Human and simian T-cell leukemia viruses type 2 (HTLV-2 and STLV-2(pan-p)) transform T cells independently of Jak/STAT activation

Superoxide-Generating Nox5α Is Functionally Required for the Human T-Cell Leukemia Virus Type 1-Induced Cell Transformation Phenotype

Human T-cell leukemia virus type 1 (HTLV-1) is associated with adult T-cell leukemia (ATL) and transforms T cells in vitro. To our knowledge, the functional role of reactive oxygen species (ROS)-generating NADPH oxidase 5 (Nox5) in HTLV-1 transformation remains undefined. Here, we found that Nox5α expression was upregulated in 88% of 17 ATL patient samples but not in normal peripheral blood T cells. Upregulation of the Nox5α variant was transcriptionally sustained by the constitutive Janus family tyrosine kinase (Jak)-STAT5 signaling pathway in interleukin-2 (IL-2)-independent HTLV-1-transformed cell lines, including MT1 and MT2, whereas it was transiently induced by the IL-2-triggered Jak-STAT5 axis in uninfected T cells. A Nox inhibitor, diphenylene iodonium, and antioxidants such as N-acetyl cysteine blocked proliferation of MT1 and MT2 cells. Ablation of Nox5α by small interfering RNAs abrogated ROS production, inhibited cellular activities, including proliferation, migration, and survival, and suppressed tumorigenicity in immunodeficient NOG mice. The findings suggest that Nox5α is a key molecule for redox-signal-mediated maintenance of the HTLV-1 transformation phenotype and could be a potential molecular target for therapeutic intervention in cancer development.

IMPORTANCE

HTLV-1 is the first human oncogenic retrovirus shown to be associated with ATL. Despite the extensive study over the years, the mechanism underlying HTLV-1-induced cell transformation is not fully understood. In this study, we addressed the expression and function of ROS-generating Nox family genes in HTLV-1-transformed cells. Our report provides the first evidence that the upregulated expression of Nox5α is associated with the pathological state of ATL peripheral blood mononuclear cells and that Nox5α is an integral component of the Jak-STAT5 signaling pathway in HTLV-1-transformed T cells. Nox5α-derived ROS are critically involved in the regulation of cellular activities, including proliferation, migration, survival, and tumorigenicity, in HTLV-1-transformed cells. These results indicate that Nox5α-derived ROS are functionally required for maintenance of the HTLV-1 transformation phenotype. The finding provides new insight into the redox-dependent mechanism of HTLV-1 transformation and raises an intriguing possibility that Nox5α serves as a potential molecular target to treat HTLV-1-related leukemia.

HTLV-1/-2 and HIV-1 co-infections: retroviral interference on host immune status

The human retroviruses HIV-1 and HTLV-1/HTLV-2 share similar routes of transmission but cause significantly different diseases. In this review we have outlined the immune mediated mechanisms by which HTLVs affect HIV-1 disease in co-infected hosts. During co-infection with HIV-1, HTLV-2 modulates the cellular microenvironment favoring its own viability and inhibiting HIV-1 progression. This is achieved when the HTLV-2 proviral load is higher than that of HIV-1, and thanks to the ability of HTLV-2 to: (i) up-regulate viral suppressive CCL3L1 chemokine expression; (ii) overcome HIV-1 capacity to activate the JAK/STAT pathway; (iii) reduce the activation of T and NK cells; (iv) modulate the host miRNA profiles. These alterations of immune functions have been mainly attributed to the effects of the HTLV-2 regulatory protein Tax and suggest that HTLV-2 exerts a protective role against HIV-1 infection. Contrary to HIV-1/HTLV-2, the effect of HIV-1/HTLV-1 co-infection on immunological and pathological conditions is still controversial. There is evidence that indicates a worsening of HIV-1 infection, while other evidence does not show clinically relevant effects in HIV-positive people. Possible differences on innate immune mechanisms and a particularly impact on NK cells are becoming evident. The differences between the two HIV-1/HTLV-1 and HIV-1/HTLV-2 co-infections are highlighted and further discussed.

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.

Preexisting infection with human T-cell lymphotropic virus type 2 neither exacerbates nor attenuates simian immunodeficiency virus SIVmac251 infection in macaques

Coinfection with human T-cell lymphotropic virus type 2 (HTLV-2) and human immunodeficiency virus type 1 (HIV-1) has been reported to have either a slowed disease course or to have no effect on progression to AIDS. In this study, we generated a coinfection animal model and investigated whether HTLV-2 could persistently infect macaques, induce a T-cell response, and impact simian immunodeficiency virus SIV(mac251)-induced disease. We found that inoculation of irradiated HTLV-2-infected T cells into Indian rhesus macaques elicited humoral and T-cell responses to HTLV-2 antigens at both systemic and mucosal sites. Low levels of HTLV-2 provirus DNA were detected in the blood, lymphoid tissues, and gastrointestinal tracts of infected animals. Exposure of HTLV-2-infected or naïve macaques to SIV(mac251) demonstrated comparable levels of SIV(mac251) viral replication, similar rates of mucosal and peripheral CD4(+) T-cell loss, and increased T-cell proliferation. Additionally, neither the magnitude nor the functional capacity of the SIV-specific T-cell-mediated immune response was different in HTLV-2/SIV(mac251) coinfected animals versus SIV(mac251) singly infected controls. Thus, HTLV-2 targets mucosal sites, persists, and importantly does not exacerbate SIV(mac251) infection. These data provide the impetus for the development of an attenuated HTLV-2-based vectored vaccine for HIV-1; this approach could elicit persistent mucosal immunity that may prevent HIV-1/SIV(mac251) infection.

