Molecular mechanisms of promoter regulation of the gp34 gene that is trans-activated by an oncoprotein Tax of human T cell leukemia virus type I

Trans-Activation of the Coactivator-Associated Arginine Methyltransferase 1 (Carm1) Gene by the Oncogene Product Tax of Human T-Cell Leukemia Virus Type 1

Human T-cell leukemia virus type 1 (HTLV-1) is the causative agent of adult T-cell leukemia/lymphoma. The oncogene product Tax of HTLV-I is thought to play crucial roles in leukemogenesis by promoting proliferation of the virus-infected cells through activation of growth-promoting genes. These genes code for growth factors and their receptors, cytokines, cell adhesion molecules, growth signal transducers, transcription factors and cell cycle regulators. We show here that Tax activates the gene coding for coactivator-associated arginine methyltransferase 1 (CARM1), which epigenetically enhances gene expression through methylation of histones. Tax activated the Carm1 gene and increased protein expression, not only in human T-cell lines but also in normal peripheral blood lymphocytes (PHA-PBLs). Tax increased R17-methylated histone H3 on the target gene IL-2Rα, concomitant with increased expression of CARM1. Short hairpin RNA (shRNA)-mediated knockdown of CARM1 decreased Tax-mediated induction of IL-2Rα and Cyclin D2 gene expression, reduced E2F activation and inhibited cell cycle progression. Tax acted via response elements in intron 1 of the Carm1 gene, through the NF-κB pathway. These results suggest that Tax-mediated activation of the Carm1 gene contributes to leukemogenic target-gene expression and cell cycle progression, identifying the first epigenetic target gene for Tax-mediated trans-activation in cell growth promotion.

The Pleiotropic Effects of YBX1 on HTLV-1 Transcription

HTLV-1 is an oncogenic human retrovirus and the etiologic agent of the highly aggressive ATL malignancy. Two viral genes, Tax and Hbz, are individually linked to oncogenic transformation and play an important role in the pathogenic process. Consequently, regulation of HTLV-1 gene expression is a central feature in the viral lifecycle and directly contributes to its pathogenic potential. Herein, we identified the cellular transcription factor YBX1 as a binding partner for HBZ. We found YBX1 activated transcription and enhanced Tax-mediated transcription from the viral 5' LTR promoter. Interestingly, YBX1 also interacted with Tax. shRNA-mediated loss of YBX1 decreased transcript and protein abundance of both Tax and HBZ in HTLV-1-transformed T-cell lines, as well as Tax association with the 5' LTR. Conversely, YBX1 transcriptional activation of the 5' LTR promoter was increased in the absence of HBZ. YBX1 was found to be associated with both the 5' and 3' LTRs in HTLV-1-transformed and ATL-derived T-cell lines. Together, these data suggest that YBX1 positively influences transcription from both the 5' and 3' promoter elements. YBX1 is able to interact with Tax and help recruit Tax to the 5' LTR. However, through interactions with HBZ, YBX1 transcriptional activation of the 5' LTR is repressed.

Human T-cell leukemia virus type 1 (HTLV-1) Tax1 oncoprotein but not HTLV-2 Tax2 induces the expression of OX40 ligand by interacting with p52/p100 and RelB

Human T-cell leukemia virus type 1 (HTLV-1) is a causative retrovirus of adult T-cell leukemia and HTLV-1-associated myelopathy. Unlike HTLV-1, the same group of retrovirus HTLV-2 has not been found to be associated with these diseases. HTLV-1 and HTLV-2 encode transforming proteins Tax1 and Tax2, and a few distinct activities of Tax1 from those of Tax2 have been proposed to contribute to the HTLV-1-specific pathogenesis of disease. One significant difference of Tax1 from Tax2 is the activation of transcription factor NF-κB2/p100/p52. We found that Tax1 but not Tax2 induces the expression of OX40 ligand (OX40L) in a human T-cell line. To induce the OX40L expression, Tax1 but not Tax2 was observed to interact with NF-κB2/p100/p52 and RelB and the distinct interaction activity was mediated by the Tax1 amino acid region of 225-232. In addition, Tax1 but not Tax2 or Tax1/225-232 interacted with p65, p50, and c-Rel; however, the interactions were much less than those noted with NF-κB2/p100/p52 and RelB. OX40L is a T-cell costimulatory molecule of the tumor necrosis factor family, and its signal plays a critical role in establishing adaptive immunity by inducing the polarized differentiation of T-cells to cells such as T helper type 2 and T follicular helper cells. Therefore, the present findings suggest that Tax1 might alter the immune response to HTLV-1 and/or differentiation of HTLV-1-infected T-cells via OX40L induction, thereby acting as a factor mediating the distinct phenotypes and pathogenesis of HTLV-1 from that of HTLV-2.

