Abstract

The insulin-like growth factor binding protein (IGFBP) family comprises six structurally distinct, but highly homologous proteins. They have been identified in serum and other biological fluids, tissue extracts, and cell culture media. We have recently cloned cDNAs encoding human IGFBP-4, -5, and -6 and have now expressed these BPs in yeast as ubiquitin (Ub)-IGFBP fusion proteins. Western ligand blotting with 125I-IGF II under nonreducing conditions of recombinant human (rh) IGFBP-containing yeast lysates revealed specific binding bands for IGFBP-4, -5, and -6 at apparent molecular masses of 24-26, 30-32, and 24-26 kDa, respectively, indicating processing of the fusion proteins. High-performance liquid chromatography-purified rhIGFBPs had virually the same amino acid composition, amino acid number, and NH2-terminal sequences as the native BPs. Except for the affinity of rhIGFBP-6 for IGF I (Ka = 8.5 x 10(8) M-1), the affinity constants of the three IGFBPs for IGF I and II lie between 1.7 and 3.3 x 10(10) M-1, i.e. 25-100 times higher than the IGF I and II affinities of the type I IGF receptor. When present in excess, rhIGFBP-4, -5, and -6 inhibited IGF I- and II-stimulated DNA and glycogen synthesis in human osteoblastic cells, but rhIGFBP-6 had only a weak inhibitory effect on IGF I in agreement with its relatively lower IGF I affinity constant. The results of this study show that the primary effect of the three rhIGFBPs is the attenuation of IGF activity and suggest that IGFBPs contribute to the control of IGF-mediated cell growth and metabolism.