A unique enhancer element for the trans activator (p40tax) of human T-cell leukemia virus type I that is distinct from cyclic AMP- and 12-O-tetradecanoylphorbol-13-acetate-responsive elements

Conformation of Tax-response elements in the human T-cell leukemia virus type I promoter

BACKGROUND

HTLV-I Tax is believed to activate viral gene expression by binding bZIP proteins (such as CREB) and increasing their affinities for proviral TRE target sites. Each 21 bp TRE target site contains an imperfect copy of the intrinsically bent CRE target site (the TRE core) surrounded by highly conserved flanking sequences. These flanking sequences are essential for maximal increases in DNA affinity and transactivation, but they are not, apparently, contacted by protein. Here we employ non-denaturing gel electrophoresis to evaluate TRE conformation in the presence and absence of bZIP proteins, and to explore the role of DNA conformation in viral transactivation.

RESULTS

Our results show that the TRE-1 flanking sequences modulate the structure and modestly increase the affinity of a CREB bZIP peptide for the TRE-1 core recognition sequence. These flanking sequences are also essential for a maximal increase in stability of the CREB-DNA complex in the presence of Tax.

CONCLUSIONS

The CRE-like TRE core and the TRE flanking sequences are both essential for formation of stable CREB-TRE-1 and Tax-CREB-TRE-1 complexes. These two DNA segments may have co-evolved into a unique structure capable of recognizing Tax and a bZIP protein.

Analysis of a novel defective HTLV-I provirus and detection of a new HTLV-I-induced cellular transcript

HTLV-I generally integrates at least one full-length copy in adult T-cell leukemia (ATL) cells. A group of patients without full-length provirus have a unique conserved truncation of the provirus which retains env-pX-3'LTR. Tumor cells of a patient from this group were genetically analyzed. Analysis of the 5' and 3' cellular flanking region adjacent to the provirus suggest that the defective provirus was integrated immediately downstream of a promoter of an unknown cellular gene. The activity of the promoter was weak but was responsive to Tax-like HTLV-I LTR. The provirus may have utilized it as a substitute for the 5'LTR and thus 3'LTR may have become an alternative promoter for the cellular gene, which may give similar viral-cellular interactions to that of general cases with full-length proviruses. Surprisingly, the 3' cellular flanking region which is thought to be controlled originally by the promoter is constitutively expressed specifically in an HTLV-I producing ATL cell line HUT1O2G, in which the corresponding region is not modified by provirus. The detection of this HTLV-I-induced transcript provides a probe to find an HTLV-I inducible unknown cellular gene that may be related to the pathogenesis of ATL.

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.

The amino terminus of Tax is required for interaction with the cyclic AMP response element binding protein

Tax of human T-cell leukemia virus type 1 was analyzed for interaction with the cyclic AMP response element binding protein (CREB) in vitro with and without Tax response element DNA. Mutations in the carboxy terminus of Tax (L296G and L320G) did not affect binding to CREB and led to supershifts. In contrast, mutants with changes in the amino-terminal cysteine-rich region lost the ability to bind to CREB. The S10A mutant protein bound moderately. Thus, the amino terminus of Tax is essential for Tax-CREB interaction.

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.

Human T-cell leukaemia virus

HTLV-I has a complex and finely regulated mechanism of replication, which can be used as a model to study both cellular and viral regulation pathways in T-cells. Understanding of the underlying mechanisms involved in the pleiotropic effects of HTLV-I in the host represents a real challenge. Immunological regulation likely plays a central role in HTLV-I induced neurological disease, uveitis, and perhaps arthritis, implicating the importance of host factors as well. Viral proteins, including tax and p12' might play a role in T-cell proliferation, but the event(s) that result in the late leukaemic phase are unknown. The lack of effective therapy against HTLV-I-induced leukaemia renders prevention of viral infection the best means to eliminate HTLV-I associated diseases. Elimination or reduction of breast feeding from seropositive mothers in Japan has already produced encouraging results. In developing countries, probably only a vaccine will prevent the spread of HTLV-I infection. The molecular epidemiology of HTLV and STLV will help understand not only the phylogeny of these viruses but also the migration of human populations in the past. Episodes of horizontal transmission in the past and probably the present, indicates that nonhuman primates are the natural reservoir of HTLVs. New related viruses will likely be discovered in monkeys (and humans) in the future.

Distinct regions in human T-cell lymphotropic virus type I tax mediate interactions with activator protein CREB and basal transcription factors

Human T-cell lymphotropic virus type I (HTLV-I) transactivator Tax augments transcription from three (cyclic AMP response element (CRE)-containing 21-bp repeats in the viral long terminal repeat and several other cis regulatory elements, including the NF-kappa B binding sites and the serum response element. Tax does not bind DNA directly; rather, it acts via cellular sequence-specific DNA binding proteins to stimulate transcription. We have shown recently that Tax forms multiprotein complexes with the heterodimeric and homodimeric forms of a ubiquitous cellular transcription factor, CREB (CRE binding protein). In vitro selection for preferred Tax-CREB binding sites indicates that the Tax-CREB complex exhibits greatly increased DNA recognition specificity and assembles preferentially on CRE motifs, TGACGT/C, flanked by long runs of G (5') and/or C (3') residues, as found in the HTLV-I 21-bp repeats. The indirect tethering of Tax to the 21-bp repeats via CREB is crucial for Tax transactivation. We now report the domain organization of Tax by characterizing its mutants. Tax mutants with alterations in the NH2 terminus, including three deletion mutants, Tax(6-353), Tax(21-353), and Tax(89-353), and two amino acid substitution mutants, M1 (H3S) and M7 (C29A, P30S), all failed to interact with CREB in vitro. In contrast, a short COOH-terminal deletion, Tax(1-319), and a Tax mutant with amino acid substitutions near the COOH end, M47 (L319R, L320S), were able to interact with CREB and the 21-bp repeats to assemble ternary Tax-CREB-DNA complexes. As demonstrated earlier, M1, M7, and M47 all failed to transactivate the HTLV-I long terminal repeat. Our data indicate that the defects in M1 and M7 result from an inability to interact with CREB. In contrast, the COOH-terminal mutations in M47 most likely inactivated the transactivation domain of Tax. As anticipated, a Tax mutant, M22 (G137A, L138S) which activated transcription from the 21-bp repeats with reduced capacity and was defective in trans activating the NF-kappa B binding sites, continued to interact with CREB in vitro, albeit with a lower level of efficiency. Finally, a glutathione S-transferase (GST)-Tax fusion protein with the GST moiety fused to the NH2 terminus of Tax failed to interact with CREB. Removal of the GST domain from GST-Tax by thrombin restores Tax's ability to assemble a ternary Tax-CREB-21-bp-repeat complex.(ABSTRACT TRUNCATED AT 400 WORDS)

