Location of the trans-activating region on the genome of human T-cell lymphotropic virus type III

Interaction of the protein transduction domain of HIV-1 TAT with heparan sulfate: binding mechanism and thermodynamic parameters

The positively charged protein transduction domain of the HIV-1 TAT protein (TAT-PTD; residues 47-57 of TAT) rapidly translocates across the plasma membrane of living cells. This property is exploited for the delivery of proteins, drugs, and genes into cells. The mechanism of this translocation is, however, not yet understood. Recent theories for translocation suggest binding of the protein transduction domain (PTD) to extracellular glycosaminoglycans as a possible mechanism. We have studied the binding equilibrium between TAT-PTD and three different glycosaminoglycans with high sensitivity isothermal titration calorimetry and provide the first quantitative thermodynamic description. The polysulfonated macromolecules were found to exhibit multiple identical binding sites for TAT-PTD with only small differences between the three species as far as the thermodynamic parameters are concerned. Heparan sulfate (HS, molecular weight, 14.2 +/- 2 kDa) has 6.3 +/- 1.0 independent binding sites for TAT-PTD which are characterized by a binding constant K0 = (6.0 +/- 0.6) x 10(5) M(-1) and a reaction enthalpy deltaHpep0 = -4.6 +/- 1.0 kcal/mol at 28 degrees C. The binding affinity, deltaGpep0, is determined to equal extent by enthalpic and entropic contributions. The HS-TAT-PTD complex formation entails a positive heat capacity change of deltaCp0 = +135 cal/mol peptide, which is characteristic of a charge neutralization reaction. This is in contrast to hydrophobic binding reactions which display a large negative heat capacity change. The stoichiometry of 6-7 TAT-PTD molecules per HS corresponds to an electric charge neutralization. Light scattering data demonstrate a maximum scattering intensity at this stoichiometric ratio, the intensity of which depends on the order of mixing of the two components. The data suggest cross-linking and/or aggregation of HS-TAT-PTD complexes. Two other glycosaminoglycans, namely heparin and chondroitin sulfate B, were also studied with isothermal titration calorimetry. The thermodynamic parameters are K0 = (6.0 +/- 0.8) x 10(5) M(-1) and kcal/mol for heparin and K0 = (2.5 +/- 0.5) x 10(5) M(-1) and kcal/mol for chondroitin sulfate B at 28 degrees C. The close thermodynamic similarity of the three binding molecules also implies a close structural relationship. The ubiquitous occurrence of glycosaminoglycans on the cell surface together with their tight and rapid interaction with the TAT protein transduction domain makes complex formation a strong candidate as the primary step of protein translocation.

Zn2+ binding to cysteine-rich domain of extracellular human immunodeficiency virus type 1 Tat protein is associated with Tat protein-induced apoptosis

The Tat protein has several functional domains, one of which is the cysteine-rich domain that is a highly conserved region in spite of the presence of many subtypes of human immunodeficiency virus type 1 (HIV-1). Although the cysteine-rich domain is a potential site for Zn(2+) binding, it is controversial whether Zn(2+) is substantially essential for the structure and activities of the Tat protein. To study the significance of Zn(2+) in the cysteine-rich domain of the Tat protein particularly released to the extracellular space, we raised the monoclonal antibody (MAb) 5A4, which has an attractive property of recognizing the Zn(2+)-binding Tat(20-41) peptide but not the apo-Tat(20-41) peptide. MAb 5A4 inhibited the trans-activation of the HIV long terminal repeat (LTR) in HeLa-CD4-LTR/beta-gal cells induced by treatment with the recombinant Tat protein, indicating that MAb 5A4 can recognize the full-length Tat protein and inhibit its trans-activity. The antibody also inhibited the apoptosis of Jurkat cells induced by treatment with the released native-Tat-protein-containing supernatant from the culture of HIV-1(JRFL)-infected cells. These results suggest that Zn(2+), whose structure is closely associated with not only the trans-activation of HIV-LTR but also the induction of apoptosis, binds to the extracellular native Tat protein. The Zn(2+)-binding cysteine-rich domain therefore can be a molecular target in the development of an anti-Tat vaccine and agents for the control of extracellular-Tat-protein-mediated pathogenesis leading to the progression of acquired immunodeficiency syndrome.

Gene therapy with viral vectors

A key factor in the success of gene therapy is the development of gene delivery systems that are capable of efficient gene transfer in a broad variety of tissues, without causing any pathogenic effect. Currently, viral vectors based on many different viruses have been developed, and their performance and pathogenicity has been evaluated in animal models. The results of these studies form the basis for the first clinical trials for correcting genetic disorders using retroviral, adenoviral, and adeno-associated viral vectors. Even though the results of these trials are encouraging, vector development is still required to improve and refine future treatment of hereditary disorders.

HIV-1 pathogenesis

Despite considerable advances in HIV science in the past 20 years, the reason why HIV-1 infection is pathogenic is still debated and the goal of eradicating HIV-1 infection remains elusive. A deeper understanding of the interplay between HIV-1 and its host and why simian immunodeficiency virus (SIV) is nonpathogenic in some natural hosts may provide a few answers.

Monocytes treated with human immunodeficiency virus Tat kill uninfected CD4(+) cells by a tumor necrosis factor-related apoptosis-induced ligand-mediated mechanism

The human immunodeficiency virus (HIV) Tat protein has a critical role in viral transcription, but this study focuses on its additional role as an extracellular effector of lymphocyte cell death. It is well known that Tat induces tumor necrosis factor-related apoptosis-induced ligand (TRAIL) in peripheral blood mononuclear cells (PBMC), and we show that the majority of TRAIL is produced by the monocyte subset of PBMC. Human monocytes and U937 monoblastoid cells did not take up soluble HIV Tat-86, as T cells did, yet produced more TRAIL than did T cells. TRAIL secretion was induced by Tat and by a cysteine-rich peptide of Tat but not by sulfhydryl-modified Tat toxoid. Although there was only a slight increase in cell surface expression of TRAIL on monocytes, sufficient TRAIL was secreted to be toxic for T cells. The cytotoxicity of Tat-stimulated monocyte medium could be blocked by a TRAIL-neutralizing antibody. T cells treated with Tat did not secrete enough TRAIL to mediate cell death in our assay. Remarkably, uninfected T cells are more susceptible to TRAIL than are HIV-infected T cells. The production of TRAIL by Tat-stimulated monocytes provides a mechanism by which HIV infection can destroy uninfected bystander cells.

Differential activation of Tat variants in mitogen-stimulated cells: implications for HIV-1 postintegration latency

Like other HIV-1 (human immunodeficiency virus type 1) proteins, Tat undergoes rapid mutation and occurs in numerous sequence variants in nature. Virus isolated from patients often has defects in Tat that lower its activity. The levels of P-TEFb, an essential cellular cofactor for Tat, are elevated by T-cell activation. To test the hypothesis that stimulation of P-TEFb levels might compensate for attenuation of Tat activity, we generated Tat constructs with a range of transactivation function. Transactivation by the Tat mutants correlated with their ability to bind to P-TEFb in vitro. Treatment of U937 cells with the phorbol ester PMA (phorbol myristate acetate) induced P-TEFb and stimulated Tat transactivation for alleles with basal transcription activity above a threshold (>5% compared to wild-type). Highly active alleles (>66% of wild-type) were stimulated to a lesser extent than those with activity in the intermediate range. Thus, attenuation of Tat function, in concert with low levels of P-TEFb activity, could serve to keep the virus in a latent state in quiescent cells yet permit viral replication after cell activation.

