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Bellon M, Nicot C. HTLV-1 Tax Tug-of-War: Cellular Senescence and Death or Cellular Transformation. Pathogens 2024; 13:87. [PMID: 38276160 PMCID: PMC10820833 DOI: 10.3390/pathogens13010087] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/19/2023] [Revised: 01/11/2024] [Accepted: 01/15/2024] [Indexed: 01/27/2024] Open
Abstract
Human T cell leukemia virus type 1 (HTLV-1) is a retrovirus associated with a lymphoproliferative disease known as adult T cell leukemia/lymphoma (ATLL). HTLV-1 infection efficiently transforms human T cells in vivo and in vitro. The virus does not transduce a proto-oncogene, nor does it integrate into tumor-promoting genomic sites. Instead, HTLV-1 uses a random mutagenesis model, resulting in cellular transformation. Expression of the viral protein Tax is critical for the immortalization of infected cells by targeting specific cellular signaling pathways. However, Tax is highly immunogenic and represents the main target for the elimination of virally infected cells by host cytotoxic T cells (CTLs). In addition, Tax expression in naïve cells induces pro-apoptotic signals and has been associated with the induction of non-replicative cellular senescence. This review will explore these conundrums and discuss the mechanisms used by the Tax viral oncoprotein to influence life-and-death cellular decisions and affect HTLV-1 pathogenesis.
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Affiliation(s)
| | - Christophe Nicot
- Department of Pathology and Laboratory Medicine, University of Kansas Medical Center, 3901 Rainbow Blvd, Kansas City, KS 66160, USA;
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Benedetti F, Curreli S, Gallo RC, Zella D. Tampering of Viruses and Bacteria with Host DNA Repair: Implications for Cellular Transformation. Cancers (Basel) 2021; 13:E241. [PMID: 33440726 PMCID: PMC7826954 DOI: 10.3390/cancers13020241] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/23/2020] [Revised: 01/06/2021] [Accepted: 01/07/2021] [Indexed: 02/07/2023] Open
Abstract
A reduced ability to properly repair DNA is linked to a variety of human diseases, which in almost all cases is associated with an increased probability of the development of cellular transformation and cancer. DNA damage, that ultimately can lead to mutations and genomic instability, is due to many factors, such as oxidative stress, metabolic disorders, viral and microbial pathogens, excess cellular proliferation and chemical factors. In this review, we examine the evidence connecting DNA damage and the mechanisms that viruses and bacteria have evolved to hamper the pathways dedicated to maintaining the integrity of genetic information, thus affecting the ability of their hosts to repair the damage(s). Uncovering new links between these important aspects of cancer biology might lead to the development of new targeted therapies in DNA-repair deficient cancers and improving the efficacy of existing therapies. Here we provide a comprehensive summary detailing the major mechanisms that viruses and bacteria associated with cancer employ to interfere with mechanisms of DNA repair. Comparing these mechanisms could ultimately help provide a common framework to better understand how certain microorganisms are involved in cellular transformation.
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Affiliation(s)
- Francesca Benedetti
- Institute of Human Virology and Global Virus Network Center, Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, MD 21201, USA;
| | - Sabrina Curreli
- Institute of Human Virology and Global Virus Network Center, Department of Medicine, University of Maryland School of Medicine, Baltimore, MD 21201, USA; (S.C.); (R.C.G.)
| | - Robert C. Gallo
- Institute of Human Virology and Global Virus Network Center, Department of Medicine, University of Maryland School of Medicine, Baltimore, MD 21201, USA; (S.C.); (R.C.G.)
| | - Davide Zella
- Institute of Human Virology and Global Virus Network Center, Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, MD 21201, USA;
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Mohanty S, Harhaj EW. Mechanisms of Oncogenesis by HTLV-1 Tax. Pathogens 2020; 9:E543. [PMID: 32645846 PMCID: PMC7399876 DOI: 10.3390/pathogens9070543] [Citation(s) in RCA: 45] [Impact Index Per Article: 11.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/31/2020] [Revised: 06/24/2020] [Accepted: 07/01/2020] [Indexed: 01/23/2023] Open
Abstract
The human T-cell lymphotropic virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia/lymphoma (ATLL), a neoplasm of CD4+CD25+ T cells that occurs in 2-5% of infected individuals after decades of asymptomatic latent infection. Multiple HTLV-1-encoded regulatory proteins, including Tax and HTLV-1 basic leucine zipper factor (HBZ), play key roles in viral persistence and latency. The HTLV-1 Tax oncoprotein interacts with a plethora of host cellular proteins to regulate viral gene expression and also promote the aberrant activation of signaling pathways such as NF-κB to drive clonal proliferation and survival of T cells bearing the HTLV-1 provirus. Tax undergoes various post-translational modifications such as phosphorylation and ubiquitination that regulate its function and subcellular localization. Tax shuttles in different subcellular compartments for the activation of anti-apoptotic genes and deregulates the cell cycle with the induction of DNA damage for the accumulation of genomic instability that can result in cellular immortalization and malignant transformation. However, Tax is highly immunogenic and therefore HTLV-1 has evolved numerous strategies to tightly regulate Tax expression while maintaining the pool of anti-apoptotic genes through HBZ. In this review, we summarize the key findings on the oncogenic mechanisms used by Tax that set the stage for the development of ATLL, and the strategies used by HTLV-1 to tightly regulate Tax expression for immune evasion and viral persistence.
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Affiliation(s)
| | - Edward W. Harhaj
- Department of Microbiology and Immunology, Penn State College of Medicine, Hershey, PA 17033, USA;
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Song Y, Rundberget JT, Evenseth LM, Xie L, Gomes T, Høgåsen T, Iguchi T, Tollefsen KE. Whole-Organism Transcriptomic Analysis Provides Mechanistic Insight into the Acute Toxicity of Emamectin Benzoate in Daphnia magna. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2016; 50:11994-12003. [PMID: 27704796 DOI: 10.1021/acs.est.6b03456] [Citation(s) in RCA: 30] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
Emamectin benzoate (EMB) is an antisea lice chemical widely used in the aquaculture that may also unintentionally affect nontarget crustaceans in the environment. Although the adverse effects of this compound are well documented in various species, the full modes of action (MoAs) are still not well characterized. The current study was therefore conducted to characterize the MoAs of EMB and link perturbations of key toxicological pathways to adverse effects in the model freshwater crustacean Daphnia magna. Effects on molting and survival were determined after 48 h exposure to EMB, whereas global transcriptional changes and the ecdysone receptor (EcR) binding potency was determined to characterize the MoA. The results showed that the molting frequency and survival of D. magna decreased in a concentration-dependent manner, and the observed changes could not be attributed to direct interactions with the EcR. Major MoAs such as activation of glutamate-gated chloride channels and gamma-aminobutyric acid signaling, disruption of neuroendocrine regulation of molting, perturbation of energy homeostasis, suppression of DNA repair and induction of programmed cell death were observed by transcriptional analysis and successfully linked to the adverse effects. This study has demonstrated that acute exposure to intermediate and high pM levels of EMB may pose hazards to nontarget crustaceans in the aquatic environment.
