Molecular interactions involved in the transactivation of the human T-cell leukemia virus type 1 promoter mediated by Tax and CREB-2 (ATF-4)

ATF4 Signaling in HIV-1 Infection: Viral Subversion of a Stress Response Transcription Factor

Cellular integrated stress response (ISR), the mitochondrial unfolded protein response (UPRmt), and IFN signaling are associated with viral infections. Activating transcription factor 4 (ATF4) plays a pivotal role in these pathways and controls the expression of many genes involved in redox processes, amino acid metabolism, protein misfolding, autophagy, and apoptosis. The precise role of ATF4 during viral infection is unclear and depends on cell hosts, viral agents, and models. Furthermore, ATF4 signaling can be hijacked by pathogens to favor viral infection and replication. In this review, we summarize the ATF4-mediated signaling pathways in response to viral infections, focusing on human immunodeficiency virus 1 (HIV-1). We examine the consequences of ATF4 activation for HIV-1 replication and reactivation. The role of ATF4 in autophagy and apoptosis is explored as in the context of HIV-1 infection programmed cell deaths contribute to the depletion of CD4 T cells. Furthermore, ATF4 can also participate in the establishment of innate and adaptive immunity that is essential for the host to control viral infections. We finally discuss the putative role of the ATF4 paralogue, named ATF5, in HIV-1 infection. This review underlines the role of ATF4 at the crossroads of multiple processes reflecting host-pathogen interactions.

YAP suppresses human T-cell leukemia virus type 1 transcription

Human T-cell leukemia virus type 1 (HTLV-1) is an oncogenic retrovirus that causes adult T-cell leukemia/lymphoma (ATL). HTLV-1 encodes Tax protein that activates transcription from viral long terminal repeats (LTR). Multiple cofactors are involved in the regulation of HTLV-1 transcription via association with Tax. Yes-associated protein (YAP), which is the key effector of Hippo pathway, is elevated and activated in ATL cells. In this study, we reported that YAP protein suppressed Tax activation of HTLV-1 5' LTR but not 3' LTR. The activation of the 5' LTR by Tax was potentiated when YAP was depleted. Moreover, overexpression of YAP repressed HTLV-1 plus-strand viral gene expression and virion production, whereas compromising YAP by RNA inference augmented the expression of HTLV-1 protein. As mechanisms of YAP-mediated viral transcription inhibition, we found that YAP interacted with Tax, and prevented the association between Tax and p300. It finally led to the inhibition of recruitment of Tax to the Tax-responsive element in the 5' LTR of HTLV-1. Taken together, our results demonstrate the negative regulatory function of YAP in Tax activation of HTLV-1 transcription. It may achieve sufficient transcriptional repression to maintain persistent infection and long-term latency of HTLV-1 in the host cells.

Pathogenicity and virulence of human T lymphotropic virus type-1 (HTLV-1) in oncogenesis: adult T-cell leukemia/lymphoma (ATLL)

Adult T-cell leukemia/lymphoma (ATLL) is an aggressive malignancy of CD4⁺ T lymphocytes caused by human T lymphotropic virus type-1 (HTLV-1) infection. HTLV-1 was brought to the World Health Organization (WHO) and researchers to address its impact on global public health, oncogenicity, and deterioration of the host immune system toward autoimmunity. In a minority of the infected population (3-5%), it can induce inflammatory networks toward HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), or hijacking the infected CD4⁺ T lymphocytes into T regulatory subpopulation, stimulating anti-inflammatory signaling networks, and prompting ATLL development. This review critically discusses the complex signaling networks in ATLL pathogenesis during virus-host interactions for better interpretation of oncogenicity and introduces the main candidates in the pathogenesis of ATLL. At least two viral factors, HTLV-1 trans-activator protein (TAX) and HTLV-1 basic leucine zipper factor (HBZ), are implicated in ATLL manifestation, interacting with host responses and deregulating cell signaling in favor of infected cell survival and virus dissemination. Such molecules can be used as potential novel biomarkers for ATLL prognosis or targets for therapy. Moreover, the challenging aspects of HTLV-1 oncogenesis introduced in this review could open new venues for further studies on acute leukemia pathogenesis. These features can aid in the discovery of effective immunotherapies when reversing the gene expression profile toward appropriate immune responses gradually becomes attainable.

Role of p38 mitogen-activated protein kinase signalling in virus replication and potential for developing broad spectrum antiviral drugs

Mitogen-activated protein kinases (MAPKs) play a key role in complex cellular processes such as proliferation, development, differentiation, transformation and apoptosis. Mammals express at least four distinctly regulated groups of MAPKs which include extracellular signal-related kinases (ERK)-1/2, p38 proteins, Jun amino-terminal kinases (JNK1/2/3) and ERK5. p38 MAPK is activated by a wide range of cellular stresses and modulates activity of several downstream kinases and transcription factors which are involved in regulating cytoskeleton remodeling, cell cycle modulation, inflammation, antiviral response and apoptosis. In viral infections, activation of cell signalling pathways is part of the cellular defense mechanism with the basic aim of inducing an antiviral state. However, viruses can exploit enhanced cell signalling activities to support various stages of their replication cycles. Kinase activity can be inhibited by small molecule chemical inhibitors, so one strategy to develop antiviral drugs is to target these cellular signalling pathways. In this review, we provide an overview on the current understanding of various cellular and viral events regulated by the p38 signalling pathway, with a special emphasis on targeting these events for antiviral drug development which might identify candidates with broad spectrum activity.

Regulation of cellular immunity by activating transcription factor 4

Activating transcription factor 4 (ATF4) is a DNA binding transcription factor belonging to the family of basic Leucine zipper proteins. ATF4 can be activated in response to multiple cellular stress signals including endoplasmic reticulum stress in the event of improper protein folding or oxidative stress because of mitochondrial dysfunction as well as hypoxia. There are multiple downstream targets of ATF4 that can coordinate the regulation between survival and apoptosis of a cell based on time and exposure to stress. ATF4, therefore, has a broad range of control that results in the modulation of immune cells of the innate and adaptive responses leading to regulation of the cellular immunity. Studies provide evidence that ATF4 can regulate immune cells such as macrophages, T cells, B cells, NK cells and dendritic cells contributing to progression of disease. Immune cells can be exposed to stressed environment in the event of a pathogen attack, infection, inflammation, or in the tumor microenvironment leading to increased ATF4 activity to regulate these responses. ATF4 can further control differentiation and maturation of different immune cell types becoming a determinant of effective immune regulation. Additionally, ATF4 has been heavily implicated in rendering effector immune cells dysfunctional that are used to target tumorigenesis. Therefore, there is a need to evaluate where the literature stands in understanding the overall role of ATF4 in regulating cellular immunity to identify therapeutic targets and generalized mechanisms for different disease progressions.

