Molecular mechanism of cell regulatory properties of vitamin D analogues in human cancer cell lines

Abstract: The active form of vitamin D, 1α,25-dihydroxyvitamin D3, is a potent modulator of the induction of differentiation and inhibition of cell growth in both malignant and normal cells. Vitamin D and its analogues exert their effects through a transcription factor, the nuclear vitamin D receptor (VDR). The VDR can form a homodimer or heterodimers with other nuclear receptors. Its main heterodimer partner is the retinoid X receptor (RXR), which is the nuclear receptor for 9-cis retinoic acid. VDR-RXR heterodimers bind to vitamin D response elements (VDREs) that are formed by directly repeated hexameric core binding motifs spaced by 3 nucleotides (DR3-type VDREs), or by inverted palindromes spaced by 9 nucleotides (IP9-type VDREs).This study addressed the cell regulatory properties of vitamin D analogues in human cancer cell lines, mainly in the breast cancer cell line MCF-7 and in the melanoma cell lines WM1341 and MeWo, It was found that of 1α,25-dihydroxyvitamin D3 and its analogues, EB1089 and KH1230 anti-proliferative effects are not mediated by repression of AP-1-mediated gene activity, which has beenobserved for the retinoids. Further observations seen in these studies were that EB1089 and ZK161422, two potent and completely different vitamin D analogues, were capable of demonstrating a promoter selectivity for an IP9-type VDRE. Interestingly, CB1093 the most potent vitamin D analogue in these studies showed a promoter selectivity for a DR3-type VDRE. Additional studies with a synthetic retinoid, CD437 and 1α,25-dihydroxyvitamin D3 resulted in a synergistic enhancement of apoptosis in WIG1341 melanoma cells. These studies demonstrated that the vitamin D analogues have potential as therapeutic agents in breast cancer and melanoma.Two novel vitamin D-regulated genes, the cyclin C and p19INK4D, involved in cell proliferation were identified by an mRNA differential display reverse transcriptase-polymerase chain reaction (DDRT-PCR) approach. Up-regulation at both mRNA and protein levels for cyclin C and p19INK4D were successfully shown in MCF-7 cells.