Mechanism of interferon-gamma down-regulation of the interleukin 4-induced CD23/Fc epsilon RII expression in human B cells: post-transcriptional modulation by interferon-gamma

Counter-regulation mechanism of IL-4 and IFN-α signal transduction through cytosolic retention of the pY-STAT6:pY-STAT2:p48 complex

IFN-α and IL-4 induce Th1 and Th2 responses, respectively, and often display antagonistic actions against each other. To elucidate the molecular mechanism of counter-regulation, we have investigated the signal interception by IFN-α and IL-4, employing a human B-cell line Ramos, sensitive to both cytokines. In these cells, IFN-α effectively inhibited IL-4-induced Fc epsilon receptor II (CD23) expression, whereas IL-4 suppressed IFN-α-mediated IRF7 expression. The counter-regulatory action by IL-4 and IFN-α proceeded with a delayed kinetics requiring 4 h. Notably, IFN-α did not affect the IL-4-induced tyrosine phosphorylation of STAT6, but induced a time-dependent cytoplasmic accumulation of phosphotyrosine(pY)-STAT6 and a corresponding decrease in nuclear pY-STAT6. By confocal analysis and co-immunoprecipitation assays, we demonstrated the colocalization and molecular interaction of IL-4-induced pY-STAT6 with IFN-α-induced pY-STAT2:p48 in the cytosol. In addition, the over-expression of STAT2 or STAT6 induced the concomitant cytosolic accumulation of pY-STAT6 or pY-STAT2, leading to the suppression of IL-4-induced CD23 or IFN-α-induced IRF7 gene expression, respectively. Our data suggest that the signals ensued by IFN-α and IL-4 induce cytoplasmic sequestration of IL-4-activated STAT6 and IFN-α-activated STAT2:p48 in B cells through the formation of pY-STAT6:pY-STAT2:p48 complex, which provides a novel mechanism by which IFN-α and IL-4 cross-regulate their signaling into the nucleus.

Interferon-gamma inhibits STAT6 signal transduction and gene expression in human airway epithelial cells

The activating and inhibitory cytokine signals that act upon epithelial cells in the human lung are critically important for controlling the production of inflammatory mediators from those cells in the context of allergic disease. The cytokines interleukin (IL)-4 and IL-13, derived from T helper (Th)-2 cells and other cell types, are potent inducers of epithelial cell expression of a host of inflammatory molecules, including the chemokines eotaxin-1, -2 and -3. Intracellular signal transduction in response to IL-4/IL-13 occurs largely through activation of signal transducer and activator of transcription 6 (STAT6). Interferon (IFN)-gamma, a Th1-type cytokine, has opposing effects to IL-4/IL-13 in various cell types, including T cells, B-cells, endothelium, and epithelium. In this study, we demonstrate that IL-4-induced STAT6 activation was inhibited profoundly by 24 h pretreatment with IFN-gamma in human primary airway epithelial cell cultures. Using Western blotting, we showed that the levels of both cytoplasmic and nuclear-localized phospho-STAT6 were reduced by IFN-gamma pretreatment, and this effect was dependent on the concentration of IFN-gamma and time of exposure to IFN-gamma. The functional activity of STAT6 was also completely inhibited by IFN-gamma: IL-4-induced luciferase activity from a STAT6-driven reporter construct was suppressed, as was IL-4-induced expression of messenger RNA (mRNA) and protein for eotaxin-3, a STAT6-dependent gene implicated in allergic inflammation. We found that mRNA for suppressor of cytokine signaling (SOCS)-1 and (SOCS)-3, known inhibitors of IL-4 signaling, and IL-13 receptor alpha2, a potential inhibitor of IL-4 signaling, were both strongly induced by IFN-gamma pretreatment. IFN-gamma also increased the rate of decay of IL-4-induced eotaxin-3 mRNA. We conclude that there are multiple mechanisms by which IFN-gamma regulates IL-4- and STAT6-dependent signaling and gene expression in airway epithelial cells. These observations have important implications for the regulation of epithelial cell activation by the balance of Th1/Th2-type cytokines in the airways in allergic disease.

