Inhibition of histone deacetylases induces bovine leukemia virus expression in vitro and in vivo

A complex network of transcription factors and epigenetic regulators involved in bovine leukemia virus transcriptional regulation

Bovine Leukemia Virus (BLV) is the etiological agent of enzootic bovine leukosis, a disease characterized by the neoplastic proliferation of B cells in cattle. While most European countries have introduced efficient eradication programs, BLV is still present worldwide and no treatment is available. A major feature of BLV infection is the viral latency, which enables the escape from the host immune system, the maintenance of a persistent infection and ultimately the tumoral development. BLV latency is a multifactorial phenomenon resulting in the silencing of viral genes due to genetic and epigenetic repressions of the viral promoter located in the 5' Long Terminal Repeat (5'LTR). However, viral miRNAs and antisense transcripts are expressed from two different proviral regions, respectively the miRNA cluster and the 3'LTR. These latter transcripts are expressed despite the viral latency affecting the 5'LTR and are increasingly considered to take part in tumoral development. In the present review, we provide a summary of the experimental evidence that has enabled to characterize the molecular mechanisms regulating each of the three BLV transcriptional units, either through cis-regulatory elements or through epigenetic modifications. Additionally, we describe the recently identified BLV miRNAs and antisense transcripts and their implications in BLV-induced tumorigenesis. Finally, we discuss the relevance of BLV as an experimental model for the closely related human T-lymphotropic virus HTLV-1.

Role of the cellular factor CTCF in the regulation of bovine leukemia virus latency and three-dimensional chromatin organization

Bovine leukemia virus (BLV)-induced tumoral development is a multifactorial phenomenon that remains incompletely understood. Here, we highlight the critical role of the cellular CCCTC-binding factor (CTCF) both in the regulation of BLV transcriptional activities and in the deregulation of the three-dimensional (3D) chromatin architecture surrounding the BLV integration site. We demonstrated the in vivo recruitment of CTCF to three conserved CTCF binding motifs along the provirus. Next, we showed that CTCF localized to regions of transitions in the histone modifications profile along the BLV genome and that it is implicated in the repression of the 5′Long Terminal Repeat (LTR) promoter activity, thereby contributing to viral latency, while favoring the 3′LTR promoter activity. Finally, we demonstrated that BLV integration deregulated the host cellular 3D chromatin organization through the formation of viral/host chromatin loops. Altogether, our results highlight CTCF as a new critical effector of BLV transcriptional regulation and BLV-induced physiopathology.

Latency Reversing Agents: Kick and Kill of HTLV-1?

Human T-cell leukemia virus type 1 (HTLV-1), the cause of adult T-cell leukemia/lymphoma (ATLL), is a retrovirus, which integrates into the host genome and persistently infects CD4⁺ T-cells. Virus propagation is stimulated by (1) clonal expansion of infected cells and (2) de novo infection. Viral gene expression is induced by the transactivator protein Tax, which recruits host factors like positive transcription elongation factor b (P-TEFb) to the viral promoter. Since HTLV-1 gene expression is repressed in vivo by viral, cellular, and epigenetic mechanisms in late phases of infection, HTLV-1 avoids an efficient CD8⁺ cytotoxic T-cell (CTL) response directed against the immunodominant viral Tax antigen. Hence, therapeutic strategies using latency reversing agents (LRAs) sought to transiently activate viral gene expression and antigen presentation of Tax to enhance CTL responses towards HTLV-1, and thus, to expose the latent HTLV-1 reservoir to immune destruction. Here, we review strategies that aimed at enhancing Tax expression and Tax-specific CTL responses to interfere with HTLV-1 latency. Further, we provide an overview of LRAs including (1) histone deacetylase inhibitors (HDACi) and (2) activators of P-TEFb, that have mainly been studied in context of human immunodeficiency virus (HIV), but which may also be powerful in the context of HTLV-1.

Regulation of Expression and Latency in BLV and HTLV

Human T-lymphotrophic virus type 1 (HTLV-1) and Bovine leukemia virus (BLV) belong to the Deltaretrovirus genus. HTLV-1 is the etiologic agent of the highly aggressive and currently incurable cancer adult T-cell leukemia (ATL) and a neurological disease HTLV-1-associated myelopathy (HAM)/tropical spastic paraparesis (TSP). BLV causes neoplastic proliferation of B cells in cattle: enzootic bovine leucosis (EBL). Despite the severity of these conditions, infection by HTLV-1 and BLV appear in most cases clinically asymptomatic. These viruses can undergo latency in their hosts. The silencing of proviral gene expression and maintenance of latency are central for the establishment of persistent infection, as well as for pathogenesis in vivo. In this review, we will present the mechanisms that control proviral activation and retroviral latency in deltaretroviruses, in comparison with other exogenous retroviruses. The 5′ long terminal repeats (5′-LTRs) play a main role in controlling viral gene expression. While the regulation of transcription initiation is a major mechanism of silencing, we discuss topics that include (i) the epigenetic control of the provirus, (ii) the cis-elements present in the LTR, (iii) enhancers with cell-type specific regulatory functions, (iv) the role of virally-encoded transactivator proteins, (v) the role of repressors in transcription and silencing, (vi) the effect of hormonal signaling, (vii) implications of LTR variability on transcription and latency, and (viii) the regulatory role of non-coding RNAs. Finally, we discuss how a better understanding of these mechanisms may allow for the development of more effective treatments against Deltaretroviruses.

