Glycosaminoglycans inhibit formation of the 140 kDa insulin-like growth factor-binding protein complex

Abstract

The 140 kDa insulin-like growth factor (IGF)-binding protein complex in human serum consists of three subunits: an acid-labile, non-IGF-binding glycoprotein (alpha-subunit), an IGF-binding glycoprotein known as BP-53 or IGFBP-3 (beta-subunit), and IGF-I or IGF-II (gamma-subunit). This study investigates the regulation, by salt and glycosaminoglycans, of ternary (alpha-beta-gamma) complex formation, measured by incubating radioiodinated alpha-subunit with a mixture of IGF-I and IGFBP-3 and precipitating bound radioactivity with an anti-IGFBP-3 antiserum. Increasing NaCl concentrations progressively decreased ternary complex formation without any effect on binary (beta-gamma) complex formation. In 0.15 M-NaCl, the association constant for the ternary complex was 0.318 +/- 0.092 nM-1, 100-fold lower than that for the binary complex. Glycosaminoglycans also inhibited ternary complex formation without affecting the binary complex. Heparin [50% inhibition at 0.27 +/- 0.08 units/ml (1.5 +/- 0.4 micrograms/ml)] was more potent than heparan sulphate (50% inhibition at 15 +/- 7 micrograms/ml), with chondroitin sulphate even less potent. The inhibition by heparin was due principally to a decrease in binding affinity, from 0.604 +/- 0.125 to 0.151 +/- 0.024 nM-1 in the presence of 0.25 units of heparin/ml, with a slight decrease in the number of apparent binding sites from 1.05 +/- 0.08 to 0.85 +/- 0.15 mol of alpha-subunit bound/mol of beta-subunit. Since the ternary IGF-binding protein complex cannot cross the capillary barrier, it is proposed that a decrease in the affinity of the complex, mediated by circulating or cell-associated glycosaminoglycans, may be important in the passage of IGFs and IGFBP-3 to the tissues.