Viral transformation of human T lymphocytes

Transactivation of human osteopontin promoter by human T-cell leukemia virus type 1-encoded Tax protein

Osteopontin (OPN) is a cytokine that contributes substantially to the growth and metastasis in a wide spectrum of malignancies. We report here that OPN gene is transactivated by Tax protein of human T-cell leukemia virus type 1 (HTLV-1). Northern blot showed enhanced OPN gene expression in cells stably expressing Tax. Co-expression of Tax increased the reporter gene expression directed by OPN promoter. Tax-induced OPN activation was abrogated by treatment with LY294002 (PI3K inhibitor) or co-transfection with AKT siRNA, suggesting PI3K/AKT pathway is involved in Tax-mediated transactivation. Reporter assay with deletion mutants showed that the 5'-partial sequence between -765 and -660 of the OPN promoter is the region responsive to Tax, and further, disrupting the AP-1 site within this region abolished the OPN induction by Tax, indicating that Tax activation of OPN promoter is likely mediated by AP-1 site. This study suggests that OPN is one of the downstream mediators of aberrantly activated PI3K/AKT signaling by Tax, which may partially contribute to HTLV-1-associated leukemogenesis.

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

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

In vivo and in vitro treatment of HTLV-1 and HTLV-2 infected cells with arsenic trioxide and interferon-alpha

Adult T-cell leukemia/lymphoma (ATLL) is a malignant lymphoproliferation of mature activated T-cells, mostly CD4, which develops after a long period of latency following Human T cell Lymphotropic virus Type 1 infection. It is characterized by the clonal integration of one or more HTLV-1 proviruses in the tumor cells. There are 4 major subtypes of ATLL: a smoldering type, a chronic type, a lymphoma type and a leukemic/acute type. The survival rate of ATLL patients, especially those who develop the acute leukemic or lymphomas forms, is very poor and such a tumor remains one of the most severe lymphoproliferations. Treatment of ATLL patients using conventional chemotherapy has very limited benefit, since HTLV-1 transformed cells are resistant to most apoptosis-inducing agents. Recently, antiretroviral therapy using the combination of zidovudine (AZT) and interferon alpha (IFN-alpha) has been shown to induce a high complete remission rate and to prolong the survival of ATLL patients. Based on the current physiopathology, other drugs such as arsenic trioxide combined to IFN-alpha have also been demonstrated to synergize in vitro for inducing apoptosis in HTLV-1 infected T cells. Such drugs have now been used in vivo for treating ATLL patients. Both in vitro and in vivo data will be discussed.

Utilization of the CBP but not the p300 co-activator by human T-lymphotropic virus type-2 Tax for p53 inhibition

We previously reported, both in transfected cells and in human T-cell leukemia virus type-2 subtype B infected cells, that the viral transactivator Tax-2B protein could inhibit p53 functions. We have now investigated the mechanism through which Tax-2B represses p53 using GFPTax-2B fusion proteins. We present evidence that Tax-2B inhibition of p53 function is not linked to CREB/ATF activation, but is uniquely correlated with the interaction of CREB binding protein (CBP), but not p300, with the C-terminus of Tax-2B. Wild type, but not a Tax-2B-M47 mutant, inhibits p53 function in adherent cells. We demonstrate that both Tax-2B and Tax-2B-M47 can bind p300, while Tax-2B-M47 is impaired for CBP binding. Importantly, transfection of increasing amounts of CBP but not p300 or p300/CBP-associated factor (P/CAF) could rescue p53 transcriptional activity in the presence of Tax-2B in nonlymphocytic cells. In lymphoid cells, Tax-2B mediated inhibition of p53 is correlated with the NF-kappaB pathway activation and could be prevented by the overexpression of an IkappaBalpha mutant. Given the similarities between the functional domains of CBP and p300, these results are intriguing and suggest that Tax-2B must bind the CR2 domain of CBP, but not that of p300 in order to repress p53.

