Impact of protein kinase PKR in cell biology: from antiviral to antiproliferative action

Interaction of double-stranded RNA-dependent protein kinase (PKR) with the death receptor signaling pathway in amyloid beta (Abeta)-treated cells and in APPSLPS1 knock-in mice

For 10 years, research has focused on signaling pathways controlling translation to explain neuronal death in Alzheimer Disease (AD). Previous studies demonstrated in different cellular and animal models and AD patients that translation is down-regulated by the activation of double-stranded RNA-dependent protein kinase (PKR). Among downstream factors of PKR, the Fas-associated protein with a death domain (FADD) and subsequent activated caspase-8 are responsible for PKR-induced apoptosis in recombinant virus-infected cells. However, no studies have reported the role of PKR in death receptor signaling in AD. The aim of this project is to determine physical and functional interactions of PKR with FADD in amyloid-beta peptide (Abeta) neurotoxicity and in APP(SL)PS1 KI transgenic mice. In SH-SY5Y cells, results showed that Abeta42 induced a large increase in phosphorylated PKR and FADD levels and a physical interaction between PKR and FADD in the nucleus, also observed in the cortex of APP(SL)PS1 KI mice. However, PKR gene silencing or treatment with a specific PKR inhibitor significantly prevented the increase in pT(451)-PKR and pS(194)-FADD levels in SH-SY5Y nuclei and completely inhibited activities of caspase-3 and -8. The contribution of PKR in neurodegeneration through the death receptor signaling pathway may support the development of therapeutics targeting PKR to limit neuronal death in AD.

Intrinsic cellular defenses against virus infection by antiviral type I interferon

Intrinsic cellular defenses are non-specific antiviral activities by recognizing pathogen-associated molecular patterns (PAMPs). Toll-like receptors (TLRs), one of the pathogen recognize receptor (PRR), sense various microbial ligands. Especially, TLR2, TLR3, TLR4, TLR7, TLR8 and TLR9 recognize viral ligands such as glycoprotein, single- or double-stranded RNA and CpG nucleotides. The binding of viral ligands to TLRs transmits its signal to Toll/interleukin-1 receptor (TIR) to activate transcription factors via signal transduction pathway. Through activation of transcription factors, such as interferon regulatory factor-3, 5, and 7 (IRF-3, 5, 7) or nuclear factor-kappaB (NF-kappaB), type I interferons are induced, and antiviral proteins such as myxovirus-resistance protein (Mx) GTPase, RNA-dependent Protein Kinase (PKR), ribonuclease L (RNase L), Oligo-adenylate Synthetase (OAS) and Interferon Stimulated Gene (ISG) are further expressed. These antiviral proteins play an important role of antiviral resistancy against several viral pathogens in infected cells and further activate innate immune responses.

Innate immune evasion strategies of influenza viruses

Influenza viruses are globally important human respiratory pathogens. These viruses cause seasonal epidemics and occasional worldwide pandemics, both of which can vary significantly in disease severity. The virulence of a particular influenza virus strain is partly determined by its success in circumventing the host immune response. This article briefly reviews the innate mechanisms that host cells have evolved to resist virus infection, and outlines the plethora of strategies that influenza viruses have developed in order to counteract such powerful defences. The molecular details of this virus-host interplay are summarized, and the ways in which research in this area is being applied to the rational design of protective vaccines and novel antivirals are discussed.

Retinol supplements antiviral action of interferon in patients with chronic hepatitis C: a prospective pilot study

Sustained virologic response with peg-interferon and ribavirin combination therapy for 48 weeks is still inadequate. Our study examined whether short-term administration of retinol clinically influences the anti-viral activity of interferon early during interferon and ribavirin combination therapy. The control group received 6 MIU of interferon α-2b every day for two weeks and then 3 times a week for 22 weeks intramuscularly plus 600 mg or 800 mg per day of ribavirin orally for 24 weeks. The retinol group, in addition to above treatment, received retinol 30,000 units per day orally for 3 weeks from one week before the start of interferon α-2b plus ribavirin combination therapy. The hepatitis C virus (HCV) RNA negativity rate at 1 week after the end of interferon α-2b and ribavirin combination therapy was 46.7% (28/60) for the retinol group and 31.7% (19/60) for the control group, which was significantly higher for the retinol group. The level of serum HCV RNA in the retinol group was significantly lower at 1 week after beginning treatment as compared to the control group (p<0.01). Furthermore, serum 2,5'AS protein at 1 week after beginning treatment was significantly higher in the retinol group (p = 0.0002). The results suggest that retinol supplement increases the antiviral effect of interferon α-2b plus ribavirin only during the administration of IFN α-2b, ribavirin and retinol in patients with chronic hepatitis C.

Vitamin D receptor deletion leads to reduced level of IkappaBalpha protein through protein translation, protein-protein interaction, and post-translational modification

Vitamin D receptor plays an essential role in the regulation of inflammation. Previous studies demonstrate that vitamin D receptor negatively modulates the proinflammatory NF-kappaB pathway. However, it is unknown how vitamin D receptor regulates IkappaBalpha, the endogenous inhibitor of NF-kappaB. Here we investigated the molecular mechanism of vitamin D receptor deletion and IkappaBalpha expression. We found that cells lacking vitamin D receptor had significantly increased levels of IkappaBalpha mRNA and simultaneously decreased levels of IkappaBalpha protein. Lacking vitamin D receptor abolished its binding to the IkappaBalpha promoter. Moreover, the levels of protein translation regulators and the rate of protein synthesis were both decreased in cells lacking vitamin D receptor. At the post-translational level, IkappaBalpha ubiquitination was enhanced, indicating increased degradation of IkappaBalpha in the absence of vitamin D receptor. We further transfected cells with a plasmid carrying either wild-type or mutant IkappaBalpha. The expression of wild-type IkappaBalpha was much higher in the cells with vitamin D receptor than in the cells without vitamin D receptor, whereas the expression of exogenous IkappaBalpha was equally high in both cell lines. In summary, vitamin D receptor deletion affects IkappaBalpha through mRNA transcription, protein translation, protein-protein interaction, post-translational modification, and protein degradation, thus reducing the level of IkappaBalpha protein. Cells lacking vitamin D receptor are known in a proinflammatory state with activation of NF-kappaB. Our study provides new insight into vitamin D receptor regulation of an inhibitor of NF-kappaB in inflammation. Deletion of vitamin D receptor contributes to the activation of NF-kappaB on multiple levels.

RNA adenosine deaminase ADAR1 deficiency leads to increased activation of protein kinase PKR and reduced vesicular stomatitis virus growth following interferon treatment

Two size forms of ADAR1 adenosine deaminase are known, one constitutively expressed (p110) and the other interferon (IFN)-induced (p150). To test the role of ADAR1 in viral infection, HeLa cells with ADAR1 stably knocked down and 293 cells overexpressing ADAR1 were utilized. Overexpression of p150 ADAR1 had no significant effect on the yield of vesicular stomatitis virus. Likewise, reduction of p110 and p150 ADAR1 proteins to less than approximately 10 to 15% of parental levels (ADAR1-deficient) had no significant effect on VSV growth in the absence of IFN treatment. However, inhibition of virus growth following IFN treatment was approximately 1 log(10) further reduced compared to ADAR1-sufficient cells. The level of phosphorylated protein kinase PKR was increased in ADAR1-deficient cells compared to ADAR1-sufficient cells following IFN treatment, regardless of viral infection. These results suggest that ADAR1 suppresses activation of PKR and inhibition of VSV growth in response to IFN treatment.

