Insulin-like growth factor (IGF) binding to cell monolayers is directly modulated by the addition of IGF-binding proteins.

Insulin-like growth factor-I (IGF-I) binds to specific receptors and IGF-binding proteins (IGFBPs) that are present on cell surfaces. The analysis of [125I]IGF-I binding to human fibroblasts is complicated by IGFBPs on the cell surface and their release into the medium during the binding assay. This release alters the distribution of [125I]IGF-I between type I IGF receptors and both soluble as well as cell surface-associated IGFBPs. In the present study we have determined the effects of three different forms of IGFBPs on [125I]IGF-I binding to cell surface binding sites of human fetal fibroblasts (GM10 cells) and porcine smooth muscle cells. Human 29,000 mol wt (Mr; IGFBP-1), bovine 34,000 Mr (IGFBP-2), and bovine 46,000 Mr (IGFBP-3) forms of IGFBP were compared. Each of the three IGFBPs inhibited [125I]IGF-I binding to the cell surface of both cell types. This effect was due to increased binding of [125I]IGF-I by the IGFBPs in the assay buffer. At equimolar concentrations, IGFBP-3 was more effective than either IGFBP-1 or IGFBP-2 in blocking cell surface binding. The addition of increasing concentrations of unlabeled IGF-I in the presence of each IGFBP showed that either IGFBP-1 or IGFBP-3, but not IGFBP-2, resulted in a paradoxical increase in [125I]IGF-I binding to the cell surface. The paradoxical increase occurred in the presence of excess insulin, indicating that unsaturated type I IGF receptors are not required to demonstrate this phenomenon. In a physiological salt solution, the order of affinity of the IGFBPs for IGF-I was IGFBP-3 greater than IGFBP-1 greater than IGFBP-2. These differences in affinity appear to account for the differences in IGF-I competition for binding that are seen when each of the three proteins is added. Thus, IGFBPs have the potential to alter the partitioning of IGF-I between cell surface-associated IGFBPs, membrane receptors, and the IGFBPs in extracellular fluids. The various forms of IGFBP affect IGF cell surface binding differently, and therefore, each may have distinct effects on IGF target cell actions.