Activation Of HIV-1 Transcription Research Articles

A Role of RNA Helicase A in cis-Acting Transactivation Response Element-mediated Transcriptional Regulation of Human Immunodeficiency Virus Type 1

RNA helicase A (RHA) has two double-stranded (ds) RNA-binding domains (dsRBD1 and dsRBD2). These domains are conserved with the cis-acting transactivation response element (TAR)-binding protein (TRBP) and dsRNA-activated protein kinase (PKR). TRBP and PKR are involved in the regulation of HIV-1 gene expression through their binding to TAR RNA. This study shows that RHA also plays an important role in TAR-mediated HIV-1 gene expression. Wild-type RHA preferably bound to TAR RNA in vitro and in vivo. Overexpression of wild type RHA strongly enhanced viral mRNA synthesis and virion production as well as HIV-1 long terminal repeat-directed reporter (luciferase) gene expression. Substitution of lysine for glutamate at residue 236 in dsRBD2 (RHA(K236E)) reduced its affinity for TAR RNA and impaired HIV-1 transcriptional activity. These results indicate that TAR RNA is a preferred target of RHA dsRBDs and that RHA enhances HIV-1 transcription in vivo in part through the TAR-binding of RHA.

Cutting edge: activation of HIV-1 transcription by the MHC class II transactivator.

Both macrophages and activated CD4+ T cells can be productively infected by HIV-1, and both cell types express MHC class II molecules. Expression of MHC class II proteins in these cells is regulated by a specific transcriptional coactivator, the class II transactivator (CIITA). In this study, we report for the first time that CIITA expression profoundly influences HIV-1 replication. Stable expression of CIITA in Jurkat cells markedly increased 1) HIV-1 replication as assessed by the p24 Ag production and 2) luciferase expression after transfection with full-length provirus or long terminal repeat constructs. Similarly, transient expression of CIITA increased provirus expression as well as long terminal repeat promoter activity in 293 and HeLa-T4 cells. In contrast, mutant forms of CIITA did not increase HIV-1 expression. This study shows that expression of CIITA increases HIV-1 replication through a transcriptional mechanism.

Requirement for a Kinase-specific Chaperone Pathway in the Production of a Cdk9/Cyclin T1 Heterodimer Responsible for P-TEFb-mediated Tat Stimulation of HIV-1 Transcription

Tat activation of HIV-1 transcription is mediated by human transcription elongation factor P-TEFb, which interacts with Tat and phosphorylates the C-terminal domain of RNA polymerase II. The catalytic subunit of the P-TEFb complex, Cdk9, has been shown to interact with cyclin T and several other proteins of unknown identity. Consequently, the exact subunit composition of active P-TEFb has not been determined. Here we report the affinity purification and identification of the Cdk9-associated proteins. In addition to forming a heterodimer with cyclin T1, Cdk9 interacted with the molecular chaperone Hsp70 or a kinase-specific chaperone complex, Hsp90/Cdc37, to form two separate chaperone-Cdk9 complexes. Although the Cdk9/cyclin T1 dimer was exceptionally stable and produced slowly in the cell, free and unprotected Cdk9 appeared to be degraded rapidly. Several lines of evidence indicate the heterodimer of Cdk9/cyclin T1 to be the mature, active form of P-TEFb responsible for phosphorylation of the C-terminal domain of RNA polymerase II interaction with the Tat activation domain, and mediation of Tat activation of HIV-1 transcription. Pharmacological inactivation of Hsp90/Cdc37 function by geldanamycin revealed an essential role for the chaperone-Cdk9 complexes in generation of Cdk9/cyclin T1. Our data suggest a previously unrecognized chaperone-dependent pathway involving the sequential actions of Hsp70 and Hsp90/Cdc37 in the stabilization/folding of Cdk9 as well as the assembly of an active Cdk9/cyclin T1 complex responsible for P-TEFb-mediated Tat transactivation.

Specific interaction of Tat with the human but not rodent P-TEFb complex mediates the species-specific Tat activation of HIV-1 transcription.

