PKR Activation Research Articles

\u03b2-adrenergic receptor inhibition enhances oncolytic herpes virus propagation through STAT3 activation in gastric cancer

BackgroundOncolytic viruses (OVs) are considered a promising therapeutic alternative for cancer. However, OVs could activate the host innate immunity, then impair the viral propagation in tumor cells. In this study, we explored the effect of propranolol, a non-selective β-blocker, on the antitumor efficacy of T1012G virus in gastric cancer models.MethodsThe proliferation of gastric cancer cells treated with monotherapy or combination treatment was detected by CCK8 cell proliferation assay. The effect of propranolol was further evaluated by in vitro viral replication assays. In vivo tumor xenograft experiments were used to observe the effect of combination therapy on gastric cancer growth in mice. The expression levels of viral proteins and interferon responsive genes were detected in the gastric cancer cell lines treated with combined treatment by western blot. The impact of propranolol on IFN-α/β-mediated inhibition of viral propagation and the expression of antiviral gene PKR was detected by viral replication assays and western blot.ResultsCell viability assay detected a 97.9% decrease of T1012G IC50 in HGC-27 when it was pretreated with propranolol along with a sevenfold increase of virus titers compared with T1012G only group (P < 0.001). Moreover, propranolol pretreatment caused sustained tumor regression (335.3 ± 36.92 mm3 vs. 1118 ± 210.0 mm3, P < 0.01) and enhanced the viral propagation (fourfold increase, P < 0.01) compared with T1012G only group. Propranolol pretreatment significantly enhanced the p-STAT3 (2.9-fold, P < 0.05) and suppressed p-PKR (65.94% ± 10.11%, P < 0.05) compared with T1012G only group. In addition, propranolol could counteract IFN-α/β-mediated inhibition of viral propagation (compared with IFNα: 5.1-fold, P < 0.001; IFNβ: 4.6-fold, P < 0.01) or enhancement of PKR activation (IFNα: 92.57% ± 1.77%, P < 0.001, IFNβ: 99.34% ± 0.13% decrease, P < 0.001).ConclusionsIn summary, β-blocker pretreatment could improve the propagation and therapeutic efficacy of T1012G in human gastric cancer by regulating STAT3-PKR signaling cascade, even in the presence of type I IFNs. These data support new strategies of improving the efficacy of OVs in gastric cancer.

ADAR1 limits stress granule formation through both translation-dependent and translation-independent mechanisms.

Stress granules (SGs) are cytoplasmic assemblies of RNA and protein that form when translation is repressed during the integrated stress response. SGs assemble from the combination of RNA-RNA, RNA-protein and protein-protein interactions between messenger ribonucleoprotein complexes (mRNPs). The protein adenosine deaminase acting on RNA 1 (ADAR1, also known as ADAR) recognizes and modifies double-stranded RNAs (dsRNAs) within cells to prevent an aberrant innate immune response. ADAR1 localizes to SGs, and since RNA-RNA interactions contribute to SG assembly and dsRNA induces SGs, we examined how ADAR1 affects SG formation. First, we demonstrate that ADAR1 depletion triggers SGs by allowing endogenous dsRNA to activate the integrated stress response through activation of PKR (also known as EIF2AK2) and translation repression. However, we also show that ADAR1 limits SG formation independently of translation inhibition. ADAR1 repression of SGs is independent of deaminase activity but is dependent on dsRNA-binding activity, suggesting a model where ADAR1 binding limits RNA-RNA and/or RNA-protein interactions necessary for recruitment to SGs. Given that ADAR1 expression is induced during viral infection, these findings have implications for the role of ADAR1 in the antiviral response. This article has an associated First Person interview with the first author of the paper.

PKR deficiency alleviates pulmonary hypertension via inducing inflammasome adaptor ASC inactivation

