A mutant form of the tax protein of bovine leukemia virus (BLV), with enhanced transactivation activity, increases expression and propagation of BLV in vitro but not in vivo

In a previous study, we identified an interesting mutant form of the Tax protein of bovine leukemia virus (BLV), designated D247G. This mutant protein strongly transactivated the long terminal repeat of BLV and was also able to transactivate the cellular proto-oncogene c-fos. This finding suggested that BLV that encode the mutant protein might propagate and induce lymphoma more efficiently than wild-type BLV. To characterize the effects of the strong transactivation activity of the mutant Tax protein, we constructed an infectious molecular clone of BLV that encoded D247G and examined the replication and propagation of the virus in vitro and in vivo. Cultured cells were transfected with the wild-type and mutant BLV, and then levels of viral proteins and particles and the propagation of viruses were compared. As expected, in vitro, mutant BLV produced more viral proteins and particles and was transmitted very effectively. We injected the wild-type and mutant BLV into sheep, which are easily infected with BLV, and monitored the proportion of BLV-positive cells in the blood and the expression of BLV RNA for 28 weeks. By contrast to the results of our analyses in vitro, we found no significant difference in the viral load or the expression of viral RNA between sheep inoculated with wild-type or mutant BLV. Our observations indicate that the mutant D247G Tax protein does not enhance the expansion of BLV and that there might be a dominant mechanism for regulation of the expression of BLV 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.

Differential requirements for activation of integrated and transiently transfected human T-cell leukemia virus type 1 long terminal repeat

Adult T-cell leukemia (ATL) cells contain integrated human T-cell leukemia virus type 1 (HTLV-1) proviruses. Although the exact sequence of events leading to the development of ATL remains incompletely resolved, expression of the integrated HTLV-1 long terminal repeat (LTR) is likely required at some point during the process of T-cell transformation. While much has been learned about the regulated expression of transiently transfected LTR reporter plasmids, an analysis of factors required for expression of chromosomally integrated HTLV-1 LTR has not been done. Here, we have constructed CHOK1 and HeLa cells that contain an integrated HTLV-1 LTR-luciferase gene. Using these cells, we have compared the requirements for activation of transiently transfected versus stably integrated HTLV-1 LTR. We observed different requirements for CREB, p300, and P/CAF in the expression of transiently transfected versus stably integrated HTLV-1 LTR. Furthermore, with dominant-negative mutants of CREB, p300, and P/CAF, we found that activation of integrated HTLV-1 LTR by an ambient stress signal, UV-C, proceeds through a path mechanistically distinct from that used by viral oncoprotein, Tax. Our findings point to additional complexities in the regulated expression of HTLV-1 proviruses compared with those hitherto revealed through transfection studies.

Stimulation of IKK-gamma oligomerization by the human T-cell leukemia virus oncoprotein Tax

Human T-cell leukemia virus type 1 oncoprotein Tax activates NF-kappaB through direct binding to IKK-gamma, the regulatory component of the IkappaB kinase complex. Mechanisms by which IKK-gamma adapts the Tax signal to the IkappaB kinase are poorly understood. Here we demonstrate that IKK-gamma forms homodimer and homotrimer both in vitro and in yeast or mammalian cells through a C-terminal domain comprising amino acids 251-419. In contrast, Tax protein targets a central region of IKK-gamma, which consists of amino acids 201-250. Interestingly, Tax stimulates the oligomerization of IKK-gamma, likely through direct binding. Taken together, our findings suggest a new model of Tax activation of NF-kappaB, in which Tax interacts with IKK-gamma to stimulate its oligomerization.

