Binding of SP1 to the immediate-early protein-responsive element of the human cytomegalovirus DNA polymerase promoter

The Dietary Flavonol Kaempferol Inhibits Epstein-Barr Virus Reactivation in Nasopharyngeal Carcinoma Cells

Kaempferol (KP, 3,4',5,7-tetrahydroxyflavone), a dietary flavonol, has anti-cancer, antioxidant, anti-inflammatory, antimicrobial, and antimutagenic functions. However, it is unknown whether kaempferol possesses anti-Epstein-Barr virus (EBV) activity. Previously, we demonstrated that inhibition of EBV reactivation represses nasopharyngeal carcinoma (NPC) tumourigenesis, suggesting the importance of identifying EBV inhibitors. In this study, Western blotting, immunofluorescence staining, and virion detection showed that kaempferol repressed EBV lytic gene protein expression and subsequent virion production. Specifically, kaempferol was found to inhibit the promoter activities of Zta and Rta (Zp and Rp) under various conditions. A survey of the mutated Zp constructs revealed that Sp1 binding regions are critical for kaempferol inhibition. Kaempferol treatment repressed Sp1 expression and decreased the activity of the Sp1 promoter, suggesting that Sp1 expression was inhibited. In conclusion, kaempferol efficiently inhibits EBV reactivation and provides a novel choice for anti-EBV therapy and cancer prevention.

Emerging Role of PYHIN Proteins as Antiviral Restriction Factors

Innate immune sensors and restriction factors are cellular proteins that synergize to build an effective first line of defense against viral infections. Innate sensors are usually constitutively expressed and capable of detecting pathogen-associated molecular patterns (PAMPs) via specific pattern recognition receptors (PRRs) to stimulate the immune response. Restriction factors are frequently upregulated by interferons (IFNs) and may inhibit viral pathogens at essentially any stage of their replication cycle. Members of the Pyrin and hematopoietic interferon-inducible nuclear (HIN) domain (PYHIN) family have initially been recognized as important sensors of foreign nucleic acids and activators of the inflammasome and the IFN response. Accumulating evidence shows, however, that at least three of the four members of the human PYHIN family restrict viral pathogens independently of viral sensing and innate immune activation. In this review, we provide an overview on the role of human PYHIN proteins in the innate antiviral immune defense and on viral countermeasures.

IFI16, a nuclear innate immune DNA sensor, mediates epigenetic silencing of herpesvirus genomes by its association with H3K9 methyltransferases SUV39H1 and GLP

IFI16, an innate immune DNA sensor, recognizes the nuclear episomal herpes viral genomes and induces the inflammasome and interferon-β responses. IFI16 also regulates cellular transcription and act as a DNA virus restriction factor. IFI16 knockdown disrupted the latency of Kaposi’s sarcoma associated herpesvirus (KSHV) and induced lytic transcripts. However, the mechanism of IFI16’s transcription regulation is unknown. Here, we show that IFI16 is in complex with the H3K9 methyltransferase SUV39H1 and GLP and recruits them to the KSHV genome during de novo infection and latency. The resulting depositions of H3K9me2/me3 serve as a docking site for the heterochromatin-inducing HP1α protein leading into the IFI16-dependent epigenetic modifications and silencing of KSHV lytic genes. These studies suggest that IFI16’s interaction with H3K9MTases is one of the potential mechanisms by which IFI16 regulates transcription and establish an important paradigm of an innate immune sensor’s involvement in epigenetic silencing of foreign DNA.

Specific transcriptional and post-transcriptional regulation of the major immediate early ICP4 gene of GaHV-2 during the lytic, latent and reactivation phases

Transcriptional and post-transcriptional mechanisms are involved in the switch between the lytic, latent and reactivation phases of the viral cycle in herpesviruses. During the productive phases, herpesvirus gene expression is characterized by a temporally regulated cascade of immediate early (IE), early (E) and late (L) genes. In alphaherpesviruses, the major product of the IE ICP4 gene is a transcriptional regulator that initiates the cascade of gene expression that is essential for viral replication. In this study, we redefine the infected cell protein 4 (ICP4) gene of the oncogenic Marek's disease virus (MDV or gallid herpesvirus 2) as a 9438 nt gene ended with four alternative poly(A) signals and controlled by two alternative promoters containing essentially ubiquitous functional response elements (GC, TATA and CCAAT boxes). The distal promoter is associated with ICP4 gene expression during the lytic and the latent phases, whereas the proximal promoter is associated with the expression of this gene during the reactivation phase. Both promoters are regulated by DNA methylation during the viral cycle and are hypermethylated during latency. Transcript analyses showed ICP4 to consist of three exons and two introns, the alternative splicing of which is associated with five predicted nested ICP4ORFs. We show that the ICP4 gene is highly and specifically regulated by transcriptional and post-transcriptional mechanisms during the three phases of the GaHV-2 viral cycle, with a clear difference in expression between the lytic phase and reactivation from latency in our model.

Cyclophilin A as a target in the treatment of cytomegalovirus infections

BACKGROUND

Viruses are obligate parasites that depend on the cellular machinery of the host to regenerate and manufacture their proteins. Most antiviral drugs on the market today target viral proteins. However, the more recent strategies involve targeting the host cell proteins or pathways that mediate viral replication. This new approach would be effective for most viruses while minimizing drug resistance and toxicity.

