Inhibiting proteasomes in human HepG2 and LNCaP cells increases endogenous androgen receptor levels

Manzamine A reduces androgen receptor transcription and synthesis by blocking E2F8-DNA interactions and effectively inhibits prostate tumor growth in mice

The androgen receptor (AR) is the main driver in the development of castration-resistant prostate cancer, where the emergence of AR splice variants leads to treatment-resistant disease. Through detailed molecular studies of the marine alkaloid manzamine A (MA), we identified transcription factor E2F8 as a previously unknown regulator of AR transcription that prevents AR synthesis in prostate cancer cells. MA significantly inhibited the growth of various prostate cancer cell lines and was highly effective in inhibiting xenograft tumor growth in mice without any pathophysiological perturbations in major organs. MA suppressed the full-length AR (AR-FL), its spliced variant AR-V7, and the AR-regulated prostate-specific antigen (PSA; also known as KLK3) and human kallikrein 2 (hK2; also known as KLK2) genes. RNA sequencing (RNA-seq) analysis and protein modeling studies revealed E2F8 interactions with DNA as a potential novel target of MA, suppressing AR transcription and its synthesis. This novel mechanism of blocking AR biogenesis via E2F8 may provide an opportunity to control therapy-resistant prostate cancer over the currently used AR antagonists designed to target different parts of the AR gene.

Differentially disrupted spinal cord and muscle energy metabolism in spinal and bulbar muscular atrophy

Prior studies showed that polyglutamine-expanded androgen receptor (AR) is aberrantly acetylated and that deacetylation of the mutant AR by overexpression of nicotinamide adenine dinucleotide-dependent (NAD+-dependent) sirtuin 1 is protective in cell models of spinal and bulbar muscular atrophy (SBMA). Based on these observations and reduced NAD+ in muscles of SBMA mouse models, we tested the therapeutic potential of NAD+ restoration in vivo by treating postsymptomatic transgenic SBMA mice with the NAD+ precursor nicotinamide riboside (NR). NR supplementation failed to alter disease progression and had no effect on increasing NAD+ or ATP content in muscle, despite producing a modest increase of NAD+ in the spinal cords of SBMA mice. Metabolomic and proteomic profiles of SBMA quadriceps muscles indicated alterations in several important energy-related pathways that use NAD+, in addition to the NAD+ salvage pathway, which is critical for NAD+ regeneration for use in cellular energy production. We also observed decreased mRNA levels of nicotinamide riboside kinase 2 (Nmrk2), which encodes a key kinase responsible for NR phosphorylation, allowing its use by the NAD+ salvage pathway. Together, these data suggest a model in which NAD+ levels are significantly decreased in muscles of an SBMA mouse model and intransigent to NR supplementation because of decreased levels of Nmrk2.

Insight into Recent Advances in Degrading Androgen Receptor for Castration-Resistant Prostate Cancer

Induced protein degradation has emerged as an innovative drug discovery approach, complementary to the classical method of suppressing protein function. The androgen receptor signaling pathway has been identified as the primary driving force in the development and progression of lethal castration-resistant prostate cancer. Since androgen receptor degraders function differently from androgen receptor antagonists, they hold the promise to overcome the drug resistance challenges faced by current therapeutics. Proteolysis-targeting chimeras (PROTACs), monomeric degraders, hydrophobic tagging, molecular glues, and autophagic degradation have demonstrated their capability in downregulating intracellular androgen receptor concentrations. The potential of these androgen receptor degraders to treat castration-resistant prostate cancer is substantiated by the advancement of six PROTACs and two monomeric androgen receptor degraders into phase I or II clinical trials. Although the chemical structures, in vitro and in vivo data, and degradation mechanisms of androgen receptor degraders have been reviewed, it is crucial to stay updated on recent advances in this field as novel androgen receptor degraders and new strategies continue to emerge. This review thus provides insight into recent advancements in this paradigm, offering an overview of the progress made since 2020.

Dual-Acting Peptides Target EZH2 and AR: A New Paradigm for Effective Treatment of Castration-Resistant Prostate Cancer

Prostate cancer starts as a treatable hormone-dependent disease, but often ends in a drug-resistant form called castration-resistant prostate cancer (CRPC). Despite the development of the antiandrogens enzalutamide and abiraterone for CRPC, which target the androgen receptor (AR), drug resistance usually develops within 6 months and metastatic CRPC (mCRPC) leads to lethality. EZH2, found with SUZ12, EED, and RbAP48 in Polycomb repressive complex 2 (PRC2), has emerged as an alternative target for the treatment of deadly mCRPC. Unfortunately, drugs targeting EZH2 have shown limited efficacy in mCRPC. To address these failures, we have developed novel, dual-acting peptide inhibitors of PRC2 that uniquely target the SUZ12 protein component, resulting in the inhibition of both PRC2 canonical and noncanonical functions in prostate cancer. These peptides were found to inhibit not only the EZH2 methylation activity, but also block its positive effect on AR gene expression in prostate cancer cells. Since the peptide effect on AR levels is transcriptional, the inhibitory peptides can block the expression of both full-length AR and its splicing variants including AR-V7, which plays a significant role in the development of drug resistance. This dual-mode action provides the peptides with the capability to kill enzalutamide-resistant CRPC cells. These peptides are also more cytotoxic to prostate cancer cells than the combination of enzalutamide and an EZH2 inhibitory drug, which was recently suggested to be an effective treatment of mCRPC disease. Our data show that such a dual-acting therapeutic approach can be more effective than the existing front-line drug therapies for treating deadly mCRPC.

The role of ubiquitination in spinal and bulbar muscular atrophy

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

Novel AR/AR-V7 and Mnk1/2 Degrader, VNPP433-3β: Molecular Mechanisms of Action and Efficacy in AR-Overexpressing Castration Resistant Prostate Cancer In Vitro and In Vivo Models

Prostate cancer (PCa) relies in part on AR-signaling for disease development and progression. Earlier, we developed drug candidate galeterone, which advanced through phase 2-clinical trials in treating castration-resistant PCa (CRPC). Subsequently, we designed, synthesized, and evaluated next-generation galeterone-analogs including VNPP433-3β which is potently efficacious against pre-clinical models of PCa. This study describes the mechanism of action of VNPP433-3β that promotes degradation of full-length AR (fAR) and its splice variant AR-V7 besides depleting MNK1/2 in in vitro and in vivo CRPC models that stably overexpresses fAR. VNPP433-3β directly engages AR within the cell and promotes proteasomal degradation of fAR and its splice variant AR-V7 by enhancing the interaction of AR with E3 ligases MDM2/CHIP but disrupting AR-HSP90 binding. Next, VNPP433-3β decreases phosphorylation of 4EBP1 and abates binding of eIF4E and eIF4G to 5' cap of mRNA by depleting MNK1/2 with consequent depletion of phosphorylated eIF4E. Finally, RNA-seq demonstrates modulation of multiple pathways that synergistically contribute to PCa inhibition. Therefore, VNPP433-3β exerts its antitumor effect by imposing 1) transcriptional regulation of AR and AR-responsive oncogenes 2) translational regulation by disrupting mRNA-5'cap-dependent translation initiation, 3) reducing AR half-life through enhanced proteasomal degradation in vitro and AR-overexpressing tumor xenografts in vivo.

