PLASMA-MEMBRANE PHOSPHOLIPID ASYMMETRY PRECEDES DNA FRAGMENTATION IN DIFFERENT APOPTOTIC CELL MODELS

Biochemical alterations occurring in many cell types during apoptosis include the loss of plasma membrane phospholipid asymmetry and nuclear DNA fragmentation. Annexin V staining detects phosphatidylserine tran slocation into the outer plasma membrane layer occurring during cell d eath, while the in situ tailing (IST or TUNEL) reaction labels the DNA strand breaks typical of apoptosis. To compare the time course of the se processes we investigated methylprednisolone-induced apoptosis of r at thymocytes, topoisomerase inhibitor-induced apoptosis in the human histiocytic lymphoma cell line U937, and serum deprivation-induced apo ptosis in the rat pheochromocytoma cell line, PC12. At all time points , FAGS analysis and quantitative fluorescence light microscopy showed a higher proportion of annexin V-positive than IST-positive cells, wit h significantly different time courses in the apoptotic cell models in vestigated (Anova test). Results were confirmed by confocal microscopy . Our data indicate that the exposure of phosphatidylserine, a potenti al phagocyte recognition signal on the cell surface of apoptotic cells in vivo, precedes DNA strand breaks during apoptosis in different cel l types.