Tat stimulation of HIV-1 transcriptional elongation is species-specific and is believed to require a specific cellular cofactor present in many human and primate cells but not in nonpermissive rodent cells. Human P-TEFb, composed of Cdk9 and cyclin T1, is a general transcription elongation factor that phosphorylates the C-terminal domain of RNA polymerase II. Previous studies have also implicated P-TEFb as a Tat-specific cellular cofactor and, in particular, human cyclin T1 as responsible for the species-specific Tat activation. To obtain functional evidence in support of these hypotheses, we generated and examined the activities of human-rodent "hybrid" P-TEFb complexes. We found that P-TEFb complexes containing human cyclin T1 complexed with either human or rodent Cdk9 supported Tat transactivation and interacted with the Tat activation domain and the HIV-1 TAR RNA element to form TAR loop-dependent ribonucleoprotein complexes. Although a stable complex containing rodent cyclin T1 and human Cdk9 was capable of phosphorylating CTD and mediating basal HIV-1 elongation, it failed to interact with Tat and to mediate Tat transactivation, indicating that the abilities of P-TEFb to support basal elongation and Tat activation can be separated. Together, our data indicated that the specific interaction of human P-TEFb with Tat/TAR, mostly through cyclin T1, is crucial for P-TEFb to mediate a Tat-specific and species-restricted activation of HIV-1 transcription. Amino acid residues unique to human Cdk9 also contributed partially to the formation of the P-TEFb-Tat-TAR complex. Moreover, the cyclin box of cyclin T1 and its immediate flanking region are largely responsible for the specific P-TEFb-Tat interaction.

Le rôle de la structure chromatinienne dans la régulation transcriptionnelle de l'expression des gènes

Le rôle de la structure chromatinienne dans la régulation transcriptionnelle de l'expression des gènes

Interleukin-10 Enhances Tumor Necrosis Factor-α Activation of HIV-1 Transcription in Latently Infected T Cells

Interleukin-10 (IL-10) is elevated in HIV-1-infected individuals and has been implicated in disease progression. We previously reported that IL-10 cooperates with tumor necrosis factor-alpha (TNF-alpha) to activate HIV-1 expression synergistically in acutely infected monocyte-derived macrophages and the chronically infected U1 promonocytic cell line. To determine whether IL-10 also cooperates with TNF-alpha to activate latent HIV-I expression in lymphocytes, we examined the effects of IL-10 on proviral expression in the chronically infected T-cell line, ACH-2. Although IL-10 inhibited HIV-1 expression acting alone, in combination with suboptimal concentrations of TNF-alpha, IL-10 increased HIV-1 steady-state mRNA expression and p24 core antigen production in ACH-2 cells. Interestingly, IL-10 concentrations that synergistically induced virus also maximally stimulated endogenous TNF-alpha expression, suggesting that cell-derived TNF-alpha may contribute to cytokine synergy. Transfection studies in ACH-2 cells indicated that IL-10 combined with TNF-alpha to activate the HIV-1 long terminal repeat (LTR). IL-10 also cooperated with TNF-alpha to activate HIV-1 LTR in 1G5 cells, a Jurkat T-cell line stably transfected with an LTR-dependent luciferase reporter gene. Pyrrolidine dithiocarbamate, a potent transcriptional inhibitor of the viral LTR, abrogated the cytokine responses in both U1 and ACH-2 cells, suggesting a common TNF-alpha-mediated transcriptional mechanism in these cell types despite their different modes of provirus latency. Taken collectively, these data suggest that IL-10 enhances suboptimal TNF-alpha activation of HIV-1 transcription in chronically infected T-cells at least in part through induction of endogenous TNF-alpha expression.

HIV transcriptional activation by the accessory protein, VPR, is mediated by the p300 co-activator.