Human T-cell leukemia virus type-I Tax induces the expression of CD83 on T cells

BACKGROUND

CD83, a cell surface glycoprotein that is stably expressed on mature dendritic cells, can be transiently induced on other hematopoietic cell lineages upon cell activation. In contrast to the membrane form of CD83, soluble CD83 appears to be immunosuppressive. In an analysis of the phenotype of leukemic CD4(+) T cells from patients with adult T-cell leukemia (ATL), we found that a number of primary CD4(+) T cells became positive for cell surface CD83 after short-term culture, and that most of these CD83(+) CD4(+) T cells were positive for human T-cell leukemia virus type-I (HTLV-I) Tax (Tax1). We hypothesized that Tax1 is involved in the induction of CD83.

RESULT

We found that CD83 was expressed selectively on Tax1-expressing human CD4(+) T cells in short-term cultured peripheral blood mononuclear cells (PBMCs) isolated from HTLV-I(+) donors, including ATL patients and HTLV-I carriers. HTLV-I-infected T cell lines expressing Tax1 also expressed cell surface CD83 and released soluble CD83. CD83 can be expressed in the JPX-9 cell line by cadmium-mediated Tax1 induction and in Jurkat cells or PBMCs by Tax1 introduction via infection with a recombinant adenovirus carrying the Tax1 gene. The CD83 promoter was activated by Tax1 in an NF-κB-dependent manner. Based on a previous report showing soluble CD83-mediated prostaglandin E2 (PGE2) production from human monocytes in vitro, we tested if PGE2 affected HTLV-I propagation, and found that PGE2 strongly stimulated expression of Tax1 and viral structural molecules.

CONCLUSIONS

Our results suggest that HTLV-I induces CD83 expression on T cells via Tax1 -mediated NF-κB activation, which may promote HTLV-I infection in vivo.

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.

Activation of mTOR by human T-cell leukemia virus type 1 Tax is important for the transformation of mouse T cells to interleukin-2-independent growth

Human T-cell leukemia virus type 1 (HTLV-1) is a causative agent of adult T-cell leukemia, and it immortalizes and transforms human T cells in both an interleukin (IL)-2-dependent and -independent manner. HTLV-1 encodes Tax, which plays crucial roles in HTLV-1-mediated immortalization and transformation of human T cells. A previous study showed that Tax can transform a mouse T-cell line, CTLL-2, from having IL-2-dependent growth to IL-2-independent growth. Given that the Akt/mTOR pathway is essential for IL-2-induced cell growth in T cells, we examined whether the Akt/mTOR pathway is involved in Tax-induced transformation to IL-2-independent growth. The stable and transient expression of Tax in CTLL-2 induced the phosphorylation of p70S6 kinase and ribosomal protein S6, downstream targets of the mTOR kinase, whereas that of Akt was only minimally induced. Studies with Tax mutants indicated that the activation of mTOR by Tax was correlated with the transformation of CTLL-2 cells to IL-2-independent growth. Rapamycin, an inhibitor of mTOR kinase, reduced the growth of Tax-transformed CTLL-2 cells. Moreover, the transduction of a constitutively active form of Akt in the CTLL-2 cells also induced IL-2-independent growth. Like CTLL-2/Tax, constitutive phosphorylation of p70S6 kinase was detected in the absence of IL-2 in all of the HTLV-1-infected human T-cell lines. These results suggest that Tax activates the mTOR pathway in T cells, and that this activation plays a crucial role in the growth of HTLV-1-infected T cells when a limited amount of IL-2 is available.

Cell surface markers in HTLV-1 pathogenesis

The phenotype of HTLV-1-transformed CD4⁺ T lymphocytes largely depends on defined viral effector molecules such as the viral oncoprotein Tax. In this review, we exemplify the expression pattern of characteristic lineage markers, costimulatory receptors and ligands of the tumor necrosis factor superfamily, cytokine receptors, and adhesion molecules on HTLV-1-transformed cells. These molecules may provide survival signals for the transformed cells. Expression of characteristic surface markers might therefore contribute to persistence of HTLV-1-transformed lymphocytes and to the development of HTLV-1-associated disease.

Distinct signal codes generate dendritic cell functional plasticity

Our adaptive immune system induces distinct responses to different pathogens because of the functional plasticity of dendritic cells (DCs); however, how DCs program unique responses remains unclear. Here, we found that the cytokine thymic stromal lymphopoietin (TSLP) potently transduced a unique T helper type 2 (T(H)2)-inducing compound signal in DCs. Whereas activation of nuclear factor kappaB (predominantly p50) drove DCs to produce OX40L to induce T(H)2 differentiation, the activation of signal transducer and activator of transcription 6 (STAT6) triggered DCs to secrete chemokines necessary for the recruitment of T(H)2 cells. In addition, TSLP signaling limited the activation of STAT4 and interferon regulatory factor 8 (IRF-8), which are essential factors for the production of the T(H)1-polarizing cytokine interleukin-12 (IL-12). By contrast, Toll-like receptor ligands and CD40 ligand did not activate STAT6 in myeloid DCs, but instead increased the abundance of STAT4 and IRF-8 to induce T(H)1 responses through the production of IL-12. Therefore, we propose that the functional plasticity of DCs relies on elaborate signal codes that are generated by different stimuli.

