HTLV-1 tax oncoprotein binds to DNA topoisomerase I and inhibits its catalytic activity

Inducible nitric oxide synthase mediates DNA double strand breaks in Human T-Cell Leukemia Virus Type 1-induced leukemia/lymphoma

Background

Adult T-cell leukemia/lymphoma (ATLL) is an aggressive and fatal malignancy of CD4⁺ T-lymphocytes infected by the Human T-Cell Virus Type 1 (HTLV-1). The molecular mechanisms of transformation in ATLL have not been fully elucidated. However, genomic instability and cumulative DNA damage during the long period of latency is believed to be essential for HTLV-1 induced leukemogenesis. In addition, constitutive activation of the NF-κB pathway was found to be a critical determinant for transformation. Whether a connection exists between NF-κB activation and accumulation of DNA damage is not clear. We recently found that the HTLV-1 viral oncoprotein, Tax, the activator of the NF-κB pathway, induces DNA double strand breaks (DSBs).

Results

Here, we investigated whether any of the NF-κB target genes are critical in inducing DSBs. Of note, we found that inducible nitric oxide synthase (iNOS) that catalyzes the production of nitric oxide (NO) in macrophages, neutrophils and T-cells is over expressed in HTLV-1 infected and Tax-expressing cells. Interestingly, we show that in HTLV-1 infected cells, iNOS expression is Tax-dependent and specifically requires the activation of the classical NF-κB and JAK/STAT pathways. A dramatic reduction of DSBs was observed when NO production was inhibited, indicating that Tax induces DSBs through the activation of NO synthesis.

Conclusions

Determination of the impact of NO on HTLV-1-induced leukemogenesis opens a new area for treatment or prevention of ATLL and perhaps other cancers in which NO is produced.

Tax impairs DNA replication forks and increases DNA breaks in specific oncogenic genome regions

BACKGROUND

Human T-cell leukemia virus type 1 (HTLV-I) is a human retrovirus associated with adult T-cell leukemia (ATL), an aggressive CD4 T-cell proliferative disease with dismal prognosis. The long latency preceding the development of the disease and the low incidence suggests that the virus itself is not sufficient for transformation and that genetic defects are required to create a permissive environment for leukemia. In fact, ATL cells are characterized by profound genetic modifications including structural and numerical chromosome alterations.

RESULTS

In this study we used molecular combing techniques to study the effect of the oncoprotein Tax on DNA replication. We found that replication forks have difficulties replicating complex DNA, fork progression is slower, and they pause or stall more frequently in the presence of Tax expression. Our results also show that Tax-associated replication defects are partially compensated by an increase in the firing of back-up origins. Consistent with these effects of Tax on DNA replication, an increase in double strand DNA breaks (DDSB) was seen in Tax expressing cells. Tax-mediated increases in DDSBs were associated with the ability of Tax to activate NF-kB and to stimulate intracellular nitric oxide production. We also demonstrated a reduced expression of human translesion synthesis (TLS) DNA polymerases Pol-H and Pol-K in HTLV-I-transformed T cells and ATL cells. This was associated with an increase in DNA breaks induced by Tax at specific genome regions, such as the c-Myc and the Bcl-2 major breakpoints. Consistent with the notion that the non-homologous end joining (NHEJ) pathway is hyperactive in HTLV-I-transformed cells, we found that inhibition of the NHEJ pathway induces significant killing of HTLV-I transformed cells and patient-derived leukemic ATL cells.

CONCLUSION

Our results suggest that, replication problems increase genetic instability in HTLV-I-transformed cells. As a result, abuse of NHEJ and a defective homologous repair (HR) DNA repair pathway can be targeted as a new therapeutic approach for the treatment of adult T-cell leukemia.

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.

