The cAMP-dependent protein kinase A and protein kinase C-beta pathways synergistically interact to activate HIV-1 transcription in latently infected cells of monocyte/macrophage lineage

HIV Expression in Infected T Cell Clones

The principal barrier to an HIV-1 cure is the persistence of infected cells harboring replication-competent proviruses despite antiretroviral therapy (ART). HIV-1 transcriptional suppression, referred to as viral latency, is foremost among persistence determinants, as it allows infected cells to evade the cytopathic effects of virion production and killing by cytotoxic T lymphocytes (CTL) and other immune factors. HIV-1 persistence is also governed by cellular proliferation, an innate and essential capacity of CD4+ T cells that both sustains cell populations over time and enables a robust directed response to immunological threats. However, when HIV-1 infects CD4+ T cells, this capacity for proliferation can enable surreptitious HIV-1 propagation without the deleterious effects of viral gene expression in latently infected cells. Over time on ART, the HIV-1 reservoir is shaped by both persistence determinants, with selective forces most often favoring clonally expanded infected cell populations harboring transcriptionally quiescent proviruses. Moreover, if HIV latency is incomplete or sporadically reversed in clonal infected cell populations that are replenished faster than they are depleted, such populations could both persist indefinitely and contribute to low-level persistent viremia during ART and viremic rebound if treatment is withdrawn. In this review, select genetic, epigenetic, cellular, and immunological determinants of viral transcriptional suppression and clonal expansion of HIV-1 reservoir T cells, interdependencies among these determinants, and implications for HIV-1 persistence will be presented and discussed.

Targeting ER stress/PKA/GSK-3β/β-catenin pathway as a potential novel strategy for hepatitis C virus-infected patients

BACKGROUND

Chronic hepatitis C virus (HCV) infection causes hepatocellular carcinoma (HCC). The HCC risk, while decreased compared with active HCV infection, persists in HCV-cured patients by direct-acting antiviral agents (DAA). We previously demonstrated that Wnt/β-catenin signaling remained activated after DAA-mediated HCV eradication. Developing therapeutic strategies to both eradicate HCV and reverse Wnt/β-catenin signaling is needed.

METHODS

Cell-based HCV long term infection was established. Chronically HCV infected cells were treated with DAA, protein kinase A (PKA) inhibitor H89 and endoplasmic reticulum (ER) stress inhibitor tauroursodeoxycholic acid (TUDCA). Western blotting analysis and fluorescence microscopy were performed to determine HCV levels and component levels involved in ER stress/PKA/glycogen synthase kinase-3β (GSK-3β)/β-catenin pathway. Meanwhile, the effects of H89 and TUDCA were determined on HCV infection.

RESULTS

Both chronic HCV infection and replicon-induced Wnt/β-catenin signaling remained activated after HCV and replicon eradication by DAA. HCV infection activated PKA activity and PKA/GSK-3β-mediated Wnt/β-catenin signaling. Inhibition of PKA with H89 both repressed HCV and replicon replication and reversed PKA/GSK-3β-mediated Wnt/β-catenin signaling in both chronic HCV infection and replicon. Both chronic HCV infection and replicon induced ER stress. Inhibition of ER stress with TUDCA both repressed HCV and replicon replication and reversed ER stress/PKA/GSK-3β-dependent Wnt/β-catenin signaling. Inhibition of either PKA or ER stress both inhibited extracellular HCV infection.

CONCLUSION

Targeting ER stress/PKA/GSK-3β-dependent Wnt/β-catenin signaling with PKA inhibitor could be a novel therapeutic strategy for HCV-infected patients to overcomes the issue of remaining activated Wnt/β-catenin signaling by DAA treatment. Video Abstract.

Cardiomyoblast caveolin expression: effects of simulated diabetes, α-linolenic acid, and cell signaling pathways

Caveolins regulate myocardial substrate handling, survival signaling, and stress resistance; however, control of expression is incompletely defined. We test how metabolic features of type 2 diabetes (T2D), and modulation of cell signaling, influence caveolins in H9c2 cardiomyoblasts. Cells were exposed to glucose (25 vs. 5 mM), insulin (100 nM), or palmitate (0.1 mM), individually or combined, and the effects of adenylate cyclase (AC) activation (50 μM forskolin), focal adhesion kinase (FAK) or protein kinase C β2 (PKCβ2) inhibition (1 μM FAK inhibitor 14 or CGP-53353, respectively) or the polyunsaturated fatty acid (PUFA) α-linolenic acid (ALA; 10 μM) were tested. Simulated T2D (elevated glucose + insulin + palmitate) depressed caveolin-1 and -3 without modifying caveolin-2. Caveolin-3 repression was primarily palmitate dependent, whereas high glucose (HG) and insulin independently increased caveolin-3 (while reducing expression when combined). Differential control was evident: baseline caveolin-3 was suppressed by FAK/PKCβ2 and insensitive to AC activities, with baseline caveolin-1 and -2 suppressed by AC and insensitive to FAK/PKCβ2. Forskolin and ALA selectively preserved caveolin-3 in T2D cells, whereas PKCβ2 and FAK inhibition increased caveolin-3 under all conditions. Despite preservation of caveolin-3, ALA did not modify nucleosome content (apoptosis marker) or transcription of proinflammatory mediators in T2D cells. In summary, caveolin-1 and -3 are strongly repressed with simulated T2D, with caveolin-3 particularly sensitive to palmitate; intrinsic PKCβ2 and FAK activities depress caveolin-3 in healthy and stressed cells; ALA and AC activation and PKCβ2 inhibition preserve caveolin-3 under T2D conditions; and caveolin-3 changes with T2D and ALA appear unrelated to inflammatory signaling or extent of apoptosis.