HTLV-1 tax-induced NF-kappaB activation is synergistically enhanced by 12-O-tetradecanoylphorbol-13-acetate: mechanism and implications for Tax oncogenicity

Nuclear factor kappa B (NF-kappaB) factors regulate a wide range of physiological and oncogenic processes. Normally, these factors are transiently activated by specific external signals which induce their dissociation from inhibitors of kappaB (IkappaB) and subsequent translocation to the nucleus where p65 links to the cyclic adenosine monophosphate response element binding protein (CBP)-p300 and P/CAF coactivators that are essential for its transcriptional activity. The pathogenic potential of human T-cell leukemia virus type 1 (HTLV-1) Tax protein is partly ascribed to its capacity to constitutively activate NF-kappaB factors because constitutive activity of these factors play an important role in the pathophysiology of adult T-cell leukemia (ATL) and tropical spastic paraparesis-HTLV-1 associated myelopathy (TSP-HAM). In assessing the possibility of modulating Tax pathogenic potential by external factors, we focused here on 12-O -tetradecanoylphorbol-13-acetate (TPA) which is a potent protein kinase C (PKC) activator. There are conflicting reports regarding the effect of TPA and PKC on NF-kappaB. Therefore, we reassessed this issue and also investigated their influence on Tax-mediated activation of these factors. We found that TPA promoted NF-kappaB nuclear translocation and the DNA binding of p65 dimers through PKC activation. However, both TPA and ectopically expressed PKC had only a marginal effect on the transcriptional competence of these dimers, indicating that the DNA binding of such dimers is insufficient by itself for gene activation. Notably, however, both TPA and the ectopic PKC displayed strong synergistic enhancement of the Tax-induced activation of the NF-kappaB transcriptional function. In contrast, TPA and the ectopic PKC only slightly elevated the low activation of the NF-kappaB transcriptional capacity by cytoplasmic Tax mutants, indicating that the nuclear translocation of Tax was essential for this synergism. Subsequent experiments suggested that TPA contributed to this synergism by increasing the pool of free p65 which Tax could link to CBP and elevate, thereby, the amount of a p65-Tax-CBP ternary complex that could bind to NF-kappaB-responsive promoters and stimulate their expression. Finally, we demonstrated that this synergism operated also in HTLV-1-infected human T-cells. Earlier reports have shown a close linkage of pathological PKC-activating conditions (e.g., infectious and inflammatory diseases) to certain malignancies. On this ground, the present study suggests that such conditions may enhance the risk for ATL and TSP-HAM in HTLV-1 carriers by increasing the Tax-induced NF-kappaB activation.

Strong induction of 4-1BB, a growth and survival promoting costimulatory receptor, in HTLV-1-infected cultured and patients' T cells by the viral Tax oncoprotein

Human T-cell leukemia virus type 1 (HTLV-1), the cause of adult T-cell leukemia, stimulates the growth of infected T cells in cultures and in nonleukemic patients. In the latter, HTLV-1 is found in long-term persisting T-cell clones. The persistence of normal T cells is controlled by the growth-stimulating and antiapoptotic functions of costimulatory receptors, while the growth-stimulating HTLV-1 functions are mediated by the viral oncoprotein Tax. Here we analyzed the impact of Tax on costimulatory receptors in T cells with repressible Tax and found that among these receptors 4-1BB (TNFRSF9/CD137/ILA) was induced most strongly. Up-regulated 4-1BB expression was a consistent feature of all HTLV-1-infected cell lines, whether patient-derived or in vitro transformed. Tax was sufficient to induce the expression of the endogenous 4-1BB gene in uninfected T cells, and it strongly activated (45-fold) the 4-1BB promoter via a single NF-kappaB site. The ligand of 4-1BB was also found on transformed T-cell lines, opening up the possibility of autostimulation. Moreover, 4-1BB expression in patients' lymphocytes ex vivo correlated with Tax expression, strongly suggesting Tax-mediated 4-1BB activation in vivo. Thus, 4-1BB up-regulation by Tax could contribute to growth, survival, and clonal expansion of the infected cells during persistence and disease.

Molecular aspects of HTLV-I infection and adult T-cell leukaemia/lymphoma

Human T-cell lymphotropic virus-I (HTLV-I) is the cause of adult T-cell leukaemia/lymphoma. Various viral proteins, especially, but not exclusively, Tax have been implicated in oncogenesis, mostly through in vitro studies. Tax transactivates a large and apparently ever expanding list of human genes through transcriptional factors. Elucidating not only the pathways but also the timing of action of HTLV proteins is important for understanding the pathogenesis and development of new treatments.

