Tat as a transcriptional activator and a potential therapeutic target for HIV-1

microRNA-21: a key modulator in oncogenic viral infections

Oncogenic viruses are associated with approximately 15% of human cancers. In viral infections, microRNAs play an important role in host-pathogen interactions. miR-21 is a highly conserved non-coding RNA that not only regulates the development of oncogenic viral diseases, but also responds to the regulation of intracellular signal pathways. Oncogenic viruses, including HBV, HCV, HPV, and EBV, co-evolve with their hosts and cause persistent infections. The upregulation of host miR-21 manipulates key cellular pathways to evade host immune responses and then promote viral replication. Thus, a better understanding of the role of miR-21 in viral infections may help us to develop effective genetically-engineered oncolytic virus-based therapies against cancer.

Hepatitis C Virus Influences HIV-1 Viral Splicing in Coinfected Patients

Coinfection with hepatitis C virus (HCV) influences HIV reservoir size. However, it is unknown whether this coinfection also induces a higher provirus transcription. Viral transcription is promoted by synergy between cellular factors such as NF-κB and the viral regulator Tat. The impact of HCV coinfection on HIV provirus transcription was analyzed in resting (r)CD4 T+ cells (CD3+CD4+CD25-CD69-HLADR-) and rCD4 T cells-depleted PBMCs (rCD4 T- PBMCs) from a multicenter cross-sectional study of 115 cART-treated HIV patients: 42 HIV+/HCV+ coinfected individuals (HIV+/HCV+), 34 HIV+ patients with HCV spontaneous clearance (HIV+/HCV−) and 39 HIV patients (HIV+). Viral transcription was assessed in total RNA through the quantification of unspliced, single spliced, and multiple spliced viral mRNAs by qPCR. Linear correlations between viral reservoir size and viral splicing were determined. A 3-fold increase of multiple spliced transcripts in rCD4 T+ cells of HIV+/HCV+ patients was found compared to HIV+ individuals (p < 0.05). As Tat is synthesized by multiple splicing, the levels of Tat were also quantified in these patients. Significant differences in single and multiple spliced transcripts were also observed in rCD4 T- PBMCs. Levels of multiple spliced mRNAs were increased in rCD4 T+ cells isolated from HIV+/HCV+ subjects, which could indicate a higher Tat activity in these cells despite their resting state.

Morphine exposure exacerbates HIV-1 Tat driven changes to neuroinflammatory factors in cultured astrocytes

Despite antiretroviral therapy human immunodeficiency virus type-1 (HIV-1) infection results in neuroinflammation of the central nervous system that can cause HIV-associated neurocognitive disorders (HAND). The molecular mechanisms involved in the development of HAND are unclear, however, they are likely due to both direct and indirect consequences of HIV-1 infection and inflammation of the central nervous system. Additionally, opioid abuse in infected individuals has the potential to exacerbate HIV-comorbidities, such as HAND. Although restricted for productive HIV replication, astrocytes (comprising 40-70% of all brain cells) likely play a significant role in neuropathogenesis in infected individuals due to the production and response of viral proteins. The HIV-1 protein Tat is critical for viral transcription, causes neuroinflammation, and can be secreted from infected cells to affect uninfected bystander cells. The Wnt/β-catenin signaling cascade plays an integral role in restricting HIV-1 infection in part by negatively regulating HIV-1 Tat function. Conversely, Tat can overcome this negative regulation and inhibit β-catenin signaling by sequestering the critical transcription factor TCF-4 from binding to β-catenin. Here, we aimed to explore how opiate exposure affects Tat-mediated suppression of β-catenin in astrocytes and the downstream modulation of neuroinflammatory genes. We observed that morphine can potentiate Tat suppression of β-catenin activity in human astrocytes. In contrast, Tat mutants deficient in secretion, and lacking neurotoxic effects, do not affect β-catenin activity in the presence or absence of morphine. Finally, morphine treatment of astrocytes was sufficient to reduce the expression of genes involved in neuroinflammation. Examining the molecular mechanisms of how HIV-1 infection and opiate exposure exacerbate neuroinflammation may help us inform or predict disease progression prior to HAND development.

Sulfonation, an underexploited area: from skeletal development to infectious diseases and cancer

Sulfonation is one of the most abundant cellular reactions modifying a wide range of xenobiotics as well as endogenous molecules which regulate important biological processes including blood clotting, formation of connective tissues, and functionality of secreted proteins, hormones, and signaling molecules. Sulfonation is ubiquitous in all tissues and widespread in nature (plants, animals, and microorganisms). Although sulfoconjugates were discovered over a century ago when, in 1875, Baumann isolated phenyl sulfate in the urine of a patient given phenol as an antiseptic, the significance of sulfonation and its roles in human diseases have been underappreciated until recent years. Here, we provide a current overview of the significance of sulfonation reactions in a variety of biological functions and medical conditions (with emphasis on cancer). We also discuss research areas that warrant further attention if we are to fully understand how deficiencies in sulfonation could impact human health which, in turn, could help define treatments to effect improvements in health.

Functional Studies of CCAAT/Enhancer Binding Protein Site Located Downstream of the Transcriptional Start Site

Previous studies have identified a CCAAT/enhancer binding protein (C/EBP) site located downstream of the transcriptional start site (DS3). The role of the DS3 element with respect to HIV-1 transactivation by Tat and viral replication has not been characterized. We have demonstrated that DS3 was a functional C/EBPβ binding site and mutation of this site to the C/EBP knockout DS3-9C variant showed lower HIV-1 long terminal repeat (LTR) transactivation by C/EBPβ. However, it was able to exhibit similar or even higher transcription levels by Tat compared to the parental LTR. C/EBPβ and Tat together further enhanced the transcription level of the parental LAI-LTR and DS3-9C LTR, with higher levels in the DS3-9C LTR. HIV molecular clone viruses carrying the DS3-9C variant LTR demonstrated a decreased replication capacity and delayed rate of replication. These results suggest that DS3 plays a role in virus transcriptional initiation and provides new insight into C/EBP regulation of HIV-1.

Changes in the cellular microRNA profile by the intracellular expression of HIV-1 Tat regulator: A potential mechanism for resistance to apoptosis and impaired proliferation in HIV-1 infected CD4+ T cells

HIV-1 induces changes in the miRNA expression profile of infected CD4+ T cells that could improve viral replication. HIV-1 regulator Tat modifies the cellular gene expression and has been appointed as an RNA silencing suppressor. Tat is a 101-residue protein codified by two exons that regulates the elongation of viral transcripts. The first exon of Tat (amino acids 1–72) forms the transcriptionally active protein Tat72, but the presence of the second exon (amino acids 73–101) results in a more competent regulatory protein (Tat101) with additional functions. Intracellular, full-length Tat101 induces functional and morphological changes in CD4+ T cells that contribute to HIV-1 pathogenesis such as delay in T-cell proliferation and protection against FasL-mediated apoptosis. But the precise mechanism by which Tat produces these changes remains unknown. We analyzed how the stable expression of intracellular Tat101 and Tat72 modified the miRNA expression profile in Jurkat cells and if this correlated with changes in apoptotic pathways and cell cycle observed in Tat-expressing cells. Specifically, the enhanced expression of hsa-miR-21 and hsa-miR-222 in Jurkat-Tat101 cells was associated with the reduced expression of target mRNAs encoding proteins related to apoptosis and cell cycle such as PTEN, PDCD4 and CDKN1B. We developed Jurkat cells with stable expression of hsa-miR-21 or hsa-miR-222 and observed a similar pattern to Jurkat-Tat101 in resistance to FasL-mediated apoptosis, cell cycle arrest in G₂/M and altered cell morphology. Consequently, upregulation of hsa-miR-21 and hsa-miR-222 by Tat may contribute to protect against apoptosis and to anergy observed in HIV-infected CD4+ T cells.

