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

Molecular targeting of PD-1 signaling pathway as a novel therapeutic approach in HTLV-1 infection

HTLV-1, the first human oncogenic retrovirus, is a type C retrovirus that belongs to the Deltaretrovirus genus. The HTLV-1 genome has 8.5 kbp length, and consists of major genes such as gag, pol, pro, env, and pX region. This retrovirus is considered as one of the most deadly infectious agent for peripheral-blood mononuclear cells (PBMC). The infection of HTLV-1 can lead to dangerous complications, such as infective dermatitis (ID), uveitis, arthritis, lymphadenitis, arthropathies, Sjögren's Syndrome (SS), and particularly HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) or Adult T-Cell Leukemia Lymphoma (ATLL). At the moment, Zidovudine (AZT) plus IFN-α is the only treatment available for HTLV-1 infections. Based on scientific studies, alongside the therapeutic regimens, intrinsic mechanisms also play a determinant role in reducing the signs of disease. Programmed cell death-1 (PD-1) signaling pathway, one of the most important checkpoints, has recently received interest, such as the development of a novel generation of anti-tumors. In the present study, we discuss the role of PD-1 signaling pathway in HTLV-1 infection as well as its application as a novel approach for treatment of HTLV-1 infections.

Silencers of HTLV-1 and HTLV-2: the pX-encoded latency-maintenance factors

Of the members of the primate T cell lymphotropic virus (PTLV) family, only the human T-cell leukemia virus type-1 (HTLV-1) causes disease in humans—as the etiological agent of adult T-cell leukemia/lymphoma (ATLL), HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), and other auto-inflammatory disorders. Despite having significant genomic organizational and structural similarities, the closely related human T-cell lymphotropic virus type-2 (HTLV-2) is considered apathogenic and has been linked with benign lymphoproliferation and mild neurological symptoms in certain infected patients. The silencing of proviral gene expression and maintenance of latency are central for the establishment of persistent infections in vivo. The conserved pX sequences of HTLV-1 and HTLV-2 encode several ancillary factors which have been shown to negatively regulate proviral gene expression, while simultaneously activating host cellular proliferative and pro-survival pathways. In particular, the ORF-II proteins, HTLV-1 p30^II and HTLV-2 p28^II, suppress Tax-dependent transactivation from the viral promoter—whereas p30^II also inhibits PU.1-mediated inflammatory-signaling, differentially augments the expression of p53-regulated metabolic/pro-survival genes, and induces lymphoproliferation which could promote mitotic proviral replication. The ubiquitinated form of the HTLV-1 p13^II protein localizes to nuclear speckles and interferes with recruitment of the p300 coactivator by the viral transactivator Tax. Further, the antisense-encoded HTLV-1 HBZ and HTLV-2 APH-2 proteins and mRNAs negatively regulate Tax-dependent proviral gene expression and activate inflammatory signaling associated with enhanced T-cell lymphoproliferation. This review will summarize our current understanding of the pX latency-maintenance factors of HTLV-1 and HTLV-2 and discuss how these products may contribute to the differences in pathogenicity between the human PTLVs.

The Botanical Glycoside Oleandrin Inhibits Human T-cell Leukemia Virus Type-1 Infectivity and Env-Dependent Virological Synapse Formation

