DNA recognition by the androgen receptor: evidence for an alternative DNA-dependent dimerization, and an active role of sequences flanking the response element on transactivation

Treatments Targeting the Androgen Receptor and Its Splice Variants in Breast Cancer

Breast cancer is a major cause of death worldwide. The complexity of endocrine regulation in breast cancer may allow the cancer cells to escape from a particular treatment and result in resistant and aggressive disease. These breast cancers usually have fewer treatment options. Targeted therapies for cancer patients may offer fewer adverse side effects because of specificity compared to conventional chemotherapy. Signaling pathways of nuclear receptors, such as the estrogen receptor (ER), have been intensively studied and used as therapeutic targets. Recently, the role of the androgen receptor (AR) in breast cancer is gaining greater attention as a therapeutic target and as a prognostic biomarker. The expression of constitutively active truncated AR splice variants in breast cancer is a possible mechanism contributing to treatment resistance. Therefore, targeting both the full-length AR and AR variants, either through the activation or suppression of AR function, depending on the status of the ER, progesterone receptor, or human epidermal growth factor receptor 2, may provide additional treatment options. Studies targeting AR in combination with other treatment strategies are ongoing in clinical trials. The determination of the status of nuclear receptors to classify and identify patient subgroups will facilitate optimized and targeted combination therapies.

Molecular Dynamics Simulations of a Chimeric Androgen Receptor Protein (SPARKI) Confirm the Importance of the Dimerization Domain on DNA Binding Specificity

The DNA binding domains of Androgen/Glucocorticoid receptors (AR/GR), members of class I steroid receptors, bind as a homo-dimer to a cis-regulatory element. These response elements are arranged as inverted repeat (IR) of hexamer “AGAACA”, separated with a 3 base pairs spacer. DNA binding domains of the Androgen receptor, AR-DBDs, in addition, selectively recognize a direct-like repeat (DR) arrangement of this hexamer. A chimeric AR protein, termed SPARKI, in which the second zinc-binding motif of AR is swapped with that of GR, however, fails to recognize DR-like elements. By molecular dynamic simulations, we identify how the DNA binding domains of the wild type AR/GR, and also the chimeric SPARKI model, distinctly interact with both IR and DR response elements. AR binds more strongly to DR than GR binds to IR elements. A SPARKI model built from the structure of the AR (SPARKI-AR) shows significantly fewer hydrogen bond interactions in complex with a DR sequence than with an IR sequence. Moreover, a SPARKI model based on the structure of the GR (SPARKI-GR) shows a considerable distortion in its dimerization domain when complexed to a DR-DNA whereas it remains in a stable conformation in a complex with an IR-DNA. The diminished interaction of SPARKI-AR with and the instability of SPARKI-GR on DR response elements agree with SPARKI's lack of affinity for these sequences. The more GR-like binding specificity of the chimeric SPARKI protein is further emphasized by both SPARKI models binding even more strongly to IR elements than observed for the DNA binding domain of the GR.

Phenotypic and molecular characteristics of androgen insensitivity syndrome patients

Androgen insensitivity syndrome (AIS), an X-linked recessive genetic disorder of sex development, is caused by mutations in the androgen receptor (AR) gene, and is characterized by partial or complete inability of specific tissues to respond to androgens in individuals with the 46, XY karyotype. This study aimed to investigate AR gene mutations and to characterize genotype–phenotype correlations. Ten patients from unrelated families, aged 2–31 years, were recruited in the study. Based on karyotype, altered hormone profile, and clinical manifestations, nine patients were preliminarily diagnosed with complete AIS and one with partial AIS. Genetic analysis of AR gene revealed the existence of 10 different mutations, of which five were novel (c.2112 C>G[p.S704R], c.2290T>A[p.Y764N], c.2626C>T[p.Q876X], c.933dupC[p.K313Qfs*28], and c.1067delC[p.A356Efs*123]); the other five were previously reported (c.1789G>A[p.A597T], c.2566C>T[p.R856C], c.2668G>A[p.V890M], c.2679C>T[p.P893L], and c.1605C>G[p.Y535X]). Regarding the distribution of these mutations, 60.0% were clustered in the ligand-binding domain of AR gene. Exons 1 and 8 of AR gene each accounted for 30.0% (3/10) of all mutations. Most of the truncation mutations were in exon 1 and missense mutations were mainly located in exons 4–8. Our study expands the spectrum of AR gene mutations and confirms the usefulness of AR gene sequencing to support a diagnosis of AIS and to enable prenatal or antenatal screening.

Androgen receptor and miR-206 regulation in prostate cancer

In the United States, prostate cancer is the second leading cause of cancer-related deaths among men with an approximately 220,000 patients diagnosed with the disease in 2015. Prostate cancer is a hormone-driven tumor, and a common therapy is androgen-deprivation therapy (ADT) that involves anti-androgen treatments and/or castration therapy. Understanding the molecular basis for androgen-independent tumors is crucial toward developing new therapies for these patients. Understanding how androgen receptor itself functions is an important step in elucidating this process. Androgen receptor (AR), NR3C4, is a nuclear hormone receptor and functions as a DNA-binding transcription factor that regulates the expression of protein-coding genes. Translocation of AR to improper gene promoter elements or DNA-binding sites can result in an alteration in gene expression and thus normal prostate function. Therefore, it is crucial to understand which AR-promoter interactions are drivers of disease, as compared to promiscuous or benign AR-binding interactions. While a large portion of our genome is considered a gene desert, it is now appreciated that these regions of the genome contain non-coding RNA genes such as microRNAs (miRNAs). These non-coding RNAs have enormous regulatory potential, as they post-transcriptionally regulate gene expression by binding to messenger RNAs (mRNAs) to promote degradation or intervention of translational processes. In this review, we focus specifically on the notion that mis-regulation of non-coding RNAs such as miRNAs by improper AR-DNA binding are an important component that promotes prostate cancer. We also highlight the role of miR-206 and the interaction of miR-206 and AR within this process, given this is a miRNA known to be regulated by hormones in both breast and prostate cancer.

