Transcriptional and post-transcriptional gene regulation of HTLV-1

MicroRNAs and long non-coding RNAs during transcriptional regulation and latency of HIV and HTLV

Human immunodeficiency virus (HIV) and human T cell leukemia virus (HTLV) have replicative and latent stages of infection. The status of the viruses is dependent on the cells that harbour them and on different events that change the transcriptional and post-transcriptional events. Non-coding (nc)RNAs are key factors in the regulation of retrovirus replication cycles. Notably, micro (mi)RNAs and long non-coding (lnc)RNAs are important regulators that can induce switches between active transcription-replication and latency of retroviruses and have important impacts on their pathogenesis. Here, we review the functions of miRNAs and lncRNAs in the context of HIV and HTLV. We describe how specific miRNAs and lncRNAs are involved in the regulation of the viruses' transcription, post-transcriptional regulation and latency. We further discuss treatment strategies using ncRNAs for HIV and HTLV long remission, reactivation or possible cure.

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.

Recent Updates on Viral Oncogenesis: Available Preventive and Therapeutic Entities

Human viral oncogenesis is a complex phenomenon and a major contributor to the global cancer burden. Several recent findings revealed cellular and molecular pathways that promote the development and initiation of malignancy when viruses cause an infection. Even, antiviral treatment has become an approach to eliminate the viral infections and prevent the activation of oncogenesis. Therefore, for a better understanding, the molecular pathogenesis of various oncogenic viruses like, hepatitis virus, human immunodeficiency viral (HIV), human papillomavirus (HPV), herpes simplex virus (HSV), and Epstein-Barr virus (EBV), could be explored, especially, to expand many potent antivirals that may escalate the apoptosis of infected malignant cells while sparing normal and healthy ones. Moreover, contemporary therapies, such as engineered antibodies antiviral agents targeting signaling pathways and cell biomarkers, could inhibit viral oncogenesis. This review elaborates the recent advancements in both natural and synthetic antivirals to control viral oncogenesis. The study also highlights the challenges and future perspectives of using antivirals in viral oncogenesis.

The Involvement of Ubiquitination and SUMOylation in Retroviruses Infection and Latency

Retroviruses, especially the pathogenic human immunodeficiency virus type 1 (HIV-1), have severely threatened human health for decades. Retroviruses can form stable latent reservoirs via retroviral DNA integration into the host genome, and then be temporarily transcriptional silencing in infected cells, which makes retroviral infection incurable. Although many cellular restriction factors interfere with various steps of the life cycle of retroviruses and the formation of viral latency, viruses can utilize viral proteins or hijack cellular factors to evade intracellular immunity. Many post-translational modifications play key roles in the cross-talking between the cellular and viral proteins, which has greatly determined the fate of retroviral infection. Here, we reviewed recent advances in the regulation of ubiquitination and SUMOylation in the infection and latency of retroviruses, focusing on both host defense- and virus counterattack-related ubiquitination and SUMOylation system. We also summarized the development of ubiquitination- and SUMOylation-targeted anti-retroviral drugs and discussed their therapeutic potential. Manipulating ubiquitination or SUMOylation pathways by targeted drugs could be a promising strategy to achieve a "sterilizing cure" or "functional cure" of retroviral infection.

How Can We Prevent Mother-to-Child Transmission of HTLV-1?

The perception of human T-cell leukemia virus type 1 (HTlV-1) infection as a "silent disease" has recently given way to concern that its presence may be having a variety of effects. HTLV-1 is known to cause adult T-cell leukemia (ATL), an aggressive cancer of peripheral CD4 T cells; however, it is also responsible for HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Most patients develop ATL as a result of HTLV-1 mother-to-child transmission. The primary route of mother-to-child transmission is through the mother's milk. In the absence of effective drug therapy, total artificial nutrition such as exclusive formula feeding is a reliable means of preventing mother-to-child transmission after birth, except for a small percentage of prenatal infections. A recent study found that the rate of mother-to-child transmission with short-term breastfeeding (within 90 days) did not exceed that of total artificial nutrition. Because these preventive measures are in exchange for the benefits of breastfeeding, clinical applications of antiretroviral drugs and immunotherapy with vaccines and neutralizing antibodies are urgently needed.

Diversity of HLA-A2-Restricted and Immunodominant Epitope Repertoire of Human T-Lymphotropic Virus Type 1 (HTLV-1) Tax Protein: Novel Insights among N-Terminal, Central and C-Terminal Regions

The present study sought to search for the immunodominance related to the N-terminal, Central and C-terminal regions of HTLV-1 Tax using novel, cutting-edge peptide microarray analysis. In addition, in silico predictions were performed to verify the presence of nine amino acid peptides present along Tax restricted to the human leukocyte antigen (HLA)-A2.02*01 haplotype, as well as to verify the ability to induce pro-inflammatory and regulatory cytokines, such as IFN-γ and IL-4, respectively. Our results indicated abundant dose-dependent reactivity for HLA-A*02:01 in all regions (N-terminal, Central and C-terminal), but with specific hotspots. Furthermore, the results of fold-change over the Tax11–19 reactivity obtained at lower concentrations of HLA-A*02:01 reveal that peptides from the three regions contain sequences that react 100 times more than Tax11–19. On the other hand, Tax11–19 has similar or superior HLA-A*02:01 reactivity at higher concentrations of this haplotype. The in silico analysis showed a higher frequency of IFN-γ-inducing peptides in the N-terminal portion, while the C-terminal portion showed a higher frequency of IL-4 inducers. Taken together, these results shed light on the search for new Tax immunodominant epitopes, in addition to the canonic Tax11–19, for the rational design of immunomodulatory strategies for HTLV-1 chronic diseases.

HTLV-1 persistence and leukemogenesis: A game of hide-and-seek with the host immune system

Human T-cell leukemia virus type 1 (HTLV-1), a retrovirus which mainly infects CD4⁺ T cells and causes adult T-cell leukemia/lymphoma (ATL), is primarily transmitted via direct cell-to-cell transmission. This feature generates a wide variety of infected clones in hosts, which are maintained via clonal proliferation, resulting in the persistence and survival of the virus. The maintenance of the pool of infected cells is achieved by sculpting the immunophenotype of infected cells and modulating host immune responses to avoid immune surveillance. Here, we review the processes undertaken by HTLV-1 to modulate and subvert host immune responses which contributes to viral persistence and development of ATL.

