NF-kappa B-mediated chromatin reconfiguration and transcriptional activation of the HIV-1 enhancer in vitro

The HIV-1 Transcriptional Program: From Initiation to Elongation Control

A large body of work in the last four decades has revealed the key pillars of HIV-1 transcription control at the initiation and elongation steps. Here, I provide a recount of this collective knowledge starting with the genomic elements (DNA and nascent TAR RNA stem-loop) and transcription factors (cellular and the viral transactivator Tat), and later transitioning to the assembly and regulation of transcription initiation and elongation complexes, and the role of chromatin structure. Compelling evidence support a core HIV-1 transcriptional program regulated by the sequential and concerted action of cellular transcription factors and Tat to promote initiation and sustain elongation, highlighting the efficiency of a small virus to take over its host to produce the high levels of transcription required for viral replication. I summarize new advances including the use of CRISPR-Cas9, genetic tools for acute factor depletion, and imaging to study transcriptional dynamics, bursting and the progression through the multiple phases of the transcriptional cycle. Finally, I describe current challenges to future major advances and discuss areas that deserve more attention to both bolster our basic knowledge of the core HIV-1 transcriptional program and open up new therapeutic opportunities.

Discovery of new acetamide derivatives of 5-indole-1,3,4-oxadiazol-2-thiol as inhibitors of HIV-1 Tat-mediated viral transcription

Human immunodeficiency virus-1 (HIV-1) encodes a transcriptional factor called Tat, which is critical for viral transcription. Tat-mediated transcription is highly ordered apart from the cellular manner; therefore, it is considered a target for developing anti-HIV-1 drugs. However, drugs targeting Tat-mediated viral transcription are not yet available. Our high-throughput screen of a compound library employing a dual-reporter assay identified a 1,3,4-oxadiazole scaffold against Tat and HIV-1 infection. Furthermore, a serial structure-activity relation (SAR) study performed with biological assays found 1,3,4-oxadiazole derivatives (9 and 13) containing indole and acetamide that exhibited potent inhibitory effects on HIV-1 infectivity, with half-maximal effective concentrations (EC50) of 0.17 (9) and 0.24 µM (13), respectively. The prominent derivatives specifically interfered with the viral transcriptional step without targeting other infection step(s) and efficiently inhibited the HIV-1 replication cycle in the T cell lines and peripheral blood mononuclear cells infected with HIV-1. Additionally, compared to the wild type, the compounds exhibited similar potency against anti-retroviral drug-resistant HIV-1 strains. In a series of mode-of-action studies, the compounds inhibited the ejection of histone H3 for facilitating viral transcription on the long-terminal repeat (LTR) promoter. Furthermore, SAHA (a histone deacetylase inhibitor) treatment abolished the compound potency, revealing that the compounds can possibly target Tat-regulated epigenetic modulation of LTR to inhibit viral transcription. Overall, our screening identified novel 1,3,4-oxadiazole compounds that inhibited HIV-1 Tat, and subsequent SAR-based optimization led to the derivatives 9 and 13 development that could be a promising scaffold for developing a new class of therapeutic agents for HIV-1 infection.

CDK8 inhibitors antagonize HIV-1 reactivation and promote provirus latency in T cells

Latent HIV-1 provirus represents the barrier toward a cure for infection and is dependent upon the host RNA Polymerase (Pol) II machinery for reemergence. Here, we find that inhibitors of the RNA Pol II mediator kinases CDK8/19, Senexin A and BRD6989, inhibit induction of HIV-1 expression in response to latency-reversing agents and T cell signaling agonists. These inhibitors were found to impair recruitment of RNA Pol II to the HIV-1 LTR. Furthermore, HIV-1 expression in response to several latency reversal agents was impaired upon disruption of CDK8 by shRNA or gene knockout. However, the effects of CDK8 depletion did not entirely mimic CDK8/19 kinase inhibition suggesting that the mediator kinases are not functionally redundant. Additionally, treatment of CD4+ peripheral blood mononuclear cells isolated from people living with HIV-1 and who are receiving antiretroviral therapy with Senexin A inhibited induction of viral replication in response to T cell stimulation by PMA and ionomycin. These observations indicate that the mediator kinases, CDK8 and CDK19, play a significant role for regulation of HIV-1 transcription and that small molecule inhibitors of these enzymes may contribute to therapies designed to promote deep latency involving the durable suppression of provirus expression. IMPORTANCE A cure for HIV-1 infection will require novel therapies that can force elimination of cells that contain copies of the virus genome inserted into the cell chromosome, but which is shut off, or silenced. These are known as latently-infected cells, which represent the main reason why current treatment for HIV/AIDS cannot cure the infection because the virus in these cells is unaffected by current drugs. Our results indicate that chemical inhibitors of Cdk8 also inhibit the expression of latent HIV provirus. Cdk8 is an important enzyme that regulates the expression of genes in response to signals to which cells need to respond and which is produced by a gene that is frequently mutated in cancers. Our observations indicate that Cdk8 inhibitors may be employed in novel therapies to prevent expression from latent provirus, which might eventually enable infected individuals to cease treatment with antiretroviral drugs.

New Concepts in Therapeutic Manipulation of HIV-1 Transcription and Latency: Latency Reversal versus Latency Prevention

Antiretroviral therapy (ART) has dramatically improved the prognosis for people living with HIV-1, but a cure remains elusive. The largest barrier to a cure is the presence of a long-lived latent reservoir that persists within a heterogenous mix of cell types and anatomical compartments. Efforts to eradicate the latent reservoir have primarily focused on latency reversal strategies. However, new work has demonstrated that the majority of the long-lived latent reservoir is established near the time of ART initiation, suggesting that it may be possible to pair an intervention with ART initiation to prevent the formation of a sizable fraction of the latent reservoir. Subsequent treatment with latency reversal agents, in combination with immune clearance agents, may then be a more tractable strategy for fully clearing the latent reservoir in people newly initiating ART. Here, we summarize molecular mechanisms of latency establishment and maintenance, ongoing efforts to develop effective latency reversal agents, and newer efforts to design latency prevention agents. An improved understanding of the molecular mechanisms involved in both the establishment and maintenance of latency will aid in the development of new latency prevention and reversal approaches to ultimately eradicate the latent reservoir.

Depicting HIV-1 Transcriptional Mechanisms: A Summary of What We Know

Despite the introduction of combinatory antiretroviral therapy (cART), HIV-1 infection cannot be cured and is still one of the major health issues worldwide. Indeed, as soon as cART is interrupted, a rapid rebound of viremia is observed. The establishment of viral latency and the persistence of the virus in cellular reservoirs constitute the main barrier to HIV eradication. For this reason, new therapeutic approaches have emerged to purge or restrain the HIV-1 reservoirs in order to cure infected patients. However, the viral latency is a multifactorial process that depends on various cellular mechanisms. Since these new therapies mainly target viral transcription, their development requires a detailed and precise understanding of the regulatory mechanism underlying HIV-1 transcription. In this review, we discuss the complex molecular transcriptional network regulating HIV-1 gene expression by focusing on the involvement of host cell factors that could be used as potential drug targets to design new therapeutic strategies and, to a larger extent, to reach an HIV-1 functional cure.

The transformation of the DNA template in RNA polymerase II transcription: a historical perspective

The discovery of RNA polymerases I, II, and III opened up a new era in gene expression. Here I provide a personal retrospective account of the transformation of the DNA template, as it evolved from naked DNA to chromatin, in the biochemical analysis of transcription by RNA polymerase II. These studies have revealed new insights into the mechanisms by which transcription factors function with chromatin to regulate gene expression.

