Transcription of herpes simplex virus immediate-early and early genes is inhibited by roscovitine, an inhibitor specific for cellular cyclin-dependent kinases

Anti-herpes simplex virus activities and mechanisms of marine derived compounds

Herpes simplex virus (HSV) is the most widely prevalent herpes virus worldwide, and the herpetic encephalitis and genital herpes caused by HSV infection have caused serious harm to human health all over the world. Although many anti-HSV drugs such as nucleoside analogues have been ap-proved for clinical use during the past few decades, important issues, such as drug resistance, toxicity, and high cost of drugs, remain unresolved. Recently, the studies on the anti-HSV activities of marine natural products, such as marine polysaccharides, marine peptides and microbial secondary metabolites are attracting more and more attention all over the world. This review discusses the recent progress in research on the anti-HSV activities of these natural compounds obtained from marine organisms, relating to their structural features and the structure-activity relationships. In addition, the recent findings on the different anti-HSV mechanisms and molecular targets of marine compounds and their potential for therapeutic application will also be summarized in detail.

BMS-265246, a Cyclin-Dependent Kinase Inhibitor, Inhibits the Infection of Herpes Simplex Virus Type 1

Herpes simplex virus type 1 (HSV-1) infections are prevalent illnesses that can cause mucocutaneous ulcerative disease, keratitis, and genital herpes. In patients with compromised immune systems, the infection can lead to serious problems, such as encephalitis. Additionally, neonatal infections can cause brain problems and even death. Current first-line antiviral drugs are nucleoside analog inhibitors that target viral polymerase, and resistant strains have emerged. As a result, new drugs with distinct action modes are needed. Recent research indicates that cyclin-dependent kinases (CDKs) are prospective antiviral targets. Thus, CDK inhibitors may be effective antiviral agents against HSV-1 infection. In this study, we examined a panel of CDK inhibitors that target CDKs in the present study. BMS-265246 (BMS), a CDK 1/2 inhibitor, was found to effectively limit HSV-1 multiplication in Vero, HepG2, and Hela cells. A mechanism of action study suggested that BMS inhibits the early stages of viral replication when added early in the viral infection. The suppression of multiple steps in viral replication by BMS was revealed when HSV-1 infected cells were treated at different time periods in the viral life cycle. Our results suggest that BMS is a potent anti-HSV-1 agent and unique in that it may interfere with multiple steps in HSV-1 replication.

Simple and rapid high-throughput assay to identify HSV-1 ICP0 transactivation inhibitors

Herpes simplex virus 1 (HSV-1) is a ubiquitous virus that results in lifelong infections due to its ability to cycle between lytic replication and latency. As an obligate intracellular pathogen, HSV-1 exploits host cellular factors to replicate and aid in its life cycle. HSV-1 expresses infected cell protein 0 (ICP0), an immediate-early regulator, to stimulate the transcription of all classes of viral genes via its E3 ubiquitin ligase activity. Here we report an automated, inexpensive, and rapid high-throughput approach to examine the effects of small molecule compounds on ICP0 transactivator function in cells. Two HSV-1 reporter viruses, KOS6β (wt) and dlx3.1-6β (ICP0-null mutant), were used to monitor ICP0 transactivation activity through the HSV-1 ICP6 promoter:lacz expression cassette. A ≥10-fold difference in β-galactosidase activity was observed in cells infected with KOS6β compared to dlx3.1-6β, demonstrating that ICP0 potently transactivates the ICP6 promoter. We established the robustness and reproducibility with a Z'-factor score of ≥0.69, an important criterium for high-throughput analyses. Approximately 19,000 structurally diverse compounds were screened and 76 potential inhibitors of the HSV-1 transactivator ICP0 were identified. We expect this assay will aid in the discovery of novel inhibitors and tools against HSV-1 ICP0. Using well-annotated compounds could identify potential novel factors and pathways that interact with ICP0 to promote HSV-1 gene expression.

Chromatin-mediated epigenetic regulation of HSV-1 transcription as a potential target in antiviral therapy

The ability to establish, and reactivate from, latent infections is central to the biology and pathogenesis of HSV-1. It also poses a strong challenge to antiviral therapy, as latent HSV-1 genomes do not replicate or express any protein to be targeted. Although the processes regulating the establishment and maintenance of, and reactivation from, latency are not fully elucidated, the current general consensus is that epigenetics play a major role. A unifying model postulates that whereas HSV-1 avoids or counteracts chromatin silencing in lytic infections, it becomes silenced during latency, silencing which is somewhat disrupted during reactivation. Many years of work by different groups using a variety of approaches have also shown that the lytic HSV-1 chromatin is distinct and has unique biophysical properties not shared with most cellular chromatin. Nonetheless, the lytic and latent viral chromatins are typically enriched in post translational modifications or histone variants characteristic of active or repressed transcription, respectively. Moreover, a variety of small molecule epigenetic modulators inhibit viral replication and reactivation from latency. Despite these successes in culture and animal models, it is not obvious how epigenetic modulation would be used in antiviral therapy if the same epigenetic mechanisms governed viral and cellular gene expression. Recent work has highlighted several important differences between the viral and cellular chromatins, which appear to be of consequence to their respective epigenetic regulations. In this review, we will discuss the distinctiveness of the viral chromatin, and explore whether it is regulated by mechanisms unique enough to be exploited in antiviral therapy.

Herpes Simplex Virus 1 ICP22 Suppresses CD80 Expression by Murine Dendritic Cells

Herpes simplex virus type 1 (HSV-1) has the ability to delay its clearance from the eye during ocular infection. Here, we show that ocular infection of mice with HSV-1 suppressed expression of the costimulatory molecule CD80 but not CD86 in the cornea. The presence of neutralizing anti-HSV-1 antibodies did not alleviate this suppression. At the cellular level, HSV-1 consistently downregulated the expression of CD80 by dendritic cells (DCs) but not by other antigen-presenting cells. Furthermore, flow cytometric analysis of HSV-1-infected corneal cells during a 7-day period reduced CD80 expression in DCs but not in B cells, macrophages, or monocytes. This suppression was associated with the presence of virus. Similar results were obtained using infected or transfected spleen cells or bone marrow-derived DCs. A combination of roscovitine treatment, transfection with immediate early genes (IE), and infection with a recombinant HSV-1 lacking the ICP22 gene shows the importance of ICP22 in downregulation of the CD80 promoter but not the CD86 promoter in vitro and in vivo At the mechanistic level, we show that the HSV-1 immediate early gene ICP22 binds the CD80 promoter and that this interaction is required for HSV-1-mediated suppression of CD80 expression. Conversely, forced expression of CD80 by ocular infection of mice with a recombinant HSV-1 exacerbated corneal scarring in infected mice. Taken together, these studies identify ICP22-mediated suppression of CD80 expression in dendritic cells as central to delayed clearance of the virus and limitation of the cytopathological response to primary infection in the eye.IMPORTANCE HSV-1-induced eye disease is a major public health problem. Eye disease is associated closely with immune responses to the virus and is exacerbated by delayed clearance of the primary infection. The immune system relies on antigen-presenting cells of the innate immune system to activate the T cell response. We found that HSV-1 utilizes a robust and finely targeted mechanism of local immune evasion. It downregulates the expression of the costimulatory molecule CD80 but not CD86 on resident dendritic cells irrespective of the presence of anti-HSV-1 antibodies. The effect is mediated by direct binding of HSV-1 ICP22, the product of an immediate early gene of HSV-1, to the promoter of CD80. This immune evasion mechanism dampens the host immune response and, thus, reduces eye disease in ocularly infected mice. Therefore, ICP22 may be a novel inhibitor of CD80 that could be used to modulate the immune response.

Selective antitumor activity of roscovitine in head and neck cancer

Radiation and chemotherapy that are commonly used to treat human cancers damage cellular DNA. DNA damage appears to be more toxic to cancer cells than normal cells, most likely due to deregulated checkpoint activation and/or deficiency in DNA repair pathways that are characteristics of many tumors. However, unwanted side effects arise as a result of DNA damage to normal cells during the treatment.

