Calcium and Its Role in the Nuclear Translocation and Activation of Cytosolic Phospholipase A2 in Cells Rendered Sensitive to TNF-Induced Apoptosis by Cycloheximide

Abstract

In these experiments, we investigated the role of calcium as a second messenger in the apoptotic activation of cytosolic phospholipase A2 (cPLA2). As our model, we used a murine fibroblast cell line (C3HA) that was induced to undergo apoptosis by a combination of TNF and cycloheximide. Using fura 2 Ca2+ imaging, we found strong evidence for an intracellular calcium response after 1 h of treatment, which correlated with the onset of phosphatidylserine externalization, but preceded effector procaspase processing by several hours. The response was strongest in the perinuclear region, where mean levels rose 83% (144 ± 14 nM in untreated cells vs 264 ± 39 nM in treated), while cells displaying morphological evidence of apoptosis had the highest levels of calcium (250–1000 nM). Verapamil blocked this response, indicating an extracellular source for the calcium. Fluorescence microscopy revealed a pattern of nuclear translocation of cPLA2 during apoptosis, which was also blocked by verapamil, indicating an important role for calcium in this process. In addition, we found that verapamil prevented the release of [3H]arachidonic acid from C3HA cells induced to undergo apoptosis by the chemotherapeutic agents vinblastine, melphalan, and cis-platinum. Together, these data suggest that calcium is important for cPLA2 activation by diverse apoptotic stimuli.