Resistance to vesicular stomatitis virus infection requires a functional cross talk between the eukaryotic translation initiation factor 2alpha kinases PERK and PKR

A-to-I RNA editing by ADAR and its therapeutic applications: From viral infections to cancer immunotherapy

ADAR deaminases catalyze adenosine-to-inosine (A-to-I) editing on double-stranded RNA (dsRNA) substrates that regulate an umbrella of biological processes. One of the two catalytically active ADAR enzymes, ADAR1, plays a major role in innate immune responses by suppression of RNA sensing pathways which are orchestrated through the ADAR1-dsRNA-MDA5 axis. Unedited immunogenic dsRNA substrates are potent ligands for the cellular sensor MDA5. Upon activation, MDA5 leads to the induction of interferons and expression of hundreds of interferon-stimulated genes with potent antiviral activity. In this way, ADAR1 acts as a gatekeeper of the RNA sensing pathway by striking a fine balance between innate antiviral responses and prevention of autoimmunity. Reduced editing of immunogenic dsRNA by ADAR1 is strongly linked to the development of common autoimmune and inflammatory diseases. In viral infections, ADAR1 exhibits both antiviral and proviral effects. This is modulated by both editing-dependent and editing-independent functions, such as PKR antagonism. Several A-to-I RNA editing events have been identified in viruses, including in the insidious viral pathogen, SARS-CoV-2 which regulates viral fitness and infectivity, and could play a role in shaping viral evolution. Furthermore, ADAR1 is an attractive target for immuno-oncology therapy. Overexpression of ADAR1 and increased dsRNA editing have been observed in several human cancers. Silencing ADAR1, especially in cancers that are refractory to immune checkpoint inhibitors, is a promising therapeutic strategy for cancer immunotherapy in conjunction with epigenetic therapy. The mechanistic understanding of dsRNA editing by ADAR1 and dsRNA sensing by MDA5 and PKR holds great potential for therapeutic applications. This article is categorized under: RNA Processing > RNA Editing and Modification RNA in Disease and Development > RNA in Disease.

Viral Hemorrhagic Septicemia Virus Activates Integrated Stress Response Pathway and Induces Stress Granules to Regulate Virus Replication

Virus infection activates integrated stress response (ISR) and stress granule (SG) formation and viruses counteract by interfering with SG assembly, suggesting an important role in antiviral defense. The infection of fish cells by Viral Hemorrhagic Septicemia Virus (VHSV), activates the innate immune recognition pathway and the production of type I interferon (IFN). However, the mechanisms by which VHSV interacts with ISR pathway regulating SG formation is poorly understood. Here, we demonstrate that fish cells respond to heat shock, oxidative stress and VHSV infection by forming SG that localized key SG marker, Ras GTPase-activating protein (SH3 domain)-binding protein 1 (G3BP1). We show that PKR-like endoplasmic reticulum kinase (PERK), but not (dsRNA)-dependent protein kinase (PKR), is required for VHSV-induced SG formation. Furthermore, in VHSV Ia infected cells, PERK activity is required for IFN production, antiviral signaling and viral replication. SG formation required active virus replication as individual VHSV Ia proteins or inactive virus did not induce SG. Cells lacking G3BP1 produced increased IFN, antiviral genes and viral mRNA, however viral protein synthesis and viral titers were reduced. We show a critical role of the activation of ISR pathway and SG formation highlighting a novel role of G3BP1 in regulating VHSV protein translation and replication.

Role of symmetry in 3,3-diphenyl-1,3-dihydroindol-2-one derivatives as inhibitors of translation initiation

The ternary complex (eIF2·GTP·Met-tRNAiMet) and the eIF4F complex assembly are two major regulatory steps in the eukaryotic translation initiation. Inhibition of the ternary complex assembly is therefore a promising target for the development of novel anti-cancer therapeutics. Building on the finding that clotrimazole (CLT), a molecular probe that depletes intracellular Ca2+ stores and subsequently induce eIF2α phosphorylation, inhibit translation initiation, and reduce preferentially the expression of oncoproteins over "housekeeping" ones,1-3 we undertook structure activity relationship (SAR) studies that identified 3,3-diarylindoline-2-one #1181 as an interesting scaffold. Compound #1181 also induce phosphorylation of eIF2α thereby reducing the availability of the ternary complex, which leads to inhibition of translation initiation.4 Our subsequent efforts focused on understanding SAR iterative lead optimization to enhance potency and improve bioavailability. Herein, we report a complementing study focusing on heavily substituted symmetric and asymmetric 3,3-(o,m-disubstituted)diarylindoline-2-ones. These compounds were evaluated by the dual luciferase reporter ternary complex assay that recapitualates phosphorylation of eIF2α in a quantitative manner. We also evaluated all compounds by sulforhodamine B assay, which measures the overall effect of compounds on cell proliferations and/or viability.

Endoplasmic Reticulum Stress in Elderly Patients with COVID-19: Potential of Melatonin Treatment

Aging processes, including immunosenescence, inflammation, inflammasome formation, genomic instability, telomeric attrition, and altered autophagy, are involved in viral infections and they may contribute to increased pathophysiological responses to the SARS-CoV-2 infection in the elderly; this poses additional risks of accelerated aging, which could be found even after recovery. Aging is associated with oxidative damage. Moreover, SARS-CoV-2 infections may increase the production of reactive oxygen species and such infections will disturb the Ca++ balance via an endoplasmic reticulum (ER) stress-mediated unfolded protein response. Although vaccine development and anti-inflammation therapy lower the severity of COVID-19, the prevalence and mortality rates are still alarming in some countries worldwide. In this review, we describe the involvement of viral proteins in activating ER stress transducers and their downstream signals and in inducing inflammation and inflammasome formation. Furthermore, we propose the potential of melatonin as an ER stress modulator, owing to its antioxidant, anti-inflammatory, and immunoregulatory effects in viral infections. Considering its strong safety profile, we suggest that additive melatonin supplementation in the elderly could be beneficial in treating COVID-19.

Clinical activity of single-dose systemic oncolytic VSV virotherapy in patients with relapsed refractory T-cell lymphoma

Clinical success with intravenous (IV) oncolytic virotherapy (OV) has to-date been anecdotal. We conducted a phase 1 clinical trial of systemic OV and investigated the mechanisms of action in responding patients. A single IV dose of vesicular stomatitis virus (VSV) interferon-β (IFN-β) with sodium iodide symporter (NIS) was administered to patients with relapsed/refractory hematologic malignancies to determine safety and efficacy across 4 dose levels (DLs). Correlative studies were undertaken to evaluate viremia, virus shedding, virus replication, and immune responses. Fifteen patients received VSV-IFNβ-NIS. Three patients were treated at DL1 through DL3 (0.05, 0.17, and 0.5 × 1011 TCID50), and 6 were treated at DL4 (1.7 × 1011 TCID50) with no dose-limiting toxicities. Three of 7 patients with T-cell lymphoma (TCL) had responses: a 3-month partial response (PR) at DL2, a 6-month PR, and a complete response (CR) ongoing at 20 months at DL4. Viremia peaked at the end of infusion, g was detected. Plasma IFN-β, a biomarker of VSV-IFNβ-NIS replication, peaked between 4 hours and 48 hours after infusion. The patient with CR had robust viral replication with increased plasma cell-free DNA, high peak IFN-β of 18 213 pg/mL, a strong anti-VSV neutralizing antibody response, and increased numbers of tumor reactive T-cells. VSV-IFNβ-NIS as a single agent was effective in patients with TCL, resulting in durable disease remissions in heavily pretreated patients. Correlative analyses suggest that responses may be due to a combination of direct oncolytic tumor destruction and immune-mediated tumor control. This trial is registered at www.clinicaltrials.gov as #NCT03017820.

The PERK/PKR-eIF2α pathway negatively regulates porcine hemagglutinating encephalomyelitis virus replication by attenuating global protein translation and facilitating stress granule formation

The replication of coronaviruses, including severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), and the recently emerged severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is closely associated with the endoplasmic reticulum (ER) of infected cells. The unfolded protein response (UPR), which is mediated by ER stress (ERS), is a typical outcome in coronavirus-infected cells and is closely associated with the characteristics of coronaviruses. However, the interaction between virus-induced ERS and coronavirus replication is poorly understood. Here, we demonstrate that infection with the betacoronavirus porcine hemagglutinating encephalomyelitis virus (PHEV) induced ERS and triggered all three branches of the UPR signaling pathway both in vitro and in vivo. In addition, ERS suppressed PHEV replication in mouse neuro-2a (N2a) cells primarily by activating the protein kinase R-like ER kinase (PERK)–eukaryotic initiation factor 2α (eIF2α) axis of the UPR. Moreover, another eIF2α phosphorylation kinase, interferon (IFN)-induced double-stranded RNA-dependent protein kinase (PKR), was also activated and acted cooperatively with PERK to decrease PHEV replication. Furthermore, we demonstrate that the PERK/PKR-eIF2α pathways negatively regulated PHEV replication by attenuating global protein translation. Phosphorylated eIF2α also promoted the formation of stress granules (SGs), which in turn repressed PHEV replication. In summary, our study presents a vital aspect of the host innate response to invading pathogens and reveals attractive host targets (e.g., PERK, PKR, and eIF2α) for antiviral drugs.

