Metabolic Actions of Insulin-Like Growth Factor-I in Normal Physiology and Diabetes

Abstract

Synopsis Insulin-like growth factor-I (IGF-I) is closely related to insulin but has distinct metabolic actions. IGF-I is an important stimulant of protein synthesis in muscle but it also stimulates free fatty acid utilization. Important indirect effects of IGF-I that influence metabolism include suppression of growth hormone secretion and at supraphysiologic concentrations suppression of insulin secretion. IGF-I actions are regulated by IGF binding proteins and in obesity and metabolic syndrome there is major dysregulation of IGF binding protein secretion resulting in alterations in the concentration of free IGF-I and IGF-I actions. In type 1 diabetes, IGF-I synthesis is markedly impaired and in type 2 diabetes multiple changes occur in IGF-I actions including sensitization to its mitogenic actions in some target tissues. Administration of IGF-I to patients with extreme insulin resistance results in improvement in glycemic control and IGF-I has been shown to be associated with lowering glucose and enhancing insulin sensitivity in both type 1 and type 2 diabetes. However diabetics are also quite sensitive to stimulation of side effects in response to IGF-I and this has greatly limited its usefulness as a hypoglycemic agent. IGF-I coordinately links growth hormone and insulin actions as well as having direct effects on intermediary metabolism.