We previously reported that a 16-kDa proteolytic fragment of IGF Binding Protein-3 (IGFBP-3), which is devoid of affinity for IGFs, inhibits the mitogenic effects of IGF-I on chick embryo fibroblasts. Here, we set out to determine if the fragment had biological effects on fibroblasts from mouse embryos homozygous for a targeted disruption of the Type 1 IGF receptor gene. In the cell clone used, bFGF (but not IGF, EGF or PDGF) was mitogenic in serum-free medium, increasing ¹⁴C-thymidine uptake by a factor of 10–15 within 24 hours and doubling cell proliferation. The 16-kDa fragment, isolated by HPLC following limited proteolysis of recombinant human (rh) IGFBP-3 by plasmin, in both assays dose-dependently (20 to 100 ng/ml) inhibited (up to 100%) maximal stimulation induced by 25 ng/ml bFGF, whereas intact IGFBP-3 had virtually no effect. Similar results were obtained with control wild-type cells. In the latter, the mitogenic activity of 1% fetal calf serum (equal to that of 25 ng/ml bFGF) was inhibited by only 25–30% by 100 ng/ml 16-kDa fragment or 200 ng/ml rhIGFBP-3. This agrees with an antagonistic action, affecting the mitogenic activity of serum that is attributable to IGFs.

The 16-kDa IGFBP-3 fragment therefore appears to be a potent inhibitor of mitogenic signals resulting from activation of both the type 1 IGF and FGF receptors.