Human T-cell lymphotropic virus type 1 Tax represses c-Myb-dependent transcription through activation of the NF-kappaB pathway and modulation of coactivator usage

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

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

MiR-150 in HTLV-1-infection and T-cell transformation

Human T-cell leukemia virus-1 (HTLV-1) is a retrovirus that persistently infects CD4+ T-cells, and is the causative agent of adult T-cell leukemia/lymphoma (ATLL), tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM) and several inflammatory diseases. T-cell transformation by HTLV-1 is driven by multiple interactions between viral regulatory proteins and host cell pathways that govern cell proliferation and survival. Studies performed over the last decade have revealed alterations in the expression of many microRNAs in HTLV-1-infected cells and ATLL cells, and have identified several microRNA targets with roles in the viral life cycle and host cell turnover. This review centers on miR-150-5p, a microRNA whose expression is temporally regulated during lymphocyte development and altered in several hematological malignancies. The levels of miR-150-5p are reduced in many HTLV-1-transformed- and ATLL-derived cell lines. Experiments in these cell lines showed that downregulation of miR-150-5p results in activation of the transcription factor STAT1, which is a direct target of the miRNA. However, data on miR-150-5p levels in freshly isolated ATLL samples are suggestive of its upregulation compared to controls. These apparently puzzling findings highlight the need for more in-depth studies of the role of miR-150-5p in HTLV-1 infection and pathogenesis based on knowledge of miR-150-5p-target mRNA interactions and mechanisms regulating its function in normal leukocytes and hematologic neoplasms.

In silico analysis of human T-lymphotropic virus type 1 complete genomes from patients with different clinical outcomes

Aims: This study aims to identify nucleotide variations in human T-lymphotropic virus type 1 (HTLV-1) proviral genome that might be related with the different clinical conditions associated to the virus. Materials & methods: 91 complete HTLV-1 genomes available in GenBank with their respective clinical information were subjected to in silico analyzes (subtyping, molecular characterization and machine learning). Results: We identified 22 mutations that seems to be important in patients’ clinical condition. The presence of some mutations demonstrated alterations in the proteins physicochemical profile, such as the P34L, present in the p12 protein. Furthermore, a correlation between mutations in long terminal repeat and pX region seems to be important for clinical manifestation. Conclusions: Some mutations have the potential to alter the conformation of viral proteins that are important for infection outcomes. Therefore, further functional studies should be performed to assess the impact of these variations on the pathogenesis and on the development of clinical manifestations associated with HTLV-1.

Gene Expression and Resistance to Glucocorticoid-Induced Apoptosis in Acute Lymphoblastic Leukemia: A Brief Review and Update

BACKGROUND

Resistance to glucocorticoid (GC)-induced apoptosis in Acute Lymphoblastic Leukemia (ALL), is considered one of the major prognostic factors for the disease. Prednisolone is a corticosteroid and one of the most important agents in the treatment of acute lymphoblastic leukemia. The mechanics of GC resistance are largely unknown and intense ongoing research focuses on this topic.

AIM

The aim of the present study is to review some aspects of GC resistance in ALL, and in particular of Prednisolone, with emphasis on previous and present knowledge on gene expression and signaling pathways playing a role in the phenomenon.

METHODS

An electronic literature search was conducted by the authors from 1994 to June 2019. Original articles and systematic reviews selected, and the titles and abstracts of papers screened to determine whether they met the eligibility criteria, and full texts of the selected articles were retrieved.

RESULTS

Identification of gene targets responsible for glucocorticoid resistance may allow discovery of drugs, which in combination with glucocorticoids may increase the effectiveness of anti-leukemia therapies. The inherent plasticity of clinically evolving cancer justifies approaches to characterize and prevent undesirable activation of early oncogenic pathways.

CONCLUSION

Study of the pattern of intracellular signal pathway activation by anticancer drugs can lead to development of efficient treatment strategies by reducing detrimental secondary effects.