Pulmonary hypertension is a progressive fatal disease that currently has no specific therapeutic approaches. In this study, dsRNA-dependent protein kinase (PKR) was considered a candidate molecule in pulmonary hypertension. We demonstrated that PKR is activated in the endothelium of experimental pulmonary hypertension models. Deletion of PKR or treatment with the PKR activation inhibitor C16 inhibited the development of pulmonary hypertension. To explore the mechanism of PKR in pulmonary hypertension, we detected its downstream signaling and found that PKR knockout represses apoptosis-associated speck-like protein containing CARD (ASC) activation to inhibit high mobility group box 1 (HMGB1) and interleukin-1 beta release. To further explore whether ASC mediates the pro-pulmonary hypertension role of PKR, we used ASC deletion mice and found that ASC deletion inhibits the development of pulmonary hypertension and the release of HMGB1 and interleukin-1 beta. Furthermore, we co-cultured pulmonary arterial endothelial cells (PAECs) and pulmonary arterial smooth muscle cells (PASMCs) and found that endothelial PKR promotes PASMCs proliferation through the release of HMGB1 and interleukin-1 beta. In conclusion, these data indicate that endothelial PKR promotes the excessive proliferation of PASMCs by inducing ASC activation to release HMGB1 and interleukin-1 beta, which lead to the development of pulmonary hypertension. Our study will provide a novel insight that PKR is a potential target in the future treatment of pulmonary hypertension.

Regulation of PKR activation and apoptosis during oxidative stress by TRBP phosphorylation.

Transactivation response element RNA-binding protein (TRBP or TARBP2) originally identified as a pro-viral cellular protein in human immunodeficiency virus (HIV) replication is also a regulator of microRNA biogenesis and cellular stress response. TRBP inhibits the catalytic activity of interferon-induced double-stranded RNA (dsRNA)-activated protein kinase (PKR) during viral infections and cell stress thereby regulating stress-induced signaling pathways. During cellular stress, PKR is catalytically activated transiently by its protein activator PACT and TRBP inhibits PKR to bring about a timely cellular recovery. We have previously established that TRBP phosphorylated after oxidative stress binds to and inhibits PKR more efficiently promoting cell survival. In this study, we investigated if phosphorylation of TRBP enhances its interaction with PACT to bring about additional PKR inhibition. Our data establishes that phosphorylation of TRBP has no effect on PACT-TRBP interaction and TRBP's inhibitory actions on PKR are mediated exclusively by its enhanced interaction with PKR. Cells lacking TRBP are more sensitive to apoptosis in response to oxidative stress and show persistent PKR activation. These results establish that PKR inhibition by stress-induced TRBP phosphorylation occurs by its direct binding to PKR and is important for preventing apoptosis due to sustained PKR activation.

Nuclear PKR in retinal neurons in the early stage of diabetic retinopathy in streptozotocin‑induced diabetic rats.

Retinal neuron apoptosis is a key component of diabetic retinopathy (DR), one of the most common complications of diabetes. Stress due to persistent hyperglycaemia and corresponding glucotoxicity represents one of the primary pathogenic mechanisms of diabetes and its complications. Apoptosis of retinal neurons serves a critical role in the pathogenesis of DR observed in patients with diabetes and streptozotocin (STZ)-induced diabetic rats. Retinal neuron apoptosis occurs one month after STZ injection, which is considered the early stage of DR. The molecular mechanism involved in the suppression of retinal neuron apoptosis during the early stage of DR remains unclear. RNA-dependent protein kinase (PKR) is a stress-sensitive pro-apoptotic kinase. Our previous study indicated that PKR-associated protein X, a stress-sensitive activator of PKR, is upregulated in the early stage of STZ-induced diabetes. In order to assess the role of PKR in DR prior to apoptosis of retinal neurons, immunofluorescence and western blotting were performed to investigate the cellular localization and expression of PKR in the retina in the early stage of STZ-induced diabetes in rats. PKR activity was indirectly assessed by expression levels of phosphorylated eukaryotic translation initiation factor 2α (p-eIF2-α) and the presence of apoptotic cells in the retina was investigated by TUNEL assay. The findings revealed that PKR was localized in the nucleus of retinal ganglion and inner nuclear layer cells from normal and diabetic rats. To the best of our knowledge, the present study is the first to demonstrate nuclear localization of PKR in retinal neurons. Immunofluorescence analysis demonstrated that PKR was expressed in the nuclei of retinal neurons at 3 and 6 days and its expression was decreased at 15 days after STZ treatment. In addition, p-eIF2-α expression and cellular localization followed the trend of PKR, suggesting that this pro-apoptotic kinase was active in the nuclei of retinal neurons. These findings are consistent with the hypothesis that nuclear translocation of PKR may be a mechanism to sequester active PKR, thus preventing upregulation of cytosolic signalling pathways that induce apoptosis in retinal neurons. Apoptotic cells were not detected in the retina in the early stage of DR. A model was proposed to explain the mechanism by which apoptosis of retinal neurons by PKR is suppressed in the early stage of DR. The possible role of mitochondrial RNA (mtRNA) and Alu RNA in this phenomenon is also discussed since it was demonstrated that the cellular stress due to prolonged hyperglycaemia induces the release of mtRNA and transcription of Alu RNA. Moreover, it mtRNA activates PKR, whereas Alu RNA inhibits the activation of this protein kinase.

