Human T-lymphotropic virus type 1 oncoprotein tax promotes unscheduled degradation of Pds1p/securin and Clb2p/cyclin B1 and causes chromosomal instability

STLV-1 Commonly Targets Neurons in the Brain of Asymptomatic Non-Human Primates

The human T-cell leukemia virus (HTLV)-1 is responsible for an aggressive neurodegenerative disease (HAM/TSP) and multiple neurological alterations. The capacity of HTLV-1 to infect central nervous system (CNS) resident cells, together with the neuroimmune-driven response, has not been well-established. Here, we combined the use of human induced pluripotent stem cells (hiPSC) and of naturally STLV-1-infected nonhuman primates (NHP) as models with which to investigate HTLV-1 neurotropism. Hence, neuronal cells obtained after hiPSC differentiation in neural polycultures were the main cell population infected by HTLV-1. Further, we report the infection of neurons with STLV-1 in spinal cord regions as well as in brain cortical and cerebellar sections of postmortem NHP. Additionally, reactive microglial cells were found in infected areas, suggesting an immune antiviral response. These results emphasize the need to develop new efficient models by which to understand HTLV-1 neuroinfection and suggest an alternative mechanism that leads to HAM/TSP.

CDC20 in and out of mitosis: a prognostic factor and therapeutic target in hematological malignancies

Cell division cycle 20 homologue (CDC20) is a well-known regulator of cell cycle, as it controls the correct segregation of chromosomes during mitosis. Many studies have focused on the biological role of CDC20 in cancer development, as alterations of its functionality have been linked to genomic instability and evidence demonstrated that high CDC20 expression levels are associated with poor overall survival in solid cancers. More recently, novel CDC20 functions have been demonstrated or suggested, including the regulation of apoptosis and stemness properties and a correlation with immune cell infiltration. Here, we here summarize and discuss the role of CDC20 inside and outside mitosis, starting from its network of interacting proteins. In the last years, CDC20 has also attracted more interest in the blood cancer field, being overexpressed and showing an association with prognosis both in myeloid and lymphoid malignancies. Preclinical findings showed that selective CDC20 and APC/C^CDC20/APC/C^CDH1 inhibitors, namely Apcin and proTAME, are effective against lymphoma and multiple myeloma cells, resulting in mitotic arrest and apoptosis and synergizing with clinically-relevant drugs. The evidence and hypothesis presented in this review provide the input for further biological and chemical studies aiming to dissect novel potential CDC20 roles and targeting strategies in hematological malignancies.

Contribution of yeast models to virus research

Abstract

Time and again, yeast has proven to be a vital model system to understand various crucial basic biology questions. Studies related to viruses are no exception to this. This simple eukaryotic organism is an invaluable model for studying fundamental cellular processes altered in the host cell due to viral infection or expression of viral proteins. Mechanisms of infection of several RNA and relatively few DNA viruses have been studied in yeast to date. Yeast is used for studying several aspects related to the replication of a virus, such as localization of viral proteins, interaction with host proteins, cellular effects on the host, etc. The development of novel techniques based on high-throughput analysis of libraries, availability of toolboxes for genetic manipulation, and a compact genome makes yeast a good choice for such studies. In this review, we provide an overview of the studies that have used yeast as a model system and have advanced our understanding of several important viruses.

Key points

• Yeast, a simple eukaryote, is an important model organism for studies related to viruses.

• Several aspects of both DNA and RNA viruses of plants and animals are investigated using the yeast model.

• Apart from the insights obtained on virus biology, yeast is also extensively used for antiviral development.

Polyploid Giant Cancer Cells, a Hallmark of Oncoviruses and a New Therapeutic Challenge

Tumors are renowned as intricate systems that harbor heterogeneous cancer cells with distinctly diverse molecular signatures, sizes and genomic contents. Among those various genomic clonal populations within the complex tumoral architecture are the polyploid giant cancer cells (PGCC). Although described for over a century, PGCC are increasingly being recognized for their prominent role in tumorigenesis, metastasis, therapy resistance and tumor repopulation after therapy. A shared characteristic among all tumors triggered by oncoviruses is the presence of polyploidy. Those include Human Papillomaviruses (HPV), Epstein Barr Virus (EBV), Hepatitis B and C viruses (HBV and HCV, respectively), Human T-cell lymphotropic virus-1 (HTLV-1), Kaposi's sarcoma herpesvirus (KSHV) and Merkel polyomavirus (MCPyV). Distinct viral proteins, for instance Tax for HTLV-1 or HBx for HBV have demonstrated their etiologic role in favoring the appearance of PGCC. Different intriguing biological mechanisms employed by oncogenic viruses, in addition to viruses with high oncogenic potential such as human cytomegalovirus, could support the generation of PGCC, including induction of endoreplication, inactivation of tumor suppressors, development of hypoxia, activation of cellular senescence and others. Interestingly, chemoresistance and radioresistance have been reported in the context of oncovirus-induced cancers, for example KSHV and EBV-associated lymphomas and high-risk HPV-related cervical cancer. This points toward a potential linkage between the previously mentioned players and highlights PGCC as keystone cancer cells in virally-induced tumors. Subsequently, although new therapeutic approaches are actively needed to fight PGCC, attention should also be drawn to reveal the relationship between PGCC and oncoviruses, with the ultimate goal of establishing effective therapeutic platforms for treatment of virus-associated cancers. This review discusses the presence of PGCCs in tumors induced by oncoviruses, biological mechanisms potentially favoring their appearance, as well as their consequent implication at the clinical and therapeutic level.

Cell Cycle Changes, DNA Ploidy, and PTTG1 Gene Expression in HTLV-1 Patients

Human T-cell lymphotropic virus type-1 (HTLV-1) is a pathogenic retrovirus that is associated with adult T-cell leukemia/lymphoma (ATL). Genetic instability is the hallmark of ATL. Cell cycle progression is needed for virus particle reproduction. HTLV-1 encoded Tax protein ultimately disrupts the mitotic spindle checkpoint, leading to incorrect chromosome segregation, resulting in aneuploidy. Cell cycle abnormalities have been described in T cells transfected with HTLV-1 virus in vitro, but not in HTLV-1 asymptomatic carriers. PTTG1 and HTLV-1 viral protein Tax exhibit a cooperative transforming activity. Overexpressed PTTG1 results in chromosome instability and aneuploidy, which has been suggested as a mechanism underlying PTTG1 transforming activity. Here we aimed to investigate cell cycle, DNA ploidy and PTTG1 mRNA expression in CD4⁺ and CD8⁺ T cells in healthy subjects (HS), HTLV-1 asymptomatic carriers and ATL patients. We have identified that HTLV-1 asymptomatic carriers have shown DNA aneuploidy and cell cycle arrest at cell cycle phase G₀/G₁ in CD4⁺ T cells. CD8⁺ T cells of HTLV-1 asymptomatic carriers also demonstrated DNA aneuploidy but without alteration in cell cycle. In ATL, CD4⁺ and CD8⁺ T cells present a higher number of cells in cell cycle S-phase and PTTG1 overexpression. These studies provide insight into malignant transformation of HTLV-1 asymptomatic carriers to ATL patients.

