Androgen receptor (AR) coregulators: an overview

NRIP, a Novel Nuclear Receptor Interaction Protein, Enhances the Transcriptional Activity of Nuclear Receptors

Transcriptional regulation by members of the nuclear hormone receptor superfamily is a modular process requiring the mediation of distinct subclasses of coregulators. In this study, we isolated a novel WD40 repeat-containing gene, human nuclear receptor interaction protein (NRIP). We found NRIP interacts with either androgen or glucocorticoid receptors from in vitro and in vivo pulldown assays. Subsequently, transient transfection and luciferase activity assays suggested that NRIP was a ligand-dependent coactivator of steroid receptors (androgen and glucocorticoid) in distinct promoters. To further clarify the function of NRIP, we found an RNA interference-3-targeted NRIP gene sequence (5'-GATGATACAGCACGAGAAC-3') that could efficiently and specifically knock down endogenous and exogenous NRIP gene expression and that significantly diminished cell proliferation in prostate (LNCaP) and cervical (C33A) cells. Therefore, NRIP may play a role in enhancing the transcriptional activity of nuclear receptors and may be a critical target for developing therapeutic agents against nuclear receptor-mediated progression of prostate and cervical cancers.

Androgen receptor corepressors: an overview

Androgens play pivotal roles in sex differentiation and development, in reproductive functions, and sexual behavior. The actions of androgens are mediated through the intracellular androgen receptor (AR), a member of the nuclear receptor (NR) superfamily, which regulates a wide range of target gene expression. Recent studies indicate that the proper transcriptional activity of AR is modulated by AR coregulators, including coactivators that can enhance AR transactivation and corepressors that can suppress AR transactivation. Here, we summarize the recent discoveries relating to AR corepressor function with the following different mechanisms: (1) corepressors that inhibit the DNA binding or nuclear translocation of AR; (2) corepressors that recruit histone deacetylases; (3) corepressors that interrupt the interaction between AR and its coactivators; (4) corepressors that interrupt the interaction between the N-terminus and C-terminus of AR; (5) corepressors that function as scaffolds for other AR coregulators; (6) corepressors that target the basal transcriptional machinery; (7) other mechanisms. The potential impact and future directions of AR corepressors are also discussed.

BAF57 governs androgen receptor action and androgen-dependent proliferation through SWI/SNF

Androgen receptor (AR) activity is required for prostate cancer development and progression. Thus, there is a major impetus to understand the regulation of AR action. We and others have previously shown that AR transactivation potential is dependent on the presence of an active SWI/SNF chromatin remodeling complex. However, the mechanisms underlying SWI/SNF regulation of the AR remained unsolved. We show here that the BAF57 subunit, an accessory component of the remodeling complex, is a critical regulator of AR function. We show that BAF57 is expressed in the luminal epithelia of the prostate and is required for AR-dependent transactivation in prostatic adenocarcinoma cells. Our data reveal that BAF57 can directly bind to the AR and is recruited to endogenous AR targets upon ligand activation. Loss of BAF57 or inhibition of BAF57 function severely compromised AR activity, as observed with both exogenous and endogenous AR targets. Rescue of BAF57 function restored AR activity, thus demonstrating a specific requirement of BAF57 for AR activity. This action of BAF57 proved to be dependent on SWI/SNF ATPase function. BAF57 has previously been implicated in nuclear receptor coactivator function, and we show that, although BAF57 facilitated coactivator activity, only a selected subset required BAF57 for coactivator function. Lastly, we demonstrate that both BAF57 and BRM are required for the proliferation of AR-dependent prostatic adenocarcinoma cells. In summary, these findings identify BAF57 as a critical modulator of the AR that is capable of altering AR activity, coactivator function, and AR-dependent proliferation.

HOXB13 induces growth suppression of prostate cancer cells as a repressor of hormone-activated androgen receptor signaling

Androgen receptor (AR) signals play a decisive role in regulating the growth and differentiation of both normal and cancerous prostate cells by triggering the regulation of target genes, in a process in which AR cofactors have critical functions. Because of the highly prostate-specific expression pattern of HOXB13, we studied the role of this homeodomain protein in prostate cells. Expression of HOXB13 was limited to AR-expressing prostate cells. Reporter transcription assay demonstrated that HOXB13 significantly suppressed hormone-mediated AR activity in a dose-responsive manner, and suppression was specific to AR with which HOXB13 physically interacts. Overexpression of HOXB13 further down-regulated the androgen-stimulated expression of prostate-specific antigen, and suppression of endogenous HOXB13 stimulated transactivation of AR. Functionally, HOXB13 suppressed growth of LNCaP prostate cancer cells, which could be counteracted by additional hormone-activated AR. On the other hand, the growth-suppressive function of HOXB13 in AR-negative CV-1 cells was not affected by AR. These results suggest that HOXB13 functions as an AR repressor to modulate the complex AR signaling and subsequent growth regulation of prostate cancer cells. In addition to the loss of HOXB13 expression, maintaining AR may be an important step for prostate cancer cells to tolerate the suppressor function of HOXB13. Altogether, our data present a novel mechanism for the HOXB13-mediated repression of AR signaling, which can be interpreted to a growth-suppressive event.

Novel Function of Androgen Receptor-associated Protein 55/Hic-5 as a Negative Regulator of Smad3 Signaling

Androgen receptor-associated protein 55 (ARA55/Hic-5) belongs to the LIM protein superfamily and is featured by three or four N-terminal LD motifs and four C-terminal zinc finger-like LIM domains. Both LD motifs and LIM domains can serve as protein-protein interaction interfaces. Recently, we found that enforced expression of ARA55 inhibits transforming growth factor-beta-mediated up-regulation of Smad binding element-luciferase reporter activity in NRP-154 and NRP-152 rat prostate and LNCaP human prostate cell lines. Moreover, ARA55 also inhibits the induction of Smad-binding element 4-luciferase and 3TP-luciferase (a plasminogen activator inhibitor-1 (PAI-1) promoter construct) reporters by constitutively active (CA)-Smad3 in these cell lines. Co-immunoprecipitation studies suggest an interaction between ARA55 and either CA-Smad3 or wild-type Smad3 in HEK293 cells that occurs through the MH2 domain of Smad3 and the C terminus of ARA55 with wild-type Smad3 having stronger affinity than CA-Smad3 to ARA55. Glutathione S-transferase pull-down assays demonstrate that this interaction can occur in a cell-free system. These results are consistent with the luciferase data showing that the C terminus of ARA55 is critical for suppression of Smad3 activity. Furthermore, using a mammalian two-hybrid system, we confirmed that ARA55 interacts with the MH2 domain of Smad3 and suppresses CA-Smad3-induced transcriptional responses. In conclusion, these results support that ARA55 selectively intercepts transforming growth factor-beta signaling through an interaction of the LIM domain of ARA55 with the MH2 domain of Smad3.

