The trans-activator tax of human T-cell leukemia virus type 1 (HTLV-1) interacts with cAMP-responsive element (CRE) binding and CRE modulator proteins that bind to the 21-base-pair enhancer of HTLV-1

Specific versus cooperative regulatory mechanisms of the cytokine genes that are clustered on the same chromosome

The genes for IL-3, IL-4, IL-5, IL-9, IL-13, and granulocyte-macrophage colony-stimulating factor (GM-CSF) are known to be clustered on human chromosome 5q and on mouse chromosome 11. IL-2 and interferon gamma (IFN-gamma) genes are located on separate chromosomes. It is well known that upon stimulation by antigen presentation, TH1 and TH2 subsets of T helper cells start to transcribe distinct sets of cytokine genes. Thus mechanisms should exist that transmit extracellular signals into the nucleus, thereby coordinately turning on transcriptional machinery in cell type-specific manners. Several different mechanisms exist in which specific as well as coordinated expression of cytokines are regulated at the transcriptional level. These include (1) regulation by proximal cis-elements, to which specific transcription factors bind, (2) regulation by distal cis-elements, such as enhancers or locus controlling elements, especially those located several kilobases away from the target gene, and (3) enhancement of transcription by viral trans-activators in a pathologic state. In this article, we review the recent studies on the above issues, with particular emphasis on our own results that support the presence of different modes of control mechanisms. We also discuss the possible approaches to the thorough understanding of the coordinated and specific regulation of cytokines.

Interaction of the human T-lymphotropic virus type 1 Tax dimer with CREB and the viral 21-base-pair repeat

Human T-lymphotropic virus type 1 Tax interacts specifically with the cellular transcription factor CREB and the viral 21-bp repeat element to form a Tax-CREB-DNA ternary complex which mediates activation of viral mRNA transcription. Analyses of Tax and Tax mutants indicate that, like CREB, Tax incorporates into the ternary complex as a dimer. The ability of Tax to form a dimer is necessary for its interaction with CREB and the 21-bp element. Analyses of several Tax mutants with amino acid substitutions spanning residues 123 to 204 indicate that intersubunit Tax dimerization correlates with its ability to assemble into the ternary complex and activate transcription. Tax also enhances the DNA binding activities of specific bZip domains in vitro. The ability of Tax to enhance DNA binding of bZip proteins can be explained in part by Tax dimerization. This activity alone is not sufficient for transactivation. A dual amino acid substitution mutant of Tax, M47 (L319R, L320S), completely abrogated for activation of the human T-lymphotropic virus type 1 long terminal repeat as a result of a defect in the transactivation domain, continues to stimulate binding of bZip proteins to DNA.

T helper cell activation and human retroviral pathogenesis

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

The oncoprotein Tax of the human T-cell leukemia virus type 1 activates transcription via interaction with cellular ATF-1/CREB factors in Saccharomyces cerevisiae

The transcription factor Tax of the oncogenic human T-cell leukemia virus type 1 is likely to be responsible for viral replication in the host organism and for the induction of proliferation in infected cells. To investigate Tax-mediated transcription in vivo, we expressed Tax as well as CREB in Saccharomyces cerevisiae. The activity of these proteins was monitored by expression of a beta-galactosidase reporter gene, which was fused to two viral 21-bp repeats located upstream of the yeast cytochrome c1 oxidase minimal promoter. Coexpression of Tax and CREB in S. cerevisiae led to a 20-fold increase in beta-galactosidase activity in comparison with that in strains expressing either Tax or CREB alone. By screening a human cDNA library, we were able to demonstrate that the Tax transactivation assay using S. cerevisiae can be successfully applied to identify other cellular proteins forming ternary complexes with Tax and 21-bp repeats in vivo. Upon transformation in S. cerevisiae, 1 of 13,500 clones tested positive. Sequencing of the cDNA insert of the rescued plasmid revealed that this DNA encoded the ATF-1 protein. beta-Galactosidase induction was comparable to that of the Tax/CREB coexpression system. This indicates that Tax-mediated transcription is critically dependent on the presence of cellular CREB or ATF-1 in vivo. Stimulation of transcription initiation required an unmasked NH2 terminus of Tax. Fusion of Tax to the yeast Gal4 protein abolished the transactivation potential of Tax. Reconstitution of the transcriptional properties of viral Tax together with the cellular proteins of the ATF-1/CREB family in S. cerevisiae allows the functional characterization of these proteins in vivo.

Evidence for the Role of Cyclic AMP-Responsive Elements in Human Virus Replication and Disease

We review the involvement of the cyclic AMP responsive DNA element (CRE) and the ATF/CREB (activating transcription factor/CRE binding protein) family of transcription factors in the regulation and pathology of clinically important viruses that infect humans, including the herpesviridae, adenoviridae, parvoviridae, hepadnaviridae, and retroviridae families. CRE sequences found in specific regulatory elements of human viruses are listed, and the functional evidence for CRE activity, in the form of DNA binding assays, mutational studies, transfection and transcriptional activation experiments, or in vitro transcription assays, is summarized. Manipulation of cellular processes is required for virus replication in human cells following infection. A primary target of many viruses is the cellular transcription machinery, and several human viruses contain transcriptional activator and repressor proteins that affect cellular transcription. Through their effect on cellular transcription, viral genes alter the pattern of cellular gene expression, and thereby affect the differentiation state and cell cycle progression of the infected cell. We summarize evidence demonstrating that the CRE and its binding proteins are involved in the activity of the viruses, implicating their function in the pathogenesis of human diseases. The targeting of specific transcription factor pathways as a potential therapeutic approach is discussed. Copyright 1996 S. Karger AG, Basel

Human T-cell leukemia virus type I Tax protein transactivates RNA polymerase III promoter in vitro and in vivo

Tax protein of the human T-cell lymphotropic virus type 1 (HTLV-I) is critical for viral replication and is a potent transcriptional activator of viral and cellular polymerase II (pol II) genes. We report here that Tax is able to transactivate a classical pol III promoter, VA-I. In cotransfection experiments, Tax is shown to increase transcription of the VA-I promoter approximately 25-fold. Moreover, Tax is able to activate VA-I transcription when added exogenously to an in vitro transcription reaction. Using Tax affinity column chromatography, we demonstrate that Tax is able to deplete a HeLa cell extract for components required for transcription of VA-I. The transcriptional activity of the Tax-depleted extract can be restored by the 0.6 phosphocellulose fraction. Interestingly, a consensus binding site for cAMP-responsive element binding protein (CREB) is located upstream of the VA-I promoter, and deletion of this element results in the loss of Tax responsiveness. When this CREB binding site is replaced by a Gal-4 binding site, the VA-I promoter can be transactivated by a Gal4-Tax fusion protein. Taken together, these results suggest that Tax may activate pol III and pol II promoter through a similar mechanism involving the CREB activation pathway. It is also possible that Tax affects pol III transcription by direct interaction with a component of the pol III transcriptional machinery.

The tax gene of human T-cell leukemia virus type 2 is essential for transformation of human T lymphocytes

The mechanism of human T-cell leukemia virus (HTLV)-mediated transformation and induction of malignancy is unknown; however, several studies have implicated the viral gene product, Tax. Conclusive evidence for the role of Tax in the HTLV malignant process has been impeded by the inability to mutate tax in the context of an infectious virus and dissociate viral replication from cellular transformation. To circumvent this problem we constructed a mutant of HTLV type 2 (HTLV-2) that replicates by a Tax-independent mechanism. For these studies, the Tax response element in the viral long terminal repeat was replaced with the cytomegalovirus immediate-early promoter enhancer (C-enh). Transcription of the chimeric HTLV-2 (HTLVC-enh) was efficiently directed by this heterologous promoter. Also, the chimeric virus transformed primary human T lymphocytes with an efficiency similar to that of wild-type HTLV-2. A tax-knockout virus, termed HTLVC-enhDeltaTax, was constructed to directly assess the importance of Tax in cellular transformation. Transfection and infection studies indicated that HTLVC-enhDeltaTax was replication competent; however, HTLVC-enhDeltaTax failed to transform primary human T lymphocytes. We conclude that Tax is essential for HTLV-mediated transformation of human T lymphocytes. Furthermore, this chimeric HTLV, that replicates in the absence of Tax, should facilitate studies to determine the precise mechanism of T-lymphocyte transformation by HTLV.

