Statistical test of models and computerised parameter estimation for aldosterone binding in rat kidney

Finerenone Impedes Aldosterone-dependent Nuclear Import of the Mineralocorticoid Receptor and Prevents Genomic Recruitment of Steroid Receptor Coactivator-1

Aldosterone regulates sodium homeostasis by activating the mineralocorticoid receptor (MR), a member of the nuclear receptor superfamily. Hyperaldosteronism leads todeleterious effects on the kidney, blood vessels, and heart. Although steroidal antagonists such as spironolactone and eplerenone are clinically useful for the treatment of cardiovascular diseases, they are associated with several side effects. Finerenone, a novel nonsteroidal MR antagonist, is presently being evaluated in two clinical phase IIb trials. Here, we characterized the molecular mechanisms of action of finerenone and spironolactone at several key steps of the MR signaling pathway. Molecular modeling and mutagenesis approaches allowed identification of Ser-810 and Ala-773 as key residues for the high MR selectivity of finerenone. Moreover, we showed that, in contrast to spironolactone, which activates the S810L mutant MR responsible for a severe form of early onset hypertension, finerenone displays strict antagonistic properties. Aldosterone-dependent phosphorylation and degradation of MR are inhibited by both finerenone and spironolactone. However, automated quantification of MR subcellular distribution demonstrated that finerenone delays aldosterone-induced nuclear accumulation of MR more efficiently than spironolactone. Finally, chromatin immunoprecipitation assays revealed that, as opposed to spironolactone, finerenone inhibits MR, steroid receptor coactivator-1, and RNA polymerase II binding at the regulatory sequence of the SCNN1A gene and also remarkably reduces basal MR and steroid receptor coactivator-1 recruitment, unraveling a specific and unrecognized inactivating mechanism on MR signaling. Overall, our data demonstrate that the highly potent and selective MR antagonist finerenone specifically impairs several critical steps of the MR signaling pathway and therefore represents a promising new generation MR antagonist.

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.

A new mode of mineralocorticoid receptor antagonism by a potent and selective nonsteroidal molecule

Limitations of current steroidal mineralocorticoid receptor (MR) antagonists have stimulated the search for a new generation of molecules. We screened for novel nonsteroidal compounds and identified MR antagonists derived from the chemical class of dihydropyridines. Chemical optimization resulted in BR-4628, which displays high in vitro and in vivo MR potency as well as selectivity with respect to the other steroid hormone receptors and the L-type calcium channel. Biochemical studies demonstrated that BR-4628 forms complexes with MR that do not promote the recruitment of transcriptional co-regulators. Docking experiments, using the crystal structure of the MR ligand-binding domain in an agonist conformation, revealed that BR-4628 accommodates in the MR ligand-binding cavity differently in comparison with the classical steroidal MR antagonists. An alanine scanning mutagenesis approach, based on BR-4628 docking, allowed identifying its anchoring mode within the ligand-binding cavity. Altogether, we propose that BR-4628 is a bulky antagonist that inactivates MR through a passive mechanism. It represents the prototype of a new class of MR antagonists.

MineralocorticoidversusGlucocorticoid Receptor Occupancy Mediating Aldosterone-Stimulated Sodium Transport in a Novel Renal Cell Line

Aldosterone controls sodium balance by regulating an epithelial sodium channel (ENaC)-mediated sodium transport along the aldosterone-sensitive distal nephron, which expresses both mineralocorticoid (MR) and glucocorticoid receptors (GR). Mineralocorticoid specificity is ensured by 11beta-hydroxysteroid dehydrogenase type 2, which metabolizes cortisol or corticosterone into inactive metabolites that are unable to bind MR and/or GR. The fractional occupancy of MR and GR by aldosterone mediating the sodium transport response in the aldosterone-sensitive distal nephron cannot be studied in vivo. For answering this question, a novel mouse cortical collecting duct cell line (mCCD(cl1)), which expresses significant levels of MR and GR and a robust aldosterone sodium transport response, was used. Aldosterone elicited a biphasic response: Low doses (K(1/2) = approximately 0.5 nM) induced a transient and early increase of sodium transport (peaking at 3 h), whereas high doses (K(1/2) = approximately 90 nM) entailed an approximately threefold larger, long-lasting response. At 3 h, the corticosterone dose-response curve was shifted to the right compared with that of aldosterone by more than two log concentrations, an effect that was fully reverted in the presence of the 11beta-hydroxysteroid dehydrogenase type 2 inhibitor carbenoxolone. Low doses of dexamethasone (0.1 to 1 nM) failed to induce an early response, but high doses elicited a long-lasting response (K(1/2) = approximately 8 nM), similar to that observed for high aldosterone concentrations. Equilibrium binding assays showed that both aldosterone and corticosterone bind to a high-affinity, low-capacity site, whereas dexamethasone binds to one site. Within the physiologic range of aldosterone concentrations, sodium transport is predicted to be controlled by MR occupancy during circadian cycles and by MR and GR occupancy during salt restriction or acute stress.

Identification of steroid ligands able to inactivate the mineralocorticoid receptor harboring the S810L mutation responsible for a severe form of hypertension

The ability of steroid ligands to inactivate the human mineralocorticoid receptor (MR(WT)) has been shown to be due to their inability to contact Asn770, a residue of the H3 helix involved in stabilizing contacts with the H11-H12 loop region. However, all steroid ligands that display antagonist properties when bound to MR(WT), have been shown to activate a mutant receptor (MR(L810)) associated with a severe form of hypertension. Biochemical studies revealed that S810L mutation induces a change in the receptor conformation and increases the steroid-receptor complexes stability. From a three-dimensional model of the MR ligand-binding domain, it is likely that the S810L mutation causes a steric hindrance between the side chains of Leu810 (H5) and Gln776 (H3) that provokes a bending of the H3 helix. As a consequence, the positioning of MR(WT) antagonists within the ligand-binding cavity is modified in such a way that they can activate the mutant MR(L810). The results from biochemical studies also revealed that 5alpha-pregnan-20-one, 4,9-androstadiene-3,17-dione and RU486, unable to bind MR(WT), acted as potent MR(L810) antagonists.

