The insulin-like growth factors (IGFs) are polypeptides structurally related to insulin that have potent growth-promoting and insulin-like effects in vitro and in vivo. Unlike insulin, IGFs associate with distinct proteins present in plasma and other biological fluids that may be key determinants of IGF bioavailability and bioactivity. At present, the factors regulating the production of the various IGF binding proteins and the precise role of the different proteins in modulating IGF action are unknown. Our central hypothesis is that IGF binding proteins are integral components of IGF physiology that define and coordinate IGF and insulin action at the level of the target cell. In this proposal, we focus on the predominant circulating IGF binding protein in adults: IGFBP-3.
The Specific Aims of this research project are (1) to determine the hormonal control of IGFBP-3 synthesis and secretion under defined conditions, (2) to ascertain the cellular mechanisms of IGFBP-3 action, (3) to assess quantitative and qualitative changes in IGFBP-3 production and function with aging in vitro, and (4) to define the regulation of IGFBP-3 in vivo. Major hypotheses to be tested are directed toward deciphering the complex relationship between IGF-I peptide and IGFBP-3: (a) IGF-I is a primary regulator of IGFBP-3, acting via both receptor-mediated and receptor- independent mechanisms, (b) IGFBP-3 regulates type I IGF receptor signalling at the level of ligand binding, and (c) Factors that alter IGFBP-3 availability can modulate IGF-I action independent of changes in IGF-I peptide or receptor concentration. An essential feature of this proposal is the development of appropriate models for evaluating the regulation and biological effects of IGFBP-3. We will employ cultured normal and SV40-transformed human fibroblasts and normal bovine fibroblasts because of their unique and complementary attributes well suited for investigations into IGFBP-3 cellular physiology. Information derived from these in vitro systems will be integrated with in vivo studies. The principal investigator has expertise in the various techniques of cellular physiology, molecular biology, and biochemistry necessary for the success of this proposal. These studies will help clarify the integrated role if IGFs and IGF binding proteins in the control of cell growth. Knowledge of the structure, function, and regulation of IGFBP-3 will have major implications for our understanding of the physiology and pathophysiology of IGF-I, as well as for applications of IGF-I in clinical medicine.