Tuning Rex rules HTLV-1 pathogenesis

HTLV-1 is an oncovirus causing ATL and other inflammatory diseases such as HAM/TSP and HU in about 5% of infected individuals. It is also known that HTLV-1-infected cells maintain a disease-free, immortalized, latent state throughout the lifetimes of about 95% of infected individuals. We believe that the stable maintenance of disease-free infected cells in the carrier is an intrinsic characteristic of HTLV-1 that has been acquired during its evolution in the human life cycle. We speculate that the pathogenesis of the virus is ruled by the orchestrated functions of viral proteins. In particular, the regulation of Rex, the conductor of viral replication rate, is expected to be closely related to the viral program in the early active viral replication followed by the stable latency in HTLV-1 infected T cells. HTLV-1 and HIV-1 belong to the family Retroviridae and share the same tropism, e.g., human CD4⁺ T cells. These viruses show significant similarities in the viral genomic structure and the molecular mechanism of the replication cycle. However, HTLV-1 and HIV-1 infected T cells show different phenotypes, especially in the level of virion production. We speculate that how the activity of HTLV-1 Rex and its counterpart HIV-1 Rev are regulated may be closely related to the properties of respective infected T cells. In this review, we compare various pathological aspects of HTLV-1 and HIV-1. In particular, we investigated the presence or absence of a virally encoded “regulatory valve” for HTLV-1 Rex or HIV-1 Rev to explore its importance in the regulation of viral particle production in infected T cells. Finally, wereaffirm Rex as the key conductor for viral replication and viral pathogenesis based on our recent study on the novel functional aspects of Rex. Since the activity of Rex is closely related to the viral replication rate, we hypothesize that the “regulatory valve” on the Rex activity may have been selectively evolved to achieve the “scenario” with early viral particle production and the subsequent long, stable deep latency in HTLV-1 infected cells.

Elucidation of the Mechanism of Host NMD Suppression by HTLV-1 Rex: Dissection of Rex to Identify the NMD Inhibitory Domain

The human retrovirus human T-cell leukemia virus type I (HTLV-1) infects human T cells by vertical transmission from mother to child through breast milk or horizontal transmission through blood transfusion or sexual contact. Approximately 5% of infected individuals develop adult T-cell leukemia/lymphoma (ATL) with a poor prognosis, while 95% of infected individuals remain asymptomatic for the rest of their lives, during which time the infected cells maintain a stable immortalized latent state in the body. It is not known why such a long latent state is maintained. We hypothesize that the role of functional proteins of HTLV-1 during early infection influences the phenotype of infected cells in latency. In eukaryotic cells, a mRNA quality control mechanism called nonsense-mediated mRNA decay (NMD) functions not only to eliminate abnormal mRNAs with nonsense codons but also to target virus-derived RNAs. We have reported that HTLV-1 genomic RNA is a potential target of NMD, and that Rex suppresses NMD and stabilizes viral RNA against it. In this study, we aimed to elucidate the molecular mechanism of NMD suppression by Rex using various Rex mutant proteins. We found that region X (aa20-57) of Rex, the function of which has not been clarified, is required for NMD repression. We showed that Rex binds to Upf1, which is the host key regulator to detect abnormal mRNA and initiate NMD, through this region. Rex also interacts with SMG5 and SMG7, which play essential roles for the completion of the NMD pathway. Moreover, Rex selectively binds to Upf3B, which is involved in the normal NMD complex, and replaces it with a less active form, Upf3A, to reduce NMD activity. These results revealed that Rex invades the NMD cascade from its initiation to completion and suppresses host NMD activity to protect the viral genomic mRNA.

Relevance of BET Family Proteins in SARS-CoV-2 Infection

The recent pandemic we are experiencing caused by the coronavirus disease 2019 (COVID-19) has put the world's population on the rack, with more than 191 million cases and more than 4.1 million deaths confirmed to date. This disease is caused by a new type of coronavirus, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). A massive proteomic analysis has revealed that one of the structural proteins of the virus, the E protein, interacts with BRD2 and BRD4 proteins of the Bromodomain and Extra Terminal domain (BET) family of proteins. BETs are essential to cell cycle progression, inflammation and immune response and have also been strongly associated with infection by different types of viruses. The fundamental role BET proteins play in transcription makes them appropriate targets for the propagation strategies of some viruses. Recognition of histone acetylation by BET bromodomains is essential for transcription control. The development of drugs mimicking acetyl groups, and thereby able to displace BET proteins from chromatin, has boosted interest on BETs as attractive targets for therapeutic intervention. The success of these drugs against a variety of diseases in cellular and animal models has been recently enlarged with promising results from SARS-CoV-2 infection studies.

Virus-Driven Carcinogenesis

Cancer arises from the accumulation of genetic and epigenetic alterations. Even in the era of precision oncology, carcinogens contributing to neoplastic process are still an important focus of research. Comprehensive genomic analyses have revealed various combinations of base substitutions, referred to as the mutational signatures, in cancer. Each mutational signature is believed to arise from specific DNA damage and repair processes, including carcinogens. However, as a type of carcinogen, tumor viruses increase the cancer risk by alternative mechanisms, including insertional mutagenesis, viral oncogenes, and immunosuppression. In this review, we summarize virus-driven carcinogenesis to provide a framework for the control of malignant cell proliferation. We first provide a brief overview of oncogenic viruses and describe their implication in virus-related tumors. Next, we describe tumor viruses (HPV, Human papilloma virus; HBV, Hepatitis B virus; HCV, Hepatitis C virus; EBV, Epstein-Barr virus; Kaposi sarcoma herpesvirus; MCV, Merkel cell polyoma virus; HTLV-1, Human T-cell lymphotropic virus, type-1) and tumor virus-related cancers. Lastly, we introduce emerging tumor virus candidates, human cytomegalovirus (CMV), human herpesvirus-6 (HHV-6) and adeno-associated virus-2 (AAV-2). We expect this review to be a hub in a complex network of data for virus-associated carcinogenesis.

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.

