PRODUCTION OF CARTILAGE OLIGOMERIC MATRIX PROTEIN (COMP) BY CULTURED HUMAN DERMAL AND SYNOVIAL FIBROBLASTS

Objective. Cartilage oligomeric matrix protein (COMP) is a large disul fide-linked pentameric protein. Each of its five subunits is approxima tely 100,000 Da in molecular weight. COMP was originally identified an d characterized in cartilage and it has been considered a marker of ca rtilage metabolism because it is currently thought not to be present i n other joint tissues, except fur tendon. To confirm the tissue specif icity of COMP expression we examined cultured human dermal fibroblasts , human foreskin fibroblasts, and normal human synovial cells for the synthesis of COMP in culture. Method: Normal synovial cells and normal human dermal foreskin fibroblasts were isolated from the correspondin g tissues by sequential enzymatic digestions and cultured in media con taining 10% fetal bovine serum until confluent. During the final 24 h of culture, the cells were labeled with S-35-methionine and S-35-cyste ine in serum- and cysteine/methionine-free medium. The newly synthesiz ed COMP molecules were immunoprecipitated from the culture media with a COMP-specific polyclonal antiserum, or with monoclonal antibodies or affinity-purified COMP antibodies. The immunoprecipitated COMP was an alyzed by electrophoresis in 5.5% polyacrylamide gels. For other exper iments. synovial cells cultured from the synovium of patients with rhe umatoid arthritis(RA) and osteoarthritis(OA) were similarly examined. Results: A comparison of the amounts of COMP produced by each cell typ e (corrected for the DNA content) revealed that synovial cells produce d greater than or equal to 9 times more COMP than chondrocytes or derm al fibroblasts. COMP could be easily detected by immunoprecipitation i n all cell types. Electrophoretic analysis revealed a distinct band wi th an apparent MW of 115-120 kDa in samples from each of the three cel l types, regardless of the antibody used. COMP expression in cultures of synoviocytes derived from OA and RA patients showed that OA and RA synovial cells produced similar amounts of monomeric COMP of identical size to those COMP monomers produced by normal synovial cells. The ad dition of TGF-beta to these cultures resulted in an increase in COMP p roduction in normal, OA and RA synovial cells (45, 116 and 115% respec tively). Conclusion: These studies demonstrate that substantial amount s of COMP are pl oduced by several mesenchymal cells including synovio cytes and dermal fibroblasts. These findings raise important concerns regarding the utility of measurements of COMP levels in serum or in sy novial fluid as markers of articular cartilage degradation because of the likelihood that a substantial proportion of COMP or COMP fragments present in serum or synovial fluid may be produced by cells other tha n articular chondrocytes.