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

Discovery of a novel androgen receptor antagonist, MEL-6, with stereoselective activity and optimization of its metabolic stability

A new chemical scaffold with antagonistic activity towards the androgen receptor (AR) was identified. The parent compound, (3-Methoxy-N-[1-methyl-2-(4-phenyl-1-piperazinyl)-2-(2-thienyl)ethyl]benzamide) referred to as MEL-6, binds in the ligand binding pocket of AR and induces an antagonistic conformation of the ligand binding domain, even in presence of the antagonist-to-agonist switch mutations W741C, T877A and F876L-T877A. MEL-6 has antiproliferative effects on several AR positive prostate cancer cell lines. We further identified AR as the specific target of MEL-6 since it demonstrates little effect on other steroid receptors. In LNCaP cells it also inhibits the androgen-regulated transcriptome. These findings identify MEL-6 as a promising candidate for treatment of patients with prostate tumors that have become resistant to current clinically used AR antagonists. Analytical studies on the chemical composition of MEL-6 identified the presence of four isomers (two enantiomeric pairs), among which one isomer is responsible for the antiandrogenic activity. We therefore developed a synthetic route towards the selective preparation of the active enantiomeric pair. Various MEL-6-like analogues had improved metabolic stability while maintaining antiandrogenic activity. Metabolite identification of MEL-6 derivatives pinpointed N-dealkylation of the piperazine as the main mode for inactivation by liver enzymes. For further structural optimization, MEL-6 derivatives were purchased or synthesized having alterations on the N-phenyl group of the piperazine, the benzoyl group and additionally substituting the thiophen-2-yl ring of MEL-6 to a phenyl ring. This optimization process resulted in compound 12b with sustained AR inhibition and a 4-fold increased half-life due to the 1-(5-chloro-2-methylphenyl)-piperazine substitution, thienyl-to-phenyl substitution and chloro in para-position of the benzoyl group.

Androgen receptor cofactors: A potential role in understanding prostate cancer

Prostate cancer (PCa) is witnessing a concerning rise in incidence annually, with the androgen receptor (AR) emerging as a pivotal contributor to its growth and progression. Mounting evidence underscores the AR's ability to recruit cofactors, influencing downstream gene transcription and thereby fueling the proliferation and metastasis of PCa cells. Although, clinical strategies involving AR antagonists provide some relief, managing castration resistant prostate cancer (CRPC) remains a formidable challenge. Thus, the need of the hour lies in unearthing new drugs or therapeutic targets to effectively combat PCa. This review encapsulates the pivotal roles played by coactivators and corepressors of AR, notably androgen receptor-associated protein (ARA) and steroid receptor Coactivators (SRC) in PCa. Our data unveils how these cofactors intricately modulate histone modifications, cell cycling, SUMOylation, and apoptosis through their interactions with AR. Among the array of cofactors scrutinised, such as ARA70β, ARA24, ARA160, ARA55, ARA54, PIAS1, PIAS3, SRC1, SRC2, SRC3, PCAF, p300/CBP, MED1, and CARM1, several exhibit upregulation in PCa. Conversely, other cofactors like ARA70α, PIASy, and NCoR/SMRT demonstrate downregulation. This duality underscores the complexity of AR cofactor dynamics in PCa. Based on our findings, we propose that manipulating cofactor regulation to modulate AR function holds promise as a novel therapeutic avenue against advanced PCa. This paradigm shift offers renewed hope in the quest for effective treatments in the face of CRPC's formidable challenges.

N/C Interactions Are Dispensable for Normal In Vivo Functioning of the Androgen Receptor in Male Mice

The androgen receptor (AR) plays a central role in the development and maintenance of the male phenotype. The binding of androgens to the receptor induces interactions between the carboxyterminal ligand-binding domain and the highly conserved 23FQNLF27 motif in the aminoterminal domain. The role of these so-called N/C interactions in AR functioning is debated. In vitro assays show that mutating the AR in the 23FQNLF27 motif (called ARNoC) attenuates the AR transactivation of reporter genes, has no effect on ligand binding, but does affect protein-protein interactions with several AR coregulators. To test the in vivo relevance of the N/C interaction, we analyzed the consequences of the genomic introduction of the ARNoC mutation in mice. Surprisingly, the ARNoC/Y mice show a normal male development, with unaffected male anogenital distance and normal accessory sex glands, male circulating androgen levels, body composition, and fertility. The responsiveness of androgen target genes in kidney, prostate, and testes was also unaffected. We thus conclude that the N/C interactions in the AR are not essential for the development of a male phenotype under normal physiological conditions.

Using biochemistry and biophysics to extinguish androgen receptor signaling in prostate cancer

Castration resistant prostate cancer (CRPC) continues to be androgen receptor (AR) driven. Inhibition of AR signaling in CRPC could be advanced using state-of-the-art biophysical and biochemical techniques. Structural characterization of AR and its complexes by cryo-electron microscopy would advance the development of N-terminal domain (NTD) and ligand-binding domain (LBD) antagonists. The structural basis of AR function is unlikely to be determined by any single structure due to the intrinsic disorder of its NTD, which not only interacts with coregulators but likely accounts for the constitutive activity of AR-splice variants (SV), which lack the LBD and emerge in CRPC. Using different AR constructs lacking the LBD, their effects on protein folding, DNA binding, and transcriptional activity could reveal how interdomain coupling explains the activity of AR-SVs. The AR also interacts with coregulators that promote chromatin looping. Elucidating the mechanisms involved can identify vulnerabilities to treat CRPC, which do not involve targeting the AR. Phosphorylation of the AR coactivator MED-1 by CDK7 is one mechanism that can be blocked by the use of CDK7 inhibitors. CRPC gains resistance to AR signaling inhibitors (ARSI). Drug resistance may involve AR-SVs, but their role requires their reliable quantification by SILAC-mass spectrometry during disease progression. ARSI drug resistance also occurs by intratumoral androgen biosynthesis catalyzed by AKR1C3 (type 5 17β-hydroxysteroid dehydrogenase), which is unique in that its acts as a coactivator of AR. Novel bifunctional inhibitors that competitively inhibit AKR1C3 and block its coactivator function could be developed using reverse-micelle NMR and fragment-based drug discovery.

