Vitamin D resistance and colon cancer prevention

Abstract

Observational studies have been largely consistent in showing an inverse association between vitamin D and an individual's risk of developing colorectal cancer. Vitamin D protection is further supported by a range of preclinical colon cancer models, including carcinogen, genetic and dietary models. A large number of mechanistic studies in both humans and rodents point to vitamin D preventing cancer by regulating cell proliferation. Counterbalancing this mostly positive data are the results of human intervention studies in which supplemental vitamin D was found to be ineffective for reducing colon cancer risk. One explanation for these discrepancies is the timing of vitamin D intervention. It is possible that colon lesions may progress to a stage where they become unresponsive to vitamin D. Such a somatic loss in vitamin D responsiveness bears the hallmarks of an epigenetic change. Here, we review data supporting the chemopreventive effectiveness of vitamin D and discuss how gene silencing and other molecular changes somatically acquired during colon cancer development may limit the protection that may otherwise be afforded by vitamin D via dietary intervention. Finally, we discuss how understanding the mechanisms by which vitamin D protection is lost might be used to devise strategies to enhance its chemopreventive actions.

Fig. 1.

Summary of VDR complexes. Unliganded VDR can associate with RXR and the transcriptional repressors nuclear receptor corepressor, silencing mediator for retinoid or thyroid-hormone receptors and HDACs. VDR or RXR ligand binding increases VDR-RXR dimer stability and causes a replacement of transcriptional repressors for coactivators. Other VDR complexes formed include a 1,25(OH)₂D3-dependent interaction with β-catenin that inhibits β-catenin activity and a 1,25(OH)₂D3-independent binding to Lef1 that can activate Lef1 promoters.

Fig. 2.

Histone deacetylases participate in VDR regulation. (A) Regulation of a VDRE-regulated luciferase gene by 1,25(OH)₂D3 (D3) and the HDAC inhibitor butyrate (BA). HT29 cells were transfected with a VDRE-regulated luciferase reporter and a constitutive cytomegalovirus-regulated lacZ control plasmid. Cells were then treated overnight with the indicated concentrations of butyrate in the presence or absence of 25 nM 1,25(OH)₂D3. Data show luciferase expression normalized to β-galactosidase activity. Analysis of variance indicates significant differences in expression between the control and the butyrate-treated groups and the butyrate-treated and D3 plus butyrate-treated groups. (B) Knockdown of HDAC expression in HCT116 cells following transfection with siRNA. HCT116 cells were transfected with Smart-pool siRNA (Dharmacon) with HDAC expression levels determined by immunoblotting 48 h following transfection. (C) Gene expression changes in HCT116 cells following HDAC knockdown. RNA was isolated 48 h after siRNA transfection and quantified for VDR, TLE1 and p21 expression. HDAC3 knockdown provided the highest level of induction for these three genes.

Fig. 3.

Colon cancer cell changes following an epithelial–mesenchymal transition. Normal colonocytes express the VDR and form tight junctions through E-cadherin (E-cad). Activation of Snail and Slug suppresses expression of VDR and E-cadherin and generates a transformed cell with increased motility. A number of changes in cancer cells or cancer tissue may facilitate the epithelial–mesenchymal transition (EMT), including cytokines or specific oncogene mutations (as shown).