Constitutive phosphorylation and turnover of I kappa B alpha in human T-cell leukemia virus type I-infected and Tax-expressing T cells

Regulation of NF-kappaB by the HTLV-1 Tax protein

The Tax protein encoded by the human T-cell leukemia virus type 1 (HTLV-1) activates viral gene expression via the ATF/CREB pathway. Tax also induces a variety of cellular genes through activation of the transcription factor NF-kappaB. The ability of Tax to activate the NF-kappaB pathway plays an essential role in HTLV-1-induced cellular transformation. This review briefly summarizes the remarkable discoveries of the past several years that have greatly advanced our knowledge on signal-mediated activation of the NF-kappaB pathway. It highlights our current understanding of how viral agents like Tax modulate cellular signaling machinery to activate the NF-kappaB pathway.

The multifaceted oncoprotein Tax: subcellular localization, posttranslational modifications, and NF-κB activation

The human T-cell lymphotropic virus type-I (HTLV-I) is the etiologic agent of adult T-cell leukemia/lymphoma (ATL) and of tropical spastic paraparesis/HTLV-I-associated myelopathy. Constitutive NF-κB activation by the viral oncoprotein Tax plays a crucial role in the induction and maintenance of cellular proliferation, transformation, and inhibition of apoptosis. In an attempt to provide a general view of the molecular mechanisms of constitutive Tax-induced NF-κB activation, we summarize in this review the recent body of literature that supports a major role for Tax posttranslational modifications, chiefly ubiquitination, and SUMOylation, in the NF-κB activity of Tax. These modifications indeed participate in the control of Tax subcellular localization and modulate its protein-protein interaction potential. Tax posttranslational modifications, which highlight the ability of HTLV-I to optimize its limited viral genome size, might represent an attractive target for the design of new therapies for ATL.

Anti-apoptotic effect of Tax: an NF-κB path or a CREB way?

The NF-κB pathway is intimately linked to the survival of mammalian cells, and its activation by Tax has consequently been considered important for human T-cell leukemia/lymphoma virus type 1 (HTLV-1)-infected cell resistance to death. Very little emphasis has been given to other mechanisms, although Tax regulates the expression and activity of several cellular genes. The finding that CREB protein is activated in HTLV-1 infected cells underlines the possibility that other mechanisms of survival may be implicated in HTLV-1 infection. Indeed, CREB activation or overexpression plays a role in normal hematopoiesis, as well as in leukemia development, and CREB is considered as a survival factor in various cell systems. A better understanding of the different molecular mechanisms used by Tax to counteract cell death will also help in the development of new therapeutic strategies for HTLV-1 associated diseases.

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

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

Treatment of adult T-cell leukemia/lymphoma: past, present, and future

Adult T-cell leukemia/lymphoma (ATLL) is a peripheral T-cell malignancy caused by human T-cell lymphotrophic virus type I. Clinical manifestations of ATLL range from smoldering to chronic, lymphoma and acute. Patients with acute and lymphoma type ATLL require therapeutic intervention. Conventional chemotherapeutic regimens used against other malignant lymphoma have been administered to ATLL patients, but the therapeutic outcomes of acute and lymphoma type ATLL remain very poor. Promising results of allogeneic stem cell transplantation (SCT) for ATLL patients have recently been reported and the treatment outcome might be improved for some ATLL patients. Besides conventional chemotherapy and SCT, interferon, zidovudine, arsenic trioxide, targeted therapy against surface molecule on ATLL cells, retinoid derivatives, and bortezomib have been administered to ATLL patients in pilot or phase I/II studies. Further studies are required to confirm the clinical benefits of these novel therapeutics. This article reviews the current status and future directions of ATLL treatment.

Human T-cell leukemia virus type 1 Tax and cellular transformation

Infection of T-cells by human T-cell leukemia virus type 1 (HTLV-1) causes a lymphoproliferative malignancy known as adult T-cell leukemia (ATL). ATL is characterized by abnormal lymphocytes, called flower cells, which have cleaved and convoluted nuclei. Tax, encoded by the HTLV-1 pX region, is a critical nonstructural protein that plays a central role in leukemogenesis; however, the mechanisms of HTLV-1 oncogenesis have not been clarified fully. In this review, we summarize current thinking on how Tax may affect ATL leukemogenesis.

Manipulation of the nuclear factor-kappaB pathway and the innate immune response by viruses

Viral and microbial constituents contain specific motifs or pathogen-associated molecular patterns (PAMPs) that are recognized by cell surface- and endosome-associated Toll-like receptors (TLRs). In addition, intracellular viral double-stranded RNA is detected by two recently characterized DExD/H box RNA helicases, RIG-I and Mda-5. Both TLR-dependent and -independent pathways engage the IkappaB kinase (IKK) complex and related kinases TBK-1 and IKKvarepsilon. Activation of the nuclear factor kappaB (NF-kappaB) and interferon regulatory factor (IRF) transcription factor pathways are essential immediate early steps of immune activation; as a result, both pathways represent prime candidates for viral interference. Many viruses have developed strategies to manipulate NF-kappaB signaling through the use of multifunctional viral proteins that target the host innate immune response pathways. This review discusses three rapidly evolving areas of research on viral pathogenesis: the recognition and signaling in response to virus infection through TLR-dependent and -independent mechanisms, the involvement of NF-kappaB in the host innate immune response and the multitude of strategies used by different viruses to short circuit the NF-kappaB pathway.

Deregulated activation of oncoprotein kinase Tpl2/Cot in HTLV-I-transformed T cells

Protein kinase Tpl2/Cot is encoded by a protooncogene that is cis-activated by retroviral insertion in murine T cell lymphomas. It has remained unclear whether this oncoprotein kinase is mutated or post-translationally activated in human cancer cells. We have shown here that Tpl2/Cot is constitutively activated in human leukemia cell lines transformed by the human T cell leukemia virus type I (HTLV-I). The kinase activity of Tpl2/Cot is normally suppressed through its physical interaction with an inhibitor, the NF-kappaB1 precursor protein p105. Interestingly, a large pool of Tpl2/Cot is liberated from p105 and exhibits constitutive kinase activity in HTLV-I-transformed T cells. In contrast to its labile property in normal cells, the pathologically activated Tpl2/Cot is remarkably stable. Further, whereas the physiological activation of Tpl2/Cot involves its long isoform, the HTLV-activated Tpl2/Cot is predominantly the short isoform. We have also shown that the HTLV-I-encoded Tax protein is able to activate Tpl2/Cot in transfected cells. Finally, Tpl2/Cot participates in the activation of NF-kappaB by Tax. These findings indicate that deregulated activation of Tpl2/Cot may occur in human cancer cells.

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.

