Androgen receptor and its splice variant, AR-V7, differentially regulate FOXA1 sensitive genes in LNCaP prostate cancer cells

Targeting PCNA/AR interaction inhibits AR-mediated signaling in castration resistant prostate cancer cells

We previously showed that proliferating cell nuclear antigen (PCNA) interacts with androgen receptor (AR) through a PIP-box (PIP-box4) at the N-terminus of AR and regulates AR activity. In this study, we further investigated PCNA/AR interaction. We identified a second PIP-box (PIP-box592) in the DNA binding domain of AR and found that dihydrotestosterone enhances the binding of full-length AR (AR-FL) but not a constitutively active variant (AR-V7) to PCNA. Treatment with R9-AR-PIP, a PIP-box4-mimicking small peptide, inhibits the PCNA/AR interaction, AR occupancy at the androgen response element (ARE) in PSA and p21 genes, and expression of AR target genes, and induces cytotoxicity in AR-positive castration-resistant prostate cancer (CRPC) cells. R9-AR-PIP also significantly inhibits transcriptional activity of AR-FL upon dihydrotestosterone stimulation and the constitutive activity of AR-V7. Moreover, R9-AR-PIP and PCNA-I1S, a small molecule PCNA inhibitor, inhibit the ARE occupancy by AR-FL and AR-Vs in CCNA2 gene that encodes cyclin A2 and cyclin A2 expression. Finally, we found that cyclin A2 is overexpressed in all CRPC cells examined, suggesting that it may contribute to the development of CRPC. These data indicate that targeting PCNA/AR interaction inhibits both AR-FL- and AR-Vs-mediated signaling and implicates it could be a novel therapeutic strategy against CRPC.

Synergistic antitumor effect of MK-1775 and CUDC-907 against prostate cancer

Due to the emergence of drug resistance, androgen receptor (AR)-targeted drugs still pose great challenges in the treatment of prostate cancer, and it is urgent to explore an innovative therapeutic strategy. MK-1775, a highly selective WEE1 inhibitor, is shown to have favorable therapeutic benefits in several solid tumor models. Recent evidence suggests that the combination of MK-1775 with DNA-damaging agents could lead to enhanced antitumor efficacy. Here, our results demonstrate that MK-1775 alone could indeed inhibit proliferation and induce apoptosis in prostate cancer. Moreover, the combination of MK-1775 and a dual PI3K and HDAC inhibitor, CUDC-907, can synergistically inhibit cell proliferation and dramatically induces apoptosis in prostate cancer cells. This effect is partially mediated by DNA damage, resulting from the downregulation of DNA damage response (DDR) proteins such as CDK, CHK, and RRM1/2. Notably, the combination of MK-1775 and CUDC-907 leads to significant antitumor effects in vivo. Our findings provide a strong basis for a promising combination strategy against prostate cancer.

Increased nuclear factor I-mediated chromatin access drives transition to androgen receptor splice variant dependence in prostate cancer

Androgen receptor (AR) splice variants, of which ARv7 is the most common, are increased in castration-resistant prostate cancer, but the extent to which they drive AR activity is unclear. We generated a subline of VCaP cells (VCaP16) that is resistant to the AR inhibitor enzalutamide (ENZ). AR activity in VCaP16 is driven by ARv7, independently of full-length AR (ARfl), and its cistrome and transcriptome mirror those of ARfl in VCaP cells. ARv7 expression increases rapidly in response to ENZ, but there is a delay in gaining chromatin binding and transcriptional activity, which is associated with increased chromatin accessibility. AR and nuclear factor I (NFI) motifs are most enriched at more accessible sites, and NFIB/X knockdown greatly diminishes ARv7 function. These findings indicate that ARv7 can drive the AR program but that its activity is dependent on adaptations that increase chromatin accessibility to enhance its intrinsically weak chromatin binding.

Looking beyond the ER, PR, and HER2: what's new in the ARsenal for combating breast cancer?

Breast cancer (BrCa) is a complex and heterogeneous disease with diverse molecular subtypes, leading to varied clinical outcomes and posing significant treatment challenges. The increasing global burden of BrCa, particularly in low- and middle-income countries, underscores the urgent need for more effective therapeutic strategies. The androgen receptor (AR), expressed in a substantial proportion of breast cancer cases, has emerged as a potential biomarker and therapeutic target. In breast cancer, AR exhibits diverse functions across subtypes, often interacting with other hormone receptors, thereby influencing tumor progression and treatment responses. This intricate interplay is further complicated by the presence of constitutively expressed AR splice variants (AR-Vs) that drive resistance to AR-targeting therapies through structural rearrangements in the domains and activation of aberrant signaling pathways. Although AR-targeting drugs, initially developed for prostate cancer (PCa), have shown promise in AR-positive breast cancer, significant gaps remain in understanding AR's precise functions and therapeutic potential. The systemic management of breast cancer is guided primarily by theranostic biomarkers; ER, PR, HER2, and Ki67 which also dictate the breast cancer classification. The ubiquitous expression of AR in BrCa and the emergence of AR-Vs can assist the management of disease complementing the standard of care. This article provides a comprehensive overview of AR and its splice variants in the context of breast cancer, highlighting their prognostic and predictive value across different subtypes looking beyond the conventional ER, PR, and HER2 status. This review also raises the possibility of using AR splice variants in predicting tumor aggressiveness. From the settings of developing nations, this may provide useful insight by integrating recent advances in AR-targeted therapies and exploring their translational potential, emphasizing the critical need for further research to optimize AR-based therapeutic strategies for breast cancer management.

AR-V7 condensates drive androgen-independent transcription in castration resistant prostate cancer

Biomolecular condensates organize cellular environments and regulate key processes such as transcription. We previously showed that full-length androgen receptor (AR-FL), a major oncogenic driver in prostate cancer (PCa), forms nuclear condensates upon androgen stimulation in androgen-sensitive PCa cells. Disrupting these condensates impairs AR-FL transcriptional activity, highlighting their functional importance. However, resistance to androgen deprivation therapy often leads to castration-resistant prostate cancer (CRPC), driven by constitutively active splice variants like AR variant 7 (AR-V7). The mechanisms underlying AR-V7's role in CRPC remain unclear. In this study, we characterized the condensate-forming ability of AR-V7 and compared its phase behavior with AR-FL across a spectrum of PCa models and in vitro conditions. Our findings indicate that cellular context can influence AR-V7's condensate-forming capacity. Unlike AR-FL, AR-V7 spontaneously forms condensates in the absence of androgen stimulation and functions independently of AR-FL in CRPC models. However, AR-V7 requires a higher concentration to form condensates, both in cellular contexts and in vitro . We further reveal that AR-V7 drives transcription via both condensate-dependent and condensate-independent mechanisms. Using an AR-V7 mutant incapable of forming condensates, while retaining nuclear localization and DNA-binding ability, we reveal that the condensate-dependent regime activates part of the oncogenic KRAS pathway in CRPC models. Genes under this condensate-dependent regime were found to harbor significantly higher numbers of AR-binding sites and exhibited boosted expression in response to AR-V7. These findings uncover a previously unrecognized role of AR-V7 condensate formation in driving oncogenic transcriptional programs and shed light on its unique contribution to CRPC progression.

Highlights

AR-V7 condensates form independently of both androgens and AR-FL in CRPC models.AR-V7 mediates condensate-dependent and independent transcriptionCondensate-dependent transcription enables boosted expression of oncogenic KRAS genesCondensate-dependent genes exhibit an exponential increase in expression, with a higher number of AR binding sites potentially playing a key role in their reliance on condensate formation.

Targeting proliferating cell nuclear antigen enhances ionizing radiation-induced cytotoxicity in prostate cancer cells

BACKGROUND

Proliferating cell nuclear antigen (PCNA) is essential for DNA replication and repair, cell growth, and survival. PCNA also enhances androgen receptor (AR) signaling in prostate cancer (PC) cells. We identified a PCNA interaction protein (PIP) box at the N-terminal domain of AR and developed a small peptide PCNA inhibitor R9-AR-PIP containing AR PIP-box. We also identified a series of small molecule PCNA inhibitors (PCNA-Is) that bind directly to PCNA and interrupt PCNA functions. The present study investigated the effects of the PCNA inhibitors on the sensitivity of PC cells to X-ray radiation.

METHODS

The effects of targeting PCNA on radio sensitivity of PC cells were investigated in four lines of castration-resistant PC (CRPC) cells with different AR expression statuses. The cells were treated with the PCNA inhibitors and X-ray radiation alone or in combination. The effects of the treatment on expression of AR target genes, DNA damage response, DNA damage, homologous recombination repair (HRR), and cytotoxicity were evaluated.