Regulatory Mechanisms of Somatostatin Expression

Somatostatin is a peptide hormone, which most commonly is produced by endocrine cells and the central nervous system. In mammals, somatostatin originates from pre-prosomatostatin and is processed to a shorter form, i.e., somatostatin-14, and a longer form, i.e., somatostatin-28. The two peptides repress growth hormone secretion and are involved in the regulation of glucagon and insulin synthesis in the pancreas. In recent years, the processing and secretion of somatostatin have been studied intensively. However, little attention has been paid to the regulatory mechanisms that control its expression. This review provides an up-to-date overview of these mechanisms. In particular, it focuses on the role of enhancers and silencers within the promoter region as well as on the binding of modulatory transcription factors to these elements. Moreover, it addresses extracellular factors, which trigger key signaling pathways, leading to an enhanced somatostatin expression in health and disease.

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

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

The eIF2α/ATF4 pathway is essential for stress-induced autophagy gene expression

In response to different environmental stresses, eIF2α phosphorylation represses global translation coincident with preferential translation of ATF4, a master regulator controlling the transcription of key genes essential for adaptative functions. Here, we establish that the eIF2α/ATF4 pathway directs an autophagy gene transcriptional program in response to amino acid starvation or endoplasmic reticulum stress. The eIF2α-kinases GCN2 and PERK and the transcription factors ATF4 and CHOP are also required to increase the transcription of a set of genes implicated in the formation, elongation and function of the autophagosome. We also identify three classes of autophagy genes according to their dependence on ATF4 and CHOP and the binding of these factors to specific promoter cis elements. Furthermore, different combinations of CHOP and ATF4 bindings to target promoters allow the trigger of a differential transcriptional response according to the stress intensity. Overall, this study reveals a novel regulatory role of the eIF2α–ATF4 pathway in the fine-tuning of the autophagy gene transcription program in response to stresses.

Complex role of microRNAs in HTLV-1 infections

Human T-lymphotropic virus 1 (HTLV-1) was the first human retrovirus to be discovered and is the causative agent of adult T-cell leukemia/lymphoma (ATL) and the neurodegenerative disease HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The importance of microRNA (miRNA) in the replicative cycle of several other viruses, as well as in the progression of associated pathologies, has been well established in the past decade. Moreover, involvement of miRNA alteration in the HTLV-1 life cycle, and in the progression of its related oncogenic and neurodegenerative diseases, has recently come to light. Several HTLV-1 derived proteins alter transcription factor functionalities, interact with chromatin remodelers, or manipulate components of the RNA interference (RNAi) machinery, thereby establishing various routes by which miRNA expression can be up- or down-regulated in the host cell. Furthermore, the mechanism of action through which dysregulation of host miRNAs affects HTLV-1 infected cells can vary substantially and include mRNA silencing via the RNA-induced silencing complex (RISC), transcriptional gene silencing, inhibition of RNAi components, and chromatin remodeling. These miRNA-induced changes can lead to increased cell survival, invasiveness, proliferation, and differentiation, as well as allow for viral latency. While many recent studies have successfully implicated miRNAs in the life cycle and pathogenesis of HTLV-1 infections, there are still significant outstanding questions to be addressed. Here we will review recent discoveries elucidating HTLV-1 mediated manipulation of host cell miRNA profiles and examine the impact on pathogenesis, as well as explore future lines of inquiry that could increase understanding in this field of study.

HTLV tax: a fascinating multifunctional co-regulator of viral and cellular pathways

Human T-cell lymphotropic virus type 1 (HTLV-1) has been identified as the causative agent of adult T-cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The virus infects between 15 and 20 million people worldwide of which approximately 2-5% develop ATL. The past 35 years of research have yielded significant insight into the pathogenesis of HTLV-1, including the molecular characterization of Tax, the viral transactivator, and oncoprotein. In spite of these efforts, the mechanisms of oncogenesis of this pleiotropic protein remain to be fully elucidated. In this review, we illustrate the multiple oncogenic roles of Tax by summarizing a recent body of literature that refines our understanding of cellular transformation. A focused range of topics are discussed in this review including Tax-mediated regulation of the viral promoter and other cellular pathways, particularly the connection of the NF-κB pathway to both post-translational modifications (PTMs) of Tax and subcellular localization. Specifically, recent research on polyubiquitination of Tax as it relates to the activation of the IkappaB kinase (IKK) complex is highlighted. Regulation of the cell cycle and DNA damage responses due to Tax are also discussed, including Tax interaction with minichromosome maintenance proteins and the role of Tax in chromatin remodeling. The recent identification of HTLV-3 has amplified the importance of the characterization of emerging viral pathogens. The challenge of the molecular determination of pathogenicity and malignant disease of this virus lies in the comparison of the viral transactivators of HTLV-1, -2, and -3 in terms of transformation and immortalization. Consequently, differences between the three proteins are currently being studied to determine what factors are required for the differences in tumorogenesis.

Localization and sub-cellular shuttling of HTLV-1 tax with the miRNA machinery

The innate ability of the human cell to silence endogenous retroviruses through RNA sequences encoding microRNAs, suggests that the cellular RNAi machinery is a major means by which the host mounts a defense response against present day retroviruses. Indeed, cellular miRNAs target and hybridize to specific sequences of both HTLV-1 and HIV-1 viral transcripts. However, much like the variety of host immune responses to retroviral infection, the virus itself contains mechanisms that assist in the evasion of viral inhibition through control of the cellular RNAi pathway. Retroviruses can hijack both the enzymatic and catalytic components of the RNAi pathway, in some cases to produce novel viral miRNAs that can either assist in active viral infection or promote a latent state. Here, we show that HTLV-1 Tax contributes to the dysregulation of the RNAi pathway by altering the expression of key components of this pathway. A survey of uninfected and HTLV-1 infected cells revealed that Drosha protein is present at lower levels in all HTLV-1 infected cell lines and in infected primary cells, while other components such as DGCR8 were not dramatically altered. We show colocalization of Tax and Drosha in the nucleus in vitro as well as coimmunoprecipitation in the presence of proteasome inhibitors, indicating that Tax interacts with Drosha and may target it to specific areas of the cell, namely, the proteasome. In the presence of Tax we observed a prevention of primary miRNA cleavage by Drosha. Finally, the changes in cellular miRNA expression in HTLV-1 infected cells can be mimicked by the add back of Drosha or the addition of antagomiRs against the cellular miRNAs which are downregulated by the virus.

Activation of LTRs from different human endogenous retrovirus (HERV) families by the HTLV-1 tax protein and T-cell activators

Human endogenous retroviruses (HERVs) represent approximately 8% of our genome. HERVs influence cellular gene expression and contribute to normal physiological processes such as cellular differentiation and morphogenesis. HERVs have also been associated with certain pathological conditions, including cancer and neurodegenerative diseases. As HTLV-1 causes adult T-cell leukemia and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) and has been shown to modulate host gene expression mainly through the expression of the powerful Tax transactivator, herein we were interested in looking at the potential modulation capacity of HTLV-1 Tax on HERV expression. In order to evaluate the promoter activity of different HERV LTRs, pHERV-LTR-luc constructs were co-transfected in Jurkat T-cells with a Tax expression vector. Tax expression potently increased the LTR activity of HERV-W8 and HERV-H (MC16). In parallel, Jurkat cells were also stimulated with different T-cell-activating agents and HERV LTRs were observed to respond to different combination of Forskolin, bpV[pic] a protein tyrosine phosphatase inhibitor, and PMA. Transfection of expression vectors for different Tax mutants in Jurkat cells showed that several transcription factors including CREB appeared to be important for HERV-W8 LTR activation. Deletion mutants were derived from the HERV-W8 LTR and the region from −137 to −123 was found to be important for LTR response following Tax expression in Jurkat cells, while a different region was shown to be required in cells treated with activators. Our results thus demonstrated that HTLV-1 Tax activates several HERV LTRs. This raises the possibility that upregulated HERV expression could be involved in diseases associated with HTLV-1 infection.

