Early phosphatidylinositol 3-kinase/Akt pathway activation limits poliovirus-induced JNK-mediated cell death

Antiviral activity of the HSP90 inhibitor VER-50589 against enterovirus 71

Enterovirus 71 (EV71) is the major pathogen responsible for hand, foot, and mouth disease (HFMD) outbreaks; to date, there is no specific anti-EV71 agent. HSP90 is a crucial host factor for the viral life cycle and an ideal therapeutic target for limiting viral proliferation. However, the specific role of HSP90 in EV71-related signaling pathways and anti-EV71 agents targeting HSP90 remains unclear. This study aimed to verify the role of HSP90 in signaling pathways involved in EV71 replication and investigate the antiviral effects of a small molecule of VER-50589, a potent HSP90 inhibitor, against EV71 both in vitro and in vivo. Viral plaque assay, western blotting, and qPCR results showed that VER-50589 diminished the plaque formation induced by EV71 and inhibited EV71 mRNA and protein synthesis. A single daily dose of VER-50589 treatment significantly improved the survival rate of EV71-infected mice (p < 0.005). Interestingly, VER-50589 also exhibits activities against a series of human enteroviruses, including Coxsackievirus B3 (CVB3), Coxsackievirus B4-5 (CVB4-5), Coxsackievirus B4-7 (CVB4-7), and Echovirus 11 (Echo11). EV71 infection activated the AKT and ERK signaling pathways, and phosphorylation of AKT and RAF/MEK/ERK was weakened by VER-50589 administration. Thus, VER-50589 exhibits robust antiviral activity by inhibiting HSP90 and mediating the AKT and RAF/MEK/ERK signaling pathways. Considering that there are no effective antivirals or vaccines for the prevention and cure of HFMD in a clinical setting, the development of an anti-EV71 agent would be a straightforward and feasible therapeutic approach.

Akt plays differential roles during the life cycles of acute and persistent murine norovirus strains in macrophages

Akt (Protein kinase B) is a key signaling protein in eukaryotic cells that controls many cellular processes such as glucose metabolism and cell proliferation for survival. As obligate intracellular pathogens, viruses modulate host cellular processes, including Akt signaling, for optimal replication. The mechanisms by which viruses modulate Akt and the resulting effects on the infectious cycle differ widely depending on the virus. In this study, we explored the effect of Akt serine 473 phosphorylation (p-Akt) during murine norovirus (MNV) infection. p-Akt increased during infection of murine macrophages with acute MNV-1 and persistent CR3 and CR6 strains. Inhibition of Akt with MK2206, an inhibitor of all three isoforms of Akt (Akt1/2/3), reduced infectious virus progeny of all three virus strains. This reduction was due to decreased viral genome replication (CR3), defective virus assembly (MNV-1), or diminished cellular egress (CR3 and CR6) in a virus strain-dependent manner. Collectively, our data demonstrate that Akt activation increases in macrophages during the later stages of the MNV infectious cycle, which may enhance viral infection in unique ways for different virus strains. The data, for the first time, indicate a role for Akt signaling in viral assembly and highlight additional phenotypic differences between closely related MNV strains. Importance Human noroviruses (HNoV) are a leading cause of viral gastroenteritis, resulting in high annual economic burden and morbidity; yet there are no small animal models supporting productive HNoV infection, or robust culture systems producing cell culture-derived virus stocks. As a result, research on drug discovery and vaccine development against norovirus infection has been challenging, and no targeted antivirals or vaccines against HNoV are approved. On the other hand, murine norovirus (MNV) replicates to high titers in cell culture and is a convenient and widespread model in norovirus research. Our data demonstrate the importance of Akt signaling during the late stage of the MNV life cycle. Notably, the effect of Akt signaling on genome replication, virus assembly and cellular egress is virus strain specific, highlighting the diversity of biological phenotypes despite small genetic variability among norovirus strains. This study is the first to demonstrate a role for Akt in viral assembly.

Differential Effects of Toll-Like Receptor Activation and Differential Mediation by MAP Kinases of Immune Responses in Microglial Cells

Microglial activation is believed to play a role in many psychiatric and neurodegenerative diseases. Based largely on evidence from other cell types, it is widely thought that MAP kinase (ERK, JNK and p38) signalling pathways contribute strongly to microglial activation following immune stimuli acting on toll-like receptor (TLR) 3 or TLR4. We report here that exposure of SimA9 mouse microglial cell line to immune mimetics stimulating TLR4 (lipopolysaccharide-LPS) or TLR7/8 (resiquimod/R848), results in marked MAP kinase activation, followed by induction of nitric oxide synthase, and various cytokines/chemokines. However, in contrast to TLR4 or TLR7/8 stimulation, very few effects of TLR3 stimulation by poly-inosine/cytidine (polyI:C) were detected. Induction of chemokines/cytokines at the mRNA level by LPS and resiquimod were, in general, only marginally affected by MAP kinase inhibition, and expression of TNF, Ccl2 and Ccl5 mRNAs, along with nitrite production, were enhanced by p38 inhibition in a stimulus-specific manner. Selective JNK inhibition enhanced Ccl2 and Ccl5 release. Many distinct responses to stimulation of TLR4 and TLR7 were observed, with JNK mediating TNF protein induction by the latter but not the former, and suppressing Ccl5 release by the former but not the latter. These data reveal complex modulation by MAP kinases of microglial responses to immune challenge, including a dampening of some responses. They demonstrate that abnormal levels of JNK or p38 signalling in microglial cells will perturb their profile of cytokine and chemokine release, potentially contributing to abnormal inflammatory patterns in CNS disease states.

Regulation of Apoptosis by Enteroviruses

Enterovirus infection can cause a variety of diseases and severely impair the health of humans, animals, poultry, and other organisms. To resist viral infection, host organisms clear infected cells and viruses via apoptosis. However, throughout their long-term competition with host cells, enteroviruses have evolved a series of mechanisms to regulate the balance of apoptosis in order to replicate and proliferate. In the early stage of infection, enteroviruses mainly inhibit apoptosis by regulating the PI3K/Akt pathway and the autophagy pathway and by impairing cell sensors, thereby delaying viral replication. In the late stage of infection, enteroviruses mainly regulate apoptotic pathways and the host translation process via various viral proteins, ultimately inducing apoptosis. This paper discusses the means by which these two phenomena are balanced in enteroviruses to produce virus-favoring conditions – in a temporal sequence or through competition with each other. This information is important for further elucidation of the relevant mechanisms of acute infection by enteroviruses and other members of the picornavirus family.