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.

Rationale for the development of alternative forms of androgen deprivation therapy

With few exceptions, the almost 30,000 prostate cancer deaths annually in the United States are due to failure of androgen deprivation therapy. Androgen deprivation therapy prevents ligand-activation of the androgen receptor. Despite initial remission after androgen deprivation therapy, prostate cancer almost invariably progresses while continuing to rely on androgen receptor action. Androgen receptor's transcriptional output, which ultimately controls prostate cancer behavior, is an alternative therapeutic target, but its molecular regulation is poorly understood. Recent insights in the molecular mechanisms by which the androgen receptor controls transcription of its target genes are uncovering gene specificity as well as context-dependency. Heterogeneity in the androgen receptor's transcriptional output is reflected both in its recruitment to diverse cognate DNA binding motifs and in its preferential interaction with associated pioneering factors, other secondary transcription factors and coregulators at those sites. This variability suggests that multiple, distinct modes of androgen receptor action that regulate diverse aspects of prostate cancer biology and contribute differentially to prostate cancer's clinical progression are active simultaneously in prostate cancer cells. Recent progress in the development of peptidomimetics and small molecules, and application of Chem-Seq approaches indicate the feasibility for selective disruption of critical protein-protein and protein-DNA interactions in transcriptional complexes. Here, we review the recent literature on the different molecular mechanisms by which the androgen receptor transcriptionally controls prostate cancer progression, and we explore the potential to translate these insights into novel, more selective forms of therapies that may bypass prostate cancer's resistance to conventional androgen deprivation therapy.

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.

Preventing the Androgen Receptor N/C Interaction Delays Disease Onset in a Mouse Model of SBMA

Spinal and bulbar muscular atrophy (SBMA) is a neurodegenerative disease caused by a polyglutamine expansion in the androgen receptor (AR) and is associated with misfolding and aggregation of the mutant AR. We investigated the role of an interdomain interaction between the amino (N)-terminal FxxLF motif and carboxyl (C)-terminal AF-2 domain in a mouse model of SBMA. Male transgenic mice expressing polyQ-expanded AR with a mutation in the FxxLF motif (F23A) to prevent the N/C interaction displayed substantially improved motor function compared with N/C-intact AR-expressing mice and showed reduced pathological features of SBMA. Serine 16 phosphorylation was substantially enhanced by the F23A mutation; moreover, the protective effect of AR F23A was dependent on this phosphorylation. These results reveal an important role for the N/C interaction on disease onset in mice and suggest that targeting AR conformation could be a therapeutic strategy for patients with SBMA.

Nemo-like kinase is a novel regulator of spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is a progressive neuromuscular disease caused by polyglutamine expansion in the androgen receptor (AR) protein. Despite extensive research, the exact pathogenic mechanisms underlying SBMA remain elusive. In this study, we present evidence that Nemo-like kinase (NLK) promotes disease pathogenesis across multiple SBMA model systems. Most remarkably, loss of one copy of Nlk rescues SBMA phenotypes in mice, including extending lifespan. We also investigated the molecular mechanisms by which NLK exerts its effects in SBMA. Specifically, we have found that NLK can phosphorylate the mutant polyglutamine-expanded AR, enhance its aggregation, and promote AR-dependent gene transcription by regulating AR-cofactor interactions. Furthermore, NLK modulates the toxicity of a mutant AR fragment via a mechanism that is independent of AR-mediated gene transcription. Our findings uncover a crucial role for NLK in controlling SBMA toxicity and reveal a novel avenue for therapy development in SBMA.

DOI:http://dx.doi.org/10.7554/eLife.08493.001

Spinal and bulbar muscular atrophy (SBMA) is an inherited disease that eventually leads to degeneration in motor neurons and weakness in muscles. It is caused by a specific genetic mutation in the gene that encodes the androgen receptor protein, which leads to the production of a mutant protein that is larger than normal. Similar mutations in other genes can lead to the development of other so-called ‘polyglutamine’ diseases such as Huntington's disease and spinocerebellar ataxia. However, the precise details of how these mutations lead to disease symptoms are not known, and there are currently no effective ways of treating these conditions.

Previous research has shown that an enzyme called Nemo-like kinase (or NLK for short) regulates the normal androgen receptor in cancer cells. NLK has kinase activity, that is, it adds phosphate molecules to other proteins to regulate their activity. Todd et al. used human cells, fruit flies, and mice as model systems to investigate whether NLK is involved in the development of SBMA.

The experiments show that NLK promotes the development of features associated with SBMA in all three models. The kinase activity of NLK is required for these features to develop. Todd et al. also found that NLK can bind to and add phosphate molecules to the mutant version of the androgen receptor protein. This causes the mutant androgen receptor proteins to accumulate and increases the ability of the mutant proteins to activate particular genes.

Todd et al.'s findings suggest that NLK promotes the development of SBMA by interacting with the mutant androgen receptor. Previous studies have shown that NLK is able to modulate the development of spinocerebellar ataxia type 1, which suggests that NLK may also play an important role in other polyglutamine diseases. The next challenge will be to fully understand the role of NLK in these diseases, which may aid future efforts to develop new treatments.

