Complex regulation of LCoR signaling in breast cancer cells.

Ligand-dependent corepressor (LCoR) is a transcriptional repressor of ligand-activated estrogen receptors (ERs) and other transcription factors that acts both by recruiting histone deacetylases and C-terminal binding proteins. Here, we first studied LCOR gene expression in breast cancer cell lines and tissues. We detected two mRNAs variants, LCoR and LCoR2 (which encodes a truncated LCoR protein). Their expression was highly correlated and localized in discrete nuclear foci. LCoR and LCoR2 strongly repressed transcription, inhibited estrogen-induced target gene expression and decreased breast cancer cell proliferation. By mutagenesis analysis, we showed that the helix-turn-helix domain of LCoR is required for these effects. Using in vitro interaction, coimmunoprecipitation, proximity ligation assay and confocal microscopy experiments, we found that receptor-interacting protein of 140 kDa (RIP140) is a LCoR and LCoR2 partner and that this interaction requires the HTH domain of LCoR and RIP140 N- and C-terminal regions. By increasing or silencing LCoR and RIP140 expression in human breast cancer cells, we then showed that RIP140 is necessary for LCoR inhibition of gene expression and cell proliferation. Moreover, LCoR and RIP140 mRNA levels were strongly correlated in breast cancer cell lines and biopsies. In addition, RIP140 positively regulated LCoR expression in human breast cancer cells and in transgenic mouse models. Finally, their expression correlated with overall survival of patients with breast cancer. Taken together, our results provide new insights into the mechanism of action of LCoR and RIP140 and highlight their strong interplay for the control of gene expression and cell proliferation in breast cancer cells.

[The androgen receptor: molecular pathology]. / Récepteur des androgènes: pathologie moléculaire.

Androgens play a crucial role in the development, maintenance and regulation of male phenotype and reproductive physiology through the androgen receptor, a transcription factor. Testosterone or dihydrotestosterone binding induces a trans-conformation of the androgen receptor and allows its translocation into the nucleus, where it recognizes specific DNA sequences. Recent developments in molecular genetics, as well as structural analysis of the androgen receptor, allow a better understanding of the structure/function relationship of this nuclear receptor. Molecular analyses of androgen insensitivity syndrome, as well as hormone-resistant prostate cancer, Kennedy's disease and isolated male infertility, have been proved useful as privileged models for this purpose. In the absence of identified AR receptor mutations in androgen insensitivity syndromes, abnormalities of transcriptional cofactor should be considered. Finally, identification of androgen-dependent genes will be helpful for evaluating the degree of the molecular defect of androgen action within target cells.

Disorders of androgen action.

Disorders of androgen action are the main cause of male pseudohermaphroditism and include 5alphaR deficiency and androgen receptor defects. 5alphaR deficiency is characterized by female genitalia with some degree of masculinization, clitoromegaly, and severely bifid scrotum corresponding to the so-called pseudovaginal perineoscrotal hypospadias. At the onset of puberty, increased muscle mass, development of pubic hair, and phallic growth are associated with the acquisition of male gender identity. Normal or increased levels of testosterone and an elevated testosterone-to-dihydrotestosterone ratio after human chorionic gonadotropin stimulation testing suggest 5alphareductase deficiency, and the diagnosis can be ascertained by identifying the mutation in the 5alphaR-2 gene. Whatever the patient's age at diagnosis, psychological evaluation with 5alphaRD is vital. Androgen receptor defects encompass two clinical expressions: the complete and partial androgen insensitivity syndromes. Complete androgen insensitivity syndrome should be suspected at birth in the presence of inguinal hernia in a girl without genital ambiguity. At puberty, the sign of alert is primary amenorrhea with normal female phenotype and harmonious mammary development but no pubic hair growth. Partial androgen insensitivity syndrome covers a wide spectrum of undervirilized phenotypes ranging from clitoromegaly at birth to infertile men. In all cases, complementary investigations should include plasma testosterone and luteinizing hormone as well as androgen-binding capacity in cultured genital skin fibroblasts. Diagnosis is confirmed by identification of the androgen receptor gene mutation. Although patients with complete androgen insensitivity syndrome are raised as females, patients with partial androgen insensitivity syndrome should be managed according to age at diagnosis, response to treatment with exogenous androgens, and the presence of an androgen gene mutation. Gonadectomy in complete androgen insensitivity syndrome should be performed before puberty, and androgen substitution may improve the development of external genitalia in some patients with partial androgen insensitivity syndrome. Psychological follow-up is necessary.

