Papillomaviruses comprise a large number of related small DNA tumor viruses with tropism for squamous epithelial cells. The papillomavirus replication cycle is strictly linked to the differentiation stages of the infected epithelial cells, and the expression of L1 and L2 capsid proteins from the viral late genes is primarily detected in the superficial layers of terminally differentiated cells. Expression of the L1 and L2 genes is blocked in nonterminally differentiated cells and the production of progeny virus is delayed until the infected cell reaches the upper strata of the squamous epithelium. This property presumably aids the papillomavirus to evade the immune surveillance of the host and allows establishment of persistent infections. Late gene expression levels are determined in part by regulatory RNA sequences on the papillomavirus mRNAs. This review focuses primarily on negativecis-acting elements on late papillomavirus mRNAs and their candidatetrans-acting factors. Identification and characterization of these components will contribute to our understanding of the regulated expression of papillomaviruses in mammalian cells.
The essential herpes simplex virus type 1 (HSV-1) immediate-early protein IE63 (ICP27) is an RNA binding protein which, at the posttranscriptional level, interacts with cellular splicing small nuclear ribonucleoprotein particles (snRNPs) and inhibits RNA splicing, promotes RNA 3′ processing, and prevents the nucleocytoplasmic transport of intron-containing mRNAs. IE63 is the only HSV IE protein with homologs not only among other α herpesviruses, but also throughout the herpesviridae family. Recent evidence shows that IE63 is a nucleocytoplasmic shuttle protein able to travel from snRNP- and RNA-rich nuclear foci to the cytoplasm. This property suggests that IE63 may facilitate the nuclear export of virus mRNAs, perhaps selectively, of HSV-1 transcripts, from virus genes which lack introns. Posttranscriptional effects of other HSV-1 functions are discussed.
The Sendai virus (SeV) P/C mRNA expresses eight different polypeptide chains using a combination of ribosomal choice and cotranscriptional editing (an internal open reading frame (ORF) is accessed by the addition of a single G residue after a short run of Gs at position 1053 on the mRNA). The longest ORF within the mRNA starts at ATG104 (the second initiation site) and encodes the 568-aa P protein, an essential viral structural protein which serves both as a cofactor for the RNA-dependant RNA polymerase (L protein) and as a part of the assembly complex. The first (ACG81), third (ATG114), fourth (ATG183) and fifth (ATG201) initiation sites are used to express a C-terminal nested set of polypeptides which are in the +1 ORF relative to P, namely C′, C, Y1, and Y2, respectively (collectively named the C proteins). Leaky scanning accounts for translational initiation at the first three start sites (a non-ATG followed by ATGs in progressively stronger contexts). Consistent with this, changing the C′ ACG to an ATG (GCCATG81G; ATG81/C′) ablates all expression from the downstream ATG104/P and ATG114/C initiation codons, whereas initiation from ATG183/Y1 and ATG201/Y2 remains normal in this background. Initiation from ATG183/Y1/ ATG201/Y2 probably takes place by discontinuous scanning via a ribosomal shunt. Scanning complexes appear to assemble at the 5′ cap and then scan the first ≈30 nt of the 5′ UTR before being translocated to an acceptor site close to the Y initiation codons. No specific 5′ UTR or donor site sequence elements are required, and translation of the Y proteins continues even when their start codons are changed to ACG.
Chicken anemia virus (CAV) is a globally distributed avian pathogen. CAV has a circular single-stranded DNA of 2.3 kb and constitutes a unique type in the newly described virus family ofCircoviridae.The CAV genome contains three partially or completely overlapping genes. CAV causes fatal cytopathogenic effects in chicken thymocytes and cultured transformed mononuclear cells via apoptosis. In transformed (chicken) cells, the synthesis of the VP3 gene product apoptin alone mimics this CAV-induced apoptosis, which is p53 independent and cannot be inhibited by the apoptosis-inhibiting proteins Bcl-2 and CrmA. Apoptin induces apoptosis in human tumor cells but, interestingly, not in normal cells. These properties suggest that apoptin may have potential medical applications.
Cytokines play a critical role in the regulation of immune responses and constitute important targets for virus immune evasion mechanisms. One strategy used by large DNA viruses is to encode proteins that mimic cytokines or cytokine receptors, which modulate the activity of cytokines during infection. Poxviruses encode a unique set of proteins that are secreted from the infected cell and function as soluble cytokine receptors or binding proteins and sequester tumor necrosis factor, interleukin-1β, or chemokines. Characterization of these poxvirus proteins is providing information on virus pathogenesis, the function of cytokines, and new strategies for immune modulation and therapeutic intervention.
