Transcriptional activation of human TR3/nur77 gene expression by human T-lymphotropic virus type I Tax protein through two AP-1-like elements

Ginsenoside 20(S)-Rh2 Induces Apoptosis and Differentiation of Acute Myeloid Leukemia Cells: Role of Orphan Nuclear Receptor Nur77

Ginsenoside 20(S)-Rh2 has been shown to induce apoptosis and differentiation of acute myeloid leukemia (AML) cells. However, the underlying molecular mechanisms are not fully understood. In our study, 20(S)-Rh2 induced the expression of orphan nuclear receptor Nur77 and death receptor proteins Fas, FasL, DR5, and TRAIL, as well as the cleavage of caspase 8 and caspase 3 in HL-60 cells. Importantly, shNur77 attenuated 20(S)-Rh2-induced apoptosis and Fas and DR5 expression. Meanwhile, 20(S)-Rh2 promoted Nur77 translocation from the nucleus to mitochondria and enhanced the interaction between Nur77 and Bcl-2, resulting in the exposure of the BH3 domain of Bcl-2 and activation of Bax. Furthermore, 20(S)-Rh2 promoted the differentiation of HL-60 cells as evidenced by Wright-Giemsa staining, NBT reduction assay, and detection of the myeloid differentiation marker CD11b by flow cytometry. Notably, shNur77 reversed 20(S)-Rh2-mediated HL-60 differentiation. Additionally, 20(S)-Rh2 also exhibited an antileukemic effect and induced Nur77 expression in NOD/SCID mice with the injection of HL-60 cells into the tail vein. Together, our studies suggest that the Nur77-mediated signaling pathway is highly involved in 20(S)-Rh2-induced apoptosis and differentiation of AML cells.

Altered SOX9 genital tubercle enhancer region in hypospadias

Human mutations in the SOX9 gene or its regulatory region can disrupt testicular development, leading to disorders of sex development (DSDs). Our previous work involving the genomic analysis of isolated DSD patients revealed a 78kb minimal sex determining region (RevSex) far upstream of SOX9 that was duplicated in 46,XX and deleted in 46,XY DSDs. It was postulated that RevSex contains a gonadal enhancer. However, the most highly conserved sub-region within RevSex, called SR4, was neither responsive to sex determining factors in vitro nor active in the gonads of transgenic mice, suggesting that SR4 may not be functioning as a testicular enhancer. Interestingly, SR4 transgenic mice showed reporter activity in the genital tubercle, the primordium of the penis and clitoris, a previously unreported domain of Sox9 expression. SOX9 protein was detected in the genital tubercle, notably in the urethral plate epithelium, preputial glands, ventral surface ectoderm and corpus cavernosa. SR4 may therefore function as a Sox9 genital tubercle enhancer, mutations of which could possibly lead to hypospadias, a birth defect seen in the DSD patients in the RevSex study. SR4 activity and the observed SOX9 expression pattern suggest that SR4 may function as a Sox9 genital tubercle enhancer. However, conditional ablation of Sox9 in the genital tubercle using Shh-Cre/+;Sox9^flox/flox mice revealed no genital tubercle abnormalities, possibly due to compensation by similar Sox factors. To conclude, we have identified a novel regulatory enhancer driving Sox9 expression during external genitalia development.

Nkx6.1-mediated insulin secretion and β-cell proliferation is dependent on upregulation of c-Fos

Understanding the molecular pathways that enhance β-cell proliferation, survival, and insulin secretion may be useful to improve treatments for diabetes. Nkx6.1 induces proliferation through the Nr4a nuclear receptors, and improves insulin secretion and survival through the peptide hormone VGF. Here we demonstrate that Nkx6.1-mediated upregulation of Nr4a1, Nr4a3, and VGF is dependent on c-Fos expression. c-Fos overexpression results in activation of Nkx6.1 responsive genes and increases β-cell proliferation, insulin secretion, and cellular survival. c-Fos knockdown impedes Nkx6.1-mediated β-cell proliferation and insulin secretion. These data demonstrate that c-Fos is critical for Nkx6.1-mediated expansion of functional β-cell mass.