IMPORTANCE Coronavirus diseases are caused by different coronaviruses of importance in humans and animals, and specific treatments are extremely limited. ERS, which can activate the UPR to modulate viral replication and the host innate response, is a frequent occurrence in coronavirus-infected cells. PHEV, a neurotropic betacoronavirus, causes nerve cell damage, which accounts for the high mortality rates in suckling piglets. However, it remains incompletely understood whether the highly developed ER in nerve cells plays an antiviral role in ERS and how ERS regulates viral proliferation. In this study, we found that PHEV infection induced ERS and activated the UPR both in vitro and in vivo and that the activated PERK/PKR-eIF2α axis inhibited PHEV replication through attenuating global protein translation and promoting SG formation. A better understanding of coronavirus-induced ERS and UPR activation may reveal the pathogenic mechanism of coronavirus and facilitate the development of new treatment strategies for these diseases.

Molecular Pathogenesis and Immune Evasion of Vesicular Stomatitis New Jersey Virus Inferred from Genes Expression Changes in Infected Porcine Macrophages

The molecular mechanisms associated with the pathogenesis of vesicular stomatitis virus (VSV) in livestock remain poorly understood. Several studies have highlighted the relevant role of macrophages in controlling the systemic dissemination of VSV during infection in different animal models, including mice, cattle, and pigs. To gain more insight into the molecular mechanisms used by VSV to impair the immune response in macrophages, we used microarrays to determine the transcriptomic changes produced by VSV infection in primary cultures of porcine macrophages. The results indicated that VSV infection induced the massive expression of multiple anorexic, pyrogenic, proinflammatory, and immunosuppressive genes. Overall, the interferon (IFN) response appeared to be suppressed, leading to the absence of stimulation of interferon-stimulated genes (ISG). Interestingly, VSV infection promoted the expression of several genes known to downregulate the expression of IFNβ. This represents an alternate mechanism for VSV control of the IFN response, beyond the recognized mechanisms mediated by the matrix protein. Although there was no significant differential gene expression in macrophages infected with a highly virulent epidemic strain compared to a less virulent endemic strain, the endemic strain consistently induced higher expression of all upregulated cytokines and chemokines. Collectively, this study provides novel insights into VSV molecular pathogenesis and immune evasion that warrant further investigation.

Understanding Hepatitis B Virus Dynamics and the Antiviral Effect of Interferon Alpha Treatment in Humanized Chimeric Mice

Whereas the mode of action of lamivudine (LAM) against hepatitis B virus (HBV) is well established, the inhibition mechanism(s) of interferon alpha (IFN-α) is less completely defined. To advance our understanding, we mathematically modeled HBV kinetics during 14-day pegylated IFN-α-2a (pegIFN), LAM, or pegIFN-plus-LAM (pegIFN+LAM) treatment of 39 chronically HBV-infected humanized uPA/SCID chimeric mice. Serum HBV DNA and intracellular HBV DNA were measured frequently. We developed a multicompartmental mathematical model and simultaneously fit it to the serum and intracellular HBV DNA data. Unexpectedly, even in the absence of an adaptive immune response, a biphasic decline in serum HBV DNA and intracellular HBV DNA was observed in response to all treatments. Kinetic analysis and modeling indicate that the first phase represents inhibition of intracellular HBV DNA synthesis and secretion, which was similar under all treatments with an overall mean efficacy of 98%. In contrast, there were distinct differences in HBV decline during the second phase, which was accounted for in the model by a time-dependent inhibition of intracellular HBV DNA synthesis, with the steepest decline observed during pegIFN+LAM treatment (1.28/day) and the slowest (0.1/day) during pegIFN monotherapy. Reminiscent of observations in patients treated with pegIFN and/or LAM, a biphasic HBV decline was observed in treated humanized mice in the absence of an adaptive immune response. Interestingly, combination treatment did not increase the initial inhibition of HBV production but rather enhanced second-phase decline, providing insight into the dynamics of HBV treatment response and the mode of action of IFN-α against HBV. IMPORTANCE Chronic hepatitis B virus (HBV) infection remains a global health care problem, as we lack sufficient curative treatment options. Elucidating the dynamics of HBV infection and treatment response at the molecular level could facilitate the development of novel, more effective HBV antivirals. Currently, the only well-established small animal HBV infection model available is the chimeric uPA/SCID mice with humanized livers; however, the HBV inhibition kinetics under pegylated IFN-α-2a (pegIFN) in this model system have not been determined in sufficient detail. In this study, viral kinetics in 39 humanized mice treated with pegIFN and/or lamivudine were monitored and analyzed using a mathematical modeling approach. We found that the main mode of action of IFN-α is blocking HBV DNA synthesis and that the majority of synthesized HBV DNA is secreted. Our study provides novel insights into HBV DNA dynamics within infected human hepatocytes.

The endoplasmic reticulum unfolded protein response - homeostasis, cell death and evolution in virus infections

Viruses elicit cell and organismic stress, and offset homeostasis. They trigger intrinsic, innate and adaptive immune responses, which limit infection. Viruses restore homeostasis by harnessing evolutionary conserved stress responses, such as the endoplasmic reticulum (ER) unfolded protein response (UPR^ER). The canonical UPR^ER restores homeostasis based on a cell-autonomous signalling network modulating transcriptional and translational output. The UPR^ER remedies cell damage, but upon severe and chronic stress leads to cell death. Signals from the UPR^ER flow along three branches with distinct stress sensors, the inositol requiring enzyme (Ire) 1, protein kinase R (PKR)-like ER kinase (PERK), and the activating transcription factor 6 (ATF6). This review shows how both enveloped and non-enveloped viruses use the UPR^ER to control cell stress and metabolic pathways, and thereby enhance infection and progeny formation, or undergo cell death. We highlight how the Ire1 axis bypasses apoptosis, boosts viral transcription and maintains dormant viral genomes during latency and persistence periods concurrent with long term survival of infected cells. These considerations open new options for oncolytic virus therapies against cancer cells where the UPR^ER is frequently upregulated. We conclude with a discussion of the evolutionary impact that viruses, in particular retroviruses, and anti-viral defense has on the UPR^ER.

Mucosal immunity and tRNA, tRF, and tiRNA

Mucosal immunity has crucial roles in human diseases such as respiratory tract infection, inflammatory bowel diseases (IBD), and colorectal cancer (CRC). Recent studies suggest that the mononuclear phagocyte system, cancer cells, bacteria, and viruses induce the mucosal immune reaction by various pathways, and can be major factors in the pathogenesis of these diseases. Transfer RNA (tRNA) and its fragments, including tRNA-derived RNA fragments (tRFs) and tRNA-derived stress-induced RNAs (tiRNAs), have emerged as a hot topic in recent years. They not only are verified as essential for transcription and translation but also play roles in cellular homeostasis and functions, such as cell metastasis, proliferation, and apoptosis. However, the specific relationship between their biological regulation and mucosal immunity remains unclear to date. In the present review, we carry out a comprehensive discussion on the specific roles of tRNA, tRFs, and tiRNAs relevant to mucosal immunity and related diseases.

Porcine reproductive and respiratory syndrome virus infection induces endoplasmic reticulum stress, facilitates virus replication, and contributes to autophagy and apoptosis

During viral infection, the host cell synthesizes high amounts of viral proteins, which often causes stress to the endoplasmic reticulum (ER). To manage abnormal ER stress, mammalian cells trigger a response called the unfolded protein response (UPR). Previous studies have indicated that porcine reproductive and respiratory syndrome virus (PRRSV), an Arterivirus that has been devastating the swine industry worldwide, can induce ER stress and activate UPR, however, the activation pathways and the biological significance requires further investigation. In this study, we demonstrated that, among the three types of UPR pathways, PRRSV infection induced PERK and IRE1 pathways, but not the ATF6 pathway. Furthermore, the induction of UPR promoted PRRSV replication. We also found that PRRSV-induced UPR, particularly the PERK pathway, was involved in the induction of autophagy, a cellular degradation process that can alleviate cell stress. Besides, we also provided insights into the ER stress-mediated apoptosis in response to PRRSV infection. PRRSV infection induced the expression of the transcription factor CHOP, which activated caspase 3 and PARP led to ER stress-mediated apoptosis. Using 3-Methyladenine (3-MA) to inhibit autophagy, the increased ER stress and cell apoptosis were observed in the PRRSV infected cell. Taken together, our results revealed the associations of ER stress, autophagy, and apoptosis during PRRSV infection, helping us to further understand how PRRSV interacts with host cells.

