HTLV-1 tax oncoprotein represses the p53-mediated trans-activation function through coactivator CBP sequestration

Small RNA Profiling in an HTLV-1-Infected Patient with Acute Adult T-Cell Leukemia-Lymphoma at Diagnosis and after Maintenance Therapy: A Case Study

Small RNAs (sRNAs) are epigenetic regulators of essential biological processes associated with the development and progression of leukemias, including adult T-cell leukemia/lymphoma (ATLL) caused by human T-cell lymphotropic virus type 1 (HTLV-1), an oncogenic human retrovirus originally discovered in a patient with adult T-cell leukemia/lymphoma. Here, we describe the sRNA profile of a 30-year-old woman with ATLL at the time of diagnosis and after maintenance therapy with the aim of correlating expression levels with response to therapy.

Implications of viral infection in cancer development

Since the identification of the first human oncogenic virus in 1964, viruses have been studied for their potential role in aiding the development of cancer. Through the modulation of cellular pathways associated with proliferation, immortalization, and inflammation, viral proteins can mimic the effect of driver mutations and contribute to transformation. Aside from the modulation of signaling pathways, the insertion of viral DNA into the host genome and the deregulation of cellular miRNAs represent two additional mechanisms implicated in viral oncogenesis. In this review, we will discuss the role of twelve different viruses on cancer development and how these viruses utilize the abovementioned mechanisms to influence oncogenesis. The identification of specific mechanisms behind viral transformation of human cells could further elucidate the process behind cancer development.

Mechanisms of Oncogenesis by HTLV-1 Tax

The human T-cell lymphotropic virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia/lymphoma (ATLL), a neoplasm of CD4+CD25+ T cells that occurs in 2-5% of infected individuals after decades of asymptomatic latent infection. Multiple HTLV-1-encoded regulatory proteins, including Tax and HTLV-1 basic leucine zipper factor (HBZ), play key roles in viral persistence and latency. The HTLV-1 Tax oncoprotein interacts with a plethora of host cellular proteins to regulate viral gene expression and also promote the aberrant activation of signaling pathways such as NF-κB to drive clonal proliferation and survival of T cells bearing the HTLV-1 provirus. Tax undergoes various post-translational modifications such as phosphorylation and ubiquitination that regulate its function and subcellular localization. Tax shuttles in different subcellular compartments for the activation of anti-apoptotic genes and deregulates the cell cycle with the induction of DNA damage for the accumulation of genomic instability that can result in cellular immortalization and malignant transformation. However, Tax is highly immunogenic and therefore HTLV-1 has evolved numerous strategies to tightly regulate Tax expression while maintaining the pool of anti-apoptotic genes through HBZ. In this review, we summarize the key findings on the oncogenic mechanisms used by Tax that set the stage for the development of ATLL, and the strategies used by HTLV-1 to tightly regulate Tax expression for immune evasion and viral persistence.

Impact of host immunity on HTLV-1 pathogenesis: potential of Tax-targeted immunotherapy against ATL

Human T-cell leukemia virus type-1 (HTLV-1) causes adult T-cell leukemia/lymphoma (ATL), HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), and other inflammatory diseases. There is no disease-specific difference in viral strains, and it is unclear how HTLV-1 causes such different diseases manifesting as lymphoproliferation or inflammation. Although some progress has been made in therapies for these diseases, the prognosis for ATL is still dismal and HAM/TSP remains an intractable disease. So far, two regulatory proteins of HTLV-1, Tax and HBZ, have been well studied and shown to have pleiotropic functions implicated in viral pathogenesis. Tax in particular can strongly activate NFκB, which is constitutively activated in HTLV-1-infected cells and considered to contribute to both oncogenesis and inflammation. However, the expression level of Tax is very low in vivo, leading to confusion in understanding its role in viral pathogenesis. A series of studies using IL-2-dependent HTLV-1-infected cells indicated that IL-10, an anti-inflammatory/immune suppressive cytokine, could induce a proliferative phenotype in HTLV-1-infected cells. In addition, type I interferon (IFN) suppresses HTLV-1 expression in a reversible manner. These findings suggest involvement of host innate immunity in the switch between lymphoproliferative and inflammatory diseases as well as the regulation of HTLV-1 expression. Innate immune responses also affect another important host determinant, Tax-specific cytotoxic T lymphocytes (CTLs), which are impaired in ATL patients, while activated in HAM/TSP patients. Activation of Tax-specific CTLs in ATL patients after hematopoietic stem cell transplantation indicates Tax expression and its fluctuation in vivo. A recently developed anti-ATL therapeutic vaccine, consisting of Tax peptide-pulsed dendritic cells, induced Tax-specific CTL responses in ATL patients and exhibited favorable clinical outcomes, unless Tax-defective ATL clones emerged. These findings support the significance of Tax in HTLV-1 pathogenesis, at least in part, and encourage Tax-targeted immunotherapy in ATL. Host innate and acquired immune responses induce host microenvironments that modify HTLV-1-encoded pathogenesis and establish a complicated network for development of diseases in HTLV-1 infection. Both host and viral factors should be taken into consideration in development of therapeutic and prophylactic strategies in HTLV-1 infection.

Comparative virology of HTLV-1 and HTLV-2

Human T cell leukemia virus type 1 (HTLV-1) was the first discovered human retrovirus and the etiologic agent of adult T-cell leukemia and HTLV-1-associated myelopathy/tropical spastic paraparesis. Shortly after the discovery of HTLV-1, human T-cell leukemia virus type 2 (HTLV-2) was isolated from a patient with hairy cell leukemia. Despite possession of similar structural features to HTLV-1, HTLV-2 has not been definitively associated with lymphoproliferative disease. Since their discovery, studies have been performed with the goal of highlighting the differences between HTLV-1 and HTLV-2. A better understanding of these differences will shed light on the specific pathogenic mechanisms of HTLV-1 and lead to novel therapeutic targets. This review will compare and contrast the two oldest human retroviruses with regards to epidemiology, genomic structure, gene products, and pathobiology.

