Towards selectively modulating mineralocorticoid receptor function: lessons from other systems

Identification of an Evolutionarily Conserved Allosteric Network in Steroid Receptors

Allosteric pathways in proteins describe networks comprising amino acid residues which may facilitate the propagation of signals between distant sites. Through inter-residue interactions, dynamic and conformational changes can be transmitted from the site of perturbation to an allosteric site. While sophisticated computational methods have been developed to characterize such allosteric pathways linking specific sites on proteins, few attempts have been made to apply these approaches toward identifying new allosteric sites. Here, we use molecular dynamics simulations and suboptimal path analysis to discover new allosteric networks in steroid receptors with a focus on evolutionarily conserved pathways. Using modern receptors and a reconstructed ancestral receptor, we identify networks connecting several sites to the activation function surface 2 (AF-2), the site of coregulator recruitment. One of these networks is conserved across the entire family, connecting a predicted allosteric site located between helices 9 and 10 of the ligand-binding domain. We investigate the basis of this conserved network as well as the importance of this site, discovering that the site lies in a region of the ligand-binding domain characterized by conserved inter-residue contacts. This study suggests an evolutionarily importance of the helix 9-helix 10 site in steroid receptors and identifies an approach that may be applied to discover previously unknown allosteric sites in proteins.

Chronic Systemic Dexamethasone Regulates the Mineralocorticoid/Glucocorticoid Pathways Balance in Rat Ocular Tissues

Central serous chorioretinopathy (CSCR) is a retinal disease affecting the retinal pigment epithelium (RPE) and the choroid. This is a recognized side-effect of glucocorticoids (GCs), administered through nasal, articular, oral and dermal routes. However, CSCR does not occur after intraocular GCs administration, suggesting that a hypothalamic-pituitary-adrenal axis (HPA) brake could play a role in the mechanistic link between CSCR and GS. The aim of this study was to explore this hypothesis. To induce HPA brake, Lewis rats received a systemic injection of dexamethasone daily for five days. Control rats received saline injections. Baseline levels of corticosterone were measured by Elisa at baseline and at 5 days in the serum and the ocular media and dexamethasone levels were measured at 5 days in the serum and ocular media. The expression of genes encoding glucocorticoid receptor (GR), mineralocorticoid receptors (MR), and the 11 beta hydroxysteroid dehydrogenase (HSD) enzymes 1 and 2 were quantified in the neural retina and in RPE/ choroid. The expression of MR target genes was quantified in the retina (Scnn1A (encoding ENac-α, Kir4.1 and Aqp4) and in the RPE/choroid (Shroom 2, Ngal, Mmp9 and Omg, Ptx3, Plaur and Fosl-1). Only 10% of the corticosterone serum concentration was measured in the ocular media. Corticosterone levels in the serum and in the ocular media dropped after 5 days of dexamethasone systemic treatment, reflecting HPA axis brake. Whilst both GR and MR were downregulated in the retina without MR/GR imbalance, in the RPE/choroid, both MR/GR and 11β-hsd2/11β-hsd1 ratio increased, indicating MR pathway activation. MR-target genes were upregulated in the RPE/ choroid but not in the retina. The psychological stress induced by the repeated injection of saline also induced HPA axis brake with a trend towards MR pathway activation in RPE/ choroid. HPA axis brake causes an imbalance of corticoid receptors expression in the RPE/choroid towards overactivation of MR pathway, which could favor the occurrence of CSCR.

Aging-like skin changes in metabolic syndrome model mice are mediated by mineralocorticoid receptor signaling

Aging is accelerated, at least in part, by pathological condition such as metabolic syndrome (MetS), and various molecular pathways such as oxidative stress are common mediators of aging and MetS. We previously developed the aging-like skin model by single ultraviolet (UV) irradiation on the MetS model mice. Recent studies revealed that mineralocorticoid receptor (MR) signaling plays a pivotal role for various tissue inflammation and damages in MetS. Although previous studies reported that MR is expressed in the skin and that overexpression of MR in the skin resulted in the skin atrophy, the physiological or pathological functions of MR in the skin are not fully elucidated. Here, we show the involvement of MR signaling in the aging-like skin changes in our own model. Elevations of oxidative stress and inflammation markers were observed in the MetS mice, and the UV-evoked aging-like skin damages were attenuated by topical antioxidant. MR expression was higher in the MetS mouse skin, and notably, expression of its effecter gene Sgk1 was significantly upregulated in the aging-like skin in the UV-irradiated MetS mice. Furthermore, topical application of MR antagonist spironolactone suppressed Sgk1 expression, oxidative stress, inflammation, and the aging-like changes in the skin. The 2-week UV onto the non-MetS mice, the more usual photoaging model, resulted in the skin damages mostly equivalent to the MetS mice with single UV, but they were not associated with upregulation of MR signaling. Our studies suggested an unexpected role of MR signaling in the skin aging in MetS status.