Stress proteins: the biological functions in virus infection, present and challenges for target-based antiviral drug development

Stress proteins (SPs) including heat-shock proteins (HSPs), RNA chaperones, and ER associated stress proteins are molecular chaperones essential for cellular homeostasis. The major functions of HSPs include chaperoning misfolded or unfolded polypeptides, protecting cells from toxic stress, and presenting immune and inflammatory cytokines. Regarded as a double-edged sword, HSPs also cooperate with numerous viruses and cancer cells to promote their survival. RNA chaperones are a group of heterogeneous nuclear ribonucleoproteins (hnRNPs), which are essential factors for manipulating both the functions and metabolisms of pre-mRNAs/hnRNAs transcribed by RNA polymerase II. hnRNPs involve in a large number of cellular processes, including chromatin remodelling, transcription regulation, RNP assembly and stabilization, RNA export, virus replication, histone-like nucleoid structuring, and even intracellular immunity. Dysregulation of stress proteins is associated with many human diseases including human cancer, cardiovascular diseases, neurodegenerative diseases (e.g., Parkinson’s diseases, Alzheimer disease), stroke and infectious diseases. In this review, we summarized the biologic function of stress proteins, and current progress on their mechanisms related to virus reproduction and diseases caused by virus infections. As SPs also attract a great interest as potential antiviral targets (e.g., COVID-19), we also discuss the present progress and challenges in this area of HSP-based drug development, as well as with compounds already under clinical evaluation.

PERK-Mediated Unfolded Protein Response Signaling Restricts Replication of the Tick-Borne Flavivirus Langat Virus

The unfolded protein response (UPR) maintains protein-folding homeostasis in the endoplasmic reticulum (ER) and has been implicated as both beneficial and detrimental to flavivirus infection. Protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK), a sensor of the UPR, is commonly associated with antiviral effects during mosquito-borne flavivirus (MBFV) infection, but its relation to tick-borne flavivirus (TBFV) infection remains largely unexplored. In this study, we identified changes in UPR and autophagic activity during Langat virus (LGTV) infection. LGTV robustly activated UPR and altered autophagic flux. Knockdown of endogenous PERK in human cells resulted in increased LGTV replication, but not that of closely related Powassan virus (POWV). Finally, on examining changes in protein levels of components associated with UPR and autophagy in the absence of PERK, we could show that LGTV-infected cells induced UPR but did not lead to expression of C/EBP homologous protein (CHOP), an important downstream transcription factor of multiple stress pathways. From these data, we hypothesize that LGTV can antagonize other kinases that target eukaryotic initiation factor 2α (eIF2α), but not PERK, implicating PERK as a potential mediator of intrinsic immunity. This effect was not apparent for POWV, a more pathogenic TBFV, suggesting it may be better equipped to mitigate the antiviral effects of PERK.

Insights Into the Role of Endoplasmic Reticulum Stress in Infectious Diseases

The endoplasmic reticulum (ER) is the major organelle in the cell for protein folding and plays an important role in cellular functions. The unfolded protein response (UPR) is activated in response to misfolded or unfolded protein accumulation in the ER. However, the UPR successfully alleviates the ER stress. If UPR fails to restore ER homeostasis, apoptosis is induced. ER stress plays an important role in innate immune signaling in response to microorganisms. Dysregulation of UPR signaling contributes to the pathogenesis of a variety of infectious diseases. In this review, we summarize the contribution of ER stress to the innate immune response to invading microorganisms and its role in the pathogenesis of infectious diseases.

Calcium Influx Caused by ER Stress Inducers Enhances Oncolytic Adenovirus Enadenotucirev Replication and Killing through PKCα Activation

Oncolytic viruses represent an emerging approach to cancer therapy. However, better understanding of their interaction with the host cancer cell and approaches to enhance their efficacy are needed. Here, we investigate the effect of chemically induced endoplasmic reticulum (ER) stress on the activity of the chimeric group B adenovirus Enadenotucirev, its closely related parental virus Ad11p, and the archetypal group C oncolytic adenovirus Ad5. We show that treatment of colorectal and ovarian cancer cell lines with thapsigargin or ionomycin caused an influx of Ca2+, leading to an upregulation in E1A transcript and protein levels. Increased E1A protein levels, in turn, increased levels of expression of the E2B viral DNA polymerase, genome replication, late viral protein expression, infectious virus particle production, and cell killing during Enadenotucirev and Ad11p, but not Ad5, infection. This effect was not due to the induction of ER stress, but rather the influx of extracellular Ca2+ and consequent increase in protein kinase C activity. These results underscore the importance of Ca2+ homeostasis during adenoviral infection, indicate a signaling pathway between protein kinase C and E1A, and raise the possibility of using Ca2+ flux-modulating agents in the manufacture and potentiation of oncolytic virotherapies.

eIF2α-CHOP-BCl-2/JNK and IRE1α-XBP1/JNK signaling promote apoptosis and inflammation and support the proliferation of Newcastle disease virus

Newcastle disease virus (NDV) causes severe infectious disease in poultry and selectively kills tumor cells, by inducing apoptosis and cytokines secretion. In this report, we study the mechanisms underlying NDV-induced apoptosis by investigating the unfolded protein response (UPR). We found that NDV infection activated all three branches of the UPR signaling (PERK-eIF2α, ATF6, and IRE1α) and triggered apoptosis, in avian cells (DF-1 and CEF) and in various human cancer cell types (HeLa, Cal27, HN13, A549, H1299, Huh7, and HepG2). Interestingly, the suppression of either apoptosis or UPR led to impaired NDV proliferation. Meanwhile, the inhibition of UPR by 4-PBA protected cells from NDV-induced apoptosis. Further study revealed that activation of PERK-eIF2α induced the expression of transcription factor CHOP, which subsequently promoted apoptosis by downregulating BCL-2/MCL-1, promoting JNK signaling and suppressing AKT signaling. In parallel, IRE1α mediated the splicing of XBP1 mRNA and resulted in the translation and nuclear translocation of XBP1s, thereby promoting the transcription of ER chaperones and components of ER-associated degradation (ERAD). Furthermore, IRE1α promoted apoptosis and cytokines secretion via the activation of JNK signaling. Knock down and overexpression studies showed that CHOP, IRE1α, XBP1, and JNK supported efficient virus proliferation. Our study demonstrates that the induction of eIF2α-CHOP-BCL-2/JNK and IRE1α-XBP1/JNK signaling cascades promote apoptosis and cytokines secretion, and these signaling cascades support NDV proliferation.

Porcine parvovirus replication is suppressed by activation of the PERK signaling pathway and endoplasmic reticulum stress-mediated apoptosis

Endoplasmic reticulum (ER) stress is associated with numerous mammalian diseases, especially viral diseases. Porcine parvovirus (PPV) is the causative agent of reproductive failure in swine. Here, we observed that the PPV infection of porcine kidney 15 and porcine testis cells resulted in the activation of ER stress sensors mediated by protein kinase R-like ER kinase (PERK), but not inositol-requiring enzyme 1 and activating transcription factor 6 (ATF6). ER stress activation obviously blocked PPV replication. Depletion of proteins, such as PERK, eukaryotic initiation factor 2, and ATF4, by small interfering RNA significantly enhanced PPV replication. Moreover, the pro-apoptotic factor C/EBP homologous protein was identified a key factor in the inhibition of PPV replication. These data demonstrate that PPV infection activates ER stress through the PERK signaling pathway and that ER stress inhibits further PPV replication by promoting apoptosis.

