Sequence-specific interaction of the Ets1 protein with the long terminal repeat of the human T-lymphotropic virus type I

ETS1 Function in Leukemia and Lymphoma

ETS proto-oncogene 1 (ETS1) is a transcription factor (TF) critically involved in lymphoid cell development and function. ETS1 expression is tightly regulated throughout differentiation and activation in T-cells, natural killer (NK) cells, and B-cells. It has also been described as an oncogene in a range of solid and hematologic cancer types. Among hematologic malignancies, its role has been best studied in T-cell acute lymphoblastic leukemia (T-ALL), adult T-cell leukemia/lymphoma (ATLL), and diffuse large B-cell lymphoma (DLBCL). Aberrant expression of ETS1 in these malignancies is driven primarily by chromosomal amplification and enhancer-driven transcriptional regulation, promoting the ETS1 transcriptional program. ETS1 also facilitates aberrantly expressed or activated transcriptional complexes to drive oncogenic pathways. Collectively, ETS1 functions to regulate cell growth, differentiation, signaling, response to stimuli, and viral interactions in these malignancies. A tumor suppressor role has also been indicated for ETS1 in select lymphoma types, emphasizing the importance of cellular context in ETS1 function. Research is ongoing to further characterize the clinical implications of ETS1 dysregulation in hematologic malignancies, to further resolve binding complexes and transcriptional targets, and to identify effective therapeutic targeting approaches.

Differential role of PKC-induced c-Jun in HTLV-1 LTR activation by 12-O-tetradecanoylphorbol-13-acetate in different human T-cell lines

We have previously shown that TPA activates HTLV-1 LTR in Jurkat T-cells by inducing the binding of Sp1-p53 complex to the Sp1 site residing within the Ets responsive region 1 (ERR-1) of the LTR and that this activation is inhibited by PKCalpha and PKCepsilon. However, in H9 T-cells TPA has been noted to activate the LTR in two consecutive stages. The first stage is activation is mediated by PKCetta and requires the three 21 bp TRE repeats. The second activation mode resembles that of Jurkat cells, except that it is inhibited by PKCdelta. The present study revealed that the first LTR activation in H9 cells resulted from PKCetta-induced elevation of non-phosphorylated c-Jun which bound to the AP-1 site residing within each TRE. In contrast, this TRE-dependent activation did not occur in Jurkat cells, since there was no elevation of non-phosphorylated c-Jun in these cells. However, we found that PKCalpha and PKCepsilon, in Jurkat cells, and PKCetta and PKCdelta, in H9 cells, increased the level of phosphorylated c-Jun that interacted with the Sp1-p53 complex. This interaction prevented the Sp1-p53 binding to ERR-1 and blocked, thereby, the ERR-1-mediated LTR activation. Therefore, this PKC-inhibited LTR activation started in both cell types after depletion of the relevant PKCs by their downregulation. In view of these variable activating mechanisms we assume that there might be additional undiscovered yet modes of HTLV-1 LTR activation which vary in different cell types. Moreover, in line with this presumption we speculate that in HTLV-1 carriers the LTR of the latent provirus may also be reactivated by different mechanisms that vary between its different host T-lymphocyte subclones. Since this reactivation may initiate the ATL process, understanding of these mechanisms is essential for establishing strategies to block the possibility of reactivating the latent virus as preventive means for ATL development in carriers.

Activation of the human T-cell leukemia virus type 1 long terminal repeat by the ternary complex factor Elk-1

Serum response factor (SRF) was recently shown to bind and activate the human T-cell leukemia virus type 1 (HTLV-1) promoter at bases -116 to -125 relative to the transcription start site. In addition to the SRF binding site (CArG box), serum response elements (SRE) also typically contain a binding site for a member of the ternary complex factor (TCF) family. Here we demonstrate the presence of two TCF binding sites upstream of the viral CArG box. Binding of the TCF family member Elk-1 to these sites was shown to activate transcription of the promoter. Based on these results, the position of the previously described viral SRE (vSRE) within the HTLV-1 promoter can be extended from -116 to -157 to include the two newly identified TCF sites. Purified Elk-1 bound to a probe containing the vSRE, and this complex formed a ternary complex with SRF. In addition, the complex formed by nuclear extract on this probe contained Elk-1, as shown by electrophoretic mobility shift assay supershift. Both of the predicted TCF sites independently bound Elk-1. Elk-1 activated transcription of the HTLV-1 long terminal repeat (LTR), and mutations within either of the TCF sites or the CArG box reduced responsiveness of the LTR to Elk-1. Chromatin immunoprecipitation demonstrated that Elk-1 associates with the HTLV-1 LTR in vivo. These results identify a functional SRE within the HTLV-1 LTR and suggest that both Elk-1 and SRF play important roles in regulating basal HTLV-1 gene expression.

Activation of HTLV-I gene transcription by protein tyrosine phosphatase inhibitors

Human T-cell leukemia virus type I (HTLV-I) transcription generally depends on the ability of the viral Tax protein to bind the CREB transcription factor and form an active complex by recruiting CBP/p300 coactivators to the long terminal repeat (LTR). Studies have demonstrated that T-cell activating agents that stimulate CREB are potent inducers of HTLV-I transcription. Herein, we demonstrate that bpV[pic], a protein tyrosine phosphatase (PTP) inhibitor activates the HTLV-I LTR in the presence and absence of Tax expression. Optimal activation occurred at 8 h and was synergistic with forskolin or PGE(2). Infected cell lines and cells transfected with HTLV-I proviral DNA were equally responsive to the synergistic effect of bpV and forskolin on HTLV-I gene expression. Activation of the LTR by bpV[pic] was T-cell receptor-independent, but required ZAP70, calcineurin activity and functional calcium entry. Inhibition of the SHP-1 PTP was suggested to be important. Transfection experiments with a CREB dominant-negative mutant and with isolated TRE1- or CREB-responsive reporter constructs and treatment with the MDL-12,330A adenylate cyclase inhibitor all supported the involvement of a CREB/ATF family member in this bpV-dependent activation of the HTLV-I LTR, although CREB itself did not seem to be involved. Analysis of HTLV-I reporter constructs containing mutated CREB-binding sites also implied the involvement of another element in this activation. These results demonstrate for the first time a powerful effect of PTP inhibitors on HTLV-I LTR activity and suggest participation of both CREB-dependent and -independent pathways in this activation.

Identification of a functional serum response element in the HTLV-I LTR

In response to various mitogenic signals, serum response factor (SRF) activates cellular gene expression after binding to its cognate target sequence (CArG box) located within a serum response element (SRE). SRF is particularly important in T cell activation, and we now report that SRF activates basal transcription from the human T-cell leukemia virus-I (HTLV-I) long terminal repeat (LTR). A DNA element, with similarity to the consensus cellular CArG box found in the c-fos promoter centered approximately 120 base pairs upstream from the viral transcription start site, has been identified and named the vCArG box. SRF activation of gene expression from the LTR was localized to the vCArG box, and mutation of this site abolished SRF responsiveness. An oligonucleotide probe containing the vCArG box bound purified SRF, and a complex formed on this probe with nuclear extract was supershifted by anti-SRF antibody. Moreover, a biotinylated probe containing the vCArG box bound SRF in avidin-biotin pull-down assays. Quantitative binding analysis yielded nanomolar affinities for both the viral and cellular CArG boxes. Chromatin immunoprecipitation experiments demonstrated that SRF is resident on the HTLV-I LTR in vivo. These data identify a functional serum response element in the HTLV-I LTR and suggest that SRF may play an important role in regulating basal HTLV-I gene expression in early infection and reactivation from latency.

