptor, SMC-MR contributes to myogenic tone and vasoconstriction, thereby contributing to systemic blood pressure. MR activation also promotes SMC proliferation, migration, production and degradation of extracellular matrix, and osteogenic differentiation by regulating target genes including connective tissue growth factor, osteopontin, bone morphogenetic protein 2, galectin-3, and matrix metallopeptidase-2. By these mechanisms, SMC-MR promotes disease progression in models of aging-associated vascular stiffness, vascular calcification, mitral and aortic valve disease, pulmonary hypertension, and heart failure. While rarely tested, when sexes were compared, the mechanisms of SMC-MR-mediated disease were sexually dimorphic. These advances support targeting SMC-MR-mediated mechanisms to prevent and treat diverse cardiovascular disorders.

Aldosterone: Essential for Life but Damaging to the Vascular Endothelium

The renin angiotensin aldosterone system is a key regulator of blood pressure. Aldosterone is the final effector of this pathway, acting predominantly via mineralocorticoid receptors. Aldosterone facilitates the conservation of sodium and, with it, water and acts as a powerful stimulus for potassium excretion. However, evidence for the pathological impact of excess mineralocorticoid receptor stimulation is increasing. Here, we discussed how in the heart, hyperaldosteronism is associated with fibrosis, cardiac dysfunction, and maladaptive hypertrophy. In the kidney, aldosterone was shown to cause proteinuria and fibrosis and may contribute to the progression of kidney disease. More recently, studies suggested that aldosterone excess damaged endothelial cells. Here, we reviewed how damage to the endothelial glycocalyx may contribute to this process. The endothelial glycocalyx is a heterogenous, negatively charged layer on the luminal surface of cells. Aldosterone exposure alters this layer. The resulting structural changes reduced endothelial reactivity in response to protective shear stress, altered permeability, and increased immune cell trafficking. Finally, we reviewed current therapeutic strategies for limiting endothelial damage and suggested that preventing glycocalyx remodelling in response to aldosterone exposure may provide a novel strategy, free from the serious adverse effect of hyperkalaemia seen in response to mineralocorticoid blockade.

Impact of Mineralocorticoid Receptor Gene NR3C2 on the Prediction of Functional Classification of Left Ventricular Remodeling and Arrhythmia after Acute Myocardial Infarction

Background: The NR3C2 gene encodes the mineralocorticoid receptor, which is present on cardiomyocytes. Prior studies reported an association between the presence of NR3C2 single-nucleotide polymorphisms (SNPs) and an increased cortisol production during a stress response such as acute myocardial infarction (AMI), which may lead to adverse cardiac remodeling. Objective: To study the impact of the NR3C2 rs2070950, rs4635799 and rs5522 gene polymorphisms on left ventricular (LV) remodeling, rhythm and conduction disorders in AMI patients. Methods: A cohort of 301 AMI patients who underwent revascularization was included. SNPs of the NR3C2 gene (rs2070950, rs4635799 and rs5522) were evaluated. A total of 127 AMI patients underwent transthoracic echocardiography follow-up after 72 h and 6 months. Results: The rs2070950 GG genotype and rs4635799 TT genotype were most common in patients who had LV end-diastolic volume increase < 20% and the same or increased LV ejection fraction, indicating a possible protective effect of these SNPs. The rs5522 TT genotype was associated with a higher frequency of arrhythmias, while the presence of at least one rs5522 C allele was associated with a lower risk of arrhythmias. Conclusion: SNPs of the NR3C2 gene appear to correlate with better ventricular remodeling and a reduced rate of arrhythmias post-AMI, possibly by limiting the deleterious effects of cortisol on cardiomyocytes.

