Metformin increases the radiosensitivity of non-small cell lung cancer cells by destabilizing NRF2

Radiation resistance is an obstacle to the successful treatment of lung cancer. Metformin, a first-line antidiabetic drug, has been studied for its potential use in radiotherapy, as several lines of evidence suggest that metformin enhances radiation sensitivity of cancer cells. However, the underlying mechanisms by which metformin exerts its radiosensitization effects on non-small cell lung cancer (NSCLC) cells remain obscure. Here, we confirmed that metformin increases the radiosensitivity of NSCLC cells and radiation-resistant NSCLC cells. Furthermore, we identified nuclear factor erythroid 2-related factor 2 (NRF2) as a critical target of radiosensitization effect of metformin, as the radiosensitization effect was abolished in NRF2 knockout cells. We also showed that metformin treatment increased the ubiquitination and proteasomal degradation of NRF2 through a KEAP1-independent mechanism. The decrease of NRF2 led to reduced transcription of downstream antioxidant-related proteins, inhibited the initiation of DNA damage repair pathways, and compromised G2/M phase arrest after radiation. In an orthotopic transplanted tumor model in nude mice, metformin treatment reduced NRF2 levels and led to fewer lung tumor nodules. Combination of irradiation further potentiated the antitumor efficacy compared to each of the single treatments. In conclusion, our results suggest that the degradation of NRF2 that is induced by metformin may play a pivotal role in radiosensitizing NSCLC cells and that metformin can be developed as a sensitizer of radiotherapy against lung cancer.