Ligand Binding Prolongs Androgen Receptor Protein Half-Life by Reducing its Degradation

Androgens are important in female reproduction, but the molecular actions of androgens in female reproductive tissues are not fully understood. We investigated the androgen-responsive transcriptome in human and mouse granulosa cells (GCs) and surprisingly found that the gene-regulation activity of androgen receptor (AR) in these cells is negligible. We then investigated extranuclear actions of AR and found that in human and mouse GCs, as well as in prostate cancer cells, dihydrotestosterone (DHT) dramatically increases the half-life of its own receptor protein. Using the human granulosa-like KGN cells, we show that this effect is not the result of increased AR gene transcription or protein synthesis, nor is it fully abrogated by proteasome inhibition. Knockdown of PTEN, which contributes to degradation of cytoplasmic AR, did not diminish AR accumulation in the presence of DHT. Using immunofluorescence cellular localization studies, we show that nuclear AR is selectively protected from degradation in the presence of DHT. Knockdown of importin 7 expression, a potential regulator of AR nuclear import, does not affect DHT-mediated nuclear accumulation of AR, suggesting importin 7-independent nuclear import of AR in GCs. Further, DNA binding is not required for this protective mechanism. In summary, we show that ligand binding sequesters AR in the nucleus through enhanced nuclear localization independent of DNA binding, thereby protecting it from proteasome degradation in the cytoplasm. This phenomenon distinguishes AR from other sex steroid receptors and may have physiological significance through a positive feedback loop in which androgen induces its own activity in male and female reproductive tissues.

Molecular mechanism of Bisphenol A on androgen receptor antagonism

Bisphenol A (BPA), one of the highest production volume chemicals, is a typical endocrine-disrupting chemical (EDC) that exhibits antiandrogenic activity. However, how BPA antagonizes androgen effects remains ambiguous. In this study, the in silico and in vitro assays were carried out to explore the molecular mechanism(s) of BPA on androgen receptor (AR) antagonism. In reporter gene assay, BPA caused a significant antagonistic effect on 5α-dihydrotestosterone (DHT)-induced AR transcriptional activity at concentrations of 10^-9 M-10^-5 M. The results of molecular docking and molecular dynamics simulations indicated the availability of BPA binding to the ligand binding domain of AR. BPA treatment prevented the inhibition of receptor degradation caused by DHT binding to AR. BPA exposure also abolished DHT-dependent dissociation of AR from its co-chaperone, 90-kDa heat shock protein (Hsp90), and resulted in the blockage of DHT-induced AR nuclear translocation. This is the first report to show that BPA inhibited the DHT-induced stabilization of AR and the DHT-induced dissociation of AR-Hsp90 complex. This study provided new evidence for further understanding the precise mechanisms of antagonism of BPA on AR.

Impaired Nuclear Export of Polyglutamine-Expanded Androgen Receptor in Spinal and Bulbar Muscular Atrophy

Spinal and bulbar muscular atrophy (SBMA) is a neuromuscular disease caused by polyglutamine (polyQ) expansion in the androgen receptor (AR). Prior studies have highlighted the importance of AR nuclear localization in SBMA pathogenesis; therefore, in this study, we sought to determine the role of AR nuclear export in the pathological manifestations of SBMA. We demonstrate here that the nuclear export of polyQ-expanded AR is impaired, even prior to the formation of intranuclear inclusions of aggregated AR. Additionally, we find that promoting AR export with an exogenous nuclear export signal substantially reduces its aggregation and blocks hormone-induced toxicity. Moreover, we show that these protective effects are conferred by destabilization of the mutant protein due to an increase in proteasomal degradation of the cytoplasmic AR. Despite a growing body of evidence that global disruption of nucleo/cytoplasmic transport occurs in ALS and HD, our data suggest that no such global disruption occurs in models of SBMA; rather, AR-specific mechanisms, including reduced phosphorylation at Serine 650, are likely responsible for the impaired nuclear export of polyQ-expanded AR.

Targeting the turnover of oncoproteins as a new avenue for therapeutics development in castration-resistant prostate cancer

The current therapeutic armamentarium for castration-resistant prostate cancer (CRPC) includes second-generation agents such as the Androgen Receptor (AR) inhibitor enzalutamide and the androgen synthesis inhibitor abiraterone acetate, immunotherapies like sipuleucel-T, chemotherapies including docetaxel and cabazitaxel and the radiopharmaceutical radium 223 dichloride. However, relapse of CRPC resistant to these therapeutic modalities occur rapidly. The mechanisms of resistance to these treatments are complex, including specific mutations or alternative splicing of oncogenic proteins. An alternative approach to treating CRPC may be to target the turnover of these molecular drivers of CRPC. In this review, the mechanisms by which protein stability of several oncoproteins such as AR, ERG, GR, CYP17A1 and MYC, will be discussed, as well as how these findings could be translated into novel therapeutic agents.

Interference with the androgen receptor protein stability in therapy-resistant prostate cancer

The androgen receptor (AR) plays a central role in the pathogenesis of prostate cancer (PCa). Most PCa cases develop eventually from an androgen-dependent stage to castration-resistant prostate cancer (CRPC) with AR-signaling still being active. Thus, inhibition of AR remains a well-established promising drug target in CRPC. However, despite the improvements of current treatment for CRPC by targeting the AR, the evolution of adaptive AR-signaling leads to therapy-resistant CRPC. Treatment failure is based mostly on the inability to keep AR under long-term restraint due to adaptive responses of AR-signaling. One underlying mechanism appears to be the increased AR protein stability. Therefore, the regulation of AR protein stability and its degradation is another interesting path that could enhance our knowledge of carcinogenesis and tumor evolution possibly leading to novel therapeutic targets. In this review, we discuss various molecular mechanisms and factors that stabilize AR protein levels directly or indirectly. We summarize novel approaches to interfere with AR stability including targeting the glucocorticoid receptor (GR), heat shock proteins, and co-chaperones as well as E3-ligases using small chimeric molecules. These novel approaches in combination with antiandrogen treatment inhibit PCa growth through the regulation of AR protein levels.

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

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

Androgen signaling expands β-cell mass in male rats and β-cell androgen receptor is degraded under high-glucose conditions

A deficient pancreatic β-cell mass increases the risk of type 2 diabetes mellitus. Here, we investigated the effects of testosterone on the development of pancreatic β-cell mass in male rats. The β-cell mass of male rats castrated at 6 wk of age was reduced to ~30% of that of control rats at 16 wk of age, and castration caused glucose intolerance. Loss of β-cell mass occurred because of decreases in islet density per pancreas and β-cell cluster size. Castration was negatively associated with the number of Ki-67-positive β-cells and positively associated with the number of TUNEL-positive β-cells. These β-cell changes could be prevented by testosterone treatment. In contrast, castration did not affect β-cell mass in male mice. Androgen receptor (AR) localized differently in mouse and rat β-cells. Testosterone enhanced the viability of INS-1 and INS-1 #6, which expresses high levels of AR, in rat β-cell lines. siRNA-mediated AR knockdown or AR antagonism with hydroxyflutamide attenuated this enhancement. Moreover, testosterone did not stimulate INS-1 β-cell viability under high d-glucose conditions. In INS-1 β-cells, d-glucose dose dependently (5.5-22.2 mM) downregulated AR protein levels both in the presence and absence of testosterone. The intracellular calcium chelator (BAPTA-AM) could prevent this decrease in AR expression. AR levels were also reduced by a calcium ionophore (A23187), but not by insulin, in the absence of the proteasome inhibitor MG132. Our results indicate that testosterone regulates β-cell mass, at least in part, by AR activation in the β-cells of male rats and that the β-cell AR is degraded under hyperglycemic conditions.