Aldosterone Suppresses Endothelial Mitochondria through Mineralocorticoid Receptor/Mitochondrial Reactive Oxygen Species Pathway

Excessive aldosterone secretion causes endothelial dysfunction, vascular inflammation, and vascular fibrosis in patients with primary aldosteronism (PA). Endothelial function is closely related to endothelial mitochondria. However, the effects of elevated aldosterone levels on endothelial mitochondria remain unclear. In this study, we used primary cultured human umbilical vein endothelial cells (HUVECs) to investigate the effects of aldosterone on endothelial mitochondria. Mineralocorticoid receptor (MR) small interfering (si)RNA or glucocorticoid receptor (GR) siRNA were used to confirm the pathway by which aldosterone exerts its effects on the mitochondria of HUVECs. The results showed that excess aldosterone suppressed mitochondrial DNA copy numbers, anti-mitochondrial protein, and SOD2 protein expression in a dose- and time-dependent manner. These effects were attenuated by treatment with MR siRNA, but not with GR siRNA. Furthermore, it was attenuated by treatment with a mitochondria-targeted antioxidant (Mito-TEMPO, associated with mitochondrial reactive oxygen species (ROS) production), but not N-acetyl-L-cysteine (associated with cytosolic ROS production), which suggests that the process was through the mitochondrial ROS pathway, but not the cytosolic ROS pathway. In conclusion, aldosterone excess suppressed endothelial mitochondria through the MR/mitochondrial ROS pathway.

Corticosteroid Receptors in Cardiac Health and Disease

Nuclear receptors play a central role in both energy metabolism and cardiomyocyte death and survival in the heart. Recent evidence suggests they may also influence cardiomyocyte endowment. Although several members of the nuclear receptor family play key roles in heart maturation (including thyroid hormone receptors) and cardiac metabolism, here, the focus will be on the corticosteroid receptors, the glucocorticoid receptor (GR) and mineralocorticoid receptor (MR). The heart is an important target for the actions of corticosteroids, yet the homeostatic role of GR and MR in the healthy heart has been elusive. However, MR antagonists are important in the treatment of heart failure, a condition associated with mitochondrial dysfunction and energy failure in cardiomyocytes leading to mitochondria-initiated cardiomyocyte death (Ingwall and Weiss, Circ Res 95:135-145, 2014; Ingwall , Cardiovasc Res 81:412-419, 2009; Zhou and Tian , J Clin Invest 128:3716-3726, 2018). In contrast, animal studies suggest GR activation in cardiomyocytes has a cardioprotective role, including in heart failure.

Id2 Represses Aldosterone-Stimulated Cardiac T-Type Calcium Channels Expression

Aldosterone excess is a cardiovascular risk factor. Aldosterone can directly stimulate an electrical remodeling of cardiomyocytes leading to cardiac arrhythmia and hypertrophy. L-type and T-type voltage-gated calcium (Ca2+) channels expression are increased by aldosterone in cardiomyocytes. To further understand the regulation of these channels expression, we studied the role of a transcriptional repressor, the inhibitor of differentiation/DNA binding protein 2 (Id2). We found that aldosterone inhibited the expression of Id2 in neonatal rat cardiomyocytes and in the heart of adult mice. When Id2 was overexpressed in cardiomyocytes, we observed a reduction in the spontaneous action potentials rate and an arrest in aldosterone-stimulated rate increase. Accordingly, Id2 siRNA knockdown increased this rate. We also observed that CaV1.2 (L-type Ca2+ channel) or CaV3.1, and CaV3.2 (T-type Ca2+ channels) mRNA expression levels and Ca2+ currents were affected by Id2 presence. These observations were further corroborated in a heart specific Id2- transgenic mice. Taken together, our results suggest that Id2 functions as a transcriptional repressor for L- and T-type Ca2+ channels, particularly CaV3.1, in cardiomyocytes and its expression is controlled by aldosterone. We propose that Id2 might contributes to a protective mechanism in cardiomyocytes preventing the presence of channels associated with a pathological state.

Role of Aldosterone and Mineralocorticoid Receptor in Cardiovascular Aging

The mineralocorticoid receptor (MR) was originally identified as a regulator of blood pressure, able to modulate renal sodium handling in response to its principal ligand aldosterone. MR is expressed in several extra-renal tissues, including the heart, vasculature, and adipose tissue. More recent studies have shown that extra-renal MR plays a relevant role in the control of cardiovascular and metabolic functions and has recently been implicated in the pathophysiology of aging. MR activation promotes vasoconstriction and acts as a potent pro-fibrotic agent in cardiovascular remodeling. Aging is associated with increased arterial stiffness and vascular tone, and modifications of arterial structure and function are responsible for these alterations. MR activation contributes to increase blood pressure with aging by regulating myogenic tone, vasoconstriction, and vascular oxidative stress. Importantly, aging represents an important contributor to the increased prevalence of cardiometabolic syndrome. In the elderly, dysregulation of MR signaling is associated with hypertension, obesity, and diabetes, representing an important cause of increased cardiovascular risk. Clinical use of MR antagonists is limited by the adverse effects induced by MR blockade in the kidney, raising the risk of hyperkalaemia in older patients with reduced renal function. Therefore, there is an unmet need for the enhanced understanding of the role of MR in aging and for development of novel specific MR antagonists in the context of cardiovascular rehabilitation in the elderly, in order to reduce relevant side effects.