At present, there are no antiretroviral drugs that inhibit incorporation of the envelope glycoprotein into newly-synthesized virus particles. The botanical glycoside, oleandrin, derived from extracts of Nerium oleander, has previously been shown to reduce the levels of the gp120 envelope glycoprotein on human immunodeficiency virus type-1 (HIV-1) particles and inhibit HIV-1 infectivity in vitro. We therefore tested whether oleandrin or an extract from N. oleander could also inhibit the infectivity of the human T-cell leukemia virus type-1 (HTLV-1): A related enveloped retrovirus and emerging tropical infectious agent. The treatment of HTLV-1+ lymphoma T-cells with either oleandrin or a N. oleander extract did not significantly inhibit viral replication or the release of p19^Gag-containing particles into the culture supernatants. However, the collected virus particles from treated cells exhibited reduced infectivity on primary human peripheral blood mononuclear cells (huPBMCs). Unlike HIV-1, extracellular HTLV-1 particles are poorly infectious and viral transmission typically occurs via direct intercellular interactions across a virological synapse. We therefore investigated whether oleandrin or a N. oleander extract could inhibit virus transmission from a GFP-expressing HTLV-1+ lymphoma T-cell-line to huPBMCs in co-culture assays. These results demonstrated that both oleandrin and the crude phytoextract inhibited the formation of virological synapses and the transmission of HTLV-1 in vitro. Importantly, these findings suggest oleandrin may have broad antiviral activity against enveloped viruses by reducing the incorporation of the envelope glycoprotein into mature particles, a stage of the infection cycle not targeted by modern HAART.

The TP53-Induced Glycolysis and Apoptosis Regulator mediates cooperation between HTLV-1 p30II and the retroviral oncoproteins Tax and HBZ and is highly expressed in an in vivo xenograft model of HTLV-1-induced lymphoma

The human T-cell leukemia virus type-1 (HTLV-1) is an oncoretrovirus that infects and transforms CD4+ T-cells and causes adult T-cell leukemia/lymphoma (ATLL) -an aggressive lymphoproliferative disease that is highly refractive to most anticancer therapies. The HTLV-1 proviral genome encodes several regulatory products within a conserved 3' nucleotide sequence, known as pX; however, it remains unclear how these factors might cooperate or dynamically interact in virus-infected cells. Here we demonstrate that the HTLV-1 latency-maintenance factor p30^II induces the TP53-induced glycolysis and apoptosis regulator (TIGAR) and counters the oxidative stress, mitochondrial damage, and cytotoxicity caused by the viral oncoproteins Tax and HBZ. The p30^II protein cooperates with Tax and HBZ and enhances their oncogenic potential in colony transformation/foci-formation assays. Further, we have shown that TIGAR is highly expressed in HTLV-1-induced tumors associated with oncogene dysregulation and increased angiogenesis in an in vivo xenograft model of HTLV-1-induced T-cell lymphoma. These findings provide the first evidence that p30^II likely collaborates as an ancillary factor for the major oncoproteins Tax and HBZ during retroviral carcinogenesis.

The human T-cell leukemia virus type-1 p30II protein activates p53 and induces the TIGAR and suppresses oncogene-induced oxidative stress during viral carcinogenesis

In normal cells, aberrant oncogene expression leads to the accumulation of cytotoxic metabolites, including reactive oxygen species (ROS), which can cause oxidative DNA-damage and apoptosis as an intrinsic barrier against neoplastic disease. The c-Myc oncoprotein is overexpressed in many lymphoid cancers due to c-myc gene amplification and/or 8q24 chromosomal translocations. Intriguingly, p53 is a downstream target of c-Myc and hematological malignancies, such as adult T-cell leukemia/lymphoma (ATL), frequently contain wildtype p53 and c-Myc overexpression. We therefore hypothesized that p53-regulated pro-survival signals may thwart the cell's metabolic anticancer defenses to support oncogene-activation in lymphoid cancers. Here we show that the Tp53-induced glycolysis and apoptosis regulator (TIGAR) promotes c-myc oncogene-activation by the human T-cell leukemia virus type-1 (HTLV-1) latency-maintenance factor p30^II, associated with c-Myc deregulation in ATL clinical isolates. TIGAR prevents the intracellular accumulation of c-Myc-induced ROS and inhibits oncogene-induced cellular senescence in ATL, acute lymphoblastic leukemia, and multiple myeloma cells with elevated c-Myc expression. Our results allude to a pivotal role for p53-regulated antioxidant signals as mediators of c-Myc oncogenic functions in viral and non-viral lymphoid tumors.