The multiple roles of the nucleocapsid in retroviral RNA conversion into proviral DNA by reverse transcriptase

Retroviruses are enveloped plus-strand RNA viruses that can cause cancer, immunodeficiency and neurological disorder in human and animals. Retroviruses have several unique properties, such as a genomic RNA in a dimeric form found in the virus, and a replication strategy called 'copy-and-paste' during which the plus-strand genomic RNA is converted into a double-stranded DNA, subsequently integrated into the cellular genome. Two essential viral enzymes, reverse transcriptase (RT) and integrase (IN), direct this 'copy-and-paste' replication. RT copies the genomic RNA generating the double-stranded proviral DNA, while IN catalyzes proviral DNA integration into the cellular DNA, then called the provirus. In that context, a major component of the virion core, the nucleocapsid protein (NC), was found to be a potent nucleic-acid chaperone that assists RT during the conversion of the genomic RNA into proviral DNA. Here we briefly review the interplay of NC with viral nucleic-acids, which enables rapid and faithful folding and hybridization of complementary sequences, and with active RT thus providing assistance to the synthesis of the complete proviral DNA. Because of its multiple roles in retrovirus replication, NC could be viewed as a two-faced Janus-chaperone acting on viral nucleic-acids and enzymes.

Recent 5-year Findings and Technological Advances in the Proteomic Study of HIV-associated Disorders

Human immunodeficiency virus-1 (HIV-1) mainly relies on host factors to complete its life cycle. Hence, it is very important to identify HIV-regulated host proteins. Proteomics is an excellent technique for this purpose because of its high throughput and sensitivity. In this review, we summarized current technological advances in proteomics, including general isobaric tags for relative and absolute quantitation (iTRAQ) and stable isotope labeling by amino acids in cell culture (SILAC), as well as subcellular proteomics and investigation of posttranslational modifications. Furthermore, we reviewed the applications of proteomics in the discovery of HIV-related diseases and HIV infection mechanisms. Proteins identified by proteomic studies might offer new avenues for the diagnosis and treatment of HIV infection and the related diseases.

Syndecan-1 increases B-lymphoid cell extravasation in response to HIV-1 Tat via αvβ3/pp60src/pp125FAK pathway

Syndecan-1 is a heparan sulfate proteoglycan (HSPG) commonly upregulated in AIDS-related B lymphoid malignancies. Tat is the main HIV-1 transactivating factor that has a major role in the pathogenesis of AIDS-related lymphomas (ARL) by engaging heparan sulfate proteoglycans (HSPGs), chemokine receptors and integrins at the lymphoid cell (LC) surface. Here B-lymphoid Namalwa cell clones that do not express or overexpress syndecan-1 (EV-Ncs and SYN-Ncs, respectively) were compared for their responsiveness with Tat: in the absence of syndecan-1, Tat induces a limited EV-Nc migration via C-X-C motif chemokine receptor 4 (CXCR4), G-proteins and Rac. Syndecan-1 overexpression increases SYN-Nc responsiveness to Tat and makes this response independent from CXCR4 and G-protein and dependent instead on pp60src phosphorylation. Tat-induced SYN-Nc migration and pp60src phosphorylation require the engagement of α_vβ₃ integrin and consequent pp125FAK phosphorylation. This complex set of Tat-driven activations is orchestrated by the direct interaction of syndecan-1 with pp60src and its simultaneous coupling with α_vβ₃. The Tat/syndecan-1/α_vβ₃ interplay is retained in vivo and is shared also by other syndecan-1⁺ B-LCs, including BJAB cells, whose responsiveness to Tat is inhibited by syndecan-1 knockdown. In conclusion, overexpression of syndecan-1 confers to B-LCs an increased capacity to migrate in response to Tat, owing to a switch from a CXCR4/G-protein/Rac to a syndecan-1/α_vβ₃/pp60src/pp125FAK signal transduction pathway that depends on the formation of a complex in which syndecan-1 interacts with Tat via its HS-chains, with α_vβ₃ via its core protein ectodomain and with pp60src via its intracellular tail. These findings have implications in ARL progression and may help in identifying new therapeutical targets for the treatment of AIDS-associated neoplasia.

Stability of HIV-1 subtype B and C Tat is associated with variation in the carboxyl-terminal region

The multifunctional trans-activator Tat is an essential regulatory protein for HIV-1 replication and is characterized by high sequence diversity. Numerous experimental studies have examined Tat in HIV-1 subtype B, but research on subtype C Tat is lacking, despite the high prevalence of infections caused by subtype C worldwide. We hypothesized that amino acid differences contribute to functional differences among Tat proteins. In the present study, we found that subtype B NL4-3 Tat and subtype C isolate HIV1084i Tat exhibited differences in stability by overexpressing the fusion protein Tat-Flag. In addition, 1084i Tat can activate LTR and NF-κB more efficiently than NL4-3 Tat. In analyses of the activities of the truncated forms of Tat, we found that the carboxyl-terminal region of Tat regulates its stability and transactivity. According to our results, we speculated that the differences in stability between B-Tat and C-Tat result in differences in transactivation ability.

Intracellular expression of Tat alters mitochondrial functions in T cells: a potential mechanism to understand mitochondrial damage during HIV-1 replication

Background

HIV-1 replication results in mitochondrial damage that is enhanced during antiretroviral therapy (ART). The onset of HIV-1 replication is regulated by viral protein Tat, a 101-residue protein codified by two exons that elongates viral transcripts. Although the first exon of Tat (aa 1–72) forms itself an active protein, the presence of the second exon (aa 73–101) results in a more competent transcriptional protein with additional functions.

Results

Mitochondrial overall functions were analyzed in Jurkat cells stably expressing full-length Tat (Tat101) or one-exon Tat (Tat72). Representative results were confirmed in PBLs transiently expressing Tat101 and in HIV-infected Jurkat cells. The intracellular expression of Tat101 induced the deregulation of metabolism and cytoskeletal proteins which remodeled the function and distribution of mitochondria. Tat101 reduced the transcription of the mtDNA, resulting in low ATP production. The total amount of mitochondria increased likely to counteract their functional impairment. These effects were enhanced when Tat second exon was expressed.

Conclusions

Intracellular Tat altered mtDNA transcription, mitochondrial content and distribution in CD4+ T cells. The importance of Tat second exon in non-transcriptional functions was confirmed. Tat101 may be responsible for mitochondrial dysfunctions found in HIV-1 infected patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-015-0203-3) contains supplementary material, which is available to authorized users.

Neuropathogenesis: rogue glia cause mayhem in the brain

Glia, including astrocytes, microglia and oligodendrocytes, are important components that maintain the architecture of the brain and in many ways contribute to the proper functioning of neurons. Glial cells vastly outnumber neurons in the brain and independently control several crucial brain functions. Impaired glial cells are the cause of several diseases, and pharmacological targeting to repair damaged glia will enable functional recovery in patients suffering from devastating neurological disorders. The interaction between glial cells and some patrolling immune cells in the brain comprise the brain-specific immune system that protects the brain from extraneous agents and repairs injured tissue. While this system can cope with minor insults and infections, when faced with significant challenges such as AIDS dementia, multiple sclerosis, Huntington’s disease, Parkinson’s disease, etc., an effective and balanced immune response that facilitates repair and protection is found wanting. Several debilitating neurological disorders are often associated with dysfunctional glial cells that have limited ability to repair the injured brain and even promote brain damage. In this discussion, specific signaling pathways in glia that are affected in AIDS dementia and periventricular white matter injury will be highlighted.

