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SETD6 Regulates E2-Dependent Human Papillomavirus Transcription. J Virol 2022; 96:e0129522. [PMID: 36300937 PMCID: PMC9682981 DOI: 10.1128/jvi.01295-22] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Human papillomaviruses (HPV) cause cervical, anogenital, and oral cancers. Brd4 plays an important role in the HPV life cycle. SETD6 was recently shown to methylate Brd4. The current study demonstrates that methylation of Brd4 by SETD6 in HPV-episomal cells is required for the activation of viral transcription. This study illustrates a novel regulatory mechanism involving E2, Brd4, and SETD6 in the HPV life cycle and provides insight into the multiple roles of Brd4 in viral pathogenesis.
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2
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Chen IP, Ott M. Viral Hijacking of BET Proteins. Viruses 2022; 14:v14102274. [PMID: 36298829 PMCID: PMC9609653 DOI: 10.3390/v14102274] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2022] [Revised: 10/09/2022] [Accepted: 10/11/2022] [Indexed: 11/29/2022] Open
Abstract
Proteins of the bromodomain and exterminal domain (BET) family mediate critical host functions such as cell proliferation, transcriptional regulation, and the innate immune response, which makes them preferred targets for viruses. These multidomain proteins are best known as transcriptional effectors able to read acetylated histone and non-histone proteins through their tandem bromodomains. They also contain other short motif-binding domains such as the extraterminal domain, which recognizes transcriptional regulatory proteins. Here, we describe how different viruses have evolved to hijack or disrupt host BET protein function through direct interactions with BET family members to support their own propagation. The network of virus-BET interactions emerges as highly intricate, which may complicate the use of small-molecule BET inhibitors-currently in clinical development for the treatment of cancer and cardiovascular diseases-to treat viral infections.
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Affiliation(s)
- Irene P. Chen
- Gladstone Institutes, San Francisco, CA 94158, USA
- Department of Medicine, University of California, San Francisco, San Francisco, CA 94143, USA
| | - Melanie Ott
- Gladstone Institutes, San Francisco, CA 94158, USA
- Department of Medicine, University of California, San Francisco, San Francisco, CA 94143, USA
- Chan Zuckerberg Biohub, San Francisco, CA 94158, USA
- Correspondence:
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3
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Lara-Ureña N, García-Domínguez M. Relevance of BET Family Proteins in SARS-CoV-2 Infection. Biomolecules 2021; 11:1126. [PMID: 34439792 PMCID: PMC8391731 DOI: 10.3390/biom11081126] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/27/2021] [Revised: 07/22/2021] [Accepted: 07/27/2021] [Indexed: 12/14/2022] Open
Abstract
The recent pandemic we are experiencing caused by the coronavirus disease 2019 (COVID-19) has put the world's population on the rack, with more than 191 million cases and more than 4.1 million deaths confirmed to date. This disease is caused by a new type of coronavirus, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). A massive proteomic analysis has revealed that one of the structural proteins of the virus, the E protein, interacts with BRD2 and BRD4 proteins of the Bromodomain and Extra Terminal domain (BET) family of proteins. BETs are essential to cell cycle progression, inflammation and immune response and have also been strongly associated with infection by different types of viruses. The fundamental role BET proteins play in transcription makes them appropriate targets for the propagation strategies of some viruses. Recognition of histone acetylation by BET bromodomains is essential for transcription control. The development of drugs mimicking acetyl groups, and thereby able to displace BET proteins from chromatin, has boosted interest on BETs as attractive targets for therapeutic intervention. The success of these drugs against a variety of diseases in cellular and animal models has been recently enlarged with promising results from SARS-CoV-2 infection studies.
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Affiliation(s)
| | - Mario García-Domínguez
- Andalusian Centre for Molecular Biology and Regenerative Medicine (CABIMER), CSIC-Universidad de Sevilla-Universidad Pablo de Olavide, Av. Américo Vespucio 24, 41092 Seville, Spain;
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4
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McBride AA, Warburton A, Khurana S. Multiple Roles of Brd4 in the Infectious Cycle of Human Papillomaviruses. Front Mol Biosci 2021; 8:725794. [PMID: 34386523 PMCID: PMC8353396 DOI: 10.3389/fmolb.2021.725794] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2021] [Accepted: 07/14/2021] [Indexed: 12/17/2022] Open
Abstract
Human Papillomaviruses (HPV) reproduce in stratified epithelia by establishing a reservoir of low- level infection in the dividing basal cells and restricting the production of viral particles to terminally differentiated cells. These small DNA viruses hijack pivotal cellular processes and pathways to support the persistent infectious cycle. One cellular factor that is key to multiple stages of viral replication and transcription is the BET (bromodomain and extra-terminal domain) protein, Brd4 (Bromodomain containing protein 4). Here we provide an overview of the multiple interactions of Brd4 that occur throughout the HPV infectious cycle.
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Affiliation(s)
- Alison A. McBride
- Laboratory of Viral Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD, United States
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5
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BRD4S interacts with viral E2 protein to limit human papillomavirus late transcription. J Virol 2021; 95:JVI.02032-20. [PMID: 33731454 PMCID: PMC8139696 DOI: 10.1128/jvi.02032-20] [Citation(s) in RCA: 10] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022] Open
Abstract
The E2 protein encoded by human papillomaviruses (HPV) is a sequence-specific DNA-binding protein that recruits viral and cellular proteins. Bromodomain-containing protein 4 (BRD4) is a highly conserved interactor for E2 proteins that has been linked to E2's functions as transcription modulator, activator of viral replication and segregation factor for viral genomes. In addition to BRD4, a short form of BRD4 (BRD4S) is expressed from the BRD4 gene which lacks the C-terminal domain of BRD4. E2 proteins interact with the C-terminal motif (CTM) of BRD4, but a recent study suggested that the phospho-dependent interaction domain (PDID) and the basic interaction domain (BID) in BRD4 also bind to E2. These domains are also present in BRD4S. We now find that HPV31 E2 interacts with the isolated PDID domain in living cells and also with BRD4S which is present in detectable amounts in HPV-positive cell lines and is recruited into HPV31 E1 and E2 induced replication foci. Overexpression and knockdown experiments surprisingly indicate that BRD4S inhibits activities of E2. In line with that, the specific knockdown of BRD4S in the HPV31-positive CIN612-9E cell line induces mainly late viral transcripts. This occurs only in undifferentiated but not differentiated cells in which the productive viral replication cycle is induced. These data suggest that the BRD4S-E2 interaction is important to prevent HPV late gene expression in undifferentiated keratinocytes which may contribute to immune evasion and HPV persistence.ImportanceHuman papillomaviruses (HPV) have coevolved with their host by using cellular factors like bromodomain-containing protein 4 (BRD4) to control viral processes such as genome maintenance, gene expression and replication. We here show that, in addition to the C-terminal motif in BRD4, the phospho-dependent interaction domain in BRD4 interacts with E2 proteins which enable the recruitment of BRD4S, the short isoform of BRD4, to E2. Knock-down and overexpression of BRD4S reveals that BRD4S is a negative regulator of E2 activities. Importantly, the knockdown of BRD4S induces mainly L1 transcripts in undifferentiated CIN612-9E cells, which maintain replicating HPV31 genomes. Our study reveals an inhibitory role of BRD4S on HPV transcription, which may serve as an immune escape mechanism by the suppression of L1 transcripts and thus contribute to the establishment of persistent HPV infections.
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6
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Della Fera AN, Warburton A, Coursey TL, Khurana S, McBride AA. Persistent Human Papillomavirus Infection. Viruses 2021; 13:v13020321. [PMID: 33672465 PMCID: PMC7923415 DOI: 10.3390/v13020321] [Citation(s) in RCA: 74] [Impact Index Per Article: 24.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/16/2021] [Revised: 02/16/2021] [Accepted: 02/17/2021] [Indexed: 12/16/2022] Open
Abstract
Simple Summary The success of HPV as an infectious agent lies not within its ability to cause disease, but rather in the adeptness of the virus to establish long-term persistent infection. The ability of HPV to replicate and maintain its genome in a stratified epithelium is contingent on the manipulation of many host pathways. HPVs must abrogate host anti-viral defense programs, perturb the balance of cellular proliferation and differentiation, and hijack DNA damage signaling and repair pathways to replicate viral DNA in a stratified epithelium. Together, these characteristics contribute to the ability of HPV to achieve long-term and persistent infection and to its evolutionary success as an infectious agent. Abstract Persistent infection with oncogenic human papillomavirus (HPV) types is responsible for ~5% of human cancers. The HPV infectious cycle can sustain long-term infection in stratified epithelia because viral DNA is maintained as low copy number extrachromosomal plasmids in the dividing basal cells of a lesion, while progeny viral genomes are amplified to large numbers in differentiated superficial cells. The viral E1 and E2 proteins initiate viral DNA replication and maintain and partition viral genomes, in concert with the cellular replication machinery. Additionally, the E5, E6, and E7 proteins are required to evade host immune responses and to produce a cellular environment that supports viral DNA replication. An unfortunate consequence of the manipulation of cellular proliferation and differentiation is that cells become at high risk for carcinogenesis.
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Phosphorylation of the Human Papillomavirus E2 Protein at Tyrosine 138 Regulates Episomal Replication. J Virol 2020; 94:JVI.00488-20. [PMID: 32350070 DOI: 10.1128/jvi.00488-20] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/18/2020] [Accepted: 04/19/2020] [Indexed: 01/15/2023] Open
Abstract
The papillomavirus (PV) E2 protein is a critical regulator of viral transcription and genome replication. We previously reported that tyrosine (Y) 138 of HPV-31 E2 is phosphorylated by the fibroblast growth factor receptor 3 (FGFR3) kinase. In this study, we generated quasiviruses containing G418-selectable HPV-31 genomes with phosphodeficient phenylalanine mutant E2 Y138F and phosphomimetic glutamic acid mutant Y138E. We observed significantly fewer early viral transcripts immediately after infection with these Y138 mutant genomes even though E2 occupancy at the viral origin was equivalent to that of wild-type E2. Keratinocytes infected with Y138F quasiviruses formed stable colonies, and the genomes were maintained as episomes, while those infected with Y138E quasiviruses did not. We previously reported that the HPV-31 E2 Y138 mutation to glutamic acid did not bind to the Brd4 C-terminal motif (CTM). Here, we demonstrate that HPV-16 E2 Y138E bound to full-length Brd4 but not to the Brd4 CTM. We conclude that association of E2 with the Brd4 CTM is necessary for viral genome replication and suggest that this interaction can be regulated by phosphorylation of E2 Y138.IMPORTANCE Papillomavirus (PV) is a double-stranded DNA tumor virus infecting the cutaneous and mucosal epithelium. The PV E2 protein associates with a number of cellular factors to mediate replication of the HPV genome. Fibroblast growth factor receptor 3 (FGFR3) regulates HPV replication through phosphorylation of tyrosine 138 in the HPV E2 protein. Employing a quasivirus infection model and selection for G418 resistant genomes, we demonstrated that Y138 is a critical residue for Brd4 association and that inability to complex with Brd4 does not support episomal replication.
