The Regulation of Collagenolysis in Bone

Jilka, Robert

Abstract

Vertebrate collagenase (E.C. 3.4.23.7) and some acid proteases are secreted from bone in culture, but the cells and enzymes which mediate hormone stimulated collagen degradation in bone are not well defined. We propose to test the hypothesis that 1) non-osteoclastic cell secretes at least some of the enzymes or factors that regulate collagenolysis upon exposure of bone to hormone and 2) lysosomal enzymes are not directly involved in the control of initiation of collagen fibril degradation. Using media from neonatal murine calvaria and bone derived cells cultured with or without parathormone or 1,25-Vitamin D3, we will attempt to define the hormone regulated enzyme or protein which is reponsible for activation of latent collagenase. This entity may act by: 1) cleavage of zymogen, 2) the conversion of procollagenase to collagenase by an activator enzyme or stochiometric activator and 3) the level of secretion of a collagenase inhibitor protein. Collagenase activity will be monitored using labeled reconstituted collagen fibrils. Using partially purified bone cell populations we will then determine which cells are responsible for procollagenase secretion and the factors which control its activity. In the long term, with the development of methods to isolate relatively pure populations of osteoclasts and other bone cells, we plan to directly define the cell type(s) responsible for collagenolysis, using biochemical and immunohistochemical methods. The role of lysosomes and osteoclasts in the regulation of collagenolysis will also be studied by examining the enzymatic and cellular details of the noncalcified collagenolytic response in calvaria and cells of osteopetrotic microphthalmic (mi/mi) mice, upon exposure to resorptive agents. Bones of these animals have apparently inactive osteoclasts and no hormone stimulated increase in lysomal enzyme release has been observed in culture. We plan to critically examine the lysosomal enzymes that could be released from such bone and also determine the effects of inhibitors of lysosomal enzyme release upon the hormone stimulated degradation of noncalcified collagen in normal and mi/mi calvaria. We anticipate that our results will provide a better understanding of the regulation of matrix destruction in bone so that disorders in which matrix destruction is impaired or excessive may be better understood and treated.