Transient neuroprotection by SRY upregulation in dopamine cells following injury in males

Emerging evidence suggest sex-specific regulation of dopamine neurons may underlie susceptibility of males to disorders such as Parkinson's disease (PD). In healthy male dopamine neurons, the Y-chromosome gene product, the sex-determining region on the Y chromosome (SRY) modulates dopamine biosynthesis and motor function. We investigated the regulation and function of SRY in a model of dopamine cell injury. Treatment with the dopaminergic toxin, 6-hydroxydopamine (6-OHDA), significantly elevated SRY mRNA expression (9-fold) in human male dopamine M17 cells. SRY up-regulation occurred via the p-quinone pathway, associated with a 3.5-fold increase in expression of GADD45γ, a DNA damage inducible factor gene and known SRY regulator. In turn, a signaling cascade involving GADD45γ/p38-MAPK/GATA activated the SRY promoter. Knockdown of SRY mRNA in 6-OHDA-treated M17 cells was deleterious, increasing levels of reactive oxygen species (ROS), pro-apoptotic marker PUMA mRNA, and cell injury (+25%, +32% and +34%, respectively). Conversely, ectopic over-expression of SRY in 6-OHDA-treated female SH-SY5Y cells was protective, decreasing ROS, PUMA, and cell injury (-40%, -46%, and -30%, respectively). However, the 6-OHDA-induced increase in SRY expression was diminished with higher concentrations of toxins or with chronic exposure to 6-OHDA. We conclude that SRY upregulation after dopamine cell injury is initially a protective response in males, but diminishes with gradual loss in dopamine cells. We speculate that dysregulation of SRY may contribute the susceptibility of males to PD.

Nur77 suppresses pulmonary artery smooth muscle cell proliferation through inhibition of the STAT3/Pim-1/NFAT pathway

The orphan nuclear receptor 4A (NR4A) family plays critical roles in the regulation of cell proliferation, differentiation, and survival in the cardiovascular system. However, the molecular mechanisms underlying the regulation of NR4A receptor expression and its role in pulmonary artery smooth muscle cell (PASMC) function remain unclear. Here, we investigated whether the NR4A family regulates PASMC proliferation, and if so, which mechanisms are involved. By using quantitative real-time RT-PCR, we showed that the orphan nuclear receptor Nur77 was the most abundant member of NR4A family expressed in rat PASMCs, as compared with the two other members, NOR-1 and Nurr1. In rat PASMCs, expression of Nur77 was robustly induced in response to several pathologic stimuli of pulmonary arterial hypertension (PAH), such as hypoxia, 5-hydroxytryptamine (5-HT), platelet-derived growth factor, and endothelin-1. Importantly, Nur77 was also significantly increased in lungs of rats with monocrotaline-induced PAH. Furthermore, we demonstrated that 5-HT markedly up-regulated Nur77 expression through the mitogen-activated protein kinases/extracellular signal-regulated kinase 1/2 pathway. Overexpression of Nur77 inhibited 5-HT-induced PASMC proliferation, as well as the expression of cyclin D1 and proliferating cell nuclear antigen. Mechanistically, we demonstrated that Nur77 specifically interacts with signal transducer and activator of transcription 3, thus inhibiting its phosphorylation and expression of its target genes, such as Pim-1, nuclear factor of activated T cells c2, and survivin in PASMCs. These results indicate that Nur77 is a novel negative-feedback regulator of PASMC proliferation through inhibition of the signal transducer and activator of transcription 3/Pim-1/nuclear factor of activated T cells axis. Modulation of Nur77 activity may potentially represent a novel therapeutic strategy for the treatment of PAH.

Nuclear receptor 4 group A member 1 determines hepatitis C virus entry efficiency through the regulation of cellular receptor and apolipoprotein E expression

Orphan nuclear receptor subfamily 4 group A member 1 (NR4A1) is a transcription factor stimulated by many factors and plays pivotal roles in metabolism, proliferation and apoptosis. In this study, the expression of NR4A1 in Huh7.5.1 cells was significantly upregulated by hepatitis C virus (HCV) infection. The silencing of NR4A1 inhibited the entry of HCV and reduced the specific infectivity of secreted HCV particles but had only minor or no effect on the genome replication and translation, virion assembly and virus release steps of the virus life cycle. Further experiments demonstrated that the silencing of NR4A1 affected virus entry through pan-downregulation of the expression of HCV receptors scavenger receptor BI, occludin, claudin-1 and epidermal growth factor receptor but not CD81. The reduced specific infectivity of HCV in the knockdown cells was due to decreased apolipoprotein E (ApoE) expression. These results explain the delayed spread of HCV in NR4A1 knockdown Huh7.5.1 cells. Thus, NR4A1 plays a role in HCV replication through regulating the expression of HCV receptors and ApoE, and facilitates HCV entry and spread.

