Tax protein of HTLV-1 inhibits CBP/p300-mediated transcription by interfering with recruitment of CBP/p300 onto DNA element of E-box or p53 binding site

Epigenetic perspectives of COVID-19: Virus infection to disease progression and therapeutic control

COVID-19 has caused numerous deaths as well as imposed social isolation and upheaval world-wide. Although, the genome and the composition of the virus, the entry process and replication mechanisms are well investigated from by several laboratories across the world, there are many unknown remaining questions. For example, what are the functions of membrane lipids during entry, packaging and exit of virus particles? Also, the metabolic aspects of the infected tissue cells are poorly understood. In the course of virus replication and formation of virus particles within the host cell, the enhanced metabolic activities of the host is directly proportional to viral loads. The epigenetic landscape of the host cells is also altered, particularly the expression/repression of genes associated with cellular metabolism as well as cellular processes that are antagonistic to the virus. Metabolic pathways are enzyme driven processes and the expression profile and mechanism of regulations of the respective genes encoding those enzymes during the course of pathogen invasion might be highly informative on the course of the disease. Recently, the metabolic profile of the patients' sera have been analysed from few patients. In view of this, and to gain further insights into the roles that epigenetic mechanisms might play in this scenario in regulation of metabolic pathways during the progression of COVID-19 are discussed and summarised in this contribution for ensuring best therapy.

Interplay between innate immunity and the viral oncoproteins Tax and HBZ in the pathogenesis and therapeutic response of HTLV-1 associated adult T cell leukemia

The Human T-cell Leukemia virus type 1 (HTLV-1) causes an array of pathologies, the most aggressive of which is adult T-cell leukemia (ATL), a fatal blood malignancy with dismal prognosis. The progression of these diseases is partly ascribed to the failure of the immune system in controlling the spread of virally infected cells. HTLV-1 infected subjects, whether asymptomatic carriers or symptomatic patients are prone to opportunistic infections. An increasing body of literature emphasizes the interplay between HTLV-1, its associated pathologies, and the pivotal role of the host innate and adoptive immune system, in shaping the progression of HTLV-1 associated diseases and their response to therapy. In this review, we will describe the modalities adopted by the malignant ATL cells to subvert the host innate immune response with emphasis on the role of the two viral oncoproteins Tax and HBZ in this process. We will also provide a comprehensive overview on the function of innate immunity in the therapeutic response to chemotherapy, anti-viral or targeted therapies in the pre-clinical and clinical settings.

Genetic Alterations in Adult T-Cell Leukemia/Lymphoma: Novel Discoveries with Clinical and Biological Significance

Adult T-cell leukemia/lymphoma (ATLL) is a refractory T-cell neoplasm that develops in human T-cell leukemia virus type-I (HTLV-1) carriers. Large-scale comprehensive genomic analyses have uncovered the landscape of genomic alterations of ATLL and have identified several altered genes related to prognosis. The genetic alterations in ATLL are extremely enriched in the T-cell receptor/nuclear factor-κB pathway, suggesting a pivotal role of deregulation in this pathway in the transformation of HTLV-1-infected cells. Recent studies have revealed the process of transformation of HTLV-1-infected cells by analyzing longitudinal samples from HTLV-1 carriers and patients with overt ATLL, an endeavor that might enable earlier ATLL diagnosis. The latest whole-genome sequencing study discovered 11 novel alterations, including CIC long isoform, which had been overlooked in previous studies employing exome sequencing. Our study group performed the targeted sequencing of ATLL in Okinawa, the southernmost island in Japan and an endemic area of HTLV-1, where the comprehensive genetic alterations had never been analyzed. We found associations of genetic alterations with HTLV-1 strains phylogenetically classified based on the tax gene, an etiological virus factor in ATLL. This review summarizes the genetic alterations in ATLL, with a focus on their clinical significance, geographical heterogeneity, and association with HTLV-1 strains.

Protein lysine acetylation and its role in different human pathologies: a proteomic approach

INTRODUCTION

Lysine acetylation is a reversible post-translational modification (PTM) regulated through the action of specific types of enzymes: lysine acetyltransferases (KATs) and lysine deacetylases (HDACs), in addition to bromodomains, which are a group of conserved domains which identify acetylated lysine residues, several of the players in the process of protein acetylation, including enzymes and bromodomain-containing proteins, have been related to the progression of several diseases. The combination of high-resolution mass spectrometry-based proteomics, and immunoprecipitation to enrich acetylated peptides has contributed in recent years to expand the knowledge about this PTM described initially in histones and nuclear proteins, and is currently reported in more than 5000 human proteins, that are regulated by this PTM.

AREAS COVERED

This review presents an overview of the main participant elements, the scenario in the development of protein lysine acetylation, and its role in different human pathologies.

EXPERT OPINION

Acetylation targets are practically all cellular processes in eukaryotes and prokaryotes organisms. Consequently, this modification has been linked to many pathologies like cancer, viral infection, obesity, diabetes, cardiovascular, and nervous system-associated diseases, to mention a few relevant examples. Accordingly, some intermediate mediators in the acetylation process have been projected as therapeutic targets.

Viral PDZ Binding Motifs Influence Cell Behavior Through the Interaction with Cellular Proteins Containing PDZ Domains

Viruses have evolved to interact with their hosts. Some viruses such as human papilloma virus, dengue virus, SARS-CoV, or influenza virus encode proteins including a PBM that interact with cellular proteins containing PDZ domains. There are more than 400 cellular protein isoforms with these domains in the human genome, indicating that viral PBMs have a high potential to influence the behavior of the cell. In this review we analyze the most relevant cellular processes known to be affected by viral PBM-cellular PDZ interactions including the establishment of cell-cell interactions and cell polarity, the regulation of cell survival and apoptosis and the activation of the immune system. Special attention has been provided to coronavirus PBM conservation throughout evolution and to the role of the PBMs of human coronaviruses SARS-CoV and MERS-CoV in pathogenesis.

Mechanisms of Oncogenesis by HTLV-1 Tax

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.

Deregulation of the Notch pathway as a common road in viral carcinogenesis

The Notch pathway is a conserved signaling pathway and a form of direct cell-cell communication related to many biological processes during development and adulthood. Deregulation of the Notch pathway is involved in many diseases, including cancer. Almost 20% of all cancer cases have an infectious etiology, with viruses responsible for at least 1.5 million new cancer cases per year. Seven groups of viruses have been classified as oncogenic: hepatitis B and C viruses (HBV and HCV respectively), Epstein-Barr virus (EBV), Kaposi sarcoma-associated herpesvirus (KSHV), human T lymphotropic virus (HTLV-1), human papillomavirus (HPV), and Merkel cell polyomavirus (MCPyV). These viruses share the ability to manipulate a variety of cell pathways that are critical in proliferation and differentiation, leading to malignant transformation. Viral proteins interact directly or indirectly with different members of the Notch pathway, altering their normal function. This review focuses exclusively on the direct interactions of viral oncoproteins with Notch elements, providing a deeper understanding of the dual behavior of the Notch pathway as activator or suppressor of neoplasia in virus-related cancers.

