HIV-1 tat transactivator recruits p300 and CREB-binding protein histone acetyltransferases to the viral promoter

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.

SIRT1 regulates Tat-induced HIV-1 transactivation through activating AMP-activated protein kinase

Transcription of human immunodeficiency virus (HIV-1) is activated by viral Tat protein which regulates HIV-long terminal repeat (LTR) transcription and elongation. HIV-1 Tat protein is a substrate for the deacetylase activity of sirtuin 1 (SIRT1). Here we investigate the signaling pathway involved in Tat-induced HIV-1 transactivation through SIRT1. Western blot analysis showed a significant reduction in AMPK activation and downstream acetyl-CoA carboxylase (ACC) activation in response to Tat treatment. NAD(+) levels and SIRT1 activity were also decreased with Tat treatment. SIRT1 activator resveratrol reversed Tat-mediated reduction in AMPK activation and downstream ACC activation; while SIRT1 inhibitor nicotinamide or knockdown of SIRT1 by siRNA potentiated Tat-mediated reduction in AMPK activation and downstream ACC activation. Consistent with this association, AMPK activator AICAR as well as resveratrol inhibited Tat-induced HIV-1 transactivation. On the contrary, AMPK inhibitor compound C, knockdown of AMPK by siRNA as well as nicotinamide or knockdown of SIRT1 by siRNA potentiated Tat-induced HIV-1 transactivation. Collectively, our data provide new insights into understanding of the molecular mechanisms of Tat-regulated transcription, suggesting that targeting SIRT1-AMPK pathway could serve as a new target for the development of new anti HIV-1 agents.

Mechanism of HIV-1 Tat RNA translation and its activation by the Tat protein

BACKGROUND

The human immunodeficiency virus type 1 (HIV-1) Tat protein is a major viral transactivator required for HIV-1 replication. In the nucleus Tat greatly stimulates the synthesis of full-length transcripts from the HIV-1 promoter by causing efficient transcriptional elongation. Tat induces elongation by directly interacting with the bulge of the transactivation response (TAR) RNA, a hairpin-loop located at the 5'-end of all nascent viral transcripts, and by recruiting cellular transcriptional co-activators. In the cytoplasm, Tat is thought to act as a translational activator of HIV-1 mRNAs. Thus, Tat plays a central role in the regulation of HIV-1 gene expression both at the level of mRNA and protein synthesis. The requirement of Tat in these processes poses an essential question on how sufficient amounts of Tat can be made early on in HIV-1 infected cells to sustain its own synthesis. To address this issue we studied translation of the Tat mRNA in vitro and in human cells using recombinant monocistronic and dicistronic RNAs containing the 5' untranslated region (5'-UTR) of Tat RNA.

RESULTS

This study shows that the Tat mRNA can be efficiently translated both in vitro and in cells. Furthermore, our data suggest that translation initiation from the Tat mRNA probably occurs by a internal ribosome entry site (IRES) mechanism. Finally, we show that Tat protein can strongly stimulate translation from its cognate mRNA in a TAR dependent fashion.

CONCLUSION

These results indicate that Tat mRNA translation is efficient and benefits from a feedback stimulation by the Tat protein. This translational control mechanism would ensure that minute amounts of Tat mRNA are sufficient to generate enough Tat protein required to stimulate HIV-1 replication.

Tat-induced FOXO3a is a key mediator of apoptosis in HIV-1-infected human CD4+ T lymphocytes

The high mutation rate of HIV is linked to the generation of viruses expressing proteins with altered function whose impact on disease progression is unknown. We investigated how HIV-1 viruses lacking Env, Vpr, and Nef affect CD4(+) T cell survival. We found that in the absence of these proteins, HIV-1-infected CD4(+) primary T cells progress to the G(0) phase of the cell cycle and to cell death, indicating that viruses expressing inactive forms of these proteins can contribute to the CD4(+) T cell decline as the wild-type virus, suggesting that other HIV proteins are responsible for inducing apoptosis. Apoptosis in these cells is triggered by the alteration of the Egr1-PTEN-Akt (early growth response-1/phosphate and tensin homolog deleted on chromosome 10/Akt) and p53 pathways, which converge on the FOXO3a (Forkhead box transcription factor O class 3a) transcriptional activator. The FOXO3a target genes Fas ligand and TRAIL, involved in the extrinsic apoptotic pathway, and PUMA, Noxa, and Bim, which are part of the intrinsic apoptotic pathway, were also up-regulated, indicating that HIV infection leads to apoptosis by the engagement of multiple apoptotic pathways. RNAi-mediated knockdown of Egr1 and FOXO3a resulted in reduced apoptosis in HIV-infected HeLa and CD4(+) T cells, providing further evidence for their critical role in HIV-induced apoptosis and G(0) arrest. We tested the possibility that Tat is responsible for the T cell apoptosis observed with these mutant viruses. The induction of Egr1 and FOXO3a and its target genes was observed in Jurkat cells transduced by Tat alone. Tat-dependent activation of the Egr1-PTEN-FOXO3a pathway provides a mechanism for HIV-1-associated CD4(+) T cell death.

HIV-1 infection and the PPARγ-dependent control of adipose tissue physiology

PPARγ is a ligand-dependent master transcription factor controlling adipocyte differentiation as well as multiple biological processes taking place in other cells present in adipose tissue depots such as macrophages. Recent research indicates that HIV-1 infection-related events may alter adipose tissue biology through several mechanisms involving PPARγ, ranging from direct effects of HIV-1-encoded proteins on adipocytes to the promotion of a proinflammatory environment that interferes with PPARγ actions. This effect of HIV-1 on adipose tissue cells can occur even in the absence of direct infection of adipocytes, as soluble HIV-1-encoded proteins such as Vpr may enter cells and inhibit PPARγ action. Moreover, repression of PPARγ actions may relieve inhibitory pathways of HIV-1 gene transcription, thus enhancing HIV-1 effects in infected cells. HIV-1 infection-mediated interference of PPARγ-dependent pathways in adipocytes and other cells inside adipose depots such as macrophages is likely to create an altered local environment that, after antiretroviral treatment, leads to lipodystrophy in HIV-1-infected and HAART-treated patients.

