Hijacking of the O-GlcNAcZYME complex by the HTLV-1 Tax oncoprotein facilitates viral transcription.

The viral Tax oncoprotein plays a key role in both Human T-cell lymphotropic virus type 1 (HTLV-1)-replication and HTLV-1-associated pathologies, notably adult T-cell leukemia. Tax governs the transcription from the viral 5'LTR, enhancing thereby its own expression, via the recruitment of dimers of phosphorylated CREB to cAMP-response elements located within the U3 region (vCRE). In addition to phosphorylation, CREB is also the target of O-GlcNAcylation, another reversible post-translational modification involved in a wide range of diseases, including cancers. O-GlcNAcylation consists in the addition of O-linked-N-acetylglucosamine (O-GlcNAc) on Serine or Threonine residues, a process controlled by two enzymes: O-GlcNAc transferase (OGT), which transfers O-GlcNAc on proteins, and O-GlcNAcase (OGA), which removes it. In this study, we investigated the status of O-GlcNAcylation enzymes in HTLV-1-transformed T cells. We found that OGA mRNA and protein expression levels are increased in HTLV-1-transformed T cells as compared to control T cell lines while OGT expression is unchanged. However, higher OGA production coincides with a reduction in OGA specific activity, showing that HTLV-1-transformed T cells produce high level of a less active form of OGA. Introducing Tax into HEK-293T cells or Tax-negative HTLV-1-transformed TL-om1 T cells is sufficient to inhibit OGA activity and increase total O-GlcNAcylation, without any change in OGT activity. Furthermore, Tax interacts with the OGT/OGA complex and inhibits the activity of OGT-bound OGA. Pharmacological inhibition of OGA increases CREB O-GlcNAcylation as well as HTLV-1-LTR transactivation by Tax and CREB recruitment to the LTR. Moreover, overexpression of wild-type CREB but not a CREB protein mutated on a previously described O-GlcNAcylation site enhances Tax-mediated LTR transactivation. Finally, both OGT and OGA are recruited to the LTR. These findings reveal the interplay between Tax and the O-GlcNAcylation pathway and identify new key molecular actors involved in the assembly of the Tax-dependent transactivation complex.

HTLV-1 Tax protein recruitment into IKKε and TBK1 kinase complexes enhances IFN-I expression.

The Tax protein expressed by human T-cell leukemia virus type 1 (HTLV-1) plays a pivotal role in the deregulation of cellular pathways involved in the immune response, inflammation, cell survival, and cancer. Many of these effects derive from Tax multiple interactions with host factors, including the subunits of the IKK-complex that are required for NF-κB activation. IKKɛ and TBK1 are two IKK-related kinases that allow the phosphorylation of interferon regulatory factors that trigger IFN type I gene expression. We observed that IKKɛ and TBK1 recruit Tax into cellular immunocomplexes. We also found that TRAF3, which regulates cell receptor signaling effectors, forms complexes with Tax. Transactivation analyses revealed that expression of Tax, in presence of IKKɛ and TBK1, enhances IFN-ß promoter activity, whereas the activation of NF-κB promoter is not modified. We propose that Tax may be recruited into the TBK1/IKKɛ complexes as a scaffolding-adaptor protein that enhances IFN-I gene expression.

A non-SUMOylated tax protein is still functional for NF-κB pathway activation.