In vivo genetic mutations define predominant functions of the human T-cell leukemia/lymphoma virus p12I protein

The human T-cell leukemia/lymphoma virus type 1 (HTLV-1) ORF-I encodes a 99-amino acid hydrophobic membrane protein, p12(I), that affects receptors in different cellular compartments. We report here that proteolytic cleavage dictates different cellular localization and functions of p12(I). The removal of a noncanonical endoplasmic reticulum (ER) retention/retrieval signal within the amino terminus of p12(I) is necessary for trafficking to the Golgi apparatus and generation of a completely cleaved 8-kDa protein. The 8-kDa protein in turn traffics to the cell surface, is recruited to the immunologic synapse following T-cell receptor (TCR) ligation, and down-regulates TCR proximal signaling. The uncleaved 12-kDa form of p12(I) resides in the ER and interacts with the beta and gamma(c) chains of the interleukin-2 receptor (IL-2R), the heavy chain of the major histocompatibility complex (MHC) class I, as well as calreticulin and calnexin. Genetic analysis of ORF-I from ex vivo samples of HTLV-1-infected patients reveals predominant amino acid substitutions within ORF-I that affect proteolytic cleavage, suggesting that ER-associated functions of p12(I) may contribute to the survival and proliferation of the infected T cells in the host.

Inhibition of constitutively active Jak-Stat pathway suppresses cell growth of human T-cell leukemia virus type 1-infected T-cell lines and primary adult T-cell leukemia cells

Background

Human T-cell leukemia virus type 1 (HTLV-1), the etiologic agent for adult T-cell leukemia (ATL), induces cytokine-independent proliferation of T-cells, associated with the acquisition of constitutive activation of Janus kinases (Jak) and signal transducers and activators of transcription (Stat) proteins. Our purposes in this study were to determine whether activation of Jak-Stat pathway is responsible for the proliferation and survival of ATL cells, and to explore mechanisms by which inhibition of Jak-Stat pathway kills ATL cells.

Results

Constitutive activation of Stat3 and Stat5 was observed in HTLV-1-infected T-cell lines and primary ATL cells, but not in HTLV-1-negative T-cell lines. Using AG490, a Jak-specific inhibitor, we demonstrated that the activation of Stat3 and Stat5 was mediated by the constitutive phosphorylation of Jak proteins. AG490 inhibited the growth of HTLV-1-infected T-cell lines and primary ATL cells by inducing G₁ cell-cycle arrest mediated by altering the expression of cyclin D2, Cdk4, p53, p21, Pim-1 and c-Myc, and by apoptosis mediated by the reduced expression of c-IAP2, XIAP, survivin and Bcl-2. Importantly, AG490 did not inhibit the growth of normal peripheral blood mononuclear cells.

Conclusion

Our results indicate that activation of Jak-Stat pathway is responsible for the proliferation and survival of ATL cells. Inhibition of this pathway may provide a new approach for the treatment of ATL.

Comparative biology of human T-cell lymphotropic virus type 1 (HTLV-1) and HTLV-2

HTLV-1 and HTLV-2 are highly related complex retroviruses that have been studied intensely for nearly three decades because of their association with neoplasia, neuropathology, and/or their capacity to transform primary human T lymphocytes. The study of HTLV also represents an attractive model that has allowed investigators to dissect the mechanism of various cellular processes, several of which may be critical steps in HTLV-mediated pathogenesis. Both HTLV-1 and HTLV-2 can efficiently immortalize and transform T lymphocytes in cell culture and persist in infected individuals or experimental animals. However, the clinical manifestations of these two viruses differ significantly. HTLV-1 is associated with adult T-cell leukemia (ATL) and a variety of immune-mediated disorders including the chronic neurological disease termed HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). In contrast, HTLV-2 is much less pathogenic with reports of only a few cases of variant hairy cell leukemia and neurological disease associated with infection. The limited number of individuals shown to harbor HTLV-2 in association with specific diseases has, to date, precluded convincing epidemiological demonstration of a definitive etiologic role of HTLV-2 in human disease. Therefore, it has become clear that comparative studies designed to elucidate the mechanisms by which HTLV-1 and HTLV-2 determine distinct outcomes are likely to provide fundamental insights into the initiation of multistep leukemogenesis.

Deregulation of cell-signaling pathways in HTLV-1 infection

Human T-lymphotropic virus type 1 (HTLV-1) infection is associated with the clonal expansion and transformation of mature T lymphocytes. While the mechanisms involved are incompletely understood the viral regulatory protein Tax plays a central role in these processes. Recent studies employing genomic and proteomic approaches have demonstrated the marked complexity of gene deregulation associated with Tax expression and confirmed the remarkable pleiotropism of this protein as evidenced by the numerous Tax-cellular protein interactions in infected cells. In this review, we summarize the role of Tax in the deregulation of selected cellular-signaling pathways. Specifically, this has focused on the influence and interaction of Tax with the AP-1 and NF-AT transcription factors, PDZ domain-containing proteins, Rho-GTPases, and the Janus kinase/signal transducer and activator of transcription and transforming growth factor-beta-signaling pathways. In addition to identifying the deregulation of events within these pathways, attempts have been made to highlight differences between HTLV-1 and -2, which may relate to differences in their pathogenic properties.

