Indole-3-Acetic Acid as a Putative Selective AhR Modulator Counteracts Skatole-Induced Dual-Hit Toxicity in Colorectal Cancer Cells

The rising incidence of colorectal cancer (CRC) in modernized societies is linked to diet-induced dysbiosis, characterized by a critical metabolic divergence: the depletion of protective indole-3-acetic acid (IAA) concurrent with the accumulation of toxic skatole (3-methylindole). However, the molecular mechanisms by which high concentrations of skatole drive malignancy-and whether IAA can counteract this toxicity-remain elusive. Here, we demonstrate that physiologically relevant concentrations of skatole (500 µM) significantly promote the proliferation of HCT-116 CRC cells through a "dual-hit" mechanism involving both aryl hydrocarbon receptor (AhR)-dependent genomic activity and AhR-independent activation of the ERK MAPK pathway. Notably, co-treatment with IAA (250 µM) effectively abrogated skatole-induced proliferation, restoring cell growth to baseline levels while sparing upstream MAPK phosphorylation. Mechanistic analysis indicates that IAA acts not merely as a competitor, but as a functional antagonist. Specifically, our findings suggest that IAA functions as a putative selective AhR modulator (SAhRM) that qualitatively reprograms AhR signaling. This modulation uncouples upstream MAPK phosphorylation from downstream cell cycle progression, effectively impeding the proliferative program even in the presence of skatole-induced stress. Furthermore, we propose a theoretical model of counter-balancing metabolic activation, hypothesizing that the oxidative environment associated with skatole metabolism may trigger the bioactivation of IAA into highly active anti-tumor derivatives. These findings suggest that restoring the gut IAA/skatole balance-either by targeting the bacterial enzyme indoleacetate decarboxylase (IAD) or via dietary resistant starch-may offer a promising precision nutrition strategy for CRC prevention.