The accessory protein, Vpr, is a virion-associated protein that is required for HIV-1 replication in macrophages and regulates viral gene expression in T cells. Vpr causes arrest of cell cycle progression at G2/M, presumably through its effect on cyclin B1.Cdc2 activity. Here, we show that the ability of Vpr to activate HIV transcription correlates with its ability to induce G2/M growth arrest, and this effect is mediated by the p300 transcriptional co-activator, which promotes cooperative interactions between the Rel A subunit of NF-kappaB and cyclin B1.Cdc2. Vpr cooperates with p300, which regulates NF-kappaB and the basal transcriptional machinery, to increase HIV gene expression. Similar effects are seen in the absence of Vpr with a kinase-deficient Cdc2, and overexpression of p300 increases levels of HIV Vpr+ replication. Taken together, these data suggest that p300, through its interactions with NF-kappaB, basal transcriptional components, and Cdks, is modulated by Vpr and regulates HIV replication. The regulation of p300 by Vpr provides a mechanism to enhance viral replication in proliferating cells after growth arrest by increasing viral transcription.

Transcriptional activation and chromatin remodeling of the HIV-1 promoter in response to histone acetylation.

After integration in the host cell genome, the HIV-1 provirus is packaged into chromatin. A specific chromatin disruption occurs in the HIV-1 promoter during transcriptional activation in response to TNF-alpha, suggesting that chromatin plays a repressive role in HIV-1 transcription and that chromatin modification(s) might result in transcriptional activation. We have treated several cell lines latently infected with HIV-1 with two new specific inhibitors of histone deacetylase, trapoxin (TPX) and trichostatin A (TSA), to cause a global hyperacetylation of cellular histones. Treatment with both drugs results in the transcriptional activation of the HIV-1 promoter and in a marked increase in virus production. Dose-response curves and kinetic analysis show a close correlation between the level of histone acetylation and HIV-1 gene expression. In contrast, both TPX and TSA have little or no effect on HIV-1 promoter activity following transient transfection of an HIV-1 promoter-reporter plasmid. Activation of HIV-1 transcription by TSA and TPX treatment occurs in the absence of NF-kappa B induction. Chromatin analysis of the HIV-1 genome shows that a single nucleosome (nuc-1) located at the transcription start and known to be disrupted following TNF-alpha treatment, is also disrupted following TPX or TSA treatment. This disruption is independent of transcription as it is resistant to alpha-amanitin. These observations further support the crucial role played by nuc-1 in the suppression of HIV-1 transcription during latency and demonstrate that transcriptional activation of HIV-1 can proceed through a chromatin modification.

Quantitative dynamics of HIV type 1 expression.

A competitive PCR and RT-PCR procedure was developed for the quantification of HIV-1 nucleic acids in infected biological samples, with particular reference to the study of the kinetics of production of differently processed viral transcripts. The procedure entails the utilization of a competitor plasmid DNA (on DNA samples) or of an in vitro transcription product obtained from this plasmid (on RNA samples) and allows the quantification of proviral DNA, viral genomic RNA, and viral single- and multispliced mRNAs. Furthermore, it permits the direct standardization of these measurements to the amount of a reference cellular gene (for DNA quantification) or of a reference cellular transcript (for RNA quantification). This quantification procedure was used to monitor the dynamics of HIV-1 transcriptional activation in the latently infected U1 monocytic cell line after stimulation with phorbol-12-myristate-13-acetate, and in experimentally infected peripheral blood lymphocytes. Despite the biological differences between the two experimental systems, in both cases production of infectious virus is accompanied by a remarkable increase in the levels of unspliced viral mRNAs (rising up to 20,000 fold in U1 cells) and by a consequent switch in the abundance of the differently spliced transcript classes. These observations reinforce the notion that the control of infection is subjected also to posttranscriptional events and prompts for quantitative evaluation of HIV-1 transcript class abundance in infected individuals to define potential markers for disease progression.