Strategies of Human T-Cell Leukemia Virus Type 1 for Persistent Infection: Implications for Leukemogenesis of Adult T-Cell Leukemia-Lymphoma

Human T-cell leukemia virus type 1 (HTLV-1) establishes persistent infection in vivo in two distinct ways: de novo infection and clonal proliferation of infected cells. Two viral genes, Tax and HTLV-1 bZIP factor (HBZ) play critical roles in viral transcription and promotion of T-cell proliferation, respectively. Tax is a potent transactivator not only for viral transcription but also for many cellular oncogenic pathways, such as the NF-κB pathway. HBZ is a suppressor of viral transcription and has the potential to change the immunophenotype of infected cells, conferring an effector regulatory T cell (eTreg)-like signature (CD4+ CD25+ CCR4+ TIGIT+ Foxp3+) and enhancing the proliferation of this subset. Reports that mice transgenic for either gene develop malignant tumors suggest that both Tax and HBZ are involved in leukemogenesis by HTLV-1. However, the immunogenicity of Tax is very high, and its expression is generally suppressed in vivo. Recently, it was found that Tax can be expressed transiently in a small subpopulation of adult T-cell leukemia-lymphoma (ATL) cells and plays a critical role in maintenance of the overall population. HBZ is expressed in almost all infected cells except for the rare Tax-expressing cells, and activates the pathways associated with cell proliferation. These findings indicate that HTLV-1 fine-tunes the expression of viral genes to control the mode of viral propagation. The interplay between Tax and HBZ is the basis of a sophisticated strategy to evade host immune surveillance and increase transmission - and can lead to ATL as a byproduct.

Regulation of Autophagy in Cells Infected With Oncogenic Human Viruses and Its Impact on Cancer Development

About 20% of total cancer cases are associated to infections. To date, seven human viruses have been directly linked to cancer development: high-risk human papillomaviruses (hrHPVs), Merkel cell polyomavirus (MCPyV), hepatitis B virus (HBV), hepatitis C virus (HCV), Epstein–Barr virus (EBV), Kaposi’s sarcoma-associated herpesvirus (KSHV), and human T-lymphotropic virus 1 (HTLV-1). These viruses impact on several molecular mechanisms in the host cells, often resulting in chronic inflammation, uncontrolled proliferation, and cell death inhibition, and mechanisms, which favor viral life cycle but may indirectly promote tumorigenesis. Recently, the ability of oncogenic viruses to alter autophagy, a catabolic process activated during the innate immune response to infections, is emerging as a key event for the onset of human cancers. Here, we summarize the current understanding of the molecular mechanisms by which human oncogenic viruses regulate autophagy and how this negative regulation impacts on cancer development. Finally, we highlight novel autophagy-related candidates for the treatment of virus-related cancers.

HTLV-1 as a Model for Virus and Host Coordinated Immunoediting

Immunoediting is a process that occurs in cancer, whereby the immune system acts to initially repress, and subsequently promote the outgrowth of tumor cells through the stages of elimination, equilibrium, and escape. Here we present a model for a virus that causes cancer where immunoediting is coordinated through synergistic viral- and host-mediated events. We argue that the initial viral replication process of the Human T cell leukemia virus type I (HTLV-1), which causes adult T cell leukemia/lymphoma (ATL) in ~5% of individuals after decades of latency, harmonizes with the host immune system to create a population of cells destined for malignancy. Furthermore, we explore the possibility for HIV to fit into this model of immunoediting, and propose a non-malignant escape phase for HIV-infected cells that persist beyond equilibrium.

Circulating miR-29c, miR-30c, miR-193a-5p and miR-885-5p: Novel potential biomarkers for HTLV-1 infection diagnosis

Human T-cell leukemia virus type 1 (HTLV-1) is an oncoretrovirus that infects 5-10 million people worldwide. Currently, different methods are used to test HTLV-1 infection. However, a biomarker that could enable an early and accurate diagnosis of HTLV-1 infection is still lacking. Here, we compared the serum miRNA expression profile in HTLV-1 infected patients versus healthy individuals to identify a potential biomarker for diagnosis of HTLV-1 infection.TaqMan miRNA microarray (TLDA) was carried out to compare the miRNA expression profile in infected versus healthy individuals. Quantitative real-time RT-PCR (qRT-PCR) was applied to validate TLDA results. Receiver-operator characteristic (ROC) curve analysis was performed to determine the diagnostic accuracy of the most highly and significantly identified deregulated miRNA(s) as potential biomarker(s). We identified deregulated expression for ten miRNAs with miR-127, miR-136, miR-142-3p, miR-221, and miR-423-5p being down-regulated whilst let-7b, miR-29c, miR-30c, miR-193a-5p, and miR-885-5p being up-regulated in infected individuals. ROC curve analyses showed an AUC (Areas Under the ROC Curve) of 0.875 (95% CI: 0.7819-0.9581; P = .0021), 0.861 (95% CI: 0.7596-0.9754; P = .003), 0.856 (95% CI: 0.689-0.895; P = .011), and 0.849 (95% CI: 0.678-0.855; P = .017) for miR-29c, miR-30c, miR-193a-5p, and miR-885-5p respectively. Combined ROC analyses using these 4 miRNAs showed a greater AUC of 0.907 (95% CI: 0.809-1; P = .000001) indicating a robust diagnostic value of these 4 miRNAs. Our findings highlight serum miR-29c, miR-30c, miR-193a-5p and miR-885-5p as novel potential biomarkers important for HTLV-1 diagnosis.

[Astute strategies of HTLV-1 with driven viral genes]

Human T-cell leukemia virus type 1 (HTLV-1) is the world's first retrovirus with pathogenicity to cause adult T-cell leukemia-lymphoma (ATL) and chronic inflammatory diseases,such as HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP) and HTLV-1 uveitis. As the virological characteristic, HTLV-1 can transmit efficiently only through cell-to-cell contact. Spread of infection and viral persistence is ingeniously driven by several viral genes as exemplified by HTLV-1 bZIP factor (HBZ) and tax. After the infection, the virus promotes proliferation and immortalization of the infected cells with acculturating immunophenotype into effector/memory T cells. In addition, HBZ enhances expression of co-inhibitory receptors on the surface of infected cells, potentially leading to suppression of host immune responses. These viral strategies can also result in unforeseen by-product, the pathogenicity of HTLV-1-associated diseases. In this review, with recent progress of HTLV-1 researches, we focus on astute regulation systems of the viral genes developed by HTLV-1.

Epigenetic Regulatory Mechanisms Induced by Resveratrol

Resveratrol (RVT) is one of the main natural compounds studied worldwide due to its potential therapeutic use in the treatment of many diseases, including cancer, diabetes, cardiovascular diseases, neurodegenerative diseases and metabolic disorders. Nevertheless, the mechanism of action of RVT in all of these conditions is not completely understood, as it can modify not only biochemical pathways but also epigenetic mechanisms. In this paper, we analyze the biological activities exhibited by RVT with a focus on the epigenetic mechanisms, especially those related to DNA methyltransferase (DNMT), histone deacetylase (HDAC) and lysine-specific demethylase-1 (LSD1).

