A critical region in the mineralocorticoid receptor for aldosterone binding and activation by cortisol: evidence for a common mechanism governing ligand binding specificity in steroid hormone receptors

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.

Cortisol resistance in the degu (Octodon degus)

Cortisol resistance has also been reported in the degu, Octodon degus, a New World hystricomorph endemic to central Chile. The degu is used as a model for studies of stress and diurnal rhythms, parental behaviour and female masculinization. Another New World hystricomorph, the guinea pig, also exhibits glucocorticoid resistance, a result of amino acid sequences that differ from other mammalian glucocorticoid receptors (GR). Mutations in the ligand-binding domain of the human GR have been identified in familial or sporadic generalised cortisol resistance as have variants in the guinea pig. To address the possibility that the high levels of cortisol observed in the degu are a result of the same or similar sequence variations observed in the guinea pig GR, we have cloned, expressed and characterised the ligand-binding domain (LBD) of the degu GR. Somewhat unexpectedly, neither the amino acids nor the region involved in the resistance observed in the guinea pig GR are relevant in the degu GR. The relative resistance to cortisol observed in the degu GR is conferred by the substitution of two isoleucine residues, which are highly conserved in the GR across species, with a valine doublet. These amino acids lie in the region between helices 5 and 6 of the GR LBD, a region known to be important in determining the affinity of ligand-binding in steroid receptors.

Structural and molecular determinants of mineralocorticoid receptor signalling

During the past decades, the mineralocorticoid receptor (MR) has evolved from a much-overlooked member of the steroid hormone receptor family to an important player, not only in volume and electrolyte homeostasis but also in pathological changes occurring in an increasing number of tissues, especially the renal and cardiovascular systems. Simultaneously, a wealth of information about the structure, interaction partners and chromatin requirements for genomic signalling of steroid hormone receptors became available. However, much of the information for the MR has been deduced from studies of other family members and there is still a lack of knowledge about MR-specific features in ligand binding, chromatin remodelling, co-factor interactions and general MR specificity-conferring mechanisms that can completely explain the differences in pathophysiological function between MR and its closest relative, the glucocorticoid receptor. This review aims to give an overview of the current knowledge of MR structure, signalling and co-factors modulating its activity.

Clinical and Molecular Perspectives of Monogenic Hypertension

Advances in molecular research techniques have enabled a new frontier in discerning the mechanisms responsible for monogenic diseases. In this review, we discuss the current research on the molecular pathways governing blood pressure disorders with a Mendelian inheritance pattern, each presenting with a unique pathophysiology. Glucocorticoid Remediable Aldosteronism (GRA) and Apparent Mineralocorticoid Excess (AME) are caused by mutations in regulatory enzymes that induce increased production of mineralocorticoids or inhibit degradation of glucocorticoids, respectively. Geller syndrome is due to a point mutation in the hormone responsive element of the promotor for the mineralocorticoid receptor, rendering the receptor susceptible to activation by progesterone, leading to hypertension during pregnancy. Pseudohypoaldosteronism type II (PHA-II), also known as Gordon's syndrome or familial hyperkalemic hypertension, is a more variable disorder typically characterized by hypertension, high plasma potassium and metabolic acidosis. Mutations in a variety of intracellular enzymes that lead to enhanced sodium reabsorption have been identified. In contrast, hypertension in Liddle's syndrome, which results from mutations in the Epithelial sodium Channel (ENaC), is associated with low plasma potassium and metabolic alkalosis. In Liddle's syndrome, truncation of one the ENaC protein subunits removes a binding site necessary protein for ubiquitination and degradation, thereby promoting accumulation along the apical membrane and enhanced sodium reabsorption. The myriad effects due to mutation in phosphodiesterase 3A (PDE3A) lead to severe hypertension underlying sodium-independent autosomal dominant hypertension with brachydactyly. How mutations in PDE3A result in the phenotypic features of this disorder are discussed. Understanding the pathologies of these monogenic hypertensive disorders may provide insight into the causes of the more prevalent essential hypertension and new avenues to unravel the complexities of blood pressure regulation.

