TNF-alpha induces tyrosine phosphorylation and recruitment of the Src homology protein-tyrosine phosphatase 2 to the gp130 signal-transducing subunit of the IL-6 receptor complex

Recently, it has been demonstrated that TNF-alpha and LPS induce the expression of suppressor of cytokine signaling 3 (SOCS3) and inhibit IL-6-induced STAT3 activation in macrophages. Inhibitor studies suggested that both induction of SOCS3 and inhibition of IL-6-induced STAT3 activation depend on the activation of p38 mitogen-activated protein kinase. Since recruitment of the tyrosine phosphatase Src homology protein tyrosine phosphatase 2 (SHP2) to the signal-transducing receptor subunit gp130 attenuates IL-6-mediated STAT-activation, we were interested in whether TNF-alpha also induces the association of SHP2 to the gp130 receptor subunit. In this study we demonstrate that stimulation of macrophages and fibroblast cell lines with TNF-alpha causes the recruitment of SHP2 to the gp130 signal-transducing subunit and leads to tyrosine phosphorylation of SHP2 and gp130. In this context the cytoplasmic SHP2/SOCS3 recruitment site of gp130 tyrosine 759 is shown to be important for the inhibitory effects of TNF-alpha, since mutation of this residue completely restores IL-6-stimulated activation of STAT3 and, consequently, of a STAT3-dependent promoter. In this respect murine fibroblasts lacking exon 3 of SHP2 are not sensitive to TNF-alpha, indicating that functional SHP2 and its recruitment to gp130 are key events in inhibition of IL-6-dependent STAT activation by TNF-alpha. Furthermore, activation of p38 mitogen-activated protein kinase is shown to be essential for the inhibitory effect of TNF-alpha on IL-6 signaling and TNF-alpha-dependent recruitment of SHP2 to gp130.     

STAT3 is indispensable to IL-27-mediated cell proliferation but not to IL-27-induced Th1 differentiation and suppression of proinflammatory cytokine production

IL-27, a member of the IL-6/IL-12 family, activates both STAT1 and STAT3 through its receptor, which consists of WSX-1 and gp130 subunits, resulting in augmentation of Th1 differentiation and suppression of proinflammatory cytokine production. In the present study, we investigated the role of STAT3 in the IL-27-mediated immune functions. IL-27 induced phosphorylation of STAT1, -2, -3 and -5 in wild-type naive CD4+ T cells, but failed to induce that of STAT3 and STAT5 in STAT3-deficient cohorts. IL-27 induced not only proinflammatory responses including up-regulation of ICAM-1, T-box expressed in T cells, and IL-12Rbeta2 and Th1 differentiation, but also anti-inflammatory responses including suppression of proinflammatory cytokine production such as IL-2, IL-4, and IL-13 even in STAT3-deficient naive CD4+ T cells. In contrast, IL-27 augmented c-Myc and Pim-1 expression and induced cell proliferation in wild-type naive CD4+ T cells but not in STAT3-deficient cohorts. Moreover, IL-27 failed to activate STAT3, augment c-Myc and Pim-1 expression, and induce cell proliferation in pro-B BaF/3 transfectants expressing mutant gp130, in which the putative STAT3-binding four Tyr residues in the YXXQ motif of the cytoplasmic region was replaced by Phe. These results suggest that STAT3 is activated through gp130 by IL-27 and is indispensable to IL-27-mediated cell proliferation but not to IL-27-induced Th1 differentiation and suppression of proinflammatory cytokine production. Thus, IL-27 may be a cytokine, which activates both STAT1 and STAT3 through distinct receptor subunits, WSX-1 and gp130, respectively, to mediate its individual immune functions.