The high-affinity Sp1 binding site in the HTLV-1 promoter contributes to Tax-independent basal expression

Transcriptional activation of human T-cell leukemia virus type 1 (HTLV-1) requires many cellular proteins and the virally encoded transcription factor Tax. Tax binds the three viral cAMP-response elements (CREs) with ATF/CREB (activating transcription factor/cAMP-response element-binding protein) and recruits the cellular coactivators CBP/p300. HTLV-1 also utilizes other cellular transcription factors that bind to the promoter to regulate transcription. One of these factors, Sp1, has been shown to bind to the viral promoter at two elements; one located within the third viral CRE, and the second located between the second and third viral CREs. The functional significance of Sp1 binding at each of these regions of the viral promoter is not completely understood. We set out to characterize Sp1 binding and to evaluate the functional significance of Sp1, both in the absence and presence of Tax. We found that Sp1 binds preferentially to the element located between the second and third viral CREs, and modestly activates transcription in vitro and in vivo. Sp1 was detected at the integrated HTLV-1 promoter in vivo. Surprisingly, point mutagenesis of the strong Sp1 binding site rendered the HTLV-1 reporter plasmid insensitive to Sp1 activation, and dramatically reduced basal transcription in vivo. These data indicate a role for Sp1 in basal level transcription of HTLV-1.

Identification and characterization of a PU.1/Spi-B binding site in the bovine leukemia virus long terminal repeat

Bovine leukemia virus (BLV) is a B-lymphotropic oncogenic retrovirus whose transcriptional promoter is located in the viral 5' long terminal repeat (LTR). To date, no B-lymphocyte-specific cis-regulatory element has been identified in this region. Since ETS proteins are known to regulate transcription of numerous retroviruses, we searched for the presence in the BLV promoter region of binding sites for PU.1/Spi-1, a B-cell- and macrophage-specific ETS family member. In this report, nucleotide sequence analysis of the viral LTR identified a PUbox located at -95/-84 bp. We demonstrated by gel shift and supershift assays that PU.1 and the related Ets transcription factor Spi-B interacted specifically with this PUbox. A 2-bp mutation (GGAA-->CCAA) within this motif abrogated PU.1/Spi-B binding. This mutation caused a marked decrease in LTR-driven basal gene expression in transient transfection assays of B-lymphoid cell lines, but did not impair the responsiveness of the BLV promoter to the virus-encoded transactivator Tax(BLV). Moreover, ectopically expressed PU.1 and Spi-B proteins transactivated the BLV promoter in a PUbox-dependent manner. Taken together, our results provide the first demonstration of regulation of the BLV promoter by two B-cell-specific Ets transcription factors, PU.1 and Spi-B. The PU.1/Spi-B binding site identified here could play an important role in BLV replication and B-lymphoid tropism.

Dual regulation of ets-activated gene expression by SP1

The human folate receptor (hFR) type gamma gene is driven by a TATA-less promoter that uses a canonical Sp1 element for basal transcription. Using nuclear extract from 293 (human embryonic) cells, we mapped a second (non-canonical) Sp1 element to which Sp1 bound with a comparable affinity and which overlaps a functional ets binding site (EBS). Mutagenesis experiments revealed that the binding of ets to the EBS activates the promoter synergistically with Sp1 bound at the downstream site; however, binding of Sp1 to the EBS does not contribute to promoter activity. A further increase in Sp1 by inducible expression in recombinant 293 cells resulted in a small but significant decrease in the hFR-gamma promoter activity, but the decrease was abolished when the EBS was deleted from the promoter. In 293 cells, which do not express hFR-gamma, the Sp1 level was relatively high whereas in the hFR-gamma-positive HL60 leukemia cells, the Sp1 level was low and the EBS predominantly bound an ets protein. To account for the above observations, we propose a model in which when the Sp1 level is low, ets out competes Sp1 for binding to the EBS and synergistically enhances the hFR-gamma promoter activity by interacting with Sp1 bound at the canonical site whereas at higher levels, Sp1 represses the promoter by competitively inhibiting the binding of ets. As a partial extension of this model to the regulation of other ets activated genes, we show that Sp1 can predictably bind to a variety of ets elements including those responsive to Ets1 and Spi.1/Pu.1. A dual concentration-dependent action of Sp1 as an activator or a repressor offers a potential mechanism contributing to tissue-specific regulation of ets-dependent genes by Sp1.

GLI-2 modulates retroviral gene expression

GLI proteins are involved in the development of mice, humans, zebrafish, Caenorhabditis elegans, Xenopus, and Drosophila. While these zinc finger-containing proteins bind to TG-rich promoter elements and are known to regulate gene expression in C. elegans and Drosophila, mechanistic understanding of how regulation is mediated through naturally occurring transcriptional promoters is lacking. One isoform of human GLI-2 appears to be identical to a factor previously called Tax helper protein (THP), thus named due to its ability to interact with a TG-rich element in the human T-lymphotropic virus type 1 (HTLV-1) enhancer thought to mediate transcriptional stimulation by the Tax protein of HTLV-1. We now demonstrate that, working through its TG-rich binding site and adjacent elements, GLI-2/THP actually suppresses gene expression driven by the HTLV-1 promoter. GLI-2/THP has no effect on the HTLV-2 promoter, activates expression from the promoters of human immunodeficiency virus types 1 and (HIV-1 and -2), and stimulates HIV-1 replication. Both effective suppression and activation of gene expression and viral replication require the first of the five zinc fingers, which is not necessary for DNA binding, to be intact. Thus, not only can GLI-2/THP either activate or suppress gene expression, depending on the promoter, but the same domain (first zinc finger) mediates both effects. These findings suggest a role for GLI-2 in retroviral gene regulation and shed further light on the mechanisms by which GLI proteins regulate naturally occurring promoters.

