The interferon-stimulated gene 54 K promoter contains two adjacent functional interferon-stimulated response elements of different strength, which act synergistically for maximal interferon-alpha inducibility

Interferon regulatory factor 3 mediates effective antiviral responses to human coronavirus 229E and OC43 infection

Interferon regulatory factors (IRFs) are key elements of antiviral innate responses that regulate the transcription of interferons (IFNs) and IFN-stimulated genes (ISGs). While the sensitivity of human coronaviruses to IFNs has been characterized, antiviral roles of IRFs during human coronavirus infection are not fully understood. Type I or II IFN treatment protected MRC5 cells from human coronavirus 229E infection, but not OC43. Cells infected with 229E or OC43 upregulated ISGs, indicating that antiviral transcription is not suppressed. Antiviral IRFs, IRF1, IRF3 and IRF7, were activated in cells infected with 229E, OC43 or severe acute respiratory syndrome-associated coronavirus 2 (SARS-CoV-2). RNAi knockdown and overexpression of IRFs demonstrated that IRF1 and IRF3 have antiviral properties against OC43, while IRF3 and IRF7 are effective in restricting 229E infection. IRF3 activation effectively promotes transcription of antiviral genes during OC43 or 229E infection. Our study suggests that IRFs may be effective antiviral regulators against human coronavirus infection.

Mouse Ifit1b is a cap1-RNA-binding protein that inhibits mouse coronavirus translation and is regulated by complexing with Ifit1c

Knockout mouse models have been extensively used to study the antiviral activity of IFIT (interferon-induced protein with tetratricopeptide repeats). Human IFIT1 binds to cap0 (m7GpppN) RNA, which lacks methylation on the first and second cap-proximal nucleotides (cap1, m7GpppNm, and cap2, m7GpppNmNm, respectively). These modifications are signatures of "self" in higher eukaryotes, whereas unmodified cap0-RNA is recognized as foreign and, therefore, potentially harmful to the host cell. IFIT1 inhibits translation at the initiation stage by competing with the cap-binding initiation factor complex, eIF4F, restricting infection by certain viruses that possess "nonself" cap0-mRNAs. However, in mice and other rodents, the IFIT1 orthologue has been lost, and the closely related Ifit1b has been duplicated twice, yielding three paralogues: Ifit1, Ifit1b, and Ifit1c. Although murine Ifit1 is similar to human IFIT1 in its cap0-RNA-binding selectivity, the roles of Ifit1b and Ifit1c are unknown. Here, we found that Ifit1b preferentially binds to cap1-RNA, whereas binding is much weaker to cap0- and cap2-RNA. In murine cells, we show that Ifit1b can modulate host translation and restrict WT mouse coronavirus infection. We found that Ifit1c acts as a stimulatory cofactor for both Ifit1 and Ifit1b, promoting their translation inhibition. In this way, Ifit1c acts in an analogous fashion to human IFIT3, which is a cofactor to human IFIT1. This work clarifies similarities and differences between the human and murine IFIT families to facilitate better design and interpretation of mouse models of human infection and sheds light on the evolutionary plasticity of the IFIT family.

Nipah and Hendra Virus Nucleoproteins Inhibit Nuclear Accumulation of Signal Transducer and Activator of Transcription 1 (STAT1) and STAT2 by Interfering with Their Complex Formation

Henipaviruses, such as Nipah (NiV) and Hendra (HeV) viruses, are highly pathogenic zoonotic agents within the Paramyxoviridae family. The phosphoprotein (P) gene products of the paramyxoviruses have been well characterized for their interferon (IFN) antagonist activity and their contribution to viral pathogenicity. In this study, we demonstrated that the nucleoprotein (N) of henipaviruses also prevents the host IFN signaling response. Reporter assays demonstrated that the NiV and HeV N proteins (NiV-N and HeV-N, respectively) dose-dependently suppressed both type I and type II IFN responses and that the inhibitory effect was mediated by their core domains. Additionally, NiV-N prevented the nuclear transport of signal transducer and activator of transcription 1 (STAT1) and STAT2. However, NiV-N did not associate with Impα5, Impβ1, or Ran, which are members of the nuclear transport system for STATs. Although P protein is known as a binding partner of N protein and actively retains N protein in the cytoplasm, the IFN antagonist activity of N protein was not abolished by the coexpression of P protein. This suggests that the IFN inhibition by N protein occurs in the cytoplasm. Furthermore, we demonstrated that the complex formation of STATs was hampered in the N protein-expressing cells. As a result, STAT nuclear accumulation was reduced, causing a subsequent downregulation of interferon-stimulated genes (ISGs) due to low promoter occupancy by STAT complexes. This novel route for preventing host IFN responses by henipavirus N proteins provides new insight into the pathogenesis of these viruses.IMPORTANCE Paramyxoviruses are well known for suppressing interferon (IFN)-mediated innate immunity with their phosphoprotein (P) gene products, and the henipaviruses also possess P, V, W, and C proteins for evading host antiviral responses. There are numerous studies providing evidence for the relationship between viral pathogenicity and antagonistic activities against IFN responses by P gene products. Meanwhile, little attention has been paid to the influence of nucleoprotein (N) on host innate immune responses. In this study, we demonstrated that both the NiV and HeV N proteins have antagonistic activity against the JAK/STAT signaling pathway by preventing the nucleocytoplasmic trafficking of STAT1 and STAT2. This inhibitory effect is due to an impairment of the ability of STATs to form complexes. These results provide new insight into the involvement of N protein in viral pathogenicity via its IFN antagonism.

