Rheumatoid synovial fibroblasts (RASFs) mediate joint inflammation and destruction in rheumatoid arthritis (RA). Endothelial protein C receptor (EPCR) is a specific receptor for the natural anticoagulant activated protein C (APC). It mediates the cytoprotective properties of APC and is expressed in rheumatoid synovial tissue. A recent report shows that group V secretory phospholipase A2 (sPLA₂V) prevents APC from binding to EPCR in endothelium and inhibits EPCR/APC function. The aim of this study was to investigate the expression and function of EPCR on RASFs.
Human synovial fibroblasts (SFs) were isolated from RA or osteoarthritis (OA) synovial tissues and treated with control, EPCR, or sPLA₂V small interfering RNA (siRNA); recombinant human APC, tumor necrosis factor-alpha (TNF-α), or sPLA₂V. RASF viability and migration/invasion were measured by 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) and collagen gel migration/invasion assays, respectively, and cartilage degradation by 1,9-dimethylmethylene blue (DMMB) assay in the presence of human OA articular cartilage explants. The expression or activation of cytokines, EPCR, cadherin-11, mitogen-activated protein (MAP) kinases, and nuclear factor-kappa-B (NF-κB) or both were detected by enzyme-linked immunosorbent assay, Western blotting, or immunostaining.
EPCR was expressed by both OASFs and RASFs but was markedly increased in RASFs. When EPCR was suppressed by siRNA or blocking antibody cell viability, cell invasion and cartilage degradation were reduced by more than 30%. Inflammatory mediators interleukin-1-beta (IL-1β), cadherin-11, and NF-κB were significantly reduced by EPCR suppression under control or TNF-α-stimulated conditions. The expression or activation (or both) of MAP kinases ERK, p38, and JNK were also markedly decreased in cells transfected with EPCR siRNA. Further analysis revealed that sPLA₂V co-localized with EPCR on RASFs. Suppression of sPLA₂V reduced cell viability and cartilage degradation and increased APC binding to RASFs. Conversely, recombinant sPLA₂V increased cartilage degradation, blocked APC binding to RASFs, and could not rescue the effects induced by EPCR suppression.
Our results demonstrate that EPCR is overexpressed by RASFs and mediates the aggressive behavior of RASFs. This function of EPCR is contrary to its cytoprotective role in other settings and is likely driven by sPLA₂V.