Human T-cell leukemia virus type-I Tax induces expression of interleukin-6 receptor (IL-6R): Shedding of soluble IL-6R and activation of STAT3 signaling

Human T-cell leukemia virus type-I (HTLV-I) encodes for the viral protein Tax, which is known to significantly disrupt transcriptional control of cytokines, cytokine receptors and other immuno-modulatory proteins in T cells. Specific dysregulation of these factors can alter the course and pathogenesis of infection. Soluble interleukin-6 receptor (sIL-6R) was shown to circulate at elevated levels in HTLV-I-infected patients, and high expressions of IL-6R and sIL-6R by HTLV-I-infected T cells were clinically and experimentally associated with Tax activity. To examine roles of Tax in expression of the IL-6R gene, the JPX-9 cell line was used, which is derived from Jurkat cell line expressing Tax cDNA. Over-expression of Tax enhanced IL-6R expression but not in Tax mutant JPX-9/M cell line. The clinical relevance of these observations was further demonstrated by ELISA using sera obtained from HTLV-I-infected patients. Our results revealed that sIL-6R levels were apparently elevated in HAM/TSP patients who were expressing Tax in their cells, while ATL patients' cells barely expressed Tax. HTLV-I-infected T-cell lines stimulated by IL-6/sIL-6R showed gp130-mediated STAT3 activity. IL-6/sIL-6R enhanced proliferation of HTLV-I-infected T cells in association with activation of STAT3. Consequently, Tax-mediated regulations of IL-6R and sIL-6R observed in HTLV-I-associated disorders may contribute to proliferation of HTLV-I-infected T cells through activation of inducible STAT3, and ultimately affect malignant growth and transformation of T cells by HTLV-I.

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.

Transcriptional regulation of parathyroid hormone-related protein promoter P3 by ETS-1 in adult T-cell leukemia/lymphoma

Parathyroid hormone-related protein (PTHrP) plays a primary role in the development of humoral hypercalcemia of malignancy seen in the majority of adult T-cell leukemia/lymphoma (ATLL) patients with human T-cell lymphotropic virus type-1 (HTLV-1) infection. HTLV-1 Tax has been shown to complex with ETS-1 and SP1 to transactivate the PTHrP P3 promoter. Previously, we established a SCID/bg mouse model of human ATL with RV-ATL cells and showed that PTHrP expression was independent of Tax. In this study, we report an inverse correlation of PTHrP with tax/rex mRNA in multiple HTLV-1-positive cell lines and RV-ATL cells. Stimulation of Jurkat T cells with PMA/ionomycin upregulated the PTHrP P3 promoter by a previously characterized Ets binding site and also induced protein/DNA complex formation identical to that observed in RV-ATL cells. Further, we provide evidence that cotransfection with Ets-1 and constitutively active Mek-1 in HTLV-1-negative transformed T cells with stimulation by PMA/ionomycin not only resulted in a robust induction of PTHrP P3 but also formed a complex with ETS-1/P3 EBS similar to that in ATLL cells. Our data demonstrate that transcriptional regulation of PTHrP in ATLL cells can be controlled by T-cell receptor signaling and the ETS and MAPK ERK pathway in a Tax-independent manner.

Potential immunogenicity of adult T cell leukemia cells in vivo

Experimental vaccines targeting human T cell leukemia virus type-I (HTLV-I) Tax have been demonstrated in a rat model of HTLV-I-induced lymphomas. However, the scarcity of HTLV-I-expression and the presence of defective HTLV-I-proviruses in adult T cell leukemia (ATL) cells have raised controversy about the therapeutic potential of HTLV-I-targeted immunotherapy in humans. We investigated the expression of HTLV-I antigens in fresh ATL cells by using both in vitro and in vivo assays. In flow cytometric analysis, we found that 3 of 5 acute-type and six of fifteen chronic-type ATL patients tested showed significant induction of HTLV-I Tax and Gag in their ATL cells in a 1-day culture. Concomitantly with HTLV-I-expression, these ATL cells expressed co-stimulatory molecules such as CD80, CD86 and OX40, and showed elevated levels of antigenicity against allogeneic T cells and HTLV-I Tax-specific cytotoxic T-lymphocytes (CTL). Representative CTL epitopes restricted by HLA-A2 or A24 were conserved in 4 of 5 acute-type ATL patients tested. Furthermore, spleen T cells from rats, which had been subcutaneously inoculated with formalin-fixed uncultured ATL cells, exhibited a strong interferon gamma-producing helper T cell responses specific for HTLV-I Tax-expressing cells. Our study indicated that ATL cells from about half the patients tested readily express HTLV-I antigens including Tax in vitro, and that ATL cells express sufficient amounts of Tax or Tax-induced antigens to evoke specific T cell responses in vivo.

