Maria Christina Zennaro

Pivotal role of the mineralocorticoid receptor in corticosteroid-induced adipogenesis

In addition to their role in controlling water and salt homeostasis, recent work suggests that aldosterone and mineralocorticoid receptors (MR) may be involved in adipocyte biology. This is of particular relevance given the role of MR as a high‐affinity receptor for both mineralocorticoids and glucocorticoids. We have thus examined the effect of aldosterone and MR on white adipose cell differentiation. When cells are cultured in a steroid‐free medium, aldosterone promotes acquisition of the adipose phenotype of 3T3‐L1 and 3T3‐F442A cells in a time‐, dose‐, and MR‐dependent manner. In contrast, late and long‐term exposure to dexamethasone inhibits adipocyte terminal maturation. The aldosterone effect on adipose maturation was accompanied by induction of PPARγ mRNA expression, which was blocked by the MR antagonist spironolactone. Under permissive culture conditions, specific MR down‐regulation by siRNAs markedly inhibited 3T3‐L1 differentiation by interfering with the transcriptional control of adipogenesis, an effect not mimicked by specific inactivation of the glucocorticoid receptor. These results demonstrate that MR represents an important proadipogenic transcription factor that may mediate both aldosterone and glucocorticoid effects on adipose tissue development. MR thus may be of pathophysiological relevance to the development of obesity and the metabolic syndrome.–Caprio, M., Fève, B., Claës, A., Viengchareun, S., Lombès, M., Zennaro, M‐C. Pivotal role of the mineralocorticoid receptor in corticosteroid‐induced adipogenesis. FASEB J. 21, 2185–2194 (2007)

WNT/β-catenin signalling is activated in aldosterone-producing adenomas and controls aldosterone production

Primary aldosteronism (PA) is the main cause of secondary hypertension, resulting from adrenal aldosterone-producing adenomas (APA) or bilateral hyperplasia. Here, we show that constitutive activation of WNT/β-catenin signalling is the most frequent molecular alteration found in 70% of APA. We provide evidence that decreased expression of the WNT inhibitor SFRP2 may be contributing to deregulated WNT signalling and APA development in patients. This is supported by the demonstration that mice with genetic ablation of Sfrp2 have increased aldosterone production and ectopic differentiation of zona glomerulosa cells. We further show that β-catenin plays an essential role in the control of basal and Angiotensin II-induced aldosterone secretion, by activating AT1R, CYP21 and CYP11B2 transcription. This relies on both LEF/TCF-dependent activation of AT1R and CYP21 regulatory regions and indirect activation of CYP21 and CYP11B2 promoters, through increased expression of the nuclear receptors NURR1 and NUR77. Altogether, these data show that aberrant WNT/β-catenin activation is associated with APA development and suggest that WNT pathway may be a good therapeutic target in PA.

The role of the mineralocorticoid receptor in adipocyte biology and fat metabolism

Aldosterone controls blood pressure by binding to the mineralocorticoid receptor (MR), a ligand-activated transcription factor which regulates critical genes controlling salt and water homeostasis in the kidney. In recent years, inappropriate MR activation has been shown to trigger deleterious responses in various tissues, including vessels, heart and brain, hence promoting vascular inflammation, cardiovascular remodeling, endothelial dysfunction, and oxidative stress. Moreover, epidemiological studies have shown a clear association between aldosterone levels and the incidence of metabolic syndrome. In particular, recent work has revealed functional MRs in adipose tissue, where they mediate the effects of aldosterone and glucocorticoids, displaying important and specific functions involving adipose differentiation, expansion and proinflammatory capacity. This recent evidence finally moved MR out of the shadow of the glucocorticoid receptor (GR), which had previously been considered the only player mediating corticosteroid action in adipose tissue. This has opened a new era of research focusing on the complexity and selectivity of MR function in adipocyte biology. The aim of this review is to summarize the latest concepts on the role of MR in white and brown adipocytes, and to discuss the potential benefits of tissue-selective MR blockade in the treatment of obesity and metabolic syndrome.