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

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.

Stimulation of virus production and induction of self-syncytium formation in human T-cell leukemia virus type I- and type II-infected T cells by 12-O-tetradecanoylphorbol-13-acetate

Treatment of human T-cell leukemia virus type I (HTLV-I)- and HTLV-II-infected T-cell lines with 12-O-tetradecanoylphorbol-13-acetate (TPA) stimulated virus release. However, this stimulation was mainly detected at 42 to 48 h of treatment, whereas later virus release declined rapidly. During the first 48 h, TPA had no effect on cell growth, but later, the number of viable cells was profoundly lower in the TPA-treated than in the untreated cultures. This shift in virus release and cell number resulted from self-fusion of a large proportion of the virus-producing cells, which seemed to consequently enter into a dying process. This fusion, which resulted in syncytium formation, was strongly inhibited by anti-HTLV-I env monoclonal antibodies. Furthermore, no self-fusion was detected in three different uninfected T-cell lines similarly treated with TPA. On the other hand, stimulation of virus production by 3-methylcholanthrene (3-MC) treatment failed to induce self-fusion in the infected cells. Moreover, no syncytium was detected when these 3-MC-treated infected cells were cocultured with any of the TPA-treated uninfected cells. The effects of TPA on virus production and syncytium formation were both abolished by three different protein kinase C inhibitors. Taken together, these data suggest that the self-fusion observed in these experiments required both enhanced virus production and protein kinase C-phosphorylated viral or/and virally induced cellular component(s).

In vitro selection of DNA elements highly responsive to the human T-cell lymphotropic virus type I transcriptional activator, Tax

The human T-cell lymphotropic virus type I (HTLV-I) transactivator, Tax, the ubiquitous transcriptional factor cyclic AMP (cAMP) response element-binding protein (CREB protein), and the 21-bp repeats in the HTLV-I transcriptional enhancer form a ternary nucleoprotein complex (L. J. Zhao and C. Z. Giam, Proc. Natl. Acad. Sci. USA 89:7070-7074, 1992). Using an antibody directed against the COOH-terminal region of Tax along with purified Tax and CREB proteins, we selected DNA elements bound specifically by the Tax-CREB complex in vitro. Two distinct but related groups of sequences containing the cAMP response element (CRE) flanked by long runs of G and C residues in the 5' and 3' regions, respectively, were preferentially recognized by Tax-CREB. In contrast, CREB alone binds only to CRE motifs (GNTGACG[T/C]) without neighboring G- or C-rich sequences. The Tax-CREB-selected sequences bear a striking resemblance to the 5' or 3' two-thirds of the HTLV-I 21-bp repeats and are highly inducible by Tax. Gel electrophoretic mobility shift assays, DNA transfection, and DNase I footprinting analyses indicated that the G- and C-rich sequences flanking the CRE motif are crucial for Tax-CREB-DNA ternary complex assembly and Tax transactivation but are not in direct contact with the Tax-CREB complex. These data show that Tax recruits CREB to form a multiprotein complex that specifically recognizes the viral 21-bp repeats. The expanded DNA binding specificity of Tax-CREB and the obligatory role the ternary Tax-CREB-DNA complex plays in transactivation reveal a novel mechanism for regulating the transcriptional activity of leucine zipper proteins like CREB.

In vitro selection of DNA elements highly responsive to the human T-cell lymphotropic virus type I transcriptional activator, Tax

The human T-cell lymphotropic virus type I (HTLV-I) transactivator, Tax, the ubiquitous transcriptional factor cyclic AMP (cAMP) response element-binding protein (CREB protein), and the 21-bp repeats in the HTLV-I transcriptional enhancer form a ternary nucleoprotein complex (L. J. Zhao and C. Z. Giam, Proc. Natl. Acad. Sci. USA 89:7070-7074, 1992). Using an antibody directed against the COOH-terminal region of Tax along with purified Tax and CREB proteins, we selected DNA elements bound specifically by the Tax-CREB complex in vitro. Two distinct but related groups of sequences containing the cAMP response element (CRE) flanked by long runs of G and C residues in the 5' and 3' regions, respectively, were preferentially recognized by Tax-CREB. In contrast, CREB alone binds only to CRE motifs (GNTGACG[T/C]) without neighboring G- or C-rich sequences. The Tax-CREB-selected sequences bear a striking resemblance to the 5' or 3' two-thirds of the HTLV-I 21-bp repeats and are highly inducible by Tax. Gel electrophoretic mobility shift assays, DNA transfection, and DNase I footprinting analyses indicated that the G- and C-rich sequences flanking the CRE motif are crucial for Tax-CREB-DNA ternary complex assembly and Tax transactivation but are not in direct contact with the Tax-CREB complex. These data show that Tax recruits CREB to form a multiprotein complex that specifically recognizes the viral 21-bp repeats. The expanded DNA binding specificity of Tax-CREB and the obligatory role the ternary Tax-CREB-DNA complex plays in transactivation reveal a novel mechanism for regulating the transcriptional activity of leucine zipper proteins like CREB.