Acetylation of the c-MYC oncoprotein is required for cooperation with the HTLV-1 p30(II) accessory protein and the induction of oncogenic cellular transformation by p30(II)/c-MYC

The human T-cell leukemia retrovirus type-1 (HTLV-1) p30(II) protein is a multifunctional latency-maintenance factor that negatively regulates viral gene expression and deregulates host signaling pathways involved in aberrant T-cell growth and proliferation. We have previously demonstrated that p30(II) interacts with the c-MYC oncoprotein and enhances c-MYC-dependent transcriptional and oncogenic functions. However, the molecular and biochemical events that mediate the cooperation between p30(II) and c-MYC remain to be completely understood. Herein we demonstrate that p30(II) induces lysine-acetylation of the c-MYC oncoprotein. Acetylation-defective c-MYC Lys→Arg substitution mutants are impaired for oncogenic transformation with p30(II) in c-myc(-/-) HO15.19 fibroblasts. Using dual-chromatin-immunoprecipitations (dual-ChIPs), we further demonstrate that p30(II) is present in c-MYC-containing nucleoprotein complexes in HTLV-1-transformed HuT-102 T-lymphocytes. Moreover, p30(II) inhibits apoptosis in proliferating cells expressing c-MYC under conditions of genotoxic stress. These findings suggest that c-MYC-acetylation is required for the cooperation between p30(II)/c-MYC which could promote proviral replication and contribute to HTLV-1-induced carcinogenesis.

The need to accessorize: molecular roles of HTLV-1 p30 and HTLV-2 p28 accessory proteins in the viral life cycle

Extensive studies of human T-cell leukemia virus (HTLV)-1 and HTLV-2 over the last three decades have provided detailed knowledge on viral transformation, host–viral interactions and pathogenesis. HTLV-1 is the etiological agent of adult T cell leukemia and multiple neurodegenerative and inflammatory diseases while HTLV-2 disease association remains elusive, with few infected individuals displaying neurodegenerative diseases similar to HTLV-1. The HTLV group of oncoretroviruses has a genome that encodes structural and enzymatic proteins Gag, Pro, and Env, regulatory proteins Tax and Rex, and several accessory proteins from the pX region. Of these proteins, HTLV-1 p30 and HTLV-2 p28 are encoded by the open reading frame II of the pX region. Like most other accessory proteins, p30 and p28 are dispensable for in vitro viral replication and transformation but are required for efficient viral replication and persistence in vivo. Both p30 and p28 regulate viral gene expression at the post-transcriptional level whereas p30 can also function at the transcriptional level. Recently, several reports have implicated p30 and p28 in multiple cellular processes, which provide novel insight into HTLV spread and survival and ultimately pathogenesis. In this review we summarize and compare what is known about p30 and p28, highlighting their roles in viral replication and viral pathogenesis.

Protective roles of epithelial cells in the survival of adult T-cell leukemia/lymphoma cells

Adult T-cell leukemia/lymphoma (ATL) is a highly invasive and intractable T-cell malignancy caused by human T-cell leukemia virus-1 infection. We demonstrate herein that normal tissue-derived epithelial cells (NECs) exert protective effects on the survival of leukemic cells, which may partially account for high resistance to antileukemic therapies in patients with ATL. Viral gene-silenced, ATL-derived cell lines (ATL cells) dramatically escaped from histone deacetylase inhibitor-induced apoptosis by direct co-culture with NECs. Adhesions to NECs suppressed p21(Cip1) expression and increased a proportion of resting G0/G1 phase cells in trichostatin A (TSA)-treated ATL cells. ATL cells adhering to NECs down-regulated CD25 expression and enhanced vimentin expression, suggesting that most ATL cells acquired a quiescent state by cell-cell interactions with NECs. ATL cells adhering to NECs displayed highly elevated expression of the cancer stem cell marker CD44. Blockade of CD44 signaling diminished the NEC-conferred resistance of ATL cells to TSA-induced apoptosis. Co-culture with NECs also suppressed the expression of NKG2D ligands on TSA-treated ATL cells, resulting in decreased natural killer cell-mediated cytotoxicity. Combined evidence suggests that interactions with normal epithelial cells augment the resistance of ATL cells to TSA-induced apoptosis and facilitate immune evasion by ATL cells.