Non-Structural Protein 2B of Human Rhinovirus 16 Activates Both PERK and ATF6 Rather Than IRE1 to Trigger ER Stress

To understand the underlying mechanisms of endoplasmic reticulum (ER) stress caused by human rhinovirus (HRV) 16 and non-structural transmembrane protein 2B, the expressions of ER chaperone glucose-regulated protein 78 (GRP78) and three signal transduction pathways, including protein kinase RNA-like ER kinase (PERK), activating transcription factor 6 (ATF6) and inositol-requiring enzyme 1 (IRE1), were evaluated after HRV16 infection and 2B gene transfection. Our results showed that both HRV16 infection and 2B gene transfection increased the expression of ER chaperone GRP78, and induced phosphorylation of PERK and cleavage of ATF6 in a time-dependent manner. Our data also revealed that the HRV16 2B protein was localized to the ER membrane. However, both HRV16 infection and HRV16 2B gene transfection did not induce ER stress through the IRE1 pathway. Moreover, our results showed that apoptosis occurred in H1-HeLa cells infected with HRV16 or transfected with 2B gene accompanied with increased expression of CHOP and cleaved caspase-3. Taken together, non-structural protein 2B of HRV16 induced an ER stress response through the PERK and ATF6 pathways rather than the IRE1 pathway.

Litopenaeus vannamei activating transcription factor 6 alpha gene involvement in ER-stress response and white spot symptom virus infection

A previous study found that inositol-requiring enzyme-1-X-box binding protein 1 (IRE1-XBP1) pathway and the protein kinase RNA (PKR)-like ER kinase-eIF2α (PERK-eIF2α) pathway of shrimp play roles in the unfolded protein response (UPR). And they also be proved that was involved in white spot symptom virus (WSSV) infection. Yet the functions of the third branch in shrimp UPR are still unclear. In this study, we showed that upon UPR activation, activating transcription factor 6 alpha (LvATF6α) of Litopenaeus vannamei was cleaved and transferred from the cytoplasm to the nucleus in 293T cells, indicating that the ATF6 pathway in shrimp is also a branch of UPR. Furthermore, LvATF6α could reduce the apoptosis rate of Drosophila Schneider 2 (S2) cells treated with actinomycin, and knock-down expression of LvATF6α increased the apoptosis rate of shrimp hemocytes. In vivo testing revealed that the short from LvATF6α (LvATF6α-s) was obviously increased after UPR activation or WSSV infection, indicating that the ATF6 pathway was activated in L. vannamei gills under such circumstances. Moreover, knock-down expression of LvATF6α could reduce the cumulative mortality and WSSV copy number in WSSV-infected shrimp. Further study revealed that WSSV may profit from shrimp ATF6 pathway activation in two aspects. First, LvATF6α-s significantly upregulated the expression of the WSSV genes (wsv023, wsv045, wsv083, wsv129, wsv222, wsv249, and wsv343). Second, LvATF6α-s inhibited apoptosis by negatively regulating the apoptosis signal-regulating kinase 1 - (c-Jun N-terminal kinase) pathway. All of these evidences suggested that the ATF6 pathway is a member of the L. vannamei UPR, and it is also engaged in WSSV infection.

IRE1α promotes viral infection by conferring resistance to apoptosis

The unfolded protein response (UPR) is an ancient cellular pathway that detects and alleviates protein-folding stresses. The UPR components X-box binding protein 1 (XBP1) and inositol-requiring enzyme 1α (IRE1α) promote type I interferon (IFN) responses. We found that Xbp1-deficient mouse embryonic fibroblasts and macrophages had impaired antiviral resistance. However, this was not because of a defect in type I IFN responses but rather an inability of Xbp1-deficient cells to undergo viral-induced apoptosis. The ability to undergo apoptosis limited infection in wild-type cells. Xbp1-deficient cells were generally resistant to the intrinsic pathway of apoptosis through an indirect mechanism involving activation of the nuclease IRE1α. We observed an IRE1α-dependent reduction in the abundance of the proapoptotic microRNA miR-125a and a corresponding increase in the amounts of the members of the antiapoptotic Bcl-2 family. The activation of IRE1α by the hepatitis C virus (HCV) protein NS4B in XBP1-proficient cells also conferred apoptosis resistance and promoted viral replication. Furthermore, we found evidence of IRE1α activation and decreased miR-125a abundance in liver biopsies from patients infected with HCV compared to those in the livers of healthy controls. Our results reveal a prosurvival role for IRE1α in virally infected cells and suggest a possible target for IFN-independent antiviral therapy.

Vitamin C modulates the metabolic and cytokine profiles, alleviates hepatic endoplasmic reticulum stress, and increases the life span of Gulo-/- mice

Suboptimal intake of dietary vitamin C (ascorbate) increases the risk of several chronic diseases but the exact metabolic pathways affected are still unknown. In this study, we examined the metabolic profile of mice lacking the enzyme gulonolactone oxidase (Gulo) required for the biosynthesis of ascorbate. Gulo^−/− mice were supplemented with 0%, 0.01%, and 0.4% ascorbate (w/v) in drinking water and serum was collected for metabolite measurements by targeted mass spectrometry. We also quantified 42 serum cytokines and examined the levels of different stress markers in liver. The metabolic profiles of Gulo^−/− mice treated with ascorbate were different from untreated Gulo^−/− and normal wild type mice. The cytokine profiles of Gulo^−/− mice, in return, overlapped the profile of wild type animals upon 0.01% or 0.4% vitamin C supplementation. The life span of Gulo^−/− mice increased with the amount of ascorbate in drinking water. It also correlated significantly with the ratios of serum arginine/lysine, tyrosine/phenylalanine, and the ratio of specific species of saturated/unsaturated phosphatidylcholines. Finally, levels of hepatic phosphorylated endoplasmic reticulum associated stress markers IRE1α and eIF2α correlated inversely with serum ascorbate and life span suggesting that vitamin C modulates endoplasmic reticulum stress response and longevity in Gulo^−/− mice.

ER Stress, UPR and Virus Infections in Plants

The endoplasmic reticulum (ER) endomembrane is a central site for protein synthesis. Perturbation of ER homeostasis can result in an accumulation of unfolded proteins within the ER lumen, causing ER stress and the unfolded protein response (UPR). In humans, ER stress and UPR are closely associated with a vast number of diseases, including viral diseases. In plants, two arms that govern the UPR signaling network have been described: one that contains two ER membrane–associated transcription factors (bZIP17 and bZIP28) and the other that encompasses a dual protein kinase (RNA-splicing factor IRE1) and its target RNA (bZIP60). Although early studies mainly focus on the essential roles of the UPR in abiotic stresses, the significance of UPR in plant diseases caused by virus infections has recently drawn much attention. This chapter summarizes the latest scenario of ER stress and UPR in virus-infected plant cells, highlights the emerging roles of the IRE1 pathway in virus infections, and outlines exciting future directions to spark more research interest in the UPR field in plants.

PKR deficiency alters E. coli-induced sickness behaviors but does not exacerbate neuroimmune responses or bacterial load

Background

Systemic inflammation induces neuroimmune activation, ultimately leading to sickness (e.g., fever, anorexia, motor impairments, exploratory deficits, and social withdrawal). In this study, we evaluated the role of protein kinase R (PKR), a serine-threonine kinase that can control systemic inflammation, on neuroimmune responses and sickness.

Methods

Wild-type (WT) PKR+/+ mice and PKR−/− mice were subcutaneously injected with live Escherichia coli (E. coli) or vehicle. Food consumption, rotarod test performance, burrowing, open field activity, object investigation, and social interaction were monitored. Plasma TNF-α and corticosterone were measured by ELISA. The percentage of neutrophils in blood was deduced from blood smears. Inflammatory gene expression (IL-1β, TNF-α, IL-6, cyclooxygenase (COX)-2, iNOS) in the liver and the brain (hypothalamus and hippocampus) were quantified by real-time PCR. Blood and lavage fluid (injection site) were collected for microbiological plate count and for real-time PCR of bacterial 16S ribosomal DNA (rDNA). Corticotrophin-releasing hormone (CRH) expression in the hypothalamus was also determined by real-time PCR.

Results

Deficiency of PKR diminished peripheral inflammatory responses following E. coli challenge. However, while the core components of sickness (anorexia and motor impairments) were similar between both strains of mice, the behavioral components of sickness (reduced burrowing, exploratory activity deficits, and social withdrawal) were only observable in PKR−/− mice but not in WT mice. Such alteration of behavioral components was unlikely to be caused by exaggerated neuroimmune activation, by an impaired host defense to the infection, or due to a dysregulated corticosterone response, because both strains of mice displayed similar neuroimmune responses, bacterial titers, and plasma corticosterone profiles throughout the course of infection. Nevertheless, the induction of hypothalamic corticotrophin-releasing hormone (CRH) by E. coli was delayed in PKR−/− mice relative to WT mice, suggesting that PKR deficiency may postpone the CRH response during systemic inflammation.