PRMT5 Is Required for Bovine Leukemia Virus Infection In Vivo and Regulates BLV Gene Expression, Syncytium Formation, and Glycosylation In Vitro

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis, which is the most common neoplastic disease of cattle and is closely related to human T-cell leukemia viruses. We investigated the role of a new host protein, PRMT5, in BLV infection. We found that PRMT5 is overexpressed only in BLV-infected cattle with a high proviral load, but not in those with a low proviral load. Furthermore, this upregulation continued to the lymphoma stage. PRMT5 expression was upregulated in response to experimental BLV infection; moreover, PRMT5 upregulation began in an early stage of BLV infection rather than after a long period of proviral latency. Second, siRNA-mediated PRMT5 knockdown enhanced BLV gene expression at the transcript and protein levels. Additionally, a selective small-molecule inhibitor of PRMT5 (CMP5) enhanced BLV gene expression. Interestingly, CMP5 treatment, but not siRNA knockdown, altered the gp51 glycosylation pattern and increased the molecular weight of gp51, thereby decreasing BLV-induced syncytium formation. This was supported by the observation that CMP5 treatment enhanced the formation of the complex type of N-glycan more than the high mannose type. In conclusion, PRMT5 overexpression is related to the development of BLV infection with a high proviral load and lymphoma stage and PRMT5 inhibition enhances BLV gene expression. This is the first study to investigate the role of PRMT5 in BLV infection in vivo and in vitro and to reveal a novel function for a small-molecule compound in BLV-gp51 glycosylation processing.

Cis-drivers and trans-drivers of bovine leukemia virus oncogenesis

The bovine leukemia virus (BLV) is a retrovirus inducing an asymptomatic and persistent infection in ruminants and leading in a minority of cases to the accumulation of B-lymphocytes (lymphocytosis, leukemia or lymphoma). Although the mechanisms of oncogenesis are still largely unknown, there is clear experimental evidence showing that BLV infection drastically modifies the pattern of gene expression of the host cell. This alteration of the transcriptome in infected B-lymphocytes results first, from a direct activity of viral proteins (i.e. transactivation of gene promoters, protein-protein interactions), second, from insertional mutagenesis by proviral integration (cis-activation) and third, from gene silencing by microRNAs. Expression of viral proteins stimulates a vigorous immune response that indirectly modifies gene transcription in other cell types (e.g. cytotoxic T-cells, auxiliary T-cells, macrophages). In principle, insertional mutagenesis and microRNA-associated RNA interference can modify the cell fate without inducing an antiviral immunity. Despite a tight control by the immune response, the permanent attempts of the virus to replicate ultimately induce mutations in the infected cell. Accumulation of these genomic lesions and Darwinian selection of tumor clones are predicted to lead to cancer.

Characterization of new RNA polymerase III and RNA polymerase II transcriptional promoters in the Bovine Leukemia Virus genome

Bovine leukemia virus latency is a viral strategy used to escape from the host immune system and contribute to tumor development. However, a highly expressed BLV micro-RNA cluster has been reported, suggesting that the BLV silencing is not complete. Here, we demonstrate the in vivo recruitment of RNA polymerase III to the BLV miRNA cluster both in BLV-latently infected cell lines and in ovine BLV-infected primary cells, through a canonical type 2 RNAPIII promoter. Moreover, by RPC6-knockdown, we showed a direct functional link between RNAPIII transcription and BLV miRNAs expression. Furthermore, both the tumor- and the quiescent-related isoforms of RPC7 subunits were recruited to the miRNA cluster. We showed that the BLV miRNA cluster was enriched in positive epigenetic marks. Interestingly, we demonstrated the in vivo recruitment of RNAPII at the 3′LTR/host genomic junction, associated with positive epigenetic marks. Functionally, we showed that the BLV LTR exhibited a strong antisense promoter activity and identified cis-acting elements of an RNAPII-dependent promoter. Finally, we provided evidence for an in vivo collision between RNAPIII and RNAPII convergent transcriptions. Our results provide new insights into alternative ways used by BLV to counteract silencing of the viral 5′LTR promoter.

Massive depletion of bovine leukemia virus proviral clones located in genomic transcriptionally active sites during primary infection

Deltaretroviruses such as human T-lymphotropic virus type 1 (HTLV-1) and bovine leukemia virus (BLV) induce a persistent infection that remains generally asymptomatic but can also lead to leukemia or lymphoma. These viruses replicate by infecting new lymphocytes (i.e. the infectious cycle) or via clonal expansion of the infected cells (mitotic cycle). The relative importance of these two cycles in viral replication varies during infection. The majority of infected clones are created early before the onset of an efficient immune response. Later on, the main replication route is mitotic expansion of pre-existing infected clones. Due to the paucity of available samples and for ethical reasons, only scarce data is available on early infection by HTLV-1. Therefore, we addressed this question in a comparative BLV model. We used high-throughput sequencing to map and quantify the insertion sites of the provirus in order to monitor the clonality of the BLV-infected cells population (i.e. the number of distinct clones and abundance of each clone). We found that BLV propagation shifts from cell neoinfection to clonal proliferation in about 2 months from inoculation. Initially, BLV proviral integration significantly favors transcribed regions of the genome. Negative selection then eliminates 97% of the clones detected at seroconversion and disfavors BLV-infected cells carrying a provirus located close to a promoter or a gene. Nevertheless, among the surviving proviruses, clone abundance positively correlates with proximity of the provirus to a transcribed region. Two opposite forces thus operate during primary infection and dictate the fate of long term clonal composition: (1) initial integration inside genes or promoters and (2) host negative selection disfavoring proviruses located next to transcribed regions. The result of this initial response will contribute to the proviral load set point value as clonal abundance will benefit from carrying a provirus in transcribed regions.