Genetic interaction between mutations in c-Myb and the KIX domains of CBP and p300 affects multiple blood cell lineages and influences both gene activation and repression

Adult blood cell production or definitive hematopoiesis requires the transcription factor c-Myb. The closely related KAT3 histone acetyltransferases CBP (CREBBP) and p300 (EP300) bind c-Myb through their KIX domains and mice homozygous for a p300 KIX domain mutation exhibit multiple blood defects. Perplexingly, mice homozygous for the same KIX domain mutation in CBP have normal blood. Here we test the hypothesis that the CBP KIX domain contributes subordinately to hematopoiesis via a genetic interaction with c-Myb. We assessed hematopoiesis in mice bearing compound mutations of c-Myb and/or the KIX domains of CBP and p300, and measured the effect of KIX domain mutations on c-Myb-dependent gene expression. We found that in the context of a p300 KIX mutation, the CBP KIX domain mutation affects platelets, B cells, T cells, and red cells. Gene interaction (epistasis) analysis provides mechanistic evidence that blood defects in KIX mutant mice are consistent with reduced c-Myb and KIX interaction. Lastly, we demonstrated that the CBP and p300 KIX domains contribute to both c-Myb-dependent gene activation and repression. Together these results suggest that the KIX domains of CBP, and especially p300, are principal mediators of c-Myb-dependent gene activation and repression that is required for definitive hematopoiesis.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

HTLV-I Tax regulates the cellular proliferation through the down-regulation of PIP3-phosphatase expressions via the NF-κB pathway

An oncogenic retrovirus, human T-cell leukemia virus type I (HTLV-I), encodes an oncoprotein, Tax, which plays critical roles in leukemogenesis of adult T-cell leukemia/lymphoma (ATLL) through the pleiotropic actions such as transcriptional regulation, cell cycle control, and transformation. We have previously reported that PTEN and SHIP- 1, PIP3 inositol phosphatases that negatively regulate the PI3-kinase signaling cascade, are disrupted in ATLL neoplasias. Overactivation of PI3-kinase signaling has an essential role in onset of ATLL. We report here that both PTEN and SHIP-1 are downregulated by Tax through the NF-κB signaling pathway. Tax expression upregulated phosphorylated Akt, a downstream serine/threonine kinase in the PI3-kinase signaling cascade. Activation of NF-κB pathway also suppressed these phosphatases. An IκBΔN mutant which inhibits the activation of NF-κB prevented PIP3 phosphatase downregulation by Tax. The underlying mechanism of NF-κB mediated suppression of PIP3 phosphatases involved sequestration of the coactivator p300 by p65. These down-regulations of PIP3 phosphatases were found to be essential for the Tax-induced cell proliferation. Thus, our results suggest that HTLV-I Tax downregulates PIP3 phosphatases through the NF-κB pathway, resulting in increased activation of the PI3-kinase signaling cascade in human T-cells and contributing to leukemogenesis.

Human T-cell lymphotropic virus: a model of NF-κB-associated tumorigenesis

Human T-cell lymphotropic virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia/lymphoma (ATL), whereas the highly related HTLV-2 is not associated with ATL or other cancers. In addition to ATL leukemogenesis, studies of the HTLV viruses also provide an exceptional model for understanding basic pathogenic mechanisms of virus-host interactions and human oncogenesis. Accumulating evidence suggests that the viral regulatory protein Tax and host inflammatory transcription factor NF-κB are largely responsible for the different pathogenic potentials of HTLV-1 and HTLV-2. Here, we discuss the molecular mechanisms of HTLV-1 oncogenic pathogenesis with a focus on the interplay between the Tax oncoprotein and NF-κB pro-oncogenic signaling. We also outline some of the most intriguing and outstanding questions in the fields of HTLV and NF-κB. Answers to those questions will greatly advance our understanding of ATL leukemogenesis and other NF-κB-associated tumorigenesis and will help us design personalized cancer therapies.

Zebrafish microRNA-126 determines hematopoietic cell fate through c-Myb

Precise regulatory mechanisms are required to appropriately modulate the cellular levels of transcription factors controlling cell fate decisions during blood cell development. Here, we show that miR-126 is a novel physiological regulator of the proto-oncogene c-myb during definitive hematopoiesis. We show that knockdown of miR-126 results in increased c-Myb levels and promotes erythropoiesis at the expense of thrombopoiesis in vivo. We further provide evidence that specification of thrombocyte versus erythrocyte cell lineages is altered by the concerted activities of the miRNAs miR-126 and miR-150. Both microRNAs are required but not sufficient individually to precisely regulate the cell fate decision between erythroid and megakaryocytic lineages during definitive hematopoiesis in vivo. These results support the notion that microRNAs not only act to provide precision to developmental programs but also are essential determinants in the control of variable potential functions of a single gene during hematopoiesis.