Aldosterone induces albuminuria via matrix metalloproteinase-dependent damage of the endothelial glycocalyx

Aldosterone contributes to end-organ damage in heart failure and chronic kidney disease. Mineralocorticoid-receptor inhibitors limit activation of the receptor by aldosterone and slow disease progression, but side effects, including hyperkalemia, limit their clinical use. Damage to the endothelial glycocalyx (a luminal biopolymer layer) has been implicated in the pathogenesis of endothelial dysfunction and albuminuria, but to date no one has investigated whether the glomerular endothelial glycocalyx is affected by aldosterone. In vitro, human glomerular endothelial cells exposed to 0.1 nM aldosterone and 145 mMol NaCl exhibited reduced cell surface glycocalyx components (heparan sulfate and syndecan-4) and disrupted shear sensing consistent with damage of the glycocalyx. In vivo, administration of 0.6 μg/g/d of aldosterone (subcutaneous minipump) and 1% NaCl drinking water increased glomerular matrix metalloproteinase 2 activity, reduced syndecan 4 expression, and caused albuminuria. Intravital multiphoton imaging confirmed that aldosterone caused damage of the glomerular endothelial glycocalyx and increased the glomerular sieving coefficient for albumin. Targeting matrix metalloproteinases 2 and 9 with a specific gelatinase inhibitor preserved the glycocalyx, blocked the rise in glomerular sieving coefficient, and prevented albuminuria. Together these data suggest that preservation of the glomerular endothelial glycocalyx may represent a novel strategy for limiting the pathological effects of aldosterone.

Mineralocorticoid Receptor and Cardiovascular Disease

Activation of the mineralocorticoid receptor (MR) in the distal nephron by its ligand, aldosterone, plays an important role in sodium reabsorption and blood pressure regulation. However, expression of the MR goes beyond the kidney. It is expressed in a variety of other tissues in which its activation could lead to tissue injury. Indeed, MR activation in the cardiovascular (CV) system has been shown to promote hypertension, fibrosis, and inflammation. Pharmacological blockade of the MR has protective effects in several animal models of CV disease. Furthermore, the use of MR antagonists is beneficial for heart failure patients, preventing mortality and morbidity. A better understanding of the implications of the MR in the setting of CV diseases is critical for refining treatments and improving patient care. The mechanisms involved in the deleterious effects of MR activation are complex and include oxidative stress, inflammation, and fibrosis. This review will discuss the pathological role of the MR in the CV system and the major mechanisms underlying it.

MicroRNA-204 Is Necessary for Aldosterone-Stimulated T-Type Calcium Channel Expression in Cardiomyocytes

Activation of the mineralocorticoid receptor (MR) in the heart is considered to be a cardiovascular risk factor. MR activation leads to heart hypertrophy and arrhythmia. In ventricular cardiomyocytes, aldosterone induces a profound remodeling of ion channel expression, in particular, an increase in the expression and activity of T-type voltage-gated calcium channels (T-channels). The molecular mechanisms immediately downstream from MR activation, which lead to the increased expression of T-channels and, consecutively, to an acceleration of spontaneous cell contractions in vitro, remain poorly investigated. Here, we investigated the putative role of a specific microRNA in linking MR activation to the regulation of T-channel expression and cardiomyocyte beating frequency. A screening assay identified microRNA 204 (miR-204) as one of the major upregulated microRNAs after aldosterone stimulation of isolated neonatal rat cardiomyocytes. Aldosterone significantly increased the level of miR-204, an effect blocked by the MR antagonist spironolactone. When miR-204 was overexpressed in isolated cardiomyocytes, their spontaneous beating frequency was significantly increased after 24 h, like upon aldosterone stimulation, and messenger RNAs coding T-channels (CaV3.1 and CaV3.2) were increased. Concomitantly, T-type calcium currents were significantly increased upon miR-204 overexpression. Specifically repressing the expression of miR-204 abolished the aldosterone-induced increase of CaV3.1 and CaV3.2 mRNAs, as well as T-type calcium currents. Finally, aldosterone and miR-204 overexpression were found to reduce REST-NRSF, a known transcriptional repressor of CaV3.2 T-type calcium channels. Our study thus strongly suggests that miR-204 expression stimulated by aldosterone promotes the expression of T-channels in isolated rat ventricular cardiomyocytes, and therefore, increases the frequency of the cell spontaneous contractions, presumably through the inhibition of REST-NRSF protein.

