HTLV-I Tax self-association in optimal trans-activation function

The tax protein-DNA interaction is essential for HTLV-I transactivation in vitro

The human T-cell leukemia virus type-1 (HTLV-I)-encoded Tax protein enhances viral gene transcription through interaction with three repeated DNA elements located in the viral promoter. These elements, called viral CREs, are composed of an off-consensus eight base-pair cyclic AMP response element (CRE), immediately flanked by sequences that are rich in guanine and cytosine residues. Recent biochemical experiments have demonstrated that in the presence of the cellular protein CREB, Tax directly binds the viral CRE G+C-rich sequences via interaction with the minor groove. To determine the functional significance of the Tax-DNA interaction, we synthesized minor groove-binding pyrrole-imidazole polyamides which bind specifically to the G+C-rich sequences in the viral CREs. At concentrations where the polyamides specifically protect the G+C-rich sequences from MPE:Fe cleavage, the polyamides block the Tax-DNA interaction. At precisely these same concentrations, the polyamides specifically inhibit Tax transactivation in vitro, without altering CREB-activated transcription or basal transcription from the same promoter. Together, these data provide strong evidence that Tax-viral CRE interaction is essential for Tax function in vitro, and suggest that targeted disruption of the Tax-DNA minor groove interaction with polyamides may provide a novel approach for inhibiting viral replication in vivo.

The zinc finger protein A20 interacts with a novel anti-apoptotic protein which is cleaved by specific caspases

A20 is a Cys2/Cys2 zinc finger protein which is induced by a variety of inflammatory stimuli and which has been characterized as an inhibitor of cell death by a yet unknown mechanism. In order to clarify its molecular mechanism of action, we used the yeast two-hybrid system to screen for proteins that interact with A20. A cDNA fragment was isolated which encoded a portion of a novel protein (TXBP151), which was recently found to be a human T-cell leukemia virus type-I (HTLV-I) Tax-binding protein. The full-length 2386 bp TXBP151 mRNA encodes a protein of 86 kDa. Like A20, overexpression of TXBP151 could inhibit apoptosis induced by tumour necrosis factor (TNF) in NIH3T3 cells. Moreover, transfection of antisense TXBP151 partially abolished the anti-apoptotic effect of A20. Furthermore, apoptosis induced by TNF or CD95 (Fas/APO-1) was associated with proteolysis of TXBP151. This degradation could be inhibited by the broad-spectrum caspase inhibitor zVAD-fmk or by expression of the cowpox virus-derived inhibitor CrmA, suggesting that TXBP151 is a novel substrate for caspase family members. TXBP151 was indeed found to be specifically cleaved in vitro by members of the caspase-3-like subfamily, viz. caspase-3, caspase-6 and caspase-7. Thus TXBP151 appears to be a novel A20-binding protein which might mediate the anti-apoptotic activity of A20, and which can be processed by specific caspases.

Role of adapter function in oncoprotein-mediated activation of NF-kappaB. Human T-cell leukemia virus type I Tax interacts directly with IkappaB kinase gamma

Mechanisms by which the human T-cell leukemia virus type I Tax oncoprotein activates NF-kappaB remain incompletely understood. Although others have described an interaction between Tax and a holo-IkappaB kinase (IKK) complex, the exact details of protein-protein contact are not fully defined. Here we show that Tax binds to neither IKK-alpha nor IKK-beta but instead complexes directly with IKK-gamma, a newly characterized component of the IKK complex. This direct interaction with IKK-gamma correlates with Tax-induced IkappaB-alpha phosphorylation and NF-kappaB activation. Thus, our findings establish IKK-gamma as a key molecule for adapting an oncoprotein-specific signaling to IKK-alpha and IKK-beta.

Isolation of full-length cDNA and chromosomal localization of human NF-kappaB modulator NEMO to Xq28

NEMO is an essential component of the IkappaB kinase complex. Others have shown that expression of mouse NEMO can complement the lack of responsiveness to NF-kappaB stimuli in two NEMO-deficient cell lines. Here we report the isolation of a full-length human NEMO cDNA. Virtual translation of human NEMO cDNA predicts a 48-kD coiled-coil protein which shares 87.9% identity and 90.5% similarity with the mouse homolog. By sequence alignment, we mapped the human NEMO gene to chromosome Xq28. We note that the NEMO and the G6PD (glucose-6-phosphate dehydrogenase) loci are arranged in a head-to-head orientation separated by no more than 800 bp. This map location is further supported by the sequence of an alternatively spliced variant of human NEMO mRNA. Thus, human NEMO is an X-linked gene closely adjacent to the G6PD locus.