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8
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Cottontail Rabbit Papillomavirus E1 and E2 Proteins Mutually Influence Their Subcellular Localizations. J Virol 2018; 92:JVI.00704-18. [PMID: 30135125 DOI: 10.1128/jvi.00704-18] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/23/2018] [Accepted: 08/15/2018] [Indexed: 01/08/2023] Open
Abstract
The papillomavirus (PV) E2 protein is a nuclear, sequence-specific DNA-binding protein that regulates transcription and nuclear retention of viral genomes. E2 also interacts with the viral E1 protein to replicate the viral genome. E2 residue K111 is highly conserved among PV and has been implicated in contributing to nuclear transport, transcription, and replication. Cottontail rabbit (Sylvilagus floridanus) PV (CRPV or SfPV1) E2 K111R, A, or Q mutations are transcription deficient and localized to the cytoplasm, comparable to other PV types. The addition of a nuclear localization signal (NLS) resulted in nuclear E2 K111 mutant proteins but did not restore transcriptional activation, and this is most likely due to an impaired binding to the cellular Brd4 protein. Surprisingly, coexpression of E1 with E2 K111 mutations resulted in their nuclear localization and, for K111A and R mutations, the activation of an E1/E2-dependent reporter construct. Interestingly, the nuclear localization of E2 K111Q mutant protein was independent from the presence of the conserved bipartite NLS in E1 and the direct interaction between E1 and E2. On the other hand, the cytoplasmic E1 NLS mutation could be targeted to the nucleus by wild-type E2, and this was dependent upon an interaction between E1 and E2. In summary, our studies have uncovered that E1 and E2 control each other's subcellular localization: direct binding of E2 to E1 can direct E1 to the nucleus independently from the E1 NLS, and E1 can direct E2 to the nucleus without an intact NLS or direct binding to E2.IMPORTANCE Papillomaviruses encode the DNA-binding E1 and E2 proteins, which form a complex and are essential for genome replication. Both proteins are targeted to the nucleus via nuclear localization signals. Our studies have uncovered that cytoplasmic mutant E1 or E2 proteins can be localized to the nucleus when E1 or E2 is also present. An interaction between E1 and E2 is necessary to target cytoplasmic E1 mutant proteins to the nucleus, but cytoplasmic E2 mutant proteins can be targeted to the nucleus without a direct interaction, which points to a novel function of E1.
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9
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Morse MA, Balogh KK, Brendle SA, Campbell CA, Chen MX, Furze RC, Harada IL, Holyer ID, Kumar U, Lee K, Prinjha RK, Rüdiger M, Seal JT, Taylor S, Witherington J, Christensen ND. BET bromodomain inhibitors show anti-papillomavirus activity in vitro and block CRPV wart growth in vivo. Antiviral Res 2018; 154:158-165. [PMID: 29653131 DOI: 10.1016/j.antiviral.2018.03.012] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/25/2017] [Revised: 03/22/2018] [Accepted: 03/27/2018] [Indexed: 12/19/2022]
Abstract
The DNA papillomaviruses infect squamous epithelium and can cause persistent, benign and sometimes malignant hyperproliferative lesions. Effective antiviral drugs to treat human papillomavirus (HPV) infection are lacking and here we investigate the anti-papillomavirus activity of novel epigenetic targeting drugs, BET bromodomain inhibitors. Bromodomain and Extra-Terminal domain (BET) proteins are host proteins which regulate gene transcription, they bind acetylated lysine residues in histones and non-histone proteins via bromodomains, functioning as scaffold proteins in the formation of transcriptional complexes at gene regulatory regions. The BET protein BRD4 has been shown to be involved in the papillomavirus life cycle, as a co-factor for viral E2 and also mediating viral partitioning in some virus types. We set out to study the activity of small molecule BET bromodomain inhibitors in models of papillomavirus infection. Several BET inhibitors reduced HPV11 E1ˆE4 mRNA expression in vitro and topical therapeutic administration of an exemplar compound I-BET762, abrogated CRPV cutaneous wart growth in rabbits, demonstrating translation of anti-viral effects to efficacy in vivo. Additionally I-BET762 markedly reduced viability of HPV16 infected W12 cells compared to non-infected C33A cells. The molecular mechanism for the cytotoxicity to W12 cells is unknown but may be through blocking viral-dependent cell-survival factors. We conclude that these effects, across multiple papillomavirus types and in vivo, highlight the potential to target BET bromodomains to treat HPV infection.
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Affiliation(s)
- Mary A Morse
- GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2NY, UK.
| | - Karla K Balogh
- The Jake Gittlen Cancer Research Foundation, H069, Department of Pathology, C7800, The Pennsylvania State University, College of Medicine, 500 University Drive, Hershey, PA 17033-0850, USA
| | - Sarah A Brendle
- The Jake Gittlen Cancer Research Foundation, H069, Department of Pathology, C7800, The Pennsylvania State University, College of Medicine, 500 University Drive, Hershey, PA 17033-0850, USA
| | - Colin A Campbell
- GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2NY, UK
| | - Mao X Chen
- GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2NY, UK
| | - Rebecca C Furze
- GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2NY, UK
| | - Isobel L Harada
- GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2NY, UK
| | - Ian D Holyer
- GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2NY, UK
| | - Umesh Kumar
- GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2NY, UK
| | - Kevin Lee
- GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2NY, UK
| | - Rab K Prinjha
- GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2NY, UK
| | - Martin Rüdiger
- GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2NY, UK
| | - Jonathan T Seal
- GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2NY, UK
| | - Simon Taylor
- GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2NY, UK
| | - Jason Witherington
- GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2NY, UK
| | - Neil D Christensen
- The Jake Gittlen Cancer Research Foundation, H069, Department of Pathology, C7800, The Pennsylvania State University, College of Medicine, 500 University Drive, Hershey, PA 17033-0850, USA
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10
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Bojilova ED, Weyn C, Antoine MH, Fontaine V. Extrachromosomal HPV-16 LCR transcriptional activation by HDACi opposed by cellular differentiation and DNA integration. Oncotarget 2018; 7:75526-75538. [PMID: 27705914 PMCID: PMC5342758 DOI: 10.18632/oncotarget.12263] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/07/2016] [Accepted: 09/13/2016] [Indexed: 12/11/2022] Open
Abstract
Histone deacetylase inhibitors (HDACi) have been shown to render HPV-carrying cells susceptible to intrinsic and extrinsic apoptotic signals. As such, these epigenetic drugs have entered clinical trials in the effort to treat cervical cancer. Here, we studied the effect of common HDACi, with an emphasis on Trichostatin A (TSA), on the transcriptional activity of the HPV-16 Long Control Region (LCR) in order to better understand the impact of these agents in the context of the HPV life cycle and infection. HDACi strongly induced transcription of the firefly luciferase reporter gene under the control of the HPV-16 LCR in a variety of cell lines. In the HaCaT keratinocyte cell line undergoing differentiation induced by TSA, we observed a reduction in LCR-controlled transcription. Three major AP-1 binding sites in the HPV-16 LCR are involved in the regulation by TSA. However, whatever the status of differentiation of the HaCaT cells, TSA induced integration of extra-chromosomal transfected DNA into the cellular genome. Although these data suggest caution using HDACi in the treatment of HR HPV infection, further in vivo studies are necessary to better assess the risk.
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Affiliation(s)
- Ekaterina Dimitrova Bojilova
- Université Libre de Bruxelles (ULB), Faculty of Pharmacy, Unit of Pharmaceutical Microbiology and Hygiene, 1050 Brussels, Belgium
| | - Christine Weyn
- Université Libre de Bruxelles (ULB), Faculty of Pharmacy, Unit of Pharmaceutical Microbiology and Hygiene, 1050 Brussels, Belgium
| | - Marie-Hélène Antoine
- Université Libre de Bruxelles (ULB) Faculty of Medicine, Laboratory of Experimental Hormonology, 1070 Brussels, Belgium
| | - Véronique Fontaine
- Université Libre de Bruxelles (ULB), Faculty of Pharmacy, Unit of Pharmaceutical Microbiology and Hygiene, 1050 Brussels, Belgium
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11
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McBride AA. Mechanisms and strategies of papillomavirus replication. Biol Chem 2017; 398:919-927. [PMID: 28315855 DOI: 10.1515/hsz-2017-0113] [Citation(s) in RCA: 78] [Impact Index Per Article: 11.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/08/2017] [Accepted: 03/16/2017] [Indexed: 12/16/2022]
Abstract
Animal and human papillomaviruses (HPVs) replicate persistently in specific types of stratified epithelia of their host. After the initial infection, the viral genome replicates at low levels in the dividing cells of the epithelium, and these cells form a reservoir of infection that can last for decades. When the infected cells differentiate, viral genomes replicate to high levels to form progeny virus that is released from the surface of the epithelium. This complex life cycle requires several different modes of viral DNA replication, but papillomaviruses are masters at hijacking key cellular processes to facilitate their own reproduction.
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12
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Brd4 Activates Early Viral Transcription upon Human Papillomavirus 18 Infection of Primary Keratinocytes. mBio 2016; 7:mBio.01644-16. [PMID: 27879331 PMCID: PMC5120138 DOI: 10.1128/mbio.01644-16] [Citation(s) in RCA: 25] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023] Open
Abstract
Human papillomaviruses (HPVs) replicate in the cutaneous and mucosal epithelia, and the infectious cycle is synchronous with the differentiation program of the host keratinocytes. The virus initially infects dividing cells in the lower layers of the epithelium, where it establishes a persistent infection. The viral genome is maintained as a low-copy-number, extrachromosomal element in these proliferating cells but switches to the late stage of the life cycle in differentiated cells. The cellular chromatin adaptor protein Brd4 is involved in several stages and processes of the viral life cycle. In concert with the viral transcriptional regulator E2, Brd4 can repress transcription from the early viral promoter. Brd4 and E2 form a complex with the viral genome that associates with host chromosomes to partition the viral genome in dividing cells; Brd4 also localizes to active sites of productive HPV DNA replication. However, because of the difficulties in producing HPV viral particles, the role of Brd4 in modulating viral transcription and replication at the initial stage of infection is unclear. In this study, we have used an HPV18 quasivirus-based genome delivery system to assess the role of Brd4 in the initial infectivity of primary human keratinocytes. We show that, upon infection of primary human keratinocytes with HPV18 quasivirus, Brd4 activates viral transcription and replication. Furthermore, this activation is independent of the functional interaction between Brd4 and the HPV18 E2 protein. HPVs lack encapsidated proteins and so rely exquisitely on host cellular factors to initiate their gene expression programs in newly infected cells. Brd4 is an important cellular chromatin adaptor molecule that normally activates host transcription initiation and elongation. In this study, we further optimize and utilize a quasivirus infection system to show that Brd4 activates HPV18 transcription at early stages of infection. HPVs are important human pathogens causing a wide range of cutaneous and tumorigenic morbidities. Therefore, specifically targeting this protein could provide a new target of therapeutic prevention of establishment of HPV infections.