Human T-cell leukemia virus type 1 Tax activates transcription of the human fra-1 gene through multiple cis elements responsive to transmembrane signals

We have shown that Tax1 of human T-cell leukemia virus type 1 stimulates the expression of several cellular immediate-early genes (M. Fujii, T. Niki, T. Mori, T. Matsuda, M. Matsui, N. Nomura, and M. Seiki, Oncogene 6:1023-1029, 1991). In this study, the 5'-flanking region of the human fra-1 gene, which is a Tax1-inducible fos-related gene, was isolated and Tax1 or serum-responsive cis elements were analyzed to obtain further insight into the mechanism of Tax1 action. The 62-bp sequence starting 46 nucleotides upstream from the translation initiation site showed 71% homology with the sequence surrounding the TATA box of the c-fos promoter. Regulatory motifs identified in the c-fos promoter, such as an Ets-binding site, E boxes, a CArG box, c-fos AP-1 sites, and two retinoblastoma control elements, were also found upstream of the c-fos homology region. A 502-bp fragment containing these motifs mediated transcriptional activation by Tax1 or by serum in a transient transfection assay. Three independent Tax1-responsive regions (TRRs) were identified, and mutations in each revealed that one of the retinoblastoma control elements in TRR1 and the c-fos AP-1 sites in TRR2 and TRR3 were essential for the activation. Although TRR2 contains a CArG box-like sequence, it was a weak binding site for p67SRF, if it bound at all, and was not required for activation. All three TRRs could also mediate the signals stimulated by serum. Thus, Tax1 appears to activate fra-1 gene expression by means of a part of the cellular machinery similar to that which mediates growth signals.

HTLV-I Tax protein stimulation of DNA binding of bZIP proteins by enhancing dimerization

The Tax protein of human T cell leukemia virus type-1 (HTLV-I) transcriptionally activates the HTLV-I promoter. This activation requires binding sites for activating transcription factor (ATF) proteins, a family of cellular proteins that contain basic region-leucine zipper (bZIP) DNA binding domains. Data are presented showing that Tax increases the in vitro DNA binding activity of multiple ATF proteins. Tax also stimulated DNA binding by other bZIP proteins, but did not affect DNA binding proteins that lack a bZIP domain. The increase in DNA binding occurred because Tax promotes dimerization of the bZIP domain in the absence of DNA, and the elevated concentration of the bZIP homodimer then facilitates the DNA binding reaction. These results help explain how Tax activates viral transcription and transforms cells.

A new regulatory element that augments the Tax-dependent enhancer of human T-cell leukemia virus type 1 and cloning of cDNAs encoding its binding proteins

The Tax protein of human T-cell leukemia virus type 1 (HTLV-1) trans activates the 21-bp enhancer of HTLV-1. A sequence of more than two copies of the 21-bp enhancer is efficiently activated by Tax, but one copy is not activated extensively. Another sequence (TRE-2, positions -163 to -117) adjacent to the 21-bp enhancer in the long terminal repeat of HTLV-1 can enhance a single copy of the 21-bp enhancer activity in trans activation by Tax. This sequence contains motifs related to the Ets- and NF-kappa B-binding sequences, but mutations at these sites indicated that neither is responsive to cooperation with the 21-bp enhancer. A deletion mutation of TRE-2 identified 25 bases at positions -158 to -134 (TRE-2S) as an essential sequence, and TRE-2S was sufficient to give maximum cooperation with one copy of the 21-bp enhancer in trans activation by Tax protein. Using TRE-2S as a probe, we screened a cDNA library of HUT102 cells by the Southwestern (DNA-protein) procedure and isolated two cDNA clones, THP-1 and -2. These two clones encode TRE-2S-binding proteins, and they differ by only an extra 17 amino acids in THP-2. Both THP proteins contain five zinc finger motifs which are strikingly similar to those of the GLI family, an amplified gene product in glyoma cells. The binding site of THP-1 and -2 was GAACCACCCA in TRE-2S, which is highly homologous to the GLI-binding site. These results suggest that binding of THP to TRE-2S may be involved in cooperation with one copy of the 21-bp enhancer in responding to Tax trans activation.

Purification by DNA affinity precipitation of the cellular factors HEB1-p67 and HEB1-p94 which bind specifically to the human T-cell leukemia virus type-I 21 bp enhancer

Transcription driven by the proviral promoter of the Human T-cell Leukemia Virus type I (HTLV-I) is tightly regulated by the Tax1 transactivator. This viral protein potently induces the enhancer activity of a 21 bp motif repeated three times in the promoter. We have previously shown that this induction results from the binding of Tax1 to this enhancer sequence and that this association is mediated by the cellular factor HEB1. In this paper we report the purification of this factor by chromatography and DNA affinity precipitation. The latter method allowed a rapid and efficient purification which led to the identification of two polypeptides with molecular masses of 94- and 67-kDa, named HEB1-p94 and HEB1-p67, respectively. DMS methylation interference and UV crosslinking experiments indicated that both proteins formed different nucleo-protein complexes, but had the same DNA specificity. Study of the interaction of these two proteins with Tax1 showed that only HEB1-p67 can specifically interact with Tax1.

HTLV-1 Tax: regulation of gene expression and disease

The human T cell leukemia virus type 1 (HTLV-1) is a retrovirus that causes adult T cell leukemia and the neurological disorder tropical spastic paraparesis. Pathology apparently results from the activities of the Tax protein, which trans-activates the expression of viral and cellular genes by binding to various enhancer-binding proteins needed for transcription. This ultimately results in cell transformation, immortalization and disease.