Prednisolone exerts late mitogenic and biphasic effects on resistant acute lymphoblastic leukemia cells: Relation to early gene expression

Resistance or sensitivity to glucocorticoids is considered to be of crucial importance for disease prognosis in childhood acute lymphoblastic leukemia. Prednisolone exerted a delayed biphasic effect on the resistant CCRF-CEM leukemic cell line, necrotic at low doses and apoptotic at higher doses. At low doses, prednisolone exerted a pre-dominant mitogenic effect despite its induction on total cell death, while at higher doses, prednisolone's mitogenic and cell death effects were counterbalanced. Early gene microarray analysis revealed notable differences in 40 genes. The mitogenic/biphasic effects of prednisolone are of clinical importance in the case of resistant leukemic cells. This approach might lead to the identification of gene candidates for future molecular drug targets in combination therapy with glucocorticoids, along with early markers for glucocorticoid resistance.

Sustained NF-kappaB activation produces a short-term cell proliferation block in conjunction with repressing effectors of cell cycle progression controlled by E2F or FoxM1

NF-kappaB transcription factors induce a host of genes involved in pro-inflammatory/stress-like responses; but the collateral effects and consequences of sustained NF-kappaB activation on other cellular gene expression programming remain less well understood. Here enforced expression of a constitutively active IKKbeta T-loop mutant (IKKbetaca) drove murine fibroblasts into transient growth arrest that subsided within 2-3 weeks of continuous culture. Proliferation arrest was associated with a G1/S phase block in immortalized and primary early passage MEFs. Molecular analysis in immortalized MEFs revealed that inhibition of cell proliferation in the initial 1-2 weeks after their IKKbetaca retroviral infection was linked to the transient, concerted repression of essential cell cycle effectors that are known targets of either E2F or FoxM1. Co-expression of a phosphorylation resistant IkappaBalpha super repressor and IKKbetaca abrogated growth arrest and cell cycle effector repression, thereby linking IKKbetaca's effects to canonical NF-kappaB activation. Transient growth arrest of IKKbetaca cells was associated with enhanced p21 (cyclin-dependent kinase inhibitor 1A) protein expression, due in part to transcriptional activation by NF-kappaB and also likely due to strong repression of Skp2 and Csk1, both of which are FoxM1 direct targets mediating proteasomal dependent p21 turnover. Ablation of p21 in immortalized MEFs reduced their IKKbetaca mediated growth suppression. Moreover, trichostatin A inhibition of HDACs alleviated the repression of E2F and FoxM1 targets induced by IKKbetaca, suggesting chromatin mediated gene silencing in IKKbetaca's short term repressive effects on E2F and FoxM1 target gene expression.

In primary effusion lymphoma cells, MYB transcriptional repression is associated with v-FLIP expression during latent KSHV infection while both v-FLIP and v-GPCR become involved during the lytic cycle

Primary effusion lymphoma (PEL) is a rare, distinct subtype of non-Hodgkin lymphoma, which is associated with Kaposi sarcoma-associated herpesvirus (KSHV) infection. Although MYB levels are high in most neoplastic B cells, we found that, unexpectedly, both PEL cells and uncultured PEL patients' samples contained very low levels of MYB mRNA when compared to B-cell leukaemia samples obtained from KSHV(-) patients. These results were further confirmed at the protein level. Both latent viral FLICE inhibitory protein (v-FLIP) and early lytic viral G protein coupled receptor (v-GPCR) KSHV proteins were found to activate nuclear factor (NF)-kappaB and transrepress a MYB promoter reporter construct. In contrast, a dominant negative inhibitor of NF-kappaB (IkappaB-alpha) mutant prevented v-FLIP and v-GPCR from inhibiting MYB functions while a v-GPCR mutant that was impaired for NF-kappaB activation could not repress the MYB construct. Transduction of a v-FLIP expressing vector or stable transfection of v-GPCR both resulted in a marked downregulation of the endogenous MYB protein expression. However, MYB expression transactivated the lytic switch Replication and Transcription Activator (RTA) promoter in transient transfection assays. Taken together, our results demonstrate that, contrary to a number of other haematological malignancies, MYB expression is not required for PEL cell proliferation. Repressing MYB expression also helps in maintaining the virus in latency.