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Affiliation(s)
- You Song
- Norwegian Institute for Water Research (NIVA), Section of Ecotoxicology and Risk Assessment , Gaustadalléen 21, N-0349 Oslo, Norway
| | - Jan Thomas Rundberget
- Norwegian Institute for Water Research (NIVA), Section of Ecotoxicology and Risk Assessment , Gaustadalléen 21, N-0349 Oslo, Norway
| | - Linn Mari Evenseth
- Department of Medical Biology, Faculty of Health Sciences, University of Tromsø-The Arctic University of Norway , NO-9037 Tromsø, Norway
| | - Li Xie
- Norwegian Institute for Water Research (NIVA), Section of Ecotoxicology and Risk Assessment , Gaustadalléen 21, N-0349 Oslo, Norway
- Norwegian University of Life Sciences (NMBU) , Faculty of Environmental Science and Technology, Department of Environmental Sciences (IMV), Centre for Environmental Radioactivity (CERAD). P.O. Box 5003, N-1432 Ås, Oslo, Norway
| | - Tânia Gomes
- Norwegian Institute for Water Research (NIVA), Section of Ecotoxicology and Risk Assessment , Gaustadalléen 21, N-0349 Oslo, Norway
- Norwegian University of Life Sciences (NMBU) , Faculty of Environmental Science and Technology, Department of Environmental Sciences (IMV), Centre for Environmental Radioactivity (CERAD). P.O. Box 5003, N-1432 Ås, Oslo, Norway
| | - Tore Høgåsen
- Norwegian Institute for Water Research (NIVA), Section of Ecotoxicology and Risk Assessment , Gaustadalléen 21, N-0349 Oslo, Norway
| | - Taisen Iguchi
- Department of Basic Biology, Faculty of Life Science, SOKENDAI, Graduate University for Advanced Studies , 5-1 Higashiyama, Myodaiji, Okazaki, Aichi 444-8787, Japan
- Okazaki Institute for Integrative Bioscience, National Institute for Basic Biology (NIBB), National Institutes of Natural Sciences , 5-1 Higashiyama, Myodaiji, Okazaki, Aichi 444-8787, Japan
| | - Knut Erik Tollefsen
- Norwegian Institute for Water Research (NIVA), Section of Ecotoxicology and Risk Assessment , Gaustadalléen 21, N-0349 Oslo, Norway
- Norwegian University of Life Sciences (NMBU) , Faculty of Environmental Science and Technology, Department of Environmental Sciences (IMV), Centre for Environmental Radioactivity (CERAD). P.O. Box 5003, N-1432 Ås, Oslo, Norway
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Nicot C. HTLV-I Tax-Mediated Inactivation of Cell Cycle Checkpoints and DNA Repair Pathways Contribute to Cellular Transformation: "A Random Mutagenesis Model". ACTA ACUST UNITED AC 2015; 2. [PMID: 26835512 DOI: 10.13188/2377-9292.1000009] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
Abstract
To achieve cellular transformation, most oncogenic retroviruses use transduction by proto-oncogene capture or insertional mutagenesis, whereby provirus integration disrupts expression of tumor suppressors or proto-oncogenes. In contrast, the Human T-cell leukemia virus type 1 (HTLV-I) has been classified in a separate class referred to as "transactivating retroviruses". Current views suggest that the viral encoded Tax protein transactivates expression of cellular genes leading to deregulated growth and transformation. However, if Tax-mediated transactivation was indeed sufficient for cellular transformation, a fairly high frequency of infected cells would eventually become transformed. In contrast, the frequency of transformation by HTLV-I is very low, likely less than 5%. This review will discuss the current understanding and recent discoveries highlighting critical functions of Tax in cellular transformation. HTLV-I Tax carries out essential functions in order to override cell cycle checkpoints and deregulate cellular division. In addition, Tax expression is associated with increased DNA damage and genome instability. Since Tax can inhibit multiple DNA repair pathways and stimulate unfaithful DNA repair or bypass checkpoints, these processes allow accumulation of genetic mutations in the host genome. Given this, a "Random Mutagenesis" transformation model seems more suitable to characterize the oncogenic activities of HTLV-I.
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Affiliation(s)
- Christophe Nicot
- Department of Pathology and Laboratory Medicine, Center for Viral Oncology, University of Kansas Medical Center, USA
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Baydoun HH, Cherian MA, Green P, Ratner L. Inducible nitric oxide synthase mediates DNA double strand breaks in Human T-Cell Leukemia Virus Type 1-induced leukemia/lymphoma. Retrovirology 2015; 12:71. [PMID: 26265053 PMCID: PMC4534112 DOI: 10.1186/s12977-015-0196-y] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/13/2015] [Accepted: 07/30/2015] [Indexed: 01/23/2023] Open
Abstract
Background Adult T-cell leukemia/lymphoma (ATLL) is an aggressive and fatal malignancy of CD4+ T-lymphocytes infected by the Human T-Cell Virus Type 1 (HTLV-1). The molecular mechanisms of transformation in ATLL have not been fully elucidated. However, genomic instability and cumulative DNA damage during the long period of latency is believed to be essential for HTLV-1 induced leukemogenesis. In addition, constitutive activation of the NF-κB pathway was found to be a critical determinant for transformation. Whether a connection exists between NF-κB activation and accumulation of DNA damage is not clear. We recently found that the HTLV-1 viral oncoprotein, Tax, the activator of the NF-κB pathway, induces DNA double strand breaks (DSBs). Results Here, we investigated whether any of the NF-κB target genes are critical in inducing DSBs. Of note, we found that inducible nitric oxide synthase (iNOS) that catalyzes the production of nitric oxide (NO) in macrophages, neutrophils and T-cells is over expressed in HTLV-1 infected and Tax-expressing cells. Interestingly, we show that in HTLV-1 infected cells, iNOS expression is Tax-dependent and specifically requires the activation of the classical NF-κB and JAK/STAT pathways. A dramatic reduction of DSBs was observed when NO production was inhibited, indicating that Tax induces DSBs through the activation of NO synthesis. Conclusions Determination of the impact of NO on HTLV-1-induced leukemogenesis opens a new area for treatment or prevention of ATLL and perhaps other cancers in which NO is produced.
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Affiliation(s)
- Hicham H Baydoun
- Division of Molecular Oncology, Department of Medicine Campus, Washington University School of Medicine, 660 South Euclid Avenue, St Louis, MO, 63110, USA.
| | - Mathew A Cherian
- Division of Molecular Oncology, Department of Medicine Campus, Washington University School of Medicine, 660 South Euclid Avenue, St Louis, MO, 63110, USA.
| | - Patrick Green
- Department of Veterinary Biosciences, The Ohio State University, Columbus, OH, USA.
| | - Lee Ratner
- Division of Molecular Oncology, Department of Medicine Campus, Washington University School of Medicine, 660 South Euclid Avenue, St Louis, MO, 63110, USA. .,Department of Molecular Microbiology, Washington University School of Medicine, St Louis, MO, USA.
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Abstract
Human T-cell leukemia virus type 1 (HTLV-1) is a retrovirus that causes cancer (Adult T cell Leukemia, ATL) and a spectrum of inflammatory diseases (mainly HTLV-associated myelopathy—tropical spastic paraparesis, HAM/TSP). Since virions are particularly unstable, HTLV-1 transmission primarily occurs by transfer of a cell carrying an integrated provirus. After transcription, the viral genomic RNA undergoes reverse transcription and integration into the chromosomal DNA of a cell from the newly infected host. The virus then replicates by either one of two modes: (i) an infectious cycle by virus budding and infection of new targets and (ii) mitotic division of cells harboring an integrated provirus. HTLV-1 replication initiates a series of mechanisms in the host including antiviral immunity and checkpoint control of cell proliferation. HTLV-1 has elaborated strategies to counteract these defense mechanisms allowing continuous persistence in humans.
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Tumor Suppressor Inactivation in the Pathogenesis of Adult T-Cell Leukemia. JOURNAL OF ONCOLOGY 2015; 2015:183590. [PMID: 26170835 PMCID: PMC4478360 DOI: 10.1155/2015/183590] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 03/14/2015] [Accepted: 05/24/2015] [Indexed: 12/12/2022]
Abstract
Tumor suppressor functions are essential to control cellular proliferation, to activate the apoptosis or senescence pathway to eliminate unwanted cells, to link DNA damage signals to cell cycle arrest checkpoints, to activate appropriate DNA repair pathways, and to prevent the loss of adhesion to inhibit initiation of metastases. Therefore, tumor suppressor genes are indispensable to maintaining genetic and genomic integrity. Consequently, inactivation of tumor suppressors by somatic mutations or epigenetic mechanisms is frequently associated with tumor initiation and development. In contrast, reactivation of tumor suppressor functions can effectively reverse the transformed phenotype and lead to cell cycle arrest or death of cancerous cells and be used as a therapeutic strategy. Adult T-cell leukemia/lymphoma (ATLL) is an aggressive lymphoproliferative disease associated with infection of CD4 T cells by the Human T-cell Leukemia Virus Type 1 (HTLV-I). HTLV-I-associated T-cell transformation is the result of a multistep oncogenic process in which the virus initially induces chronic T-cell proliferation and alters cellular pathways resulting in the accumulation of genetic defects and the deregulated growth of virally infected cells. This review will focus on the current knowledge of the genetic and epigenetic mechanisms regulating the inactivation of tumor suppressors in the pathogenesis of HTLV-I.