Host Genetics, Innate Immune Responses, and Cellular Death Pathways in Poliomyelitis Patients

Purpose

Poliovirus (PV) is one of the most studied viruses. Despite efforts to understand PV infection within the host, fundamental questions remain unanswered. These include the mechanisms determining the progression to viremia, the pathogenesis of neuronal infection and paralysis in only a minority of patients. Because of the rare disease phenotype of paralytic poliomyelitis (PPM), we hypothesize that a genetic etiology may contribute to the disease course and outcome.

Methods

We used whole-exome sequencing (WES) to investigate the genetic profile of 18 patients with PPM. Functional analyses were performed on peripheral blood mononuclear cells (PBMCs) and monocyte-derived macrophages (MdMs).

Results

We identified rare variants in host genes involved in interferon signaling, viral replication, apoptosis, and autophagy. Upon PV infection of MdMs, we observed a tendency toward increased viral burden in patients compared to controls, suggesting reduced control of PV infection. In MdMs from patients, the IFNβ response correlated with the viral burden.

Conclusion

We suggest that genetic variants in innate immune defenses and cell death pathways contribute to the clinical presentation of PV infection. Importantly, this study is the first to uncover the genetic profile in patients with PPM combined with investigations of immune responses and viral burden in primary cells.

ASK family in infection and inflammatory disease

Living organisms are continuously exposed to pathogens such as viruses and bacteria. Soon after a limited number of germline-encoded receptors, called pathogen recognition receptors, sense pathogen-associated molecular patterns, hosts trigger innate immune responses, including production of type Ⅰ interferons, proinflammatory cytokines, and cellular apoptosis, to limit propagation of invading pathogens. Importantly, these host responses are also activated during inflammatory diseases, irrespective of pathogen infection, and often play a causal role in pathogenesis and progression of these diseases, thereby implying an intimate link between immune responses and inflammatory disease. The apoptosis signal-regulating kinase (ASK) family belongs to the larger MAP3K family that controls various stress responses. Here, I summarize the critical roles of members of the ASK family during infection and inflammatory disease, and discuss the relationship between these two noxious conditions.

The CREB3-Herp signalling module limits the cytosolic calcium concentration increase and apoptosis induced by poliovirus

Poliovirus (PV)-induced apoptosis seems to play a major role in central nervous system (CNS) tissue injury, a crucial feature of the pathogenesis of poliomyelitis. We have previously shown that calcium (Ca2+) flux from the endoplasmic reticulum (ER) to the cytosol during PV infection is involved in apoptosis induction in human neuroblastoma cells. We show here that PV infection is associated with a transient upregulation of Herp (homocysteine-induced ER protein), a protein known to promote the degradation of ER-resident Ca2+ channels. Herp gene transcription is controlled by the transcription factor CREB3 (cAMP response element-binding protein 3). We found that the CREB3/Herp pathway limited the increase in cytosolic Ca2+ concentration and apoptosis early in PV infection. This may reduce the extent of PV-induced damage to the CNS during poliomyelitis.

MicroRNA profile analysis of host cells before and after wild human rotavirus infection

Rotavirus infection is an important cause of acute gastroenteritis in children, but the interaction between rotavirus and host cells is not completely understood. We isolated a wildtype (wt) rotavirus strain, ZTR-68(P [8] G1), which is derived from an infant with diarrhea in southwest China in 2010. In this study, we investigated host cellular miRNA expression profiles changes in response to ZTR-68 in early stage of infection to investigate the role of miRNAs upon rotavirus infection. Differentially expressed miRNAs were identified by deep sequencing and qRT-PCR and the function of their targets predicted by Gene Ontology (GO) function and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway annotation. A total of 36 candidate miRNAs were identified. Comparative analysis indicated that 29 miRNAs were significantly down-regulated and 7 were up-regulated after infection. The data were provided contrasting the types of microRNAs in two different permissive cell lines (HT29 and MA104). The target assays results showed that mml-miR-7 and mml-miR-125a are involved in anti-rotavirus and virus-host interaction in host cells. These results offer clues for identifying potential candidates in vector-based antiviral strategies. J. Med. Virol. 88:1497-1510, 2016. © 2016 Wiley Periodicals, Inc.

Death waits for no man--does it wait for a virus? How enteroviruses induce and control cell death

Enteroviruses (EVs) are the most common human viral pathogens. They cause a variety of pathologies, including myocarditis and meningoencephalopathies, and have been linked to the onset of type I diabetes. These pathologies result from the death of cells in the myocardium, central nervous system, and pancreas, respectively. Understanding the role of EVs in inducing cell death is crucial to understanding the etiologies of these diverse pathologies. EVs both induce and delay host cell death, and their exquisite control of this balance is crucial for their success as human viral pathogens. Thus, EVs are tightly involved with cell death signaling pathways and interact with host cell signaling at multiple points. Here, we review the literature detailing the mechanisms of EV-induced cell death. We discuss the mechanisms by which EVs induce cell death, the signaling pathways involved in these pathways, and the strategies by which EVs antagonize cell death pathways. We also discuss the role of cell death in both the resulting pathology in the host and in the facilitation of viral spread.

