Vesicular stomatitis virus infection induces a nuclear DNA-binding factor specific for the interferon-stimulated response element

TRIM56 overexpression restricts porcine epidemic diarrhoea virus replication in Marc-145 cells by enhancing TLR3-TRAF3-mediated IFN-β antiviral response

Infection with the porcine epidemic diarrhoea virus (PEDV) causes severe enteric disease in suckling piglets, causing massive economic losses in the swine industry worldwide. Tripartite motif-containing 56 (TRIM56) has been shown to augment type I IFN response, but whether it affects PEDV replication remains uncharacterized. Here we investigated the role of TRIM56 in Marc-145 cells during PEDV infection. We found that TRIM56 expression was upregulated in cells infected with PEDV. Overexpression of TRIM56 effectively reduced PEDV replication, while knockdown of TRIM56 resulted in increased viral replication. TRIM56 overexpression significantly increased the phosphorylation of IRF3 and NF-κB P65, and enhanced the IFN-β antiviral response, while silencing TRIM56 did not affect IRF3 activation. TRIM56 overexpression increased the protein level of TRAF3, the component of the TLR3 pathway, thereby significantly activating downstream IRF3 and NF-κB signalling. We demonstrated that TRIM56 overexpression inhibited PEDV replication and upregulated expression of IFN-β, IFN-stimulated genes (ISGs) and chemokines in a dose-dependent manner. Moreover, truncations of the RING domain, N-terminal domain or C-terminal portion on TRIM56 were unable to induce IFN-β expression and failed to restrict PEDV replication. Together, our results suggested that TRIM56 was upregulated in Marc-145 cells in response to PEDV infection. Overexpression of TRIM56 inhibited PEDV replication by positively regulating the TLR3-mediated antiviral signalling pathway. These findings provide evidence that TRIM56 plays a positive role in the innate immune response during PEDV infection.

MCPIP1 negatively regulate cellular antiviral innate immune responses through DUB and disruption of TRAF3-TBK1-IKKε complex

IFNβ innate immune plays an essential role in antiviral immune. Previous reports suggested that many important regulatory proteins in innate immune pathway may be modified by ubiquitin and that many de-ubiquitination (DUB) proteins may affect immunity. Monocyte chemotactic protein-inducing protein 1 (MCPIP1), one of the CCCH Zn finger-containing proteins, was reported to have DUB function, but its effect on IFNβ innate immune was not fully understood. In this study, we uncovered a novel mechanism that may explain how MCPIP1 efficiently inhibits IFNβ innate immune. It was found that MCPIP1 negatively regulates the IFNβ expression activated by RIG-I, STING, TBK1, IRF3. Furthermore, MCPIP1 inhibits the nuclear translocation of IRF3 upon stimulation with virus, which plays a key role in type I IFN expression. Additionally, MCPIP1 interacts with important modulators of IFNβ expression pathway including IPS1, TRAF3, TBK1 and IKKε. Meanwhile, the interaction between the components in TRAF3-TBK1-IKKε complex was disrupted by MCPIP1. These results collectively suggest MCPIP1 as an innate immune regulator encoded by the host and point to a new mechanism through which MCPIP1 negatively regulates IRF3 activation and type I IFNβ expression.

The papain-like protease of porcine epidemic diarrhea virus negatively regulates type I interferon pathway by acting as a viral deubiquitinase

