ISGF3 gamma p48, a specificity switch for interferon activated transcription factors

Conserved and divergent arms of the antiviral response in the duplicated genomes of salmonid fishes

Antiviral innate immunity is orchestrated by the interferon system, which appeared in ancestors of jawed vertebrates. Interferon upregulation induces hundreds of interferon-stimulated-genes (ISGs) with effector or regulatory functions. Here we investigated the evolutionary diversification of ISG responses through comparison of two salmonid fishes, accounting for the impact of sequential whole genome duplications ancestral to teleosts and salmonids. We analysed the transcriptomic response of the IFN pathway in the head kidney of rainbow trout and Atlantic salmon, which separated 25-30 Mya. We identified a large set of ISGs conserved in both species and cross-referenced them with zebrafish and human ISGs. In contrast, around one-third of salmonid ISG lacked orthologs in human, mouse, chicken or frog, and often between rainbow trout and Atlantic salmon, revealing a fast-evolving, lineage-specific arm of the antiviral response. This study also provides a key resource for in-depth functional analysis of ISGs in salmonids of commercial significance.

Subversion of Programed Cell Death by Poxviruses

Poxviruses have been long regarded as potent inhibitors of apoptotic cell death. More recently, they have been shown to inhibit necroptotic cell death through two distinct strategies. These strategies involve either blocking virus sensing by the host pattern recognition receptor, ZBP1 (also called DAI) or by influencing receptor interacting protein kinase (RIPK)3 signal transduction by inhibition of activation of the executioner of necroptosis, mixed lineage kinase-like protein (MLKL). Vaccinia virus E3 specifically blocks ZBP1 → RIPK3 → MLKL necroptosis, leaving virus-infected cells susceptible to the TNF death-receptor signaling (e.g., TNFR1 → FADD → RIPK1 → RIPK3 → MLKL), and, potentially, TLR3 → TRIF → RIPK3 → MLKL necroptosis. While E3 restriction of necroptosis appears to be common to many poxviruses that infect vertebrate hosts, another modulatory strategy not observed in vaccinia or variola virus manifests through subversion of MLKL activation. Recently described viral mimics of MLKL in other chordopoxviruses inhibit all three modes of necroptotic cell death. As with inhibition of apoptosis, the evolution of potentially redundant viral mechanisms to inhibit programmed necroptotic cell death emphasizes the importance of this pathway in the arms race between pathogens and their hosts.

Evasion of interferon-mediated immune response by arteriviruses

IFN is the most potent antiviral cytokine required for the innate and adaptive immune responses, and its expression can help the host defend against viral infection. Arteriviruses have evolved strategies to antagonize the host cell’s innate immune responses, interfering with IFN expression by interfering with RIG, blocking PRR, obstructing IRF-3/7, NF-κB, and degrading STAT1 signaling pathways, thereby assisting viral immune evasion. Arteriviruses infect immune cells and may result in persistence in infected hosts. In this article, we reviewed the strategies used by Arteriviruses to antagonize IFN production and thwart IFN-activated antiviral signaling, mainly including structural and nonstructural proteins of Arteriviruses encoding IFN antagonists directly or indirectly to disrupt innate immunity. This review will certainly provide a better insight into the pathogenesis of the arthritis virus and provide a theoretical basis for developing more efficient vaccines.

Comparative Analysis of Six IRF Family Members in Alveolar Epithelial Cell-Intrinsic Antiviral Responses

Host cell-intrinsic antiviral responses are largely mediated by pattern-recognition receptor (PRR) signaling and the interferon (IFN) system. The IFN regulatory factor (IRF) family of transcription factors takes up a central role in transcriptional regulation of antiviral innate immunity. IRF3 and IRF7 are known to be key players downstream of PRRs mediating the induction of type I and III IFNs. IFN signaling then requires IRF9 for the expression of the full array of interferon stimulated genes (ISGs) ultimately defining the antiviral state of the cell. Other members of the IRF family clearly play a role in mediating or modulating IFN responses, such as IRF1, IRF2 or IRF5, however their relative contribution to mounting a functional antiviral response is much less understood. In this study, we systematically and comparatively assessed the impact of six members of the IRF family on antiviral signaling in alveolar epithelial cells. We generated functional knockouts of IRF1, -2, -3, -5, -7, and -9 in A549 cells, and measured their impact on the expression of IFNs and further cytokines, ISGs and other IRFs, as well as on viral replication. Our results confirmed the vital importance of IRF3 and IRF9 in establishing an antiviral state, whereas IRF1, 5 and 7 were largely dispensable. The previously described inhibitory activity of IRF2 could not be observed in our experimental system.

Evolution of the IRF Family in Salmonids

Interferon regulatory factors (IRFs) as a family, are major regulators of the innate antiviral response in vertebrates principally involved in regulating the expression of interferons (IFNs) and interferon-stimulated genes (ISGs). To date, nine IRFs have been identified in mammals with a 10th member also found in several avian and fish species. Through genome mining and phylogenetic analysis, we identified and characterised 23 irf genes in 6 salmonid species. This larger repertoire of IRF in salmonids results from two additional whole-genome duplications which occurred in early teleosts and salmonids, respectively. Synteny analysis was then used to identify and confirm which paralogues belonged to each subgroup and a new nomenclature was assigned to the salmonid IRFs. Furthermore, we present a full set of Real-Time PCR primers for all rainbow trout IRFs, confirmed by sequencing to ensure paralogue specificity. RT PCR was then used to examine the response of all trout irf genes in vivo, following Vibrio anguillarum and poly I:C stimulation, indicating potential functional divergence between paralogues. Overall, this study presents a comprehensive overview of the IRF family in salmonids and highlights some novel roles for the salmonid-specific IRFs in immunity.

Signal Integration of IFN-I and IFN-II With TLR4 Involves Sequential Recruitment of STAT1-Complexes and NFκB to Enhance Pro-inflammatory Transcription

Atherosclerosis is a chronic inflammatory disease of the blood vessels, characterized by atherosclerotic lesion formation. Vascular Smooth Muscle Cells (VSMC), macrophages (MΦ), and dendritic cells (DC) play a crucial role in vascular inflammation and atherosclerosis. Interferon (IFN)α, IFNγ, and Toll-like receptor (TLR)4 activate pro-inflammatory gene expression and are pro-atherogenic. Gene expression regulation of many pro-inflammatory genes has shown to rely on Signal Integration (SI) between IFNs and TLR4 through combinatorial actions of the Signal Transducer and Activator of Transcription (STAT)1 complexes ISGF3 and γ-activated factor (GAF), and Nuclear Factor-κB (NFκB). Thus, IFN pre-treatment (“priming”) followed by LPS stimulation leads to enhanced transcriptional responses as compared to the individual stimuli. To characterize the mechanism of priming-induced IFNα + LPS- and IFNγ + LPS-dependent SI in vascular cells as compared to immune cells, we performed a comprehensive genome-wide analysis of mouse VSMC, MΦ, and DC in response to IFNα, IFNγ, and/or LPS. Thus, we identified IFNα + LPS or IFNγ + LPS induced genes commonly expressed in these cell types that bound STAT1 and p65 at comparable γ-activated sequence (GAS), Interferon-stimulated response element (ISRE), or NFκB sites in promoter proximal and distal regions. Comparison of the relatively high number of overlapping ISRE sites in these genes unraveled a novel role of ISGF3 and possibly STAT1/IRF9 in IFNγ responses. In addition, similar STAT1-p65 co-binding modes were detected for IFNα + LPS and IFNγ + LPS up-regulated genes, which involved recruitment of STAT1 complexes preceding p65 to closely located GAS/NFκB or ISRE/NFκB composite sites already upon IFNα or IFNγ treatment. This STAT1-p65 co-binding significantly increased after subsequent LPS exposure and correlated with histone acetylation, PolII recruitment, and amplified target gene transcription in a STAT1-p65 co-bound dependent manner. Thus, co-binding of STAT1-containing transcription factor complexes and NFκB, activated by IFN-I or IFN-II together with LPS, provides a platform for robust transcriptional activation of pro-inflammatory genes. Moreover, our data offer an explanation for the comparable effects of IFNα or IFNγ priming on TLR4-induced activation in vascular and immune cells, with important implications in atherosclerosis.

