Two distinct calcium-dependent mitochondrial pathways are involved in oxidized LDL-induced apoptosis.

Objective: Oxidized low-density lipoprotein (oxLDL)-induced apoptosis of vascular endothelial cells may contribute to plaque erosion and rupture. We aimed to clarify the relationship between the oxLDL-induced calcium signal and induction of apoptotic pathways.

Methods And Results: Apoptosis was evaluated by biochemical methods, including studies of enzyme activities, protein processing, release of proapoptotic factors, chromatin cleavage, and especially by morphological methods that evaluate apoptosis/necrosis by SYTO-13/propidium iodide fluorescent labeling. The oxLDL-induced sustained calcium rise activated 2 distinct calcium-dependent mitochondrial apoptotic pathways in human microvascular endothelial cells. OxLDLs induced calpain activation and subsequent Bid cleavage and cytochrome C release, which were blocked by calpeptin. Cyclosporin-A inhibited cytochrome C release, possibly by inhibiting the opening of the mitochondrial permeability transition pore (mPTP). Calcineurin, another cyclosporin-sensitive step, was not implicated, because oxLDLs inhibited calcineurin and FK-506 treatment was ineffective. Cytochrome C release in turn induced caspase-3 activation. In addition, oxLDLs triggered release and nuclear translocation of mitochondrial apoptosis-inducing factor through a mechanism dependent on calcium but independent of calpains, mPTP, and caspases.

Conclusions: OxLDL-induced apoptosis involves 2 distinct calcium-dependent pathways, the first mediated by calpain/mPTP/cytochrome C/caspase-3 and the second mediated by apoptosis-inducing factor, which is cyclosporin-insensitive and caspase-independent.