Roles of the translationally controlled tumour protein (TCTP) and the double-stranded RNA-dependent protein kinase, PKR, in cellular stress responses

Translationally controlled tumour protein (TCTP) is a highly conserved protein present in all eukaryotic organisms. Various cellular functions and molecular interactions have been ascribed to this protein, many related to its growth-promoting and antiapoptotic properties. TCTP levels are highly regulated in response to various cellular stimuli and stresses. We have shown recently that the double-stranded RNA-dependent protein kinase, PKR, is involved in translational regulation of TCTP. Here we extend these studies by demonstrating that TCTP is downregulated in response to various proapoptotic treatments, in particular agents that induce Ca(++) stress, in a PKR-dependent manner. This regulation requires phosphorylation of protein synthesis factor eIF2alpha. Since TCTP has been characterized as an antiapoptotic and Ca(++)-binding protein, we asked whether it is involved in protecting cells from Ca(++)-stress-induced apoptosis. Overexpression of TCTP partially protects cells against thapsigargin-induced apoptosis, as measured using caspase-3 activation assays, a nuclear fragmentation assay, using fluorescence-activated cell sorting analysis, and time-lapse video microscopy. TCTP also protects cells against the proapoptotic effects of tunicamycin and etoposide, but not against those of arsenite. Our results imply that cellular TCTP levels influence sensitivity to apoptosis and that PKR may exert its proapoptotic effects at least in part through downregulation of TCTP via eIF2alpha phosphorylation.

Expression of the interferon-inducible proteins MxA and IFI16 in liver allografts

AIMS

To test the hypothesis that the activation of the interferon (IFN) system pathways might link hepatitis C virus (HCV) recurrence in the liver allograft with acute cellular rejection.

METHODS AND RESULTS

In this retrospective study, allograft biopsy specimens from 28 adult patients (14 HCV+ and 14 HCV-) who had undergone their first liver transplantation were analysed. Eleven biopsy specimens showed acute cellular rejection (Banff rejection activity index score > or =3). Specimens were immunostained for two IFN-inducible proteins, MxA and IFI16, and for CD45. The predominant MxA reactivity pattern was hepatocytic, whereas IFI16 was expressed in both the hepatocellular and inflammatory compartments. Moderate to strong MxA expression in hepatocytes was associated positively with rejection score (P < 0.01), donor's age < or =45 years (P < 0.05) and aspartate aminotransferase levels >40 U/l on the day of biopsy (P < 0.05), and inversely with infiltration of portal triads by IFI16+/CD45+ cells (P < 0.005) and time to progression beyond Ishak stage 2 of recurrent hepatitis C (P < 0.01). On multivariate analysis, MxA expression in hepatocytes was independently associated with allograft rejection and donor's age.

CONCLUSIONS

Acute allograft rejection and recurrence of HCV infection in the liver allograft appear to intersect in the IFN system pathways.

InvAluable junk: the cellular impact and function of Alu and B2 RNAs

The short interspersed elements (SINEs) Alu and B2 are retrotransposons that litter the human and mouse genomes, respectively. Given their abundance, the manner in which these elements impact the host genome and what their biological functions might be is of significant interest. Finding that Alu and B2 SINEs are transcribed, both as distinct RNA polymerase III transcripts and as part of RNA polymerase II transcripts, and that these SINE encoded RNAs indeed have biological functions has refuted the historical notion that SINEs are merely "junk DNA." This article reviews currently known cellular functions of both RNA polymerase II and RNA polymerase III transcribed Alu and B2 RNAs. These RNAs, in different forms, control gene expression by participating in processes as diverse as mRNA transcriptional control, A-to-I editing, nuclear retention, and alternative splicing. Future studies will likely reveal additional contributions of Alu and B2 RNAs as regulators of gene expression.

Activation of the Antiviral Kinase PKR and Viral Countermeasures

The interferon-induced double-stranded (ds)RNA-dependent protein kinase (PKR) limits viral replication by an eIF2α-mediated block of translation. Although many negative-strand RNA viruses activate PKR, the responsible RNAs have long remained elusive, as dsRNA, the canonical activator of PKR, has not been detected in cells infected with such viruses. In this review we focus on the activating RNA molecules of different virus families, in particular the negative-strand RNA viruses. We discuss the recently identified non-canonical activators 5′-triphosphate RNA and the vRNP of influenza virus and give an update on strategies of selected RNA and DNA viruses to prevent activation of PKR.

Dual functions of Rift Valley fever virus NSs protein: inhibition of host mRNA transcription and post-transcriptional downregulation of protein kinase PKR

Rift Valley fever virus (RVFV), which belongs to the genus Phlebovirus, family Bunyaviridae, is a negative-stranded RNA virus carrying a single-stranded, tripartite RNA genome. RVFV is an important zoonotic pathogen transmitted by mosquitoes and causes large outbreaks among ruminants and humans in Africa and the Arabian Peninsula. Human patients develop an acute febrile illness, followed by a fatal hemorrhagic fever, encephalitis, or ocular diseases. A viral nonstructural protein, NSs, is a major viral virulence factor. Past studies showed that NSs suppresses the transcription of host mRNAs, including interferon-beta mRNAs. Here we demonstrated that the NSs protein induced post-transcriptional downregulation of dsRNA-dependent protein kinase (PKR), to prevent phosphorylation of eIF2alpha and promoted viral translation in infected cells. These two biological activities of the NSs most probably have a synergistic effect in suppressing host innate immune functions and facilitate efficient viral replication in infected mammalian hosts.

Sequence-non-specific effects of RNA interference triggers and microRNA regulators

RNA reagents of diverse lengths and structures, unmodified or containing various chemical modifications are powerful tools of RNA interference and microRNA technologies. These reagents which are either delivered to cells using appropriate carriers or are expressed in cells from suitable vectors often cause unintended sequence-non-specific immune responses besides triggering intended sequence-specific silencing effects. This article reviews the present state of knowledge regarding the cellular sensors of foreign RNA, the signaling pathways these sensors mobilize and shows which specific features of the RNA reagents set the responsive systems on alert. The representative examples of toxic effects caused in the investigated cell lines and tissues by the RNAs of specific types and structures are collected and may be instructive for further studies of sequence-non-specific responses to foreign RNA in human cells.

Dynamic refolding of IFN-gamma mRNA enables it to function as PKR activator and translation template

Interferon-gamma mRNA activates the RNA-dependent protein kinase PKR, which in turn strongly attenuates translation of interferon-gamma mRNA. Unlike riboswitches restricted to noncoding regions, the interferon-gamma RNA domain that activates PKR comprises the 5' UTR and 26 translated codons. Extensive interferon-gamma coding sequence is thus dedicated to activating PKR and blocking interferon-gamma synthesis. This implies that the PKR activator is disrupted by ribosomes during translation initiation and must refold promptly to restore PKR activation. The activator structure harbors an essential kink-turn, probably to allow formation of a pseudoknot that is critical for PKR activation. Three indispensable short helices, bordered by orientation-sensitive base pairs, align with the pseudoknot stem, generating RNA helix of sufficient length to activate PKR. Through gain-of-function mutations, we show that the RNA activator can adopt alternative conformations that activate PKR. This flexibility promotes efficient refolding of interferon-gamma mRNA, which is necessary for its dual function as translation template and activator of PKR, and which thus prevents overexpression of this inflammatory cytokine.

