A Novel Paradigm for Aryl Hydrocarbon Receptor Signaling

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 DESCRIPTION (provided by applicant): The Aryl hydrocarbon Receptor (AhR) is a mediator of xenobiotic toxicity, best recognized for conveying the deleterious human health effects following exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) and related environmental contaminants. Mechanistically, the AhR is known to function as a ligand-activated transcription factor that binds to a canonical xenobiotic response element (XRE) in association with its heterodimerization partner, the AhR nuclear translocator (Arnt) protein. However, within the repertoire of AhR target genes identified in recent years, many lack a clearly defined XRE, highlighting the growing realization that AhR-mediated gene expression appears to involve additional mechanisms distinct from the well-characterized process involving the XRE. We recently identified a non-consensus XRE (NC-XRE) that recruits the AhR in conjunction with a novel DNA binding partner, the Krüppel-like factor 6 (KLF6) tumor suppressor. DNA binding and functional studies confirmed that NC-XRE binding by the AhR-KLF6 complex is independent of the Arnt protein. The toxicological ramification of TCDD-induced AhR-KLF6 activity is revealed in liver regeneration studies. Our previous work established that TCDD inhibits normal liver regeneration following injury, by suppressing G1 phase cyclin-dependent kinase (CDK) activity, specifically CDK2. Increased CDK2 activity is necessary for G1 phase cell cycle progression, and inhibition of CDK2 activity leads to cell cycle arrest. New data demonstrate that the TCDD-induced inhibition of liver regeneration is directly tied to AhR-dependent expression of the CDK2 inhibitor, p21Cip1. The results confirm that p21Cip1 is an AhR target gene, and that its induction relies on NC-XRE-mediated AhR-KLF6 activity. A preliminary analysis of the AhR-KLF6 protein-DNA interaction revealed that deletion of the AhR's DNA binding domain required for XRE binding did not prevent receptor binding to the NC-XRE. Hence, we hypothesize that AhR-NC-XRE binding and function is fundamentally different from signaling through the XRE. In keeping with our long-term objective to understand the pathophysiology of AhR activity, this proposal will examine two specific aims designed to characterize this novel NC-XRE regulatory complex as a platform for future studies designed to explore AhR-KLF6 signaling. Specific aim 1 describes a detailed characterization and functional analysis of the NC-XRE protein-DNA complex. Specific Aim 2 will use ChIP-seq and RNA-seq to provide a global genomic assessment of the functional NC-XRE sites regulated by the AhR/KLF6 transcriptional complex. Identification of this novel AhR complex represents a paradigm shift in our understanding of AhR biology and TCDD toxicity.