| Vitamin D has long been associated with a protective role against cancer. Its therapeutic index in the prevention and treatment of cancer is restricted due to dose-limiting toxicity that is related to its role in calcium mobilization. In order to identify new chemotherapeutic targets in the vitamin D pathway, we are studying its downstream effects. We have identified two novel mechanisms whereby the vitamin D receptor (VDR) and its cognate ligand, 1,25(OH) 2D3, may protect against aberrant gene regulation and cell cycling.;Mouse mammary tumor cells and human colorectal cancer cells (CaCo 2) were assayed for their response to 1,25(OH)2D3. VDR-null cells have less TCF-4 than cells with wildtype VDR. TCF-4, a member of the TCF/LEF family of transcriptional repressors, is trans-activated by beta-catenin, a well-known oncogene. CaCo2 cells responded strongly to 1,25(OH)2D 3 by increasing TCF-4 at the mRNA and protein levels. This is likely effected through a vitamin D-sensitive intermediary, as detailed analysis of putative VDREs did not support a direct mechanism. VDR/1,25(OH)2D 3-mediated up-regulation of TCF-4 also increases beta-catenin activity on an exogenous reporter. We propose a mechanism whereby this increase in TCF-4 may be protective in colorectal cancer that includes beta-catenin/VDR pathway interactions as published in the literature along with our novel findings.;In the process of using the mouse mammary tumor cells from VDR-null animals, we observed a lack of regulation of the classic vitamin D responsive gene, CYP24A1, which is involved in the metabolism of 1,25(OH)2D 3 into inactive metabolites. This gene is 'silenced' by an epigenetic DNA methylation event, as treatment with a DNA methyltransferase inhibitor restored responsiveness. In our studies, the proximal CpG island-containing promoter of CYP24A1, is not the target of this methylation event. We propose that this is either due to an indirect methylation event, a methylation event in the distal promoter or enhancer regions of CYP24A1, or a result of dynamic methylation in this region that allows demethylation and re-expression of this gene.;In this work, we identify a novel target of the vitamin D pathway that may shed light onto the mechanism whereby vitamin D prevents cancer. We also generate a new hypothesis, as well as show evidence to reinforce previously identified, but poorly supported hypotheses in the field of epigenetics. |