Results Update from the DRIVE PK Study: Effects of AG-348, a Pyruvate Kinase Activator, in Patients with Pyruvate Kinase Deficiency

Results Update from the DRIVE PK Study: Effects of AG-348, a Pyruvate Kinase Activator, in Patients with Pyruvate Kinase Deficiency

Opposing forces fight over the same ground to regulate interferon signaling.

The current SARS-CoV-2 pandemic has spurred new interest in interferon signaling in response to viral pathogens. Much of what we know about the signaling molecules and associated signal transduction induced during the host cellular response to viral pathogens has been gained from research conducted from the 1990's to the present day, but certain intricacies of the mechanisms involved, still remain unclear. In a recent study by Vaughn et al. the authors examine one of the main mechanisms regulating interferon induction following viral infection, the RIG-I/MAVS/IRF3 pathway, and find that similar to PKR both DICER interacting proteins, PACT and TRBP, regulate RIG-I signaling in an opposing manner. More specifically, the reported findings demonstrate, like others, that PACT stimulates RIG-I-mediated signaling in a manner independent of PACT dsRNA-binding ability or phosphorylation at sites known to be important for PACT-dependent PKR activation. In contrast, they show for the first time that TRBP inhibits RIG-I-mediated signaling. RIG-I inhibition by TRBP did not require phosphorylation of sites shown to be important for inhibiting PKR, nor did it involve PACT or PKR, but it did require the dsRNA-binding ability of TRBP. These findings open the door to a complex co-regulation of RIG-I, PKR, MDA5, miRNA processing, and interferon induction.

FT-4202, an oral PKR activator, has potent antisickling effects and improves RBC survival and Hb levels in SCA mice

Sickle cell anemia (SCA) results from an abnormal sickle hemoglobin (HbS). HbS polymerizes upon deoxygenation, resulting in red blood cell (RBC) sickling and membrane damage that cause vaso-occlusions and hemolysis. Sickle RBCs contain less adenosine triphosphate and more 2,3-diphosphoglycerate than normal RBCs, which allosterically reduces hemoglobin (Hb) oxygen (O2) affinity (ie, increases the partial pressure of oxygen at which hemoglobin is 50% saturated with oxygen [P50]), potentiating HbS polymerization. Herein, we tested the effect of investigational agent FT-4202, an RBC pyruvate kinase (PKR) activator, on RBC sickling and membrane damage by administering it to Berkeley SCA mice. Two-week oral FT-4202 administration was well tolerated, decreasing HbS P50 to levels similar to HbA and demonstrating beneficial biological effects. In FT-4202–treated animals, there was reduced sickling in vivo, demonstrated by fewer irreversibly sickled cells, and improved RBC deformability, assessed at varying shear stress. Controlled deoxygenation followed by reoxygenation of RBCs obtained from the blood of FT-4202–treated mice showed a shift in the point of sickling to a lower partial pressure of oxygen (pO2). This led to a nearly 30% increase in RBC survival and a 1.7g/dL increase in Hb level in the FT-4202–treated SCA mice. Overall, our results in SCA mice suggest that FT-4202 might be a potentially useful oral antisickling agent that warrants investigation in patients with SCA.

PKR activity modulation by phosphomimetic mutations of serine residues located three aminoacids upstream of double-stranded RNA binding motifs

Eukaryotic translation initiation factor 2 alpha kinase 2 (EIF2AK2), better known as PKR, plays a key role in the response to viral infections and cellular homeostasis by regulating mRNA translation. Upon binding dsRNA, PKR is activated through homodimerization and subsequent autophosphorylation on residues Thr446 and Thr451. In this study, we identified a novel PKR phosphorylation site, Ser6, located 3 amino acids upstream of the first double-stranded RNA binding motif (DRBM1). Another Ser residue occurs in PKR at position 97, the very same position relative to the DRBM2. Ser or Thr residues also occur 3 amino acids upstream DRBMs of other proteins such as ADAR1 or DICER. Phosphoinhibiting mutations (Ser-to-Ala) introduced at Ser6 and Ser97 spontaneously activated PKR. In contrast, phosphomimetic mutations (Ser-to-Asp) inhibited PKR activation following either poly (I:C) transfection or virus infection. These mutations moderately affected dsRNA binding or dimerization, suggesting a model where negative charges occurring at position 6 and 97 tighten the interaction of DRBMs with the kinase domain, thus keeping PKR in an inactive closed conformation even in the presence of dsRNA. This study provides new insights on PKR regulation mechanisms and identifies Ser6 and Ser97 as potential targets to modulate PKR activity for therapeutic purposes.

