Transmembrane Protein TMEM59L Modulates 5‐ FU Resistance via PTPRN ‐Mediated DNA Damage Repair in Colorectal Cancer

ABSTRACT Background Chemotherapy resistance in colorectal cancer (CRC) is often mediated by enhanced DNA damage repair (DDR). Transmembrane protein TMEM59L is implicated in cancer progression, but its role in CRC chemoresistance is unclear. We investigated whether TMEM59L regulates 5‐fluorouracil (5‐FU) sensitivity through PTPRN‐mediated DDR. Aims This study aimed to investigate the role of TMEM59L in regulating PTPRN‐mediated DNA damage repair and its impact on 5‐FU sensitivity in colorectal cancer, with the goal of identifying potential therapeutic targets to overcome chemoresistance. Methods and Results Bioinformatics analyses assessed TMEM59L/PTPRN expression and prognosis in CRC cohorts. Gain‐ and loss‐of‐function experiments were conducted in CRC cell lines (HCT116, SW480) and their 5‐FU‐resistant derivatives. Cell proliferation, migration, invasion, apoptosis, DNA damage, and reactive oxygen species (ROS) were measured. Protein interactions were examined by Western blot and immunofluorescence. A xenograft model in nude mice validated the TMEM59L/PTPRN axis on tumor growth, stemness, and EMT markers. TMEM59L expression was elevated in metastatic lesions and associated with poor CRC patient survival. Functionally, TMEM59L promoted malignant behaviors and epithelial–mesenchymal transition. It was upregulated in 5‐FU‐resistant cells and non‐responsive patients. TMEM59L knockdown sensitized cells to 5‐FU, increasing ROS, DNA damage, and apoptosis, while its overexpression induced resistance. Mechanistically, TMEM59L regulated 5‐FU‐induced DNA damage and ROS through PTPRN. PTPRN overexpression reversed the sensitization caused by TMEM59L silencing. In vivo, TMEM59L knockdown enhanced 5‐FU's antitumor effect, which was counteracted by PTPRN overexpression. Conclusion The TMEM59L/PTPRN axis is a key regulator of DDR and 5‐FU resistance in CRC. TMEM59L promotes chemoresistance via PTPRN by enhancing DNA repair and reducing ROS‐mediated apoptosis. Targeting this pathway may offer a novel strategy to overcome 5‐FU resistance.