Genomic and rapid effects of aldosterone: what we know and do not know thus far

Aldosterone is the most known mineralocorticoid hormone synthesized by the adrenal cortex. The genomic pathway displayed by aldosterone is attributed to the mineralocorticoid receptor (MR) signaling. Even though the rapid effects displayed by aldosterone are long known, our knowledge regarding the receptor responsible for such event is still poor. It is intense that the debate whether the MR or another receptor-the "unknown receptor"-is the receptor responsible for the rapid effects of aldosterone. Recently, G protein-coupled estrogen receptor-1 (GPER-1) was elegantly shown to mediate some aldosterone-induced rapid effects in several tissues, a fact that strongly places GPER-1 as the unknown receptor. It has also been suggested that angiotensin receptor type 1 (AT1) also participates in the aldosterone-induced rapid effects. Despite this open question, the relevance of the beneficial effects of aldosterone is clear in the kidneys, colon, and CNS as aldosterone controls the important water reabsorption process; on the other hand, detrimental effects displayed by aldosterone have been reported in the cardiovascular system and in the kidneys. In this line, the MR antagonists are well-known drugs that display beneficial effects in patients with heart failure and hypertension; it has been proposed that MR antagonists could also play an important role in vascular disease, obesity, obesity-related hypertension, and metabolic syndrome. Taken altogether, our goal here was to (1) bring a historical perspective of both genomic and rapid effects of aldosterone in several tissues, and the receptors and signaling pathways involved in such processes; and (2) critically address the controversial points within the literature as regarding which receptor participates in the rapid pathway display by aldosterone.

Cardiac Fibrosis and Arrhythmogenesis

Myocardial injury, mechanical stress, neurohormonal activation, inflammation, and/or aging all lead to cardiac remodeling, which is responsible for cardiac dysfunction and arrhythmogenesis. Of the key histological components of cardiac remodeling, fibrosis either in the form of interstitial, patchy, or dense scars, constitutes a key histological substrate of arrhythmias. Here we discuss current research findings focusing on the role of fibrosis, in arrhythmogenesis. Numerous studies have convincingly shown that patchy or interstitial fibrosis interferes with myocardial electrophysiology by slowing down action potential propagation, initiating reentry, promoting after-depolarizations, and increasing ectopic automaticity. Meanwhile, there has been increasing appreciation of direct involvement of myofibroblasts, the activated form of fibroblasts, in arrhythmogenesis. Myofibroblasts undergo phenotypic changes with expression of gap-junctions and ion channels thereby forming direct electrical coupling with cardiomyocytes, which potentially results in profound disturbances of electrophysiology. There is strong evidence that systemic and regional inflammatory processes contribute to fibrogenesis (i.e., structural remodeling) and dysfunction of ion channels and Ca2+ homeostasis (i.e., electrical remodeling). Recognizing the pivotal role of fibrosis in the arrhythmogenesis has promoted clinical research on characterizing fibrosis by means of cardiac imaging or fibrosis biomarkers for clinical stratification of patients at higher risk of lethal arrhythmia, as well as preclinical research on the development of antifibrotic therapies. At the end of this review, we discuss remaining key questions in this area and propose new research approaches. © 2017 American Physiological Society. Compr Physiol 7:1009-1049, 2017.