Human T-lymphotropic Virus 1 (HTLV-1) induces a persistent infection that remains generally asymptomatic. Nevertheless, in a small proportion of individuals and after a long latency, HTLV-1 infection leads to leukemia or lymphoma. Onset of clinical manifestations correlates with a persistently elevated number of infected cells. Because the vast majority of cells are infected at early stages, primary infection is a crucial period for HTLV-1 persistence and pathogenesis. Since HTLV-1 is transmitted through breast feeding and because systematic population screenings are rare, there is a lack of available samples at early infection. Therefore, we addressed this question in a closely related animal model by inoculating cows with Bovine Leukemia Virus (BLV). We show that the vast majority of cells becoming infected during the first weeks of infection and do not survive later on. We also demonstrate that the initial host selection occurring during primary infection will specifically target cells that carry a provirus inserted in genomic transcribed regions. This conclusion thus highlights a key role exerted by the host immune system during primary infection and indicates that antiviral treatments would be optimal when introduced straight after infection.

Chemoresistance to Valproate Treatment of Bovine Leukemia Virus-Infected Sheep; Identification of Improved HDAC Inhibitors

We previously proved that a histone deacetylase inhibitor (valproate, VPA) decreases the number of leukemic cells in bovine leukemia virus (BLV)-infected sheep. Here, we characterize the mechanisms initiated upon interruption of treatment. We observed that VPA treatment is followed by a decrease of the B cell counts and proviral loads (copies per blood volume). However, all sheep eventually relapsed after different periods of time and became refractory to further VPA treatment. Sheep remained persistently infected with BLV. B lymphocytes isolated throughout treatment and relapse were responsive to VPA-induced apoptosis in cell culture. B cell proliferation is only marginally affected by VPA ex vivo. Interestingly, in four out of five sheep, ex vivo viral expression was nearly undetectable at the time of relapse. In two sheep, a new tumoral clone arose, most likely revealing a selection process exerted by VPA in vivo. We conclude that the interruption of VPA treatment leads to the resurgence of the leukemia in BLV-infected sheep and hypothesize that resistance to further treatment might be due to the failure of viral expression induction. The development of more potent HDAC inhibitors and/or the combination with other compounds can overcome chemoresistance. These observations in the BLV model may be important for therapies against the related Human T-lymphotropic virus type 1.

Animal models on HTLV-1 and related viruses: what did we learn?

Retroviruses are associated with a wide variety of diseases, including immunological, neurological disorders, and different forms of cancer. Among retroviruses, Oncovirinae regroup according to their genetic structure and sequence, several related viruses such as human T-cell lymphotropic viruses types 1 and 2 (HTLV-1 and HTLV-2), simian T cell lymphotropic viruses types 1 and 2 (STLV-1 and STLV-2), and bovine leukemia virus (BLV). As in many diseases, animal models provide a useful tool for the studies of pathogenesis, treatment, and prevention. In the current review, an overview on different animal models used in the study of these viruses will be provided. A specific attention will be given to the HTLV-1 virus which is the causative agent of adult T-cell leukemia/lymphoma (ATL) but also of a number of inflammatory diseases regrouping the HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP), infective dermatitis and some lung inflammatory diseases. Among these models, rabbits, monkeys but also rats provide an excellent in vivo tool for early HTLV-1 viral infection and transmission as well as the induced host immune response against the virus. But ideally, mice remain the most efficient method of studying human afflictions. Genetically altered mice including both transgenic and knockout mice, offer important models to test the role of specific viral and host genes in the development of HTLV-1-associated leukemia. The development of different strains of immunodeficient mice strains (SCID, NOD, and NOG SCID mice) provide a useful and rapid tool of humanized and xenografted mice models, to test new drugs and targeted therapy against HTLV-1-associated leukemia, to identify leukemia stem cells candidates but also to study the innate immunity mediated by the virus. All together, these animal models have revolutionized the biology of retroviruses, their manipulation of host genes and more importantly the potential ways to either prevent their infection or to treat their associated diseases.

Inflammation and cancer: a comparative view

Rudolph Virchow first speculated on a relationship between inflammation and cancer more than 150 years ago. Subsequently, chronic inflammation and associated reactive free radical overload and some types of bacterial, viral, and parasite infections that cause inflammation were recognized as important risk factors for cancer development and account for one in four of all human cancers worldwide. Even viruses that do not directly cause inflammation can cause cancer when they act in conjunction with proinflammatory cofactors or when they initiate or promote cancer via the same signaling pathways utilized in inflammation. Whatever its origin, inflammation in the tumor microenvironment has many cancer-promoting effects and aids in the proliferation and survival of malignant cells and promotes angiogenesis and metastasis. Mediators of inflammation such as cytokines, free radicals, prostaglandins, and growth factors can induce DNA damage in tumor suppressor genes and post-translational modifications of proteins involved in essential cellular processes including apoptosis, DNA repair, and cell cycle checkpoints that can lead to initiation and progression of cancer.

