Osteopontin (OPN) is a proinflammatory protein with a critical role in leukocyte migration. Although OPN has been implicated in rheumatoid arthritis (RA), its underlying mechanism remains unknown. In this study, we investigated the role and molecular mechanism of OPN in the induction of 2 key chemokines, monocyte chemoattractant protein 1 (MCP-1) and macrophage inflammatory protein 1beta (MIP-1beta), in RA.
Enzyme-linked immunosorbent assay and quantitative polymerase chain reaction were used to determine chemokine expression. Leukocyte migration in the presence of OPN was measured by chemotaxis assay. Signaling and molecular events were analyzed by immunoblotting and chromatin immunoprecipitation.
The effect of OPN on inflammatory cell migration was mediated through its unique property of inducing the expression of MCP-1 and MIP-1beta in CD14+ monocytes. The concentration of OPN was significantly elevated in RA patients and appeared to correlate with the serum levels of inflammation markers and increased expression of MCP-1 or MIP-1beta in monocytes in RA patients. Endogenous production of OPN in RA synovial fluid was attributable to increased production of MCP-1 or MIP-1beta, and this effect could be blocked by an anti-OPN antibody. Furthermore, the structural motif responsible for this property resided within residues 50-83 of human OPN, sparing the known RGD or SVVYGLR sequences. It was evident that the effect of OPN on chemokine expression was mediated through both the NF-kappaB and MAPK pathways, involving the activation of IKKbeta, p38, and JNK.
These results support a unique role of OPN in leukocyte migration, in the context of perpetuation of rheumatoid synovitis through the induction of MCP-1 and MIP-1beta.