Porcine epidemic diarrhea virus (PEDV) is the cause of an economically important swine disease. Previous studies suggested that PEDV does not elicit a robust IFN response, but the mechanism(s) used to evade or block this innate immune response was not known. In this study, we found that PEDV infection blocked synthetic dsRNA-induced IFN-β production by interfering with the activation of interferon regulatory factor 3 (IRF3). We identified PEDV replicase encoded papain-like protease 2 (PLP2) as an IFN antagonist that depends on catalytic activity for its function. We show that levels of ubiquitinated proteins are reduced during PEDV infection and that PEDV PLP2 has deubiquitinase (DUB) activity that recognizes and processes both K-48 and K-63 linked polyubiquitin chains. Furthermore, we found that PEDV PLP2 strongly inhibits RIG-I- and STING-activated IFN expression and that PEDV PLP2 can be co-immunoprecipitated with and deubiquitinates RIG-I and STING, the key components of the signalling pathway for IFN expression. These results show that PEDV infection suppresses production of IFN-β and provides evidence indicating that the PEDV papain-like protease 2 acts as a viral DUB to interfere with the RIG-I- and STING-mediated signalling pathway.

Gamma interferon triggers interaction between ICSBP (IRF-8) and TEL, recruiting the histone deacetylase HDAC3 to the interferon-responsive element

ICSBP (IRF-8) is a transcription factor of the IRF family expressed only in the immune system. It is induced in macrophages by gamma interferon (IFN-gamma) and contributes to macrophage functions. By interacting with Ets family protein PU.1, ICSBP binds to the IRF/Ets composite element and stimulates transcription. ICSBP binds to another DNA element, the IFN-stimulated response element (ISRE), a common target of the IRF family. Limited knowledge as to how ICSBP and other IRF proteins regulate ISRE-dependent transcription in IFN-gamma-activated macrophages is available. By mass-spectrometric analysis of ISRE-bound proteins in macrophages, we identified TEL, another Ets member, as a factor recruited to the element in an IFN-gamma-dependent manner. In vitro analysis with recombinant proteins indicated that this recruitment is due to a direct interaction between ICSBP and TEL, which is enhanced by the presence of ISRE. Significantly, the interaction with TEL in turn resulted in the recruitment of the histone deacetytase HDAC3 to the ISRE, causing increased repression of IFN-gamma-mediated reporter activity through the ISRE. This repression may provide a negative-feedback mechanism operating after the initial transcriptional activation by IFN-gamma. By associating with two different Ets family proteins, ICSBP exerts a dual function in IFN-gamma-dependent gene regulation in an immune system-specific manner.

IRF-3-dependent, NFkappa B- and JNK-independent activation of the 561 and IFN-beta genes in response to double-stranded RNA

Double-stranded (ds) RNA induces transcription of the 561 gene by activating IFN regulatory factor (IRF) transcription factors, whereas similar induction of the IFN-beta gene is thought to require additional activation of NFkappaB and AP-1. In mutant P2.1 cells, dsRNA failed to activate NFkappaB, IRF-3, p38, or c-Jun N-terminal kinase, and transcription of neither 561 mRNA nor IFN-beta mRNA was induced. The defect in the IRF-3 pathway was traced to a low cellular level of this protein because of its higher rate of degradation in P2.1 cells. As anticipated, in several clonal derivatives of P2.1 cells expressing different levels of transfected IRF-3, activation of IRF-3 and induction of 561 mRNA by dsRNA was restored fully, although the defects in other responses to dsRNA persisted. Surprisingly, IFN-beta mRNA also was induced strongly in these cells in response to dsRNA, demonstrating that the activation of NFkappaB and AP-1 is not required. This conclusion was confirmed in wild-type cells overexpressing IRF-3 by blocking NFkappaB activation with the IkappaB superrepressor and AP-1 activation with a p38 inhibitor. Therefore, IRF-3 activation by dsRNA is sufficient to induce the transcription of genes with simple promoters such as 561 as well as complex promoters such as IFN-beta.

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.