Discovery Proteomics Identifies a Molecular Link between the Coatomer Protein Complex I and Androgen Receptor-dependent Transcription

Aberrant androgen receptor (AR)-dependent transcription is a hallmark of human prostate cancers. At the molecular level, ligand-mediated AR activation is coordinated through spatial and temporal protein-protein interactions involving AR-interacting proteins, which we designate the “AR-interactome.” Despite many years of research, the ligand-sensitive protein complexes involved in ligand-mediated AR activation in prostate tumor cells have not been clearly defined. Here, we describe the development, characterization, and utilization of a novel human LNCaP prostate tumor cell line, N-AR, which stably expresses wild-type AR tagged at its N terminus with the streptavidin-binding peptide epitope (streptavidin-binding peptide-tagged wild-type androgen receptor; SBP-AR). A bioanalytical workflow involving streptavidin chromatography and label-free quantitative mass spectrometry was used to identify SBP-AR and associated ligand-sensitive cytosolic proteins/protein complexes linked to AR activation in prostate tumor cells. Functional studies verified that ligand-sensitive proteins identified in the proteomic screen encoded modulators of AR-mediated transcription, suggesting that these novel proteins were putative SBP-AR-interacting proteins in N-AR cells. This was supported by biochemical associations between recombinant SBP-AR and the ligand-sensitive coatomer protein complex I (COPI) retrograde trafficking complex in vitro. Extensive biochemical and molecular experiments showed that the COPI retrograde complex regulates ligand-mediated AR transcriptional activation, which correlated with the mobilization of the Golgi-localized ARA160 coactivator to the nuclear compartment of prostate tumor cells. Collectively, this study provides a bioanalytical strategy to validate the AR-interactome and define novel AR-interacting proteins involved in ligand-mediated AR activation in prostate tumor cells. Moreover, we describe a cellular system to study how compartment-specific AR-interacting proteins influence AR activation and contribute to aberrant AR-dependent transcription that underlies the majority of human prostate cancers.

Autophagic degradation of the androgen receptor mediated by increased phosphorylation of p62 suppresses apoptosis in hypoxia

Prostate cancer grows under hypoxic conditions. Hypoxia decreases androgen receptor (AR) protein levels. However, the molecular mechanism remains unclear. Here, we report that p62-mediated autophagy degrades AR protein and suppresses apoptosis in prostate cancer LNCaP cells in hypoxia. In LNCaP cells, hypoxia decreased AR at the protein level, but not at the mRNA level. Hypoxia-induced AR degradation was inhibited not only by knockdown of LC3, a key component of the autophagy machinery, but also by knockdown of p62. Depletion of p62 enhanced hypoxia-induced poly(ADP-ribose) polymerase cleavage and caspase-3 cleavage, markers of apoptosis, whereas simultaneous knockdown of p62 and AR suppressed hypoxia-induced apoptosis. Hypoxia increased the formation of a cytosolic p62-AR complex and enhanced sequestration of AR from the nucleus. Formation of this complex was promoted by the increased phosphorylation of serine 403 in the ubiquitin-associated domain of p62 during hypoxia. An antioxidant and an AMP-activated protein kinase (AMPK) inhibitor reduced hypoxia-induced p62 phosphorylation at serine 403 and suppressed hypoxia-induced complex formation between AR and p62. These results demonstrate that hypoxia enhances the complex formation between p62 and AR by promoting phosphorylation of p62 at serine 403, probably through activating AMPK, and that p62-mediated autophagy degrades AR protein for cell survival in hypoxia.

Bisbibenzyls, novel proteasome inhibitors, suppress androgen receptor transcriptional activity and expression accompanied by activation of autophagy in prostate cancer LNCaP cells

CONTEXT

Bisbibenzyl compounds have gained our interests for their potential antitumor activity in malignant cell-types.

OBJECTIVE

The objective of this study is to investigate the effect of bisbibenzyl compounds riccardin C (RC), marchantin M (MM), and riccardin D (RD) on androgen receptor (AR) in prostate cancer (PCa) cells.

MATERIALS AND METHODS

After exposure to 10 μM of the compounds for 24 h, cell cycle and cell survival analyses were performed using FACS and MTT assay to confirm the effect of these bisbibenzyls on PCa LNCaP cells. Changes in the AR expression and function, as the result of exposure to the compounds, were investigated using real-time PCR, ELISA, transient transfection, western blotting (WB), immunoprecipitation, and immunofluorescence staining (IF). Chemical-induced autophagy was examined by WB, IF, and RNAi.

RESULTS

RC, MM, and RD reduced the viability of LNCaP cells accompanied with arrested cell cycle in the G0/G1 phase and induction of apoptosis. Further investigation revealed that these compounds significantly inhibited AR expression at mRNA and protein levels, leading to the suppression of AR transcriptional activity. Moreover, inhibition of proteasome activity by bisbibenzyls, which in turn caused the induction of autophagy, as noted by induction of LC3B expression, conversion, and accumulation of punctate dots in treated cells. Co-localization of AR/LC3B and AR/Ub suggested that autophagy contributed to the degradation of polyubiquitinated-AR when proteasome activity was suppressed by the bisbibenzyls.

DISCUSSION AND CONCLUSION

Suppression of proteasome activity and induction of autophagy were involved in bisbibenzyl-mediated modulation of AR activities and apoptosis, suggesting their potential in treating PCa.

A novel prostate cancer therapeutic strategy using icaritin-activated arylhydrocarbon-receptor to co-target androgen receptor and its splice variants

Persistent androgen receptor (AR) signaling is the key driving force behind progression and development of castration-resistant prostate cancer (CRPC). In many patients, AR COOH-terminal truncated splice variants (ARvs) play a critical role in contributing to the resistance against androgen depletion therapy. Unfortunately, clinically used antiandrogens like bicalutamide (BIC) and enzalutamide (MDV), which target the ligand binding domain, have failed to suppress these AR variants. Here, we report for the first time that a natural prenylflavonoid, icaritin (ICT), can co-target both persistent AR and ARvs. ICT was found to inhibit transcription of key AR-regulated genes, such as KLK3 [prostate-specific antigen (PSA)] and ARvs-regulated genes, such as UBE2C and induce apoptosis in AR-positive prostate cancer (PC) cells. Mechanistically, ICT promoted the degradation of both AR and ARvs by binding to arylhydrocarbon-receptor (AhR) to mediate ubiquitin-proteasomal degradation. Therefore, ICT impaired AR transactivation in PC cells. Knockdown of AhR gene restored AR stability and partially prevented ICT-induced growth suppression. In clinically relevant murine models orthotopically implanted with androgen-sensitive and CRPC cells, ICT was able to target AR and ARvs, to inhibit AR signaling and tumor growth with no apparent toxicity. Our results provide a mechanistic framework for the development of ICT, as a novel lead compound for AR-positive PC therapeutics, especially for those bearing AR splice variants.

Androgen receptor is the key transcriptional mediator of the tumor suppressor SPOP in prostate cancer

Somatic missense mutations in the substrate-binding pocket of the E3 ubiquitin ligase adaptor SPOP are present in up to 15% of human prostate adenocarcinomas, but are rare in other malignancies, suggesting a prostate-specific mechanism of action. SPOP promotes ubiquitination and degradation of several protein substrates, including the androgen receptor (AR) coactivator SRC-3. However, the relative contributions that SPOP substrates may make to the pathophysiology of SPOP-mutant (mt) prostate adenocarcinomas are unknown. Using an unbiased bioinformatics approach, we determined that the gene expression profile of prostate adenocarcinoma cells engineered to express mt-SPOP overlaps greatly with the gene signature of both SRC-3 and AR transcriptional output, with a stronger similarity to AR than SRC-3. This finding suggests that in addition to its SRC-3-mediated effects, SPOP also exerts SRC-3-independent effects that are AR-mediated. Indeed, we found that wild-type (wt) but not prostate adenocarcinoma-associated mutants of SPOP promoted AR ubiquitination and degradation, acting directly through a SPOP-binding motif in the hinge region of AR. In support of these results, tumor xenografts composed of prostate adenocarcinoma cells expressing mt-SPOP exhibited higher AR protein levels and grew faster than tumors composed of prostate adenocarcinoma cells expressing wt-SPOP. Furthermore, genetic ablation of SPOP was sufficient to increase AR protein levels in mouse prostate. Examination of public human prostate adenocarcinoma datasets confirmed a strong link between transcriptomic profiles of mt-SPOP and AR. Overall, our studies highlight the AR axis as the key transcriptional output of SPOP in prostate adenocarcinoma and provide an explanation for the prostate-specific tumor suppressor role of wt-SPOP.