Adenovirus (Ad) gene expression involves regulation both of RNA processing and RNA transport. The major late transcription unit (MLTU) in particular requires complex differential splicing and polyadenylation to produce its full array of mRNA and the pattern of processing changes as infection proceeds. Many MLTU mRNAs cannot reach the cytoplasm effectively without viral functions, probably because they are recognized by host nuclear retention mechanisms. Infection produces changes to the activity of cellular splicing factors and polyadenylation factors so altering the relative recognition of competing RNA processing sites as infection proceeds. In addition, three Ad proteins, E1B 55K, E4 Orf3, and E4 Orf6 are involved in posttranscriptional regulation. These directly or indirectly alter the pattern of MLTU splicing, facilitate late RNA export, and alter the distribution of cellular antigens within the nucleus.
The herpesvirus family of viruses is large, containing over 100 different members, of which 8 infect humans. In this short review, we examine the induction and inhibition of apoptosis by herpesviruses, focusing primarily on the human viruses.
Adenovirus infection induces apoptosis by multiple paradigms that involve viral proteins coded by three different early gene regions,E1A, E3,andE4.These cell death programs are antagonized by a different set of viral proteins coded by early gene blocksE1B, E3,andE4.The E1A proteins activate the cellular transcription factor E2F and also increase accumulation of the p53 tumor suppressor protein, both proteins with known apoptotic activity. The E1B-19K protein, a distantly related member of the BCL-2 family of antiapoptosis proteins, efficiently suppresses both p53-dependent and -independent apoptosis induced during adenovirus infection. Two other proteins, E1B-55K and E4-36K, which inactivate p53 by physical protein complex formation suppress p53-dependent apoptosis. The mechanisms by which the E1B-19K protein acts are not fully known, but at least one appears to involve antagonizing the activity of various cellular pro-apoptotic proteins such as BAX, BAK, and BIK. One of the two major E1A proteins, 289R, also appears to induce cell death via transactivation of the cellular transcription factor NF-κB as well as two early gene blocks,E3andE4.NF-κB is believed to induce expression of TNF and various inflammatory cytokines in virus-infected cells. TheE3andE4gene blocks each code for a cell death protein, E3-11.6K (ADP) and E4-orf4. The E1A proteins also sensitize infected cells for TNF-induced apoptosis and this activity is linked to the cell cycle regulatory activities of E1A. In addition to the E1B-19K protein, three E3 proteins, 14.7K, 10.4K, and 14.5K, specifically protect cells against TNF-induced cell death. One of the mechanisms by which the E3-14.7K and E3-10.4K/14.5K complex suppress TNF-induced toxicity appears to be mediated by the release of arachidonic acid by the cytosolic phospholipase A2. The E3 proteins also inhibit Fas-agonist-induced apoptosis by endosome-mediated internalization and degradation of Fas from the cell surface. Suppression of E1A-induced apoptosis in nonpermissive cells by E1B-19K, E1B-55K, and E4-36K proteins leads to oncogenic transformation.
Human adenoviruses cause lytic and persistent respiratory, enteric, and other infections. Adenoviruses can transform primary rodent cells and certain serotypes of human adenoviruses (such as adenovirus type 12 from subgenus A) induce tumors in newborn rodents. In both cases, adenovirus gene products mediate evasion of the cellular immune system by infected and tumor cells. In viral infection, a product of the early regionE3gene, a glycoprotein termed E3-19K, binds to and retains newly synthesised major histocompatibility complex (MHC) class I molecules in the endoplasmic reticulum thus rendering infected cells resistant to lysis by cytotoxic T lymphocytes. In addition, other products of theE3region confer on infected cells resistance to tumor necrosis factor-α-mediated lysis. Not all human adenovirus serotypes encode an E3-19K protein: viruses from subgenus A (such as adenovirus 12) and F (the enteric adenoviruses 40 and 41) do not encode an E3-19K molecule. In the case of Ad12, products of the viral E1A gene repress MHC class I heavy chain gene transcription. This leads to loss of MHC class I molecules from the surface of adenovirus 12-transformed cells and contributes to their evasion from cytotoxic T lymphocytes. Considerable progress has been made toward identifying the targets for E1A-mediated repression of the class I heavy chain promoter. In addition, adenovirus 12 mediates transcriptional repression of other genes in the MHC complex involved in antigen presentation, namely the transporter associated with antigen presentation (TAP) genes and MHC-encoded proteasome components, the low molecular weight proteins termed LMPs. Overall, adenoviruses display a variety of differing mechanisms for posttranslational (E3-19K) and transcriptional (E1A) repression of MHC class I expression that operate in all human viral serotypes studied, suggesting that evasion of cytotoxic T cell lysis forms an important part of the infection and oncogenic transformation strategies adopted by human adenoviruses.
