Metformin reverses 5-FU resistance induced by radiotherapy through mediating folate metabolism in colorectal cancer.

PURPOSE: Radiation therapy has revolutionized the treatment of primary or liver metastases in colorectal cancer (CRC). In colorectal cancer, conventional fractionation (1.8 ~ 2.0 Gy daily) is typically used for treatment. Nevertheless, there is a paucity of research investigating the potential implications of radiation therapy-induced alterations in the expression levels of regulatory genes on resistance to chemotherapy agents. Herein, we explored the mechanism by which conventional fractionation drives 5-fluorouracil (5-FU) resistance and metformin (Met) rescued 5-FU resistance in CRC. METHODS AND MATERIALS: RNA sequencing, differential genes expression analysis was performed to identify the 5-FU resistance genes after irradiation (according to the convention of cell irradiation, 2 Gy × 8 scheme was selected). Drug sensitivity assay, immunofluorescence staining, folate analogs concentration measurement was used to explore the biological function of histocompatibility minor 13 (HM13) and γ-Glutamyl Hydrolase (GGH). Combined chemosensitivity test and xenograft mouse model has been used to gain insights into the underlying clinical value of the combination of 5-FU and Met. RESULTS: The conventional fractionation scheme (2 Gy × 8) induced resistance to 5-FU in the CRC cell line HCT-15, accompanied by an elevated RNA expression level of peptidase HM13. Mechanistically, the increased expression of HM13 caused an abnormal shearing of the N-terminal signal peptide of γ-Glutamyl Hydrolase (GGH), which resulted in decreased intracellular content of 5, 10-methylenetetrahydrofolate (5,10-CH(2)-THF). CONCLUSION: We revealed a new mechanism of 5-FU resistance induced by irradiated with 2 Gy × 8 through the HM13-GGH-5,10-CH(2)-THF axis. The synergistic effect of Met and 5-FU can rescue 5-FU resistance after conventional fractionated irradiation. In summary, this work will help to reveal the mechanisms of IR-induced 5-FU resistance, which is important for finding new therapeutic targets and improving the efficacy of chemotherapy regimens after radiotherapy.