Oncosis is associated with exposure of phosphatidylserine residues on the outside layer of the plasma membrane: A reconsideration of the specificity of the annexin V/propidium iodide assay

Abstract

Following a lethal injury, two modes of cell death can be distinguished, apoptosis and primary necrosis. Cells pass through a prelethal stage characterized by a preservation of membrane integrity, in which they shrink (apoptosis) or swell (oncosis, the early phase of primary necrosis). During apoptosis, a loss of phospholipid asymmetry leads to exposure of phosphatidylserine (PS) residues on the outer leaflet of the plasma membrane. We examined whether the external PS exposure, initially supposed to be specific for apoptosis, was also observed in oncotic cells. Human peripheral lymphocytes, Jurkat T cells, U937 cells, or HeLa cells were submitted to either apoptotic or oncotic stimuli. PS external exposure was assessed after binding of FITC-conjugated annexin V as was the loss of membrane integrity after propidium iodide (PI) uptake. Morphological examination was performed by optical or electron microscopy. Similarly to apoptotic cells, oncotic cells expose external PS residues while preserving membrane integrity. Consequently, oncotic cells exhibit the annexin V+ PI- phenotype, previously considered to be specific for apoptotic cells. This study concludes that the annexin V/PI assay does not discriminate between apoptosis and oncosis and that it can be a useful tool to study oncosis by flow cytometry.