DOI:http://dx.doi.org/10.7554/eLife.08493.002

Engineered repressors are potent inhibitors of androgen receptor activity

Prostate cancer growth is dependent upon the Androgen Receptor (AR) pathway, hence therapies for this disease often target this signalling axis. Such therapies are successful in the majority of patients but invariably fail after a median of 2 years and tumours progress to a castrate resistant stage (CRPC). Much evidence exists to suggest that the AR remains key to CRPC growth and hence remains a valid therapeutic target. Here we describe a novel method to inhibit AR activity, consisting of an interaction motif, that binds to the AR ligand-binding domain, fused to repression domains. These ‘engineered repressors’ are potent inhibitors of AR activity and prostate cancer cell growth and importantly inhibit the AR under circumstances in which conventional therapies would be predicted to fail, such as AR mutation and altered cofactor levels.

Looking at nuclear receptors from a new angle

While the structures of the DNA- and ligand-binding domains of many nuclear receptors have been determined in great detail; the mechanisms by which these domains interact and possibly 'communicate' is still under debate. The first crystal structures of receptor dimers bound to ligand, DNA and coactivator peptides provided new insights in this matter. The observed binding modes revealed exciting new interaction surfaces between the different nuclear receptor domains. Such interfaces are proposed to be the route through which allosteric signals from the DNA are passed on to the ligand-binding domain and the activating functions of the receptor. The structural determinations of DNA-bound receptor dimers in solution, however, revealed an extended structure of the receptors. Here, we discuss these apparent contradictory structural data and their possible implications for the functioning of nuclear receptors.

Ligand-independent androgen receptor antagonism caused by the newly developed pesticide pyrifluquinazon (PFQ)

Androgen receptor (AR) is an essential component to activate AR dependent gene transcriptions. Despite wide acceptance of pharmacological controls on transcriptional pathway depending on competitive inhibitions of ligand binding, only a few examples, AR antagonism via ligand-independent mechanisms, have been recognized. Pyrifluquinazon(PFQ), a newly developed pesticide, induced representative AR antagonism against rats in in vivo and in vitro. Intriguingly, this AR antagonism was not based on inhibition of ligand binding. Instead, the evidence suggested that the AR antagonism was induced as a consequence of decline of cellular AR protein level. This study demonstrated that AR N-terminal region could be an essential element for a ligand-independent mechanism underling the AR antagonism by PFQ. Our findings should provide a novel insight into the regulation of AR-mediated transcription.

An androgen receptor mutation in the MDA-MB-453 cell line model of molecular apocrine breast cancer compromises receptor activity

Recent evidence indicates that the estrogen receptor-α-negative, androgen receptor (AR)-positive molecular apocrine subtype of breast cancer is driven by AR signaling. The MDA-MB-453 cell line is the prototypical model of this breast cancer subtype; its proliferation is stimulated by androgens such as 5α-dihydrotestosterone (DHT) but inhibited by the progestin medroxyprogesterone acetate (MPA) via AR-mediated mechanisms. We report here that the AR gene in MDA-MB-453 cells contains a G-T transversion in exon 7, resulting in a receptor variant with a glutamine to histidine substitution at amino acid 865 (Q865H) in the ligand binding domain. Compared with wild-type AR, the Q865H variant exhibited reduced sensitivity to DHT and MPA in transactivation assays in MDA-MB-453 and PC-3 cells but did not respond to non-androgenic ligands or receptor antagonists. Ligand binding, molecular modeling, mammalian two-hybrid and immunoblot assays revealed effects of the Q865H mutation on ligand dissociation, AR intramolecular interactions, and receptor stability. Microarray expression profiling demonstrated that DHT and MPA regulate distinct transcriptional programs in MDA-MB-453 cells. Gene Set Enrichment Analysis revealed that DHT- but not MPA-regulated genes were associated with estrogen-responsive transcriptomes from MCF-7 cells and the Wnt signaling pathway. These findings suggest that the divergent proliferative responses of MDA-MB-453 cells to DHT and MPA result from the different genetic programs elicited by these two ligands through the AR-Q865H variant. This work highlights the necessity to characterize additional models of molecular apocrine breast cancer to determine the precise role of AR signaling in this breast cancer subtype.

Evidence for DNA-binding domain--ligand-binding domain communications in the androgen receptor

DNA binding as well as ligand binding by nuclear receptors has been studied extensively. Both binding functions are attributed to isolated domains of which the structure is known. The crystal structure of a complete receptor in complex with its ligand and DNA-response element, however, has been solved only for the peroxisome proliferator-activated receptor γ (PPARγ)-retinoid X receptor α (RXRα) heterodimer. This structure provided the first indication of direct interactions between the DNA-binding domain (DBD) and ligand-binding domain (LBD). In this study, we investigated whether there is a similar interface between the DNA- and ligand-binding domains for the androgen receptor (AR). Despite the structural differences between the AR- and PPARγ-LBD, a combination of in silico modeling and docking pointed out a putative interface between AR-DBD and AR-LBD. The surfaces were subjected to a point mutation analysis, which was inspired by known AR mutations described in androgen insensitivity syndromes and prostate cancer. Surprisingly, AR-LBD mutations D695N, R710A, F754S, and P766A induced a decrease in DNA binding but left ligand binding unaffected, while the DBD-residing mutations K590A, K592A, and E621A lowered the ligand-binding but not the DNA-binding affinity. We therefore propose that these residues are involved in allosteric communications between the AR-DBD and AR-LBD.