A Positive Feedback Amplifier Circuit That Regulates Interferon (IFN)-Stimulated Gene Expression and Controls Type I and Type II IFN Responses

Interferon (IFN)-I and IFN-II both induce IFN-stimulated gene (ISG) expression through Janus kinase (JAK)-dependent phosphorylation of signal transducer and activator of transcription (STAT) 1 and STAT2. STAT1 homodimers, known as γ-activated factor (GAF), activate transcription in response to all types of IFNs by direct binding to IFN-II activation site (γ-activated sequence)-containing genes. Association of interferon regulatory factor (IRF) 9 with STAT1–STAT2 heterodimers [known as interferon-stimulated gene factor 3 (ISGF3)] or with STAT2 homodimers (STAT2/IRF9) in response to IFN-I, redirects these complexes to a distinct group of target genes harboring the interferon-stimulated response element (ISRE). Similarly, IRF1 regulates expression of ISGs in response to IFN-I and IFN-II by directly binding the ISRE or IRF-responsive element. In addition, evidence is accumulating for an IFN-independent and -dependent role of unphosphorylated STAT1 and STAT2, with or without IRF9, and IRF1 in basal as well as long-term ISG expression. This review provides insight into the existence of an intracellular amplifier circuit regulating ISG expression and controlling long-term cellular responsiveness to IFN-I and IFN-II. The exact timely steps that take place during IFN-activated feedback regulation and the control of ISG transcription and long-term cellular responsiveness to IFN-I and IFN-II is currently not clear. Based on existing literature and our novel data, we predict the existence of a multifaceted intracellular amplifier circuit that depends on unphosphorylated and phosphorylated ISGF3 and GAF complexes and IRF1. In a combinatorial and timely fashion, these complexes mediate prolonged ISG expression and control cellular responsiveness to IFN-I and IFN-II. This proposed intracellular amplifier circuit also provides a molecular explanation for the existing overlap between IFN-I and IFN-II activated ISG expression.

Infectious pancreatic necrosis virus infection of fish cell lines: Preliminary analysis of gene expressions related to extracellular matrix remodeling and immunity

The pathogenic infectious pancreatic necrosis virus (IPNV) causes high economic losses in fish farming. This virus can modulate several cellular processes during infection, but little is known about the infection mechanism. To investigate gene activation in response to IPNV, CHSE/F and SHK-1 cell line were infected with a cytopathic Sp field isolate of IPNV, and the expression profiles of proinflammatory, antiviral cytokine, and extracellular matrix markers were analyzed. IPNV induced the production of perlecan, fibulin-1, matrix metalloproteinase-2, 14-3-3β, interleukin-1β, Mx1, and interferon regulatory factors-1, -3, and -9. Interestingly, IPNV-mediated activity was blocked by pharmacological inhibitors of the NF-κB signaling pathway. These results, together with in silico analyses showing the presence of several regulatory consensus-target motifs, suggest that IPNV regulates gene expressions in fish through the activation of several key transcription factors. Collectively, these data indicate that IPNV is a viral regulator of expression for extracellular-matrix and immune markers, even during early infection. Finally, this is the first report in fish to find IPNV modulating the activation of interleukin-1β production primarily through the NF-κB pathway.

Hematopoietic Stem Cell Regulation by Type I and II Interferons in the Pathogenesis of Acquired Aplastic Anemia

Aplastic anemia (AA) occurs when the bone marrow fails to support production of all three lineages of blood cells, which are necessary for tissue oxygenation, infection control, and hemostasis. The etiology of acquired AA is elusive in the vast majority of cases but involves exhaustion of hematopoietic stem cells (HSC), which are usually present in the bone marrow in a dormant state, and are responsible for lifelong production of all cells within the hematopoietic system. This destruction is immune mediated and the role of interferons remains incompletely characterized. Interferon gamma (IFNγ) has been associated with AA and type I IFNs (alpha and beta) are well documented to cause bone marrow aplasia during viral infection. In models of infection and inflammation, IFNγ activates HSCs to differentiate and impairs their ability to self-renew, ultimately leading to HSC exhaustion. Recent evidence demonstrating that IFNγ also impacts the HSC microenvironment or niche, raises new questions regarding how IFNγ impairs HSC function in AA. Immune activation can also elicit type I interferons, which may exert effects both distinct from and overlapping with IFNγ on HSCs. IFNα/β increase HSC proliferation in models of sterile inflammation induced by polyinosinic:polycytidylic acid and lead to BM aplasia during viral infection. Moreover, patients being treated with IFNα exhibit cytopenias, in part due to BM suppression. Herein, we review the current understanding of how interferons contribute to the pathogenesis of acquired AA, and we explore additional potential mechanisms by which interferons directly and indirectly impair HSCs. A comprehensive understanding of how interferons impact hematopoiesis is necessary in order to identify novel therapeutic approaches for treating AA patients.

The unique role of STAT2 in constitutive and IFN-induced transcription and antiviral responses

In the canonical pathway of IFN-I-mediated signaling, phosphorylation of STAT1 and STAT2 leads to heterodimerization and interaction with IRF9. This complex, also known as IFN-stimulated gene factor 3 (ISGF3), then translocates into the nucleus and binds the IFN-I-stimulated response element (ISRE) leading to the activation of transcription of over 300 interferon stimulated genes (ISGs). In addition, STAT1 homodimers [known as γ-activated factor (GAF)] are formed and translocate to the nucleus, where they target genes containing the γ-activated sequence (GAS). The primary function of ISGF3 is to mediate a rapid and robust IFN-I activated response by regulating transient transcription of antiviral ISGs. This requires the quick assembly of ISGF3 from its pre-existing components STAT1, STAT2 and IRF9 and transport to the nucleus to bind ISRE-containing ISGs. The exact events that take place in formation, nuclear translocation and DNA-binding of active ISGF3 are still not clear. Over the years many studies have provided evidence for the existence of a multitude of alternative STAT2-containing (ISRE or GAS-binding) complexes involved in IFN-I signaling, emphasizing the importance of STAT2 in the regulation of specific IFN-I-induced transcriptional programs, independent of its involvement in the classical ISGF3 complex. This review describes the unique role of STAT2 in differential complex formation of unphosphorylated and phosphorylated ISGF3 components that direct constitutive and IFN-I-stimulated transcriptional responses. In addition, we highlight the existence of a STAT1-independent IFN-I signaling pathway, where STAT2/IRF9 can potentially substitute for the role of ISGF3 and offer a back-up response against viral infection.

The emerging role of interferon regulatory factor 9 in the antiviral host response and beyond

The host response to viral infections relies on tightly regulated and intricate signaling pathways involving type I interferons (IFN-Is). The IFN-Is mediate their antiviral effects predominantly through a signaling factor complex that comprises the transcription factors, interferon regulatory factor 9 (IRF9) and the signal transducers and activators of transcription (STAT) 1 and STAT2. While STAT1 and STAT2 have been studied extensively, the biological significance of IRF9 is only beginning to emerge. Recent studies have revealed a unique role for IRF9 as a conductor of the cellular responses to IFN-Is. Intriguingly, novel roles for IRF9 outside of the antiviral response are also being identified. Thus IRF9 may have a more extensive influence on cellular processes than previously recognized, ranging from antiviral immune responses to oncogenesis and gut homeostasis. In this review, we will focus on the distinct and emerging roles of IRF9 in the antiviral host response and beyond.