Neuropathologies in transgenic mice expressing human immunodeficiency virus type 1 Tat protein under the regulation of the astrocyte-specific glial fibrillary acidic protein promoter and doxycycline

The human immunodeficiency virus type 1 (HIV-1) Tat protein is a key pathogenic factor in a variety of acquired immune deficiency syndrome (AIDS)-associated disorders. A number of studies have documented the neurotoxic property of Tat protein, and Tat has therefore been proposed to contribute to AIDS-associated neurological diseases. Nevertheless, the bulk of these studies are performed in in vitro neuronal cultures without taking into account the intricate cell-cell interaction in the brain, or by injection of recombinant Tat protein into the brain, which may cause secondary stress or damage to the brain. To gain a better understanding of the roles of Tat protein in HIV-1 neuropathogenesis, we attempted to establish a transgenic mouse model in which Tat expression was regulated by both the astrocyte-specific glial fibrillary acidic protein promoter and a doxycycline (Dox)-inducible promoter. In the present study, we characterized the phenotypic and neuropathogenic features of these mice. Both in vitro and in vivo assays confirmed that Tat expression occurred exclusively in astrocytes and was Dox-dependent. Tat expression in the brain caused failure to thrive, hunched gesture, tremor, ataxia, and slow cognitive and motor movement, seizures, and premature death. Neuropathologies of these mice were characterized by breakdown of cerebellum and cortex, brain edema, astrocytosis, degeneration of neuronal dendrites, neuronal apoptosis, and increased infiltration of activated monocytes and T lymphocytes. These results together demonstrate that Tat expression in the absence of HIV-1 infection is sufficient to cause neuropathologies similar to most of those noted in the brain of AIDS patients, and provide the first evidence in the context of a whole organism to support a critical role of Tat protein in HIV-1 neuropathogenesis. More importantly, our data suggest that the Dox inducible, brain-targeted Tat transgenic mice offer an in vivo model for delineating the molecular mechanisms of Tat neurotoxicity and for developing therapeutic strategies for treating HIV-associated neurological disorders.

RNA interference directed against viral and cellular targets inhibits human immunodeficiency Virus Type 1 replication

Human immunodeficiency virus type 1 (HIV-1) gene expression is regulated by both cellular transcription factors and Tat. The ability of Tat to stimulate transcriptional elongation is dependent on its binding to TAR RNA in conjunction with cyclin T1 and CDK9. A variety of other cellular factors that bind to the HIV-1 long terminal repeat, including NF-kappaB, SP1, LBP, and LEF, are also important in the control of HIV-1 gene expression. Although these factors have been demonstrated to regulate HIV-1 gene expression by both genetic and biochemical analysis, in most cases a direct in vivo demonstration of their role on HIV-1 replication has not been established. Recently, the efficacy of RNA interference in mammalian cells has been shown utilizing small interfering RNAs (siRNAs) to result in the specific degradation of host mRNAs and decreases the levels of their corresponding proteins. In this study, we addressed whether siRNAs directed against either HIV-1 tat or reverse transcriptase or the NF-kappaB p65 subunit could specifically decrease the levels of these proteins and thus alter HIV-1 replication. Our results demonstrate the specificity of siRNAs for decreasing the expression of these viral and cellular proteins and inhibiting HIV-1 replication. These studies suggest that RNA interference is useful in exploring the biological role of cellular and viral regulatory factors involved in the control of HIV-1 gene expression.

Activation of HTLV-I transcription in the presence of Tax is independent of the acetylation of CREB-2 (ATF-4)

We have previously demonstrated that the bZIP transcription factor CREB-2, also called ATF-4, trans-activates, in association with the viral protein Tax, the human T-cell leukemia virus type I (HTLV-I) promoter. In this study, we have examined whether CREB-2 acetylation affects transcriptional activation mediated by Tax. We present evidence that CREB-2 is acetylated in vitro and in vivo. CREB-2 is acetylated in two regions: the basic domain of the bZIP (from amino acid residue 270 to 300) and the short basic domain (from 342 to 351) located downstream from the bZIP. We also demonstrate that CREB-2 is acetylated by p300/CBP but not by p/CAF. Moreover, replacement of lysine by arginine in the basic domains decreases the trans-activating capacity of CREB-2. However, in the presence of Tax, the HTLV-I transcription remains fully activated by these CREB-2 mutants. Although we cannot totally exclude that the mutations could also affect CREB-2 structure and activity independent of acetylation, our results suggest that activation of the viral promoter in the presence of Tax is independent of the CREB-2 acetylation.

Human retroviruses after 20 years: a perspective from the past and prospects for their future control

Among viruses the human retroviruses may be of special interest to immunologists, because they target cells of the immune system, particularly mature CD4+ T cells, impair their function and cause them to grow abnormally (human T-cell leukemia virus, HTLV) or to die (human immunodeficiency virus, HIV). Human retroviruses cause disease ranging from neurological disorders and leukemias (HTLV-1) to AIDS (acquired immunodeficiency virus) (HIV) and promote development of several types of malignancies (HIV). They share many common features, but their contrasts are greater, especially the far greater replication and variation of HIV associated with its greater genomic complexity. Both have evolved striking redundancy for mechanisms which promote their survival. Thus, HTLV has redundant mechanisms for promoting growth of provirus containing T cells needed for virus continuity, because it is chiefly through its cellular DNA provirus that HTLV replicates and not through production of virions. Conversely, HIV has redundancy in its mechanisms for promoting virion replication and escape from the host immune system. It is via these redundant mechanisms that they produce disease: leukemias from mechanisms promoting T-cell proliferation (HTLV-1) and AIDS from mechanisms promoting virus replication and T-cell death (HIV). The practical challenges for the future are clear. For HTLV-1, education and control of breastfeeding. For HIV, the formidable tasks now ahead in part demand new kinds of talent, talents that will foster greater insights into the development of therapy for the developing countries, new forms of less toxic therapies for all infected persons, a continued and expanded commitment to education, and a persistent 'never say die' commitment to the development of a truly preventive vaccine with all the scientific and nonscientific challenges that these objectives face.

Angiogenic effects of extracellular human immunodeficiency virus type 1 Tat protein and its role in the pathogenesis of AIDS-associated Kaposi's sarcoma

The Tat protein of human immunodeficiency virus (HIV) type 1 is a transactivator of viral gene expression that is required for virus replication and spread. Moreover, Tat is released by acutely HIV-infected cells via a leaderless secretory pathway and in a biologically active form that exerts effects on both HIV-infected and uninfected cells from different organs and systems. This review focuses on the activities of extracellular Tat protein on endothelial cells, on angiogenesis, and on the pathogenesis of AIDS-associated angioproliferative diseases such as Kaposi's sarcoma. In particular, we discuss results from different groups indicating that Tat mimics the proangiogenic activities of extracellular matrix molecules and that it enhances the effects of angiogenic factors.

Spt5 cooperates with human immunodeficiency virus type 1 Tat by preventing premature RNA release at terminator sequences

The human immunodeficiency virus type 1 (HIV-1) Tat protein activates transcription elongation by stimulating the Tat-activated kinase (TAK/p-TEFb), a protein kinase composed of CDK9 and its cyclin partner, cyclin T1. CDK9 is able to hyperphosphorylate the carboxyl-terminal domain (CTD) of the large subunit of RNA polymerase during elongation. In addition to TAK, the transcription elongation factor Spt5 is required for the efficient activation of transcriptional elongation by Tat. To study the role of Spt5 in HIV transcription in more detail, we have developed a three-stage Tat-dependent transcription assay that permits the isolation of active preinitiation complexes, early-stage elongation complexes, and Tat-activated elongation complexes. Spt5 is recruited in the transcription complex shortly after initiation. After recruitment of Tat during elongation through the transactivation response element RNA, CDK9 is activated and induces hyperphosphorylation of Spt5 in parallel to the hyperphosphorylation of the CTD of RNA polymerase II. However, immunodepletion experiments demonstrate that Spt5 is not required for Tat-dependent activation of the kinase. Chase experiments using the Spt5-depleted extracts demonstrate that Spt5 is not required for early elongation. However, Spt5 plays an important role in late elongation by preventing the premature dissociation of RNA from the transcription complex at terminator sequences and reducing the amount of polymerase pausing at arrest sites, including bent DNA sequences. This novel biochemical function of Spt5 is analogous to the function of NusG, an elongation factor found in Escherichia coli that enhances RNA polymerase stability on templates and shows sequence similarity to Spt5.

Hiv-1 Tat can substantially enhance the capacity of NIK to induce IkappaB degradation

The human immunodeficiency virus type 1 (HIV-1) Tat is a virally encoded protein that dramatically up-regulates viral replication through interactions with the HIV-1 5' long terminal repeat (LTR) and cellular transcription factors. The HIV-1 LTR is divided into three major regions: modulatory, core and TAR. The modulatory region contains numerous cis-acting sequences for the binding of transcription factors including NF-kappaB, NF-AT, and AP-1. In several reports, Tat has been found to induce NF-kappaB activation of the HIV-1 LTR, while in other studies Tat has been reported to have no effect on activation of NF-kappaB. These discrepancies may arise from differences in experimental conditions such as the source of Tat (exogenous versus endogenous), the detection methods for NF-kappaB activation (DNA binding capability versus IkappaB degradation), and the types of reporters used (HIV-1 versus non-HIV-1 derived). To reconcile these differences we examined the effect of endogenous Tat on NF-kappaB activation, on IkappaB degradation and its interaction with upstream MAP3Ks. We demonstrate that although an 80% reduction in Tat-induced HIV-1 LTR activity can be detected if the kappaB binding sites are mutated, surprisingly endogenous Tat (expressed intracellularly by transfection) lacks direct effect on IkappaB degradation. Further analysis demonstrates that although Tat alone lacks direct effect on IkappaBalpha degradation or dissociation from NF-kappaB, Tat can substantially enhance the capacity of NF-kappaB-inducing kinase (NIK), but not MEKK1, to accelerate degradation of IkappaB. We propose a model to explain these collective experimental findings.

