Ligand-selective AHR regulation in benzo[a]pyrene-induced colonic barrier injury: Indole-driven functional reprogramming and microbiota-dependent tryptophan defense.

Benzo[a]pyrene (BaP) is a representative environmentally persistent polycyclic aromatic hydrocarbon (PAH) that continues to enter the human diet through food-chain accumulation and food processing. The aryl hydrocarbon receptor (AHR) is a shared sensor for xenobiotic BaP and microbiota-derived tryptophan (Trp) metabolites, but how opposing ligands acting through the same receptor dictate divergent epithelial outcomes remains unclear. Here, integrating receptor engagement, AHR chromatin occupancy, and epithelial barrier function, we show that indole, a microbial Trp metabolite, counteracts BaP-induced colonic barrier injury through ligand-selective AHR regulation. AHR antagonism and knockdown attenuated BaP-induced barrier disruption and AHR signaling dysregulation, supporting AHR involvement in BaP toxicity. Indole and BaP occupied the same AHR ligand-binding pocket but displayed divergent binding modes and kinetics. Consistently, indole remodeled BaP-driven AHR chromatin occupancy and shifted enriched regulatory programs from xenobiotic metabolism toward epithelial junction, barrier maintenance, and cytoskeletal integrity. Functionally, indole restored tight-junction architecture, barrier permeability, and normalized AHR and cytochrome P450 1A1 expression under BaP challenge. In vivo, Trp and indole supplementation protected against BaP-induced colonic injury under intact microbiota. Under antibiotic-treated conditions, indole remained protective whereas Trp protection was markedly diminished, indicating that microbial conversion is required for Trp-dependent defense. These findings establish ligand-selective AHR reprogramming as a mechanism by which microbial Trp metabolites counteract BaP at the receptor interface, and identify the Trp-microbiota-indole axis as a targetable endogenous defense against health risks posed by persistent dietary pollutants.