Inhibition of anti-viral stress granule formation by coronavirus endoribonuclease nsp15 ensures efficient virus replication.

Cytoplasmic stress granules (SGs) are generally triggered by stress-induced translation arrest for storing mRNAs. Recently, it has been shown that SGs exert anti-viral functions due to their involvement in protein synthesis shut off and recruitment of innate immune signaling intermediates. The largest RNA viruses, coronaviruses, impose great threat to public safety and animal health; however, the significance of SGs in coronavirus infection is largely unknown. Infectious Bronchitis Virus (IBV) is the first identified coronavirus in 1930s and has been prevalent in poultry farm for many years. In this study, we provided evidence that IBV overcomes the host antiviral response by inhibiting SGs formation via the virus-encoded endoribonuclease nsp15. By immunofluorescence analysis, we observed that IBV infection not only did not trigger SGs formation in approximately 80% of the infected cells, but also impaired the formation of SGs triggered by heat shock, sodium arsenite, or NaCl stimuli. We further demonstrated that the intrinsic endoribonuclease activity of nsp15 was responsible for the interference of SGs formation. In fact, nsp15-defective recombinant IBV (rIBV-nsp15-H238A) greatly induced the formation of SGs, along with accumulation of dsRNA and activation of PKR, whereas wild type IBV failed to do so. Consequently, infection with rIBV-nsp15-H238A strongly triggered transcription of IFN-β which in turn greatly affected rIBV-nsp15-H238A replication. Further analysis showed that SGs function as an antiviral hub, as demonstrated by the attenuated IRF3-IFN response and increased production of IBV in SG-defective cells. Additional evidence includes the aggregation of pattern recognition receptors (PRRs) and signaling intermediates to the IBV-induced SGs. Collectively, our data demonstrate that the endoribonuclease nsp15 of IBV interferes with the formation of antiviral hub SGs by regulating the accumulation of viral dsRNA and by antagonizing the activation of PKR, eventually ensuring productive virus replication. We further demonstrated that nsp15s from PEDV, TGEV, SARS-CoV, and SARS-CoV-2 harbor the conserved function to interfere with the formation of chemically-induced SGs. Thus, we speculate that coronaviruses employ similar nsp15-mediated mechanisms to antagonize the host anti-viral SGs formation to ensure efficient virus replication.

Porcine circovirus type 2 exploits cap to inhibit PKR activation through interaction with Hsp40

Porcine circovirus type 2 is the main pathogen of porcine circovirus disease, which has caused enormous economic losses to the pig industry worldwide. The PKR signaling pathway is important for the cellular antiviral response, but its role in the process of PCV2 infection is unknown. In this study, we first found that dsRNA was produced and that PKR was activated in PCV2 infection. However, interestingly, the activation of PKR was inhibited when the Cap protein was exogenously expressed in PAMs, and this inhibition was reversed by the expression of DNAJC7. The interaction between Cap and DNAJC7 was confirmed by laser confocal microscopy, coimmunoprecipitation and GST pull-down, and it was found that PCV2 infection or the expression of Cap protein could induce DNAJC7 to migrate to the nucleus and release P58IPK, an inhibitor of PKR activation. Downregulating the expression of DNAJC7 by a specific inhibitor or recombinant lentivirus-mediated shRNA, inhibited the replication of the PCV2 genome and the production of virions, which was consistent with the increase of DNAJC7 expression in multiple tissues of weaned piglets infected with PCV2. These data indicate that although PKR was activated by PCV2 infection, the activation was inhibited by Cap through an interaction with DNAJC7. These results help to understand the molecular mechanism of immune escape after PCV2 infection.