The HIV-1 transactivator protein Tat is a potent inducer of the human DNA repair enzyme beta-polymerase

OBJECTIVE

This study examines the effects of the HIV-1 regulatory proteins, Tat and Rev, on the expression of the DNA polymerase beta (beta-pol) gene, which encodes a key protein in the DNA base-excision repair pathway. The rationale for these experiments is to examine the potential involvement of base-excision repair protein deregulation in HIV-1-related lymphomas.

DESIGN

Expression of beta-pol mRNA was examined in AIDS-related lymphomas and non-AIDS-related lymphomas and as a function of HIV-1 infection of B cells in culture. The effect of Tat or Rev over-expression on beta-pol promoter expression was tested by transient co-transfection assays with a beta-pol promoter reporter plasmid and a Tat or Rev over-expression plasmid.

METHODS

Northern blot analysis was used to quantitate beta-pol expression in lymphoma and cells. Raji cells were co-transfected with a chloramphenicol acetyltransferase (CAT) reporter plasmid and a plasmid over-expressing Tat or Rev. CAT activity was measured in transfected cells.

RESULTS

beta-Pol mRNA was > 10-fold higher in AIDS-related than in non-AIDS B-lineage lymphomas. beta-Pol expression was up-regulated in a B-cell line upon infection with HIV-1, and increased in Raji cells upon recombinant expression of the Tat gene. The beta-pol promoter was transactivated (fourfold induction) by Tat, but not by Rev. Tat-dependent transactivation required a binding site for the transcription factor Sp1 in the beta-pol promoter.

CONCLUSION

These results suggest that HIV-1 Tat can interact with cellular transcription factors to increase the steady-state level of beta-pol in B cells. Tat-mediated induction of beta-pol may alter DNA stability in AIDS-related lymphomas.

HIV-1 Tat protects CD4+ Jurkat T lymphoblastoid cells from apoptosis mediated by TNF-related apoptosis-inducing ligand

We have here investigated the effect of TNF-related apoptosis-inducing ligand (TRAIL), a new member of the TNF cytokine superfamily, on the survival of Jurkat lymphoblastoid cell lines stably transfected with plasmids expressing the wild-type or mutated (Cys22) human immunodeficiency virus type 1 (HIV-1) tat gene. Jurkat cells transfected with wild-type tat were resistant to TRAIL-mediated apoptosis, while Jurkat cells mock-transfected with the control plasmid or with a mutated nonfunctional tat cDNA were highly susceptible to TRAIL-mediated apoptosis. Also, pretreatment with low concentrations (10-100 ng/ml) of extracellular synthetic Tat protein partially protected Jurkat cells from TRAIL-mediated apoptosis. Taken together, these results demonstrated that endogenously expressed tat and, to a lesser extent, extracellular Tat block TRAIL-mediated apoptosis. Since it has been shown that primary lymphoid T cells purified from HIV-1-infected individuals are more susceptible than those purified from normal individuals to TRAIL-mediated apoptosis, our findings underscore a potentially important role of Tat in protecting HIV-1-infected cells from TRAIL-mediated apoptosis.

Molecular interactions involved in the transactivation of the human T-cell leukemia virus type 1 promoter mediated by Tax and CREB-2 (ATF-4)

The human T-cell leukemia virus type 1 (HTLV-1) Tax protein activates viral transcription through three 21-bp repeats located in the U3 region of the HTLV-1 long terminal repeat and called Tax-responsive elements (TxREs). Each TxRE contains nucleotide sequences corresponding to imperfect cyclic AMP response elements (CRE). In this study, we demonstrate that the bZIP transcriptional factor CREB-2 is able to bind in vitro to the TxREs and that CREB-2 binding to each of the 21-bp motifs is enhanced by Tax. We also demonstrate that Tax can weakly interact with CREB-2 bound to a cellular palindromic CRE motif such as that found in the somatostatin promoter. Mutagenesis of Tax and CREB-2 demonstrates that both N- and C-terminal domains of Tax and the C-terminal region of CREB-2 are required for direct interaction between the two proteins. In addition, the Tax mutant M47, defective for HTLV-1 activation, is unable to form in vitro a ternary complex with CREB-2 and TxRE. In agreement with recent results suggesting that Tax can recruit the coactivator CREB-binding protein (CBP) on the HTLV-1 promoter, we provide evidence that Tax, CREB-2, and CBP are capable of cooperating to stimulate viral transcription. Taken together, our data highlight the major role played by CREB-2 in Tax-mediated transactivation.

Molecular cloning of a novel human I-mfa domain-containing protein that differently regulates human T-cell leukemia virus type I and HIV-1 expression

Regulation of viral genome expression is the result of complex cooperation between viral proteins and host cell factors. We report here the characterization of a novel cellular factor sharing homology with the specific cysteine-rich C-terminal domain of the basic helix-loop-helix repressor protein I-mfa. The synthesis of this new factor, called HIC for Human I-mfa domain-Containing protein, is controlled at the translational level by two different codons, an ATG and an upstream non-ATG translational initiator, allowing the production of two protein isoforms, p32 and p40, respectively. We show that the HIC protein isoforms present different subcellular localizations, p32 being mainly distributed throughout the cytoplasm, whereas p40 is targeted to the nucleolus. Moreover, in trying to understand the function of HIC, we have found that both isoforms stimulate in T-cells the expression of a luciferase reporter gene driven by the human T-cell leukemia virus type I-long terminal repeat in the presence of the viral transactivator Tax. We demonstrate by mutagenesis that the I-mfa-like domain of HIC is involved in this regulation. Finally, we also show that HIC is able to down-regulate the luciferase expression from the human immunodeficiency virus type 1-long terminal repeat induced by the viral transactivator Tat. From these results, we propose that HIC and I-mfa represent two members of a new family of proteins regulating gene expression and characterized by a particular cysteine-rich C-terminal domain.

The human immunodeficiency virus type 1 Tat protein up-regulates the promoter activity of the beta-chemokine monocyte chemoattractant protein 1 in the human astrocytoma cell line U-87 MG: role of SP-1, AP-1, and NF-kappaB consensus sites

It has been shown that the human immunodeficiency virus type 1 (HIV-1) Tat protein can specifically enhance expression and release of monocyte chemoattractant protein 1 (MCP-1) from human astrocytes. In this study, we show evidence that Tat-induced MCP-1 expression is mediated at the transcriptional level. Transient transfection of an expression construct encoding the full-length Tat into the human glioblastoma-astrocytoma cell line U-87 MG enhances reporter gene activity from cotransfected deletion constructs of the MCP-1 promoter. HIV-1 Tat exerts its effect through a minimal construct containing 213 nucleotides upstream of the translational start site. Site-directed mutagenesis studies indicate that an SP1 site (located between nucleotides -123 and -115) is critical for both constitutive and Tat-enhanced expression of the human MCP-1 promoter, as mutation of this SP1 site significantly diminished reporter gene expression in both instances. Gel retardation experiments further demonstrate that Tat strongly enhances the binding of SP1 protein to its DNA element on the MCP-1 promoter. Moreover, we also observe an increase in the binding activities of transcriptional factors AP1 and NF-kappaB to the MCP-1 promoter following Tat treatment. Mutagenesis studies show that an upstream AP1 site and an adjacent NF-kappaB site (located at -128 to -122 and -150 to -137, respectively) play a role in Tat-mediated transactivation. In contrast, a further upstream AP1 site (-156 to -150) does not appear to be crucial for promoter activity. We postulate that a Tat-mediated increase in SP1 binding activities augments the binding of AP1 and NF-kappaB, leading to synergistic activation of the MCP-1 promoter.