The Anaphase Promoting Complex/Cyclosome (APC/C): A Versatile E3 Ubiquitin Ligase

In the present chapter we discuss the essential roles of the human E3 ubiquitin ligase Anaphase Promoting Complex/Cyclosome (APC/C) in mitosis as well as the emerging evidence of important APC/C roles in cellular processes beyond cell division control such as regulation of genomic integrity and cell differentiation of the nervous system. We consider the potential incipient role of APC/C dysregulation in the pathophysiology of the neurological disorder Alzheimer's disease (AD). We also discuss how certain Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA) viruses take control of the host's cell division regulatory system through harnessing APC/C ubiquitin ligase activity and hypothesise the plausible molecular mechanisms underpinning virus manipulation of the APC/C. We also examine how defects in the function of this multisubunit protein assembly drive abnormal cell proliferation and lastly argue the potential of APC/C as a promising therapeutic target for the development of innovative therapies for the treatment of chronic malignancies such as cancer.

Breaking Bad: How Viruses Subvert the Cell Cycle

Interactions between the host and viruses during the course of their co-evolution have not only shaped cellular function and the immune system, but also the counter measures employed by viruses. Relatively small genomes and high replication rates allow viruses to accumulate mutations and continuously present the host with new challenges. It is therefore, no surprise that they either escape detection or modulate host physiology, often by redirecting normal cellular pathways to their own advantage. Viruses utilize a diverse array of strategies and molecular targets to subvert host cellular processes, while evading detection. These include cell-cycle regulation, major histocompatibility complex-restricted antigen presentation, intracellular protein transport, apoptosis, cytokine-mediated signaling, and humoral immune responses. Moreover, viruses routinely manipulate the host cell cycle to create a favorable environment for replication, largely by deregulating cell cycle checkpoints. This review focuses on our current understanding of the molecular aspects of cell cycle regulation that are often targeted by viruses. Further study of their interactions should provide fundamental insights into cell cycle regulation and improve our ability to exploit these viruses.

The role of placental growth factor (PlGF) and its receptor system in retinal vascular diseases

Placental growth factor (PlGF) is a member of the vascular endothelial growth factor (VEGF) family. Upon binding to VEGF- and neuropilin-receptor sub-types, PlGF modulates a range of neural, glial and vascular cell responses that are distinct from VEGF-A. As PlGF expression is selectively associated with pathological angiogenesis and inflammation, its blockade does not affect the healthy vasculature. PlGF actions have been extensively described in tumor biology but more recently there has been accumulating preclinical evidence that indicates that this growth factor could have an important role in retinal diseases. High levels of PlGF have been found in aqueous humor, vitreous and/or retina of patients exhibiting retinopathies, especially those with diabetic retinopathy (DR) and neovascular age-related macular degeneration (nvAMD). Expression of this growth factor seems to correlate closely with many of the key pathogenic features of early and late retinopathy in preclinical models. For example, studies using genetic modification and/or pharmacological treatment to block PlGF in the laser-induced choroidal neovascularization (CNV) model, oxygen-induced retinopathy model, as well as various murine diabetic models, have shown that PlGF deletion or inhibition can reduce neovascularization, retinal leakage, inflammation and gliosis, without affecting vascular development or inducing neuronal degeneration. Moreover, an inhibitory effect of PlGF blockade on retinal scarring in the mouse CNV model has also been recently demonstrated and was found to be unique for PlGF inhibition, as compared to various VEGF inhibition strategies. Together, these preclinical results suggest that anti-PlGF therapy might have advantages over anti-VEGF treatment, and that it may have clinical applications as a standalone treatment or in combination with anti-VEGF. Additional clinical studies are clearly needed to further elucidate the role of PlGF and its potential as a therapeutic target in ocular diseases.

HTLV-I Tax-Mediated Inactivation of Cell Cycle Checkpoints and DNA Repair Pathways Contribute to Cellular Transformation: "A Random Mutagenesis Model"

To achieve cellular transformation, most oncogenic retroviruses use transduction by proto-oncogene capture or insertional mutagenesis, whereby provirus integration disrupts expression of tumor suppressors or proto-oncogenes. In contrast, the Human T-cell leukemia virus type 1 (HTLV-I) has been classified in a separate class referred to as "transactivating retroviruses". Current views suggest that the viral encoded Tax protein transactivates expression of cellular genes leading to deregulated growth and transformation. However, if Tax-mediated transactivation was indeed sufficient for cellular transformation, a fairly high frequency of infected cells would eventually become transformed. In contrast, the frequency of transformation by HTLV-I is very low, likely less than 5%. This review will discuss the current understanding and recent discoveries highlighting critical functions of Tax in cellular transformation. HTLV-I Tax carries out essential functions in order to override cell cycle checkpoints and deregulate cellular division. In addition, Tax expression is associated with increased DNA damage and genome instability. Since Tax can inhibit multiple DNA repair pathways and stimulate unfaithful DNA repair or bypass checkpoints, these processes allow accumulation of genetic mutations in the host genome. Given this, a "Random Mutagenesis" transformation model seems more suitable to characterize the oncogenic activities of HTLV-I.

Tumor Suppressor Inactivation in the Pathogenesis of Adult T-Cell Leukemia

Tumor suppressor functions are essential to control cellular proliferation, to activate the apoptosis or senescence pathway to eliminate unwanted cells, to link DNA damage signals to cell cycle arrest checkpoints, to activate appropriate DNA repair pathways, and to prevent the loss of adhesion to inhibit initiation of metastases. Therefore, tumor suppressor genes are indispensable to maintaining genetic and genomic integrity. Consequently, inactivation of tumor suppressors by somatic mutations or epigenetic mechanisms is frequently associated with tumor initiation and development. In contrast, reactivation of tumor suppressor functions can effectively reverse the transformed phenotype and lead to cell cycle arrest or death of cancerous cells and be used as a therapeutic strategy. Adult T-cell leukemia/lymphoma (ATLL) is an aggressive lymphoproliferative disease associated with infection of CD4 T cells by the Human T-cell Leukemia Virus Type 1 (HTLV-I). HTLV-I-associated T-cell transformation is the result of a multistep oncogenic process in which the virus initially induces chronic T-cell proliferation and alters cellular pathways resulting in the accumulation of genetic defects and the deregulated growth of virally infected cells. This review will focus on the current knowledge of the genetic and epigenetic mechanisms regulating the inactivation of tumor suppressors in the pathogenesis of HTLV-I.

Tax impairs DNA replication forks and increases DNA breaks in specific oncogenic genome regions

BACKGROUND

Human T-cell leukemia virus type 1 (HTLV-I) is a human retrovirus associated with adult T-cell leukemia (ATL), an aggressive CD4 T-cell proliferative disease with dismal prognosis. The long latency preceding the development of the disease and the low incidence suggests that the virus itself is not sufficient for transformation and that genetic defects are required to create a permissive environment for leukemia. In fact, ATL cells are characterized by profound genetic modifications including structural and numerical chromosome alterations.

RESULTS

In this study we used molecular combing techniques to study the effect of the oncoprotein Tax on DNA replication. We found that replication forks have difficulties replicating complex DNA, fork progression is slower, and they pause or stall more frequently in the presence of Tax expression. Our results also show that Tax-associated replication defects are partially compensated by an increase in the firing of back-up origins. Consistent with these effects of Tax on DNA replication, an increase in double strand DNA breaks (DDSB) was seen in Tax expressing cells. Tax-mediated increases in DDSBs were associated with the ability of Tax to activate NF-kB and to stimulate intracellular nitric oxide production. We also demonstrated a reduced expression of human translesion synthesis (TLS) DNA polymerases Pol-H and Pol-K in HTLV-I-transformed T cells and ATL cells. This was associated with an increase in DNA breaks induced by Tax at specific genome regions, such as the c-Myc and the Bcl-2 major breakpoints. Consistent with the notion that the non-homologous end joining (NHEJ) pathway is hyperactive in HTLV-I-transformed cells, we found that inhibition of the NHEJ pathway induces significant killing of HTLV-I transformed cells and patient-derived leukemic ATL cells.

