Attenuation of Neointimal Vascular Smooth Muscle Cellularity in Atheroma by Plasminogen Activator Inhibitor Type 1 (PAI-1)

Abstract

S U M M A R YRupture of vulnerable atheroma often underlies acute coronary syndromes. Vulnerable plaques exhibit a paucity of vascular smooth muscle cells (VSMCs) in the cap. Therefore, decreased VSMC migration into the neointima may predispose to vulnerability. The balance between cell surface plasminogen activator activity and its inhibition [mediated primarily by plasminogen activator inhibitor type 1 (PAI-1)] modulates migration of diverse types of cells. We sought to determine whether increased expression of PAI-1 would decrease migration of VSMCs in vitro and neointimal cellularity in vivo in apolipoprotein E knockout (ApoE Ϫ / Ϫ ) mice fed a high-fat diet. Increased vessel wall expression of PAI-1 in transgenic mice was induced with the SM22 ␣ promoter. VSMC migration through Matrigel in vitro was quantified with laser scanning cytometry. Expression of PAI-1 was increased threefold in the aortic wall of SM22-PAI transgene-positive mice. Neointimal cellularity of vascular lesions was decreased by 26% ( p ϭ 0.01; n ϭ 5 each) in ApoE Ϫ / Ϫ mice with the SM22-PAI transgene compared with ApoE Ϫ / Ϫ mice. VSMCs explanted from transgenepositive mice exhibited twofold greater expression of PAI-1 and their migration was attenuated by 27% ( p ϭ 0.03). Accordingly, increased expression of PAI-1 protein by VSMCs reduces their migration in vitro and their contribution to neointimal cellularity in vivo. A therogenesis is a multifactorial and prolonged process (Ross 1993). Atherosclerotic plaques particularly prone to rupture, so-called vulnerable plaques, are pivotal in the genesis of acute coronary syndromes (Davies and Thomas 1985;Falk et al. 1995;Libby 1995;Sobel 1999;Sobel et al. 2003). Such plaques have a relative paucity of vascular smooth muscle cells (VSMCs), thin fibrous caps, lipid-laden cores, and high ratios of lipid and extracellular matrix (ECM) to VSMCs. Macrophages and T-lymphocytes, especially in shoulder regions of vulnerable plaques and implicated in plaque rupture (Libby 1995), are relatively prominent cellular constituents, whereas VSMCs are not (Davies and Thomas 1985;Falk et al. 1995;Libby 1995;Sobel 1999;Sobel et al. 2003).We have hypothesized that limitation of activation of intramural plasminogen activators by their primary inhibitor, plasminogen activator inhibitor type 1 (PAI-1), is one factor predisposing to evolution of vulnerable plaques by promoting or retarding neointimal migration of VSMCs, leading to a paucity of such cells and consequent elaboration of thin rather than thick fibrous plaques (Sobel 1999;Sobel et al. 2003). The present study was performed to determine whether decreased migration of VSMCs, characterized in vitro, would ensue if VSMC expression of PAI-1 were increased with the use of the promoter region of the VSMC gene SM-22 ␣ . In addition, we determined whether neointimal cellularity would be diminished in vivo by overexpression of VSMC PAI-1 during the evolution of atheroma in apolipoprotein E (ApoE)-deficient mice. A specific objective was to characterize the effect...

Augmentation of Proliferation of Vascular Smooth Muscle Cells by Plasminogen Activator Inhibitor Type 1

“…8,20 In the present studies, greater proliferation was demonstrated in VSMCs from PAI-1 transgenic mice (SM22-PAI ϩ ). After 6 days in culture, the average number of SM22-PAI ϩ VSMCs was 2.3Ϯ0.4-fold greater than control VSMCs ( Figure 1A …”

“…Their observation of decreased neointimal formation after injury when PAI-1 expression was restored with adenoviral gene transfer 29 is consistent with our previous observation that increased expression of PAI-1 inhibits VSMC contribution to neointimal formation presumably by inhibiting migration. 8 Differing effects of PAI-1 on the in vitro proliferation of aortic endothelial cells 33 and VSMCs 34 are likely to be a reflection of differences in cell type, culture conditions, genotypes, and experimental design. Nevertheless, these results are consistent with the observation that increased intracellular expression PAI-1 promotes proliferation of VSMCs.…”

“…VSMCs were obtained by explantation from the aortas of SM22-PAI ϩ mice that exhibit a 3-fold increased expression of PAI-1 8,20 and negative control littermates, and grown in Dulbecco Modified Eagle Medium (DMEM; Gibco-BRL) supplemented with 20% fetal bovine serum as we described previously. 20 The identity of smooth muscle cells was confirmed by Western blot and flow cytometry with smooth muscle cell specific ␣-actin antibody.…”

“…6,7 We have shown that increased expression of plasminogen activator inhibitor type 1 (PAI-1) limits migration of VSMCs, a phenomenon that may result in promoting generation of plaques more prone to rupture. 8 The present study was designed to determine whether increased PAI-1 influences the proliferation of VSMCs.…”

The association between plasminogen activator inhibitor type 1 (PAI-1) levels, PAI-1 4G/5G polymorphism, and myocardial infarction: a Mendelian randomization meta-analysis

“…Besides modulating thrombolysis, fibrinolytic activity likely influences the progression of atherosclerotic lesions [3]. Overexpression of PAI-1 may also promote the development of weak plaques with thin fibrous caps by inhibiting both u-PA receptor-and integrin-mediated cell adhesion and migration [4,5]. Therefore, increased PAI-1 levels may play a crucial role in the occurrence of MI by affecting both atheroma and thrombosis.…”