Free cholesterol loading of macrophages is associated with widespread mitochondrial dysfunction and activation of the mitochondrial apoptosis pathway

Macrophage death in advanced atherosclerotic lesions leads to lesional necr osis and possibly plaque rupture and acute vascular occlusion. Among the li kely causes of lesional macrophage death is intracellular accumulation of e xcess free cholesterol (FC), which is known to occur in vivo. We recently s howed that FC loading of macrophages causes apoptosis, similar to 50% of wh ich is mediated by activation of cell-surface FasL and triggering of the Fa s pathway (Yao, P. M., and Tabas, I. (2000) J. Biol. Chem. 275, 23807-23813 ). To elucidate other pathways of death in FC-loaded macrophages, we invest igated mitochondrial transmembrane potential (Delta psi (m)) and the mitoch ondrial apoptosis pathway in FC-loaded mouse peritoneal macrophages. Starti ng between 3 and 6 h of FC loading, Delta psi (m) was markedly decreased in the majority of macrophages and was independent of the Fas pathway. The de crease in Delta psi (m) by FC loading was not prevented by GSH, thus distin guishing it from 7-ketocholesterol-induced mitochondrial dysfunction. Cytoc hrome c release into the cytosol was noted by 4 h of FC loading, and activa tion of caspase-9 and effector caspases was observed at 6 h. Finally, we fo und that both cellular and mitochondrial levels of the pro-apoptotic protei n Bax were increased severalfold as early as 4 h after FC loading. Thus, FC loading, perhaps via increased levels of Bax and/or cholesterol overloadin g of mitochondria, triggers cytochrome c release and activation of caspase- 9 and the effector caspases, leading to macrophage apoptosis. These finding s and our previous data support a model in which FC loading of macrophages promotes a dual program of caspase-mediated death.