UNLABELLED: Whether NF-κB promoter transactivation by the human T-cell leukemia virus type 1 (HTLV-1) Tax protein requires Tax SUMOylation is still a matter of debate. In this study, we revisited the role of Tax SUMOylation using a strategy based on the targeting of Ubc9, the unique E2 SUMO-conjugating enzyme. We show that either a catalytically inactive form of Ubc9 (Ubc9-C93S) or Ubc9 small interfering RNA (siRNA) dramatically reduces Tax conjugation to endogenous SUMO-1 or SUMO-2/3, demonstrating that as expected, Tax SUMOylation is under the control of the catalytic activity of Ubc9. We further report that a non-SUMOylated Tax protein produced in 293T cells is still able to activate either a transfected or an integrated NF-κB reporter promoter and to induce expression of an NF-κB-regulated endogenous gene. Importantly, blocking Ubc9 activity in T cells also results in the production of a non-SUMOylated Tax that is still fully functional for the activation of a NF-κB promoter. These results provide the definitive evidence that Tax SUMOylation is not required for NF-κB-driven gene induction. IMPORTANCE: Human T-cell leukemia virus type 1 is able to transform CD4(+) T lymphocytes. The viral oncoprotein Tax plays a key role in this process by promoting cell proliferation and survival, mainly through permanent activation of the NF-κB pathway. Elucidating the molecular mechanisms involved in NF-κB pathway activation by Tax is therefore a key issue to understand HTLV-1-mediated transformation. Tax SUMOylation was initially proposed to be critical for Tax-induced NF-κB promoter activation, which was challenged by our later observation that a low-level-SUMOylated Tax mutant was still functional for activation of NF-κB promoters. To clarify the role of Tax SUMOylation, we set up a new approach based on the inhibition of the SUMOylation machinery in Tax-expressing cells. We show that blocking the SUMO-conjugating enzyme Ubc9 abolishes Tax SUMOylation and that a non-SUMOylated Tax still activates NF-κB promoters in either adherent cells or T cells.

Alpha interferon restricts human T-lymphotropic virus type 1 and 2 de novo infection through PKR activation.

Type I interferon (IFN-I) inhibits the replication of different viruses. However, the effect of IFN-I on the human T-lymphotropic virus type 1 (HTLV-1) viral cycle is controversial. Here, we investigated the consequences of IFN-α addition for different steps of HTLV-1 and HTLV-2 infection. We first show that alpha interferon (IFN-α) efficiently impairs HTLV-1 and HTLV-2 de novo infection in a T cell line and in primary lymphocytes. Using pseudotyped viruses expressing HTLV-1 envelope, we then show that cell-free infection is insensitive to IFN-α, demonstrating that the cytokine does not affect the early stages of the viral cycle. In contrast, intracellular levels of Gag, Env, or Tax protein are affected by IFN-α treatment in T cells, primary lymphocytes, or 293T cells transfected with HTLV-1 or HTLV-2 molecular clones, demonstrating that IFN-α acts during the late stages of infection. We show that IFN-α does not affect Tax-mediated transcription and acts at a posttranscriptional level. Using either small interfering RNA (siRNA) directed against PKR or a PKR inhibitor, we demonstrate that PKR, whose expression is induced by interferon, plays a major role in IFN-α-induced HTLV-1/2 inhibition. These results indicate that IFN-α has a strong repressive effect on the HTLV-1 and HTLV-2 viral cycle during de novo infection of cells that are natural targets of the viruses.

Protein phosphatase 2A as a potential target for treatment of adult T cell leukemia.

The aim of this study was to establish the role of serine/threonine protein phosphatase 2A (PP2A) in the survival of leukemic cells from patients with adult T cell leukemia (ATL), associated with human T cell leukemia virus type 1 (HTLV-1). In HTLV-1-infected T cell lines and ATL cells, okadaic acid (OkA), a potent PP2A inhibitor, induced decrease in cell viability and G1 cell cycle arrest by decreasing the expression levels of cyclin D2, cyclin-dependent kinase 4 and cyclin-dependent kinase 6, phosphorylation of pRb, and upregulation of p21, p27 and GADD45α. OkA-induced apoptosis was also due to the suppression of expression of Bcl-2, Bcl-x(L) and XIAP, and the activation of caspases-3, -8 and -9, and caspase-3 downstream mammalian STE20-like kinase 1 and H2AX. OkA inhibited nuclear factor-kappa B DNA binding and activated mitogen-activated protein (MAP) kinases. Other new PP2A-specific inhibitors, cytostatin and rubratoxin A, also induced decrease in cell viability through caspase-dependent mechanism. MAP kinase inhibitors confirmed the role of p38 MAP kinase in PP2A inhibitors-induced apoptosis. OkA resulted in the generation of reactive oxygen species, and exogenous antioxidant prevented activation of the indicated caspases. Finally, PP2A knockdown inhibited cell growth. The results showed that PP2A inhibition caused reactive oxygen species generation and affected distinct signaling pathways, resulting in the activation of H2AX and subsequent apoptotic cell death. These results suggest that PP2A is a potentially useful target in the treatment of ATL.

