Vitamin D and Wnt/β-catenin Pathway in Colon Cancer: Role and Regulation of DICKKOPF Genes

Abstract

Colorectal cancer is a major health problem worldwide. Aberrant activation of the Wingless-type mouse mammary tumour virus integration site family (Wnt)/β-catenin signalling pathway due to mutation of adenomatous polyposis coli (APC), β-catenin (CTNNB1) or AXIN genes is the most common and initial alteration in sporadic colorectal tumours. Numerous epidemiological and experimental studies have indicated a protective action of vitamin D against colorectal cancer. Previous work has demonstrated that the most active vitamin D metabolite, 1α,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) inhibits β-catenin transcriptional activity by promoting vitamin D receptor (VDR) binding to β-catenin and the induction of E-cadherin expression. Recently, 1,25(OH)₂D₃ has been shown to distinctly regulate two genes encoding the extracellular Wnt inhibitors DICKKOPF-1 and DICKKOPF-4 (DKK-1, DKK-4). By an indirect transcriptional mechanism, 1,25(OH)₂D₃ increases the expression of DKK-1 RNA and protein, which acts as a tumour suppressor in human colon cancer cells harbouring endogenous mutations in the Wnt/β-catenin pathway. In contrast, 1,25(OH)₂D₃ represses DKK-4 transcription by inducing direct VDR binding to its promoter. Unexpectedly, DKK-4 is a target of the Wnt/β-catenin pathway and is up-regulated in colorectal tumours, and it has been shown to increase cell migration and invasion and to promote a proangiogenic phenotype. Together, these results show that 1,25(OH)₂D₃ exerts a complex set of regulatory actions leading to the inhibition of the Wnt/β-catenin pathway in colon cancer cells that is in line with its protective effect against this neoplasia.