In some inherited disorders of the skeleton, including osteopetrosis and osteosclerosis, excess bone accumulates whereas in osteoporosis bone loss significantly exceeds bone gain. Historically, bone formation was associated with the osteoblast and resorption with the osteoclast. However, data has accumulated which indicates that the osteoblast takes an active role in the resorption process. This cell has been shown to be the primary target for a variety of resorption process. This cell has been shown to be the primary target for a variety of resorption agents (e.g., parathyroid hormone, PTH, and 1, 25(OH)2D3) and to be capable of producing neutral proteases, such as plasminogen activator and collagenase (C'ase). We have recently shown that PTH stimulates secretion of the latter enzyme by the rat osteoblastic cell line, UMR 106-01, with maximal concentrations of C'ase appearing in the extracellular medium 12-24 h after addition of the hormone. These subsequently decline becoming almost undetectable by 96 h. Further experiments revealed that the disappearance of previously secreted enzyme was cell-mediated and suggested the action of a membrane receptor for C'ase. We have now demonstrated the existence of such a receptor on UMR cells which may be analogous to those reported for serine proteases. These appear to be responsible for either activation and/or endocytosis of the secreted enzyme. This has led us to formulate the hypothesis that the C'ase receptor on osteoblastic cells is involved in the regulation of the amounts of extracellular bone C'ase and that down- regulation of such a moiety (e.g., by treatment of cells with PTH) may dictate the abundance and function of the enzyme in the extracellular milieu. Thus, the aims of the present proposal are to further characterize the receptor and to test this hypothesis.
These aims will be accomplished by: 1) delineating the characteristics of the binding reaction, including determining whether receptor number or affinity changes after PTH treatment, 2) assessing the role of the receptor in the cell-mediated turnover of the ligand, C'ase, and whether an alteration in the receptor modifies this process, 3) determining the biochemical properties of the receptor, both by cross-linking 125I-labelled C'ase to the receptor and by 125I-labelling the receptor, and finally, 4) using this information and techniques to purify the receptor and obtain peptide sequence data. The results of this work should aid in ascertaining the multiple sites of regulation in the osteoblast of elaborated C'ase and shed some light on the comprehensive role of this cell in the remodelling process (both matrix synthesis and degradation). In so doing, the data should also provide some insight into those many skeletal disorders where bone remodelling (? aberrant C'ase expression/uptake) has gone awry.
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