Sp1-p53 heterocomplex mediates activation of HTLV-I long terminal repeat by 12-O-tetradecanoylphorbol-13-acetate that is antagonized by protein kinase C

We have previously demonstrated that 12-O-tetradecanoylphorbol-13-acetate (TPA) activates human T-cell leukemia virus type-I long terminal repeat (LTR) in Jurkat cells by a protein kinase C (PKC)-independent mechanism involving a posttranslational activation of Sp1 binding to an Sp1 site located within the Ets responsive region-1 (ERR-1). By employing the PKC inhibitor, bisindolylmaleimide I and cotransfecting the reporter LTR construct with a vector expressing PKC-alpha, we demonstrated, in the present study, that this effect of TPA was not only independent of, but actually antagonized by, PKC. Electrophoretic mobility shift assays together with antibody-mediated supershift and immuno-coprecipitation analyses, revealed that the posttranslational activation of Sp1 was exerted by inducing the formation of Sp1-p53 heterocomplex capable of binding to the Sp1 site in ERR-1. Furthermore, we demonstrated that Jurkat cells contain both wild-type (w.t.) and mutant forms of p53 and we detected both of them in this complex at variable combinations; some molecules of the complex contained either the w.t. or the mutant p53 separately, whereas others contained the two of them together. Finally, we showed that the Sp1-p53 complexes could bind also to an Sp1 site present in the promoter of another gene such as the cyclin-dependent kinase inhibitor p21(WAF-1), but not to consensus recognition sequences of the w.t. p53. Therefore, we speculate that there might be several other PKC-independent biological effects of TPA which result from interaction of such Sp1-p53 complexes with Sp1 recognition sites residing in the promoters of a wide variety of cellular and viral genes.

Ets target genes: past, present and future

Ets is a family of transcription factors present in species ranging from sponges to human. All family members contain an approximately 85 amino acid DNA binding domain, designated the Ets domain. Ets proteins bind to specific purine-rich DNA sequences with a core motif of GGAA/T, and transcriptionally regulate a number of viral and cellular genes. Thus, Ets proteins are an important family of transcription factors that control the expression of genes that are critical for several biological processes, including cellular proliferation, differentiation, development, transformation, and apoptosis. Here, we tabulate genes that are regulated by Ets factors and describe past, present and future strategies for the identification and validation of Ets target genes. Through definition of authentic target genes, we will begin to understand the mechanisms by which Ets factors control normal and abnormal cellular processes.

Virus inactivation in a proportion of human T-cell leukaemia virus type I-infected T-cell clones arises through naturally occurring mutations

Human T-cell leukaemia virus type I (HTLV-I) is the aetiological agent of adult T-cell leukaemia/lymphoma and tropical spastic paraparesis/HTLV-I-associated myelopathy (TSP/HAM). The trans-activating protein (Tax) of HTLV-I is strongly implicated in cellular proliferation. We examined the tax gene and 5' long terminal repeat (LTR) sequences in eight naturally infected T-cell clones derived from TSP/HAM-affected individuals who were either productively (proliferate spontaneously) or silently (do not proliferate spontaneously) infected. In two silently infected clones point mutations within the proviruses resulted in truncation of the Tax protein. One clone harboured both a deleterious tax gene mutation and a point mutation in an enhancer element of the 5' LTR. Sequence changes, immunological escape mutation, integration site context and host cell phenotype may all contribute to the high proportion of latently or silently infected T-cells found in vivo in virus carriers.

Characterization of the human endothelial nitric-oxide synthase promoter

Understanding transcription initiation of the endothelial nitric-oxide synthase (eNOS) gene appears pivotal to gaining a comprehensive view of NO biology in the blood vessel wall. The present study therefore focused upon a detailed dissection of the functionally important cis-DNA elements and the multiprotein complexes implicated in the cooperative control of constitutive expression of the human eNOS gene in vascular endothelium. Two tightly clustered cis-regulatory regions were identified in the proximal enhancer of the TATA-less eNOS promoter using deletion analysis and linker-scanning mutagenesis: positive regulatory domains I (-104/-95 relative to transcription initiation) and II (-144/-115). Analysis of trans-factor binding and functional expression studies revealed a surprising degree of cooperativity and complexity. The nucleoprotein complexes that form upon these regions in endothelial cells contained Ets family members, Sp1, variants of Sp3, MAZ, and YY1. Functional domain studies in Drosophila Schneider cells and endothelial cells revealed examples of positive and negative protein-protein cooperativity involving Sp1, variants of Sp3, Ets-1, Elf-1, and MAZ. Therefore, multiprotein complexes are formed on the activator recognition sites within this 50-base pair region of the human eNOS promoter in vascular endothelium.

A novel ets-related transcription factor, ERT/ESX/ESE-1, regulates expression of the transforming growth factor-beta type II receptor

A 2.5-kilobase cDNA clone that encodes a 371-amino acid novel transcription factor was isolated from a human placenta cDNA library using a yeast one-hybrid system. The novel ets-related transcription factor (ERT) showed a homology with the ETS DNA-binding domain. Using constructs of the transforming growth factor-beta (TGF-beta) type II receptor (RII) promoter linked to the luciferase gene, we have demonstrated that ERT activates transcription of the TGF-beta RII gene through the 5'-TTTCCTGTTTCC-3' response element spanning nucleotides +13 to +24 and multiple additional ETS binding sites between -1816 and -82 of the TGF-beta RII promoter. A specific interaction between ERT and the ETS binding sites was also demonstrated using an electrophoretic mobility shift assay. Deletion mapping of ERT protein suggests that the transactivation domain resides in the amino terminus while the DNA-binding domain is localized to the carboxyl-terminal region. Our results suggest that ERT might be a major transcription factor involved in the transcriptional regulation of the TGF-beta RII gene.

HUB1, a novel Krüppel type zinc finger protein, represses the human T cell leukemia virus type I long terminal repeat-mediated expression

We have shown that human T-cell leukemia virus type I (HTLV-I) gene expression is negatively regulated by the U5 repressive element (U5RE) of its long terminal repeat (LTR). To isolate factors binding to U5RE, we screened a cDNA expression library by south-western blotting with a U5RE probe. Screening 2 x10(6) clones gave a positive clone with a 3.8 kb insert encoding a novel 671 residue polypeptide, named HTLV-I U5RE binding protein 1 (HUB1), with five zinc finger domains and a Krüppel-associated box like domain; HUB1 may be related to a repressor belonging to the Krüppel type zinc finger protein. A 4.0 kb mRNA for HUB1 is ubiquitously expressed among all human tissues tested. HUB1 recognizes the TCCACCCC sequence as a core motif and exerts a strong repressive effect on HTLV-I LTR-mediated expression. A new repressive domain, named HUB1 repressive (HUR) domain, was identified, rather than the Krüppel-associated box like domain. The N-terminal region upstream of HUR domain seemed to be also indispensable to the repression. Thus, we propose that HUB1 is a new type repressor and plays an important role in the HTLV-I U5-mediated repression.

A molecular basis for species differences in Thy-1 expression patterns

Thy-1 is a membrane glycoprotein that displays species-specific differences in its pattern of expression. Although it is expressed on thymocytes and splenocytes in mice, it is only expressed on thymocytes in rats. Based on previous studies suggesting that the third intron of the mouse Thy-1 gene is required for its expression in thymocytes, in vivo footprinting analysis was performed on the third introns of both the mouse and rat Thy-1 genes, and led to the identification of homologous 36 bp "footprinted" regions. The mouse 36 bp region was found to be capable of specifically binding an Ets-1-like nuclear factor present in both mouse thymocytes and splenocytes. In contrast, the homologous 36 bp region of the rat which differs from the mouse 36 bp region by three nucleotides resulting in the loss of the Ets-1 binding site, is unable to bind a similar Ets-1-like factor present in rat thymocytes. Instead, this region of the rat third intron binds another nuclear factor which is present in rat thymocytes but not splenocytes. These observations suggest that the differential expression of the mouse and rat Thy-1 genes in thymocytes and splenocytes is the result of differential expression of nuclear factors that bind to this 36 bp region.

