Apoptosis in atherosclerosis: Induction of apoptosis by oxidized low density lipoprotein and oxysterols

In an attempt to understand the role of apoptosis in the development of atherosclerosis, we examined lesions developed in the aortas of apo E- and LDL receptor-deficient mice, murine models of atherosclerosis, and determined frequency, spatial distribution and cell types of apoptotic cells in each lesion. Terminal deoxunucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) and nuclear staining with propidium iodide were used to demonstrate apoptotic cells. Most of the TUNEL-positive cells were filled with fat and distributed in close proximity to lipid pools. The TUNEL-positive cells in the intimal side of the lipid cores were macrophages, while same of those in the adventitial side were: smooth muscle cells. These observations suggest that apoptosis is involved in the active turnover of foam cells of both macrophage- and smooth muscle cell-lineage. Oxidized LDL existing in hyperlipidemia-induced atherosclerotic lesions is reported to be cytotoxic, and oxysterols are presumed to mediate its cytotoxicity. To elucidate the molecular mechanism, we established murine macrophage-like P388-D1 cells which over-express Bcl-2 protein by retorvirus-mediated gene transfer. Oxysterols (7-ketocholesterol, 25-hydroxycholesterol) induced nuclear condensation and oligonucleosomal DNA fragmentation, which were partially inhibited by Bcl-2 over-expression. Though CPP32 inhibitor suppressed the cell death in control cells. it showed no additive protection in the cells over-expressing Bcl-2. These findings indicate that oxysterols induce apoptosis via Bcl-2-inhibitable and -uninhibitable pathways, and the former depends on CPP32 activation.