Activation of the IL-2 gene promoter by HTLV-I tax involves induction of NF-AT complexes bound to the CD28-responsive element

The tax gene product of the type I human T-cell leukemia virus (HTLV-I) is a potent transcriptional activator of various growth-related cellular genes, including that encoding interleukin-2 (IL-2). Tax activation of many of these target genes appears to be mediated by the NF-kappa B/Rel and CREB/ATF family of cellular transcription factors. However, the mechanism by which Tax transactivates the IL-2 gene remains unclear. In the present study, we demonstrate that neither NF-kappa B/Rel nor CREB/ATF is sufficient for Tax-mediated activation of the IL-2 promoter. Two novel nuclear protein complexes are induced by Tax and specifically bind to an IL-2 gene enhancer, the CD28-responsive element (CD28RE). Immunobiochemical analyses suggest that these DNA binding complexes contain at least two members of the nuclear factor of activated T cells, NF-ATp and NF-ATc. However, the CD28 binding NF-AT complexes do not contain Jun and Fos family proteins that have been proposed to serve as NF-AT partners in the activation of the IL-2 NF-AT motif. Transient transfection studies demonstrate that the in vivo expressed NF-ATp binds to the CD28RE probe and enhances Tax-mediated activation of this critical IL-2 enhancer. We demonstrate further that binding of NF-AT to CD28RE is critical for Tax activation of the IL-2 promoter. Together, these results suggest a novel mechanism of Tax-mediated activation of the IL-2 gene, which involves the induction of NF-AT-containing CD28RE binding complexes.

Requirements for interleukin 2 promoter transactivation by the Tax protein of human T-cell leukemia virus type 1

The Tax protein of human T-cell leukemia virus type 1 (HTLV-1) upregulates the expression of several cellular genes by activating members of both the NF-kappaB and bZIP families of transcription factors. Recent studies demonstrate that the CD28 response element (CD28RE) of the interleukin 2 (IL-2) promoter is the site upregulated by Tax in stimulated T cells. Although some reports suggest that this site is transactivated by NF-kappaB family members, others disagree, leaving the identity of the transcription factor(s) binding the CD28RE unclear. The studies presented here further characterize the response of the IL-2 promoter and CD28RE to the HTLV-1 Tax protein and demonstrate that the TATA-proximal AP-1 binding site of the IL-2 promoter is also necessary for Tax transactivation in stimulated Jurkat cells. In contrast to its upregulation of the IL-2 promoter which requires T-cell stimulation, Tax transactivates the isolated CD28RE-AP-1 element without stimulation but is greatly synergized by calcium ionophore and phorbol ester. Additionally, transactivation of the IL-2 promoter requires the Tax activation domain involved in upregulation of bZIP-enhanced transcription while the NF-kappaB-activating domain of Tax is dispensable. Interestingly, both domains appear to be necessary for the activation of the isolated CD28RE-AP-1 sequence in the context of a heterologous promoter construct. This strongly suggests that activation of NF-kappaB is insufficient to activate transcription via the CD28RE-AP-1 element of the IL-2 promoter and that a different transcription factor, upregulated via the activation domain of the HTLV-1 Tax protein, may be involved.

c-sis/PDGF-B promoter transactivation by the Yax protein of human T-cell leukemia virus type 1

The human c-sis proto-oncogene promoter is transactivated by the human T-cell leukemia virus type 1 Tax protein in human Jurkat T-cells. Transactivation was >7-fold in Jurkat cells stably expressing the Tax protein (Jurkat-Tax) than in Jurkat E6.1 cells and was further enhanced in Jurkat-Tax cells stimulated with 12-O-tetradecanoylphorbol-13-acetate and the calcium ionophore, ionomycin. Deletion analysis showed that a 167-base pair promoter fragment retained full Tax responsiveness. Insertion of this minimal Tax-responsive region into a heterologous, minimal promoter resulted in approximately a 7-fold increase of transcriptional activation in the presence of Tax. Linker-scanning insertion analysis of this region identified Tax-responsive elements at nucleotides -64 to -45 (TRE1) and -34 to -15 (TATA box region). TRE1 contains a consensus binding site for the Sp family of transcription factors. The TATA box region corresponds to the TATA box and its 3'-neighboring sequence. Gel-shift and antibody supershift analysis of TRE1-binding proteins in unstimulated Jurkat E6.1 and Jurkat-Tax nuclear extracts identified Sp1 and Sp3 as the main TRE1 binding factors. Nuclear extracts from stimulated Jurkat E6.1 and Jurkat-Tax cells identified an additional TRE1 binding factor, Egr-1. These studies define a novel mechanism whereby Tax transactivates the c-sis promoter.

Complex formation between CREB and Tax enhances the binding affinity of CREB for the human T-cell leukemia virus type 1 21-base-pair repeats

The regulation of human T-cell leukemia virus type 1 (HTLV-1) gene expression is dependent on three cis-acting elements, known as the 21-bp repeats, in the long terminal repeat. Each of the 21-bp repeats contains a nonpalindromic cyclic AMP response element (CRE) sequence which is capable of binding members of the ATF/CREB family of transcription factors. The HTLV-1 transactivator protein Tax is able to markedly stimulate the in vitro binding of CREB to the CRE sites present in each of the 21-bp repeats but not to CRE sites present in cellular promoters. The ability to Tax to stimulate CREB binding to different CRE sites correlates with the ability of Tax to activate gene expression from these sites. We wished to determine how sequence differences between the somatostatin CRE and the 21-bp repeat were involved in this different response to Tax. Scatchard analysis indicated that CREB bound to the somatostatin CRE with a single class of high-affinity binding while CREB bound to the 21-bp repeats with a biphasic binding pattern, indicating the presence of both low- and high-affinity binding. Tax increased the affinity of CREB binding but not that of another ATF/CREB protein, CREB2, to the 21-bp repeat. However, Tax did not increase affinity of binding of CREB to the somatostatin CRE. To determine the mechanism by which Tax increased dCREB binding affinity, immobilized oligonucleotides corresponding to either the 21-bp repeat or the somatostatin CRE were used to demonstrate that Tax formed a highly specific complex with CREB on the 21-bp repeat but not on the somatostatin CRE. These results indicate that formation of a complex between Tax and CREB results in specific high-affinity binding of this ternary complex to the HTLV-1 21 bp repeats.

Sp1 family proteins recognize the U5 repressive element of the long terminal repeat of human T cell leukemia virus type I through binding to the CACCC core motif

We have identified several nuclear proteins binding to the U5 repressive element (U5RE) at the U5 region of the human T cell leukemia virus type I (HTLV-I) long terminal repeat (LTR). In gel mobility shift assays with the U5RE DNA probe, Jurkat T cell nuclear proteins generated five different complexes, named U5RE binding protein complexes (U5RP)-A1, -A2, -A3, -B, and -C. Only U5RP-C was affected by pretreatment with an excess of poly(dI-dC) and was immunodepressed by anti-Ku/p80 antibodies, suggesting that U5RP-C is a nonspecific complex involving Ku antigen. UV cross-linking showed at least six nuclear proteins involved in the other complexes, including U5RP-A1, -A2, -A3, and -B. The sequence of the binding core element of these specific complexes, determined by competition assays and gel mobility shift assays using a series of the U5RE mutants, is CACCC which is identical to that for the Sp1 transcription factor. LTR with a mutant U5RE, which has no ability to bind with the nuclear proteins, showed stronger promoter activity than LTR with the wild U5RE, suggesting that the specific interaction of these U5RE-binding proteins might result in the U5-mediated repression. U5RP-A1 was supershifted by anti-Sp1 antibodies and U5RP-A2 and -B were supershifted by anti-Sp3 antibodies, suggesting that Sp1 or Sp3 is involved in U5RP-A1 or U5RP-A2 and -B, respectively. Although the other nuclear proteins remain to be characterized, these findings suggest that U5RE-binding proteins in U5RP-A1, -A2, -A3, and -B are involved in HTLV-I gene repression.