Stimulation of HTLV-I expression by subtoxic dose of 3-methylcholanthrene

In vivo infection by HTLV-I is characterized by the lack of virus expression in the host's infected cells and in most cases it is clinically inapparent. The present study shows that exposure of HTLV-I-infected human T-cells to a maximal non-toxic dose of 3-MC remarkably enhances virus expression. This stimulatory effect was demonstrated by showing elevated levels of viral RNA and proteins within the cells and increased virus release to the culture medium. The enhanced virus expression was found to increase the ability of the treated cells to infect normal human peripheral lymphocytes in co-culture. Such data point to the possibility that initiation of a pathogenic process in HTLV-I-infected individuals might result from activating the expression of the latent provirus by environmental carcinogens.

Cloning of a cDNA encoding a DNA-binding protein TAXREB302 that is specific for the tax-responsive enhancer of HTLV-I

The transcriptional activator, Tax, of human T-cell leukemia virus (HTLV-I) has been considered to interact with cellular proteins to act on target enhancer motifs. Using oligodeoxyribonucleotides containing the tax-responsive element (TAXRE) of the HTLV-I enhancer, we have cloned multiple cDNAs coding for TAXRE-binding proteins (TAXREB), and determined the cDNA and the deduced 200-amino-acid sequences for TAXREB302. The recombinant protein binds to the enhancer DNA by specific interaction to the CRE-like sequence. A single 1.8-kb species of mRNA was detected in cultured cells, as well as in normal human tissues, especially brain and skeletal muscle. The 22-kDa native protein was detected in the cultured-cell lysate by immunoblotting analysis. TAXREB302 does not have structural features common to the CRE-binding protein or activating transcription factor (CREB/ATF) family, but has homology to chicken erythroid transcription factor (Eryf1 or GATA-1), suggesting a possible protein-protein interaction.

Isolation of a cDNA clone encoding DNA-binding protein (TAXREB107) that binds specifically to domain C of the tax-responsive enhancer element in the long terminal repeat of human T-cell leukemia virus type I

Five cDNA clones for TAXREB proteins that bind to the tax-responsive enhancer element of human T-cell leukemia virus type I (HTLV-I) were isolated from a Jurkat cell cDNA library. The beta-galactosidase fusion proteins of three of these clones specifically recognized the domain C within the enhancer. One of the three cDNAs, encoding TAXREB107, contained an open reading frame with 288 amino acid residues. RNA blot analysis showed that the level of mRNA for TAXREB107 increased transiently in Jurkat cells on treatment with TPA. Immunoblot analysis showed that polyclonal antibody against TAXREB107 specifically recognized a 34-kD protein in Jurkat cells. TAXREB107 may participate in tax-mediated trans-activation of transcription.

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

The trans-activator protein Tax of human T-cell leukemia virus type 1 (HTLV-1) activates the viral 21-base-pair (bp) enhancer in the long terminal repeat and has been suggested to associate indirectly with the enhancer DNA. To demonstrate this, we used DNA-affinity precipitation assay and detected the Tax protein in 21-bp DNA-protein complexes isolated from HTLV-1-infected cells. To identify cellular components in the complexes, we tested various 21-bp DNA-binding proteins by gel electrophoretic mobility-shift assay. Each binding protein gave a shifted band of each 21-bp DNA-protein complex, and exogenously added Tax protein further shifted these bands of cAMP-responsive element (CRE) binding protein (CREB) and CRE modulator but did not shift other bands. Anti-Tax antibodies blocked formation of the complex, indicating complex formations of [Tax-CREB(or CRE modulator)-21-bp DNA]. The formations of these complexes paralleled the functional activities of Tax mutants. Furthermore, the Tax-CREB complex was detected in a nuclear extract of HTLV-1-infected cells, and the Tax-CREB-21-bp-DNA complex was demonstrated as a major component of Tax complexes containing the 21-bp DNA probe. These observations indicate that Tax protein binds to CREB and CRE modulator and the complexes then bind to the 21-bp enhancer, suggesting that the complex binding to the enhancer mediates trans-activation of transcription.

Strong trans activation of the human cytomegalovirus major immediate-early enhancer by p40tax of human T-cell leukemia virus type I via two repetitive tax-responsive sequence elements

The immediate-early 1 and 2 gene locus of human cytomegalovirus (HCMV) that encodes trans-activator proteins with effects on both homologous and heterologous promoters is expressed under control of a complex enhancer/promoter regulatory region. This enhancer contains four types of repetitive sequence elements with 17, 18, 19, and 21 bp that bind cellular transcription factors. Although the HCMV enhancer acts as a powerful stimulator of transcription in most cell types examined, human T cells do not support strong activity. The present study demonstrates that the tax gene product of human T-cell leukemia virus type I trans activates the major enhancer of HCMV more than 60-fold in the T-cell line Jurkat. When a series of chloramphenicol acetyltransferase expression plasmids containing synthetic oligonucleotides with the 17-, 18-, 19-, or 21-bp motif upstream of a minimal immediate-early 1 and 2 gene promoter was tested, two of the four repeat motifs could be identified as Tax-responsive elements. Both the 18- and the 19-bp motifs were able to act as strong Tax-responsive elements even when they were present as single copies. Thus, in addition to interacting with human immunodeficiency virus, HCMV is able to interact with a second retrovirus of clinical importance.