HIV-1 Vpr and p21 restrict LINE-1 mobility

Long interspersed element-1 (LINE-1, L1) composes ∼17% of the human genome. However, genetic interactions between L1 and human immunodeficiency virus type 1 (HIV-1) remain poorly understood. In this study, we found that HIV-1 suppresses L1 retrotransposition. Notably, HIV-1 Vpr strongly inhibited retrotransposition without inhibiting L1 promoter activity. Since Vpr is known to regulate host cell cycle, we examined the possibility whether Vpr suppresses L1 retrotransposition in a cell cycle dependent manner. We showed that the inhibitory effect of a mutant Vpr (H71R), which is unable to arrest the cell cycle, was significantly relieved compared with that of wild-type Vpr, suggesting that Vpr suppresses L1 mobility in a cell cycle dependent manner. Furthermore, a host cell cycle regulator p21^Waf1 strongly suppressed L1 retrotransposition. The N-terminal kinase inhibitory domain (KID) of p21 was required for this inhibitory effect. Another KID-containing host cell cycle regulator p27^Kip1 also strongly suppressed L1 retrotransposition. We showed that Vpr and p21 coimmunoprecipitated with L1 ORF2p and they suppressed the L1 reverse transcriptase activity in LEAP assay, suggesting that Vpr and p21 inhibit ORF2p-mediated reverse transcription. Altogether, our results suggest that viral and host cell cycle regulatory machinery limit L1 mobility in cultured cells.

Apolipoprotein E is an HIV-1-inducible inhibitor of viral production and infectivity in macrophages

Apolipoprotein E (ApoE) belongs to a class of cellular proteins involved in lipid metabolism. ApoE is a polymorphic protein produced primarily in macrophages and astrocytes. Different isoforms of ApoE have been associated with susceptibility to various diseases including Alzheimer's and cardiovascular diseases. ApoE expression has also been found to affect susceptibility to several viral diseases, including Hepatitis C and E, but its effect on the life cycle of HIV-1 remains obscure. In this study, we initially found that HIV-1 infection selectively up-regulated ApoE in human monocyte-derived macrophages (MDMs). Interestingly, ApoE knockdown in MDMs enhanced the production and infectivity of HIV-1, and was associated with increased localization of viral envelope (Env) proteins to the cell surface. Consistent with this, ApoE over-expression in 293T cells suppressed Env expression and viral infectivity, which was also observed with HIV-2 Env, but not with VSV-G Env. Mechanistic studies revealed that the C-terminal region of ApoE was required for its inhibitory effect on HIV-1 Env expression. Moreover, we found that ApoE and Env co-localized in the cells, and ApoE associated with gp160, the precursor form of Env, and that the suppression of Env expression by ApoE was cancelled by the treatment with lysosomal inhibitors. Overall, our study revealed that ApoE is an HIV-1-inducible inhibitor of viral production and infectivity in macrophages that exerts its anti-HIV-1 activity through association with gp160 Env via the C-terminal region, which results in subsequent degradation of gp160 Env in the lysosomes.

HTLV Deregulation of the NF-κB Pathway: An Update on Tax and Antisense Proteins Role

Human T-cell lymphotropic virus type 1 (HTLV-1) is the causative agent of adult T-cell leukemia (ATL), an aggressive CD4+/CD25+ T-cell malignancy and of a severe neurodegenerative disease, HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP). The chronic activation or deregulation of the canonical and non-canonical nuclear factor kappa B (NF-κB) pathways play a crucial role in tumorigenesis. The HTLV-1 Tax-1 oncoprotein is a potent activator of the NF-κB transcription factors and the NF-κB response is required for promoting the development of HTLV-1 transformed cell lines. The homologous retrovirus HTLV-2, which also expresses a Tax-2 transforming protein, is not associated with ATL. In this review, we provide an updated synopsis of the role of Tax-1 in the deregulation of the NF-κB pathway, highlighting the differences with the homologous Tax-2. Special emphasis is directed toward the understanding of the molecular mechanisms involved in NF-κB activation resulting from Tax interaction with host factors affecting several cellular processes, such as cell cycle, apoptosis, senescence, cell proliferation, autophagy, and post-translational modifications. We also discuss the current knowledge on the role of the antisense viral protein HBZ in down-regulating the NF-κB activation induced by Tax, and its implication in cellular senescence. In addition, we review the recent studies on the mechanism of HBZ-mediated inhibition of NF-κB activity as compared to that exerted by the HTLV-2 antisense protein, APH-2. Finally, we discuss recent advances aimed at understanding the role exerted in the development of ATL by the perturbation of NF-κB pathway by viral regulatory proteins.

Comparison of the seleno-transcriptome expression between human non-cancerous mammary epithelial cells and two human breast cancer cell lines

Breast cancer is the second most common cause of mortality in women; therefore, the identification of novel putative markers is required to improve its diagnosis and prognosis. Selenium is known to protect mammary epithelial cells from oxidative DNA damage, and to inhibit the initiation phase of carcinogenesis by stimulating DNA repair and apoptosis regulation. Consequently, the present study has focused attention on the selenoprotein family and their involvement in breast cancer. The present study performed a global analysis of the seleno-transcriptome expression in human breast cancer MCF-7 and MDA-MB231 cell lines compared with healthy breast MCF-10A cells using reverse transcription-quantitative polymerase chain reaction. The present data revealed the presence of differently expressed genes in MCF-7 and MDA-MB231 cells compared with MCF-10A cells: Four downregulated [glutathione peroxidase (GPX)1, GPX4, GPX5 and GPX7] and three upregulated (deiodinase iodothyronine, type II, GPX2 and GPX3) genes. Additionally, interactomic investigation were performed by the present study to evaluate the association between the downregulated and upregulated genes, and to identify putative HUB nodes, which represent the centers of association between the genes that are capable of direct control over the gene networks. Network analysis revealed that all differentially regulated genes, with the exception of selenoprotein T, are implicated in the same network that presents three HUB nodes interconnected to the selenoprotein mRNAs, including TP53, estrogen receptor 1 and catenin-β1 (CTNNB1). Overall, these data demonstrated for the first time, a profile of seleno-mRNAs specific for human breast cells, indicating that these genes alter their expression on the basis of the ER-positivity or negativity of breast cancer cells.