Transcriptional profiling indicates cAMP-driven reversal of HIV latency in monocytes occurs via transcription factor SP-1

Latent HIV reservoir is a major barrier to absolute HIV cure. Studies on latency reversal agents (LRA) have by far focused mainly on CD4+ T-lymphocytes, while myeloid reservoirs remain under-represented despite their persistence and key contribution to HIV pathogenesis. cAMP has been shown to increase HIV-1 transcription in latently-infected monocytes/macrophages. In this communication, we explored the potential of commercially available pharmacological drugs and phosphodiesterase inhibitors to reactivate HIV in latently-infected monocytic cell-line, U1. We showed that increased levels of intracellular cAMP reverse HIV latency in vitro, which is specific to cells of the myeloid lineage. High throughput RNA-seq analysis revealed that cAMP modulates transcriptional profile of latently HIV-infected cells and provides favourable cellular environment for HIV to produce viral proteins. This reactivation of latent HIV was inhibited by Mithramycin A, a selective Sp1 inhibitor, indicating that the reversal of HIV latency in monocytes is driven by transcription factor Sp1.

Natural product-derived compounds in HIV suppression, remission, and eradication strategies

While combination antiretroviral therapy (cART) has successfully converted HIV to a chronic but manageable infection in many parts of the world, HIV continues to persist within latent cellular reservoirs, which can become reactivated at any time to produce infectious virus. New therapies are therefore needed not only for HIV suppression but also for containing or eliminating HIV reservoirs. Compounds derived from plant, marine, and other natural products have been found to combat HIV infection and/or target HIV reservoirs, and these discoveries have substantially guided current HIV therapy-based studies. Here we summarize the role of natural product-derived compounds in current HIV suppression, remission, and cure strategies.

Identification of Novel HIV-1 Latency-Reversing Agents from a Library of Marine Natural Products

Natural products originating from marine and plant materials are a rich source of chemical diversity and unique antimicrobials. Using an established in vitro model of HIV-1 latency, we screened 257 pure compounds from a marine natural product library and identified 4 (psammaplin A, aplysiatoxin, debromoaplysiatoxin, and previously-described alotaketal C) that induced expression of latent HIV-1 provirus in both cell line and primary cell models. Notably, aplysiatoxin induced similar levels of HIV-1 expression as prostratin but at up to 900-fold lower concentrations and without substantial effects on cell viability. Psammaplin A enhanced HIV-1 expression synergistically when treated in combination with the protein kinase C (PKC) activator prostratin, but not the histone deacetylase inhibitor (HDACi) panobinostat, suggesting that psammaplin A functions as a latency-reversing agent (LRA) of the HDACi class. Conversely, aplysiatoxin and debromoaplysiatoxin synergized with panobinostat but not prostratin, suggesting that they function as PKC activators. Our study identifies new compounds from previously untested marine natural products and adds to the repertoire of LRAs that can inform therapeutic “shock-and-kill”-based strategies to eliminate latent HIV-infected reservoirs.

Molecular Mechanisms for cAMP-Mediated Immunoregulation in T cells - Role of Anchored Protein Kinase A Signaling Units

The cyclic AMP/protein kinase A (cAMP/PKA) pathway is one of the most common and versatile signal pathways in eukaryotic cells. A-kinase anchoring proteins (AKAPs) target PKA to specific substrates and distinct subcellular compartments providing spatial and temporal specificity for mediation of biological effects channeled through the cAMP/PKA pathway. In the immune system, cAMP is a potent negative regulator of T cell receptor-mediated activation of effector T cells (Teff) acting through a proximal PKA/Csk/Lck pathway anchored via a scaffold consisting of the AKAP Ezrin holding PKA, the linker protein EBP50, and the anchoring protein phosphoprotein associated with glycosphingolipid-enriched microdomains holding Csk. As PKA activates Csk and Csk inhibits Lck, this pathway in response to cAMP shuts down proximal T cell activation. This immunomodulating pathway in Teff mediates clinically important responses to regulatory T cell (Treg) suppression and inflammatory mediators, such as prostaglandins (PGs), adrenergic stimuli, adenosine, and a number of other ligands. A major inducer of T cell cAMP levels is PG E2 (PGE2) acting through EP2 and EP4 prostanoid receptors. PGE2 plays a crucial role in the normal physiological control of immune homeostasis as well as in inflammation and cancer immune evasion. Peripherally induced Tregs express cyclooxygenase-2, secrete PGE2, and elicit the immunosuppressive cAMP pathway in Teff as one tumor immune evasion mechanism. Moreover, a cAMP increase can also be induced by indirect mechanisms, such as intercellular transfer between T cells. Indeed, Treg, known to have elevated levels of intracellular cAMP, may mediate their suppressive function by transferring cAMP to Teff through gap junctions, which we speculate could also be regulated by PKA/AKAP complexes. In this review, we present an updated overview on the influence of cAMP-mediated immunoregulatory mechanisms acting through localized cAMP signaling and the therapeutical increasing prospects of AKAPs disruptors in T-cell immune function.

