Identification of a family of low-affinity insulin-like growth factor binding proteins (IGFBPs): Characterization of connective tissue growth factor as a member of the IGFBP superfamily

• DOI: 10.1073/pnas.94.24.12981
• Publication Date: 2002-07-26T14:40:52Z
• Authors: Ho-Seong Kim; Srinivasa R. Nagalla; Youngman Oh; Elizabeth Wilson; Charles T. Roberts; Ron G. Rosenfeld
• Keywords: 0303 health sciences; Sequence Homology, Amino Acid; Molecular Sequence Data; Connective Tissue Growth Factor; Immediate-Early Proteins; 3. Good health; Insulin-Like Growth Factor Binding Proteins; Epitopes; Nephroblastoma Overexpressed Protein; 03 medical and health sciences; Somatomedins; Multigene Family; Humans; Intercellular Signaling Peptides and Proteins; Amino Acid Sequence; RNA, Messenger; Cloning, Molecular; Growth Substances; Baculoviridae; Protein Binding

The insulin-like growth factor (IGF) binding proteins (IGFBPs) modulate the actions of the insulin-like growth factors in endocrine, paracrine, and autocrine settings. Additionally, some IGFBPs appear to exhibit biological effects that are IGF independent. The six high-affinity IGFBPs that have been characterized to date exhibit 40–60% amino acid sequence identity overall, with the most conserved sequences in their NH2and COOH termini. We have recently demonstrated that the product of the mac25/IGFBP-7 gene, which shows significant conservation in the NH2terminus, including an “IGFBP motif” (GCGCCXXC), exhibits low-affinity IGF binding. The closely related mammalian genes connective tissue growth factor (CTGF) gene,nov, andcyr61encode secreted proteins that also contain the conserved sequences and IGFBP motifs in their NH2termini. To ascertain if these genes, along with mac25/IGFBP-7, encode a family of low-affinity IGFBPs, we assessed the IGF binding characteristics of recombinant human CTGF (rhCTGF). The ability of baculovirus-synthesized rhCTGF to bind IGFs was demonstrated by Western ligand blotting, affinity cross-linking, and competitive affinity binding assays using125I-labeled IGF-I or IGF-II and unlabeled IGFs. CTGF, like mac25/IGFBP-7, specifically binds IGFs, although with relatively low affinity. On the basis of these data, we propose that CTGF represents another member of the IGFBP family (IGFBP-8) and that the CTGF gene, mac25/IGFBP-7,nov, andcyr61are members of a family of low-affinity IGFBP genes. These genes, along with those encoding the high-affinity IGFBPs 1–6, together constitute an IGFBP superfamily whose products function in IGF-dependent or IGF-independent modes to regulate normal and neoplastic cell growth.