Molecular evolution of the switch for progesterone and spironolactone from mineralocorticoid receptor agonist to antagonist

The mineralocorticoid receptor (MR) is highly conserved across vertebrate evolution. In terrestrial vertebrates, the MR mediates sodium homeostasis by aldosterone and also acts as a receptor for cortisol. Although the MR is present in fish, they lack aldosterone. The MR binds progesterone and spironolactone as antagonists in human MR but as agonists in zebrafish MR. We have defined the molecular basis of these divergent responses using MR chimeras between the zebrafish and human MR coupled with reciprocal site-directed mutagenesis and molecular dynamic (MD) simulation based on the crystal structures of the MR ligand-binding domain. Substitution of a leucine by threonine in helix 8 of the ligand-binding domain of the zebrafish MR confers the antagonist response. This leucine is conserved across fish species, whereas threonine (serine in rodents) is conserved in terrestrial vertebrate MR. MD identified an interaction of the leucine in helix 8 with a highly conserved leucine in helix 1 that stabilizes the agonist conformation including the interaction between helices 3 and 5, an interaction which has previously been characterized. This switch in the MR coincides with the evolution of terrestrial vertebrates and of aldosterone synthesis. It was perhaps mandatory if the appearance of aldosterone as a specific mediator of the homeostatic salt retention was to be tolerated. The conformational changes also provide insights into the structural basis of agonism versus antagonism in steroid receptors with potential implications for drug design in this important therapeutic target.

30 YEARS OF THE MINERALOCORTICOID RECEPTOR: Coregulators as mediators of mineralocorticoid receptor signalling diversity

The cloning of the mineralocorticoid receptor (MR) 30 years ago was the start of a new era of research into the regulatory processes of MR signalling at target genes in the distal nephron, and subsequently in many other tissues. Nuclear receptor (NR) signalling is modified by interactions with coregulatory proteins that serve to enhance or inhibit the gene transcriptional responses. Over 400 coregulatory proteins have been described for the NR super family, many with functional roles in signalling, cellular function, physiology and pathophysiology. Relatively few coregulators have however been described for the MR although recent studies have demonstrated both ligand and/or tissue selectivity for MR-coregulator interactions. A full understanding of the cell, ligand and promoter-specific requirements for MR-coregulator signalling is an essential first step towards the design of small molecular inhibitors of these protein-protein interactions. Tissue-selective steroidal or non-steroidal modulators of the MR are also a desired therapeutic goal. Selectivity, as for other steroid hormone receptors, will probably depend on differential expression and recruitment of coregulatory proteins.

Evolution of corticosteroid specificity for human, chicken, alligator and frog glucocorticoid receptors

We investigated the evolution of the response of human, chicken, alligator and frog glucocorticoid receptors (GRs) to dexamethasone, cortisol, cortisone, corticosterone, 11-deoxycorticosterone, 11-deoxycortisol and aldosterone. We find significant differences among these vertebrates in the transcriptional activation of their full length GRs by these steroids, indicating that there were changes in the specificity of the GR for steroids during the evolution of terrestrial vertebrates. To begin to study the role of interactions between different domains on the GR in steroid sensitivity and specificity for terrestrial GRs, we investigated transcriptional activation of truncated GRs containing their hinge domain and ligand binding domain (LBD) fused to a GAL4 DNA binding domain (GAL4-DBD). Compared to corresponding full length GRs, transcriptional activation of GAL4-DBD_GR-hinge/LBD constructs required higher steroid concentrations and displayed altered steroid specificity, indicating that interactions between the hinge/LBD and other domains are important in glucocorticoid activation of these terrestrial GRs.

