INSULIN-LIKE GROWTH-FACTOR BINDING-PROTEIN (IGFBP-3), AN INHIBITOR OF SERUM GROWTH-FACTORS OTHER THAN IGF-I AND IGF-II

Our results show that an insulin-like growth factor binding protein, IGFBP-3, purified from rat serum, is an inhibitor of chick embryo fibroblast (CEF) growth. It abolished DNA synthesis in CEF stimulated by IGF-I as well as by human serum. Rat IGFBP-3 and IDF45 (an inhibitory diffusible factor secreted by mouse cells) had the same activities, confirming that they have an intrinsic capacity to inhibit serum stimulation and may be considered as growth inhibitors. Our data show that inhibition by IGFBP-3 of serum stimulation was not simply the result of its inhibition of IGF present in the serum: 1) While anti-IGF-I IgG was able to completely inhibit stimulation induced by added IGF-I, it did not decrease stimulation induced by 1% human serum. Anti-IGF-II IgG inhibited the stimulation induced by added IGF-II, but only 25% decreased the stimulation induced by 0.7% serum. The percent inhibition was not significantly increased when the concentration of serum was decreased to 0.2%, which induced 140% stimulation of DNA synthesis; 2) stimulation by 0.2% serum was much more inhibited by IGFBP-3 than by IgG anti IGF-II; 3) after separation of IGF-I and IGF-II from serum by chromatography of acidified serum proteins on BioGel P150, the remaining serum proteins (with a molecular mass greater than 45 kDa) which were depleted in IGF-I and -II (verified by RIA determination) still stimulated DNA synthesis, and this stimulation was 80% inhibited by IGFBP-3.