Human T-cell leukemia virus type-1-encoded protein HBZ represses p53 function by inhibiting the acetyltransferase activity of p300/CBP and HBO1

Adult T-cell leukemia (ATL) is an often fatal malignancy caused by infection with the complex retrovirus, human T-cell Leukemia Virus, type 1 (HTLV-1). In ATL patient samples, the tumor suppressor, p53, is infrequently mutated; however, it has been shown to be inactivated by the viral protein, Tax. Here, we show that another HTLV-1 protein, HBZ, represses p53 activity. In HCT116 p53+/+ cells treated with the DNA-damaging agent, etoposide, HBZ reduced p53-mediated activation of p21/CDKN1A and GADD45A expression, which was associated with a delay in G2 phase-arrest. These effects were attributed to direct inhibition of the histone acetyltransferase (HAT) activity of p300/CBP by HBZ, causing a reduction in p53 acetylation, which has be linked to decreased p53 activity. In addition, HBZ bound to, and inhibited the HAT activity of HBO1. Although HBO1 did not acetylate p53, it acted as a coactivator for p53 at the p21/CDKN1A promoter. Therefore, through interactions with two separate HAT proteins, HBZ impairs the ability of p53 to activate transcription. This mechanism may explain how p53 activity is restricted in ATL cells that do not express Tax due to modifications of the HTLV-1 provirus, which accounts for a majority of patient samples.

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

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

Molecular Studies of HTLV-1 Replication: An Update

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

p53 modulates the activity of the GLI1 oncogene through interactions with the shared coactivator TAF9

The GLI1 oncogene and p53 tumor suppressor gene function in an inhibitory loop that controls stem cell and tumor cell numbers. Since GLI1 and p53 both interact with the coactivator TATA Binding Protein Associated Factor 9 (TAF9), we hypothesized that competition between these transcription factors for TAF9 in cancer cells may contribute to the inhibitory loop and directly affect GLI1 function and cellular phenotype. We showed that TAF9 interacts with the oncogenic GLI family members GLI1 and GLI2 but not GLI3 in cell-free pull-down assays and with GLI1 in rhabdomyosarcoma and osteosarcoma cell lines. Removal of the TAF9-binding acidic alpha helical transactivation domain of GLI1 produced a significant reduction in the ability of GLI1 to transform cells. We then introduced a point mutation into GLI1 (L1052I) that eliminates TAF9 binding and a point mutation into GLI3 (I1510L) that establishes binding. Wild-type and mutant GLI proteins that bind TAF9 showed enhanced transactivating and cell transforming activity compared with those that did not. Therefore, GLI-TAF9 binding appears important for oncogenic activity. We then determined whether wild-type p53 down-regulates GLI function by sequestering TAF9. We showed that p53 binds TAF9 with greater affinity than does GLI1 and that co-expression of p53 with GLI1 or GLI2 down-regulated GLI-induced transactivation, which could be abrogated using mutant forms of GLI1 or p53. This suggests that p53 sequesters TAF9 from GLI1, which may contribute to inhibition of GLI1 activity by p53 and potentially impact therapeutic success of agents targeting GLI-TAF9 interactions in cancer.

Modes of Human T Cell Leukemia Virus Type 1 Transmission, Replication and Persistence

Human T-cell leukemia virus type 1 (HTLV-1) is a retrovirus that causes cancer (Adult T cell Leukemia, ATL) and a spectrum of inflammatory diseases (mainly HTLV-associated myelopathy—tropical spastic paraparesis, HAM/TSP). Since virions are particularly unstable, HTLV-1 transmission primarily occurs by transfer of a cell carrying an integrated provirus. After transcription, the viral genomic RNA undergoes reverse transcription and integration into the chromosomal DNA of a cell from the newly infected host. The virus then replicates by either one of two modes: (i) an infectious cycle by virus budding and infection of new targets and (ii) mitotic division of cells harboring an integrated provirus. HTLV-1 replication initiates a series of mechanisms in the host including antiviral immunity and checkpoint control of cell proliferation. HTLV-1 has elaborated strategies to counteract these defense mechanisms allowing continuous persistence in humans.

A novel one-class SVM based negative data sampling method for reconstructing proteome-wide HTLV-human protein interaction networks

Protein-protein interaction (PPI) prediction is generally treated as a problem of binary classification wherein negative data sampling is still an open problem to be addressed. The commonly used random sampling is prone to yield less representative negative data with considerable false negatives. Meanwhile rational constraints are seldom exerted on model selection to reduce the risk of false positive predictions for most of the existing computational methods. In this work, we propose a novel negative data sampling method based on one-class SVM (support vector machine, SVM) to predict proteome-wide protein interactions between HTLV retrovirus and Homo sapiens, wherein one-class SVM is used to choose reliable and representative negative data, and two-class SVM is used to yield proteome-wide outcomes as predictive feedback for rational model selection. Computational results suggest that one-class SVM is more suited to be used as negative data sampling method than two-class PPI predictor, and the predictive feedback constrained model selection helps to yield a rational predictive model that reduces the risk of false positive predictions. Some predictions have been validated by the recent literature. Lastly, gene ontology based clustering of the predicted PPI networks is conducted to provide valuable cues for the pathogenesis of HTLV retrovirus.

Viruses and the DNA Damage Response: Activation and Antagonism

Viruses must interact with their hosts in order to replicate; these interactions often provoke the evolutionarily conserved response to DNA damage, known as the DNA damage response (DDR). The DDR can be activated by incoming viral DNA, during the integration of retroviruses, or in response to the aberrant DNA structures generated upon replication of DNA viruses. Furthermore, DNA and RNA viral proteins can induce the DDR by promoting inappropriate S phase entry, by modifying cellular DDR factors directly, or by unintentionally targeting host DNA. The DDR may be antiviral, although viruses often require proximal DDR activation of repair and recombination factors to facilitate replication as well as downstream DDR signaling suppression to ensure cell survival. An unintended consequence of DDR attenuation during infection is the long-term survival and proliferation of precancerous cells. Therefore, the molecular basis for DDR activation and attenuation by viruses remains an important area of study that will likely provide key insights into how viruses have evolved with their hosts.

HTLV-1 and leukemogenesis: virus-cell interactions in the development of adult T-cell leukemia

Human T-cell lymphotropic virus type 1 (HTLV-1) was originally discovered in the early 1980s. It is the first retrovirus to be unambiguously linked causally to a human cancer. HTLV-1 currently infects approximately 20 million people worldwide. In this chapter, we review progress made over the last 30 years in our understanding of HTLV-1 infection, replication, gene expression, and cellular transformation.

