Mechanism of Action of a Novel Analog of Vitamin D3 1 Alpha-hydroxy-24- ethyl Cholecalciferol (D5) in Normal and Transformed Human Breast Epithelial Cells

摘要

It is now well established that the active metabolite of vitamin D3, 1alpha, 25 (OH)2D3, regulates cell growth and differentiation in various in vitro models. However, its clinical use is precluded due to its hypercalcemic activity in vivo. Hence, several less calcemic vitamin D3 analogs have been synthesized and evaluated for their chemopreventive and therapeutic efficacy in experimental carcinogenesis models. We have previously reported an analog of vitamin D3, 1-hydroxy-24-ethyl Cholecalciferol (D5) to be antiproliferative and inducer of differentiation in carcinogen-transformed mouse mammary gland organ culture (MMOC) and breast cancer cells in vitro with little or no calcemic activity in vivo. Moreover, D5 showed a selective growth inhibitory action on transformed MCF-12F cells while no growth inhibitory effect was observed on normal MCR-12F cells. Similarly, OS induced growth arrest and/or differentiation in hormone responsive breast cancer BT-474 and MCF-7 cells. D5 induced its growth inhibitory effect by modulating expression of various cell cycle related proteins. Among the major cell cycle check points that were up regulated by D5, were CDKIs p21 and p27 in BT-474 cells. In the MCF 12F(exp MNU) cell model, D5 reduced expression of epidermal growth factor receptor, over expression of which is associated with hormone resistance. There was no effect of DS on EGFR expression in the breast cancer BT-474 and MCF-7 cells. Thus, the effect of D5 appears to be cell model specific. On the other hand, in the BT-474 cell model, D5 down regulated the transcription and expression of estrogen receptor alpha (ERalpha) . Down regulation of ERalpha was followed by decreased expression of progesterone receptor and the trefoil factor pS2 suggesting anti-estrogenic action of D5.