Interaction of the 72-kilodalton human cytomegalovirus IE1 gene product with E2F1 coincides with E2F-dependent activation of dihydrofolate reductase transcription

Non-covalent Interaction With SUMO Enhances the Activity of Human Cytomegalovirus Protein IE1

Viruses interact with the host cellular pathways to optimize cellular conditions for replication. The Human Cytomegalovirus (HCMV) Immediate-Early protein 1 (IE1) is the first viral protein to express during infection. It is a multifunctional and conditionally essential protein for HCMV infection. SUMO signaling regulates several cellular pathways that are also targets of IE1. Consequently, IE1 exploits SUMO signaling to regulate these pathways. The covalent interaction of IE1 and SUMO (IE1-SUMOylation) is well studied. However, the non-covalent interactions between SUMO and IE1 are unknown. We report two SUMO-Interacting Motifs (SIMs) in IE1, one at the end of the core domain and another in the C-terminal domain. NMR titrations showed that IE1-SIMs bind to SUMO1 but not SUMO2. Two critical functions of IE1 are inhibition of SUMOylation of Promyelocytic leukemia protein (PML) and transactivation of viral promoters. Although the non-covalent interaction of IE1 and SUMO is not involved in the inhibition of PML SUMOylation, it contributes to the transactivation activity. The transactivation activity of IE1 was previously correlated to its ability to inhibit PML SUMOylation. Our results suggest that transactivation and inhibition of PML SUMOylation are independent activities of IE1.

Activation of E2F-dependent transcription by the mouse cytomegalovirus M117 protein affects the viral host range

Cytomegaloviruses (CMVs) have a highly restricted host range as they replicate only in cells of their own or closely related species. To date, the molecular mechanisms underlying the CMV host restriction remain poorly understood. However, it has been shown that mouse cytomegalovirus (MCMV) can be adapted to human cells and that adaptation goes along with adaptive mutations in several viral genes. In this study, we identify MCMV M117 as a novel host range determinant. Mutations in this gene enable the virus to cross the species barrier and replicate in human RPE-1 cells. We show that the M117 protein is expressed with early kinetics, localizes to viral replication compartments, and contributes to the inhibition of cellular DNA synthesis. Mechanistically, M117 interacts with members of the E2F transcription factor family and induces E2F target gene expression in murine and human cells. While the N-terminal part of M117 mediates E2F interaction, the C-terminal part mediates self-interaction. Both parts are required for the activation of E2F-dependent transcription. We further show that M117 is dispensable for viral replication in cultured mouse fibroblasts and endothelial cells, but is required for colonization of mouse salivary glands in vivo. Conversely, inactivation of M117 or pharmacological inhibition of E2F facilitates MCMV replication in human RPE-1 cells, whereas replacement of M117 by adenovirus E4orf6/7, a known E2F activator, prevents it. These results indicate that E2F activation is detrimental for MCMV replication in human cells. In summary, this study identifies MCMV M117 as a novel E2F activator that functions as a host range determinant by precluding MCMV replication in human cells.

Feedback-mediated signal conversion promotes viral fitness

A fundamental signal-processing problem is how biological systems maintain phenotypic states (i.e., canalization) long after degradation of initial catalyst signals. For example, to efficiently replicate, herpesviruses (e.g., human cytomegalovirus, HCMV) rapidly counteract cell-mediated silencing using transactivators packaged in the tegument of the infecting virion particle. However, the activity of these tegument transactivators is inherently transient-they undergo immediate proteolysis but delayed synthesis-and how transient activation sustains lytic viral gene expression despite cell-mediated silencing is unclear. By constructing a two-color, conditional-feedback HCMV mutant, we find that positive feedback in HCMV's immediate-early 1 (IE1) protein is of sufficient strength to sustain HCMV lytic expression. Single-cell time-lapse imaging and mathematical modeling show that IE1 positive feedback converts transient transactivation signals from tegument pp71 proteins into sustained lytic expression, which is obligate for efficient viral replication, whereas attenuating feedback decreases fitness by promoting a reversible silenced state. Together, these results identify a regulatory mechanism enabling herpesviruses to sustain expression despite transient activation signals-akin to early electronic transistors-and expose a potential target for therapeutic intervention.

Lausannevirus Encodes a Functional Dihydrofolate Reductase Susceptible to Proguanil

Lausannevirus belongs to the family Marseilleviridae within the group of nucleocytoplasmic large DNA viruses (NCLDVs). These giant viruses exhibit unique features, including a large genome, ranging from 100 kb to 2.5 Mb and including from 150 to more than 2,500 genes, as well as the presence of genes coding for proteins involved in transcription and translation. The large majority of Lausannevirus open reading frames have unknown functions. Interestingly, a bifunctional dihydrofolate reductase-thymidylate synthase (DHFR-TS) is encoded in the Lausannevirus genome. The enzyme plays central roles in DNA precursor biosynthesis. DHFR is the pharmacological target of antifolates, such as trimethoprim, pyrimethamine, and proguanil. First, the functionality of Lausannevirus DHFR-TS was demonstrated by the successful complementation of a DHFR-deficient Saccharomyces cerevisiae strain with a plasmid expressing the heterologous gene. Additionally, using this heterologous expression system, we demonstrated the in vitro susceptibility of Lausannevirus DHFR-TS to proguanil and its resistance to pyrimethamine and trimethoprim. Proguanil may provide a unique and useful treatment if Lausannevirus proves to be a human pathogen. To our knowledge, this is the first time that a DHFR-TS has been described and characterized in an NCLDV.

Human Cytomegalovirus Immediate-Early 1 Protein Rewires Upstream STAT3 to Downstream STAT1 Signaling Switching an IL6-Type to an IFNγ-Like Response

The human cytomegalovirus (hCMV) major immediate-early 1 protein (IE1) is best known for activating transcription to facilitate viral replication. Here we present transcriptome data indicating that IE1 is as significant a repressor as it is an activator of host gene expression. Human cells induced to express IE1 exhibit global repression of IL6- and oncostatin M-responsive STAT3 target genes. This repression is followed by STAT1 phosphorylation and activation of STAT1 target genes normally induced by IFNγ. The observed repression and subsequent activation are both mediated through the same region (amino acids 410 to 445) in the C-terminal domain of IE1, and this region serves as a binding site for STAT3. Depletion of STAT3 phenocopies the STAT1-dependent IFNγ-like response to IE1. In contrast, depletion of the IL6 receptor (IL6ST) or the STAT kinase JAK1 prevents this response. Accordingly, treatment with IL6 leads to prolonged STAT1 instead of STAT3 activation in wild-type IE1 expressing cells, but not in cells expressing a mutant protein (IE1dl410-420) deficient for STAT3 binding. A very similar STAT1-directed response to IL6 is also present in cells infected with a wild-type or revertant hCMV, but not an IE1dl410-420 mutant virus, and this response results in restricted viral replication. We conclude that IE1 is sufficient and necessary to rewire upstream IL6-type to downstream IFNγ-like signaling, two pathways linked to opposing actions, resulting in repressed STAT3- and activated STAT1-responsive genes. These findings relate transcriptional repressor and activator functions of IE1 and suggest unexpected outcomes relevant to viral pathogenesis in response to cytokines or growth factors that signal through the IL6ST-JAK1-STAT3 axis in hCMV-infected cells. Our results also reveal that IE1, a protein considered to be a key activator of the hCMV productive cycle, has an unanticipated role in tempering viral replication.

Our previous work has shown that the human cytomegalovirus (hCMV) major immediate-early 1 protein (IE1) modulates host cell signaling pathways involving proteins of the signal transducer and activator of transcription (STAT) family. IE1 has also long been known to facilitate viral replication by activating transcription. In this report we demonstrate that IE1 is as significant a repressor as it is an activator of host gene expression. Many genes repressed by IE1 are normally induced via STAT3 signaling triggered by interleukin 6 (IL6) or related cytokines, whereas many genes activated by IE1 are normally induced via STAT1 signaling triggered by interferon gamma (IFNγ). Our results suggest that the repression of STAT3- and the activation of STAT1-responsive genes by IE1 are coupled. By targeting STAT3, IE1 rewires upstream STAT3 to downstream STAT1 signaling. Consequently, genes normally induced by IL6 are repressed while genes normally induced by IFNγ become responsive to IL6 in the presence of IE1. We also demonstrate that, by switching an IL6 to an IFNγ-like response, IE1 tempers viral replication. These results suggest an unanticipated dual role for IE1 in either promoting or limiting hCMV propagation and demonstrate how a key viral regulatory protein merges two central cellular signaling pathways to divert cytokine responses relevant to hCMV pathogenesis.