Semen Exosomes Promote Transcriptional Silencing of HIV-1 by Disrupting NF-κB/Sp1/Tat Circuitry

Exosomes play various roles in host responses to cancer and infective agents, and semen exosomes (SE) inhibit HIV-1 infection and transmission, although the mechanism(s) by which this occurs is unclear. Here, we show that SE block HIV-1 proviral transcription at multiple transcriptional checkpoints, including transcription factor recruitment to the long terminal repeat (LTR), transcription initiation, and elongation. Biochemical and functional studies show that SE inhibit HIV-1 LTR-driven viral gene expression and virus replication. Through partitioning of the HIV-1 RNA, we found that SE reduced the optimal expression of various viral RNA species. Chromatin immunoprecipitation-real-time quantitative PCR (ChIP-RT-qPCR) and electrophoretic mobility shift assay (EMSA) analysis of infected cells identified the human transcription factors NF-κB and Sp1, as well as RNA polymerase (Pol) II and the viral protein transcriptional activator (Tat), as targets of SE. Of interest, SE inhibited HIV-1 LTR activation mediated by HIV-1 or Tat, but not by the mitogen phorbol myristate acetate (PMA) or tumor necrosis factor alpha (TNF-α). SE inhibited the DNA binding activities of NF-κB and Sp1 and blocked the recruitment of these transcription factors and Pol II to the HIV-1 LTR promoter. Importantly, SE directly blocked NF-κB, Sp1, and Pol II binding to the LTR and inhibited the interactions of Tat/NF-κB and Tat/Sp1, suggesting that SE-mediated inhibition of the functional quadripartite complex NF-κB-Sp1-Pol II-Tat may be a novel mechanism of proviral transcription repression. These data provide a novel molecular basis for SE-mediated inhibition of HIV-1 and identify Tat as a potential target of SE.IMPORTANCE HIV is most commonly transmitted sexually, and semen is the primary vector. Despite progress in studies of HIV pathogenesis and the success of combination antiretroviral therapy in controlling viral replication, current therapy cannot completely control sexual transmission. Thus, there is a need to identify effective methods of controlling HIV replication and transmission. Recently, it was shown that human semen contains exosomes that protect against HIV infection in vitro In this study, we identified a mechanism by which semen exosomes inhibited HIV-1 RNA expression. We found that semen exosomes inhibit recruitment of transcription factors NF-κB and Sp1, as well as RNA Pol II, to the promoter region in the 5' long terminal repeat (LTR) of HIV-1. The HIV-1 early protein transcriptional activator (Tat) was a target of semen exosomes, and semen exosomes inhibited the binding and recruitment of Tat to the HIV-1 LTR.

Bioinformatics and Translation Elongation

Codon usage depends on mutation bias, tRNA-mediated selection, and the need for high efficiency and accuracy in translation. One codon in a synonymous codon family is often strongly over-used, especially in highly expressed genes, which often leads to a high dN/dS ratio because dS is very small. Many different codon usage indices have been proposed to measure codon usage and codon adaptation. Sense codon could be misread by release factors and stop codons misread by tRNAs, which also contribute to codon usage in rare cases. This chapter outlines the conceptual framework on codon evolution, illustrates codon-specific and gene-specific codon usage indices, and presents their applications. A new index for codon adaptation that accounts for background mutation bias (Index of Translation Elongation) is presented and contrasted with codon adaptation index (CAI) which does not consider background mutation bias. They are used to re-analyze data from a recent paper claiming that translation elongation efficiency matters little in protein production. The reanalysis disproves the claim.

Bioinformatics and Drug Discovery

Bioinformatic analysis can not only accelerate drug target identification and drug candidate screening and refinement, but also facilitate characterization of side effects and predict drug resistance. High-throughput data such as genomic, epigenetic, genome architecture, cistromic, transcriptomic, proteomic, and ribosome profiling data have all made significant contribution to mechanismbased drug discovery and drug repurposing. Accumulation of protein and RNA structures, as well as development of homology modeling and protein structure simulation, coupled with large structure databases of small molecules and metabolites, paved the way for more realistic protein-ligand docking experiments and more informative virtual screening. I present the conceptual framework that drives the collection of these high-throughput data, summarize the utility and potential of mining these data in drug discovery, outline a few inherent limitations in data and software mining these data, point out news ways to refine analysis of these diverse types of data, and highlight commonly used software and databases relevant to drug discovery.

IUGR increases chromatin-remodeling factor Brg1 expression and binding to GR exon 1.7 promoter in newborn male rat hippocampus

Intrauterine growth restriction (IUGR) increases the risk for neurodevelopment delay and neuroendocrine reprogramming in both humans and rats. Neuroendocrine reprogramming involves the glucocorticoid receptor (GR) gene that is epigenetically regulated in the hippocampus. Using a well-characterized rodent model, we have previously shown that IUGR increases GR exon 1.7 mRNA variant and total GR expressions in male rat pup hippocampus. Epigenetic regulation of GR transcription may involve chromatin remodeling of the GR gene. A key chromatin remodeler is Brahma-related gene-1(Brg1), a member of the ATP-dependent SWItch/Sucrose NonFermentable (SWI/SNF) chromatin remodeling complex. Brg1 regulates gene expression by affecting nucleosome repositioning and recruiting transcriptional components to target promoters. We hypothesized that IUGR would increase hippocampal Brg1 expression and binding to GR exon 1.7 promoter, as well as alter nucleosome positioning over GR promoters in newborn male pups. Further, we hypothesized that IUGR would lead to accumulation of specificity protein 1 (Sp1) and RNA pol II at GR exon 1.7 promoter. Indeed, we found that IUGR increased Brg1 expression and binding to GR exon 1.7 promoter. We also found that increased Brg1 binding to GR exon 1.7 promoter was associated with accumulation of Sp1 and RNA pol II carboxy terminal domain pSer-5 (a marker of active transcription). Furthermore, the transcription start site of GR exon 1.7 was located within a nucleosome-depleted region. We speculate that changes in hippocampal Brg1 expression mediate GR expression and subsequently trigger neuroendocrine reprogramming in male IUGR rats.

A hardwired HIV latency program

Biological circuits can be controlled by two general schemes: environmental sensing or autonomous programs. For viruses such as HIV, the prevailing hypothesis is that latent infection is controlled by cellular state (i.e., environment), with latency simply an epiphenomenon of infected cells transitioning from an activated to resting state. However, we find that HIV expression persists despite the activated-to-resting cellular transition. Mathematical modeling indicates that HIV's Tat positive-feedback circuitry enables this persistence and strongly controls latency. To overcome the inherent crosstalk between viral circuitry and cellular activation and to directly test this hypothesis, we synthetically decouple viral dependence on cellular environment from viral transcription. These circuits enable control of viral transcription without cellular activation and show that Tat feedback is sufficient to regulate latency independent of cellular activation. Overall, synthetic reconstruction demonstrates that a largely autonomous, viral-encoded program underlies HIV latency—potentially explaining why cell-targeted latency-reversing agents exhibit incomplete penetrance.

Using the ENCODE Resource for Functional Annotation of Genetic Variants

This article illustrates the use of the Encyclopedia of DNA Elements (ENCODE) resource to generate or refine hypotheses from genomic data on disease and other phenotypic traits. First, the goals and history of ENCODE and related epigenomics projects are reviewed. Second, the rationale for ENCODE and the major data types used by ENCODE are briefly described, as are some standard heuristics for their interpretation. Third, the use of the ENCODE resource is examined. Standard use cases for ENCODE, accessing the ENCODE resource, and accessing data from related projects are discussed. Although the focus of this article is the use of ENCODE data, some of the same approaches can be used with data from other projects.

