Search tips
Search criteria

Results 1-15 (15)

Clipboard (0)

Select a Filter Below

Year of Publication
Document Types
1.  Chronic exposure to the cytolethal distending toxins of Gram-negative bacteria promotes genomic instability and altered DNA damage response 
Cellular microbiology  2012;15(1):98-113.
Epidemiological evidence links chronic bacterial infections to the increased incidence of certain types of cancer but the molecular mechanisms by which bacteria contribute to tumour initiation and progression are still poorly characterized. Here we show that chronic exposure to the genotoxin cytolethal distending toxin (CDT) of Gram-negative bacteria promotes genomic instability and acquisition of phenotypic properties of malignancy in fibroblasts and colon epithelial cells. Cells grown for more than 30 weeks in the presence of sublethal doses of CDT showed increased mutation frequency, and accumulation of chromatin and chromosomal aberrations in the absence of significant alterations of cell cycle distribution, decreased viability or senescence. Cell survival was dependent on sustained activity of the p38 MAP kinase. The ongoing genomic instability was associated with impaired activation of the DNA damage response and failure to efficiently activate cell cycle checkpoints upon exposure to genotoxic stress. Independently selected sublines showed enhanced anchorage-independent growth as assessed by the formation of colonies in semisolid agarose. These findings support the notion that chronic infection by CDT-producing bacteria may promote malignant transformation, and point to the impairment of cellular control mechanisms associated with the detection and repair of DNA damage as critical events in the process.
PMCID: PMC4136655  PMID: 22998585
2.  The MAPK Signaling Cascade is a Central Hub in the Regulation of Cell Cycle, Apoptosis and Cytoskeleton Remodeling by Tripeptidyl-Peptidase II 
Tripeptidyl-peptidase II (TPPII) is a serine peptidase highly expressed in malignant Burkitt’s lymphoma cells (BL). We have previously shown that overexpression of TPPII correlates with chromosomal instability, centrosomal and mitotic spindle abnormalities and resistance to apoptosis induced by spindle poisons. Furthermore, TPPII knockdown by RNAi was associated with endoreplication and the accumulation of polynucleated cells that failed to complete cell division, indicating a role of TPPII in the cell cycle. Here we have applied a global approach of gene expression analysis to gain insights on the mechanism by which TPPII regulates this phenotype. mRNA profiling of control and TPPII knockdown BL cells identified one hundred and eighty five differentially expressed genes. Functional categorization of these genes highlighted major physiological functions such as apoptosis, cell cycle progression, cytoskeleton remodeling, proteolysis, and signal transduction. Pathways and protein interactome analysis revealed a significant enrichment in components of MAP kinases signaling. These findings suggest that TPPII influences a wide network of signaling pathways that are regulated by MAPKs and exerts thereby a pleiotropic effect on biological processes associated with cell survival, proliferation and genomic instability.
PMCID: PMC2733081  PMID: 19787088
TPPII; MAPK signaling; centrosome; cell cycle; cytoskeleton
3.  Caspase-1 Promotes Epstein-Barr Virus Replication by Targeting the Large Tegument Protein Deneddylase to the Nucleus of Productively Infected Cells 
PLoS Pathogens  2013;9(10):e1003664.
The large tegument proteins of herpesviruses contain N-terminal cysteine proteases with potent ubiquitin and NEDD8-specific deconjugase activities, but the function of the enzymes during virus replication remains largely unknown. Using as model BPLF1, the homologue encoded by Epstein-Barr virus (EBV), we found that induction of the productive virus cycle does not affect the total level of ubiquitin-conjugation but is accompanied by a BPLF1-dependent decrease of NEDD8-adducts and accumulation of free NEDD8. Expression of BPLF1 promotes cullin degradation and the stabilization of cullin-RING ligases (CRLs) substrates in the nucleus, while cytoplasmic CRLs and their substrates are not affected. The inactivation of nuclear CRLs is reversed by the N-terminus of CAND1, which inhibits the binding of BPLF1 to cullins and prevents efficient viral DNA replication. Targeting of the deneddylase activity to the nucleus is dependent on processing of the catalytic N-terminus by caspase-1. Inhibition of caspase-1 severely impairs viral DNA synthesis and the release of infectious virus, pointing a previously unrecognized role of the cellular response to danger signals triggered by EBV reactivation in promoting virus replication.
