Calcitriol is the hormonally active form of
vitamin D and has anti-proliferative and pro-apoptotic effects.
Calcitriol and its precursor
calcidiol (25(
OH)D3) are degraded by the
1,25-dihydroxyvitamin D3 24-hydroxylase (
CYP24A1). This
enzyme is overexpressed in
colorectal tumors, however, the mechanisms of this overexpression remain to be elucidated.
CYP24A1 mRNA level differs among
colorectal cancer cell lines and range from almost undetectable to high. Since DNA methylation and
histone acetylation regulate
CYP24A1 gene expression in
prostate cancer cell lines, we investigated whether epigenetic mechanisms could explain the differences in basal expression of
CYP24A1 in
colon cancer cells.
Methyltransferase inhibitor
5-aza-2'-deoxycytidine (DAC) treatment resulted in an over 50-fold induction of
CYP24A1 mRNA expression in Coga1A and HT-29 cells but in no response in Caco2/AQ and Coga13 cells. This finding is supported by a strong increase in
CYP24A1 activity after DAC treatment in Coga1A (35%). In addition,
calcitriol and DAC had synergistic effects on
CYP24A1 gene transcription. Interestingly, the
CYP24A1 promoter was not methylated in Coga1A and HT-29 (<5%), while in Caco2/AQ it was 62% methylated. This suggests that DNA demethylation must activate genes upstream of
CYP24A1 rather than act on the gene itself. However, transcriptional regulators of
CYP24A1 such as
vitamin D receptor (VDR),
retinoid X receptor (RXR), specificity
protein 1 (SP1), or
mediator complex subunit 1 (MED1) were not upregulated. We conclude that in
colon cancer cells,
CYP24A1 gene expression is inducible by
methyltransferase and some
histone deacetylase inhibitors in a cell line-dependent manner. This effect does not correlate with the methylation state of the promoter and therefore must affect genes upstream of
CYP24A1. This article is part of a Special Issue '
Vitamin D Workshop'.