Intragenic transcriptional cis-activation of the human immunodeficiency virus 1 does not result in allele-specific inhibition of the endogenous gene

BACKGROUND

The human immunodeficiency virus type 1 (HIV-1) favors integration in active genes of host chromatin. It is believed that transcriptional interference of the viral promoter over the endogenous gene or vice versa might occur with implications in HIV-1 post-integrative transcriptional latency.

RESULTS

In this work a cell line has been transduced with a HIV-based vector and selected for Tat-inducible expression. These cells were found to carry a single silent integration in sense orientation within the second intron of the HMBOX1 gene. The HIV-1 Tat transactivator induced the viral LTR and repressed HMBOX1 expression independently of vector integration. Instead, single-cell quantitative in situ hybridization revealed that allele-specific transcription of HMBOX1 carrying the integrated provirus was not affected by the transactivation of the viral LTR in cis.

CONCLUSION

A major observation of the work is that the HIV-1 genome has inserted in genes that are also repressed by Tat and this could be an advantage for the virus during transcriptional reactivation. In addition, it has also been observed that transcription of the provirus and of the endogenous gene in which it is integrated may coexist at the same time in the same genomic location.

Reversal of HIV-1 latency with anti-microRNA inhibitors

Human immunodeficiency virus type 1 (HIV-1) latency is achieved when host cells contain integrated proviral DNA but do not produce viral particles. The virus remains in resting CD4 T-lymphocytes, evading host immune surveillance and antiviral drugs. When resting cells are activated, infectious viral particles are produced. Latency is critical for the survival of all HIV-1 strains in vivo. Recently, it has been reported that a cluster of cellular microRNAs (miRNAs) enriched specifically in resting CD4+ T-cells suppresses translation of most HIV-1-encoded proteins in the cytoplasm, sustaining HIV-1 escape from the host immune response. Complementary antisense miRNA inhibitors block the inhibitory effect of miRNAs and drive viral production from the resting T-lymphocytes without activating the cells. Therefore, inhibition of these HIV-1-specific cellular miRNAs is of great therapeutic significance for eliminating the HIV-1 reservoir in HIV-1-infected individuals receiving suppressive highly active antiretroviral therapy (HAART).

CDK13, a new potential human immunodeficiency virus type 1 inhibitory factor regulating viral mRNA splicing

The human immunodeficiency virus type 1 (HIV-1) Tat is a 14-kDa viral protein that acts as a potent transactivator by binding to the transactivation-responsive region, a structured RNA element located at the 5' end of all HIV-1 transcripts. Tat transactivates viral gene expression by inducing the phosphorylation of the C-terminal domain of RNA polymerase II through several Tat-activated kinases and by recruiting chromatin-remodeling complexes and histone-modifying enzymes to the HIV-1 long terminal repeat. Histone acetyltransferases, including p300 and hGCN5, not only acetylate histones but also acetylate Tat at lysine positions 50 and 51 in the arginine-rich motif. Acetylated Tat at positions 50 and 51 interacts with a specialized protein module, the bromodomain, and recruits novel factors having this particular domain, such as P/CAF and SWI/SNF. In addition to having its effect on transcription, Tat has been shown to be involved in splicing. In this study, we demonstrate that Tat interacts with cyclin-dependent kinase 13 (CDK13) both in vivo and in vitro. We also found that CDK13 increases HIV-1 mRNA splicing and favors the production of the doubly spliced protein Nef. In addition, we demonstrate that CDK13 acts as a possible restriction factor, in that its overexpression decreases the production of the viral proteins Gag and Env and subsequently suppresses virus production. Using small interfering RNA against CDK13, we show that silencing of CDK13 leads to a significant increase in virus production. Finally, we demonstrate that CDK13 mediates its effect on splicing through the phosphorylation of ASF/SF2.

The HIV-1 transcriptional activator Tat has potent nucleic acid chaperoning activities in vitro

The human immunodeficiency virus type 1 (HIV-1) is a primate lentivirus that causes the acquired immunodeficiency syndrome (AIDS). In addition to the virion structural proteins and enzyme precursors, that are Gag, Env and Pol, HIV-1 encodes several regulatory proteins, notably a small nuclear transcriptional activator named Tat. The Tat protein is absolutely required for virus replication since it controls proviral DNA transcription to generate the full-length viral mRNA. Tat can also regulate mRNA capping and splicing and was recently found to interfere with the cellular mi- and siRNA machinery. Because of its extensive interplay with nucleic acids, and its basic and disordered nature we speculated that Tat had nucleic acid-chaperoning properties. This prompted us to examine in vitro the nucleic acid-chaperoning activities of Tat and Tat peptides made by chemical synthesis. Here we report that Tat has potent nucleic acid-chaperoning activities according to the standard DNA annealing, DNA and RNA strand exchange, RNA ribozyme cleavage and trans-splicing assays. The active Tat(44–61) peptide identified here corresponds to the smallest known sequence with DNA/RNA chaperoning properties.

HIV-1 latency in actively dividing human T cell lines

Background

Eradication of HIV-1 from an infected individual cannot be achieved by current drug regimens. Viral reservoirs established early during the infection remain unaffected by anti-retroviral therapy and are able to replenish systemic infection upon interruption of the treatment. Therapeutic targeting of viral latency will require a better understanding of the basic mechanisms underlying the establishment and long-term maintenance of HIV-1 in resting memory CD4 T cells, the most prominent reservoir of transcriptional silent provirus. However, the molecular mechanisms that permit long-term transcriptional control of proviral gene expression in these cells are still not well understood. Exploring the molecular details of viral latency will provide new insights for eventual future therapeutics that aim at viral eradication.

Results

We set out to develop a new in vitro HIV-1 latency model system using the doxycycline (dox)-inducible HIV-rtTA variant. Stable cell clones were generated with a silent HIV-1 provirus, which can subsequently be activated by dox-addition. Surprisingly, only a minority of the cells was able to induce viral gene expression and a spreading infection, eventhough these experiments were performed with the actively dividing SupT1 T cell line. These latent proviruses are responsive to TNFα treatment and alteration of the DNA methylation status with 5-Azacytidine or genistein, but not responsive to the regular T cell activators PMA and IL2. Follow-up experiments in several T cell lines and with wild-type HIV-1 support these findings.

Conclusion

We describe the development of a new in vitro model for HIV-1 latency and discuss the advantages of this system. The data suggest that HIV-1 proviral latency is not restricted to resting T cells, but rather an intrinsic property of the virus.