Different inactivating mutations of the mineralocorticoid receptor in fourteen families affected by type I pseudohypoaldosteronism

We have analyzed the human mineralocorticoid receptor (hMR) gene in 14 families with autosomal dominant or sporadic pseudohypoaldosteronism (PHA1), a rare form of mineralocorticoid resistance characterized by neonatal renal salt wasting and failure to thrive. Six heterozygous mutations were detected. Two frameshift mutations in exon 2 (insT1354, del8bp537) and one nonsense mutation in exon 4 (C2157A, Cys645stop) generate truncated proteins due to premature stop codons. Three missense mutations (G633R, Q776R, L979P) differently affect hMR function. The DNA binding domain mutant R633 exhibits reduced maximal transactivation, although its binding characteristics and ED(50) of transactivation are comparable with wild-type hMR. Ligand binding domain mutants R776 and P979 present reduced or absent aldosterone binding, respectively, which is associated with reduced or absent ligand-dependent transactivation capacity. Finally, P979 possesses a transdominant negative effect on wild-type hMR activity, whereas mutations G633R and Q776R probably result in haploinsufficiency in PHA1 patients. We conclude that hMR mutations are a common feature of autosomal dominant PHA1, being found in 70% of our familial cases. Their absence in some families underscores the importance of an extensive investigation of the hMR gene and the role of precise diagnostic procedures to allow for identification of other genes potentially involved in the disease.

Mineralocorticoid effects in the kidney: correlation between alphaENaC, GILZ, and Sgk-1 mRNA expression and urinary excretion of Na+ and K+

Aldosterone exerts its effects through interactions with two types of binding sites, the mineralocorticoid (MR) and the glucocorticoid (GR) receptors. Although both receptors are known to be involved in the anti-natriuretic response to aldosterone, the mechanisms of signal transduction leading to modulation of electrolyte transport are not yet fully understood. This study measured the Na(+) and K(+) urinary excretion and the mRNA levels of three known aldosterone-induced transcripts, the serum and glucocorticoid-induced kinase (Sgk-1), the alpha subunit of the epithelial Na(+) channel (alphaENaC), and the glucocorticoid-induced-leucine-zipper protein (GILZ) in the whole kidney and in isolated cortical collecting tubules of adrenalectomized rats treated with low doses of aldosterone and/or dexamethasone. The resulting plasma concentrations of both steroids were close to 1 nmol/L. Aldosterone, given with or without dexamethasone, induced anti-natriuresis and kaliuresis, whereas dexamethasone alone did not. GILZ and alphaENaC transcripts were higher after treatment with either or both hormones, whereas the mRNA abundance of Sgk-1 was increased in the cortical collecting tubule by aldosterone but not by dexamethasone. We conclude the increased expression of Sgk-1 in the cortical collecting tubules is a primary event in the early antinatriuretic and kaliuretic responses to physiologic concentrations of aldosterone. Induction of alphaENaC and/or GILZ mRNAs may play a permissive role in the enhancement of the early and/or late responses; these effects may be necessary for a full response but do not by themselves promote early changes in urinary Na(+) and K(+) excretion.

Overexpression, purification, and crystal structure of native ER alpha LBD

Several crystal structures of human estrogen receptor alpha ligand-binding domain (hERalpha LBD) complexed with agonist or antagonist molecules have previously been solved. The proteins had been modified in cysteine residues (carboxymethylation) or renatured in urea to circumvent aggregation and denaturation problems. In this work, high-level protein expression and purification together with crystallization screening procedure yielded high amounts of soluble protein without renaturation or modifications steps. The native protein crystallizes in the space group P3(2) 21 with three molecules in the asymmetric unit. The overall structure is very similar to that previously reported for the hERalpha LBD with cysteine carboxymethylated residues thus validating the modification approach. The present strategy can be adapted to other cases where the solubility and the proper folding is a difficulty.

Crystal structure of a mutant hERalpha ligand-binding domain reveals key structural features for the mechanism of partial agonism

The crystal structure of a triple cysteine to serine mutant ERalpha ligand-binding domain (LBD), complexed with estradiol, shows that despite the presence of a tightly bound agonist ligand, the protein exhibits an antagonist-like conformation, similar to that observed in raloxifen and 4-hydroxytamoxifen-bound structures. This mutated receptor binds estradiol with wild type affinity and displays transcriptional activity upon estradiol stimulation, but with limited potency (about 50%). This partial activity is efficiently repressed in antagonist competition assays. The comparison with available LBD structures reveals key features governing the positioning of helix H12 and highlights the importance of cysteine residues in promoting an active conformation. Furthermore the present study reveals a hydrogen bond network connecting ligand binding to protein trans conformation. These observations support a dynamic view of H12 positioning, where the control of the equilibrium between two stable locations determines the partial agonist character of a given ligand.

