A 36-kilodalton cellular transcription factor mediates an indirect interaction of human T-cell leukemia/lymphoma virus type I TAX1 with a responsive element in the viral long terminal repeat

T-cell receptor/CD28 engagement when combined with prostaglandin E2 treatment leads to potent activation of human T-cell leukemia virus type 1

Infection with human T-cell leukemia virus type 1 (HTLV-1) is characterized by long latency periods, indicating that viral gene expression is under tight control. There is presently little information available regarding the nature of extracellular stimuli that can transactivate the regulatory elements of HTLV-1 (i.e., long terminal repeat [LTR]). To gain insight into the biological importance of externally induced activation pathways in virus gene expression, primary and established T cells were transfected with HTLV-1-based reporter gene vectors and then were treated with agents that cross-linked the T-cell receptor (TCR) or the costimulatory CD28 molecule with prostaglandin E(2) (PGE(2)). We demonstrated that a potent induction of HTLV-1 LTR-driven reporter gene activity was seen only when the three agents were used in combination. Interestingly, similar observations were made when using C91/PL, a cell line that carries integrated HTLV-1 proviral DNA. This TCR-CD28-PGE(2)-mediated increase in virus transcription was dependent on protein kinase A activation and induction of the cAMP response element binding protein. Experiments with a mutated reporter construct further revealed the importance of the Tax-responsive elements in the HTLV-1 LTR in the observed up regulation of virus gene expression when TCR/CD28 engagement was combined with PGE(2) treatment. The protein tyrosine kinases p56(lck) and the transmembrane tyrosine phosphatase CD45 were all found to be involved in TCR-CD28-PGE(2)-directed increase in HTLV-1 LTR activity. This study presents new information on the possible mechanisms underlying reactivation of this retrovirus.

Anti-Rex aptamers as mimics of the Rex-binding element

RNA molecules that bind tightly and specifically to a Rex fusion protein have been isolated from a conformationally constrained pool of random sequence RNAs. The anti-Rex aptamers effectively mimic several features of the wild-type Rex-binding element (XBE). The highest-affinity aptamers effectively compete with the wild-type XBE for binding to the RNA-binding domain of Rex, an arginine-rich motif (ARM), but do not bind to the functionally analogous Rev protein or its ARM. However, characteristic sequence and structural motifs found in some of the anti-Rex aptamers may provide insights into how the Rex protein can interact with other viral RNAs, such as the Rev-responsive element. The anti-Rex aptamers can functionally substitute for the XBE in vivo, a result which supports a previously proposed model for mRNA transport in which the viral genome serves as a platform for assembling a nucleoprotein complex that can co-opt the cellular transport apparatus. Overall, these studies suggest that anti-Rex aptamers may serve as RNA decoys of the Rex protein.

Activation of human T-cell leukemia virus type 1 tax gene expression in chronically infected T cells

Expression of human T-cell leukemia virus type 1 (HTLV-1) is regulated both by the HTLV-1 Tax transactivator and by cellular transcriptional factors binding to the viral long terminal repeat (LTR), suggesting that cellular signals may play a role in regulating viral expression. Treatment of cells chronically infected with HTLV-1, which express low levels of HTLV-1 RNAs and Tax protein, with phorbol esters (i.e., phorbol12-myristate 13- acetate [PMA]), phytohemagglutinin (PHA), sodium butyrate, or combinations of cytokines resulted in induction of HTLV- 1 gene expression. PMA or PHA treatment following cotransfection of HTLV-1 Tax expression plasmids resulted in synergistic activation of HTLV-1 LTR-directed gene expression, apparently involving tyrosine ki- nase- mediated pathways. These results suggest that cellular activation stimuli may cooperate with HTLV-1 Tax to enhance expression of integrated HTLV-1 genomes and thus may play a role in the pathogenesis of HTLV-1 disease.

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.

cis-Acting inhibitory elements within the pol-env region of human T-cell leukemia virus type 1 possibly involved in viral persistence

Human T-cell leukemia virus type 1 (HTLV-1) remains latent throughout the life of the carrier, with cells containing the provirus and viral gene expression efficiently down-regulated. On a molecular level, exactly how viruses are down-regulated in vivo remains unresolved. We described here the possibility that down-regulation results from the presence of inhibitory elements within the gag-env region of the provirus in fresh peripheral blood mononuclear cells from carriers. In vitro experiments then revealed that potent cis-acting inhibitory elements (CIEs) are indeed contained in two discrete fragments from the pol region and weaker ones in the env region. The effect of CIEs is relieved by the HTLV-1 posttranscriptional regulator Rex through binding to the Rex-responsive element (RxRE), suggesting that Rex might interfere with pre-mRNA degradation and/or activate the export of mRNA molecules harboring both of the inhibitory elements and RxRE on the same RNA molecule. Thus, we propose the hypothesis that such functions of CIEs may be involved in HTLV-1 persistence.