METHODS

Cytomegalovirus replication, latency, and immune response are mediated by the intermediate early protein 2, the main protein that determines the effectiveness of drugs in cytomegalovirus inhibition. This review explains how intermediate early protein 2 can modify the action of cyclosporin A, an immunosuppressive, and antiviral drug. It also links all the pathways mediated by cyclosporin A, cytomegalovirus replication, and its encoded proteins.

RESULTS

Intermediate early protein 2 can influence the cellular cyclophilin A pathway, affecting cyclosporin A as a mediator of viral replication or anti-cytomegalovirus drug.

CONCLUSION

Cyclosporin A has a dual function in cytomegalovirus pathogenesis. It has the immunosuppressive effect that establishes virus replication through the inhibition of T-cell function. It also has an anti-cytomegalovirus effect mediated by intermediate early protein 2. Both of these functions involve cyclophilin A pathway.

Human cytomegalovirus IE1 downregulates Hes1 in neural progenitor cells as a potential E3 ubiquitin ligase

Congenital human cytomegalovirus (HCMV) infection is the leading cause of neurological disabilities in children worldwide, but the mechanisms underlying these disorders are far from well-defined. HCMV infection has been shown to dysregulate the Notch signaling pathway in human neural progenitor cells (NPCs). As an important downstream effector of Notch signaling, the transcriptional regulator Hairy and Enhancer of Split 1 (Hes1) is essential for governing NPC fate and fetal brain development. In the present study, we report that HCMV infection downregulates Hes1 protein levels in infected NPCs. The HCMV 72-kDa immediate-early 1 protein (IE1) is involved in Hes1 degradation by assembling a ubiquitination complex and promoting Hes1 ubiquitination as a potential E3 ubiquitin ligase, followed by proteasomal degradation of Hes1. Sp100A, an important component of PML nuclear bodies, is identified to be another target of IE1-mediated ubiquitination. A C-terminal acidic region in IE1, spanning amino acids 451 to 475, is required for IE1/Hes1 physical interaction and IE1-mediated Hes1 ubiquitination, but is dispensable for IE1/Sp100A interaction and ubiquitination. Our study suggests a novel mechanism linking downregulation of Hes1 protein to neurodevelopmental disorders caused by HCMV infection. Our findings also complement the current knowledge of herpesviruses by identifying IE1 as the first potential HCMV-encoded E3 ubiquitin ligase.

Congenital human cytomegalovirus (HCMV) infection is the leading cause of neurological disabilities in children, but the underlying pathogenesis of this infection remains unclear. Hes1, an important effector of Notch signaling, governs the fate of neural progenitor cells (NPCs) and fetal brain development. Here we demonstrate that: (1) HCMV infection results in loss of Hes1 protein in NPCs; (2) the HCMV immediate-early 1 protein (IE1) mediates Hes1 protein downregulation through direct interaction, which requires amino acids 451–475; (3) IE1 assembles a Hes1 ubiquitination complex and mediates Hes1 ubiquitination; and (4) IE1 also assembles an Sp100A ubiquitination complex and mediates Sp100A ubiquitination, but does not require amino acids 451–475. These results suggest that HCMV IE1 is a potential E3 ubiquitin ligase. Downregulation of Hes1 by HCMV infection and IE1 implies a novel mechanism linking Hes1 depletion to virus-induced neuropathogenesis.

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

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

Differential effects of Sp cellular transcription factors on viral promoter activation by varicella-zoster virus (VZV) IE62 protein

The immediate early (IE) 62 protein is the major varicella-zoster virus (VZV) regulatory factor. Analysis of the VZV genome revealed 40 predicted GC-rich boxes within 36 promoters. We examined effects of ectopic expression of Sp1-Sp4 on IE62- mediated transactivation of three viral promoters. Ectopic expression of Sp3 and Sp4 enhanced IE62 activation of ORF3 and gI promoters while Sp3 reduced IE62 activation of ORF28/29 promoter and VZV DNA replication. Sp2 reduced IE62 transactivation of gI while Sp1 had no significant influence on IE62 activation with any of these viral promoters. Electrophoretic mobility shift assays (EMSA) confirmed binding of Sp1 and Sp3 but not Sp2 and Sp4 to the gI promoter. Sp1-4 bound to IE62 and amino acids 238-258 of IE62 were important for the interaction with Sp3 and Sp4 as well as Sp1. This work shows that Sp family members have differential effects on IE62-mediated transactivation in a promoter-dependent manner.

Cytomegalovirus immune evasion of myeloid lineage cells

Cytomegalovirus (CMV) evades the immune system in many different ways, allowing the virus to grow and its progeny to spread in the face of an adverse environment. Mounting evidence about the antiviral role of myeloid immune cells has prompted the research of CMV immune evasion mechanisms targeting these cells. Several cells of the myeloid lineage, such as monocytes, dendritic cells and macrophages, play a role in viral control, but are also permissive for CMV and are naturally infected by it. Therefore, CMV evasion of myeloid cells involves mechanisms that qualitatively differ from the evasion of non-CMV-permissive immune cells of the lymphoid lineage. The evasion of myeloid cells includes effects in cis, where the virus modulates the immune signaling pathways within the infected myeloid cell, and those in trans, where the virus affects somatic cells targeted by cytokines released from myeloid cells. This review presents an overview of CMV strategies to modulate and evade the antiviral activity of myeloid cells in cis and in trans.