Functional and structural characterization of the insertion region in the ligand binding domain of the vitamin D nuclear receptor

Vitamin D nuclear receptor mediates the genomic actions of the active form of vitamin D, 1,25(OH)2D3. This hormone is involved in calcium and phosphate metabolism and cell differentiation. Compared to other nuclear receptors, VDR presents a large insertion region at the N-terminal part of the ligand binding domain between helices H1 and H3, encoded by an additional exon. This region is poorly conserved in VDR in different species and is not well ordered as observed by secondary structure prediction. We engineered a VDR ligand binding domain mutant by removing this insertion region. Here we report its biochemical and biophysical characterization. The mutant protein exhibits the same ligand binding, dimerization with retinoid X receptor and transactivation properties as the wild-type VDR, suggesting that the insertion region does not affect these main functions. Solution studies by small angle X-ray scattering shows that the conformation in solution of the VDR mutant is similar to that observed in the crystal and that the insertion region in the VDR wild-type is not well ordered.

Crucial role of the H11-H12 loop in stabilizing the active conformation of the human mineralocorticoid receptor

The crystal structures of ligand-free and agonist-associated ligand-binding domain (LBD) of nuclear receptors (NRs) reveal that the amphipathic helix H12 is folded back toward the LBD core in the agonist-associated conformation, allowing the binding of coactivators. We used alanine scanning mutagenesis to explore the role of the residues of the loop connecting H11 and H12 in the activation of the human mineralocorticoid receptor (hMR), a member of the NRs family. H950A retained the ligand binding and transcriptional activities of the wild-type receptor and interacted with coactivators. In contrast F956A had no receptor functions. Aldosterone bound to the mutant hMRs (L952A, K953A, V954A, E955A, P957A) with nearly the same affinity as to the wild-type receptor and caused a receptor conformational change in these mutant hMRs as it does for the wild-type receptor. But the aldosterone-induced transcriptional activity of the mutant hMRs was lower (L952A, E955A, P957A) than that of the wild-type receptor or completely abolished (K953A, V954A) and their interaction with coactivators was impaired (E955A) or suppressed (L952A, K953A, V954A, P957A). In the light of a hMR-LBD model based on the structure of the progesterone-associated receptor-LBD, we propose that the integrity of the H11-H12 loop is crucial for folding the receptor into a ligand-binding competent state and for establishing the network of contacts that stabilize the active receptor conformation.

The crystal structure of the nuclear receptor for vitamin D bound to its natural ligand

The action of 1 alpha, 25-dihydroxyvitamin D3 is mediated by its nuclear receptor (VDR), a ligand-dependent transcription regulator. We report the 1.8 A resolution crystal structure of the complex between a VDR ligand-binding domain (LBD) construct lacking the highly variable VDR-specific insertion domain and vitamin D. The construct exhibits the same binding affinity for vitamin D and transactivation ability as the wild-type protein, showing that the N-terminal part of the LBD is essential for its structural and functional integrity while the large insertion peptide is dispensable. The structure reveals the active conformation of the bound ligand and allows understanding of the different binding properties of some synthetic analogs.

Specific hydroxylations determine selective corticosteroid recognition by human glucocorticoid and mineralocorticoid receptors

The ligand binding domains of the human mineralocorticoid receptor (hMR) and glucocorticoid receptor (hGR) display a high sequence homology. Aldosterone and cortisol, the major mineralocorticoid and glucocorticoid hormones, are very closely related, leading to the cross-binding of these hormones to both receptors. The present study reports on the mechanism by which hMR and hGR are activated preferentially by their cognate hormones. We found that the ability of corticosteroids to stimulate the receptor's transactivation function is depending on the stability of the steroid-receptor complexes. In the light of a hMR structural model we propose that contacts through the corticosteroid C21 hydroxyl group are sufficient to stabilize hMR but not hGR and that additional contacts through the C11- and C17-hydroxyl groups are required for hGR.

Corticosteroid-dependent sodium transport in a novel immortalized mouse collecting duct principal cell line

The final control of sodium balance takes place in the cortical collecting duct (CCD) of the nephron, where corticosteroid hormones regulate sodium reabsorption by acting through mineralocorticoid (MR) and/or glucocorticoid (GR) receptors. A clone of principal CCD cells (mpkCCDc14) has been established that is derived from a transgenic mouse (SV40 large T antigen under the control of the SV40 enhancer/L-type pyruvate kinase promoter). Cells grown on filters form polarized monolayers with high electrical transepithelial resistance (R(T) approximately 4700 ohm x cm2) and potential difference (P(D) approximately -50 mV) and have an amiloride-sensitive electrogenic sodium transport, as assessed by the short-circuit current method (Isc approximately 11 microA/cm2). Reverse transcription-PCR experiments using rat MR primers, [3H]aldosterone, and [3H]dexamethasone binding and competition studies indicated that the mpkCCDc14 cells exhibit specific MR and GR. Aldosterone increased Isc in a dose- (10(-10) to 10(-6) M) and time-dependent (2 to 72 h) manner, whereas corticosterone only transiently increased Isc (2 to 6 h). Consistent with the expression of 11beta-hydroxysteroid dehydrogenase type 2, which metabolizes glucocorticoids to inactive 11-dehydroderivates, carbenoxolone potentiated the corticosterone-stimulated Isc. Aldosterone (5x10(-7) M)-induced Isc (fourfold) was associated with a three- to fivefold increase in alpha-ENaC mRNA (but not in those for beta- or gamma-ENaC) and three- to 10-fold increases in alpha-ENaC protein synthesis. In conclusion, this new immortalized mammalian CCD clonal cell line has retained a high level of epithelial differentiation and sodium transport stimulated by aldosterone and therefore represents a useful mammalian cell system for identifying the genes controlled by aldosterone.

