Inhibition of mineralocorticoid-mediated transcription by NF-kappaB

Structure-function relationships of the aldosterone receptor

The cellular response to the adrenal steroid aldosterone is mediated by the mineralocorticoid receptor (MR), a member of the nuclear receptor superfamily of ligand-dependent transcription factors. The MR binds more than one physiological ligand with binding at the MR determined by pre-receptor metabolism of glucocorticoid ligands by 11β hydroxysteroid dehydrogenase type 2. The MR has a wide tissue distribution with multiple roles beyond the classical role in electrolyte homeostasis including cardiovascular function, immune cell signaling, neuronal fate and adipocyte differentiation. The MR has three principal functional domains, an N-terminal ligand domain, a central DNA binding domain and a C-terminal, ligand binding domain, with structures having been determined for the latter two domains but not for the whole receptor. MR signal-transduction can be best viewed as a series of interactions which are determined by the conformation conferred on the receptor by ligand binding. This conformation then determines subsequent intra- and inter-molecular interactions. These interactions include chromatin, coregulators and other transcription factors, and additional less well characterized cytoplasmic non-genomic effects via crosstalk with other signaling pathways. This chapter will provide a review of MR structure and function, and an analysis of the critical interactions involved in MR-mediated signal transduction, which contribute to ligand- and tissue-specificity. Understanding the relevant mechanisms for selective MR signaling in terms of these interactions opens the possibility of novel therapeutic approaches for the treatment of MR-mediated diseases.

Novel interactions of the mineralocorticoid receptor

The mineralocorticoid receptor (MR) differs from the other steroid receptors in that it responds to two physiological ligands, aldosterone and cortisol. In epithelial tissues, aldosterone selectivity is determined by 11β-hydroxysteroid dehydrogenase type II. In other tissues cortisol is the primary ligand; in some tissues cortisol may act as an antagonist. To better target MR, an understanding of the structural determinants of tissue and ligand-specific MR activation is required. Our focus is on interactions of the ligand-binding domain (LBD) with ligand, the N-terminal domain and putative co-regulatory molecules. Molecular modelling has identified a region in the LBD of the MR and indeed other steroid receptors that critically defines ligand-specificity for aldosterone and cortisol, yet is not part of the ligand-binding pocket. An interaction between the N-terminus and LBD observed in the MR is aldosterone-dependent but is unexpectedly antagonised by cortisol. The structural basis of this interaction has been defined. We have identified proteins which interact in the presence of either aldosterone or cortisol but not both. These have been confirmed as coactivators of the full-length hMR. The structural basis of this interaction has been determined for tesmin, a ligand-discriminant coactivator of the MR. The successful identification of the structural basis of antagonism and of ligand-specific interactions of the MR may provide the basis for the development of novel MR ligands with tissue specificity.

Mineralocorticoid receptors in the heart: lessons from cell-selective transgenic animals

The clinical impact of cardiovascular disease cannot be underestimated. Equally, the importance of cost-effective management of cardiac failure is a pressing issue in the face of an ageing population and the increasing incidence of metabolic disorders worldwide. Targeting the mineralocorticoid receptor (MR) offers one approach for the treatment of heart failure with current strategies for novel MR therapeutics focusing on harnessing their cardio-protective benefits, but limiting the side effects of existing agents. It is now well accepted that activation of the MR in the cardiovascular system promotes tissue inflammation and fibrosis and has negative consequences for cardiac function and patient outcomes following cardiac events. Indeed, blockade of the MR using one of the two available antagonists (spironolactone and eplerenone) provides significant cardio-protective effects in the clinical and experimental setting. Although the pathways downstream of MR that translate receptor activation into tissue inflammation, fibrosis and dysfunction are still being elucidated, a series of recent studies using cell-selective MR (NR3C2)-null or MR-overexpressing mice have offered many new insights into the role of MR in cardiovascular disease and the control of blood pressure. Dissecting the cell-specific roles of MR signalling in the heart and vasculature to identify those pathways that are critical for MR-dependent responses is an important step towards achieving cardiac-selective therapeutics. The goal of this review is to discuss recent advances in this area that have emerged from the study of tissue-selective MR-null mice, and other targeted transgenic models and their relevance to clinical disease.