RESULTS

We found that the androgen response element (ARE) occupancy of the DNA damage response gene PARP1 by AR is significantly attenuated by PCNA-I1S or R9-AR-PIP combined with X-ray radiation, while X-ray radiation alone does not enhance the ARE occupancy. PCNA-I1S or R9-AR-PIP alone significantly inhibits occupancy of the AR-occupied regions (AROR) in PRKDC and XRCC2 genes. R9-AR-PIP and PCNA-I1S inhibit expression of AR-Vs target gene cyclin A2 and show the additive effects with radiation in AR-positive CRPC cells. Targeting PCNA by PCNA-I1S and R9-AR-PIP downregulates expression of DNA damage response genes EXO1, Rad54L, Rad51, and/or PARP1 and shows the additive effects with radiation as compared with their respective controls in AR-positive CRPC LNCaP-AI, 22Rv1, and R1-D567 cells, but not in AR-negative PC-3 cells. R9-AR-PIP and PCNA-I1S elevate the levels of phospho-DNA-PKcs(S2056) and γH2AX, indicating DNA damage in response to radiation in AR-positive cells. The HRR is significantly attenuated by PCNA inhibitors PCNA-I1S, R9-AR-PIP, and T2AA in all four CRPC cells examined, and inhibited by Enzalutamide (Enz) only in 22RV1 cells. The cytotoxicity induced by X-ray radiation in androgen-dependent LNCaP cells is enhanced by Enz and a lower concentration of R9-AR-PIP in the colony formation assay. R9-AR-PIP at higher concentration reduces the colony formation and has an additive effect with X-ray radiation in all AR expressing cells, regardless of AR-FL and AR-Vs, but does not significantly alter the colony formation in AR-negative PC-3 cells. PCNA-I1S attenuates colony formation and has an additive effect with ionizing radiation in all four CRPC cells, regardless of AR expression status.

CONCLUSION

These data provide a strong rationale for the therapy studies using PCNA-I1S or R9-AR-PIP in combination with X-ray radiation against CRPC tumors in preclinical models.

The loss of hepatitis B virus receptor NTCP/SLC10A1 in human liver cancer cells is due to epigenetic silencing

Human Na+-taurocholate cotransporting polypeptide (hNTCP) is predominantly expressed in hepatocytes, maintaining bile salt homeostasis and serving as a receptor for hepatitis B virus (HBV). hNTCP expression is downregulated during hepatocellular carcinoma (HCC) development. In this study, we investigated the molecular mechanisms underlying hNTCP dysregulation using HCC tissues and cell lines, and primary human hepatocytes (PHHs). Firstly, we observed a significant reduction of hNTCP in HCC tumors compared to adjacent and normal tissues. Additionally, hNTCP mRNA levels were markedly lower in HepG2 cells compared to PHHs, which was corroborated at the protein level by immunoblotting. Sanger sequencing confirmed identical sequences for hNTCP promoter, exons, and mRNA coding sequences between PHH and HepG2 cells, indicating no mutations or splicing alterations. We then assessed the epigenetic status of hNTCP. The hNTCP promoter, with low CG content, showed no significant methylation differences between PHH and HepG2 cells. Chromatin immunoprecipitation coupled with qPCR (ChIP-qPCR) revealed a loss of activating histone posttranslational modification (PTM) H3K27ac near the hNTCP transcription start site (TSS) in HepG2 cells. This loss was also confirmed in HCC tumor cells compared to adjacent and background cells. Treating HepG2 cells with histone deacetylase inhibitors enhanced H3K27ac accumulation and glucocorticoid receptor (GR) binding at the hNTCP TSS, significantly increasing hNTCP mRNA and protein levels, and rendering the cells susceptible to HBV infection. In summary, histone PTM-related epigenetic mechanisms play a critical role in hNTCP dysregulation in liver cancer cells, providing insights into hepatocarcinogenesis and its impact on chronic HBV infection.

IMPORTANCE

HBV is a hepatotropic virus that infects human hepatocytes expressing the viral receptor hNTCP. Without effective antiviral therapy, chronic HBV infection poses a high risk of liver cancer. However, most liver cancer cell lines, including HepG2 and Huh7, do not support HBV infection due to the absence of hNTCP expression, and the mechanism underlying this defect remains unclear. This study demonstrates a significant reduction of hNTCP in hepatocellular carcinoma samples and HepG2 cells compared to normal liver tissues and primary human hepatocytes. Despite identical hNTCP genetic sequences, epigenetic analyses revealed a loss of the activating histone modification H3K27ac near the hNTCP transcription start site in cancer cells. Treatment with histone deacetylase inhibitors restored H3K27ac levels, reactivated hNTCP expression, and rendered HepG2 cells susceptible to HBV infection. These findings highlight the role of epigenetic modulation in hNTCP dysregulation, offering insights into hepatocarcinogenesis and its implications for chronic HBV infection.

Increased chromatin accessibility mediated by nuclear factor I drives transition to androgen receptor splice variant dependence in castration-resistant prostate cancer

Androgen receptor (AR) splice variants, of which ARv7 is the most common, are increased in prostate cancer (PC) that develops resistance to androgen signaling inhibitor drugs, but the extent to which these variants drive AR activity, and whether they have novel functions or dependencies, remain to be determined. We generated a subline of VCaP PC cells (VCaP16) that is resistant to the AR inhibitor enzalutamide (ENZ) and found that AR activity was independent of the full-length AR (ARfl), despite its continued high-level expression, and was instead driven by ARv7. The ARv7 cistrome and transcriptome in VCaP16 cells mirrored that of the ARfl in VCaP cells, although ARv7 chromatin binding was weaker, and strong ARv7 binding sites correlated with higher affinity ARfl binding sites across multiple models and clinical samples. Notably, although ARv7 expression in VCaP cells increased rapidly in response to ENZ, there was a long lag before it gained chromatin binding and transcriptional activity. This lag was associated with an increase in chromatin accessibility, with the AR and nuclear factor I (NFI) motifs being most enriched at these more accessible sites. Moreover, the transcriptional effects of combined NFIB and NFIX knockdown versus ARv7 knockdown were highly correlated. These findings indicate that ARv7 can drive the AR program, but that its activity is dependent on adaptations that increase chromatin accessibility to enhance its intrinsically weak chromatin binding.

Characterization of allosteric modulators that disrupt androgen receptor co-activator protein-protein interactions to alter transactivation-Drug leads for metastatic castration resistant prostate cancer

Three series of compounds were prioritized from a high content screening campaign that identified molecules that blocked dihydrotestosterone (DHT) induced formation of Androgen Receptor (AR) protein-protein interactions (PPIs) with the Transcriptional Intermediary Factor 2 (TIF2) coactivator and also disrupted preformed AR-TIF2 PPI complexes; the hydrobenzo-oxazepins (S1), thiadiazol-5-piperidine-carboxamides (S2), and phenyl-methyl-indoles (S3). Compounds from these series inhibited AR PPIs with TIF2 and SRC-1, another p160 coactivator, in mammalian 2-hybrid assays and blocked transcriptional activation in reporter assays driven by full length AR or AR-V7 splice variants. Compounds inhibited the growth of five prostate cancer cell lines, with many exhibiting differential cytotoxicity towards AR positive cell lines. Representative compounds from the 3 series substantially reduced both endogenous and DHT-enhanced expression and secretion of the prostate specific antigen (PSA) cancer biomarker in the C4-2 castration resistant prostate cancer (CRPC) cell line. The comparatively weak activities of series compounds in the H3-DHT and/or TIF2 box 3 LXXLL-peptide binding assays to the recombinant ligand binding domain of AR suggest that direct antagonism at the orthosteric ligand binding site or AF-2 surface respectively are unlikely mechanisms of action. Cellular enhanced thermal stability assays (CETSA) indicated that compounds engaged AR and reduced the maximum efficacy and right shifted the EC50 of DHT-enhanced AR thermal stabilization consistent with the effects of negative allosteric modulators. Molecular docking of potent representative hits from each series to AR structures suggest that S1-1 and S2-6 engage a novel binding pocket (BP-1) adjacent to the orthosteric ligand binding site, while S3-11 occupies the AR binding function 3 (BF-3) allosteric pocket. Hit binding poses indicate spaces and residues adjacent to the BP-1 and BF-3 pockets that will be exploited in future medicinal chemistry optimization studies. Small molecule allosteric modulators that prevent/disrupt AR PPIs with coactivators like TIF2 to alter transcriptional activation in the presence of orthosteric agonists might evade the resistance mechanisms to existing prostate cancer drugs and provide novel starting points for medicinal chemistry lead optimization and future development into therapies for metastatic CRPC.

Beyond Prostate Cancer: An Androgen Receptor Splice Variant Expression in Multiple Malignancies, Non-Cancer Pathologies, and Development

Multiple studies have demonstrated the importance of androgen receptor (AR) splice variants (SVs) in the progression of prostate cancer to the castration-resistant phenotype and their utility as a diagnostic. However, studies on AR expression in non-prostatic malignancies uncovered that AR-SVs are expressed in glioblastoma, breast, salivary, bladder, kidney, and liver cancers, where they have diverse roles in tumorigenesis. AR-SVs also have roles in non-cancer pathologies. In granulosa cells from women with polycystic ovarian syndrome, unique AR-SVs lead to an increase in androgen production. In patients with nonobstructive azoospermia, testicular Sertoli cells exhibit differential expression of AR-SVs, which is associated with impaired spermatogenesis. Moreover, AR-SVs have been identified in normal cells, including blood mononuclear cells, neuronal lipid rafts, and the placenta. The detection and characterization of AR-SVs in mammalian and non-mammalian species argue that AR-SV expression is evolutionarily conserved and that AR-SV-dependent signaling is a fundamental regulatory feature in multiple cellular contexts. These discoveries argue that alternative splicing of the AR transcript is a commonly used mechanism that leads to an expansion in the repertoire of signaling molecules needed in certain tissues. Various malignancies appropriate this mechanism of alternative AR splicing to acquire a proliferative and survival advantage.