HIV-1 Tat protein variants: critical role for the cysteine region in synaptodendritic injury

HIV-1 enters the central nervous system early in infection; although HIV-1 does not directly infect neurons, HIV-1 may cause a variety of neurological disorders. Neuronal loss has been found in HIV-1, but synaptodendritic injury is more closely associated with the neurocognitive disorders of HIV-1. The HIV-1 transactivator of transcription (Tat) protein causes direct and indirect damage to neurons. The cysteine rich domain (residues 22-37) of Tat is important for producing neuronal death; however, little is known about the effects of the Tat protein functional domains on the dendritic network. The ability of HIV-1 Tat 1-101 Clades B and C, Tat 1-86 and Tat 1-72 proteins, as well as novel peptides (truncated 47-57, 1-72δ31-61, and 1-86 with a mutation at Cys22) to produce early synaptodendritic injury (24h), relative to later cell death (48h), was examined using cell culture. Treatment of primary hippocampal neurons with Tat proteins 1-72, 1-86 and 1-101B produced a significant early reduction in F-actin labeled puncta, implicating that these peptides play a role in synaptodendritic injury. Variants with a mutation, deletion, or lack of a cysteine rich region (1-86[Cys22], 1-101C, 1-72δ31-61, or 47-57) did not cause a significant reduction in F-actin rich puncta. Tat 1-72, 1-86, and 1-101B proteins did not significantly differ from one another, indicating that the second exon (73-86 or 73-101) does not play a significant role in the reduction of F-actin puncta. Conversely, peptides with a mutation, deletion, or lack of the cysteine rich domain (22-37) failed to produce a loss of F-actin puncta, indicating that the cysteine rich domain plays a key role in synaptodendritic injury. Collectively, these results suggest that for Tat proteins, 1) synaptodendritic injury occurs early, relative to cell death, and 2) the cysteine rich domain of the first exon is key for synaptic loss. Preventing such early synaptic loss may attenuate HIV-1 associated neurocognitive disorders.

The presence of HIV-1 Tat protein second exon delays fas protein-mediated apoptosis in CD4+ T lymphocytes: a potential mechanism for persistent viral production

HIV-1 replication is efficiently controlled by the regulator protein Tat (101 amino acids) and codified by two exons, although the first exon (1-72 amino acids) is sufficient for this process. Tat can be released to the extracellular medium, acting as a soluble pro-apoptotic factor in neighboring cells. However, HIV-1-infected CD4(+) T lymphocytes show a higher resistance to apoptosis. We observed that the intracellular expression of Tat delayed FasL-mediated apoptosis in both peripheral blood lymphocytes and Jurkat cells, as it is an essential pathway to control T cell homeostasis during immune activation. Jurkat-Tat cells showed impairment in the activation of caspase-8, deficient release of mitochondrial cytochrome c, and delayed activation of both caspase-9 and -3. This protection was due to a profound deregulation of proteins that stabilized the mitochondrial membrane integrity, such as heat shock proteins, prohibitin, or nucleophosmin, as well as to the up-regulation of NF-κB-dependent anti-apoptotic proteins, such as BCL2, c-FLIPS, XIAP, and C-IAP2. These effects were observed in Jurkat expressing full-length Tat (Jurkat-Tat101) but not in Jurkat expressing the first exon of Tat (Jurkat-Tat72), proving that the second exon, and particularly the NF-κB-related motif ESKKKVE, was necessary for Tat-mediated protection against FasL apoptosis. Accordingly, the protection exerted by Tat was independent of its function as a regulator of both viral transcription and elongation. Moreover, these data proved that HIV-1 could have developed strategies to delay FasL-mediated apoptosis in infected CD4(+) T lymphocytes through the expression of Tat, thus favoring the persistent replication of HIV-1 in infected T cells.

Short communication: a single step assay for rapid evaluation of inhibitors targeting HIV type 1 Tat-mediated long terminal repeat transactivation

Human immunodeficiency virus (HIV) long terminal repeat (LTR) promoter-mediated gene expression is regulated by the viral Tat protein that relieves a block to viral transcription elongation after binding with a viral hairpin loop RNA structure called the trans-activation-responsive region (TAR). Tat protein significantly up-regulates viral genome transcription and hence it has long been considered a potential target for antiretrovirals. Here we report the construction of a plasmid containing an HIV-1 LTR-driven reporter cassette with a colinear tat gene under control of a viral promoter and thus conditionally configured for constitutive expression of reporter genes. Inhibition of luciferase reporter expression in a cell line harboring the plasmid in the presence of tat-targeted shRNA confirmed the specificity of the assay and a dose-dependent reporter activity inhibition by the fluoroquinoline derivative K-37, a class of small RNA binding molecule that inhibits Tat and other RNA-dependent transactivations, further validated the method. Subsequently we also made a lentiviral vector (LV) containing the same transcription units and derived a stable cell line using the said LV and similar dose-dependent inhibition was documented using K-37. This quick and sensitive reporter-based method is the simplest screening assay for putative inhibitors of HIV-1 Tat-induced LTR-driven gene expression requiring test material addition as the only manipulation.

Molecular Understanding of HIV-1 Latency

The introduction of highly active antiretroviral therapy (HAART) has been an important breakthrough in the treatment of HIV-1 infection and has also a powerful tool to upset the equilibrium of viral production and HIV-1 pathogenesis. Despite the advent of potent combinations of this therapy, the long-lived HIV-1 reservoirs like cells from monocyte-macrophage lineage and resting memory CD4+ T cells which are established early during primary infection constitute a major obstacle to virus eradication. Further HAART interruption leads to immediate rebound viremia from latent reservoirs. This paper focuses on the essentials of the molecular mechanisms for the establishment of HIV-1 latency with special concern to present and future possible treatment strategies to completely purge and target viral persistence in the reservoirs.

HIV-1 Tat binds to SH3 domains: cellular and viral outcome of Tat/Grb2 interaction

The Src-homology 3 (SH3) domain is one of the most frequent protein recognition modules (PRMs), being represented in signal transduction pathways and in several pathologies such as cancer and AIDS. Grb2 (growth factor receptor-bound protein 2) is an adaptor protein that contains two SH3 domains and is involved in receptor tyrosine kinase (RTK) signal transduction pathways. The HIV-1 transactivator factor Tat is required for viral replication and it has been shown to bind directly or indirectly to several host proteins, deregulating their functions. In this study, we show interaction between the cellular factor Grb2 and the HIV-1 trans-activating protein Tat. The binding is mediated by the proline-rich sequence of Tat and the SH3 domain of Grb2. As the adaptor protein Grb2 participates in a wide variety of signaling pathways, we characterized at least one of the possible downstream effects of the Tat/Grb2 interaction on the well-known IGF-1R/Raf/MAPK cascade. We show that the binding of Tat to Grb2 impairs activation of the Raf/MAPK pathway, while potentiating the PKA/Raf inhibitory pathway. The Tat/Grb2 interaction affects also viral function by inhibiting the Tat-mediated transactivation of HIV-1 LTR and viral replication in infected primary microglia.

Heparan Sulfate Proteoglycans in Infection

To cause infections, microbial pathogens elaborate a multitude of factors that interact with host components. Using these host–pathogen interactions to their advantage, pathogens attach, invade, disseminate, and evade host defense mechanisms to promote their survival in the hostile host environment. Many viruses, bacteria, and parasites express adhesins that bind to cell surface heparan sulfate proteoglycans (HSPGs) to facilitate their initial attachment and subsequent cellular entry. Some pathogens also secrete virulence factors that modify HSPG expression. HSPGs are ubiquitously expressed on the cell surface of adherent cells and in the extracellular matrix. HSPGs are composed of one or several heparan sulfate (HS) glycosaminoglycan chains attached covalently to specific core proteins. For most intracellular pathogens, cell surface HSPGs serve as a scaffold that facilitates the interaction of microbes with secondary receptors that mediate host cell entry. Consistent with this mechanism, addition of HS or its pharmaceutical functional mimic, heparin, inhibits microbial attachment and entry into cultured host cells, and HS-binding pathogens can no longer attach or enter cultured host cells whose HS expression has been reduced by enzymatic treatment or chemical mutagenesis. In pathogens where the specific HS adhesin has been identified, mutant strains lacking HS adhesins are viable and show normal growth rates, suggesting that the capacity to interact with HSPGs is strictly a virulence activity. The goal of this chapter is to provide a mechanistic overview of our current understanding of how certain microbial pathogens subvert HSPGs to promote their infection, using specific HSPG–pathogen interactions as representative examples.

