PP2A targeting by viral proteins: a widespread biological strategy from DNA/RNA tumor viruses to HIV-1

HBV Remodels PP2A Complexes to Rewire Kinase Signaling in Hepatocellular Carcinoma

Hepatitis B virus (HBV) infections promote liver cancer initiation by inducing inflammation and cellular stress. Despite a primarily indirect effect on oncogenesis, HBV is associated with a recurrent genomic phenotype in hepatocellular carcinoma (HCC), suggesting that it impacts the biology of established HCC. Characterization of the interaction of HBV with host proteins and the mechanistic contributions of HBV to HCC initiation and maintenance could provide insights into HCC biology and uncover therapeutic vulnerabilities. In this study, we used affinity purification mass spectrometry to comprehensively map a network of 145 physical interactions between HBV and human proteins in HCC. A subset of the host factors targeted by HBV proteins were preferentially mutated in non-HBV-associated HCC, suggesting that their interaction with HBV influences HCC biology. HBV interacted with proteins involved in mRNA splicing, mitogenic signaling, and DNA repair, with the latter set interacting with the HBV oncoprotein X (HBx). HBx remodeled the PP2A phosphatase complex by excluding striatin regulatory subunits from the PP2A holoenzyme, and the HBx effects on PP2A caused Hippo kinase activation. In parallel, HBx activated mTOR complex 2, which can prevent YAP degradation. mTOR complex 2-mediated upregulation of YAP was observed in human HCC specimens and mouse HCC models and could be targeted with mTOR kinase inhibitors. Thus, HBV interaction with host proteins rewires HCC signaling rather than directly activating mitogenic pathways, providing an alternative paradigm for the cellular effects of a tumor-promoting virus. Significance: Integrative proteomic and genomic analysis of HBV/host interactions illuminated modifiers of hepatocellular carcinoma behavior and key signaling mechanisms in advanced disease, which suggested that HBV may have therapeutically actionable effects.

Merkel Cell Polyomavirus: Oncogenesis in a Stable Genome

Merkel cell polyomavirus (MCV) is the causative agent for the majority of Merkel cell carcinoma (MCC) cases. Polyomavirus-associated MCC (MCCP) is characterized by the integration of MCV DNA into the tumor genome and a low tumor mutational burden. In contrast, nonviral MCC (MCCN) is characterized by a high tumor mutational burden induced by UV damage. Since the discovery of MCV, much work in the field has focused on understanding the molecular mechanisms of oncogenesis driven by the MCV tumor (T) antigens. Here, we review our current understanding of how the activities of large T (LT) and small T (ST) promote MCC oncogenesis in the absence of genomic instability. We highlight how both LT and ST inhibit tumor suppressors to evade growth suppression, an important cancer hallmark. We discuss ST interactions with cellular proteins, with an emphasis on those that contribute to sustaining proliferative signaling. Finally, we examine active areas of research into open questions in the field, including the origin of MCC and mechanisms of viral integration.

Characteristics of Immune Memory and Effector Activity to Cancer-Expressed MHC Class I Phosphopeptides Differ in Healthy Donors and Ovarian Cancer Patients

Elevated immunity to cancer-expressed antigens can be detected in people with no history of cancer and may contribute to cancer prevention. We have previously reported that MHC-restricted phosphopeptides are cancer-expressed antigens and targets of immune recognition. However, the extent to which this immunity reflects prior or ongoing phosphopeptide exposures was not investigated. In this study, we found that preexisting immune memory to cancer-expressed phosphopeptides was evident in most healthy donors, but the breadth among donors was highly variable. Although three phosphopeptides were recognized by most donors, suggesting exposures to common microbial/infectious agents, most of the 205 tested phosphopeptides were not recognized by peripheral blood mononuclear cells (PBMC) from any donor and the remainder were recognized by only 1 to 3 donors. In longitudinal analyses of 2 donors, effector immune response profiles suggested active reexposures to a subset of phosphopeptides. These findings suggest that the immunogens generating most phosphopeptide-specific immune memory are rare infectious agents or incipient cancer cells with distinct phosphoproteome dysregulations, and that repetitive immunogenic exposures occur in individual donors. Phosphopeptide-specific immunity in PBMCs and tumor-infiltrating lymphocytes from ovarian cancer patients was limited, regardless of whether the phosphopeptide was expressed on the tumor. However, 4 of 10 patients responded to 1 to 2 immunodominant phosphopeptides, and 1 showed an elevated effector response to a tumor-expressed phosphopeptide. As the tumors from these patients displayed many phosphopeptides, these data are consistent with lack of prior exposure or impaired ability to respond to some phosphopeptides and suggest that enhancing phosphopeptide-specific T-cell responses could be a useful approach to improve tumor immunotherapy.

PP2A Phosphatase as an Emerging Viral Host Factor

Protein phosphatase 2A (PP2A) is one of the most ubiquitous cellular proteins and is responsible for the vast majority of Ser/Thr phosphatase activity in eukaryotes. PP2A is a heterotrimer, and its assembly, intracellular localization, enzymatic activity, and substrate specificity are subject to dynamic regulation. Each of its subunits can be targeted by viral proteins to hijack and modulate its activity and downstream signaling to the advantage of the virus. Binding to PP2A is known to be essential to the life cycle of many viruses and seems to play a particularly crucial role for oncogenic viruses, which utilize PP2A to transform infected cells through controlling the cell cycle and apoptosis. Here we summarise the latest developments in the field of PP2A viral targeting; in particular recent discoveries of PP2A hijacking through molecular mimicry of a B56-specific motif by several different viruses. We also discuss the potential as well as shortcomings for therapeutic intervention in the face of our current understanding of viral PP2A targeting.

MHC Phosphopeptides: Promising Targets for Immunotherapy of Cancer and Other Chronic Diseases

Major histocompatibility complex-associated peptides have been considered as potential immunotherapeutic targets for many years. MHC class I phosphopeptides result from dysregulated cell signaling pathways that are common across cancers and both viral and bacterial infections. These antigens are recognized by central memory T cells from healthy donors, indicating that they are considered antigenic by the immune system and that they are presented across different individuals and diseases. Based on these responses and the similar dysregulation, phosphorylated antigens are promising candidates for prevention or treatment of different cancers as well as a number of other chronic diseases.

Proteomic analysis of host cellular proteins co-immunoprecipitated with duck enteritis virus gC

Duck enteritis virus (DEV), the causative agent of duck viral enteritis, causes a contagious, lethal viral disease in Anseriformes (waterfowls). In virus infection, host-virus interaction plays a crucial role in virus replication and pathogenesis. In our previous study, mRFP was fused with the C-terminus of DEV glycoprotein C (gC) to construct a fluorescent-tag DEV virus rgCRFP. In the current study, fluorescent fusion protein (gC-mRFP) was used as the proteomic probe. Co-immunoprecipitation and mass spectrometric analysis of proteins from rgCRFP-infected chicken embryo fibroblasts using commercial anti-RFP antibody led to the identification of a total of 21 gC interacting host proteins. Out of these 21 proteins, the interaction of seven host proteins (GNG2, AR1H1, PPP2CA, UBE2I, MCM5, NUBP1, HN1) with DEV gC protein was validated using membrane-bound split-ubiquitin yeast two-hybrid system (MbYTH) and bimolecular fluorescence complementation (BiFC) analyses. It indicated direct interaction between these proteins with DEV gC protein. This study has furthered the current understanding of DEV virus infection and pathogenesis. SIGNIFICANCE: gC is an crucial glycoprotein of duck enteritis virus that plays an important role in the viral life cycle. Uncovering the interaction between virus-host is very important to elucidate the pathogenic mechanism of the virus. In this study, host factors interacting with DEV gC have been discerned. And seven host proteins (GNG2, AR1H1, PPP2CA, UBE2I, MCM5, NUBP1, HN1) have been further validated to interact with DEV gC using MbYTH and BiFC analyses. These outcomes could shed light on how DEV manipulates the cellular machinery, which could further our understanding of DEV pathogenesis.