Tax-1 and Tax-2 similarities and differences: focus on post-translational modifications and NF-κB activation

Although human T cell leukemia virus type 1 and 2 (HTLV-1 and HTLV-2) share similar genetic organization, they have major differences in their pathogenesis and disease manifestation. HTLV-1 is capable of transforming T lymphocytes in infected patients resulting in adult T cell leukemia/lymphoma whereas HTLV-2 is not clearly associated with lymphoproliferative diseases. Numerous studies have provided accumulating evidence on the involvement of the viral transactivators Tax-1 versus Tax-2 in T cell transformation. Tax-1 is a potent transcriptional activator of both viral and cellular genes. Tax-1 post-translational modifications and specifically ubiquitylation and SUMOylation have been implicated in nuclear factor-kappaB (NF-κB) activation and may contribute to its transformation capacity. Although Tax-2 has similar protein structure compared to Tax-1, the two proteins display differences both in their protein–protein interaction and activation of signal transduction pathways. Recent studies on Tax-2 have suggested ubiquitylation and SUMOylation independent mechanisms of NF-κB activation. In this present review, structural and functional differences between Tax-1 and Tax-2 will be summarized. Specifically, we will address their subcellular localization, nuclear trafficking and their effect on cellular regulatory proteins. A special attention will be given to Tax-1/Tax-2 post-translational modification such as ubiquitylation, SUMOylation, phosphorylation, acetylation, NF-κB activation, and protein–protein interactions involved in oncogenecity both in vivo and in vitro.

Interferon-α (IFN-α) suppresses HTLV-1 gene expression and cell cycling, while IFN-α combined with zidovudine induces p53 signaling and apoptosis in HTLV-1-infected cells

BACKGROUND

Human T-cell leukemia virus type-1 (HTLV-1) is the causative retrovirus of adult T-cell leukemia/lymphoma (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). HTLV-1 gene expression is maintained at low levels in vivo by unknown mechanisms. A combination therapy of interferon-α (IFN-α) and zidovudin (AZT) shows therapeutic effects in ATL patients, although its mechanism is also obscure. We previously found that viral gene expression in IL-2-dependent HTLV-1-infected T-cells (ILTs) derived from ATL patients was markedly suppressed by stromal cells through a type I IFN response. Here, we investigated the effects of IFN-α with or without AZT on viral gene expression and cell growth in ILTs.

RESULTS

ILTs expressed variable but lower amounts of HTLV-1 Tax protein than HTLV-1-transformed HUT102 cells. Following the addition of IFN-α, the amounts of HTLV-1 p19 in the supernatants of these cells decreased in three days, while HTLV-1 gene expression decreased only in ILTs but not HUT102 cells. IFN-α also suppressed the spontaneous HTLV-1 induction in primary ATL cells cultured for 24 h. A time course study using ILTs revealed that the levels of intracellular Tax proteins decreased in the first 24 h after addition of IFN-α, before the reduction in HTLV-1 mRNA levels. The initial decreases of Tax protein following IFN-α treatment were observed in 6 of 7 ILT lines tested, although the reduction rates varied among ILT lines. An RNA-dependent protein kinase (PKR)-inhibitor reversed IFN-mediated suppression of Tax in ILTs. IFN-α also induced cell cycle arrest at the G0/G1 phase and suppressed NF-κB activities in these cells. AZT alone did not affect HTLV-1 gene expression, cell viability or NF-κB activities. AZT combined with IFN-α markedly induced cell apoptosis associated with phosphorylation of p53 and induction of p53-responsive genes in ILTs.

CONCLUSIONS

IFN-α suppressed HTLV-1 gene expression at least through a PKR-mediated mechanism, and also induced cell cycle arrest in ILTs. In combination with AZT, IFN-α further induced p53 signaling and cell apoptosis in these cells. These findings suggest that HTLV-1-infected cells at an IL-2-dependent stage retain susceptibility to type I IFN-mediated regulation of viral expression, and partly explain how AZT/IFN-α produces therapeutic effects in ATL.

p53 dysfunction precedes the activation of nuclear factor-κB during disease progression in mice expressing Tax, a human T-cell leukemia virus type 1 oncoprotein

Transgenic (Tg) mice expressing Tax, a human T-cell leukemia virus type 1 (HTLV-1) oncoprotein, develop mature T-cell leukemia/lymphoma. The leukemic cells in Tg mice expressing Tax show p53 dysfunction and nuclear factor-κB (NF-κB) activation, similar to that seen in adult T-cell leukemia/lymphoma (ATLL) cells from patients infected with HTLV-1. However, it is unclear when these effects occur in HTLV-1 carriers during the development of ATLL. Here, we examined p53 function and NF-κB activity before the onset of leukemia in Tax-expressing Tg (Tax-Tg) mice between 4 and 25 months of age. At 4-10 months of age, 71% of mice showed p53 inactivation, without evidence for NF-κB activation, even though tax expression was consistent from 4 to 25 months of age. The decline in p53 function resulted from decreased p53 accumulation after DNA damage. From 11 months of age onward, 75% of mice showed p53 dysfunction and 37.5% showed constitutive NF-κB activation with the components of p50 and RelB. An NF-κB inhibitor, dehydroxymethylepoxyquinomicin (DHMEQ), reduced NF-κB activity (i.e. p50/RelB) but did not restore p53 function. In vivo, treatment with DHMEQ until 24 months of age prevented the onset of T-cell leukemia in Tax-Tg mice. These results suggest that the Tax-induced decline in p53 function, which is independent of NF-κB activation in the early stage, might be the first stage in the onset of ATLL. NF-κB activity is involved in the later stages of ATLL onset.

Wip1 and p53 contribute to HTLV-1 Tax-induced tumorigenesis

Background

Human T-cell Leukemia Virus type 1 (HTLV-1) infects 20 million individuals world-wide and causes Adult T-cell Leukemia/Lymphoma (ATLL), a highly aggressive T-cell cancer. ATLL is refractory to treatment with conventional chemotherapy and fewer than 10% of afflicted individuals survive more than 5 years after diagnosis. HTLV-1 encodes a viral oncoprotein, Tax, that functions in transforming virus-infected T-cells into leukemic cells. All ATLL cases are believed to have reduced p53 activity although only a minority of ATLLs have genetic mutations in their p53 gene. It has been suggested that p53 function is inactivated by the Tax protein.

