Insulin-like growth factors (IGFs) are important mediators of growth and differentiation of osteoblasts; however, six high-affinity IGF-binding proteins (IGFBPs 1-6) antagonize their binding to cell surface type-1 IGF receptors. Proteases produced by bone cells can degrade IGFBPs into low-affinity fragments, allowing for IGF-IGF receptor interactions to occur. We have identified pregnancy-associated plasma protein-A (PAPP-A) as an IGFBP-4 proteinase and MMP-13 (collagenase-3) as an IGFBP-5 proteinase, and both are produced by the osteoblast cell line, MC3T3-E1. Thus, proteolytic processing of these IGFBPs may modulate IGF action during osteoblastogenesis. We have created recombinant mutant forms of IGFBP-4 and IGFBP-5 that are resistant to proteolysis, and have used these constructs to establish stably transfected MC3T3-E1 cell lines overexpressing protease resistant IGFBP mutants. Herein, we propose to use these reagents in order to explore how IGFBP proteolysis influences osteoblast growth and differentiation. We will compare how protease-resistant forms of IGFBP-4 or -5 impact osteoblastogenesis compared to cleavable forms, and if inhibiting or down-regulating IGFBP-degrading proteinases (i.e., PAPP-A and MMP-13) influences this process. In vivo, we will examine how protease-resistant or sensitive forms of IGFBP-4 or -5 alter new bone formation, and if PAPP-A can overcome osteoinhibitory effects of transgenic overproduction of IGFBP-4 on new bone formation. To explore how MMP-13 may mediate anabolic affects of PTH on bone cells, we will determine how blunting of PTH-mediated up-regulation of MMP-13 may alter IGF action in osteoblasts in vitro. We will also explore how MMP-13 resistant forms of IGFBP-5 impact PTH actions on osteoblast cultures. In vivo studies will determine if systemic PTH administration alters the IGF-IGFBP-IGF protease axis in bone and if deletion of MMP-13 affects PTH and IGF-mediated new bone formation. Together, these studies will provide new insights into mechanisms controlling IGF bioavailability in bone, and how up-stream mediators of bone formation, such as PTH, may exert their affects on osteoblastogenesis through regulating IGF action.
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