Isobutyric Acid Promotes Immune Evasion in Colorectal Cancer via Increased PD-L1 Expression.

Introduction: Isobutyric acid (IBA), a short-chain fatty acid, has been unequivocally demonstrated to exert significant influence on the progression of colorectal cancer (CRC). Nevertheless, a comprehensive understanding of its intricate regulatory mechanisms remains elusive.

Methods: Employing advanced techniques such as western blot, RT-qPCR, and flow cytometry, we systematically investigated the impact of IBA on the expression of PD-L1 in CRC cells. Concurrently, employing RNA silencing technology and small-molecule inhibitors, we delved into the molecular intricacies underlying the regulatory axis of IBA involving ROCK1/c-Myc/PD-L1. Furthermore, through flow cytometry analysis, we examined the alterations in the tumor immune microenvironment following anti-PD-L1 antibody therapy in a murine tumor model treated with IBA.

Results: Elevated levels of IBA were found to robustly activate PD-L1 expression in CRC cells both in vitro and in vivo, concomitantly reshaping the tumor immune microenvironment. Subsequent mechanistic investigations unveiled that IBA, through its interaction and activation of ROCK1, promotes the activation of c-Myc, thereby enhancing the transcription of PD-L1. Silencing of ROCK1 and application of ROCK1 inhibitors effectively reversed the regulatory effects of IBA on PD-L1. Additionally, IBA inhibited the activity of infiltrating CD8 T cells, resulting in diminished antitumor immunity and attenuating the sensitivity to anti-PD-L1 therapy.

Conclusion: Our study elucidates a novel mechanism by which IBA inhibits the sensitivity of CRC to anti-PD-L1 antibody therapy. Emphasizing IBA and its downstream pathways as potential therapeutic targets for immune therapy resistance mechanisms, our findings provide a novel theoretical foundation for overcoming immune therapy resistance.