Regulation of IFN regulatory factor 4 expression in human T cell leukemia virus-I-transformed T cells

IFN regulatory factor (IRF)-4 is a lymphoid/myeloid-restricted member of the IRF transcription factor family that plays an essential role in the homeostasis and function of mature lymphocytes. IRF-4 expression is tightly regulated in resting primary T cells and is transiently induced at the mRNA and protein levels after activation by Ag-mimetic stimuli such as TCR cross-linking or treatment with phorbol ester and calcium ionophore (PMA/ionomycin). However, IRF-4 is constitutively upregulated in human T cell leukemia virus type I (HTLV-I) infected T cells as a direct gene target for the HTLV-I Tax oncoprotein. In this study we demonstrate that chronic IRF-4 expression in HTLV-I-infected T lymphocytes is associated with a leukemic phenotype, and we examine the mechanisms by which continuous production of IRF-4 is achieved in HTLV-I-transformed T cells. IRF-4 expression in HTLV-1-infected cells is driven through activation of the NF-kappaB and NF-AT pathways, resulting in the binding of p50, p65, and c-Rel to the kappaB1 element and p50, c-Rel, and NF-ATp to the CD28RE element within the -617 to -209 region of the IRF-4 promoter. Furthermore, mutation of either the kappaB1 or CD28RE sites blocks Tax-mediated transactivation of the human IRF-4 promoter in T cells. These experiments constitute the first detailed analysis of human IRF-4 transcriptional regulation within the context of HTLV-I infection and transformation of CD4(+) T lymphocytes.

Human T-cell lymphotropic virus type 1 tax inhibits transforming growth factor-beta signaling by blocking the association of Smad proteins with Smad-binding element

The human T-cell lymphotropic virus type 1 (HTLV-1) oncoprotein Tax is implicated in various clinical manifestations associated with infection by HTLV-1, including an aggressive and fatal T-cell malignancy. Because many human HTLV-1-infected T-cell lines are resistant to the growth inhibitory activity of transforming growth factor beta (TGF-beta), we examined the possibility that the HTVL-1-Tax oncoprotein regulates TGF-beta signaling. We show that Tax significantly decreases transcriptional activity and growth inhibition in response to TGF-beta. Tax inhibits TGF-beta-induced plasminogen activator inhibitor-1 expression and Smad2 phosphorylation. Competitive interaction studies show that Tax inhibits TGF-beta signaling, in part, by disrupting the interaction of the Smads with the transcriptional co-activator p300. Tax directly interacts with Smad2, Smad3, and Smad4; the Smad MH2 domain binds to Tax. Furthermore, Tax inhibits Smad3.Smad4 complex formation and its DNA binding. These results suggest that suppression of Smad-mediated signaling by Tax may contribute to HTLV-1-associated leukemogenesis.

Activation of HTLV-I transcription in the presence of Tax is independent of the acetylation of CREB-2 (ATF-4)

We have previously demonstrated that the bZIP transcription factor CREB-2, also called ATF-4, trans-activates, in association with the viral protein Tax, the human T-cell leukemia virus type I (HTLV-I) promoter. In this study, we have examined whether CREB-2 acetylation affects transcriptional activation mediated by Tax. We present evidence that CREB-2 is acetylated in vitro and in vivo. CREB-2 is acetylated in two regions: the basic domain of the bZIP (from amino acid residue 270 to 300) and the short basic domain (from 342 to 351) located downstream from the bZIP. We also demonstrate that CREB-2 is acetylated by p300/CBP but not by p/CAF. Moreover, replacement of lysine by arginine in the basic domains decreases the trans-activating capacity of CREB-2. However, in the presence of Tax, the HTLV-I transcription remains fully activated by these CREB-2 mutants. Although we cannot totally exclude that the mutations could also affect CREB-2 structure and activity independent of acetylation, our results suggest that activation of the viral promoter in the presence of Tax is independent of the CREB-2 acetylation.