Mitochondrial adenine nucleotide translocator 3 is regulated by IL-4 and IFN-γ via STAT-dependent pathways

IL-4 and IFN-gamma are prototypical Th2 and Th1 cytokines, respectively. They reciprocally regulate a number of genes involved in Th1 vs Th2 immune balance. Using DD-PCR analysis, adenine nucleotide translocase (ANT) 3, an enzyme which exchanges ATP and ADP through mitochondrial membrane, has been identified as a novel target counter-regulated by IL-4 and IFN-gamma. We have observed that IL-4 and IFN-gamma each up-regulates ANT3 in T cells both at mRNA and protein levels, while cotreatment of IL-4 and IFN-gamma counter-regulates ANT3 expression. In contrast, other isoforms of ANT were not affected by IL-4 or IFN-gamma. Emplyoing transfection and overexpression of STAT6 and STAT1 in STAT-deficient cells, we demonstrate that induction of ANT3 by IL-4 and IFN-gamma proceeds via pathways involving STAT6 and STAT1, respectively. Furthermore, regulation of ANT3 expression by IL-4 and IFN-gamma correlated with the modulation T cell survival by these cytokines from dex-induced apoptosis. Considering the critical role of mitochondrial ANTs in energy metabolism and apoptosis, ANT3 regulation by IL-4 and IFN-gamma may have a functional implication in cytokine-mediated T cell survival.

Identification of DC21 as a novel target gene counter-regulated by IL-12 and IL-4

The Th1 vs. Th2 balance is critical for the maintenance of immune homeostasis. Therefore, the genes that are selectively-regulated by the Th1 and Th2 cytokines are likely to play an important role in the Th1 and Th2 immune responses. In order to search for and identify the novel target genes that are differentially regulated by the Th1/Th2 cytokines, the human PBMC mRNAs differentially expressed upon the stimulation with IL-4 or IL-12, were screened by employing the differential display polymerase chain reaction. Among a number of clones selected, DC21 was identified as a novel target gene that is regulated by IL-4 and IL-12. The DC21 gene expression was up-regulated either by IL-4 or IL-12, yet counterregulated by co-treatment with IL-4 and IL-12. DC21 is a dendritic cell protein with an unknown function. The sequence analysis and conserved-domain search revealed that it has two AU-rich motifs in the 3'UTR, which is a target site for the regulation of mRNA stability by cytokines, and that it belongs to the N-acetyltransferase family. The induction of DC21 by IL-12 peaked around 8-12 h, and lasted until 24 h. LY294002 and SB203580 significantly suppressed the IL-12-induced DC21 gene expression, which implies that PI3K and p38/JNK are involved in the IL-12 signal transduction pathway that leads to the DC21 expression. Furthermore, tissue blot data indicated that DC21 is highly expressed in tissues with specialized-resident macrophages, such as the lung, liver, kidney, and placenta. Together, these data suggest a possible role for DC21 in the differentiation and maturation of dendritic cells regulated by IL-4 and IL-12.

Pim serine/threonine kinases regulate the stability of Socs-1 protein

Studies of SOCS-1-deficient mice have implicated Socs-1 in the suppression of JAK-STAT (Janus tyrosine kinase-signal transducers and activators of transcription) signaling and T cell development. It has been suggested that the levels of Socs-1 protein may be regulated through the proteasome pathway. Here we show that Socs-1 interacts with members of the Pim family of serine/threonine kinases in thymocytes. Coexpression of the Pim kinases with Socs-1 results in phosphorylation and stabilization of the Socs-1 protein. The protein levels of Socs-1 are significantly reduced in the Pim-1(-/-), Pim-2(-/-) mice as compared with wild-type mice. Similar to Socs-1(-/-) mice, thymocytes from Pim-1(-/-), Pim-2(-/-) mice showed prolonged Stat6 phosphorylation upon IL-4 stimulation. These data suggest that the Pim kinases may regulate cytokine-induced JAK-STAT signaling through modulation of Socs-1 protein levels.

