Modifications of Insulin-Like Growth Factor Binding Proteins and Their Role in Controlling IGF Actions

THE Insulin-like growth factor binding proteins (IGFBPs) are a family of six proteins that are found in all physiologic fluids These proteins have an extremely high affinity for IGF-I and IGF-II (e.g. 10J9 to 10-10 M), therefore, they bind these ligands tightly in interstitial fluids. Because their affinity is greater than the type I IGF receptor for each ligand, they act to partition the IGFs among three sites which include soluble forms of IGFBPs in interstitial fluids, IGFBPs that are bound to cell surfaces or extracellular matrix (ECM) and type I IGF receptors. When IGFBPs adhere to cell surfaces