Cytosolic Phospholipase A2 Modulates TLR2 Signaling in Synoviocytes

Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic synovitis leading to destruction of cartilage and bone. PLA2 enzymes are key players in inflammation regulating the release of unsaturated fatty acids such as arachidonic acid (AA), a precursor of pro-inflammatory eicosanoids. Several lines of evidence point to toll-like receptors (TLRs) as drivers of synovitis and joint destruction in RA. However, few studies have addressed the implication of PLA2 activity downstream TLR activation in the synovium. Here, we aimed to characterize PLA2 enzyme involvement in TLR2-induced signaling in synovial fibroblast-like cells. TLRs1-7 and a range of sPLA2, iPLA2 and cPLA2 enzymes were found to be transcriptionally expressed in cultured synoviocytes. Activation of TLR2/1 and TLR2/6 led to phosphorylation of cPLA2 alpha at Ser(505), and induced AA release and PGE(2) production; effects that were attenuated by cPLA2 alpha inhibitors. In contrast, sPLA2 inhibitors did not affect AA or PGE(2) release. cPLA2 alpha inhibitors furthermore attenuated TLR-induced expression of IL-6, IL-8 and COX2. COX1/2 inhibitors attenuated TLR2/6-induced IL-6 transcription and protein production comparable to cPLA2 alpha inhibition. Moreover, exogenously PGE(2) added alone induced IL-6 production and completely rescued IL-6 transcription when added simultaneously with FSL-1 in the presence of a cPLA2 alpha inhibitor. Our results demonstrate for the first time that cPLA2 alpha is involved in TLR2/1-and TLR2/6-induced AA release, PGE(2) production and pro-inflammatory cytokine expression in synoviocytes, possibly through COX/PGE(2)-dependent pathways. These findings expand our understanding of cPLA2 alpha as a modulator of inflammatory molecular mechanisms in chronic diseases such as RA.