Immediate-Early (IE) gene regulation of cytomegalovirus: IE1- and pp71-mediated viral strategies against cellular defenses

Three crucial hurdles hinder studies on human cytomegalovirus (HCMV): strict species specificity, differences between in vivo and in vitro infection, and the complexity of gene regulation. Ever since the sequencing of the whole genome was first accomplished, functional studies on individual genes have been the mainstream in the CMV field. Gene regulation has therefore been elucidated in a more detailed fashion. However, viral gene regulation is largely controlled by both cellular and viral components. In other words, viral gene expression is determined by the virus-host interaction. Generally, cells respond to viral infection in a defensive pattern; at the same time, viruses try to counteract the cellular defense or else hide in the host (latency). Viruses evolve effective strategies against cellular defense in order to achieve replicative success. Whether or not they are successful, cellular defenses remain in the whole viral replication cycle: entry, immediate-early (IE) gene expression, early gene expression, DNA replication, late gene expression, and viral egress. Many viral strategies against cellular defense, and which occur in the immediate-early time of viral infection, have been documented. In this review, we will summarize the documented biological functions of IE1 and pp71 proteins, especially with regard to how they counteract cellular intrinsic defenses.

The DNA damage response induced by infection with human cytomegalovirus and other viruses

Viruses use different strategies to overcome the host defense system. Recent studies have shown that viruses can induce DNA damage response (DDR). Many of these viruses use DDR signaling to benefit their replication, while other viruses block or inactivate DDR signaling. This review focuses on the effects of DDR and DNA repair on human cytomegalovirus (HCMV) replication. Here, we review the DDR induced by HCMV infection and its similarities and differences to DDR induced by other viruses. As DDR signaling pathways are critical for the replication of many viruses, blocking these pathways may represent novel therapeutic opportunities for the treatment of certain infectious diseases. Lastly, future perspectives in the field are discussed.

The human cytomegalovirus IE1 protein: past and present developments

ABSTRACT: 

Human cytomegalovirus (HCMV), a member of the β-herpesvirus subfamily, is an important pathogen that infects the majority of the human population. The evolutionary success of HCMV largely depends on its ability to evade host defense systems and establish a lifelong persistence after primary infection. In fact, HCMV has dedicated a considerable part of its gene products to manipulate or disable immune effector processes. This review focuses on the major immediate–early protein IE1 – a multifunctional key regulator that has the capacity to counteract the first host defense activities. We summarize the known structural and mechanistic features by which IE1 modulates innate immune mechanisms as well as other cellular processes, and discuss how the individual functions of IE1 contribute to the success of a lytic HCMV infection.

Selective degradation of host MicroRNAs by an intergenic HCMV noncoding RNA accelerates virus production

Virulence of human cytomegalovirus (HCMV) clinical isolates correlates with carriage of a 15 kb segment in the UL/b' region of the viral genome, which is absent from attenuated strains. The mechanisms by which this segment contributes to HCMV virulence remain obscure. We observed that intergenic RNA sequences within the 15 kb segment function as a microRNA (miRNA) decay element (miRDE) and direct the selective, sequence-specific turnover of mature miR-17 and miR-20a encoded within the host miR-17-92 cluster. Unlike canonical miRNA-mRNA interactions, the miRNA-miRDE interactions did not repress miRDE expression. miRNA binding site mutations retargeted miRDE to other miR-17-92 cluster miRNAs, which are otherwise resistant to miRDE-mediated decay. miRDE function was required to accelerate virus production in the context of lytic HCMV infection. These results indicate a role for viral noncoding RNA in regulating cellular miRNAs during HCMV pathogenesis and suggest that noncoding RNAs may play a role in mature miRNA turnover.

Human cytomegalovirus IE2 protein interacts with transcription activating factors

The human cytomegalovirus (HCMV) IE86 cDNA was cloned into pGEX-2T and fusion protein GST-IE86 was expressed inE. coli. SDS-PAGE and Western blot assay indicated that fusion protein GST-IE86 with molecular weight of 92 ku is soluble in the supernatant of cell lysate. Protein GST and fusion protein GST-IE86 were purified by affinity chromatography. The technology of co-separation and specific affinity chromatography was used to study the interactions of HCMV IE86 protein with some transcriptional regulatory proteins and transcriptional factors. The results indicated that IE86 interacts separately with transcriptional factor TFIIB and promoter DNA binding transcription trans-activating factors SP1, AP1 and AP2 to form a heterogenous protein complex. These transcriptional trans-activating factors, transcriptional factor and IE86 protein were adsorbed and retained in the affinity chromatography simultaneously. But IE86 protein could not interact with NF-kappaB, suggesting that the function of IE86 protein that can interact with transcriptional factor and transcriptional trans-activating factors has no relevance to protein glycosylation. IE86 protein probably has two domains responsible for binding transcriptional trans-activating regulatory proteins and transcriptional factors respectively, thus activating the transcription of many genes. The interactions accelerated the assembly of the transcriptional initiation complexes.

Transcriptional activation of endoplasmic reticulum chaperone GRP78 by HCMV IE1-72 protein

Glucose-regulated protein 78 (GRP78), a key regulator of endoplasmic reticulum (ER) stress, facilitates cancer cell growth and viral replication. The mechanism leading to grp78 gene activation during viral infection is largely unknown. In this study, we show that the immediate-early 1 (IE1-72) protein of the human cytomegalovirus (HCMV) is essential for HCMV-mediated GRP78 activation. IE1-72 upregulated grp78 gene expression depending on the ATP-binding site, the zinc-finger domain and the putative leucine-zipper motif of IE1-72, as well as the ER stress response elements (ERSEs) on the grp78 promoter. The purified IE1-72 protein bound to the CCAAT box within ERSE in vitro, whereas deletion mutants of IE1-72 deficient in grp78 promoter stimulation failed to do so. Moreover, IE1-72 binding to the grp78 promoter in infected cells accompanied the recruitment of TATA box-binding protein-associated factor 1 (TAF1), a histone acetyltransferase, and the increased level of acetylated histone H4, an indicator of active-state chromatin. These results provide evidence that HCMV IE1-72 activates grp78 gene expression through direct promoter binding and modulation of the local chromatin structure, indicating an active viral mechanism of cellular chaperone induction for viral growth.

Human cytomegalovirus IE1 protein elicits a type II interferon-like host cell response that depends on activated STAT1 but not interferon-γ

Human cytomegalovirus (hCMV) is a highly prevalent pathogen that, upon primary infection, establishes life-long persistence in all infected individuals. Acute hCMV infections cause a variety of diseases in humans with developmental or acquired immune deficits. In addition, persistent hCMV infection may contribute to various chronic disease conditions even in immunologically normal people. The pathogenesis of hCMV disease has been frequently linked to inflammatory host immune responses triggered by virus-infected cells. Moreover, hCMV infection activates numerous host genes many of which encode pro-inflammatory proteins. However, little is known about the relative contributions of individual viral gene products to these changes in cellular transcription. We systematically analyzed the effects of the hCMV 72-kDa immediate-early 1 (IE1) protein, a major transcriptional activator and antagonist of type I interferon (IFN) signaling, on the human transcriptome. Following expression under conditions closely mimicking the situation during productive infection, IE1 elicits a global type II IFN-like host cell response. This response is dominated by the selective up-regulation of immune stimulatory genes normally controlled by IFN-γ and includes the synthesis and secretion of pro-inflammatory chemokines. IE1-mediated induction of IFN-stimulated genes strictly depends on tyrosine-phosphorylated signal transducer and activator of transcription 1 (STAT1) and correlates with the nuclear accumulation and sequence-specific binding of STAT1 to IFN-γ-responsive promoters. However, neither synthesis nor secretion of IFN-γ or other IFNs seems to be required for the IE1-dependent effects on cellular gene expression. Our results demonstrate that a single hCMV protein can trigger a pro-inflammatory host transcriptional response via an unexpected STAT1-dependent but IFN-independent mechanism and identify IE1 as a candidate determinant of hCMV pathogenicity.