Conclusions

Taken together, our findings show that (1) loss of PKR could alter E. coli-induced sickness behaviors and (2) this was unlikely to be due to exacerbated neuroimmune activation, (3) elevated bacterial load, or (4) dysregulation in the corticosterone response. Further studies can address the role of PKR in the CRH response together with its consequence on sickness.

Inhibition of viral protein translation by indomethacin in vesicular stomatitis virus infection: role of eIF2α kinase PKR

Indomethacin, a cyclooxygenase‐1 and ‐2 inhibitor widely used in the clinic for its potent anti‐inflammatory/analgesic properties, possesses antiviral activity against several viral pathogens; however, the mechanism of antiviral action remains elusive. We have recently shown that indomethacin activates the double‐stranded RNA (dsRNA)‐dependent protein kinase R (PKR) in human colon cancer cells. Because of the important role of PKR in the cellular defence response against viral infection, herein we investigated the effect of indomethacin on PKR activity during infection with the prototype rhabdovirus vesicular stomatitis virus. Indomethacin was found to activate PKR in an interferon‐ and dsRNA‐independent manner, causing rapid (< 5 min) phosphorylation of eukaryotic initiation factor‐2 α‐subunit (eIF2α). These events resulted in shutting off viral protein translation and blocking viral replication (IC₅₀ = 2 μM) while protecting host cells from virus‐induced damage. Indomethacin did not affect eIF2α kinases PKR‐like endoplasmic reticulum‐resident protein kinase (PERK) and general control non‐derepressible‐2 (GCN2) kinase, and was unable to trigger eIF2α phosphorylation in the presence of PKR inhibitor 2‐aminopurine. In addition, small‐interfering RNA‐mediated PKR gene silencing dampened the antiviral effect in indomethacin‐treated cells. The results identify PKR as a critical target for the antiviral activity of indomethacin and indicate that eIF2α phosphorylation could be a key element in the broad spectrum antiviral activity of the drug.

Coronavirus-induced ER stress response and its involvement in regulation of coronavirus-host interactions

Coronavirus replication is structurally and functionally associated with the endoplasmic reticulum (ER), a major site of protein synthesis, folding, modification and sorting in the eukaryotic cells. Disturbance of ER homeostasis may occur under various physiological or pathological conditions. In response to the ER stress, signaling pathways of the unfolded protein response (UPR) are activated. UPR is mediated by three ER transmembrane sensors, namely the PKR-like ER protein kinase (PERK), the inositol-requiring protein 1 (IRE1) and the activating transcriptional factor 6 (ATF6). UPR facilitates adaptation to ER stress by reversible translation attenuation, enhancement of ER protein folding capacity and activation of ER-associated degradation (ERAD). In cells under prolonged and irremediable ER stress, UPR can also trigger apoptotic cell death. Accumulating evidence has shown that coronavirus infection causes ER stress and induces UPR in the infected cells. UPR is closely associated with a number of major signaling pathways, including autophagy, apoptosis, the mitogen-activated protein (MAP) kinase pathways, innate immunity and pro-inflammatory response. Therefore, studies on the UPR are pivotal in elucidating the complicated issue of coronavirus-host interaction. In this paper, we present the up-to-date knowledge on coronavirus-induced UPR and discuss its potential involvement in regulation of innate immunity and apoptosis.

Roles of the translation initiation factor eIF2α serine 51 phosphorylation in cancer formation and treatment

Cells respond to various forms of stress by activating anti-proliferative pathways, which allow them to correct the damage caused by stress before re-entering proliferation. If the damage, however, is beyond repair, stressed cells are eliminated from the host by undergoing death. The balance between cell survival and death is essential for cancer formation and is determined by several key pathways that impact on different stages of gene expression. In recent years, it has become evident that phosphorylation of the alpha (α) subunit of the translation initiation factor eIF2 at serine 51 (eIF2αS51P) is an important determinant of cell fate in response to stress. Induction of eIF2αS51P is mediated by a family of four kinases namely, HRI, PKR, PERK and GCN2, each of which responds to distinct forms of stress. Increased eIF2αS51P results in a global inhibition of protein synthesis but at the same time enhances the translation of select mRNAs encoding for proteins that control cell adaptation to stress. Short-term induction of eIF2αS51P has been associated with cell survival whereas long-term induction with cell death. Studies in mouse and human models of cancer have provided compelling evidence that eIF2αS51P plays an essential role in stress-induced tumorigenesis. Increased eIF2αS51P exhibits cell autonomous as well as immune regulatory effects, which can influence tumor growth and the efficacy of anti-tumor therapies. The findings suggest that eIF2αS51P may be of prognostic value and a suitable target for the design and implementation of effective anti-tumor therapies. This article is part of a Special Issue entitled: Translation and Cancer.

Coronavirus infection, ER stress, apoptosis and innate immunity

The replication of coronavirus, a family of important animal and human pathogens, is closely associated with the cellular membrane compartments, especially the endoplasmic reticulum (ER). Coronavirus infection of cultured cells was previously shown to cause ER stress and induce the unfolded protein response (UPR), a process that aims to restore the ER homeostasis by global translation shutdown and increasing the ER folding capacity. However, under prolonged ER stress, UPR can also induce apoptotic cell death. Accumulating evidence from recent studies has shown that induction of ER stress and UPR may constitute a major aspect of coronavirus–host interaction. Activation of the three branches of UPR modulates a wide variety of signaling pathways, such as mitogen-activated protein (MAP) kinase activation, autophagy, apoptosis, and innate immune response. ER stress and UPR activation may therefore contribute significantly to the viral replication and pathogenesis during coronavirus infection. In this review, we summarize the current knowledge on coronavirus-induced ER stress and UPR activation, with emphasis on their cross-talking to apoptotic signaling.

Triggering unfolded protein response by 2-Deoxy-D-glucose inhibits porcine epidemic diarrhea virus propagation

The unfolded protein response (UPR) is cyto-protective machinery elicited towards an influx of large amount of protein synthesis in the endoplasmic reticulum (ER). Extensive studies suggest that the UPR can also be activated during virus infection. In the present studies, we first evaluated if porcine epidemic diarrhea virus (PEDV) infection activated the UPR pathways. Electron microscopy analysis demonstrated the morphology changes of ER post-PEDV infection. Western blot and real-time PCR identified the differences of UPR genes in response to PEDV infection. The results suggested that PEDV infection induced UPR in Vero cells. Meanwhile, we silenced the expression of PKR-like ER kinase (PERK) by shRNA, we found that the knockdown of PERK increased virus loads in the cells, which was consistent with the result on 4-phenylbutyrate (4-PBA) treatment. We next determined whether 2-Deoxy-d-glucose (2-DG), an ER stress inducer, possessed antiviral activity against PEDV infection. Plaque formation assay, RT-PCR and Western blot analysis suggested that 2-DG might inhibit virus infection by affecting viral protein translation during the early stage of virus infection. Interestingly, we also found that 2-DG treatment could affect virus assembly, which is similar to previous studies on influenza virus. All these results support the therapeutic potential of using 2-DG or glucose/mannose analogs to induce the UPR to block virus replication.

Danger signalling during cancer cell death: origins, plasticity and regulation

Accumulating data indicates that following anti-cancer treatments, cancer cell death can be perceived as immunogenic or tolerogenic by the immune system. The former is made possible due to the ability of certain anti-cancer modalities to induce immunogenic cell death (ICD) that is associated with the emission of damage-associated molecular patterns (DAMPs), which assist in unlocking a sequence of events leading to the development of anti-tumour immunity. In response to ICD inducers, activation of endoplasmic reticulum (ER) stress has been identified to be indispensable to confer the immunogenic character of cancer cell death, due to its ability to coordinate the danger signalling pathways responsible for the trafficking of vital DAMPs and subsequent anti-cancer immune responses. However, in recent times, certain processes apart from ER stress have emerged (e.g., autophagy and possibly viral response-like signature), which have the ability to influence danger signalling. In this review, we discuss the molecular nature, emerging plasticity in the danger signalling mechanisms and immunological impact of known DAMPs in the context of immunogenic cancer cell death. We also discuss key effector mechanisms modulating the interface between dying cancer cells and the immune cells, which we believe are crucial for the therapeutic relevance of ICD in the context of human cancers, and also discuss the influence of experimental conditions and animal models on these.