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Currer R, Van Duyne R, Jaworski E, Guendel I, Sampey G, Das R, Narayanan A, Kashanchi F. HTLV tax: a fascinating multifunctional co-regulator of viral and cellular pathways. Front Microbiol 2012; 3:406. [PMID: 23226145 PMCID: PMC3510432 DOI: 10.3389/fmicb.2012.00406] [Citation(s) in RCA: 103] [Impact Index Per Article: 8.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/11/2012] [Accepted: 11/12/2012] [Indexed: 12/18/2022] Open
Abstract
Human T-cell lymphotropic virus type 1 (HTLV-1) has been identified as the causative agent of adult T-cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The virus infects between 15 and 20 million people worldwide of which approximately 2-5% develop ATL. The past 35 years of research have yielded significant insight into the pathogenesis of HTLV-1, including the molecular characterization of Tax, the viral transactivator, and oncoprotein. In spite of these efforts, the mechanisms of oncogenesis of this pleiotropic protein remain to be fully elucidated. In this review, we illustrate the multiple oncogenic roles of Tax by summarizing a recent body of literature that refines our understanding of cellular transformation. A focused range of topics are discussed in this review including Tax-mediated regulation of the viral promoter and other cellular pathways, particularly the connection of the NF-κB pathway to both post-translational modifications (PTMs) of Tax and subcellular localization. Specifically, recent research on polyubiquitination of Tax as it relates to the activation of the IkappaB kinase (IKK) complex is highlighted. Regulation of the cell cycle and DNA damage responses due to Tax are also discussed, including Tax interaction with minichromosome maintenance proteins and the role of Tax in chromatin remodeling. The recent identification of HTLV-3 has amplified the importance of the characterization of emerging viral pathogens. The challenge of the molecular determination of pathogenicity and malignant disease of this virus lies in the comparison of the viral transactivators of HTLV-1, -2, and -3 in terms of transformation and immortalization. Consequently, differences between the three proteins are currently being studied to determine what factors are required for the differences in tumorogenesis.
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Affiliation(s)
- Robert Currer
- National Center for Biodefense and Infectious Diseases, George Mason University Manassas, VA, USA
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HTLV-I tax increases genetic instability by inducing DNA double strand breaks during DNA replication and switching repair to NHEJ. PLoS One 2012; 7:e42226. [PMID: 22916124 PMCID: PMC3423393 DOI: 10.1371/journal.pone.0042226] [Citation(s) in RCA: 48] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/29/2012] [Accepted: 07/02/2012] [Indexed: 01/23/2023] Open
Abstract
BACKGROUND Appropriate responses to damaged DNA are indispensible for preserving genome stability and preventing cancer. Tumor viruses often target DNA repair machinery to achieve transformation. The Human T-cell leukemia virus type I (HTLV-I) is the only known transforming human retrovirus and the etiological agent of Adult T-cell Leukemia (ATLL). Although HTLV-I-transformed leukemic cells have numerous genetic lesions, the precise role of the viral tax gene in this process is not fully understood. RESULTS Our results show a novel function of HTLV-I oncoprotein Tax as an inducer of genomic DNA double strand breaks (DDSB) during DNA replication. We also found that Tax acts as a potent inhibitor of homologous recombination (HR) DNA repair through the activation of the NF-kB pathway. These results were confirmed using HTLV-I molecular clones expressing Tax at physiological levels in a natural context. We further found that HTLV-I- and Tax-transformed cells are not more susceptible to DNA damaging agents and repair DNA lesions at a rate similar to that of normal cells. Finally, we demonstrated that during S phase, Tax-associated DDSB are preferentially repaired using the error-prone non-homologous end joining (NHEJ) pathway. CONCLUSIONS This study provides new insights in Tax effects on DNA repair and genome instability. Although it may not be self sufficient, the creation of DNA breaks and subsequent abnormal use of the non-conservative NHEJ DNA repair during the S phase in HTLV-I-infected Tax-expressing cells may cooperate with other factors to increase genetic and genome instability and favor transformation.
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Lodewick J, Lamsoul I, Bex F. Move or die: the fate of the Tax oncoprotein of HTLV-1. Viruses 2011; 3:829-57. [PMID: 21994756 PMCID: PMC3185767 DOI: 10.3390/v3060829] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/14/2011] [Revised: 05/31/2011] [Accepted: 06/01/2011] [Indexed: 12/12/2022] Open
Abstract
The HTLV-1 Tax protein both activates viral replication and is involved in HTLV-1-mediated transformation of T lymphocytes. The transforming properties of Tax include altering the expression of select cellular genes via activation of cellular pathways and perturbation of both cell cycle control mechanisms and apoptotic signals. The recent discovery that Tax undergoes a hierarchical sequence of posttranslational modifications that control its intracellular localization provides provocative insights into the mechanisms regulating Tax transcriptional and transforming activities.
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Affiliation(s)
- Julie Lodewick
- Institut de Recherches Microbiologiques J-M Wiame, Université Libre de Bruxelles, B-1070 Bruxelles, Belgium.
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Human T-cell leukemia virus type 1 (HTLV-1) and leukemic transformation: viral infectivity, Tax, HBZ and therapy. Oncogene 2010; 30:1379-89. [PMID: 21119600 DOI: 10.1038/onc.2010.537] [Citation(s) in RCA: 201] [Impact Index Per Article: 14.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
The human T-cell leukemia virus type 1 (HTLV-1) was the first retrovirus discovered to be causative of a human cancer, adult T-cell leukemia. The transforming entity of HTLV-1 has been attributed to the virally-encoded oncoprotein, Tax. Unlike the v-onc proteins encoded by other oncogenic animal retroviruses that transform cells, Tax does not originate from a c-onc counterpart. In this article, we review progress in our understanding of HTLV-1 infectivity, cellular transformation, anti-sense transcription and therapy, 30 years after the original discovery of this virus.
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Jeang KT, Giam CZ, Majone F, Aboud M. HTLV-1 Tax: Linking transformation, DNA damage and apoptotic T-cell death. J Biol Chem 2010; 279:31991-4. [PMID: 15090550 DOI: 10.1074/jbc.r400009200] [Citation(s) in RCA: 140] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
The human T-cell leukemia virus type I (HTLV-1) is the causative agent of adult T-cell leukemia (ATL), an aggressive CD4-positive T-cell neoplasia. The HTLV-1 proto-oncogene Tax, a potent transcriptional activator of cellular and viral genes, is thought to play a pivotal role in the transforming properties of the virus by deregulating intracellular signaling pathways. During the course of HTLV-1 infection, the dysregulation of cell-cycle checkpoints and the suppression of DNA damage repair is tightly linked to the activity of the viral oncoprotein Tax. Tax activity is associated with production of reactive oxygen intermediates (ROS), chromosomal instability and DNA damage, apoptotic cell death and cellular transformation. Changes in the intracellular redox status induced by Tax promote DNA damage. Tax-mediated DNA damage is believed to be essential in initiating the transformation process by subjecting infected T cells to genetic changes that eventually promote the neoplastic state. Apoptosis and immune surveillance would then exert the necessary selection pressure for eliminating the majority of virally infected cells, while escape variants acquiring a mutator phenotype would constitute a subpopulation of genetically altered cells prone to neoplasia. While the potency of Tax-activity seems to be a determining factor for the observed effects, the cooperation of Tax with other viral proteins determines the fate and progression of HTLV-1-infected cells through DNA damage, apoptosis, survival and transformation.
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Affiliation(s)
- Kuan-Teh Jeang
- Laboratory of Molecular Microbiology, Nattional Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA.
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Human T-cell leukemia virus type 1 Tax relieves repression of proliferating cell nuclear antigen gene expression. J Virol 2008; 82:11714-22. [PMID: 18799587 DOI: 10.1128/jvi.00356-08] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023] Open
Abstract
Human T-cell leukemia virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia. The transforming ability of Tax, the viral oncoprotein, is believed to depend on interactions with cell cycle regulators and on transactivation of genes that control cellular proliferation, including proliferating cell nuclear antigen (PCNA), a cofactor associated with DNA replication and repair. Tax associates with cellular transcription factors to alter their affinity for cognate DNA elements, leading to increased or decreased transcription from that promoter. Although it has been demonstrated that Tax transactivates the PCNA promoter, the mechanism of transcriptional activation is unknown. Here we report a cellular complex that binds specifically to a novel site within the minimal Tax-responsive element of the TATAA-less PCNA promoter. Mutation at this binding site or Tax expression inhibited complex formation and increased promoter activity, suggesting that the complex is a transcriptional repressor. The activation of PCNA gene expression by Tax and consequential decrease in nucleotide excision repair mediated by PCNA overexpression could contribute to the reduced DNA repair capacity and genomic instability observed in HTLV-1-infected cells.
