Gene Induction Pathways Mediated by Distinct IRFs during Viral Infection

Role of Cyclophilin B in Activation of Interferon Regulatory Factor-3

IRF-3 is a member of the interferon regulatory factors (IRFs) and plays a principal role in the induction of interferon-beta (IFN-beta) by virus infection. Virus infection results in the phosphorylation of IRF-3 by IkappaB kinase epsilon and TANK-binding kinase 1, leading to its dimerization and association with the coactivators CREB-binding protein/p300. The IRF-3 holocomplex translocates to the nucleus, where it induces IFN-beta. In the present study, we examined the molecular mechanism of IRF-3 activation. Using bacterial two-hybrid screening, we isolated molecules that interact with IRF-3. One of these was cyclophilin B, a member of the immunophilins with a cis-trans peptidyl-prolyl isomerase activity. A GST pull-down assay suggested that one of the autoinhibition domains of IRF-3 and the peptidyl-prolyl isomerase domain of cyclophilin B are required for the binding. A knockdown of cyclophilin B expression by RNA interference resulted in the suppression of virus-induced IRF-3 phosphorylation, leading to the inhibition of the subsequent dimerization, association with CREB-binding protein, binding to the target DNA element, and induction of IFN-beta. These findings indicate that cyclophilin B plays a critical role in IRF-3 activation.

Distinct poly(I-C) and virus-activated signaling pathways leading to interferon-beta production in hepatocytes

Innate cellular antiviral defenses are likely to influence the outcome of infections by many human viruses, including hepatitis B and C viruses, agents that frequently establish persistent infection leading to chronic hepatitis, cirrhosis, and liver cancer. However, little is known of the pathways by which hepatocytes, the cell type within which these hepatitis agents replicate, sense infection, and initiate protective responses. We show that cultured hepatoma cells, including Huh7 cells, do not activate the interferon (IFN)-beta promoter in response to extracellular poly(I-C). In contrast, the addition of poly(I-C) to culture media activates the IFN-beta promoter and results in robust expression of IFN-stimulated genes (ISG) in PH5CH8 cells, which are derived from non-neoplastic hepatocytes transformed with large T antigen. Small interfering RNA knockdown of TLR3 or its adaptor, Toll-interleukin-1 receptor domain-containing adaptor inducing IFN-beta (TRIF), blocked extracellular poly(I-C) signaling in PH5CH8 cells, whereas poly(I-C) responsiveness could be conferred on Huh7 hepatoma cells by ectopic expression of Toll-like receptor 3 (TLR3). In contrast to poly(I-C), both cell types signal the presence of Sendai virus infection through a TLR3-independent intracellular pathway requiring expression of retinoic acid-inducible gene I (RIG-I), a putative cellular RNA helicase. Silencing of RIG-I expression impaired only the response to Sendai virus and not extracellular poly(I-C). We conclude that hepatocytes contain two distinct antiviral signaling pathways leading to expression of type I IFNs, one dependent upon TLR3 and the other dependent on RIG-I, with little cross-talk between these pathways.

Analysis of genes induced by Sendai virus infection of mutant cell lines reveals essential roles of interferon regulatory factor 3, NF-kappaB, and interferon but not toll-like receptor 3

Sendai virus (SeV) infection causes the transcriptional induction of many cellular genes that are also induced by interferon (IFN) or double-stranded RNA (dsRNA). We took advantage of various mutant cell lines to investigate the putative roles of the components of the IFN and dsRNA signaling pathways in the induction of those genes by SeV. Profiling the patterns of gene expression in SeV-infected cells demonstrated that Toll-like receptor 3, although essential for gene induction by dsRNA, was dispensable for gene induction by SeV. In contrast, Jak1, which mediates IFN signaling, was required for the induction of a small subset of genes by SeV. NF-kappaB and interferon regulatory factor 3 (IRF-3), the two major transcription factors activated by virus infection, were essential for the induction of two sets of genes by SeV. As expected, some of the IRF-3-dependent genes, such as ISG56, were more strongly induced by SeV in IRF-3-overexpressing cells. Surprisingly, in those cells, a number of NF-kappaB-dependent genes, such as the A20 gene, were induced poorly. Using a series of cell lines expressing increasing levels of IRF-3, we demonstrated that the degree of induction of A20 mRNA, upon SeV infection, was inversely proportional to the cellular level of IRF-3, whereas that of ISG56 mRNA was directly proportional. Thus, IRF-3 can suppress the expression of NF-kappaB-dependent genes in SeV-infected cells.

Regulation of the transcriptional activity of the IRF7 promoter by a pathway independent of interferon signaling

Genes containing an interferon (IFN)-stimulated response element (ISRE) can be divided into two groups according to their inducibility by IFN and virus infection: one induced only by IFN and the other induced by both IFN and virus infection. Although it is now clear that IFN regulatory factor 7 (IRF7) is a multifunctional gene essential for induction of type I IFNs, regulation of the IRF7 promoter (IRF7p) is poorly understood. The IRF7 gene includes two IFN responsive elements, an IRF-binding element (IRFE) in the promoter region and an ISRE in the first intron, and is induced by the IFN-triggered Jak-STAT pathway by binding of the IFN-stimulated gene factor 3 (ISGF3) complex to the ISRE. In this study, we demonstrate that IRF3 and IRF7, which with the coactivators CREB-binding protein and P300 form the virus-activated factor (VAF) complex upon Sendai virus infection, bind to the IRF7 ISRE and IRFE and can directly activate IRF7 transcription. Promoter reporter assays show that both the ISRE and IRFE are responsive to activation by IRF7 and IRF3. In cells transiently expressing IRF7 or/and IRF3, the VAF level and binding of VAF are clearly increased after Sendai virus infection. Studies with Jak1 kinase inactive 293 cells that were stably transfected with a Jak1 kinase dead dominant negative construct, and the mutant cell lines SAN (IFNalpha-/beta-), U2A (IRF9-), U4A (Jak1-), and DKO (IRF1-/IRF2-) show that the IRF7 transcription activated directly by VAF is distinct from and independent of the IFN signaling pathway. Thus, IRF7 transcription is autoregulated by binding of the IRF7-containing VAF to its own ISRE and IRFE. The results show two distinct mechanisms for the activation of the IRF7 promoter, by IFN and by virus infection. A regulatory network between type I IFNs and IRF7 is proposed. The distinct pathways may reflect special roles for an efficient antiviral response at different stages of virus infection.