Survivin an important determinant for prognosis in adult T-cell leukemia: a novel biomarker in practical hemato-oncology

Survivin is a 16.5-kDa protein that belongs to the inhibitor of apoptosis (IAP) family. It is expressed at high levels in the G2/M phase and is rapidly down-regulated after cell-cycle arrest. It was suggested that survivin plays a pivotal role in linking cell death and cell proliferation. Although present during fetal development, survivin disappears in terminally differentiated adult tissues. Its expression is aberrantly enhanced in transformed cell lines, and in all the most common human cancers. Adult T-cell leukemia (ATL), which is abundant with Fas (Apo-1/CD95), has the characteristic feature of high tumor burden, suggesting that ATL cells probably prolong their lives as a result of escape from apoptosis. Survivin is prominently and consistently expressed in all cases of ATL and ATL cell lines. Its mRNA expression levels among the subtypes of ATL and ATL cell lines are characteristic and informative, low in chronic type, low to high in acute type and extremely high in ATL cell lines. In addition, when the survivin mRNA expression is higher, the survival of the patient is shorter. Its overexpression may account for a growth advantage in vivo and subsequently the malignant behavior of ATL. So quantification of survivin mRNA is important for clinical laboratory examinations. Among all of the current clinical tests for survivin mRNA quantification, the real time PCR is desirable. Despite some technological problems of standardization, quantification of survivin mRNA was shown to be a biological marker for clinical stages or minimal residual disease (MRD).

Human T-cell lymphotropic virus oncoprotein Tax represses TGF-beta 1 signaling in human T cells via c-Jun activation: a potential mechanism of HTLV-I leukemogenesis

Human T-cell leukemia virus I is the etiologic agent of adult T-cell leukemia (ATL), an aggressive T-cell malignancy. The viral oncoprotein Tax, through the activation of nuclear factorkappaB (NF-kappaB), CCAAT-enhancer binding protein (CREB), and activated protein-1 (AP-1) pathways, is a transcriptional regulator of critical genes for T-cell homeostasis. In ATL cells, activated AP-1 complexes induce the production of transforming growth factor beta1 (TGF-beta1). TGF-beta1 is an inhibitor of T-cell proliferation and cytotoxicity. Here we show that, in contrast to normal peripheral T cells, ATL cells are resistant to TGF-beta1-induced growth inhibition. The retroviral transduction of the Tax protein in peripheral T cells resulted in the loss of TGF-beta1 sensitivity. Transient transfection of Tax in HepG2 cells specifically inhibited Smad/TGF-beta1 signaling in a dose-dependent manner. In the presence of Tax transfection, increasing amounts of Smad3 restored TGF-beta1 signaling. Tax mutants unable to activate NF-kappaB or CREB pathways were also able to repress Smad3 transcriptional activity. Next we have demonstrated that Tax inhibits TGF-beta1 signaling by reducing the Smad3 DNA binding activity. However, Tax did not decrease the expression and the nuclear translocation of Smad3 nor did it interact physically with Smad3. Rather, Tax induced c-Jun N-terminal kinase (JNK) activity and c-Jun phosphorylation, leading to the formation of Smad3/c-Jun complexes. Whereas c-Jun alone abrogates Smad3 DNA binding, cotransfection of Tax and of a dominant-negative form of JNK or a c-Jun antisense-restored Smad3 DNA binding activity and TGF-beta1 responsiveness. In ATL and in normal T cells transduced by Tax, c-Jun was constitutively phosphorylated. Thus, we describe a new function of Tax, as a repressor of TGF-beta1 signaling through JNK/c-Jun constitutive activation, which may play a critical role in ATL leukemogenesis.