An unhealthy relationship: viral manipulation of the nuclear receptor superfamily

The nuclear receptor (NR) superfamily is a diverse group of over 50 proteins whose function is to regulate the transcription of a vast array of cellular genes. These proteins are able to tune transcription over an extremely dynamic range due to the fact that they may act as either transcriptional activators or repressors depending on promoter context and ligand status. Due to these unique properties, diverse families of viruses have evolved strategies to exploit NRs in order to regulate expression of their own genes and to optimize the cellular milieu to facilitate the viral lifecycle. While the specific NRs targeted by these viruses vary, the strategies used to target them are common. This is accomplished at the cis-level by incorporation of nuclear receptor response elements into the viral genome and at the trans-level by viral proteins that target NRs directly or indirectly to modulate their function. The specific NR(s) targeted by a particular virus are likely to be reflective of the tissue tropism of the virus in question. Thus, the essential role played by NRs in the replication cycles of such diverse viruses underscores the importance of understanding their functions in the context of specific infections. This knowledge will allow appropriate considerations to be made when treating infected individuals with hormone-associated diseases and will potentially assist in the rational design of novel antiviral therapeutics.

Enrichment of Nur77 mediated by retinoic acid receptor β leads to apoptosis of human hepatocellular carcinoma cells induced by fenretinide and histone deacetylase inhibitors

The synthetic retinoid fenretinide is one of the most promising clinically tested retinoids. Previously, we have shown that fenretinide induces apoptosis of Huh7 cells, but HepG2 cells are relatively resistant to fenretinide-induced apoptosis. This study examines the interactive role of fenretinide and histone deacetylase inhibitors (HDACi) in inducing apoptosis of human hepatocellular carcinoma (HCC) cells and the underlying mechanism. Trichostatin A and scriptaid can either enhance fenretinide-induced apoptosis in the fenretinide sensitive HCC cells (Huh7 and Hep3B) or sensitize the fenretinide resistant cells (HepG2) to become sensitive to the apoptotic effect of fenretinide in a cancer cell-specific manner. The sensitivity of cells to fenretinide-induced apoptosis was not associated with reactive oxygen species production nor with antioxidant gene expression. However, the level of retinoic acid receptor β (RARβ) and Nur77 (NR4A1) was important for inducing apoptosis. Upon fenretinide and HDACi treatment, the expression of RARβ and Nur77 were induced and colocalized in the cytosol. The induction of Nur77 protein level, but not the messenger RNA level, was RARβ-dependent. In addition, RARβ interacted with Nur77. Nur77 was essential for fenretinide-induced and HDACi-induced apoptosis of Huh7 cells. Induction of the expression, the interaction, and the nuclear export of RARβ and Nur77 mediate fenretinide-induced and HDACi-induced apoptosis.

CONCLUSION

Our findings suggest that targeting Nur77 and RARβ simultaneously provides an effective way to induce HCC cell death.

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.

Adenovirus-induced alterations in host cell gene expression prior to the onset of viral gene expression

In this report, we have studied gene expression profiles in human primary lung fibroblasts (IMR-90) during the very early phase of an adenovirus infection. Eight out of twelve genes with known functions encoded transcription factors linked to two major cellular processes; inhibition of cell growth (ATF3, ATF4, KLF4, KLF6 and ELK3) and immune response (NR4A1 and CEBPB), indicating that the earliest consequences of an adenovirus infection are growth arrest and induction of an immune response. A time course analysis showed that the induction of these immediate-early response genes was transient and suppressed after the onset of the adenovirus early gene expression.