Ubiquitination of the common cytokine receptor gammac and regulation of expression by an ubiquitination/deubiquitination machinery

The common cytokine receptor gamma(c) is shared by the interleukin-2, -4, -7, -9, -15, and -21 receptors, and is essential for lymphocyte proliferation and survival. The regulation of gamma(c) receptor expression level is therefore critical for the ability of cells to respond to these cytokines. We previously reported that gamma(c) is efficiently constitutively internalized and addressed towards a degradation endocytic compartment. We show that gamma(c) is ubiquitinated and also associated to ubiquitinated proteins. We report that the ubiquitin-ligase c-Cbl induces gamma(c) down-regulation. In addition, the ubiquitin-hydrolase, DUB-2, counteracts the effect of c-Cbl on gamma(c) expression. We show that an increase in DUB-2 expression correlates with an increased gamma(c) half-life, resulting in the up-regulation of the receptor. Altogether, we show that gamma(c) is the target of an ubiquitination mechanism and its expression level can be regulated through the activities of a couple of ubiquitin-ligase/ubiquitin-hydrolase enzymes, namely c-Cbl/DUB-2.

A 10-amino acid domain within human T-cell leukemia virus type 1 and type 2 tax protein sequences is responsible for their divergent subcellular distribution

Human T-cell leukemia virus type 1 and type 2 (HTLV-1/2) are related retroviruses that infect T-lymphocytes. Whereas HTLV-1 infection can cause leukemia, HTLV-2 has not been demonstrated to be the agent of a hematological malignant disease. Nevertheless, the virally encoded Tax-1 and Tax-2 transactivators display a high percentage of similarity. Tax-1 is a shuttling protein that contains a noncanonical nuclear localization signal as well as a nuclear export signal. The presence of the nuclear localization signal and the nuclear export signal domains in the Tax-2 sequence has not been determined. The distribution of Tax-2 in infected cells is not known but has been assumed to be similar to that of Tax-1. By using a Tax-2-specific antibody, we report here that Tax-2 is located predominantly in the cytoplasm of the HTLV-2 immortalized or transformed infected T-cells. These results were confirmed after transient transfection of untagged Tax-1 and Tax-2 constructs, histidine tag Tax1/Tax2, GFP-Tax, and Tax-GFP fusion constructs in several cell lines. We show that this unanticipated localization is not due to a default in the Tax-2 nuclear localization signal functions nor to major differences in Tax-2 versus Tax-1 binding to the IKKgamma/NEMO protein. In addition, we demonstrate that inhibiting the proteasome results in a relocalization of Tax-1 in the cytoplasm, similar to that of Tax-2. By using a series of Tax-1/Tax-2 chimeras, we determined that the minimal domain that is necessary for Tax-2 peculiar distribution encompasses amino acids 90-100. Finally, we show a high correlation between intracellular localization of Tax and their NF-kappaB or CREB transactivating ability.

DUB-3, a Cytokine-inducible Deubiquitinating Enzyme That Blocks Proliferation

Previous studies have identified the DUB family of cytokine-regulated murine deubiquitinating enzymes, which play a role in the control of cell proliferation and survival. Through data base analyses and cloning, we have identified a human cDNA (DUB-3) that shows significant homology to the known murine DUB family members. Northern blotting has shown expression of this gene in a number of tissues including brain, liver, and muscle, with two transcripts being apparent (1.6 and 1.7 kb). In addition, expression was observed in cell lines including those derived from a number of hematopoietic tumors such as the Burkitt's lymphoma cell line RAJI. We have also demonstrated that DUB-3, which was shown to be an active deubiquitinating enzyme, is induced in response to interleukin-4 and interleukin-6 stimulation. Finally, we have demonstrated that constitutive expression of DUB-3 blocks proliferation and can initiate apoptosis in both IL-3-dependent Ba/F3 cells and NIH3T3 fibroblasts. These findings suggest that human DUB-3, like the murine DUB family members, is transiently induced in response to cytokines and can, when constitutively expressed, block growth factor-dependent proliferation.

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.

Human T-cell lymphotropic virus type 1 p12I enhances interleukin-2 production during T-cell activation

Human T-cell lymphotropic virus type 1 (HTLV-1) causes adult T-cell leukemia/lymphoma (ATLL) and a variety of lymphoproliferative disorders. The early virus-cell interactions that determine a productive infection remain unclear. However, it is well recognized that T-cell activation is required for effective retroviral integration into the host cell genome and subsequent viral replication. The HTLV-1 pX open reading frame I encoding protein, p12(I), is critical for the virus to establish persistent infection in vivo and for infection in quiescent primary lymphocytes in vitro. p12(I) localizes in the endoplasmic reticulum (ER) and cis-Golgi apparatus, increases intracellular calcium and activates nuclear factor of activated T cells (NFAT)-mediated transcription. To clarify the function of p12(I), we tested the production of IL-2 from Jurkat T cells and peripheral blood mononuclear cells (PBMC) expressing p12(I). Lentiviral vector expressed p12(I) in Jurkat T cells enhanced interleukin-2 (IL-2) production in a calcium pathway-dependent manner during T-cell receptor (TCR) stimulation. Expression of p12(I) also induced higher NFAT-mediated reporter gene activities during TCR stimulation in Jurkat T cells. In contrast, p12 expression in PBMC elicited increased IL-2 production in the presence of phorbal ester stimulation, but not during TCR stimulation. Finally, the requirement of ER localization for p12(I)-mediated NFAT activation was demonstrated and two positive regions and two negative regions in p12(I) were identified for the activation of this transcription factor by using p12(I) truncation mutants. These results are the first to indicate that HTLV-1, an etiologic agent associated with lymphoproliferative diseases, uses a conserved accessory protein to induce T-cell activation, an antecedent to efficient viral infection.