Making sense out of antisense transcription in human T-cell lymphotropic viruses (HTLVs)

Retroviral gene expression generally depends on a full-length transcript that initiates in the 5′ long terminal repeat (LTR), which is either unspliced or alternatively spliced. We and others have demonstrated the existence of an antisense transcript initiating in the 3′ LTR of the Human T-cell Leukemia Virus type 1 (HTLV-1) that is involved in the production of HBZ (HTLV-1 basic leucine zipper (bZIP) factor). HBZ is a Fos-like factor capable of inhibiting Tax-mediated activation of the HTLV-1 LTR by interacting with the cellular transcription factor cAMP-response element-binding protein (CREB) and the pleiotropic cellular coactivators p300/CBP. HBZ can also activate cellular transcription through its interaction with p300/CBP. Interestingly, HBZ has also been found to promote T-lymphocyte proliferation. By down-regulating viral expression and by stimulating T-cell proliferation, HBZ could be essential in the establishment of a chronic infection. Antisense transcription also occurs in the closely related HTLV-2 retrovirus as well as in the recently discovered HTLV-3 and HTLV-4. These antisense transcripts are also involved in the production of retroviral proteins that we have termed Antisense Protein of HTLVs (APH). Like HBZ, the APH proteins are localized in the nucleus of transfected cells and repress Tax-mediated viral transcription.

Human T-lymphotropic virus type 1 transcription and chromatin-remodeling complexes

Human T-lymphotropic virus type 1 (HTLV-1) encodes the viral protein Tax, which is believed to act as a viral transactivator through its interactions with a variety of transcription factors, including CREB and NF-kappaB. As is the case for all retroviruses, the provirus is inserted into the host DNA, where nucleosomes are deposited to ensure efficient packaging. Nucleosomes act as roadblocks in transcription, making it difficult for RNA polymerase II (Pol II) to proceed toward the 3' end of the genome. Because of this, a variety of chromatin remodelers can act to modify nucleosomes, allowing for efficient transcription. While a number of covalent modifications are known to occur on histone tails in HTLV-1 infection (i.e., histone acetyltransferases [HATs], histone deacetylases [HDACs], and histone methyltransferases [HMTs]), evidence points to the use of chromatin remodelers that use energy from ATP hydrolysis to remodel nucleosomes. Here we confirm that BRG1, which is the core subunit of eight chromatin-remodeling complexes, is essential not only for Tax transactivation but also for viral replication. This is especially evident when wild-type infectious clones of HTLV-1 are used. BRG1 associates with Tax at the HTLV-1 long terminal repeat (LTR), and coexpression of BRG1 and Tax results in increased rates of transcription. The interaction of BRG1 with Tax additionally recruits the basal transcriptional machinery and removes some of the core histones from the nucleosome at the start site (Nuc 1). When using the BRG1-deficient cell lines SW13, C33A, and TSUPR1, we observed little viral transcription and no viral replication. Importantly, while these three cell lines do not express detectable levels of BRG1, much of the SWI/SNF complex remains assembled in the cells. Knockdown of BRG1 and associated SWI/SNF subunits suggests that the BRG1-utilizing SWI/SNF complex PBAF is responsible for HTLV-1 nucleosome remodeling. Finally, HTLV-1 infection of cell lines with a knockdown in BRG1 or the PBAF complex results in a significant reduction in viral production. Overall, we concluded that BRG1 is required for Tax transactivation and HTLV-1 viral production and that the PBAF complex appears to be responsible for nucleosome remodeling.

HTLV-1-Tax and ICAM-1 act on T-cell signal pathways to polarize the microtubule-organizing center at the virological synapse

Human T-lymphotropic virus type 1 (HTLV-1) spreads directly between lymphocytes and other cells via a specialized cell-cell contact, termed the virological synapse. The formation of the virological synapse is accompanied by the orientation of the microtubule-organizing center (MTOC) in the infected T cell toward the cell contact region with the noninfected target cell. We previously demonstrated that the combination of intracellular Tax protein expression and the stimulation of the intercellular adhesion molecule-1 (ICAM-1) on the cell surface is sufficient to trigger MTOC polarization in the HTLV-1-infected T cell. However, the mechanism by which Tax and ICAM-1 cause the MTOC polarization is not fully understood. Here we show that the presence of Tax at the MTOC region and its ability to stimulate cyclic AMP-binding protein-dependent pathways are both required for MTOC polarization in the HTLV-1-infected T cell at the virological synapse. Furthermore, we show that the MTOC polarization induced by ICAM-1 engagement depends on activation of the Ras-MEK-ERK signaling pathway. Our findings indicate that efficient MTOC polarization at the virological synapse requires Tax-mediated stimulation of T-cell activation pathways in synergy with ICAM-1 cross-linking. The results also reveal differences in the signaling pathways used to trigger MTOC polarization between the immunologic synapse and the virological synapse.

CCAAT/enhancer-binding proteins and the pathogenesis of retrovirus infection

Previous studies indicate that two upstream CCAAT/enhancer-binding protein (C/EBP) sites and C/EBPbeta are required for subtype B HIV-1 gene expression in cells of the monocyte-macrophage lineage. The mechanisms of C/EBP regulation of HIV-1 transcription and replication remain unclear. This review focuses on studies concerning the role of C/EBP factors in HIV-1, human T-cell leukemia virus type 1, and SIV transcription in various cell types and tissues cultured in vitro, animal models and during human infection. The structure and function of the C/EBPbeta gene and the related protein isoforms are discussed along with the transcription factors, coactivators, viral proteins, cytokines and chemokines that affect C/EBP function.

Upregulation of human T-cell leukemia virus type 1 antisense transcription by the viral tax protein

Several studies have recently demonstrated the existence of human T-cell leukemia virus type 1 (HTLV-1) antisense transcripts, which allow the synthesis of the newly described HBZ protein. Although previous reports have been aimed at understanding the potential role of the HBZ protein in HTLV-1 pathogenesis, little is known as to how this viral gene is regulated. Here, using our K30-3'asLuc reporter construct, we show that the viral Tax protein upregulates antisense transcription through its action on the TRE sequences located in the 3' long terminal repeat. Generation of stable clones in 293T cells demonstrated that Tax-induced HBZ expression is importantly influenced by the integration site in the host genome. The cellular DNA context could thus affect the level of HBZ mRNA expression in infected cells.