Role of the HTLV-1 viral factors in the induction of apoptosis

Adult T-cell leukemia (ATL) and HTLV-1-associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP) are the two main diseases that are caused by the HTLV-1 virus. One of the features of HTLV-1 infection is its resistance against programmed cell death, which maintains the survival of cells to oncogenic transformation and underlies the viruses' therapeutic resistance. Two main genes by which the virus develops cancer are Tax and HBZ; playing an essential role in angiogenesis in regulating viral transcription and modulating multiple host factors as well as apoptosis pathways. Here we have reviewed by prior research how the apoptosis pathways are suppressed by the Tax and HBZ and new drugs which have been designed to deal with this suppression.

Molecular Studies of HTLV-1 Replication: An Update

Human T-cell leukemia virus type 1 (HTLV-1) was the first human retrovirus discovered. Studies on HTLV-1 have been instrumental for our understanding of the molecular pathology of virus-induced cancers. HTLV-1 is the etiological agent of an adult T-cell leukemia (ATL) and can lead to a variety of neurological pathologies, including HTLV-1-associated-myelopathy/tropical spastic paraparesis (HAM/TSP). The ability to treat the aggressive ATL subtypes remains inadequate. HTLV-1 replicates by (1) an infectious cycle involving virus budding and infection of new permissive target cells and (2) mitotic division of cells harboring an integrated provirus. Virus replication initiates host antiviral immunity and the checkpoint control of cell proliferation, but HTLV-1 has evolved elegant strategies to counteract these host defense mechanisms to allow for virus persistence. The study of the molecular biology of HTLV-1 replication has provided crucial information for understanding HTLV-1 replication as well as aspects of viral replication that are shared between HTLV-1 and human immunodeficiency virus type 1 (HIV-1). Here in this review, we discuss the various stages of the virus replication cycle—both foundational knowledge as well as current updates of ongoing research that is important for understanding HTLV-1 molecular pathogenesis as well as in developing novel therapeutic strategies.

KATs in cancer: functions and therapies

Post-translational acetylation of lysines is most extensively studied in histones, but this modification is also found in many other proteins and is implicated in a wide range of biological processes in both the cell nucleus and the cytoplasm. Like phosphorylation, acetylation patterns and levels are often altered in cancer, therefore small molecule inhibition of enzymes that regulate acetylation and deacetylation offers much potential for inhibiting cancer cell growth, as does disruption of interactions between acetylated residues and ‘reader’ proteins. For more than a decade now, histone deacetylase (HDAC) inhibitors have been investigated for their ability to increase acetylation and promote expression of tumor suppressor genes. However, emerging evidence suggests that acetylation can also promote cancer, in part by enhancing the functions of oncogenic transcription factors. In this review we focus on how acetylation of both histone and non-histone proteins may drive cancer, and we will discuss the implications of such changes on how patients are assigned to therapeutic agents. Finally, we will explore what the future holds in the design of small molecule inhibitors for modulation of levels or functions of acetylation states.

Herpes simplex virus 1-encoded tegument protein VP16 abrogates the production of beta interferon (IFN) by inhibiting NF-κB activation and blocking IFN regulatory factor 3 to recruit its coactivator CBP

Host cells activate innate immune signaling pathways to defend against invading pathogens. To survive within an infected host, viruses have evolved intricate strategies to counteract host immune responses. Herpesviruses, including herpes simplex virus type 1 (HSV-1), have large genomes and therefore have the capacity to encode numerous proteins that modulate host innate immune responses. Here we define the contribution of HSV-1 tegument protein VP16 in the inhibition of beta interferon (IFN-β) production. VP16 was demonstrated to significantly inhibit Sendai virus (SeV)-induced IFN-β production, and its transcriptional activation domain was not responsible for this inhibition activity. Additionally, VP16 blocked the activation of the NF-κB promoter induced by SeV or tumor necrosis factor alpha treatment and expression of NF-κB-dependent genes through interaction with p65. Coexpression analysis revealed that VP16 selectively blocked IFN regulatory factor 3 (IRF-3)-mediated but not IRF-7-mediated transactivation. VP16 was able to bind to IRF-3 but not IRF-7 in vivo, based on coimmunoprecipitation analysis, but it did not affect IRF-3 dimerization, nuclear translocation, or DNA binding activity. Rather, VP16 interacted with the CREB binding protein (CBP) coactivator and efficiently inhibited the formation of the transcriptional complexes IRF-3-CBP in the context of HSV-1 infection. These results illustrate that VP16 is able to block the production of IFN-β by inhibiting NF-κB activation and interfering with IRF-3 to recruit its coactivator CBP, which may be important to the early events leading to HSV-1 infection.

HTLV-1 bZIP factor enhances TGF-β signaling through p300 coactivator

Human T-cell leukemia virus type 1 (HTLV-1) is an oncogenic retrovirus that is etiologically associated with adult T-cell leukemia. The HTLV-1 bZIP factor (HBZ), which is encoded by the minus strand of the provirus, is involved in both regulation of viral gene transcription and T-cell proliferation. We showed in this report that HBZ interacted with Smad2/3, and enhanced transforming growth factor-β (TGF-β)/Smad transcriptional responses in a p300-dependent manner. The N-terminal LXXLL motif of HBZ was responsible for HBZ-mediated TGF-β signaling activation. In a serial immunoprecipitation assay, HBZ, Smad3, and p300 formed a ternary complex, and the association between Smad3 and p300 was markedly enhanced in the presence of HBZ. In addition, HBZ could overcome the repression of the TGF-β response by Tax. Finally, HBZ expression resulted in enhanced transcription of Pdgfb, Sox4, Ctgf, Foxp3, Runx1, and Tsc22d1 genes and suppression of the Id2 gene; such effects were similar to those by TGF-β. In particular, HBZ induced Foxp3 expression in naive T cells through Smad3-dependent TGF-β signaling. Our results suggest that HBZ, by enhancing TGF-β signaling and Foxp3 expression, enables HTLV-1 to convert infected T cells into regulatory T cells, which is thought to be a critical strategy for virus persistence.

