Mechanistic Insights into the Events That Lead to Synergistic Induction of Interleukin 6 Transcription upon Activation of the Aryl Hydrocarbon Receptor and Inflammatory Signaling

The aryl hydrocarbon receptor (AHR) is the ligand-activated transcription factor responsible for mediating the toxicological effects of dioxin and xenobiotic metabolism. However, recent evidence has implicated the AHR in additional, nonmetabolic physiological processes, including immune regulation. Certain tumor cells are largely nonresponsive to cytokine-mediated induction of the pro-survival cytokine interleukin (IL) 6. We have demonstrated that multiple nonresponsive tumor lines are able to undergo synergistic induction of IL6 following combinatorial treatment with IL1 beta and the AHR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin. Such data implicate the AHR in tumor expansion, although the mechanistic basis for the AHR-dependent synergistic induction of IL6 has not been determined. Here, we demonstrate that ligand-activated AHR is involved in priming the IL6 promoter through binding to nonconsensus dioxin response elements located upstream of the IL6 start site. Such binding appears to render the promoter more permissive to IL1 beta-induced binding of NF-kappa B components. The nature of the AHR-dependent increases in IL6 promoter transcriptional potential has been shown to involve a reorganization of repressive complexes as exemplified by the presence of HDAC1 and HDAC3. Dismissal of these HDACs correlates with post-translational modifications of promoter-bound NF-kappa B components in a time-dependent manner. Thus the AHR plays a role in derepressing the IL6 promoter, leading to synergistic IL6 expression in the presence of inflammatory signals. These observations may explain the association between enhanced expression of AHR and tumor aggressiveness. It is likely that AHR-mediated priming is not restricted to the IL6 promoter and may contribute to the expression of a variety of genes, which do not have consensus dioxin response elements.