Genetic characterization of transactivation of the human T-cell leukemia virus type 1 promoter: Binding of Tax to Tax-responsive element 1 is mediated by the cyclic AMP-responsive members of the CREB/ATF family of transcription factors

To achieve a better understanding of the mechanism of transactivation by Tax of human T-cell leukemia virus type 1 Tax-responsive element 1 (TRE-1), we developed a genetic approach with Saccharomyces cerevisiae. We constructed a yeast reporter strain containing the lacZ gene under the control of the CYC1 promoter associated with three copies of TRE-1. Expression of either the cyclic AMP response element-binding protein (CREB) or CREB fused to the GAL4 activation domain (GAD) in this strain did not modify the expression of the reporter gene. Tax alone was also inactive. However, expression of the reporter gene was induced by coexpression of Tax and CREB. This effect was stronger with the GAD-CREB fusion protein. Analysis of different CREB mutants with this genetic system indicated that the C-terminal 92 amino acid residues, which include the basic domain and the leucine zipper, are necessary and sufficient to mediate transactivation by Tax. To identify cellular proteins binding to TRE-1 in a Tax-dependent manner, this strain was also used to screen a library of human cDNAs fused to GAD. Of five positive clones isolated from 0.75 x 10(6) yeast colonies, four were members of the CREB/activating transcription factor (ATF) family: CREB, two isoforms of the cyclic AMP-responsive element modulator (CREM), and ATF-1. Interestingly, these three proteins can be phosphorylated by protein kinase A and thus form a particular subgroup within the CREB/ATF family. Expression of ATF-2 in S. cerevisiae did not activate TRE-1 in the presence of Tax. This shows that in a eukaryotic nucleus, Tax specifically interacts with the basic domain-leucine zipper region of ATF-1, CREB, and CREM. The fifth clone identified in this screening corresponded to the Ku autoantigen p70 subunit. When fused to GAD, the C-terminal region of Ku was able to activate transcription via TRE-1 but this activation was not dependent on Tax.

HTLV-1 Tax protein interacts with cyclin-dependent kinase inhibitor p16INK4A and counteracts its inhibitory activity towards CDK4

Tax, a regulatory protein of human T-cell leukemia virus type 1 (HTLV-1), is an oncoprotein which immortalizes human T cells and induces tumors in transgenic mice. These effects may be due to its interaction with cellular proteins, consisting of several transcription factors including CREB, NF-kappa B and SRF, and the transcriptional inhibitor, I kappa B. Here, we found that Tax binds to a cyclin-dependent kinase inhibitor, p16INK4A, which has ankyrin motifs similar to I kappa B. p16INK4A binds to the cyclin-dependent kinases, CDK4 and CDK6, and inhibits their activity, resulting in suppression of G1 phase progression. The binding of Tax to p16INK4a induced a reduction in the p16INK4A-CDK4 complex, with subsequent activation of CDK4 kinase. Tax also suppressed p16INK4A-mediated inhibition of U2OS cell growth. The p16INK4A gene was frequently deleted in many T-cell lines, but not in HTLV-1-infected T-cell lines. Taking these findings together, the functional inactivation of p16INK4A by Tax through protein-protein interaction is suggested to contribute to cellular immortalization and transformation induced by HTLV-1 infection.

The CREB, ATF-1, and ATF-2 transcription factors from bovine leukemia virus-infected B lymphocytes activate viral expression

Efficient transcription and replication of the bovine leukemia virus (BLV) genome require both the viral long terminal repeat (LTR) and the virus-coded transcriptional activator Tax, which functions through a 21-bp sequence (Tax-responsive element [TxRE]) which is repeated three times within the LTR. Since Tax does not bind directly to DNA, host cell transcription factors play a central role in BLV expression. Electrophoretic mobility shift assays with nuclear extracts prepared with infected bovine B lymphocytes revealed five TxRE-specific complexes (C1, C2, C3, C4, and C5). Here, by using a UV-induced indirect labeling technique (UV cross-linking) in conjunction with mobility shift assays, eight major polypeptides of 31, 33, 42, 46, 51, 57, 87, and 119 kDa were identified within these five complexes. Immunoprecipitation experiments identified the 57- and 119-kDa proteins as cyclic AMP response element-binding (CREB) proteins, the 46- and 51-kDa proteins as activating transcription factor-1 (ATF-1), and the 87-kDa as protein ATF-2. All of these proteins (except the ATF-1 protein of 51 kDa) belong to the complex C1, which is the major complex identified in freshly isolated BLV-infected lymphocytes from cattle with persistent lymphocytosis. In transient-cotransfection experiments, these three transcription factors were able to activate LTR-directed gene expression in the presence of protein kinase A or Ca2+/calmodulin-dependent protein kinase IV. CREB protein, ATF-1, and ATF-2 thus appear to be the major transcription factors involved in the early stages of viral expression.

cAMP-response element-binding protein induces directed DNA bending of the HTLV-I 21-base pair repeat

Gene expression from the human T-cell leukemia virus type I (HTLV-I) long terminal repeat (LTR) is mediated by three cis-acting regulatory elements known as 21-base pair (bp) repeats in addition to the transactivator protein Tax. Each of the 21-bp repeats contain nucleotide sequences which are homologous to a cAMP response element (CRE) which bind members of the ATF/CREB family of transcription factors. In this study, we investigated whether CREB alone or in the presence of Tax was able to induce DNA structural changes when bound to CRE sites in the HTLV-I 21 bp, the cellular somatostatin promoter, or a hybrid CRE construct comprised of both the somatostatin and 21-bp repeat sequences. Circular permutation analysis indicated that CREB was able to induce DNA flexure upon binding to each of these elements. However, phasing analysis, which is a more sensitive method to determine the degree and orientation of directed DNA bending, demonstrated that CREB induced DNA bending of the HTLV-I 21-bp repeat and the hybrid CRE but not the somatostatin CRE. The addition of Tax did not change CREB-mediated bending of the 21-bp repeat or the hybrid CRE although it markedly increased the amount of CREB bound to each of these DNA elements. These results indicate that sequence motifs flanking the CRE in the 21-bp repeat are critical for inducing DNA structural changes and that these changes are likely important in mediating Tax activation of the HTLV-I LTR.

Enhanced E-cadherin expression and increased calcium-dependent cell-cell adhesion in human T-cell leukemia virus type I Tax-expressing PC12 cells

Human T-cell leukemia virus type I (HTLV-I) Tax protein induces the expression of host cellular genes, some of which are crucial in cell proliferation and differentiation. We examined the mechanisms by which HTLV-I Tax protein induces phenotypic changes in PC12 cells. We demonstrated that the HTLV-I Tax gene induces epithelioid changes and increases cell-cell contact in PC12 cells. No change in the expression of the neural cell adhesion molecule was observed between HTLV-I Tax-expressing PC12 cells and PC12 cells transfected with a control plasmid. However, HTLV-I Tax-expressing PC12 cells demonstrated a marked change in the abundance and distribution of E-cadherin, which was concentrated at regions of cellular contact and accompanied by changes in calcium-dependent cell adhesion. Although E-cadherin is expressed at low levels in PC12 and PC12 transfected with a control plasmid cells, the steady state level of E-cadherin in tax-expressing PC12 cells increases significantly, apparently as a result of regulation at the transcriptional level. Diminished expression of Tax protein in Tax-expressing PC12 cells exposed to antisense oligonucleotides for the Tax gene suppresses E-cadherin expression and decreases cell-cell adhesion. These findings imply that HTLV-I Tax protein enhanced E-cadherin expression modulates calcium-dependent cell-cell adhesion mechanisms.