Histone deacetylase inhibitors, valproic acid and trichostatin-A induce apoptosis and affect acetylation status of p53 in ERG-positive prostate cancer cells

An ETS family member, ETS Related Gene (ERG) is involved in the Ewing family of tumors as well as leukemias. Rearrangement of the ERG gene with the TMPRSS2 gene has been identified in the majority of prostate cancer patients. Additionally, overexpression of ERG is associated with unfavorable prognosis in prostate cancer patients similar to leukemia patients. Histone acetyltransferases (HATs) and histone deacetylases (HDACs) regulate transcription as well as epigenetic status of genes through acetylation of both histones and transcription factors. Deregulation of HATs and HDACs is frequently seen in various cancers, including prostate cancer. Many cellular oncogenes as well as tumor viral proteins are known to target either or both HATs and HDACs. Several studies have demonstrated that there are alterations of HDAC activity in prostate cancer cells. Recently, we found that ERG binds and inhibits HATs, which suggests that ERG is involved in deregulation of protein acetylation. Additionally, it has been shown that ERG is associated with a higher expression of HDACs. In this study, we tested the effect of the HDAC inhibitors valproic acid (VPA) and trichostatin-A (TSA) on ERG-positive prostate cancer cells (VCaP). We found that VPA and TSA induce apoptosis, upregulate p21/Waf1/CIP1, repress TMPRSS2-ERG expression and affect acetylation status of p53 in VCaP cells. These results suggest that HDAC inhibitors might restore HAT activity through two different ways: by inhibiting HDAC activity and by repressing HAT targeting oncoproteins such as ERG.

ATF3, an HTLV-1 bZip factor binding protein, promotes proliferation of adult T-cell leukemia cells

Background

Adult T-cell leukemia (ATL) is an aggressive malignancy of CD4⁺ T-cells caused by human T-cell leukemia virus type 1 (HTLV-1). The HTLV-1 bZIP factor (HBZ) gene, which is encoded by the minus strand of the viral genome, is expressed as an antisense transcript in all ATL cases. By using yeast two-hybrid screening, we identified activating transcription factor 3 (ATF3) as an HBZ-interacting protein. ATF3 has been reported to be expressed in ATL cells, but its biological significance is not known.

Results

Immunoprecipitation analysis confirmed that ATF3 interacts with HBZ. Expression of ATF3 was upregulated in ATL cell lines and fresh ATL cases. Reporter assay revealed that ATF3 could interfere with the HTLV-1 Tax's transactivation of the 5' proviral long terminal repeat (LTR), doing so by affecting the ATF/CRE site, as well as HBZ. Suppressing ATF3 expression inhibited proliferation and strongly reduced the viability of ATL cells. As mechanisms of growth-promoting activity of ATF3, comparative expression profiling of ATF3 knockdown cells identified candidate genes that are critical for the cell cycle and cell death, including cell division cycle 2 (CDC2) and cyclin E2. ATF3 also enhanced p53 transcriptional activity, but this activity was suppressed by HBZ.

Conclusions

Thus, ATF3 expression has positive and negative effects on the proliferation and survival of ATL cells. HBZ impedes its negative effects, leaving ATF3 to promote proliferation of ATL cells via mechanisms including upregulation of CDC2 and cyclin E2. Both HBZ and ATF3 suppress Tax expression, which enables infected cells to escape the host immune system.

Molecular approach to human leukemia: isolation and characterization of the first human retrovirus HTLV-1 and its impact on tumorigenesis in adult T-cell leukemia

Molecular biology of mouse and chicken retroviruses had identified oncogenes and provided a revolutionary concept in understanding of cancers. A human retrovirus was established during 1980-1982 in linkage with a unique human leukemia, concurrently in Japan and USA. This review covers our efforts on the discovery of new retrovirus, Human T-cell Leukemia Virus Type 1 (HTLV-1), first introducing to a new class of retroviruses with a unique regulatory factors, Tax and Rex. Then it is followed by analyses of molecular interaction of the vial Tax with cellular machineries involved in the pathogenesis of Adult T-cell Leukemia (ATL). And then a probable mechanism of pathogenesis of ATL is proposed including recent findings on HBZ after our efforts.

Induction of p21(CIP1/WAF1) expression by human T-lymphotropic virus type 1 Tax requires transcriptional activation and mRNA stabilization

HTLV-1 Tax can induce senescence by up-regulating the levels of cyclin-dependent kinase inhibitors p21^CIP1/WAF1 and p27^KIP1. Tax increases p27^KIP1 protein stability by activating the anaphase promoting complex/cyclosome (APC/C) precociously, causing degradation of Skp2 and inactivation of SCF^Skp2, the E3 ligase that targets p27^KIP1. The rate of p21^CIP1/WAF1 protein turnover, however, is unaffected by Tax. Rather, the mRNA of p21^CIP1/WAF1 is greatly up-regulated. Here we show that Tax increases p21 mRNA expression by transcriptional activation and mRNA stabilization. Transcriptional activation of p21^CIP1/WAF1 by Tax occurs in a p53-independent manner and requires two tumor growth factor-β-inducible Sp1 binding sites in the -84 to -60 region of the p21^CIP1/WAF1 promoter. Tax binds Sp1 directly, and the CBP/p300-binding activity of Tax is required for p21^CIP1/WAF1 trans-activation. Tax also increases the stability of p21^CIP1/WAF1 transcript. Several Tax mutants trans-activated the p21 promoter, but were attenuated in stabilizing p21^CIP1/WAF1 mRNA, and were less proficient in increasing p21^CIP1/WAF1 expression. The possible involvement of Tax-mediated APC/C activation in p21^CIP1/WAF1 mRNA stabilization is discussed.

The state of the art in the pathogenesis of ATL and new potential targets associated with HTLV-1 and ATL

Almost 30 years have passed since adult T-cell leukemia (ATL) was identified as a new disease entity in Japan. During this period, its causative agent, human T-cell leukemia virus (HTLV-1), was discovered, and a crucial role of the viral product Tax in ATL leukemogenesis was demonstrated. Recently, another HTLV-1 product, HBZ, which is encoded on the negative strand, was found, and it has now become a subject of intensive research because of its possible activity in cell proliferation. It is, however, impossible to elucidate the whole process of ATL leukemogenesis by studying only HTLV-1, and aberrations of cellular genes such as tumor suppressor genes are also profoundly involved in the later stages of ATL development. In contrast with the progress in the understanding of ATL pathogenesis, more progress in developing therapy for ATL is needed, and there has been only slight improvement in the prognosis. Recently, unique therapeutic approaches targeting molecules and/or mechanisms involved in the pathogenesis have been explored, and some of them produced encouraging results that might lead to breakthrough therapies. One of these approaches, the use of monoclonal antibody against chemokine receptor CCR4, is now ongoing as a multicenter clinical trial in Japan. Here we review the state of the art regarding our understanding of ATL leukemogenesis and new potential molecular targets in ATL therapy.