Antiadipogenic Effects of the Mineralocorticoid Receptor Antagonist Drospirenone: Potential Implications for the Treatment of Metabolic Syndrome

The mineralocorticoid receptor (MR) mediates aldosterone- and glucocorticoid-induced adipocyte differentiation. Drospirenone (DRSP) is a potent synthetic antimineralocorticoid with progestogenic and antiandrogenic properties, which is widely used for contraception and hormone replacement therapy. We investigated its potential role on adipocyte differentiation. The effects of DRSP were studied in murine preadipocyte cell lines and primary cultures of human preadipocytes. Differentiation markers and mechanisms underlying phenotypic variations in response to DRSP were explored. Early exposure to DRSP during differentiation led to a marked dose-dependent inhibition of adipose differentiation and triglyceride accumulation in 3T3-L1 and 3T3-F442A cells. DRSP also markedly inhibited adipose conversion of human primary preadipocytes derived from visceral (mesenteric and epicardial) and subcutaneous fat. This effect was MR-dependent and did not involve the glucocorticoid, androgen, or progesterone receptors. DRSP inhibited clonal expansion of preadipocytes and decreased expression of PPARγ, a key transcriptional mediator of adipogenesis, but had no effect on lipolysis, glucose uptake, and PPARγ binding to its ligands. DRSP exerts a potent antiadipogenic effect that is related to an alteration of the transcriptional control of adipogenesis via an antagonistic effect on the MR. Selective MR blockade therefore has promise as a novel therapeutic option for the control of excessive adipose tissue deposition and its related metabolic complications.

Mineralocorticoid receptors in the metabolic syndrome

The mineralocorticoid receptor (MR) mediates aldosterone effects on salt homeostasis and blood pressure regulation. MR activation also promotes inflammation, cardiovascular remodelling and endothelial dysfunction, and affects adipose tissue differentiation and function. Some of these effects derive from MR activation by glucocorticoids. Recent epidemiological studies show that the incidence of metabolic syndrome increases across quartiles of aldosterone, implicating the MR as a central player in metabolic homeostasis, involving electrolyte, water and energy balance. This review summarizes the current understanding of MR-mediated effects in diverse tissues and the role of aldosterone as a cardiometabolic risk factor, and discusses the possible relationship between inappropriate MR activation (by both mineralocorticoids and glucocorticoids) and the development of metabolic syndrome.

Inactivating mutations of the mineralocorticoid receptor in Type I pseudohypoaldosteronism.

Type I pseudohypoaldosteronism (PHA1) is a rare form of mineralocorticoid resistance characterized by neonatal renal salt wasting and failure to thrive. Typical biochemical features include high levels of plasma aldosterone and renin, hyponatremia and hyperkalemia. Different mutations of the human mineralocorticoid receptor (hMR) gene have been identified in subjects affected by the autosomal dominant or sporadic form of the disease. Our laboratory has investigated a large number of subjects with familial and sporadic PHA1. Several different mutations have been detected, which are localized in different coding exons of the hMR gene. These mutations either create truncated proteins, either affect specific amino acids involved in receptor function. In this paper, we review hMR mutations described to date in PHA1 and their functional characterization. We discuss the absence of mutations in some kindreds and the role of precise phenotypic and biological examination of patients to allow for identification of other genes potentially involved in the disease.

The Functional c.-2G>C Variant of the Mineralocorticoid Receptor Modulates Blood Pressure, Renin, and Aldosterone Levels

The mineralocorticoid receptor (MR) is essential in the regulation of volemia and blood pressure. Rare mutations in the MR gene cause type 1 pseudohypoaldosteronism and hypertension. In this study we characterized the common MR polymorphism c.-2G>C (rs2070951) in vitro and tested its influence on parameters related to blood pressure regulation and the renin-angiotensin system. In vitro studies showed that the G allele was associated with decreased MR protein levels and reduced transcriptional activation compared with the C allele. Association studies were performed with several outcome variables in 3 independent cohorts: a mild hypertensive group subjected to a salt-sensitivity test, a healthy normotensive group included in a crossover study to receive both a high and low Na/K diet, and a large cohort (The Netherlands Study of Depression and Anxiety), in which blood pressure was measured. Subjects with the GG genotype had significantly higher plasma renin levels both in the mild hypertensive group and in normal volunteers compared with homozygous C carriers. The GG genotype was also correlated with higher plasma aldosterone levels in healthy subjects. In both the mild hypertensive group and The Netherlands Study of Depression and Anxiety cohort the genotype GG was associated with higher systolic blood pressure in males. In conclusion, the G allele of the common functional genetic polymorphism c.-2G>C in the MR gene associates with increased activation of the renin-angiotensin-aldosterone axis and with increased blood pressure, probably related to decreased MR expression.