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.

Retroviral interference on STAT activation in individuals coinfected with human T cell leukemia virus type 2 and HIV-1

Human T cell leukemia virus (HTLV) type-2 is a human retrovirus whose infection has not been tightly linked to human diseases. However, the fairly high prevalence of this infection among HIV-1-positive individuals indicates the importance of better understanding the potential interference of HTLV-2 infection on HIV-1 infection and AIDS. We previously demonstrated that one signature of PBMC freshly derived from HIV-1-infected individuals is the constitutive activation of a C-terminal truncated STAT5 (STAT5Delta). Therefore, we analyzed the potential activation of STATs in HTLV-2 monoinfected and HTLV-2/HIV-1 dually infected individuals. We observed that PBMC of HTLV-2-infected individuals do not show STAT activation unless they are cultivated ex vivo, in the absence of any mitogenic stimuli, for at least 8 h. The emergence of STAT activation, namely of STAT1, in culture was mostly related to the secretion of IFN-gamma. Of note, this phenomenon is not only a characteristic feature of HTLV-2-infected individuals but also occurred with PBMC of HIV-1(+) individuals. Surprisingly, HTLV-2/HIV-1 coinfection resulted in low/absent STAT activation in vivo that paralleled a diminished secretion of IFN-gamma after ex vivo cultivation. Our findings indicate that both HTLV-2 and HIV-1 infection prime T lymphocytes for STAT1 activation, but they also highlight an interference exerted by HTLV-2 on HIV-1-induced STAT1 activation. Although the nature of such a phenomenon is unclear at the present, these findings support the hypothesis that HTLV-2 may interfere with HIV-1 infection at multiple levels.

Human T-cell leukemia virus type 2 induces survival and proliferation of CD34(+) TF-1 cells through activation of STAT1 and STAT5 by secretion of interferon-gamma and granulocyte macrophage-colony-stimulating factor

Human T-cell leukemia-lymphoma virus (HTLV) type-2 can induce the survival and proliferation of CD34(+) TF-1 cells deprived of interleukin (IL)-3. This effect did not require productive infection and occurred when HTLV-2 was produced from T cells (CMo), but not from B cells (BMo), unless the latter virus was complexed with anti-HLA-DR monoclonal antibodies (mAbs). Cellular and molecular mechanisms triggered by HTLV-2 interaction with TF-1 cells were here investigated. Activation of signal transducer and activator of transcription (STAT) 5 protein occurred in TF-1 cells incubated either with IL-3 or with HTLV-2/CMo; in addition the virus, but not IL-3, activated STAT1. The effect of HTLV-2 required several hours, suggesting dependence on the induction of cellular factors. By screening a panel of secreted factors, granulocyte macrophage-colony-stimulating factor (GM-CSF), interferon (IFN)-gamma, and stem cell factor (SCF) were found induced by HTLV-2 in TF-1 cells. Of note is the fact that these molecules induce a variety of biologic effects through the activation of STAT proteins, including STAT1 and STAT5. Neutralization experiments indicated that GM-CSF and IFN-gamma, but not SCF, were responsible for HTLV-2-induced STAT activation, whereas anti-GM-CSF antibodies greatly inhibited TF-1 cell proliferation. Finally, incubation of BMo virus with anti-HLA-DR mAb rescued TF-1 cell survival in the absence of IL-3. Thus, HTLV-2 interaction with CD34(+) precursor cells may lead to the expression of cytokines that, by inducing autocrine activation of STATs, may influence the host's regenerative capacity and immune response to HTLV-2 and to other infectious agents.

HTLV-1 cell lines differ in constitutively activated signaling pathways that can be altered by cytokine exposure

Examination of signaling pathways used by HTLV-1-infected rabbit cell lines revealed differences between one, RH/K30, that mediates asymptomatic infection and another, RH/K34, that causes lethal experimental leukemia. Both lines are IL-2 independent; RH/K30 produces IL-4 while RH/K34 produces IL-10. Examination of the Jak/STAT (Janus kinase/signal transducer and activator of transcription) activation of the lines revealed constitutive phosphorylation of Jak1 in both STAT6 phosphorylation, not previously reported for HTLV-1 cells, was observed in RH/K30; STAT1 and STAT3 were phosphorylated in RH/K34. Treatment with cytokines altered the activation of the STAT proteins: IL-2 induced STAT5 phosphorylation in both lines. Supernatant from RH/K34 or IL-10 induced STAT3 phosphorylation in RH/K30 cells. Supernatant from RH/K30 or IL-4 induced STAT6 phosphorylation in RH/K34 cells, which could be reversed with a Jak kinase inhibitor--AG-490.

