title: Role of epigenetic mechanisms in differential regulation of the dioxin-inducible human CYP1A1 and CYP1B1 genes.
abstract: The aryl hydrocarbon receptor (AhR) mediates induction of CYP1A1 and CYP1B1 by 2,3,7,8-tetrachlorodibenzo-ρ-dioxin (dioxin) via binding to xenobiotic-responsive elements (XREs) in their enhancer regions. CYP1A1 and CYPIB1 were both inducible by dioxin in human MCF-7 cells. However, only CYP1A1 was inducible in human HepG2 cells. Further experiments focused on providing an explanation for this last observation. Dioxin induced the recruitment of AHR and the transcriptional coactivators p300 and p300/cAMP response element-binding protein binding protein-associated factor (PCAF) to the CYP1B1 enhancer in HepG2 cells but failed to induce recruitment of RNA polymerase II (polII) or the TATA binding protein (TBP) and acetylations of histones 3 and 4 or methylation of histone 3 at the promoter. Because p300 was required for dioxin induction of the aforementioned histone modifications at the CYP1B1 promoter and for induction of CYP1B1 transcription (in MCF-7 cells), the recruitments of p300 and AhR, although necessary, are not sufficient for eliciting the above responses to dioxin. Cytosine residues within CpG dinucleotides at the enhancer, including those within the XREs, were partially methylated, whereas those at the promoter were fully methylated. Treatment of HepG2 cells with 5-aza-2'-deoxycytidine led to partial demethylation of the promoter, restored polII and TBP binding, and CYP1B1 inducibility. Thus, the deficiency of CYP1B1 induction in HepG2 cells is ascribable to cytosine methylation at the promoter, which prevents recruitment of TBP and polII. It is noteworthy that our data indicate that stable recruitment of p300 and PCAF to the CYP1B1 gene does not require their tethering to the promoter and to the enhancer.
authors.affiliation: Departmental of Pathology and Laboratory Medicine, David Geffen School of Medicine, UCLA, 650 Charles Young Drive, Los Angeles, CA 90095, USA.
authors.forename: Sudheer R
authors.initials: SR
authors.lastname: Beedanagari
authors.forename: Robert T
authors.initials: RT
authors.lastname: Taylor
authors.forename: Peter
authors.initials: P
authors.lastname: Bui
authors.forename: Feng
authors.initials: F
authors.lastname: Wang
authors.forename: Derek W
authors.initials: DW
authors.lastname: Nickerson
authors.forename: Oliver
authors.initials: O
authors.lastname: Hankinson
mesh_terms: D001189:Aryl Hydrocarbon Hydroxylases; D045744:Cell Line, Tumor; D019363:Cytochrome P-450 CYP1A1 / Q000096:biosynthesis* / Q000235:genetics; D065633:Cytochrome P-450 CYP1B1; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis* / Q000235:genetics; D004147:Dioxins / Q000494:pharmacology; D004790:Enzyme Induction / Q000187:drug effects / Q000235:genetics; D044127:Epigenesis, Genetic / Q000187:drug effects* / Q000502:physiology; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects; D056945:Hep G2 Cells; D006801:Humans; D000072317:Polychlorinated Dibenzodioxins / Q000494:pharmacology*
chemical_list: D004147:Dioxins; D000072317:Polychlorinated Dibenzodioxins; D003577:Cytochrome P-450 Enzyme System; D001189:Aryl Hydrocarbon Hydroxylases; C585496:CYP1B1 protein, human; D019363:Cytochrome P-450 CYP1A1; D065633:Cytochrome P-450 CYP1B1