Human T Lymphotropic Virus Type I (HTLV-I) Oncogenesis: Molecular Aspects of Virus and Host Interactions in Pathogenesis of Adult T cell Leukemia/Lymphoma (ATL)

The study of tumor viruses paves the way for understanding the mechanisms of virus pathogenesis, including those involved in establishing infection and dissemination in the host tumor affecting immune-compromised patients. The processes ranging from viral infection to progressing malignancy are slow and usually insufficient for establishment of transformed cells that develop cancer in only a minority of infected subjects. Therefore, viral infection is usually not the only cause of cancer, and further environmental and host factors, may be implicated. HTLV-I, in particular, is considered as an oncovirus cause of lymphoproliferative disease such as adult T cell leukemia/lymphoma (ATL) and disturbs the immune responses which results in HTLV-I associated meylopathy/tropical spastic parapresis (HAM/TSP). HTLV-I infection causes ATL in a small proportion of infected subjects (2-5%) following a prolonged incubation period (15-30 years) despite a strong adaptive immune response against the virus. Overall, these conditions offer a prospect to study the molecular basis of tumorgenicity in mammalian cells. In this review, the oncogencity of HTLV-I is being considered as an oncovirus in context of ATL.

HTLV-1 Rex: the courier of viral messages making use of the host vehicle

The human T-cell leukemia virus type 1 (HTLV-1) is a retrovirus causing an aggressive T-cell malignancy, adult T-cell leukemia (ATL). Although HTLV-1 has a compact RNA genome, it has evolved elaborate mechanisms to maximize its coding potential. The structural proteins Gag, Pro, and Pol are encoded in the unspliced form of viral mRNA, whereas the Env protein is encoded in singly spliced viral mRNA. Regulatory and accessory proteins, such as Tax, Rex, p30II, p12, and p13, are translated only from fully spliced mRNA. For effective viral replication, translation from all forms of HTLV-1 transcripts has to be achieved in concert, although unspliced mRNA are extremely unstable in mammalian cells. It has been well recognized that HTLV-1 Rex enhances the stability of unspliced and singly spliced HTLV-1 mRNA by promoting nuclear export and thereby removing them from the splicing site. Rex specifically binds to the highly structured Rex responsive element (RxRE) located at the 3' end of all HTLV-1 mRNA. Rex then binds to the cellular nuclear exporter, CRM1, via its nuclear export signal domain and the Rex-viral transcript complex is selectively exported from the nucleus to the cytoplasm for effective translation of the viral proteins. Yet, the mechanisms by which Rex inhibits the cellular splicing machinery and utilizes the cellular pathways beneficial to viral survival in the host cell have not been fully explored. Furthermore, physiological impacts of Rex against homeostasis of the host cell via interactions with numerous cellular proteins have been largely left uninvestigated. In this review, we focus on the biological importance of HTLV-1 Rex in the HTLV-1 life cycle by following the historical path in the literature concerning this viral post-transcriptional regulator from its discovery to this day. In addition, for future studies, we discuss recently discovered aspects of HTLV-1 Rex as a post-transcriptional regulator and its use in host cellular pathways.

Human T-lymphotropic virus proteins and post-translational modification pathways

Cell life from the cell cycle to the signaling transduction and response to stimuli is finely tuned by protein post-translational modifications (PTMs). PTMs alter the conformation, the stability, the localization, and hence the pattern of interactions of the targeted protein. Cell pathways involve the activation of enzymes, like kinases, ligases and transferases, that, once activated, act on many proteins simultaneously, altering the state of the cell and triggering the processes they are involved in. Viruses enter a balanced system and hijack the cell, exploiting the potential of PTMs either to activate viral encoded proteins or to alter cellular pathways, with the ultimate consequence to perpetuate through their replication. Human T-lymphotropic virus type 1 (HTLV-1) is known to be highly oncogenic and associates with adult T-cell leukemia/lymphoma, HTLV-1-associated myelopathy/tropical spastic paraparesis and other inflammatory pathological conditions. HTLV-1 protein activity is controlled by PTMs and, in turn, viral activity is associated with the modulation of cellular pathways based on PTMs. More knowledge is acquired about the PTMs involved in the activation of its proteins, like Tax, Rex, p12, p13, p30, HTLV-I basic leucine zipper factor and Gag. However, more has to be understood at the biochemical level in order to counteract the associated fatal outcomes. This review will focus on known PTMs that directly modify HTLV-1 components and on enzymes whose activity is modulated by viral proteins.

