Regulation of insulin-like growth factor I actions by insulin-like growth factor binding protein-5

The insulin-like growth factors are present in the pericellular space complexed to insulin-like growth factor binding proteins. Since these binding proteins generally have a higher affinity for the ligand than the cell surface, type-I IGF receptor, they control the ability of IGF-I to access receptors, and thus indirectly regulate IGF actions. The variables that regulate the affinity of the binding proteins, and therefore their capacity to control receptor access, include proteolytic cleavage, binding to cell surfaces and to extracellular matrix, as well as the abundance of each protein in the pericellular environment and their affinities for IGF-I or IGF-II. We have determined that insulin-like growth factor binding protein-5 (IGFBP-5) associates with extracellular matrix (ECM) with high affinity. Furthermore, we have shown that several specific proteins within the ECM, such as plasminogen activator inhibitor-1 (PAI-1), bind to IGFBP-5. When associated with ECM, its affinity is reduced by 8 fold, and it is protected from proteolytic cleavage. In contrast, when IGFBP-5 is present in culture medium, it is rapidly cleaved to a non-IGF binding 22 kDa fragment. Specific basic amino acids between amino acids 201 and 218 regulate the ability of IGFBP-5 to bind to ECM. The residues Arginine 207 and 211 appear to be the most important mediators of binding. Mutagenesis of these residues results in decreased binding to extracellular matrix, and decreased cellular responsiveness to IGF-I. In contrast to ECM-associated IGFBP-5, the form of this protein that is present in the pericellular fluid has a high affinity for IGF-I. We determined its cleavage site and mutated it to create a protease resistant form. Addition of this form to smooth muscle cell cultures resulted in significant inhibition of replication. Therefore, ongoing proteolytic cleavage of IGFBP-5 may be necessary for full expression of IGF-I actions. In summary, several variables in the pericellular environment are capable of modulating the ability of IGFBPs to alter IGF actions. Understanding how these variables alter IGF actions could be an important key to understanding the mechanism by which this growth factor stimulates anabolic effects.