Insulin-like growth factor binding protein-5 (IGFBP-5) interacts with thrombospondin-1 to induce negative regulatory effects on IGF-I actions

• Published in: Journal of cellular physiology 2005-05, Vol.203 (2), p.328-334
• Main Authors: Moralez, Anna M., Maile, Laura A., Clarke, Jane, Busby Jr, Walker H., Clemmons, David R.
• Format: Article
• Language: English
• Subjects: Animals; Antigens, CD - metabolism; Antigens, Differentiation - metabolism; CD47 Antigen; Cell Movement - drug effects; Cell Movement - physiology; Cells, Cultured; Dose-Response Relationship, Drug; Down-Regulation - drug effects; Down-Regulation - physiology; Insulin-Like Growth Factor Binding Protein 5 - metabolism; Insulin-Like Growth Factor Binding Protein 5 - pharmacology; Insulin-Like Growth Factor I - antagonists & inhibitors; Insulin-Like Growth Factor I - metabolism; Integrin alphaVbeta3 - drug effects; Integrin alphaVbeta3 - metabolism; MAP Kinase Signaling System - drug effects; MAP Kinase Signaling System - physiology; Membrane Glycoproteins - metabolism; Muscle, Smooth, Vascular - drug effects; Muscle, Smooth, Vascular - metabolism; Mutation - genetics; Neural Cell Adhesion Molecule L1 - metabolism; Phosphorylation - drug effects; Protein Binding - physiology; Receptor, IGF Type 1 - metabolism; Receptors, Immunologic - metabolism; Sus scrofa; Thrombospondin 1 - metabolism; Thrombospondin 1 - pharmacology
• ISSN: 0021-9541; 1097-4652
• DOI: 10.1002/jcp.20343

Insulin‐like growth factor binding protein‐5 (IGFBP‐5) and thrombospondin‐1 (TS‐1) are both present in extracellular matrix (ECM). Both proteins have been shown to bind to one another with high affinity. The purpose of these studies was to determine how the interaction between IGFBP‐5 and TS‐1 modulates IGF‐I actions in porcine aortic smooth muscle cells (pSMC) in culture. The addition of increasing concentrations of TS‐1 to pSMC cultures enhanced the protein synthesis and cell migration responses to IGF‐I; whereas the addition of IGFBP‐5 alone resulted in minimal changes. In contrast, the addition of IGFBP‐5 to cultures that were also exposed to IGF‐I and TS‐1 resulted in inhibition of protein synthesis. When the cell migration response was assessed, the response to IGF‐I plus TS‐1 was also significantly inhibited by the addition of IGFBP‐5, whereas 1.0 μg/ml of IGFBP‐5 alone had no effect on the response to IGF‐I.To determine the molecular mechanism by which this inhibition occurred, a mutant form of IGFBP‐5 that does not bind to IGF‐I was tested. This mutant was equipotent compared to native IGFBP‐5 in its ability to inhibit both protein synthesis and cell migration responses to IGF‐I plus TS‐1 thus excluding the possibility that IGFBP‐5 was inhibiting the response to TS‐1 and IGF‐I by inhibiting IGF‐I binding to the IGF‐I receptor. To determine if an interaction between TS‐1 and IGFBP‐5 was the primary determinant of the inhibitory effect of IGFBP‐5, an IGFBP‐5 mutant that bound poorly to TS‐1 was utilized. The addition of 1.0 μg/ml of this mutant did not inhibit the protein synthesis or cell migration responses to IGF‐I plus TS‐1. To determine the mechanism by which IGFBP‐5 binding to TS‐1 inhibited cellular responses to TS‐1 plus IGF‐I, TS‐1 binding to integrin associated protein (IAP) was assessed. The addition of IGFBP‐5 (1.0 μg/ml) inhibited TS‐1–IAP association. In contrast, a mutant form of IGFBP‐5 that bound poorly to TS‐1 had a minimal effect on TS‐1 binding to IAP. Further analysis showed that IGFBP‐5 addition altered the ability of TS‐1 to modulate the SHPS‐1/IAP interaction. When the IGFBP‐5 mutant that did not bind to IGF‐I was incubated with TS‐1 and IGF‐I, it inhibited the capacity of TS‐1 to enhance the IGF‐I receptor phosphorylation and MAP kinase activation in response to IGF‐I. In contrast, the IGFBP‐5 mutant that did not bind to TS‐1 had no effect on IGF‐I stimulated IGF‐I receptor phosphorylation or MAP kinase activation. These res