PI3K/AKT inhibition induces caspase-dependent apoptosis in HTLV-1-transformed cells

The phosphatidylinositol-3-kinase (PI3K) and AKT (protein kinase B) signaling pathways play an important role in regulating cell cycle progression and cell survival. In previous studies, we demonstrated that AKT is activated in HTLV-1-transformed cells and that Tax activation of AKT is linked to p53 inhibition and cell survival. In the present study, we extend these observations to identify regulatory pathways affected by AKT in HTLV-1-transformed cells. We demonstrate that inhibition of AKT reduces the level of phosphorylated Bad, an important member of the pro-apoptotic family of proteins. Consistent with the decrease of phosphorylated Bad, cytochrome c is released from the mitochondria and caspase-9 is activated. Pretreatment of the cells with caspase-9 specific inhibitor z-LEHD-FMK or pan caspase inhibitor Ac-DEVD-CHO prevented LY294002-induced apoptosis. Of interest, p53 siRNA prevents LY294002-induced apoptosis in HTLV-1-transformed cells, suggesting that p53 reactivation is linked to apoptosis. In conclusion, the AKT pathway is involved in targeting multiple proteins which regulate caspase- and p53-dependent apoptosis in HTLV-1-transformed cells. Since AKT inhibitors simultaneously inhibit NF-kappaB and activate p53, these drugs should be promising candidates for HTLV-1-associated cancer therapy.

Human T-cell leukemia virus oncoprotein tax represses nuclear receptor-dependent transcription by targeting coactivator TAX1BP1

Human T-cell leukemia virus type 1 oncoprotein Tax is a transcriptional regulator that interacts with a large number of host cell factors. Here, we report the novel characterization of the interaction of Tax with a human cell protein named Tax1-binding protein 1 (TAX1BP1). We show that TAX1BP1 is a nuclear receptor coactivator that forms a complex with the glucocorticoid receptor. TAX1BP1 and Tax colocalize into intranuclear speckles that partially overlap with but are not identical to the PML oncogenic domains. Tax binds TAX1BP1 directly, induces the dissociation of TAX1BP1 from the glucocorticoid receptor-containing protein complex, and represses the coactivator function of TAX1BP1. Genetic knockout of Tax1bp1 in mice abrogates the influence of Tax on the activation of nuclear receptors. We propose that Tax-TAX1BP1 interaction mechanistically explains the previously reported repression of nuclear receptor activity by Tax.

Human T-cell leukemia virus type 1 Tax protein down-regulates pre-T-cell receptor alpha gene transcription in human immature thymocytes

The human pre-T-cell receptor alpha (TCRalpha; pTalpha) gene encodes a polypeptide which associates with the TCRbeta chain and CD3 molecules to form the pre-TCR complex. The surface expression of the pre-TCR is pTalpha dependent, and signaling through this complex triggers an early alphabeta T-cell developmental checkpoint inside the thymus, known as beta-selection. E2A transcription factors, which are involved at multiple stages of T-cell development, regulate the transcription of the pTalpha gene. Here we show that the regulatory protein Tax of the human T-cell leukemia virus type 1 (HTLV-1) efficiently suppresses the E47-mediated activation of the pTalpha promoter. Furthermore, we report that in Tax lentivirally transduced human MOLT-4 T cells, which constitutively express the pTalpha gene, the amount of pTalpha transcripts decreases. Such a decrease is not observed in MOLT-4 cells transduced by a vector encoding the Tax mutant K88A, which is unable to interact with p300. These data underline that Tax inhibits pTalpha transcription by recruiting this coactivator. Finally, we show that the expression of Tax in human immature thymocytes results in a decrease of pTalpha gene transcription but does not modify the level of E47 transcripts. These observations indicate that Tax, by silencing E proteins, down-regulates pTalpha gene transcription during early thymocyte development. They further provide evidence that Tax can interfere with an important checkpoint during T-cell differentiation in the thymus.

Histone acetyltransferase (HAT) activity of p300 modulates human T lymphotropic virus type 1 p30II-mediated repression of LTR transcriptional activity

Human T-lymphotropic virus type-1 (HTLV-1) is a deltaretrovirus that causes adult T cell leukemia/lymphoma, and is implicated in a variety of lymphocyte-mediated inflammatory disorders. HTLV-1 provirus has regulatory and accessory genes in four pX open reading frames. HTLV-1 pX ORF-II encodes two proteins, p13II and p30II, which are incompletely defined in virus replication or pathogenesis. We have demonstrated that pX ORF-II mutations block virus replication in vivo and that ORF-II encoded p30II, a nuclear-localizing protein that binds with CREB-binding protein (CBP)/p300, represses CREB and Tax responsive element (TRE)-mediated transcription. Herein, we have identified p30II motifs important for p300 binding and in regulating TRE-mediated transcription in the absence and presence of HTLV-1 provirus. Within amino acids 100-179 of p30II, a region important for repression of LTR-mediated transcription, we identified a single lysine residue at amino acid 106 (K3) that significantly modulates the ability of p30II to repress TRE-mediated transcription. Exogenous p300, in a dose-responsive manner, reverses p30II-dependent repression of TRE-mediated transcription, in the absence or presence of the provirus, In contrast to wild type p300, p300 HAT mutants (defective in histone acetyltransferase activity) only partially rescued p30(II)-mediated LTR repression. Deacetylation by histone deacetylase-1 (HDAC-1) enhanced p30II-mediated LTR repression, while inhibition of deacetylation by trichostatin A decreases p30(II)-mediated LTR repression. Collectively, our data indicate that HTLV-1 p30II modulates viral gene expression in a cooperative manner with p300-mediated acetylation.

HTLV-I viral escape and host genetic changes in the development of adult T cell leukemia

In the pathogenesis of adult T cell leukemia (ATL), an oncogenetic role of the human T cell lymphotropic virus type I (HTLV-I) Tax protein, viral escape from the host immune system, and host genetic changes have been proposed as contributory factors. We examined the premature stop codons in tax gene as one of the mutations that may lead to escape of HTLV-I from the cytotoxic T lymphocyte (CTL) response in HTLV-I carriers, to test whether a putative CTL escape mutant can emerge in the early stage of ATL development and whether HTLV-I infected cells with such a mutation can proliferate subsequently. We also examined deletion of cyclin-dependent kinase inhibitor 4 (INK4) genes and mutation of p53 gene in combination with changes in the HTLV-I genome in acute type ATL to test whether host genetic changes promoted the malignant transformation of ATL cells that carry putative CTL escape mutations. The premature stop codon in tax gene existed in many non-ATL HTLV-I carriers as a minor population but not in the commonest HTLV-I sequence of the individual. This minor population with a premature stop codon did not expand subsequently in 3 asymptomatic carriers tested. There were cases who had a mutation or deletion in HTLV-I who also have either deletion of INK4 genes or mutation in p53 gene. Our findings suggest that CTL escape mutation can occur at an early stage of ATL development, and that certain host genetic changes favor the development of the aggressive form of ATL.