Characterization of nuclear proteins that bind to the regulatory TGATTGGC motif in the human immunodeficiency virus type 1 long terminal repeat

We have recently elucidated the nature and function of transcription factors present in Jurkat, glial and neuronal cells that interact with modulatory region B, the nuclear receptor responsive element, in the long terminal repeat of human immunodeficiency virus type 1 (HIV-1). Considering the key role that the combination of host cell proteins plays in HIV-1 gene transcription, it appears essential to characterize proteins interacting with the adjacent region A. In vitro experiments revealed that the 5'-TGATTGGC-3'motif of region A is the target for at least three distinct proteins, one belonging to the nuclear factor I family, while two others are related to the cAMP response element binding (CREB) protein family. One of these proteins, present in DNA-protein complex C2, is formed by distinct polypeptides of relative molecular mass 43 000 and 50 000. We have purified the 43 kDa protein, which is distinct from CREB-43, and have shown that renatured p43 is able to specifically interact with site A. Transient expression experiments with vectors containing wild-type or mutant motif A revealed that basal HIV-1 gene transcription in Jurkat cells is regulated by antagonistic effects of the site A binding proteins.

Human T-cell leukemia virus type 1 Tax transactivates the human proliferating cell nuclear antigen promoter

The human T-cell leukemia virus type 1 (HTLV-1) transforming protein, Tax, is a potent transactivator of both viral and cellular gene expression. The ability of Tax to transform cells is believed to depend on its transactivation of cellular-growth-regulatory genes. Expression of proliferating cell nuclear antigen (PCNA) is intimately linked to cell growth and DNA replication and repair. By testing a series of PCNA promoter deletion constructs, we have demonstrated that the PCNA promoter can be transactivated by Tax. The smallest construct that was activated did not include the ATF/CRE binding site at nucleotide -50, and mutations in the ATF/CRE element in the context of a larger promoter were still activated by Tax. In addition, a Tax mutant that is defective for activation of the CRE pathway retained the ability to activate the -397 promoter construct. When a series of linker scanner mutations that span the region from nucleotide -45 to -7 were assayed, mutations in and around a repeat sequence were found to abolish Tax transactivation. Multimerized copies of either half of the repeat were Tax responsive. A single protein complex was shown to bind specifically to the Tax-responsive region, and the binding of this complex was enhanced in the presence of Tax. These results demonstrate that the PCNA promoter contains a Tax-responsive element located between nucleotides -45 and -7 whose sequence is different from those of other, previously identified Tax-responsive elements. The ability of Tax to activate the PCNA promoter may play an important role in cellular transformation by HTLV-1.

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

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

Transcription factor AP-4 participates in activation of bovine leukemia virus long terminal repeat by p34 Tax

Three 21bp repeats can be found in the bovine leukemia virus long terminal repeat, which are crucial for the LTR directed gene expression by the trans activator protein Tax. Previous studies demonstrated that the major target of the Tax directed activation are the CRE-like elements in the center of these repeats. In this work we report that another motif of the 21bp repeats is also required for the Tax activation. Gel retardation--with the wild type or mutant 21bp repeats--revealed that cellular factors from HeLa cells were specifically bound to the center (CRE-like element) and the 3' region of the repeats, which contains a CAGCTG consensus AP-4 binding site. In vivo analysis using the synthetic 21bp repeats indicated that beyond the consensus CRE-like motif, the AP-4 site is also essential for Tax activation. To determine the role of AP-4 in BLV Tax trans activation, we used the AP-4 cDNA in antisense transient assays. In the in vivo experiments the antisense AP-4 RNA resulted in strongly decreased Tax activation. On the basis of these results we conclude that AP-4 is a good candidate of cellular factors involved in BLV Tax trans activation.

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

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

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

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

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.