Author Summary
Viruses rely on the host cell for replication and have evolved sophisticated strategies to manipulate and harness the cellular metabolic pathways and defense responses. A better knowledge of these viral strategies will provide new targets for antiviral therapies. The N-terminus of the large tegument proteins of herpesviruses encodes an ubiquitin and NEDD8-specific deconjugase, but the function of the enzyme during virus replication is largely unknown. Here we report that, endogenously expressed BPLF1, the homolog encoded by Epstein-Barr virus (EBV), promotes a dramatic decrease of NEDD8-conjugates and the accumulation of free NEDD8 in cells entering the productive virus cycle. BPLF1 exerts its deneddylase activity in the nucleus, which promotes the accumulation of cullin-RING ligase (CRL) substrates that are required for efficient virus replication. Targeting of the viral enzyme to the nucleus is dependent on processing of the catalytic N-terminus by caspase-1. Inhibition of caspase-1 severely impairs viral DNA synthesis and the release of infectious virus, pointing to an unexpected role of the cellular response to danger signals triggered by EBV reactivation in promoting virus replication.
PMCID: PMC3795028  PMID: 24130483
4.  Epitope-dependent Selection of Highly Restricted or Diverse T Cell Receptor Repertoires in Response to Persistent Infection by Epstein-Barr Virus 
The T cell receptor (TCR) repertoires of cytotoxic responses to the immunodominant and subdominant HLA A11–restricted epitopes in the Epstein-Barr virus (EBV) nuclear antigen-4 were investigated in four healthy virus carriers. The response to the subdominant epitope (EBNA4 399-408, designated AVF) was highly restricted with conserved Vβ usage and identical length and amino acid motifs in the third complementarity-determining regions (CDR3), while a broad repertoire using different combinations of TCR-α/β V and J segments and CDR3 regions was selected by the immunodominant epitope (EBNA4 416-424, designated IVT). Distinct patterns of interaction with the A11–peptide complex were revealed for each AVF- or IVT-specific TCR clonotype by alanine scanning mutagenesis analysis. Blocking of cytotoxic function by antibodies specific for the CD8 coreceptor indicated that, while AVF-specific TCRs are of high affinity, the oligoclonal response to the IVT epitope includes both low- and high-affinity TCRs. Thus, comparison of the memory response to two epitopes derived from the same viral antigen and presented through the same MHC class I allele suggests that immunodominance may correlate with the capacity to maintain a broad TCR repertoire.
PMCID: PMC2198955  PMID: 9207000
5.  Interaction of Gamma-Herpesvirus Genome Maintenance Proteins with Cellular Chromatin 
PLoS ONE  2013;8(5):e62783.
The capacity of gamma-herpesviruses to establish lifelong infections is dependent on the expression of genome maintenance proteins (GMPs) that tether the viral episomes to cellular chromatin and allow their persistence in latently infected proliferating cells. Here we have characterized the chromatin interaction of GMPs encoded by viruses belonging to the genera Lymphocryptovirus (LCV) and Rhadinovirus (RHV). We found that, in addition to a similar diffuse nuclear localization and comparable detergent resistant interaction with chromatin in transfected cells, all GMPs shared the capacity to promote the decondensation of heterochromatin in the A03-1 reporter cell line. They differed, however, in their mobility measured by fluorescence recovery after photobleaching (FRAP), and in the capacity to recruit accessory molecules required for the chromatin remodeling function. While the AT-hook containing GMPs of LCVs were highly mobile, a great variability was observed among GMPs encoded by RHV, ranging from virtually immobile to significantly reduced mobility compared to LCV GMPs. Only the RHV GMPs recruited the bromo- and extra terminal domain (BET) proteins BRD2 and BRD4 to the site of chromatin remodeling. These findings suggest that differences in the mode of interaction with cellular chromatin may underlie different strategies adopted by these viruses for reprogramming of the host cells during latency.