Activation of PI3K/Akt pathway limits JNK-mediated apoptosis during EV71 infection

Apoptosis is frequently induced to inhibit virus replication during infection of Enterovirus 71 (EV71). On the contrary, anti-apoptotic pathway, such as PI3K/Akt pathway, is simultaneously exploited by EV71 to accomplish the viral life cycle. The relationship by which EV71-induced apoptosis and PI3K/Akt signaling pathway remains to be elucidated. In this study, we demonstrated that EV71 infection altered Bax conformation and triggered its redistribution from the cytosol to mitochondria in RD cells. Subsequently, cytochrome c was released from mitochondria to cytosol. We also found that c-Jun NH2-terminal kinase (JNK) was activated during EV71 infection. The JNK specific inhibitor significantly inhibited Bax activation and cytochrome c release, suggesting that EV71-induced apoptosis was involved into a JNK-dependent manner. Meanwhile, EV71-induced Akt phosphorylation involved a PI3K-dependent mechanism. Inhibition of the PI3K/Akt pathway enhanced JNK phosphorylation and the JNK-mediated apoptosis upon EV71 infection. Moreover, PI3K/Akt pathway phosphorylated apoptosis signal-regulating kinase 1 (ASK1) and negatively regulated the ASK1 activity. Knockdown of ASK1 significantly decreased JNK phosphorylation, which implied that ASK1 phosphorylation by Akt inhibited ASK1-mediated JNK activation. Collectively, these data reveal that activation of the PI3K/Akt pathway limits JNK-mediated apoptosis by phosphorylating and inactivating ASK1 during EV71 infection.

Rotavirus infection induces G1 to S phase transition in MA104 cells via Ca⁺²/Calmodulin pathway

Viruses, obligate cellular parasites rely on host cellular functions and target the host cell cycle for their own benefit. In this study, effect of rotavirus infection on cell cycle machinery was explored. We found that rotavirus (RV) infection in MA104 cells induces the expression of cyclins and cyclin dependent kinases and down-regulates expression of CDK inhibitors, resulting in G1 to S phase transition. The rotavirus induced S phase accumulation was found to be concurrent with induction in expression of calmodulin and activation of CaMKI which is reported as inducer of G1-S phase transition. This cell cycle manipulation was found to be Ca(+2)/Calmodulin pathway dependent. The physiological relevance of G1 to S phase transition was established when viral gene expressions as well as viral titers were found to be increased in S phase synchronized cells and decreased in G0/G1 phase synchronized cells compared to unsynchronized cells during rotavirus infection.

Regulatory role of ASK1 in porcine circovirus type 2-induced apoptosis

Porcine circovirus type 2 (PCV2), a major causative agent of postweaning multisystemic wasting syndrome which is an emerging and important swine disease, can induce apoptotic responses in vitro and in vivo. However, the molecular mechanism of PCV2-induced apoptosis is not well understood. Apoptosis signal-regulating kinase 1 (ASK1), serves as an upstream enzyme that activates the JNK and p38 MAPK pathways, has been shown to play a target role in the regulation of apoptosis. Here, we showed that PCV2 infection induced ASK1 phosphorylation which preceded activation of JNK1/2 and p38 signaling in the cultured cells, and that the phosphatidylinostol 3-kinase (PI3K)/Akt signaling triggered by PCV2 infection limited the extent of JNK1/2 and p38 activation and thereby apoptotic cell death. Furthermore, inhibition of JNK and p38 activation is associated with PI3K-dependent negative-regulation of ASK1 in the PCV2-infected cells. These results indicate an important regulatory role of ASK1 in PCV2-induced apoptotic responses.

Molecular mechanism behind rotavirus NSP1-mediated PI3 kinase activation: interaction between NSP1 and the p85 subunit of PI3 kinase

Our previous study had reported on the interaction of rotavirus NSP1 with cellular phosphoinositide 3-kinase (PI3K) during activation of the PI3K pathway (P. Bagchi et al., J. Virol. 84:6834-6845, 2010). In this study, we have analyzed the molecular mechanism behind this interaction. Results showed that this interaction is direct and that both α and β isomers of the PI3K regulatory subunit p85 and full-length NSP1 are important for this interaction, which results in efficient activation of the PI3K/Akt pathway during rotavirus infection.

Differential apoptosis gene expressions of rhabdomyosarcoma cells in response to enterovirus 71 infection

Background

Enterovirus 71 (EV71) infection can induce the apoptosis of infected cells. The aim of this study is to explore the effect of EV71 infection on apoptosis mechanisms in virus-infected human rhabdomyosarcoma (RD) cells.

Methods

The apoptosis of RD cells was examined using annexin V-FITC/PI by flow cytometry and cytokines were detected by ELISA. Cellular RNA was extracted and transcribed to cDNA. PCR array was employed to analyze the expressions of 84 apoptotic genes from EV71-infected RD cells at 8 and 20 h postinfection, respectively. In addition, the expressions of FasL, caspase, AKT2, JNK1/2, c-Jun and NF-κB proteins were detected by western blotting.

Results

Flow cytometry demonstrated that the apoptosis or death of EV71-infected RD cells was increased by 37.1% with a multiplicity of infection (MOI) of 5 at 20 h postinfection. The production of IL-4, IL-10 and TNF-α was enhanced by the subsequent EV71 infection. PCR array revealed significant changes in the expressions of apoptotic genes. Among 84 genes, 42 genes were down-regulated after EV71 infection at 8 h, whereas 32 genes were up-regulated at 20 h postinfection. Moreover, the ligands of TNF superfamily such as FasL, CD40L and TNF-α were significantly up-regulated and enhanced the expressions of apoptosis-related cysteine peptidases, including caspase-10, -8, -7 and -3. In addition, EV71 infection induces the phosphorylation of AKT2, JNK1/2, c-Jun and NF-κB at 20 h postinfection.

Conclusion

PCR array for the determination of apoptosis gene expressions is an informative assay in elucidating biological pathways. During the early stage of EV71 infection, the apoptotic process of RD cells is significantly delayed. EV71 infection can also induce the expressions of FasL, TNF-α and CD40L, which contribute to the apoptosis of RD cells.

Identification and functional analysis of the novel ORF4 protein encoded by porcine circovirus type 2

Porcine circovirus type 2 (PCV2) is the primary causative agent of porcine circovirus-associated diseases in pigs. To date, viral proteins Cap, Rep, Rep', and ORF3, encoded by the PCV2 genome, have been described. Here, transcription and translation of a novel viral gene within the PCV2 genome (designated ORF4) was determined and functionally analyzed in vitro and in vivo. Northern blot analysis indicated that the RNA transcribed from the ORF4 gene is about 180 bp in length and overlaps ORF3 in the same direction. Site-directed mutagenesis confirmed that the viral ORF4 protein is not essential for virus replication in PK-15 cells and in mice infected with an ORF4-deficient PCV2 (PCV2Δ). PCV2Δ triggered higher activity levels of caspase-3 and -8 than wild-type PCV2 (wPCV2) in PK-15 cells. The antigenic epitopes of two mouse monoclonal antibodies (MAbs) raised against the viral ORF4 protein were mapped to the same 19KSSASPR25 peptide. Expression of ORF4 was confirmed using the specific MAbs in wPCV2-infected PK-15 cells and mice. Mice infected with PCV2Δ had a higher serum viral load (genomic copies) and more severe lymphoid tissue damage in the spleen than those infected with wPCV2. Meanwhile, flow-cytometric analysis indicated that the PCV2Δ infection caused a significant decrease of CD4(+) and CD8(+) T lymphocytes. Our results demonstrate that ORF4 is a newly discovered viral protein that is not essential for PCV2 replication but plays a role in suppressing caspase activity and regulating CD4(+) and CD8(+) T lymphocytes during PCV2 infection.

