Progesterone potentiates the growth inhibitory effects of calcitriol in endometrial cancer via suppression of CYP24A1

// Amber A. Bokhari 1 , Laura R. Lee 1 , Dewayne Raboteau 1 , Jane Turbov 2 , Isabel V. Rodriguez 2 , John Wesley Pike 3 , Chad A. Hamilton 1, 4, 5 , George Larry Maxwell 5, 6 , Gustavo C. Rodriguez 2 , Viqar Syed 1, 5, 7 1 Department of Obstetrics and Gynecology, Uniformed Services University of the Health Sciences, Bethesda, MD, USA 2 Division of Gynecologic Oncology, North Shore University Health System, University of Chicago, Evanston, IL, USA 3 Department of Biochemistry, University of Wisconsin, Madison, WI, USA 4 Division of Gynecologic Oncology, and Gynecologic Cancer Translational Research Center of Excellence, Walter Reed National Military Medical Center, Bethesda, MD, USA 5 John P. Murtha Cancer Center at Water Reed National Military Medical Center, Bethesda, MD, USA 6 Department of Obstetrics and Gynecology and Women’s Health Integrated Research Center, Inova Fairfax Hospital, Falls Church, VA, USA 7 Department of Molecular and Cell Biology, Uniformed Services University of the Health Sciences, Bethesda, MD, USA Correspondence to: Viqar Syed, email: viqar.syed@usuhs.edu Keywords: chemoprevention, cell proliferation, progesterone, calcitriol, vitamin D receptor Received: June 01, 2016 Accepted: October 03, 2016 Published: October 18, 2016 ABSTRACT Here, we evaluated the expression of CYP24A1, a protein that inactivates vitamin D in tissues. CYP24A1 expression was increased in advanced-stage endometrial tumors compared to normal tissues. Similarly, endometrial cancer cells expressed higher levels of CYP24A1 than immortalized endometrial epithelial cells. RT-PCR and Western blotting were used to examine CYP24A1 mRNA and protein levels in endometrial cancer cells after 8, 24, 72, and 120 h of exposure to progesterone, progestin derivatives and calcitriol, either alone or in combination. Progestins inhibited calcitriol-induced expression of CYP24A1 and splice variant CYP24SV mRNA and protein in cancer cells. Furthermore, actinomycin D, but not cycloheximide, blocked calcitriol-induced CYP24A1 splicing. siRNA-induced knockdown of CYP24A1 expression sensitized endometrial cancer cells to calcitriol-induced growth inhibition. These data suggest that CYP24A1 overexpression reduces the antitumor effects of calcitriol in cancer cells and that progestins may be beneficial for maintaining calcitriol’s anti-endometrial cancer activity.