Interaction of human dipeptidyl peptidase IV and human immunodeficiency virus type-1 transcription transactivator in Sf9 cells

Background

Dipeptidyl peptidase IV (DPPIV) also known as the T cell activation marker CD26 is a multifunctional protein which is involved in various biological processes. The association of human-DPPIV with components of the human immunodeficiency virus type-1 (HIV1) is well documented and raised some discussions. Several reports implicated the interaction of human-DPPIV with the HIV1 transcription transactivator protein (HIV1-Tat) and the inhibition of the dipeptidyl peptidase activity of DPPIV by the HIV1-Tat protein. Furthermore, enzyme kinetic data implied another binding site for the HIV1-Tat other than the active centre of DPPIV. However, the biological significance of this interaction of the HIV1-Tat protein and human-DPPIV has not been studied, yet. Therefore, we focused on the interaction of HIV1-Tat protein with DPPIV and investigated the subsequent biological consequences of this interaction in Spodoptera frugiperda cells, using the BAC-TO-BAC baculovirus system.

Results

The HIV1-Tat protein (Tat-BRU) co-localized and co-immunoprecipitated with human-DPPIV protein, following co-expression in the baculovirus-driven Sf9 cell expression system. Furthermore, tyrosine phosphorylation of DPPIV protein was up-regulated in Tat/DPPIV-co-expressing cells after 72 h culturing and also in DPPIV-expressing Sf9 cells after application of purified recombinant Tat protein. As opposed to the expression of Tat alone, serine phosphorylation of the Tat protein was decreased when co-expressed with human-DPPIV protein.

Conclusions

We show for the first time that human-DPPIV and HIV1-Tat co-immunoprecipitate. Furthermore, our findings indicate that the interaction of HIV1-Tat and human-DPPIV may be involved in signalling platforms that regulate the biological function of both human-DPPIV and HIV1-Tat.

Molecular mechanisms of HIV-1 persistence in the monocyte-macrophage lineage

The introduction of the highly active antiretroviral therapy (HAART) has greatly improved survival. However, these treatments fail to definitively cure the patients and unveil the presence of quiescent HIV-1 reservoirs like cells from monocyte-macrophage lineage. A purge, or at least a significant reduction of these long lived HIV-1 reservoirs will be needed to raise the hope of the viral eradication. This review focuses on the molecular mechanisms responsible for viral persistence in cells of the monocyte-macrophage lineage. Controversy on latency and/or cryptic chronic replication will be specifically evoked. In addition, since HIV-1 infected monocyte-macrophage cells appear to be more resistant to apoptosis, this obstacle to the viral eradication will be discussed. Understanding the intimate mechanisms of HIV-1 persistence is a prerequisite to devise new and original therapies aiming to achieve viral eradication.

Regulation of CCR5 expression in human placenta: insights from a study of mother-to-child transmission of HIV in Malawi

Background

Human promoter polymorphisms in the chemokine co-receptor 5 gene (CCR5) have been noted for association with mother-to-child transmission of HIV (HIV MTCT) as well as reduced receptor expression in vitro, but have not been clearly associated with CCR5 expression in vivo. Placental expression of CCR5 may be influenced by such polymorphisms as well as other in vivo regulatory factors.

Methodology/Principal Findings

We evaluated the associations between infant CCR5 polymorphisms, measures of maternal infection, and placental expression of CCR5 among mother-infant pairs in Blantyre, Malawi. RNA was extracted from placental tissue and used in multiplex real-time PCR to quantify gene expression. Through linear regression, we observed that CCR5-2554T (β = −0.67, 95% CI = −1.23, −0.11) and -2132T (β = −0.75, 95% CI = −0.131, −0.18) were significantly associated with reduced placental expression of CCR5. An incremental increase in CCR5 expression was observed for incremental increases in expression of two heparan sulfate genes involved in viral infection, HS3ST3A1 (β = 0.27, 95% CI = 0.18, 0.35) and HS3ST3B1 (β = 0.11, 95% CI = 0.06, 0.18). Among HIV infected mothers, an incremental increase in maternal HIV viral load was also associated with higher CCR5 expression (β = 0.76, 95% CI = 0.12, 1.39). Maternal HIV status had no overall effect (β = 0.072, 95% CI = −0.57, −0.72). Higher CCR5 expression was observed for mothers with malaria but was not statistically significant (β = 0.37, 95% CI = −0.43, 1.18).

Conclusions/Significance

These results provide in vivo evidence for genetic and environmental factors involved in the regulation of CCR5 expression in the placenta. Our findings also suggest that the measurement of placental expression of CCR5 alone is not an adequate indicator of the risk of mother-to-child transmission of HIV.

HIV-1 regulation of latency in the monocyte-macrophage lineage and in CD4+ T lymphocytes

The introduction in 1996 of the HAART raised hopes for the eradication of HIV-1. Unfortunately, the discovery of latent HIV-1 reservoirs in CD4+ T cells and in the monocyte-macrophage lineage proved the optimism to be premature. The long-lived HIV-1 reservoirs constitute a major obstacle to the eradication of HIV-1. In this review, we focus on the establishment and maintenance of HIV-1 latency in the two major targets for HIV-1: the CD4+ T cells and the monocyte-macrophage lineage. Understanding the cell-type molecular mechanisms of establishment, maintenance, and reactivation of HIV-1 latency in these reservoirs is crucial for efficient therapeutic intervention. A complete viral eradication, the holy graal for clinicians, might be achieved by strategic interventions targeting latently and productively infected cells. We suggest that new approaches, such as the combination of different kinds of proviral activators, may help to reduce dramatically the size of latent HIV-1 reservoirs in patients on HAART.

Emergence of a complex relationship between HIV-1 and the microRNA pathway

Recent experimental evidences support the existence of an increasingly complex and multifaceted interaction between viruses and the microRNA-guided RNA silencing machinery of human cells. The discovery of small interfering RNAs (siRNAs), which are designed to mediate cleavage of specific messenger RNAs (mRNAs), prompted virologists to establish therapeutic strategies based on siRNAs with the aim to suppress replication of several viruses, including human immunodeficiency virus type 1 (HIV-1). It has been appreciated only recently that viral RNAs can also be processed endogenously by the microRNA-generating enzyme Dicer or recognized by cellular miRNAs, in processes that could be viewed as an adapted antiviral defense mechanism. Known to repress mRNA translation through recognition of specific binding sites usually located in their 3' untranslated region, miRNAs of host or viral origin may exert regulatory effects towards host and/or viral genes and influence viral replication and/or the host response to viral infection. This article summarizes our current state of knowledge on the relationship between HIV-1 and miRNA-guided RNA silencing, and discusses the different aspects of their interaction.

Modulation of microtubule assembly by the HIV-1 Tat protein is strongly dependent on zinc binding to Tat

Background

During HIV-1 infection, the Tat protein plays a key role by transactivating the transcription of the HIV-1 proviral DNA. In addition, Tat induces apoptosis of non-infected T lymphocytes, leading to a massive loss of immune competence. This apoptosis is notably mediated by the interaction of Tat with microtubules, which are dynamic components essential for cell structure and division. Tat binds two Zn²⁺ ions through its conserved cysteine-rich region in vitro, but the role of zinc in the structure and properties of Tat is still controversial.

Results

To investigate the role of zinc, we first characterized Tat apo- and holo-forms by fluorescence correlation spectroscopy and time-resolved fluorescence spectroscopy. Both of the Tat forms are monomeric and poorly folded but differ by local conformational changes in the vicinity of the cysteine-rich region. The interaction of the two Tat forms with tubulin dimers and microtubules was monitored by analytical ultracentrifugation, turbidity measurements and electron microscopy. At 20°C, both of the Tat forms bind tubulin dimers, but only the holo-Tat was found to form discrete complexes. At 37°C, both forms promoted the nucleation and increased the elongation rates of tubulin assembly. However, only the holo-Tat increased the amount of microtubules, decreased the tubulin critical concentration, and stabilized the microtubules. In contrast, apo-Tat induced a large amount of tubulin aggregates.