Neutrophil gelatinase-associated lipocalin is a novel mineralocorticoid target in the cardiovascular system

Mineralocorticoid receptor (MR) activation may be deleterious to the cardiovascular system, and MR antagonists improve morbidity and mortality of patients with heart failure. However, mineralocorticoid signaling in the heart remains largely unknown. Using a pan-genomic transcriptomic analysis, we identified neutrophil gelatinase-associated lipocalin (NGAL or lipocalin 2) as a strongly induced gene in the heart of mice with conditional and targeted MR overexpression in cardiomyocytes (whereas induction was low in glucocorticoid receptor-overexpressing mice). NGAL mRNA levels were enhanced after hormonal stimulation by the MR ligand aldosterone in cultured cardiac cells and in the heart of wild-type mice. Mineralocorticoid pathological challenge induced by nephrectomy/aldosterone/salt treatment upregulated NGAL expression in the heart and aorta and its plasma levels. We show evidence for MR binding to an NGAL promoter, providing a mechanism for NGAL regulation. We propose that NGAL may be a marker of mineralocorticoid-dependent injury in the cardiovascular system in mice.

Interactions of the mineralocorticoid receptor--within and without

The mineralocortoid receptor (MR) regulates salt homeostasis in the kidneys and plays a range of other roles in the heart, vasculature, brain and adipose tissue. It interacts with both mineralocorticoids and glucocorticoids to mediate transcription of target genes. The ability of the MR to exert tissue- and ligand-specific effects relies on its interactions with a range of binding partners, including the chaperone proteins, coregulators, other transcription factors, DNA and modifying proteins. Interactions within the domains of the MR also modulate the overall transcriptional complex. This review will discuss the current understanding of interactions involving the MR and highlight their relevance to ligand- or tissue-specificity as well as their suitability as therapeutic targets.

Luteinizing hormone induces expression of 11beta-hydroxysteroid dehydrogenase type 2 in rat Leydig cells

BACKGROUND

Leydig cells are the primary source of testosterone in male vertebrates. The biosynthesis of testosterone in Leydig cells is strictly dependent on luteinizing hormone (LH). On the other hand, it can be directly inhibited by excessive glucocorticoid (Corticosterone, CORT, in rats) which is beyond the protective capability of 11beta-Hydroxysteroid dehydrogenase type 1 (11beta-HSD1) and type 2 (11beta-HSD2; encoded by gene Hsd11b2 in rats) in Leydig cells. Our previous study found that LH increases 11beta-HSD1 expression in rat Leydig cells, but the effect of LH on the expression and activity of 11beta-HSD2 is not investigated yet.

METHODS

The Leydig cells were isolated from male Sprague-Dawley rats (90 days of age). After Leydig cells were incubated either for 24 h with various concentrations of LH (2.5, 5, 10 and 20 ng/mL) or for different time periods (2, 8, 12 and 24 h) with 20 ng/mL LH, the mRNA expression of 11beta-HSD2 was measured by real-time PCR. 11beta-HSD2 protein levels in Leydig cells were assayed by Western Blot and 11beta-HSD2 enzyme activity was determined by calculating the ratio of conversion of [3H]CORT to [3H]11-dehydrocorticosterone by 24 h after stimulation with 20 ng/ml LH. Four reporter gene plasmids containing various lengths of Hsd11b2 promoter region were constructed and transfected into mouse Leydig tumor cells to investigate the effect of LH on Hsd11b2 transcription. A glucocorticoid-responsive reporter gene plasmid, GRE-Luc, was constructed. To evaluate influence of LH on intracellular glucocorticoid level, rat Leydig cells were transfected with GRE-Luc, and luciferase activities were measured after incubation with CORT alone or CORT plus LH.

RESULTS

We observed dose- and temporal-dependent induction of rat 11beta-HSD2 mRNA expression in Leydig cells subject to LH stimulation. The protein and enzyme activity of 11beta-HSD2 and the luciferase activity of reporter gene driven by promoter regions of Hsd11b2 were increased by LH treatment. LH decreased the glucocorticoid-induced luciferase activity of GRE-Luc reporter gene.

