GPx4 is the controller of a specific form of programmed cell death executed by lipid peroxidation products

Abstract

The recent identification of a routine of non-apoptotic controlled cell death recapitulates in a physiological perspective the long history of oxygen toxicity and lipid peroxidation. Due to the critical role of iron, this form of controlled non-apoptotic cell death had been named ferroptosis1. Ferroptosis executes cell death in major neurodegenerative diseases and ischemia-reoxygenation and accounts for of cytotoxicity of drug candidates for cancer treatment2. Moreover, evidence on embryo development indicates that ferroptosis is also involved in tissue homeostasis3. What we know is that oxygen, phospholipids containing ω-6 fatty acids, iron and a lipoxygenase active on membrane lipids are required. We also know that ferroptosis can only be executed by inactivation the selenoperoxidase GPx44. The reduction to hydroperoxy derivatives of lipid hydroperoxides inserted in membranes is, indeed, the critical anti-peroxidant reaction5. Seemingly, in the emerging scenario, oxygen metabolically activated in mitochondria, slowly but continuously generates species competent for free radical oxidation of a polyunsaturated fatty acid, evolving into a lipid hydroperoxide. This activates a lipoxygenase active on membrane lipids to produce more hydroperoxides. By decomposition of lipid hydroperoxides, iron propagates chain reactions and generates the electrophiles (aldehydes?) alleged executing cell death. All these events can only, take place when the reaction of GPx4 becomes limiting. The mechanism of GPx4 reaction has been analyzed up to quantum-mechanical level and calculated structures have been corroborated by MS6. The interaction of GPx4 with membrane phospholipid has been elucidated by Surface Plasmon Resonance (SPR), supported by molecular dynamics (MD) analysis. We know now how GPx4 works on membranes. A strong electrostatic interaction takes place between specific amino acid residues in the cationic area on the surface of GPx4 and polar head of phospholipids. This binding drives the orientation of the hydroperoxidic group flipping out of the membrane to precisely interact with the redox center of the enzyme7. Redox catalysis is operated by proton tunneling leading to the formation of a charge-separated species6. Interaction of the oxidized selenium with two GSH molecules reacting in sequence lessens the interaction with polar head of phospholipids and permits the “surfing” of the enzyme on the membrane surface to catch and reduce lipid hydroperoxides7. In conclusion, metabolism of oxygen in mitochondria provides metabolic energy but can also kill the cells when not sufficiently protected by GPx4 and GSH. About the mechanisms of induction of ferroptosis under physiological conditions we know that GSH concentration can be controlled by the efficiency of the import of cystine for the synthesis of GSH. What is still largely unknown, instead, is how expression/activity of GPx4 could be controlled under physiological conditions.