A Mechanism-Based Targeted Screen To Identify Epstein-Barr Virus-Directed Antiviral Agents.

Epstein-Barr virus (EBV) is one of nine human herpesviruses that persist latently to establish permanent residence in their hosts. Periodic activation into the lytic/replicative phase allows such viruses to propagate and spread, but can also cause disease in the host. This lytic phase is also essential for EBV to cause infectious mononucleosis and cancers, including B lymphocyte-derived Burkitt lymphoma and immunocompromise-associated lymphoproliferative diseases/lymphomas as well as epithelial cell-derived nasopharyngeal cell carcinoma. In the absence of anti-EBV agents, however, therapeutic options for EBV-related diseases are limited. In earlier work, we discovered that through the activities of the viral protein kinase conserved across herpesviruses and two cellular proteins, ATM and KAP1, a lytic cycle amplification loop is established, and disruption of this loop disables the EBV lytic cascade. We therefore devised a high-throughput screening assay, screened a small-molecule-compound library, and identified 17 candidates that impair the release of lytically replicated EBV. The identified compounds will (i) serve as lead compounds or may be modified to inhibit EBV and potentially other herpesviruses, and (ii) be developed into anticancer agents, as functions of KAP1 and ATM are tightly linked to cancer. Importantly, our screening strategy may also be used to screen additional compound libraries for antiherpesviral and anticancer drugs.IMPORTANCE Epstein-Barr virus, which is nearly ubiquitous in humans, is causal to infectious mononucleosis, chronic active EBV infection, and lymphoid and epithelial cancers. However, EBV-specific antiviral agents are not yet available. To aid in the identification of compounds that may be developed as antivirals, we pursued a mechanism-based approach. Since many of these diseases rely on EBV's lytic phase, we developed a high-throughput assay that is able to measure a key step that is essential for successful completion of EBV's lytic cascade. We used this assay to screen a library of small-molecule compounds and identified inhibitors that may be pursued for their anti-EBV and possibly even antiherpesviral potential, as this key mechanism appears to be common to several human herpesviruses. Given the prominent role of this mechanism in both herpesvirus biology and cancer, our screening assay may be used as a platform to identify both antiherpesviral and anticancer drugs.

Structural and functional insights into the regulation of the lysis-lysogeny decision in viral communities.

Communication is vital for all organisms including microorganisms, which is clearly demonstrated by the bacterial quorum-sensing system. However, the molecular mechanisms underlying communication among viruses (phages) via the quorum-sensing-like 'arbitrium' system remain unclear. Viral or host densities are known to be related to an increased prevalence of lysogeny; however, how the switch from the lytic to the lysogenic pathway occurs is unknown. Thus, we sought to reveal mechanisms of communication among viruses and determine the lysogenic dynamics involved. Structural and functional analyses of the phage-derived SAIRGA and GMPRGA peptides and their corresponding receptors, phAimR and spAimR, indicated that SAIRGA directs the lysis-lysogeny decision of phi3T by modulating conformational changes in phAimR, whereas GMPRGA regulates the lysis-lysogeny pathway by stabilizing spAimR in the dimeric state. Although temperate viruses are thought to share a similar lytic-lysogenic cycle switch model, our study suggests the existence of alternative strain-specific mechanisms that regulate the lysis-lysogeny decision. Collectively, these findings provide insights into the molecular mechanisms underlying communication among viruses, offering theoretical applications for the treatment of infectious viral diseases.

Structural basis of the arbitrium peptide-AimR communication system in the phage lysis-lysogeny decision.

A bacteriophage can replicate and release virions from a host cell in the lytic cycle or switch to a lysogenic process in which the phage integrates itself into the host genome as a prophage. In Bacillus cells, some types of phages employ the arbitrium communication system, which contains an arbitrium hexapeptide, the cellular receptor AimR and the lysogenic negative regulator AimX. This system controls the decision between the lytic and lysogenic cycles. However, both the mechanism of molecular recognition between the arbitrium peptide and AimR and how downstream gene expression is regulated remain unknown. Here, we report crystal structures for AimR from the SPbeta phage in the apo form and the arbitrium peptide-bound form at 2.20 Å and 1.92 Å, respectively. With or without the peptide, AimR dimerizes through the C-terminal capping helix. AimR assembles a superhelical fold and accommodates the peptide encircled by its tetratricopeptide repeats, which is reminiscent of RRNPP family members from the quorum-sensing system. In the absence of the arbitrium peptide, AimR targets the upstream sequence of the aimX gene; its DNA binding activity is prevented following peptide binding. In summary, our findings provide a structural basis for peptide recognition in the phage lysis-lysogeny decision communication system.