A role for androgen receptor variant 7 in sensitivity to therapy: Involvement of IGFBP-2 and FOXA1

Prostate cancer (PCa) is one of the leading causes of cancer-related deaths in men. Localised PCa can be treated effectively, but most patients relapse/progress to more aggressive disease. One possible mechanism underlying this progression is alternative splicing of the androgen receptor, with AR variant 7(ARV7) considered to play a major role. Using viability assays, we confirmed that ARV7-positive PCa cells were less sensitive to treatment with cabazitaxel and an anti-androgen-enzalutamide. Also, using live-holographic imaging, we showed that PCa cells with ARV7 exhibited an increased rate of cell division, proliferation, and motility, which could potentially contribute to a more aggressive phenotype. Furthermore, protein analysis demonstrated that ARV7 knock-down was associated with a decrease in insulin-like growth factor-2 (IGFBP-2) and forkhead box protein A1(FOXA1). This correlation was confirmed in-vivo using PCa tissue samples. Spearman rank correlation analysis showed significant positive associations between ARV7 and IGFBP-2 or FOXA1 in tissue from patients with PCa. This association was not present with the AR. These data suggest an interplay of FOXA1 and IGFBP-2 with ARV7-mediated acquisition of an aggressive prostate cancer phenotype.

Androgen receptor transcriptional activity is required for heregulin-1β-mediated nuclear localization of the HER3/ErbB3 receptor tyrosine kinase

Prostate cancer is initially regulated by the androgen receptor (AR), a ligand-activated, transcription factor, and is in a hormone-dependent state (hormone-sensitive prostate cancer (HSPC)), but eventually becomes androgen-refractory (castration-resistant prostate cancer (CRPC)) because of mechanisms that bypass the AR, including by activation of ErbB3, a member of the epidermal growth factor receptor family. ErbB3 is synthesized in the cytoplasm and transported to the plasma membrane for ligand binding and dimerization, where it regulates downstream signaling, but nuclear forms are reported. Here, we demonstrate in prostatectomy samples that ErbB3 nuclear localization is observed in malignant, but not benign prostate, and that cytoplasmic (but not nuclear) ErbB3 correlated positively with AR expression but negatively with AR transcriptional activity. In support of the latter, androgen depletion upregulated cytoplasmic, but not nuclear ErbB3, while in vivo studies showed that castration suppressed ErbB3 nuclear localization in HSPC, but not CRPC tumors. In vitro treatment with the ErbB3 ligand heregulin-1β (HRG) induced ErbB3 nuclear localization, which was androgen-regulated in HSPC but not in CRPC. In turn, HRG upregulated AR transcriptional activity in CRPC but not in HSPC cells. Positive correlation between ErbB3 and AR expression was demonstrated in AR-null PC-3 cells where stable transfection of AR restored HRG-induced ErbB3 nuclear transport, while AR knockdown in LNCaP reduced cytoplasmic ErbB3. Mutations of ErbB3's kinase domain did not affect its localization but was responsible for cell viability in CRPC cells. Taken together, we conclude that AR expression regulated ErbB3 expression, its transcriptional activity suppressed ErbB3 nuclear translocation, and HRG binding to ErbB3 promoted it.

α-Mangostin Promotes In Vitro and In Vivo Degradation of Androgen Receptor and AR-V7 Splice Variant in Prostate Cancer Cells

A major limitation of current prostate cancer pharmacotherapy approaches is the inability of these compounds to target androgen receptor variants or mutants that develop during prostate cancer progression. The demand for novel therapeutics to prevent, slow, and treat prostate cancer is significant because FDA approved anti-androgens are associated with adverse events and can eventually drive drug-resistant prostate cancer. This study evaluated α-mangostin for its novel ability to degrade the androgen receptor and androgen receptor variants. α-Mangostin is one of more than 70 isoprenylated xanthones isolated from Garcinia mangostana that we have been evaluating for their anticancer potential. Prostate cancer cells treated with α-mangostin exhibited decreased levels of wild-type and mutated androgen receptors. Immunoblot, immunoprecipitation, and transfection experiments demonstrated that the androgen receptor was ubiquitinated and subsequently degraded via the proteasome, which we hypothesize occurs with the assistance of BiP, an ER chaperone protein that we have shown to associate with the androgen receptor. We also evaluated α-mangostin for its antitumor activity and promotion of androgen receptor degradation in vivo. In summary, our study demonstrates that androgen receptor degradation occurs through the novel activation of BiP and suggests a new therapeutic approach for prostate cancer.

A compendium of Androgen Receptor Variant 7 target genes and their role in Castration Resistant Prostate Cancer

Persistent androgen receptor (AR) signalling is the main driver of prostate cancer (PCa). Truncated isoforms of the AR called androgen receptor variants (AR-Vs) lacking the ligand binding domain often emerge during treatment resistance against AR pathway inhibitors such as Enzalutamide. This review discusses how AR-Vs drive a more aggressive form of PCa through the regulation of some of their target genes involved in oncogenic pathways, enabling disease progression. There is a pressing need for the development of a new generation of AR inhibitors which can repress the activity of both the full-length AR and AR-Vs, for which the knowledge of differentially expressed target genes will allow evaluation of inhibition efficacy. This review provides a detailed account of the most common variant, AR-V7, the AR-V7 regulated genes which have been experimentally validated, endeavours to understand their relevance in aggressive AR-V driven PCa and discusses the utility of the downstream protein products as potential drug targets for PCa treatment.

Mechanisms and targeting of proteosome-dependent androgen receptor degradation in prostate cancer

The androgen receptor (AR) remains to be a key target for the treatment of prostate cancer, including the majority of castration-resistant prostate cancer (CRPC). AR is stabilized in CRPC and the ubiquitin-proteasome system (UPS) plays a major role in AR degradation. Targeting AR for degradation provides a potential approach to overcome the resistance of CRPC to current AR antagonists, including the next generation AR signaling inhibitors. Different types of AR degraders have been developed, including the proteolysis-targeting chimeras (PROTACs), selective AR degraders (SARDs), and novel AR degraders, with several AR PROTACs currently in clinical trials. The present mini-review discusses the regulation of AR degradation by the UPS, the potential role of a novel nuclear degradation signal in AR, and different types of AR degraders.

FOXA1 in prostate cancer

Most prostate cancers initially respond to androgen deprivation therapy (ADT). With the long-term application of ADT, localized prostate cancer will progress to castration-resistant prostate cancer (CRPC), metastatic CRPC (mCRPC), and neuroendocrine prostate cancer (NEPC), and the transcriptional network shifted. Forkhead box protein A1 (FOXA1) may play a key role in this process through multiple mechanisms. To better understand the role of FOXA1 in prostate cancer, we review the interplay among FOXA1-targeted genes, modulators of FOXA1, and FOXA1 with a particular emphasis on androgen receptor (AR) function. Furthermore, we discuss the distinct role of FOXA1 mutations in prostate cancer and clinical significance of FOXA1. We summarize possible regulation pathways of FOXA1 in different stages of prostate cancer. We focus on links between FOXA1 and AR, which may play different roles in various types of prostate cancer. Finally, we discuss FOXA1 mutation and its clinical significance in prostate cancer. FOXA1 regulates the development of prostate cancer through various pathways, and it could be a biomarker for mCRPC and NEPC. Future efforts need to focus on mechanisms underlying mutation of FOXA1 in advanced prostate cancer. We believe that FOXA1 would be a prognostic marker and therapeutic target in prostate cancer.

Androgen receptor variant-7 regulation by tenascin-c induced src activation

BACKGROUND

Bone metastatic prostate cancer does not completely respond to androgen-targeted therapy and generally evolves into lethal castration resistant prostate cancer (CRPC). Expression of AR-V7- a constitutively active, ligand independent splice variant of AR is one of the critical resistant mechanisms regulating metastatic CRPC. TNC is an extracellular matrix glycoprotein, crucial for prostate cancer progression, and associated with prostate cancer bone metastases. In this study, we investigated the mechanisms that regulate AR-V7 expression in prostate cancer cells interacting with osteogenic microenvironment including TNC.

METHODS

Prostate cancer/preosteoblast heterotypical organoids were evaluated via immunofluorescence imaging and gene expression analysis using RT-qPCR to assess cellular compartmentalization, TNC localization, and to investigate regulation of AR-V7 in prostate cancer cells by preosteoblasts and hormone or antiandrogen action. Prostate cancer cells cultured on TNC were assessed using RT-qPCR, Western blotting, cycloheximide chase assay, and immunofluorescence imaging to evaluate (1) regulation of AR-V7, and (2) signaling pathways activated by TNC. Identified signaling pathway induced by TNC was targeted using siRNA and a small molecular inhibitor to investigate the role of TNC-induced signaling activation in regulation of AR-V7. Both AR-V7- and TNC-induced signaling effectors were targeted using siRNA, and TNC expression assessed to evaluate potential feedback regulation.

