Persistence of atherosclerotic plaque but reduced aneurysm formation in mice with stromelysin-1 (MMP-3) gene inactivation

To investigate a potential role for stromelysin-1 (MMP-3) in the development and progression of atherosclerotic lesions and aneurysm formation, mice with a deficiency of apolipoprotein E (ApoE(-/-):MMP-3(+/+))) or with a combined deficiency of apoE and MMP-3 (ApoE(-/-):MMP-3(-/-)) were kept on a cholesterol-rich diet for 30 weeks. Atherosclerotic lesions throughout the thoracic aorta were significantly larger in ApoE(-/-):MMP-3(-/-) than in ApoE(-/-):MMP-3(+/+) mice (P<0.05) and contained more fibrillar collagen (P<0.01). Aneurysms in the thoracic and abdominal aortas were less frequent in ApoE(-/-):MMP-3(-/-) than in ApoE(-/-):MMP-3(+/+) mice (8.5 +/- 1.7% vs 14 +/- 2.1% of sections, mean +/- SD, P<0.01). Immunocytochemistry revealed enhanced accumulation of macrophages in atherosclerotic lesions of ApoE(-/-):MMP-3(+/+) mice (P<0.01) and expression of urokinase-type plasminogen activator (u-PA) and MMP-3 colocalizing with macrophages. Zymography confirmed the presence of u-PA and MMP-3 activity in extracts of atherosclerotic aortas. These data suggest that plasmin, generated by macrophage-secreted u-PA, activates pro-MMP-3 produced by accumulated macrophages. MMP-3 activity may then contribute to a reduction of plaque size, possibly by degradation of matrix components, and promote aneurysm formation by degradation of the elastica lamina.