Here, we show that roscovitine, a cyclin-dependent kinase (CDK) inhibitor that inhibits CDK-1, CDK-2, CDK-5, CDK-7, and CDK-9 due to competitive binding to the ATP site on the kinases, causes significant DNA damage followed by p53-dependent cell death in human papilloma virus (HPV)-positive, but not in HPV-negative, head and neck cancer cells. Since HPV positivity was a molecular marker for increased sensitivity of cells to roscovitine, we reasoned that systemic roscovitine administration would not be toxic to healthy HPV-negative tissue. Indeed, low roscovitine doses significantly inhibited the growth of HPV-associated xenografted tumors in mice without causing any detectable side effects.

Given that inhibition of CDKs has been shown to inhibit replication of several viruses, we suggest that roscovitine treatment may represent a selective and safe targeted therapeutic option against HPV-positive head and neck cancer.

Timing Is Everything

N. Drayman et al. in their recent article (mBio 8:e01612-17, 2017, https://doi.org/10.1128/mBio.01612-17) have used dynamic proteomics and machine learning to show that the cell cycle state of any individual cell affects the outcome of a productive herpes simplex virus 1 (HSV-1) infection. Cells infected from early G₁ through S were most permissive for expression of genes from the HSV-1 genome, whereas cells infected in late G₂ to mitosis were much less so. Most of the infected cells that underwent mitosis became permanently nonpermissive for HSV-1 gene expression afterward. The cell cycle stage accounted for 60% of the success of infection, and cell density and motility accounted for most of the rest. To successfully reactivate, HSV-1 must express its genes in neurons and cells of the spinosum and granulosum epidermis strata. These cells are permanently in the cell cycle stages most permissive for HSV-1 gene expression, and none reenters mitosis, thus maximizing the efficiency of a successful HSV-1 reactivation before the adaptive immunity can control it.

The ERK-1 function is required for HSV-1-mediated G1/S progression in HEP-2 cells and contributes to virus growth

The herpes simplex virus 1 is able to readdress different cellular pathways including cell cycle to facilitate its replication and spread. During infection, the progression of the cell cycle from G1 to S phase makes the cellular replication machinery accessible to viral DNA replication. In this work we established that HSV-1, in asynchronized HEp-2 cells, strictly controls cell cycle progression increasing S-phase population from 9 hours post infection until the end of HSV-1 replication. The G1/S phases progression depends on two important proteins, cyclin E and CDK2. We demonstrate that their phosphorylated status and then their activity during the infection is strongly correlated to viral replication events. In addition, HSV-1 is able to recruit and distribute ERK1/2 proteins in a spatio-temporal fashion, highlighting its downstream regulatory effects on cellular processes. According with this data, using chemical inhibitor U0126 and ERK dominant negative cells we found that the lack of ERK1 activity affects cyclin E protein accumulation, viral gene transcription and percentage of the cells in S phase, during the viral replication. These data suggested a complex interaction between ERK, cell cycle progression and HSV-1 replication.

Luteolin inhibits Epstein-Barr virus lytic reactivation by repressing the promoter activities of immediate-early genes

The lytic reactivation of Epstein-Barr virus (EBV) has been reported to be strongly associated with several human diseases, including nasopharyngeal carcinoma (NPC). Inhibition of the EBV lytic cycle has been shown to be of great benefit in the treatment of EBV-associated diseases. The administration of dietary compounds is safer and more convenient than other approaches to preventing EBV reactivation. We screened several dietary compounds for their ability to inhibit EBV reactivation in NPC cells. Among them, the flavonoid luteolin showed significant inhibition of EBV reactivation. Luteolin inhibited protein expression from EBV lytic genes in EBV-positive epithelial and B cell lines. It also reduced the numbers of EBV-reactivating cells detected by immunofluorescence analysis and reduced the production of virion. Furthermore, luteolin reduced the activities of the promoters of the immediate-early genes Zta (Zp) and Rta (Rp) and also inhibited Sp1-luc activity, suggesting that disruption of Sp1 binding is involved in the inhibitory mechanism. CHIP analysis revealed that luteolin suppressed the activities of Zp and Rp by deregulating Sp1 binding. Taken together, luteolin inhibits EBV reactivation by repressing the promoter activities of Zp and Rp, suggesting luteolin is a potential dietary compound for prevention of virus infection.

Discovery and development of Seliciclib. How systems biology approaches can lead to better drug performance

Seliciclib (R-Roscovitine) was identified as an inhibitor of CDKs and has undergone drug development and clinical testing as an anticancer agent. In this review, the authors describe the discovery of Seliciclib and give a brief summary of the biology of the CDKs Seliciclib inhibits. An overview of the published in vitro and in vivo work supporting the development as an anti-cancer agent, from in vitro experiments to animal model studies ending with a summary of the clinical trial results and trials underway is presented. In addition some potential non-oncology applications are explored and the potential mode of action of Seliciclib in these areas is described. Finally the authors argue that optimisation of the therapeutic effects of kinase inhibitors such as Seliciclib could be enhanced using a systems biology approach involving mathematical modelling of the molecular pathways regulating cell growth and division.

Inhibition of post-transcriptional RNA processing by CDK inhibitors and its implication in anti-viral therapy

Cyclin-dependent kinases (CDKs) are key regulators of the cell cycle and RNA polymerase II mediated transcription. Several pharmacological CDK inhibitors are currently in clinical trials as potential cancer therapeutics and some of them also exhibit antiviral effects. Olomoucine II and roscovitine, purine-based inhibitors of CDKs, were described as effective antiviral agents that inhibit replication of a broad range of wild type human viruses. Olomoucine II and roscovitine show high selectivity for CDK7 and CDK9, with important functions in the regulation of RNA polymerase II transcription. RNA polymerase II is necessary for viral transcription and following replication in cells. We analyzed the effect of inhibition of CDKs by olomoucine II on gene expression from viral promoters and compared its effect to widely-used roscovitine. We found that both roscovitine and olomoucine II blocked the phosphorylation of RNA polymerase II C-terminal domain. However the repression of genes regulated by viral promoters was strongly dependent on gene localization. Both roscovitine and olomoucine II inhibited expression only when the viral promoter was not integrated into chromosomal DNA. In contrast, treatment of cells with genome-integrated viral promoters increased their expression even though there was decreased phosphorylation of the C-terminal domain of RNA polymerase II. To define the mechanism responsible for decreased gene expression after pharmacological CDK inhibitor treatment, the level of mRNA transcription from extrachromosomal DNA was determined. Interestingly, our results showed that inhibition of RNA polymerase II C-terminal domain phosphorylation increased the number of transcribed mRNAs. However, some of these mRNAs were truncated and lacked polyadenylation, which resulted in decreased translation. These results suggest that phosphorylation of RNA polymerase II C-terminal domain is critical for linking transcription and posttrancriptional processing of mRNA expressed from extrachromosomal DNA.

Discovery of the antiviral activities of pharmacologic cyclin-dependent kinase inhibitors: from basic to applied science

'Owing to the strong potential as antivirals exhibited to date by PCIs, cellular proteins are now starting to be considered more often as valid targets for antiviral drugs'

Potential of protein kinase inhibitors for treating herpesvirus-associated disease

Herpesviruses are ubiquitous human pathogens that establish lifelong persistent infections. Clinical manifestations range from mild self-limiting outbreaks such as childhood rashes and cold sores to the more severe and life-threatening outcomes of disseminated infection, encephalitis, and cancer. Nucleoside analog drugs that target viral DNA replication provide the primary means of treatment. However, extended use of these drugs can result in selection for drug-resistant strains, particularly in immunocompromised patients. In this review we will present recent observations about the participation of cellular protein kinases in herpesvirus biology and discuss the potential for targeting these protein kinases as well as the herpesvirus-encoded protein kinases as an anti-herpesvirus therapeutic strategy.