Physical interaction of human T-cell leukemia virus type 1 Tax with cyclin-dependent kinase 4 stimulates the phosphorylation of retinoblastoma protein

The Tax oncoprotein of human T-cell leukemia virus type 1 (HTLV-1) induces leukemia in transgenic mice and permanent T-cell growth in vitro. In transformed lymphocytes, it acts as an essential growth factor. Tax stimulates the cell cycle in the G(1) phase by activating the cyclin-dependent kinase (CDK) CDK4 and CDK6 holoenzyme complexes. Here we show that Tax directly interacts with CDK4. This binding to CDK4 was specific, since Tax did not bind to either CDK2 or CDK1. The interaction with CDK4/cyclin D complexes was observed in vitro, in transfected fibroblasts, in HTLV-1-infected T cells, and in adult T-cell leukemia-derived cultures. Binding studies with several point and deletion mutants indicated that the N terminus of Tax mediates the interaction with CDK4. The Tax/CDK complex represented an active holoenzyme which capably phosphorylates the Rb protein in vitro and is resistant to repression by the inhibitor p21(CIP). Binding-deficient Tax mutants failed to activate CDK4, indicating that direct association with Tax is required for enhanced kinase activity. Tax also increased the association of CDK4 with its positive cyclin regulatory subunit. Thus, protein-protein contact between Tax and the components of the cyclin D/CDK complexes provides a further mechanistic explanation for the mitogenic and immortalizing effects of this HTLV-1 oncoprotein.

Arsenic trioxide induces apoptosis in human T-cell leukemia virus type 1- and type 2-infected cells by a caspase-3-dependent mechanism involving Bcl-2 cleavage

Treatment of patients with adult T-cell leukemia-lymphoma (ATLL) using conventional chemotherapy has limited benefit because human T-cell leukemia virus type 1 (HTLV-1) cells are resistant to most apoptosis-inducing agents. The recent report that arsenic trioxide induces apoptosis in HTLV-1-transformed cells prompted investigation of the mechanism of action of this drug in HTLV-1 and HTLV-2 interleukin-2-independent T cells and in HTLV-1-immortalized cells or in ex vivo ATLL samples. Fluorescence-activated cell sorter analysis, fluorescence microscopy, and measures of mitochondrial membrane potential (Delta Psi m) demonstrated that arsenic trioxide alone was sufficient to induce programmed cell death in all HTLV-1 and -2 cells tested and in ATLL patient samples. I kappa B-alpha phosphorylation strongly decreased, and NF-kappa B translocation to the nucleus was abrogated. Expression of the antiapoptotic protein Bcl-X(L), whose promoter is NF-kappa B dependent, was down-regulated. The collapse of Delta Psi m and the release of cytochrome c to the cytosol resulted in the activation of caspase-3, as demonstrated by the cleavage of PARP. A specific caspase-3 inhibitor (Ac-DEVD-CHO) could reverse this phenotype. The antiapoptotic factor Bcl-2 was then cleaved, converting it to a Bax-like death effector. These results demonstrated that arsenic trioxide induces apoptosis in HTLV-1- and -2-infected cells through activation of the caspase pathway.

Oncogenic retroviruses in animals and humans

Retrovirology emerged as a branch of science at the beginning of the last century. However, a deeper insight into the pathology of retroviruses and retrovirus-induced cancers could only be gained after the advent of modern biochemical and molecular biological techniques in the 1970s and 1980s. This study gives an overview of the known and well-characterised exogenous oncogenic animal retroviruses and the only human oncoretrovirus discovered thus far, HTLV-1. It briefly reviews retrovirus genetics, mechanisms of oncogenic transformation and malignant diseases caused by retroviruses.

Autocrine antiapoptotic stimulation of cultured adult T-cell leukemia cells by overexpression of the chemokine I-309