Human T-cell lymphotropic virus type 1 and its oncogenesis

Human T-cell lymphotropic virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia/lymphoma (ATL), a rapidly progressing clonal malignancy of CD4+ T lymphocytes. Exploring the host-HTLV-1 interactions and the molecular mechanisms underlying HTLV-1-mediated tumorigenesis is critical for developing efficient therapies against the viral infection and associated leukemia/lymphoma. It has been demonstrated to date that several HTLV-1 proteins play key roles in the cellular transformation and immortalization of infected T lymphocytes. Of note, the HTLV-1 oncoprotein Tax inhibits the innate IFN response through interaction with MAVS, STING and RIP1, causing the suppression of TBK1-mediated phosphorylation of IRF3/IRF7. The HTLV-1 protein HBZ disrupts genomic integrity and inhibits apoptosis and autophagy of the target cells. Furthermore, it is revealed that HBZ enhances the proliferation of ATL cells and facilitates evasion of the infected cells from immunosurveillance. These studies provide insights into the molecular mechanisms by which HTLV-1 mediates the formation of cancer as well as useful strategies for the development of new therapeutic interventions against ATL. In this article, we review the recent advances in the understanding of the pathogenesis, the underlying mechanisms, clinical diagnosis and treatment of the disease caused by HTLV-1 infection. In addition, we discuss the future direction for targeting HTLV-1-associated cancers and strategies against HTLV-1.

RNA stability regulates human T cell leukemia virus type 1 gene expression in chronically-infected CD4 T cells

Regulation of expression of HTLV-1 gene products from integrated proviruses plays an important role in HTLV-1-associated disease pathogenesis. Previous studies have shown that T cell receptor (TCR)- and phorbol ester (PMA) stimulation of chronically infected CD4 T cells increases the expression of integrated HTLV-1 proviruses in latently infected cells, however the mechanism remains unknown. Analysis of HTLV-1 RNA and protein species following PMA treatment of the latently HTLV-1-infected, FS and SP T cell lines demonstrated rapid induction of tax/rex mRNA. This rapid increase in tax/rex mRNA was associated with markedly enhanced tax/rex mRNA stability while the stability of unspliced or singly spliced HTLV-1 RNAs did not increase. Tax/rex mRNA in the HTLV-1 constitutively expressing cell lines exhibited high basal stability even without PMA treatment. Our data support a model whereby T cell activation leads to increased HTLV-1 gene expression at least in part through increased tax/rex mRNA stability.

Repression of Human T-lymphotropic virus type 1 Long Terminal Repeat sense transcription by Sp1 recruitment to novel Sp1 binding sites

Human T-lymphotropic Virus type 1 (HTLV-1) infection is characterized by viral latency in the majority of infected cells and by the absence of viremia. These features are thought to be due to the repression of viral sense transcription in vivo. Here, our in silico analysis of the HTLV-1 Long Terminal Repeat (LTR) promoter nucleotide sequence revealed, in addition to the four Sp1 binding sites previously identified, the presence of two additional potential Sp1 sites within the R region. We demonstrated that the Sp1 and Sp3 transcription factors bound in vitro to these two sites and compared the binding affinity for Sp1 of all six different HTLV-1 Sp1 sites. By chromatin immunoprecipitation experiments, we showed Sp1 recruitment in vivo to the newly identified Sp1 sites. We demonstrated in the nucleosomal context of an episomal reporter vector that the Sp1 sites interfered with both the sense and antisense LTR promoter activities. Interestingly, the Sp1 sites exhibited together a repressor effect on the LTR sense transcriptional activity but had no effect on the LTR antisense activity. Thus, our results demonstrate the presence of two new functional Sp1 binding sites in the HTLV-1 LTR, which act as negative cis-regulatory elements of sense viral transcription.

Identification of TBK1 and IKKε, the non-canonical IκB kinases, as crucial pro-survival factors in HTLV-1-transformed T lymphocytes

Persistent activation of NF-κB is a prerequisite for development of adult T cell leukemia-lymphoma (ATL) caused by human T cell leukemia virus type 1 (HTLV-1). HTLV-1 genome encodes a viral transforming protein named Tax, which constitutively activates the canonical IκB kinases (IKK), the central regulator of NF-κB signaling. However, the role of the non-canonical IκB kinases, TBK1 and IKKε, in the pathogenesis of HTLV-1-associated leukemia has not been evaluated. We here show that TBK1/IKKε are crucial pro-survival molecules by maintaining persistent activity of Stat3. Consistent with this finding, silencing Stat3 by the specific shRNA or by the chemical inhibitor ruxolitinib results in drastic impediment of leukemia cell growth. We further find that in HTLV-1-transformed T cells expressing Tax, TBK1 co-localizes with the canonical IκB kinases and Tax in the lipid raft microdomains. The wild type Tax, but not the Tax mutant defective in activating the canonical IKK, promotes the lipid raft translocation of TBK1. This phenomenon correlates with Tax activation of both NF-κB and Stat3. Tax does not interact directly with TBK1/IKKε, and it rather engages a molecular crosstalk between the canonical IKKs and TBK1/IKKε. Our data, therefore, demonstrate a key role of TBK1/IKKε in the survival and proliferation of HTLV-1-transformed T cells and implicate a potential therapy targeting TBK1/IKKε and Stat3 in controlling HTLV-1-mediated oncogenesis.

Suppression of Type I Interferon Production by Human T-Cell Leukemia Virus Type 1 Oncoprotein Tax through Inhibition of IRF3 Phosphorylation

Infection with human T-cell leukemia virus type 1 (HTLV-1) is associated with adult T-cell leukemia (ATL) and tropical spastic paraparesis. Type I interferons (IFNs) are key effectors of the innate antiviral response, and IFN-α combined with the nucleoside reverse transcriptase inhibitor zidovudine is considered the standard first-line therapy for ATL. HTLV-1 oncoprotein Tax is known to suppress innate IFN production and response but the underlying mechanisms remain to be fully established. In this study, we report on the suppression of type I IFN production by HTLV-1 Tax through interaction with and inhibition of TBK1 kinase that phosphorylates IRF3. Induced transcription of IFN-β was severely impaired in HTLV-1-transformed ATL cells and freshly infected T lymphocytes. The ability to suppress IRF3 activation was ascribed to Tax. The expression of Tax alone sufficiently repressed the induction of IFN production by RIG-I plus PACT, cGAMP synthase plus STING, TBK1, IKKε, IRF3, and IRF7, but not by IRF3-5D, a dominant-active phosphomimetic mutant. This suggests that Tax perturbs IFN production at the step of IRF3 phosphorylation. Tax mutants deficient for CREB or NF-κB activation were fully competent in the suppression of IFN production. Coimmunoprecipitation experiments confirmed the association of Tax with TBK1, IKKε, STING, and IRF3.In vitrokinase assay indicated an inhibitory effect of Tax on TBK1-mediated phosphorylation of IRF3. Taken together, our findings suggested a new mechanism by which HTLV-1 oncoprotein Tax circumvents the production of type I IFNs in infected cells. Our findings have implications in therapeutic intervention of ATL.

