Role for Akt/Protein Kinase B and Activator Protein-1 in Cellular Proliferation Induced by the Human T-cell Leukemia Virus Type 1 Tax Oncoprotein

Human T-cell leukemia virus type 1 is an oncogenic retrovirus etiologically causal of adult T-cell leukemia. The virus encodes a Tax oncoprotein, which functions in transcriptional regulation, cell cycle control, and transformation. Because adult T-cell leukemia is a highly virulent cancer that is resistant to numerous chemotherapeutic treatments, to understand better this disease it is important to comprehend how human T-cell leukemia virus type 1 promotes cellular growth and survival. Most of the existing data point to Tax activation of NF-kappaB as important for cellular proliferation and transformation. We show here that Tax, in the absence of NF-kappaB signaling, can activate activator protein-1 to promote cellular proliferation and survival. Tax is shown to activate activator protein-1 through the phosphatidylinositol 3-kinase/Akt pathway.

Insights into gene expression changes impacting B-cell transformation: cross-species microarray analysis of bovine leukemia virus tax-responsive genes in ovine B cells

Large-animal models for leukemia have the potential to aid in the understanding of networks that contribute to oncogenesis. Infection of cattle and sheep with bovine leukemia virus (BLV), a complex retrovirus related to human T-cell leukemia virus type 1 (HTLV-1), is associated with the development of B-cell leukemia. Whereas the natural disease in cattle is characterized by a low tumor incidence, experimental infection of sheep leads to overt leukemia in the majority of infected animals, providing a model for studying the pathogenesis associated with BLV and HTLV-1. Tax(BLV), the major oncoprotein, initiates a cascade of events leading toward malignancy, although the basis of transformation is not fully understood. We have taken a cross-species ovine-to-human microarray approach to identify Tax(BLV)-responsive transcriptional changes in two sets of cultured ovine B cells following retroviral vector-mediated delivery of Tax(BLV). Using cDNA-spotted microarrays comprising 10,336 human genes/expressed sequence tags, we identified a cohort of differentially expressed genes, including genes related to apoptosis, DNA transcription, and repair; proto-oncogenes; cell cycle regulators; transcription factors; small Rho GTPases/GTPase-binding proteins; and previously reported Tax(HTLV-1)-responsive genes. Interestingly, genes known to be associated with human neoplasia, especially B-cell malignancies, were extensively represented. Others were novel or unexpected. The results suggest that Tax(BLV) deregulates a broad network of interrelated pathways rather than a single B-lineage-specific regulatory process. Although cross-species approaches do not permit a comprehensive analysis of gene expression patterns, they can provide initial clues for the functional roles of genes that participate in B-cell transformation and pinpoint molecular targets not identified using other methods in animal models.

Requirement of the human T-cell leukemia virus (HTLV-1) tax-stimulated HIAP-1 gene for the survival of transformed lymphocytes

Human T cell leukemia virus type 1 (HTLV-1), the cause of adult T cell leukemia (ATL), induces clonal expansion of infected T-cells in nonleukemic individuals and immortalizes T cells in vitro. The resistance against apoptotic stimuli of these cells hints at a viral survival function in addition to a proliferation-stimulating activity. Here we describe the up-regulation of the antiapoptotic HIAP-1/CIAP-2 gene as a consistent phenotype of HTLV-1-transformed and ATL-derived cultures and its stimulation by the viral oncoprotein Tax. Cotransfections revealed a 60-fold increase of HIAP-1 promoter activity mediated by Tax mainly via nuclear factor-kappaB (NF-kappaB) activation. To address the relevance of virally increased HIAP-1 levels for the survival of HTLV-1-transformed cells, its expression was RNA interference (RNAi) suppressed using a lentiviral transduction system. This resulted in a dramatic reduction of cell growth, a strong induction of apoptosis rates, and increased caspases 3/7 activity, which is known to be suppressed by HIAP-1. Thus, the Tax-mediated HIAP-1 overexpression is required to suppress endogenous apoptosis and, therefore, is essential for the survival of HTLV-1-transformed lymphocytes. Moreover, this points to HIAP-1 as an important target of the HTLV-1-mediated NF-kappaB activation.