Recent developments in antiandrogens and selective androgen receptor modulators

The androgens testosterone and dihydrotestosterone play an essential role in the development and maintenance of primary and secondary male characteristics. Androgens bind to a specific androgen receptor (AR), a ligand-dependent transcription factor which controls the expression of a large number of downstream target genes. The AR is an essential player in early and late prostate cancer, and may also be involved in some forms of breast cancer. It also represents a drug target for the treatment of hypogonadism. Recent studies furthermore indicate that targeting the AR in pathologies such as frailty syndrome, cachexia or polycystic ovary syndrome may have clinical benefit. Numerous AR ligands with very different pharmacological properties have been identified in the last 40 years and helped to treat several of these diseases. However, progress still needs to be made in order to find compounds with an improved profile with regard to efficacy, differentiation and side-effects. This will only be achieved through a better understanding of the mechanisms involved in normal and aberrant AR signaling.

Identification and characterization of MEL-3, a novel AR antagonist that suppresses prostate cancer cell growth

Antiandrogens are an important component of prostate cancer therapy as the androgen receptor (AR) is the key regulator of prostate cancer growth and survival. Current AR antagonists, such as bicalutamide and hydroxyflutamide, have a low affinity for the AR and as a result block AR signaling insufficiently. Moreover, many patients develop a resistance for bicalutamide or hydroxyflutamide during therapy or show a clinical improvement after withdrawal of the antiandrogen. New and more effective AR antagonists are needed to ensure follow-up of these patients. We therefore developed a screening system to identify novel AR antagonists from a collection of compounds. MEL-3 [8-(propan-2-yl)-5,6-dihydro-4H-pyrazino[3,2,1-jk]carbazole] was selected as potent inhibitor of the AR and was further characterized in vitro. On different prostate cancer cell lines MEL-3 displayed an improved therapeutic profile compared with bicalutamide. Not only cell growth was inhibited but also the expression of androgen-regulated genes: PSA and FKBP5. Prostate cancer is often associated with mutated ARs that respond to a broadened spectrum of ligands including the current antiandrogens used in the clinic, hydroxyflutamide and bicalutamide. The activity of two mutant receptors (AR T877A and AR W741C) was shown to be reduced in presence of MEL-3, providing evidence that MEL-3 can potentially be a follow-up treatment for bicalutamide- and hydroxyflutamide-resistant patients. The mechanism of action of MEL-3 on the molecular level was further explored by comparing the structure-activity relationship of different chemical derivatives of MEL-3 with the in silico docking of MEL-3 derivatives in the binding pocket of the AR.

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.

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.

Regulation of androgen receptor-dependent transcription by coactivator MED1 is mediated through a newly discovered noncanonical binding motif

Nuclear receptor (NR) activation by cognate ligand generally involves allosteric realignment of C-terminal α-helices thus generating a binding surface for coactivators containing canonical LXXLL α-helical motifs. The androgen receptor (AR) is uncommon among NRs in that ligand triggers an intramolecular interaction between its N- and C-terminal domains (termed the N/C interaction) and that coactivators can alternatively bind to surfaces in the AR N-terminal or hinge regions. The evolutionary conserved Mediator complex plays a key coregulatory role in steroid hormone-dependent transcription and is chiefly targeted to NRs via the LXXLL-containing MED1 subunit. Whereas MED1 has been demonstrated to serve as a key transcriptional coactivator for AR, the mechanisms by which AR recruits MED1 have remained unclear. Here we show that MED1 binds to a distinct AR N-terminal region termed transactivation unit-1 (Tau-1) via two newly discovered noncanonical α-helical motifs located between MED1 residues 505 and 537. Neither of the two MED1 LXXLL motifs is required for AR binding, whereas loss of the intramolecular AR N/C interaction decreases MED1 binding. We further demonstrate that mitogen-activated protein kinase phosphorylation of MED1 enhances the AR-MED1 interaction in prostate cancer cells. In sum, our findings reveal a novel AR-coactivator binding mechanism that may have clinical implications for AR activity in prostate cancer.

The role of androgen receptor mutations in prostate cancer progression

Prostate tumour growth is almost always dependent upon the androgen receptor pathway and hence therapies aimed at blocking this signalling axis are useful tools in the management of this disease. Unfortunately such therapies invariably fail; and the tumour progresses to an “androgen-independent” stage. In such cases androgen receptor expression is almost always maintained and much evidence exists to suggest that it may still be driving growth. One mechanism by which the receptor is thought to remain active is mutation. This review summarises the present data on androgen receptor mutations in prostate cancer, and how such substitutions offer a growth advantage by affecting cofactor interactions or by reducing ligand specificity. Such alterations appear to have a subsequent effect upon gene expression suggesting that tumours may “behave” differently dependent upon the ligand promoting growth and if a mutation is present.

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.

Inhibition of androgen receptor functions by gelsolin FxxFF peptide delivered by transfection, cell-penetrating peptides, and lentiviral infection

BACKGROUND

Prostate cancer (PC) growth is dependent on the androgen-androgen receptor (AR) axis. Because current androgen ablation therapies of PC lead to resistance, novel approaches to block AR activity are urgently needed.

METHODS

We inhibited AR function beyond the level of hormone binding by blockade of the coactivator groove in the ligand-binding domain (LBD) using a high-affinity gelsolin FxxFF peptide. Following peptide selection, the effect of the gelsolin FxxFF peptide on AR functions was determined in Hep3B cells that were transiently transfected with pM-peptide expression vectors or were incubated with synthetic gelsolin FxxFF peptide coupled to the TAT cell-penetrating peptide. Lentiviruses expressing the gelsolin FxxFF peptide were used to study endogenous AR target gene expression in LNCaP cells.