Compounds That Target Novel Cellular Components Involved in HIV-1 Transcription

Therapeutic intervention designed to block expression of human immunodeficiency virus (HIV) at a cellular level may slow the clinical progression of HIV-1 disease. Cellular models of latent (OM-10.1 and U1) and chronic (8E5) HIV infection were used to evaluate two benzothiophene derivatives, PD 121871 and PD 144795, for an ability to inhibit HIV activation and expression. The benzothiophene derivatives were effective at micromolar concentrations in preventing tumor necrosis factor alpha (TNF alpha)-induced HIV-1 expression in OM 10.1 and U1 cultures. These compounds inhibited the activation of HIV-1 transcription; however, this inhibition was selective in that another TNF alpha-induced response, the transcription of autocrine TNF alpha, was unaffected. Constitutive HIV-1 expression by chronically infected 8E5 cells was also significantly reduced when treated with these experimental compounds. In TNF alpha-treated OM-10.1 cultures, the inhibition of HIV-1 transcription by these compounds was not due to a block of nuclear factor-kappa B induction. The benzothiophene derivatives also inhibited HIV-1 activation by phorbol ester treatment of OM-10.1 promyelocytes, although no inhibition of cellular differentiation toward a macrophage-like phenotype was observed. Furthermore, these experimental compounds induced a state of HIV-1 latency in cytokine-activated OM-10.1 cultures even when maintained under constant TNF alpha stimulation. The benzothiophene derivatives did not inhibit the activity of the HIV-1 trans-activator, Tat, when evaluated in transient transfection assays. The benzothiophene derivatives appear to inhibit a critical cellular component, distinct from nuclear factor-kappa B, involved in HIV transcription and may serve to identify new therapeutic targets to restrict HIV expression.

Induction of Sp1 Phosphorylation and NF-κB-Independent HIV Promoter Domain Activity in T Lymphocytes Stimulated by Okadaic Acid

In contrast to the purely enhancer-dependent effect of cytokines such as TNF on the activity of the HIV regulatory region (LTR), we observed that okadaic acid (OKA) activates HIV transcription through both the enhancer, responding to the factor NF-κB, and the promoter domain of the LTR. The inducibility of HIV LTR-driven luciferase expression constructs in lymphoblastoid cells stimulated by OKA depended on both functional Sp1 binding elements and the ability of the TATA box to bind the protein TBP. In both transformed and normal lymphocytes, OKA stimulation induced intense phosphorylation of the constitutively expressed Sp1 protein in the nucleus, a property of OKA not shared by TNF, phorbol ester, or PHA and interleukin 2. Responsiveness of LTR constructs deleted of κB elements to HIV Tat expression was increased upon OKA but not TNF stimulation. Our results suggest that SP1 phosphorylation induced by OKA, a selective inhibitor of the serine threonine phosphatase PP2A, facilitates the formation of a transcription complex involving general transcription factors, HIV Tat, and Sp1 proteins. The formation of this complex would increase, independently of an in synergy with NF-κB, the low basal activity of the HIV LTR observed in normal T lymphocytes.

TAR-Independent Activation of HIV-1 Requires the Activation Domain but Not the RNA-Binding Domain of Tat

The Tat protein of HIV-1 is a potent activator of transcription directed by the viral long terminal repeat. In most cell types this activation requires a specific interaction between Tat and an RNA target termed TAR in the 5′-leader sequence of HIV-1 mRNAs. We have previously reported that in astrocytic cells Tat is capable of activating transcription in a TAR-independent manner through an alternative Tat-responsive element in the LTR (J. P. Taylor et al., EMBO J. 11(9), 3395-3403, 1992). In this report we demonstrate that TAR-independent activation by Tat can effectively bypass competition by decoy TAR RNA molecules. Studies with site-directed mutants demonstrate that the RNA-binding domain of Tat is dispensable for TAR-independent activation of HIV-1. In contrast, the requirement for specific components of the Tat activation domain suggests that common targets exist for this viral trans-activator to exert its activity in TAR-independent and TAR-dependent transactivation pathways of HIV-1 transcriptional activation.