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13
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Dreer M, Fertey J, van de Poel S, Straub E, Madlung J, Macek B, Iftner T, Stubenrauch F. Interaction of NCOR/SMRT Repressor Complexes with Papillomavirus E8^E2C Proteins Inhibits Viral Replication. PLoS Pathog 2016; 12:e1005556. [PMID: 27064408 PMCID: PMC4827801 DOI: 10.1371/journal.ppat.1005556] [Citation(s) in RCA: 36] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/19/2015] [Accepted: 03/17/2016] [Indexed: 11/19/2022] Open
Abstract
Infections with high-risk human papillomaviruses (HR-HPV) such as HPV16 and 31 can lead to ano-genital and oropharyngeal cancers and HPV types from the beta genus have been implicated in the development of non-melanoma skin cancer. HPV replicate as nuclear extrachromosomal plasmids at low copy numbers in undifferentiated cells. HPV16 and 31 mutants have indicated that these viruses express an E8^E2C protein which negatively regulates genome replication. E8^E2C shares the DNA-binding and dimerization domain (E2C) with the essential viral replication activator E2 and the E8 domain replaces the replication/transcription activation domain of E2. The HR-HPV E8 domain is required for inhibiting viral transcription and the replication of the viral origin mediated by viral E1 and E2 proteins. We show now that E8^E2C also limits replication of HPV1, a mu-PV and HPV8, a beta-PV, in normal human keratinocytes. Proteomic analyses identified all NCoR/SMRT corepressor complex components (HDAC3, GPS2, NCoR, SMRT, TBL1 and TBLR1) as co-precipitating host cell proteins for HPV16 and 31 E8^E2C proteins. Co-immunoprecipitation and co-localization experiments revealed that NCoR/SMRT components interact with HPV1, 8, 16 and 31 E8^E2C proteins in an E8-dependent manner. SiRNA knock-down experiments confirm that NCoR/SMRT components are critical for both the inhibition of transcription and HPV origin replication by E8^E2C proteins. Furthermore, a dominant-negative NCoR fragment activates transcription and replication only from HPV16 and 31 wt but not from mutant genomes encoding NCoR/SMRT-binding deficient E8^E2C proteins. In summary, our data suggest that the repressive function of E8^E2C is highly conserved among HPV and that it is mediated by an E8-dependent interaction with NCoR/SMRT complexes. Our data also indicate for the first time that NCoR/SMRT complexes not only are involved in inhibiting cellular and viral transcription but also in controlling the replication of HPV origins. Human papillomaviruses (HPV) have been shown to cause ano-genital and oropharyngeal cancers and have been also implicated in non-melanoma skin cancer. HPV have a two-stage replication cycle: in undifferentiated keratinocytes only a low level of genome replication without virus production can be observed whereas in differentiated keratinocytes high-level genome replication and virus production takes place. Previous studies have suggested that some HPV encode an E8^E2C protein that limits genome replication in undifferentiated cells. We now demonstrate that E8^E2C proteins from phylogenetically diverse HPV types interact with NCoR/SMRT corepressor complexes to limit viral transcription and genome replication. While NCoR/SMRT complexes are known to mediate the transcription repression functions of a wide variety of host transcription factors, this is the first evidence that NCoR/SMRT proteins also are involved in the repression of the replication of viral origins.
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Affiliation(s)
- Marcel Dreer
- University Hospital Tuebingen, Institute for Medical Virology and Epidemiology of Viral Diseases, Division of Experimental Virology, Tuebingen, Germany
| | - Jasmin Fertey
- University Hospital Tuebingen, Institute for Medical Virology and Epidemiology of Viral Diseases, Division of Experimental Virology, Tuebingen, Germany
| | - Saskia van de Poel
- University Hospital Tuebingen, Institute for Medical Virology and Epidemiology of Viral Diseases, Division of Experimental Virology, Tuebingen, Germany
| | - Elke Straub
- University Hospital Tuebingen, Institute for Medical Virology and Epidemiology of Viral Diseases, Division of Experimental Virology, Tuebingen, Germany
| | - Johannes Madlung
- Proteome Center Tuebingen, University of Tuebingen, Tuebingen, Germany
| | - Boris Macek
- Proteome Center Tuebingen, University of Tuebingen, Tuebingen, Germany
| | - Thomas Iftner
- University Hospital Tuebingen, Institute for Medical Virology and Epidemiology of Viral Diseases, Division of Experimental Virology, Tuebingen, Germany
| | - Frank Stubenrauch
- University Hospital Tuebingen, Institute for Medical Virology and Epidemiology of Viral Diseases, Division of Experimental Virology, Tuebingen, Germany
- * E-mail:
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14
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Aydin I, Schelhaas M. Viral Genome Tethering to Host Cell Chromatin: Cause and Consequences. Traffic 2016; 17:327-40. [PMID: 26787361 DOI: 10.1111/tra.12378] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/18/2015] [Revised: 01/14/2016] [Accepted: 01/14/2016] [Indexed: 12/26/2022]
Abstract
Viruses are small infectious agents that replicate in cells of a host organism and that evolved to use cellular machineries for all stages of the viral life cycle. Here, we critically assess current knowledge on a particular mechanism of persisting viruses, namely, how they tether their genomes to host chromatin, and what consequences arise from this process. A group of persisting DNA viruses, i.e. gamma-herpesviruses and papillomaviruses (PV), uses this tethering strategy to maintain their genomes in the nuclei during cell division. Thus, these viruses face the challenge of viral genome loss during mitosis, as they are transported with the host chromosomes to the nascent daughter nuclei. Incidentally, another group of viruses, certain retroviruses and PV, have adopted this tethering strategy to deliver their genomes into the nuclei of dividing cells during cell entry. By exploiting a phase in the cell cycle when the nuclear envelope is disassembled, viruses bypass the need to engage with the nuclear import machinery. Recent reports suggest that tethering may induce severe cellular consequences that involve activation of mitotic checkpoints, causing missegregation of host chromosomes and genomic instability, which may contribute to cancer.
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Affiliation(s)
- Inci Aydin
- Cell Biology of Virus Infection Unit, Institutes of Molecular Virology and Medical Biochemistry, ZMBE, University of Münster, Münster, Germany.,Cells in Motion, CiM, Cluster of Excellence EXC 1003, Münster, Germany
| | - Mario Schelhaas
- Cell Biology of Virus Infection Unit, Institutes of Molecular Virology and Medical Biochemistry, ZMBE, University of Münster, Münster, Germany.,Cells in Motion, CiM, Cluster of Excellence EXC 1003, Münster, Germany
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15
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Delcuratolo M, Fertey J, Schneider M, Schuetz J, Leiprecht N, Hudjetz B, Brodbeck S, Corall S, Dreer M, Schwab RM, Grimm M, Wu SY, Stubenrauch F, Chiang CM, Iftner T. Papillomavirus-Associated Tumor Formation Critically Depends on c-Fos Expression Induced by Viral Protein E2 and Bromodomain Protein Brd4. PLoS Pathog 2016; 12:e1005366. [PMID: 26727473 PMCID: PMC4699637 DOI: 10.1371/journal.ppat.1005366] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/18/2015] [Accepted: 12/04/2015] [Indexed: 12/27/2022] Open
Abstract
We investigated the mechanism of how the papillomavirus E2 transcription factor can activate promoters through activator protein (AP)1 binding sites. Using an unbiased approach with an inducible cell line expressing the viral transcription factor E2 and transcriptome analysis, we found that E2 induces the expression of the two AP1 components c-Fos and FosB in a Brd4-dependent manner. In vitro RNA interference confirmed that c-Fos is one of the AP1 members driving the expression of viral oncogenes E6/E7. Mutation analysis and in vivo RNA interference identified an essential role for c-Fos/AP1 and also for the bromodomain protein Brd4 for papillomavirus-induced tumorigenesis. Lastly, chromatin immunoprecipitation analysis demonstrated that E2 binds together with Brd4 to a canonical E2 binding site (E2BS) in the promoter of c-Fos, thus activating c-Fos expression. Thus, we identified a novel way how E2 activates the viral oncogene promoter and show that E2 may act as a viral oncogene by direct activation of c-Fos involved in skin tumorigenesis. Human Papillomaviruses (HPV) are the etiological agents of cervical cancer and of skin cancer in individuals with the inherited disease epidermodysplasia verruciformis (EV). While the role of the viral oncogenes E6/E7 as drivers of tumorigenesis in cervical cancer has been firmly established, the contribution of the early viral genes in skin cancer is less clear. For EV-associated HPV8 and for the skin cancer model system using cottontail rabbit PV, an important role of the viral E2 protein in tumorigenesis was suggested earlier and regulation of cellular genes by E2 through different mechanisms was demonstrated. We show now that the viral E2 and cellular Brd4 act together to induce the cellular gene c-Fos, which as a member of the AP-1 complex, is involved in the regulation of cellular genes and the viral promoter driving the expression of viral oncogenes. As c-Fos has also been shown to be essential for skin cancer, E2 contributes to tumorigenesis via expression of E6/E7 as well as by increasing c-Fos.
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Affiliation(s)
- Maria Delcuratolo
- Division of Experimental Virology, Institute of Medical Virology, University Hospital Tübingen, Tübingen, Germany
| | - Jasmin Fertey
- Division of Experimental Virology, Institute of Medical Virology, University Hospital Tübingen, Tübingen, Germany
| | - Markus Schneider
- Division of Experimental Virology, Institute of Medical Virology, University Hospital Tübingen, Tübingen, Germany
| | - Johanna Schuetz
- Division of Experimental Virology, Institute of Medical Virology, University Hospital Tübingen, Tübingen, Germany
| | - Natalie Leiprecht
- Division of Experimental Virology, Institute of Medical Virology, University Hospital Tübingen, Tübingen, Germany
| | - Benjamin Hudjetz
- Division of Experimental Virology, Institute of Medical Virology, University Hospital Tübingen, Tübingen, Germany
| | - Stephan Brodbeck
- Division of Experimental Virology, Institute of Medical Virology, University Hospital Tübingen, Tübingen, Germany
| | - Silke Corall
- Division of Experimental Virology, Institute of Medical Virology, University Hospital Tübingen, Tübingen, Germany
| | - Marcel Dreer
- Division of Experimental Virology, Institute of Medical Virology, University Hospital Tübingen, Tübingen, Germany
| | - Roxana Michaela Schwab
- Division of Experimental Virology, Institute of Medical Virology, University Hospital Tübingen, Tübingen, Germany
| | - Martin Grimm
- Department of Oral and Maxillofacial Surgery, University Hospital Tübingen, Tübingen, Germany
| | - Shwu-Yuan Wu
- University of Texas Southwestern Medical Center, Simmons Comprehensive Cancer Center, Department of Biochemistry, Department of Pharmacology, Dallas, Texas, United States of America
| | - Frank Stubenrauch
- Division of Experimental Virology, Institute of Medical Virology, University Hospital Tübingen, Tübingen, Germany
| | - Cheng-Ming Chiang
- University of Texas Southwestern Medical Center, Simmons Comprehensive Cancer Center, Department of Biochemistry, Department of Pharmacology, Dallas, Texas, United States of America
| | - Thomas Iftner
- Division of Experimental Virology, Institute of Medical Virology, University Hospital Tübingen, Tübingen, Germany
- * E-mail:
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16
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Role of Ostm1 Cytosolic Complex with Kinesin 5B in Intracellular Dispersion and Trafficking. Mol Cell Biol 2015; 36:507-21. [PMID: 26598607 DOI: 10.1128/mcb.00656-15] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/30/2015] [Accepted: 11/17/2015] [Indexed: 01/05/2023] Open
Abstract
In humans and in mice, mutations in the Ostm1 gene cause the most severe form of osteopetrosis, a major bone disease, and neuronal degeneration, both of which are associated with early death. To gain insight into Ostm1 function, we first investigated by sequence and biochemical analysis an immature 34-kDa type I transmembrane Ostm1 protein with a unique cytosolic tail. Mature Ostm1 is posttranslationally processed and highly N-glycosylated and has an apparent mass of ∼60 kDa. Analysis the subcellular localization of Ostm1 showed that it is within the endoplasmic reticulum, trans-Golgi network, and endosomes/lysosomes. By a wide protein screen under physiologic conditions, several novel cytosolic Ostm1 partners were identified and validated, for which a direct interaction with the kinesin 5B heavy chains was demonstrated. These results determined that Ostm1 is part of a cytosolic scaffolding multiprotein complex, imparting an adaptor function to Ostm1. Moreover, we uncovered a role for the Ostm1/KIF5B complex in intracellular trafficking and dispersion of cargos from the endoplasmic reticulum to late endosomal/lysosomal subcellular compartments. These Ostm1 molecular and cellular functions could elucidate all of the pathophysiologic mechanisms underlying the wide phenotypic spectrum of Ostm1-deficient mice.
