Abstract
AIMS: Increased prevalence of acute myocardial infarction related to diabetes and insulin resistance is associated with an elevated risk of unstable atherosclerotic plaques, which are characterized by reduced vascular smooth muscle cells (VSMCs) and extracellular matrix (ECM) and increased inflammation. Thus, insulin resistance may reduce plaque stability, as deleting insulin receptors (IRs) in VSMCs decreases their proliferation and enhances apoptosis. METHODS AND RESULTS: Direct effects of insulin on VSMCs to alter plaque composition were studied using mice with double knockout of ApoE and IR genes in VSMCs with SMIRKO/ApoE-/-, Myh11-CreERT2EYFP+/ApoE-/-, and Myh11-CreERT2EYFP+IRKO/ApoE-/- mice, which were also used for lineage tracing studies. Compared with ApoE-/- mice, SMIRKO/ApoE-/- mice exhibited more atherosclerotic plaques, which contained less VSMCs and collagen but increased levels of VSMC apoptosis and necrotic areas. Lineage tracing studies showed that Icam1+ Vcam1+ VSMC was inflammatory, which increased in the aortas of Myh11-CreERT2EYFP+IRKO/ApoE-/- mice compared with control mice. Isolated VSMCs lacking IRs expressed higher inflammatory cytokines than cells with IRs. Cell-based studies indicated that insulin's anti-apoptotic and pro-proliferative effects in VSMCs were mediated via activation of the IR/Akt pathway, which were decreased in VSMCs from SMIRKO or high-fat diet mice. An analysis of the IR targets that regulated inflammatory cytokines in VSMCs showed that thrombospondin 1 (Thbs1) and Mmp2 were consistently increased with a loss of IRs. Insulin inhibited Thbs1 expression, but not Mmp2 expression, through p-Akt/p-FoxO1 pathways in VSMCs from ApoE-/- mice, and was impaired in cells from SMIRKO/ApoE-/- mice. Thbs1 further induced Icam1 and Mmp2 expressions in VSMCs. CONCLUSION: Insulin via IRs has significant actions in VSMCs to decrease inflammation, apoptosis, and ECM turnover via the activation of Akt and FoxO1 pathways. The inhibition of insulin actions and related pathways related to insulin resistance and diabetes may contribute to the formation of unstable atherosclerotic plaques.