Release of human immunodeficiency virus by THP-1 cells and human macrophages is regulated by cellular adherence and activation

Macrophage adherence, an important regulatory signal, has the potential to affect human immunodeficiency virus (HIV) production either directly or by priming monocytes to respond to other activating signals. We have investigated the role of adherence as an activator of HIV-1 transcription and release. The effects of adherence on HIV-1 transcription were examined by using THP-1 cells, a human monocytic cell line, transfected with HIV long terminal repeat (LTR)-chloramphenicol acetyltransferase (CAT) constructs. The effects of adherence on release of HIV-1 were investigated in both HIV-1-infected THP-1 cells and human peripheral blood monocyte-derived macrophages (MDM). Adherence of lipopolysaccharide (LPS)-stimulated THP-1 cells to either tissue culture plastic or endothelial cells was crucial for enhanced HIV-1 transcription as measured by LTR-CAT expression. Such increased LTR-CAT expression did not occur with an HIV LTR construct containing mutated NF-kappa B binding sites. In contrast, release of whole HIV, measured by reverse transcriptase (RT) activity in tissue culture medium, was reduced upon adherence of stimulated HIV-1-infected THP-1 cells without suppression of HIV LTR-CAT transcription or p24 release. This finding suggested that activation of adherent monocytic cells interfered with HIV assembly and release. Although the reduction of RT activity following activation of HIV-1-infected MDM was independent of adhesion, adherence alone of nonstimulated HIV-infected MDM to endothelial cells was sufficient to induce a reduction in RT release. This study demonstrates that LPS stimulation of monocytic cells enhances HIV LTR transcription under adherent conditions. In contrast, activation of adherent monocytic cells infected with HIV reduced viral release.

Frequency of dual infections of corneas with HIV-1 and HHV-6.

In the current study, 35 pairs of corneas from asymptomatic carriers of HIV-1 and ten pairs from AIDS patients were analyzed for the presence of HIV-1 and HHV-6. The tissues were evaluated for viral antigens, transcripts, DNA sequences and intact and infectious virus. Three corneas from two asymptomatic carriers of HIV-1 and three corneas from two AIDS patients were culture positive for HIV-1. One of the three HIV-1 positive corneas from an asymptomatic HIV-1 carrier also was culture positive for HHV-6. Two of the tissue culture positive corneas from asymptomatic HIV-1 carriers and two from AIDS patients also tested positive for HIV-1 transcriptional activity by in situ hybridization. The label denoting the transcriptional activity was limited to stromal keratocytes. Most significantly, we were able to demonstrate the presence of HIV-1 particle(s) in sections and cultured PBMC from one of the HIV-1 culture positive corneas. PBMC from the same cornea also contained herpes virus particles. This report strengthens our earlier findings that HIV-1 and HHV-6 can invade corneal tissue, which emphasizes the importance of vigorous screening of corneal donors, specifically donors with HIV-1 exposure.

Induction of NF-χB during monocyte differentiation is associated with activation of HIV-gene expression

Cells of the monocyte-macrophage lineage are important targets of HIV infection. We report here that the phenotypic differentiation of monocyte cell lines induced by phorbol esters or tumour necrosis factor α (TNFα) is associated with expression of nuclear factor kappa B (NF-χB). In parallel with such differentiation, HIV transcription, monitored using an HIV long terminal repeat reporter gene construct, is activated in such cells under the influence of enhanced NF-χB expression. Also, in a promonocyte cell line chronically infected with HIV, NF-χB expression and HIV transcription were enhanced on stimulation with phorbol ester or TNFα. Thus, stimulation of monocyte cell lines by phorbol esters or TNFα induces cell differentiation and activates HIV transcription. Such a process may have fundamental implications in AIDS pathogenesis in vivo and may be important in disease progression induced by opportunistic infections directly or indirectly involving macrophages.

