Evidence for an anti-parallel orientation of the ligand-activated human androgen receptor dimer

Regulating Androgen Receptor Function in Prostate Cancer: Exploring the Diversity of Post-Translational Modifications

Androgen receptor (AR) transcriptional activity significantly influences prostate cancer (PCa) progression. In addition to ligand stimulation, AR transcriptional activity is also influenced by a variety of post-translational modifications (PTMs). A number of oncogenes and tumor suppressors have been observed leveraging PTMs to influence AR activity. Subjectively targeting these post-translational modifiers based on their impact on PCa cell proliferation is a rapidly developing area of research. This review elucidates the modifiers, contextualizes the effects of these PTMs on AR activity, and connects these cellular interactions to the progression of PCa.

Interactions governing transcriptional activity of nuclear receptors

The key players in transcriptional regulation are transcription factors (TFs), proteins that bind specific DNA sequences. Several mechanisms exist to turn TFs ‘on’ and ‘off’, including ligand binding which induces conformational changes within TFs, subsequently influencing multiple inter- and intramolecular interactions to drive transcriptional responses. Nuclear receptors are a specific family of ligand-regulated TFs whose activity relies on interactions with DNA, coregulator proteins and other receptors. These multidomain proteins also undergo interdomain interactions on multiple levels, further modulating transcriptional outputs. Cooperation between these distinct interactions is critical for appropriate transcription and remains an intense area of investigation. In this review, we report and summarize recent findings that continue to advance our mechanistic understanding of how interactions between nuclear receptors and diverse partners influence transcription.

The role of ubiquitination in spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is a neurodegenerative and neuromuscular genetic disease caused by the expansion of a polyglutamine-encoding CAG tract in the androgen receptor (AR) gene. The AR is an important transcriptional regulator of the nuclear hormone receptor superfamily; its levels are regulated in many ways including by ubiquitin-dependent degradation. Ubiquitination is a post-translational modification (PTM) which plays a key role in both AR transcriptional activity and its degradation. Moreover, the ubiquitin-proteasome system (UPS) is a fundamental component of cellular functioning and has been implicated in diseases of protein misfolding and aggregation, including polyglutamine (polyQ) repeat expansion diseases such as Huntington’s disease and SBMA. In this review, we discuss the details of the UPS system, its functions and regulation, and the role of AR ubiquitination and UPS components in SBMA. We also discuss aspects of the UPS that may be manipulated for therapeutic effect in SBMA.

Structural and molecular determinants of mineralocorticoid receptor signalling

During the past decades, the mineralocorticoid receptor (MR) has evolved from a much-overlooked member of the steroid hormone receptor family to an important player, not only in volume and electrolyte homeostasis but also in pathological changes occurring in an increasing number of tissues, especially the renal and cardiovascular systems. Simultaneously, a wealth of information about the structure, interaction partners and chromatin requirements for genomic signalling of steroid hormone receptors became available. However, much of the information for the MR has been deduced from studies of other family members and there is still a lack of knowledge about MR-specific features in ligand binding, chromatin remodelling, co-factor interactions and general MR specificity-conferring mechanisms that can completely explain the differences in pathophysiological function between MR and its closest relative, the glucocorticoid receptor. This review aims to give an overview of the current knowledge of MR structure, signalling and co-factors modulating its activity.

Steroid receptor-coregulator transcriptional complexes: new insights from CryoEM

Steroid receptors activate gene transcription through recruitment of a number of coregulators to facilitate histone modification, chromatin remodeling, and general transcription machinery stabilization. Understanding the structures of full-length steroid receptor and coregulatory complexes has been difficult due to their large molecular sizes and dynamic structural conformations. Recent developments in cryo-electron microscopy (cryoEM) technology and proteomics have advanced the structural studies of steroid receptor complexes. Here, we will review the insights we learned from cryoEM studies of the estrogen and androgen receptor transcriptional complexes. Despite similar domain organizations, the two receptors have different coregulator interaction modes. The cryoEM structures now have revealed the fundamental differences between the two receptors and their functional mechanisms.

An androgen-independent mechanism underlying the androgenic effects of 3-methylcholanthrene, a potent aryl hydrocarbon receptor agonist

Aryl hydrocarbon receptor (AhR) and androgen receptor (AR) are ligand-activated transcription factors with profound cross-talk between their signal transduction pathways. Previous studies have shown that AhR agonists activate the transcription of AR-regulated genes in an androgen-independent manner; however, the underlying mechanism remains unclear. To decipher this mechanism, we evaluated the effects of 3-methylcholanthrene (3MC), a potent AhR agonist, on the transcription of AR-regulated genes in three AR-expressing cell lines. 3MC induced the expression of not only three representative AR-regulated chromosomal genes but also the exogenous AR-responsive luciferase reporter gene. No significant difference in the 3MC-induced luciferase activity was detected in the presence of SKF-525A, a non-specific inhibitor of CYP enzymes. The androgenic effects of 3MC were diminished by AhR and AR knockdown. Following 3MC treatment, the amount of nuclear AhR and AR increased synchronously. Co-immunoprecipitation revealed that AhR and AR formed a complex in the nucleus of cells treated with 3MC. AR was recruited to the proximal promoter and distal enhancer regions of the PSA gene upon the addition of 3MC. We propose that AhR activated by 3MC forms a complex with unliganded AR which translocates from the cytoplasm to the nucleus. Nuclear AR now binds the transcriptional regulatory region of AR-regulated genes and activates the transcription.

Structural Insights of Transcriptionally Active, Full-Length Androgen Receptor Coactivator Complexes

Steroid receptors activate gene transcription by recruiting coactivators to initiate transcription of their target genes. For most nuclear receptors, the ligand-dependent activation function domain-2 (AF-2) is a primary contributor to the nuclear receptor (NR) transcriptional activity. In contrast to other steroid receptors, such as ERα, the activation function of androgen receptor (AR) is largely dependent on its ligand-independent AF-1 located in its N-terminal domain (NTD). It remains unclear why AR utilizes a different AF domain from other receptors despite that NRs share similar domain organizations. Here, we present cryoelectron microscopy (cryo-EM) structures of DNA-bound full-length AR and its complex structure with key coactivators, SRC-3 and p300. AR dimerization follows a unique head-to-head and tail-to-tail manner. Unlike ERα, AR directly contacts a single SRC-3 and p300. The AR NTD is the primary site for coactivator recruitment. The structures provide a basis for understanding assembly of the AR:coactivator complex and its domain contributions for coactivator assembly and transcriptional regulation.