Activation of the phosphatidylinositol 3-kinase/Akt signaling pathway during porcine circovirus type 2 infection facilitates cell survival and viral replication

Virus infection activates host cellular signaling pathways, including the phosphatidylinositol 3-kinase (PI3K)/Akt pathway, which regulates diverse cellular activities related to cell growth, survival, and apoptosis. The present study demonstrated for the first time that porcine circovirus type 2 (PCV2), a major causative agent of postweaning multisystemic wasting syndrome, which is an emerging and important swine disease, can transiently induce the PI3K/Akt pathway in cultured cells at an early step during PCV2 infection. Activation of the PI3K/Akt signal was also induced by UV-irradiated PCV2, indicating that virus replication was not required for this induction. Inhibition of PI3K activation leads to reduced virus yield, which is associated with decreased viral DNA replication and lower virus protein expression. However, inhibition of PI3K activation greatly enhanced apoptotic responses as evidenced by the cleavage of poly-ADP ribose polymerase and caspase-3 as well as DNA fragmentation using terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling staining during the early stage of PCV2 infection. Furthermore, the pancaspase inhibitor zVAD.fmk alleviated the reduction in Akt phosphorylation levels by inhibiting PI3K activation, indicating that the signaling promotes cell survival and thereby favors viral replication. These results reveal that an antiapoptotic role for the PI3K/Akt pathway induced by PCV2 infection to suppress premature apoptosis for improved virus growth after infection, extending our understanding of the molecular mechanism of PCV2 infection.

Inflammatory cause of metabolic syndrome via brain stress and NF-κB

Metabolic syndrome, a network of medical disorders that greatly increase the risk for developing metabolic and cardiovascular diseases, has reached epidemic levels in many areas of today's world. Despite this alarming medicare situation, scientific understandings on the root mechanisms of metabolic syndrome are still limited, and such insufficient knowledge contributes to the relative lack of effective treatments or preventions for related diseases. Recent interdisciplinary studies from neuroendocrinology and neuroimmunology fields have revealed that overnutrition can trigger intracellular stresses to cause inflammatory changes mediated by molecules that control innate immunity. This type of nutrition-related molecular inflammation in the central nervous system, particularly in the hypothalamus, can form a common pathogenic basis for the induction of various metabolic syndrome components such as obesity, insulin resistance, and hypertension. Proinflammatory NF-κB pathway has been revealed as a key molecular system for pathologic induction of brain inflammation, which translates overnutrition and resulting intracellular stresses into central neuroendocrine and neural dysregulations of energy, glucose, and cardiovascular homeostasis, collectively leading to metabolic syndrome. This article reviews recent research advances in the neural mechanisms of metabolic syndrome and related diseases from the perspective of pathogenic induction by intracellular stresses and NF-κB pathway of the brain.

Hypothalamic inflammation: a double-edged sword to nutritional diseases

The hypothalamus is one of the master regulators of various physiological processes, including energy balance and nutrient metabolism. These regulatory functions are mediated by discrete hypothalamic regions that integrate metabolic sensing with neuroendocrine and neural controls of systemic physiology. Neurons and nonneuronal cells in these hypothalamic regions act supportively to execute metabolic regulations. Under conditions of brain and hypothalamic inflammation, which may result from overnutrition-induced intracellular stresses or disease-associated systemic inflammatory factors, extracellular and intracellular environments of hypothalamic cells are disrupted, leading to central metabolic dysregulations and various diseases. Recent research has begun to elucidate the effects of hypothalamic inflammation in causing diverse components of metabolic syndrome leading to diabetes and cardiovascular disease. These new understandings have provocatively expanded previous knowledge on the cachectic roles of brain inflammatory response in diseases, such as infections and cancers. This review describes the molecular and cellular characteristics of hypothalamic inflammation in metabolic syndrome and related diseases as opposed to cachectic diseases, and also discusses concepts and potential applications of inhibiting central/hypothalamic inflammation to treat nutritional diseases.

Influenza A virus neuraminidase protein enhances cell survival through interaction with carcinoembryonic antigen-related cell adhesion molecule 6 (CEACAM6) protein

The influenza virus neuraminidase (NA) protein primarily aids in the release of progeny virions from infected cells. Here, we demonstrate a novel role for NA in enhancing host cell survival by activating the Src/Akt signaling axis via an interaction with carcinoembryonic antigen-related cell adhesion molecule 6/cluster of differentiation 66c (C6). NA/C6 interaction leads to increased tyrosyl phosphorylation of Src, FAK, Akt, GSK3β, and Bcl-2, which affects cell survival, proliferation, migration, differentiation, and apoptosis. siRNA-mediated suppression of C6 resulted in a down-regulation of activated Src, FAK, and Akt, increased apoptosis, and reduced expression of viral proteins and viral titers in influenza virus-infected human lung adenocarcinoma epithelial and normal human bronchial epithelial cells. These findings indicate that influenza NA not only aids in the release of progeny virions, but also cell survival during viral replication.