Interferon-stimulated response element (ISRE)-binding protein complex DRAF1 is activated in Sindbis virus (HR)-infected cells

To elucidate the host cell defense mechanisms in response to Sindbis viral infection, we have started to characterize interferon (IFN)-stimulated response element (ISRE)-binding proteins activated in infected cells that are involved in the transcriptional induction of IFN type I-inducible genes. Using electromobility shift assays (EMSA), we detected several protein complexes with a human IFN-stimulated gene 15 (ISG15) ISRE in extracts from virus-infected L929 cells that were absent in extracts from uninfected cells. Comigration with Newcastle disease virus-activated ISRE-binding complexes, ISRE-binding specificity, supershift experiments, and conditions of formation indicate that the complexes activated by Sindbis viral infection in L929 cells correspond to DRAF1 and ISG factor 3 (ISGF3). Transfection of L929 cells with poly rI:rC induced only ISGF3. DRAF1 could be detected in Sindbis virus-infected mouse embryo fibroblasts derived from IFNR type I and type II KO mice. Viral RNA synthesis is required for activation of DRAF1.

Differential Expression and Distinct Functions of IFN Regulatory Factor 4 and IFN Consensus Sequence Binding Protein in Macrophages

IFN regulatory factor 4 (IRF4) and IFN consensus sequence binding protein (ICSBP) are highly homologous members of the growing family of IRF proteins. ICSBP expression is restricted to lymphoid and myeloid cells, whereas IRF4 expression has been reported to be lymphoid-restricted. We present evidence that primary murine and human macrophages express IRF4, thereby extending its range of expression to myeloid cells. Here, we provide a comparative analysis of IRF4 and ICSBP expression and function in distinct cell types. These IRF proteins can form specific complexes with the Ets-like protein PU.1, and can activate transcription via binding to PU.1/IRF composite sequences. EMSA analysis revealed that murine macrophages contained both IRF4/PU.1 and ICSBP/PU.1 complexes, analogous to B cells. Over-expression of ICSBP in these macrophages activated transcription of a PU.1/IRF-dependent promoter, whereas over-expression of IRF4 had no effect on this promoter. In contrast, over-expression of either IRF4 or ICSBP in both macrophages and NIH-3T3 fibroblasts suppressed transcription of the PU.1-independent H-2Ld MHC class I promoter. In NIH-3T3 fibroblasts, IRF4 and ICSBP also synergized with exogenous PU.1 to activate transcription of a PU.1/IRF-dependent promoter. Furthermore, both IRF4 and ICSBP activated transcription of the IL-1β promoter in both cell types. While this promoter is PU.1-dependent, it lacks any known PU.1/IRF composite binding sites. Synergistic activation of the IL-1β promoter by these IRF proteins and PU.1 was found to require PU.1 serine 148. Together, these data demonstrate that IRF4 and ICSBP are dichotomous regulators of transcription in macrophages.

Regulation of apoptosis in myeloid cells by interferon consensus sequence-binding protein

Mice with a null mutation of the gene encoding interferon consensus sequence-binding protein (ICSBP) develop a disease with marked expansion of granulocytes and macrophages that frequently progresses to a fatal blast crisis, thus resembling human chronic myelogenous leukemia (CML). One important feature of CML is decreased responsiveness of myeloid cells to apoptotic stimuli. Here we show that myeloid cells from mice deficient in ICSBP exhibit reduced spontaneous apoptosis and a significant decrease in sensitivity to apoptosis induced by DNA damage. In contrast, apoptosis in thymocytes from ICSBP-deficient mice is unaffected. We also show that overexpression of ICSBP in the human U937 monocytic cell line enhances the rate of spontaneous apoptosis and the sensitivity to apoptosis induced by etoposide, lipopolysaccharide plus ATP, or rapamycin. Programmed cell death induced by etoposide was specifically blocked by peptides inhibitory for the caspase-1 or caspase-3 subfamilies of caspases. Studies of proapoptotic genes showed that cells overexpressing ICSBP have enhanced expression of caspase-3 precursor protein. In addition, analyses of antiapoptotic genes showed that overexpression of ICSBP results in decreased expression of Bcl-X(L). These data suggest that ICSBP modulates survival of myeloid cells by regulating expression of apoptosis-related genes.