HTLV-1-mediated immunopathological CNS disease

HAM/TSP is a chronic inflammatory disease of the spinal cord. It is rather rare in HTLV-1-infected individuals. Immunogenetic factors of the HLA complex have been identified that support or prevent the development of the disease. In HAM/TSP patients a characteristic constellation of high proviral loads and increased cellular and humoral immune responses have been established. Immune dysfunction in HAM/TSP patients might be partly explained by HTLV-1 tax p40 transactivation of cellular genes in infected CD4+ T lymphocytes. The oligoclonal expansion of infected T lymphocytes, the variation of tax p40 within HTLV-1 carriers, and the regulation of proviral gene expression are possible determinants for disease development and need to be clarified in future studies.

Inhibition of the tax-dependent human T-lymphotropic virus type I replication in persistently infected cells by the fluoroquinolone derivative k-37

In the search for anti-human T-lymphotropic virus type-I (HTLV-I) compounds, we have evaluated several compounds for their inhibitory effects on HTLV-I replication in cell cultures. Among the test compounds, the fluoroquinolone derivative 7-(3,4-dehydro-4-phenyl-1-piperidinyl)-1,4-dihydro-6-fluoro-1-methyl-8- trifluoromethyl-4-oxoquinoline-3-carboxylic acid (K-37) was found to be a potent and selective inhibitor of HTLV-I replication in persistently infected cells, such as MT-2 and MT-4. When the cells were cultured in the presence of various concentrations of the compound, the 50% effective concentrations of K-37 for HTLV-I p19 antigen production were 0.44 and 0.24 microM in MT-2 and MT-4 cells, respectively. K-37 did not affect the viability and proliferation of these cells at these concentrations, and its 50% cytotoxic concentrations to MT-2 and MT-4 cells were 5.7 and 1.1 microM, respectively. The compound also showed selective inhibition of HTLV-I production in peripheral blood mononuclear cells obtained from patients with HTLV-I-associated myelopathy/tropical spastic paraparesis. Quantitative reverse transcription-polymerase chain reaction analysis revealed that K-37 selectively suppressed viral mRNA synthesis in MT-2 cells in a dose-dependent fashion. Furthermore, K-37 could inhibit the endogenous Tax-induced HTLV-I long terminal repeat (LTR)-driven reporter gene expression in MT-2 cells. Western blot analysis confirmed the reduced expression of Tax in MT-2 cells exposed to K-37. In contrast, when Tax was introduced into cells not infected with HTLV-I with a plasmid under the control of human cytomegalovirus promoter, the compound did not affect Tax-induced HTLV-I LTR-driven reporter gene expression. These results suggest that the inhibition occurred at the level of HTLV-I LTR-driven Tax expression.

HTLV-1 proviruses encoding non-functional TAX in adult T-cell leukemia

Adult T-cell leukemia (ATL) is associated with prior infection with human T-cell leukemia virus type 1 (HTLV-1). TAX, the major transactivator of HTLV-1, has been implicated in the immortalization of infected T-cells, but molecular mechanisms of in vivo malignant cell transformation induced by HTLV-1 remain unclear. To investigate the role of TAX in the monoclonal proliferation of ATL cells, we determined the nucleotide sequence of tax DNA clones obtained from 6 ATL patients and analysed the biological function of their products. We found that ATL cells from 2 of these patients possessed tax with a nonsense or frame-shift mutation resulting in the premature termination of its protein product, which was no longer functional. This strongly argued against an indispensable role of TAX for the maintenance of ATL cells in vivo. On the other hand, the frequency of nucleotide substitutions found in non-functional tax DNA clones from these patients was significantly lower than those in functional tax DNA clones from the others, suggesting a role for TAX in the genome instability of infected cells. Although mismatch repair defects in the microsatellite markers, including those in hMSH3, hMSH6, BAX, TGF-beta RII, and E2F4 genes, were infrequent, we found an increase in the number of CAG repeats of the E2F4 microsatellite marker in 1 patient. These findings indicate that while TAX may be a necessary prerequisite for malignant transformation of infected cells, it is not essential for the maintenance of ATL cells in vivo.