System among the corticosteroids: specificity and molecular dynamics

Understanding how structural features determine specific biological activities has often proved elusive. With over 161,000 steroid structures described, an algorithm able to predict activity from structural attributes would provide manifest benefits. Molecular simulations of a range of 35 corticosteroids show striking correlations between conformational mobility and biological specificity. Thus steroid ring A is important for glucocorticoid action, and is rigid in the most specific (and potent) examples, such as dexamethasone. By contrast, ring C conformation is important for the mineralocorticoids, and is rigid in aldosterone. Other steroids that are less specific, or have mixed functions, or none at all, are more flexible. One unexpected example is 11-deoxycorticosterone, which the methods predict (and our activity studies confirm) is not only a specific mineralocorticoid, but also has significant glucocorticoid activity. These methods may guide the design of new corticosteroid agonists and antagonists. They will also have application in other examples of ligand-receptor interactions.

General molecular biology and architecture of nuclear receptors

Nuclear receptors (NRs) regulate and coordinate multiple processes by integrating internal and external signals, thereby maintaining homeostasis in front of nutritional, behavioral and environmental challenges. NRs exhibit strong similarities in their structure and mode of action: by selective transcriptional activation or repression of cognate target genes, which can either be controlled through a direct, DNA binding-dependent mechanism or through crosstalk with other transcriptional regulators, NRs modulate the expression of gene clusters thus achieving coordinated tissue responses. Additionally, non genomic effects of NR ligands appear mediated by ill-defined mechanisms at the plasma membrane. These effects mediate potential therapeutic effects as small lipophilic molecule targets, and many efforts have been put in elucidating their precise mechanism of action and pathophysiological roles. Currently, numerous nuclear receptor ligand analogs are used in therapy or are tested in clinical trials against various diseases such as hypertriglyceridemia, atherosclerosis, diabetes, allergies and cancer and others.

Steroid receptors and microRNAs: relationships revealed

MicroRNAs (miRNAs) are small non-coding RNAs that serve as post-transcriptional regulators of gene expression. They work predominantly by binding to complementary sequences in target messenger RNA (mRNA) 3' untranslated regions (UTRs) where they prevent translation or cause degradation of the message. Steroid hormone receptors (SHRs) are ligand-activated transcription factors that regulate genes in steroid responsive tissues. Recent studies demonstrate that SHRs regulate miRNAs, and in turn, miRNAs can regulate SHR expression and function. Mounting evidence indicates that miRNAs are intimately involved with SHRs, as they are with other transcription factors, often in double negative feedback loops. Investigators are just beginning to expose the details of these complex relationships and reveal the extent to which miRNAs are involved with SHRs in normal physiology and the pathobiology of steroid hormone responsive tissues.

Diuretics in clinical practice. Part I: mechanisms of action, pharmacological effects and clinical indications of diuretic compounds

IMPORTANCE OF THE FIELD

Diuretics are among the most important drugs of our therapeutic armamentarium and have been broadly used for > 50 years, providing important help towards the treatment of several diseases. Although all diuretics act primarily by impairing sodium reabsorption in the renal tubules, they differ in their mechanism and site of action and, therefore, in their specific pharmacological properties and clinical indications. Loop diuretics are mainly used for oedematous disorders (i.e., cardiac failure, nephrotic syndrome) and for blood pressure and volume control in renal disease; thiazides and related agents are among the most prescribed drugs for hypertension treatment; aldosterone-blockers are traditionally used for primary or secondary aldosteronism; and other diuretic classes have more specific indications.

AREAS COVERED IN THIS REVIEW

This article discusses the mechanisms of action, pharmacological effects and clinical indications of the various diuretic classes used in everyday clinical practice, with emphasis on recent knowledge suggesting beneficial effects of certain diuretics on clinical conditions distinct from the traditional indications of these drugs (i.e., heart protection for aldosterone blockers).

WHAT THE READER WILL GAIN

Reader will gain insights into the effective use of diuretic agents for various medical conditions, representing their established or emerging therapeutic indications.