Roles of an IkappaB kinase-related pathway in human cytomegalovirus-infected vascular smooth muscle cells: a molecular link in pathogen-induced proatherosclerotic conditions

Viral and bacterial pathogens have long been suspected to affect atherogenesis directly. However, mechanisms linking innate immunity to chronic inflammatory diseases such as atherosclerosis are still poorly defined. Here we show that infection of primary human aortic smooth muscle cells (HAOSMC) with human cytomegalovirus (HCMV) leads to activation of the novel IkappaB kinase (IKK)-related kinase, Tank-binding kinase-1 (TBK1), a major effector of the cellular innate immune response. We demonstrate that part of the HCMV inflammatory response is most likely mediated via this novel kinase because the canonical IKK complex was only poorly activated upon infection of HAOSMC. An increase in TBK1 phosphotransferase activity led to a strong activation of the interferon regulatory factor (IRF)-3 transcription factor as measured by its C-terminal phosphorylation, dimerization, and DNA binding activity. In addition to TBK1, HAOSMC also express another IKK-related kinase isoform, IKKepsilon, albeit at a lower level. Nevertheless, both isoforms were required for full activation of IRF-3 by HCMV. The transcripts of proatherosclerotic genes Ccl5 (encoding for the chemokine RANTES (regulated upon activation, normal T cell expressed and secreted)) and Cxcl10 (encoding for the chemokine IP-10 (interferon-gamma-inducible protein 10)) were induced in an IRF-3-dependent manner after HCMV infection of smooth muscle cells. In addition, cytokine arrays analysis showed that RANTES and IP-10 were the predominant chemokines present in the supernatant of HCMV-infected HAOSMC. Activation of the TBK1/IRF-3 pathway was independent of epidermal growth factor receptor and pertussis toxin-sensitive G protein-coupled receptor activation. Our results thus add additional molecular clues to a possible role of HCMV as a modulator of atherogenesis through the induction of a proinflammatory response that is, in part, dependent of an IKK-related kinase pathway.

Regulation of hepatitis C virus replication by interferon regulatory factor 1

Cellular antiviral responses are mediated partly by the expression of interferon-stimulated genes, triggered by viral genomes, their transcripts and replicative intermediates. Persistent replication of a hepatitis C virus (HCV) replicon suggests that the replicon does not elicit cellular innate antiviral responses. In the present study, we investigated regulatory factors of the interferon-mediated antiviral system in cells expressing an HCV replicon. Luciferase reporter assays revealed that the baseline activity of the interferon-stimulated response element (ISRE) was significantly lower in cells harboring the replicon than in naive cells. Among the proteins involved in the IFN/Jak/STAT pathway and in ISRE activity, the expression level of interferon regulatory factor 1 (IRF-1) was found to be significantly lower in cells harboring the replicon. Transfection of an IRF-1 expression construct into cells harboring the replicon caused an increase of ISRE activity, accompanied by suppression of expression of the HCV replicon. Moreover, in cured Huh7 cells from which the HCV replicon had been eliminated, the expression levels of IRF-1 and ISRE activity also were suppressed, demonstrating that the decrease of IRF-1 is attributable, not to active suppression by the viral proteins, but to adaptation of cells that enables replication of the HCV subgenome. The high permissiveness of the cured cells for the replicon was abolished by transgenic supplementation of IRF-1 expression. Taken together, IRF-1 is one of the key host factors that regulate intracellular HCV replication through modulation of interferon-stimulated-gene-mediated antiviral responses.

Rabies virus stimulates nitric oxide production and CXC chemokine ligand 10 expression in macrophages through activation of extracellular signal-regulated kinases 1 and 2

Macrophages represent an essential part of innate immunity, and the viral infection of macrophages results in the release of multiple proinflammatory mediators, such as nitric oxide (NO), cytokines, and chemokines. This study was undertaken to define the molecular mechanism of macrophage activation in response to rabies virus (RV) infection. In RAW264 murine macrophage cells, a well-characterized macrophage model, RV replication was strictly restricted, whereas cell proliferation was significantly enhanced upon RV inoculation. Transcriptional analyses for the expression of inducible forms of NO synthase (iNOS), cytokines, and chemokines revealed that RV virions potentiate the gene expression of iNOS and CXC chemokine ligand 10 (CXCL10), a major chemoattractant of T helper cell type 1. However, RV stimulation had little or no effect on the expression profiles of proinflammatory cytokines and other types of chemokines. In macrophages stimulated with UV-inactivated RV virions, as well as infectious viruses, the phosphorylation of extracellular signal-regulated kinase (ERK) 1 and 2, members of the mitogen-activated protein kinase family, was significantly induced. Specific inhibitors of MAPK/ERK kinase reduced the RV-induced production of NO and CXCL10. Furthermore, the RV-induced activation of the ERK1/2 pathway was severely impaired by the neutralization of the endosomal and lysosomal pH environment with lysosomotropic agents, indicating that endocytosis is a key step leading to the activation of ERK1/2 signaling. Taken together, these results suggest that the ERK1/2-mediated signaling pathway plays a cardinal role in the selective activation of macrophages in response to RV virions, thereby regulating cellular functions during virus infection.

Herpes simplex virus 1 has multiple mechanisms for blocking virus-induced interferon production

In response to viral infection, host cells elicit a number of responses, including the expression of alpha/beta interferon (IFN-alpha/beta). In these cells, IFN regulatory factor-3 (IRF-3) undergoes a sequence of posttranslational modifications that allow it to act as a potent transcriptional coactivator of specific IFN genes, including IFN-beta. We investigated the mechanisms by which herpes simplex virus 1 (HSV-1) inhibits the production of IFN-beta mediated by the IRF-3 signaling pathway. Here, we show that HSV-1 infection can block the accumulation of IFN-beta triggered by Sendai virus (SeV) infection. Our results indicate that HSV-1 infection blocks the nuclear accumulation of activated IRF-3 but does not block the initial virus-induced phosphorylation of IRF-3. The former effect was at least partly mediated by increased turnover of IRF-3 in HSV-1-infected cells. Using mutant viruses, we determined that the immediate-early protein ICP0 was necessary for the inhibition of IRF-3 nuclear accumulation. Expression of ICP0 also had the ability to reduce IFN-beta production induced by SeV infection. ICP0 has been shown previously to play a role in HSV-1 sensitivity to IFN and in the inhibition of antiviral gene production. However, we observed that an ICP0 mutant virus still retained the ability to inhibit the production of IFN-beta. These results argue that HSV-1 has multiple mechanisms to inhibit the production of IFN-beta, providing additional ways in which HSV-1 can block the IFN-mediated host response.