Characterization of the interaction between androgen receptor and a new transcriptional inhibitor, SHP.

SHP (short heterodimer partner) is an orphan nuclear receptor, first described for its interaction with nuclear receptors. This study explores a new way of inhibiting the androgen-signaling pathway. We demonstrated that SHP inhibited up to 97% of AR-induced activity. Characterization of AR/SHP interaction provided evidence of a clear ligand dependency. We also showed that the LXXI/LL motifs previously found on SHP mediated the interaction with the AR ligand-binding domain (AR-LBD), the motif responsible for the interaction being slightly different from that found with ER. The AR N-terminal domain (AR-NTD), in contrast to that of other nuclear receptors, accounts for most of the entire receptor transactivation potential. SHP also interacted with AR-NTD, thus stabilizing the interaction with AR. We demonstrated that SHP inhibited both AR-LBD and NTD-dependent transactivation, which evidenced for the first time a protein capable of inhibiting a steroid receptor amino-terminal-dependent transactivation. We further characterized the SHP mechanism of action by showing that SHP reversed AR coactivator-mediated activation. Conversely, FHL2 and TIF2 counteracted SHP-mediated inhibition of AR. SHP evidences a new way of inhibiting AR activity by competing with AR coactivators. This new type of inhibitor could dictate the activity of nuclear receptors, depending on the equilibrium between activators and inhibitors.

Isolation and characterization of AINT: a novel ARNT interacting protein expressed during murine embryonic development.

Basic helix-loop-helix-PER-ARNT-SIM (bHLH-PAS) proteins form dimeric transcription factors to mediate diverse biological functions including xenobiotic metabolism, hypoxic response, circadian rhythm and central nervous system midline development. The Ah receptor nuclear translocator protein (ARNT) plays a central role as a common heterodimerization partner. Herein, we describe a novel, embryonically expressed, ARNT interacting protein (AINT) that may be a member of a larger coiled-coil PAS interacting protein family. The AINT C-terminus mediates interaction with the PAS domain of ARNT in yeast and interacts in vitro with ARNT and ARNT2 specifically. AINT localizes to the cytoplasm and overexpression leads to non-nuclear localization of ARNT. A dynamic pattern of AINT mRNA expression during embryogenesis and cerebellum ontogeny supports a role for AINT in development.

Folding requirements of the ligand-binding domain of the human mineralocorticoid receptor.

The effects of aldosterone are mediated by the mineralocorticoid receptor (MR), a ligand-dependent transcription factor. We investigated the structural determinants for ligand binding to the receptor using a series of human MR (hMR) deletion mutants. These proteins were produced in vitro in rabbit reticulocyte lysate and analyzed for their ability to bind agonists, antagonists, and the heat shock protein hsp90, which is a prerequisite for ligand binding to hMR. Studies on N terminus-truncated hMRs showed that the ligand-binding domain (LBD: amino acids 734-984) has a lower affinity for aldosterone than the entire receptor [dissociation constant (Kd) 2.9 vs. 0.47 nM] and does not interact with hsp90. Addition of the five-amino acid sequence (729-733) upstream from the LBD is necessary for interaction with hsp90, but a larger region is needed for high aldosterone affinity. Deletions at the C-terminal end of the hMR greatly reduced both agonist and antagonist binding: deletion of the last three amino acids reduced the affinity for aldosterone to 1/20 that of the entire protein, and deletion of the last four amino acids completely abolished binding, although the interaction with hsp90 was not affected. These effects can be explained by misfolding of the receptor, since limited proteolysis assays showed that deletions at the C-terminal end of hMR affect the accessibility of the cleavage sites within the DNA-binding domain and the N-terminal part of the hinge region to trypsin. Thus, our results support the idea that a short sequence upstream of the LBD is essential for the interaction of hMR with hsp90 and that the C terminus of hMR and hsp90 are both essential for folding of the receptor in a high-affinity hormone-binding state.