Galeterone prevents androgen receptor binding to chromatin and enhances degradation of mutant androgen receptor

PURPOSE

Galeterone inhibits the enzyme CYP17A1 and is currently in phase II clinical trials for castration-resistant prostate cancer (CRPC). Galeterone is also a direct androgen receptor (AR) antagonist and may enhance AR degradation. This study was undertaken to determine the molecular basis for AR effects and their therapeutic potential.

EXPERIMENTAL DESIGN

Effects of galeterone on AR expression and activities were examined in prostate cancer cell lines.

RESULTS

Similar to the AR antagonist enzalutamide, but in contrast to bicalutamide, galeterone did not induce binding of a constitutively active VP16-AR fusion protein to reporter genes and did not induce AR recruitment to endogenous androgen-regulated genes based on chromatin immunoprecipitation. Galeterone at low micromolar concentrations that did not induce cellular stress responses enhanced AR protein degradation in LNCaP and C4-2 cells, which express a T878A mutant AR, but not in prostate cancer cells expressing wild-type AR. Further transfection studies using stable LNCaP and PC3 cell lines ectopically expressing wild-type or T878A-mutant ARs confirmed that galeterone selectively enhances degradation of the T878A-mutant AR.

CONCLUSIONS

Similar to enzalutamide, galeterone may be effective as a direct AR antagonist in CRPC. It may be particularly effective against prostate cancer cells with the T878A AR mutation but may also enhance degradation of wild-type AR in vivo through a combination of direct and indirect mechanisms. Finally, these findings show that conformational changes in AR can markedly enhance its degradation and thereby support efforts to develop further antagonists that enhance AR degradation.

The interplay of AMP-activated protein kinase and androgen receptor in prostate cancer cells

AMP-activated protein kinase (AMPK) has recently emerged as a potential target for cancer therapy due to the observation that activation of AMPK inhibits tumor cell growth. It is well-known that androgen receptor (AR) signaling is a major driver for the development and progression of prostate cancer and that downregulation of AR is a critical step in the induction of apoptosis in prostate cancer cells. However, little is known about the potential interaction between AMPK and AR signaling pathways. In the current study, we showed that activation of AMPK by metformin caused decrease of AR protein level through suppression of AR mRNA expression and promotion of AR protein degradation, demonstrating that AMPK activation is upstream of AR downregulation. We also showed that inhibition of AR function by an anti-androgen or its siRNA enhanced AMPK activation and growth inhibition whereas overexpression of AR delayed AMPK activation and increased prostate cancer cellular resistance to metformin treatment, suggesting that AR suppresses AMPK signaling-mediated growth inhibition in a feedback mechanism. Our findings thus reveal a novel AMPK-AR regulatory loop in prostate cancer cells and should have a potential clinical significance.

On the origins of the androgen receptor low molecular weight species

Prostate cancer (CaP), a commonly diagnosed malignancy, is readily treated by androgen ablation. This treatment temporarily halts the disease, but castration-resistant neoplasms that are refractory to current therapies emerge. While these neoplasms are no longer dependent on physiological levels of androgens, they remain reliant on the expression of the androgen receptor (AR). There are multiple mechanisms by which CaP cells circumvent androgen ablation therapies. These include AR mutations that broaden ligand specificity, AR overexpression, AR activation by growth factors and cytokines, overexpression of AR co-activators, altered steroid metabolism, and a locus-wide histone transcriptional activation of some AR targets. This review focuses on a more recently described mechanism: the expression of low molecular weight AR species that are missing the ligand-binding domain and function independently of ligand to drive proliferation. The etiology, biological activity, unique features, predictive value, and therapeutic implication of these androgen receptor isoforms are discussed in depth.

Androgen receptor drives transcription of rat PACAP in gonadotrope cells

Gonadotropin expression is precisely regulated within the hypothalamic-pituitary-gonadal axis through the complex interaction of neuropeptides, gonadal steroids. and both gonadal- and pituitary-derived peptides. In the anterior pituitary gland, the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) modulates gonadotropin biosynthesis and secretion, acting both alone and in conjunction with GnRH. Steroid hormone feedback also influences gonadotropin expression via both direct and indirect mechanisms. Evidence from nonpituitary tissues suggests that PACAP may be a target for gonadal steroid regulation. In the present study, we show that androgen markedly stimulates rat (r) PACAP promoter-reporter activity in the LβT2 mature mouse gonadotrope cell line. 5'-Serial deletion analysis of reporter constructs identifies 2 regions of androgen responsiveness located at (-915 to -818) and (-308 to -242) of the rPACAP promoter. Androgen receptor (AR) binds directly to DNA cis-elements in each of these regions in vitro. Site-directed mutagenesis of 3 conserved hormone response element half-sites straddling the (-308 to -242) region dramatically blunts androgen-dependent PACAP promoter activity and prevents AR binding at the mutated promoter element. Chromatin immunoprecipitation demonstrates that endogenous AR binds the homologous region on mouse chromatin in LβT2 cells in both the presence and absence of androgen. These data demonstrate that androgen stimulates PACAP gene expression in the pituitary gonadotrope via direct binding of AR to a specific cluster of evolutionarily conserved hormone response elements in the proximal rPACAP gene promoter. Thus, androgen regulation of pituitary PACAP expression may provide an additional layer of control over gonadotropin expression within the hypothalamic-pituitary-gonadal axis.

RBCK1, an E3 ubiquitin ligase, interacts with and ubiquinates the human pregnane X receptor

The pregnane X receptor (PXR, NR1I2) plays a pivotal role in the disposition and detoxification of numerous foreign and endogenous chemicals by increasing transcription of numerous target genes, including phase I and II drug-metabolizing enzymes and transporters. In the present study, yeast two-hybrid screening identified an E3 ubiquitin ligase, RBCK1 (Ring-B-box-coiled-coil protein interacting with protein kinase C-1), as a human pregnane X receptor (hPXR)-interacting protein. Coimmunoprecipitation studies confirmed the interaction between RBCK1 and hPXR when both were ectopically expressed in AD-293 cells. Domain mapping studies showed that the interaction between RBCK1 and hPXR involves all RBCK1 domains. We further demonstrate that RBCK1 ubiquitinates hPXR, and this may target hPXR for degradation by the ubiquitin-proteasome pathway. Simultaneous ectopic overexpression of RBCK1 and PXR decreased PXR levels in AD-293 cells, and this decrease was inhibited by the proteasomal inhibitor MG-132 (carbobenzoxy-Leu-Leu-leucinal). Furthermore, overexpression of RBCK1 decreased endogenous levels of PXR in HepG2 cells. Of importance, ectopic overexpression and silencing of endogenous RBCK1 in primary human hepatocytes resulted in a decrease and increase, respectively, in endogenous PXR protein levels and in the induction of PXR target genes by rifampicin. These results suggest that RBCK1 is important for the ubiquitination of PXR and may play a role in its proteasomal degradation.