Chromatin disruption in the promoter of bovine leukemia virus during transcriptional activation

Bovine leukemia virus expression relies on its chromatin organization after integration into the host cell genome. Proviral latency, which results from transcriptional repression in vivo, represents a viral strategy to escape the host immune system and likely allows for tumor progression. Here, we discriminated two types of latency: an easily reactivable latent state of the YR2 provirus and a ‘locked’ latent state of the L267 provirus. The defective YR2 provirus was characterized by the presence of nuclease hypersensitive sites at the U3/R junction and in the R/U5 region of the 5′-long terminal repeat (5′-LTR), whereas the L267 provirus displayed a closed chromatin configuration at the U3/R junction. Reactivation of viral expression in YR2 cells by the phorbol 12-myristate 13-acetate (PMA) plus ionomycin combination was accompanied by a rapid but transient chromatin remodeling in the 5′-LTR, leading to an increased PU.1 and USF-1/USF-2 recruitment in vivo sustained by PMA/ionomycin-mediated USF phosphorylation. In contrast, viral expression was not reactivated by PMA/ionomycin in L267 cells, because the 5′-LTR U3/R region remained inaccessible to nucleases and hypermethylated at CpG dinucleotides. Remarkably, we elucidated the BLV 5′-LTR chromatin organization in PBMCs isolated from BLV-infected cows, thereby depicting the virus hiding in vivo in its natural host.

Preventive and therapeutic strategies for bovine leukemia virus: lessons for HTLV

Bovine leukemia virus (BLV) is a retrovirus closely related to the human T-lymphotropic virus type 1 (HTLV-1). BLV is a major animal health problem worldwide causing important economic losses. A series of attempts were developed to reduce prevalence, chiefly by eradication of infected cattle, segregation of BLV-free animals and vaccination. Although having been instrumental in regions such as the EU, these strategies were unsuccessful elsewhere mainly due to economic costs, management restrictions and lack of an efficient vaccine. This review, which summarizes the different attempts previously developed to decrease seroprevalence of BLV, may be informative for management of HTLV-1 infection. We also propose a new approach based on competitive infection with virus deletants aiming at reducing proviral loads.

Keep it in the subfamily: the conserved alphaherpesvirus US3 protein kinase

The US3 protein kinase is conserved over the alphaherpesvirus subfamily. Increasing evidence shows that, although the kinase is generally not required for virus replication in cell culture, it plays a pivotal and in some cases an essential role in virus virulence in vivo. The US3 protein is a multifunctional serine/threonine kinase that is involved in viral gene expression, virion morphogenesis, remodelling the actin cytoskeleton and the evasion of several antiviral host responses. In the current review, both the well conserved and virus-specific functions of alphaherpesvirus US3 protein kinase orthologues will be discussed.

DNA cytosine methylation in the bovine leukemia virus promoter is associated with latency in a lymphoma-derived B-cell line: potential involvement of direct inhibition of cAMP-responsive element (CRE)-binding protein/CRE modulator/activation transcription factor binding

Bovine leukemia virus (BLV) proviral latency represents a viral strategy to escape the host immune system and allow tumor development. Besides the previously demonstrated role of histone deacetylation in the epigenetic repression of BLV expression, we showed here that BLV promoter activity was induced by several DNA methylation inhibitors (such as 5-aza-2'-deoxycytidine) and that overexpressed DNMT1 and DNMT3A, but not DNMT3B, down-regulated BLV promoter activity. Importantly, cytosine hypermethylation in the 5'-long terminal repeat (LTR) U3 and R regions was associated with true latency in the lymphoma-derived B-cell line L267 but not with defective latency in YR2 cells. Moreover, the virus-encoded transactivator Tax(BLV) decreased DNA methyltransferase expression levels, which could explain the lower level of cytosine methylation observed in the L267(LTaxSN) 5'-LTR compared with the L267 5'-LTR. Interestingly, DNA methylation inhibitors and Tax(BLV) synergistically activated BLV promoter transcriptional activity in a cAMP-responsive element (CRE)-dependent manner. Mechanistically, methylation at the -154 or -129 CpG position (relative to the transcription start site) impaired in vitro binding of CRE-binding protein (CREB) transcription factors to their respective CRE sites. Methylation at -129 CpG alone was sufficient to decrease BLV promoter-driven reporter gene expression by 2-fold. We demonstrated in vivo the recruitment of CREB/CRE modulator (CREM) and to a lesser extent activating transcription factor-1 (ATF-1) to the hypomethylated CRE region of the YR2 5'-LTR, whereas we detected no CREB/CREM/ATF recruitment to the hypermethylated corresponding region in the L267 cells. Altogether, these findings suggest that site-specific DNA methylation of the BLV promoter represses viral transcription by directly inhibiting transcription factor binding, thereby contributing to true proviral latency.

Identification of cellular proteins that maintain retroviral epigenetic silencing: evidence for an antiviral response

Integrated retroviral DNA is subject to epigenetic gene silencing, resulting in loss of expression of viral genes as well as reporter or therapeutic genes transduced by retroviral vectors. Possible mediators of such silencing include the histone deacetylase (HDAC) family of cellular proteins. We previously isolated HeLa cell populations that harbored silent avian sarcoma virus-based green fluorescent protein (GFP) vectors that could be reactivated by treatment with HDAC inhibitors. Here, we developed a small interfering RNA (siRNA)-based approach to identify specific host factors that participate in the maintenance of silencing. Knockdown of HDAC1, the transcriptional repressor Daxx (a binding partner of HDAC1), or heterochromatin protein 1 gamma resulted in robust and specific GFP reporter gene reactivation. Analyses of cell clones and diverse GFP vector constructs revealed that the roles of HDAC1 and Daxx in retroviral silencing are largely independent of the integration site or the promoter controlling the silent GFP reporter gene. Previous findings from our laboratory and those of others have suggested that Daxx and HDAC proteins may act broadly as part of an antiviral response to repress viral gene transcription. Expression of presumptive viral "countermeasure" proteins that are known to inhibit Daxx or HDACs (pp71, IE2, and Gam1) resulted in the reactivation of GFP reporter gene expression. This study has identified individual host factors that maintain retroviral silencing and supports the proposal that these factors participate in an antiviral response. Furthermore, our results indicate that siRNAs can be used as specific reagents to interrupt the maintenance of epigenetic silencing.

