Proteasome inhibition restored the reduced expression of fibroblast growth factor receptor 3 in rheumatoid arthritis synovial fibroblasts

Background. Synovial hyperplasia is a hallmark of rheumatoid arthritis (RA). This might be associated with an imbalance of growth-promoting and apoptotic pathways, among them fibroblast growth factors (FGFs) and their receptors (FGFRs) . The aim was to investigate differences in FGFRs and to explore the factors that might explain the differences between RA and osteoarthritis (OA) synovial fibroblasts. Methods. To assess FGFRs expression, immunohistochemistry, flow cytometry and RT-qPCR were performed. The cells were treated with TNFa, FGF2, a demethylation agent, PKA-mimics or inhibitors, hypoxia-mimics or proteasome inhibitors. Proliferation was measured using the CCK8 assay and 20S proteasome activity by a fluorescent assay. Results. In RA, FGFR3 protein was decreased in the synovial lining layer (p<0.005) and in cultured RA synovial fibroblasts (RASF) (n=10, p < 0.01). Transcription was unchanged and DNA demethylation decrease its expression. Exposure to TNFa or FGF2 had no effect on FGFR3. PKA-modulation and hypoxia-mimics induced transient changes only. Most interesting, in RASF, the proteasome inhibitor MG-132 restored FGFR3 expression (p<0.001) to levels measured in normal or OA synovial fibroblasts. MG-132 abolished the enhanced proliferative response of RASF to FGF2 (p<0.005). Increased 20S proteasome activity correlated (r=- 0.63, p<0.05) with decreased expression of FGFR3. Conclusions. The expression of FGFR3 is reduced in RA partially due to an increased degradation in proteasomes. This leads to an imbalance in the FGF-related signal pathways and may contribute to synovial hyperplasia. Proteasome inhibitors could represent a novel therapeutic strategy in RA, particularly to prevent synovial hyperplasia.