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

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

Selective infection of human T-lymphotropic virus type 1 (HTLV-1)-infected cells by chimeric human immunodeficiency viruses containing HTLV-1 tax response elements in the long terminal repeat

Previous studies have suggested that the human immunodeficiency virus long terminal repeat (HIV LTR) enhancer/promoter sequences contribute to the replication ability of HIV in different T-cell lines; mutation of these sequences can alter HIV tropism. We have utilized site-specific mutagenesis to generate variants of HIV that exhibit specific tropism for human T-lymphotropic virus type 1 (HTLV-1) Tax-expressing CD4+ T cells. The wild-type HIV LTR NF-kappa B and Sp1 sites in an infectious molecular clone of HIV type 1 were replaced with sequences derived from the 21-bp Tax response elements (TRE) from the HTLV-1 LTR to generate TRE-containing chimeric HIVs (TRE-HIVs). The TRE-HIVs exhibit selective replication and cell killing in HTLV-infected human CD4+ T cells, but not in HTLV-negative T cells. Transient transfections suggested that Tax-TRE interactions could account for the observed replication specificity. The TRE-containing HIV LTRs were synergistically activated by the HIV Tat and HTLV-1 Tax transactivators. These results demonstrate that it is possible to specifically target HIV replication and cytotoxicity to HTLV-1+, CD4+ human T cells, on the basis of Tax-TRE interactions, and provide a model for the development of specific, cytotoxic, retroviral gene therapy vectors for HTLV-1-infected cells based on alterations of the LTR transcriptional regulatory elements. They also suggest that HIV Tat can cooperate with heterologous transcriptional activators, such as Tax, which act through upstream binding sites without directly binding to DNA.

Chimeric proteins composed of Jun and CREB define domains required for interaction with the human T-cell leukemia virus type 1 Tax protein

The regulation of human T-cell leukemia virus type 1 (HTLV-1) long terminal repeat gene expression is dependent on three cis-acting elements known as 21-bp repeats and the transactivator protein Tax. Mutagenesis has demonstrated that sequences in each of the 21-bp repeats can be divided into three domains designated A, B, and C. Tax stimulates the binding of CREB to the B domain, which is essential for Tax activation of HTLV-1 gene expression. In this study, we demonstrate that Tax will stimulate the binding of CREB to the HTLV-1 21-bp repeats but does not stimulate CREB binding to the consensus cyclic AMP response element (CRE) element found in the somatostatin promoter. However, Tax stimulates CREB binding to a consensus CRE in the context of the 21-bp repeats, indicating the importance of these sequences in stimulating CREB binding. To determine the mechanism by which Tax stimulates CREB binding and determine potential interactions between Tax and CREB, we used the mammalian two-hybrid system in conjunction with in vitro binding and gel retardation assays. Two-hybrid analysis indicated that mutations in either the basic or leucine zipper region of CREB prevented interactions with Tax. Since several studies have demonstrated that Tax will also stimulate the binding of a variety of different basic region-leucine zipper proteins to their cognate binding sites, we assayed whether chimeric proteins composed of portions of CREB and another basic region-leucine zipper protein, Jun, could be used to map domains required for interactions with Tax. These studies were possible because we did not detect in vivo or in vitro interactions between Tax and Jun. The amino acid sequence of the CREB basic region and a portion of its leucine zipper were required for both in vivo and in vitro interactions with Tax and increased binding of CREB to the 21-bp repeats in response to Tax. These studies define the domains in CREB required for both in vivo and in vitro interactions by the HTLV-1 Tax protein.

HTLV-1 oncoprotein Tax deregulates transcription of cellular genes through multiple mechanisms

Infection of a human retrovirus HTLV-1 induces adult T cell leukemia and a neurological disease, HAM/TSP. Regulatory protein Tax of HTLV-1 is thought to contribute to the pathogenesis. We have studied the mechanism of transcriptional activation induced by Tax protein and identified two independent mechanisms: (a) binding to the enhancer-binding proteins, CREB, CREM, NF-kappa B and SRF, resulting in the activation of these factors through indirect binding to each specific DNA sequence, and (b) binding to I kappa B protein resulting in the suppression of the negative regulator I kappa B, which binds to NF-kappa B. In addition to these factors, a new protein GLI/THP is also involved in the transactivation. On the basis of these mechanisms, gene regulations in vitro and in vivo in HTLV-1-infected cells are discussed.

Characterization of a transcriptional attenuator within the 5' R region of the human T cell leukemia virus type 1

Several regulatory sequences have been characterized in the HTLV-I promoter. We report here identification of a sequence element downstream of the transcriptional start site within the first 52 nucleotides of the 5' R region, which acts negatively on the activity of the HTLV-I promoter. Determination of the half-lives of the RNAs either including or lacking this sequence element showed that the observed effect intervenes at the transcriptional level. This negative element does not affect basal activity of the HTLV-I TATA box, but down-regulates transcription induced by strong activators. Thus, we propose that this so-called negative regulatory sequence functions as an attenuator of transcription.

Constitutive activation of different Jak tyrosine kinases in human T cell leukemia virus type 1 (HTLV-1) tax protein or virus-transformed cells

HTLV-1 infection causes an adult T cell leukemia in humans. The viral encoded protein tax, is thought to play an important role in oncogenesis. Our previous data obtained from a tax transgenic mouse model revealed that tax transforms mouse fibroblasts but not thymocytes, despite comparable levels of tax expression in both tissues. Constitutive tyrosine phosphorylation of a 130-kD protein(s) was observed in the tax transformed fibroblast B line and in HTLV-1 transformed human lymphoid lines, but not in thymocytes from Thy-tax transgenic mice. Phosphotyrosine immunoprecipitation followed by Western blot analysis with a set of Jak kinase specific antibodies, identified p130 as Jak2 in the tax transformed mouse fibroblastic cell line and Jak3 in HTLV-1 transformed human T cell lines. Phosphorylation of Jak2 in tax transformed cells resulted from high expression of IL-6. Tyrosine phosphorylation of this protein could also be induced in Balb/c3T3 cells using a supernatant from the B line, which was associated with induction of cell proliferation. Both phosphorylation and proliferation were inhibited by IL-6 neutralizing antibodies. Constitutive phosphorylation of Jak kinases may facilitate tumor growth in both HTLV-1 infected human T cells and the transgenic mouse model.

Transactivation of the human T-cell lymphotropic virus type 1 Tax1-responsive 21-base-pair repeats requires Holo-TFIID and TFIIA

The human T-cell lymphotropic virus type 1 (HTLV-1) is the etiological agent for adult T-cell leukemia and tropical spastic paraparesis/HTLV-1-associated myelopathy. The HTLV-1 Tax1 gene product has been shown to transactivate transcription of viral and cellular promoters. To examine the biochemical mechanism of Tax1 transactivation, we have developed an in vitro transactivation assay in which wild-type Tax1 is able to specifically transactivate a polymerase II promoter through upstream Tax1-responsive elements. The in vitro system utilizes the HTLV-1 21-bp repeats cloned upstream of the ovalbumin promoter and G-free cassette. Purified Tax1 specifically transactivates this template 5- to 10-fold in a concentration-dependent manner. No transactivation of the ovalbumin promoter (pLovTATA) template control was observed. Tax1 transactivation was inhibited by low concentrations of alpha-amanitin and was effectively neutralized by anti-Tax1 but not control sera. Consistent with in vivo transactivating activity, Tax1 NF-kappa B mutant M22, but not cyclic AMP-responsive element-binding protein mutant M47, transactivated the template containing the tandem 21-bp repeat. In a reconstituted in vitro transcription assay, Tax1 transactivation was dependent upon basal transcription factors TFIIB, TFIIF, Pol II, TFIID, and TFIIA. TATA-binding protein did not functionally substitute for TFIID in the transactivation assay by Tax1 but was sufficient for basal transcription. Finally, we have used anti-TFIIA antibody (p55) to ask if Tax1 transactivation required TFIIA activity. Addition of TFIIA antibody to in vitro transcription reactions, as well as depletion of TFIIA by preclearing with antibody, showed that TFIIA was required for Tax1 transactivation. Only a slight (twofold) drop of basal transcription was observed. Tax1 transactivation was restored when purified HeLa TFIIA was added back into the reconstituted system. We propose that Tax1 utilizes a transactivation pathway involving the activator regulated basal transcription factors TFIID and TFIIA.