The human cytomegalovirus major immediate-early proteins as antagonists of intrinsic and innate antiviral host responses

The major immediate-early (IE) gene of human cytomegalovirus (CMV) is believed to have a decisive role in acute infection and its activity is an important indicator of viral reactivation from latency. Although a variety of gene products are expressed from this region, the 72-kDa IE1 and the 86-kDa IE2 nuclear phosphoproteins are the most abundant and important. Both proteins have long been recognized as promiscuous transcriptional regulators. More recently, a critical role of the IE1 and IE2 proteins in counteracting non-adaptive host cell defense mechanisms has been revealed. In this review we will briefly summarize the available literature on IE1- and IE2-dependent mechanisms contributing to CMV evasion from intrinsic and innate immune responses.

Physical requirements and functional consequences of complex formation between the cytomegalovirus IE1 protein and human STAT2

Our previous work has shown that efficient evasion from type I interferon responses by human cytomegalovirus (hCMV) requires expression of the 72-kDa immediate-early 1 (IE1) protein. It has been suggested that IE1 inhibits interferon signaling through intranuclear sequestration of the signal transducer and activator of transcription 2 (STAT2) protein. Here we show that physical association and subnuclear colocalization of IE1 and STAT2 depend on short acidic and serine/proline-rich low-complexity motifs in the carboxy-terminal region of the 491-amino-acid viral polypeptide. These motifs compose an essential core (amino acids 373 to 420) and an adjacent ancillary site (amino acids 421 to 445) for STAT2 interaction that are predicted to form part of a natively unstructured domain. The presence of presumably "disordered" carboxy-terminal domains enriched in low-complexity motifs is evolutionarily highly conserved across all examined mammalian IE1 orthologs, and the murine cytomegalovirus IE1 protein appears to interact with STAT2 just like the human counterpart. A recombinant hCMV specifically mutated in the IE1 core STAT2 binding site displays hypersensitivity to alpha interferon, delayed early viral protein accumulation, and attenuated growth in fibroblasts. However, replication of this mutant virus is specifically restored by knockdown of STAT2 expression. Interestingly, complex formation with STAT2 proved to be entirely separable from disruption of nuclear domain 10 (ND10), another key activity of IE1. Finally, our results demonstrate that IE1 counteracts the antiviral interferon response and promotes viral replication by at least two distinct mechanisms, one depending on sequestration of STAT2 and the other one likely involving ND10 interaction.

Human cytomegalovirus infection downregulates the expression of glial fibrillary acidic protein in human glioblastoma U373MG cells: identification of viral genes and protein domains involved

Human cytomegalovirus (HCMV) has tropism for glial cells, among many other cell types. It was reported previously that the stable expression of HCMV immediate-early protein 1 (IE1) could dramatically reduce the RNA level of glial fibrillary acidic protein (GFAP), an astroglial cell-specific intermediate filament protein, which is progressively lost with an increase in glioma malignancy. To understand this phenomenon in the context of virus infection, a human glioblastoma cell line, U373MG, was infected with HCMV (strain AD169 or Towne). The RNA level of GFAP was reduced by more than 10-fold at an m.o.i. of 3 at 48 h post-infection, whilst virus treated with neutralizing antibody C23 or with UV light had a much-reduced effect. Treatment of infected cells with ganciclovir did not prevent HCMV-mediated downregulation of GFAP. Although the expression of GFAP RNA is downregulated in IE1-expressing cells, a mutant HCMV strain lacking IE1 still suppressed GFAP, indicating that other IE proteins may be involved. IE2 is also proposed to be involved in GFAP downregulation, as an adenoviral vector expressing IE2 could also reduce the RNA level of GFAP. Data from the mutational analysis indicated that HCMV infection might affect the expression of this structural protein significantly, primarily through the C-terminal acidic region of the IE1 protein.

Anti-Human Cytomegalovirus Activity of Constituents from Sasa Albo-Marginata (Kumazasa in Japan)

Background:

Effective new anti-human cytomegalovirus (HCMV) agents and regimens need to be developed. We examined the anti-HCMV properties of crude extract (True World Extract of Bambuseae sasa [TWEBS]) and five compounds ( p-coumaric acid, 3-hydroxy-4-methoxyben-zaldehyde [vanillin], p-hydroxybenzaldehyde, 3-hydroxypyridine and 4',5,7-trihydroxy-3',5'-dimethoxyflavone [tricin]), isolated from Sasa albo-marginata, a bamboo known in Japan as Sasa.

Methods:

Among TWEBS and five compounds screened in a plaque reduction assay, four showed anti-HCMV activity in the MRC-5 human embryonic fibroblast cell line. The anti-HCMV mechanisms of the TWEBS was examined by western blot analysis using primary antibody specific for an immediate early (IE) antigen of HCMV, for a structural late antigen of HCMV and for β-actin.

Results:

Treatment of cells with ⩾0.001% of TWEBS inhibited the observable cytopathic effects of HCMV on infected cells. Western blot analysis demonstrated that TWEBS decreased the expression of IE antigen and late antigen of HCMV in the infected cells. Next, we examined the anti-HCMV properties of five compounds isolated from TWEBS. In a viral plaque reduction assay, tricin showed dose-dependent inhibitory properties with a 50% effective concentration of 0.17 µug/ml (selective index =1,205.8).

Conclusions:

The hot water extract (TWEBS) of Sasa albo-marginata, with tricin isolated from it, has anti-HCMV activity in MRC-5 cells. TWEBS and/or tricin are a novel compound with potential anti-HCMV activity. Future studies should evaluate these findings in vivo.

Phenotypes of major immediate-early gene mutants of mouse cytomegalovirus

Immediate-early (IE) genes are the first genes to be transcribed during the lytic replication cycle of cytomegaloviruses (CMV), and encode nonstructural proteins, which are assumed to have mainly regulatory functions. The IE proteins may play important roles in the pathogenesis of CMV in vivo, for instance during the establishment of latency and during reactivation. We constructed mouse CMV mutants with disruptions in the major IE genes, ie1 and ie3, to study the roles of these genes in the context of the viral infection. Here we summarize the current results on the characterization of these mutants and give a perspective of the future research in this field.

The S phase of the cell cycle and its perturbation by human cytomegalovirus

Human cytomegalovirus (HCMV) is a complex human herpesvirus that is known to productively infect a wide range of cell types. In addition, it has been suggested to contribute to some proliferative disorders, particularly atherosclerosis. Consistent with this, a number of studies have shown that HCMV profoundly affects normal cell cycle control. Specifically, the virus can stimulate early entry into S phase thus ensuring adequate resources for viral DNA replication. Importantly, however, the virus concomitantly inhibits potentially competing cellular DNA synthesis allowing cellular precursors to be used for viral but not cellular DNA replication. The mechanisms by which HCMV perturbs S phase entry involve interactions between the virus and the cellular replication machinery such that formation of competent pre-replication complexes (Pre-RC) at cellular origins of replication is restricted in infected cells.

Subversion of cell cycle regulatory pathways

Human cytomegalovirus (HCMV) has evolved numerous strategies to commandeer the host cell for producing viral progeny. The virus manipulates host cell cycle pathways from the early stages of infection to stimulate viral DNA replication at the expense of cellular DNA synthesis. At the same time, cell cycle checkpoints are by-passed, preventing apoptosis and allowing sufficient time for the assembly of infectious virus.

Anticytomegalovirus activity of pristimerin, a triterpenoid quinone methide isolated from Maytenus heterophylla (Eckl. & Zeyh.)

We examined the anticytomegalovirus properties of four compounds: pristimerin, the pristimerin analogue, lupeol and 2-acetylphenol-1-beta-D-glucopyranosyl (1 --> 6)-beta-D-xylpyranoside (acetophenol glycoside), isolated from Maytenus heterophylla, a Kenyan medicinal plant. The effects were studied on human cytomegalovirus (HCMV) replication in the human embryonic fibroblast cell line, MRC-5. In a viral plaque-reduction assay, pristimerin showed dose-dependent inhibitory properties with a 50% inhibitory concentration of 0.53 microg/ml (selective index = 27.9). The cells treated with pristimerin inhibited the cytopathic effects in HCMV-infected cells. Moreover, pristimerin suppressed viral replication without affecting the cell growth. Pristimerin inhibited the synthesis of viral DNA but had no virucidal effect on cell-free HCMV. Furthermore, Western blot analysis demonstrated that pristimerin decreased the amount of immediate early (IE) antigen (especially IE2) expression in the infected cells. These results suggest that pristimerin is a unique compound with potential anti-HCMV activity.