Acetylation of conserved lysines in the catalytic core of cyclin-dependent kinase 9 inhibits kinase activity and regulates transcription

Promoter clearance and transcriptional processivity in eukaryotic cells are fundamentally regulated by the phosphorylation of the carboxy-terminal domain of RNA polymerase II (RNAPII). One of the kinases that essentially performs this function is P-TEFb (positive transcription elongation factor b), which is composed of cyclin-dependent kinase 9 (CDK9) associated with members of the cyclin T family. Here we show that cellular GCN5 and P/CAF, members of the GCN5-related N-acetyltransferase family of histone acetyltransferases, regulate CDK9 function by specifically acetylating the catalytic core of the enzyme and, in particular, a lysine that is essential for ATP coordination and the phosphotransfer reaction. Acetylation markedly reduces both the kinase function and transcriptional activity of P-TEFb. In contrast to unmodified CDK9, the acetylated fraction of the enzyme is specifically found in the insoluble nuclear matrix compartment. Acetylated CDK9 associates with the transcriptionally silent human immunodeficiency virus type 1 provirus; upon transcriptional activation, it is replaced by the unmodified form, which is involved in the elongating phase of transcription marked by Ser2-phosphorylated RNAPII. Given the conservation of the CDK9 acetylated residues in the catalytic task of virtually all CDK proteins, we anticipate that this mechanism of regulation might play a broader role in controlling the function of other members of this kinase family.

The histone chaperone protein Nucleosome Assembly Protein-1 (hNAP-1) binds HIV-1 Tat and promotes viral transcription

Background

Despite the large amount of data available on the molecular mechanisms that regulate HIV-1 transcription, crucial information is still lacking about the interplay between chromatin conformation and the events that regulate initiation and elongation of viral transcription. During transcriptional activation, histone acetyltransferases and ATP-dependent chromatin remodeling complexes cooperate with histone chaperones in altering chromatin structure. In particular, human Nucleosome Assembly Protein-1 (hNAP-1) is known to act as a histone chaperone that shuttles histones H2A/H2B into the nucleus, assembles nucleosomes and promotes chromatin fluidity, thereby affecting transcription of several cellular genes.

Results

Using a proteomic screening, we identified hNAP-1 as a novel cellular protein interacting with HIV-1 Tat. We observed that Tat specifically binds hNAP1, but not other members of the same family of factors. Binding between the two proteins required the integrity of the basic domain of Tat and of two separable domains of hNAP-1 (aa 162–290 and 290–391). Overexpression of hNAP-1 significantly enhanced Tat-mediated activation of the LTR. Conversely, silencing of the protein decreased viral promoter activity. To explore the effects of hNAP-1 on viral infection, a reporter HIV-1 virus was used to infect cells in which hNAP-1 had been either overexpressed or knocked-down. Consistent with the gene expression results, these two treatments were found to increase and inhibit viral infection, respectively. Finally, we also observed that the overexpression of p300, a known co-activator of both Tat and hNAP-1, enhanced hNAP-1-mediated transcriptional activation as well as its interaction with Tat.

Conclusion

Our study reveals that HIV-1 Tat binds the histone chaperone hNAP-1 both in vitro and in vivo and shows that this interaction participates in the regulation of Tat-mediated activation of viral gene expression.

Regulation of HIV-1 latency by T-cell activation

HIV-infected patients harbor approximately 10(5)-10(6) memory CD4 T-cells that contain fully integrated but transcriptionally silent HIV proviruses. While small in number, these latently infected cells form a drug-insensitive reservoir that importantly contributes to the life-long persistence of HIV despite highly effective antiviral therapy. In tissue culture, latent HIV proviruses can be activated when their cellular hosts are exposed to select proinflammatory cytokines or their T-cell receptors are ligated. However, due to a lack of potency and/or dose-limiting toxicity, attempts to purge virus from this latent reservoir in vivo with immune-activating agents, such as anti-CD3 antibodies and IL-2, have failed. A deeper understanding of the molecular underpinnings of HIV latency is clearly required, including determining whether viral latency is actively reinforced by transcriptional repressors, defining which inducible host transcription factors most effectively antagonize latency, and elucidating the role of chromatin in viral latency. Only through such an improved understanding will it be possible to identify combination therapies that might allow complete purging of the latent reservoir and to realize the difficult and elusive goal of complete eradication of HIV in infected patients.

Fruit-specific expression of the human immunodeficiency virus type 1 tat gene in tomato plants and its immunogenic potential in mice

The human immunodeficiency virus type 1 (HIV-1) Tat protein is considered a potential candidate vaccine antigen. In an effort to design a strategy for noninvasive vaccination against HIV-1, we developed transgenic tomatoes expressing the Tat protein. Two independent plants testing positive in transgene detection analysis were selected and grown to maturity. Monoclonal antibodies against Tat recognized a protein of the expected size. Interestingly, expression of Tat seemed to be toxic to the plant, as in all cases the fruit exhibited underdeveloped reproductive structures and no seeds. Nine groups of 10 pathogen-free BALB/c male mice were primed either orally, intraperitoneally, or intramuscularly with 10 mg of tomato fruit extract derived from transgenic or wild-type plants and with 10 microg of Tat86 recombinant protein. Mice were immunized at days 0, 14, and 28, and given boosters after 15 weeks; sera were drawn 7 days after each booster, and the antibody titer was determined by enzyme-linked immunosorbent assay. All three immunization approaches induced the development of a strong anti-Tat immunological response, which increased over time. Isotype subclass determination showed the presence of mucosal (immunoglobulin A) immunity soon after the beginning of the oral immunization protocol, and the data were confirmed by the presence of anti-Tat antibodies in fecal pellets and in vaginal washes. We also demonstrated that sera from immunized mice inhibited with high efficiency recombinant Tat-dependent transactivation of the HIV-1 long terminal repeat promoter. This neutralization activity might be relevant for the suppression of extracellular Tat activities, which play an important role in HIV disease development.