Activation of NF-kappa B by the Tax protein of HTLV-1

The Tax protein, encoded by the human T cell leukemia virus HTLV-1, is responsible for transcriptional activation of the viral genome through conserved 21bp repeats located in its promoter. Tax also activates the transcription of cellular genes such as interleukin 2, interleukin 2 receptor (IL2R), GM-CSF, vimentin, c-fos, c-jun as well as the major histocompatibility complex class I genes. Tax does not bind DNA directly, but seems to activate transcription indirectly by enhancing the activity of the transcription factors that recognize responsive elements located in the promoters of the Tax-responsive genes, or by forming ternary complexes with these factors and DNA. One class of target sites for Tax are the kappa B sequences which are bound by members of the rel/NF-kappa B family. It has been previously shown that Tax is able to induce nuclear translocation of NF-kappa B. The activity of the NF-kappa B transcription factor is normally controlled through cytoplasmic retention by either of two types of molecules: the inhibitor I kappa B alpha/MAD3 or the p105 and p100 precursors of the p50 and p52 DNA-binding subunits. Treatment of cells with classical NF-kappa B inducers like TNF, IL-1, PMA or LPS results in MAD-3 degradation followed by nuclear translocation of NF-kappa B. On the other hand, the mechanisms involved in the dissociation of the cytoplasmic p105/p100-containing complexes are largely unknown. We demonstrate here that Tax can induce translocation of members of the NF-kappa B family retained in the cytoplasm through interaction with either p105 or p100. On the other hand Tax induces no apparent degradation of MAD-3. These results suggest that Tax activates NF-kappa B essentially through the p105/p100-retention pathway.

Protein domains involved in both in vivo and in vitro interactions between human T-cell leukemia virus type I tax and CREB

Gene expression from the human T-cell leukemia virus type I (HTLV-I) long terminal repeat (LTR) is mediated by three cis-acting regulatory elements known as 21-bp repeats and the transactivator protein Tax. The 21-bp repeats can be subdivided into three motifs known as A, B, and C, each of which is important for maximal gene expression in response to Tax. The B motif contains nucleotide sequences known as a cyclic AMP response element (CRE) or tax-response element which binds members of the ATF/CREB family of transcription factors. Though mutations of this element in the HTLV-I LTR eliminate tax activation, Tax will not activate most other promoters containing these CRE sites. In this study, we investigated the mechanism by which Tax activates gene expression in conjunction with members of the ATF/CREB family. We found that Tax enhanced the binding of one member of the ATF/CREB family, CREB 1, to each of the three HTLV-I LTR 21-bp repeats but not another member designated CRE-BP1 or CREB2. Tax enhanced the binding of CREB1 to nonpalindromic CRE binding sites such as those found in the HTLV-I LTR, but Tax did not enhance the binding of CREB1 to palindromic CRE binding sites such as found in the somatostatin promoter. This finding may help explain the failure of Tax to activate promoters containing consensus CRE sites. These studies were extended by use of the mammalian two-hybrid system. Tax was demonstrated to interact directly with CREB1 but not with other bZIP proteins, including CREB2 and Jun. Site-directed mutagenesis of both Tax and CREB1 demonstrated that the amino terminus of Tax and both the basic and the leucine zipper regions of CREB1 were required for direct interactions between these proteins both in vivo and in vitro. This interaction occurred in vivo and thus did not require the presence of the HTLV-I 21-bp repeats, as previously suggested. These results define the domains required for interaction between Tax and CREB that are likely critical for the activation of HTLV-I gene expression.

A molecular mechanism for human T-cell leukemia virus latency and Tax transactivation

The human T-cell leukemia virus type I (HTLV-I) is the causative agent of an aggressive T-cell malignancy in humans. While the virus appears to maintain a state of latency in most infected cells, high level virion production is an essential step in the HTLV-I life cycle. The virally-encoded Tax protein, a potent activator of gene expression, is believed to control the switch from latency to replication. Tax stimulation of HTLV-I transcription is mediated through cellular activating transcription factor/cAMP response element binding proteins, which bind the three 21-base pair (bp) repeat viral enhancer elements. In this report, we show that viral latency may result from a highly unstable interaction between CREB and the HTLV-I 21-bp repeats, resulting in rapid dissociation of CREB from the viral promoter. In the presence Tax, the dissociation rate of CREB from a 21-bp repeat element is decreased. This stabilization is highly specific, requiring the amino-terminal region of CREB and appropriate 21-bp repeat sequences. We suggest that Tax stabilization of CREB binding to the viral promoter leads to an increase in gene expression, possibly providing the switch from latency to high level replication of the virus.

Coupling of a signal response domain in I kappa B alpha to multiple pathways for NF-kappa B activation

The eukaryotic transcription factor NF-kappa B plays a central role in the induced expression of human immunodeficiency virus type 1 and in many aspects of the genetic program mediating normal T-cell activation and growth. The nuclear activity of NF-kappa B is tightly regulated from the cytoplasmic compartment by an inhibitory subunit called I kappa B alpha. This cytoplasmic inhibitor is rapidly phosphorylated and degraded in response to a diverse set of NF-kappa B-inducing agents, including T-cell mitogens, proinflammatory cytokines, and viral transactivators such as the Tax protein of human T-cell leukemia virus type 1. To explore these I kappa B alpha-dependent mechanisms for NF-kappa B induction, we identified novel mutants of I kappa B alpha that uncouple its inhibitory and signal-transducing functions in human T lymphocytes. Specifically, removal of the N-terminal 36 amino acids of I kappa B alpha failed to disrupt its ability to form latent complexes with NF-kappa B in the cytoplasm. However, this deletion mutation prevented the induced phosphorylation, degradative loss, and functional release of I kappa B alpha from NF-kappa B in Tax-expressing cells. Alanine substitutions introduced at two serine residues positioned within this N-terminal regulatory region of I kappa B alpha also yielded constitutive repressors that escaped from Tax-induced turnover and that potently inhibited immune activation pathways for NF-kappa B induction, including those initiated from antigen and cytokine receptors. In contrast, introduction of a phosphoserine mimetic at these sites rectified this functional defect, a finding consistent with a causal linkage between the phosphorylation status and proteolytic stability of this cytoplasmic inhibitor. Together, these in vivo studies define a critical signal response domain in I kappa B alpha that coordinately controls the biologic activities of I kappa B alpha and NF-kappa B in response to viral and immune stimuli.

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

Human T-cell leukemia virus type I (HTLV-I) encodes a strong transcriptional activator, Tax, that stimulates transcription indirectly through the viral long terminal repeat and also activates a number of cellular genes via association with host transcription factors. The NF-kappa B/Rel pathway is a target for Tax trans-activation, and Tax has been correlated with increased NF-kappa B-binding activity and NF-kappa B-dependent gene expression in HTLV-I-infected cells. In this study we demonstrate that constitutive phosphorylation and increased turnover of the regulatory I kappa B alpha protein in HTLV-I-infected MT-2 and C8166 cells and Tax-expressing 19D cells contribute to constitutive NF-kappa B-binding activity, which consists primarily of c-Rel, p52(NFKB2), and p50(NFKB1). I kappa B alpha mRNA expression is also increased 7- to 20-fold in these cells, although the steady-state level of I kappa B alpha protein is reduced in HTLV-I-infected and Tax-expressing T cells. These results indicate that the viral Tax protein, by indirectly mediating phosphorylation of I kappa B, may target I kappa B alpha for rapid degradation, thus leading to constitutive NF-kappa B activity.

Mechanisms of Tax Regulation of Human T Cell Leukemia Virus Type I Gene Expression

During the last several years, the human T cell leukemia virus type I (HTLV-I) has become recognized as an important cause for public health concern throughout the world. HTLV-I is the causative agent of a variety of clinical diseases, including an aggressive lymphoproliferative disorder named adult T cell leukemia. HTLV-I induces pathogenicity in the infected host cell through the synthesis of a virally encoded protein called Tax. Expression of Tax is critical to the life cycle of the virus, as the protein greatly increases the efficiency of HTLV-I gene transcription and replication. Furthermore, Tax has been shown to deregulate the transcription of many cellular genes, leading to the hypothesis that the presence of Tax promotes unchecked growth in the HTLV-I-infected cell. The mechanism of Tax trans-activation of HTLV-I gene expression is not known. Tax does not bind directly to the Tax-responsive promoter elements of the virus, but appears to function through interaction with certain cellular DNA binding proteins, including activating transcription factor 2 and cAMP response element binding protein that recognize these sequences. This review summarizes some of the recent work in the field aimed at elucidating the mechanism of Tax trans-activation of HTLV-I gene expression. Copyright 1995 S. Karger AG, Basel

Autoantigen Ku protein is involved in DNA binding proteins which recognize the U5 repressive element of human T-cell leukemia virus type I long terminal repeat