RESULTS

Utilizing heterotypical organoids, we show that TNC is an integral component of prostate cancer interaction with preosteoblasts. Interaction with preosteoblasts upregulated both TNC and AR-V7 expression in prostate cancer cells which was suppressed by testosterone but elevated by antiandrogen enzalutamide. Interestingly, the results demonstrate that TNC-induced Src activation regulated AR-V7 expression, post-translational stability, and nuclear localization in prostate cancer cells. Treatment with TNC neutralizing antibody, Src knockdown, and inhibition of Src kinase activity repressed AR-V7 transcript and protein. Reciprocally, both activated Src and AR-V7 were observed to upregulate autocrine TNC gene expression in prostate cancer cells.

CONCLUSION

Overall, the findings reveal that prostate cancer cell interactions with the cellular and ECM components in the osteogenic microenvironment plays critical role in regulating AR-V7 associated with metastatic CRPC. Video Abstract.

Development of a High-Throughput Screening Assay for Small-Molecule Inhibitors of Androgen Receptor Splice Variants

The role of the androgen receptor (AR) in the progression of prostate cancer (PCa) is well established and competitive inhibition of AR ligand binding domain (LBD) has been the mainstay of antiandrogen therapies for advanced and metastatic disease. However, the efficacy of such drugs is often limited by the emergence of resistance, mediated through point mutations and receptor splice variants lacking the AR-LBD. As a result, the prognosis for patients with malignant, castrate-resistant disease remains poor. The amino terminal domain (NTD) of the AR has been shown to be critical for AR function. Its modular activation function (AF-1) is important for both gene regulation and participation in protein–protein interactions. However, due to the intrinsically disordered structure of the domain, its potential as a candidate for therapeutic intervention has been generally overlooked. In this article, we describe the design and development of a functional cell-based assay aimed at identifying small-molecule inhibitors of the AR-NTD. We demonstrate the suitability of the assay for high-throughput screening platforms and validate two initial hits emerging from a small, targeted, library screen in PCa cells.

Cistrome and transcriptome analysis identifies unique androgen receptor (AR) and AR-V7 splice variant chromatin binding and transcriptional activities

The constitutively active androgen receptor (AR) splice variant, AR-V7, plays an important role in resistance to androgen deprivation therapy in castration resistant prostate cancer (CRPC). Studies seeking to determine whether AR-V7 is a partial mimic of the AR, or also has unique activities, and whether the AR-V7 cistrome contains unique binding sites have yielded conflicting results. One limitation in many studies has been the low level of AR variant compared to AR. Here, LNCaP and VCaP cell lines in which AR-V7 expression can be induced to match the level of AR, were used to compare the activities of AR and AR-V7. The two AR isoforms shared many targets, but overall had distinct transcriptomes. Optimal induction of novel targets sometimes required more receptor isoform than classical targets such as PSA. The isoforms displayed remarkably different cistromes with numerous differential binding sites. Some of the unique AR-V7 sites were located proximal to the transcription start sites (TSS). A de novo binding motif similar to a half ARE was identified in many AR-V7 preferential sites and, in contrast to conventional half ARE sites that bind AR-V7, FOXA1 was not enriched at these sites. This supports the concept that the AR isoforms have unique actions with the potential to serve as biomarkers or novel therapeutic targets.

Cellular specificity of androgen receptor, coregulators, and pioneer factors in prostate cancer

Androgen signalling, through the transcription factor androgen receptor (AR), is vital to all stages of prostate development and most prostate cancer progression. AR signalling controls differentiation, morphogenesis, and function of the prostate. It also drives proliferation and survival in prostate cancer cells as the tumour progresses; given this importance, it is the main therapeutic target for disseminated disease. AR is also essential in the surrounding stroma, for the embryonic development of the prostate and controlling epithelial glandular development. Stromal AR is also important in cancer initiation, regulating paracrine factors that excite cancer cell proliferation, but lower stromal AR expression correlates with shorter time to progression/worse outcomes. The profile of AR target genes is different between benign and cancerous epithelial cells, between castrate-resistant prostate cancer cells and treatment-naïve cancer cells, between metastatic and primary cancer cells, and between epithelial cells and fibroblasts. This is also true of AR DNA-binding profiles. Potentially regulating the cellular specificity of AR binding and action are pioneer factors and coregulators, which control and influence the ability of AR to bind to chromatin and regulate gene expression. The expression of these factors differs between benign and cancerous cells, as well as throughout disease progression. The expression profile is also different between fibroblast and mesenchymal cell types. The functional importance of coregulators and pioneer factors in androgen signalling makes them attractive therapeutic targets, but given the contextual expression of these factors, it is essential to understand their roles in different cancerous and cell-lineage states.

Upregulation of miR-3195, miR-3687 and miR-4417 is associated with castration-resistant prostate cancer

PURPOSE

Prostate cancer (PCa) is a leading cause of cancer-related death. Upon androgen-deprivation therapy, the disease may progress further to castration-resistant PCa (CRPC) with a poor prognosis. MicroRNAs (miRNAs) are small non-coding RNAs, which play crucial roles in gene regulation. The aim of our study is to find CRPC-associated miRNAs and to evaluate their functional role.

METHODS

In this study, 23 benign prostatic hyperplasia (BPH), 76 primary PCa, and 35 CRPC specimens were included. Total RNA extracted from tissue sections was used for miRNA profiling on the Affymetrix GSC 3000 platform. Subsequently, stem-loop RT-qPCR analysis was performed to validate the expression levels of selected miRNAs. PCa cell lines were transfected with miRNA mimics or inhibitors to evaluate the effects on cell proliferation, cell migration and cell invasion.

RESULTS

In our profiling study, several miRNAs were found to be deregulated in CRPC compared to primary PCa tissue, of which miR-205 (- 4.5-fold; p = 0.0009), miR-92b (- 3.1 fold; p < 0.0001) were downregulated and miR-3195 (5.6-fold; p < 0.0001), miR-3687 (8.7-fold; p = 0.0006) and miR-4417 (5.0-fold; p = 0.0005) were most upregulated. While KLK3, miR-21 and miR-141 expression levels in androgen-treated VCaP and LNCaP cells were increased, the expression levels of miR-3687 and miR-4417 were reduced. None of the miRNAs were androgen-regulated in the AR-negative PC3 cell line. Overexpression of miR-3687 reduced cell migration and cell invasion, whilst miR-3195 enhanced cell migration.

CONCLUSION

We have identified several novel deregulated miRNAs in CRPC tissue, including two microRNAs that are potentially involved in tumor invasion. Our data support the hypothesized involvement of miRNAs in PCa tumorigenesis and progression to CRPC. The applicability of these miRNAs as novel biomarkers for CRPC remains to be further investigated.

NF-κB Blockade with Oral Administration of Dimethylaminoparthenolide (DMAPT), Delays Prostate Cancer Resistance to Androgen Receptor (AR) Inhibition and Inhibits AR Variants

NF-κB activation has been linked to prostate cancer progression and is commonly observed in castrate-resistant disease. It has been suggested that NF-κB-driven resistance to androgen-deprivation therapy (ADT) in prostate cancer cells may be mediated by aberrant androgen receptor (AR) activation and AR splice variant production. Preventing resistance to ADT may therefore be achieved by using NF-κB inhibitors. However, low oral bioavailability and high toxicity of NF-κB inhibitors is a major challenge for clinical translation. Dimethylaminoparthenolide (DMAPT) is an oral NF-κB inhibitor in clinical development and has already shown favorable pharmacokinetic and pharmacodyanamic data in patients with heme malignancies, including decrease of NF-κB in circulating leuchemic blasts. Here, we report that activation of NF-κB/p65 by castration in mouse and human prostate cancer models resulted in a significant increase in AR variant-7 (AR-V7) expression and modest upregulation of AR. In vivo castration of VCaP-CR tumors resulted in significant upregulation of phosphorylated-p65 and AR-V7, which was attenuated by combination with DMAPT and DMAPT increased the efficacy of AR inhibition. We further demonstrate that the effects of DMAPT-sensitizing prostate cancer cells to castration were dependent on the ability of DMAPT to inhibit phosphorylated-p65 function. IMPLICATIONS: Our study shows that DMAPT, an oral NF-κB inhibitor in clinical development, inhibits phosphorylated-p65 upregulation of AR-V7 and delays prostate cancer castration resistance. This provides rationale for the development of DMAPT as a novel therapeutic strategy to increase durable response in patients receiving AR-targeted therapy.

Increased transcription and high translation efficiency lead to accumulation of androgen receptor splice variant after androgen deprivation therapy

Upregulation of androgen receptor splice variants (AR-Vs), especially AR-V7, is associated with castration resistance of prostate cancer. At the RNA level, AR-V7 upregulation is generally coupled with increased full-length AR (AR-FL); consequently, AR-V7 and AR-Vs collectively constitute a minority of the AR population. However, Western blotting showed that the relative abundance of AR-V proteins is much higher in many castration-resistant prostate cancers (CRPCs). To address the mechanism underlying this discrepancy, we analyzed RNA-seq data from ~350 CRPC samples and found a positive correlation between all canonical and alternative AR splicing. This indicates that increased alternative splicing is not at the expense of canonical splicing. Instead, androgen deprivation releases AR-FL from repressing the transcription of the AR gene to induce coordinated increase of AR-FL and AR-V mRNAs. At the protein level, however, androgen deprivation induces AR-FL, but not AR-V, degradation. Moreover, AR-V7 is translated much faster than AR-FL. Thus, androgen-deprivation-induced AR-gene transcription and AR-FL protein decay, together with efficient AR-V7 translation, explain the discrepancy between the relative AR-V mRNA and protein abundances in many CRPCs, highlighting the inevitability of AR-V induction after endocrine therapy.

