Lipid Lowering Promotes Accumulation of Mature Smooth Muscle Cells Expressing Smooth Muscle Myosin Heavy Chain Isoforms in Rabbit Atheroma

Abstract

Abstract-Smooth muscle cells (SMCs) in the atherosclerotic intima characteristically differ from those in the arterial media, for example, by reduced expression of SMC differentiation/maturation markers such as smooth muscle myosin heavy chain isoforms (SM1 and SM2). This study tested the hypothesis that lipid lowering promotes maturation of intimal SMCs in 33 rabbits subjected to balloon injury and cholesterol feeding (0.3%) for 4 months (Baseline group, nϭ15); some of which then were switched to a low-cholesterol diet for 8 months (Low group at 8 months, nϭ3) or 16 months (Low group at 16 months, nϭ10). The remaining rabbits continued to consume a high-cholesterol diet for 16 months (High group, nϭ5). We monitored SMC phenotype by expression of immunoreactive ␣-smooth muscle actin, SM1, and SM2. ␣-Actin is an early marker, and SM1 and SM2 are late markers for SMC differentiation/maturation. Only fully differentiated or mature SMCs express SM2. Data are reported as the percentage of the ␣-actin-positive intimal area occupied by smooth muscle myosin-positive SMCs determined by color image analysis of immunostained sections. Levels of SM1 and SM2, highly expressed by SMCs in the normal aortic media (nϭ5) decreased in the aortic intima of the Baseline and High groups, indicating a less mature phenotype. In contrast, SM1 and SM2 increased in the Low (16 months) group, indicating that intimal SMCs exhibit a more mature phenotype after lipid lowering. Electron microscopy also showed the presence of mature intimal SMCs with abundant myofilaments. Furthermore, lipid lowering reduced levels of platelet-derived growth factor-B in the arterial intima, a factor known to suppress smooth muscle myosin expression. These data demonstrate that lipid lowering favors accumulation of mature SMCs in the atherosclerotic intima in association with reduced levels of platelet-derived growth factor-B expression. Intimal SMCs in the Low group also displayed reduced expression of matrix metalloproteinases-3 and -9 compared with the Baseline and High groups. These findings shed new light on the effects of lipid lowering at the level of the vascular wall, which may influence the biology of the atheroma. (Circ Res. 1998;83:1015-1026.)

Lipid Lowering Reduces Oxidative Stress and Endothelial Cell Activation in Rabbit Atheroma

“…Thus, despite these limitations, rabbit studies described above establish the concept that lipid lowering influences important aspects of atherogenesis. 14,15,17 Indeed, our present observations demonstrate that dietary lipid lowering can improve several indices of oxidative stress and concomitantly normalize certain crucial EC functions involved in the progression of vulnerable and thrombogenic atherosclerotic lesions. Recent clinical studies have suggested that statin therapy reduces inflammation by mechanisms independent of their lipid lowering effect.…”

“…14,15,17 Briefly, we created aortic atheroma by balloon injury followed by an atherogenic diet for 4 months. (Figure 1).…”

“…Transmission electron microscopic study on aortic rings obtained from three animals in each group was performed as previously reported. 17 The percentage of the intimal area immunopositive for oxLDL or apoB-100 and percentage positive endothelium for VCAM-1 or eNOS within CD31-positive endothelium were determined using a computer-based color image analysis system. 14 -17 Statistical testing used one-way ANOVA followed by post hoc testing (Fisher test).…”

“…14 -16 We also showed that a diminished number of macrophages expressing platelet-derived growth factor B chain may favor the accumulation of mature smooth muscle cells (SMCs) observed in atheroma after lipid lowering. 17 However, the mechanisms by which lipid lowering reduces macrophage accumulation and other aspects of inflammation in atheroma remained obscure. This study therefore tested the hypothesis that lipid lowering can limit oxidative stress and functions of ECs involved in vascular inflammation.…”

Monoclonal Antibody Against Vascular Cell Adhesion Molecule-1 Inhibits Neointimal Formation After Periadventitial Carotid Artery Injury in Genetically Hypercholesterolemic Mice

“…Sections from mice 3 days after injury were used for computer-assisted image analysis as a semiquantitative assessment of immunohistochemical stains as previously described. 13 Briefly, images were captured and analyzed by use of Optimas 6.1 (Optimas System, Bioscan). Color detection was accomplished by sampling, and threshold masking defined the positive area.…”