We have identified and analyzed a 27-nucleotide sequence (U5 repressive element, designated as U5RE) at the U5 region of the human T-cell leukemia virus type I (HTLV-I) long terminal repeat (LTR) which is required for HTLV-I basal transcriptional repression. The basal promoter strength of constructs that contained deletions in the U5 region of the LTR was analyzed by chloramphenicol acetyltransferase (CAT) assays following transfection of HeLa cells or Jurkat T-cells in the presence or absence of viral transactivator tax protein. We consistently observed a 2- to 5-fold increase in basal promoter activity when sequences between +277 to +306 were deleted. In vivo competition experiments suggested that the U5 DNA fragment from +269 to +295 contains a functional repressive element (U5RE). Using gel mobility shift assays, we have purified a highly enriched fraction that could specifically bind U5RE. This DNA affinity column fraction contained three major detectable proteins on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with silver staining: 110-, 80- and 70-kDa proteins. The 110-kDa protein appeared to be a novel DNA-binding protein whose characteristics are still obscure, while the 70- and 80-kDa proteins were shown to be related to the human autoantigen Ku, the Ku (p70/p80) complex, as demonstrated by amino acid sequencing and immunological analyses. As Ku is known to be involved in transcriptional regulation, the specific interaction of Ku with U5RE raises intriguing possibilities for its function in HTLV-I basal transcriptional repression.

Tax induces nuclear translocation of NF-kappa B through dissociation of cytoplasmic complexes containing p105 or p100 but does not induce degradation of I kappa B alpha/MAD3

The activity of the NF-kappa B transcription factor is controlled through cytoplasmic retention by either of two types of molecules: the inhibitor I kappa B alpha/MAD3 or the p105 and p100 precursors of the p50 and p52 DNA-binding subunits. Treatment of cells with classical NF-kappa B inducers such as tumor necrosis factor, interleukin-1, phorbol myristate acetate, and lipopolysaccharide results in MAD3 degradation followed by nuclear translocation of NF-kappa B. On the other hand, the mechanisms involved in the dissociation of the cytoplasmic p105/p100-containing complexes are largely unknown. The Tax protein encoded by human T-cell leukemia virus type 1 is a potent activator of viral and cellular gene transcription. It does not bind DNA directly but seems to activate transcription indirectly either by enhancing the activities of the transcription factors that recognize responsive elements located in the promoters of the Tax-responsive genes or by forming ternary complexes with these factors and DNA. It has been previously shown that Tax is able to induce nuclear translocation of NF-kappa B. We demonstrate here that Tax can induce translocation of members of the NF-kappa B family retained in the cytoplasm through their interaction with either p105 or p100. On the other hand, Tax induces no apparent degradation of MAD3, although experiments using cycloheximide indicate that it decreases the half-life of MAD3. However, this activity is shared by a mutant of Tax which is unable to activate NF-kappa B. These results suggest that Tax activates NF-kappa B essentially through the p105/p100 retention pathway.

Human T-cell leukemia virus type I Tax activation of NF-kappa B/Rel involves phosphorylation and degradation of I kappa B alpha and RelA (p65)-mediated induction of the c-rel gene

The tax gene product of human T-cell leukemia virus type I (HTLV-I) is a potent transcriptional activator that both stimulates viral gene expression and activates an array of cellular genes involved in T-cell growth. Tax acts indirectly by inducing or modifying the action of various host transcription factors, including members of the NF-kappa B/Rel family of enhancer-binding proteins. In resting T cells, many of these NF-kappa B/Rel factors are sequestered in the cytoplasm by various ankyrin-rich inhibitory proteins, including I kappa B alpha. HTLV-I Tax expression leads to the constitutive nuclear expression of biologically active NF-kappa B and c-Rel complexes; however, the biochemical mechanism(s) underlying this response remains poorly understood. In this study, we demonstrate that Tax-stimulated nuclear expression of NF-kappa B in both HTLV-I-infected and Tax-transfected human T cells is associated with the phosphorylation and rapid proteolytic degradation of I kappa B alpha. In contrast to prior in vitro studies, at least a fraction of the phosphorylated form of I kappa B alpha remains physically associated with the NF-kappa B complex in vivo but is subject to rapid degradation, thereby promoting the nuclear translocation of the active NF-kappa B complex. We further demonstrate that Tax induction of nuclear c-Rel expression is activated by the RelA (p65) subunit of NF-kappa B, which activates transcription of the c-rel gene through an intrinsic kappa B enhancer element. In normal cells, the subsequent accumulation of nuclear c-Rel acts to inhibit its own continued production, indicating the presence of an autoregulatory loop. However, the pathologic action HTLV-I Tax leads to the deregulated and sustained nuclear expression of both NF-kappa B and c-Rel, a response that may contribute to HTLV-I-induced T-cell transformation.

Human T-cell leukemia virus type I Tax activation of NF-kappa B/Rel involves phosphorylation and degradation of I kappa B alpha and RelA (p65)-mediated induction of the c-rel gene

The tax gene product of human T-cell leukemia virus type I (HTLV-I) is a potent transcriptional activator that both stimulates viral gene expression and activates an array of cellular genes involved in T-cell growth. Tax acts indirectly by inducing or modifying the action of various host transcription factors, including members of the NF-kappa B/Rel family of enhancer-binding proteins. In resting T cells, many of these NF-kappa B/Rel factors are sequestered in the cytoplasm by various ankyrin-rich inhibitory proteins, including I kappa B alpha. HTLV-I Tax expression leads to the constitutive nuclear expression of biologically active NF-kappa B and c-Rel complexes; however, the biochemical mechanism(s) underlying this response remains poorly understood. In this study, we demonstrate that Tax-stimulated nuclear expression of NF-kappa B in both HTLV-I-infected and Tax-transfected human T cells is associated with the phosphorylation and rapid proteolytic degradation of I kappa B alpha. In contrast to prior in vitro studies, at least a fraction of the phosphorylated form of I kappa B alpha remains physically associated with the NF-kappa B complex in vivo but is subject to rapid degradation, thereby promoting the nuclear translocation of the active NF-kappa B complex. We further demonstrate that Tax induction of nuclear c-Rel expression is activated by the RelA (p65) subunit of NF-kappa B, which activates transcription of the c-rel gene through an intrinsic kappa B enhancer element. In normal cells, the subsequent accumulation of nuclear c-Rel acts to inhibit its own continued production, indicating the presence of an autoregulatory loop. However, the pathologic action HTLV-I Tax leads to the deregulated and sustained nuclear expression of both NF-kappa B and c-Rel, a response that may contribute to HTLV-I-induced T-cell transformation.

Serum response factor has functional roles both in indirect binding to the CArG box and in the transcriptional activation function of human T-cell leukemia virus type I Tax

We previously reported that Tax1 of human T-cell leukemia virus type I interacts directly with serum response factor (SRF) and that Tax1 activates the transcription of several cellular immediate-early genes through the SRF binding site (CArG box). This activation required the transcriptional activation function of Tax1, identified as an activity of GALTax (a chimeric Tax1 with the yeast transcription factor GAL4) at the GAL4-binding site. In this study, we examined whether SRF plays a role in the transcriptional activation function of Tax1. Expression of Tax1 suppressed the GALTax activity at the GAL4 site as a result of squelching, and the suppressed activity was recovered by the overexpression of SRF, suggesting that SRF is a factor that is required for GALTax activity and that is inhibited by competition with Tax1. The expression of antisense SRF RNA specifically inhibited GALTax activity to less than 20%. Deletion of the Tax1 interaction domain of SRF at its C terminus converted SRF from an activator of GALTax to an inhibitor. These results suggest that SRF is an essential component of the transcriptional activation of Tax1 in addition to a mediator of CArG box binding.