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.

Interferon γ-inducible protein (IFI) 16 transcriptionally regulates type i interferons and other interferon-stimulated genes and controls the interferon response to both DNA and RNA viruses

The interferon γ-inducible protein 16 (IFI16) has recently been linked to the detection of nuclear and cytosolic DNA during infection with herpes simplex virus-1 and HIV. IFI16 binds dsDNA via HIN200 domains and activates stimulator of interferon genes (STING), leading to TANK (TRAF family member-associated NF-κB activator)-binding kinase-1 (TBK1)-dependent phosphorylation of interferon regulatory factor (IRF) 3 and transcription of type I interferons (IFNs) and related genes. To better understand the role of IFI16 in coordinating type I IFN gene regulation, we generated cell lines with stable knockdown of IFI16 and examined responses to DNA and RNA viruses as well as cyclic dinucleotides. As expected, stable knockdown of IFI16 led to a severely attenuated type I IFN response to DNA ligands and viruses. In contrast, expression of the NF-κB-regulated cytokines IL-6 and IL-1β was unaffected in IFI16 knockdown cells, suggesting that the role of IFI16 in sensing these triggers was unique to the type I IFN pathway. Surprisingly, we also found that knockdown of IFI16 led to a severe attenuation of IFN-α and the IFN-stimulated gene retinoic acid-inducible gene I (RIG-I) in response to cyclic GMP-AMP, a second messenger produced by cyclic GMP-AMP synthase (cGAS) as well as RNA ligands and viruses. Analysis of IFI16 knockdown cells revealed compromised occupancy of RNA polymerase II on the IFN-α promoter in these cells, suggesting that transcription of IFN-stimulated genes is dependent on IFI16. These results indicate a broader role for IFI16 in the regulation of the type I IFN response to RNA and DNA viruses in antiviral immunity.

The intracellular DNA sensor IFI16 gene acts as restriction factor for human cytomegalovirus replication

Human interferon (IFN)-inducible IFI16 protein, an innate immune sensor of intracellular DNA, modulates various cell functions, however, its role in regulating virus growth remains unresolved. Here, we adopt two approaches to investigate whether IFI16 exerts pro- and/or anti-viral actions. First, the IFI16 gene was silenced using specific small interfering RNAs (siRNA) in human embryo lung fibroblasts (HELF) and replication of DNA and RNA viruses evaluated. IFI16-knockdown resulted in enhanced replication of Herpesviruses, in particular, Human Cytomegalovirus (HCMV). Consistent with this, HELF transduction with a dominant negative form of IFI16 lacking the PYRIN domain (PYD) enhanced the replication of HCMV. Second, HCMV replication was compared between HELFs overexpressing either the IFI16 gene or the LacZ gene. IFI16 overexpression decreased both virus yield and viral DNA copy number. Early and late, but not immediate-early, mRNAs and proteins were strongly down-regulated, thus IFI16 may exert its antiviral effect by impairing viral DNA synthesis. Constructs with the luciferase reporter gene driven by deleted or site-specific mutated forms of the HCMV DNA polymerase (UL54) promoter demonstrated that the inverted repeat element 1 (IR-1), located between −54 and −43 relative to the transcription start site, is the target of IFI16 suppression. Indeed, electrophoretic mobility shift assays and chromatin immunoprecipitation demonstrated that suppression of the UL54 promoter is mediated by IFI16-induced blocking of Sp1-like factors. Consistent with these results, deletion of the putative Sp1 responsive element from the HCMV UL44 promoter also relieved IFI16 suppression. Together, these data implicate IFI16 as a novel restriction factor against HCMV replication and provide new insight into the physiological functions of the IFN-inducible gene IFI16 as a viral restriction factor.

Only recently, intrinsic cellular-based defense mechanisms which give cells the capacity to resist pathogens have been discovered as an essential component of immunity. However, unlike the innate and adaptive branches of the immune system, intrinsic immune defenses are mediated by cellular restriction factors that are constitutively expressed and active even before a pathogen enters the cell. The protein family HIN-200 may act as sensors of foreign DNA and modulate various functions such as growth, apoptosis, and senescence. Here we show that, in the absence of functional IFI16, the replication of some Herpesviruses and in particular of Human Cytomegalovirus (HCMV) is significantly enhanced. Accordingly, IFI16 overexpression strongly inhibited HCMV replication. Accumulation of viral DNA copies was down-regulated along with expression of early and late viral gene expression suggesting that IFI16 inhibits viral DNA synthesis. Using transient transfection, luciferase, gel shift assay, and chromatin immunoprecipitation, we demonstrate that IFI16 suppresses the transcriptional activity of the viral DNA polymerase gene (UL54) and the UL44 gene, also required for viral DNA synthesis. The finding that the nuclear DNA sensor IFI16 controls virus growth represents an important step forward in understanding the intrinsic mechanisms that drive viral infections sustained by DNA viruses such as Herpesviruses.