IMPORTANCE

Human T-cell leukemia virus type 1 (HTLV-1) is the cause of adult T-cell leukemia (ATL), an aggressive and fatal blood cancer, as well as another chronic disabling disease of the spinal cord. Treatments are unsatisfactory, and options are limited. A combination of antiviral cellular protein alpha interferon and zidovudine, which is an inhibitor of a viral enzyme called reverse transcriptase, has been recommended as the standard first-line therapy for ATL. Exactly how HTLV-1 interacts with the cellular machinery for interferon production and action is not well understood. Our work sheds light on the mechanism of action for the inhibition of interferon production by an HTLV-1 oncogenic protein called Tax. Our findings might help to improve interferon-based anti-HTLV-1 and anti-ATL therapy.

Molecular Mechanisms of HTLV-1 Cell-to-Cell Transmission

The tumorvirus human T-cell lymphotropic virus type 1 (HTLV-1), a member of the delta-retrovirus family, is transmitted via cell-containing body fluids such as blood products, semen, and breast milk. In vivo, HTLV-1 preferentially infects CD4⁺ T-cells, and to a lesser extent, CD8⁺ T-cells, dendritic cells, and monocytes. Efficient infection of CD4⁺ T-cells requires cell-cell contacts while cell-free virus transmission is inefficient. Two types of cell-cell contacts have been described to be critical for HTLV-1 transmission, tight junctions and cellular conduits. Further, two non-exclusive mechanisms of virus transmission at cell-cell contacts have been proposed: (1) polarized budding of HTLV-1 into synaptic clefts; and (2) cell surface transfer of viral biofilms at virological synapses. In contrast to CD4⁺ T-cells, dendritic cells can be infected cell-free and, to a greater extent, via viral biofilms in vitro. Cell-to-cell transmission of HTLV-1 requires a coordinated action of steps in the virus infectious cycle with events in the cell-cell adhesion process; therefore, virus propagation from cell-to-cell depends on specific interactions between cellular and viral proteins. Here, we review the molecular mechanisms of HTLV-1 transmission with a focus on the HTLV-1-encoded proteins Tax and p8, their impact on host cell factors mediating cell-cell contacts, cytoskeletal remodeling, and thus, virus propagation.

HTLV-1 Rex Tunes the Cellular Environment Favorable for Viral Replication

Human T-cell leukemia virus type-1 (HTLV-1) Rex is a viral RNA binding protein. The most important and well-known function of Rex is stabilizing and exporting viral mRNAs from the nucleus, particularly for unspliced/partially-spliced mRNAs encoding the structural proteins essential for viral replication. Without Rex, these unspliced viral mRNAs would otherwise be completely spliced. Therefore, Rex is vital for the translation of structural proteins and the stabilization of viral genomic RNA and, thus, for viral replication. Rex schedules the period of extensive viral replication and suppression to enter latency. Although the importance of Rex in the viral life-cycle is well understood, the underlying molecular mechanism of how Rex achieves its function has not been clarified. For example, how does Rex protect unspliced/partially-spliced viral mRNAs from the host cellular splicing machinery? How does Rex protect viral mRNAs, antigenic to eukaryotic cells, from cellular mRNA surveillance mechanisms? Here we will discuss these mechanisms, which explain the function of Rex as an organizer of HTLV-1 expression based on previously and recently discovered aspects of Rex. We also focus on the potential influence of Rex on the homeostasis of the infected cell and how it can exert its function.

Molecular Studies of HTLV-1 Replication: An Update

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

The Emerging Role of miRNAs in HTLV-1 Infection and ATLL Pathogenesis

Human T-cell leukemia virus (HTLV)-1 is a human retrovirus and the etiological agent of adult T-cell leukemia/lymphoma (ATLL), a fatal malignancy of CD4/CD25+ T lymphocytes. In recent years, cellular as well as virus-encoded microRNA (miRNA) have been shown to deregulate signaling pathways to favor virus life cycle. HTLV-1 does not encode miRNA, but several studies have demonstrated that cellular miRNA expression is affected in infected cells. Distinct mechanisms such as transcriptional, epigenetic or interference with miRNA processing machinery have been involved. This article reviews the current knowledge of the role of cellular microRNAs in virus infection, replication, immune escape and pathogenesis of HTLV-1.

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

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

SIRT1 Suppresses Human T-Cell Leukemia Virus Type 1 Transcription

Human T-cell leukemia virus type 1 (HTLV-1)-associated diseases are poorly treatable, and HTLV-1 vaccines are not available. High proviral load is one major risk factor for disease development. HTLV-1 encodes Tax oncoprotein, which activates transcription from viral long terminal repeats (LTR) and various types of cellular promoters. Counteracting Tax function might have prophylactic and therapeutic benefits. In this work, we report on the suppression of Tax activation of HTLV-1 LTR by SIRT1 deacetylase. The transcriptional activity of Tax on the LTR was largely ablated when SIRT1 was overexpressed, but Tax activation of NF-κB was unaffected. On the contrary, the activation of the LTR by Tax was boosted when SIRT1 was depleted. Treatment of cells with resveratrol shunted Tax activity in a SIRT1-dependent manner. The activation of SIRT1 in HTLV-1-transformed T cells by resveratrol potently inhibited HTLV-1 proviral transcription and Tax expression, whereas compromising SIRT1 by specific inhibitors augmented HTLV-1 mRNA expression. The administration of resveratrol also decreased the production of cell-free HTLV-1 virions from MT2 cells and the transmission of HTLV-1 from MT2 cells to uninfected Jurkat cells in coculture. SIRT1 associated with Tax in HTLV-1-transformed T cells. Treatment with resveratrol prevented the interaction of Tax with CREB and the recruitment of CREB, CRTC1, and p300 to Tax-responsive elements in the LTR. Our work demonstrates the negative regulatory function of SIRT1 in Tax activation of HTLV-1 transcription. Small-molecule activators of SIRT1 such as resveratrol might be considered new prophylactic and therapeutic agents in HTLV-1-associated diseases.