Conclusion

Our data suggest that holo-Tat corresponds to the active form, responsible for the Tat-mediated apoptosis.

HIV-1 Tat and AIDS-associated cancer: targeting the cellular anti-cancer barrier?

The acquired immunodeficiency syndrome (AIDS) is accompanied by a significant increase in the incidence of neoplasms. Several causative agents have been proposed for this phenomenon. These include immunodeficiency and oncogenic DNA viruses and the HIV-1 protein Tat. Cancer in general is closely linked to genomic instability and DNA repair mechanisms. The latter maintains genomic stability and serves as a cellular anti-cancer barrier. Defects in DNA repair pathway are associated with carcinogenesis.

This review focuses on newly discovered connections of the HIV-1 protein Tat, as well as cellular co-factors of Tat, to double-strand break DNA repair. We propose that the Tat-induced DNA repair deficiencies may play a significant role in the development of AIDS-associated cancer.

Identification of functional microRNAs released through asymmetrical processing of HIV-1 TAR element

The interaction between human immunodeficiency virus type 1 (HIV-1) and RNA silencing pathways is complex and multifaceted. Essential for efficient viral transcription and supporting Tat-mediated transactivation of viral gene expression, the trans-activation responsive (TAR) element is a structured RNA located at the 5′ end of all transcripts derived from HIV-1. Here, we report that this element is a source of microRNAs (miRNAs) in cultured HIV-1-infected cell lines and in HIV-1-infected human CD4+ T lymphocytes. Using primer extension and ribonuclease (RNase) protection assays, we delineated both strands of the TAR miRNA duplex deriving from a model HIV-1 transcript, namely miR-TAR-5p and miR-TAR-3p. In vitro RNase assays indicate that the lack of a free 3′ extremity at the base of TAR may contribute to its low processing reactivity in vivo. Both miR-TAR-5p and miR-TAR-3p down-regulated TAR miRNA sensor activity in a process that required an integral miRNA-guided RNA silencing machinery. miR-TAR-3p exerted superior gene downregulatory effects, probably due to its preferential release from HIV-1 TAR RNA by the RNase III Dicer. Our study suggests that the TAR element of HIV-1 transcripts releases functionally competent miRNAs upon asymmetrical processing by Dicer, thereby providing novel insights into viral miRNA biogenesis.

The presence of the TAR RNA structure alters the programmed -1 ribosomal frameshift efficiency of the human immunodeficiency virus type 1 (HIV-1) by modifying the rate of translation initiation

HIV-1 uses a programmed -1 ribosomal frameshift to synthesize the precursor of its enzymes, Gag-Pol. The frameshift efficiency that is critical for the virus replication, is controlled by an interaction between the ribosome and a specific structure on the viral mRNA, the frameshift stimulatory signal. The rate of cap-dependent translation initiation is known to be altered by the TAR RNA structure, present at the 5' and 3' end of all HIV-1 mRNAs. Depending upon its concentration, TAR activates or inhibits the double-stranded RNA-dependent protein kinase (PKR). We investigated here whether changes in translation initiation caused by TAR affect HIV-1 frameshift efficiency. CD4+ T cells and 293T cells were transfected with a dual-luciferase construct where the firefly luciferase expression depends upon the HIV-1 frameshift. Translation initiation was altered by adding TAR in cis or trans of the reporter mRNA. We show that HIV-1 frameshift efficiency correlates negatively with changes in the rate of translation initiation caused by TAR and mediated by PKR. A model is presented where changes in the rate of initiation affect the probability of frameshifting by altering the distance between elongating ribosomes on the mRNA, which influences the frequency of encounter between these ribosomes and the frameshift stimulatory signal.

Heparin-mimicking sulfonic acid polymers as multitarget inhibitors of human immunodeficiency virus type 1 Tat and gp120 proteins

Human immunodeficiency virus (HIV) Tat and gp120 intriguingly share the feature of being basic peptides that, once released by HIV(+) cells, bind to polyanionic heparan sulfate proteoglycans (HSPGs) on target uninfected cells, contributing to the onset of AIDS-associated pathologies. To identify multitarget anti-HIV prodrugs, we investigated the gp120 and Tat antagonist potentials of a series of polyanionic synthetic sulfonic acid polymers (SSAPs). Surface plasmon resonance revealed that SSAPs inhibit with a competitive mechanism of action the binding of Tat and gp120 to surface-immobilized heparin, an experimental condition that resembles binding to cellular HSPGs. Accordingly, SSAPs inhibited HSPG-dependent cell internalization and the transactivating activity of Tat. Little is known about the binding of free gp120 to target cells. Here, we identified two classes of gp120 receptors expressed on endothelial cells, one of which was consistent with an HSPG-binding, low-affinity/high-capacity receptor that is inhibited by free heparin. SSAPs inhibited the binding of free gp120 to endothelial cells, as well as its capacity to induce apoptosis in the same cells. In all the assays, poly(4-styrenesulfonic acid) (PSS) proved to be the most potent antagonist of Tat and gp120. Accordingly, PSS bound both proteins with high affinity. In conclusion, SSAPs represent an interesting class of compounds that bind both gp120 and Tat and inhibit their HSPG-dependent cell surface binding and pathological effects. As these activities contribute to both AIDS progression and associated pathologies, SSAPs can be considered prototypic molecules for the development of multitarget drugs for the treatment of HIV infection and AIDS-associated pathologies.

Delivery of 2-5A cargo into living cells using the Tat cell penetrating peptide: 2-5A-tat

2',5'-Oligoadenylate tetramer (2-5A) has been chemically conjugated to short HIV-1 Tat peptides to provide 2-5A-tat chimeras. Two different convergent synthetic approaches have been employed to provide such 2-5A-tat bioconjugates. One involved generation of a bioconjugate through reaction of a cysteine terminated Tat peptide with a alpha-chloroacetyl derivative of 2-5A. The second synthetic strategy was based upon a cycloaddition reaction of an azide derivative of 2-5A with a Tat peptide bearing an alkyne function. Either bioconjugate of 2-5A-tat was able to activate human RNase L. The union of 2-5A and Tat peptide provided an RNase L-active chimeric nucleopeptide with the ability to be taken up by cells by virtue of the Tat peptide and to activate RNase L in intact cells. This strategy provides a valuable vehicle for the entry of the charged 2-5A molecule into cells and may provide a means for targeted destruction of HIV RNA in vivo.

Anti-HIV activity of stilbene-related heterocyclic compounds

Viral transcription has not been routinely targeted in the development of new antiviral drugs. This crucial step of the viral cycle depends on the concerted action of cellular and viral proteins such as NF-kappaB and Tat. In the present study, stilbene-related heterocyclic compounds including benzalphthalide, phthalazinone, imidazoindole and pyrimidoisoindole derivatives are tested for their anti-HIV activity. Original assays based on recombinant viruses were used to evaluate HIV replication inhibition and stably transfected cell lines were used to evaluate inhibition of Tat and NF-kappaB proteins. Some of the stilbene-related heterocyclic compounds analysed displayed anti-HIV activity through interference with NF-kappaB and Tat function. Moreover, compounds inhibiting both targets displayed a stronger activity on viral replication.

Integrin alphavbeta3 as a target for blocking HIV-1 Tat-induced endothelial cell activation in vitro and angiogenesis in vivo

OBJECTIVE

The transactivating factor (Tat) of HIV-1 binds to alphavbeta3 integrin present on endothelial cells contributing to neovascularization. Here, we investigated the biological consequences of Tat/alphavbeta3 interaction and the antagonist effect of an Arg-Gly-Asp (RGD)-based peptidomimetic.

