['Interleukin-32 (IL-32) is a recently discovered cytokine that appears to play a critical role in human rheumatoid arthritis (RA). It is highly expressed in synovium and fibroblast-like synoviocytes (FLS) from RA patients, but not in patients with osteoarthritis (OA). This study was undertaken to assess IL-32 levels in RA synovial fluid (SF) and to investigate the secretion and regulation of IL-32 in RA FLS.', 'FLS and SF were obtained from the joints of RA patients. The secretion and expression of IL-32 and activation of signaling molecules were examined by enzyme-linked immunosorbent assay, immunoblotting, immunoprecipitation, reverse transcriptase-polymerase chain reaction, and small interfering RNA (siRNA) transfection.', 'IL-32 levels were high in RA SF compared with OA SF. Furthermore, RA FLS expressed and secreted IL-32 when stimulated with tumor necrosis factor alpha (TNFalpha). TNFalpha-induced expression of IL-32 was significantly suppressed, in a dose-dependent manner, by inhibitors of Syk, protein kinase Cdelta (PKCdelta), and JNK and by knockdown of these kinases and c-Jun with siRNA. We also observed that PKCdelta mediated the activation of JNK and c-Jun, and experiments using specific inhibitors and siRNA demonstrated that Syk was the upstream kinase for the activation of PKCdelta.'] Is tumor necrosis factor alpha-induced interleukin-32 positively regulated via the Syk/protein kinase Cdelta/JNK pathway in rheumatoid synovial fibroblasts?