Characterization of insulin-like growth factor II binding to human fibroblast monolayer cultures.

Two major somatomedin peptides have been isolated from human plasma, somatomedin-C/insulin-like growth factor I (SMC/IGF-I) and insulin-like growth factor II (IGF-II). Also, two types of SM/IGF receptors have been defined. Type I receptors have a higher affinity for SMC/IGF-I than IGF-II, and insulin binds to this receptor at high concentrations. Type II receptors have a higher affinity for IGF-II than SMC/IGF-I, and insulin does not bind to this receptor site. In this study, we characterized the binding of IGF-II to human monolayer fibroblast cultures, and the affinity and specificity of this binding. We also compared the binding of IGF-II to the binding of insulin and SMC/IGF-I to these cells. The receptors for IGF-II on normal human fibroblast monolayers fit the criteria for type II SM/IGF receptors, and there were more type II receptors on these cells than either insulin receptors or type I SM/IGF receptors. The type II receptors on human fibroblasts did not demonstrate autoregulation by homologous hormone, unlike the type I SM/IGF and insulin receptors. In addition, they were not changed by acute or chronic exposure to insulin. It is so far unclear what biological function IGF-II plays in vivo. This human fibroblast system will be a valuable experimental model for the study of IGF-II receptors and their relationship to the biological actions of IGF-II.