Comparing the rules of engagement of androgen and glucocorticoid receptors

Despite the diverse physiological activities of androgens and glucocorticoids, the corresponding receptors are very close members of the nuclear-receptor super family. Their action mechanisms show striking similarities, since both receptors recognize very similar DNA-response elements and recruit the same coactivators to their target genes. The specificity of the responses lies mainly in the tissue-specific expression of the receptors and in their ligand specificity. In cells, where both receptors are expressed, the mechanisms leading to the difference in target genes are less obvious. They lie in part in subtle variations of the DNA-binding sites, in cooperativity with other transcription factors and in differential allosteric signals from the DNA and ligand to other receptor domains. We will highlight the different suggestions that might explain the DNA sequence selectivity and will compare the possible allosteric routes between the response elements and the different functions in the transactivation process. The interplay of androgen and glucocorticoid receptors is also highly relevant in clinical settings, where both receptors are therapeutically targeted. We will discuss the possibility that the glucocorticoid and androgen receptors can play partially redundant roles in castration-resistant prostate cancer.

Stepwise androgen receptor dimerization

Androgen-regulated gene expression is a highly coordinated dynamic process mediated by androgen receptor (AR) ligand binding and DNA binding, and by specific AR protein-protein interactions. The latter include DNA-binding domain (D-box) interactions in AR homodimers, and the interaction of the FQNLF motif in the AR N-terminal domain and the coactivator groove in the ligand-binding domain (N/C interaction). We have studied these interactions in AR homodimerization using quantitative imaging techniques. We found that the initial cytoplasmic intramolecular AR N/C interaction after ligand binding is followed by a D-box-dimerization-dependent transition to intermolecular N/C interaction in a proportion of nuclear ARs. The consecutive steps leading to homodimerization are initiated prior to DNA binding. Our data indicate the presence of nuclear pools of both AR homodimers and monomers. On the basis of AR-regulated reporter assays we propose specificity in regulation of gene expression by AR homodimers and monomers mediated by AR domain interactions. Moreover, our findings elucidate important steps in the spatiotemporal organization of AR intra- and inter-molecular interactions.

Structural basis for nuclear hormone receptor DNA binding

The gene family of nuclear receptors is characterized by the presence of a typical, well conserved DNA-binding domain. In general, two zinc coordinating modules are folded such that an α-helix is inserted in the major groove of the DNA-helix displaying a sequence similar to one of two hexameric consensus motifs. Both zinc molecules coordinate four cysteines. Although the DNA-binding domains as well as the hormone response elements are very similar, each nuclear receptor will affect transcription of a specific set of target genes. This is in part due to some important receptor-specific variations on the general theme of DNA interaction. For most nuclear receptors, the DNA-binding domain dimerizes on DNA, which explains why most hormone response elements consist of a repeat of two hexamers. The hexamer dimers can be organized either as direct, inverted or everted repeats with spacers of varying lengths. The DNA can be bound by homodimers, heterodimers and for some orphan receptors, as monomer. Another key element for DNA binding by nuclear receptors is the carboxy-terminal extension of the DNA-binding domain extending into the hinge region. This part not only co-determines sequence specificity, but also affects other functions of the receptors like nuclear translocation, intranuclear mobility and transactivation potential. Moreover, allosteric signals passing through towards other receptor domains, explain why to some extent, the DNA elements can also be considered as controlling ligands.

Structural and functional analysis of domains of the progesterone receptor

Steroid hormone receptors are multi-domain proteins composed of conserved well-structured regions, such as ligand (LBD) and DNA binding domains (DBD), plus other naturally unstructured regions including the amino-terminal domain (NTD) and the hinge region between the LBD and DBD. The hinge is more than just a flexible region between the DBD and LBD and is capable of binding co-regulatory proteins and the minor groove of DNA flanking hormone response elements. Because the hinge can directly participate in DNA binding it has also been termed the carboxyl terminal extension (CTE) of the DNA binding domain. The CTE and NTD are dynamic regions of the receptor that can adopt multiple conformations depending on the environment of interacting proteins and DNA. Both regions have important regulatory roles for multiple receptor functions that are related to the ability of the CTE and NTD to form multiple active conformations. This review focuses on studies of the CTE and NTD of progesterone receptor (PR), as well as related work with other steroid/nuclear receptors.

Influence of hormone response elements (HREs) on ecdysteroid receptor concentration

Transcriptional activity of nuclear receptors is the result of transactivation capability and receptor protein concentration. The concentration of ecdysteroid receptor (EcR) constitutively expressed in vertebrate cells varies depending on the isoforms. Besides ligand binding and heterodimerization with ultraspiracle (USP), which stabilizes receptor protein concentration, degradation is regulated by interaction of the receptor complex with different ecdysteroid response elements (EcREs). Coexpression of EcREs significantly reduces ecdysteroid receptor concentration depending on the type of EcRE. Transcriptional activity and interaction with hormone response elements (HREs) as determined by Electrophoretic Mobility Shift Assay (EMSA) are often inversely related to receptor protein concentration. The complex regulation of receptor protein concentration offers an additional opportunity to regulate transcriptional activity in an isoform- and target cell-specific manner and allows the temporal limitation of hormone action.

The N-terminus of ecdysteroid receptor isoforms and ultraspiracle interacts with different ecdysteroid response elements in a sequence specific manner to modulate transcriptional activity

The functional insect ecdysteroid receptor is comprised of two nuclear receptors, the ecdysteroid receptor (EcR) and the RXR homologue, ultraspiracle (USP), which form a heterodimer. The dimer recognizes various hormone response elements and the effect of these elements on transcriptional activity of EcR isoforms was determined in vertebrate cells transfected with EcR and USP. Only constitutive activity mediated by the core response elements was preserved after elimination of nonspecific binding sites on the DNA of the vector. The constitutive transcriptional activity was regulated in a complex manner by the N-termini of both EcR and USP, the DBD of USP and the type and number of hormone response elements (HRE). Cooperative effects at oligomeric response elements particularly DR1 depended on the type of ecdysteroid response element and the N-termini of EcR and USP. The DBD of USP abolishes or attenuates synergistic effects. The data show that in the absence of hormone, transcriptional activity is regulated in a complex manner that offers additional possibilities for ecdysteroid receptor mediated gene regulation during development.