RESULTS

pM-Gelsolin FxxFF efficiently interfered with AR N/C interaction and specifically inhibited AR-regulated reporter gene activity. The peptide did not inhibit progesterone receptor (PR) and glucocorticoid receptor (GR) activity, nor constitutively active gene promoters. The peptide also specifically blocked in vitro interactions of AR LBD with peptides. Like the gelsolin FxxFF peptide expressed by an expression vector, synthetic TAT-gelsolin FxxFF peptide efficiently blocked AR N/C interaction and inhibited full-length AR-regulated reporter gene activity. It hardly affected PR and GR activity, but the effect on constitutively active promoters was variable. Lentiviral gelsolin FxxFF peptide inhibited expression of KLK2 and NDRG1, but hardly affected PSA and TMPRSS2.

CONCLUSIONS

Our results show that the AR coactivator groove may function as a target to overcome therapeutic failure that arises during current androgen ablation therapies.

Analysis of interdomain interactions of the androgen receptor

High-affinity binding of testosterone or dihydrotestosterone to the androgen receptor (AR) triggers the androgen-dependent AR NH2- and carboxyl-terminal (N/C) interaction between the AR NH2-terminal FXXLF motif and the activation function 2 (AF2) hydrophobic binding surface in the ligand-binding domain. The functional importance of the AR N/C interaction is supported by naturally occurring loss-of-function AR AF2 mutations where AR retains high-affinity androgen binding but is defective in AR FXXLF motif binding. Ligands with agonist activity in vivo such as testosterone, dihydrotestosterone, and the synthetic anabolic steroids induce the AR N/C interaction and increase AR transcriptional activity in part by slowing the dissociation rate of bound ligand and stabilizing AR against degradation. AR ligand-binding domain competitive antagonists inhibit the agonist-dependent AR N/C interaction. Although the human AR N/C interaction is important for transcriptional activity, it has an inhibitory effect on transcriptional activity from AF2 by competing for p160 coactivator LXXLL motif binding. The primate-specific AR coregulatory protein, melanoma antigen gene protein-A11 (MAGE-A11), modulates the AR N/C interaction through a direct interaction with the AR FXXLF motif. Inhibition of AF2 transcriptional activity by the AR N/C interaction is relieved by AR FXXLF motif binding to the F-box region of MAGE-11. Described here are methods to measure the androgen-dependent AR N/C interdomain interaction and the influence of transcriptional coregulators.

Native functions of the androgen receptor are essential to pathogenesis in a Drosophila model of spinobulbar muscular atrophy

Spinobulbar muscular atrophy (SBMA) is a neurodegenerative disease caused by expansion of a polyglutamine tract in the androgen receptor (AR). This mutation confers toxic function to AR through unknown mechanisms. Mutant AR toxicity requires binding of its hormone ligand, suggesting that pathogenesis involves ligand-induced changes in AR. However, whether toxicity is mediated by native AR function or a novel AR function is unknown. We systematically investigated events downstream of ligand-dependent AR activation in a Drosophila model of SBMA. We show that nuclear translocation of AR is necessary, but not sufficient, for toxicity and that DNA binding by AR is necessary for toxicity. Mutagenesis studies demonstrated that a functional AF-2 domain is essential for toxicity, a finding corroborated by a genetic screen that identified AF-2 interactors as dominant modifiers of degeneration. These findings indicate that SBMA pathogenesis is mediated by misappropriation of native protein function, a mechanism that may apply broadly to polyglutamine diseases.

Functional screening of FxxLF-like peptide motifs identifies SMARCD1/BAF60a as an androgen receptor cofactor that modulates TMPRSS2 expression

Androgen receptor (AR) transcriptional activity is tightly regulated by interacting cofactors and cofactor complexes. The best described cofactor interaction site in the AR is the hormone-induced coactivator binding groove in the ligand-binding domain, which serves as a high-affinity docking site for FxxLF-like motifs. This study aimed at identifying novel AR cofactors by in silico selection and functional screening of FxxLF-like peptide motifs. Candidate interacting motifs were selected from a proteome-wide screening and from a supervised screening focusing on components of protein complexes involved in transcriptional regulation. Of the 104 peptides tested, 12 displayed moderate to strong in vivo hormone-dependent interactions with AR. For three of these, ZBTB16/PLZF, SMARCA4/BRG1, and SMARCD1/BAF60a, the full-length protein was tested for interaction with AR. Of these, BAF60a, a subunit of the SWI/SNF chromatin remodeling complex, displayed hormone-dependent interactions with AR through its FxxFF motif. Vice versa, recruitment of BAF60a by the AR required an intact coactivator groove. BAF60a depletion by small interfering RNA in LNCaP cells demonstrated differential effects on expression of endogenous AR target genes. AR-driven expression of TMPRSS2 was almost completely blocked by BAF60a small interfering RNA. In summary, our data demonstrate that BAF60a directly interacts with the coactivator groove in the AR ligand-binding domain via its FxxFF motif, thereby selectively activating specific AR-driven promoters.

Comparison of the molecular consequences of different mutations at residue 754 and 690 of the androgen receptor (AR) and androgen insensitivity syndrome (AIS) phenotype

OBJECTIVE

Androgen insensitivity syndrome (AIS) is associated with mutations throughout the androgen receptor (AR) gene. Different mutations at the same codon have been identified in individuals with various phenotypes suggesting the nature of the codon substituted may influence the degree of AIS. We investigated if phenotype could be predicted by comparing the functionality of AR mutations with those at the same codon of known phenotype.

PATIENTS

We identified patients from the Cambridge Disorders of Sex Development Database with the AR substitutions: Phe754Ser with microphallus without hypospadias and Asp690Val with complete AIS. Mutations Phe754Leu, Phe754Val and Asp690deletion (Asp690del) have previously been reported to be associated with different degrees of AIS.