Isoform-specific Activities of Androgen Receptor and its Splice Variants in Prostate Cancer Cells

Androgen receptor (AR) signaling continues to drive castration-resistant prostate cancer (CRPC) in spite of androgen deprivation therapy (ADT). Constitutively active shorter variants of AR, lacking the ligand binding domain, are frequently expressed in CRPC and have emerged as a potential mechanism for prostate cancer to escape ADT. ARv7 and ARv567es are 2 of the most commonly detected variants of AR in clinical samples of advanced, metastatic prostate cancer. It is not clear if variants of AR merely act as weaker substitutes for AR or can mediate unique isoform-specific activities different from AR. In this study, we employed LNCaP prostate cancer cell lines with inducible expression of ARv7 or ARv567es to delineate similarities and differences in transcriptomics, metabolomics, and lipidomics resulting from the activation of AR, ARv7, or ARv567es. While the majority of target genes were similarly regulated by the action of all 3 isoforms, we found a clear difference in transcriptomic activities of AR versus the variants, and a few differences between ARv7 and ARv567es. Some of the target gene regulation by AR isoforms was similar in the VCaP background as well. Differences in downstream activities of AR isoforms were also evident from comparison of the metabolome and lipidome in an LNCaP model. Overall our study implies that shorter variants of AR are capable of mediating unique downstream activities different from AR and some of these are isoform specific.

Eighty Years of Targeting Androgen Receptor Activity in Prostate Cancer: The Fight Goes on

Simple Summary

Prostate cancer is the second most common cancer in men world-wide, with nearly 1.3 million new cases each year, and over the next twenty years the incidence and death rate are predicted to nearly double. For decades, this lethal disease has been more or less successfully treated using hormonal therapy, which has the ultimate aim of inhibiting androgen signalling. However, prostate tumours can evade such hormonal therapies in a number of different ways and therapy resistant disease, so-called castration-resistant prostate cancer (CRPC) is the major clinical problem. Somewhat counterintuitively, the androgen receptor remains a key therapy target in CRPC. Here, we explain why this is the case and summarise both new hormone therapy strategies and the recent advances in knowledge of androgen receptor structure and function that underpin them.

Abstract

Prostate cancer (PCa) is the most common cancer in men in the West, other than skin cancer, accounting for over a quarter of cancer diagnoses in US men. In a seminal paper from 1941, Huggins and Hodges demonstrated that prostate tumours and metastatic disease were sensitive to the presence or absence of androgenic hormones. The first hormonal therapy for PCa was thus castration. In the subsequent eighty years, targeting the androgen signalling axis, where possible using drugs rather than surgery, has been a mainstay in the treatment of advanced and metastatic disease. Androgens signal via the androgen receptor, a ligand-activated transcription factor, which is the direct target of many such drugs. In this review we discuss the role of the androgen receptor in PCa and how the combination of structural information and functional screenings is continuing to be used for the discovery of new drug to switch off the receptor or modify its function in cancer cells.

Androgen receptor and its splice variant, AR-V7, differentially induce mRNA splicing in prostate cancer cells

Prostate cancer (PCa) is dependent on the androgen receptor (AR). Advanced PCa is treated with an androgen deprivation therapy-based regimen; tumors develop resistance, although they typically remain AR-dependent. Expression of constitutively active AR variants lacking the ligand-binding domain including the variant AR-V7 contributes to this resistance. AR and AR-V7, as transcription factors, regulate many of the same genes, but also have unique activities. In this study, the capacity of the two AR isoforms to regulate splicing was examined. RNA-seq data from models that endogenously express AR and express AR-V7 in response to doxycycline were used. Both AR isoforms induced multiple changes in splicing and many changes were isoform-specific. Analyses of two endogenous genes, PGAP2 and TPD52, were performed to examine differential splicing. A novel exon that appears to be a novel transcription start site was preferentially induced by AR-V7 in PGAP2 although it is induced to a lesser extent by AR. The previously described AR induced promoter 2 usage that results in a novel protein derived from TPD52 (PrLZ) was not induced by AR-V7. AR, but not AR-V7, bound to a site proximal to promoter 2, and induction was found to depend on FOXA1.

A detailed characterization of stepwise activation of the androgen receptor variant 7 in prostate cancer cells

Expression of the andrgogen receptor splice variant 7 (AR-V7) is frequently detected in castrate resistant prostate cancer and associated with resistance to AR-targeted therapies. While we have previously noted that homodimerization is required for the transcriptional activity of AR-V7 and that AR-V7 can also form heterodimers with the full-length AR (AR-FL), there are still many gaps of knowledge in AR-V7 stepwise activation. In the present study, we show that neither AR-V7 homodimerization nor AR-V7/AR-FL heterodimerization requires cofactors or DNA binding. AR-V7 can enter the nucleus as a monomer and drive a transcriptional program and DNA-damage repair as a homodimer. While forming a heterodimer with AR-FL to induce nuclear localization of unliganded AR-FL, AR-V7 does not need to interact with AR-FL to drive gene transcription or DNA-damage repair in prostate cancer cells that co-express AR-V7 and AR-FL. These data indicate that AR-V7 can function independently of its interaction with AR-FL in the true castrate state or “absence of ligand”, providing support for the utility of targeting AR-V7 in improving outcomes of patients with castrate resistant prostate cancer.

Quadruple-negative breast cancer: novel implications for a new disease

Based on the androgen receptor (AR) expression, triple-negative breast cancer (TNBC) can be subdivided into AR-positive TNBC and AR-negative TNBC, also known as quadruple-negative breast cancer (QNBC). QNBC characterization and treatment is fraught with many challenges. In QNBC, there is a greater paucity of prognostic biomarkers and therapeutic targets than AR-positive TNBC. Although the prognostic role of AR in TNBC remains controversial, many studies revealed that a lack of AR expression confers a more aggressive disease course. Literature characterizing QNBC tumor biology and uncovering novel biomarkers for improved management of the disease remains scarce. In this comprehensive review, we summarize the current QNBC landscape and propose avenues for future research, suggesting potential biomarkers and therapeutic strategies that warrant investigation.

Molecular Origin, Expression Regulation, and Biological Function of Androgen Receptor Splicing Variant 7 in Prostate Cancer

The problem of resistance to therapy in prostate cancer (PCa) is multifaceted. Key determinants of drug resistance include tumor burden and growth kinetics, tumor heterogeneity, physical barriers, immune system and microenvironment, undruggable cancer drivers, and consequences of therapeutic pressures. With regard to the fundamental importance of the androgen receptor (AR) in all stages of PCa from tumorigenesis to progression, AR is postulated to have a continued critical role in castration-resistant prostate cancer (CRPC). Suppression of AR signaling mediated by the full-length AR (AR-FL) is the therapeutic goal of all AR-directed therapies. However, AR-targeting agents ultimately lead to AR aberrations that promote PCa progression and drug resistance. Among these AR aberrations, androgen receptor variant 7 (AR-V7) is gaining attention as a potential predictive marker for as well as one of the resistance mechanisms to the most current anti-AR therapies in CRPC. Meanwhile, development of next-generation drugs that directly or indirectly target AR-V7 signaling is urgently needed. In the present review of the current literature, we have summarized the origin, alternative splicing, expression induction, protein conformation, interaction with coregulators, relationship with AR-FL, transcriptional activity, and biological function of AR-V7 in PCa development and therapeutic resistance. We hope this review will help further understand the molecular origin, expression regulation, and role of AR-V7 in the progression of PCa and provide insight into the design of novel selective inhibitors of AR-V7 in PCa treatment.

Androgen Receptor and Poly(ADP-ribose) Glycohydrolase Inhibition Increases Efficiency of Androgen Ablation in Prostate Cancer Cells

There is mounting evidence of androgen receptor signaling inducing genome instability and changing DNA repair capacity in prostate cancer cells. Expression of genes associated with base excision repair (BER) is increased with prostate cancer progression and correlates with poor prognosis. Poly(ADP-ribose) polymerase (PARP) and poly(ADP-ribose) glycohydrolase (PARG) are key enzymes in BER that elongate and degrade PAR polymers on target proteins. While PARP inhibitors have been tested in clinical trials and are a promising therapy for prostate cancer patients with TMPRSS2-ERG fusions and mutations in DNA repair genes, PARG inhibitors have not been evaluated. We show that PARG is a direct androgen receptor (AR) target gene. AR is recruited to the PARG locus and induces PARG expression. Androgen ablation combined with PARG inhibition synergistically reduces BER capacity in independently derived LNCaP and LAPC4 prostate cancer cell lines. A combination of PARG inhibition with androgen ablation or with the DNA damaging drug, temozolomide, significantly reduces cellular proliferation and increases DNA damage. PARG inhibition alters AR transcriptional output without changing AR protein levels. Thus, AR and PARG are engaged in reciprocal regulation suggesting that the success of androgen ablation therapy can be enhanced by PARG inhibition in prostate cancer patients.