MSKs are required for the transcription of the nuclear orphan receptors Nur77, Nurr1 and Nor1 downstream of MAPK signalling

MSK (mitogen- and stress-activated protein kinase) 1 and MSK2 are kinases activated downstream of either the ERK (extracellular-signal-regulated kinase) 1/2 or p38 MAPK (mitogen-activated protein kinase) pathways in vivo and are required for the phosphorylation of CREB (cAMP response element-binding protein) and histone H3. Here we show that the MSKs are involved in regulating the transcription of the immediate early gene Nur77. Stimulation of mouse embryonic fibroblasts with PMA, EGF (epidermal growth factor), TNF (tumour necrosis factor) or anisomycin resulted in induction of the Nur77 mRNA. The induction of Nur77 by TNF and anisomycin was abolished in MSK1/2 double-knockout cells, whereas induction was significantly reduced in response to PMA or EGF. The MSK responsive elements were mapped to two AP (activator protein)-1-like elements in the Nur77 promoter. The induction of Nur77 was also blocked by A-CREB, suggesting that MSKs control Nur77 transcription by phosphorylating CREB bound to the two AP-1-like elements. Consistent with the decrease in Nur77 mRNA levels in the MSK1/2-knockout cells, it was also found that MSKs were required for the induction of Nur77 protein by PMA and TNF. MSKs were also found to be required for the transcription of two genes related to Nur77, Nurr1 and Nor1, which were also transcribed in a CREB- or ATF1 (activating transcription factor-1)-dependent manner. Downstream of anisomycin signalling, a second ERK-dependent pathway, independent of MSK and CREB, was also required for the transcription of Nurr1 and Nor1.

Deregulation of cell-signaling pathways in HTLV-1 infection

Human T-lymphotropic virus type 1 (HTLV-1) infection is associated with the clonal expansion and transformation of mature T lymphocytes. While the mechanisms involved are incompletely understood the viral regulatory protein Tax plays a central role in these processes. Recent studies employing genomic and proteomic approaches have demonstrated the marked complexity of gene deregulation associated with Tax expression and confirmed the remarkable pleiotropism of this protein as evidenced by the numerous Tax-cellular protein interactions in infected cells. In this review, we summarize the role of Tax in the deregulation of selected cellular-signaling pathways. Specifically, this has focused on the influence and interaction of Tax with the AP-1 and NF-AT transcription factors, PDZ domain-containing proteins, Rho-GTPases, and the Janus kinase/signal transducer and activator of transcription and transforming growth factor-beta-signaling pathways. In addition to identifying the deregulation of events within these pathways, attempts have been made to highlight differences between HTLV-1 and -2, which may relate to differences in their pathogenic properties.

Interleukin-13 overexpression by tax transactivation: a potential autocrine stimulus in human T-cell leukemia virus-infected lymphocytes

The human T-cell leukemia virus type 1 (HTLV-1) Tax oncoprotein induces growth transformation and is critical for the pathogenesis of the HTLV-1-induced adult T-cell leukemia (ATL). It stimulates the cell cycle and transactivates cellular genes. Here we show that the expression of interleukin-13 (IL-13) is upregulated as a consequence of Tax in HTLV-1-transformed T cells and ATL-derived cultures. IL-13 exerts proliferative and antiapoptotic functions and is linked to leukemogenesis, since it stimulates Hodgkin lymphoma cells by an autocrine mechanism. Overexpression of IL-13 RNA and protein was confirmed in HTLV-1-positive and Tax-transformed cells. Induction of endogenous IL-13 levels in tax-transfected Jurkat cells and in conditional Tax-expressing transformed T lymphocytes suggested that Tax can replace signals required for IL-13 synthesis. For functional analysis, the IL-13 promoter and deletion variants were cloned into luciferase reporter plasmids. Experiments with transfected human T lymphocytes revealed a 16-fold stimulation of the IL-13 promoter by Tax. Experiments with Tax mutants indicated that none of the classical transactivation pathways (SRF, CREB, and NF-kappaB) is sufficient for the transactivation; at least two different Tax functions are required for full transactivation. The IL-13 promoter is stimulated via two elements; one is a NF-AT binding P element, and the other is a putative AP-1 site. The following observations suggest that IL-13 may stimulate HTLV-1-transformed cells by an autocrine mechanism: (i) the HTLV-1-transformed cells express the IL-13 receptor on their surface, and (ii) STAT6, a downstream effector of IL-13 signaling, is constitutively activated. Thus, in summary, Tax, by transactivating the promoter, induces IL-13 overexpression that possibly leads to an autocrine stimulation of HTLV-1-infected cells.