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17
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McKinney CC, Hussmann KL, McBride AA. The Role of the DNA Damage Response throughout the Papillomavirus Life Cycle. Viruses 2015; 7:2450-69. [PMID: 26008695 PMCID: PMC4452914 DOI: 10.3390/v7052450] [Citation(s) in RCA: 61] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/27/2015] [Accepted: 05/13/2015] [Indexed: 12/25/2022] Open
Abstract
The DNA damage response (DDR) maintains genomic integrity through an elaborate network of signaling pathways that sense DNA damage and recruit effector factors to repair damaged DNA. DDR signaling pathways are usurped and manipulated by the replication programs of many viruses. Here, we review the papillomavirus (PV) life cycle, highlighting current knowledge of how PVs recruit and engage the DDR to facilitate productive infection.
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Affiliation(s)
- Caleb C McKinney
- Laboratory of Viral Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892, USA.
| | - Katherine L Hussmann
- Laboratory of Viral Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892, USA.
| | - Alison A McBride
- Laboratory of Viral Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892, USA.
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18
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Chang SW, Liu WC, Liao KY, Tsao YP, Hsu PH, Chen SL. Phosphorylation of HPV-16 E2 at serine 243 enables binding to Brd4 and mitotic chromosomes. PLoS One 2014; 9:e110882. [PMID: 25340539 PMCID: PMC4207782 DOI: 10.1371/journal.pone.0110882] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/25/2014] [Accepted: 09/25/2014] [Indexed: 01/16/2023] Open
Abstract
The papillomavirus E2 protein is involved in the maintenance of persistent infection and known to bind either to cellular factors or directly to mitotic chromosomes in order to partition the viral genome into the daughter cells. However, how the HPV-16 E2 protein acts to facilitate partitioning of the viral genome remains unclear. In this study, we found that serine 243 of HPV-16 E2, located in the hinge region, is crucial for chromosome binding during mitosis. Bromodomain protein 4 (Brd4) has been identified as a cellular binding target through which the E2 protein of bovine papillomavirus type 1 (BPV-1) tethers the viral genome to mitotic chromosomes. Mutation analysis showed that, when the residue serine 243 was substituted by glutamic acid or aspartic acid, whose negative charges mimic the effect of constitutive phosphorylation, the protein still can interact with Brd4 and colocalize with Brd4 in condensed metaphase and anaphase chromosomes. However, substitution by the polar uncharged residues asparagine or glutamine abrogated Brd4 and mitotic chromosome binding. Moreover, following treatment with the inhibitor JQ1 to release Brd4 from the chromosomes, Brd4 and E2 formed punctate foci separate from the chromosomes, further supporting the hypothesis that the association of the HPV-16 E2 protein with the chromosomes is Brd4-dependent. In addition, the S243A E2 protein has a shorter half-life than the wild type, indicating that phosphorylation of the HPV-16 E2 protein at serine 243 also increases its half-life. Thus, phosphorylation of serine 243 in the hinge region of HPV-16 E2 is essential for interaction with Brd4 and required for host chromosome binding.
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Affiliation(s)
- Szu-Wei Chang
- Graduate Institute of Microbiology, College of Medicine, National Taiwan University, Taipei, Taiwan
| | - Wei-Chen Liu
- Graduate Institute of Microbiology, College of Medicine, National Taiwan University, Taipei, Taiwan
| | - Kuan-Yu Liao
- Graduate Institute of Microbiology, College of Medicine, National Taiwan University, Taipei, Taiwan
| | - Yeou-Ping Tsao
- Department of Ophthalmology, Mackay Memorial Hospital, Taipei, Taiwan
| | - Pang-Hung Hsu
- Department of Bioscience and Biotechnology, College of Life Sciences, National Taiwan Ocean University, Keelung, Taiwan
| | - Show-Li Chen
- Graduate Institute of Microbiology, College of Medicine, National Taiwan University, Taipei, Taiwan
- * E-mail:
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19
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Helfer CM, Yan J, You J. The cellular bromodomain protein Brd4 has multiple functions in E2-mediated papillomavirus transcription activation. Viruses 2014; 6:3228-49. [PMID: 25140737 PMCID: PMC4147693 DOI: 10.3390/v6083228] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/26/2014] [Revised: 08/03/2014] [Accepted: 08/12/2014] [Indexed: 12/17/2022] Open
Abstract
The cellular bromodomain protein Brd4 functions in multiple processes of the papillomavirus life cycle, including viral replication, genome maintenance, and gene transcription through its interaction with the viral protein, E2. However, the mechanisms by which E2 and Brd4 activate viral transcription are still not completely understood. In this study, we show that recruitment of positive transcription elongation factor b (P-TEFb), a functional interaction partner of Brd4 in transcription activation, is important for E2’s transcription activation activity. Furthermore, chromatin immunoprecipitation (ChIP) analyses demonstrate that P-TEFb is recruited to the actual papillomavirus episomes. We also show that E2’s interaction with cellular chromatin through Brd4 correlates with its papillomavirus transcription activation function since JQ1(+), a bromodomain inhibitor that efficiently dissociates E2-Brd4 complexes from chromatin, potently reduces papillomavirus transcription. Our study identifies a specific function of Brd4 in papillomavirus gene transcription and highlights the potential use of bromodomain inhibitors as a method to disrupt the human papillomavirus (HPV) life cycle.
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Affiliation(s)
- Christine M Helfer
- Department of Microbiology, University of Pennsylvania, Perelman School of Medicine, Philadelphia, PA 19104, USA.
| | - Junpeng Yan
- Department of Microbiology, University of Pennsylvania, Perelman School of Medicine, Philadelphia, PA 19104, USA.
| | - Jianxin You
- Department of Microbiology, University of Pennsylvania, Perelman School of Medicine, Philadelphia, PA 19104, USA.
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20
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Jang MK, Shen K, McBride AA. Papillomavirus genomes associate with BRD4 to replicate at fragile sites in the host genome. PLoS Pathog 2014; 10:e1004117. [PMID: 24832099 PMCID: PMC4022725 DOI: 10.1371/journal.ppat.1004117] [Citation(s) in RCA: 83] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2014] [Accepted: 03/28/2014] [Indexed: 12/11/2022] Open
Abstract
It has long been recognized that oncogenic viruses often integrate close to common fragile sites. The papillomavirus E2 protein, in complex with BRD4, tethers the viral genome to host chromatin to ensure persistent replication. Here, we map these targets to a number of large regions of the human genome and name them Persistent E2 and BRD4-Broad Localized Enrichments of Chromatin or PEB-BLOCs. PEB-BLOCs frequently contain deletions, have increased rates of asynchronous DNA replication, and are associated with many known common fragile sites. Cell specific fragile sites were mapped in human C-33 cervical cells by FANCD2 ChIP-chip, confirming the association with PEB-BLOCs. HPV-infected cells amplify viral DNA in nuclear replication foci and we show that these form adjacent to PEB-BLOCs. We propose that HPV replication, which hijacks host DNA damage responses, occurs adjacent to highly susceptible fragile sites, greatly increasing the chances of integration here, as is found in HPV-associated cancers. Papillomavirus cause persistent, but mostly self-limiting, infections of the host epithelium. However, a subset of oncogenic papillomaviruses is the causative agent of certain human cancers. In persistent infection the viral genomes are tethered to host chromosomes to maintain and partition the extrachromosomal viral genomes to daughter cells. However, in cancers viral DNA is often found integrated close to common fragile sites, regions prone to breakage, amplification and deletion. We show that the viral E2 and cellular BRD4 proteins are associated with fragile regions of the human genome and nucleate viral replication foci at these sites. This is a resourceful strategy for a virus that uses the host DNA damage response to amplify viral DNA. However, the outcome may be increased accidental integration of viral DNA, which in the case of the oncogenic viruses can promote carcinogenesis.
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Affiliation(s)
- Moon Kyoo Jang
- Laboratory of Viral Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland, United States of America
| | - Kui Shen
- Bioinformatics and Computational Biosciences Branch, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland, United States of America
| | - Alison A. McBride
- Laboratory of Viral Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland, United States of America
- * E-mail:
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21
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Gupta SS, Maetzig T, Maertens GN, Sharif A, Rothe M, Weidner-Glunde M, Galla M, Schambach A, Cherepanov P, Schulz TF. Bromo- and extraterminal domain chromatin regulators serve as cofactors for murine leukemia virus integration. J Virol 2013; 87:12721-36. [PMID: 24049186 PMCID: PMC3838128 DOI: 10.1128/jvi.01942-13] [Citation(s) in RCA: 112] [Impact Index Per Article: 10.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/15/2013] [Accepted: 09/09/2013] [Indexed: 01/03/2023] Open
Abstract
Retroviral integrase (IN) proteins catalyze the permanent integration of proviral genomes into host DNA with the help of cellular cofactors. Lens epithelium-derived growth factor (LEDGF) is a cofactor for lentiviruses, including human immunodeficiency virus type 1 (HIV-1), and targets lentiviral integration toward active transcription units in the host genome. In contrast to lentiviruses, murine leukemia virus (MLV), a gammaretrovirus, tends to integrate near transcription start sites. Here, we show that the bromodomain and extraterminal domain (BET) proteins BRD2, BRD3, and BRD4 interact with gammaretroviral INs and stimulate the catalytic activity of MLV IN in vitro. We mapped the interaction site to a characteristic structural feature within the BET protein extraterminal (ET) domain and to three amino acids in MLV IN. The ET domains of different BET proteins stimulate MLV integration in vitro and, in the case of BRD2, also in vivo. Furthermore, two small-molecule BET inhibitors, JQ1 and I-BET, decrease MLV integration and shift it away from transcription start sites. Our data suggest that BET proteins might act as chromatin-bound acceptors for the MLV preintegration complex. These results could pave a way to redirecting MLV DNA integration as a basis for creating safer retroviral vectors.