Increased PrLZ-mediated androgen receptor transactivation promotes prostate cancer growth at castration-resistant stage

Most advanced prostate cancers (PCa) will develop into the castration-resistant stage following androgen deprivation therapy, yet the molecular mechanisms remain unclear. In this study, we found PrLZ, a newly identified Prostate Leucine Zipper gene that is highly expressed in PCa could interact with the androgen receptor (AR) directly leading to enhance AR transactivation in the castration-resistant condition. PrLZ might enhance AR transactivation via a change of AR conformation that leads to promotion of AR nuclear translocation and suppression of AR degradation via modulating the proteasome pathway, which resulted in increased prostate-specific antigen expression and promoted PCa growth at the castration-resistant stage. Clinical PCa sample survey from same-patient paired specimens found increased PrLZ expression in castration-resistant PCa following the classical androgen deprivation therapy. Targeting the AR-PrLZ complex via ASC-J9® or PrLZ-siRNA resulted in suppression of PCa growth in various human PCa cells and in vivo mouse PCa models. Together, these data not only strengthen PrLZ roles in the transition from androgen dependence to androgen independence during the castration-resistant stage, but they may also provide a new potential therapy to battle PCa at the castration-resistant stage.

Nuclear export signal of androgen receptor (NESAR) regulation of androgen receptor level in human prostate cell lines via ubiquitination and proteasome-dependent degradation

Androgen receptor (AR) plays a key role in prostate development and carcinogenesis. Increased expression and/or stability of AR is associated with sensitization of prostate cancer cells to low levels of androgens, leading to castration resistance. Hence, understanding the mechanisms regulating AR protein stability is clinically relevant and may lead to new approaches to prevent and/or treat prostate cancer. Using fluorescence microscopy, Western blot, and pulse chase assay, we showed that nuclear export signal (NES)(AR), a nuclear export signal in the ligand binding domain (LBD) of AR, can significantly enhance the degradation of fusion protein constructs in PC3 prostate cancer cells. The half-life of GFP-NES(AR) was less than 3 h, which was 10 times shorter than that of green fluorescent protein (GFP) control. Further analysis showed that NES(AR) can signal for polyubiquitination and that degradation of NES(AR)-containing fusion proteins can be blocked by proteasome inhibitor MG132. Ubiquitination of GFP-AR or GFP-LBD was suppressed in the presence of dihydrotestosterone, which is known to suppress NES(AR) while inducing nuclear localization signal 2 in AR or LBD, suggesting that the export activity of NES(AR) is required for NES(AR)-mediated polyubiquitination. Treatment with MG132 also induced aggresome formation of NES(AR)-containing fusion proteins in perinuclear regions of the transfected PC3 cells, indicating a role for NES(AR) in inducing unfolded protein responses. The above observations suggest that NES(AR) plays a key role in AR ubiquitination and proteasome-dependent degradation in prostate cancer cells.

PAGE4 positivity is associated with attenuated AR signaling and predicts patient survival in hormone-naive prostate cancer

Aberrant activation of the androgen receptor (AR) plays a key role during prostate cancer (PCa) development and progression to castration-resistant prostate cancer (CR-PCa) after androgen deprivation therapy, the mainstay systemic treatment for PCa. New strategies to abrogate AR activity and biomarkers that predict aggressive tumor behavior are essential for improved therapeutic intervention. PCa tissue microarrays herein reveal that prostate-associated gene 4 (PAGE4), an X-linked cancer/testis antigen, is highly up-regulated in the epithelium of preneoplastic lesions compared with benign epithelium, but subsequently decreases with tumor progression. We show that AR signaling is attenuated in PAGE4-expressing cells both in vitro and in vivo, most likely via impaired androgen-induced AR nuclear translocation and subsequently reduced AR protein stabilization and phosphorylation at serines 81 and 213. Consistently, epithelial PAGE4 protein levels inversely correlated with AR activation status in hormone-naive and CR-PCa clinical specimens. Moreover, PAGE4 impaired the development of CR-PCa xenografts, and strong PAGE4 immunoreactivity independently predicted favorable patient survival in hormone-naive PCa. Collectively, these data suggest that dysregulation of epithelial PAGE4 modulates AR signaling, thereby promoting progression to advanced lethal PCa and highlight the potential value of PAGE4 as a prognostic and therapeutic target.

The curcumin analog ca27 down-regulates androgen receptor through an oxidative stress mediated mechanism in human prostate cancer cells

BACKGROUND

The androgen receptor (AR) plays a critical role in prostate cancer development and progression. Therefore, the inhibition of AR function is an established therapeutic intervention. Since the expression of the AR is retained and often increased in progressive disease, AR protein down-regulation is a promising therapeutic approach against prostate cancer. We show here that the curcumin analog 27 (ca27) down-regulates AR expression in several prostate cancer cell lines.

METHODS

ca27 at low micromolar concentrations was tested for its effect on AR expression, AR activation, and induction of oxidative stress in human LNCaP, C4-2, and LAPC-4 prostate cancer cells.

RESULTS

ca27 induced the down-regulation of AR protein expression in LNCaP, C4-2, and LAPC-4 cells within 12 hr. Further, ca27 led to the rapid induction of reactive oxygen species (ROS). To further support this finding, ca27 treatment led to the activation of the cellular redox sensor NF-E2-related factor 2 (Nrf2) and the induction of the Nrf2-regulated genes NAD(P)H quinone oxidoreductase 1 and aldoketoreductase 1C1. We show that ROS production preceded AR protein loss and that ca27-mediated down-regulation of the AR was attenuated by the antioxidant, N-acetyl cysteine.

CONCLUSIONS

ca27 induces ROS and mediates AR protein down-regulation through an oxidative stress mechanism of action. Our results suggest that ca27 represents a novel agent for the elucidation of mechanisms of AR down-regulation, which could lead to effective new anti-androgenic strategies for the treatment of advanced prostate cancer.

Androgen deprivation causes truncation of the C-terminal region of androgen receptor in human prostate cancer LNCaP cells

The androgen receptor (AR) acts as a ligand-dependent transcription factor, whereas mutant AR lacking the C-terminal ligand-binding domain functions in a ligand-independent manner. In the present study we report that the C-terminal truncated AR, which we named AR-NH1 (the N-terminal fragment of AR cleaved in the neighborhood of helix 1 of the ligand-binding domain), is produced in LNCaP prostatic carcinoma cells. The AR-NH1 of ~90 kDa was observed in an androgen-independent LNCaP subline and was further accumulated by the proteasome inhibitor MG132. MG132 treatment caused the accumulation of AR-NH1 even in parent LNCaP cells. AR-NH1 was produced in the absence of ligand or in the presence of the AR antagonist bicalutamide, whereas AR agonists suppressed its production. AR-NH1 was detected with different AR antibodies recognizing amino acid residues 1-20 and 300-316 and was also generated from exogenous AR. Both siRNA-mediated AR knockdown and treatment with a serine protease inhibitor (4-(2-aminoethyl)-benzenesulfonyl fluoride) reduced AR-NH1 levels. According to the predicted cleavage site (between amino acid residues 660-685) and its nuclear localization, it is assumed that AR-NH1 functions as a constitutively active transcription factor. These data suggest that AR-NH1 is produced under hormone therapy and contributes to the development of castration-resistant prostate cancer due to its ligand-independent transcriptional activity.

Androgen regulated HN1 leads proteosomal degradation of androgen receptor (AR) and negatively influences AR mediated transactivation in prostate cells

We recently reported that hematological and neurological expressed 1 (HN1) is a ubiquitously expressed, EGF-regulated gene. Expression of HN1 in prostate cell lines down-regulates PI3K-dependent Akt activation. Here, we investigate whether HN1 is regulated by androgens through the putative androgen response elements (AREs) found in its promoter. Knockdown of HN1 expression by siRNA silencing leads to an increase in Akt((S473)) phosphorylation, resulting in the translocation of androgen receptor (AR) to the nucleus; these effects can be abrogated by the non-specific Akt inhibitor LY294002 but not by the ERK inhibitor PD98059. Furthermore, HN1 overexpression correlates with an increase in ubiquitination-mediated degradation (a consequence of the decrease in S213/210 phosphorylation of AR), ultimately resulting in the down-regulation of AR-mediated expression of the KLK3, KLK4, NKX3.1 and STAMP2 genes. We also found that HN1 overexpression suppresses colony formation as well as R1881-mediated growth in LNCaP cells, while it has the opposite effect (increasing colony formation but not proliferation) in PC-3 and DU145 cells. Therefore, we suggest that HN1 maintains a balance between the androgen-regulated nuclear translocation of AR and steady-state Akt phosphorylation, predominantly in the absence of androgens. If so, the balance between cell growth and EGF- and AR-signaling must be tightly regulated by HN1. This work has important implications for prostate cancer research, as AR, EGFR and HN1 are known to be highly expressed in prostate adenocarcinomas.