Association of Mineralocorticoid Receptor Polymorphism I180V With Left Ventricular Hypertrophy in Resistant Hypertension

OBJECTIVES

Genetic polymorphisms on mineralocorticoid receptor gene (NC3C2) are associated with variability of mineralocorticoid receptor (MR) function and cardiovascular implications. We sought to investigate whether I180V (rs5522) and MRc.-2G_C (rs2070951) polymorphisms in NR3C2 gene are associated with resistance to antihypertensive treatment and target-organ damage in resistant hypertensive (RHTN) patients.

METHODS

One hundred and eighty-one RHTN and 122 mild to moderate hypertensive (HTN) patients were enrolled in this study. Genotypes were obtained by allelic discrimination assay using real-time polymerase chain reaction. We determined pulse wave velocity (PWV), microalbuminuria, and left ventricular mass index to assess target-organ damage. We compared clinical and laboratorial characteristics of AA vs. G carriers for rs5522 and AC vs. GG vs. CG for rs2070951.

RESULTS

We did not found differences in allele, genotype, and haplotype frequencies for both polymorphisms between HTN and RHTN subjects. We found increased levels of aldosterone and ambulatory blood pressure (BP) in G carriers only for rs5522. Left ventricular hypertrophy (LVH) was more prevalent in G carriers than AA homozygous for rs5522 but not for rs2070951 in RHTN. On the other hand, microalbuminuria and PWV were similar among genotypes for both polymorphisms. No differences were observed between the haplotypes, except for higher aldosterone concentration in GG compared to AG and AC haplotypes.

CONCLUSION

Our study suggests that rs5522 polymorphism might affect cardiac remodeling and aldosterone levels in RHTN subjects.

Cardiovascular actions of mineralocorticoid receptor antagonists in patients with chronic kidney disease: A systematic review and meta-analysis of randomized trials

INTRODUCTION

The safety and actions of mineralocorticoid receptor antagonists on surrogate markers of cardiovascular disease as well as major patient level cardiovascular end-points in patients with chronic kidney disease are unclear.

METHODS

MEDLINE, EMBASE, Trip Database, Cochrane Central Register of Controlled Trials, Cochrane Renal Group specialized register, Current Controlled Trials and clinicaltrials.gov were searched for relevant trials.

RESULTS

Twenty-nine trials (1581 patients) were included. Overall, mineralocorticoid receptor antagonists lowered both systolic and diastolic blood pressure (-5.24, 95% confidence interval (CI) -8.65, -1.82 mmHg; p=0.003 and -1.96, 95% CI -3.22, -0.69 mmHg; p=0.002 respectively). There were insufficient data to perform a meta-analysis of other cardiovascular effects. However, a systematic review of the studies included suggested a consistent improvement in surrogate markers of cardiovascular disease. Overall, the use of mineralocorticoid receptor antagonists was associated with an increased serum potassium (0.23, 95% CI 0.13, 0.33 mmol/l; p<0.0001) and higher risk ratio (1.76, 95% CI 1.20, 2.57; p=0.001) of hyperkalemia. Data on long-term cardiovascular outcomes and mortality were not available in any of the trials.

CONCLUSIONS

The long-term effects of mineralocorticoid receptor antagonists on cardiovascular events, mortality and safety need to be established.

The neuronal mineralocorticoid receptor: from cell survival to neurogenesis

Mineralocorticoid receptor (MR), a hormone-activated transcription factor belonging to the nuclear receptor superfamily, exerts widespread actions in many tissues such as tight epithelia, the cardiovascular system, adipose tissues and macrophages. In the mammalian brain, MR is present in the limbic areas where it is highly expressed in neurons of the hippocampus and mostly absent in other regions while the glucocorticoid receptor (GR) expression is ubiquitous. MR binds both aldosterone and glucocorticoids, the latter having a ten-fold higher affinity for MR than for the closely related GR. However, owing to the minimal aldosterone transfer across the blood brain barrier and the absence of neuronal 11β hydroxysteroid dehydrogenase type 2 as an intracellular gate-keeper, neuronal MR appears to be fully occupied even at low physiological glucocorticoid levels while GR activation only occurs at high glucocorticoid concentrations, i.e. at the peak of the circadian rhythm or under stress. This defined a one hormone/two receptors system that works in balance, modulating a large spectrum of actions in the central nervous system. MR and GR are involved in the stress responses, the regulation of neuron excitability, long term potentiation, neuroprotection and neurogenesis in the dentate gyrus. MR thus constitutes a key factor in the arising of higher cognitive functions such as memorization, learning and mood. This review presents an overview of various roles of MR in the central nervous system which are somewhat less studied than that of GR, in the light of recent data obtained using cellular models, animal models and clinical investigations.

