Microparticles stimulate the synthesis of prostaglandin E2 via induction of cyclooxygenase 2 and microsomal prostaglandin E synthase 1

Objective

Microparticles are small vesicles that are released from activated or dying cells and that occur abundantly in the synovial fluid of patients with rheumatoid arthritis (RA). The goal of these studies was to elucidate the mechanisms by which microparticles activate synovial fibroblasts to express a proinflammatory phenotype.

Methods

Microparticles from monocytes and T cells were isolated by differential centrifugation. Synovial fibroblasts were cocultured with increasing numbers of microparticles. Gene expression was analyzed by real‐time polymerase chain reaction and confirmed by Western blotting and enzyme immunoassay. Arachidonic acid labeled with tritium was used to study the transport of biologically active lipids by microparticles. The roles of NF‐κB and activator protein 1 (AP‐1) signaling were analyzed with electrophoretic mobility shift assay and transfection with small interfering RNA and IκB expression vectors.

Results

Microparticles strongly induced the synthesis of cyclooxygenase 2 (COX‐2), microsomal prostaglandin E synthase 1 (mPGES‐1), and prostaglandin E₂ (PGE₂). In contrast, no up‐regulation of COX‐1, mPGES‐2, cytosolic PGES, or phospholipase A₂ was observed. The induction of PGE₂ was blocked by selective inhibition of COX‐2. Microparticles activated NF‐κB, AP‐1, p38, and JNK signaling in synovial fibroblasts. Inhibition of NF‐κB, AP‐1, and JNK signaling reduced the stimulatory effects. Arachidonic acid was transported from leukocytes to fibroblasts by microparticles. Arachidonic acid derived from microparticles was converted to PGE₂ by synovial fibroblasts.

Conclusion

These results demonstrate that microparticles up‐regulate the production of PGE₂ in synovial fibroblasts by inducing COX‐2 and mPGES‐1. These data provide evidence for a novel mechanism by which microparticles may contribute to inflammation and pain in RA.