The deubiquitinating enzyme DUB-2 prolongs cytokine-induced signal transducers and activators of transcription activation and suppresses apoptosis following cytokine withdrawal

Cytokines, such as interleukin-2 (IL-2), activate intracellular signaling pathways via rapid tyrosine phosphorylation of their receptors, resulting in the activation of many genes involved in cell growth and survival. The deubiquitinating enzyme DUB-2 is induced in response to IL-2 but as yet its function has not been determined. The results of this study show that DUB-2 is expressed in human T-cell lymphotropic virus-I (HTLV-1)-transformed T cells that exhibit constitutive activation of the IL-2 JAK/STAT (signal transducers and activators of transcription) pathway, and when expressed in Ba/F3 cells DUB-2 markedly prolonged IL-2-induced STAT5 phosphorylation. Although DUB-2 did not enhance IL-2-mediated proliferation, when withdrawn from growth factor, cells expressing DUB-2 had sustained STAT5 phosphorylation and enhanced expression of IL-2-induced genes cis and c-myc. Moreover, DUB-2 expression markedly inhibited apoptosis induced by cytokine withdrawal allowing cells to survive. Taken together these data suggest that DUB-2 can enhance signaling through the JAK/STAT pathway, prolong lymphocyte survival, and, when constitutively expressed, may contribute to the activation of the JAK/STAT pathway observed in some transformed cells.

HTLV-1 p12(I) protein enhances STAT5 activation and decreases the interleukin-2 requirement for proliferation of primary human peripheral blood mononuclear cells

The p12(I) protein, encoded by the pX open reading frame I of the human T-lymphotropic virus type 1 (HTLV-1), is a hydrophobic protein that localizes to the endoplasmic reticulum and the Golgi. Although p12(I) contains 4 minimal proline-rich, src homology 3-binding motifs (PXXP), a characteristic commonly found in proteins involved in signaling pathways, it has not been known whether p12(I) has a role in modulating intracellular signaling pathways. This study demonstrated that p12(I) binds to the cytoplasmic domain of the interleukin-2 receptor (IL-2R) beta chain that is involved in the recruitment of the Jak1 and Jak3 kinases. As a result of this interaction, p12(I) increases signal transducers and activators of transcription 5 (STAT5) DNA binding and transcriptional activity and this effect depends on the presence of both IL-2R beta and gamma(c) chains and Jak3. Transduction of primary human peripheral blood mononuclear cells (PBMCs) with a human immunodeficiency virus type 1-based retroviral vector expressing p12(I) also resulted in increased STAT5 phosphorylation and DNA binding. However, p12(I) could increase proliferation of human PBMCs only after stimulation of T-cell receptors by treatment of cells with low concentrations of alphaCD3 and alphaCD28 antibodies. In addition, the proliferative advantage of p12(I)-transduced PBMCs was evident mainly at low concentrations of IL-2. Together, these data indicate that p12(I) may confer a proliferative advantage on HTLV-1-infected cells in the presence of suboptimal antigen stimulation and that this event may account for the clonal proliferation of infected T cells in vivo. (Blood. 2001;98:823-829)

Molecular biology and pathogenesis of the human T-cell leukaemia/lymphotropic virus Type-1 (HTLV-1)

Retroviruses are associated with a variety of diseases, including immunological and neurological disorders, and various forms of cancer. In humans, the Human T-cell Leukaemia/Lymphotropic virus type 1 (HTLV-1), which belongs to the Oncovirus family, is the aetiological agent of two diverse diseases: Adult T-cell leukaemia/lymphoma (ATLL) (Poiesz et al. 1980; Hinuma et al. 1981; Yoshida et al. 1982), as well as the neurological disorder tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM) (Gessain et al. 1985; Rodgers-Johnson et al. 1985; Osame et al. 1986). HTLV-1 is the only human retrovirus known to be the aetiological agent of cancer. A genetically related virus, HTLV-2, has been identified and isolated (Kalyanaraman et al. 1982). However, there has been no demonstration of a definitive aetiological role for HTLV-2 in human disease to date. Simian T-cell lymphotropic viruses types 1 and 2 (STLV-1 and -2) and bovine leukaemia virus (BLV) have also been classified in same group, Oncoviridae, based upon their similarities in genetic sequence and structure to HTLV-1 and -2 (Burny et al. 1988; Dekaban et al. 1995; Slattery et al. 1999). This article will focus on HTLV-1, reviewing its discovery, molecular biology, and its role in disease pathogenesis.