CONCLUSION

The results of the present study suggest that LH increases the expression and enzyme activity of 11beta-HSD2, and therefore enhances capacity for oxidative inactivation of glucocorticoid in rat Leydig cells in vitro.

Spironolactone prevents diabetic nephropathy through an anti-inflammatory mechanism in type 2 diabetic rats

Aldosterone induces myocardial fibrosis and vascular inflammation via proinflammatory and profibrotic cytokines. The effect of spironolactone on renal inflammation and renal function was investigated in type 2 diabetic rats. For define the molecular mechanism of spironolactone, the effect of spironolactone on the synthesis of monocyte chemotactic peptide-1 (MCP-1) and its upstream transcription factor, NF-kappaB, was evaluated in cultured mesangial cells and proximal tubular cells. There were no changes in blood glucose concentration or BP after spironolactone treatment. Spironolactone treatment significantly reduced urinary albumin excretion and ameliorated glomerulosclerosis. Urinary levels of MCP-1 were significantly increased concurrently with renal expression of MCP-1, macrophage migration inhibitory factor, and macrophage infiltration. Spironolactone treatment significantly inhibited urinary excretion of MCP-1 as well as renal MCP-1 and migration inhibitory factor expression and macrophage infiltration. In addition, aldosterone induced upregulation of MCP-1 expression and NF-kappaB transcriptional activity in cultured cells, and spironolactone reduced both NF-kappaB activation and MCP-1 synthesis. Furthermore, NF-kappaB inhibition abolished aldosterone-induced MCP-1 production. Overall, these findings suggest that aldosterone-induced NF-kappaB activation leads to activation of proinflammatory cytokines, ultimately leading to renal injury in this model. These data suggest that mineralocorticoid blockade may be a potential therapeutic target in diabetic nephropathy.

Mechanisms of Mineralocorticoid Action

Sodium transport in epithelial tissues is regulated by the physiological mineralocorticoid aldosterone. The response to aldosterone is mediated by the mineralocorticoid receptor (MR), for which the crystal structure of the ligand-binding domain has recently been established. The classical mode of action for this receptor involves the regulation of gene transcription. Several genes have now been shown to be regulated by aldosterone in epithelial tissues. Of these, the best characterized is serum- and glucocorticoid-regulated kinase, which increases sodium influx through the epithelial sodium channel. Turnover of these channels in the cell membrane is mediated by Nedd4-2, a ubiquitin protein ligase; serum- and glucocorticoid-regulated kinase interacts with and phosphorylates Nedd4-2, thereby rendering it unable to bind the sodium channels. In nonepithelial tissues, particularly the cardiovascular system, aldosterone also has direct effects, activating an inflammatory cascade, leading to cardiac fibrosis. A critical role for the MR in cardiovascular disease has now been demonstrated by the beneficial response to MR blockade in 2 large clinical trials in patients with cardiac failure. It is these nonepithelial actions of MR activation that need to be exploited for the development of antagonists that target the cardiovascular system while avoiding the undesirable side effects of renal MR blockade.