CONCLUSION

Our results suggest that, replication problems increase genetic instability in HTLV-I-transformed cells. As a result, abuse of NHEJ and a defective homologous repair (HR) DNA repair pathway can be targeted as a new therapeutic approach for the treatment of adult T-cell leukemia.

Human T cell leukaemia virus type 2 tax protein mediates CC-chemokine expression in peripheral blood mononuclear cells via the nuclear factor kappa B canonical pathway

Retroviral co-infections with human immunodeficiency virus type-1 (HIV-1) and human T cell leukaemia virus type 1 (HTLV-1) or type 2 (HTLV-2) are prevalent in many areas worldwide. It has been observed that HIV-1/HTLV-2 co-infections are associated with slower rates of CD4(+) T cell decline and delayed progression to AIDS. This immunological benefit has been linked to the ability of Tax2, the transcriptional activating protein of HTLV-2, to induce the expression of macrophage inflammatory protein (MIP)-1α/CCL3, MIP-1β/CCL4 and regulated upon activation normal T cell expressed and secreted (RANTES)/CCL5 and to down-regulate the expression of the CCR5 co-receptor in peripheral blood mononuclear cells (PBMCs). This study aimed to assess the role of Tax2-mediated activation of the nuclear factor kappa B (NF-κB) signalling pathway on the production of the anti-viral CC-chemokines MIP-1α, MIP-1β and RANTES. Recombinant Tax1 and Tax2 proteins, or proteins expressed via adenoviral vectors used to infect cells, were tested for their ability to activate the NF-κB pathway in cultured PBMCs in the presence or absence of NF-κB pathway inhibitors. Results showed a significant release of MIP-1α, MIP-1β and RANTES by PBMCs after the activation of p65/RelA and p50. The secretion of these CC-chemokines was significantly reduced (P < 0·05) by canonical NF-κB signalling inhibitors. In conclusion, Tax2 protein may promote innate anti-viral immune responses through the activation of the canonical NF-κB pathway.

Control the host cell cycle: viral regulation of the anaphase-promoting complex

Viruses commonly manipulate cell cycle progression to create cellular conditions that are most beneficial to their replication. To accomplish this feat, viruses often target critical cell cycle regulators in order to have maximal effect with minimal input. One such master regulator is the large, multisubunit E3 ubiquitin ligase anaphase-promoting complex (APC) that targets effector proteins for ubiquitination and proteasome degradation. The APC is essential for cells to progress through anaphase, exit from mitosis, and prevent a premature entry into S phase. These far-reaching effects of the APC on the cell cycle are through its ability to target a number of substrates, including securin, cyclin A, cyclin B, thymidine kinase, geminin, and many others. Recent studies have identified several proteins from a number of viruses that can modulate APC activity by different mechanisms, highlighting the potential of the APC in driving viral replication or pathogenesis. Most notably, human cytomegalovirus (HCMV) protein pUL21a was recently identified to disable the APC via a novel mechanism by targeting APC subunits for degradation, both during virus infection and in isolation. Importantly, HCMV lacking both viral APC regulators is significantly attenuated, demonstrating the impact of the APC on a virus infection. Work in this field will likely lead to novel insights into viral replication and pathogenesis and APC function and identify novel antiviral and anticancer targets. Here we review viral mechanisms to regulate the APC, speculate on their roles during infection, and identify questions to be addressed in future studies.

Induction of CC-chemokines with antiviral function in macrophages by the human T lymphotropic virus type 2 transactivating protein, Tax2

Recent data provide evidence that co-infection with human immunodeficiency virus type 1 (HIV-1) and human T lymphotropic virus type 2 (HTLV-2) delays progression to AIDS compared to isolated HIV-1 infection. These results were linked to expression of the HTLV-2 transcriptional activating gene known as Tax2. Preliminary studies in lymphocytic systems suggest that Tax2 is responsible for induction of CC-chemokines, which play a major role in innate immune responses against HIV-1. In this study, the effect of Tax2 on CC-chemokines (MIP-1α/CCL3, MIP-1β/CCL4, and RANTES/CCL5) in monocyte-derived macrophages (MDMs) was evaluated. An immortalized human monocytic cell line (U937) and donor-derived MDMs were used to evaluate these interactions. These cells were cultured in vitro, allowed to mature into macrophages for 14 d, and treated with Tax2 or Tax1 (the transcriptional activator of HTLV-1) at three concentrations (1, 10, and 100 pM) daily thereafter. Extracellular bacterial extract (EBE) lacking the vector and untreated samples served as controls. An additional group of donor-derived MDMs were transduced with an adenovirus vector that expressed either Tax2 or green fluorescent protein (GFP). Liposomal transfection agents alone were used as controls. Supernatants were collected from each sample on multiple days post-maturation and evaluated for MIP-1α, MIP-1β, and RANTES, by enzyme-linked immunosorbent assay. Analysis of variance and Tukey's Honestly Significant Difference tests were used to analyze the results. In all systems, cells exposed to either Tax2 or Tax1 expressed significantly (p<0.01) higher concentrations of CC-chemokines than controls. There was no significant difference in chemokine expression between Tax1-treated and Tax2-treated samples, between EBE-treated and EBE-untreated samples, or between GFP-transduced MDMs and controls. This suggests that HTLV-2 could alter innate immune responses in macrophagic reservoirs of HIV-1 in HIV-1/HTLV-2 co-infected individuals, and could guide the development of HIV-1 treatments.

HTLV-I tax increases genetic instability by inducing DNA double strand breaks during DNA replication and switching repair to NHEJ

BACKGROUND

Appropriate responses to damaged DNA are indispensible for preserving genome stability and preventing cancer. Tumor viruses often target DNA repair machinery to achieve transformation. The Human T-cell leukemia virus type I (HTLV-I) is the only known transforming human retrovirus and the etiological agent of Adult T-cell Leukemia (ATLL). Although HTLV-I-transformed leukemic cells have numerous genetic lesions, the precise role of the viral tax gene in this process is not fully understood.

RESULTS

Our results show a novel function of HTLV-I oncoprotein Tax as an inducer of genomic DNA double strand breaks (DDSB) during DNA replication. We also found that Tax acts as a potent inhibitor of homologous recombination (HR) DNA repair through the activation of the NF-kB pathway. These results were confirmed using HTLV-I molecular clones expressing Tax at physiological levels in a natural context. We further found that HTLV-I- and Tax-transformed cells are not more susceptible to DNA damaging agents and repair DNA lesions at a rate similar to that of normal cells. Finally, we demonstrated that during S phase, Tax-associated DDSB are preferentially repaired using the error-prone non-homologous end joining (NHEJ) pathway.

CONCLUSIONS

This study provides new insights in Tax effects on DNA repair and genome instability. Although it may not be self sufficient, the creation of DNA breaks and subsequent abnormal use of the non-conservative NHEJ DNA repair during the S phase in HTLV-I-infected Tax-expressing cells may cooperate with other factors to increase genetic and genome instability and favor transformation.

