[The androgen receptor: molecular pathology]

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.

Molecular action of androgens

The androgen receptor (AR), which mediates androgen action in the cell, belongs to the superfamily of nuclear receptors, a large group of transcription factors. Recent studies have described how the AR acts on specific target genes. The receptor's specificity of action depends on its regulation at different levels: expression in the cells, ligand binding and DNA-specific sequence recognition by structurally conserved domains and regulation by transcriptional factors in an integrated response. We propose, here, an overview of recent works on the molecular regulation of androgen-dependent genes by AR.

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.

Disorders linked to insufficient androgen action in male children

Virilization of the external genitalia in the male fetus requires testosterone and dihydrotestosterone (DHT), which is formed from testosterone by the action of the enzyme, 5alpha-reductase type 2 (5alphaR-2). Mediation of the effects of both testosterone and DHT requires a functional androgen receptor (AR) located in the cytoplasmic compartment of target cells. DHT (or testosterone) binding induces a conformational change which facilitates AR nuclear transport, phosphorylation and dimerization, ultimately regulating of the rate of transcription of androgen-dependent genes. Any event which impairs DHT formation (mutation within the 5alphaR-2 gene or 5alphaR-2 inhibitors) or normal function of the AR (mutation in the AR gene, antiandrogens) may result in insufficient androgen action in the male fetus and in subsequent undervirilization in the newborn. Hypospadias may be due to a defect in androgen action due to mutation of the 5alphaR-2 or of the AR gene. Mutation of unidentified genes is likely to underlie this displacement of the urethral meatus from the tip to the ventral side of the phallus. An aetiological role for environmental chemical products has been postulated, since ethnic as well as geographical differences in the incidence of hypospadias have been noted. Increasing evidence has been gathered indicating that widely used industrial and agricultural chemicals have deleterious effects on normal male sexual differentiation. Cryptorchidism and micropenis may represent an intersex phenotype, even if they are isolated. Aetiological factors include 5alphaR-2 gene mutation, AR gene mutation or environmental hormonal disruptors. In conclusion, several phenotypes have been attributed to insufficient androgen action during fetal life. Whereas mutations in the 5alphaR-2 gene and AR gene are natural, attention should be focused on environmental endocrine disruptors that are able to mimic steroid 5alpha-reductase deficiency or partial androgen insensitivity syndrome.

The human estrogen receptor alpha dimer binds a single SRC-1 coactivator molecule with an affinity dictated by agonist structure

Nuclear receptors act as ligand-inducible transcription factors. Agonist binding leads to interaction with coactivator proteins, and to the assembly of the general transcription machinery. In addition to structural information, a thorough understanding of transcriptional activation by the nuclear receptors requires the characterization of the thermodynamic parameters governing these protein/protein interactions. In this study we have quantitatively characterized the interactions of full-length baculovirus expressed human estrogen receptor alpha (ERalpha), as well as ERalpha hormone binding domain (ERHBD) with a fragment of the coactivator protein SRC-1 (amino acid residues 570 to 780). Fluorescence anisotropy and fluorescence correlation spectroscopy of fluorescently labeled SRC-1(570-780) demonstrate unambiguously that the stoichiometry of the SRC-1/ERalpha/estradiol complex is one coactivator molecule per ERalpha dimer. The affinity of the estradiol or estriol bound ERalpha/SRC-1 complexes was found to be significantly higher than that observed in the presence of estrone. No binding was observed in the absence of ligand or in the presence of antagonists. Distinct anisotropy values for the ERalpha-SRC-1 complexes with different agonists suggest distinct conformations of the complexes depending upon agonist structure.

Specific recognition of androgens by their nuclear receptor. A structure-function study

Androgens, like progestins, are 3-ketosteroids with structural differences restricted to the 17beta substituent in the steroid D-ring. To better understand the specific recognition of ligands by the human androgen receptor (hAR), a homology model of the ligand-binding domain (LBD) was constructed based on the progesterone receptor LBD crystal structure. Several mutants of residues potentially involved in the specific recognition of ligands in the hAR were constructed and tested for their ability to bind agonists. Their transactivation capacity in response to agonist (R1881) and antagonists (cyproterone acetate, hydroxyflutamide, and ICI 176344) was also measured. Substitution of His(874) by alanine, only marginally impairs the ligand-binding and transactivation capacity of the hAR receptor. In contrast, mutations of Thr(877) and, to a greater extent, Asn(705) perturb ligand recognition, alter transactivation efficiency, and broaden receptor specificity. Interestingly, the N705A mutant acquires progesterone receptor (PR) properties for agonist ligands but, unlike wild type AR and PR, loses the capacity to repress transactivation with nonsteroidal antagonists. Models of the hAR.LBD complexes with several ligands are presented, which suggests new directions for drug design.