Nature's therapy for COVID-19: Targeting the vital non-structural proteins (NSP) from SARS-CoV-2 with phytochemicals from Indian medicinal plants

BACKGROUND

Containing COVID-19 is still a global challenge. It has affected the "normal" world by targeting its economy and health sector. The effect is shifting of focus of research from life threatening diseases like cancer. Thus, we need to develop a medical solution at the earliest. The purpose of this present work was to understand the efficacy of 22 rationally screened phytochemicals from Indian medicinal plants obtained from our previous work, following drug-likeness properties, against 6 non-structural-proteins (NSP) from SARS-CoV-2.

METHODS

100 ns molecular dynamics simulations were performed, and relative binding free energies were computed by MM/PBSA. Further, principal component analysis, dynamic cross correlation and hydrogen bond occupancy were analyzed to characterize protein-ligand interactions. Biological pathway enrichment analysis was also carried out to elucidate the therapeutic targets of the phytochemicals in comparison to SARS-CoV-2.

RESULTS

The potential binding modes and favourable molecular interaction profile of 9 phytochemicals, majorly from Withania somnifera with lowest free binding energies, against the SARS-CoV-2 NSP targets were identified. It was understood that phytochemicals and 2 repurposed drugs with steroidal moieties in their chemical structures formed stable interactions with the NSPs. Additionally, human target pathway analysis for SARS-CoV-2 and phytochemicals showed that cytokine mediated pathway and phosphorylation pathways were with the most significant p-value.

CONCLUSIONS

To summarize this work, we suggest a global approach of targeting multiple proteins of SARS-CoV-2 with phytochemicals as a natural alternative therapy for COVID-19. We also suggest that these phytochemicals need to be tested experimentally to confirm their efficacy.

TSLP-Driven Chromatin Remodeling and Trained Systemic Immunity after Neonatal Respiratory Viral Infection

Our studies have previously shown a role for persistent TSLP production in the lungs of mice after early-life respiratory syncytial virus (RSV) infection that leads to an altered immune phenotype, including accumulation of "inflammatory" dendritic cells (DC). This study investigates the role of TSLP driving systemic trained immunity in DC in early-life RSV-infected mice. Bone marrow-derived DCs (BMDC) from early-life RSV-infected mice at 4 wk postinfection showed enhanced expression of costimulatory molecules and cytokines, including Tslp, that regulate immune cell function. The adoptive transfer of BMDC grown from early-life RSV-infected mice was sufficient to exacerbate allergic disease development. The addition of recombinant TSLP during differentiation of BMDC from naive mice induced a similar altered phenotype as BMDC grown from early-life RSV-infected mice, suggesting a role for TSLP in the phenotypic changes. To assess the role of TSLP in these changes, global transcriptomic characterization of TSLPR^-/- BMDC infected with RSV was performed and showed a higher upregulation of type 1 IFN genes and concomitant downregulation of inflammatory genes. Assay for transposase-accessible chromatin using sequencing analysis demonstrated that TSLPR^-/- BMDC had a parallel gain in physical chromatin accessibility near type 1 genes and loss in accessibility near genes related to RSV pathology, with IFN regulatory factor 4 (IRF4) and STAT3 predicted as top transcription factors binding within differentially accessible regions in wild-type. Importantly, these studies show that in the absence of TSLP signaling, BMDC are able to mount an appropriate type 1 IFN-associated antiviral response to RSV. In summary, RSV-induced TSLP alters chromatin structure in DC to drive trained innate immunity and activates pathogenic gene programs in mice.

Cryo-EM structure of the deltaretroviral intasome in complex with the PP2A regulatory subunit B56γ

Human T-cell lymphotropic virus type 1 (HTLV-1) is a deltaretrovirus and the most oncogenic pathogen. Many of the ~20 million HTLV-1 infected people will develop severe leukaemia or an ALS-like motor disease, unless a therapy becomes available. A key step in the establishment of infection is the integration of viral genetic material into the host genome, catalysed by the retroviral integrase (IN) enzyme. Here, we use X-ray crystallography and single-particle cryo-electron microscopy to determine the structure of the functional deltaretroviral IN assembled on viral DNA ends and bound to the B56γ subunit of its human host factor, protein phosphatase 2 A. The structure reveals a tetrameric IN assembly bound to two molecules of the phosphatase via a conserved short linear motif. Insight into the deltaretroviral intasome and its interaction with the host will be crucial for understanding the pattern of integration events in infected individuals and therefore bears important clinical implications.

Identification of a PP2A gene in Bombyx mori with antiviral function against B. mori nucleopolyhedrovirus

Ser/Thr protein phosphatase 2A (PP2A) is one of the type 2 protein phosphatases, which is required for many intracellular physiological processes and pathogen infection. However, the function of PP2A is unclear in silkworm, Bombyx mori. Here, we cloned and identified BmPP2A, a PP2A gene from B. mori, which has two HEAT domains and a high similarity to PP2A from other organisms. Our results showed that BmPP2A is localized in the cytoplasm and highly expressed in silkworm epidermis and midgut, and that Bombyx mori nucleopolyhedrovirus (BmNPV) infection induces down-regulation of BmPP2A expression. Furthermore, up-regulation of BmPP2A via overexpression significantly inhibited BmNPV multiplication. In contrast, down-regulation of BmPP2A via RNA interference and okadaic acid (a PP2A inhibitor) treatment allowed robust BmNPV replication. This is the first report of PP2A having an antiviral effect in silkworm and provides insights into the function of BmPP2A, a potential anti-BmNPV mechanism, and a possible target for the breeding of silkworm-resistant strains.

Merkel Cell Polyomavirus and Merkel Cell Carcinoma

Viruses are the cause of approximately 15% of all human cancers. Both RNA and DNA human tumor viruses have been identified, with Merkel cell polyomavirus being the most recent one to be linked to cancer. This virus is associated with about 80% of Merkel cell carcinomas, a rare, but aggressive cutaneous malignancy. Despite its name, the cells of origin of this tumor may not be Merkel cells. This review provides an update on the structure and life cycle, cell tropism and epidemiology of the virus and its oncogenic properties. Putative strategies to prevent viral infection or treat virus-positive Merkel cell carcinoma patients are discussed.

Structural basis of host protein hijacking in human T-cell leukemia virus integration

Integration of the reverse-transcribed viral DNA into host chromosomes is a critical step in the life-cycle of retroviruses, including an oncogenic delta(δ)-retrovirus human T-cell leukemia virus type-1 (HTLV-1). Retroviral integrase forms a higher order nucleoprotein assembly (intasome) to catalyze the integration reaction, in which the roles of host factors remain poorly understood. Here, we use cryo-electron microscopy to visualize the HTLV-1 intasome at 3.7-Å resolution. The structure together with functional analyses reveal that the B56γ (B'γ) subunit of an essential host enzyme, protein phosphatase 2 A (PP2A), is repurposed as an integral component of the intasome to mediate HTLV-1 integration. Our studies reveal a key host-virus interaction underlying the replication of an important human pathogen and highlight divergent integration strategies of retroviruses.