A Novel NF-κB Pathway Involving IKKβ and p65/RelA Ser-536 Phosphorylation Results in p53 Inhibition in the Absence of NF-κB Transcriptional Activity

Nuclear factor kappaB (NF-kappaB) plays an important role in regulating cellular transformation and apoptosis. The human T-cell lymphotropic virus type I protein, Tax, which is critical for viral transformation, modulates the transcription of several cellular genes through activation of NF-kappaB. We have demonstrated previously that Tax inhibits p53 activity through the p65/RelA subunit of NF-kappaB. We now present evidence that suggests that the upstream kinase IKKbeta plays an important role in Tax-induced p53 inhibition through phosphorylation of p65/RelA at Ser-536. First, mouse embryo fibroblast (MEF) IKKbeta-/-cells did not support Tax-mediated p53 inhibition, whereas MEFs lacking IKKalpha allowed Tax inhibition of p53. Second, transfection of IKKbeta wild type (WT), but not a kinase-dead mutant, into IKKbeta-/-cells rescued p53 inhibition by Tax. Third, the IKKbeta-specific inhibitor SC-514 decreased the ability of Tax to inhibit p53. Fourth, we show that phosphorylation of p65/RelA at Ser-536 is important for Tax inhibition of p53 using MEF p65/RelA-/-cells transfected with p65/RelA WT or mutant plasmids. Moreover, Tax induced p65/RelA Ser-536 phosphorylation in WT or IKKalpha-/- cells but failed to induce the phosphorylation of p65/RelA Ser-536 in IKKbeta-/-cells, suggesting a link between IKKbeta and p65/RelA phosphorylation. Consistent with this observation, blocking IKKbeta kinase activity by SC-514 decreases the phosphorylation of p65/RelA at Ser-536 in the presence of Tax in human T-cell lymphotropic virus type I-transformed cells. Finally, the ability of Tax to inhibit p53 is distinguished from the NF-kappaB transcription activation pathway. Our work, therefore, describes a novel Tax-NF-kappaB p65/RelA pathway that functions to inhibit p53 but does not require NF-kappaB transcription activity.

Mouse serum factor(s) down-modulate the CD4 and CXCR4 molecules on human T cells conferring resistance to HIV infection in NOG mice

Human cells have developed innate immunity, exploiting several means to block virus infection, and viruses have evolved diverse strategies to resist these. We show here that the human immunodeficiency virus 1 (HIV-1) could neither progressively infect engrafted human leukemic T cells nor repress their growth in NOG mice. However, ED-40515(-) cells infected with HIV-1 before inoculation were found to significantly delay the onset of tumor growth and increased the survival period of NOG mice. ED-40515(-) tumor cells showed resistance to HIV-1 which was apparently correlated with the down-regulation of CD4 and CXCR4 molecules in NOG mice. Serum from three different mouse strains, including NOG, retained a suppressive effect on the CD4 molecule of ED-40515(-) cells in vitro. ED-40515(-) cells obtained from mice re-expressed CD4 and CXCR4 molecules upon in vitro culture and were again successfully infected with HIV-1. These findings indicate that HIV-1 may initially successfully delay or regress tumor growth in NOG mice, but eventually fails to do so because of the evolution of HIV-resistant cells due to a rapid down-modulation of CD4 and CXCR4. Our data also demonstrated that some unknown soluble factor(s) present in mouse serum was responsible for conferring resistance to HIV infection to human T cells.

Paradoxical effects of a stress signal on pro- and anti-apoptotic machinery in HTLV-1 Tax expressing cells

Adult T-cell leukemia (ATL) and HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP) are associated with Human T-cell lymphotropic virus type 1 (HTLV-1) infection. The viral transactivator, Tax is able to mediate the cell cycle progression by targeting key regulators of the cell cycle such as p21/waf1, p16/ink4a, p53, cyclins D1-3/cdk complexes, and the mitotic spindle checkpoint MAD apparatus, thereby deregulating cellular DNA damage and checkpoint control. Genome expression profiling of infected cells exemplified by the development of DNA microarrays represents a major advance in genome-wide functional analysis. Utilizing cDNA microarray analysis, we have observed an apparent opposing and paradoxical regulatory network of host cell gene expression upon the introduction of DNA damage stress signal. We find the apparent induction of cell cycle inhibitors, and pro- as well as anti-apoptotic gene expression is directly linked to whether cells are at either G1, S, or G2/M phases of the cell cycle. Specifically, a G1/S block is induced by p21/waf1 and p16/ink4a, while pro-apoptotic expression at S, and G2/M is associated with caspase activation, and anti-apoptotic gene expression is associated with up regulation of Bcl-2 family member, namely bfl-1 gene. Therefore, the microarray results indicating expression of both pro- and anti-apoptotic genes could easily be explained by the particular stage of the cell cycle. Mechanism and the functional outcome of induction for both pathways are discussed.

The contribution of NF-kappa B activity to spontaneous proliferation and resistance to apoptosis in human T-cell leukemia virus type 1 Tax-induced tumors

Human T-cell leukemia virus type I is the etiologic agent of adult T-cell leukemia/lymphoma. The Tax protein of this virus is thought to contribute to cellular transformation and tumor development. In this report, we have used a Tax transgenic mouse model of tumorigenesis to study the contribution of nuclear factor (NF)-kappa B activity to spontaneous tumor cell proliferation and resistance to apoptosis. We have demonstrated elevated expression levels of NF-kappa B--inducible cytokines, including interleukin (IL)-6, IL-10, IL-15, and interferon (IFN)-gamma, in freshly isolated primary tumors from Tax transgenic mice. Inhibitors of NF-kappa B activity, sodium salicylate and cyclopentenone prostaglandins (prostaglandin A(1) and 15-deoxy-Delta(12,14)-prostaglandin J(2)), blocked spontaneous proliferation of Tax transgenic mouse spleen cells. In addition, Tax-induced tumor cells, which are resistant to irradiation-induced apoptosis, became sensitive to apoptosis in the presence of sodium salicylate and prostaglandins. These results strongly suggest that Tax-mediated induction of NF-kappa B activity contributes to tumorigenesis in vivo. (Blood. 2001;98:1200-1208)

Hepatitis B virus X mutants derived from human hepatocellular carcinoma retain the ability to abrogate p53-induced apoptosis

Chronic hepatitis B virus (HBV) infection and the integration of its X gene (HBx) are closely associated with the development of hepatocellular carcinoma (HCC). The integrated HBx frequently is truncated or contains point mutations. Previous studies indicated that these HBx mutants have a diminished co-transactivational activity. We have compared the effects of wild-type (wt) HBx and its naturally occurring mutants derived from human HCCs on transcriptional co-transactivation, apoptosis and interactive effects with p53. We demonstrated that overexpression of mutant, but not wt HBx, is defective in transcriptional co-transactivation of the NF-kappaB-driven luciferase reporter. By using a microinjection technique, the HBx mutants were shown to have an attenuated pro-apoptotic activity. This deficiency may be attributed to multiple mutations in the co-transactivation domain of HBx, that leads to decreased stability of the translated product. However, wt or mutant HBx bind to p53 in vitro and retain their ability to block p53-mediated apoptosis in vivo, which has been implicated as its major tumor suppressor function. The abrogation of p53-mediated apoptosis by integrated HBx mutants may provide a selective clonal advantage for preneoplastic or neoplastic hepatocytes and contribute to hepatocellular carcinogenesis.

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).