Rapamycin-induced inhibition of HTLV-I LTR activity is rescued by c-Myb

Background

Rapamycin is an immunosuppressive which represses translation of transcripts harbouring a polypyrimidine motif downstream of the mRNA cap site through the mammalian target of rapamycin complex. It inhibits the abnormal autologous proliferation of T-cell clones containing a transcriptionally active human T-lymphotropic virus, type I (HTLV-I) provirus, generated from infected subjects. We showed previously that this effect is independent of the polypyrimidine motifs in the viral long terminal repeat (LTR) R region suggesting that HTLV-I transcription, and not translation, is being affected. Here we studied whether rapamycin is having an effect on a specific transcription factor pathway. Further, we investigated whether mRNAs encoding transcription factors involved in HTLV-I transcriptional activation, specifically CREB, Ets and c-Myb, are implicated in the rapamycin-sensitivity of the HTLV-I LTR.

Results

An in vitro analysis of the role of SRE- and NF-κB-mediated transcription highlighted the latter as rapamycin sensitive. Over-expression of c-Myb reversed the rapamycin effect.

Conclusion

The sensitivity of HTLV-I transcription to rapamycin may be effected through an NF-κB-pathway associated with the rapamycin-sensitive mTORC1 cellular signalling network.

Human T-cell leukemia virus oncoprotein tax represses nuclear receptor-dependent transcription by targeting coactivator TAX1BP1

Human T-cell leukemia virus type 1 oncoprotein Tax is a transcriptional regulator that interacts with a large number of host cell factors. Here, we report the novel characterization of the interaction of Tax with a human cell protein named Tax1-binding protein 1 (TAX1BP1). We show that TAX1BP1 is a nuclear receptor coactivator that forms a complex with the glucocorticoid receptor. TAX1BP1 and Tax colocalize into intranuclear speckles that partially overlap with but are not identical to the PML oncogenic domains. Tax binds TAX1BP1 directly, induces the dissociation of TAX1BP1 from the glucocorticoid receptor-containing protein complex, and represses the coactivator function of TAX1BP1. Genetic knockout of Tax1bp1 in mice abrogates the influence of Tax on the activation of nuclear receptors. We propose that Tax-TAX1BP1 interaction mechanistically explains the previously reported repression of nuclear receptor activity by Tax.

Human T-cell leukemia virus type I p30 nuclear/nucleolar retention is mediated through interactions with RNA and a constituent of the 60 S ribosomal subunit

Human T-cell leukemia virus type I is the etiological agent of adult T-cell leukemia/lymphoma, an aggressive and fatal lymphoproliferative malignancy. The virus has evolved strategies to escape immune clearance by remaining latent in most infected cells in vivo. We demonstrated previously that virally encoded p30 protein is a potent post-transcriptional inhibitor of virus replication (Nicot, C., Dundr, M., Johnson, J. M., Fullen, J. R., Alonzo, N., Fukumoto, R., Princler, G. L., Derse, D., Misteli, T., and Franchini, G. (2004) Nat. Med. 10, 197-201). p30 is unable to shuttle out of the nucleus in heterokaryon assays, suggesting the existence of specific retention signals. Because suppression of virus replication relies on nuclear retention of the tax/rex mRNA by p30, determining the retention features of p30 will offer hints to break latency in infected cells and insights into new therapeutic approaches. In this study, we used live cell imaging technologies to study the kinetics of p30 and to delineate its retention signals and their function in virus replication. Notably, this is the first study to identify p30 nucleolar retention domains. Using mutants of p30 that localized in different cellular compartments, we show that post-transcriptional control of virus replication by p30 occurs in the nucleoplasm. We further demonstrate that p30 nuclear/nucleolar retention is dependent upon de novo RNA transcripts and interactions with components of the ribosomal machinery.

