BACKGROUND. Liquid biopsies have demonstrated that the constitutively active androgen receptor splice variant-7 (AR-V7) associates with reduced response and overall survival from endocrine therapies in castration-resistant prostate cancer (CRPC). However, these studies provide little information pertaining to AR-V7 expression in prostate cancer (PC) tissue. METHODS. Following generation and validation of a potentially novel AR-V7 antibody for IHC, AR-V7 protein expression was determined for 358 primary prostate samples and 293 metastatic biopsies. Associations with disease progression, full-length androgen receptor (AR-FL) expression, response to therapy, and gene expression were determined. RESULTS. We demonstrated that AR-V7 protein is rarely expressed (<1%) in primary PC but is frequently detected (75% of cases) following androgen deprivation therapy, with further significant (P = 0.020) increase in expression following abiraterone acetate or enzalutamide therapy. In CRPC, AR-V7 expression is predominantly (94% of cases) nuclear and correlates with AR-FL expression (P ≤ 0.001) and AR copy number (P = 0.026). However, dissociation of expression was observed, suggesting that mRNA splicing remains crucial for AR-V7 generation. AR-V7 expression was heterogeneous between different metastases from a patient, although AR-V7 expression was similar within a metastasis. Moreover, AR-V7 expression correlated with a unique 59-gene signature in CRPC, including HOXB13, a critical coregulator of AR-V7 function. Finally, AR-V7–negative disease associated with better prostate-specific antigen (PSA) responses (100% vs. 54%, P = 0.03) and overall survival (74.3 vs. 25.2 months, hazard ratio 0.23 [0.07–0.79], P = 0.02) from endocrine therapies (pre-chemotherapy). CONCLUSION. This study provides impetus to develop therapies that abrogate AR-V7 signaling to improve our understanding of AR-V7 biology and to confirm the clinical significance of AR-V7. FUNDING. Work at the University of Washington and in the Plymate and Nelson laboratories is supported by the Department of Defense Prostate Cancer Research Program (W81XWH-14-2-0183, W81XWH-12-PCRP-TIA, W81XWH-15-1-0430, and W81XWH-13-2-0070), the Pacific Northwest Prostate Cancer SPORE (P50CA97186), the Institute for Prostate Cancer Research, the Veterans Affairs Research Program, the NIH/National Cancer Institute (P01CA163227), and the Prostate Cancer Foundation. Work in the de Bono laboratory was supported by funding from the Movember Foundation/Prostate Cancer UK (CEO13-2-002), the US Department of Defense (W81XWH-13-2-0093), the Prostate Cancer Foundation (20131017 and 20131017-1), Stand Up To Cancer (SU2C-AACR-DT0712), Cancer Research UK (CRM108X-A25144), and the UK Department of Health through an Experimental Cancer Medicine Centre grant (ECMC-CRM064X).

The androgen receptor splice variant-7 (AR-V7) has been implicated in the development of castration-resistant prostate cancer (CRPC) and resistance to abiraterone and enzalutamide. To develop a validated assay for detection of AR-V7 protein in tumour tissue and determine its expression and clinical significance as patients progress from hormone-sensitive prostate cancer (HSPC) to CRPC. Following monoclonal antibody generation and validation, we retrospectively identified patients who had HSPC and CRPC tissue available for AR-V7 immunohistochemical (IHC) analysis. Nuclear AR-V7 expression was determined using IHC H score (HS) data. The change in nuclear AR-V7 expression from HSPC to CRPC and the association between nuclear AR-V7 expression and overall survival (OS) was determined. Nuclear AR-V7 expression was significantly lower in HSPC (median HS 50, interquartile range [IQR] 17.5-90) compared to CRPC (HS 135, IQR 80-157.5; p<0.0001), and in biopsy tissue taken before (HS 80, IQR 30-136.3) compared to after (HS 140, IQR 105-167.5; p=0.007) abiraterone or enzalutamide treatment. Lower nuclear AR-V7 expression at CRPC biopsy was associated with longer OS (hazard ratio 1.012, 95% confidence interval 1.004-1.020; p=0.003). While this monoclonal antibody primarily binds to AR-V7 in PC biopsy tissue, it may also bind to other proteins. We provide the first evidence that nuclear AR-V7 expression increases with emerging CRPC and is prognostic for OS, unlike antibody staining for the AR N-terminal domain. These data indicate that AR-V7 is important in CRPC disease biology; agents targeting AR splice variants are needed to test this hypothesis and further improve patient outcome from CRPC. In this study we found that levels of the protein AR-V7 were higher in patients with advanced prostate cancer. A higher level of AR-V7 identifies a group of patients who respond less well to certain prostate cancer treatments and live for a shorter period of time.