Human alpha-galactosidase A: high plasma activity expressed by the -30G-->A allele

Human alpha-galactosidase A (EC 3.2.1.22; alpha-Gal A) is the lysosomal exoglycosidase responsible for the hydrolysis of terminal alpha-galactosyl residues from glycoconjugates and is the defective enzyme causing Fabry disease (McKusick 301500). An unusally elevated level of plasma alpha-Gal A activity (> 2.5 times the normal mean) was detected in two unrelated normal males and the elevated activities were inherited as X-linked traits in their families. Sequencing of the alpha-Gal A coding region, intron/exon boundaries and 5'-flanking region from the proband identified a single mutation, a G-->A transition 30 nt upstream from the initiation of translation codon in exon 1. The -30G-->A mutation occurred in a putative NF kappa B/Ets consensus binding site that was recently shown to inhibit protein binding to the 5'-untranslated region of the gene, providing a possible explanation for its high activity. To further characterize the mutation, the mRNA and protein expressed by this variant allele were studied. Purified plasma and lymphoblast alpha-Gal A activity from individuals with the -30G-->A mutation had normal physical and kinetic properties. In vitro translation of mRNAs from the cloned normal and high plasma activity alleles resulted in similar levels of alpha-Gal A protein, indicating that this mutation did not enhance translation. These findings suggest that the -30G-->A mutation in the 5'-untranslated region of the alpha-Gal A gene enhances transcription, presumably by interfering with the binding of negatively-acting transcription factors which normally decrease alpha-Gal A expression in various cells. Preliminary studies of the frequency of the -30G-->A mutation in 395 unrelated normal males of mixed ancestry revealed two additional unrelated individuals who had high plasma enzymatic activity and the mutation, confirming the effect of this mutation on enzyme expression and suggesting that about 0.5% of normal individuals have high plasma alpha-Gal A activity due to this variant allele.

The long terminal repeats of human immunodeficiency virus type-1 and human T-cell leukemia virus type-I are activated by 12-O-tetradecanoylphorbol-13-acetate through different pathways

The LTRs of HIV-1 and HTLV-I have been shown by several laboratories to be activated by 12-O-tetradecanoylphorbol-13-acetate (TPA). This agent is a potent activator of protein kinase C (PKC). However, long exposure to TPA downregulates PKC in many cell types. We demonstrated that TPA treatment of Jurkat cells for more than 24 hr resulted in a sever depletion of this enzyme. Therefore, to explore the role of PKC in the effect of TPA on these LTRs, we transfected Jurkat cells with HIV-1 LTR-CAT or HTLV-I LTR-CAT construct after 72 hr of TPA pretreatment. While this TPA pretreatment considerably reduced the HIV-1 LTR basal expression, it strongly stimulated the expression of HTLV-I LTR. Furthermore, when TPA was added after transfection, a strong stimulation of HIV-1 LTR was observed, which could be abrogated by PKC inhibitors like H7 and chelerythryn. However, under these conditions TPA stimulated HTLV-I LTR to a lesser extent than did the long-term TPA pretreatment. Moreover, this stimulation was enhanced by the PKC inhibitors. Thus our data indicate that while the effect of TPA on HIV-1 LTR is strictly dependent on PKC activity, its effect on HTLV-I LTR is exerted via a different pathway that not only does not require PKC activation but rather seems to be antagonized by the activated PKC. Using a deletion mutant of HTLV-I LTR we mapped the PKC-independent effect of TPA to the c-ets responsive region 1 (ERR-1) located in U3 of this LTR.

Human T-cell leukemia viruses: epidemiology, biology, and pathogenesis

The human T-cell lymphotropic viruses type I and type II are closely related human retroviruses that have similar biological properties, genetic organization and tropism for T lymphocytes. Along with the simian T-cell lymphoma virus type I, they define the group of retroviruses known as the primate T-cell leukemia/lymphoma viruses. Initially identified in 1980, the human T-cell lymphotropic virus type I has been implicated as the etiologic agent of adult T-cell leukemia/lymphoma and of a degenerative neurologic disorder known as tropical spastic paraparesis or human T-cell lymphotropic virus type I-associated myelopathy. The intriguing link between human T-cell lymphotropic virus type, T-cell malignancy, and a totally unrelated and non-overlapping neurological disorder suggests divergent and unique pathogenetic mechanisms. This review will address the epidemiology, molecular biology, and pathogenesis of human T-cell leukemia viruses.

DEK, an autoantigen involved in a chromosomal translocation in acute myelogenous leukemia, binds to the HIV-2 enhancer

The product of the dek oncogene is the 43-kDa DEK nuclear protein. DEK was first identified in a fusion with the CAN nucleoporin protein in a specific subtype of acute myelogenous leukemia. DEK has also been shown to be an autoantigen in patients with pauciarticular onset juvenile rheumatoid arthritis. Further, the last 65 amino acids of DEK can partially reverse the mutation-prone phenotype of cells from patients with ataxia-telangiectasia. However, in spite of these significant disease associations, the function of DEK has remained unclear. The HIV-2 peri-ets (pets) site is a TG-rich element found between the two Elf-1 binding sites in the HIV-2 enhancer. The pets element mediates transcriptional activation whether the enhancer is stimulated by phorbol 12-myristate 13-acetate (PMA) alone, phytohemagluttinin (PHA) alone, PMA plus PHA, soluble antibodies to the T cell receptor, immobilized antibodies to the T cell receptor, or by antigen. Previously, we purified and characterized the pets factor, demonstrating that it is a 43-kDa nuclear protein. We now describe the identification of DEK as this 43-kDa pets factor. Using a modified Southwestern screening procedure, we find that DEK can recognize the pets element. We demonstrate the ability of recombinant DEK to bind specifically to the pets site using the electrophoretic mobility shift assay (EMSA) and DNase I footprinting. "Supershift" EMSA further confirms that DEK is the dominant protein binding to the pets site in T cell extracts. Our findings show that DEK is a site-specific DNA binding protein that is likely involved in transcriptional regulation and signal transduction. This has implications for multiple pathogenic processes, including hematologic malignancies, arthritis, ataxia-telangiectasia, and AIDS caused by HIV-2.

