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 necrosis and possibly plaque rupture and acute vascular occlusion. Among the likely causes of lesional macrophage death is intracellular accumulation of excess free cholesterol (FC), which is known to occur in vivo. We recently showed that FC loading of macrophages causes apoptosis, similar to 50% of which is mediated by activation of cell-surface FasL and triggering of the Fas 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 investigated mitochondrial transmembrane potential (Delta psi (m)) and the mitochondrial apoptosis pathway in FC-loaded mouse peritoneal macrophages. Starting 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 decrease in Delta psi (m) by FC loading was not prevented by GSH, thus distinguishing it from 7-ketocholesterol-induced mitochondrial dysfunction. Cytochrome c release into the cytosol was noted by 4 h of FC loading, and activation of caspase-9 and effector caspases was observed at 6 h. Finally, we found that both cellular and mitochondrial levels of the pro-apoptotic protein Bax were increased severalfold as early as 4 h after FC loading. Thus, FC loading, perhaps via increased levels of Bax and/or cholesterol overloading of mitochondria, triggers cytochrome c release and activation of caspase-9 and the effector caspases, leading to macrophage apoptosis. These findings and our previous data support a model in which FC loading of macrophages promotes a dual program of caspase-mediated death.