Cathepsin K Is a Critical Protease in Synovial Fibroblast-Mediated Collagen Degradation

Abstract

Synovial fibroblasts (SFs) play a critical role in the pathogenesis of rheumatoid arthritis (RA) and are directly involved in joint destruction. Both SF-resident matrix metalloproteases and cathepsins have been implicated in cartilage degradation although their identities and individual contributions remain unclear. The aims of this study were to investigate the expression of cathepsin K in SFs, the correlation between cathepsin K expression and disease severity, and the contribution of cathepsin K to fibroblast-mediated collagen degradation. Immunostaining of joint specimens of 21 patients revealed high expression of cathepsin K in SFs in the synovial lining and the stroma of synovial villi, and to a lesser extent in CD68-positive cells of the synovial lining. Cathepsin K-positive SFs were consistently observed at sites of cartilage and bone degradation. Expression levels of cathepsin K in the sublining and vascularized areas of inflamed synovia showed a highly significant negative correlation with results derived from the Hannover Functional Capacity Questionnaire (r = 0.78, P = 0.003; and r = 0.70, P = 0.012, respectively) as a measure of the severity of RA in individual patients. For comparison, there was no correlation between Hannover Functional Capacity Questionnaire and cathepsin S whose expression is limited to CD-68-positive macrophage-like synoviocytes. The expression of cathepsin K was also demonstrated in primary cell cultures of RA-SFs. Co-cultures of SFs on cartilage disks revealed the ability of fibroblast-like cells to phagocytose collagen fibrils whose intralysosomal hydrolysis was prevented in the presence of a potent cathepsin K inhibitor but not by an inhibitor effective against cathepsins L, B, and S. The selective and critical role of cathepsin K in articular cartilage and subchondral bone erosion was further corroborated by the finding that cathepsin K has a potent aggrecan-degrading activity and that cathepsin K-generated aggrecan cleavage products specifically potentiate the collagenolytic activity of cathepsin K toward type I and II collagens. This study demonstrates for the first time a critical role of cathepsin K in cartilage degradation by SFs in RA that is comparable to its well-known activity in osteoclasts.

The role of proteases in pathologies of the synovial joint

“…An up-regulation of cathepsin K in synovial fibroblast-like cells has also been reported in both rheumatoid and osteoarthritis (Hou et al, 2001(Hou et al, , 2002Hummel et al, 1998). In rheumatoid arthritis cathepsin K-positive cells were observed in areas of high proliferation within the synovium and at sites of cartilage and bone degradation (Hou et al, 2001).…”

“…An up-regulation of cathepsin K in synovial fibroblast-like cells has also been reported in both rheumatoid and osteoarthritis (Hou et al, 2001(Hou et al, , 2002Hummel et al, 1998). In rheumatoid arthritis cathepsin K-positive cells were observed in areas of high proliferation within the synovium and at sites of cartilage and bone degradation (Hou et al, 2001). Ex vivo studies demonstrated engulfment and internalization of type II collagen by fibroblast-like synoviocytes at sites of cartilage erosion and its accumulation within lysosomes in the presence of a cathepsin K inhibitor (Hou et al, 2001).…”

“…These include a number of the MMPs (Cawston, 1995), the aspartic protease cathepsin D ) and the cysteine proteases cathepsins B, L, K and S (Hou et al, 2001;Nguyen, Mort, & Roughley, 1990). Within cartilage the apparent protection of collagen proteolysis conferred by aggrecan and the requirement of aggrecanase-mediated aggrecan IGD cleavage for pathological progression may reflect in part an inability of the tissue to activate MMP whilst aggrecan aggregates are intact.…”

“…All of the collagenases are present in cartilage pathology and may play a role in type II collagen degradation (Kevorkian et al, 2004). Triple helical collagen may also be cleaved by the cysteine protease cathepsin K, which is unique in its ability to cleave the triple helical region at multiple sites and is reportedly up-regulated in chondrocytes in a mouse osteoarthritis model and in the synovium of patients with rheumatoid arthritis (Garnero et al, 1998;Hou et al, 2001Hou et al, , 2002Morko, Söderström, Säämänen, Salminen, & Vuorio, 2004). Cathepsin K has an acidic pH optimum although interaction with chondroitin sulphate has been shown to stabilise the protein at neutral pH and may promote accumulation of cathepsin K within cartilage (Li, Hou, & Brömme, 2000).…”

“…In rheumatoid arthritis cathepsin K-positive cells were observed in areas of high proliferation within the synovium and at sites of cartilage and bone degradation (Hou et al, 2001). Ex vivo studies demonstrated engulfment and internalization of type II collagen by fibroblast-like synoviocytes at sites of cartilage erosion and its accumulation within lysosomes in the presence of a cathepsin K inhibitor (Hou et al, 2001). These studies suggest that fibroblastlike synoviocytes expressing cathepsin K are important in the clearance of endocytosed collagen at sites of cartilage erosion.…”