Identification of a negative element in the human vimentin promoter: modulation by the human T-cell leukemia virus type I Tax protein

The vimentin gene is a member of the intermediate filament multigene family and encodes a protein expressed, in vivo, in all mesenchymal derivatives and, in vitro, in cell types of various origin. We have previously demonstrated that the expression of this growth-regulated gene could be trans activated by the 40-kDa Tax protein of HTLV-I (human T-cell leukemia virus type I) and that responsiveness to this viral protein was mediated by the presence of an NF-kappa B binding site located between -241 and -210 bp upstream of the mRNA cap site (A. Lilienbaum, M. Duc Dodon, C. Alexandre, L. Gazzolo, and D. Paulin, J. Virol. 64:256-263, 1990). These previous assays, performed with deletion mutants of the vimentin promoter linked to the chloramphenicol acetyltransferase gene, also revealed the presence of an upstream negative region between -529 and -241 bp. Interestingly, the inhibitory activity exerted by this negative region was overcome after cotransfection of a Tax-expressing plasmid. In this study, we further characterize the vimentin negative element and define the effect of the Tax protein on the inhibitory activity of this element. We first demonstrate that a 187-bp domain (-424 to -237 bp) behaves as a negative region when placed upstream either of the NF-kappa B binding site of vimentin or of a heterologous enhancer such as that present in the desmin gene promoter. The negative effect can be further assigned to a 32-bp element which is indeed shown to repress the basal or induced activity of the NF-kappa B binding site.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of a negative element in the human vimentin promoter: modulation by the human T-cell leukemia virus type I Tax protein

The vimentin gene is a member of the intermediate filament multigene family and encodes a protein expressed, in vivo, in all mesenchymal derivatives and, in vitro, in cell types of various origin. We have previously demonstrated that the expression of this growth-regulated gene could be trans activated by the 40-kDa Tax protein of HTLV-I (human T-cell leukemia virus type I) and that responsiveness to this viral protein was mediated by the presence of an NF-kappa B binding site located between -241 and -210 bp upstream of the mRNA cap site (A. Lilienbaum, M. Duc Dodon, C. Alexandre, L. Gazzolo, and D. Paulin, J. Virol. 64:256-263, 1990). These previous assays, performed with deletion mutants of the vimentin promoter linked to the chloramphenicol acetyltransferase gene, also revealed the presence of an upstream negative region between -529 and -241 bp. Interestingly, the inhibitory activity exerted by this negative region was overcome after cotransfection of a Tax-expressing plasmid. In this study, we further characterize the vimentin negative element and define the effect of the Tax protein on the inhibitory activity of this element. We first demonstrate that a 187-bp domain (-424 to -237 bp) behaves as a negative region when placed upstream either of the NF-kappa B binding site of vimentin or of a heterologous enhancer such as that present in the desmin gene promoter. The negative effect can be further assigned to a 32-bp element which is indeed shown to repress the basal or induced activity of the NF-kappa B binding site.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression and characterization of the trans-activating protein Tax of human T-cell leukemia virus type I in Saccharomyces cerevisiae

The trans-activator protein Tax of human T-cell leukemia virus type I (HTLV-I) stimulates transcription of the viral genome from the long terminal repeat. With a reporter HIS4TATA::lacZ fusion gene, the transcriptional activity of the Tax-responsive element in the long terminal repeat was tested in Saccharomyces cerevisiae. We found that fragments containing the 21-bp repeat of the HTLV-I enhancer stimulate synthesis of beta-galactosidase activity 15- to 20-fold. To test the ability of the Tax protein to trans activate the HTLV-I enhancer in yeast cells, the pX region of HTLV-I, encoding the Tax protein, was cloned under the control of the yeast GAL1 promoter. The expressed Tax protein is localized in the nucleus and associated with the yeast nuclear matrix fraction. In yeast cells that contained the integrated tax gene, two- to sixfold stimulation of expression from the HTLV-I enhancer was detected at the early stages of tax induction. This in vivo reconstitution system provides a new approach for examining the host factor(s), the signal transduction mechanism(s), and the role of nuclear architecture involved in Tax-mediated trans activation.

Mechanism of action of regulatory proteins encoded by complex retroviruses

Complex retroviruses are distinguished by their ability to control the expression of their gene products through the action of virally encoded regulatory proteins. These viral gene products modulate both the quantity and the quality of viral gene expression through regulation at both the transcriptional and posttranscriptional levels. The most intensely studied retroviral regulatory proteins, termed Tat and Rev, are encoded by the prototypic complex retrovirus human immunodeficiency virus type 1. However, considerable information also exists on regulatory proteins encoded by human T-cell leukemia virus type I, as well as several other human and animal complex retroviruses. In general, these data demonstrate that retrovirally encoded transcriptional trans-activators can exert a similar effect by several very different mechanisms. In contrast, posttranscriptional regulation of retroviral gene expression appears to occur via a single pathway that is probably dependent on the recruitment of a highly conserved cellular cofactor. These two shared regulatory pathways are proposed to be critical to the ability of complex retroviruses to establish chronic infections in the face of an ongoing host immune response.

