Resistance to UV-induced apoptosis in Chinese-hamster ovary cells overexpressing phosphatidylserine synthases

Biochem J. 2004 Aug 1;381(Pt 3):609-18. doi: 10.1042/BJ20031857.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Apoptosis / radiation effects*
  • CHO Cells / enzymology
  • CHO Cells / metabolism*
  • CHO Cells / radiation effects*
  • Caspases / metabolism
  • Cell Line
  • Cricetinae
  • Cricetulus
  • Nitrogenous Group Transferases / biosynthesis*
  • Phospholipids / biosynthesis
  • Sequence Homology, Amino Acid
  • Serine / biosynthesis
  • Ultraviolet Rays*


  • Phospholipids
  • Serine
  • Nitrogenous Group Transferases
  • Ptdss2 protein, mouse
  • phospholipid serine base exchange enzyme
  • Caspases