PMCID: PMC3646995  PMID: 23667520
6.  The Epstein–Barr virus nuclear antigen-1 reprograms transcription by mimicry of high mobility group A proteins 
Nucleic Acids Research  2013;41(5):2950-2962.
Viral proteins reprogram their host cells by hijacking regulatory components of protein networks. Here we describe a novel property of the Epstein–Barr virus (EBV) nuclear antigen-1 (EBNA1) that may underlie the capacity of the virus to promote a global remodeling of chromatin architecture and cellular transcription. We found that the expression of EBNA1 in transfected human and mouse cells is associated with decreased prevalence of heterochromatin foci, enhanced accessibility of cellular DNA to micrococcal nuclease digestion and decreased average length of nucleosome repeats, suggesting de-protection of the nucleosome linker regions. This is a direct effect of EBNA1 because targeting the viral protein to heterochromatin promotes large-scale chromatin decondensation with slow kinetics and independent of the recruitment of adenosine triphosphate–dependent chromatin remodelers. The remodeling function is mediated by a bipartite Gly-Arg rich domain of EBNA1 that resembles the AT-hook of High Mobility Group A (HMGA) architectural transcription factors. Similar to HMGAs, EBNA1 is highly mobile in interphase nuclei and promotes the mobility of linker histone H1, which counteracts chromatin condensation and alters the transcription of numerous cellular genes. Thus, by regulating chromatin compaction, EBNA1 may reset cellular transcription during infection and prime the infected cells for malignant transformation.
PMCID: PMC3597695  PMID: 23358825
7.  Thioredoxin 80-Activated-Monocytes (TAMs) Inhibit the Replication of Intracellular Pathogens 
PLoS ONE  2011;6(2):e16960.
Thioredoxin 80 (Trx80) is an 80 amino acid natural cleavage product of Trx, produced primarily by monocytes. Trx80 induces differentiation of human monocytes into a novel cell type, named Trx80-activated-monocytes (TAMs).
Principal Findings
In this investigation we present evidence for a role of TAMs in the control of intracellular bacterial infections. As model pathogens we have chosen Listeria monocytogenes and Brucella abortus which replicate in the cytosol and the endoplasmic reticulum respectively. Our data indicate that TAMs efficiently inhibit intracellular growth of both L. monocytogenes and B. abortus. Further analysis shows that Trx80 activation prevents the escape of GFP-tagged L. monocytogenes into the cytosol, and induces accumulation of the bacteria within the lysosomes. Inhibition of the lysosomal activity by chloroquine treatment resulted in higher replication of bacteria in TAMs compared to that observed in control cells 24 h post-infection, indicating that TAMs kill bacteria by preventing their escape from the endosomal compartments, which progress into a highly degradative phagolysosome.
Our results show that Trx80 potentiates the bactericidal activities of professional phagocytes, and contributes to the first line of defense against intracellular bacteria.
PMCID: PMC3041819  PMID: 21365006
8.  The Translation Initiation Factor 3f (eIF3f) Exhibits a Deubiquitinase Activity Regulating Notch Activation 
PLoS Biology  2010;8(11):e1000545.
The translation initiation factor complex eIF3f has an intrinsic deubiquitinase activity and regulates the Notch signaling pathway.
