Diet-induced hypercholesterolemia increases endothelin-1 release by aortic endothelial cells

Effect of simvastatin on left ventricular mass in hypercholesterolemic rabbits

Epidemiological studies showed that hypercholesterolemia is associated with higher left ventricular mass. Endothelin signaling is activated in hyperlipidemic animals and may contribute to progressive ventricular hypertrophy. Simvastatin has been shown to inhibit endothelin-1. However, the behavior of simvastatin on ventricular hypertrophy in hyperlipidemic animals is not well understood. In this study, we evaluated the hemodynamic, biochemical, and morphological responses to simvastatin in cholesterol-fed (1%) rabbits. The left ventricular weight increased 8 wk after cholesterol feeding compared with that in normocholesterolemic rabbits. Simvastatin at a clinical therapeutic dose (1.2 mg x kg(-1) x day(-1)) significantly decreased left ventricular weight by 14% and left ventricular myocyte sizes by 14% as isolated by enzymatic dissociation. Hypercholesterolemia upregulated ventricular preproendothelin-1 mRNA as assessed by real-time quantitative RT-PCR and elevated production of cardiac endothelin-1 concentration. The increased endothelin-1 responses can be inhibited after simvastatin administration. Left ventricular mass indexed by body weight positively correlated with tissue endothelin-1 levels (P = 0.0003). In Langendorff-perfused rabbit hearts, hyperlipidemia led to significant QT prolongation compared with normocholesterolemia, which can be reversed by administering simvastatin. In contrast, simvastatin-induced beneficial effects were reversed by the addition of mevalonate. The addition of bosentan, a nonspecific endothelin receptor blocker, improved the response in hypercholesterolemic rabbits and did not have additional beneficial effects in simvastatin-treated rabbits. The results of the present study suggest that the antihypertropic and electrocardiographic effects of simvastatin at a clinical therapeutic dose are mediated through inhibition of tissue endothelin-1 expression, which is linked to mevalonate metabolism, and result in an amelioration of cardiomyocyte hypertrophy development by an atherogenic diet.

Corresponding distributions of increased endothelin-B receptor expression and increased endothelin-1 expression in the aorta of apolipoprotein E-deficient mice with advanced atherosclerosis

Endothelin (ET)-1 causes proliferation of vascular smooth muscle cells (VSMC). Although it has been reported that stimulation of ET(B) receptors as well as ET(A) receptors promote proliferation of VSMC, the precise distribution of each receptor subtype in atherosclerotic vessels is unknown. Previous studies demonstrated that apolipoprotein E (apoE)-deficient mice have hypercholesterolaemia and develop severe atherosclerosis. To investigate the pathophysiological roles of vascular ET system in atherosclerosis, we examined preproET-1 messenger ribonucleic acid expression in the aorta of apoE-deficient mice, and performed immunohistochemical staining for ET-1 and each ET receptor subtype (ET(A) and ET(B) receptors) in the atherosclerotic lesions of these mice. The level of preproET-1 mRNA in the aorta was significantly higher in the apoE-deficient mice than in the control mice. Strong ET-1 staining was observed in the macrophage-foam cells, intimal and medial VSMC in the atherosclerotic lesions of the apoE-deficient mice. In addition, in the atherosclerotic lesions, strong ET(B) receptor staining was observed in the macrophage-foam cells, intimal and medial VSMC, which distribution corresponded closely to that of ET-1. ET(A) receptor staining was observed in the medial VSMC of both groups, but not in the macrophage-foam cells of the apoE-deficient mice. ET(A) receptor staining in the medial VSMC was stronger in the apoE-deficient mice than in the control mice. These results suggest that the vascular ET system, including ET-1 and ET receptors, is activated in the atherosclerotic lesions of apoE-deficient mice. Since the distribution of strong ET(B) receptor staining corresponded closely to that of ET-1, it is suggested that the ET system, mediated by ET(B) receptors, has an important role in the pathophysiology of the atherosclerotic lesions of apoE-deficient mice.