Deletion of P58(IPK), the Cellular Inhibitor of the Protein Kinases PKR and PERK, Causes Bone Changes and Joint Degeneration in Mice

OBJECTIVE

Protein kinase-like endoplasmic reticulum kinase (PERK) and protein kinase R (PKR) are implicated in endoplasmic reticulum stress-induced arthritis and pro-inflammatory cytokine-mediated cartilage degradation in vitro, respectively. We determined whether knockout of the cellular inhibitor of PERK and PKR, P58(IPK) causes joint degeneration in vivo and whether these molecules are activated in human osteoarthritis (OA).

MATERIALS AND METHODS

Sections of knee joints from P58(IPK)-null and wild-type mice aged 12-13 and 23-25 months were stained with toluidine blue and scored for degeneration using the osteoarthritis research society international (OARSI) system. Bone changes were assessed by radiology and high-resolution micro-computed tomography of hind limbs. Sections from the medial tibial plateaus of two human knees, removed in total knee replacement surgery for OA, were immunolabelled for phosphorylated PERK and PKR and P58(IPK).

RESULTS

Knockout mice exhibited narrower tibiae (p = 0.0031) and smaller epiphyses in tibiae (p = 0.0004) and femora (p = 0.0214). Older knockout mice had reduced total volume inside the femoral periosteal envelope (p = 0.023), reduced tibial (p = 0.03), and femoral (p = 0.0012) bone volumes (BV) and reduced femoral BV fraction (p = 0.025). Compared with wild-types, younger P58(IPK)-null mice had increased OARSI scores in medial femoral condyles (p = 0.035). Thirty four percent of null mice displayed severe joint degeneration with complete articular cartilage loss from the medial compartment and heterotopic chondro-osseous tissue in the medial joint capsule. Phosphorylated PERK and PKR were localized throughout human osteoarthritic tibial plateaus but, in particular, in areas exhibiting the most degeneration. There was limited expression of P58(IPK).

CONCLUSION

This study is the first to reveal a critical role for P58(IPK) in maintaining joint integrity in vivo, implicating the PKR and PERK stress signaling pathways in bony changes underlying the pathogenesis of joint degeneration.

Explorations of substituted urea functionality for the discovery of new activators of the heme-regulated inhibitor kinase

Heme-regulated inhibitor kinase (HRI), a eukaryotic translation initiation factor 2 alpha (eIF2α) kinase, plays critical roles in cell proliferation, differentiation, and adaptation to cytoplasmic stress. HRI is also a critical modifier of hemoglobin disorders such as β-thalassemia. We previously identified N,N'-diarylureas as potent activators of HRI suitable for studying the biology of this important kinase. To expand the repertoire of chemotypes that activate HRI, we screened a ∼1900 member N,N'-disubstituted urea library in the surrogate eIF2α phosphorylation assay, identifying N-aryl,N'-cyclohexylphenoxyurea as a promising scaffold. We validated hit compounds as a bona fide HRI activators in secondary assays and explored the contributions of substitutions on the N-aryl and N'-cyclohexylphenoxy groups to their activity by studying focused libraries of complementing analogues. We tested these N-aryl,N'-cyclohexylphenoxyureas in the surrogate eIF2α phosphorylation and cell proliferation assays, demonstrating significantly improved bioactivities and specificities. We consider these compounds to represent lead candidates for the development of potent and specific HRI activators.

Dual activators of protein kinase R (PKR) and protein kinase R-like kinase PERK identify common and divergent catalytic targets

Chemical genetics has evolved into a powerful tool for studying gene function in normal and pathobiology. PKR and PERK, two eukaryotic translation initiation factor 2 alpha (eIF2α) kinases, play critical roles in the maintenance of cellular hemostasis, metabolic stability, and anti-viral defenses. Both kinases interact with and phosphorylate additional substrates including tumor suppressor p53 and nuclear protein 90. Loss of function of both kinases has been studied by reverse genetics and with recently identified inhibitors. In contrast, no activating probes for studying the catalytic activity of these kinases are available. We identified 3-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-5,7-dihydroxy-4H-chromen-4-one (DHBDC) as a specific dual activator of PKR and PERK by screening a chemical library of 20 000 small molecules in a dual luciferase surrogate eIF2α phosphorylation assay. We present here extensive biological characterization and a preliminary structure-activity relationship of DHBDC, which phosphorylates eIF2α by activating PKR and PERK but no other eIF2α kinases. These agents also activate downstream effectors of eIF2α phosphorylation by inducing CEBP homologue protein, suppressing cyclin D1 expression, and inhibiting cancer cell proliferation, all in a manner dependent on PKR and PERK. Consistent with the role of eIF2α phosphorylation in viral infection, DHBDC inhibits the proliferation of human hepatitis C virus. Finally, DHBDC induces the phosphorylation of IκBα and activates the NF-κB pathway. Surprisingly, activation of the NF-κB pathway is dependent on PERK but independent of PKR activity. These data indicate that DHBDC is an invaluable probe for elucidating the role of PKR and PERK in normal and pathobiology.

Cannabinoid-induced enhanced interaction and protein levels of serotonin 5-HT(2A) and dopamine D₂ receptors in rat prefrontal cortex

Recent evidence suggests that non-selective cannabinoid receptor agonists may regulate serotonin 2A (5-HT(2A)) receptor neurotransmission in brain. The molecular mechanisms of this regulation are unknown, but could involve cannabinoid-induced enhanced interaction between 5-HT(2A) and dopamine D2 (D₂) receptors. Here, we present experimental evidence that Sprague-Dawley rats treated with a non-selective cannabinoid receptor agonist (CP55,940, 50 µg/kg, 7 days, i.p.) showed enhanced co-immunoprecipitation of 5-HT(2A) and D₂ receptors and enhanced membrane-associated expression of D₂ and 5-HT(2A) receptors in prefrontal cortex (PFCx). Furthermore, 5-HT(2A) receptor mRNA levels were increased in PFCx, suggesting a cannabinoid-induced upregulation of 5-HT(2A) receptors. To date, two cannabinoids receptors have been found in brain, CB1 and CB2 receptors. We used selective cannabinoid agonists in a neuronal cell line to study mechanisms that could mediate this 5-HT(2A) receptor upregulation. We found that selective CB2 receptor agonists upregulate 5-HT(2A) receptors by a mechanism that seems to involve activation of Gα(i) G-proteins, ERK1/2, and AP-1 transcription factor. We hypothesize that the enhanced cannabinoid-induced interaction between 5-HT(2A) and D₂ receptors and in 5-HT(2A) and D₂ receptors protein levels in the PFCx might provide a molecular mechanism by which activation of cannabinoid receptors might be contribute to the pathophysiology of some cognitive and mood disorders.

Virus-induced ER stress and the unfolded protein response

The accumulation of unfolded or misfolded proteins in the lumen of the endoplasmic reticulum (ER) results in ER stress that triggers cytoprotective signaling pathways, termed the unfolded protein response (UPR), to restore and maintain homeostasis in the ER or to induce apoptosis if ER stress remains unmitigated. The UPR signaling network encompasses three core elements, i.e., PKR-like ER kinase (PERK), activating transcription factor 6 (ATF6), and inositol-requiring protein-1 (IRE1). Activation of these three branch pathways of the UPR leads to the translation arrest and degradation of misfolded proteins, the expression of ER molecular chaperones, and the expansion of the ER membrane to decrease the load of proteins and increase the protein-folding capacity in the ER. Recently, the essential roles of the UPR have been implicated in a number of mammalian diseases, particularly viral diseases. In virus-infected cells, the cellular translation machinery is hijacked by the infecting virus to produce large amounts of viral proteins, which inevitably perturbs ER homeostasis and causes ER stress. This review summarizes current knowledge about the UPR signaling pathways, highlights two identified UPR pathways in plants, and discuss progress in elucidating the UPR in virus-infected cells and its functional roles in viral infection.

Increased susceptibility to DNA virus infection in mice with a GCN2 mutation

The downregulation of translation through eIF2α phosphorylation is a cellular response to diverse stresses, including viral infection, and is mediated by the GCN2 kinase, protein kinase R (PKR), protein kinase-like endoplasmic reticulum kinase (PERK), and heme-regulated inhibitor kinase (HRI). Although PKR plays a major role in defense against viruses, other eIF2α kinases also may respond to viral infection and contribute to the shutdown of protein synthesis. Here we describe the recessive, loss-of-function mutation atchoum (atc) in Eif2ak4, encoding GCN2, which increased susceptibility to infection by the double-stranded DNA viruses mouse cytomegalovirus (MCMV) and human adenovirus. This mutation was identified by screening macrophages isolated from mice carrying N-ethyl-N-nitrosourea (ENU)-induced mutations. Cells from Eif2ak4(atc/atc) mice failed to phosphorylate eIF2α in response to MCMV. Importantly, homozygous Eif2ak4(atc) mice showed a modest increase in susceptibility to MCMV infection, demonstrating that translational arrest dependent on GCN2 contributes to the antiviral response in vivo.

