The appearance of phosphatidylserine (PS) on the outer surface of apoptotic cells is an important signal for their ingestion. In platelets, elevation of intracellular Ca 2÷ with thapsigargin can trigger large amounts of PS exposure within minutes. We detected PS exposure in U937 promonocytes and J urkat T-cells after incubation with thapsigargin, but in only 10% of the cells, and it took up to 6 h to occur. Tumor necrosis factor and anti-Fas antibody rapidly trigger apoptosis in these cells, and chelation of extracellular Ca 2+ with 5 mM EGTA inhibited PS exposure by 65% and 50%, respectively. Chelation of intracellular Ca 2+ with BAPTA-AM had no effect. Other parameters of apoptosis, including cell blebbing, shrinkage, nuclear fragmentation, activation of the ICE-like proteases, and fodrin cleavage, were not inhibited by extracelhilar EGTA. We conclude that while an elevation of intracellular Ca 2+ is an ineffective trigger of apoptosis in the cells investigated, extracellular Ca 2+ is required for efficient PS exposure during apoptosis.Key words: Apoptosis; Phosphatidylserine; Ca2+; l'hapsigargin; Tumor necrosis factor; Anti-fas
~. IntroductionCells undergoing apoptosis are ingested by both neighbour~ng cells and professional phagocytes. This prevents any initammation and tissue damage that could occur upon lysis of ~he dying cell. One important mechanism for the recognition , ~f apoptotic cells is the expression of phosphatidylserine (PS) ~n their outer surface [I]. PS is normally constrained to the nner leaflet of the plasma membrane [2], but disruption of ,his asymmetry has been reported in a variety of cell types mdergoing apoptosis [3][4][5][6][7][8][9].It has been known for many years that PS is exposed on the ,,urface of activated platelets, where it plays an important role n the clotting cascade [10]. PS can also be exposed by red ~lood cells and is involved in their clearance from circulation 11]. Platelet activation causes rapid increase in intracellular 2a 2+, and pharmacological modulation with Ca 2+ ionophores ~r Ca2+-ATPase inhibitors can stimulate the exposure of large lmounts of PS by platelets within minutes [12]. Ca 2+ inacti- vates the aminophospholipid translocase which normally constrains PS to the inner leaflet [13], but this alone is insufficient for exposure. Another protein is proposed to mediate the scrambling of membrane phospholipids [14], although there is debate as to whether there is a general phospholipid scrambling or a specific translocation of PS [15]. Reconstituted membranes have been used to study these mechanisms [16], and recently a 37 kDa protein has been extracted from red cell ghosts that can transport labelled phospholipids upon triggering with CaA variety of studies implicate Ca 2+ in the regulation of apoptosis [18]. Disruption of Ca 2+ homeostasis can trigger apoptosis [19], an increase in the concentration of intracellular Ca 2~ has been reported in some apoptotic models [20], and Ca 2+ chelators have been observed to block apoptosis [21]. In other models...