Ah receptor ligands and their impacts on gut resilience: structure-activity effects

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD, dioxin) and structurally related halogenated aromatics modulate gene expression and induce biochemical and toxic responses that are mediated by initial binding to the aryl hydrocarbon receptor (AhR). The AhR also binds structurally diverse compound including pharmaceuticals, endogenous biochemicals, health-promoting phytochemicals, and microbial metabolites. Many of these AhR ligands do not induce TCDD-like toxic responses and some AhR ligands such as microbial metabolites of tryptophan play a role in maintaining gut health and protecting against intestinal inflammation and cancer. Many AhR ligands exhibit tissue- and response-specific AhR agonist or antagonist activities, and act as selective AhR modulators (SAhRMs) and this SAhRM-like activity has also been observed in AhR-ligand-mediated effects in the intestine. This review summarizes studies showing that several AhR ligands including phytochemicals and TCDD protect against dextran sodium sulfate-induced intestinal inflammation. In contrast, AhR ligands such as oxazole compounds enhance intestinal inflammation suggesting that AhR-mediated gut health can be enhanced or decreased by selective AhR modulators and this needs to be considered in development of AhR ligands for therapeutic applications in treating intestinal inflammation.