Comparison of cathepsins K and S expression within the rheumatoid and osteoarthritic synovium
Authors: Hou Wu‐Shiun , Li Weijie , Keyszer Gernot , Weber Ekkehard , Levy Roger , Klein Michael J. , Gravallese Ellen M. , Goldring Steven R. , Brömme Dieter 
Published: October 2002

Objective

To determine and compare the expression of cathepsins K and S proteins in joints with rheumatoid arthritis (RA) and osteoarthritis (OA) and to determine the effect of interleukin‐1β (IL‐1β) and tumor necrosis factor α (TNFα) on the expression of cathepsin K in fibroblast‐like synoviocytes.

Method

Expression and localization of cathepsins K and S were determined by immunohistochemistry in the synovium of 10 RA‐ and 8 OA‐affected joints. Northern and Western blot analyses were performed to analyze cathepsin K and S expression in primary fibroblast‐like synoviocyte cultures from RA and OA patients. The effect of IL‐1β and TNFα on the expression and secretion of cathepsin K in primary cultures of synoviocytes was determined by real‐time polymerase chain reaction and Western blot analysis. Staining of in situ activity was used to identify active cathepsin K enzyme in primary synovial fibroblast cultures.

Results

Cathepsin K and S protein expression was identified in the synovium from patients with RA and OA. Cathepsin K protein was localized in synovial fibroblasts, stromal multinucleated giant cells, and, to a lesser degree, in CD68+ macrophage‐like synoviocytes. Of note is the expression of cathepsin K in synovial fibroblasts and mononuclear macrophage‐like cells at sites of cartilage erosion in RA and in interdigitating cells of lymphocyte‐rich areas. In contrast, cathepsin S expression was restricted to CD68+ macrophage‐like synoviocytes, interdigitating cells, and endothelial cells of blood vessels. Cathepsin K protein expression in the interstitial areas and perivascular regions of RA‐derived synovial specimens was 2–5 times higher than in OA samples (P < 0.001), whereas the expression of cathepsin S did not significantly differ in these diseases. Cathepsin K expression levels in normal synovium were low and restricted to fibroblast‐like cells. Of note, cathepsin K also was expressed in repairing fibrocartilage in 1 OA specimen. Primary cell cultures of RA‐ and OA‐derived synovial fibroblasts expressed comparable amounts of cathepsin K at the transcript and protein levels. Both cell cultures secreted mature cathepsin K as well as procathepsin K, and expressed active cathepsin K in cytosolic vesicles. In contrast, neither RA‐ nor OA‐derived fibroblasts expressed detectable levels of cathepsin S. IL‐1β and TNFα stimulated the transcript (7–8‐fold) and protein expression (2‐fold) of cathepsin K (P < 0.05) in primary synovial fibroblast cultures, without differences in expression between RA‐ and OA‐derived synovial fibroblasts.

Conclusion

The presence of cathepsin K polypeptide in synovial fibroblasts and macrophage‐like cells in normal, OA, and RA synovia suggests a constitutive expression of this protease and a role in synovial remodeling. The comparable increase in cathepsin K expression after stimulation of RA‐ and OA‐derived synovial fibroblasts with IL‐1β and TNFα further suggests that the expression of cathepsin K is independent of cellular alterations leading to the invasive phenotype of RA‐synovial fibroblasts. However, the overexpression of cathepsin K in RA synovia due to an increase in the number of cathepsin K–expressing cells identifies this enzyme as a candidate protease for the pathologic degradation of articular cartilage. Cathepsin S expression in macrophage‐like synoviocytes suggests dual activity in antigen presentation and matrix degradation in the inflamed synovia.