Recent progress on the role and molecular mechanism of chicken ovalbumin upstream promoter-transcription factor II in cancer

Chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII) is an orphan receptor that regulates the expression of genes involved in development and homeostasis. COUP-TFII is also dysregulated in cancer, where it plays important roles in oncogenesis and malignant progression. Recent studies have also investigated altered microRNA-mediated regulation of COUP-TFII in cancer. Although many investigators have studied the expression and clinical significance of COUP-TFII in several cancer types, there remain many controversies regarding its role in these diseases. In this review, we will describe the functions and underlying molecular mechanisms of COUP-TFII in several cancers, especially colorectal, gastric, breast, and prostate cancer; additionally, we will briefly summarize what is known about microRNA-mediated regulation of COUP-TFII.

Posttranslational regulation of androgen dependent and independent androgen receptor activities in prostate cancer

Prostate cancer (PCa) is the most commonly diagnosed cancer among men in western countries. Androgen receptor (AR) signaling plays key roles in the development of PCa. Androgen deprivation therapy (ADT) remains the standard therapy for advanced PCa. In addition to its ligand androgen, accumulating evidence indicates that posttranscriptional modification is another important mechanism to regulate AR activities during the progression of PCa, especially in castration resistant prostate cancer (CRPC). To date, a number of posttranscriptional modifications of AR have been identified, including phosphorylation (e.g. by CDK1), acetylation (e.g. by p300 and recognized by BRD4), methylation (e.g. by EZH2), ubiquitination (e.g. by SPOP), and SUMOylation (e.g. by PIAS1). These modifications are essential for the maintenance of protein stability, nuclear localization and transcriptional activity of AR. This review summarizes posttranslational modifications that influence androgen-dependent and -independent activities of AR, PCa progression and therapy resistance. We further emphasize that in addition to androgen, posttranslational modification is another important way to regulate AR activity, suggesting that targeting AR posttranslational modifications, such as proteolysis targeting chimeras (PROTACs) of AR, represents a potential and promising alternate for effective treatment of CRPC. Potential areas to be investigated in the future in the field of AR posttranslational modifications are also discussed.

Molecular Mechanisms and Therapeutics for SBMA/Kennedy's Disease

Spinal and bulbar muscular atrophy (SBMA) is a neuromuscular disease caused by a polyglutamine (polyQ) expansion in the androgen receptor (AR). Despite the fact that the monogenic cause of SBMA has been known for nearly 3 decades, there is no effective treatment for this disease, underscoring the complexity of the pathogenic mechanisms that lead to a loss of motor neurons and muscle in SBMA patients. In the current review, we provide an overview of the system-wide clinical features of SBMA, summarize the structure and function of the AR, discuss both gain-of-function and loss-of-function mechanisms of toxicity caused by polyQ-expanded AR, and describe the cell and animal models utilized in the study of SBMA. Additionally, we summarize previously conducted clinical trials which, despite being based on positive results from preclinical studies, proved to be largely ineffective in the treatment of SBMA; nonetheless, these studies provide important insights as researchers develop the next generation of therapies.

From gene to therapy in spinal and bulbar muscular atrophy: Are we there yet?

Abnormal polyglutamine expansions in the androgen receptor (AR) cause a muscular condition, known as Kennedy's disease or spinal and bulbar muscular atrophy (SBMA). The disease is transmitted in an X-linked fashion and is clinically characterized by weakness, atrophy and fasciculations of the limb and bulbar muscles as a result of a toxic gain-of-function of the mutant protein. Notably, affected males also show signs of androgen insensitivity, such as gynaecomastia and reduced fertility. The characterization of the natural history of the disease, the increasing understanding of the mechanism of pathogenesis and the elucidation of the functions of normal and mutant AR have offered a momentum for developing a rational therapeutic strategy for this disease. In this special issue on androgens and AR functions, we will review the molecular, biochemical, and cellular mechanisms underlying the pathogenesis of SBMA. We will discuss recent advances on therapeutic approaches and opportunities for this yet incurable disease, ranging from androgen deprivation, to gene silencing, to an expanding repertoire of peripheral targets, including muscle. With the advancement of these strategies into the clinic, it can be reasonably anticipated that the landscape of treatment options for SBMA and other neuromuscular conditions will change rapidly in the near future.

Structure of the homodimeric androgen receptor ligand-binding domain

The androgen receptor (AR) plays a crucial role in normal physiology, development and metabolism as well as in the aetiology and treatment of diverse pathologies such as androgen insensitivity syndromes (AIS), male infertility and prostate cancer (PCa). Here we show that dimerization of AR ligand-binding domain (LBD) is induced by receptor agonists but not by antagonists. The 2.15-Šcrystal structure of homodimeric, agonist- and coactivator peptide-bound AR-LBD unveils a 1,000-Ų large dimerization surface, which harbours over 40 previously unexplained AIS- and PCa-associated point mutations. An AIS mutation in the self-association interface (P767A) disrupts dimer formation in vivo, and has a detrimental effect on the transactivating properties of full-length AR, despite retained hormone-binding capacity. The conservation of essential residues suggests that the unveiled dimerization mechanism might be shared by other nuclear receptors. Our work defines AR-LBD homodimerization as an essential step in the proper functioning of this important transcription factor.

The androgen receptor is crucial for the development and physiology of reproductive organs. Here the authors present the structure of the androgen receptor ligand-binding domain bound to dihydrotestosterone, identifying a homodimerization interface that is crucial for receptor activity in vivo.

Epigenomic Regulation of Androgen Receptor Signaling: Potential Role in Prostate Cancer Therapy

Androgen receptor (AR) signaling remains the major oncogenic pathway in prostate cancer (PCa). Androgen-deprivation therapy (ADT) is the principle treatment for locally advanced and metastatic disease. However, a significant number of patients acquire treatment resistance leading to castration resistant prostate cancer (CRPC). Epigenetics, the study of heritable and reversible changes in gene expression without alterations in DNA sequences, is a crucial regulatory step in AR signaling. We and others, recently described the technological advance Chem-seq, a method to identify the interaction between a drug and the genome. This has permitted better understanding of the underlying regulatory mechanisms of AR during carcinogenesis and revealed the importance of epigenetic modifiers. In screening for new epigenomic modifiying drugs, we identified SD-70, and found that this demethylase inhibitor is effective in CRPC cells in combination with current therapies. The aim of this review is to explore the role of epigenetic modifications as biomarkers for detection, prognosis, and risk evaluation of PCa. Furthermore, we also provide an update of the recent findings on the epigenetic key processes (DNA methylation, chromatin modifications and alterations in noncoding RNA profiles) involved in AR expression and their possible role as therapeutic targets.