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Dayaram T, Marriott SJ. Effect of transforming viruses on molecular mechanisms associated with cancer. J Cell Physiol 2008; 216:309-14. [PMID: 18366075 DOI: 10.1002/jcp.21439] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023]
Abstract
Viruses have been linked to approximately 20% of all human tumors worldwide. These transforming viruses encode viral oncoproteins that interact with cellular proteins to enhance viral replication. The transcriptional and post-transcriptional effects of these viral oncoproteins ultimately result in cellular transformation. Historically, viral research has been vital to the discovery of oncogenes and tumor suppressors with more current research aiding in unraveling some mechanisms of carcinogenesis. Interestingly, since transforming viruses affect some of the same pathways that are dysregulated in human cancers, their study enhances our understanding of the multistep process of tumorigenesis. This review will examine the cellular mechanisms targeted by oncogenic human viruses and the processes by which these effects contribute to transformation. In particular, we will focus on three transforming viruses, human T-cell leukemia virus type-I, hepatitis B virus and human papillomavirus. These viruses all encode specific oncogenes that promote cell cycle progression, inhibit DNA damage checkpoint responses and prevent programmed cell death in an effort to promote viral propagation. While the transforming properties of these viruses are probably unintended consequences of replication strategies, they provide excellent systems in which to study cancer development.
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Affiliation(s)
- Tajhal Dayaram
- Interdepartmental Program in Cell and Molecular Biology, Department of Molecular Virology and Microbiology, Baylor College of Medicine, One Baylor Plaza, Houston, Texas 77030, USA
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Laybourn P. The ups and downs of Tax and histones in adult T-cell leukemogenesis. Future Oncol 2008; 4:311-7. [PMID: 18518755 DOI: 10.2217/14796694.4.3.311] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
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17
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Schavinsky-Khrapunsky Y, Priel E, Aboud M. Dose-dependent dual effect of HTLV-1 tax oncoprotein on p53-dependent nucleotide excision repair in human T-cells. Int J Cancer 2008; 122:305-16. [PMID: 17918160 DOI: 10.1002/ijc.23091] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
In this study we investigated the effect of Tax on nucleotide excision repair (NER) in human T-cell lines by using the host cell repair analysis of UVC-irradiated reporter plasmid. This analysis revealed a p53-dependent NER activity in wild type (w.t.) p53-containing T-cells and p53-independent NER in w.t. p53-lacking T-cells. Notably, in the w.t. p53-containing cells Tax exerted a dose-dependent dual effect on NER. While low Tax doses markedly stimulated this repair, high Tax doses strongly reduced it. Further experiments demonstrated that the low Tax doses enhanced, in these cells, the level and the transcriptional function of their w.t. p53 protein. On the other hand, although the high Tax doses further increased the level of p53, they functionally inactivated its accumulating molecules. Both of these Tax effects on p53 proved to be mediated by Tax-induced NF-kappaB-related mechanisms. Together, these data suggest that by NF-kappaB activation Tax elevates the level of the cellular w.t. p53. However, while at low Tax doses the elevating w.t. p53 molecules are functionally active and capable of stimulating NER, intensifying further the NF-kappaB activation by the high Tax doses concomitantly evokes certain mechanism(s) which functionally inactivates the accumulating p53 protein. In contrast to this dual effect on the p53-dependent NER, Tax displayed only an inhibitory effect on the p53-independent NER by its high doses, whereas its low doses had no effect on this repair. The mechanisms of the NF-kappaB-associated effects on the level and function of the cellular w.t.p53 and of the p53-independent NER noted in our experimental systems are further investigated in our laboratory.
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Affiliation(s)
- Yana Schavinsky-Khrapunsky
- The Shraga Segal Department of Microbiology and Immunology, Faculty of Health Sciences and Cancer Research Center, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel
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Peloponese JM, Kinjo T, Jeang KT. Human T-cell leukemia virus type 1 Tax and cellular transformation. Int J Hematol 2007; 86:101-6. [PMID: 17875521 DOI: 10.1532/ijh97.07087] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
Infection of T-cells by human T-cell leukemia virus type 1 (HTLV-1) causes a lymphoproliferative malignancy known as adult T-cell leukemia (ATL). ATL is characterized by abnormal lymphocytes, called flower cells, which have cleaved and convoluted nuclei. Tax, encoded by the HTLV-1 pX region, is a critical nonstructural protein that plays a central role in leukemogenesis; however, the mechanisms of HTLV-1 oncogenesis have not been clarified fully. In this review, we summarize current thinking on how Tax may affect ATL leukemogenesis.
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Affiliation(s)
- Jean-Marie Peloponese
- Molecular Virology Section, Laboratory of Molecular Microbiology, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20892-0460, USA
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19
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Merling R, Chen C, Hong S, Zhang L, Liu M, Kuo YL, Giam CZ. HTLV-1 Tax mutants that do not induce G1 arrest are disabled in activating the anaphase promoting complex. Retrovirology 2007; 4:35. [PMID: 17535428 PMCID: PMC1894815 DOI: 10.1186/1742-4690-4-35] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/31/2007] [Accepted: 05/29/2007] [Indexed: 11/17/2022] Open
Abstract
HTLV-1 Tax is a potent activator of viral transcription and NF-κB. Recent data indicate that Tax activates the anaphase promoting complex/cyclosome (APC/C) ahead of schedule, causing premature degradation of cyclin A, cyclin B1, securin, and Skp2. Premature loss of these mitotic regulators is accompanied by mitotic aberrations and leads to rapid senescence and cell cycle arrest in HeLa and S. cerevisiae cells. Tax-induced rapid senescence (tax-IRS) of HeLa cells is mediated primarily by a dramatic stabilization of p27KIP and is also accompanied by a great surge in the level of p21CIP1mRNA and protein. Deficiencies in p27KIP prevent Tax-IRS. A collection of tax point mutants that permit normal growth of S. cerevisiae have been isolated. Like wild-type tax, many of them (C23W, A108T, L159F, and L235F) transactivate both the HTLV-LTR and the NF-κB reporters. One of them, V19M, preferentially activates NF-κB, but is attenuated for LTR activation. None of the mutants significantly elevated the levels of p21CIP1and p27KIP1, indicating that the dramatic surge in p21CIP1/WAF1and p27KIP 1induced by Tax is brought about by a mechanism distinct from NF-κB or LTR activation. Importantly, the ability of these mutants to activate APC/C is attenuated or abrogated. These data indicate that Tax-induced rapid senescence is causally associated with APC/C activation.
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Affiliation(s)
- Randall Merling
- Department of Microbiology and Immunology, Uniformed Services University of the Health Sciences, 4301 Jones Bridge Rd., Bethesda, MD 20814, USA
| | - Chunhua Chen
- Department of Microbiology and Immunology, Uniformed Services University of the Health Sciences, 4301 Jones Bridge Rd., Bethesda, MD 20814, USA
| | - Sohee Hong
- Department of Microbiology and Immunology, Uniformed Services University of the Health Sciences, 4301 Jones Bridge Rd., Bethesda, MD 20814, USA
| | - Ling Zhang
- Department of Microbiology and Immunology, Uniformed Services University of the Health Sciences, 4301 Jones Bridge Rd., Bethesda, MD 20814, USA
| | - Meihong Liu
- Department of Microbiology and Immunology, Uniformed Services University of the Health Sciences, 4301 Jones Bridge Rd., Bethesda, MD 20814, USA
| | - Yu-Liang Kuo
- Department of Microbiology and Immunology, Uniformed Services University of the Health Sciences, 4301 Jones Bridge Rd., Bethesda, MD 20814, USA
| | - Chou-Zen Giam
- Department of Microbiology and Immunology, Uniformed Services University of the Health Sciences, 4301 Jones Bridge Rd., Bethesda, MD 20814, USA
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20
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Banerjee P, Rochford R, Antel J, Canute G, Wrzesinski S, Sieburg M, Feuer G. Proinflammatory cytokine gene induction by human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2 Tax in primary human glial cells. J Virol 2007; 81:1690-700. [PMID: 17121800 PMCID: PMC1797548 DOI: 10.1128/jvi.01513-06] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/14/2006] [Accepted: 11/14/2006] [Indexed: 01/04/2023] Open
Abstract
Infection with human T-cell leukemia virus type 1 (HTLV-1) can result in the development of HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), a chronic inflammatory disease of the central nervous system (CNS). HTLV-2 is highly related to HTLV-1 at the genetic level and shares a high degree of sequence homology, but infection with HTLV-2 is relatively nonpathogenic compared to HTLV-1. Although the pathogenesis of HAM/TSP remains to be fully elucidated, previous evidence suggests that elevated levels of the proinflammatory cytokines in the CNS are associated with neuropathogenesis. We demonstrate that HTLV-1 infection in astrogliomas results in a robust induction of interleukin-1beta (IL-1beta), IL-1alpha, tumor necrosis factor alpha (TNF-alpha), TNF-beta, and IL-6 expression. HTLV encodes for a viral transcriptional transactivator protein named Tax that also induces the transcription of cellular genes. To investigate and compare the effects of Tax1 and Tax2 expression on the dysregulation of proinflammatory cytokines, lentivirus vectors were used to transduce primary human astrocytomas and oligodendrogliomas. The expression of Tax1 in primary human astrocytomas and oligodendrogliomas resulted in significantly higher levels of proinflammatory cytokine gene expression compared to Tax2. Notably, Tax1 expression uniquely sensitized primary human astrocytomas to apoptosis. A Tax2/Tax1 chimera encoding the C-terminal 53 amino acids of the Tax1 fused to the Tax2 gene (Tax(221)) demonstrated a phenotype that resembled Tax1, with respect to proinflammatory cytokine gene expression and sensitization to apoptosis. The patterns of differential cytokine induction and sensitization to apoptosis displayed by Tax1 and Tax2 may reflect differences relating to the heightened neuropathogenicity associated with HTLV-1 infection and the development of HAM/TSP.