Kinetic analysis of human T-cell leukemia virus type I Tax-mediated activation of NF-kappa B

The human T-cell leukemia virus type I (HTLV-I) Tax protein induces the expression of cellular genes, at least in part, by activating the endogenous NF-kappa B transcription factors. Induced expression of cellular genes is thought to be important for transformation of T cells to continued growth, a prelude to the establishment of adult T-cell leukemia. However, neither underlying mechanisms nor kinetics of the Tax-mediated activation of NF-kappa B are understood. We have analyzed a permanently transfected Jurkat T-cell line in which the expression of Tax is entirely dependent on addition of heavy metals. The initial NF-kappa B binding activity seen after induction of Tax is due almost exclusively to p50/p65 heterodimers. At later times, NF-kappa B complexes containing c-Rel and/or p52 accumulate. The early activation of p50/p65 complexes is a posttranslational event, since neither mRNA nor protein levels of NF-kappa B subunits had increased at that time. We demonstrate for the first time a Tax-induced proteolytic degradation of the NF-kappa B inhibitor, I kappa B-alpha, which may trigger the initial nuclear translocation of NF-kappa B. As nuclear NF-kappa B rapidly and potently stimulates resynthesis of I kappa B-alpha, the steady-state level of I kappa B-alpha does not significantly change. Thus, the dramatic Tax-induced increase in the I kappa B-alpha turnover may continually weaken inhibition and activate NF-kappa B. Additional, distinct actions of Tax may contribute further to the high levels of NF-kappa B activity seen.

Kinetic analysis of human T-cell leukemia virus type I Tax-mediated activation of NF-kappa B

The human T-cell leukemia virus type I (HTLV-I) Tax protein induces the expression of cellular genes, at least in part, by activating the endogenous NF-kappa B transcription factors. Induced expression of cellular genes is thought to be important for transformation of T cells to continued growth, a prelude to the establishment of adult T-cell leukemia. However, neither underlying mechanisms nor kinetics of the Tax-mediated activation of NF-kappa B are understood. We have analyzed a permanently transfected Jurkat T-cell line in which the expression of Tax is entirely dependent on addition of heavy metals. The initial NF-kappa B binding activity seen after induction of Tax is due almost exclusively to p50/p65 heterodimers. At later times, NF-kappa B complexes containing c-Rel and/or p52 accumulate. The early activation of p50/p65 complexes is a posttranslational event, since neither mRNA nor protein levels of NF-kappa B subunits had increased at that time. We demonstrate for the first time a Tax-induced proteolytic degradation of the NF-kappa B inhibitor, I kappa B-alpha, which may trigger the initial nuclear translocation of NF-kappa B. As nuclear NF-kappa B rapidly and potently stimulates resynthesis of I kappa B-alpha, the steady-state level of I kappa B-alpha does not significantly change. Thus, the dramatic Tax-induced increase in the I kappa B-alpha turnover may continually weaken inhibition and activate NF-kappa B. Additional, distinct actions of Tax may contribute further to the high levels of NF-kappa B activity seen.

Involvement of the cyclic AMP-responsive element binding protein in bovine leukemia virus expression in vivo

The TAR element (Tax-responsive element; also called TxRE) is a major determinant of the regulation of bovine leukemia virus (BLV) expression. In order to gain insight into the mechanisms of viral expression, complexes formed between proteins and the TAR enhancer DNA were analyzed by gel retardation assays. We report here that nuclear lysates from ex vivo-isolated B lymphocytes contain proteins that specifically bind to TAR. An antibody directed toward the cyclic AMP-responsive element binding (CREB) protein supershifted a complex (C1) present only in BLV-infected B lymphocytes. The CREB protein thus appears to be a major transcription factor involved in BLV expression in vivo.

Transcriptional suppression of the human T-cell leukemia virus type I long terminal repeat occurs by an unconventional interaction of a CREB factor with the R region

To analyze regulation of the human T-cell leukemia virus type I (HTLV-I) long terminal repeat (LTR), cell lines were generated from LTR-tax x LTR-beta-galactosidase (beta-Gal) doubly transgenic mouse fibroblastic tumors. The HTLV-I LTR directs expression of both the tax and lacZ genes, and Tax up-modulates both promoters in primary cells. However, once cells were transformed by tax, beta-Gal but not tax expression was suppressed. Supertransformation of these cells with v-src suppressed both beta-Gal and tax expression. This suppression was reversed by treatment with the tyrosine kinase inhibitor herbimycin A or protein kinase A inhibitor H8. Electrophoretic mobility shift assays demonstrated augmented binding in the R but not U3 region. This binding was competitively inhibited by a high-affinity CREB oligodeoxynucleotide and super-shifted with a specific CREB antibody. Treatment of cells with the cyclic AMP analog dibutyryl cyclic AMP also transiently increased the R region binding dramatically. In vitro DNase I footprint analysis identified a protein-binding sequence in the R region which corresponded with suppression. However, this target sequence lacked a conventional CREB-binding site. A 70.5-kDa DNA-binding protein was partially purified by affinity chromatography, along with a 49-kDa protein which reacted with CREB-specific sera. These data demonstrate that HTLV-I LTR suppression is associated with CREB factor binding in the R region, probably by direct interaction with a 70.5-kDa protein, and provide a novel mechanism for maintenance of viral latency.

Molecular cloning of a novel human cDNA encoding a zinc finger protein that binds to the interleukin-3 promoter

The CT/GC-rich region (-76 to -47) is one transcriptional regulatory region of the interleukin-3 (IL-3) gene which confers basic transcriptional activity and responds to trans-activation by human T-cell leukemia virus type I-encoded Tax. We isolated three types of cDNAs encoding Cys2/His2-type zinc finger proteins that bind to this region. Two were identical to known transcription factors, EGR1 and EGR2, and the other clone, named DB1, encoded a novel protein of 516 amino acids with six zinc finger motifs. DB1 mRNA was present in human tissues, ubiquitously. Two constitutive transcripts of 4.0 and 4.8 kb in length were present in Jurkat cells. Electrophoretic mobility shift assay, with specific antibodies, showed that DB1 constitutively binds to this region whereas EGR1 binds in a T-cell activation-dependent manner. Overexpression of DB1 in Jurkat cells had no detectable effect on the transcription activity of the IL-3 promoter, in a transient-transfection assay. EGR1 and EGR2 increased IL-3 promoter activity when the transfected cells were stimulated with phorbol-12-myristate-13-acetate and A23187. When DB1 was cotransfected with a Tax expression vector, transcription activity of the IL-3 promoter induced by Tax was significantly increased, while EGR1 and EGR2 were without effect. These results suggest that EGR1 has a role in inducible transcription of the IL-3 gene, while DB1 sustains basal transcriptional activity and also cooperates with Tax to activate the IL-3 promoter.

Quantitative studies of the effect of HTLV-I Tax protein on CREB protein--DNA binding

The human T-cell leukemia virus type I (HTLV-I) Tax protein increases the DNA binding activity of a number of different host cell transcription factors, including the cyclic AMP response element binding protein (CREB). We have performed quantitative studies of CREB binding in the presence and absence of Tax in an attempt to gain insight into the mechanism of the Tax effect. Enhancement of binding occurred over a wide range of CREB concentrations, but sharply increased at the lowest concentrations tested. The data are best explained by a two-step binding model where Tax changes the apparent equilibrium constants for both a CREB-CREB dimerization step and a (CREB)2-DNA binding step. We used the model to perform a quantitative analysis of the binding of CREB to DNA that had been mutated at positions flanking the core CREB recognition site. Results suggest that there are altered or more extensive DNA-protein contacts at these positions in the presence of Tax. We also used the model to analyze differences in the interaction of Tax with nonphosphorylated and protein kinase A-phosphorylated CREB protein. There was no significant change in the behavior of CREB upon phosphorylation.

Transcriptional suppression of the human T-cell leukemia virus type I long terminal repeat occurs by an unconventional interaction of a CREB factor with the R region

To analyze regulation of the human T-cell leukemia virus type I (HTLV-I) long terminal repeat (LTR), cell lines were generated from LTR-tax x LTR-beta-galactosidase (beta-Gal) doubly transgenic mouse fibroblastic tumors. The HTLV-I LTR directs expression of both the tax and lacZ genes, and Tax up-modulates both promoters in primary cells. However, once cells were transformed by tax, beta-Gal but not tax expression was suppressed. Supertransformation of these cells with v-src suppressed both beta-Gal and tax expression. This suppression was reversed by treatment with the tyrosine kinase inhibitor herbimycin A or protein kinase A inhibitor H8. Electrophoretic mobility shift assays demonstrated augmented binding in the R but not U3 region. This binding was competitively inhibited by a high-affinity CREB oligodeoxynucleotide and super-shifted with a specific CREB antibody. Treatment of cells with the cyclic AMP analog dibutyryl cyclic AMP also transiently increased the R region binding dramatically. In vitro DNase I footprint analysis identified a protein-binding sequence in the R region which corresponded with suppression. However, this target sequence lacked a conventional CREB-binding site. A 70.5-kDa DNA-binding protein was partially purified by affinity chromatography, along with a 49-kDa protein which reacted with CREB-specific sera. These data demonstrate that HTLV-I LTR suppression is associated with CREB factor binding in the R region, probably by direct interaction with a 70.5-kDa protein, and provide a novel mechanism for maintenance of viral latency.