Restoration of FKBP51 protein promotes the progression of castration resistant prostate cancer

Background

As deregulation of androgen receptor (AR) signaling target genes is associated with tumorigenesis and the development of prostate cancer (PCa), AR signaling is the primary therapeutic target for PCa. Although patients initially responses to first-line androgen deprivation therapies (ADTs), most of them with advanced PCa progress to lethal castration-resistant prostate cancer (CRPC). Recent studies have suggested the molecular mechanisms by which AR elicit the robust up-regulation of the FKBP51 gene. We suggest that restored expression of FKBP51 gene, modulated by androgen receptor splicing variant 7 (AR-V7) which replaces full length androgen receptor (AR-FL) in androgen ablation status, promotes CRPC progression through activating NF-κB signaling.

Methods

Immunohistochemistry assays were used to detect the expression of AR-V7, FKBP51 and NF-κB signaling correlated proteins in CRPC tissues. An androgen ablation resistant PCa cell line model established by Long-term culturing in androgen depleted medium, named androgen-independent LNCaP (LNCaP-AI) cells, were used to dynamically monitor FKBP51 expression during the process of androgen dependent PCa cells transforming into androgen-independent cells, as well as its association with NF-κB signal pathway. LNCaP-AI cell line was determined to express AR-V7 protein continuously. Luciferase reporter assays and DNA pull down were used to determine the association between AR-V7 and FKBP51.

Results

Our results suggested that CRPC patients with AR-V7 high expression tend to have higher expression of FKBP51 and enhanced NF-κB signaling compared with AR-V7 negative patients. Knockdown of AR-V7 or FKBP51 in LNCaP-AI cells attenuated the level of p-NF-κB (Ser536) and androgen-resistant cells growth. Luciferase reporter assays and DNA pull down results indicated that FKBP51 was transcriptionally promoted by AR-V7 in absence of androgen, which enhanced NF-κB signaling.

Conclusions

Because of upregulation of AR-V7 in androgen-independent PCa cells, increasing of FKBP51 induced NF-κB signaling, leading to progression of CRPC.

Androgen Receptor and Its Splicing Variant 7 Expression in Peripheral Blood Mononuclear Cells and in Circulating Tumor Cells in Metastatic Castration-Resistant Prostate Cancer

Androgen receptor (AR) signaling remains crucial in castration-resistant prostate cancer (CRPC). Since it is also essential in immune cells, we studied whether the expression of AR full-length (ARFL) and its splicing variant ARV7 in peripheral blood mononuclear cells (PBMC) predicts systemic treatment response in mCRPC in comparison with circulating-tumor cells (CTC). We measured ARFL and ARV7 mRNA in PBMC and CTC from patients prior to receiving abiraterone (AA), enzalutamide (E), or taxanes by a pre-amplification plus quantitative reverse-transcription PCR. They were also tested in LNCaP-ARV7-transfected and in 22RV1 docetaxel-resistant (22RV1DR) cells. We studied 171 PBMC from 136 patients and from 24 non-cancer controls, and 47 CTC from 22 patients. High PBMC ARV7 levels correlated with worse AA/E and better taxane response. In taxane-treated patients high PBMC ARFL also correlated with longer progression-free survival (PFS). High ARV7 and ARFL expression were independently associated with better biochemical-PFS. Conversely, high CTC ARV7 and ARFL correlated with shorter radiological-PFS and overall survival, respectively. High ARV7 in 22RV1DR and LNCaP-ARV7 cells correlated with taxane resistance. In conclusion, ARFL and ARV7 at PBMC or CTC have a different predictive role in the taxane response, suggesting a potential influence of the AR pathway from PBMC in such response modulation.

LRIG1 is a pleiotropic androgen receptor-regulated feedback tumor suppressor in prostate cancer

LRIG1 has been reported to be a tumor suppressor in gastrointestinal tract and epidermis. However, little is known about the expression, regulation and biological functions of LRIG1 in prostate cancer (PCa). We find that LRIG1 is overexpressed in PCa, but its expression correlates with better patient survival. Functional studies reveal strong tumor-suppressive functions of LRIG1 in both AR+ and AR- xenograft models, and transgenic expression of LRIG1 inhibits tumor development in Hi-Myc and TRAMP models. LRIG1 also inhibits castration-resistant PCa and exhibits therapeutic efficacy in pre-established tumors. We further show that 1) AR directly transactivates LRIG1 through binding to several AR-binding sites in LRIG1 locus, and 2) LRIG1 dampens ERBB expression in a cell type-dependent manner and inhibits ERBB2-driven tumor growth. Collectively, our study indicates that LRIG1 represents a pleiotropic AR-regulated feedback tumor suppressor that functions to restrict oncogenic signaling from AR, Myc, ERBBs, and, likely, other oncogenic drivers.

Orally Bioavailable Androgen Receptor Degrader, Potential Next-Generation Therapeutic for Enzalutamide-Resistant Prostate Cancer

PURPOSE

Androgen receptor (AR)-targeting prostate cancer drugs, which are predominantly competitive ligand-binding domain (LBD)-binding antagonists, are inactivated by common resistance mechanisms. It is important to develop next-generation mechanistically distinct drugs to treat castration- and drug-resistant prostate cancers.

EXPERIMENTAL DESIGN

Second-generation AR pan antagonist UT-34 was selected from a library of compounds and tested in competitive AR binding and transactivation assays. UT-34 was tested using biophysical methods for binding to the AR activation function-1 (AF-1) domain. Western blot, gene expression, and proliferation assays were performed in various AR-positive enzalutamide-sensitive and -resistant prostate cancer cell lines. Pharmacokinetic and xenograft studies were performed in immunocompromised rats and mice.

RESULTS

UT-34 inhibits the wild-type and LBD-mutant ARs comparably and inhibits the in vitro proliferation and in vivo growth of enzalutamide-sensitive and -resistant prostate cancer xenografts. In preclinical models, UT-34 induced the regression of enzalutamide-resistant tumors at doses when the AR is degraded; but, at lower doses, when the AR is just antagonized, it inhibits, without shrinking, the tumors. This indicates that degradation might be a prerequisite for tumor regression. Mechanistically, UT-34 promotes a conformation that is distinct from the LBD-binding competitive antagonist enzalutamide and degrades the AR through the ubiquitin proteasome mechanism. UT-34 has a broad safety margin and exhibits no cross-reactivity with G-protein-coupled receptor kinase and nuclear receptor family members.

CONCLUSIONS

Collectively, UT-34 exhibits the properties necessary for a next-generation prostate cancer drug.

Assays to Interrogate the Ability of Compounds to Inhibit the AF-2 or AF-1 Transactivation Domains of the Androgen Receptor

Prostate cancer is the leading cause of cancer and second leading cause of cancer-related death in men in the United States. Twenty percent of patients receiving the standard of care androgen deprivation therapy (ADT) eventually progress to metastatic and incurable castration-resistant prostate cancer (CRPC). Current FDA-approved drugs for CRPC target androgen receptor (AR) binding or androgen production, but only provide a 2- to 5-month survival benefit due to the emergence of resistance. Overexpression of AR coactivators and the emergence of AR splice variants, both promote continued transcriptional activation under androgen-depleted conditions and represent drug resistance mechanisms that contribute to CRPC progression. The AR contains two transactivation domains, activation function 2 (AF-2) and activation function 1 (AF-1), which serve as binding surfaces for coactivators involved in the transcriptional activation of AR target genes. Full-length AR contains both AF-2 and AF-1 surfaces, whereas AR splice variants only have an AF-1 surface. We have recently prosecuted a high-content screening campaign to identify hit compounds that can inhibit or disrupt the protein-protein interactions (PPIs) between AR and transcriptional intermediary factor 2 (TIF2), one of the coactivators implicated in CRPC disease progression. Since an ideal inhibitor/disruptor of AR-coactivator PPIs would target both the AF-2 and AF-1 surfaces, we describe here the development and validation of five AF-2- and three AF-1-focused assays to interrogate and prioritize hits that disrupt both transactivation surfaces. The assays were validated using a test set of seven known AR modulator compounds, including three AR antagonists and one androgen synthesis inhibitor that are FDA-approved ADTs, two investigational molecules that target the N-terminal domain of AR, and an inhibitor of the Hsp90 (heat shock protein) molecular chaperone.

Androgen receptor splicing variant 7: Beyond being a constitutively active variant

In prostate cancer development, the androgen receptor (AR) signaling plays a crucial role during both formation of early prostate lesions and progression to the lethal, incurable castration resistant stage. Accordingly, numerous approaches have been developed to inhibit AR activity including androgen deprivation therapy, application of the AR antagonists as well as the use of taxanes. However, these treatments, although effective initially, resistance inevitably occur for most of the patients within several years and limiting the therapeutic efficacy. Of note, alterations and reactivation of the AR signaling pathway have been demonstrated as the major reasons for the observed resistance. Accumulating evidences have suggested that synthesis of AR splicing variants, in particular, the constitutively active AR-V7, is one of the most important mechanisms that contribute to the abnormal AR signaling. In addition, clinical data also highlight the potential of using AR-V7 as a predictive biomarker and a therapeutic target in metastatic castration resistant prostate cancer (mCRPC). In this review, we summarize the recent findings concerning the specific role of AR-V7 in CRPC progression, drug resistance and its potential value in clinical assessment.