Biochemical analysis of PKR activation by PACT

Many extracellular stresses cause inhibition of translation initiation by triggering phosphorylation of the initiation factor, eIF-2alpha. A major protein kinase responsible for this phosphorylation is PKR, a latent kinase which itself needs to be activated by autophosphorylation. In stressed cells, this activation occurs when PACT, a PKR-binding protein, is phosphorylated and activates PKR. We have previously demonstrated that the presence of specific residues in domain 3 of PACT is necessary for its ability to activate PKR in vivo. Here, we analyze the biochemical properties of the inactive PACT mutants by assessing their ability to bind and activate PKR in vitro. Among the essential residues, two serines need to be phosphorylated in vivo for PACT's ability to activate PKR. We substituted those serines with aspartic acids, mimics of phosphoserines, and investigated the properties of the corresponding mutant PACTs. In vitro, they activate PKR more efficiently because they bind to PKR more tightly. These results indicate that stress-induced phosphorylation of specific serine residues in domain 3 of PACT increases its affinity for PKR, which leads to better activation of PKR and resultant eIF-2alpha phosphorylation.

Adenosine deaminase ADAR1 increases gene expression at the translational level by decreasing protein kinase PKR-dependent eIF-2alpha phosphorylation

ADAR1 (adenosine deaminase acting on RNA) catalyzes the deamination of adenosine to inosine on RNA substrates with double-stranded character. Here, we show that coexpression of ADAR1 in mammalian cells markedly increases plasmid-based gene expression in transfected cells. The enhanced expression was independent of the nature of the promoter (viral and cellular) used to drive gene expression, of the protein reporter (luciferase and RRP) tested, and of the human cell line examined (293T and HeLa). Exogenous protein levels were increased by approximately 20-fold to approximately 50-fold when ADAR1 was coexpressed, whereas RNA transcript levels changed by less than 2-fold. The activation of PKR (protein kinase regulated by RNA) protein kinase and the phosphorylation of translation initiation factor eIF-2alpha seen following plasmid DNA transfection were both greatly reduced in ADAR1-transfected cells. Stable knockdown of the PKR kinase increased reporter gene expression in the absence, but not in the presence, of ADAR1 coexpression. Both size forms of ADAR1-the p150-inducible form and the p110-like constitutive form-enhanced plasmid-based gene expression. Taken together, these results indicate that the ADAR1 deaminase increases exogenous gene expression at the translational level by decreasing PKR-dependent eIF-2alpha phosphorylation.

Modified vaccinia virus Ankara can activate NF-kappaB transcription factors through a double-stranded RNA-activated protein kinase (PKR)-dependent pathway during the early phase of virus replication

Modified vaccinia virus Ankara (MVA), which is a promising replication-defective vaccine vector, is unusual among the orthopoxviruses in activating NF-kappaB transcription factors in cells of several types. In human embryonic kidney (HEK 293T) cells, the MVA-induced depletion of IkappaBalpha required to activate NF-kappaB is inhibited by UV-inactivation of the virus, and begins before viral DNA replication. In HEK 293T, CHO, or RK13 cells, expression of the cowpox virus CP77 early gene, or the vaccinia virus K1L early gene suppresses MVA-induced IkappaBalpha depletion. In mouse embryonic fibroblasts (MEFs), MVA induction of IkappaBalpha depletion is dependent on the expression of mouse or human double-stranded RNA-activated protein kinase (PKR). These results demonstrate that events during the early phase of MVA replication can induce PKR-mediated processes contributing both to the activation of NF-kappaB signaling, and to processes that may restrict viral replication. This property may contribute to the efficacy of this vaccine virus.

PKR activity is required for acute leukemic cell maintenance and growth: a role for PKR-mediated phosphatase activity to regulate GSK-3 phosphorylation

Recent reports demonstrate that PKR is constitutively active in a variety of tumors and is required for tumor maintenance and growth. Here we report acute leukemia cell lines contain elevated levels of p-T451 PKR and PKR activity as compared to normal controls. Inhibition of PKR with a specific inhibitor, as well as overexpression of a dominant-negative PKR, inhibited cell proliferation and induced cell death. Interestingly, PKR inhibition using the specific inhibitor resulted in a time-dependent augmentation of AKT S473 and GSK-3alpha S21 phosphorylation, which was confirmed in patient samples. Increased phosphorylation of AKT and GSK-3alpha was not dependent on PI3K activity. PKR inhibition augmented levels of p-S473 AKT and p-S21/9 GSK-3alpha/beta in the presence of the PI3K inhibitor, LY294002, but was unable to augment GSK-3alpha or beta phosphorylation in the presence of the AKT inhibitor, A443654. Pre-treatment with the PKR inhibitor blocked the ability of A443654 and LY294002 to promote phosphorylation of eIF2alpha, indicating the mechanism leading to AKT phosphorylation and activation did not require eIF2alpha phosphorylation. The effects of PKR inhibition on AKT and GSK-3 phosphorylation were found to be, in part, PP2A-dependent. These data indicate that, in acute leukemia cell lines, constitutive basal activity of PKR is required for leukemic cell homeostasis and growth and functions as a negative regulator of AKT, thereby increasing the pool of potentially active GSK-3.

Cytoplasmic nucleic acid sensors in antiviral immunity

The innate immune system uses pattern recognition receptors (PRRs) to sense invading microbes and initiate a rapid protective response. PRRs bind and are activated by structural motifs, such as nucleic acids or bacterial and fungal cell wall components, collectively known as pathogen-associated molecular patterns. PRRs that recognize pathogen-derived nucleic acids are present in vesicular compartments and in the cytosol of most cell types. Here, we review recent studies of these cytosolic sensors, focusing on the nature of the ligands for DNA-dependent activator of interferon (DAI)-regulatory factors, absent in melanoma 2 (AIM2), and the retinoic acid-inducible gene I-like helicase (RLH) family of receptors, the basis of ligand recognition and the signaling pathways triggered by the activation of these receptors. An increased understanding of these molecular aspects of innate immunity will guide the development of novel antiviral therapeutics.

Crosstalk between components of the innate immune system: promoting anti-microbial defenses and avoiding immunopathologies

Because it reaches full functional efficacy rapidly upon encounter with a pathogen, the innate immune system is considered as the first line of defense against infections. The sensing of microbes or of transformed or infected cells, through innate immune recognition receptors (referred to as activating I2R2), initiates pro-inflammatory responses and innate immune effector functions. Other I2R2 with inhibitory properties bind self-ligands constitutively expressed in host. However, this dichotomy in the recognition of foreign or induced self versus constitutive self by I2R2 is not always respected in certain non-infectious conditions reminiscent of immunopathologies. In this review, we discuss that immune mechanisms have evolved to avoid inappropriate inflammatory disorders in individuals. Molecular crossregulation exists between components of I2R2 signaling pathways, and intricate interactions between cells from both innate and adaptive immune systems set the bases of controlled immune responses. We also pinpoint that, like T or B cells, some cells of the innate immune system must go through education processes to prevent autoreactivity. In addition, we illustrate how gene expression profiling of immune cell types is a useful tool to find functional homologies between cell subsets of different species and to speculate about unidentified functions of these cells in the responses to pathogen infections.

