The Plasma Membrane Is the Site of Selective Phosphatidylserine Oxidation During Apoptosis: Role of Cytochrome c

Abstract

Phosphatidylserine (PS) externalization, a functional end point of apoptosis that triggers phagocytic recognition of dying cells, may be modulated by oxidative stress in biological membranes. We previously observed selective oxidation of PS during apoptosis, but the intracellular location and molecular mechanisms responsible for PS oxidation remain to be described. Peroxidation in individual classes of cellular phospholipids was monitored in whole cells and various subcellular fractions obtained from HL-60 cells undergoing apoptosis in response to tert-butyl hydroperoxide (t-BuOOH) after metabolic acylation of phospholipids with the oxidation-sensitive fluorescent fatty acid, cis-parinaric acid. Nonrandom selective oxidation of PS was observed in whole cells, as well as in plasma membrane. PS in mitochondria appeared selectively resistant to oxidation during apoptosis. All phospholipids in nuclear membranes appeared resistant to oxidation after t-BuOOH treatment. Selective PS oxidation was accompanied by cytochrome c release and PS externalization. PS oxidation and externalization were followed by caspase activation and other end points of apoptosis. HL-60 cells "loaded" with exogenous cytochrome c by mild sonication showed selective oxidation of PS in both the absence and presence of t-BuOOH. Cytochrome c/hydrogen peroxide could effectively oxidize purified PS but not phosphatidylcholine in a cell-free model system. Selective plasma membrane-based PS oxidation and subsequent externalization during oxidant-induced apoptosis may be mediated through the redox activity of cytochrome c.