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Submitted on October 18, 2002
Accepted on March 20, 2003
1 Tissue BioRegeneration and Integration Program, School of Life Sciences, Queensland University of Technology, 2 George Street, Brisbane, Qld, 4001, Australia.; Kolling Institute of Medical Research, University of Sydney, Royal North Shore Hospital, Pacific Highway, St Leonards, NSW, 2065, Australia.
* To whom correspondence should be addressed. E-mail: j.kricker{at}qut.edu.au.
Previous studies demonstrated that IGF-II binds directly to vitronectin (VN) while IGF-I binds poorly. However, binding of VN to integrins has been demonstrated to be essential for a range of IGF-I stimulated biological effects including IGF binding protein-5 (IGFBP-5) production, IGF type-1 receptor autophosphorylation and cell migration. Thus we hypothesized that a link between IGF-I and VN must occur and may be mediated through IGFBPs. This was tested using competitive binding assays with VN and [125I]-labeled IGFs in the absence and presence of IGFBPs. IGFBP-4, IGFBP-5 and non-glycosylated IGFBP-3 were shown to significantly enhance binding of IGF-I to VN, while IGFBP-2 and glycosylated IGFBP-3 had a smaller effect. Furthermore binding studies with analogs indicate that glycosylation status and the heparin-binding domain of IGFBP-3 are important in this interaction. To examine the functional significance of IGFs binding to VN, cell migration in MCF7 cells was measured and found to be enhanced when VN was pre-bound to IGF-I in the presence of IGFBP-5. The effect required IGF:IGFBP:VN complex formation; this was demonstrated by use of a non-IGFBP-binding IGF-I analog. Together, these data indicate the importance of IGFBPs in modulating IGF-I binding to VN and that this binding has functional consequences in cells.
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