Functional Significance of Vitamin D Receptor FokI Polymorphism in Human Breast Cancer Cells

Background: The FokI vitamin D receptor (VDR) polymorphism results in different translation initiation sites on VDR. In the VDRff variant, initiation of translation occurs at the first ATG site, giving rise to a full length VDR protein of 427 amino acids. Conversely, in the VDRFF variant, translation begins at the second ATG site, resulting in a truncated protein with three less amino acids. Epidemiological studies have paradoxically implicated this polymorphism with increased breast cancer risk. 1 alpha,25 (OH)(2)D-3, the active metabolite of vitamin D, is known to inhibit cell proliferation, induce apoptosis and potentiate differentiation in human breast cancer cells. It is well documented that 1 alpha,25 (OH)(2)D-3 downregulates estrogen receptor alpha expression and inhibits estrogen mediated signaling in these cells. The functional significance of the VDR FokI polymorphism in vitamin D action is undefined. Methods/Findings: To elucidate the functional role of FokI polymorphism in breast cancer, MCF-7-Vector, MCF-7-VDRff and MCF-7-VDRFF stable cell lines were established from parental MCF-7 cells as single-cell clones. In response to 1 alpha,25 (OH)(2)D-3 treatments, cell growth was inhibited by 60% in VDRFF cells compared to 28% in VDRff cells. The induction of the vitamin D target gene CYP24A1 mRNA was 1.8 fold higher in VDRFF cells than in VDRff cells. Estrogen receptor-a protein expression was downregulated by 62% in VDRFF cells compared to 25% in VDRff cells. VDR protein stability was greater in MCF-7-VDRFF cells in the presence of cycloheximide. PCR array analyses of VDRff and VDRFF cells revealed increased basal expression levels of pro-inflammatory genes Cyclooxygenase-2, Interleukin-8 and Chemokine (C-C Motif) Ligand 2 in MCF-7-VDRff cells by 14, 52.7 and 5 fold, respectively. Conclusions/Significance: These results suggest that a VDRff genotype may play a role in amplifying aggressive breast cancer, paving the way for understanding why some breast cancer cells respond inefficiently to vitamin D treatment.