FT-4202, an Allosteric Activator of Pyruvate Kinase-R, Demonstrates Proof of Mechanism and Proof of Concept after a Single Dose and after Multiple Daily Doses in a Phase 1 Study of Patients with Sickle Cell Disease

FT-4202, an Allosteric Activator of Pyruvate Kinase-R, Demonstrates Proof of Mechanism and Proof of Concept after a Single Dose and after Multiple Daily Doses in a Phase 1 Study of Patients with Sickle Cell Disease

Ex-Vivo FT-4202 Treatment Improves Hemoglobin Oxygen Affinity and Membrane Health in Red Blood Cells of Patients with Hemoglobin SS and Hemoglobin SC Disease Irrespective of Prior Hydroxyurea Use

The two most common genotypes causing sickle cell disease (SCD) are homozygous bsbs producing hemoglobin S (HbSS), and compound heterozygous bsbc producing HbS and HbC (HbSC). HbS polymerizes in both, when deoxygenated. Patients with HbSC have distinct hematologic characteristics such as higher Hb levels but lower fetal hemoglobin (HbF); increased corpuscular Hb concentration, due to pronounced red blood cell (RBC) K+ loss and dehydration, contributes to HbS polymerization. While hydroxyurea (HU) as a HbF-inducing agent clearly improves HbSS phenotype, targeting of other pathogenetic mechanisms in SCD may offer additional benefit in both HbSS and HbSC patients. The RBC metabolism depends on anaerobic glycolysis for ATP production. Key RBC functions are intricately linked to metabolism: hemoglobin-oxygen (Hb-O2) affinity is modulated by 2,3-diphosphoglycerate (2,3-DPG), RBC membrane health depends on adequate synthesis of ATP, and heme-iron is maintained in the functional reduced state by the synthesis of antioxidants, such as GSH, supplying NADH to methemoglobin reductase. Erythrocyte pyruvate kinase (PKR) performs a rate limiting step in RBC glycolysis, converting phosphoenolpyruvate to pyruvate and generating ATP. Insufficient PKR activity due to any of a variety of causes, including excessive energy demand, can alter the abundance of metabolic products and intermediates, including ATP and 2,3-DPG, adversely affecting RBC function. We assessed baseline 2,3-DPG and ATP levels in whole blood of patients with HbSS (N=18) by LC/MS analysis. As well-reported, hemoglobin (Hb) levels were significantly decreased in patients with HbSS compared to healthy volunteers (9.68 ± 0.32 g/dL vs 14.88 ± 0.27 g/dL, p&amp;lt;0.0001). After normalizing to the decreased Hb levels, 2,3-DPG levels were significantly increased (4842.78 ± 182.98 vs 4327.73 ± 126.60 µg/g Hb, p&amp;lt;0.05) and ATP levels were significantly decreased (1621.27 ± 133.02 vs 1995.37 ± 60.80 µg/g Hb, p&amp;lt;0.05) in whole blood specimens of patients with HbSS compared to healthy volunteers. FT-4202 is a potent, selective, and orally bioavailable allosteric activator of PKR which is in Phase 1 clinical trial for the treatment of SCD (NCT03815695). We have previously shown that FT-4202 decreases 2,3-DPG within 24 hr of single dose and increases ATP after 14 days of multiple dosing in healthy volunteers (Blood, 2019, 134, Supplement_1:616). Here, we hypothesized that, by reducing 2,3-DPG and increasing ATP, FT-4202 may improve oxygen affinity and membrane health in both HbSS and HbSC RBCs irrespective of hydroxyurea use and HbF expression. Whole blood samples in K2EDTA were collected from donors with confirmed HbSS or HbSC disease in accordance with an approved IRB protocol in Cincinnati Children's Hospital. % HbF and % F-cells were measured for each sample. Packed RBCs were washed, resuspended to 20% v/v (20% hematocrit), and then incubated for 4 hr at 37oC with 20 μM FT-4202 or equivalent volume of vehicle control DMSO. P50 representing the partial pressure of oxygen at which Hb is 50% saturated with oxygen, was determined using non-linear regression analysis of hemoglobin-oxygen equilibrium curves obtained on a HEMOX Analyzer (TCS Scientific). RBC deformability as a function of oxygen pressure, or Oxygenscan, was measured using Lorrca® (RR Mechatronics) (Rab et al., 2019, Am J Hematol). Point of Sickling (PoS) was calculated as the pO2 at which RBC deformability is decreased by 5% of maximum value during deoxygenation. The P50 and PoS values in FT-4202 versus DMSO-treated HbSC or HbSS RBCs from patients treated with or without HU were analyzed by paired Wilcoxon test. FT-4202 treatment significantly decreased P50 in both HU-treated and not HU-treated HbSS RBCs, as well as in HbSC RBCs, as detailed in Table below showing mean ± se (*p&amp;lt;0.05, **p&amp;lt;0.002, ***p&amp;lt;0.0005 for FT-4202 versus vehicle). The decrease in P50 was significantly higher in not HU- versus HU-treated HbSS RBCs (###p&amp;lt;0.001 by two-way ANOVA) likely due to the increased HbF with baseline decreased P50 in HU-treated RBCs (23.6 ± 2.3 %HbF) versus non-HU treated RBCs (10 ± 1.9 %HbF). PoS was decreased after FT-4202 treatment both in HbSS and HbSC RBCs (p&amp;lt;0.05). These data suggest that increasing flux through the glycolytic pathway by FT-4202 treatment may offer dual beneficial effects of increased Hb-O2 affinity and membrane flexibility in HbSC as well as in HbSS RBCs irrespective of HU use. Figure Disclosures Fulzele: FORMA Therapeutics, Inc: Current Employment, Other: Shareholder of Forma Therapeutics. Guichard:FORMA Therapeutics, Inc: Current Employment, Other: Shareholder of Forma Therapeutics; AstraZeneca: Other: Shareholder. Kalfa:Agios Pharmaceuticals, Inc: Consultancy, Research Funding; Forma Therapeutics, Inc: Research Funding.

