Oxidized Low-Density Lipoprotein Induces Apoptosis of Human Endothelial Cells by Activation of CPP32-Like Proteases

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Abstract
Background Oxidized LDL (oxLDL) is believed to play a key role as a triggering molecule that causes injury to the endothelium as an early event in atherogenesis. However, the mechanisms by which oxLDL injures endothelial cells are entirely unknown. We speculate that oxLDL may activate a cellular suicide pathway that leads to apoptosis. Methods and Results Human umbilical venous endothelial cells (HUVEC) were incubated with increasing doses of native or oxLDL for 18 hours. Apoptosis of HUVEC was measured with an ELISA specific for histone-associated DNA fragments and confirmed with DNA laddering. Native LDL had no effect, but incubation with oxLDL dose-dependently induced apoptosis of HUVEC. Induction of apoptosis by oxLDL was associated with increased CPP32-like protease activity, which is the major enzyme that initiates the proteolytical cascade leading to cell death. Specific inhibition of CPP32 activity completely abrogated oxLDL-induced apoptosis. The antioxidants N-acetylcysteine and the combination of vitamins C and E prevented oxLDL-induced apoptosis, abrogated the enhancement of CPP32-like protease activity, and inhibited the proteolytic cleavage of CPP32 into its active subunit p17. Conclusions oxLDL activates the suicide pathway leading to apoptosis of endothelial cells by enhancing CPP32-like protease activity. The oxLDL-mediated activation of CPP32 appears to involve the elaboration of reactive oxygen species. Activation of the cell death effector CPP32 by oxLDL may provide a mechanistic clue to the “response-to-injury” hypothesis of atherogenesis.