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 programmed cell death and plays an important role in the recognition and removal of apoptotic cells. In this study with U937 cells, PtdSer synthesis from [(3)H]serine was stimulated and newly synthesized PtdSer was transferred preferentially to cell-free medium vesicles (CFMV) from cells when apoptosis was induced with a topoisomerase I inhibitor, camptothecin (CAM). When CAM-induced apoptosis was blocked by a caspase inhibitor, z-VAD-fmk, stimulation of PtdSer synthesis and movement to CFMV were abolished. In contrast, changes in synthesis and transport of sphingomyelin (SM) or phosphatidylethanolamine (PtdEtn) were minor; total phosphatidylcholine (PtdCho) synthesis was below control levels. All phospholipids appeared in CFMV but PtdSer displayed 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 observed in apoptotic cells induced by UV irradiation or tumor necrosis factor-alpha/cycloheximide treatment. Thus, PtdSer biosynthesis stimulated during apoptosis in U937 cells was specific for this phospholipid and was correlated with caspase-mediated exposure of PtdSer at the cell surface and preferential movement to vesicles during apoptosis.
Yu
A
A
Atlantic Research Centre, Departments of Pediatrics and Biochemistry and Molecular Biology, 5849 University Avenue, Dalhousie University, B3H 4H7, Halifax, NS, Canada.
Byers
D M
DM
Ridgway
N D
ND
McMaster
C R
CR
Cook
H W
HW
D017209:Apoptosis; D002166:Camptothecin; D061945:Caspase Inhibitors; D020169:Caspases / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D002474:Cell-Free System; D004791:Enzyme Inhibitors; D005089:Exocytosis; D006801:Humans; D010718:Phosphatidylserines / Q000096:biosynthesis / Q000378:metabolism*; D014407:Tumor Cells, Cultured; D020298:U937 Cells; D014466:Ultraviolet Rays
D061945:Caspase Inhibitors; D004791:Enzyme Inhibitors; D010718:Phosphatidylserines; D020169:Caspases; D002166:Camptothecin