HTLV-I basic leucine zipper factor gene mRNA supports proliferation of adult T cell leukemia cells

Human T cell leukemia virus type I (HTLV-I) causes adult T cell leukemia (ATL) in 2-5% of carriers after a long latent period. An HTLV-I encoded protein, Tax, induces proliferation and inhibits apoptosis, resulting in clonal proliferation of infected cells. However, tax gene expression in ATL cells is disrupted by several mechanisms, including genetic changes in the tax gene and DNA methylation/deletion of the 5' long terminal repeat (LTR). Because Tax is the major target of cytotoxic T-lymphocytes in vivo, loss of Tax expression should enable ATL cells to escape the host immune system. The 5' LTR of HTLV-I is frequently hypermethylated or deleted in ATL cells, whereas the 3' LTR remains unmethylated and intact, suggesting the involvement of the 3' LTR in leukemogenesis. Here we show that a gene encoded by the minus strand of the HTLV-I proviral genome, HTLV-I basic leucine zipper factor (HBZ), is transcribed from 3'-LTR in all ATL cells. Suppression of HBZ gene transcription by short interfering RNA inhibits proliferation of ATL cells. In addition, HBZ gene expression promotes proliferation of a human T cell line. Analyses of T cell lines transfected with mutated HBZ genes showed that HBZ promotes T cell proliferation in its RNA form, whereas HBZ protein suppresses Tax-mediated viral transcription through the 5' LTR. Thus, the single HBZ gene has bimodal functions in two different molecular forms. The growth-promoting activity of HBZ RNA likely plays an important role in oncogenesis by HTLV-I.

Functional inactivation of p53 by human T-cell leukemia virus type 1 Tax protein: mechanisms and clinical implications

Human T-cell leukemia virus type 1 (HTLV-I) has been implicated with the etiology of adult T-cell leukemia (ATL) and certain other clinical disorders. Although the leukemogenic mechanism of HTLV-1 is not fully understood yet, the viral Tax protein is widely regarded as a key factor in this mechanism. Tax can modulate the synthesis or function of many regulatory factors which control a wide range of normal and oncogenic cellular processes and therefore, it acts as a potent oncoprotein. In the last few years, special attention has been attracted to Tax interference with the transactivation function of p53, a tumor-suppressor protein that is involved in regulation of the cell-cycle and apoptosis and in maintaining the cellular genome integrity. p53 is mutated in approximately 60% of all human tumors. In contrast, mutant p53 is found in only small percentage of ATL patients. Nevertheless, p53 is inactive in the leukemic cells of most ATL patients and in most HTLV-1 transformed cells. By inactivating p53, Tax can immortalize the HTLV-1-infected cells and destabilize their genome. Consequently, such cells can progress toward the ultimate leukemic state by a stepwise accumulation of oncogenic mutations and other types of chromosomal aberrations. Furthermore, since p53 exists in most ATL patients in its wild-type form, its reactivation by therapeutic drugs might be an effective approach for ATL therapy. Several mechanisms have been proposed so far for Tax-induced p53 inactivation. Understanding the exact mechanism of this Tax effect is essential for designing effective means for this therapeutic approach. In this review article, we discuss the various mechanisms proposed for Tax interference with p53 functions and their clinical and therapeutic implications.

Impact of HTLV-I Tax on cell cycle progression and the cellular DNA damage repair response

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.

Activation of NF-kappaB by HTLV-I and implications for cell transformation

T-cell transformation by the human T-cell leukemia virus type I (HTLV-I) involves deregulation of cellular transcription factors, including members of the NF-kappaB family. In normal T cells, NF-kappaB activation occurs transiently in response to immune stimuli, which is required for antigen-stimulated T-cell proliferation and survival. However, HTLV-I induces persistent activation of NF-kappaB, causing deregulated expression of a large array of cellular genes, which in turn contributes to the induction of T-cell transformation. The HTLV-I transforming protein Tax functions as an intracellular stimulator of IkappaB kinase (IKK), a cellular kinase mediating NF-kappaB activation by diverse stimuli. Tax physically interacts with IKK and renders this inducible kinase constitutively active. By assembling different Tax/IKK complexes, Tax targets the persistent activation of both canonical and noncanonical NF-kappaB signaling pathways. Whereas Tax plays a primary role in HTLV-I-mediated NF-kappaB activation, recent studies reveal that the IKK/NF-kappaB signaling pathway is also activated in freshly isolated adult T-cell leukemia (ATL) cells that often lack detectable Tax expression. The mechanism underlying this Tax-independent pathway of NF-kappaB activation remains poorly understood. Clarifying the precise nature and consequences of the constitutive NF-kappaB activation in ATL cells is important for developing rational therapeutic strategies for this T-cell malignancy.

Malignant transformation of thymoma in recipient rats by heterotopic thymus transplantation from HTLV-I transgenic rats

Transgenic rats expressing the pX gene of human T lymphocyte virus type-I (HTLV-I) under control of the rat lymphocyte-specific protein tyrosine kinase type-I promoter (lck-pX rats) developed benign epithelial thymomas. When the thymuses of newborn lck-pX rats were transplanted into the subcapsular space of the kidney in other thymectomized lck-pX rats, similar tumors developed in the transplanted thymuses. Following the tumor growth, dissemination in the abdominal cavity and distant metastasis occurred. The tumors were histopathologically similar to the original thymomas, but prominent nuclear atypia and high mitotic activity were present. The Ki-67 index was twice as high as that in the originals. The tumors were transplantable into the subcutis of lck-pX rats, although transplantation of the originals never succeeded. All evidence indicated that malignant transformation of thymoma was induced by the heterotopic transplantation. Expression of the pX transgene in the transformed tumors were significantly reduced. Among host genes, the expression of p16ink4a/ARF, which was significantly upregulated in the originals, was never detected in the transformed tumors. Genomic Southern blots and PCR suggest that homozygous deletion of the p16ink4a/ARF gene may play important roles in malignant transformation in this model. Our model described here is a useful unique model for in vivo malignant transformation.

The bZIP transcription factor ATFx binds human T-cell leukemia virus type 1 (HTLV-1) Tax and represses HTLV-1 long terminal repeat-mediated transcription

The human T-cell leukemia virus type 1 (HTLV-1) viral protein Tax is a transactivator of transcription driven by the cognate viral long terminal repeat (LTR). Tax exerts its effect through three nonidentical copies of the Tax-responsive element (TxRE), a member of the asymmetric cyclic AMP response element (CRE) family of enhancer sequences. Transactivation is mediated via interaction of Tax with members of the CREB/ATF family bound to TxRE. We have identified a cellular repressor of transcription, activating transcription factor x (ATFx), as a novel Tax-binding protein. In addition to binding directly to Tax we show by electrophoretic mobility shift assay that ATFx binds to the TxRE enhancer element via the bZIP domain. The functional impact of this bridging interaction results in repression of both basal and Tax-induced transcription from the HTLV-1 LTR. ATFx is unique among ATF family of proteins in that it is cell cycle regulated and exerts a tight repressive control over apoptotic signaling. We propose that recruitment of ATFx to the HTLV-1 LTR serves to link viral transcription with critical events in cellular homeostasis.

