The androgen-signaling axis plays a pivotal role in the pathogenesis of prostate cancer. Since the landmark discovery by Huggins and Hodges, gonadal depletion of androgens has remained a mainstay of therapy for advanced disease. However, progression to castration-resistant prostate cancer (CRPC) typically follows and is largely the result of restored androgen signaling. Efforts to understand the mechanisms behind CRPC have revealed new insights into dysregulated androgen signaling and intratumoral androgen synthesis, which has ultimately led to the development of several novel androgen receptor (AR)-directed therapies for CRPC. However, emergence of resistance to these newer agents has also galvanized new directions in investigations of prereceptor and postreceptor AR regulation. Here, we review our current understanding of AR signaling as it pertains to the biology and natural history of prostate cancer.

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Androgen Signaling in Prostate Cancer

Current treatments for castration-resistant prostate cancer (CRPC) that target androgen receptor (AR) signaling improve patient survival, yet ultimately fail. Here, we provide novel insights into treatment response for the antiandrogen abiraterone by analyses of a genetically engineered mouse (GEM) model with combined inactivation of Trp53 and Pten, which are frequently comutated in human CRPC. These NPp53 mice fail to respond to abiraterone and display accelerated progression to tumors resembling treatment-related CRPC with neuroendocrine differentiation (CRPC-NE) in humans. Cross-species computational analyses identify master regulators of adverse response that are conserved with human CRPC-NE, including the neural differentiation factor SOX11, which promotes neuroendocrine differentiation in cells derived from NPp53 tumors. Furthermore, abiraterone-treated NPp53 prostate tumors contain regions of focal and/or overt neuroendocrine differentiation, distinguished by their proliferative potential. Notably, lineage tracing in vivo provides definitive and quantitative evidence that focal and overt neuroendocrine regions arise by transdifferentiation of luminal adenocarcinoma cells. These findings underscore principal roles for TP53 and PTEN inactivation in abiraterone resistance and progression from adenocarcinoma to CRPC-NE by transdifferentiation.Significance: Understanding adverse treatment response and identifying patients likely to fail treatment represent fundamental clinical challenges. By integrating analyses of GEM models and human clinical data, we provide direct genetic evidence for transdifferentiation as a mechanism of drug resistance as well as for stratifying patients for treatment with antiandrogens. Cancer Discov; 7(7); 736-49. ©2017 AACR.See related commentary by Sinha and Nelson, p. 673This article is highlighted in the In This Issue feature, p. 653.

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Transdifferentiation as a Mechanism of Treatment Resistance in a Mouse Model of Castration-Resistant Prostate Cancer

Despite many recent advances in the therapy for metastatic castration-resistant prostate cancer (mCRPC), the disease remains incurable, although men suffering from this disease are living considerably longer. In this review, we discuss the current treatment options available for this disease, such as taxane-based chemotherapy, the novel hormone therapies abiraterone and enzalutamide, and treatments such as radium-223 and sipuleucel-T. We also highlight the need for ongoing research in this field, because, despite numerous recent advances, the prognosis for mCRPC remains poor. Furthermore, as a growing body of evidence shows the increasing heterogeneity of the disease, and highlights the ongoing need for disease molecular stratification and validation/qualification of predictive biomarkers, we explore this burgeoning research space that is likely to transform how we treat this disease. We describe putative predictive biomarkers, including androgen receptor splice variants, phosphatase and tensin homolog (PTEN) loss, homologous recombination repair defects, including BRCA2 loss, and mismatch repair defects. The development of next-generation sequencing techniques and the routine biopsy of metastatic disease have driven significant advances in our understanding of the genomics of cancer, and are now poised to transform our treatment of this disease.

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Treatment of Advanced Prostate Cancer—A Review of Current Therapies and Future Promise

