Lipoprotein(a) Promotes Smooth Muscle Cell Proliferation and Dedifferentiation in Atherosclerotic Lesions of Human Apo(a) Transgenic Rabbits

Abstract

Elevated plasma lipoprotein(a) [Lp(a)]levels constitute an independent risk factor for the development of atherosclerosis. However, the mechanism underlying Lp(a) atherogenicity is unclear. Recently, we demonstrated that Lp(a) may potentially be proatherogenic in transgenic rabbits expressing human apolipoprotein(a) [apo(a)]. In this study, we further investigated atherosclerotic lesions of transgenic rabbits by morphometry and immunohistochemistry. On a cholesterol diet, human apo(a) transgenic rabbits had more extensive atherosclerotic lesions of the aorta, carotid artery, iliac artery, and coronary artery than did nontransgenic littermate rabbits as defined by increased intimal lesion area. Enhanced lesion development in transgenic rabbits was characterized by increased accumulation of smooth muscle cells, that was often associated with the Lp(a) deposition. To explore the possibility that Lp(a) may be involved in the smoothmuscle cell phenotypic modulation, we stained the lesions using a panel of monoclonal antibodies against smooth-muscle myosin heavy-chain isoforms (SM1, SM2, and SMemb) and basic transcriptional element binding protein-2 (BTEB2). We found that a large number of smooth muscle cells located in the apo(a)-containing areas of transgenic rabbits were positive for SMemb and BTEB2, suggesting that these smooth muscle cells were either immature or in the state of activation. In addition, transgenic rabbits showed delayed fibrinolytic activity accompanied by increased plasma plasminogen activator inhibitor-1. Lipoprotein(a) [Lp(a)] closely resembles low-density lipoprotein (LDL) in both lipid composition and the presence of apolipoprotein (apo) B-100 (apoB-100). Lp(a) is distinguished from LDL by the presence of an additional protein designated as apolipoprotein(a) [apo(a)], which is complexed to apoB-100 by disulfide linkage.

Inflammatory Reactions in the Pathogenesis of Atherosclerosis

“…In addition to oxLDL, Lp (a), another atherogenic lipoprotein, may also lead to an inflammatory process by inducing the expression of adhesion molecules on endothelial cells, the chemotaxis of monocytes, and the proliferation of smooth muscle cells (38). In one of our recent studies using transgenic rabbits expressing human apo (a), we demonstrated that Lp (a) in transgenic rabbits which do not have endogenous apo (a) showed increased aortic atherosclerosis in comparison with control rabbits.…”

“…Atherosclerotic lesions in transgenic rabbits are characterized by increased cellular proliferation, and Lp (a) is often deposited in the lesions. We initially hypothesized that Lp (a) may stimulate smooth muscle cell dedifferentiation in addition to proliferation and found that a large number of intimal cells are positively stained with SMemb, a monoclonal antibody against the smooth muscle myosin heavy chain isoform (38). In addition, Lp (a) has the ability to inhibit fibrinolytic activity by increasing plasma plasminogen activator-1 production (38).…”

“…We initially hypothesized that Lp (a) may stimulate smooth muscle cell dedifferentiation in addition to proliferation and found that a large number of intimal cells are positively stained with SMemb, a monoclonal antibody against the smooth muscle myosin heavy chain isoform (38). In addition, Lp (a) has the ability to inhibit fibrinolytic activity by increasing plasma plasminogen activator-1 production (38). More recently, we demonstrated that Lp (a) enhances advanced atherosclerosis and vascular calcification in WHHL transgenic rabbits (39).…”

“…The biological functions of Lp (a) and whether it plays a role in lesion development remain unknown, but recent studies in our laboratory using human apolipoprotein (a) [apo (a)] transgenic rabbits have revealed that Lp (a) may act as a pro-inflammatory mediator that augments the lesion formation (5,6). In addition to the foam cells in the lesions, T lymphocytes are scattered around macrophages and foam cells (Fig.…”

Overexpression of lipoprotein lipase in transgenic rabbits leads to increased small dense LDL in plasma and promotes atherosclerosis

“…The intimal lesions were measured using a computerized image analysis system and expressed as microscopic lesion areas. 28 To study cellular components, lipoprotein and LPL deposits in the lesions, we performed immunohistochemical staining using the following monoclonal antibodies against macrophages (RAM-11), smooth muscle a-actin (HHF35), oxidized LDL (FOH1a/ DLH3), 29 and LPL (5D2). 30 …”