The multifaceted oncoprotein Tax: subcellular localization, posttranslational modifications, and NF-κB activation

The human T-cell lymphotropic virus type-I (HTLV-I) is the etiologic agent of adult T-cell leukemia/lymphoma (ATL) and of tropical spastic paraparesis/HTLV-I-associated myelopathy. Constitutive NF-κB activation by the viral oncoprotein Tax plays a crucial role in the induction and maintenance of cellular proliferation, transformation, and inhibition of apoptosis. In an attempt to provide a general view of the molecular mechanisms of constitutive Tax-induced NF-κB activation, we summarize in this review the recent body of literature that supports a major role for Tax posttranslational modifications, chiefly ubiquitination, and SUMOylation, in the NF-κB activity of Tax. These modifications indeed participate in the control of Tax subcellular localization and modulate its protein-protein interaction potential. Tax posttranslational modifications, which highlight the ability of HTLV-I to optimize its limited viral genome size, might represent an attractive target for the design of new therapies for ATL.

Move or die: the fate of the Tax oncoprotein of HTLV-1

The HTLV-1 Tax protein both activates viral replication and is involved in HTLV-1-mediated transformation of T lymphocytes. The transforming properties of Tax include altering the expression of select cellular genes via activation of cellular pathways and perturbation of both cell cycle control mechanisms and apoptotic signals. The recent discovery that Tax undergoes a hierarchical sequence of posttranslational modifications that control its intracellular localization provides provocative insights into the mechanisms regulating Tax transcriptional and transforming activities.

HTLV-1 Tax: Linking transformation, DNA damage and apoptotic T-cell death

The human T-cell leukemia virus type I (HTLV-1) is the causative agent of adult T-cell leukemia (ATL), an aggressive CD4-positive T-cell neoplasia. The HTLV-1 proto-oncogene Tax, a potent transcriptional activator of cellular and viral genes, is thought to play a pivotal role in the transforming properties of the virus by deregulating intracellular signaling pathways. During the course of HTLV-1 infection, the dysregulation of cell-cycle checkpoints and the suppression of DNA damage repair is tightly linked to the activity of the viral oncoprotein Tax. Tax activity is associated with production of reactive oxygen intermediates (ROS), chromosomal instability and DNA damage, apoptotic cell death and cellular transformation. Changes in the intracellular redox status induced by Tax promote DNA damage. Tax-mediated DNA damage is believed to be essential in initiating the transformation process by subjecting infected T cells to genetic changes that eventually promote the neoplastic state. Apoptosis and immune surveillance would then exert the necessary selection pressure for eliminating the majority of virally infected cells, while escape variants acquiring a mutator phenotype would constitute a subpopulation of genetically altered cells prone to neoplasia. While the potency of Tax-activity seems to be a determining factor for the observed effects, the cooperation of Tax with other viral proteins determines the fate and progression of HTLV-1-infected cells through DNA damage, apoptosis, survival and transformation.

HTLV-1 Tax: Linking transformation, DNA damage and apoptotic T-cell death

The human T-cell leukemia virus type I (HTLV-1) is the causative agent of adult T-cell leukemia (ATL), an aggressive CD4-positive T-cell neoplasia. The HTLV-1 proto-oncogene Tax, a potent transcriptional activator of cellular and viral genes, is thought to play a pivotal role in the transforming properties of the virus by deregulating intracellular signaling pathways. During the course of HTLV-1 infection, the dysregulation of cell-cycle checkpoints and the suppression of DNA damage repair is tightly linked to the activity of the viral oncoprotein Tax. Tax activity is associated with production of reactive oxygen intermediates (ROS), chromosomal instability and DNA damage, apoptotic cell death and cellular transformation. Changes in the intracellular redox status induced by Tax promote DNA damage. Tax-mediated DNA damage is believed to be essential in initiating the transformation process by subjecting infected T cells to genetic changes that eventually promote the neoplastic state. Apoptosis and immune surveillance would then exert the necessary selection pressure for eliminating the majority of virally infected cells, while escape variants acquiring a mutator phenotype would constitute a subpopulation of genetically altered cells prone to neoplasia. While the potency of Tax-activity seems to be a determining factor for the observed effects, the cooperation of Tax with other viral proteins determines the fate and progression of HTLV-1-infected cells through DNA damage, apoptosis, survival and transformation.

Hematopoietic stem cells and retroviral infection

Retroviral induced malignancies serve as ideal models to help us better understand the molecular mechanisms associated with the initiation and progression of leukemogenesis. Numerous retroviruses including AEV, FLV, M-MuLV and HTLV-1 have the ability to infect hematopoietic stem and progenitor cells, resulting in the deregulation of normal hematopoiesis and the development of leukemia/lymphoma. Research over the last few decades has elucidated similarities between retroviral-induced leukemogenesis, initiated by deregulation of innate hematopoietic stem cell traits, and the cancer stem cell hypothesis. Ongoing research in some of these models may provide a better understanding of the processes of normal hematopoiesis and cancer stem cells. Research on retroviral induced leukemias and lymphomas may identify the molecular events which trigger the initial cellular transformation and subsequent maintenance of hematologic malignancies, including the generation of cancer stem cells. This review focuses on the role of retroviral infection in hematopoietic stem cells and the initiation, maintenance and progression of hematological malignancies.

HTLV-I Tax-dependent and -independent events associated with immortalization of human primary T lymphocytes

Human T-cell leukemia virus type I (HTLV-I)-associated malignancies are seen in a small percentage of infected persons. Although in vitro immortalization by HTLV-I virus is very efficient, we report that Tax has poor oncogenic activity in human primary T cells and that immortalization by Tax is rare. Sustained telomerase activity represents one of the oncogenic steps required for Tax-mediated immortalization. Tax expression was required for the growth of primary T cells, but was not sufficient to propel T cells into cell cycle in the absence of exogenous interleukin-2 (IL-2). Tax was sufficient to activate the phosphoinositide-3 kinase (PI3K)/Akt pathway as shown by down regulation of Src homology phosphatase-1 and increased phosphorylation of Akt. We also found disruption of putative tumor suppressors IL-16 and translocated promoter region (TPR) in Tax-immortalized and HTLV-I-transformed cell lines. Our results confirmed previous observations that Tax activates the anaphase-promoting complex. However, Tax did not affect the mitotic spindle checkpoint, which was also functional in HTLV-I-transformed cells. These data provide a better understanding of Tax functions in human T cells, and highlight the limitations of Tax, suggesting that other viral proteins are key to T-cell transformation and development of adult T-cell leukemia.

Viral manipulation of DNA repair and cell cycle checkpoints

Recognition and repair of DNA damage is critical for maintaining genomic integrity and suppressing tumorigenesis. In eukaryotic cells, the sensing and repair of DNA damage are coordinated with cell cycle progression and checkpoints, in order to prevent the propagation of damaged DNA. The carefully maintained cellular response to DNA damage is challenged by viruses, which produce a large amount of exogenous DNA during infection. Viruses also express proteins that perturb cellular DNA repair and cell cycle pathways, promoting tumorigenesis in their quest for cellular domination. This review presents an overview of strategies employed by viruses to manipulate DNA damage responses and cell cycle checkpoints as they commandeer the cell to maximize their own viral replication. Studies of viruses have identified key cellular regulators and revealed insights into molecular mechanisms governing DNA repair, cell cycle checkpoints, and transformation.

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.