Androgen-receptor coregulators mediate the suppressive effect of androgen signals on vitamin D receptor activity

Overexpression of androgen receptors (AR) in PC-3 cell, and treatment of 5alpha-dihydrotestosterone in LNCaP cells lead to the suppression of VDR transactivation. Competition for shared coregulators between AR and VDR is one possible mechanism to explain the suppressive effect of androgen-AR signals on VDR activity. Among the AR coregulators we tested, ARA54, ARA70, supervillin, and gelsolin were found to enhance VDR transactivation. Further characterization of the interaction between ARA54 or ARA70 and VDR demonstrated a direct interaction between VDR and ARA70, but no association between ARA54 and VDR. The LXXLL motif of ARA70 is essential for interaction with VDR and partially responsible for its function as a coactivator of VDR. The suppression of VDR transactivation by AR signal was restored by overexpression of ARA70, but not ARA54. Together, ARA70 and ARA54 modulate VDR transactivation, and the competition for ARA70 mediates the suppressive effect of androgen-AR on VDR transactivation.

Tob proteins suppress steroid hormone receptor-mediated transcriptional activation

Although sex steroid hormones have significant effects on bone metabolism, the molecular mechanisms of these actions have not been fully elucidated yet. We examined the functional relationship between steroid hormone receptors and Tob, a member of an anti-proliferative protein family and a negative regulator of osteoblast proliferation and differentiation. Luciferase assay using promoters carrying hormone-responsive elements revealed that both Tob1 and Tob2 proteins but not PC3 suppressed steroid hormone receptor-dependent transcriptional activation in MC3T3-E1 osteoblastic cells. Mutated Tob proteins carrying amino acid substitutions at an LXXLL motif also showed the same degree of inhibition of the transcriptional activation as the wild type. By observation of androgen receptor (AR)-tagged with green fluorescent protein under a confocal laser scanning microscope, we found that Tob1 inhibits the nuclear foci formation of dihydrotestosterone-bound AR. These results indicate that Tob family proteins may negatively regulate sex steroid hormone action in bone formation.

A type I DnaJ homolog, DjA1, regulates androgen receptor signaling and spermatogenesis

Two type I DnaJ homologs DjA1 (DNAJA1; dj2, HSDJ/hdj-2, rdj1) and DjA2 (DNAJA2; dj3, rdj2) work similarly as a cochaperone of Hsp70s in protein folding and mitochondrial protein import in vitro. To study the in vivo role of DjA1, we generated DjA1-mutant mice. Surprisingly, loss of DjA1 in mice led to severe defects in spermatogenesis that involve aberrant androgen signaling. Transplantation experiments with green fluorescent protein-labeled spermatogonia into DjA1(-/-) mice revealed a primary defect of Sertoli cells in maintaining spermiogenesis at steps 8 and 9. In Sertoli cells of DjA1(-/-) mice, the androgen receptor markedly accumulated with enhanced transcription of several androgen-responsive genes, including Pem and testin. Disruption of Sertoli-germ cell adherens junctions was also evident in DjA1(-/-) mice. Experiments with DjA1(-/-) fibroblasts and primary Sertoli cells indicated aberrant androgen receptor signaling. These results revealed a critical role of DjA1 in spermiogenesis and suggest that DjA1 and DjA2 are not functionally equivalent in vivo.

Xenopus laevis CYP17 regulates androgen biosynthesis independent of the cofactor cytochrome b5

The enzyme CYP17 primarily regulates androgen production by mediating four reactions: conversion of pregnenolone and progesterone to 17-hydroxypregnenolone and 17-hydroxyprogesterone, respectively (17alpha-hydroxylase activity), followed by conversion of the 17-hydroxylated steroids to dehydroepiandrosterone and androstenedione, respectively (17,20-lyase activity). Most mammalian CYP17 isoforms have high 17alpha-hydroxylase relative to 17,20-lyase activities and preferentially mediate one of the two 17,20-lyase reactions. In contrast, Xenopus laevis CYP17 potently regulates all four reactions in the frog ovary. CYP17 isoforms generally rely on the cofactor cytochrome b(5) for the 17,20-lyase reaction, suggesting that the high lyase activity of Xenopus CYP17 might be due to a lesser dependence on b(5). The kinetics of Xenopus CYP17 expressed in yeast microsomes were therefore examined in the absence and presence of Xenopus on human b(5). Xenopus CYP17 mediated both 17,20-lyase reactions in the absence of b(5), confirming that the activity did not require b(5). However, both Xenopus and human b(5) slightly enhanced Xenopus CYP17-mediated lyase activity, indicating that the enzyme was still at least partially responsive to b(5). Surprisingly, only the human b(5) cofactor enhanced human CYP17-mediated lyase activity, implying that the human enzyme had more specific cofactor requirements than Xenopus CYP17. Studies using human/Xenopus chimeric b(5) proteins revealed that human b(5) residues 16-41 were important for the specific regulation of the lyase activity of HuCYP17, possibly serving as an interacting domain with the enzyme. CYP17 may therefore have evolved from a general producer of sex steroids in lower vertebrates to a more tightly regulated producer of both sex steroids and glucocorticoids in mammals.

Inhibition of the Akt, cyclooxygenase-2, and matrix metalloproteinase-9 pathways in combination with androgen deprivation therapy: Potential therapeutic approaches for prostate cancer

Prostate cancer cells are generally dependent on androgen stimulation mediated by the androgen receptor (AR) for growth and survival, and, therefore, hormonal manipulation, such as castration and/or the use of AR antagonists, results in a regression of the cancer. However, this treatment very rarely leads to the "cure" of advanced disease, and cancers eventually become androgen-independent. A number of genes/pathways have been reported to be activated in prostate cancer, most of which are possibly associated with disease progression. In this article, among them, we focus on Akt (also known as protein kinase B), cyclooxygenase (COX)-2, and matrix metalloproteinase (MMP)-9, whose activities or expressions have been found to be regulated by androgens/AR. Previous studies by us and others, with androgen-sensitive prostate cancer cell lines, have demonstrated that androgen deprivation results in activation/overexpression of Akt, COX-2, and MMP-9 in cells. This suggests that androgen deprivation in clinical settings activates the Akt, COX-2, and MMP-9 pathways in prostate cancer, which may increase cell growth and in turn promote the transition to the androgen-independent state. We hypothesize that androgen deprivation, in combination with inhibition of the Akt, COX-2, and MMP-9 pathways, delays the androgen-independent transition and has more beneficial effects than hormonal therapy alone.

