Loss of Insulin Actions on Arterial Smooth Muscle Cells May Enhance Unstable Atherosclerotic Plaque Formation

Abstract

Formation of unstable atherosclerotic plaque, a major cause of cardiovascular mortality, is accelerated in insulin resistance and diabetes. Previous studies have suggested that endothelial dysfunction and systemic hyperlipidemia due to loss of insulin actions can accelerated its formation. However, the mechanism for insulin resistance to accelerate the various pathologies of unstable plaque, such as decreases in VSMC and extra cellular matrix (ECM) and increased inflammatory cells, have not been defined. Since insulin actions on the vascular wall are diminished in insulin resistant states, we assessed the loss of insulin action in VSMC and its effect on the formation of atherosclerotic plaque. Insulin receptor (IR) was specifically deleted from VSMC by breeding SM22 a promoter Cre and IR Flox mice to produce SMIRKO mice which were mated with ApoE-/- mice to generate SMIRKO/ ApoE-/- mice. Cultured VSMC from SMIRKO/ApoE-/- mice showed decreased insulin signaling and stimulated proliferation by >50%, but not to PDGF. Weight, blood pressure, glucose, and systemic insulin tolerance were comparable between ApoE-/- and SMIRKO/ApoE-/- mice; which were fed with normal chow to mimic pathophysiological hyperlipidemia encountered clinically. Surprisingly, SMIRKO/ ApoE-/- mice developed significantly more atherosclerotic aortic plaques (Sudan IV staining for lipid content) compared to ApoE-/- mice with parallel increases of macrophage content and inflammatory cytokines such as IL18 and ICAM1. In contrast, SMC as measured by SM22α staining was decreased with elevation of necrotic plaques (p<0.05). Further, proliferative index of VSMC as measured by Ki67 staining and expression of hyaluronan synthase-2 were decreased. Thus, surprisingly, the loss of insulin’s specific actions on VSMC to increase proliferation increased inflammatory cell influx as observed in insulin resistance, may accelerated formation of unstable plaques in diabetes. Disclosure Q. Li: None. J. Fu: None. K. Park: None. C. Rask-Madsen: Research Support; Self; Novo Nordisk A/S. G.L. King: Research Support; Self; Sanofi-Aventis.