Role of Pro-637 and Gln-642 in human glucocorticoid receptors and Ser-843 and Leu-848 in mineralocorticoid receptors in their differential responses to cortisol and aldosterone

Mineralocorticoid receptors (MR) and glucocorticoid receptors (GR) are descended from a common ancestral corticoid receptor. The basis for specificities of human MR for aldosterone and human GR for glucocorticoids, such as cortisol, bearing 17α-hydroxyl-groups, is incompletely understood. Differences in MR at S843 and L848 and GR at the corresponding P637 and Q642 have been proposed as important in their different responses to glucocorticoids with 17α-hydroxyl-groups. We investigated the impact of these residues on binding affinity (Ki) and transcriptional activation (EC50) of mutants MR-S843P, MR-L848Q and MR-S843P/L848Q and mutants GR-P637S, GR-Q642L and GR-P637S/Q642L in the presence of different corticosteroids. Aldosterone, cortisol and corticosterone had similar affinities for wild-type MR and all mutants, while dexamethasone had increased affinity for the three mutants. However, transactivation of MR-S843P and MR-S843P/L848Q by all four steroids was significantly lower than for wild-type MR. In contrast, transactivation of MR-L848Q tended to be 3-fold higher for cortisol and corticosterone and increased 7-fold for dexamethasone, indicating that MR-L848Q has an increased response to glucocorticoids, while retaining a strong response to aldosterone. Compared to wild-type GR, GR-P637S and GR-Q642L had increased affinities and significantly increased transcriptional activity with aldosterone and corticosterone, and GR-P637S had similar transcriptional activity with cortisol and dexamethasone, while GR-Q642L and GR-P637S/Q642L had a significant decrease in transcriptional activity with cortisol and dexamethasone. 3D-models of these MR and GR mutants revealed that dexamethasone and aldosterone, respectively, fit nicely into the steroid-binding pocket, consistent with the affinity of dexamethasone for MR mutants and aldosterone for GR mutants.

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.

Aldosterone activates transcription factor Nrf2 in kidney cells both in vitro and in vivo

AIMS

An increased kidney cancer risk was found in hypertensive patients, who frequently exhibit hyperaldosteronism, known to contribute to kidney injury, with oxidative stress playing an important role. The capacity of kidney cells to up-regulate transcription factor nuclear factor-erythroid-2-related factor 2 (Nrf2), a key regulator of the cellular antioxidative defense, as a prevention of aldosterone-induced oxidative damage was investigated both in vitro and in vivo.

RESULTS

Aldosterone activated Nrf2 and increased the expression of enzymes involved in glutathione (GSH) synthesis and detoxification. This activation depended on the mineralocorticoid receptor (MR) and oxidative stress. In vitro, Nrf2 activation, GSH amounts, and target gene levels decreased after 24 h, while oxidant levels remained high. Nrf2 activation could not protect cells against oxidative DNA damage, as aldosterone-induced double-strand breaks and 7,8-dihydro-8-oxo-guanine (8-oxodG) lesions steadily rose. The Nrf2 activator sulforaphane enhanced the Nrf2 response both in vitro and in vivo, thereby preventing aldosterone-induced DNA damage. In vivo, Nrf2 activation further had beneficial effects on the aldosterone-caused blood pressure increase and loss of kidney function.

INNOVATION

This is the first study showing the activation of Nrf2 by aldosterone. Moreover, the results identify sulforaphane as a substance that is capable of preventing aldosterone-induced damage both in vivo and in vitro.

CONCLUSION

Aldosterone-induced Nrf2 adaptive response cannot neutralize oxidative actions of chronically increased aldosterone, which, therefore could be causally involved in the increased cancer incidence of hypertensive individuals. Enhancing the cellular antioxidative defense with sulforaphane might exhibit beneficial effects.

Beneficial effects of proanthocyanidins in the cardiac alterations induced by aldosterone in rat heart through mineralocorticoid receptor blockade