An interaction between the human T cell leukemia virus type 1 basic leucine zipper factor (HBZ) and the KIX domain of p300/CBP contributes to the down-regulation of tax-dependent viral transcription by HBZ

Activation of human T cell leukemia virus type 1 (HTLV-1) transcription is established through the formation of protein complexes on the viral promoter that are essentially composed of the cellular basic leucine zipper (bZIP) transcription factor cAMP-response element-binding protein (CREB (or certain other members of the ATF/CREB family), the HTLV-1-encoded transactivator Tax, and the pleiotropic cellular coactivators p300/CBP. HTLV-1 bZIP factor (HBZ) is a protein encoded by HTLV-1 that contains a bZIP domain and functions to repress HTLV-1 transcription. HBZ has been shown to repress viral transcription by dimerizing with CREB, which occurs specifically through the bZIP domain in each protein, and preventing CREB from binding to the DNA. However, we previously found that HBZ causes only partial removal of CREB from a chromosomally integrated viral promoter, and more importantly, an HBZ mutant lacking the COOH-terminal bZIP domain retains the ability to repress viral transcription. These results suggest that an additional mechanism contributes to HBZ-mediated repression of HTLV-1 transcription. In this study, we show that HBZ binds directly to the p300 and CBP coactivators. Two LXXLL-like motifs located within the NH(2)-terminal region of HBZ are important for this interaction and specifically mediate binding to the KIX domain of p300/CBP. We provide evidence that this interaction interferes with the ability of Tax to bind p300/CBP and thereby inhibits the association of the coactivators with the viral promoter. Our findings demonstrate that HBZ utilizes a bipartite mechanism to repress viral transcription.

Two specific drugs, BMS-345541 and purvalanol A induce apoptosis of HTLV-1 infected cells through inhibition of the NF-kappaB and cell cycle pathways

Human T-cell leukemia virus type-1 (HTLV-1) induces adult T-cell leukemia/lymphoma (ATL/L), a fatal lymphoproliferative disorder, and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), a chronic progressive disease of the central nervous system after a long period of latent infection. Although the mechanism of transformation and leukemogenesis is not fully elucidated, there is evidence to suggest that the viral oncoprotein Tax plays a crucial role in these processes through the regulation of several pathways including NF-κB and the cell cycle pathways. The observation that NF-κB, which is strongly induced by Tax, is indispensable for the maintenance of the malignant phenotype of HTLV-1 by regulating the expression of various genes involved in cell cycle regulation and inhibition of apoptosis provides a possible molecular target for these infected cells. To develop potential new therapeutic strategies for HTLV-1 infected cells, in this present study, we initially screened a battery of NF-κB and CDK inhibitors (total of 35 compounds) to examine their effects on the growth and survival of infected T-cell lines. Two drugs namely BMS-345541 and Purvalanol A exhibited higher levels of growth inhibition and apoptosis in infected cell as compared to uninfected cells. BMS-345541 inhibited IKKβ kinase activity from HTLV-1 infected cells with an IC₅₀ (the 50% of inhibitory concentration) value of 50 nM compared to 500 nM from control cells as measured by in vitro kinase assays. The effects of Purvalanol A were associated with suppression of CDK2/cyclin E complex activity as previously shown by us. Combination of both BMS-345541 and Purvalanol A showed a reduced level of HTLV-1 p19 Gag production in cell culture. The apparent apoptosis in these infected cells were associated with increased caspase-3 activity and PARP cleavage. The potent and selective apoptotic effects of these drugs suggest that both BMS-345541 and Purvalanol A, which target both NF-κB and CDK complex and the G1/S border, might be promising new agents in the treatment of these infected patients.

XBP-1, a novel human T-lymphotropic virus type 1 (HTLV-1) tax binding protein, activates HTLV-1 basal and tax-activated transcription

X-box binding protein 1 (XBP-1), a basic leucine zipper transcription factor, plays a key role in the cellular unfolded protein response (UPR). There are two XBP-1 isoforms in cells, spliced XBP-1S and unspliced XBP-1U. XBP-1U has been shown to bind to the 21-bp Tax-responsive element of the human T-lymphotropic virus type 1 (HTLV-1) long terminal repeat (LTR) in vitro and transactivate HTLV-1 transcription. Here we identify XBP-1S as a transcription activator of HTLV-1. Compared to XBP-1U, XBP-1S demonstrates stronger activating effects on both basal and Tax-activated HTLV-1 transcription in cells. Our results show that both XBP-1S and XBP-1U interact with Tax and bind to the HTLV-1 LTR in vivo. In addition, elevated mRNA levels of the gene for XBP-1 and several UPR genes were detected in the HTLV-1-infected C10/MJ and MT2 T-cell lines, suggesting that HTLV-1 infection may trigger the UPR in host cells. We also identify Tax as a positive regulator of the expression of the gene for XBP-1. Activation of the UPR by tunicamycin showed no effect on the HTLV-1 LTR, suggesting that HTLV-1 transcription is specifically regulated by XBP-1. Collectively, our study demonstrates a novel host-virus interaction between a cellular factor XBP-1 and transcriptional regulation of HTLV-1.

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?

The PDZ domain-binding motif of the human T cell leukemia virus type 1 tax protein induces mislocalization of the tumor suppressor hScrib in T cells

Interactions with cellular PDZ domain-containing proteins obviously contribute to the tumorigenic potential of several viral oncoproteins. In this regard, the oncogenic potential of the human T cell leukemia virus type 1 Tax protein correlates with its binding capacity to the tumor suppressor hDlg. Recent results show that hDlg in T cells is associated to a network of scaffolding proteins including another PDZ domain-containing protein termed hScrib. Interestingly, previous studies have revealed complementary activities of both proteins in the control of epithelial cell polarity. Here, we demonstrate that Tax can bind to hScrib and that the resulting Tax/hScrib complex is present in human T cell leukemia virus type 1-infected T cells. By confocal microscopy, we show that Tax modifies the localization of hScrib in transfected COS cells as well as in infected T cell lines and targets hScrib to particular spots exhibiting a granular distribution, mainly distributed in the cytoplasm. Given that Tax sequesters hScrib to these particular structures, we postulate that Tax might inhibit hScrib activity. Providing further support to this idea, we find that transient overexpression of hScrib attenuates T cell receptor-induced NFAT activity but that the presence of Tax counteracts this negative effect on the NFAT pathway. The fact that hDlg and hScrib are both targeted by Tax underlies their importance in T cell function.