Activation of the mammalian Notch receptor after ligand binding relies on a succession of events including metalloprotease-cleavage, endocytosis, monoubiquitination, and eventually processing by the gamma-secretase, giving rise to a soluble, transcriptionally active molecule. The Notch1 receptor was proposed to be monoubiquitinated before its gamma-secretase cleavage; the targeted lysine has been localized to its submembrane domain. Investigating how this step might be regulated by a deubiquitinase (DUB) activity will provide new insight for understanding Notch receptor activation and downstream signaling. An immunofluorescence-based screening of an shRNA library allowed us to identify eIF3f, previously known as one of the subunits of the translation initiation factor eIF3, as a DUB targeting the activated Notch receptor. We show that eIF3f has an intrinsic DUB activity. Knocking down eIF3f leads to an accumulation of monoubiquitinated forms of activated Notch, an effect counteracted by murine WT eIF3f but not by a catalytically inactive mutant. We also show that eIF3f is recruited to activated Notch on endocytic vesicles by the putative E3 ubiquitin ligase Deltex1, which serves as a bridging factor. Finally, catalytically inactive forms of eIF3f as well as shRNAs targeting eIF3f repress Notch activation in a coculture assay, showing that eIF3f is a new positive regulator of the Notch pathway. Our results support two new and provocative conclusions: (1) The activated form of Notch needs to be deubiquitinated before being processed by the gamma-secretase activity and entering the nucleus, where it fulfills its transcriptional function. (2) The enzyme accounting for this deubiquitinase activity is eIF3f, known so far as a translation initiation factor. These data improve our knowledge of Notch signaling but also open new avenues of research on the Zomes family and the translation initiation factors.
Author Summary
The highly conserved signaling pathway involving the transmembrane receptor Notch is essential for development, and misregulation of this pathway is linked to many diseases. We previously proposed that the Notch1 receptor is monoubiquitinated during its activation. With the aim of identifying a deubiquinating enzyme that could regulate Notch activation, we demonstrated that eIF3f, known previously as part of the multiprotein translation initiation factor eIF3 complex, harbors an enzymatic activity that acts on Notch. The activated form of Notch is able to interact with eIF3f only in the presence of the E3 ubiquitin ligase Deltex, and Notch needs to be deubiquitinated before it can be cleared and its intracellular domain can enter the nucleus and fulfill its transcriptional function. Our results further decipher the molecular mechanisms of Notch signaling activation, showing that ubiquitination and deubiquitination events are required. Additionally, we show that beyond acting as a translation initiation factor, eIF3f fulfills other functions and has an intrinsic enzymatic activity.
PMCID: PMC2990700  PMID: 21124883
9.  Transcription Profiling of Epstein-Barr Virus Nuclear Antigen (EBNA)-1 Expressing Cells Suggests Targeting of Chromatin Remodeling Complexes 
PLoS ONE  2010;5(8):e12052.
The Epstein-Barr virus (EBV) encoded nuclear antigen (EBNA)-1 regulates virus replication and transcription, and participates in the remodeling of the cellular environment that accompanies EBV induced B-cell immortalization and malignant transformation. The putative cellular targets of these effects of EBNA-1 are largely unknown. To address this issue we have profiled the transcriptional changes induced by short- and long-term expression of EBNA-1 in the EBV negative B-cell lymphoma BJAB. Three hundred and nineteen cellular genes were regulated in a conditional transfectant shortly after EBNA-1 induction while a ten fold higher number of genes was regulated upon continuous EBNA-1 expression. Promoter analysis of the differentially regulated genes demonstrated a significant enrichment of putative EBNA-1 binding sites suggesting that EBNA-1 may directly influence the transcription of a subset of genes. Gene ontology analysis of forty seven genes that were consistently regulated independently on the time of EBNA-1 expression revealed an unexpected enrichment of genes involved in the maintenance of chromatin architecture. The interaction network of the affected gene products suggests that EBNA-1 may promote a broad rearrangement of the cellular transcription landscape by altering the expression of key components of chromatin remodeling complexes.
PMCID: PMC2919392  PMID: 20706582
10.  Epstein-Barr Virus Encodes Three Bona Fide Ubiquitin-Specific Proteases▿ ‡  
Journal of Virology  2008;82(21):10477-10486.