Green fluorescence protein as a transcriptional reporter for the long terminal repeats of the human immunodeficiency virus type 1

Using the enhanced green fluorescence protein (EGFP), a transient reporter expression system was established to assess the transcriptional activity of the long terminal repeats (LTR) of primary isolates of the human immunodeficiency virus type 1 (HIV-1). Consistent with the conventional chloramphenicol acetyl transferase (CAT) reporter, EGFP expression, under the direction of HIV-1 LTR, was readily detected in the transient transfection and was elevated by co-transfection of HIV-1 tat-expression vector. Comparing to CAT, however, EGFP expression system has two advantages: (i) Using a fluorescence activated cell sorter (FACS), it was possible to simultaneously measure transfection efficiency and fluorescence intensity of the transfected live cells without the necessity of co-transfection of a reference plasmid for comparing the transcriptional activity of two promoters; and (ii) EGFP expression was readily detected at a DNA concentration where CAT activity was not detectable possibly because the transfectants could be 'gated'. On the other hand, at a higher concentration of DNA, CAT signal became more prominent than that of EGFP, possibly because the enzymatic activity of CAT 'amplified' the signal. EGFP fluorescence detected by FACS was a direct measurement of the expressed chromophore. It is concluded that the system is rapid, reproducible, convenient and useful for quantitative analysis of transcription.

Tackling Tat

Activation of cellular genes typically involves control of transcription initiation by DNA-binding regulatory proteins. The human immunodeficiency virus transactivator protein, Tat, provides the first example of the regulation of viral gene expression through control of elongation by RNA polymerase II. In the absence of Tat, initiation from the long terminal repeat is efficient, but transcription is impaired because the promoter engages poorly processive polymerases that disengage from the DNA template prematurely. Activation of transcriptional elongation occurs following the recruitment of Tat to the transcription machinery via a specific interaction with an RNA regulatory element called TAR, a 59-residue RNA leader sequence that folds into a specific stem-loop structure. After binding to TAR RNA, Tat stimulates a specific protein kinase called TAK (Tat-associated kinase). This results in hyperphosphorylation of the large subunit of the RNA polymerase II carboxyl- terminal domain. The kinase subunit of TAK, CDK9, is analogous to a component of a positive acting elongation factor isolated from Drosophila called pTEFb. Direct evidence for the role of TAK in transcriptional regulation of the HIV long terminal repeat comes from experiments using inactive mutants of the CDK9 kinase expressed in trans to inhibit transcription. A critical role for TAK in HIV transcription is also demonstrated by selective inhibition of Tat activity by low molecular mass kinase inhibitors. A second link between TAK and transactivation is the observation that the cyclin component of TAK, cyclin T1, also participates in TAR RNA recognition. It has been known for several years that mutations in the apical loop region of TAR RNA abolish Tat activity, yet this region of TAR is not required for binding by recombinant Tat protein in vitro, suggesting that the loop region acts as a binding site for essential cellular co-factors. Tat is able to form a ternary complex with TAR RNA and cyclin T1 only when a functional loop sequence is present on TAR.

Transiently transfected and stably integrated HIV-1 LTR responds differentially to the silencing activity of the Krüppel-associated box (KRAB) transcriptional repressor domain

It has been demonstrated previously that the transcriptional repressor domain called the Krüppel-associated box (KRAB), conserved in a large number of Krüppel-type zinc finger proteins, fused to Tat transdominant negative mutants, is able to silence HIV-1 long terminal repeat (LTR)-driven gene expression in transient transfection assays. In the present study chimeric Tat mutant-KRAB retroviral expression vectors were used to control HIV-1 replication in acutely infected cells. It was found that while transient and stable expression of Tat mutant-KRAB chimeric proteins represses HIV-1 LTR-driven gene transcription in transient assays, stable expression of Tat mutant-KRAB chimeric molecules does not confer resistance to HIV-1 infection in Jurkat T lymphocytic cell lines. The results provide further evidence that transient transfection may underestimate the role of chromosomal structure in transcriptional regulation and highlight the caveat of direct extrapolation of transient results for designing gene therapy strategies for efficient control of HIV-1 infection.

Induction of the Tat-binding protein 1 gene accompanies the disabling of oncogenic erbB receptor tyrosine kinases

Conversion of a malignant phenotype into a more normal one can be accomplished either by down-regulation of erbB family surface receptors or by creating inactive erbB heterodimers on the cell surface. In this report, we report the identification and cloning of differentially expressed genes from antibody-treated vs. untreated fibroblasts transformed by oncogenic p185(neu). We repeatedly isolated a 325-bp cDNA fragment that, as determined by Northern analysis, was expressed at higher levels in anti-p185(neu)-treated tumor cells but not in cells expressing internalization defective p185(neu) receptors. This cDNA fragment was identical in amino acid sequence to the recently cloned mouse Tat binding protein-1 (mTBP1), which has 98.4% homology to the HIV tat-binding protein-1 (TBP1). TBP1 mRNA levels were found to be elevated on inhibition of the oncogenic phenotype of transformed cells expressing erbB family receptors. TBP1 overexpression diminished cell proliferation, reduced the ability of the parental cells to form colonies in vitro, and almost completely inhibited transforming efficiency in athymic mice when stably expressed in human tumor cells containing erbB family receptors. Collectively, these results suggest that the attenuation of erbB receptor signaling seems to be associated with activation/induction or recovery of a functional tumor suppressor-like gene, TBP1. Disabling erbB tyrosine kinases by antibodies or by trans-inhibition represents an initial step in triggering a TBP1 pathway.

Human immunodeficiency virus type 1 Tat-dependent activation of an arrested RNA polymerase II elongation complex

The human immunodeficiency virus type 1 (HIV-1) Tat protein is a transcriptional activator that is essential for efficient viral gene expression and replication. Tat increases the level of full-length transcripts from the HIV-1 promoter by dramatically enhancing the elongation efficiency of the RNA polymerase II complexes assembled on this promoter. Tat could potentially activate the transcription machinery during initiation, elongation, or both. We used an immobilized HIV-1 promoter template with a reversible lac repressor (LacR) elongation block inserted downstream to dissect the stages in transcription affected by Tat. Transcription complexes assembled in the absence of Tat and blocked by LacR cannot be activated by incubation with Tat alone. These complexes can, however, be activated if Tat is added in combination with cellular factors. In this system, Tat also promoted the assembly of preinitiation complexes capable of elongating efficiently, suggesting that Tat can associate with transcription complex at an early stage. These data indicate that Tat can activate elongation of RNA polymerase by modifying an already elongating transcription complex. The data also suggest the possibility that Tat can interact with initiation complexes.

Interactions of HIV-1 TAR RNA with Tat-derived peptides discriminated by on-line acoustic wave detector

The human immunodeficiency virus type I is strongly regulated at the transcriptional level through the interaction of an 86-amino acid protein (Tat) with a viral messenger RNA transcript. Accordingly, the binding of this protein and other cellular factors to the RNA has constituted a significant target for the development of anti-HIV drugs. In the present work, we describe the detection of the binding of two Tat-derived peptides, of 12 and 40 amino acids in length, with chemically synthesized RNA by an acoustic wave sensor. Immobilization of the nucleic acid to the sensor surface, which was incorporated in an on-line system, was effected using the biotin-neutravidin interaction. As expected, the changes in series resonance frequency and motional resistance for the two peptides indicate reversible interactions in both cases that can be further characterized by the calculation of kinetic off-rates. Of particular interest is the nature of the two frequency-based signals, which are in opposite directions for the two peptides. These results together with those obtained for the surface interactions of neutravidin and biotinylated RNA confirm that the thickness shear mode sensor, mass-response model involving the well-known Sauerbrey expression is invalid when applied to operation in liquids.

Transactivation of a ribosomal gene by simian virus 40 large-T antigen requires at least three activities of the protein

Simian virus 40 large-T antigen transactivates the ribosomal genes which are transcribed by RNA polymerase (pol I), as well as genes that are dependent on either pol II or pol III. This report identifies regions and activities of T antigen that are required to transactivate a pol I-dependent rat ribosomal gene promoter. By using the rat ribosomal gene (rDNA) promoter linked to a chloramphenicol acetyltransferase gene, we show that at least three separable T-antigen regions are necessary to achieve wild-type levels of transactivation of rDNA in transiently transfected monkey cells. One activity depends on the region of T antigen shared with small-t antigen (T/t common region). A second activity maps to amino acids 109 to 626 and is highly sensitive to mutational inactivation. Complementation analyses suggest that at least one activity in this region is independent of and must be in cis with the activity within the T/t common region. In addition, a functional nuclear localization signal is required for maximal T-antigen-mediated transactivation of rat rDNA. The three activities work in concert to override cellular species-specific controls and transactivate the rat ribosomal gene promoter. Finally, we provide evidence that although the tumor suppressor protein Rb has been shown to repress a pol I-dependent promoter, transactivation of the rat rDNA promoter does not depend on T antigen's ability to bind the tumor suppressor product Rb.