TAKE HOME MESSAGE

Knowledge of the pharmacologic properties and mechanisms of action of diuretic agents is a prerequisite for the successful choice and effective clinical use of these compounds.

Marinobufagenin interferes with the function of the mineralocorticoid receptor

Marinobufagenin (MBG) is a cardiotonic steroid of the bufadienolide class of compounds which has the ability to inhibit the ubiquitous enzyme, Na+/K+-ATPase, resulting in natriuresis. The involvement of MBG in the pathogenesis of volume expansion-mediated forms of hypertension has been suggested for some time, and we have proposed that MBG participates in the hypertension noted in preeclampsia. We examined the hypothesis that MBG might contribute to these forms of hypertension by promoting the activity of the mineralocorticoid receptor (MR). However, our data demonstrate that instead, MBG interferes with the functioning of the MR by inhibiting the transcriptional activity of the receptor, and this is reflected in a reduced interaction between the SRC-3 coactivator and the MR. Thus, the ability of MBG to cause a natriuresis may be due, not only to inhibition of Na+/K+-ATPase activity, but also to its ability to interfere with MR-dependent expression of the Na/K/H exchanger in the late distal nephron.

Rapid effects of aldosterone on clonal human vascular smooth muscle cells

It has been increasingly appreciated that aldosterone elicits acute vascular effects through nongenomic signaling pathways. Our previous studies demonstrated that aldosterone attenuated phenylephrine-mediated constriction in intact vessels [via phosphatidylinositol 3-kinase-dependent nitric oxide synthase activation] but enhanced vasoconstrictor responses in endothelium-denuded arteries. To determine the mechanism of this vasoconstrictor response, we assessed the effect of aldosterone on myosin light-chain phosphorylation and contraction in clonal adult human vascular smooth muscle cells. Acute aldosterone exposure mediated dose-dependent myosin light-chain phosphorylation, inhibited by spironolactone and phosphatidylinositol 3-kinase inhibition. These rapid effects of aldosterone were mimicked by estradiol and hydrocortisone and were also inhibitable by both spironolactone and eplerenone. In parallel to its effects on myosin light-chain phosphorylation, aldosterone mediated dose-dependent contraction responses that were inhibited by spironolactone. Comparable contractile responses were seen with both 17beta-estradiol and hydrocortisone. In total, these data are consistent with a mechanism of acute aldosterone-mediated contraction common to both glucocorticoids and estrogen. Steroid-mediated vasoconstriction may represent an important pathobiological mechanism of vascular disease, especially in the setting of preexisting endothelial dysfunction.

Aldosterone and the conquest of land

The sequence of the phylogenetic events that preceded the appearance of aldosterone in vertebrates is described, starting from the ancestral conversion of cytochrome P450s from oxygen detoxification to xenobiotic detoxification and synthesis of oxygenated endobiotics with useful functions in intercellular signalling, such as steroid hormones. At the end of the Silurian period [438-408 million yr ago, (Mya)], a complete set of cytochrome P450s for corticoid synthesis was presumably already available, except for mitochondrial cytochrome P450c18 or aldosterone synthase encoded by CYP11B2. This gene arose by duplication of the CYP11B gene in the sarcopterygian or lobe-finned fish/tetrapod line after its divergence from the actinopterygian or ray-finned fish line 420 Mya, but before the beginning of the colonization of land by tetrapods in the late Devonian period, around 370 Mya. The fact that aldosterone is already present in Dipnoi, which occupy an evolutionary transition between water- and air-breathing but are fully aquatic, suggests that the role of this steroid was to potentiate the corticoid response to hypoxia, rather than to prevent dehydration out of the water. In terrestrial amphibians, there is no differentiation between the secretion rates and gluco- and mineralocorticoid effects of aldosterone and corticosterone. In sauropsids, plasma aldosterone concentrations are much lower than in amphibians, but regulation of salt/water balance is dependent upon both aldosterone and corticosterone, though sometimes with opposed actions. In terrestrial mammals, aldosterone acquires a specific mineralocorticoid function, because its interaction with the mineralocorticoid receptor is protected by the coexpression of the enzyme 11beta-hydroxysteroid dehydrogenase type 2, which inactivates both cortisol and corticosterone. There is evidence that aldosterone can be also synthesized extra-adrenally in brain neurons and cardiac myocytes, which lack this protection and where the effects of aldosterone oppose those of glucocorticoids. In conclusion, the phylogenetic history of aldosterone documents the erratic progression of evolutionary changes in the course of the strenuous struggle for environmental resources and survival.