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.

CD40 and OX40 ligand are increased on stimulated asthmatic airway smooth muscle

BACKGROUND

Severe, persistent asthma is characterized by airway smooth muscle hyperplasia, inflammatory cell infiltration into the smooth muscle, and increased expression of many cytokines, including IL-4, IL-13, IL-1beta, and TNF-alpha. These cytokines have the potential to alter the expression of surface receptors such as CD40 and OX40 ligand on the airway smooth muscle cell.

OBJECTIVE

To examine whether cytokines alter expression of CD40 and OX40 ligand on airway smooth muscle cells and identify any differences in response between asthmatic and nonasthmatic airway smooth muscle cells.

METHODS

We used flow cytometry and immunohistochemistry to detect CD40 and OX40 ligand on airway smooth muscle cells cultured in the presence of TNF-alpha, IL-1beta, IL-4, or IL-13. Prostaglandin E 2 levels were assessed by ELISA.

RESULTS

TNF-alpha increased expression of both CD40 and OX40 ligand on both asthmatic and nonasthmatic airway smooth muscle cells. The level of expression was significantly greater on the asthmatic cells. IL-1beta alone had no effect, but it attenuated the TNF-induced expression of both CD40 and OX40 ligand. The mechanism of inhibition was COX-dependent for CD40 and was COX-independent but cyclic AMP-dependent for OX40 ligand. IL-4 and IL-13 had no effect.

CONCLUSION

Our study has demonstrated that TNF-alpha and IL-1beta have the potential to modulate differentially the interactions between cells present in the inflamed airways of a patient with asthma and therefore to contribute to the regulation of airway inflammation and remodeling.

Homotypic cell-cell adhesion induced by human T cell leukemia virus type 1 tax protein in T cell lines

Cell-cell adhesion is involved in the processes of cell growth, activation and migration, and inflammation. T cells infected with human T cell leukemia virus type 1 (HTLV-1) exhibit a high degree of homotypic cell-cell adhesion in vitro. In this study, we investigated the involvement of the viral protein Tax in such process. Expression of Tax in an interleukin (IL)-2-dependent mouse T cell line (CTLL-2) increased homotypic cell-cell adhesion; however, less cell adhesion was induced by Tax than that observed in HTLV-1-infected T cell lines. Moreover, Tax induced cell-cell adhesion in a human T cell line, in which the expression of Tax is inducible. Microscopic examination also revealed Tax-induced morphologic changes, including rounding of CTLL-2 cells, increased cell volume, and increased nucleus size. Taken together, our results suggest that Tax induces cell-cell adhesion and morphologic changes in HTLV-1-infected cells. Tax may thus play a role in persistent HTLV-1 infection and the pathogenesis of associated disease.

Human T cell leukemia virus type-I Tax activates human macrophage inflammatory protein-3 alpha/CCL20 gene transcription via the NF-kappa B pathway

Infection by human T cell leukemia virus type (HTLV)-I is associated with several diseases, including adult T cell leukemia and HTLV-I-associated myelopathy/tropical spastic paraparesis. Leukocytes are attracted to the sites of inflammation by chemotactic factors. Macrophage inflammatory protein (MIP)-3 alpha/CCL20 is a recently isolated member of the CC subfamily of chemokines and has been proposed as a crucial factor to elicit inflammatory reactions. We now report that endogenous MIP-3 alpha mRNA levels are elevated in HTLV-I-infected T cell lines and in a human T cell line following the induced expression of the HTLV-I-encoded transactivator, Tax. Analysis of the human MIP-3 alpha promoter revealed that this gene is activated by Tax, via the activation of nuclear factor (NF)-kappa B, whose responsive element, -82-kappa B, is located at a position between -82 and -91 relative to the putative transcription start site. With an electromobility shift assay we further demonstrated that the -82-kappa B element was bound by the Tax-activated p50/p65 heterodimers of NF-kappa B. Expression of the specific receptor of MIP-3 alpha, CCR6, was also increased in HTLV-I-infected T cell lines, suggesting an autocrine and/or paracrine mechanism to establish the pathogenesis of HTLV-I-associated diseases.

