Characterization of insulin-like growth factor (IGF) binding proteins and their role in modulating IGF-I action in BHK cells.
We have found that over one-half of the total cell surface 125I-insulin-like growth factor I (IGF-I) binding to BHK cells represents binding to IGF binding proteins (IGFBPs) rather than to the IGF-I receptor. In addition to a number of secreted IGFBPs, we have now characterized two cell-associated IGFBPs with unique characteristics. The cell-associated IGFBPs have molecular weights of 30,000 (30K) and 25,000 (25K), as determined by the Western ligand blot technique. IGFBP-30K is located at the cell surface and can be readily labeled by affinity cross-linking with 125I-IGF-I. Surface expression of IGFBP-30K increases 5.4 +/- 1.2-fold (n = 11) with serum starvation. This induction is fully evident by 4 h, plateauing by 24 h, and is completely inhibitable by cycloheximide. The fasting-induced increase in IGFBP-30K is inhibited by IGF-I and by des-IGF-I and, to a lesser extent, by insulin. Unlike cell-associated IGFBP-30K, secretion of IGFBP was stimulated (6.8 +/- 0.5-fold, n = 2) by IGF-I, whereas IGFBP secretion was inhibited 54% by insulin. These results demonstrate coordinate regulation of IGFBP by serum starvation and IGF-I, such that at low concentrations of IGF-I, cell surface binding protein increases whereas binding protein secretion decreases. At high concentrations of IGF-I, IGFBP secretion increases and cell surface IGF-I receptor, as well as IGFBP, decreases. Taken together, these regulatory events regulate the availability of IGF-I for biologic signalling.
D000345:Affinity Labels; D000818:Animals; D001667:Binding, Competitive; D015153:Blotting, Western; D002352:Carrier Proteins / Q000187:drug effects / Q000302:isolation & purification / Q000378:metabolism*; D002455:Cell Division / Q000187:drug effects; D002460:Cell Line; D006224:Cricetinae; D016895:Culture Media, Serum-Free; D004305:Dose-Response Relationship, Drug; D004591:Electrophoresis, Polyacrylamide Gel; D007328:Insulin / Q000494:pharmacology; D018970:Insulin-Like Growth Factor Binding Protein 1; D007334:Insulin-Like Growth Factor I / Q000378:metabolism* / Q000494:pharmacology*; D007668:Kidney; D007700:Kinetics; D008647:Mesocricetus; D008970:Molecular Weight; D017526:Receptor, IGF Type 1 / Q000302:isolation & purification / Q000378:metabolism*
D000345:Affinity Labels; D002352:Carrier Proteins; D016895:Culture Media, Serum-Free; D007328:Insulin; D018970:Insulin-Like Growth Factor Binding Protein 1; D007334:Insulin-Like Growth Factor I; D017526:Receptor, IGF Type 1