Phosphoinositide-3 kinase-PKB/Akt pathway activation is involved in fibroblast Rat-1 transformation by human T-cell leukemia virus type I tax

Activated phosphoinositide 3-kinase (PI3K) and its downstream target Akt are essential for the fibroblast transformation induced by many viral products. Tax, encoded by human T-cell leukemia virus type I (HTLV-I), has been demonstrated to induce the transformation of rat fibroblast Rat-1 cell through NF-kappaB activation. By stable transfection of Rat-1 cells with expressing constructs of Tax and its mutant M47, which is defective in HTLV-I LTR transactivation, we selected their transformed clones, which have characteristics of NF-kappaB activation and colony formation beyond the cell monolayer (a malignant phenotype). However, these two characteristics in the transformed clones of Tax and M47 disappear after these cells have been treated with wortmannin, a specific inhibitor of PI3K. Further, increased activity of the PI3K/Akt is observed in the transformed clones of Tax and M47 as compared to the clones of empty vector Neo and the M148, which is defective in NF-kappaB activation and cell transformation. Increased activity of PI5K is present in the transformed clones of both Tax and M47 and in the M148 clone as compared to that in the Neo cell. It is known that the efficiency of Tax-induced cell transformation is not high; a minority of Tax-expressing clones show transformation, although the majority of Tax-expressing clones show activated NF-kappaB. A Tax-expressing, nontransformed clone after transfection with an active form of the catalytic subunit of PI3K, p110alpha, becomes transformed. Consistent with these results, a Tax highly-expressing human T-cell line MT2 exhibits both higher polyphosphoinositide turnover and higher activities of PI3K and PI5K than those of Jurkat or MT1 and HTLV-I-negative and a Tax-unexpressing cell line, respectively. These results demonstrate that the activation of the PI3K/Akt signaling pathway, excepting for the NF-kappaB, is also required for the cell transformation induced by Tax.

Role of NF-kappaB and myc proteins in apoptosis induced by hepatitis B virus HBx protein

Chronic infection with hepatitis B virus (HBV) promotes a high level of liver disease and cancer in humans. The HBV HBx gene encodes a small regulatory protein that is essential for viral replication and is suspected to play a role in viral pathogenesis. HBx stimulates cytoplasmic signal transduction pathways, moderately stimulates a number of transcription factors, including several nuclear factors, and in certain settings sensitizes cells to apoptosis by proapoptotic stimuli, including tumor necrosis factor alpha (TNF-alpha) and etopocide. Paradoxically, HBx activates members of the NF-kappaB transcription factor family, some of which are antiapoptotic in function. HBx induces expression of Myc protein family members in certain settings, and Myc can sensitize cells to killing by TNF-alpha. We therefore examined the roles of NF-kappaB, c-Myc, and TNF-alpha in apoptotic killing of cells by HBx. RelA/NF-kappaB is shown to be induced by HBx and to suppress HBx-mediated apoptosis. HBx also induces c-Rel/NF-kappaB, which can promote apoptotic cell death in some contexts or block it in others. Induction of c-Rel by HBx was found to inhibit its ability to directly mediate apoptotic killing of cells. Thus, HBx induction of NF-kappaB family members masks its ability to directly mediate apoptosis, whereas ablation of NF-kappaB reveals it. Investigation of the role of Myc protein demonstrates that overexpression of Myc is essential for acute sensitization of cells to killing by HBx plus TNF-alpha. This study therefore defines a specific set of parameters which must be met for HBx to possibly contribute to HBV pathogenesis.

Domain-specific interaction with the I kappa B kinase (IKK)regulatory subunit IKK gamma is an essential step in tax-mediated activation of IKK

The human T-cell leukemia virus type 1 Tax oncoprotein deregulates the NF-kappa B signaling pathway by persistently stimulating a key signal transducer, the I kappa B kinase (IKK). Tax physically associates with the IKK regulatory subunit, IKK gamma, although the underlying biochemical mechanism and functional significance remain unclear. We show that the Tax-IKK gamma interaction requires two homologous leucine zipper domains located within IKK gamma. These leucine zipper domains are unique for the presence of a conserved upstream region that is essential for Tax binding. Site-directed mutagenesis analysis revealed that a leucine-repeat region of Tax is important for IKK gamma binding. Interestingly, all the Tax mutants defective in IKK gamma binding failed to engage the IKK complex or stimulate IKK activity, and these functional defects can be rescued by fusing the Tax mutants to IKK gamma. These results provide mechanistic insights into how Tax specifically targets and functionally activates the cellular kinase IKK.

Mechanisms of NF-kappaB activation by the HTLV type 1 tax protein

The Tax protein encoded by the human T cell leukemia virus type I virus (HTLV-1) activates the expression of both viral genes and cellular genes involved in T lymphocyte growth and proliferation. One of the critical cellular pathways activated by Tax is NF-kappaB. NF-kappaB is normally sequestered in the cytoplasm, bound to a family of inhibitory proteins known as I-kappaB. In contrast to the transient activation of the NF-kappaB pathway seen in response to cytokines, Tax results in constitutive nuclear levels of NF-kappaB. Tax activation of the NF-kappaB pathway is mediated by its ability to enhance the phosphorylation and subsequent degradation of I-kappaB. The persistent activation of the NF-kappaB pathway by Tax is believed to be one of the major events involved in HTLV-1-mediated cellular transformation of T lymphocytes. This review summarizes data exploring the role of Tax in activating the NF-kappaB pathway and discusses our studies to determine the mechanism by which Tax activates the NF-kappaB pathway.

Somatic mutagenesis studies of NF-kappa B signaling in human T cells: evidence for an essential role of IKK gamma in NF-kappa B activation by T-cell costimulatory signals and HTLV-I Tax protein

NF-kappa B plays a pivotal role in normal T-cell activation and may also mediate human T-cell leukemia virus (HTLV)-induced T-cell transformation. Activation of NF-kappa B by both T-cell costimulatory signals and the HTLV Tax protein involves stimulation of I kappa B kinase (IKK). As a genetic approach to dissect the intermediate steps involved in NF-kappa B activation in human T cells, we performed somatic cell mutagenesis to isolate signaling-defective mutant Jurkat T-cell lines. One of the mutant cell lines was shown to have a specific blockade in the IKK signaling pathway but remained competent in the c-Jun N-terminal kinase and MAP kinase pathways. Interestingly, this mutant cell line lacks expression of IKK gamma, a non-catalytic component of the IKK complex. Expression of exogenous IKK gamma in the mutant cells restored NF-kappa B activation by both the T-cell costimulation agents and Tax. These findings provide genetic evidence for the requirement of IKK gamma in NF-kappa B signaling triggered by both T-cell costimulatory signals and HTLV-I Tax protein.

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.

IKKgamma serves as a docking subunit of the IkappaB kinase (IKK) and mediates interaction of IKK with the human T-cell leukemia virus Tax protein

The tax gene product of human T-cell leukemia virus type I induces activation of transcription factor NF-kappaB, which contributes to deregulated expression of various cellular genes. Tax expression triggers persistent phosphorylation and degradation of the NF-kappaB inhibitory proteins IkappaBalpha and IkappaBbeta, resulting in constitutive nuclear expression of NF-kappaB. Recent studies demonstrate that Tax activates the IkappaB kinase (IKK), although the underlying mechanism remains unclear. In this report, we show that Tax physically interacts with a regulatory component of the IKK complex, the NF-kappaB essential modulator or IKKgamma (NEMO/IKKgamma). This molecular interaction appears to be important for recruiting Tax to the IKK catalytic subunits, IKKalpha and IKKbeta. Expression of NEMO/IKKgamma greatly promotes binding of Tax to IKKalpha and IKKbeta and stimulates Tax-mediated IKK activation. Interestingly, a mutant form of Tax defective in IKK activation exhibited a markedly diminished level of NEMO/IKKgamma association. These findings suggest that the physical interaction of Tax with NEMO/IKKgamma may play an important role in Tax-mediated IKK activation.