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.

Heat shock enhances CMV-IE promoter-driven metabotropic glutamate receptor expression and toxicity in transfected cells

In CHO-K1 cells, heat shock strongly activated reporter-gene expression driven by the cytomegalovirus immediate-early (CMV-IE) promoter from adenoviral and plasmid vectors. Heat shock treatment (2h at 42.5 °C) significantly enhanced the promoter DNA-binding activity in nuclear extracts. In CHO cells expressing mGluR1a and mGluR5a receptors under the control of the CMV promoter, heat shock increased receptor protein expression, mRNA levels and receptor function estimated by measurement of PI hydrolysis, intracellular Ca²+ and cAMP. Hyperthermia increased average amplitudes of Ca²+ responses, the number of responding cells, and revealed the toxic properties of mGluR1a receptor. Heat shock also effectively increased the expression of EGFP. Hence, heat shock effects on mGluR expression and function in CHO cells may be attributed to the activation of the CMV promoter. Moreover, this effect was not limited to CHO cells as heat shock also increased EGFP expression in PC-12 and HEK293 cells. Heat shock treatment may be a useful tool to study the function of proteins expressed in heterologous systems under control of the CMV promoter. It may be especially valuable for increasing protein expression in transient transfections, for enhancing receptor expression in drug screening applications and to control the expression of proteins endowed with toxic properties. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

The multifaceted interferon-inducible p200 family proteins: from cell biology to human pathology

The interferon-inducible p200 family proteins consist of a group of homologous human and mouse proteins that have an N-terminal Pyrin domain and 1 or 2 partially conserved 200 amino acid long C-terminal domains (designated the HIN domain or p200 X domain). These proteins display multifaceted activity due to their ability to bind to various target proteins (eg, transcription factors, signaling proteins, and tumor suppressor proteins) and modulate different cell functions. In addition to a role in interferon biology, increasing evidence supports a role for these proteins as regulators of various cell functions, including proliferation, differentiation, apoptosis, senescence, inflammasome assembly, and control of organ transplants. As a consequence, alterations in their expression and function may be of relevance in the pathogenesis of human diseases, such as systemic autoimmune syndromes, tumors, and degenerative diseases. This review summarizes the literature describing these data, highlights some of the important findings derived from recent studies, and speculates about future perspectives.

Human cytomegalovirus pUL83 stimulates activity of the viral immediate-early promoter through its interaction with the cellular IFI16 protein

The human cytomegalovirus (HCMV) virion protein pUL83 (also termed pp65) inhibits the expression of interferon-inducible cellular genes. In this work we demonstrate that pUL83 is also important for efficient induction of transcription from the viral major immediate-early promoter. Infection with a mutant virus containing a premature translation termination codon in the UL83 open reading frame (ORF) (UL83Stop) resulted in decreased transcription from the major immediate-early promoter in a time- and multiplicity-dependent manner. Expression of pUL83 alone is capable of transactivating the promoter in a reporter assay, and pUL83 associates with the promoter in infected cells. To investigate the mechanism by which the protein regulates the major immediate-early promoter, we utilized a mutant virus expressing an epitope-tagged pUL83 from its own promoter to identify protein binding partners for pUL83 during infection. We identified and confirmed the interaction of pUL83 with cellular IFI16 family members throughout the course of HCMV infection. pUL83 recruits IFI16 to the major immediate-early promoter, and IFI16 binding at the promoter is dependent upon the presence of pUL83. Consistent with the results obtained with the UL83Stop virus, infection of IFI16 knockdown cells with wild-type virus resulted in decreased levels of immediate-early transcripts compared to those of control cells. These data identify a previously unknown role for pUL83 in the initiation of the human cytomegalovirus gene expression cascade.

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

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

RPA nucleic acid-binding properties of IFI16-HIN200

InterFeron-gamma Inducible protein 16 (IFI16) belongs to the interferon inducible HIN200 protein family that contains transcriptional regulators linked to cell cycle regulation and differentiation. All family members contain at most two domains of 200 amino acids, called HIN200, each containing two Oligonucleotide/Oligosaccharide Binding (OB) folds. IFI16 is involved in transcriptional repression and is a component of the DNA repair multi-protein complex known as BASC, which forms after UV-induced DNA damage. In this study, we used fold recognition and biophysical approaches as a tool to infer and validate functions to the HIN200 domain. Since the best template to model IFI16-HIN200 is Replication Protein A (RPA) in complex with single-stranded nucleic acids, we tested six RPA nucleic acid-binding characteristics for IFI16-HIN200. Our results indicate that IFI16-HIN200 is an RPA-like, OB-fold, nucleic acid-binding protein that binds to ssDNA with higher affinity than to dsDNA, recognizes ssDNA in the same orientation as RPA, oligomerizes upon ssDNA binding, wraps and stretches ssDNA, but does not destabilize dsDNA. We finally propose a framework model explaining how the HIN200 domain could prevent ssDNA from re-annealing.