Human T-cell leukemia virus type I (HTLV-I) infection and the onset of adult T-cell leukemia (ATL)

The clinical entity of adult T-cell leukemia (ATL) was established around 1977, and human T-cell leukemia virus type 1 (HTLV-I) was subsequently identified in 1980. In the 25 years since the discovery of HTLV-I, HTLV-I infection and its associated diseases have been extensively studied, and many of their aspects have been clarified. However, the detailed mechanism of leukemogenesis remains unsolved yet, and the prognosis of ATL patients still poor because of its resistance to chemotherapy and immunodeficiency. In this review, I highlight the recent progress and remaining enigmas in HTLV-I infection and its associated diseases, especially ATL.

Preferential selection of human T-cell leukemia virus type I provirus integration sites in leukemic versus carrier states

Human T-cell leukemia virus type I (HTLV-I) is a causative agent of neoplastic disease, adult T-cell leukemia (ATL). Although the encoding viral proteins play an important role in oncogenesis, the role of the HTLV-I proviral integration site remains unsolved. We determined the integration sites of HTLV-I proviruses in ATL cells and HTLV-I-infected cells in asymptomatic carriers. In carrier and ATL cells, HTLV-I provirus was integrated into the transcriptional unit at frequencies of 26.8% (15/56) and 33.9% (20/59), respectively, which were equivalent to the frequency calculated based on random integration (33.2%). In addition, HTLV-I provirus was prone to integration near the transcriptional start sites in leukemic cells (P = .006), and the transcriptional direction of the provirus was in accordance with that of integrated cellular genes in 70% of cases. More importantly, the integration sites in the carrier cells favored the alphoid repetitive sequences (11/56; 20%) whereas in leukemic cells they disfavored these sequences (2/59; 3.4%). Taken together, during natural course from carrier to onset of ATL, HTLV-I-infected cells with integration sites favorable for viral gene transcription are susceptible to malignant transformation due to increased viral gene expression.

High levels of cytoplasmic HTLV-1 Tax mutant proteins retain a Tax-NF-κB-CBP ternary complex in the cytoplasm

The oncogenic potential of HTLV-1 Tax protein is partially ascribed to its capacity to activate NF-kappaB. The current view is that Tax acts first in the cytoplasm to dissociate NF-kappaB factors from the IkappaB proteins and enable their nuclear translocation, then Tax links p65(RelA), within the nucleus, to CBP/p300 and P/CAF, which are essential for its optimal transcriptional activity. Our present study challenges the paradigm that Tax-p65(RelA)-CBP/p300 assembly occurs in the nucleus. Using Tax mutants defective for nuclear localization we show that at low levels these mutants induce the nuclear translocation of NF-kappaB factors but not their transcriptional activity, whereas at high levels they trap CBP and free p65(RelA) in the cytoplasm and block, thereby, their transcriptional function. In contrast, wild-type (w.t.) Tax strongly stimulated NF-kappaB-dependent gene expression in all tested experimental settings. These data suggest that the Tax-p65(RelA)-CBP ternary complex is established in the cytoplasm rather than in the nucleus. When this complex is formed with w.t. Tax, the entire moiety translocates into the nucleus and exerts high transcriptional activity. However, if the complex is formed with the cytoplasmic Tax mutants, the resulting moiety is retained in the cytoplasm and is, therefore, devoid of transcriptional activity.

Impact of transforming viruses on cellular mutagenesis, genome stability, and cellular transformation

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.

Identification of Aberrantly Methylated Genes in Association with Adult T-Cell Leukemia

In this study, we identified 53 aberrantly hypermethylated DNA sequences in adult T-cell leukemia (ATL) cells using methylated CpG island amplification/representational difference analysis method. We also observed a proportionate increase in the methylation density of these regions with disease progression. Seven genes, which were expressed in normal T cells, but suppressed in ATL cells, were identified near the hypermethylated regions. Among these silenced genes, Kruppel-like factor 4 (KLF4) gene is a cell cycle regulator and early growth response 3 (EGR3) gene is a critical transcriptional factor for induction of Fas ligand (FasL) expression. Treatment with 5-aza-2'-deoxycytidine resulted in the recovery of their transcription, indicating that their silencing might be associated with DNA hypermethylation. To study their functions in ATL cells, we transfected recombinant adenovirus vectors expressing KLF4 and EGR3 genes. Expression of KLF4 induced apoptosis of ATL cells whereas enforced expression of EGR3 induced the expression of FasL gene, resulting in apoptosis. Thus, suppressed expression of EGR3 enabled ATL cells to escape from activation-induced cell death mediated by FasL. Our results showed that the methylated CpG island amplification/representational difference analysis method allowed the isolation of hypermethylated DNA regions specific to leukemic cells and thus shed light on the roles of DNA methylation in leukemogenesis.

Utilization of the CBP but not the p300 co-activator by human T-lymphotropic virus type-2 Tax for p53 inhibition

We previously reported, both in transfected cells and in human T-cell leukemia virus type-2 subtype B infected cells, that the viral transactivator Tax-2B protein could inhibit p53 functions. We have now investigated the mechanism through which Tax-2B represses p53 using GFPTax-2B fusion proteins. We present evidence that Tax-2B inhibition of p53 function is not linked to CREB/ATF activation, but is uniquely correlated with the interaction of CREB binding protein (CBP), but not p300, with the C-terminus of Tax-2B. Wild type, but not a Tax-2B-M47 mutant, inhibits p53 function in adherent cells. We demonstrate that both Tax-2B and Tax-2B-M47 can bind p300, while Tax-2B-M47 is impaired for CBP binding. Importantly, transfection of increasing amounts of CBP but not p300 or p300/CBP-associated factor (P/CAF) could rescue p53 transcriptional activity in the presence of Tax-2B in nonlymphocytic cells. In lymphoid cells, Tax-2B mediated inhibition of p53 is correlated with the NF-kappaB pathway activation and could be prevented by the overexpression of an IkappaBalpha mutant. Given the similarities between the functional domains of CBP and p300, these results are intriguing and suggest that Tax-2B must bind the CR2 domain of CBP, but not that of p300 in order to repress p53.

Genetic and epigenetic inactivation of tax gene in adult T-cell leukemia cells

To clarify the status of tax gene, we analyzed human T-cell leukemia virus type-I (HTLV-I) associated cell lines and fresh adult T-cell leukemia (ATL) cells. We compared 2 types of HTLV-I associated cell lines: one was derived from leukemic cells (leukemic cell line) and the other from nonleukemic cells (nonleukemic cell line). Although all nonleukemic cell lines expressed Tax, it could not be detected in 3 of 5 leukemic cell lines, in which nonsense mutation or deletion (60 bp) of tax genes, and DNA methylation in 5'-LTR were identified as the responsible changes. We found such genetic changes of the tax gene in 5 of 47 fresh ATL cases (11%). The tax gene transcripts could be detected in 14 of 41 fresh ATL cases (34%) by RT-PCR. In ATL cases with genetic changes that could not produce Tax protein, the tax gene was frequently transcribed, suggesting that such cells do not need the transcriptional silencing. Although DNA methylation of 5'-LTR was detected in the fresh ATL cases (19 of 28 cases; 68%), the complete methylation associated with transcriptional silencing was observed only in 4 cases. Since partial methylation could not silence the transcription, and the tax gene transcription was not detected in 27 of 41 cases (66%), the epigenetic change(s) other than DNA methylation is considered to play an important role in the silencing.

