Substitution of specific amino acids in insulin-like growth factor (IGF) binding protein 5 alters heparin binding and its change in affinity for IGF-I response to heparin

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 bind to insulin-like growth factor-I and -II with very high affinity. Because their affinity constants are between two- and 50-fold greater than the IGF-I receptor, they control the distribution of the IGFs among soluble IGFBPs in interstitial fluids, IGFBPs bound to cell surfaces or extracellular matrix (ECM) and cell surface receptors. Although there are six forms of insulin-like growth factor binding proteins, most interstitial fluids contain only three or four forms, and usually only one or two predominate. The proteins differ significantly in their biochemical characteristics, and this accounts for many of the differences that have been observed in their biological actions. Several different types of protease cleave these binding proteins. Proteolytic cleavage generally inactivates the binding proteins or reduces their ability to bind to IGF-I or -II substantially. Several cell types have been shown to secrete these proteases; therefore, the factors that regulate protease activity can control binding protein actions indirectly. Other post-translational modifications, such as glycosylation and phosphorylation, have been shown to alter IGF binding protein activity. While binding protein actions have been studied extensively in vitro, many of the in vivo activities of these proteins remain to be defined.

Characterization of glycosaminoglycan-binding domains present in insulin-like growth factor-binding protein-3

Matrix metalloproteinase 3 cleaves insulin-like growth factor-binding protein-3 (IGFBP-3) into six fragments, four of which bind heparin-Sepharose (Fowlkes, J. L., Enghild, J. J., Suzuki, K., and Nagase, H. (1994) J. Biol. Chem. 269, 25742-25746). Sequence analysis of IGFBP-3 heparin-binding fragments shows that all fragments contain at least one of two highly basic, putative heparin-binding consensus sequences present in IGFBP-3. Epitope-specific antibodies generated against synthetic peptides containing these domains recognized IGFBP-3, yet were significantly inhibited from binding in the presence of heparin, demonstrating that these regions of IGFBP-3 contain functional heparin-binding domains. IGFBP-3 peptides containing one of the two heparin-binding consensus sequences bound heparin in a solid phase binding assay in a dose-dependent and saturable manner. However, the IGFBP-3 peptide containing the heparin-binding consensus sequence 149KKGHA153 bound heparin with approximately 4-fold less affinity than the IGFBP-3 peptide containing the longer heparin-binding consensus sequence 219YKKKQCRP226. Examination of several well characterized glycosaminoglycans to inhibit the binding of heparin to both heparin-binding IGFBP-3 peptides revealed that the most potent inhibitors were heparin, heparan sulfate, and dermatan sulfate; chondroitin sulfate A and hyaluronic acid were intermediate in their inhibitory activities; and chondroitin sulfate C caused no inhibition. These studies identify and characterize the glycosaminoglycan-binding domains in IGFBP-3, providing a basis for the better understanding of IGFBP-3-glycosaminoglycan interactions at the cellular and extracellular interface.

Role of insulin-like growth factor binding proteins in the control of IGF actions

The insulin-like growth factor binding proteins have been shown to modify IGF actions. IGFBP-5 binds to extracellular matrix (ECM) and its ability to potentiate IGF activity is dependent upon the amount that is ECM associated. To determine the specific regions of IGFBP-5 that are required for ECM association, site directed mutagenesis has been used to prepare several forms of IGFBP-5. Mutants that have had the amino acids between positions 201 and 218 altered have been useful. Mutation of the lysine 211 resulted in no change in the affinity of IGFBP-5 for ECM or heparin Sepharose; however, it resulted in a major reduction in affinity for IGF-I following heparin binding. Other mutations which disrupted heparin binding also resulted in loss of this affinity shift. Most distruptive were mutations of amino acids 211, 214, 217 and 218 and 202, 206 and 207. Mutation of residues 201 plus 202 had some effect, but substitution for 207, 211, 217 and 218 had no effect. When binding to intact ECM was analyzed, similar results were obtained. This suggests that amino acids 202, 206 and 214 are definitely involved in heparin and ECM binding. When binding to proteoglycans such as tenascin and heparin sulfate proteoglycan was analyzed, similar results were obtained. IGFBP-5 also binds to other proteins in ECM, including type IV collagen and plasminogen activator inhibitor-I. Specific antisera for plasminogen activator inhibitor-1 can coprecipitate IGFBP-5. IGFBPs are degraded by specific proteases. Three proteases that degrade IGFBP-2, -4 and -5 have been characterized. They are serine proteases that cleave these proteins at basic residues. Although several well characterized serine proteases cleave IGFBP-4 or -5, the proteases in cell conditioned media appear to be distinct.