Mast Cell Hyperplasia Is Associated With Aldosterone Hypersecretion in a Subset of Aldosterone-Producing Adenomas

CONTEXT Adrenal mast cells can stimulate aldosterone secretion through the local release of serotonin (5-HT) and activation of the 5-HT4 receptor (5-HT4). In aldosterone-producing adenomas (APAs), 5-HT4 receptor is overexpressed and the administration of 5-HT4 receptor agonists to patients with APA increases plasma aldosterone levels. These data and the well-documented role of mast cells in tumorigenesis suggest that mast cells may be involved in the pathophysiology of APA. OBJECTIVE The study aimed at investigating the occurrence of mast cells in a series of APA tissues and to examine the influence of mast cells on aldosterone secretion. DESIGN The occurrence of mast cells in APAs was investigated by immunohistochemistry. Mast cell densities were compared with clinical data. The influence of mast cells on aldosterone production was studied by using cultures of human mast cell and adrenocortical cell lines. RESULTS In APA tissues, the density of mast cells was found to be increased in comparison with normal adrenals. Mast cells were primarily observed in adrenal cortex adjacent to adenomas or in the adenomas themselves, distinguishing two groups of APAs. A subset of adenomas was found to contain a high density of intratumoral mast cells, which was correlated with aldosterone synthase expression and in vivo aldosterone secretory parameters. Administration of conditioned medium from cultures of human mast cell lines to human adrenocortical cells induced a significant increase in aldosterone synthase (CYP11B2) mRNA expression and aldosterone production. CONCLUSION APA tissues commonly contain numerous mast cells that may influence aldosterone secretion through the local release of regulatory factors.

Dkk3 is a component of the genetic circuitry regulating aldosterone biosynthesis in the adrenal cortex

Primary aldosteronism (PA, autonomous aldosterone production from the adrenal cortex) causes the most common form of secondary arterial hypertension (HT), which is also the most common curable form of HT. Recent studies have highlighted an important role of mutations in genes encoding potassium channels in the pathogenesis of PA, both in human disease and in animal models. Here, we have exploited the unique features of the hyperaldosteronemic phenotype of Kcnk3 null mice, which is dependent on sexual hormones, to identify genes whose expression is modulated in the adrenal gland according to the dynamic hyperaldosteronemic phenotype of those animals. Genetic inactivation of one of the genes identified by our strategy, dickkopf-3 (Dkk3), whose expression is increased by calcium influx into adrenocortical cells, in the Kcnk3 null background results in the extension of the low-renin, potassium-rich diet insensitive hyperaldosteronemic phenotype to the male sex. Compound Kcnk3/Dkk3 animals display an increased expression of Cyp11b2, the rate-limiting enzyme for aldosterone biosynthesis in the adrenal zona glomerulosa (ZG). Our data show that Dkk3 can act as a modifier gene in a mouse model for altered potassium channel function and suggest its potential involvement in human PA syndromes.

The Mineralocorticoid Receptor in Endothelial Physiology and Disease : Novel Concepts in the Understanding of Erectile Dysfunction

Aldosterone is a steroid hormone that controls blood pressure by binding to the mineralocorticoid receptor (MR), a ligand-activated transcription factor, and regulating genes that play a role in salt and water homeostasis in the kidney. Dysregulation of the mineralocorticoid system reveals its crucial role in various human diseases including hypertension, atherosclerosis, cardiac failure, mineralocorticoid resistance, and disorders of the nervous system. Recently, experimental animal models of mineralocorticoid/salt-induced hypertension and atherosclerosis have revealed an epithelial, pro-inflammatory role for MR activation. Extensive investigation has begun to elucidate the mechanisms underlying the vascular effects of MR activation which involve its direct role in cardiomyocytes, vascular smooth muscle cells, and endothelial cells. More specifically, in patients with cardiovascular risk factors and disease, including diabetes, hypertension, and/or congestive heart failure, an excess of MR activation has been shown to have a negative impact on endothelial function hence disrupting the physiological balance between vasoconstriction and vasodilation. Such a mechanism may play a role in the pathogenesis of erectile dysfunction (ED), a condition that occurs frequently in patients with increased cardiovascular risk and involves endothelial dysregulation of vascular relaxation. The aim of this review is to summarize the latest concepts in MR signaling, with particular attention to the endothelium, and to discuss the potential benefits of tissue-selective MR blockade in treating subsets of ED patients, such as those with congestive heart failure and hypertension, in which the MR system may be over activated.