The impact of proteostasis dysfunction secondary to environmental and genetic causes on neurodegenerative diseases progression and potential therapeutic intervention

Aggregation of particular proteins in the form of inclusion bodies or plaques followed by neuronal death is a hallmark of neurodegenerative proteopathies such as primary Parkinsonism, Alzheimer's disease, Lou Gehrig's disease, and Huntington's chorea. Complex polygenic and environmental factors implicated in these proteopathies. Accumulation of proteins in these disorders indicates a substantial disruption in protein homeostasis (proteostasis). Proteostasis or cellular proteome homeostasis is attained by the synchronization of a group of cellular mechanisms called the proteostasis network (PN), which is responsible for the stability of the proteome and achieves the equilibrium between synthesis, folding, and degradation of proteins. In this review, we will discuss the different types of PN and the impact of PN component dysfunction on the four major neurodegenerative diseases mentioned earlier. Graphical abstract.

NAD(P)H Quinone Dehydrogenase 1 Ablation Inhibits Activation of the Phosphoinositide 3-Kinase/Akt Serine/Threonine Kinase and Mitogen-Activated Protein Kinase/Extracellular Signal-Regulated Kinase Pathways and Blocks Metabolic Adaptation in Hepatocellular Carcinoma

Cancer cells undergo metabolic adaptation to sustain uncontrolled proliferation. Aerobic glycolysis and glutaminolysis are two of the most essential characteristics of cancer metabolic reprogramming. Hyperactivated phosphoinositide 3-kinase (PI3K)/Akt serine/threonine kinase (Akt) and mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) signaling pathways play central roles in cancer cell metabolic adaptation given that their downstream effectors, such as Akt and c-Myc, control most of the glycolytic and glutaminolysis genes. Here, we report that the cytosolic flavoprotein, NAD(P)H quinone dehydrogenase 1 (Nqo1), is strongly overexpressed in mouse and human hepatocellular carcinoma (HCC). Knockdown of Nqo1 enhanced activity of the serine/threonine phosphatase, protein phosphatase 2A, which operates at the intersection of the PI3K/Akt and MAPK/ERK pathways and dephosphorylates and inactivates pyruvate dehydrogenase kinase 1, Akt, Raf, mitogen-activated protein kinase kinase, and ERK1/2. Nqo1 ablation also induced the expression of phosphatase and tensin homolog, a dual protein/lipid phosphatase that blocks PI3K/Akt signaling, through the ERK/cAMP-responsive element-binding protein/c-Jun pathway. Together, Nqo1 ablation triggered simultaneous inhibition of the PI3K/Akt and MAPK/ERK pathways, suppressed the expression of glycolysis and glutaminolysis genes and blocked metabolic adaptation in liver cancer cells. Conversely, Nqo1 overexpression caused hyperactivation of the PI3K/Akt and MAPK/ERK pathways and promoted metabolic adaptation. Conclusion: In conclusion, Nqo1 functions as an upstream activator of both the PI3K/Akt and MAPK/ERK pathways in liver cancer cells, and Nqo1 ablation blocked metabolic adaptation and inhibited liver cancer cell proliferation and HCC growth in mice. Therefore, our results suggest that Nqo1 may function as a therapeutic target to inhibit liver cancer cell proliferation and inhibit HCC.

Protein phosphatase 2A as a therapeutic target in inflammation and neurodegeneration

Protein phosphatase 2A (PP2A) is a highly complex heterotrimeric enzyme that catalyzes the selective removal of phosphate groups from protein serine and threonine residues. Emerging evidence suggests that it functions as a tumor suppressor by constraining phosphorylation-dependent signalling pathways that regulate cellular transformation and metastasis. Therefore, PP2A-activating drugs (PADs) are being actively sought and investigated as potential novel anti-cancer treatments. Here we explore the concept that PP2A also constrains inflammatory responses through its inhibitory effects on various signalling pathways, suggesting that PADs may be effective in the treatment of inflammation-mediated pathologies.

Antitumor and antibacterial properties of virally encoded cationic sequences

Objective: The objective of this study was to test our Viral Quinta Columna Strategy (VQCS), a new biological hypothesis predicting that specific multifunctional virally encoded cationic domains may have the capacity to penetrate human cells and interact with PP2A proteins to deregulate important human intracellular pathways, and may display LL37 cathelicidin-like antagonistic effects against multiple pathogens such as bacteria or viruses.

Methods: We comparatively analyzed the host defense properties of adenodiaphorins and of some specific cationic sequences encoded by different viruses using two distinct biological models: U87G, a well-characterized cell tumor model; and a group B Streptococcus agalactiae NEM316 ΔdltA, highly sensitive to LL37 cathelicidin.

Results: We found that the adenovirus type 2 E4orf4 is a cell-permeable protein containing a new E4orf4_64–95 protein transduction domain, named large adenodiaphorin or LadD_64–95. Interestingly, the host defense LL37 peptide is the unique cathelicidin in humans. In this context, we also demonstrated that similarly to LL37 LadD_64–95, several virally encoded cationic sequences including the C-terminus HIV-1 89.6 Vpr_77–92, shorter adenodiaphorins AdD_67–84/AdD/_69–84/AdD_69–83, as well as HIV-2 Tat_67–90 and JC polyomavirus small t_115–134, displayed similar toxicity against Gram-positive S. agalactiae NEM316 ΔdltA strain. Finally, LadD_64–95, adenodiaphorin AdD_67–84, AdD_69–84, and LL37 and LL_17–32 cathelicidin peptides also inhibited the survival of human U87G glioblastoma cells.

Conclusion: In this study, we demonstrated that specific cationic sequences encoded by four different viruses displayed antibacterial activities against S. agalactiae NEM316 ΔdltA strain. In addition, HIV-1 Vpr_71–92 and adenovirus 2 E4orf4_64–95, two cationic penetrating sequences that bind PP2A, inhibited the survival of U87G glioblastoma cells. These results illustrate the host defense properties of virally encoded sequences and could represent an initial step for future complete validation of the VQCS hypothesis.

SIV-Mediated Synaptic Dysfunction Is Associated with an Increase in Synapsin Site 1 Phosphorylation and Impaired PP2A Activity

Although the reduction of viral loads in people with HIV undergoing combination antiretroviral therapy has mitigated AIDS-related symptoms, the prevalence of neurological impairments has remained unchanged. HIV-associated CNS dysfunction includes impairments in memory, attention, memory processing, and retrieval. Here, we show a significant site-specific increase in the phosphorylation of Syn I serine 9, site 1, in the frontal cortex lysates and synaptosome preparations of male rhesus macaques infected with simian immunodeficiency virus (SIV) but not in uninfected or SIV-infected antiretroviral therapy animals. Furthermore, we found that a lower protein phosphatase 2A (PP2A) activity, a phosphatase responsible for Syn I (S9) dephosphorylation, is primarily associated with the higher S9 phosphorylation in the frontal cortex of SIV-infected macaques. Comparison of brain sections confirmed higher Syn I (S9) in the frontal cortex and greater coexpression of Syn I and PP2A A subunit, which was observed as perinuclear aggregates in the somata of the frontal cortex of SIV-infected macaques. Synaptosomes from SIV-infected animals were physiologically tested using a synaptic vesicle endocytosis assay and FM4-64 dye showing a significantly higher baseline depolarization levels in synaptosomes of SIV⁺-infected than uninfected control or antiretroviral therapy animals. A PP2A-activating FDA-approved drug, FTY720, decreased the higher synaptosome depolarization in SIV-infected animals. Our results suggest that an impaired distribution and lower activity of serine/threonine phosphatases in the context of HIV infection may cause an indirect effect on the phosphorylation levels of essential proteins involving in synaptic transmission, supporting the occurrence of specific impairments in the synaptic activity during SIV infection.SIGNIFICANCE STATEMENT Even with antiretroviral therapy, neurocognitive deficits, including impairments in attention, memory processing, and retrieval, are still major concerns in people living with HIV. Here, we used the rhesus macaque simian immunodeficiency virus model with and without antiretroviral therapy to study the dynamics of phosphorylation of key amino acid residues of synapsin I, which critically impacts synaptic vesicle function. We found a significant increase in synapsin I phosphorylation at serine 9, which was driven by dysfunction of serine/threonine protein phosphatase 2A in the nerve terminals. Our results suggest that an impaired distribution and lower activity of serine/threonine phosphatases in the context of HIV infection may cause an indirect effect on the phosphorylation levels of essential proteins involved in synaptic transmission.

