Production of an Insulin-Like Growth Factor (IGF)- Inducible IGF-Binding Protein by Human Skin Fibroblasts*

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Abstract
Neonatal human skin fibroblasts produce insulin-like growth factor-binding proteins (IGFBPs) that have the potential to modulate the actions of the growth factors. We have examined the IGFBPs secreted by monolayer cultures of neonatal fibroblasts by ligand blotting with [125I]IGF-II and immunoblotting with antisera raised against three well characterized IGFBPs: IGFBP-1, IGFBP-2, and IGFBP-3. As detected by ligand blotting, medium from untreated fibroblasts contained IGFBP-3, a second IGFBP which appeared as a doublet of 29-31K, and a smaller protein of 22K. Within 10 h of the addition of 50 ng/ml IGF-I, a markedly increased level of production of the 29-31K IGFBP doublet was detectable, with levels increasing 8- to 9-fold by 24 h compared to that in untreated samples. IGF-I was approximately twice as potent as IGF-II at inducing 29-31K IGFBP, with a half-maximal response at 15.4 .+-. 2.7 ng/ml IGF-I and 26.6 .+-. 1.6 ng/ml IGF-II (n = 3). Insulin tested at 1 .mu.g/ml did not induce 29-31K IGFBP. Neither GH nor the acid-labile subunit of the circulating high mol wt IGFBP complex induced 29-31K IGFBP or affected its induction by IGF-I or IGF-II. Immunoblotting demonstrated that IGF-inducible IGFBP did not react with antibodies to IGFBP-1, IGFBP-2, or IGFBP-3. These results indicate that IGF-I and IGF-II induce an IGFBP that is different from previously characterized human IGFBPs.

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