Aldosterone administration in rats results in several cardiac alterations. Previous studies have demonstrated that proanthocyanidins, phenolic bioactive compounds, have cardioprotective effects. We studied the potential beneficial effects of the proanthocyanidin-rich almond skin extract (PASE) on the cardiac alterations induced by aldosterone-salt treatment, their effects in mineralocorticoid receptor activity and we sought to confirm proanthocyanidins as the specific component of the extract involved in the beneficial cardiac effects. Male Wistar rats received aldosterone (1 mg/Kg/day) +1% NaCl for 3 weeks. Half of the animals in each group were simultaneously treated with either PASE (100 mg/Kg/day) or spironolactone (200 mg/Kg/day). The ability of PASE to act as an antagonist of the mineralocorticoid receptor was examined using a transactivation assay. High performance liquid chromatography was used to identify and to isolate proanthocyanidins. Hypertension and diastolic dysfunction induced by aldosterone were abolished by treatment with PASE. Expression of the aldosterone mediator SGK-1, together with fibrotic, inflammatory and oxidative mediators were increased by aldosterone-salt treatment; these were reduced by PASE. Aldosterone-salt induced transcriptional activity of the mineralocorticoid receptor was reduced by PASE. HPLC confirmed proanthocyanidins as the compound responsible for the beneficial effects of PASE. The effects of PASE were comparable to those seen with the mineralocorticoid antagonist, spironolactone. The observed responses in the aldosterone-salt treated rats together with the antagonism of transactivation at the mineralocorticoid receptor by PASE provides evidence that the beneficial effect of this proanthocyanidin-rich almond skin extract is via as a mineralocorticoid receptor antagonist with proanthocyanidins identified as the compounds responsible for the beneficial effects of PASE.

The helix 1-3 loop in the glucocorticoid receptor LBD is a regulatory element for FKBP cochaperones

The heat-shock protein 90 (Hsp90) cochaperone FK506-binding protein 52 (FKBP52) upregulates, whereas FKBP51 inhibits, hormone binding and nuclear targeting of the glucocorticoid receptor (GR). Decreased cortisol sensitivity in the guinea pig is attributed to changes within the helix 1 to helix 3 (H1-H3) loop of the guinea pig GR (gpGR) ligand-binding domain. It has been proposed that this loop serves as a contact point for FKBP52 and/or FKBP51 with receptor. We examined the role of the H1-H3 loop in GR activation by FKBP52 using a Saccharomyces cerevisiae model. The activity of rat GR (rGR) containing the gpGR H1-H3 loop substitutions was still potentiated by FKBP52, confirming the loop is not involved in primary FKBP52 interactions. Additional assays also excluded a role for other intervening loops between ligand-binding domain helices in direct interactions with FKBP52 associated with enhanced receptor activity. Complementary studies in FKBP51-deficient mouse embryo fibroblasts and HEK293 cells demonstrated that substitution of the gpGR H1-H3 loop residues into rGR dramatically increased receptor repression by FKBP51 without enhancing receptor-FKBP51 interaction and did not alter recruitment of endogenous Hsp90 and the p23 cochaperone to receptor complexes. FKBP51 suppression of the mutated rGR did not require FKBP51 peptidylprolyl cis-trans isomerase activity and was not disrupted by mutation of the FK1 proline-rich loop thought to mediate reciprocal FKBP influences on receptor activity. We conclude that the gpGR-specific mutations within the H1-H3 loop confer global changes within the GR-Hsp90 complex that favor FKBP51 repression over FKBP52 potentiation, thus identifying the loop as an important target for GR regulation by the FKBP cochaperones.

Molecular cloning and characterization of the corticoid receptors from the American alligator

Steroid hormones are essential for health in vertebrates. Corticosteroids, for example, have a regulatory role in many physiological functions, such as osmoregulation, respiration, immune responses, stress responses, reproduction, growth, and metabolism. Although extensively studied in mammals and some non-mammalian species, the molecular mechanisms of corticosteroid hormone (glucocorticoids and mineralocorticoids) action are poorly understood in reptiles. Here, we have evaluated hormone receptor-ligand interactions in the American alligator (Alligator mississippiensis), following the isolation of cDNAs encoding a glucocorticoid receptor (GR) and a mineralocorticoid receptor (MR). The full-length alligator GR (aGR) and aMR cDNAs were obtained using 5' and 3' rapid amplification cDNA ends (RACE). The deduced amino acid sequences exhibited high identity to the chicken orthologs (aGR: 83%; aMR: 90%). Using transient transfection assays of mammalian cells, both aGR and aMR proteins displayed corticosteroid-dependent activation of transcription from keto-steroid hormone responsive, murine mammary tumor virus promoters. We further compared the ligand-specifity of human, chicken, Xenopus, and zebrafish GR and MR. We found that the alligator and chicken GR/MR have very similar amino acid sequences, and this translates to very similar ligand specificity. This is the first report of the full-coding regions of a reptilian GR and MR, and the examination of their transactivation by steroid hormones.