Epidermal growth factor receptor is not required for human cytomegalovirus entry or signaling

Human cytomegalovirus (HCMV) can bind, fuse, and initiate gene expression in a diverse range of vertebrate cell types. This broad cellular tropism suggests that multiple receptors and/or universally distributed receptors mediate HCMV entry. Our laboratory has recently discovered that certain beta1 and beta3 integrin heterodimers are critical mediators of HCMV entry into permissive fibroblasts (A. L. Feire, H. Koss, and T. Compton, Proc. Natl. Acad. Sci. USA 101:15470-15475, 2004). It has also been reported that epidermal growth factor receptor (EGFR) is necessary for HCMV-mediated signaling and entry (X. Wang, S. M. Huong, M. L. Chiu, N. Raab-Traub, and E. E. Huang, Nature 424:456-461, 2003). Integrins are known to signal synergistically with growth factor receptors, and this coordination was recently reported for EGFR and beta3 integrins in the context of HCMV entry (X. Wang, D. Y. Huang, S. M. Huong, and E. S. Huang, Nat. Med. 11:515-521, 2005). However, EGFR-negative cell lines, such as hematopoietic cells, are known to be infected by HCMV. Therefore, we wished to confirm a role for EGFR in HCMV entry and then examine any interaction between beta1 integrins and EGFR during the entry process. Surprisingly, we were unable to detect any role for EGFR in the process of HCMV entry into fibroblast, epithelial, or endothelial cell lines. Additionally, HCMV did not activate the EGFR kinase in fibroblast cell lines. We first examined HCMV entry into two EGFR-positive or -negative cell lines but observed no increase in entry when EGFR was expressed to high levels. Physically blocking EGFR with a neutralizing antibody in fibroblast, epithelial, or endothelial cell lines or blocking EGFR kinase signaling with a chemical inhibitor in fibroblast cells did not inhibit virus entry. Lastly, we were unable to detect phosphorylation of EGFR in fibroblasts cells in response to HCMV stimulation. Our findings demonstrate that EGFR does not play a significant role in HCMV entry or signaling. These results suggest that specific integrin heterodimers either act alone as the primary entry receptors or interact in conjunction with an additional receptor(s), other than EGFR, to facilitate virus entry.

Experimental study on the action of allitridin against human cytomegalovirus in vitro: Inhibitory effects on immediate-early genes

Garlic (Allium sativum) extraction has been reported having anti-HCMV efficacy. This study was aimed to investigate the effect of allitridin (diallyl trisulfide, a compound from A. sativum extraction) on the replication of HCMV and the expression of viral immediate-early genes. In HCMV plaque-reduction assay, allitridin appeared a dose-dependent inhibitory ability with EC(50) value of 4.2 microg/ml (selective index, SI=16.7). Time-of-addition and time-of-removal studies showed that allitridin inhibited HCMV replication in earlier period of viral cycle before viral DNA synthesis. Both immediate early gene (ie1) transcription and IEA (IE(1)72 and IE(2)86) expression was suppressed by allitridin, but not by GCV in a single HCMV cycle format. In addition, allitridin appeared stronger inhibition on IE(2)86 than on IE(1)72. Decrease of viral DNA load in infected cells was also detected under allitridin treatment, probably due to an indirect consequence of the reduction in ie gene transcription. In summary, this study indicated that allitridin has anti-HCMV activity and the mechanism is associated with suppression of ie gene transcription.

Evidence that the human cytomegalovirus IE2-86 protein binds mdm2 and facilitates mdm2 degradation

Levels of the p53 tumor suppressor protein are increased in human cytomegalovirus (HCMV)-infected cells and may be important for HCMV pathogenesis. In normal cells p53 levels are kept low due to an autoregulatory feedback loop where p53 activates the transcription of mdm2 and mdm2 binds and ubiquitinates p53, targeting p53 for proteasomal degradation. Here we report that, in contrast to uninfected cells, mdm2 was undetectable upon treatment of infected fibroblasts with the proteasome inhibitor MG132. Cellular depletion of mdm2 was reproducible in p53-null cells transfected with the HCMV IE2-86 protein, but not with IE172, independently of the endogenous mdm2 promoter. IE2-86 also prevented the emergence of presumably ubiquitinated species of p53. The regions of IE2-86 important for mdm2 depletion were those containing the sequences corresponding to the putative zinc finger and C-terminal acidic motifs. mdm2 and IE2-86 coimmunoprecipitated in transfected and infected cell lysates and in a cell-free system. IE2-86 blocked mdm2's p53-independent transactivation of the cyclin A promoter in transient-transfection experiments. Pulse-chase experiments revealed that IE2-86 but not IE1-72 or several loss-of-function IE2-86 mutants increased the half-life of p53 and reduced the half-life of mdm2. Short interfering RNA-mediated depletion of IE2-86 restored the ability of HCMV-infected cells to accumulate mdm2 in response to proteasome inhibition. Taken together, the data suggest that specific interactions between IE2-86 and mdm2 cause proteasome-independent degradation of mdm2 and that this may be important for the accumulation of p53 in HCMV-infected cells.

Human cytomegalovirus IE1-72 activates ataxia telangiectasia mutated kinase and a p53/p21-mediated growth arrest response

Human cytomegalovirus (HCMV) encodes several proteins that can modulate components of the cell cycle machinery. The UL123 gene product, IE1-72, binds the Rb-related, p107 protein and relieves its repression of E2F-responsive promoters; however, it is unable to induce quiescent cells to enter S phase in wild-type (p53(+/+)) cells. IE1-72 also induces p53 accumulation through an unknown mechanism. We present here evidence suggesting that IE1-72 may activate the p53 pathway by increasing the levels of p19(Arf) and by inducing the phosphorylation of p53 at Ser15. Phosphorylation of this residue by IE1-72 expression alone or HCMV infection is found to be dependent on the ataxia-telangiectasia mutated kinase. IE2-86 expression leads to p53 phosphorylation and may contribute to this phenotype in HCMV-infected cells. We also found that IE1-72 promotes p53 nuclear accumulation by abrogating p53 nuclear shuttling. These events result in the stimulation of p53 activity, leading to a p53- and p21-dependent inhibition of cell cycle progression from G(1) to S phase in cells transiently expressing IE1-72. Thus, like many of the small DNA tumor viruses, the first protein expressed upon HCMV infection activates a p53 response by the host cell.

Downregulation of GFAP, TSP-1, and p53 in human glioblastoma cell line, U373MG, by IE1 protein from human cytomegalovirus

Human cytomegalovirus (HCMV) is a member of the beta-herpesvirus family, which has tropism for glial cells. It was recently reported that HCMV might play important roles in the pathogenesis of malignant glioma. In this study, we investigated the effects of the HCMV IE1 protein on the gene expression profile in the human glioblastoma cell line, U373MG by employing cDNA microarray technology. Using DNA chips containing approximately 1,000 human cDNAs, RNA samples from U373MG cells stably expressing IE1 were compared with those from the control cells lacking IE1 cDNA. Fluorescence intensities of 13 genes were significantly decreased in IE1-expressing cells, while one gene was found to be upregulated. Among these 14 genes, we chose to work further on glial fibrillary acidic protein (GFAP), thrombospondin-1 (TSP-1), and p53, because of their previously known involvement in tumorigenesis. The mRNA levels of all these genes were found to be decreased in IE1-expressing glioblastoma cells by real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR) as well as Northern blot analysis. The decreased expression of these genes was also observed at protein levels as measured by immunocytochemistry or fluorescence-activated cell sorting (FACS) analysis. Our data strongly suggested that HCMV IE1 could modulate the expression of cellular genes that might play important roles in the pathogenesis of glial tumors.