Negative modulation of androgen receptor transcriptional activity by Daxx

The transcriptional activity of the androgen receptor (AR) modulated by positive or negative regulators plays a critical role in controlling the growth and survival of prostate cancer cells. Although numerous positive regulators have been identified, negative regulators of AR are less well understood. We report here that Daxx functions as a negative AR coregulator through direct protein-protein interactions. Overexpression of Daxx suppressed AR-mediated promoter activity in COS-1 and LNCaP cells and AR-mediated prostate-specific antigen expression in LNCaP cells. Conversely, downregulation of endogenous Daxx expression by RNA interference enhances androgen-induced prostate-specific antigen expression in LNCaP cells. In vitro and in vivo interaction studies revealed that Daxx binds to both the amino-terminal and the DNA-binding domain of the AR. Daxx proteins interfere with the AR DNA-binding activity both in vitro and in vivo. Moreover, sumoylation of AR at its amino-terminal domain is involved in Daxx interaction and trans-repression. Together, these findings not only provide a novel role of Daxx in controlling AR transactivation activity but also uncover the mechanism underlying sumoylation-dependent transcriptional repression of the AR.

Androgen deprivation therapy for prostate cancer: current status and future prospects

Androgens play a major role in promoting the development and progression of prostate cancer. As a result, androgen ablation or blockade of androgen action through the androgen receptor (AR) has been the cornerstone of treatment of advanced prostate cancer. Different strategies involving this hormonal therapy produce a significant clinical response in most of the patients, but most responders eventually lose dependency, resulting in mortality. Thus, whether hormonal therapy contributes to the improvement of overall survival rates, especially in patients with advanced prostate cancer, remains controversial. However, patients with advanced disease clearly have a benefit from androgen deprivation-based treatment for palliating their symptoms and for improving the quality of their lives. In order to improve overall survival, novel treatment strategies that prolong the androgen-dependent state and that are useful for androgen-independent disease based on specific molecular mechanisms need to be identified.

Hormone multifunctionalities: a theory of endocrine signaling, command and control

A theory is presented outlining how organisms can function and benefit from multifunctionality of hormones in order to enhance greatly the information-carrying potential of endocrine signaling. Hormones are produced continuously as micropulses, and intermittently as larger pulses. It is generally believed that micropulses generate fluctuating basal hormone concentrations, which may consistently elicit particular responses among diverse variables. Evidence is discussed suggesting that in contrast to the hormone micropulses, the larger endogenous hormone pulses may elicit responses which may differ from one pulse to another and may therefore serve different physiological functions. In this paper we postulate that an endogenous hormone pulse is a specific form of a multisignal message that serves a certain physiological function. Different pulses of a hormone may be signals of diverse multisignal messages that serve different functions. A multisignal message may elicit congruous responses by selectively enhancing some actions and suppressing other actions of the component signals. Various roles of signals of multisignal messages are discussed, as well as processes that may be involved in the diversity and selectivity of actions of different pulses of a hormone. Hormones also are converted into other hormones; we analyze how precursor and derived hormones may function independently of each other, and how precursor hormones may give rise to permissive effects. Mechanisms involved in therapeutic and adverse effects of hormone administrations are analyzed, and a strategy is suggested for developing more selective hormonal therapies.

Analysis of Wnt gene expression in prostate cancer: mutual inhibition by WNT11 and the androgen receptor

The Wnt signaling pathway is aberrantly activated in many tumor types, including those of the prostate, in which beta-catenin accumulates in cell nuclei and acts as a transcriptional coregulator for the androgen receptor. Because activating mutations in the beta-catenin gene are rare in prostate cancer, we have looked for altered expression of other components of the Wnt signaling pathway in prostate cancer cells. Here we determined the expression levels of Wnt family genes in cultured human prostate cells and prostate cancer cell lines. We found that WNT11 expression is elevated in hormone-independent prostate cancer cell lines. Additional analysis indicated that WNT11 expression is also elevated in high-grade prostatic tumors and in hormone-independent xenografts. Growth of hormone-dependent LNCaP cells in hormone-depleted media led to increased WNT11 expression, which was repressed by the synthetic androgen R1881. This repression was inhibited by the antiandrogen bicalutamide, suggesting that androgens negatively regulate WNT11 expression through the androgen receptor. Expression of WNT11 inhibited androgen receptor transcriptional activity and cell growth in androgen-dependent cells but not in androgen-independent cells. WNT11 inhibited activation of the canonical Wnt pathway by WNT3A in HEK 293 cells and inhibited basal beta-catenin/Tcf transcriptional activity in LNCaP cells. However, expression of stabilized beta-catenin did not prevent the inhibition of androgen receptor transcriptional activity by WNT11. Our observations are consistent with a model in which androgen depletion activates WNT11-dependent signals that inhibit androgen-dependent but not androgen-independent cell growth.

Androgen receptor (AR) NH2- and COOH-terminal interactions result in the differential influences on the AR-mediated transactivation and cell growth

Early reports showed that androgen receptor (AR) NH2- and COOH-terminal (N-C) interaction was important for full AR function. However, the influence of these interactions on the AR in vivo effects remains unclear. Here we tested some AR-associated peptides and coregulators to determine their influences on AR N-C interaction, AR transactivation, and AR coregulator function. The results showed that AR coactivators such as ARA70N, gelsolin, ARA54, and SRC-1 can enhance AR transactivation but showed differential influences on the N-C interaction. In contrast, AR corepressors ARA67 and Rad9 can suppress AR transactivation, with ARA67 enhancing and Rad9 suppressing AR N-C interaction. Furthermore, liganded AR C terminus-associated peptides can block AR N-C interaction, but only selective peptides can block AR transactivation and coregulator function. We found all the tested peptides can suppress prostate cancer LNCaP cell growth at different levels in the presence of 5alpha-dihydrotestosterone, but only the tested FXXLF-containing peptides, not FXXMF-containing peptides, can suppress prostate cancer CWR22R cell growth. Together, these results suggest that the effects of AR N-C interactions may not always correlate with similar effects on AR-mediated transactivation and/or AR-mediated cell growth. Therefore, drugs designed by targeting AR N-C interaction as a therapeutic intervention for prostate cancer treatment may face unpredictable in vivo effects.

Microcephaly, jejunal atresia, aberrant right bronchus, ocular anomalies, and XY sex reversal

We present a patient with microcephaly, jejunal atresia, aberrant right tracheobronchial tree, mild left blepharoptosis, and corectopia (irregular pupil), left sectoral iris stromal hypoplasia and peripheral anterior synechia, and 46,XY sex reversal. Testosterone and dihydrotestosterone (DHT) levels were within normal limits for a male infant at 3 weeks of age. Gonadectomy at age 18 months revealed immature testis tissue and no evidence of Müllerian structures. PCR amplification of the androgen receptor (AR) gene and flanking genomic regions revealed no evidence for deletion. Array-comparative genomic hybridization (array-CGH) for assessment of gene dosage in other regions of the genome was normal. This patient represents a multiple anomaly disorder similar to intestinal atresia-ocular anomalies-microcephaly syndrome (MIM#243605) but incorporating 46,XY sex reversal with testicular tissue, demonstrating a defect in the sexual differentiation pathway.