An RNA interference screen identifies the Deubiquitinase STAMBPL1 as a critical regulator of human T-cell leukemia virus type 1 tax nuclear export and NF-κB activation.

The human T-cell leukemia virus type 1 (HTLV-1) Tax oncoprotein actively shuttles between the nucleus, where it interacts with transcriptional and splicing regulatory proteins, and the cytoplasm, where it activates NF-κB. Posttranslational modifications of Tax such as ubiquitination regulate its subcellular localization and hence its function; however, the regulation of Tax trafficking and NF-κB activation by host factors is poorly understood. By screening a deubiquitinating (DUB) enzyme small interfering RNA (siRNA) library, we identified the metalloprotease STAM-binding protein-like 1 (STAMBPL1) as a positive regulator of Tax-mediated NF-κB activation. Overexpression of wild-type STAMBPL1, but not a catalytically inactive mutant, enhanced Tax-mediated NF-κB activation, whereas silencing of STAMBPL1 with siRNA impaired Tax activation of both the canonical and noncanonical NF-κB signaling pathways. STAMBPL1 regulated Tax-induced NF-κB signaling indirectly by controlling Tax nuclear/cytoplasmic transport and was required for DNA damage-induced Tax nuclear export. Together, these results reveal that the deubiquitinase STAMBPL1 is a key regulator of Tax trafficking and function.

HTLV-1 positive and negative T cells cloned from infected individuals display telomerase and telomere genes deregulation that predominate in activated but untransformed CD4+ T cells.

Untransformed HTLV-1 positive CD4(+) cells from infected individuals are selected for expressing tax and displaying morphological features consistent with telomere dysfunctions. We show that in resting HTLV-1 positive CD4(+) cells cloned from patients, hTERT expression parallels tax expression and cell cycling. Upon activation, these cells dramatically augment tax expression, whereas their increase in telomerase activity is about 20 times lower than that of their uninfected counterpart. Activated HTLV-1 positive CD4(+) but not uninfected CD4(+) or CD8(+) clones also repress the transcription of TRF1, TPP1, TANK1, POT1, DNA-PKc and Ku80. Both infected and uninfected lymphocytes from infected individuals shared common telomere gene deregulations toward a pattern consistent with premature senescence. ATLL cells displayed the highest telomerase activity (TA) whereas recovered a telomere gene transcriptome close to that of normal CD4(+) cells. In conclusion HTLV-1-dependent telomere modulations seem involved in clonal expansion, immunosuppression, tumor initiation and progression.

Elevated cyclic AMP levels in T lymphocytes transformed by human T-cell lymphotropic virus type 1.

Human T-cell lymphotropic virus type 1 (HTLV-1), the cause of adult T-cell leukemia/lymphoma (ATLL), transforms CD4(+) T cells to permanent growth through its transactivator Tax. HTLV-1-transformed cells share phenotypic properties with memory and regulatory T cells (T-reg). Murine T-reg-mediated suppression employs elevated cyclic AMP (cAMP) levels as a key regulator. This led us to determine cAMP levels in HTLV-1-transformed cells. We found elevated cAMP concentrations as a consistent feature of all HTLV-1-transformed cell lines, including in vitro-HTLV-1-transformed, Tax-transformed, and patient-derived cells. In transformed cells with conditional Tax expression, high cAMP levels coincided with the presence of Tax but were lost without it. However, transient ectopic expression of Tax alone was not sufficient to induce cAMP. We found specific downregulation of the cAMP-degrading phosphodiesterase 3B (PDE3B) in HTLV-1-transformed cells, which was independent of Tax in transient expression experiments. This is in line with the notion that PDE3B transcripts and cAMP levels are inversely correlated. Overexpression of PDE3B led to a decrease of cAMP in HTLV-1-transformed cells. Decreased expression of PDE3B was associated with inhibitory histone modifications at the PDE3B promoter and the PDE3B locus. In summary, Tax transformation and its continuous expression contribute to elevated cAMP levels, which may be regulated through PDE3B suppression. This shows that HTLV-1-transformed cells assume biological features of long-lived T-cell populations that potentially contribute to viral persistence.