Mycobacterial and HIV infections up-regulated human zinc finger protein 134, a novel positive regulator of HIV-1 LTR activity and viral propagation

BACKGROUND

Concurrent occurrence of HIV and Tuberculosis (TB) infections influence the cellular environment of the host for synergistic existence. An elementary approach to understand such coalition at the molecular level is to understand the interactions of the host and the viral factors that subsequently effect viral replication. Long terminal repeats (LTR) of HIV genome serve as a template for binding trans-acting viral and cellular factors that regulate its transcriptional activity, thereby, deciding the fate of HIV pathogenesis, making it an ideal system to explore the interplay between HIV and the host.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, using biotinylated full length HIV-1 LTR sequence as bait followed by MALDI analyses, we identified and further characterized human-Zinc-finger-protein-134 (hZNF-134) as a novel positive regulator of HIV-1 that promoted LTR-driven transcription and viral production. Over-expression of hZNF-134 promoted LTR driven luciferase activity and viral transcripts, resulting in increased virus production while siRNA mediated knockdown reduced both the viral transcripts and the viral titers, establishing hZNF-134 as a positive effector of HIV-1. HIV, Mycobacteria and HIV-TB co-infections increased hZNF-134 expressions in PBMCs, the impact being highest by mycobacteria. Corroborating these observations, primary TB patients (n = 22) recorded extraordinarily high transcript levels of hZNF-134 as compared to healthy controls (n = 16).

CONCLUSIONS/SIGNIFICANCE

With these observations, it was concluded that hZNF-134, which promoted HIV-1 LTR activity acted as a positive regulator of HIV propagation in human host. High titers of hZNF-134 transcripts in TB patients suggest that up-regulation of such positive effectors of HIV-1 upon mycobacterial infection can be yet another mechanism by which mycobacteria assists HIV-1 propagation during HIV-TB co-infections. hZNF-134, an uncharacterized host protein, thus assumes a novel regulatory role during HIV-host interactions. Our study provides new insights into the emerging role of zinc finger proteins in HIV-1 pathogenesis.

Coregulation of HIV-1 dependency factors in individuals heterozygous to the CCR5-delta32 deletion

Background

CCR5-delta32 heterozygous individuals are susceptible to HIV-1. However, it is not clear if there is a relevant protective effect against transmission and a beneficial effect in terms of HIV progression which cannot be attributed to CCR5 surface density alone. Therefore we investigated HIV-1 dependency factors (HDF) which might be differently regulated in CCR5 wild type (WT) and CCR5-delta32 heterozygous individuals.

Methods

We examined CD34+ hematopoietic progenitor cells derived from bone marrow samples from 19 healthy volunteers, 12 individuals with CCR5 WT and 7 with heterozygous CCR5-delta32 deletion. Samples were analyzed using a global gene expression oligonucleotide microarray (HG-U133plus 2.0, Affymetrix Inc.).

Results

A total of 205 genes were found with altered expression (3fold difference, present call rate of 75%, p < 0.05) and 7 of these had a connection to HIV-1 pathogenesis. In 4 genes: TOP1, CXCR2, SREBF2, and TAP we found a different regulation which was consistent with a supposed beneficial effect for CCR5-delta32 heterozygotes.

Conclusion

The CCR5-delta32 deletion is associated with other HDFs in HIV-1 pathogenesis as a possible explanation for beneficial effects regarding the deletion leading to a variant expression profile in heterozygous carriers of this mutation.

Herpes simplex virus type 1 infection activates the Epstein-Barr virus replicative cycle via a CREB-dependent mechanism

The reactivation of latent Epstein-Barr virus (EBV) to lytic replication is important in pathogenesis and requires virus-host cellular interactions. However, the mechanism underlying the reactivation of EBV is not yet fully understood. In the present study, herpes simplex virus type 1 (HSV-1) was shown to induce the reactivation of latent EBV by triggering BZLF1 expression. The BZLF1 promoter (Zp) was not activated by HSV-1 essential glycoprotein-induced membrane fusion. Nevertheless, Zp was activated within 6 h post HSV-1 infection in virus entry-dependent and replication-independent manners. Using a panel of Zp deletion mutants, HSV-1 was shown to promote Zp through a cyclic adenosine monophosphate (cAMP) response element (CRE) located in ZII. The phosphorylated cAMP response element-binding (phos-CREB) protein, the cellular transactivator that binds to CRE, also increased after HSV-1 infection. By transient transfection, cAMP-dependent protein kinase A and HSV-1 US3 protein were found to be capable of activating Zp in CREB- and CRE-dependent manners. The relationship between EBV activation and HSV-1 infection revealed a possible common mechanism that stimulated latent EBV into lytic cycles in vivo.

Urokinase plasminogen activator inhibits HIV virion release from macrophage-differentiated chronically infected cells via activation of RhoA and PKCε

BACKGROUND

HIV replication in mononuclear phagocytes is a multi-step process regulated by viral and cellular proteins with the peculiar feature of virion budding and accumulation in intra-cytoplasmic vesicles. Interaction of urokinase-type plasminogen activator (uPA) with its cell surface receptor (uPAR) has been shown to favor virion accumulation in such sub-cellular compartment in primary monocyte-derived macrophages and chronically infected promonocytic U1 cells differentiated into macrophage-like cells by stimulation with phorbol myristate acetate (PMA). By adopting this latter model system, we have here investigated which intracellular signaling pathways were triggered by uPA/uPAR interaction leading the redirection of virion accumulation in intra-cytoplasmic vesicles.

RESULTS

uPA induced activation of RhoA, PKCδ and PKCε in PMA-differentiated U1 cells. In the same conditions, RhoA, PKCδ and PKCε modulated uPA-induced cell adhesion and polarization, whereas only RhoA and PKCε were also responsible for the redirection of virions in intracellular vesicles. Distribution of G and F actin revealed that uPA reorganized the cytoskeleton in both adherent and polarized cells. The role of G and F actin isoforms was unveiled by the use of cytochalasin D, a cell-permeable fungal toxin that prevents F actin polymerization. Receptor-independent cytoskeleton remodeling by Cytochalasin D resulted in cell adhesion, polarization and intracellular accumulation of HIV virions similar to the effects gained with uPA.

