Coactivator dynamics in aryl hydrocarbon receptor complex mediated transcription

Many polycyclic and halogenated aromatic hydrocarbon compounds (PAH and HAH) are potent carcinogens. The aryl hydrocarbon receptor (AHR) binds these xenobiotic chemicals and translocates into the nucleus where it dimerizes with the aryl hydrocarbon receptor nuclear translocator (ARNT) to form the aryl hydrocarbon receptor complex (AHRC). The dimer then regulates transcription of a battery of target genes including the cytochrome P450-1 gene subfamily, which contributes to the carcinogenic response to PAHs. Most, if not all the toxic effects of the HAH, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TODD) are mediated by the AHR/ARNT. The potency of TCDD being a tumor promoter is believed largely to be attributable to sustained gene activation mediated by AHRC due to the compound's resistance to metabolism and long half-life. Taken together, transcription regulation mediated by the AHRC is essential for fully understanding the mechanism of carcinogenesis by these xenobiotic compounds.; We demonstrate that mammalian chromatin remodeling complexes including the BRG-1 containing SWI/SNF complex and p300/p160 coactivator complexes that contain histone acetyl transferase (HAT) activity are recruited by AHR/ARNT to facilitate chromatin remodeling. In cell lines lacking BRG-1 expression, AHR/ARNT mediated CYP1A1 gene expression is lost. This absence of CYP1A1 expression can be rescued by ectopic expression of wild type but not ATPase abolished mutant BRG-1. Chromatin immuno-precipitation (ChIP) experiments show that BRG-1 can be recruited to the endogenous CYP1A1 enhancer in an AHR/ARNT dependent manner. In vitro interaction analysis shows that AHR is capable of interacting with BRG-1 through its Glutamine-rich transcription activation domain.; The thyroid hormone receptor (TR)-associated protein complex (TRAP) has been shown to participate in the transcriptional activation by a diverse set of transcription factors including certain nuclear hormone receptors. Its TRAP220 subunit has been shown to mediate interaction with several nuclear hormone receptors. Here we show that in the murine Hepa-1 and human HepG2 cell lines, wild type TRAP220 as well as the TRAP220 with mutated steroid hormone receptor interacting motifs (LxxLL) enhance AHR/ARNT mediated transcription. ChIP assays revealed that TRAP220 is recruited to the murine CYP1A1 enhancer in a TCDD dependent fashion. Time course ChIP assays show that TRAP/Mediator is persistently associated with the murine CYP1A1 enhancer after TCDD treatment and behaves in a similar kinetic manner as p300 and pCIP. Using RNA interference (RNAi), we show that selectively depletion of TRAP220 reduces AHR/ARNT mediated CYP1A1 transcription in HepG2 cells.; Collectively, these lines of evidence provide the basic framework of the regulation of AHR/ARNT dependent transcription.