title: Regulation of phosphatidylserine exposure at the cell surface by the serine--base exchange enzyme system during CD95-induced apoptosis.
abstract: Early in the apoptotic process, CD95 induces a translocation of phosphatidylserine (PtdSer) from the inner to the outer leaflet of the cellular plasma membrane. In mammalian cells, PtdSer is only synthesized through a calcium-dependent exchange of the polar head group of pre-existing phospholipids, either phosphatidylcholine or phosphatidylethanolamine, by a serine. Using a pharmacological approach, we examined the influence of PtdSer synthesis on CD95-induced PtdSer exposure at the surface of Jurkat cells. We found that CD3/TCR triggering or thapsigargin treatment of Jurkat cells was accompanied both by a decreased PtdSer synthesis and by a strong reduction of CD95-induced PtdSer at the cell surface, as monitored by fluorescence-activated cell sorting (FACS) analysis of annexin V-fluorescein isothiocyanate (FITC)-labeled cells. PtdSer synthesis through the serine-base exchange enzyme system thus appeared as one of the mechanisms implicated in the recently discovered CD3/TCR-induced down-regulation of CD95-induced apoptosis. Conversely, increasing the activity of the serine-base exchange enzyme system with different drugs, either the K+ channel blocker quinine, the cationic amphiphil stearylamine, or three different calmodulin antagonists, chlorpromazine, trifluoperazine, and N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide (W7), resulted in an increased appearance of PtdSer at the surface of CD95-treated cells. Both PtdSer synthesis and CD95-induced annexin V-FITC reactivity were abrogated in ATP-depleted cells. Also, modifying the membrane potential with valinomycin (hyperpolarization) or either gramicidin or KCl (depolarization) demonstrated a strong relationship between PtdSer synthesis and annexin V-FITC reactivity in CD95-treated cells. Together, our results indicate that CD95-induced exposure of PtdSer at the cell surface could be regulated by the activity of the serine-base exchange enzyme system.
authors.affiliation: INSERM U343 and IFR 50, Hôpital de l'Archet, Nice, France.
authors.forename: C
authors.initials: C
authors.lastname: Pelassy
authors.forename: J P
authors.initials: JP
authors.lastname: Breittmayer
authors.forename: C
authors.initials: C
authors.lastname: Aussel
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D017209:Apoptosis; D006801:Humans; D019169:Jurkat Cells; D007700:Kinetics; D008213:Lymphocyte Activation; D008564:Membrane Potentials; D019884:Nitrogenous Group Transferases / Q000378:metabolism*; D010718:Phosphatidylserines / Q000096:biosynthesis / Q000502:physiology*; D012694:Serine; D013601:T-Lymphocytes / Q000187:drug effects / Q000502:physiology; D019014:fas Receptor / Q000378:metabolism*
chemical_list: D010718:Phosphatidylserines; D019014:fas Receptor; D012694:Serine; D000255:Adenosine Triphosphate; D019884:Nitrogenous Group Transferases; C022370:phospholipid serine base exchange enzyme