CONCLUSIONS

These findings illustrate the potential contribution of the uPA/uPAR system in the generation and/or maintenance of intra-cytoplasmic vesicles that actively accumulate virions, thus sustaining the presence of HIV reservoirs of macrophage origin. In addition, our observations also provide evidences that pathways controlling cytoskeleton remodeling and activation of PKCε bear relevance for the design of new antiviral strategies aimed at interfering with the partitioning of virion budding between intra-cytoplasmic vesicles and plasma membrane in infected human macrophages.

Control of Kaposi's sarcoma-associated herpesvirus reactivation induced by multiple signals

The ability to control cellular functions can bring about many developments in basic biological research and its applications. The presence of multiple signals, internal as well as externally imposed, introduces several challenges for controlling cellular functions. Additionally the lack of clear understanding of the cellular signaling network limits our ability to infer the responses to a number of signals. This work investigates the control of Kaposi's sarcoma-associated herpesvirus reactivation upon treatment with a combination of multiple signals. We utilize mathematical model-based as well as experiment-based approaches to achieve the desired goals of maximizing virus reactivation. The results show that appropriately selected control signals can induce virus lytic gene expression about ten folds higher than a single drug; these results were validated by comparing the results of the two approaches, and experimentally using multiple assays. Additionally, we have quantitatively analyzed potential interactions between the used combinations of drugs. Some of these interactions were consistent with existing literature, and new interactions emerged and warrant further studies. The work presents a general method that can be used to quantitatively and systematically study multi-signal induced responses. It enables optimization of combinations to achieve desired responses. It also allows identifying critical nodes mediating the multi-signal induced responses. The concept and the approach used in this work will be directly applicable to other diseases such as AIDS and cancer.

Mice with disrupted type I protein kinase A anchoring in T cells resist retrovirus-induced immunodeficiency

Type I protein kinase A (PKA) is targeted to the TCR-proximal signaling machinery by the A-kinase anchoring protein ezrin and negatively regulates T cell immune function through activation of the C-terminal Src kinase. RI anchoring disruptor (RIAD) is a high-affinity competitor peptide that specifically displaces type I PKA from A-kinase anchoring proteins. In this study, we disrupted type I PKA anchoring in peripheral T cells by expressing a soluble ezrin fragment with RIAD inserted in place of the endogenous A-kinase binding domain under the lck distal promoter in mice. Peripheral T cells from mice expressing the RIAD fusion protein (RIAD-transgenic mice) displayed augmented basal and TCR-activated signaling, enhanced T cell responsiveness assessed as IL-2 secretion, and reduced sensitivity to PGE(2)- and cAMP-mediated inhibition of T cell function. Hyperactivation of the cAMP-type I PKA pathway is involved in the T cell dysfunction of HIV infection, as well as murine AIDS, a disease model induced by infection of C57BL/6 mice with LP-BM5, a mixture of attenuated murine leukemia viruses. LP-BM5-infected RIAD-transgenic mice resist progression of murine AIDS and have improved viral control. This underscores the cAMP-type I PKA pathway in T cells as a putative target for therapeutic intervention in immunodeficiency diseases.

Cholera toxin inhibits HIV-1 replication in human colorectal epithelial HT-29 cells through adenylate cyclase activation

Mixed feeding, combining breast milk and nonhuman milk and/or solid food, is a common practice in developing countries that increases the risk of vertical HIV-1 transmission. It also enhances the risk of infection by waterborne microorganisms such as Vibrio cholerae, a diarrhoea-causing pathogen that frequently infects children below 18 months of age. Although both HIV-1 and V. cholerae affect young children and target intestinal epithelial cells, no information is currently available on possible interactions between these two pathogens. In this study, we show for the first time that cholera toxin (CTx), at a concentration as low as 100 pg/ml, inhibits HIV-1 infection of HT-29, a human colorectal epithelial cell line. The CTx-mediated inhibitory effect does not result from a down-regulation of receptor/co-receptor expression or a modulation of viral transcription. Nevertheless, additional experiments indicate that a yet to be identified early step in the virus life cycle is targeted by CTx since the enterotoxin similarly reduces infection of HT-29 cells with AMLV-I, HTLV-I and HIV-1 pseudotyped viruses while exerting no effect on infection with VSV-G pseudotypes. Furthermore, our results indicate that the CTx-dependent suppression is not due to the cholera toxin subunit B but linked instead to the action of cholera toxin subunit A (CTA). Altogether our data indicate that the CTA subunit of CTx is negatively affecting an early event in HIV-1 replication in human colon cancer HT-29 cells.

Reverse Transcriptase and Cellular Factors: Regulators of HIV-1 Reverse Transcription

There is ample evidence that synthesis of HIV-1 proviral DNA from the viral RNA genome during reverse transcription requires host factors. However, only a few cellular proteins have been described in detail that affect reverse transcription and interact with reverse transcriptase (RT). HIV-1 integrase is an RT binding protein and a number of IN-binding proteins including INI1, components of the Sin3a complex, and Gemin2 affect reverse transcription. In addition, recent studies implicate the cellular proteins HuR, AKAP149, and DNA topoisomerase I in reverse transcription through an interaction with RT. In this review we will consider interactions of reverse transcription complex with viral and cellular factors and how they affect the reverse transcription process.