Human T-cell leukemia virus type I tax protein induces the expression of anti-apoptotic gene Bcl-xL in human T-cells through nuclear factor-kappaB and c-AMP responsive element binding protein pathways

Human T-cell leukemia virus type I (HTLV-I) is the etiologic agent of adult T-cell leukemia (ATL), which is an aggressive form of human T-cell malignancy. The viral protein, Tax, immortalizes human T-cells and inhibits various types of apoptosis, and is thought to play crucial roles in the development of ATL. We have recently demonstrated that Tax induces the constitutive expression of the anti-apoptotic protein, Bcl-xL, in a mouse T-cell line. The mouse, however, is not a natural host of HTLV-I, and HTLV-I does not induce this malignancy in mice. We thus examined whether Tax also activates the expression of Bcl-xL in human T-cells. Expression of Tax in a human T-cell line, Jurkat, induced the expression of the Bcl-xL gene, but did not significantly affect the expression of the other apoptosis-related genes, Bcl-2 and Bax. Transient transfection assays showed that Tax stimulated human Bcl-xL promoter activity in Jurkat cells. Deletion of the two potential nuclear factor (NF)-kappaB binding sites in the human Bcl-xL promoter significantly decreased Tax-induced transactivation. In addition to NF-kappaB, Tax activates transcription through the c-AMP responsive element binding protein (CREB). Tax mutants segregating these two pathways showed that both the NF-kappaB and CREB pathways of Tax are required for maximum activation of a human Bcl-xL promoter, nevertheless, NF-kappaB alone was sufficient for that of a mouse Bcl-xL promoter. Northern blot analysis showed that all the human T-cell lines expressing Tax had higher levels of Bcl-xL mRNA than HTLV-I-uninfected ones. Furthermore, the sample from one patient with ATL expressed higher levels of Bcl-xL mRNA compared with levels from uninfected peripheral blood mononuclear cells. Our results suggest that Tax induces the expression of Bc-xL through the NF-kappaB and CREB pathways in HTLV-I-infected human T-cells, and then inhibits apoptosis, and such inhibition is necessary for the infected cells to advance to the leukemia in vivo.

Genome-wide expression changes induced by HTLV-1 Tax: evidence for MLK-3 mixed lineage kinase involvement in Tax-mediated NF-kappaB activation

The Tax protein of human T-lymphotropic virus type 1 (HTLV-1), an oncoprotein that transactivates viral and cellular genes, plays a key role in HTLV-1 replication and pathogenesis. We used cDNA microarrays to examine Tax-mediated transcriptional changes in the human Jurkat T-cell lines JPX-9 and JPX-M which express Tax and Tax-mutant protein, respectively, under the control of an inducible promoter. Approximately 300 of the over 2000 genes examined were differentially expressed in the presence of Tax. These genes were grouped according to their function and are discussed in the context of existing findings in the literature. There was strong agreement between our results and genes previously reported as being Tax-responsive. Genes that were differentially expressed in the presence of Tax included those related to apoptosis, the cell cycle and DNA repair, signaling factors, immune modulators, cytokines and growth factors, and adhesion molecules. Functionally, we provide evidence that one of these genes, the mixed-lineage kinase MLK-3, is involved in Tax-mediated NF-kappa-B signaling. Our current results provide additional insights into Tax-mediated signaling.

Human T-cell leukemia virus type I tax protein induces the expression of anti-apoptotic gene Bcl-xL in human T-cells through nuclear factor-kappaB and c-AMP responsive element binding protein pathways

Human T-cell leukemia virus type I (HTLV-I) is the etiologic agent of adult T-cell leukemia (ATL), which is an aggressive form of human T-cell malignancy. The viral protein, Tax, immortalizes human T-cells and inhibits various types of apoptosis, and is thought to play crucial roles in the development of ATL. We have recently demonstrated that Tax induces the constitutive expression of the anti-apoptotic protein, Bcl-xL, in a mouse T-cell line. The mouse, however, is not a natural host of HTLV-I, and HTLV-I does not induce this malignancy in mice. We thus examined whether Tax also activates the expression of Bcl-xL in human T-cells. Expression of Tax in a human T-cell line, Jurkat, induced the expression of the Bcl-xL gene, but did not significantly affect the expression of the other apoptosis-related genes, Bcl-2 and Bax. Transient transfection assays showed that Tax stimulated human Bcl-xL promoter activity in Jurkat cells. Deletion of the two potential nuclear factor (NF)-kappaB binding sites in the human Bcl-xL promoter significantly decreased Tax-induced transactivation. In addition to NF-kappaB, Tax activates transcription through the c-AMP responsive element binding protein (CREB). Tax mutants segregating these two pathways showed that both the NF-kappaB and CREB pathways of Tax are required for maximum activation of a human Bcl-xL promoter, nevertheless, NF-kappaB alone was sufficient for that of a mouse Bcl-xL promoter. Northern blot analysis showed that all the human T-cell lines expressing Tax had higher levels of Bcl-xL mRNA than HTLV-I-uninfected ones. Furthermore, the sample from one patient with ATL expressed higher levels of Bcl-xL mRNA compared with levels from uninfected peripheral blood mononuclear cells. Our results suggest that Tax induces the expression of Bc-xL through the NF-kappaB and CREB pathways in HTLV-I-infected human T-cells, and then inhibits apoptosis, and such inhibition is necessary for the infected cells to advance to the leukemia in vivo.