In vitro cellular tropism of human T cell leukemia virus type 2

Human T cell leukemia virus type 1 (HTLV-1) and 2 (HTLV-2) are distinct oncogenic retroviruses that infect several cell types, but display their biologic/pathogenic activity only in T lymphocytes. HTLV-1 is associated with adult T cell leukemia, a malignancy of mature CD4(+) T cells, and a chronic neurological disorder termed HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). HTLV-2 is less pathogenic and has been associated with a few cases of a variant of hairy cell leukemia and neurological disease. Previous studies have indicated that in vivo HTLV-1 has a preferential tropism for CD4(+) T cells, whereas HTLV-2 in vivo tropism is less clear, but appears to favor CD8(+) T cells. The molecular mechanism that determines the cellular tropism of HTLV-1 and HTLV-2 has not been precisely determined. However, one study by our group has provided evidence that HTLV-1-enhanced viral transcription in CD4(+) T cells may be responsible for its tropism. In an effort to understand HTLV-2 tropism we tested the ability of HTLV-2 to infect, replicate in, and transform purified CD4(+) or CD8(+) T cells in cell culture. After cocultures of purified primary human CD4(+) and CD8(+) T cells with an HTLV-2-producer cell line we measured viral transcription by reverse transcription PCR analysis, virus production by p19(gag) ELISA, proviral integration by DNA slot-blot analysis, surface phenotype by FACS analysis, and cellular transformation. We also measured HTLV-2 long terminal repeat-directed transcription in the presence and absence of Tax in purified CD4(+) and CD8(+) T cells, using transient transfection assays. Our data indicate that CD4(+) and CD8(+) T cells are equally susceptible to HTLV-2 infection. We observed no significant difference in viral transcription based on mRNA and virus production in CD4(+) and CD8(+) T cell cocultures. Although LTR transcription was enhanced 12- to 16-fold in the presence of Tax, there was no significant difference in CD4(+) or CD8(+) T cells. Interestingly, we show that HTLV-2 preferentially transforms CD8(+) T cells in culture. Together, our data indicate that, unlike HTLV-1, HTLV-2 cell tropism is not due to inhibition of viral infection and inefficient gene expression in CD4(+) versus CD8(+) T cells, and likely involves unique interactions with viral and CD8(+) T cell-specific proteins.

Functional uncoupling of the Janus kinase 3-Stat5 pathway in malignant growth of human T cell leukemia virus type 1-transformed human T cells

Human T cell leukemia virus type 1 (HTLV-1) transforms cytokine-dependent T lymphocytes and causes adult T cell leukemia. Janus tyrosine kinase (Jak)3 and transcription factors Stat5a and Stat5b are essential for the proliferation of normal T cells and are constitutively hyperactivated in both HTLV-1-transformed human T cell lines and lymphocytes isolated from HTLV-1-infected patients; therefore, a critical role for the Jak3-Stat5 pathway in the progression of this disease has been postulated. We recently reported that tyrphostin AG-490 selectively blocked IL-2 activation of Jak3/Stat5 and growth of murine T cell lines. Here we demonstrate that disruption of Jak3/Stat5a/b signaling with AG-490 (50 microM) blocked the proliferation of primary human T lymphocytes, but paradoxically failed to inhibit the proliferation of HTLV-1-transformed human T cell lines, HuT-102 and MT-2. Structural homologues of AG-490 also inhibited the proliferation of primary human T cells, but not HTLV-1-infected cells. Disruption of constitutive Jak3/Stat5 activation by AG-490 was demonstrated by inhibition of 1) tyrosine phosphorylation of Jak3, Stat5a (Tyr(694)), and Stat5b (Tyr(699)); 2) serine phosphorylation of Stat5a (Ser(726)) as determined by a novel phosphospecific Ab; and 3) Stat5a/b DNA binding to the Stat5-responsive beta-casein promoter. In contrast, AG-490 had no effect on DNA binding by p50/p65 components of NF-kappaB, a transcription factor activated by the HTLV-1-encoded phosphoprotein, Tax. Collectively, these data suggest that the Jak3-Stat5 pathway in HTLV-1-transformed T cells has become functionally redundant for proliferation. Reversal of this functional uncoupling may be required before Jak3/Stat5 inhibitors will be useful in the treatment of this malignancy.

The MHC class I heavy chain is a common target of the small proteins encoded by the 3' end of HTLV type 1 and HTLV type 2

Human T cell leukemia/lymphotropic virus types 1 and 2 are two immunologically and phylogenetically related retroviruses that differ in their pathogenicity in vivo. The overall genetic structure of HTLV-1 and -2 is similar. Each contains a unique region at the 3' end of the genome, designated the pX region. p12(I) is a membrane-associated protein encoded by the open reading frame I (ORF I) region of HTLV-1, which lies within the pX region. A corresponding protein, p10(I) is encoded by the ORF I region of HTLV-2 and an additional protein, p11(V), is encoded by ORF V, which overlaps the HTLV-2 ORF I region. As with HTLV-1, the small proteins encoded by the pX region of HTLV-2 appear to be dispensable for viral replication and cellular transformation in vitro. However, the small open reading frames of both viruses are important for viral replication in vivo, which suggests they may play an important role during the viral life cycle. This study was undertaken to investigate and compare the cellular targets of the p10(I), p11(V), and p12(I) putative proteins.

Bcl-XL is up-regulated by HTLV-I and HTLV-II in vitro and in ex vivo ATLL samples

Human T lymphotropic virus type I (HTLV-I) is the etiological agent of adult T-cell lymphocytic leukemia (ATLL), whereas HTLV-II has not been associated with hematopoietic malignancies. The control of apoptotic pathways has emerged as a critical step in the development of many cancer types. As a result, the underlying mechanism of long-term survival of HTLV-I and HTLV-II was studied in infected T cells in vitro and in ex vivo ATLL samples. Results indicate that HTLV-I– and HTLV-II–infected T cells in vitro express high levels of the antiapoptotic protein Bcl compared with other human leukemic T cell lines or uninfected peripheral blood mononuclear cells. The levels of proapoptotic proteins Bax, BAD, and Bak were not significantly altered. HTLV-I and HTLV-II viral transactivators, Tax1 and Tax2, are known to increase expression of cellular genes. These proteins were tested for increased transcription from the human Bcl2 and Bcl-XL promoters. Whereas no effect was observed on the Bcl2 promoter, both Tax1 and Tax2 increased transcription of the Bcl-XL promoter in T cells, although Tax1 appeared to be more efficient than Tax2. The biological significance of these observations was validated by the finding of an increased expression of Bcl-XL in ex vivo ATLL cells, especially from patients unresponsive to various chemotherapy regimens. Altogether, these data suggest that overexpression of Bcl-XL in vivomay be in part responsible for the resistance of ATLL cells to chemotherapy. In addition, inefficient activation of the Bcl-XL promoter by Tax2 may result in a shorter survival time of HTLV-II–infected cells in vivo and a diminished risk of leukemia development.