Antitumor effect of diphtheria toxin A-chain gene-containing cationic liposomes conjugated with monoclonal antibody directed to tumor-associated antigen of bovine leukemia cells

Monoclonal antibody c143 against tumor-associated antigen (TAA) expressed on bovine leukemia cells was conjugated to cationic liposomes carrying a plasmid pLTR-DT which contained a gene for diphtheria toxin A-chain (DT-A) under the control of the long terminal repeat (LTR) of bovine leukemia virus (BLV) in the multicloning site of pUC-18. The specificity and antitumor effects of the conjugates were examined in vitro and in vivo using TAA-positive bovine B-cell lymphoma line as the target tumor. In vitro studies with the TAA-positive cell line indicated that luciferase gene-containing cationic liposomes associated with the c143 anti-TAA monoclonal antibody caused about 2-fold increase in luciferase activity compared with cationic liposomes having no antibody, and also that the c143-conjugated cationic liposomes containing pLTR-DT exerted selective growth-inhibitory effects on the TAA-positive B-cell line. Three injections of pLTR-DT-containing cationic liposomes coupled with c143 into tumor-bearing nude mice resulted in significant inhibition of the tumor growth. The antitumor potency of the c143-conjugated cationic liposomes containing pLTR-DT was far greater than that of normal mouse IgG-coupled cationic liposomes containing pLTR-DT as assessed in terms of tumor size. These results suggest that cationic liposomes bearing c143 are an efficient transfection reagent for BLV-infected B-cells lymphoma cells, and that the delivery of the pLTR-DT gene into BLV-infected B-cells by the use of such liposomes may become a useful technique for gene therapy of bovine leukosis.

Naturally occurring human immunodeficiency virus type 1 long terminal repeats have a frequently observed duplication that binds RBF-2 and represses transcription

Approximately 38% of human immunodeficiency virus type 1 (HIV-1)-infected patients within the Vancouver Lymphadenopathy-AIDS Study have proviruses bearing partial 15- to 34-nucleotide duplications upstream of the NF-kappaB binding sites within the 5' long terminal repeat (LTR). This most frequent naturally occurring length polymorphism (MFNLP) of the HIV-1 5' LTR encompasses potential binding sites for several candidate transcription factors, including TCF-1alpha/hLEF, c-Ets, AP-4, and Ras-responsive binding factor 2 (RBF-2) (M. C. Estable et al., J. Virol. 70:4053-4062, 1996). RBF-2 and an apparently related factor, RBF-1, bind to at least four cis elements within the LTR which are required for full transcriptional responsiveness to protein-tyrosine kinases and v-Ras (B. Bell and I. Sadowski, Oncogene 13:2687-2697, 1996). Here we demonstrate that representative MFNLPs from two patients specifically bind RBF-2. In both cases, deletion of the MFNLP caused elevated LTR-directed transcription in cells expressing RBF-2 but not in cells with undetectable RBF-2. RBF-1, but not RBF-2, appears to contain the Ets transcription factor family member GABPalpha/GABPbeta1. Taken together with the fact that every MFNLP from a comparative study of over 500 LTR sequences from 42 patients contains a predicted binding site for RBF-2, our data suggest that the MFNLP is selected in vivo because it provides a duplicated RBF-2 cis element, which may limit transcription in monocytes and activated T cells.

A novel complex of proteins binds the HIV-1 promoter upon virus interaction with CD4

HIV-1Lai13EM is a mutant isolate which is less sensitive than the parental HIV-1Lai strain to an in vitro treatment with 13B8-2 anti-CD4 monoclonal antibody (mAb) that generally inhibits transcription of HIV-1 and HIV-2. In contrast to other clade B viruses, this isolate carries a point mutation G > A at position -188 of the viral promoter. The fact that HIV-1NDK, a clade D virus insensitive to 13B8-2 mAb, also carries an A nucleotide at this position has brought our attention to the sequence surrounding position -188. Here we analyzed whether a DNA-binding molecule interacts with this region. Electrophoretic mobility shift assays performed with the -201/-175 HIV-1Lai wild-type sequence or the sequence containing a point mutation G > A at position -188 demonstrated their ability to bind a heterotrimeric complex induced in CEM cells by stimulation with heat-inactivated HIV-1.

The basic domain in HIV-1 Tat protein as a target for polysulfonated heparin-mimicking extracellular Tat antagonists

Heparin binds extracellular HIV-1 Tat protein and modulates its HIV long terminal repeat (LTR)-transactivating activity (M. Rusnati, D. Coltrini, P. Oreste, G. Zoppetti, A. Albini, D. Noonan, F. d'Adda di Fagagna, M. Giacca, and M. Presta (1997) J. Biol. Chem. 272, 11313-11320). On this basis, the glutathione S-transferase (GST)-TatR49/52/53/55/56/57A mutant, in which six arginine residues within the basic domain of Tat were mutagenized to alanine residues, was compared with GST-Tat for its capacity to bind immobilized heparin. Dissociation of the GST-TatR49/52/53/55/56/57A.heparin complex occurred at ionic strength significantly lower than that required to dissociate the GST-Tat.heparin complex. Accordingly, heparin binds immobilized GST-Tat and GST-TatR49/52/53/55/56/57A with a dissociation constant equal to 0.3 and 1.0 microM, respectively. Also, the synthetic basic domain Tat-(41-60) competes with GST-Tat for heparin binding. Suramin inhibits [3H]heparin/Tat interaction, 125I-GST-Tat internalization, and the LTR-transactivating activity of extracellular Tat in HL3T1 cells and prevents 125I-GST-Tat binding and cell proliferation in Tat-overexpressing T53 cells. The suramin derivative 14C-PNU 145156E binds immobilized GST-Tat with a dissociation constant 5 times higher than heparin and is unable to bind GST-TatR49/52/53/55/56/57A. Although heparin was an antagonist more potent than suramin, modifications of the backbone structure in selected suramin derivatives originated Tat antagonists whose potency was close to that shown by heparin. In conclusion, suramin derivatives bind the basic domain of Tat, prevent Tat/heparin and Tat/cell surface interactions, and inhibit the biological activity of extracellular Tat. Our data demonstrate that tailored polysulfonated compounds represent potent extracellular Tat inhibitors of possible therapeutic value.

PITALRE, the catalytic subunit of TAK, is required for human immunodeficiency virus Tat transactivation in vivo

The human cdc2-related kinase PITALRE is the catalytic component of TAK, the Tat-associated kinase. Previously, we have proposed that TAK is a cellular factor that mediates Tat transactivation function. Here we demonstrate that transient overexpression of PITALRE specifically squelches Tat-1 activation of both a transfected and an integrated human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR), suggesting that PITALRE mediates Tat function as a multiprotein complex. A catalytic mutant of PITALRE, D167N, was found to be more efficient than wild-type PITALRE in squelching Tat transactivation. Neither wild-type PITALRE nor D167N was able to squelch transactivation of the human T-cell leukemia type 1 LTR by the Tax protein. Additionally, we show that artificial targeting of PITALRE to a nascent RNA element, in the absence of Tat, activated HIV-1 LTR expression. These results indicate that a PITALRE-containing complex mediates transactivation by Tat and suggest that Tat proteins function by localizing such a PITALRE-containing complex to the site of the transcribing provirus.

Glutathione depletion associated with the HIV-1 TAT protein mediates the extracellular appearance of acidic fibroblast growth factor

Primary murine embryonic fibroblasts transfected with HIV-1 TAT demonstrated decreased levels of high energy phosphates (ATP, GTP, UTP/CTP), adenine nucleotides (ATP, ADP, AMP), and both NAD+/NADH redox pairs, resulting in a substantial loss of redox poise. A greater than 50% decrease in intracellular reduced glutathione (GSH) concentration was accompanied by the extracellular appearance of acidic fibroblast growth factor (FGF-1). Addition of either N-acetyl-L-cysteine or glutathione ester (GSE), but not L-2-oxothiazolidine 4-carboxylate, partially restored intracellular GSH levels and resulted in loss of extracellular FGF-1. Treatment of FGF-1-transduced cells with buthionine sulfoximine (BSO) resulted in a time- and dose-dependent decrease in total cellular GSH concentration that was accompanied by the extracellular appearance of FGF-1. Inclusion of GSE during BSO treatment eliminated the extracellular appearance of FGF-1. BSO treatment of cells transfected with a mutant form of FGF-1, in which all three cysteine residues were replaced with serines, also decreased total cellular GSH concentration but failed to induce the extracellular appearance of FGF-1. Collectively, these results suggest that HIV-1 TAT induces a condition of oxidative stress, which mediates cellular secretion of FGF-1, an observation relevant to the pathophysiologic development and progression of AIDS-associated Kaposi's sarcoma.