Activation of the ubiquitin proteolytic system in murine acquired immunodeficiency syndrome affects IkappaBalpha turnover

Murine acquired immunodeficiency syndrome (MAIDS) is a complex immunopathology caused by a defective murine leukemia virus (LP-BM5) that mainly targets B-lymphocytes. Lymphadenophathy, splenomegaly, hypergammaglobulinemia and progressive immunodeficiency are prominent features of MAIDS. Previously, we showed that the ubiquitin proteolytic system was upregulated in infected lymph nodes [Crinelli, R., Fraternale, A., Casabianca, A. & Magnani, M. (1997) Eur. J. Biochem. 247, 91-97]. In this report, we demonstrate that increased 26S proteasome activity is responsible for accelerated turnover of the IkappaBalpha inhibitor in lymph node extracts derived from animals with MAIDS. The molecular mechanisms mediating IkappaBalpha proteolysis involved constitutive phosphorylation of IkappaBalpha at Ser32 and Ser36 and subsequent ubiquitination, suggesting persistent activation of an NF-kappaB inducing pathway. Interestingly, enhanced IkappaBalpha degradation did not result in enhanced NF-kappaB DNA binding activity, but rather in a different subunit composition. The modulation of NF-kappaB/IkappaB system may affect multiple immunoregulatory pathways and may in part explain the mechanisms leading to the profound immune dysregulation involved in MAIDS pathogenesis.

NFkappaB activation is required for interferon regulatory factor-1-mediated interferon beta induction

The interferon regulatory factor 1 (IRF-1) acts as a transcriptional inducer of the interferon beta (IFN-beta) gene and interferon-stimulated genes. Here we report that IRF-1-mediated IFN-beta induction depends on NFkappaB activity. IRF-1 by itself initiates NFkappaB activation by inducing a reduction in cellular MAD3/IkappaBalpha, an inhibitor of NFkappaB. After nuclear translocation, NFkappaB synergizes with IRF-1 on the cis-elements positive regulatory domain (PRD)II and PRDI/III to induce transcription of the IFN-beta gene. In contrast with IFN-beta transcription induced by dsRNA or virus, c-Jun/ATF-2 binding to PRDIV is not involved. Recombinant MAD3/IkappaBalpha is phosphorylated in vitro by extracts from IRF-1-expressing cells. IRF-1-dependent MAD3/IkappaBalpha degradation is not detectable in cells expressing a dominant negative mutant of the protein kinase PKR, suggesting that PKR mediates MAD3/IkappaBalpha degradation.

Binding of c-Rel to STAT5 target sequences in HTLV-I-transformed T cells

The type I human T-cell leukemia virus (HTLV-I) induces abnormal growth and subsequent transformation of T cells, which is associated with the development of an acute T-cell malignancy termed adult T-cell leukemia. A characteristic of HTLV-I-transformed T cells is the constitutive nuclear expression of NF-kappaB/Rel family of transcription factors, which appears to be essential for the growth of these transformed cells. Although NF-kappaB/Rel factors are known to induce the expression of T-cell growth factor interleukin (IL)-2, it is unclear how they participate in the IL-2-independent growth of HTLV-I-transformed cells. In this study, we show that certain NF-kappaB/Rel members, predominantly c-Rel, interact with enhancer sequences for STAT5, a key transcription factor mediating IL-2-induced T-cell proliferation. Reporter gene assays reveal that the binding of c-Rel to the STAT5 site present in the Fc gammaR1 gene leads to potent transactivation of this enhancer. Binding of c-Rel to the Fc gammaR1 STAT site also occurs in human peripheral blood T cells immortalized with HTLV-I in vitro and is correlated with enhanced levels of proliferation of these cells. These results raise the possibility that NF-kappaB/Rel may participate in the growth control of HTLV-I-transformed T cells by regulating genes driven by both kappaB and certain STAT enhancers.

Hepatitis C virus core protein enhances NF-kappaB signal pathway triggering by lymphotoxin-beta receptor ligand and tumor necrosis factor alpha

Our previous study indicated that the core protein of hepatitis C virus (HCV) can associate with tumor necrosis factor receptor (TNFR)-related lymphotoxin-beta receptor (LT-betaR) and that this protein-protein interaction plays a modulatory effect on the cytolytic activity of recombinant form LT-betaR ligand (LT-alpha1beta2) but not tumor necrosis factor alpha (TNF-alpha) in certain cell types. Since both TNF-alpha/TNFR and LT-alpha1beta2/LT-betaR are also engaged in transcriptional activator NF-kappaB activation or c-Jun N-terminal kinase (JNK) activation, the biological effects of the HCV core protein on these regards were elucidated in this study. As demonstrated by the electrophoretic mobility shift assay, the expression of HCV core protein prolonged or enhanced the TNF-alpha or LT-alpha1beta2-induced NF-kappaB DNA-binding activity in HuH-7 and HeLa cells. The presence of HCV core protein in HeLa or HuH-7 cells with or without cytokine treatment also enhanced the NF-kappaB-dependent reporter plasmid activity, and this effect was more strongly seen with HuH-7 cells than with HeLa cells. Western blot analysis suggested that this modulation of the NF-kappaB activity by the HCV core protein was in part due to elevated or prolonged nuclear retention of p50 or p65 species of NF-kappaB in core protein-producing cells with or without cytokine treatment. Furthermore, the HCV core protein enhanced or prolonged the IkappaB-beta degradation triggering by TNF-alpha or LT-alpha1beta2 both in HeLa and HuH-7 cells. In contrast to that of IkappaB-beta, the increased degradation of IkappaB-alpha occurred only in LT-alpha1beta2-treated core-producing HeLa cells and not in TNF-alpha-treated cells. Therefore, the HCV core protein plays a modulatory effect on NF-kappaB activation triggering by both cytokines, though the mechanism of NF-kappaB activation, in particular the regulation of IkappaB degradation, is rather cell line and cytokine specific. Studies also suggested that the HCV core protein had no effect on TNF-alpha-stimulated JNK activity in both HeLa and HuH-7 cells. These findings, together with our previous study, strongly suggest that among three signaling pathways triggered by the TNF-alpha-related cytokines, the HCV core protein potentiates NF-kappaB activation in most cell types, which in turn may contribute to the chronically activated, persistent state of HCV-infected cells.

