CYSTATIN-C, AN INHIBITOR OF BONE-RESORPTION PRODUCED BY OSTEOBLASTS

The effects of human cystatin C on bone resorption, enzyme release; os teoclast generation, bone cell proliferation and bone matrix protein b iosynthesis have been examined in different in vitro systems. The effe cts of cystatin C were compared with those of calcitonin and E 64 Epox ysuccinyl-L-leucyl-amido-(4-guanidino)butane). Recombinant human cysta tin C and E 64 dose dependently inhibited the mobilization of Ca-45 an d the release of H-3 (from [H-3]-proline-labelled bones) in mouse calv ariae stimulated to resorb by parathyroid hormone (PTH) or 1,25(OH)(2) -vitamin D-3. Cystatin C and E 64 also inhibited the release of Ca-45 from bones stimulated by thrombin, interleukin-l and prostaglandin E-2 in PTH-stimulated bones, the inhibitory action of cystatin C and E 64 on Ca-45 release was observed after 6-9 h, whereas the inhibitory eff ect on 3H release was seen after just 2 h. In contrast. calcitonin cau sed an inhibition of both Ca-45 and H-3 release which was seen after 2 h. The PTH-stimulated release of the lysosomal enzymes was not affect ed by cystatin C and E 64, whereas calcitonin caused a significant inh ibition. in contrast to calcitonin, cystatin C did not affect PTH-stim ulated enhancement of osteoclast generation in the mouse calvariae. Us ing Western blot analysis and radioimmunoassay, we demonstrated that m ouse calvarial bones and MC3T3-E1 cells produce cystatin C. These data show that cystatin C is synthesized by bone cells and that recombinan t human cystatin C inhibits bone resorption in vitro without affecting bone cell proliferation, bone matrix formation or osteoclast generati on. The mechanism seems to be due primarily to inhibition of the activ ity of osteoclastic proteolytic enzymes released into the resorption l acunae.