Genomic organization and functional characterization of the chemokine receptor CXCR4, a major entry co-receptor for human immunodeficiency virus type 1

CXCR4 is both a chemokine receptor and entry co-receptor for T-cell line-adapted human immunodeficiency virus type 1. The genomic organization and promoter function for the entire transcription unit of CXCR4 were determined. The gene contains 2 exons of 103 and 1563 base pairs (bp) interrupted by a 2132-bp intron precisely between codons 5 and 6 of the coding sequences. A transcription start site was identified 88 bp upstream of the initiation codon, and a polyadenylate addition site was identified 22 bp 3' to a polyadenylation signal. Transient expression assays defined a minimal promoter at positions -114 to +43 relative to the transcription start site. This region contains a TATA box, a nuclear respiratory factor-1 (NRF-1) site, and two GC boxes. Specific factor binding to the NRF-1 site and GC boxes were demonstrated by gel mobility shifts and DNase I footprinting. Site-directed mutagenesis showed that the NRF-1 site is crucial for promoter activity providing the first evidence for the regulation of a signal transduction gene by NRF-1. Sequences between -691 and -191 repress CXCR4 promoter activity. Further study of these regulatory elements will be important to understanding how CXCR4 functions as both a chemokine receptor and human immunodeficiency virus type 1 entry co-receptor.

HIV Tat protein requirements for transactivation and repression of transcription are separable

The HIV Tat protein, primarily characterized as a transcriptional activator of the viral long terminal repeat (LTR), is also a potent repressor of major histocompatibility complex (MHC) class I transcription. In the present study, we demonstrate that these two functional activities are distinct and mediated by discrete, but overlapping, structural domains of Tat. Tat repressor activity depends on C-terminal sequences, whereas transactivation depends on N-terminal sequences; both functions require core sequences. The repressor activity requires a domain encompassing the region encoded by the second exon of the Tat gene, beginning at amino acid 73, with a C-terminal limit between amino acids 80 and 83. Tat repressor function also depends on the presence of a lysine at position 41, located within the core of the protein. Tat repressor activity is independent of two N-terminal domains essential for transactivation: the acidic segment and the cysteine-rich region. Conversely, Tat transactivation is independent of the second exon-encoded region of Tat. As further support for this novel model of separable Tat functions, we show that in murine fibroblasts, Tat represses class I promoter activity, but does not transactivate the HIV LTR. We propose that distinct structural domains mediate the two functionally distinct activities associated with the Tat protein.

Increasing the ratio of PP2A core enzyme to holoenzyme inhibits Tat-stimulated HIV-1 transcription and virus production

We demonstrated previously that PP2A exists in many cell types as two abundant forms: (1) holoenzyme composed of two regulatory subunits, A and B, and a catalytic subunit C; and (2) core enzyme consisting of the A and C subunits. These two forms have different substrate specificities. Since published data suggested that HIV-1 transcription may be regulated by a cellular protein phosphatase, it was of interest to determine whether changing the ratio between PP2A core and holoenzyme affects HIV-1 gene expression. This question was addressed by expression in COS cells of an N-terminal mutant of the A subunit, A delta 5, which binds the C but not the B subunit. This resulted in an increase in the amount of core enzyme and a decrease in the amount of holoenzyme concomitant with the expected change in phosphatase activity. Tat-stimulated transcription from the HIV-1 LTR was inhibited 5-fold by mutant A delta 5, whereas mRNA synthesis directed by the actin promoter was not affected. Furthermore, virus production in COS, HeLa, and Jurkat T cells was inhibited 45-, 5-, and 3-fold, respectively, by mutant A delta 5. These results demonstrate that the balance between PP2A holoenzyme and core enzyme is important for HIV-1 gene expression and virus production.

HIV-1 Tat protein exits from cells via a leaderless secretory pathway and binds to extracellular matrix-associated heparan sulfate proteoglycans through its basic region

OBJECTIVE

To analyze the mechanisms of release and the extracellular fate of the HIV-1 Tat protein and to determine the Tat domain binding to the extracellular matrix.

DESIGN AND METHODS

Release of Tat was studied by pulse-chase experiments with Tat-transfected COS-1 cells in the presence or absence of different serum concentrations, temperatures and drugs inhibiting the classical secretion pathway or endo-exocytosis, such as brefeldin A and methylamine. The binding of extracellular Tat to heparan sulfate proteoglycans (HSPG) was determined by using trypsin, heparin or heparinase in pulse-chase experiments, by gel shift and competition assays with radiolabeled heparin, and by heparin-affinity chromatography. The mapping of the Tat binding site to heparin was defined by functional assays of rescue of Tat-defective HIV-1 proviruses.

RESULTS

Tat is released in the absence of cell death or permeability changes. Tat release is dependent upon the temperature and serum concentration, and it is not blocked by brefeldin A or methylamine. After release, a portion of the protein remains in a soluble form whereas the other binds to extracellular matrix (ECM)-associated HSPG. The HSPG-bound Tat can be retrieved into a soluble form by heparin, heparinase or trypsin. Binding to heparin is competed out by heparin-binding factors such as basic fibroblast growth factor (bFGF), and it is mediated by the Tat basic region which forms a specific complex with heparin which blocks HIV-1 rescue by exogenous Tat and allows purification of a highly biologically active protein.

CONCLUSIONS

These results demonstrate that Tat exits from intact cells through a leaderless secretion pathway which shares several features with that of acid FGF or bFGF. The released Tat binds to HSPG through its basic region and this determines its storage into the ECM, as occurs for bFGF.

Oligomerization of HIV-1 Rev mutants in the cytoplasm and during nuclear import

Oligomerization of Rev molecules has been shown to be required for Rev function. In addition to a Western blot assay monitoring dimer formation, a new in vivo assay analyzing formation of Rev heteromers in the cytoplasm and during nuclear import is presented here. The oligomerization assay is based upon the ability of Rev mutants with an intact nuclear localization signal (NLS) to interact specifically with mutants with a defective NLS and translocate such mutants to the nuclear compartments. Several of the mutants previously reported to be oligomerization defective were found to mediate nuclear and nucleolar localization of the NLS mutant. The Rev mutant previously named M4 was the only mutant tested that did not translocate the mutant with a defective NLS to the nucleus. Furthermore, the predominantly cytoplasmic localization of the M4 mutant suggests that oligomerization is important for effective nuclear import of Rev.

The human immunodeficiency virus type 1 Tat protein potentiates zidovudine-induced cellular toxicity in transgenic mice

3'-Azido-2',3'-dideoxythymidine (AZT, zidovudine) is the principal antiretroviral agent in the treatment of AIDS. Although beneficial, AZT remains restricted for human usage because of its severe toxic effects. We examined the AZT sensitivity in transgenic mice expressing HIV-1 one-exon-encoded 72 amino acid Tat (Tat72) and full-length 86 amino acid Tat (Tat86) proteins. Administration of AZT (1 mg/ml) in drinking water for 1 week resulted in a three- to fourfold decrease in hematopoietic progenitors from bone marrow in Tat mice compared to AZT-treated nontransgenic controls as determined by erythroid and granulocyte/macrophage colony-forming unit assays. In liver and thymus, two of the tissues examined, AZT treatment of Tat mice resulted in as much as 80-90% suppression of Mn-superoxide dismutase (Mn-SOD) activity. Other parameters associated with loss of Mn-SOD such as increase in carbonyl proteins and decrease of sulfhydryl content were also significantly enhanced by AZT in Tat mice. Our in vivo study suggests that AZT therapy is associated with oxidative damage affecting cellular functions in several tissues and that Tat is one of the contributory factors in AZT-induced toxicities. The findings of AZT-induced oxidative damage may help to improve the therapeutic index of AZT and other related drugs in AIDS patients.