Postintegrative gene silencing within the Sleeping Beauty transposition system

The Sleeping Beauty (SB) transposon represents an important vehicle for in vivo gene delivery because it can efficiently and stably integrate into mammalian genomes. In this report, we examined transposon expression in human cells using a novel nonselective fluorescence-activated cell sorter-based method and discovered that SB integrates approximately 20 times more frequently than previously reported within systems that were dependent on transgene expression and likely subject to postintegrative gene silencing. Over time, phenotypic analysis of clonal integrants demonstrated that SB undergoes additional postintegrative gene silencing, which varied based on the promoter used for transgene expression. Molecular and biochemical studies suggested that transposon silencing was influenced by DNA methylation and histone deacetylation because both 5-aza-2'-deoxycytidine and trichostatin A partially rescued transgene silencing in clonal cell lines. Collectively, these data reveal the existence of a multicomponent postintegrative gene silencing network that efficiently targets invading transposon sequences for transcriptional silencing in mammalian cells.

Transcriptional regulation of the bovine leukemia virus promoter by the cyclic AMP-response element modulator tau isoform

Bovine leukemia virus (BLV) expression is controlled at the transcriptional level through three Tax(BLV)-responsive elements (TxREs) responsive to the viral transactivator Tax(BLV). The cAMP-responsive element (CRE)-binding protein (CREB) has been shown to interact with CRE-like sequences present in the middle of each of these TxREs and to play critical transcriptional roles in both basal and Tax(BLV)-transactivated BLV promoter activity. In this study, we have investigated the potential involvement of the cAMP-response element modulator (CREM) in BLV transcriptional regulation, and we have demonstrated that CREM proteins were expressed in BLV-infected cells and bound to the three BLV TxREs in vitro. Chromatin immunoprecipitation assays using BLV-infected cell lines demonstrated in the context of chromatin that CREM proteins were recruited to the BLV promoter TxRE region in vivo. Functional studies, in the absence of Tax(BLV), indicated that ectopic CREMtau protein had a CRE-dependent stimulatory effect on BLV promoter transcriptional activity. Cross-link of the B-cell receptor potentiated CREMtau transactivation of the viral promoter. Further experiments supported the notion that this potentiation involved CREMtau Ser-117 phosphorylation and recruitment of CBP/p300 to the BLV promoter. Although CREB and Tax(BLV) synergistically transactivated the BLV promoter, CREMtau repressed this Tax(BLV)/CREB synergism, suggesting that a modulation of the level of Tax(BLV) transactivation through opposite actions of CREB and CREMtau could facilitate immune escape and allow tumor development.

Suppression of viral gene expression in bovine leukemia virus-associated B-cell malignancy: interplay of epigenetic modifications leading to chromatin with a repressive histone code

Ovine leukemia/lymphoma resulting from bovine leukemia virus infection of sheep offers a large animal model for studying mechanisms underlying leukemogenesis. Silencing of viral information including Tax, the major contributor to the oncogenic potential of the virus, is critical if not mandatory for tumor progression. In this study, we have identified epigenetic mechanisms that govern the complete suppression of viral expression, using a lymphoma-derived B-cell clone carrying a silent provirus. Silencing was not relieved by injection of the malignant B cells into sheep. However, exogenous expression of Tax or treatment with either the DNA methyltransferase inhibitor 5'azacytidine or the histone deacetylase (HDAC) inhibitor trichostatin A rescued viral expression, as demonstrated by in vivo infectivity trials. Comparing silent and reactivated provirus, we found mechanistic connections between chromatin conformation and tumor-associated transcriptional repression. Silencing is associated with DNA methylation and decreased accessibility of promoter sequences. HDAC1 and the transcriptional corepressor mSin3A are associated with the inactive but not the reactivated promoter. Silencing correlates with a repressed chromatin structure marked by histone H3 and H4 hypoacetylation, a loss of methylation at H3 lysine 4, and an increase of H3 lysine 9 methylation. These observations point to the critical role of epigenetic mechanisms in tumor-specific virus/oncogene silencing, a potential strategy to evade immune response and favor the propagation of the transformed cell.

Mechanisms of leukemogenesis induced by bovine leukemia virus: prospects for novel anti-retroviral therapies in human

In 1871, the observation of yellowish nodules in the enlarged spleen of a cow was considered to be the first reported case of bovine leukemia. The etiological agent of this lymphoproliferative disease, bovine leukemia virus (BLV), belongs to the deltaretrovirus genus which also includes the related human T-lymphotropic virus type 1 (HTLV-1). This review summarizes current knowledge of this viral system, which is important as a model for leukemogenesis. Recently, the BLV model has also cast light onto novel prospects for therapies of HTLV induced diseases, for which no satisfactory treatment exists so far.