Viral product trafficking to mitochondria, mechanisms and roles in pathogenesis

A wide variety of viruses cause significant morbidity and mortality in humans. However, targeted antiviral therapies have been developed for only a subset of these viruses, with the majority of currently licensed antiviral drugs targeting viral entry, replication or exit steps during the viral life cycle. Due to increasing emergence of antiviral drug resistant viruses, the isolation of multiple viral subtypes, and toxicities of existing therapies, there remains an urgent need for the timely development of novel antiviral agents, including those targeting host factors essential for viral replication. This review summarizes viral products that target mitochondria and their effects on common mitochondria regulated pathways. These viral products and the mitochondrial pathways affected by them provide potential novel targets for the rational design of antiviral drugs. Viral products alter oxidative balance, mitochondrial permeability transition pore, mitochondrial membrane potential, electron transport and energy production. Moreover, viruses may cause the Warburg Effect, in which metabolism is reprogrammed to aerobic glycolysis as the main source of energy. Finally, viral products affect proapoptotic and antiapoptotic signaling, as well as mitochondrial innate immune signaling. Because of their importance for the generation of metabolic intermediates and energy as well as cell survival, mitochondrial pathways are targeted by multiple independent viral products. Structural modifications of existing drugs targeted to mitochondrial pathways may lead to the development of novel antiviral drugs with improved efficacy and reduced toxicity.

HijAkt: The PI3K/Akt pathway in virus replication and pathogenesis

As obligate parasites of cellular processes, viruses must take over cellular macromolecular machinery. It is also becoming clear that viruses routinely control intracellular signaling pathways through the direct or indirect control of kinases and phosphatases. This control of cellular phosphoproteins is important to promote a variety of viral processes, from control of entry to nuclear function to the stimulation of viral protein synthesis. This review focuses on the takeover of the cellular phosphatidylinositol-3-kinase (PI3K)/Akt signaling pathway by a variety of retroviruses, DNA viruses, and RNA viruses, highlighting the functions ascribed to virus activation of PI3K and Akt activity. This review also describes the role that the PI3K/Akt pathway plays in the host response, noting that it that can trigger anti- as well as proviral functions.

Encephalomyocarditis virus 2A protein is required for viral pathogenesis and inhibition of apoptosis

The encephalomyocarditis virus (EMCV), a Picornaviridae virus, has a wide host spectrum and can cause various diseases. EMCV virulence factors, however, are as yet ill defined. Here, we demonstrate that the EMCV 2A protein is essential for the pathogenesis of EMCV. Infection of mice with the B279/95 strain of EMCV resulted in acute fatal disease, while the clone C9, derived by serial in vitro passage of the B279/95 strain, was avirulent. C9 harbored a large deletion in the gene encoding the 2A protein. This deletion was incorporated into the cDNA of a pathogenic EMCV1.26 strain. The new virus, EMCV1.26Δ2A, was capable of replicating in vitro, albeit more slowly than EMCV1.26. Only mice inoculated with EMCV1.26 triggered death within a few days. Mice infected with EMCV1.26Δ2A did not exhibit clinical signs, and histopathological analyses showed no damage in the central nervous system, unlike EMCV1.26-infected mice. In vitro, EMCV1.26Δ2A presented a defect in viral particle release correlating with prolonged cell viability. Unlike EMCV1.26, which induced cytopathic cell death, EMCV1.26Δ2A induced apoptosis via caspase 3 activation. This strongly suggests that the 2A protein is required for inhibition of apoptosis during EMCV infection. All together, our data indicate that the EMCV 2A protein is important for the virus in counteracting host defenses, since Δ2A viruses were no longer pathogenic and were unable to inhibit apoptosis in vitro.

Infectious bursal disease virus activates the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway by interaction of VP5 protein with the p85α subunit of PI3K

Phosphatidylinositol 3-kinase (PI3K)/Akt signaling is commonly activated upon virus infection and has been implicated in the regulation of diverse cellular functions such as proliferation and apoptosis. The present study demonstrated for the first time that infectious bursal disease virus (IBDV), the causative agent of a highly contagious disease in chickens, can induce Akt phosphorylation in cultured cells, by a mechanism that is dependent on PI3K. Inhibition of PI3K activation greatly enhanced virus-induced cytopathic effect and apoptotic cell death as evidenced by cleavage of poly-ADP ribose polymerase and activation of caspase-3. Investigations into the mechanism of PI3K/Akt activation revealed that IBDV activates PI3K/Akt signaling through binding of the non-structural protein VP5 to regulatory subunit p85α of PI3K resulting in the suppression of premature apoptosis and improved virus growth after infection. The results presented here provide a basis for understanding molecular mechanism of IBDV infection.

Influenza virus and cell signaling pathways

Influenza viruses comprise a major class of human respiratory pathogens, responsible for causing morbidity and mortality worldwide. Influenza A virus, due to its segmented RNA genome, is highly subject to mutation, resulting in rapid formation of variants. During influenza infection, viral proteins interact with host proteins and exploit a variety of cellular pathways for their own benefit. Influenza virus inhibits the synthesis of these cellular proteins and facilitates expression of its own proteins for viral transcription and replication. Infected cell pathways are hijacked by an array of intracellular signaling cascades such as NF-κB signaling, PI3K/Akt pathway, MAPK pathway, PKC/PKR signaling and TLR/RIG-I signaling cascades. This review presents a research update on the subject and discusses the impact of influenza viral infection on these cell signaling pathways.

A monoclonal antibody (Mc178-Ab) targeted to the ecto-ATP synthase β-subunit-induced cell apoptosis via a mechanism involving the MAPKase and Akt pathways

Ecto-ATP synthase has been considered to be an effective target for cancer recently. As inhibitors of ecto-ATP synthase were found to be cytotoxic for tumor cells, a monoclonal antibody (Mc178-Ab) against ecto-ATP synthase was generated in our previous study that exhibited both anti-angiogenic and anti-tumorigenic effects. However, the mechanism of action of Mc178-Ab and its downstream pathways for anti-tumor effects remain unclear. In this research, we intended to investigate the mechanism of the anti-tumor action of Mc178-Ab. The expressions of cell surface ATP synthase on A549 and CHO cells were confirmed by flow cytometry and confocal microscope. Proliferation and apoptosis were examined after the treatment with Mc178-Ab. In order to examine the activity of ecto-ATP synthase changed by Mc178-Ab, extracellular ATP generation and intracellular pH levels were assessed. The phosphorylation of the signaling molecules, MAPKase and Akt, was analyzed by western blot. Cell proliferation was blocked, and apoptosis was induced in A549 cells treated with Mc178-Ab, as determined by MTT assay and flow cytometry analysis of Annexin-V/PI staining separately. The intracellular pH level and extracellular ATP generation were also decreased after Mc178-Ab treatment. Finally, western blot data revealed that the phosphorylation of JNK and p38 was increased, while the phosphorylation of ERK and Akt was decreased in A549 cells treated with Mc178-Ab. Compared with A549 cells, Mc178-Ab had less effect on CHO cells. The decreased intracellular pH levels and the altered concentration of extracellular ATP may contribute to the mechanisms of the effect of Mc178-Ab on A549 and CHO cells. The results also suggested that the anti-tumor effect of Mc178-Ab was associated with MAPKase and Akt pathways.