Distinct temporal roles for the promyelocytic leukaemia (PML) protein in the sequential regulation of intracellular host immunity to HSV-1 infection.

Detection of viral nucleic acids plays a critical role in the induction of intracellular host immune defences. However, the temporal recruitment of immune regulators to infecting viral genomes remains poorly defined due to the technical difficulties associated with low genome copy-number detection. Here we utilize 5-Ethynyl-2'-deoxyuridine (EdU) labelling of herpes simplex virus 1 (HSV-1) DNA in combination with click chemistry to examine the sequential recruitment of host immune regulators to infecting viral genomes under low multiplicity of infection conditions. Following viral genome entry into the nucleus, PML-nuclear bodies (PML-NBs) rapidly entrapped viral DNA (vDNA) leading to a block in viral replication in the absence of the viral PML-NB antagonist ICP0. This pre-existing intrinsic host defence to infection occurred independently of the vDNA pathogen sensor IFI16 (Interferon Gamma Inducible Protein 16) and the induction of interferon stimulated gene (ISG) expression, demonstrating that vDNA entry into the nucleus alone is not sufficient to induce a robust innate immune response. Saturation of this pre-existing intrinsic host defence during HSV-1 ICP0-null mutant infection led to the stable recruitment of PML and IFI16 into vDNA complexes associated with ICP4, and led to the induction of ISG expression. This induced innate immune response occurred in a PML-, IFI16-, and Janus-Associated Kinase (JAK)-dependent manner and was restricted by phosphonoacetic acid, demonstrating that vDNA polymerase activity is required for the robust induction of ISG expression during HSV-1 infection. Our data identifies dual roles for PML in the sequential regulation of intrinsic and innate immunity to HSV-1 infection that are dependent on viral genome delivery to the nucleus and the onset of vDNA replication, respectively. These intracellular host defences are counteracted by ICP0, which targets PML for degradation from the outset of nuclear infection to promote vDNA release from PML-NBs and the onset of HSV-1 lytic replication.

Regulation of gammaherpesvirus lytic replication by endoplasmic reticulum stress-induced transcription factors ATF4 and CHOP.

The stress-induced unfolded protein response (UPR) in the endoplasmic reticulum (ER) involves various signaling cross-talks and controls cell fate. B-cell receptor (BCR) signaling, which can trigger UPR, induces gammaherpesvirus lytic replication and serves as a physiological mechanism for gammaherpesvirus reactivation in vivo However, how the UPR regulates BCR-mediated gammaherpesvirus infection is unknown. Here, we demonstrate that the ER stressors tunicamycin and thapsigargin inhibit BCR-mediated murine gammaherpesvirus 68 (MHV68) lytic replication by inducing expression of the UPR mediator Bip and blocking activation of Akt, ERK, and JNK. Both Bip and the downstream transcription factor ATF4 inhibited BCR-mediated MHV68 lytic gene expression, whereas UPR-induced C/EBP homologous protein (CHOP) was required for and promoted BCR-mediated MHV68 lytic replication by suppressing upstream Bip and ATF4 expression. Bip knockout was sufficient to rescue BCR-mediated MHV68 lytic gene expression in CHOP knockout cells, and this rescue was blocked by ectopic ATF4 expression. Furthermore, ATF4 directly inhibited promoter activity of the MHV68 lytic switch transactivator RTA. Altogether, we show that ER stress-induced CHOP inhibits Bip and ATF4 expression and that ATF4, in turn, plays a critical role in CHOP-mediated regulation of BCR-controlled MHV68 lytic replication. We conclude that ER stress-mediated UPR and BCR signaling pathways are interconnected and form a complex network to regulate the gammaherpesvirus infection cycle.