Quantitative characterization of the interaction between purified human estrogen receptor alpha and DNA using fluorescence anisotropy

In an effort to better define the molecular mechanisms of the functional specificity of human estrogen receptor alpha, we have carried out equilibrium binding assays to study the interaction of the receptor with a palindromic estrogen response element derived from the vitellogenin ERE. These assays are based on the observation of the fluorescence anisotropy of a fluorescein moiety covalently bound to the target oligonucleotide. The low anisotropy value due to the fast tumbling of the free oligonucleotide in solution increases substantially upon binding the receptor to the labeled ERE. The quality of our data are sufficient to ascertain that the binding is clearly cooperative in nature, ruling out a simple monomer interaction and implicating a dimerization energetically coupled to DNA binding in the nanomolar range. The salt concentration dependence of the affinity reveals formation of high stoichiometry, low specificity complexes at low salt concentration. Increasing the KCl concentration above 200 mM leads to specific binding of ER dimer. We interpret the lack of temperature dependence of the apparent affinity as indicative of an entropy driven interaction. Finally, binding assays using fluorescent target EREs bearing mutations of each of the base pairs in the palindromic ERE half-site indicate that the energy of interaction between ER and its target is relatively evenly distributed throughout the site.

A stable prostatic bioluminescent cell line to investigate androgen and antiandrogen effects

We developed a new stable prostatic cell line expressing the human androgen receptor (AR) and the AR-responsive reporter gene to generate a powerful tool for investigating androgen action and for rapid screening of agonists and antagonists. The AR-deficient PC-3 cells were stably transfected with pSG(5)-puro-hAR and pMMTV-neo-Luc. After selection with puromycin and neomycin, one highly inducible clone was isolated and named PALM, for PC-3-Androgen receptor-Luciferase-MMTV. The expression of hAR was confirmed by western blot and steroid-binding assays on the whole cells. The transcriptional activity of the clone was measured after incubation of cells with increasing concentrations of synthetic R1881 or natural androgens (DHT and testosterone). The three agonists had the same maximal activity at 0.1 microM and the fold induction was equal to 20. The agonist and antagonist activities of the steroidal antiandrogens (cyproterone acetate and RU2956) and the non-steroidal antiandrogens (nilutamide, bicalutamide, inocoterone and hydroxyflutamide) measured with the PALM cells were in good correlation with the results obtained with transiently transfected cells. The selectivity in steroid transactivation was demonstrated with estradiol, progesterone, cortisol, dexamethasone and aldosterone. Spironolactone and RU486 showed partial agonist and antagonist activities, whereas R5020 presented only a partial antagonist activity. We here demonstrate that this stable transfectant provides an accurate tool for studying wild-type human AR activation and its regulation by androgens and antiandrogens in a human prostatic epithelial cell, which is routinely available and remains androgen-responsive in vitro.

Transcriptional interferences between normal or mutant androgen receptors and the activator protein 1--dissection of the androgen receptor functional domains

We investigated the interferences of the normal or mutated androgen receptor with the activator protein-1 (AP-1) by assessing their effects on transcriptional activity in CV-1 cells. A luciferase reporter gene was constructed downstream from either a promoter for the mouse vas deferens protein, or a trimerized 12-O-tetradecanoyl phorbol-13-acetate-response element site whose transcriptions are activated by androgen and 12-O-tetradecanoyl phorbol-13-acetate, a potent AP-1 activator. The blockade of dephosphorylation by protein phosphatases identifies the protein phosphatases that modulate the AP-1/androgen receptor cross-talk. Using engineered or naturally occurring androgen receptor mutants that are responsible for complete or partial androgen insensitivity syndromes, we defined the subregions involved in the cross-talk of the androgen receptor with the AP-1 factors. First, it appears that the 188 first amino acids of the N-terminal domain of the androgen receptor are necessary to obtain a full transrepression. Second, a functional and intact ligand binding domain is critical for the modulation of androgen/AP-1 pathway interactions. Third, normal DNA binding capacity of the androgen receptor is not required. Two mutants at positions 568 and 581 of the DNA binding domain demonstrate that the transactivation and transrepression functions of the androgen receptor can be dissociated. Collectively, these data indicate that several segments of the androgen receptor are involved in cross-talk with the AP-1 pathway. Mutations within the DNA binding domain of the androgen receptor highly impair these interferences.

