Evidence for an antagonist specific receptor that does not bind mineralocorticoid agonists

Regulation of sgk by aldosterone and its effects on the epithelial Na(+) channel

Aldosterone is the major corticosteroid regulating Na(+) absorption in tight epithelia and acts primarily by activating the epithelial Na(+) channel (ENaC) through unknown induced proteins. Recently, it has been reported that aldosterone induces the serum- and glucocorticoid-dependent kinase sgk and that coexpressing ENaC with this kinase in Xenopus laevis oocytes increases the amiloride-sensitive Na(+) current (Chen SY, Bhargava A, Mastroberardino L, Meijer OC, Wang J, Buse P, Firestone GL, Verrey F, and Pearce D. Proc Natl Acad Sci USA 96: 2514-2519, 1999). The present study was done to further characterize regulation of sgk by aldosterone in native mammalian epithelia and to examine its effect on ENaC. With both in vivo and in vitro protocols, an almost fivefold increase in the abundance of sgk mRNA has been demonstrated in rat kidney and colon but not in lung. Induction of sgk by aldosterone was detected in kidney cortex and medulla, whereas the papilla expressed a constitutively high level of the kinase. The increase in sgk mRNA was detected as early as 30 min after the hormonal application and was independent of de novo protein synthesis. The observed aldosterone dose-response relationships suggest that the response is mediated, at least in part, by occupancy of the mineralocorticoid receptor. Coexpressing sgk and ENaC in Xenopus oocytes evoked a fourfold increase in the amiloride-blockable Na(+) channel activity. A point mutation in the beta-subunit known to impair regulation of the channel by Nedd4 (Y618A) had no significant effect on the response to sgk.

HIV-1 viral protein R (Vpr) as a regulator of the target cell

Among the putative 'accessory genes' of HIV-1, the 96 amino acid virion-associated Vpr gene product has been described to have several novel biological activities. These include cytoplasmic-to-nuclear translocation thus empowering HIV to infect and replicate in nondividing cells and to function to increase viral replication, particularly in monocytes. Along with these viral effects, we describe the dramatic biological changes induced by HIV-1 Vpr in the target cells of HIV infection including induction of changes in transcriptional patterns and complete inhibition of proliferation which collectively is termed differentiation. These changes occur in the absence of other viral gene products and suggest that Vpr mediates its proviral effects partially or perhaps solely through modulation of the state of the target cell rather than directly on the virus. The inhibition of proliferation in T-cell lines has been proposed by several groups to demonstrate that the inhibition of proliferation specifically arrests the cell cycle further supporting the notion that Vpr activity is directed at cellular targets. We have recently described a role for Vpr in modulating the glucocorticoid pathway, a pathway involved in the regulation of the state of the cell in cytoplasmic-to-nuclear translocation and in the modulation of host cell transcription. Importantly, certain antiglucocorticoids have been shown to modulate Vpr activity in vitro. These results demonstrate that the cell contains specific receptor(s) molecule(s) through which Vpr mediates its activity and that these molecules have implications for cell biology in general. These results collectively demonstrate that Vpr represents a unique target for anti-HIV drug development and has significance for HIV-1 disease progression.

The glucocorticoid receptor type II complex is a target of the HIV-1 vpr gene product

The vpr gene of human immunodeficiency virus type 1 (HIV-1) encodes a 15-kDa virion-associated protein that functions as a regulator of cellular processes linked to the HIV life cycle. We report the interaction of a 41-kDa cytosolic viral protein R interacting protein 1 (Rip-1) with Vpr in vitro. Rip-1 displays a wide tissue distribution, including relevant targets of HIV infection. Vpr protein induced nuclear translocation of Rip-1, as did glucocorticoid receptor (GR)-II-stimulating steroids. Importantly, Vpr and Rip-1 coimmunoprecipitated with the human GR as part of an activated receptor complex. Vpr complementation of a vpr mutant virus was also mimicked by GR-II-stimulating steroids. Vpr and GR-II actions were inhibited by mifepristone, a GR-II pathway inhibitor. Together these data directly link the activity of the vpr gene product to the glucocorticoid steroid pathway and provide a biochemical mechanism for the cellular and viral activity of Vpr, as well as suggest that a unique class of antivirals, which includes mifepristone (RU486), may influence HIV-1 replication.