Targeted Therapies in Liver Fibrosis: Combining the Best Parts of Platelet-Derived Growth Factor BB and Interferon Gamma

Cytokines, growth factors, and other locally produced mediators play key roles in the regulation of disease progression. During liver fibrosis, these mediators orchestrate the balance between pro- and antifibrotic activities as exerted by the hepatic cells. Two important players in this respect are the profibrotic mediator platelet-derived growth factor BB (PDGF-BB) and the antifibrotic cytokine interferon gamma (IFNγ). PDGF-BB, produced by many resident and infiltrating cells, causes extensive proliferation, migration, and contraction of hepatic stellate cells (HSCs) and myofibroblasts. These cells are the extracellular matrix-producing hepatic cells and they highly express the PDGFβ receptor. On the other hand, IFNγ is produced by natural killer cells in fibrotic livers and is endowed with proinflammatory, antiviral, and antifibrotic activities. This cytokine attracted much attention as a possible therapeutic compound in fibrosis. However, clinical trials yielded disappointing results because of low efficacy and adverse effects, most likely related to the dual role of IFNγ in fibrosis. In our studies, we targeted the antifibrotic IFNγ to the liver myofibroblasts. For that, we altered the cell binding properties of IFNγ, by delivery of the IFNγ-nuclear localization sequence to the highly expressed PDGFβ receptor using a PDGFβ receptor recognizing peptide, thereby creating a construct referred to as “Fibroferon” (i.e., fibroblast-targeted interferon γ). In recent years, we demonstrated that HSC-specific delivery of IFNγ increased its antifibrotic potency and improved its general safety profile in vivo, making Fibroferon highly suitable for the treatment of (fibrotic) diseases associated with elevated PDGFβ receptor expression. The present review summarizes the knowledge on these two key mediators, PDGF-BB and IFNγ, and outlines how we used this knowledge to create the cell-specific antifibrotic compound Fibroferon containing parts of both of these mediators.

Interferons, signal transduction pathways, and the central nervous system

The interferon (IFN) family of cytokines participates in the development of innate and acquired immune defenses against various pathogens and pathogenic stimuli. Discovered originally as a proteinaceous substance secreted from virus-infected cells that afforded immunity to neighboring cells from virus infection, these cytokines are now implicated in various human pathologies, including control of tumor development, cell differentiation, and autoimmunity. It is now believed that the IFN system (IFN genes and the genes induced by them, and the factors that regulate these processes) is a generalized alarm of cellular stress, including DNA damage. IFNs exert both beneficial and deleterious effects on the central nervous system (CNS). Our knowledge of the IFN-regulated processes in the CNS is far from being clear. In this article, we reviewed the current understanding of IFN signal transduction pathways and gene products that might have potential relevance to diseases of the CNS.

Type I interferon-mediated immune response against influenza A virus is attenuated in the absence of p53

Influenza A virus (IAV) infection induces secretion of type I interferon (IFN) and activation of p53, which play essential roles in the host defense against tumor development and viral infection. In this study, we knocked down p53 expression by RNA interference. The expression levels of IFN-stimulated genes (ISGs) including IFN regulatory factor (IRF) 5, IRF9, ISG15, ISG20, guanylate-binding protein 1, retinoic acid-inducible gene-I and 2'-5'-oligoadenylate synthetase 1 were significantly attenuated in response to IAV infection and IFN-α stimulation in p53-knockdown cells. This attenuated expression of ISGs was associated with enhanced replication of IAV. Pretreatment of p53-knockdown cells with IFN-α failed to inhibit IAV replication, indicating impaired antiviral activity. These findings indicate that p53 plays an essential role in the enhancement of the type I IFN-mediated immune response against IAV infection.

Further evidence suggesting a role for variation in ARHGAP29 variants in nonsyndromic cleft lip/palate

BACKGROUND

Nonsyndromic cleft lip with or without cleft palate (NSCL/P) is a common birth defect of complex etiology. Several genes have been implicated in the etiology of NSCL/P, although only a few have been replicated across datasets.

METHODS

ARHGAP29 was suggested as a candidate gene for NSCL/P as it is located in close proximity to ABCA4 (1p22), a gene previously identified in a genome-wide association study of NSCL/P.

RESULTS

Rare, potentially damaging, coding variants in ARHGAP29 were found in NSCL/P cases, and its expression was detected during murine craniofacial development. In this study, we investigated whether variations in ARHGAP29 were associated with NSCL/P in our family based dataset. Five single-nucleotide polymorphisms (SNPs) flanking and within ARHGAP29 were genotyped in our NSCL/P datasets consisting of simplex and multiplex families of non-Hispanic white (NHW, primarily European) and Hispanic ethnicities. Results showed strong association of three ARHGAP29 SNPs with NSCL/P in the NHW families. Two intronic SNPs (rs1541098 and rs3789688) showed strong association with NSCL/P in all NHW families (p = 0.0005 and p = 0.0002, respectively), and simplex NHW families (p = 0.003 for both SNPs). A SNP in the 3' untranslated region (rs1576593) also showed strong association with NSCL/P in all NHW families (p = 0.002), and the multiplex subset (p = 0.002). ARHGAP29 SNP haplotypes were also associated with NSCL/P. Evidence of gene-gene interaction was found between ARHGAP29 and additional cleft susceptibility genes.

CONCLUSION

This study further supports ARHGAP29 as a candidate gene for human NSCL/P in families of Caucasian descent.

STAT1 as a central mediator of IFNγ and TLR4 signal integration in vascular dysfunction

Atherosclerosis is characterized by early endothelial dysfunction and altered vascular smooth muscle cells (VSMCs) contractility. The forming atheroma is a site of excessive production of cytokines and inflammatory ligands by various cell types that mediate inflammation and immune responses. Key factors contributing to early stages of plaque development are IFNγ and TLR4. This review provides insight in the differential STAT1-dependent signal integration between IFNγ and TLR4 signals in vascular cells and atheroma interacting immune cells. This results in increased leukocyte attraction and adhesion and VSMC proliferation and migration, which are important characteristics of EC dysfunction and early triggers of atherosclerosis.

Hepatitis C virus impairs TLR3 signaling and inhibits IFN-λ 1 expression in human hepatoma cell line

Toll-like receptor 3 (TLR3) activation plays an important role in the innate immune responses to viral infections. We show here that the activation of TLR3 signaling pathway by poly I:C, a synthetic mimic of dsRNA, could induce high-level expression of interferon (IFN)-λ1 in a hepatoma cell line. The induced IFN-λ1 contributed to poly I:C-mediated inhibition of hepatitis C virus (HCV) Japanese fulminant hepatitis-1 (JFH-1) replication in Huh7 cells. This inhibitory effect of poly I:C on HCV replication, however, was compromised by HCV infection of Huh7 cells. Investigation of the mechanisms showed that HCV infection suppressed the expression of poly I:C-induced IFN-λ1 and IFN-stimulated genes [IFN-stimulated gene 56 (ISG-56), myxovirus resistance A (MxA) and 2'-5'-oligoadenylate synthetase 1 (OAS-1))], the key antiviral elements in IFN signaling pathway. Among the HCV nonstructural (NS) proteins tested, NS3/4A, NS5A and NS5B had the ability to inhibit poly I:C-induced IFN-λ1 expression in Huh7 cells. These observations provide the experimental evidence that HCV and its proteins impair TLR3 signaling and inhibit intracellular IFN-λ1/ISG expression in a hepatoma cell line, which may account for HCV persistence in the liver.

Association between Interferon-Inducible Protein 6 (IFI6) Polymorphisms and Hepatitis B Virus Clearance

CD8+ T cells are key factors mediating hepatitis B virus (HBV) clearance. However, these cells are killed through HBV-induced apoptosis during the antigen-presenting period in HBV-induced chronic liver disease (CLD) patients. Interferon-inducible protein 6 (IFI6) delays type I interferon-induced apoptosis in cells. We hypothesized that single nucleotide polymorphisms (SNPs) in the IFI6 could affect the chronicity of CLD. The present study included a discovery stage, in which 195 CLD patients, including chronic hepatitis B (HEP) and cirrhosis patients and 107 spontaneous recovery (SR) controls, were analyzed. The genotype distributions of rs2808426 (C > T) and rs10902662 (C > T) were significantly different between the SR and HEP groups (odds ratio [OR], 6.60; 95% confidence interval [CI], 1.64 to 26.52, p = 0.008 for both SNPs) and between the SR and CLD groups (OR, 4.38; 95% CI, 1.25 to 15.26; p = 0.021 and OR, 4.12; 95% CI, 1.18 to 14.44; p = 0.027, respectively). The distribution of diplotypes that contained these SNPs was significantly different between the SR and HEP groups (OR, 6.58; 95% CI, 1.63 to 25.59; p = 0.008 and OR, 0.15; 95% CI, 0.04 to 0.61; p = 0.008, respectively) and between the SR and CLD groups (OR, 4.38; 95% CI, 1.25 to 15.26; p = 0.021 and OR, 4.12; 95% CI, 1.18 to 14.44; p = 0.027, respectively). We were unable to replicate the association shown by secondary enrolled samples. A large-scale validation study should be performed to confirm the association between IFI6 and HBV clearance.

