Oxidized LDLs Induce Massive Apoptosis of Cultured Human Endothelial Cells Through a Calcium-Dependent Pathway

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Abstract
Oxidized LDLs are thought to play a central role in atherogenesis. Among their wide variety of biological properties, oxidized LDLs exhibit a cytotoxic effect on cultured vascular cells. Toxic doses of mildly oxidized LDLs elicited massive apoptosis in both primary and immortalized cultures of endothelial cells as shown by characteristic morphological and biochemical changes. Cytoplasmic and nucleic modifications (eg, chromatin condensation and nucleus fragmentation) were visualized by using electron and fluorescence microscopy of intact cells labeled by the fluorescent DNA probe SYTO-11. DNA fragmentation was quantified by ultracentrifugation of chromatin fragments, evaluated in situ by using the TUNEL (Terminal transferase-mediated dUTP-biotinnickendlabeling) procedure, and visualized by electrophoresis of radiolabeled DNA fragments showing the characteristic apoptotic ladder. Apoptotic cells became rapidly detached and underwent postapoptotic necrosis that led to cell disintegration. Apoptosis was subsequent to a sustained and delayed peak of cytosolic calcium. Both the calcium peak and apoptosis were blocked by chelating the extracellular calcium with EGTA or by inhibiting the calcium influx by the calcium-channel blockers nifedipine and nisoldipine, thus suggesting that the apoptotic process induced by oxidized LDLs is clearly calcium dependent. Aurintricarboxylic acid, an inhibitor of endonucleases, also blocked the apoptotic process without blocking the calcium peak. These results suggest that toxic doses of mildly oxidized LDLs induce massive apoptosis of endothelial cells through a calcium-dependent mechanism and that this apoptotic process can be prevented by inhibiting the rise of cytosolic calcium or by inhibiting cellular endonucleases by aurintricarboxylic acid.

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