Communication between viruses guides lysis-lysogeny decisions.

Temperate viruses can become dormant in their host cells, a process called lysogeny. In every infection, such viruses decide between the lytic and the lysogenic cycles, that is, whether to replicate and lyse their host or to lysogenize and keep the host viable. Here we show that viruses (phages) of the SPbeta group use a small-molecule communication system to coordinate lysis-lysogeny decisions. During infection of its Bacillus host cell, the phage produces a six amino-acids-long communication peptide that is released into the medium. In subsequent infections, progeny phages measure the concentration of this peptide and lysogenize if the concentration is sufficiently high. We found that different phages encode different versions of the communication peptide, demonstrating a phage-specific peptide communication code for lysogeny decisions. We term this communication system the 'arbitrium' system, and further show that it is encoded by three phage genes: aimP, which produces the peptide; aimR, the intracellular peptide receptor; and aimX, a negative regulator of lysogeny. The arbitrium system enables a descendant phage to 'communicate' with its predecessors, that is, to estimate the amount of recent previous infections and hence decide whether to employ the lytic or lysogenic cycle.

Isolation of Phage via Induction of Lysogens.

Most bacterial cells carry prophage genomes either integrated into the host DNA or present as repressed plasmids. Methods are described for the induction of prophages using Mitomycin C, and for the isolation of prophage-cured bacterial cell lines.

Effect of glutathione on UV induction of prophage lambda.

The effect of glutathione (GSH) on the ultraviolet (UV) induction of lambda prophage was investigated in lysogenic Escherichia coli. The data showed that extracellular GSH could inhibit the UV induction of lambda prophage. The inhibitory rates were concentration dependent, and the maximal rate obtained was 94% with 3.0 M GSH. The effect was also measured in three different lambda lysogens: a wild-type strain (wt), an isogenic GSH-deficient strain, and an isogenic strain producing increased amounts of GSH. The result showed that when subjected to UV irradiation (254 nm, 60 J m(-2)), GSH-deficient strain was approximately fivefold more sensitive to be lysed than wt, whereas the strain with higher intracellular GSH levels was only 28% susceptible to be lysed. With electron spin resonance and spin trapping techniques, we observed that free radical signals occurred in the suspensions of UV irradiated lysogenic cells and the intensity of signals was influenced by GSH levels. These results indicate that GSH can significantly inhibit the UV induction of lambda prophage, and that this effect is correlated to its capacity to scavenge free radicals generated after UV irradiation.

Induction of vaginal Lactobacillus phages by the cigarette smoke chemical benzo[a]pyrene diol epoxide.

Because smoking increases a woman's risk of contracting bacterial vaginosis (BV), which is manifested by a reduction of vaginal lactobacilli and an overgrowth of anaerobic bacteria, chemicals contained in cigarette smoke were analyzed in vitro to determine their role in reducing lactobacilli. The result showed that trace amounts of benzo[a]pyrene diol epoxide (BPDE), which can be found in vaginal secretion of women who smoke, significantly increased phage induction in lactobacilli. This finding implies that smoking may reduce vaginal lactobacilli by promoting phage induction.

Induction of Epstein-Barr virus lytic cycle by tumor-promoting and non-tumor-promoting phorbol esters requires active protein kinase C.