The type 2 dengue virus envelope protein interacts with small ubiquitin-like modifier-1 (SUMO-1) conjugating enzyme 9 (Ubc9)

Dengue viruses are mosquito-borne flaviviruses and may cause the life-threatening dengue hemorrhagic fever and dengue shock syndrome. Its envelope protein is responsible mainly for the virus attachment and entry to host cells. To identify the human cellular proteins interacting with the envelope protein of dengue virus serotype 2 inside host cells, we have performed a screening with the yeast-two-hybrid-based "Functional Yeast Array". Interestingly, the small ubiquitin-like modifier-1 conjugating enzyme 9 protein, modulating cellular processes such as those regulating signal transduction and cell growth, was one of the candidates interacting with the dengue virus envelope protein. With co-precipitation assay, we have demonstrated that it indeed could interact directly with the Ubc9 protein. Site-directed mutagenesis has demonstrated that Ubc9 might interact with the E protein via amino acid residues K51 and K241. Furthermore, immunofluorescence microscopy has shown that the DV2E-EGFP proteins tended to progress toward the nuclear membrane and co-localized with Flag-Ubc9 proteins around the nuclear membrane in the cytoplasmic side, and DV2E-EGFP also shifted the distribution of Flag-Ubc9 from evenly in the nucleus toward concentrating around the nuclear membrane in the nucleic side. In addition, over-expression of Ubc9 could reduce the plaque formation of the dengue virus in mammalian cells. This is the first report that DV envelope proteins can interact with the protein of sumoylation system and Ubc9 may involve in the host defense system to prevent virus propagation.

Histone deacetylase inhibitors increase virus gene expression but decrease CD8+ cell antiviral function in HTLV-1 infection

The dynamics of human T-lymphotropic virus type-1 (HTLV-1) provirus expression in vivo are unknown. There is much evidence to suggest that HTLV-1 gene expression is restricted: this restricted gene expression may contribute to HTLV-1 persistence by limiting the ability of the HTLV-1–specific CD8+ cell immune response to clear infected cells. In this study, we tested the hypothesis that derepression of HTLV-1 gene expression would allow an increase in CD8+ cell–mediated lysis of HTLV-1–infected cells. Using histone deacetylase enzyme inhibitors (HDIs) to hyperacetylate histones and increase HTLV-1 gene expression, we found that HDIs doubled Tax expression in naturally infected lymphocytes after overnight culture. However, the rate of CD8+ cell–mediated lysis of Tax-expressing cells ex vivo was halved. HDIs appeared to inhibit the CD8+ cell–mediated lytic process itself, indicating a role for the microtubule-associated HDAC6 enzyme. These observations indicate that HDIs may reduce the efficiency of cytotoxic T-cell (CTL) surveillance of HTLV-1 in vivo. The impact of HDIs on HTLV-1 proviral load in vivo cannot be accurately predicted because of the widespread effects of these drugs on cellular processes; we therefore recommend caution in the use of HDIs in nonmalignant cases of HTLV-1 infection.

Histone deacetylase inhibitors induce apoptosis with minimal viral reactivation in cells infected with Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) latently infects tumor cells in patients with Kaposi's sarcoma and primary effusion lymphoma (PEL). The purpose of this study was to determine whether histone deacetylase inhibitors (HDAI) could induce apoptosis, with minimal viral replication, in cells latently infected with KSHV. Four HDAI (depsipeptide, suberoylanilide hydroxamic acid, MS-275, and trichostatin A) were studied in two PEL B cell lines (BCBL-1, BC-3). As expected, histone hyperacetylation was readily induced in all PEL cells exposed to HDAI. HDAI also triggered KSHV reactivation in a time- and dose-dependent manner. Flow cytometric and transmission electron microscopic studies, however, revealed that reactivation occurred in only a minor percentage (3-14%) of treated cells. Importantly, and in contrast to viral reactivation, HDAI induced apoptotic cell death in a dose-dependent manner in a large percentage (up to 90%) of KSHV-infected cells. In summary, all four HDAI tested induced histone hyperacetylation in all cells, KSHV reactivation in a minority of cells, and apoptotic cell death in a majority of cells latently infected with KSHV. These findings suggest that HDAI may be a therapeutic option for patients with KSHV-mediated diseases by rendering cells infected with KSHV susceptible to apoptosis.

ICP0 and the US3 protein kinase of herpes simplex virus 1 independently block histone deacetylation to enable gene expression

SK-N-SH cells exposed to low ratios of ICP0-null (DeltaICP0) mutants of herpes simplex virus per cell express the viral alpha proteins, but the progression to beta and gamma gene expression does not ensue. In these restrictive cells, post-alpha gene expression can be induced after exposure of the infected cells to sodium butyrate, an indication that VP16 brought into cells by the virus and the alpha gene products made after infection cannot block the silencing of viral post-alpha genes by histone deacetylases (HDACs). This observation is consistent with evidence reported earlier that ICP0 dissociates HDAC1/2 from the CoREST/REST complex. In permissive U2OS cells, replication is independent of the ratio of DeltaICP0 mutant per cell. To determine whether other viral genes are involved in blocking HDACs, we used a surrogate system consisting of baculoviruses carrying viral or cellular genes driven by CMV immediate-early promoter. Expression of these genes requires blocking of histone deacetylation. We report that (i) cotransduced U(S)3 or U(S)3.5 protein kinase substitutes for sodium butyrate in enabling the expression of a reporter gene in restrictive cells and enhancing it in permissive cells; (ii) HDAC1 is phosphorylated concomitant with the expression of reporter genes; and (iii) the amounts and appearance of HDAC1 are altered in transduced cells expressing U(S)3 protein kinase in the absence of other viral proteins. We conclude that the U(S)3 protein kinase blocks histone deacetylation by a mechanism distinct from that of ICP0 and that debilitated histone deacetylation contributes to the permissiveness of U2OS cells for DeltaICP0 mutants.