Characterization of the promoter region of the src family gene lyn and its trans activation by human T-cell leukemia virus type I-encoded p40tax

The src family gene lyn is expressed preferentially in B lymphocytes but very little in normal T lymphocytes. Transcription of the lyn gene in T lymphocytes was shown to be induced by the p40tax protein encoded by human T-cell lymphotropic virus type I. For determination of the mechanism of p40tax-mediated trans activation, the transcriptional promoter region of the lyn gene was characterized. By endonuclease S1 mapping, the transcriptional initiation sites were identified within the 770-bp EcoRI-SacI fragment of the 5'-terminal portion of the human lyn gene. This fragment showed promoter activity when placed upstream of the bacterial chloramphenicol acetyltransferase gene and transfected into various cell lines. Nucleotide sequence analysis revealed that the lyn promoter region contained four GC box-like sequences but not a TATA or CCAAT box. In addition, it contained sequences characteristic of a cyclic AMP-responsive element, octamer-binding motif, PEA3-like motifs, and NF kappa B-binding motif-like sequence. Mutational analysis suggested that the octamer-binding motif sequence is of primary importance for the lyn promoter activity but that the other elements are not. Cotransfection of various chloramphenicol acetyltransferase constructs containing different length of the lyn promoter together with p40tax expression plasmids into Jurkat T cells showed that the sequence responsible for p40tax-induced transcription is present around the transcription initiation sites.

Characterization of the promoter region of the src family gene lyn and its trans activation by human T-cell leukemia virus type I-encoded p40tax

The src family gene lyn is expressed preferentially in B lymphocytes but very little in normal T lymphocytes. Transcription of the lyn gene in T lymphocytes was shown to be induced by the p40tax protein encoded by human T-cell lymphotropic virus type I. For determination of the mechanism of p40tax-mediated trans activation, the transcriptional promoter region of the lyn gene was characterized. By endonuclease S1 mapping, the transcriptional initiation sites were identified within the 770-bp EcoRI-SacI fragment of the 5'-terminal portion of the human lyn gene. This fragment showed promoter activity when placed upstream of the bacterial chloramphenicol acetyltransferase gene and transfected into various cell lines. Nucleotide sequence analysis revealed that the lyn promoter region contained four GC box-like sequences but not a TATA or CCAAT box. In addition, it contained sequences characteristic of a cyclic AMP-responsive element, octamer-binding motif, PEA3-like motifs, and NF kappa B-binding motif-like sequence. Mutational analysis suggested that the octamer-binding motif sequence is of primary importance for the lyn promoter activity but that the other elements are not. Cotransfection of various chloramphenicol acetyltransferase constructs containing different length of the lyn promoter together with p40tax expression plasmids into Jurkat T cells showed that the sequence responsible for p40tax-induced transcription is present around the transcription initiation sites.

Multistep regulation of enhancer activity of the 21-base-pair element of human T-cell leukemia virus type I

We examined the regulatory mechanisms of binding and transcriptional enhancement of the 21-bp core element of the enhancer of human T-cell leukemia virus type I (HTLV-I) in response to forskolin, 12-O-tetradecanoylphorbol-13-acetate (TPA), and a viral transactivator, p40tax. The 21-bp core element has been shown to bind to a cyclic AMP-responsive element binding protein (CREB)-like molecule at the site of an imperfect palindrome containing the TGAC motif. Experiments with oligonucleotides with mutations in the imperfect palindrome demonstrated that one TGAC motif is necessary and sufficient for both the binding of the CREB-related factor and transcriptional activity in response to forskolin in a human T-cell line, Jurkat. We also found that binding of the CREB-like factor to the 21-bp core element was enhanced by treatment with TPA, with little effect on transcriptional activity; in contrast, forskolin and p40tax did not facilitate binding, though they enhanced transcription. The combination of forskolin and TPA synergistically induced the transcription activity of the element, showing a hierarchical mechanism of regulation of the HTLV-I core enhancer element to levels sufficient for formation of the factor-enhancer complex and for activation of the complex. Added to those findings, our results indicate that the modes of activation by forskolin and p40tax are different from each other.

Overlap of the p53-responsive element and cAMP-responsive element in the enhancer of human T-cell leukemia virus type I

The wild-type p53 protein suppresses transformation, but certain missense mutants of p53 can transform cells. Although the wild-type p53 protein contains a transcriptional activation domain, no p53-responsive element has been identified. Here, we identified the p53-responsive element within the Tax-responsive element [21-base-pair (bp) enhancer] of human T-cell leukemia virus type I. Mutation analysis of the 21-bp enhancer indicated that the 16-bp sequence containing the cAMP-responsive element and its surrounding sequence was responsible for p53-induced transactivation. This 16-bp sequence was demonstrated to bind specifically to wild-type human p53 protein in vitro. Using a series of deletion mutants of p53, we showed that almost the entire region of p53 is needed for the transactivating capacity. Furthermore, the transforming mutants of p53 were unable to act as transcriptional activators. The p53-responsive element identified here should be useful to analyze the mechanism by which p53 regulates expression of a set of genes with a negative effect on cellular growth.

In vitro activation of transcription by the human T-cell leukemia virus type I Tax protein

The human T-cell leukemia virus type I (HTLV-I) regulatory protein Tax activates transcription of the proviral long terminal repeats and a number of cellular promoters. We have developed an in vitro system to characterize the mechanism by which Tax interacts with the host cell transcription machinery. Tax was purified from cells infected with a baculovirus expression vector. Addition of these Tax preparations to nuclear extracts from uninfected human T lymphocytes activated transcription of the HTLV-I long terminal repeat approximately 10-fold. Transcription-stimulatory activity copurified with the immunoreactive 40-kDa Tax polypeptide on gel filtration chromatography, and, as expected, the effect of recombinant Tax was diminished in HTLV-I-infected T-lymphocyte extracts containing endogenous Tax. Tax-mediated transactivation in vivo has been previously shown to require 21-bp-repeat Tax-responsive elements (TxREs) in the promoter DNA. Stimulation of transcription in vitro was also strongly dependent on these sequences. To investigate the mechanism of Tax transactivation, cellular proteins that bind the 21-bp-repeat TxREs were prepared by DNA affinity chromatography. Recombinant Tax markedly increased the formation of a specific host protein-DNA complex detected in an electrophoretic mobility shift assay. These data suggest that Tax activates transcription through a direct interaction with cellular proteins that bind to the 21-bp-repeat TxREs.

