GALNT5 drives colorectal cancer progression and chemoresistance via PI3K/Akt/ABCC1 axis

BACKGROUND: Polypeptide N-acetylgalactosaminyltransferase 5 (GALNT5), a member of the GALNT enzyme family, has been implicated in the pathogenesis of various malignancies. However, its precise role in colorectal cancer (CRC) progression and its contribution to acquired resistance to the FOLFOX components (oxaliplatin and 5-fluorouracil) remains poorly understood. This study aimed to elucidate the biological functions of GALNT5 in CRC pathogenesis and chemoresistance. METHODS: GALNT5 expression in CRC cell lines was evaluated through integrated bioinformatics analyses, RNA sequencing (RNA-Seq), RT-qPCR, and Western blotting. Isogenic CRC sublines resistant to oxaliplatin (LoVo/L) or 5-fluorouracil (LoVo/5FU) were established separately by chronic exposure to each drug. RNA-Seq was employed to identify downstream signaling pathways and effector genes modulated by GALNT5. VVL lectin pull-down assays were performed to confirm that GALNT5 mediates O-GalNAc glycosylation of EGFR-an upstream activator of the PI3K/Akt signaling pathway. Functional assays, including CCK-8 and Transwell, were conducted to assess cell proliferation, apoptosis, and invasion. The involvement of the PI3K pathway was examined using the PI3K agonist 740Y-P, while expression changes of the downstream effector ABCC1 were analyzed both in vitro and in vivo. RESULTS: GALNT5 was significantly upregulated in CRC cell lines compared to the normal colonic epithelial cell line NCM460. Knockdown of GALNT5 markedly suppressed proliferation and invasion, while promoting apoptosis in both parental and chemoresistant CRC cells. Silencing GALNT5 enhanced the sensitivity of resistant cells to oxaliplatin (L-OHP) and 5-fluorouracil (5FU), an effect that was reversible upon PI3K pathway activation, indicating the involvement of the PI3K/Akt axis. Mechanistically, GALNT5 activated the PI3K/Akt axis through O-GalNAc glycosylation of EGFR. Furthermore, GALNT5 depletion led to downregulation of the multidrug resistance-associated protein ABCC1 in vitro and in vivo. CONCLUSION: GALNT5 facilitates malignant progression in CRC by promoting O-GalNAc glycosylation of EGFR and thereby activating the PI3K/Akt pathway, and confers resistance to L-OHP and 5FU through upregulation of ABCC1. These findings suggest GALNT5 as a potential diagnostic biomarker and a promising therapeutic target for overcoming chemoresistance in CRC.