Molecular basis for gene-specific transactivation by nuclear receptors

Nuclear receptors (NRs) are key transcriptional regulators of metazoan physiology and metabolism. Different NRs bind to similar or even identical core response elements; however, they regulate transcription in a highly receptor- and gene-specific manner. These differences in gene activation can most likely be accounted for by mechanisms involving receptor-specific interactions with DNA as well as receptor-specific interactions with protein complexes binding to adjacent and distant DNA sequences. Here, we review key molecular aspects of transactivation by NRs with special emphasis on the recent advances in the molecular mechanisms responsible for receptor- and gene-specific transcriptional activation. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

A 629RKLKK633 motif in the hinge region controls the androgen receptor at multiple levels

The androgen receptor protein has specific domains involved in DNA binding, ligand binding, and transactivation, whose activities need to be integrated during transcription activation. The hinge region, more particular a (629)RKLKK(633) motif, seems to play a crucial role in this process. Indeed, although the motif is not part of the DNA-binding domain, its positive residues are involved in optimal DNA binding and nuclear translocation as shown by mutation analysis. When the mutated ARs are forced into the nucleus, however, the residues seem to play different roles in transactivation. Moreover, we show by FRAP analysis that during activation, the AR is distributed in the nucleus in a mobile and two immobile fractions, and that mutations in the (629)RKLKK(633) motif affect the distribution of the AR over these three intranuclear fractions. Taken together, the (629)RKLKK(633) motif is a multifunctional motif that integrates nuclear localization, receptor stability, DNA binding, transactivation potential and intranuclear mobility.

Inhibition of cyclin D1 expression by androgen receptor in breast cancer cells--identification of a novel androgen response element

Cyclin D1 gene (CCND1) is a critical mitogen-regulated cell-cycle control element whose transcriptional modulation plays a crucial role in breast cancer growth and progression. Here we demonstrate that the non-aromatizable androgen 5-α-dihydrotestosterone (DHT) inhibits endogenous cyclin D1 expression, as evidenced by reduction of cyclin D1 mRNA and protein levels, and decrease of CCND1-promoter activity, in MCF-7 cells. The DHT-dependent inhibition of CCND1 gene activity requires the involvement and the integrity of the androgen receptor (AR) DNA-binding domain. Site directed mutagenesis, DNA affinity precipitation assay, electrophoretic mobility shift assay and chromatin immunoprecipitation analyses indicate that this inhibitory effect is ligand dependent and it is mediated by direct binding of AR to an androgen response element (CCND1-ARE) located at −570 to −556-bp upstream of the transcription start site, in the cyclin D1 proximal promoter. Moreover, AR-mediated repression of the CCND1 involves the recruitment of the atypical orphan nuclear receptor DAX1 as a component of a multiprotein repressor complex also embracing the participation of Histone Deacetylase 1. In conclusion, identification of the CCND1-ARE allows defining cyclin D1 as a specific androgen target gene in breast and might contribute to explain the molecular basis of the inhibitory role of androgens on breast cancer cells proliferation.

The rules of DNA recognition by the androgen receptor

The androgen receptor (AR) and glucocorticoid, progestagen, and mineralocorticoid receptors all recognize classical DNA response elements that are organized as inverted repeats of 5'-AGAACA-3'-like motifs with a three-nucleotide spacer. Next to such elements, the AR also recognizes a second type of androgen response element (ARE), the so-called selective AREs, which resemble more the direct repeats of the same hexamer. In this work, we show that not only the AR but also the progestagen receptor can recognize the selective AREs, whereas neither glucocorticoid nor mineralocorticoid receptor can. Recently, genomic AR-binding fragments have been postulated to contain AR-binding sites that diverge considerably from the classical ARE consensus. Extensive mutational analyses of these candidate motifs, however, reinstalls the values of the consensus sequence for the AREs as mentioned above, the importance of their dimeric nature and the presence of exactly three-nucleotide spacing. We developed a position-specific probability matrix that was used to predict with higher accuracy new AREs in different AR-binding regions. So far, all AR-binding genomic fragments that were analyzed contain AREs defined as receptor-dimer binding motifs with the ability to confer responsiveness to a reporter gene.

Identification of novel truncated androgen receptor (AR) mutants including unreported pre-mRNA splicing variants in the 22Rv1 hormone-refractory prostate cancer (PCa) cell line

Advanced prostate cancer (PCa) has emerged as a public health concern due to population aging. Although androgen deprivation has proven efficacy in this condition, most advanced PCa patients will have to face failure of androgen deprivation as a treatment. Mutations in the androgen receptor (AR) from tumor cells have been shown to induce androgen independency both in PCa cell lines and in the clinic. We have investigated the molecular events leading to androgen independency in the 22Rv1 cell line, a commonly used preclinical model of PCa. Besides AR mutants that have been described so far, including nonsense mutations, recent data have focused on AR pre-mRNA aberrant splicing as a new mechanism leading to constitutively active truncated AR variants. In this article, we describe two novel variants arising from aberrant splicing of AR pre-mRNA, characterized by long mRNA transcripts that encode truncated, constitutively active proteins. We also describe several new nonsense mutants that share ligand independency and transcriptional activity. Finally, we show that alongside these mutants, 22Rv1 cells also express a mutant AR lacking exon 3 tandem duplication, a major feature of this cell line. By describing unreported AR mutants in the 22Rv1 cell line, our data emphasize the complexity and heterogeneity of molecular events that occur in preclinical models, and supposedly in the clinic. Future work on the 22Rv1 cell line should take into account the concomitant expression of various AR mutants.