Androgen receptor enhancer usage and the chromatin regulatory landscape in human prostate cancers

The androgen receptor (AR) is commonly known as a key transcription factor in prostate cancer development, progression and therapy resistance. Genome-wide chromatin association studies revealed that transcriptional regulation by AR mainly depends on binding to distal regulatory enhancer elements that control gene expression through chromatin looping to gene promoters. Changes in the chromatin epigenetic landscape and DNA sequence can locally alter AR-DNA-binding capacity and consequently impact transcriptional output and disease outcome. The vast majority of reports describing AR chromatin interactions have been limited to cell lines, identifying numerous other factors and interacting transcription factors that impact AR chromatin interactions. Do these factors also impact AR cistromics - the genome-wide chromatin-binding landscape of AR - in vivo? Recent technological advances now enable researchers to identify AR chromatin-binding sites and their target genes in human specimens. In this review, we provide an overview of the different factors that influence AR chromatin binding in prostate cancer specimens, which is complemented with knowledge from cell line studies. Finally, we discuss novel perspectives on studying AR cistromics in clinical samples.

Role of androgen receptor splice variants, their clinical relevance and treatment options

PURPOSE

In this review, we summarize the importance of AR variants with a particular focus on clinically relevant members of this family.

METHODS

A non-systematic literature review was performed based on Medline and PubMed.

RESULTS

Endocrine therapy represents the central paradigm for the management of prostate cancer. Eventually, in response to androgen ablation therapy, several resistance mechanisms against the endocrine therapy might develop that can circumvent the therapy approaches. One specific resistance mechanism that has gained increasing attention is the generation of alternatively spliced variants of the androgen receptor, with AR-V7 being the most prominent. More broadly, AR-V7 is one member of a group of alternatively spliced AR variants that share a common feature, the missing ligand-binding domain. These ΔLBD androgen receptor variants have shown the capability to induce androgen receptor-mediated gene transcription even under conditions of androgen deprivation and to drive cancer progression.

CONCLUSION

The methods used for detecting AR-Vs, at least on the mRNA level, are well-advanced and harbor the potential to be introduced into clinical diagnostics. It is important to note, that the testing, especially of AR-V7 has its limitations in predicting treatment response. More promising is the great number of active clinical trials aimed at reducing the AR-Vs, and using this to re-sensitize CRPC towards endocrine treatment might provide additional treatment options for CRPC patients in the future.

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.

Hsp70 Binds to the Androgen Receptor N-terminal Domain and Modulates the Receptor Function in Prostate Cancer Cells

The androgen receptor (AR) is a key driver and therapeutic target in androgen-sensitive prostate cancer, castration-resistant prostate cancer (CRPC), and CRPC resistant to abiraterone and enzalutamide, two second-generation inhibitors of AR signaling. Because current AR inhibitors target a functioning C-terminal ligand-binding domain (LBD), the identification and characterization of cofactors interacting with the N-terminal domain (NTD) of AR may lead to new approaches to target AR signaling in CRPC. Using a pull-down approach coupled with proteomics, we have identified Hsp70 as a cofactor for the NTD of AR in prostate cancer cells. Hsp70 inhibition using siRNA or small molecules indicated that Hsp70 played an important role in the expression and transactivation of endogenous AR. Prostate-specific antigen (PSA) promoter/enhancer-driven luciferase assays showed that Hsp70 was also required for transactivation of AR mutant lacking LBD. Furthermore, clonogenic assays showed that an Hsp70 inhibitor, either alone or in synergy with enzalutamide, can inhibit the proliferation of 22Rv1, a widely used enzalutamide-resistant CRPC prostate cancer cell line. These findings suggest that Hsp70 is a potential therapeutic target for the treatment of enzalutamide-resistant CRPC.

Inositol polyphosphate 4-phosphatase type II regulation of androgen receptor activity

Activation and transcriptional reprogramming of AR in advanced prostate cancer frequently coincides with the loss of two tumor suppressors, INPP4B and PTEN, which are highly expressed in human and mouse prostate epithelium. While regulation of AR signaling by PTEN has been described by multiple groups, it is not known whether the loss of INPP4B affects AR activity. Using prostate cancer cell lines we showed that INPP4B regulates AR transcriptional activity and the oncogenic signaling pathways Akt and PKC. Analysis of gene expression in prostate cancer patient cohorts showed a positive correlation between INPP4B expression and both AR mRNA levels and AR transcriptional output. Using an Inpp4b^-/- mouse model, we demonstrated that INPP4B suppresses Akt and PKC signaling pathways and modulates AR transcriptional activity in normal mouse prostate. Remarkably, PTEN protein levels and phosphorylation of S380 were the same in Inpp4b^-/- and WT males, suggesting that the observed changes were due exclusively to the loss of INPP4B. Our data show that INPP4B modulates AR activity in normal prostate and its loss contributes to the AR-dependent transcriptional profile in prostate cancer.

Prognostic value and potential function of splicing events in prostate adenocarcinoma

Prostate adenocarcinoma (PRAD) is one of the most common types of malignancy in males and at present, effective prognostic indicators are limited. The development of PRAD has been associated with abnormalities in alternative splicing (AS), a requisite biological process of gene expression in eukaryotic cells; however, the prognostic value of AS products and splicing events remains to be elucidated. In the present study, the data of splicing events and the clinical information of PRAD patients were obtained from The Cancer Genome Atlas (TCGA)SpliceSeq and TCGA databases, respectively. A prognostic index (PI) was generated from disease-free survival-associated splicing events (DFS-SEs), which were identified by univariate/multivariate Cox regression analysis. A total of 6,909 DFS-SEs were identified in PRAD. The corresponding genes for the DFS-SEs were significantly enriched in mitochondria and their associated pathways according to Gene Ontology annotation and in the pathways of fatty acid metabolism, oxidative phosphorylation and Huntington's disease according to a Kyoto Encyclopedia of Genes and Genomes pathway analysis. The PI for mutually exclusive exons had the greatest ability to predict the probability of five-year disease-free survival of patients with PRAD, with an area under the time-dependent receiver-operating characteristic curve of 0.7606. Patients with PRAD, when divided into a 'low' and a 'high' group based on their median PI for exon skip values, exhibited a marked difference in disease-free survival (low vs. high, 3,588.45±250.51 vs. 1,531.08±136.50 days; P=7.43×10⁻⁹). A correlation network between DFS-SEs of splicing factors and non-splicing factors was constructed to determine the potential mechanisms in PRAD, which included the potential regulatory interaction between the splicing event of splicing factor RNA binding motif protein 5-alternate terminator (AT)-64957 and the splicing event of non-splicing factor heterochromatin protein 1 binding protein 3-AT-939. In conclusion, the PIs derived from DFS-SEs are valuable prognostic factors for patients with PRAD, and the function of splicing events in PRAD deserves further exploration.

Minimal residual disease in prostate cancer patients after primary treatment: theoretical considerations, evidence and possible use in clinical management

Minimal residual disease is that not detected by conventional imaging studies and clinically the patient remains disease free. However, with time these dormant cells will awaken and disease progression occurs, resulting in clinically and radiological detectable metastatic disease. This review addresses the concept of tumor cell dissemination from the primary tumor, the micrometastatic niche and tumor cell survival and finally the clinical utility of detecting and characterizing these tumor cells in order to guide management decisions in treating patients with prostate cancer.

Targeting the Hsp40/Hsp70 Chaperone Axis as a Novel Strategy to Treat Castration-Resistant Prostate Cancer

Castration-resistant prostate cancer (CRPC) is characterized by reactivation of androgen receptor (AR) signaling, in part by elevated expression of AR splice variants (ARv) including ARv7, a constitutively active, ligand binding domain (LBD)-deficient variant whose expression has been correlated with therapeutic resistance and poor prognosis. In a screen to identify small-molecule dual inhibitors of both androgen-dependent and androgen-independent AR gene signatures, we identified the chalcone C86. Binding studies using purified proteins and CRPC cell lysates revealed C86 to interact with Hsp40. Pull-down studies using biotinylated-C86 found Hsp40 present in a multiprotein complex with full-length (FL-) AR, ARv7, and Hsp70 in CRPC cells. Treatment of CRPC cells with C86 or the allosteric Hsp70 inhibitor JG98 resulted in rapid protein destabilization of both FL-AR and ARv, including ARv7, concomitant with reduced FL-AR- and ARv7-mediated transcriptional activity. The glucocorticoid receptor, whose elevated expression in a subset of CRPC also leads to androgen-independent AR target gene transcription, was also destabilized by inhibition of Hsp40 or Hsp70. In vivo, Hsp40 or Hsp70 inhibition demonstrated single-agent and combinatorial activity in a 22Rv1 CRPC xenograft model. These data reveal that, in addition to recognized roles of Hsp40 and Hsp70 in FL-AR LBD remodeling, ARv lacking the LBD remain dependent on molecular chaperones for stability and function. Our findings highlight the feasibility and potential benefit of targeting the Hsp40/Hsp70 chaperone axis to treat prostate cancer that has become resistant to standard antiandrogen therapy.Significance: These findings highlight the feasibility of targeting the Hsp40/Hsp70 chaperone axis to treat CRPC that has become resistant to standard antiandrogen therapy. Cancer Res; 78(14); 4022-35. ©2018 AACR.