11 Beta-hydroxysteroid dehydrogenase type 1 is induced in human monocytes upon differentiation to macrophages

11beta-hydroxysteroid dehydrogenases (11beta-HSD) perform prereceptor metabolism of glucocorticoids through interconversion of the active glucocorticoid, cortisol, with inactive cortisone. Although the immunosuppressive and anti-inflammatory activities of glucocorticoids are well documented, the expression of 11beta-HSD enzymes in immune cells is not well understood. Here we demonstrate that 11beta-HSD1, which converts cortisone to cortisol, is expressed only upon differentiation of human monocytes to macrophages. 11beta-HSD1 expression is concomitant with the emergence of peroxisome proliferator activating receptor gamma, which was used as a surrogate marker of monocyte differentiation. The type 2 enzyme, 11beta-HSD2, which converts cortisol to cortisone, was not detectable in either monocytes or cultured macrophages. Incubation of monocytes with IL-4 or IL-13 induced 11beta-HSD1 activity by up to 10-fold. IFN-gamma, a known functional antagonist of IL-4 and IL-13, suppressed the induction of 11beta-HSD1 by these cytokines. THP-1 cells, a human macrophage-like cell line, expressed 11beta-HSD1 and low levels of 11beta-HSD2. The expression of 11beta-HSD1 in these cells is up-regulated 4-fold by LPS. In summary, we have shown strong expression of 11beta-HSD1 in cultured human macrophages and THP-1 cells. The presence of the enzyme in these cells suggests that it may play a role in regulating the immune function of these cells.

IL-4-activated STAT-6 inhibits IFN-gamma-induced CD40 gene expression in macrophages/microglia

The antagonism between the cytokines IFN-gamma and IL-4 is well documented, but the mechanism by which IL-4 inhibits IFN-gamma-induced gene expression is not clearly understood. CD40 is a type I transmembrane protein that is critical for proper functioning of the immune system. We have previously shown that IFN-gamma is the most potent inducer of CD40 expression by macrophages and microglia. In this report, we describe the molecular mechanisms by which IL-4 inhibits IFN-gamma-induced CD40 expression. IL-4 suppresses IFN-gamma-induced CD40 gene expression in both macrophages and microglia, and such inhibition is dependent on the activation of STAT-6. Nuclear run-on and transfection studies indicate that IL-4-mediated repression is at the transcriptional level. Furthermore, IL-4 inhibition of IFN-gamma-induced CD40 expression is specific, since IL-4 does not inhibit IFN-gamma-induced IFN-responsive factor-1 gene expression. Site-directed mutagenesis studies demonstrate that two STAT binding sites, named proximal and distal IFN-gamma-activated sequences, in the human CD40 promoter are important for IL-4 inhibition of IFN-gamma-induced CD40 promoter activity. Moreover, EMSAs indicate that IL-4-activated STAT-6 binds to these two STAT binding sites. These results suggest that IL-4 inhibition of IFN-gamma-induced CD40 gene expression is mediated by direct STAT-6 binding to the CD40 promoter.

IFN-gamma and IFN-alpha posttranscriptionally down-regulate the IL-4-induced IL-4 receptor gene expression

As Th1 and Th2 cytokines, IFN-gamma/alpha and IL-4 counterregulate diverse immune functions. In particular, IFN-gamma and IFN-alpha have been reported to markedly suppress the IL-4-induced IgE production and type II IgE receptor (FcepsilonRII/CD23) expression. Because modulation of IL-4R may be an important mechanism in the regulation of IL-4 response, we have investigated the effect of IFN-gamma/alpha on IL-4R expression and signal transduction mechanisms involved in this process. In human mononuclear cells and B cells isolated from tonsil or peripheral blood, IL-4 up-regulates IL-4R(alpha) expression at surface protein and mRNA levels, and the IL-4-induced IL-4R(alpha) is significantly down-regulated by both IFN-gamma and IFN-alpha to a similar extent. The inhibitory effects of IFN-gamma/alpha on the IL-4R mRNA expression require a lag period of about 8 h, and are sensitive to cycloheximide treatment, which suggests that the suppressive effect of IFNs on IL-4R gene expression is a secondary response requiring de novo synthesis of IFN-induced factors. Under such conditions that the inhibitory effects of IFNs are observed, IFNs do not affect the IL-4-induced STAT6 activation and IL-4R transcription, as analyzed by EMSA and nuclear run-on assays, respectively. Subsequently, mRNA stability studies have indicated that the action of IFN-gamma/alpha is primarily mediated by an accelerated decay of IL-4-induced IL-4R mRNA. Thus, it appears that, as already shown in the case of the IL-4-induced FcepsilonRII regulation, posttranscriptional inhibition of IL-4-inducible genes by mRNA destabilization is a common mechanism by which type I and II IFNs antagonize the IL-4 response in human immune cells.