Inhibition of Apoptosis by Human T-Lymphotropic Virus Type-1 Tax Protein

Deregulation of the apoptotic process can lead to pathophysiological changes that result in either degenerative diseases or cancer. Although the transactivator Tax has been established as an essential effector of human T-lymphotropic virus type-1 (HTLV-1)-mediated diseases, which include both a neurodegenerative pathology and leukemia/lymphoma, its exact role(s) in the pathogenesis remains to be clarified. Because the apoptotic potential of Tax is still being debated, we addressed this question by testing the susceptibility of Tax(-) and Tax(+) cells to apoptosis under conditions of growth factor withdrawal and Bax overexpression. Results showed that Tax(+) cells are protected from apoptosis triggered by depletion of growth factors. This protective effect seems to be the result of a block in the apoptotic program regulated by mitochondria. Furthermore, in an attempt to elucidate which transcriptional pathway activated by Tax is important in the observed Tax-induced resistance, we found that CREB/ATF activity plays a relevant role in protecting cells from apoptosis induced by Bax overexpression. All together, these data might suggest that the ability of Tax to inhibit certain apoptotic stimuli could be important in its role as a viral transforming protein.

HTLV-1 Tax protects against CD95-mediated apoptosis by induction of the cellular FLICE-inhibitory protein (c-FLIP)

The HTLV-1 transactivator protein Tax is essential for malignant transformation of CD4 T cells, ultimately leading to adult T-cell leukemia/lymphoma (ATL). Malignant transformation may involve development of apoptosis resistance. In this study we investigated the molecular mechanisms by which HTLV-1 Tax confers resistance toward CD95-mediated apoptosis. We show that Tax-expressing T-cell lines derived from HTLV-1-infected patients express elevated levels of c-FLIP(L) and c-FLIP(S). The levels of c-FLIP correlated with resistance toward CD95-mediated apoptosis. Using an inducible system we demonstrated that both resistance toward CD95-mediated apoptosis and induction of c-FLIP are dependent on Tax. In addition, analysis of early cleavage of the BH3-only Bcl-2 family member Bid, a direct caspase-8 substrate, revealed that apoptosis is inhibited at a CD95 death receptor proximal level in Tax-expressing cells. Finally, using siRNA we directly showed that c-FLIP confers Tax-mediated resistance toward CD95-mediated apoptosis. In conclusion, our data suggest an important mechanism by which expression of HTLV-1 Tax may lead to immune escape of infected T cells and, thus, to persistent infection and transformation.

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.

Presence of a functional but dispensable nuclear export signal in the HTLV-2 Tax protein

BACKGROUND

Human T-cell leukemia virus type 1 and type 2 are related human retroviruses. HTLV-1 is the etiological agent of the Adult T-cell Leukemia/Lymphoma and of the Tropical Spastic Paraparesis/HTLV-1 Associated Myelopathy, whereas, HTLV-2 infection has not been formally associated with any T-cell malignancy. HTLV-1 and 2 genomes encode, respectively, the Tax1 and Tax2 proteins whose role is to transactivate the viral promoter. HTLV-1 and HTLV-2 Tax sequences display 28% divergence at the amino acid level. Tax1 is a shuttling protein that possesses both a non canonical nuclear import (NLS) and a nuclear export (NES) signal. We have recently demonstrated that Tax1 and Tax2 display different subcellular localization and that residues 90-100 are critical for this process. We investigate in the present report, whether Tax2 also possesses a functional NES.