Androgen receptor inhibits estrogen receptor-alpha activity and is prognostic in breast cancer

There is emerging evidence that the balance between estrogen receptor-alpha (ER(alpha)) and androgen receptor (AR) signaling is a critical determinant of growth in the normal and malignant breast. In this study, we assessed AR status in a cohort of 215 invasive ductal breast carcinomas. AR and (ER(alpha)) were coexpressed in the majority (80-90%) of breast tumor cells. Kaplan-Meier product limit analysis and multivariate Cox regression showed that AR is an independent prognostic factor in (ER(alpha))-positive disease, with a low level of AR (less than median of 75% positive cells) conferring a 4.6-fold increased risk of cancer-related death (P = 0.002). Consistent with a role for AR in breast cancer outcome, AR potently inhibited (ER(alpha))transactivation activity and 17beta-estradiol-stimulated growth of breast cancer cells. Transfection of MDA-MB-231 breast cancer cells with either functionally impaired AR variants or the DNA-binding domain of the AR indicated that the latter is both necessary and sufficient for inhibition of (ER(alpha)) signaling. Consistent with molecular modeling, electrophoretic mobility shift assays showed binding of the AR to an estrogen-responsive element (ERE). Evidence for a functional interaction of the AR with an ERE in vivo was provided by chromatin immunoprecipitation data, revealing recruitment of the AR to the progesterone receptor promoter in T-47D breast cancer cells. We conclude that, by binding to a subset of EREs, the AR can prevent activation of target genes that mediate the stimulatory effects of 17beta-estradiol on breast cancer cells.

Bioactivity-based screening of antibiotics and hormones

Bioactivity-based screening methods are relatively cheap, quick and easy to use tools. Especially with respect to antimicrobial residues and compounds with hormonal activity, they form a very cost-effective alternative to physical chemical methods in large-scale surveillance and monitoring programs, where their main purpose is to identify samples that require additional chemical confirmation. A major advantage is their intrinsic capability to detect unknown compounds and new hazards. This review shows an overview of the available methods and their potential and limitations for regulatory control.

Targeting the BAF57 SWI/SNF subunit in prostate cancer: a novel platform to control androgen receptor activity

The androgen receptor (AR) is critical for disseminated prostate cancer proliferation and survival. AR activity is targeted either through prevention of ligand synthesis or through the use of antagonists that bind the COOH-terminal ligand-binding domain. Although initially effective, treatment fails due to restored AR activity in the presence of therapeutics. Thus, new means must be developed to target AR activity. The SWI/SNF chromatin remodeling complex is critical for AR transcriptional activity, and the BAF57 SWI/SNF subunit facilitates direct interaction with the receptor. Although selected SWI/SNF subunit expression is reduced in prostate cancer, we show that BAF57 is retained in human disease and is elevated in a subset of tumors. Functional analyses showed that BAF57 contributes uniquely to androgen-mediated stimulation of transcription without compromising the effectiveness of AR antagonists. Subsequent studies revealed that BAF57 is recruited to the AR DNA-binding domain/hinge region, which occurs concomitant with receptor activation. These data provided the basis for a novel inhibitor derived from BAF57 [BAF57 inhibitory peptide (BIPep)], which blocked AR residence on chromatin and resultant AR-dependent gene activation. Importantly, BIPep expression was sufficient to inhibit androgen-dependent prostate cancer cell proliferation in AR-positive cells. In summary, these data identify blockade of AR-BAF57 interaction as a novel means to target agonist-induced AR function in prostate cancer, and provide the first evidence that abrogation of SWI/SNF function can be developed as a point of therapeutic intervention in prostate cancer.

The contribution of different androgen receptor domains to receptor dimerization and signaling

The androgen receptor (AR) is a ligand-activated transcription factor of the nuclear receptor superfamily that plays a critical role in male physiology and pathology. Activated by binding of the native androgens testosterone and 5alpha-dihydrotestosterone, the AR regulates transcription of genes involved in the development and maintenance of male phenotype and male reproductive function as well as other tissues such as bone and muscle. Deregulation of AR signaling can cause a diverse range of clinical conditions, including the X-linked androgen insensitivity syndrome, a form of motor neuron disease known as Kennedy's disease, and male infertility. In addition, there is now compelling evidence that the AR is involved in all stages of prostate tumorigenesis including initiation, progression, and treatment resistance. To better understand the role of AR signaling in the pathogenesis of these conditions, it is important to have a comprehensive understanding of the key determinants of AR structure and function. Binding of androgens to the AR induces receptor dimerization, facilitating DNA binding and the recruitment of cofactors and transcriptional machinery to regulate expression of target genes. Various models of dimerization have been described for the AR, the most well characterized interaction being DNA-binding domain- mediated dimerization, which is essential for the AR to bind DNA and regulate transcription. Additional AR interactions with potential to contribute to receptor dimerization include the intermolecular interaction between the AR amino terminal domain and ligand-binding domain known as the N-terminal/C-terminal interaction, and ligand-binding domain dimerization. In this review, we discuss each form of dimerization utilized by the AR to achieve transcriptional competence and highlight that dimerization through multiple domains is necessary for optimal AR signaling.

Diverse roles of androgen receptor (AR) domains in AR-mediated signaling

Androgens control male sexual development and maintenance of the adult male phenotype. They have very divergent effects on their target organs like the reproductive organs, muscle, bone, brain and skin. This is explained in part by the fact that different cell types respond differently to androgen stimulus, even when all these responses are mediated by the same intracellular androgen receptor. To understand these tissue- and cell-specific readouts of androgens, we have to learn the many different steps in the transcription activation mechanisms of the androgen receptor (NR3C4). Like all nuclear receptors, the steroid receptors have a central DNA-binding domain connected to a ligand-binding domain by a hinge region. In addition, all steroid receptors have a relatively large amino-terminal domain. Despite the overall structural homology with other nuclear receptors, the androgen receptor has several specific characteristics which will be discussed here. This receptor can bind two types of androgen response elements (AREs): one type being similar to the classical GRE/PRE-type elements, the other type being the more divergent and more selective AREs. The hormone-binding domain has low intrinsic transactivation properties, a feature that correlates with the low affinity of this domain for the canonical LxxLL-bearing coactivators. For the androgen receptor, transcriptional activation involves the alternative recruitment of coactivators to different regions in the amino-terminal domain, as well as the hinge region. Finally, a very strong ligand-induced interaction between the amino-terminal domain and the ligand-binding domain of the androgen receptor seems to be involved in many aspects of its function as a transcription factor. This review describes the current knowledge on the structure-function relationships within the domains of the androgen receptor and tries to integrate the involvement of different domains, subdomains and motifs in the functioning of this receptor as a transcription factor with tissue- and cell-specific readouts.

