Ferroptosis is determined by chloride ions

Ferroptosis has emerged as a deliberate type of programmed cell death that manifests marked importance ubiquitously in health and diseases. However, after a decade of research, the mechanisms of ferroptosis execution remain unclear. Here we identify chloride ions (Cl - ) as essential determinants of ferroptosis. Water homeostasis manipulated by extracellular solute concentration disrupts ferroptotic cell death. Hyperosmotic stress attenuates ferroptosis and endues cells with high lipid peroxidation. Analyses of a fluorescent chloride probe show that Cl - fluxes into the cytoplasm during ferroptosis, substantiating a role for Cl - to drive water flow. Depletion of extracellular chloride ions ([Cl - ] o ) from culture media congruously confers resistance to ferroptosis. The [Cl - ] o -depleted ferroptotic cells fall into two populations: cells with low lipid peroxidation; cells with high lipid peroxidation but not cell swelling or cell rupture. Contrarily, solitary [Cl - ] o overload is sufficient to elicit ferroptosis without canonical ferroptosis inducers. Further experiments show that ferroptotic cells depolarize and [Cl - ] o is positively correlated with this process. Membrane depolarization upregulates the level of lipid peroxidation, suggesting that membrane potential may be a universal mechanism governing ferroptosis. Together, our findings reveal that ferroptosis is determined by chloride ions.