The p300/CBP-associated factor (PCAF) is a cofactor of ATF4 for amino acid-regulated transcription of CHOP

When an essential amino acid is limited, a signaling cascade is triggered that leads to increased translation of the ‘master regulator’, activating transcription factor 4 (ATF4), and resulting in the induction of specific target genes. Binding of ATF4 to the amino acid response element (AARE) is an essential step in the transcriptional activation of CHOP (a CCAAT/enhancer-binding protein-related gene) by amino acid deprivation. We set out to identify proteins that interact with ATF4 and that play a role in the transcriptional activation of CHOP. Using a tandem affinity purification (TAP) tag approach, we identified p300/CBP-associated factor (PCAF) as a novel interaction partner of ATF4 in leucine-starved cells. We show that the N-terminal region of ATF4 is required for a direct interaction with PCAF and demonstrate that PCAF is involved in the full transcriptional response of CHOP by amino acid starvation. Chromatin immunoprecipitation analysis revealed that PCAF is engaged on the CHOP AARE in response to amino acid starvation and that ATF4 is essential for its recruitment. We also show that PCAF stimulates ATF4-driven transcription via its histone acetyltransferase domain. Thus PCAF acts as a coactivator of ATF4 and is involved in the enhancement of CHOP transcription following amino acid starvation.

TORC1 and TORC2 coactivators are required for tax activation of the human T-cell leukemia virus type 1 long terminal repeats

Human T-cell leukemia virus type 1 (HTLV-1) Tax protein activates viral transcription from the long terminal repeats (LTR). Mechanisms through which Tax activates LTR have been established, but coactivators of this process remain to be identified and characterized. Here we show that all three members of the TORC family of transcriptional regulators are coactivators of Tax for LTR-driven expression. TORC coactivation requires CREB, but not ATF4 or other bZIP factors. Tax physically interacts with TORC1, TORC2, and TORC3 (TORC1/2/3), and the depletion of TORC1/2/3 inhibited Tax activity. TORC coactivation can be further enhanced by transcriptional coactivator p300. In addition, coactivators in the p300 family are required for full activity of Tax independently of TORC1/2/3. Thus, both TORC and p300 families of coactivators are essential for optimal activation of HTLV-1 transcription by Tax.

Site-specific phosphorylation differentiates active from inactive forms of the human T-cell leukemia virus type 1 Tax oncoprotein

The human T-cell leukemia virus type 1 oncoprotein Tax is a phosphoprotein with a predominately nuclear subcellular localization that accomplishes multiple functions via protein-protein interactions. It has been proposed that regulation of this protein's pleiotropic functions may be accomplished through phosphorylation of specific amino acid residues. We have conducted a phosphoryl mapping of mammalian-expressed Tax protein using a combination of affinity purification, liquid chromatography tandem mass spectrometry, and site-directed substitution mutational analysis. We achieved physical coverage of 77% of the Tax sequence and identified four novel sites of phosphorylation at Thr-48, Thr-184, Thr-215, and Ser-336. Previously identified potential serine phosphorylation sites at Ser-10, Ser-77, and Ser-274 could not be confirmed by mass spectrometry. The functional significance of these novel phosphorylation events was evaluated by mutational analysis and subsequent evaluation for activity via both CREB and NF-kappaB-responsive promoters. Our results demonstrate that phosphorylation at Thr-215 is associated with loss of both Tax functions, phosphorylation at Thr-48 was specifically deficient for activation via NF-kappaB, and phosphorylation at Thr-184 and Ser-336 had no effect on these Tax functions. Semiquantitation of phosphopeptides revealed that the majority of Tax was phosphorylated at Thr-48, Thr-184, Thr-215, and Ser-336, whereas only a minor population of Tax was phosphorylated at either Ser-300 or Ser-301. These results suggest that both positive and negative phosphorylation signals result in the maintenance of a subfraction of Tax as "active" protein.

HBZ, a new important player in the mystery of adult T-cell leukemia

Adult T-cell leukemia (ATL) was first described in 1977. A link between ATL and human T-cell leukemia virus type 1 (HTLV-1) was clearly established in the early 1980s. Over the years, many aspects of HTLV-1-induced cellular dysfunctions have been clarified. However, the detailed mechanism behind ATL occurrence remains unsolved. Presently, we are still unable to explain the absence of viral Tax protein (thought to play a central role in T-cell transformation) in more than 50% of ATL cells. A novel HTLV-1 HBZ protein, encoded on the negative strand, was characterized by our group and is currently the subject of intensive research efforts to determine its function in viral replication and/or pathophysiology. Recently, 4 studies reported on the existence of different HBZ isoforms and have investigated on their function in both ATL cells or animal models. One report suggests that the HBZ gene might have a bimodal function (at the mRNA and protein levels), which could represent an uncharacterized strategy to regulate viral replication and proliferation of infected T cells.

Transcription of human zinc finger ZNF268 gene requires an intragenic promoter element

Human ZNF268 gene is a typical Krüppel-associated box/C2H2 zinc finger gene whose homolog has been found only in higher mammals and not in lower mammals such as mouse. Its expression profiles have suggested that it plays a role in the differentiation of blood cells during early human embryonic development and the pathogenesis of leukemia. To gain additional insight into the molecular mechanisms controlling the expression of the ZNF268 gene and to provide the necessary tools for further genetic studies of leukemia, we have mapped the 5'-end of the human ZNF268 mRNA by reverse transcription-PCR and primer extension assays. We then cloned the 5'-flanking genomic DNA containing the putative ZNF268 gene promoter and analyzed its function in several different human and mouse tissue culture cell lines. Interestingly, our studies show that the ZNF268 gene lacks a typical eukaryotic promoter that is present upstream of the transcription start site and directs a basal level of transcription. Instead, the functional promoter requires an essential element that is located within the first exon of the gene. Deletion and mutational analysis reveals the requirement for a cAMP response-element-binding protein (CREB)-binding site within this element for promoter function. Gel mobility shift and chromatin immunoprecipitation assays confirm that CREB-2 binds to the site in vitro and in vivo. Furthermore, overexpression of CREB-2 enhances the promoter activity. These results demonstrate that the human ZNF268 gene promoter is atypical and requires an intragenic element located within the first exon that mediates the effect of CREB for its activity.

CCAAT/enhancer-binding proteins modulate human T cell leukemia virus type 1 long terminal repeat activation

CCAAT/enhancer-binding protein (C/EBP) basic region/leucine zipper (bZIP) transcription factors have been shown to form heterodimers with cAMP-responsive element binding protein 2 (CREB-2), a transcription factor involved in regulating basal and Tax-mediated transactivation of the human T cell leukemia virus type 1 (HTLV-1) long terminal repeat (LTR). In cells of the monocyte-macrophage lineage (proposed to play a role in HTLV-1 pathogenesis as an accessory target cell), several members of the C/EBP family are expressed at high levels and may have functional impact on both basal and Tax-mediated transactivation of the HTLV-1 LTR. Basal activation of the HTLV-1 LTR was enhanced by overexpression of C/EBPbeta, C/EBPdelta, or C/EBPepsilon, whereas transactivation of the LTR by Tax was inhibited by overexpression of C/EBPalpha and C/EBPbeta. Inhibition of Tax-mediated transactivation of the HTLV-1 LTR was co-activator-independent, did not require C/EBP binding to the Tax-responsive elements, and may involve heterodimerization with CREB factors.