Nur77 activated by hypoxia-inducible factor-1alpha overproduces proopiomelanocortin in von Hippel-Lindau-mutated renal cell carcinoma

Mutation in the von Hippel-Lindau (VHL) protein associated with renal cell carcinoma causes hypoxia-inducible factor (HIF) to stabilize and consequently to induce various HIF-targeting proteins. In this study, we found that proopiomelanocortin (POMC), an adrenocorticotropic hormone precursor, is up-regulated constitutively in VHL-mutated renal cell carcinoma. A critical transcription factor responsible for POMC overproduction was identified as Nur77, a member of the orphan steroid receptor superfamily. Little is known about how VHL mutation leads to activation of Nur77. We report that Nur77 is directly regulated by HIF. We show that HIF-1alpha, but not HIF-2alpha, binds to a putative HIF responsive element in the Nur77 promoter, activating the expression of Nur77. Mutation or deletion of the HIF binding site in the Nur77 promoter abrogates activation of a luciferase reporter gene under the control of Nur77 promoter by HIF-1alpha. The treatment of Nur77 antisense oligonucleotide reduces POMC transcription under hypoxic conditions. We confirmed that Nur77 and POMC are up-regulated in VHL-mutated renal cell carcinoma. In this study, we provide the first molecular evidence that Nur77 activated by HIF under hypoxic conditions regulates production of the peptide hormone precursor POMC.

Identification of transcription factor binding sites upstream of human genes regulated by the phosphatidylinositol 3-kinase and MEK/ERK signaling pathways

We have taken an integrated approach in which expression profiling has been combined with the use of small molecule inhibitors and computational analysis of transcription factor binding sites to characterize regulatory sequences of genes that are targets of specific signaling pathways in growth factor-stimulated human cells. T98G cells were stimulated with platelet-derived growth factor (PDGF) and analyzed by DNA microarrays, which identified 74 immediate-early gene transcripts. Cells were then treated with inhibitors to identify subsets of genes that are targets of the phosphatidylinositol 3-kinase (PI3K) and MEK/ERK signaling pathways. Four groups of PDGF-induced genes were defined: independent of PI3K and MEK/ERK signaling, dependent on PI3K signaling, dependent on MEK/ERK signaling, and dependent on both pathways. The upstream regions of all genes in the four groups were scanned using TRANSFAC for putative cis-elements as compared with a background set of non-induced genes. Binding sites for 18 computationally predicted transcription factors were over-represented in the four groups of co-expressed genes compared with the background sequences (p < 0.01). Many of the cis-elements identified were conserved in orthologous mouse genes, and many of the predicted elements and their cognate transcription factors were consistent with previous experimental data. In addition, chromatin immunoprecipitation assays experimentally verified nine predicted SRF binding sites in T98G cells, including a previously unknown SRF site upstream of DUSP5. These results indicate that groups of human genes regulated by discrete intracellular signaling pathways share common cis-regulatory elements.

Transcriptional activation of immediate-early gene ETR101 by human T-cell leukaemia virus type I Tax

Human T-cell leukaemia virus type I (HTLV-I) Tax regulates viral and cellular gene expression through interactions with multiple cellular transcription pathways. This study describes the finding of immediate-early gene ETR101 expression in HTLV-I-infected cells and its regulation by Tax. ETR101 was persistently expressed in HTLV-I-infected cells but not in HTLV-I uninfected cells. Expression of ETR101 was dependent upon Tax expression in the inducible Tax-expressing cell line JPX-9 and also in Jurkat cells transiently transfected with Tax-expressing vectors. Tax transactivated the ETR101 gene promoter in a transient transfection assay. A series of deletion and mutation analyses of the ETR101 gene promoter indicated that a 35 bp region immediately upstream of the TATA-box sequence, which contains a consensus cAMP response element (CRE) and a G+C-rich sequence, is the critical responsive element for Tax activation. Site-directed mutagenesis analysis of the 35 bp region suggested that both the consensus CRE motif and its upstream G+C-rich sequence were critical for Tax transactivation. Electrophoretic mobility shift analysis (EMSA) using the 35 bp sequence as probe showed the formation of a specific protein-DNA complex in HTLV-I-infected cell lines. EMSA with specific antibodies confirmed that the CREB transcription factor was responsible for formation of this specific protein-DNA complex. These results suggested that Tax directly transactivated ETR101 gene expression, mainly through a CRE sequence via the CREB transcription pathway.