Involvement of the N-terminal region of the human mineralocorticoid receptor hormone-binding domain in agonist and antagonist binding as revealed by a new monoclonal antibody.

To gain a better understanding of the mechanism of binding to the human mineralocorticoid receptor (hMR), we developed a new monoclonal antibody (mAb) raised against the hormone-binding domain (HBD). For this purpose, mice were immunized with a fusion protein including the sequence Thr729-Lys984 of hMR. After ELISA screening, mAb 18C7 was selected for its specificity towards the HBD. This antibody recognized both the denatured and native MR forms, as well as the hetero-oligomeric MR form and the transformed MR state. By using several HBD subfragments, the mAb 18C7 epitope was located in the N-terminal region of the HBD from Thr729 to Leu765. We then studied the effect of the antibody on aldosterone and progesterone binding to the hMR. When 18C7 was incubated with liganded MR, it was able to partly displace (20%) the hormone from its binding site. When 18C7 was incubated with MR before aldosterone or progesterone, the antibody inhibited 75-80% of the binding. The effect of 18C7 on the binding was similar with both hormones. A sucrose gradient analysis indicated the simultaneous presence of two kinds of receptor complexes: the steroid-MR complex and the antibody-MR complex. After its associated proteins, especially the heat-shock protein hsp90, had been cross-linked with the hMR by dimethylpimelimidate, 18C7 was still able to react with the receptor. Our results indicated that the epitope recognized by 18C7 was directly implicated in hormone binding. The lack of steroid binding of HBD mutants with the Thr729-Leu765 sequence deleted [Jalaguier, Mesnier, Léger and Auzou (1996) J. Steroid Biochem. Mol. Biol. 57, 43-50] supports this hypothesis. Because of the similar behaviours of aldosterone and progesterone, we conclude that the N-terminal Thr729-Leu765 region of the HBD is similarly involved in the binding of both hormones.

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.

Human mineralocorticoid receptor interacts with actin under mineralocorticoid ligand modulation.

The human mineralocorticoid receptor of the steroid receptor family contains a modular structure with domain E which is considered to be a hormone binding domain. Recombinant protein approaches enabled us to clearly determine that this domain is also able to interact with F-actin (Kd about 2 microM) and G-actin. Moreover, it was revealed that this mineralocorticoid receptor domain/actin interaction was modulated by specific mineralocorticoid ligands. Agonist (aldosterone) steroid binding almost totally (91%) abolished the interaction with F-actin, while antagonist (progesterone) binding allowed more than 30% of this binding. Steroid modulation of the interaction between domain E and actin indicated that this actin binding is specific and could be essential for cellular mineralocorticoid receptor activity.

GABA inhibition of somatostatin gene expression in cultured hypothalamic neurones.

GABAA receptors mediate the inhibition of somatostatin release in hypothalamic neurones. To study the possible effect of GABA on somatostatin biosynthesis, somatostatin and preprosomatostatin mRNA levels were evaluated after exposure of hypothalamic neurones to muscimol or bicuculline. Muscimol (50 microM) decreased preprosomatostatin mRNA levels, by 25% after 4 h and 30% after 24 h treatment. Bicuculline (50 microM and 100 microM) increased preprosomatostatin mRNA levels by 1.4 and 1.5 fold after 4 h and by 1.3 and 1.7 fold after 24 h treatment. Somatostatin content was not modified after muscimol or bicuculline exposure. Total DNA content, used to control cellular viability, was not modified under any experimental conditions. Our findings suggest that the GABAergic inhibition of mRNA levels could be a consequence of the GABA inhibition of somatostatin release, thus allowing limited changes in peptide steady-state levels.