Human cytomegalovirus immediate-early two protein region involved in negative regulation of the major immediate-early promoter

Polyvalent Diazonium Polymers Provide Efficient Protection of Oncolytic Adenovirus Enadenotucirev from Neutralizing Antibodies while Maintaining Biological Activity In Vitro and In Vivo

Oncolytic viruses offer many advantages for cancer therapy when administered directly to confined solid tumors. However, the systemic delivery of these viruses is problematic because of the host immune response, undesired interactions with blood components, and inherent targeting to the liver. Efficacy of systemically administered viruses has been improved by masking viral surface proteins with polymeric materials resulting in modulation of viral pharmacokinetic profile and accumulation in tumors in vivo. Here we describe a new class of polyvalent reactive polymer based on poly( N-(2-hydroxypropyl)methacrylamide) (polyHPMA) with diazonium reactive groups and their application in the modification of the chimeric group B oncolytic virus enadenotucirev (EnAd). A series of six copolymers with different chain lengths and density of reactive groups was synthesized and used to coat EnAd. Polymer coating was found to be extremely efficient with concentrations as low as 1 mg/mL resulting in complete (>99%) ablation of neutralizing antibody binding. Coating efficiency was found to be dependent on both chain length and reactive group density. Coated viruses were found to have reduced transfection activity both in vitro and in vivo, with greater protection against neutralizing antibodies resulting in lower transgene production. However, in the presence of neutralizing antibodies, some in vivo transgene expression was maintained for coated virus compared to the uncoated control. The decrease in transgene expression was found not to be solely due to lower cellular uptake but due to reduced unpackaging of the virus within the cells and reduced replication, indicating that the polymer coating does not cause permanent inactivation of the virus. These data suggest that virus activity may be modulated by the appropriate design of coating polymers while retaining protection against neutralizing antibodies.

Murine cytomegalovirus IE3-dependent transcription is required for DAI/ZBP1-mediated necroptosis

DNA-dependent activator of interferon regulatory factors/Z-DNA binding protein 1 (DAI/ZBP1) is a crucial sensor of necroptotic cell death induced by murine cytomegalovirus (MCMV) in its natural host. Here, we show that viral capsid transport to the nucleus and subsequent viral IE3-dependent early transcription are required for necroptosis. Necroptosis induction does not depend on input virion DNA or newly synthesized viral DNA A putative RNA-binding domain of DAI/ZBP1, Zα2, is required to sense virus and trigger necroptosis. Thus, MCMV IE3-dependent transcription from the viral genome plays a crucial role in activating DAI/ZBP1-dependent necroptosis. This implicates RNA transcripts generated by a large double-stranded DNA virus as a biologically relevant ligand for DAI/ZBP1 during natural viral infection.

UL84-independent replication of human cytomegalovirus strains conferred by a single codon change in UL122

The UL84 gene of human cytomegalovirus (HCMV) is thought to be involved in the initiation of viral DNA replication, and is essential for replication of strains AD169 and Towne. Hence, discovery that strain TB40-BAC4 is viable in the absence of UL84 presented an enigma requiring an explanation. Data reported here show that strain TR also tolerated loss of UL84, whereas strains FIX, Merlin, Ph, and Toledo did not. UL84-independent growth required the viral replication origin. The genetic locus in TB40 that controls UL84 dependence was mapped to codon 388 of the UL122 gene, which encodes the immediate early 2 (IE2) 86kD protein. Introduction of this TB40-BAC4 variant (H388D) into FIX and Toledo clones converted these strains to UL84 independence. These results provide genetic evidence in virus-infected cells that supports the hypothesis that UL122 participates in the initiation of viral DNA replication by a mechanism involving transcription-mediated activation of the origin.

The 6-Aminoquinolone WC5 inhibits different functions of the immediate-early 2 (IE2) protein of human cytomegalovirus that are essential for viral replication

The human cytomegalovirus (HCMV) immediate-early 2 (IE2) protein is a multifunctional factor essential for viral replication. IE2 modulates both viral and host gene expression, deregulates cell cycle progression, acts as an immunomodulator, and antagonizes cellular antiviral responses. Based on these facts, IE2 has been proposed as an important target for the development of innovative antiviral approaches. We previously identified the 6-aminoquinolone WC5 as a promising inhibitor of HCMV replication, and here, we report the dissection of its mechanism of action against the viral IE2 protein. Using glutathione S-transferase (GST) pulldown assays, mutagenesis, cell-based assays, and electrophoretic mobility shift assays, we demonstrated that WC5 does not interfere with IE2 dimerization, its interaction with TATA-binding protein (TBP), and the expression of a set of cellular genes that are stimulated by IE2. On the contrary, WC5 targets the regulatory activity exerted by IE2 on different responsive viral promoters. Indeed, WC5 blocked the IE2-dependent negative regulation of the major immediate-early promoter by preventing IE2 binding to the crs element. Moreover, WC5 reduced the IE2-dependent transactivation of a series of indicator constructs driven by different portions of the early UL54 gene promoter, and it also inhibited the transactivation of the murine CMV early E1 promoter by the IE3 protein, the murine cytomegalovirus (MCMV) IE2 homolog. In conclusion, our results indicate that the overall anti-HCMV activity of WC5 depends on its ability to specifically interfere with the IE2-dependent regulation of viral promoters. Importantly, our results suggest that this mechanism is conserved in murine CMV, thus paving the way for further preclinical evaluation in an animal model.

Interaction network of proteins associated with human cytomegalovirus IE2-p86 protein during infection: a proteomic analysis

Human cytomegalovirus protein IE2-p86 exerts its functions through interaction with other viral and cellular proteins. To further delineate its protein interaction network, we generated a recombinant virus expressing SG-tagged IE2-p86 and used tandem affinity purification coupled with mass spectrometry. A total of 9 viral proteins and 75 cellular proteins were found to associate with IE2-p86 protein during the first 48 hours of infection. The protein profile at 8, 24, and 48 h post infection revealed that UL84 tightly associated with IE2-p86, and more viral and cellular proteins came into association with IE2-p86 with the progression of virus infection. A computational analysis of the protein-protein interaction network indicated that all of the 9 viral proteins and most of the cellular proteins identified in the study are interconnected to varying degrees. Of the cellular proteins that were confirmed to associate with IE2-p86 by immunoprecipitation, C1QBP was further shown to be upregulated by HCMV infection and colocalized with IE2-p86, UL84 and UL44 in the virus replication compartment of the nucleus. The IE2-p86 interactome network demonstrated the temporal development of stable and abundant protein complexes that associate with IE2-p86 and provided a framework to benefit future studies of various protein complexes during HCMV infection.

Human pathogens and the host cell SUMOylation system

Since posttranslational modification (PTM) by the small ubiquitin-related modifiers (SUMOs) was discovered over a decade ago, a huge number of cellular proteins have been found to be reversibly modified, resulting in alteration of differential cellular pathways. Although the molecular consequences of SUMO attachment are difficult to predict, the underlying principle of SUMOylation is altering inter- and/or intramolecular interactions of the modified substrate, changing localization, stability, and/or activity. Unsurprisingly, many different pathogens have evolved to exploit the cellular SUMO modification system due to its functional flexibility and far-reaching functional downstream consequences. Although the extensive knowledge gained so far is impressive, a definitive conclusion about the role of SUMO modification during virus infection in general remains elusive and is still restricted to a few, yet promising concepts. Based on the available data, this review aims, first, to provide a detailed overview of the current state of knowledge and, second, to evaluate the currently known common principles/molecular mechanisms of how human pathogenic microbes, especially viruses and their regulatory proteins, exploit the host cell SUMO modification system.