Corticosteroid receptor antagonists: a current perspective

This review aims to highlight a selection of antagonists for the mineralocorticoid and glucocorticoid receptors. Concepts of these receptor systems are described, as is the mechanism of action of these steroids in the brain and periphery. Examples of commonly available and newly synthesized antimineralocorticoids and antiglucocorticoids are given, together with their pharmacological profiles and, when appropriate, clinical and therapeutic applications.

Corticosterone is a preferable ligand for measuring rat brain corticosteroid receptors: competition by RU 28362 and RU 26752 for dexamethasone binding in rat hippocampal cytosol

It is unclear whether in vitro corticosteroid receptor binding assays have used inappropriately high concentrations of synthetic corticosteroid competitors, thereby potentially introducing error into estimates of type I (mineralocorticoid) and type II (glucocorticoid) receptor binding. To determine more accurately the concentration of blockers necessary to discriminate between these two sites, we have derived Ki values for the competition of dexamethasone, RU 28362 and RU 26752 for [3H]corticosterone and [3H]dexamethasone binding in rat hippocampus. Non-specific binding of both radioligands was defined with unlabeled dexamethasone to exclude transcortin. The type II agonist RU 28362 competed for only a portion of [3H]corticosterone binding, exhibiting a Ki of 0.5 nM for this binding. In contrast, RU 28362 fully competed all binding of a saturating concentration of [3H]dexamethasone, even though [3H]dexamethasone also recognized type I receptors, defined as specific [3H]corticosterone binding in the presence of 80 nM RU 28362. RU 28362 competition for [3H]dexamethasone binding exhibited characteristics of a 2-site interaction, with Kis of 0.3 and 194 nM. The type I receptor antagonist RU 26752 competed less effectively for [3H]corticosterone and [3H]dexamethasone binding, but nonetheless competed fully within a 1000-fold concentration range. Even at a level less than 125 x its Ki for type I binding, RU 26752 still inhibited virtually all type II receptor binding by [3H]corticosterone. We conclude that type I and II receptors in rat brain are best distinguished using [3H]corticosterone as the labelling ligand, with cold RU 28362 and dexamethasone to eliminate binding to type II and transcortin sites, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Aldosterone antagonists destabilize the mineralocorticosteroid receptor

To elucidate the mechanism of action of aldosterone antagonists, we studied the interaction of spironolactone with the chick mineralocorticosteroid receptor (MR). Intestinal cytosol contains specific spironolactone-binding sites (Kd approximately 3 nM; max. no. of binding sites approximately 100 fmol/mg of protein) that have been identified as MRs by competition experiments with steroid ligands and with the monoclonal anti-idiotypic antibody H10E that interacts with aldosterone-binding domain of the MR. Binding studies indicate that aldosterone and spironolactone bind to the MR through a common site that encompasses the epitope recognized by H10E. At 4 degrees C, spironolactone dissociates much more rapidly from the cytosol 8-9 S form of MR (t1/2 38 min) than does aldosterone (t1/2 3240 min). A high dissociation rate was also observed for progesterone, a natural aldosterone antagonist (t1/2 84 min). The covalent linkage of the 90 kDa heat shock protein (hsp90) to the ligand-binding subunit of MR with dimethyl pimelimidate did not notably modify the rate of dissociation of spironolactone from the receptor (t1/2 96 min), excluding the possibility that the rapid dissociation rate of the antagonist was related to hsp90 release. The effects of aldosterone and the two anti-mineralocorticosteroids on the 8-9 S heterooligomeric structure of the MR differed strikingly. Using low-salt density-gradient centrifugation analysis, aldosterone-labelled receptors were recovered as 8-9S complexes, whereas 4 S entities were detected after spironolactone and progesterone binding. This indicated that, under the experimental conditions used, aldosterone antagonists facilitate hsp90 release and thus do not stabilize the non-DNA-binding 8-9S form of MR. We propose that the combination of rapid dissociation of the ligand and a weakened hsp90-receptor interaction is involved in the anti-mineralococorticosteroid activity of aldosterone antagonists.

