Retroviral oncoprotein Tax induces processing of NF-kappaB2/p100 in T cells: evidence for the involvement of IKKalpha.

IkappaB kinase (IKK) is a key mediator of NF-kappaB activation induced by various immunological signals. In T cells and most other cell types, the primary target of IKK is a labile inhibitor of NF-kappaB, IkappaBalpha, which is responsible for the canonical NF-kappaB activation. Here, we show that in T cells infected with the human T-cell leukemia virus (HTLV), IKKalpha is targeted to a novel signaling pathway that mediates processing of the nfkappab2 precursor protein p100, resulting in active production of the NF-kappaB subunit, p52. This pathogenic action is mediated by the HTLV-encoded oncoprotein Tax, which appears to act by physically recruiting IKKalpha to p100, triggering phosphorylation-dependent ubiquitylation and processing of p100. These findings suggest a novel mechanism by which Tax modulates the NF-kappaB signaling pathway.

NF-kappaB-inducing kinase regulates the processing of NF-kappaB2 p100.

Processing of the nf(kappa)b2 gene product p100 to generate p52 is an important step in NF-kappaB regulation. We show that this step is negatively regulated by a processing-inhibitory domain (PID) within p100 and positively regulated by the NF-kappaB-inducing kinase (NIK). While the PID suppresses the constitutive processing of p100, NIK induces p100 processing by stimulating site-specific phosphorylation and ubiquitination of this precursor protein. Further, a natural mutation of the gene encoding NIK in alymphoplasia (aly) mice cripples the function of NIK in p100 processing, causing a severe defect in p52 production. These data suggest that NIK is a specific kinase regulating p100 processing and explain why the aly and nf(kappa)b2 knockout mice exhibit similar immune deficiencies.

Activation of I-kappaB kinase by the HTLV type 1 Tax protein: mechanistic insights into the adaptor function of IKKgamma.

The Tax protein encoded by human T cell leukemia virus type 1 (HTLV-1) induces constitutive nuclear expression of the transcription factor NF-kappaB, causing aberrant expression of a large array of cellular genes. Tax activates NF-kappaB by stimulating the activity of the I-kappaB kinase (IKK), which in turn leads to phosphorylation and degradation of the NF-kappaB inhibitor I-kappaBalpha. In normal T cells, IKK activation occurs transiently on cellular stimulation through the T cell receptor (TCR) and the CD28 costimulatory molecule. However, this inducible kinase is constitutively activated in Tax-expressing and HTLV-1-infected T cells, which contributes to the deregulated nuclear expression of NF-kappaB. As a genetic approach to dissect the pathways mediating IKK activation by Tax and T cell activation signals, somatic cell mutagenesis was performed to isolate signaling-defective mutant Jurkat T cell lines. One of the mutant cell lines was shown to have a defect in NF-kappaB activation by both T cell mitogens and Tax. Interestingly, this mutant cell line lacks expression of the IKK regulatory protein, IKKgamma. Expression of exogenous IKKgamma in the mutant cells restored NF-kappaB activation, thus confirming the essential role of this regulatory factor in IKK activation by the cellular and viral stimuli. Mechanistic studies have shown that Tax physically interacts with IKKgamma via specific domains, including two homologous leucine zipper motifs present in IKKgamma. The Tax/IKKgamma interaction serves to recruit Tax to the IKK catalytic subunits, IKKalpha and IKKbeta, and this recruitment appears to be an essential mechanism by which Tax stimulates the activity of IKK.

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.

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.

Regulation of RelA subcellular localization by a putative nuclear export signal and p50.