The HBZ factor of human T-cell leukemia virus type I dimerizes with transcription factors JunB and c-Jun and modulates their transcriptional activity

The human T-cell leukemia virus type I (HTLV-I)-encoded Tax protein activates transcription from the viral promoter via association with the cellular basic leucine zipper factor cAMP-response element-binding protein-2. Tax is also able to induce cellular transformation of T lymphocytes probably by modulating transcriptional activity of cellular factors, including nuclear factor-kappaB, E2F, activator protein-1 (AP-1), and p53. Recently, we characterized in HTLV-I-infected cells the presence of a novel viral protein, HBZ, encoded by the complementary strand of the HTLV-I RNA genome (Gaudray, G., Gachon, F., Basbous, J., Biard-Piechaczyk, M., Devaux, C., and Mesnard, J.-M. (2002) J. Virol. 76, 12813-12822). HBZ is a nuclear basic leucine zipper protein that down-regulates Tax-dependent viral transcription by inhibiting the binding of cAMP-response element-binding protein-2 to the HTLV-I promoter. In searching for other cellular targets of HBZ, we identified two members of the Jun family, JunB and c-Jun. Co-immunoprecipitation and cellular colocalization confirmed that HBZ interacts in vivo with JunB and c-Jun. When transiently introduced into CEM cells with a reporter gene containing the AP-1 site from the collagenase promoter, HBZ suppressed transactivation by c-Jun. On the other hand, the combination of HBZ with Jun-B had higher transcriptional activity than JunB alone. Consistent with the structure of its basic domain, we demonstrate that HBZ decreases the DNA-binding activity of c-Jun and JunB. Last, we show that c-Jun is no longer capable of activating the basal expression of the HTLV-I promoter in the presence of HBZ in vivo. Our results support the hypothesis that HBZ could be a negative modulator of the Tax effect by controlling Tax expression at the transcriptional level and by attenuating activation of AP-1 by Tax.

Orphan nuclear receptor Nur77 is involved in caspase-independent macrophage cell death

Activation-induced cell death in macrophages has been observed, but the mechanism remains largely unknown. Activation-induced cell death in macrophages can be independent from caspases, and the death of activated macrophages can even be triggered by the pan-caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone (zVAD). Here, we show that this type of macrophage death can occur in the septic mouse model and that toll-like receptor (TLR)-2 or TLR4 signaling is required in this process. We conclude that Nur77 is involved in the macrophage death because Nur77 expression correlates with cell death, and cell death is reduced significantly in Nur77-deficient macrophages. The extracellular signal-regulated kinase pathway, which is downstream of TLR2 or TLR4, and myocyte-specific enhancer binding factor 2 (MEF2) transcription factor activity, which is up-regulated by zVAD, are required for Nur77 induction and macrophage death. Reporter gene analysis suggests that Nap, Ets, Rce, and Sp1 sites in the Nur77 promoter are regulated by TLR4 signaling and that MEF2 sites in the Nur77 promoter are regulated by zVAD treatment. MEF2 transcription factors are constitutively expressed and degraded in macrophages, and zVAD increases MEF2 transcription factor activity by preventing the proteolytic cleavage and degradation of MEF2 proteins. This paper delineates the dual signaling pathways that are required for Nur77 induction in macrophages and demonstrates a role of Nur77 in caspase-independent cell death.

Functional activities of the human T-cell leukemia virus type I Tax oncoprotein: cellular signaling through NF-kappa B

Human T-cell leukemia virus type I (HTLV-I) is the etiological agent for adult T-cell leukemia (ATL), as well as for tropical spastic paraparesis (TSP) and HTLV-I associate myelopathy (HAM). A biological understanding of the involvement of HTLV-I and in ATL has focused significantly on the workings of the virally-encoded 40 kDa phospho-oncoprotein, Tax. Tax is a transcriptional activator. Its ability to modulate the expression and function of many cellular genes has been reasoned to be a major contributory mechanism explaining HTLV-I-mediated transformation of cells. In activating cellular gene expression, Tax impinges upon several cellular signal-transduction pathways, including those for CREB/ATF and NF-kappa B. In this paper, we review aspects of Tax's transcriptional potential with particular focus on recent evidence linking Tax to IKK (I kappa B-kinase)-complex and MAP3Ks (mitogen-activated protein kinase kinase kinases).