The HTLV-I p30 Interferes with TLR4 Signaling and Modulates the Release of Pro- and Anti-inflammatory Cytokines from Human Macrophages

Whereas adaptive immunity has been extensively studied, very little is known about the innate immunity of the host to HTLV-I infection. HTLV-I-infected ATL patients have pronounced immunodeficiency associated with frequent opportunistic infections, and in these patients, concurrent infections with bacteria and/or parasites are known to increase risks of progression to ATL. The Toll-like receptor-4 (TLR4) activation in response to bacterial infection is essential for dendritic cell maturation and links the innate and adaptive immune responses. Recent reports indicate that TLR4 is targeted by viruses such as RSV, HCV, and MMTV. Here we report that HTLV-I has also evolved a protein that interferes with TLR4 signaling; p30 interacts with and inhibits the DNA binding and transcription activity of PU.1 resulting in the down-regulation of the TLR4 expression from the cell surface. Expression of p30 hampers the release of pro-inflammatory cytokines MCP-1, TNF-alpha, and IL-8 and stimulates release of anti-inflammatory IL-10 following stimulation of TLR4 in human macrophage. Finally, we found that p30 increases phosphorylation and inactivation of GSK3-beta a key step for IL-10 production. Our study suggests a novel function of p30, which may instigate immune tolerance by reducing activation of adaptive immunity in ATL patients.

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

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

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

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

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

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

Interaction of HTLV-1 Tax and methyl-CpG-binding domain 2 positively regulates the gene expression from the hypermethylated LTR

Epigenetic regulation of gene expression is critical in the maintenance of cellular homeostasis. Dysregulation of normal epigenetic transcription occurs in abnormal physiological conditions, such as those seen in cancer cells and cells infected with parasites, making the mechanism underlying abnormal epigenetic transcription of great interest. Gene expression of human T-cell leukemia virus type 1 (HTLV-1) is regulated by a viral transcriptional stimulator, Tax. We herein report a novel mechanism of transcription from the HTLV-1 long terminal repeat (LTR) that is regulated by Tax. In this study, we determined that Tax is able to activate transcription from the LTR, even when it was heavily methylated. In addition, the methyl-CpG-binding domain 2 (MBD2) protein played an important role in Tax-mediated transcriptional activation. We demonstrated the importance of a physical interaction between Tax and MBD2 in enhancing the transcriptional activity of Tax against the methylated LTR. Furthermore, we identified the formation of a protein complex composed of MBD2 and Tax bound to the methylated LTR. We propose a new model of epigenetic regulation by MBD2 acting in concert with a virally encoded transactivator, Tax. Our observation provides insight into the epigenetic regulation of gene expression and the diverse mechanisms of transcriptional regulation using methylated promoters.

HTLV-I Tax induces a novel interaction between p65/RelA and p53 that results in inhibition of p53 transcriptional activity

Nuclear factor kappaB (NF-kappaB) activation plays a critical role in oncogenesis by human T-cell lymphotrophic virus type I (HTLV-I), the etiologic agent of adult T-cell leukemia (ATL), and is indispensable for maintenance of the malignant phenotype. In T lymphocytes, Tax-mediated p53 inhibition is dependent on Tax activation of the NF-kappaB pathway and is linked to p53 phosphorylation. We now report that blocking NF-kappaB transcriptional activation in HTLV-I-transformed cells restores p53 activity. Further, using mouse embryo fibroblast (MEF) null cells and antisense oligonucleotides to inhibit expression of NF-kappaB family members, we demonstrate that the p65 subunit of NF-kappaB is uniquely involved in p53 inhibition. Coimmunoprecipitation assays demonstrate an interaction between p65 and p53 in HTLV-I-transformed cells. In transient transfection assays, we demonstrate that Tax induces the p53-p65 interaction. Phosphorylation of p53 at serines 15 and 392 is critical for complex formation. Importantly, Tax-mediated p53 inhibition correlates with p65 and p53 interaction. By using chromatin immunoprecipitation (ChIP) assays, we find that in HTLV-I-transformed cells p53 and p65 form a complex on the inactive, p53-responsive murine double minute 2 (MDM2) promoter. Consistent with reduced transcriptional activity, transcription factor IID (TFIID) binding is not observed. These studies identify a unique mechanism for p53 regulation by the p65/RelA subunit of NF-kappaB.