Nuclear factor kappaB (NF-kappaB) represents a family of dimeric DNA binding proteins, the pleotropic form of which is a heterodimer composed of RelA and p50 subunits. The biological activity of NF-kappaB is controlled through its subcellular localization. Inactive NF-kappaB is sequestered in the cytoplasm by physical interaction with an inhibitor, IkappaBalpha. Signal-mediated IkappaBalpha degradation triggers the release and subsequent nuclear translocation of NF-kappaB. It remains unknown whether the NF-kappaB shuttling between the cytoplasm and nucleus is subjected to additional steps of regulation. In this study, we demonstrated that the RelA subunit of NF-kappaB exhibits strong cytoplasmic localization activity even in the absence of IkappaBalpha inhibition. The cytoplasmic distribution of RelA is largely mediated by a leucine-rich sequence homologous to the recently characterized nuclear export signal (NES). This putative NES is both required and sufficient to mediate cytoplasmic localization of RelA as well as that of heterologous proteins. Furthermore, the cytoplasmic distribution of RelA is sensitive to a nuclear export inhibitor, leptomycin B, suggesting that RelA undergoes continuous nuclear export. Interestingly, expression of p50 prevents the cytoplasmic expression of RelA, leading to the nuclear accumulation of both RelA and p50. Together, these results suggest that the nuclear and cytoplasmic shuttling of RelA is regulated by both an intrinsic NES-like sequence and the p50 subunit of NF-kappaB.

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.

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.

Gene expression profiles in HTLV-I-immortalized T cells: deregulated expression of genes involved in apoptosis regulation.

Human T-cell leukemia virus type I (HTLV-I) is the etiologic agent of adult T-cell leukemia, an acute and often fatal T-cell malignancy. A key step in HTLV-I-induced leukemigenesis is induction of abnormal T-cell growth and survival. Unlike antigen-stimulated T cells, which cease proliferation after a finite number of cell division, HTLV-I-infected T cells proliferate indefinitely (immortalized), thus facilitating occurrence of secondary genetic changes leading to malignant transformation. To explore the molecular basis of HTLV-I-induced abnormal T-cell survival, we compared the gene expression profiles of normal and HTLV-I-immortalized T cells using 'gene array'. These studies revealed a strikingly altered expression pattern of a large number of genes along with HTLV-I-mediated T-cell immortalization. Interestingly, many of these deregulated genes are involved in the control of programmed cell death or apoptosis. These findings indicate that disruption of the cellular apoptosis-regulatory network may play a role in the HTLV-I-mediated oncogenesis.

IkappaB kinases serve as a target of CD28 signaling.

Optimal T cell activation and interleukin-2 production requires a second signal in addition to antigen-mediated T cell receptor (TCR) signaling. The CD28 molecule has been demonstrated to act as an effective costimulatory molecule upon binding by B7.1 or B7.2 present on antigen-presenting cells. The CD28 signal acts in concert with the TCR signal to significantly augment activation of the NF-kappaB family of transcription factors. The interleukin-2 gene is regulated by NF-kappaB among other transcription factors, in part, via a CD28 responsive element (CD28RE) present in the IL-2 promoter. Enhanced activation of NF-kappaB by CD28 is mediated by rapid phosphorylation and proteasome-mediated degradation of the NF-kappaB inhibitory proteins IkappaB alpha and IkappaB beta, which allows for accelerated nuclear expression of the liberated NF-kappaB. Herein, we provide evidence that the catalytic activities of two recently identified IkappaB kinases, IKKalpha and IKKbeta, are significantly elevated when T cells are stimulated through CD28 in addition to mitogen treatment. Catalytically inactive forms of IKKs are able to block the in vivo phosphorylation of IkappaB alpha induced by mitogen and CD28. Furthermore, CD28-mediated reporter gene transactivation of the CD28RE/AP-1 composite element is consistently attenuated by the IKK mutants. These findings suggest that cellular signaling pathways initiated at the TCR and CD28 converge at or upstream of IKK, resulting in more robust kinase activity and enhanced and prolonged NF-kappaB activation.

Bcl-3 expression and nuclear translocation are induced by granulocyte-macrophage colony-stimulating factor and erythropoietin in proliferating human erythroid precursors.

