IGFBP-1, an insulin like growth factor binding protein, is a cell growth inhibitor

A novel cell growth inhibitor, IDF45 (inhibitory diffusible factor), was recently purified to apparent homogeneity. It is a bifunctional molecule: able to bind Insulin like growth factor (IGF) and to 100% inhibit DNA synthesis stimulated by serum in fibroblasts. It was of interest to verify whether other members of the IGF-binding protein (IGFBP) family show the same bifunctional growth inhibitory properties. In this paper we show that purified IGFBP-1 derived from amniotic fluid is a cell growth inhibitor. In chick embryo fibroblasts, it inhibited DNA synthesis stimulated by serum. However the stimulation was maximally 60% inhibited and half of the inhibition was observed with 100ng/ml IGFBP-1. So the specific activity of IGFBP-1 is lower than that of IDF45. IGFBP-1 also reversibly prevented the CEF growth. In the same cells IGFBP-1 inhibited DNA synthesis stimulated by IGF-I. We demonstrated that the same protein IGFBP-1 is able to inhibit DNA synthesis stimulated by serum and by IGF-I. The possibility that IGFBP-1 is a bifunctional molecule is discussed.