Protein kinase A-dependent step(s) in hepatitis C virus entry and infectivity

Viruses exploit signaling pathways to their advantage during multiple stages of their life cycle. We demonstrate a role for protein kinase A (PKA) in the hepatitis C virus (HCV) life cycle. The inhibition of PKA with H89, cyclic AMP (cAMP) antagonists, or the protein kinase inhibitor peptide reduced HCV entry into Huh-7.5 hepatoma cells. Bioluminescence resonance energy transfer methodology allowed us to investigate the PKA isoform specificity of the cAMP antagonists in Huh-7.5 cells, suggesting a role for PKA type II in HCV internalization. Since viral entry is dependent on the host cell expression of CD81, scavenger receptor BI, and claudin-1 (CLDN1), we studied the role of PKA in regulating viral receptor localization by confocal imaging and fluorescence resonance energy transfer (FRET) analysis. Inhibiting PKA activity in Huh-7.5 cells induced a reorganization of CLDN1 from the plasma membrane to an intracellular vesicular location(s) and disrupted FRET between CLDN1 and CD81, demonstrating the importance of CLDN1 expression at the plasma membrane for viral receptor activity. Inhibiting PKA activity in Huh-7.5 cells reduced the infectivity of extracellular virus without modulating the level of cell-free HCV RNA, suggesting that particle secretion was not affected but that specific infectivity was reduced. Viral particles released from H89-treated cells displayed the same range of buoyant densities as did those from control cells, suggesting that viral protein association with lipoproteins is not regulated by PKA. HCV infection of Huh-7.5 cells increased cAMP levels and phosphorylated PKA substrates, supporting a model where infection activates PKA in a cAMP-dependent manner to promote virus release and transmission.

NF-kappaB-repressing factor inhibits elongation of human immunodeficiency virus type 1 transcription by DRB sensitivity-inducing factor

Human immunodeficiency virus type 1 (HIV-1) is able to establish a latent infection during which the integrated provirus remains transcriptionally silent. In response to specific stimuli, the HIV-1 long terminal repeat (LTR) is highly activated, enhancing both transcriptional initiation and elongation. Here, we have identified a specific binding sequence of the nuclear NF-kappaB-repressing factor (NRF) within the HIV-1 LTR. The aim of this work was to define the role of NRF in regulating the LTR. Our data show that the endogenous NRF is required for transcriptional activation of the HIV-1 LTR in stimulated cells. In unstimulated cells, however, NRF inhibits HIV-1 LTR activity at the level of transcription elongation. Binding of NRF to the LTR in unstimulated cells prevents recruitment of elongation factor DRB sensitivity-inducing factor and formation of processive elongation complexes by hyperphosphorylated RNA polymerase II. Our data suggest that NRF interrupts the regulatory coupling of LTR binding factors and transcription elongation events. This inhibitory mechanism might contribute to transcriptional quiescence of integrated HIV-1 provirus.

Involvement of histamine H1 and H2 receptors in the regulation of STAT-1 phosphorylation: inverse agonism exhibited by the receptor antagonists

Signal transducer and activator of transcription-1 (STAT1) is a latent signal transducer protein which, on phosphorylation, is translocated from the cytoplasm to the nucleus and is subsequently activated. This study was designed to determine the involvement of histamine receptors in histamine-mediated effect on STAT1 phosphorylation. It is known that the actions of histamine mediated through H1 and H2 receptors are dependent on their respective downstream pathways, Ca(2+)-PKC and cAMP-PKA. In this study, we investigated the significance of PKA in STAT1 phosphorylation. C57BL/6 mouse splenocytes were isolated and treated with histamine (10(-7)-10(-4) M) and then activated with PMA (phorbol 12 myristate 13-acetate) plus ionomycin. The phosphorylated STAT1 levels were analyzed by immunoblotting. Histamine receptor agonists amthamine and betahistine, histamine receptor antagonists pyrilamine maleate, tripelennamine, ranitidine, cimetidine and thioperamide, cAMP agonists N(6), 2'-0-dibutyryladenosine-3',5'-cyclic monophosphate sodium salt (db-cAMP) and forskolin, protein kinase A inhibitors N-(2-[p-bromocinnamylamino]ethyl)-5-isoquinoline-sulfonamide (H89) and Rp diastereomer of adenosine cyclic 3',5'-phosphorothioate (RpcAMPs) and tyrosine kinase inhibitor tyrphostin were used to identify the upstream signal transduction pathways. We observed that histamine augmented the phosphorylation of STAT1 through both H1 and H2 receptors. Furthermore, H1 and H2 receptor antagonists displayed inverse agonism. Ca(2+)-PKC-induced phosphorylation of STAT1 was completely inhibited by H89 and significantly inhibited by RpcAMPs. DbcAMP and forskolin augmented the Ca(2+)-PKC-induced STAT1 phosphorylation thus suggesting a convergent crosstalk between the two histamine receptor signaling pathways, PKA and PKC.