A novel role of the interferon-inducible protein IFI16 as inducer of proinflammatory molecules in endothelial cells

The human IFI16 gene is an interferon-inducible gene implicated in the regulation of endothelial cell proliferation and tube morphogenesis. Immunohistochemical analysis has demonstrated that this gene is highly expressed in endothelial cells in addition to hematopoietic tissues. In this study, gene array analysis of human umbilical vein endothelial cells overexpressing IFI16 revealed an increased expression of genes involved in immunomodulation, cell growth, and apoptosis. Consistent with these observations, IFI16 triggered expression of adhesion molecules such as ICAM-1 and E-selectin or chemokines such as interleukin-8 or MCP-1. Treatment of cells with short hairpin RNA targeting IFI16 significantly inhibited ICAM-1 induction by interferon (IFN)-gamma demonstrating that IFI16 is required for proinflammatory gene stimulation. Moreover, functional analysis of the ICAM-1 promoter by deletion- or site-specific mutation demonstrated that NF-kappaB is the main mediator of IFI16-driven gene induction. NF-kappaB activation appears to be triggered by IFI16 through a novel mechanism involving suppression of IkappaBalpha mRNA and protein expression. Support for this finding comes from the observation that IFI16 targeting with specific short hairpin RNA down-regulates NF-kappaB binding activity to its cognate DNA and inhibits ICAM-1 expression induced by IFN-gamma. Using transient transfection and luciferase assay, electrophoretic mobility shift assay, and chromatin immunoprecipitation, we demonstrate indeed that activation of the NF-kappaB response is mediated by IFI16-induced block of Sp1-like factor recruitment to the promoter of the IkappaBalpha gene, encoding the main NF-kappaB inhibitor. Activation of NF-kappaB accompanied by induction of proinflammatory molecules was also observed when IkappaBalpha expression was down-regulated by specific small interfering RNA, resulting in an outcome similar to that observed with IFI16 overexpression. Taken together, these data implicate IFI16 as a novel regulator of endothelial proinflammatory activity and provide new insights into the physiological functions of the IFN-inducible gene IFI16.

Ethanol feeding alters death signaling in the pancreas

OBJECTIVES

Alcohol abuse is a major cause of pancreatitis, which is associated with death of parenchymal cells. The goal of this study was to explore the effects of ethanol on cell death pathways in the pancreas.

METHODS

Adult male Wistar rats were fed with ethanol diets using the Lieber-DeCarli method. Caspase-8, caspase-3, and cathepsin B expression and activity in the pancreas of these animals as well as the signals that regulate their expression were studied using Western blot analysis and specific assays for biochemical enzyme activity.

RESULTS

In the pancreas from rats fed with ethanol, the protein expression and activity of caspase-8 decreased by 48% and 45%, respectively, and caspase-3 activity decreased by 39%. In contrast, cathepsin B protein expression and activity increased with ethanol feeding by 189% and 143%, respectively. Evaluation of the transcriptional regulatory system for caspase-8 and cathepsin B showed that the ethanol effects on these pathways were largely transcriptional.

CONCLUSIONS

Our findings show effects of ethanol on the expression of several signals involved in cell death in the pancreas through alteration of transcriptional regulators. The decrease in caspase expression and increase in cathepsin B expression indicate that ethanol feeding may prevent apoptosis and promote necrosis of pancreatic tissue with stresses that cause pancreatitis.

Sp1/Sp3 and the myeloid zinc finger gene MZF1 regulate the human N-cadherin promoter in osteoblasts

To determine the molecular mechanisms by which N-cadherin transcription is regulated, we cloned and sequenced a 3681-bp of the 5'-flanking region of the human N-cadherin gene. Deletion analysis of the proximal region identified a minimal 318-bp region with strong promoter activity in human osteoblasts. The cryptic promoter is characterized by high GC content and a GA-rich binding core that may bind zing finger transcription factors. Electrophoretic mobility shift assays (EMSA), competition and supershift EMSA revealed that an Sp1/Sp3 binding site acts as a basal regulatory element of the promoter in osteoblasts. Incubation of osteoblast nuclear extracts with -163/-131 wild-type probe containing the GA-rich binding core revealed another specific complex, which was not formed with a -163/-131 probe mutated in the GA repeat. EMSA identified the nuclear factor involved as myeloid zinc finger-1 (MZF1). Mutation analysis showed that Sp1/Sp3 and MZF1 binding sites contribute to basal promoter activity. Cotransfection analyses showed that Sp1 and MZF1 overexpression increases whereas Sp3 antagonizes Sp1-induced N-cadherin promoter activity in osteoblasts. RT-PCR analysis showed that human osteoblastic cells express MZF1 and that Sp1/MZF1 overexpression increased N-cadherin expression. These results indicate that Sp1/Sp3 and MZF1 are important transcription factors regulating N-cadherin promoter activity and expression in osteoblasts.