Interferon Regulatory Factor 8 (IRF8) Impairs Induction of Interferon Induced with Tetratricopeptide Repeat Motif (IFIT) Gene Family Members

The chromosomally clustered interferon-induced with tetratricopeptide repeat motif (IFIT) gene family members share structural features at the gene and protein levels. Despite these similarities, different IFIT genes have distinct inducer- and cell type-specific induction patterns. Here, we investigated the mechanism for the observed differential induction of the mouse Ifit1, Ifit2, and Ifit3 genes in B cells and demonstrated that the repressive effect of the transcription factor interferon regulatory factor 8 (IRF8), which is highly expressed in B cells, played an essential role in this regulation. Although IRF8 could impair induction of all three IFIT genes following stimulation of retinoic acid-inducible gene I (RIG-I), it could selectively impair the induction of the Ifit1 gene following IFN stimulation. The above properties could be imparted to IRF8-non-expressing cells by ectopic expression of the protein. Induction of reporter genes, driven by truncated Ifit1 promoters, identified the regions that mediate the repression, and a chromatin immunoprecipitation assay revealed that more IRF8 bound to the IFN-stimulated response element of the Ifit1 gene than to those of the Ifit2 and the Ifit3 genes. Mutational analyses of IRF8 showed that its ability to bind DNA, interact with other proteins, and undergo sumoylation were all necessary to selectively repress Ifit1 gene induction in response to IFN. Our study revealed a new role for IRFs in differentially regulating the induction patterns of closely related IFN-stimulated genes that are located adjacent to one another in the mouse genome.

Visualizing infection spread: dual-color fluorescent reporting of virus-host interactions

Although the molecular mechanisms by which host cells defend themselves against viral infection have been studied in great depth, and countermeasures viruses employ to suppress such defensive responses have been widely documented, relatively little attention has been devoted toward elucidating how such interactions between virus and host are resolved over multiple rounds of infection. Here, we describe the design, synthesis, and validation of a dual-color fluorescent reporter system to study how viral infections spread through a host cell monolayer and how the cellular innate immune system mounts an antiviral response. We employed recombinant, red fluorescent protein expressing mutants of a prototypical RNA virus, vesicular stomatitis virus to enable identification and tracking of infected cells. Further, we generated stable reporter cells that use green fluorescent protein to report on the expression of IFIT2, an interferon stimulated gene involved in the interference of viral protein translation, and a marker of antiviral defense activation. The presence of the fluorescent protein reporters had minimal effects on the normal behavior of the cells or viruses. Moreover, expression of the virus and cell reporters correlated with the kinetics of viral replication and activation of an anti-viral response, respectively. This two-color system enabled us to track and quantify in live cells how viral replication and activation of host defensive responses play out over multiple rounds of infection. Initial study of propagating infections demonstrated that antiviral activation over multiple rounds was critical for slowing and ultimately halting the spread of infection.

Effect of pistachio oil on gene expression of IFN-induced protein with tetratricopeptide repeats 2: a biomarker of inflammatory response

When incorporated into the diet, pistachios have a beneficial effect on lipid and lipoprotein profiles. However, little is known about potential anti-inflammatory properties. This study was conducted to determine whether pistachio oil and an organic extract from pistachio oil extract (PE) regulated expression of inflammation-related genes. A mouse macrophage cell line (RAW 264.7 cells) was treated with pistachio oil and gene expression microarray analyses were performed. Pistachio oil significantly affected genes involved in immune response, defense response to bacteria, and gene silencing, of which INF-induced protein with tetratricopeptide repeats 2 (Ifit-2) was the most dramatically reduced. PE reduced the LPS-induced Ifit-2 by 78% and the bioactive molecules contained in PE, linoleic acid, and beta-sitosterol recapitulated this inhibition. Promoter analysis identified two adjacent IFN-stimulated response elements, which lie between -110 and -85bp of the 5'-flanking region of the Ifit-2 promoter, as being responsive to LPS activation and inhibition by PE. Our results indicate that pistachio oil and bioactive molecules present therein decrease Ifit-2 expressions, and due to the sensitivity of this effect, this gene is a potential biomarker for monitoring diet-induced changes in inflammation.

Forced IFIT-2 expression represses LPS induced TNF-alpha expression at posttranscriptional levels

Background

Interferon induced tetratricopeptide repeat protein 2 (IFIT-2, P54) belongs to the type I interferon response genes and is highly induced after stimulation with LPS. The biological function of this protein is so far unclear. Previous studies indicated that IFIT-2 binds to the initiation factor subunit eIF-3c, affects translation initiation and inhibits protein synthesis. The aim of the study was to further characterize the function of IFIT-2.