BK Polyomavirus Activates the DNA Damage Response To Prolong S Phase

BK polyomavirus (PyV) is a major source of kidney failure in transplant recipients. The standard treatment for patients with lytic BKPyV infection is to reduce immunosuppressive therapy, which increases the risk of graft rejection. PyVs are DNA viruses that rely upon host replication proteins for viral genome replication. A hallmark of PyV infection is activation of the DNA damage response (DDR) to prevent severe host and viral DNA damage that impairs viral production by an unknown mechanism. Therefore, we sought to better understand why BKPyV activates the DDR through the ATR and ATM pathways and how this prevents DNA damage and leads to increased viral production. When ATR was inhibited in BKPyV-infected primary kidney cells, severe DNA damage occurred due to premature Cdk1 activation, which resulted in mitosis of cells that were actively replicating host DNA in S phase. Conversely, ATM was required for efficient entry into S phase and to prevent normal mitotic entry after G₂ phase. The synergistic activation of these DDR kinases promoted and maintained BKPyV-mediated S phase to enhance viral production. In contrast to BKPyV infection, DDR inhibition did not disrupt cell cycle control in uninfected cells. This suggests that DDR inhibitors may be used to specifically target BKPyV-infected cells.IMPORTANCE BK polyomavirus (BKPyV) is an emerging pathogen that reactivates in immunosuppressed organ transplant patients. We wanted to understand why BKPyV-induced activation of the DNA damage response (DDR) enhances viral titers and prevents host DNA damage. Here, we show that the virus activates the DNA damage response in order to keep the infected cells in S phase to replicate the viral DNA. The source of DNA damage was due to actively replicating cells with uncondensed chromosomes entering directly into mitosis when the DDR was inhibited in BKPyV-infected cells. This study clarifies the previously enigmatic role of the DDR during BKPyV infection by demonstrating that the virus activates the DDR to maintain the cells in S phase in order to promote viral replication and that disruption of this cell cycle arrest can lead to catastrophic DNA damage for the host.

MicroRNA Profile of HCV Spontaneous Clarified Individuals, Denotes Previous HCV Infection

Factors involved in the spontaneous cleareance of a hepatitis C (HCV) infection are related to both HCV and the interaction with the host immune system, but little is known about the consequences after a spontaneous resolution. The main HCV extrahepatic reservoir is the peripheral blood mononuclear cells (PBMCs), and their transcriptional profile provides us information of innate and adaptive immune responses against an HCV infection. MicroRNAs regulate the innate and adaptive immune responses, and they are actively involved in the HCV cycle. High Throughput sequencing was used to analyze the miRNA profiles from PBMCs of HCV chronic naïve patients (CHC), individuals that spontaneously clarified HCV (SC), and healthy controls (HC). We did not find any differentially expressed miRNAs between SC and CHC. However, both groups showed similar expression differences (21 miRNAs) with respect to HC. This miRNA signature correctly classifies HCV-exposed (CHC and SC) vs. HC, with the has-miR-21-3p showing the best performance. The potentially targeted molecular pathways by these 21 miRNAs mainly belong to fatty acids pathways, although hippo signaling, extracellular matrix (ECM) interaction, proteoglycans-related, and steroid biosynthesis pathways were also altered. These miRNAs target host genes involved in an HCV infection. Thus, an HCV infection promotes molecular alterations in PBMCs that can be detected after an HCV spontaneous resolution, and the 21-miRNA signature is able to identify HCV-exposed patients (either CHC or SC).

ER stress regulating protein phosphatase 2A-B56γ, targeted by hepatitis B virus X protein, induces cell cycle arrest and apoptosis of hepatocytes

Hepatitis B virus X (HBx) protein contributes to the progression of hepatitis B virus (HBV)-related hepatic injury and diseases, but the exact mechanism remains unclear. Protein phosphatase 2 A (PP2A) is a major serine/threonine phosphatase involved in regulating many cellular phosphorylation signals that are important for regulation of cell cycle and apoptosis. Does HBx target to PP2A-B56γ and therefore affect HBx-induced hepatotoxicity? In the present study, the expression of B56γ positively correlated with the level of HBx in HBV-infected primary human hepatocytes in human-liver-chimeric mice, HBx-transgenic mice, HBV-infected cells, and HBx-expressing hepatic cells. B56γ promoted p53/p21-dependent cell cycle arrest and apoptosis. Mechanistically, B56γ was transactivated by AP-1, which was under the regulation of endoplasmic reticulum (ER) stress induced CREBH signaling in HBx-expressing hepatic cells. B56γ dephosphorylated p-Thr55-p53 to trigger p53/p21 pathway-dependent cell cycle G1 phase arrest, resulting in apoptosis of hepatic cells. In conclusion, this study provides a novel insight into a mechanism of B56γ mediating cell cycle arrest and apoptosis of HBx-expressing hepatic cells and a basis for B56γ being a potential therapeutic target for HBV-infected hepatic cells.

Dysregulation of the MEK/ERK/MNK1 signalling cascade by middle T antigen of the trichoydsplasia spinulosa polyomavirus

BACKGROUND

Trichodysplasia spinulosa (TS) is a disfiguring folliculocentric cutaneous disease caused by infection with the trichodysplasia spinulosa polyomavirus (TSPyV). The TSPyV genome contains splice variants encoding the middle tumour (mT) antigen, although the potential role for TSPyV mT antigen in disease development remains unknown.

OBJECTIVE

The current study was designed to investigate the mechanistic properties of TSPyV mT antigen, which may further our understanding of TS pathogenesis and provide insight into potential therapies.

METHODS

A lentiviral packaging system was used to create an inducible cell line expressing TSPyV mT antigen. Proteins were extracted, separated by SDS-PAGE and subjected to Western blot analysis. Co-immunoprecipitation experiments and mutational analyses were also performed to evaluate protein-protein interactions of mT antigen.

RESULTS

We describe a novel mechanism of action for mT antigen that involves hyperactivation of MEK, ERK and MNK1. Our findings suggest that dysregulation of these key signalling molecules depends upon TSPyV mT antigen interaction with protein phosphatase 2A (PP2A) via intact Zn binding motifs.

CONCLUSION

Given that PP2A interaction and MEK/ERK/MNK1 phosphorylation are associated with high levels of cell proliferation and inflammation, our findings provide new evidence that TSPyV mT antigen may contribute to the pro-proliferative conditions that lead to TS development.