Paired Stat6 C-Terminal Transcription Activation Domains Required Both for Inhibition of an IFN-Responsive Promoter and Trans-Activation

The cytokines IL-4 and IFN-γ exert biologically antagonistic effects that in part reflect opposing influences on gene transcription. While the molecular mechanisms for IL-4-mediated transcription activation have been extensively studied, little is known about molecular mechanisms required for IL-4 inhibition of IFN-γ signaling. We have investigated IL-4 inhibition of the IFN-γ-inducible promoter for IFN regulatory factor-1 (IRF-1). In a cell line with low endogenous Stat6, increasing levels of activated Stat6 at constant doses of IFN-γ and IL-4 leads to inhibition of the IRF-1 promoter. The Stat1-dependent IFN-γ activation sequence element of the IRF-1 promoter is a target for Stat6-mediated inhibition despite apparently normal Stat1 DNA binding. However, our data are inconsistent with competition between Stat1 and Stat6 for access to the IRF-1 IFN-γ activation sequence or for an essential coactivator as a mechanism for this Stat6-mediated inhibition. Instead, the data demonstrate that a threshold of Stat6 transcription activation domains is required for IL-4-dependent inhibition. The findings provide evidence of a novel mechanism in which the Stat6 transcription activation domains play a critical role in the IL-4-mediated inhibition of an IFN-γ-inducible promoter.

Hepatitis B virus RNA-binding proteins associated with cytokine-induced clearance of viral RNA from the liver of transgenic mice

Hepatitis B virus (HBV) gene expression is downregulated in the liver of HBV transgenic mice by a posttranscriptional mechanism that is triggered by the local production of gamma interferon (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha) during intrahepatic inflammation (hepatitis). The molecular basis for this antiviral effect is unknown. In this study, we identified three HBV RNA-binding liver nuclear proteins (p45, p39, and p26) the relative abundance of which correlates with the abundance of HBV RNA in response to the induction of IFN-gamma and TNF-alpha. All three proteins bind to a 91-bp element located at the 5' end of a previously defined posttranscriptional regulatory element that is thought to mediate the nuclear export of HBV RNA. The presence of p45 correlates directly with the presence of HBV RNA, being detectable under baseline conditions when the viral RNA is abundant and undetectable when the viral RNA disappears in response to IFN-gamma and TNF-alpha. In contrast, p26 is inversely related to HBV RNA, being detectable only when the viral RNA disappears following cytokine activation. Finally, p39 is constitutively expressed, and its abundance and mobility appear to be slightly increased by cytokine activation. These results suggest a model in which hepatocellular HBV RNA content might be controlled by the stabilizing and/or destabilizing influences of these RNA-binding proteins whose activity is regulated by cytokine-induced signaling pathways.

Interleukin-4 and interferon-gamma: the quintessence of a mutual antagonistic relationship

The two cytokines interleukin (IL)-4 and interferon (IFN)-gamma play major roles in the generation and regulation of immune responses. Central in this respect is their mutually antagonistic functions. First, IL-4 promotes T helper cell type 2 (Th2) differentiation and stability and inhibits Th1-cell differentiation. A direct role of IFN-gamma in Th1-cell differentiation is debatable, whereas inhibition of Th2-cell differentiation and roles in Th1-cell stabilization are well established functions of IFN-gamma. Secondly, IL-4 and IFN-gamma also affect antibody class switch and expression of Fc receptors differentially, which strongly affect the effector mechanisms following antibody production. Thirdly, macrophage activities induced or enhanced by IFN-gamma, such as expression of certain cytokines, surface molecules and enzymes, are antagonized by IL-4. Together, these functions of IL-4 and IFN-gamma place the two cytokines at cardinal positions in the regulation of immune reactions. In this review the known molecular mechanisms underlying the observed functions of IL-4 and IFN-gamma are presented and discussed.