Comparative host protein interactions with HTLV-1 p30 and HTLV-2 p28: insights into difference in pathobiology of human retroviruses

Background

Human T lymphotropic virus type-1 (HTLV-1) and type 2 (HTLV-2) are closely related human retroviruses, but have unique disease associations. HTLV-1 is the causative agent of an aggressive T-cell leukemia known as adult T-cell leukemia (ATL), HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP), and other inflammatory diseases. HTLV-2 infection has not been clearly associated with any disease condition. Although both viruses can transform T cells in vitro, the HTLV-1 provirus is mainly detected in CD4+ T cells whereas HTLV-2 is mainly detected in CD8+ T cells of infected individuals. HTLV-1 and HTLV-2 encode accessory proteins p30 and p28, respectively, which share partial amino acid homology and are required for viral persistence in vivo. The goal of this study was to identify host proteins interacting with p30 and p28 in order to understand their role in pathogenesis.

Results

Affinity-tag purification coupled with mass spectrometric (MS) analyses revealed 42 and 22 potential interacting cellular partners of p30 and p28, respectively. Of these, only three cellular proteins, protein arginine methyltransferase 5 (PRMT5), hnRNP K and 60 S ribosomal protein L8 were detected in both p30 and p28 fractions. To validate the proteomic results, four interacting proteins were selected for further analyses using immunoblot assays. In full agreement with the MS analysis two cellular proteins REGγ and NEAF-interacting protein 30 (NIP30) selectively interacted with p30 and not with p28; heterogeneous nuclear ribonucleoprotein H1 (hnRNP H1) bound to p28 and not to p30; and PRMT5 interacted with both p30 and p28. Further studies demonstrated that reduced levels of PRMT5 resulted in decreased HTLV-2 viral gene expression whereas the viral gene expression of HTLV-1 was unchanged.

Conclusion

The comparisons of p30 and p28 host protein interaction proteome showed striking differences with some degree of overlap. PRMT5, one of the host proteins that interacted with both p30 and p28 differentially affected HTLV-1 and HTLV-2 viral gene expression suggesting that PRMT5 is involved at different stages of HTLV-1 and HTLV-2 biology. These findings suggest that distinct host protein interaction profiles of p30 and p28 could, in part, be responsible for differences in HTLV-1 and HTLV-2 pathobiology. This study provides new avenues of investigation into mechanisms of viral infection, tropism and persistence.

Human T lymphotropic virus type 1 regulatory and accessory gene transcript expression and export are not rex dependent

Human T lymphotropic virus type 1 (HTLV-1) requires regulated gene expression from unspliced and alternatively spliced transcripts for efficient replication and persistence. HTLV-1 Rex is known to facilitate cytoplasmic export of unspliced, gag/pol and incompletely spliced env mRNAs, but its contribution to the expression of other viral transcripts has not been experimentally assessed. In this study, we utilized HTLV-1 proviral clones, cellular fractionation, and real-time reverse transcriptase PCR to determine the role of Rex on the expression and export of all viral mRNAs. Our results indicate that the steady-state levels of the different viral mRNAs are modulated by Rex, which we attribute to a redistribution of completely spliced mRNAs toward incompletely spliced mRNAs. Furthermore, we confirmed the positive effect of Rex on the unspliced gag/pol mRNA and singly spliced env mRNA, resulting in increased cytoplasmic expression. However, the cytoplasmic export of the alternatively spliced HTLV-1 mRNAs encoding the accessory proteins and the antisense Hbz mRNA are independent of direct Rex regulation. This is consistent with the conclusion that viral mRNAs that contain the cis-acting repressive sequence (CRS) and/or a fully functional splice donor site require a Rex/RxRE interaction for efficient cytoplasmic expression.