Active participation of cellular chaperone Hsp90 in regulating the function of rotavirus nonstructural protein 3 (NSP3)

Heat shock protein 90 (Hsp90) has been reported to positively regulate rotavirus replication by modulating virus induced PI3K/Akt and NFκB activation. Here, we report the active association of Hsp90 in the folding and stabilization of rotavirus nonstructural protein 3 (NSP3). In pCD-NSP3-transfected cells, treatment with Hsp90 inhibitor (17-N,N-dimethylethylenediamine-geldanamycin (17DMAG)) resulted in the proteasomal degradation of NSP3. Sequence analysis and deletion mutations revealed that the region spanning amino acids 225-258 within the C-terminal eIF4G-binding domain of NSP3 is a putative Hsp90 binding region. Co-immunoprecipitation and mammalian two-hybrid experiments revealed direct interaction of the C-terminal 12-kDa domain of Hsp90 (C90) with residues 225-258 of NSP3. NSP3-Hsp90 interaction is important for the formation of functionally active mature NSP3, because full-length NSP3 in the presence of the Hsp90 inhibitor or NSP3 lacking the amino acid 225-258 region did not show NSP3 dimers following in vitro coupled transcription-translation followed by chase. Disruption of residues 225-258 within NSP3 also resulted in poor RNA binding and eIF4G binding activity. In addition, inhibition of Hsp90 by 17DMAG resulted in reduced nuclear translocation of poly(A)-binding protein and translation of viral proteins. These results highlight the crucial role of Hsp90 chaperone in the regulation of assembly and functionality of a viral protein during the virus replication and propagation in host cells.

Enter the kill zone: initiation of death signaling during virus entry

Infection of host cells by a variety of viruses results in programmed cell death or apoptosis. In many cases, early events in virus replication that occur prior to synthesis of viral proteins and replication of viral genomes directly or indirectly activate signaling pathways that culminate in cell death. Using examples of viruses for which prodeath signaling is better defined, this review will describe how cell entry steps including virus attachment to receptors, virus uncoating in endosomes, and events that occur following membrane penetration lead to apoptosis. The relevance and physiologic consequences of early induction of prodeath signaling to viral pathogenesis also will be discussed.

Viral security proteins: counteracting host defences

Viral reproduction involves not only replication but also interactions with host defences. Although various viral proteins can take part in counteracting innate and adaptive immunity, many viruses possess a subset of proteins that are specifically dedicated to counter-defensive activities. These proteins are sometimes referred to as 'virulence factors', but here we argue that the term 'security proteins' is preferable, for several reasons.

The concept of security proteins of RNA-containing viruses can be considered using the leader (L and L^*) and 2A proteins of picornaviruses as examples. The picornaviruses are a large group of human and animal viruses that include important pathogens such as poliovirus, hepatitis A virus and foot-and-mouth disease virus. The genomes of different picornaviruses have a similar organization, in which the genes for L and 2A occupy fixed positions upstream and downstream of the capsid genes, respectively.

Both L and 2A are extremely heterogeneous with respect to size, sequence and biochemical properties. The similarly named proteins can be completely unrelated to each other in different viral genera, and the variation can be striking even among members of the same genus. A subset of picornaviruses lacks L altogether.

The properties and functions of L and 2A of many picornaviruses are unknown, but in those viruses that have been investigated sufficiently it has been found that these proteins can switch off various aspects of host macromolecular synthesis and specifically suppress mechanisms involved in innate immunity. Thus, notwithstanding their unrelatedness, the security proteins carry out similar biological functions. It is proposed that other picornavirus L and 2A proteins that have not yet been investigated should also be primarily involved in security activities.

The L, L^* and 2A proteins are dispensable for viral reproduction, but their elimination or inactivation usually renders the viruses less pathogenic. The phenotypic changes associated with inactivation of security proteins are much less pronounced in cells or organisms that have innate immunity deficiencies. In several examples, the decreased fitness of a virus in which a security protein has been inactivated could be rescued by the experimental introduction of an unrelated security protein.

It can be argued that L and 2A were acquired by different picornaviruses independently, and possibly by exploiting different mechanisms, late in the evolution of this viral family.

It is proposed that the concept of security proteins is of general relevance and can be applied to viruses other than picornaviruses. The hallmarks of security proteins are: structural and biochemical unrelatedness in related viruses or even absence in some of them; dispensability of the entire protein or its functional domains for viral viability; and, for mutated versions of the proteins, fewer detrimental effects on viral reproduction in immune-compromised hosts than in immune-competent hosts.

Viral security proteins are structurally and biochemically unrelated proteins that function to counteract host defences. Here, Agol and Gmyl consider the impact of the picornavirus security proteins on viral reproduction, pathogenicity and evolution.

Interactions with host defences are key aspects of viral infection. Various viral proteins perform counter-defensive functions, but a distinct class, called security proteins, is dedicated specifically to counteracting host defences. Here, the properties of the picornavirus security proteins L and 2A are discussed. These proteins have well-defined positions in the viral polyprotein, flanking the capsid precursor, but they are structurally and biochemically unrelated. Here, we consider the impact of these two proteins, as well as that of a third security protein, L^*, on viral reproduction, pathogenicity and evolution. The concept of security proteins could serve as a paradigm for the dedicated counter-defensive proteins of other viruses.