Differential patterns of human cytomegalovirus gene expression in various T-cell lines carrying human T-cell leukemia-lymphoma virus type I: role of Tax-activated cellular transcription factors

Replication of human cytomegalovirus (HCMV) was investigated in various T-cell lines expressing the tax gene product of human T-cell leukemia-lymphoma virus type I (HTLV-I). Differential patterns of HCMV replication were found in HTLV-I-carrying cell lines. HCMV gene expression was restricted to the immediate-early genes in MT-2 and MT-4 cells, whereas full replication cycle of the virus was observed in C8166-45 cells. Productive HCMV infection induced a cytopathic effect resulting in the lysis of infected cells. The results of electrophoretic mobility shift assay (EMSA) showed high levels of NF-kappaB-, CREB/ATF-1-, and SRF-specific DNA binding activity in all Tax-positive cell lines. In contrast, SP1 activity could be detected only in C8166-45 cells. Using an inducible system (Jurkat cell line JPX-9), a dramatic increase in NF-kappaB, CREB/ATF-1, SRF, and SP1 binding activity, as well as productive HCMV infection, were observed upon Tax expression. Overexpression of SP1 in MT-2 and MT-4 cells converted HCMV infection from an abortive to a productive one. These data suggest that the stimulatory effect of Tax protein on HCMV in T cells is accomplished through at least five host-related transcription factor pathways. The results of this study provide possible mechanisms whereby HCMV infections might imply suppression of adult T-cell leukemia.

Synergistic induction of intercellular adhesion molecule-1 by the human cytomegalovirus transactivators IE2p86 and pp71 is mediated via an Sp1-binding site

Human cytomegalovirus (HCMV) infection of transplant recipients is frequently associated with allograft vasculopathy and rejection. One potential mechanism is vascular injury from HCMV-triggered, immunologically mediated processes. HCMV infection has been shown to increase the expression of intercellular adhesion molecule-1 (ICAM-1). The objective of this study was to determine the molecular basis of HCMV-enhanced ICAM-1 gene expression. Transient transfection experiments identified the IE2p86 protein as a potent activator of the ICAM-1 promoter. The tegument protein pp71 showed a strong synergistic effect on IE2p86-mediated ICAM-1 promoter activation. Mutagenesis experiments defined a DNA element from -110 to -42 relative to the transcription start site as responsive for IE2p86. Further point mutations within this DNA element identified an Sp1-binding site that was essential for strong synergistic activation by IE2p86 and pp71. To confirm the activation of ICAM-1 gene expression, human fibroblasts (HFF) as well as endothelial cells (HUVEC) were infected with recombinant IE2p86- and pp71-expressing baculoviruses, respectively. In FACS analysis, a synergistic induction of ICAM-1 was detectable when cells were co-infected with the two recombinant baculoviruses. These findings clearly demonstrate that IE2p86 and pp71 are crucial regulatory factors for HCMV-induced ICAM-1 upregulation.

Expression of IFI 16 in epithelial cells and lymphoid tissues

IFI 16 is a member of the HIN-200 protein family named for their haemopoietic expression, interferon-inducibility and nuclear localisation. These proteins have been characterised as transcriptional regulators that modulate the cell cycle. IFI 16 is expressed in some haemopoietic lineages including CD34+ progenitor cells, mature lymphocytes and monocytes, but is absent from granulocytes, erythrocytes and megakaryocytes. We present a wider study of IFI 16 expression in normal human tissues using a monoclonal antibody specifically recognising the C-terminus of IFI 16. As expected, IFI 16 was detected in the nuclei of lymphocytes in the spleen, thymus, lymph node and palatine tonsil, but was also found in epithelial cells in these tissues. Interestingly, IFI 16 protein was also expressed in non-lymphoid tissues including trachea, gastrointestinal tract, skin and testis, but was absent from others including heart and brain. In each tissue, IFI 16 was predominantly expressed in surface epithelial cells and staining was strongest in basal epithelial layers. Therefore, IFI 16 expression is not restricted to cells of the immune system, but is also expressed in epithelial cells. In contrast to the perceived role of HIN-200 proteins as suppressors of cell growth, maximal expression of IFI 16 was in cells with high proliferative potential.

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.

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.

Dominance of virus over host factors in cross-species activation of human cytomegalovirus early gene expression

Human cytomegalovirus (HCMV) exhibits a highly restricted host range. In this study, we sought to examine the relative significance of host and viral factors in activating early gene expression of the HCMV UL54 (DNA polymerase) promoter in murine cells. Appropriate activation of the UL54 promoter at early times is essential for viral DNA replication. To study how the HCMV UL54 promoter is activated in murine cells, a transgenesis system based on yeast artificial chromosomes (YACs) was established for HCMV. A 178-kb YAC, containing a subgenomic fragment of HCMV encompassing the majority of the unique long (UL) region, was constructed by homologous recombination in yeast. This HCMV YAC backbone is defective for viral growth and lacks the major immediate-early (IE) gene region, thus permitting the analysis of essential cis-acting sequences when complemented in trans. To quantitatively measure the level of gene expression, we generated HCMV YACs containing a luciferase reporter gene inserted downstream of either the UL54 promoter or, as a control for late gene expression, the UL86 promoter, which directs expression of the major capsid protein. To determine the early gene activation pathway, point mutations were introduced into the inverted repeat 1 (IR1) element of the UL54 promoter of the HCMV YAC. In the transgenesis experiments, HCMV YACs and derivatives generated in yeast were introduced into NIH 3T3 murine cells by polyethylene glycol-mediated fusion. We found that infection of YAC, but not plasmid, transgenic lines with HCMV was sufficient to fully recapitulate the UL54 expression program at early times of infection, indicating the importance of remote regulatory elements in influencing regulation of the UL54 promoter. Moreover, YACs containing a mutant IR1 in the UL54 promoter led to reduced ( approximately 30-fold) reporter gene expression levels, indicating that HCMV major IE gene activation of the UL54 promoter is fully permissive in murine cells. In comparison with HCMV, infection of YAC transgenic NIH 3T3 lines with murine cytomegalovirus (MCMV) resulted in lower (more than one order of magnitude) efficiency in activating UL54 early gene expression. MCMV is therefore not able to fully activate HCMV early gene expression, indicating the significance of virus over host determinants in the cross-species activation of key early gene promoters. Finally, these studies show that YAC transgenesis can be a useful tool in functional analysis of viral proteins and control of gene expression for large viral genomes.

