Background: Osteopontin is a proinflammatory protein which production is shown to increase in rheumatoid arthritis (RA) and osteoarthritis (OA). OPNs cytokine properties are dependent on phosphorylation, which is controlled by extracellular tartrate-resistant acid phosphatase (TRACP), also a biomarker for RA. Our objective was to assess the phosphorylation status of OPN in RA and OA, and its correlation with TRACP levels.
Methods: Synovial fluid was obtained from 16 RA patients (8 seropositive and 8 seronegative) and four OA patients. Western blot method was used to analyse OPNs phosphorylation and TRACP levels in synovia. The protein membranes were exposed to chemiluminescence films and band optical densities were quantified. Immunohistochemistry was done to find TRACP secreting cells in synovial tissue.
Results: Immunoblotting showed the presence of phosphorylated OPN and TRACP in both RA and OA synovia. Full length OPN was more heavily phosphorylated in RA (seropositive OD 0.200±0.018) than in OA (OD 0.153±0.017) (P=0.011). Thrombin cleaved C-terminal end of OPN was also more phosphorylated in RA (P=0.003). TRACP correlated negatively with phospho-OPN. RA patients had less TRACP in their synovia (seropositive OD 0.287±0.071) than OA patients (OD 0.398±0.076) (P=0.015). TRACP secreting cells were found all along the synovial epithelium with immunohistochemistry.
Conclusion: Phosphorylated OPN in synovia leads to an increased macrophage activation and inflammation in RA patient joints when compared with OA patient joints. Phospho-OPN is also shown to increase the production of cartilage degrading enzymes in chondrocytes, and bone resorption in osteoclasts. Although it is shown that TRACP production increases in RA when compared with healthy subjects, the enzyme levels are not high enough to dephosphorylate enough OPN as in OA levels of phospho-OPN are lower.
Disclosure: The authors declared no competing interests.