Human T-lymphotropic type 1 virus p30 inhibits homologous recombination and favors unfaithful DNA repair

Whereas oncogenic retroviruses are common in animals, human T-lymphotropic virus 1 (HTLV-1) is the only transmissible retrovirus associated with cancer in humans and is etiologically linked to adult T-cell leukemia. The leukemogenesis process is still largely unknown, but relies on extended survival and clonal expansion of infected cells, which in turn accumulate genetic defects. A common feature of human tumor viruses is their ability to stimulate proliferation and survival of infected pretumoral cells and then hide by establishing latency in cells that have acquired a transformed phenotype. Whereas disruption of the DNA repair is one of the major processes responsible for the accumulation of genomic abnormalities and carcinogenesis, the absence of DNA repair also poses the threat of cell-cycle arrest or apoptosis of virus-infected cells. This study describes how the HTLV-1 p30 viral protein inhibits conservative homologous recombination (HR) DNA repair by targeting the MRE11/RAD50/NBS1 complex and favors the error-prone nonhomologous-end-joining (NHEJ) DNA-repair pathway instead. As a result, HTLV-1 p30 may facilitate the accumulation of mutations in the host genome and the cumulative risk of transformation. Our results provide new insights into how human tumor viruses may manipulate cellular DNA-damage responses to promote cancer.

Molecular approach to human leukemia: isolation and characterization of the first human retrovirus HTLV-1 and its impact on tumorigenesis in adult T-cell leukemia

Molecular biology of mouse and chicken retroviruses had identified oncogenes and provided a revolutionary concept in understanding of cancers. A human retrovirus was established during 1980-1982 in linkage with a unique human leukemia, concurrently in Japan and USA. This review covers our efforts on the discovery of new retrovirus, Human T-cell Leukemia Virus Type 1 (HTLV-1), first introducing to a new class of retroviruses with a unique regulatory factors, Tax and Rex. Then it is followed by analyses of molecular interaction of the vial Tax with cellular machineries involved in the pathogenesis of Adult T-cell Leukemia (ATL). And then a probable mechanism of pathogenesis of ATL is proposed including recent findings on HBZ after our efforts.

The HTLV-1 Tax interactome

The Tax1 oncoprotein encoded by Human T-lymphotropic virus type I is a major determinant of viral persistence and pathogenesis. Tax1 affects a wide variety of cellular signalling pathways leading to transcriptional activation, proliferation and ultimately transformation. To carry out these functions, Tax1 interacts with and modulates activity of a number of cellular proteins. In this review, we summarize the present knowledge of the Tax1 interactome and propose a rationale for the broad range of cellular proteins identified so far.

HTLV-1 and apoptosis: role in cellular transformation and recent advances in therapeutic approaches

A universal cellular defense mechanism against viral invasion is the elimination of infected cells through apoptotic cell death. To counteract host defenses many viruses have evolved complex apoptosis evasion strategies. The oncogenic human retrovirus HTLV-1 is the etiological agent of adult-T-cell leukemia/lymphoma (ATLL) and the neurodegenerative disease known as HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The poor prognosis in HTLV-1-induced ATLL is linked to the resistance of neoplastic T cells against conventional therapies and the immuno-compromised state of patients. Nevertheless, several studies have shown that the apoptotic pathway is largely intact and can be reactivated in ATLL tumor cells to induce specific killing. A better understanding of the molecular mechanisms employed by HTLV-1 to counteract cellular death pathways remains an important challenge for future therapies and the treatment of HTLV-1-associated diseases.

The human T-cell leukemia virus type 1 (HTLV-1): New insights into the clinical aspects and molecular pathogenesis of adult t-cell leukemia/lymphoma (ATLL) and tropical spastic paraparesis/HTLV-associated myelopathy (TSP/HAM)

Human T-cell leukemia virus type 1 (HTLV-1) was the first human retrovirus to be identified in the early 1980s. The isolation and identification of a related virus, HTLV-2, and the distantly related human immunodeficiency virus (HIV) immediately followed. Of the three retroviruses, two are associated definitively with specific diseases, HIV, with acquired immune deficiency syndrome (AIDS) and HTLV-1, with adult T-cell leukemia/lymphoma (ATLL) and tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM). While an estimated 10-20 million people worldwide are infected with HTLV-I, infection is endemic in the Caribbean, parts of Africa, southwestern Japan, and Italy. Approximately 4% of HTLV-I infected individuals develop ATLL, a disease with a poor prognosis. The clinical manifestations of infection and the current biology of HTLV viruses with emphasis on HTLV-1 are discussed in detail. The implications for improvements in diagnosis, treatment, intervention, and vaccination are included, as well as a discussion of the emergence of HTLV-1 and -2 as copathogens among HIV-1-infected individuals.

