Discrete alterations of the insulin-like growth factor I molecule which alter its affinity for insulin-like growth factor-binding proteins result in changes in bioactivity

Insulin-like growth factor (IGF)-binding proteins (BPs) bind IGF-I and IGF-II with high affinity. They are present in extracellular fluids and modulate the interactions of their ligands with the type 1 IGF cell surface receptor. These studies utilized IGF-I analogs that have reduced binding affinity for either the type 1 IGF receptor or binding proteins to study the ligand specificity of IGF-BP-1 and the role of IGF-BP-1 in modulating the biological activity of IGF-I. The data indicate that the regions of IGF-I which are responsible for binding to IGF-BP-1 and to human serum-binding proteins are distinct but overlapping and are clearly distinct from the type I receptor binding sites. In the absence of exogenously added IGF-BP-1, the analogs with reduced affinity for IGF-BP-1 are more potent than IGF-I in stimulating DNA synthesis by porcine aortic smooth muscle cells. In contrast, when cells are concomitantly exposed to IGF-BP-1, two of the analogs with reduced affinity for binding protein give only 40-65% of the maximal IGF-I response. [Leu24, 1-62]IGF-I, which has a 100-fold reduced affinity for the type 1 IGF receptor, gave a value that was 62% of the maximal IGF-BP-1 potentiated response. A second biological response, that of stimulating binding protein secretion by IGF-I, was also examined. [Leu24, 1-62]IGF-I is more potent than IGF-I whereas the activity of the analogs with lower affinity for IGF-BP-1 is significantly reduced. Thus, the ability to activate DNA synthesis and binding protein secretion maximally in the presence of IGF-BP-1 is dependent on the affinity of IGFs for both type 1 receptors and binding proteins.