Androgen receptor regulates expression of skeletal muscle-specific proteins and muscle cell types

C2C12 myoblasts expressing the androgen receptor (AR) were used to analyze the role of androgen-AR signaling pathway in skeletal muscle development. Marked up-regulation of AR expression was observed in differentiated myotubes. A nuclear run-on transcription assay demonstrated that transcription of the AR gene is increased during skeletal muscle cell differentiation. Regulation of skeletal muscle-specific protein expression by the androgen-AR signaling pathway was further analyzed using quadriceps skeletal muscle from wild-type (WT) and AR knock-out (ARKO) male mice. A histological analysis of quadriceps skeletal muscle indicates no morphological differences between ARKO and WT mice. However, the androgen-AR signaling pathway increases expression of slow-twitch-specific skeletal muscle proteins and downregulates fast-twitch-specific skeletal muscle proteins, resulting in an increase of slow-twitch muscle fiber type cells in quadriceps muscle.

Expression of androgen receptor coregulators in prostate cancer

PURPOSE

The androgen receptor (AR)-mediated signaling pathway seems to be essentially involved in the development and progression of prostate cancer. In vitro studies have shown that altered expression of AR coregulators may significantly modify transcriptional activity of AR, suggesting that these coregulators could also contribute to the progression of prostate cancer. Here, our goal was to assess alterations in the expression of the AR coregulators in prostate cancer in vivo.

EXPERIMENTAL DESIGN

The expression of 16 AR coactivators and corepressors (SRC1, beta-catenin, TIF2, PIAS1, PIASx, ARIP4, BRCA1, AIB1, AIB3, CBP, STAT1, NCoR1, AES, cyclin D1, p300, and ARA24) was measured in prostate cancer cell lines, xenografts, and clinical prostate tumor specimens by using real-time quantitative reverse transcription-PCR. In addition, gene copy number of SRC1 was analyzed by fluorescence in situ hybridization.

RESULTS

Both AR-positive and AR-negative cell lines and xenografts expressed the coregulators. Most of the coregulators studied were expressed at equal levels in benign prostatic hyperplasia and untreated and hormone-refractory carcinomas. However, the expression of PIAS1 and SRC1 was significantly (P = 0.048 and 0.017, respectively) lower in hormone-refractory prostate tumors than in untreated prostate tumors. No overexpression of the coregulators was found in the clinical material. Paradoxically, the SRC1 gene was found to be amplified and highly expressed in a LuCaP 70 prostate cancer xenograft.

CONCLUSIONS

These findings suggest that the decreased expression of PIAS1 and SRC1 could be involved in the progression of prostate cancer. In addition, gene amplification of SRC1 in one of the xenografts implies that, in some tumors, genetic alteration of SRC1 may provide a growth advantage.

The role of the androgen receptor in the development of prostatic hyperplasia and prostate cancer

The androgen receptor (AR) is an androgen-inducible transcription factor characterized by a modular primary structure, with each module representing a distinct functional unit. After its interaction with androgens, the cytoplasmic AR is activated and translocated to the nucleus where it binds to target genes at the androgen responsive element(s) and recruits coregulators to form a multiprotein complex that interacts with transcriptional mediators and the basal transcription machinery to regulate gene transcription. Androgens play an essential role in the morphogenesis and physiology of the normal prostate. The etiology of benign prostatic hyperplasia (BPH) and prostatic neoplasia, which can progress to adenocarcinoma, is androgen-dependent, and reduction/obliteration of androgen action in the prostate has been the therapy of choice for BPH and prostate cancer. After androgen withdrawal and antiandrogen treatment, the androgen responsive prostate cancer cells cease to proliferate and undergo apoptosis, causing tumor regression. However, relapses are seen invariably, when tumors emerge as androgen-independent and apoptosis-resistant. Gene amplification and amino acid substitutions in the AR are detected at a high frequency in recurrent tumors. These changes confer growth advantage to the tumor cells due to either hypersensitivity of AR to low, castrate-level androgens or a realignment of the receptor conformation, leading to altered ligand specificity that enables antiandrogens, adrenal androgens and non-androgen steroids act agonistically to increase AR activity. Persistence of signaling by the wild-type AR in therapy-resistant tumors is due to the increased receptor activity caused by cross talk of AR with multiple intracellular signaling cascades, especially the growth factor activated MAP kinase/ERK and PI3 kinase/Akt pathways. Ablation of AR function using antisense oligodeoxynucleotides, ribozymes or small interference RNAs (RNAi) holds promise as future approaches to the successful treatment of hormone-refractory, apoptosis-resistant prostate tumors.

PC-3 cells with enhanced androgen receptor signaling: a model for clonal selection in prostate cancer

BACKGROUND

Two sublines of the human prostate cancer cell line, PC-3, which is widely used as a model of prostate cancer progression, have been reported: PC-3(AR-) that do not express androgen receptor (AR), and PC-3AR+ that have measurable AR RNA but little protein.

METHODS

We assayed the geneotype, karyotype, AR expression, and physical characteristics of the two PC-3 sublines, and compared their ability to elicit a transactivation response from ectopic AR in the presence and absence of specific AR coregulators.

RESULTS

PC-3(AR-) and PC-3AR+ cells are genotypically and karyotypically similar, but exhibit salient differences in their morphology, growth rate, and expression of AR RNA. Whereas endogenous AR expression in PC-3AR+ cells does not result in sufficient protein to confer androgen responsiveness in culture, ectopic AR consistently elicited a much greater transactivation response in PC-3AR+ than in PC-3(AR-) cells, without altered sensitivity to activation by native ligand or AR coregulators including GRIP1, BRCA1, and Zac1. Moreover, phenotypic differences of AR variants implicated in prostate cancer susceptibility and progression were only observed in PC-3AR+ cells. Higher levels of known AR coregulator proteins detected in PC-3AR+ compared with PC-3(AR-) cells likely contribute to these differences.

CONCLUSIONS

These studies provide new evidence that the androgen-signaling axis can be sensitized in prostate cancer cells, and have important implications for the analysis and interpretation of AR structure and function in in vitro cell systems.