Results

Using genetically altered mice, we report here that Tax expression does not achieve a functional equivalence of p53 inactivation as that seen with genetic mutation of p53 (i.e. a p53^−/− genotype). Thus, we find statistically significant differences in tumorigenesis between Tax⁺p53^+/+versus Tax⁺p53^−/− mice. We also find a role contributed by the cellular Wip1 phosphatase protein in tumor formation in Tax transgenic mice. Notably, Tax⁺Wip1^−/− mice show statistically significant reduced prevalence of tumorigenesis compared to Tax⁺Wip1^+/+ counterparts.

Conclusions

Our findings provide new insights into contributions by p53 and Wip1 in the in vivo oncogenesis of Tax-induced tumors in mice.

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.

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.

Development of ultra-super sensitive immunohistochemistry and its application to the etiological study of adult T-cell leukemia/lymphoma

Antigen retrieval (AR) and ultra-super sensitive immunohistochemistry (ultra-IHC) have been established for application to archival human pathology specimens. The original ultra-IHC was the ImmunoMax method or the catalyzed signal amplification system (ImmunoMax/CSA method), comprising the streptavidin-biotin complex (sABC) method and catalyzed reporter deposition (CARD) reaction with visualization of its deposition. By introducing procedures to diminish non-specific staining in the original ultra-IHC method, we developed the modified ImmunoMax/CSA method with AR heating sections in an AR solution (heating-AR). The heating-AR and modified ImmunoMax/CSA method visualized expression of the predominantly simple present form of HTLV-1 proviral DNA pX region p40Tax protein (Tax) in adult T-cell leukemia/lymphoma (ATLL) cells in archival pathology specimens in approximately 75% of cases. The simple present form of Tax detected exhibited a close relation with ATLL cell proliferation. We also established a new simplified CSA (nsCSA) system by replacing the sABC method with the secondary antibody- and horse radish peroxidase-labeled polymer reagent method, introducing the pretreatments blocking non-specific binding of secondary antibody reagent, and diminishing the diffusion of deposition in the CARD reaction. Combined with AR treating sections with proteinase K solution (enzymatic-AR), the nsCSA system visualized granular immunostaining of the complex present form of Tax in a small number of ATLL cells in most cases, presenting the possibility of etiological pathological diagnosis of ATLL and suggesting that the complex present form of Tax-positive ATLL cells were young cells derived from ATLL stem cells. The heating-AR and ultra-IHC detected physiological expression of the p53 protein and its probable phosphorylation by Tax in peripheral blood mononuclear cells of peripheral blood tissue specimens from HTLV-1 carriers, as well as physiological and pathological expression of the molecules involved with G1 phase progression and G1–S phase transition (E2F-1, E2F-4, DP-1, and cyclin E) in ATLL and peripheral T-cell lymphoma cells. The ultra-IHC with AR is useful for etiological pathological diagnosis of ATLL since HTLV-1 pathogenicity depends on that of Tax, and can be a useful tool for studies translating advanced molecular biology and pathology to human pathology.

The DNA damage response in viral-induced cellular transformation

The DNA damage response (DDR) has emerged as a critical tumour suppressor pathway responding to cellular DNA replicative stress downstream of aberrant oncogene over-expression. Recent studies have now implicated the DDR as a sensor of oncogenic virus infection. In this review, we discuss the mechanisms by which tumour viruses activate and also suppress the host DDR. The mechanism of tumour virus induction of the DDR is intrinsically linked to the need for these viruses to promote an S-phase environment to replicate their nucleic acid during infection. However, inappropriate expression of viral oncoproteins can also activate the DDR through various mechanisms including replicative stress, direct interaction with DDR components and induction of reactive oxygen species. Given the growth-suppressive consequences of activating the DDR, tumour viruses have also evolved mechanisms to attenuate these pathways. Aberrant expression of viral oncoproteins may therefore promote tumourigenesis through increased somatic mutation and aneuploidy due to DDR inactivation. This review will focus on the interplay between oncogenic viruses and the DDR with respect to cellular checkpoint control and transformation.

[Combined infection with HTLV-1 and Strongyloides stercoralis]

Infection of carriers of strongyloides by the human oncogenic retrovirus HTLV-1 significantly augments the number of larval parasites in the stools and impairs the action of anti-helminthic agents, resulting in an increase in immediate and longer term failure of therapy. The proliferation of cytokine type 1 secreting lymphocytes, the preferred target for viral infection, shifts the Th1/Th2 balance in favour of a Th1 response with a consequent increase in the production of gamma interferon (INF-γ). In addition to other effects, this causes a decrease in the secretion of cytokines IL-4, IL-5 and IL-13, which results in substantial reduction in total and specific IgE; failure of activation of eosinophils or stagnation in or reduction of their numbers; and an increased risk of development of a severe form of strongyloidiasis. This risk is clearly correlated with the level of anti-HTLV-1 antibodies and the amplitude of the proviral load of peripheral lymphocytes. The polyclonal expansion of infected CD4 cells might be partly due to the activation of the IL-2/IL-2R system by parasite antigens together with the action of the virus type 1 Tax protein. The fact that adult T cell leukaemia arises significantly earlier and more often in individuals with combined infection is an argument in favour of the parasite's role as a leukaemogenic co-factor. In practice it is, therefore, appropriate to initiate all available measures to eliminate parasites from co-infected hosts although this does present difficulties, and one should not reject the possibility of a diagnosis of strongyloidiasis in the absence of hypereosinophilia. In all cases of chronic strongyloidiasis without hypereosinophilia, co-infection with HTLV-1 should be looked for routinely. The same applies to carriers of strongyloides with repeated treatment failures. Finally, corticosteroids and immunosuppressants should be used only with care in HTLV-1-positive patients who seem not to be co-infected, even if they have received precautionary therapy.

Human T-cell leukemia virus type 1 (HTLV-1) and leukemic transformation: viral infectivity, Tax, HBZ and therapy

The human T-cell leukemia virus type 1 (HTLV-1) was the first retrovirus discovered to be causative of a human cancer, adult T-cell leukemia. The transforming entity of HTLV-1 has been attributed to the virally-encoded oncoprotein, Tax. Unlike the v-onc proteins encoded by other oncogenic animal retroviruses that transform cells, Tax does not originate from a c-onc counterpart. In this article, we review progress in our understanding of HTLV-1 infectivity, cellular transformation, anti-sense transcription and therapy, 30 years after the original discovery of this virus.