Berberine suppresses androgen receptor signaling in prostate cancer

The androgen receptor (AR) is critical in the normal development and function of the prostate, as well as in prostate carcinogenesis. Androgen deprivation therapy is the mainstay in the treatment of advanced prostate cancer; however, after an initial response, the disease inevitably progresses to castration-resistant prostate cancer (CRPC). Recent evidence suggests that continued AR activation, sometimes in a ligand-independent manner, is commonly associated with the development of CRPC. Thus, novel agents targeting the AR are urgently needed as a strategic step in developing new therapies for this disease state. In this study, we investigated the effect of berberine on AR signaling in prostate cancer. We report that berberine decreased the transcriptional activity of AR. Berberine did not affect AR mRNA expression, but induced AR protein degradation. Several ligand-binding, domain-truncated AR splice variants have been identified, and these variants are believed to promote the development of CRPC in patients. Interestingly, we found that these variants were more susceptible to berberine-induced degradation than the full-length AR. Furthermore, although the growth of LNCaP xenografts in nude mice was inhibited by berberine, and AR expression was reduced in the tumors, the morphology and AR expression in normal prostates were not affected. This study is the first to show that berberine suppresses AR signaling and suggests that berberine, or its derivatives, presents a promising agent for the prevention and/or treatment of prostate cancer.

Regulation of androgen receptor and prostate cancer growth by cyclin-dependent kinase 5

Prostate cancer is the most frequently diagnosed male malignancy. The normal prostate development and prostate cancer progression are mediated by androgen receptor (AR). Recently, the roles of cyclin-dependent kinase 5 (Cdk5) and its activator, p35, in cancer biology are explored one after another. We have previously demonstrated that Cdk5 may regulate proliferation of thyroid cancer cells. In addition, we also identify that Cdk5 overactivation can be triggered by drug treatments and leads to apoptosis of prostate cancer cells. The aim of this study is to investigate how Cdk5 regulates AR activation and growth of prostate cancer cells. At first, the data show that Cdk5 enables phosphorylation of AR at Ser-81 site through direct biochemical interaction and, therefore, results in the stabilization of AR proteins. The Cdk5-dependent AR stabilization causes accumulation of AR proteins and subsequent activation. Besides, the positive regulations of Cdk5-AR on cell growth are also determined in vitro and in vivo. S81A mutant of AR diminishes its interaction with Cdk5, reduces its nuclear localization, fails to stabilize its protein level, and therefore, decreases prostate cancer cell proliferation. Prostate carcinoma specimens collected from 177 AR-positive patients indicate the significant correlations between the protein levels of AR and Cdk5 or p35. These findings demonstrate that Cdk5 is an important modulator of AR and contributes to prostate cancer growth. Therefore, Cdk5-p35 may be suggested as diagnostic and therapeutic targets for prostate cancer in the near future.

Calmodulin protects androgen receptor from calpain-mediated breakdown in prostate cancer cells

Although inactivation of the androgen receptor (AR) by androgen-ablation or anti-androgen treatment has been frontline therapy for disseminated prostate cancer for over 60 years, it is not curative because castration-resistant prostate cancer cells retain AR activity. Therefore, curative strategy should include targeted elimination of AR protein. Since AR binds to calmodulin (CaM), and since CaM-binding proteins are targets of calpain (Cpn)-mediated proteolysis, we studied the role of CaM and Cpn in AR breakdown in prostate cancer cells. Whereas the treatment of prostate cancer cells individually with anti-CaM drug or calcimycin, which increases intracellular Ca(++) and activates Cpn, led to minimal AR breakdown, combined treatment led to a precipitous decrease in AR protein levels. This decrease in AR protein occurred without noticeable changes in AR mRNA levels, suggesting an increase in AR protein turnover rather than inhibition of AR mRNA expression. Thus, CaM inactivation seems to sensitize AR to Cpn-mediated breakdown in prostate cancer cells. Consistent with this possibility, purified recombinant human AR (rhAR) underwent proteolysis in the presence of purified Cpn, and the addition of purified CaM to the incubation blocked rhAR proteolysis. Together, these observations demonstrate that AR is a Cpn target and AR-bound CaM plays an important role in protecting AR from Cpn-mediated breakdown in prostate cancer cells. These observations raise an intriguing possibility that anti-CaM drugs in combination with Cpn-activating agents may offer a curative strategy for the treatment of prostate cancer, which relies on AR for growth and survival.

The significance of Her2 on androgen receptor protein stability in the transition of androgen requirement in prostate cancer cells

Androgen ablation therapy is the most common strategy for suppressing prostate cancer progression; however, tumor cells eventually escape androgen dependence and progress to an androgen-independent phase. The androgen receptor (AR) plays a pivotal role in this transition. To address this transition mystery in prostate cancer, we established an androgen-independent prostate cancer cell line (LNCaPdcc), by long-term screening of LNCaP cells in androgen-deprived conditions, to investigate changes of molecular mechanisms before and after androgen withdrawal. We found that LNCaPdcc cells displayed a neuroendocrine morphology, less aggressive growth, and lower expression levels of cell cycle-related factors, although the cell cycle distribution was similar to parental LNCaP cells. Notably, higher protein expression of AR, phospho-Ser(81)-AR, and PSA in LNCaPdcc cells were observed. The nuclear distribution and protein stability of AR increased in LNCaPdcc cells. In addition, cell proliferation results exhibited the biphasic nature of the androgen (R1881) effect in two cell lines. On the other hand, LNCaPdcc cells expressed higher levels of Her2, phospho-Tyr(1221/1222)-Her2, ErbB3, and ErbB4 proteins than parental LNCaP cells. These two cell lines exhibited distinct responses to Her2 activation (by heregulin treatment) on Her2 phosphorylation and Her2 inhibition (by AG825 or Herceptin treatments) on proliferation. In addition, the Her2 inhibitor more effectively caused AR degradation and diminished AR Ser(81) phosphorylation in LNCaPdcc cells. Taken together, our data demonstrate that Her2 plays an important role in the support of AR protein stability in the transition of androgen requirement in prostate cancer cells. We hope these findings will provide novel insight into the treatment of hormone-refractory prostate cancer.

Dynamics of corticosteroid receptors: lessons from live cell imaging

Adrenal corticosteroids (cortisol in humans or corticosterone in rodents) exert numerous effects on the central nervous system that regulates the stress response, mood, learning and memory, and various neuroendocrine functions. Corticosterone (CORT) actions in the brain are mediated via two receptor systems: the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR). It has been shown that GR and MR are highly colocalized in the hippocampus. These receptors are mainly distributed in the cytoplasm without hormones and translocated into the nucleus after treatment with hormones to act as transcriptional factors. Thus the subcellular dynamics of both receptors are one of the most important issues. Given the differential action of MR and GR in the central nervous system, it is of great consequence to clarify how these receptors are trafficked between cytoplasm and nucleus and their interactions are regulated by hormones and/or other molecules to exert their transcriptional activity. In this review, we focus on the nucleocytoplasmic and subnuclear trafficking of GR and MR in neural cells and non-neural cells analyzed by using molecular imaging techniques with green fluorescent protein (GFP) including fluorescence recovery after photobleaching (FRAP) and fluorescence resonance energy transfer (FRET), and discuss various factors affecting the dynamics of these receptors. Furthermore, we discuss the future directions of in vivo molecular imaging of corticosteroid receptors at the whole brain level.