SENP1 enhances androgen receptor-dependent transcription through desumoylation of histone deacetylase 1

SUMO (also called Sentrin) is a ubiquitin-like protein that plays an important role in regulating protein function and localization. It is known that several nuclear receptors are modified by SUMO; however, the effect of desumoylation in regulating nuclear receptor function has not been elucidated. Here we show that androgen receptor (AR)-mediated transcription is markedly enhanced by SENP1, a member of SUMO-specific protease family. SENP1's ability to enhance AR-dependent transcription is not mediated through desumoylation of AR, but rather through its ability to deconjugate histone deacetylase 1 (HDAC1), thereby reducing its deacetylase activity. HDAC1's repressive effect on AR-dependent transcription could be reversed by SENP1 and by deletion of its sumoylation sites. RNA interference depletion of endogenous HDAC1 also reduced SENP1's effect. Thus, SENP1 could regulate AR-dependent transcription through desumoylation of HDAC1. These studies provide insights on the potential role of desumoylation in the regulation of nuclear receptor activity.

Molecular basis for the antiandrogen withdrawal syndrome

In patients with prostate cancer who manifest disease progression during combined androgen blockade therapy, discontinuation of antiandrogen treatment might result in prostate-specific antigen decline, often associated with clinical improvement. The response called antiandrogen withdrawal syndrome is thus acknowledged as a general phenomenon. However, molecular mechanisms responsible for this syndrome are not completely understood. This article outlines the proposed mechanisms, including alterations of androgen receptor gene and its coregulatory proteins and activation of the signal transduction pathway, and the potential therapeutic approaches based on the specific mechanisms.

The LATS2/KPM Tumor Suppressor Is a Negative Regulator of the Androgen Receptor

The androgen receptor (AR) is a member of the steroid receptor superfamily that plays critical roles in the development and maintenance of the male reproductive system and in prostate cancer. Actions of AR are controlled by interaction with several classes of coregulators. In this study, we have identified LATS2/KPM as a novel AR-interacting protein. Human LATS1 and LATS2 are tumor suppressors that are homologs of Drosophila warts/lats. The interaction surface of LATS2 is mapped to the central region of the protein, whereas the AR ligand binding domain is sufficient for this interaction. LATS2 functions as a modulator of AR by inhibiting androgen-regulated gene expression. The mechanism of LATS2-mediated repression of AR activity appears to involve the inhibition of AR NH2- and COOH-terminal interaction. Chromatin immunoprecipitation assays in human prostate carcinoma cells reveal that LATS2 and AR are present in the protein complex that binds at the promoter and enhancer regions of prostate-specific antigen, and overexpression of LATS2 results in a reduction in androgen-induced expression of endogenous prostate-specific antigen mRNA. Immunohistochemistry shows that LATS2 and AR are localized within the prostate epithelium and that LATS2 expression is lower in human prostate tumor samples than in normal prostate. The results suggest that LATS2 may play a role in AR-mediated transcription and contribute to the development of prostate cancer.

Subfertility and defective folliculogenesis in female mice lacking androgen receptor

The roles of the androgen receptor (AR) in female fertility and ovarian function remain largely unknown. Here we report on the generation of female mice lacking AR (AR(-/-)) and the resulting influences on the reproductive system. Female AR(-/-) mice appear normal but show longer estrous cycles and reduced fertility. The ovaries from sexually mature AR(-/-) females exhibited a marked reduction in the number of corpora lutea. After superovulation treatment, the AR(-/-) ovaries produced fewer oocytes and also showed fewer corpora lutea. During the periovulatory period, an intensive granulosa apoptosis event occurs in the AR(-/-) preovulatory follicles, concurrent with the down-regulation of p21 and progesterone receptor expression. Furthermore, the defective conformation of the cumulus cell-oocyte complex from the AR(-/-) females implies a lower fertilization capability of the AR(-/-) oocytes. In addition to insufficient progesterone production, the diminished endometrial growth in uteri in response to exogenous gonadotropins indicates that AR(-/-) females exhibit a luteal phase defect. Taken together, these data provide in vivo evidence showing that AR plays an important role in female reproduction.

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.

Actin monomer enhances supervillin-modulated androgen receptor transactivation

Actin-binding protein, supervillin, has been identified as an androgen receptor (AR) coregulator. Although actin has been suggested to participate in transcription regulation, the mechanism is not clear. Here we demonstrate signals involved in the cytoskeleton dynamic can modulate the coregulator function of supervillin. Three actin isoforms cooperate with supervillin in additive manner to further enhance AR transactivation. Latrunculin B toxin, an actin chelator, reduces the availability of monomer actin and attenuates supervillin function. Rac, the small G-protein kinase, is well studied in reorganization of cytoskeleton. The overexpression of constitutive-active Rac triggers the membrane ruffling site and reduces the coregulator activity of supervillin. Together, the availability of actin monomer affects supervillin-modulated AR transactivation.

Androgens and bone

Loss of estrogens or androgens increases the rate of bone remodeling by removing restraining effects on osteoblastogenesis and osteoclastogenesis, and also causes a focal imbalance between resorption and formation by prolonging the lifespan of osteoclasts and shortening the lifespan of osteoblasts. Conversely, androgens, as well as estrogens, maintain cancellous bone mass and integrity, regardless of age or sex. Although androgens, via the androgen receptor (AR), and estrogens, via the estrogen receptors (ERs), can exert these effects, their relative contribution remains uncertain. Recent studies suggest that androgen action on cancellous bone depends on (local) aromatization of androgens into estrogens. However, at least in rodents, androgen action on cancellous bone can be directly mediated via AR activation, even in the absence of ERs. Androgens also increase cortical bone size via stimulation of both longitudinal and radial growth. First, androgens, like estrogens, have a biphasic effect on endochondral bone formation: at the start of puberty, sex steroids stimulate endochondral bone formation, whereas they induce epiphyseal closure at the end of puberty. Androgen action on the growth plate is, however, clearly mediated via aromatization in estrogens and interaction with ERalpha. Androgens increase radial growth, whereas estrogens decrease periosteal bone formation. This effect of androgens may be important because bone strength in males seems to be determined by relatively higher periosteal bone formation and, therefore, greater bone dimensions, relative to muscle mass at older age. Experiments in mice again suggest that both the AR and ERalpha pathways are involved in androgen action on radial bone growth. ERbeta may mediate growth-limiting effects of estrogens in the female but does not seem to be involved in the regulation of bone size in males. In conclusion, androgens may protect men against osteoporosis via maintenance of cancellous bone mass and expansion of cortical bone. Such androgen action on bone is mediated by the AR and ERalpha.