Orf-I and orf-II-encoded proteins in HTLV-1 infection and persistence

The 3′ end of the human T-cell leukemia/lymphoma virus type-1 (HTLV-1) genome contains four overlapping open reading frames (ORF) that encode regulatory proteins. Here, we review current knowledge of HTLV-1 orf-I and orf-II protein products. Singly spliced mRNA from orf-I encodes p12, which can be proteolytically cleaved to generate p8, while differential splicing of mRNA from orf-II results in production of p13 and p30. These proteins have been demonstrated to modulate transcription, apoptosis, host cell activation and proliferation, virus infectivity and transmission, and host immune responses. Though these proteins are not essential for virus replication in vitro, p8, p12, p13, and p30 have an important role in the establishment and maintenance of HTLV-1 infection in vivo.

Murine leukemia retrovirus integration induces the formation of transcription factor complexes on palindromic sequences in the signal transducer and activator of transcription factor 5a gene during the development of pre-B lymphomagenesis

Murine leukemia retrovirus (MLV) vectors are highly effective tools for introducing a foreign gene into a target host genome. However, it remains unclear how integrated retroviral promoter activity is influenced by the upstream or downstream sequences and how the host cell phenotype is influenced by the integrated promoter activity. Herein, we analyzed a set of pre-B lymphoma clones in which the MLV genome was integrated into the signal transducer and activator of transcription factor 5a (Stat5a) gene. Among the clones, the lymphoma clones with a provirus integrating into the middle position of the palindromic target sequences showed significantly higher transcription of the Stat5a gene; and p300 and other transcriptional factors formed complexes, with binding to the proviral-host junctional DNA segment. By using a luciferase assay, the upstream and downstream sequences of the provirus contributed to the up-regulation of proviral promoter activity. In concomitance with the higher Stat5a transcription, the immunoglobulin gene recombination was arrested. Antiapoptotic activity was significantly higher, with an increase in Bcl-xL, one of the targets of STAT5A, when IL-7 was supplied. Thus, a minute difference between MLV integration sites can lead to large differences in the host phenotype through the formation of transcription factor complexes on the proviral-host junctional DNA segment, suggesting that caution is necessary in monitoring integration sites when working with MLV vectors.

Human T-lymphotropic virus type 1 p30 interacts with REGgamma and modulates ATM (ataxia telangiectasia mutated) to promote cell survival

Human T-lymphotropic virus type 1 (HTLV-1) is a causative agent of adult T cell leukemia/lymphoma and a variety of inflammatory disorders. HTLV-1 encodes a nuclear localizing protein, p30, that selectively alters viral and cellular gene expression, activates G(2)-M cell cycle checkpoints, and is essential for viral spread. Here, we used immunoprecipitation and affinity pulldown of ectopically expressed p30 coupled with mass spectrometry to identify cellular binding partners of p30. Our data indicate that p30 specifically binds to cellular ATM (ataxia telangiectasia mutated) and REGγ (a nuclear 20 S proteasome activator). Under conditions of genotoxic stress, p30 expression was associated with reduced levels of ATM and increased cell survival. Knockdown or overexpression of REGγ paralleled p30 expression, suggesting an unexpected enhancement of p30 expression in the presence of REGγ. Finally, size exclusion chromatography revealed the presence of p30 in a high molecular mass complex along with ATM and REGγ. On the basis of our findings, we propose that HTLV-1 p30 interacts with ATM and REGγ to increase viral spread by facilitating cell survival.

