Expression of osteopontin messenger RNA and protein in rheumatoid arthritis - Effects of osteopontin on the release of collagenase 1 from articular chondrocytes and synovial fibroblasts

Objective. Osteopontin (OPN) is an extracellular matrix protein that has be en implicated in the interactions between tumor cells and host matrix, incl uding those involved in invasion and spread of tumor cells. Because joint d estruction in rheumatoid arthritis (RA) is mediated by the invasive growth of synovial tissue through its attachment to cartilage, we examined the exp ression of OPN in the synovia of patients with RA and the effect of OPN on the production of collagenase 1 in rheumatoid synovial fibroblasts and arti cular chondrocytes, Methods. The expression of OPN messenger RNA (mRNA) and protein in synovia from 10 RA patients was examined by in situ hybridization and immunohistoch emistry. Synovial fibroblasts from RA patients and articular chondrocytes f rom patients without joint disease were cultured in the presence of various concentrations of OPN, and levels of collagenase 1 in the culture supernat ants were measured by enzyme-linked immunosorbent assay. Results, The expression of OPN mRNA and protein was observed in 9 of 10 spe cimens obtained from patients with RA. OPN was expressed in the synovial li ning and sublining layer and at the interface of cartilage and invading syn ovium, Double labeling revealed that the majority of OPN-expressing cells w ere positive for the fibroblast-specific enzyme prolyl 4-hydroxylase and ne gative for the macrophage marker CD68, while only a few, single OPN-express ing cells were positive for CD68 at sites of synovial invasion into cartila ge. OPN staining was not observed in lymphocytic infiltrates or leukocyte c ommon antigen (CD45)positive cells. Three of 3 cultures of human articular chondrocytes secreted detectable basal amounts of collagenase, with a dose- dependent increase upon OPN stimulation, while synovial fibroblast cultures produced much lower levels of collagenase, with only 2 of 4 fibroblast cul tures responding in a dose-dependent manner. Conclusion. These findings suggest that OPN produced by synovial fibroblast s in the synovial lining layer and at sites of cartilage invasion not only mediates attachment of these cells to cartilage, but also contributes to ma trix degradation in RA by stimulating the secretion of collagenase 1 in art icular chondrocytes.