The carboxyl-terminal region of human cytomegalovirus IE1491aa contains an acidic domain that plays a regulatory role and a chromatin-tethering domain that is dispensable during viral replication

The human cytomegalovirus major immediate-early (alpha) protein IE1(491aa) plays an important role in controlling viral gene expression at low multiplicities of infection. With a transient complementation assay, full-length IE1(491aa) enhanced the growth of ie1 mutant virus CR208 20-fold better than a deletion mutant lacking 71 carboxyl-terminal amino acids (IE1(1-420aa)). A 16-amino-acid domain between amino acids 476 and 491 was both necessary and sufficient for chromatin-tethering activity; however, this domain was completely dispensable for complementation of CR208 replication. The proximal 55-amino-acid acidic domain (amino acids 421 to 475) was found to be most important for function. A deletion mutant lacking only this domain retained chromatin-tethering activity but failed to complement mutant virus. Interestingly, serine phosphorylation (at amino acids 399, 402, 406, 423, 428, 431, 448, 451, and 455) was not required for complementation. These results show that IE1(491aa) is composed of at least two domains that support replication, a region located between amino acids 1 and 399 that complements ie1 mutant virus replication to low levels and an acidic domain between amino acids 421 and 479 that dramatically enhances complementation.

Human cytomegalovirus proteins encoded by UL37 exon 1 protect infected fibroblasts against virus-induced apoptosis and are required for efficient virus replication

Human cytomegalovirus (HCMV) strain AD169 mutants carrying transposon insertions or large deletions in UL37 exon 1 (UL37x1) were recovered from modified bacterial artificial chromosomes by reconstitution in human fibroblasts expressing the adenovirus anti-apoptotic protein E1B19K. UL37x1 mutant growth was severely compromised in normal fibroblasts, with minimal release of infectious progeny. Growth in E1B19K-expressing cells was restored, but did not reach wild-type levels. Normal fibroblasts infected by UL37x1 mutants underwent apoptosis spontaneously between 48 and 96 h after infection. Apoptosis was inhibited by treatment of cells with the broad-spectrum caspase inhibitor z-Val-Ala-Asp(OMe)-fluoromethylketone, resulting in substantially increased release of virus. Inhibition of viral DNA replication by phosphonoformate or ganciclovir also inhibited apoptosis, implying that death was triggered by late viral functions or by replication and packaging of the viral genome. Immunofluorescent staining showed that although viral proteins accumulated normally during delayed-early phase and viral DNA replication compartments formed, viral late proteins were detected only rarely, suggesting that spontaneous apoptosis occurs early in late phase. These results demonstrate that anti-apoptotic proteins encoded by HCMV UL37x1 [pUL37x1 (vMIA), gpUL37 and gpUL37(M)] prevent apoptosis that would otherwise be initiated by the replication programme of the virus and are required for efficient and sustainable virus replication.

Human cytomegalovirus immediate-early 1 protein facilitates viral replication by antagonizing histone deacetylation

The human cytomegalovirus 72-kDa immediate-early (IE)1 and 86-kDa IE2 proteins are expressed at the start of infection, and they are believed to exert much of their function through promiscuous transcriptional activation of viral and cellular gene expression. Here, we show that the impaired growth of an IE1-deficient mutant virus in human fibroblasts is efficiently rescued by histone deacetylase (HDAC) inhibitors of three distinct chemical classes. In the absence of IE1 expression, the viral major IE and UL44 early promoters exhibited decreased de novo acetylation of histone H4 during the early phase of infection, and the hypoacetylation correlated with reduced transcription and accumulation of the respective gene products. Consistent with these findings, IE1 interacts specifically with HDAC3 within infected cells. We also demonstrate an interaction between IE2 and HDAC3. We propose that the ability to modify chromatin is fundamental to transcriptional activation by IE1 and, likely, IE2 as well.

Cellular stress and signal transduction responses to human cytomegalovirus infection

Human cytomegalovirus (HCMV) receptor-ligand interactions and viral entry excite cellular responses such as receptor tyrosine kinase and mitogen-activated protein kinase signaling, cytoskeletal rearrangement, and the induction of transcription factors, prostaglandins, and cytokines. Bi-phasic stimulation of these pathways, excepting interferon, facilitates productive viral infection and likely contributes to viral pathogenesis.

Cyclin-dependent kinase activity is required at early times for accurate processing and accumulation of the human cytomegalovirus UL122-123 and UL37 immediate-early transcripts and at later times for virus production

Human cytomegalovirus (HCMV) infection leads to dysregulation of multiple cell cycle-regulatory proteins. In this study, we examined the effects of inhibition of cyclin-dependent kinase (cdk) activity on viral replication. With the drug Roscovitine, a specific inhibitor of cyclin-dependent kinases 1, 2, 5, 7, and 9, we have shown that during the first 6 h of infection, cyclin-dependent kinase-dependent events occurred that included the regulated processing and accumulation of the immediate-early (IE) UL122-123 transcripts and UL36-37 transcripts. Altered processing of UL122-123 led to a loss of IE1-72 and an increase in IE2-86. The ratio of spliced to unspliced UL37 transcripts also changed. These effects did not require de novo protein synthesis or degradation of proteins by the proteasome. Addition of Roscovitine at the beginning of the infection was also associated with inhibition of expression of selected viral early gene products, viral DNA replication, and late viral gene expression. When Roscovitine was added after the first 6 h of infection, the effects on IE gene expression were no longer observed and viral replication proceeded through the late phase, but viral titers were reduced. The reduction in viral titer was observed even when Roscovitine was first added at 48 h postinfection, indicating that cyclin-dependent kinase activity is required at both IE and late times. Flavopiridol, another specific inhibitor of cyclin-dependent kinases, had similar effects on IE and early gene expression. These results underscore the importance of accurate RNA processing and reiterate the significant role of cell cycle-regulatory factors in HCMV infection.

SUMOylation of the human cytomegalovirus 72-kilodalton IE1 protein facilitates expression of the 86-kilodalton IE2 protein and promotes viral replication

The 72-kDa immediate-early 1 protein (IE1-72kDa) of human cytomegalovirus has been previously shown to be posttranslationally modified by covalent conjugation to the ubiquitin-related protein SUMO-1. Using an infectious bacterial artificial chromosome clone of human cytomegalovirus, we constructed a mutant virus (BADpmIE1-K450R) that is deficient for SUMOylation of IE1-72 kDa due to a single amino acid exchange in the SUMO-1 attachment site. Compared to wild-type virus, this mutant grew more slowly and generated a reduced yield in infected human fibroblasts, indicating that SUMO modification is required for the full activity of IE1-72 kDa. The lack of SUMOylation did not affect the intranuclear localization of IE1-72 kDa, including its ability to target to and disrupt PML bodies and to bind to mitotic chromatin. Likewise, SUMOylation-deficient IE1-72 kDa activated several viral promoters as efficiently as the wild-type protein. However, the failure to modify IE1-72 kDa resulted in substantially reduced levels of the IE2 transcript and its 86-kDa protein (IE2-86 kDa). These observations suggest that SUMO modification of IE1-72 kDa contributes to efficient HCMV replication by promoting the accumulation of IE2-86 kDa.

Ability of the human cytomegalovirus IE1 protein to modulate sumoylation of PML correlates with its functional activities in transcriptional regulation and infectivity in cultured fibroblast cells

In one of the earliest events in human cytomegalovirus (HCMV)-infected cells, the major immediate-early (IE) protein IE1 initially targets to and then disrupts the nuclear structures known as PML oncogenic domains (PODs) or nuclear domain 10. Recent studies have suggested that modification of PML by SUMO is essential to form PODs and that IE1 both binds to PML and may disrupt PODs by preventing or removing SUMO adducts on PML. In this study, we showed that in contrast to herpes simplex virus type 1 (HSV-1) IE110 (ICP0), the loss of sumoylated forms of PML by cotransfected IE1 was resistant to the proteasome inhibitor MG132 and that IE1 did not reduce the level of unmodified PML. Reduced sumoylation of PML was also observed in U373 cells after infection with wild-type HCMV and proved to require IE1 protein expression. Mutational analysis revealed that the central hydrophobic domain of IE1, including Leu174, is required for both PML binding and loss of PML sumoylation and confirmed that all IE1 mutants tested that were deficient in these functions also failed both to target to PODs and to disrupt PODs. These same mutants were also inactive in several reporter gene transactivation assays and in inhibition of PML-mediated repression. Importantly, a viral DNA genome containing an IE1 gene with a deletion [IE1(Delta290-320)] that was defective in these activities was not infectious when transfected into permissive fibroblast cells, but the mutant IE1(K450R), which is defective in IE1 sumoylation, remained infectious. Our mutational analysis strengthens the idea that interference by IE1 with both the sumoylation of PML and its repressor activity requires a physical interaction with PML that also leads to disruption of PODs. These activities of IE1 also correlate with several unusual transcriptional transactivation functions of IE1 and may be requirements for efficient initiation of the lytic cycle in vivo.