Cyanobacterial Toxins of the Laurentian Great Lakes, Their Toxicological Effects, and Numerical Limits in Drinking Water

Cyanobacteria are ubiquitous phototrophic bacteria that inhabit diverse environments across the planet. Seasonally, they dominate many eutrophic lakes impacted by excess nitrogen (N) and phosphorus (P) forming dense accumulations of biomass known as cyanobacterial harmful algal blooms or cyanoHABs. Their dominance in eutrophic lakes is attributed to a variety of unique adaptations including N and P concentrating mechanisms, N₂ fixation, colony formation that inhibits predation, vertical movement via gas vesicles, and the production of toxic or otherwise bioactive molecules. While some of these molecules have been explored for their medicinal benefits, others are potent toxins harmful to humans, animals, and other wildlife known as cyanotoxins. In humans these cyanotoxins affect various tissues, including the liver, central and peripheral nervous system, kidneys, and reproductive organs among others. They induce acute effects at low doses in the parts-per-billion range and some are tumor promoters linked to chronic diseases such as liver and colorectal cancer. The occurrence of cyanoHABs and cyanotoxins in lakes presents challenges for maintaining safe recreational aquatic environments and the production of potable drinking water. CyanoHABs are a growing problem in the North American (Laurentian) Great Lakes basin. This review summarizes information on the occurrence of cyanoHABs in the Great Lakes, toxicological effects of cyanotoxins, and appropriate numerical limits on cyanotoxins in finished drinking water.

DENV up-regulates the HMG-CoA reductase activity through the impairment of AMPK phosphorylation: A potential antiviral target

Dengue is the most common mosquito-borne viral disease in humans. Changes of lipid-related metabolites in endoplasmic reticulum of dengue virus (DENV) infected cells have been associated with replicative complexes formation. Previously, we reported that DENV infection inhibits HMGCR phosphorylation generating a cholesterol-enriched cellular environment in order to favor viral replication. In this work, using enzymatic assays, ELISA, and WB we found a significant higher activity of HMGCR in DENV infected cells, associated with the inactivation of AMPK. AMPK activation by metformin declined the HMGCR activity suggesting that AMPK inactivation mediates the enhanced activity of HMGCR. A reduction on AMPK phosphorylation activity was observed in DENV infected cells at 12 and 24 hpi. HMGCR and cholesterol co-localized with viral proteins NS3, NS4A and E, suggesting a role for HMGCR and AMPK activity in the formation of DENV replicative complexes. Furthermore, metformin and lovastatin (HMGCR inhibitor) altered this co-localization as well as replicative complexes formation supporting that active HMGCR is required for replicative complexes formation. In agreement, metformin prompted a significant dose-dependent antiviral effect in DENV infected cells, while compound C (AMPK inhibitor) augmented the viral genome copies and the percentage of infected cells. The PP2A activity, the main modulating phosphatase of HMGCR, was not affected by DENV infection. These data demonstrate that the elevated activity of HMGCR observed in DENV infected cells is mediated through AMPK inhibition and not by increase in PP2A activity. Interestingly, the inhibition of this phosphatase showed an antiviral effect in an HMGCR-independent manner. These results suggest that DENV infection increases HMGCR activity through AMPK inactivation leading to higher cholesterol levels in endoplasmic reticulum necessary for replicative complexes formation. This work provides new information about the mechanisms involved in host lipid metabolism during DENV replicative cycle and identifies new potential antiviral targets for DENV replication.

DENV replicative complexes formation is associated with changes of lipid-related metabolites in endoplasmic reticulum, such as an increase in cholesterol synthesis. This increase correlates with a significant augment in the activity of HMGCoA reductase (the limiting enzyme in cholesterol synthesis), favoring a cholesterol-enriched cellular environment. The augment in the activity of the HMGCR observed in infected cells is caused by a decrease in the phosphorylation level of the HMGCR, associated with the inactivation of AMPK. In agreement, AMPK activation by metformin reduces HMGCR activity and affects viral replication. The role HMGCR and AMPK activity in DENV replicative complexes formation was confirmed by the co-localization of HMGCR and cholesterol with the viral proteins NS3, NS4A and E. Furthermore, metformin and lovastatin (HMGCR inhibitor) treatments altered this co-localization as well as replicative complexes formation supporting that active HMGCR is required for replicative complexes formation. The results show that during DENV infection, an increase in the HMGCR activity occurs through AMPK inactivation, leading to higher cholesterol levels in endoplasmic reticulum necessary for replicative complexes formation. This work provides new information about the mechanisms involved in host lipid metabolism during DENV replicative cycle and identifies potential new antiviral targets for DENV replication.

Isolation and characterization of a novel putative human polyomavirus

The small double-stranded DNA polyomaviruses (PyVs) form a family of 73 species, whose natural hosts are primarily mammals and birds. So far, 13 PyVs have been isolated in humans, and some of them have clearly been associated with several diseases, including cancer. In this study, we describe the isolation of a novel PyV in human skin using a sensitive degenerate PCR protocol combined with next-generation sequencing. The new virus, named Lyon IARC PyV (LIPyV), has a circular genome of 5269 nucleotides. Phylogenetic analyses showed that LIPyV is related to the raccoon PyV identified in neuroglial tumours in free-ranging raccoons. Analysis of human specimens from cancer-free individuals showed that 9 skin swabs (9/445; 2.0%), 3 oral gargles (3/140; 2.1%), and one eyebrow hair sample (1/439; 0.2%) tested positive for LIPyV. Future biological and epidemiological studies are needed to confirm the human tropism and provide insights into its biological properties.

Protein Phosphatase 2A: a Double-Faced Phosphatase of Cellular System and Its Role in Neurodegenerative Disorders

Protein phosphatase 2A (PP2A), a ubiquitously expressed serine/threonine phosphatase, is a vitally important phosphatase for the cellular system. Structurally, it is constituted of three different subunits, namely catalytic subunit (PP2Ac), structural scaffold subunit (PP2A-A), and regulatory subunit (PP2A-B). All subunits have various isoforms, and catalytic and scaffold subunits are ubiquitously expressed, whereas regulatory subunits are more specific to tissue and cell type. It is the numerous possibilities of PP2A holoenzyme assembly with varying isoform components that make it possess a dual nature of activator or the inhibitory character in different signaling pathways, namely neural developmental pathways, Akt/protein kinase B pathway, NF-kB pathway, MAPK pathway, apoptosis pathway, and cell cycle progression to name a few. Importantly, the expression of PP2A in the brain is highest among the serine phosphatases and is known to actively participate in the neural development process. However, the exact mechanism of action of PP2A is still debated and enunciating the holoenzyme components, especially the regulatory subunit of PP2A involved in regulating neural developmental process is still poorly understood. In this review, we try to throw some light on the involvement of various PP2A holoenzyme forms in the process of neurogenesis and progression of neurodegenerative diseases.

Molecular mechanisms supporting a pathogenic role for human polyomavirus 6 small T antigen: Protein phosphatase 2A targeting and MAPK cascade activation

BRAF inhibitors are highly effective therapies in treating a subset of melanomas but are associated with induction of secondary cutaneous squamous cell carcinoma (cSCC). Recently, Human Polyomavirus 6 (HPyV6) was found to actively express viral proteins in BRAF inhibitor-induced cSCCs; however, the specific cellular mechanisms by which HPyV6 may facilitate neoplastic cell growth require further investigation. The current study describes a novel pathogenic mechanism of action for HPyV6 small tumor (sT) antigen which involves binding to protein phosphatase 2A (PP2A) via its WFG motif and zinc binding sites. Our findings demonstrate an important role of HPyV6 sT for activation of PP2A's downstream oncogenic pathways (MEK/ERK/c-Jun), which may underlie the pathogenesis of BRAF inhibitor-induced neoplasms. J. Med. Virol. 89:742-747, 2017. © 2016 Wiley Periodicals, Inc.

