Metformin reverses chemoresistance in non-small cell lung cancer via accelerating ubiquitination-mediated degradation of Nrf2.

The therapeutic efficacy of cisplatin-based chemotherapy for non-small cell lung cancer (NSCLC) is limited by drug resistance. In NSCLC, hyperactivation of nuclear factor erythroid 2-related factor 2 (Nrf2) counteracts oxidative stress to promote chemoresistance. Metformin-mediated downregulation of Nrf2 plays a pivotal role in overcoming drug resistance in NSCLC cells. Therefore, a deeper understanding of the molecular mechanisms of combination therapy and the role of Nrf2 in chemotherapeutic response is critical to clinical translation. The effects of combination therapy with metformin and cisplatin on cell proliferation and apoptosis, intracellular reactive oxygen species (ROS) levels, and xenograft tumor formation were analyzed in NSCLC cells. Co-immunoprecipitation (co-IP) and Phos-tag assays were used to explore the mechanism of metformin-mediated Nrf2 suppression. Immunohistochemical (IHC) staining was performed to detect Nrf2 expression in matched tumor samples before and after neoadjuvant chemotherapy. Metformin was observed to synergistically augment cisplatin-induced cytotoxicity by strongly inhibiting the level of Nrf2, thereby weakening the antioxidant system and detoxification ability of Nrf2 and enhancing ROS-mediated apoptosis in NSCLC. The synergistic antitumor effect of combination therapy is blocked by treatment with the ROS scavenger N-acetyl cysteine (NAC) as well as overexpression of Nrf2 and its downstream antioxidant protein. Mechanistically, metformin extensively dephosphorylates Nrf2 by attenuating the interaction between Nrf2 and extracellular signal-regulated kinases 1/2 (ERK1/2), which then restores its polyubiquitination and accelerates its proteasomal degradation. Moreover, for the first time, an association of non-decreased Nrf2 expression in patients after neoadjuvant chemotherapy with poor survival and chemoresistance in NSCLC was revealed. Our findings illustrate the mechanism of metformin-mediated Nrf2 degradation through posttranslational modifications (PTMs), which weakens the ROS defense system in NSCLC. Fluctuations in Nrf2 expression have a strong predictive ability for chemotherapeutic response in neoadjuvant NSCLC patients. Targeting of the Nrf2 pathway could be a therapeutic strategy for overcoming chemoresistance, with metformin as the first choice for this strategy.