Epstein-Barr virus nuclear antigen 2 disrupts mitotic checkpoint and causes chromosomal instability

The Epstein-Barr virus nuclear antigen 2 (EBNA2) plays a key role in transformation of B-lymphocytes mediated by Epstein-Barr virus (EBV) and can induce tumor formation in transgenic mice. However, the precise mechanism underlying EBNA2-mediated tumorigenesis remains elusive. Here, we report that EBNA2 can compromise mitotic spindle checkpoint (MSC) induced by the spindle inhibitor nocodazole and cause chromosomal instability (CIN) in HEp-2, U2-OS and BJAB cells. When EBNA2-expressing cells were treated with nocodazole, they exited mitosis prematurely and initiated another round of DNA synthesis. Nucleolocalization of EBNA2 was essential for EBNA2 to compromise MSC and to cause CIN. The metaphase chromosome spread data indicated that the EBNA2-expressing U2-OS cells showed a more heterogenous chromosome number distribution than the vector-transfected and parental cells. The median chromosome number for EBNA2-expressing, vector-transfected and parental U2-OS cells is 75, 65 and 64, respectively. EBNA2 was shown to be able to downregulate mitotic arrest deficient 2 (MAD2) approximately 2- to 3-fold and upregulate polo-like kinase 1 (PLK1) approximately 2-fold. The dysregulation of MAD2 and PLK1 may lead to activation of anaphase promoting complex/cyclosome and premature degradation of securin. Indeed, we found that when MSC was induced by nocodazole, securin was prematurely degraded in EBNA2-expressing cells. Finally, we show that EBNA2 could induce micronuclei and multinuclei formation in HEp-2 and U2-OS cells. Together, these studies reveal a new function of EBNA2 in cell-cycle regulation and may shed light on the role of EBNA2 in EBV-mediated tumorigenesis.

HTLV-1 Tax oncoprotein subverts the cellular DNA damage response via binding to DNA-dependent protein kinase

Human T-cell leukemia virus type-1 is the causative agent for adult T-cell leukemia. Previous research has established that the viral oncoprotein Tax mediates the transformation process by impairing cell cycle control and cellular response to DNA damage. We showed previously that Tax sequesters huChk2 within chromatin and impairs the response to ionizing radiation. Here we demonstrate that DNA-dependent protein kinase (DNA-PK) is a member of the Tax.Chk2 nuclear complex. The catalytic subunit, DNA-PKcs, and the regulatory subunit, Ku70, were present. Tax-containing nuclear extracts showed increased DNA-PK activity, and specific inhibition of DNA-PK prevented Tax-induced activation of Chk2 kinase activity. Expression of Tax induced foci formation and phosphorylation of H2AX. However, Tax-induced constitutive signaling of the DNA-PK pathway impaired cellular response to new damage, as reflected in suppression of ionizing radiation-induced DNA-PK phosphorylation and gammaH2AX stabilization. Tax co-localized with phospho-DNA-PK into nuclear speckles and a nuclear excluded Tax mutant sequestered endogenous phospho-DNA-PK into the cytoplasm, suggesting that Tax interaction with DNA-PK is an initiating event. We also describe a novel interaction between DNA-PK and Chk2 that requires Tax. We propose that Tax binds to and stabilizes a protein complex with DNA-PK and Chk2, resulting in a saturation of DNA-PK-mediated damage repair response.

The HTLV-1 Tax interactome

The Tax1 oncoprotein encoded by Human T-lymphotropic virus type I is a major determinant of viral persistence and pathogenesis. Tax1 affects a wide variety of cellular signalling pathways leading to transcriptional activation, proliferation and ultimately transformation. To carry out these functions, Tax1 interacts with and modulates activity of a number of cellular proteins. In this review, we summarize the present knowledge of the Tax1 interactome and propose a rationale for the broad range of cellular proteins identified so far.

Human T-cell leukemia virus type 1 infection leads to arrest in the G1 phase of the cell cycle

Infection by the human T-cell leukemia virus type 1 (HTLV-1) is thought to cause dysregulated T-cell proliferation, which in turn leads to adult T-cell leukemia/lymphoma. Early cellular changes after HTLV-1 infection have been difficult to study due to the poorly infectious nature of HTLV-1 and the need for cell-to-cell contact for HTLV-1 transmission. Using a series of reporter systems, we show that HeLa cells cease proliferation within one or two division cycles after infection by HTLV-1 or transduction of the HTLV-1 tax gene. HTLV-1-infected HeLa cells, like their tax-transduced counterparts, expressed high levels of p21(CIP1/WAF1) and p27(KIP1), developed mitotic abnormalities, and became arrested in G(1) in senescence. In contrast, cells of a human osteosarcoma lineage (HOS) continued to divide after HTLV-1 infection or Tax expression, albeit at a reduced growth rate and with mitotic aberrations. Unique to HOS cells is the dramatic reduction of p21(CIP1/WAF1) and p27(KIP1) expression, which is in part associated with the constitutive activation of the phosphatidylinositol-3-kinase (PI3K)-protein kinase B (Akt) pathway. The loss of p21(CIP1/WAF1) and p27(KIP1) in HOS cells apparently allows HTLV-1- and Tax-induced G(1) arrest to be bypassed. Finally, HTLV-1 infection and Tax expression also cause human SupT1 T cells to arrest in the G(1) phase of the cell cycle. These results suggest that productive HTLV-1 infection ordinarily leads to Tax-mediated G(1) arrest. However, T cells containing somatic mutations that inactivate p21(CIP1/WAF1) and p27(KIP1) may continue to proliferate after HTLV-1 infection and Tax expression. These infected cells can expand clonally, accumulate additional chromosomal abnormalities, and progress to cancer.

Cross talk between expression of the human T-cell leukemia virus type 1 Tax transactivator and the oncogenic bHLH transcription factor TAL1

The human T-cell leukemia virus type 1 (HTLV-1) Tax transactivator is known to induce or repress various cellular genes, several of them encoding transcription factors. As Tax is known to deregulate various basic bHLH factors, we looked more specifically at its effect on TAL1 (T-cell acute lymphoblastic leukemia 1), also known as SCL (stem cell leukemia). Indeed, TAL1 is deregulated in a high percentage of T-cell acute lymphoblastic leukemia cells, and its oncogenic properties are well-established. Here we show that Tax induces transcription of this proto-oncogene by stimulating the activity of the TAL1 gene promoter 1b, through both the CREB and NF-kappaB pathways. It was also observed that TAL1 upregulates HTLV-1 promoter activity, in either the presence or the absence of Tax. The viral promoter is inhibited in trans by expression of the E2A protein E47, and TAL1 is able to abrogate this inhibition. These data show the existence of a positive feedback loop between Tax and TAL1 expression and support the notion that this proto-oncogene participates in generation of adult T-cell leukemia/lymphoma by increasing the amount of the Tax oncoprotein but also possibly by its own transforming activities.

Viruses associated with human cancer

It is estimated that viral infections contribute to 15-20% of all human cancers. As obligatory intracellular parasites, viruses encode proteins that reprogram host cellular signaling pathways that control proliferation, differentiation, cell death, genomic integrity, and recognition by the immune system. These cellular processes are governed by complex and redundant regulatory networks and are surveyed by sentinel mechanisms that ensure that aberrant cells are removed from the proliferative pool. Given that the genome size of a virus is highly restricted to ensure packaging within an infectious structure, viruses must target cellular regulatory nodes with limited redundancy and need to inactivate surveillance mechanisms that would normally recognize and extinguish such abnormal cells. In many cases, key proteins in these same regulatory networks are subject to mutation in non-virally associated diseases and cancers. Oncogenic viruses have thus served as important experimental models to identify and molecularly investigate such cellular networks. These include the discovery of oncogenes and tumor suppressors, identification of regulatory networks that are critical for maintenance of genomic integrity, and processes that govern immune surveillance.