Changes in T cell phenotype and activated MAPKs are correlated to impaired cellular responses to antigens and glucocorticoids during HTLV-I infection.

Lymphocytes of human T-lymphotropic virus type-I (HTLV-I) infected patients were previously found tolerant to mitogenic stimuli as well as glucocorticoid treatment. These data suggest that common signaling events are impaired during this infection. The underlying mechanisms of these phenomena may include changes in cellular composition, cytokine milieu and the differential activation of mitogen-activated protein kinases (MAPKs). We investigated the role of (i) p38 and ERK MAPKs, (ii) lymphocyte subpopulations, (iii) and cytokines implicated in antigen or glucocorticoid-induced immunomodulation. Twenty-one asymptomatic carriers (AC), 19 patients with HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP) and 21 healthy subjects took part in this study. Lymphocytes were isolated and cultured in vitro to assess lymphocyte proliferation and sensitivity to dexamethasone. The expression of phospho-MAPKs, lymphocyte subsets and cytokines were assessed by flow cytometry. Patients with HAM/TSP had a higher p38/ERK ratio (p<0.05) associated with a reduced response to mitogens (phytohaemagglutinin or PMA+ionomycin) (p<0.001) and higher sensitivity to dexamethasone (p<0.05). HAM/TSP patients presented increased frequency of activated T cells and CD8(+)CD28(-) regulatory T cells, being negatively related to the mitogenic response. These data suggest that multiple underlying mechanisms could be involved with HTLV-related changes in cellular response to mitogens and glucocorticoids.

Central role of PI3K in transcriptional activation of hTERT in HTLV-I-infected cells.

The persistence of human T-cell leukemia/lymphoma virus-I (HTLV-I)-infected cells is dependent upon clonal expansion and up-regulation of telomerase (hTERT). We have previously found that in interleukin (IL)-2-independent transformed HTLV-I cells, Tax strongly activates the hTERT promoter through nuclear factor-kappaB (NF-kappaB)-mediated Sp1 and c-Myc activation. In IL-2-dependent cells and adult T-cell leukemia/lymphoma (ATLL) patient samples, however, Tax expression is very low to undetectable, yet these cells retain strong telomerase activity. This suggests the existence of compensatory mechanisms in IL-2-dependent cells and ATLL patients. In this study, we demonstrate that telomerase activity is significantly decreased upon IL-2 withdrawal in immortalized HTLV-I cell lines. Inhibition of PI3K or AKT signaling pathways reduced telomerase activity in HTLV-I cells. We found that IL-2/IL-2R signaling was associated with a PI3K-dependent/AKT-independent transcriptional up-regulation of the endogenous hTERT promoter. We found that activation of the PI3K pathway mediated cytoplasmic retention of the Wilms tumor (WTI) protein, which strongly suppressed the hTERT promoter. The importance of this regulatory pathway for telomerase expression is underscored by findings that the PI3K pathway is commonly found activated in cancer cells.

Reduction of human T-cell leukemia virus type-1 infection in mice lacking nuclear factor-kappaB-inducing kinase.