Structural and Biochemical Mechanisms for the Specificity of Hormone Binding and Coactivator Assembly by Mineralocorticoid Receptor

Mineralocorticoid receptor (MR) controls sodium homeostasis and blood pressure through hormone binding and coactivator recruitment. Here, we report a 1.95 A crystal structure of the MR ligand binding domain containing a single C808S mutation bound to corticosterone and the fourth LXXLL motif of steroid receptor coactivator-1 (SRC1-4). Through a combination of biochemical and structural analyses, we demonstrate that SRC1-4 is the most potent MR binding motif and mutations that disrupt the MR/SRC1-4 interactions abolish the ability of the full-length SRC1 to coactivate MR. The structure also reveals a compact steroid binding pocket with a unique topology that is primarily defined by key residues of helices 6 and 7. Mutations swapping a single residue at position 848 from helix H7 between MR and glucocorticoid receptor (GR) switch their hormone specificity. Together, these findings provide critical insights into the molecular basis of hormone binding and coactivator recognition by MR and related steroid receptors.

Eplerenone blocks nongenomic effects of aldosterone on the Na+/H+ exchanger, intracellular Ca2+ levels, and vasoconstriction in mesenteric resistance vessels

There is increasing evidence for rapid nongenomic effects of aldosterone. Aldosterone has been demonstrated to alter intracellular pH and calcium in isolated cells. However, few studies have correlated these effects with aldosterone-mediated physiological responses. Therefore, we studied rapid effects of aldosterone on vascular reactivity, intracellular Ca2+, and pH in resistance vessels. Furthermore, we explored whether the new antimineralocorticoid drug eplerenone could effectively block nongenomic aldosterone-mediated effects. The vasoconstrictor action of aldosterone was examined directly by determining the diameter of small resistance mesenteric vessels (160-200 microm resting diameter), simultaneously with intracellular pH or Ca2+. Aldosterone (10 nm) caused a rapid constriction of resistance vessels (8.1% +/- 1.0% reduction in the diameter below control conditions, P < 0.05). Aldosterone potentiated phenylephrine-mediated constriction in small and large mesenteric vessels. Aldosterone induced a rapid increase of intracellular Ca2+ and cellular alkalinization. Vasoconstrictor action of aldosterone and nongenomic effects on the sodium-proton exchanger (NHE1) activity or intracellular Ca2+ responses was abolished by eplerenone. The vasoconstrictor response of aldosterone was related to phosphatidylinositol 3-kinase (PI3-K): the hormone decreased protein kinase B phosphorylation; pharmacological inhibition of PI3-K (10 microm LY294002 or 1 microm wortmannin) increased arterial contractility. Inhibitors of ERK 1/2 phosphorylation (15 microm PD98059) had no effect on aldosterone-mediated vasoconstriction. Inhibition of protein kinase C with 1 microm bi-sindolylmaleimide I and/or inhibition of NHE1 with 100 microm amiloride abolished aldosterone vasoconstrictor action of resistance mesenteric arteries. We conclude that aldosterone-mediated increase in vascular tone is related to a nongenomic mechanism that involves protein kinase C, PI3-K, and NHE1 activity. Eplerenone is an effective blocker of nongenomic effects of aldosterone in vascular tissue.

Conformational adaptation of nuclear receptor ligand binding domains to agonists: potential for novel approaches to ligand design

Ligands occupy the core of nuclear receptor (NR) ligand binding domains (LBDs) and modulate NR function. X-ray structures of NR LBDs reveal most NR agonists fill the enclosed pocket and promote packing of C-terminal helix 12 (H12), whereas the pockets of unliganded NR LBDs differ. Here, we review evidence that NR pockets rearrange to accommodate different agonists. Some thyroid hormone receptor (TR) ligands with 5' extensions designed to perturb H12 act as antagonists, but many are agonists. One mode of adaptation is seen in a TR/thyroxine complex; the pocket expands to accommodate a 5' iodine extension. Crystals of other NR LBDs reveal that the pocket can expand or contract and some agonists do not fill the pocket. A TRbeta structure in complex with an isoform selective drug (GC-24) reveals another mode of adaptation; the LBD hydrophobic interior opens to accommodate a bulky 3' benzyl extension. We suggest that placement of extensions on NR agonists will highlight unexpected areas of flexibility within LBDs that could accommodate extensions; thereby enhancing the selectivity of agonist binding to particular NRs. Finally, agonists that induce similar LBD structures differ in their activities and we discuss strategies to reveal subtle structural differences responsible for these effects.