Transcriptional control by NF-κB: elongation in focus

The NF-κB family of transcription factors governs the cellular reaction to a variety of extracellular signals. Following stimulation, NF-κB activates genes involved in inflammation, cell survival, cell cycle, immune cell homeostasis and more. This review focuses on studies of the past decade that uncover the transcription elongation process as a key regulatory stage in the activation pathway of NF-κB. Of interest are studies that point to the elongation phase as central to the selectivity of target gene activation by NF-κB. Particularly, the cascade leading to phosphorylation and acetylation of the NF-κB subunit p65 on serine 276 and lysine 310, respectively, was shown to mediate the recruitment of Brd4 and P-TEFb to many pro-inflammatory target genes, which in turn facilitate elongation and mRNA processing. On the other hand, some anti-inflammatory genes are refractory to this pathway and are dependent on the elongation factor DSIF for efficient elongation and mRNA processing. While these studies have advanced our knowledge of NF-κB transcriptional activity, they have also raised unresolved issues regarding the specific genomic and physiological contexts by which NF-κB utilizes different mechanisms for activation.

Role of nuclear factor-ĸB in breast and colorectal cancer

The purpose of this review article is to highlight articles and new research regarding the link between NF-ĸB and several cancers. This review presents the most up-to-date NF-ĸB research and how it links this important transcription factor with hematology and oncology. It was written by conducting a thorough search of Pubmed as well as several journals such as Cancer, Nature, Science, Cell and those of one of the authors. The articles relating to the link between NF-ĸB and cancer were used to write this review. The results of this study clarified that there is a critical link between NF-ĸB and cancer. NF-ĸB has often been implicated in a variety of different diseases and it plays a variety of roles in cell survival, differentiation, and proliferation of cells. In cancer, NF-ĸB plays a pivotal role by facilitating oncogenesis as well as metastasis. A thorough understanding of NF-ĸB and its role in cancer can lead to future studies and drug development which could provide a novel option in the treatment of this disease.

Immune response to nanomaterials: implications for medicine and literature review

Nanomaterials, substances below 100 nm, are increasingly used in medical diagnosis and treatment every day. The use of such materials has helped deliver drugs across the blood-brain barrier, alleviate allergy symptoms, specifically target cancer or HIV cells, and more. However, the tunable characteristics of such materials have not been perfected. The different materials, sizes, shapes, and structures have different responses on the body. This paper will investigate the successful treatments made with nanoparticles and some general health effects. A review of the literature revealed an inflammatory response and an increased production of reactive oxidative species (ROS) to be common immune responses to nanomaterial use. The mechanisms by which the inflammatory response and ROS production occur will also be discussed.

The BAF complex and HIV latency

The persistence of a reservoir of transcriptionally competent but latent virus in the presence of antiviral regimens presents the main impediment to a curative therapy against HIV. Therefore it is critical to understand the molecular mechanisms, which lead to the establishment and maintenance of HIV latency, and which contribute to the reversal of this process and mediate HIV transcriptional activation in response to T cell activation signals. Here I discuss features of the nucleosomal landscape of the HIV promoter or 5'LTR in controlling HIV transcription. I emphasize on the emerging understanding of the role of the ATP dependent SWI/SNF chromatin remodelling complexes in modulating the chromatin architecture at the HIV LTR and how this leads to a tight regulation of LTR transcription.

Curaxins: anticancer compounds that simultaneously suppress NF-κB and activate p53 by targeting FACT

Effective eradication of cancer requires treatment directed against multiple targets. The p53 and nuclear factor κB (NF-κB) pathways are dysregulated in nearly all tumors, making them attractive targets for therapeutic activation and inhibition, respectively. We have isolated and structurally optimized small molecules, curaxins, that simultaneously activate p53 and inhibit NF-κB without causing detectable genotoxicity. Curaxins demonstrated anticancer activity against all tested human tumor xenografts grown in mice. We report here that the effects of curaxins on p53 and NF-κB, as well as their toxicity to cancer cells, result from "chromatin trapping" of the FACT (facilitates chromatin transcription) complex. This FACT inaccessibility leads to phosphorylation of the p53 Ser(392) by casein kinase 2 and inhibition of NF-κB-dependent transcription, which requires FACT activity at the elongation stage. These results identify FACT as a prospective anticancer target enabling simultaneous modulation of several pathways frequently dysregulated in cancer without induction of DNA damage. Curaxins have the potential to be developed into effective and safe anticancer drugs.

Transcriptional control of HIV replication by multiple modulators and their implication for a novel antiviral therapy

Transcriptional regulation is critical for the human immunodeficiency virus 1 (HIV-1) life cycle and is the only step at which the virus amplifies the content of its genetic information. Numerous known and still unknown transcriptional factors, both host and viral, regulate HIV-1 gene expression and latency. This article is a comprehensive review of transcription factors involved in HIV-1 gene expression and presents the significant implications of nuclear factor kappa B (NF-κB) and the HIV-1 transactivator of transcription (Tat) protein. We include recent findings on chromatin remodeling toward HIV transcription and its therapeutic implication is also discussed. The current status of small-molecular-weight compounds that affect HIV transcription is also described.

The Role of RNA Polymerase II Elongation Control in HIV-1 Gene Expression, Replication, and Latency

HIV-1 usurps the RNA polymerase II elongation control machinery to regulate the expression of its genome during lytic and latent viral stages. After integration into the host genome, the HIV promoter within the long terminal repeat (LTR) is subject to potent downregulation in a postinitiation step of transcription. Once produced, the viral protein Tat commandeers the positive transcription elongation factor, P-TEFb, and brings it to the engaged RNA polymerase II (Pol II), leading to the production of viral proteins and genomic RNA. HIV can also enter a latent phase during which factors that regulate Pol II elongation may play a role in keeping the virus silent. HIV, the causative agent of AIDS, is a worldwide health concern. It is hoped that knowledge of the mechanisms regulating the expression of the HIV genome will lead to treatments and ultimately a cure.

Involvement of JNK and NF-κB pathways in lipopolysaccharide (LPS)-induced BAG3 expression in human monocytic cells

Lipopolysaccharide (LPS) is an outer-membrane glycolipid component of Gram-negative bacteria known for its fervent ability to activate monocytic cells and for its potent proinflammatory capabilities. Bcl-2-associated athanogene 3 (BAG3) is a survival protein that has been shown to be stimulated during cell response to stressful conditions, such as exposure to high temperature, heavy metals, proteasome inhibition, and human immunodeficiency virus 1 (HIV-1) infection. In addition, BAG3 regulates replication of Varicella-Zoster Virus (VZV) and Herpes Simplex Virus (HSV) replication, suggesting that BAG3 could participate in the host response to infection. In the current study, we found that LPS increased the expression of BAG3 in a dose- and time-dependent manner. Actinomycin D completely blocked the LPS-induced BAG3 accumulation, as well as LPS activated the proximal promoter of BAG3 gene, supported that the induction by LPS occurred at the level of gene transcription. LPS-induced BAG3 expression was blocked by JNK or NF-κB inhibition, suggesting that JNK and NF-κB pathways participated in BAG3 induction by LPS. In addition, we also found that induction of BAG3 was implicated in monocytic cell adhesion to extracellular matrix induced by LPS. Overall, the data support that BAG3 is induced by LPS via JNK and NF-κB-dependent signals, and involved in monocytic cell-extracellular matrix interaction, suggesting that BAG3 may have a role in the host response to LPS stimulation.