IMPORTANCE

Human T-cell leukemia virus type 1 (HTLV-1) causes a highly lethal blood cancer or a chronic debilitating disease of the spinal cord. Treatments are unsatisfactory, and vaccines are not available. Disease progression is associated with robust expression of HTLV-1 genes. Suppressing HTLV-1 gene expression might have preventive and therapeutic benefits. It is therefore critical that host factors controlling HTLV-1 gene expression be identified and characterized. This work reveals a new host factor that suppresses HTLV-1 gene expression and a natural compound that activates this suppression. Our findings not only provide new knowledge of the host control of HTLV-1 gene expression but also suggest a new strategy of using natural compounds for prevention and treatment of HTLV-1-associated diseases.

HTLV-1 Tax-mediated inhibition of FOXO3a activity is critical for the persistence of terminally differentiated CD4+ T cells

The mechanisms involved in the persistence of activated CD4+ T lymphocytes following primary human T leukemia/lymphoma virus type 1 (HTLV-1) infection remain unclear. Here, we demonstrate that the HTLV-1 Tax oncoprotein modulates phosphorylation and transcriptional activity of the FOXO3a transcription factor, via upstream activation of the AKT pathway. De novo HTLV-1 infection of CD4+ T cells or direct lentiviral-mediated introduction of Tax led to AKT activation and AKT-dependent inactivation of FOXO3a, via phosphorylation of residues Ser253 and Thr32. Inhibition of FOXO3a signalling led to the long-term survival of a population of highly activated, terminally differentiated CD4+Tax+CD27negCCR7neg T cells that maintained the capacity to disseminate infectious HTLV-1. CD4+ T cell persistence was reversed by chemical inhibition of AKT activity, lentiviral-mediated expression of a dominant-negative form of FOXO3a or by specific small interfering RNA (siRNA)-mediated silencing of FOXO3a. Overall this study provides new mechanistic insight into the strategies used by HTLV-1 to increase long-term maintenance of Tax+CD4+ T lymphocytes during the early stages of HTLV-1 pathogenesis.

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.

HTLV-1 clonality in adult T-cell leukaemia and non-malignant HTLV-1 infection

Human T lymphotropic virus type 1 (HTLV-1) causes a range of chronic inflammatory diseases and an aggressive malignancy of T lymphocytes known as adult T-cell leukaemia/lymphoma (ATLL). A cardinal feature of HTLV-1 infection is the presence of expanded clones of HTLV-1-infected T cells, which may persist for decades. A high viral burden (proviral load) is associated with both the inflammatory and malignant diseases caused by HTLV-1, and it has been believed that the oligoclonal expansion of infected cells predisposes to these diseases. However, it is not understood what regulates the clonality of HTLV-1 in vivo, that is, the number and abundance of HTLV-1-infected T cell clones. We review recent advances in the understanding of HTLV-1 infection and disease that have come from high-throughput quantification and analysis of HTLV-1 clonality in natural infection.

Mechanisms of pathogenesis induced by bovine leukemia virus as a model for human T-cell leukemia virus

Bovine leukemia virus (BLV) and human T-cell leukemia virus type 1 (HTLV-1) make up a unique retrovirus family. Both viruses induce chronic lymphoproliferative diseases with BLV affecting the B-cell lineage and HTLV-1 affecting the T-cell lineage. The pathologies of BLV- and HTLV-induced infections are notably similar, with an absence of chronic viraemia and a long latency period. These viruses encode at least two regulatory proteins, namely, Tax and Rex, in the pX region located between the env gene and the 3′ long terminal repeat. The Tax protein is a key contributor to the oncogenic potential of the virus, and is also the key protein involved in viral replication. However, BLV infection is not sufficient for leukemogenesis, and additional events such as gene mutations must take place. In this review, we first summarize the similarities between the two viruses in terms of genomic organization, virology, and pathology. We then describe the current knowledge of the BLV model, which may also be relevant for the understanding of leukemogenesis caused by HTLV-1. In addition, we address our improved understanding of Tax functions through the newly identified BLV Tax mutants, which have a substitution between amino acids 240 and 265.

Bromodomain and extraterminal (BET) protein inhibition suppresses human T cell leukemia virus 1 (HTLV-1) Tax protein-mediated tumorigenesis by inhibiting nuclear factor κB (NF-κB) signaling

The etiology of human T cell leukemia virus 1 (HTLV-1)-mediated adult T cell leukemia is associated with the ability of viral oncoprotein Tax to induce sustained NF-κB activation and the expression of many NF-κB target genes. Acetylation of the RelA subunit of NF-κB and the subsequent recruitment of bromodomain-containing factor Brd4 are important for the expression of NF-κB target genes in response to various stimuli. However, their contributions to Tax-mediated NF-κB target gene expression and tumorigenesis remain unclear. Here we report that Tax induced the acetylation of lysine 310 of RelA and the binding of Brd4 to acetylated RelA to facilitate Tax-mediated transcriptional activation of NF-κB. Depletion of Brd4 down-regulated Tax-mediated NF-κB target gene expression and cell proliferation. Inhibiting the interaction of Brd4 and acetylated RelA with the bromodomain extraterminal protein inhibitor JQ1 suppressed the proliferation of Tax-expressing rat fibroblasts and Tax-positive HTLV-1-infected cells and Tax-mediated cell transformation and tumorigenesis. Moreover, JQ1 attenuated the Tax-mediated transcriptional activation of NF-κB, triggering the polyubiquitination and proteasome-mediated degradation of constitutively active nuclear RelA. Our results identify Brd4 as a key regulator for Tax-mediated NF-κB gene expression and suggest that targeting epigenetic regulators such as Brd4 with the bromodomain extraterminal protein inhibitor might be a potential therapeutic strategy for cancers and other diseases associated with HTLV-1 infection.

Genome-wide structural modelling of TCR-pMHC interactions

Background

The adaptive immune response is antigen-specific and triggered by pathogen recognition through T cells. Although the interactions and mechanisms of TCR-peptide-MHC (TCR-pMHC) have been studied over three decades, the biological basis for these processes remains controversial. As an increasing number of high-throughput binding epitopes and available TCR-pMHC complex structures, a fast genome-wide structural modelling of TCR-pMHC interactions is an emergent task for understanding immune interactions and developing peptide vaccines.

Results

We first constructed the PPI matrices and iMatrix, using 621 non-redundant PPI interfaces and 398 non-redundant antigen-antibody interfaces, respectively, for modelling the MHC-peptide and TCR-peptide interfaces, respectively. The iMatrix consists of four knowledge-based scoring matrices to evaluate the hydrogen bonds and van der Waals forces between sidechains or backbones, respectively. The predicted energies of iMatrix are high correlated (Pearson's correlation coefficient is 0.6) to 70 experimental free energies on antigen-antibody interfaces. To further investigate iMatrix and PPI matrices, we inferred the 701,897 potential peptide antigens with significant statistic from 389 pathogen genomes and modelled the TCR-pMHC interactions using available TCR-pMHC complex structures. These identified peptide antigens keep hydrogen-bond energies and consensus interactions and our TCR-pMHC models can provide detailed interacting models and crucial binding regions.