The host response to West Nile Virus infection limits viral spread through the activation of the interferon regulatory factor 3 pathway

Recent outbreaks of West Nile Virus (WNV) have been associated with an increase in morbidity and mortality in humans, birds, and many other species. We have initiated studies to define the molecular mechanisms by which a recent pathogenic isolate of WNV evades the host cell innate antiviral response. Biochemical and microarray analyses demonstrated that WNV induced the expression of beta interferon (IFN-beta) and several IFN-stimulated genes late in infection of cultured human cells. The late expression of these antiviral genes was due to the delayed activation of the transcription factor IFN regulatory factor 3 (IRF-3). Despite this host response, WNV was still able to replicate efficiently. The effect of the IRF-3 pathway on WNV replication was assessed by examining virus replication and spread in cultures of wild-type or IRF-3-null mouse embryo fibroblasts. The absence of IRF-3 was marked by a significant increase in plaque size and a sustained production of infectious particles. Although the activation of the IRF-3 pathway was not sufficient to block virus replication, our results suggest that IRF-3 target genes function to constrain WNV infection and limit cell-to-cell virus spread.

Oxidized Low Density Lipoprotein Blocks Lipopolysaccharide-induced Interferon β Synthesis in Human Macrophages by Interfering with IRF3 Activation

In response to lipopolysaccharide (LPS) exposure, macrophages activate the transcription of a large number of pro-inflammatory genes by way of signaling pathways downstream of the LPS receptor, Toll-Like Receptor 4. Many of these genes are expressed sequentially in time, with early synthesis events resulting in the secretion of soluble factors that drive the transcription of genes expressed later in the activation cycle. In this study we show that human blood-derived macrophages pretreated with oxidized low density lipoprotein (OxLDL) fail to transcribe and secrete interferon beta (IFNbeta) immediately following LPS stimulation. As such, the normal downstream activation of Stat1 is blocked, and numerous IFNbeta/Stat1-activated genes, including the chemokines IP10 and ITAC, are weakly expressed or not expressed at all in these cells. Inspection of the LPS-induced activation state of several transcription factors known to play a prominent role in IFNbeta transcription reveals that, although NFkappaB, c-Jun, and ATF-2 activation appears normal, the LPS-induced activation of IFNbeta regulatory factor 3 (IRF3), as measured by DNA-binding activity and association with the coactivator CBP, is inhibited in the OxLDL pre-treated cells. These IRF3 activities have been shown to be essential for the initiation of transcription of the IFNbeta gene, and the loss of these activities presumably accounts for the lack of LPS-induced IFN beta transcription seen in the OxLDL pre-treated cells.

The roles of two IkappaB kinase-related kinases in lipopolysaccharide and double stranded RNA signaling and viral infection

Viral infection and stimulation with lipopolysaccharide (LPS) or double stranded RNA (dsRNA) induce phosphorylation of interferon (IFN) regulatory factor (IRF)-3 and its translocation to the nucleus, thereby leading to the IFN-beta gene induction. Recently, two IkappaB kinase (IKK)-related kinases, inducible IkappaB kinase (IKK-i) and TANK-binding kinase 1 (TBK1), were suggested to act as IRF-3 kinases and be involved in IFN-beta production in Toll-like receptor (TLR) signaling and viral infection. In this work, we investigated the physiological roles of these kinases by gene targeting. TBK1-deficient embryonic fibroblasts (EFs) showed dramatic decrease in induction of IFN-beta and IFN-inducible genes in response to LPS or dsRNA as well as after viral infection. However, dsRNA-induced expression of these genes was residually detected in TBK1-deficient cells and intact in IKK-i-deficient cells, but completely abolished in IKK-i/TBK1 doubly deficient cells. IRF-3 activation, in response not only to dsRNA but also to viral infection, was impaired in TBK1-deficient cells. Together, these results demonstrate that TBK1 as well as, albeit to a lesser extent, IKK-i play a crucial role in the induction of IFN-beta and IFN-inducible genes in both TLR-stimulated and virus-infected EFs.

Early induction of interferon-responsive mRNAs in Creutzfeldt-Jakob disease

Foreign infectious agents typically evoke a host immune response. In scrapie and Creutzfeldt-Jakob disease (CJD), no immune response has been detectable. However, many latent or persistent viruses evade immune recognition but still activate inflammatory pathways. Unique microglial responses in late CJD infection that could be part of a host defense mechanism were previously delineated, although changes secondary to neurodegeneration could not be excluded. Data here show these microglial transcriptional changes are detectable in CJD brain beginning at 30 days after innoculation. In addition, 10 other interferon-sensitive genes were similarly upregulated at very early stages of infection. These responses occurred well before abnormal prion protein (PrP) and clinical signs of CJD were detectable. Further analyses in very pure microglia from CJD brain suggested the CJD agent activated signaling pathways distinct from those induced by amyloidogenic proteins (including abnormal PrP). Although increases in interferon-alpha or -beta transcript levels were not seen in cultures or in whole brain, CJD microglia exhibited a potentiated interferon response when challenged with double-stranded RNA. The induction of interferon-sensitive genes without appreciable interferon synthesis was strikingly similar to that seen in some viral infections. These data suggest the CJD agent is recognized as a foreign virus-like entity. Moreover, the early reactive gene expression profiles described here may be useful in preclinical diagnosis.

Differential protein synthesis and expression levels in normal and neoplastic human prostate cells and their regulation by type I and II interferons

Protein expression and de novo synthesis in normal and prostate cancer cell lines derived from the same patient were compared by proteomic analysis, and the effects of INFalpha and INFgamma (INF=interferon) determined. The expressions of several INF-inducible proteins, including MxA, Nmi, PA28a and IFP53, were downregulated in the cancer cells. INFgamma induced a more than twofold increase or decrease in the synthesis rates of almost twice as many proteins in the cancer cell line. The positive regulator of INF-induced transcription ISGF3gamma was upregulated in the cancer cells and inversely regulated by INFalpha and INFgamma in the normal and cancer cells. Moreover, ISGF3gamma's induction by INFgamma in the cancer cells was more enhanced by simultaneous stimulation with EGF, than its induction in the normal cells. In all, 31 differentially regulated proteins were identified by mass spectrometry analysis, several of which are involved in chaperone-assisted protein folding in the endoplasmic reticulum (ER) or in regulated protein degradation. Our results suggest that the exclusion of proteins by the ER quality control system, crosstalk between the EGF- and INF-induced signalling pathways and the regulation of INF-inducible genes are all altered in the prostate cancer cells. The combination of upregulated activity in the growth-promoting PI3K/Akt pathway, suppression of Nmi and overexpression of hnRNP-K and c-myc proteins may explain why the prostate cancer cells were found to be more resistant to the growth inhibitory effects of INFgamma.