RESULTS

We first used a NES prediction method to determine whether the Tax2 protein might contain a NES and the results do suggest the presence of a NES sequence in Tax2. Using Green Fluorescent Protein-NES (GFP-NES) fusion proteins, we demonstrate that the Tax2 sequence encompasses a functional NES (NES2). As shown by microscope imaging, NES2 is able to mediate translocation of GFP from the nucleus, without the context of a full length Tax protein. Furthermore, point mutations or leptomycin B treatment abrogate NES2 function. However, within the context of full length Tax2, similar point mutations in the NES2 leucine rich stretch do not modify Tax2 localization. Finally, we also show that Tax1 NES function is dependent upon the positioning of the nuclear export signal "vis-à-vis" GFP.

CONCLUSION

HTLV-2 Tax NES is functional but dispensable for the protein localization in vitro.

Induction of cell cycle arrest by human T-cell lymphotropic virus type 1 Tax in hematopoietic progenitor (CD34+) cells: modulation of p21cip1/waf1 and p27kip1 expression

Human T-cell lymphotropic virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia, an aggressive CD4(+) malignancy. Although HTLV-2 is highly homologous to HTLV-1, infection with HTLV-2 has not been associated with lymphoproliferative disorders. Lentivirus-mediated transduction of CD34(+) cells with HTLV-1 Tax (Tax1) induced G(0)/G(1) cell cycle arrest and resulted in the concomitant suppression of multilineage hematopoiesis in vitro. Tax1 induced transcriptional upregulation of the cdk inhibitors p21(cip1/waf1) (p21) and p27(kip1) (p27), and marked suppression of hematopoiesis in immature (CD34(+)/CD38(-)) hematopoietic progenitor cells in comparison to CD34(+)/CD38(+) cells. HTLV-1 infection of CD34(+) cells also induced p21 and p27 expression. Tax1 also protected CD34(+) cells from serum withdrawal-mediated apoptosis. In contrast, HTLV-2 Tax (Tax2) did not detectably alter p21 or p27 gene expression, failed to induce cell cycle arrest, failed to suppress hematopoiesis in CD34(+) cells, and did not protect cells from programmed cell death. A Tax2/Tax1 chimera encoding the C-terminal 53 amino acids of Tax1 fused to Tax2 (Tax(221)) displayed a phenotype in CD34(+) cells similar to that of Tax1, suggesting that unique domains encoded within the C terminus of Tax1 may account for the phenotypes displayed in human hematopoietic progenitor cells. These remarkable differences in the activities of Tax1 and Tax2 in CD34(+) hematopoietic progenitor cells may underlie the sharp differences observed in the pathogenesis resulting from infection with HTLV-1 and HTLV-2.

HTLV-1 Tax nucleocytoplasmic shuttling, interaction with the secretory pathway, extracellular signaling, and implications for neurologic disease

The human T cell leukemia virus type 1 (HTLV-1) oncoprotein Tax interacts with numerous cellular pathways promoting both the survival and pathogenesis of the virus in the human population. Tax has been studied extensively with respect to its role in transcriptional transactivation and its involvement in the up-regulation of a number of cellular genes during the process of oncogenic transformation. These processes are dependent on Tax localization to the nucleus where it interacts with a number of cellular transcription factors during its course of nuclear action. However, there is mounting evidence suggesting that Tax may shuttle between the nucleus and cytoplasm, localize to several cytoplasmic organelles with subsequent secretion from both Tax-transfected cells as well as HTLV-1-infected cells. In addition, the presence of cell-free Tax in cerebral spinal fluid (CSF) was recently demonstrated to occur during all stages of HAM/TSP. This has brought about an increased interest in the cytoplasmic localization of Tax and the implications this localization may have with respect to the progression of HTLV-1-associated disease processes. This review addresses the functional implications relevant to the localization and accumulation of Tax in the cytoplasm including the Tax amino acid signals and cellular protein interactions that may regulate this process. Specifically, we have discussed three important processes associated with the cytoplasmic localization of Tax. First, the process of Tax shuttling between the nucleus and cytoplasm will be described and how this process may be involved in regulating different transcriptional activation pathways. Second, cytoplasmic localization of Tax will be discussed with relevance to Tax secretion and the interaction of Tax with proteins in the cellular secretory pathway. Finally, the secretion of Tax and the effects of extracellular Tax on HTLV-1 pathogenesis will be addressed.