Single-Chain Probes for Illuminating Androgenicity of Chemicals

The present protocol introduces a single-chain probe carrying a functional peptide in the N-terminal domain of the androgen receptor (AR NTD) for illuminating androgenicity of ligands. In the single-chain probe, a functional peptide in the AR NTD was genetically fused to the ligand-binding domain of AR (AR LBD) via a flexible linker, and then sandwiched between the N- and C-terminal fragments of split-firefly luciferase (FLuc) dissected at D415. This single-chain probe exerts (1) a high signal-to-background ratio and (2) sensitive discrimination between agonists and antagonists, where the dimerization of AR LBD is not involved. The present protocol guides a fundamental methodology on how to discriminate weak protein-protein (peptide) binding, and provides a new insight into the intramolecular folding inside monomeric AR.

Structures of androgen receptor bound with ligands: advancing understanding of biological functions and drug discovery

INTRODUCTION

Androgen receptor (AR) is a ligand-dependent transcription factor and a member of the nuclear receptor superfamily. It plays a vital role in male sexual development and regulates gene expression in various tissues, including prostate. Androgens are compounds that exert their biological effects via interaction with AR. Binding of androgens to AR initiates conformational changes in AR that affect binding of co-regulator proteins and DNA. AR agonists and antagonists are widely used in a variety of clinical applications (i.e. hypogonadism and prostate cancer therapy).

AREAS COVERED

This review provides a close look at structures of AR-ligand complexes and mutations in the receptor that have been revealed, discusses current challenges in the field, and sheds light on future directions.

EXPERT OPINION

AR is one of the primary targets for the treatment of prostate cancer, as AR antagonists inhibit prostate cancer growth. However, these drugs are not effective for long-term treatment and lead to castration-resistant prostate cancer. The structures of AR-ligand complexes are an invaluable scientific asset that enhances our understanding of biological functions and mechanisms of androgenic and anti-androgenic chemicals as well as promotes the discovery of superior drug candidates.

Post-translational Down-regulation of Melanoma Antigen-A11 (MAGE-A11) by Human p14-ARF Tumor Suppressor

X-linked primate-specific melanoma antigen-A11 (MAGE-A11) is a human androgen receptor (AR) coactivator and proto-oncogene expressed at low levels in normal human reproductive tract tissues and at higher levels in castration-resistant prostate cancer where it is required for androgen-dependent cell growth. In this report, we show that MAGE-A11 is targeted for degradation by human p14-ARF, a tumor suppressor expressed from an alternative reading frame of the p16 cyclin-dependent kinase inhibitor INK4a/ARF gene. MAGE-A11 degradation by the proteasome was mediated by an interaction with p14-ARF and was independent of lysine ubiquitination. A dose-dependent inverse relationship between MAGE-A11 and p14-ARF correlated with p14-ARF inhibition of the MAGE-A11-induced increase in androgen-dependent AR transcriptional activity and constitutive activity of a splice variant-like AR. Reciprocal stabilization between MAGE-A11 and AR did not protect against degradation promoted by p14-ARF. p14-ARF prevented MAGE-A11 interaction with the E2F1 oncoprotein and inhibited the MAGE-A11-induced increase in E2F1 transcriptional activity. Post-translational down-regulation of MAGE-A11 promoted by p14-ARF was independent of HDM2, the human homologue of mouse double minute 2, an E3 ubiquitin ligase inhibited by p14-ARF. However, MAGE-A11 had a stabilizing effect on HDM2 in the absence or presence of p14-ARF and cooperated with HDM2 to increase E2F1 transcriptional activity in the absence of p14-ARF. We conclude that degradation of MAGE-A11 promoted by the human p14-ARF tumor suppressor contributes to low levels of MAGE-A11 in nontransformed cells and that higher levels of MAGE-A11 associated with low p14-ARF increase AR and E2F1 transcriptional activity and promote the development of castration-resistant prostate cancer.

Novel interactions of the mineralocorticoid receptor

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

Characterization of niphatenones that inhibit androgen receptor N-terminal domain

Androgen ablation therapy causes a temporary reduction in tumor burden in patients with advanced prostate cancer. Unfortunately the malignancy will return to form lethal castration-recurrent prostate cancer (CRPC). The androgen receptor (AR) remains transcriptionally active in CRPC in spite of castrate levels of androgens in the blood. AR transcriptional activity resides in its N-terminal domain (NTD). Possible mechanisms of continued AR transcriptional activity may include, at least in part, expression of constitutively active splice variants of AR that lack the C-terminal ligand-binding domain (LBD). Current therapies that target the AR LBD, would not be effective against these AR variants. Currently no drugs are clinically available that target the AR NTD which should be effective against these AR variants as well as full-length AR. Niphatenones were originally isolated and identified in active extracts from Niphates digitalis marine sponge. Here we begin to characterize the mechanism of niphatenones in blocking AR transcriptional activity. Both enantiomers had similar IC50 values of 6 µM for inhibiting the full-length AR in a functional transcriptional assay. However, (S)-niphatenone had significantly better activity against the AR NTD compared to (R)-niphatenone. Consistent with niphatenones binding to and inhibiting transactivation of AR NTD, niphatenones inhibited AR splice variant. Niphatenone did not affect the transcriptional activity of the related progesterone receptor, but slightly decreased glucocorticoid receptor (GR) activity and covalently bound to GR activation function-1 (AF-1) region. Niphatenone blocked N/C interactions of AR without altering either AR protein levels or its intracellular localization in response to androgen. Alkylation with glutathione suggests that niphatenones are not a feasible scaffold for further drug development.