Phosphorylation of the human T-cell leukemia virus type 1 transactivator tax on adjacent serine residues is critical for tax activation

The Tax transactivator protein of human T-cell leukemia virus type 1 (HTLV-1) plays a central role in the activation of viral gene expression. In addition, Tax is capable of activating the expression of specific cellular genes and is involved in the transformation of T-lymphocytes resulting in the development of adult T-cell leukemia. Tax is a phosphoprotein that colocalizes in nuclear bodies with RNA polymerase II, splicing complexes, and specific transcription factors including members of the ATF/CREB and NF-kappaB families. In this study, we identified adjacent serine residues at positions 300 and 301 in the carboxy terminus of Tax as the major sites for phosphorylation. Phosphorylation of at least one of these serine residues is required for Tax localization in nuclear bodies and for Tax-mediated activation of gene expression via both the ATF/CREB and NF-kappaB pathways. Introduction of amino acid substitutions which are phosphoserine mimetics at positions 300 and 301 restored the ability of a phosphorylation-defective Tax mutant to form nuclear bodies and to activate gene expression. These studies define sites for regulatory phosphorylation events in Tax which are critical for its ability to activate gene transcription.

Association between HTLV-1 Tax and I kappa B alpha is dependent on the I kappa B alpha phosphorylation state

Biological, molecular, and epidemiological data have demonstrated that human T cell leukemia virus type 1 (HTLV-1) encoded Tax protein plays a central role in the initiation of T cell malignancy. The 40-kDa Tax oncoprotein serves as a potent transcriptional activator that induces viral gene expression driven by the HTLV-1 long terminal repeats and also stimulates multiple cellular genes involved in T cell activation, cell cycle regulation, and gene activation. Since Tax has been shown to interact directly and indirectly with the NF-kappa B/I kappa B regulatory proteins, we examined the significance of an in vivo association between Tax and the I kappa B alpha inhibitor. Using GST affinity chromatography, Tax was shown to interact with the I kappa B alpha ankyrin repeats which are essential for interaction with the NF-kappa B/Rel proteins. In vivo, using I kappa B alpha mutants and co-immunoprecipitation, a preferential interaction between HTLV-1 Tax and N-terminally hypophosphorylated I kappa B alpha was detected. Tax also enhanced binding of I kappa B alpha to the proteasome subunit HsN3, resulting in a Tax-enhanced, constitutive degradation of wild-type and mutated forms of I kappa B alpha in the absence of phosphorylation and ubiquitination. Binding of I kappa B alpha to proteasome subunit HC9 was also observed, but this interaction occurred independently of Tax. Taken together, these results suggest a role for Tax as a viral chaperone resulting in the enhanced constitutive turnover of I kappa B alpha. The association of Tax with hypophosphorylated I kappa B alpha may prevent I kappa B alpha from binding to NF-kappa B and also target I kappa B alpha to the proteasome for degradation via a phosphorylation-independent pathway.

Human T-cell leukemia virus type 1 Tax induction of NF-kappaB involves activation of the IkappaB kinase alpha (IKKalpha) and IKKbeta cellular kinases

Tax corresponds to a 40-kDa transforming protein from the pathogenic retrovirus human T-cell leukemia virus type 1 (HTLV-1) that activates nuclear expression of the NF-kappaB/Rel family of transcription factors by an unknown mechanism. Tax expression promotes N-terminal phosphorylation and degradation of IkappaB alpha, a principal cytoplasmic inhibitor of NF-kappaB. Our studies now demonstrate that HTLV-1 Tax activates the recently identified cellular kinases IkappaB kinase alpha (IKKalpha) and IKKbeta, which normally phosphorylate IkappaB alpha on both of its N-terminal regulatory serines in response to tumor necrosis factor alpha (TNF-alpha) and interleukin-1 (IL-1) stimulation. In contrast, a mutant of Tax termed M22, which does not induce NF-kappaB, fails to activate either IKKalpha or IKKbeta. Furthermore, endogenous IKK enzymatic activity was significantly elevated in HTLV-1-infected and Tax-expressing T-cell lines. Transfection of kinase-deficient mutants of IKKalpha and IKKbeta into either human Jurkat T or 293 cells also inhibits NF-kappaB-dependent reporter gene expression induced by Tax. Similarly, a kinase-deficient mutant of NIK (NF-kappaB-inducing kinase), which represents an upstream kinase in the TNF-alpha and IL-1 signaling pathways leading to IKKalpha and IKKbeta activation, blocks Tax induction of NF-kappaB. However, plasma membrane-proximal elements in these proinflammatory cytokine pathways are apparently not involved since dominant negative mutants of the TRAF2 and TRAF6 adaptors, which effectively block signaling through the cytoplasmic tails of the TNF-alpha and IL-1 receptors, respectively, do not inhibit Tax induction of NF-kappaB. Together, these studies demonstrate that HTLV-1 Tax exploits a distal part of the proinflammatory cytokine signaling cascade leading to induction of NF-kappaB. The pathological alteration of this cytokine pathway leading to NF-kappaB activation by Tax may play a central role in HTLV-1-mediated transformation of human T cells, clinically manifested as the adult T-cell leukemia.

Regulation of gene expression by HTLV-I Tax protein

The human T-cell leukemia virus type I or HTLV-I is the causative agent of adult T-cell leukemia. A protein encoded by HTLV-I, Tax, activates viral gene expression and is essential for transforming T-lymphocytes. Tax activates HTLV-I gene expression via interactions with the ATF/CREB proteins and the coactivators CBP/p300 which assemble as a multiprotein complex on regulatory elements known as 21-bp repeats in the HTLV-I LTR. Tax can also activate expression from cellular genes including the interleukin-2 (IL-2) and the IL-2 receptor genes via increases in nuclear levels of NF-kappaB. Tax modulation of gene expression via the ATF/CREB and NF-kappaB pathways is linked to its transforming properties. This review discusses the mechanisms by which Tax regulates viral and cellular gene expression.

Molecular mechanism of tumorigenesis in mice transgenic for the human T cell leukemia virus Tax gene

The infection by human T lymphotropic virus type I is associated with adult T cell leukemia and several inflammatory degenerative disorders, including tropical spastic paraparesis. To investigate the role of the Tax protein in the development of diseases linked to human T lymphotropic virus type I infection, we generated two lines of transgenic mice carrying the tax gene under the control of the viral promoter. The expression of the transgene was low in these mice and was restricted to the central nervous system and testis. Mice from both lines developed various types of tumors, including fibrosarcomas and adenocarcinomas. Tax was expressed at a high level in fibrosarcomas and in cell lines derived from these tumors. In tumor-derived cells, the expression of Tax led to an increased degradation of IkappaB alpha and IkappaB beta and caused stable nuclear translocation of nuclear factor-kappaB. This translocation was essential for cell proliferation, as shown by expressing a nondegradable form of IkappaBbeta in these cells. Therefore, Tax-induced cell transformation in mice correlates with the degradation of IkappaB alpha and IkappaB beta and with the constitutive activation of NF-kappaB.

NF-kappaB-inducing kinase and IkappaB kinase participate in human T-cell leukemia virus I Tax-mediated NF-kappaB activation

The tax gene product of human T-cell leukemia virus I induces aberrant expression of various cellular genes, which contributes to transformation of host cells. Induction of many Tax target genes is mediated through transcription factor NF-kappaB. Here we show that Tax triggers activation of cellular protein kinases, IkappaB kinase alpha (IKKalpha) and IKKbeta, which phosphorylate the NF-kappaB inhibitory protein IkappaB alpha, resulting in its degradation and NF-kappaB activation. Constitutive IKK activation occurs in both Tax-transfected and human T-cell leukemia virus I-infected T cells. We further demonstrate that Tax-mediated NF-kappaB signaling also requires the NF-kappaB-inducing kinase (NIK). Consistently, inactive forms of either IKKs or NIK attenuate Tax-mediated NF-kappaB activation. Therefore, Tax activates NF-kappaB by targeting cellular signaling molecules, including both IKKs and NIK.