Discovery of HTLV-1, the first human retrovirus, its unique regulatory mechanisms, and insights into pathogenesis

I briefly review the discovery and characterization of the first human retrovirus, human T-cell leukemia virus type 1, focusing on contributions from Japanese researchers. The unique regulatory mechanisms for the viral regulation with Tax and Rex, etiology of ATL and possible leukemogenic mechanism with Tax are also discussed briefly.

Proteomic analysis of complexes formed by human topoisomerase I

Human topoisomerase I is a nuclear enzyme that catalyses DNA relaxation and phosphorylation of SR proteins. Topoisomerase I participates in several protein-protein interactions. We performed a proteomic analysis of protein partners of topoisomerase I. Two methods were applied to proteins of the nuclear extract of HeLa cells: a co-immunoprecipitation and an affinity chromatography combined with mass spectrometry. Complexes formed by topoisomerase I with its protein partners were immunoprecipitated by scleroderma anti-topoisomerase I antibodies. To identify binding sites for the protein partners, baits corresponding to fragments of topoisomerase I were constructed and used in the affinity chromatography. The N-terminal domain and the cap region of the core domain appeared to be the main regions that bound proteins. We identified 36 nuclear proteins that were associated with topoisomerase I. The proteins were mainly involved in RNA metabolism. We found 29 new and confirmed 7 previously identified protein partners of topoisomerase I. More than 40% proteins that associate with the cap region contain two closely spaced RRM domains. Docking calculations identified the RRM domains as a possible site for the interaction of these proteins with the cap region.

In vivo and in vitro treatment of HTLV-1 and HTLV-2 infected cells with arsenic trioxide and interferon-alpha

Adult T-cell leukemia/lymphoma (ATLL) is a malignant lymphoproliferation of mature activated T-cells, mostly CD4, which develops after a long period of latency following Human T cell Lymphotropic virus Type 1 infection. It is characterized by the clonal integration of one or more HTLV-1 proviruses in the tumor cells. There are 4 major subtypes of ATLL: a smoldering type, a chronic type, a lymphoma type and a leukemic/acute type. The survival rate of ATLL patients, especially those who develop the acute leukemic or lymphomas forms, is very poor and such a tumor remains one of the most severe lymphoproliferations. Treatment of ATLL patients using conventional chemotherapy has very limited benefit, since HTLV-1 transformed cells are resistant to most apoptosis-inducing agents. Recently, antiretroviral therapy using the combination of zidovudine (AZT) and interferon alpha (IFN-alpha) has been shown to induce a high complete remission rate and to prolong the survival of ATLL patients. Based on the current physiopathology, other drugs such as arsenic trioxide combined to IFN-alpha have also been demonstrated to synergize in vitro for inducing apoptosis in HTLV-1 infected T cells. Such drugs have now been used in vivo for treating ATLL patients. Both in vitro and in vivo data will be discussed.

Human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2 tax oncoproteins modulate cell cycle progression and apoptosis

