Role of epigenetic mechanisms in regulation of dioxin-induced transcription of the mouse and human CYP1A1 and CYP1B1 genes

Dioxins are classified as environmental toxicants and carcinogens. One of the most investigated dioxins is TCDD (2,3,7,8-tetrachlorodibenzo- p-dioxin), which mediates various biological effects via the Aryl Hydrocarbon Receptor pathway (AHR). AHR pathway induces several genes, including all the CYP1 family genes. Dioxin differentialy induces the expression of the mouse and human CYP1A1 and CYP1B1 genes in different cancer and normal cell lines. The goal of this study was to delineate the molecular mechanisms involved in dioxin-induced transcriptional regulation of the CYP1A1 and CYP1B1 genes utilizing both in vitro and in vivo models.;In this study we demonstrated that the CYP1B1 gene was silenced in the hepatic cancer cell line, HepG2 due to hypermethylation of its promoter. We also demonstrated that 5-AzadC treatment reactivates the CYP1B1 mRNA expression and the hypermethylation of the promoter is also reversed. Further we demonstrated that four chromatin modifications AcH3K9, AcH3K14, AcH4, and me3H3K4 strongly correlate with the dioxin-induction of the CYP1A1 and CYP1B1 genes. Subsequently we demonstrated that knockdown of the coactivator protein p300 significantly inhibits the dioxin-induction of the CYP1A1 and CYP1B1 genes and also inhibits the four chromatin modifications correlating with the inducibility of these genes. Together our results demonstrate that both chromatin modifications and DNA methylation play an important role in the transcriptional regulation of these genes. In similar studies using mouse hepatoma cell line Hepa-1 and mouse fibroblast cell line C3H 10T1/2, we demonstrated that that both DNA methylation and histone acetylation work in concert to silence the Cyp1b1 gene in Hepa-1 and the Cyp1a1 gene in C3H 10T1/2 cells.;Further, we also demonstrated that resveratrol inhibits dioxin induction of the CYP1 family members either by directly or indirectly inhibiting the recruitment of the transcription factors AHR and ARNT to the xenobiotic response elements (XREs) of the corresponding genes. The reduced transcriptional factor binding at their enhancers then results in reduced polII recruitment at the promoters of these genes.