The Werner Syndrome Helicase Is a Cofactor for HIV-1 Long Terminal Repeat Transactivation and Retroviral Replication

The Werner syndrome helicase (WRN) participates in DNA replication, double strand break repair, telomere maintenance, and p53 activation. Mutations of wrn cause Werner syndrome (WS), an autosomal recessive premature aging disorder associated with cancer predisposition, atherosclerosis, and other aging related symptoms. Here, we report that WRN is a novel cofactor for HIV-1 replication. Immortalized human WRN(-/-) WS fibroblasts, lacking a functional wrn gene, are impaired for basal and Tat-activated HIV-1 transcription. Overexpression of wild-type WRN transactivates the HIV-1 long terminal repeat (LTR) in the absence of Tat, and WRN cooperates with Tat to promote high-level LTR transactivation. Ectopic WRN induces HIV-1 p24(Gag) production and retroviral replication in HIV-1-infected H9(HIV-1IIIB) lymphocytes. A dominant-negative helicase-minus mutant, WRN(K577M), inhibits LTR transactivation and HIV-1 replication. Inhibition of endogenous WRN, through co-expression of WRN(K577M), diminishes recruitment of p300/CREB-binding protein-associated factor (PCAF) and positive transcription elongation factor b (P-TEFb) to Tat/transactivation response-RNA complexes, and immortalized WRN(-/-) WS fibroblasts exhibit comparable defects in recruitment of PCAF and P-TEFb to the HIV-1 LTR. Our results demonstrate that WRN is a novel cellular cofactor for HIV-1 replication and suggest that the WRN helicase participates in the recruitment of PCAF/P-TEFb-containing transcription complexes. WRN may be a plausible target for antiretroviral therapy.

Suv39H1 and HP1gamma are responsible for chromatin-mediated HIV-1 transcriptional silencing and post-integration latency

HIV-1 gene expression is the major determinant regulating the rate of virus replication and, consequently, AIDS progression. Following primary infection, most infected cells produce virus. However, a small population becomes latently infected and constitutes the viral reservoir. This stable viral reservoir seriously challenges the hope of complete viral eradication. Viewed in this context, it is critical to define the molecular mechanisms involved in the establishment of transcriptional latency and the reactivation of viral expression. We show that Suv39H1, HP1gamma and histone H3Lys9 trimethylation play a major role in chromatin-mediated repression of integrated HIV-1 gene expression. Suv39H1, HP1gamma and histone H3Lys9 trimethylation are reversibly associated with HIV-1 in a transcription-dependent manner. Finally, we show in different cellular models, including PBMCs from HIV-1-infected donors, that HIV-1 reactivation could be achieved after HP1gamma RNA interference.

Recruitment and activation of RSK2 by HIV-1 Tat

The transcriptional activity of the integrated HIV provirus is dependent on the chromatin organization of the viral promoter and the transactivator Tat. Tat recruits the cellular pTEFb complex and interacts with several chromatin-modifying enzymes, including the histone acetyltransferases p300 and PCAF. Here, we examined the interaction of Tat with activation-dependent histone kinases, including the p90 ribosomal S6 kinase 2 (RSK2). Dominant-negative RSK2 and treatment with a small-molecule inhibitor of RSK2 kinase activity inhibited the transcriptional activity of Tat, indicating that RSK2 is important for Tat function. Reconstitution of RSK2 in cells from subjects with a genetic defect in RSK2 expression (Coffin-Lowry syndrome) enhanced Tat transactivation. Tat interacted with RSK2 and activated RSK2 kinase activity in cells. Both properties were lost in a mutant Tat protein (F38A) that is deficient in HIV transactivation. Our data identify a novel reciprocal regulation of Tat and RSK2 function, which might serve to induce early changes in the chromatin organization of the HIV LTR.

Interaction of papillomavirus E2 protein with the Brm chromatin remodeling complex leads to enhanced transcriptional activation

Papillomavirus E2 is a sequence-specific DNA binding protein that regulates transcription and replication of the viral genome. The transcriptional activities of E2 are typically evaluated by transient transfection of nonreplicating E2-dependent reporters. We sought to address whether E2 activates transcription in an episomal context and its potential interaction with the chromatin remodeling proteins. Using an Epstein-Barr virus-based episomal reporter, we demonstrate that E2 stimulates transcription from an E2-dependent promoter in a chromatin context. This activation is enhanced by the presence of proteins associated with SWI/SNF complexes, which are ATP-dependent chromatin remodeling enzymes. We show that exogenous expression of the Brm ATPase enhances E2 activity in SWI/SNF-deficient cell lines and that the amino-terminal transactivation domain of E2 mediates association with the Brm complex in vivo. Using chromatin immunoprecipitation assays, we demonstrate that Brm enhances promoter occupancy by E2 in an episomal context. Our results demonstrate that E2 activates transcription from an episomal reporter system and reveal a novel property of E2 in collaborating with the Brm chromatin remodeling complex in enhancing transcriptional activation.

Novel function of prothymosin alpha as a potent inhibitor of human immunodeficiency virus type 1 gene expression in primary macrophages

CD8(+) T lymphocytes control human immunodeficiency virus type 1 (HIV-1) infection by a cytotoxic major histocompatibility complex-restricted pathway as well as by secretion of noncytotoxic soluble inhibitory factors. Several components of CD8(+) cell supernatants have been identified that contribute to the latter activity. In this study we report that prothymosin alpha (ProTalpha), a protein found in the cell culture medium of the herpesvirus saimiri-transformed CD8(+) T-cell line, K#1 50K, has potent HIV-1-inhibitory activity. Depletion of native ProTalpha from an HIV-1-inhibitory fraction of CD8(+) cell supernatants removes the inhibitory activity, supporting its role in inhibition via soluble mediators. ProTalpha is an abundant, acidic peptide that has been reported to be localized in the nucleus and associated with cell proliferation and activation of transcription. In this report we demonstrate that ProTalpha suppresses HIV-1 replication, its activity is target cell specific, and inhibition occurs following viral integration. Native and recombinant ProTalpha protein potently inhibit HIV-1 long terminal repeat (LTR)-driven gene expression in macrophages. Furthermore studies using different promoters in lentiviral vectors (cytomegalovirus and phosphoglycerate kinase) revealed that suppression of viral replication by ProTalpha is not HIV LTR specific.

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.