Genome guardian p53 and viral infections

Because virus infections elicit various cellular responses that inhibit viral replication and growth, viruses must intervene to attenuate antiviral measures in order to thrive. The genome guardian p53 plays a central part not only in DNA damage responses, inducing cell cycle arrest or apoptosis, but also in the innate host immune control of viral infections by orchestrating diverse signaling pathways originating from many different cellular receptors and sensors. Many viruses have acquired sophisticated mechanisms to regulate p53 functions by deploying subversive proteins and modulating its post-transcriptional status. In this review, we overview the mechanisms by which DNA and RNA viruses manage p53 signaling in favor of their continued survival.

Negative regulation of the type I interferon signaling pathway by synthetic Toll-like receptor 7 ligands

Ten Toll-like receptor (TLR) family members have been reported in humans. Here, the endoplasmatic receptors TLR9, TLR8, TLR7, and TLR3 respond to nucleic acids and derivatives or to small molecules (TLR7 and 8). Another cytoplasmic RNA receptor, retinoic acid inducible gene I (RIG-I), is stimulated by 5' triphosphate double-stranded RNA. We discovered that TLR7 small-molecule agonists inhibit nucleic acid-mediated TLR3, TLR7, TLR9, or RIG-I-dependent interferon-α (IFN-α) immune response. Other cytokines and chemokines stimulated by nucleic acid agonists remained unaffected. The observed blockage of TLR3, TLR7, TLR9, and RIG-I-mediated IFN-α response appears to be driven by a competitive mechanism at the type I IFN pathway. Besides type I IFN, IFN response genes such as IFIT-1, Mx1, OAS1, or IRF7 were affected, which indicates that the key element driving the inhibition is located in the type I IFN pathway. Indeed, the heterotrimeric complex formation of phosphor-signal transducer and activator of transcription factor 1 (STAT1), phosphor-STAT2, and IRF9 (called ISGF3, IFN-stimulated gene factor 3) is inhibited through the TLR7 small-molecule agonists by phosphor-STAT2 blockage. These findings provide novel insights into the use of synthetic TLR7 or TLR7/8 small molecules as ligands for immune activation and suppression.

STAT1 as a novel therapeutical target in pro-atherogenic signal integration of IFNγ, TLR4 and IL-6 in vascular disease

Inflammation participates importantly in host defenses against infectious agents and injury, but it also contributes to the pathophysiology of atherosclerosis. Recruitment of blood leukocytes to the injured vascular endothelium characterizes the initiation and progression of atherosclerosis and involves many inflammatory mediators, modulated by cells of both innate and adaptive immunity. The pro-inflammatory cytokine, interferon (IFN)-γ derived from T cells, is vital for both innate and adaptive immunity and is also expressed at high levels in atherosclerotic lesions. As such IFN-γ plays a crucial role in the pathology of atherosclerosis through activation of signal transducer and activator of transcription (STAT) 1. Toll-like receptors (TLRs) are innate immune pattern recognition receptors (PRRs) expressed on a variety of cells, and thus initiate and sustain the inflammatory response in atherosclerosis. More recent studies have revealed that STAT1 is involved in the signaling events mediated by TLR4, leading to increased expression of several pro-inflammatory and pro-atherogenic mediators. By upregulating members of the Suppressors Of Cytokine Signaling (SOCS) family that regulate cellular responsiveness to immune signals, IFNγ and TLR4-activated pathways have also shown to inhibit IL-6 STAT3-dependent anti-inflammatory signaling and potentially shift IL-6 to a STAT1 activating pro-inflammatory cytokine. Consequently, STAT1 has been identified as a point of convergence for the cross-talk between the pro-atherogenic IFN-γ, TLR4 and IL-6 activated pathways in immune as well as vascular cells, as such amplifying pro-inflammatory signals. This results in augmented smooth muscle cell (SMC) and leukocyte migration, leukocyte to endothelial cell (EC) adhesion and foam cell formation, and could encompass a novel mechanism involved in the initiation and progression of atherosclerosis. Therefore, application of small inhibitory compounds that specifically interact with the SH2-phosphotyrosine pocket of STAT1, proposed here as a novel working mechanism for the known STAT1 inhibitor fludarabine, could be a promising tool in the development of a therapeutical strategy for atherosclerosis.

T cell-, interleukin-12-, and gamma interferon-driven viral clearance in measles virus-infected brain tissue

Genetic studies with immunocompetent mice show the importance of both T cells and gamma interferon (IFN-γ) for survival of a measles virus (MV) challenge; however, the direct role of T cells and IFN-γ within the MV-infected brain has not been addressed. Organotypic brain explants represent a successful ex vivo system to define central nervous system (CNS)-specific mechanisms of leukocyte migration, activation, and MV clearance. Within the heterogeneous, brain-derived, primed leukocyte population which reduced MV RNA levels in brain explants by 60%, CD3 T cells are the active antiviral cells, as purified CD3-positive cells are highly antiviral and CD3-negative leukocytes are unable to reduce the viral load. Neutralization of CCL5 and CXCL10 decreases leukocyte migration to areas of infection by 70%. However, despite chemokines directing the migration of T cells to infected neurons, chemokine neutralization revealed that migration is not required for viral clearance, suggesting a cytokine-mediated antiviral mechanism. In accordance with our hypothesis, the ability of leukocytes to clear the virus is abrogated when explants are treated with anti-IFN-γ neutralizing antibodies. IFN-γ applied to infected slices in the absence of primed leukocytes reduces the viral load by more than 80%; therefore, in brain tissue, IFN-γ is both necessary and sufficient to clear MV. Secretion of IFN-γ is stimulated by interleukin-12 (IL-12) in the brain, as neutralization of IL-12 results in loss of antiviral activity and stimulation of leukocytes with IL-12/IL-18 enhances their immune effector function of viral clearance. MV-primed leukocytes can reduce both West Nile and mouse hepatitis viral RNAs, indicating that cytokine-mediated viral clearance occurs in an antigen-independent manner. The IFN-γ signal is transduced within the brain explant by the Jak/STAT signaling pathway, as inhibition of Jak kinases results in a loss of antiviral activity driven by either brain-derived leukocytes or recombinant IFN-γ. These results reveal that primed T cells directly act to clear MV infection of the brain by using a noncytolytic IL-12- and IFN-γ-dependent mechanism in the CNS and that this mechanism relies upon Jak/STAT signaling.

Combining theoretical analysis and experimental data generation reveals IRF9 as a crucial factor for accelerating interferon α-induced early antiviral signalling

Type I interferons (IFN) are important components of the innate antiviral response. A key signalling pathway activated by IFNα is the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway. Major components of the pathway have been identified. However, critical kinetic properties that facilitate accelerated initiation of intracellular antiviral signalling and thereby promote virus elimination remain to be determined. By combining mathematical modelling with experimental analysis, we show that control of dynamic behaviour is not distributed among several pathway components but can be primarily attributed to interferon regulatory factor 9 (IRF9), constituting a positive feedback loop. Model simulations revealed that increasing the initial IRF9 concentration reduced the time to peak, increased the amplitude and enhanced termination of pathway activation. These model predictions were experimentally verified by IRF9 over-expression studies. Furthermore, acceleration of signal processing was linked to more rapid and enhanced expression of IFNα target genes. Thus, the amount of cellular IRF9 is a crucial determinant for amplification of early dynamics of IFNα-mediated signal transduction.