Cooperative interaction of Zhangfei and ATF4 in transactivation of the cyclic AMP response element

Zhangfei (ZF) is a basic region-leucine zipper protein that has been implicated in herpesvirus infection cycle and related cellular processes. Here we show both in vivo and in vitro data demonstrating that ZF is a novel cellular binding partner of activating transcription factor 4 (ATF4) (or CREB2). We found that ZF competed with ATF4 to form ATF4-ZF heterodimeric complexes through the bZIP regions. ZF enhanced ATF4 binding to the cAMP response element (CRE), and augmented activation of a CRE reporter by ATF4, in response to MEK1 activation. These results suggest an important role of ZF in the MEK1-ATF4 signaling pathway.

p300 Modulates ATF4 Stability and Transcriptional Activity Independently of Its Acetyltransferase Domain

ATF4 plays a crucial role in the cellular response to stress and multiple stress responses pathways converge to the translational up-regulation of ATF4. ATF4 is a substrate of the SCF(betaTrCP) ubiquitin ligase that binds to betaTrCP through phosphorylation on a DSGXXXS motif. We show here that ATF4 stability is also modulated by the histone acetyltransferase p300, which induces ATF4 stabilization by inhibiting its ubiquitination. Despite p300 acetylates ATF4, we found that p300-mediated ATF4 stabilization is independent of p300 catalytic activity, using either the inactive form of p300 or the acetylation mutant ATF4-K311R. ATF4 deleted of its p300 binding domain is no more stabilized by p300 nor recruited into nuclear speckles. In consequence of ATF4 stabilization, both p300 and the catalytically inactive enzyme increase ATF4 transcriptional activity.

Transcriptional and post-transcriptional gene regulation of HTLV-1

Adult T-cell leukemia (ATL) is an aggressive hematologic malignancy caused by human T-cell leukemia virus type I (HTLV-1). Tax, encoded by the HTLV-1 pX region, has been recognized by its pleiotropic actions to play a critical role in leukemogenesis. Three highly conserved 21-bp repeat elements located within the long terminal repeat, commonly referred to as Tax-responsive element 1 (TRE-1), are critical to Tax-mediated viral transcriptional activation through complex interaction with cyclic AMP-responsive element binding protein (CREB), CBP/p300 and PCAF. Tax has also been shown to activate transcription from a number of critical cellular genes through the NF-kappaB and serum-responsive factor pathways. Tax transactivation has been attributed to the protein's interaction with transcription factors, chromatin remodeling complexes, cell cycle and repair genes. In this review, we will discuss some of the latest findings on this fascinating viral activator and highlight its regulation of cellular factors including CREB, p300/CBP and their effect on RNA polymerase II and chromatin remodeling, as well as its role in cytoplasmic and nuclear function. We will highlight the possible contribution of each factor, discuss Tax's critical peptide domains and highlight its post-transcriptional modifications. It is quite obvious that, collectively, Tax's effects on a wide variety of cellular targets cooperate in promoting cell proliferation and leukemogenesis. In addition, the post-transcriptional effects of Rex play an important role in virus replication. Understanding these interactions at a molecular level will facilitate the targeted development of drugs to effectively inhibit or treat ATL.

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

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

Nuclear localization of HTLV-I bZIP factor (HBZ) is mediated by three distinct motifs

The genome of the human T-cell leukemia virus type I (HTLV-I) codes for a basic leucine zipper protein, HBZ, capable of repressing JUN activity and viral transcription. Transient expression in mammalian cells showed that HBZ was targeted to the nucleus, where it accumulated in nuclear speckles. By using a complementary set of deletion mutants, we report here that the nuclear targeting of HBZ is mediated by three distinct nuclear localization signals and that at least two are necessary for the translocation of HBZ to the nucleus. Moreover, the resulting mutant proteins distribute throughout the nucleoplasm and/or into the nucleoli, whereas the wild-type HBZ exclusively accumulates in nuclear speckles, suggesting that the integrity of the protein is required for its speckle localization. We also demonstrate that the HBZ-containing speckles do not correspond to Cajal bodies, splicing factor compartments, or promyelocytic leukemia oncoprotein bodies. Unexpectedly, by using immunogold electron microscopy, we found HBZ localized to heterochromatin. Until now, such characteristics had never been described for a transcription factor and could explain the inhibitory activity of HBZ.

Enhanced activation of tax-dependent transcription of human T-cell leukemia virus type I (HTLV-I) long terminal repeat by TORC3

Tax, a protein encoded by the env-pX gene of human T-cell leukemia virus type I (HTLV-I), interacts with various host cell transcription factors. Tax activates transcription from the long terminal repeat (LTR) of HTLV-I through association with cyclic AMP-responsive element-binding protein (CREB). Here, we present evidence that transducer of regulated cyclic AMP-response element-binding protein 3 (TORC3), a co-activator of CREB, is involved in Tax-induced transcriptional activation from the HTLV-I LTR. By using a luciferase assay system, we show that TORC3 alone can enhance transcription from the HTLV-I LTR, as well as from a cellular cyclic AMP-response element (CRE). Interestingly, we find that co-expression of TORC3 and Tax dramatically increased transcriptional activation at the HTLV-I LTR. We also show by glutathione S-transferase pull-down and co-immunoprecipitation experiments that TORC3 interacts with Tax. Using deletion mutant analysis, we identify the Tax interaction domain of TORC3 as a region spanning from amino acid 1 to 103, which contains a coiled-coil domain. These results provide important clues toward understanding the molecular mechanism of Tax-dependent transcriptional activation of the HTLV-I LTR.

Role of Tax protein in human T-cell leukemia virus type-I leukemogenicity

HTLV-1 is the etiological agent of adult T-cell leukemia (ATL), the neurological syndrome TSP/HAM and certain other clinical disorders. The viral Tax protein is considered to play a central role in the process leading to ATL. Tax modulates the expression of many viral and cellular genes through the CREB/ATF-, SRF- and NF-κB-associated pathways. In addition, Tax employs the CBP/p300 and p/CAF co-activators for implementing the full transcriptional activation competence of each of these pathways. Tax also affects the function of various other regulatory proteins by direct protein-protein interaction. Through these activities Tax sets the infected T-cells into continuous uncontrolled replication and destabilizes their genome by interfering with the function of telomerase and topoisomerase-I and by inhibiting DNA repair. Furthermore, Tax prevents cell cycle arrest and apoptosis that would otherwise be induced by the unrepaired DNA damage and enables, thereby, accumulation of mutations that can contribute to the leukemogenic process. Together, these capacities render Tax highly oncogenic as reflected by its ability to transform rodent fibroblasts and primary human T-cells and to induce tumors in transgenic mice. In this article we discuss these effects of Tax and their apparent contribution to the HTLV-1 associated leukemogenic process. Notably, however, shortly after infection the virus enters into a latent state, in which viral gene expression is low in most of the HTLV-1 carriers' infected T-cells and so is the level of Tax protein, although rare infected cells may still display high viral RNA. This low Tax level is evidently insufficient for exerting its multiple oncogenic effects. Therefore, we propose that the latent virus must be activated, at least temporarily, in order to elevate Tax to its effective level and that during this transient activation state the infected cells may acquire some oncogenic mutations which can enable them to further progress towards ATL even if the activated virus is re-suppressed after a while. We conclude this review by outlining an hypothetical flow of events from the initial virus infection up to the ultimate ATL development and comment on the risk factors leading to ATL development in some people and to TSP/HAM in others.

