Progestins inhibit calcitriol-induced CYP24A1 and synergistically inhibit ovarian cancer cell viability: An opportunity for chemoprevention

Objectives Previously we have shown in endometrial cells that progesterone (P4) and calcitriol (CAL, 1,25(OH)₂D₃) synergistically promote apoptosis and that progestins induce expression of the vitamin D receptor. In the current study we examined the progestin/vitamin D combination in ovarian cells and searched for other progestin-related effects on vitamin D metabolism that may underlie the novel interaction between progestins and vitamin D, including whether progestins inhibit CYP24A1, the enzyme that renders CAL inactive. Methods We investigated the impact of P4 on CAL-induced CYP24A1 expression in cancer cell lines expressing progesterone receptors (PRs), [OVCAR-5, OVCAR-3-PGR (PR-transfected OVCAR-3 ovarian line), and T47D-WT, T47D-A and T47D-B (breast lines expressing PRs or individual PR isoforms)] or lines that do not express PRs (OVCAR-3 and T47D-Y). We examined CYP24A1 expression using RT-PCR and western blotting, and apoptosis by TUNEL. We also investigated P4 inhibition of Cyp24a1 in ovaries from CAL-treated mice. Results CAL treatment induced CYP24A1 expression. When co-treated with P4, cell lines expressing PRs showed marked inhibition of CYP24A1 expression (p < 0.001), along with increased apoptosis (p < 0.01); cells not expressing PRs did not. Mouse ovaries showed a significant reduction in CAL-induced Cyp24a1 mRNA (p < 0.001) and protein (p < 0.01) in response to P4. Conclusions We show for the first time that progestins and vitamin D synergistically reduce cell viability and induce apoptosis in ovarian cells and that progestins PR-dependently inhibit CAL-induced CYP24A1, thus extending CAL activity. The combination of progestins and vitamin D deserves further consideration as a strategy for inhibiting ovarian carcinogenesis.