Identification of androgen-selective androgen-response elements in the human aquaporin-5 and Rad9 genes

The AR (androgen receptor) is known to influence the expression of its target genes by binding to different sets of AREs (androgen-response elements) in the DNA. One set consists of the classical steroid-response elements which are partial palindromic repeats of the 5'-TGTTCT-3' steroid-receptor monomer-binding element. The second set contains motifs that are AR-specific and that are proposed to be partial direct repeats of the same motif. On the basis of this assumption, we used an in silico approach to identify new androgen-selective AREs in the regulatory regions of known androgen-responsive genes. We have used an extension of the NUBIScan algorithm to screen a collection of 85 known human androgen-responsive genes compiled from literature and database searches. We report the evaluation of the most promising hits resulting from this computational search by in vitro DNA-binding assays using full-size ARs and GRs (glucocorticoid receptors) as well as their isolated DBDs (DNA-binding domains). We also describe the ability of some of these motifs to confer androgen-, but not glucocorticoid-, responsiveness to reporter-gene expression. The elements found in the aquaporin-5 and the Rad9 (radiation-sensitive 9) genes showed selective AR versus GR binding in band-shift assays and a strong activity and selectivity in functional assays, both as isolated elements and in their original contexts. Our data indicate the validity of the hypothesis that selective AREs are recognizable as direct 5'-TGTTCT-3' repeats, and extend the list of currently known selective elements.

Detailed functional studies on androgen receptor mild mutations demonstrate their association with male infertility

CONTEXT

Mutations in the androgen receptor (AR) gene can cause the androgen insensitivity syndrome (AIS). For complete and severe partial AIS, well-characterized in vitro functional assays can be used for genotype-phenotype correlation; however, for mild forms of AIS, as associated with male infertility, experimental evidence is scarce or lacking. In particular, optimal in vitro functional tests informative about the genotype-phenotype relation have not been described.

OBJECTIVE

The objective of this study was to investigate the association among genotype and phenotype for AR mutations found in infertile males by conventional functional assays and additional in-depth studies performed with several gene reporters.

DESIGN

To this aim, we selected four AR missense mutations associated with isolated male infertility (L547F and two novel mutations A474V and S650G) or partial AIS (Y571H). After introduction of the specific mutations in AR expression plasmid, we performed classical in vitro studies (Western immunoblotting, electrophoretic mobility shift assay, hormone-response curves) and transactivation assays with different reporter constructs (MMTV, Sc-ARU-TK, TAT-GRE- 2X, Slp-ARU-TK and PEM).

RESULTS AND CONCLUSIONS

Our results showed that standard functional tests provide sufficient information only for severe AR mutations, whereas for AR mutations found in mild AIS patients with male infertility, only an extensive analysis with different in vitro systems, and in particular with PEM promoter, can give information on the functionality of the AR and therefore on the pathogenicity of the mutations and on genotype-phenotype correlation.

A new highly androgen specific yeast biosensor, enabling optimisation of (Q)SAR model approaches

Recently we constructed recombinant yeast cells that express the human androgen receptor (hAR) and yeast enhanced green fluorescent protein (yEGFP), the latter in response to androgens. When exposed to 17beta-testosterone, the concentration where half-maximal activation is reached (EC50) was 50 nM. Relative androgenic potencies (RAP), defined as the ratio between the EC50 of 17beta-testosterone and the EC50 of the compound, were 1.7, 1.2 and 0.008 for 19-nortestosterone, tetrahydrogestrinone and 17beta-estradiol respectively. Steroids representative for other hormone receptors, like estrone, 17alpha-ethynylestradiol, and diethylstilbestrol for the estrogen receptor and corticosterone and dexamethasone for the glucocorticoid receptor, showed no agonistic response. Only compounds known to exert androgenic effects give a response. Determined RAPs were in line with results obtained from optimised QSAR model calculations and demonstrated that Saccharomyces cerevisiae showed no metabolism of test compounds and displayed no crosstalk from endogenous hormone receptors. The suitability of this bioassay to verify the outcomes of (Q)SAR models to predict the activities of different steroids was further examined by studies with steroid isomers and a number of designer steroids, confirming that the 17beta-hydroxyl group, 3-keto group and 5alpha-steroidal framework are extremely important for the activity of the androgenic steroid.

A new highly specific and robust yeast androgen bioassay for the detection of agonists and antagonists

Public concern about the presence of natural and anthropogenic compounds which affect human health by modulating normal endocrine functions is continuously growing. Fast and simple high-throughput screening methods for the detection of hormone activities are thus indispensable. During the last two decades, a panel of different in vitro assays has been developed, mainly for compounds with an estrogenic mode of action. Here we describe the development of an androgen transcription activation assay that is easy to use in routine screening. Recombinant yeast cells were constructed that express the human androgen receptor and yeast enhanced green fluorescent protein (yEGFP), the latter in response to androgens. Compared with other reporters, the yEGFP reporter protein is very convenient because it is directly measurable in intact living cells, i.e., cell wall disruption and the addition of a substrate are not needed. When yeast was exposed to 17beta-testosterone, the concentration where half-maximal activation is reached (EC(50)) was 50 nM. The relative androgenic potencies, defined as the ratio between the EC(50) of 17beta-testosterone and the EC(50) of the compound, of 5alpha-dihydrotestosterone, methyltrienolone, and 17beta-boldenone are 2.3, 1.4, and 0.15 respectively. The results presented in this paper demonstrate that this new yeast androgen bioassay is fast, sensitive, and very specific and also suited to detect compounds that have an antiandrogenic mode of action.

