The calcium ionophore ionomycin induces apoptosis-like eventsin the human embryonic kidney cell line at early times. Plasmamembrane blebbing, mitochondrial depolarization, externalizationof phosphatidylserine, and nuclear permeability changes can all beobserved within 15 min of treatment. However, there is no activa-tion of caspases or chromatin condensation. Expression of a fusionprotein containing the enhanced green fluorescent protein (EGFP)and human cytosolic Group IVA phospholipase A 2 (EGFP-cPLA 2 ) in these cells prevents ionomycin-induced phosphatidyl-serine externalization and death. Cells expressing the cPLA 2 mutant D43N, which does not bind calcium, retain their suscepti-bility to ionomycin-induced cell death. Both nonexpressing andEGFP-D43N-cPLA 2 -expressing human embryonic kidney cellscan be spared from ionomycin-induced cell death by pretreatingthem with exogenous arachidonic acid. Moreover, during calciumoverload, mitochondrial depolarization is significantly lower in theEGFP-cPLA 2 -expressing cells than in cells expressing normalamounts of cPLA 2 . These results suggest that early cell deathevents promoted by an overload of calcium can be prevented by thepresence of high levels of arachidonic acid.
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