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Affiliation(s)
| | - Tobias Maetzig
- Institute of Experimental Hematology, Hannover Medical School, Hannover, Germany
| | - Goedele N. Maertens
- Division of Medicine, St. Mary's Campus, Imperial College London, London, United Kingdom
| | - Azar Sharif
- Division of Medicine, St. Mary's Campus, Imperial College London, London, United Kingdom
| | - Michael Rothe
- Institute of Experimental Hematology, Hannover Medical School, Hannover, Germany
| | | | - Melanie Galla
- Institute of Experimental Hematology, Hannover Medical School, Hannover, Germany
| | - Axel Schambach
- Institute of Experimental Hematology, Hannover Medical School, Hannover, Germany
- Division of Hematology/Oncology, Children's Hospital Boston, Harvard Medical School, Boston, Massachusetts, USA
| | - Peter Cherepanov
- Division of Medicine, St. Mary's Campus, Imperial College London, London, United Kingdom
- Chromatin Structure and Mobile DNA Laboratory, Cancer Research UK, Herts, United Kingdom
| | - Thomas F. Schulz
- Institute of Virology, Hannover Medical School, Hannover, Germany
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22
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Sakakibara N, Chen D, Jang MK, Kang DW, Luecke HF, Wu SY, Chiang CM, McBride AA. Brd4 is displaced from HPV replication factories as they expand and amplify viral DNA. PLoS Pathog 2013; 9:e1003777. [PMID: 24278023 PMCID: PMC3836737 DOI: 10.1371/journal.ppat.1003777] [Citation(s) in RCA: 63] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/17/2013] [Accepted: 10/04/2013] [Indexed: 12/19/2022] Open
Abstract
Replication foci are generated by many viruses to concentrate and localize viral DNA synthesis to specific regions of the cell. Expression of the HPV16 E1 and E2 replication proteins in keratinocytes results in nuclear foci that recruit proteins associated with the host DNA damage response. We show that the Brd4 protein localizes to these foci and is essential for their formation. However, when E1 and E2 begin amplifying viral DNA, Brd4 is displaced from the foci and cellular factors associated with DNA synthesis and homologous recombination are recruited. Differentiated HPV-infected keratinocytes form similar nuclear foci that contain amplifying viral DNA. We compare the different foci and show that, while they have many characteristics in common, there is a switch between early Brd4-dependent foci and mature Brd4-independent replication foci. However, HPV genomes encoding mutated E2 proteins that are unable to bind Brd4 can replicate and amplify the viral genome. We propose that, while E1, E2 and Brd4 might bind host chromatin at early stages of infection, there is a temporal and functional switch at later stages and increased E1 and E2 levels promote viral DNA amplification, displacement of Brd4 and growth of a replication factory. The concomitant DNA damage response recruits proteins required for DNA synthesis and repair, which could then be utilized for viral DNA replication. Hence, while Brd4 can enhance replication by concentrating viral processes in specific regions of the host nucleus, this interaction is not absolutely essential for HPV replication. Papillomaviruses have a remarkable infection cycle that depends on the development of a stratified epithelium. The virus infects the lower, dividing layers of the epithelium and viral genomes replicate at low copy number, and are maintained in these cells, for long periods of time. As infected cells differentiate and move to the surface of the epithelium, they switch on high level viral DNA replication, synthesize capsid proteins and form new viral particles. Viral replication takes place in nuclear foci and is dependent on the E1 and E2 replication proteins. Brd4 is a cellular chromatin binding protein that interacts with E2 and is important for transcriptional regulation of papillomaviruses. In this study we examine the role of Brd4 at different stages in the formation of viral replication foci. In the absence of viral DNA replication, Brd4 links the viral proteins to host chromatin. However, when viral genomes begin to amplify to high levels, Brd4 is displaced from nuclear foci and is not required for replication.
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Affiliation(s)
- Nozomi Sakakibara
- Laboratory of Viral Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland, United States of America
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23
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Helfer CM, Wang R, You J. Analysis of the papillomavirus E2 and bromodomain protein Brd4 interaction using bimolecular fluorescence complementation. PLoS One 2013; 8:e77994. [PMID: 24205059 PMCID: PMC3808292 DOI: 10.1371/journal.pone.0077994] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/28/2013] [Accepted: 09/14/2013] [Indexed: 11/19/2022] Open
Abstract
The human papillomavirus (HPV) vaccines effectively protect against new infections of up to four HPV subtypes. However, these vaccines are not protective against many other clinically relevant HPV subtypes and are ineffective at treating established HPV infections. There is therefore a significant need for antiviral treatments for persistent HPV infections. A promising anti-HPV drug target is the interaction between the HPV E2 protein and cellular bromodomain-containing protein 4 (Brd4) since this protein complex mediates several processes important for the viral life cycle including viral genome maintenance, replication, and transcription. Using bimolecular fluorescence complementation (BiFC) technology, we demonstrate the E2 and Brd4 interaction on both interphase chromatin and mitotic chromosomes throughout mitosis. The E2-Brd4 BiFC was significantly diminished by mutating the Brd4 binding sites in E2 or by a dominant negative inhibitor of the E2-Brd4 interaction, demonstrating the potential of BiFC for identifying inhibitors of this important virus-host interaction. Importantly, when Brd4 was released from chromatin using the bromodomain inhibitor JQ1(+), the E2-Brd4 interacting complex relocated into foci that no longer associate with mitotic chromosomes, pointing to JQ1(+) as a promising antiviral inhibitor of HPV genome maintenance during HPV persistent infection.
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Affiliation(s)
- Christine M. Helfer
- Department of Microbiology, University of Pennsylvania Perelman School of Medicine, Philadelphia, Pennsylvania, United States of America
| | - Ranran Wang
- Department of Microbiology, University of Pennsylvania Perelman School of Medicine, Philadelphia, Pennsylvania, United States of America
| | - Jianxin You
- Department of Microbiology, University of Pennsylvania Perelman School of Medicine, Philadelphia, Pennsylvania, United States of America
- * E-mail:
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24
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Abstract
The papillomavirus E2 proteins are pivotal to the viral life cycle and have well characterized functions in transcriptional regulation, initiation of DNA replication and partitioning the viral genome. The E2 proteins also function in vegetative DNA replication, post-transcriptional processes and possibly packaging. This review describes structural and functional aspects of the E2 proteins and their binding sites on the viral genome. It is intended to be a reference guide to this viral protein.
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Affiliation(s)
- Alison A McBride
- Laboratory of Viral Diseases, National Institute of Allergy and Infectious Disease, National Institutes of Health, Bethesda, MD 20892, USA.
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25
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McBride AA, Jang MK. Current understanding of the role of the Brd4 protein in the papillomavirus lifecycle. Viruses 2013; 5:1374-94. [PMID: 23722886 PMCID: PMC3717712 DOI: 10.3390/v5061374] [Citation(s) in RCA: 37] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/02/2013] [Revised: 05/21/2013] [Accepted: 05/21/2013] [Indexed: 12/19/2022] Open
Abstract
The Brd4 protein is an epigenetic reader that is central to regulation of cellular transcription and mitotic bookmarking. The transcription and replication proteins of many viruses interact with Brd4. We describe the multiple roles of Brd4 in the papillomavirus lifecycle.
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Affiliation(s)
- Alison A McBride
- Laboratory of Viral Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892, USA.
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26
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Gagnon D, Sénéchal H, D'Abramo CM, Alvarez J, McBride AA, Archambault J. Genetic analysis of the E2 transactivation domain dimerization interface from bovine papillomavirus type 1. Virology 2013; 439:132-9. [PMID: 23490049 DOI: 10.1016/j.virol.2013.02.012] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2012] [Revised: 02/11/2013] [Accepted: 02/13/2013] [Indexed: 11/29/2022]
Abstract
The bovine papillomavirus type 1 (BPV1) E2 protein binds as a dimer to the viral genome to promote its transcription, replication and maintenance in keratinocytes. Although BPV1 E2 dimerizes primarily through its DNA-binding domain, it was shown previously that its transactivation domain (TAD) can also dimerize in vitro through formation of a disulfide bond between cysteine 57 (C57) of adjacent monomers and of an ion pair between arginine 172 (R172) and aspartic acid 175 (D175). The function of this TAD dimerization interface in vivo remains unknown. Here, we report the effects of substituting C57, R172 and D175 by alanine on the transactivation activity of BPV E2 as well as on its ability to support viral DNA replication using a novel luciferase-based assay. Results for this mutational analysis suggest that the TAD dimerization interface is not essential for either process but may contribute to the DNA replication activity of BPV1 E2.
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Affiliation(s)
- David Gagnon
- Molecular Virology Laboratory, Institut de Recherches Cliniques de Montréal (IRCM) and Department of Biochemistry, Université de Montréal, Montreal, QC, Canada
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27
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Abstract
The p300, CBP, and pCAF lysine acetyltransferase (KAT) proteins have been reported to physically interact with bovine (BPV) and human (HPV) papillomavirus E2 proteins. While overexpression of these KAT proteins enhances E2-dependent transcription, the mechanism has not been determined. Using RNA interference (RNAi) to deplete these factors, we demonstrated that E2 transcriptional activity requires physiological levels of p300, CBP, and pCAF. Each protein appears to have a unique function in E2-dependent transcription, since overexpression of one KAT failed to compensate for RNAi knockdown of another KAT. Using an in vitro acetylation assay, we identified highly conserved lysines that are targeted by p300 for acetylation. The conservative changes of lysines at positions 111 and 112 to arginine were of particular interest. The K111R and the K111R/K112R mutants showed reduced transcriptional activity that was not responsive to p300 overexpression, while the K112R mutant retained activity. p300 and CBP were detected at the viral promoter; however, pCAF was not. We propose a model by which E2 transcriptional activity is controlled by p300-mediated acetylation of lysine 111. This model represents a novel mechanism regulating papillomavirus gene expression.
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Lace MJ, Ushikai M, Yamakawa Y, Anson JR, Ishiji T, Turek LP, Haugen TH. The truncated C-terminal E2 (E2-TR) protein of bovine papillomavirus (BPV) type-1 is a transactivator that modulates transcription in vivo and in vitro in a manner distinct from the E2-TA and E8^E2 gene products. Virology 2012; 429:99-111. [DOI: 10.1016/j.virol.2012.03.020] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/11/2011] [Revised: 01/09/2012] [Accepted: 03/30/2012] [Indexed: 10/28/2022]
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McBride AA, Sakakibara N, Stepp WH, Jang MK. Hitchhiking on host chromatin: how papillomaviruses persist. BIOCHIMICA ET BIOPHYSICA ACTA 2012; 1819:820-5. [PMID: 22306660 PMCID: PMC3357461 DOI: 10.1016/j.bbagrm.2012.01.011] [Citation(s) in RCA: 39] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/01/2011] [Revised: 01/07/2012] [Accepted: 01/10/2012] [Indexed: 10/14/2022]
Abstract
Persistent viruses need mechanisms to protect their genomes from cellular defenses and to ensure that they are efficiently propagated to daughter host cells. One mechanism by which papillomaviruses achieve this is through the association of viral genomes with host chromatin, mediated by the viral E2 tethering protein. Association of viral DNA with regions of active host chromatin ensures that the virus remains transcriptionally active and is not relegated to repressed heterochromatin. In addition, viral genomes are tethered to specific regions of host mitotic chromosomes to efficiently partition their DNA to daughter cells. Vegetative viral DNA replication also initiates at specific regions of host chromatin, where the viral E1 and E2 proteins initiate a DNA damage response that recruits cellular DNA damage and repair proteins to viral replication foci for efficient viral DNA synthesis. Thus, these small viruses have capitalized on interactions with chromatin to efficiently target their genomes to beneficial regions of the host nucleus. This article is part of a Special Issue entitled: Chromatin in time and space.