Photoaffinity labelling of the human mineralocorticoid receptor with steroids having a reactive group at position 3, 18 or 21

The ability of a glucocorticoid (triamcinolone acetonide: TA) and three progesterone derivatives with photoreactive groups at different positions (promegestone: R5020; 18-oxo-18-vinylprogesterone: 18OVP; 21-diazoprogesterone: 21DP) to bind covalently to the human mineralocorticoid receptor (hMR) expressed in Sf9 insect cells was assessed. Sedimentation gradient analysis and exchange assays with aldosterone showed that [3H]TA, a partial mineralocorticoid agonist, and [3H]R5020, a pure antimineralocorticoid, were covalently bound to hMR after UV irradiation, with a labelling efficiency of approx. 3-5%. UV irradiation did not alter the heterooligomeric structure of the hMR, since the irradiated [3H]TA- and [3H]R5020-hMR complexes sedimented at approx. 9-10 S, as did the non-irradiated complexes. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis revealed a band labelled by [3H]TA or [3H]R5020, having a molecular mass of 120 kDa. This band was not detected in the presence of an excess of the corresponding unlabelled steroid or when the cytosol was recovered from non-infected Sf9 cells. Electrophoresis of a truncated hMR (hMRDelta(1-351)) photolabelled with [3H]TA revealed a 80 kDa band, compatible with the molecular mass of the truncated hMR. Limited chymotrypsin proteolysis of the [3H]TA photolabelled hMR generated a 30 kDa fragment covalently associated with [3H]TA. As the 30 kDa fragment generated by chymotrypsin has been shown to encompass the entire ligand-binding domain of the hMR (B. Couette, J. Fagart, S. Jalaguier, M. Lombès, A. Souque, M.E. Rafestin-Oblin, Biochem. J. 315 (1996) 421-427), the present experiments provide evidence that [3H]TA is covalently bound to the ligand binding domain of the hMR. Exchange assays with [3H]A also revealed that unlabelled 18OVP and 21DP, two mineralocorticoid agonists bearing photoreactive groups at skeleton positions crucial for the ligand-MR interaction, are covalently bound to hMR with an approx. 30-35% labelling efficiency.

Antagonism in the human mineralocorticoid receptor

Key residues of the human mineralocorticoid receptor (hMR) involved in the recognition of agonist and antagonist ligands were identified by alanine-scanning mutagenesis based on a homology model of the hMR ligand-binding domain. They were tested for their transactivation capacity and ability to bind agonists (aldosterone, cortisol) and antagonists (progesterone, RU26752). The three-dimensional model reveals two polar sites located at the extremities of the elongated hydrophobic ligand-binding pocket. Mutations of Gln776 and Arg817 in site I reduce the affinity of hMR for both agonists and antagonists and affect the capacity of hMR to activate transcription, suggesting that the C3-ketone group, common to all ligands, is anchored by these two residues conserved within the nuclear steroid receptor family. In contrast, mutations of Asn770 and Thr945 in the opposite site only affect the binding of agonists bearing the C21-hydroxyl group. The binding of hMR antagonists that exhibit a smaller size and faster off-rate kinetics compared with agonists is not affected. In the light of the hMR homology model, a new mechanism of antagonism is proposed in which the AF2-AD core region is destabilized by the loss of contacts between the antagonist and the helix H12 region.

Ligand-induced conformational change in the human mineralocorticoid receptor occurs within its hetero-oligomeric structure

To determine the first steps involved in the mechanism of action of aldosterone and its antagonists, we analysed the ligand-induced structural changes of the human mineralocorticoid receptor (hMR) translated in vitro. Limited chymotrypsin digestion of the receptor generated a 30 kDa fragment. Following binding of a ligand to hMR, the 30 kDa fragment became resistant to chymotrypsin proteolysis, indicating a change in the receptor conformation. Differences in sensitivity to chymotrypsin of the 30 kDa fragment were observed after binding of agonists and antagonists to hMR, suggesting that these two classes of ligands induced different hMR conformations. Several lines of evidence allowed us to identify the 30 kDa fragment as the subregion encompassing the C-terminal part of the hinge region and the ligand-binding domain (LBD) or hMR (hMR 711-984). (1) The 30 kDa fragment is not recognized by FD4, an antibody directed against the N-terminal region of hMR. (2) Aldosterone remains associated with the 30 kDa fragment after chymotrypsin proteolysis of the aldosterone-hMR complex. (3) A truncated hMR, lacking the last 40 C-terminal amino acids (hMR 1-944), yields a 26 kDa proteolytic fragment. In addition, we showed that the unbound and the aldosterone-bound 30 kDa fragment were both associated with heat-shock protein (hsp) 90, indicating that the ligand-induced conformational change takes place within the hetero-oligomeric structure and that the 711-984 region is sufficient for hsp90-MR interaction. We conclude that the ligand-induced conformational change of the receptor is a crucial step in mineralocorticoid action. It occurs within the LBD, precedes the release of hsp90 from the receptor and is dependent upon the agonist/antagonist nature of the ligand.

Sulfhydryl groups are involved in the binding of agonists and antagonists to the human mineralocorticoid receptor

To investigate the role of sulfhydryl groups in the interaction of agonists and antagonists with the human mineralocorticoid receptor (hMR) the effect of methyl methanethiosulfonate (MMTS) on free and liganded-hMR was examined. hMR was expressed in insect cells (Sf9) using the baculovirus system. Treatment of cytosol with MMTS at 4 degrees C inhibited the binding to hMR of both [3H]aldosterone and [3H]RU26752 (a synthetic aldosterone antagonist). At 4 degrees C, the sensitivity to MMTS of the liganded-hMR complexes was dependent upon the nature of the ligands: agonists (aldosterone, corticosterone and cortisol) rendered the hMR resistant to MMTS, whereas antagonists (progesterone and RU26752) did not protect the receptor against MMTS inactivation. Analysis of the dose- and time-dependent effects of MMTS revealed that the free hMR and the RU26752-hMR complexes displayed a similar sensitivity to MMTS and that MMTS increased the dissociation of RU26752 from the hMR. At 4 degrees C the aldosterone-hMR complexes were not affected by MMTS treatment, whereas at 20 degrees C MMTS increased the dissociation of aldosterone from hMR. This effect was unrelated to the dissociation of hsp90 from hMR, because the sensitivity of the aldosterone-hmR complexes to MMTS remained unchanged after covalent linkage between hsp90 and the receptor. Our results suggest that agonists and antagonists modify the receptor conformation in distinct ways that render cysteine residues of the ligand binding domain more or less accessible to the MMTS action.

