Metformin Inhibits Ferroptotic Cell Death by Suppressing Cysteine Deprivation-Induced Mitochondrial Dysfunction

Ferroptosis, an iron-dependent form of regulated cell death driven by excessive lipid peroxidation, has emerged as a promising therapeutic strategy for cancer. Metformin, a widely used biguanide antidiabetic drug, has recently been reported to induce ferroptosis and exert anticancer effects in various malignancies. However, metformin or its analogue phenformin did not induce ferroptotic cell death in H1299 non-small cell lung cancer cells and HEYA8 ovarian cancer cells for up to 24 h. Instead, both drugs attenuated cysteine deprivation-induced ferroptotic cell death, which also appeared to involve necroptotic signaling. This effect was accompanied by reductions in reactive oxygen species generation, iron accumulation, and lipid peroxidation at both the cellular and mitochondrial levels. Notably, metformin and phenformin suppressed cysteine deprivation-induced mitochondrial membrane hyperpolarization and increased oxygen consumption, suggesting suppression of mitochondrial bioenergetic activity associated with cysteine deprivation-induced ferroptosis. Cysteine deprivation markedly reduced glutathione (GSH) levels and glutathione peroxidase 4 (GPX4) expression. Although metformin alone also reduced intracellular GSH levels, metformin did not restore GSH levels or GPX4 expression under cysteine deprivation despite suppressing ferroptosis, indicating that its anti-ferroptotic effects are independent of the canonical GSH-GPX4 axis. Collectively, these findings underscore that the use of biguanides should be carefully considered when applying cysteine deprivation-based ferroptosis-inducing strategies in cancer.