Interaction of human T-cell lymphotropic virus type I Tax, Ets1, and Sp1 in transactivation of the PTHrP P2 promoter

We have previously shown that the parathyroid hormone-related protein (PTHrP) promoter contains binding sites for transcription factors Ets1 and Sp1 and that human T-cell lymphotropic virus type I (HTLV-I) Tax cooperates with Ets1 to transactivate the PTHrP P2 promoter. Using the yeast two-hybrid interaction system, we now provide evidence that Tax interacts with Ets1. Moreover, a double mutation (D22A,C23S) in the Tax protein that abrogated the Tax/Ets1 interaction also inhibited the Tax/Ets1 cooperative effect, suggesting that the interaction between Tax and Ets1 is important for transactivation of the PTHrP promoter. In coimmunoprecipitation assays, we find that Tax facilitates the interaction between Ets1 and Sp1, forming a ternary complex. When the Sp1 site in the PTHrP promoter was mutated, the Tax/Ets1 cooperative effect was dramatically decreased. This suggests that Sp1 plays an important role in the Ets1-dependent Tax transactivation of the PTHrP P2 promoter. Finally, we demonstrate that Gal4-Tax is a strong activator of the Gal PTHrP promoter, implying that Tax contributes directly to the transcriptional activation of the promoter. We propose a model in which the Tax/Ets1 cooperative effect on the PTHrP P2 promoter is based on the ability of Tax, Ets1, and Sp1 to form a ternary complex on the template DNA. Tax facilitates the interaction of Ets1/Sp1 and participates directly in the transcription initiation process.

Function of the human T-cell leukemia virus type 1 21-base-pair repeats in basal transcription

The human T-cell leukemia virus type 1 (HTLV-1) promoter contains three copies of an imperfect 21-bp repeat called Tax-responsive element (TRE1). To examine the role of individual TRE1 sequences in basal transcription of the HTLV-1 promoter, site-directed mutations were generated in all possible combinations of one, two, or all three TRE1 elements in the viral long terminal repeat (LTR) and tested in vivo for transcriptional activity. Mutation of the middle TRE1 resulted in the greatest reduction in basal activity. Electrophoretic mobility shift analysis demonstrated that the protein complexes bound to each of the three TRE1 sequences were not identical. The complexes formed with the TATA-distal and middle TRE1s were dependent on the core cyclic AMP response element (CRE) found in all three TRE1s, while the cellular transcription factor Sp1 bound the TATA-proximal TRE1 in a CRE-independent manner. Sp1 binding produced a footprint on the viral LTR which covered the 5' region of the proximal TRE1. Mixing experiments demonstrated that the bindings of CREB and Sp1 to the proximal TRE1 were mutually exclusive. Sp1 was able to activate transcription both from the complete LTR and from the proximal TRE1 alone. These studies demonstrate that the TRE1 elements in the HTLV-1 LTR are functionally nonequivalent and suggest that Sp1 can influence HTLV-1 basal transcription.

Characterization of NERF, a novel transcription factor related to the Ets factor ELF-1

We have cloned the gene for a novel Ets-related transcription factor, new Ets-related factor (NERF), from human spleen, fetal liver, and brain. Comparison of the deduced amino acid sequence of NERF with those of other members of the Ets family reveals that the level of homology to ELF-1, which is involved in the regulation of several T- and B-cell-specific genes, is highest. Homologies are clustered in the putative DNA binding domain in the middle of the protein, a basic domain just upstream of this domain, and several shorter stretches of homology towards the amino terminus. The presence of two predominant NERF transcripts in various fetal and adult human tissues is due to at least three alternative splice products, NERF-1a, NERF-1b, and NERF-2, which differ in their amino termini and their expression in different tissues. Only NERF-2 and ELF-1, and not NERF-1a and NERF-1b, function as transcriptional activators of the lyn and blk gene promoters, although all isoforms of NERF bind with affinities similar to those of ELF-1 to a variety of Ets binding sites in, among others, the blk, lck, lyn, mb-1, and immunoglobulin H genes and are expressed at similar levels. Since NERF and ELF-1 are coexpressed in B and T cells, both might be involved in the regulation of the same genes.

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.

Genomic footprinting of HTLV type I and HIV type 1 in human T cell lines

Genomic footprinting of integrated HTLV-I and HIV-1 confirmed many aspects of retroviral transcriptional regulation deduced from previous studies. However, many notable differences were seen. HTLV-I genomic protein-binding patterns corresponded more closely to elements defined by transient transfection expression studies than to those mapped by in vitro protein-binding studies. HIV-1 genomic footprinting showed activation-related binding to adjacent NF-KB/SP1 sites and a large (90 bp) region transversing the R/U5 boundary, but minimal protein binding to NFAT, NRE, LBP-1, and CTF/NF1 sites relative to previous in vitro footprinting studies.

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.

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

Transcription of the human T-cell lymphotropic virus type I promoter by an alpha-amanitin-resistant polymerase

The human T-lymphotropic virus type I (HTLV-I) promoter contains the structural features of a typical RNA polymerase II (pol II) template. The promoter contains a TATA box 30 bp upstream of the transcription initiation site and binding sites for several pol II transcription factors, and long poly(A)+ RNA is synthesized from the integrated HTLV-I proviral DNA in vivo. Consistent with these characteristics, HTLV-I transcription activity was reconstituted in vitro by using TATA-binding protein, TFIIA, recombinant TFIIB, TFIIE, and TFIIF, TFIIH, and pol II. Transcription of the HTLV-I promoter in the reconstituted system requires RNA pol II. In HeLa whole cell extracts, however, the HTLV-I long terminal repeat also contains an overlapping transcription unit (OTU). HTLV-I OTU transcription is initiated at the same nucleotide site as the RNA isolated from the HTLV-I-infected cell line MT-2 but was not inhibited by the presence of alpha-amanitin at concentrations which inhibited the adenovirus major late pol II promoter (6 micrograms/ml). HTLV-I transcription was inhibited when higher concentrations of alpha-amanitin (60 micrograms/ml) were used, in the range of a typical pol III promoter (VA-I). Neutralization and depletion experiments with three distinct pol II antibodies demonstrate that RNA pol II is not required for HTLV-I OTU transcription. Antibodies to basal transcription factors TATA-binding protein and TFIIB, but not TFIIIC, inhibited HTLV-I OTU transcription. These observations suggest that the HTLV-I long terminal repeat contains overlapping promoters, a typical pol II promoter and a unique pol III promoter which requires a distinct set of transcription factors.

Subtype analysis of HTLV-1 in patients with HTLV-1 uveitis

The hypothesis that HTLV-1 uveitis, a recently identified disease entity associated with human T-cell leukemia virus type I (HTLV-1), is caused by a specific subtype of the virus was tested. The nucleotide sequences of the long terminal repeat of HTLV-1 from five patients with HTLV-1 uveitis (HU) and four with adult T-cell leukemia were phylogenetically analyzed. Our results showed that both subtypes which had been identified in Japan were associated with HU, indicating that there was no difference in pathogenicity between these phylogenetic subtypes. One of the subtypes was more frequently isolated in Okinawa than in Kyushu, suggesting a bias in the prevalence of each subtype among the inhabitants of these two areas of Japan.