Different polymorphisms of the mineralocorticoid receptor gene are associated with either glucocorticoid or mineralocorticoid levels in hypertension

OBJECTIVE

Both aldosterone and cortisol can activate the mineralocorticoid receptor (MR). Polymorphisms in the MR gene have been inconsistently shown to be associated with risk of hypertension and aldosterone and cortisol levels. The purpose of this project was to investigate the association of MR gene variants with serum aldosterone and a previously identified hypertension subgroup with higher urinary free cortisol (UFC) levels (high-mode UFC) in a rigorously phenotyped Caucasian hypertensive cohort.

MATERIALS AND METHODS

A haplotype-based tagging single nucleotide polymorphism (htSNP) association study was conducted in the HyperPATH cohort of 570 hypertensive Caucasian subjects on a salt-controlled diet. Haplotypes generated from 74 htSNP representing the common genetic variations of the entire MR gene were analyzed by comparing high- vs. normal-mode UFC groups and the association with serum aldosterone levels.

RESULTS

Of the observed 20 haplotype blocks, there were three main linkage disequilibrium (LD) regions with high recombination rates between adjacent regions. Overlaying gene structure on this LD map revealed that block 1-8 corresponded to exon 5-9 [ligand binding domain (LBD)], blocks 9-18 to exon 3-4 [DNA binding domain (DBD)], and block 19-20 to exon 1-2 (N-terminal domain). Haplotype association results showed that DBD-aligned LD blocks were associated with high-mode UFC status (global P values, 0.0004 to 0.05). The LBD-aligned LD blocks showed significant associations with serum aldosterone levels.

CONCLUSIONS

These findings imply that there may be differential functional importance of the DBD and LBD of the MR in the regulation of glucocorticoid and aldosterone levels in hypertensive subjects.

Mechanisms of ligand specificity 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 the activity of 11β-hydroxysteroid dehydrogenase type 2, while in other tissues, including the heart and regions of the central nervous system, cortisol is the primary ligand for the MR where it may act as an antagonist. Clinical trials have demonstrated the potential of MR antagonists in the treatment of cardiovascular disease, though their use has been limited by concurrent hyperkalaemia. In order to better target the MR, an understanding of the structural determinants of tissue- and ligand-specific MR activation is needed. Interactions of the MR have been identified, which exhibit ligand discrimination and/or specificity. These interactions include those of the ligand-binding domain with ligand, with the N-terminal domain and with putative co-regulatory molecules. Agonist and antagonist binding have been characterised using chimeras between the human MR and the glucocorticoid receptor or the zebra fish MR together with molecular modelling. The interaction between the N-terminus and the C-terminus is aldosterone dependent but is unexpectedly antagonised by cortisol and deoxycorticosterone in the human MR. Nuclear receptor-mediated transactivation is critically dependent on, and modulated by, co-regulatory molecules. Proteins that interact with the MR in the presence of either aldosterone or cortisol, but not both, have been identified. The successful identification of ligand-specific interactions of the MR may provide the basis for the development of novel MR ligands with tissue specificity.

Structural determinants of ligand binding to the mineralocorticoid receptor

The first and critical step in the mechanism of aldosterone action is its binding to the mineralocorticoid receptor (MR), a member of the nuclear receptor superfamily. Over the last 40 years, numerous studies have attempted to determine the structural determinants of ligand-binding to MR. An initial set of data showed that hsp90 is bound to the receptor via specific regions and maintains it in a ligand-binding competent state. Site-directed mutagenesis and functional studies guided by a 3D model of the MR ligand-binding domain (LBD) made it possible to identify the residues responsible for the high affinity and selectivity for aldosterone, and to characterize the mechanisms of MR activation and inactivation. The recent determination of the X-ray crystal structures of the LBD of the wild-type MR and MR(S810L), which is responsible for a familial form of hypertension, has made it possible to elucidate the peculiar mechanism of activation of MR(S810L) and established a clear structure/activity relationship for steroidal and non-steroidal MR antagonists.

