Involvement of extracellular calcium in phosphatidylserine exposure during apoptosis.

The appearance of phosphatidylserine (PS) on the outer surface of apoptotic cells is an important signal for their ingestion. In platelets, elevation of intracellular Ca2+ with thapsigargin can trigger large amounts of PS exposure within minutes. We detected PS exposure in U937 promonocytes and Jurkat 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 Ca2+ with 5 mM EGTA inhibited PS exposure by 65% and 50%, respectively. Chelation of intracellular Ca2+ 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 extracellular EGTA. We conclude that while an elevation of intracellular Ca2+ is an ineffective trigger of apoptosis in the cells investigated, extracellular Ca2+ is required for efficient PS exposure during apoptosis.