Expression of gp34 (OX40 ligand) and OX40 on human T cell clones

gp34, which we previously cloned, is a ligand of OX40 (CD134), a costimulatory molecule involved in T cell activation. To elucidate the role of human OX40 / OX40L interaction, we examined the expression of gp34 (OX40L) and OX40 in normal human hematopoietic cells by using flow cytometry. OX40 expression is observed on activated T cells, while OX40L is expressed in antigen-presenting cells. However, cytotoxic T lymphocyte (CTL) clones specific for Epstein-Barr virus (EBV)-transformed autologous lymphoblastic cell lines (LCLs) induced both OX40 and OX40L expression after antigen or T cell receptor (TCR) stimulation. This study suggests a possible function of OX40L / OX40, through T cell-T cell interaction, in the reactivation of memory T cells in an autocrine manner, with implications for the pathogenesis of viral infections and neoplasms.

The HTLV-I tax protein transcriptionally modulates OX40 antigen expression

OX40 is a member of the TNF receptor family, expressed on activated T cells. It is the only costimulatory T cell molecule known to be specifically up-regulated in human T cell leukemia virus type-I (HTLV-I)-producing cells. In a T cell line, OX40 surface expression was shown to be induced by HTLV-I Tax alone. To understand molecular mechanisms of OX40 gene regulation and modulation by HTLV-I Tax, we have cloned the human OX40 gene and analyzed its 5'-flanking region. By reporter gene analysis with progressive 5' deletions from nucleotides -1259 to -64, we have defined a 157-bp DNA fragment as a minimal promoter for constitutive expression. In addition, we show that in the OX40+ cell line, Co, Tax is able to further increase OX40 surface expression. Up-regulation of OX40 promoter activity by Tax requires two upstream NF-kappaB sites, which are not active in the constitutive OX40 expression. Their deletion abrogates Tax responsiveness in reporter gene analysis. The site-directed mutagenesis of each NF-kappaB site demonstrates that cooperative NF-kappaB binding is a prerequisite for Tax-directed activity as neither site alone is sufficient for a full Tax responsiveness of the OX40 promoter. Upon Tax expression, both sites bind p65 and c-Rel. These data provide new insight into the direct regulation of OX40 by Tax and add to our understanding of the possible role of the OX40/OX40 ligand system in the proliferation of HTLV-I+ T cells.

Cell type-specific E2F activation and cell cycle progression induced by the oncogene product Tax of human T-cell leukemia virus type I

The transactivator protein Tax of human T-cell leukemia virus type I plays an important role in the development of adult T-cell leukemia probably through modulation of growth regulatory molecules including p16(INK4a). The molecular mechanism of leukemogenesis induced by Tax has yet to be elucidated. We analyzed Tax function in the cell cycle using an interleukin-2 (IL-2)-dependent human T-cell line (Kit 225) that can undergo cell cycle arrest at G(0)/G(1) phase by deprivation of IL-2. Tax activated endogenous E2F activity in IL-2-starved Kit 225 cells, resulting in activation of E2F site-carrying promoters of genes involved in G(1) to S phase transition in a cell type-dependent and p16(INK4a)-independent manner. The ability of Tax mutants to activate E2F coincided with that to activate nuclear factors kappaB and AT, sole expression of which, however, did not activate E2F, suggesting involvement of another pathway in activation of E2F. Introduction of Tax by a recombinant adenovirus induced cell cycle progression to G(2)/M phase in resting Kit 225 cells accompanied by endogenous cyclin D2 gene expression. Similarly, Tax-induced cell cycle progression was seen with peripheral blood lymphocytes prestimulated with phytohemagglutinin. Analyses with Tax mutants did not allow Tax-induced cell cycle progression to be differentiated from Tax-dependent activation of E2F, suggesting that Tax induces cell cycle progression presumably through activation of E2F. Nevertheless, infection with an E2F1-expressing virus, which is sufficient for induction of S phase in serum-starved fibroblasts, was not sufficient for either E2F activation or cell cycle progression in IL-2-starved Kit 225 cells, implying differential regulation of E2F activation and cell cycle progression in T-cells that is activated by Tax.

Impairment of antigen-presenting cell function in mice lacking expression of OX40 ligand

OX40 expressed on activated T cells is known to be an important costimulatory molecule on T cell activation in vitro. However, the in vivo functional significance of the interaction between OX40 and its ligand, OX40L, is still unclear. To investigate the role of OX40L during in vivo immune responses, we generated OX40L-deficient mice and a blocking anti-OX40L monoclonal antibody, MGP34. OX40L expression was demonstrated on splenic B cells after CD40 and anti-immunoglobulin (Ig)M stimulation, while only CD40 ligation was capable of inducing OX40L on dendritic cells. OX40L-deficient and MGP34-treated mice engendered apparent suppression of the recall reaction of T cells primed with both protein antigens and alloantigens and a significant reduction in keyhole limpet hemocyanin-specific IgG production. The impaired T cell priming was also accompanied by a concomitant reduction of both T helper type 1 (Th1) and Th2 cytokines. Furthermore, antigen-presenting cells (APCs) derived from the mutant mice revealed an impaired intrinsic APC function, demonstrating the importance of OX40L in both the priming and effector phases of T cell activation. Collectively, these results provide convincing evidence that OX40L, expressed on APCs, plays a critical role in antigen-specific T cell responses in vivo.