Mineralocorticoid receptor mutations differentially affect individual gene expression profiles in pseudohypoaldosteronism type 1

CONTEXT

Type 1 pseudohypoaldosteronism (PHA1), a primary form of mineralocorticoid resistance, is due to inactivating mutations of the NR3C2 gene, coding for the mineralocorticoid receptor (MR).

OBJECTIVE

The objective of the study was to assess whether different NR3C2 mutations have distinct effects on the pattern of MR-dependent transcriptional regulation of aldosterone-regulated genes.

DESIGN AND METHODS

Four MR mutations affecting residues in the ligand binding domain, identified in families with PHA1, were tested. MR proteins generated by site-directed mutagenesis were analyzed for their binding to aldosterone and were transiently transfected into renal cells to explore the functional effects on the transcriptional activity of the receptors by cis-trans-cotransactivation assays and by measuring the induction of endogenous gene transcription.

RESULTS

Binding assays showed very low or absent aldosterone binding for mutants MR(877Pro), MR(848Pro), and MR(947stop) and decreased affinity for aldosterone of MR(843Pro). Compared with wild-type MR, the mutations p.Leu843Pro and p.Leu877Pro displayed half-maximal aldosterone-dependent transactivation of reporter genes driven by mouse mammary tumor virus or glucocorticoid response element-2 dependent promoters, whereas MR(848Pro) and MR(947stop) nearly or completely lost transcriptional activity. Although MR(848Pro) and MR(947stop) were also incapable of inducing aldosterone-dependent gene expression of endogenous sgk1, GILZ, NDRG2, and SCNN1A, MR(843Pro) retained complete transcriptional activity on sgk1 and GILZ gene expression, and MR(877Pro) negatively affected the expression of sgk1, NDRG2, and SCNN1A.

CONCLUSIONS

Our data demonstrate that MR mutations differentially affect individual gene expression in a promoter-dependent manner. Investigation of differential gene expression profiles in PHA1 may allow a better understanding of the molecular substrate of phenotypic variability and to elucidate pathogenic mechanisms underlying the disease.

Characterization of the zebrafish (Danio rerio) mineralocorticoid receptor

Comparison between evolutionarily distant receptors can provide critical insights into both structure and function. Sequence comparison between the mineralocorticoid receptors (MR) of the zebrafish (zMR) and human (hMR) reveals a high degree of sequence conservation in the major functional domains. We isolated a zMR cDNA to contrast the transcriptional response to a range of ligands and to establish whether a teleost MR exhibits the amino/carboxyl-terminal interaction (N/C-interaction) previously reported for the hMR. Aldosterone, deoxycorticosterone (DOC) and cortisol induced zMR transcriptional activity with similar efficacy to that observed with the hMR. The hMR antagonist, spironolactone, acted as an agonist with the zMR. The zMR exhibited an N/C-interaction in response to aldosterone but, in contrast to the hMR, cortisol and DOC predominantly stimulated the interaction in the zMR. Conservation of the N/C-interaction between evolutionarily distant MR provides evidence of functional significance.