Human T-cell lymphotropic virus type 1 (HTLV-1) causes adult T-cell leukemia and inflammatory disorders. Aberrant activation of nuclear factor-kappaB (NF-kappaB) has been linked to HTLV-1 pathogenesis and to various kinds of cancers, including adult T-cell leukemia. NF-kappaB-inducing kinase (NIK) is critical for non-canonical activation of NF-kappaB and for the development of lymphoid organs. HTLV-1 activates NF-kappaB by the non-canonical pathway, but examination of the role of NIK in proliferation of HTLV-1-infected cells in vivo has been hindered by lack of a suitable animal model. Alymphoplasia (aly/aly) mice bear a mutation of NIK, resulting in defects in the development of lymphoid organs and severe deficiencies in both humoral and cell-mediated immunity. In the present study we therefore used a mouse model of HTLV-1 infection with aly/aly mice. The number of HTLV-1-infected cells in the reservoir organs in aly/aly mice was significantly smaller than in the control group 1 month after infection. In addition, aly/aly mice did not maintain provirus for 1 year and antibodies against HTLV-1 were undetectable. These results demonstrate that the absence of functional NIK impairs primary HTLV-1 proliferation and abolishes the maintenance of provirus. Interestingly, clonal proliferation of HTLV-1-infected mouse cells was not detected in aly/aly mice, which is consistent with the lack of HTLV-1 persistence. These observations imply that the clonal proliferation of HTLV-1-infected cells in secondary lymphoid organs might be important for HTLV-1 persistence.

The protease of human T-cell leukemia virus type-1 is a potential therapeutic target.

Human T-cell leukemia virus type-1 (HTLV-1) is associated with a number of human diseases. Although the mechanism by which the virus causes diseases is still not known, studies indicate that viral replication is critical for the development of HTLV-1 associated myelopathy, and initial studies suggested that blocking replication with reverse transcriptase inhibitors had a therapeutic effect. Therefore, based on the success of HIV-1 protease inhibitors, the HTLV-1 protease is also a potential target for chemotherapy. Furthermore, mutated residues in HIV-1 protease that confer drug resistance are frequently seen in equivalent positions of other retroviral proteases, like HTLV-1 protease. Therefore, comparison of HTLV-1 and HIV-1 proteases is expected to aid the rational design of broad spectrum inhibitors effective against various retroviral proteases, including the mutant HIV-1 enzymes appearing in drug resistance. This review describes the characteristics of HTLV-1 protease, makes comparison with HIV-1 protease, and discusses the status of inhibitor development for the HTLV-1 protease.

Constitutive and induced activation of JAK/Stat pathway in leukemogenic and asymptomatic human T-cell lymphoptropic virus type 1 (HTLV-1) transformed rabbit cell lines.

We have shown that Vav and C-cbl are activated in the leukemogenic HTLV-I transformed rabbit T cell line RH/K34 but not in the asymptomatic one RH/K30. We extended these observations and investigated the activation of JAKs (Janus Kinase) and the STATs (signal transducers and activators of transcription) pathway in these cell lines. We found that Tyk2 and Stat3 are constitutively tyrosine phosphorylated in the leukemogenic cell line. Phosphorylation of Tyk2 can be induced in RH/K30 by treatment with IL-10, interferon alpha (INFalpha) and by the supernatant of RH/K34 which contain both these cytokines. Stat3 tyrosine phosphorylation can be induced in RH/K30 by treatment with IL-10. Transfection of RL-5, a rabbit T-cell line, with the RH/K34 viral clone transiently increased the expression of serine/threonine phosphorylated Stat3. Differences were also observed on induced Stat5 phosphorylation. These results highlight the relation between the virulence of HTLV-I and the activation of the Jak/Stat pathway.

Phenotypic and genotypic comparisons of human T-cell leukemia virus type 1 reverse transcriptases from infected T-cell lines and patient samples.