Cellular and molecular aspects of the renal effects of diuretic agents

In the past few years, increased knowledge of the nature of transport proteins and their molecular regulation in the translocation of ions across kidney membranes has emerged. We are beginning to better understand the characteristics of the interaction of diuretics with these transport proteins. It is likely that this knowledge will permit further insight into nephron function regulation.

Effects of antimineralocorticoid RU 26752 on steroid-induced hypertension in rats

The effect of mineralocorticoid antagonist RU 26752 on the development and maintenance of hypertension produced by long-term administration of mineralocorticoid agonist aldosterone has been investigated. Uninephrectomized, saline-drinking male Sprague-Dawley rats were subcutaneously implanted with either placebo (control) pellets or pellets containing 100 micrograms aldosterone, 50 mg RU 26752, or 100 micrograms aldosterone plus 50 mg RU 26752. Aldosterone treatment resulted in an increase in blood pressure to 165 +/- 5 mmHg over the control value of 105 +/- 2 mmHg within 3 wk of experimental period. RU 26752 given alone had no observable hypertensinogenic effect. However, RU 26752 administered with aldosterone significantly prevented the hypertension produced by aldosterone alone. RU 26752 when given with aldosterone was able to prevent the aldosterone-induced increase in saline consumption, increase urine output, and reduce urinary Na+ excretion. The results presented suggest that long-term administration of antimineralocorticoid RU 26752 in vivo to Sprague-Dawley rats prevents the aldosterone-induced hypertension.

Purification and characterization of the activated mineralocorticoid receptor from rat myocardium

Cardiac cytosol from adrenalectomized rats was radiolabelled with 10 nM tritiated RU 26752, R 5020 or aldosterone, to saturate the mineralocorticoid receptor (MCR) in the presence of 1 microM RU 38486 to block the glucocorticoid and progestin receptors. Free steroids were removed by charcoal treatment and the radiolabelled cytosol was passed through a phosphocellulose column. The MCR peak in the phosphocellulose eluate was activated at 25 degrees C for 45 min, adsorbed onto the DNA-cellulose and finally extracted once each with buffers containing 1 M potassium chloride or 25 mM magnesium chloride. The pooled DNA-cellulose extracts, reequilibrated with 10 nM [3H]RU 26752, were resolved as a single, homogeneous band of 78 kDa upon polyacrylamide gel electrophoresis. Ion-exchange analysis of the purified MCR on DEAE-cellulose-52 revealed a single peak in the 0.017 M sodium phosphate region with both RU 26752 and R 5020, but aldosterone dissociated during this procedure. Molecular filtration on Ultrogel AcA-44 columns revealed a major 145 kDa peak, with some smaller components of 40 and 80 kDa. These hydrodynamic properties of the purified MCR are at variance with those of the native receptor in crude myocardial cytosol, and suggest that some post-translational modifications in vivo may be required for the expression of MCR-mediated responses.

Purification and characterization of the activated mineralocorticoid receptor from rat kidney

1. The mineralcorticoid receptor (MR) from rat kidney was purified within 8 hr by the following, successive steps: stabilization with synthetic, tritiated steroids (RU 26752 or R 5020), phosphocellulose passage, heat activation (25 degrees C), and DNA-cellulose batch elution. 2. The purified preparation was resolved as a single, 75 KDa band on SDS-PAGE electrophoresis although the exact degree of purity was difficult to assess by the charcoal assay due to denaturation. 3. The natural hormone, aldosterone, was unsuitable for receptor purification and characterization. 4. The MR purified with different ligands behaved identically during ion exchange and gel permeation analyses, suggesting post-translational modifications of the native receptor in whole cytosol that exhibits molecular heterogeneity.