Exposure to the tiglian 12-O-tetradecanoylphorbol-13-acetate (TPA) represents one of the most efficient and widely used protocols for inducing Epstein-Barr virus (EBV)-infected cells from latent into lytic cycle. Since TPA is both a potent tumor promoter and a potent activator of the cellular protein kinase C (PKC), we sought to determine whether either of these activities was closely linked to EBV lytic cycle induction. A panel of TPA structural analogs, encompassing tiglians with different spectra of biological activities, was assayed on a number of EBV-positive B-lymphoid cell lines. Lytic cycle induction correlated with the capacity to activate PKC, not with tumor promoter status; some nonpromoting tiglians were as efficient as TPA in inducing lytic cycle antigen expression. We then sought more direct evidence for an involvement of PKC in the induction process. In initial experiments, 1-(5-isoquinolinyl sulphonyl)-2-methylpiperazine (H-7), the best available pharmacological inhibitor of PKC, completely blocked the induction of the lytic cycle by TPA and its active analogs. This is consistent with, but does not prove, a requirement for active PKC in the induction process, since H-7 targets PKC preferentially but also has some effects on other kinases. We therefore turned to the synthetic pseudosubstrate peptide PKC(19-36) as a means of specific PKC inhibition and to the closely related but inactive peptide PKC(19-Ser-25-36) as a control. Using the technique of scrape loading to deliver the peptides into cells of an adherent EBV-positive target line, we found that the pseudosubstrate peptide PKC(19-36) completely and specifically blocked tiglian-induced entry of the cells into the lytic cycle. The evidence both from TPA analogs and from enzyme inhibition studies therefore indicates that the pathway linking TPA treatment to lytic cycle induction involves active PKC. Interestingly, inhibition of PKC had no effect upon the spontaneous entry into lytic cycle which occurs in naturally productive cell lines, suggesting that spontaneous entry is signalled by another route.

Lysogeny in Leuconostoc oenos.

Thirty strains of Leuconostoc oenos were exposed to mitomycin C to induce lysogenic bacteriophages. Lysis curves typical for lysogenic strains were obtained with 19 strains. Indicator strans were found for 17 of these phages. Five were characterized by electron microscopy, lytic spectrum, molecular masses of the proteins, sequencing of five N-terminal amino acids of the two major proteins and DNA analysis (restriction patterns, cross hybridization). The results revealed a very close relationship between the phages. Hybridization experiments between the DNAs of the temperate phages and the appropriate lysogenic strains revealed phage-related sequences in the DNA of the lysogenic strain.

[The modifying effect of glycerine and cysteamine on prophage lambda induction in E. coli cells under gamma irradiation]. / Modifitsiruiushchee vliianie glitserina i tsisteamina na induktsiiu profaga lambda u kletok E. coli pri gamma-obluchenii.

In studying the modifying influence of glycerol and cysteamine of gamma radiation induction of lambda prophage it was shown that dose curves for lambda prophage induced upon irradiation in normal conditions and in the presence of glycerol are those with a maximum. In the presence of cysteamine, the prophage induction is inhibited significantly.

Interferon-mediated inhibition of retroviral infection: use of a defective retrovirus carrying a drug-resistance gene.

We here report the results of an investigation of the effect of interferon on the establishment of new infections by a retrovirus. For this study, we used an infectious but replication-incompetent retrovirus carrying a drug-resistance gene and assayed for infectivity by measuring drug-resistant colony formation. Mouse interferon-beta inhibited retroviral infection of mouse CG1 cells in a dose-dependent manner. However, a higher dose of interferon was needed for eliciting the antiretroviral effects than for action against vesicular stomatitis virus. The degree of antiretroviral effect was comparable over at least a 100-fold range of multiplicity of infection and the effect was most pronounced when the cells were continuously treated with interferon before infections and during infection and drug-selection.

Prophage induction by 4 antimalaria drugs (daraprim, fansidar, nivaquine and camoquine) and in combination with aflatoxin B1.

The abilities of 4 antimalaria drugs (Daraprim, Fansidar, Nivaquine and Camoquine) on their own and in combination with aflatoxin B1 to induce prophage in tester strains E. coli D21 and D22 were studied. The 4 drugs were found to induce prophage in the tester strains; Daraprim and Fansidar without the need for activation by liver mixed function oxidases (S9 microsomal fraction), while Nivaquine and Camoquine did require activation by this system. Aflatoxin B1 was used as a positive control in all the tests. The combined effects of each of the drugs with aflatoxin B1 were lower than that of the individual drugs acting alone. From these results, it may be concluded that the drugs may, on their own, pose a carcinogenic hazard to the population in the Nigerian environment exposed to them. In addition, the depression of prophage induction observed when the drugs were combined with aflatoxin B1 may be indicative of a common target site of action in the tester strains.

The screening, diagnosis and evaluation of genotoxic agents with batteries of bacterial tests.