Transcriptional activity of estrogen-related receptor γ (ERRγ) is stimulated by the phytoestrogen equol

Estrogen-related receptor γ (ERRγ) is an orphan nuclear receptor lacking identified natural ligands. The synthetic estrogen receptor ligands 4-hydroxytamoxifen and diethylstilbestrol have, however, been shown to bind to and abolish the constitutive transcriptional activity of ERRγ. Certain phytoestrogens were recently reported to act as agonists of the related ERRα. We investigated whether phytoestrogens also modulated the transcriptional activity of ERRγ. We analyzed a selection of phytoestrogens for their potential agonistic or antagonistic activity on ERRγ. In transiently transfected PC-3 and U2-OS cells equol stimulated the transcriptional activity of ERRγ and enhanced its interaction with the coactivator GRIP1. The agonistic effect of equol was abolished by 4-hydroxytamoxifen. Equol induced a conformational change in the ERRγ ligand-binding domain. Based on structural models of the ERRγ ligand-binding domain, we were able to introduce mutations that modulated the agonistic potential of equol. Finally, equol enhanced the growth inhibitory effect of ERRγ on the prostate cancer PC-3 cells. In conclusion, we have demonstrated that the phytoestrogen equol acts as an ERRγ agonist.

11-deoxycortisol is a corticosteroid hormone in the lamprey

Corticosteroid hormones are critical for controlling metabolism, hydromineral balance, and the stress response in vertebrates. Although corticosteroid hormones have been well characterized in most vertebrate groups, the identity of the earliest vertebrate corticosteroid hormone has remained elusive. Here we provide evidence that 11-deoxycortisol is the corticosteroid hormone in the lamprey, a member of the agnathans that evolved more than 500 million years ago. We used RIA, HPLC, and mass spectrometry analysis to determine that 11-deoxycortisol is the active corticosteroid present in lamprey plasma. We also characterized an 11-deoxycortisol receptor extracted from sea lamprey gill cytosol. The receptor was highly specific for 11-deoxycortisol and exhibited corticosteroid binding characteristics, including DNA binding. Furthermore, we observed that 11-deoxycortisol was regulated by the hypothalamus-pituitary axis and responded to acute stress. 11-deoxycortisol implants reduced sex steroid concentrations and up-regulated gill Na+, K+-ATPase, an enzyme critical for ion balance. We show here that 11-deoxycortisol functioned as both a glucocorticoid and a mineralocorticoid in the lamprey. Our findings indicate that a complex and highly specific corticosteroid signaling pathway evolved at least 500 million years ago with the arrival of the earliest vertebrate.

The human glucocorticoid receptor: molecular basis of biologic function

The characterization of the subfamily of steroid hormone receptors has enhanced our understanding of how a set of hormonally derived lipophilic ligands controls cellular and molecular functions to influence development and help achieve homeostasis. The glucocorticoid receptor (GR), the first member of this subfamily, is a ubiquitously expressed intracellular protein, which functions as a ligand-dependent transcription factor that regulates the expression of glucocorticoid-responsive genes. The effector domains of the GR mediate transcriptional activation by recruiting coregulatory multi-subunit complexes that remodel chromatin, target initiation sites, and stabilize the RNA-polymerase II machinery for repeated rounds of transcription of target genes. This review summarizes the basic aspects of the structure and actions of the human (h) GR, and the molecular basis of its biologic functions.

The mineralocorticoid receptor: insights into its molecular and (patho)physiological biology

The last decade has witnessed tremendous progress in the understanding of the mineralocorticoid receptor (MR), its molecular mechanism of action, and its implications for physiology and pathophysiology. After the initial cloning of MR, and identification of its gene structure and promoters, it now appears as a major actor in protein-protein interaction networks. The role of transcriptional coregulators and the determinants of mineralocorticoid selectivity have been elucidated. Targeted oncogenesis and transgenic mouse models have identified unexpected sites of MR expression and novel roles for MR in non-epithelial tissues. These experimental approaches have contributed to the generation of new cell lines for the characterization of aldosterone signaling pathways, and have also facilitated a better understanding of MR physiology in the heart, vasculature, brain and adipose tissues. This review describes the structure, molecular mechanism of action and transcriptional regulation mediated by MR, emphasizing the most recent developments at the cellular and molecular level. Finally, through insights obtained from mouse models and human disease, its role in physiology and pathophysiology will be reviewed. Future investigations of MR biology should lead to new therapeutic strategies, modulating cell-specific actions in the management of cardiovascular disease, neuroprotection, mineralocorticoid resistance, and metabolic disorders.