Role of the environmental sensor, AhR on colitis

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The environmental sensor, aryl hydrocarbon receptor (AhR) serves as a ligand for pollutants as well as for plant, microbial and endogenous compounds. Following AhR ligation, the activated AhR regulates gene expression through the binding of AhR/ARNT complex to specific DNA motifs known as Dioxin Response Elements (DREs). Studies from our lab and elsewhere have shown that some AhR ligands have potent immunosuppressive properties. Inflammatory bowel disease (IBD) results from chronic inflammation in the gastrointestinal tract that affects 1.5 million people in the US. The pathogenesis of IBD involves complex interactions between gut microbiota, immune response, environmental and dietary factors, and genetic/epigenetic regulation. Recently, we made an exciting observation that the AhR ligand and plant product, I3C ameliorates colitis in mice, which was associated with anti-inflammatory effects, regulation of gut dysbiosis, and enhanced expression of β-defensins (mBD1,2,3) by Colonic Epithelial Cells (CEC). β-defensins constitute antimicrobial peptides (AMPs) that resist microbial colonization of epithelial surfaces in the colonic tissue. β- defensins may also mediate anti-inflammatory effects. In fact, studies have shown defective expression of intestinal AMPs particularly defensins in IBD patients. We were excited to uncover DREs in the promoters of mouse β-defensins (mBD1, 2, and 3). In the current study, we will test the central hypothesis that dietary indoles (I3C) attenuate colitis through AhR activation leading to increased expression of mBDs by CECs via pathways involving DREs, and/or epigenetic regulation resulting in modulation of microbiota and prevention of epithelial barrier damage. Furthermore, we propose that mBDs induced by I3C play a critical role in restoring healthy gut microbiota, preventing intestinal barrier damage and suppressing colonic inflammation through induction of Tregs. Aim 1 will test the mechanisms of mBD induction by dietary indoles. We will use reporter assay, promoter bashing and electrophoretic mobility shift assay to determine whether I3C activated AhR directly binds to the DREs to induce mBDs. We will also determine the effect of I3C on the mBD expression by using AhR cKO mice with AhR deletion in IEC, ILC3 and Tregs. In Aim 2, we will study epigenetic regulation of β-defensins by dietary indoles. To that end, we will test whether I3C regulates mBD expression by altering histone modification and decreasing DNA methylation. We will specifically determine if I3C regulates the SATB1-mediated histone deacetylation and chromatin remodeling. In Aim 3, we will test whether administration of mBDs offers protection from colitis by regulating gut dysbiosis, preventing CEC barrier damage, enhancing Treg subsets and decreasing Th17 subpopulations to attenuate colonic inflammation. Finally, we will use mBD KO mice to test whether mBDs are required for I3C-mediated protection from colitis. The proposed studies are highly significant because they will identify novel mechanisms through which dietary indoles suppress colitis by altering the microbiota, through activation of AhR leading to increased expression of host-derived AMPs, specifically β-defensins.