Human T-cell leukemia virus type 2 post-transcriptional control protein p28 is required for viral infectivity and persistence in vivo

Background

Human T-cell leukemia virus (HTLV) type 1 and type 2 are related but distinct pathogenic complex retroviruses. HTLV-1 is associated with adult T-cell leukemia and a variety of immune-mediated disorders including the chronic neurological disease termed HTLV-1-associated myelopathy/tropical spastic paraparesis. In contrast, HTLV-2 displays distinct biological differences and is much less pathogenic, with only a few reported cases of leukemia and neurological disease associated with infection. In addition to the structural and enzymatic proteins, HTLV encodes regulatory (Tax and Rex) and accessory proteins. Tax and Rex positively regulate virus production and are critical for efficient viral replication and pathogenesis. Using an over-expression system approach, we recently reported that the accessory gene product of the HTLV-1 and HTLV-2 open reading frame (ORF) II (p30 and p28, respectively) acts as a negative regulator of both Tax and Rex by binding to and retaining their mRNA in the nucleus, leading to reduced protein expression and virion production. Further characterization revealed that p28 was distinct from p30 in that it was devoid of major transcriptional modulating activity, suggesting potentially divergent functions that may be responsible for the distinct pathobiologies of HTLV-1 and HTLV-2.

Results

In this study, we investigated the functional significance of p28 in HTLV-2 infection, proliferation, and immortaliztion of primary T-cells in culture, and viral survival in an infectious rabbit animal model. An HTLV-2 p28 knockout virus (HTLV-2Δp28) was generated and evaluated. Infectivity and immortalization capacity of HTLV-2Δp28 in vitro was indistinguishable from wild type HTLV-2. In contrast, we showed that viral replication was severely attenuated in rabbits inoculated with HTLV-2Δp28 and the mutant virus failed to establish persistent infection.

Conclusion

We provide direct evidence that p28 is dispensable for viral replication and cellular immortalization of primary T-lymphocytes in cell culture. However, our data indicate that p28 function is critical for viral survival in vivo. Our results are consistent with the hypothesis that p28 repression of Tax and Rex-mediated viral gene expression may facilitate survival of these cells by down-modulating overall viral gene expression.

Human T-lymphotropic virus type-1 p30 alters cell cycle G2 regulation of T lymphocytes to enhance cell survival

Background

Human T-lymphotropic virus type-1 (HTLV-1) causes adult T-cell leukemia/lymphoma and is linked to a number of lymphocyte-mediated disorders. HTLV-1 contains both regulatory and accessory genes in four pX open reading frames. pX ORF-II encodes two proteins, p13 and p30, whose roles are still being defined in the virus life cycle and in HTLV-1 virus-host cell interactions. Proviral clones of HTLV-1 with pX ORF-II mutations diminish the ability of the virus to maintain viral loads in vivo. p30 expressed exogenously differentially modulates CREB and Tax-responsive element-mediated transcription through its interaction with CREB-binding protein/p300 and while acting as a repressor of many genes including Tax, in part by blocking tax/rex RNA nuclear export, selectively enhances key gene pathways involved in T-cell signaling/activation.

Results

Herein, we analyzed the role of p30 in cell cycle regulation. Jurkat T-cells transduced with a p30 expressing lentivirus vector accumulated in the G2-M phase of cell cycle. We then analyzed key proteins involved in G2-M checkpoint activation. p30 expression in Jurkat T-cells resulted in an increase in phosphorylation at serine 216 of nuclear cell division cycle 25C (Cdc25C), had enhanced checkpoint kinase 1 (Chk1) serine 345 phosphorylation, reduced expression of polo-like kinase 1 (PLK1), diminished phosphorylation of PLK1 at tyrosine 210 and reduced phosphorylation of Cdc25C at serine 198. Finally, primary human lymphocyte derived cell lines immortalized by a HTLV-1 proviral clone defective in p30 expression were more susceptible to camptothecin induced apoptosis. Collectively these data are consistent with a cell survival role of p30 against genotoxic insults to HTLV-1 infected lymphocytes.

Conclusion

Collectively, our data are the first to indicate that HTLV-1 p30 expression results in activation of the G2-M cell cycle checkpoint, events that would promote early viral spread and T-cell survival.