Adenovirus induction of an interferon-regulatory factor during entry into the late phase of infection

Virus infection of animal cells can induce intracellular antiviral responses mediated by the induction of interferon-regulatory transcription factors (IRFs), which bind to and control genes directed by the interferon-stimulated response element (ISRE). The purpose of this study was to determine whether adenovirus (Ad) induces IRFs during infection, because they might play a role in promoting viral pathogenesis. Here we show that after the late phase of infection, Ad induces a transcription factor related to the IRF family of factors. The IRF is induced shortly after Ad entry into late phase and is shown to stimulate ISRE-directed transcription, to require activation by protein tyrosine kinase signalling, and to be induced several hours prior to the inhibition of cell protein synthesis. Inhibition of tyrosine kinase activity blocks Ad induction and activation of the IRF. Attempts to identify the Ad-induced factor immunologically and by photo-UV cross-linking indicate that it is likely a novel member of the IRF family. Finally, several independent lines of evidence also suggest that Ad induction of the IRF might correlate with the ability of the virus to block host cell protein synthesis later during infection.

Effect of vesicular stomatitis virus matrix protein on transcription directed by host RNA polymerases I, II, and III

The matrix (M) protein of vesicular stomatitis virus (VSV) functions in virus assembly and inhibits host-directed gene expression independently of other viral components. Experiments in this study were carried out to determine the ability of M protein to inhibit transcription directed by each of the three host RNA polymerases (RNA polymerase I [RNAPI], RNAPII, and RNAPIII). The effects of wild-type (wt) VSV, v6 (a VSV mutant isolated from persistently infected cells), and tsO82 viruses on poly(A)+ and poly(A)- RNA synthesis were measured by incorporation of [3H]uridine. v6 and tsO82 viruses, which contain M-gene mutations, had a decreased ability to inhibit synthesis of both poly(A)+ and poly(A)- RNA. Nuclear runoff analysis showed that VSV inhibited transcription of 18S rRNA and alpha-tubulin genes, which was dependent on RNAPI and RNAPII, respectively, but infection with wt virus enhanced transcription of 5S rRNA by RNAPIII. The effect of M protein alone on transcription by RNAPI-, RNAPII-, and RNAPIII-dependent promoters was measured by cotransfection assays. M protein inhibited transcription from RNAPI- and RNAPII-dependent promoters in the absence of other viral gene products. RNAPIII-dependent transcription of the adenovirus VA promoters was also inhibited by M protein. However, as observed during wt VSV infection, M protein enhanced endogenous 5S rRNA transcription, indicating that the inhibition of transcription by RNAPIII was dependent on the nature of the promoter.

Cytomegalovirus activates interferon immediate-early response gene expression and an interferon regulatory factor 3-containing interferon-stimulated response element-binding complex

Interferon establishes an antiviral state in numerous cell types through the induction of a set of immediate-early response genes. Activation of these genes is mediated by phosphorylation of latent transcription factors of the STAT family. We found that infection of primary foreskin fibroblasts with human cytomegalovirus (HCMV) causes selective transcriptional activation of the alpha/beta-interferon-responsive ISG54 gene. However, no activation or nuclear translocation of STAT proteins was detected. Activation of ISG54 occurs independent of protein synthesis but is prevented by protein tyrosine kinase inhibitors. Further analysis revealed that HCMV infection induced the DNA binding of a novel complex, tentatively called cytomegalovirus-induced interferon-stimulated response element binding factor (CIF). CIF is composed, at least in part, of the recently identified interferon regulatory factor 3 (IRF3), but it does not contain the STAT1 and STAT2 proteins that participate in the formation of interferon-stimulated gene factor 3. IRF3, which has previously been shown to possess no intrinsic transcriptional activation potential, interacts with the transcriptional coactivator CREB binding protein, but not with p300, to form CIF. Activating interferon-stimulated genes without the need for prior synthesis of interferons might provide the host cell with a potential shortcut in the activation of its antiviral defense.