ADAR1 interacts with PKR during human immunodeficiency virus infection of lymphocytes and contributes to viral replication

The interferon-induced protein kinase RNA activated (PKR) is activated after virus infection. This activation is transient during the human immunodeficiency virus type 1 (HIV-1) infection of lymphocytes, and the protein is not activated at the peak of infection. We observed that interferon-induced adenosine deaminase acting on RNA 1-p150 (ADAR1-p150) and ADAR1-p110 expression increases while the virus replicates actively. Furthermore, both forms of ADAR1 show enhanced interactions with PKR at the peak of HIV infection, suggesting a role for this protein in the regulation of PKR activation. We observed that ADAR1-p150, as previously shown for the TAR RNA binding protein (TRBP), reverses the PKR inhibition of HIV expression and production in HEK 293T cells. This activity requires the Z-DNA binding motif and the three double-stranded RNA binding domains but not the catalytic domain. In astrocytic cells, ADAR1-p150 increased HIV expression and production to an extent similar to that of TRBP. Small interfering RNAs against ADAR1-p150 moderately decreased HIV production. These results indicate that two interferon-induced proteins, ADAR1 and PKR, have antagonistic functions on HIV production. They suggest that ADAR1 and TRBP belong to a multiprotein complex that inhibits PKR during the HIV infection of lymphocytes.

PKR, the double stranded RNA-dependent protein kinase as a critical target in Alzheimer's disease

Amyloid β-peptide (Aβ) deposits and neurofibrillary tangles are key hallmarks in Alzheimer's disease (AD). Aβ stimulates many signal transducers involved in the neuronal death. However, many mechanisms remain to be elucidated because no definitive therapy of AD exists. Some studies have focused on the control of translation which involves eIF2 and eIF4E, main eukaryotic factors of initiation. The availability of these factors depends on the activation of the double-stranded RNA-dependent protein kinase (PKR) and the mammalian target of rapamycin (mTOR), respectively. mTOR positively regulates the translation while PKR results in a protein synthesis shutdown. Many studies demonstrated that the PKR signalling pathway is up-regulated in cellular and animal models of AD and in the brain of AD patients. Interestingly, our results showed that phosphorylated PKR and eIF2α levels were significantly increased in lymphocytes of AD patients. These modifications were significantly correlated with cognitive and memory test scores performed in AD patients. On the contrary, the mTOR signalling pathway is down-regulated in cellular and animal models of AD. Recently, we showed that p53, regulated protein in development and DNA damage response 1 and tuberous sclerosis complex 2 could represent molecular links between PKR and mTOR signalling pathways. PKR could be an early biomarker of the neuronal death and a critical target for a therapeutic programme in AD.

Herpesvirus-encoded G protein-coupled receptors as modulators of cellular function

Human herpesviruses (HHVs) are widespread pathogens involved in proliferative diseases, inflammatory conditions, and cardiovascular diseases. During evolution, homologs of human chemokine receptors were integrated into the HHV genomes. In addition to binding endogenous chemokines, these viral G protein-coupled receptors (vGPCRs) have acquired the ability to signal in a constitutive manner. Ligand-induced and ligand-independent and autocrine and paracrine signaling properties of vGPCRs modify the functions of the expressing cells and lead to transformation and escape from immune surveillance. Furthermore, cross-talk or heterodimerization with endogenous chemokine receptors represent other ways for vGPCRs to modify intracellular signaling and cellular functions. As such, these viral receptors seem to play a prominent role during viral pathogenesis and life cycle and thus represent innovative antiviral therapies.

Preclinical evaluation of a replication-deficient intranasal DeltaNS1 H5N1 influenza vaccine

BACKGROUND

We developed a novel intranasal influenza vaccine approach that is based on the construction of replication-deficient vaccine viruses that lack the entire NS1 gene (DeltaNS1 virus). We previously showed that these viruses undergo abortive replication in the respiratory tract of animals. The local release of type I interferons and other cytokines and chemokines in the upper respiratory tract may have a "self-adjuvant effect", in turn increasing vaccine immunogenicity. As a result, DeltaNS1 viruses elicit strong B- and T- cell mediated immune responses.

METHODOLOGY/PRINCIPAL FINDINGS

We applied this technology to the development of a pandemic H5N1 vaccine candidate. The vaccine virus was constructed by reverse genetics in Vero cells, as a 5:3 reassortant, encoding four proteins HA, NA, M1, and M2 of the A/Vietnam/1203/04 virus while the remaining genes were derived from IVR-116. The HA cleavage site was modified in a trypsin dependent manner, serving as the second attenuation factor in addition to the deleted NS1 gene. The vaccine candidate was able to grow in the Vero cells that were cultivated in a serum free medium to titers exceeding 8 log(10) TCID(50)/ml. The vaccine virus was replication deficient in interferon competent cells and did not lead to viral shedding in the vaccinated animals. The studies performed in three animal models confirmed the safety and immunogenicity of the vaccine. Intranasal immunization protected ferrets and mice from being infected with influenza H5 viruses of different clades. In a primate model (Macaca mulatta), one dose of vaccine delivered intranasally was sufficient for the induction of antibodies against homologous A/Vietnam/1203/04 and heterologous A/Indonesia/5/05 H5N1 strains.

CONCLUSION/SIGNIFICANCE

Our findings show that intranasal immunization with the replication deficient H5N1 DeltaNS1 vaccine candidate is sufficient to induce a protective immune response against H5N1 viruses. This approach might be attractive as an alternative to conventional influenza vaccines. Clinical evaluation of DeltaNS1 pandemic and seasonal influenza vaccine candidates are currently in progress.

Influenza B virus ribonucleoprotein is a potent activator of the antiviral kinase PKR

Activation of the latent kinase PKR is a potent innate defense reaction of vertebrate cells towards viral infections, which is triggered by recognition of viral double-stranded (ds) RNA and results in a translational shutdown. A major gap in our understanding of PKR's antiviral properties concerns the nature of the kinase activating molecules expressed by influenza and other viruses with a negative strand RNA genome, as these pathogens produce little or no detectable amounts of dsRNA. Here we systematically investigated PKR activation by influenza B virus and its impact on viral pathogenicity. Biochemical analysis revealed that PKR is activated by viral ribonucleoprotein (vRNP) complexes known to contain single-stranded RNA with a 5′-triphosphate group. Cell biological examination of recombinant viruses showed that the nucleo-cytoplasmic transport of vRNP late in infection is a strong trigger for PKR activation. In addition, our analysis provides a mechanistic explanation for the previously observed suppression of PKR activation by the influenza B virus NS1 protein, which we show here to rely on complex formation between PKR and NS1's dsRNA binding domain. The high significance of this interaction for pathogenicity was revealed by the finding that attenuated influenza viruses expressing dsRNA binding-deficient NS1 proteins were rescued for high replication and virulence in PKR-deficient cells and mice, respectively. Collectively, our study provides new insights into an important antiviral defense mechanism of vertebrates and leads us to suggest a new model of PKR activation by cytosolic vRNP complexes, a model that may also be applicable to other negative strand RNA viruses.

