Overexpression of Urokinase by Plaque Macrophages Causes Histological Features of Plaque Rupture and Increases Vascular Matrix Metalloproteinase Activity in Aged Apolipoprotein E–Null Mice

Abstract

Background The mechanisms of atherosclerotic plaque rupture are poorly understood. Urokinase-type plasminogen activator (uPA) is expressed at elevated levels by macrophages in advanced human plaques. Patients with evidence of increased plasminogen activation have an elevated risk of major cardiovascular events. We used atherosclerotic mice to test the hypothesis that increased macrophage uPA expression in advanced plaques would cause histologic features similar to those in ruptured human plaques. Methods and Results Bone marrow from transgenic mice with increased macrophage uPA expression or nontransgenic controls (all Apoe−/−) was transplanted into 35-wk-old Apoe−/− recipients, and innominate lesions and aortae examined 8 – 13 wk later. Donor macrophages accumulated in innominate lesions adjacent to plaque caps and in aortae, increasing uPA expression at both sites. Recipients of uPA-overexpressing macrophages had an increased prevalence of intraplaque hemorrhage (61 vs 13%; P = 0.002) as well as increased lesion fibrin staining and fibrous cap disruption (P = 0.06 for both). Transplantation of uPA-overexpressing macrophages increased aortic matrix metalloproteinase (MMP) activity (40%; P = 0.02). This increase was independent of MMP-9. Conclusions In advanced plaques of Apoe−/− mice, macrophage uPA overexpression causes intraplaque hemorrhage and fibrous cap disruption, features associated with human plaque rupture. uPA overexpression also increases vascular MMP activity. These data provide a mechanism that connects macrophage uPA expression, MMP activity, and plaque rupture features in mice. The data also suggest that elevated plaque plasminogen activator expression and plasminogen activation in humans may be causally linked to plaque rupture and cardiovascular events.

Mechanisms of Urokinase Plasminogen Activator (uPA)-mediated Atherosclerosis

“…therefore, would be more likely to contribute to atherogenesis), we repeated the qRT-PCR of S100A9 and S100A8 using aortic RNA from 45-week-old nontransgenic Apoe Ϫ/Ϫ recipients of SR-uPA ϩ/0 or nontransgenic bone marrow (available from a previous study) (16). Both transcripts were significantly more abundant in the aortae of SR-uPA ϩ/0 bone marrow recipients (p Յ 0.004 for both; Fig.…”

“…Moreover, patients with evidence of increased Plg activation have an elevated risk for accelerated atherosclerosis and major cardiovascular events (8 -10), and uPA expression is elevated in aneurysmal human aortae (11,12). Important roles for uPA in atherosclerosis and aneurysm formation are supported by recent studies in our laboratory showing that 15-week-old Apoe Ϫ/Ϫ mice with macrophage-targeted uPA overexpression (SR-uPA ϩ/0 mice, with scavenger receptor promoter-driven expression of a uPA transgene) have significantly accelerated plaque growth and aneurysm forma-tion (13)(14)(15), older SR-uPA ϩ/0 Apoe Ϫ/Ϫ mice have increased atherosclerotic plaque rupture (16), and recipients of bone marrow transplants from uPA null mice have less atherosclerosis than recipients of bone marrow from uPA-expressing mice (14).…”

“…Therefore S100A8/A9 could mediate enhanced accumulation of both lipids and macrophages in lesions of SR-uPA ϩ/0 mice. S100A8/A9 are also associated with plaque rupture in both humans and diabetic mice (53,61), and older SR-uPA ϩ/0 mice have significantly increased plaque rupture (16). It will be interesting to generate SR-uPA ϩ/0 mice that are deficient in S100A8/A9 and test whether accelerated atherosclerosis and plaque rupture in SR-uPA ϩ/0 mice are dependent on S100A8/A9.…”

“…Model of pathways through which macrophage uPA expression could accelerate atherosclerosis. The model is constructed based on data from the present study and others showing that macrophage overexpression of uPA promotes early lesion lipid accumulation as well as later lesion macrophage accumulation, increases macrophage migration, enhances vascular matrix metalloproteinase (MMP) activity, accelerates lesion progression, and stimulates macrophage S100A8/A9 expression (13)(14)(15)(16). 1, uPA-expressing macrophages begin to accumulate within an early lesion.…”

Molecular Biology of Atherosclerosis

“…Macrophage-specific over-expression in apoE Ϫ/Ϫ mice increased plaque (142%) and greatly increased coronary lumen narrowing with 62% stenosis in transgenics versus just 2.6% in controls-with many myocardial infarctions and early mortality (354). Later studies showed uPA over-expression effect was dependent on plasminogen and could be evoked by BMT into older apoE Ϫ/Ϫ mice (789,948). BMT with uPA KO led to less atherosclerosis only in older mice with established lesions (953).…”