Functional and structural analysis of R607Q and R608K androgen receptor substitutions associated with male breast cancer

We previously described an androgen receptor (AR) point mutation located in the DNA-binding domain (DBD), adjacent to another AR substitution. Both were observed in two unrelated families with male breast cancer (MBC) and partial androgen insensitivity syndrome. This work was designed to determine the potential role of these two residues by in vitro study of the consequences of these two substitutions on biological functions and their structural impact at the atomic level. Mutant ARs revealed normal androgen-binding affinities and weaker DNA binding to an isolated androgen-responsive element. In cotransfection assays the mutant ARs displayed a reduced transactivation efficiency at 0.3 x 10(-10) M. Neither binding to an estrogen-responsive element nor transactivation efficiency of an ERE reporter gene was observed. Molecular modeling revealed that Arg607 and Arg608 were partially surface-exposed and located in adjacent areas in the AR-DBD complex with DNA. This is in favor of a protein-protein interaction. It is conceivable that such an interaction could be affected by mutation of one of these two arginines.

A novel substitution (Leu707Arg) in exon 4 of the androgen receptor gene causes complete androgen resistance

A wide spectrum of androgen receptor (AR) gene mutations has been reported in complete androgen insensitivity syndromes. The molecular basis of androgen resistance was investigated in a female newborn with complete testicular feminization. Sequencing identified a point mutation in exon 4 responsible for a leucine (CTG) to arginine (CGG) replacement at codon 707. This novel mutation is located in the amino-terminal part of the ligand-binding domain of the AR. To determine the functional properties of the mutated AR and to establish the correlation with the clinical phenotype of androgen resistance, the mutation was reproduced in AR wild-type complementary DNA, and the plasmid was transfected into AR-free mammalian cells. In vitro studies showed that the mutant AR was functionally defective as an androgen-binding molecule. Electrophoretic mobility shift assay revealed that the binding of mutated AR to DNA was reduced. Finally, the mutant was unable to induce the transcriptional activation of androgen-responsive reporter gene. This amino acid defect in the primary sequence probably involves the rupture of hydropathicity in a region that is conserved among members of the steroid receptor subfamily. Our data substantiate the major contribution of leucine 707 to normal AR function and demonstrate that its substitution by an arginine caused the complete androgen insensitivity in this patient. Our findings also contribute to the elaboration of the structure-function map of the AR based on naturally occurring mutations.

Molecular modeling and in vitro investigations of the human androgen receptor DNA-binding domain: application for the study of two mutations

In two families with complete androgen insensitivity, we have identified naturally occurring point mutations in the human androgen receptor gene that encode amino acid substitutions within the DNA-binding domain. The two amino acid substitutions, a valine to phenylalanine and a leucine to proline, occur at positions 581 and 616, respectively, of the androgen receptor. The mutations were recreated by site-directed mutagenesis. Expression of the mutants androgen receptors in COS 7 and CV 1 cells revealed a normal size 110-kDa receptor protein on Western blots, normal androgen binding capacities and affinities, but absence of binding to target DNA on electrophoretic mobility shift assays. In cotransfection assays, the mutant ARs failed to activate transcription of an androgen-responsive reporter gene. To study the possible structural impact of these mutations, three-dimensional models of the normal androgen receptor and of each mutant were built by homology with the glucocorticoid receptor. Analysis of the models predicts that mutation Val581Phe would affect interaction between the protein and DNA, whereas the Leu616Pro mutation would more likely be involved in destabilizing the protein structure or protein dimerization. Taken together, the experimental investigations and the molecular modeling studies of the mutant androgen receptors observed in families with complete androgen insensitivity syndrome, highlight the role of Val-581 and Leu-616 in receptor structure and function.