Human T-cell lymphotropic virus type I Tax activates lung resistance-related protein expression in leukemic clones established from an adult T-cell leukemia patient

OBJECTIVE

We examined the significance of human T-cell lymphotropic virus type I (HTLV-I) Tax protein-induced resistance to anticancer drugs and the relationship between Tax and multidrug resistance proteins.

MATERIALS AND METHODS

S1T cell, a leukemic non-Tax-producing T-cell clone established from an adult T-cell leukemia (ATL) patient, S1TcTax05 and S1TcTax10 clones, transfected with Tax stably expressing cDNA, and S1Tneo, transfected with a neomycin-resistant gene, were examined for Tax-related anticancer drug resistance. Resistance of those cells to the anticancer drugs doxorubicin, etoposide, cisplatin, and vindesine was tested with the MTT method. Expression of multidrug resistance protein mRNAs (MDR1, MRP1, cMOAT/MRP2, and LRP) was analyzed with reverse transcriptase polymerase chain reaction (RT-PCR). Doxorubicin subcellular distribution in those cells was examined by fluorescence microscopy.

RESULTS

S1TcTax05 and S1TcTax10 showed resistance to doxorubicin, etoposide, and vindesine, but not to cisplatin as compared with S1T or S1Tneo. RT-PCR demonstrated that MRP1 mRNA was expressed, but MDR1, cMOAT, and LRP mRNAs were not in S1T or S1Tneo. Marked expression of LRP mRNA was detected, but no change of MDR1, MRP1, or cMOAT mRNA expression in Tax-expressing S1TcTax05 and S1TcTax10. Fluorescence microscopy demonstrated that doxorubicin was distributed mainly in the cytoplasm of S1TcTax05 and S1TcTax10, and in the nucleus of S1T and S1Tneo.

CONCLUSIONS

These findings suggest that Tax-related drug resistance of ATL cells is due to LRP and not MDR1, as reported previously. These findings in cells derived from an ATL patient suggest a novel mechanism for drug resistance in Tax-expressing ATL cells.

Human T-lymphotropic virus type 1 oncoprotein tax promotes S-phase entry but blocks mitosis

Human T-lymphotropic virus type 1 (HTLV-1) Tax exerts pleiotropic effects on multiple cellular regulatory processes to bring about NF-kappaB activation, aberrant cell cycle progression, and cell transformation. Here we report that Tax stimulates cellular G(1)/S entry but blocks mitosis. Tax expression in naive cells transduced with a retroviral vector, pBabe-Tax, leads to a significant increase in the number of cells in the S phase, with an accompanying rise in the population of cells with a DNA content of 4N or more. In all cell types tested, including BHK-21, mouse NIH 3T3, and human diploid fibroblast WI-38, Tax causes an uncoupling of DNA synthesis from cell division, resulting in the formation of multinucleated giant cells and cells with decondensed, highly convoluted and lobulated nuclei that are reminiscent of the large lymphocytes with cleaved or cerebriform nuclei seen in HTLV-1-positive individuals. This contrasts with the Tax-transformed cell lines, PX1 (fibroblast) and MT4 (lymphocyte), which produce Tax at high levels, but without the accompanying late-stage cell cycle abnormalities. PX1 and MT4 may have been selected to harbor somatic mutations that allow a bypass of the Tax-induced block in mitosis.

Comparison of CREB- and NF-kappaB-mediated transactivation by human T lymphotropic virus type II (HTLV-II) and type I (HTLV-I) tax proteins