METHODS AND RESULTS

Binding of Tat to endothelial alphavbeta3 triggers focal adhesion kinase and nuclear factor-kappaB activation, leading to endothelial cell proliferation, membrane ruffling, and motility in vitro and neovascularization in vivo. The RGD-peptidomimetic SCH221153 inhibits Tat/alphavbeta3 interaction in a solid phase binding assay and endothelial cell adhesion to immobilized Tat with a potency higher than that of RGD-containing peptides. Accordingly, SCH221153 inhibits Tat/alphavbeta3-dependent focal adhesion kinase and nuclear factor-kappaB activation, proliferation, membrane ruffling, and motility in endothelial cells. Finally, SCH221153 inhibits the angiogenic response triggered by Tat in the chick-embryo chorioallantoic membrane without affecting physiological vascularization. SCH221153 exerts these inhibitory effects without affecting the interaction of Tat with endothelial heparan sulfate proteoglycans or with the vascular endothelial growth factor receptor-2/kinase domain-containing receptor. In all the assays the negative control SCH216687 was ineffective.

CONCLUSIONS

These data provide new insights on the mechanism of endothelial cell activation by Tat and point to RGD peptidomimetics as prototypes for the development of novel Tat antagonists.

The C terminus of HIV-1 Tat modulates the extent of CD178-mediated apoptosis of T cells

HIV infection and the progression to AIDS are characterized by the depletion of CD4(+) T cells through apoptosis of the uninfected bystander cells and the direct killing of HIV-infected cells. This is mediated in part by the human immunodeficiency virus, type 1 Tat protein, which is secreted by virally infected cells and taken up by uninfected cells and CD178 gene expression, which is critically involved in T cell apoptosis. The differing ability of HIV strains to induce death of infected and uninfected cells may play a role in the clinical and biological differences displayed by HIV strains. We chemically synthesized the 86-residue truncated short variant of Tat and its full-length form. We show that the trans-activation ability of Tat at the long terminal repeat does not correlate with T cell apoptosis but that the ability of Tat to up-regulate CD178 mRNA expression and induce apoptosis in T cells is critically dependent on the C terminus of Tat. Moreover, the greater 86-residue Tat-induced apoptosis is via the extrinsic pathway of CD95-CD178.

αvβ3-integrin-dependent activation of focal adhesion kinase mediates NF-κB activation and motogenic activity by HIV-1 Tat in endothelial cells

Once in the extracellular environment, the transactivator protein HIV-1 Tat exerts several pleiotropic effects by interacting with different cellular receptors, including integrin αvβ3. Real-time surface plasmon resonance analysis reveals that Tat/αVβ3 interaction occurs with rapid kinetics (association and dissociation rates equal to 1.16×107 M-1 s-1 and 3.78×10-1 s-1, respectively) and high affinity (dissociation constant = 32 nM). Through this interaction, substratum-immobilized Tat promotes adhesion and motogenic activity in endothelial cells. Also, αvβ3/Tat interaction triggers the activation of focal adhesion kinase, RhoA and pp60src. Overexpression of the dominant negative form of focal adhesion kinase, but not of an inactive Leu1034Ser substitution mutant isoform, impairs the activation of focal adhesion kinase and RhoA, but not that of pp60src, without affecting endothelial cell adhesion and spreading. αvβ3/Tat interaction triggers the activation of NF-κB in endothelial cells in a focal adhesion kinase-, RhoA- and pp60src-dependent manner, as shown in dominant negative focal adhesion kinase transfectants or using specific pharmacological inhibitors. Finally, the activation of focal adhesion kinase, RhoA, NF-κB and pp60src are required to mediate the motogenic activity of Tat in endothelial cells.

Since Tat accumulates in an immobilized form in the extracellular matrix, these results provide new biochemical and biological insights about αvβ3/Tat interaction exploitable for the design of anti-Tat strategies.

Regulation of human immunodeficiency virus type 1 gene expression by clade-specific Tat proteins

The major group of human immunodeficiency virus type 1 (HIV-1) strains that comprise the current global pandemic have diversified during their worldwide spread into at least 10 distinct subtypes, or clades. Subtype C predominates in sub-Saharan Africa and is responsible for the majority of worldwide HIV-1 infections, subtype B predominates in North America and Europe, and subtype E is prevalent in Southeast Asia. Significant amino acid variations have been observed among the clade-specific Tat proteins. For the present study, we examined clade-specific interactions between Tat, transactivation-responsive (TAR) element, and P-TEFb proteins and how these interactions may modulate the efficiency of HIV-1 transcription. Clade-specific Tat proteins significantly modified viral gene expression. Tat proteins derived from HIV-1 clades C and E were strong transactivators of long terminal repeat (LTR) activity; Tat E also had a longer half-life than the other Tat proteins and interacted more efficiently with the stem-loop TAR element. Chimeric Tat proteins harboring the Tat E activation domain were strong transactivators of LTR expression. While Tat B, C, and E were able to rescue a Tat-defective HIV-1 proviral clone, Tat E was significantly more efficient at rescue than Tat C, possibly due to the relative stability of the Tat protein. Swapping the activation domains of Tat B, C, and E identified the cyclin T1 association domain as a critical determinant of the transactivation efficiency and of Tat-defective HIV-1 provirus rescue.

Tat-neutralizing versus Tat-protecting antibodies in rhesus macaques vaccinated with Tat peptides

The human immunodeficiency virus (HIV) regulatory protein Tat represents an attractive target for developing vaccine strategies. Both humoral and cellular responses against Tat might reduce disease progression by interfering with the deleterious functions of extracellularly secreted protein and by reducing viral replication. We have immunized Rhesus macaques intramuscularly and intranasally with a cocktail of three Tat peptides encompassing residues 1-20, 1-61 and 44-61 administrated in the presence of Montanide ISA 720 as adjuvant. The monkeys were challenged by the intrarectal route with 10 MID50 of SHIV BX08. All immunized macaques but one gave a good cross-reactive antibody response to Tat but the proliferative response and levels of IL-2, IFN-gamma and TNF-alpha secretion of peripheral blood mononuclear cells (PBMCs) recalled ex vivo with active Tat protein were weak. After viral challenge one peptide-vaccinated macaque only remained free of virus. The presence in the serum of vaccinated animals of neutralizing antibodies able to inhibit Tat transactivation activity or Tat-induced apoptosis was not correlated to protection.

DTL, the Drosophila homolog of PIMT/Tgs1 nuclear receptor coactivator-interacting protein/RNA methyltransferase, has an essential role in development

We describe a novel Drosophila gene, dtl (Drosophila Tat-like), which encodes a 60-kDa protein with RNA binding activity and a methyltransferase (MTase) domain. Dtl has an essential role in Drosophila development. The homologs of DTL recently described include PIMT (peroxisome proliferator-activated receptor-interacting protein with a methyltransferase domain), an RNA-binding protein that interacts with and enhances the nuclear receptor coactivator function, and TGS1, the methyltransferase involved in the formation of the 2,2,7-trimethylguanosine (m3G) cap of non-coding small RNAs. DTL is expressed throughout all of the developmental stages of Drosophila. The dtl mRNA has two ORFs (uORF and dORF). The product of dORF is the 60-kDa PIMT/TGS1 homolog protein that is translated from an internal AUG located 538 bp downstream from the 5' end of the message. This product of dtl is responsible for the formation of the m3G cap of small RNAs of Drosophila. Trimethylguanosine synthase activity is essential in Drosophila. The deletion in the dORF or point mutation in the putative MTase active site results in a reduced pool of m3G cap-containing RNAs and lethality in the early pupa stage. The 5' region of the dtl message also has the coding capacity (uORF) for a 178 amino acid protein. For complete rescue of the lethal phenotype of dtl mutants, the presence of the entire dtl transcription unit is required. Transgenes that carry mutations within the uORF restore the MTase activity but result in only partial rescue of the lethal phenotype. Interestingly, two transgenes bearing a mutation in uORF or dORF in trans can result in complete rescue.