CDK INHIBITORY NUCLEOSIDE ANALOGS PREVENT TRANSCRIPTION FROM VIRAL GENOMES

Targeting viral proteins has lead to many successful antivirals. Yet, such antivirals rapidly select for resistance, tend to be active against only a few related viruses, and require previous characterization of the target proteins. Alternatively, antivirals may be targeted to cellular proteins. Replication of many viruses requires cellular CDKs and pharmacological CDK inhibitors (PCIs), such as the purine-based roscovitine (Rosco), are proving safe in clinical trials against cancer. Rosco inhibits replication of wild-type or (multi-)drug resistant HIV, HCMV, EBV, VZV, and HSV-1 and 2. However, the antiviral mechanisms of purine PCIs remain unknown. Our objective is to characterize these mechanisms using HSV as a model We have shown that Rosco prevents initiation of transcription from viral, but not cellular, genomes. This inhibition is promoter independent, but genome dependent, and requires no viral proteins. This is a novel antiviral mechanism and a previously unknown activity for purine PCIs.

Analysis of the cell cycle regulatory protein (E2F1) after infection of cultured cells with bovine herpesvirus 1 (BHV-1) or herpes simplex virus type 1 (HSV-1)

The E2F family of cellular transcription factors controls cell cycle progression and cell death. During cell cycle progression, activated cyclin-dependent kinases phosphorylate the retinoblastoma (Rb) protein, causing the release and activation of E2F family members. Previous studies demonstrated that bovine herpes virus 1 (BHV-1) productive infection increases E2F1 protein levels, the bICP0 early promoter is activated more than 100 fold by E2F1 or E2F2, and silencing E2F1 reduced the efficiency of productive infection. In this study, the effect of herpes simplex virus type 1 (HSV-1) productive infection on E2F protein levels and regulation of E2F dependent transcription was compared to BHV-1 infection in the same permissive cell line, rabbit skin (RS) cells. Silencing E2F1 with a specific siRNA reduced HSV-1 productive infection approximately 10 fold in RS cells, and total E2F1 protein levels increased during productive infection. In contrast to RS cells infected with BHV-1, a fraction of total E2F1 protein was localized to the cytoplasm in HSV-1 infected RS cells. Furthermore, E2F1 did not efficiently trans-activate the HSV-1 ICP0 or ICP4 promoter. When RS cells were transfected with an E2F reporter construct or the cyclin D1 promoter and then infected with BHV-1, promoter activity increased after infection. In contrast, HSV-1 infection of RS cells had little effect on E2F dependent transcription and cyclin D1 promoter activity was reduced. In summary, these studies indicated that silencing E2F1 reduced the efficiency of HSV-1 and BHV-1 productive infection. However, only BHV-1 productive infection induced E2F dependent transcription.

Virus reactivation: a panoramic view in human infections

Viruses are obligate intracellular parasites, relying to a major extent on the host cell for replication. An active replication of the viral genome results in a lytic infection characterized by the release of new progeny virus particles, often upon the lysis of the host cell. Another mode of virus infection is the latent phase, where the virus is 'quiescent' (a state in which the virus is not replicating). A combination of these stages, where virus replication involves stages of both silent and productive infection without rapidly killing or even producing excessive damage to the host cells, falls under the umbrella of a persistent infection. Reactivation is the process by which a latent virus switches to a lytic phase of replication. Reactivation may be provoked by a combination of external and/or internal cellular stimuli. Understanding this mechanism is essential in developing future therapeutic agents against viral infection and subsequent disease. This article examines the published literature and current knowledge regarding the viral and cellular proteins that may play a role in viral reactivation. The focus of the article is on those viruses known to cause latent infections, which include herpes simplex virus, varicella zoster virus, Epstein-Barr virus, human cytomegalovirus, human herpesvirus 6, human herpesvirus 7, Kaposi's sarcoma-associated herpesvirus, JC virus, BK virus, parvovirus and adenovirus.

Role of a cdk5-associated protein, p35, in herpes simplex virus type 1 replication in vivo

Previous studies have shown that herpes simplex virus type 1 (HSV-1) replication is inhibited by the cyclin-dependent kinase (cdk) inhibitor roscovitine. One roscovitine-sensitive cdk that functions in neurons is cdk5, which is activated in part by its binding partner, p35. Because HSV establishes latent infections in sensory neurons, we sought to determine the role p35 plays in HSV-1 replication in vivo. For these studies, wild-type (wt) and p35−/− mice were infected with HSV-1 using the mouse ocular model of HSV latency and reactivation. The current results indicate that p35 is an important determinant of viral replication in vivo.

MHV-68 Open Reading Frame 20 is a nonessential gene delaying lung viral clearance

Recently, it has been demonstrated that the MHV-68 ORF20-encoded gene product induces cell-cycle arrest at the G2/M phase, followed by apoptosis. To study the role of this conserved gene in vivo, two independent ORF20-deficient MHV-68 viruses and their revertants were constructed. As the replication in vitro of both mutants followed similar kinetics to that of the wild-type and revertant viruses, ORF20 is therefore a nonessential virus gene. No cell cycle arrest could be observed upon infection of cells with wild type MHV-68 or mutant viruses. In addition, no major differences were detected between mock- and virus-infected cells when protein and inactivation levels of the mitotic promoter factor cdc2/cyclinB were analyzed. Following intranasal infection, the recovery of mutant, revertant and wild-type viruses in the lungs was similar. With the ORF20-deficient viruses, however, there was a significant delay of four days in clearance of virus from the lungs. Surprisingly, the magnitude and cell population distribution in the exudates of the lung was essentially similar to mice infected with wild-type, revertant or ORF20-deleted viruses. Subsequent establishment of latency was normal for both mutants, demonstrating that ORF20 does not play a critical role in establishment of a persistent infection. These results indicate that while expression of ORF20 may impact on the pathogenicity of the infection, the observed induction of G2/M arrest in ORF20-expressing cells may not be the primary function of ORF20 in the context of viral infection.

Immunomodulatory cytokines determine the outcome of Japanese encephalitis virus infection in mice

Japanese encephalitis virus (JEV) induces an acute infection of the central nervous system, the pathogenic mechanism of which is not fully understood. To investigate host response to JEV infection, 14-day-old mice were infected via the extraneural route, which resulted in encephalitis and death. Mice that received JEV immune splenocyte transfer were protected from extraneural JEV infection. Pathology and gene expression profiles were then compared in brains of mice that either succumbed to JEV infection or were protected from infection by JEV immune cell transfer. Mice undergoing progressive JEV infection had increased expression of proinflammatory cytokines, chemokines, and signal transducers associated with the interferon (IFN) pathway. In contrast, mice receiving immune cell transfer had increased production of the Th2 cytokine IL-4, and of IL-10, with subdued expression of IFN-gamma. We observed IL-10 to be an important factor in determining clinical outcome in JEV infection. Data obtained by microarray analysis were further confirmed by quantitative RT-PCR. Together, these data suggest that JEV infection causes an unregulated inflammatory response that can be countered by the expression of immunomodulatory cytokines in mice that survive lethal infection.

The inhibitor of cyclin-dependent kinases, olomoucine II, exhibits potent antiviral properties

BACKGROUND

Olomoucine II, the most recent derivative of roscovitine, is an exceptionally potent pharmacological inhibitor of cyclin-dependent kinase activities. Here, we report that olomoucine II is also an effective antiviral agent.

METHODS

Antiviral activities of olomoucine II were tested on a range of human viruses in in vitro assays that evaluated viral growth and replication.

RESULTS

Olomoucine II inhibited replication of a broad range of wild-type human viruses, including herpes simplex virus, human adenovirus type-4 and human cytomegalovirus. Olomoucine II also inhibited replication of vaccinia virus and herpes simplex virus mutants resistant to conventional acyclovir treatment. This report is the first demonstration of a poxvirus being sensitive to a cyclin-dependent kinase inhibitor. The antiviral effects of olomoucine II could be observed at lower concentrations than with roscovitine, although both were short-term. A remarkable observation was that olomoucine II, when used in combination with the DNA polymerase inhibitor cidofovir, was able to almost completely eliminate the spread of infectious adenovirus type-4 progeny from infected cells.

CONCLUSIONS

Our results show that when targeting two complementary antiviral mechanisms, strongly additive effects could be observed.