Modulation of recombinant alpha 6 beta 2 gamma 2 GABAA receptors by neuroactive steroids

The sensitivity of the recombinant alpha 6 beta 2 gamma 2 GABAA receptor expressed in HEK 293 cells to neuroactive steroids was studied. The steroids 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha-OH-DHP), pregnenolone sulfate and 3 alpha-OH-DHP sulfate have different modulatory effects on [3H]muscimol or [3H]Ro15-4513 binding to the alpha 6 beta 2 gamma 2 than to the alpha 1 beta 2 gamma 2 receptor. Binding of both radioactive ligands to the alpha 6 beta 2 gamma 2 receptor was maximally potentiated with each steroid used (10 nM) and decreased with further increases in steroid concentration. Using whole-cell recording, the GABA response of clusters of transfected HEK 293 cells was strongly potentiated by 3 or 10 nM 3 alpha-OH-DHP. In contrast, this response was reduced by 100 nM 3 alpha-OH-DHP. This latter effect appears to be related to the acceleration of the GABA response desensitization, observed in isolated cells. 3 alpha-OH-DHP (10 or 100 nM) was able to activate a response in the absence of GABA. It is proposed that the interaction of neuroactive steroids with the alpha 6 beta 2 gamma 2 receptor involves at least two distinct binding sites. One of them might be located close to the GABA binding site.

Mineralocorticoid receptors in SV40-transformed tubule cell lines derived from rabbit kidney

The presence of mineralocorticoid (MR) and glucocorticoid (GR) receptors was investigated in two renal tubular cell lines, derived from primary cultures of isolated rabbit kidney cortical cells infected with the wild-type SV40 virus, which exhibit thick ascending limb (RC.SV2) and collecting tubule (RC.SV3) phenotypes (Vandewalle et al. J. Cell. Physiol. 141, 1989, 203-221). MR and GR were quantified, in cell monolayers and cell cytosolic fractions, with [3H]aldosterone, [3H]dexamethasone and [3H]RU486, an antiglucocorticoid with no affinity for MR. Cytosolic receptors from RC.SV2 and RC.SV3 cells labeled with [3H]aldosterone, [3H]dexamethasone or [3H]RU486 sedimented at approximately 8 S in a 15-40% glycerol gradient. All steroids displaced bound [3H]dexamethasone to the same extent, suggesting that dexamethasone bound to both MR and GR: under the conditions of assay, [3H]aldosterone binds exclusively to MR, and [3H]RU486 to GR. In both RC.SV2 and RC.SV3 cells, [3H]aldosterone bound to one class of high affinity sites (Kd 0.14-0.8 nM; Nmax 8 to 22 fmol/mg protein). In both cell lines, the number of high affinity binding sites for [3H]dexamethasone ranged from 9 to 18 fmol/mg protein with an affinity of 0.5-1.3 nM. Compared to renal cortex, the most striking observation was a marked decrease in [3H]dexamethasone binding in primary cultures and SV40-transformed cells. These results indicate that MR and GR are expressed in two established mammalian kidney tubular cell lines providing new models of cultured renal cells for studies on the physiological effects of corticosteroid hormones.

Immunohistochemical and biochemical evidence for a cardiovascular mineralocorticoid receptor

The presence of mineralocorticoid receptors (MRs) and their physicochemical characteristics were investigated in the heart and blood vessels of rabbits. Immunohistochemical methods using the monoclonal anti-idiotypic antibody H10E, which interacts with the steroid binding domain of MRs, revealed the presence of immunoreactive material in the heart and large blood vessels. In the heart, a positive staining was observed in myocytes and endothelial cells of atria and ventricles. In vessels, MRs were detected in the aorta and pulmonary artery. They were localized in endothelial and vascular smooth muscle cells. No staining was present in the small vascular bed, arterioles, and capillaries. In all these studies, the mineralocorticoid specificity of the staining was assessed by in situ competition experiments with aldosterone and RU486, a glucocorticoid antagonist. The presence of MRs in the heart and vessels was further demonstrated by specific aldosterone binding to one class of high affinity binding sites in the cytosol of the adrenalectomized rabbit heart (Kd, 0.25 nM; maximum MR concentration, 15-20 fmol/mg protein), whose mineralocorticoid specificity has been clearly established by competition studies. Sedimentation gradient analyses revealed that the cardiovascular MR is an 8.5S hetero-oligomer that includes the heat shock protein 90. The physicochemical characteristics of the cardiovascular MRs are virtually identical to those of the renal MRs. Altogether, our results clearly demonstrate the presence of MRs in the cardiovascular system. This supports the possibility of direct aldosterone actions in the heart and blood vessels.

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.