The function of the transactivator protein Tax from HTLV-II subtype A, subtype B, Brazilian subtype C, and African subtype D isolates was compared to that of Tax from an HTLV-I isolate. HTLV-II subtypes A, B, and C were less active in the transactivation of a NF-kappaB reporter compared to HTLV-I Tax in 293T but not Jurkat T cells. In both cell types there were no significant differences between the functions of HTLV-II B, C, and D and HTLV-I Tax proteins on either a full-length HTLV-I LTR or a 21-bp repeat reporter, suggesting that there is equivalent CREB-mediated transactivation between the viruses and these subtypes. In contrast, Tax of some but not all HTLV-II subtype A isolates, including the prototype Mo, had a greatly decreased ability to transactivate, and this could be directly correlated with a decrease in protein expression. Employment of cDNA clones encoding both Rex and Tax demonstrated that Rex was unable to rescue the expression or activity of the IIA Mo isolate. These studies demonstrate that with the exception of some HTLV-IIA subtypes there are no significant differences in Tax transactivation via the CREB and NF-kappaB pathways between the two viruses and suggest that the HTLV-II Tax may have a pathogenic potential equivalent to that of HTLV-I.

Upstream stimulatory factors binding to an E box motif in the R region of the bovine leukemia virus long terminal repeat stimulates viral gene expression

The bovine leukemia virus (BLV) promoter is located in its 5'-long terminal repeat and is composed of the U3, R, and U5 regions. BLV transcription is regulated by cis-acting elements located in the U3 region, including three 21-bp enhancers required for transactivation of the BLV promoter by the virus-encoded transactivator Tax(BLV). In addition to the U3 cis-acting elements, both the R and U5 regions contain stimulatory sequences. To date, no transcription factor-binding site has been identified in the R region. Here sequence analysis of this region revealed the presence of a potential E box motif (5'-CACGTG-3'). By competition and supershift gel shift assays, we demonstrated that the basic helix-loop-helix transcription factors USF1 and USF2 specifically interacted with this R region E box motif. Mutations abolishing upstream stimulatory factor (USF) binding caused a reproducible decrease in basal or Tax-activated BLV promoter-driven gene expression in transient transfection assays of B-lymphoid cell lines. Cotransfection experiments showed that the USF1 and USF2a transactivators were able to act through the BLV R region E box. Taken together, these results physically and functionally characterize a USF-binding site in the R region of BLV. This E box motif located downstream of the transcription start site constitutes a new positive regulatory element involved in the transcriptional activity of the BLV promoter and could play an important role in virus replication.

Human T-cell leukemia virus type 2 (HTLV-2) Tax protein transforms a rat fibroblast cell line but less efficiently than HTLV-1 Tax

Human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2 are retroviruses with similar biological properties. Whereas HTLV-1 is the causative agent of an aggressive T-cell leukemia, HTLV-2 has been associated with only a few cases of lymphoproliferative disorders. Tax1 and Tax2 are the transcriptional activators of HTLV-1 and HTLV-2, respectively. Here we show that Tax2 transformed a Rat-1 fibroblast cell line to form colonies in soft agar, but the size and number of the colonies were lower than those of Tax1. Use of a chimeric Tax protein showed that the C-terminal amino acids 300 to 353 were responsible for the high transforming activity of Tax1. Activation of cellular genes by Tax1 through transcription factor NF-kappa B is reportedly essential for the transformation of Rat-1 cells. Tax2 also activated the transcription through NF-kappa B in Rat-1 cells, and such activity was equivalent to that induced by Tax1. Thus, the high transforming activity of Tax1 is mediated by mechanisms other than NF-kappa B activation. Our results showed that Tax2 has a lower transforming activity than Tax1 and suggest that the high transforming activity of Tax1 is involved in the leukemogenic property of HTLV-1.

Molecular mechanisms in lymphocyte activation and growth

Transcription factor NF-kappaB is biochemically coupled to the T cell antigen receptor (TCR) and activated transiently during an adaptive immune response. The author's laboratory is investigating the signal-dependent regulation of NF-kappaB, its downstream gene targets, and its function in lymphocyte biology. Our studies have revealed novel enzymatic checkpoints in the NF-kappaB signaling pathway and constitutive repressors of NF-kappaB that might be clinically applicable for therapeutic control of the immune system. We have also found that the Tax transforming protein encoded by human T cell leukemia virus type 1 (HTLV1) binds to and persistently activates an inducible protein kinase in the TCR/NF-kappaB axis. This viral/host interaction appears to trigger the inappropriate expression of NF-kappaB and the development of HTLV1-associated disease.