RNA duplexes in transcriptional regulation

Transcriptional regulation by small RNA molecules, including small interfering RNA and microRNA, has emerged as an important gene expression modulator. The regulatory pathways controlling gene expression, post-transcriptional gene silencing and transcriptional gene silencing (TGS) have been demonstrated in yeast, plants and more recently in human cells. In this review, we discuss the currents models of transcriptional regulation and the main components of the RNA-induced silencing complex and RNA-induced transcriptional silencing complex machinery, as well as confounding off-target effects and gene activation. We also discuss RNA-mediated TGS within the NF-κB motif of the human immunodeficiency virus type 1 5' long tandem repeat promoter region and the associated epigenetic modifications. Finally, we outline the current RNA interference (RNAi) delivery methods and describe the current status of human trials investigating potential RNAi therapeutics for several human diseases.

HIV promoter integration site primarily modulates transcriptional burst size rather than frequency

Mammalian gene expression patterns, and their variability across populations of cells, are regulated by factors specific to each gene in concert with its surrounding cellular and genomic environment. Lentiviruses such as HIV integrate their genomes into semi-random genomic locations in the cells they infect, and the resulting viral gene expression provides a natural system to dissect the contributions of genomic environment to transcriptional regulation. Previously, we showed that expression heterogeneity and its modulation by specific host factors at HIV integration sites are key determinants of infected-cell fate and a possible source of latent infections. Here, we assess the integration context dependence of expression heterogeneity from diverse single integrations of a HIV-promoter/GFP-reporter cassette in Jurkat T-cells. Systematically fitting a stochastic model of gene expression to our data reveals an underlying transcriptional dynamic, by which multiple transcripts are produced during short, infrequent bursts, that quantitatively accounts for the wide, highly skewed protein expression distributions observed in each of our clonal cell populations. Interestingly, we find that the size of transcriptional bursts is the primary systematic covariate over integration sites, varying from a few to tens of transcripts across integration sites, and correlating well with mean expression. In contrast, burst frequencies are scattered about a typical value of several per cell-division time and demonstrate little correlation with the clonal means. This pattern of modulation generates consistently noisy distributions over the sampled integration positions, with large expression variability relative to the mean maintained even for the most productive integrations, and could contribute to specifying heterogeneous, integration-site-dependent viral production patterns in HIV-infected cells. Genomic environment thus emerges as a significant control parameter for gene expression variation that may contribute to structuring mammalian genomes, as well as be exploited for survival by integrating viruses.

Innate and adaptive factors regulating human immunodeficiency virus type 1 genomic activation

Over the past decade, antiretroviral therapy targeting the viral entry process, reverse transcriptase, integrase, and protease, has prolonged the lives of people infected with human immunodeficiency virus type 1 (HIV-1). However, despite the development of more effective therapeutic strategies, reservoirs of viral infection remain. This review discusses molecular mechanisms surrounding the development of latency from the site of integration to pre- and post-integration maintenance of latency, including epigenetic factors. In addition, an overview of innate and adaptive cells important to HIV-1 infection are examined from the viewpoint of cytokines released and cytokines that act on these cells to explore an overall understanding of HIV-1 proviral genome activation. Finally, this review is discussed from the viewpoint of how an understanding of the interplay of all of these factors will help guide the next generation of therapies.

TAp63 is a transcriptional target of NF-kappaB

The p53 homologue p63 encodes multiple protein isoforms either with (TA) or without (DeltaN) the N-terminal transactivation domain. Accumulating evidence indicates that TAp63 plays an important role in various biological processes, including cell proliferation, differentiation, and apoptosis. However, how TAp63 is regulated remains largely unclear. In this study, we demonstrate that NF-kappaB induces TAp63 gene expression. The responsible elements for NF-kappaB-mediated TAp63 induction are located within the region from -784 to -296 bp in the TAp63 promoter, which contains two NF-kappaB binding sites. Ectopic expression of RelA stimulates TAp63 promoter-driven reporter activity and increases endogenous TAp63 mRNA levels. Inhibition of NF-kappaB by IkappaBalpha super-repressor or with a chemical inhibitor leads to down regulation of TAp63 mRNA expression and activity. In addition, mutations in the critical NF-kappaB-binding sites significantly abolish the effects of NF-kappaB on TAp63. Activation of NF-kappaB by TNFalpha enhances p50/RelA binding to the NF-kappaB binding sites. Furthermore, we show that an Sp1 site adjacent to the NF-kappaB sites plays a role in NF-kappaB-mediated upregulation of TAp63. Taken together, these data reveal that TAp63 is a transcriptional target of NF-kappaB, which may play a role in cell proliferation, differentiation and survival upon NF-kappaB activation by various stimuli.

The DNA sequence-dependence of nucleosome positioning in vivo and in vitro

The contribution of histone-DNA interactions to nucleosome positioning in vivo is currently a matter of debate. We argue here that certain nucleosome positions, often in promoter regions, in yeast may be, at least in part, specified by the DNA sequence. In contrast other positions may be poorly specified. Positioning thus has both statistical and DNA-determined components. We further argue that the relative affinity of the octamer for different DNA sequences can vary and therefore the interaction of histones with the DNA is a 'tunable' property.

Transcription of human resistin gene involves an interaction of Sp1 with peroxisome proliferator-activating receptor gamma (PPARgamma)

BACKGROUND

Resistin is a cysteine rich protein, mainly expressed and secreted by circulating human mononuclear cells. While several factors responsible for transcription of mouse resistin gene have been identified, not much is known about the factors responsible for the differential expression of human resistin.

METHODOLOGY/PRINCIPAL FINDING

We show that the minimal promoter of human resistin lies within approximately 80 bp sequence upstream of the transcriptional start site (-240) whereas binding sites for cRel, CCAAT enhancer binding protein alpha (C/EBP-alpha), activating transcription factor 2 (ATF-2) and activator protein 1 (AP-1) transcription factors, important for induced expression, are present within sequences up to -619. Specificity Protein 1(Sp1) binding site (-276 to -295) is also present and an interaction of Sp1 with peroxisome proliferator activating receptor gamma (PPARgamma) is necessary for constitutive expression in U937 cells. Indeed co-immunoprecipitation assay demonstrated a direct physical interaction of Sp1 with PPARgamma in whole cell extracts of U937 cells. Phorbol myristate acetate (PMA) upregulated the expression of resistin mRNA in U937 cells by increasing the recruitment of Sp1, ATF-2 and PPARgamma on the resistin gene promoter. Furthermore, PMA stimulation of U937 cells resulted in the disruption of Sp1 and PPARgamma interaction. Chromatin immunoprecipitation (ChIP) assay confirmed the recruitment of transcription factors phospho ATF-2, Sp1, Sp3, PPARgamma, chromatin modifier histone deacetylase 1 (HDAC1) and the acetylated form of histone H3 but not cRel, C/EBP-alpha and phospho c-Jun during resistin gene transcription.

CONCLUSION

Our findings suggest a complex interplay of Sp1 and PPARgamma along with other transcription factors that drives the expression of resistin in human monocytic U937 cells.