Manipulation of the ubiquitin proteasome system (UPS) is emerging as a common theme in viral pathogenesis. Some viruses have been shown to encode functional homologs of UPS enzymes, suggesting that a systematic identification of these products may provide new insights into virus-host cell interactions. Ubiquitin-specific proteases, collectively known as deubiquitinating enzymes (DUBs), regulate the activity of the UPS by hydrolyzing ubiquitin peptide or isopeptide bonds. The prediction of viral DUBs based on sequence similarity with known enzymes is hampered by the diversity of viral genomes. In this study sequence alignments, pattern searches, and hidden Markov models were developed for the conserved C- and H-boxes of the known DUB families and used to search the open reading frames (ORFs) of Epstein-Barr virus (EBV), a large gammaherpesvirus that has been implicated in the pathogenesis of a broad spectrum of human malignancies of lymphoid and epithelial cell origin. The searches identified a limited number of EBV ORFs that contain putative DUB catalytic domains. DUB activity was confirmed by functional assays and mutation analysis for three high scoring candidates, supporting the usefulness of this bioinformatics approach in predicting distant homologues of cellular enzymes.
PMCID: PMC2573217  PMID: 18715931
11.  A Bacterial Cytotoxin Identifies the RhoA Exchange Factor Net1 as a Key Effector in the Response to DNA Damage 
PLoS ONE  2008;3(5):e2254.
Exposure of adherent cells to DNA damaging agents, such as the bacterial cytolethal distending toxin (CDT) or ionizing radiations (IR), activates the small GTPase RhoA, which promotes the formation of actin stress fibers and delays cell death. The signalling intermediates that regulate RhoA activation and promote cell survival are unknown.
Principal Findings
We demonstrate that the nuclear RhoA-specific Guanine nucleotide Exchange Factor (GEF) Net1 becomes dephosphorylated at a critical inhibitory site in cells exposed to CDT or IR. Expression of a dominant negative Net1 or Net1 knock down by iRNA prevented RhoA activation, inhibited the formation of stress fibers, and enhanced cell death, indicating that Net1 activation is required for this RhoA-mediated responses to genotoxic stress. The Net1 and RhoA-dependent signals involved activation of the Mitogen-Activated Protein Kinase p38 and its downstream target MAPK-activated protein kinase 2.
Our data highlight the importance of Net1 in controlling RhoA and p38 MAPK mediated cell survival in cells exposed to DNA damaging agents and illustrate a molecular pathway whereby chronic exposure to a bacterial toxin may promote genomic instability.
PMCID: PMC2386254  PMID: 18509476
12.  Functional Inactivation of EBV-Specific T-Lymphocytes in Nasopharyngeal Carcinoma: Implications for Tumor Immunotherapy 
PLoS ONE  2007;2(11):e1122.
Nasopharyngeal carcinoma (NPC) is an Epstein-Barr virus (EBV) associated malignancy with high prevalence in Southern Chinese. In order to assess whether defects of EBV-specific immunity may contribute to the tumor, the phenotype and function of circulating T-cells and tumor infiltrating lymphocytes (TILs) were investigated in untreated NPC patients. Circulating naïve CD3+CD45RA+ and CD4+CD25− cells were decreased, while activated CD4+CD25+ T-cells and CD3−CD16+ NK-cells were increased in patients compared to healthy donors. The frequency of T-cells recognizing seven HLA-A2 restricted epitopes in LMP1 and LMP2 was lower in the patients and remained low after stimulation with autologous EBV-carrying cells. TILs expanded in low doses of IL-2 exhibited an increase of CD3+CD4+, CD3+CD45RO+ and CD4+CD25+ cells and 2 to 5 fold higher frequency of LMP1 and LMP2 tetramer positive cells compared to peripheral blood. EBV-specific cytotoxicity could be reactivated from the blood of most patients, whereas the TILs lacked cytotoxic activity and failed to produce IFNγ upon specific stimulation. Thus, EBV-specific rejection responses appear to be functionally inactivated at the tumor site in NPC.