A comparative study of the androgenic properties of progesterone and the progestins, medroxyprogesterone acetate (MPA) and norethisterone acetate (NET-A)

The importance of investigating the molecular mechanism of action of medroxyprogesterone acetate (MPA) and norethisterone acetate (NET-A), two clinically important progestins used in hormone therapy (HT), has been highlighted by clinical evidence showing that MPA and norethisterone (NET) increase the risk of the development of breast cancer in HRT users, and that MPA may increase susceptibility to- and transmission of HIV-1. The aim of this study was to compare the molecular mechanisms of action of MPA, NET-A and progesterone (Prog) via the androgen receptor (AR) in a cell line model that can minimize confounding factors such as the presence of other steroid receptors. This study is the first to determine accurate apparent Ki values for Prog, MPA and NET-A toward the human AR in COS-1 cells. The results reveal that these ligands have a similar binding affinity for the AR to that of the natural androgen 5α-dihydrotestosterone (DHT) (Ki's for DHT, Prog, MPA and NET-A are 29.4, 36.6, 19.4 and 21.9 nM, respectively). Moreover, in both transactivation and transrepression transcriptional assays we demonstrate that, unlike Prog, MPA and NET-A are efficacious AR agonists, with activities comparable to DHT. One of the most novel findings of our study is that NET-A, like DHT, induces the ligand-dependent interaction between the NH2- and COOH-terminal domains (N/C-interaction) of the AR independent of promoter-context, while MPA does not induce the N/C interaction on a classical ARE and does so only weakly on an AR-selective ARE. This suggests that MPA and NET-A may exert differential promoter-specific actions via the AR in vivo. Consistent with this, molecular modeling suggests that MPA and NET-A induce subtle differences in the structure of the AR ligand binding domain. Taken together, the results from this study suggest that unlike Prog, both MPA and NET-A used in hormonal therapy are likely to compete with DHT and exert significant and promoter-specific off-target transcriptional effects via the AR, possibly contributing to some of the observed side-effects with the clinical use of MPA and NET-A.

Androgen receptor phosphorylation: biological context and functional consequences

The androgen receptor (AR) is a ligand-regulated transcription factor that belongs to the family of nuclear receptors. In addition to regulation by steroid, the AR is also regulated by post-translational modifications generated by signal transduction pathways. Thus, the AR functions not only as a transcription factor but also as a node that integrates multiple extracellular signals. The AR plays an important role in many diseases, including complete androgen insensitivity syndrome, spinal bulbar muscular atrophy, prostate and breast cancer, etc. In the case of prostate cancer, dependence on AR signaling has been exploited for therapeutic intervention for decades. However, the effectiveness of these therapies is limited in advanced disease due to restoration of AR signaling. Greater understanding of the molecular mechanisms involved in AR action will enable the development of improved therapeutics to treat the wide range of AR-dependent diseases. The AR is subject to regulation by a number of kinases through post-translational modifications on serine, threonine, and tyrosine residues. In this paper, we review the AR phosphorylation sites, the kinases responsible for these phosphorylations, as well as the biological context and the functional consequences of these phosphorylations. Finally, what is known about the state of AR phosphorylation in clinical samples is discussed.

Feed-forward inhibition of androgen receptor activity by glucocorticoid action in human adipocytes

We compared transcriptomes of terminally differentiated mouse 3T3-L1 and human adipocytes to identify cell-specific differences. Gene expression and high content analysis (HCA) data identified the androgen receptor (AR) as both expressed and functional, exclusively during early human adipocyte differentiation. The AR agonist dihydrotestosterone (DHT) inhibited human adipocyte maturation by downregulation of adipocyte marker genes, but not in 3T3-L1. It is interesting that AR induction corresponded with dexamethasone activation of the glucocorticoid receptor (GR); however, when exposed to the differentiation cocktail required for adipocyte maturation, AR adopted an antagonist conformation and was transcriptionally repressed. To further explore effectors within the cocktail, we applied an image-based support vector machine (SVM) classification scheme to show that adipocyte differentiation components inhibit AR action. The results demonstrate human adipocyte differentiation, via GR activation, upregulates AR but also inhibits AR transcriptional activity.

Antiandrogen flutamide protects male mice from androgen-dependent toxicity in three models of spinal bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is a late-onset, progressive neurodegenerative disease linked to a polyglutamine (polyQ) expansion in the androgen receptor (AR). Men affected by SBMA show marked muscle weakness and atrophy, typically emerging midlife. Given the androgen-dependent nature of this disease, one might expect AR antagonists to have therapeutic value for treating SBMA. However, current work from animal models suggests otherwise, raising questions about whether polyQ-expanded AR exerts androgen-dependent toxicity through mechanisms distinct from normal AR function. In this study, we asked whether the nonsteroidal AR antagonist flutamide, delivered via a time-release pellet, could reverse or prevent androgen-dependent AR toxicity in three different mouse models of SBMA: the AR97Q transgenic (Tg) model, a knock-in (KI) model, and a myogenic Tg model. We find that flutamide protects mice from androgen-dependent AR toxicity in all three SBMA models, preventing or reversing motor dysfunction in the Tg models and significantly extending the life span in KI males. Given that flutamide effectively protects against androgen-dependent disease in three different mouse models of SBMA, our data are proof of principle that AR antagonists have therapeutic potential for treating SBMA in humans and support the notion that toxicity caused by polyQ-expanded AR uses at least some of the same mechanisms as normal AR before diverging to produce disease and muscle atrophy.

Preclinical pharmacology of FL442, a novel nonsteroidal androgen receptor modulator

The preclinical profiles of two most potent compounds of our recently published cycloalkane[d]isoxazole pharmacophore-based androgen receptor (AR) modulators, FL442 (4-(3a,4,5,6,7,7a-hexahydro-benzo[d]isoxazol-3-yl)-2-(trifluoromethyl)benzonitrile) and its nitro analog FL425 (3-(4-nitro-3-(trifluoromethyl)phenyl)-3a,4,5,6,7,7a-hexahydrobenzo[d]isoxazole), were explored to evaluate their druggability for the treatment of AR dependent prostate cancer. The studies revealed that both compounds are selective to AR over other closely related steroid hormone receptors and that FL442 exhibits equal inhibition efficiency towards the androgen-responsive LNCaP prostate cancer cell line as the most widely used antiandrogen bicalutamide and the more recently discovered enzalutamide. Notably, FL442 maintains antiandrogenic activity with enzalutamide-activated AR mutant F876L. In contrast to bicalutamide, FL442 does not stimulate the VCaP prostate cancer cells which express elevated levels of the AR. Distribution analyses showed that [(14)CN]FL442 accumulates strongly in the mouse prostate. In spite of its low plasma concentration obtained by intraperitoneal administration, FL442 significantly inhibited LNCaP xenograft tumor growth. These findings provide a preclinical proof for FL442 as a promising AR targeted candidate for a further optimization.

Sex hormone receptor repertoire in breast cancer

Classification of breast cancer as endocrine sensitive, hormone dependent, or estrogen receptor (ER) positive refers singularly to ER α . One of the oldest recognized tumor targets, disruption of ER α -mediated signaling, is believed to be the mechanistic mode of action for all hormonal interventions used in treating this disease. Whereas ER α is widely accepted as the single most important predictive factor (for response to endocrine therapy), the presence of the receptor in tumor cells is also of prognostic value. Even though the clinical relevance of the two other sex hormone receptors, namely, ER β and the androgen receptor remains unclear, two discordant phenomena observed in hormone-dependent breast cancers could be causally related to ER β -mediated effects and androgenic actions. Nonetheless, our understanding of regulatory molecules and resistance mechanisms remains incomplete, further compromising our ability to develop novel therapeutic strategies that could improve disease outcomes. This review focuses on the receptor-mediated actions of the sex hormones in breast cancer.

