Abstract
Immunotherapy including anti-PD-1 demonstrated therapeutic promise to colorectal cancer (CRC) patients, but tumor cell resistance limits their efficacy. Butyrate may influence therapeutic outcomes by modulating tumor metabolism, but it remains unclear whether butyrate influences CRC cell resistance to anti-PD-1 therapy. We aimed to investigate whether butyrate promotes resistance to anti-PD-1 therapy in CRC and underlying metabolic and immunologic mechanisms. CRC murine models were established by subcutaneously inoculating MC38 cells or butyrate/anti-PD-1-administered tumor cells of mice, followed by treatment with butyrate, anti-PD-1, or a combination. Therapeutic efficacy was assessed by tumor growth and survival outcomes. In vitro, HCT116 cells were exposed to monotherapy or co-therapy regimens. Carnitine Palmitoyltransferase 1A (CPT1A) knockdown was conducted by shRNA transfection both in vivo and in vitro. Fatty acid oxidation (FAO) was determined by oxygen consumption rate and CPT1A expression. CD8+ T cell cytotoxicity assays and CD8 expression in tumors were performed to evaluate immune cell infiltration. The addition of butyrate into anti-PD-1 treatment combination did not improve survival or reduce tumor volume compared to anti-PD-1 alone, with a marked activation of CPT1A observed in treated tumor tissues. Butyrate significantly elevated FAO, contributing to elevated oxygen consumption rate and reduced CD8+ T cell cytotoxicity. However, in sh-CPT1A models, the combination therapy significantly improved antitumor efficacy and restored CD8+ T cell infiltration. Furthermore, CRC patient samples resistant to anti-PD-1 therapy exhibited elevated CPT1A levels. Butyrate-induced CPT1A-mediated FAO promotes resistance to anti-PD-1 therapy in CRC, suggesting that targeting CPT1A might enhance the efficacy of immunotherapy.