DESIGN

We characterized the functional properties of Phe754Ser, Phe754Leu, Phe754Val, Asp690Val and Asp690del receptor mutants in vitro and used the crystal structure of the AR ligand binding domain to model the mutations.

RESULTS

The receptor mutants Phe754Ser, Phe754Leu and Phe754Val bound androgen with decreasing affinity, while Asp690Val showed reduced affinity compared to Asp690del. A similar pattern of reduced activation was seen on androgen responsive elements. We suggest how the mutations could affect AR structure, resulting in the observed phenotypes.

CONCLUSIONS

The relative functional properties of Phe754 and Asp690 mutant AR receptors correlate broadly with their specific phenotypes. Therefore, comparing the molecular consequences of novel mutations with others at the same codon may be a useful aid to AIS patient management, particularly for sex of rearing decisions when prediction of functionality is important.

The androgen receptor gene polyglycine repeat polymorphism is associated with memory performance in healthy Chinese individuals

Cognitive functions such as memory are quantitative traits in human, and have both genetic and environmental influences. Testosterone has been implicated in the modulation of memory function. Therefore, genetic variation which influences testosterone signaling may modulate memory function. The principal receptor for testosterone is the androgen receptor, the gene for which maps to the X chromosome. In the present study, we hypothesized that common variation in two functional polymorphisms in the androgen receptor gene, the polyglutamine (CAG) and/or polyglycine (GGN) repeats, would influence memory function in healthy subjects. Variation in length of either repeat modulates the function of the AR gene, either by changing the amount of protein produced, by altering transactivation of the receptor or by producing toxic polyglycine or polyglutamine fragments. In order to test this hypothesis, we analyzed 449 healthy Chinese individuals. CAG repeats were not associated with memory performance. However we observed a significant association between GGN repeats and Immediate Logical Memory (chi(2)=23.6, d.f.=7, p=0.001) and Delayed Logical Memory (chi(2)=16.3, d.f.=7, p=0.022). The association of GGN repeats with Immediate Logical Memory remained significant after 6000 permutation corrections (p=0.013). There was also a sex difference, as association between GGN repeats and memory was observed only in females (p=0.002 for Immediate and p=0.014 for Delayed Logical Memory), but not in males (p=0.31 and 0.83, respectively). We conclude that functional variation of the androgen receptor gene is able to modulate memory function in women.

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.

Functional characterization of the native NH2-terminal transactivation domain of the human androgen receptor: binding kinetics for interactions with TFIIF and SRC-1a

The androgen receptor (AR) is a ligand-activated transcription factor that mediates the actions of the steroid hormones testosterone and dihydrotestosterone at the level of gene transcription. The main transactivation function is modular in structure, maps to the N-terminal domain (NTD), and is termed AF1. This region of the AR is structurally flexible and functions in multiple protein-protein interactions with coregulatory proteins and components of the general transcription machinery. Using surface plasmon resonance, the binding kinetics for the interaction of AR-AF1 with the large subunit of the general transcription factor TFIIF, termed RAP74, and the coactivator SRC-1a were measured. AR-AF1 interacts with both the NTD and CTD of RAP74 and the CTD of SRC-1a. The dissociation constants ( Kd) for the binding of polypeptides derived from RAP74 are in the submicromolar range, while a peptide from SRC-1a bound with a Kd of 14 microM. Significantly, the individual NTD and CTD of RAP74 interacted with AR-AF1 with distinct binding kinetics, with the NTD exhibiting slower on and off rates. TFIIF is involved in transcription initiation and elongation, and the CTD of RAP74 binds to the RNA polymerase II enzyme, the general transcription factor TFIIB, and a CTD phosphatase, FCP1. We have mutated hydrophobic residues in the RAP74-CTD structure to disrupt secondary structure elements and show that binding of AR-AF1 depends upon helix 3 in the winged-helix domain of the RAP74-CTD polypeptide. Altogether, a model is suggested for AR-AF1-dependent transactivation of receptor-target genes.

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.

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.

The androgen receptor DNA-binding domain determines androgen selectivity of transcriptional response

The AR (androgen receptor) is a hormone-dependent transcription factor that translates circulating androgen hormone levels into a physiological cellular response by directly regulating the expression of its target genes. It is the key molecule in e.g. the development and maintenance of the male sexual characteristics, spermatocyte production and prostate gland development and growth. It is also a major factor in the onset and maintenance of prostate cancer and a first target for pharmaceutical action against the further proliferation of prostate cancer cells. The AR is a member of the steroid hormone receptors, a group of steroid-inducible transcription factors sharing an identical consensus DNA-binding motif. The problem of how specificity in gene activation is achieved among the different members of this nuclear receptor subfamily is still unclear. In this report, we describe our investigations on how the AR can specifically activate its target genes, while the other steroid hormone receptors do not, despite having the same consensus monomeric DNA-binding motif. In this respect, we describe how the AR interacts with a newly identified class of steroid-response elements to which only the AR and not, for example, the glucocorticoid receptor can bind.