Enzalutamide, an androgen receptor signaling inhibitor, induces tumor regression in a mouse model of castration-resistant prostate cancer

BACKGROUND

Enzalutamide (formerly MDV3100 and available commercially as Xtandi), a novel androgen receptor (AR) signaling inhibitor, blocks the growth of castration-resistant prostate cancer (CRPC) in cellular model systems and was shown in a clinical study to increase survival in patients with metastatic CRPC. Enzalutamide inhibits multiple steps of AR signaling: binding of androgens to AR, AR nuclear translocation, and association of AR with DNA. Here, we investigate the effects of enzalutamide on AR signaling, AR-dependent gene expression and cell apoptosis.

METHODS

The expression of AR target gene prostate-specific antigen (PSA) was measured in LnCaP and C4-2 cells. AR nuclear translocation was assessed in HEK-293 cells stably transfected with AR-yellow fluorescent protein. The in vivo effects of enzalutamide were determined in a mouse xenograft model of CRPC. Differential gene expression in LNCaP cells was measured using Affymetrix human genome microarray technology.

RESULTS

We found that unlike bicalutamide, enzalutamide lacked AR agonistic activity at effective doses and did not induce PSA expression or AR nuclear translocation. Additionally, it is more effective than bicalutamide at inhibiting agonist-induced AR nuclear translocation. Enzalutamide induced the regression of tumor volume in a CRPC xenograft model and apoptosis in AR-over-expressing prostate cancer cells. Finally, gene expression profiling in LNCaP cells indicated that enzalutamide opposes agonist-induced changes in genes involved in processes such as cell adhesion, angiogenesis, and apoptosis.

CONCLUSIONS

These results indicate that enzalutamide efficiently inhibits AR signaling, and we suggest that its lack of AR agonist activity may be important for these effects.

Posttranslational modification of the androgen receptor in prostate cancer

The androgen receptor (AR) is important in the development of the prostate by regulating transcription, cellular proliferation, and apoptosis. AR undergoes posttranslational modifications that alter its transcription activity, translocation to the nucleus and stability. The posttranslational modifications that regulate these events are of utmost importance to understand the functional role of AR and its activity. The majority of these modifications occur in the activation function-1 (AF1) region of the AR, which contains the transcriptional activation unit 1 (TAU1) and 5 (TAU5). Identification of the modifications that occur to these regions may increase our understanding of AR activation in prostate cancer and the role of AR in the progression from androgen-dependent to castration-resistant prostate cancer (CRPC). Most of the posttranslational modifications identified to date have been determined using the full-length AR in androgen dependent cells. Further investigations into the role of posttranslational modifications in androgen-independent activation of full-length AR and constitutively active splicing variants are warranted, findings from which may provide new therapeutic options for CRPC.

Melanoma antigen-A11 (MAGE-A11) enhances transcriptional activity by linking androgen receptor dimers

Prostate cancer growth and progression depend on androgen receptor (AR) signaling through transcriptional mechanisms that require interactions with coregulatory proteins, one of which is the primate-specific steroid receptor coregulator melanoma antigen-A11 (MAGE-A11). In this report, we provide evidence how increased expression of MAGE-A11 during prostate cancer progression enhances AR signaling and prostate cancer growth. MAGE-A11 protein levels were highest in castration-recurrent prostate cancer. The cyclic AMP-induced increase in androgen-dependent and androgen-independent AR transcriptional activity correlated with an increase in MAGE-A11 and was inhibited by silencing MAGE-A11 expression. MAGE-A11 mediated synergistic AR transcriptional activity in LAPC-4 prostate cancer cells. The ability of MAGE-A11 to rescue transcriptional activity of complementary inactive AR mutants and promote coimmunoprecipitation between unlike forms of AR suggests that MAGE-A11 links transcriptionally active AR dimers. A model for the AR·MAGE-A11 multidimeric complex is proposed in which one AR FXXLF motif of the AR dimer engages in the androgen-dependent AR NH(2)- and carboxyl-terminal interaction, whereas the second FXXLF motif region of the AR dimer interacts with dimeric MAGE-A11. The AR·MAGE-A11 multidimeric complex accounts for the dual functions of the AR FXXLF motif in the androgen-dependent AR NH(2)- and carboxyl-terminal interaction and binding MAGE-A11 and for synergy between reported AR splice variants and full-length AR. We conclude that the increased expression of MAGE-A11 in castration-recurrent prostate cancer, which is enhanced by cyclic AMP signaling, increases AR-dependent growth of prostate cancer by MAGE-A11 forming a molecular bridge between transcriptionally active AR dimers.

The chicken ovalbumin upstream promoter-transcription factor II negatively regulates the transactivation of androgen receptor in prostate cancer cells

Androgen receptor (AR) is involved in the development and progression of prostate cancers. However, the mechanisms by which this occurs remain incompletely understood. In previous reports, chicken ovalbumin upstream promoter-transcription factor II (COUP-TF II) has been suggested to play a role in the development of cancers. In the present study, we explored a putative role of COUP-TF II in prostate cancers by investigating its effect on cell proliferation and a cross-talk between COUP-TF II and AR. Overexpression of COUP-TF II results in the inhibition of androgen-dependent proliferation of prostate cancer cells. Further studies show that COUP-TF II functions as a corepressor of AR. It represses AR transactivation on target promoters containing the androgen response element (ARE) in a dose-dependent manner. In addition, COUP-TF II interacts physically with AR in vitro and in vivo. It binds to both the DNA binding domain (DBD) and the ligand-binding domain (LBD) of AR and disrupts the N/C terminal interaction of AR. Furthermore, COUP-TF II competes with coactivators such as ARA70, SRC-1, and GRIP1 to modulate AR transactivation as well as inhibiting the recruitment of AR to its ARE-containing target promoter. Taken together, our findings suggest that COUP-TF II is a novel corepressor of AR, and provide an insight into the role of COUP-TF II in prostate cancers.