The expanding role of Tax in transcription

The viral transactivator of HTLV-I, Tax, has long been shown to target the earliest steps of transcription by forming quaternary complexes with sequence specific transcription factors and histone-modifying enzymes in the LTR of HTLV-I. However, a new study suggests that Tax preferentially transactivates the 21-bp repeats through CREB1 and not other bZIP proteins. The additional transactivation of Tax-responsive promoters subsequent to initiation is also presented. This result highlights a potentially novel role of Tax following TBP recruitment (i.e. initiation) and may expand the mechanism of Tax transactivation in promoter clearance and transcriptional elongation.

Specific TATAA and bZIP requirements suggest that HTLV-I Tax has transcriptional activity subsequent to the assembly of an initiation complex

Background

Human T-cell leukemia virus type I (HTLV-I) Tax protein is a transcriptional regulator of viral and cellular genes. In this study we have examined in detail the determinants for Tax-mediated transcriptional activation.

Results

Whereas previously the LTR enhancer elements were thought to be the sole Tax-targets, herein, we find that the core HTLV-I TATAA motif also provides specific responsiveness not seen with either the SV40 or the E1b TATAA boxes. When enhancer elements which can mediate Tax-responsiveness were compared, the authentic HTLV-I 21-bp repeats were found to be the most effective. Related bZIP factors such as CREB, ATF4, c-Jun and LZIP are often thought to recognize the 21-bp repeats equivalently. However, amongst bZIP factors, we found that CREB, by far, is preferred by Tax for activation. When LTR transcription was reconstituted by substituting either κB or serum response elements in place of the 21-bp repeats, Tax activated these surrogate motifs using surfaces which are different from that utilized for CREB interaction. Finally, we employed artificial recruitment of TATA-binding protein to the HTLV-I promoter in "bypass" experiments to show for the first time that Tax has transcriptional activity subsequent to the assembly of an initiation complex at the promoter.

Conclusions

Optimal activation of the HTLV-I LTR by Tax specifically requires the core HTLV-I TATAA promoter, CREB and the 21-bp repeats. In addition, we also provide the first evidence for transcriptional activity of Tax after the recruitment of TATA-binding protein to the promoter.

HBZ interacts with JunD and stimulates its transcriptional activity

Human T-cell leukemia virus type I (HTLV-I) bZIP factor (HBZ) is a viral basic leucine zipper protein that was originally described as a partner of cAMP response element binding protein-2 and as a repressor of HTLV-I viral transcription. In addition, HBZ is able to interact with the activator protein-1 (AP-1) transcription factors c-Jun and JunB, the interaction with c-Jun leading to a transcriptional repression of AP-1-regulated genes. Here we show that HBZ also interacts with JunD in vitro and in vivo, and that this association occurs via the bZIP domain of the two proteins. Moreover, we show that HBZ can activate JunD-dependent transcription and that its amino-terminus is required.

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

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

The central region of human T-cell leukemia virus type 1 Tax protein contains distinct domains involved in subunit dimerization

The Tax protein of human T-cell leukemia virus type 1 (HTLV-1) can form homodimers. Tax dimerization contributes to optimal Tax activity involved in transactivation of the HTLV-1 promoter. The mechanisms used to form specific Tax dimers are poorly understood because the domains that mediate such interactions have not been clearly characterized. Here we have used different approaches (the two-hybrid assay in yeast, the glutathione S-transferase pull-down assay, and the Spot method) to study Tax-Tax interactions. Our results indicate that the integrity of the sequence of Tax, except for the last 16 amino acids (residues 338 to 353), is critical, suggesting that Tax dimerization is dictated more by secondary structure than by primary structure. We were, however, able to delimit a central region involved in Tax self-association that encompasses the residues 127 to 228. This region can be divided into three subdomains of dimerization: DD1 (residues 127 to 146), DD2 (residues 181 to 194), and DD3 (residues 213 to 228). Moreover, the Tax mutants M22 (T130A and L131S) and M29 (K189A and R190S), with amino acid substitutions located in DD1 and DD2, respectively, were found to be impaired in Tax self-association.

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

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

Protein profile of tax-associated complexes

Infection with human T-cell leukemia virus type 1 (HTLV-1) results in adult T-cell leukemia and HTLV-1-associated myelopathy/tropical spastic paraparesis. Tax, a 40-kDa protein, regulates viral and cellular transcription, host signal transduction, the cell cycle, and apoptosis. Tax has been shown to modulate cellular CREB and NFkappaB pathways; however, to date, its role in binding to various host cellular proteins involved in tumorigenesis has not been fully described. In this study, we describe the Tax-associated proteins and their functions in cells using several approaches. Tax eluted from a sizing column mostly at an apparent molecular mass of 1800 kDa. Following Tax immunoprecipitation, washes with high salt buffer, two-dimensional gel separation, and mass spectrometric analysis, a total of 32 proteins was identified. Many of these proteins belong to the signal transduction and cytoskeleton pathways and transcription/chromatin remodeling. A few of these proteins, including TXBP151, have been shown previously to bind to Tax. The interaction of Tax with small GTPase-cytoskeleton proteins, such as ras GAP1m, Rac1, Cdc42, RhoA, and gelsolin, indicates how Tax may regulate migration, invasion, and adhesion in T-cell cancers. Finally, the physical and functional association of Tax with the chromatin remodeling SWI/SNF complex was assessed using in vitro chromatin remodeling assays, chromatin remodeling factor BRG1 mutant cells, and RNA interference experiments. Collectively, Tax is able to bind and regulate many cellular proteins that regulate transcription and cytoskeletal related pathways, which might explain the pleiotropic effects of Tax leading to T-cell transformation and leukemia in HTLV-1-infected patients.

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

Human T-cell leukemia/lymphoma virus type 1 (HTLV-1) causes neoplastic transformation of human T-cells in a small number of infected individuals several years from infection. Several viral proteins act in concert to increase the responsiveness of T-cells to extracellular stimulation, modulate proapoptotic and antiapoptotic gene signals, enhance T-cell survival, and avoid immune recognition of the infected T-cells. The virus promotes T-cell proliferation by usurping several signaling pathways central to immune T-cell function. Viral proteins modulate the downstream effects of antigen stimulation and receptor-ligand interaction, suggesting that extracellular signals are important for HTLV-1 oncogenesis. Environmental factors such as chronic antigen stimulation are therefore important, as also suggested by epidemiological data. The ability of a given individual to respond to specific antigens is determined genetically. Thus, genetic and environmental factors, together with the virus, contribute to disease development. As in the case of other virus-associated cancers, HTLV-1-induced leukemia/lymphoma can be prevented by avoiding viral infection or by intervention during the asymptomatic phase with approaches able to interrupt the vicious cycle of virus-induced proliferation of a subset of T-cells. This review focuses on current knowledge of the mechanisms regulating HTLV-1 replication and the T-cell pathways that are usurped by viral proteins to induce and maintain clonal proliferation of infected T-cells in vitro. The relevance of these laboratory findings will be related to clonal T-cell proliferation and adult T-cell leukemia/lymphoma development in vivo.