PMCID: PMC2048575  PMID: 17987110
13.  Hepatitis C Virus Core Protein Induces an Anergic State Characterized by Decreased Interleukin-2 Production and Perturbation of Mitogen-Activated Protein Kinase Responses 
Journal of Virology  2005;79(4):2230-2239.
Alterations of cytokine responses are thought to favor the establishment of persistent hepatitis C virus (HCV) infections, enhancing the risk of liver cirrhosis and hepatocellular carcinoma. Here we demonstrate that the expression of the HCV core (C) protein in stably transfected T cells correlates with a selective reduction of interleukin-2 (IL-2) promoter activity and IL-2 production in response to T-cell receptor triggering, whereas the activation of IL-4, IL-10, gamma interferon, and tumor necrosis factor alpha was moderately increased. This altered cytokine expression profile was associated with a perturbation of mitogen-activated protein (MAP) kinase responses. Extracellular regulated kinase and p38 were constitutively phosphorylated in C-expressing cells, while triggering of the costimulatory c-Jun N-terminal kinase (JNK) signaling cascade and activation of the CD28 response element within the IL-2 promoter appeared to be impaired. The perturbations of MAP kinase phosphorylation could be eliminated by cyclosporine A-mediated inhibition of nuclear factor of activated T cells, suggesting that the inactivation of JNK signaling and hyporesponsiveness to IL-2 induction were downstream consequences of C-induced Ca2+ flux in a manner that mimics the induction of clonal anergy.
PMCID: PMC546561  PMID: 15681425
14.  Mutant ubiquitin found in neurodegenerative disorders is a ubiquitin fusion degradation substrate that blocks proteasomal degradation 
The Journal of Cell Biology  2002;157(3):417-427.
Loss of neurons in neurodegenerative diseases is usually preceded by the accumulation of protein deposits that contain components of the ubiquitin/proteasome system. Affected neurons in Alzheimer's disease often accumulate UBB+1, a mutant ubiquitin carrying a 19–amino acid C-terminal extension generated by a transcriptional dinucleotide deletion. Here we show that UBB+1 is a potent inhibitor of ubiquitin-dependent proteolysis in neuronal cells, and that this inhibitory activity correlates with induction of cell cycle arrest. Surprisingly, UBB+1 is recognized as a ubiquitin fusion degradation (UFD) proteasome substrate and ubiquitinated at Lys29 and Lys48. Full blockade of proteolysis requires both ubiquitination sites. Moreover, the inhibitory effect was enhanced by the introduction of multiple UFD signals. Our findings suggest that the inhibitory activity of UBB+1 may be an important determinant of neurotoxicity and contribute to an environment that favors the accumulation of misfolded proteins.
PMCID: PMC2173284  PMID: 11980917
proteasome; neurodegeneration; aggregate; tauopathies; polyglutamine disorders
15.  Cell-Based Fluorescence Assay for Human Immunodeficiency Virus Type 1 Protease Activity 
The human immunodeficiency virus type 1 (HIV-1) protease is essential for production of infectious virus and is therefore a major target for the development of drugs against AIDS. Cellular proteins are also cleaved by the protease, which explains its cytotoxic activity and the consequent failure to establish convenient cell-based protease assays. We have exploited this toxicity to develop a new protease assay that relies on transient expression of an artificial protease precursor harboring the green fluorescent protein (GFP-PR). The precursor is activated in vivo by autocatalytic cleavage, resulting in rapid elimination of protease-expressing cells. Treatment with therapeutic doses of HIV-1 protease inhibitors results in a dose-dependent accumulation of the fluorescent precursor that can be easily detected and quantified by flow cytometric and fluorimetric assays. The precursor provides a convenient and noninfectious model for high-throughput screenings of substances that can interfere with the activity of the protease in living cells.
PMCID: PMC90701  PMID: 11502538

Results 1-15 (15)