Apoptosis in atherosclerosis: pathological and pharmacological implications.

Normal embryonic development, tissue differentiation and repair in the eukaryote requires a tightly regulated apoptosis, or programmed cell death. Apoptosis also plays an essential role in different pathological processes including atherosclerosis, in which it affects all cell types in the atherosclerotic lesion, including endothelial cells, vascular smooth muscle cells, and macrophages. During atherosclerosis progression, pro- and anti-apoptotic signals abound in the evolving lesion. Apoptosis limits the number of a particular cell type that accumulates in the lesion and slows down the overall progression of the lesion. On the other hand, it contributes to the production of unstable plaques. Many pharmacological agents used to treat cardiovascular and lipid disorders have pro- or/and anti-apoptotic effects. Pharmaceuticals that modulate apoptosis in specific types of cell can potentially serve as anti-atherogenic agents. However, to develop agents for clinical use requires a thorough knowledge of the pathophysiology of apoptosis in atheromatous lesions, a highly cell-specific process. Here we review our current understanding of the process to provide a background for future pharmacological research in the area.

P-Selectin or intercellular adhesion molecule (ICAM)-1 deficiency substantially protects against atherosclerosis in apolipoprotein E-deficient mice.

The expression of leukocyte and endothelial cell adhesion molecules (CAMs) is essential for the emigration of leukocytes during an inflammatory response. The importance of the inflammatory response in the development of atherosclerosis is indicated by the increased expression of adhesion molecules, proinflammatory cytokines, and growth factors in lesions and lesion-prone areas and by protection in mice deficient in various aspects of the inflammatory response. We have quantitated the effect of deficiency for intercellular adhesion molecule (ICAM)-1, P-selectin, or E-selectin on atherosclerotic lesion formation at 20 wk of age in apolipoprotein (apo) E(-/-) (deficient) mice fed a normal chow diet. All mice were apo E(-/-) and CAM(+/+) or CAM(-/-) littermates, and no differences were found in body weight or cholesterol levels among the various genotypes during the study. ICAM-1(-/-) mice had significantly less lesion area than their ICAM-1(+/+) littermates: 4.08 +/- 0.70 mm(2) for -/- males vs. 5.87 +/- 0.66 mm(2) for +/+ males, and 3.95 +/- 0. 65 mm(2) for -/- females vs. 5.59 +/- 1.131 mm(2) for +/+ females, combined P < 0.0001. An even greater reduction in lesion area was observed in P-selectin(-/-) mice: 3.06 +/- 1.04 mm(2) for -/- males vs. 5.09 +/- 1.22 mm(2) for +/+ males, and 2.85 +/- 1.26 mm(2) for -/- females compared with 5.60 +/- 1.19 mm(2) for +/+ females, combined P < 0.001. The reduction in lesion area for the E-selectin null mice, although less than that seen for ICAM-1 or P-selectin, was still significant (4.54 +/- 2.14 mm(2) for -/- males vs. 5.92 +/- 0.63 mm(2) for +/+ males, and 4.38 +/- 0.85 mm(2) for -/- females compared with 5.94 +/- 1.44 mm(2) for +/+ females, combined P < 0.01). These results, coupled with the closely controlled genetics of this study, indicate that reductions in the expression of P-selectin, ICAM-1, or E-selectin provide direct protection from atherosclerotic lesion formation in this model.

The absence of p53 accelerates atherosclerosis by increasing cell proliferation in vivo.

The tumor suppressor protein p53 is an essential molecule in cell proliferation and programmed cell death (apoptosis), and has been postulated to play a principal part in the development of atherosclerosis. We have examined the effect of p53 inactivation on atherogenesis in apoE-knockout mice, an animal model for atherosclerosis. We found that, compared with p53+/+/apoE-/- mice, p53-/-/apoE-/- mice developed considerably accelerated aortic atherosclerosis in the presence of a similar serum cholesterol in response to a high-fat diet. Furthermore, the atherosclerotic lesions in p53-/-/apoE-/- mice had a significant (approximately 280%) increase in cell proliferation rate and an insignificant (approximately 180%) increase in apoptosis compared with those in p53+/+/apoE-/- mice. Our observations indicate that the role of p53 in atherosclerotic lesion development might be associated with its function in cell replication control, and that p53-independent mechanisms can mediate the apoptotic response in atherosclerosis.

Macrophage-mediated 15-lipoxygenase expression protects against atherosclerosis development.

Oxidative modification of LDL increases its atherogenicity, and 15-lipoxygenase (15-LO) has been implicated in the process. To address this issue, we generated transgenic rabbits that expressed 15-LO in a macrophage-specific manner and studied their susceptibility to atherosclerosis development when they were fed a high-fat, high-cholesterol (HFHC) diet (Teklad 0533 rabbit diet 7009 with 10% corn oil and 0.25% cholesterol) for 13.5 wk. Transgenic and nontransgenic rabbits developed similar degrees of hypercholesterolemia and had similar levels of triglyceride, VLDL, LDL, and HDL. Quantitative morphometric analysis of the aortic atherosclerosis indicated that the transgenic animals (n = 19) had significantly smaller lesion areas (9.8+/-6.5%, mean+/-SD) than their littermate controls (n = 14, 17.8+/-15.0%) (P < 0.05). In a subgroup (n = 9) of transgenic rabbits that received the HFHC diet plus the antioxidant N',N '-diphenyl-phenylenediamine (1%), the extent of lesion involvement (9.8+/-7.5%) did not differ from the subgroup (n = 10) that received the regular HFHC diet (9.7+/-5.9%). Since the results were unexpected, we repeated the experiments. Again, we found that the nontransgenic littermates (n = 12) had more extensive lesions (11.6+/-10.6%) than the transgenic rabbits (n = 13; 9.5+/-7.8%), although the difference was not significant. In a third set of experiments, we crossed 15-LO transgenic rabbits with Watanabe heritable hyperlipidemic (WHHL) rabbits and found that the lesion area in the 15-LO transgenic/heterozygous WHHL rabbits (n = 14) was only about one third (7.7+/-5.7%) that found in nontransgenic heterozygous WHHL littermate controls (n = 11, 20.7+/-19.4%) (P < 0.05). These data suggest that overexpression of 15-LO in monocytes/macrophages protects against lipid deposition in the vessel wall during early atherogenesis in these rabbit models of atherosclerosis.