Transcription factor binding and histone modifications on the integrated proviral promoter in human T-cell leukemia virus-I-infected T-cells

The human T-cell leukemia virus (HTLV-I)-encoded Tax protein is a potent transcriptional activator that stimulates expression of the integrated provirus. Biochemical studies indicate that Tax, together with cellular transcription factors, interacts with viral cAMP-response element enhancer elements to recruit the pleiotropic coactivators CREB-binding protein and p300. Histone acetylation by these coactivators has been shown to play a major role in activating HTLV-I transcription from chromatin templates in vitro. However, the extent of histone modification and the precise identity of the cellular regulatory proteins bound at the HTLV-I promoter in vivo is not known. Chromatin immunoprecipitation analysis was used to investigate factor binding and histone modification at the integrated HTLV-I provirus in infected T-cells (SLB-1). These studies reveal the presence of Tax, a variety of ATF/CREB and AP-1 family members (CREB, CREB-2, ATF-1, ATF-2, c-Fos, and c-Jun), and both p300 and CREB-binding protein at the HTLV-I promoter. Consistent with the binding of these coactivators, we observed histone H3 and H4 acetylation at three regions within the proviral genome. Histone deacetylases were also present at the viral promoter and, following their inhibition, we observe an increase in histone H4 acetylation on the HTLV-I promoter and a concomitant increase in viral RNA. Together, these results suggest that a variety of transcriptional activators, coactivators, and histone deacetylases participate in the regulation of HTLV-I transcription in infected T-cells.

The complementary strand of the human T-cell leukemia virus type 1 RNA genome encodes a bZIP transcription factor that down-regulates viral transcription

The RNA genome of the human T-cell leukemia virus type 1 (HTLV-1) codes for proteins involved in infectivity, replication, and transformation. We report in this study the characterization of a novel viral protein encoded by the complementary strand of the HTLV-1 RNA genome. This protein, designated HBZ (for HTLV-1 bZIP factor), contains a N-terminal transcriptional activation domain and a leucine zipper motif in its C terminus. We show here that HBZ is able to interact with the bZIP transcription factor CREB-2 (also called ATF-4), known to activate the HTLV-1 transcription by recruiting the viral trans-activator Tax on the Tax-responsive elements (TxREs). However, we demonstrate that the HBZ/CREB-2 heterodimers are no more able to bind to the TxRE and cyclic AMP response element sites. Taking these findings together, the functional inactivation of CREB-2 by HBZ is suggested to contribute to regulation of the HTLV-1 transcription. Moreover, the characterization of a minus-strand gene protein encoded by HTLV-1 has never been reported until now.

Cooperativity in transcription factor binding to the coactivator CREB-binding protein (CBP). The mixed lineage leukemia protein (MLL) activation domain binds to an allosteric site on the KIX domain

The interactions between cAMP-response element-binding protein (CREB)-binding protein (CBP) and gene-specific transcription factors play an important role in activation of transcription from numerous genes. Cooperative interactions between CBP and multiple transcriptional activators may provide a mechanism for synergistic increases in transcriptional activation. Here we report the characterization of ternary complexes formed by the KIX domain of CBP and the transactivation domain of the trithorax group protein mixed lineage leukemia protein (MLL), together with either the phosphorylated kinase-inducible domain (pKID) of CREB or the activation domain from c-Myb. Isothermal titration calorimetry experiments show that KIX in complex with the MLL activation domain binds the c-Myb activation domain and pKID domain with 2-fold higher affinity than does the KIX domain alone. Thus, the activation domains of Myb and MLL or of pKID and MLL bind cooperatively to KIX. The thermodynamics of these interactions imply different mechanisms of binding cooperativity for the two ternary complexes; the KIX.MLL.pKID complex is stabilized by entropy increases, whereas the enhancement of Myb binding in the presence of the MLL activation domain is due to more favorable enthalpy. NMR experiments show that the MLL-binding site on KIX is distinct from the surface that binds the pKID and c-Myb activation domains. Our data indicate that KIX can directly mediate cooperative interactions between pairs of transcriptional regulatory proteins. In the case of MLL and c-Myb, both proteins are involved in proliferation of hematopoietic cells and leukemogenesis, and synergistic interactions mediated by CBP may play a functional role.

Activation of HTLV-I transcription in the presence of Tax is independent of the acetylation of CREB-2 (ATF-4)

We have previously demonstrated that the bZIP transcription factor CREB-2, also called ATF-4, trans-activates, in association with the viral protein Tax, the human T-cell leukemia virus type I (HTLV-I) promoter. In this study, we have examined whether CREB-2 acetylation affects transcriptional activation mediated by Tax. We present evidence that CREB-2 is acetylated in vitro and in vivo. CREB-2 is acetylated in two regions: the basic domain of the bZIP (from amino acid residue 270 to 300) and the short basic domain (from 342 to 351) located downstream from the bZIP. We also demonstrate that CREB-2 is acetylated by p300/CBP but not by p/CAF. Moreover, replacement of lysine by arginine in the basic domains decreases the trans-activating capacity of CREB-2. However, in the presence of Tax, the HTLV-I transcription remains fully activated by these CREB-2 mutants. Although we cannot totally exclude that the mutations could also affect CREB-2 structure and activity independent of acetylation, our results suggest that activation of the viral promoter in the presence of Tax is independent of the CREB-2 acetylation.

Inhibition of HTLV-1 transcription by cyclin dependent kinase inhibitors

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

Cloning of mouse genomic ribosomal protein L6 gene and analysis of its promoter

Ribosomal protein (Rp) L6 is also defined as Taxreb107 (Tax responsive element binding protein 107) for its binding activity to the long terminal repeats of human T cell leukemia virus (HTLV)-I. We cloned the genomic gene of mouse RpL6/Taxreb107 and analyzed its exon/intron structures. The promoter of RpL6/Taxreb107 contains recognition sites for multiple transcription factors including nuclear factor (NF)-kappa B. Luciferase reporter assay showed that the RpL6/Taxreb107 promoter has a constitutive activity in transfected cells, and the constitutive activity depends on the intact promoter. Expression of HTLV-I viral protein Tax mildly but reproducibly induced RpL6/Taxreb107 mRNA and promoter activity. We provide evidence suggesting that induction of RpL6/Taxreb107 by Tax is at least partially mediated by the NF-kappa B site in the promoter of RpL6/Taxreb107. Taken together, Tax up-regulates RpL6/Taxreb107 and this may provide a feedback mechanism to facilitate proliferation of HTLV-I-infected cells and production of viral particles.