Bcl-3 is a proto-oncogene involved in the chromosomal translocation t(14;19) found in some patients with chronic lymphocytic leukemia. It shares structural similarities with and is a member of the IkappaB family of proteins. In this report, involvement of Bcl-3 in hematopoietic growth factor-stimulated erythroid proliferation and differentiation was examined. In TF-1 cells, an erythroleukemia cell line, granulocyte-macrophage colony-stimulating factor (GM-CSF) and erythropoietin (Epo) greatly enhanced Bcl-3 expression at both the protein and mRNA levels in association with stimulation of proliferation. Bcl-3 protein was also highly expressed in early burst-forming unit-erythroid (BFU-E)-derived erythroid precursors (day 7) and decreased during maturation (days 10 and 14), suggesting that Bcl-3 is involved in normal erythroid proliferation. In these hematopoietic cells, Bcl-3 was hyperphosphorylated. GM-CSF and Epo modulated the subcellular localization of Bcl-3. Upon stimulation of TF-1 cells with GM-CSF or Epo, the nuclear translocation of Bcl-3 was dramatically enhanced. Overexpression of Bcl-3 in TF-1 cells by transient transfection along with the NF-kappaB factors p50 or p52 resulted in significant induction of an human immunodeficiency virus-type 1 (HIV-1) kappaB-TATA-luceriferase reporter plasmid, demonstrating that Bcl-3 has a positive role in transactivation of kappaB-containing genes in erythroid cells. Stimulation with GM-CSF enhanced c-myb mRNA expression in these cells. Bcl-3 in nuclear extracts of TF-1 cells bound to a kappaB enhancer in the c-myb promoter together with NF-kappaB2/p52 and this binding activity was enhanced by GM-CSF stimulation. Furthermore, cotransfection of Bcl-3 with p52 or p50 in TF-1 cells resulted in significant activation of a c-myb kappaB-TATA-luceriferase reporter plasmid. These findings suggest that Bcl-3 may participate in the transcriptional regulation of certain kappaB-containing genes involved in hematopoiesis, including c-myb.

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.

Involvement of NF-AT in type I human T-cell leukemia virus Tax-mediated Fas ligand promoter transactivation.

Human T-cell leukemia virus type I (HTLV-I)-infected T-cells constitutively express surface Fas ligand (FasL), which may serve as a mechanism of viral pathogenesis. HTLV-I induces transcription of FasL gene through the viral transactivator Tax, although the underlying molecular mechanism remains unclear. In the present study, we have analyzed both the cis-activating element and transactivating factors involved in Tax activation of the FasL promoter. We show that the 486-base pair upstream region of the human FasL gene is sufficient for Tax-mediated transactivation in Jurkat T-cells. Interestingly, a palindromic DNA sequence (GGAAACTTCC) covering the consensus NF-ATp binding site (GGAAA), is required for Tax activation of FasL promoter. The involvement of NF-AT in this transactivation event is suggested by the finding that Tax fails to activate the FasL promoter in a Jurkat T-cell line defective in capacitative calcium entry, a signaling mechanism involved in NF-AT activation. Furthermore, activation of FasL promoter by Tax is largely attenuated in the nonlymphoid F9 embryonal and COS kidney cells deficient in NF-ATp expression. DNA-protein interaction assays reveal that the NF-AT-like motif in the FasL promoter is bound by both NF-ATp and NF-AT4 in Jurkat T-cells expressing Tax. The binding of NF-ATp, although not NF-AT4, to this enhancer also occurs along with HTLV-I-mediated infection of human peripheral blood T-cells. Furthermore, exogenously transfected NF-ATp binds to the NF-AT-like enhancer and participates in Tax-mediated FasL promoter transactivation. These results suggest an important role for proteins of the NF-AT family in HTLV-I Tax-mediated FasL gene transactivation.