Activation of transcription of the human cytomegalovirus early UL4 promoter by the Ets transcription factor binding element

The human cytomegalovirus (HCMV) early UL4 promoter has served as a useful model for studying the activation of early viral gene expression. Previous transient-transfection experiments detected cis-acting elements (the NF-Y site and site 2) upstream of the transcriptional start site (L. Huang and M. F. Stinski, J. Virol. 69:7612-7621, 1995). The roles of two of these sites, the NF-Y site and site 2, in the context of the viral genome were investigated further by comparing mRNA levels from the early UL4 promoter in human foreskin fibroblasts infected by recombinant viruses with either wild-type or mutant cis-acting elements. Steady-state mRNA levels from the UL4 promoter with a mutation in the NF-Y site were comparable to that of wild type. A mutation in an Elk-1 site plus putative IE86 protein binding sites decreased the steady-state mRNA levels compared to the wild type at early times after infection. Electrophoretic mobility shift assays and antibody supershifts detected the binding of cellular transcription factor Elk-1 to site 2 DNA with infected nuclear extracts but not with mock-infected nuclear extracts. The role of cellular transcription factors activated by the mitogen activated protein kinase/extracellular signal-regulated kinase pathway in activating transcription from early viral promoters is discussed.

Major product pp43 of human cytomegalovirus U(L)112-113 gene is a transcriptional coactivator with two functionally distinct domains

Human cytomegalovirus U(L)112-113 encodes four phosphoproteins, pp84, pp50, pp43, and pp34, with common amino-termini. A previous report by Kerry et al. (J. Virol. 70, 373-382, 1996) demonstrated that U(L)112-113 products activate U(L)54 promoter in cooperation with immediate-early (IE) proteins. In this study, we identified a domain required for transcriptional activation in the pp43 protein, which consisted of two distinct regions: domain I (amino acids 272-296) and domain II (amino acids 297-306). Domain I contained two long glycine stretches, and domain II was a short proline-containing region. Both of domains were required for IE2-dependent transcriptional activation. The pp43 mutant that had domain I but lacked domain II acted as a dominant negative mutant and suppressed most of the IE2-dependent activation, indicating the importance of coactivation by pp43 in this transcriptional activation. The major protein pp43 also weakly activated the promoter through IR1 element in a manner independent of IE2. Only domain I was required for this IE2-independent activation. These domains were common in pp84, pp50, and pp43 but did not exist in pp34, which did not activate transcription alone. These results suggest that the major product, pp43, of U(L)112-113 has two functionally distinct domains and plays an important role in mediating IE2-dependent transcriptional activation.

Effects of human cytomegalovirus major immediate-early proteins in controlling the cell cycle and inhibiting apoptosis: studies with ts13 cells

The major immediate-early (MIE) gene of human cytomegalovirus (HCMV) encodes several MIE proteins (MIEPs) produced as a result of alternative splicing and polyadenylation of the primary transcript. Previously we demonstrated that the HCMV MIEPs expressed from the entire MIE gene could rescue the temperature-sensitive (ts) transcriptional defect in the ts13 cell line. This defect is caused by a ts mutation in TAFII250, the 250-kDa TATA binding protein-associated factor (TAF). These and other data suggested that the MIEPs perform a TAF-like function in complex with the basal transcription factor TFIID. In addition to the transcriptional defect, the ts mutation in ts13 cells results in a defect in cell cycle progression which ultimately leads to apoptosis. Since all of these defects can be rescued by wild-type TAFII250, we asked whether the MIEPs could rescue the cell cycle defect and/or affect the progression to apoptosis. We have found that the MIEPs, expressed from the entire MIE gene, do not rescue the cell cycle block in ts13 cells grown at the nonpermissive temperature. However, despite the maintenance of the cell cycle block, the ts13 cells which express the MIEPs are resistant to apoptosis. MIEP mutants, which have previously been shown to be defective in rescuing the ts transcriptional defect, maintained the ability to inhibit apoptosis. Hence, the MIEP functions which affect transcription appear to be separable from the functions which inhibit apoptosis. We discuss these data in the light of the HCMV life cycle and the possibility that the MIEPs promote cellular transformation by a "hit-and-run" mechanism.