A splicing variant of the androgen receptor detected in a metastatic prostate cancer exhibits exclusively cytoplasmic actions

The androgen receptor (AR) is a ligand-activated transcription factor that displays genomic actions characterized by binding to androgen-response elements in the promoter of target genes as well as nongenomic actions that do not require nuclear translocation and DNA binding. In this study, we report exclusive cytoplasmic actions of a splicing variant of the AR detected in a metastatic prostate cancer. This AR variant, named AR23, results from an aberrant splicing of intron 2, wherein the last 69 nucleotides of the intronic sequence are retained, leading to the insertion of 23 amino acids between the two zinc fingers in the DNA-binding domain. We show that the nuclear entry of AR23 upon dihydrotestosterone (DHT) stimulation is impaired. Alternatively, DHT-activated AR23 forms cytoplasmic and perinuclear aggregates that partially colocalize with the endoplasmic reticulum and are devoid of genomic actions. However, in LNCaP cells, this cytoplasmic DHT-activated AR23 remains partially active as evidenced by the activation of transcription from androgen-responsive promoters, the stimulation of NF-kappaB transcriptional activity and by the decrease of AP-1 transcriptional activity. Our data reveal novel cytoplasmic actions for this splicing AR variant, suggesting a contribution in prostate cancer progression.

The hinge region regulates DNA binding, nuclear translocation, and transactivation of the androgen receptor

The androgen receptor (AR) encoding gene can undergo mutations during the development and treatment of prostate cancer. Even in hormone-independent stages, mutations in the receptor paradoxically seem to result in an increased AR function. Two such point mutations have been described in the part of the AR involved in DNA binding and nuclear translocation, namely the hinge region. Despite a decreased nuclear translocation, these mutant ARs display increased transactivating potencies. Through detailed analysis of the hinge region, we found that deletion of residues 629 to 636 resulted in a stronger androgen response on different reporters, although this mutant displays an extremely low in vitro affinity for androgen response elements. This superactivity is independent of nuclear localization and can be inhibited by antiandrogens. Surprisingly, the AR activation functions, AF1 and AF2, are not dramatically affected when the inhibitory region (629-RKLKKLGN-636) is deleted, although cotransfected p160 coactivator TIF2 had a stronger potentiating effect in the absence of this motif. The ligand-dependent interaction between the amino-terminal domain and the ligand-binding domain (N/C interaction) plays an important role in transactivation by the AR. We found that this interaction is strongly enhanced by deletion of the inhibitory region. In conclusion, the description of prostate cancer mutations has led to the discovery of a complex role of the hinge region in nuclear localization, DNA binding, coactivator recruitment, and N/C interaction of the AR.

Identification of androgen response elements in the insulin-like growth factor I upstream promoter

Testosterone stimulates the expression of IGF-I in cells and tissues that include prostate, muscle and muscle satellite cells, and the uterus. Here, the molecular mechanisms of this effect of testosterone were explored. Testosterone increased IGF-I mRNA levels in HepG2 and LNCaP cells and stimulated the activity of reporter genes controlled by 1.6 kb of the upstream promoter of the human IGF-I gene. An androgen-responsive region that was located between -1320 and -1420 bases upstream of the first codon was identified by truncation studies. The androgen-responsive region was found to contain two sequences resembling known androgen receptor (AR)-binding sites from the Pem1 gene. Reporter genes incorporating these sequences were strongly stimulated by androgens. Each of the androgen-responsive elements (AREs) bound recombinant AR-DNA-binding domain in gel-shift experiments; binding was greatly enhanced by sequences flanking the apparent AR-binding half-sites. Testosterone induced recruitment of AR to sequences of genomic DNA containing these AREs. The two AREs were activated 5-fold more by AR than glucocorticoid receptor. Collectively, these findings indicate the presence of two AREs within the IGF-I upstream promoter that act in cis to activate IGF-I expression. These AREs seem likely to contribute to the up-regulation of the IGF-I gene in prostate tissues, HepG2 cells, and potentially other tissues.

Loss of androgen receptor binding to selective androgen response elements causes a reproductive phenotype in a knockin mouse model

Androgens influence transcription of their target genes through the activation of the androgen receptor (AR) that subsequently interacts with specific DNA motifs in these genes. These DNA motifs, called androgen response elements (AREs), can be classified in two classes: the classical AREs, which are also recognized by the other steroid hormone receptors; and the AR-selective AREs, which display selectivity for the AR. For in vitro interaction with the selective AREs, the androgen receptor DNA-binding domain is dependent on specific residues in its second zinc-finger. To evaluate the physiological relevance of these selective elements, we generated a germ-line knockin mouse model, termed SPARKI (SPecificity-affecting AR KnockIn), in which the second zinc-finger of the AR was replaced with that of the glucocorticoid receptor, resulting in a chimeric protein that retains its ability to bind classical AREs but is unable to bind selective AREs. The reproductive organs of SPARKI males are smaller compared with wild-type animals, and they are also subfertile. Intriguingly, however, they do not display any anabolic phenotype. The expression of two testis-specific, androgen-responsive genes is differentially affected by the SPARKI mutation, which is correlated with the involvement of different types of response elements in their androgen responsiveness. In this report, we present the first in vivo evidence of the existence of two functionally different types of AREs and demonstrate that AR-regulated gene expression can be targeted based on this distinction.