Regulation of HIV-1 transcription in cells of the monocyte-macrophage lineage

Human immunodeficiency virus type 1 (HIV-1) has been shown to replicate productively in cells of the monocyte-macrophage lineage, although replication occurs to a lesser extent than in infected T cells. As cells of the monocyte-macrophage lineage become differentiated and activated and subsequently travel to a variety of end organs, they become a source of infectious virus and secreted viral proteins and cellular products that likely initiate pathological consequences in a number of organ systems. During this process, alterations in a number of signaling pathways, including the level and functional properties of many cellular transcription factors, alter the course of HIV-1 long terminal repeat (LTR)-directed gene expression. This process ultimately results in events that contribute to the pathogenesis of HIV-1 infection. First, increased transcription leads to the upregulation of infectious virus production, and the increased production of viral proteins (gp120, Tat, Nef, and Vpr), which have additional activities as extracellular proteins. Increased viral production and the presence of toxic proteins lead to enhanced deregulation of cellular functions increasing the production of toxic cellular proteins and metabolites and the resulting organ-specific pathologic consequences such as neuroAIDS. This article reviews the structural and functional features of the cis-acting elements upstream and downstream of the transcriptional start site in the retroviral LTR. It also includes a discussion of the regulation of the retroviral LTR in the monocyte-macrophage lineage during virus infection of the bone marrow, the peripheral blood, the lymphoid tissues, and end organs such as the brain. The impact of genetic variation on LTR-directed transcription during the course of retrovirus disease is also reviewed.

p53 sumoylation: mechanistic insights from reconstitution studies

Sumoylation represents a cascade of enzymatic reactions mediated by SUMO-activating enzyme (SAE1/SAE2 heterodimer), SUMO-conjugating enzyme Ubc9, and SUMO E3 ligases that include five protein inhibitors of activated STATs (PIAS1, PIAS3, PIASy, PIASxalpha and PIASxbeta), and culminates in the formation of an isopeptide bond between the C-terminal glycine of a small ubiquitin-related modifier (SUMO) and the lysine residue of a protein substrate. Conjugation of a SUMO moiety, ranging from 92 (for SUMO-2) to 97 (for SUMO-1) amino acids, not only increases the molecular size but also alters the property and function of the modified protein. Although sumoylation has been observed with many cellular proteins and the majority of transcription factors including the p53 tumor suppressor, this covalent modification is normally detectable only in a small population, often less than 5%, of a given substrate in vivo. This low abundance of SUMO-modified proteins, due to the presence of sentrin/SUMO-specific proteases (SENPs) that actively cleave the reversible SUMO linkage, has posed a challenge to define the biological effect of SUMO in living cells. Nevertheless, the recent development of reconstituted modification and chromatin-dependent transcription assays has provided unique insights into the molecular action of SUMO in modifying protein function. The availability of these reconstitution systems has unraveled the interplay between sumoylation and acetylation in regulating the DNA binding and transcriptional activity of p53 tetramers and further allow the identification of transcriptional corepressors, such as mSin3A, CoREST1/LSD1 and Mi-2/NuRD implicated in SUMO-dependent gene silencing events.

Expression of latent human immunodeficiency type 1 is induced by novel and selective histone deacetylase inhibitors

OBJECTIVES

A family of histone deacetylases (HDACs) mediates chromatin remodeling, and repression of gene expression. Deacetylation of histones within the HIV-1 long terminal repeat (LTR) by HDACs plays a key role in the maintenance of latency, whereas acetylation of histones about the LTR is linked to proviral expression and escape of HIV from latency. Global HDAC inhibition may adversely affect host gene expression, leading to cellular toxicities. Potent inhibitors selective for HDACs that maintain LTR repression could be ideal antilatency therapeutics.

METHODS

We investigated the ability of selective HDAC inhibitors to de-repress the HIV-1 LTR in both a cell line model of latency and in resting CD4 T cells isolated from patients who were aviremic on antiretroviral therapy (ART).

RESULTS

We found that inhibition of class I HDACs increased acetylation of histones at the LTR, but that LTR chromatin was unaffected by class II HDAC inhibitors. In a latently infected cell line, inhibitors selective for class I HDACs were more efficient activators of the LTR than inhibitors that target class II HDACs. Class I HDAC inhibitors were strikingly efficient inducers of virus outgrowth from resting CD4 T cells of aviremic patients, whereas HIV was rarely recovered from patient's cells exposed to class II HDAC inhibitors.

CONCLUSIONS

Further development of selective HDAC inhibitors as part of a clinical strategy to target persistent HIV infection is warranted.

Chromatin dynamics associated with HIV-1 Tat-activated transcription

Chromatin remodeling is an essential event for HIV-1 transcription. Over the last two decades this field of research has come to the forefront, as silencing of the HIV-1 provirus through chromatin modifications has been linked to latency. Here, we focus on chromatin remodeling, especially in relation to the transactivator Tat, and review the most important and newly emerging studies that investigate remodeling mechanisms. We begin by discussing covalent modifications that can alter chromatin structure including acetylation, deacetylation, and methylation, as well as topics addressing the interplay between chromatin remodeling and splicing. Next, we focus on complexes that use the energy of ATP to remove or secure nucleosomes and can additionally act to control HIV-1 transcription. Finally, we cover recent literature on viral microRNAs which have been shown to alter chromatin structure by inducing methylation or even by remodeling nucleosomes.

Astrocyte elevated gene-1 (AEG-1) is induced by lipopolysaccharide as toll-like receptor 4 (TLR4) ligand and regulates TLR4 signalling

Astrocyte elevated gene-1 (AEG-1) is induced by human immunodeficiency virus 1 (HIV-1) infection and involved in tumour progression, migration and invasion as a nuclear factor-kappaB (NF-kappaB) -dependent gene. The involvement of AEG-1 on lipopolysaccharide (LPS) -induced proinflammatory cytokine production was examined. AEG-1 was induced via NF-kappaB activation in LPS-stimulated U937 human promonocytic cells. AEG-1 induced by LPS subsequently regulated NF-kappaB activation. The prevention of AEG-1 expression inhibited LPS-induced tumour necrosis factor-alpha and prostaglandin E(2) production. The AEG-1 activation was not induced by toll-like receptor ligands other than LPS. Therefore, AEG-1 was suggested to be a LPS-responsive gene and involved in LPS-induced inflammatory response.

Expression of latent HIV induced by the potent HDAC inhibitor suberoylanilide hydroxamic acid

Histone deacetylases (HDACs) act on histones within the nucleosome-bound promoter of human immunodeficiency virus type 1 (HIV-1) to maintain proviral latency. HDAC inhibition leads to promoter expression and the escape of HIV from latency. We evaluated the ability of the potent inhibitor recently licensed for use in oncology, suberoylanilide hydroxamic acid (SAHA; Vorinostat), selective for Class I HDACs, to induce HIV promoter expression in cell lines and virus production from the resting CD4(+) T cells of antiretroviral-treated, aviremic HIV-infected patients. In J89, a Jurkat T cell line infected with a single HIV genome encoding the enhanced green fluorescence protein (EGFP) within the HIV genome, SAHA induced changes at nucleosome 1 of the HIV promoter in chromatin immunoprecipitation (ChIP) assays in concert with EGFP expression. In the resting CD4(+) T cells of antiretroviral-treated, aviremic HIV-infected patients clinically achievable exposures to SAHA induced virus outgrowth ex vivo. These results suggest that potent, selective HDAC inhibitors may allow improved targeting of persistent proviral HIV infection, and define parameters for in vivo studies using SAHA.