Novel FXXFF and FXXMF motifs in androgen receptor cofactors mediate high affinity and specific interactions with the ligand-binding domain

Upon hormone binding, a hydrophobic coactivator binding groove is induced in the androgen receptor (AR) ligand-binding domain (LBD). This groove serves as high affinity docking site for alpha-helical FXXLF motifs present in the AR N-terminal domain and in AR cofactors. Study of the amino acid requirements at position +4 of the AR FXXLF motif revealed that most amino acid substitutions strongly reduced or completely abrogated AR LBD interaction. Strong interactions were still observed following substitution of Leu+4 by Phe or Met residues. Leu+4 to Met or Phe substitutions in the FXXLF motifs of AR cofactors ARA54 and ARA70 were also compatible with strong AR LBD binding. Like the corresponding FXXLF motifs, interactions of FXXFF and FXXMF variants of AR and ARA54 motifs were AR specific, whereas variants of the less AR-selective ARA70 motif displayed increased AR specificity. A survey of currently known AR-binding proteins revealed the presence of an FXXFF motif in gelsolin and an FXXMF motif in PAK6. In vivo fluorescence resonance energy transfer and functional protein-protein interaction assays showed direct, efficient, and specific interactions of both motifs with AR LBD. Mutation of these motifs abrogated interaction of gelsolin and PAK6 proteins with AR. In conclusion, we have demonstrated strong interaction of FXXFF and FXXMF motifs to the AR coactivator binding groove, thereby mediating specific binding of a subgroup of cofactors to the AR LBD.

A Role of the Amino-Terminal (N) and Carboxyl-Terminal (C) Interaction in Binding of Androgen Receptor to Chromatin

The N-terminal domain of AR is known to engage a hormone-dependent interaction with its C-terminal ligand-binding domain, and this N/C interaction is known to modulate AR transcriptional activity. Using Xenopus oocytes as a model system to study transcriptional regulation in chromatin, we found that two previously reported N/C interaction-defective AR mutants, one with deletion of 23FQNLF27(ARDeltaF) and one with a Gly 21 to Glu mutation (ARG21E), were surprisingly inactive in activating transcription from various reporters assembled into chromatin. Further study using chromatin immunoprecipitation assay revealed that these mutants failed to bind both mouse mammary tumor virus-long terminal repeat and prostate-specific antigen enhancer assembled into chromatin. This defect is specific to chromatin because both mutants could bind to a consensus AR response element in vitro and activate transcription driven by mouse mammary tumor virus-long terminal repeat in transient transfection as effective as the wild-type AR. To further substantiate this novel finding, we established 293 cell lines that stably expressed either AR or ARDeltaF mutant in an inducible manner. Using these cell lines, we confirmed by using chromatin immunoprecipitation assay that AR but not ARDeltaF could bind to the endogenous prostate-specific antigen enhancer. Furthermore, we found that the ARDeltaF mutant interacts poorly with Brg1, the ATPase subunit of the chromatin-remodeling factor SWI/SNF. Taken together, our study reveals a novel role of AR N/C interaction in control of AR chromatin binding and suggests a working model that the proper N/C interaction is required for AR to recruit SWI/SNF complex, which in turn remodels chromatin to allow AR to bind to AR response elements in chromatin.

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.

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.

Partial androgen insensitivity with phenotypic variation caused by androgen receptor mutations that disrupt activation function 2 and the NH(2)- and carboxyl-terminal interaction

Partial androgen insensitivity with sex phenotype variation in two unrelated families was associated with missense mutations in the androgen receptor (AR) gene that disrupted the AR NH(2)-terminal/carboxy terminal interaction. Each mutation caused a single amino acid change within the region of the ligand-binding domain that forms activation function 2 (AF2). In one family, the mutation I737T was in alpha helix 4 and in the other F725L was between helices 3 and 4. Neither mutation altered androgen binding as determined by assays of mutant AR in the patient's cultured genital skin fibroblasts or of recombinant mutant receptors transfected into COS cells. In transient cotransfection assays in CV1 cells, transactivation with the AR mutants at low concentrations of DHT was reduced several fold compared with wild-type AR but increased at higher concentrations. Defects in NH(2)-terminal/carboxy terminal interactions were identified in mammalian two hybrid assays. In similar assays, there was reduced binding of the p160 coactivators TIF2/SRC2 and SRC1 to the mutant AR ligand binding domains (LBD). In the family with AR I737T, sex phenotype varied from severely defective masculinization in the proband to a maternal great uncle whose only manifestation of AIS was severe gynecomastia. He was fertile and passed the mutation to two daughters. The proband of the F725L family was also incompletely masculinized but was raised as a male while his half-sibling by a different father was affected more severely and reared as a female. These studies indicate that the function of an AR AF2 mutant in male development can vary greatly depending on the genetic background.

Melanoma antigen gene protein MAGE-11 regulates androgen receptor function by modulating the interdomain interaction

Gene activation by steroid hormone receptors involves the recruitment of the steroid receptor coactivator (SRC)/p160 coactivator LXXLL motifs to activation function 2 (AF2) in the ligand binding domain. For the androgen receptor (AR), AF2 also serves as the interaction site for the AR NH(2)-terminal FXXLF motif in the androgen-dependent NH(2)-terminal and carboxyl-terminal (N/C) interaction. The relative importance of the AR AF2 site has been unclear, since the AR FXXLF motif interferes with coactivator recruitment by competitive inhibition of LXXLL motif binding. In this report, we identified the X chromosome-linked melanoma antigen gene product MAGE-11 as an AR coregulator that specifically binds the AR NH(2)-terminal FXXLF motif. Binding of MAGE-11 to the AR FXXLF alpha-helical region stabilizes the ligand-free AR and, in the presence of an agonist, increases exposure of AF2 to the recruitment and activation by the SRC/p160 coactivators. Intracellular association between AR and MAGE-11 is supported by their coimmunoprecipitation and colocalization in the absence and presence of hormone and by competitive inhibition of the N/C interaction. AR transactivation increases in response to MAGE-11 and the SRC/p160 coactivators through mechanisms that include but are not limited to the AF2 site. MAGE-11 is expressed in androgen-dependent tissues and in prostate cancer cell lines. The results suggest MAGE-11 is a unique AR coregulator that increases AR activity by modulating the AR interdomain interaction.