Properties and distribution of binding sites for the mineralocorticoid receptor antagonist [3H]ZK 91587 in brain

We have studied the binding of the synthetic antimineralocorticoid [3H]ZK 91587 to soluble receptors in brain of adrenalectomized rats. It was observed that [3H]ZK 91587 labeled a single receptor class with high affinity (Kd 1.3 nM) and low capacity (51.1 fmol/mg prot.) in cytosol of hippocampus (HIPPO). The ligand was efficiently displaced in vitro from the receptor by aldosterone (IC50 2.0 nM) and corticosterone (2.3), while dexamethasone showed less potency (IC50 5.1 nM) and the pure antiglucocorticoid RU 28362 competed weakly (161 nM). Furthermore, there was a widespread distribution of binding sites all over the brain for this compound, but with CA1 and CA3 regions of HIPPO, some amygdaloid nuclei and lateral septum containing most of the binding sites, as revealed by binding assays employing 16 different microdissected brain regions. Finally, the receptor labeled with [3H]ZK 91587 was readily displaced by administration of aldosterone in vivo in physiological amounts, from 5 whole brain regions examined, but preferentially from preoptic area, amygdala and HIPPO. It is concluded that [3H]ZK 91587 is a useful ligand for further studies on putative mineralocorticoid responsive cells in brain, due to its high affinity, stability and lack of cross reactivity with glucocorticoid receptors. Its brain distribution is similar to that previously obtained using [3H]aldosterone in the presence of RU 28362 to block ligand binding to the glucocorticoid receptor.

Analysis of the mineralocorticoid receptor in rat heart with the aid of two new spirolactone derivatives

Two derivatives of spirolactone, synthesized in an effort to eliminate the obnoxious side effects of the native molecule, were employed to dissect various aspects of the MR structure and function in rat heart. The introduction of a propyl residue in position 7 of spirolactone produced a molecule (RU 26752) that exhibited an increased affinity for the agonist specific MR, and furthermore revealed an antagonist-specific MR population in the target organ heart but absent from nontarget lung and liver. The specificity for both sites increased when a methoxycarbonyl group was introduced in the 7 position (ZK 91587). RU 26752 labilized the MR at 35 degrees C but did not interfere with thermal activation assessed on DNA-cellulose and sucrose density gradients. ZK 91587 was even more effective in labilizing the MR and did not permit thermal activation at all. Whereas only one ionic species was observed with RU 26752 on DE-52 columns, two were evident with ZK 91587. Both antimineralocorticoids were bound to populations of two molecular sizes on Ultrogel columns. Thus, the nature of chemical substitution in the 7 position of spirolactone dramatically alters the receptor-mediated antisteroid action of the resulting molecule. Such differences may permit distinction between agonist versus the antagonist-specific receptor conformations, and could possibly be exploited for the eventual purification of the mineralocorticoid receptor from various organs.

Different mechanisms for the receptor mediated antimineralocorticoid action of two new spirolactone derivatives

The introduction of a methoxycarbonyl residue in position 7 of spirolactone produced a molecule (ZK 91587) that exhibited dramatically increased affinity for the rat renal mineralocorticoid receptor (MR) that was lacking in RU 26752 with a propyl group in this same 7 position. The binding of 3H-ZK 91587 was specific to MR in renal cytosol and was not obtained with either serum or cytosol from non-target organs such as liver and lung. The RU 26752-receptor complex was more unstable than aldosterone-MR complex at 35 degrees C but underwent complete thermal activation on DNA cellulose. Contrarily, ZK 91587 did not permit thermal activation at all and also rendered the MR highly labile at 35 degrees C. Unactivated aldosterone and RU 26752-MR complexes sedimented largely in the 7 S region during sucrose density gradient centrifugation, and this shifted to 4 S following thermal activation. Paradoxically, under these very conditions, the molybdate stabilized, nonactivated, ZK 91587-MR complex was distributed almost equally into 7 S and 4 S regions which was not altered further by the activation process. If it is assumed that ZK 91587 exerts an antagonist action by inhibiting MR activation, or by MR labilization at body temperature, RU 26752 would seem to act at a step beyond the activation process. These form new tools to dissect receptor structure and function and necessitate a reevaluation of current notions regarding hormone action.

Activation of mineralocorticoid agonist and antagonist specific receptors from rat kidney

The kinetics of saturation, as well as of denaturation, confirm the existence of two distinct mineralocorticoid receptor populations one each for the agonist aldosterone (MR2) and the antagonist RU 26752 (MR3) in rat kidney. Receptor activation in vitro was dependent upon the buffer, progressed just as well in the presence of the agonist and the antagonist, and was inhibited by molybdate. These necessitate a reassessment of both the importance of receptor activation in vitro and its possible contribution to hormone action in vivo.