Detection of membrane and soluble interleukin-6 receptor in lymphoid malignancies

We studied the membrane expression of the gp80 chain of IL-6 receptor (IL-6R) by quantitative flow cytometry in chronic lymphocytic leukaemia (CLL) and leukaemic centrocytic lymphoma using a panel of seven monoclonal antibodies. IL-6R was detected in 18/26 CLL cases and 4/7 lymphoma cases, with a mean antigen density < 3000 molecules/cell. Multiple labelling experiments confirmed the IL-6R expression by neoplastic cells. Specific mRNA was found by RT-PCR in neoplastic cells. A specific ELISA test was designed using two anti-IL-6 receptor MAbs to measure the serum soluble IL-6R (sIL-6R) in CLL (n = 48). B-cell non-Hodgkin's lymphoma (NHL; n = 40), and monoclonal gammopathy (MG; n = 32). SIL-6R was higher in CLL (170 +/- 12.6 ng/ml) in NHL (160 +/- 12 ng/ml) and MG patients (183 +/- 23 ng/ml) than in age-matched controls (100 +/- 5.6 ng/ml; P < 0.001) and higher in high-grade than low-grade NHL. No correlation was noted with a previous treatment. Among CLL cases the patients classified as stage B according to the Binet's staging of the disease had the highest sIL-6R values, thus suggesting a link with tumour cell mass.

Complete androgen insensitivity syndrome due to a new frameshift deletion in exon 4 of the androgen receptor gene: functional analysis of the mutant receptor

We studied the androgen receptor gene in a large kindred with complete androgen insensitivity syndrome and negative receptor-binding activity, single-strand conformation polymorphism (SSCP) analysis and sequencing identified a 13 base pair deletion within exon 4. This was responsible for a predictive frameshift in the open reading frame and introduction of a premature stop codon at position 783 instead of 919. The deletion was reproduced in androgen receptor wildtype cDNA and transfected into mammalian cells. Western blot showed a smaller androgen receptor of 94 kDa for the transfected mutated cDNA instead of 110 kDa. Androgen-binding assay of the mutated transfected cells assessed the lack of androgen-binding. Gel retardation assay demonstrated the ability of the mutant to bind target DNA; however, the mutant was unable to transactivate a reporter gene. Although the role of the partial deletion in the lack of androgen action was expected, in vitro analyses highlight the role of the abnormal C-terminal portion in the inhibition of the receptor transregulatory activity of the protein causing androgen resistance in this family.

Molecular genetics of androgen insensitivity syndromes

The androgen receptor belongs to the family of nuclear receptors and contains three functional domains: a carboxy-terminal hormone binding region, a central cystein rich DNA binding region and an amino-terminal region involved in the expression of androgen regulated genes. Cloning of the complementary DNA encoding the androgen receptor enabled the characterization of the molecular defects associated with androgen insensitivity syndromes, X-linked disorders resulting from androgen action defects in target cells. Moreover, androgen receptor gene alterations have been recently described in two unrelated diseases: male breast cancer and spinal and bulbar muscular atrophy. Our group have identified 16 androgen receptor gene alterations in patients with androgen insensitivity syndrome, an amino acid substitution in a patient with a partial androgen insensitivity syndrome and a breast cancer. In 2 families, the molecular diagnosis of spinal and bulbar muscular atrophy has been performed.

Mutations of androgen receptor gene in androgen insensitivity syndromes

The androgen receptor belongs to the family of steroid-thyroid hormone-retinoid nuclear receptors. It contains 3 major domains: a hormone-binding region, a DNA-binding region and an amino-terminal region. Cloning of the cDNA encoding the androgen receptor and elucidation of the androgen receptor gene structure enabled the characterization of the molecular defects associated with androgen insensitivity. Mutations of the androgen receptor in 46,XY individuals cause a spectrum of androgen insensitivity syndromes, ranging from female phenotype (testicular feminization) to minor degrees of undervirilization or infertility. Reports on androgen receptor gene structure in patients with complete or partial forms of androgen insensitivity demonstrate that gene deletions are very rare. Several categories of mutations have been reported and are reviewed in this paper. Nucleotide substitutions in the androgen-binding domain or the N-terminal region that cause insertion of premature termination codons result in failure to form a functional protein. Missense mutations within the androgen-binding domain are responsible for a decrease or absence of receptor-binding activity. Mutations within the DNA-binding domain are associated with a positive receptor-binding form of androgen insensitivity. Analysis of described mutations indicates that they are spread throughout the gene, either associated with partial or complete androgen insensitivity. Furthermore, the same point mutation was reported to be associated with variable phenotypic expression of androgen insensitivity syndrome. It is thus difficult to define a genotype/phenotype relationship. However, mutations causing androgen insensitivity will certainly yield important new insights into the molecular basis of androgen action.