An Adaptive, Randomized, Placebo-Controlled, Double-Blind, Multi-Center Study of Oral FT-4202, a Pyruvate Kinase Activator in Patients with Sickle Cell Disease (PRAISE)

An Adaptive, Randomized, Placebo-Controlled, Double-Blind, Multi-Center Study of Oral FT-4202, a Pyruvate Kinase Activator in Patients with Sickle Cell Disease (PRAISE)

Dystonia 16 (DYT16) mutations in PACT cause dysregulated PKR activation and eIF2α signaling leading to a compromised stress response

Dystonia 16 (DYT16) is caused by mutations in PACT, the protein activator of interferon-induced double-stranded RNA-activated protein kinase (PKR). PKR regulates the integrated stress response (ISR) via phosphorylation of the translation initiation factor eIF2α. This post-translational modification attenuates general protein synthesis while concomitantly triggering enhanced translation of a few specific transcripts leading either to recovery and homeostasis or cellular apoptosis depending on the intensity and duration of stress signals. PKR plays a regulatory role in determining the cellular response to viral infections, oxidative stress, endoplasmic reticulum (ER) stress, and growth factor deprivation. In the absence of stress, both PACT and PKR are bound by their inhibitor transactivation RNA-binding protein (TRBP) thereby keeping PKR inactive. Under conditions of cellular stress these inhibitory interactions dissociate facilitating PACT-PACT interactions critical for PKR activation. While both PACT-TRBP and PKR-TRBP interactions are pro-survival, PACT-PACT and PACT-PKR interactions are pro-apoptotic. In this study we evaluate if five DYT16 substitution mutations alter PKR activation and ISR. Our results indicate that the mutant DYT16 proteins show stronger PACT-PACT interactions and enhanced PKR activation. In DYT16 patient derived lymphoblasts the enhanced PACT-PKR interactions and heightened PKR activation leads to a dysregulation of ISR and increased apoptosis. More importantly, this enhanced sensitivity to ER stress can be rescued by luteolin, which disrupts PACT-PKR interactions. Our results not only demonstrate the impact of DYT16 mutations on regulation of ISR and DYT16 etiology but indicate that therapeutic interventions could be possible after a further evaluation of such strategies.

EIF2B as a Target for Viral Evasion of PKR-Mediated Translation Inhibition.