Persistent activation of NF-kappaB by the tax transforming protein of HTLV-1: hijacking cellular IkappaB kinases

Biochemical coupling of transcription factor NF-kappaB to antigen and co-stimulatory receptors is required for the temporal control of T-cell proliferation. In contrast to its transitory activation during normal growth-signal transduction, NF-kappaB is constitutively deployed in T-cells transformed by the type 1 human T-cell leukemia virus (HTLV-1). This viral/host interaction is mediated by the HTLV-1-encoded Tax protein, which has potent oncogenic properties. As reviewed here, Tax activates NF-kappaB primarily via a pathway leading to the chronic phosphorylation and degradation of IkappaBalpha, a cytoplasmic inhibitor of NF-kappaB. To access this pathway, Tax associates stably with a cytokine-inducible IkappaB kinase (IKK), which contains both catalytic (IKKalpha and IKKbeta) and noncatalytic (IKKgamma) subunits. Unlike their transiently induced counterparts in cytokine-treated cells, Tax-associated forms of IKKalpha and IKKbeta are persistently activated in HTLV-1-infected T cells. Acquisition of the deregulated IKK phenotype is contingent on the presence of IKKgamma, which functions as a molecular adaptor in the assembly of pathologic Tax/IkappaB kinase complexes. These findings highlight a key mechanistic role for IKK in the Tax/NF-kappaB signaling axis and define new intracellular targets for the therapeutic control of HTLV-1-associated disease.

Expression of Human Inducible Nitric Oxide Synthase Gene in T-Cell Lines Infected With Human T-Cell Leukemia Virus Type-I and Primary Adult T-Cell Leukemia Cells

We examined the expression of messenger RNA (mRNA) of the human inducible nitric oxide synthase (hiNOS) gene in a panel of human T-cell lines. Reverse transcriptase-polymerase chain reaction showed that human T-cell leukemia virus type-I (HTLV-I)–infected T-cell lines (MT-1, SLB-1, and C5/MJ) expressed mRNA for the hiNOS, but TL-Om1 or uninfected Jurkat, H9, and CCRF-CEM did not. The MT-1, SLB-1, and C5/MJ cell lines are infected with HTLV-I and express the viral transactivator Tax, whereas TL-Om1 cells, although derived from adult T-cell leukemia (ATL) leukemic cells, do not express Tax. There was, thus, a correlation between Tax and hiNOS mRNA expression. The transcriptional regulatory region of the hiNOS gene was activated by Tax in Jurkat, in which endogenous hiNOS is induced by Tax. Deletion analysis showed that the region of hiNOS encompassing nucleotides −159 to −111 contained the minimum Tax-responsive elements. Mutations in the NF-κB element at position −115 and −106 bp in the hiNOS promoter were still activated by Tax, and a Tax mutant defective for activation of the NF-κB pathway retained the ability to activate the hiNOS promoter. In addition, overexpression of the dominant-negative mutants of IκB and I κBβ failed to reduce Tax-induced activation of hiNOS gene. Furthermore, hiNOS mRNA was detected in leukemic cells from ATL patients. Our results show that the hiNOS promoter contains a minimum Tax-responsive element located between nucleotides −159 and −111, and imply that the expression of the hiNOS gene is involved in the pathogenesis of HTLV-I–associated diseases.

Gene expression profiles in HTLV-I-immortalized T cells: deregulated expression of genes involved in apoptosis regulation

Human T-cell leukemia virus type I (HTLV-I) is the etiologic agent of adult T-cell leukemia, an acute and often fatal T-cell malignancy. A key step in HTLV-I-induced leukemigenesis is induction of abnormal T-cell growth and survival. Unlike antigen-stimulated T cells, which cease proliferation after a finite number of cell division, HTLV-I-infected T cells proliferate indefinitely (immortalized), thus facilitating occurrence of secondary genetic changes leading to malignant transformation. To explore the molecular basis of HTLV-I-induced abnormal T-cell survival, we compared the gene expression profiles of normal and HTLV-I-immortalized T cells using 'gene array'. These studies revealed a strikingly altered expression pattern of a large number of genes along with HTLV-I-mediated T-cell immortalization. Interestingly, many of these deregulated genes are involved in the control of programmed cell death or apoptosis. These findings indicate that disruption of the cellular apoptosis-regulatory network may play a role in the HTLV-I-mediated oncogenesis.