Resistance to UV-induced apoptosis in Chinese-hamster ovary cells overexpressing phosphatidylserine synthases.
Externalization of PtdSer (phosphatidylserine) is an important event in signalling removal of apoptotic cells. In contrast with previous work [Yu, Byers, Ridgway, McMaster and Cook (2000) Biochim. Biophys. Acta 1487, 296-308] with U937 cells showing that specific stimulation of PtdSer biosynthesis during apoptosis was caspase dependent, PtdSer biosynthesis in CHO (Chinese-hamster ovary)-K1 increased 2.5-fold during UV-induced apoptosis but was not reversed by a caspase inhibitor, Z-VAD-FMK (benzyloxycarbonyl-Val-Ala-DL-Asp-fluoromethylketone). Also, in CHO-K1 cells, stimulation of synthesis was less specific for PtdSer as similar levels of stimulation were observed for sphingomyelin biosynthesis. Involvement of PtdSer synthase isoforms was tested in CHO-K1 cells overexpressing PSS I (PtdSer synthase I) and PSS II. Both types of transformed cells showed resistance to UV-induced apoptosis based on the decreased levels of caspase 3 activation and morphology changes; externalization of PtdSer was reduced with UV treatment even though expression of endogenous scramblase increased slightly. Serine-labelling experiments showed that PSS I- or PSS II-expressing cells had higher basal levels of PtdSer biosynthesis compared with vector control cells. When cells were exposed to UV light to induce apoptosis, PtdSer biosynthesis was further stimulated 1.5- and 2-fold in PSS I- and PSS II-expressing cells respectively compared with UV-treated vector cells. Caspase activation was not required, as Z-VAD-FMK did not change PtdSer synthesis. Although enhanced PtdSer synthesis was supposed to facilitate apoptosis, cells overexpressing PSS I and II were actually resistant to UV-induced apoptosis. Whereas enhanced PtdSer synthesis was associated with apoptosis, potential anti-apoptotic effects were observed when excess activity of these synthetic enzymes was present. This suggests a tightly regulated role for PtdSer synthesis and/or an important dependence on compartmentation of PSS enzymes in association with scramblase facilitated enrichment of this phospholipid at the cell surface.
Yu
Anan
A
Department of Pediatrics, Atlantic Research Centre, Dalhousie University, Halifax, NS, Canada B3H 4H7.
McMaster
Christopher R
CR
Byers
David M
DM
Ridgway
Neale D
ND
Cook
Harold W
HW
D000818:Animals; D017209:Apoptosis / Q000528:radiation effects*; D016466:CHO Cells / Q000201:enzymology / Q000378:metabolism* / Q000528:radiation effects*; D020169:Caspases / Q000378:metabolism; D002460:Cell Line; D006224:Cricetinae; D003412:Cricetulus; D019884:Nitrogenous Group Transferases / Q000096:biosynthesis*; D010743:Phospholipids / Q000096:biosynthesis; D017386:Sequence Homology, Amino Acid; D012694:Serine / Q000096:biosynthesis; D014466:Ultraviolet Rays
D010743:Phospholipids; D012694:Serine; D019884:Nitrogenous Group Transferases; C111493:Ptdss2 protein, mouse; C022370:phospholipid serine base exchange enzyme; D020169:Caspases