Analysis of the hormone-binding domain of steroid receptors using chimeras generated by homologous recombination

Structure-function relationships of the aldosterone receptor

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

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

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

Novel interactions of the mineralocorticoid receptor

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

The aldosterone-mineralocorticoid receptor pathway exerts anti-inflammatory effects in endotoxin-induced uveitis

We have previously shown that the eye is a mineralocorticoid-sensitive organ and we now question the role of mineralocorticoid receptor (MR) in ocular inflammation. The endotoxin-induced uveitis (EIU), a rat model of human intraocular inflammation, was induced by systemic administration of lipopolysaccharide (LPS). Evaluations were made 6 and 24 hours after intraocular injection of aldosterone (simultaneous to LPS injection). Three hours after onset of EIU, the MR and the glucocorticoid metabolizing enzyme 11-beta hydroxysteroid dehydrogenase type 2 (11β-HSD2) expression were down-regulated in iris/ciliary body and the corticosterone concentration was increased in aqueous humor, altering the normal MR/glucocorticoid receptor (GR) balance. At 24 hours, the GR expression was also decreased. In EIU, aldosterone reduced the intensity of clinical inflammation in a dose-dependent manner. The clinical benefit of aldosterone was abrogated in the presence of the MR antagonist (RU26752) and only partially with the GR antagonist (RU38486). Aldosterone reduced the release of inflammatory mediators (6 and 24 hours: TNF-α, IFN-γ, MIP-1α) in aqueous humor and the number of activated microglia/macrophages. Aldosterone partly prevented the uveitis-induced MR down-regulation. These results suggest that MR expression and activation in iris/ciliary body could protect the ocular structures against damages induced by EIU.

Mechanisms of ligand specificity of the mineralocorticoid receptor

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

New in vitro tools to study human constitutive androstane receptor (CAR) biology: discovery and comparison of human CAR inverse agonists

The human constitutive androstane receptor (CAR, NR1I3) is one of the key regulators of xenobiotic and endobiotic metabolism. The unique properties of human CAR, such as the high constitutive activity and the complexity of signaling, as well as the lack of functional and predictive cell-based assays to study the properties of the receptor, have hindered the discovery of selective human CAR ligands. Here we report a novel human CAR inverse agonist, 1-[(2-methylbenzofuran-3-yl)methyl]-3-(thiophen-2-ylmethyl) urea (S07662), which suppresses human CAR activity, recruits the corepressor NCoR in cell-based assays, and attenuates the phenytoin- and 6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime (CITCO)-induced expression of CYP2B6 mRNA in human primary hepatocytes. The properties of S07662 are also compared with those of known human CAR inverse agonists by using an array of different in vitro and in silico assays. The identified compound S07662 can be used as a chemical tool to study the biological functions of human CAR and also as a starting point for the development of new drugs for various conditions involving the receptor.

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

The amino acids that confer aldosterone binding specificity to the mineralocorticoid receptor (MR) remain to be determined. We had previously analyzed a panel of chimeras created between the MR and the glucocorticoid receptor and determined that amino acids 804-874 of the MR ligand binding domain are critical for aldosterone binding. In the present study a further series of chimeras was created within this region. The chimeras were analyzed by a transactivation assay and [(3)H]aldosterone binding, and the critical region was narrowed down to amino acids 820-844. Site-directed mutagenesis was used to create single and multiple amino acid substitutions in this region. These studies identified 12 of the 16 amino acids that differ in the MR and the glucocorticoid receptor in this region as being critical to conferring aldosterone responsivity. The amino acids that differ in the region 820-844 lie on the surface of the molecule and, therefore, it appears that MR ligand binding selectivity is conferred by residues that do not form part of the ligand binding pocket. Other studies have found that the corresponding regions of the androgen and glucocorticoid receptors are critical for the binding of natural and synthetic ligands, suggesting a common mechanism governing ligand binding specificity. The new chimeras also displayed, as previously reported, a dissociation between cortisol binding and transactivation and, intriguingly, only those that bound aldosterone with high affinity were activated by cortisol, suggesting a common mechanism that underlies specificity of aldosterone binding and the ability of cortisol to activate the MR.

Glucocorticoid resistance in two key models of acute lymphoblastic leukemia occurs at the level of the glucocorticoid receptor

Glucocorticoids (GCs) specifically induce apoptosis in malignant lymphoblasts and are thus pivotal in the treatment of acute lymphoblastic leukemia (ALL). However, GC-resistance is a therapeutic problem with an unclear molecular mechanism. We generated approximately 70 GC-resistant sublines from a GC-sensitive B- and a T-ALL cell line and investigated their mechanisms of resistance. In response to GCs, all GC-resistant subclones analyzed by real-time polymerase chain reaction (PCR) showed a deficient up-regulation of the GC-receptor (GR) and its downstream target, GC-induced leucine zipper. This deficiency in GR up-regulation was confirmed by Western blotting and on retroviral overexpression of GR in resistant subclones GC-sensitivity was restored. All GC-resistant subclones were screened for GR mutations using denaturing high-pressure liquid chromatography (DHPLC), DNA-fingerprinting, and fluorescence in situ hybridization (FISH). Among the identified mutations were some previously not associated with GC resistance: A484D, P515H, L756N, Y663H, L680P, and R714W. This approach revealed three genotypes, complete loss of functional GR in the mismatch repair deficient T-ALL model, apparently normal GR genes in B-ALLs, and heterozygosity in both. In the first genotype, deficiency in GR up-regulation was fully explained by mutational events, in the second by a putative regulatory defect, and in the third by a combination thereof. In all instances, GC-resistance occurred at the level of the GR in both models.