HTLV-1 and adult T-cell leukemia: insights into viral transformation of cells 30 years after virus discovery

Human T-cell leukemia virus type 1 (HTLV-1), the etiological agent of adult T-cell leukemia, was the first human retrovirus to be isolated. It is now the 30(th) anniversary of the initial discovery of HTLV-1. This review discusses recent insights into the role of the HTLV-1 Tax oncoprotein in cellular proliferation and the abrogation of cellular checkpoints that lead to disease progression.

Human T Lymphotropic Virus Type 1 (HTLV-1): Molecular Biology and Oncogenesis

Human T lymphotropic viruses (HTLVs) are complex deltaretroviruses that do not contain a proto-oncogene in their genome, yet are capable of transforming primary T lymphocytes both in vitro and in vivo. There are four known strains of HTLV including HTLV type 1 (HTLV-1), HTLV-2, HTLV-3 and HTLV-4. HTLV-1 is primarily associated with adult T cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). HTLV-2 is rarely pathogenic and is sporadically associated with neurological disorders. There have been no diseases associated with HTLV-3 or HTLV-4 to date. Due to the difference in the disease manifestation between HTLV-1 and HTLV-2, a clear understanding of their individual pathobiologies and the role of various viral proteins in transformation should provide insights into better prognosis and prevention strategies. In this review, we aim to summarize the data accumulated so far in the transformation and pathogenesis of HTLV-1, focusing on the viral Tax and HBZ and citing appropriate comparisons to HTLV-2.

Mapping the interactions of the p53 transactivation domain with the KIX domain of CBP

Molecular interactions between the tumor suppressor p53 and the transcriptional coactivators CBP/p300 are critical for the regulation of p53 transactivation and stability. The transactivation domain (TAD) of p53 binds directly to several CBP/p300 domains (TAZ1, TAZ2, NCBD, and KIX). Here we map the interaction between the p53 TAD and the CBP KIX domain using isothermal titration calorimetry and NMR spectroscopy. KIX is a structural domain in CBP/p300 that can simultaneously bind two polypeptide ligands, such as the activation domain of MLL and the kinase-inducible activation domain (pKID) of CREB, using distinct interaction surfaces. The p53 TAD consists of two subdomains (AD1 and AD2); peptides corresponding to the isolated AD1 and AD2 subdomains interact with KIX with relatively low affinity, but a longer peptide containing both subdomains binds KIX tightly. In the context of the full-length p53 TAD, AD1 and AD2 bind synergistically to KIX. Mapping of the chemical shift perturbations onto the structure of KIX shows that isolated AD1 and AD2 peptides bind to both the MLL and pKID sites. Spin-labeling experiments show that the complex of the full-length p53 TAD with KIX is disordered, with the AD1 and AD2 subdomains each interacting with both the MLL and pKID binding surfaces. Phosphorylation of the p53 TAD at Thr18 or Ser20 increases the KIX binding affinity. The affinity is further enhanced by simultaneous phosphorylation of Thr18 and Ser20, and the specificity of the interaction is increased. The p53 TAD simultaneously occupies the two distinct sites that have been identified on the CBP KIX domain and efficiently competes for these sites with other known KIX-binding transcription factors.

SIK1 couples LKB1 to p53-dependent anoikis and suppresses metastasis

Resistance to anoikis, the subtype of apoptosis triggered by lack of adhesion, contributes to malignant transformation and the development of metastasis. Although several lines of evidence suggest that p53 plays a critical role in anoikis, the pathway(s) that connect cell detachment to p53 remain undefined. Here, through the use of a kinome-wide loss-of-function screen, we identify the serine-threonine kinase SIK1 (salt-inducible kinase 1) as a regulator of p53-dependent anoikis. Inactivation of SIK1 compromised p53 function in anoikis and allowed cells to grow in an anchorage-independent manner. In vivo, SIK1 loss facilitated metastatic spread and survival of disseminated cells as micrometastases in lungs. The presence of functional SIK1 was required for the activity of the kinase LKB1 in promoting p53-dependent anoikis and suppressing anchorage-independent growth, Matrigel invasion, and metastatic potential. In human cancers, decreased expression of the gene encoding SIK1 closely correlated with development of distal metastases in breast cancers from three independent cohorts. Together, these findings indicate that SIK1 links LKB1 to p53-dependent anoikis and suppresses metastasis.

Advances in genomic research on hepatitis C virus with a useful tool, replicon system

The research for hepatitis C virus (HCV) has long delayed by missing of in vitro culture system. Since the development of replicon system, a replication system of subgenomic HCV RNAs in a hepatoma cell line, has been reported, many virological and clinical findings have been discovered. Recently, in addition of subgenomic replication system, hepatitis C virus full-length RNA replication has been possible, and a few cell culture systems producing viral particles have been produced. These developments enabled us to investigate the life cycle or intracellular circumstance of HCV production. By screening of newly synthesized drugs with this replicon system, several possible medicines have been established and clinical researches are now running. Among them, VX950 and SCH503034 are nearest to clinical use. Other possible agents for reducing viral replication such as cyclophyllin inhibitors, inhibitors of sphingomyelin synthesis, HMG-CoA reductase inhibitors, or RNA-dependent RNA polymerase inhibitors have been also investigated. Furthermore the mechanism for development of hepatocellular carcinoma in the HCV infected liver has been vigorously studied using the HCV replicon system.

Acetylation of the human T-cell leukemia virus type 1 Tax oncoprotein by p300 promotes activation of the NF-kappaB pathway

The oncogenic potential of the HTLV-1 Tax protein involves activation of the NF-kappaB pathway, which depends on Tax phosphorylation, ubiquitination and sumoylation. We demonstrate that the nuclei of Tax-expressing cells, including HTLV-1 transformed T-lymphocytes, contain a pool of Tax molecules acetylated on lysine residue at amino acid position 346 by the transcriptional coactivator p300. Phosphorylation of Tax on serine residues 300/301 was a prerequisite for Tax localization in the nucleus and correlated with its subsequent acetylation by p300, whereas sumoylation, resulting in the formation of Tax nuclear bodies in which p300 was recruited, favored Tax acetylation. Overexpression of p300 markedly increased Tax acetylation and the ability of a wild type HTLV-1 provirus, but not of a mutant provirus carrying an acetylation deficient Tax gene, to activate gene expression from an integrated NF-kappaB-controlled promoter. Thus, Tax acetylation favors NF-kappaB activation and might play an important role in HTLV-1-induced cell transformation.