CREB-2, a cellular CRE-dependent transcription repressor, functions in association with Tax as an activator of the human T-cell leukemia virus type 1 promoter

The Tax protein of the human T-cell leukemia virus type 1 (HTLV-1) has been implicated in human T-cell immortalization. The primary function of Tax is to transcriptionally activate the HTLV-1 promoter, but Tax is also known to stimulate expression of cellular genes. It has been reported to associate with several transcription factors, as well as proteins not involved in transcription. To better characterize potential cellular targets of Tax present in infected cells, a Saccharomyces cerevisiae two-hybrid screening was performed with a cDNA library constructed from the HTLV-1-infected MT2 cell line. From this study, we found 158 positive clones representing seven different cDNAs. We focused our attention on the cDNA encoding the transcription factor CREB-2. CREB-2 is an unconventional member of the ATF/CREB family in that it lacks a protein kinase A (PKA) phosphorylation site and has been reported to negatively regulate transcription from the cyclic AMP response element of the human enkephalin promoter. In this study, we demonstrate that CREB-2 cooperates with Tax to enhance viral transcription and that its basic-leucine zipper C-terminal domain is required for both in vitro and in vivo interactions with Tax. Our results confirm that the activation of the HTLV-1 promoter through Tax and factors of the ATF/CREB family is PKA independent.

Regulation of the spatiotemporal pattern of expression of the glutamine synthetase gene

Glutamine synthetase, the enzyme that catalyzes the ATP-dependent conversion of glutamate and ammonia into glutamine, is expressed in a tissue-specific and developmentally controlled manner. The first part of this review focuses on its spatiotemporal pattern of expression, the factors that regulate its levels under (patho)physiological conditions, and its role in glutamine, glutamate, and ammonia metabolism in mammals. Glutamine synthetase protein stability is more than 10-fold reduced by its product glutamine and by covalent modifications. During late fetal development, translational efficiency increases more than 10-fold. Glutamine synthetase mRNA stability is negatively affected by cAMP, whereas glucocorticoids, growth hormone, insulin (all positive), and cAMP (negative) regulate its rate of transcription. The signal transduction pathways by which these factors may regulate the expression of glutamine synthetase are briefly discussed. The second part of the review focuses on the evolution, structure, and transcriptional regulation of the glutamine synthetase gene in rat and chicken. Two enhancers (at -6.5 and -2.5 kb) were identified in the upstream region and two enhancers (between +156 and +857 bp) in the first intron of the rat glutamine synthetase gene. In addition, sequence analysis suggests a regulatory role for regions in the 3' untranslated region of the gene. The immediate-upstream region of the chicken glutamine synthetase gene is responsible for its cell-specific expression, whereas the glucocorticoid-induced developmental appearance in the neural retina is governed by its far-upstream region.

Specific regions of contact between human T-cell leukemia virus type I Tax protein and DNA identified by photocross-linking

The human T-cell leukemia virus type I Tax protein forms a ternary complex on DNA in association with a host factor, the cyclic AMP response element-binding protein (CREB). An understanding of the precise geometry of this complex has been elusive. We have used photocross-linking to investigate Tax-DNA contacts. Our data show that Tax contacts the DNA at two symmetric positions 14 nucleotides apart on either side of the Tax responsive element. The presence of symmetric, widely separated regions of contact suggests that at least two molecules of Tax are present in the complex. Mapping the contacts onto a three-dimensional model of the CREB-DNA binary complex shows that they lie on the same face of the DNA near the regions where the N termini of the CREB bZIP domains enter the major groove. This location correlates well with previous evidence that CREB amino acid residues immediately N-terminal to the bZIP domain are crucial for the formation of the ternary complex. The limited number of cross-links observed suggests that contacts are primarily with the phosphate backbone and does not support the idea that a major structural element of the Tax protein inserts into the major or minor grooves of the DNA.

Human T cell leukemia virus type 1 oncoprotein Tax targets the human mitotic checkpoint protein MAD1

In searching for cellular targets of the HTLV-I oncoprotein Tax, we identified TXBP181, which we characterized as the human homolog of yeast mitotic checkpoint MAD1 protein. Evidence supporting TXBP181 as HsMAD1 includes sequence conservation with yeast MAD1, hyperphosphorylation during S/G2/M phases and upon treatment of cells with nocodazole, and binding to HsMAD2. HsMAD1 functions as a homodimer. It localizes to the centrosome during metaphase and to the spindle midzone and the midbody during anaphase and telophase. Expression of either Tax or a transdominant-negative TXBP181 results in multinucleated cells, a phenotype consistent with a loss of HsMAD1 function. We propose a model of viral transformation in which Tax targets TXBP181, thereby abrogating a mitotic checkpoint.