Enhancement of humoral immunity to SIVenv following simultaneous inoculation of mice by three recombinant adenoviruses encoding SIVenv/poliovirus chimeras, Tat and Rev

A means of inducing gene expression by simultaneous infection with three recombinant adenoviruses (Ad) is described. The simian immunodeficiency virus (SIV) envelope-coding region was placed under the control of the human immunodeficiency virus type 1 (HIV-1) Tat and Rev proteins provided in trans by distinct Ad vectors (Ad-tat; Ad-rev). Coinfection of cells with the three recombinant adenoviruses led to induction of high levels of SIV env mRNA and protein synthesis, while inoculation of mice elicited anti-Env antibodies. Insertion of the poliovirus VP1 neutralization epitope (C3) in the V1 hypervariable region of SIV envelope not only proved to be highly immunogenic for the poliovirus epitope but also enhanced the kinetics of anti-SIV Env antibody production. By contrast, insertion in V4 elicited no anti-C3 response and only normal anti-Env responses.

HIV-1 Tat induces the expression of the interleukin-6 (IL6) gene by binding to the IL6 leader RNA and by interacting with CAAT enhancer-binding protein beta (NF-IL6) transcription factors

Human immunodeficiency virus type 1 (HIV-1) infection is associated with severe psoriasis, B cell lymphoma, and Kaposi's sarcoma. A deregulated production of interleukin-6 (IL6) has been implicated in the pathogenesis of these diseases. The molecular mechanisms underlying the abnormal IL6 secretion of HIV-1-infected cells may include transactivation of the IL6 gene by HIV-1. Here we report the molecular mechanisms of Tat activity on the expression of the IL6 gene. By using 5' deletion mutants of pIL6Pr-CAT and using IL6:HIV-1-LTR hybrid constructs where discrete regions of the IL6 promoter replaced the TAR sequence in HIV-1 LTR, we identified a short sequence of the 5'-untranslated region of the IL6 mRNA that is required for Tat to trans-activate the IL6 promoter. This sequence acquires a stem-loop structure and includes a UCU sequence that binds to Tat and is necessary for full trans-activation. In addition, we provide the evidence that Tat can function by enhancing the CAAT enhancer-binding protein (C/EBP) DNA binding activity and is able to complex with in vitro translated C/EBPbeta, which is a major mediator of IL6 promoter function. By using the yeast two-hybrid system and immunoprecipitation, we observed that the interaction of Tat with C/EBP proteins also occurred in vivo. The data are consistent with the possibility that Tat may function on heterologous genes by interacting with RNA structures possibly present in a large number of cellular and viral genes. In addition, Tat may function by protein-protein interactions, leading to the generation of heterodimers with specific transcription factors.

Upregulation of urokinase-type plasminogen activator by endogenous and exogenous HIV-1 Tat protein in tumour cell lines derived from BK virus/tat-transgenic mice

OBJECTIVE

To demonstrate that Tat modulates the plasminogen-dependent proteolytic activity of tumour cell lines derived from BK virus (BKV)/tat-transgenic mice by affecting the production of plasminogen activators (PA) and the PA inhibitor (PAI)-1 and to demonstrate that this occurs through mechanism(s) that are distinct from those responsible for transactivating activity of extracellular Tat.

DESIGN AND METHODS

To assess whether endogenous Tat is responsible for PA activity in T53 adenocarcinoma cells, cell cultures were transfected with antisense Tat cDNA and evaluated for cell-associated PA activity by a plasmin chromogenic assay. The assay was also used to evaluate PA activity in T53 cells and T111 leiomyosarcoma cells stimulated by extracellular Tat. The type(s) of PA produced were identified by sodium dodecyl sulphate-polyacrylamide gel electrophoresis zymography. The levels of PAI-1 were evaluated by Western blotting. Tat transactivating activity was measured by a chloramphenicol acetyltransferase (CAT) enzyme-linked immunosorbent assay in HL3T1 cells containing integrated copies of an HIV-1 long terminal repeat (LTR)-CAT plasmid.

RESULTS

Transfection of T53 cells with antisense Tat cDNA results in the decrease of Tat production and PA activity. Exogenously added Tat increases PA levels in T53 and in T111 cells. PA activity was identified as urokinase-type PA (uPA). Tat also increases the production of PAI-1 in T111 but not in T53 cells. Chloroquine and heparin have different affects on the LTR-CAT-transactivating and the PA-inducing activities of Tat. The fusion protein glutathione-S-transferase-Tat and the mutant Tat-1e, lacking the second Tat exon, cause LTR-CAT transactivation without stimulating uPA upregulation.

CONCLUSIONS

Tat affects the fibrinolytic activity of tumour cell lines derived from BKV/tat-transgenic mice by modulating the production of both uPA and PAI-1 via autocrine and paracrine mechanisms of action. The capacity of Tat to modulate the plasminogen-dependent proteolytic activity of these tumour cell lines may contribute to their metastatic potential. The uPA-inducing activity of Tat depends upon specific biological and structural features of the Tat protein that are distinct from those responsible for its LTR-CAT-transactivating activity, suggesting distinct mechanisms of induction for the two biological responses.

Interaction of HIV-1 Tat protein with heparin. Role of the backbone structure, sulfation, and size

Human immunodeficiency virus type 1 (HIV-1) Tat protein is released from infected cells. Extracellular Tat enters the cell where it stimulates the transcriptional activity of HIV-long terminal repeat (LTR) and of endogenous genes. Heparin modulates the angiogenic (Albini, A., Benelli, R., Presta, M., Rusnati, M., Ziche, M., Rubartelli, A., Paglialunga, G., Bussolino, F., and Noonan, D. (1996) Oncogene 12, 289-297) and transcriptional (Mann, D. A., and Frankel, A. D. (1991) EMBO J. 10, 1733-1739) activity of extracellular Tat. Here we demonstrate that heparin binds specifically to recombinant HIV-1 Tat produced as glutathione S-transferase (GST) fusion protein and immobilized on glutathione-agarose beads. Heparin and heparan sulfate (HS), but not dermatan sulfate, chondroitin sulfates A and C, hyaluronic acid, and K5 polysaccharide, competed with 3H-labeled heparin for binding to immobilized GST-Tat and inhibited HIV-LTR transactivation induced by extracellular GST-Tat. Selective 2-O-, 6-O-, total-O-desulfation, or N-desulfation/N-acetylation dramatically reduced the capacity of heparin to bind GST-Tat. Totally-O-desulfated and 2-O-desulfated heparins also showed a reduced capacity to inhibit the transactivating activity of GST-Tat. Very low molecular weight heparins showed a significant decrease in their capacity to bind GST-Tat and to inhibit its LTR transactivating activity when compared with conventional 13.6-kDa heparin. However, when 3.0-kDa heparin was affinity chromatographed on immobilized GST-Tat to isolate binding and non-binding subfractions, the Tat-bound fraction was >/=1,000 times more potent than the unbound fraction in inhibiting the transactivating activity of GST-Tat. The results demonstrate that Tat interacts in a size-dependent manner with heparin/HS and that high affinity Tat-heparin interaction requires at least some 2-O-, 6-O-, and N-positions to be sulfated. The Tat binding activity of the glycosaminoglycans tested correlates with their capacity to affect the transactivating activity of extracellular Tat, indicating the possibility to design specific heparin/HS-like structures with Tat-antagonist activity.

An RGD containing peptide from HIV-1 Tat-(65-80) modulates protooncogene expression in human bronchoalveolar carcinoma cell line, A549

Tat (transactivator of transcription) is essential for HIV-1 replication in vivo and in vitro. Tat-(65-80), an RGD containing domain, has been shown to regulate proliferative function of a variety of cell lines, including a human adenocarcinoma cell line, A549. The exact cellular and molecular mechanisms by which these effects are mediated, remain unknown. To evaluate the hypothesis that Tat-(65-80) modulates the expression of immediate early genes (IEG) c-jun, c-myc, c-fos and the tumor suppressor gene p53, serum starved A549 cells were incubated with Tat-(65-80) or heat-inactivated Tat-(65-80) at 10 ng/ml. Total cellular RNA was isolated from the cells at various time points (0-24 hours). In each case, 5 micrograms of RNA was reverse transcribed in 20 microliters of reaction volume. Equal amounts of cDNA were subjected to polymerase chain reaction (PCR) and analyzed by electrophoresis. The photographic negatives of the ethidium bromide stained gels were quantitated by densitometric scanning and normalized to corresponding beta-actin PCR products. Treatment with Tat-(65-80) showed a twofold induction of c-jun at 0.5 h. Peak expression occurred at 60 minutes and remained above baseline at 24 hours (h). c-myc was increased at 0.5 h, reached a twofold increase at 2 h and remained above baseline at 24 h. c-fos increased seven fold at 0.5 h and declined subsequently to baseline at 8 h. p-53 gene was reduced fivefold at 0.5 h and remained downregulated thereafter. These results show that Tat-(65-80) can modulate growth related genes in human lung epithelial cells.