Interferon regulatory factor 3 and CREB-binding protein/p300 are subunits of double-stranded RNA-activated transcription factor DRAF1

Cells respond to viral infection or double-stranded RNA with the transcriptional induction of a subset of alpha/beta interferon-stimulated genes by a pathway distinct from the interferon signal pathway. The transcriptional induction is mediated through a DNA sequence containing the alpha/beta interferon-stimulated response element (ISRE). We previously identified a novel transcription factor, designated double-stranded RNA-activated factor 1 (DRAF1), that recognizes this response element. The DNA-binding specificity of DRAF1 correlates with transcriptional induction, thereby distinguishing it as a positive regulator of alpha/beta interferon-stimulated genes. Two of the components of DRAF1 have now been identified as interferon regulatory factor 3 (IRF-3) and the transcriptional coactivator CREB-binding protein (CBP)/p300. We demonstrate that IRF-3 preexists in the cytoplasm of uninfected cells and translocates to the nucleus following viral infection. Translocation of IRF-3 is accompanied by an increase in serine and threonine phosphorylation. Coimmunoprecipitation analyses of endogenous proteins demonstrate an association of IRF-3 with the transcriptional coactivators CBP and p300 only subsequent to infection. In addition, antibodies to the IRF-3, CBP, and p300 molecules react with DRAF1 bound to the ISRE target site of induced genes. The cellular response that leads to DRAF1 activation and specific gene expression may serve to increase host survival during viral infection.

Virus infection induces the assembly of coordinately activated transcription factors on the IFN-beta enhancer in vivo

We have identified a virus-activated factor (VAF) that binds to a regulatory element shared by different virus-inducible genes. We provide evidence that VAF contains two members of the interferon regulatory factor (IRF) family of transcriptional activator proteins (IRF-3 and IRF-7), as well as the transcriptional coactivator proteins p300 and CBP. Remarkably, VAF, as well as recombinant IRF-3 and IRF-7 proteins, binds very weakly to the interferon-beta (IFN-beta) gene promoter in vitro. However, in virus-infected cells, both proteins are recruited to the endogenous IFN-beta promoter as part of a protein complex that includes ATF-2/c-Jun and NF-kappa B. These observations provide a unique example of the coordinate activation of multiple transcriptional activator proteins and their highly cooperative assembly into a transcriptional enhancer complex in vivo.

High Affinity Receptor for IgG (FcγRI/CD64) Gene and STAT Protein Binding to the IFN-γ Response Region (GRR) Are Regulated Differentially in Human Neutrophils and Monocytes by IL-10

Since IL-10 has been shown to up-regulate the expression of the high affinity receptor for IgG (FcγRI/CD64) in human monocytes, we examined whether the cytokine exerts a similar action toward polymorphonuclear neutrophils (PMN). Unexpectedly, we found that in neutrophils, IL-10 failed to induce either the mRNA accumulation or the surface expression of FcγRI. Consistent with these findings, stimulation of PMN with IFN-γ, but not with IL-10, resulted in the induction of specific DNA-binding activities to the IFN-γ response region (GRR), a regulatory element located in the FcγRI gene promoter, required for transcriptional activation. In electrophoretic mobility shift assays (EMSAs), we confirmed that in PBMC, IL-10 induces the binding to the GRR of both STAT1 and STAT3, two members of the STAT family. In neutrophils, however, these activators did not bind to the GRR in response to IL-10, despite the fact that both STAT1 and STAT3 are expressed in these cells. On the other hand, IFN-γ was an efficient inducer of STAT1 binding to the GRR in both PMN and PBMC. The lack of inducible GRR-binding activity in IL-10-treated PMN could not be ascribed to a lack of IL-10R, and did not appear to reflect an inhibitory effect of the cytokine. Taken together, our data suggest that IL-10 is unable to induce FcγRI gene expression in neutrophils because the intracellular signaling pathway triggered by the cytokine is impaired at the level of, or upstream of, STAT1 and/or STAT3 activation.