Mammalian peroxiredoxin isoforms can reduce hydrogen peroxide generated in response to growth factors and tumor necrosis factor-alpha

Mammalian tissues express three immunologically distinct peroxiredoxin (Prx) proteins (Prx I, II, and III), which are the products of distinct genes. With the use of recombinant proteins Prx I, II, and III, all have now been shown to possess peroxidase activity and to rely on Trx as a source of reducing equivalents for the reduction of H2O2. Prx I and II are cytosolic proteins, whereas Prx III is localized in mitochondria. Transient overexpression of Prx I or II in cultured cells showed that they were able to eliminate the intracellular H2O2 generated in response to growth factors. Moreover, the activation of nuclear factor kappaB (NFkappaB) induced by extracellularly added H2O2 or tumor necrosis factor-alpha was blocked by overproduction of Prx II. These results suggest that, together with glutathione peroxidase and catalase, Prx enzymes likely play an important role in eliminating peroxides generated during metabolism. In addition, Prx I and II might participate in the signaling cascades of growth factors and tumor necrosis factor-alpha by regulating the intracellular concentration of H2O2.

Human T-cell leukemia virus type 1 Tax requires direct access to DNA for recruitment of CREB binding protein to the viral promoter

Efficient human T-cell leukemia virus type 1 (HTLV-1) replication and viral gene expression are dependent upon the virally encoded oncoprotein Tax. To activate HTLV-1 transcription, Tax interacts with the cellular DNA binding protein cyclic AMP-responsive element binding protein (CREB) and recruits the coactivator CREB binding protein (CBP), forming a nucleoprotein complex on the three viral cyclic AMP-responsive elements (CREs) in the HTLV-1 promoter. Short stretches of dG-dC-rich (GC-rich) DNA, immediately flanking each of the viral CREs, are essential for Tax recruitment of CBP in vitro and Tax transactivation in vivo. Although the importance of the viral CRE-flanking sequences is well established, several studies have failed to identify an interaction between Tax and the DNA. The mechanistic role of the viral CRE-flanking sequences has therefore remained enigmatic. In this study, we used high resolution methidiumpropyl-EDTA iron(II) footprinting to show that Tax extended the CREB footprint into the GC-rich DNA flanking sequences of the viral CRE. The Tax-CREB footprint was enhanced but not extended by the KIX domain of CBP, suggesting that the coactivator increased the stability of the nucleoprotein complex. Conversely, the footprint pattern of CREB on a cellular CRE lacking GC-rich flanking sequences did not change in the presence of Tax or Tax plus KIX. The minor-groove DNA binding drug chromomycin A3 bound to the GC-rich flanking sequences and inhibited the association of Tax and the Tax-CBP complex without affecting CREB binding. Tax specifically cross-linked to the viral CRE in the 5'-flanking sequence, and this cross-link was blocked by chromomycin A3. Together, these data support a model where Tax interacts directly with both CREB and the minor-groove viral CRE-flanking sequences to form a high-affinity binding site for the recruitment of CBP to the HTLV-1 promoter.

Accessory factor-bZIP-DNA interactions

DNA binding and transcriptional activation by basic region leucine zipper (bZIP) proteins can be enhanced by accessory factors. In the past year, three accessory factor-bZIP-DNA complexes have been investigated extensively. Experiments have probed the effect of accessory factors on bZIP-DNA specificity, the residues that participate in stabilizing these complexes, the contribution of DNA conformation to accessory factor-bZIP interactions, and the relative stabilization of bZIP dimerization and DNA binding by accessory factors.

Mechanism of transdominant inhibition of CCR5-mediated HIV-1 infection by ccr5delta32