Hypercalcemia, parathyroid hormone-related protein expression and human T-cell leukemia virus infection

Adult T-cell leukemia (ATL) associated with HTLV-1 infection is characterized by the development of hypercalcemia in over two thirds of patients. Dysregulation of cellular gene transcription by viral proteins is an emerging paradigm for molecular pathogenesis of disease. A recent example is the parathyroid hormone-related protein (PTHrP) gene, which has been implicated in the hypercalcemia of ATL, and is transactivated by the HTLV-1 tax and HTLV-11 tax proteins. PTHrP is expressed at high levels in leukemia cells derived from ATL patients, as well as in asymptomatic HTLV-1 positive carriers. This article reviews the interaction of the HTLV-1 transcriptional regulator tax with the PTHrP promoter. Tax mediates its effects on PTHrP via cellular transcription factors AP-2 and AP-1, and transactivation via an AP-2 motif represents a novel interaction of tax with a cellular transcription factor.

ERP, a new member of the ets transcription factor/oncoprotein family: cloning, characterization, and differential expression during B-lymphocyte development

The ets gene family encodes a group of proteins which function as transcription factors under physiological conditions and, if aberrantly expressed, can cause cellular transformation. We have recently identified two regulatory elements in the murine immunoglobulin heavy-chain (IgH) enhancer, pi and microB, which exhibit striking similarity to binding sites for ets-related proteins. To identify ets-related transcriptional regulators expressed in pre-B lymphocytes that may interact with either the pi or the microB site, we have used a PCR approach with degenerate oligonucleotides encoding conserved sequences in all members of the ets family. We have cloned the gene for a new ets-related transcription factor, ERP (ets-related protein), from the murine pre-B cell line BASC 6C2 and from mouse lung tissue. The ERP protein contains a region of high homology with the ETS DNA-binding domain common to all members of the ets transcription factor/oncoprotein family. Three additional smaller regions show homology to the ELK-1 and SAP-1 genes, a subgroup of the ets gene family that interacts with the serum response factor. Full-length ERP expresses only negligible DNA-binding activity by itself. Removal of the carboxy terminus enables ERP to interact with a variety of ets-binding sites including the E74 site, the IgH enhancer pi site, and the lck promoter ets site, suggesting a carboxy-terminal negative regulatory domain. At least three ERP-related transcripts are expressed in a variety of tissues. However, within the B-cell lineage, ERP is highly expressed primarily at early stages of B-lymphocyte development, and expression declines drastically upon B-cell maturation, correlating with the enhancer activity of the IgH pi site. These data suggest that ERP might play a role in B-cell development and in IgH gene regulation.

DNA-binding and transcriptional activation properties of the EWS-FLI-1 fusion protein resulting from the t(11;22) translocation in Ewing sarcoma

The 5' half of the EWS gene has recently been described to be fused to the 3' regions of genes encoding the DNA-binding domain of several transcriptional regulators, including ATF1, FLI-1, and ERG, in several human tumors. The most frequent occurrence of this situation results from the t(11;22)(q24;q12) chromosome translocation specific for Ewing sarcoma (ES) and related tumors which joins EWS sequences to the 3' half of FLI-1, which encodes a member of the Ets family of transcriptional regulators. We show here that this chimeric gene encodes an EWS-FLI-1 nuclear protein which binds DNA with the same sequence specificity as the wild-type parental FLI-1 protein. We further show that EWS-FLI-1 is an efficient sequence-specific transcriptional activator of model promoters containing FLI-1 (Ets)-binding sites, a property which is strictly dependent on the presence of its EWS domain. Comparison of the properties of the N-terminal activation domain of FLI-1 to those of the EWS domain of the fusion protein indicates that EWS-FLI-1 has altered transcriptional activation properties compared with FLI-1. These results suggest that EWS-FLI-1 contributes to the transformed phenotype of ES tumor cells by inducing the deregulated and/or unscheduled activation of genes normally responsive to FLI-1 or to other close members of the Ets family. ES and related tumors are characterized by an elevated level of c-myc expression. We show that EWS-FLI-1 is a transactivator of the c-myc promoter, suggesting that upregulation of c-myc expression is under control of EWS-FLI-1.

ERP, a new member of the ets transcription factor/oncoprotein family: cloning, characterization, and differential expression during B-lymphocyte development

The ets gene family encodes a group of proteins which function as transcription factors under physiological conditions and, if aberrantly expressed, can cause cellular transformation. We have recently identified two regulatory elements in the murine immunoglobulin heavy-chain (IgH) enhancer, pi and microB, which exhibit striking similarity to binding sites for ets-related proteins. To identify ets-related transcriptional regulators expressed in pre-B lymphocytes that may interact with either the pi or the microB site, we have used a PCR approach with degenerate oligonucleotides encoding conserved sequences in all members of the ets family. We have cloned the gene for a new ets-related transcription factor, ERP (ets-related protein), from the murine pre-B cell line BASC 6C2 and from mouse lung tissue. The ERP protein contains a region of high homology with the ETS DNA-binding domain common to all members of the ets transcription factor/oncoprotein family. Three additional smaller regions show homology to the ELK-1 and SAP-1 genes, a subgroup of the ets gene family that interacts with the serum response factor. Full-length ERP expresses only negligible DNA-binding activity by itself. Removal of the carboxy terminus enables ERP to interact with a variety of ets-binding sites including the E74 site, the IgH enhancer pi site, and the lck promoter ets site, suggesting a carboxy-terminal negative regulatory domain. At least three ERP-related transcripts are expressed in a variety of tissues. However, within the B-cell lineage, ERP is highly expressed primarily at early stages of B-lymphocyte development, and expression declines drastically upon B-cell maturation, correlating with the enhancer activity of the IgH pi site. These data suggest that ERP might play a role in B-cell development and in IgH gene regulation.

DNA-binding and transcriptional activation properties of the EWS-FLI-1 fusion protein resulting from the t(11;22) translocation in Ewing sarcoma

The 5' half of the EWS gene has recently been described to be fused to the 3' regions of genes encoding the DNA-binding domain of several transcriptional regulators, including ATF1, FLI-1, and ERG, in several human tumors. The most frequent occurrence of this situation results from the t(11;22)(q24;q12) chromosome translocation specific for Ewing sarcoma (ES) and related tumors which joins EWS sequences to the 3' half of FLI-1, which encodes a member of the Ets family of transcriptional regulators. We show here that this chimeric gene encodes an EWS-FLI-1 nuclear protein which binds DNA with the same sequence specificity as the wild-type parental FLI-1 protein. We further show that EWS-FLI-1 is an efficient sequence-specific transcriptional activator of model promoters containing FLI-1 (Ets)-binding sites, a property which is strictly dependent on the presence of its EWS domain. Comparison of the properties of the N-terminal activation domain of FLI-1 to those of the EWS domain of the fusion protein indicates that EWS-FLI-1 has altered transcriptional activation properties compared with FLI-1. These results suggest that EWS-FLI-1 contributes to the transformed phenotype of ES tumor cells by inducing the deregulated and/or unscheduled activation of genes normally responsive to FLI-1 or to other close members of the Ets family. ES and related tumors are characterized by an elevated level of c-myc expression. We show that EWS-FLI-1 is a transactivator of the c-myc promoter, suggesting that upregulation of c-myc expression is under control of EWS-FLI-1.

