Dioxin-induced transcriptional regulation of CYP1A1 and CYP1B1 by the aryl hydrocarbon receptor complex

875 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced activation of cytochrome P450 1A1 (CYP1A1) and cytochrome P450 1B1 (CYP1B1) is mediated by the Aryl Hydrocarbon Receptor complex (AHRC). The AHRC consists of a heterodimer formed between the Aryl Hydrocarbon Receptor (AHR) and the Aryl Hydrocarbon Receptor Nuclear Translocator (ARNT). The aim of this study was to investigate the kinetics of AHR and coactivator recruitment to two AHRC target genes CYP1A1 and CYP1B1 by TCDD in the human breast carcinoma cell line MCF-7. We also seek to identify coactivator proteins that are essential for CYP1A1 and/ or CYP1B1 induction. We hypothesize that the differences in TCDD-induction of CYP1A1 and CYP1B1 in these cells are based on the differences in promoter/enhancer structure of the two genes and the coactivator proteins that are recruited to each gene. The chromatin immunoprecipitation assay was employed to determine the kinetics of TCDD-induced protein recruitment to the CYP1A1 and CYP1B1 promoter and enhancer regions. We found that TCDD treatment leads to rapid recruitment of AHR to the enhancer region of both genes. As opposed to other published reports, we observed no “cycling” of the AHR on and off of the enhancer region of either gene up to 105 minutes of TCDD treatment. Nor did we observe binding of the AHR to the promoter region of either gene. Conversely, RNA polymerase II is recruited to the promoter region of both genes and not to the enhancer. RNA interference was utilized to identify coactivator proteins that are essential for cytochrome P450 induction by TCDD. We found the coactivator proteins p300 and Steroid Receptor Interacting Protein-2 (SRC-2) to be essential for both CYP1A1 and CYP1B1 maximal TCDD-induction. While the nucleosome remodeling factors Brahma/SWI2 Related Gene 1 (BRG-1) and Brahma (BRM) appear to be required for CYP1A1 induction and not CYP1B1. These results indicate that the promoter regions of CYP1A1 and CYP1B1 require a different set of transcriptional coactivators and thus may explain their different induction levels in these MCF-7 breast carcinomas.