Prostate cancer patients treated with androgen deprivation therapy (ADT) eventually develop castrate-resistant prostate cancer (CRPC). 1,25-dihydroxyvitamin D3 (1,25D3), is a potential adjuvant therapy that confers anti-proliferative and pro-differentiation effects in vitro, but has had mixed results in clinical trials. The impact of the tumor microenvironment on 1,25D3 therapy in CRPC patients has not been assessed. Transforming growth factor-β (TGF-β), which is associated with the development of tumorigenic “reactive stroma” in prostate cancer, induced VDR expression in the human WPMY-1 prostate stromal cell line. Similarly, TGF-β enhanced 1,25D3-induced upregulation of CYP24A1, which metabolizes of 1,25D3 and thereby limits VDR activity. Ablation of Hic-5, a TGF-β-inducible nuclear receptor co-regulator, inhibited basal VDR expression, 1,25D3-induced CYP24A1 expression and metabolism of 1,25D3 and TGF-β-enhanced CYP24A1 expression. Luciferase reporter mapping of the CYP24A1 promoter identified a Hic-5-responsive sequence 392-451 bp upstream of the transcription start site (TSS). Ectopic expression of Hic-5 sensitized LNCaP prostate tumor cells to growth-inhibitory effects of 1,25D3 at a lower concentration by a pathway independent of CYP24A1. The sensitivity of Hic-5-expressing LNCaP cells to 1,25D3-induced growth inhibition was accentuated in co-culture with Hic-5-ablated WPMY-1 cells. Therefore, my findings suggest that the search for mechanisms to sensitize prostate cancer cells to the anti-proliferative effects of VDR ligands needs to account for the impact of VDR activity in the tumor microenvironment. By acting as a co-regulator with distinct effects on VDR transactivation in prostate cancer and stromal cells, Hic-5 could exert diverse effects on adjuvant therapy designed to exploit VDR activity in prostate cancer