Expression of endothelin-1 after endothelial denudation of thoracic aortas in experimental hypercholesterolemic rats

Although endothelin-1 (ET-1) is involved in balloon-induced neointima formation, the role of ET-1 in balloon-induced neointima formation in hypercholesterolemia is unclear. In addition, it remains to be determined whether ET-1 is produced by endothelial cells or vascular smooth muscle cells, or both. We investigated tissue immunoreactive ET-1 levels by immunoblot analysis, localization of ET-1 immunoreactivity by immunohistochemistry, and expression of preproET-1 mRNA by in situ hybridization in balloon-induced neointima formation in experimental hypercholesterolemic rats. Serum total cholesterol levels were significantly higher (p< 0.01) in the 5%cholesterol-diet group (194 +/- 17 mg/dl, n=20) than in the normal-diet group (64 +/- 2 mg/dl, n=20). Before and after endothelial denudation, plasma ET-1 levels and tissue immunoreactive ET-1 levels were significantly higher in cholesterol-diet rats. The expression of preproET-1 mRNA by in situ hybridization was observed in the nuclei of endothelial cells, but not medial smooth muscle cells in normal- or cholesterol diet rats. After endothelial denudation, plasma ET-1 levels and serum total cholesterol levels did not change in either the normal- or the cholesterol-diet rats. Tissue level of ET-1 tended to increase at 3 days after denudation in normal-diet rats (1.0 +/- 0.1 vs 2.6 +/- 0. 2 density ratio, p< 0.05), although endothelial cells had not yet regenerated. The expression of preproET-1 mRNA by in situ hybridization was not observed at 3 days after endothelial denudation in either endothelial or medial smooth muscle cells in normal-diet rats. Four weeks after denudation, regeneration of endothelial cells was almost complete, and an intimal hyperplasia was observed. Tissue ET-1 levels were significantly elevated 4 weeks after endothelial denudation in normal-diet rats (1.0 +/- 0.1 vs 7.6 +/- 0.2 density ratio, p< 0.05). The expression of preproET-1 mRNA by in situ hybridization was observed in the nuclei of regenerated endothelial cells after endothelial denudation, and in smooth muscle cells migrating into the intima, but was not observed in medial smooth muscle cells in normal-diet rats. A similar pattern was observed in cholesterol-diet rats. We concluded that ET-1 was involved in neointima formation and that ET-1 was produced by both endothelial and neointimal smooth muscle cells, but not medial smooth muscle cells after endothelial denudation in experimental hypercholesterolemic rats.

Role of endothelin-1 in atherosclerosis

Increased evidence has shown that endothelin-1 (ET-1) derived from the arterial cells is involved in the development of atherosclerosis. ET-1 and ET receptors are upregulated in both human and experimental animal atherosclerotic lesions. Plasma ET-1 levels are significantly elevated in hypercholestolemic subjects and cholesterol-fed animals. We hypothesized that plasma lipoproteins such as LDL and HDL retained in the arterial wall can affect ET-1 production and secretion, thereby sustaining vascular functions. Using a two-chamber culture system, we have demonstrated that endothelial cells (ECs) show a polar secretion of ET-1; the majority of ET-1 are secreted toward the basal side of the vessels. Furthermore, we found that LDL enhances whereas HDL inhibits the ET-1 secretion from ECs in a polarized pattern. In order to demonstrate ET receptor distribution in the lesion, we recently studied both human and apoE-KO mice. Our study showed that there is an increased expression of ETB receptors in foamy macrophages in the lesions. More importantly, medial smooth muscle cells (SMCs) beneath the foam cell lesions exhibited a higher intensity of ETB receptor immunoreactivity than those located in foam cell-free areas. In such an area, ET-1 immunoreactivity is also increased. These results suggest that accumulation of foamy macrophages may modulate the shift of ET receptor subtypes from ETA to ETB in SMCs and an enhanced ET system mediated by ETB receptors may play a pivotal role in the progression of atherosclerosis. This notion has been further supported by a recent finding that administration of ET receptor antagonists resulted in a significant reduction of atherosclerosis in apoE-KO mice.

Increased immunoreactivity of endothelin-1 and endothelin B receptor in human atherosclerotic lesions. A possible role in atherogenesis

This study was designed to analyze the distribution and localization of endothelin-1 (ET-1) and ET receptors (ET(A) and ET(B)) at different stages of human atherosclerotic lesions by immunohistochemistry. Compared with ET(A) receptors, there was increased immunoreactivity of ET-1 and ET(B) receptor in both unfoamy and foamy macrophages and T lymphocytes in fatty streak and fibrous plaque lesions. In addition, medial SMCs located just beneath the foam cell lesions revealed a higher intensity of ET(B) receptor immunoreactivity than those located beneath the normal-looking intima without foam cells. In fibrous plaques, intimal SMCs near foam cells showed an increased density of ET receptors with predominant ET(B) immunoreactivity. In the areas where SMCs showed ET(B) receptor, ET-1 immunoreactivity was also enhanced. These results suggest that accumulation of foamy macrophages and T lymphocytes may modulate the switching of ET receptor subtypes from ET(A) to ET(B) in vascular SMCs. and that the enhanced ET system mediated by ET(B) receptors may play active roles in the progression of atherosclerosis.