12th International Conference on Human Retrovirology: HTLV and Related Retroviruses

The 12^th International Conference on Human Retrovirology: HTLV and Related Retroviruses, was held at the Half Moon Hotel in Montego Bay, Jamaica, from June 22^nd to June 25^th 2005. The scientific conference, sponsored by the International Retrovirology Association, is held biennially at rotating international venues around the world. The meeting brings together basic scientists, epidemiologists and clinical researchers to discuss findings to prevent HTLV infection or develop new therapies against HTLV-mediated diseases. The Association fosters the education and training of young scientists to bring new approaches to the complex problems of HTLV research, such as translational research to bring findings from the laboratory into clinical trials that benefit HTLV-infected patients. The breadth and quality of research presentations and workshops at the 12^th International Conference indicate that these goals are being accomplished. As HTLV research enters its third decade a new generation of scientists face many challenges. However, HTLV scientists and clinicians displayed exciting new approaches and discoveries during plenary talks and poster sessions. The conference encouraged research in HTLV infections and disease, fostered collaborations, and stimulated new partnerships between clinicians and scientists to encourage clinical trials and novel therapeutic interventions.

Lethal cutaneous disease in transgenic mice conditionally expressing type I human T cell leukemia virus Tax

Type I human T cell leukemia virus (HTLV-I) is etiologically linked with adult T cell leukemia, an aggressive and usually fatal expansion of activated CD4+ T lymphocytes that frequently traffic to skin. T cell transformation induced by HTLV-I involves the action of the 40-kDa viral Tax transactivator protein. Tax both stimulates the HTLV-I long terminal repeat and deregulates the expression of select cellular genes by altering the activity of specific host transcription factors, including cyclic AMP-responsive element-binding protein (CREB)/activating transcription factor, NF-kappaB/Rel, and serum response factor. To study initiating events involved in HTLV-I Tax-induced T cell transformation, we generated "Tet-off" transgenic mice conditionally expressing in a lymphocyte-restricted manner (EmuSR alpha promoter-enhancer) either wild-type Tax or mutant forms of Tax that selectively compromise the NF-kappaB (M22) or CREB/activating transcription factor (M47) activation pathways. Wild-type Tax and M47 Tax-expressing mice, but not M22-Tax expressing mice, developed progressive alopecia, hyperkeratosis, and skin lesions containing profuse activated CD4 T cell infiltrates with evidence of deregulated inflammatory cytokine production. In addition, these animals displayed systemic lymphadenopathy and splenomegaly. These findings suggest that Tax-mediated activation of NF-kappaB plays a key role in the development of this aggressive skin disease that shares several features in common with the skin disease occurring during the preleukemic stage in HTLV-I-infected patients. Of note, this skin disease completely resolved when Tax transgene expression was suppressed by administration of doxycycline, emphasizing the key role played by this viral oncoprotein in the observed pathology.

Engagement of specific T-cell surface molecules regulates cytoskeletal polarization in HTLV-1–infected lymphocytes

Cell-cell contact is required for efficient transmission of human T-lymphotropic virus type 1 (HTLV-1). An HTLV-1–infected cell polarizes its microtubule-organizing center (MTOC) toward the cell-cell junction; HTLV-1 core (Gag) complexes and the HTLV-1 genome accumulate at the point of contact and are then transferred to the uninfected cell. However, the mechanisms involved in this cytoskeletal polarization and transport of HTLV-1 complexes are unknown. Here, we tested the hypothesis that engagement of a specific T-cell surface ligand is synergistic with HTLV-1 infection in causing polarization of the MTOC to the cell contact region. We show that antibodies to intercellular adhesion molecule-1 (ICAM-1; CD54) caused MTOC polarization at a higher frequency in HTLV-1–infected cells. ICAM-1 is upregulated on HTLV-1–infected cells, and, in turn, ICAM-1 on the cell surface upregulates HTLV-1 gene expression. We propose that a positive feedback loop involving ICAM-1 and HTLV-1 Tax protein facilitates the formation of the virologic synapse and contributes to the T-cell tropism of HTLV-1. In contrast, MTOC polarization induced in T cells by antibodies to CD3 or CD28 was significantly inhibited by HTLV-1 infection.