Development of cell lines that provide tightly controlled temporal translation of the human cytomegalovirus IE2 proteins for complementation and functional analyses of growth-impaired and nonviable IE2 mutant viruses

The human cytomegalovirus (HCMV) IE2 86 protein is essential for viral replication. Two other proteins, IE2 60 and IE2 40, which arise from the C-terminal half of IE2 86, are important for later stages of the infection. Functional analyses of IE2 86 in the context of the infection have utilized bacterial artificial chromosomes as vectors to generate mutant viruses. One limitation is that many mutations result in debilitated or nonviable viruses. Here, we describe a novel system that allows tightly controlled temporal expression of the IE2 proteins and provides complementation of both growth-impaired and nonviable IE2 mutant viruses. The strategy involves creation of cell lines with separate lentiviruses expressing a bicistronic RNA with a selectable marker as the first open reading frame (ORF) and IE2 86, IE2 60, or IE2 40 as the second ORF. Induction of expression of the IE2 proteins occurs only following DNA recombination events mediated by Cre and FLP recombinases that delete the first ORF. HCMV encodes Cre and FLP, which are expressed at immediate-early (for IE2 86) and early-late (for IE2 40 and IE2 60) times, respectively. We show that the presence of full-length IE2 86 alone provides some complementation for virus production, but the correct temporal expression of IE2 86 and IE2 40 together has the most beneficial effect for early-late gene expression and synthesis of infectious virus. This approach for inducible protein translation can be used for complementation of other mutations as well as controlled expression of toxic cellular and microbial proteins.

Functional roles of the human cytomegalovirus essential IE86 protein

The IE86 protein of human cytomegalovirus (HCMV) is unique among viral and cellular proteins because it negatively autoregulates its own expression, activates the viral early and late promoters, and both activates and inhibits cellular promoters. It promotes cell cycle progression from Go/G1 to G1/S and arrests cell cycle progression at the G1/S interface or at G2/M. The IE86 protein is essential because it creates a cellular environment favorable for viral replication. The multiple functions of the IE86 protein during the replication of HCMV are reviewed.

The autoregulatory and transactivating functions of the human cytomegalovirus IE86 protein use independent mechanisms for promoter binding

The functions of the human cytomegalovirus (HCMV) IE86 protein are paradoxical, as it can both activate and repress viral gene expression through interaction with the promoter region. Although the mechanism for these functions is not clearly defined, it appears that a combination of direct DNA binding and protein-protein interactions is involved. Multiple sequence alignment of several HCMV IE86 homologs reveals that the amino acids (534)LPIYE(538) are conserved between all primate and nonprimate CMVs. In the context of a bacterial artificial chromosome (BAC), mutation of both P535 and Y537 to alanines (P535A/Y537A) results in a nonviable BAC. The defective HCMV BAC does not undergo DNA replication, although the P535A/Y537A mutant IE86 protein appears to be stably expressed. The P535A/Y537A mutant IE86 protein is able to negatively autoregulate transcription from the major immediate-early (MIE) promoter and was recruited to the MIE promoter in a chromatin immunoprecipitation (ChIP) assay. However, the P535A/Y537A mutant IE86 protein was unable to transactivate early viral genes and was not recruited to the early viral UL4 and UL112 promoters in a ChIP assay. From these data, we conclude that the transactivation and repressive functions of the HCMV IE86 protein can be separated and must occur through independent mechanisms.

Functional interaction between human herpesvirus 6 immediate-early 2 protein and ubiquitin-conjugating enzyme 9 in the absence of sumoylation

The immediate-early 2 (IE2) protein of human herpesvirus 6 is a potent transactivator of cellular and viral promoters. To better understand the biology of IE2, we generated a LexA-IE2 fusion protein and screened, using the yeast two-hybrid system, a Jurkat T-cell cDNA library for proteins that could interact with IE2. The most frequently isolated IE2-interacting protein was the human ubiquitin-conjugating enzyme 9 (Ubc9), a protein involved in the small ubiquitin-like modifier (SUMO) conjugation pathway. Using deletion mutants of IE2, we mapped the IE2-Ubc9-interacting region to residues 989 to 1037 of IE2. The interaction was found to be of functional significance to IE2, as Ubc9 overexpression significantly repressed promoter activation by IE2. The C93S Ubc9 mutant exhibited a similar effect on IE2, indicating that the E2 SUMO-conjugating function of Ubc9 is not required for its repressive action on IE2. No consensus sumoylation sites or evidence of IE2 conjugation to SUMO could be demonstrated under in vivo or in vitro conditions. Moreover, expression levels and nuclear localization of IE2 were not altered by Ubc9 overexpression, suggesting that Ubc9's repressive function likely occurs at the transcriptional complex level. Overall, our results indicate that Ubc9 influences IE2's function and provide new information on the complex interactions that occur between herpesviruses and the sumoylation pathway.

Mapping of human herpesvirus 6 immediate-early 2 protein transactivation domains

The immediate-early 2 (IE2) protein of human herpesvirus 6 (HHV-6) is a potent transactivator of multiple cellular and viral promoters. Deletion mutants of HHV-6 variant A IE2 allowed us to map functional transactivation domains acting on complex and minimal promoter sequences. This mapping showed that both the N-terminal and C-terminal domains of IE2 are required for efficient transactivation, and that deletion of the C-terminal (1397-1466) tail of IE2 drastically reduces both transactivation and the intranuclear distribution of IE2. Moreover, we determined that the ATF/CRE binding site within the HHV-6A polymerase promoter is not required for efficient transactivation by IE2, whereas the R3 repeat region of the putative immediate-early promoter of HHV-6A is responsive to and positively regulated by IE2. These results contrast sharply to that of human cytomegalovirus (HCMV) IE2, which down-regulates its promoter. Our characterization of HHV-6 IE2 transactivating activity provides a better understanding of the complex interactions of this protein with the viral and cellular transcription machinery and highlights significant differences with the IE2 protein of HCMV.

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.

Prolonged activation of NF-kappaB by human cytomegalovirus promotes efficient viral replication and late gene expression

Infection of fibroblasts by human cytomegalovirus (HCMV) rapidly activates the NF-kappaB signaling pathway, which we documented promotes efficient transactivation of the major immediate-early promoter (DeMeritt, I.B., Milford, L.E., Yurochko, A.D. (2004). Activation of the NF-kappaB pathway in human cytomegalovirus-infected cells is necessary for efficient transactivation of the major immediate-early promoter. J. Virol. 78, 4498-4507). Because a second, sustained increase in NF-kappaB activity following the initial phase of NF-kappaB activation was also observed, we investigated the role that this prolonged NF-kappaB activation played in viral replication and late gene expression. We first investigated HCMV replication in cells in which NF-kappaB activation was blocked by pretreatment with NF-kappaB inhibitors: HCMV replication was significantly decreased in these cultures. A decrease in replication was also observed when NF-kappaB was inhibited up to 48 h post-infection, suggesting a previously unidentified role for NF-kappaB in the regulation of the later class of viral genes.

The putative zinc finger of the human cytomegalovirus IE2 86-kilodalton protein is dispensable for DNA binding and autorepression, thereby demarcating a concise core domain in the C terminus of the protein

The IE2 86-kDa gene product is an essential regulatory protein of human cytomegalovirus (HCMV) with several functions, including transactivation, negative autoregulation, and cell cycle regulation. In order to understand the physiological significance of each of the IE2 functions, discriminating mutants of IE2 are required that can be tested in a viral background. However, no such mutants of IE2 are available, possibly reflecting structural peculiarities of the large and ill-defined C-terminal domain of IE2. Here, we revisited the C-terminal domain by analyzing IE2 mutants for transactivation, DNA binding, autoregulation, and cell cycle regulation in parallel. We found it to contain an unexpectedly concise core domain (amino acids 450 to 544) that is defined by its absolute sensitivity to any kind of mutation. In contrast, the region adjacent to the core (amino acids 290 to 449) generally tolerates mutations much better. Although it contributes more specific sequence information to distinct IE2 activities, none of the mutations analyzed abolished any particular function. The core is demarcated from the adjacent region by the putative zinc finger region (amino acids 428 to 452). Surprisingly, the deletion of the putative zinc finger region from IE2 revealed that this region is entirely dispensable for any of the IE2 functions tested here in transfection assays. Our work supports the view that the 100 amino acids of the core domain hold the key to most functions of IE2. A systematic, high-density mutational analysis of this region may identify informative mutants discriminating between various IE2 functions that can then be tested in a viral background.

