Osteopontin is expressed by adult human osteoarthritic chondrocytes: protein and mRNA analysis of normal and osteoarthritic cartilage

Osteopontin, a sulfated phosphoprotein with cell binding and matrix binding properties, is expressed in a variety of tissues. In the embryonic growth plate, osteopontin expression was found in bone-forming cells and in hypert rophic chondrocytes. In this study, the expression of osteopontin was analy zed in normal and osteoarthritic human knee cartilage. Immunohistochemistry , using a monoclonal anti-osteopontin antibody was negative on normal carti lage. These results were confirmed in Western blot experiments, using parti ally purified extracts of normal knee cartilage. No osteopontin gene expres sion was observed in chondrocytes of adult healthy cartilage, however, in t he subchondral bone plate, expression of osteopontin mRNA was detected in t he osteoblasts. In cartilage from patients with osteoarthritis, osteopontin could be detected by immunohistochemistry, Western blot analysis, in situ hybridization, and Northern blot analysis. A qualitative analysis indicated that osteopontin protein deposition and mRNA expression increase with the severity of the osteoarthritic lesions and the disintegration of the cartil aginous matrix. Osteopontin expression in the cartilage was limited to the chondrocytes of the upper deep zone, showing cellular and territorial depos ition. The strongest osteopontin detection was found in deep zone chondrocy tes and in clusters of proliferating chondrocytes from samples with severe osteoarthritic lesions. These data show the expression of osteopontin in ad ult human osteoarthritic chondrocytes, suggesting that chondrocyte differen tiation and the expression of differentiation markers in osteoarthritic car tilage resembles that of epiphyseal growth plate chondrocytes. (C) 2000 Els evier Science B.V./International Society of Matrix Biology. All rights rese rved.