Gender-dependent expression of murine Irf5 gene: implications for sex bias in autoimmunity

Molecular mechanisms that contribute to sex bias in the development of systemic lupus erythematosus (SLE), an autoimmune disease, remain unknown. We found that the expression levels of interferon regulatory factor 5 (IRF5), a lupus susceptibility factor, depend on gender of mice. We found that steady-state levels of the Irf5 mRNA were relatively higher in splenic cells from certain autoimmune-prone mice (for example, NZB and NZB/W F(1)) than in non-autoimmune C57BL/6 mice. Additionally, levels of Irf5 mRNA and protein were higher in females than in strain and age-matched males. Accordingly, splenic cells from estrogen receptor-alpha (ERα) knockout, when compared with the wild-type (ERα(+/+)), female mice expressed relatively lower levels of Irf5 mRNA and the treatment of splenic cells with 17β-estradiol increased the levels. Furthermore, splenic B cells from the female mice had relatively more IRF5 protein in the nucleus than the male mice. Collectively, our observations demonstrate a gender bias in the expression and sub-cellular localization of the murine IRF5.

Structural and functional studies of STAT1 from Atlantic salmon (Salmo salar)

BACKGROUND

Type I and type II interferons (IFNs) exert their effects mainly through the JAK/STAT pathway, which is presently best described in mammals. STAT1 is involved in signaling pathways induced by both types of IFNs. It has a domain-like structure including an amino-terminus that stabilizes interaction between STAT dimers in a promoter-binding situation, a coiled coil domain facilitating interactions to other proteins, a central DNA-binding domain, a SH2 domain responsible for dimerization of phosphorylated STATs and conserved phosphorylation sites within the carboxy terminus. The latter is also the transcriptional activation domain.

RESULTS

A salmon (Salmo salar) STAT1 homologue, named ssSTAT1a, has been identified and was shown to be ubiquitously expressed in various cells and tissues. The ssSTAT1a had a domain-like structure with functional motifs that are similar to higher vertebrates. Endogenous STAT1 was shown to be phosphorylated at tyrosine residues both in salmon leukocytes and in TO cells treated with recombinant type I and type II IFNs. Also ectopically expressed ssSTAT1 was phosphorylated in salmon cells upon in vitro stimulation by the IFNs, confirming that the cloned gene was recognized by upstream tyrosine kinases. Treatment with IFNs led to nuclear translocation of STAT1 within one hour. The ability of salmon STAT1 to dimerize was also shown.

CONCLUSIONS

The structural and functional properties of salmon STAT1 resemble the properties of mammalian STAT1.

Dual Role of p53 in Innate Antiviral Immunity

Tumor suppressor p53 is widely known as 'the guardian of the genome' due to its ability to prevent the emergence of transformed cells by the induction of cell cycle arrest and apoptosis. However, recent studies indicate that p53 is also a direct transcriptional target of type I interferons (IFNs) and thus, it is activated by these cytokines upon viral infection. p53 has been shown to contribute to virus-induced apoptosis, therefore dampening the ability of a wide range of viruses to replicate and spread. Interestingly, recent studies also indicate that several IFN-inducible genes such as interferon regulatory factor 9 (IRF9), IRF5, IFN-stimulated gene 15 (ISG15) and toll-like receptor 3 (TLR3) are in fact, p53 direct transcriptional targets. These findings indicate that p53 may play a key role in antiviral innate immunity by both inducing apoptosis in response to viral infection, and enforcing the type I IFN response, and provide a new insight into the evolutionary reasons why many viruses encode p53 antagonistic proteins.

Molecular mechanisms of IFN-gamma to up-regulate MHC class I antigen processing and presentation

IFN-gamma up-regulates MHC class I expression and antigen processing and presentation on cells, since IFN-gamma can induce multiple gene expressions that are related to MHC class I antigen processing and presentation. MHC class I antigen presentation-associated gene expression is initiated by IRF-1. IRF-1 expression is initiated by phosphorylated STAT1. IFN-gamma binds to IFN receptors, and then activates JAK1/JAK2/STAT1 signal transduction via phosphorylation of JAK and STAT1 in cells. IFN-gamma up-regulates MHC class I antigen presentation via activation of JAK/STAT1 signal transduction pathway. Mechanisms of IFN-gamma to enhance MHC class I antigen processing and presentation were summarized in this literature review.

Critical role for constitutive type I interferon signaling in the prevention of cellular transformation

Interferons-alpha/beta, which are produced upon viral infection, are key soluble factors for the establishment of an antiviral state, but are also produced at low levels in the absence of infection. Herein, we demonstrate that a weak signal by these constitutively produced IFN-alpha/beta show a preventive role in cellular transformation. Ifnar1-deficient (Ifnar1(-/-)) MEF, which are devoid of IFN-alpha/beta signal, undergo a spontaneous transformation during long-term cell culture. Similar to Irf1(-/-) MEF, primary Ifnar1(-/-) MEF become tumorigenic in nude mice by the expression of activated c-Ha-Ras oncoprotein. However, Ifnar1(-/-) MEF do not show any abnormal growth properties. A similar observation is made in Ifnb(-/-) MEF that fail to produce constitutive IFN-alpha/beta, whereas such a transforming property is not found in MEF that lack any of the IFN receptor downstream molecules including Stat1, IRF9 and IRF1. Furthermore, Ifnar1(-/-) mice develop chemically-induced skin papilloma more severely than wild-type mice. In addition, the expression levels of IFNAR1 mRNA are significantly decreased in human gastric cancer tissues. These results suggest a cell-intrinsic role of the weak signal by constitutively produced IFN-alpha/beta to prevent cells from transformation, which may be mediated by a hitherto-unknown pathway(s) downstream of the IFN-alpha/beta receptor.

Transcriptional role of p53 in interferon-mediated antiviral immunity

Tumor suppressor p53 is activated by several stimuli, including DNA damage and oncogenic stress. Previous studies (Takaoka, A., S. Hayakawa, H. Yanai, D. Stoiber, H. Negishi, H. Kikuchi, S. Sasaki, K. Imai, T. Shibue, K. Honda, and T. Taniguchi. 2003. Nature. 424:516–523) have shown that p53 is also induced in response to viral infections as a downstream transcriptional target of type I interferon (IFN) signaling. Moreover, many viruses, including SV40, human papillomavirus, Kaposi's sarcoma herpesvirus, adenoviruses, and even RNA viruses such as polioviruses, have evolved mechanisms designated to abrogate p53 responses. We describe a novel p53 function in the activation of the IFN pathway. We observed that infected mouse and human cells with functional p53 exhibited markedly decreased viral replication early after infection. This early inhibition of viral replication was mediated both in vitro and in vivo by a p53-dependent enhancement of IFN signaling, specifically the induction of genes containing IFN-stimulated response elements. Of note, p53 also contributed to an increase in IFN release from infected cells. We established that this p53-dependent enhancement of IFN signaling is dependent to a great extent on the ability of p53 to activate the transcription of IFN regulatory factor 9, a central component of the IFN-stimulated gene factor 3 complex. Our results demonstrate that p53 contributes to innate immunity by enhancing IFN-dependent antiviral activity independent of its functions as a proapoptotic and tumor suppressor gene.

Interleukin-29 uses a type 1 interferon-like program to promote antiviral responses in human hepatocytes

Interleukin-28A (IL-28A), IL-28B and IL-29 are a family of class II cytokines that stimulate antiviral responses through a heterodimeric receptor that is distinct from the type I interferon (IFN) receptor. To better understand how this newly described family of cytokines regulates the antiviral state, we compared various cellular responses elicited by IL-29 and IFN-alpha. Here we show that these cytokines stimulate similar patterns of signal transducer and activator of transcription 1 (STAT-1), -2, -3, and -5 phosphorylation and nearly identical patterns of gene expression when analyzed in two distinct cell types by microarray analysis. Interestingly, the IL-29 receptor is preferentially expressed on primary hepatocytes within normal liver and pegylated forms of IL-29 and IFN-alpha induced equivalent 2'5' oligoadenylate synthetase (OAS) and MX1 gene expression in this cell type. Pegylated IL-29 also produced a significant reduction in human hepatitis B and hepatitis C viral load in vitro and reduced the cytopathic effect caused by the fully replicating flavivirus, West Nile virus. In conclusion, IL-29 and IFN-alpha stimulate identical antiviral responses despite their utilization of different receptors. This fact, combined with significant receptor expression in hepatitis virus-infected livers, suggests that IL-29 may have therapeutic value against chronic viral hepatitis in human patients.