Lysine 419 targets human glucocorticoid receptor for proteasomal degradation

Glucocorticoid receptors (GRs) are members of a highly conserved family of ligand dependent transcription factors which following hormone binding undergo homologous down-regulation reducing the levels of receptor protein. This decline in human GR (hGR) is due in part to a decrease in protein receptor stability that may limit cellular responsiveness to ligand. To examine the role of the proteasome protein degradation pathway in steroid-dependent hGR responsiveness, we utilized the proteasomal inhibitors MG-132, beta-lactone, and epoxomicin. HeLa cells and COS cells were treated with proteasome inhibitors in the presence of the GR agonist dexamethasone (Dex), or were pretreated with proteasomal inhibitor and then Dex. Dexamethasone induced glucocorticoid responsive reporter activity significantly over untreated controls, whereas cells treated with proteasomal inhibitors and Dex together showed 2-3-fold increase in activity. Protein sequence analysis of the hGR protein identified several candidate protein degradation motifs including a PEST element. Mutagenesis of this element at lysine 419 was done and mutant K419A hGR failed to undergo ligand dependent down-regulation. Mutant K419A hGR displayed 2-3-fold greater glucocorticoid responsive reporter activity in the presence of Dex than wild type hGR. These differences in transcriptional activity were not due to altered subcellular localization, since when the mutant K419A hGR was fused with the green fluorescent protein (GFP) it was found to move in and out of the nucleus similarly to wild type hGR. Together these results suggest that the proteasome and the identified PEST degradation motif limit steroid-dependent human glucocorticoid receptor signaling.

Trihydrophobin 1 attenuates androgen signal transduction through promoting androgen receptor degradation

The androgen-signaling pathway plays critical roles in normal prostate development, benign prostatic hyperplasia, established prostate cancer, and in prostate carcinogenesis. In this study, we report that trihydrophobin 1 (TH1) is a potent negative regulator to attenuate the androgen signal-transduction cascade through promoting androgen receptor (AR) degradation. TH1 interacts with AR both in vitro and in vivo, decreases the stability of AR, and promotes AR ubiquitination in a ligand-independent manner. TH1 also associates with AR at the active androgen-responsive prostate-specific antigen (PSA) promoter in the nucleus of LNCaP cells. Decrease of endogenous AR protein by TH1 interferes with androgen-induced luciferase reporter expression and reduces endogenous PSA expression. Taken together, these results indicate that TH1 is a novel regulator to control the duration and magnitude of androgen signal transduction and might be directly involved in androgen-related developmental, physiological, and pathological processes.

Proteasomal degradation unleashes the pro-death activity of androgen receptor

Androgen receptor (AR) is able to promote stress-induced cell death independently of its transcription activity in androgen-independent prostate cancer cells. Yet, the underlying mechanism is incompletely understood. Here, we report that stress-induced proteasomal degradation of AR contributes to its pro-death activity. Upon exposure to ultraviolet light and staurosporine, AR underwent proteasomal degradation. Blockade of AR degradation significantly suppressed stress-induced apoptosis in androgen-independent prostate cancer cells. Ectopic expression of the AR N-terminal (AR-N) domain, which lacks DNA- and ligand-binding abilities, led to cell death without any additional death stimuli. Truncation analysis revealed that AR-N domain contains several sub-domains that regulate the pro-death activity of AR, specifically the first 105 amino acids, which function as a minimal pro-death domain acting upstream of caspases. The pro-apoptotic activity of AR N-terminal fragments was suppressed by ectopic expression of Bcl-2 or selected caspase inhibitors. Thus, our results reveal a novel mechanism by which AR promotes stress-induced cell death in androgen-independent prostate cancer cells.

Growth inhibition of androgen-responsive prostate cancer cells with brefeldin A targeting cell cycle and androgen receptor

Background

Androgen ablation is one of the viable therapeutic options for patients with primary hormone (androgen)-dependent prostate cancer. However, an antibiotic brefeldin A (BFA) has been shown to exhibit the growth inhibitory effect on human cancer cells. We thus investigated if BFA might inhibit proliferation of androgen-responsive prostate cancer LNCaP cells and also explored how it would be carried out, focusing on cell cycle and androgen receptor (AR).

Methods

Androgen-mediated cellular events in LNCaP cells were induced using 5α-dihydrotestosterone (DHT) as an androgenic mediator. Effects of BFA on non-DHT-stimulated or DHT-stimulated cell growth were assessed. Its growth inhibitory mechanism(s) was further explored; performing cell cycle analysis on a flow cytometer, assessing AR activity by AR binding assay, and analyzing AR protein expression using Western blot analysis.

Results

DHT (1 nM) was capable of stimulating LNCaP cell growth by ~40% greater than non-stimulated controls, whereas BFA (30 ng/ml) completely inhibited such DHT-stimulated proliferation. Cell cycle analysis showed that this BFA-induced growth inhibition was associated with a ~75% reduction in the cell number in the S phase and a concomitant increase in the G₁ cell number, indicating a G₁ cell cycle arrest. This was further confirmed by the modulations of specific cell cycle regulators (CDK2, CDK4, cyclin D₁, and p21^WAF1), revealed by Western blots. In addition, the growth inhibition induced by BFA was accompanied by a profound (~90%) loss in AR activity, which would be presumably attributed to the significantly reduced cellular AR protein level.

Conclusions

This study demonstrates that BFA has a potent growth inhibitory activity, capable of completely inhibiting DHT (androgen)-stimulated LNCaP proliferation. Such inhibitory action of BFA appears to target cell cycle and AR: BFA led to a G₁ cell cycle arrest and the down-regulation of AR activity/expression, possibly accounting for its primary growth inhibitory mechanism. Thus, it is conceivable that BFA may provide a more effective therapeutic modality for patients with hormone-dependent prostate cancer.

Emerging roles of the ubiquitin proteasome system in nuclear hormone receptor signaling

Nuclear receptor (NR)-mediated transcription is intimately tied to the ubiquitin proteasome system (UPS). The UPS targets numerous NR and coregulator proteins, regulating their stability and altering their transcriptional activities through the posttranslational placement of ubiquitin marks on them. Differences in the manner in which ubiquitin is attached to target proteins or itself have distinct regulatory consequences. Protein monoubiquitination, polyubiquitination, the site of ubiquitin attachment to a target protein, and the type of polyubiquitin chain linkage all lead to different biological outcomes and have an important regulatory function in NR-mediated transcription. Consistent with its role in protein degradation, the UPS is able to limit the biological actions of both NRs and coregulators by reducing their protein concentrations in the cell. However, in spite of its destructive capabilities, the UPS can play a positive role in facilitating NR-mediated transcription as well. In addition, ubiquitin-like modifications such as SUMOylation also modify and regulate NRs and coregulators. The UPS forms a key biological system that underlies a sophisticated postranslational regulatory scheme from which complex and dynamic regulation of NR-mediated transcription can occur.

Transcriptional repression and inhibition of nuclear translocation of androgen receptor by diallyl trisulfide in human prostate cancer cells

PURPOSE

The present study was undertaken to determine the effect of diallyl trisulfide (DATS), a promising cancer chemopreventive constituent of garlic, on androgen receptor (AR) protein expression and function using prostate cancer cells.

EXPERIMENTAL DESIGN

The protein levels of AR and prostate-specific antigen (PSA) were determined by immunoblotting. The effect of DATS treatment on AR mRNA level and AR promoter activity was determined by quantitative reverse transcription-PCR and luciferase reporter assay, respectively. Expression of AR protein in poorly differentiated carcinoma and normal prostate of transgenic adenocarcinoma of mouse prostate (TRAMP) mice was determined by immunohistochemistry. Confocal microscopy was done to determine nuclear translocation of AR. Cell viability was determined by trypan blue dye exclusion assay.