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Affiliation(s)
- Prabal Banerjee
- Department of Microbiology and Immunology, SUNY Upstate Medical University, 750 East Adams Street, Syracuse, NY 13210, USA
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21
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Gatza ML, Marriott SJ. Genotoxic stress and cellular stress alter the subcellular distribution of human T-cell leukemia virus type 1 tax through a CRM1-dependent mechanism. J Virol 2006; 80:6657-68. [PMID: 16775353 PMCID: PMC1488944 DOI: 10.1128/jvi.02270-05] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/27/2005] [Accepted: 04/14/2006] [Indexed: 12/17/2022] Open
Abstract
Human T-cell leukemia virus type 1 Tax is a predominantly nuclear viral oncoprotein that colocalizes with cellular proteins in nuclear foci known as Tax speckled structures (TSS). Tax is also diffusely distributed throughout the cytoplasm, where it interacts with and affects the functions of cytoplasmic cellular proteins. Mechanisms that regulate the distribution of Tax between the cytoplasm and nucleus remain to be identified. Since Tax has been shown to promote genome instability by perturbing cell cycle progression and DNA repair mechanisms following DNA damage, we examined the effect of genotoxic stress on the subcellular distribution and interacting partners of Tax. Tax localization was altered in response to various forms of cellular stress, resulting in an increase in cytoplasmic Tax and a decrease in Tax speckled structures. Concomitantly, colocalization of Tax with sc35 (a TSS protein) decreased following stress. Tax translocation required the CRM1 nuclear export pathway, and a transient interaction between Tax and CRM1 was observed following stress. These results suggest that the subcellular distribution of Tax and the interactions between Tax and cellular proteins respond dynamically to cellular stress. Changes in Tax distribution and interacting partners are likely to affect cellular processes that regulate cellular transformation.
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Affiliation(s)
- Michael L Gatza
- Department of Molecular Virology and Microbiology, Baylor College of Medicine, One Baylor Plaza MS-385, Houston, TX 77030, USA
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22
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Marriott SJ, Semmes OJ. Impact of HTLV-I Tax on cell cycle progression and the cellular DNA damage repair response. Oncogene 2005; 24:5986-95. [PMID: 16155605 DOI: 10.1038/sj.onc.1208976] [Citation(s) in RCA: 150] [Impact Index Per Article: 7.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023]
Abstract
Human T-cell lymphotropic virus type I (HTLV-I) is the etiologic agent of adult T-cell leukemia (ATL), a rapidly progressing, clonal malignancy of CD4+ T lymphocytes. Fewer than one in 20 infected individuals typically develop ATL and the onset of this cancer occurs after decades of relatively symptom-free infection. Leukemic cells from ATL patients display extensive and varied forms of chromosomal abnormalities and this genomic instability is thought to be a major contributor to the development of ATL. HTLV-I encodes a regulatory protein, Tax, which is necessary and sufficient to transform cells and is therefore considered to be the viral oncoprotein. Tax interacts with numerous cellular proteins to reprogram cellular processes including, but not limited to, transcription, cell cycle regulation, DNA repair, and apoptosis. This review presents an overview of the impact of HTLV-I infection in general, and Tax expression in particular, on cell cycle progression and the repair of DNA damage. The contribution of these activities to genome instability and cellular transformation will be discussed.
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Affiliation(s)
- Susan J Marriott
- Baylor College of Medicine, Department of Molecular Virology and Microbiology, One Baylor Plaza, Houston, TX 77030, USA.
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23
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Gatza ML, Chandhasin C, Ducu RI, Marriott SJ. Impact of transforming viruses on cellular mutagenesis, genome stability, and cellular transformation. ENVIRONMENTAL AND MOLECULAR MUTAGENESIS 2005; 45:304-325. [PMID: 15645440 DOI: 10.1002/em.20088] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
It is estimated that 15% of all cancers are etiologically linked to viral infection. Specific cancers including adult T-cell leukemia, hepatocellular carcinoma, and uterine cervical cancer are associated with infection by human T-cell leukemia virus type I, hepatitis B virus, and high-risk human papilloma virus, respectively. In these cancers, genomic instability, a hallmark of multistep cancers, has been explicitly linked to the expression of oncoproteins encoded by these viruses. This review discusses mechanisms utilized by these viral oncoproteins, Tax, HBx, and E6/E7, to mediate genomic instability and cellular transformation.
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Affiliation(s)
- Michael L Gatza
- Interdepartmental Program in Cell and Molecular Biology, Baylor College of Medicine, Houston, Texas 77030, USA
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24
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Abstract
Epidemiological studies have demonstrated that the relative percentage of malignant lymphoid proliferations varies widely according to geographical location and ethnic populations. HTLV-I is the etiological agent of adult T-cell leukemia/lymphoma (ATLL) and is also associated with cutaneous T-cell lymphoma (CTCL). However, a definite role of HTLV-I in mycosis fungoides (MF) and/or Sezary syndrome (SS) remains controversial. While most HTLV-I-infected individuals remain asymptomatic carriers, 1-5% will develop ATLL, an invariably fatal expansion of virus-infected CD4+ T cells. This low incidence and the long latency period preceding occurrence of the disease suggest that additional factors are involved in development of ATLL. In this review, diagnosis, clinical features, and molecular pathogenesis of HTLV-I are discussed.
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Affiliation(s)
- Christophe Nicot
- Department of Microbiology, Immunology and Molecular Genetics, University of Kansas Medical Center, Kansas City, Kansas 66160, USA.
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25
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Abstract
Viruses of the retrovirus and herpesvirus families are aetiological agents of human leukaemias and lymphomas. The human T-cell leukaemia virus type 1 causes adult T-cell leukaemia and the Epstein-Barr virus is associated with Burkitt's lymphoma, lymphomas in immunosuppressed people, and Hodgkin lymphoma. The discovery of human herpesvirus type 8 has led to the identification of a rare and unusual group of virus-associated lymphoproliferative diseases. Individuals infected with the human immunodeficiency virus are at greatly increased risk of developing lymphoma but here the mechanism of lymphomagenesis is indirect. Recent data suggest that hepatitis C virus infection is also associated with an increased incidence of lymphoma, whereas data relating to SV40 remain controversial.
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Affiliation(s)
- Ruth F Jarrett
- LRF Virus Centre, Institute of Comparative Medicine, University of Glasgow, UK.