Human cytomegalovirus IE1 protein enhances herpes simplex virus type 1-induced syncytial formation in U373MG cells

Co-infection of herpes simplex virus type 1 (HSV-1) and human cytomegalovirus (HCMV) is not uncommon in immunocompromised hosts. Importantly, organ transplant recipients concurrently infected with HSV-1 and HCMV have a worse clinical outcome than recipients infected with a single virus. However, factors regulating the pathologic response in HSV-1, HCMV co-infected tissues are unclear. We investigated the potential biologic role of HCMV gene product immediate early 1 (IE1) protein in HSV-1-induced syncytial formation in U373MG cells. We utilized a co-infection model by infecting HSV-1 to U373MG cells constitutively expressing HCMV IE1 protein, UMG1-2. Syncytial formation was assessed by enumerating nuclei number per syncytium and number of syncytia. HSV-1-induced syncytial formation was enhanced after 24 hr in UMG1-2 cells compared with U373MG controls. The amplified phenotype in UMG1-2 cells was effectively suppressed by roscovitine in addition to inhibitors of viral replication. This is the first study to provide histological evidence of the contribution of HCMV IE1 protein to enhanced cytopathogenic responses in active HSV-1 infection.

Inhibition of the cyclin-dependent kinases at the beginning of human cytomegalovirus infection specifically alters the levels and localization of the RNA polymerase II carboxyl-terminal domain kinases cdk9 and cdk7 at the viral transcriptosome

We previously reported that defined components of the host transcription machinery are recruited to human cytomegalovirus immediate-early (IE) transcription sites, including cdk9 and cdk7 (S. Tamrakar, A. J. Kapasi, and D. H. Spector, J. Virol. 79:15477-15493, 2005). In this report, we further document the complexity of this site, referred to as the transcriptosome, through identification of additional resident proteins, including viral UL69 and cellular cyclin T1, Brd4, histone deacetylase 1 (HDAC1), and HDAC2. To examine the role of cyclin-dependent kinases (cdks) in the establishment of this site, we used roscovitine, a specific inhibitor of cdk1, cdk2, cdk7, and cdk9, that alters processing of viral IE transcripts and inhibits expression of viral early genes. In the presence of roscovitine, IE2, cyclin T1, Brd4, HDAC1, and HDAC2 accumulate at the transcriptosome. However, accumulation of cdk9 and cdk7 was specifically inhibited. Roscovitine treatment also resulted in decreased levels of cdk9 and cdk7 RNA. There was a corresponding reduction in cdk9 protein but only a modest decrease in cdk7 protein. However, overexpression of cdk9 does not compensate for the effects of roscovitine on cdk9 localization or viral gene expression. Delaying the addition of roscovitine until 8 h postinfection prevented all of the observed effects of the cdk inhibitor. These data suggest that IE2 and multiple cellular factors needed for viral RNA synthesis accumulate within the first 8 h at the viral transcriptosome and that functional cdk activity is required for the specific recruitment of cdk7 and cdk9 during this time interval.

Pharmacological cdk inhibitor R-Roscovitine suppresses JC virus proliferation

The human Polyomavirus JC virus (JCV) utilizes cellular proteins for viral replication and transcription in the host cell nucleus. These cellular proteins represent potential targets for antiviral drugs against the JCV. In this study, we examined the antiviral effects of the pharmacological cyclin-dependent kinase (cdk) inhibitor R-Roscovitine, which has been shown to have antiviral activity against other viruses. We found that Roscovitine significantly inhibited the viral production and cytopathic effects of the JCV in a JCV-infected cell line. Roscovitine attenuated the transcriptional activity of JCV late genes, but not early genes, and also prevented viral replication via inhibiting phosphorylation of the viral early protein, large T antigen. These data suggest that the JCV requires cdks to transcribe late genes and to replicate its own DNA. That Roscovitine exhibited antiviral activity in JCV-infected cells suggests that Roscovitine might have therapeutic utility in the treatment of progressive multifocal leukoencephalopathy (PML).

Emerin is hyperphosphorylated and redistributed in herpes simplex virus type 1-infected cells in a manner dependent on both UL34 and US3

Cells infected with wild-type herpes simplex virus type 1 (HSV-1) show disruption of the organization of the nuclear lamina that underlies the nuclear envelope. This disruption is reflected in changes in the localization and phosphorylation of lamin proteins. Here, we show that HSV-1 infection causes relocalization of the LEM domain protein emerin. In cells infected with wild-type virus, emerin becomes more mobile in the nuclear membrane, and in cells infected with viruses that fail to express UL34 protein (pUL34) and US3 protein (pUS3), emerin no longer colocalizes with lamins, suggesting that infection causes a loss of connection between emerin and the lamina. Infection causes hyperphosphorylation of emerin in a manner dependent upon both pUL34 and pUS3. Some emerin hyperphosphorylation can be inhibited by the protein kinase Cdelta (PKCdelta) inhibitor rottlerin. Emerin and pUL34 interact physically, as shown by pull-down and coimmunoprecipitation assays. Emerin expression is not, however, necessary for infection, since virus growth is not impaired in cells derived from emerin-null transgenic mice. The results suggest a model in which pUS3 and PKCdelta that has been recruited by pUL34 hyperphosphorylate emerin, leading to disruption of its connections with lamin proteins and contributing to the disruption of the nuclear lamina. Changes in emerin localization, nuclear shape, and lamin organization characteristic of cells infected with wild-type HSV-1 also occur in cells infected with recombinant virus that does not make viral capsids, suggesting that these changes occur independently of capsid envelopment.

Five years of progress on cyclin-dependent kinases and other cellular proteins as potential targets for antiviral drugs

In 1997-1998, the pharmacological cyclin-dependent kinase (CDK) inhibitors (PCIs) were independently discovered to inhibit replication of human cytomegalovirus, herpes simplex virus type 1 and HIV-1. The results from small clinical trials against cancer were then suggesting that PCIs could be safe enough to be used clinically. It was thus hypothesized that PCIs could have the potential to be developed as novel antivirals targeting cellular proteins. Consequently, Antiviral Chemistry & Chemotherapy published in 2001 the first review on the potential of CDKs, and cellular proteins in general, as potential targets for antivirals. The viral functions inhibited by PCIs, or their cellular targets, were then just starting to be characterized. The antiviral spectrum of PCIs and their effects on viral disease were still mostly untested. Even their actual specificity was not yet completely characterized. In addition, cellular proteins were not accepted as valid targets for antivirals. Significant progress has been made in the last 5 years in understanding the antiviral activities of PCIs and the potential roles of cellular proteins in general as targets for antivirals. The first clinical trials of the antiviral activities of PCIs and other inhibitors of cellular protein kinases have now been scheduled. Herein, we review the progress made since the publication of the first review on PCIs as potential antiviral drugs and on CDKs, and cellular proteins in general, as potential targets for antiviral drugs. We also highlight the major issues that still need to be addressed before PCIs or other drugs targeting cellular proteins can be developed as clinical antivirals.

Cyclin-dependent kinase activity is required for efficient expression and posttranslational modification of human cytomegalovirus proteins and for production of extracellular particles

We have previously shown that the addition of the cyclin-dependent kinase (cdk) inhibitor Roscovitine at the beginning of infection of cells with human cytomegalovirus (HCMV) significantly disrupts immediate-early gene expression and the progression of the infection. In the present study, we have examined the effects of cdk inhibition on late viral events by delaying addition of Roscovitine until 24 h postinfection. Although viral DNA replication was inhibited two- to threefold by treatment of infected cells with Roscovitine, the drop did not correspond to the 1- to 2-log-unit decrease in virus titer. Quantification of viral DNA in the supernatant from cells revealed that there was a significant reduction in the production or release of extracellular particles. We observed a lag in the expression of several viral proteins but there was a significant decrease in the steady-state levels of IE2-86. Likewise, the steady-state level of the essential tegument protein UL32 (pp150) was reduced. The levels of pp150 and IE2-86 mRNA were not greatly affected by treatment with Roscovitine and thus did not correlate with the reduced levels of protein. In contrast, the expression of the tegument protein ppUL69 was higher in drug-treated samples, and the protein accumulated in a hyperphosphorylated form. ppUL69 localized to intranuclear aggregates that did not overlap with viral replication centers in cells treated with Roscovitine. Taken together, these data indicate that cdk activity is required at multiple steps during HCMV infection, including the expression, modification, and localization of virus-encoded proteins.