The immunity-related GTPase Irgm3 relieves endoplasmic reticulum stress response during coxsackievirus B3 infection via a PI3K/Akt dependent pathway

The IRG protein Irgm3 preserves cell survival during coxsackievirus B3 (CVB3) infection. However, the molecular mechanisms are not clear. Here, we examined the effect of Irgm3 expression on ER stress triggered by pharmacological agents or CVB3 infection. In Tet-On/Irgm3 HeLa cells, Irgm3 expression suppressed either chemical- or CVB3-induced upregulation of glucose-regulated protein 78. Further, Irgm3 strongly inhibited the activation of both the PERK and ATF6 pathways of ER stress responses, which further led to the diminished phosphorylation of eIF2α, reduced cleavage/activation of transcription factor SREBP1 and attenuated induction of proapoptotic genes CHOP and GADD34. These data were further supported by experiments using Irgm3 knockout mouse embryonic fibroblasts, in which the ER stress induced by CVB3 was not relieved due to the lack of Irgm3 expression. In addition, the tunicamycin-triggered ER stress promoted the subsequent CVB3 infection. The effect of Irgm3 on ER stress and CVB3 infection was diminished by the PI3K inhibitor, LY294002, while inhibitors of ERK, JNK and p38 had no effect. These data were further corroborated by transfection of cells with a dominant negative Akt. Taken together, these data suggest that Irgm3 relieves the ER stress response via a PI3K/Akt dependent mechanism, which contributes to host defence against CVB3 infection.

Potential of vesicular stomatitis virus as an oncolytic therapy for recurrent and drug-resistant ovarian cancer

In the last decade, we have gained significant understanding of the mechanism by which vesicular stomatitis virus (VSV) specifically kills cancer cells. Dysregulation of translation and defective innate immunity are both thought to contribute to VSV oncolysis. Safety and efficacy are important objectives to consider in evaluating VSV as a therapy for malignant disease. Ongoing efforts may enable VSV virotherapy to be considered in the near future to treat drug-resistant ovarian cancer when other options have been exhausted. In this article, we review the development of VSV as a potential therapeutic approach for recurrent or drug-resistant ovarian cancer.

Aquatic birnavirus-induced ER stress-mediated death signaling contribute to downregulation of Bcl-2 family proteins in salmon embryo cells

Aquatic birnavirus induces mitochondria-mediated cell death, but whether connects to endoplasmic reticulum (ER) stress is still unknown. In this present, we characterized that IPNV infection triggers ER stress-mediated cell death via PKR/eIF2α phosphorylation signaling for regulating the Bcl-2 family protein expression in fish cells. The IPNV infection can induce ER stress as follows: (1) ER stress sensor ATF6 cleavaged; (2) ER stress marker GRP78 upregulation, and (3) PERK/eIF2α phosphorylation. Then, the IPNV-induced ER stress signals can induce the CHOP expression at early (6 h p.i.) and middle replication (12 h p.i.) stages. Moreover, IPNV-induced CHOP upregulation dramatically correlates to apparently downregulate the Bcl-2 family proteins, Bcl-2, Mcl-1 and Bcl-xL at middle replication stage (12 h p.i.) and produces mitochondria membrane potential (MMP) loss and cell death. Furthermore, with GRP78 synthesis inhibitor momitoxin (VT) and PKR inhibitor 2-aminopurine (2-AP) treatment for blocking GRP78 expression and eIF2α phosphorylation, PKR/PERK may involve in eIF2α phosphorylation/CHOP upregulation pathway that enhances the downstream regulators Bcl-2 family proteins expression and increased cell survival. Taken together, our results suggest that IPNV infection activates PKR/PERK/eIF2α ER stress signals for regulating downstream molecules CHOP upregulation and Bcl-2 family downregulation that led to induce mitochondria-mediated cell death in fish cells, which may provide new insight into RNA virus pathogenesis and disease.

Betanodavirus up-regulates chaperone GRP78 via ER stress: roles of GRP78 in viral replication and host mitochondria-mediated cell death

Whether viral pathogens that induce ER stress responses benefit the host or the virus remains controversial. In this study we show that betanodavirus induced ER stress responses up-regulate GRP78, which regulates the viral replication and host cellular mitochondrial-mediated cell death. Betanodavirus (redspotted grouper nervous necrosis virus, RGNNV) infection resulted in the following increased ER stress responses in fish GF-1 grouper fin cells: (1) IRE-1 and ATF-6 sensors at 48 h post-infection (p.i.) that up-regulated chaperone protein GRP78; (2) activation of caspase-12; and (3) PERK phosphorylation and down-regulation of Bcl-2. Analyses of GRP78 functions during viral replication using either loss-of-function or gain-of-function approaches showed that GRP78 over-expression also enhanced viral replication and induced cell death. Then, we found that zfGRP78 localization gradually increased in mitochondria after RGNNV infection by EGFP tagging approach. Furthermore, zfGRP78 can interact with viral RNA-dependent RNA polymerase (RdRp) by using immunofluorescent and immunoprecipitation assays. Finally, we found that blocking GRP78-mediated ER signals can reduce the viral death factors protein α and protein B2 expression and decrease the Bcl-2 down-regulation mediated mitochondria-dependent cell death, which also enhances host cellular viability. Taken together, our results suggest that RGNNV infection and expression can trigger ER stress responses, which up-regulate the chaperone GRP78 at early replication stage. Then, GRP78 can interact with RdRp that may enhance the viral replication for increasing viral death factors' expressions at middle-late replication stage, which can enhance mitochondrial-mediated cell death pathway and viral spreading. These results may provide new insights into the mechanism of ER stress-mediated cell death in RNA viruses.

Role of p38 protein kinase in the ligand-independent ubiquitination and down-regulation of the IFNAR1 chain of type I interferon receptor

Phosphorylation-dependent ubiquitination and degradation of the IFNAR1 chain of type I interferon (IFN) receptor is a robust and specific mechanism that limits the magnitude and duration of IFNα/β signaling. Besides the ligand-inducible IFNAR1 degradation, the existence of an "inside-out" signaling that accelerates IFNAR1 turnover in the cells undergoing the endoplasmic reticulum (ER) stress and activated unfolded protein responses has been recently described. The latter pathway does not require either presence of ligands (IFNα/β) or catalytic activity of Janus kinases (JAK). Instead, this pathway relies on activation of the PKR-like ER kinase (PERK) and ensuing specific priming phosphorylation of IFNAR1. Here, we describe studies that identify the stress activated p38 protein kinase as an important regulator of IFNAR1 that acts downstream of PERK. Results of the experiments using pharmacologic p38 kinase inhibitors, RNA interference approach, and cells from p38α knock-out mice suggest that p38 kinase activity is required for priming phosphorylation of IFNAR1 in cells undergoing unfolded protein response. We further demonstrate an important role of p38 kinase in the ligand-independent stimulation of IFNAR1 ubiquitination and degradation and ensuing attenuation of IFNα/β signaling and anti-viral defenses. We discuss the distinct importance of p38 kinase in regulating the overall responses to type I IFN in cells that have been already exposed to IFNα/β versus those cells that are yet to encounter these cytokines.

West Nile virus differentially modulates the unfolded protein response to facilitate replication and immune evasion

For intracellular survival it is imperative that viruses have the capacity to manipulate various cellular responses, including metabolic and biosynthetic pathways. The unfolded protein response (UPR) is induced by various external and internal stimuli, including the accumulation of misfolded proteins in the endoplasmic reticulum (ER). Our previous studies have indicated that the replication and assembly of the flavivirus West Nile virus strain Kunjin virus (WNV(KUN)) is intimately associated with the ER. Thus, we sought to determine whether the UPR was induced during WNV(KUN) infection. WNV(KUN) induces UPR signaling during replication, which is coordinated with peak replication. Interestingly, signaling is biased toward the ATF6/IRE-1 arm of the response, with high levels of Xbp-1 activation but negligible eukaryotic translation initiation factor 2α phosphorylation and downstream transcription. We show that the PERK-mediated response may partially regulate replication, since external UPR stimulation had a limiting effect on early replication events and cells deficient for PERK demonstrated increased replication and virus release. Significantly, we show that the WNV(KUN) hydrophobic nonstructural proteins NS4A and NS4B are potent inducers of the UPR, which displayed a high correlation in inhibiting Jak-STAT signaling in response to alpha interferon (IFN-α). Sequential removal of the transmembrane domains of NS4A showed that reducing hydrophobicity decreased UPR signaling and restored IFN-α-mediated activation. Overall, these results suggest that WNV(KUN) can stimulate the UPR to facilitate replication and that the induction of a general ER stress response, regulated by hydrophobic WNV(KUN) proteins, can potentiate the inhibition of the antiviral signaling pathway.