Molecular cloning of a novel human cDNA encoding a zinc finger protein that binds to the interleukin-3 promoter

The CT/GC-rich region (-76 to -47) is one transcriptional regulatory region of the interleukin-3 (IL-3) gene which confers basic transcriptional activity and responds to trans-activation by human T-cell leukemia virus type I-encoded Tax. We isolated three types of cDNAs encoding Cys2/His2-type zinc finger proteins that bind to this region. Two were identical to known transcription factors, EGR1 and EGR2, and the other clone, named DB1, encoded a novel protein of 516 amino acids with six zinc finger motifs. DB1 mRNA was present in human tissues, ubiquitously. Two constitutive transcripts of 4.0 and 4.8 kb in length were present in Jurkat cells. Electrophoretic mobility shift assay, with specific antibodies, showed that DB1 constitutively binds to this region whereas EGR1 binds in a T-cell activation-dependent manner. Overexpression of DB1 in Jurkat cells had no detectable effect on the transcription activity of the IL-3 promoter, in a transient-transfection assay. EGR1 and EGR2 increased IL-3 promoter activity when the transfected cells were stimulated with phorbol-12-myristate-13-acetate and A23187. When DB1 was cotransfected with a Tax expression vector, transcription activity of the IL-3 promoter induced by Tax was significantly increased, while EGR1 and EGR2 were without effect. These results suggest that EGR1 has a role in inducible transcription of the IL-3 gene, while DB1 sustains basal transcriptional activity and also cooperates with Tax to activate the IL-3 promoter.

Novel interactions between human T-cell leukemia virus type I Tax and activating transcription factor 3 at a cyclic AMP-responsive element

Human proenkephalin gene transcription is transactivated by human T-cell leukemia virus type I (HTLV-I) Tax in human Jurkat T lymphocytes. This transactivation was further enhanced in Jurkat cells treated with concanavalin A, cyclic AMP, or 12-O-tetradecanoylphorbol-13-acetate. Deletion and cis-element transfer analyses of the human proenkephalin promoter identified a cyclic AMP-responsive AP-1 element (-92 to -86) as both necessary and sufficient to confer Tax-dependent transactivation. Different AP-1 or cyclic AMP-responsive element-binding protein (CREB)/activating transcription factor (ATF) proteins which bind this element were expressed in murine teratocarcinoma F9 cells to identify those capable of mediating Tax-dependent transactivation of human proenkephalin gene transcription. Although CREB, c-Fos, c-Jun, and JunD did not have significant effects, JunB inhibited the Tax-dependent transactivation. In contrast, ATF3 dramatically induced Tax-dependent transactivation, which was further enhanced by protein kinase A. Electrophoretic mobility shift assays with recombinant fusion proteins expressed and purified from bacteria indicate that the DNA-binding activity of ATF3 is also dramatically enhanced by Tax. Chimeric fusion proteins consisting of the DNA-binding domain of the yeast transcription factor Gal4 and the amino-terminal domain (residues 1 to 66) of ATF3 were able to mediate Tax-dependent transactivation of a Gal4-responsive promoter, which suggests a direct involvement of this region of ATF3. Recombinant fusion proteins of glutathione S-transferase with either the amino- or carboxy-terminal (residues 139 to 181) domain of ATF3 were able to specifically interact with Tax. Furthermore, specific antisera directed against Tax coimmunoprecipitated ATF3 only in the presence of Tax.

Identification of human T-cell lymphotropic virus type I 21-base-pair repeat-specific and glial cell-specific DNA-protein complexes

The human T-cell lymphotropic virus type I (HTLV-I)-encoded protein, Tax, is capable of trans-activating HTLV-I transcription by interacting with specific sequences in the HTLV-I long terminal repeat (LTR) which comprise an inducible enhancer containing three imperfect tandem repeats of a 21-bp sequence. There is no evidence that purified Tax can bind to DNA in the absence of cellular factors, suggesting that Tax most likely regulates transcription via interaction with cellular factors. Since HTLV-I is a documented agent of adult T-cell leukemia and tropical spastic paraparesis, disorders of the immune and nervous systems, respectively, characterization of cellular factors of lymphoid and neuroglial origin which interact with the 21-bp repeat elements is essential to understanding of the mechanisms involved in basal and Tax-mediated transcription in cells of immune and nervous system origin. Utilizing electrophoretic mobility shift (EMS) analyses, we have detected both 21-bp repeat-specific and glial cell-specific DNA-protein complexes. Several 21-bp repeat-specific DNA-protein complexes were detected when nuclear extracts derived from cells of lymphoid (Jurkat, SupT1, and H9), neuronal (IMR-32 and SK-N-MC), and glial (U-373 MG, Hs683, and U-118) origin were used in reactions with each of the three 21-bp repeat elements. In addition, a glial cell-specific DNA-protein complex was detected when nuclear extracts derived from U-373 MG, Hs683, and U-118 glial cell lines reacted with the promoter-distal and central 21-bp repeat elements. Furthermore, EMS analyses performed with nuclear extracts derived from lymphocytic and glial cell origin and a 223-bp fragment of the HTLV-I long terminal repeat encompassing the three 21-bp repeat elements (designated Tax-responsive elements 1 and 2, TRE-1/-2) have also resulted in the detection of glial cell type-specific DNA-protein complexes. Competition EMS analyses with oligonucleotides containing transcription factor binding site sequences indicate the involvement of a cyclic AMP response element binding protein in the formation of DNA-protein complexes which form with all three 21-bp repeat elements and the glial cell-specific DNA-protein complex as well as the involvement of Sp1 or an Sp1-related factor in the formation of the 21-bp repeat III-specific DNA-protein complexes.

Novel interactions between human T-cell leukemia virus type I Tax and activating transcription factor 3 at a cyclic AMP-responsive element

Human proenkephalin gene transcription is transactivated by human T-cell leukemia virus type I (HTLV-I) Tax in human Jurkat T lymphocytes. This transactivation was further enhanced in Jurkat cells treated with concanavalin A, cyclic AMP, or 12-O-tetradecanoylphorbol-13-acetate. Deletion and cis-element transfer analyses of the human proenkephalin promoter identified a cyclic AMP-responsive AP-1 element (-92 to -86) as both necessary and sufficient to confer Tax-dependent transactivation. Different AP-1 or cyclic AMP-responsive element-binding protein (CREB)/activating transcription factor (ATF) proteins which bind this element were expressed in murine teratocarcinoma F9 cells to identify those capable of mediating Tax-dependent transactivation of human proenkephalin gene transcription. Although CREB, c-Fos, c-Jun, and JunD did not have significant effects, JunB inhibited the Tax-dependent transactivation. In contrast, ATF3 dramatically induced Tax-dependent transactivation, which was further enhanced by protein kinase A. Electrophoretic mobility shift assays with recombinant fusion proteins expressed and purified from bacteria indicate that the DNA-binding activity of ATF3 is also dramatically enhanced by Tax. Chimeric fusion proteins consisting of the DNA-binding domain of the yeast transcription factor Gal4 and the amino-terminal domain (residues 1 to 66) of ATF3 were able to mediate Tax-dependent transactivation of a Gal4-responsive promoter, which suggests a direct involvement of this region of ATF3. Recombinant fusion proteins of glutathione S-transferase with either the amino- or carboxy-terminal (residues 139 to 181) domain of ATF3 were able to specifically interact with Tax. Furthermore, specific antisera directed against Tax coimmunoprecipitated ATF3 only in the presence of Tax.