Functional domain and motif analyses of androgen receptor coregulator ARA70 and its differential expression in prostate cancer

Androgen receptor (AR)-associated coregulator 70 (ARA70) was the first identified AR coregulator. However, its molecular mechanism and biological relevance to prostate cancer remain unclear. Here we show that ARA70 interacts with and promotes AR activity via the consensus FXXLF motif within the ARA70-N2 domain (amino acids 176-401). However, it does not promote AR activity via the classic LXXLL motif located at amino acids 92-96, although this classic LXXLL motif is important for ARA70 to interact with other receptors, such as PPARgamma. The molecular mechanisms by which ARA70 enhances AR transactivation involve the increase of AR expression, protein stability, and nuclear translocation. Furthermore, ARA70 protein is more frequently detected in prostate cancer specimens (91.74%) than in benign tissues (64.64%, p < 0.0001). ARA70 expression is also increased in high-grade prostate cancer tissues as well as the hormone-refractory LNCaP xenografts and prostate cancer cell lines. Because ARA70 can promote the antiandrogen hydroxyflutamide (HF)-enhanced AR transactivation, the increased ARA70 expression in hormone-refractory prostate tumors may confer the development of HF withdrawal syndrome, commonly diagnosed in patients with the later stages of prostate cancer. Because ARA70-N2 containing the AR-interacting FXXLF motif without coactivation function can suppress HF-enhanced AR transactivation in the hormone-refractory LNCaP cells, using the ARA70-N2 inhibitory peptide at the hormone refractory stage to battle the HF withdrawal syndrome may become an alternative strategy to treat prostate cancer.

Androgen receptor immunoreactivity in skeletal muscle: enrichment at the neuromuscular junction

Potential cellular targets of androgen action within skeletal muscle of the rat were determined by comparing the cellular distribution of androgen receptor (AR)-positive nuclei in the highly androgen-responsive levator ani (LA) muscle with that of the relatively androgen-unresponsive extensor digitorum longus (EDL) muscle. We found that androgen responsiveness correlates with AR expression in muscle fibers and not in fibroblasts. Results indicate that a much higher percentage of myonuclei in the LA are AR(+) than in the EDL (74% vs. 7%), correlating with differences in androgen responsiveness. Both muscles contain an equivalent proportion of AR(+) fibroblasts (approximately 62%). AR(+) nuclei were not observed in terminal Schwann cells in either muscle. These results suggest that ARs within LA muscle fibers mediate the androgen-dependent survival and growth of the LA muscle and its motoneurons. We also observed an unexpected enrichment of AR(+) myonuclei and fibroblasts proximate to neuromuscular junctions, suggesting that ARs at muscle synapses may selectively regulate synapse-specific genes important for the survival and growth of motoneurons. Although castration reduced the proportion of AR(+) fibroblasts in both muscles, the proportion of AR(+) myonuclei was reduced only in the LA. As expected, testosterone treatment prevented these effects of castration but, unexpectedly, increased the proportion of AR(+) myonuclei in the EDL to above normal. These results suggest that how AR expression in skeletal muscle is influenced by androgens depends not only on the particular muscle but on the particular cell type within that muscle.

Androgen receptor: a key molecule in the progression of prostate cancer to hormone independence

Despite earlier detection and recent advances in surgery and radiation, prostate cancer is second only to lung cancer in male cancer deaths in the United States. Hormone therapy in the form of medical or surgical castration remains the mainstay of systemic treatment in prostate cancer. Over the last 15 years with the clinical use of prostate specific antigen (PSA), there has been a shift to using hormone therapy earlier in the disease course and for longer duration. Despite initial favorable response to hormone therapy, over a period of time these tumors will develop androgen-independence that results in death. The androgen receptor (AR) is central to the initiation and growth of prostate cancer and to its response to hormone therapy. Analyses have shown that AR continues to be expressed in androgen-independent tumors and AR signaling remains intact as demonstrated by the expression of the AR regulated gene, PSA. Androgen-independent prostate cancers have demonstrated a variety of AR alterations that are either not found in hormone naïve tumors or found at lower frequency. These changes include AR amplification, AR point mutation, and changes in expression of AR co-regulatory proteins. These AR changes result in a "super AR" that can respond to lower concentrations of androgens or to a wider variety of agonistic ligands. There is also mounting evidence that AR can be activated in a ligand independent fashion by compounds such as growth factors or cytokines working independently or in combination. These growth factors working through receptor tyrosine kinase pathways may promote AR activation and growth in low androgen environments. The clinical significance of these AR alterations in the development and progression of androgen-independent prostate cancer remains to be determined. Understanding the changes in AR signaling in the evolution of androgen-independent prostate cancer will be key to the development of more effective hormone therapy.

Infertility with defective spermatogenesis and hypotestosteronemia in male mice lacking the androgen receptor in Sertoli cells

Androgens and the androgen receptor (AR) play important roles in male fertility, although the detailed mechanisms, particularly how androgen/AR influences spermatogenesis in particular cell types, remain unclear. Using a Cre-Lox conditional knockout strategy, we generated a tissue-specific knockout mouse with the AR gene deleted only in Sertoli cells (S-AR(-/y)). Phenotype analyses show the S-AR(-/y) mice were indistinguishable from WT AR mice (B6 AR(+/y)) with the exception of testes, which were significantly atrophied. S-AR(-/y) mice were infertile, with spermatogenic arrest predominately at the diplotene premeiotic stage and almost no sperm detected in the epididymides. S-AR(-/y) mice also have lower serum testosterone concentrations and higher serum leuteinizing hormone concentrations than B6 AR(+/y) mice. Further mechanistic studies demonstrated that S-AR(-/y) mice have defects in the expression of anti-Müllerian hormone, androgen-binding protein, cyclin A1, and sperm-1, which play important roles in the control of spermatogenesis and/or steroidogenesis. Together, our Sertoli cell-specific AR knockout mice provide in vivo evidence of the need for functional AR in Sertoli cells to maintain normal spermatogenesis and testosterone production, and ensure normal male fertility.

Tip110, the Human Immunodeficiency Virus Type 1 (HIV-1) Tat-interacting Protein of 110 kDa as a Negative Regulator of Androgen Receptor (AR) Transcriptional Activation

Upon binding to androgen, androgen receptor (AR) can activate expression of target genes through its direct binding to the androgen-responsive elements (AREs), which are located within the target gene promoters and/or enhancers. A number of cellular proteins have been identified as co-regulators to regulate this transactivation process. One common structural feature among these co-regulators is the presence of the LXXLL motif (X, any amino acid), the so-called nuclear receptor (NR) box, through which binding of these regulatory proteins to AR occurs. We have recently shown that Tip110 functions to potentiate the transactivation activity of human immunodeficiency virus type I (HIV-1) Tat protein. In this study, we report that Tip110 is a potent AR-binding protein that can suppress AR activity. Tip110 bound to AR in an NR box-dependent manner and inhibited AREs-mediated reporter gene expression. The inhibitory effects were abolished by removal of the NR box. Moreover, knock-down of the constitutive Tip110 expression significantly augmented AR transcriptional activation. In agreement with these findings, Tip110 overexpression blocked the prostate-specific antigen (PSA) gene, a well characterized target gene of AR from expression in LNCaP cells. Further analysis revealed that Tip110 prevented the complex formation between AR and AREs. Taken together, these results indicate that Tip110 is a negative regulator of AR transcriptional activation, and may be directly involved in AR-related developmental, physiological, and pathological processes.

