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

Chick embryo liver cells were incubated in serum‐free and hormone‐free medium and shown to produce a mitogen starting after the first 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 behaviour on ion‐exchange chromatography and on sodium dodecylsulfate/poly acrylamide 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 slay thus be used to study synthesis and regulation of IGF and its binding protein.