Cellular repressor inhibits human cytomegalovirus transcription from the UL127 promoter

The region of the human cytomegalovirus (HCMV) genome between the UL127 promoter and the major immediate-early (MIE) enhancer is referred to as the unique region. The role of this region during a viral infection is not known. In wild-type HCMV-infected permissive fibroblasts, there is no transcription from the UL127 promoter at any time during productive infection. Our investigators previously reported that the region upstream of the UL127 TATA box repressed expression from the UL127 promoter (C. A. Lundquist et al., J. Virol. 73:9039-9052, 1999). The region was reported to contain functional NF1 DNA binding sites (L. Hennighausen and B. Fleckenstein, EMBO J. 5:1367-1371, 1986). Sequence analysis of this region detected additional consensus binding sites for three transcriptional regulatory proteins, FoxA (HNF-3), suppressor of Hairy wing, and CAAT displacement protein. The cis-acting elements in the unique region prevented activation of the early UL127 promoter by the HCMV MIE proteins. In contrast, deletion of the region permitted very high activation of the UL127 promoter by the viral MIE proteins. Mutation of the NF1 sites had no effect on the basal activity of the promoter. To determine the role of the other sites in the context of the viral genome, recombinant viruses were generated in which each putative repressor site was mutated and the effect on the UL127 promoter was analyzed. Mutation of the putative Fox-like site resulted in a significant increase in expression from the viral early UL127 promoter. Insertion of wild-type Fox-like sites between the HCMV immediate-early (IE) US3 TATA box and the upstream NF-kappaB-responsive enhancer (R2) also significantly decreased gene expression, but mutated Fox-like sites did not. The wild-type Fox-like site inhibits activation of a viral IE enhancer-containing promoter. Cellular protein, which is present in uninfected or infected permissive cell nuclear extracts, binds to the wild-type Fox-like site but not to mutated sites. Reasons for repression of UL127 gene transcription during productive infection are discussed.

Small internal deletions in the human cytomegalovirus IE2 gene result in nonviable recombinant viruses with differential defects in viral gene expression

The human cytomegalovirus (HCMV) IE2 86-kDa protein is a key viral transactivator and an important regulator of HCMV infections. We used the HCMV genome cloned as a bacterial artificial chromosome (BAC) to construct four HCMV mutants with disruptions in regions of IE2 86 that are predicted to be important for its transactivation and autoregulatory functions. Three of these mutants have mutations that remove amino acids 356 to 359, 427 to 435, and 505 to 511, which disrupts a region of IE2 86 implicated in the activation of HCMV early promoters, a predicted zinc finger domain, and a putative helix-loop-helix motif, respectively, while the fourth carries three arginine-to-alanine substitution mutations in the region of amino acids 356 to 359. The resulting recombinant viruses are not viable, and by using quantitative real-time reverse transcription-PCR and immunofluorescence we have determined the location of the block in their replicative cycles. The IE2 86 Delta 356-359 mutant is able to support early gene expression, as indicated by the presence of UL112-113 transcripts and UL112-113 and UL44 proteins in cells transfected with the mutant BAC. This mutant does not express late genes and behaves nearly indistinguishably from the IE2 86R356/7/9A substitution mutant. Both exhibit detectable upregulation of major immediate-early transcripts at early times. The IE2 86 Delta 427-435 and IE2 86 Delta 505-511 recombinant viruses do not activate the early genes examined and are defective in repression of the major immediate-early promoter. These two mutants also induce the expression of selected delayed early (UL89) and late genes at early times in the infection. We conclude that these three regions of IE2 86 are necessary for productive infections and for differential control of downstream viral gene expression.

Characterisation of transcripts from the human cytomegalovirus genes TRL7, UL20a, UL36, UL65, UL94, US3 and US34

The genome of human cytomegalovirus (HCMV) has been studied extensively in some regions, but not others. In this study, transcripts of the genome were further characterised for open reading frames (ORFs) TRL7, UL36, UL65, UL94, US3 and US34, and for the previously unrecognised ORF, UL20a. Reverse transcription-PCR demonstrated the presence of spliced transcripts from the putative glycoprotein gene, UL20a, at early and late times post-infection. US3 full-length and spliced transcripts, including a previously unidentified transcript (US3ii), were described at immediate early times. Sequencing of the complete ORFs of UL20a and US3 from 21 clinical isolates showed that US3 is well conserved in all isolates (97-100% identity), whereas UL20a shows more variation at the nucleotide level, with 90-100% identity. The limits of transcription, and splice donor and acceptor sequences for UL20a and US3 were conserved in all isolates, indicating likely conservation of mRNA splicing patterns. Sequencing a late cDNA library identified the limits of transcription for ORFs TRL7, UL94 and US34 and transcription from the TRL7 ORF was confirmed by northern blotting. Transcripts were found that were congruent with UL36 and UL65, but these differed in the limits previously predicted for these ORFs. These findings show the variation between predicted and actual transcription and indicate the complex nature of transcription from HCMV ORFs.

Characterization of the immediate-early 2 protein of human herpesvirus 6, a promiscuous transcriptional activator

In the present work we report the cloning of a full-length cDNA encoding the immediate-early (IE) 2 protein from human herpesvirus 6 (HHV-6) variant A (GS strain). The transcript is 4690 nucleotides long and composed of 5 exons. Translation initiation occurs within the third exon and proceeds to the end of U86. Kinetic studies indicate that the 5.5-kb IE2 mRNA is expressed under IE condition, within 2-4 h of infection. IE2 transcripts from both variants A and B are expressed under similar kinetics with IE2 transcripts accumulating up to 96 h postinfection. Although several large transcripts (>5.5 kb) hybridized with the IE2 probe, suggesting multiple transcription initiation sites, a single form of the IE2 protein, in excess of 200 kDa, was detected by Western blot. Within cells, the IE2 protein was detected (8-48 h) as intranuclear granules while at later time points (72-120 h), the IE2 protein coalesced into a few large immunoreactive patches. Transfection of cells with an IE2 expression vector (pBK-IE2A) failed to reproduce the patch-like distribution, suggesting that other viral proteins are necessary for this process to occur. Last, IE2 was found to behave as a promiscuous transcriptional activator. Cotransfection experiments in T cells indicate that IE2 can induce the transcription of a complex promoter, such as the HIV-LTR, as well as simpler promoters, whose expression is driven by a unique set of responsive elements (CRE, NFAT, NF-kB). Moreover, minimal promoters having a single TATA box or no defined eukaryotic regulatory elements were significantly activated by IE2, suggesting that IE2 is likely to play an important role in initiating the expression of several HHV-6 genes. In all, the work presented represents the first report on the successful cloning, expression, and functional characterization of the major regulatory IE2 gene/protein of HHV-6.

The human cytomegalovirus major immediate-early enhancer determines the efficiency of immediate-early gene transcription and viral replication in permissive cells at low multiplicity of infection

To determine the effect of the human cytomegalovirus (CMV) major immediate-early (MIE) enhancer or promoter on the efficiency of viral replication in permissive human cells, we constructed recombinant viruses with their human MIE promoter, enhancer, and promoter plus enhancer replaced with the murine CMV components. After a low multiplicity of infection (MOI) (0.01 PFU/cell), recombinant human CMV with the murine CMV promoter replicated like the wild type but recombinant virus with the murine enhancer replicated less efficiently. Immediate-early (IE) viral protein pIE72 (UL123), early viral protein (UL44), and viral DNA synthesis were significantly decreased. The effect of the human CMV enhancer substitution with the murine CMV enhancer was also demonstrated in different cell types by using recombinant virus with the UL127 promoter, driving the expression of green fluorescent protein (GFP). After an MOI of 1, GFP expression was high with the human CMV enhancer and significantly lower with the murine CMV enhancer. Even though at a high MOI (10 PFU/cell), the murine CMV enhancer was as efficient as the human CMV enhancer for the transcription of IE genes in human foreskin fibroblast cells, at lower MOIs, the murine CMV enhancer was less efficient. Proximal and distal chimeras of the human and murine enhancers also replicated less efficiently at a low MOI and expressed lower levels of GFP from the UL127 promoter. These experiments demonstrate that the entire human CMV enhancer has evolved for the efficient expression of the viral IE and early genes in human cells. Possible functions of the human CMV enhancer and promoter at a low MOI are discussed.