Results

Stimulation of RAW264.7 macrophages with LPS or IFN-γ leads to the expression of IFIT-2 in a type I interferon dependent manner. By using stably transfected RAW264.7 macrophages overexpressing IFIT-2 we found that IFIT-2 inhibits selectively LPS induced expression of TNF-α, IL-6, and MIP-2 but not of IFIT-1 or EGR-1. In IFIT-2 overexpressing cells TNF-α mRNA expression was lower after LPS stimulation due to reduced mRNA stability. Further experiments suggest that characteristics of the 3'UTR of transcripts discriminate whether IFIT-2 has a strong impact on protein expression or not.

Conclusion

Our data suggest that IFIT-2 may affect selectively LPS induced protein expression probably by regulation at different posttranscriptional levels.

Inhibition of IFN-alpha/beta signaling by two discrete peptides within measles virus V protein that specifically bind STAT1 and STAT2

The V protein of measles virus (MV-V) is a potent inhibitor of IFN-alpha/beta signaling pathway. We previously reported that when physically dissociated, the N-terminal and C-terminal regions of MV-V (PNT and VCT, respectively) could independently impair signal transduction. The PNT region inhibited IFN-alpha/beta signaling by interacting with at least two components of this pathway: Jak1 and STAT1. Here we report a direct interaction between the VCT of MV-V and STAT2, a third component of IFN-alpha/beta transduction machinery. This interaction with STAT2 is carried by the cysteine-constrained peptide of 49 amino acids localized in the VCT region, and is essential to the inhibition of IFN-alpha/beta signaling. In parallel, we also mapped STAT1 binding site in the PNT region and identified a minimal peptide of only 11 amino acids. IFN-alpha/beta signaling was impaired in human cells treated with this MV-V peptide fused to a cell-penetrating sequence. Finally, we show that signaling downstream of IFN-lambda, a recently identified cytokine that also relies on STAT1, STAT2 and Jak1 to transduce, is blocked by MV-V. Altogether, our results illustrate how a single viral protein has evolved to achieve a robust inhibition of the antiviral response by interacting with several signaling molecules.

IFN-induced protein with tetratricopeptide repeats 2 inhibits migration activity and increases survival of oral squamous cell carcinoma

The function of the IFN-stimulated gene family protein, IFN-induced protein with tetratricopeptide repeats 2 (IFIT2), is poorly understood. Here, we report that IFIT2 colocalizes with cytokeratin 18 in oral squamous cell carcinoma (OSCC) cells. Treatment of OSCC cells with IFN-beta significantly increased the expression of IFIT2 and remarkably inhibited cell migration. To further explore the effect of IFIT2 on cell migration, IFIT2 expression was either silenced with a small interfering RNA or increased by ectopic expression. IFIT2 knockdown in OSCC cells led to a significantly higher level of migration in vitro (P < 0.05) compared with control cells; by contrast, IFIT2 overexpression led to a significantly lower level of migration in vitro (P < 0.05). Immunohistochemically, 71.4% of OSCC tissues had elevated IFIT2 protein levels compared with noncancerous matched tissues. Elevated IFIT2 protein expression was positively associated with tumor differentiation status and inversely associated with nodal stage in OSCC specimens (P < 0.05). Higher IFIT2 protein levels in tumor tissues were also associated with better patient survival (P < 0.01). Our present study shows an inverse correlation between IFIT2 expression and cell migration, suggesting that IFIT2 plays an important role in inhibiting this process and that its expression may be associated with better prognosis in patients with OSCC.

Inhibition of retinoic acid-inducible gene I-mediated induction of beta interferon by the NS1 protein of influenza A virus

The retinoic acid-inducible gene I product (RIG-I) has been identified as a cellular sensor of RNA virus infection resulting in beta interferon (IFN-beta) induction. However, many viruses are known to encode viral products that inhibit IFN-beta production. In the case of influenza A virus, the viral nonstructural protein 1 (NS1) prevents the induction of the IFN-beta promoter by inhibiting the activation of transcription factors, including IRF-3, involved in IFN-beta transcriptional activation. The inhibitory properties of NS1 appear to be due at least in part to its binding to double-stranded RNA (dsRNA), resulting in the sequestration of this viral mediator of RIG-I activation. However, the precise effects of NS1 on the RIG-I-mediated induction of IFN-beta have not been characterized. We now report that the NS1 of influenza A virus interacts with RIG-I and inhibits the RIG-I-mediated induction of IFN-beta. This inhibition was apparent even when a mutant RIG-I that is constitutively activated (in the absence of dsRNA) was used to trigger IFN-beta production. Coexpression of RIG-I, its downstream signaling partner, IPS-1, and NS1 resulted in increased levels of RIG-I and NS1 within an IPS-1-rich, solubilization-resistant fraction after cell lysis. These results suggest that RIG-I, IPS-1, and NS1 become part of the same complex. Consistent with this idea, NS1 was also found to inhibit IFN-beta promoter activation by IPS-1 overexpression. Our results indicate that, in addition to sequestering dsRNA, the NS1 of influenza A virus binds to RIG-I and inhibits downstream activation of IRF-3, preventing the transcriptional induction of IFN-beta.

