Steroid hormone synthetic pathways in prostate cancer

While androgen deprivation therapy (ADT) remains the primary treatment for metastatic prostate cancer (PCa) since the seminal recognition of the disease as androgen-dependent by Huggins and Hodges in 1941, therapy is uniformly marked by progression to castration-resistant prostate cancer (CRPC) over a period of about 18 months, with an ensuing median survival of 1 to 2 years. Importantly, castration does not eliminate androgens from the prostate tumor microenvironment. Castration resistant tumors are characterized by elevated tumor androgens that are well within the range capable of activating the AR and AR-mediated gene expression, and by steroid enzyme alterations which may potentiate de novo androgen synthesis or utilization of circulating adrenal androgens. The dependence of CRPC on intratumoral androgen metabolism has been modeled in vitro and in vivo, and residual intratumoral androgens are implicated in nearly every mechanism by which AR-mediated signaling promotes castration-resistant disease. These observations suggest that tissue based alterations in steroid metabolism contribute to the development of CRPC and underscore these metabolic pathways as critical targets of therapy. Herein, we review the accumulated body of evidence which strongly supports intracrine (tumoral) androgen synthesis as an important mechanism underlying PCa progression. We first discuss the presence and significance of residual prostate tumor androgens in the progression of CRPC. We review the classical and non-classical pathways of androgen metabolism, and how dysregulated expression of these enzymes is likely to potentiate tumor androgen production in the progression to CRPC. Next we review the in vitro and in vivo data in human tumors, xenografts, and cell line models which demonstrate the capacity of prostate tumors to utilize cholesterol and adrenal androgens in the production of testosterone (T) and dihydrotestosterone (DHT), and briefly review the potential role of exogenous influences on this process. Finally, we discuss the emerging data regarding mechanisms of response and resistance to potent ligand synthesis inhibitors entering clinical practice, and conclude by discussing the implications of these findings for future therapy.