Human fibroblasts transfected with cytomegalovirus immediate-early genes show increased MHC class I expression and are targets for natural killer cell-mediated cytotoxicity

Natural killer (NK) cells are an important line of defense against viral infections, such as those caused by cytomegalovirus (CMV), but in the context of solid organ transplantation NK responses to CMV-infected graft cells might be deleterious to the graft survival. To gain a better understanding of NK responses to CMV-infected human lung fibroblasts (HLF), we transfected HLF with a plasmid expressing CMV immediate-early (IE) genes under the control of the CMV major IE promoter and compared major histocompatibility complex (MHC) class I expression and NK-mediated lysis of transfected cells, CMV-infected cells, and appropriate controls. HLF transfected with CMV IE genes showed increased MHC Class I expression and triggered NK-mediated cytotoxicity at the same level as CMV-infected HLF and at significantly higher levels than mock-infected or mock-transfected controls. Transfection of CMV genes provides an experimental model for molecular studies of CMV- and allograft-specific cell-mediated immunity and modulation.

Identification of additional IE2-p86-responsive cis-repressive sequences within the human cytomegalovirus major immediate early gene promoter

Human cytomegalovirus (HCMV) is a ubiquitous human pathogen that is the leading viral cause of birth defects and also causes significant morbidity and mortality in immunosuppressed individuals. The immediate early (IE) genes, IE1-p72 and IE2-p86, are the first HCMV genes expressed after infection under the control of a strong transcriptional enhancer-promoter, the major IE promoter (MIEP). Gene expression mediated by the predominant IE2-p86 is believed to be essential for the progression of viral production, as well as for the development of HCMV-associated pathogenesis. To gain further understanding of the transcriptional activity of IE2-p86, we attempted to isolate its downstream target genes within the HCMV genome. By a modified approach coupling the methods of cyclic amplification and selection of targets and selection and amplification of binding sites, several HCMV genomic fragments were identified based on their ability to bind to IE2-p86. Two additional IE2-p86-responsive elements other than the cis-repressive sequence (CRS) were identified within the MIEP and were termed -240 and -170 boxes. These two cis elements resemble the CRS in their sequences, as they contain the CG(N)(10)CG motif. The binding of IE2-p86 to these two distal CRS-like sequences was further confirmed by DNase I footprinting analysis and electrophoretic mobility shift assay. Promoter activity analysis in the transient expression system suggested that these two cis elements act functionally as IE2-p86-responsive repressive sequences to cooperate with the CRS to suppress MIEP expression.

Role of regulatory elements and the MAPK/ERK or p38 MAPK pathways for activation of human cytomegalovirus gene expression

A series of recombinant viruses with either site-specific mutations or various deletions of the early UL4 promoter of human cytomegalovirus were used to determine the roles of regulatory elements and the effects of the mitogen-activated protein kinase (MAPK) pathways. Viral gene expression was regulated by upstream cis-acting sites and by basic promoter elements that respond to the MAPK signal transduction pathways. Inhibitors of either the mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathway or the p38 MAPK pathway affected expression equally with either wild-type or mutant early UL4 promoters in the viral genome, indicating that the effects of the inhibitors are not exclusive for a single transcription factor. The minimal responsive element is the TATA box-containing early viral promoter.

Viable human cytomegalovirus recombinant virus with an internal deletion of the IE2 86 gene affects late stages of viral replication

Using bacterial artificial chromosome (BAC) technology, we have constructed and characterized a human cytomegalovirus recombinant virus with a mutation in the exon specific for the major immediate-early region 2 (IE2) gene product. The resulting IE2 86-kDa protein (IE2 86) has an internal deletion of amino acids 136 to 290 and is fused at the carboxy terminus to enhanced green fluorescent protein (EGFP). The deletion also removes the promoter and initiator methionine for the p40 form of IE2 and initiator methionine for the p60 form of the protein, and therefore, these late gene products are not produced. The mutant virus IE2 86 Delta SX-EGFP is viable but exhibits altered growth characteristics in tissue culture compared with a full-length wild-type (wt) IE2 86-EGFP virus or a revertant virus. When cells are infected with the mutant virus at a low multiplicity of infection (MOI), there is a marked delay in the production of infectious virus. This is associated with slower cell-to-cell spread of the virus. By immunofluorescence and Western blot analyses, we show that the early steps in the replication of the mutant virus are comparable to those for the wt. Although there is significantly less IE2 protein in the cells infected with the mutant, there is only a modest lag in the initial accumulation of IE1 72 and viral early proteins, and viral DNA replication proceeds normally. The mutation also has only a small effect on the synthesis of the viral major capsid protein. The most notable molecular defect in the mutant virus infection is that the steady-state levels of the pp65 (UL83) and pp28 (UL99) matrix proteins are greatly reduced. In the case of UL83, but not UL99, there is also a corresponding decrease in the amount of mRNA present in cells infected with the mutant virus.

Effect of the R1 element on expression of the US3 and US6 immune evasion genes of human cytomegalovirus

Human cytomegalovirus (HCMV) has several gene products that are important for escape from immune surveillance. These viral gene products downregulate the expression of HLA molecules on the cell surface. The viral US3 and US6 gene products are expressed at immediate-early and early times after infection, respectively. There are two regulatory regions between the US3 and the US6 transcription units. The first region is an NF-kappaB responsive enhancer that promotes the immediate-early expression of the US3 gene and is designated the R2 enhancer. Upstream of the R2 enhancer is a region designated the R1 element that in transient transfection assays behaves as a silencer by repressing the effect of the enhancer on downstream gene expression (A. R. Thrower et al., J. Virol. 1996, 70, 91; Y.-J. Chan et al., J. Virol. 1996, 70, 5312). We constructed recombinant viruses with wild-type or mutated R1 elements. The expression of the US3 gene at 6 h after infection and the US6 gene at 24 h was higher when the R1 element was present. The R1 element in the context of the viral genome is not a silencer of US3 or US6 gene expression. The R1 element has multiple effects on the US3 and US6 RNAs. It enhances the level of US3 and US6 mRNA; it determines the 3'-end cleavage and polyadenylation of US6 RNA, and it stabilizes read-through viral RNAs. The potential mechanisms of R1 enhancement of US3 and US6 gene expression are discussed.

Identification of a novel transcriptional repressor encoded by human cytomegalovirus

The expression of human cytomegalovirus (HCMV) genes during viral replication is precisely regulated, with the interactions of both transcriptional activators and repressors determining the level of gene expression. One gene of HCMV, the US3 gene, is transcriptionally repressed early in infection. Repression of US3 expression requires viral infection and protein synthesis and is mediated through a DNA sequence, the transcriptional repressive element. In this report, we identify the protein that represses US3 transcription as the product of the HCMV UL34 open reading frame. The protein encoded by UL34 (pUL34) binds to the US3 transcriptional repressive element in yeast and in vitro. pUL34 localizes to the nucleus and alone is sufficient for repression of US3 expression. The data presented here, along with earlier data (B. J. Biegalke, J. Virol. 72:5457-5463, 1998), suggests that pUL34 binding of the transcriptional repressive element prevents transcription initiation complex formation.

Inhibition of cytomegalovirus immediate early gene expression: a therapeutic option?

The replication cycle of the human cytomegalovirus (HCMV) is characterized by the expression of immediate early (IE), early (E), and late (L) gene regions. Current antiviral strategies are directed against the viral DNA polymerase expressed during the early phase of infection. The regulation of the IE-1 and IE-2 gene expression is the key to latency and active replication due to their transactivating and repressing functions. There is growing evidence that the pathogenic features of HCMV are largely due to the abilities of IE-1 and IE-2 to transactivate cellular genes. Consequently, current drugs used to inhibit HCMV infection would have no impact on IE-1 and IE-2-induced effects that are produced before the early phase. Moreover, when HCMV DNA replication is inhibited, IE gene products accumulate in infected cells causing disturbances of host cell functions. This review summarizes the biological functions of HCMV-IE gene expression, their relevance in pathogenesis, as well as efforts to develop novel treatment strategies directed against HCMV-IE expression.

Human cytomegalovirus immediate-early gene 2 expression leads to JCV replication in nonpermissive cells via transcriptional activation of JCV T antigen

Human papovavirus JCV is the causative agent of the demyelinating brain disease progressive multifocal leukoencephalopathy (PML) that typically develops as a complication of impaired immunocompetence. JCV displays a strong tropism for glial cells which is correlated by glial-specific transcriptional regulation of viral gene expression. In a previous report HCMV was shown to overcome the restricted cell specificity of JCV by inducing DNA replication of a PML-derived JCV strain in human fibroblasts which are nonpermissive for the replication of JCV alone. Here we show that productive JCV replication is induced by HCMV in human glioblastoma cells. Both in fibroblasts and in glioblastoma cells, the HCMV immediate-early transactivator 2 (IE2) is sufficient to mediate JCV replication. Furthermore, IE2 induces DNA replication of several structurally different brain- or kidney-derived JCV variants. IE2-induced JCV DNA replication is accompanied by the induction of JCV T antigen expression due to stimulation of the JCV early promoter. Our results indicate that stimulation of JCV early gene expression by HCMV-IE2 is sufficient to overcome the restricted cell specificity of JCV.