PDZ domain-binding motif of human T-cell leukemia virus type 1 Tax oncoprotein is essential for the interleukin 2 independent growth induction of a T-cell line

Background

Human T-cell leukemia virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia (ATL), whereas HTLV type 2 (HTLV-2), is not associated with ATL or any other leukemia. HTLV-1 encodes the transforming gene tax1, whose expression in an interleukin (IL)-2-dependent T-cell line (CTLL-2) induces IL-2-independent growth.

Results

In this study, we demonstrated that IL-2-independent growth induction by Tax1 was abrogated by mutations of the PDZ domain-binding motif (PBM) at the Tax1 C-terminus. HTLV-2 Tax2, which shares 75% amino acid identity with Tax1 but does not have a PBM, was not able to induce IL-2-independent growth of CTLL-2.

Conclusion

Our results suggest that Tax1, through interaction with PDZ domain protein(s) induces IL-2-independent growth, which may be a factor in multi-step leukemogenesis caused by HTLV-1.

Evidence for NF-kappaB- and CBP-independent repression of p53's transcriptional activity by human T-cell leukemia virus type 1 Tax in mouse embryo and primary human fibroblasts

The human T-cell leukemia virus type 1 (HTLV-1) Tax oncoprotein can repress the transcriptional activity of the tumor suppressor protein p53. However, it remains controversial whether Tax requires NF-kappaB factors/activity and/or p300/CBP in order to inactivate p53 function. To address this issue, we have investigated Tax's effect on p53's transcriptional activation in IkappaB-kinase-deficient mouse embryonic fibroblasts (MEFs); some of which are entirely silent for Tax-induced NF-kappaB activity. We found that, in IKKalpha-/-, IKKbeta-/-, and IKKgamma-/- MEFs, p53 activation of a prototypic responsive plasmid (pG13-luciferase) was repressed by wild-type Tax. Curiously, p53's activity in MEFs was also repressed by a p300/CBP-binding deficient Tax protein. Our results highlight the complex nature of Tax-mediated repression of p53- activity, which requires further investigation.

Human T-lymphotropic virus, type 1, tax protein triggers microtubule reorientation in the virological synapse

We showed recently that the human T-lymphotropic virus, type 1 (HTLV-1), spreads directly from cell to cell via a virological synapse. The HTLV-1 virological synapse resembles the immunological synapse; each is a specialized contact between a lymphocyte and another cell that contains organized protein microdomains, and each involves repolarization of the T-cell microtubule cytoskeleton. However, formation of the virological synapse is not triggered by T-cell receptor-mediated antigen recognition. On the basis of our previous data, we postulated that formation of the viral synapse was triggered by a conjunction of two signals, one from HTLV-1 infection of the T-cell and one from cell-cell contact. We have recently identified ICAM-1 engagement as a cell-contact signal that causes the microtubule polarization associated with the virological synapse. Here we used confocal microscopy of T-lymphocytes naturally infected with HTLV-1 or transfected with individual HTLV-1 genes to investigate the role of the viral transcriptional transactivator protein Tax. Polarization of the microtubules was induced by cell-cell contact or by cross-linking T-cell surface molecules with monoclonal antibodies adsorbed to latex beads. We show that Tax, which is mainly found in the nucleus, is also present at two specific extranuclear sites as follows: around the microtubule organizing center in association with the cis-Golgi and in the cell-cell contact region. We show that expression of Tax provides an intracellular signal that synergizes with ICAM-1 engagement to cause the T-cell microtubule polarization observed at the virological synapse.