Upon viral infection of vertebrate cells, a vigorous innate defense response is initiated via the recognition of viral double-stranded (ds) RNA by the protein kinase PKR, resulting in the cessation of protein synthesis and subsequent blockage of viral propagation. The activation of PKR's potent antiviral response against influenza and other viruses with a negative strand RNA genome has presented a conundrum, however, as previous attempts failed to detect dsRNA in cells infected with these viruses. Here, we identify genomic RNA within the ribonucleoprotein (RNP) of influenza viruses as a non-canonical activator of the latent kinase PKR. Cell biological examinations revealed that the transfer of viral RNP from the nucleus to the cytoplasm provides a strong stimulus for PKR activation. Moreover, we provide insight into mechanisms of pathogenesis by showing PKR and the NS1 protein of influenza B virus forms a complex in infected cells, which inhibits PKR activation. This interaction seems to be crucial for viral pathogenicity, as a strong attenuation of NS1 mutant viruses was largely rescued in PKR-deficient mice and cells. Taken together, these findings suggest a new model for the induction and inhibition of PKR by influenza virus that may also apply to viruses with a similar genome structure.

PKR acts early in infection to suppress Semliki Forest virus production and strongly enhances the type I interferon response

The double-stranded RNA-activated protein kinase (PKR) is a key regulator of protein translation, interferon (IFN) expression and cell survival. Upon infection of vertebrate cells in continuous culture, the alphavirus Semliki Forest virus (SFV) initiates apoptosis and IFN synthesis. To determine the effect of PKR on SFV infection, we studied the course of infection in wild-type (wt) mice, mice with a genetic deletion of PKR (PKR-/-) and mouse embryo fibroblasts (MEFs) derived from these mice. In MEFs, PKR delayed virus protein synthesis, production of infectious virus and caspase-3-activated cell death and reduced the yield of infectious virus by 90%. Small interfering RNA suppression of PKR levels in NIH-3T3 cells also reduced virus production and apoptosis. In MEFs, PKR was not required for initiation of IFN-beta gene transcription, but contributed strongly to the magnitude of this response. Levels of IFN-beta transcripts in PKR-/- MEFs at 8 h were 80% lower than those in wt MEFs and levels of functional IFN at 24 h were 95% lower. Following infection of wt and PKR-/- mice, SFV4 and SFV A7(74) were avirulent. PKR increased levels of serum IFN and the rate of clearance of infectious virus from the brain. In summary, in response to SFV, PKR exerts an early antiviral effect that delays virus protein production and release of infectious virus and, whilst PKR is not required for induction of apoptosis or activation of the type I IFN response, it strongly augments the type I IFN response and contributes to clearance of infectious virus from the mouse brain.

Vaccinia virus E3 suppresses expression of diverse cytokines through inhibition of the PKR, NF-kappaB, and IRF3 pathways

The vaccinia virus double-stranded RNA binding protein E3 has been demonstrated to inhibit the expression of cytokines, including beta interferon (IFN-beta) and tumor necrosis factor alpha (TNF-alpha). However, few details regarding the molecular mechanisms of this inhibition have been described. Using real-time PCR arrays, we found that E3 suppressed the induction of a diverse array of cytokines representing members of the IFN, interleukin (IL), TNF, and transforming growth factor cytokine families. We discovered that the factor(s) responsible for the induction of IL-6, TNF-alpha, and inhibin beta A (INHBA) was associated with the early and late phases of virus infection. In contrast, the factor(s) which regulates IFN-beta induction was associated with the late phase of replication. We have found that expression of these cytokines can be induced by transfection of cells with RNA isolated from vaccinia virus-infected cells. Moreover, we provide evidence that E3 antagonizes both PKR-dependent and PKR-independent pathways to regulate cytokine expression. PKR-dependent activation of p38 and NF-kappaB was required for vaccinia virus-induced INHBA expression, whereas induction of TNF-alpha required only PKR-dependent NF-kappaB activation. In contrast, induction of IL-6 and IFN-beta was largely PKR independent. IL-6 induction is regulated by NF-kappaB, while IFN-beta induction is mediated by IFN-beta promoter stimulator 1 and IFN regulatory factor 3/NF-kappaB. Collectively, these results indicate that E3 suppresses distinct but interlinked host signaling pathways to inhibit the expression of a diverse array of cytokines.

Neuronal phosphorylated RNA-dependent protein kinase in Creutzfeldt-Jakob disease

The mechanisms of neuronal apoptosis in Creutzfeldt-Jakob disease (CJD) and their relationship to accumulated prion protein (PrP) are unclear. A recent cell culture study showed that intracytoplasmic PrP may induce phosphorylated RNA-dependent protein kinase (PKR(p))-mediated cell stress. The double-stranded RNA protein kinase PKR is a proapoptotic and stress kinase that accumulates in degenerating neurons in Alzheimer disease. To determine whether neuronal apoptosis in human CJD is associated with activation of the PKR(p) signaling pathway, we assessed in situ end labeling and immunocytochemistry for PrP, glial fibrillary acidic protein, CD68, activated caspase 3, and phosphorylated PKR (Thr451) in samples of frontal, occipital, and temporal cortex, striatum, and cerebellum from 6 patients with sporadic CJD and 5 controls. Neuronal immunostaining for activated PKR was found in all CJD cases. The most staining was in nuclei and, in contrast to findings in Alzheimer disease, cytoplasmic labeling was not detected. Both the number and distribution of PKR(p)-positive neurons correlated closely with the extent of neuronal apoptosis, spongiosis, astrocytosis, and microglial activation and with the phenotype and disease severity. There was no correlation with the type, topography, or amount of extracellular PrP deposits. These findings suggest that neuronal apoptosis in human CJD may result from PKR(p)-mediated cell stress and are consistent with recent studies supporting a pathogenic role for intracellular or transmembrane PrP.

Interferon gamma enhances clonal expansion and survival of CD4+ T cells

Interferon-gamma (IFN-gamma) serves numerous functions in the regulation of the immune response. During the early phase of the immune response IFN-gamma is produced by natural killer and natural killer T cells. Although the effects of this cytokine on antigen presenting cells and other cell types are known, its direct role on CD4(+) T cells remains unclear. We demonstrate that CD4(+) T cells exposed to IFN-gamma proliferate more vigorously than the controls in response to signals through the antigen receptor. The increased proliferation of IFN-gamma-treated CD4(+) T cells is not due to enhanced signaling through the antigen receptor, but is accounted for by their increased survival. Our data suggest that enhanced survival of IFN-gamma-treated CD4(+)T cells is independent of signal transducer and activator of transcription 1 (STAT 1), a transcription factor that controls the expression of a variety of IFN-gamma-targeted genes. In addition, we demonstrate that independent of STAT 1, IFN-gamma treatment increases the expression of double-stranded RNA-dependent protein kinase, a kinase involved in regulating protein synthesis. Taken together, our findings suggest a direct role of IFN-gamma on unstimulated CD4(+) T cells that is likely to enhance the advent of adaptive immunity by augmenting their survival during the initiation of the immune response.