Inhibition of the type I interferon response by the nucleoprotein of the prototypic arenavirus lymphocytic choriomeningitis virus

The prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) is a formidable battle horse for the study of viral immunology, as well as viral persistence and associated diseases. Investigations with LCMV have uncovered basic mechanisms by which viruses avoid elimination by the host adaptive immune response. In this study we show that LCMV also disables the host innate defense by interfering with beta interferon (IFN-beta) production in response to different stimuli, including infection with Sendai virus and liposome-mediated DNA transfection. Inhibition of IFN production in LCMV-infected cells was caused by an early block in the IFN regulatory factor 3 (IRF-3) activation pathway. This defect was restored in cells cured of LCMV, indicating that one or more LCMV products are responsible for the inhibition of IRF-3 activation. Using expression plasmids encoding individual LCMV proteins, we found that expression of the LCMV nucleoprotein (NP) was sufficient to inhibit both IFN production and nuclear translocation of IRF-3. To our knowledge, this is the first evidence of an IFN-counteracting viral protein in the Arenaviridae family. Inhibition of IFN production by the arenavirus NP is likely to be a determinant of virulence in vivo.

Proximal genomic localization of STAT1 binding and regulated transcriptional activity

BACKGROUND

Signal transducer and activator of transcription (STAT) proteins are key regulators of gene expression in response to the interferon (IFN) family of anti-viral and anti-microbial cytokines. We have examined the genomic relationship between STAT1 binding and regulated transcription using multiple tiling microarray and chromatin immunoprecipitation microarray (ChIP-chip) experiments from public repositories.

RESULTS

In response to IFN-gamma, STAT1 bound proximally to regions of the genome that exhibit regulated transcriptional activity. This finding was consistent between different tiling microarray platforms, and between different measures of transcriptional activity, including differential binding of RNA polymerase II, and differential mRNA transcription. Re-analysis of tiling microarray data from a recent study of IFN-gamma-induced STAT1 ChIP-chip and mRNA expression revealed that STAT1 binding is tightly associated with localized mRNA transcription in response to IFN-gamma. Close relationships were also apparent between STAT1 binding, STAT2 binding, and mRNA transcription in response to IFN-alpha. Furthermore, we found that sites of STAT1 binding within the Encyclopedia of DNA Elements (ENCODE) region are precisely correlated with sites of either enhanced or diminished binding by the RNA polymerase II complex.

CONCLUSION

Together, our results indicate that STAT1 binds proximally to regions of the genome that exhibit regulated transcriptional activity. This finding establishes a generalized basis for the positioning of STAT1 binding sites within the genome, and supports a role for STAT1 in the direct recruitment of the RNA polymerase II complex to the promoters of IFN-gamma-responsive genes.

Ebola virus VP35 protein binds double-stranded RNA and inhibits alpha/beta interferon production induced by RIG-I signaling

The Ebola virus (EBOV) VP35 protein blocks the virus-induced phosphorylation and activation of interferon regulatory factor 3 (IRF-3), a transcription factor critical for the induction of alpha/beta interferon (IFN-alpha/beta) expression. However, the mechanism(s) by which this blockage occurs remains incompletely defined. We now provide evidence that VP35 possesses double-stranded RNA (dsRNA)-binding activity. Specifically, VP35 bound to poly(rI) . poly(rC)-coated Sepharose beads but not control beads. In contrast, two VP35 point mutants, R312A and K309A, were found to be greatly impaired in their dsRNA-binding activity. Competition assays showed that VP35 interacted specifically with poly(rI) . poly(rC), poly(rA) . poly(rU), or in vitro-transcribed dsRNAs derived from EBOV sequences, and not with single-stranded RNAs (ssRNAs) or double-stranded DNA. We then screened wild-type and mutant VP35s for their ability to target different components of the signaling pathways that activate IRF-3. These experiments indicate that VP35 blocks activation of IRF-3 induced by overexpression of RIG-I, a cellular helicase recently implicated in the activation of IRF-3 by either virus or dsRNA. Interestingly, the VP35 mutants impaired for dsRNA binding have a decreased but measurable IFN antagonist activity in these assays. Additionally, wild-type and dsRNA-binding-mutant VP35s were found to have equivalent abilities to inhibit activation of the IFN-beta promoter induced by overexpression of IPS-1, a recently identified signaling molecule downstream of RIG-I, or by overexpression of the IRF-3 kinases IKKepsilon and TBK-1. These data support the hypothesis that dsRNA binding may contribute to VP35 IFN antagonist function. However, additional mechanisms of inhibition, at a point proximal to the IRF-3 kinases, most likely also exist.

Atorvastatin affects leukocyte gene expression in dyslipidemia patients: in vivo regulation of hemostasis, inflammation and apoptosis

BACKGROUND

The beneficial effect of HMG-CoA reductase inhibitors (statins) on coronary artery disease has been linked to mechanisms beyond their lipid-lowering effect. However the existence of direct, lipid-independent effects of statin in humans is still controversial.

OBJECTIVE

To investigate the early effect of atorvastatin on peripheral blood mononuclear cells (PBMC) in dyslipidemic patients using gene arrays.

PATIENTS AND METHODS

Eleven male patients with primary hyperlipidemia received 20 mg atorvastatin daily for 4 weeks. Blood was collected at baseline, 12 h, 36 h, 1 and 4 weeks after the start of treatment.