Transduction of CD34+ hematopoietic progenitor cells with an antitat gene protects T-cell and macrophage progeny from AIDS virus infection

Transduction of hematopoietic stem cells with genes that inhibit human immunodeficiency virus (HIV) replication has the potential to reconstitute immune function in individuals with AIDS. We evaluated the ability of an autoregulated gene, antitat, to inhibit replication of simian immunodeficiency virus (SIV) and HIV type 1 (HIV-1) in hematopoietic cells derived from transduced progenitor cells. The antitat gene expresses an antiviral RNA encoding polymeric Tat activation response elements in combination with an antisense tat moiety under the control of the HIV-1 long terminal repeat. CD34+ hematopoietic progenitor cells were transduced with a retroviral vector containing the antitat gene and then cultured under conditions that support in vitro differentiation of T cells or macrophage-like cells. Rhesus macaque CD4+ T cells and macrophage-like cells derived from CD34+ bone marrow cells transduced with the antitat gene were highly resistant to challenge with SIV, reflecting a 2- to 3-log reduction in peak SIV replication compared with controls. Similarly, human CD4+ T cells derived from CD34+ cord blood cells transduced with antitat were also resistant to infection with HIV-1. No evidence for toxicity of the antitat gene was observed in any of five different lineages derived from transduced hematopoietic cells. These results demonstrate that a candidate therapeutic gene introduced into hematopoietic progenitor cells can retain the ability to inhibit AIDS virus replication following T-cell differentiation and support the potential use of the antitat gene for stem cell gene therapy.

Exon 1 leader sequences downstream of U5 are important for efficient human immunodeficiency virus type 1 gene expression

In previous studies, little attention has been paid to maintaining the native HIV-1 leader sequence in reporter constructs analyzing the human immunodeficiency virus type 1 (HIV-1) promoter activity. To investigate a possible influence of the leader sequence on HIV-1-driven gene expression in the presence as well as in the absence of Tat, an expression vector was designed for transcripts consisting of the native HIV-1 tat 1.4 mRNA leader followed by the open reading frame for the bacterial chloramphenicol acetyltransferase (CAT). Deletion mutants with mutations within the leader sequence downstream of U5 (lsdU5) were constructed, as well as a mutant containing a mutation with a reverse orientation of this region. Quantification of CAT protein in HeLa-T4+ cells transiently transfected with wild-type and mutant leader constructs showed that the exon 1-derived lsdU5 region has an influence on basal as well as Tat-induced protein expression. The dramatic decrease in the level of CAT protein upon deletion of lsdU5 was paralleled by a drop in the steady-state level of CAT mRNA. Deletion of the exon 1-derived lsdU5 region also decreased the expression of mRNAs containing authentic HIV-1 sequences instead of CAT. The effect observed with the reporter constructs was not due to the loss of binding sites for nuclear factors, as could be shown with DBF1 and Sp1 mutant constructs. Nuclear run-on transcription assays showed that the presence or absence of lsdU5 did not influence the rate of transcription. This indicates that the exon 1 lsdU5 element functions at the posttranscriptional level in the processing, nucleocytoplasmic export, or stabilization of HIV-1 transcripts.

The Tat protein of human immunodeficiency virus type 1 is a substrate and inhibitor of the interferon-induced, virally activated protein kinase, PKR

We demonstrate that the interferon-induced, double-stranded (ds) RNA-activated kinase, PKR, is able to bind to and phosphorylate the human immunodeficiency virus type 1 (HIV-1) trans-activating protein, Tat. Furthermore, Tat can inhibit the activation and activity of the kinase. Phosphorylation of Tat by PKR is dependent on the prior activation of PKR by dsRNA and occurs on serine and threonine residues adjacent to the basic region important for TAR RNA binding and Tat function. Activated PKR efficiently phosphorylates both the two-exon form of Tat (Tat-86) and the single exon form (Tat-72). Mutagenesis indicates that the interaction between PKR and Tat requires the RNA-binding region of Tat. Tat competes with eukaryotic initiation factor 2, a well-characterized substrate of PKR, for phosphorylation by activated PKR. Tat also inhibits the autophosphorylation of PKR by dsRNA. This biochemical evidence of an intimate relationship between Tat, an important regulator of HIV transcription, and PKR, a pleiotropic cellular regulator, may provide insights into HIV-1 pathogenesis and, more generally, virus/host interactions.

Molecular determinants for cellular uptake of Tat protein of human immunodeficiency virus type 1 in brain cells

We measured the cellular uptake of 125I-labeled full-length Tat (amino acids 1 to 86) (125I-Tat(1-86)) and 125I-Tat(1-72) (first exon) in human fetal astrocytes, neuroblastoma cells, and human fetal neurons and demonstrated that the uptake of 125I-Tat(1-72) without the second exon was much lower than that of 125I-Tat(1-86) (P < 0.01). This suggests an important role for the C-terminal region of Tat for its cellular uptake. 125I-Tat uptake could be inhibited by dextran sulfate and competitively inhibited by unlabeled Tat but not by overlapping 15-mer peptides, suggesting that Tat internalization is charge and conformationally dependent. Interestingly, one of 15-mer peptides, Tat(28-42), greatly enhanced 125I-Tat uptake. These findings are important for understanding the neuropathogenesis of human immunodeficiency virus type 1 infection and in the potential application of Tat for drug delivery to cells.

The presence of host-derived HLA-DR1 on human immunodeficiency virus type 1 increases viral infectivity

Human immunodeficiency virus type 1 (HIV-1) incorporates several host cell components when budding out of the infected cell. One of the most abundant host-derived molecules acquired by HIV-1 is the HLA-DR determinant of the major histocompatibility complex class II (MHC-II) molecules. The fact that CD4 is the natural ligand of MHC-II prompted us to determine if such virally embedded cellular components can affect the biology of the virus. Herein, we report for the first time that the incorporation of cellular HLA-DR1 within HIV-1 enhances its infectivity. This observation was made possible with virions bearing or not bearing on their surfaces host-derived HLA-DR1 glycoproteins. Such virus stocks were prepared by a transient-expression system based on transfection of 293T cells with a recombinant luciferase-encoding HIV-1 molecular clone along with plasmids encoding the alpha and beta chains of HLA-DR1. Cell-free virions recovered from transfected cells were shown to have efficiently incorporated host-derived HLA-DR1 glycoproteins. Infectivity was increased by a factor of 1.6 to 2.3 for virions bearing on their surfaces host-derived HLA-DR1. The observed enhancement of HIV-1 infectivity was independent of the virus stocks used and was seen in several T-lymphoid cell lines, in a premonocytoid cell line, and in primary peripheral blood mononuclear cells. Finally, we determined that the presence of virion-bound cellular HLA-DR1 is associated with faster kinetics of virus infection. Taken together, these results suggest that HLA-DR-1-bearing HIV-1 particles had a greater infectivity per picogram of viral p24 protein than HLA-DR1-free virions.

A simple method for measurement of cell-substrate attachment forces: application to HIV-1 Tat

In order to understand the importance of cell attachment to HIV-1 Tat, we quantified the strength of cell attachment to immobilized Tat in microtiter plate wells by the application of buoyant force. By replacing the attachment medium with dense medium, and subjecting the attached cells in the microtiter plates to centrifugal force in the conventional upright position, weakly binding and strongly binding cells could be discriminated (and separated) by varying the centrifugal speed. The strength of attachment of HT1080 cells to Tat was compared with that of the well-known extracellular matrix (ECM) proteins fibronectin and vitronectin. We observed that all three proteins mediated significant attachment of HT1080 cells both at 4 degrees C and 37 degrees C. However, unlike the ECM proteins, Tat was unable to engage in higher strength binding when the temperature was raised to 37 degrees C. The relatively weak binding of HT1080 cells to Tat (in the order of 3.0 mudynes/picomole of coated Tat) and lack of strengthening of binding to Tat at physiologic temperature suggests that this protein does not mimic adhesion molecule function. We anticipate that the methodology developed and described here will be useful in a wide variety of cell-matrix and cell-cell interaction studies.