Human T-cell lymphotropic virus type 1 infection of CD34+ hematopoietic progenitor cells induces cell cycle arrest by modulation of p21(cip1/waf1) and survivin

Human T-cell lymphotropic virus type 1 (HTLV-1) is an oncogenic retrovirus and the etiologic agent of adult T-cell leukemia (ATL), an aggressive CD4(+) malignancy. HTLV-2 is highly homologous to HTLV-1; however, infection with HTLV-2 has not been associated with lymphoproliferative diseases. Although HTLV-1 infection of CD4(+) lymphocytes induces cellular replication and transformation, infection of CD34(+) human hematopoietic progenitor cells (HPCs) strikingly results in G(0)/G(1) cell cycle arrest and suppression of in vitro clonogenic colony formation by induction of expression of the cdk inhibitor p21(cip1/waf1) (p21) and concurrent repression of survivin. Immature CD34(+)/CD38(-) hematopoietic stem cells (HSCs) were more susceptible to alterations of p21 and survivin expression as a result of HTLV-1 infection, in contrast to more mature CD34(+)/CD38(+) HPCs. Knockdown of p21 expression in HTLV-1-infected CD34(+) HPCs partially abrogated cell cycle arrest. Notably, HTLV-2, an HTLV strain that is not associated with leukemogenesis, does not significantly modulate p21 and survivin expression and does not suppress hematopoiesis from CD34(+) HPCs in vitro. We speculate that the remarkable differences in the activities displayed by CD34(+) HPCs following infection with HTLV-1 or HTLV-2 suggest that HTLV-1 uniquely exploits cell cycle arrest mechanisms to establish a latent infection in hematopoietic progenitor/hematopoietic stem cells and initiates preleukemic events in these cells, which eventually results in the manifestation of ATL.

Human T-cell leukemia virus type 1 Tax modulates interferon-alpha signal transduction through competitive usage of the coactivator CBP/p300

We describe here Tax protein of human T-cell leukemia virus type 1 (HTLV-1) as an interferon (IFN)-alpha antagonist counteracting the transactivation function of IFN-stimulated gene factor 3 (ISGF3). Co-expression of Tax, but not the Tax mutant unable to bind to CBP, significantly inhibited the reporter gene expression directed by IFN-stimulated regulatory elements, despite that the formation of DNA-binding ISGF3 complex was unaffected. Gene activation induced by STAT2 transcription domain was also inhibited by expression of Tax. Furthermore, Tax-mediated transcriptional inhibition was reversed by overexpression of p300. These observations indicate that Tax interferes with IFN-alpha-induced JAK-STAT pathway by competition with STAT2 for CBP/p300 binding. Consistently, GST pull-down assay showed that Tax dose-dependently inhibited binding of STAT2 to p300. This study suggests that Tax may prevent IFN-alpha from exerting its antiviral, antiproliferative and proapoptotic effects, thereby contributing to persistent viral infection and HTLV-1-associated oncogenesis.

Human T-cell leukemia virus type 1 Tax and cellular transformation

Infection of T-cells by human T-cell leukemia virus type 1 (HTLV-1) causes a lymphoproliferative malignancy known as adult T-cell leukemia (ATL). ATL is characterized by abnormal lymphocytes, called flower cells, which have cleaved and convoluted nuclei. Tax, encoded by the HTLV-1 pX region, is a critical nonstructural protein that plays a central role in leukemogenesis; however, the mechanisms of HTLV-1 oncogenesis have not been clarified fully. In this review, we summarize current thinking on how Tax may affect ATL leukemogenesis.

Soy-derived isoflavones inhibit the growth of adult T-cell leukemia cells in vitro and in vivo

Adult T-cell leukemia occurs in human T-lymphotropic virus type I-infected individuals and is endemic to the south-western area of Kyushu in Japan. In this communication, we examined the effect of soy isoflavones on the growth of adult T-cell leukemia cells in vitro and in vivo. In the in vitro study, daidzein and genistein but not glycitein significantly inhibited the proliferation of ED-40515 and Hut102 cells in a dose-dependent manner. Among the isoflavones studied, genistein had the highest growth-inhibitory effect; however, genistein did not exert an apparent growth-inhibitory effect on Jurkat and Molt-4 cells, which were non-adult T-cell leukemia cells. Genistein prevented the G(1)/S or G(2)/M transition at 3 and 10 or 30 microM, respectively. Genistein upregulated p21 protein expression together with p53 accumulation. In addition, treatment with 30 microM genistein strongly induced phosphorylation of checkpoint kinase (CHK) 2 and p53 at serines 15, 20 and 37. Caffeine, an inhibitor of ataxia-telangiectasia mutated protein kinase, alleviated the genistein-induced p53 and CHK2 phosphorylation, suggesting the involvement of DNA damage at 30 microM. However, marked phosphorylation of CHK2 and p53 could not be detected at 3 and 10 microM genistein. These data indicate that genistein has biphasic growth-inhibitory properties. The in vivo studies demonstrated that soy-derived isoflavones significantly inhibit ED-40515 cell growth and infiltration into various organs in non-obese diabetic severe combined-immunodeficiency common gamma-chain knockout mice. Taken together, it is evident that soy isoflavones might serve as a promising compound for the treatment of adult T-cell leukemia.

Negative regulation of the SH2-homology containing protein-tyrosine phosphatase-1 (SHP-1) P2 promoter by the HTLV-1 Tax oncoprotein

Expression of SH(2)-homology-containing protein-tyrosine phosphatase-1 (SHP-1), a candidate tumor suppressor, is repressed in human T-cell leukemia virus type-1 (HTLV-1)-transformed lymphocyte cell lines, adult T-cell leukemia (ATL) cells, and in other hematologic malignancies. However, the mechanisms underlying regulation and repression of SHP-1 remain unclear. Herein, we cloned the putative full-length, hematopoietic cell-specific SHP-1 P2 promoter and identified the "core" promoter regions. HTLV-1 Tax profoundly represses P2 promoter activity and histone deacetylase-1 (HDAC1) potentiates such inhibition. NF-kappaB was implicated as both a rate-limiting factor for basal P2 promoter activity and important for Tax-induced promoter silencing (TIPS). Chromatin immunoprecipitation studies demonstrated that NF-kappaB dissociates from the SHP-1 P2 promoter following the binding of Tax and HDAC1. This is in agreement with coimmunoprecipitation studies where NF-kappaB competed with HDAC1 for association with Tax protein. We propose that in TIPS, Tax recruits HDAC1 to the SHP-1 P2 promoter and forms an inhibitory complex that results in deacetylation and dissociation of NF-kappaB from the promoter and attenuation of SHP-1 expression. TIPS provides a possible first step toward HTLV-1 leukemogenesis through its down-modulation of this key immediate early negative regulator of IL-2 signaling.