HCMV infection: modulating the cell cycle and cell death

Human cytomegalovirus (HCMV) is a member of the Herpesviridae family and is recognized as a significant pathogen to certain subgroups of the human population. It has become apparent that HCMV manipulation of the host cell cycle as well as the immune response promotes the replication and propagation of the virus. The ability of HCMV to modulate components of the host immune system and the response to infection most likely contributes to the pathology associated with this virus. This review will address the mechanisms HCMV has adapted to modulate the cell cycle to promote viral replication as well as the different ways it can prevent the "death" of an infected cell.

Interactions between human cytomegalovirus IE1-72 and cellular p107: functional domains and mechanisms of up-regulation of cyclin E/cdk2 kinase activity

Previous work has demonstrated that the human cytomegalovirus IE1-72 protein is able to bind to the N terminus of p107, and IE1-72 alone is sufficient for alleviation of p107-mediated cell growth suppression. However, the mechanism of this alleviation is unclear. Here, we show that IE1-72 can alleviate p107 inhibition of cyclin E/cdk2 kinase activity. We cotransfected various IE1-72 and p107 constructs into C33A cells and demonstrated that IE1-72 could activate the kinase activity of cyclin E/cdk2. Conversely, IE2-86 did not activate this activity, suggesting that the interaction between p107 and IE1-72 and the subsequent kinase activation are specific. By the use of a series of deletion and point mutants of IE1-72 and p107, we observed that a mutation of the loop region of helix-loop-helix-turn-helix in exon 3 of IE1-72 as well as a mutation of the leucine zipper-2 region in exon 4 of IE1-72 abolished binding to p107. In addition, these two IE1-72 mutants did not alleviate p107 inhibition of cyclin E/cdk2 kinase activity and also failed to alleviate p107 inhibition of the E2F-responsive promoter. Meanwhile, deletion of the N-terminal aa 1 to 175 of p107 abolished both p107 binding with IE1-72 and p107 inhibition of cyclin E/cdk2 kinase activity. This result confirms that the N-terminus aa 1 to 175 region of p107 is a common region where both IE1-72 protein and cyclin E/cdk2 bind. We propose a mechanism in which binding of IE1-72 to p107 displaces cyclin E/cdk2 from p107. Once released from p107, cyclin E/cdk2 is able to function as an active kinase.

Human cytomegalovirus proteins PP65 and IEP72 are targeted to distinct compartments in nuclei and nuclear matrices of infected human embryo fibroblasts

The cellular distribution of the human cytomegalovirus (HCMV)-specific UL83 phosphoprotein (pp65) and UL123 immediate-early protein (IEp72) in lytically infected human embryo fibroblasts was studied by means of indirect immunofluorescence and confocal microscopy. Both proteins were found to have a nuclear localization, but they were concentrated in different compartments within the nuclei. The pp65 was located predominantly in the nucleoli; this was already evident with the parental viral protein, which was targeted to the above nuclear compartment very soon after infection. The nucleolar localization of pp65 was also observed at later stages of the HCMV infectious cycle. After chromatin extraction (in the so-called in situ nuclear matrices), a significant portion of the pp65 remained associated with nucleoli within the first hour after infection, then gradually redistributed in a perinucleolar area, as well as throughout the nucleus, with a granular pattern. A quite different distribution was observed for IEp72 at very early stages after infection of human embryo fibroblasts with HCMV; indeed, this viral protein was found in bright foci, clearly observable in both non-extracted nuclei and in nuclear matrices. At later stages of infection, IEp72 became almost homogeneously distributed within the whole nucleus, while the foci increased in size and were more evenly spread; in several infected cells some of them lay within nucleoli. This peculiar nuclear distribution of IEp72 was preserved in nuclear matrices as well. The entire set of data is discussed in terms of the necessity of integration for HCMV-specific products into the pre-existing nuclear architecture, with the possibility of subsequent adaptation of nuclear compartments to fit the needs of the HCMV replicative cycle.

The human cytomegalovirus

Human cytomegalovirus (HCMV), a betaherpesvirus, represents the major infectious cause of birth defects, as well as an important pathogen for immunocompromised individuals. The viral nucleocapsid containing a linear double-stranded DNA of 230 kb is surrounded by a proteinaceous tegument, which is itself enclosed by a loosely applied lipid bilayer. Expression of the HCMV genome is controlled by a cascade of transcriptional events that leads to the synthesis of three categories of viral proteins designated as immediate-early, early, and late. Clinical manifestations can be seen following primary infection, reinfection, or reactivation. About 10% of infants are infected by the age of 6 months following transmission from their mothers via the placenta, during delivery, or by breastfeeding. HCMV is a significant post-allograft pathogen and contributes to graft loss independently from graft rejection. Histopathologic examination of necropsy tissues demonstrates that the virus enters via the epithelium of the upper alimentary, respiratory, or genitourinary tracts. Hematogenous spreading is typically followed by infection of ductal epithelial cells. Infections are kept under control by the immune system. However, total HCMV clearance is rarely achieved, and the viral genome remains at selected sites in a latent state. Virological and molecular detection of HCMV, as well as serological demonstration of a specific immune response, are used for diagnosis. Treatment of HCMV infections is difficult because there are few options. The presently available drugs produced a significant clinical improvement, but suffer from poor oral bioavailability, low potency, development of resistance in clinical practice, and dose-limiting toxicities.

Cell cycle arrest by human cytomegalovirus 86-kDa IE2 protein resembles premature senescence

Primary human embryo lung fibroblasts and adult diploid fibroblasts infected by the human cytomegalovirus (HCMV) display beta-galactosidase (beta-Gal) activity at neutral pH (senescence-associated beta-Gal [SA-beta-Gal] activity) and overexpression of the plasminogen activator inhibitor type 1 (PAI-1) gene, two widely recognized markers of the process designated premature cell senescence. This activity is higher when cells are serum starved for 48 h before infection, a process that speeds and facilitates HCMV infection but that is insufficient by itself to induce senescence. Fibroblasts infected by HCMV do not incorporate bromodeoxyuridine, a prerequisite for the formal definition of senescence. At the molecular level, cells infected by HCMV, beside the accumulation of large amounts of the cell cycle regulators p53 and pRb, the latter in its hyperphosphorylated form, display a strong induction of the cyclin-dependent kinase inhibitor (cdki) p16(INK4a), a direct effector of the senescence phenotype in fibroblasts, and a decrease of the cdki p21(CIP1/WAF). Finally, a replicative senescence state in the early phases of infection significantly increased the number of cells permissive to virus infection and enhanced HCMV replication. HCMV infection assays carried out in the presence of phosphonoformic acid, which inhibits the virus DNA polymerase and the expression of downstream genes, indicated that immediate-early and/or early (alpha) genes are sufficient for the induction of SA-beta-Gal activity. When baculovirus vectors expressing HCMV IE1-72 or IE2-86 proteins were inoculated into fibroblasts, the increase of p16(INK4a) (observed predominantly with IE2-86) was similar to that observed with the whole virus, as was the induction of SA-beta-Gal activity, suggesting that the viral IE2 gene leads infected cells into senescence. Altogether our results demonstrate for the first time that HCMV, after arresting the cell cycle and inhibiting apoptosis, triggers the cellular senescence program, probably through the p16(INK4a) and p53 pathways.

Human cytomegalovirus infection induces cellular thymidylate synthase gene expression in quiescent fibroblasts

Productive infection of non-proliferating cells by cytomegalovirus (CMV) requires the coordinated stimulation of host biochemical pathways that prepare cells to synthesize DNA. Here we illustrate the ability of human CMV (HCMV) to stimulate cellular thymidylate synthase (TS) gene expression in quiescent human embryonic lung fibroblasts. TS mRNA and protein levels are nearly undetectable in quiescent cells, but are greatly increased following HCMV infection. Inhibition of TS activity was shown to impair HCMV DNA synthesis, demonstrating that TS upregulation is required for efficient HCMV replication in quiescent cells. The increase in TS gene expression was due to an increase in gene transcription, since the expression of a reporter gene driven by the human TS promoter was strongly induced by HCMV infection. Deletion analysis of the human TS promoter identified two positive elements that are important for this increased transcription. We have previously shown that murine CMV (MCMV) stimulates the mouse TS promoter by a mechanism that depends on the presence of an E2F element in the promoter region. However, deletion of the two potential E2F binding sites in the human TS promoter did not prevent the virus-induced increase in TS promoter activity. Our data suggest that HCMV activates human TS gene transcription by mechanisms that are independent of E2F and different from those used by MCMV to stimulate the mouse TS promoter.