Comparative Analysis of Salivary Gland Proteomes of Two Glossina Species that Exhibit Differential Hytrosavirus Pathologies

Glossina pallidipes salivary gland hypertrophy virus (GpSGHV; family Hytrosaviridae) is a dsDNA virus exclusively pathogenic to tsetse flies (Diptera; Glossinidae). The 190 kb GpSGHV genome contains 160 open reading frames and encodes more than 60 confirmed proteins. The asymptomatic GpSGHV infection in flies can convert to symptomatic infection that is characterized by overt salivary gland hypertrophy (SGH). Flies with SGH show reduced general fitness and reproductive dysfunction. Although the occurrence of SGH is an exception rather than the rule, G. pallidipes is thought to be the most susceptible to expression of overt SGH symptoms compared to other Glossina species that are largely asymptomatic. Although Glossina salivary glands (SGs) play an essential role in GpSGHV transmission, the functions of the salivary components during the virus infection are poorly understood. In this study, we used mass spectrometry to study SG proteomes of G. pallidipes and G. m. morsitans, two Glossina model species that exhibit differential GpSGHV pathologies (high and low incidence of SGH, respectively). A total of 540 host proteins were identified, of which 23 and 9 proteins were significantly up- and down-regulated, respectively, in G. pallidipes compared to G. m. morsitans. Whereas 58 GpSGHV proteins were detected in G. pallidipes F₁ progenies, only 5 viral proteins were detected in G. m. morsitans. Unlike in G. pallidipes, qPCR assay did not show any significant increase in virus titers in G. m. morsitans F₁ progenies, confirming that G. m. morsitans is less susceptible to GpSGHV infection and replication compared to G. pallidipes. Based on our results, we speculate that in the case of G. pallidipes, GpSGHV employs a repertoire of host intracellular signaling pathways for successful infection. In the case of G. m. morsitans, antiviral responses appeared to be dominant. These results are useful for designing additional tools to investigate the Glossina-GpSGHV interactions.

PP2A: The Wolf in Sheep's Clothing?

Protein Phosphatase 2A (PP2A) is a major serine/threonine phosphatase in cells. It consists of a catalytic subunit (C), a structural subunit (A), and a regulatory/variable B-type subunit. PP2A has a critical role to play in homeostasis where its predominant function is as a phosphatase that regulates the major cell signaling pathways in cells. Changes in the assembly, activity and substrate specificity of the PP2A holoenzyme have a direct role in disease and are a major contributor to the maintenance of the transformed phenotype in cancer. We have learned a lot about how PP2A functions from specific mutations that disrupt the core assembly of PP2A and from viral proteins that target PP2A and inhibit its effect as a phosphatase. This prompted various studies revealing that restoration of PP2A activity benefits some cancer patients. However, our understanding of the mechanism of action of this is limited because of the complex nature of PP2A holoenzyme assembly and because it acts through a wide variety of signaling pathways. Information on PP2A is also conflicting as there are situations whereby inactivation of PP2A induces apoptosis in many cancer cells. In this review we discuss this relationship and we also address many of the pertinent and topical questions that relate to novel therapeutic strategies aimed at altering PP2A activity.

Restricted protein phosphatase 2A targeting by Merkel cell polyomavirus small T antigen

Merkel cell polyomavirus (MCV) is a newly discovered human cancer virus encoding a small T (sT) oncoprotein. We performed MCV sT FLAG-affinity purification followed by mass spectroscopy (MS) analysis, which identified several protein phosphatases (PP), including PP2A A and C subunits and PP4C, as potential cellular interacting proteins. PP2A targeting is critical for the transforming properties of nonhuman polyomaviruses, such as simian virus 40 (SV40), but is not required for MCV sT-induced rodent cell transformation. We compared similarities and differences in PP2A binding between MCV and SV40 sT. While SV40 sT coimmunopurified with subunits PP2A Aα and PP2A C, MCV sT coimmunopurified with PP2A Aα, PP2A Aβ, and PP2A C. Scanning alanine mutagenesis at 29 sites across the MCV sT protein revealed that PP2A-binding domains lie on the opposite molecular surface from a previously described large T stabilization domain (LSD) loop that binds E3 ligases, such as Fbw7. MCV sT-PP2A interactions can be functionally distinguished by mutagenesis from MCV sT LSD-dependent 4E-BP1 hyperphosphorylation and viral DNA replication enhancement. MCV sT has a restricted range for PP2A B subunit substitution, inhibiting only the assembly of B56α into the phosphatase holoenzyme. In contrast, SV40 sT inhibits the assembly of B55α, B56α and B56ε into PP2A. We conclude that MCV sT is required for Merkel cell carcinoma growth, but its in vitro transforming activity depends on LSD interactions rather than PP2A targeting.

IMPORTANCE

Merkel cell polyomavirus is a newly discovered human cancer virus that promotes cancer, in part, through expression of its small T (sT) oncoprotein. Animal polyomavirus sT oncoproteins have been found to cause experimental tumors by blocking the activities of a group of phosphatases called protein phosphatase 2A (PP2A). Our structural analysis reveals that MCV sT also displaces the B subunit of PP2A to inhibit PP2A activity. MCV sT, however, only displaces a restricted subset of PP2A B subunits, which is insufficient to cause tumor cell formation in vitro. MCV sT instead transforms tumor cells through another region called the large T stabilization domain. The PP2A targeting and transforming activities lie on opposite faces of the MCV sT molecule and can be genetically separated from each other.

Emerging roles of interferon-stimulated genes in the innate immune response to hepatitis C virus infection

Infection with hepatitis C virus (HCV), a major viral cause of chronic liver disease, frequently progresses to steatosis and cirrhosis, which can lead to hepatocellular carcinoma. HCV infection strongly induces host responses, such as the activation of the unfolded protein response, autophagy and the innate immune response. Upon HCV infection, the host induces the interferon (IFN)-mediated frontline defense to limit virus replication. Conversely, HCV employs diverse strategies to escape host innate immune surveillance. Type I IFN elicits its antiviral actions by inducing a wide array of IFN-stimulated genes (ISGs). Nevertheless, the mechanisms by which these ISGs participate in IFN-mediated anti-HCV actions remain largely unknown. In this review, we first outline the signaling pathways known to be involved in the production of type I IFN and ISGs and the tactics that HCV uses to subvert innate immunity. Then, we summarize the effector mechanisms of scaffold ISGs known to modulate IFN function in HCV replication. We also highlight the potential functions of emerging ISGs, which were identified from genome-wide siRNA screens, in HCV replication. Finally, we discuss the functions of several cellular determinants critical for regulating host immunity in HCV replication. This review will provide a basis for understanding the complexity and functionality of the pleiotropic IFN system in HCV infection. Elucidation of the specificity and the mode of action of these emerging ISGs will also help to identify novel cellular targets against which effective HCV therapeutics can be developed.

Emerging roles of interferon-stimulated genes in the innate immune response to hepatitis C virus infection

Infection with hepatitis C virus (HCV), a major viral cause of chronic liver disease, frequently progresses to steatosis and cirrhosis, which can lead to hepatocellular carcinoma. HCV infection strongly induces host responses, such as the activation of the unfolded protein response, autophagy and the innate immune response. Upon HCV infection, the host induces the interferon (IFN)-mediated frontline defense to limit virus replication. Conversely, HCV employs diverse strategies to escape host innate immune surveillance. Type I IFN elicits its antiviral actions by inducing a wide array of IFN-stimulated genes (ISGs). Nevertheless, the mechanisms by which these ISGs participate in IFN-mediated anti-HCV actions remain largely unknown. In this review, we first outline the signaling pathways known to be involved in the production of type I IFN and ISGs and the tactics that HCV uses to subvert innate immunity. Then, we summarize the effector mechanisms of scaffold ISGs known to modulate IFN function in HCV replication. We also highlight the potential functions of emerging ISGs, which were identified from genome-wide siRNA screens, in HCV replication. Finally, we discuss the functions of several cellular determinants critical for regulating host immunity in HCV replication. This review will provide a basis for understanding the complexity and functionality of the pleiotropic IFN system in HCV infection. Elucidation of the specificity and the mode of action of these emerging ISGs will also help to identify novel cellular targets against which effective HCV therapeutics can be developed.