Dual role of alpha-defensin-1 in anti-HIV-1 innate immunity

Alpha-defensins are abundant antimicrobial peptides in polymorphonuclear leukocytes and play an important role in innate immunity. We have previously shown that alpha-defensin-1 can inhibit HIV-1 replication following viral entry. Here we examined the molecular mechanism(s) of alpha-defensin-1-mediated HIV-1 inhibition. Alpha-defensin-1 had a direct effect on HIV-1 virions at a low MOI in the absence of serum. The direct effect on HIV-1 virions was abolished by the presence of serum or an increase in virus particles. Studying the kinetics of the HIV life cycle revealed that alpha-defensin-1 inhibited steps following reverse transcription and integration. Analysis of PKC phosphorylation in primary CD4+ T cells in response to alpha-defensin-1 indicated that alpha-defensin-1 inhibited PKC activity. Pretreatment of infected CD4+ T cells with a PKC activator, bryostatin 1, partially reversed alpha-defensin-1-mediated HIV inhibition. Like alpha-defensin-1, the PKC isoform-selective inhibitor Go6976 blocked HIV-1 infection in a dose-dependent manner. Furthermore, kinetic studies and analysis of HIV-1 products indicated that alpha-defensin-1 and Go6976 blocked HIV-1 infection at similar stages in its life cycle, including nuclear import and transcription. Taken together, our studies demonstrate that, in the absence of serum, alpha-defensin-1 may act directly on the virus, but, in the presence of serum, its effects are on the cell, where it inhibits HIV-1 replication. At least 1 of the cellular effects associated with HIV inhibition is interference with PKC signaling in primary CD4+ T cells. Studying the complex function of alpha-defensin-1 in innate immunity against HIV has implications for prevention as well as therapeutics.

Dual role of alpha-defensin-1 in anti-HIV-1 innate immunity

Alpha-defensins are abundant antimicrobial peptides in polymorphonuclear leukocytes and play an important role in innate immunity. We have previously shown that alpha-defensin-1 can inhibit HIV-1 replication following viral entry. Here we examined the molecular mechanism(s) of alpha-defensin-1-mediated HIV-1 inhibition. Alpha-defensin-1 had a direct effect on HIV-1 virions at a low MOI in the absence of serum. The direct effect on HIV-1 virions was abolished by the presence of serum or an increase in virus particles. Studying the kinetics of the HIV life cycle revealed that alpha-defensin-1 inhibited steps following reverse transcription and integration. Analysis of PKC phosphorylation in primary CD4+ T cells in response to alpha-defensin-1 indicated that alpha-defensin-1 inhibited PKC activity. Pretreatment of infected CD4+ T cells with a PKC activator, bryostatin 1, partially reversed alpha-defensin-1-mediated HIV inhibition. Like alpha-defensin-1, the PKC isoform-selective inhibitor Go6976 blocked HIV-1 infection in a dose-dependent manner. Furthermore, kinetic studies and analysis of HIV-1 products indicated that alpha-defensin-1 and Go6976 blocked HIV-1 infection at similar stages in its life cycle, including nuclear import and transcription. Taken together, our studies demonstrate that, in the absence of serum, alpha-defensin-1 may act directly on the virus, but, in the presence of serum, its effects are on the cell, where it inhibits HIV-1 replication. At least 1 of the cellular effects associated with HIV inhibition is interference with PKC signaling in primary CD4+ T cells. Studying the complex function of alpha-defensin-1 in innate immunity against HIV has implications for prevention as well as therapeutics.

Phosphorylation of HIV Nef by cAMP-dependent protein kinase

Nef, a multifunctional accessory protein of human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV), is important for disease progression. Nef downmodulates CD4 and MHC class I expression, alters host-cell signal transduction pathways, and enhances viral replication. We have identified a novel interaction between Nef and cAMP-dependent kinase (PKA). N-terminal serine residues Ser6,9 of HIVNL4-3 Nef and Ser10 of SIVmac239 Nef were phosphorylated by PKA in a cell-free system; intracellularly, only Ser9 of HIVNL4-3 Nef was phosphorylated by PKA. Mutation of Ser9 to alanine in the context of full-length HIVNL4-3 lowered HIV replication in resting peripheral blood mononuclear cells (PBMC) compared to parental virus. As this mutation played a major role in abrogating the Nef effect on HIV replication in unstimulated primary cells, we postulate that Nef phosphorylation by PKA is an important step in the viral life cycle in resting cells.

Antiviral therapy with non-selective β-blockers: preliminary experimental and clinical corroboration

Treatment with non-selective beta-blockers has been proposed to have an indirect antiviral activity acting via an enhanced performance of the immune system, and the mechanism of this activity has been laid out earlier. Experimental and clinical findings are presented that corroborate the hypothesis that inhibiting the immunosuppressive and stress-related cAMP-PKA pathway will enhance the immune system's ability to recognize foreign antigen and to access its vast repertory in an improved way, resulting in an indirect antiviral activity. Other drugs having an inhibitory effect on the cAMP-PKA pathway in cells of the immune system and therefore expected to have a comparable activity spectrum with different specific side-effects are presented, for example aspirin. Additionally the so far unexplained anticancer activity of aspirin is related to the same mechanism of an enhanced performance of the immune system.

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.

Identification of T cell-signaling pathways that stimulate latent HIV in primary cells

Eradication of HIV infection depends on the elimination of a small, but stable population of latently infected T cells. After the discontinuation of therapy, activation of latent virus can rekindle infection. To purge this reservoir, it is necessary to define cellular signaling pathways that lead to activation of latent HIV. We used the SCID-hu (Thy/Liv) mouse model of HIV latency to analyze a broad array of T cell-signaling pathways and show in primary, quiescent cells that viral induction depends on the activation of two primary intracellular signaling pathways, protein kinase C or nuclear factor of activated T cells (NF-AT). In contrast, inhibition or activation of other important T cell stimulatory pathways (such as mitogen-activated protein kinase, calcium flux, or histone deacetylation) do not significantly induce virus expression. We found that the activation of NF-kappaB is critical to viral reactivation; however, all pathways that stimulate NF-kappaBdonot reactivate latent virus. Our studies further show that inhibition of NF-kappaB does not prevent activation of HIV by NF-AT, indicating that these pathways can function independently to activate the HIV LTR. Thus, we define several molecular pathways that trigger HIV reactivation from latency and provide evidence that latent HIV infection is maintained by the functional lack of particular transcription factors in quiescent cells.