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

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

Tax relieves transcriptional repression by promoting histone deacetylase 1 release from the human T-cell leukemia virus type 1 long terminal repeat

Expression of human T-cell leukemia virus type 1 (HTLV-1) is regulated by the viral transcriptional activator Tax. Tax activates viral transcription through interaction with the cellular transcription factor CREB and the coactivators CBP/p300. In this study, we have analyzed the role of histone deacetylase 1 (HDAC1) on HTLV-1 gene expression from an integrated template. First we show that trichostatin A, an HDAC inhibitor, enhances Tax expression in HTLV-1-transformed cells. Second, using a cell line containing a single-copy HTLV-1 long terminal repeat, we demonstrate that overexpression of HDAC1 represses Tax transactivation. Furthermore, a chromatin immunoprecipitation assay allowed us to analyze the interaction of transcription factors, coactivators, and HDACs with the basal and activated HTLV-1 promoter. We demonstrate that HDAC1 is associated with the inactive, but not the Tax-transactivated, HTLV-1 promoter. In vitro and in vivo glutathione S-transferase-Tax pull-down and coimmunoprecipitation experiments demonstrated that there is a direct physical association between Tax and HDAC1. Importantly, biotinylated chromatin pull-down assays demonstrated that Tax inhibits and/or dissociates the binding of HDAC1 to the HTLV-1 promoter. Our results provide evidence that Tax interacts directly with HDAC1 and regulates binding of the repressor to the HTLV-1 promoter.

Suppression of PTEN expression by NF-kappa B prevents apoptosis

NF-kappa B is a heterodimeric transcription activator consisting of the DNA binding subunit p50 and the transactivation subunit p65/RelA. NF-kappa B prevents cell death caused by tumor necrosis factor (TNF) and other genotoxic insults by directly inducing antiapoptotic target genes. We report here that the tumor suppressor PTEN, which functions as a negative regulator of phosphatidylinositol (PI)-3 kinase/Akt-mediated cell survival pathway, is down regulated by p65 but not by p50. Moreover, a subset of human lung or thyroid cancer cells expressing high levels of endogenous p65 showed decreased expression of PTEN that could be rescued by specific inhibition of the NF-kappa B pathway with I kappa B overexpression as well as with small interfering RNA directed against p65. Importantly, TNF, a potent inducer of NF-kappa B activity, suppressed PTEN gene expression in IKK beta(+/+) cells but not in IKK beta(-/-) cells, which are deficient in the NF-kappa B activation pathway. These findings indicated that NF-kappa B activation was necessary and sufficient for inhibition of PTEN expression. The promoter, RNA, and protein levels of PTEN are down-regulated by NF-kappa B. The mechanism underlying suppression of PTEN expression by NF-kappa B was independent of p65 DNA binding or transcription function and involved sequestration of limiting pools of transcriptional coactivators CBP/p300 by p65. Restoration of PTEN expression inhibited NF-kappa B transcriptional activity and augmented TNF-induced apoptosis, indicating a negative regulatory loop involving PTEN and NF-kappa B. PTEN is, thus, a novel target whose suppression is critical for antiapoptosis by NF-kappa B.

Interaction of retroviral Tax oncoproteins with tristetraprolin and regulation of tumor necrosis factor-alpha expression

BACKGROUND

The Tax oncoproteins are transcriptional regulators of viral expression involved in pathogenesis induced by complex leukemogenic retroviruses (or delta-retroviruses, i.e., primate T-cell leukemia viruses and bovine leukemia virus). To better understand the molecular pathways leading to cell transformation, we aimed to identify cellular proteins interacting with Tax.