Interaction between C/EBPbeta and Tax down-regulates human T-cell leukemia virus type I transcription

The human T-cell leukemia virus type I (HTLV-I) Tax protein trans-activates viral transcription through three imperfect tandem repeats of a 21-bp sequence called Tax-responsive element (TxRE). Tax regulates transcription via direct interaction with some members of the activating transcription factor/CRE-binding protein (ATF/CREB) family including CREM, CREB, and CREB-2. By interacting with their ZIP domain, Tax stimulates the binding of these cellular factors to the CRE-like sequence present in the TxREs. Recent observations have shown that CCAAT/enhancer binding protein beta (C/EBPbeta) forms stable complexes on the CRE site in the presence of CREB-2. Given that C/EBPbeta has also been found to interact with Tax, we analyzed the effects of C/EBPbeta on viral Tax-dependent transcription. We show here that C/EBPbeta represses viral transcription and that Tax is no more able to form a stable complex with CREB-2 on the TxRE site in the presence of C/EBPbeta. We also analyzed the physical interactions between Tax and C/EBPbeta and found that the central region of C/EBPbeta, excluding its ZIP domain, is required for direct interaction with Tax. It is the first time that Tax is described to interact with a basic leucine-zipper (bZIP) factor without recognizing its ZIP domain. Although unexpected, this result explains why C/EBPbeta would be unable to form a stable complex with Tax on the TxRE site and could then down-regulate viral transcription. Lastly, we found that C/EBPbeta was able to inhibit Tax expression in vivo from an infectious HTLV-I molecular clone. In conclusion, we propose that during cell activation events, which stimulate the Tax synthesis, C/EBPbeta may down-regulate the level of HTLV-I expression to escape the cytotoxic-T-lymphocyte response.

Leukemogenesis of adult T-cell leukemia

Adult T-cell leukemia (ATL) is one of the most aggressive hematologic malignancies and is caused by human T-cell leukemia virus type I (HTLV-I). Tax, encoded by the HTLV-I pX region, has been recognized by its pleiotropic actions as a critical accessory protein playing a central role in leukemogenesis. However, fresh ATL cells frequently lose Tax protein expression via several mechanisms, such as genetic and epigenetic changes in the provirus. Furthermore, there is a long latency period before the onset of ATL, indicating the multistep mechanisms of leukemogenesis. Therefore, additional factors, including other viral proteins, genetic and epigenetic changes of the host genome, and alterations in the gene expression and immune systems of the host cells, may be implicated in ATL leukemogenesis. This review summarizes recent advances in the understanding of ATL leukemogenesis.

T-Cell Control by Human T-Cell Leukemia/Lymphoma Virus Type 1

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.

The HBZ factor of human T-cell leukemia virus type I dimerizes with transcription factors JunB and c-Jun and modulates their transcriptional activity

The human T-cell leukemia virus type I (HTLV-I)-encoded Tax protein activates transcription from the viral promoter via association with the cellular basic leucine zipper factor cAMP-response element-binding protein-2. Tax is also able to induce cellular transformation of T lymphocytes probably by modulating transcriptional activity of cellular factors, including nuclear factor-kappaB, E2F, activator protein-1 (AP-1), and p53. Recently, we characterized in HTLV-I-infected cells the presence of a novel viral protein, HBZ, encoded by the complementary strand of the HTLV-I RNA genome (Gaudray, G., Gachon, F., Basbous, J., Biard-Piechaczyk, M., Devaux, C., and Mesnard, J.-M. (2002) J. Virol. 76, 12813-12822). HBZ is a nuclear basic leucine zipper protein that down-regulates Tax-dependent viral transcription by inhibiting the binding of cAMP-response element-binding protein-2 to the HTLV-I promoter. In searching for other cellular targets of HBZ, we identified two members of the Jun family, JunB and c-Jun. Co-immunoprecipitation and cellular colocalization confirmed that HBZ interacts in vivo with JunB and c-Jun. When transiently introduced into CEM cells with a reporter gene containing the AP-1 site from the collagenase promoter, HBZ suppressed transactivation by c-Jun. On the other hand, the combination of HBZ with Jun-B had higher transcriptional activity than JunB alone. Consistent with the structure of its basic domain, we demonstrate that HBZ decreases the DNA-binding activity of c-Jun and JunB. Last, we show that c-Jun is no longer capable of activating the basal expression of the HTLV-I promoter in the presence of HBZ in vivo. Our results support the hypothesis that HBZ could be a negative modulator of the Tax effect by controlling Tax expression at the transcriptional level and by attenuating activation of AP-1 by Tax.

Human T-cell leukemia virus type I and adult T-cell leukemia

Human T-cell leukemia virus type I (HTLV-I) causes adult T-cell leukemia (ATL) in about 5% of carriers after a long latent period. After its infection, HTLV-I promotes the clonal proliferation of HTLV-I infected cells in vivo by actions of encoded viral proteins, including Tax. However, leukemic cells frequently lack the expression of Tax by the genetic and epigenetic changes of HTLV-I provirus, suggesting that Tax is not always necessary after transformation. Alternatively, ATL cells without Tax protein could escape from the host immune system since Tax is the major target of cytotoxic lymphocytes. During the latent period, alterations of host genome accumulate, finally leading to onset of ATL.

Cellular transformation by the HTLV-I Tax protein, a jack-of-all-trades

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.

P53 facilitates degradation of human T-cell leukaemia virus type I Tax-binding protein through a proteasome-dependent pathway

Human T-cell leukaemia virus type 1 (HTLV-I), the aetiological agent of adult T-cell leukaemia (ATL) and tropical spastic paraparesis (TSP/HAM), transforms human T-cells in vivo and in vitro. The Tax protein of HTLV-I is essential for cellular transformation as well as viral and cellular gene transactivation. The interaction of Tax with cellular proteins is critical for these functions. We previously isolated and characterized a novel Tax-binding protein, TRX (TAX1BP2), by screening a Jurkat T-cell cDNA library. In the present study, we present evidence that the tumour suppressor p53 targets the TRX protein for proteasome degradation. Pulse-chase experiments revealed that p53 enhanced the degradation of TRX protein and reduced the half-life from 2.0 to 0.25 h. p53 mutants R248W and R273H enhance TRX degradation suggesting a transcriptionally independent mechanism. Both HTLV-I Tax and the proteasome-specific inhibitor MG132 inhibited p53-mediated TRX protein degradation. These results suggest that TRX degradation is mediated through activation of the proteasome protein degradation pathway independent of transcriptional function of p53. Our results provide the first experimental evidence that Tax inhibits transcription-dependent and independent functions of p53.

