Preferential externalization of newly synthesized phosphatidylserine in apoptotic U937 cells is dependent on caspase-mediated pathways

Externalization of phosphatidylserine (PtdSer) is a common feature of progr ammed cell death and plays an important role in the recognition and removal of apoptotic cells. In this study with U937 cells, PtdSer synthesis from [ H-3]serine was Stimulated and newly synthesized PtdSer was transferred pref erentially to cell-free medium vesicles (CFMV) from cells when apoptosis wa s induced with a topoisomerase I inhibitor, camptothecin (CAM). When CAM-in duced apoptosis was blocked by a caspase inhibitor, z-VAD-fmk, stimulation of PtdSer synthesis and movement to CFMV were abolished. In contrast, chang es in synthesis and transport of sphingomyelin (SM) or phosphatidylethanola mine (PtdEtn) were minor; total phosphatidylcholine (PtdCho) synthesis was below control levels. All phospholipids appeared in CFMV but PtdSer display ed a 6-fold increase relative to controls compared to 3-fold for SM, 2-fold for PtdCho and 1.8-fold for PtdEtn. Even greater effects on specificity of PtdSer synthesis, movement to CFMV and inhibition by z-VAD-fmk were observ ed in apoptotic cells induced by UV irradiation or tumor necrosis factor-al pha/cycloheximide treatment. Thus, PtdSer biosynthesis stimulated during ap optosis in U937 cells was specific for this phospholipid and was correlated with caspase-mediated exposure of PtdSer at the cell surface and preferent ial movement to vesicles during apoptosis. (C) 2000 Elsevier Science B.V. A ll rights reserved.