Divergent Roles of PIK3C3 in KRAS ‐Wild‐Type and KRAS ‐Mutant Advanced Colorectal Cancer: Implications for 5‐Fluorouracil Sensitivity and Prognosis

INTRODUCTION: Phosphatidylinositol 3-kinase catalytic subunit type 3 (PIK3C3) is central to autophagy, which supports cancer adaptation to environmental stresses and may contribute to drug resistance. KRAS-mutant cancers have been reported to require autophagy for survival. We investigated the influence of KRAS mutation status on the roles of PIK3C3 in colorectal cancer (CRC) cell viability and chemosensitivity in vitro, as well as its association with patient prognosis. METHODS: The clinical significance of PIK3C3 expression in relation to KRAS mutation status was evaluated by immunohistochemistry in patients with advanced CRC. Cell viability, colony formation, and spheroid formation assays were performed using CRC cell lines. RESULTS: In KRAS-wild-type (KRAS-WT) Caco-2 and Colo320 cells, PIK3C3 and the KRAS/ERK pathway appeared to positively regulate each other. In contrast, PIK3C3 inhibition did not reduce p-ERK levels or induce significant p62 accumulation in KRAS-mutant SW480 and HCT116 cells. PIK3C3 knockdown reduced cell viability but increased resistance to 5-fluorouracil (5-FU) in Caco-2 and Colo320 cells, whereas the opposite effect was observed in SW480 and HCT116 cells. Transfection with a KRAS-mutant plasmid sensitized Caco-2 cells to 5-FU accompanied by reduced thymidylate synthase (TS) and β-catenin levels following PIK3C3 inhibition. Clinically, high PIK3C3 expression was associated with KRAS mutation and poorer survival in patients with advanced CRC. Its adverse short-term prognostic association was more pronounced in KRAS-mutant CRC than in KRAS-WT CRC. CONCLUSION: Based on preclinical in vitro findings, PIK3C3 appears to support KRAS-WT and KRAS-mutant CRCs in different ways. Notably, PIK3C3 inhibition enhances 5-FU sensitivity in KRAS-mutant CRC cells, potentially through TS regulation, and warrants further investigation as a therapeutic strategy.