Induction of expression of bovine leukemia virus (BLV) in blood taken from BLV-infected cows without removal of plasma

The expression of bovine leukemia virus (BLV) is blocked at the transcriptional level during the so-called latency period. However, when peripheral blood mononuclear cells and B lymphocytes are isolated from BLV-infected animals and incubated in the presence of activating reagents, such as phorbol ester, the expression of BLV is markedly enhanced. Such "reactivation" is thought to play a crucial role in the spread of BLV from infected to uninfected cattle. In the present study, we found that the expression of BLV in samples of whole blood from BLV-infected cattle was activated immediately upon incubation at 37 degrees C and that such activation did not require the addition of any exogenous factors except for anticoagulants or the removal of blood cells from plasma. The expression of BLV was repressed by an inhibitor of protein kinase C (PKC), namely, H-7, and by a membrane-permeable chelator of Ca2+ ions, BAPTA/AM. We also found that several isotypes of PKC were translocated immediately from the cytoplasm to the membrane fraction upon incubation of whole blood at 37 degrees C. Our data suggest that the actual collection of blood and pathways that involve PKC and Ca2+ might play important roles in the reactivation of expression of BLV in blood from infected cattle.

Valproate activates bovine leukemia virus gene expression, triggers apoptosis, and induces leukemia/lymphoma regression in vivo

Leukemogenic viruses like human T-lymphotropic virus and bovine leukemia virus (BLV) presumably persist in the host partly by latent integration of the provirus in a fraction of infected cells, leading to accumulative increase in the outgrowth of transformed cells. Furthermore, viral infection also correlates with a blockade of the apoptotic mechanisms concomitant with an apparent latency of the host cell. Conceptually, induction of viral or cellular gene expression could thus also be used as a therapeutic strategy against retroviral-associated leukemia. Here, we provide evidence that valproate, an inhibitor of deacetylases, activates BLV gene expression in transient transfection experiments and in short-term cultures of primary B-lymphocytes. In vivo, valproate injection into newly BLV-inoculated sheep did not abrogate primary infection. However, valproate treatment, in the absence of any other cytotoxic drug, was efficient for leukemia/lymphoma therapy in the sheep model leading to decreased lymphocyte numbers (respectively from 25.6, 35.7, and 46.5 x 10(3) cells per mm3 to 1.0, 10.6, and 24.3 x 10(3) cells per mm3 in three leukemic sheep) and tumor regression (from >700 cm3 to undetectable). The concept of a therapy that targets the expression of viral and cellular genes might be a promising treatment of adult T cell leukemia or tropical spastic paraparesis/human T-lymphotropic virus-associated myelopathy, diseases for which no satisfactory treatment exists so far.

Overlapping CRE and E box motifs in the enhancer sequences of the bovine leukemia virus 5' long terminal repeat are critical for basal and acetylation-dependent transcriptional activity of the viral promoter: implications for viral latency

Bovine leukemia virus (BLV) infection is characterized by viral latency in a large proportion of cells containing an integrated provirus. In this study, we postulated that mechanisms directing the recruitment of deacetylases to the BLV 5' long terminal repeat (LTR) could explain the transcriptional repression of viral expression in vivo. Accordingly, we showed that BLV promoter activity was induced by several deacetylase inhibitors (such as trichostatin A [TSA]) in the context of episomal LTR constructs and in the context of an integrated BLV provirus. Moreover, treatment of BLV-infected cells with TSA increased H4 acetylation at the viral promoter, showing a close correlation between the level of histone acetylation and transcriptional activation of the BLV LTR. Among the known cis-regulatory DNA elements located in the 5' LTR, three E box motifs overlapping cyclic AMP responsive elements (CREs) in U3 were shown to be involved in transcriptional repression of BLV basal gene expression. Importantly, the combined mutations of these three E box motifs markedly reduced the inducibility of the BLV promoter by TSA. E boxes are susceptible to recognition by transcriptional repressors such as Max-Mad-mSin3 complexes that repress transcription by recruiting deacetylases. However, our in vitro binding studies failed to reveal the presence of Mad-Max proteins in the BLV LTR E box-specific complexes. Remarkably, TSA increased the occupancy of the CREs by CREB/ATF. Therefore, we postulated that the E box-specific complexes exerted their negative cooperative effect on BLV transcription by steric hindrance with the activators CREB/ATF and/or their transcriptional coactivators possessing acetyltransferase activities. Our results thus suggest that the overlapping CRE and E box elements in the BLV LTR were selected during evolution as a novel strategy for BLV to allow better silencing of viral transcription and to escape from the host immune response.

Comparison of bovine leukemia virus (BLV) and CMV promoter-driven reporter gene expression in BLV-infected and non-infected cells

Background

Viral promoters are used in mammalian expression vectors because they generally have strong activity in a wide variety of cells of differing tissues and species.

Methods

The utility of the BLV LTR/promoter (BLVp) for use in mammalian expression vectors was investigated through direct comparison to the CMV promoter (CMVp). Promoter activity was measured using luciferase assays of cell lines from different tissues and species stably transduced with BLVp or CMVp driven luciferase vectors including D17, FLK, BL3.1 and primary bovine B cells. Cells were also modified through the addition of BLV Tax expression vectors and/or BLV infection as well as treatment with trichostatin A (TSA).