RESULTS

Human microarrays containing 12 650 genes were used to study the effect of atorvastatin on PBMC gene expression at all time-points. Two hundred and forty genes were significantly regulated by atorvastatin treatment, several of which are involved in hemostasis, inflammation and other processes critical to atherosclerosis. Different patterns of response over time suggested both lipid-dependent and independent effects of atorvastatin on gene expression.

CONCLUSIONS

This study demonstrates for the first time that atorvastatin regulates gene expression in PBMC in man before changes in the lipid profile are detectable in serum. Using blood leukocytes as a pharmacogenomic surrogate, we have identified new in vivo targets of atorvastatin treatment.

Mouse p56 blocks a distinct function of eukaryotic initiation factor 3 in translation initiation

Members of the p56 family of mammalian proteins are strongly induced in virus-infected cells and in cells treated with interferons or double-stranded RNA. Previously, we have reported that human p56 inhibits initiation of translation by binding to the "e" subunit of eukaryotic initiation factor 3 (eIF3) and subsequently interfering with the eIF3/eIF2.GTP.Met-tRNAi (ternary complex) interaction. Here we report that mouse p56 also interferes with eIF3 functions and inhibits translation. However, the murine protein binds to the "c" subunit, not the "e" subunit, of eIF3. Consequently, it has only a marginal effect on eIF3.ternary complex interaction. Instead, the major inhibitory effect of mouse p56 is manifested at a different step of translation initiation, namely the binding of eIF4F to the 40 S ribosomal subunit.eIF3.ternary complex. Thus, mouse and human p56 proteins block different functions of eIF3 by binding to its different subunits.

A recombinant influenza A virus expressing an RNA-binding-defective NS1 protein induces high levels of beta interferon and is attenuated in mice

Previously we found that the amino-terminal region of the NS1 protein of influenza A virus plays a key role in preventing the induction of beta interferon (IFN-beta) in virus-infected cells. This region is characterized by its ability to bind to different RNA species, including double-stranded RNA (dsRNA), a known potent inducer of IFNs. In order to investigate whether the NS1 RNA-binding activity is required for its IFN antagonist properties, we have generated a recombinant influenza A virus which expresses a mutant NS1 protein defective in dsRNA binding. For this purpose, we substituted alanines for two basic amino acids within NS1 (R38 and K41) that were previously found to be required for RNA binding. Cells infected with the resulting recombinant virus showed increased IFN-beta production, demonstrating that these two amino acids play a critical role in the inhibition of IFN production by the NS1 protein during viral infection. In addition, this virus grew to lower titers than wild-type virus in MDCK cells, and it was attenuated in mice. Interestingly, passaging in MDCK cells resulted in the selection of a mutant virus containing a third mutation at amino acid residue 42 of the NS1 protein (S42G). This mutation did not result in a gain in dsRNA-binding activity by the NS1 protein, as measured by an in vitro assay. Nevertheless, the NS1 R38AK41AS42G mutant virus was able to replicate in MDCK cells to titers close to those of wild-type virus. This mutant virus had intermediate virulence in mice, between those of the wild-type and parental NS1 R38AK41A viruses. These results suggest not only that the IFN antagonist properties of the NS1 protein depend on its ability to bind dsRNA but also that they can be modulated by amino acid residues not involved in RNA binding.

Inhibition of interferon signaling by dengue virus

Dengue virus is a worldwide-distributed mosquito-borne flavivirus with a positive strand RNA genome. Its transcribed polyprotein is cleaved by host- and virus-encoded peptidases into 10 proteins, some of which are of unknown function. Although dengue virus-infected cells seem to be resistant to the antiviral action of IFN, the viral products that mediate this resistance are unknown. Therefore, we have analyzed the ability of the 10 dengue virus-encoded proteins to antagonize the IFN response. We found that expression in human A549 cells of the dengue virus nonstructural proteins NS2A, NS4A, or NS4B enhances replication of an IFN-sensitive virus. Moreover, expression of NS4B and, to a lesser extent, of NS2A and NS4A proteins results in down-regulation of IFN-beta-stimulated gene expression. Cells expressing NS4B or infected with dengue virus do not exhibit nuclear signal transducer and activator of transcription (STAT) 1 on treatment with IFN-beta or IFN-gamma, indicating that NS4B might be involved in blocking IFN signaling during dengue virus infections. This protein, encoded by a positive strand RNA virus, is implicated as an IFN-signaling inhibitor.

Differentiation therapy of human cancer: basic science and clinical applications

Current cancer therapies are highly toxic and often nonspecific. A potentially less toxic approach to treating this prevalent disease employs agents that modify cancer cell differentiation, termed 'differentiation therapy.' This approach is based on the tacit assumption that many neoplastic cell types exhibit reversible defects in differentiation, which upon appropriate treatment, results in tumor reprogramming and a concomitant loss in proliferative capacity and induction of terminal differentiation or apoptosis (programmed cell death). Laboratory studies that focus on elucidating mechanisms of action are demonstrating the effectiveness of 'differentiation therapy,' which is now beginning to show translational promise in the clinical setting.