In vitro activation of transcription by the human T-cell leukemia virus type I Tax protein

The human T-cell leukemia virus type I (HTLV-I) regulatory protein Tax activates transcription of the proviral long terminal repeats and a number of cellular promoters. We have developed an in vitro system to characterize the mechanism by which Tax interacts with the host cell transcription machinery. Tax was purified from cells infected with a baculovirus expression vector. Addition of these Tax preparations to nuclear extracts from uninfected human T lymphocytes activated transcription of the HTLV-I long terminal repeat approximately 10-fold. Transcription-stimulatory activity copurified with the immunoreactive 40-kDa Tax polypeptide on gel filtration chromatography, and, as expected, the effect of recombinant Tax was diminished in HTLV-I-infected T-lymphocyte extracts containing endogenous Tax. Tax-mediated transactivation in vivo has been previously shown to require 21-bp-repeat Tax-responsive elements (TxREs) in the promoter DNA. Stimulation of transcription in vitro was also strongly dependent on these sequences. To investigate the mechanism of Tax transactivation, cellular proteins that bind the 21-bp-repeat TxREs were prepared by DNA affinity chromatography. Recombinant Tax markedly increased the formation of a specific host protein-DNA complex detected in an electrophoretic mobility shift assay. These data suggest that Tax activates transcription through a direct interaction with cellular proteins that bind to the 21-bp-repeat TxREs.

A cyclic AMP-responsive DNA-binding protein (CREB2) is a cellular transactivator of the bovine leukemia virus long terminal repeat

To gain insight into the cellular regulation of bovine leukemia virus (BLV) trans activation, a lambda-gt11 cDNA library was constructed with mRNA isolated from a BLV-induced tumor and the recombinant proteins were screened with an oligonucleotide corresponding to the tax activation-responsive element (TAR). Two clones (called TAR-binding protein) were isolated from 750,000 lambda-gt11 plaques. The binding specificity was confirmed by Southwestern (DNA-protein) and gel retardation assays. Nucleotide sequence analysis revealed that TAR-binding protein is very similar to the CREB2 protein. It contains a leucine zipper structure required for dimerization, a basic amino acid domain, and multiple potential phosphorylation sites. A vector expressing CREB2 was transfected into D17 osteosarcoma cells. In the absence of the tax transactivator, the CREB2 protein and the cyclic AMP-dependent protein kinase A activate the BLV long terminal repeat at a basal expression level: trans activation reached 10% of the values obtained in the presence of tax alone. These data demonstrate that CREB2 is a cellular factor able to induce BLV long terminal repeat expression in the absence of tax protein and could thus be involved in the early stages of viral infection. In addition, we observed that in vitro tax-induced trans activation can be activated or inhibited by CREB2 depending on the presence or absence of protein kinase A. These data suggest that the cyclic AMP pathway plays a role in the regulation of viral expression in BLV-infected animals.

trans activation of nerve growth factor in transgenic mice containing the human T-cell lymphotropic virus type I tax gene

Three lines of transgenic mice containing the human T-cell lymphotropic virus type I (HTLV-I) tax gene develop neurofibromas composed of perineural fibroblasts (S. H. Hinrichs, M. Nerenberg, R. K. Reynolds, G. Khoury, and G. Jay, Science 237:1340-1343, 1987; M. Nerenberg, S. H. Hinrichs, R. K. Reynolds, G. Khoury, and G. Jay, Science 237:1324-1327, 1987). Tumors and tumor cell lines derived from these mice produce neurite outgrowth from PC-12 cells and nerve growth factor (NGF), as determined by RNA (Northern) blot analysis and enzyme-linked immunosorbent assays. In vitro cotransfection studies demonstrate that Tax is able to trans activate the NGF promoter in NIH 3T3 fibroblast cells. The major cis-acting tax-responsive element in the NGF promoter (AGGGTGTGACGA) has 92% homology with a tax-responsive element contained within the 21-bp repeats of the HTLV-I long terminal repeat. The receptor for NGF is also expressed in the transgenic tumor cells, suggesting that Tax may activate an autocrine mechanism through the upregulation of NGF.

Identification and functional characterisation of the cellular activating transcription factor 43 (ATF-43) protein

The promoter motif CGTCA binds multiple cellular factors that mediate a variety of inducible events, including positive responses to raised cellular levels of cAMP and to the Adenovirus E1a protein. To date, at least ten mammalian cDNA clones have been isolated that encode distinct proteins capable of binding to this motif. However, in most cases the precise stimuli that may regulate these different factors have yet to be determined. We have previously shown that the abundant Hela protein ATF-43 forms a complex in vivo with the cyclic AMP response element binding protein (CREB). In this report we definitively show that ATF-43 is the product of the two published cDNA clones, ATF1 and TREB 36. We confirm that ATF1 efficiently heterodimerises with CREB and demonstrate that even though ATF1 and CREB homodimers, as well as the ATF1/CREB heterodimer efficiently bind to the CGTCA motif, the resulting DNA-protein complexes have significantly different stabilities. A region outside the DNA binding domain of ATF1 contributes to the instability of its interaction with DNA. We further show that despite ATF1's homology to CREB, it responds poorly to activation by protein kinase A. In light of our finding that in Hela cells the majority of CREB protein is heterodimerised with ATF1, we speculate on the functional significance of such heterodimers.