The homeobox protein MSX2 interacts with tax oncoproteins and represses their transactivation activity

Bovine leukemia virus (BLV) tax is an essential gene involved in the transcriptional activation of viral expression. Tax is also believed to be implicated in leukemogenesis because of its ability to immortalize primary cells in vitro. To gain insight into the molecular pathways mediating the activities of this important gene, we identified cellular proteins interacting with Tax. By means of a two-hybrid approach, we show that Tax specifically interacts with MSX2, a general repressor of gene expression. GST pull-down experiments and co-immunoprecipitation assays further confirmed binding specificity. Furthermore, the N-terminal residues 1-79 of MSX2 are required for binding, whereas the C-terminal residues 201-267 of MSX2 do not play a critical role. Whereas the oncogenic potential of Tax in primary cells was only slightly affected by overexpression of MSX2, the other function of Tax, namely LTR-dependent transcriptional activation, was inhibited by MSX2 in human HeLa and bovine B-lymphoblastoid (BL3) cell lines. This MSX2 repression function can be counteracted by overexpression of transcription factors CREB2 and RAP74. The Tax/MSX2 interplay thus results in repression of viral transcriptional activation possibly acting as a regulatory feedback loop. Importantly, this viral gene silencing is not strictly associated with a concomitant loss of Tax oncogenicity as measured by its ability to immortalize primary cells. And interestingly, MSX2 also interacts with and inhibits the transactivation function of the related Tax1 protein encoded by the Human T-cell leukemia virus type 1 (HTLV-1).

Chronic graft-versus-host disease-like autoimmune disorders spontaneously occurred in rats with neonatal thymus atrophy

We earlier reported that the human T cell leukemia virus type-1 pX gene transduced into rat thymic epithelial cells had an impact on biology of the cells. We report here that FW-pX rats born by mating of F344 transgenic rats expressing the pX gene without tissue specificity with nontransgenic Wistar rats developed disorders, including atrophy of the thymus, lymphocytopenia, and inflammatory cell infiltration into multiple organs, similar to events in chronic graft-vs.-host disease (GVHD). Vanishment of thymic epithelial cells especially in the cortex and marked depletion of CD4 CD8 double-positive thymocytes were evident in the neonatal thymus in these rats. The relative abundance of CD8 compared to CD4 T cells may be related to dominant infiltration of CD8 T cells into the affected organs. Additionally, adoptive transfer of FW-pX splenocytes could induce lymphocytic infiltration into sublethally irradiated wild-type syngeneic recipients. Analysis of the expression level of the Foxp3 gene in peripheral blood mononuclear cells revealed that the numbers of immunoregulatory T cells were less in FW-pX rats than in wild-type rats. The collective evidence suggested that the FW-pX rats spontaneously developed chronic GVHD-like autoimmune diseases, following abortive differentiation of T cells in the thymus in early days of the newborn. This rat model may shed light on the pathogenesis of chronic GVHD and also other systemic autoimmune diseases, the etiology of which is unknown.

Regulation of the GATA3 promoter by human T-cell lymphotropic virus type I Tax protein

The Human T-cell leukemia virus type I (HTLV-I) non-structural protein Tax plays a crucial role in cellular transformation. It activates the transcription factors of various cellular genes and interacts with cellular proteins. There is limited data available on the interaction between specific T-cell transcription factor GATA3 and Tax. Implications for the significance of GATA3 in T-cell development and function, T helper2 (Th2) differentiation, and a role of GATA3 during the immune response have been reported. To determine the effect of the Tax protein on GATA3 gene expression, we investigated the interaction between this protein and the GATA3 promoter and repressor regions. Results demonstrated an interaction between Tax and the GATA3 promoter via the transcription factor Sp1 and a role for Tax in the negative regulation of GATA3 expression, through its interaction with the repressor ZEB. This interaction may be involved in the pathophysiology of adult T-cell leukemia/lymphoma (ATL) and tropical spastic paraparesis/HTLV-I-associated myelopathy (TSP/HAM).

