Binding of insulin-like growth factor (IGF) I or II to IGF-binding protein-2 enables it to bind to heparin and extracellular matrix.
Insulin-like growth factor (IGF)-binding protein-2 (IGFBP-2) is secreted by several cell types that also secrete IGF-I or IGF-II. The binding of IGF-I or IGF-II to IGFBP-2 has been shown to alter their actions. Although IGFBP-2 is not an abundant component of the extracellular matrix (ECM), it is easily localized by immunohistochemical staining in basement membranes of several epithelial cell types. We have previously shown that IGFBP-5 associates with glycosaminoglycans and binds to proteoglycans that are contained in ECM and basement membranes. In those studies we were unable to demonstrate an association of IGFBP-2 with glycosaminoglycans. In this study we report that IGFBP-2 binds to heparin if IGF-I or IGF-II is also included in the incubation buffer. IGFBP-1, -3, -4, or -5 did not have increased heparin binding in the presence of IGF-I or IGF-II. The binding of IGFBP-2 to heparin was detectable using an IGF-I to IGFBP-2 molar ratio of 2:1. Binding to heparin-Sepharose could be inhibited by soluble heparin or heparan sulfate, but not by glycosaminoglycans that do not contain O-linked sulfate groups in the 2 or 3 position of the iduronic acid ring. Binding was also inhibited by a synthetic IGFBP-5 peptide that contained a heparin-binding domain, but not by a peptide with an identical charge to mass ratio that does not bind to heparin. Binding appeared to be physiologically significant, as IGFBP-2 bound to human fibroblast ECM if IGF-I or IGF-II was added. IGF-II was more potent than IGF-I in facilitating the binding interaction, and des(1-3)-IGF-I or insulin had no effect, suggesting that IGF binding to IGFBP-2 is required for this effect to be detected. In summary, IGFBP-2 binding to glycosaminoglycans is dependent upon binding of IGF-I and IGF-II to IGFBP-2. This suggests that IGFBP-2 undergoes a conformational change that exposes a glycosaminoglycan-binding domain. This could provide a mechanism for focally concentrating IGFBP-2 in the pericellular environment.
Department of Medicine, University of North Carolina School of Medicine, Chapel Hill 27599, USA.
T
T
Arai
W
W
Busby
D R
DR
Clemmons
D000595:Amino Acid Sequence; D000818:Animals; D001485:Basement Membrane / Q000378:metabolism; D002417:Cattle; D005109:Extracellular Matrix / Q000378:metabolism*; D005347:Fibroblasts; D006493:Heparin / Q000378:metabolism*; D006801:Humans; D018971:Insulin-Like Growth Factor Binding Protein 2 / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism*; D018972:Insulin-Like Growth Factor Binding Protein 3 / Q000378:metabolism; D018974:Insulin-Like Growth Factor Binding Protein 4 / Q000378:metabolism; D018969:Insulin-Like Growth Factor Binding Proteins / Q000378:metabolism; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D007335:Insulin-Like Growth Factor II / Q000378:metabolism*; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry / Q000494:pharmacology; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000378:metabolism
D018971:Insulin-Like Growth Factor Binding Protein 2; D018972:Insulin-Like Growth Factor Binding Protein 3; D018974:Insulin-Like Growth Factor Binding Protein 4; D018969:Insulin-Like Growth Factor Binding Proteins; D010446:Peptide Fragments; D011994:Recombinant Proteins; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II; D006493:Heparin