Curcumin regulates signal transducer and activator of transcription (STAT) expression in K562 cells

Signal transducers and activators of transcription (STATs) play important roles in numerous cellular events as for example differentiation, inflammation or immune response. Furthermore, constitutive STAT activation can be observed in a high number of tumors. In our hands, curcumin treatment induced a decrease of nuclear STAT3, -5a and -5b, without affecting neither STAT1, nor the phosphorylation state of STAT1, -3 or -5 in the K562 cell line. Most interestingly, the decrease of nuclear STAT5a and -5b after curcumin treatment was accompanied by an increase of truncated STAT5 isoforms, indicating that curcumin is able to induce the cleavage of STAT5 into its dominant negative variants lacking the STAT5 C-terminal region. Interferon (IFN)-beta and -gamma treatment induced IFN-stimulated responsive element (ISRE) transcriptional activity, which was efficiently inhibited by curcumin pre-treatment. In parallel, IFN-gamma treatment induced an increase of the amount of nuclear STAT1 and -3, as well as their phosphorylated isoforms. Again, curcumin pre-treatment inhibited these increases. Finally, curcumin treatment inhibited Jak2 mRNA expression as well as cyclin D1 and v-src gene expression in K562 chronic leukaemia cells.

Interferon signalling network in innate defence

Interferons (IFNs) elicit multifaceted effects in host innate defence. Accumulating evidence revealed that not only the first identified Jak-Stat pathway but also other newly found signalling pathways are required for the induction of versatile responses by IFNs. In particular, type I IFNs are inducible by viral infection through the recognition of pathogen-associated molecules by pattern recognition receptors, and the induction of multiple IFN-stimulated genes through the activation of type I IFN signalling confers antiviral and immunomodulatory activities. Any step in this process is often targeted by viruses for their immuno-evasion. The regulatory function of constitutive IFN-alpha/beta signalling has been recognized in terms of its boosting effect on cellular responsiveness in host defence systems. Further comprehensive understanding of IFN signalling may offer a better direction to unravelling the complex signalling networks in the host defence system, and may contribute to their more effective therapeutic applications.

Stats: multifaceted regulators of transcription

The high-affinity binding interactions between interferons (IFNs) and their cognate cell surface receptors lead to the activation of receptor-associated Janus protein tyrosine kinases (Jaks) and subsequent phosphorylation and activation of a group of transcription factors, the signal transducers and activators of transcription (Stats). Upon IFN-induced activation, these Stat proteins form homodimeric and heterodimeric complexes that translocate to the nucleus and bind specific elements within the promoters of IFN-stimulated genes (ISGs). In addition to the well-studied IFN-induced ISG factor 3 (ISGF3) and Stat1:1 complexes, IFNs induce the formation of a number of other Stat-containing complexes, including Stat3:3 and Stat5:5 homodimers, as well as Stat2:1 and Stat5:CrkL heterodimers, that also mediate gene transcription. Moreover, emerging evidence suggests that particular amino acid residues within the individual Stat proteins contribute to different aspects of Stat function. These residues modulate the transcriptional activation potential of Stat-containing complexes and thereby influence the expression of ISGs. Indeed, the Stat proteins function in a multifaceted manner to regulate the expression of proteins that mediate IFN responses.

CCAAT/enhancer binding proteins and interferon signaling pathways

Interferons (IFNs) regulate a number of host responses, including innate and adaptive immunity against viruses, microbes, and neoplastic cells. These responses are dependent on the expression of IFN-stimulated genes (ISGs). Given the diversities in these responses and their kinetics, it is conceivable that a number of different factors are required for controlling them. Here, we describe one such pathway wherein transcription factor CAAAT/enhancer binding protein-beta (C/EBP-beta) is controlled via IFN-gamma-induced MAPK signaling pathways. At least two IFN-gamma-induced MAPK signals converge on to C/EBP-beta for inducing transcription. One of these, driven by extracellular signal-regulated kinases (ERKs), phosphorylates the C/EBP-beta protein in its regulatory domain. The second, driven by the mixed-lineage kinases (MLKs), induces a dephosphorylation leading to the recruitment of transcriptional coactivators.

Suppression of hepatocellular carcinoma growth in mice via leptin, is associated with inhibition of tumor cell growth and natural killer cell activation

BACKGROUND/AIMS

Leptin exerts potent immune modulatory properties. The aim of this study was to determine leptin's anti-tumor effect in a murine model of hepatocellular carcinoma (HCC).

METHODS

In vivo, athymic nude mice were transplanted with Hep3B cells, followed by daily leptin administration for 6 weeks.

RESULTS

Leptin administration induced a significant reduction in tumor size, improved survival rate, and was associated with a significant increase in peripheral natural killer (NK) cell number. Splenocytes from leptin-treated mice featured decreased expression of CIS mRNA. SCID mice featured a similar leptin-associated tumor suppression. In contrast, NK-deficient SCID-beige mice developed larger tumors which were unresponsive to leptin. NK cells incubated in vitro with increasing doses of leptin demonstrated increased cytotoxicity and proliferation. Incubation of leptin with hepatoma cells induced a dose-dependent reduction in proliferation, suggesting a direct anti-tumor effect. Leptin induced increased mRNA expression of STAT2 and SOCS1 on HCC cell lines.

CONCLUSIONS

Leptin administration induces a significant suppression of human HCC. This effect is mediated by induction of natural killer cell proliferation and activation, along with direct inhibition of tumor growth. Decreased NK expression of inhibitory CIS and over-expression of the antiproliferative STAT2 and SOCS1 proteins in HCC lines may underline the anti-cancerous effects of leptin.

Development of monoclonal antibodies against GBF1 and their use in studying its functions

Interferon-gamma (IFN-gamma) regulates a number of cellular genes using a variety of cellular signaling pathways. Previously, we identified a novel IFN-regulated element, IFN-gamma-activated transcriptional element (GATE), in the promoter of the murine IFN regulatory factor-9 (IRF-9) gene. This element binds to novel factors. We have recently characterized a novel regulatory factor, GATE binding factor 1 (GBF1), which promoted IFN-gamma-induced transcription. Although GBF1 was a potent inducer of transcription, it did not bind to DNA well in vitro. To understand its role in IFN-gamma-induced actions, we raised monoclonal antibodies (mAb) against GBF1. These antibodies are highly useful in Western, immunoprecipitation, and immunocytochemical analyses. Employing these antibodies, we show that GBF1 is recruited to the endogenous IRF-9 promoter. We also show GBF1 interacts with CAAAT/enhancer binding protein-beta (C/EBP-beta), the other GATE binding factor. Furthermore, other cytokines, such as interleukin-1 (IL-1) and IL-6, induced the expression of GBF1. These antibodies may be useful tools for investigating the role of GBF1 in cytokine-induced responses.

IFN-gamma-stimulated transcriptional activation by IFN-gamma-activated transcriptional element-binding factor 1 occurs via an inducible interaction with CAAAT/enhancer-binding protein-beta

IFN-gamma-activated transcriptional element (GATE)-binding factor 1 (GBF1) was identified as a transactivator that induces gene expression through GATE, a novel IFN-inducible element. Although it can induce gene expression, it is an extremely weak DNA-binding protein on its own. GATE also binds another transcription factor, C/EBP-beta. Therefore, we explored whether GBF1 physically interacts with C/EBP-beta to induce IFN-gamma-regulated transcription. In response to IFN-gamma, C/EBP-beta undergoes phosphorylation at a critical ERK1/2 phosphorylation motif. Mutational inactivation of this motif and/or interference with the ERK1/2 activation prevented the IFN-gamma-induced interactions between GBF1 and C/EBP-beta. A 37-aa long peptide derived from the GBF1 protein can associate with C/EBP-beta in an IFN-inducible manner. These results identify a converging point for two transactivators that exert their effects through a single response element. Together, our studies identify a novel regulatory mechanism that controls IFN-induced transcription.