Cytoplasmic forms of human T-cell leukemia virus type 1 Tax induce NF-kappaB activation

Human T-cell leukemia virus type 1 (HTLV-1) Tax targets I-kappaB alpha and I-kappaB beta for phosphorylation, ubiquitination, and proteasome-mediated degradation, causing the nuclear translocation of NF-kappaB/Rel proteins and transcription induction of many cellular genes. The mechanism by which a nuclear protein such as Tax stimulates I-kappaB phosphorylation and degradation remains unclear. Here, we describe two cytoplasmic mutants of Tax, designated TaxDeltaN81 and TaxDeltaN109, from which the domains important for cyclic AMP response element binding factor (CREB) and serum response factor (SRF) binding and nuclear transport have been removed. These mutants were unable to trans activate from the HTLV-1 21-bp repeats or the serum response element in the c-fos promoter. In contrast, they activated NF-kappaB reporters, suggesting that activation of NF-kappaB by Tax occurs in the cytoplasm. Incorporation of the nuclear localization signal (NLS) of the simian virus 40 large T antigen into TaxDeltaN81 and TaxDeltaN109 redirected both proteins predominantly to the nucleus yet did not restore trans activation via CREB or SRF. The NLS fusion had little effect on TaxDeltaN81 but reduced NF-kappaB trans activation by TaxDeltaN109, possibly because of its proximity to the NF-kappaB-activating domain of Tax. In contrast to wild-type Tax, the cytoplasmic TaxDeltaN mutants are not cytotoxic. Stable expression of TaxDeltaN109 in HeLa cells resulted in a significant reduction in the intracellular level of I-kappaB alpha, with the constitutive presence of NF-kappaB in the nucleus and concomitant activation of the NF-kappaB enhancer. These results are suggestive of a potential application of the TaxDeltaN109-like mutants in targeting I-kappaB degradation and NF-kappaB activation. Interestingly, a Tax species with a molecular mass similar to that of TaxDeltaN109 was identified in many HTLV-1-transformed T cells, suggesting that TaxDeltaN109-like species might play a role in HTLV-1-induced leukemogenesis.

The major component of IkappaBalpha proteolysis occurs independently of the proteasome pathway in respiratory syncytial virus-infected pulmonary epithelial cells

Previously we showed that infection of human type II airway epithelial (A549) cells with purified respiratory syncytial virus (pRSV) induced interleukin-8 transcription by a mechanism involving cytokine-inducible cytoplasmic-nuclear translocation of the RelA transcription factor. In unstimulated cells, RelA is tethered in the cytoplasm by association with the IkappaB inhibitor and can be released only following IkappaB degradation. In this study, we examined the spectrum of IkappaB isoform expression and kinetics of proteolysis of the isoforms in A549 cells following pRSV infection. In contrast to the rapid and robust activation of RelA DNA binding that peaked within 15 min of treatment produced by the prototypic activator tumor necrosis factor alpha (TNF-alpha), pRSV produced a weaker increase in RelA binding that began at 3 h and did not peak until 24 h after infection. A549 cells expressed the IkappaB inhibitory subunits IkappaBalpha, IkappaBbeta, and p105; however, following either stimulus, only the IkappaBalpha and IkappaBbeta steady-state levels declined in parallel with the increase in RelA DNA-binding activity. The >120-min half-life of IkappaBalpha in control cells was shortened to 5 min in TNF-alpha-stimulated cells and to 90 min in pRSV-infected cells. Although IkappaBalpha was resynthesized within 30 min following recombinant human TNFalpha treatment due to a robust 25-fold increase of IkappaBalpha mRNA expression (the RelA:IkappaBalpha positive feedback loop), following pRSV infection, there was no reaccumulation of IkappaBalpha protein, as infected cells produced only a 3-fold increase in IkappaBalpha mRNA at 24 h, indicating the RelA:IkappaBalpha positive feedback loop was insufficient to restore control IkappaBalpha levels. IkappaBalpha proteolysis induced by TNF-alpha occurred through the 26S proteasome, as both 26S proteasome activity and IkappaBalpha proteolysis were blocked by specific inhibitors lactacystin, MG-132, and ZLLF-CHO. Although total proteasome activity in 24-h pRSV-infected lysates increased twofold, its activity was >90% inhibited by the proteasome inhibitors; surprisingly, however, IkappaBalpha proteolysis was not. We conclude that RSV infection produces IkappaBalpha proteolysis through a mechanism primarily independent of the proteasome pathway.

Differential transcriptional activation by human T-cell leukemia virus type 1 Tax mutants is mediated by distinct interactions with CREB binding protein and p300

The human T-cell leukemia virus type 1 Tax protein transforms human T lymphocytes, which can lead to the development of adult T-cell leukemia. Tax transformation is related to its ability to activate gene expression via the ATF/CREB and the NF-kappaB pathways. Transcriptional activation of these pathways is mediated by the actions of the related coactivators CREB binding protein (CBP) and p300. In this study, immunocytochemistry and confocal microscopy were used to localize CBP and p300 in cells expressing wild-type Tax or Tax mutants that are able to selectively activate gene expression from either the NF-kappaB or ATF/CREB pathway. Wild-type Tax colocalized with both CBP and p300 in nuclear bodies which also contained ATF-1 and the RelA subunit of NF-kappaB. However, a Tax mutant that selectively activates gene expression from only the ATF/CREB pathway colocalized with CBP but not p300, while a Tax mutant that selectively activates gene expression from only the NF-kappaB pathway colocalized with p300 but not CBP. In vitro and in vivo protein interaction studies indicated that the integrity of two independent domains of Tax delineated by these mutants was involved in the direct interaction of Tax with either CBP or p300. These studies are consistent with a model in which activation of either the NF-kappaB or the ATF/CREB pathway by specific Tax mutants is mediated by distinct interactions with related coactivator proteins.

Viral transactivating proteins

Many viruses utilize the cellular transcription apparatus to express their genomes, and they encode transcriptional regulatory proteins that modulate the process. Here we review the current understanding of three viral regulatory proteins. The adenovirus E1A protein acts within the nucleus to regulate transcription through its ability to bind to other proteins. The herpes simplex type 1 virus VP16 protein acts within the nucleus to control transcription by binding to DNA in conjunction with cellular proteins. The human T-cell leukemia virus Tax protein influences transcription through interactions with cellular proteins in the nucleus as well as the cytoplasm.

