Early investigations from our laboratory revealed that skeletal cells secrete growth factors with important effects on bone cell function. Over the past decade significant advances were made in the identification and study of growth factors synthesized by skeletal cells, and research conducted in our laboratory revealed that insulin-like growth factors (IGF) I is one of the most prevalent skeletal growth factors, having important effects on bone formation. IGF I is likely to act as an autologous regulator of bone metabolism and to interact with systemic hormones, other growth factors, and IGF binding proteins (IGFBP). During the past five years, studies supported by this grant led to a better understanding of the role of IGF I in bone remodeling. We identified the agents that regulate the synthesis of IGF I and found that parathyroid hormone (PTH) and cAMP inducers are stimulatory and that basic fibroblasts growth factor (bFGF) and platelet derived growth factor (PDGF) BB are inhibitory. We established that IGF I stimulates bone formation and it inhibits bone collagen degradation although the mechanism of this effect is still unknown. We determined that osteoblast enriched cells from fetal rat calvariae (Ob cells) express IGFBP-2,3,4,5 and 6 and that IGFBP-3 and 5, like IGF I, are cAMP dependent and IGFBP-5 is IGF I dependent, suggesting a role for IGFBP 3 and 5 in the local control of IGF I action. During the course of the next five years we wish to extend these studies to enhance our understanding of the role of IGF I and IGFBPs in bone cell function.
The first aim of the proposed research is to study the mechanism of action of PTH, bFGF and PDGF on skeletal IGF I synthesis. This will be done by determining effects on the transcriptional regulation of IGF I and by defining the IGF I gene elements involved in its regulation in Ob cells and osteoblast cell lines.
The second aim of these investigations is to characterize the protective effects of IGF I on bone collagen degradation and define the mechanisms of IGF I action, by examining IGF I effects on collagenase mRNA and concentrations, and on the synthesis of collagenase inhibitors.
The third aim i s to study the regulation of IGFBP-3 and 5 by cAMP inducers and IGF I and define the role of these IGFBPs in bone formation. This will be done by examining IGFBP-3 and 5 concentrations, using specific radioimmunoassays, mRNA expression, using rat cDNA clones, and studies to test changes in IGFBP-3 and 5 stability. We also plan to examine the distribution of IFGBP-3 and 5 and their interactions with IGF I as they relate to effects on bone cell function, and determine whether these are modified when IGFBPs are free or attached to the cell matrix. This work is valuable because IGF I has important effects on bone cell function and it is our hypothesis that its synthesis and actions are strictly regulated. Our current understanding of bone cell physiology and metabolic bone disease is limited, and this research may provide future therapeutic alternatives for metabolic bone disorders.
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