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Abstract
Neointimal hyperplasia is a major event in atherosclerosis and restenosis after angioplasty. It is attributable, in part, to exaggerated proliferation of vascular smooth muscle cells (VSMCs). Despite the use of drug-eluting stents to limit intimal hyperplasia, in-stent restenosis still remains a major clinical problem. Several lines of evidence suggest that insulin resistance increases the risk of vascular proliferative disease. In this regard, the contribution of systemic versus vessel wall-specific insulin resistance toward dysregulated VSMC phenotype remains unclear. Pioglitazone (PIO), a classical insulin sensitizer that belongs to the family of PPARg agonists, reduces neointimal hyperplasia after coronary angioplasty in diabetic and nondiabetic subjects. However, the molecular mechanisms by which PIO regulates VSMC phenotype have not been fully elucidated. In particular, the likely intermediary role of AMP-activated protein kinase (AMPK) toward PIO inhibition of VSMC proliferation remains unclear. The objectives of our study were to examine the role of dysregulated insulin receptor signaling in VSMC proliferation and to identify the molecular mechanisms by which PIO prevents neointima formation after arterial injury. Using human aortic VSMCs in vitro, we demonstrated that high fructose treatment dysregulates proximal insulin receptor signaling events. However, high fructose did not affect platelet-derived growth factor (PDGF)-induced proliferative signaling. These findings suggest that systemic rather than VSMC-specific dysregulation of insulin signaling plays a major role in enhancing atherosclerosis and neointimal hyperplasia. Next, we demonstrated that PIO inhibits PDGF-induced key proliferative signaling events in VSMCs through AMPK-dependent and AMPK-independent mechanisms. In particular, PIO activates AMPK to induce raptor phosphorylation, which diminishes PDGF-induced mTOR activity. In addition, PIO inhibits the basal phosphorylation of ERK, independent of AMPK, thereby decreasing cyclin D1 expression and Rb phosphorylation. Furthermore, AMPK-dependent inhibition of mTOR and AMPK-independent inhibition of ERK signaling occur regardless of PPARg expression/activation in VSMCs. Using arterial injury model in vivo, we demonstrated that an AMPK inhibitor (compound C) partially reverses PIO-mediated inhibition of neointima formation. Collectively, our findings suggest that local delivery of PIO at the lesion site may limit restenosis after angioplasty without inducing PPARg- mediated systemic adverse effects