Regulation of the interleukin-2 CD28-responsive element by NF-ATp and various NF-kappaB/Rel transcription factors.

The CD28 costimulatory signal enhances antigen-mediated induction of interleukin-2 (IL-2) gene transcription through activation of an enhancer termed the CD28-responsive element (CD28RE). Although various nuclear proteins have been shown to bind to CD28RE, their in vivo functions in the regulation of this enhancer remain elusive. In this report, we show that CD28RE binds distinct transcription factors in cells treated with different mitogenic stimuli. Stimulation of the T-cell receptor (TCR) complex in the absence of a CD28 costimulatory signal induces a member of the nuclear factor of the activated T cells, NF-ATp; however, this treatment fails to activate the CD28RE enhancer activity. Significant activation of CD28RE was detected when the cells were treated with both the TCR stimulators and an anti-CD28 monoclonal antibody (anti-CD28), which induces the NF-kappaB/Rel enhancer binding proteins in addition to NF-ATp. The costimulatory activity of anti-CD28 can be further enhanced by a phorbol ester. Kinetic analyses demonstrate that activation of endogenous IL-2 gene transcription is correlated with the binding of CD28RE by NF-ATp and different NF-kappaB/Rel species. Transient-transfection studies reveal that expression of either NF-ATp or the p50-RelA NF-kappaB heterodimer leads to the potent transactivation of both the CD28RE enhancer and the intact IL-2 promoter in mitogen-stimulated cells. Remarkably, coexpression of these two families of enhancer-binding proteins in Jurkat T cells results in the transactivation of the CD28RE enhancer even in the absence of any cellular stimuli. Together, these results suggest that activation of IL-2 gene transcription by the TCR- and CD28-mediated signals involves the interaction of CD28RE with NF-ATp and various NF-kappaB/Rel transcription factors.

The serine/threonine phosphatase inhibitor calyculin A induces rapid degradation of IkappaBbeta. Requirement of both the N- and C-terminal sequences.

Signal-initiated activation of the transcription factor NF-kappaB is mediated through proteolysis of its cytoplasmic inhibitory proteins IkappaBalpha and IkappaBbeta. While most NF-kappaB inducers trigger the degradation of IkappaBalpha, only certain stimuli are able to induce the degradation of IkappaBbeta. The degradation of IkappaBalpha is targeted by its site-specific phosphorylations, although the mechanism underlying the degradation of IkappaBbeta remains elusive. In the present study, we have analyzed the effect of phosphatase inhibitors on the proteolysis of IkappaBbeta. We show that the serine/threonine phosphatase inhibitor calyculin A induces the hyperphosphorylation and subsequent degradation of IkappaBbeta in both human Jurkat T cells and the murine 70Z-3 preB cells, which is associated with the nuclear expression of active NF-kappaB. The calyculin A-mediated degradation of IkappaBbeta is further enhanced by the cytokine tumor necrosis factor-alpha (TNF-alpha), although TNF-alpha alone is unable to induce the degradation of IkappaBbeta. Mutational analyses have revealed that the inducible degradation of IkappaBbeta induced by calyculin A, and TNF-alpha requires two N-terminal serines (serines 19 and 23) that are homologous to the inducible phosphorylation sites present in IkappaBalpha. Furthermore, the C-terminal 51 amino acid residues, which are rich in serines and aspartic acids, are also required for the inducible degradation of IkappaBbeta. These results suggest that the degradation signal of IkappaBbeta may be controlled by the opposing actions of protein kinases and phosphatases and that both the N- and C-terminal sequences of IkappaBbeta are required for the inducible degradation of this NF-kappaB inhibitor.

Constitutive dephosphorylation and activation of a member of the nuclear factor of activated T cells, NF-AT1, in Tax-expressing and type I human T-cell leukemia virus-infected human T cells.