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Azran I, Schavinsky-Khrapunsky Y, Aboud M. Role of Tax protein in human T-cell leukemia virus type-I leukemogenicity. Retrovirology 2004; 1:20. [PMID: 15310405 PMCID: PMC514576 DOI: 10.1186/1742-4690-1-20] [Citation(s) in RCA: 123] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/26/2004] [Accepted: 08/13/2004] [Indexed: 11/17/2022] Open
Abstract
HTLV-1 is the etiological agent of adult T-cell leukemia (ATL), the neurological syndrome TSP/HAM and certain other clinical disorders. The viral Tax protein is considered to play a central role in the process leading to ATL. Tax modulates the expression of many viral and cellular genes through the CREB/ATF-, SRF- and NF-κB-associated pathways. In addition, Tax employs the CBP/p300 and p/CAF co-activators for implementing the full transcriptional activation competence of each of these pathways. Tax also affects the function of various other regulatory proteins by direct protein-protein interaction. Through these activities Tax sets the infected T-cells into continuous uncontrolled replication and destabilizes their genome by interfering with the function of telomerase and topoisomerase-I and by inhibiting DNA repair. Furthermore, Tax prevents cell cycle arrest and apoptosis that would otherwise be induced by the unrepaired DNA damage and enables, thereby, accumulation of mutations that can contribute to the leukemogenic process. Together, these capacities render Tax highly oncogenic as reflected by its ability to transform rodent fibroblasts and primary human T-cells and to induce tumors in transgenic mice. In this article we discuss these effects of Tax and their apparent contribution to the HTLV-1 associated leukemogenic process. Notably, however, shortly after infection the virus enters into a latent state, in which viral gene expression is low in most of the HTLV-1 carriers' infected T-cells and so is the level of Tax protein, although rare infected cells may still display high viral RNA. This low Tax level is evidently insufficient for exerting its multiple oncogenic effects. Therefore, we propose that the latent virus must be activated, at least temporarily, in order to elevate Tax to its effective level and that during this transient activation state the infected cells may acquire some oncogenic mutations which can enable them to further progress towards ATL even if the activated virus is re-suppressed after a while. We conclude this review by outlining an hypothetical flow of events from the initial virus infection up to the ultimate ATL development and comment on the risk factors leading to ATL development in some people and to TSP/HAM in others.
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Affiliation(s)
- Inbal Azran
- Department of Microbiology and Immunology and Cancer Research Center, Faculty of Health Sciences, Ben Gurion University of the Negev, Beer Sheva 84105, Israel
| | - Yana Schavinsky-Khrapunsky
- Department of Microbiology and Immunology and Cancer Research Center, Faculty of Health Sciences, Ben Gurion University of the Negev, Beer Sheva 84105, Israel
| | - Mordechai Aboud
- Department of Microbiology and Immunology and Cancer Research Center, Faculty of Health Sciences, Ben Gurion University of the Negev, Beer Sheva 84105, Israel
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Park HU, Jeong JH, Chung JH, Brady JN. Human T-cell leukemia virus type 1 Tax interacts with Chk1 and attenuates DNA-damage induced G2 arrest mediated by Chk1. Oncogene 2004; 23:4966-74. [PMID: 15107832 DOI: 10.1038/sj.onc.1207644] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Checkpoint kinase 1 (Chk1) mediates diverse cellular responses to genotoxic stress, regulating the network of genome-surveillance pathways that coordinate cell cycle progression with DNA repair. Chk1 is essential for mammalian development and viability, and has been shown to be important for both S and G(2) checkpoints. We now present evidence that the HTLV-1 Tax protein interacts directly with Chk1 and impairs its kinase activities in vitro and in vivo. The direct and physical interaction of Chk1 and Tax was observed in HTLV-1-infected T cells (C81, HuT 102 and MT-2) and transfected fibroblasts (293 T) by coimmunoprecipitation and by in vitro GST pull-down assays. Interestingly, Tax inhibited the kinase activity of Chk1 protein in in vitro and in vivo kinase assays. Consistent with these results, Tax inhibited the phosphorylation-dependent degradation of Cdc25A and G(2) arrest in response to gamma-irradiation (IR) in a dose-dependent manner in vivo. The G(2) arrest did not require Chk2 or p53. These studies provide the first example of a viral transforming protein targeting Chk1 and provide important insights into checkpoint pathway regulation.
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Affiliation(s)
- Hyeon Ung Park
- Virus Tumor Biology Section, Laboratory of Cellular Oncology, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892, USA
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28
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Sinha-Datta U, Horikawa I, Michishita E, Datta A, Sigler-Nicot JC, Brown M, Kazanji M, Barrett JC, Nicot C. Transcriptional activation of hTERT through the NF-kappaB pathway in HTLV-I-transformed cells. Blood 2004; 104:2523-31. [PMID: 15226182 DOI: 10.1182/blood-2003-12-4251] [Citation(s) in RCA: 98] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
In immortal cells, the existence of a mechanism for the maintenance of telomere length is critical. In most cases this is achieved by the reactivation of telomerase, a cellular reverse transcriptase that prevents telomere shortening. Here we report that the telomerase gene (hTERT) promoter is up-regulated during transmission of human T-cell lymphotropic virus type-I (HTLV-I) to primary T cells in vitro and in ex vivo adult T-cell leukemia/lymphoma (ATLL) samples, but not asymptomatic carriers. Although Tax impaired induction of human telomerase reverse transcriptase (hTERT) mRNA in response to mitogenic stimulation, transduction of Tax into primary lymphocytes was sufficient to activate and maintain telomerase expression and telomere length when cultured in the absence of any exogenous stimulation. Transient transfection assays revealed that Tax stimulates the hTERT promoter through the nuclear factor kappaB (NF-kappaB) pathway. Consistently, Tax mutants inactive for NF-kappaB activation could not activate the hTERT or sustain telomere length in transduced primary lymphocytes. Analysis of the hTERT promoter occupancy in vivo using chromatin immunoprecipitation assays suggested that an increased binding of c-Myc and Sp1 is involved in the NF-kappaB-mediated activation of the hTERT promoter. This study establishes the role of Tax in regulation of telomerase expression, which may cooperate with other functions of Tax to promote HTLV-I-associated adult T-cell leukemia.
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MESH Headings
- Cell Line, Transformed
- Cells, Cultured
- Chromatin/genetics
- Chromatin/metabolism
- DNA-Binding Proteins
- Gene Products, tax/genetics
- Gene Products, tax/metabolism
- Human T-lymphotropic virus 1/physiology
- Humans
- Leukemia-Lymphoma, Adult T-Cell/enzymology
- Leukemia-Lymphoma, Adult T-Cell/genetics
- Leukemia-Lymphoma, Adult T-Cell/virology
- Lymphocyte Activation/drug effects
- Models, Genetic
- NF-kappa B/metabolism
- Promoter Regions, Genetic/genetics
- Proto-Oncogene Proteins c-myc/metabolism
- RNA, Messenger/genetics
- RNA, Messenger/metabolism
- Sp1 Transcription Factor/metabolism
- T-Lymphocytes/cytology
- T-Lymphocytes/drug effects
- T-Lymphocytes/metabolism
- T-Lymphocytes/virology
- Telomerase/genetics
- Telomerase/metabolism
- Transcription, Genetic/genetics
- Transcriptional Activation/genetics
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Affiliation(s)
- Uma Sinha-Datta
- University of Kansas Medical Center, Department of Microbiology, Immunology, and Molecular Genetics, 3025 Wahl Hall West, 3901 Rainbow Blvd, Kansas City, KS 66160, USA
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29
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Twizere JC, Kruys V, Lefèbvre L, Vanderplasschen A, Collete D, Debacq C, Lai WS, Jauniaux JC, Bernstein LR, Semmes OJ, Burny A, Blackshear PJ, Kettmann R, Willems L. Interaction of retroviral Tax oncoproteins with tristetraprolin and regulation of tumor necrosis factor-alpha expression. J Natl Cancer Inst 2004; 95:1846-59. [PMID: 14679154 DOI: 10.1093/jnci/djg118] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023] Open
Abstract
BACKGROUND The Tax oncoproteins are transcriptional regulators of viral expression involved in pathogenesis induced by complex leukemogenic retroviruses (or delta-retroviruses, i.e., primate T-cell leukemia viruses and bovine leukemia virus). To better understand the molecular pathways leading to cell transformation, we aimed to identify cellular proteins interacting with Tax. METHODS We used a yeast two-hybrid system to identify interacting cellular proteins. Interactions between Tax and candidate interacting cellular proteins were confirmed by glutathione S-transferase (GST) pulldown assays, co-immunoprecipitation, and confocal microscopy. Functional interactions between Tax and one interacting protein, tristetraprolin (TTP), were assessed by analyzing the expression of tumor necrosis factor-alpha (TNF-alpha), which is regulated by TTP, in mammalian cells (HeLa, D17, HEK 293, and RAW 264.7) transiently transfected with combinations of intact and mutant Tax and TTP. RESULTS We obtained seven interacting cellular proteins, of which one, TTP, was further characterized. Tax and TTP were found to interact specifically through their respective carboxyl-terminal domains. The proteins colocalized in the cytoplasm in a region surrounding the nucleus of HeLa cells. Furthermore, coexpression of Tax was associated with nuclear accumulation of TTP. TTP is an immediate-early protein that inhibits expression of TNF-alpha at the post-transcriptional level. Expression of Tax reverted this inhibition, both in transient transfection experiments and in stably transfected macrophage cell lines. CONCLUSION Tax, through its interactions with the TTP repressor, indirectly increases TNF-alpha expression. This observation is of importance for the cell transformation process induced by leukemogenic retroviruses, because TNF-alpha overexpression plays a central role in pathogenesis.