Comparative analysis of HIV-1 Tat variants

HIV-1 Tat protein is a crucial element for viral replication; therefore, its inhibition might be exploited against the AIDS infection. To gain insights on the natural variability of this protein, we present a comparative investigation on the relationship between the primary sequences and the experimentally available three-dimensional structures from the HIV-1 Tat variants Z2, BRU, and MAL. Our computational tools include sequence conservation algorithms, structural analysis, electrostatic modeling, and molecular dynamics (MD) simulations. We find that two regions located between residues 10-18 and 41-52 display the highest primary sequence conservation, while the most conserved region among the available structures corresponds approximately to the segment between positions approximately 44 and 50. Furthermore, in spite of their large structural divergence, Tat variants share a common mode for long-range intramolecular interactions. Finally, the flexibility of the Z2, BRU, and MAL variants, as emerging from multinanosecond MD simulations, is rather similar. Based on this work, we conclude that the turnlike region between amino acids 44 and 50 is structurally most conserved, emerging as an important motif for pharmaceutical targeting aimed toward inhibiting Tat action.

The SWI/SNF chromatin-remodeling complex is a cofactor for Tat transactivation of the HIV promoter

Tat is a critical viral transactivator essential for human immunodeficiency virus (HIV) gene expression. Activation involves binding to an RNA stem-loop structure and recruitment of the positive transcription elongation factor b. Tat also induces the remodeling of a single nucleosome in the HIV promoter. However, the mechanism of this remodeling has remained unclear. Knockdown of INI-1 and BRG-1, two components of the SWI/SNF chromatin-remodeling complex, suppressed Tat-mediated transactivation. Cells lacking INI-1 (G401 and MON) or BRG-1 (C33A) exhibited defective transactivation by Tat that was restored upon INI-1 and BRG-1 expression, respectively. Tat was co-immunoprecipitated with several SWI/SNF subunits, including INI-1, BRG-1, and beta-actin. The SWI/SNF complex interacted with the integrated HIV promoter in a Tat-dependent manner. We also found that INI-1 and BRG-1 synergized with the p300 acetyltransferase to activate the HIV promoter. This synergism depended on the acetyltransferase activity of p300 and on Tat Lys(50) and Lys(51). In conclusion, Tat-mediated activation of the HIV promoter requires the SWI/SNF complex in synergy with the coactivator p300.

Acetylated Tat regulates human immunodeficiency virus type 1 splicing through its interaction with the splicing regulator p32

The human immunodeficiency virus type 1 (HIV-1) potent transactivator Tat protein mediates pleiotropic effects on various cell functions. Posttranslational modification of Tat affects its activity during viral transcription. Tat binds to TAR and subsequently becomes acetylated on lysine residues by histone acetyltransferases. Novel protein-protein interaction domains on acetylated Tat are then established, which are necessary for both sustained transcriptional activation of the HIV-1 promoter and viral transcription elongation. In this study, we investigated the identity of proteins that preferentially bound acetylated Tat. Using a proteomic approach, we identified a number of proteins that preferentially bound AcTat, among which p32, a cofactor of splicing factor ASF/SF-2, was identified. We found that p32 was recruited to the HIV-1 genome, suggesting a mechanism by which acetylation of Tat may inhibit HIV-1 splicing needed for the production of full-length transcripts. Using Tat from different clades, harboring a different number of acetylation sites, as well as Tat mutated at lysine residues, we demonstrated that Tat acetylation affected splicing in vivo. Finally, using confocal microscopy, we found that p32 and Tat colocalize in vivo in HIV-1-infected cells.

Melatonin suppresses macrophage cyclooxygenase-2 and inducible nitric oxide synthase expression by inhibiting p52 acetylation and binding

Melatonin has been shown to be produced by nonpineal cells and possess anti-inflammatory actions in animal models. In the present study, we tested the hypothesis that melatonin suppresses the expression of proinflammatory genes such as cyclooxygenase-2 (COX2) and inducible nitric oxide synthase (INOS) by a common transcriptional mechanism. Melatonin but not tryptophan or serotonin inhibited lipopolysaccharide (LPS)-induced COX-2 and iNOS protein levels and promoter activities in RAW 264.7 cells in a time- and concentration-dependent manner. LPS or LPS plus interferon-gamma (IFNgamma) increased binding of all 5 isoforms of NF-kappaB to COX-2 and iNOS promoters. Melatonin selectively inhibited p52 binding without affecting p100 expression, p52 generation from p100, or p52 nuclear translocation. p52 acetylation was enhanced by LPS, which was abrogated by melatonin. Melatonin inhibited p300 histone acetyltransferase (HAT) activity and abrogated p300-augmented COX-2 and iNOS expression. HAT inhibitors suppressed LPS-induced p52 binding and acetylation to an extent similar to melatonin, and melatonin did not potentiate the effect of HAT inhibitors. These results suggest that melatonin inhibits COX-2 and iNOS transcriptional activation by inhibiting p300 HAT activity, thereby suppressing p52 acetylation, binding, and transactivation.

Requirement for SWI/SNF chromatin-remodeling complex in Tat-mediated activation of the HIV-1 promoter

Activation of the human immunodeficiency virus type-1 (HIV-1) promoter in infected cells requires the sequential recruitment of several cellular factors to facilitate the formation of a processive elongation complex. The nucleosomal reorganization of the HIV-1 long terminal repeat (LTR) observed upon Tat stimulation suggests that chromatin-remodeling complexes could play a role during this process. Here, we reported that Tat interacts directly with Brm, a DNA-dependent ATPase subunit of the SWI/SNF chromatin-remodeling complex, to activate the HIV-1 LTR. Inhibition of Brm via small interfering RNAs impaired Tat-mediated transactivation of an integrated HIV-1 promoter. Furthermore, Brm is recruited in vivo to the HIV-1 LTR in a Tat-dependent manner. Interestingly, we found that Tat/Brm interaction is regulated by Tat lysine 50 acetylation. These data show the requirement of Tat-mediated recruitment of SWI/SNF chromatin-remodeling complex to HIV-1 promoter in the activation of the LTR.

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.

