Induction of RANKL expression and osteoclast maturation by the binding of fibroblast growth factor 2 to heparan sulfate proteoglycan on rheumatoid synovial fibroblasts

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Abstract
Objective Rheumatoid arthritis (RA) is characterized by progressive joint destruction. The aim of this study was to clarify the relevance of RA synovial fibroblasts (RASFs) and fibroblast growth factor 2 (FGF‐2), which is produced abundantly by RASFs, to the osteoclastogenesis and bone resorption in RA. Methods Synovial fibroblasts were prepared from the synovial tissues of 10 patients with active RA and 7 patients with osteoarthritis (OA). The expression of RANKL, intercellular adhesion molecule 1 (ICAM‐1), FGF receptor 1 (FGFR‐1), and heparan sulfate proteoglycan (HSPG) on synovial fibroblasts was measured by FACScan. Osteoclast formation in cocultures of RASFs and peripheral blood mononuclear cells (PBMCs) was evaluated by tartrate‐resistant acid phosphatase staining and a pit‐formation assay using dentin slices. Results FGF‐2 induced the expression of both RANKL and ICAM‐1 on RASFs more so than on OA synovial fibroblasts (OASFs). FGF‐2–induced up‐regulation of RANKL and ICAM‐1 was inhibited by anti–FGF‐2 antibody. Although FGFR‐1 was equally expressed on RASFs and OASFs, HSPG was highly expressed on RASFs. Up‐regulation of RANKL by FGF‐2 on RASFs was diminished by the removal of heparan sulfate with heparitinase. Osteoclast formation from PBMCs induced by RASFs was inhibited by the addition of either heparitinase, anti–ICAM‐1 antibody, anti–FGF‐2 antibody, or osteoprotegerin. FGF‐2–induced RANKL on RASFs and osteoclast formation were suppressed by an inhibitor of ERK. Conclusion FGF‐2 was transferred to FGFR‐1 through binding to HSPG, which is characteristically expressed on RASFs, resulting in RANKL‐ and ICAM‐1–mediated maturation of osteoclasts via ERK activation. Thus, we propose that FGF‐2 not only augments the proliferation of RASFs, but also is involved in osteoclast maturation, which leads to bone destruction in RA.

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