title: Regulation of vascular smooth muscle cell responses to insulin-like growth factor (IGF)-I by local IGF-binding proteins.
abstract: Insulin-like growth factor (IGF)-I is a pleiotropic hormone that regulates vascular smooth muscle cell (VSMC) migration, proliferation, apoptosis, and differentiation. These actions are mediated by the IGF-I receptor. How activation of the same receptor by the same ligand leads to these diverse cellular responses is not well understood. Here we describe a novel mechanism specifying VSMC responses to IGF-I stimulation, distinctive for the pivotal roles of local IGF-binding proteins (IGFBPs). The role of local IGFBPs was indicated by comparing the activities of IGF-I and des-1-3-IGF-I, an IGF-I analog with reduced binding affinity to IGFBPs. Compared with IGF-I, des-1-3-IGF-I was more potent in stimulating DNA synthesis but much less potent in inducing directed migration of VSMCs. When the effects of individual IGFBPs were tested, IGFBP-2 and IGFBP-4 were found to inhibit IGF-I-stimulated DNA synthesis and migration. IGFBP-5 had an inhibitory effect on IGF-I-stimulated DNA synthesis, but it strongly potentiated IGF-I-induced VSMC migration. By using a non-IGF-binding IGFBP-5 mutant and an IGF-I-neutralizing antibody, it was demonstrated that IGFBP-5 also stimulates VSMC migration in an IGF-independent manner. This effect of IGFBP-5 was inhibited by soluble heparin and by treating cells with heparinase. Mutation of the heparin-binding motif of IGFBP-5 reduced its migration promoting activity. These findings suggest that local IGFBPs are important determinants of cellular responses to IGF-I stimulation, and a key player in this paradigm is IGFBP-5. IGFBP-5 not only modulates IGF-I actions, but it also stimulates cell migration by interacting with cell-surface heparan sulfate proteoglycans.
authors.affiliation: Department of Molecular, Cellular, and Developmental Biology, the University of Michigan, Ann Arbor, Michigan 48109, USA.
authors.forename: Tzefu
authors.initials: T
authors.lastname: Hsieh
authors.forename: Rebecca E
authors.initials: RE
authors.lastname: Gordon
authors.forename: David R
authors.initials: DR
authors.lastname: Clemmons
authors.forename: Walker H
authors.initials: WH
authors.lastname: Busby
authors.forename: Cunming
authors.initials: C
authors.lastname: Duan
mesh_terms: D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D002465:Cell Movement; D002633:Chemotaxis; D004247:DNA / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D006025:Glycosaminoglycans / Q000378:metabolism; D019812:Heparan Sulfate Proteoglycans / Q000378:metabolism; D019763:Heparin Lyase / Q000378:metabolism; D018971:Insulin-Like Growth Factor Binding Protein 2 / Q000378:metabolism; D018974:Insulin-Like Growth Factor Binding Protein 4 / Q000378:metabolism; D018975:Insulin-Like Growth Factor Binding Protein 5 / Q000378:metabolism; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D008024:Ligands; D009131:Muscle, Smooth, Vascular / Q000166:cytology*; D009154:Mutation; D011485:Protein Binding; D013552:Swine / Q000378:metabolism; D013997:Time Factors; D014162:Transfection
chemical_list: D006025:Glycosaminoglycans; D019812:Heparan Sulfate Proteoglycans; D018971:Insulin-Like Growth Factor Binding Protein 2; D018974:Insulin-Like Growth Factor Binding Protein 4; D018975:Insulin-Like Growth Factor Binding Protein 5; D008024:Ligands; D007334:Insulin-Like Growth Factor I; D004247:DNA; D019763:Heparin Lyase