NADPH oxidase links endoplasmic reticulum stress, oxidative stress, and PKR activation to induce apoptosis

The endoplasmic reticulum (ER) is a cellular membrane organelle that plays important roles in virus replication and maturation. Accumulating evidence indicates that virus infection often disturbs ER homeostasis and leads to ER stress, which is associated with a variety of prevalent diseases. To cope with the deleterious effects of virus-induced ER stress, cells activate critical signaling pathways including the unfolded protein response (UPR) and intrinsic mitochondrial apoptosis, which have complex effects on virus replication and pathogenesis. In this review, we present a comprehensive summary of recent research in this field, which revealed that about 36 viruses trigger ER stress and differentially activate ER stress-related signaling pathways. We also highlight the strategies evolved by viruses to modulate ER stress-related signaling networks including immune responses in order to ensure their survival and pathogenesis. Together, the knowledge gained from this field will shed light on unveiling the mechanisms of virus replication and pathogenesis and provide insight for future research as well as antiviral development.

Regulation of basal cellular physiology by the homeostatic unfolded protein response

The extensive membrane network of the endoplasmic reticulum (ER) is physically juxtaposed to and functionally entwined with essentially all other cellular compartments. Therefore, the ER must sense diverse and constantly changing physiological inputs so it can adjust its numerous functions to maintain cellular homeostasis. A growing body of new work suggests that the unfolded protein response (UPR), traditionally charged with signaling protein misfolding stress from the ER, has been co-opted for the maintenance of basal cellular homeostasis. Thus, the UPR can be activated, and its output modulated, by signals far outside the realm of protein misfolding. These findings are revealing that the UPR causally contributes to disease not just by its role in protein folding but also through its broad influence on cellular physiology.

Endoplasmic reticulum stress is induced and modulated by enterovirus 71

Picornavirus infection alters the endoplasmic reticulum (ER) membrane but it is unclear whether this induces ER stress. Infection of rhabdomyosarcoma cells with enterovirus 71 (EV71), a picornavirus, caused overexpression of the ER-resident chaperone proteins, BiP and calreticulin, and phosphorylation of eIF2alpha, but infection with UV-inactivated virus did not, indicating that ER stress was induced by viral replication and not by viral attachment or entry. Silencing (si)RNA knockdown demonstrated that phosphorylation of eIF2alpha was dependent on PKR: eIF2alpha phosphorylation was reduced by siPKR but not by siPERK. We provided evidence showing that PERK is upstream of PKR and is thus able to negatively regulate the PKR-eIF2alpha pathway. Pulse-chase experiments revealed that EV71 infection inhibited translation and activation of ATF6. Expression of BiP at the protein level was activated by a virus-dependent, ATF6-independent mechanism. EV71 upregulated XBP1 mRNA level, but neither IRE1-mediated XBP1 splicing nor its active spliced protein was detected, and its downstream gene, EDEM, was not activated. Epigenetic BiP overexpression alleviated EV71-induced ER stress and reduced viral protein expression and replication. Our results suggest that EV71 infection induces ER stress but modifies the outcome to assist viral replication.

Vesicular stomatitis virus oncolysis is potentiated by impairing mTORC1-dependent type I IFN production

Oncolytic viruses constitute a promising therapy against malignant gliomas (MGs). However, virus-induced type I IFN greatly limits its clinical application. The kinase mammalian target of rapamycin (mTOR) stimulates type I IFN production via phosphorylation of its effector proteins, 4E-BPs and S6Ks. Here we show that mouse embryonic fibroblasts and mice lacking S6K1 and S6K2 are more susceptible to vesicular stomatitis virus (VSV) infection than their WT counterparts as a result of an impaired type I IFN response. We used this knowledge to employ a pharmacoviral approach to treat MGs. The highly specific inhibitor of mTOR rapamycin, in combination with an IFN-sensitive VSV-mutant strain (VSV(DeltaM51)), dramatically increased the survival of immunocompetent rats bearing MGs. More importantly, VSV(DeltaM51) selectively killed tumor, but not normal cells, in MG-bearing rats treated with rapamycin. These results demonstrate that reducing type I IFNs through inhibition of mTORC1 is an effective strategy to augment the therapeutic activity of VSV(DeltaM51).

Explicit targeting of transformed cells by VSV in ovarian epithelial tumor-bearing Wv mouse models

OBJECTIVE

Current treatment options for epithelial ovarian cancer are limited and therapeutic development for recurrent and drug-resistant ovarian cancer is an urgent agenda. We investigated the potential use of genetically engineered Vesicular Stomatitis Virus (VSV) to treat ovarian cancer patients who fail to respond to available therapies. Specifically, we examined the toxicity to hosts and specificity of targeting ovarian tumors using a Wv ovarian tumor model.

METHODS

We first tested recombinant VSV for oncolytic activity in a panel of human ovarian epithelial cancer, immortalized, and primary ovarian surface epithelial cells in culture. Then, we tested VSV oncolytic therapy using the immune competent Wv mice that develop tubular adenomas, benign tumor lesions derived from ovarian surface epithelial cells.

RESULTS

The expression of GFP encoded by the recombinant VSV genome was detected in about 5% of primary ovarian surface epithelial cells (3 lines) up to 30 days without significantly altering the growth pattern of the cells, suggesting the lack of toxicity to the normal ovarian surface epithelial cells. However, VSV-GFP was detected in the majority (around 90%) of cells that are either "immortalized" by SV40 antigen expression or cancer lines. Some variation in killing time courses was observed, but all the transformed cell lines were killed within 3 days. We found that regardless of the inoculation route (intra bursal, IP, or IV), VSV specifically infected and replicated in the in situ ovarian tumors in the Wv mice without significant activity in any other organs and tissues, and showed no detectable toxicity. The epithelial tumor lesions were greatly reduced in VSV-targeted ovarian tumors in the Wv mice.

CONCLUSIONS

VSV oncolytic activity depends on a cell autonomous property distinguishing primary and transformed cells. The efficient oncolytic activity of VSV for the "immortalized" non-tumorigenic ovarian surface epithelial cells suggests that the selective specificity extends from pre-neoplastic to overt cancer cells. The results demonstrated the explicit targeting of ovarian epithelial tumors by VSV in immune competent, ovarian tumor-bearing mouse models, and further support the utility of VSV as an effective and safe anti-cancer agent.

Severe acute respiratory syndrome coronavirus triggers apoptosis via protein kinase R but is resistant to its antiviral activity

In this study, infection of 293/ACE2 cells with severe acute respiratory syndrome coronavirus (SARS-CoV) activated several apoptosis-associated events, namely, cleavage of caspase-3, caspase-8, and poly(ADP-ribose) polymerase 1 (PARP), and chromatin condensation and the phosphorylation and hence inactivation of the eukaryotic translation initiation factor 2alpha (eIF2alpha). In addition, two of the three cellular eIF2alpha kinases known to be virus induced, protein kinase R (PKR) and PKR-like endoplasmic reticulum kinase (PERK), were activated by SARS-CoV. The third kinase, general control nonderepressible-2 kinase (GCN2), was not activated, but late in infection the level of GCN2 protein was significantly reduced. Reverse transcription-PCR analyses revealed that the reduction of GCN2 protein was not due to decreased transcription or stability of GCN2 mRNA. The specific reduction of PKR protein expression by antisense peptide-conjugated phosphorodiamidate morpholino oligomers strongly reduced cleavage of PARP in infected cells. Surprisingly, the knockdown of PKR neither enhanced SARS-CoV replication nor abrogated SARS-CoV-induced eIF2alpha phosphorylation. Pretreatment of cells with beta interferon prior to SARS-CoV infection led to a significant decrease in PERK activation, eIF2alpha phosphorylation, and SARS-CoV replication. The various effects of beta interferon treatment were found to function independently on the expression of PKR. Our results show that SARS-CoV infection activates PKR and PERK, leading to sustained eIF2alpha phosphorylation. However, virus replication was not impaired by these events, suggesting that SARS-CoV possesses a mechanism to overcome the inhibitory effects of phosphorylated eIF2alpha on viral mRNA translation. Furthermore, our data suggest that viral activation of PKR can lead to apoptosis via a pathway that is independent of eIF2alpha phosphorylation.