Cell cycle dysregulation by HTLV-I: role of the tax oncoprotein

HTLV-I is a human retrovirus which is the etiological agent for adult T-cell leukemia. The virus encodes a 40 kDa oncoprotein, Tax, which has no cellular counterpart. Findings from several laboratories over the past decade have shown that over-expression of the Tax oncoprotein is wholly sufficient to transform animal cells. Emerging evidence supports that Tax transforms cells through dysregulation of several cell cycle checkpoints. Here, we review extant data on how Tax targets cyclins, inhibitors of cyclin dependent kinase, as well as cellular sentries for DNA-damage.

Constitutive expression of the cyclooxygenase-2 gene in T-cell lines infected with human T cell leukemia virus type I

Cyclooxygenase-2 (COX-2), an inducible enzyme that catalyzes the formation of prostaglandins and other eicosanoids from arachidonic acid, is constitutively expressed in several human carcinomas. COX-2 expression, however, has not been extensively studied in leukemia. Human T cell leukemia virus type I (HTLV-I) is the etiologic agent of adult T cell leukemia, an aggressive form of human T cell malignancy. We studied COX-2 mRNA expression in various human T-cell lines. Northern blot analysis revealed that COX-2 mRNA steady-state levels were high in 4 of 7 T-cell lines infected with HTLV-I. COX-2 mRNA, however, was not expressed in any of 3 HTLV-I-negative T-cell lines. We also confirmed COX-2 expression in 6 of 7 HTLV-I-positive T-cell lines by reverse transcription-PCR. HTLV-I Tax is known to increase the expression of cellular genes and thus we assayed Tax for its ability to increase transcription from the COX-2 promoter. Although Tax increased transcription of the COX-2 promoter in a T-cell line, Tax expression did not induce COX-2 mRNA expression, indicating that Tax alone is not sufficient for significant accumulation of COX-2 mRNA, which probably requires an additional viral protein. To evaluate the potential role of COX-2 in T cells, the HTLV-I-infected T-cell lines were treated with NS398, a selective COX-2 inhibitor. NS398 treatment inhibited proliferation and induced apoptosis of HTLV-I-infected T-cell lines and downregulated Bcl-2 and Bcl-x(L) mRNA expression, followed by chromosomal DNA fragmentation. Our data suggest that COX-2 is expressed selectively in T-cell lines infected with HTLV-I and that this gene may play a role in cell survival.

Roles of CCAAT/enhancer-binding protein in transcriptional regulation of the human IL-5 gene

Interleukin-5 (IL-5) plays a critical role in the pathogenesis of eosinophil-associated allergic disorders, such as asthma. IL-5 may also play a major role in the development of eosinophilia-associated lymphoproliferative disorders caused by human T lymphotropic virus type I (HTLV-I). In this study, we have investigated the control mechanisms for IL-5 production and found that ectopic expression of NF-IL6 (C/EBPbeta) increases endogenous IL-5 mRNA expression. The IL-5 promoter contains four C/EBP consensus sequences. We show here that one of the C/EBP site at - 235 promoter region binds to NF-IL6 protein with high affinity and interacts with NF-IL6 and NF-IL6beta (C/EBPdelta) in Jurkat T cells. Mutations within the C/EBP sequence reduced the promoter activity in response to T cell activation by more than 50 %. In addition, we show that in vivo inducible expression of Tax protein in Jurkat T cells stably transfected with Tax further increased ionomycin plus phorbol ester stimulated IL-5 promoter activity. The effect of Tax on IL-5 promoter activity was abolished when the C/EBP site was mutated. Thus, the C/EBP site may be also involved in HTLV-I Tax-mediated up-regulation of IL-5 gene expression. Our data suggest that C/EBP proteins may regulate IL-5 gene expression in response to different stimulation signals.

