Abstract
The prevalence of atherosclerosis and incidence of cardiovascular events increase with advancing age. Increased generation and/or decreased scavenging of reactive oxygen species (ROS) in the vascular wall are major determinants of vascular disease progression. We previously showed that SOD2 deficiency in aging increases vascular oxidative stress and reduces aortic compliance, while young ApoE-/-/SOD2+/- mice show increased mitochondrial DNA damage and accelerated atherogenesis. Here we investigated the effects of aging and SOD2 deficiency on atherosclerosis and vascular wall morphology in ApoE-/- mice. Compared with the control ApoE-/-, aged ApoE-/-/SOD2+/- mice had increased total vascular wall (P < 0.001) and mitochondrial ROS levels (P < 0.05) and 70% higher atherosclerotic lesion area (P < 0.001). Pulse wave velocity (P < 0.001) and aortic wall collagen content (P < 0.05) were significantly increased, while atherosclerotic plaque fibrous cap thickness (P < 0.01) and vascular smooth muscle cell (VSMC)-positive area (P < 0.01) were decreased in aged ApoE-/-/SOD2+/- mice. Immunofluorescence analysis of atherosclerotic lesions revealed higher levels of calpain 2, a cysteine protease implicated in apoptosis and cleaved caspase-3 localized in smooth muscle-positive areas of the plaque in aged ApoE-/-/SOD2+/- mice. VSMC isolated from aged SOD2+/- had higher levels of mitochondrial oxidative stress and dysfunction compared with VSMC from wild-type mice, as determined by increased superoxide generation and mitochondrial protein carbonylation and lower complex I and III activity. The calpain 2 protein levels were significantly higher in the SOD2+/- VSMCs compared with the wild-type cells and its expression and activity increased after TNF-α treatment, causing apoptosis. TNF-α increased MMP2 activity in SOD2+/- VSMC in Calpain 2-dependent manner. In conclusion, enhanced mitochondrial oxidative stress in aging increases aortic stiffening, atherosclerotic burden and plaque instability in advanced atherosclerosis, in part, via the activation of calpain 2 and MMP2. Targeting mitochondrial oxidative stress and calpain 2 activity may be a viable strategy to prevent aging-associated atherosclerosis complications.
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