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Affiliation(s)
- Jean-Claude Twizere
- Biologie cellulaire et moléculaire, Faculté Universitaire des Sciences Agronomiques, Gembloux, Belgium
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30
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Shan B, Xu J, Zhuo Y, Morris CA, Morris GF. Induction of p53-dependent activation of the human proliferating cell nuclear antigen gene in chromatin by ionizing radiation. J Biol Chem 2003; 278:44009-17. [PMID: 12947108 DOI: 10.1074/jbc.m302671200] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023] Open
Abstract
A human fibroblast cell line with conditional p53 expression displayed a p53-dependent increase in both the protein and mRNA levels of proliferating cell nuclear antigen (PCNA) after exposure to ionizing radiation (IR). The combination of p53 induction and IR cooperated to activate a transiently expressed human PCNA promoter-reporter gene via a p53-responsive element. Chromatin immunoprecipitation assays with antibodies specific for p53 or p300/CREB-binding protein revealed specific p53-dependent enrichment of PCNA promoter sequences in immunoprecipitates of sheared chromatin prepared from irradiated cells. Maximal and specific association of acetylated histone H4 with the PCNA promoter also depended on p53 induction and exposure to IR. These data demonstrate p53 binding to a target site in the PCNA promoter, recruitment of p300/CREB-binding protein, and localized acetylation of histone H4 in an IR-dependent manner. These molecular events are likely to play a role in mediating activation of PCNA gene expression by p53 during the cellular response to DNA damage. The analyses indicate that the combination of p53 induction and IR activate the PCNA gene via mechanisms similar to that of p21/wild-type p53-activated factor but to a lesser extent. This differential regulation of PCNA and p21/wild-type p53-activated factor may establish the proper ratio of the two proteins to coordinate DNA repair with cell cycle arrest.
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Affiliation(s)
- Bin Shan
- Programs in Molecular and Cellular Biology and Lung Biology, Department of Pathology, Tulane Cancer Center and Tulane/Xavier Center for Bioenvironmental Research, New Orleans, Louisiana 70112, USA
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31
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Franchini G, Fukumoto R, Fullen JR. T-Cell Control by Human T-Cell Leukemia/Lymphoma Virus Type 1. Int J Hematol 2003; 78:280-96. [PMID: 14686485 DOI: 10.1007/bf02983552] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
Human T-cell leukemia/lymphoma virus type 1 (HTLV-1) causes neoplastic transformation of human T-cells in a small number of infected individuals several years from infection. Collective evidence from in vitro studies indicates that several viral proteins act in concert to increase the responsiveness of T-cells to extracellular stimulation, modulate proapoptotic and antiapoptotic gene signals, enhance T-cell survival, and avoid immune recognition of the infected T-cells. The virus promotes T-cell proliferation by usurping several signaling pathways central to immune T-cell function, such as antigen stimulation and receptor-ligand interaction, suggesting that extracellular signals are important for HTLV-1 oncogenesis. Environmental factors such as chronic antigen stimulation may therefore be of importance, as also suggested by epidemiological data. Thus genetic and environmental factors together with the virus contribute to disease development. This review focuses on current knowledge of the mechanisms regulating HTLV-1 replication and the T-cell pathways that are usurped by viral proteins to induce and maintain clonal proliferation of infected T-cells. The relevance of these laboratory findings is related to clonal T-cell proliferation and adult T-cell leukemia/lymphoma development in vivo.
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Affiliation(s)
- Genoveffa Franchini
- Basic Research Laboratory, National Cancer Institute, Bethesda, Maryland 20892-5055, USA.
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Tripp A, Liu Y, Sieburg M, Montalbano J, Wrzesinski S, Feuer G. Human T-cell leukemia virus type 1 tax oncoprotein suppression of multilineage hematopoiesis of CD34+ cells in vitro. J Virol 2003; 77:12152-64. [PMID: 14581552 PMCID: PMC254283 DOI: 10.1128/jvi.77.22.12152-12164.2003] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/10/2003] [Accepted: 08/13/2003] [Indexed: 11/20/2022] Open
Abstract
Human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2 are highly related viruses that differ in disease manifestation. HTLV-1 is the etiologic agent of adult T-cell leukemia and lymphoma, an aggressive clonal malignancy of human CD4-bearing T lymphocytes. Infection with HTLV-2 has not been conclusively linked to lymphoproliferative disorders. We previously showed that human hematopoietic progenitor (CD34(+)) cells can be infected by HTLV-1 and that proviral sequences were maintained after differentiation of infected CD34(+) cells in vitro and in vivo. To investigate the role of the Tax oncoprotein of HTLV on hematopoiesis, bicistronic lentiviral vectors were constructed encoding the HTLV-1 or HTLV-2 tax genes (Tax1 and Tax2, respectively) and the green fluorescent protein marker gene. Human hematopoietic progenitor (CD34(+)) cells were infected with lentivirus vectors, and transduced cells were cultured in a semisolid medium permissive for the development of erythroid, myeloid, and primitive progenitor colonies. Tax1-transduced CD34(+) cells displayed a two- to fivefold reduction in the total number of hematopoietic clonogenic colonies that arose in vitro, in contrast to Tax2-transduced cells, which showed no perturbation of hematopoiesis. The ratio of colony types that developed from Tax1-transduced CD34(+) cells remained unaffected, suggesting that Tax1 inhibited the maturation of relatively early, uncommitted hematopoietic stem cells. Since previous reports have linked Tax1 expression with initiation of apoptosis, lentiviral vector-mediated transduction of Tax1 or Tax2 was investigated in CEM and Jurkat T-cell lines. Ectopic expression of either Tax1 or Tax2 failed to induce apoptosis in T-cell lines. These data demonstrate that Tax1 expression perturbs development and maturation of pluripotent hematopoietic progenitor cells, an activity that is not displayed by Tax2, and that the suppression of hematopoiesis is not attributable to induction of apoptosis. Since hematopoietic progenitor cells may serve as a latently infected reservoir for HTLV infection in vivo, the different abilities of HTLV-1 and -2 Tax to suppress hematopoiesis may play a role in the respective clinical outcomes after infection with HTLV-1 or -2.
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Affiliation(s)
- Adam Tripp
- Department of Microbiology and Immunology, SUNY Upstate Medical University, Syracuse, New York 13210, USA
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Gatza ML, Watt JC, Marriott SJ. Cellular transformation by the HTLV-I Tax protein, a jack-of-all-trades. Oncogene 2003; 22:5141-9. [PMID: 12910251 DOI: 10.1038/sj.onc.1206549] [Citation(s) in RCA: 72] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
The human T-cell leukemia virus type I (HTLV-I) is an oncogenic retrovirus that is responsible for adult T-cell leukemia and a neurological disease, HTLV-I-associated myelopathy/tropical spastic paraparesis. HTLV-I encodes an oncogenic protein, Tax, which affects a variety of cellular functions prompting it to be referred to as a jack-of-all trades. The ability of Tax to both transcriptionally regulate cellular gene expression and to functionally inactivate proteins involved in cell-cycle progression and DNA repair provide the basis for Tax-mediated transformation and leukemogenesis. This review will concentrate on the effects of Tax on the dysregulation of the G(1)/S and G(2)/M checkpoints as well as the suppression of DNA damage repair leading to cellular transformation.