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Affiliation(s)
| | | | - Wesley H. Stepp
- Laboratory of Viral Diseases, NIAID, NIH, Bethesda, Maryland, USA
| | - Moon Kyoo Jang
- Laboratory of Viral Diseases, NIAID, NIH, Bethesda, Maryland, USA
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Muller M, Jacob Y, Jones L, Weiss A, Brino L, Chantier T, Lotteau V, Favre M, Demeret C. Large scale genotype comparison of human papillomavirus E2-host interaction networks provides new insights for e2 molecular functions. PLoS Pathog 2012; 8:e1002761. [PMID: 22761572 PMCID: PMC3386243 DOI: 10.1371/journal.ppat.1002761] [Citation(s) in RCA: 49] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/20/2011] [Accepted: 05/04/2012] [Indexed: 11/30/2022] Open
Abstract
Human Papillomaviruses (HPV) cause widespread infections in humans, resulting in latent infections or diseases ranging from benign hyperplasia to cancers. HPV-induced pathologies result from complex interplays between viral proteins and the host proteome. Given the major public health concern due to HPV-associated cancers, most studies have focused on the early proteins expressed by HPV genotypes with high oncogenic potential (designated high-risk HPV or HR-HPV). To advance the global understanding of HPV pathogenesis, we mapped the virus/host interaction networks of the E2 regulatory protein from 12 genotypes representative of the range of HPV pathogenicity. Large-scale identification of E2-interaction partners was performed by yeast two-hybrid screenings of a HaCaT cDNA library. Based on a high-confidence scoring scheme, a subset of these partners was then validated for pair-wise interaction in mammalian cells with the whole range of the 12 E2 proteins, allowing a comparative interaction analysis. Hierarchical clustering of E2-host interaction profiles mostly recapitulated HPV phylogeny and provides clues to the involvement of E2 in HPV infection. A set of cellular proteins could thus be identified discriminating, among the mucosal HPV, E2 proteins of HR-HPV 16 or 18 from the non-oncogenic genital HPV. The study of the interaction networks revealed a preferential hijacking of highly connected cellular proteins and the targeting of several functional families. These include transcription regulation, regulation of apoptosis, RNA processing, ubiquitination and intracellular trafficking. The present work provides an overview of E2 biological functions across multiple HPV genotypes. Over 100 types of human papillomaviruses are responsible for widespread infections in humans. They cause a wide range of pathologies, ranging from inapparent infections to benign lesions, hyperplasia or cancers. Such heterogeneity results from variable interplay among viral and host cell proteins. Aiming to identify specific features that distinguish different pathological genotypes, we mapped the virus-host interaction networks of the regulatory E2 proteins from a set of 12 genotypes representative of HPV diversity. The E2-host interaction profiles recapitulate HPV phylogeny, thus providing a valuable framework for understanding the role of E2 in HPV infection of different pathological traits. The E2 proteins tend to bind to highly connected cellular proteins, indicating a profound effect on the host cell. These interactions predominantly impact on a subset of cellular processes, like transcriptional regulation, apoptosis, RNA metabolism, ubiquitination or intracellular transport. This work improves the global understanding of HPV-associated pathologies, and provides a framework to select interactions that can be used as targets for the development of new therapeutics.
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Affiliation(s)
- Mandy Muller
- Unité de Génétique, Papillomavirus et Cancer Humain (GPCH), Institut Pasteur, Paris, France
- University Paris Diderot, Sorbonne Paris cite, Cellule Pasteur, Paris, France
| | - Yves Jacob
- Unité de Génétique, Papillomavirus et Cancer Humain (GPCH), Institut Pasteur, Paris, France
| | - Louis Jones
- Groupe Logiciels et banques de données, Institut Pasteur, Paris, France
| | | | | | | | | | - Michel Favre
- Unité de Génétique, Papillomavirus et Cancer Humain (GPCH), Institut Pasteur, Paris, France
| | - Caroline Demeret
- Unité de Génétique, Papillomavirus et Cancer Humain (GPCH), Institut Pasteur, Paris, France
- * E-mail:
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Abstract
The bromodomain is a highly conserved motif of 110 amino acids that is bundled into four anti-parallel α-helices and found in proteins that interact with chromatin, such as transcription factors, histone acetylases and nucleosome remodelling complexes. Bromodomain proteins are chromatin 'readers'; they recruit chromatin-regulating enzymes, including 'writers' and 'erasers' of histone modification, to target promoters and to regulate gene expression. Conventional wisdom held that complexes involved in chromatin dynamics are not 'druggable' targets. However, small molecules that inhibit bromodomain and extraterminal (BET) proteins have been described. We examine these developments and discuss the implications for small molecule epigenetic targeting of chromatin networks in cancer.
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Affiliation(s)
- Anna C Belkina
- Cancer Research Center, Nutrition Obesity Research Center, Departments of Medicine and Pharmacology, Boston University School of Medicine, 72 East Concord Street, Boston, MA 02118, USA
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32
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Ballestas ME, Kaye KM. The latency-associated nuclear antigen, a multifunctional protein central to Kaposi's sarcoma-associated herpesvirus latency. Future Microbiol 2012; 6:1399-413. [PMID: 22122438 DOI: 10.2217/fmb.11.137] [Citation(s) in RCA: 74] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023] Open
Abstract
Latency-associated nuclear antigen (LANA) is encoded by the Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) open reading frame 73. LANA is expressed during latent KSHV infection of cells, including tumor cells, such as primary effusion lymphoma, KS and multicentric Castleman's disease. Latently infected cells have multiple extrachromosomal copies of covalently closed circular KSHV genomes (episomes) that are stably maintained in proliferating cells. LANA's best characterized function is that of mediating episome persistence. It does so by binding terminal repeat sequences to the chromosomal matrix, thus ensuring episome replication with each cell division and efficient DNA segregation to daughter nuclei after mitosis. To achieve these functions, LANA associates with different host cell proteins, including chromatin-associated proteins and proteins involved in DNA replication. In addition to episome maintenance, LANA has transcriptional regulatory effects and affects cell growth. LANA exerts these functions through interactions with different cell proteins.
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Affiliation(s)
- Mary E Ballestas
- Department of Pediatrics, Division of Infectious Diseases, University of Alabama in Birmingham, School of Medicine, Children's Harbor Building, Room 148, 1600 6th Ave South, Birmingham, AL 35233, USA
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Gao C, Pan MM, Lei YJ, Tian LQ, Jiang HY, Li XL, Shi Q, Tian C, Yuan YK, Fan GX, Dong XP. A point mutation in the DNA-binding domain of HPV-2 E2 protein increases its DNA-binding capacity and reverses its transcriptional regulatory activity on the viral early promoter. BMC Mol Biol 2012; 13:5. [PMID: 22333459 PMCID: PMC3307029 DOI: 10.1186/1471-2199-13-5] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2011] [Accepted: 02/15/2012] [Indexed: 01/26/2023] Open
Abstract
Background The human papillomavirus (HPV) E2 protein is a multifunctional DNA-binding protein. The transcriptional activity of HPV E2 is mediated by binding to its specific binding sites in the upstream regulatory region of the HPV genomes. Previously we reported a HPV-2 variant from a verrucae vulgaris patient with huge extensive clustered cutaneous, which have five point mutations in its E2 ORF, L118S, S235P, Y287H, S293R and A338V. Under the control of HPV-2 LCR, co-expression of the mutated HPV E2 induced an increased activity on the viral early promoter. In the present study, a series of mammalian expression plasmids encoding E2 proteins with one to five amino acid (aa) substitutions for these mutations were constructed and transfected into HeLa, C33A and SiHa cells. Results CAT expression assays indicated that the enhanced promoter activity was due to the co-expressions of the E2 constructs containing A338V mutation within the DNA-binding domain. Western blots analysis demonstrated that the transiently transfected E2 expressing plasmids, regardless of prototype or the A338V mutant, were continuously expressed in the cells. To study the effect of E2 mutations on its DNA-binding activity, a serial of recombinant E2 proteins with various lengths were expressed and purified. Electrophoresis mobility shift assays (EMSA) showed that the binding affinity of E2 protein with A338V mutation to both an artificial probe with two E2 binding sites or HPV-2 and HPV-16 promoter-proximal LCR sequences were significantly stronger than that of the HPV-2 prototype E2. Furthermore, co-expression of the construct containing A338V mutant exhibited increased activities on heterologous HPV-16 early promoter P97 than that of prototype E2. Conclusions These results suggest that the mutation from Ala to Val at aa 338 is critical for E2 DNA-binding and its transcriptional regulation.
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Affiliation(s)
- Chen Gao
- State Key Laboratory for Infectious Disease Prevention and Control, National Institute for Viral Disease Control and Prevention, Chinese Center for Disease Control and Prevention, Changbai Rd 155, Beijing 102206, People's Republic of China
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Inhibition of human papillomavirus DNA replication by an E1-derived p80/UAF1-binding peptide. J Virol 2012; 86:3486-500. [PMID: 22278251 DOI: 10.1128/jvi.07003-11] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The papillomavirus E1 helicase is recruited by E2 to the viral origin, where it assembles into a double hexamer that orchestrates replication of the viral genome. We previously identified the cellular WD40 repeat-containing protein p80/UAF1 as a novel interaction partner of E1 from anogenital human papillomavirus (HPV) types. p80 was found to interact with the first 40 residues of HPV type 31 (HPV31) E1, and amino acid substitutions within this domain abrogated the maintenance of the viral episome in keratinocytes. In this study, we report that these p80-binding substitutions reduce by 70% the ability of E1 to support transient viral DNA replication without affecting its interaction with E2 and assembly at the origin in vivo. Microscopy studies revealed that p80 is relocalized from the cytoplasm to discrete subnuclear foci by E1 and E2. Chromatin immunoprecipitation assays further revealed that p80 is recruited to the viral origin in an E1- and E2-dependent manner. Interestingly, overexpression of a 40-amino-acid-long p80-binding peptide, derived from HPV31 E1, was found to inhibit viral DNA replication by preventing the recruitment of endogenous p80 to the origin. Mutant peptides defective for p80 interaction were not inhibitory, demonstrating the specificity of this effect. Characterization of this E1 peptide by nuclear magnetic resonance (NMR) showed that it is intrinsically disordered in solution, while mapping studies indicated that the WD repeats of p80 are required for E1 interaction. These results provide additional evidence for the requirement for p80 in anogenital HPV DNA replication and highlight the potential of E1-p80 interaction as a novel antiviral target.
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35
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D'Abramo CM, Archambault J. Small molecule inhibitors of human papillomavirus protein - protein interactions. Open Virol J 2011; 5:80-95. [PMID: 21769307 PMCID: PMC3137155 DOI: 10.2174/1874357901105010080] [Citation(s) in RCA: 53] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/08/2011] [Revised: 05/23/2011] [Accepted: 06/13/2011] [Indexed: 02/06/2023] Open
Abstract
Human papillomaviruses (HPV) have now been identified as a necessary cause of benign and malignant lesions of the differentiating epithelium, particularly cervical cancer, the second most prevalent cancer in women worldwide. While two prophylactic HPV vaccines and screening programs are available, there is currently no antiviral drug for the treatment of HPV infections and associated diseases. The recent progress toward the identification and characterization of specific molecular targets for small molecule-based approaches provides prospect for the development of effective HPV antiviral compounds. Traditionally, antiviral therapies target viral enzymes. HPV encode for few proteins, however, and rely extensively on the infected cell for completion of their life cycle. This article will review the functions of the viral E1 helicase, which encodes the only enzymatic function of the virus, of the E2 regulatory protein, and of the viral E6 and E7 oncogenes in viral replication and pathogenesis. Particular emphasis will be placed on the recent progress made towards the development of novel small molecule inhibitors that specifically target and inhibit the functions of these viral proteins, as well as their interactions with other viral and/or cellular proteins.
