Insulin-like growth factors.

The insulin-like growth factors (IGF-I and IGF-II) play important roles in the regulation of growth and metabolism. While the liver is the main source of circulating IGFs, their production by numerous extrahepatic tissues suggests the existence of autocrine and paracrine modes of action in addition to typical endocrine mechanisms. The actions of the IGFs are mediated through their activation of specific cell surface receptors, primarily the IGF-I receptor, although some effects may be mediated through the IGF-II receptor and the insulin receptor. The stability of the IGFs and their interaction with their receptors are mediated by specific IGF binding proteins (IGF-BPs) which are found in the circulation and in extracellular fluids. Thus, the overall biological actions of the IGFs can be regulated by control of ligand biosynthesis, modulation of receptor levels and postreceptor signalling pathways, and changes in the levels and activity of IGF-BPs.