20(S)-protopanaxadiol-aglycone downregulation of the full-length and splice variants of androgen receptor

As a public health problem, prostate cancer engenders huge economic and life-quality burden. Developing effective chemopreventive regimens to alleviate the burden remains a major challenge. Androgen signaling is vital to the development and progression of prostate cancer. Targeting androgen signaling via blocking the production of the potent ligand dihydrotestosterone has been shown to decrease prostate cancer incidence. However, the potential of increasing the incidence of high-grade prostate cancers has been a concern. Mechanisms of disease progression after the intervention may include increased expression of androgen receptor (AR) in prostate tissue and expression of the constitutively active AR splice variants (AR-Vs) lacking the ligand-binding domain. Thus, novel agents targeting the receptor, preferentially both the full-length and AR-Vs, are urgently needed. In the present study, we show that ginsenoside 20(S)-protopanaxadiol-aglycone (PPD) effectively downregulates the expression and activity of both the full-length AR and AR-Vs. The effects of PPD on AR and AR-Vs are manifested by an immediate drop in proteins followed by a reduction in transcripts, attributed to PPD induction of proteasome-mediated degradation and inhibition of the transcription of the AR gene. We further show that although PPD inhibits the growth as well as AR expression and activity in LNCaP xenograft tumors, the morphology and AR expression in normal prostates are not affected. This study is the first to show that PPD suppresses androgen signaling through downregulating both the full-length AR and AR-Vs, and provides strong rationale for further developing PPD as a promising agent for the prevention and/or treatment of prostate cancer.

RWDD1 interacts with the ligand binding domain of the androgen receptor and acts as a coactivator of androgen-dependent transactivation

During embryogenesis, the development of the male genital is dependent on androgens. Their actions are mediated by the androgen receptor (AR), which functions as a transcription factor. To identify AR coregulators that support AR action during the critical time window of androgen-dependent development in the genital tubercle of male mice, we performed yeast two-hybrid screenings with cDNA libraries of genital tubercles from male mouse embryos using human AR as bait. RWD domain containing 1 (RWDD1) was identified as an AR-interacting protein from three independent libraries of the embryonic days E15, E16 and E17. The interaction between the AR and RWDD1 was confirmed in vitro and in vivo and the ligand binding domain of the AR was shown to be sufficient to mediate the interaction. RWDD1 enhanced AR-dependent transactivation in reporter assays with promoters of different complexity and in different cell lines. These results suggest that RWDD1 functions as a coactivator of androgen-dependent transcription.

A competitive inhibitor that reduces recruitment of androgen receptor to androgen-responsive genes

The androgen receptor (AR) has a critical role in the growth and progression of androgen-dependent and castration-resistant prostate cancers. To identify novel inhibitors of AR transactivation that block growth of prostate cancer cells, a luciferase-based high-throughput screen of ~160,000 small molecules was performed in cells stably expressing AR and a prostate-specific antigen (PSA)-luciferase reporter. CPIC (1-(3-(2-chlorophenoxy) propyl)-1H-indole-3-carbonitrile) was identified as a small molecule that blocks AR transactivation to a greater extent than other steroid receptors. CPIC inhibited AR-mediated proliferation of androgen-sensitive prostate cancer cell lines, with minimal toxicity in AR-negative cell lines. CPIC treatment also reduced the anchorage-independent growth of LAPC-4 prostate cancer cells. CPIC functioned as a pure antagonist by inhibiting the expression of AR-regulated genes in LAPC-4 cells that express wild-type AR and exhibited weak agonist activity in LNCaP cells that express the mutant AR-T877A. CPIC treatment did not reduce AR levels or alter its nuclear localization. We used chromatin immunoprecipitation to identify the site of action of CPIC. CPIC inhibited recruitment of androgen-bound AR to the PSA promoter and enhancer sites to a greater extent than bicalutamide. CPIC is a new therapeutic inhibitor that targets AR-mediated gene activation with potential to arrest the growth of prostate cancer.

Post-translational modification of the androgen receptor

Regulation of the androgen receptor (AR) by its cognate ligand is well established, but how post-translational modification modulates AR activity is only emerging. The AR is subject to modification by phosphorylation, acetylation, methylation, SUMOylation, and ubiquitination. As several of the enzymes that modify the AR are altered in prostate cancer, defining the context and physiological effects of these modifications could provide insight into mechanisms that underpin human disease. Here, we review how post-translational modification contributes to AR function as a transcription factor with particular emphasis on phosphorylation and dephosphorylation mechanisms.

Allosteric modulators of steroid hormone receptors: structural dynamics and gene regulation

Steroid hormones are synthesized from cholesterol primarily in the adrenal gland and the gonads and play vital roles in normal physiology, the control of development, differentiation, metabolic homeostasis, and reproduction. The actions of these small lipophilic molecules are mediated by intracellular receptor proteins. It is just over 25 yr since the first cDNA for steroid receptors were cloned, a development that led to the birth of a superfamily of ligand-activated transcription factors: the nuclear receptors. The receptor proteins share structurally and functionally related ligand binding and DNA-binding domains but possess distinct N-terminal domains and hinge regions that are intrinsically disordered. Since the original cloning experiments, considerable progress has been made in our understanding of the structure, mechanisms of action, and biology of this important class of ligand-activated transcription factors. In recent years, there has been interest in the structural plasticity and function of the N-terminal domain of steroid hormone receptors and in the allosteric regulation of protein folding and function in response to hormone, DNA response element architecture, and coregulatory protein binding partners. The N-terminal domain can exist as an ensemble of conformers, having more or less structure, which prime this region of the receptor to rapidly respond to changes in the intracellular environment through hormone binding and posttranslation modifications. In this review, we address the question of receptor structure and function dynamics with particular emphasis on the structurally flexible N-terminal domain, intra- and interdomain communications, and the allosteric regulation of receptor action.

Mechanisms of ligand specificity of the mineralocorticoid receptor

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

Androgen receptor exon 1 mutation causes androgen insensitivity by creating phosphorylation site and inhibiting melanoma antigen-A11 activation of NH2- and carboxyl-terminal interaction-dependent transactivation

Naturally occurring germ line mutations in the X-linked human androgen receptor (AR) gene cause incomplete masculinization of the external genitalia by disrupting AR function in males with androgen insensitivity syndrome. Almost all AR missense mutations that cause androgen insensitivity syndrome are located in the highly structured DNA and ligand binding domains. In this report we investigate the functional defect associated with an AR exon 1 missense mutation, R405S, that caused partial androgen insensitivity. The 46,XX heterozygous maternal carrier had a wild-type Arg-405 CGC allele but transmitted an AGC mutant allele coding for Ser-405. At birth, the 46,XY proband had a bifid scrotum, hypospadias, and micropenis consistent with clinical stage 3 partial androgen insensitivity. Androgen-dependent transcriptional activity of AR-R405S expressed in CV1 cells was less than wild-type AR and refractory in androgen-dependent AR NH(2)- and carboxyl interaction transcription assays that depend on the coregulator effects of melanoma antigen-A11. This mutation created a Ser-405 phosphorylation site evident by the gel migration of an AR-R405S NH(2)-terminal fragment as a double band that converted to the wild-type single band after treatment with λ-phosphatase. Detrimental effects of the R405S mutation were related to the proximity of the AR WXXLF motif (433)WHTLF(437) required for melanoma antigen-A11 and p300 to stimulate transcriptional activity associated with the AR NH(2)- and carboxyl-terminal interaction. We conclude that the coregulator effects of melanoma antigen-A11 on the AR NH(2)- and carboxyl-terminal interaction amplify the androgen-dependent transcriptional response to p300 required for normal human male sex development in utero.