Androgens regulate protein kinase Cdelta transcription and modulate its apoptotic function in prostate cancer cells

Activation of protein kinase Cdelta (PKCdelta), a member of the novel PKC family, leads to apoptosis in several cell types. Although the molecular bases of PKCdelta activation are being unfolded, limited information is available on the mechanisms that control its expression. Here, we report that in prostate cancer cells PKCdelta is tightly regulated by androgens at the transcriptional level. Steroid depletion from the culture medium causes a pronounced down-regulation of PKCdelta protein and mRNA in androgen-sensitive LNCaP prostate cancer cells, an effect that is rescued by the androgen R1881 in an androgen receptor (AR)-dependent manner. Analysis of the PKCdelta promoter revealed a putative androgen responsive element (ARE) located 4.7 kb upstream from the transcription start site. Luciferase reporter assays show that this element is highly responsive to androgens, and mutations in key nucleotides in the AR-binding consensus abolish reporter activity. Furthermore, using chromatin immunoprecipitation assays, we determined that the AR binds in vivo to the PKCdelta ARE in response to androgen stimulation. Functional studies revealed that, notably, androgens modulate phorbol 12-myristate 13-acetate (PMA)-induced apoptosis in LNCaP cells, an effect that is dependent on PKCdelta. Indeed, androgen depletion or AR RNA interference severely impaired the apoptotic function of PKCdelta or the activation of p38, a downstream effector of PKCdelta in LNCaP cells--effects that can be rescued by restoring PKCdelta levels using an adenoviral delivery approach. Our studies identified a novel hormonal mechanism for the control of PKCdelta expression via transcriptional regulation that fine-tunes the magnitude of PKCdelta apoptotic responses.

c-Jun enhancement of androgen receptor transactivation is associated with prostate cancer cell proliferation

Androgens and the androgen receptor (AR) are involved in the growth and progression of prostate cancer. Our previous studies suggest that the proto-oncoprotein c-Jun is an AR coactivator that stimulates AR transactivation by mediating receptor dimerization and subsequent DNA binding. To study the physiological relevance of this c-Jun activity on AR, we have generated stable LNCaP cell lines expressing different levels of c-Jun. These cell lines exhibit a direct correlation between endogenous c-Jun levels and AR transcriptional activity and expression of endogenous androgen-regulated genes. Disruption by antisense RNA of endogenous c-Jun expression in LNCaP cells strongly compromises the androgen-dependent proliferation of these cells. In contrast, expression of a c-Jun mutant, which is fully active in coactivation of AR but deficient in AP-1 transactivation, significantly enhances androgen-dependent proliferation. This finding indicates that the coactivation function of c-Jun is sufficient for regulating androgen-induced growth of LNCaP cells. c-Jun also enhances AR transactivtion in androgen-independent LNCaP cells, which closely mimic hormone-refractory prostate cancer cells in gene expression and growth behavior. Importantly, siRNA-mediated repression of endogenous c-Jun expression results in markedly reduced growth of these cells, strongly suggesting an important biological role for c-Jun in hormone-refractory prostate cancer.

Interplay between two hormone-independent activation domains in the androgen receptor

The androgen receptor (AR) plays a key role in prostate cancer development, as well as its treatments, even for the hormone-refractory state. Here, we report that an earlier described lysine-to-arginine mutation at position 179 in AR leads to a more potent AR. We show that two activation domains (Tau-1 and Tau-5) are necessary and sufficient for the full activity of AR and the intrinsic activity of the AR-NTD. Two alpha-helices surrounding the Lys179 define the core of Tau-1, which can act as an autonomous activation function, independent of p160 coactivators. Furthermore, we show that although the recruitment of p160 coactivators is mediated through Tau-5, this event is attenuated by core Tau-1. This better definition of the mechanisms of action of both Tau-1 and Tau-5 is instrumental for the design of alternative therapeutic strategies against prostate cancer.

Transfection with steroid-responsive reporter constructs shows glucocorticoid rather than androgen responsiveness in cultured Sertoli cells

It remains unclear why it has proven so difficult to identify androgen target genes in cultured Sertoli cells. Given the lack of useful endogenous reporter genes, we studied the androgen and glucocorticoid responsiveness of these cells by transfection with three different steroid-responsive reporter constructs. The constructs were driven by the tyrosine aminotransferase steroid-responsive region (TAT-GRE4x-Luc), the mouse mammary tumor virus promoter (MMTV-Luc) and the Pem homeobox gene proximal promoter respectively (Pem-Luc). These constructs can be activated either by both the glucocorticoid receptor (GR) and the androgen receptor (AR) (TAT-GRE4x-Luc and MMTV-Luc) or selectively by the AR (Pem-Luc). Despite high transfection efficiency (30-40%) none of the constructs could be activated by treatment of the Sertoli cells with testosterone, 5alpha-dihydrotestosterone or synthetic androgens. Even pretreatment with follicle-stimulating hormone to raise AR levels (from 31 up to 82fmol/mg protein) did not result in androgen responsiveness. In contrast, treatment with dexamethasone markedly stimulated TAT-GRE4x-Luc and MMTV-Luc activity. GR levels reached a value of 172fmol/mg protein in the cultured cells and both AR and GR displayed homogeneous distribution by immunocytochemical evaluation. Androgen responsiveness was restored and glucocorticoid responsiveness was increased by cotransfection with AR or GR expression constructs. Under cotransfection conditions, 1nM of testosterone (a concentration that is some 100 times lower than that estimated to be present in the testis) was sufficient to stimulate the TAT-GRE4x-Luc maximally. Our data indicate that cultured Sertoli cells respond better to glucocorticoids than to androgens and that one of the factors limiting androgen responsiveness is the availability of AR. Other factors limiting the transactivation capacity of the (endogenous) AR, however, cannot be excluded.

Expression of androgen and estrogen receptors in sertoli cells: studies using the mouse SK11 cell line

Sertoli cells (Sc) play a major role in the establishment and maintenance of spermatogenesis. In the adult testis, Sc contain androgen receptor (AR) and estrogen receptor (ER)-beta but exhibit a loss of steroid responsiveness when maintained in primary culture. In the present study, we demonstrated that a transformed murine cell line (SK11) has retained a Sc phenotype and remains steroid responsive. SK11 cells expressed mRNAs found in Sc (aromatase, sulfated glycoprotein-1, sulfated glycoprotein-2, GATA-1, Sry-type high-mobility-group box transcription factor-9, testatin, dosage-sensitive sex reversal-adrenal hypoplasia congenita critical region on the X chromosome, gene 1) including those for AR and ERbeta but not ERalpha. AR and ERbeta were immunolocalized to cell nuclei, and their ability to activate gene expression was investigated using transient transfections with reporter constructs containing either 3xERE or pem-androgen-responsive element promoters. Expression of the 3xERE reporter was induced after incubation with 17beta-estradiol (E2), 5alpha-androstane-3-beta, 17beta-diol (3betaAdiol), or testosterone (T); up-regulation of the pem-androgen-responsive element reporter was detected only in the presence of T or dihydrotestosterone. Activation of the ERE reporter did not occur after targeted knockdown of ERbeta mRNA. Expression of AR and ERbeta mRNAs was increased after incubation of cells with T or E2, respectively. In conclusion, we have demonstrated that the SK11 Sc cell line contains functional AR and ERbeta and that treatment of the cells with their respective steroids results in an increase in the amount of their mRNAs. Our results suggest that E2 or 3betaAdiol acting via ERbeta might modulate Sc function in vivo and that SK11 cells provide a useful model that can be used to complement studies using Sc selective gene ablation.

