Osteoclastic Bone Degradation and the Role of Different Cysteine Proteinases and Matrix Metalloproteinases: Differences Between Calvaria and Long Bone

Abstract

Osteoclastic bone degradation involves the activity of cathepsin K. We found that in addition to this enzyme other, yet unknown, cysteine proteinases participate in digestion. The results support the notion that osteoclasts from different bone sites use different enzymes to degrade the collagenous bone matrix. Introduction:The osteoclast resorbs bone by lowering the pH in the resorption lacuna, which is followed by secretion of proteolytic enzymes. One of the enzymes taken to be essential in resorption is the cysteine proteinase, cathepsin K. Some immunolabeling and enzyme inhibitor data, however, suggest that other cysteine proteinases and/or proteolytic enzymes belonging to the group of matrix metalloproteinases (MMPs) may participate in the degradation. In this study, we investigated whether, in addition to cathepsin K, other enzymes participate in osteoclastic bone degradation. Materials and Methods:In bones obtained from mice deficient for cathepsin K, B, or L or a combination of K and L, the bone-resorbing activity of osteoclasts was analyzed at the electron microscopic level. In addition, bone explants were cultured in the presence of different selective cysteine proteinase inhibitors and an MMP inhibitor, and the effect on resorption was assessed. Because previous studies showed differences in resorption by calvarial osteoclasts compared with those present in long bones, in all experiments, the two types of bone were compared. Finally, bone extracts were analyzed for the level of activity of cysteine proteinases and the effect of inhibitors hereupon. Results: The analyses of the cathepsin-deficient bone explants showed that, in addition to cathepsin K, calvarial osteoclasts use other cysteine proteinases to degrade bone matrix. It was also shown that, in the absence of cathepsin K, long bone osteoclasts use MMPs for resorption. Cathepsin L proved to be involved in the MMP-mediated resorption of bone by calvarial osteoclasts; in the absence of this cathepsin, calvarial osteoclasts do not use MMPs for resorption. Selective inhibitors of cathepsin K and other cysteine proteinases showed a stronger effect on calvarial resorption than on long bone resorption. Conclusions: Our findings suggest that (1) cathepsin K-deficient long bone osteoclasts compensate the lack of this enzyme by using MMPs in the resorption of bone matrix; (2) cathepsin L is involved in MMP-mediated resorption by calvarial osteoclasts; (3) in addition to cathepsin K, other, yet unknown, cysteine proteinases are likely to participate in skull bone degradation; and finally, (4) the data provide strong additional support for the existence of functionally different bone-site specific osteoclasts.

A Cathepsin K Inhibitor Reduces Breast Cancer–Induced Osteolysis and Skeletal Tumor Burden

“…Osteoclasts resorb bone by secreting protons and cathepsin K. Protons dissolve bone mineral and provide an acidic microenvironment, enabling cathepsin K to degrade the demineralized collagenous matrix (7). Matrix metalloproteinases are also involved in this resorptive process (18,19). However, cathepsin K is a key proteinase in bone resorption (7,8).…”

Interface membrane fibroblasts around aseptically loosened endoprostheses express MMP‐13

“…29 The MMPdriven matrix solubilization, which is found in long bones, could also happen in pathological conditions as they are found in aseptical loosening of endoprostheses. A highly significant correlation between collagen degradation and the level of MMP-13 found in interface tissues indicates that this enzymes participates in pathological periprosthetic tissue destruction.…”

Specialized roles for cysteine cathepsins in health and disease

“…Activated extracellular cathepsin L is capable of processing ECM proteins, such as fibronectin, laminin, and type I, IV, and XVIII collagen, even at neutral pH (76)(77)(78). Cysteine cathepsins, such as cathepsin S and cathepsin B, are highly abundant in the left ventricular myocardium of patients with hypertensive heart failure and therefore have been implicated in turnover of the ECM and cardiac remodeling in this disease (68,79).…”