AR-v7 liquid biopsy for treatment stratification in prostate cancer: how close are we?

PURPOSE OF REVIEW

Recent clinical introduction of the novel antiandrogen, Enzalutamide (Enza), CYP17 inhibitor, Abiraterone (Abi), and the second-generation chemotherapeutic, Cabazitaxel, has increased survival of patients with advanced, metastatic castration-resistant prostate cancer (mCRPC). However, de novo and acquired resistance rates are high. A liquid biopsy that can rapidly, sensitively and robustly identify which patients will respond to treatment in a minimally invasive manner is urgently required to permit switch to a potentially more efficacious drug regimen, thus increasing survival whilst avoiding debilitating side effects associated with unnecessary treatment. This review will highlight recent developments in detection of AR-v7 in circulating mRNA/whole blood and circulating tumour cells (CTCs) as a liquid biopsy for patient-stratification in mCRPC.

RECENT FINDINGS

Continued androgen receptor (AR) activity in mCRPC has been linked to the expression of a number of truncated but constitutively active AR isoforms. One such variant, AR-v7, can drive drug resistance in preclinical models and is correlated with disease progression whilst showing dynamic response to AR-targeting treatments when assessed in blood. It has thus been proposed as an Abi/Enza treatment-response biomarker.

SUMMARY

AR-v7 liquid biopsy has the potential to transform clinical management of mCRPC and increase patient survival. This review will explore recent efforts to validate AR-v7 as a robust, clinically informative biomarker. I will also address potential limitations of detection and quantification that could frustrate its adoption into routine clinical practise.

The potential of AR-V7 as a therapeutic target

INTRODUCTION

The androgen receptor variant AR-V7 is gaining attention as a potential predictive marker for as well as one of the resistance mechanisms to the most current anti-androgen receptor (AR) therapies in castration-resistant prostate cancer (CRPC). Accordingly, development of next-generation drugs that directly or indirectly target AR-V7 signaling is urgently needed. Areas covered: We review proposed mechanisms of drug resistance in relation to AR-V7 status, the mechanisms of generation of AR-V7, and its transcriptome, cistrome, and interactome. Pharmacological agents that interfere with these processes are being developed to counteract pan AR and AR-V7-specific signaling. Also, we address the current status of the preclinical and clinical studies targeting AR-V7 signaling. Expert opinion: AR-V7 is considered a true therapeutic target, however, it remains to be determined if AR-V7 is a principal driver or merely a bystander requiring heterodimerization with co-expressed full-length AR or other variants to drive CRPC progression. While untangling AR-V7 biology, multiple strategies are being developed to counteract drug resistance, including selective blockade of AR-V7 signaling as well as inhibition of pan-AR signaling. Ideally anti-AR therapies will be combined with agents preventing activation and enrichment of AR negative tumor cells that are otherwise depressed by AR activity axis.

DBC1 promotes castration-resistant prostate cancer by positively regulating DNA binding and stability of AR-V7

Constitutively active AR-V7, one of the major androgen receptor (AR) splice variants lacking the ligand-binding domain, plays a key role in the development of castration-resistant prostate cancer (CRPC) and anti-androgen resistance. However, our understanding of the regulatory mechanisms of AR-V7-driven transcription is limited. Here we report DBC1 as a key regulator of AR-V7 transcriptional activity and stability in CRPC cells. DBC1 functions as a coactivator for AR-V7 and is required for the expression of AR-V7 target genes including CDH2, a mesenchymal marker linked to CRPC progression. DBC1 is required for recruitment of AR-V7 to its target enhancers and for long-range chromatin looping between the CDH2 enhancer and promoter. Mechanistically, DBC1 enhances DNA-binding activity of AR-V7 by direct interaction and inhibits CHIP E3 ligase-mediated ubiquitination and degradation of AR-V7 by competing with CHIP for AR-V7 binding, thereby stabilizing and activating AR-V7. Importantly, DBC1 depletion suppresses the tumorigenic and metastatic properties of CRPC cells. Our results firmly establish DBC1 as a critical AR-V7 coactivator that plays a key role in the regulation of DNA binding and stability of AR-V7 and has an important physiological role in CRPC progression.

Role of Androgen Receptor Variants in Prostate Cancer: Report from the 2017 Mission Androgen Receptor Variants Meeting

CONTEXT

Although a number of studies have demonstrated the importance of constitutively active androgen receptor variants (AR-Vs) in prostate cancer, questions still remain about the precise role of AR-Vs in the progression of castration-resistant prostate cancer (CRPC).

OBJECTIVE

Key stakeholders and opinion leaders in prostate cancer convened on May 11, 2017 in Boston to establish the current state of the field of AR-Vs.

EVIDENCE ACQUISITION

The meeting "Mission Androgen Receptor Variants" was the second of its kind sponsored by the Prostate Cancer Foundation (PCF). This invitation-only event was attended by international leaders in the field and representatives from sponsoring organizations (PCF and industry sponsors). Eighteen faculty members gave short presentations, which were followed by in-depth discussions. Discussions focused on three thematic topics: (1) potential of AR-Vs as biomarkers of therapeutic resistance; (2) role of AR-Vs as functionally active CRPC progression drivers; and (3) utility of AR-Vs as therapeutic targets in CRPC.

EVIDENCE SYNTHESIS

The three meeting organizers synthesized this meeting report, which is intended to summarize major data discussed at the meeting and identify key questions as well as strategies for addressing these questions. There was a critical consensus that further study of the AR-Vs is an important research focus in CRPC. Contrasting views and emphasis, each supported by data, were presented at the meeting, discussed among the participants, and synthesized in this report.

CONCLUSIONS

This article highlights the state of knowledge and outlines the most pressing questions that need to be addressed to advance the AR-V field.

PATIENT SUMMARY

Although further investigation is needed to delineate the role of androgen receptor (AR) variants in metastatic castration-resistant prostate cancer, advances in measurement science have enabled development of blood-based tests for treatment selection. Detection of AR variants (eg, AR-V7) identified a patient population with poor outcomes to existing AR-targeting therapies, highlighting the need for novel therapeutic agents currently under development.

High throughput microscopy identifies bisphenol AP, a bisphenol A analog, as a novel AR down-regulator

Prostate cancer remains a deadly disease especially when patients become resistant to drugs that target the Androgen Receptor (AR) ligand binding domain. At this stage, patients develop recurring castrate-resistant prostate cancers (CRPCs). Interestingly, CRPC tumors maintain dependency on AR for growth; moreover, in CRPCs, constitutively active AR splice variants (e.g., AR-V7) begin to be expressed at higher levels. These splice variants lack the ligand binding domain and are rendered insensitive to current endocrine therapies. Thus, it is of paramount importance to understand what regulates the expression of AR and its splice variants to identify new therapeutic strategies in CRPCs. Here, we used high throughput microscopy and quantitative image analysis to evaluate effects of selected endocrine disruptors on AR levels in multiple breast and prostate cancer cell lines. Bisphenol AP (BPAP), which is used in chemical and medical industries, was identified as a down-regulator of both full length AR and the AR-V7 splice variant. We validated its activity by performing time-course, dose-response, Western blot and qPCR analyses. BPAP also reduced the percent of cells in S phase, which was accompanied by a ~60% loss in cell numbers and colony formation in anchorage-independent growth assays. Moreover, it affected mitochondria size and cell metabolism. In conclusion, our high content analysis-based screening platform was used to classify the effect of compounds on endogenous ARs, and identified BPAP as being capable of causing AR (both full-length and variants) down-regulation, cell cycle arrest and metabolic alterations in CRPC cell lines.

Clinical relevance of androgen receptor alterations in prostate cancer

Prostate cancer (PC) remains a leading cause of cancer-related deaths among men worldwide, despite continuously improved treatment strategies. Patients with metastatic disease are treated by androgen deprivation therapy (ADT) that with time results in the development of castration-resistant prostate cancer (CRPC) usually established as metastases within bone tissue. The androgen receptor (AR) transcription factor is the main driver of CRPC development and of acquired resistance to drugs given for treatment of CRPC, while a minority of patients have CRPC that is non-AR driven. Molecular mechanisms behind epithelial AR reactivation in CRPC include AR gene amplification and overexpression, AR mutations, expression of constitutively active AR variants, intra-tumoural and adrenal androgen synthesis and promiscuous AR activation by other factors. This review will summarize AR alterations of clinical relevance for patients with CRPC, with focus on constitutively active AR variants, their possible association with AR amplification and structural rearrangements as well as their ability to predict patient resistance to AR targeting drugs. The review will also discuss AR signalling in the tumour microenvironment and its possible relevance for metastatic growth and therapy.