Cathepsin B in osteoclasts

Active cathepsin B has been found in cell extract and medium of human osteoblast-like cells and MG-63 cells. The released form is stable at neutral and alkaline pH and, in both cell types, intracellular and extracellular cathepsin B activities are increased by interleukin-1 beta (IL- 1h) and parathyroid hormone (PTH). To evaluate the physiological role of cathepsin B in osteoblasts, we investigated the production and secretion of this enzyme in normal human synovial fibroblasts and modulation by IL-1h and PTH. Lactate secretion concurrent with release of cathepsin B and comparable responses in osteoblasts were also examined. Our data show that synovial fibroblasts respond differently to treatment with the two agents, suggesting a cell-specific regulation of cathepsin B and possible involvement in osteoblast physiology. Cathepsin B involvement was then evaluated in the activation of plasminogen activator (PA) in MG-63 cells using two specific inhibitors of cathepsin B, CA074 and CA074-Me, in constitutive conditions and after treatment with IL-1h. As results of PA activity obtained in the presence of IL-h were in contrast with previous reports, we examined the activities of PA, pro-PA activated with trypsin, and plasmin in cell extract and media of MG-63 cells after 24-h treatment with IL-1h. Results show that in normal conditions and in the presence of IL-1h, cathepsin B is involved in the activation of PA. Moreover, IL-1h stimulates PA, pro-PA activated by trypsin, and plasmin activity in medium, whereas in cell extract it stimulates pro-PA activated by trypsin and plasmin activity. IL-1h has no effect on cell extract-associated PA.