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Affiliation(s)
- Michael L Gatza
- Interdepartmental Program in Cell and Molecular Biology, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030, USA
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Franchini G, Nicot C, Johnson JM. Seizing of T Cells by Human T-Cell Leukemia⧸Lymphoma Virus Type 1. Adv Cancer Res 2003; 89:69-132. [PMID: 14587871 DOI: 10.1016/s0065-230x(03)01003-0] [Citation(s) in RCA: 63] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
Human T-cell leukemia/lymphoma virus type 1 (HTLV-1) causes neoplastic transformation of human T-cells in a small number of infected individuals several years from infection. Several viral proteins act in concert to increase the responsiveness of T-cells to extracellular stimulation, modulate proapoptotic and antiapoptotic gene signals, enhance T-cell survival, and avoid immune recognition of the infected T-cells. The virus promotes T-cell proliferation by usurping several signaling pathways central to immune T-cell function. Viral proteins modulate the downstream effects of antigen stimulation and receptor-ligand interaction, suggesting that extracellular signals are important for HTLV-1 oncogenesis. Environmental factors such as chronic antigen stimulation are therefore important, as also suggested by epidemiological data. The ability of a given individual to respond to specific antigens is determined genetically. Thus, genetic and environmental factors, together with the virus, contribute to disease development. As in the case of other virus-associated cancers, HTLV-1-induced leukemia/lymphoma can be prevented by avoiding viral infection or by intervention during the asymptomatic phase with approaches able to interrupt the vicious cycle of virus-induced proliferation of a subset of T-cells. This review focuses on current knowledge of the mechanisms regulating HTLV-1 replication and the T-cell pathways that are usurped by viral proteins to induce and maintain clonal proliferation of infected T-cells in vitro. The relevance of these laboratory findings will be related to clonal T-cell proliferation and adult T-cell leukemia/lymphoma development in vivo.
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Affiliation(s)
- Genoveffa Franchini
- National Cancer Institute, Basic Research Laboratory, Bethesda, Maryland 20892, USA
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Marriott SJ, Lemoine FJ, Jeang KT. Damaged DNA and miscounted chromosomes: human T cell leukemia virus type I tax oncoprotein and genetic lesions in transformed cells. J Biomed Sci 2002; 9:292-8. [PMID: 12145525 DOI: 10.1007/bf02256583] [Citation(s) in RCA: 42] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
Genetic instability is a recurring theme in human cancers. Although the molecular mechanisms mediating this effect commonly observed in transformed cells are not completely understood, it has been proposed to involve either the loss of DNA repair capabilities or the loss of chromosomal stability. The transforming retrovirus human T cell leukemia virus type I (HTLV-I) encodes a viral oncoprotein Tax, which is believed to cause the genomic instability characteristic of HTLV-I-infected cells. This review focuses on the ability of HTLV-I Tax to disrupt the cellular processes of DNA repair and chromosomal segregation. The consequences of these effects as well as the evolutionary advantage this may provide to HTLV-I are discussed.
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Affiliation(s)
- Susan J Marriott
- Department of Molecular Virology and Microbiology, Baylor College of Medicine, Houston, TX 77030, USA.
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Liang MH, Geisbert T, Yao Y, Hinrichs SH, Giam CZ. Human T-lymphotropic virus type 1 oncoprotein tax promotes S-phase entry but blocks mitosis. J Virol 2002; 76:4022-33. [PMID: 11907241 PMCID: PMC136099 DOI: 10.1128/jvi.76.8.4022-4033.2002] [Citation(s) in RCA: 63] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/18/2001] [Accepted: 01/07/2002] [Indexed: 11/20/2022] Open
Abstract
Human T-lymphotropic virus type 1 (HTLV-1) Tax exerts pleiotropic effects on multiple cellular regulatory processes to bring about NF-kappaB activation, aberrant cell cycle progression, and cell transformation. Here we report that Tax stimulates cellular G(1)/S entry but blocks mitosis. Tax expression in naive cells transduced with a retroviral vector, pBabe-Tax, leads to a significant increase in the number of cells in the S phase, with an accompanying rise in the population of cells with a DNA content of 4N or more. In all cell types tested, including BHK-21, mouse NIH 3T3, and human diploid fibroblast WI-38, Tax causes an uncoupling of DNA synthesis from cell division, resulting in the formation of multinucleated giant cells and cells with decondensed, highly convoluted and lobulated nuclei that are reminiscent of the large lymphocytes with cleaved or cerebriform nuclei seen in HTLV-1-positive individuals. This contrasts with the Tax-transformed cell lines, PX1 (fibroblast) and MT4 (lymphocyte), which produce Tax at high levels, but without the accompanying late-stage cell cycle abnormalities. PX1 and MT4 may have been selected to harbor somatic mutations that allow a bypass of the Tax-induced block in mitosis.
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Affiliation(s)
- Min-Hui Liang
- Department of Microbiology and Immunology, Uniformed Services University of the Health Sciences, Bethesda, Maryland 20814, USA
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Mahieux R, Pise-Masison C, Gessain A, Brady JN, Olivier R, Perret E, Misteli T, Nicot C. Arsenic trioxide induces apoptosis in human T-cell leukemia virus type 1- and type 2-infected cells by a caspase-3-dependent mechanism involving Bcl-2 cleavage. Blood 2001; 98:3762-9. [PMID: 11739184 DOI: 10.1182/blood.v98.13.3762] [Citation(s) in RCA: 79] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Treatment of patients with adult T-cell leukemia-lymphoma (ATLL) using conventional chemotherapy has limited benefit because human T-cell leukemia virus type 1 (HTLV-1) cells are resistant to most apoptosis-inducing agents. The recent report that arsenic trioxide induces apoptosis in HTLV-1-transformed cells prompted investigation of the mechanism of action of this drug in HTLV-1 and HTLV-2 interleukin-2-independent T cells and in HTLV-1-immortalized cells or in ex vivo ATLL samples. Fluorescence-activated cell sorter analysis, fluorescence microscopy, and measures of mitochondrial membrane potential (Delta Psi m) demonstrated that arsenic trioxide alone was sufficient to induce programmed cell death in all HTLV-1 and -2 cells tested and in ATLL patient samples. I kappa B-alpha phosphorylation strongly decreased, and NF-kappa B translocation to the nucleus was abrogated. Expression of the antiapoptotic protein Bcl-X(L), whose promoter is NF-kappa B dependent, was down-regulated. The collapse of Delta Psi m and the release of cytochrome c to the cytosol resulted in the activation of caspase-3, as demonstrated by the cleavage of PARP. A specific caspase-3 inhibitor (Ac-DEVD-CHO) could reverse this phenotype. The antiapoptotic factor Bcl-2 was then cleaved, converting it to a Bax-like death effector. These results demonstrated that arsenic trioxide induces apoptosis in HTLV-1- and -2-infected cells through activation of the caspase pathway.
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Affiliation(s)
- R Mahieux
- Unité d'Epidémiologie et Physiopathologie des Virus Oncogènes, Institut Pasteur, Paris, France.
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Torgeman A, Ben-Aroya Z, Grunspan A, Zelin E, Butovsky E, Hallak M, Löchelt M, Flügel RM, Livneh E, Wolfson M, Kedar I, Aboud M. Activation of HTLV-I long terminal repeat by stress-inducing agents and protection of HTLV-I-infected T-cells from apoptosis by the viral tax protein. Exp Cell Res 2001; 271:169-79. [PMID: 11697893 DOI: 10.1006/excr.2001.5363] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
HTLV-I is etiologically implicated with tropical spastic paraparesis/HTLV-I associated myelopathy, adult T-cell leukemia and certain other diseases. However, after infection the virus enters into a dormant state, whereas the characteristics of the HTLV-I related diseases indicate that their genesis requires activation of the dormant virus by a Tax-independent mechanism. In the present study we demonstrate that a variety of stress-inducing agents (TPA, cisplatin, etoposide, taxol, and 3-methylcholanthrene) are capable of Tax-independent activation of HTLV-I LTR and that this activation is detected mainly in cells that are undergoing through the apoptotic process. Furthermore, it is demonstrated that both apoptosis induction and HTLV-I LTR activation are inhibited by Bcl-2 and by PKC, indicating that these two processes are mechanistically cross-linked. In addition, using an HTLV-I producing human T-cell line which permanently express the negatively transdominant tax mutant, Delta58tax, under the Tet-Off control system, we prove that the virally encoded Tax protein protects the host cells from apoptosis. Together, these data suggest that activation of the dormant virus in the carriers' infected T-cells by certain stress-inducing conditions and protecting these cells from the consequent apoptotic death by the viral Tax protein emerging after this activation, might be the basis for switching the virus from latency to a pathogenic phase.
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Affiliation(s)
- A Torgeman
- Department of Microbiology and Immunology, Ben Gurion University of the Negev, Beer Sheva, 84105, Israel
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