Mineralocorticoid and SGK1-sensitive inflammation and tissue fibrosis

Effects of mineralocorticoids are not restricted to regulation of epithelial salt transport, extracellular volume and blood pressure; mineralocorticoids also influence a wide variety of seemingly unrelated functions such as inflammation and fibrosis. The present brief review addresses the role of mineralocorticoids in the orchestration of these latter processes. Mineralocorticoids foster inflammation as well as vascular, cardiac, renal and peritoneal fibrosis. Mechanisms involved in mineralocorticoid-sensitive inflammation and fibrosis include the serum- and glucocorticoid-inducible kinase 1 (SGK1), which is genomically upregulated by mineralocorticoids and transforming growth factor β (TGF-β), and stimulated by mineralocorticoid-sensitive phosphatidylinositide 3-kinase. SGK1 upregulates the inflammatory transcription factor nuclear factor-κB, which in turn stimulates the expression of diverse inflammatory mediators including connective tissue growth factor. Moreover, SGK1 inhibits the degradation of the TGF-β-dependent transcription factors Smad2/3. Mineralocorticoids foster the development of TH17 cells, which is compromised following SGK1 deletion. Excessive SGK1 expression is observed in a wide variety of fibrosing diseases including lung fibrosis, diabetic nephropathy, glomerulonephritis, obstructive kidney disease, experimental nephrotic syndrome, obstructive nephropathy, liver cirrhosis, fibrosing pancreatitis, peritoneal fibrosis, Crohn's disease and celiac disease. The untoward inflammatory and fibrosing effects of mineralocorticoids could be blunted or even reversed by mineralocorticoid receptor blockers, which may thus be considered in the treatment of inflammatory and/or fibrosing disease.

Impact of aldosterone on osteoinductive signaling and vascular calcification

Vascular calcification is frequently found already in early stages of chronic kidney disease (CKD) patients and is associated with high cardiovascular risk. The process of vascular calcification is not considered a passive phenomenon but involves, at least in part, phenotypical transformation of vascular smooth muscle cells (VSMCs). Following exposure to excessive extracellular phosphate concentrations, VSMCs undergo a reprogramming into osteo-/chondroblast-like cells. Such 'vascular osteoinduction' is characterized by expression of osteogenic transcription factors and triggered by increased phosphate concentrations. A key role in this process is assigned to cellular phosphate transporters, most notably the type III sodium-dependent phosphate transporter Pit1. Pit1 expression is stimulated by mineralocorticoid receptor activation. Therefore, aldosterone participates in the phenotypical transformation of VSMCs. In preclinical models, aldosterone antagonism reduces vascular osteoinduction. Patients with CKD suffer from hyperphosphatemia predisposing to vascular osteogenic transformation, potentially further fostered by concomitant hyperaldosteronism. Clearly, additional research is required to define the role of aldosterone in the regulation of osteogenic signaling and the consecutive vascular calcification in CKD, but more generally also other diseases associated with excessive vascular calcification and even in individuals without overt disease.

On the pleotropic actions of mineralocorticoids

Classical effects of mineralocorticoids include stimulation of Na(+) reabsorption and K(+) secretion in the kidney and other epithelia including colon and several glands. Moreover, mineralocorticoids enhance the excretion of Mg(2+) and renal tubular H(+) secretion. The renal salt retention following mineralocorticoid excess leads to extracellular volume expansion and hypertension. The increase of blood pressure following mineralocorticoid excess is, however, not only the result of volume expansion but may result from stiff endothelial cell syndrome impairing the release of vasodilating nitric oxide. Beyond that, mineralocorticoids are involved in the regulation of a wide variety of further functions, including cardiac fibrosis, platelet activation, neuronal function and survival, inflammation as well as vascular and tissue fibrosis and calcification. Those functions are briefly discussed in this short introduction to the special issue. Beyond that, further contributions of this special issue amplify on mineralocorticoid-induced sodium appetite and renal salt retention, the role of mineralocorticoids in the regulation of acid-base balance, the involvement of aldosterone and its receptors in major depression, the mineralocorticoid stimulation of inflammation and tissue fibrosis and the effect of aldosterone on osteoinductive signaling and vascular calcification. Clearly, still much is to be learned about the various ramifications of mineralocorticoid-sensitive physiology and pathophysiology.