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.

The human cytomegalovirus IE86 protein can block cell cycle progression after inducing transition into the S phase of permissive cells

Human cytomegalovirus (HCMV) infection of permissive cells has been reported to induce a cell cycle halt. One or more viral proteins may be involved in halting progression at different stages of the cell cycle. We investigated how HCMV infection, and specifically IE86 protein expression, affects the cell cycles of permissive and nonpermissive cells. We used a recombinant virus that expresses the green fluorescent protein (GFP) to determine the effects of HCMV on the cell cycle of permissive cells. Fluorescence by GFP allowed us to select for only productively infected cells. Replication-defective adenovirus vectors expressing the IE72 or IE86 protein were also used to efficiently transduce 95% or more of the cells. The adenovirus-expressed IE86 protein was determined to be functional by demonstrating negative autoregulation of the major immediate-early promoter and activation of an early viral promoter in the context of the viral genome. To eliminate adenovirus protein effects, plasmids expressing GFP for fluorescent selection of only transfected cells and wild-type IE86 protein or a mutant IE86 protein were tested in permissive and nonpermissive cells. HCMV infection induced the entry of U373 cells into the S phase. All permissive cells infected with HCMV were blocked in cell cycle progression and could not divide. After either transduction or transfection and IE86 protein expression, the number of all permissive or nonpermissive cell types in the S phase increased significantly, but the cells could no longer divide. The IE72 protein did not have a significant effect on the S phase. Since IE86 protein inhibits cell cycle progression, the IE2 gene in a human fibroblast IE86 protein-expressing cell line was sequenced. The IE86 protein in these retrovirus-transduced cells has mutations in a critical region of the viral protein. The locations of the mutations and the function of the IE86 protein in controlling cell cycle progression are discussed.

Covalent modification of the transactivator protein IE2-p86 of human cytomegalovirus by conjugation to the ubiquitin-homologous proteins SUMO-1 and hSMT3b

The 86-kDa IE2 protein (IE2-p86) of human cytomegalovirus (HCMV) is a potent transactivator of viral as well as cellular promoters. Several lines of evidence indicate that this broad transactivation spectrum is mediated by protein-protein interactions. To identify novel cellular binding partners, we performed a yeast two-hybrid screen using a N-terminal deletion mutant of IE2-p86 comprising amino acids 135 to 579 as a bait. Here, we report the isolation of two ubiquitin-homologous proteins, SUMO-1 and hSMT3b, as well as their conjugating activity hUBC9 (human ubiquitin-conjugating enzyme 9) as specific interaction partners of HCMV IE2. The polypeptides SUMO-1 and hSMT3b have previously been shown to be covalently coupled to a subset of nuclear proteins such as the nuclear domain 10 (ND10) proteins PML and Sp100 in a manner analogous to ubiquitinylation, which we call SUMOylation. By Western blot analysis, we were able to show that the IE2-p86 protein can be partially converted to a 105-kDa isoform in a dose-dependent manner after cotransfection of an epitope-tagged SUMO-1. Immunoprecipitation experiments of the conjugated isoforms using denaturing conditions further confirmed the covalent coupling of SUMO-1 or hSMT3b to IE2-p86 both after transient transfection and after lytic infection of human primary fibroblasts. Moreover, we defined two modification sites within IE2, located in an immediate vicinity at amino acid positions 175 and 180, which appear to be used alternatively for coupling. By using a SUMOylation-defective mutant, we showed that the targeting of IE2-p86 to ND10 occurs independent of this modification. However, a strong reduction of IE2-mediated transactivation of two viral early promoters and a heterologous promoter was observed in cotransfection analysis with the SUMOylation-defective mutant. This suggests a functional relevance of covalent modification by ubiquitin-homologous proteins for IE2-mediated transactivation, possibly by providing an additional interaction motif for cellular cofactors.

A strong negative transcriptional regulatory region between the human cytomegalovirus UL127 gene and the major immediate-early enhancer

The region of the human cytomegalovirus (CMV) genome between the UL127 open reading frame and the major immediate-early (MIE) enhancer is referred to as the unique region. DNase I protection analysis with human cell nuclear extracts demonstrated multiple protein binding sites in this region of the viral genome (P. Ghazal, H. Lubon, C. Reynolds-Kohler, L. Hennighausen, and J. A. Nelson, Virology 174:18-25, 1990). However, the function of this region in the context of the viral genome is not known. In wild-type human CMV-infected human fibroblasts, cells permissive for viral replication, there is little to no transcription from UL127. We determined that the unique region prevented transcription from the UL127 promoter but had no effect on the divergent MIE promoter. In transient-transfection assays, the basal level of expression from the UL127 promoter increased significantly when the wild-type unique sequences were mutated. In recombinant viruses with similar mutations in the unique region, expression from the UL127 promoter occurred only after de novo viral protein synthesis, typical of an early viral promoter. A 111-bp deletion-substitution of the unique sequence caused approximately a 20-fold increase in the steady-state level of RNA from the UL127 promoter and a 245-fold increase in the expression of a downstream indicator gene. This viral negative regulatory region was also mutated at approximately 50-bp regions proximal and distal to the UL127 promoter. Although some repressive effects were detected in the distal region, mutations of the region proximal to the UL127 promoter had the most significant effects on transcription. Within the proximal and distal regions, there are potential cis sites for known eucaryotic transcriptional repressor proteins. This region may also bind unknown viral proteins. We propose that the unique region upstream of the UL127 promoter and the MIE enhancer negatively regulates the expression from the UL127 promoter in permissive human fibroblast cells. This region may be a regulatory boundary preventing the effects of the very strong MIE enhancer on this promoter.

Human cytomegalovirus US3 gene expression is regulated by a complex network of positive and negative regulators

One immediate early gene of human cytomegalovirus, the US3 gene, causes retention of major histocompatibility locus class I heavy chain proteins in the endoplasmic reticulum and is postulated to have a role in viral pathogenicity. Expression of the US3 gene is regulated by a number of cis-acting elements. In addition, numerous viral proteins are involved in regulating US3 gene expression. US3 transcription was activated modestly by a virion protein, ppUL82. The immediate early proteins encoded by UL122-123 (IE1 and IE2) further activate US3 expression, with the activation enhanced by expression of pTRS1. Other proteins, the immediate early protein encoded by UL37ex1/UL38 and the early protein, pUL84, inhibited IE1 and IE2 activation of US3 expression. US3 transcription is regulated both positively and negatively by a complex network of viral proteins, the interaction of which contributes to precise regulation of US3 gene expression. The ability of pUL37ex1/UL38 to repress expression of the immediate early US3 gene while activating early gene expression suggests that pUL37ex1/UL38 may function to switch viral gene expression from immediate early to early genes.

Human cytomegalovirus and human herpesvirus 6 genes that transform and transactivate

This review is an update on the transforming genes of human cytomegalovirus (HCMV) and human herpesvirus 6 (HHV-6). Both viruses have been implicated in the etiology of several human cancers. In particular, HCMV has been associated with cervical carcinoma and adenocarcinomas of the prostate and colon. In vitro transformation studies have established three HCMV morphologic transforming regions (mtr), i.e., mtrI, mtrII, and mtrIII. Of these, only mtrII (UL111A) is retained and expressed in both transformed and tumor-derived cells. The transforming and tumorigenic activities of the mtrII oncogene were localized to an open reading frame (ORF) encoding a 79-amino-acid (aa) protein. Furthermore, mtrII protein bound to the tumor suppressor protein p53 and inhibited its ability to transactivate a p53-responsive promoter. In additional studies, the HCMV immediate-early protein IE86 (IE2; UL122) was found to interact with cell cycle-regulatory proteins such as p53 and Rb. However, IE86 exhibited transforming activity in vitro only in cooperation with adenovirus E1A. HHV-6 is a T-cell-tropic virus associated with AIDS-related and other lymphoid malignancies. In vitro studies identified three transforming fragments, i.e., SalI-L, ZVB70, and ZVH14. Of these, only SalI-L (DR7) was retained in transformed and tumor-derived cells. The transforming and tumorigenic activities of SalI-L have been localized to a 357-aa ORF-1 protein. The ORF-1 protein was expressed in transformed cells and, like HCMV mtrII, bound to p53 and inhibited its ability to transactivate a p53-responsive promoter. HHV-6 has also been proposed to be a cofactor in AIDS because both HHV-6 and human immunodeficiency virus type 1 (HIV-1) have been demonstrated to coinfect human CD4(+) T cells, causing accelerated cytopathic effects. Interestingly, like the transforming proteins of DNA tumor viruses such as simian virus 40 and adenovirus, ORF-1 was also a transactivator and specifically up-regulated the HIV-1 long terminal repeat when cotransfected into CD4(+) T cells. Finally, based on the interactions of HCMV and HHV-6 transforming proteins with tumor suppressor proteins, a scheme is proposed for their role in oncogenesis.

