Chemical and biological characterization of chicken insulin-like growth factor-II

Chicken insulin-like growth factor-II (cIGF-II) has been characterized by amino acid sequencing, by its receptor and binding protein interactions and by its biological activities in cultured cells in order to help define the significance of the peptide in the growth process. Chicken IGF-II has an N-terminal region and several amino acid substitutions in the mid-peptide region that differ from the mammalian growth factor. Nevertheless, cIGF-II was indistinguishable from ovine IGF-II in all assay systems, including those involving chicken receptors and chicken binding proteins. Thus the amino acid substitutions did not modify the biological activities. In chick embryo fibroblasts, labelled bovine IGF-II or cIGF-II bound to a receptor with size and specificity properties expected for a type 1 IGF receptor, except that IGF-I competition for binding was less than IGF-II competition. No evidence for a type 2 receptor was obtained. The relatively lower biological activity of IGF-I compared with IGF-II in chick embryo fibroblasts contrasts with the much higher potency of IGF-I in rat L6 myoblasts. This difference can be explained by a combination of an inhibitory, IGF-II-specific binding protein produced only by the rat cells as well as the unusual receptor specificity of the chicken cells.