Mode and role of cell death during progression of atherosclerotic lesions in hypercholesterolemic rabbits.

Two cell types, macrophages and smooth muscle cells (SMCs), play important roles in the development of atherosclerotic lesions. Both contribute to the formation of the lesions not only by their presence but also by taking in or releasing extracellular substrates during life and at death. The present study aimed to elucidate their turnover, focusing on the detailed description of the modes of death in each cell type, and the roles of their death in the progression from early into advanced atherosclerotic lesions. Ascending aortas were obtained from New Zealand white male rabbits fed a diet with 1% cholesterol for 3 months (3-M group, n= 6) and 6 months (6-M group, n = 6). They were histologically examined, and the cell death was checked by in situ nick end-labeling (TUNEL), using a Taq polymerase-based in situ ligation assay with/without combination of immunohistochemistry, electron microscopy (EM), and TUNEL at the EM level. Intimal hyperplasia and luminal stenosis advanced with increased dietary interval, and the aortic intima of the 3-M group consisted of histological types I-III atherosclerotic lesions, whereas that of the 6-M group included types III-V. Along with the progression, the cellular population decreased, but the area of fibrosis increased. The percentage area of macrophages declined (from 60% +/- 5% to 23% +/- 2%), but that of SMCs increased (from 5% +/- 1% to 10% +/- 2%). The positive cells for in situ ligation were less frequent in the 6-M group (0.05% +/- 0.01%) than in the 3-M group (0.2% +/- 0.04%), which was due to a decrease in SMCs positive for in situ ligation. The frequency of TUNEL-positive cells was higher than that of in situ ligation-positive cells in both groups, suggesting that cell death involved not only apoptosis but also oncosis. This was confirmed using EM: cell death occurred via both apoptosis and oncosis. EM-TUNEL positively labeled not only apoptotic but also some oncotic nuclei. Death of macrophages and SMCs involves both apoptosis and oncosis in the aortic intima of hypercholesterolemic rabbits. Decline in the dying rate of SMCs might be associated with the formation of SMC-rich and collagen-rich lesions in the late advanced stage of atherosclerosis, although such a cause-effect relationship is to be further confirmed.