The Ebola virus VP35 protein functions as a type I IFN antagonist

An assay has been developed that allows the identification of molecules that function as type I IFN antagonists. Using this assay, we have identified an Ebola virus-encoded inhibitor of the type I IFN response, the Ebola virus VP35 protein. The assay relies on the properties of an influenza virus mutant, influenza delNS1 virus, which lacks the NS1 ORF and, therefore, does not produce the NS1 protein. When cells are infected with influenza delNS1 virus, large amounts of type I IFN are produced. As a consequence, influenza delNS1 virus replicates poorly. However, high-efficiency transient transfection of a plasmid encoding a protein that interferes with type I IFN-induced antiviral functions, such as the influenza A virus NS1 protein or the herpes simplex virus protein ICP34.5, rescues growth of influenza delNS1 virus. When plasmids expressing individual Ebola virus proteins were transfected into Madin Darby canine kidney cells, the Ebola virus VP35 protein enhanced influenza delNS1 virus growth more than 100-fold. VP35 subsequently was shown to block double-stranded RNA- and virus-mediated induction of an IFN-stimulated response element reporter gene and to block double-stranded RNA- and virus-mediated induction of the IFN-beta promoter. The Ebola virus VP35 therefore is likely to inhibit induction of type I IFN in Ebola virus-infected cells and may be an important determinant of Ebola virus virulence in vivo.

IL-10 attenuates IFN-alpha-activated STAT1 in the liver: involvement of SOCS2 and SOCS3

Interleukin-10 (IL-10) has been used in the treatment of viral hepatitis in interferon-alpha (IFN-alpha) non-responders while patients who have high levels of IL-10 are poorly responsive to IFN-alpha. The mechanism underlying such controversial functions of IL-10 remains unknown. Here we demonstrated that injection of IL-10 into mice attenuated IFN-alpha-induced signal transducer and activator transcription factor (STAT)1 tyrosine phosphorylation in the liver. Reverse transcriptase-polymerase chain reaction assay demonstrated that mouse liver expressed high levels of IL-10 receptor 2 (IL-10R2) but low levels of IL-10R1. Injection of IL-10 into mice activated STAT3 but not STAT1 tyrosine phosphorylation and induced suppressor of cytokine signal 2 (SOCS2), SOCS3, and cytokine-inducible SH2 protein (CIS) mRNA expression in the liver. Furthermore, overexpression of SOCS2 or SOCS3 inhibited IFN-alpha-induced reporter activity in hepatic cells. These findings suggest that IL-10 inhibits IFN-alpha-activated STAT1 in the liver, at least in part, by inducing SOCS2, SOCS3, and CIS expression, which may be responsible for the resistance of IFN-alpha therapy in patients who have high levels of IL-10 and recommends that IL-10 treatment for viral hepatitis should be cautious.

Influenza A virus lacking the NS1 gene replicates in interferon-deficient systems

The NS1 protein is the only nonstructural protein encoded by influenza A virus. It has been proposed that the NS1 performs several regulatory functions during the viral replication cycle, including the regulation of synthesis, transport, splicing, and translation of mRNAs. Through the use of reverse genetics, a viable transfectant influenza A virus (delNS1) which lacks the NS1 gene has been generated. Our results indicate that the NS1 of influenza A virus is an auxiliary (virulence) factor which plays a crucial role in inhibiting interferon-mediated antiviral responses of the host.

IFI60/ISG60/IFIT4, a new member of the human IFI54/IFIT2 family of interferon-stimulated genes

We report the cloning and sequencing of a full-length cDNA encoding a new member of the human IFI54 (HGMW-approved symbol IFIT2) gene family, designated IFI60 (HGMW-approved symbol IFIT4). The upstream regulatory region of IFI60 shows conservation in structure with that of the IFI54 and IFI56 (HGMW-approved symbol IFIT1) genes, each containing two interferon-stimulated response elements upstream of a conserved TATA box. We have established a partial gene map of the IFI54 gene family by analysis of YAC library clones. All four members of the human family are clustered together at chromosome 10q23.3. It is proposed that the four members of the IFI54 gene family evolved by a series of duplication events from a common gene of origin.

Maximal induction of p69 2', 5'-oligoadenylate synthetase in Daudi cells requires cooperation between an ISRE and two IRF-1-like elements

The human 2', 5'-oligoadenylate (2-5A) synthetases are members of a family interferon (IFN)-inducible anti-viral proteins. Three size classes of these enzymes: small (p40, p46), intermediate (p69, p71) and large (p100), are encoded by three genes that exhibit differential constitutive and IFN-inducible expression. Since the 5'-regulatory region of the 69 kDa isoform contains multiple putative control elements, deletion analysis and site directed mutagenesis were done to identify key regulatory motifs. The region located between bp -972 and -452 from the translational start site contains elements that slightly repress constitutive and IFN-inducible transcription. The region from bp -366 to -117 contains two positive regulatory elements that differ slightly from consensus IFN-regulated factor 1 (IRF-1) binding sites. In mobility shift assays, the proximal IRF-1 site weakly binds a novel factor found in both control and IFN-treated cells. The region from bp -117 to -10 contains a functional interferon stimulated response element (ISRE) that contributes to constitutive expression and confers IFN-inducibility on a heterologous promoter. The ISRE specifically binds an IFN-inducible factor that contains signal transducer activator of transcription (STAT) 1alpha. The ISRE and the two IRF-1-like sites cooperatively interact to control transcription. These three elements are sufficient for constitutive and IFN-inducible expression, since expression from reporter constructs containing mutations in all three elements is low in both control and IFN-treated cells.

