Indole-3-Acetic Acid and Skatole Exert Opposing Effects on MDR1 Proteostasis in Human Colonic Epithelial Cells: A Molecular Basis for the Gut Microbial Metabolic Switch

). While gut microbiota metabolize dietary tryptophan into diverse indole derivatives that function as aryl hydrocarbon receptor (AhR) ligands, their differential regulation of MDR1 remains an unresolved AhR paradox. Here, we investigated the mechanisms by which two distinct metabolites, indole-3-acetic acid (IAA) and skatole, regulate MDR1 expression in human colonic epithelial Caco-2 cells. We observed that IAA selectively enhances MDR1 protein stability via an AhR-dependent pathway without inducing de novo transcription, suggesting a mechanism we term enhanced proteostasis mediated by the AhR-Hsp90 complex. Conversely, skatole, a toxic dysbiotic metabolite linked to red meat intake, triggered a time-dependent depletion of MDR1 and potently abrogated the protective efficacy of IAA. Our findings are consistent with a model in which skatole acts as a putative structural disruptor, potentially destabilizing the chaperone complex essential for MDR1 integrity. This destruction is facilitated by a key bacterial enzyme, indoleacetate decarboxylase (IAD), which is a pH-dependent metabolic switch in the gut. The modern Western diet, characterized by high protein and low fiber content, elevates colonic pH, thereby activating IAD to convert protective IAA into toxic skatole. These findings provide a molecular framework for the red meat-microbiome-barrier failure axis and highlight the restoration of the IAA/skatole balance through dietary intervention as a promising therapeutic strategy.