Structural features discriminate androgen receptor N/C terminal and coactivator interactions

Human androgen receptor (AR) transcriptional activity involves interdomain and coactivator interactions with the agonist-bound AR ligand binding domain (LBD). Structural determinants of the AR NH(2)- and carboxyl-terminal interaction between the AR NH(2)-terminal FXXLF motif and activation function 2 (AF2) in the LBD were shown previously by crystallography. In this report, we provide evidence for a region in AR LBD helix 12 outside the AF2 binding cleft that facilitates interactions with the FXXLF and LXXLL motifs. Mutagenesis of glutamine 902 to alanine in AR LBD helix 12 (Q902A) disrupted AR FXXLF motif binding to AF2, but enhanced coactivator LXXLL motif binding. Functional compensation for defective FXXLF motif binding by AR-Q902A was suggested by the slower dissociation rate of bound androgen. Functional importance of glutamine 902 was indicated by the charged residue germline mutation Q902R that caused partial androgen insensitivity, and a similar somatic mutation Q902K reported in prostate cancer, both of which increased the androgen dissociation rate and decreased AR transcriptional activity. High affinity equilibrium androgen binding was retained by alanine substitution mutations at Tyr-739 in AR LBD helix 5 or Lys-905 in helix 12 structurally adjacent to AF2, whereas transcriptional activity decreased and the androgen dissociation increased. Deleterious effects of these loss of function mutations were rescued by the helix stabilizing AR prostate cancer somatic mutation H874Y. Sequence NH(2)-terminal to the AR FXXLF motif contributed to the AR NH(2)- and carboxyl-terminal interaction based on greater AR-2-30 FXXLF motif peptide binding to the agonist-bound AR LBD than a shorter AR-20-30 FXXLF motif peptide. We conclude that helix 12 residues outside the AF2 binding cleft modulate AR transcriptional activity by providing flexibility to accommodate FXXLF or LXXLL motif binding.

Neurotoxic effects of androgens in spinal and bulbar muscular atrophy

Expansion of polyglutamine tracts in nine different genes causes selective neuronal degeneration through unknown mechanisms. Expansion of polyglutamine in the androgen receptor is responsible for spinal and bulbar muscular atrophy (SBMA), a neuromuscular disorder characterized by the loss of lower motor neurons in the brainstem and spinal cord. A unique feature of SBMA in the family of polyglutamine diseases is sex specificity. SBMA fully manifests only in males. SBMA is a disease triggered by the binding of polyglutamine androgen receptor to its natural ligand testosterone. Recent evidence has emerged showing that the expanded polyglutamine tract itself is not the only determinant of disease pathogenesis. There is evidence that both the native structure and function of the disease protein strongly influence the pathogenicity of mutant protein. Here, we review recent progress in the understanding of disease pathogenesis and advancements towards development of potential therapeutic strategies for SBMA.

Gain in transcriptional activity by primate-specific coevolution of melanoma antigen-A11 and its interaction site in androgen receptor

Male sex development and growth occur in response to high affinity androgen binding to the androgen receptor (AR). In contrast to complete amino acid sequence conservation in the AR DNA and ligand binding domains among mammals, a primate-specific difference in the AR NH(2)-terminal region that regulates the NH(2)- and carboxyl-terminal (N/C) interaction enables direct binding to melanoma antigen-A11 (MAGE-11), an AR coregulator that is also primate-specific. Human, mouse, and rat AR share the same NH(2)-terminal (23)FQNLF(27) sequence that mediates the androgen-dependent N/C interaction. However, the mouse and rat AR FXXLF motif is flanked by Ala(33) that evolved to Val(33) in primates. Human AR Val(33) was required to interact directly with MAGE-11 and for the inhibitory effect of the AR N/C interaction on activation function 2 that was relieved by MAGE-11. The functional importance of MAGE-11 was indicated by decreased human AR regulation of an androgen-dependent endogenous gene using lentivirus short hairpin RNAs and by the greater transcriptional strength of human compared with mouse AR. MAGE-11 increased progesterone and glucocorticoid receptor activity independently of binding an FXXLF motif by interacting with p300 and p160 coactivators. We conclude that the coevolution of the AR NH(2)-terminal sequence and MAGE-11 expression among primates provides increased regulatory control over activation domain dominance. Primate-specific expression of MAGE-11 results in greater steroid receptor transcriptional activity through direct interactions with the human AR FXXLF motif region and indirectly through steroid receptor-associated p300 and p160 coactivators.

Characterization of the zebrafish (Danio rerio) mineralocorticoid receptor

Comparison between evolutionarily distant receptors can provide critical insights into both structure and function. Sequence comparison between the mineralocorticoid receptors (MR) of the zebrafish (zMR) and human (hMR) reveals a high degree of sequence conservation in the major functional domains. We isolated a zMR cDNA to contrast the transcriptional response to a range of ligands and to establish whether a teleost MR exhibits the amino/carboxyl-terminal interaction (N/C-interaction) previously reported for the hMR. Aldosterone, deoxycorticosterone (DOC) and cortisol induced zMR transcriptional activity with similar efficacy to that observed with the hMR. The hMR antagonist, spironolactone, acted as an agonist with the zMR. The zMR exhibited an N/C-interaction in response to aldosterone but, in contrast to the hMR, cortisol and DOC predominantly stimulated the interaction in the zMR. Conservation of the N/C-interaction between evolutionarily distant MR provides evidence of functional significance.