IκB Kinase-β (IKKβ) Modulation of Epithelial Sodium Channel Activity

Using the yeast two-hybrid system, we identified a number of proteins that interacted with the carboxyl termini of murine epithelial sodium channel (ENaC) subunits. Initial screens indicated an interaction between the carboxyl terminus of beta-ENaC and IkappaB kinase-beta (IKKbeta), the kinase that phosphorylates Ikappabeta and results in nuclear targeting of NF-kappaB. A true two-hybrid reaction employing full-length IKKbeta and the carboxyl termini of all three subunits confirmed a strong interaction with beta-ENaC, a weak interaction with gamma-ENaC, and no interaction with alpha-ENaC. Co-immunoprecipitation studies for IKKbeta were performed in a murine cortical collecting duct cell line that endogenously expresses ENaC. Immunoprecipitation with beta-ENaC, but not gamma-ENaC, resulted in co-immunoprecipitation of IKKbeta. To examine the direct effects of IKKbeta on ENaC activity, co-expression studies were performed using the two-electrode voltage clamp technique in Xenopus oocytes. Oocytes were injected with cRNAs for alphabetagamma-ENaC with or without cRNA for IKKbeta. Co-injection of IKKbeta significantly increased the amiloride-sensitive current above controls. Using cell surface ENaC labeling, we determined that an increase of ENaC in the plasma membrane accounted for the increase in current. The injection of kinase-dead IKKbeta (K44A) in ENaC-expressing oocytes resulted in a significant decrease in current. Treatment of mpkCCD(c14) cells with aldosterone increased whole cell amounts of IKKbeta. Because this result suggested that aldosterone might activate NF-kappaB, mpkCCD(c14) cells were transiently transfected with a luciferase reporter gene responsive to NF-kappaB activation. Both aldosterone and tumor necrosis factor-alpha (TNFalpha) stimulation caused a similar and significant increase in luciferase activity as compared with controls. We conclude that IKKbeta interacts with ENaC by up-regulating ENaC at the plasma membrane and that the presence of IKKbeta is at very least permissive to ENaC function. These studies also suggest a previously unexpected interaction between the NF-kappaB transcription pathway and steroid regulatory pathways in epithelial cells.

Mineralocorticoid receptor binding, structure and function

The isolation of aldosterone 50 years ago was a critical first step in elucidating the mechanism by which corticosteroids regulate electrolyte homeostasis. The broad principles of this mechanism involving an intracellular receptor acting on specific genes to induce the expression/repression of aldosterone-induced proteins (AIP) were established 30 years ago. The cloning of the mineralocorticoid receptor (MR) has enabled studies of the subcellular mechanisms of aldosterone action, including the molecular dissection of structure-function relationships in the receptor. We have exploited the close structural and functional similarity of the MR with the glucocorticoid receptor to identify the regions in the MR that confer ligand-binding specificity. The critical region is located, not as might be expected in the ligand-binding pocket but rather on the surface of the molecule. These studies have been extended to an analysis of the interactions between the N-terminal and ligand-binding domains of the MR. In the last decade, AIP have been identified; the regulation of the genes encoding these AIP are discussed.

Aldosterone: a risk factor for vascular disease

Blockade of the renin-angiotensin-aldosterone system with angiotensin-converting enzyme (ACE) inhibitors or angiotensin receptor antagonists has resulted in beneficial effects in essential hypertensive patients. However, occurrence of cardiovascular events has not been appropriately controlled beyond a certain percentage. One reason could be the effects of aldosterone, the final component of the system. The aldosterone escape phenomenon could explain undesirable outcomes observed in hypertensive patients even under treatment with ACE inhibitors or angiotensin antagonists. Aldosterone has direct effects on the vasculature and has been associated with vascular smooth muscle cell hypertrophy, endothelial dysfunction, cardiac fibrosis, proteinuria, and renal vascular injury. Animal models and clinical trials have proven the benefit of aldosterone receptor antagonism. With increased recognition of the prevalence of hyperaldosteronism in patients thought to have "essential" hypertension, the use of drugs that block aldosterone action may become more widespread and protect the vasculature from the deleterious effects of aldosterone.

New ideas about aldosterone signaling in epithelia

The systemic actions of aldosterone are well documented; however, in comparison, our understanding of the cellular and molecular mechanisms by which aldosterone orchestrates these actions is rudimentary. Aldosterone exerts most of its physiological actions by modifying gene expression. It is now apparent that aldosterone represses almost as many genes as it induces. Several aldosterone-sensitive genes, including serum and glucocorticoid-inducible kinase (sgk) and small, monomeric Kirsten Ras GTP-binding protein (Ki-ras) have recently been identified. The molecular mechanisms and elements bestowing corticosteroid sensitivity on these and many other genes are becoming clear. Induction of Ki-Ras and Sgk is necessary and sufficient for some portion of aldosterone action in epithelia. These two signaling factors are components of a converging pathway with phosphatidylinositol 3-kinase positioned between them that enables both stabilizing the epithelial Na(+) channel (ENaC) in the open state as well as increasing the number of ENaC in the apical membrane. This aldosterone-induced signaling pathway contains many potential sites for feedback regulation and cross talk from other cascades and potentially impinges directly on the activity of transport proteins and/or cellular differentiation to modify electrolyte transport.