RNA binding by the tat and rev proteins of HIV-1

HIV-1 tat protein binds specifically to HIV-1 TAR RNA. A Scatchard analysis of tat binding has shown that the purified protein forms a one-to-one complex with HIV-1 TAR RNA with a dissociation constant of K d = 12 nM. Tat binding in vitro is dependent upon the presence of 3 non-base paired U residues which produce a ‘bulge’ in the TAR RNA stem-loop structure. Deletion of the uridine residues in the bulge or substitution with guanine residues produced RNAs with a 6 to 8-fold lower affinity than wild-type TAR. By contrast, mutations that alter the sequence of the 6 nucleotide-long loop at the tip of TAR RNA structure, and mutations which alter the sequence of the stem whilst preserving Watson-Crick base pairing, do not affect tat binding significantly. There is a direct correlation between the ability of tat to bind to TAR RNA and to activate HIV transcription. Viral LTRs encoding TAR sequences known to bind tat weakly, are not stimulated efficiently by tat in vivo. HIV-1 regulator of virion expression (rev) protein binds specifically to RNA transcripts containing the 223 nucleotide-long RRE sequence with an apparent dissociation constant of 1–3 nM. The minimum binding site for rev is a ‘bubble’ containing 2 G residues on one side and the sequence AGU on the other. Rev is able to bind efficiently to this restricted site in the context of the RRE sequence as well as in the context of a stable RNA duplex with a sequence unrelated to that found in the RRE. After binding of a rev protein to the high affinity site on the RRE, additional protein molecules can co-assemble along the length of the mRNA precursors carrying RRE sequences by virtue of protein-protein and low affinity protein-RNA interactions to form long filamentous structures which may be detected by electron microscopy.

HIV-1 tat protein stimulates transcription by binding to a U-rich bulge in the stem of the TAR RNA structure.

The HIV-1 trans-activator protein, tat, is an RNA binding protein with a high affinity for a U-rich bulge near the tip of the stem in the RNA stem-loop structure encoded by the trans-activation responsive region (TAR). A Scatchard analysis of tat binding has shown that the purified protein forms a one-to-one complex with HIV-1 TAR RNA with a dissociation constant of Kd = 12 nM. Deletion of the uridine residues in the bulge or substitution with guanine residues produced RNAs with a 6- to 8-fold lower affinity than wild-type TAR. Introduction of a point mutation expected to destabilize base pairing in nearby residues of the TAR stem-loop structure reduced tat binding 10-fold. In contrast, mutations that alter the sequence of the six nucleotide long loop at the tip of TAR RNA structure, and mutations which alter the sequence of the stem whilst preserving Watson-Crick base pairing, do not affect tat binding significantly. There is a direct correlation between the ability of tat to bind to TAR RNA and to activate HIV transcription. Viral LTRs carrying TAR sequences encoding any of the mutations known to produce transcripts which bind tat weakly, are not stimulated efficiently by tat in vivo.

Selective induction of toxicity to human cells expressing human immunodeficiency virus type 1 Tat by a conditionally cytotoxic adenovirus vector.

The human immunodeficiency viruses (HIVs) primarily infect CD4+ T lymphocytes, leading eventually to the development of a systemic immune dysfunction termed acquired immunodeficiency syndrome (AIDS). An attractive strategy to combat HIV-mediated pathogenesis would be to eliminate the initial pool of infected cells and thus prevent disease progression. We have engineered a replication-defective, conditionally cytotoxic adenovirus vector, Ad-tk, whose action is dependent on the targeted expression of the herpes simplex virus type 1 thymidine kinase gene (tk), cloned downstream of the HIV-1 long terminal repeat, in human cells expressing the HIV-1 transcriptional activator Tat. Infection of Tat-expressing human HeLa or Jurkat cells with Ad-tk resulted in high-level tk expression, which was not deleterious to the viability of these cells. However, in the presence of the antiherpetic nucleoside analog ganciclovir, Ad-tk infection resulted in a massive reduction in the viability of these Tat-expressing cell lines. As adenoviruses are natural passengers of the human lymphoid system, our results suggest adenovirus vector-based strategies for the targeted expression, under the control of cis-responsive HIV regulatory elements, of cytotoxic agents in HIV-infected cells for the therapy of HIV-mediated pathogenesis.