Angiogenesis, the formation of neovessels from the endothelium of preexisting vessels, is stimulated by soluble angiogenic factors. Insulin-like growth factor I (IGF-I) stimulates myogenesis and induces nerve regeneration after injury, and it has been shown to stimulate angiogenesis. However, the in vivo angiogenic effects of IGF-I in regenerating and diabetic muscle have yet to be described. Therefore, we studied the effects of human IGF-I (hIGF-I) delivered by a plasmid-mediated therapy on angiogenesis in mouse models of these two conditions. Plasmid hIGF-I was delivered to the injured tibialis muscle by direct intramuscular injection followed by electroporation. Initial experiments compared two muscle-specific hIGF-I-expressing constructs containing either a skeletal actin 3′UTR (pAV2001) or a human growth hormone (GH) 3′UTR (pAV2002). Skeletal actin 3′UTR mediates sequestration of hIGF-I in the muscle and was more active, while the GH 3′UTR mediated release of IGF-I into the circulation. Treatment of regenerating muscle with pAV2001 and sequestration of IGF-I in muscle led to increased expression of vascular endothelial growth factor (VEGF) and VEGF receptors fetal liver kinase-1 and FmS-like tyrosine kinase receptor-1, as well as platelet endothelial cell adhesion molecule-1, on endothelial cells. These results indicate that IGF-I can amplify angiogenic responses in regenerating muscle. In a mouse diabetic model, plasmid-mediated IGF-I therapy reversed diabetic microangiopathy, as shown by increased angiogenesis and arterial flow as analyzed by Doppler imaging. These studies show that plasmid IGF-I delivery and sequestration in muscle can augment angiogenesis in regenerating muscle and increase blood flow and angiogenesis in the diabetic limb.