The human T-cell lymphoma/leukemia viruses

The primate T-cell lymphoma/leukemia viruses belong to an oncogenic genus of complex retroviruses. Members of this genus have been shown to be pathogenic in man. The human T-cell lymphoma/leukemia virus (HTLV) type I has been linked in the etiology of T-cell malignancies and "autoimmune-like" neurologic and rheumatic disorders; a related virus, HTLV-II, is becoming increasingly associated with similar disorders. Cell transformation is thought to be caused predominantly by the effects of the viral regulatory protein, Tax. An additional induced host cell molecule, adult T-cell lymphoma-derived factor, may contribute to cell immortalization. Like the DNA tumor viruses, HTLV activates transcription of cellular proto-oncogenes and inhibits cellular mechanisms of tumor suppression, cell cycle arrest, and apoptosis. However, individuals who are able to mount a strong cell-mediated immune response and limit viral entry into uninfected cells do not develop associated malignancies. Unfortunately, HTLV-induced malignancies are difficult to treat with conventional chemotherapy, and disease progression is often rapid with a median survival of less than 2 years. There are, however, some novel approaches that have yet to be fully tested that may have greater efficacy in the treatment of HTLV-induced diseases. In the future, better screening and detection methods, along with new vaccines and therapies, may contribute to the increased prevention and control of HTLV infection and its associated diseases.

Human T-cell leukemia virus type 1 Tax protein stimulates the interferon-responsive enhancer element via NF-kappaB activity

Gene expression in Rat-1 fibroblast cells transformed by Tax from human T-cell leukemia virus type 1 was studied using the reverse transcriptase polymerase chain reaction differential display technique. The analysis revealed eight genes that were upregulated and one gene that was suppressed in Tax-transformed cells. Interestingly, at least four of the upregulated genes were interferon-stimulated genes. Promoter analysis of the 2',5'-oligoadenylate synthetase gene, which was activated in both Tax-transformed Rat-1 cells and primary adult T-cell leukemia cells, demonstrated that Tax indirectly activates its interferon-responsive enhancer element in a nuclear factor-kappaB pathway-dependent manner, indicating a close association of interferon signaling with the transformation by Tax.

Distinct nuclear body components, PML and SMRT, regulate the trans-acting function of HTLV-1 Tax oncoprotein

Several viruses target cellular promyelocytic leukemia (PML)-nuclear bodies (PML-NBs) to induce their disruption, marked morphological changes in these structures or the relocation to PML-NB components to the cytoplasm of infected cells. PML conversely interferes with viral replication. We demonstrate that PML acts as a coactivator for the human T-cell leukemia virus type 1 (HTLV-1) Tax oncoprotein without direct binding. Tax was identified within interchromatin granule clusters (IGCs)/RNA splicing bodies (SBs), not PML-NBs; Tax expression did not affect PML-NB formation. Moreover, PML and CBP/p300 cooperatively activated Tax-mediated HTLV-1-LTR-dependent gene expression. Interestingly, two PML mutants, PML-RAR and PMLDelta216-331, which fail to form PML-NBs, could also coactivate Tax-mediated trans-acting function but had no effect on retinoic acid receptor (RAR)- or p53-dependent gene expression. In contrast, SMRT (silencing mediator for retinoic acid and thyroid hormone receptors), a nuclear corepressor found within the matrix-associated deacetylase (MAD) nuclear body, relocalized into Tax-associated nuclear bodies upon coexpression with Tax. SMRT coactivated the trans-acting function of Tax through direct binding. Coexpression of SMRT and PML resulted in an additive activation of Tax trans-acting function. Thus, crosstalk between distinct nuclear bodies may control Tax function.

The HTLV-1 tax oncoprotein attenuates DNA damage induced G1 arrest and enhances apoptosis in p53 null cells

Transformation of cells by the human T cell leukemia virus type 1 occurs via mechanisms unique among oncogenic retroviruses. A prevailing hypothesis for HTLV-1-mediated cellular transformation is that expression of the viral transactivator, Tax, induces genomic instability. Tax-mediated failure in the cellular repair response is one possible mechanism for loss in genomic integrity. Here we have examined the in vivo repair response of Tax-expressing cells to determine the underlying defects that contribute to loss of genomic integrity. In these studies we examined the effects of de novo Tax-expression in naive "pre-neoplastic" REF52 cells. DNA-damage-induced p53 stabilization and concomitant transient stabilization of p21 were clearly evident in Tax-expressing cells. Likewise, the damage-induced apoptotic response of Tax-expressing cells was normal. However, the damage-induced G1 checkpoint was abrogated in either p53+ or p53- cellular backgrounds. Although nucleotide excision repair (NER) of asynchronous Tax-expressing cells was impaired, cell-cycle-independent assessment of NER in the global excision repair assay demonstrated comparable NER activity in Tax-expressing cells, suggesting that the failure of G1 checkpoint contributes to NER deficiency. Interestingly, we observed a dramatic increase in apoptosis and UV sensitivity of Tax-expressing p53-/- cells when compared to Tax-expressing p53+/+ cells. These data demonstrate that Tax-mediated cellular genomic instability arises from attenuation of cell-cycle checkpoint and imply a clonal dependence on p53 status separate from genomic integrity.

Seizing of T Cells by Human T-Cell Leukemia⧸Lymphoma Virus Type 1

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.

Inhibition of HTLV-1 transcription by cyclin dependent kinase inhibitors

HTLV-1 is the etiologic agent for adult T-cell leukemia/lymphoma (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), where viral replication and transformation are largely dependent upon modification of regulatory and host cell cycle proteins. The mechanism of HTLV-1 transformation appears to be distinct from that of many known chronic or acute leukemia viruses and is related to the viral activator Tax. Here we show that cyclin E, can associate tightly with the coactivator p300 and Pol II complex in HTLV-1 infected cells. The cyclin E associated complex is kinase active and phosphorylates the carboxy terminal domain of RNA Pol II. More importantly, p21/Waf1, a well-known cdk inhibitor at the G1/S border, inhibits transcription of HTLV-1 in both transfections and in in vitro transcription assays. Finally, specific cdk chemical inhibitors, functionally similar to cellular cdkIs, such as p21/Waf1 which inhibits cyclin E/cdk2 activity, also inhibit transcription of the HTLV-1 promoter. In particular, Purvalanol A, with an IC50 of 0.035 microm inhibits activated, but not basal transcription, as well as HTLV-1 infected cells. Collectively, the role of cyclin E/cdk2 in HTLV-1 infected cells and its involvement in RNA Pol II phosphorylation is discussed.

Damaged DNA and miscounted chromosomes: human T cell leukemia virus type I tax oncoprotein and genetic lesions in transformed cells

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.