Annexin I surface binding sites and their regulation on human fibroblast‐like synoviocytes

Objective Annexin I is a glucocorticoid‐inducible protein whose expression in rheumatoid synovium and inhibitory actions in animal models of arthritis suggests its involvement in human arthritis. The present study explored the potential for annexin I to mediate its antiinflammatory actions via specific cell‐surface binding sites on human fibroblast‐like synoviocytes (FLS). Methods Annexin I binding sites on cultured FLS from patients with osteoarthritis (OA) and rheumatoid arthritis (RA) were determined by ligand‐binding flow cytometry. Phospholipase A2 (PLA2) activity was determined by arachidonic acid release. Results FLS exhibited saturable, concentration‐dependent cell‐surface annexin I binding, with >99% of the OA FLS exhibiting binding at an annexin I concentration of 10 μM. Annexin I binding of RA FLS was significantly lower than that of OA FLS. FLS annexin I binding sites were not affected by elastase or a specific elastase inhibitor, and elastase release did not differ between RA and OA cells. In contrast, collagenase significantly increased annexin I binding sites on OA FLS and approached a significant effect on RA FLS. Tumor necrosis factor α increased annexin I binding sites on OA and RA FLS. Similarly, interleukin‐1β significantly increased annexin I binding on OA FLS; but the increased binding on RA FLS was not significant. Dexamethasone exerted no significant effect on OA or RA FLS annexin I binding sites. Treatment of RA FLS with an annexin I N‐terminal peptide significantly inhibited RA FLS PLA2 activity. Conclusion This is the first description of the expression, regulation, and function of cell surface annexin I binding sites on FLS. Reduced annexin I binding sites in RA FLS may impair the sensitivity of certain proinflammatory events to glucocorticoids.