Direct and quantitative single-cell analysis of human immunodeficiency virus type 1 reactivation from latency

The ability of human immunodeficiency virus type 1 (HIV-1) to establish latent infections in cells has received renewed attention owing to the failure of highly active antiretroviral therapy to eradicate HIV-1 in vivo. Despite much study, the molecular bases of HIV-1 latency and reactivation are incompletely understood. Research on HIV-1 latency would benefit from a model system that is amenable to rapid and efficient analysis and through which compounds capable of regulating HIV-1 reactivation may be conveniently screened. We describe a novel reporter system that has several advantages over existing in vitro systems, which require elaborate, expensive, and time-consuming techniques to measure virus production. Two HIV-1 molecular clones (NL4-3 and 89.6) were engineered to express enhanced green fluorescent protein (EGFP) under the control of the viral long terminal repeat without removing any viral sequences. By using these replication-competent viruses, latently infected T-cell (Jurkat) and monocyte/macrophage (THP-1) lines in which EGFP fluorescence and virus expression are tightly coupled were generated. Following reactivation with agents such as tumor necrosis factor alpha, virus expression and EGFP fluorescence peaked after 4 days and over the next 3 weeks each declined in a synchronized manner, recapitulating the establishment of latency. Using fluorescence microscopy, flow cytometry, or plate-based fluorometry, this system allows immediate, direct, and quantitative real-time analysis of these processes within single cells or in bulk populations of cells. Exploiting the single-cell analysis abilities of this system, we demonstrate that cellular activation and virus reactivation following stimulation with proinflammatory cytokines can be uncoupled.

Up-regulation of HIV coreceptor CXCR4 expression in human T lymphocytes is mediated in part by a cAMP-responsive element

The chemokine and HIV receptor CXCR4 has been shown to play a role in chemotaxis and HIV-1 entry into T cells. Dibutyryl cAMP (DcAMP), an analog of cAMP, has been shown to increase CXCR4 cell surface expression and HIV-1 infectivity, but the molecular mechanism(s) responsible is unknown. Here we show that DcAMP treatment of purified human T lymphocytes increased transcription of CXCR4 mRNA as well as cell surface and intracellular CXCR4 protein expression. DcAMP-mediated stimulation of human PBL increased T-trophic HIV-1 (X4) fusion and viral replication as measured by syncytia formation and p24 levels, respectively. To determine the region(s) of the CXCR4 promoter required for cAMP responsiveness, truncations and point mutations of the CXCR4 promoter (nucleotides -1098 to +59) fused to luciferase were constructed and transiently transfected into human PBL. Deletional analysis demonstrated that the -1098 to -93 region of the CXCR4 promoter construct could be eliminated; the residual (-93 to +59) promoter retained cAMP responsiveness. Site-directed mutagenesis of a putative cAMP-responsive element (CRE) in the 5' UTR (+41 to +49) significantly and specifically attenuated the ability of DcAMP to drive the minimal CXCR4 promoter. Electrophoretic mobility shift assays demonstrated the formation of a complex between the CREB transcription factor and the putative CXCR4 CRE site. Our findings demonstrate a CRE element within the CXCR4 promoter that regulates CXCR4 transcription in response to changes in cAMP signaling. The cAMP-dependent up-regulation of CXCR4 mRNA results in increased CXCR4 intracellular and cell surface protein expression as well as increased HIV infectivity.

Prostaglandin E(2)-mediated activation of HIV-1 long terminal repeat transcription in human T cells necessitates CCAAT/enhancer binding protein (C/EBP) binding sites in addition to cooperative interactions between C/EBPbeta and cyclic adenosine 5'-monophosphate response element binding protein

Previous work indicates that treatment of human T cells with PGE(2) results in an increase of HIV-1 long terminal repeat (LTR) transcriptional activity. The noticed PGE(2)-mediated activation of virus gene activity required the participation of specific intracellular second messengers such as calcium and two transcription factors, i.e., NF-kappaB and CREB. We report in this work that the nuclear transcription factor CCAAT/enhancer binding protein (C/EBP) is also important for PGE(2)-dependent up-regulation of HIV-1 LTR-driven gene activity. The implication of C/EBP was shown by using a trans-dominant negative inhibitor of C/EBP (i.e., liver-enriched transcriptional inhibitory protein) and several molecular constructs carrying site-directed mutations in the C/EBP binding sites located within the HIV-1 LTR. Mutated HIV-1 LTR constructs also revealed the involvement of the two most proximal C/EBP binding sites. Data from cotransfection experiments with vectors coding for dominant negative mutants and gel mobility shift assays indicated that PGE(2)-mediated induction of HIV-1 LTR activity results from a cooperative interaction between C/EBPbeta and CREB, two members of the basic leucine zipper family of transcription factors. Altogether these findings indicate that treatment of human T cells with PGE(2) induces HIV-1 LTR activity through a complex interplay between C/EBPbeta and CREB. Such a combinatorial regulation may represent a mechanism that permits a fine regulation of HIV-1 expression by PGE(2) in human T cells.