Activation of host antiviral RNA-sensing factors necessary for herpes simplex virus type 1-activated transcription of host cell fucosyltransferase genes FUT3, FUT5, and FUT6 and subsequent expression of sLe(x) in virus-infected cells

Herpes simplex virus type 1 (HSV-1) induces expression of a selectin receptor, the carbohydrate epitope sialyl Lewis X (sLe(x)), at the surface of infected cells. The molecular background to this phenomenon is that a viral immediate early RNA interacts with as yet unidentified host factors, eventually resulting in transcription of three dormant host fucosyltransferase genes (FUT3, FUT5, and FUT6), whose gene products are rate-limiting for synthesis of sLe(x). The aim of the present study was to define the immediate targets for the viral RNA in this process. We found that the Protein Kinase R (PKR) inhibitors 2-aminopurine (2-AP) and C16 inhibited FUT3, FUT5, and FUT6 expression as well as HSV-1-induced expression of sLe(x), indicating a primary role of PKR as a viral RNA target. The PKR-dependent activation of the FUT genes seemed neither to involve PKR effects on translation nor to involve NF-kappaB- or JNK-dependent activation. IMD-0354, known as an inhibitor of the NF-kappaB-activating factor IKK-2, induced FUT transcription via a novel IKK-2-independent mechanism, irrespective of whether the cells were virus-infected or not. Altogether, the results suggested that PKR is the primary target for HSV-1 early RNA during induction of FUT3, FUT5, and FUT6, and that the subsequent steps in the transcriptional activation of these host genes involve a hitherto unknown IMD-0354, yet IKK-2-independent, pathway.

Gamma interferon-induced interferon regulatory factor 1-dependent antiviral response inhibits vaccinia virus replication in mouse but not human fibroblasts

Vaccinia virus (VACV) replicates in mouse and human fibroblasts with comparable kinetics and efficiency, yielding similar titers of infectious progeny. Here we demonstrate that gamma interferon (IFN-gamma) but not IFN-alpha or IFN-beta pretreatment of mouse fibroblasts prior to VACV infection induces a long-lasting antiviral state blocking VACV replication. In contrast, high doses of IFN-gamma failed to establish an antiviral state in human fibroblasts. In mouse fibroblasts, IFN-gamma impeded the viral replication cycle at the level of late gene transcription and blocked the multiplication of VACV genomes. The IFN-gamma-induced antiviral state invariably prevented the growth of different VACV strains but was not effective against the replication of ectromelia virus. The IFN-gamma effect required intact IFN-gamma receptor signaling prior to VACV infection through Janus kinase 2 (Jak2) and signal transducer and activator of transcription 1 (STAT1). The permissive state of IFN-gamma-treated human cells was unrelated to the VACV-encoded IFN decoy receptors B8 and B18 and associated with a complete disruption of STAT1 homodimer formation and DNA binding. Unlike human fibroblasts, mouse cells responded with long-lasting STAT1 activation which was preserved after VACV infection. The deletion of the IFN regulatory factor 1 (IRF-1) gene from mouse cells rescued efficient VACV replication, demonstrating that IRF-1 target genes have a critical role in VACV control. These data have implications for the understanding of VACV pathogenesis and identify an incongruent IFN-gamma response between the human host and the mouse model.

Conserved herpesviral kinase promotes viral persistence by inhibiting the IRF-3-mediated type I interferon response

A conserved herpesviral kinase, designated ORF36 in murine gamma-herpesvirus 68 (MHV-68), plays multiple vital roles in the viral life cycle. Here, we show by screening mutant viruses that ORF36 counteracts the antiviral type I interferon (IFN) response. ORF36 specifically binds to the activated form of interferon regulatory factor 3 (IRF-3) in the nucleus, inhibiting IRF-3 interaction with the cotranscriptional activator CBP and thereby suppressing the recruitment of RNA polymerase II to the interferon beta promoter. The anti-IFN function of ORF36 is conserved among herpesvirus subfamilies, although the conserved kinase activity is not absolutely required for this function. MHV-68 lacking ORF36 induces a greater interferon response and is attenuated in vitro and in vivo, where acute viral infection in the lung and latency in the spleen are compromised. Our data suggest that herpesviruses have evolved within their conserved kinase an anti-IFN activity critical for evasion of host immunity and for persistence.

IL-10-dependent S100A8 gene induction in monocytes/macrophages by double-stranded RNA

The S100 calcium-binding proteins S100A8 and S100A9 are elevated systemically in patients with viral infections. The S100A8-S100A9 complex facilitated viral replication in human CD4(+) T lymphocytes latently infected with HIV-1- and S100A8-induced HIV-1 transcriptional activity. Mechanisms inducing the S100 genes and the potential source of these proteins following viral activation are unknown. In this study, we show that S100A8 was induced in murine macrophages, and S100A8 and S100A9 in human monocytes and macrophages, by polyinosinic:polycytidylic acid, a dsRNA mimetic. Induction was at the transcriptional level and was IL-10 dependent. Similar to LPS-induced S100A8, induction by dsRNA was dependent on p38 and ERK MAPK. Protein kinase R (PKR) mediates antiviral defense and participates in MyD88-dependent/independent signaling triggered by TLR4 or TLR3. Like IL-10, S100 induction by polyinosinic:polycytidylic acid and by LPS was inhibited by the specific PKR inhibitor 2-aminopurine, indicating a novel IL-10, PKR-dependent pathway. Other mediators such as IFN-beta, which synergized with dsRNA, may also be involved. C/EBPbeta bound the defined promoter region in response to dsRNA. S100A8 was expressed in lungs of mice infected with influenza virus and was maximal at day 8 with strong immunoreactivity in epithelial cells lining the airways and in mononuclear cells and declined early in the recovery phase, implying down-regulation by mediator(s) up-regulated during resolution of the infection. IL-10 is implicated in viral persistence. Since S100A8/S100A9 levels are likely to be maintained in conditions where IL-10 is raised, these proteins may contribute to viral persistence in patients infected by some RNA viruses.

Rift Valley fever virus NSs protein promotes post-transcriptional downregulation of protein kinase PKR and inhibits eIF2alpha phosphorylation

Rift Valley fever virus (RVFV) (genus Phlebovirus, family Bunyaviridae) is a negative-stranded RNA virus with a tripartite genome. RVFV is transmitted by mosquitoes and causes fever and severe hemorrhagic illness among humans, and fever and high rates of abortions in livestock. A nonstructural RVFV NSs protein inhibits the transcription of host mRNAs, including interferon-β mRNA, and is a major virulence factor. The present study explored a novel function of the RVFV NSs protein by testing the replication of RVFV lacking the NSs gene in the presence of actinomycin D (ActD) or α-amanitin, both of which served as a surrogate of the host mRNA synthesis suppression function of the NSs. In the presence of the host-transcriptional inhibitors, the replication of RVFV lacking the NSs protein, but not that carrying NSs, induced double-stranded RNA-dependent protein kinase (PKR)–mediated eukaryotic initiation factor (eIF)2α phosphorylation, leading to the suppression of host and viral protein translation. RVFV NSs promoted post-transcriptional downregulation of PKR early in the course of the infection and suppressed the phosphorylated eIF2α accumulation. These data suggested that a combination of RVFV replication and NSs-induced host transcriptional suppression induces PKR-mediated eIF2α phosphorylation, while the NSs facilitates efficient viral translation by downregulating PKR and inhibiting PKR-mediated eIF2α phosphorylation. Thus, the two distinct functions of the NSs, i.e., the suppression of host transcription, including that of type I interferon mRNAs, and the downregulation of PKR, work together to prevent host innate antiviral functions, allowing efficient replication and survival of RVFV in infected mammalian hosts.