Calcium flux between the endoplasmic reticulum and mitochondrion contributes to poliovirus-induced apoptosis

We show that poliovirus (PV) infection induces an increase in cytosolic calcium (Ca(2+)) concentration in neuroblastoma IMR5 cells, at least partly through Ca(2+) release from the endoplasmic reticulum lumen via the inositol 1,4,5-triphosphate receptor (IP(3)R) and ryanodine receptor (RyR) channels. This leads to Ca(2+) accumulation in mitochondria through the mitochondrial Ca(2+) uniporter and the voltage-dependent anion channel (VDAC). This increase in mitochondrial Ca(2+) concentration in PV-infected cells leads to mitochondrial dysfunction and apoptosis.

Enhancing therapeutic efficacy by targeting non-oncogene addicted cells with combinations of signal transduction inhibitors and chemotherapy

The effects of inhibition of the Raf/MEK/ERK and PI3K/Akt/mTOR signaling pathways and chemotherapeutic drugs on cell cycle progression and drug sensitivity were examined in cytokine-dependent FL5.12 hematopoietic cells. We examined their effects, as these cells resemble normal hematopoietic precursor cells as they do not exhibit "oncogene-addicted" growth, while they do display "cytokine-addicted" proliferation as cytokine removal resulted in apoptosis in greater than 80% of the cells within 48 hrs. When cytokine-dependent FL5.12 cells were cultured in the presence of IL-3, which stimulated multiple proliferation and anti-apoptotic cascades, MEK, PI3K and mTOR inhibitors transiently suppressed but did not totally inhibit cell cycle progression or induce apoptosis while chemotherapeutic drugs such as doxorubicin and paclitaxel were more effective in inducing cell cycle arrest and apoptosis. Doxorubicin induced a G(1) block, while paclitaxel triggered a G(2)/M block. Doxorubicin was more effective in inducing cell death than paclitaxel. Furthermore the effects of doxorubicin could be enhanced by addition of MEK, PI3K or mTOR inhibitors. Cytokine-dependent cells which proliferate in vitro and are not "oncogene-addicted" may represent a pre-malignant stage, more refractory to treatment with targeted therapy. However, these cells are sensitive to chemotherapeutic drugs. It is important to develop methods to inhibit the growth of such cytokine-dependent cells as they may resemble the leukemia stem cell and other cancer initiating cells. These results demonstrate the enhanced effectiveness of targeting early hematopoietic progenitor cells with combinations of chemotherapeutic drugs and signal transduction inhibitors.

Rotavirus nonstructural protein 1 suppresses virus-induced cellular apoptosis to facilitate viral growth by activating the cell survival pathways during early stages of infection

Following virus infection, one of the cellular responses to limit the virus spread is induction of apoptosis. In the present study, we report role of rotavirus nonstructural protein 1 (NSP1) in regulating apoptosis by activating prosurvival pathways such as phosphatidylinositol 3-kinase (PI3K)/Akt and NF-kappaB (nuclear factor kappaB) during early hours of infections (2 to 8 hpi). The NSP1 mutant strain A5-16 induces weak and transient activation of Akt (protein kinase B) and p65 NF-kappaB compared to the isogenic wild-type strain A5-13 in MA104 or HT29 cells. The weak NF-kappaB promoter activity or Akt phosphorylation after A5-16 infection could be complemented in cells transfected with plasmid expressing NSP1 after infection with the rotavirus A5-16 strain. In cells either infected with A5-13 or transfected with pcD-NSP1, coimmunoprecipitation of NSP1 with phosphoinositide 3-kinase (PI3K) was observed, indicating that strong activation of PI3K/Akt could be due to its interaction with NSP1. In addition, after infection with same multiplicity of infection, A5-16 showed reduced number of viral particles compared to the A5-13 strain at the end of the replication cycle. A lower growth rate could be due to weak induction of PI3K/Akt and NF-kappaB, since the A5-13 strain also showed reduced growth in the presence of PI3K or NF-kappaB inhibitors. This effect was interferon independent; however, it was partly due to significantly higher caspase-3 activity, poly-ADP ribose polymerase (PARP) cleavage, and apoptosis during earlier stages of infection with the NSP1 mutant. Thus, our data suggest that NSP1 positively supports rotavirus growth by suppression of premature apoptosis for improved virus growth after infection.

Akt inhibitor Akt-IV blocks virus replication through an Akt-independent mechanism

Many viruses activate the phosphatidylinositol 3'-kinase (PI3k)/Akt intracellular signaling pathway to promote viral replication. We have analyzed whether a rapidly replicating rhabdovirus, vesicular stomatitis virus (VSV), requires the PI3k/Akt signaling pathway for its replication. Through the use of chemical inhibitors of PI3k and Akt, we show that VSV replication and cytopathic effects do not require activation of these kinases. Inhibitors that block the activating phosphorylations of Akt at threonine 308 (Thr308) and serine 473 (Ser473) did not inhibit VSV protein expression or the induction of the cytopathic effects of VSV. One compound, Akt inhibitor Akt-IV, inhibited the replication of VSV, respiratory syncytial virus, and vaccinia virus but increased the phosphorylation of Akt at positions Thr308 and Ser473 and did not inhibit Akt kinase activity in vitro. Together, our data suggest that the PI3k/Akt pathway is of limited relevance to the replication of VSV but that Akt inhibitor Akt-IV is a novel broad-spectrum antiviral compound with a mechanism differing from that of its previously reported effect on the PI3k/Akt pathway. Identification of other targets for this compound may define a new approach for blocking virus replication.

Interactions between viral and prokaryotic pathogens in a mixed infection with cardiovirus and mycoplasma

In the natural environment, animal and plant viruses often share ecological niches with microorganisms, but the interactions between these pathogens, although potentially having important implications, are poorly investigated. The present report demonstrates, in a model system, profound mutual effects of mycoplasma and cardioviruses in animal cell cultures. In contrast to mycoplasma-free cells, cultures contaminated with Mycoplasma hyorhinis responded to infection with encephalomyocarditis virus (EMCV), a picornavirus, but not with poliovirus (also a picornavirus), with a strong activation of a DNase(s), as evidenced by the TUNEL (terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling) immunofluorescence assay and electrophoretic analysis of host DNA. This degradation was reminiscent of that observed upon apoptosis but was caspase independent, judging by the failure of the specific pan-caspase inhibitor Q-VD-OPh to prevent it. The electrophoretic mobility of the enzyme responsible for DNA degradation and dependence of its activity on ionic conditions strongly suggested that it was represented by a DNase(s) of mycoplasma origin. In cells not infected with EMCV, the relevant DNase was dormant. The possibility is discussed that activation of the mycoplasma DNase might be linked to a relatively early increase in permeability of plasma membrane of the infected cells caused by EMCV. This type of unanticipated virus-mycoplasma "cooperation" may exemplify the complexity of pathogen-host interactions under conditions when viruses and microorganisms are infecting the same host. In the course of the present study, it was also demonstrated that pan-caspase inhibitor zVAD(OMe).fmk strongly suppressed cardiovirus polyprotein processing, illustrating an additional pitfall in investigations of viral effects on the apoptotic system of host cells.