RNA-activated protein kinase (PKR) is a major innate immune factor that senses viral double-stranded RNA (dsRNA) and phosphorylates eukaryotic initiation factor (eIF) 2α. Phosphorylation of the α subunit converts the eIF2αβγ complex into a stoichiometric inhibitor of eukaryotic initiation factor eIF2B, thus halting mRNA translation. To escape this protein synthesis shutoff, viruses have evolved countermechanisms such as dsRNA sequestration, eIF-independent translation by an internal ribosome binding site, degradation of PKR, or dephosphorylation of PKR or of phospho-eIF2α. Here, we report that sandfly fever Sicilian phlebovirus (SFSV) confers such a resistance without interfering with PKR activation or eIF2α phosphorylation. Rather, SFSV expresses a nonstructural protein termed NSs that strongly binds to eIF2B. Although NSs still allows phospho-eIF2α binding to eIF2B, protein synthesis and virus replication are unhindered. Hence, SFSV encodes a unique PKR antagonist that acts by rendering eIF2B resistant to the inhibitory action of bound phospho-eIF2α.IMPORTANCE RNA-activated protein kinase (PKR) is one of the most powerful antiviral defense factors of the mammalian host. PKR acts by phosphorylating mRNA translation initiation factor eIF2α, thereby converting it from a cofactor to an inhibitor of mRNA translation that strongly binds to initiation factor eIF2B. To sustain synthesis of their proteins, viruses are known to counteract this on the level of PKR or eIF2α or by circumventing initiation factor-dependent translation altogether. Here, we report a different PKR escape strategy executed by sandfly fever Sicilian virus (SFSV), a member of the increasingly important group of phleboviruses. We found that the nonstructural protein NSs of SFSV binds to eIF2B and protects it from inactivation by PKR-generated phospho-eIF2α. Protein synthesis is hence maintained and the virus can replicate despite ongoing full-fledged PKR signaling in the infected cells. Thus, SFSV has evolved a unique strategy to escape the powerful antiviral PKR.

Grass carp (Ctenopharyngodon idella) PACT induces cell apoptosis and activates NF-кB via PKR

As a dsRNA-dependent and interferon-induced protein kinase, PKR is involved in antiviral immune response and apoptosis mediated by various cytokines. In mammalian cells, PKR can also be activated in the absence of dsRNA. A PKR activator, PACT (PKR activating protein), also referred to as RAX (PKR-associated protein X) plays an important role. In recent years, with the increasing recognition of fish interferon system, PKR and PACT have been gradually revealed in fish. However, the function of fish PACT is unclear. In our previous work, we suggested that grass carp (Ctenopharyngodon idella) PACT must be involved in IRF2 and ATF4-mediated stress response pathways. In the present study, we found that the expression of C. idella PACT (CiPACT) and CiPKR were significantly up-regulated under the stimulation of LPS. It indicated that CiPACT and CiPKR may play an important role in response to LPS stimulation. In addition, the response time of CiPACT to LPS is earlier than that of CiPKR. It has also shown that overexpression of CiPACT in CIK cells can significantly enhance the level of p-eIF2α, induces apoptosis and translocation of Cip65 to nucleus from cytoplasm. To further understand the mechanism, we carried out the co-immunoprecipitation assay. It proved that the interaction of CiPACT and CiPKR made the phosphorylation of CiPKR. Overexpression of CiPACT induced the down-regulation of intracellular expression of bcl-2 and up-regulation of bax. However, in CiPKR knocked-down cells the expression of bcl-2 and bax were just the opposite. Therefore, the mechanism of fish PACT induces apoptosis and activates NF-кB is dependent on PKR.

Effect of inhibiting the activity of double-stranded RNA-dependent protein kinase in sepsis mice

Objective: To observe the effects of 2-aminopurine (2-AP), a double-stranded RNA-dependent protein kinase (PKR) inhibitor, on organ function, plasma inflammatory factor expression and 7 days mortality in sepsis mice induced by cecal ligation puncture (CLP). Methods: Forty specific specific pathogen free C57BL/6 mice were randomly divided into sham group (n=10), CLP group (n=10), CLP+2-AP group (n=10) and 2-AP group (n=10). CLP was used to establish sepsis mice models.Peripheral blood serum was collected 24 hours after operation, alanine aminotransferase (ALT), aspartate aminotransferase (AST), serum creatinine (Cr), blood urea nitrogen (BUN) and inflammatory factors (IL-1β, IL-10 and TNF-α) were detected; peripheral blood and peritoneal lavage fluid were taken for bacterial clearance detection. Another 60 C57BL/6 mice were selected to observe the 7-day survival rate according to the above groups (n=15). Independent sample t test was used to compare the measurement data between groups. Results: The levels of ALT, AST, Cr and BUN in CLP Group and CLP+2-AP group were significantly higher than those in sham group (all P<0.001). The levels of ALT and AST in CLP+2-AP group were significantly lower than those in CLP Group (t=27.88, 11.33, both P<0.001); the levels of Cr and BUN in CLP+2-AP group were significantly lower than those in CLP Group (t=11.02, 7.15, bothP<0.001). Compared with sham group, the levels of pro-inflammatory (IL-1β and TNF-α) and anti-inflammatory (IL-10) cytokines in CLP group were significantly higher (all P<0.001); the levels of IL-1β and IL-10 in CLP+2-AP group were significantly lower (all P<0.001), but the levels of TNF-α in CLP+2-AP group were not significantly lower (P=0.33). The 7-day survival rate was 100% in sham group, 13.3% in CLP+2-AP group, 86.7% in 2-AP group and 20.0% in CLP+2-AP group. Inhibition of PKR activation slightly improved the trend of 7-days survival rate of CLP model mice (analysis by mantel Cox test, χ(2)=0.0012, P=0.97). Conclusion: In sepsis mice model, inhibition of PKR activity can reduce the expression of inflammatory factors in plasma, decrease bacterial load in blood and abdominal cavity, and protect organ function, which could suggest that inhibition of PKR activity has potential application in sepsis treatment.