Metabolic reprogramming is associated with re/activation and antagonism of androgen receptor (AR) signaling that drives prostate cancer (PCa) progression to castration resistance, respectively. In particular, AR signaling influences the fates of citrate that uniquely characterizes normal and malignant prostatic metabolism (i.e., mitochondrial export and extracellular secretion in normal prostate, mitochondrial retention and oxidation to support oxidative phenotype of primary PCa, and extra-mitochondrial interconversion into acetyl-CoA for fatty acid synthesis and epigenetics in the advanced PCa). The emergence of castration-resistant PCa (CRPC) involves reactivation of AR signaling, which is then further targeted by androgen synthesis inhibitors (abiraterone) and AR-ligand inhibitors (enzalutamide, apalutamide, and daroglutamide). However, based on AR dependency, two distinct metabolic and cellular adaptations contribute to development of resistance to these agents and progression to aggressive and lethal disease, with the tumor ultimately becoming highly glycolytic and with imaging by a tracer of tumor energetics, ¹⁸F-fluorodoxyglucose (¹⁸F-FDG). Another major resistance mechanism involves a lineage alteration into AR-indifferent carcinoma such a neuroendocrine which is diagnostically characterized by robust ¹⁸F-FDG uptake and loss of AR signaling. PCa is also characterized by metabolic alterations such as fatty acid and polyamine metabolism depending on AR signaling. In some cases, AR targeting induces rather than suppresses these alterations in cellular metabolism and energetics, which can be explored as therapeutic targets in lethal CRPC.

One critical mechanism through which prostate cancer (PCa) adapts to treatments targeting androgen receptor (AR) signaling is the emergence of ligand-binding domain–truncated and constitutively active AR splice variants, particularly AR-V7. While AR-V7 has been intensively studied, its ability to activate distinct biological functions compared with the full-length AR (AR-FL), and its role in regulating the metastatic progression of castration-resistant PCa (CRPC), remain unclear. Our study found that, under castrated conditions, AR-V7 strongly induced osteoblastic bone lesions, a response not observed with AR-FL overexpression. Through combined ChIP-seq, ATAC-seq, and RNA-seq analyses, we demonstrated that AR-V7 uniquely accesses the androgen-responsive elements in compact chromatin regions, activating a distinct transcription program. This program was highly enriched for genes involved in epithelial-mesenchymal transition and metastasis. Notably, we discovered that SOX9 , a critical metastasis driver gene, was a direct target and downstream effector of AR-V7. Its protein expression was dramatically upregulated in AR-V7–induced bone lesions. Moreover, we found that Ser81 phosphorylation enhanced AR-V7’s pro-metastasis function by selectively altering its specific transcription program. Blocking this phosphorylation with CDK9 inhibitors impaired the AR-V7–mediated metastasis program. Overall, our study has provided molecular insights into the role of AR splice variants in driving the metastatic progression of CRPC.

Since Huggins defined the androgen-sensitive nature of prostate cancer (PCa), suppression of systemic testosterone (T) has remained the most effective initial therapy for advanced disease although progression inevitably occurs. From the inception of clinical efforts to suppress androgen receptor (AR) signaling by reducing AR ligands, it was also recognized that administration of T in men with castration-resistant prostate cancer (CRPC) could result in substantial clinical responses. Data from preclinical models have reproducibly shown biphasic responses to T administration, with proliferation at low androgen concentrations and growth inhibition at supraphysiological T concentrations. Many questions regarding the biphasic response of PCa to androgen treatment remain, primarily regarding the mechanisms driving these responses and how best to exploit the biphasic phenomenon clinically. Here we review the preclinical and clinical data on high dose androgen growth repression and discuss cellular pathways and mechanisms likely to be involved in mediating this response. Although meaningful clinical responses have now been observed in men with PCa treated with high dose T, not all men respond, leading to questions regarding which tumor characteristics promote response or resistance, and highlighting the need for studies designed to determine the molecular mechanism(s) driving these responses and identify predictive biomarkers.

Clinical trials of high-dose androgen (HDA) therapy for prostate cancer (PC) have shown promising efficacy but are limited by lack of criteria to identify likely responders. To elucidate factors that govern the growth-repressive effects of HDAs, we applied an unbiased integrative approach using genetic screens and transcriptional profiling of PC cells with or without demonstrated phenotypic sensitivity to androgen-mediated growth repression. Through this comprehensive analysis, we identified genetic events and related signaling networks that determine the response to both HDA and androgen withdrawal. We applied these findings to develop a gene signature that may serve as an early indicator of treatment response and identify men with tumors that are amenable to HDA therapy.