Imaging immediate-early and strict-late promoter activity during oncolytic herpes simplex virus type 1 infection and replication in tumors

An increasing number of oncolytic viruses have been developed and studied for cancer therapy. In response to needs for non-invasive monitoring and imaging of oncolytic virotherapy, several different approaches, including a positron emission tomography-based method, a method using secreted marker peptides, and optical imaging-based methods, have been reported. Among these modalities, we utilized the luciferase-based bioluminescent assay/imaging systems to determine the kinetics and dynamics of a productive viral infection. The replication cycle of herpes simplex virus type 1 (HSV-1) is punctuated by a temporal cascade of three classes of viral genes: immediate-early (IE), early (E) and late (L) genes. U(L)39- and gamma(1)34.5-deleted, replication-conditional HSV-1 mutants that express firefly luciferase under the control of the IE4/5 or strict-late gC promoters were generated. These oncolytic viruses were examined in cultured cells and a mouse tumor model. IE promoter- and strict-late promoter-mediated luciferase expression was confirmed to indicate viral infection and replication, respectively. Incorporation of a strict-late promoter-driven luciferase cassette into oncolytic HSV-1 vectors would be useful for assessing tumor oncolysis in preclinical tumor treatment studies.

The products of the herpes simplex virus type 1 immediate-early US1/US1.5 genes downregulate levels of S-phase-specific cyclins and facilitate virus replication in S-phase Vero cells

Herpes simplex virus type 1 ICP22-/U(S)1.5- mutants initiate viral gene expression in all cells; however, in most cell types, the replication process stalls due to an inability to express gamma2 late proteins. Although the function of ICP22/U(S)1.5 has not been established, it has been suggested that these proteins activate, induce, or repress the activity of cellular proteins during infection. In this study, we hypothesized that cell cycle-associated proteins are targets of ICP22/U(S)1.5. For this purpose, we first isolated and characterized an ICP22-/U(S)1.5- mutant virus, 22/n199. Like other ICP22-/U(S)1.5- mutants, 22/n199 replicates in a cell-type-specific manner and fails to induce efficient gamma2 late gene expression in restrictive cells. Although synchronization of restrictive human embryonic lung cells in each phase of the cell cycle did not overcome the growth restrictions of 22/n199, synchronization of permissive Vero cells in S phase rendered them less able to support 22/n199 plaque formation and replication. Consistent with this finding, expression of cellular S-phase cyclins was altered in an ICP22/U(S)1.5-dependent manner specifically when S-phase Vero cells were infected. Collectively, these observations support the notion that ICP22/U(S)1.5 deregulates the cell cycle upon infection of S-phase permissive cells by altering expression of key cell cycle regulatory proteins either directly or indirectly.

Phosphorylation of the VP16 transcriptional activator protein during herpes simplex virus infection and mutational analysis of putative phosphorylation sites

VP16 is a virion phosphoprotein of herpes simplex virus and a transcriptional activator of the viral immediate-early (IE) genes. We identified four novel VP16 phosphorylation sites (Ser18, Ser353, Ser411, and Ser452) at late times in infection but found no evidence of phosphorylation of Ser375, a residue reportedly phosphorylated when VP16 is expressed from a transfected plasmid. A virus carrying a Ser375Ala mutation of VP16 was viable in cell culture but with a slow growth rate. The association of the mutant VP16 protein with IE gene promoters and subsequent IE gene expression was markedly reduced during infection, consistent with prior transfection and in vitro results. Surprisingly, the association of Oct-1 with IE promoters was also diminished during infection by the mutant strain. We propose that Ser375 is important for the interaction of VP16 with Oct-1, and that the interaction is required to enable both proteins to bind to IE promoters.

Roscovitine inhibits activation of promoters in herpes simplex virus type 1 genomes independently of promoter-specific factors

Flavopiridol, roscovitine, and other inhibitors of Cyclin-Dependent Kinases (CDK) inhibit the replication of a variety of viruses in vitro while proving nontoxic in human clinical trials of their effects against cancer. Consequently, these and other Pharmacological CDK inhibitors (PCIs) have been proposed as potential antivirals. Flavopiridol potently inhibits all tested CDKs and inhibits the transcription of most cellular and viral genes. In contrast, roscovitine and other purine PCIs inhibit with high potency only CDK1, CDK2, CDK5, and CDK7, and they specifically inhibit the expression of viral but not cellular genes. The levels at which purine PCIs inhibit gene expression are unknown, as are the factors which determine their specificity for expression of viral but not cellular genes. We show herein that roscovitine prevents the initiation of transcription of herpes simplex virus type 1 (HSV-1) genes but has no effect on transcription elongation. We further show that roscovitine does not inhibit the initiation or elongation of cellular transcription and that its inhibitory effects are specific for promoters in HSV-1 genomes. Therefore, we have identified a novel biological activity for PCIs, i.e., their ability to prevent the initiation of transcription. We have also identified genome location as one of the factors that determine whether the transcription of a given gene is inhibited by roscovitine. The activities of roscovitine on viral transcription resemble one of the antiherpesvirus activities of alpha interferon and could be used as a model for the development of novel antivirals. The genome-specific effects of roscovitine may also be important for its development against virus-induced cancers.

Use of a multi-virus array for the study of human viral and retroviral pathogens: gene expression studies and ChIP-chip analysis

Background

Since the discovery of human immunodeficiency virus (HIV-1) twenty years ago, AIDS has become one of the most studied diseases. A number of viruses have subsequently been identified to contribute to the pathogenesis of HIV and its opportunistic infections and cancers. Therefore, a multi-virus array containing eight human viruses implicated in AIDS pathogenesis was developed and its efficacy in various applications was characterized.

Results

The amplified open reading frames (ORFs) of human immunodeficiency virus type 1, human T cell leukemia virus types 1 and 2, hepatitis C virus, Epstein-Barr virus, human herpesvirus 6A and 6B, and Kaposi's sarcoma-associated herpesvirus were spotted on glass slides and hybridized to DNA and RNA samples. Using a random priming method for labeling genomic DNA or cDNA probes, we show specific detection of genomic viral DNA from cells infected with the human herpesviruses, and effectively demonstrate the inhibitory effects of a cellular cyclin dependent kinase inhibitor on viral gene expression in HIV-1 and KSHV latently infected cells. In addition, we coupled chromatin immunoprecipitation with the virus chip (ChIP-chip) to study cellular protein and DNA binding.

Conclusions

An amplicon based virus chip representing eight human viruses was successfully used to identify each virus with little cross hybridization. Furthermore, the identity of both viruses was correctly determined in co-infected cells. The utility of the virus chip was demonstrated by a variety of expression studies. Additionally, this is the first demonstrated use of ChIP-chip analysis to show specific binding of proteins to viral DNA, which, importantly, did not require further amplification for detection.

Roscovitine, a cyclin-dependent kinase inhibitor, prevents replication of varicella-zoster virus

Understanding the interactions between varicella-zoster virus (VZV) and host cells can be addressed by using small molecule inhibitors of cellular enzymes. Roscovitine (Rosco) is a purine derivative that inhibits cyclin-dependent kinase 1 (cdk1), cdk2, cdk5, cdk7, and cdk9, which are key regulators of the cell cycle and transcription. Herpesviruses are known to interact with cell cycle proteins; thus, the antiviral effects of Rosco on VZV growth were evaluated. In a plaque reduction assay, 25 micro M Rosco prevented VZV replication, and the antiviral effect was reversible for at least up to 24 h posttreatment. Rosco also reduced expression of the major transactivator, IE62, over 48 h. Confocal microscopy studies indicated that Rosco caused the immediate-early proteins ORF4 and IE62 to abnormally localize in infected cells and prevented cell-cell spread of VZV over 48 h. Rosco was found to inhibit VZV DNA synthesis as measured by real-time PCR, and this technique was used to estimate the 50% effective concentration (EC(50)) of 14 micro M. This value was close to the EC(50) estimate of 12 micro M determined from plaque reduction assays. At 25 micro M, Rosco was not cytotoxic over 48 h in a neutral red uptake assay, and proliferation was slowed as the cells accumulated in a G(2)-like state. These results demonstrate the importance of cdk's in VZV replication and suggest that cdk inhibitors could serve as useful VZV antivirals.