Distinct recognition modes of FXXLF and LXXLL motifs by the androgen receptor

Among nuclear receptors, the androgen receptor (AR) is unique in that its ligand-binding domain (LBD) interacts with the FXXLF motif in the N-terminal domain, resembling coactivator LXXLL motifs. We compared AR- and estrogen receptor alpha-LBD interactions of the wild-type AR FXXLF motif and coactivator transcriptional intermediary factor 2 LXXLL motifs and variants of these motifs. Random mutagenesis revealed a key role for the F residues in FXXLF motifs in high-affinity and selective AR LBD interaction. The FXXLF motif in full-length AR and transcriptional intermediary factor 2 LXXLL motifs competed for an overlapping binding site. A computer model of the AR LBD/AR FXXLF complex showed that the bulky F residues are buried in a deep coactivator-binding groove. The corresponding groove in estrogen receptor alpha LBD is considerably shallower, explaining lack of binding of any of the FXXLF motifs tested. FXXLF and LXXLL motif interaction depended on different charged amino acid residues in the AR LBD present at opposite ends of the coactivator groove. In conclusion, our data demonstrate the importance of a deep hydrophobic groove and alternative usage of charged amino acids in specifying peptide binding to the AR LBD.

Induced α-Helix Structure in AF1 of the Androgen Receptor upon Binding Transcription Factor TFIIF†

In recent years, it has become clear that in many proteins, significant regions are encoded by amino acid sequences that do not automatically fold into their fully condensed, functional structures. Characterization of the conformational propensities and function of the nonglobular protein sequences represents a major challenge. Striking among proteins with unfolded regions are numbers of transcription factors, including steroid receptors. In many cases, the unfolded or partially folded regions of such proteins take shape when the protein interacts with its proper binding partner(s), that is, the molecules to which it must bind to carry out its function. The AF1 domain of the androgen receptor (AR) shows little structure, when expressed as a recombinant peptide. It has been shown previously that AF1 interacts with transcription factor TFIIF in vitro. Using Fourier transform infrared (FTIR), we tested whether this interaction can induce structure in the AR AF1. Our results demonstrate that the recombinant AR AF1 can acquire significantly higher helical content after interacting with RAP74, a subunit of the TFIIF complex. We further show that this induced conformation in the AR AF1 is well-suited for its interaction with SRC-1.

Differential modulation of androgen receptor action by deoxyribonucleic acid response elements

In addition to the steroid response elements (SREs), which are recognized by several steroid receptors, a second class of DNA elements exhibiting selectivity for the androgen receptor (AR) and named androgen response elements (AREs) has been identified. Here we provide evidence for the differential role of these element classes in modulating AR function. AR complexes attached to response elements representative of each class were purified. Limited protease digests of ARE- or SRE-bound AR complexes led to the generation of different patterns, in line with differential accessibilities. In transactivation assays, mutations in the AR dimerization interface of the DNA-binding domain had various effects, depending on the response elements tested. The R598D mutant displayed much enhanced activity on SREs, whereas far less effect was seen on the selective AREs. The A596T mutant had reduced activity on AREs but not on SREs. Ectopic expression of the coactivators transcriptional intermediary factor 2 (TIF2) and ARA55 stimulated AR activity to different extents, depending on the response element. When using cysteine-rich secretory protein 1 (CRISP-1) SRE as reference, the most significant difference was observed with Pem ARE-2. A differential response of each element class was furthermore observed in the presence of two enzymes involved in the sumoylation pathway. Ubiquitin-conjugating enzyme 9 (Ubc9) overexpression enhanced AR action conveyed by SREs, whereas little effect was seen on Pem ARE-1 and repression on Pem ARE-2. Protein inhibitor of activated STAT (PIAS)xalpha overexpression had little influence on SRE-mediated AR activity but was repressive when using AREs. Altogether, these results demonstrate that DNA response elements play an important modulatory role in transmitting AR action and may be determinative for specificity of gene expression in cell or tissue types.

The use of phage display technique for the isolation of androgen receptor interacting peptides with (F/W)XXL(F/W) and FXXLY new signature motifs

Early studies suggested that the signature motif, LXXLL, within steroid hormone receptor p160 coregulators may play important roles for the mediation of receptor-coregulator interaction. Interestingly, several androgen receptor (AR) coregulators, such as ARA70 and ARA55, may not use such a unique motif to mediate their coregulator activity. Here we apply the phage display technique to identify some new signature motifs, (F/W)XXL(F/W) and FXXLY (where F is phenylalanine, W is tryptophan, L is leucine, Y is tyrosine, and X is any amino acid) that can influence the interaction between AR and AR coregulators. Sequence analyses found that several AR coregulators, such as ARA70, ARA55, ARA54, and FHL2, contain FXXL(F/Y) motifs. Both glutathione S-transferase pull-down assays and transient transfection reporter assays demonstrate that these AR coregulators may use the FXXL(F/Y) motif to interact with AR and exert their AR coregulator activity. Exchanging the amino acid of Phe, Trp, or Tyr in this newly identified signature motif cluster may influence these peptides to interact with AR. The motif-containing peptides, as well as ARA70 or ARA54, may require selective flanking sequences for the better interaction with AR. In addition to influencing the AR transactivation, these motifs in AR-interacting peptides/proteins were also able to influence the AR N-/C-terminal interaction. Together, our data suggest that AR interacting peptides and/or AR coregulators may utilize the (F/W)XXL(F/W) and FXXLY motifs to mediate their interaction with AR and exert their influences on the AR transactivation.

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.