Quantitative proteomics identifies host factors modulated during acute hepatitis E virus infection in the swine model

Hepatitis E virus (HEV) causes acute enterically transmitted hepatitis. In industrialized countries, it is a zoonotic disease, with swine being the major reservoir of human HEV contamination. The occurrence and severity of the disease are variable, with clinical symptoms ranging from asymptomatic to self-limiting acute hepatitis, chronic infection, or fulminant hepatitis. In the absence of a robust cell culture system or small-animal models, the HEV life cycle and pathological process remain unclear. To characterize HEV pathogenesis and virulence mechanisms, a quantitative proteomic analysis was carried out to identify cellular factors and pathways modulated during acute infection of swine. Three groups of pigs were inoculated with three different strains of swine HEV to evaluate the possible role of viral determinants in pathogenesis. Liver samples were analyzed by a differential proteomic approach, two-dimensional difference in gel electrophoresis, and 61 modulated proteins were identified by mass spectroscopy. The results obtained show that the three HEV strains replicate similarly in swine and that they modulate several cellular pathways, suggesting that HEV impairs several cellular processes, which can account for the various types of disease expression. Several proteins, such as heterogeneous nuclear ribonucleoprotein K, apolipoprotein E, and prohibitin, known to be involved in other viral life cycles, were upregulated in HEV-infected livers. Some differences were observed between the three strains, suggesting that HEV's genetic variability may induce variations in pathogenesis. This comparative analysis of the liver proteome modulated during infection with three different strains of HEV genotype 3 provides an important basis for further investigations on the factors involved in HEV replication and the mechanism of HEV pathogenesis.

IMPORTANCE

Hepatitis E virus (HEV) is responsible for acute hepatitis, with clinical symptoms ranging from asymptomatic to self-limiting acute hepatitis, chronic infection, or fulminant hepatitis. In industrialized countries, HEV is considered an emerging zoonotic disease, with swine being the principal reservoir for human contamination. The viral and cellular factors involved in the replication and/or pathogenesis of HEV are still not fully known. Here we report that several cellular pathways involved in cholesterol and lipid metabolism or cell survival were modulated during HEV infection in the swine model. Moreover, we observed a difference between the different swine strains, suggesting that HEV's genetic variability could play a role in pathogenesis. We also identified some proteins known to be involved in other viral cycles. Our study provides insight into the mechanisms modulated during HEV infection and constitutes a useful reference for future work on HEV pathogenesis and virulence.

Protein phosphatase 2A dysfunction in Alzheimer's disease

Protein phosphatase 2A (PP2A) is a large family of enzymes that account for the majority of brain Ser/Thr phosphatase activity. While PP2A enzymes collectively modulate most cellular processes, sophisticated regulatory mechanisms are ultimately responsible for ensuring isoform-specific substrate specificity. Of particular interest to the Alzheimer’s disease (AD) field, alterations in PP2A regulators and PP2A catalytic activity, subunit expression, methylation and/or phosphorylation, have been reported in AD-affected brain regions. “PP2A” dysfunction has been linked to tau hyperphosphorylation, amyloidogenesis and synaptic deficits that are pathological hallmarks of this neurodegenerative disorder. Deregulation of PP2A enzymes also affects the activity of many Ser/Thr protein kinases implicated in AD. This review will more specifically discuss the role of the PP2A/Bα holoenzyme and PP2A methylation in AD pathogenesis. The PP2A/Bα isoform binds to tau and is the primary tau phosphatase. Its deregulation correlates with increased tau phosphorylation in vivo and in AD. Disruption of PP2A/Bα-tau protein interactions likely contribute to tau deregulation in AD. Significantly, alterations in one-carbon metabolism that impair PP2A methylation are associated with increased risk for sporadic AD, and enhanced AD-like pathology in animal models. Experimental studies have linked deregulation of PP2A methylation with down-regulation of PP2A/Bα, enhanced phosphorylation of tau and amyloid precursor protein, tau mislocalization, microtubule destabilization and neuritic defects. While it remains unclear what are the primary events that underlie “PP2A” dysfunction in AD, deregulation of PP2A enzymes definitely affects key players in the pathogenic process. As such, there is growing interest in developing PP2A-centric therapies for AD, but this may be a daunting task without a better understanding of the regulation and function of specific PP2A enzymes.

Induction of cancer-specific cell death by the adenovirus E4orf4 protein

The adenovirus E4orf4 protein is a multifunctional viral regulator that contributes to temporal regulation of the progression of viral infection. When expressed alone, outside the context of the virus, E4orf4 induces p53-independent cell-death in transformed cells. Oncogenic transformation of primary cells in tissue culture sensitizes them to cell killing by E4orf4, indicating that E4orf4 research may have implications for cancer therapy. It has also been reported that E4orf4 induces a caspase-independent, non-classical apoptotic pathway, which maintains crosstalk with classical caspase-dependent pathways. Furthermore, several E4orf4 activities in the nucleus and in the cytoplasm and various protein partners contribute to cell killing by this viral protein. In the following chapter I summarize the current knowledge of the unique mode of E4orf4-induced cell death and its underlying mechanisms. Although several explanations for the cancer-specificity of E4orf4-induced toxicity have been proposed, a better grasp of the mechanisms responsible for E4orf4-induced cell death is required to elucidate the differential sensitivity of normal and cancer cells to E4orf4.

The glutathione peroxidase 1-protein tyrosine phosphatase 1B-protein phosphatase 2A axis. A key determinant of airway inflammation and alveolar destruction

Protein phosphatase-2A (PP2A) is a primary serine-threonine phosphatase that modulates inflammatory responses in asthma and chronic obstructive pulmonary disease (COPD). Despite its importance, the mechanisms that regulate lung PP2A activity remain to be determined. The redox-sensitive enzyme protein tyrosine phosphatase-1B (PTP1B) activates PP2A by dephosphorylating the catalytic subunit of the protein at tyrosine 307. This study aimed to identify how the interaction between the intracellular antioxidant glutathione peroxidase-1 (GPx-1) and PTP1B affected lung PP2A activity and airway inflammation. Experiments using gene silencing techniques in mouse lung or human small airway epithelial cells determined that knocking down PTP1B expression blocked GPx-1's activation of PP2A and negated the anti-inflammatory effects of GPx-1 protein in the lung. Similarly, the expression of human GPx-1 in transgenic mice significantly increased PP2A and PTP1B activities and prevented chronic cigarette smoke-induced airway inflammation and alveolar destruction. GPx-1 knockout mice, however, exhibited an exaggerated emphysema phenotype, correlating with a nonresponsive PP2A pathway. Importantly, GPx-1-PTP1B-PP2A signaling becomes inactivated in advanced lung disease. Indeed, PTP1B protein was oxidized in the lungs of subjects with advanced emphysema, and cigarette smoke did not increase GPx-1 or PTP1B activity within epithelial cells isolated from subjects with COPD, unlike samples of healthy lung epithelial cells. In conclusion, these findings establish that the GPx-1-PTP1B-PP2A axis plays a critical role in countering the inflammatory and proteolytic responses that result in lung-tissue destruction in response to cigarette smoke exposure.

Expression and mechanism of regulation of PP2A/Pr65 in ameloblastoma

OBJECTIVE

This study aimed to investigate the expression of PP2A/PR65 protein in ameloblastoma and the molecular mechanisms underlying the regulation of PP2A/PR65. The association between PP2A/PR65 and the clinicopathological characteristics of tumor specimens in ameloblastoma were to provide a theoretical basis for the diagnosis, therapy and prognosis of ameloblastoma.