Dicer as a Repressor of Antiviral Responses in Mouse Embryonic Stem Cells

Recent studies have demonstrated that pluripotent stem cells (PSCs), including human and mouse embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) have underdeveloped antiviral innate immunity. Specifically, they fail to express IFN in response to viral stimuli. This finding has led to the hypothesis that the lack of IFN response is an intrinsic property of PSCs. It has also inspired the investigation of the underlying molecular mechanisms. In an effort to conduct a holistic analysis of antiviral responses in ESCs, we have developed an assay that can simultaneously measure RNA‐mediated gene expression and RNA‐induced antiviral responses. In this system, GFP is expressed from functionalized synthetic GFP‐mRNA and is used to measure the cellular translation activity, whereas dsRNA and 5’‐triphosphate‐ssRNA act as viral RNA analogs. Transfection of ESCs with these synthetic RNAs can potently activate all common antiviral signaling pathways, including RNA interference (RNAi), dsRNA activated protein kinase (PKR), OAS‐RNase L, and IFN responses. Our data revealed that RNAi activity was detected in ESCs, but not in Dicer knockout ESCs (D‐/‐ESCs) as expected since Dicer is a key enzyme in miRNA and siRNA processing. The antiviral responses mediated by PKR, OAS‐RNase L, and IFN were either not detected or detected at low levels in ESCs, but they increased dramatically in D‐/‐ESCs, as indicated by increased activity of PKR (measured by low level GFP expression and PKR phosphorylation) and RNase L (determined by RNA degradation). Furthermore, the components of the IFN signaling pathway are expressed at higher basal levels in D‐/‐ESCs than in normal ESCs and are further up‐regulated by RNA transfection. The most notable effect of RNA transfection into D‐/‐ESCs is the induction of cell death and decreased cell proliferation, which is at least partly attributed to PKR activation and increased expression of the cell cycle inhibitors p21 and p19. Our findings suggest that ESCs are exquisitely sensitive to cellular damages caused by antiviral response and that Dicer acts as a repressor of antiviral responses in ESCs under normal conditions.Support or Funding InformationThis work was in part supported by the National Institute of General Medical Sciences (R15GM128196‐01). We thank Mississippi‐IDeA Network of Biomedical Research Excellence for the use of the imaging facility (funded by the National Institute of General Medical Sciences P20 GM103476‐11).

Non-replicating Vaccinia Virus TianTan Strain (NTV) Translation Arrest of Viral Late Protein Synthesis Associated With Anti-viral Host Factor SAMD9.

NTV is a highly attenuated virus that was created by genetically deleting 26 genes related to host range and virulence from TianTan strain. Since NTV is highly attenuated, it has been used widely as an optimizing viral vector. In this study, we explored the biological characteristics in vitro and the host restriction mechanism of NTV. Most cell lines do not support sufficient dissemination and replication of NTV, and in non-permissive cell line HeLa, the replication block of NTV occurred at the translation stage of viral late protein expression. Lack of PKR activity was not sufficient to rescue expression of viral late proteins and replication, even though the phosphorylation level of eIF2α increased in NTV-infected HeLa cells. Moreover, the translation inhibition of NTV in HeLa cells was dependent upon a SAMD9 signaling pathway, as demonstrated by silencing SAMD9 expression with siRNA and observing the colocalization of SAMD9 and AVGs. Reinserting C7L or K1L into NTV rescued the late viral protein expression and replication of NTV in HeLa cells. Among the genes deleted in NTV, C7L or/and K1L gene was mainly responsible for its replication defect. Protein C7 interacted with SAMD9, which antagonized the antiviral response of SAMD9 to ensure viral protein translation and replication of NTV in non-permissive cell lines. Our finding will serve as a baseline for modification of NTV in future application.