The hinge region of the androgen receptor plays a role in proteasome-mediated transcriptional activation

To investigate the function of the hinge region in transcriptional activation by the androgen receptor, we compared the actions of the wild-type receptor with a mutant receptor, deleted of amino acids 628-646 of the hinge. The role of the proteasome on the expression and activity of these two proteins was investigated. The deletion mutant demonstrated a threefold increase in transcriptional activity when compared to the wild-type receptor protein. Furthermore, we found that hormone-dependent stabilization of the receptor protein was more enhanced for the deletion mutant. In addition, experiments using the proteasome inhibitor, MG132, demonstrated that the deletion mutant is more sensitive to proteasome-mediated degradation than the wild-type receptor. However, inhibition of the proteasome had a negative effect on the transcriptional activity of the deletion mutant. Taken together, our results suggest that the hinge region not only plays an important role in controlling the transactivation potential of the androgen receptor but also in determining the influence of the proteasome on androgen receptor-mediated transcriptional activation.

The role of DNA response elements as allosteric modulators of steroid receptor function

Steroid receptors are ligand-activated transcription factors which control the expression of their target genes by binding to specific DNA elements. Consensus response elements have been delineated for the glucocorticoid, androgen, progesterone and mineralocorticoid receptors on one hand (steroid response element, SRE) and for the estrogen receptor on the other hand (estrogen response element, ERE). Small variations in these sequences not only affect the binding but may also have a dramatic impact on the transcriptional activity of steroid receptors. It has now become obvious that DNA response elements do not merely tether regulatory proteins to control regions of target genes but may additionally impart conformational changes onto the DNA-binding domain as well as to neighbouring domains of steroid receptors. This in turn will create unique platforms for selective recruitment of cofactors and possibly for induction of modifications in local chromatin architecture. An additional level of complexity is added by the frequent presence of multiple response elements in gene promoter regions. The allosteric effects of DNA response elements on steroid receptors may be essential for differential gene expression and this offers interesting perspectives for the identification of selective modulators.

Structure of the murine tenascin-R gene and functional characterisation of the promoter

The tenascin-R (TN-R) gene encodes a multidomain extracellular matrix glycoprotein belonging to the tenascin family. It is detectable mainly in oligodendrocytes and neuronal subpopulations of the central nervous system. In this report, we describe the structure of the 5'-region of the mouse TN-R gene and characterise the activity of its promoter. By in silico cloning and genome walking, we have deduced the organisation of the gene and identified the promoter sequence by 5'-RACE technology. TN-R transcripts in adult mouse brain contain non-coding exons 1 and 2 as demonstrated by the reverse transcriptase-polymerase chain reaction. The promoter displays its activity in cultured cells of neural origin, but not in a fibroblast-like cell line or an undifferentiated teratocarcimoma cell line. As for the human and rat genes, the elements required for the full and cell type-specific activity of the promoter are contained in exon 1 and 167 bp upstream of this exon. The mouse TN-R promoter sequence is similar to that of rat and human in that it displays similarly unusual features: it lacks any classical TATA-box or CAAT-box, GC-rich regions or initiator elements. The promoter contains consensus sequences for binding of a variety of transcription factors, notably p53/p73 and glucocorticoid receptors.

Implications of a polyglutamine tract in the function of the human androgen receptor

The androgen receptor (AR) is a ligand-dependent transcription factor and belongs to the nuclear receptor family. The AR gene contains a long polymorphic CAG repeat, coding for a polyglutamine tract. In the full size AR, the deletion of the polyglutamine tract results in an increase in the transactivation through canonical AREs. However, this effect is clearly dependent on the response elements, since it is not observed on selective elements. In our assays, a deletion of the repeat positively affected the interactions of the ligand-binding domain with the amino-terminal domain as well as the recruitment of the p160 coactivator SRC-1e to the amino-terminal domain of the AR. This is reflected by an enhanced coactivation of the AR by SRC-1e.

Dual function of an amino-terminal amphipatic helix in androgen receptor-mediated transactivation through specific and nonspecific response elements

Steroid receptors are transcription factors that, upon binding to their response elements, regulate the expression of several target genes via direct protein interactions with transcriptional coactivators. For the androgen receptor, additional interactions between the amino- and carboxyl-terminal regions have been reported. The first amino acids of the amino-terminal domain are necessary for this amino/carboxyl-terminal interaction. Deletion of a FQNLF core sequence in this region blunts the interaction, as does a G21E mutation. We investigated the effect of the aforementioned mutations in the context of the full size androgen receptor on a series of selective and nonselective androgen response elements. Strikingly, the FQNLF deletion strongly reduced the androgen receptor capacity to transactivate through nonselective motifs but did not affect its activity on selective elements. Although the G21E mutation strongly impairs the amino/carboxyl-terminal interaction, it does not significantly influence androgen receptor activity on either selective or nonselective elements. Surprisingly, this mutation leads to an increased binding of the amino-terminal domain to the glutamine-rich region of the steroid receptor coactivator-1 of the p160 family. Taken together, these data suggest that the amino-terminal amino acids of the androgen receptor play a key role in determining its transcriptional activity by modulating the interaction with the ligand-binding domain as well as interaction with p160 coactivators.