Conclusions

Experimental results demonstrate that our method can achieve high precision for predicting binding affinity and potential peptide antigens. We believe that iMatrix and our template-based method can be useful for the binding mechanisms of TCR-pMHC complexes and peptide vaccine designs.

HSP90 protects the human T-cell leukemia virus type 1 (HTLV-1) tax oncoprotein from proteasomal degradation to support NF-κB activation and HTLV-1 replication

Human T-cell leukemia virus type 1 (HTLV-1) is the causative agent of adult T-cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The HTLV-1 genome encodes the Tax protein that plays essential regulatory roles in HTLV-1 replication and oncogenic transformation of T lymphocytes. Despite intensive study of Tax, how Tax interfaces with host signaling pathways to regulate virus replication and drive T-cell proliferation and immortalization remains poorly understood. To gain new insight into the mechanisms of Tax function and regulation, we used tandem affinity purification and mass spectrometry to identify novel cellular Tax-interacting proteins. This screen identified heat shock protein 90 (HSP90) as a new binding partner of Tax. The interaction between HSP90 and Tax was validated by coimmunoprecipitation assays, and colocalization between the two proteins was observed by confocal microscopy. Treatment of HTLV-1-transformed cells with the HSP90 inhibitor 17-DMAG elicited proteasomal degradation of Tax in the nuclear matrix with concomitant inhibition of NF-κB and HTLV-1 long terminal repeat (LTR) activation. Knockdown of HSP90 by lentiviral shRNAs similarly provoked a loss of Tax protein in HTLV-1-transformed cells. Finally, treatment of HTLV-1-transformed cell lines with 17-DMAG suppressed HTLV-1 replication and promoted apoptotic cell death. Taken together, our results reveal that Tax is a novel HSP90 client protein and HSP90 inhibitors may exert therapeutic benefits for ATL and HAM/TSP patients.

Human T-cell leukemia virus type 1: replication, proliferation and propagation by Tax and HTLV-1 bZIP factor

Human T-cell leukemia virus type 1 (HTLV-1) spreads primarily by cell-to-cell transmission. Therefore, HTLV-1 promotes the proliferation of infected cells to facilitate transmission. In HTLV-1 infected individuals, the provirus is present mainly in effector/memory T cells and Foxp3+ T cells. Recent study suggests that this immunophenotype is acquired by infected cells through the function of HTLV-1 bZIP factor (HBZ). Tax, which is encoded by the plus strand, is crucial for viral replication and de novo infection, while HBZ, encoded by the minus strand, is important for proliferation of infected cells. Importantly, HBZ and Tax have opposing functions in most transcription pathways. HBZ and Tax cooperate in elaborate ways to permit viral replication, proliferation of infected cells and propagation of the virus.

Highlights on distinctive structural and functional properties of HTLV Tax proteins

Human T cell leukemia viruses (HTLVs) are complex human retroviruses of the Deltaretrovirus genus. Four types have been identified thus far, with HTLV-1 and HTLV-2 much more prevalent than HTLV-3 or HTLV-4. HTLV-1 and HTLV-2 possess strictly related genomic structures, but differ significantly in pathogenicity, as HTLV-1 is the causative agent of adult T cell leukemia and of HTLV-associated myelopathy/tropical spastic paraparesis, whereas HTLV-2 is not associated with neoplasia. HTLVs code for a protein named Tax that is responsible for enhancing viral expression and drives cell transformation. Much effort has been invested to dissect the impact of Tax on signal transduction pathways and to identify functional differences between the HTLV Tax proteins that may explain the distinct oncogenic potential of HTLV-1 and HTLV-2. This review summarizes our current knowledge of Tax-1 and Tax-2 with emphasis on their structure, role in activation of the NF-κB (nuclear factor kappa-B) pathway, and interactions with host factors.

The MHC-II transactivator CIITA, a restriction factor against oncogenic HTLV-1 and HTLV-2 retroviruses: similarities and differences in the inhibition of Tax-1 and Tax-2 viral transactivators

The activation of CD4(+) T helper cells is strictly dependent on the presentation of antigenic peptides by MHC class II (MHC-II) molecules. MHC-II expression is primarily regulated at the transcriptional level by the AIR-1 gene product CIITA (class II transactivator). Thus, CIITA plays a pivotal role in the triggering of the adaptive immune response against pathogens. Besides this well known function, we recently found that CIITA acts as an endogenous restriction factor against HTLV-1 (human T cell lymphotropic virus type 1) and HTLV-2 oncogenic retroviruses by targeting their viral transactivators Tax-1 and Tax-2, respectively. Here we review our findings on CIITA-mediated inhibition of viral replication and discuss similarities and differences in the molecular mechanisms by which CIITA specifically counteracts the function of Tax-1 and Tax-2 molecules. The dual function of CIITA as a key regulator of adaptive and intrinsic immunity represents a rather unique example of adaptation of host-derived factors against pathogen infections during evolution.

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

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

Group I p21-activated kinases facilitate Tax-mediated transcriptional activation of the human T-cell leukemia virus type 1 long terminal repeats

Background

Human T-cell leukemia virus type 1 (HTLV-1) is the causative agent of adult T-cell leukemia and tropical spastic paraparesis. HTLV-1 encodes transactivator protein Tax that interacts with various cellular factors to modulate transcription and other biological functions. Additional cellular mediators of Tax-mediated transcriptional activation of HTLV-1 long terminal repeats (LTR) remain to be identified and characterized.

Results

In this study, we investigated the regulatory role of group I p21-activated kinases (Paks) in Tax-induced LTR activation. Both wild-type and kinase-dead mutants of Pak3 were capable of potentiating the activity of Tax to activate LTR transcription. The effect of Paks on the LTR was attributed to the N-terminal regulatory domain and required the action of CREB, CREB-regulating transcriptional coactivators (CRTCs) and p300/CREB-binding protein. Paks physically associated with Tax and CRTCs. Paks were recruited to the LTR in the presence of Tax. siRNAs against either Pak1 or Pak3 prevented the interaction of Tax with CRTC1 and the recruitment of Tax to the LTR. These siRNAs also inhibited LTR-dependent transcription in HTLV-1-transformed MT4 cells and in cells transfected with an infectious clone of HTLV-1.

Conclusion

Group I Paks augment Tax-mediated transcriptional activation of HTLV-1 LTR in a kinase-independent manner.