Human T cell leukemia virus type I Tax activates CD40 gene expression via the NF-kappa B pathway

The human T cell leukemia virus type I (HTLV-I) is an oncogenic retrovirus that is etiologically linked to the genesis of adult T cell leukemia (ATL) as well as HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Emerging evidence suggests that the pathogenicity of HTLV-I involves deregulated activation of immune cells, especially T lymphocytes, although the underlying mechanism remains unclear. In this study, we demonstrate that HTLV-I Tax induces the aberrant expression of CD40, a member of the tumor necrosis factor receptor (TNFR) family that plays an important role in lymphocyte activation and differentiation. In a panel of HTLV-I-transformed T cell lines analyzed, CD40 expression was highly elevated compared to HTLV-I-negative T cells. Using Tax mutants and a genetically manipulated T cell system, we demonstrated that Tax-induced CD40 expression required the NF-kappaB signaling pathway. In addition, ligation of CD40 on T cells with recombinant CD40L elicited NF-kappaB activation, suggesting that the CD40 pathway is intact and may participate in a positive regulatory loop in T cells. CD40 ligation strongly synergized with Tax to activate NF-kappaB, suggesting that CD40 signals may costimulate Tax-mediated NF-kappaB activation, particularly when Tax is expressed at low levels. Collectively, these results indicate that CD40 is a novel Tax-regulated gene, and the regulation of CD40 by Tax may play a role in cellular activation and HTLV-I-induced disease pathogenesis.

Activation of HTLV-I gene transcription by protein tyrosine phosphatase inhibitors

Human T-cell leukemia virus type I (HTLV-I) transcription generally depends on the ability of the viral Tax protein to bind the CREB transcription factor and form an active complex by recruiting CBP/p300 coactivators to the long terminal repeat (LTR). Studies have demonstrated that T-cell activating agents that stimulate CREB are potent inducers of HTLV-I transcription. Herein, we demonstrate that bpV[pic], a protein tyrosine phosphatase (PTP) inhibitor activates the HTLV-I LTR in the presence and absence of Tax expression. Optimal activation occurred at 8 h and was synergistic with forskolin or PGE(2). Infected cell lines and cells transfected with HTLV-I proviral DNA were equally responsive to the synergistic effect of bpV and forskolin on HTLV-I gene expression. Activation of the LTR by bpV[pic] was T-cell receptor-independent, but required ZAP70, calcineurin activity and functional calcium entry. Inhibition of the SHP-1 PTP was suggested to be important. Transfection experiments with a CREB dominant-negative mutant and with isolated TRE1- or CREB-responsive reporter constructs and treatment with the MDL-12,330A adenylate cyclase inhibitor all supported the involvement of a CREB/ATF family member in this bpV-dependent activation of the HTLV-I LTR, although CREB itself did not seem to be involved. Analysis of HTLV-I reporter constructs containing mutated CREB-binding sites also implied the involvement of another element in this activation. These results demonstrate for the first time a powerful effect of PTP inhibitors on HTLV-I LTR activity and suggest participation of both CREB-dependent and -independent pathways in this activation.

The HTLV-I Tax oncoprotein: hyper-tasking at the molecular level

HTLV-I is a complex retrovirus that encodes a transcriptional activator, Tax, which regulates expression of the viral promoter. Tax has been shown to be both necessary and sufficient to effect immortalization and transformation of cells in culture and tumorigenesis in animal models. Tax exerts its influence through protein-protein interactions with a variety of molecular targets, including transcription factors and cofactors, histone modifying enzymes and post-translational modifying enzymes. Through these interactions, Tax disrupts cellular regulatory cascades and checkpoints designed to control a variety of systems. The result is untimely activation or repression of gene expression, inappropriate protein modifications, incorrect cell cycling, loss of adequate DNA repair capacity, and potential release of the cell from tumor suppression. Whereas for the virus these functions of Tax provide a means for successful completion of its life cycle, for the cell, they result at best in anarchy, and at worst in death of both the cell and the organism of which that cell is a part.