Interferon (IFN) consensus sequence-binding protein, a transcription factor of the IFN regulatory factor family, regulates immune responses in vivo through control of interleukin 12 expression

Mice with a null mutation of the gene encoding interferon consensus sequence-binding protein (ICSBP) develop a chronic myelogenous leukemia-like syndrome and mount impaired responses to certain viral and bacterial infections. To gain a mechanistic understanding of the contributions of ICSBP to humoral and cellular immunity, we characterized the responses of control and ICSBP-/- mice to infection with influenza A (flu) and Leishmania major (L. major). Mice of both genotypes survived infections with flu, but differed markedly in the isotype distribution of antiflu antibodies. In sera of normal mice, immunoglobulin (Ig)G2a antibodies were dominant over IgG1 antibodies, a pattern indicative of a T helper cell type 1 (Th1)-driven response. In sera of ICSBP-/- mice, however, IgG1 antibodies dominated over IgG2a antibodies, a pattern indicative of a Th2-driven response. The dominance of IgG1 and IgE over IgG2a was detected in the sera of uninfected mice as well. A seeming Th2 bias of ICSBP-deficient mice was also uncovered in their inability to control infection with L. major, where resistance is known to be dependent on IL-12 and IFN-gamma as components of a Th1 response. Infected ICSBP-deficient mice developed fulminant, disseminated leishmaniasis as a result of failure to mount a Th1-mediated curative response, although T cells remained capable of secreting IFN-gamma and macrophages of producing nitric oxide. Compromised Th1 differentiation in ICSBP-/- mice could not be attributed to hyporesponsiveness of CD4(+) T cells to interleukin (IL)-12; however, the ability of uninfected and infected ICSBP-deficient mice to produce IL-12 was markedly impaired. This indicates that ICSBP is a deciding factor in Th responses governing humoral and cellular immunity through its role in regulating IL-12 expression.

Transcription factor binding sites downstream of the human immunodeficiency virus type 1 transcription start site are important for virus infectivity

When transcriptionally active, the human immunodeficiency virus (HIV) promoter contains a nucleosome-free region encompassing both the promoter/enhancer region and a large region (255 nucleotides [nt]) downstream of the transcription start site. We have previously identified new binding sites for transcription factors downstream of the transcription start site (nt 465 to 720): three AP-1 sites (I, II, and III), an AP3-like motif (AP3-L), a downstream binding factor (DBF) site, and juxtaposed Sp1 sites. Here, we show that the DBF site is an interferon-responsive factor (IRF) binding site and that the AP3-L motif binds the T-cell-specific factor NF-AT. Mutations that abolish the binding of each factor to its cognate site are introduced in an infectious HIV-1 molecular clone to study their effect on HIV-1 transcription and replication. Individual mutation of the DBF or AP3-L site as well as the double mutation AP-1(III)/AP3-L did not affect HIV-1 replication compared to that of the wild-type virus. In contrast, proviruses carrying mutations in the Sp1 sites were totally defective in terms of replication. Virus production occurred with slightly delayed kinetics for viruses containing combined mutations in the AP-1(III), AP3-L, and DBF sites and in the AP3-L and DBF-sites, whereas viruses mutated in the AP-1(I,II,III) and AP3-L sites and in the AP-1(I,II,III), AP3-L, and DBF sites exhibited a severely defective replicative phenotype. No RNA-packaging defect could be measured for any of the mutant viruses as determined by quantification of their HIV genomic RNA. Measurement of the transcriptional activity of the HIV-1 promoter after transient transfection of the HIV-1 provirus DNA or of long terminal repeat-luciferase constructs showed a positive correlation between the transcriptional and the replication defects for most mutants.