Aldosterone augments Na+-induced reduction of cardiac norepinephrine reuptake

Impairment of the cardiac norepinephrine (NE) reuptake by the neuronal NE transporter contributes to enhanced cardiac NE net release in congestive heart failure. Elevated plasma levels of aldosterone (AL) promote sympathetic overstimulation in failing hearts by unclear mechanisms. Our aim was to evaluate if elevated AL and/or alterations in Na(+) intake regulate cardiac NE reuptake. To test the effects of AL and Na(+) on cardiac NE reuptake, Wistar rats were fed a normal-salt (NS) diet (0.2% NaCl), a low-salt (LS) diet (0.015% NaCl), or a high-salt (HS) diet (8% NaCl). Another group of animals received AL infusion alone (0.75 μg/h) or AL infusion plus HS diet. Specific cardiac [(3)H]NE uptake via the NE transporter in a Langendorff preparation and AL plasma levels were measured at different time points between 5 and 42 days of treatment. To compare these findings from healthy animals with a disease model, Dahl salt-sensitive rats were investigated as a model of congestive heart failure with endogenously elevated AL. In summary, neither exogenous nor endogenous elevations of AL alone were sufficient to reduce cardiac NE reuptake. Only the HS diet induced a reduction of NE reuptake by 26%; additional infusion of AL augmented this effect to a further reduction of NE reuptake by 36%. In concordance, Dahl salt-sensitive rats treated with a HS diet displayed elevated AL and a marked reduction of NE reuptake. We conclude that exogenous or endogenous AL elevations alone do not reduce cardiac NE reuptake, but AL serves as an additional factor that negatively regulates cardiac NE reuptake in concert with HS intake.

The involvement of aldosterone on vascular insulin resistance: implications in obesity and type 2 diabetes

Aldosterone, a mineralocorticoid hormone produced at the adrenal glands, controls corporal hydroelectrolytic balance and, consequently, has a key role in blood pressure adjustments. Aldosterone also has direct effects in many organs, including the vasculature, leading to many cellular events that influence proliferation, migration, inflammation, redox balance and apoptosis. Aldosterone effects depend on its binding to mineralocorticoid receptors (MR). Aldosterone binding to MR triggers two pathways, the genomic pathway and the non-genomic pathway. In the vasculature e.g., activation of the non-genomic pathway by aldosterone induces rapid effects that involve activation of kinases, phosphatases, transcriptional factors and NAD(P)H oxidases. Aldosterone also plays a crucial role on systemic and vascular insulin resistance, i.e. the inability of a tissue to respond to insulin. Insulin has a critical role on cell function and vascular insulin resistance is considered an early contributor to vascular damage. Accordingly, aldosterone impairs insulin receptor (IR) signaling by altering the phosphatidylinositol 3-kinase (PI3K)/nitric oxide (NO) pathway and by inducing oxidative stress and crosstalk between the IR and the insulin-like growth factor-1 receptor (IGF-1R). This mini-review focuses on the relationship between aldosterone and vascular insulin resistance. Evidence indicating MR antagonists as therapeutic tools to minimize vascular injury associated with obesity and diabetes type 2 is also discussed.

Mineralocorticoid receptor modulators: a patent review (2007 - 2012)

INTRODUCTION

Considered for years as a 'renal hormone' leading to the control of renal excretion of minerals (explaining the term 'mineralocorticoid' used to describe this hormone), aldosterone is now recognized as a key factor in several diseases including hypertension, heart failure, arrhythmia, metabolic and kidney diseases, to cite only a few of them.

AREAS COVERED

In this review article, the authors aim to cover information provided by patents of the years 2007 through 2012. The rationale of writing this article is to cover the most important patents that can progress the field with new important discoveries.

EXPERT OPINION

The recognition of its proinflammatory and fibrogenic effects and the discovery of extrarenal sites of expression of its receptor (the mineralocorticoid receptor or MR) support a broader implication in diseases of MR activation than previously anticipated and the possible novel therapeutic indications of MR antagonists.