Cooperative binding of Stat1-2 heterodimers and ISGF3 to tandem DNA elements

Interferon (IFN)-alpha-activated Stat1 homodimers and Stat1-2 heterodimers bind to GAS elements, whereas the transcription factor ISGF3, which contains Stat1, Stat2 and p48, binds to ISREs. We now find that Stat1-2 dimers can form heterotetramers on tandem GAS sites and that the heterotetramers have a much higher binding affinity for a double GAS site than do heterodimers for a single site, suggesting cooperativity mediated through protein-protein interactions. Stat1-2 heterotetramers can also be detected with a single GAS site, again indicating cooperativity mediated through protein-protein interactions. Deleting 40 amino acid residues from the N-terminus of Stat1 abolished Stat1-Stat2 heterotetramer formation, but did not affect heterodimer formation and an N-terminal peptide containing the first 120 residues of Stat2 inhibited heterotetramer formation but did not affect heterodimer formation. Thus, the N-terminal regions of both Stat1 and Stat2 are important for cooperative DNA binding, and heterodimers probably interact with each other through these regions. Cooperative binding of ISGF3 was also observed using the tandem ISREs from the IFN-alpha responsive promoter of the 6-16 gene. A more abundant and larger complex was formed with a probe containing two ISREs than with a probe containing a single ISRE. The N-terminal regions of both Stat1 and Stat2 are important for the cooperative binding of ISGF3 to tandem ISREs but not to a single site. The cooperative DNA-binding activities of ISGF3 and Stat1-2 dimers are likely to contribute to the transcriptional activation of those IFN-alpha-responsive genes that have tandem DNA elements.

Characterization and Mapping to Human Chromosome 8q24.3 of Ly-6-Related Gene 9804 Encoding an Apparent Homologue of Mouse TSA-1

The 9804 gene, which encodes a human Ly-6 protein most similar to mouse differentiation Ag TSA-1/Sca-2, has also been called RIG-E. Like mouse TSA-1, it has a broad tissue distribution with varied expression levels in normal human tissues and tumor cell lines. Like some members of the murine Ly-6 family, the 9804 gene is responsive to IFNs, particularly IFN-α. Overlapping genomic fragments spanning the 9804 gene (5543 bp) have been isolated and characterized. The gene organization is analogous to that of known mouse Ly-6 genes. The first exon, 2296 bp upstream from exon II, is entirely untranslated. The three coding exons (II, III, and IV) are separated by short introns of 321 and 131 bp, respectively. Primers were developed for specific amplification of 9804 gene fragments. Screening of human-hamster somatic cell hybrids and yeast artificial chromosomes (YACs) indicated that the gene is distal to c-Myc, located in the q arm of human chromosome 8. No positives were detected from the Centre d′Etude du Polymorphisme Humain mega-YAC A or B panels, nor from bacterial artificial chromosome libraries; two positive cosmids (c101F1 and c157F6) were isolated from a human chromosome 8 cosmid library (LA08NC01). Fluorescence in situ hybridization of metaphase spreads of chromosome 8, containing hybrid cell line 706-B6 clone 17 (CL-17) with cosmid c101F1, placed the 9804 gene close to the telomere at 8q24.3. This mapping is significant, since the region shares a homology with a portion of mouse chromosome 15, which extends into band E where Ly-6 genes reside. Moreover, the gene encoding E48, the homologue of mouse Ly-6 molecule ThB, has also been mapped to 8q24.

A novel interferon-inducible gene expressed during myeloid differentiation

The acute promyelocytic leukemia cell line, NB4, can be induced to differentiate to mature granulocytes by retinoic acid treatment. A novel retinoic acid-inducible cDNA clone, designated RI58, was isolated from a cDNA library constructed from retinoic acid-treated NB4 cells by differential hybridization. RI58 cDNA encodes a protein of 58kDa which has a similarity in its amino acids sequence to interferon (IFN)-inducible proteins. In addition, RI58 was induced by recombinant human IFN-alpha (rhIFN-alpha) in NB4 cells. RI58 was detectable within 4 hours post-stimulation with rhIFN-alpha, while it took as long as 1day after retinoic acid stimulation. Culture supernatant from retinoic acid-treated NB4 cells also induced RI58 expression similarly as rhIFN-alpha. This activity in culture supernatant was inhibited by anti-leukocyte IFN antiserum which showed specific reactivity to rhIFN-alpha. These results indicate that RI58 is induced by retinoic acid stimulation through autocrinally secreted IFN-alpha from NB4 cells. In the retinoic acid-treated NB4 cells, the expression of RI58 was increased along the process of differentiation. On the other hand, it was expressed constitutively in untreated non-hematopoietic cell lines and mature hematopoietic cell lines.