Differences between aldosterone and its antagonists in binding kinetics and ligand-induced hsp90 release from mineralocorticosteroid receptor

We have previously reported that mineralocorticosteroid receptor (MR) is a 8-9 S heterooligomeric complex that includes the 90 kDa heat shock protein (hsp90). To elucidate how antagonist-receptor complexes are biologically inactive in terms of transcriptional regulation, we analyzed the binding of mineralocorticosteroid agonists and antagonists with MR and the ligand-induced transformation of its heterooligomeric structure. This study was performed in the cytosol of adrenalectomized rat kidney and of COS cells transiently transfected with human MR cDNA. Although aldosterone antagonists (SC9420 and RU26752) bind MR with the same affinity as aldosterone, they dissociate much more rapidly from the 8-9 S form of both rat and human MR than does aldosterone. Using sedimentation gradient analysis, we showed that the interaction between hsp90 and the steroid binding subunit of MR is highly dependent upon the nature of the steroid ligand since the binding of aldosterone antagonists results in an easy release of hsp90. We propose that both rapid dissociation of ligand and weakened hsp90-receptor interaction play a key role in the mechanism of mineralocorticosteroid antagonism. In the COS cell model, cortisol, described as a weak mineralocorticosteroid agonist, dissociates also more rapidly from human MR than does aldosterone. Our results suggest that ligand binding kinetics and ligand dependent modification in receptor structure are important modulators of MR function as a transcriptional regulatory factor.

Mutations in the "zinc fingers" or in the N-terminal region of the DNA binding domain of the human glucocorticosteroid receptor facilitate its salt-induced transformation, but do not modify hormone binding

While the effects of the ligand (hormone) binding domain (LBD) on other receptor domain functions are known, the effects of other domains on LBD functions have not been studied. In this work, we examined the importance of the structural integrity of other domains of the human glucocorticosteroid receptor (hGR) on LBD activity (stability of 8S complexes, binding of hormone, and transformation from the 8S to the 4S form). Several mutations introduced outside the LBD affect neither the formation of stable 8S heterooligomeric complexes nor the hGR binding affinity for the agonist triamcinolone acetonide (TA) or the antagonist RU486. However, some of them led to an easier salt-induced transformation of the 8S-hGR into a 4S form. Deletion of the second zinc finger of the DNA binding domain (DBD) facilitated 8S dissociation whether the ligand was TA or RU486. Deletion of the first zinc finger facilitated dissociation only in the presence of RU486, while replacement of PRO 416 (in the N-terminal region of the DBD) by ARG destabilized the 8S form only in the presence of TA. Variations in the salt-sensitivity of the mutated 8S GR complexes as a function of the ligand suggest that the DBD may interact functionally (if not physically) with the LBD. This interaction (possibly mediated by hsp90) could be influenced by minor structural differences between agonist and antagonist-GR complexes.

Corticosteroid receptors in cells derived from rat brain microvessels: mRNA identification and aldosterone binding

B7 is a cell clone derived from rat brain microvessels. Expression of an amiloride-sensitive cationic channel has been recently established in these cells. In this study, the polymerase chain reaction (PCR) was used to amplify definite segments of mineralocorticoid and glucocorticoid receptor mRNA in B7 cells. Aldosterone binding was also characterized. Two classes of sites were detected. Aldosterone exhibited a high affinity for type I sites [dissociation constant (Kd) approximately 0.3 nM] and a lower one for type II sites (Kd approximately 20 nM). RU 28362, a highly specific glucocorticoid agonist, did not compete for type I sites. RU 28362 and dexamethasone were better competitors for type II sites than aldosterone. The sedimentation coefficients of aldosterone type I and type II complexes were approximately 9S. These characteristics are close to the one exhibited by aldosterone type I and type II receptors in rat kidney and other target tissues. In intact B7 cells, aldosterone binding expressed as number of acceptor sites per cell was higher (approximately 41,000 for type II and 8,800 for type I) than in the soluble cellular extract (approximately 18,000 for type II and 1,000 for type I).

Cooperation between structural elements in hormono-regulated transcription from the mouse mammary tumor virus promoter

The mouse mammary tumor virus (MMTV) promoter is under the control of several types of regulatory agents. The proximal promoter within the long terminal repeat (LTR), from -200 to the CAP site and its regulation by steroid hormones have been extensively studied. However the precise role of sequences located upstream of this region remain unclear. We have constructed MMTV LTR deletion mutants coupled to the luciferase reporter gene and assayed their activities after transient transfection into transformed mammary epithelial cells (34i) and immortalized fibroblasts (NIH-3T3). In the absence of hormone, the MMTV promoter is almost silent, and deletions in the LTR have no significant effect on basal activity. In the presence of hormone, deletions spanning from the 5'-end to -455 have only slight effects on luciferase levels. In contrast, deletion of the region spanning from -450 to -201 leads to a dramatic decrease in transcription. A substantial decrease, more marked in 34i cells, is also clear when 90bp between -290 and -201 are deleted. At least one element cooperating positively with the glucocorticoid response element (GRE) is present between -223 and -201, as supported by the results of substitution mutation experiments. In 34i cell line, dexamethasone stimulates the MMTV LTR transcriptional activity to a level comparable to that of SV40. In contrast, in NIH-3T3 cells, MMTV promoter inducibility is weak. This results from a glucocorticoid receptor content 10-fold lower in NIH-3T3 cells than in 34i cells. Transfection of a glucocorticoid receptor expression plasmid allows recovery of a high inducibility of the MMTV promoter. This was true with all the MMTV LTR mutants studied here and suggests that NIH-3T3 cells possess all the factors necessary to cooperate with the steroid hormone in order to achieve a high transcriptional activity.

