Abstract-Aging is associated with an increased risk for atherosclerosis. A possible cause is low numbers and dysfunction of endothelial progenitor cells (EPC) which insufficiently repair damaged vascular walls. We hypothesized that decreased levels of insulin-like growth factor-1 (IGF-1) during age contribute to dysfunctional EPC. We measured the effect of growth hormone (GH), which increases endogenous IGF-1 levels, on EPC in mice and human subjects. We compared EPC number and function in healthy middle-aged male volunteers (57.4Ϯ1.4 years) before and after a 10 day treatment with recombinant GH (0.4 mg/d) with that of younger and elderly male subjects (27.5Ϯ0.9 and 74.1Ϯ0.9 years). Middle-aged and elderly subjects had lower circulating CD133 ϩ /VEGFR-2 ϩ EPC with impaired function and increased senescence. GH treatment in middle-aged subjects elevated IGF-1 levels (126.0Ϯ7.2 ng/mL versus 241.1Ϯ13.8 ng/mL; PϽ0.0001), increased circulating EPC with improved colony forming and migratory capacity, enhanced incorporation into tube-like structures, and augmented endothelial nitric oxide synthase expression in EPC comparable to that of the younger group. EPC senescence was attenuated, whereas telomerase activity was increased after GH treatment. Treatment of aged mice with GH (7 days) or IGF-1 increased IGF-1 and EPC levels and improved EPC function, whereas a two day GH treatment did not alter IGF-1 or EPC levels. Ex vivo treatment of EPC from elderly individuals with IGF-1 improved function and attenuated cellular senescence. IGF-1 stimulated EPC differentiation, migratory capacity and the ability to incorporate into forming vascular networks in vitro via the IGF-1 receptor. IGF-1 increased telomerase activity, endothelial nitric oxide synthase expression, phosphorylation and activity in EPC in a phosphoinositide-3-kinase/Akt dependent manner. Small interference RNA-mediated knockdown of endothelial nitric oxide synthase in EPC abolished the IGF-1 effects. Growth hormone-mediated increase in IGF-1 reverses age-related EPC dysfunction and may be a novel therapeutic strategy against vascular disorders with impairment of EPC. Patients with reduced EPC levels are at increased risk for cardiovascular events and death. 3,4 Recent studies suggest augmentation of circulating EPC to result in improved coronary collateral development in coronary artery disease. 5 Increasing age is associated with decreased number 6 and impaired function of EPC, 7 which may facilitate atherosclerotic processes. Regulation of EPC mobilization, differentiation and function is complex, but specific growth hormones and cytokines are explicitly involved. 8 Insulin-like growth factor-1 (IGF-1) enhances migration, tube formation and angiogenesis of mature endothelial cells 9 and increases te- Low serum IGF-1 levels, common in the elderly, are associated with an increased risk for ischemic heart disease. 12 Restoration of IGF-1 in elderly individuals by growth hormone therapy may have significant beneficial health effects. 13 In growth hormone de...

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