Summary Background HSD3B1 (1245A>C) has been mechanistically linked to castration-resistant prostate cancer because it encodes an altered enzyme that augments dihydrotestosterone synthesis from non-gonadal precursors. We postulated that men inheriting the HSD3B1 (1245C) allele would exhibit resistance to androgen-deprivation therapy (ADT). Methods In this multicohort study, we determined HSD3B1 genotype retrospectively in men treated with ADT for post-prostatectomy biochemical failure and correlated genotype with long-term clinical outcomes. We used data and samples from prospectively maintained prostate cancer registries at the Cleveland Clinic (Cleveland, OH, USA; primary study cohort) and the Mayo Clinic (Rochester, MN, USA; post-prostatectomy and metastatic validation cohorts). In the post-prostatectomy cohorts, patients of any age were eligible if they underwent prostatectomy between Jan 1, 1996, and Dec 31, 2009 (at the Cleveland Clinic; primary cohort), or between Jan 1, 1987, and Dec 31, 2011 (at the Mayo Clinic; post-prostatectomy cohort) and were treated with ADT for biochemical failure or for non-metastatic clinical failure. In the metastatic validation cohort, patients of any age were eligible if they were enrolled at Mayo Clinic between Sept 1, 2009, and July 31, 2013, with metastatic castration-resistant prostate cancer. The primary endpoint was progression-free survival according to HSD3B1 genotype. We did prespecified multivariable analyses to assess the independent predictive value of HSD3B1 genotype on outcomes. Findings We included and genotyped 443 patients: 118 in the primary cohort (who underwent prostatectomy), 137 in the post-prostatectomy validation cohort, and 188 in the metastatic validation cohort. In the primary study cohort, median progression-free survival diminished as a function of the number of variant alleles inherited: 6·6 years (95% CI 3·8–not reached) in men with homozygous wild-type genotype, 4·1 years (3·0–5·5) in men with heterozygous variant genotype, and 2·5 years (0·7 to not reached) in men with homozygous variant genotype (p=0·011). Relative to the homozygous wild-type genotype, inheritance of two copies of the variant allele was predictive of decreased progression-free survival (hazard ratio [HR] 2·4 [95% CI 1·1–5·3], p=0·029), as was inheritance of one copy of the variant allele (HR 1·7 [1·0–2·9], p=0·041). Findings were similar for distant metastasis-free survival and overall survival. The effect of the HSD3B1 genotype was independently confirmed in the validation cohorts. Interpretation Inheritance of the HSD3B1 (1245C) allele that enhances dihydrotestosterone synthesis is associated with prostate cancer resistance to ADT. HSD3B1 could therefore potentially be a powerful genetic biomarker capable of distinguishing men who are a priori likely to fare favourably with ADT from those who harbour disease liable to behave more aggressively, and who therefore might warrant early escalated therapy. Funding Prostate Cancer Foundation, National Institutes of Health, US Department of Defense, Howard Hughes Medical Institute, American Cancer Society, Conquer Cancer Foundation of the American Society of Clinical Oncology, Cleveland Clinic Research Programs Committee and Department of Radiation Oncology, Gail and Joseph Gassner Development Funds.

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HSD3B1 and resistance to androgen-deprivation therapy in prostate cancer: a retrospective, multicohort study

Bone metastasis is the leading cause of death in prostate cancer patients, for which there is currently no effective treatment. Since the bone microenvironment plays an important role in this process, attentions have been directed to the interactions between cancer cells and the bone microenvironment, including osteoclasts, osteoblasts, and bone stromal cells. Here, we explained the mechanism of interactions between prostate cancer cells and metastasis-associated cells within the bone microenvironment and further discussed the recent advances in targeted therapy of prostate cancer bone metastasis. This review also summarized the effects of bone microenvironment on prostate cancer metastasis and the related mechanisms, and provides insights for future prostate cancer metastasis studies.

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Interactions between cancer cells and bone microenvironment promote bone metastasis in prostate cancer

Abiraterone blocks androgen synthesis and prolongs survival in patients with castration-resistant prostate cancer, which is otherwise driven by intratumoral androgen synthesis. Abiraterone is metabolized in patients to Δ(4)-abiraterone (D4A), which has even greater anti-tumour activity and is structurally similar to endogenous steroidal 5α-reductase substrates, such as testosterone. Here, we show that D4A is converted to at least three 5α-reduced and three 5β-reduced metabolites in human serum. The initial 5α-reduced metabolite, 3-keto-5α-abiraterone, is present at higher concentrations than D4A in patients with prostate cancer taking abiraterone, and is an androgen receptor agonist, which promotes prostate cancer progression. In a clinical trial of abiraterone alone, followed by abiraterone plus dutasteride (a 5α-reductase inhibitor), 3-keto-5α-abiraterone and downstream metabolites were depleted by the addition of dutasteride, while D4A concentrations rose, showing that dutasteride effectively blocks production of a tumour-promoting metabolite and permits D4A accumulation. Furthermore, dutasteride did not deplete the three 5β-reduced metabolites, which were also clinically detectable, demonstrating the specific biochemical effects of pharmacological 5α-reductase inhibition on abiraterone metabolism. Our findings suggest a previously unappreciated and biochemically specific method of clinically fine-tuning abiraterone metabolism to optimize therapy.

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Mohamad E. Abazeed

Papers

Redirecting abiraterone metabolism to fine-tune prostate cancer anti-androgen therapy.