Interleukin-4/STAT6 Represses STAT1 and NF-κB-dependent Transcription through Distinct Mechanisms

Abstract

Cytokines mediate much of the cell to cell communication that is necessary for control of immune-mediated inflammation. In many circumstances, this is achieved through interplay between distinct classes of cytokines that promote either cooperative or antagonistic effects (1-3). The T helper cell type I (Th1) 1 -derived cytokine IFN␥ promotes the development of cell-mediated immunity (3-6) and often functions cooperatively with TNF to regulate pro-inflammatory gene expression in a variety of cell types (7,8). In contrast, the Th2-derived cytokine IL-4 promotes humoral immunity and differentiation of Th2 cells (1,3,5,6,9) in part via antagonistic effects on Th1-cytokine-driven gene expression (3, 5, 10 -12).Intracellular signaling events linked with response to cytokine-receptor interaction have been extensively studied. Signal transducers and activators of transcription (STATs) are latent cytoplasmic transcription factors that are phosphorylated at a single tyrosine residue via members of the Jak kinase family following ligand-receptor interaction, assemble in dimeric form, translocate to the nucleus, and bind to specific DNA sequence motifs (13-15). Prototypically, IFN␥ activates STAT1 homodimers that bind to IFN␥ activation sequences (GAS) composed of two inverted repeats separated by three intervening nucleotides (N3 site, TTCnnnGAA) and promote transcription of IFN␥-responsive genes (14, 16). Likewise, IL-4 leads to activation of STAT6 (17-19), which can stimulate gene transcription by binding to a sequence that resembles the GAS motif but instead contains four intervening nucleotides between the inverted repeat (N4 site, TTCnnnnGAA) (20 -24). Interestingly, STAT6 can also bind N3 GAS elements but with little or no transactivation potential (23,25,26).Although the negative regulatory role of IL-4 in immune inflammatory reactions has been well delineated (3, 5, 10 -12, 27-34), the molecular mechanisms involved remain incompletely understood. We have previously demonstrated that IL-4 does not inhibit tyrosine phosphorylation, nuclear translocation, nor DNA binding activity of STAT1 and that STAT6 is required for inhibition of IFN␥-inducible gene transcription in mouse macrophages (26,35). Because the cis-elements required for IFN␥-induced transcription (ISRE, GAS, and ␥RE) are also sufficient for IL-4-mediated inhibition and because STAT6 homodimers are capable of binding to these sites without transactivating function (26,35,36), it has been suggested that inhibition might involve competition between transactivating STAT1 and non-transactivating STAT6 for the N3 sites found in the promoters of sensitive genes (26). An analogous mechanism has been reported for inhibition of E-selectin gene transcription by IL-4, where STAT6 competes with NF-B for an overlapping binding site (37). A second hypothesis is that IL-4-activated STAT6 competes with STAT1 for limiting quantities of transcriptional coactivators such as the CREB-binding protein (CBP) (35). Previous studies have shown that repression of AP-1-dependent tr...

IL-4 Suppresses Dendritic Cell Response to Type I Interferons

“…This indicates that the cross-talk between the two signaling pathways occurs at the level of STATs or upstream of them. Previous reports show that in macrophages/monocytes, IL-4-activated STAT6 negatively regulates IFN-stimulated STAT1-dependent transcription (19), possibly through the interaction of the transactivation domain of STAT6, suggesting the importance of transcriptional coactivators and corepressors (43). Although some argue that IL-4 is necessary for STAT1 induction in DCs (44), our results are in agreement with Ohmori and Hamilton (43), who demonstrated that IL-4 suppresses STAT1 transcription via STAT6 in transfected fibroblasts.…”

Differential Roles of C-terminal Activation Motifs in the Establishment of Stat6 Transcriptional Specificity

“…work has shown that a Stat-binding element activated by IFN-␥ and Stat1 was repressed by IL-4 and Stat6 even though the Stat proteins bound equally well to the cis-acting element (8,9). The specificity of Stat-mediated trans-activation in instances such as these cannot be explained by differences in DNA binding or cytokine receptor activation.…”

“…For example Stat1␣ and Stat6 both bind to the Stat binding site (SBS) of an IL-4-inducible CD23 promoter, but only Stat6 activated by IL-4 induces CD23 (7). Conversely, IFN-␥-induced Stat1␣ activates the IRF-1 promoter through a Stat binding site to which Stat6 binds equally well, yet IL-4-induced Stat6 fails to activate this promoter and instead inhibits the action of IFN-␥ by a competition-independent mechanism (8,9). The finding that DNA binding specificity cannot account for these differences between Stat1␣ and Stat6 function raises the question through what mechanism(s) a particular Stat factor mediates promoter specificity when binding to a consensus DNA element.…”

Inhibition of IFN-γ-Induced Class II Transactivator Expression by a 19-kDa Lipoprotein fromMycobacterium tuberculosis: A Potential Mechanism for Immune Evasion

“…Some experiments used the IRFGAS2X luciferase reporter construct containing Stat1 binding sites from the IRF-1 promoter (20). Cells were rested for 2 h and incubated for 24 h with or without 30 nM MTB 19-kDa lipoprotein, and for 20 h with or without IFN-␥ in the continued presence or absence of the lipoprotein.…”