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.

Identification and characterization of a T-cell-specific enhancer adjacent to the murine CD4 gene

Expression of the CD4 and CD8 glycoproteins is a tightly regulated process tied to the maturation of functionally distinct classes of thymocytes. Therefore, understanding of the mechanism of expression of the genes encoding CD4 and CD8 is likely to yield important insight into regulation of the differentiated functions of T cells. Here, we report the identification of a T-cell-specific enhancer in a DNase I-hypersensitive region about 13 kb 5' of the transcription initiation site of the murine CD4 gene. Within the minimal enhancer element, at least three nuclear protein binding sites were identified by DNase I footprint analysis. One site contains the consensus motif for TCF-1 alpha/LEF-1, a recently identified HMG box transcription factor primarily expressed in pre-B and T cells. By Southwestern (DNA-protein) blotting and binding competition analyses, the protein binding to this site was found to be indistinguishable from TCF-1 alpha/LEF-1. Mutagenesis of this site resulted in loss of factor binding but had a relatively minor effect on enhancer activity. In contrast, mutations in another site, containing two consensus binding motifs for basic helix-loop-helix proteins, abolished factor binding and dramatically reduced enhancer activity. None of the protein binding sites had activity on its own, suggesting that the CD4 enhancer requires the interaction of multiple regulatory sites.

Negative regulation of human immunodeficiency virus type 1 expression in monocytes: role of the 65-kDa plus 50-kDa NF-kappa B dimer

Although monocytic cells can provide a reservoir for viral production in vivo, their regulation of human immunodeficiency virus type 1 (HIV-1) transcription can be either latent, restricted, or productive. These differences in gene expression have not been molecularly defined. In THP-1 cells with restricted HIV expression, there is an absence of DNA-protein binding complex formation with the HIV-1 promoter-enhancer associated with markedly less viral RNA production. This absence of binding was localized to the NF-kappa B region of the HIV-1 enhancer; the 65-kDa plus 50-kDa NF-kappa B heterodimer was preferentially lost. Adding purified NF-kappa B protein to nuclear extracts from cells with restricted expression overcomes this lack of binding. In addition, treatment of these nuclear extracts with sodium deoxycholate restored their ability to form the heterodimer, suggesting the presence of an inhibitor of NF-kappa B activity. Furthermore, treatment of nuclear extracts from these cells that had restricted expression with lipopolysaccharide increased viral production and NF-kappa B activity. Antiserum specific for NF-kappa B binding proteins, but not c-rel-specific antiserum, disrupted heterodimer complex formation. Thus, both NF-kappa B-binding complexes are needed for optimal viral transcription. Binding of the 65-kDa plus 50-kDa heterodimer to the HIV-1 enhancer can be negatively regulated in monocytes, providing one mechanism restricting HIV-1 gene expression.

Identification and characterization of a T-cell-specific enhancer adjacent to the murine CD4 gene

Expression of the CD4 and CD8 glycoproteins is a tightly regulated process tied to the maturation of functionally distinct classes of thymocytes. Therefore, understanding of the mechanism of expression of the genes encoding CD4 and CD8 is likely to yield important insight into regulation of the differentiated functions of T cells. Here, we report the identification of a T-cell-specific enhancer in a DNase I-hypersensitive region about 13 kb 5' of the transcription initiation site of the murine CD4 gene. Within the minimal enhancer element, at least three nuclear protein binding sites were identified by DNase I footprint analysis. One site contains the consensus motif for TCF-1 alpha/LEF-1, a recently identified HMG box transcription factor primarily expressed in pre-B and T cells. By Southwestern (DNA-protein) blotting and binding competition analyses, the protein binding to this site was found to be indistinguishable from TCF-1 alpha/LEF-1. Mutagenesis of this site resulted in loss of factor binding but had a relatively minor effect on enhancer activity. In contrast, mutations in another site, containing two consensus binding motifs for basic helix-loop-helix proteins, abolished factor binding and dramatically reduced enhancer activity. None of the protein binding sites had activity on its own, suggesting that the CD4 enhancer requires the interaction of multiple regulatory sites.

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.