Human T-cell leukemia virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia and lymphoma, an aggressive clonal malignancy of human CD4-bearing T lymphocytes. HTLV-2, although highly related to HTLV-1 at the molecular level, has not been conclusively linked to development of lymphoproliferative disorders. Differences between the biological activities of the respective tax gene products (Tax1 and Tax2) may be one factor which accounts for the differential pathogenicities associated with infection. To develop an in vitro model to investigate and compare the effects of constitutive expression of Tax1 and Tax2, Jurkat T-cell lines were infected with lentivirus vectors encoding Tax1 and Tax2 in conjunction with green fluorescent protein, and stably transduced clonal cell lines were generated by serial dilution in the absence of drug selection. Jurkat cells that constitutively express Tax1 and Tax2 (Tax1/Jurkat and Tax2/Jurkat, respectively) showed notably reduced kinetics of cellular replication, and Tax1 inhibited cellular replication to a higher degree in comparison to Tax2. Tax1 markedly activated transcription from the cdk inhibitor p21(cip1/waf1) promoter in comparison to Tax2, suggesting that upregulation of p21(cip1/waf1) may account for the differential inhibition of cellular replication kinetics displayed by Tax1/Jurkat and Tax2/Jurkat cells. The presence of binucleated and multinucleated cells, reminiscent of large lymphocytes with cleaved or cerebriform nuclei often seen in HTLV-1- and -2-seropositive patients, was noted in cultures expressing Tax1 and Tax2. Although Tax1 and Tax2 expression mediated elevated resistance to apoptosis in Jurkat cells after serum deprivation, Tax1 was unique in protection from apoptosis after exposure to camptothecin and etoposide, inhibitors of topoisomerase I and II, respectively. Characterization of the unique phenotypes displayed by Tax1 and Tax2 in vitro will provide information as to the relative roles of these oncoproteins and their contribution to HTLV-1 and -2 pathogenesis in vivo.

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

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

Transcriptional activation of hTERT through the NF-kappaB pathway in HTLV-I-transformed cells

In immortal cells, the existence of a mechanism for the maintenance of telomere length is critical. In most cases this is achieved by the reactivation of telomerase, a cellular reverse transcriptase that prevents telomere shortening. Here we report that the telomerase gene (hTERT) promoter is up-regulated during transmission of human T-cell lymphotropic virus type-I (HTLV-I) to primary T cells in vitro and in ex vivo adult T-cell leukemia/lymphoma (ATLL) samples, but not asymptomatic carriers. Although Tax impaired induction of human telomerase reverse transcriptase (hTERT) mRNA in response to mitogenic stimulation, transduction of Tax into primary lymphocytes was sufficient to activate and maintain telomerase expression and telomere length when cultured in the absence of any exogenous stimulation. Transient transfection assays revealed that Tax stimulates the hTERT promoter through the nuclear factor kappaB (NF-kappaB) pathway. Consistently, Tax mutants inactive for NF-kappaB activation could not activate the hTERT or sustain telomere length in transduced primary lymphocytes. Analysis of the hTERT promoter occupancy in vivo using chromatin immunoprecipitation assays suggested that an increased binding of c-Myc and Sp1 is involved in the NF-kappaB-mediated activation of the hTERT promoter. This study establishes the role of Tax in regulation of telomerase expression, which may cooperate with other functions of Tax to promote HTLV-I-associated adult T-cell leukemia.

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.

The HTLV-1 tax oncoprotein attenuates DNA damage induced G1 arrest and enhances apoptosis in p53 null cells

Transformation of cells by the human T cell leukemia virus type 1 occurs via mechanisms unique among oncogenic retroviruses. A prevailing hypothesis for HTLV-1-mediated cellular transformation is that expression of the viral transactivator, Tax, induces genomic instability. Tax-mediated failure in the cellular repair response is one possible mechanism for loss in genomic integrity. Here we have examined the in vivo repair response of Tax-expressing cells to determine the underlying defects that contribute to loss of genomic integrity. In these studies we examined the effects of de novo Tax-expression in naive "pre-neoplastic" REF52 cells. DNA-damage-induced p53 stabilization and concomitant transient stabilization of p21 were clearly evident in Tax-expressing cells. Likewise, the damage-induced apoptotic response of Tax-expressing cells was normal. However, the damage-induced G1 checkpoint was abrogated in either p53+ or p53- cellular backgrounds. Although nucleotide excision repair (NER) of asynchronous Tax-expressing cells was impaired, cell-cycle-independent assessment of NER in the global excision repair assay demonstrated comparable NER activity in Tax-expressing cells, suggesting that the failure of G1 checkpoint contributes to NER deficiency. Interestingly, we observed a dramatic increase in apoptosis and UV sensitivity of Tax-expressing p53-/- cells when compared to Tax-expressing p53+/+ cells. These data demonstrate that Tax-mediated cellular genomic instability arises from attenuation of cell-cycle checkpoint and imply a clonal dependence on p53 status separate from genomic integrity.

