The overall goal of this research is to extend our understanding of the mechanisms of action of hormones on bone. The approaches include a) biochemical studies of second messenger pathways in bone and changes in these pathways in response to calcemic hormones and b) pharmacological studies utilizing agents that affect these metabolic pathways in other tissues to determine their influence on bone resorption. The proposed studies would examine the functional significance of effects of parathyroid hormone (PTH) and other calcemic hormones on bone phospholipid metabolism. Studies to date indicate that concentrations of PTH and thrombin that are equieffective in stimulating resorption in vitro show marked differences their stimulation of inositol phosphate production, thrombin being the more effective agent. The proposed studies would further examine the effects of PTH and thrombin on bone phospholipid and calcium metabolism. Changes in inositol phosphates, inositol phospholipids, and diacylglycerol would be more fully characterizes in bone organ cultures, normal bone cells and osteosarcoma-derived cells. The cells would also be used to determine whether these agents differ in their effects on intracellular calcium. Possible bases for the observed differences in inositol phosphate production would be explored, including altered phosphatase activity, and the relative increases in cyclic AMP elicited by PTH and thrombin. The possibility that the changes in inositol phosphates are related to bone formation rather than resorption will be examined. Studies similar to those carried out with PTH and thrombin would be extended to other stimulators of resorption, including growth factors and cytokines. Pharmacological agents whose effects on resorption could be due to altered cellular calcium metabolism (e.g. tamoxifen, cyclosporine, and milrinone) will be tested for their effects on the Ca/phosphatidylinositol pathway. Agents shown to affect the pathways of phosphatidylinositol metabolism and protein kinase C (including RO 59 022 and bryostatin) will be examined for their effects on bone resorption and formation. Upgrading of presently available HPLC capabilities would permit measuremnt of newly recognized and potentially important inositol phosphate metabolites. The described studies should increase our understanding of the role of the calcium second messenger systems in the action of hormones on bone.
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