Induction of tumor necrosis factor-alpha (TNF-alpha) by interleukin-12 p40 monomer and homodimer in microglia and macrophages

The present study was undertaken to explore the role of interleukin-12 (IL-12) p40 in the expression of TNF-alpha in microglia. Interestingly, we have found that IL-12 p70, p402 (the p40 homodimer) and p40 (the p40 monomer) dose-dependently induced the production of TNF-alpha and the expression of TNF-alpha mRNA in BV-2 microglial cells. In addition to BV-2 microglial cells, p70, p402 and p40 also induced the production of TNF-alpha in mouse primary microglia and peritoneal macrophages. As the activation of both NF-kappaB and CCAAT/enhancer binding protein beta (C/EBPbeta) is important for the expression of TNF-alpha in microglial cells, we investigated the effect of p40 on the activation of NF-kappaB as well as C/EBPbeta. Activation of NF-kappaB as well as C/EBPbeta by p40 and inhibition of p40-induced expression of TNF-alpha by Deltap65, a dominant-negative mutant of p65, and DeltaC/EBPbeta, a dominant-negative mutant of C/EBPbeta, suggests that p40 induces the expression of TNF-alpha through the activation of NF-kappaB and C/EBPbeta. In addition, we show that p40 induced the activation of both extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK). Interestingly, PD98059, an inhibitor of ERK, inhibited p40-induced expression of TNF-alpha through the inhibition of C/EBPbeta, but not that of NF-kappaB, whereas SB203580, an inhibitor of p38 MAPK, inhibited p40-induced expression of TNF-alpha through the inhibition of both NF-kappaB and C/EBPbeta. This study delineates a novel biological function of p40 in inducing TNF-alpha in microglia and macrophages.     

Melanoma differentiation-associated gene-7/IL-24 gene enhances NF-kappa B activation and suppresses apoptosis induced by TNF

Melanoma differentiation-associated gene-7 (mda-7), also referred to as IL-24, is a novel growth regulatory cytokine that has been shown to regulate the immune system by inducing the expression of inflammatory cytokines, such as TNF, IL-1, and IL-6. Whether the induction of these cytokines by MDA-7 is mediated through activation of NF-kappaB or whether it regulates cytokine signaling is not known. In the present report we investigated the effect of MDA-7 on NF-kappaB activation and on TNF-induced NF-kappaB activation and apoptosis in human embryonic kidney 293 cells. Stable or transient transfection with mda-7 into 293 cells failed to activate NF-kappaB. However, TNF-induced NF-kappaB activation was significantly enhanced in mda-7-transfected cells, as indicated by DNA binding, p65 translocation, and NF-kappaB-dependent reporter gene expression. Mda-7 transfection also potentiated NF-kappaB reporter activation induced by TNF receptor-associated death domain and TNF receptor-associated factor-2. Cytoplasmic MDA-7 with deleted signal sequence was as effective as full-length MDA-7 in potentiating TNF-induced NF-kappaB reporter activity. Secretion of MDA-7 was not required for the potentiation of TNF-induced NF-kappaB activation. TNF-induced expression of the NF-kappaB-regulated gene products cyclin D1 and cyclooxygenase-2, were significantly up-regulated by stable expression of MDA-7. Furthermore, MDA-7 expression abolished TNF-induced apoptosis, and suppression of NF-kappaB by IkappaBalpha kinase inhibitors enhanced apoptosis. Overall, our results indicate that stable or transient MDA-7 expression alone does not substantially activate NF-kappaB, but potentiates TNF-induced NF-kappaB activation and NF-kappaB-regulated gene expression. Potentiation of NF-kappaB survival signaling by MDA-7 inhibits TNF-mediated apoptosis.     

Role of very-late antigen-4 (VLA-4) in myelin basic protein-primed T cell contact-induced expression of proinflammatory cytokines in microglial cells

The presence of neuroantigen-primed T cells recognizing self-myelin antigens within the CNS is necessary for the development of demyelinating autoimmune disease like multiple sclerosis. This study was undertaken to investigate the role of myelin basic protein (MBP)-primed T cells in the expression of proinflammatory cytokines in microglial cells. MBP-primed T cells alone induced specifically the microglial expression of interleukin (IL)-1beta, IL-1alpha tumor necrosis factor alpha, and IL-6, proinflammatory cytokines that are primarily involved in the pathogenesis of MS. This induction was primarily dependent on the contact between MBP-primed T cells and microglia. The activation of microglial NF-kappaB and CCAAT/enhancer-binding protein beta (C/EBPbeta) by MBP-primed T cell contact and inhibition of contact-mediated microglial expression of proinflammatory cytokines by dominant-negative mutants of p65 and C/EBPbeta suggest that MBP-primed T cells induce microglial expression of cytokines through the activation of NF-kappaB and C/EBPbeta. In addition, we show that MBP-primed T cells express very late antigen-4 (VLA-4), and functional blocking antibodies to alpha4 chain of VLA-4 (CD49d) inhibited the ability of MBP-primed T cells to induce microglial proinflammatory cytokines. Interestingly, the blocking of VLA-4 impaired the ability of MBP-primed T cells to induce microglial activation of only C/EBPbeta but not that of NF-kappaB. This study illustrates a novel role of VLA-4 in regulating neuroantigen-primed T cell-induced activation of microglia through C/EBPbeta     

Regulation of tumor necrosis factor-α expression by CD40 ligation in BV-2 microglial cells

Ligation of CD40 has been shown to induce/stimulate the expression of tumor necrosis factor-alpha (TNF-alpha) in microglial cells. This study delineates the mechanism by which CD40 ligation regulates the expression of TNF-alpha in BV-2 microglial cells. There was very little induction of TNF-alpha by ligation of CD40 alone by either cross-linking antibodies against CD40 or recombinant CD40 ligand (CD154). The absence of any increase in TNF-alpha production by CD40 ligation alone even in CD40-overexpressed BV-2 microglial cells suggest that signal transduced by the ligation of CD40 alone is not sufficient for strong induction of TNF-alpha. However, CD40 ligation markedly induced the production of TNF-alpha as well as the expression of TNF-alpha mRNA in interferon-gamma (IFN-gamma)-stimulated BV-2 glial cells. Ligation of CD40 in CD40-overexpressed cells markedly enhanced the expression of TNF-alpha in the presence of IFN-gamma. To understand the mechanism of CD40 ligation-mediated induction/stimulation of TNF-alpha, we investigated the role of nuclear factor-kappaB (NF-kappaB) and C/EBPbeta. IFN-gamma alone was able to induce the activation of NF-kappaB as well as C/EBPbeta. However, CD40 ligation alone in the presence or absence of CD40 overexpression induced the activation of only NF-kappaB and not that of C/EBPbeta, suggesting that the activation of NF-kappaB alone by CD40 ligation is not sufficient to induce the expression of TNF-alpha and that the activation of C/EBPbeta is also necessary for strong induction of TNF-alpha. Consistently, a dominant-negative mutant of p65 (Delta(p65)) and that of C/EBPbeta (DeltaC/EBPbeta) inhibited the expression of TNF-alpha in BV-2 microglial cells stimulated with the combination of IFN-gamma and CD40 ligand. Taken together, these studies suggest that activation of both NF-kappaB and C/EBPbeta is important for strong induction of TNF-alpha and that CD40 ligation regulates the expression of TNF-alpha by modulating the activation of only NF-kappaB but not that of C/EBPbeta.