Detection of differential expression of mouse interferon-alpha subtypes by polymerase chain reaction using specific primers

Specific primers for nine mouse interferon-alpha (IFN-alpha) subtypes, namely, IFN-alpha1, IFN-alpha1-9, IFN-alpha2, IFN-alpha4, IFN-alpha5, IFN-alpha7, IFN-alpha6/8, IFN-alpha11, and IFN-alphaB, were designed and evaluated on Poly(I).Poly(C)-induced and influenza virus-infected L929 cells. Specificity of the primers was confirmed in a cross-polymerase chain reaction (cross-PCR). IFN-alpha1, IFN-alpha1-9, IFN-alpha4, IFN-alpha6/8, IFN-alpha11, and IFN-alphaB were found to be induced in L929 cells 6-9 h after Poly(I).Poly(C) treatment. The amplification of a particular subtype was not biased in the presence of excess of other templates. Differential expression of the IFN-alpha subtypes was observed in influenza A/NWS/33- and B/Lee/40-infected L929 cells. A/NWS/33 virus was found to upregulate the gene expression of IFN-alpha1, IFN-alpha4, IFN-alpha6/8, IFN-alpha11, and IFN-alphaB in L929 cells as early as 6 h after infection. In B/Lee/40-infected L929 cells, only IFN-alpha4 was upregulated. Our results suggest that the designed primers will serve as a useful tool in analyzing the expression of IFN-alpha subtypes in various systems and hence for the evaluation of their function.

Innate cellular response to virus particle entry requires IRF3 but not virus replication

Mammalian cells respond to virus infections by eliciting both innate and adaptive immune responses. One of the most effective innate antiviral responses is the production of alpha/beta interferon and the subsequent induction of interferon-stimulated genes (ISGs), whose products collectively limit virus replication and spread. Following viral infection, interferon is produced in a biphasic fashion that involves a number of transcription factors, including the interferon regulatory factors (IRFs) 1, 3, 7, and 9. In addition, virus infection has been shown to directly induce ISGs in the absence of prior interferon production through the activation of IRF3. This process is believed to require virus replication and results in IRF3 hyperphosphorylation, nuclear localization, and proteasome-mediated degradation. Previously, we and others demonstrated that herpes simplex virus type 1 (HSV-1) induces ISGs and an antiviral response in fibroblasts in the absence of both interferon production and virus replication. In this report, we show that the entry of enveloped virus particles from diverse virus families elicits a similar innate response. This process requires IRF3, but not IRF1, IRF7, or IRF9. Following virus replication, the large DNA viruses HSV-1 and vaccinia virus effectively inhibit ISG mRNA accumulation, whereas the small RNA viruses Newcastle disease virus, Sendai virus, and vesicular stomatitis virus do not. In addition, we found that IRF3 hyperphosphorylation and degradation do not correlate with ISG and antiviral state induction but instead serve as a hallmark of productive virus replication, particularly following a high-multiplicity infection. Collectively, these data suggest that virus entry triggers an innate antiviral response mediated by IRF3 and that subsequent virus replication results in posttranslational modification of IRF3, such as hyperphosphorylation, depending on the nature of the incoming virus.

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.

Molecular cloning and characterization of crucian carp (Carassius auratus L.) interferon regulatory factor 7

Interferon (IFN) can induce an antiviral state via interferon-regulatory transcription factors (IRFs), which bind to and control genes directed by the interferon-stimulated response element (ISRE). Here we describe a fish IRF, termed CaIRF7, cloned from a subtractive cDNA library which is constructed with mRNAs obtained from crucian carp (Carassius auratus L.) blastulae embryonic (CAB) cells infected by UV-inactivated GCHV and mock-infected cells. CaIRF7 cDNA was found to be 1816 bp in length, with a 42 bp 5'UTR and a 508 bp 3'UTR. The open reading frame translates into 421 amino acids in which a DNA-binding domain (DBD) containing the repeated tryptophan motif and IRFs association domain have been identified. Like chicken GgIRF3, CaIRF7 was most similar to mammalian IRF7 with 27 to 30% identity overall and some 37% identity in their DBDs. A single transcript of 1.9 kb was detected in virally induced CAB cells by virtual Northern blotting. RT-PCR analysis revealed a wide tissue distribution of CaIRF7 constitutive expression, with detectable transcript in non-infected CAB cells and various tissues of healthy crucian carp. In addition, CaIRF7 expression was differentially increased by stimulation of the CAB cells with active GCHV, UV-inactivated GCHV or CAB IFN, indicating that the activation of CaIRF7 was directly regulated by IFN.

VSV strains with defects in their ability to shutdown innate immunity are potent systemic anti-cancer agents

Ideally, an oncolytic virus will replicate preferentially in malignant cells, have the ability to treat disseminated metastases, and ultimately be cleared by the patient. Here we present evidence that the attenuated vesicular stomatitis strains, AV1 and AV2, embody all of these traits. We uncover the mechanism by which these mutants are selectively attenuated in interferon-responsive cells while remaining highly lytic in 80% of human tumor cell lines tested. AV1 and AV2 were tested in a xenograft model of human ovarian cancer and in an immune competent mouse model of metastatic colon cancer. While highly attenuated for growth in normal mice, both AV1 and AV2 effected complete and durable cures in the majority of treated animals when delivered systemically.

Selective contribution of IFN-alpha/beta signaling to the maturation of dendritic cells induced by double-stranded RNA or viral infection

A complex mechanism may be operational for dendritic cell (DC) maturation, wherein Toll-like receptor and other signaling pathways may be coordinated differently depending on the nature of the pathogens, in order for DC maturation to be most effective to a given threat. Here, we show that IFN-alpha/beta signaling is selectively required for the maturation of DCs induced by double-stranded RNA or viral infection in vitro. Interestingly, the maturation is still observed in the absence of either of the two target genes of IFN-alpha/beta, TLR3 and PKR (double-stranded-RNA-dependent protein kinase R), indicating the complexity of the IFN-alpha/beta-induced transcriptional program in DCs. We also show that the DCs stimulated in vivo by these agents can migrate into the T cell zone of the spleen but fail to mature without the IFN signal. The immune system may have acquired the selective utilization of this cytokine system, which is essential for innate antiviral immunity, to effectively couple with the induction of adaptive immunity.