Induction of antiviral state in fish cells by Japanese flounder, Paralichthys olivaceus, interferon regulatory factor-1

Interferon regulatory factor-1 (IRF-1) mediates an antiviral state in cells by regulating the expression of the interferon (IFN-alpha/beta) system. To elucidate the role of IRF-1 in fish during virus infections, we constructed a recombinant plasmid of the Japanese flounder, Paralichthys olivaceus IRF-1 (JF IRF-1) under the control of the cytomegalovirus (CMV) immediate/early enhancer promoter. The antiviral mechanism of JF IRF-1 was studied using transfection experiments in a homologous cell line. Here, we show that cell supernatants obtained from transiently transfected cells enhanced cell viability of a heterologous cell line upon incubation, reduced the titers of hirame rhabdovirus (HIRRV) and viral hemorrhagic septicemia virus (VHSV), and possessed cytokine-like activity, as shown by their ability to protect cells against virus infections. The supernatants also inhibited the replication of the rhabdoviruses during the early stages of infection as indicated by the reduction of viral titers in the presence of the supernatants obtained from the transfected cells. Further analysis showed that the cell culture supernatants contain cytokine-like substances that possess acid-labile and temperature-resistant properties. These results indicate that JF IRF-1 induces an antiviral state in cells by mediating the production of cytokine-like substances. Thus, JF IRF-1 might be useful as an adjuvant in the development of DNA vaccines against commercially important viral pathogens in Japanese flounder aquaculture.

Activation of mitogen-activated protein kinase kinase (MKK) 3 and MKK6 by type I interferons

There is accumulating evidence that the p38 MAP kinase pathway plays important roles in Type I interferon (IFN) signaling, but the mechanisms regulating p38 activation during engagement of the Type I IFN receptor remain to be defined. We sought to identify the events that lead to activation of the p38 MAP kinase in response to Type I IFNs. Our data demonstrate that treatment of sensitive cell lines with IFNalpha results in activation of both MAP kinase kinase 3 (MKK3) and MAP kinase kinase 6 (MKK6). Such IFN-inducible activation of MKK3 and MKK6 is essential for downstream phosphorylation and activation of the p38 MAP kinase, as shown by studies using mouse embryonic fibroblasts (MEFs) with targeted disruption of the Mkk3 and Mkk6 genes (MKK3-/- MKK6-/-). Similarly, IFN-dependent activation of the downstream effectors of p38, MAPKAPK-2 and MAPKAPK-3, is not detectable in cells lacking Mkk3 and Mkk6, demonstrating that the function of these MAP kinase kinases is required for full activation of the p38 pathway. To define the functional relevance of MKK3/6 engagement in Type I IFN signaling, IFN-inducible gene transcription was evaluated in the MKK3/MKK6 double knock-out cells. IFNalpha- and IFNbeta-dependent transcription via either interferon-stimulated response element or IFNgamma activated site elements was defective in MKK3 -/-/MKK6 -/- MEFs in luciferase reporter assays. In addition, IFN-dependent induction of two genes known to be of importance in the generation of IFN responses, Isg15 and Irf-9, was diminished in the absence of Mkk3 and Mkk6. The effects of Mkk3 and Mkk6 on IFN-dependent transcription were unrelated to any effects on the phosphorylation and activation of STAT proteins, indicating the presence of a STAT-independent mechanism. Altogether, our findings demonstrate that MKK3 and MKK6 are rapidly activated during engagement of the Type I IFN receptor and play important roles in Type I IFN signaling and the generation of IFN responses.

Identification of a novel GC-rich binding protein that binds to an indispensable element for constitutive IRF-4 promoter activity in B cells

Interferon regulatory factor-4 (IRF-4) is a lymphoid-specific transcription factor that plays crucial roles in the development and the functions of immune cells. B lymphocytes express IRF-4 constitutively. In this report, we investigated the transcriptional control of IRF-4 in B lymphocytes. Successive deletions of the IRF-4 promoter from -4799 revealed that the region between -51 and -28 (5'-CGCCCGCCCCAGGCCCCGCCCCA-3') was required for the basal promoter activity. Mutations in the distal and proximal sites of this GC-rich sequence resulted in 62 and 81% reductions in the IRF-4 promoter activity, respectively. EMSA observations revealed the formation of a protein complex with the corresponding DNA, which was sensitive to mutations in the GC-rich sequences. UV photocrosslinking assays identified a novel 60 kDa protein with a similar sequence preference. The possible involvement of this factor in the regulation of IRF-4 gene expression is discussed.

Induction of interferon-stimulated gene expression and antiviral responses require protein deacetylase activity

Histone deacetylase (HDAC) activity, commonly correlated with transcriptional repression, was essential for transcriptional induction of IFN-stimulated genes (ISG). Inhibition of HDAC function led to global impairment of ISG expression, with little effect on basal expression. HDAC function was not required for signal transducer and activator of transcription tyrosine phosphorylation, nuclear translocation, or assembly on chromatin, but it was needed for full activity of the signal transducer and activator of transcription transactivation domain. HDAC function was also required for gene induction driven by the IFN regulatory factor 3 transcription factor activated by virus infection, and it was essential for establishment of an antiviral response against Flaviviridae, Rhabdoviridae, and Picornaviridae. Requirement for HDAC function in transcriptional activation may represent a general mechanism for rapid stimulation of ISG transcription.

Alternate interferon signaling pathways

More than a half a century ago, interferons (IFN) were identified as antiviral cytokines. Since that discovery, IFN have been in the forefront of basic and clinical cytokine research. The pleiotropic nature of these cytokines continues to engage a large number of investigators to define their actions further. IFN paved the way for discovery of Janus tyrosine kinase (JAK)-signal transducing activators of transcription (STAT) pathways. A number of important tumor suppressive pathways are controlled by IFN. Several infectious pathogens counteract IFN-induced signaling pathways. Recent studies indicate that IFN activate several new protein kinases, including the MAP kinase family, and downstream transcription factors. This review not only details the established IFN signaling paradigms but also provides insights into emerging alternate signaling pathways and mechanisms of pathogen-induced signaling interference.

Cellular responses to ErbB-2 overexpression in human mammary luminal epithelial cells: comparison of mRNA and protein expression

Microarray analysis offers a powerful tool for studying the mechanisms of cellular transformation, although the correlation between mRNA and protein expression is largely unknown. In this study, a microarray analysis was performed to compare transcription in response to overexpression of the ErbB-2 receptor tyrosine kinase in a model mammary luminal epithelial cell system, and in response to the ErbB-specific growth factor heregulin β1. We sought to validate mRNA changes by monitoring changes at the protein level using a parallel proteomics strategy, and report a surprisingly high correlation between transcription and translation for the subset of genes studied. We further characterised the identified targets and relate differential expression to changes in the biological properties of ErbB-2-overexpressing cells. We found differential regulation of several key cell cycle modulators, including cyclin D2, and downregulation of a large number of interferon-inducible genes, consistent with increased proliferation of the ErbB-2-overexpressing cells. Furthermore, differential expression of genes involved in extracellular matrix modelling and cellular adhesion was linked to altered adhesion of these cells. Finally, we provide evidence for enhanced autocrine activation of MAPK signalling and the AP-1 transcription complex. Together, we have identified changes that are likely to drive proliferation and anchorage-independent growth of ErbB-2- overexpressing cancer cells.