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.

Characterization of BTBD1 and BTBD2, two similar BTB-domain-containing Kelch-like proteins that interact with Topoisomerase I

BACKGROUND

Two-hybrid screening for proteins that interact with the core domain of human topoisomerase I identified two novel proteins, BTBD1 and BTBD2, which share 80% amino acid identities.

RESULTS

The interactions were confirmed by co-precipitation assays demonstrating the physical interaction of BTBD1 and BTBD2 with 100 kDa topoisomerase I from HeLa cells. Deletion mapping using two-hybrid and GST-pulldown assays demonstrated that less than the C-terminal half of BTBD1 is sufficient for binding topoisomerase I. The topoisomerase I sequences sufficient to bind BTBD2 were mapped to residues 215 to 329. BTBD2 with an epitope tag localized to cytoplasmic bodies. Using truncated versions that direct BTBD2 and TOP1 to the same cellular compartment, either the nucleus or the cytoplasm, co-localization was demonstrated in co-transfected Hela cells. The supercoil relaxation and DNA cleavage activities of topoisomerase I in vitro were affected little or none by co-incubation with BTBD2. Northern analysis revealed only a single sized mRNA for each BTBD1 and BTBD2 in all human tissues tested. Characterization of BTBD2 mRNA revealed a 255 nucleotide 90% GC-rich region predicted to encode the N-terminus. BTBD1 and BTBD2 are widely if not ubiquitously expressed in human tissues, and have two paralogs as well as putative orthologs in C. elegans and D. melanogaster.

CONCLUSIONS

BTBD1 and BTBD2 belong to a small family of uncharacterized proteins that appear to be specific to animals. Epitope-tagged BTBD2 localized to cytoplasmic bodies. The characterization of BTBD1 and BTBD2 and their interaction with TOP1 is underway.

Transcriptional consequences of topoisomerase inhibition

In principle, the generation, transmission, and dissipation of supercoiling forces are determined by the arrangement of the physical barriers defining topological boundaries and the disposition of enzymes creating (polymerases and helicases, etc.) or releasing (topoisomerases) torsional strain in DNA. These features are likely to be characteristic for individual genes. By using topoisomerase inhibitors to alter the balance between supercoiling forces in vivo, we monitored changes in the basal transcriptional activity and DNA conformation for several genes. Every gene examined displayed an individualized profile in response to inhibition of topoisomerase I or II. The expression changes elicited by camptothecin (topoisomerase I inhibitor) or adriamycin (topoisomerase II inhibitor) were not equivalent. Camptothecin generally caused transcription complexes to stall in the midst of transcription units, while provoking little response at promoters. Adriamycin, in contrast, caused dramatic changes at or near promoters and prevented transcription. The response to topoisomerase inhibition was also context dependent, differing between chromosomal or episomal c-myc promoters. In addition to being well-characterized DNA-damaging agents, topoisomerase inhibitors may evoke a biological response determined in part from transcriptional effects. The results have ramifications for the use of these drugs as antineoplastic agents.

Multiple viral strategies of HTLV-1 for dysregulation of cell growth control

The human T cell leukemia virus-1 (HTLV-1) is a retrovirus that causes adult T cell leukemia (ATL) and neurological disorder, the tropical spastic paraparesis (HAM/TSP). The pathogenesis apparently results from the pleiotropic function of Tax protein, which is a key regulator of viral replication. Tax exerts (a) trans-activation and -repression of transcription of different sets of cellular genes through binding to groups of transcription factors and coactivators, (b) dysregulation of cell cycle through binding to inhibitors of CDK4/6, and (c) inhibition of some tumor suppressor proteins. These effects on a wide variety of cellular targets seem to cooperate in promoting cell proliferation. This is an effective viral strategy to amplify its proviral genome through replication of infected cells; ultimately it results in cell transformation and leukemogenesis.