USF2 is connected to GAAAATATGATA element and associates with androgen receptor-dependent transcriptional regulation in prostate

BACKGROUND

We have previously identified a GAAAATATGATA binding site (pros) of a transcription factor involved in prostatic and androgen-dependent gene regulation. We now purified the potential factors interacting with the pros and characterized their co-operation with the androgen receptor (AR).

METHODS

Sequence-specific DNA affinity chromatography, mass-spectrometry, electromobility shift assays, supershifts, glutathione-S-transferase pull-downs, and transient transfections.

RESULTS

Several proteins bound to the pros site, but only upstream stimulatory factor 2 (USF2) was confirmed to be part of the transcription factor complex. Weak interaction was detected between AR and the transcription factor complex. Physical proximity between the androgen response element (ARE) and the pros was shown to be important for their co-operation. In the presence of pros and androgen, AR achieves its maximal efficiency even at low concentrations.

CONCLUSIONS

The protein complex binding to the GAAAATATGATA site does not have a significant independent function, but may interact with AR if GAAAATATGATA is physically close to the ARE and enhances the transactivation function of AR.

Modulation of androgen receptor-dependent transcription by resveratrol and genistein in prostate cancer cells

BACKGROUND

The androgen receptor (AR) is a ligand-activated transcription factor that mediates the biological responses of androgens in the prostate gland. This study focuses on the chemopreventive agents, resveratrol and genistein, on AR-mediated transcription in prostate cancer cells.

RESULTS

We found that resveratrol and genistein activated AR-driven gene expression at low concentrations, whereas they repressed the AR-dependent reporter gene activity at high concentrations. We determined that resveratrol and genistein induced AR-driven gene expression by activating the Raf-MEK-ERK kinase pathway. The ERK1 kinase phosphorylated the AR on multiple sites in vitro, but this phosphorylation event did not contribute to the resveratrol-induced AR transactivation.

CONCLUSIONS

In vitro and in vivo studies have indicated that resveratrol and genistein are promising chemopreventive agents. Given the clear evidence that AR pathways are involved in the development and progression of prostate cancer, these data showed that the ability to modulate AR function would contribute the observed chemopreventive activity of resveratrol and genistein.

Update on hormone-refractory prostate cancer

PURPOSE OF REVIEW

Androgen deprivation therapy is the cornerstone treatment for men with de novo or recurrent metastatic prostate cancer. Unfortunately, androgen deprivation therapy is primarily palliative, with nearly all men progressing to an androgen-independent state. Hormone-refractory prostate cancer presents significant management challenges and is the focus of this review.

RECENT FINDINGS

Investigations into the pathophysiology of hormone-refractory prostate cancer, the exploration of chemotherapeutic combinations, novel biological targets, skeletal protectants, and radiopharmaceuticals, as well as new prognostic tools are expanding the clinician's armamentarium and improving patient outcomes.

SUMMARY

Bisphosphonates and chemotherapy are providing effective palliative approaches. Phase II trials of taxane-based regimens show higher response rates and longer survival than has typically been achieved with existing standards. Two completed randomized phase III studies to be reported in mid-2004 will more definitively answer the question of whether currently available chemotherapy can improve survival.

An examination of how different mutations at arginine 855 of the androgen receptor result in different androgen insensitivity phenotypes

Two substitutions at an identical location in the ligand-binding domain (LBD) of the human androgen receptor (AR), R855C and R855H, are associated with complete androgen insensitivity syndrome (AIS) and partial AIS, respectively. Kinetic analysis of the mutant receptors in genital skin fibroblasts and in transfected cells revealed very low total binding (Bmax) and increased rate constants of dissociation (k) for the R855C mutant; and normal Bmax and k, with slightly elevated equilibrium affinity constants (Kd), but decreased transactivational capacity for the R855H mutant. Further analysis of the R855H mutant revealed both thermolability and decreased N/C-terminal inter-actions in the presence and absence of the co-activator transcriptional intermediary factor 2. To establish the nature of these functional differences we have used molecular dynamic modeling to create four-dimensional models of each of the mutant receptors. Molecular dynamic modeling produced profoundly different models for each of the mutants: in modeling of R855C a surprisingly significant distant alteration in the position of helix 12 of the helix 12 positioning of the AR ligand binding domain (AR-LBD) occurs, which would predict severe ligand binding abnormalities and complete AIS; in modeling of R855H, no dramatic effect on the position of helix 12 was seen; thus, binding properties of the receptor are not compromised. Molecular dynamics four-dimensional modeling clearly supports the biochemical and kinetic studies of both mutants. Such novel computational modeling may lead to a better understanding of the structure-function relationships and the molecular mechanics of ligand binding not only of the AR-LBD but also of other nuclear receptors.

Human checkpoint protein hRad9 functions as a negative coregulator to repress androgen receptor transactivation in prostate cancer cells

Positive responses to combined androgen elimination therapy and radiation therapy have been well documented in the treatment of prostate cancer patients. The detailed mechanisms how androgen-androgen receptor (AR) cross talks to the radiation-related signal pathways, however, remain largely unknown. Here we report the identification of hRad9, a key member of the checkpoint Rad protein family, as a coregulator to suppress androgen-AR transactivation in prostate cancer cells. In vivo and in vitro interaction assays using Saccharomyces cerevisiae two-hybrid, mammalian two-hybrid, glutathione S-transferase pull-down, and coimmunoprecipitation methods prove that AR can interact with the C terminus of hRad9 via its ligand binding domain. The FXXLF motif within the C terminus of hRad9 interrupts the androgen-induced interaction between the N terminus and C terminus of AR. This interaction between AR and hRad9 may result in the suppression of AR transactivation, demonstrated by the repressed AR transactivation in androgen-induced luciferase reporter assay and the reduced endogenous prostate-specific antigen expression in Western blot assay. Addition of small interfering RNA of hRad9 can reverse hRad9 suppression effects, which suggests that hRad9 functions as a repressor of AR transactivation in vivo. Together, our data provide the first linkage between androgen-AR signals and radiation-induced responses. Further studies of the influence of hRad9 on prostate cancer growth may provide potential new therapeutic approaches.