Does the HBZ gene represent a new potential target for the treatment of adult T-cell leukemia?

Links between human T-cell leukemia virus type 1 and adult T-cell leukemia (ATL) were first suspected in 1980. Provirus integration has since been found in all ATL cells. Although the viral Tax protein is involved in the proliferation of the infected cells during the preleukemic stage, Tax expression is not systematically detected in primary leukemic cells. Recent studies found that the viral HBZ gene was always expressed in leukemic cells, suggesting its involvement in the progression of the infected cells toward malignancy. How could this new discovery be translated into possible new avenues for the prevention or treatment of ATL?

HTLV-1 Tax mutants that do not induce G1 arrest are disabled in activating the anaphase promoting complex

HTLV-1 Tax is a potent activator of viral transcription and NF-κB. Recent data indicate that Tax activates the anaphase promoting complex/cyclosome (APC/C) ahead of schedule, causing premature degradation of cyclin A, cyclin B1, securin, and Skp2. Premature loss of these mitotic regulators is accompanied by mitotic aberrations and leads to rapid senescence and cell cycle arrest in HeLa and S. cerevisiae cells. Tax-induced rapid senescence (tax-IRS) of HeLa cells is mediated primarily by a dramatic stabilization of p27^KIP and is also accompanied by a great surge in the level of p21^CIP1mRNA and protein. Deficiencies in p27^KIP prevent Tax-IRS. A collection of tax point mutants that permit normal growth of S. cerevisiae have been isolated. Like wild-type tax, many of them (C23W, A108T, L159F, and L235F) transactivate both the HTLV-LTR and the NF-κB reporters. One of them, V19M, preferentially activates NF-κB, but is attenuated for LTR activation. None of the mutants significantly elevated the levels of p21^CIP1and p27^KIP1, indicating that the dramatic surge in p21^CIP1/WAF1and p27^{KIP 1}induced by Tax is brought about by a mechanism distinct from NF-κB or LTR activation. Importantly, the ability of these mutants to activate APC/C is attenuated or abrogated. These data indicate that Tax-induced rapid senescence is causally associated with APC/C activation.

Evidence for cooperative transforming activity of the human pituitary tumor transforming gene and human T-cell leukemia virus type 1 Tax

Aneuploidy is frequent in cancers. Recently it was found that pituitary tumor transforming gene (PTTG; also called Pds1p or securin) is overexpressed in many different tumors. Human T-cell leukemia virus type 1 (HTLV-1) is a retrovirus that primarily infects CD4+ T lymphocytes and causes adult T-cell leukemia. Here, we report that overexpression of human PTTG cooperated with the HTLV-I Tax oncoprotein in cellular transformation. Coexpression of Tax and PTTG enhanced chromosomal instability and neoplastic changes to levels greater than overexpression of either factor singularly. Cells that overexpressed both PTTG and Tax induced tumors more robustly in nude mice than cells that expressed either PTTG alone or Tax alone.

Mechanism limiting centrosome duplication to once per cell cycle

The centrosome organizes the microtubule cytoskeleton and consists of a pair of centrioles surrounded by pericentriolar material. Cells begin the cell cycle with a single centrosome, which duplicates once before mitosis. During duplication, new centrioles grow orthogonally to existing ones and remain engaged (tightly opposed) with those centrioles until late mitosis or early G1 phase, when they become disengaged. The relationship between centriole engagement/disengagement and centriole duplication potential is not understood, and the mechanisms that control these processes are not known. Here we show that centriole disengagement requires the protease separase at anaphase, and that this disengagement licences centriole duplication in the next cell cycle. We describe an in vitro system using Xenopus egg extract and purified centrioles in which both centriole disengagement and centriole growth occur. Centriole disengagement at anaphase is independent of mitotic exit and Cdk2/cyclin E activity, but requires the anaphase-promoting complex and separase. In contrast to disengagement, new centriole growth occurs in interphase, is dependent on Cdk2/cyclin E, and requires previously disengaged centrioles. This suggests that re-duplication of centrioles within a cell cycle is prevented by centriole engagement itself. We propose that the 'once-only' control of centrosome duplication is achieved by temporally separating licensing in anaphase from growth of new centrioles during S phase. The involvement of separase in both centriole disengagement and sister chromatid separation would prevent premature centriole disengagement before anaphase onset, which can lead to multipolar spindles and genomic instability.

Involvement of HTLV-I Tax and CREB in aneuploidy: a bioinformatics approach

Background

Adult T-cell leukemia (ATL) is a complex and multifaceted disease associated with human T-cell leukemia virus type 1 (HTLV-I) infection. Tax, the viral oncoprotein, is considered a major contributor to cell cycle deregulation in HTLV-I transformed cells by either directly disrupting cellular factors (protein-protein interactions) or altering their transcription profile. Tax transactivates these cellular promoters by interacting with transcription factors such as CREB/ATF, NF-κB, and SRF. Therefore by examining which factors upregulate a particular set of promoters we may begin to understand how Tax orchestrates leukemia development.

Results

We observed that CTLL cells stably expressing wild-type Tax (CTLL/WT) exhibited aneuploidy as compared to a Tax clone deficient for CREB transactivation (CTLL/703). To better understand the contribution of Tax transactivation through the CREB/ATF pathway to the aneuploid phenotype, we performed microarray analysis comparing CTLL/WT to CTLL/703 cells. Promoter analysis of altered genes revealed that a subset of these genes contain CREB/ATF consensus sequences. While these genes had diverse functions, smaller subsets of genes were found to be involved in G2/M phase regulation, in particular kinetochore assembly. Furthermore, we confirmed the presence of CREB, Tax and RNA Polymerase II at the p97Vcp and Sgt1 promoters in vivo through chromatin immunoprecipitation in CTLL/WT cells.

Conclusion

These results indicate that the development of aneuploidy in Tax-expressing cells may occur in response to an alteration in the transcription profile, in addition to direct protein interactions.

Activation of the anaphase promoting complex by HTLV-1 tax leads to senescence

The human T-lymphotropic virus type 1 (HTLV-1) Tax binds the anaphase promoting complex (APC) and activates it ahead of schedule. Here, we show that APC activation by Tax induces rapid senescence (tax-IRS) independently of p53 and pRB. In response to tax, cyclin A, cyclin B1, securin, and Skp2 becomes polyubiquitinated and degraded starting in S phase. This is followed by a surge in p21(CIP1/WAF1) and p27(KIP1) in mid to late S and G2/M leading to a permanent G1 arrest. Tax-positive HTLV-1-transformed T-cell lines express elevated levels of p21(CIP1/WAF1), but low levels of p27(KIP1). Finally, Tax can be stably expressed in p27(KIP1)-null NIH3T3 cells. These results indicate that APC activation by Tax causes inactivation of SCF(Skp2) and stabilization of p21(CIP1/WAF1) and p27(KIP1). The build-up of p21(CIP1/WAF1) and especially p27(KIP1) commits cells to senescence. Evading tax-IRS through a loss of p27(KIP1) function is likely to be critical for cell transformation by Tax and development of adult T-cell leukemia after HTLV-1 infection. Finally, activation of APC ahead of schedule may be exploited to arrest cancer cell growth.