trans activation of nerve growth factor in transgenic mice containing the human T-cell lymphotropic virus type I tax gene

Three lines of transgenic mice containing the human T-cell lymphotropic virus type I (HTLV-I) tax gene develop neurofibromas composed of perineural fibroblasts (S. H. Hinrichs, M. Nerenberg, R. K. Reynolds, G. Khoury, and G. Jay, Science 237:1340-1343, 1987; M. Nerenberg, S. H. Hinrichs, R. K. Reynolds, G. Khoury, and G. Jay, Science 237:1324-1327, 1987). Tumors and tumor cell lines derived from these mice produce neurite outgrowth from PC-12 cells and nerve growth factor (NGF), as determined by RNA (Northern) blot analysis and enzyme-linked immunosorbent assays. In vitro cotransfection studies demonstrate that Tax is able to trans activate the NGF promoter in NIH 3T3 fibroblast cells. The major cis-acting tax-responsive element in the NGF promoter (AGGGTGTGACGA) has 92% homology with a tax-responsive element contained within the 21-bp repeats of the HTLV-I long terminal repeat. The receptor for NGF is also expressed in the transgenic tumor cells, suggesting that Tax may activate an autocrine mechanism through the upregulation of NGF.

The indirect association of human T-cell leukemia virus tax protein with DNA results in transcriptional activation

trans-activator p40tax of human T-cell leukemia virus type I activates specific enhancers and stimulates transcription of the viral and some cellular genes but does not bind directly to the enhancer sequences. We demonstrated here that a fusion protein of p40tax and GAL4 DNA binding domain activated transcription dependent on the GAL4-binding site in the reporter plasmid. Mutants of p40tax were also tested in the fusion protein, and their activities were found to be in parallel with those of their free forms on the original target long terminal repeat. This activation with GAL4 fusion protein was interfered with by the free form of p40tax. These results suggest that p40tax associates with DNA through interaction with DNA binding protein(s) and also interacts with another transcription factor(s) to elicit the activation.

In vivo genomic variability of human T-cell leukemia virus type I depends more upon geography than upon pathologies

To investigate the geography- and disease-associated genomic variation of human T-cell leukemia virus type I (HTLV-I), we studied ex vivo DNA from peripheral blood lymphocytes from nine patients by polymerase chain reaction and direct DNA sequencing. For each viral strain, 1,917 bp was sequenced, including parts of the long terminal repeat, the env gene, and the px II, px III, and px IV coding frames of the px region. The number of genomic variations observed in the U3 region of the long terminal repeat was higher than that seen in the env and px genes. Very few mutations were present in the px II and px III genes. In contrast, the px IV open reading frame exhibited numerous single point mutations. While no specific mutation could be linked to any pathology (adult T-cell leukemia/lymphoma or tropical spastic paraparesis/HTLV-I-associated myelopathy), variations among HTLV-I isolates from different geographic areas (Ivory Coast, Caribbean, and Japan) existed. The Ivory Coast HTLV-I appeared to represent a group by itself.

A eukaryotic nuclear protein of 130 kDa binds to a bacterial cAMP responsive element

It has been known that one of the signal transduction mechanisms in Escherichia coli is mediated by cAMP which binds to the receptor protein (CAP), and that CAP complexed with cAMP facilitates gene expression by binding to the specific sequences. To identify a molecular mechanism in eukaryotes similar to a cAMP-mediated pathway in E. coli, the function of the CAP binding site of lac gene in E. coli and the protein(s) interacting with it were examined in a mammalian system. From transient expression studies of the fusion gene between the chloramphenicol acetyltransferase and lac genes, it was found that the lacCAP binding site could act as an enhancer activity on the SV40 promoter, and also as an additive enhancer activity to the SV40 enhancer in HeLa cells. However, the activity was not stimulated by cpt-cAMP (a highly stable analogue of cAMP) in HeLa cells, although it was induced in PC12 cells. These results suggest that a bacterial cAMP responsive element may function also in eukaryotes as a cis-acting element in a cell type dependent manner. Results from gel mobility shift assays showed that a protein(s) exists that specifically binds to the lacCAP binding site in eukaryotic nuclear extracts. As one of the proteins binding to the above site, we have identified a 130 kDa protein by using the Southwestern method. Although a function of the 130 kDa protein has not yet been understood, there is a possibility that the 130 kDa protein may play a role in the regulation of cAMP-dependent gene expression.

Transcriptional block of HTLV-I LTR by sequence-specific methylation

HTLV-I infection is characterized by low viremia and restricted viral expression. While the mechanisms regulating viral latency are poorly understood, it is believed that interactions between viral and host cellular factor(s) are involved. Several lines of evidence indicate that HTLV-I provirus may be methylated in primary ATL leukemic cells. To determine whether methylation of the viral promoting sequences was sufficient to inhibit gene transcription, we methylated the HTLV-I LTR enzymatically at the HpaII (CCGG) sites. HTLV-I LTR contains several HpaII methylase-sensitive sites, and some involve one of the three 21-bp motifs, responsible for tax induction, as well as sequences that respond to phorbol 12-myristate 13-acetate (PMA). We found that CpG site-specific methylation of HTLV-I LTR sequences inhibits their transcriptional activation mediated by both tax product and PMA. This transcriptional block, however, was overcome when tax product and PMA were added together, thus indicating that tax and PMA act synergistically in bypassing the transcriptional block exerted by methylation.