Phenotype of a herpes simplex virus type 1 mutant that fails to express immediate-early regulatory protein ICP0

Herpes simplex virus type 1 (HSV-1) immediate-early (IE) regulatory protein ICP0 is required for efficient progression of infected cells into productive lytic infection, especially in low-multiplicity infections of limited-passage human fibroblasts. We have used single-cell-based assays that allow detailed analysis of the ICP0-null phenotype in low-multiplicity infections of restrictive cell types. The major conclusions are as follows: (i) there is a threshold input multiplicity above which the mutant virus replicates normally; (ii) individual cells infected below the threshold multiplicity have a high probability of establishing a nonproductive infection; (iii) such nonproductively infected cells have a high probability of expressing IE products at 6 h postinfection; (iv) even at 24 h postinfection, IE protein-positive nonproductively infected human fibroblast cells exceed the number of cells that lead to plaque formation by up to 2 orders of magnitude; (v) expression of individual IE proteins in a proportion of the nonproductively infected cells is incompletely coordinated; (vi) the nonproductive cells can also express early gene products at low frequencies and in a stochastic manner; and (vii) significant numbers of human fibroblast cells infected at low multiplicity by an ICP0-deficient virus are lost through cell death. We propose that in the absence of ICP0 expression, HSV-1 infected human fibroblasts can undergo a great variety of fates, including quiescence, stalled infection at a variety of different stages, cell death, and, for a minor population, initiation of formation of a plaque.

Effects of pharmacological cyclin-dependent kinase inhibitors on viral transcription and replication

Cyclin-dependent kinases (CDKs) are required for replication of adeno-, papilloma- and other viruses that replicate only in dividing cells. Surprisingly, CDKs are also required for replication of HIV-1, HSV-1, and other viruses that can replicate in non-dividing cells. Since two low-molecular weight pharmacological CDK inhibitors (PCIs), flavopiridol (Flavo) and roscovitine (Rosco), appear to be non-toxic in human clinical trials against cancer, these drugs have been proposed as potential antiviral drugs. Rosco preferentially inhibits CDKs involved in cell cycle regulation (CDK1, 2, and 7) or neuronal functions (CDK5), whereas Flavo preferentially inhibits CDKs involved in cell cycle (CDK1, 2, 4, 7) or transcription (CDK7, and 9). As potential antivirals, PCIs display several advantages: (i) they are active against many different viruses, including drug-resistant strains of HIV-1 and HSV-1; (ii) PCI-resistant mutants of HIV-1 or HSV-1 have not been identified; and (iii) the antiviral effects of PCIs and conventional antivirals appear to be additive (as expected from drugs that target independent pathways). Moreover, PCIs target both the etiological agents (i.e., the virus) and the pathogenic mechanisms (i.e., unrestricted cell division) of the many diseases that include both a CDK-requiring virus and unrestricted cell division (e.g., Kaposi's sarcoma, cervical carcinoma, HIV-associated nephropathy-HIVAN). This is nicely illustrated in a recent study which demonstrated the efficacy of Flavo in a mouse model of HIVAN. Herein, we will review the involvement of CDKs in viral replication and the antiviral properties of the most extensively characterized PCIs, with special emphasis on the mechanisms of inhibition of viral transcription.

Potent, selective and cell-mediated inhibition of human herpesvirus 6 at an early stage of viral replication by the non-nucleoside compound CMV423

CMV423 (2-chloro-3-pyridin-3-yl-5,6,7,8-tetrahydroindolizine-1-carboxamide) is a new antiviral agent with potent and selective in vitro activity against the beta-herpesvirus human cytomegalovirus (HCMV), but not against alpha- or gamma-herpesviruses. Here we report that its activity also extends to human herpesvirus 6 (HHV-6) and 7 (HHV-7). When compared in vitro to ganciclovir and foscarnet (the standard drugs recommended for treatment of HHV-6 infections), CMV423 showed a superior selectivity, due to its high activity (antiviral IC(50): 53nM) and low cytotoxicity (CC(50): 144microM), both in continuous cell lines and in CBLCs infected with HHV-6. From mechanistic experiments at the level of viral mRNA and protein expression, we learned that CMV423 targets an event following viral entry but preceding viral DNA replication. Its antiviral action was dependent on the cell line used, implying involvement of a cellular component. When compared to a panel of known protein kinase inhibitors, CMV423 was found to share anti-HHV-6 characteristics with herbimycin A, which affects tyrosine kinase activity through heat shock protein 90 (Hsp90) inhibition. We demonstrated that high concentrations of CMV423 have an inhibitory effect on the total cellular protein tyrosine kinase activity, and that CMV423 and herbimycin A, when combined, act synergistically against HHV-6. The activities of cyclin-dependent kinases, protein kinases A and C, and the HHV-6-encoded pU69 kinase were not affected. We, therefore, conclude that CMV423 exerts its activity against HHV-6 through inhibition of a cellular process that is critical at early stages of viral replication and that may affect protein tyrosine kinase activity.

Inhibition of S-phase cyclin-dependent kinase activity blocks expression of Epstein-Barr virus immediate-early and early genes, preventing viral lytic replication

The induction of lytic replication of the Epstein-Barr virus (EBV) completely arrests cell cycle progression, in spite of elevation of S-phase cyclin-dependent kinase (CDK) activity, thereby causing accumulation of hyperphosphorylated forms of retinoblastoma (Rb) protein (A. Kudoh, M. Fujita, T. Kiyono, K. Kuzushima, Y. Sugaya, S. Izuta, Y. Nishiyama, and T. Tsurumi, J. Virol. 77:851-861, 2003). Thus, the EBV lytic program appears to promote specific cell cycle-associated activity involved in the progression from G1 to S phase. We have proposed that this provides a cellular environment that is advantageous for EBV productive infection. Purvalanol A and roscovitine, inhibitors of S-phase CDKs, blocked the viral lytic replication when cells were treated at the early stage of lytic infection, while well-characterized inhibitors of enzymes, such as mitogen-activated protein kinase, phosphatidylinositol 3-kinase, and protein kinase C, known to be involved in BZLF1 gene expression did not. Inhibition of CDK activity resulted in the accumulation of the hypophosphorylated form of Rb protein and inhibition of expression of EBV immediate-early and early proteins. Cycloheximide block-and-release experiments clearly demonstrated that even in the presence of enough amounts of the BZLF1 protein, purvalanol A blocked expression of lytic viral proteins at transcription level. Furthermore, reporter gene experiments confirmed that BZLF1-induced activation of early EBV promoters was impaired in the presence of the CDK inhibitor. We conclude here that the EBV lytic program promotes specific cell cycle-associated activity involved in the progression from G1 to S phase because the S-phase-like cellular environment is essential for the expression of immediate-early and early genes supplying the viral replication proteins and hence for lytic viral replication.