Adult T-cell leukemia (ATL) is an aggressive malignancy of CD4(+) T cells caused by the human T-cell leukemia virus type 1 (HTLV-1). The viral leukemogenesis is critically dependent on its oncoprotein Tax because the protein as well as the virus can immortalize primary human lymphocytes to permanent growth. As a transcriptional transactivator, Tax can stimulate the expression of distinct cellular genes. Alterations in the expression levels of unknown growth-relevant genes may contribute to the changed growth properties of Tax-immortalized and leukemic cells. To identify genes that are linked to Tax transformation and ATL leukemogenesis, this study systematically compared the gene expression of cultured cells from patients with acute ATL with that of stimulated peripheral blood T lymphocytes. Several overexpressed RNAs that encode signal transduction functions were identified. These include a dual-specific protein phosphatase (PAC1), an interferon-inducible factor (ISG15), a basic helix-loop-helix transcription factor (DEC-1), and the secreted antiapoptotic chemokine I-309. The ATL cell culture supernatants contained an antiapoptotic activity that could be specifically inhibited by antibodies directed against I-309. Inhibition of I-309 receptor (CCR8) signaling by pertussis toxin increased the apoptosis rate of ATL cell cultures in the presence and absence of external apoptotic stimuli. Both the I-309--specific antiapoptotic activity and the proapoptotic effect of inhibitors of I-309 signaling suggest the existence of an antiapoptotic autocrine loop in ATL cells. Thus, the overexpression of this chemokine may inhibit apoptosis in ATL cells and could substantially contribute to their growth. (Blood. 2001;98:1150-1159)

A human T-cell leukemia virus Tax variant incapable of activating NF-kappaB retains its immortalizing potential for primary T-lymphocytes

The human T-cell leukemia virus type 1 (HTLV-1) transactivator (Tax) has been shown to interfere with regulated cellular proliferation. Many studies have focused on the ability of Tax to transform rodent fibroblasts; however, none has defined the molecular requirements for Tax transformation of human lymphoid cells. We show here that tax induces permanent growth of human primary T-lymphocytes by using a transformation/immortalization defective rhadinovirus vector. The cells phenotypically resemble HTLV-immortalized lymphocytes and contain episomally persisting recombinant rhadinoviral sequences, which stably express functional Tax protein. As Tax can activate major cellular signal transducing pathways including NF-kappaB and cAMP-responsive element binding protein (CREB), we asked for the relevance of these routes in the immortalization of human lymphocytes. By using Tax mutants that either activate exclusively CREB/activating transcription factor or are defective in activating this signaling pathway, we delineated that Tax can induce immortalization of primary human T-lymphocytes through a mechanism independent of NF-kappaB activation.

Stimulation of cyclin-dependent kinase activity and G1- to S-phase transition in human lymphocytes by the human T-cell leukemia/lymphotropic virus type 1 Tax protein

The human T-cell leukemia/lymphotropic virus type 1 (HTLV-1) induces a malignant lymphocytic disease. The HTLV-1 transactivator protein, Tax, is believed to be crucial for the development of the disease since it is transforming in vitro and induces tumors in transgenic animals. Although the transcriptional modulation of viral and cellular gene expression by Tax has been analyzed thoroughly, it has remained unclear how the Tax functions act on the cell cycle of primary T cells. To investigate the mechanism of Tax-mediated T-cell stimulation, we transduced primary human cord blood T cells with a conditional, tetracycline repressor-based tax expression system. Permanent Tax expression results in an abnormal proliferation of T cells which closely resemble HTLV-1-infected lymphocytes. Suppression of Tax synthesis stopped lymphocyte growth and caused cell cycle arrest in the G1 phase. Upon reinduction of tax expression, the arrested cells entered the S phase. This showed that Tax has mitogenic activity, which is required for stimulating the G1- to S-phase transition of immortalized lymphocytes. In mammalian cells, the G1-phase progression is controlled by the serial activation of several cyclin-dependent kinases (Cdks), starting with Cdk4 and Cdk6. In the presence of Tax, both Cdk4 and Cdk6 were activated. The suppression of Tax synthesis, however, resulted in a significant reduction of the Cdk4 and Cdk6 activities but did not influence the expression of Cdk4, Cdk6, or cognate D-type cyclin proteins. These data suggest that Tax induces Cdk4 and Cdk6 activity in primary human T lymphocytes; this Cdk activation is likely to account for the mitogenic Tax effect and for the abnormal T-cell proliferation of HTLV-1-infected lymphocytes.