Insights on HIV-1 Tat:P/CAF bromodomain molecular recognition from in vivo experiments and molecular dynamics simulations

Structural and functional studies indicate that, through its bromodomain, the cellular acetyltransferase P/CAF binds the acetylated Tat protein of human immunodeficiency virus type 1 (HIV-1) and promotes transcriptional activation of the integrated provirus. Based on the NMR structure of P/CAF complexed with an acetylated Tat peptide, here we use molecular dynamics simulations to construct a model describing the interaction between full length Tat and the P/CAF bromodomain. Our calculations show that the protein-protein interface involves hydrophobic interactions between the P/CAF ZA loop and the Tat core domain. In particular, tyrosines 760 and 761 of P/CAF, two residues that are highly conserved in most known bromodomains, play an essential role for the binding. Fluorescence resonance energy transfer (FRET) experiments performed in this work demonstrate that P/CAF proteins in which these tyrosines are mutated into hydrophilic residues neither bind to Tat inside the cells nor mediate Tat transactivation. The combination of theoretical and in vivo studies provides new insights into the specificity of bromodomain recognition.

The HIV-1 Tat protein enhances megakaryocytic commitment of K562 cells by facilitating CREB transcription factor coactivation by CBP

The human immunodeficiency virus type 1 (HIV-1) Tat protein regulates transcription factor functions and alters cellular gene expression. Because hematopoietic progenitor cell (HPC) differentiation requires activation of lineage-specific transcription factors, Tat may affect hematopoiesis in HIV-1-infected micro-environments. We have monitored the molecular effects of Tat on megakaryocytic differentiation in the HPC line, K562. Flow cytometry analysis of CD61 indicated that phorbol myristate acetate (PMA) (16 nM) stimulated megakaryocytic commitment of K562 cells was increased (3- to 4-fold) following exposure to Tat (1-100 ng/ml). Activation of the megakaryocytic transcription factor cAMP regulatory element binding protein (CREB) and its coactivation by the CREB binding protein (CBP) was subsequently monitored. CREB phosphorylation and DNA binding were measured by Western immunodetection and electrophoretic mobility shift assays (EMSA), respectively. Within 2 hrs after stimulation, Tat increased both CREB phosphorylation and DNA binding by 7- to 10-fold. Transient cotransfection with CREB reporter and CBP expression plasmids demonstrated that Tat treatment increases (3- to 4-fold) both PMA-stimulated and CBP-mediated transcription via the cAMP regulatory element. Histone acetyl transferase (HAT) activity was increased (8- to 10-fold) in Tat-stimulated cells, which suggested increased chromosomal accessibility of transcription factors. Two-hybrid cotransfection assays using reporter plasmid containing the GAL4 DNA-binding domain and expression plasmid coding for the GAL4-CBP fusion protein, showed that Tat increases (2-fold) CBP-mediated coactivation of CREB. Both reverse transcription-polymerase chain reaction (RT-PCR) and Western blot analysis showed that Tat treatment increases CBP gene expression (7- to 9-fold) and protein levels (5- to 7-fold) within 6-12 hrs after stimulation. Our findings indicated that Tat treatment increases both CREB function and CREB coactivation by CBP, which may facilitate megakaryocytic commitment of K562 cells. Induction of this molecular signaling by HIV-1 Tat protein may have relevance in understanding the HIV-induced hematologic manifestations and possibly in regulation of viral infectivity parameters in progenitor cell reservoirs.

Latency: the hidden HIV-1 challenge

Eradication of HIV-1 from an infected individual cannot be achieved by current regimens. Viral reservoirs established early during the infection remain unaffected by anti-retroviral therapy for a long time and are able to replenish systemic infection upon interruption of the treatment. Therapeutic targeting of viral latency will require a better understanding of the basic mechanisms underlying the establishment and long-term maintenance of HIV-1 in resting memory CD4 T cells, the most prominent reservoir of transcriptionally silent provirus. Since the molecular mechanisms that permit long term transcriptional control of proviral gene expression in these cells are still obscure, this review aims at summarizing the various aspects of the problem that need to be considered. In particular, this review will focus the attention on the control of transcription imposed by chromatin through various epigenetic mechanisms. Exploring the molecular details of viral latency will provide new insights for eventual future therapeutics that aim at viral eradication.

Tat gets the "green" light on transcription initiation

Human immunodeficiency virus type 1 (HIV-1) Tat transactivation is an essential step in the viral life cycle. Over the past several years, it has become widely accepted that Tat exerts its transcriptional effect by binding the transactivation-responsive region (TAR) and enhancing transcriptional elongation. Consistent with this hypothesis, it has been shown that Tat promotes the binding of P-TEFb, a transcription elongation factor composed of cyclin T1 and cdk9, and the interaction of Tat with P-TEFb and TAR leads to hyperphosphorylation of the C-terminal domain (CTD) of RNA Pol II and increased processivity of RNA Pol II. A recent report, however, has generated renewed interest that Tat may also play a critical role in transcription complex (TC) assembly at the preinitiation step. Using in vivo chromatin immunoprecipitation assays, the authors reported that the HIV TC contains TBP but not TBP-associated factors. The stimulatory effect involved the direct interaction of Tat and P-TEFb and was evident at the earliest step of TC assembly, the TBP-TATA box interaction. In this article, we will review this data in context of earlier data which also support Tat's involvement in transcriptional complex assembly. Specifically, we will discuss experiments which demonstrated that Tat interacted with TBP and increased transcription initiation complex stability in cell free assays. We will also discuss studies which demonstrated that over expression of TBP alone was sufficient to obtain Tat activated transcription in vitro and in vivo. Finally, studies using self-cleaving ribozymes which suggested that Tat transactivation was not compatible with pausing of the RNA Pol II at the TAR site will be discussed.

p300 modulates HIV-1 gp120-induced apoptosis in human proximal tubular cells: associated with alteration of TGF-beta and Smad signaling

p300 is a key protein, which determines acceleration or deceleration of signal transduction. Recently, renal proximal tubular cells have not only been found to be a harboring site for HIV-1 but have also been shown to undergo apoptosis in response to HIV-1 exposure. Both HIV-1 and its envelop glycoprotein, i.e. gp120, triggered tubular cell apoptosis in the same magnitude. In the present study, we evaluated the role of p300 in gp120-induced tubular cell apoptosis and associated downstream signaling. We have demonstrated that by transient transfection assays, p300 significantly increases susceptibility of human proximal renal tubular HK-2 cells to apoptosis triggered by HIV-1 gp120. A mutant p300, missing the E1A/TFIIB binding site, fails to produce such sensitization potential. Smad7 and an anti-TGF-beta antibody rescue the p300 sensitization. Furthermore, p300 and HIV-1 gp120 synergistically increase TGF-beta, ATF-2 and activating protein-1 (AP-1) expression. In addition, HIV-1 gp120 results in phosphorylation of Smad2 and decreases c-Jun. These findings suggest that p300 acts as a potent transcriptional cofactor in HIV-1 gp120-induced apoptosis via TGF-beta and Smad signaling.