Essential role of IRF-3 in lipopolysaccharide-induced interferon-beta gene expression and endotoxin shock

Type I interferons (IFN-alpha/beta) affect many aspects of immune responses. Many pathogen-associated molecules, including bacterial lipopolysaccharide (LPS) and virus-associated double-stranded RNA, induce IFN gene expression through activation of distinct Toll-like receptors (TLRs). Although much has been studied about the activation of the transcription factor IRF-3 and induction of IFN-beta gene by the LPS-mediated TLR4 signaling, definitive evidence is missing about the actual role of IRF-3 in LPS responses in vitro and in vivo. Using IRF-3 deficient mice, we show here that IRF-3 is indeed essential for the LPS-mediated IFN-beta gene induction. Loss of IRF-3 also affects the expression of profile of other cytokine/chemokine genes. We also provide evidence that the LPS/TLR4 signaling activates IRF-7 to induce IFN-beta, if IRF-7 is induced by IFNs prior to LPS simulation. Finally, the IRF-3-deficient mice show resistance to LPS-induced endotoxin shock. These results place IRF-3 as a molecule central to LPS/TLR4 signaling.

The Ebola virus VP35 protein inhibits activation of interferon regulatory factor 3

The Ebola virus VP35 protein was previously found to act as an interferon (IFN) antagonist which could complement growth of influenza delNS1 virus, a mutant influenza virus lacking the influenza virus IFN antagonist protein, NS1. The Ebola virus VP35 could also prevent the virus- or double-stranded RNA-mediated transcriptional activation of both the beta IFN (IFN-beta) promoter and the IFN-stimulated ISG54 promoter (C. Basler et al., Proc. Natl. Acad. Sci. USA 97:12289-12294, 2000). We now show that VP35 inhibits virus infection-induced transcriptional activation of IFN regulatory factor 3 (IRF-3)-responsive mammalian promoters and that VP35 does not block signaling from the IFN-alpha/beta receptor. The ability of VP35 to inhibit this virus-induced transcription correlates with its ability to block activation of IRF-3, a cellular transcription factor of central importance in initiating the host cell IFN response. We demonstrate that VP35 blocks the Sendai virus-induced activation of two promoters which can be directly activated by IRF-3, namely, the ISG54 promoter and the ISG56 promoter. Further, expression of VP35 prevents the IRF-3-dependent activation of the IFN-alpha4 promoter in response to viral infection. The inhibition of IRF-3 appears to occur through an inhibition of IRF-3 phosphorylation. VP35 blocks virus-induced IRF-3 phosphorylation and subsequent IRF-3 dimerization and nuclear translocation. Consistent with these observations, Ebola virus infection of Vero cells activated neither transcription from the ISG54 promoter nor nuclear accumulation of IRF-3. These data suggest that in Ebola virus-infected cells, VP35 inhibits the induction of antiviral genes, including the IFN-beta gene, by blocking IRF-3 activation.

Is flavivirus resistance interferon type I-independent?

Interferon type I comprises a group of major virus-inducible host antiviral factors that control infection with a great number of human and animal viruses. They are ubiquitously expressed cytokines that interfere with virus replication within different cell types by activating a number of host genes and several parallel antiviral pathways. Two major intracellular actors of IFN-I-induced antiviral states are ribonucleic acid-dependent protein kinase and 2'-5'-oligoadenylate synthetases/RNase L, both being induced by IFN-I and activated by viral double stranded ribonucleic acid. In addition, Mx proteins and ribonucleic acid-specific adenosine deaminase have also been implicated in IFN-I-induced antiviral responses to some RNA viruses. Viruses, in turn, have evolved different strategies to escape a control imposed by IFN-I and by IFN-I-induced antiviral factors. The fatal outcome of virus infection as well as the efficiency of IFN-I-based antiviral therapies in its prevention, are determined by complex interactions between viral virulence factors and cellular antiviral IFN-I inducible factors. In the light of these facts and current knowledge on IFN-I involvement in flavivirus infection, I discuss a possible role of IFN-I signalling in resistance to flavivirus infection in a model of congenic mouse strains that express different levels of susceptibility/resistance to common flaviviruses. Specifically, this review emphasizes importance of fully operative 2'-5'-oligoadenylate synthetases/RNase L pathway for the IFN-I-induced stimulation of flavivirus resistance conferred by Flv.

Impairment of interferon-induced IRF-7 gene expression due to inhibition of ISGF3 formation by trichostatin A

Two members of the signal transducer and activator of transcription family, STAT1 and STAT2, form, together with interferon regulatory factor 9 (IRF-9), the ISGF3 complex that activates the expression of the interferon-stimulated genes (ISG). The ISGF3 complex also participates in the virus-induced alpha/beta interferon (IFN-alpha/beta) gene amplification cascade by up-regulating IRF-7 gene expression. Here, we show that treatment of cells with trichostatin A (TSA), a deacetylase inhibitor, inhibits the virus-induced activation of IFN-alpha/beta promoters and dramatically reduces the ability of different ISG promoters to respond to IFN stimulation. Impairment of IFN-alpha/beta and ISG expression by TSA in infected cells is due to the blockage of interferon-stimulated ISGF3 complex formation, which leads to the abolition of IRF-7 gene expression. We also show that the TSA-dependent inhibition of ISGF3 is related to impaired nuclear accumulation of STAT2. Our data suggest that an acetylation/deacetylation mechanism participates in the regulation of cellular distribution and function of STAT2 in IFN-alpha/beta signaling.

Regulation of interferon regulatory factor-3 by the hepatitis C virus serine protease

Persistent infections with hepatitis C virus (HCV) are likely to depend on viral inhibition of host defenses. We show that the HCV NS3/4A serine protease blocks the phosphorylation and effector action of interferon regulatory factor-3 (IRF-3), a key cellular antiviral signaling molecule. Disruption of NS3/4A protease function by mutation or a ketoamide peptidomimetic inhibitor relieved this blockade and restored IRF-3 phosphorylation after cellular challenge with an unrelated virus. Furthermore, dominant-negative or constitutively active IRF-3 mutants, respectively, enhanced or suppressed HCV RNA replication in hepatoma cells. Thus, the NS3/4A protease represents a dual therapeutic target, the inhibition of which may both block viral replication and restore IRF-3 control of HCV infection.

Interferon regulatory factor-7 synergizes with other transcription factors through multiple interactions with p300/CBP coactivators

Interferon regulatory factor (IRF)-7 is activated in response to virus infection and stimulates the transcription of a set of cellular genes involved in host antiviral defense. The mechanism by which IRF-7 is activated and cooperates with other transcription factors is not fully elucidated. Activation of IRF-7 results from a conformational change triggered by the virus-dependent phosphorylation of its C terminus. This conformational change leads to dimerization, nuclear accumulation, DNA-binding, and transcriptional transactivation. Here we show that activation of IRF-7, like that of IRF-3, is dependent on modifications of two distinct sets of Ser/Thr residues. Moreover, we show that different virus-inducible cis-acting elements display requirements for specific IRFs. In particular, the virus-responsive element of the ISG15 gene promoter can be activated by either IRF-3 or IRF-7 alone, whereas the P31 element of the interferon-beta gene is robustly activated only when IRF-3, IRF-7, and the p300/CBP coactivators are all present. Furthermore, we find that IRF-7 interacts with four distinct regions of p300/CBP. These interactions not only stimulate the intrinsic transcriptional activity of IRF-7, but they are also indispensable for its ability to strongly synergize with other transcription factors, including c-Jun and IRF-3.