In vivo replication of recombinant murine cytomegalovirus driven by the paralogous major immediate-early promoter-enhancer of human cytomegalovirus

Transcription of the major immediate-early (MIE) genes of cytomegaloviruses (CMV) is driven by a strong promoter-enhancer (MIEPE) complex. Transactivator proteins encoded by these MIE genes are essential for productive infection. Accordingly, the MIEPE is a crucial control point, and its regulation by activators and repressors is pertinent to virus replication. Since the MIEPE contains multiple regulatory elements, it was reasonable to assume that specific sequence motifs are irreplaceable for specifying the cell-type tropism and replication pattern. Recent work on murine CMV infectivity (A. Angulo, M. Messerle, U. H. Koszinowski, and P. Ghazal, J. Virol. 72:8502-8509, 1998) has documented the proposed enhancing function of the enhancer in that its resection or its replacement by a nonregulatory stuffer sequence resulted in a significant reduction of infectivity, even though replication competence was maintained by a basal activity of the spared authentic MIE promoter. Notably, full capacity for productive in vitro infection of fibroblasts was restored in recombinant viruses by the human CMV enhancer. Using two-color in situ hybridization with MIEPE-specific polynucleotide probes, we demonstrated that a murine CMV recombinant in which the complete murine CMV MIEPE is replaced by the paralogous human CMV core promoter and enhancer (recombinant virus mCMVhMIEPE) retained the potential to replicate in vivo in all tissues relevant to CMV disease. Notably, mCMVhMIEPE was also found to replicate in the liver, a site at which transgenic hCMV MIEPE is silenced. We conclude that productive in vivo infection with murine CMV does not strictly depend on a MIEPE type-specific regulation.

Murine cytomegalovirus immediate-early promoter directs astrocyte-specific expression in transgenic mice

Murine cytomegalovirus (MCMV), which causes acute, latent, and persistent infection of the natural host, is used as an animal model of human cytomegalovirus (HCMV) infection. Transcription of MCMV immediate-early (IE) genes is required for expression of the early and late genes and is dependent on host cell transcription factors. Cell-type-specific expression activity of the MCMV IE promoter was analyzed in transgenic mice generated with the major IE (MIE) enhancer/promoter involving nucleotides -1343 to -6 (1338 bp) connected to the reporter gene lacZ. Distinct expression was observed in the brain, kidneys, stomach, and skeletal muscles. Weak expression was observed in a portion of the parenchymal cells of the salivary glands and pancreas, and expression was hardly detected in the lungs, intestine, or immune and hematopoietic organs such as the thymus, spleen, lymph nodes, and bone marrow. The spectrum of organs positive for expression was narrower than that of the HCMV MIE promoter-lacZ transgenic mice reported previously and showed a greater degree of cell-type specificity. Interestingly, astrocyte-specific expression of the transgene was observed in the brain and primary glial cultures from the transgenic mice by combination of beta-galactosidase (beta-Gal) expression and immunostaining for cell markers. However, the transgene was not expressed in neurons, oligodendroglia, microglia, or endothelial cells. Furthermore, the beta-Gal expression in glial cultures was stimulated significantly by MCMV infection or by addition of calcium ionophore. These observations indicated that expression activity of the MCMV IE promoter is strictly cell-type specific, especially astrocyte-specific in the brain. This specific pattern of activity is similar to that of natural HCMV infection in humans.

A cis repression sequence adjacent to the transcription start site of the human cytomegalovirus US3 gene is required to down regulate gene expression at early and late times after infection

Human cytomegalovirus has two enhancer-containing immediate-early (IE) promoters with a cis repression sequence (CRS) positioned immediately upstream of the transcription start site, designated the major IE (MIE) promoter and the US3 promoter. The role of the CRS upstream of the US3 transcription start site in the context of the viral genome was determined by comparing the levels of transcription from these two enhancer-containing promoters in recombinant viruses with a wild-type or mutant CRS. Upstream of the CRS of the US3 promoter was either the endogenous enhancer (R2) or silencer (R1). The downstream US3 gene was replaced with the indicator gene chloramphenicol acetyltransferase (CAT). Infected permissive human fibroblast cells or nonpermissive, undifferentiated monocytic THP-1 cells were analyzed for expression from the US3 promoter containing either the wild-type or mutant CRS. With the wild-type CRS, the maximum level of transcription in permissive cells was detected within 4 to 6 h after infection and then declined. With the mutant CRS and the R2 enhancer upstream, expression from the US3 promoter continued to increase throughout the viral replication cycle to levels 20- to 40-fold higher than for the wild type. In nonpermissive or permissive monocytic THP-1 cells, expression from the US3 promoter was also significantly higher when the CRS was mutated. Less expression was obtained when only the R1 element was present, but expression was higher when the CRS was mutated. Thus, the CRS in the enhancer-containing US3 promoter appears to allow for a short burst of US3 gene expression followed by repression at early and late times after infection. Overexpression of US3 may be detrimental to viral replication, and its level of expression must be stringently controlled. The role of the CRS and the viral IE86 protein in controlling enhancer-containing promoters is discussed.

Characterization of the transcriptional repressive element of the human cytomegalovirus immediate-early US3 gene

Transcriptional repression is utilized by human cytomegalovirus to regulate expression of the immediate-early US3 gene. Sequences located 3' of the US3 TATA box are required for down regulation of expression. Mutagenesis of US3 sequences identified a 10-nucleotide region that is essential for transcriptional repression. In addition to the 10-nucleotide element, an additional region, which includes the US3 initiator element, was needed to confer repression on a heterologous promoter. Thus, a 19-nucleotide element (-18 to +1 relative to the transcription start site) functioned as a transcriptional repressive element (tre). The tre repressed transcription in a position-dependent but orientation-independent manner. In vivo footprinting experiments demonstrated that transcriptional repression is associated with a decrease in protein interactions with the US3 promoter and surrounding sequences. The data presented here suggest that the association of an as yet unidentified repressor protein with the tre represses transcription by inhibiting assembly of the transcription initiation complex on the US3 promoter.

Defective growth correlates with reduced accumulation of a viral DNA replication protein after low-multiplicity infection by a human cytomegalovirus ie1 mutant

To investigate the importance of the IE1 p72 regulatory protein during human cytomegalovirus replication, a recombinant virus unable to synthesize IE1 p72 was constructed. The Towne strain mutant CR208 lacked exon 4 of the major immediate-early gene and was isolated and complemented in an IE1-expressing immortalized human fibroblast line (ihfie1.3). Replication of CR208 in primary human fibroblasts was completed after an input multiplicity of 10 PFU/cell but was severely-impaired at 0.1 PFU/cell. CR208 formed plaques with lower efficiency on primary fibroblasts than on ihfiel.3 cells, and the relationship between the CR208 inoculum size and the resulting number of undersized plaques was nonlinear, indicating that multiple particles of CR208 were required to initiate lytic replication in a single primary fibroblast. After infection of primary fibroblasts with CR208 at 5 PFU/cell, a normal pattern of viral antigens was detected, although IE1 p72 was absent. During lower-multiplicity infections, IE2 protein was consistently detected at similar levels in a similar proportion of CR208-infected cells relative to the case for a Towne infection, but many fewer CR208-infected cells contained the ppUL44 polymerase accessory protein when evaluated at 24 or 48 h after infection. Furthermore, fibroblasts infected with CR208 at a low multiplicity failed to form viral DNA replication compartments, despite having expressed IE2 p86. These low-multiplicity growth and expression defects were corrected in two rescued derivatives of CR208 able to synthesize IE1 p72. One rescued virus (CR249) carried a deletion removing the large intron between exons 1 and 2 of the ie1-ie2 locus, revealing that this intron was dispensable for growth in cell culture.