Induction of IP-10 chemokine promoter by measles virus: comparison with interferon-gamma shows the use of the same response element but with differential DNA-protein binding profiles

Measles virus (MV) and interferon (IFN)-gamma induced IP-10 chemokine mRNA in U373 glioblastoma cells. The minimal response element for both MV and IFN-gamma was localized between nucleotide -231 and -153 of muIP-10 promoter, which contains an IFN-stimulated response element (ISRE) and the distal NF-kappa Bd site. Mutation of individual elements showed that ISRE and NF-kappa Bd were required to function together. DNA-protein binding profiles with the minimal response element showed that IFN-gamma induced a complex consisting of STAT1 while MV induced a complex consisting of p50 and p65 in the absence of new protein synthesis. IFN-gamma and MV also induced IRF-1 DNA binding activity which persisted for longer time periods with IFN-gamma stimulation. Despite the functional requirement of both ISRE and NF-kappa Bd elements, different combinations of DNA binding factors are used in the induction of IP-10 by MV or IFN-gamma.

Cloning of chicken interferon regulatory factor-2 (IRF-2) cDNA: expression and mapping of the IRF-2 gene

A cDNA clone encoding a member of the avian interferon regulatory factor (IRF) family homologous to mammalian IRF-2 was isolated from cDNA library from poly[rI:rC]-induced chicken embryo fibroblasts (CEF). The deduced amino acid sequence shows a characteristic DNA binding domain of 124 amino acids at the amino-terminal end with 96.8% identity to human and 96% to mouse IRF-2. Identities in the C-terminal part are 77.5% and 77%, respectively. Identity to all other known members of the chicken IRF (Ch-IRF) family is distinctly lower. In C32 cells, an IRF-2 mRNA of 2.4 kb is constitutively expressed in very low amounts but is inducible by Ch-IFN in the absence or presence of cycloheximide. The Ch-IRF-2 gene is a single copy gene and was mapped by fluorescence in situ hybridization to the long arm of chromosome 4.

The vesicular stomatitis virus matrix protein inhibits transcription from the human beta interferon promoter

In cells infected by wild-type (wt) vesicular stomatitis virus (VSV) Indiana, host transcription is severely inhibited. DNA cotransfection studies have implicated the VSV matrix (M) protein in this process (B. L. Black and D. S. Lyles, J. Virol. 66:4058-4064, 1992). The M protein inhibited transcription not only from viral promoters in plasmids but also from the chromosomally integrated human immunodeficiency virus type 1 (HIV-1) provirus promoter (S.-Y. Paik, A. C. Banerjea, G. G. Harmison, C.-J. Chen, and M. Schubert, J. Virol. 69:3529-3537, 1995). In this study, we investigated the effect of wt VSV M protein on expression of a reporter gene under control of a cellular promoter (beta-interferon [IFN-beta] promoter), using double transient transfections in BHK and COS-1 cells. The cellular IFN-beta promoter was as susceptible to the inhibitory effect of the M protein as the viral promoters used previously. Viral proteins N, P, and G had no significant effect on reporter gene expression. The M protein gene from VSV mutant T1026R1, which is defective in host transcription inhibition, was cloned and sequenced, and its effect on reporter gene expression was tested. The mutant M protein had a methionine-to-arginine change at position 51 in the protein sequence and did not inhibit transcription from either the IFN-beta promoter or viral promoters. This VSV mutant is a good inducer of IFN, as opposed to the wt virus, which suppresses IFN induction. These results show that the M protein inhibits transcription from cellular as well as viral promoters and that the M protein does not regulate the IFN promoter any differently from viral promoters. While the M protein may play a role in IFN gene regulation, other viral or cellular factors that provide specificity to the induction process must also be involved.