Sequence variation within the dominant amino terminus epitope affects antibody binding and neutralization of human immunodeficiency virus type 1 Tat protein

Tat is among the required regulatory genes of human immunodeficiency virus type 1 (HIV-1). Tat functions both within infected cells as a transcription factor and as an extracellular factor that binds and alters bystander cells. Some functions of extracellular Tat can be neutralized by immune serum or monoclonal antibodies. In order to understand the antibody response to Tat, we are defining antibody epitopes and the effects of natural Tat sequence variation on antibody recognition. The dominant Tat epitope in macaque sera is within the first 15 amino acids of the protein amino terminus. Together with a subdominant response to amino acids 57 to 60, these two regions account for most of the macaque response to linear Tat epitopes and both regions are also sites for the binding of neutralizing antibodies. However, the dominant and subdominant epitope sequences differ among virus strains, and this natural variation can preclude antibody binding and Tat neutralization. We also examined serum samples from 31 HIV-positive individuals that contained Tat binding antibodies; 23 of the 31 sera recognized the amino terminus peptide. Similar to binding in macaques, human antibody binding to the amino terminus was affected by variations at positions 7 and 12, sequences that are distinct for clade B compared to other viral clades. Tat-neutralizing antibodies to the dominant amino terminus epitope are affected by HIV clade variation.

The glutamine-rich region of the HIV-1 Tat protein is involved in T-cell apoptosis

Human immunodeficiency virus (HIV) infection and the progression to AIDS are characterized by the depletion of CD4(+) T-cells. HIV-1 infection leads to apoptosis of uninfected bystander cells and the direct killing of HIV-infected cells. This is mediated, in part, by the HIV-1 Tat protein, which is secreted by virally infected cells and taken up by uninfected cells. We chemically synthesized two 86-residue subtype D Tat proteins, Ug05RP and Ug11LTS, from two Ugandan patients who were clinically categorized as either rapid progressor or long-term survivor, with non-conservative mutations located essentially in the glutamine-rich region. Structural heterogeneities were revealed by CD, which translate into differing trans-activational and apoptotic effects. CD data analysis and molecular modeling indicated that the short alpha-helix observed in subtype D Tat proteins from rapid progressor patients such as Tat Mal and Tat Ug05RP is not present in Ug11LTS. We show that Tat Ug05RP is more efficient than Tat Ug11LTS in its trans-activational role and in inducing apoptosis in binding tubulin via the mitochondrial pathway. The glutamine-rich region of Tat appears to be involved in the Tat-mediated apoptosis of T-cells.

Regulation of HIV-1 gene transcription: from lymphocytes to microglial cells

Transcription is a crucial step for human immunodeficiency virus type 1 (HIV-1) expression in all infected host cells, from T lymphocytes, thymocytes, monocytes, macrophages, and dendritic cells in the immune system up to microglial cells in the central nervous system. To maximize its replication, HIV-1 adapts transcription of its integrated proviral genome by ideally exploiting the specific cellular environment and by forcing cellular stimulatory events and impairing transcriptional inhibition. Multiple cell type-specific interplays between cellular and viral factors perform the challenge for the virus to leave latency and actively replicate in a great diversity of cells, despite the variability of its long terminal repeat region in different HIV strains. Knowledge about the molecular mechanisms underlying transcriptional regulatory events helps in the search for therapeutic agents that target the step of transcription in anti-HIV strategies.

Interaction between HIV-1 Tat and pRb2/p130: a possible mechanism in the pathogenesis of AIDS-related neoplasms

Tat protein is an early nonstructural protein necessary for virus replication, which is secreted by infected cells and taken up by uninfected cells. Extensive evidence indicates that Tat may be a cofactor in the development of AIDS-related neoplasms. The molecular mechanism underlying Tat's oncogenic activity may include deregulation of cellular genes. Among these genes, it has recently been shown that pRb2/p130 oncosuppressor protein is one of the targets in the interaction between HIV gene product Tat and host proteins. However, whether the HIV-1 gene product Tat may inactivate the oncosuppressive function of pRb2/p130 has not yet been elucidated. Here, we show that mRNA levels of pRb2/p130 increase in the presence of Tat, whereas no change in the phosphorylation status of pRb2/p130 is observed. In addition, Tat can inhibit the growth control activity exerted by pRb2/p130 in the T98G cell line. Finally, Tat does not compete with E2F-4 in binding to pRb2/p130. The interaction between Tat and pRb2/p130 seems to result in the deregulation of the control exerted by pRb2/p130 on the cell cycle. Taken together, these results open a window on the role of pRb2/p130 in AIDS-related oncogenesis.

Additive activity between the trans-activation response RNA-binding protein, TRBP2, and cyclin T1 on HIV type 1 expression and viral production in murine cells

Tat-mediated trans-activation of the HIV-1 long terminal repeat (LTR) occurs through the phosphorylation of the carboxy-terminal domain of the RNA polymerase II. The kinase complex, pTEFb, composed of cyclin T1 (CycT1) and CDK9, mediates this process. The trans-activation response (TAR) RNA-binding protein 2 (TRBP2) increases HIV-1 LTR expression through TAR and protein kinase R (PKR) binding, but not through interactions with the Tat-CycT1-CDK9 complex. TRBP2 and the Tat-CycT1-CDK9 complex have overlapping binding sites on TAR RNA. TRBP2 and CycT1 increased Tat trans-activation in NIH 3T3 cells with additive effects. Upon transfection of HIV-1 pLAI, pNL4-3, pMAL, and pAD molecular clones, reverse transcriptase (RT) activity and p24 concentration were decreased 200- to 900-fold in NIH 3T3 cells compared with HeLa cells in both cells and supernatants. In murine cells, cotransfection of the HIV clones with CycT1 or TRBP2 increased modestly the expression of RT activity in cell extracts. The analysis of Gag expression in murine cells transfected with CycT1 compared with human cells showed a 20-fold decrease in expression and a strong processing defect. The expression of both CycT1 and TRBP2 had a more than additive activity on RT function in cell extracts and on viral particle production in supernatant of murine cells. These results suggest an activity of CycT1 and TRBP2 at different steps in HIV-1 expression and indicate the requirement for another posttranscriptional factor in murine cells for full HIV replication.

Specificity and effect on apoptosis of Tat antibodies from vaccinated and SHIV-infected rhesus macaques and HIV-infected individuals

Recent contributions have demonstrated that actively secreted Tat protein plays an important functional role in human immunodeficiency virus-1 (HIV-1) infection and that Tat antibodies might interfere with disease progression by blocking the protein extracellularly. In this context we have studied the recognition of several Tat mutants as well as various synthetic Tat fragments by anti-Tat monoclonal antibodies and by IgG antibodies from a large collection of slow and fast-progressor infected individuals. We have also tested the sera from simian/human immunodeficiency virus (SHIV)-infected macaques with these Tat peptides. Important differences were found between long-term non-progressors and fast-progressors, and between human and monkey sera in terms of antibody specificity. Rabbits and macaques were immunised with several Tat peptides and we found that certain antibody subsets from immunised animals recognised the cognate protein Tat and had the capacity to inhibit Tat-induced apoptosis of T cells. Such antibodies might be important for controlling Tat-induced death in cells uninfected by HIV-1.