Negative regulation of a heterologous promoter by human cytomegalovirus immediate-early protein IE2

The HCMV IE2 protein promiscuously activates transcription of many viral and cellular genes. IE2 also negatively autoregulates its own expression by binding to a strategically positioned IE2 binding site, called CRS, located immediately downstream of the TATA box of the HCMV major IE promoter. Here we show that IE2 is able to repress transcription driven by a heterologous promoter, RSV LTR. Repression of RSV LTR by IE2 is completely dependent of DNA sequences downstream of the TATA box of RSV LTR. A DNA sequence, 5'-CGATACAATAAACG-3', evidently matching the proposed CRS consensus sequence, is located between nucleotides -13 and +1 (relative to the transcription start site) of RSV LTR. Three lines of evidence support the notion that this RSV CRS element is involved in the IE2-mediated repression of RSV LTR. First, introduction of mutation to the RSV CRS element renders to the mutant RSV LTR resistance to IE2-mediated repression. Second, a mutant IE2 defective in DNA binding cannot downregulate transcription from RSV LTR. Third, IE2 specifically binds to the wild-type, but not the mutant, RSV CRS element in vitro. These data, in conjunction with previous works, demonstrate that IE2 can passively repress transcription of homologous and heterologous promoters that contain a CRS element.

Identification of domains within the human cytomegalovirus major immediate-early 86-kilodalton protein and the retinoblastoma protein required for physical and functional interaction with each other

The human cytomegalovirus major immediate-early 86-kDa protein (IE2 86) plays an important role in the trans activation and regulation of HCMV gene expression. Previously, we demonstrated that IE2 86 contains three regions (amino acids [aa] 86 to 135, 136 to 290, and 291 to 364) that can independently bind to in vitro-translated Rb when IE2 86 is produced as a glutathione S-transferase fusion protein (M. H. Sommer, A. L. Scully, and D. H. Spector, J. Virol. 68:6223-6231, 1994). In this report, we have elucidated the regions of Rb involved in binding to IE2 86 and have further analyzed the functional nature of the interaction between these two proteins. We find that two domains on Rb, the A/B pocket and the carboxy terminus, can each independently form a complex with IE2 86. In functional assays, we demonstrate that IE2 86 and another IE protein, IE1 72, can counter the enlarged flat cell phenotype, but not the G1/S block, which results from expression of wild-type Rb in the human osteosarcoma cell line Saos-2. Mutational analysis reveals that there are two domains on IE2 86 that can independently affect Rb function. One region (aa 241 to 369) includes the major Rb-binding domain, while the second maps to the amino-terminal region (aa 1 to 85) common to both IE2 86 and IE1 72. These data show that Rb and IE2 86 physically and functionally interact with each other via at least two separate domains and provide further support for the hypothesis that IE2 86 may exert its pleiotropic effects through the formation of multimeric protein complexes.

IE2 protein is insufficient for transcriptional repression of the human cytomegalovirus US3 promoter

Expression of the human cytomegalovirus (HCMV) US3 gene is regulated in part by transcriptional repression mediated through a cis-repressive region located between the TATA box and the transcriptional start site. The US3 cis-repressive element has extensive sequence identity with a similar repressive element in UL122-123 (the major immediate-early gene complex). Repression of UL122-123 is mediated through the interaction of the IE2 protein with cis-repressive sequences. In spite of the similarity of the two repressive elements, IE2 activated rather than repressed transcription from the US3 promoter. Additionally, IE1 or IE1 and IE2 in combination also activated US3 expression. These data demonstrate that regulation of HCMV immediate-early genes is quite complex and involves a number of proteins.

The UL84 protein of human cytomegalovirus acts as a transdominant inhibitor of immediate-early-mediated transactivation that is able to prevent viral replication

The 86-kilodalton immediate-early (IE) 2 protein (IE2-p86) of human cytomegalovirus (HCMV) is a multifunctional regulator of HCMV gene expression which appears to be essential for triggering the lytic replicative cycle. IE2-p86 functions as a promiscuous transactivator of both viral and cellular gene expression and can repress transcription from its own promoter. In this study we demonstrate that a viral early protein, termed pUL84, which is able to interact with IE2-p86 both in vivo and in vitro, modulates IE2-p86 in a specific manner. First, pUL84 acts as a transdominant inhibitor of IE2-p86-mediated transactivation of both homologous and heterologous promoters. Second, negative autoregulation by IE2-p86 is augmented in the presence of pUL84. Using two in vivo assays, we obtained evidence that expression of pUL84 during the IE phase of the viral replicative cycle leads to an inhibition of viral early gene expression which prevents replication of HCMV and results in a persistent infection of UL84-positive cell lines. Transdominant inhibition of a viral IE function by a protein expressed during the later phases of replication appears to be a novel principle used by herpesviruses which could account for the slow replication of HCMV and may be useful in the development of new antiviral strategies.

Human cytomegalovirus IE2 86-kilodalton protein binds p53 but does not abrogate G1 checkpoint function

Physical interactions between human cytomegalovirus (HCMV) immediate-early (IE) proteins and key cell cycle regulatory proteins have been suggested as a mechanism whereby this herpesvirus modifies cellular control of proliferation. Observed similarities to interactions of other DNA virus proteins (human papillomavirus type 16 E6 and E7, simian virus 40 large T antigen, and adenovirus type 5 E1A and E1B) with cell cycle modulatory proteins such as p53 and Rb have suggested that HCMV IE proteins may likewise alter the G1-to-S phase transition. The IE2 region gene product IE86 has been shown to specifically bind p53, potentially modifying p53 G1 checkpoint function. To examine this possibility, p53-mediated G1 arrest in the presence of IE86 was assessed. Retroviral constructs were created to facilitate the stable expression of IE86 and IE72, another IE protein implicated in HCMV-mediated alteration of cell cycle progression. Western analysis and immunoprecipitation confirmed IE protein expression and binding of IE86 to p53, respectively. Chloramphenicol acetyltransferase assays examining the ability of IE86 to repress activity from the HCMV major IE promoter or activate the HCMV early promoter for the 2.2-kb class of RNAs demonstrated the functional integrity of the IE86 protein. Induction of DNA damage in normal, uninfected fibroblasts (FB) or FB expressing IE86 by actinomycin D (Act D) resulted in increased p53 levels, a predominance of the hypophosphorylated form of Rb, and increased expression of both p21(CIP1/WAF1) and mdm-2. Fluorescence-activated cell sorting revealed that both uninfected and IE86-expressing FB experienced dramatic G1 arrest following exposure to Act D. The clear demonstration of these p53-dependent responses in the presence of IE86 indicates that binding to this viral protein does not compromise the ability of p53 to elicit growth arrest following DNA damage.