A 40-cM region on chromosome 14 plays a critical role in the development of virus persistence, demyelination, brain pathology and neurologic deficits in a murine viral model of multiple sclerosis

Theiler virus persists and induces immune-mediated demyelination in susceptible mice and serves as a model of multiple sclerosis. Previously, we identified 4 markers--D14Mit54, D14Mit60, D14Mit61, and D14Mit90--in a 40-cM region of chromosome 14 that are associated with demyelination in a cross between susceptible DBA/2 and resistant B10.D2 mice. We generated congenic-inbred mice to examine the contribution of this 40-cM region to disease. DBA Chr.14B10 mice, containing the chromosomal segment marked by the microsatellite polymorphisms, developed less spinal cord demyelination than did DBA/2 mice. More demyelination was found in the reciprocal congenic mouse B10.D2 Chr.14D2 than in the B10.D2 strain. Introduction of the DBA/2 chromosomal region onto the B10.D2 genetic background resulted in more severe disease in the striatum and cortex relative to B10.D2 mice. The importance of the marked region of chromosome 14 is indicated by the decrease in neurological performance using the Rotarod test during chronic disease in B10.D2 Chr.14D2 mice in comparison to B10.D2 mice. Viral replication was increased in B10.D2 Chr.14D2 mice as determined by quantitative real-time RT-PCR. These results indicate that the 40-cM region on chromosome 14 of DBA/2 mice contributes to viral persistence, subsequent demyelination, and loss of neurological function.

Interferon Regulatory Factor-3-mediated Activation of the Interferon-sensitive Response Element by Toll-like receptor (TLR) 4 but Not TLR3 Requires the p65 Subunit of NF-κ

Interferon regulatory factor (IRF) 3 is a transcription factor that binds the interferon-sensitive response element (ISRE) and is activated by Toll-like receptor 3 (TLR3) and TLR4. We have found that a dominant negative form of I kappa B kinase 2 and a mutant form of I kappa B, which acts as a super-repressor of NF-kappa B, blocked activation of the ISRE by the TLR4 ligand lipopolysaccharide but not the TLR3 ligand poly(I-C). TLR4 failed to activate the ISRE in mouse embryonic fibroblasts bearing a targeted deletion of p65, whereas the response to TLR3 in these cells was normal. The p65 subunit of NF-kappa B was detected in the lipopolysaccharide-activated but not poly(I-C)-activated ISRE-binding complex. Finally, p65 promoted transactivation of gene expression by IRF-3. These results therefore indicate that IRF-3-mediated activation of the ISRE by TLR4 but not TLR3 requires the p65 subunit of NF-kappa B.

Oncogenic Ki-ras inhibits the expression of interferon-responsive genes through inhibition of STAT1 and STAT2 expression

Endogenous interferon gamma (IFNgamma) promotes the host response to primary tumors, and IFNgamma-insensitive tumors display increased tumorigenicity and can evade tumor surveillance mechanisms. Here we demonstrate that activating mutations of Ki-ras are sufficient to inhibit the expression of STAT1 and STAT2, transcription factors required for signaling by IFNs, providing a potential mechanism for the insensitivity of tumors to IFNs. We demonstrated that colon cancer cell lines with Ki-ras mutations display reduced expression of IFN-responsive genes compared with the cell lines that have retained wild type Ras and that inactivation of the mutant Ki-ras allele in the HCT116 colon cancer cell line is sufficient to restore the expression of STAT1, STAT2, and IRF-9. Accordingly, the expression of 27 interferon-inducible genes was reduced in HCT116 cells compared with the isogenic clones with targeted deletion of the mutant Ki-ras allele, Hkh2 and Hke-3. The expression of IFNgamma receptors did not differ among the isogenic cell lines. IFNgamma stimulated transcription of a STAT1-dependent reporter gene was impaired by RasV12, demonstrating a transmodulation of IFN/STAT signaling by activated Ras. Finally, we demonstrated that the expression of RasV12 in 293T cells is sufficient to inhibit the endogenous expression of STAT1 and STAT2, confirming the negative regulation of IFN signaling by oncogenic Ras. Our data demonstrate that the signaling initiated by activated Ki-ras interferes with the IFN/STAT signaling pathway and modulates the responsiveness of cancer cells to interferons. Furthermore, the data suggest that tumors harboring activating Ki-ras mutations may escape tumor surveillance mechanisms due to reduced responsiveness to IFNgamma.

Interferon-gamma: an overview of signals, mechanisms and functions

Interferon-gamma (IFN-gamma) coordinates a diverse array of cellular programs through transcriptional regulation of immunologically relevant genes. This article reviews the current understanding of IFN-gamma ligand, receptor, signal transduction, and cellular effects with a focus on macrophage responses and to a lesser extent, responses from other cell types that influence macrophage function during infection. The current model for IFN-gamma signal transduction is discussed, as well as signal regulation and factors conferring signal specificity. Cellular effects of IFN-gamma are described, including up-regulation of pathogen recognition, antigen processing and presentation, the antiviral state, inhibition of cellular proliferation and effects on apoptosis, activation of microbicidal effector functions, immunomodulation, and leukocyte trafficking. In addition, integration of signaling and response with other cytokines and pathogen-associated molecular patterns, such as tumor necrosis factor-alpha, interleukin-4, type I IFNs, and lipopolysaccharide are discussed.

Integration of interferon-alpha/beta signalling to p53 responses in tumour suppression and antiviral defence

Swift elimination of undesirable cells is an important feature in tumour suppression and immunity. The tumour suppressor p53 and interferon-alpha and -beta (IFN-alpha/beta) are essential for the induction of apoptosis in cancerous cells and in antiviral immune responses, respectively, but little is known about their interrelationship. Here we show that transcription of the p53 gene is induced by IFN-alpha/beta, accompanied by an increase in p53 protein level. IFN-alpha/beta signalling itself does not activate p53; rather, it contributes to boosting p53 responses to stress signals. We show examples in which p53 gene induction by IFN-alpha/beta contributes to tumour suppression. Furthermore, we show that p53 is activated in virally infected cells to evoke an apoptotic response and that p53 is critical for antiviral defence of the host. Our study reveals a hitherto unrecognized link between p53 and IFN-alpha/beta in tumour suppression and antiviral immunity, which may have therapeutic implications.

Transcriptional upregulation of interferon-induced protein kinase, PKR, in breast cancer

PKR (double-stranded RNA activated protein kinase) is overexpressed and overactive in human breast carcinoma (BC) cells. Here, we report that BC cells also have higher PKR mRNA levels and exhibit increased transcription from the PKR promoter. Mutational analysis of the PKR promoter indicated that the interferon stimulation response element (ISRE) is responsible for the increased transcription in BC cells. By gel retardation assay, ISRE-protein complexes formed by BC and non-transformed nuclear extracts were compared. A BC-specific ISRE-protein complex resembles the multimeric factor, ISGF3.

Inhibition of response to alpha interferon by Mycobacterium tuberculosis

We previously reported that infection by Mycobacterium tuberculosis, the causative agent of tuberculosis, leads to secretion of alpha/beta interferon (IFN-alpha/beta). While IFN-alpha/beta ordinarily stimulates formation of signal transducer and stimulator of transcription-1 (STAT-1) homodimers and IFN-stimulated gene factor-3 (ISGF-3), only ISGF-3 is found in infected human monocytes and macrophages. We have now investigated the basis for this unusual profile of transcription factor activation and its consequences for regulation of transcription, as well as the impact of infection on response to IFN-alpha. After infection, IFN-alpha stimulation of STAT-1 homodimers is inhibited in monocytes and macrophages, while stimulation of ISGF-3 increases in monocytes but tends to decline in macrophages. Effects of infection on the abundance of ISGF-3 subunits, STAT-1, STAT-2, and interferon regulatory factor 9, and on tyrosine phosphorylation of STAT-1 and STAT-2 explain the observed changes in DNA-binding activity, which correlate with increased or inhibited transcription of genes regulated by ISGF-3 and STAT-1. Infection by Mycobacterium bovis BCG does not inhibit IFN-alpha-stimulated tyrosine phosphorylation of STAT-1, formation of homodimers, or transcription of genes regulated by STAT-1 homodimers, suggesting that inhibition of the response to IFN-alpha/beta by M. tuberculosis is an aspect of pathogenicity. Thus, this well-known feature of infection by pathogenic viruses may also be a strategy employed by pathogenic bacteria.