Mi2beta shows chromatin enzyme specificity by erasing a DNase I-hypersensitive site established by ACF

ATP-dependent chromatin-remodeling enzymes are linked to changes in gene expression; however, it is not clear how the multiple remodeling enzymes found in eukaryotes differ in function and work together. In this report, we demonstrate that the ATP-dependent remodeling enzymes ACF and Mi2beta can direct consecutive, opposing chromatin-remodeling events, when recruited to chromatin by different transcription factors. In a cell-free system based on the immunoglobulin heavy chain gene enhancer, we show that TFE3 induces a DNase I-hypersensitive site in an ATP-dependent reaction that requires ACF following transcription factor binding to chromatin. In a second step, PU.1 directs Mi2beta to erase an established DNase I-hypersensitive site, in an ATP-dependent reaction subsequent to PU.1 binding to chromatin, whereas ACF will not support erasure. Erasure occurred without displacing the transcription factor that initiated the site. Other tested enzymes were unable to erase the DNase I-hypersensitive site. Establishing and erasing the DNase I-hypersensitive site required transcriptional activation domains from TFE3 and PU.1, respectively. Together, these results provide important new mechanistic insight into the combinatorial control of chromatin structure.

Control of stochastic gene expression by host factors at the HIV promoter

The HIV promoter within the viral long terminal repeat (LTR) orchestrates many aspects of the viral life cycle, from the dynamics of viral gene expression and replication to the establishment of a latent state. In particular, after viral integration into the host genome, stochastic fluctuations in viral gene expression amplified by the Tat positive feedback loop can contribute to the formation of either a productive, transactivated state or an inactive state. In a significant fraction of cells harboring an integrated copy of the HIV-1 model provirus (LTR-GFP-IRES-Tat), this bimodal gene expression profile is dynamic, as cells spontaneously and continuously flip between active (Bright) and inactive (Off) expression modes. Furthermore, these switching dynamics may contribute to the establishment and maintenance of proviral latency, because after viral integration long delays in gene expression can occur before viral transactivation. The HIV-1 promoter contains cis-acting Sp1 and NF-κB elements that regulate gene expression via the recruitment of both activating and repressing complexes. We hypothesized that interplay in the recruitment of such positive and negative factors could modulate the stability of the Bright and Off modes and thereby alter the sensitivity of viral gene expression to stochastic fluctuations in the Tat feedback loop. Using model lentivirus variants with mutations introduced in the Sp1 and NF-κB elements, we employed flow cytometry, mRNA quantification, pharmacological perturbations, and chromatin immunoprecipitation to reveal significant functional differences in contributions of each site to viral gene regulation. Specifically, the Sp1 sites apparently stabilize both the Bright and the Off states, such that their mutation promotes noisy gene expression and reduction in the regulation of histone acetylation and deacetylation. Furthermore, the NF-κB sites exhibit distinct properties, with κB site I serving a stronger activating role than κB site II. Moreover, Sp1 site III plays a particularly important role in the recruitment of both p300 and RelA to the promoter. Finally, analysis of 362 clonal cell populations infected with the viral variants revealed that mutations in any of the Sp1 sites yield a 6-fold higher frequency of clonal bifurcation compared to that of the wild-type promoter. Thus, each Sp1 and NF-κB site differentially contributes to the regulation of viral gene expression, and Sp1 sites functionally “dampen” transcriptional noise and thereby modulate the frequency and maintenance of this model of viral latency. These results may have biomedical implications for the treatment of HIV latency.

After HIV genome integration into the host chromosome, the viral promoter coordinates a complex set of inputs to control the establishment of viral latency, the onset of viral gene expression, and the ensuing gene expression levels. Among these inputs are chromatin structure at the site of integration, host transcription factors, and the virally encoded transcriptional regulator Tat. Importantly, transcriptional noise from host and viral transcriptional regulators may play a critical role in the decision between replication versus latency, because stochastic fluctuations in gene expression are amplified by a Tat-mediated positive transcriptional feedback loop. To evaluate the individual contributions of key transcription factor binding elements in gene expression dynamics, we employ model HIV viruses with mutations introduced into numerous promoter elements. Extensive analysis of gene expression dynamics and transcription factor recruitment to the viral promoter reveals that each site differentially contributes to viral gene expression and to the establishment of a low expression state that may contribute to viral latency. This systems-level approach elucidates the synergistic contributions of host and viral factors to the dynamics, magnitudes, and stochastic effects in viral gene expression, as well as provides insights into mechanisms that contribute to proviral latency.

Closed chromatin architecture is induced by an RNA duplex targeting the HIV-1 promoter region

In some mammalian systems small interfering RNAs (siRNA) targeting homologous sequences in promoter regions of genes induce transcriptional gene silencing (TGS). We have previously reported the induction of TGS by an siRNA (prom-A siRNA) targeting the tandem NF-kappaB-binding motifs within the human immunodeficiency virus, type 1 (HIV-1), promoter region. Here we report that induction of TGS by prom-A siRNA is accompanied by immediate and sustained local recruitment of Argonaute-1 (Ago1), histone deacetylase-1 (HDAC1), and induction of dimethylation of histone 3 at lysine 9 (H3K9me2), processes known to be associated with transcriptional silencing. Elevated levels of H3K9me2 and HDAC1 spread upstream of the target sequence, and elevated H3K9me2 levels also spread downstream into the coding region. Moreover, this siRNA induces an immediate change in DNA accessibility to restriction enzyme digestion in the region of the transcription initiation site of the HIV-1. This change in accessibility is because of the relocation of a nucleosome known to be associated with this region of the integrated pro-virus. Although there is a theoretical possibility that the observed viral suppression could be mediated by the PTGS mechanism with this siRNA acting at the 3 (R)-long term repeat of the virus, we demonstrate that this siRNA, and three other U3 targeted siRNAs, are inefficient inducers of PTGS. These data strongly suggest that siRNA targeting the promoter region acts predominantly at a site within the 5 (R)-long term repeat of HIV to induce transcriptional silencing and alterations to chromatin structure of the HIV promoter region that extend well beyond the immediate siRNA target site. These induced changes are consistent with those described in latent HIV-1 infection.

Inactivation of AP1 proteins by a nuclear serine protease precedes the onset of radiation-induced fibrosing alveolitis

Radiation-induced lung damage comprises inflammation (alveolitis) as well as disturbed regulation of cell differentiation and proliferation (fibrosis). The transcriptional regulation of this process is poorly understood. One key transcription factor involved in the regulation of proliferation and differentiation is AP1 (activator protein 1). The present study examined changes in the DNA-binding activity of AP1 after irradiation and defined the underlying molecular mechanisms in an animal model. The right lungs of Fischer rats received a single radiation dose of 20 Gy. Lung tissue was tested for AP1 DNA-binding activity, AP1 mRNA, and levels of AP1 proteins as well as for c-Jun specific proteolytic activity. After an initial increase, the AP1 DNA-binding activity was completely lost starting at 5.5 weeks after irradiation, which is 2.5 weeks before the onset of fibrosing alveolitis. This was not caused by reduction of mRNA levels or size. Instead, a selective nuclear cleavage of c-Jun by a serine protease caused the loss of AP1 activity. Considering the central role of AP1 in cell proliferation and differentiation and the strict timely correlation to the onset of the disease, the complete loss of AP1 function is likely to play a critical role in radiation-induced fibrosing alveolitis.