Cellular kinase inhibitors that suppress enterovirus replication have a conserved target in viral protein 3A similar to that of enviroxime

Previously, we identified a cellular kinase inhibitor, GW5074, that inhibits poliovirus (PV) and enterovirus 71 replication strongly, although its target has remained unknown. To identify the target of GW5074, we searched for cellular kinase inhibitors that have anti-enterovirus activity similar or related to that of GW5074. With this aim, we performed screenings to identify cellular kinase inhibitors that could inhibit PV replication cooperatively with GW5074 or synthetically in the absence of GW5074. We identified MEK1/2 inhibitors (SL327 and U0126), an EGFR inhibitor (AG1478) and a phosphatidylinositol 3-kinase inhibitor (wortmannin) as compounds with a cooperative inhibitory effect with GW5074, and an Akt1/2 inhibitor (Akt inhibitor VIII) as a compound with a synthetic inhibitory effect with MEK1/2 inhibitors and AG1478. Individual treatment with the identified kinase inhibitors did not affect PV replication significantly, but combined treatment with MEK1/2 inhibitor, AG1478 and Akt1/2 inhibitor suppressed the replication synthetically. The effect of AG1478 in this synthetic inhibition was compensated by other receptor tyrosine kinase inhibitors (IGF-1R inhibitor II and Flt3 inhibitor II). We isolated mutants resistant to Flt3 inhibitor II and GW5074 and found that these mutants had cross-resistance to each treatment. These mutants had a common mutation in viral protein 3A that results in an amino acid change at position 70 (Ala to Thr), a mutation that was previously identified in mutants resistant to a potent anti-enterovirus compound, enviroxime. These results suggest that cellular kinase inhibitors and enviroxime have a conserved target in viral protein 3A to suppress enterovirus replication.

Antiapoptotic activity of the cardiovirus leader protein, a viral "security" protein

Apoptosis is a common antiviral defensive mechanism that potentially limits viral reproduction and spread. Many viruses possess apoptosis-suppressing tools. Here, we show that the productive infection of HeLa cells with encephalomyocarditis virus (a cardiovirus) was not accompanied by full-fledged apoptosis (although the activation of caspases was detected late in infection) but rather elicited a strong antiapoptotic state, as evidenced by the resistance of infected cells to viral and nonviral apoptosis inducers. The development of the antiapoptotic state appeared to depend on a function(s) of the viral leader (L) protein, since its mutational inactivation resulted in the efflux of cytochrome c from mitochondria, the early activation of caspases, and the appearance of morphological and biochemical signs of apoptosis in a significant proportion of infected cells. Infection with both wild-type and L-deficient viruses induced the fragmentation of mitochondria, which in the former case was not accompanied with cytochrome c efflux. Although the exact nature of the antiapoptotic function(s) of cardioviruses remains obscure, our results suggested that it includes previously undescribed mechanisms operating upstream and possibly downstream of the mitochondrial level, and that L is involved in the control of these mechanisms. We propose that cardiovirus L belongs to a class of viral proteins, dubbed here security proteins, whose roles consist solely, or largely, in counteracting host antidefenses. Unrelated L proteins of other picornaviruses as well as their highly variable 2A proteins also may be security proteins. These proteins appear to be independent acquisitions in the evolution of picornaviruses, implying multiple cases of functional (though not structural) convergence.

A new player in a deadly game: influenza viruses and the PI3K/Akt signalling pathway

Upon influenza A virus infection of cells, a wide variety of antiviral and virus‐supportive signalling pathways are induced. Phosphatidylinositol‐3‐kinase (PI3K) is a recent addition to the growing list of signalling mediators that are activated by these viruses. Several studies have addressed the role of PI3K and the downstream effector protein kinase Akt in influenza A virus‐infected cells. PI3K/Akt signalling is activated by diverse mechanisms in a biphasic manner and is required for multiple functions during infection. While the kinase supports activation of the interferon regulatory factor‐3 during antiviral interferon induction, it also exhibits virus supportive functions. In fact, PI3K not only regulates a very early step during viral entry but also results in suppression of premature apoptosis at later stages of infection. The latter function is dependent on the expression of the viral non‐structural protein‐1 (A/NS1). It has been shown that PI3K activation occurs by direct interaction of A/NS1 with the p85 regulatory subunit and interaction sites of A/NS1 and p85 have now been mapped in detail. Here, we summarize the current knowledge on influenza virus‐induced PI3K signalling and how this pathway supports viral propagation.

Cloning of a centaurin-alpha1 like gene MjCent involved in WSSV infection from shrimp Marsupeneaus japonicus

Centaurin-alpha1 specifically binds phosphatidylinositol 3,4,5-trisphosphate (PI (3,4,5)P3) and is a GTPase-activating protein (GAP) of ADP-ribosylation factor (ARF6). It actively engages in phosphatidylinositol 3-kinase (PI3-K) mediated cell signal transduction. Here, for the first time, we have identified a virus related centaurin-alpha1 homologue named MjCent from the shrimp, Marsupeneaus japonicus, an economically important crustacean in the aquaculture industry. MjCent has one conserved ArfGAP and two Pleckstrin homology domains (PH domains). As shown by RT-PCR and immunofluorescence, MjCent appeared in every tissue examined and was localized mainly in the cell cytoplasm. Further investigation with real-time quantitative PCR showed that MjCent was significantly up-regulated during white spot syndrome virus (WSSV) infection, but notably decreased in virus-resistant shrimps. This suggests a close relationship between MjCent and WSSV invasion and host defense of the shrimp, M. japonicus.