NF-kappaB signaling pathway governs TRAIL gene expression and human T-cell leukemia virus-I Tax-induced T-cell death

The Tax oncoprotein encoded by human T-cell leukemia virus induces both T-cell activation and apoptosis. The mechanism by which Tax induces apoptosis has remained unclear. Using genetically manipulated T-cell lines, we demonstrate that Tax-induced T-cell death is dependent on NF-kappaB signaling. Tax fails to induce apoptosis in T cells lacking IkappaB kinase gamma (IKKgamma), an essential component of the NF-kappaB signaling pathway. This defect was rescued when the mutant cells were reconstituted with exogenous IKKgamma. We further demonstrate that the Tax-induced T-cell death is mediated by TNF (tumor necrosis factor)-related apoptosis-inducing ligand (TRAIL), because this event can be effectively inhibited by a TRAIL-blocking antibody. Consistent with this functional aspect, Tax stimulates the expression of TRAIL mRNA. Finally, we provide genetic evidence demonstrating that the NF-kappaB signaling pathway is essential for TRAIL gene induction by both Tax and T-cell activation signals. These studies reveal a novel function of the NF-kappaB signaling pathway and suggest a key mechanism by which Tax induces T-cell death.

The role of human T cell lymphotropic virus type I tax in the development of cutaneous T cell lymphoma

Although it has been well established that the human T cell lymphotropic virus type I (HTLV-I) causes adult T cell leukemia/lymphoma (ATLL) in regions of the world where this virus is endemic, its role in the pathogenesis of cutaneous T cell lymphoma (CTCL) in the Western world has been less well established. Most patients with CTCL are negative for antibodies to the structural proteins of HTLV-I, and thus a causative role for this virus is usually dismissed. However, the Tax sequence of HTLV-I has been found in the peripheral blood mononuclear cells of practically all patients with CTCL. Such patients express Tax mRNA and have antibodies to p40Tax, the protein encoded by this sequence. Sequence analysis of a 159-bp region of Tax extracted from CTCL cells proved to be homologous with the same region prepared from a cell line infected with prototypic HTLV-I. By in situ PCR, Tax has been demonstrated in the lymphocytes infiltrating the skin as well as in the keratinocytes of such patients. Apart from the pathophysiologic and clinical interest of these studies, these observations may have therapeutic implications. In vitro, the proliferation of HTLV-I-transformed cells can be inhibited by antisense to HTLV-I Tax. Since Tax has not been identified in the normal human genome, antisense to Tax deserves serious consideration as a treatment modality for patients whose cells have been demonstrated to harborTax.

Accelerated G(1) phase progression induced by the human T cell leukemia virus type I (HTLV-I) Tax oncoprotein

Tax, the human T cell leukemia virus type I oncoprotein, plays a crucial role in viral transformation and the development of the virally associated disease adult T cell leukemia. Because oncogenesis involves alterations in cell growth, it is important to examine the effects of Tax on cell cycle progression. Using a synchronized cell system, we have found that Tax expression accelerates G(1) phase progression and S phase entry with concomitant DNA replication. This accelerated progression is accompanied by an earlier onset of cdk2 kinase activity. In contrast to the shortening of G(1) phase, the length of S phase is unaffected by Tax expression. As a result of a more rapid cell cycle progression, cells expressing Tax exhibit faster growth kinetics and display an altered cell cycle distribution. Additionally, the decreased time allowed for growth in the presence of Tax results in a decreased cell size. Tax-associated acceleration of cell cycle progression may play a role in the ability of this viral oncoprotein to mediate cellular transformation and promote the development of human T cell leukemia virus type I-associated diseases.

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.

Transcriptional regulation by the phosphorylation-dependent factor CREB

The transcription factor CREB -- for 'cyclic AMP response element-binding protein' -- functions in glucose homeostasis, growth-factor-dependent cell survival, and has been implicated in learning and memory. CREB is phosphorylated in response to various signals, but how is specificity achieved in these signalling pathways?