An interdomain interaction of the androgen receptor is required for its aggregation and toxicity in spinal and bulbar muscular atrophy

Polyglutamine expansion within the androgen receptor (AR) causes spinal and bulbar muscular atrophy (SBMA) and is associated with misfolded and aggregated species of the mutant AR. We showed previously that nuclear localization of the mutant AR was necessary but not sufficient for SBMA. Here we show that an interdomain interaction of the AR that is central to its function within the nucleus is required for AR aggregation and toxicity. Ligands that prevent the interaction between the amino-terminal FXXLF motif and carboxyl-terminal AF-2 domain (N/C interaction) prevented toxicity and AR aggregation in an SBMA cell model and rescued primary SBMA motor neurons from 5α-dihydrotestosterone-induced toxicity. Moreover, genetic mutation of the FXXLF motif prevented AR aggregation and 5α-dihydrotestosterone toxicity. Finally, selective androgen receptor modulators, which prevent the N/C interaction, ameliorated AR aggregation and toxicity while maintaining AR function, highlighting a novel therapeutic strategy to prevent the SBMA phenotype while retaining AR transcriptional function.

Regression of castrate-recurrent prostate cancer by a small-molecule inhibitor of the amino-terminus domain of the androgen receptor

Castration-recurrent prostate cancer (CRPC) is suspected to depend on androgen receptor (AR). The AF-1 region in the amino-terminal domain (NTD) of AR contains most, if not all, of the transcriptional activity. Here we identify EPI-001, a small molecule that blocked transactivation of the NTD and was specific for inhibition of AR without attenuating transcriptional activities of related steroid receptors. EPI-001 interacted with the AF-1 region, inhibited protein-protein interactions with AR, and reduced AR interaction with androgen-response elements on target genes. Importantly, EPI-001 blocked androgen-induced proliferation and caused cytoreduction of CRPC in xenografts dependent on AR for growth and survival without causing toxicity.

Structural and functional analysis of amino-terminal enhancer of split in androgen-receptor-driven transcription

We previously demonstrated that the Groucho protein AES (amino-terminal enhancer of split) functions as a co-repressor of the AR (androgen receptor). It physically interacts with the N-terminal domain of AR and inhibits AR-driven transcription, but the molecular mechanism of its action remained unclear. In the present paper we report that the AES protein contains one inhibitory domain, and one positive and one negative regulatory domain. The negative regulatory domain inhibits AES dimerization and AES-mediated inhibition of AR-driven transcription through an interaction with the inhibitory domain. The positive regulatory domain blocked this interaction and relieved the inhibitory effect. In addition, we discovered mechanisms by which AES regulates AR transcriptional activity, which included disruption of the interaction between the AR N-terminal and C-terminal domains, and inhibition of AR-DNA interaction. Although AES broadly inhibited the activity of androgen-dependent luciferase reporters in a transient transfection assay, it selectively regulated the expression of endogenous androgen-dependent genes in prostate cancer cells.

Melanoma antigen gene protein-A11 (MAGE-11) F-box links the androgen receptor NH2-terminal transactivation domain to p160 coactivators

Androgen-dependent transcriptional activity by the androgen receptor (AR) and its coregulators is required for male reproductive development and function. In humans and other primates, melanoma antigen gene protein-A11 (MAGE-11) is an AR selective coregulator that increases AR transcriptional activity. Here we show that the interaction between AR and MAGE-11 is mediated by AR NH(2)-terminal FXXLF motif binding to a highly conserved MAGE-11 F-box in the MAGE homology domain, and is modulated by serum stimulation of mitogen-activated protein kinase phosphorylation of MAGE-11 Ser-174. The MAGE-11-dependent increase in AR transcriptional activity is mediated by a direct interaction between MAGE-11 and transcriptional intermediary factor 2 (TIF2) through the NH(2)-terminal region of TIF2, and by a MAGE-11 FXXIF motif interaction with an F-box-like region in activation domain 1 of TIF2. The results suggest that MAGE-11 functions as a bridging factor to recruit AR coactivators through a novel FXX(L/I)F motif-F-box interaction paradigm.

Structural and functional characterization of the interdomain interaction in the mineralocorticoid receptor

The mineralocorticoid receptor (MR) plays a central role in electrolyte homeostasis and in cardiovascular disease. We have previously reported a ligand-dependent N/C-interaction in the MR. In the present study we sought to fully characterize the MR N/C-interaction. By using a range of natural and synthetic MR ligands in a mammalian two-hybrid assay we demonstrate that in contrast to aldosterone, which strongly induces the interaction, the physiological ligands deoxycorticosterone and cortisol weakly promote the interaction but predominantly inhibit the aldosterone-mediated N/C-interaction. Similarly, progesterone and dexamethasone antagonize the interaction. In contrast, the synthetic agonist 9alpha-fludrocortisol robustly induces the interaction. The ability of the N/C interaction to discriminate between MR agonists suggests a subtle conformational difference in the ligand-binding domain induced by these agonists. We also demonstrate that the N/C interaction is not cell specific, consistent with the evidence from a glutathione-S-transferase pull-down assay, of a direct protein-protein interaction between the N- and C-terminal domains of the MR. Examination of a panel of deletions in the N terminus suggests that several regions may be critical to the N/C-interaction. These studies have identified functional differences between physiological MR ligands, which suggest that the ligand-specific dependence of the N/C-interaction may contribute to the differential activation of the MR that has been reported in vivo.

Inhibition of androgen receptor transcriptional activity as a novel mechanism of action of arsenic

Environmental sodium arsenite is a toxin that is associated with male infertility due to decreased and abnormal sperm production. Arsenic trioxide (ATO), another inorganic trivalent semimetal, is an effective therapy for acute promyelocytic leukemia, and there is investigation of its possible efficacy in prostate cancer. However, the mechanism of arsenic action in male urogenital tract tissues is not clear. Because the androgen receptor (AR) plays an important role in spermatogenesis and prostate cancer, we explored the possibility that trivalent arsenic regulates AR function. We found that arsenic inhibited AR transcriptional activity in prostate cancer and Sertoli cells using reporter gene assays testing several androgen response element-containing regions and by assessing native target gene expression. Arsenic inhibition of AR activity was not due to down-regulation of AR protein levels, decreased hormone binding to AR, disruption of AR nuclear translocation, or interference with AR-DNA binding in vitro. However, chromatin immunoprecipitation studies revealed that arsenic inhibited AR recruitment to an AR target gene enhancer in vivo. Consistent with a deficiency in AR-chromatin binding, arsenic disrupted AR amino and carboxyl termini interaction. Furthermore, ATO caused a significant decrease in prostate cancer cell proliferation that was more pronounced in cells expressing AR compared with cells depleted of AR. In addition, inhibition of AR activity by ATO and by the AR antagonist, bicalutamide, was additive. Thus, arsenic-induced male infertility may be due to inhibition of AR activity. Further, because AR is an important target in prostate cancer therapy, arsenic may serve as an effective therapeutic option.