STUDY DESIGN

Streptavidin-peroxidase (S-P) immunohistochemical staining was used to detect PP2A/Pr65 expression changes in a total of 68 cases of ameloblastoma, six ameloblastic carcinomas, 21 squamous cell carcinomas and seven normal oral mucosas. Western blot was used to analyze PP2A/PR65 protein expression in 15 cases of ameloblastoma and three cases of normal oral mucosa.

RESULTS

Of the 68 cases analyzed, four cases were negative, 25 cases were weakly positive, 20 cases were moderately positive and 19 cases were strongly positive. In six cases of ameloblastic carcinoma, three cases were weak positive, one case was positive, two cases were strongly positive and none were negative. In 21 cases of squamous cell carcinomas, three cases were negative, 17 cases were weakly positive, one case was moderately positive and none were strongly positive. Western blot analysis showed that, PP2A/Pr65 protein expression was lower in ameloblastoma tissue compared with normal oral mucosa.

CONCLUSIONS

Reduced expression of PP2A/PR65 in ameloblastoma compared with normal oral mucosa indicates that PP2A/PR65 is involved in the occurrence and development of ameloblastoma.

A role for protein phosphatase 2A in regulating p38 mitogen activated protein kinase activation and tumor necrosis factor-alpha expression during influenza virus infection

Influenza viruses of avian origin continue to pose pandemic threats to human health. Some of the H5N1 and H9N2 virus subtypes induce markedly elevated cytokine levels when compared with the seasonal H1N1 virus. We previously showed that H5N1/97 hyperinduces tumor necrosis factor (TNF)-alpha through p38 mitogen activated protein kinase (MAPK). However, the detailed mechanisms of p38MAPK activation and TNF-alpha hyperinduction following influenza virus infections are not known. Negative feedback regulations of cytokine expression play important roles in avoiding overwhelming production of proinflammatory cytokines. Here we hypothesize that protein phosphatases are involved in the regulation of cytokine expressions during influenza virus infection. We investigated the roles of protein phosphatases including MAPK phosphatase-1 (MKP-1) and protein phosphatase type 2A (PP2A) in modulating p38MAPK activation and downstream TNF-alpha expressions in primary human monocyte-derived macrophages (PBMac) infected with H9N2/G1 or H1N1 influenza virus. We demonstrate that H9N2/G1 virus activated p38MAPK and hyperinduced TNF-alpha production in PBMac when compared with H1N1 virus. H9N2/G1 induced PP2A activity in PBMac and, with the treatment of a PP2A inhibitor, p38MAPK phosphorylation and TNF-alpha production were further increased in the virus-infected macrophages. However, H9N2/G1 did not induce the expression of PP2A indicating that the activation of PP2A is not mediated by p38MAPK in virus-infected PBMac. On the other hand, PP2A may not be the targets of H9N2/G1 in the upstream of p38MAPK signaling pathways since H1N1 also induced PP2A activation in primary macrophages. Our results may provide new insights into the control of cytokine dysregulation.

Silencing of the human SET gene in vitro with lentivirus-mediated RNA interference

In our previous study, SET was identified as one of the differentially expressed proteins that was associated with tetra-arsenic tetra-sulfide (As4S4)-induced NB4-R1 [retinoic acid-resistant acute promyelocytic leukemia (APL) cell line] apoptosis. However, the mechanism through which SET regulates pathways during this process remains unclear. The aim of this study was to construct lentivirus-mediated short hairpin RNA (shRNA) against SET and investigate the effect of SET on As4S4-induced retinoic acid-resistant APL cell apoptosis. In the present study, 4 different oligonucleotides targeting the human SET gene were synthesized and cloned into the eukaryotic expression plasmid pGCSIL-GFP. The recombinant vectors were introduced into NB4-R1 cells. The silencing efficiency was measured by real-time quantitative PCR (RT-qPCR) and western blotting. Our results showed that the 4 recombinant RNA interference (RNAi) vectors were constructed successfully. Fluorescence microscopy demonstrated that infection efficiency ranged from 70 to 90%. Infection with the 4 different RNAi vectors significantly knocked down the expression of SET by 52.8, 69.1, 48.9 and 90.3% at the mRNA level, and 92.5, 96.3, 91.7 and 98.4% at the protein level, respectively. We attempt to clarify the mechanism of As4S4 treatment on retinoic acid-resistant APL.

Quantification of the host response proteome after mammalian reovirus T1L infection

All viruses are dependent upon host cells for replication. Infection can induce profound changes within cells, including apoptosis, morphological changes, and activation of signaling pathways. Many of these alterations have been analyzed by gene arrays to measure the cellular "transcriptome." We used SILAC (stable isotope labeling by amino acids in cell culture), combined with high-throughput 2-D HPLC/mass spectrometry, to determine relative quantitative differences in host proteins at 6 and 24 hours after infecting HEK293 cells with reovirus serotype 1 Lang (T1L). 3,076 host proteins were detected at 6 hpi, of which 132 and 68 proteins were significantly up or down regulated, respectively. 2,992 cellular proteins, of which 104 and 49 were up or down regulated, respectively, were identified at 24 hpi. IPA and DAVID analyses indicated proteins involved in cell death, cell growth factors, oxygen transport, cell structure organization and inflammatory defense response to virus were up-regulated, whereas proteins involved in apoptosis, isomerase activity, and metabolism were down-regulated. These proteins and pathways may be suitable targets for intervention to either attenuate virus infection or enhance oncolytic potential.

Nucleolar protein trafficking in response to HIV-1 Tat: rewiring the nucleolus

The trans-activator Tat protein is a viral regulatory protein essential for HIV-1 replication. Tat trafficks to the nucleoplasm and the nucleolus. The nucleolus, a highly dynamic and structured membrane-less sub-nuclear compartment, is the site of rRNA and ribosome biogenesis and is involved in numerous cellular functions including transcriptional regulation, cell cycle control and viral infection. Importantly, transient nucleolar trafficking of both Tat and HIV-1 viral transcripts are critical in HIV-1 replication, however, the role(s) of the nucleolus in HIV-1 replication remains unclear. To better understand how the interaction of Tat with the nucleolar machinery contributes to HIV-1 pathogenesis, we investigated the quantitative changes in the composition of the nucleolar proteome of Jurkat T-cells stably expressing HIV-1 Tat fused to a TAP tag. Using an organellar proteomic approach based on mass spectrometry, coupled with Stable Isotope Labelling in Cell culture (SILAC), we quantified 520 proteins, including 49 proteins showing significant changes in abundance in Jurkat T-cell nucleolus upon Tat expression. Numerous proteins exhibiting a fold change were well characterised Tat interactors and/or known to be critical for HIV-1 replication. This suggests that the spatial control and subcellular compartimentaliation of these cellular cofactors by Tat provide an additional layer of control for regulating cellular machinery involved in HIV-1 pathogenesis. Pathway analysis and network reconstruction revealed that Tat expression specifically resulted in the nucleolar enrichment of proteins collectively participating in ribosomal biogenesis, protein homeostasis, metabolic pathways including glycolytic, pentose phosphate, nucleotides and amino acids biosynthetic pathways, stress response, T-cell signaling pathways and genome integrity. We present here the first differential profiling of the nucleolar proteome of T-cells expressing HIV-1 Tat. We discuss how these proteins collectively participate in interconnected networks converging to adapt the nucleolus dynamic activities, which favor host biosynthetic activities and may contribute to create a cellular environment supporting robust HIV-1 production.