Fusobacterium nucleatum-derived 3-indolepropionic acid promotes colorectal cancer progression via aryl hydrocarbon receptor activation in macrophages

An increasing body of research indicates that Fusobacterium nucleatum (F. nucleatum) significantly influences the onset and progression of colorectal cancer (CRC). Our previous study has shown that F. nucleatum exerts pro-tumorigenic effects through aryl hydrocarbon receptor (AhR) activation. However, the role of its microbial metabolites in regulating immune responses remains unclear. Here, we report for the first time that F. nucleatum-derived 3-Indolepropionic acid (IPA) activates AhR in macrophages, driving M2 polarization and tumor-promoting immunosuppression. We discovered that culture supernatant of F. nucleatum (CSF) robustly activates AhR in macrophages. In co-culture systems, CSF upregulated the expression of the M2 marker CD206 and elevated mRNA levels of CD163, TGF-β, IL-10, and VEGF. In a subcutaneous allograft model, CSF induced an elevated number of CD206⁺ macrophages and decreased presence of CD8⁺ T cells within the tumor microenvironment, thereby promoting tumor growth. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) revealed IPA as a novel major AhR-activating metabolite in CSF. Strikingly, IPA recapitulated CSF's effects in promoting tumor cell migration and immunosuppression, both in vitro and in vivo. Critically, the AhR inhibitor CH223191 abolished both IPA-mediated M2 polarization and tumor growth. Our study revealed a novel mechanism by which F. nucleatum-derived IPA reprograms macrophages through AhR activation to fuel CRC progression, providing potential therapeutic targets for CRC treatment and prognosis improvement.