It is well established that cell-free infection with human T-cell leukemia virus type 1 (HTLV-1) is less efficient than that with other retroviruses, though the specific infectivities of only a limited number of HTLV-1 isolates have been quantified. Earlier work indicated that a post-entry step in the infectious cycle accounted for the poor cell-free infectivity of HTLV-1. To determine whether variations in the pol gene sequence correlated with virus infectivity, we sequenced and phenotypically tested pol genes from a variety of HTLV-1 isolates derived from primary sources, transformed cell lines, and molecular clones. The pol genes and deduced amino acid sequences from 23 proviruses were sequenced and compared with 14 previously published sequences, revealing a limited number of amino acid variations among isolates. The variations appeared to be randomly dispersed among primary isolates and proviruses from cell lines and molecular clones. In addition, there was no correlation between reverse transcriptase sequence and the disease phenotype of the original source of the virus isolate. HTLV-1 pol gene fragments encoding reverse transcriptase were amplified from a variety of isolates and were subcloned into HTLV-1 vectors for both single-cycle infection and spreading-infection assays. Vectors carrying pol genes that matched the consensus sequence had the highest titers, and those with the largest number of variations from the consensus had the lowest titers. The molecular clone from CS-1 cells had four amino acid differences from the consensus sequence and yielded infectious titers that were approximately eight times lower than those of vectors encoding a consensus reverse transcriptase.

Telomerase activity and telomere length as prognostic factors of adult T-cell leukemia.

For the oncogenesis of many malignancies, it is crucial to prevent the shortening of the telomeres by the action of telomerase. In this study, clinical data and disease outcomes were analyzed in conjunction with the telomerase activity (TA) and telomere length (TL) of peripheral blood mononuclear cells. The study was carried out in 22 patients with adult T-cell leukemia (ATL) (7 chronic and 15 acute types) and in 13 asymptomatic human T-lymphotropic virus type 1 (HTLV-1) carriers. The mean values of TA in acute and chronic type patients were 13.8 and 1.6 total product generated (TPG) units, respectively, as determined by telomeric repeat amplification assays. The mean TA values in HTLV-1 carriers and healthy volunteers were 1.8 and 0.7 TPG, respectively. The mean TA value in acute type patients was significantly higher than in the three other subject groups. The mean TL values in patients with acute and chronic types were 5.39 and 4.38 Kb, respectively, while the mean TL values in HTLV-1 carriers and healthy volunteers were 7.69 and 7.06 Kb, respectively. The mean TL values in all ATL patients and in non-ATL subjects were 5.2 and 7.3 Kb, respectively. The former value is significantly shorter than the latter (p < 0.01). Neither TA nor TL of ATL cells showed any significant association with the number of ATL cells, serum soluble interleukin-2 receptor, or serum lactate dehydrogenase in the peripheral blood of acute type patients. This suggests that the levels of TA and TL did not reflect the ATL tumor load. The median survival period of acute ATL patients with high TA and shortened TL was 0.47 years, however, which was significantly shorter than that of acute ATL patients with low TA and normal TL (4.21 years) (p < 0.002). These data suggest that high TA and shortened TL were associated with poorer prognosis, and that TA and TL may be novel markers for the prognosis of ATL patients.

Expression of mitogen activated protein kinases in labial salivary glands of patients with Sjögren's syndrome.

OBJECTIVE: The expression of CD40 and CD40 ligand (CD40L) in mononuclear cells (MNCs) infiltrating the salivary glands of patients with Sjögren's syndrome (SS) has recently been reported. This study determined the expression of mitogen activated protein kinase (MAP kinase) superfamilies, which act as downstream effector molecules of CD40, in MNCs infiltrating labial salivary tissues in SS patients. METHODS: Six HTLV-I seronegative SS patients and 10 HTLV-I seropositive patients including five HTLV-I associated myelopathy (HAM) patients were examined. The expression of MAP kinase superfamilies in labial salivary glands was examined by immunohistochemistry containing the mirror section technique. RESULTS: Both active forms of c-Jun N-terminal kinase (JNK) and p38 were found in salivary infiltrating MNCs of SS patients. Only minimal expression of the active form of extracellular signal regulated kinase (ERK) was observed in these tissues, however, co-expression of active JNK and active p38 was confirmed by the mirror section technique. Furthermore, these protein kinases were co-expressed in CD40(+) MNCs. No difference in expression levels of active JNK and p38 was found in patients who were positive or negative for anti-HTLV-I antibody. CONCLUSION: These results indicate that JNK and p38, but not ERK, function as downstream effector molecules of CD40 in salivary infiltrating MNCs in SS patients, and suggest that these molecules may be involved in the pathological process of chronic sialadenitis in SS.