Aldosterone receptors in A6 cells: physicochemical characterization and autoradiographic study

The A6 cell line is derived from the kidney of Xenopus laevis. Aldosterone increases sodium transport across A6 cell epithelia. In the present study, aldosterone binding characteristics were studied in A6 cell cytosol. Both type I (mineralocorticoid) and type II (glucocorticoid) receptors are present in the cytosolic fraction of these cells. Aldosterone and corticosterone had a high affinity for type I sites (Kd = 1.25 and 0.16 nM, respectively) and a lower affinity for type II sites (Kd = 39 and 10 nM, respectively). Testosterone and estradiol did not compete for aldosterone binding. RU 26988, a highly specific glucocorticoid agonist, competed with aldosterone for type II but not for type I sites. Hydrodynamic parameters of both type I and type II corticosterone receptor complexes were identical. Their Stokes radius was approximately 6 nm, as estimated by high-performance size-exclusion chromatography, and their sedimentation coefficient determined by ultracentrifugation on glycerol gradients was approximately 9s. The molecular mass calculated from these parameters was approximately 200 kDa, a value that is very close to the value estimated for nontransformed mineralocorticoid and glucocorticoid receptors of other species. The [3H]aldosterone labeling of intact A6 cells was examined by autohistoradiography. At every concentration tested (2, 20, and 50 nM), all cells were found to be specifically labeled in both cytoplasm and nucleus. At 20 nM, in the presence of an excess of RU 26988, labeling was also detected. At every concentration the labeling data was compatible with a Gaussian distribution, indicating that A6 cells correspond to a homogeneous population with regard to aldosterone binding and that probably both type I and type II sites are present in the same cells.

Evidence for a single active site in the human angiotensin I-converting enzyme from inhibitor binding studies with [3H] RU 44 403: role of chloride

Binding of the potent radiolabelled competitive inhibitor 3H RU 44403 to pure human kidney angiotensin-I converting enzyme was examined in equilibrium and non equilibrium conditions. Equilibrium dialysis experiments indicate that, despite the duplicated structure of the enzyme and the presence of two putative active sites, 3H RU 44403 interacts with a single high affinity (Kd = 0.44 +/- 0.05 x 10-9 M, n = 3) binding site. This suggests that only one of the two putative active sites is functional, and can bind substrates or inhibitors. Sodium chloride plays an essential role in the enzyme-inhibitor interaction. The formation of the complex is only slightly influenced by NaCl, but the kinetic of dissociation is dramatically dependent on NaCl concentration. In a Nacl free medium the complex is unstable and dissociates rapidly. These results are consistent with the hypothesis that chloride ion influences isomerization of the complex toward a more stable form.

Affinity of corticosteroids for mineralocorticoid and glucocorticoid receptors of the rabbit kidney: effect of steroid substitution

Corticosteroid derivatives coupled in the C3, C7 or C17 position with a long aliphatic chain were synthesized in order to select a suitable ligand for the preparation of a biospecific affinity adsorbent for mineralocorticoid receptor purification. The affinity of these derivatives for mineralocorticoid receptors (MR) and glucocorticoid receptors (GR) was explored in rabbit kidney cytosol. In this model, aldosterone bound to a single class of receptors with high affinity (Kd 1 nM) and mineralocorticoid specificity. RU26988, a highly specific ligand for GR, did not compete for these sites. The C7 and C17 positions were found to be of crucial importance in the steroid's interaction with the mineralocorticoid receptors, since the linkage of a long side chain in these positions induced complete loss of affinity. Hence, deoxycorticosterone no longer bound to MR after 17 beta substitution with a 9-carbon aliphatic chain. This loss of affinity was not observed for glucocorticoids. The 17 beta nonylamide derivative of dexamethasone still competed for GR. Increasing the length of the C7 side of the spirolactone SC26304 suppressed its affinity for MR. Finally, C3 was an appropriate position for steroid substitution. The 3-nonylamide of carboxymethyloxime deoxycorticosterone bound to MR but not to GR, and therefore constitutes a suitable ligand for the preparation of a mineralocorticoid adsorbent.

Cross-reactivity of a monoclonal antiglucocorticoid receptor antibody BuGR1 with glucocorticoid and mineralocorticoid receptors of various species

The reactivity of a monoclonal antibody BuGR1, raised against glucocorticoid receptors of rat liver, with glucocorticoid and mineralocorticoid receptors of mammalian (rabbit) and amphibian (A6 cells) origin was examined. The glucocorticoid receptors of rabbit kidney and liver and of A6 cells were labeled with tritiated dexamethasone. The mineralocorticoid receptors were labeled with tritiated aldosterone in the presence or absence of RU26988, depending on whether aldosterone was bound to glucocorticoid receptors (A6 cells) or not (rabbit kidney), in addition to its binding to mineralocorticoid receptors. BuGR1 did not recognize mineralocorticoid receptors of A6 cells and rabbit kidney. BuGR1 cross-reacted with glucocorticoid receptors of rabbit liver and kidney but not of A6 cells, suggesting that the domain of glucocorticoid receptors recognized by BuRG1 could be present only in the mammalian species. The findings indicate that BuGR1 shows species differences as well as receptor class specificity.

Cytoplasmic and nuclear uptake of aldosterone in toad bladder: a mathematical modeling approach

The mechanism of aldosterone uptake in the epithelial cells of toad bladder was studied using mathematical modeling. Two complementary approaches were used. The first involved analysis of cytosolic aldosterone binding at steady state according to models defined by the sum of independent noninteractive binding sites. The best model describing the experimental data corresponded to two specific binding sites with mean dissociation constant values of 0.20 and 60 nM for types 1 and 2, respectively. The second approach was based on the analysis of cytoplasmic and nuclear aldosterone uptake kinetics at 25 and 0 degrees C in intact bladder. Two models (A and B) were studied. They both implied the existence of two types of aldosterone binding sites as precursors of the corresponding chromatin bound complexes. In model A, nuclear translocation of the two types of receptors was assumed to obey first-order kinetics. In model B, the translocation process for type 1 sites involved a time lag leading to delayed binding to chromatin. Both models were found to fit the experimental data satisfactorily. The fit obtained for model B appeared to be better at low aldosterone concentrations.