Lipopolysaccharide stimulates p38-dependent induction of antiviral genes in neutrophils independently of paracrine factors

Lipopolysaccharide (LPS) induces neutrophils to synthesize and secrete pro-inflammatory cytokines and chemokines, which are regulated at both the transcriptional and translational level. We reported previously that neutrophils stimulated with LPS induce expression of genes typically expressed in response to stimulation with antiviral type I interferons (IFN), such as myxovirus resistance-1 (MX1). However, we present evidence that this response of neutrophils to lipopolysaccharide occurs in the absence of interferon-dependent signaling. Lipopolysaccharide-stimulated neutrophils do not phosphorylate the interferon-associated transcription factors signal transducer and activator of transcription-1 and -3, and medium from lipopolysaccharide-stimulated cells was unable to induce MX1 gene expression, suggesting a soluble factor is not involved. Furthermore, LPS did not alter expression of IFNA and IFNB genes. In contrast to neutrophils, LPS-stimulated human monocyte-derived macrophages induced the expression of MX1, but IFNB was induced, and medium from LPS-stimulated monocyte-derived macrophages supported MX1 induction. An inhibitor of p38 kinase blocked induction of MX1 by lipopolysaccharide, but not IFNalpha, in neutrophils, and induction of MX1 was dependent on protein synthesis. LPS, but not IFNalpha, substantially activated p38. In contrast, the induction of MX1 by LPS in monocyte-derived macrophages was insensitive to p38 inhibition, although p38 is phosphorylated in LPS-stimulated but not IFNalpha-stimulated monocyte-derived macrophages. The expression of MX1 in neutrophils and monocyte-derived macrophages is mediated by TLR4 but not TLR2. The data presented here indicate that lipopolysaccharide activates novel interferon-independent signaling pathways in neutrophils and that induction of antiviral genes is a consequence of exposure of neutrophils to bacterial products.

Super-activated interferon-regulatory factors can enhance plasmid immunization

We have been investigating the adjuvant properties of two super-activated interferon-regulatory factors (IRFs), IRF-3(5D) and IRF7/3A, identified in our previous studies of structure-function relationships, for enhancing plasmid vaccines. Intramuscular injection of plasmid cocktails encoding IRF-3(5D) and IRF7/3A molecules elicited cytotoxic T cell responses in over 80% of mice following a single immunization compared to a 20% response-rate using a control cocktail. Most interestingly, greater than 60% of mice immunized with the super-activated IRFs developed antigen-specific antibodies compared to 0% of the mice in the control group. Finally, vaccines which incorporated the super-activated IRFs provided greater protection against challenge with a recombinant vaccinia virus. These results support further investigation of the potential of these agents as adjuvants for genetic immunization.

Transcriptional activity of interferon regulatory factor (IRF)-3 depends on multiple protein-protein interactions

Virus infection results in the activation of a set of cellular genes involved in host antiviral defense. IRF-3 has been identified as a critical transcription factor in this process. The activation mechanism of IRF-3 is not fully elucidated, yet it involves a conformational change triggered by the virus-dependent phosphorylation of its C-terminus. This conformational change leads to nuclear accumulation, DNA binding and transcriptional transactivation. Here we show that two distinct sets of Ser/Thr residues of IRF-3, on phosphorylation, synergize functionally to achieve maximal activation. Remarkably, we find that activated IRF-3 lacks transcriptional activity, but activates transcription entirely through the recruitment of the p300/CBP coactivators. Moreover, we show that two separate domains of IRF-3 interact with several distinct regions of p300/CBP. Interference with any of these interactions leads to a complete loss of transcriptional activity, suggesting that a bivalent interaction is essential for coactivator recruitment by IRF-3.

The role of interferon regulatory factors in the cardiac response to viral infection

Reovirus-induced murine myocarditis provides an excellent model for the human disease. Cardiac tissue damage varies between reovirus strains, and is caused by a direct viral cytopathogenic effect. One determinant of virus-induced cardiac tissue damage is the cardiac interferon-beta (IFN-beta) response to viral infection. Nonmyocarditic reoviruses induce more IFN-beta and/or are more sensitive to the antiviral effects of IFN-beta in cardiac cells than myocarditis reoviruses. The roles of interferon regulatory factors (IRFs) in the cardiac response to viral infection are reviewed, and results suggest possible cardiac-specific variations in IRF-3 and IRF-1 function. In addition, data are presented indicating that the role of IRF-2 in regulation of IFN-beta expression is cell type-specific and differs between skeletal and cardiac muscle cells. Together, results suggest that the heart may provide a unique environment for IRF function, critical for protection against virus-induced cardiac damage.

Differential involvement of IFN-beta in Toll-like receptor-stimulated dendritic cell activation

Toll-like receptor (TLR) can activate dendritic cells (DC) through common signaling pathways requiring a cytoplasmic adapter, MyD88. However, the signaling is differentially regulated among TLR family members. TLR4 can activate MyD88-deficient bone marrow-derived DC (BMDC), and lead to induction of IFN-inducible genes and up-regulation of co-stimulatory molecules such as CD40, implying that the MyD88-independent signaling pathway functions downstream of TLR4. Because these effects can also be induced by type I IFN, we have analyzed whether type I IFN is involved in TLR4-induced responses. In response to lipopolysaccharide (LPS), IFN-beta gene expression was augmented in both wild-type and MyD88-deficient BMDC. Expression of all IFN-inducible genes except immune-responsive gene 1 (IRG1) was abolished and CD40 up-regulation was decreased in LPS-stimulated BMDC lacking either IFN-alpha/beta receptor (IFN-alpha/betaR) or signal transducer and activator of transcription 1 (STAT-1). Similar to the LPS response, TLR9 signaling can also induce expression of IFN-beta and IFN-inducible genes, and up-regulation of CD40. However, all these effects were MyD88 dependent. Thus, in TLR4 signaling, IFN-beta expression can be induced either by the MyD88-dependent or -independent pathway, whereas, in TLR9 signaling, it is dependent on MyD88. In CpG DNA-stimulated DC, expression of IFN-inducible genes except IRG1 was dependent on type I IFN signaling as in LPS-stimulated DC. However, in contrast to TLR4 signaling, TLR9 signaling requires type I IFN signaling for CD40 up-regulation. Taken together, this study demonstrates differential involvement of type I IFN in TLR4- and TLR9-induced effects on DC.