Granulocyte colony-stimulating factor induces the binding of STAT1 and STAT3 to the IFNgamma response region within the promoter of the Fc(gamma)RI/CD64 gene in human neutrophils

Granulocyte colony-stimulating factor (G-CSF) has been recently shown to induce the high-affinity Fc receptor for IgG (Fc(gammaRI)/CD64) in human polymorphonuclear neutrophils (PMN). To elucidate the molecular mechanisms whereby G-CSF exerts this effect, we examined whether the cytokine induces the binding of transcription factors to the IFNgamma response region (GRR), a well characterized regulatory element in the Fc(gammaRI) promoter that is responsible for the transcriptional induction of this gene. Using electrophoretic mobility shift assays, we show that in human PMN, G-CSF activates a GRR-binding complex which contains members of the signal transducer and activator of transcription (STAT) family of proteins, namely STAT1 and STAT3. In keeping with this result, treatment of neutrophils with G-CSF led to tyrosine phosphorylation of STAT3, as determined by immunoprecipitation followed by immunoblotting with antiphosphotyrosine antibodies. This is the first demonstration that in human neutrophils, the induction by G-CSF of Fc(gammaRI) gene expression may be mediated by the binding of STAT1 and STAT3 to the GRR sequence.

Inhibition of human immunodeficiency virus type 1 and vaccinia virus infection by a dominant negative factor of the interferon regulatory factor family expressed in monocytic cells

ICSBP is a member of the interferon (IFN) regulatory factor (IRF) family that regulates expression of type I interferon (IFN) and IFN-regulated genes. To study the role of the IRF family in viral infection, a cDNA for the DNA-binding domain (DBD) of ICSBP was stably transfected into U937 human monocytic cells. Clones that expressed DBD exhibited a dominant negative phenotype and did not elicit antiviral activity against vesicular stomatitis virus (VSV) infection upon IFN treatment. Most notably, cells expressing DBD were refractory to infection by vaccinia virus (VV) and human immunodeficiency virus type 1 (HIV-1). The inhibition of VV infection was attributed to defective virion assembly, and that of HIV-1 to low CD4 expression and inhibition of viral transcription in DBD clones. HIV-1 and VV were found to have sequences in their regulatory regions similar to the IFN-stimulated response element (ISRE) to which IRF family proteins bind. Accordingly, these viral sequences and a cellular ISRE bound a shared factor(s) expressed in U937 cells. These observations suggest a novel host-virus relationship in which the productive infection of some viruses is regulated by the IRF-dependent transcription pathway through the ISRE.

Identification of a member of the interferon regulatory factor family that binds to the interferon-stimulated response element and activates expression of interferon-induced genes

A family of interferon (IFN) regulatory factors (IRFs) have been shown to play a role in transcription of IFN genes as well as IFN-stimulated genes. We report the identification of a member of the IRF family which we have named IRF-3. The IRF-3 gene is present in a single copy in human genomic DNA. It is expressed constitutively in a variety of tissues and no increase in the relative steady-state levels of IRF-3 mRNA was observed in virus-infected or IFN-treated cells. The IRF-3 gene encodes a 50-kDa protein that binds specifically to the IFN-stimulated response element (ISRE) but not to the IRF-1 binding site PRD-I. Overexpression of IRF-3 stimulates expression of the IFN-stimulated gene 15 (ISG15) promoter, an ISRE-containing promoter. The murine IFNA4 promoter, which can be induced by IRF-1 or viral infection, is not induced by IRF-3. Expression of IRF-3 as a Gal4 fusion protein does not activate expression of a chloramphenicol acetyltransferase reporter gene containing repeats of the Gal4 binding sites, indicating that this protein does not contain the transcription transactivation domain. The high amino acid homology between IRF-3 and ISG factor 3 gamma polypeptide (ISGF3 gamma) and their similar binding properties indicate that, like ISGF3 gamma, IRF-3 may activate transcription by complex formation with other transcriptional factors, possibly members of the Stat family. Identification of this ISRE-binding protein may help us to understand the specificity in the various Stat pathways.