Constitutive expression of the HTLV-I pX and env regions in Jurkat T-cells induces differential activation of SRE, CRE and NF kappa B pathways

Human T-cell leukemia virus type I (HTLV-I) causes adult T-cell leukemia/lymphoma (ATLL). HTLV Tax, the viral transcriptional activator, can activate a variety of cellular genes. HTLV-mediated T-cell transformation, however, may involve additional viral proteins expressed from singly- as well as doubly-spliced viral mRNA. To determine the combined effect of these viral proteins on cellular gene expression in Jurkat T-cells, we derived stable transfectants that constitutively express the HTLV-I pX and env regions (J3.9). J3.9 cells show substantially increased mRNA levels of egr-1 and c-jun but no induction of either CD25 or GM-CSF by Northern blotting. This pattern corresponded to the activation of an egr-1 but not a GM-CSF promoter-driven reporter construct in transient gene expression assays. In DNA electrophoretic mobility shift assays (EMSA), nuclear extract from J3.9 cells has significantly increased binding to CRE and SRE but not nuclear factor kappa B (NF kappa B) DNA oligos, as compared to J-Neo cell extract. These results suggest that low level expression of pX and env region gene products in Jurkat T-cells stimulates persistent activation of CRE- and SRE- but not NF kappa B-induced cellular genes.

The human T-cell leukemia virus type 1 transactivator protein Tax colocalizes in unique nuclear structures with NF-kappaB proteins

The Tax protein of human T-cell leukemia virus type 1 (HTLV-1) is a potent activator of viral transcription. Tax also activates the expression of specific cellular genes involved in the control of T-lymphocyte growth via effects on cellular transcription factors, including members of the NF-kappaB/cRel family. Immunocytochemistry and electron microscopy were used to characterize the intracellular localization of Tax and identify cellular factors which are the potential targets for its transcriptional activity. These studies indicated that Tax localizes in discrete nuclear foci in T lymphocytes transformed by HTLV-1 and in cells transduced with Tax expression vectors. The Tax-containing foci are complex nuclear structures comprising a central core in which Tax colocalizes with splicing factor Sm. In addition to splicing factors Sm and SC-35, the Tax-containing nuclear structures also contain transcriptional components, including the largest subunit of RNA polymerase II and cyclin-dependent kinase CDK8. The inclusion of the two subunits of NF-kappaB, p50 and RelA, and the presence of the mRNA from a gene specifically activated by Tax through NF-kappaB binding sites suggest that these unique nuclear structures participate in Tax-mediated activation of gene expression via the NF-kappaB pathway.

HTLV-I Tax and Cytokeratin: Tax-Expressing Cells Show Morphological Changes in Keratin-Containing Cytoskeletal Networks

Human T cell leukemia virus type I (HTLV-I) has been linked to the development of an aggressive lymphoproliferative disorder (adult T cell leukemia), a chronic neurodegenerative presentation (HTLV-I-associated myelopathy/tropical spastic paraparesis) and numerous less well-defined inflammatory conditions. The viral regulatory protein Tax has been implicated in cellular transformation events leading to the onset of adult T cell leukemia. Details on the stepwise processes through which Tax induces morphological changes in cells are poorly understood. We show here that Tax can bind to a class of intermediate filaments, the cytokeratins (Ker). Tax interacts with the 1B helical coil of keratin 8, a domain critical for higher-order intermediate filament matrix formation. Expression of Tax in epithelial cells visibly altered the structural pattern of the Ker network. In a T lymphocyte cell line, induction of Tax expression resulted in increased cellular adherence/invasion of Matrigel filters. We propose that one aspect of Tax function is the induction of morphological changes in cellular cytoskeletal structures. This finding for Tax-expressing cells might be one factor contributing directly to the pathogenesis of HTLV-I disease(s). Copyright 1997 S. Karger AG, Basel

T helper cell activation and human retroviral pathogenesis

T helper (Th) cells are of central importance in regulating many critical immune effector mechanisms. The profile of cytokines produced by Th cells correlates with the type of effector cells induced during the immune response to foreign antigen. Th1 cells induce the cell-mediated immune response, while Th2 cells drive antibody production. Th cells are the preferential targets of human retroviruses. Infections with human T-cell leukemia virus (HTLV) or human immunodeficiency virus (HIV) result in the expansion of Th cells by the action of HTLV (adult T-cell leukemia) or the progressive loss of T cells by the action of HIV (AIDS). Both retrovirus infections impart a high-level activation state in the host immune cells as well as systemically. However, diverging responses to this activation state have contrasting effects on the Th-cell population. In HIV infection, Th-cell loss has been attributed to several mechanisms, including a selective elimination of cells by apoptosis. The induction of apoptosis in HIV infection is complex, with many different pathways able to induce cell death. In contrast, infection of Th cells with HTLV-1 affords the cell a protective advantage against apoptosis. This advantage may allow the cell to escape immune surveillance, providing the opportunity for the development of Th-cell cancer. In this review, we will discuss the impact of Th-cell activation and general immune activation on human retrovirus expression with a focus upon Th-cell function and the progression to disease.

A cooperative interaction of human T-cell leukemia virus type 1 Tax with the p21 cyclin-dependent kinase inhibitor activates the human immunodeficiency virus type 1 enhancer

Interactions between the Tax transactivator of human T-cell leukemia virus type 1 (HTLV-1) and a cell cycle regulatory protein have been examined. We report cooperative stimulation of human immunodeficiency virus type 1 gene expression by Tax and a regulator of cell cycle progression, the p21 cyclin-dependent kinase inhibitor (CKI). This cooperativity results from the effect of p21 on transcriptional coactivation by Tax-induced NF-kappaB. This effect was abrogated by a mutation in Tax which specifically eliminated NF-kappaB induction, was inhibitable by IkappaB-alpha, and was markedly reduced in human immunodeficiency virus reporter plasmids with mutant kappaB sites. These studies demonstrate that transcriptional activation by Tax is influenced by cell cycle regulatory proteins.

Hepatitis B virus HBx protein activates transcription factor NF-kappaB by acting on multiple cytoplasmic inhibitors of rel-related proteins

The HBx protein is a small polypeptide encoded by mammalian hepadnaviruses that is essential for viral infectivity and is thought to play a role in development of hepatocellular carcinoma during chronic hepatitis B virus infection. HBx is a transactivator that stimulates Ras signal transduction pathways in the cytoplasm and certain transcription elements in the nucleus. To better understand the activities of HBx protein and its mechanism of action, we have explored the manner by which HBx activates the transcription factor NF-kappaB during transient expression. We show that HBx induces prolonged formation, in a Ras-dependent manner, of transcriptionally active NF-kappaB DNA-binding complexes, which make up the family of Rel-related proteins, p50, p52, RelA, and c-Rel. HBx was found to activate NF-kappaB through two distinct cytoplasmic pathways by acting on both the 37-kDa IkappaBalpha inhibitor and the 105-kappaDa NF-kappaB1 precursor inhibitor protein, known as p105. HBx induces phosphorylation of IkappaBalpha, a three- to fourfold reduction in IKBalpha stability, and concomitant nuclear accumulation of NF-kappaB DNA-binding complexes, similar to that reported for human T-cell leukemia virus type 1 Tax protein. In addition, HBx mediates a striking reduction in cytoplasmic p105 NF-kappaB1 inhibitor and p50 protein levels and release of RelA protein that was sequestered by the p105 inhibitor, concomitant with nuclear accumulation of NF-kappaB complexes. HBx mediated only a slight reduction in the cytoplasmic levels of NF-kappaB2 p100 protein, an additional precursor inhibitor of NF-kappaB, which is thought to be less efficiently processed or less responsive to release of NF-kappaB. No evidence was found for HBx activation of NF-kappaB by targeting acidic sphingomyelinase- controlled pathways. Studies also suggest that stimulation of NF-kappaB by HBx does not involve activation of Ras via the neutral sphingomyelin-ceramide pathway. Thus, HBx protein is shown to activate the NF-kappaB family of Rel-related proteins by acting on two distinct NF-kappaB cytoplasmic inhibitors.

