Evidence for the Synthesis of a Somatomedin Similar to Insulin‐Like Growth Factor I by Chick Embryo Liver Cells

Abstract

Chick embryo liver cells were incubated in serum-free and hormone-free medium and shown to produce a mitogen starting after the 1st day in culture. This mitogen was purified by gel filtration and ion-exchange chromatography and identified by several assay systems as a somatomedin similar to human insulin-like growth factor I (IGF-I). This somatomedin stimulates thymidine incorporation into DNA of chick fibroblasts, it reacts in an IGF-specific protein binding assay and it cross-reacts immunologically with antibodies prepared against human IGF-I, but not with those against IGF-II. Its behavior on ion-exchange chromatography and on sodium dodecyl sulfate/polyacrylamide gel electrophoresis is indistinguishable from that of authentic human IGF-I. Concomitantly with the mitogenic activity, chick liver cells produce an IGF-binding protein. Addition of purified human IGF-I to the liver cells inhibits production of the binding protein. Chick liver cells may thus be used to study synthesis and regulation of IGF and its binding protein.