Involvement of transcription factor YB-1 in human T-cell lymphotropic virus type I basal gene expression

Sequences which control basal human T-cell lymphotropic virus type I (HTLV-I) transcription likely play an important role in initiation and maintenance of virus replication. We previously identified and analyzed a 45-nucleotide sequence (downstream regulatory element 1 [DRE 1]), +195 to +240, at the boundary of the R/U5 region of the long terminal repeat which is required for HTLV-I basal transcription. We identified a protein, p37, which specifically bound to DRE 1. An affinity column fraction, containing p37, stimulated HTLV-I transcription approximately 12-fold in vitro. We now report the identification of a cDNA clone (15B-7), from a Jurkat expression library, that binds specifically to the DRE 1 regulatory sequence. Binding of the cDNA fusion protein, similarly to the results obtained with purified Jurkat protein, was decreased by introduction of site-specific mutations in the DRE 1 regulatory sequence. In vitro transcription and translation of 15B-7 cDNA produced a fusion protein which bound specifically to the HTLV-I +195 to +240 oligonucleotide. The partial cDNA encodes a protein which is homologous to the C-terminal 196 amino acids of the 36-kDa transcription factor, YB-1. Cotransfection of a YB-1 expression plasmid increases HTLV-I basal transcription approximately 14-fold in Jurkat T lymphocytes. On the basis of the molecular weight, DNA-binding characteristics, and in vivo transactivation activity, we suggest that the previously identified DRE 1-binding protein, p37, is YB-1.

Transcriptional activation of the human T-lymphotropic virus type I long terminal repeat by functional interaction of Tax1 and Ets1

Transcription regulation of the oncogenic retrovirus human T-lymphotropic virus type I (HTLV-I) involves the composite activity of both viral and cellular transcription factors. The HTLV-I transforming protein, Tax1, modulates the activity of several cellular transcription factors, upregulating the level of viral gene expression. In addition, cellular transcription factors, such as Ets1, independently bind to the viral long terminal repeat in a sequence-specific manner and activate transcription. It was of interest to analyze the possible interaction of Tax1 and Ets1 in viral gene regulation. We now report that Tax1 and Ets1 increase expression from the HTLV-I promoter in a cooperative manner. The level of expression was increased 5- to 10-fold above the combined individual effect of Tax1 and Ets1. S1 nuclease analysis demonstrated that the cooperative effect was due to an increase in the levels of steady-state RNA. The functional interaction between Tax1 and Ets1 required the presence of the Tax1-responsive 21-bp repeat element TRE-1 and the Ets1-responsive element ERR-1. These results suggested the possible interaction of Ets1 with transcriptional regulatory proteins that bind to the 21-bp repeats. This interaction is demonstrated by decreased electrophoretic mobility of specific 21-bp repeat gel shift complexes in the presence of Ets1. Furthermore, interaction of Ets1 with the 21-bp repeat-binding proteins enhances the relative efficiency of binding to the DNA. This cooperative interaction between Ets1 and proteins which bind to the Tax1-responsive 21-bp repeats suggests a possible role for Ets1 in the regulation of viral gene expression.

Pi, a pre-B-cell-specific enhancer element in the immunoglobulin heavy-chain enhancer

We have identified a new immunoglobulin heavy-chain enhancer element, designated pi, between the microE2 and microE3 elements. The pi enhancer element is transcriptionally active primarily during early stages of B-cell development but becomes virtually inactive during B-cell maturation at about the stage of immunoglobulin kappa light-chain gene rearrangement. Mutational analysis suggests that the pi element is crucial for immunoglobulin heavy-chain enhancer activity at the pre-B-cell stage but is almost irrelevant for enhancer activity at the mature B-cell or plasma-cell stage. The activity of the pi enhancer element correlates with the presence of an apparently pre-B-cell-specific protein-DNA complex. The similarity of the pi site to recognition sequences for members of the ets gene family suggests that the protein(s) interacting with the pi site most likely are ets-related transcription factors.

Characterization of the human interleukin-2 receptor beta-chain gene promoter: regulation of promoter activity by ets gene products

The interleukin-2 receptor (IL-2R) beta chain (IL-2R beta) is an essential signaling component of high- and intermediate-affinity IL-2Rs. Our laboratory previously reported that a DNA fragment containing 857 bp of 5'-flanking sequence of the human IL-2R beta gene exhibited promoter activity. We have now further characterized the promoter and delineated cis-acting regulatory regions. The region downstream of -363 is critical for basal and phorbol myristate acetate-inducible IL-2R beta promoter activity and contains at least three enhancer-like regions. Among them, the -56 to -34 enhancer was the most potent and had high-level activity in two T-cell lines but not in nonlymphoid HeLaS3 and MG63 cells. This enhancer contains a GGAA Ets binding site which bound two Ets family proteins, Ets-1 and GA-binding protein in vitro. Mutation of the Ets motif strongly diminished both promoter and enhancer activities. We conclude that this Ets binding site plays a key role in regulating basal and phorbol myristate acetate-inducible IL-2R beta promoter activity and may also contribute to tissue-specific expression of the IL-2R beta gene.

Characterization of the human interleukin-2 receptor beta-chain gene promoter: regulation of promoter activity by ets gene products

The interleukin-2 receptor (IL-2R) beta chain (IL-2R beta) is an essential signaling component of high- and intermediate-affinity IL-2Rs. Our laboratory previously reported that a DNA fragment containing 857 bp of 5'-flanking sequence of the human IL-2R beta gene exhibited promoter activity. We have now further characterized the promoter and delineated cis-acting regulatory regions. The region downstream of -363 is critical for basal and phorbol myristate acetate-inducible IL-2R beta promoter activity and contains at least three enhancer-like regions. Among them, the -56 to -34 enhancer was the most potent and had high-level activity in two T-cell lines but not in nonlymphoid HeLaS3 and MG63 cells. This enhancer contains a GGAA Ets binding site which bound two Ets family proteins, Ets-1 and GA-binding protein in vitro. Mutation of the Ets motif strongly diminished both promoter and enhancer activities. We conclude that this Ets binding site plays a key role in regulating basal and phorbol myristate acetate-inducible IL-2R beta promoter activity and may also contribute to tissue-specific expression of the IL-2R beta gene.

Pi, a pre-B-cell-specific enhancer element in the immunoglobulin heavy-chain enhancer

We have identified a new immunoglobulin heavy-chain enhancer element, designated pi, between the microE2 and microE3 elements. The pi enhancer element is transcriptionally active primarily during early stages of B-cell development but becomes virtually inactive during B-cell maturation at about the stage of immunoglobulin kappa light-chain gene rearrangement. Mutational analysis suggests that the pi element is crucial for immunoglobulin heavy-chain enhancer activity at the pre-B-cell stage but is almost irrelevant for enhancer activity at the mature B-cell or plasma-cell stage. The activity of the pi enhancer element correlates with the presence of an apparently pre-B-cell-specific protein-DNA complex. The similarity of the pi site to recognition sequences for members of the ets gene family suggests that the protein(s) interacting with the pi site most likely are ets-related transcription factors.