Interferon regulatory factor-1, interferon-beta, and reovirus-induced myocarditis

Viral myocarditis is an important human disease, and reovirus-induced myocarditis in mice provides an excellent model to study direct viral damage to the heart. Previously, we showed that reovirus induction of and sensitivity to interferon-beta (IFN-beta) is an important determinant of viral pathogenicity in the heart and that the transcription factor interferon regulatory factor-3 (IRF-3) is required for reovirus induction of IFN-beta in primary cardiac myocyte cultures. Given several lines of evidence suggesting a possible distinctive environment for IRFs in the heart, we have now focused on IRF-1. Previous studies demonstrated that viruses, double-stranded-RNA (dsRNA), and IFN-alpha/beta can each induce IRF-1 and that IRF-1 plays a role in dsRNA, but perhaps not viral, induction of IFN-alpha/beta. Importantly, none of these studies used a virus with a dsRNA genome (such as reovirus), none of them used a highly differentiated nonlymphoid cell type, and none of them addressed whether viral induction of IRF-1 is direct or is mediated through viral induction of IFN-beta. Indeed, as recently as this year it has been assumed that viral induction of IRF-1 is direct. Here, we found that reovirus induced IRF-1 in primary cardiac myocyte cultures, but that IRF-1 was not required for reovirus induction of IFN-beta. Surprisingly, we found that reovirus failed to induce IRF-1 in the absence of the IFN-alpha/beta response. This provides the first evidence that viruses may not induce IRF-1 directly. Finally, nonmyocarditic reovirus strains induced more cardiac lesions in mice deficient for IRF-1 than they did in wildtype mice, directly demonstrating a protective role for IRF-1. Together, the results indicate that while IRF-1 is downstream of the IFN-beta response, it plays an important protective role against viral myocarditis.

IFN-stimulated gene 15 is synergistically activated through interactions between the myelocyte/lymphocyte-specific transcription factors, PU.1, IFN regulatory factor-8/IFN consensus sequence binding protein, and IFN regulatory factor-4: characterization of a new subtype of IFN-stimulated response element

Type I IFNs cause the induction of a subset of genes termed IFN-stimulated genes (ISGs), which harbor a specific DNA element, IFN-stimulated response element (ISRE). This ISRE confers the responsiveness to the IFN signal through the binding of a family of transcription factors designated IFN regulatory factors (IRFs). Some IRFs can bind to the DNA alone, such as IRF-1, which elicits transcriptional activation, or IRF-2, which leads to transcriptional repression. In addition, these factors associate with IRF-8/IFN consensus sequence binding protein (ICSBP), an immune cell-restricted IRF, and the assembled heterocomplexes lead to synergistic repression of ISRE elements. ISG15 is a prototype ISG that contains a well-characterized ISRE. Here we show that PU.1, an ETS member essential for myeloid/lymphoid cell differentiation, forms heterocomplexes with the immune-restricted IRFs, IRF-8\/ICSBP and IRF-4, which lead to transcriptional activation of ISG15. These data allowed the characterization of a subset of ISREs designated ETS/IRF response element (EIRE), which are differentially regulated in immune cells. EIREs are unique in their ability to recruit different factors to an assembled enhanceosomes. In nonimmune cells the factors will mainly include IRF members, while cell type-restricted factors, such as PU.1, IRF-8\/ICSBP, and IRF-4, will be recruited in immune cells. IRF heterocomplex formation leads to transcriptional repression, and conversely, PU.1/IRFs heterocomplex formation leads to transcriptional activation. The fact that IRF-8\/ICSBP is an IFN-gamma-induced factor explains why some of the EIREs are also induced by type II IFN. Our results lay the molecular basis for the unique regulation of ISGs, harboring EIRE, in immune cells.

Transcriptional profiling of interferon regulatory factor 3 target genes: direct involvement in the regulation of interferon-stimulated genes

Ubiquitously expressed interferon regulatory factor 3 (IRF-3) is directly activated after virus infection and functions as a key activator of the immediate-early alpha/beta interferon (IFN) genes, as well as the RANTES chemokine gene. In the present study, a tetracycline-inducible expression system expressing a constitutively active form of IRF-3 (IRF-3 5D) was combined with DNA microarray analysis to identify target genes regulated by IRF-3. Changes in mRNA expression profiles of 8,556 genes were monitored after Tet-inducible expression of IRF-3 5D. Among the genes upregulated by IRF-3 were transcripts for several known IFN-stimulated genes (ISGs). Subsequent analysis revealed that IRF-3 directly induced the expression of ISG56 in an IFN-independent manner through the IFN-stimulated responsive elements (ISREs) of the ISG56 promoter. These results demonstrate that, in addition to its role in the formation of a functional immediate-early IFN-beta enhanceosome, IRF-3 is able to discriminate among ISRE-containing genes involved in the establishment of the antiviral state as a direct response to virus infection.

The interferon antiviral response: from viral invasion to evasion

One of the initial responses of an organism to infection by pathogenic viruses is the synthesis of antiviral cytokines such as the type I interferons (interferon-alpha/beta), interleukins, and other proinflammatory cytokines and chemokines. Interferons provide a first line of defence against virus infections by generating an intracellular environment that restricts virus replication and signals the presence of a viral pathogen to the adaptive arm of the immune response. Interferons stimulate cells in the local environment to activate a network of interferon-stimulated genes, which encode proteins that have antiviral, antiproliferative and immunomodulatory activities. The present review focuses on recent reports that describe the activation of multiple signalling pathways following virus infection, new candidate genes that are implicated in the establishment of the antiviral state, and the strategies used by viruses and their specific viral products to antagonize and evade the host antiviral response.

Mechanisms of inhibition of the host interferon alpha/beta-mediated antiviral responses by viruses

Complex multicellular organisms have evolved sophisticated mechanisms to prevent and control infection by pathogens. Among these mechanisms, the type I interferon or interferon alpha/beta system represents one of the first lines of defense against viral infections. Typically, viral infection induces the synthesis and secretion of interferon alpha/beta by the infected cell, which in turn activates signaling pathways leading to an antiviral state. As a counter measure, many viruses have developed intriguing mechanisms to evade the interferon alpha/beta system of the host. In this review, we will summarize recent research developments in this interesting field of virus-host cell interactions.