METHODS

We used a yeast two-hybrid system to identify interacting cellular proteins. Interactions between Tax and candidate interacting cellular proteins were confirmed by glutathione S-transferase (GST) pulldown assays, co-immunoprecipitation, and confocal microscopy. Functional interactions between Tax and one interacting protein, tristetraprolin (TTP), were assessed by analyzing the expression of tumor necrosis factor-alpha (TNF-alpha), which is regulated by TTP, in mammalian cells (HeLa, D17, HEK 293, and RAW 264.7) transiently transfected with combinations of intact and mutant Tax and TTP.

RESULTS

We obtained seven interacting cellular proteins, of which one, TTP, was further characterized. Tax and TTP were found to interact specifically through their respective carboxyl-terminal domains. The proteins colocalized in the cytoplasm in a region surrounding the nucleus of HeLa cells. Furthermore, coexpression of Tax was associated with nuclear accumulation of TTP. TTP is an immediate-early protein that inhibits expression of TNF-alpha at the post-transcriptional level. Expression of Tax reverted this inhibition, both in transient transfection experiments and in stably transfected macrophage cell lines.

CONCLUSION

Tax, through its interactions with the TTP repressor, indirectly increases TNF-alpha expression. This observation is of importance for the cell transformation process induced by leukemogenic retroviruses, because TNF-alpha overexpression plays a central role in pathogenesis.

T-Cell Control by Human T-Cell Leukemia/Lymphoma Virus Type 1

Human T-cell leukemia/lymphoma virus type 1 (HTLV-1) causes neoplastic transformation of human T-cells in a small number of infected individuals several years from infection. Collective evidence from in vitro studies indicates that several viral proteins act in concert to increase the responsiveness of T-cells to extracellular stimulation, modulate proapoptotic and antiapoptotic gene signals, enhance T-cell survival, and avoid immune recognition of the infected T-cells. The virus promotes T-cell proliferation by usurping several signaling pathways central to immune T-cell function, such as antigen stimulation and receptor-ligand interaction, suggesting that extracellular signals are important for HTLV-1 oncogenesis. Environmental factors such as chronic antigen stimulation may therefore be of importance, as also suggested by epidemiological data. Thus genetic and environmental factors together with the virus contribute to disease development. This review focuses on current knowledge of the mechanisms regulating HTLV-1 replication and the T-cell pathways that are usurped by viral proteins to induce and maintain clonal proliferation of infected T-cells. The relevance of these laboratory findings is related to clonal T-cell proliferation and adult T-cell leukemia/lymphoma development in vivo.

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

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

RelB-p50 NF-kappa B complexes are selectively induced by cytomegalovirus immediate-early protein 1: differential regulation of Bcl-x(L) promoter activity by NF-kappa B family members

The NF-kappa B/Rel family has been implicated in control of transcription of the Bcl-x(L) gene, a target which mediates cell survival signals. The cytomegalovirus (CMV) immediate-early protein 1 (IE1) was previously shown to induce NF-kappa B activity. Here, we report that in both vascular smooth muscle cells (SMCs) and NIH 3T3 cells, surprisingly, IE1 failed to induce Bcl-x(L) promoter activity, although it induced activity of E8-CAT, a reporter construct driven by two copies of the NF-kappa B element upstream of the c-myc promoter (upstream regulatory element [URE]). Thus, the subunit nature of the NF-kappa B/Rel factors induced by IE1 was examined using immunofluorescence and immunoblotting. IE1 was found to selectively induce nuclear RelB and p50 in SMCs and NIH 3T3 cells. An increase in RelB protein mediated by IE1 could, in part, be related to an increase in steady-state relB mRNA levels. Consistent with this subunit identification, IE1 was unable to induce E8-CAT activity in relB(-/-) murine embryonic fibroblast cells. In cotransfection analysis of SMCs and NIH 3T3 cells, RelB and p50 proteins failed to induce Bcl-x(L) promoter activity while inducing E8-CAT. Furthermore, the NF-kappa B element of the Bcl-x(L) promoter only weakly bound RelB-p50 complexes compared to the URE NF-kappa B element. Overall, these findings demonstrate in SMCs and NIH 3T3 cells that the CMV IE1 protein selectively induces RelB and p50, which fail to activate the Bcl-x(L) promoter, indicating a strong specificity of binding and activity for the RelB member of the NF-kappa B family. Furthermore, our results implicate RelB in CMV infection of cells such as vascular SMCs.