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Affiliation(s)
- C M D'Abramo
- Laboratory of Molecular Virology, Institut de Recherches Cliniques de Montréal and Department of Biochemistry, Université de Montréal, Montreal, Quebec, Canada
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36
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The Brd4 extraterminal domain confers transcription activation independent of pTEFb by recruiting multiple proteins, including NSD3. Mol Cell Biol 2011; 31:2641-52. [PMID: 21555454 DOI: 10.1128/mcb.01341-10] [Citation(s) in RCA: 401] [Impact Index Per Article: 30.8] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023] Open
Abstract
Bromodomain protein 4 (Brd4) plays critical roles in development, cancer progression, and virus-host pathogenesis. To gain mechanistic insight into the various biological functions of Brd4, we performed a proteomic analysis to identify and characterize Brd4-associated cellular proteins. We found that the extraterminal (ET) domain, whose function has to date not been determined, interacts with NSD3, JMJD6, CHD4, GLTSCR1, and ATAD5. These ET-domain interactions were also conserved for Brd2 and Brd3, the other human BET proteins tested. We demonstrated that GLTSCR1, NSD3, and JMJD6 impart a pTEFb-independent transcriptional activation function on Brd4. NSD3 as well as JMJD6 is recruited to regulated genes in a Brd4-dependent manner. Moreover, we found that depletion of Brd4 or NSD3 reduces H3K36 methylation, demonstrating that the Brd4/NSD3 complex regulates this specific histone modification. Our results indicate that the Brd4 ET domain through the recruitment of the specific effectors regulates transcriptional activity. In particular, we show that one of these effectors, NSD3, regulates transcription by modifying the chromatin microenvironment at Brd4 target genes. Our study thus identifies the ET domain as a second important transcriptional regulatory domain for Brd4 in addition to the carboxyl-terminal domain (CTD) that interacts with pTEFb.
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37
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In vivo analysis of the cell cycle dependent association of the bovine papillomavirus E2 protein and ChlR1. Virology 2011; 414:1-9. [PMID: 21489590 DOI: 10.1016/j.virol.2011.03.015] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2010] [Revised: 01/14/2011] [Accepted: 03/17/2011] [Indexed: 11/22/2022]
Abstract
It has been shown that the genomes of episomally maintained DNA viruses are tethered to host cell chromosomes during cell division, facilitating maintenance in dividing cells. The papillomavirus E2 protein serves this mechanism of viral genome persistence by simultaneously associating with chromatin and the viral genome during mitosis. Several host cell proteins are reported to be necessary for the association of E2 with chromatin including the cohesion establishment factor ChlR1. Here we use fluorescence resonance energy transfer (FRET) technology to confirm the interaction between BPV-1 E2 and ChlR1. Furthermore, we use synchronised live cells to study the temporal nature of this dynamic protein interaction and show that ChlR1 and E2 interact during specific phases of the cell cycle. These data provide evidence that the association of E2 with ChlR1 contributes to a loading mechanism during DNA replication rather than direct tethering during mitotic division.
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38
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A conserved amphipathic helix in the N-terminal regulatory region of the papillomavirus E1 helicase is required for efficient viral DNA replication. J Virol 2011; 85:5287-300. [PMID: 21450828 DOI: 10.1128/jvi.01829-10] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The papillomavirus E1 helicase, with the help of E2, assembles at the viral origin into a double hexamer that orchestrates replication of the viral genome. The N-terminal region (NTR) of E1 is essential for DNA replication in vivo but dispensable in vitro, suggesting that it has a regulatory function. By deletion analysis, we identified a conserved region of the E1 NTR needed for efficient replication of viral DNA. This region is predicted to form an amphipathic α-helix (AH) and shows sequence similarity to portions of the p53 and herpes simplex virus (HSV) VP16 transactivation domains known as transactivation domain 2 (TAD2) and VP16C, which fold into α-helices upon binding their target proteins, including the Tfb1/p62 (Saccharomyces cerevisiae/human) subunit of general transcription factor TFIIH. By nuclear magnetic resonance (NMR) spectroscopy and isothermal titration calorimetry (ITC), we found that a peptide spanning the E1 AH binds Tfb1 on the same surface as TAD2/VP16C and with a comparable affinity, suggesting that it does bind as an α-helix. Furthermore, the E1 NTRs from several human papillomavirus (HPV) types could activate transcription in yeast, and to a lesser extent in mammalian cells, when fused to a heterologous DNA-binding domain. Mutation of the three conserved hydrophobic residues in the E1 AH, analogous to those in TAD2/VP16C that directly contact their target proteins, decreased transactivation activity and, importantly, also reduced by 50% the ability of E1 to support transient replication of DNA in C33A cells, at a step following assembly of the E1-E2-ori preinitiation complex. These results demonstrate the existence of a conserved TAD2/VP16C-like AH in E1 that is required for efficient replication of viral DNA.
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39
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The E2 protein of human papillomavirus type 8 increases the expression of matrix metalloproteinase-9 in human keratinocytes and organotypic skin cultures. Med Microbiol Immunol 2011; 200:127-35. [PMID: 21274725 DOI: 10.1007/s00430-011-0183-4] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/28/2010] [Indexed: 10/18/2022]
Abstract
Non-melanoma skin cancer (NMSC) is the most frequent human cancer of Caucasian populations. Although the ultraviolet irradiation is a key contributor to the establishment of this keratinocyte malignancy, the infection by some types of human papillomavirus (HPV) has also been implicated in NMSC development. Cancers occur as a result of a complex series of interactions between the cancer cell and its surrounding matrix. The matrix metalloproteinases (MMPs) play a role in degrading the extracellular matrix. MMP9 is an important gelatinase involved in processes such as cell migration, invasion and metastasis. In this report, we demonstrated by EMSA experiments that the MMP9 promoter contains a binding site for the transcriptional regulator E2 of HPV8. Transient reporter gene assays showed that HPV8-E2 activated the MMP9 promoter in a dose-dependent manner in human epidermal keratinocytes. An E2 transactivation-defective mutant (I73L) as well as a DNA-binding deficient mutant (R433K) demonstrated no activation of the MMP9 promoter, suggesting that both an intact transactivation and DNA-binding domain are required for E2 activation of the MMP9-promoter. The functional role of the E2 binding site within the MMP9 promoter was also confirmed by mutating the E2 binding site. In organotypic cultures of human skin, an overexpression of MMP9 was observed in suprabasal layers of the HPV8 E2-expressing epidermis thus confirming the results of the monolayer cultures. These results demonstrate that the early gene E2 of HPV8 is able to increase the expression of MMP9 by direct activation of the MMP9-promoter.
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40
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Nuclear export of human papillomavirus type 31 E1 is regulated by Cdk2 phosphorylation and required for viral genome maintenance. J Virol 2010; 84:11747-60. [PMID: 20844047 DOI: 10.1128/jvi.01445-10] [Citation(s) in RCA: 44] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
The initiator protein E1 from human papillomavirus (HPV) is a helicase essential for replication of the viral genome. E1 contains three functional domains: a C-terminal enzymatic domain that has ATPase/helicase activity, a central DNA-binding domain that recognizes specific sequences in the origin of replication, and a N-terminal region necessary for viral DNA replication in vivo but dispensable in vitro. This N-terminal portion of E1 contains a conserved nuclear export signal (NES) whose function in the viral life cycle remains unclear. In this study, we provide evidence that nuclear export of HPV31 E1 is inhibited by cyclin E/A-Cdk2 phosphorylation of two serines residues, S92 and S106, located near and within the E1 NES, respectively. Using E1 mutant proteins that are confined to the nucleus, we determined that nuclear export of E1 is not essential for transient viral DNA replication but is important for the long-term maintenance of the HPV episome in undifferentiated keratinocytes. The findings that E1 nuclear export is not required for viral DNA replication but needed for genome maintenance over multiple cell divisions raised the possibility that continuous nuclear accumulation of E1 is detrimental to cellular growth. In support of this possibility, we observed that nuclear accumulation of E1 dramatically reduces cellular proliferation by delaying cell cycle progression in S phase. On the basis of these results, we propose that nuclear export of E1 is required, at least in part, to limit accumulation of this viral helicase in the nucleus in order to prevent its detrimental effect on cellular proliferation.
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Effective formation of the segregation-competent complex determines successful partitioning of the bovine papillomavirus genome during cell division. J Virol 2010; 84:11175-88. [PMID: 20810736 DOI: 10.1128/jvi.01366-10] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
Effective segregation of the bovine papillomavirus type 1 (BPV1), Epstein-Barr virus (EBV), and Kaposi's sarcoma-associated human herpesvirus type 8 (KSHV) genomes into daughter cells is mediated by a single viral protein that tethers viral genomes to host mitotic chromosomes. The linker proteins that mediate BPV1, EBV, and KSHV segregation are E2, LANA1, and EBNA1, respectively. The N-terminal transactivation domain of BPV1 E2 is responsible for chromatin attachment and subsequent viral genome segregation. Because E2 transcriptional activation and chromatin attachment functions are not mutually exclusive, we aimed to determine the requirement of these activities during segregation by analyzing chimeric E2 proteins. This approach allowed us to separate the two activities. Our data showed that attachment of the segregation protein to chromatin is not sufficient for proper segregation. Rather, formation of a segregation-competent complex which carries multiple copies of the segregation protein is required. Complementation studies of E2 functional domains indicated that chromatin attachment and transactivation functions must act in concert to ensure proper plasmid segregation. These data indicate that there are specific interactions between linker molecules and transcription factors/complexes that greatly increase segregation-competent complex formation. We also showed, using hybrid E2 molecules, that restored segregation function does not involve interactions with Brd4.
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42
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Abstract
The papillomavirus E2 open reading frame encodes the full-length E2 protein as well as an alternatively spliced product called E8;E2C. E8;E2C has been best studied for the high-risk human papillomaviruses, where it has been shown to regulate viral genome levels and, like the full-length E2 protein, to repress transcription from the viral promoter that directs the expression of the viral E6 and E7 oncogenes. The repression function of E8;E2C is dependent on the 12-amino-acid N-terminal sequence from the E8 open reading frame (ORF). In order to understand the mechanism by which E8;E2C mediates transcriptional repression, we performed an unbiased proteomic analysis from which we identified six high-confidence candidate interacting proteins (HCIPs) for E8;E2C; the top two are NCoR1 and TBLR1. We established an interaction of E8;E2C with an NCoR1/HDAC3 complex and demonstrated that this interaction requires the wild-type E8 open reading frame. Small interfering RNA (siRNA) knockdown studies demonstrated the involvement of NCoR1/HDAC3 in the E8;E2C-dependent repression of the viral long control region (LCR) promoter. Additional genetic work confirmed that the papillomavirus E2 and E8;E2C proteins repress transcription through distinct mechanisms.