Bacterial tests can be used for several related purposes: the screening for genotoxic agents, genetic analysis of the mode of action of genotoxins and attempts to predict their effects in mammals. We examine various aspects of the assembly of tests into batteries with emphasis on the genetic properties of target bacterial cells. We discuss the problems of carcinogenicity prediction, the identification of particular types of DNA lesions, the study of mutagenic specificity and the elucidation of metabolic steps towards the ultimate genotoxin.

Evaluation of the Escherichia coli K12 inductest for detection of potential chemical carcinogens.

46 chemicals of diverse classes and structures, including 30 known animal carcinogens, were evaluated for prophage-inducing ability using the Escherichia coli inductest with lysogenic strain GY5027 envA - uvrB- and indicator strain GY4015 ampR . The inductest detected 9 of 30 known carcinogens as genotoxic agents, including 3 polycyclic hydrocarbons, 2 aflatoxins, and 2 antitumor antimicrobials. Among the 21 carcinogens ineffective as prophage inducers were 3 aromatic amines (other than 2-aminoanthracene), 3 azo-aminoazo compounds, 2 methanesulfonates, and 2 nitro aromatics. In contrast, 18 and 17 of the 30 animal carcinogens were detected as genotoxic agents in the Salmonella/Ames test and E. coli WP2/ WP100 rec assay, respectively. The threshold sensitivity of the inductest was less than that of the Salmonella/Ames test for chemicals genotoxic in both tests. The ineffectiveness of the inductest as a routine test for detecting potential chemical carcinogens may be related to the nature of the DNA damage lesions formed by various genotoxic agents.

Biochemical prophage induction assay: a rapid test for antitumor agents that interact with DNA.

A biochemical (colorimetric) assay of bacteriophage lambda induction was utilized in the detection, identification, and purification of DNA-interacting natural products with potential antitumor activity. A set of 142 standard antibiotics, composed principally of natural products with established antitumor activity and/or defined mechanisms of action, was tested in the assay. As expected, most inducers were direct inhibitors of DNA synthesis. A few other types of inducer, with probable indirect effects on DNA synthesis, were found after prolonged incubation: one class of RNA synthesis inhibitor, a dihydrofolate reductase inhibitor; and two inhibitors of bacterial cell wall synthesis. The biochemical induction assay was semiautomated for use as a prescreen in the search for novel antitumor agents in 10,724 actinomycete fermentation broths. Approximately 1% of the cultures produced compounds that were active in the assay; some appear to be novel. None required metabolic activation (via rat liver S9) for inducing activity. The biochemical induction assay was adapted for bioautography (the detection of inducing chemicals chromatographed on thin-layer plates) and for strain improvement programs (selection of isolates with enhanced inducing activity). The speed of the assay (2 to 5 hr) made it useful for monitoring antitumor agent production and purification. The sensitivity of the assay could be varied, depending on the length of the incubation period. Microbes, nutrients, and toxic solvents did not usually interfere with the detection of inducing activity.

Biogenesis of membrane lipoproteins in Escherichia coli.

Globomycin-resistant mutants of Escherichia coli have been isolated and partially characterized. Approximately 2-5% of these mutants synthesize structurally altered Braun's lipoprotein. The majority of these mutants contain unprocessed and unmodified prolipoprotein. One mutant is found to contain modified, processed, but structurally altered lipoprotein. Mutants containing lipid-deficient prolipoprotein or lipoprotein also show increased resistance to globomycin. These results suggest that the inhibition of processing of modified prolipoprotein by globomycin may require fully modified prolipoprotein as the biochemical target of this novel antibiotic. Our failure to isolate mutant containing cleaved but unmodified lipoprotein among globomycin-resistant mutants is consistent with the possibility that modification of prolipoprotein precedes the removal of signal sequence by a unique signal peptidase. Recent evidence indicates that the minor lipoproteins in the cell envelope of E. coli are also synthesized as lipid-containing prolipoproteins and the processing of these prolipoproteins is inhibited by globomycin. These results suggest the existence of modifying enzymes in E. coli which would transfer glyceryl and fatty acyl moieties to cysteine residues located in the proper sequences of the precursor proteins. This speculation is confirmed by our demonstration that Bacillus licheniformis penicillinase synthesized in E. coli as well as in B. licheniformis is a lipoprotein containing glyceride-cysteine at its NH2-terminus.