Abnormal centrosome amplification in cells through the targeting of Ran-binding protein-1 by the human T cell leukemia virus type-1 Tax oncoprotein

Human T cell leukemia virus type-1 (HTLV-1) is an oncogenic retrovirus etiologically causal of adult T cell leukemia. The virus encodes a Tax oncoprotein that functions in transcriptional regulation, cell cycle control, and transformation. Because adult T cell leukemia like many other human cancers is a disease of genomic instability with frequent gains and losses of chromosomes, to understand this disease it is important to comprehend how HTLV-1 engenders aneuploidy in host cells. In this regard, loss of cell cycle checkpoints permits tolerance of aneuploidy but does not explain how aneuploidy is created. We show here that HTLV-1 Tax causes abnormal centrosome fragmentation in the mitotic phase of the cell cycle. We report that Tax directly binds Ran and Ran-binding protein-1, locates to centrosomes/spindle poles, and causes supernumerary centrosomes.

Induction of cell cycle arrest by human T-cell lymphotropic virus type 1 Tax in hematopoietic progenitor (CD34+) cells: modulation of p21cip1/waf1 and p27kip1 expression

Human T-cell lymphotropic virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia, an aggressive CD4(+) malignancy. Although HTLV-2 is highly homologous to HTLV-1, infection with HTLV-2 has not been associated with lymphoproliferative disorders. Lentivirus-mediated transduction of CD34(+) cells with HTLV-1 Tax (Tax1) induced G(0)/G(1) cell cycle arrest and resulted in the concomitant suppression of multilineage hematopoiesis in vitro. Tax1 induced transcriptional upregulation of the cdk inhibitors p21(cip1/waf1) (p21) and p27(kip1) (p27), and marked suppression of hematopoiesis in immature (CD34(+)/CD38(-)) hematopoietic progenitor cells in comparison to CD34(+)/CD38(+) cells. HTLV-1 infection of CD34(+) cells also induced p21 and p27 expression. Tax1 also protected CD34(+) cells from serum withdrawal-mediated apoptosis. In contrast, HTLV-2 Tax (Tax2) did not detectably alter p21 or p27 gene expression, failed to induce cell cycle arrest, failed to suppress hematopoiesis in CD34(+) cells, and did not protect cells from programmed cell death. A Tax2/Tax1 chimera encoding the C-terminal 53 amino acids of Tax1 fused to Tax2 (Tax(221)) displayed a phenotype in CD34(+) cells similar to that of Tax1, suggesting that unique domains encoded within the C terminus of Tax1 may account for the phenotypes displayed in human hematopoietic progenitor cells. These remarkable differences in the activities of Tax1 and Tax2 in CD34(+) hematopoietic progenitor cells may underlie the sharp differences observed in the pathogenesis resulting from infection with HTLV-1 and HTLV-2.

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.

A human T-cell lymphotropic virus type 1 enhancer of Myc transforming potential stabilizes Myc-TIP60 transcriptional interactions

The human T-cell lymphotropic virus type 1 (HTLV-1) infects and transforms CD4+ lymphocytes and causes adult T-cell leukemia/lymphoma (ATLL), an aggressive lymphoproliferative disease that is often fatal. Here, we demonstrate that the HTLV-1 pX splice-variant p30II markedly enhances the transforming potential of Myc and transcriptionally activates the human cyclin D2 promoter, dependent upon its conserved Myc-responsive E-box enhancer elements, which are associated with increased S-phase entry and multinucleation. Enhancement of c-Myc transforming activity by HTLV-1 p30II is dependent upon the transcriptional coactivators, transforming transcriptional activator protein/p434 and TIP60, and it requires TIP60 histone acetyltransferase (HAT) activity and correlates with the stabilization of HTLV-1 p30II/Myc-TIP60 chromatin-remodeling complexes. The p30II oncoprotein colocalizes and coimmunoprecipitates with Myc-TIP60 complexes in cultured HTLV-1-infected ATLL patient lymphocytes. Amino acid residues 99 to 154 within HTLV-1 p30II interact with the TIP60 HAT, and p30II transcriptionally activates numerous cellular genes in a TIP60-dependent or TIP60-independent manner, as determined by microarray gene expression analyses. Importantly, these results suggest that p30II functions as a novel retroviral modulator of Myc-TIP60-transforming interactions that may contribute to adult T-cell leukemogenesis.

Human T-Cell Leukemia Virus Type I at Age 25: A Progress Report: Figure 1

It has been 25 years since the discovery of human T-cell leukemia virus type I (HTLV-I) and its role in adult T-cell leukemia. Here, in brief, we review the current state of our understanding of HTLV-I epidemiology, viral biology, pathogenesis, and treatment. We discuss how HTLV-I may transform cells through destabilization of cellular genomic integrity and induction of cellular tolerance for chromosomal errors.

Repression of DNA repair mechanisms in IRF-4-expressing and HTLV-I-infected T lymphocytes

Human T cell leukemia virus (HTLV) is the causative agent of adult T cell leukemia (ATL), an aggressive and fatal leukemia of CD4+ T lymphocytes in which interferon regulatory factor-4 (IRF-4) becomes constitutively expressed, concomitant with major alterations in host gene expression. When constitutively expressed in uninfected T lymphocytes, IRF-4 caused reduced expression of critical DNA repair genes, including Rad51, XRCC1, Ung1, RPA, and proliferative cell nuclear antigen (PCNA), a transcriptional phenotype with striking similarities to the profile observed in HTLV-infected T lymphocytes. Concomitant with the inhibition of gene expression and defects in the DNA repair pathways, increased sensitivity of T lymphocytes to various genotoxic stresses that challenged all major DNA repair pathways were detected. Together, these results support a role for IRF- 4 in the repression of DNA repair activity and an increase in the risk of mutations. IRF-4 may thus represent a previously unidentified endogenous transcriptional repressor of DNA repair mechanisms.

Current views in HTLV-I-associated adult T-cell leukemia/lymphoma

Epidemiological studies have demonstrated that the relative percentage of malignant lymphoid proliferations varies widely according to geographical location and ethnic populations. HTLV-I is the etiological agent of adult T-cell leukemia/lymphoma (ATLL) and is also associated with cutaneous T-cell lymphoma (CTCL). However, a definite role of HTLV-I in mycosis fungoides (MF) and/or Sezary syndrome (SS) remains controversial. While most HTLV-I-infected individuals remain asymptomatic carriers, 1-5% will develop ATLL, an invariably fatal expansion of virus-infected CD4+ T cells. This low incidence and the long latency period preceding occurrence of the disease suggest that additional factors are involved in development of ATLL. In this review, diagnosis, clinical features, and molecular pathogenesis of HTLV-I are discussed.

The HTLV-I Tax oncoprotein targets the retinoblastoma protein for proteasomal degradation

Human T-cell leukemia virus type-I (HTLV-I), the etiologic agent of adult T-cell leukemia (ATL), is estimated to affect 10-20 million people worldwide. The transforming ability of HTLV-I has been largely attributed to the viral protein Tax, which modulates the activity of several well-known cell cycle regulators. An important cell cycle regulator, the retinoblastoma (Rb) protein, is often inactivated in many cancers including virally induced cancers. Upon examination of Rb status, we observed a decrease in Rb protein expression in HTLV-1-infected cell lines as well as in ex vivo ATL patient samples. Transient transfection assays indicated that decreased Rb protein levels were Tax dependent. Here, we demonstrate for the first time that Tax directly associates with Rb. This interaction was localized within the B pocket of Rb and the C-terminus of Tax (aa 245-353). Within the C-terminus of Tax, we have identified an LXCXE-like motif, that when mutated resulted in the loss of Tax/Rb interaction. Furthermore, through the use of proteasome inhibitors, such as MG-132, in vivo and proteasome degradation assays in vitro, we found that Tax destabilizes the hypo-phosphorylated (active) form of Rb via the proteasome pathway. Therefore, we propose a model whereby Tax targets Rb to the proteasome by acting as a molecular bridge bringing Rb into contact with the proteasome for degradation.