Differences in phosphorylation of the IL-2R associated JAK/STAT proteins between HTLV-I(+), IL-2-independent and IL-2-dependent cell lines and uncultured leukemic cells from patients with adult T-cell lymphoma/leukemia.

To determine activation status of the IL-2R-associated (Jak/STAT) pathway in the HTLV-I infected cells, we examined tyrosine phosphorylation of Jak3, STAT3, and STAT5 in several HTLV-I(+) T-cell lines and in uncultured leukemic T cells isolated from patients with adult T-cell lymphoma/leukemia (ATLL). Constitutive basal phosphorylation of Jak3 and, usually, STAT3 and STAT5 was detected in all four IL-2-independent cell lines tested, but in none of the three IL-2-dependent cell lines. Similarly, there was no detectable basal phosphorylation of Jak3 and STAT5 in the leukemic cells from ATLL patients (0/8 and 0/3, respectively). However, stimulation with IL-2 resulted in Jak3 and STAT5 phosphorylation in both leukemic ATLL cells and IL-2-dependent lines. Furthermore, expression of SHP-1 phosphatase which is a negative regulator of cytokine receptor signaling, was lost in most IL-2 independent cell lines (3/4) but not in the leukemic ATLL cells (0/3). Finally, the HTLV-I(+) T-cell lines (313) but not the control, HTLV-I(-) T-cell lines were resistant to rapamycin and its novel analog RAD. We conclude that (1) HTLV-I infection per se does not result in a constitutive phosphorylation of the Jak3, STAT3, and STAT5 proteins; (2) malignant transformation in at least some cases of ATLL does not require the constitutive, but may require IL-2-induced, activation of the IL-2R Jak/STAT pathway; and (3) there are major differences in T-cell immortalization mechanism(s) which appear to involve SHP-1 and target molecules for rapamycin and RAD.

The tax oncoprotein of human T-cell leukemia virus type 1 associates with and persistently activates IkappaB kinases containing IKKalpha and IKKbeta.

The Tax oncoprotein of human T-cell leukemia virus type 1 (HTLV1) chronically activates transcription factor NF-kappaB by a mechanism involving degradation of IkappaBalpha, an NF-kappaB-associated cytoplasmic inhibitor. Tax-induced breakdown of IkappaBalpha requires phosphorylation of the inhibitor at Ser-32 and Ser-36, which is also a prerequisite for the transient activation of NF-kappaB in cytokine-treated T lymphocytes. However, it remained unclear how Tax interfaces with the cellular NF-kappaB/IkappaB signaling machinery to generate a chronic rather than a transient NF-kappaB response. We now demonstrate that Tax associates with cytokine-inducible IkappaB kinase (IKK) complexes containing catalytic subunits IKKalpha and IKKbeta, which mediate phosphorylation of IkappaBalpha at Ser-32 and Ser-36. Unlike their transiently activated counterparts in cytokine-treated cells, Tax-associated forms of IKK are constitutively active in either Tax transfectants or HTLV1-infected T lymphocytes. Moreover, point mutations in Tax that ablate its IKK-binding function also prevent Tax-mediated activation of IKK and NF-kappaB. Together, these findings suggest that the persistent activation of NF-kappaB in HTLV1-infected T-cells is mediated by a direct Tax/IKK coupling mechanism.

Changes in astrocytic glutamate catabolism enzymes following neuronal degeneration or viral infection.

Functional changes in astrocytes are among the earliest cellular responses to a wide variety of insults to the central nervous system (CNS). Such responses significantly contribute to maintaining CNS homeostasis. In this context, by controlling energetic metabolism and overall excitability of the CNS, the modulation of glutamate uptake and catabolism in astrocytes is crucial. Here, we review specific modulations of the expression of glutamate catabolizing enzymes (glutamate dehydrogenase and glutamine synthetase) in response to CNS insults (degeneration of serotonergic neurons or viral infection by a human retrovirus, HTLV-I). The cellular and molecular mechanisms involved in the control of the glutamate catabolism are discussed in relation to neurological disorders.