Recruitment of Tat to heterochromatin protein HP1 via interaction with CTIP2 inhibits human immunodeficiency virus type 1 replication in microglial cells

The Tat protein of human immunodeficiency virus type 1 (HIV-1) plays a key role as inducer of viral gene expression. We report that Tat function can be potently inhibited in human microglial cells by the recently described nuclear receptor cofactor chicken ovalbumin upstream promoter transcription factor-interacting protein 2 (CTIP2). Overexpression of CTIP2 leads to repression of HIV-1 replication, as a result of inhibition of Tat-mediated transactivation. In contrast, the related CTIP1 was unable to affect Tat function and viral replication. Using confocal microscopy to visualize Tat subcellular distribution in the presence of the CTIPs, we found that overexpression of CTIP2, and not of CTIP1, leads to disruption of Tat nuclear localization and recruitment of Tat within CTIP2-induced nuclear ball-like structures. In addition, our studies demonstrate that CTIP2 colocalizes and associates with the heterochromatin-associated protein HP1alpha. The CTIP2 protein harbors two Tat and HP1 interaction interfaces, the 145-434 and the 717-813 domains. CTIP2 and HP1alpha associate with Tat to form a three-protein complex in which the 145-434 CTIP2 domain interacts with the N-terminal region of Tat, while the 717-813 domain binds to HP1. The importance of this Tat binding interface and of Tat subnuclear relocation was confirmed by analysis of CTIP2 deletion mutants. Our findings suggest that inhibition of HIV-1 expression by CTIP2 correlates with recruitment of Tat within CTIP2-induced structures and relocalization within inactive regions of the chromatin via formation of the Tat-CTIP2-HP1alpha complex. These data highlight a new mechanism of Tat inactivation through subnuclear relocalization that may ultimately lead to inhibition of viral pathogenesis.

Suppression of an intrinsic strand transfer activity of HIV-1 Tat protein by its second-exon sequences

The Tat protein of human immunodeficiency virus type 1 (HIV-1) has been shown to restrict premature reverse transcription at late stages of virus infection and to thus ensure the integrity of the viral RNA genome for packaging. To gain further insights into the roles of Tat in HIV-1 reverse transcription, we have assessed its effects on the first-strand transfer during the synthesis of minus-strand DNA through use of a reconstituted cell-free system. The results demonstrated that a form of Tat, containing only the first exon (Tat72), was able to enhance the first-strand transfer as efficiently as did the viral nucleocapsid protein. Coincidentally, this form of Tat was unable to inhibit the production of minus-strand strong-stop DNA. Further studies with various mutated forms of Tat showed that its Cys-rich region, rather than the core and Arg-rich domains, was essential for this strand transfer activity. Moreover, this activity of Tat is largely independent of the TAR RNA structure. Although full-length Tat protein (Tat86) was also able to promote strand transfer, this activity was limited by a strong overall inhibition of reverse transcription because of the presence of the second Tat exon. Other nucleic-acid-binding proteins (e.g., single-strand DNA-binding protein) were employed as negative controls and were unable to promote strand transfer in these assays. We propose that Tat possesses nucleic acid chaperone activity and can promote the first-strand transfer during HIV-1 reverse transcription; however, these activities are restricted by the second Tat exon, and the roles of these Tat activities in viral replication remain to be elucidated.

Differential regulation of HIV-1 clade-specific B, C, and E long terminal repeats by NF-kappaB and the Tat transactivator

The major group of human immunodeficiency viruses (HIV-1) that comprise the current global pandemic have diversified during their worldwide spread and may be divided into at least 10 distinct subtypes or clades, A through J. Subtype B predominates in North America and Europe, subtype E predominates in Southeast Asia, and subtype C predominates in sub-Saharan Africa. Functional distinctions in long terminal repeat (LTR) architecture among HIV subtypes have been identified, thus raising the possibility that regulatory divergence among the subtypes of HIV-1 has occurred. In addition to the transcriptional specificity of the HIV-1 LTR, productive HIV-1 replication is also dependent upon the viral Tat protein. Therefore, we sought to investigate whether interactions between host signaling pathways and the NF-kappaB regions of different HIV-1 subtypes, together with subtype-specific interactions between Tat, TAR, and cellular proteins, modulate the efficiency of HIV-1 clade-specific gene transcription. We demonstrate that the NF-kappaB sites of subtypes B and E both bind NF-kappaB-related complexes. However, the duplicated kappaB sites of the C subtype do not compete for NF-kappaB binding. Also, clade E Tat protein possesses the highest transactivation capacity, regardless of the LTR context. Furthermore, preliminary evidence suggests that the acetylation of subtype-specific Tat proteins may correlate with their transactivation efficiency.

Cell surface heparan sulfate and its roles in assisting viral infections

Heparan sulfate, a highly sulfated polysaccharide, is present on the surface of mammalian cells and in the extracellular matrix in large quantities. The sulfated monosaccharide sequences within heparan sulfate determine the protein binding specificity and regulate biological functions. Numerous viruses and parasites utilize cell surface heparan sulfate as receptors to infect target cells. Due to the structural complexity of heparan sulfate, it was considered a nonspecific cell surface receptor by interacting with the positive motifs of viral proteins. However, recent studies reveal that heparan sulfate plays multiple roles in assisting viral infection, and the activities in promoting viral infections require unique monosaccharide sequences, suggesting that heparan sulfate could serve as a specific receptor for viral infection. The currently available techniques for the structural analysis of heparan sulfate provide essential information about the specific roles of heparan sulfate in assisting viral infections. The knowledge accumulated in this fast growing field will permit us to have a better understanding of the mechanism of viral infection and will lead to the development of new antiviral agents.

Two dimerization domains in the trans-activation response RNA-binding protein (TRBP) individually reverse the protein kinase R inhibition of HIV-1 long terminal repeat expression

Trans-activation response (TAR) RNA-binding protein (TRBP) is a cellular protein that binds to the human immunodeficiency virus-1 (HIV-1) TAR element RNA. It has two double-stranded RNA binding domains (dsRBDs), but only one is functional for TAR binding. TRBP interacts with the interferon-induced protein kinase R (PKR) and inhibits its activity. We used the yeast two-hybrid assay to map the interaction sites between the two proteins. We show that TRBP and PKR-N (178 first amino acids of PKR) interact with PKR wild type and inhibit the PKR-induced yeast growth defect in this assay. We characterized two independent PKR-binding sites in TRBP. These sites are located in each dsRBD in TRBP, indicating that PKR-TRBP interaction does not require the RNA binding activity present only in dsRBD2. TRBP and its fragments that interact with PKR reverse the PKR-induced suppression of HIV-1 long terminal repeat expression. In addition, TRBP activates the HIV-1 long terminal repeat expression to a larger extent than the addition of each domain. These data suggest that TRBP activates gene expression in PKR-dependent and PKR-independent manners.

Pentosan polysulfate as an inhibitor of extracellular HIV-1 Tat

HIV-1 Tat protein, released from HIV-infected cells, may act as a pleiotropic heparin-binding growth factor. From this observation, extracellular Tat has been implicated in the pathogenesis of AIDS and of AIDS-associated pathologies. Here we demonstrate that the heparin analog pentosan polysulfate (PPS) inhibits the interaction of glutathione S-transferase (GST)-Tat protein with heparin immobilized to a BIAcore sensor chip. Competition experiments showed that Tat-PPS interaction occurs with high affinity (K(d) = 9.0 nm). Also, GST.Tat prevents the binding of [(3)H]heparin to GST.Tat immobilized to glutathione-agarose beads. In vitro, PPS inhibits GST.Tat internalization and, consequently, HIV-1 long terminal repeat transactivation in HL3T1 cells. Also, PPS inhibits cell surface interaction and mitogenic activity of GST.Tat in murine adenocarcinoma T53 Tat-less cells. In all assays, PPS exerts its Tat antagonist activity with an ID(50) equal to approximately 1.0 nm. In vivo, PPS inhibits the neovascularization induced by GST.Tat or by Tat-overexpressing T53 cells in the chick embryo chorioallantoic membrane. In conclusion, PPS binds Tat protein and inhibits its cell surface interaction, internalization, and biological activity in vitro and in vivo. PPS may represent a prototypic molecule for the development of novel Tat antagonists with therapeutic implications in AIDS and AIDS-associated pathologies, including Kaposi's sarcoma.