The intestinal epithelial layer is susceptible to damage by chemical, physiological and mechanical stress. While it is essential to maintain the integrity of epithelium, the biochemical pathways that contribute to the barrier function have not been completely investigated. Here we demonstrate an aryl hydrocarbon receptor (AHR)‐dependent mechanism facilitating the production of the antimicrobial peptide AMP regenerating islet‐derived protein 3 gamma (REG3G), which is essential for intestinal homeostasis. Genetic ablation of AHR in mice impairs pSTAT3‐mediated REG3G expression and increases bacterial numbers of Segmented filamentous bacteria ( SFB) and Akkermansia muciniphila in the small intestine. Studies with tissue‐specific conditional knockout mice revealed that the presence of AHR in the epithelial cells of the small intestine is not required for the production of REG3G through the phosphorylated STAT3‐mediated pathway. However, immune‐cell‐specific AHR activity is necessary for normal expression of REG3G in all regions of the small intestine. A diet rich in broccoli, capable of inducing AHR activity, increases REG3G production when compared to a semi‐purified diet that is devoid of ligands that can potentially activate the AHR, thus highlighting the importance of AHR in antimicrobial function. Overall, these data suggest that homeostatic antimicrobial REG3G production is increased by an AHR pathway intrinsic to the immune cells in the small intestine.

Background/Objective: The aryl hydrocarbon receptor (AHR) is an important mediator of intestinal homeostasis. The AHR senses certain classes of phytochemicals, including many flavonoids and tryptophan metabolites generated in the intestinal tract. Several in vitro studies demonstrate the presence of AHR ligands in numerous plants commonly consumed by humans. However, it has not been established that these foods can activate the AHR in vivo. The aim of this study was to evaluate how phytochemicals in foods can lead to AHR activation in vivo through modulating CYP1A1 activity. Methods: Freeze-dried spinach, corn, red potatoes, kidney beans, parsley, onion, carrots, bell peppers, and broccoli were fed to C57BL6/J female mice at 15% w/w in a semi-purified diet to evaluate the AHR activation potential. In vitro CYP1A1 microsomal assays were utilized to establish specific phytochemicals as CYP1A1 substrates. Results: Broccoli, onion, and carrots increased expression of the AHR target gene Cyp1a1 in the duodenum. Broccoli consumption led to the formation of the potent AHR ligand indolo[3,2-b]carbazole (ICZ), which is also a CYP1A1 substrate. Relative to the other vegetables, parsley contained a high concentration of apiin, a diglycoside of the flavone apigenin. Mice were fed a diet with either 10% parsley, 10% broccoli, or both vegetables. Parsley consumption increased broccoli-mediated Cyp1a1 induction in the duodenum, liver, and lung. Apigenin is a CYP1A1 substrate that can attenuate ICZ metabolism in vitro and increase broccoli-mediated Cyp1a1 expression in the lung. Conclusions: These results suggest that phytochemical competition for intestinal AHR binding and CYP1A1 metabolism modulates systemic AHR activity.