Persistent activation of NF-kappa B/Rel by human T-cell leukemia virus type 1 tax involves degradation of I kappa B beta

Activation of the eukaryotic NF-kappaB/Rel transcription factors by various cytokines and mitogens is a transient event, reflecting the fact that these inducers trigger the degradation and resynthesis of the dynamic NF-kappaB/Rel inhibitor IkappaBalpha. However, the tax gene product of the human T-cell leukemia virus type 1 (HTLV-1) is known to induce the persistent nuclear expression of various NF-kappaB/Rel factors, especially the c-Rel proto-oncoprotein, although the underlying mechanism remains unclear. In the present study, we demonstrate that Tax induces the degradation Of IkappaBbeta, another NF-kappaB/Rel cytoplasmic inhibitor that differs from IkappaBalpha in signal responses. Unlike that observed with IkappaBalpha, the degradation Of IkappaBbeta is not associated with its rapid resynthesis, apparently because of the failure of Tax to stimulate IkappaBbeta gene transcription. Thus, expression of Tax in Jurkat T cells leads to the gradual depletion of IkappaBbeta, which is correlated with the induction of c-Rel-containing kappaB binding complexes. Remarkably, in the three HTLV-1-infected T-cell lines investigated, little or no detectable amount of IkappaBbeta was found. We further demonstrate that Tax is able to override the cytoplasmic retention of c-Rel by 1kappaBbeta in transiently transfected cells. Together, these studies suggest that Tax-mediated inactivation Of IkappaBbeta may play a role in the persistent nuclear expression of c-Rel induced by HTLV-I infection.

Inactivation of IkappaBbeta by the tax protein of human T-cell leukemia virus type 1: a potential mechanism for constitutive induction of NF-kappaB

In resting T lymphocytes, the transcription factor NF-kappaB is sequestered in the cytoplasm via interactions with members of the I kappa B family of inhibitors, including IkappaBalpha and IkappaBbeta. During normal T-cell activation, IkappaBalpha is rapidly phosphorylated, ubiquitinated, and degraded by the 26S proteasome, thus permitting the release of functional NF-kappaB. In contrast to its transient pattern of nuclear induction during an immune response, NF-kappaB is constitutively activated in cells expressing the Tax transforming protein of human T-cell leukemia virus type I (HTLV-1). Recent studies indicate that HTLV-1 Tax targets IkappaBalpha to the ubiquitin-proteasome pathway. However, it remains unclear how this viral protein induces a persistent rather than transient NF-kappaB response. In this report, we provide evidence that in addition to acting on IkappaBalpha, Tax stimulates the turnover Of IkappaBbeta via a related targeting mechanism. Like IkappaBalpha, Tax-mediated breakdown of IkappaBbeta in transfected T lymphocytes is blocked either by cell-permeable proteasome inhibitors or by mutation Of IkappaBbeta at two serine residues present within its N-terminal region. Despite the dual specificity of HTLV-1 Tax for IkappaBalpha and IkappaBbeta at the protein level, Tax selectively stimulates NF-kappaB-directed transcription of the IkappaBalpha gene. Consequently, IkappaBbeta protein expression is chronically downregulated in HTLV-1-infected T lymphocytes. These findings with IkappaBbeta provide a potential mechanism for the constitutive activation of NF-kappaB in Tax-expressing cells.

Human T-cell lymphotropic virus type 1 Tax1 activation of NF-kappa B: involvement of the protein kinase C pathway

Human T-cell lymphotropic virus type 1 Tax1 induces the activation and nuclear localization of the cellular transcription factor, NF-kappa B. Treatment of cells with calphostin C, a protein kinase C (PKC) inhibitor, blocked induction of NF-kappa B DNA binding activity in human T-cell lymphotropic virus type 1-transformed C81 cells and Tax1-stimulated murine pre-B cells, suggesting that PKC was an important intermediate in the NF-kappa B induction pathway. We further demonstrate that Tax1 associates with, and activates, PKC. PKC was coimmunoprecipitated with anti- Tax1 sera from Tax1-expressing MT4 extracts and Jurkat extracts in the presence of exogenous Tax1 protein. In addition, the glutathione-S-transferase-Tax1 protein bound specifically to the alpha, delta, and eta PKC isoenzymes synthesized in rabbit reticulocyte lysates. The addition of Tax1 to in vitro kinase reaction mixtures leads to the phosphorylation of Tax1 and an 18-fold increase in the autophosphorylation of PKC. Transfection of Jurkat cells with wild-type Tax1 stimulated membrane translocation of PKC. In contrast, Tax1 mutant M22, which fails to stimulate NF-kappa B-dependent transcription, failed to stimulate membrane translocation of PKC. Tax1 did not directly increase PKC phosphorylation of I kappa B alpha. Our results are consistent with a model in which Tax1 interacts with PKC and stimulates membrane translocation and triggering of the PKC pathway. Subsequent steps in the PKC cascade likely stimulate phosphorylation of I kappa B alpha.

Interaction of HTLV-I Tax with the human proteasome: implications for NF-kappa B induction

The human T-cell leukemia virus type I (HTLV-I) has been etiologically associated with the development of the adult T-cell leukemia (ATL) as well as degenerative neurologic syndrome termed tropical spastic paraparesis (TSP). HTLV-I encodes a potent transactivator protein termed Tax that appears to play an important role in the process of T-cell immortalization. Even though the mechanisms by which Tax induces transformation are still unknown, it seems likely that the ability of Tax to alter the expression of many cellular genes plays an important part in this process. Tax does not bind directly to DNA but rather deregulates the activity of cellular transcription factors. One family of host transcription factors whose activity is altered by Tax includes NF-kappa B/Rel. These transcription factors are post-transcriptionally regulated by their assembly with a second family of inhibitory proteins termed I kappa B that serve to sequester the NF-kappa B/Rel complexes in the cytoplasm. Upon cellular activation, I kappa B alpha is phosphorylated, polyubiquitinated, and degraded in the proteasome. This proteolytic event liberates NF-kappa B, permitting its rapid translocation into the nucleus where it binds to its cognate enhancer elements. Similarly, the p105 precursor of the NF-kappa B p50 subunit is also post-translationally processed in the proteasome. The mechanisms by which Tax activates NF-kappa B remain unclear, and findings presented in the literature are often controversial. We identified a physical interaction between Tax and the HsN3 subunit of the human proteasome. This raises the intriguing possibility that physical association of the HsN3 proteasome subunit with HTLV-I Tax coupled with the independent interaction of Tax with either p100 or p65-I kappa B alpha targets these cytoplasmic NF-kappa B/Rel complexes to the proteasome for processing.