Bcl-XL is up-regulated by HTLV-I and HTLV-II in vitro and in ex vivo ATLL samples

Human T lymphotropic virus type I (HTLV-I) is the etiological agent of adult T-cell lymphocytic leukemia (ATLL), whereas HTLV-II has not been associated with hematopoietic malignancies. The control of apoptotic pathways has emerged as a critical step in the development of many cancer types. As a result, the underlying mechanism of long-term survival of HTLV-I and HTLV-II was studied in infected T cells in vitro and in ex vivo ATLL samples. Results indicate that HTLV-I– and HTLV-II–infected T cells in vitro express high levels of the antiapoptotic protein Bcl compared with other human leukemic T cell lines or uninfected peripheral blood mononuclear cells. The levels of proapoptotic proteins Bax, BAD, and Bak were not significantly altered. HTLV-I and HTLV-II viral transactivators, Tax1 and Tax2, are known to increase expression of cellular genes. These proteins were tested for increased transcription from the human Bcl2 and Bcl-XL promoters. Whereas no effect was observed on the Bcl2 promoter, both Tax1 and Tax2 increased transcription of the Bcl-XL promoter in T cells, although Tax1 appeared to be more efficient than Tax2. The biological significance of these observations was validated by the finding of an increased expression of Bcl-XL in ex vivo ATLL cells, especially from patients unresponsive to various chemotherapy regimens. Altogether, these data suggest that overexpression of Bcl-XL in vivomay be in part responsible for the resistance of ATLL cells to chemotherapy. In addition, inefficient activation of the Bcl-XL promoter by Tax2 may result in a shorter survival time of HTLV-II–infected cells in vivo and a diminished risk of leukemia development.

Human T-cell leukemia virus type 2 tax mutants that selectively abrogate NFkappaB or CREB/ATF activation fail to transform primary human T cells

Human T-cell leukemia virus (HTLV) Tax protein has been implicated in the HTLV oncogenic process, primarily due to its pleiotropic effects on cellular genes involved in growth regulation and cell cycle control. To date, several approaches attempting to correlate Tax activation of the CREB/activating transcription factor (ATF) or NFkappaB/Rel transcriptional activation pathway to cellular transformation have yielded conflicting results. In this study, we use a unique HTLV-2 provirus (HTLV(c-enh)) that replicates by a Tax-independent mechanism to directly assess the role of Tax transactivation in HTLV-mediated T-lymphocyte transformation. A panel of well-characterized tax-2 mutations is utilized to correlate the respective roles of the CREB/ATF or NFkappaB/Rel signaling pathway. Our results demonstrate that viruses expressing tax-2 mutations that selectively abrogate NFkappaB/Rel or CREB/ATF activation display distinct phenotypes but ultimately fail to transform primary human T lymphocytes. One conclusion consistent with our results is that the activation of NFkappaB/Rel provides a critical proliferative signal early in the cellular transformation process, whereas CREB/ATF activation is required to promote the fully transformed state. However, complete understanding will require correlation of Tax domains important in cellular transformation to those Tax domains important in the modulation of gene transcription, cell cycle control, induction of DNA damage, and other undefined activities.

Proliferation response to interleukin-2 and Jak/Stat activation of T cells immortalized by human T-cell lymphotropic virus type 1 is independent of open reading frame I expression

Human T-cell lymphotropic virus type 1 (HTLV-1), a complex retrovirus, encodes a hydrophobic 12-kD protein from pX open reading frame (ORF) I that localizes to cellular endomembranes and contains four minimal SH3 binding motifs (PXXP). We have demonstrated the importance of ORF I expression in the establishment of infection and hypothesize that p12(I) has a role in T-cell activation. In this study, we tested interleukin-2 (IL-2) receptor expression, IL-2-mediated proliferation, and Jak/Stat activation in T-cell lines immortalized with either wild-type or ORF I mutant clones of HTLV-1. All cell lines exhibited typical patterns of T-cell markers and maintained mutation fidelity. No significant differences between cell lines were observed in IL-2 receptor chain (alpha, beta, or gamma(c)) expression, in IL-2-mediated proliferation, or in IL-2-induced phosphorylated forms of Stat3, Stat5, Jak1, or Jak3. The expression of ORF I is more likely to play a role in early HTLV-1 infection, such as in the activation of quiescent T cells in vivo.