Cloning of a gene (RIG-G) associated with retinoic acid-induced differentiation of acute promyelocytic leukemia cells and representing a new member of a family of interferon-stimulated genes

In a cell line (NB4) derived from a patient with acute promyelocytic leukemia, all-trans-retinoic acid (ATRA) and interferon (IFN) induce the expression of a novel gene we call RIG-G (for retinoic acid-induced gene G). This gene codes for a 58-kDa protein containing 490 amino acids with several potential sites for post-translational modification. In untreated NB4 cells, the expression of RIG-G is undetectable. ATRA treatment induces the transcriptional expression of RIG-G relatively late (12-24 hr) in a protein synthesis-dependent manner, whereas IFN-alpha induces its expression early (30 min to 3 hr). Database search has revealed a high-level homology between RIG-G and several IFN-stimulated genes in human (ISG54K, ISG56K, and IFN-inducible and retinoic acid-inducible 58K gene) and some other species, defining a well conserved gene family. The gene is composed of two exons and has been mapped by fluorescence in situ hybridization to chromosome 10q24, where two other human IFN-stimulated gene members are localized. A synergistic induction of RIG-G expression in NB4 cells by combined treatment with ATRA and IFNs suggests that a collaboration exists between their respective signaling pathways.

Stat2 is a transcriptional activator that requires sequence-specific contacts provided by stat1 and p48 for stable interaction with DNA

Transcriptional responses to interferon (IFN) are mediated by tyrosine phosphorylation and nuclear translocation of transcription factors of the signal transducer and activator of transcription (Stat) family. The Stat1 protein is required for all transcriptional responses to IFN (both type I and type II). Responses to type I IFN (alpha and beta) also require Stat2 and the IFN regulatory factor family protein p48, which form a heterotrimeric transcription complex with Stat1 termed ISGF3. Stat1 homodimers formed in response to IFN-gamma treatment can also interact with p48 and function as transcriptional activators. We now show that Stat2 is capable of forming a stable homodimer that interacts with p48, can be recruited to DNA, and can activate transcription, raising a question of why Stat1 is required. Analysis of the transcriptional competence, affinity, and specificity of Stat2-p48 complexes compared with other Stat protein-containing transcription factor complexes suggests distinct roles for each component. Although Stat2 is a potent transactivator, it does not interact stably with DNA in complex with p48 alone. Adding Stat1 increases the affinity and alters the sequence selectivity of p48-DNA interactions by contacting a half-site of its palindromic recognition motif adjacent to a p48 interaction sequence. Thus, ISGF3 assembly involves p48 functioning as an adaptor protein to recruit Stat1 and Stat2 to an IFN-alpha-stimulated response element, Stat2 contributes a potent transactivation domain but is unable to directly contact DNA, while Stat1 stabilizes the heteromeric complex by contacting DNA directly.

Pip, a lymphoid-restricted IRF, contains a regulatory domain that is important for autoinhibition and ternary complex formation with the Ets factor PU.1

Pip is a lymphoid-restricted IRF transcription factor that is recruited to composite elements within immunoglobulin light-chain gene enhancers through a specific interaction with the Ets factor PU.1. We have examined the transcriptional regulatory properties of Pip as well as the requirements for its interaction with PU.1 and DNA to form a ternary complex. We demonstrate that Pip is a dichotomous regulator; it specifically stimulates transcription in conjunction with PU.1, but represses alpha/beta-interferon-inducible transcription in the absence of PU.1. Thus, during B-cell activation and differentiation, Pip may function both as an activator to promote B cell-specific gene expression and as a repressor to inhibit the antiproliferative effects of alpha/beta-interferons. Mutational analysis of Pip reveals a carboxy-terminal segment that is important for autoinhibition of DNA binding and ternary complex formation. A domain of Pip containing this segment confers autoinhibition and PU.1-dependent binding activity to the DNA-binding domain of the related IRF family member, p48. On the basis of these and other data we propose a model for PU.1/Pip ternary complex formation.

Combinatorial association and abundance of components of interferon-stimulated gene factor 3 dictate the selectivity of interferon responses

Genes containing the interferon-stimulated response element (ISRE) enhancer have been characterized as transcriptionally responsive primarily to type I interferons (IFN alpha/beta). Induction is due to activation of a multimeric transcription factor, interferon-stimulated gene factor 3 (ISGF3), which is activated by IFN alpha/beta but not by IFN gamma. We found that ISRE-containing genes were induced by IFN gamma as well as by IFN alpha in Vero cells. The IFN gamma response was dependent on the ISRE and was accentuated by preexposure of cells to IFN alpha, a treatment that increases the abundance of ISGF3 components. Overexpression of ISGF3 polypeptides showed that the IFN gamma response depended on the DNA-binding protein ISGF3 gamma (p48) as well as on the 91-kDa protein STAT91 (Stat1 alpha). The transcriptional response to IFN alpha required the 113-kDa protein STAT113 (Stat2) in addition to STAT91 and p48. Mutant fibrosarcoma cells deficient in each component of ISGF3 were used to confirm that IFN gamma induction of an ISRE reporter required p48 and STAT91, but not STAT113. A complex containing p48 and phosphorylated STAT91 but lacking STAT113 bound the ISRE in vitro. IFN gamma-induced activation of this complex, preferentially formed at high concentrations of p48 and STAT91, may explain some of the overlapping responses to IFN alpha and IFN gamma.