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.

Human T-cell leukaemia virus type 1 (HTLV-1) infectivity and cellular transformation

It has been 30 years since a 'new' leukaemia termed adult T-cell leukaemia (ATL) was described in Japan, and more than 25 years since the isolation of the retrovirus, human T-cell leukaemia virus type 1 (HTLV-1), that causes this disease. We discuss HTLV-1 infectivity and how the HTLV-1 Tax oncoprotein initiates transformation by creating a cellular environment favouring aneuploidy and clastogenic DNA damage. We also explore the contribution of a newly discovered protein and RNA on the HTLV-1 minus strand, HTLV-1 basic leucine zipper factor (HBZ), to the maintenance of virus-induced leukaemia.

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

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

Human T-cell leukemia virus type 1 Tax attenuates gamma-irradiation-induced apoptosis through physical interaction with Chk2

Checkpoint kinase 2 (Chk2) is known to mediate diverse cellular responses to genotoxic stress. The fundamental role of Chk2 is to regulate the network of genome-surveillance pathways that coordinate cell-cycle progression with DNA repair and cell survival or death. Defects in Chk2 contribute to the development of both hereditary and sporadic human cancers. We now present evidence that the human T-cell leukemia virus type-1 (HTLV-1) Tax protein directly interacts with Chk2 and the kinase activity of Chk2 is inhibited by Tax. The physical interaction of Chk2 and Tax was observed by co-immunoprecipitation assays in HTLV-1-infected T cells (C81) as well as GST pull-down assays using purified proteins. Binding and kinase activity inhibition studies with Tax deletion mutants indicated that at least two domains of Tax mediate the interaction with Chk2. We have analysed the functional consequence of de novo expression of Tax upon the cellular DNA-damage-induced apoptosis, which is mediated by Chk2. Using transient transfection and TUNEL assay, we found that gamma-irradiation-induced apoptosis was decreased in 293T and HCT-116 (p53(-/-)) cells expressing HTLV-1 Tax. Our studies demonstrate an important potential target of Tax in cellular transformation.

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

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

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

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

The HTLV-1 Tax protein binding domain of cyclin-dependent kinase 4 (CDK4) includes the regulatory PSTAIRE helix

Background

The Tax oncoprotein of human T-cell leukemia virus type 1 (HTLV-1) is leukemogenic in transgenic mice and induces permanent T-cell growth in vitro. It is found in active CDK holoenzyme complexes from adult T-cell leukemia-derived cultures and stimulates the G1- to-S phase transition by activating the cyclin-dependent kinase (CDK) CDK4. The Tax protein directly and specifically interacts with CDK4 and cyclin D2 and binding is required for enhanced CDK4 kinase activity. The protein-protein contact between Tax and the components of the cyclin D/CDK complexes increases the association of CDK4 and its positive regulatory subunit cyclin D and renders the complex resistant to p21^CIP inhibition. Tax mutants affecting the N-terminus cannot bind cyclin D and CDK4.

Results

To analyze, whether the N-terminus of Tax is capable of CDK4-binding, in vitro binding -, pull down -, and mammalian two-hybrid analyses were performed. These experiments revealed that a segment of 40 amino acids is sufficient to interact with CDK4 and cyclin D2. To define a Tax-binding domain and analyze how Tax influences the kinase activity, a series of CDK4 deletion mutants was tested. Different assays revealed two regions which upon deletion consistently result in reduced binding activity. These were isolated and subjected to mammalian two-hybrid analysis to test their potential to interact with the Tax N-terminus. These experiments concurrently revealed binding at the N- and C-terminus of CDK4. The N-terminal segment contains the PSTAIRE helix, which is known to control the access of substrate to the active cleft of CDK4 and thus the kinase activity.

Conclusion

Since the N- and C-terminus of CDK4 are neighboring in the predicted three-dimensional protein structure, it is conceivable that they comprise a single binding domain, which interacts with the Tax N-terminus.

Molecular mechanisms of cellular transformation by HTLV-1 Tax

The HTLV Tax protein is crucial for viral replication and for initiating malignant transformation leading to the development of adult T-cell leukemia. Tax has been shown to be oncogenic, since it transforms and immortalizes rodent fibroblasts and human T-lymphocytes. Through CREB, NF-kappaB and SRF pathways Tax transactivates cellular promoters including those of cytokines (IL-13, IL-15), cytokine receptors (IL-2Ralpha) and costimulatory surface receptors (OX40/OX40L) leading to upregulated protein expression and activated signaling cascades (e.g. Jak/STAT, PI3Kinase, JNK). Tax also stimulates cell growth by direct binding to cyclin-dependent kinase holenzymes and/or inactivating tumor suppressors (e.g. p53, DLG). Moreover, Tax silences cellular checkpoints, which guard against DNA structural damage and chromosomal missegregation, thereby favoring the manifestation of a mutator phenotype in cells.

Evidence for NF-kappaB- and CBP-independent repression of p53's transcriptional activity by human T-cell leukemia virus type 1 Tax in mouse embryo and primary human fibroblasts

The human T-cell leukemia virus type 1 (HTLV-1) Tax oncoprotein can repress the transcriptional activity of the tumor suppressor protein p53. However, it remains controversial whether Tax requires NF-kappaB factors/activity and/or p300/CBP in order to inactivate p53 function. To address this issue, we have investigated Tax's effect on p53's transcriptional activation in IkappaB-kinase-deficient mouse embryonic fibroblasts (MEFs); some of which are entirely silent for Tax-induced NF-kappaB activity. We found that, in IKKalpha-/-, IKKbeta-/-, and IKKgamma-/- MEFs, p53 activation of a prototypic responsive plasmid (pG13-luciferase) was repressed by wild-type Tax. Curiously, p53's activity in MEFs was also repressed by a p300/CBP-binding deficient Tax protein. Our results highlight the complex nature of Tax-mediated repression of p53- activity, which requires further investigation.

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.