Distinct DNA targets for trans-activation by HTLV-1 tax and adenovirus E1A

The HTLV-1 LTR is trans-activated by both the HTLV-1 tax (p40x) and adenovirus E1A gene products. Previous experiments have localized tax-responsive cis-elements to three 21-bp repeat units within the promoter, as well as a fourth region located between the middle and proximal repeats. A sequence TGACG, resembling the ATF/CREB recognition element, is located at the center of each of these repeat units. Mutation of this ATF/CREB site in the 21-bp repeats impairs both tax and E1A-dependent trans-activation. However, assays of a variety of promoter mutants demonstrate that sequences required for E1A and tax induction differ, suggesting that these two viral trans-activators target different factors. In addition, although the adenovirus E4 promoter also contains three ATF/CREB sites involved in E1A activation, tax does not activate this promoter. Finally, we also find that the TATAA element of the HTLV-1 LTR contributes to E1A-dependent activation but not tax activation. We concluded that although both trans-activators exhibit similarities in their activation properties, the targets for activation must differ.

Isolation of cDNAs for DNA-binding proteins which specifically bind to a tax-responsive enhancer element in the long terminal repeat of human T-cell leukemia virus type I

One of the gene products of human T-cell leukemia virus type I (HTLV-I), p40tax, activates its own viral transcription in trans through tax-responsive enhancers in viral long terminal repeats. Five species of cDNA clones for proteins that bind to the tax-responsive enhancer element in HTLV-I were isolated from the Jurkat cell library. The beta-galactosidase fusion protein prepared from the lysogen of a clone specifically recognized the cyclic AMP-responsive element in HTLV-I enhancer. The nucleotide sequence of a full-length cDNA clone (TAXREB67) had a coding capacity of 351 amino acids, which contained a basic motif followed by a leucine zipper structure near the carboxy terminus. Its mRNA was detected in human cell lines, including HTLV-I-infected or noninfected hematopoietic cell lines. The mRNA level in Jurkat cells was decreased temporarily by increasing cyclic AMP concentration but increased by increasing Ca2+ concentration. Polyclonal antibodies against the fusion protein specifically recognized a 52-kDa protein in Jurkat cells. Analyses of the function of this protein and its interactions with other cellular factors will be useful to help understand the regulatory mechanism through tax-responsive enhancers in HTLV-I.

Induction of the HTLV-I LTR by Jun occurs through the Tax-responsive 21-bp elements

The HTLV-I LTR is known to be induced by a variety of cellular signals. Tax protein is one potent viral trans-activator of LTR-directed transcription. We demonstrate here that Jun is another transcription factor that can strongly modulate the activity of this LTR. Using deletion and competition studies, the minimal portion of the LTR for Jun activation was found to coincide with the Tax-responsive 21-bp elements. In binding experiments, nuclear factors that bound to the HTLV-I 21-bp sequence were competed by an excess of AP-1 motif oligonucleotide. Although the Tax-responsive elements do not contain a strictly conserved AP-1 motif, these findings suggest that they function as AP-1 sites. We found, however, that in cells depleted for AP-1 activity (F9 teratocarcinoma), Tax activation of the HTLV-I LTR was maintained. Thus while Jun/AP-1 may be involved in the basal expression of the HTLV-I LTR, it may not be essential for Tax-mediated activation.

The cellular transcription factor CREB corresponds to activating transcription factor 47 (ATF-47) and forms complexes with a group of polypeptides related to ATF-43

Promoter elements containing the sequence motif CGTCA are important for a variety of inducible responses at the transcriptional level. Multiple cellular factors specifically bind to these elements and are encoded by a multigene family. Among these factors, polypeptides termed activating transcription factor 43 (ATF-43) and ATF-47 have been purified from HeLa cells and a factor referred to as cyclic AMP response element-binding protein (CREB) has been isolated from PC12 cells and rat brain. We demonstrated that CREB and ATF-47 are identical and that CREB and ATF-43 form protein-protein complexes. We also found that the cis requirements for stable DNA binding by ATF-43 and CREB are different. Using antibodies to ATF-43 we have identified a group of polypeptides (ATF-43) in the size range from 40 to 43 kDa. ATF-43 polypeptides are related by their reactivity with anti-ATF-43, DNA-binding specificity, complex formation with CREB, heat stability, and phosphorylation by protein kinase A. Certain cell types vary in their ATF-43 complement, suggesting that CREB activity is modulated in a cell-type-specific manner through interaction with ATF-43. ATF-43 polypeptides do not appear simply to correspond to the gene products of the ATF multigene family, suggesting that the size of the ATF family at the protein level is even larger than predicted from cDNA-cloning studies.

The cellular transcription factor CREB corresponds to activating transcription factor 47 (ATF-47) and forms complexes with a group of polypeptides related to ATF-43

Promoter elements containing the sequence motif CGTCA are important for a variety of inducible responses at the transcriptional level. Multiple cellular factors specifically bind to these elements and are encoded by a multigene family. Among these factors, polypeptides termed activating transcription factor 43 (ATF-43) and ATF-47 have been purified from HeLa cells and a factor referred to as cyclic AMP response element-binding protein (CREB) has been isolated from PC12 cells and rat brain. We demonstrated that CREB and ATF-47 are identical and that CREB and ATF-43 form protein-protein complexes. We also found that the cis requirements for stable DNA binding by ATF-43 and CREB are different. Using antibodies to ATF-43 we have identified a group of polypeptides (ATF-43) in the size range from 40 to 43 kDa. ATF-43 polypeptides are related by their reactivity with anti-ATF-43, DNA-binding specificity, complex formation with CREB, heat stability, and phosphorylation by protein kinase A. Certain cell types vary in their ATF-43 complement, suggesting that CREB activity is modulated in a cell-type-specific manner through interaction with ATF-43. ATF-43 polypeptides do not appear simply to correspond to the gene products of the ATF multigene family, suggesting that the size of the ATF family at the protein level is even larger than predicted from cDNA-cloning studies.