Bradykinin-induced IL-6 expression through bradykinin B2 receptor, phospholipase C, protein kinase Cδ and NF-κB pathway in human synovial fibroblasts

Bradykinin (BK) is an inflammatory mediator, and shows elevated levels in regions of severe injury and inflammatory diseases. It has been shown to induce interleukin-6 (IL-6) expression in inflammatory responses in rheumatoid arthritis. We investigated the signaling pathway involved in IL-6 production caused by BK in synovial fibroblasts. BK caused concentration- and time-dependent increases in IL-6 production. By using pharmacological inhibitors or genetic inhibition of the BK receptor, siRNA revealed that B2 but not B1 BK receptors are involved in BK-mediated up-regulation of IL-6. BK-mediated IL-6 production was attenuated by phospholipase C inhibitor (U73122), protein kinase Cδ inhibitor (rottlerin), NF-κB inhibitor (PDTC), IκB protease inhibitor (TPCK) and NF-κB inhibitor peptide. Stimulation of synovial fibroblasts with BK activated IκB kinase α/β (IKK α/β), IκBα phosphorylation, IκBα degradation, p65 phosphorylation at Ser276, p65 and p50 translocation from the cytosol to the nucleus and κB-luciferase activity. BK mediated an increase of IKK α/β and IκBα phosphorylation, κB-luciferase activity and p65 and p50 binding to the NF-κB element was inhibited by B2 BK receptor antagonist (HOE140), U73122 and rottlerin. Our results suggest that BK increased IL-6 production in synovial fibroblasts via the B2 BK receptor/PI-PLC/PKCδ/and NF-κB signaling pathway.