Human chemokine receptor 5 (CCR5) functions as a co-receptor for Human immunodeficiency virus (HIV-1) infection. CCR5 is a seven-transmembrane cell surface receptor. Recently, a naturally occurring mutation of CCR5, ccr5Delta32, has been described. A small number of Caucasians are homozygously ccr5Delta32/ccr5Delta32, while a larger number of individuals are heterozygously CCR5/ccr5Delta32. The ccr5Delta32/ccr5Delta32 genotype has been linked to a phenotype that is "highly" protected from HIV-1 infection. On the other hand, several studies have shown that the CCR5/ccr5Delta32 genotype confers "relative" protection from AIDS with onset of disease being delayed by 2-4 years. Although it is known that peripheral blood lymphocytes from heterozygous individuals (CCR5/ccr5Delta32) support ex vivo HIV-1 replication at a reduced level compared with CCR5/CCR5 cells, the molecular basis for this observation is unknown. Here we report on events that post-translationally modify CCR5. We show that CCR5 progresses through the endoplasmic reticulum prior to appearing on the cell surface. Mature CCR5 can be post-translationally modified by phosphorylation and/or co-translationally by multimerization. By contrast, mutant ccr5Delta32, although retaining the capacity for multimerization, was incapable of being phosphorylated. ccr5Delta32 heterocomplexes with CCR5, and this interaction retains CCR5 in the endoplasmic reticulum resulting in reduced cell surface expression. Thus, co-expression in cells of ccr5Delta32 with CCR5 produces a trans-inhibition by the former of ability by the latter to support HIV-1 infection. Taken together, our findings suggest CCR5/ccr5Delta32 heterodimerization as a molecular explanation for the delayed onset of AIDS in CCR5/ccr5Delta32 individuals.

Regulatory role for a novel human thioredoxin peroxidase in NF-kappaB activation

Reduction-oxidation (redox) plays a critical role in NF-kappaB activation. Diverse stimuli appear to utilize reactive oxygen species (e.g. hydrogen peroxide) as common effectors for activating NF-kappaB. Antioxidants govern intracellular redox status, and many such molecules can reduce H2O2. However, functionally, it does appear that different antioxidants are variously selective for redox regulation of certain transcription factors such as NF-kappaB. For NF-kappaB, thioredoxin has been described to be a more potent antioxidant than either glutathione or N-acetylcysteine. Thioredoxin peroxidase is the immediate enzyme that links reduction of H2O2 to thioredoxin. Several putative human thioredoxin peroxidases have been identified using recursive sequence searches/alignments with yeast or prokaryotic enzymes. None has been characterized in detail for intracellular function(s). Here, we describe a new human thioredoxin peroxidase, antioxidant enzyme AOE372, identified by virtue of its protein-protein interaction with the product of a proliferation association gene, pag, which is also a thiol-specific antioxidant. In human cells, AOE372 defines a redox pathway that specifically regulates NF-kappaB activity via a modulation of IkappaB-alpha phosphorylation in the cytoplasm. We show that AOE372 activity is regulated through either homo- or heterodimerization with other thiol peroxidases, implicating subunit assortment as a mechanism for regulating antioxidant specificities. AOE372 function suggests thioredoxin peroxidase as an immediate regulator of H2O2-mediated activation of NF-kappaB.

A human suppressor of c-Jun N-terminal kinase 1 activation by tumor necrosis factor alpha

Tumor necrosis factor alpha (TNFalpha) has pleiotropic effects on cellular metabolism. One of the signaling paths from the TNFalpha receptor induces a stress-activated protein kinase cascade. Components within this TNFalpha kinase cascade include mitogen-activated protein kinase/extracellular signal-regulated kinase kinase kinase 1 (MEKK1) and stress-activated protein kinase/extracellular signal-regulated kinase kinase (SEK), which regulate the activity of c-Jun N-terminal kinase 1 (JNK1). Currently, molecules upstream of MEKK1 that link TNFalpha receptor to downstream kinases are not well understood. Besides TNFalpha, many other stimuli including several oncoproteins can activate JNK1. In most cases, the signaling cascade(s) leading from oncoproteins to JNK1 is poorly elucidated. We report here that the human T-cell lymphotrophic virus, type I (HTLV-I) oncoprotein, Tax, can activate JNK1. We isolated a novel human cell factor, G-protein pathway suppressor 2 (GPS2), by its ability to bind the HTLV-I oncoprotein, and we show that this factor can potently suppress Tax activation of JNK1. In trying to understand the mechanism of GPS2 activity, we found that it also suppressed TNFalpha activation of JNK1 but not TNFalpha activation of p38 kinase nor phorbol activation of extracellular signal-regulated kinase 2. Because GPS2 has minimal effect on MEKK1- or SEK-regulated JNK1 activity, it could act at a point between the TNFalpha receptor and MEKK1 in the initial step(s) of this kinase cascade. Alternatively, it is not excluded that GPS2 could work in a parallel pathway that leads from TNFalpha to JNK1. GPS2 represents a new molecule that could contribute important insights toward how cytokine- and oncoprotein-mediated signal transduction might converge.