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.

Human cytomegalovirus immediate early proteins and cell growth control

It is widely accepted that small DNA tumor viruses, such as adenovirus, simian virus 40 and papillomavirus, push infected cells into S-phase to facilitate the replication of their genome. Until recently, it was believed that the large DNA viruses (i.e. herpesviruses) functioned very differently in this regard by inducing a G(1) arrest in infected cells as part of their replication process. However, studies over the last 6-8 years have uncovered striking parallels (and differences) between the functions of the major immediate early (IE) proteins of at least one herpesvirus, human cytomegalovirus (HCMV) and IE equivalents encoded by small DNA tumor viruses, such as adenovirus. Similarities between the HCMV major IE proteins and adenovirus IE proteins include targeting of members of the RB and p53 families and an ability of these viral factors to induce S-phase in quiescent cells. However, unlike the small DNA tumor virus proteins, individual HCMV IE proteins target different RB family members. HCMV also encodes several other IE gene products as well as virion tegument proteins that act early during infection to prevent an infected cell from replicating its host genome and from undergoing apoptosis. Here, we review the specifics of several HCMV IE proteins, two virion components, and their functions in relation to cell growth control.

Absence of IE1 p72 protein function during low-multiplicity infection by human cytomegalovirus results in a broad block to viral delayed-early gene expression

Human cytomegalovirus (HCMV) ie1 deletion mutant CR208 is profoundly growth deficient after low-multiplicity infection of primary fibroblasts. Previously, we showed that many fewer cells infected with CR208 at low multiplicity accumulated the delayed-early (DE) protein ppUL44 than accumulated the immediate-early 2 (IE2) p86 protein, indicating a high frequency of abortive infections. We now demonstrate that accumulation of all DE proteins tested was defective after low-multiplicity infection in the absence of IE1 p72. Accumulation of the DE proteins pUL57, pUL98, and pUL69 followed a pattern very similar to that of ppUL44 during low-multiplicity CR208 infection. Accumulation of the ppUL112-113 proteins occurred in a greater proportion of cells than other DE proteins during low-multiplicity CR208 infection, but was still deficient relative to wild-type virus. We also show for the first time that steady-state levels of many DE RNAs were reduced during low-multiplicity CR208 infection and that by in situ hybridization of the abundant cytoplasmic 2.7-kb TRL4 DE (beta2.7) RNA, a viral DE RNA followed a defective pattern of accumulation similar to that of ppUL44. Furthermore, transfected DE promoter-reporter constructs were found in transient assays to be considerably less responsive to CR208 infection than to infection by wild-type Towne virus. Our results indicate a general defect in DE gene expression following low-multiplicity HCMV infection in the absence of functional IE1 p72, most probably mediated by reduced transcription of DE genes and by the reduced accumulation of DE RNAs.

Novel immediate-early protein IE19 of human cytomegalovirus activates the origin recognition complex I promoter in a cooperative manner with IE72

The major immediate-early (MIE) gene of human cytomegalovirus (HCMV) expresses IE86, IE72, IE55, and IE18 mRNA by differential splicing. Reverse transcription-PCR with IE72-specific primers generated an 0.65-kb cDNA from HCMV-infected fibroblast RNA, which does not correspond to any known MIE cDNA. Nucleotide sequencing revealed that the 0.65-kb cDNA is from exons 1, 2, and 3 and part of exon 4, indicating that it is derived from a novel alternatively spliced mRNA of the MIE gene. The cDNA encodes a 172-amino-acid polypeptide, termed IE19, which corresponds to an IE72 variant with an internal deletion from Val(86) to Pro(404) and appears as a band at 38 kDa on a sodium dodecyl sulfate-polyacrylamide gel. IE19 mRNA was expressed at a low level in the immediate-early, early, and late period of viral infection. IE19 was localized in nuclei, and a transient-expression assay revealed that IE19 enhances IE72-dependent activation of the HsOrc1 promoter, which is identified here as an IE72 target promoter. Another MIE protein, IE86, activated the same promoter but only weakly compared to IE72, and coexpression of IE19 did not alter the IE86-mediated transcriptional activation. In addition, IE19 did not enhance the IE72-dependent activation of the HCMV UL54 promoter. These results suggest that IE19 is a transcriptional coactivator that works with IE72.

SUMO-1 modification of human cytomegalovirus IE1/IE72

Human cytomegalovirus (HCMV) immediate-early protein IE1/IE72 is involved in undermining many cellular processes including cell cycle regulation, apoptosis, nuclear architecture, and gene expression. The multifunctional nature of IE72 suggests that posttranslational modifications may modulate its activities. IE72 is a phosphoprotein and has intrinsic kinase activity (S. Pajovic, E. L. Wong, A. R. Black, and J. C. Azizkhan, Mol. Cell. Biol. 17:6459-6464, 1997). We now demonstrate that IE72 is covalently conjugated to the small ubiquitin-like modifier (SUMO-1). SUMO-1 is an 11.5-kDa protein that is conjugated to multiple proteins and has been reported to exhibit multiple effects, including modulation of protein stability, subcellular localization, and gene expression. A covalently modified protein migrating at approximately 92 kDa, which is stabilized by a SUMO-1 hydrolase inhibitor, is revealed by Western blotting with anti-IE72 of lysates from cells infected with HCMV or cells expressing IE72. SUMO modification of IE72 was confirmed by immunoprecipitation with anti-IE72 and anti-SUMO-1 followed by Western blotting with anti-SUMO-1 and anti-IE72, respectively. Lysine 450 is within a sumoylation consensus site (I,V,L)KXE; changing lysine 450 to arginine by point mutation abolishes SUMO-1 modification of IE72. Inhibition of protein phosphatase 1 and 2A, which increases the phosphorylation of IE72, suppresses the formation of SUMO-1-IE72 conjugates. Both wild-type IE72 and IE72(K450R) localize to nuclear PML oncogenic domains and disrupt them. Studies of protein stability, transactivation, and complementation of IE72-deficient HCMV (CR208) have revealed no significant differences between wild-type IE72 and IE72(K450R).

Proteasome-independent disruption of PML oncogenic domains (PODs), but not covalent modification by SUMO-1, is required for human cytomegalovirus immediate-early protein IE1 to inhibit PML-mediated transcriptional repression

Human cytomegalovirus (HCMV) major immediate-early protein IE1 is an abundant 72-kDa nuclear phosphoprotein that is thought to play an important role in efficient triggering of the lytic cycle, especially at low multiplicity of infection. The best-known properties of IE1 at present are its transient targeting to punctate promyelocytic leukemia protein (PML)-associated nuclear bodies (PML oncogenic domains [PODs] or nuclear domain 10 [ND10]), with associated displacement of the cellular PML tumor suppressor protein into a diffuse nucleoplasmic form and its association with metaphase chromosomes. Recent studies have shown that the targeting of PML (and associated proteins such as hDaxx) to PODs is dependent on modification of PML by ubiquitin-like protein SUMO-1. In this study, we provide direct evidence that IE1 is also covalently modified by SUMO-1 in both infected and cotransfected cells, as well as in in vitro assays, with up to 30% of the protein representing the covalently conjugated 90-kDa form in stable U373/IE1 cell lines. Lysine 450 was mapped as the major SUMO-1 conjugation site, but a point mutation of this lysine residue in IE1 did not interfere with its targeting to and disruption of the PODs. Surprisingly, unlike PML or IE2, IE1 did not interact with either Ubc9 or SUMO-1 in yeast two-hybrid assays, suggesting that some additional unknown intranuclear cofactors must play a role in IE1 sumoylation. Interestingly, stable expression of either exogenous PML or exogenous Flag-SUMO-1 in U373 cell lines greatly enhanced both the levels and rate of in vivo IE1 sumoylation during HCMV infection. Unlike the disruption of PODs by the herpes simplex virus type 1 IE110(ICP0) protein, the disruption of PODs by HCMV IE1 proved not to involve proteasome-dependent degradation of PML. We also demonstrate here that the 560-amino-acid PML1 isoform functions as a transcriptional repressor when fused to the GAL4 DNA-binding domain and that wild-type IE1 inhibits the repressor function of PML1 in transient cotransfection assays. Furthermore, both IE1(1-346) and IE1(L174P) mutants, which are defective in displacing PML from PODs, failed to inhibit the repression activity of PML1, whereas the sumoylation-negative IE1(K450R) mutant derepressed as efficiently as wild-type IE1. Taken together, our results suggest that proteasome-independent disruption of PODs, but not IE1 sumoylation, is required for efficient IE1 inhibition of PML-mediated transcriptional repression.