Allele-specific chromatin remodeling of the tumor necrosis factor-α promoter

The -863 C/A polymorphism in the tumor necrosis factor-alpha (TNF-alpha) promoter has been suggested to influence TNF-alpha expression. Here we elucidated the molecular mechanisms underlying the allele-specific regulation of TNF-alpha gene expression under basal and LPS-stimulated conditions in THP-1 cells and in human primary macrophages. We show that the binding of two NF-kappaB complexes, the p50/p50 homodimer and the p50/p65 heterodimer, was upregulated upon LPS stimulation. Both complexes bound to the C-allele whereas the A-allele only bound the p50/p65 complex. Two DNase I hypersensitive sites appeared in the TNF-alpha promoter after LPS stimulation of THP-1 cells. DNase I hypersensitivity of the TNF-alpha promoter was also analyzed in human monocytes prepared from individuals of different -863C/A genotype. Hypersensitivity was increased in the promoter harboring the mutant A-allele, particularly after LPS stimulation. In summary, binding of transcription factor NF-kappaB to the TNF-alpha promoter is associated with allele-specific remodeling of chromatin structure.

The regulation of HIV-1 transcription: molecular targets for chemotherapeutic intervention

The regulation of transcription of the human immunodeficiency virus (HIV) is a complex event that requires the cooperative action of both viral and cellular components. In latently infected resting CD4(+) T cells HIV-1 transcription seems to be repressed by deacetylation events mediated by histone deacetylases (HDACs). Upon reactivation of HIV-1 from latency, HDACs are displaced in response to the recruitment of histone acetyltransferases (HATs) by NF-kappaB or the viral transcriptional activator Tat and result in multiple acetylation events. Following chromatin remodeling of the viral promoter region, transcription is initiated and leads to the formation of the TAR element. The complex of Tat with p-TEFb then binds the loop structures of TAR RNA thereby positioning CDK9 to phosphorylate the cellular RNA polymerase II. The Tat-TAR-dependent phosphorylation of RNA polymerase II plays an important role in transcriptional elongation as well as in other post-transcriptional events. As such, targeting of Tat protein (and/or cellular cofactors) provide an interesting perspective for therapeutic intervention in the HIV replicative cycle and may afford lifetime control of the HIV infection.

Effect of SWI/SNF chromatin remodeling complex on HIV-1 Tat activated transcription

Background

Human immunodeficiency virus type 1 (HIV-1) is the etiologic agent of acquired immunodeficiency virus (AIDS). Following entry into the host cell, the viral RNA is reverse transcribed into DNA and subsequently integrated into the host genome as a chromatin template. The integrated proviral DNA, along with the specific chromatinized environment in which integration takes place allows for the coordinated regulation of viral transcription and replication. While the specific roles of and interplay between viral and host proteins have not been fully elucidated, numerous reports indicate that HIV-1 retains the ability for self-regulation via the pleiotropic effects of its viral proteins. Though viral transcription is fully dependent upon host cellular factors and the state of host activation, recent findings indicate a complex interplay between viral proteins and host transcription regulatory machineries including histone deacetylases (HDACs), histone acetyltransferases (HATs), cyclin dependent kinases (CDKs), and histone methyltransferases (HMTs).

Results

Here, we describe the effect of Tat activated transcription at the G₁/S border of the cell cycle and analyze the interaction of modified Tat with the chromatin remodeling complex, SWI/SNF. HIV-1 LTR DNA reconstituted into nucleosomes can be activated in vitro using various Tat expressing extracts. Optimally activated transcription was observed at the G₁/S border of the cell cycle both in vitro and in vivo, where chromatin remodeling complex, SWI/SNF, was present on the immobilized LTR DNA. Using a number of in vitro binding as well as in vivo chromatin immunoprecipitation (ChIP) assays, we detected the presence of both BRG1 and acetylated Tat in the same complex. Finally, we demonstrate that activated transcription resulted in partial or complete removal of the nucleosome from the start site of the LTR as evidenced by a restriction enzyme accessibility assay.

Conclusion

We propose a model where unmodified Tat is involved in binding to the CBP/p300 and cdk9/cyclin T₁ complexes facilitating transcription initiation. Acetylated Tat dissociates from the TAR RNA structure and recruits bromodomain-binding chromatin modifying complexes such as p/CAF and SWI/SNF to possibly facilitate transcription elongation.

Synergistic functions of SII and p300 in productive activator-dependent transcription of chromatin templates

We have reconstituted a highly purified RNA polymerase II transcription system containing chromatin templates assembled with purified histones and assembly factors, the histone acetyltransferase p300, and components of the general transcription machinery that, by themselves, suffice for activated transcription (initiation and elongation) on DNA templates. We show that this system mediates activator-dependent initiation, but not productive elongation, on chromatin templates. We further report the purification of a chromatin transcription-enabling activity (CTEA) that, in a manner dependent upon p300 and acetyl-CoA, strongly potentiates transcription elongation through several contiguous nucleosomes as must occur in vivo. The transcription elongation factor SII is a major component of CTEA and strongly synergizes with p300 (histone acetylation) at a step subsequent to preinitiation complex formation. The purification of CTEA also identified HMGB2 as a coactivator that, while inactive on its own, enhances SII and p300 functions.

Are viral-encoded microRNAs mediating latent HIV-1 infection?

The Human Immunodeficiency Virus type 1 (HIV-1), a member of the lentivirus subfamily, infects both dividing and nondividing cells and, following reverse transcription of the viral RNA genome, integrates into the host chromatin where it enters into a latent state. Many of the factors governing viral latency remain unresolved and current antiviral treatment regimens are largely ineffective at eliminating cellular reservoirs of latent virus. The recent identification of microRNA (miRNA) encoding sequences embedded in the HIV-1 genome, and the discovery of functional virus-derived miRNAs, suggests a role for RNA Interference (RNAi) in the regulation of HIV-1 gene expression. Recently, the mammalian RNAi machinery was shown to regulate gene expression epigenetically by transcriptional modulation, providing a direct link between RNAi and a mechanism for inducing latency. Interestingly, both HIV-1 Tat, and the host TAR RNA-binding protein (TRBP), bind to the transactivating response (TAR) RNA of HIV-1 and affect the function of RNAi in human cells. Specifically, TRBP, a cofactor in Tat-TAR interactions, is a vital component of Dicer-mediated dsRNA processing. These novel observations support a central role for HIV-1 and associated host factors in regulating cellular RNAi and viral gene expression through RNA directed processes. Thus, HIV-1 may have evolved mechanisms to exploit the RNAi pathway at both the transcriptional and posttranscriptional level to affect and/or maintain a latent infection.

Sp1 and Sp3 regulate the basal transcription of receptor activator of nuclear factor kappa B ligand gene in osteoblasts and bone marrow stromal cells

Receptor activator of nuclear factor kappa B ligand (RANKL), a potent regulator of osteoclast formation and function, is expressed by osteoblasts and bone marrow stromal cells. However, the molecular mechanism underlying RANKL expression in osteoblast/stromal cells remains largely unclear. Here, we characterized the molecular mechanism controlling RANKL basal transcription in osteoblast/stromal cells. We cloned a 1,103-bp murine RANKL promoter (from -953 to +150, relative to the transcription start site). Using a series of deletion mutants of the 1,103-bp promoter, we identified a 100-bp region (-154 to -54) mediating RANKL basal transcription in both osteoblasts and bone marrow stromal cells. Electrophoretic mobility shift assay (EMSA) using five overlapping oligonucleotides (Probes 1-5) spanning the 100-bp region showed that Probes 1 and 2 specifically bound nuclear proteins with high affinity from both cell types. Computer analysis revealed that Probes 1 and 2 contain a putative Sp1-binding site. Supershift assays with Sp1 and Sp3 antibodies confirmed that the nuclear proteins binding to Probes 1 and 2 are Sp1 and Sp3. Functionally, the mutation of the Sp1/Sp3 site in Probe 1 profoundly reduced the basal promoter activity while the mutation of the one in Probe 2 resulted in moderate reduction in the basal promoter activity. Moreover, the mutation of both sites abrogated the RANKL basal promoter activity, indicating that Sp1 and Sp3 play a key role in the RANKL basal transcription in osteoblasts and bone marrow stromal cells.

