Neutrophils are the most abundant leukocytes in the circulation and provide a primary innate immune defense function against bacterial pathogens before the development of a specific immune response. Activation of TLR4 signaling pathways by lipopolysaccharide (LPS) leads to the production of a broad array of cytokines and mediators that coordinate the immune response. The TLR4-mediated inflammatory response is tightly regulated, since excessive activation of TLR4 may induce a variety of inflammatory diseases, so the negative regulation of TLR-triggered inflammatory response attracts much attention. Glia maturation factor gamma (GMFG), a novel factor in actin cytoskeleton reorganization, is preferentially expressed in inflammatory cells, but its function in leucocytes is undefined. In this study, we investigated whether GMFG participates in the molecular events underlying the inflammatory reaction and how GMFG mediated neutrophils inflammatory response to LPS. We show here that small interfering RNA-mediated knockdown of GMFG expression in leukocytes exhibited greater than 80% inhibition of the endogenous expression levels of proinflammatory cytokines and chemokines, including TNF-alpha, IL-1beta, IL-6, and IL-8 using quantitative RT-PCR. These decreased cytokine productions could be partially restored by stimulation with TLR4 agonist LPS, but exhibit significantly lower level in silencing of GMFG cells compared to non-targeting silencing control cells (inhibition by 71%+/-3.9% of TNF-alpha, 68%+/-6.9% of IL-1beta, 50%+/-4.3% of IL-6 and 40%+/-3.1% of IL-8). Knockdown of GMFG expression results in decreased phospho-p38 MAPK levels as well as suppressed NF-kappaB by Western blot analysis. These data suggest that endogenous expression of GMFG is required for basal cytokine and chemokine response and their induced response to LPS stimulation. Moreover, immunofluorescence analysis revealed that gene silencing of GMFG inhibited LPS-induced nuclear translocation of NF-kappaB in myeloid HL-60 and THP-1 cells. In contrast, transient overexpression of GMFG significantly enhanced endogenous level of proinflammatory cytokines by 1.82.2 fold, phosphor-p38 MAPK and nuclear translocation of NF-kappaB in the absence of LPS stimulation. These results suggest that GMFG-mediated regulation of inflammatory cytokines occurs through activation of p38 MAPK and NF-kappaB signaling pathway. Taken together, our findings identify GMFG as a novel factor that participates in the regulation of TLR4-mediated control of cytokine and chemokine expression through the p38 MAPK and NF-kappaB pathway. These data, if confirmed in in vivo experiments, implicates GMFG as a potential target for therapeutic intervention in neutrophil-mediated autoimmune disease.
