Turnover of cholesterol in aorta of rabbits previously fed a cholesterol-enriched diet

Cell composition, replication, and apoptosis in atherosclerotic plaques after 6 months of cholesterol withdrawal

Unstable human atherosclerotic plaques are characterized by a thin fibrous cap that contains few smooth muscle cells (SMCs) and numerous foam cells of macrophagic origin. Apoptosis of SMCs in the fibrous cap could destabilize the plaque and promote plaque rupture. In an experimental approach, we have studied apoptotic cell death and related proteins in atherosclerotic plaques of cholesterol-fed rabbits and examined the effects of cholesterol withdrawal. The induced atherosclerotic plaques at the thoracic aorta were composed of both fibromuscular tissue and foam cells. The presence of SMCs overlying macrophage accumulation was reminiscent of the structure of human atherosclerotic plaques. The plaques showed signs of cell replication and apoptotic cell death (1.8+/-0.5% terminal deoxynucleotidyl transferase end-labeling [TUNEL]-positive nuclei). Cell replication was confined mostly to the macrophages, whereas 34% of the TUNEL-labeled cells were SMCs. Both the macrophages and SMCs in the plaques expressed BAX, a proapoptotic protein of the BCL-2 family. After 6 months of cholesterol withdrawal, the thickness of the plaques in all localizations of the aorta was unchanged, but apoptosis was nearly absent (<0.1% of nuclei). Moreover, macrophages disappeared from the plaques, whereas the SMCs that remained present lost their lipid accumulation and strongly reduced their BAX expression. These changes were associated with a reduction of cell replication and increased deposition of fibrillar collagen fibers in the plaques, which pointed to plaque stabilization. In conclusion, the cell composition but not the thickness of atherosclerotic plaques was profoundly altered after a 6-month cholesterol withdrawal period. These changes were associated with a strong reduction of cell replication and apoptotic cell death. Moreover, the expression of the proapoptotic factor, BAX, was reduced in the remaining cells, which were mainly SMCs. These findings could help to explain the benefit of lipid-lowering therapy on plaque stabilization.

Regression of atheroma in the rabbit

Ten studies in the literature concerning regression of rabbit atheroma were re-examined. In studies where cholesterol content was referred to weight, a degree of regression was noted in 3/4 studies. Such regression might at least partly have resulted from the dilution effect of the atheroma contents when results were expressed on a weight basis. By contrast, when results were referred to length or protein, partial regression was seen in only 1/4 studies. Mild atheroma induced by short-term cholesterol feeding did seem to regress in 2/2 studies.

A nicotinic acid ester and experimental atherosclerosis: no significant effect of treatment with niceritrol (pentaerythritoltetranicotinate) on cholesterol in aorta of rabbits previously fed with cholesterol-enriched diet

The influence of niceritrol on cholesterol in serum, liver and inner aorta of 115 rabbits was studied during a period where a cholesterol-enriched diet was given and during two succeeding periods after which cholesterol addition to the diet was discontinued. Niceritrol given together with cholesterol for 6 weeks reduced significantly the concentration of cholesterol in serum and liver, but not in the inner aorta. During 16 weeks decrease of serum cholesterol after discontinuation of cholesterol feeding no significant effect of niceritrol was observed on the decrease in serum and liver cholesterol or on the concentration of cholesterol in inner aorta. When serum cholesterol had normalized 16 weeks after discontinuation of cholesterol feeding, addition of niceritrol to the diet for the following 16 weeks did not significantly affect the concentration of cholesterol in liver and inner aorta. Animals in all groups were injected intravenously with an equal amount of [3H] cholesterol 3 weeks before discontinuing cholesterol feeding. Niceritrol did not significantly affect the amount of accumulated labelled cholesterol in inner aorta. The present results indicated that niceritrol had no significant effects on metabolism of cholesterol in inner aorta of the hypercholesterolemic and previously hypercholesterolemic rabbits.