Elevated leptin levels in temporomandibular joint osteoarthritis promote proinflammatory cytokine IL-6 expression in synovial fibroblasts

Background Leptin, through binding to its special receptor (Ob-Rb), has potent effects on immunity and inflammation. This study measured the levels of leptin in the synovial fluid of patients with temporomandibular joint osteoarthritis (TMJ-OA) and healthy controls, determined the expression of Ob-Rb and explored the effects and signalling pathways involved in leptin-induced proinflammatory cytokine interleukin (IL)-6 production in TMJ synovial fibroblasts (TMJ-SFs). Methods Synovial fluid samples were obtained from 16 patients with TMJ-OA and seven healthy controls. Leptin levels were measured in synovial fluid using enzyme-linked immunosorbent assay (ELISA). Ob-Rb expression was determined by quantitative real-time polymerase chain reaction (qRT-PCR) and Western blot in cultured TMJ-SFs. Small interfering RNA (siRNA) was transfected into the TMJ-SFs to down-regulate the expression of Ob-Rb. qRT-PCR and ELISA were used to determine the levels of proinflammatory cytokine IL-6 in leptin-stimulated TMJ-SFs. The involved signalling pathways that mediate the leptin-stimulated production of IL-6 were investigated using specific signalling inhibitor analyses. Results Compared with healthy controls, patients with TMJ-OA had significantly higher concentrations of leptin in their synovial fluid. The expression levels of Ob-Rb mRNA and proteins were detected in the TMJ-SFs. Leptin can stimulate the mRNA and protein expression of IL-6 in TMJ-SFs by binding with Ob-Rb. The leptin-induced production of IL-6 by the TMJ-SFs significantly decreased after exposure to siRNA targeting Ob-Rb. Inhibiting JAK2/STAT3, p38 MAPK or PI3K/Akt substantially decreased leptin-induced IL-6 production. Conclusion Leptin may up-regulate IL-6 production in vitro by binding with Ob-Rb in TMJ-SFs via the activation of the JAK2/STAT3, p38 MAPK or PI3K/Akt signalling pathways.