Androgen receptor in prostate cancer

The normal development and maintenance of the prostate is dependent on androgen acting through the androgen receptor (AR). AR remains important in the development and progression of prostate cancer. AR expression is maintained throughout prostate cancer progression, and the majority of androgen-independent or hormone refractory prostate cancers express AR. Mutation of AR, especially mutations that result in a relaxation of AR ligand specificity, may contribute to the progression of prostate cancer and the failure of endocrine therapy by allowing AR transcriptional activation in response to antiandrogens or other endogenous hormones. Similarly, alterations in the relative expression of AR coregulators have been found to occur with prostate cancer progression and may contribute to differences in AR ligand specificity or transcriptional activity. Prostate cancer progression is also associated with increased growth factor production and an altered response to growth factors by prostate cancer cells. The kinase signal transduction cascades initiated by mitogenic growth factors modulate the transcriptional activity of AR and the interaction between AR and AR coactivators. The inhibition of AR activity through mechanisms in addition to androgen ablation, such as modulation of signal transduction pathways, may delay prostate cancer progression.

Developmentally essential protein flightless I is a nuclear receptor coactivator with actin binding activity

Hormone-activated nuclear receptors (NR) activate transcription by recruiting multiple coactivator complexes to the promoters of target genes. One important coactivator complex includes a p160 coactivator (e.g., GRIP1, SRC-1, or ACTR) that binds directly to activated NR, the histone acetyltransferase p300 or CBP, and the arginine-specific histone methyltransferase CARM1. We previously demonstrated that the coactivator function of CARM1 depends both on the methyltransferase activity and on additional unknown proteins that bind to CARM1. In this study a yeast two-hybrid screen for proteins that bind CARM1 identified the protein Flightless I (Fli-I), which has essential roles in Drosophila and mouse development. Fli-I bound to CARM1, GRIP1, and NRs and cooperated synergistically with CARM1 and GRIP1 to enhance NR function. Fli-I bound poorly to and did not cooperate with PRMT1, a CARM1-related protein arginine methyltransferase that also functions as an NR coactivator. The synergy between GRIP1, CARM1, and Fli-I required the methyltransferase activity of CARM1. The C-terminal AD1 (binding site for p300/CBP) and AD2 (binding site for CARM1) activation domains of GRIP1 contributed to the synergy but were less stringently required than the N-terminal region of GRIP1, which is the binding site for Fli-I. Endogenous Fli-I was recruited to the estrogen-regulated pS2 gene promoter of MCF-7 cells in response to the hormone, and reduction of endogenous Fli-I levels by small interfering RNA reduced hormone-stimulated gene expression by the endogenous estrogen receptor. A fragment of Fli-I that is related to the actin binding protein gelsolin enhanced estrogen receptor activity, and mutations that reduced actin binding also reduced the coactivator function of this Fli-I fragment. These data suggest that Fli-I may facilitate interaction of the p160 coactivator complex with other coactivators or coactivator complexes containing actin or actin-like proteins.

ARA67/PAT1 functions as a repressor to suppress androgen receptor transactivation

The androgen receptor (AR) may recruit multiple coregulators for proper or optimal transactivation. Here we report the identification and characterization of ARA67/PAT1 as an AR coregulator from a prostate cDNA library. ARA67/PAT1 was screened out as an AR N terminus interacting protein. Interaction mapping shows that the cooperation of multiple domains within ARA67/PAT1 may be required for the maximal interaction with AR. ARA67/PAT1 functions as a repressor with better suppressive effects on AR compared to glucocorticoid receptor and estrogen receptor. Further mechanism dissection reveals that the interrupted AR cytoplasmic-nuclear shuttling may play a major role in ARA67/PAT1 mediated suppression on AR. Together, these results suggest that ARA67/PAT1 may function as a novel repressor that can modulate AR function in prostate cancer.

Mechanistic relationship between androgen receptor polyglutamine tract truncation and androgen-dependent transcriptional hyperactivity in prostate cancer cells

Androgen receptor (AR) signaling pathways mediate critical events in normal and neoplastic prostate growth. Shortening of the polymorphic N-terminal polyglutamine (poly(Q)) tract of the AR gene leads to transcriptional hyperactivity and has been correlated with an increased risk of prostate cancer. The underlying mechanisms for these effects are poorly understood. We show here that androgen-dependent cellular proliferation and transcription in prostate cancer cells is inversely correlated to the length of the AR poly(Q) region. We further show that AR proteins containing a shortened poly(Q) region functionally respond to lower concentrations of androgens than wild type AR. Whereas DNA binding activity is relatively unaffected by AR poly(Q) variation, we found that ligand binding affinity and the ligand-induced NH(2)- to COOH-terminal intramolecular interaction is enhanced when the poly(Q) region is shortened. Importantly, we show that AR proteins containing a shortened poly(Q) region associate in vivo with higher levels of specific p160 coactivators and components of the SWI/SNF chromatin remodeling complex as compared with the wild type AR. Collectively, our findings suggest that the AR transcriptional hyperactivity associated with shortened poly(Q) length stems from altered ligand-induced conformational changes that enhance coactivator recruitment.

G Proteins in Cancer: The Prostate Cancer Paradigm

Signal transduction research investigating mechanisms of androgen-independent prostate cancer cell proliferation has historically focused on the role of androgen and peptide growth factor receptors. More recent work has raised the idea that intracellular signaling mechanisms triggered by extracellular hormonal factors acting through heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) can also mediate and sustain this pathologic process. Prostate cancer patients with advanced disease express elevated levels of GPCRs and GPCR ligands, suggesting that the GPCR system is activated in the cancerous gland and may contribute to tumor growth. Importantly, inhibition of G protein signaling attenuates prostate cancer cell growth in animal models. The nature of intracellular signaling pathways mediating mitogenic effects of GPCRs in prostate cancer is poorly defined, although the G protein-dependent activation of the Ras-to-mitogen-activated protein kinase pathway has emerged as a critical regulatory event. Activated GPCRs may also exert their mitogenic effects in the prostate by activating the androgen receptor.

Androgen suppresses PML protein expression in prostate cancer CWR22R cells

The ability of PML to modulate key suppressive pathways in tumor cells suggests that PML may act as a tumor suppressor. The detailed mechanism of how PML functions in prostate cancer progression, however, remains unknown. Here we demonstrate that in the presence of androgen, PML protein expression can be suppressed in CWR22R prostate cancer cells. Further studies reveal that PML can selectively suppress AR transactivation and PML protein expression positively correlates with increased p21 protein level and enhances p53 transcription ability in CWR22R cells. We also found that PML strongly inhibits CWR22R cell colony formation, while PML siRNA enhances AR activity and CWR22R cell colony formation. Together our results suggest that PML may suppress prostate cancer cell growth by inhibiting AR transactivation and/or enhancing p53 activity.