The tax gene product of the type I human T-cell leukemia virus (HTLV-I) transactivates interleukin-2 (IL-2) gene through activation of an enhancer termed CD28 responsive element (CD28RE). Tax activation of the CD28RE is partially mediated by a member of the nuclear factor of activated T cells, NF-AT1. We have previously shown that NF-AT1 is constitutively active in Jurkat T cells stably transfected with the Tax cDNA, although the underlying molecular mechanism and physiological relevance of this finding remain unclear. In this report, we demonstrate that the active form of NF-AT1 is also present in the nuclei of HTLV-I-transformed T cells that express the Tax protein. Interestingly, the constitutive activation of NF-AT1 in these T cells is associated with its dephosphorylation. Furthermore, the dephosphorylated NF-AT1 can be rapidly rephosphorylated when the cells are incubated with cyclosporin A, an immunosuppressant inhibiting the serine/threonine phosphatase calcineurin. These results suggest that activation of NF-AT1 in Tax-expressing and HTLV-I-transformed T cells results from its dephosphorylation, which in turn may be due to deregulation of calcineurin.

CD28 mediates a potent costimulatory signal for rapid degradation of IkappaBbeta which is associated with accelerated activation of various NF-kappaB/Rel heterodimers.

Optimal activation of T cells requires at least two signals delivered by the T-cell receptor complex and costimulatory molecules such as CD28. The CD28 signaling participates in the transcription of the interleukin-2 gene through activation of an enhancer termed the CD28-responsive element (CD28RE). Stimulation of CD28 enhances mitogen-mediated induction of CD28RE-binding proteins including members of the NF-kappaB/Rel transcription factor family, although the underlying mechanism remains elusive. In this report, we show that CD28 costimulation leads to biphasic induction of NF-kappaB/Rel heterodimers, including early-phase induction of p50/RelA and c-Rel/RelA and late-phase induction of p50/c-Rel. Interestingly, activation of these NF-kappaB/Rel complexes by the CD28 signal is associated with the rapid degradation of both IkappaBalpha and IkappaBbeta, two major cytoplasmic inhibitors of NF-kappaB/Rel. Although IkappaBalpha degradation can be induced by phorbol ester alone, degradation of IkappaBbeta is largely dependent on the CD28 costimulatory signal. We further demonstrate that CD28-mediated transactivation of the CD28RE enhancer is potently inhibited by an N-terminal truncation mutant of IkappaBbeta that is incapable of responding to the degradation signals. Together, these results suggest that the CD28 costimulatory signal augments activation of NF-kappaB/Rel by promoting degradation of IkappaBbeta as well as enhancing degradation of IkappaBalpha and that induction of NF-kappaB/Rel serves as an essential step in the signal-mediated activation of the CD28RE enhancer.

Inhibition of p105 processing by NF-kappaB proteins in transiently transfected cells.

Regulation of the transcription factor NF-kappaB involves proteasome-mediated processing of the NF-kappaB1 p105 precursor protein, which generates the p50 subunit of NF-kappaB. The processing of p105 occurs constitutively in vivo but can be markedly enhanced by various cellular activation agents, although the underlying regulatory mechanism is not yet clear. In the present study, we demonstrate that signal-mediated induction of p105 processing in human T cells is associated with de novo synthesis of this precursor protein. Transient transfection studies performed in COS7 cells revealed that the newly synthesized p105 protein appears to be more rapidly processed compared to its accumulated form that is already associated with the processed product p50. Interestingly, the processing rate of p105 is markedly inhibited in cells co-transfected with p50 or other NF-kappaB subunits, including RelA and c-Rel, that physically interact with p105. These findings suggest that the processing of p105 is subject to negative regulation by the various NF-kappaB subunits. We further demonstrate that p105 undergoes degradation in lipopolysaccharide-stimulated human monocytic cells. However, the inducible degradation of p105 is not coupled with the generation of p50. Together, these studies demonstrate that the processing and inducible degradation of p105 are differentially regulated.