Interplay between NFBP and NF-kappaB modulates tat activation of the LTR

Interplay of the HIV-1 regulatory protein, Tat, with several cellular factors plays an important role in transcriptional regulation of the viral promoter, the long terminal repeat (LTR). Special attention has been paid to NF-kappaB, a family of inducible transcription factors, which interact with a specific DNA motif within the LTR. Here, we report on the physical and functional interaction of NFBP, a recently identified protein that interacts with the P65 subunit of NF-kappaB, with HIV-1 Tat. NFBP colocalizes with Tat in the nucleus and nucleoli, recognizes the amino acid residues 37 to 48 of Tat, and its interaction is modulated by RNA molecules. The interaction of NFBP with Tat modulates the synergism between Tat and P65 in activating LTR transcription. In the absence of the kappaB-binding sites, NFBP augments the TAR-dependent activation by Tat, yet it interferes with the synergistic effect of P65 and Tat on LTR transcription.

Acetylation of HIV-1 integrase by p300 regulates viral integration

Integration of HIV-1 into the human genome, which is catalyzed by the viral protein integrase (IN), preferentially occurs near transcriptionally active genes. Here we show that p300, a cellular acetyltransferase that regulates chromatin conformation through the acetylation of histones, also acetylates IN and controls its activity. We have found that p300 directly binds IN both in vitro and in the cells, as also specifically demonstrated by fluorescence resonance energy transfer technique analysis. This interaction results in the acetylation of three specific lysines (K264, K266, K273) in the carboxy-terminus of IN, a region that is required for DNA binding. Acetylation increases IN affinity to DNA, and promotes the DNA strand transfer activity of the protein. In the context of the viral replication cycle, point mutations in the IN acetylation sites abolish virus replication by specifically impairing its integration capacity. This is the first demonstration that HIV-1 IN activity is specifically regulated by post-translational modification.

Induction of OGG1 gene expression by HIV-1 Tat

To identify the cellular gene target for Tat, we performed gene expression profile analysis and found that Tat up-regulates the expression of the OGG1 (8-oxoguanine-DNA glycosylase-1) gene, which encodes an enzyme responsible for repairing the oxidatively damaged guanosine, 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG). We observed that Tat induced OGG1 gene expression by enhancing its promoter activity without changing its mRNA stability. We found that the upstream AP-4 site within the OGG1 promoter is responsible and that Tat interacted with AP-4 and removed AP-4 from the OGG1 promoter by in vivo chromatin immunoprecipitation assay. Thus, Tat appears to activate OGG1 expression by sequestrating AP-4. Interestingly, although Tat induces oxidative stress known to generate 8-oxo-dG, which causes the G:C to T:A transversion, we observed that the amount of 8-oxo-dG was reduced by Tat. When OGG1 was knocked down by small interfering RNA, Tat increased the amount of 8-oxo-dG, thus confirming the role of OGG1 in preventing the formation of 8-oxo-dG. These findings collectively indicate the possibility that Tat may play a role in maintenance of the genetic integrity of the proviral and host cellular genomes by up-regulating OGG1 as a feed-forward mechanism.

The multifactorial nature of HIV-1 latency

HIV-1 can avoid host immune responses and antiretroviral drugs through the latent infection of resting memory CD4(+) T cells. Recently, latent viral genomes have been shown to reside within the introns of active host genes. Therefore, latency is not simply due to an inaccessibility of the integrated proviruses to the transcriptional machinery. Rather, latency might result from insufficient nuclear levels of the crucial activation-dependent host transcription factors required to overcome the transcriptional interference that is an automatic consequence of the nature of HIV-1 integration sites. In addition, resting cells lack sufficient levels of HIV-1 Tat and Tat-associated activation-dependent host factors that are necessary for processive transcription. Defects at consecutive steps of transcriptional initiation and elongation enable HIV-1 to remain hidden within resting CD4(+) T cells.

Delivery of bioactive molecules into the cell: the Trojan horse approach

In recent years, vast amounts of data on the mechanisms of neural de- and regeneration have accumulated. However, only in disproportionally few cases has this led to efficient therapies for human patients. Part of the problem is to deliver cell death-averting genes or gene products across the blood-brain barrier (BBB) and cellular membranes. The discovery of Antennapedia (Antp)-mediated transduction of heterologous proteins into cells in 1992 and other "Trojan horse peptides" raised hopes that often-frustrating attempts to deliver proteins would now be history. The demonstration that proteins fused to the Tat protein transduction domain (PTD) are capable of crossing the BBB may revolutionize molecular research and neurobiological therapy. However, it was only recently that PTD-mediated delivery of proteins with therapeutic potential has been achieved in models of neural degeneration in nerve trauma and ischemia. Several groups have published the first positive results using protein transduction domains for the delivery of therapeutic proteins in relevant animal models of human neurological disorders. Here, we give an extensive review of peptide-mediated protein transduction from its early beginnings to new advances, discuss their application, with particular focus on a critical evaluation of the limitations of the method, as well as alternative approaches. Besides applications in neurobiology, a large number of reports using PTD in other systems are included as well. Because each protein requires an individual purification scheme that yields sufficient quantities of soluble, transducible material, the neurobiologist will benefit from the experiences of other researchers in the growing field of protein transduction.

Cell cycle arrest in G2 induces human immunodeficiency virus type 1 transcriptional activation through histone acetylation and recruitment of CBP, NF-kappaB, and c-Jun to the long terminal repeat promoter

In human immunodeficiency virus type 1 (HIV-1)-infected cells, a cell cycle arrest in G(2) increases viral expression and may represent a strategy for the virus to optimize its expression. In latently infected cells, balance between viral silencing and reactivation relies on the nucleosomal organization of the integrated long terminal repeat (LTR). It is shown here that nucleosome nuc-1, which is located downstream of the TATA box, is specifically modified when latently infected cells are arrested in G(2) by chemical inducers. Notably, histones H3 and H4 are hyperacetylated, and this modification is associated with an increased LTR-driven transcription. nuc-1 hyperacetylation is also associated with the recruitment of histone acetyltransferase CBP and transcription factors NF-kappaB and c-Jun. NF-kappaB and/or c-Jun binding to the LTR in G(2)-arrested cells appears to be required for CBP recruitment as well as for nuc-1 remodeling and viral reactivation.