Cyclic nucleotide phosphodiesterases (PDE) 3 and 4 in normal, malignant, and HTLV-I transformed human lymphocytes

Intracellular cyclic AMP, determined in part by cyclic nucleotide phosphodiesterases (PDEs), regulates proliferation and immune functions in lymphoid cells. Total PDE, PDE3, and PDE4 activities were measured in phytohemagglutinin (PHA)-activated peripheral blood mononuclear cells (PBMC-PHA), normal natural killer (NK) cells, Jurkat and Kit225-K6 leukemic T-cells, T-cell lines transformed with human T-lymphotropic virus (HTLV)-I (a retrovirus that causes adult T-cell leukemia/lymphoma) and HTLV-II (a nonpathogenic retrovirus), normal B-cells, and B-cells transformed with Epstein-Barr virus (EBV). All cells exhibited PDE3 and PDE4 activities but in different proportions. In EBV-transformed B cells, PDE4 was much higher than PDE3. HTLV-I+ T-cells differed significantly from other T-lymphocyte-derived cells in also having a higher proportion of PDE4 activities, which apparently were not related to selective induction of any one PDE4 mRNA (judged by reverse transcription-polymerase chain reaction) or expression of the HTLV-I regulatory protein Tax. In MJ cells (an HTLV-I+ T-cell line), Jurkat cells, and PBMC-PHA cells, the tyrosine kinase inhibitor herbimycin A strongly inhibited PDE activity. Growth of MJ cells was inhibited by herbimycin A and a protein kinase C (PKC) inhibitor, and was arrested in G1 by rolipram, a specific PDE4 inhibitor. Proliferation of several HTLV-I+ T-cell lines, PBMC-PHA, and Jurkat cells was inhibited differentially by forskolin (which activates adenylyl cyclase), the selective PDE inhibitors cilostamide and rolipram, and the nonselective PDE inhibitors pentoxifylline and isobutyl methylxanthine. These results suggest that PDE4 isoforms may be functionally up-regulated in HTLV-I+ T-cells and may contribute to the virus-induced proliferation, and that PDEs could be therapeutic targets in immune/inflammatory and neoplastic diseases.

Limiting amounts of p27Kip1 correlates with constitutive activation of cyclin E-CDK2 complex in HTLV-I-transformed T-cells

Human T-cells immortalized (interleukin-2 [IL-2] dependent) by the human T-cell lymphotropic/leukemia virus type I (HTLV-I), in time, become transformed (IL-2 independent). To understand the biochemical basis of this transition, we have used the sibling HTLV-I-infected T-cell lines, N1186 (IL-2 dependent) and N1186-94 (IL-2 independent), as models to assess the responses to antiproliferative signals. In N1186 cells arrested in G1 after serum/interleukin-2 (IL-2) deprivation, downregulation of the cyclin E-CDK2 kinase activity correlated with decreased phosphorylation of CDK2 and accumulation of p27Kip1 bound to the cyclin E-CDK2 complex, as seen in normal activated PBMCs (peripheral blood mononuclear cells). In contrast, N1186-94 cells failed to arrest in G1 upon serum starvation, displayed constitutive cyclin E-associated kinase activity, and, although CDK2 was partially dephosphorylated, the amount of p27Kip1 bound to the complex did not increase. This observation, extended to two other IL-2-dependent as well as to three IL-2-independent HTLV-I-infected T-cell lines, suggests that the lack of cyclin E-CDK2 kinase downregulation found in the late phase of HTLV-I transformation may correlate with insufficient amounts of p27Kip1 associated with the cyclin E-CDK2 complex. Reconstitution experiments demonstrated that the addition of p27Kip1 to lysates from N1186-94 starved cells resulted in the downregulation of cyclin E-associated kinase activity supporting the notion that the unresponsiveness of the cyclin E-CDK2 complex to growth inhibitory signals may be due to inadequate amounts of p27Kip1 assembled with the complex in HTLV-I-transformed T-cells. In fact, the amount of p27Kip1 protein was lower in most HTLV-I-transformed (IL-2-independent) than in the immortalized (IL-2-dependent) HTLV-I-infected T-cells. Furthermore, specific inhibitors of the phosphatidylinositol 3-kinase (P13K) induced an increase of p27Kip1 protein levels, which correlated with G1 arrest, in both IL-2-dependent and IL-2-independent HTLV-I-infected T-cells. Altogether, these results suggest that maintaining a low level of expression of p27Kip1 is a key event in HTLV-I transformation.

Binding of c-Rel to STAT5 target sequences in HTLV-I-transformed T cells

The type I human T-cell leukemia virus (HTLV-I) induces abnormal growth and subsequent transformation of T cells, which is associated with the development of an acute T-cell malignancy termed adult T-cell leukemia. A characteristic of HTLV-I-transformed T cells is the constitutive nuclear expression of NF-kappaB/Rel family of transcription factors, which appears to be essential for the growth of these transformed cells. Although NF-kappaB/Rel factors are known to induce the expression of T-cell growth factor interleukin (IL)-2, it is unclear how they participate in the IL-2-independent growth of HTLV-I-transformed cells. In this study, we show that certain NF-kappaB/Rel members, predominantly c-Rel, interact with enhancer sequences for STAT5, a key transcription factor mediating IL-2-induced T-cell proliferation. Reporter gene assays reveal that the binding of c-Rel to the STAT5 site present in the Fc gammaR1 gene leads to potent transactivation of this enhancer. Binding of c-Rel to the Fc gammaR1 STAT site also occurs in human peripheral blood T cells immortalized with HTLV-I in vitro and is correlated with enhanced levels of proliferation of these cells. These results raise the possibility that NF-kappaB/Rel may participate in the growth control of HTLV-I-transformed T cells by regulating genes driven by both kappaB and certain STAT enhancers.