Methylnicotinamide shields the intestinal epithelial barrier following radiotherapy in colorectal cancer through AhR/RNF43-dependent Wnt suppression and Occludin/ZO-1 enhancement

Background A frequent adverse effect of radiotherapy in colorectal cancer (CRC) patients is radiation-induced intestinal injury (RIII), which compromises the intestinal epithelial barrier's functionality. Emerging evidence highlights the importance of multiple tryptophan-derived metabolites in preserving gut barrier integrity. However, methylnicotinamide (MNA), a metabolite originating from tryptophan, in the context of RIII are not well understood. This study sought to determine if MNA can mitigate radiation-triggered intestinal epithelial barrier impairment and to clarify the associated molecular mechanisms. Methods A RIII in vitro model was established by irradiating Caco-2 cells with X-rays; in vitro functional assays were employed to evaluate the protective effect of MNA against radiation-induced intestinal injury, and the role of the aryl hydrocarbon receptor (AhR)/RNF43/Wnt signaling axis in this process was investigated. Additionally, the AhR antagonist CH223191 was used for pharmacological inhibition to validate the causal role of this signaling axis in the MNA-mediated protective effect. Results In an in vitro Caco-2 model of radiation-induced intestinal injury, pretreatment with MNA (6–24 ng/mL) attenuated barrier dysfunction and apoptosis induced by 10 Gy X-ray irradiation in a dose-dependent manner, as evidenced by upregulated expression and restored membrane localization of the tight junction proteins Occludin and zonula occludens-1 (ZO-1), with the highest concentration (24 ng/mL) restoring cell function to normal levels. Mechanistically, MNA activated AhR signaling (as evidenced by increased CYP1A1 expression) and upregulated RNF43 in an AhR-dependent manner. Importantly, CH223191 treatment abolished MNA-mediated protection against radiation-induced barrier dysfunction and apoptosis. Conclusion This study demonstrates that MNA protects the integrity of the intestinal epithelial barrier from radiation-induced disruption through the AhR/RNF43/Wnt/β-catenin signaling pathway, thereby maintaining the levels of Occludin and ZO-1. These results reveal a new mechanism through which MNA operates and highlight AhR and RNF43 as possible therapeutic targets for preventing and treating intestinal damage induced by radiation.