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43
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Interaction of the betapapillomavirus E2 tethering protein with mitotic chromosomes. J Virol 2010; 84:543-57. [PMID: 19846509 DOI: 10.1128/jvi.01908-09] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
During persistent papillomavirus infection, the viral E2 protein tethers the viral genome to the host cell chromosomes, ensuring maintenance and segregation of the viral genome during cell division. However, E2 proteins from different papillomaviruses interact with distinct chromosomal regions and targets. The tethering mechanism has been best characterized for bovine papillomavirus type 1 (BPV1), where the E2 protein tethers the viral genome to mitotic chromosomes in complex with the cellular bromodomain protein, Brd4. In contrast, the betapapillomavirus human papillomavirus type 8 (HPV8) E2 protein binds to the repeated ribosomal DNA genes that are found on the short arm of human acrocentric chromosomes. In this study, we show that a short 16-amino-acid peptide from the hinge region and the C-terminal DNA binding domain of HPV8 E2 are necessary and sufficient for interaction with mitotic chromosomes. This 16-amino-acid region contains an RXXS motif that is highly conserved among betapapillomaviruses, and both arginine 250 and serine 253 residues within this motif are required for mitotic chromosome binding. The HPV8 E2 proteins are highly phosphorylated, and serine 253 is a site of phosphorylation. The HPV8 E2 chromosome binding sequence also has sequence similarity with chromosome binding regions in the gammaherpesvirus EBNA and LANA tethering proteins.
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44
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Abrogation of the Brd4-positive transcription elongation factor B complex by papillomavirus E2 protein contributes to viral oncogene repression. J Virol 2010; 84:76-87. [PMID: 19846528 DOI: 10.1128/jvi.01647-09] [Citation(s) in RCA: 55] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023] Open
Abstract
The cellular bromodomain protein Brd4 is a major interacting partner of the papillomavirus (PV) E2 protein. Interaction of E2 with Brd4 contributes to viral episome maintenance. The E2-Brd4 interaction also plays an important role in repressing viral oncogene expression from the integrated viral genome in human PV (HPV)-positive cancer cells. However, the underlying mechanism is not clearly understood. In host cells, Brd4 recruits positive transcription elongation factor b (P-TEFb) to stimulate RNA polymerase II phosphorylation during cellular and viral gene expression. P-TEFb associates with the C terminus of Brd4, which largely overlaps with the E2 binding site on Brd4. In this study, we demonstrate that E2 binding to Brd4 inhibits the interaction of endogenous Brd4 and P-TEFb. P-TEFb is essential for viral oncogene E6/E7 transcription in both HeLa and CaSki cells that contain integrated HPV genomes. E2 binding to Brd4 abrogates the recruitment of P-TEFb to the integrated viral chromatin template, leading to inactivation of P-TEFb and repression of the viral oncogene E6/E7. Furthermore, dissociation of the Brd4-P-TEFb complex from the integrated viral chromatin template using a Brd4 bromodomain dominant-negative inhibitor also hampers HPV E6/E7 oncogene expression. Our data support that Brd4 recruitment of P-TEFb to the viral chromatin template is essential for viral oncogene expression. Abrogation of the interaction between P-TEFb and Brd4 thus provides a mechanism for E2-mediated repression of the viral oncogenes from the integrated viral genomes in cancer cells.
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45
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Ottinger M, Smith JA, Schweiger MR, Robbins D, Powell MLC, You J, Howley PM. Cell-type specific transcriptional activities among different papillomavirus long control regions and their regulation by E2. Virology 2009; 395:161-71. [PMID: 19836046 DOI: 10.1016/j.virol.2009.09.027] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/11/2008] [Revised: 12/26/2008] [Accepted: 09/23/2009] [Indexed: 10/20/2022]
Abstract
This study systematically examined the viral long control region (LCR) activities and their responses to E2 for human papillomavirus (HPV) types 11, 16, and 18 as well as bovine papillomavirus 1 (BPV1) in a number of different cell types, including human cervical cancer cell lines, human oral keratinocytes, BJ fibroblasts, as well as CV1 cells. The study revealed cell- and virus-type specific differences among the individual LCRs and their regulation by E2. In addition, the integration of the LCR into the host genome was identified as a critical determinant for LCR activity and its response to E2. Collectively, these data indicate a more complex level of transcriptional regulation of the LCR by cellular and viral factors than previously appreciated, including a comparatively low LCR activity and poor E2 responsiveness for HPV16 in most human cells. This study should provide a valuable framework for future transcriptional studies in the papillomavirus field.
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Affiliation(s)
- Matthias Ottinger
- Department of Pathology, Harvard Medical School, 77 Avenue Louis Pasteur, Boston, MA 02115, USA
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Chiang CM. Brd4 engagement from chromatin targeting to transcriptional regulation: selective contact with acetylated histone H3 and H4. F1000 BIOLOGY REPORTS 2009; 1:98. [PMID: 20495683 PMCID: PMC2873783 DOI: 10.3410/b1-98] [Citation(s) in RCA: 96] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
Bromodomain-containing protein 4 (Brd4) contains two tandem bromodomains (BD1 and BD2) that bind preferentially to acetylated lysine residues found in histones and nonhistone proteins. This molecular recognition allows Brd4 to associate with acetylated chromatin throughout the cell cycle and regulates transcription at targeted loci. Recruitment of positive transcription elongation factor b, and possibly the general initiation cofactor Mediator as well, plays an important role in Brd4-regulated transcription. Selective contacts with acetyl-lysines in nucleosomal histones and chromatin-binding factors likely provide a molecular switch modulating the steps from chromatin targeting to transcriptional regulation, thus further expanding the ‘acetylation code’ for combinatorial regulation in eukaryotes.
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Affiliation(s)
- Cheng-Ming Chiang
- Simmons Comprehensive Cancer Center, Department of Biochemistry, and Department of Pharmacology, University of Texas Southwestern Medical Center 5323 Harry Hines Boulevard, Dallas, Texas 75390 USA
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You J. Papillomavirus interaction with cellular chromatin. BIOCHIMICA ET BIOPHYSICA ACTA-GENE REGULATORY MECHANISMS 2009; 1799:192-9. [PMID: 19786128 DOI: 10.1016/j.bbagrm.2009.09.009] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/27/2009] [Revised: 09/14/2009] [Accepted: 09/17/2009] [Indexed: 11/26/2022]
Abstract
High-risk human papillomavirus (HPV) infection is the primary risk factor for cervical cancer. HPVs establish persistent infection by maintaining their genomes as extrachromosomal elements (episomes) that replicate along with host DNA in infected cells. The productive life cycle of HPV is intimately tied to the differentiation program of host squamous epithelium. This review examines the involvement of host chromatin in multiple aspects of the papillomavirus life cycle and the malignant progression of infected host cells. Papillomavirus utilizes host mitotic chromosomes as vehicles for transmitting its genetic materials across the cell cycle. By hitchhiking on host mitotic chromosomes, the virus ensures accurate segregation of the replicated viral episomes to the daughter cells during host cell division. This strategy allows persistent maintenance of the viral episome in the infected cells. In the meantime, the virus subverts the host chromatin-remodeling factors to promote viral transcription and efficient propagation of viral genomes. By associating with the host chromatin, papillomavirus redirects the normal cellular control of chromatin to create a cellular environment conducive to both its own survival and malignant progression of host cells. Comprehensive understanding of HPV-host chromatin interaction will offer new insights into the HPV life cycle as well as chromatin regulation. This virus-host interaction will also provide a paradigm for investigating other episomal DNA tumor viruses that share a similar mechanism for interacting with host chromatin.
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Affiliation(s)
- Jianxin You
- Department of Microbiology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA.
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Lehoux M, D'Abramo CM, Archambault J. Molecular mechanisms of human papillomavirus-induced carcinogenesis. Public Health Genomics 2009; 12:268-80. [PMID: 19684440 DOI: 10.1159/000214918] [Citation(s) in RCA: 79] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023] Open
Abstract
Approximately 20% of all cancers are associated with infectious agents. Among them, human papillomaviruses (HPVs) are very common and are now recognized as the etiological agent of cervical cancer, the second most common cancer in women worldwide, and they are increasingly linked with other forms of dysplasia. Carcinogenesis is a complex and multistep process requiring the acquisition of several genetic and/or epigenetic alterations. HPV-induced neoplasia, however, is in part mediated by the intrinsic functions of the viral proteins. In order to replicate its genome, HPV modulates the cell cycle, while deploying mechanisms to escape the host immune response, cellular senescence and apoptosis. As such, HPV infection leads directly and indirectly to genomic instability, further favouring transforming genetic events and progression to malignancy. This review aims to summarize our current understanding of the molecular mechanisms exploited by HPV to induce neoplasia, with an emphasis on the role of the 2 viral oncoproteins E6 and E7. Greater understanding of the role of HPV proteins in these processes will ultimately aid in the development of antiviral therapies, as well as unravel general mechanisms of oncogenesis.
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Affiliation(s)
- Michaël Lehoux
- Laboratory of Molecular Virology, Institut de Recherches Cliniques de Montréal, Montreal, Que., H2W 1R7 Canada
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Regulation of aurora B expression by the bromodomain protein Brd4. Mol Cell Biol 2009; 29:5094-103. [PMID: 19596781 DOI: 10.1128/mcb.00299-09] [Citation(s) in RCA: 58] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023] Open
Abstract
The bromodomain protein Brd4 plays critical roles in cellular proliferation and cell cycle progression. In this study, we investigated the involvement of Brd4 in cell cycle regulation and observed aberrant chromosome segregation and failures in cytokinesis in cancer cells as well as in primary keratinocytes in which Brd4 has been knocked down by RNA interference. Suppression of Brd4 protein levels in proliferating cells decreased Aurora B protein and transcript levels and abolished its chromosomal distribution. In contrast, exogenous Brd4 expression stimulated Aurora B promoter reporter activity and upregulated endogenous Aurora B expression. Aurora B kinase is a chromosomal passenger protein that is essential for chromosome segregation and cytokinesis. Either overexpression of Aurora B or its inactivation can induce defects in centrosome function, spindle assembly, chromosome alignment, and cytokinesis in various cancer cells. The impaired regulation of Aurora B expression in human cells by Brd4 knockdown or overexpression coincided with mitotic catastrophe and multinucleation that are typically observed when Aurora B is inactivated or overexpressed. Overall, our data suggest that Brd4 is essential for the maintenance of the cell cycle progression mediated at least in part through the control of transcription of the Aurora B kinase cell cycle regulatory gene.
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Abstract
The papillomavirus (PV) E2 protein is an important regulator of the viral life cycle. It has diverse roles in viral transcription, DNA replication, and genome maintenance. Our laboratory has previously identified the cellular bromodomain protein Brd4 as a key interacting partner of E2. Brd4 mediates the transcriptional activation function of E2 and plays an important role in viral genome maintenance in dividing cells. E2 interacts with the C-terminal domain (CTD) of Brd4, and the CTD functions in a dominant-negative manner through binding E2 and interfering with E2's interaction with the full-length Brd4 protein. Previous studies have shown that PV E2 proteins are short lived; however, the mechanisms regulating their stability and degradation have not yet been well established. In this study, we explored the role of Brd4 in the regulation of bovine PV 1 (BPV1) and human PV 16 (HPV16) E2 stability. Expression of the Brd4 CTD dramatically increases E2 levels. Both BPV1 E2 and HPV16 E2 are regulated by ubiquitylation, and Brd4 CTD expression blocks this ubiquitylation, thus stabilizing the E2 protein. Furthermore, we have identified the cullin-based E3 ligases and specifically cullin-3 as potential components of the ubiquitylation machinery that targets both BPV1 and HPV16 E2 for ubiquitylation. Expression of the Brd4 CTD blocks the interaction between E2 and the cullin-3 complex. In addition to Brd4's role in mediating E2 transcription and genome tethering activities, these data suggest a potential role for Brd4 in regulating E2 stability and protein levels within PV-infected cells.
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