Modulation of aldosterone receptors in rat kidney: effects of steroid treatment and potassium diet

The numbers of type I and type II aldosterone receptors in the kidney cytosol of adrenalectomized rats were estimated after animals were treated with various steroids, or fed with high or low potassium diets. Oestradiol and 5 beta-pregnane-3,20 dione, which exhibited no affinity for aldosterone receptors, did not modify the levels of type I or type II receptors. Cortisol, corticosterone, progesterone and spirolactones, which all competed with aldosterone for both types of receptor, reduced the number of type I sites, as does aldosterone itself. Steroid treatment has no appreciable effect on type II receptors. We conclude that type I receptors are modulated by steroids able to bind to aldosterone receptors and that steroid-receptor interaction is an essential step in the receptor modulation process. The effects of potassium on aldosterone receptor modulation were tested in adrenalectomized rats on hypo- or hyperkalaemic diets. No change in receptor levels was observed in the rats on a low potassium diet, but the number of type I receptors increased in animals on a high potassium diet. However, the effects of potassium on receptor modulation were of lesser magnitude than those of aldosterone agonists and antagonists.

Effects of thyroid hormones and aldosterone on mineralocorticoid binding sites in the toad bladder

In the urinary bladder of the toad Bufo marinus triiodothyronine selectively inhibits the late effect of aldosterone on Na+ transport. We have investigated whether T3 might mediate its antimineralocorticoid action by controlling: i) the level of aldosterone binding sites in the soluble (cytosolic) pool isolated from tissues treated with T3 (60 nM) for up to 20 hr of incubation; ii) the kinetics of uptake of 3H-aldosterone into cytoplasmic and nuclear fractions after 2 or 20 hr of exposure to T3. The number and the affinity of Type I (high affinity, low capacity) and Type II (low affinity, high capacity) cytosolic binding sites (measured at 0 degrees C) did not vary significantly after 18 hr of exposure to T3, while aldosterone-dependent Na+ transport was significantly inhibited. In addition, T3 did not modify the kinetics of uptake (90 min) of 3H-aldosterone into cytoplasmic and nuclear fractions of toad bladder incubated in vitro at 25 degrees C. By contrast, aldosterone itself was able to down-regulate its cytosolic and nuclear binding sites after an 18-hr exposure to the steroid hormone (10 or 80 nM). T3 slightly (20%) but significantly potentiated the down regulation of nuclear binding sites. In conclusion, T3 does not appear to have major effects on the regulation of the aldosterone receptor, which could explain in a simple manner its antimineralocorticoid action.

Aldosterone binding in the human colon carcinoma cell line HT29: correlation with cell differentiation

The purpose of the present study is to investigate aldosterone binding to human colon carcinoma HT29 cells. These cells grow as undifferentiated epithelial cells when cultured under standard conditions (Dulbecco's MEM; 10% FCS). Modification of the culture medium induced two types of differentiation: (1) enterocytic differentiation when glucose (25 mM) is replaced by galactose (5 mM) (2) mucus secreting cells in FCS free medium. Aldosterone specific binding was detected in the cytosolic fraction of enterocyte-differentiated cells. This binding corresponded to a single class of sites with affinity, specificity and anion-exchange chromatographic behaviour similar to those observed in the epithelial crypts of human colon. These putative aldosterone receptors were also detected in mucus secreting cells that are derived from the enterocyte-differentiated cells. Enterocytic differentiation appears thus to be a necessary step for the "induction" of aldosterone receptors in HT29 cells.

Mineralocorticoid receptors in the epithelial cells of human colon and ileum

Specific binding of [3H]aldosterone to the cytosolic fraction of epithelial cells was studied in the human colon and terminal ileum. Analysis of [3H]aldosterone binding to the epithelial cells of ascending colon, caecum and ileum as a function of [3H]aldosterone concentration revealed only one class of specific receptors with an affinity constant of about 2 nmol/l. [3H]aldosterone binding was approximately the same in the sigmoid, descending and transverse colon and in the caecum, but slightly lower in the ascending colon and ileum. The specificity of the [3H]aldosterone binding was the same along the colon. The relative order of potency in inhibiting [3H]aldosterone binding was: aldosterone = SC 26304 = dexamethasone much greater than dihydrotestosterone greater than estradiol = RU 26988.

Binding and antimineralocorticoid activities of spirolactones in toad bladder

The role of the soluble pool (cytoplasmic or cytosolic) of [3H]-aldosterone binding sites in the toad bladder was assessed by the use of two spirolactones, prorenone and spironolactone as a reference drug. Prorenone fulfills all the criteria for a specific competitive antagonist of aldosterone for its effect on Na+ transport. Compared with spironolactone (Ki approximately equal to 1 microM), prorenone was about eightfold less potent (Ki approximately equal to 8 microM). Competition for [3H]aldosterone binding sites by spironolactone and prorenone revealed an order of potency (spironolactone greater than prorenone) that corresponded to their antagonist activities in the Na+ transport assay. There was a linear correlation between the effects of the two spirolactones on the aldosterone-stimulated Na+ transport and their ability to displace [3H]aldosterone from its binding sites in the soluble pool. Finally [3H]prorenone binding sites were detected in the soluble pool but an insignificant number of antagonist-receptor complexes were found associated with the nuclear pool. Our study indicates that the aldosterone binding sites of the soluble pool are indeed mineralocorticoid receptors, which are probably the first intracellular mediators leading to an increased Na+ reabsorption.