HTLV-I Tax directly binds the Cdc20-associated anaphase-promoting complex and activates it ahead of schedule

Expression of the human T lymphotropic virus type I (HTLV-I) transactivator/oncoprotein, Tax, leads to faulty mitosis as reflected by chromosome aneuploidy, cytokinesis failure, and formation of micro- and multinucleated cells. Here we show that HTLV-I-transformed T cells progress through S/G(2)/M phases of the cell cycle with a delay. This delay is correlated with a decrease in the levels of cyclin A, cyclin B1, and securin. In tax-expressing cells, the Cdc20-associated anaphase promoting complex (APC(Cdc20)), an E3 ubiquitin ligase that controls metaphase to anaphase transition, becomes active before cellular entry into mitosis as evidenced by premature cyclin B1 polyubiquitination and degradation during S/G(2). Consistent with the notion that Tax activates APC(Cdc20) directly, Tax is found to coimmunoprecipitate with Cdc20 and Cdc27/APC3. The APC(Cdc20) activity prematurely activated by Tax remains sensitive to spindle checkpoint inhibition. Unscheduled activation of APC(Cdc20) by Tax provides an explanation for the mitotic abnormalities in HTLV-I-infected cells and is likely to play an important role in the development of adult T cell leukemia.

Cooperation between different forms of the human papillomavirus type 1 E4 protein to block cell cycle progression and cellular DNA synthesis

Posttranslational modification-oligomerization, phosphorylation, and proteolytic cleavage-of the human papillomavirus (HPV) E4 protein occurs as the infected keratinocytes migrate up through the suprabasal wart layers. It has been postulated that these events modify E4 function during the virus life cycle. In HPV type 1 (HPV1)-induced warts, N-terminal sequences are progressively cleaved from the full-length E4 protein (E1(wedge)E4) of 17 kDa to produce a series of polypeptides of 16, 11 and 10 kDa. Here, we have shown that in human keratinocytes, a truncated protein (E4-16K), equivalent to the 16-kDa species, mediated a G(2) arrest in the cell cycle that was dependent on a threonine amino acid in a proline-rich domain of the protein. Reconstitution of cyclin B1 expression in E4-16K cells reversed the G(2) arrest. Expression of E4-16K also induced chromosomal rereplication, and this was associated with aberrant nuclear morphology. Perturbation of the mitotic cell cycle was a biological activity specific to the truncated protein. However, coexpression of the full-length E1(wedge)E4 protein and the truncated E4-16K protein inhibited normal cellular proliferation and cellular DNA rereplication but did not prevent cells from arresting in G(2). Our findings provide the first evidence to support the hypothesis that proteolytic cleavage of the E1(wedge)E4 protein modifies its function. Also, different forms of the HPV1 E4 protein cooperate to negatively influence keratinocyte proliferation. We predict that these distinct biological activities of E4 act to support efficient amplification of the viral genome in suprabasal keratinocytes.

Ubiquitination of human T-cell leukemia virus type 1 tax modulates its activity

Human T-cell leukemia virus type 1 (HTLV-1) encodes a 40-kDa Tax phosphoprotein. Tax is a transcriptional activator which modulates expression of the viral long terminal repeat and transcription of many cellular genes. Because Tax is a critical HTLV-1 factor which mediates viral transformation of T cells during the genesis of adult T-cell leukemia, it is important to understand the processes which can activate or inactivate Tax function. Here, we report that ubiquitination of Tax is a posttranscriptional mechanism which regulates Tax function. We show that ubiquitination does not target Tax for degradation by the proteasome. Rather, ubiquitin addition modifies Tax in a proteasome-independent manner from an active to a less-active transcriptional form.

A functional analysis reveals dependence on the anaphase-promoting complex for prolonged life span in yeast

Defects in anaphase-promoting complex (APC) activity, which regulates mitotic progression and chromatin assembly, results in genomic instability, a hallmark of premature aging and cancer. We investigated whether APC-dependent genomic stability affects aging and life span in yeast. Utilizing replicative and chronological aging assays, the APC was shown to promote longevity. Multicopy expression of genes encoding Snf1p (MIG1) and PKA (PDE2) aging-pathway components suppressed apc5CA phenotypes, suggesting their involvement in APC-dependent longevity. While it is known that PKA inhibits APC activity and reduces life span, a link between the Snf1p-inhibited Mig1p transcriptional modulator and the APC is novel. Our mutant analysis supports a model in which Snf1p promotes extended life span by inhibiting the negative influence of Mig1p on the APC. Consistent with this, we found that increased MIG1 expression reduced replicative life span, whereas mig1Delta mutations suppressed the apc5CA chronological aging defect. Furthermore, Mig1p and Mig2p activate APC gene transcription, particularly on glycerol, and mig2Delta, but not mig1Delta, confers a prolonged replicative life span in both APC5 and acp5CA cells. However, glucose repression of APC genes was Mig1p and Mig2p independent, indicating the presence of an uncharacterized factor. Therefore, we propose that APC-dependent genomic stability is linked to prolonged longevity by the antagonistic regulation of the PKA and Snf1p pathways.

Role of accessory proteins of HTLV-1 in viral replication, T cell activation, and cellular gene expression

Human T-cell lymphotropic virus type 1 (HTLV-1), causes adult T cell leukemia/lymphoma (ATLL), and initiates a variety of immune mediated disorders. The viral genome encodes common structural and enzymatic proteins characteristic of all retroviruses and utilizes alternative splicing and alternate codon usage to make several regulatory and accessory proteins encoded in the pX region (pX ORF I to IV). Recent studies indicate that the accessory proteins p12I, p27I, p13II, and p30II, encoded by pX ORF I and II, contribute to viral replication and the ability of the virus to maintain typical in vivo expression levels. Proviral clones that are mutated in either pX ORF I or II, while fully competent in cell culture, are severely limited in their replicative capacity in a rabbit model. These HTLV-1 accessory proteins are critical for establishment of viral infectivity, enhance T-lymphocyte activation and potentially alter gene transcription and mitochondrial function. HTLV-1 pX ORF I expression is critical to the viral infectivity in resting primary lymphocytes suggesting a role for the calcineurin-binding protein p12I in lymphocyte activation. The endoplasmic reticulum and cis-Golgi localizing p12I activates NFAT, a key T cell transcription factor, through calcium-mediated signaling pathways and may lower the threshold of lymphocyte activation via the JAK/STAT pathway. In contrast p30II localizes to the nucleus and represses viral promoter activity, but may regulate cellular gene expression through p300/CBP or related co-activators of transcription. The mitochondrial localizing p13II induces morphologic changes in the organelle and may influence energy metabolism infected cells. Future studies of the molecular details HTLV-1 "accessory" proteins interactions will provide important new directions for investigations of HTLV-1 and related viruses associated with lymphoproliferative diseases. Thus, the accessory proteins of HTLV-1, once thought to be dispensable for viral replication, have proven to be directly involved in viral spread in vivo and represent potential targets for therapeutic intervention against HTLV-1 infection and disease.