The mosquito-borne bunyavirus Rift Valley fever virus (RVFV) devastates both humans and domestic animals; it causes abortions in ruminants and complications such as hemorrhage, encephalitis, or retinal vasculitis in humans. A major RVFV virulence factor, NSs, disables host cell mRNA synthesis. Here we describe our new evidence that showed NSs working in a second way; in addition to inhibiting host cell transcription, NSs kept translation active in infected cells. It is well-established that activated protein kinase PKR phosphorylates a translation factor, eIF2α, and then this phosphorylated eIF2α suppresses translation. We found that NSs decreased PKR abundance and prevented eIF2α phosphorylation in infected cells, allowing efficient viral translation and replication. In contrast, when cells were infected with an RVFV mutant lacking NSs in the presence of transcriptional inhibitors that mimic the transcription inhibition function of NSs, the PKR reduction did not occur and phoshorylated eIF2α was accumulated, resulting in the inhibition of virus gene expression and replication. Thus, NSs functions in two ways to help RVFV replicate in mammalian hosts: its newly identified PKR downregulation function secures efficient viral translation, and its host transcription inhibition function suppresses the expression of host innate antiviral functions.

Protein components of the microRNA pathway and human diseases

MicroRNAs (miRNAs) are key regulators of messenger RNA (mRNA) translation known to be involved in a wide variety of cellular processes. In fact, their individual importance is reflected in the diseases that may arise upon the loss, mutation or dysfunction of specific miRNAs. It has been appreciated only recently that diseases may also develop when the protein components of the miRNA machinery itself are affected. The core enzymes of the major protein complexes involved in miRNA biogenesis and function, such as the ribonucleases III (RNases III) Drosha and Dicer as well as Argonaute 2 (Ago2), appear to be essential. However, the accessory proteins of the miRNA pathway, such as the DiGeorge syndrome critical region gene 8 (DGCR8) protein, Exportin-5 (Exp-5), TAR RNA binding protein (TRBP) and fragile X mental retardation protein (FMRP), are each related, in various ways, to specific genetic diseases.

Interferons and viral infections

Interferons represent a family of cytokines, which is of central importance in the innate immune response to virus infections. All interferons act as secreted ligands of specific cell surface receptors, eliciting the transcription of hundreds of interferon-stimulated genes whose protein products have antiviral activity, as well as antimicrobial, antiproliferative/antitumor, and immunomodulatory effects. Expression of type I and III interferons is induced in virtually all cell types upon recognition of viral molecular patterns, especially nucleic acids, by cytoplasmic and endosomal receptors, whereas type II interferon is induced by cytokines such as IL-12, and its expression is restricted to immune cells such as T cells and NK cells. The effectiveness of the interferon system in counteracting viral infections is reflected by the multitude of inhibitors of interferon induction or interferon action that are encoded by many viruses, preventing their eradication and resulting in the continued coexistence of viruses and vertebrates. The unique biological functions of interferons have led to their therapeutic use in the treatment of diseases such as hepatitis, multiple sclerosis, and certain leukemias.

Modulation of innate immune signalling pathways by viral proteins

In recent years an explosion of information on the various strategies viruses employ to penetrate and hijack the host cell has led to an increased understanding of both viruses themselves and the host immune response. Despite their simplicity viruses have evolved a number of strategies to not only evade the host immune response but also modulate immune signalling to favour their replication and survival within the cell. The innate immune response provides the host with an early reaction against viruses. This response relies heavily upon the recognition of pathogen-associated molecular patterns (PAMPs) by a number of host pattern recognition receptors (PRRs), leading to activation of innate signalling pathways and altered gene expression. In this chapter we outline the signalling pathways that respond to viral infection and the various methods that viruses utilize to evade detection and modulate the innate immune response to favour their survival.

Double-stranded RNA-dependent protein kinase-dependent apoptosis induction by a novel small compound

The interferon-induced, double-stranded RNA-dependent protein kinase (PKR) can play critical roles in inhibiting virus replication and inducing apoptosis. To develop new agents that may inhibit viral replication or induce apoptosis in cancer cells via the PKR signaling pathway, we screened a chemical library for compounds that have differential cytotoxic effects on wild-type [mouse embryonic fibroblast (MEF)/PKR(+/+)] and PKR-knockout [MEF/PKR(-/-)] mouse embryonic fibroblast cells. We identified a synthetic compound, BEPP [1H-benzimidazole1-ethanol,2,3-dihydro-2-imino-a-(phenoxymethyl)-3-(phenylmethyl)-,monohydrochloride], that induces a cytotoxic effect more effectively in MEF/PKR(+/+) cells than in MEF/PKR(-/-) cells. BEPP also relatively effectively inhibited the growth of a human lung cancer cell line overexpressing PKR, compared with other cancer cell lines. In sensitive cells, BEPP induced apoptosis with activation of caspase-3. Treatment with BEPP led to increased phosphorylation of PKR and eIF2alpha, increased expression of BAX, and decreased expression of Bcl-2. BEPP-induced apoptosis was PKR dependent and was blocked by the adenovector expressing the dominant-negative PKR. Furthermore, pretreatment of HeLa cells at a noncytotoxic dose of BEPP effectively inhibited Vaccinia virus replication. Together, our results suggest that BEPP and its analogs may induce PKR-dependent apoptosis and inhibition of viral replication and that they can be a potential anticancer or anti-virus agent.

The RNA-activated protein kinase enhances the induction of interferon-beta and apoptosis mediated by cytoplasmic RNA sensors

Detection of foreign RNA by the innate immune system can trigger the induction of type I interferon (IFN) and apoptosis. Important antiviral defense pathways that result in type I IFN production following the recognition of foreign double-stranded RNA (dsRNA) include the RIG-I family helicases and IPS-1 adaptor cytosolic pathway and the Toll-like receptor 3 and TIR domain-containing adaptor-inducing IFN-beta (TRIF) adaptor membrane-associated pathway, both of which activate IFN regulatory factor 3 (IRF3). In addition to triggering an innate immune response, dsRNAs are widely used to mediate gene-selective silencing in mammalian cells by the RNA interference pathway. We investigated the ability of short interfering RNAs, including T7 phage polymerase-synthesized RNA (PRNA), which like some viral RNAs contains a 5'-triphosphate, to selectively silence gene expression and to cause induction of IFN-beta and apoptosis. We found that PRNA-mediated gene silencing and associated nonspecific pro-apoptotic and IFN-inducing effects were dependent on the cell line and RNA length. Double-stranded PRNAs 50 nucleotides long as well as polyinosinic-polycytidylic acid activated the RNA-dependent protein kinase (PKR) and induced significant levels of IFN-beta and apoptosis, whereas shorter PRNAs and chemically synthesized dsRNAs did not. Effector caspase activation and apoptosis following RNA transfection was enhanced by pretreatment with IFN, and removal of the 5'-phosphate from PRNAs decreased induction of both IFN-beta and apoptosis. PKR, in addition to IPS-1 and IRF3 but not TRIF, was required for maximal type I IFN-beta induction and the induction of apoptosis by both transfected PRNAs and polyinosinic-polycytidylic acid.