Measles virus-induced modulation of host-cell gene expression

The influence of measles virus (MV) infection on gene expression by human peripheral blood mononuclear cells (PBMCs) was examined with cDNA microarrays. The mRNA levels of more than 3000 cellular genes were compared between uninfected PBMCs and cells infected with either the Edmonston MV strain or a wild-type MV isolate. The MV-induced upregulation of individual genes identified by microarray analyses was confirmed by RT-PCR. In the present study, a total of 17 genes was found to be upregulated by MV infection. The Edmonston strain grew better in the PBMC cultures than the wild-type MV, and the Edmonston strain was a stronger inducer of the upregulated host cell genes than the wild-type virus. The anti-apoptotic B cell lymphoma 3 (Bcl-3) protein and the transcription factor NF-kappaB p52 subunit were upregulated in infected PBMCs both at the mRNA and at the protein level. Several genes of the interferon system including that for interferon regulatory factor 7 were upregulated by MV. The genes for a number of chaperones, transcription factors and other proteins of the endoplasmic reticulum stress response were also upregulated. These included the gene for the pro-apoptotic and growth arrest-inducing CHOP/GADD153 protein. Thus, the present study demonstrated the activation by MV of cellular mechanisms and pathways that may play a role in the pathogenesis of measles.

Lipopolysaccharide activates the expression of ISG15-specific protease UBP43 via interferon regulatory factor 3

UBP43 is a protease that specifically removes a ubiquitin-like protein, ISG15, from its targets. Highest levels of UBP43 expression are detected in macrophages and in cell lines of monocytic lineage. Macrophages are important in host defense against bacterial and viral infections. The lipopolysaccharide (LPS) of the bacterial cell wall can mimic bacteria and activate monocytes/macrophages to provoke inflammatory responses. Here, we report that LPS strongly activates UBP43 expression in macrophages, which is paralleled by changes in UBP43 protein levels. Two interferon regulatory factor (IRF) binding sites in the UBP43 promoter are responsible for the induction of UBP43 expression by LPS, as well as for basal UBP43 promoter activity. We have identified two members of the IRF family (IRF-2 and IRF-3) that specifically bind to these sites. IRF-3 plays a primary role in the LPS-inducible activation of the UBP43 gene and IRF-2 confers a basal transcriptional activity to the UBP43 promoter. Furthermore, we demonstrate that LPS treatment increases the amount of ISG15-conjugates in macrophages. Coordinated induction of ISG15 and UBP43 suggests that ISG15 conjugation is a dynamic process and that a critical balance of ISG15-modification should be maintained during innate immune response.

Toll-like receptors as adjuvant receptors

The mammalian Toll-like receptors (TLRs) are expressed on macrophages and dendritic cells, which are primarily involved in innate immunity. At present, ligands for several of the TLRs, such as TLR2, TLR3, TLR4, TLR5, TLR6, and TLR9, have been identified. Most of these ligands are derived from pathogens, but not found in the host, suggesting that the TLRs are critical to sensing invading microorganisms. Pathogen recognition by TLRs provokes rapid activation of innate immunity by inducing production of proinflammatory cytokines and upregulation of costimulatory molecules. Activated innate immunity subsequently leads to effective adaptive immunity. In this regard, the TLRs are considered to be adjuvant receptors. Distinct TLRs can exert distinct, but overlapping sets of biological effects. Accumulating evidence indicates that this can be attributed to both the common and unique aspects of the signaling mechanisms that mediate TLR family responses. For example, TLR2 and TLR9 require MyD88 as an essential signal transducer, whereas TLR4 can induce costimulatory molecule upregulation in a MyD88-independent manner. Understanding the TLR system should offer invaluable opportunity for manipulating host immune responses.

Double-stranded RNA decreases IGF-I gene expression in a protein kinase R-dependent, but type I interferon-independent, mechanism in C6 rat glioma cells

We previously demonstrated that Poly (IC) decreased the growth of C6 cultures in association with reduced IGF-I synthesis and secretion. In this study we characterized the mechanism(s) by which Poly (IC) decreased IGF-I mRNA in C6 cells. Both Poly (IC) and type I interferon (IFN) decreased IGF-I mRNA. Cycloheximide and a blocking antibody against IFN did not alter the Poly (IC)-mediated inhibition of IGF-I mRNA, but prevented IFN from reducing IGF-I mRNA. Poly (IC) did not alter the stability of IGF-I mRNA. Poly (IC) decreased the abundance of IGF-I pre-mRNA in C6 nuclei, but did not inhibit proximal IGF-I exon 1 promoter/luciferase fusion constructs in transient transfection assays. Poly (IC) activated double-stranded RNA-activated protein kinase (PKR) at 5 min and increased PKR protein levels at 48 and 72 h. Exogenous IGF-I did not prevent Poly (IC) from activating PKR, but inhibited the Poly (IC)-mediated increase in PKR protein levels. The PKR inhibitor 2-aminopurine prevented the Poly (IC) stimulation of eIF2-alpha phosphorylation and the Poly (IC)-mediated decrease in IGF-I mRNA. We conclude that Poly (IC) decreases IGF-I gene transcription in a mechanism that requires the activation of preexisting PKR, but not the induction of IFN or PKR proteins in C6 cells.

The interferon-alpha/beta system in antiviral responses: a multimodal machinery of gene regulation by the IRF family of transcription factors

The efficient induction of interferons alpha and beta (IFN-alpha/beta) in virus-infected cells is central to the antiviral response of a host and is regulated mainly at the level of gene transcription. Once produced, IFN-alpha/beta transmit signals to the cell interior via a specific receptor complex to induce an antiviral response. Recently, the auto-amplification mechanism of the IFN-alpha/beta system that follows viral infection has been identified. This mechanism is mediated by transcription factors of the IFN regulatory factor family and, in fact, may have evolved to render the system more robust in antiviral responses.

Overlapping and distinct mechanisms regulating IRF-3 and IRF-7 function

Recent molecular, biochemical, and gene disruption studies have demonstrated the essential role of interferon (IFN) regulatory factor-3, (IRF-3) and IRF-7 in the activation of type I IFN gene expression and the induction of the antiviral state. Both transcription factors share structural and functional properties, as well as a common mechanism of activation through C-terminal phosphorylation. The purpose of this review is to summarize recent investigations indicating that similar signalling pathways are likely involved in the activation of IRF-3 and IRF-7. Moreover, unique biochemical events, such as coactivator association and differential recognition of cis-acting elements, also illustrate the capacity of IRF-3 and IRF-7 to selectively regulate type I IFN and IFN-stimulated gene (ISG) expression.