The four and a half LIM family members are novel interactants of the human T-cell leukemia virus type 1 Tax oncoprotein

Human T-cell leukemia virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia (ATL). The viral regulatory protein Tax1 plays a pivotal role in T-cell transformation and ATL development. Previous studies in our laboratory, using the yeast 2-hybrid approach to screen a T-cell library for Tax1-interacting partners, identified the cellular Four and a Half Lim domain protein 3 (FHL3) as a possible Tax1-interacting candidate. FHL3 is a member of the FHL family of proteins, which function as transcriptional coactivators and cytoskeleton regulators and have a role in cancer progression and development. The aim of this study was to investigate the physical and functional interaction between Tax1 and members of the FHL family of proteins. We show that Tax1 and FHL3 interact both in vitro and in vivo. Furthermore, both FHL1 and -2 also interact with Tax1. We have demonstrated that FHL3 enhances Tax1-mediated activation of the viral long terminal repeat (LTR) without affecting basal activity and that FHL1 to -3 regulate NF-κB activation by Tax1 in a cell-specific manner. In addition, we have found that the interaction between Tax1 and FHL1 to -3 affects the localization of these proteins, leading to their redistribution in cells. Tax1 also affected FHL3 cytoskeleton function by increasing FHL3-mediated cell spreading. Overall, our results suggest that the interaction between Tax1 and the FHL family alters both the transactivating activity and the subcellular localization of Tax1 and provide new insights into molecular mechanisms that underlie the oncogenic nature of this HTLV-1 protein.

Viral interference with host mRNA surveillance, the nonsense-mediated mRNA decay (NMD) pathway, through a new function of HTLV-1 Rex: implications for retroviral replication

Nonsense-mediated mRNA decay (NMD) is an essential and conserved cellular mRNA quality control mechanism. RNA signals to express viral genes from overlapping open reading frames potentially initiate NMD, nevertheless it is not clear whether viral RNAs are sensitive to NMD or if viruses have evolved mechanisms to evade NMD. Here we demonstrate that the genomic and full-length mRNAs of Human-T-cell Leukemia Virus type-I (HTLV-1), a retrovirus responsible for Adult T-cell Leukemia (ATL), are sensitive to NMD. They exhibit accelerated turnover in NMD-activated cells, while siRNA-mediated knockdown of NMD-master-regulator, UPF1, promotes enhanced stability of them. These effects on RNA stability were recapitulated by a reporter construct encoding the HTLV-1 translational frameshift signal of gag-pol. In agreement with the RNA stability, viral protein expression from the integrated provirus was inversely correlated with cellular NMD activity. We further demonstrated that the viral RNA-binding protein, Rex, approves the stability of viral RNA by inhibiting NMD. Significantly, Rex establishes a general block to NMD, as both NMD-responsive reporter transcripts and natural host-encoded NMD substrates were stabilized in the presence of Rex. Thus, we suggest that Rex not only stabilizes viral transcripts, but also perturbs cellular mRNA metabolism and host cell homeostasis via inhibition of NMD.

[HTLV-1: Recent topics in epidemiologic, basic and clinical research]

Human T-cell leukemia virus type 1 (HTLV-1) belongs to Delta Retorviridae, and induces a malignancy of CD4+CD25+ T-cells, adult T-cell leukemia (ATL), and several chronic inflammatory diseases, such as HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP) and HTLV-1 uveitis. A nationwide survey of HTLV-1-infected subjects, which was recently conducted by Japanese government, revealed that the numbers of HTLV-1 carriers and patients with HTLV-1-associated diseases have not decreased much over the last two decades in Japan. In contrast, novel findings on HTLV-1 dynamics in vivo and molecular mechanisms of its pathogenesis are accumulating by detailed analysis of newly identified viral and cellular factors, novel technologies such as next-generation sequencing, and appropriate animal models for HTLV-1 research. In this review, we summarize the recent progress of HTLV-1 research.

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.

Overview on HTLV-1 p12, p8, p30, p13: accomplices in persistent infection and viral pathogenesis

The human T-lymphotropic virus type-1 (HTLV-1) is etiologically linked to adult T cell leukemia/lymphoma and tropical spastic paraparesis/HTLV-1-associated myelopathy. While the role of Tax and Rex in viral replication and pathogenesis has been extensively studied, recent evidence suggests that additional viral proteins are essential for the virus life cycle in vivo. In this review, we will summarize possible molecular mechanisms evoked in the literature to explain how p12, p8, p30, and p13 facilitate persistent viral infection of the host. We will explore several stratagems used by HTLV-1 accessory genes to escape immune surveillance, to establish latency, and to deregulate cell cycle and apoptosis to participate in virus-mediated cellular transformation.

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.

HTLV-1 Rex: the courier of viral messages making use of the host vehicle

The human T-cell leukemia virus type 1 (HTLV-1) is a retrovirus causing an aggressive T-cell malignancy, adult T-cell leukemia (ATL). Although HTLV-1 has a compact RNA genome, it has evolved elaborate mechanisms to maximize its coding potential. The structural proteins Gag, Pro, and Pol are encoded in the unspliced form of viral mRNA, whereas the Env protein is encoded in singly spliced viral mRNA. Regulatory and accessory proteins, such as Tax, Rex, p30II, p12, and p13, are translated only from fully spliced mRNA. For effective viral replication, translation from all forms of HTLV-1 transcripts has to be achieved in concert, although unspliced mRNA are extremely unstable in mammalian cells. It has been well recognized that HTLV-1 Rex enhances the stability of unspliced and singly spliced HTLV-1 mRNA by promoting nuclear export and thereby removing them from the splicing site. Rex specifically binds to the highly structured Rex responsive element (RxRE) located at the 3' end of all HTLV-1 mRNA. Rex then binds to the cellular nuclear exporter, CRM1, via its nuclear export signal domain and the Rex-viral transcript complex is selectively exported from the nucleus to the cytoplasm for effective translation of the viral proteins. Yet, the mechanisms by which Rex inhibits the cellular splicing machinery and utilizes the cellular pathways beneficial to viral survival in the host cell have not been fully explored. Furthermore, physiological impacts of Rex against homeostasis of the host cell via interactions with numerous cellular proteins have been largely left uninvestigated. In this review, we focus on the biological importance of HTLV-1 Rex in the HTLV-1 life cycle by following the historical path in the literature concerning this viral post-transcriptional regulator from its discovery to this day. In addition, for future studies, we discuss recently discovered aspects of HTLV-1 Rex as a post-transcriptional regulator and its use in host cellular pathways.