Results

Results indicate the BLV promoter, while having low basal activity compared to the CMV promoter, can be induced to high-levels of activity similar to the CMV promoter in all cells tested. Tax or BLV infection specifically enhanced BLVp activity with no effect on CMVp activity. In contrast, the non-specific activator, TSA, enhanced both BLVp and CMVp activity.

Conclusion

Based on these data, we conclude the BLV promoter could be very useful for transgene expression in mammalian expression vectors.

Deacetylase inhibitors and the viral transactivator TaxBLV synergistically activate bovine leukemia virus gene expression via a cAMP-responsive element- and cAMP-responsive element-binding protein-dependent mechanism

Efficient bovine leukemia virus (BLV) transcription requires the virus-encoded transactivator Tax(BLV), which acts through three Tax(BLV)-responsive elements located in the 5' long terminal repeat. It has been proposed that the binding of the CRE-binding protein (CREB) and the activating transcription factor (ATF) to the three imperfect cAMP-responsive elements (CREs) located in each Tax(BLV)-responsive element mediates Tax(BLV) transactivation. Here we demonstrated that deacetylase inhibitors (HDACis) synergistically enhanced the transcriptional activation of the BLV promoter by Tax(BLV) in a CRE-dependent manner. Tax(BLV) was acetylated in vivo at its N(alpha) terminus but not at internal lysine residues. Rather, HDACi potentiation of Tax(BLV) transactivation was mediated by an HDACi indirect action that requires new protein synthesis. Mechanistically, using a dominant-negative form of CREB, we showed that Tax(BLV) and HDACi synergistically activated BLV gene expression via a CREB-dependent mechanism. Moreover, electrophoretic mobility shift assay and Western blot experiments revealed that HDACi increased the in vitro DNA binding activity of CREB/ATF but did not alter CREB/ATF intranuclear presence. Remarkably, chromatin immunoprecipitation assays demonstrated that HDACi treatment increased the level of CREB bound to the BLV promoter in vivo. Our results together suggest that an increase in CREB/ATF occupancy of the viral CREs in response to HDACi potentiates Tax(BLV) transactivation of the BLV promoter.

Reduced cell turnover in bovine leukemia virus-infected, persistently lymphocytotic cattle

Although nucleotide analogs like bromodeoxyuridine have been extensively used to estimate cell proliferation in vivo, precise dynamic parameters are scarce essentially because of the lack of adequate mathematical models. Besides recent developments on T cell dynamics, the turnover rates of B lymphocytes are largely unknown particularly in the context of a virally induced pathological disorder. Here, we aim to resolve this issue by determining the rates of cell proliferation and death during the chronic stage of the bovine leukemia virus (BLV) infection, called bovine persistent lymphocytosis (PL). Our methodology is based on direct intravenous injection of bromodeoxyuridine in association with subsequent flow cytometry. By this in vivo approach, we show that the death rate of PL B lymphocytes is significantly reduced (average death rate, 0.057 day(-1) versus 0.156 day(-1) in the asymptomatic controls). Concomitantly, proliferation of the PL cells is also significantly restricted compared to the controls (average proliferation rate, 0.0046 day(-1) versus 0.0085 day(-1)). We conclude that bovine PL is characterized by a decreased cell turnover resulting both from a reduction of cell death and an overall impairment of proliferation. The cell dynamic parameters differ from those measured in sheep, an experimental model for BLV infection. Finally, cells expressing p24 major capsid protein ex vivo were not BrdU positive, suggesting an immune selection against proliferating virus-positive lymphocytes. Based on a comparative leukemia approach, these observations might help to understand cell dynamics during other lymphoproliferative disease such as chronic lymphocytic leukemia or human T-cell lymphotropic virus-induced adult T-cell leukemia in humans.

Latency of viral expression in vivo is not related to CpG methylation in the U3 region and part of the R region of the long terminal repeat of bovine leukemia virus

Bovine leukemia virus (BLV) is silent in most cells detectable in vivo, and the repression of its expression allows BLV to evade the host's immune response. In this study, we examined whether CpG methylation of DNA might be involved in the regulation of the expression of BLV in vivo. To investigate the effects of CpG methylation on the activity of the long terminal repeat (LTR) of BLV, we measured the transactivation activity of this region after treatment with the CpG methyltransferase SssI by using a luciferase reporter system. The activity of methylated LTR was significantly lower than that of nonmethylated LTR. Therefore, we examined the extent of CpG methylation of the U3 region and part of the R region of the LTR in BLV-infected cattle and in experimentally BLV-infected sheep at various clinical stages by the bisulfite genomic sequencing method. We detected no or minimal CpG methylation at all stages examined in cattle and sheep, and our results indicate that CpG methylation probably does not participate in the silencing of BLV in vivo.

The adenovirus protein Gam1 interferes with sumoylation of histone deacetylase 1

The adenovirus early gene product Gam1 is crucial for virus replication and induces certain cellular genes by inactivating histone deacetylase 1 (HDAC1). We demonstrate that Gam1 (i) destroys promyelocitic leukemia nuclear bodies, (ii) delocalizes SUMO-1 into the cytoplasm and (iii) influences the SUMO-1 pathway. In addition, we show that Gam1 counteracts HDAC1 sumoylation both in vivo and in vitro. Sumoylation of HDAC1 does not seem to be absolutely required for HDAC1 biological activity but is part of a complex regulatory circuit that also includes phosphorylation of the deacetylase. Our data demonstrate that Gam1 is a viral protein that can affect simultaneously two signaling pathways: sumoylation and acetylation.