2,3,7,8-Tetrachlorodibenzo-p-dioxin (
TCDD;
dioxin) is a toxic environmental contaminant that works through
dioxin response elements (DRE) to activate gene expression. We tested the hypothesis that
cancer-related epigenetic changes suppress
dioxin activation of the
cytochrome P4501A1 (
CYP1A1) gene.
5-Aza-2'-deoxycytidine (5-aza-CdR), an inhibitor of DNA methylation, increases
TCDD-inducible
CYP1A1 mRNA expression in cancerous LNCaP cells but not in noncancerous PWR-1E and RWPE-1 cells (all human prostate cell lines).
Bisulfite DNA sequencing shows that the
TCDD-responsive
CYP1A1 enhancer is highly methylated in LNCaP cells but not in RWPE-1 cells. In vivo footprinting experiments reveal that unmethylated DRE sites do not bind
protein in response to
TCDD in LNCaP cells, whereas inducible DRE occupancy occurs in RWPE-1 cells. Pretreatment of LNCaP cells with 5-aza-CdR partially restores
TCDD-inducible DRE occupancy, showing that DNA methylation indirectly suppresses DRE occupancy.
Chromatin immunoprecipitation experiments reveal that LNCaP cells lack trimethyl
histone H3 lysine 4, a mark of active genes, on the
CYP1A1 regulatory region, whereas this
histone modification is prevalent in PWR-1E and RWPE-1 cells. We also analyzed
CYP1A1 enhancer methylation in human prostate tissue
DNA. We do not detect
CYP1A1 enhancer methylation in 30
DNA samples isolated from noncancerous prostate tissue. In contrast, 11 of 30 prostate
tumor DNA samples have detectable
CYP1A1 enhancer methylation, indicating that it is hypermethylated in prostate
tumors. This is the first report that shows that
CYP1A1 is aberrantly hypermethylated in human
prostate cancer and has an altered, inaccessible
chromatin structure that suppresses its
dioxin responsiveness.