The differential requirement for cyclin-dependent kinase activities distinguishes two functions of herpes simplex virus type 1 ICP0

Herpes simplex virus type 1 (HSV-1) ICP0 directs the degradation of cellular proteins associated with nuclear structures called ND10, a function thought to be closely associated with its broad transactivating activity. Roscovitine (Rosco), an inhibitor of cyclin-dependent kinases (cdks), inhibits the replication of HSV-1, HSV-2, human cytomegalovirus, varicella-zoster virus, and human immunodeficiency virus type 1 by inhibiting specific steps or activities of viral regulatory proteins, indicating the broad and pleiotropic effects that cdks have on the replication of these viruses. We previously demonstrated that Rosco inhibits the transactivating activity of ICP0. In the present study, we asked whether Rosco also affects the ability of ICP0 to direct the degradation of ND10-associated proteins. For this purpose, WI-38 cells treated with cycloheximide (CHX) were mock infected or infected with wild-type HSV-1 or an ICP0(-) mutant (7134). After release from the CHX block, the infections were allowed to proceed for 2 h in the presence or absence of Rosco at a concentration known to inhibit ICP0's transactivating activity. The cells were then examined for the presence of ICP0 and selected ND10-associated antigens (promyelocytic leukemia antigen [PML], sp100, hDaxx, and NDP55) by immunofluorescence. Staining for the ND10-associated antigens was detected in </=20% of KOS-infected cells in the presence or absence of Rosco, demonstrating that Rosco-sensitive kinases are not required for ICP0's ability to direct the dispersal or degradation of these antigens. In contrast, >90% of 7134- and mock-infected cells stained positive for all ND10-associated antigens in the presence or absence of Rosco. Similar results were obtained with a non-ND10-associated antigen, DNA-PK(cs), a known target of ICP0-directed degradation. The results of the PML and DNA-PK(cs) immunofluorescence studies correlated with a decrease in the levels of these proteins as determined by Western blotting. Thus, the differential requirement for Rosco-sensitive cdk activities distinguishes ICP0's ability to direct the dispersal or degradation of cellular proteins from its transactivating activity.

Protein kinases conserved in herpesviruses potentially share a function mimicking the cellular protein kinase cdc2

Herpesviruses encode protein kinases. A subset of these proteins, represented by HSV-1 UL13, are conserved throughout all members of the Herpesviridae, and here, are designated CHPKs (conserved herpesvirus protein kinases). In addition to conserved gene products like CHPKs, herpesviruses encode genes specific to respective herpesviruses. When acting upon conserved viral gene products or cellular factors, CHPKs may play conserved roles in the life cycles of herpesviruses. CHPKs may also express unique functions within the infectious process of individual herpesviruses when specific viral gene products are targeted. CHPKs demonstrate specific activity in multiple herpesvirus infections, functioning in the regulation of viral gene expression in HSV-1, tissue tropism in VZV, and viral DNA synthesis, encapsidation and egress from the nucleus in HCMV. The HCMV CHPK, however, can partially substitute for the HSV-1 CHPK. Representative CHPKs from all Herpesviridae subfamilies can also facilitate the hyperphosphorylation of the cellular translation factor, EF-1delta. This indicates that CHPKs have conserved functions. Recent data have shown that both CHPKs and a cellular protein kinase, cdc2, phosphorylate the same amino acid residues of target proteins. Thus, CHPKs may mimic cdc2 function in infected cells.

The effect of roscovitine on herpetic keratitis

To determine the effect of roscovitine, a potent antiviral in tissue culture administered intramuscularly to rabbits or by eye drops to mice for the treatment of herpetic keratitis this study was commenced.New Zealand white rabbits infected with McKrae strain herpesvirus (HSV-1) were treated twice a day with 10mg Roscovitine or vehicle from day 3 to 7, or 1% trifluridine eye drops five times a day. Severity of keratitis was graded daily by a masked observer. ICR strain mice were randomized into 14 groups. Both corneas of the mice were scarified with a 25-gauge needle, and were inoculated with the KOS strain of HSV-1. Roscovitine was dissolved in Cremophor((R)) and tissue culture medium. Group 1A, 1B, 2A, and 2B mice were treated eight times daily with either 800 micro M Roscovitine or its vehicle. Trifluridine treated animals (groups 3A and 3B) received topical treatment five times daily. In groups 1A-3B, the treatment was begun two days prior to or two days after viral inoculation. Mice were examined on days 2, 3, 5, and 9 after infection. In rabbits, the severity of keratitis in animals treated with intramuscular roscovitine was not significantly different from that in vehicle-treated animals except on day 7 (p=0.0460). In mice, there was no significant difference between roscovitine and vehicle treatment at any time point studied. However, the trifluridine treated mice had significantly lower scores compared to the roscovitine or vehicle-treated mice. Although roscovitine dramatically suppresses viral replication in tissue culture studies, in vivo this drug failed to alter the course of HSV keratitis in rabbits or mice. Considering high cost of roscovitine and the poor efficacy in these experiments, we feel that roscovitine is not feasible antiviral agent for today.

Stimulation of bovine herpesvirus-1 productive infection by the adenovirus E1A gene and a cell cycle regulatory gene, E2F-4

Identifying cellular genes that promote bovine herpesvirus-1 (BHV-1) productive infection is important, as BHV-1 is a significant bovine pathogen. Previous studies demonstrated that BHV-1 DNA is not very infectious unless cotransfected with a plasmid expressing bICP0, a viral protein that stimulates expression of all classes of viral promoters. Based on these and other studies, we hypothesize that the ability of bICP0 to interact with and modify the function of cellular proteins stimulates virus transcription. If this prediction is correct, cellular proteins that activate virus transcription could, in part, substitute for bICP0 functions. The adenovirus E1A gene and bICP0 encode proteins that are potent activators of viral gene expression, they do not specifically bind DNA and both proteins interact with chromatin-remodelling enzymes. Because of these functional similarities, E1A was tested initially to see if it could stimulate BHV-1 productive infection. E1A consistently stimulates BHV-1 productive infection, but not as efficiently as bICP0. The ability of E1A to bind Rb family members plays a role in stimulating productive infection, suggesting that E2F family members activate productive infection. E2F-4, but not E2F-1, E2F-2 or E2F-5, activates productive infection with similar efficiency as E1A. Next, E2F family members were examined for their ability to activate the BHV-1 immediate-early (IE) transcription unit 1 (IEtu1) promoter, as it regulates IE expression of bICP0 and bICP4. E2F-1 and E2F-2 strongly activate the IEtu1 promoter, but not a BHV-1 IEtu2 promoter or a herpes simplex virus type 1 ICP0 promoter construct. These studies suggest that E2F family members can stimulate BHV-1 productive infection.

Identification of homeodomain proteins, PBX1 and PREP1, involved in the transcription of murine leukemia virus

Cyclin-dependent kinase inhibitors (CDKIs) have been shown to block human immunodeficiency virus and herpes simplex virus. It is hypothesized that CDKIs block viral replication by inhibiting transcription of specific cellular genes. Here we find that three CDKIs, flavopiridol, purvalanol A, and methoxy-roscovitine, block Moloney murine leukemia virus (MLV) transcription events. Using gene expression microarray technology to examine the inhibitory effects of CDKIs, we observed a cellular gene, the pre-B-cell leukemia transcription factor 1 (Pbx1) gene, down-regulated by CDKI treatment. The PBX consensus element (PCE), TGATTGAC, is conserved in the long terminal repeats of several murine retroviruses, including Moloney MLV. Mutations in the PCE completely inhibited viral transcription whereas overexpression of PBX1 and a PBX1-associated protein, PREP1, enhanced viral transcription. The interaction between the PCE and PBX1-PREP1 proteins was confirmed by gel shift experiments. Blocking PBX1 protein synthesis resulted in a significant decrease in viral transcription. Collectively, our results represent the first work demonstrating that the homeodomain proteins PBX1 and PREP1 are cellular factors involved in Moloney MLV transcription regulation.