Involvement of bovine leukemia virus in induction and inhibition of apoptosis

In a previous study, we identified an interesting mutant form of the Tax protein of bovine leukemia virus (BLV), designated D247G, that has an enhanced capacity to transactivate the long terminal repeat (LTR) of BLV and the cellular proto-oncogene c-fos when compared with wild-type Tax (wt-Tax). We demonstrate here that an infectious strain of BLV containing the mutant D247G form of Tax also differs in its capacity to modulate cell survival both positively and negatively. When peripheral blood mononuclear cells (PBMCs) infected with wild-type or mutant BLV are cultured ex vivo with staurosporine, an agent known to induce a mitochondrial caspase cascade pathway regulating apoptosis, the rate of apoptosis is reduced to a greater extent in cells infected with mutant BLV than wild-type BLV, consistent with previous observations in cultures without staurosporine. The increase in survival was associated with an increase in expression of mRNA of bcl-xl but not bcl-2 and bax ex vivo. In contrast, when a tissue culture-adapted cell line, 293T, was transiently transfected with either wild-type or mutant BLV, apoptosis was induced. The increase in the rate of apoptosis was higher in cells transfected with mutant BLV. The same difference was noted in cells transiently transfected with wild-type and mutant D247G Tax, suggesting that the observed positive and negative modulation of cell survival is attributed to the functional characteristics of mutant D247G Tax.

Longer dinucleotide repeat polymorphism in matrix metalloproteinase-9 (MMP-9) gene promoter which correlates with higher HTLV-I Tax mediated transcriptional activity influences the risk of HTLV-I associated myelopathy/tropical spastic paraparesis (HAM/TSP)

Matrix metalloproteinase-9 (MMP-9) has been reported to be expressed in various inflammatory disorders including human T cell lymphotropic virus type I (HTLV-I) associated myelopathy/tropical spastic paraparesis (HAM/TSP). HTLV-I-infected T-cells expressed high levels of MMP-9 via viral transactivator Tax mediated activation of the MMP-9 promoter. To investigate whether the d(CA) repeat polymorphism in MMP-9 promoter affects the risk of developing HAM/TSP, we compared the allele frequencies between 200 HAM/TSP patients and 200 HTLV-I seropositive asymptomatic carriers (HCs). The longer d(CA) repeat alleles of MMP-9 promoter, which was associated with higher Tax-mediated transcriptional activity, was more frequently observed in HAM/TSP patients than HCs (p<0.01 by Mann-Whitney U-test). The length alteration of this d(CA) repeat in the MMP-9 promoter may cause phenotypic differences among HTLV-I infected infiltrating cells and may thereby be in part responsible for the development of HAM/TSP.

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.

Polymorphism in the interleukin-10 promoter affects both provirus load and the risk of human T lymphotropic virus type I-associated myelopathy/tropical spastic paraparesis

To investigate non-human leukocyte antigen candidate genes that influence the outcome of human T cell lymphotropic virus (HTLV) type I infection, we analyzed 6 single-nucleotide polymorphisms in the interleukin (IL)-10 promoter region in 280 patients with HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP) and 255 HTLV-I-seropositive asymptomatic carriers from an area where HTLV-I is endemic. The IL-10 -592 A allele, which shows lower HTLV-I Tax-induced transcriptional activity than the C allele in the Jurkat T cell line, was associated with a >2-fold reduction in the odds of developing HAM/TSP (P=.011; odds ratio [OR], 0.50 [95% confidence interval, 0.30-0.86]) by reducing the provirus load in the whole cohort (P=.009, analysis of variance). Given the OR and the observed frequency of IL-10 -592 A, we demonstrate that this allele prevents approximately 44.7% (standard deviation, +/-13.1%) of potential cases of HAM/TSP, which indicates that it defines one component of the genetic susceptibility to HAM/TSP in the cohort.