Subnuclear distribution of the largest subunit of the human origin recognition complex during the cell cycle

In eukaryotes, initiation of DNA replication requires the activity of the origin recognition complex (ORC). The largest subunit of this complex, Orc1p, has a critical role in this activity. Here we have studied the subnuclear distribution of the overexpressed human Orc1p during the cell cycle. Orc1p is progressively degraded during S-phase according to a spatio-temporal program and it never colocalizes with replication factories. Orc1p is resynthesized in G1. In early G1, the protein is distributed throughout the cell nucleus, but successively it preferentially associates with heterochromatin. This association requires a functional ATP binding site and a protein region partially overlapping the bromo-adjacent homology domain at the N-terminus of Orc1p. The same N-terminal region mediates the in vitro interaction with heterochromatin protein 1 (HP1). Fluorescence resonance energy transfer (FRET) experiments demonstrate the interaction of human Orc1p and HP1 in vivo. Our data suggest a role of HP1 in the recruitment but not in the stable association of Orc1p with heterochromatin. Indeed, the subnuclear distribution of Orc1p is not affected by treatments that trigger the dispersal of HP1.

Analysis of human immunodeficiency virus type 1 transcriptional elongation in resting CD4+ T cells in vivo

A stable latent reservoir for human immunodeficiency virus type 1 (HIV-1) in resting memory CD4+ T cells presents a barrier to eradication of the infection even in patients on highly active antiretroviral therapy. Potential mechanisms for latency include inaccessibility of the integrated viral genome, absence of key host transcription factors, premature termination of HIV-1 RNAs, and abnormal splicing patterns. To differentiate among these mechanisms, we isolated extremely pure populations of resting CD4+ T cells from patients on highly active antiretroviral therapy. These cells did not produce virus but retained the capacity to do so if appropriately stimulated. Products of HIV-1 transcription were examined in purified resting CD4+ T cells. Although short, prematurely terminated HIV-1 transcripts have been suggested as a marker for latently infected cells, the production of short transcripts had not been previously demonstrated in purified populations of resting CD4+ T cells. By separating RNA into polyadenylated and nonpolyadenylated fractions, we showed that resting CD4+ T cells from patients on highly active antiretroviral therapy produce abortive transcripts that lack a poly(A) tail and that terminate prior to nucleotide 181. Short transcripts dominated the pool of total HIV-1 transcripts in resting CD4+ T cells. Processive, polyadenylated HIV-1 mRNAs were also present at a low level. Both unspliced and multiply spliced forms were found. Taken together, these results show that the nonproductive nature of the infection in resting CD4+ T cells from patients on highly active antiretroviral therapy is not due to absolute blocks at the level of either transcriptional initiation or elongation but rather relative inefficiencies at multiple steps.

Retinoic acid inhibition of chromatin remodeling at the human immunodeficiency virus type 1 promoter. Uncoupling of histone acetylation and chromatin remodeling

All-trans retinoic acid (RA) represses HIV-1 transcription and replication in cultured monocytic cells and in primary monocyte-derived macrophages. Here we examine the role of histone acetylation and chromatin remodeling in RA-mediated repression. RA pretreatment of latently infected U1 promonocytes inhibits HIV-1 expression in response to the histone deacetylase (HDAC) inhibitor, trichostatin A (TSA). TSA is thought to activate HIV-1 transcription by inducing histone hyperacetylation within a regulatory nucleosome, nuc-1, positioned immediately downstream from the transcription start site. Acetylation of nuc-1 is thought to be a critical step in activation that precedes nuc-1 remodeling and, subsequently, transcriptional initiation. Here we demonstrate that TSA treatment induces H3 and H4 hyperacetylation and nuc-1 remodeling. Although RA pretreatment inhibits nuc-1 remodeling and HIV-1 transcription, it has no effect on histone acetylation. This suggests that acetylation and remodeling are not obligatorily coupled. We also show that growth of U1 cells in retinoid-deficient medium induces nuc-1 remodeling and HIV-1 expression but does not induce histone hyperacetylation. These findings suggest that remodeling, not histone hyperacetylation, is the limiting step in transcriptional activation in these cells. Together, these data suggest that RA signaling maintains the chromatin structure of the HIV-1 promoter in a transcriptionally non-permissive state that may contribute to the establishment of latency in monocyte/macrophages.

Multiple actions of the human immunodeficiency virus type-1 Tat protein on microglial cell functions

The human immunodeficiency virus type-1 (HIV-1) regulatory protein Tat is produced in the early phase of infection and is essential for virus replication. Together with other viral products, Tat has been implicated in the pathogenesis of HIV-1-associated dementia (HAD). As HIV-1 infection in the brain is very limited and macrophage/microglial cells are the only cellular type productively infected by the virus, it has been proposed that many of the viral neurotoxic effects are mediated by microglial products. We and others have shown that Tat affects the functional state of microglial cells, supporting the hypothesis that activated microglia play a role in the neuropathology associated with HIV-1 infection. This review describes the experimental evidence indicating that Tat stimulates microglia to synthesize potentially neurotoxic molecules, including proinflammatory cytokines and free radicals, and interferes with molecular mechanisms controlling cAMP levels, intracellular [Ca2+], and ion channel expression.

HIV-1 gene expression: lessons from provirus and non-integrated DNA

Replication of HIV-1 involves a series of obligatory steps such as reverse transcription of the viral RNA genome into double-stranded DNA, and subsequent integration of the DNA into the human chromatin. Integration is an essential step for HIV-1 replication; yet the natural process of HIV-1 infection generates both integrated and high levels of non-integrated DNA. Although proviral DNA is the template for productive viral replication, the non-integrated DNA has been suggested to be active for limited viral gene synthesis. In this review, the regulation of viral gene expression from proviral DNA will be summarized and issues relating to non-integrated DNA as a template for transcription will be discussed, as will the possible function of pre-integration transcription in HIV-1 replication cycle.