Detection of the Human Cytomegalovirus 2.0-kb Immediate Early Gene 1 Transcripts in Permissive and Nonpermissive Infections by RNA in situ Hybridization

The immediate early gene 1 (IE1) is the first gene to be expressed following the entry of the human cytomegalovirus (HCMV) into the cell and it does not require prior protein synthesis for its expression. Therefore, the IE1 gene is a potential candidate for the development of probes to detect HCMV in various states of infection. Using strand-specific (32)P- or digoxigenin-labeled riboprobes derived from an exon-specific subgenomic fragment of the HCMV Towne IE1 gene, we performed Northern blot analysis and RNA in situ hybridization on HCMV-infected human (permissive cells) and mouse (nonpermissive cells) fibroblasts and on 10 formalin-fixed paraffin-embedded sections of human tissue. By Northern blot analysis and by in situ hybridization, expression of the 2.0-kb IE1 gene was found in permissive as well as in nonpermissive infections. Specific nuclear and cytoplasmic hybridization was found at 5, 10, 24 and 72 h after infection in human fibroblasts. In comparison, hybridization was first detected at 10 h after infection in mouse fibroblasts. Hybridization with the IE1 probe was detected in cells with and without cytopathic changes in the formalin-fixed paraffin-embedded HCMV-infected human tissues. Hybridization patterns of the IE1 riboprobe were compared to those of the HCMV 2.7-kb major early beta-riboprobe which we have previously described [Am J Pathol 141:1247-1254;1992]. Although both riboprobes hybridize to their respective target sequences in the consecutive tissue sections, the patterns of hybridization are different. On occasion, sections of HCMV-infected human tissue showing no specific hybridization for the 2.7-kb riboprobe will show specific in situ hybridization when using the IE1 riboprobe. Our results suggest that RNA in situ hybridization with a probe directed at the IE1 transcripts is an effective method of detecting early and late stages of both permissive and nonpermissive HCMV infections. Copyright 1997 S. Karger AG, Basel

The human cytomegalovirus IE1-72 protein interacts with the cellular p107 protein and relieves p107-mediated transcriptional repression of an E2F-responsive promoter

The Rb-related p107 protein has been implicated as an important control element in proper cell cycle progression. The p107 protein is thought to restrict cellular proliferation in part through its interaction with the E2F family of transcription factors and is, therefore, a specific target for regulation by several DNA viruses. Here, we demonstrate that p107 protein levels are induced in a biphasic manner in human fibroblasts during productive infection by the human cytomegalovirus (HCMV). Expression patterns of p107 protein levels during HCMV infection of human embryonic lung cells (HELs) demonstrate a sustained induction from early to late times of infection. We also demonstrate that the HCMV immediate-early protein IE1-72 complexes in vivo with the p107 protein and that this interaction can be reconstituted in an in vitro system by using reticulocyte-translated protein. Our data demonstrate that the interaction between p107 and the IE1-72 protein occurs at times of infection that temporally match the second tier of p107 protein induction and the phosphorylation pattern of the IE1-72 protein. Furthermore, we show here that the ability of p107 to transcriptionally repress E2F-responsive promoters can be overcome by expression of the IE1-72 protein. This effect appears to be specific, since the IE1-72 protein is not capable of relieving Rb-mediated repression of an E2F-responsive promoter. Finally, our data demonstrate that HCMV infection can induce cellular proliferation in quiescent cells and that IE1-72 expression alone can, to a degree, drive a similar progression through the cell cycle. These data suggest that IE1-72-mediated transactivation of E2F-responsive promoters through alleviation of p107 transcriptional repression may play a key role in the cell cycle progression stimulated by HCMV infection.

Cytomegalovirus induced up-regulation of LFA-3 (CD58) and ICAM-1 (CD54) is a direct viral effect that is not prevented by ganciclovir or foscarnet treatment

Cytomegalovirus (CMV) is a major pathogen in transplant recipients and AIDS patients, and the virus may also play a role in allograft rejection. Previous work from this laboratory demonstrated increased cell surface expression of the adhesion molecules ICAM-1 (CD54) and LFA-3 (CD58) following CMV infection in vitro. We investigated whether the induction of adhesion molecules by CMV was a direct viral effect or secondary to cytokine induction. Cytokines known to up-regulate ICAM-1, such as TNFalpha or IL-1beta, were not detected in the supernatants of infected fibroblasts, and neutralizing antibodies against these cytokines did not abrogate the induction of either ICAM-1 or LFA-3 by CMV. Infected cell supernatants had increased levels of IL-6, IL-8 and IFNbeta however, the addition of recombinant forms of these cytokines did not affect adhesion molecule expression. Neither virus-free infected cell supernatants nor UV-inactivated virus up-regulated adhesion molecules, demonstrating that the induction of ICAM-1 and LFA-3 by CMV was a direct effect requiring infectious virus. Effective antiviral treatment with ganciclovir or foscarnet accentuated rather than abrogated the up-regulation of adhesion molecules, suggesting that CMV immediate early/early gene expression, which is not blocked by such treatment, was responsible for the adhesion molecule induction. Thus, despite effective antiviral therapy in the transplant recipient, CMV infected cells may continue to provide a focus of proinflammatory activity, which could contribute to immunopathology and/or accentuate graft rejection or graft-versus-host disease in vivo.

CREB and CREB-binding proteins play an important role in the IE2 86-kilodalton protein-mediated transactivation of the human cytomegalovirus 2.2-kilobase RNA promoter

The human cytomegalovirus (HCMV) immediate-early region 2 86-kDa protein (IE2 86) is the major transactivator of the promoter for the 2.2-kb class of early RNAs (open reading frame UL 112-113). Previously, we reported that a DNA segment on this promoter between nucleotides (nt) -113 and -59 was critical for activation by IE2 86 in vivo and could be bound by IE2 86 in vitro (R. Schwartz, M. H. Sommer, A. Scully, and D. H. Spector, J. Virol. 68:5613-5622, 1994). With a set of site-specific mutations within nt -84 to -61, we have localized the essential cis-acting sequences to nt -72 to -61, which contain an ATF/CREB-binding site. The IE2 86-binding site between nt -113 and -85 is not essential for activation of the promoter by IE2 86 in transient-expression assays, but its presence can enhance the level of activation mediated through the sequences located between nt -84 and -59. Electrophoretic mobility shift assays with a segment containing nt -84 to -59 and nuclear extracts from human cells permissive for the HCMV infection revealed a complex band pattern. However, by supershift analysis with specific antibodies, we were able to identify CREB as the major ATF/CREB family member in the protein-DNA complexes. Further evidence that CREB is a target for IE2 86-mediated induction, is provided by the finding that IE2 86 activates the somatostatin promoter to high levels. Although the binding of IE2 86 to nonphosphorylated full-length CREB or deltaCREB is minimal, IE2 86 does form complexes with p300 and the CREB-binding protein (CBP), which in turn bind to CREB and can serve as adaptor proteins for CREB function. In addition, the in vivo functional relevance of the interaction between IE2 86 and CBP is indicated by the ability of IE2 86 to enhance transcriptional activation mediated by a GAL4-CBP fusion protein brought to a promoter by GAL4-binding sites.

Cellular or viral protein binding to a cytomegalovirus promoter transcription initiation site: effects on transcription

We have previously shown that the IE2 protein of human cytomegalovirus (CMV) represses its own synthesis by binding to the major immediate-early promoter (M. P. Macias and M. F. Stinski, Proc. Natl. Acad. Sci. USA 90:707-711, 1993). The binding of a viral protein (IE2) and a cellular protein in the region of the transcription start site was investigated by site-specific mutational analysis and electrophoretic mobility shift assay. The viral protein and the cellular protein require different but adjacent core DNA sequence elements for binding. In situ chemical footprinting analysis of DNA-protein interactions with purified CMV IE2 protein or HeLa cell nuclear extracts demonstrated binding sites that overlap the transcription start site. The IE2 protein footprint was between bp -15 and +2, relative to the transcription start site, and the cellular protein was between bp -16 and +7. The ability of the unknown human cellular protein of approximately 150 kDa to bind the CMV major immediate-early promoter correlates with an increase in the level of transcription efficiency. Mutations in the core DNA sequence element for cellular protein binding significantly reduced the level of in vitro transcription efficiency. Mutations upstream and downstream of the core sequence moderately reduced the transcription efficiency level. Negative autoregulation of the CMV promoter by the viral IE2 protein may involve both binding to the DNA template and interference with the function of a cellular protein that binds to the transcription start site and enhances transcription efficiency.

Repression by a differentiation-specific factor of the human cytomegalovirus enhancer

We detected a novel nuclear protein, MRF, that binds to multiple sites on the modulator which is located upstream of the human cytomegalovirus major immediate early gene enhancer. The expression of MRF is differentiation specific; the DNA binding activity is present in nuclear extracts from undifferentiated Tera-2 and THP-1 cells, but significantly reduced after these cells are induced to differentiate. In undifferentiated cells the enhancer activity is repressed by the modulator and upon differentiation the enhancer becomes active. Competitive binding assays demonstrate that MRF requires the presence of multiple A+T stretches for binding to DNA, rather than binding to a specific DNA sequence. Mutations of these stretches in the modulator reduce the binding activity of MRF, as well as the repressing activity on the enhancer. These results suggest that MRF may act as a repressor of enhancer function. We propose that MRF binds over the entire modulator and exerts repressor activity.