Transcriptional Activators Are Dispensable for Transcription in the Absence of Spt6-Mediated Chromatin Reassembly of Promoter Regions

The packaging of the eukaryotic genome into chromatin is likely to have a profound influence on transcription from the underlying genes. We have previously shown that the disassembly of promoter nucleosomes is obligatory for activation of the yeast PHO5 and PHO8 genes. Here, we show that the PHO5 promoter nucleosomes are reassembled concomitant with transcriptional repression and displacement of the TATA binding protein and RNA polymerase II (RNA Pol II). We identify the histone H3-H4 chaperone Spt6 as the factor that mediates nucleosome reassembly onto the PHO5, PHO8, ADH2, ADY2, and SUC2 promoters during transcriptional repression. Furthermore, promoter nucleosome reassembly is essential for transcriptional repression. In the absence of Spt6-mediated nucleosome reassembly, the activators Pho4 and Pho2 are displaced from the PHO5 promoter in repressing conditions, yet transcription is sustained. As such, these studies demonstrate that activators are not required for transcription in the absence of competing chromatin reassembly.

NF-kappaB p50 promotes HIV latency through HDAC recruitment and repression of transcriptional initiation

Cells latently infected with HIV represent a currently insurmountable barrier to viral eradication in infected patients. Using the J-Lat human T-cell model of HIV latency, we have investigated the role of host factor binding to the kappaB enhancer elements of the HIV long terminal repeat (LTR) in the maintenance of viral latency. We show that NF-kappaB p50-HDAC1 complexes constitutively bind the latent HIV LTR and induce histone deacetylation and repressive changes in chromatin structure of the HIV LTR, changes that impair recruitment of RNA polymerase II and transcriptional initiation. Knockdown of p50 expression with specific small hairpin RNAs reduces HDAC1 binding to the latent HIV LTR and induces RNA polymerase II recruitment. Similarly, inhibition of histone deacetylase (HDAC) activity with trichostatin A promotes binding of RNA polymerase II to the latent HIV LTR. This bound polymerase complex, however, remains non-processive, generating only short viral transcripts. Synthesis of full-length viral transcripts can be rescued under these conditions by expression of Tat. The combination of HDAC inhibitors and Tat merits consideration as a new strategy for purging latent HIV proviruses from their cellular reservoirs.

Strategies for the reconstitution of chromatin

In eukaryotes, chromatin is the natural form of DNA in the nucleus. For hundreds of millions of years, DNA-binding factors have evolved with chromatin. It is therefore more desirable to study the molecular mechanisms of DNA-directed processes with chromatin than with naked DNA templates. To this end, it is necessary to reconstitute DNA and histones into chromatin. Fortunately, there are a variety of methods by which a nonspecialist can prepare chromatin of high quality. Here, we describe strategies and techniques for the reconstitution of chromatin in vitro.

RON Receptor Tyrosine Kinase, a Negative Regulator of Inflammation, Inhibits HIV-1 Transcription in Monocytes/Macrophages and Is Decreased in Brain Tissue from Patients with AIDS

Activation of macrophages and microglia cells after HIV-1 infection and their production of inflammatory mediators contribute to HIV-associated CNS diseases. The mechanisms that initiate and maintain inflammation after HIV-1 infection in the brain have not been well studied. Furthermore, it is not understood why in HIV-associated CNS disease, macrophages and microglia are biased toward inflammation rather than production of mediators that control inflammation. We have focused on the receptor tyrosine kinase RON, a critical negative regulator of macrophage function and inflammation, to determine whether this receptor regulates HIV-1 expression. Overexpressing RON in monocytes/macrophages demonstrates that RON inhibits HIV-1 proviral transcription in part by decreasing the binding activity of NF-kappaB to the HIV-1 long terminal repeat. Because macrophages and microglia cells are a critical reservoir for HIV-1 in the CNS, we examined brain tissues for RON expression and detected RON in astrocytes, cortical neurons, and monocytoid cells. RON was detected in all control patients who were HIV seronegative (n = 7), whereas six of nine brain samples obtained from AIDS patients exhibited reduced RON protein. These data suggest that RON initiates signaling pathways that negatively regulate HIV-1 transcription in monocytes/macrophages and that HIV-1 suppresses RON function by decreasing protein levels in the brain to assure efficient replication. Furthermore, HIV-1 infection would compromise the ability of RON to protect against inflammation and consequent CNS damage.

Differential regulation of human immunodeficiency virus type 2 and simian immunodeficiency virus promoter activity

Promoter activity of the HIV-1 long terminal repeat (LTR) is largely dependent on intact NF-kB and SpI binding sites in the U3 region. In contrast, upstream LTR sequences allow efficient simian immunodeficiency virus (SIVmac) transcription in the absence of the core enhancer promoter region. In the present study, we investigated whether the regulation of HIV-2 Rod LTR activity is more reminiscent of HIV-1 having the same host or of SIVmac239 belonging to the same phylogenetic group. Viral promoter activity was studied in the context of the integrated provirus using both single cycle assays with pseudotyped luciferase reporter viruses and replication-competent HIV-2 LTR mutants. Our results demonstrate that intact SpI binding sites are important for both HIV-2 and SIVmac LTR activity in T cells and monocyte-derived macrophages. In contrast, deletion of the NF-kB binding site or of upstream regulatory sequences impaired HIV-2 Rod LTR activity but had little effect on SIVmac239 promoter function. Thus, similar to HIV-1, regulation of HIV-2 LTR promoter activity shows a low degree of functional redundancy possibly suggesting a specific adaptation to the human host.

Prostratin antagonizes HIV latency by activating NF-kappaB

A subset of quiescent memory CD4 T cells harboring integrated but transcriptionally silent proviruses poses a currently insurmountable barrier to the eradication of the human immunodeficiency virus (HIV) in infected patients. Induction of HIV gene expression in these latently infected cells by immune activating agents has been proposed as one approach to confer sensitivity to antiretroviral therapy. Interest has recently focused on the non-tumor-promoting phorbol ester, prostratin, as a potential agent to activate latent HIV proviruses. Using multiple Jurkat T cell lines containing integrated but transcriptionally latent HIV proviruses (J-Lat cells), we now demonstrate that prostratin effectively activates HIV gene expression in these latently infected cells. We further show that prostratin acts by stimulating IKK-dependent phosphorylation and degradation of IkappaBalpha, leading to the rapid nuclear translocation of NF-kappaB and activation of the HIV-1 long terminal repeat in a kappaB enhancer-dependent manner. In contrast, NFAT and AP-1 are not induced by prostratin. Using chromatin immunoprecipitation assays to identify host transcription factors recruited to the latent HIV-1 promoter in living cells, we find that prostratin induces RelA binding. Analysis of potential upstream signal transducers demonstrates that prostratin stimulates membrane translocation of classical, novel, and atypical protein kinase C (PKC) isoforms. Studies with isoform-specific PKC inhibitors suggest that the novel PKCs play a particularly prominent role in the prostratin response. These findings provide new insights into the molecular pathway through which prostratin antagonizes HIV latency highlighting a central role for the action of NF-kappaB.