RESULTS

Exposure of prostate cancer cells (LNCaP, C4-2, and TRAMP-C1) to DATS resulted in a concentration-dependent decrease in protein level of AR, which was accompanied by suppression of intracellular and secreted levels of PSA. Structure-activity studies revealed critical roles for allyl groups and the oligosulfide chain length in DATS-mediated down-modulation of AR protein. Quantitative reverse transcription-PCR showed a dose-dependent decrease in AR mRNA level, which correlated with inhibition of AR promoter activity. DATS treatment inhibited synthetic androgen (R1881)-stimulated nuclear translocation of AR in LNCaP/C4-2 cells and proliferation of LNCaP cells. Oral gavage of 2 mg/day DATS (three times per week for 13 weeks) markedly suppressed AR protein level in poorly differentiated prostate cancer in TRAMP mice.

CONCLUSION

The present study shows, for the first time, that DATS treatment suppresses AR function in prostate cancer cells.

Androgen receptor phosphorylation and activity are regulated by an association with protein phosphatase 1

Androgen receptor (AR) is phosphorylated at multiple sites in response to ligand binding, but the functional consequences and mechanisms regulating AR phosphorylation remain to be established. We observed initially that okadaic acid, an inhibitor of the major PPP family serine/threonine phosphatases PP2A and protein phosphatase 1 (PP1), had cell type-dependent effects on AR expression. More specific inhibitors of PP2A (fostriecin) and PP1 (tautomycin and siRNA against the PP1alpha catalytic subunit) demonstrated that PP1 and protein phosphatase 2A had opposite effects on AR protein and transcriptional activity. PP1 inhibition enhanced proteasome-mediated AR degradation, while PP1alpha overexpression increased AR expression and markedly enhanced AR transcriptional activity. Coprecipitation experiments demonstrated an AR-PP1 interaction, while immunofluorescence and nuclear-cytoplasmic fractionation showed androgen-stimulated nuclear translocation of both AR and PP1 in prostate cancer cells. Studies with phosphospecific AR antibodies showed that PP1 inhibition dramatically increased phosphorylation of Ser-650, a site in the AR hinge region shown to mediate nuclear export. Significantly, PP1 inhibition caused a marked decrease in nuclear localization of the wild-type AR, but did not alter total or nuclear levels of a S650A mutant AR. These findings reveal a critical role of PP1 in regulating AR protein stability and nuclear localization through dephosphorylation of Ser-650. Moreover, AR may function as a PP1 regulatory subunit and mediate PP1 recruitment to chromatin, where it can modulate transcription and splicing.

Degradation and beyond: control of androgen receptor activity by the proteasome system

The androgen receptor (AR) is a transcription factor belonging to the family of nuclear receptors which mediates the action of androgens in the development of urogenital structures. AR expression is regulated post-translationally by the ubiquitin/proteasome system. This regulation involves more complex mechanisms than typical degradation. The ubiquitin/proteasome system may regulate AR via mechanisms that do not engage in receptor turnover. Given the critical role of AR in sexual development, this complex regulation is especially important. Deregulation of AR signalling may be a causal factor in prostate cancer development. AR is the main target in prostate cancer therapies. Due to the critical role of the ubiquitin/proteasome system in AR regulation, current research suggests that targeting AR degradation is a promising approach.

Calpain-mediated androgen receptor breakdown in apoptotic prostate cancer cells

Since androgen receptor (AR) plays an important role in prostate cancer development and progression, androgen-ablation has been the frontline therapy for treatment of advanced prostate cancer even though it is rarely curative. A curative strategy should involve functional and structural elimination of AR from prostate cancer cells. We have previously reported that apoptosis induced by medicinal proteasome-inhibitory compound celastrol is associated with a decrease in AR protein levels. However celastrol-stimulated events contributing to this AR decrease have not been elucidated. Here, we report that a variety of chemotherapeutic agents, including proteasome inhibitors, a topoisomerase inhibitor, DNA-damaging agents and docetaxel that cause cell death, decrease AR levels in LNCaP prostate cancer cells. This decrease in AR protein levels was not due to the suppression of AR mRNA expression in these cells. We observed that a proteolytic activity residing in cytosol of prostate cancer cells is responsible for AR breakdown and that this proteolytic activity was stimulated upon induction of apoptosis. Interestingly, proteasome inhibitor celastrol- and chemotherapeutic drug VP-16-stimulated AR breakdown was attenuated by calpain inhibitors calpastatin and N-acetyl-L-leucyl-L-leucyl-L-methioninal. Furthermore, AR proteolytic activity pulled down by calmodulin-agarose beads from celastrol-treated PC-3 cells showed immunoreactivity to a calpain antibody. Taken together, these results demonstrate calpain involvement in proteasome inhibitor-induced AR breakdown, and suggest that AR degradation is intrinsic to the induction of apoptosis in prostate cancer cells.

A feedback loop between the androgen receptor and a NEDD4-binding protein, PMEPA1, in prostate cancer cells

PMEPA1 was identified originally as a highly androgen-inducible gene with prostate-abundant expression that was restricted to prostatic epithelial cells. PMEPA1 protein is a NEDD4 (ubiquitin-protein isopeptide ligase)-binding protein, which negatively regulates prostate cancer cell growth. In this study we establish that PMEPA1 is a direct transcriptional target of the androgen receptor (AR). We also demonstrate that PMEPA1 negatively regulates AR protein levels in different cell culture models. Transient expression of PMEPA1 down-regulates AR protein levels and AR transcriptional targets in prostate cancer cells. Conversely, knockdown of PMEPA1 leads to elevated levels of AR protein, AR transcriptional targets (prostate-specific antigen), and increased cell cycle S phase. We define that the PMEPA1-dependent down-regulation of AR is because of AR ubiquitination and proteasome-mediated degradation. The mutant PMEPA1 (PY1/2 motif mutation) that is impaired in NEDD4 recruitment shows attenuated AR ubiquitination and AR protein down-regulation. These data support the hypothesis that PMEPA1 negatively regulates the stability of AR protein by enhancing AR ubiquitination and proteasome-mediated degradation through NEDD4. The effect of PMEPA1 on AR ubiquitination and degradation appears to be MDM2-independent. Thus, the PMEPA1-AR degradation pathway may represent a new androgen-dependent mechanism for regulating AR levels in prostate epithelial cells. These findings underscore that the decreased PMEPA1 expression frequently noted in prostate cancers may lead to increased AR functions and strengthen the biological role of PMEPA1 in prostate cancers.

Resveratrol down-regulates the androgen receptor at the post-translational level in prostate cancer cells

Androgen receptor (AR) functions as a transcriptional factor for the development and progression of prostate cancer. Resveratrol is known to inhibit the function of AR and to repress AR expression at the transcriptional level. This study focuses on the effects of resveratrol on the AR function and the post-translational AR level. Resveratrol repressed the transcriptional activities of a mutant AR lacking the ligand-binding domain, a constitutive active form of AR, and wild-type AR in a concentration-dependent manner in human prostate cancer PC-3 cells, indicating that resveratrol does not inhibit the transcriptional activity of AR through binding to the ligand-binding domain of AR. Furthermore, the half-life of AR protein was approximately 4 h in resveratrol-treated AR-positive prostate cancer LNCaP cells, compared to approximately 13 h in control cells, as determined by cycloheximide chase. These results indicate that resveratrol down-regulates AR protein through a post-translational mechanism and suggest that the inhibitory effect of resveratrol on AR function is partly attributable to a decrease in the post-translational AR level.