Transcriptional regulation of the human cytomegalovirus US11 early gene

The human cytomegalovirus (HCMV) US11 early gene encodes a protein involved in the down-regulation of major histocompatibility complex class I cell surface expression in HCMV-infected cells. Consequently, this gene is thought to play an important role in HCMV evasion of immune recognition. In this study, we examined the transcriptional regulation of US11 gene expression. Analysis of deletions within the US11 promoter suggests that two sequence elements are important for activation by the viral immediate-early (IE) proteins. Deletion of a CREB site located at -83 relative to the cap site resulted in a reduction in promoter activity to 50% of the wild-type level. Deletion of an additional ATF site immediately upstream of the TATA box resulted in abrogation of responsiveness to the IE proteins. To confirm the role of the CREB and ATF sites within the US11 promoter, mutagenesis of these two sites, both individually and in combination, was carried out. Results indicate that both the CREB element and the ATF site were required for full promoter activity, with the ATF site critical for US11 promoter activation. The loss of transcriptional activation correlated with a loss of cellular proteins binding to the mutated US11 promoter elements. In combination with the viral IE proteins, the HCMV tegument protein pp71 (UL82) was found to up-regulate the US11 promoter by six- to sevenfold in transient assays. These results suggest that pp71 may contribute to the activation of the US11 promoter at early times after infection. Up-regulation by pp71 required the presence of the CREB and ATF sites within the US11 promoter for full activation. The role of the ATF and CREB elements in regulating US11 gene expression during viral infection was then assessed. The US11 gene is not required for replication of HCMV in tissue culture. This property was exploited to generate US11 promoter mutants regulating expression of the endogenous US11 gene in the natural genomic context. We generated recombinant HCMV that contained the US11 promoter with mutations in either the CREB or ATF element or both regulating the expression of the endogenous US11 gene. Northern blot analysis of infected cell mRNA revealed that mutation of the CREB element reduced US11 mRNA expression to approximately 25% of that of the wild-type promoter, with identical kinetics of expression. Mutation of the ATF site alone reduced US11 mRNA levels to 6% of that of the wild-type promoter, with mRNA detectable only at 8 h after infection. Mutation of both the CREB and ATF elements in the US11 promoter reduced US11 gene expression to undetectable levels. These results demonstrate that the CREB and ATF sites cooperate to regulate the US11 promoter in HCMV-infected cells.

The human interferon-inducible protein, IFI 16, is a repressor of transcription

IFI 16 is a member of a family of interferon-inducible proteins, including the human MNDA (myeloid nuclear differentiation antigen), the recently identified AIM-2 (absent in melanoma), and the homologous murine molecules, p202, p204, and D3. IFI 16 contains a domain at the amino terminus capable of binding double-stranded DNA and a bipartite nuclear localization signal. No molecular or biological function has been assigned to any of the human family members, although a role in transcription regulation has been proposed. In the present study, we show IFI 16 fused to the GAL4 DNA binding domain can function as a transcriptional repressor. IFI 16-mediated repression is not dependent on the position or distance of IFI 16 binding, relative to the site of transcription initiation, and it can significantly repress when only one GAL4 DNA element is present in the promoter. We mapped the transcriptional repression domains to the 200 amino acid repeat regions common to all human and mouse family members. We also demonstrate that wild type IFI 16 can repress transcription of a reporter gene containing the minimal promoter region of the human cytomegalovirus UL54 gene. Thus, IFI 16 is a transcriptional repressor, with a modular structure typical of many known transcription regulators.

Transcription factor Sp1 mediates cell-specific trans-activation of the human cytomegalovirus DNA polymerase gene promoter by immediate-early protein IE86 in glioblastoma U373MG cells

Human cytomegalovirus (HCMV) gene expression is highly cell and tissue specific. Cell factor-mediated regulatory interactions are involved in regulating the restricted expression of the HCMV major immediate-early (IE) gene (J. F. Baskar, P. P. Smith, G. Nilaver, R. A. Jupp, S. Hoffmann, N. J. Peffer, D. J. Tenney, A. M. Colberg-Poley, P. Ghazal, and J. A. Nelson, 70:3207-3213, 1996). To gain an understanding of HCMV early gene activation, we studied the effect of each of the three major IE proteins, IE72, IE86, and IE55, on the HCMV DNA polymerase gene (pol; UL54) promoter. In transient-expression assays, the IE86 protein alone was able to transactivate the pol promoter, but IE72 and IE55 were not, in permissive U373MG cells. However, we were unable to detect IE86-mediated transactivation in nonpermissive HeLa or C33-A cells. Using electrophoretic mobility shift assays (EMSAs), we found that expression of the IE86 protein in U373MG cells resulted in specific binding of a DNA complex to an inverted-repeat element, IR1, of the pol promoter. Antibody supershifting and EMSA-Western blotting experiments further showed that IE86 and the cellular transcription factor Sp1 were components of the IR1 DNA-binding complex. Furthermore, we found that binding of DNA by Sp1 was dramatically increased in the presence of IE86. Interestingly, this IE86-induced DNA-binding activity of Sp1 was inhibited by a repressor activity presented in HeLa cells. In summary, our study suggests that a viral regulatory protein can modulate the DNA binding activity of a cellular transcription factor, resulting in cell-specific transactivation of viral genes.