Prostaglandin E(2) inhibits replication of HIV-1 in macrophages through activation of protein kinase A

Since macrophages are a source of increased PGE(2) in AIDS, we investigated the role of PGE(2) in the replication of HIV-1 in these cells. PGE(2) inhibited HIV-1 replication measured by reverse transcriptase in human monocyte-derived macrophage (MDM). Treatment of MDM with the PGE(1) analog misoprostol, the adenylate cyclase activator forskolin, and the cyclic AMP analog dibutyryl-cyclic AMP (db-cAMP) suppressed HIV replication. The protein kinase A (PKA) activator 8-bromo-cyclic AMP also inhibited HIV-1 replication. Similar results were observed with the entry-independent, latently HIV-infected U1 cells. There was a parallel decrease in HIV-1 mRNA levels following PGE(2) treatment. Co-transfection of the HIV-1 promoter LTR.luciferase, with the vector CMV.Calpha, which expresses the PKA catalytic unit increasing PKA activity, reduced HIV-1 promoter activity. Inhibition of PKA activity with the pMT.RAB vector, a mutant regulatory unit of PKA, augmented HIV-1 promoter activity. In summary, PGE(2) inhibits HIV-1 gene expression in MDM through a PKA-dependent mechanism.

Modulation of HIV-1 enhancer activity and virus production by cAMP

The effect of cAMP on the transcriptional activity of the HIV-1 long terminal repeat/enhancer was investigated and compared to the effect of cAMP on virus replication. In culture cAMP repressed virus replication in vivo using different cell types. Transient transfection studies with HIV-1 enhancer-derived luciferase reporter gene constructs identified the minimal DNA sequence mediating the negative regulatory effect of cAMP on HIV-1 transcription. A single nuclear factor kappaB element from the HIV-1 enhancer mediates the repressive effect on transcription. AP-2 is not involved in cAMP repression. Stable transfection of Jurkat T cells with the co-activators CREB binding protein (CBP) and p300 completely abolished the cAMP repressive effect, supporting the hypothesis that elevation of intracellular cAMP increases phosphorylation of CREB, which then competes with phosphorylated p65 and Ets-1 for limiting amounts of CBP/p300 thereby mediating the observed repressive effect on transcription. These findings suggest an important role of cAMP on HIV-1 transcription.

Impaired response to HAART in HIV-infected individuals with high autonomic nervous system activity

Neurotransmitters can accelerate HIV-1 replication in vitro, leading us to examine whether differences in autonomic nervous system (ANS) activity might promote residual HIV-1 replication in patients treated with highly active antiretroviral therapy. Patients who showed constitutively high levels of ANS activity before highly active antiretroviral therapy experienced poorer suppression of plasma viral load and poorer CD4(+) T cell recovery over 3-11 months of therapy. ANS activity was not related to demographic or behavioral characteristics that might influence pathogenesis. However, the ANS neurotransmitter norepinephrine enhanced replication of both CCR5- and CXCR4-tropic strains of HIV-1 in vitro via chemokine receptor up-regulation and enhanced viral gene expression, suggesting that neural activity may directly promote residual viral replication.

Human immunodeficiency virus type-1 and chemokines: beyond competition for common cellular receptors

The chemokines and their receptors have been receiving exceptional attention in recent years following the discoveries that some chemokines could specifically block human immunodeficiency virus type 1 (HIV-1) infection and that certain chemokine receptors were the long-sought coreceptors which, along with CD4, are required for the productive entry of HIV-1 and HIV-2 isolates. Several chemokine receptors or orphan chemokine receptor-like molecules can support the entry of various viral strains, but the clinical significance of the CXCR4 and CCR5 coreceptors appear to overshadow a critical role for any of the other coreceptors and all HIV-1 and HIV-2 strains best employ one or both of these coreceptors. Binding of the HIV-1 envelope glycoprotein gp120 subunit to CD4 and/or an appropriate chemokine receptor triggers conformational changes in the envelope glycoprotein oligomer that allow it to facilitate the fusion of the viral and host cell membranes. During these interactions, gp120 appears to be capable of inducing a variety of signaling events, all of which are still not defined in detail. In addition, the more recently observed dichotomous effects, of both inhibition and enhancement, that chemokines and their receptor signaling events elicit on the HIV-1 entry and replication processes has once again highlighted the intricate and complex balance of factors that govern the pathogenic process. Here, we will review and discuss these new observations summarizing the potential significance these processes may have in HIV-1 infection. Understanding the complexities and significance of the signaling processes that the chemokines and viral products induce may substantially enhance our understanding of HIV-1 pathogenesis, and perhaps facilitate the discovery of new ways for the prevention and treatment of HIV-1 disease.

Protein kinase C-beta and oxygen deprivation. A novel Egr-1-dependent pathway for fibrin deposition in hypoxemic vasculature

Fibrin deposition is a salient feature of hypoxemic vasculature and results from induction of tissue factor. Such tissue factor expression in an oxygen deficient environment is driven by the transcription factor Early Growth Response (Egr)-1. Using homozygous null mice for the protein kinase C beta-isoform gene (PKCbeta null), PKCbeta is shown to be upstream of Egr-1 in this oxygen deprivation-mediated pathway for triggering procoagulant events. Whereas wild-type mice exposed to hypoxia (6%) displayed a robust increase in tissue factor transcripts and antigen, and vascular fibrin deposition, PKCbeta null animals showed a markedly blunted response. Consistent with a central role for Egr-1 in hypoxia-induced expression of tissue factor, PKCbeta null mice subjected to oxygen deprivation displayed at most a minor elevation in Egr-1 transcripts, antigen, and intensity of the gel shift band by electrophoretic mobility shift assay, compared with normoxic animals. These data firmly establish PKCbeta as a trigger for events leading to induction of Egr-1 and tissue factor under hypoxic conditions, and provide insight into a biologic cascade whereby oxygen deprivation recruits targets of PKCbeta and Egr-1, thereby amplifying the cellular response.

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

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

Interleukin-10 enhances tumor necrosis factor-alpha activation of HIV-1 transcription in latently infected T cells

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