The human cytomegalovirus immediate early 2 protein dissociates cellular DNA synthesis from cyclin-dependent kinase activation

Passage through the restriction point late in G1 normally commits cells to replicate their DNA. Here we show that the previously reported cell cycle block mediated by the human cytomegalovirus (HCMV) immediate early 2 (IE2) protein uncouples this association. First, IE2 expression leads to elevated levels of cyclin E-associated kinase activity via transcriptional activation of the cyclin E gene. This contributes to post-restriction point characteristics of IE2-expressing cells. Then these cells fail to undergo substantial DNA replication although they have entered S phase, and the induction of DNA replication observed after overexpression of cyclin E or D can be antagonized by IE2 without impinging on cyclin-associated kinase activities. These data suggest that IE2 secures restriction-point transition of cells before it stops them from replicating their genome. Our results fit well with HCMV physiology and support the view that IE2 is part of a viral activity which, on the one hand, promotes cell cycle-dependent expression of cellular replication factors but, on the other hand, disallows competitive cellular DNA synthesis.

Expression of an altered ribonucleotide reductase activity associated with the replication of murine cytomegalovirus in quiescent fibroblasts

Ribonucleotide reductase (RNR) is an essential enzyme for the de novo synthesis of both cellular and viral DNA and catalyzes the conversion of ribonucleoside diphosphates into the corresponding deoxyribonucleoside diphosphates. The enzyme consists of two nonidentical subunits, termed R1 and R2, whose expression is very low in resting cells and maximal in S-phase cells. Here we show that murine cytomegalovirus (MCMV) replication depends on ribonucleotide reduction since it is prevented by the RNR inhibitor hydroxyurea. MCMV infection of quiescent fibroblasts markedly induces both mRNA and protein corresponding to the cellular R2 subunit, whereas expression of the cellular R1 subunit does not appear to be up-regulated. The increase in R2 gene expression is due to an increase in gene transcription, since the activity of a reporter gene driven by the mouse R2 promoter is induced following virus infection. Cotransfection experiments revealed that expression of the viral immediate-early 1 protein was sufficient to mediate the increase in R2 promoter activity. It was found that the viral gene M45, encoding a putative homologue of the R1 subunit, is expressed 24 and 48 h after infection. Meanwhile, we observed an expansion of the deoxyribonucleoside triphosphate pool between 24 and 48 h after infection; however, neither CDP reduction nor viral replication was inhibited by treatment with 10 mM thymidine. These findings indicate the induction of an RNR activity with an altered allosteric regulation compared to the mouse RNR following MCMV infection and suggest that the virus R1 homologue may complex with the induced cellular R2 protein to reconstitute a new RNR activity.

Role of human cytomegalovirus immediate-early proteins in cell growth control

Human cytomegalovirus (HCMV) is a ubiquitous herpesvirus that has been implicated in several disorders, including an association between HCMV reactivation and the overproliferation of arterial smooth muscle cells observed in restenosis. Although HCMV can mediate a growth-arrest phenotype in infected cells, the virus can also promote an environment conducive to proliferation. Here, we present evidence that the HCMV immediate-early (IE) proteins, IE1-72 and IE2-86, may be responsible for inducing this proliferative environment by altering cell cycle control. We find that expression of either of these IE proteins can alter the cell cycle distribution of randomly cycling cells towards S and G(2)/M phases. Additionally, we find that expression of IE2-86, but not IE1-72, induces quiescent cells into S phase and delays cell cycle exit. In the absence of p53, IE1-72 expression can induce S phase and delay cell cycle exit. We also demonstrate that p53 protein levels increase in fibroblasts following the expression of IE1-72. The observed accumulation of p53 protein in IE1-72-expressing cells may account for the inability of IE1-72 to induce S phase and delay cell cycle exit. Our data suggest that expression of HCMV IE1-72 and IE2-86 is sufficient to alter the cell cycle to generate an environment conducive to proliferation.

E2F proteins are posttranslationally modified concomitantly with a reduction in nuclear binding activity in cells infected with herpes simplex virus 1

The transition from G(1) to S phase in the cell cycle requires sequential activation of cyclin-dependent kinase 4 (cdk4) and cdk2, which phosphorylate the retinoblastoma protein, causing the release of E2F. Free E2F upregulates the transcription of genes involved in S phase and cell cycle progression. Recent studies from this and other laboratories have shown that herpes simplex virus 1 stabilizes cyclin D3 early in infection and that early events in viral replication are sensitive to inhibitors of some cdks. On the other hand cdk2 is not activated. Here we report studies on the status of members of E2F family in cycling HEp-2 and HeLa cells and quiescent serum-starved, contact-inhibited human lung fibroblasts. The results show that (i) at 8 h postinfection or thereafter, E2F-1 and E2F-5 were posttranslationally modified and/or translocated from nucleus to the cytoplasm, (ii) E2F-4 was hyperphophorylated, and (iii) overall, E2F binding to cognate DNA sites was decreased at late times after infection. These results concurrent with those cited above indicate that late in infection activation of S-phase genes is blocked both by posttranslational modification and translocation of members of E2F family to inactive compartments and by the absence of active cdk2. The observation that E2F were also posttranslationally modified in quiescent human lung fibroblasts that were not in S phase at the time of infection suggests that specific viral gene products are responsible for modification of the members of E2F family and raises the possibility that in infected cells, activation of the S phase gene is an early event in viral infection and is then shut off at late times. This is consistent with the timing of stabilization of cyclin D3 and the events blocked by inhibitors of cdks.

The human cytomegalovirus 86-kilodalton major immediate-early protein interacts physically and functionally with histone acetyltransferase P/CAF

The major immediate-early proteins of human cytomegalovirus (HCMV) play a pivotal role in controlling viral and cellular gene expression during productive infection. As well as negatively autoregulating its own promoter, the HCMV 86-kDa major immediate early protein (IE86) activates viral early gene expression and is known to be a promiscuous transcriptional regulator of cellular genes. IE86 appears to act as a multimodal transcription factor. It is able to bind directly to target promoters to activate transcription but is also able to bridge between upstream binding factors such as CREB/ATF and the basal transcription complex as well as interacting directly with general transcription factors such as TATA-binding protein and TFIIB. We now show that IE86 is also able to interact directly with histone acetyltransferases during infection. At least one of these factors is the histone acetyltransferase CBP-associated factor (P/CAF). Furthermore, we show that this interaction results in synergistic transactivation by IE86 of IE86-responsive promoters. Recruitment of such chromatin-remodeling factors to target promoters by IE86 may help explain the ability of this viral protein to act as a promiscuous transactivator of cellular genes.

Murine cytomegalovirus stimulates cellular thymidylate synthase gene expression in quiescent cells and requires the enzyme for replication

Herpesviruses accomplish DNA replication either by expressing their own deoxyribonucleotide biosynthetic genes or by stimulating the expression of the corresponding cellular genes. Cytomegalovirus (CMV) has adopted the latter strategy to allow efficient replication in quiescent cells. In the present report, we show that murine CMV (MCMV) infection of quiescent fibroblasts induces both mRNA and protein corresponding to the cellular thymidylate synthase (TS) gene, which encodes the enzyme that catalyzes the de novo synthesis of thymidylic acid. The increase in TS gene expression was due to an increase in gene transcription, since the activity of a reporter gene driven by the mouse TS promoter was induced following MCMV infection. Mutagenesis of the potential E2F-responsive element immediately upstream from the TS essential promoter region abolished the virus-mediated stimulation of the TS promoter, suggesting that the transactivating activity of MCMV infection was E2F dependent. Cotransfection experiments revealed that expression of the viral immediate-early 1 protein was sufficient to mediate the increase in TS promoter activity. Finally, MCMV replication and viral DNA synthesis were found to be inhibited by ZD1694, a quinazoline-based folate analog that inhibits TS activity. These results demonstrate that upregulation of cellular TS expression is required for efficient MCMV replication in quiescent cells.