Ferroptosis is a newly characterized form of cell death that is caused by the accumulation of intolerable levels of lipid peroxides in the cell via an iron-mediated Fenton reaction. The resulting lipid peroxides that are generated in membrane phospholipids cause ferroptotic cell death by disturbing the integrity of the plasma membrane. Glutathione (GSH), a tripeptide redox molecule that contains a cysteine (Cys) unit in the center, plays a pivotal role in protection against ferroptosis. Cys is metabolically produced by the transsulfuration pathway in conjunction with methionine (Met) metabolism and may fulfil the requirement in some organs under normal physiological conditions. In addition to protein synthesis, Met is the precursor for S-adenosylmethionine (SAM), which donates a methyl group to several acceptor molecules such as DNA, RNA, proteins, and phospholipids. Cell growth is arrested in Met-free medium, which characteristically occurs in cancer cells. Because Met is an essential amino acid, a defect in Met supply could impair the protein synthesis required for cell cycle progression.
　Despite the significant roles of Met in Cys metabolism and cancer biology, the interplay between Met metabolism and ferroptosis in cancer cells has not been fully investigated. We had speculated that a combined deprivation of Met and cystine induces ferroptosis more effectively in vitro. In the current study, however, we found the Met/cystine double deprivation strongly prevented the execution of ferroptosis under conditions of intracellular Cys/GSH starvation, which led to the survival of HeLa cells as well as Hepa 1–6 cells. Supplementation of SAM resulted in the increased production of peroxidized lipids and induced ferroptosis in cells under double deprived conditions. On the other hand, SAM supplementation also increased DNA methylation and allowed cell cycle progression to resume. These collective results reveal the pivotal roles of lipid peroxides, the concentrations of which are elevated during cell cycle progression, in ferroptosis execution under Cys starvation conditions.