Transactivation of the P2 promoter of parathyroid hormone-related protein by human T-cell lymphotropic virus type I Tax1: evidence for the involvement of transcription factor Ets1

Expression of the parathyroid hormone-related protein (PTHrP), a protein that plays a primary role in the development of the humoral hypercalcemia of malignancy, is regulated by two distinct promoters, P1 and P2. PTHrP is overexpressed in lymphocytes from adult T-cell leukemia patients. We now demonstrate that in the human T-cell lymphotropic virus type I-transformed cell line MT-2, RNA synthesis is initiated primarily at the P2 promoter. Furthermore, in cotransfection experiments, Tax1 transactivates the P2 promoter 10- to 12-fold. By using deletion and site-specific point mutations, we have identified a promoter-proximal sequence (positions -72 to -40) which is important for Tax1 transactivation. The PTHrP promoter-proximal element contains two potential overlapping Ets1 binding sites, EBS I and EBS II. Gel shift analysis demonstrated that Ets1 binds specifically to both EBS I and EBS II. Mutation of the consensus GGAA core motif in EBS I abolished binding and Tax1 transactivation in Jurkat T lymphocytes. In Ets1-deficient cells, cotransfection of Tax1 and Ets1 expression plasmids stimulates PTHrP promoter activity. In the absence of Ets1, minimal transactivation of the PTHrP promoter is observed. These data suggest that Ets1 binds to EBS I and cooperates with Tax1 to transactivate the PTHrP P2 promoter.

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.

Characterization of a hepatoma mRNA transcribed from a third promoter of a 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase-encoding gene and controlled by ets oncogene-related products

6-Phosphofructo-2-kinase (EC 2.7.1.105)/fructose-2,6-bis-phosphatase (EC 3.1.3.46) catalyzes the synthesis and degradation of fructose 2,6-bisphosphate, a ubiquitous stimulator of glycolysis. The liver (L-type) and muscle (M-type) mRNAs for this bifunctional enzyme arise from distinct promoters of the same gene. We have now characterized in rat hepatoma FTO2B cells another mRNA, which is transcribed from a third promoter of that gene. This F-type mRNA is present in fetal rat liver and muscle, in rat placenta, and in several established rat cell lines. The F promoter contains no TATA box but contains several binding sites for Sp1 and for members of the ets oncogene family. Transfection of FTO2B cells with constructs containing the intact or mutagenized F promoter showed that its activity depends mainly on one of these sites. This site bound a heteromeric FTO2B cell protein indistinguishable from the ets-related GA binding protein alpha/ankyrin-repeats GA binding protein beta transcription factor.

Activation of the human T-cell leukemia virus type I enhancer is mediated by binding sites for Elf-1 and the pets factor

Infection with human T-cell leukemia virus type I (HTLV-I) is associated with adult T-cell lymphoma/leukemia. This disease occurs in only a small minority of people infected with HTLV-I and manifests itself many years after infection. Therefore, it appears that a fine balance exists between HTLV-I and the host T-cell factors with which it interacts. HTLV-I encodes a transactivating protein, Tax, which activates viral transcription via cellular mechanisms which are incompletely understood. As viral gene expression is negligible during latency, it is doubtful that Tax controls the initial transition to the replicative state. Tax-independent cellular factors which control HTLV-I transcription, and presumably latency, have received little study. Recently, the product of the chicken proto-oncogene ets-1 has been shown to bind to the HTLV-I enhancer and modestly activate transcription in certain cell types (S. C. Gitlin, R. Bosselut, A. Gégonne, J. Ghysdael, and J. N. Brady, J. Virol. 65:5513-5523, 1991). However, the functional significance of the ets-binding site in the intact enhancer has not previously been shown. We now demonstrate that site-specific mutation of the purine-rich ets-binding site significantly diminishes inducible enhancer function, but not Tax response, in the human Jurkat T-cell line. Similarly, mutation of the peri-ets (pets) site, not previously noted in the HTLV-I enhancer, markedly inhibits inducible enhancer function but not Tax response. Further, we show that the predominant protein binding the purine-rich HTLV-I enhancer element in human T cells is not ets-1 but Elf-1, a member of the ets family which is very similar to the Drosophila morphogen E74. Regulation of HTLV-I through Elf-1/pets enhancer motifs resembles that seen with human immunodeficiency virus type 2 (D. M. Markovitz, M. Smith, J. Hilfinger, M. C. Hannibal, B. Petryniak, and G. J. Nabel, J. Virol. 66:5479-5484, 1992; J. M. Leiden, C.-W. Wang, B. Petryniak, M. Smith, D. M. Markovitz, G. J. Nabel, and C. B. Thompson, J. Virol. 66:5890-5897, 1992), another human pathogenic retrovirus with a relatively long incubation period.

Differential regulation of the human immunodeficiency virus type 2 enhancer in monocytes at various stages of differentiation

We have demonstrated that stimulation of the human immunodeficiency virus type 2 (HIV-2) enhancer in T cells is dependent upon at least four cis-acting elements, including two purine-rich binding sites, PuB1 and PuB2, which are capable of binding members of the ets family of proto-oncogenes, the pets (peri-ets) site, which lies just upstream of the PuB2 site, and a single kappa B site (D. M. Markovitz, M. Smith, J. M. Hilfinger, M. C. Hannibal, B. Petryniak, and G. J. Nabel, J. Virol. 66:5479-5484, 1992). In this study, we examined the regulation of the HIV-2 enhancer in cells of monocytic lineage. We found that in immature monocytic cell lines, the HIV-2 enhancer is markedly induced by phorbol esters and that all four cis-acting elements are required for activation. In mature monocytic cells, constitutive activity is high, with only modest stimulation following phorbol ester treatment. Mutation of any of the four cis-acting elements resulted in greatly reduced basal expression in mature monocytes. This is in contrast to HIV-1, in which developmentally controlled expression of the enhancer in monocytes is mediated largely through the kappa B sites alone [G. E. Griffin, K. Leung, T. M. Folks, S. Kunkel, and G. J. Nabel, Nature (London) 339:70-73, 1989]. Further, we demonstrated that although both Elf-1, an ets family member with significant similarity to the drosophila developmental regulatory protein E74, and Pu.1, a monocyte- and B-cell-specific member of the ets family, bind the purine-rich enhancer region, Elf-1 is the protein which binds predominantly in vivo. A nuclear factor(s) which binds the pets site, an element which has been described only in HIV-2, was detected in extracts of all of the monocytic cells tested. These findings indicate that the mechanism by which cellular factors regulate HIV-2 enhancer function in monocytic cells differs significantly from that of HIV-1 and may offer a partial explanation for the differences in the biological and clinical characteristics of the two viruses.