Distinct modulation by interferon-gamma (IFN-gamma) of CD23 expression on B and T lymphocytes of atopic subjects

The low-affinity IgE receptor (FcepsilonRII/CD23) plays a role in IgE production. Cytokines participating in IgE synthesis also modulate CD23 expression on lymphocytes, but whether this modulation is different in atopic subjects remains unclear. We studied CD23 expression on B and T lymphocytes in 10 asthmatic patients with Dermatophagoides pteronyssinus hypersensitivity and 10 healthy non-atopic subjects. Studies were performed by flow cytometry, in phytohaemagglutinin (PHA) or IL-4-stimulated mononuclear cell cultures, alone or in the presence of IFN-gamma. Soluble CD23 (sCD23) released in the culture supernatants was measured by enzyme-linked immunoassay. Both PHA and IL-4 induced the expression of CD23 on lymphocytes of atopic and non-atopic subjects. Whereas PHA increased both the percentage and mean fluorescence intensity of CD23+ B and T cells, IL-4 alone did not increase the percentage of CD23+ T cells. The effects of IFN-gamma were different in both groups, since it was able to reduce the percentage of PHA-stimulated CD23+ T cells only in non-atopic individuals. In non-atopic subjects more than atopic, levels of sCD23 were increased in the supernatants of PHA and IL-4 cultures. These results show that the modulation of CD23 expression is different on B and T cells, and that IFN-gamma acts differently in atopic and non-atopic individuals.

Cellular responses to interferon-gamma

Interferons are cytokines that play a complex and central role in the resistance of mammalian hosts to pathogens. Type I interferon (IFN-alpha and IFN-beta) is secreted by virus-infected cells. Immune, type II, or gamma-interferon (IFN-gamma) is secreted by thymus-derived (T) cells under certain conditions of activation and by natural killer (NK) cells. Although originally defined as an agent with direct antiviral activity, the properties of IFN-gamma include regulation of several aspects of the immune response, stimulation of bactericidal activity of phagocytes, stimulation of antigen presentation through class I and class II major histocompatibility complex (MHC) molecules, orchestration of leukocyte-endothelium interactions, effects on cell proliferation and apoptosis, as well as the stimulation and repression of a variety of genes whose functional significance remains obscure. The implementation of such a variety of effects by a single cytokine is achieved by complex patterns of cell-specific gene regulation: Several IFN-gamma-regulated genes are themselves components of transcription factors. The IFN-gamma response is itself regulated by interaction with responses to other cytokines including IFN-alpha/beta, TNF-alpha, and IL-4. Over 200 genes are now known to be regulated by IFN-gamma and they are listed in a World Wide Web document that accompanies this review. However, much of the cellular response to IFN-gamma can be described in terms of a set of integrated molecular programs underlying well-defined physiological systems, for example the induction of efficient antigen processing for MHC-mediated antigen presentation, which play clearly defined roles in pathogen resistance. A promising approach to the complexity of the IFN-gamma response is to extend the analysis of the less understood IFN-gamma-regulated genes in terms of molecular programs functional in pathogen resistance.

Interferon-gamma regulates collagen and fibronectin gene expression by transcriptional and post-transcriptional mechanisms

Interferon-gamma (IFN-gamma) regulates the expression of collagen and fibronectin genes by molecular mechanisms not completely understood. We investigated the effects of IFN-gamma on the expression of the genes encoding alpha 1 (I) procollagen (COL1A1), alpha 1 (III) procollagen (COL3A1), and fibronectin (FN) in cultured normal human lung fibroblasts. Labeled newly synthesized proteins were analysed by electrophoresis, mRNA levels and stability by Northern hybridizations, and transcription rates by in vitro assays. IFN-gamma caused a reduction in the production of alpha 1 (I) and alpha 1 (III) procollagens and of fibronectin. The reduction in the production of procollagen chains was shown to result from a combination of IFN-gamma-induced inhibition of the transcription rates of the COL1A1 and COL3A1 genes and destabilization of the corresponding transcripts. IFN-gamma increased the transcription rate of FN, but also decreased the stability of the corresponding transcripts. The net results indicate that the regulation of the expression of extracellular matrix genes by IFN-gamma is a complex process that involves changes in gene transcription rates, alterations in mRNAs stability, and possibly, modulation of the rates of translation.

Receptors for IgE

Following advances during the past 5 years in our understanding of the molecular structure of receptors for IgE, progress has been made in elucidating the structure and function of IgE receptors and the signalling events through these receptors. IgE is not the only ligand for some of these receptors, leading to their having unexpected and interesting biological activities.