Conserved protein kinases encoded by herpesviruses and cellular protein kinase cdc2 target the same phosphorylation site in eukaryotic elongation factor 1delta

Earlier studies have shown that translation elongation factor 1delta (EF-1delta) is hyperphosphorylated in various mammalian cells infected with representative alpha-, beta-, and gammaherpesviruses and that the modification is mediated by conserved viral protein kinases encoded by herpesviruses, including UL13 of herpes simplex virus type 1 (HSV-1), UL97 of human cytomegalovirus, and BGLF4 of Epstein-Barr virus (EBV). In the present study, we attempted to identify the site in EF-1delta associated with the hyperphosphorylation by the herpesvirus protein kinases. Our results are as follows: (i) not only in infected cells but also in uninfected cells, replacement of the serine residue at position 133 (Ser-133) of EF-1delta by alanine precluded the posttranslational processing of EF-1delta, which corresponds to the hyperphosphorylation. (ii) A purified chimeric protein consisting of maltose binding protein (MBP) fused to a domain of EF-1delta containing Ser-133 (MBP-EFWt) is specifically phosphorylated in in vitro kinase assays by purified recombinant UL13 fused to glutathione S-transferase (GST) expressed in the baculovirus system. In contrast, the level of phosphorylation by the recombinant UL13 of MBP-EFWt carrying an alanine replacement of Ser-133 (MBP-EFS133A) was greatly impaired. (iii) MBP-EFWt is also specifically phosphorylated in vitro by purified recombinant BGLF4 fused to GST expressed in the baculovirus system, and the level of phosphorylation of MBP-EFS133A by the recombinant BGLF4 was greatly reduced. (iv) The sequence flanking Ser-133 of EF-1delta completely matches the consensus phosphorylation site for a cellular protein kinase, cdc2, and in vitro kinase assays revealed that purified cdc2 phosphorylates Ser-133 of EF-1delta. (v) As observed with EF-1delta, the casein kinase II beta subunit (CKIIbeta) was specifically phosphorylated by UL13 in vitro, while the level of phosphorylation of CKIIbeta by UL13 was greatly diminished when a serine residue at position 209, which has been reported to be phosphorylated by cdc2, was replaced with alanine. These results indicate that the conserved protein kinases encoded by herpesviruses and a cellular protein kinase, cdc2, have the ability to target the same amino acid residues for phosphorylation. Our results raise the possibility that the viral protein kinases mimic cdc2 in infected cells.

Herpes simplex virus type 1 and bovine herpesvirus 1 latency

Primary infection by herpes simplex virus type 1 (HSV-1) can cause clinical symptoms in the peripheral and central nervous system, upper respiratory tract, and gastrointestinal tract. Recurrent ocular shedding leads to corneal scarring that can progress to vision loss. Consequently, HSV-1 is the leading cause of corneal blindness due to an infectious agent. Bovine herpesvirus 1 (BHV-1) has similar biological properties to HSV-1 and is a significant health concern to the cattle industry. Latency of BHV-1 and HSV-1 is established in sensory neurons of trigeminal ganglia, but latency can be interrupted periodically, leading to reactivation from latency and spread of infectious virus. The ability of HSV-1 and BHV-1 to reactivate from latency leads to virus transmission and can lead to recurrent disease in individuals latently infected with HSV-1. During latency, the only abundant HSV-1 RNA expressed is the latency-associated transcript (LAT). In latently infected cattle, the latency-related (LR) RNA is the only abundant transcript that is expressed. LAT and LR RNA are antisense to ICP0 or bICP0, viral genes that are crucial for productive infection, suggesting that LAT and LR RNA interfere with productive infection by inhibiting ICP0 or bICP0 expression. Numerous studies have concluded that LAT expression is important for the latency-reactivation cycle in animal models. The LR gene has recently been demonstrated to be required for the latency-reactivation cycle in cattle. Several recent studies have demonstrated that LAT and the LR gene inhibit apoptosis (programmed cell death) in trigeminal ganglia of infected animals and transiently transfected cells. The antiapoptotic properties of LAT map to the same sequences that are necessary for promoting reactivation from latency. This review summarizes our current knowledge of factors regulating the latency-reactivation cycle of HSV-1 and BHV-1.

Explant-induced reactivation of herpes simplex virus occurs in neurons expressing nuclear cdk2 and cdk4

Herpes simplex virus (HSV) establishes productive (lytic) infections in nonneuronal cells and nonproductive (latent) infections in neurons. It has been proposed that HSV establishes latency because quiescent neurons lack cellular factors required for productive infection. It has been further proposed that these putative factors are induced following neuronal stress, as a requirement for HSV reactivation. To date, the identity of these putative cellular factors remains unknown. We have demonstrated that cyclin-dependent kinase (cdk) 1, 2, or 7 is required for HSV replication in nonneuronal cells. Interestingly, cdks 1 and 2 are not expressed in quiescent neurons but can be induced in stressed neurons. Thus, cdks may be among the cellular proteins required for HSV reactivation whose neuronal expression is differentially regulated during stress. Herein, we determined that neuronal expression of nuclear cdk2, cdk4, and cyclins E and D2 (which activate cdks 2 and 4, respectively) was induced following explant cultivation, a stressful stimulus that induces HSV reactivation. In contrast, neuronal expression of cdk7 and cytoplasmic cdk4 decreased during explant cultivation, whereas cdk3 was detected in the same small percentage of neurons before and after explant cultivation and cdks 1, 5, and 6 were not detected in neuronal cell bodies. HSV-1 reactivated specifically in neurons expressing nuclear cdk2 and cdk4, and an inhibitor specific for cdk2 inhibited HSV-1 reactivation. We conclude that neuronal levels of cdk2 are among the factors that determine the outcome of HSV infections of neurons.

Inhibition of cyclin-dependent kinase 1 by purines and pyrrolo[2,3-d]pyrimidines does not correlate with antiviral activity

We have previously shown that a series of nonnucleoside pyrrolo[2,3-d]pyrimidines selectively inhibit the replication of herpes simplex virus type 1 (HSV-1) and human cytomegalovirus (HCMV). These compounds act at the immediate-early or early stage of HCMV replication and have antiviral properties somewhat similar to those of roscovitine and olomoucine, specific inhibitors of cyclin-dependent kinases (cdks). In the present study we examine the hypothesis that pyrrolo[2,3-d]pyrimidines exert their antiviral effects by inhibition of cellular cdks. Much higher concentrations of a panel of pyrrolo[2,3-d]pyrimidine nucleoside analogs with antiviral activity were required to inhibit recombinant cdk1/cyclin B compared to the submicromolar concentrations required to inhibit HCMV and HSV-1 replication. 4,6-Diamino-5-cyano-7-(2-phenylethyl)pyrrolo[2,3-d]pyrimidine (compound 1369) was the best inhibitor of cdk1 and cyclin B, with a 50% inhibitory concentration (IC(50); 14 microM) similar to that of roscovitine; it was competitive with respect to ATP (K(i) = 14 microM). The potency of compound 1369 against cdk1 and cyclin B was similar to its cytotoxicity (IC(50)s, 32 to 100 microM) but not its antiviral efficacy (IC(50)s, 0.02 to 0.3 microM). Thus, our results indicated the null hypothesis. In contrast, roscovitine was only weakly active against HSV-1 (IC(50), 38 microM) and HCMV (IC(50), 40 microM). These values were similar to those derived by cytotoxicity and cell growth inhibition assays, thereby suggesting that roscovitine is not a selective antiviral. Therefore, we propose that inhibition of cdk1 and cyclin B is not responsible for selective antiviral activity and that pyrrolo[2,3-d]pyrimidines constitute novel pharmacophores which compete with ATP to inhibit cdk1 and cyclin B.

Inhibition of HTLV-1 transcription by cyclin dependent kinase inhibitors

HTLV-1 is the etiologic agent for adult T-cell leukemia/lymphoma (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), where viral replication and transformation are largely dependent upon modification of regulatory and host cell cycle proteins. The mechanism of HTLV-1 transformation appears to be distinct from that of many known chronic or acute leukemia viruses and is related to the viral activator Tax. Here we show that cyclin E, can associate tightly with the coactivator p300 and Pol II complex in HTLV-1 infected cells. The cyclin E associated complex is kinase active and phosphorylates the carboxy terminal domain of RNA Pol II. More importantly, p21/Waf1, a well-known cdk inhibitor at the G1/S border, inhibits transcription of HTLV-1 in both transfections and in in vitro transcription assays. Finally, specific cdk chemical inhibitors, functionally similar to cellular cdkIs, such as p21/Waf1 which inhibits cyclin E/cdk2 activity, also inhibit transcription of the HTLV-1 promoter. In particular, Purvalanol A, with an IC50 of 0.035 microm inhibits activated, but not basal transcription, as well as HTLV-1 infected cells. Collectively, the role of cyclin E/cdk2 in HTLV-1 infected cells and its involvement in RNA Pol II phosphorylation is discussed.