Essential role of PACT-mediated PKR activation in tunicamycin-induced apoptosis

Cellular stresses such as disruption of calcium homeostasis, inhibition of protein glycosylation, and reduction of disulfide bonds result in accumulation of misfolded proteins in the endoplasmic reticulum (ER) and lead to cell death by apoptosis. Tunicamycin, which is an inhibitor of protein glycosylation, induces ER stress and apoptosis. In this study, we examined the involvement of double-stranded RNA (dsRNA)-activated protein kinase (PKR) and its protein activator PACT in tunicamycin-induced apoptosis. We demonstrate for the first time that PACT is phosphorylated in response to tunicamycin and is responsible for PKR activation by direct interaction. Furthermore, PACT-induced PKR activation is essential for tunicamycin-induced apoptosis, since PACT as well as PKR null cells are markedly resistant to tunicamycin and show defective eIF2alpha phosphorylation and C/EBP homologous protein (CHOP, also known as GADD153) induction especially at low concentrations of tunicamycin. Reconstitution of PKR and PACT expression in the null cells renders them sensitive to tunicamycin, thus demonstrating that PACT-induced PKR activation plays an essential function in induction of apoptosis.

Activation of pattern recognition receptor-mediated innate immunity inhibits the replication of hepatitis B virus in human hepatocyte-derived cells

Recognition of virus infections by pattern recognition receptors (PRRs), such as Toll-like receptors (TLRs), retinoic acid-inducible gene I (RIG-I), and melanoma differentiation associated gene 5 (MDA5), activates signaling pathways, leading to the induction of inflammatory cytokines that limit viral replication. To determine the effects of PRR-mediated innate immune response on hepatitis B virus (HBV) replication, a 1.3mer HBV genome was cotransfected into HepG2 or Huh7 cells with plasmid expressing TLR adaptors, myeloid differentiation primary response gene 88 (MyD88), and TIR-domain-containing adaptor-inducing beta interferon (TRIF), or RIG-I/MDA5 adaptor, interferon promoter stimulator 1 (IPS-1). The results showed that expressing each of the three adaptors dramatically reduced the levels of HBV mRNA and DNA in both HepG2 and Huh7 cells. However, HBV replication was not significantly affected by treatment of HBV genome-transfected cells with culture media harvested from cells transfected with each of the three adaptors, indicating that the adaptor-induced antiviral response was predominantly mediated by intracellular factors rather than by secreted cytokines. Analyses of involved signaling pathways revealed that activation of NF-kappaB is required for all three adaptors to elicit antiviral response in both HepG2 and Huh7 cells. However, activation of interferon regulatory factor 3 is only essential for induction of antiviral response by IPS-1 in Huh7 cells, but not in HepG2 cells. Furthermore, our results suggest that besides NF-kappaB, additional signaling pathway(s) are required for TRIF to induce a maximum antiviral response against HBV. Knowing the molecular mechanisms by which PRR-mediated innate defense responses control HBV infections could potentially lead to the development of novel therapeutics that evoke the host cellular innate antiviral response to control HBV infections.

PKR regulates B56(alpha)-mediated BCL2 phosphatase activity in acute lymphoblastic leukemia-derived REH cells

Protein phosphatase 2A (PP2A) is a heterotrimer comprising catalytic, scaffold, and regulatory (B) subunits. There are at least 21 B subunit family members. Thus PP2A is actually a family of enzymes defined by which B subunit is used. The B56 family member B56alpha is a phosphoprotein that regulates dephosphorylation of BCL2. The stress kinase PKR has been shown to phosphorylate B56alpha at serine 28 in vitro, but it has been unclear how PKR might regulate the BCL2 phosphatase. In the present study, PKR regulation of B56alpha in REH cells was examined, because these cells exhibit robust BCL2 phosphatase activity. PKR was found to be basally active in REH cells as would be predicted if the kinase supports B56alpha-mediated dephosphorylation of BCL2. Suppression of PKR promoted BCL2 phosphorylation with concomitant loss of B56alpha phosphorylation at serine 28 and inhibition of mitochondrial PP2A activity. PKR supports stress signaling in REH cells, as suppression of PKR promoted chemoresistance to etoposide. Suppression of PKR promoted B56alpha proteolysis, which could be blocked by a proteasome inhibitor. However, the mechanism by which PKR supports B56alpha protein does not involve PKR-mediated phosphorylation of the B subunit at serine 28 but may involve eIF2alpha activation of AKT. Phosphorylation of serine 28 by PKR promotes mitochondrial localization of B56alpha, because wild-type but not mutant S28A B56alpha promoted mitochondrial PP2A activity. Cells expressing wild-type B56alpha but not S28A B56alpha were sensitized to etoposide. These results suggest that PKR regulates B56alpha-mediated PP2A signaling in REH cells.

The multifunctional NS1 protein of influenza A viruses

The non-structural (NS1) protein of influenza A viruses is a non-essential virulence factor that has multiple accessory functions during viral infection. In recent years, the major role ascribed to NS1 has been its inhibition of host immune responses, especially the limitation of both interferon (IFN) production and the antiviral effects of IFN-induced proteins, such as dsRNA-dependent protein kinase R (PKR) and 2'5'-oligoadenylate synthetase (OAS)/RNase L. However, it is clear that NS1 also acts directly to modulate other important aspects of the virus replication cycle, including viral RNA replication, viral protein synthesis, and general host-cell physiology. Here, we review the current literature on this remarkably multifunctional viral protein. In the first part of this article, we summarize the basic biochemistry of NS1, in particular its synthesis, structure, and intracellular localization. We then discuss the various roles NS1 has in regulating viral replication mechanisms, host innate/adaptive immune responses, and cellular signalling pathways. We focus on the NS1-RNA and NS1-protein interactions that are fundamental to these processes, and highlight apparent strain-specific ways in which different NS1 proteins may act. In this regard, the contributions of certain NS1 functions to the pathogenicity of human and animal influenza A viruses are also discussed. Finally, we outline practical applications that future studies on NS1 may lead to, including the rational design and manufacture of influenza vaccines, the development of novel antiviral drugs, and the use of oncolytic influenza A viruses as potential anti-cancer agents.

Pharmacotherapy of chronic viral hepatitis and hepatocellular carcinoma

BACKGROUND

Hepatitis B virus (HBV) is a major etiologic agent of chronic liver disease (CLD) and hepatocellular carcinoma. Drugs have been developed and shown to be effective against HBV replication. These treatments are often associated with the resolution of CLD. However, they are too expensive, not well tolerated, and result in the development of resistance when given as mono or salvage therapies. In addition, most of these drugs target only the virus polymerase.

OBJECTIVE

To revitalize the field, drugs with other targets and combination therapies need to be developed.

METHODS

Major advances in HBV and liver cancer drug development over the past decade, focusing on Phase III trials and FDA-approved compounds, are presented.

RESULTS/DISCUSSION

A number of potent nucleoside/nucleotide analogs are now available for treatment, but for the long-term management of CLD, the development of combination therapies will probably be required. Development of compounds with new virus targets will enhance the utility of combination therapies. Development of compounds to host targets altered prior to or after the development of liver cancer, as demonstrated by sorafenib, need to be developed. The goal is to devise drug cocktails that will yield sustained virus responses and halt disease progression and tumor development.