Metformin induces microRNA-34a to downregulate the Sirt1/Pgc-1α/Nrf2 pathway, leading to increased susceptibility of wild-type p53 cancer cells to oxidative stress and therapeutic agents

Sirtuin 1 (Sirt1) plays an important role in cellular redox balance and resistance to oxidative stress. Sirt1 exhibits oncogenic properties in wild-type p53 cancer cells, whereas it acts as a tumor suppressor in p53-mutated cancer cells. Here, we investigated the effects of metformin on Sirt1 expression in several cancer cell lines. Using human cancer cell lines that exhibit differential expression of p53, we found that metformin reduced Sirt1 protein levels in cancer cells bearing wild-type p53, but did not affect Sirt1 protein levels in cancer cell lines harboring mutant forms of p53. Metformin-induced p53 protein levels in wild-type p53 cancer cells resulted in upregulation of microRNA (miR)-34a. The use of a miR-34a inhibitor confirmed that metformin-induced miR-34a was required for Sirt1 downregulation. Metformin suppressed peroxisome proliferator-activated receptor γ (PPARγ) coactivator-1α (Pgc-1α) expression and its downstream target Nrf2 in MCF-7 cells. Genetic tools demonstrated that the reduction of Sirt1 and Pgc-1α by metformin caused Nrf2 downregulation via suppression of PPARγ transcriptional activity. Metformin reduced heme oxygenase-1 and superoxide dismutase 2 but upregulated catalase expression in MCF-7 cells. Metformin-treated MCF-7 cells had no increase in basal levels of reactive oxygen species but were more susceptible to oxidative stress. Furthermore, upregulation of death receptor 5 by metformin-mediated Sirt1 downregulation enhanced the sensitivity of wild-type p53 cancer cells to TRAIL-induced apoptosis. Our results demonstrated that metformin induces miR-34a to suppress the Sirt1/Pgc-1α/Nrf2 pathway and increases susceptibility of wild-type p53 cancer cells to oxidative stress and TRAIL-induced apoptosis.