Adenovirus E1A promotes apoptosis by interacting with and inhibiting negative regulators of cell cycle control. Binding of E1A to, and inhibition of, the transcriptional coadaptor p300 promotes accumulation of the p53 tumor suppressor protein which induces apoptosis. By inhibiting p300, E1A prevents the transcriptional activation ofmdm-2,the product of which interacts with and promotes the degradation of p53. Thus the E1A–p300 interaction disables the negative feedback loop to control p53 levels, which left unrestrained, cause apoptosis rather than growth arrest. The E1B 19K protein functions analogously to Bcl-2 to inhibit apoptosis by E1A, p53, and multiple other stimuli. The E1B 19K protein functions by at least two independent mechanisms to inhibit apoptosis. First, the E1B 19K protein binds to the pro-apoptotic Bax protein to prevent loss of mitochondrial membrane potential, caspase activation, and apoptosis. Second, the E1B 19K protein inhibits caspase interaction by interfering with the function of adaptor molecules such as FADD and Ced-4 that interact with and activate caspases. By inhibiting FADD-dependent activation of the caspase FLICE, the E1B 19K protein can disable both the TNF-α- and the Fas-mediated death signaling pathways which play an important role in immune surveillance against virus infection and cancer. The E1B 19K protein binds to Ced-4, and presumably mammalian Ced-4 homologues, and thereby prevents caspase activation. Thus, the study of the mechanism of regulation of apoptosis by the adenovirus transforming proteins has revealed important regulatory steps in death signaling pathways.
Poxviruses express a variety of proteins that are able to modulate the innate cellular apoptotic response triggered by virus infection. Poxviruses are the only DNA viruses to replicate exclusively in the cytoplasm of infected cells, and to date, members of this family have been shown to encode a wide variety of proteins that block or delay apoptosis, including caspase inhibitors, other serpins, death domain effectors, bcl-2/CED-9 homologs, modulators of the FAS/TNF pathway, and inhibitors of PKR. It is predicted that this list of poxvirus apoptosis modulators will continue to grow in the coming years and should provide an increasingly rich and diverse family of apoptosis regulators.
Herpesviruses, such as human and murine cytomegalovirus, possess an impressive array of genes believed to assist in virus survival against the host immune response. In this review, we cover the rapidly growing area of cytomegalovirus evasion of cellular immunity, specifically cytotoxic T lymphocytes and natural killer cells. The proposed mechanisms of action of viral proteins involved in blocking peptide presentation to CD8(+) T cells, namely, interference with peptide generation, inhibition of peptide assembly with class I MHC and retention/destabilization of class I MHC complexes, are described. In addition, recent evidence implicating the viral class I MHC-like proteins as inhibitors of natural killer cell-mediated clearance is reviewed, (C) 1998 Academic Press.
The hepadnaviruses utilize multiple nested promoters on the same DNA strand to transcribe several mRNA species, all of which terminate at the same polyadenylation site. While most studies of hepadnaviral gene expression have concentrated on transcriptional initiation, it has become clear that there exist viralcis-elements that are important in posttranscriptional events. This review will describe recent data on transcriptional elongation, transcriptional termination, and RNA export in hepadnaviruses and compare them with the corresponding processes in the distantly related retroviruses.
Historically, vaccinia viral enzymes have provided fundamental insights into general enzymological processes. Reasons for this include their amenity to genetic approaches, the relative ease with which they can be purified in adequate quantities, their genetic location within a relatively small, intronless, completely sequenced genome, and the recognizable sequence similarity often observed with corresponding cellular enzymes. Mechanisms by which the ubiquitous poly(A) tail is added to mRNA 3′ ends are not fully characterized in any organism. Concurrently with the characterization of the metazoan, yeast, andEscherichia colipoly(A) polymerases, some recent biochemical and crystallographic studies of the vaccinia enzyme have provided glimpses of how a heterodimeric poly(A) polymerase might elongate the poly(A) tail.
The adenovirus E3 transcription unit encodes proteins named E3-14.7K, RID, and E3-gp19K that prevent killing of infected cells by the host immune system. Tumor necrosis factor (TNF), a cytokine secreted by activated monocytes and cytotoxic T lymphocytes (CTL), can induce apoptosis when it engages the TNF receptor on target cells. E3-14.7K and RID independently prevent TNF-induced apoptosis. Fas ligand, which is expressed on activated CTL and natural killer cells, induces apoptosis when it engages its receptor, Fas, on target cells. RID blocks apoptosis through Fas by stimulating the clearance of Fas from the infected cell surface and its degradation in lysosomes. CTL induce apoptosis when the T cell receptor engages the MHC class I antigen–peptide complex on target cells. E3-gp19K inhibits killing by CTL by blocking transport of MHC class I antigens to the infected cell surface. After virus replication is complete, the cell lyses and releases virus particles; this cell lysis is mediated by the E3-coded adenovirus death protein.
Poxviruses express a variety of proteins which can function to inhibit apoptosis in infected cells, allowing virus replication to continue and conferring a broad host range. Some poxvirus antiapoptosis proteins act by sequestering or inactivating inducers of apoptosis such as dsRNA and superoxide anions. Others interfere with signaling by receptors including those belonging to the TNF receptor superfamily that would otherwise activate a proteolytic cascade that terminates with the cleavage of death substrates. The cowpox virus crmA protein directly inhibits cysteine proteinases within the cascade and can also block apoptosis triggered by the serine proteinase granzyme B. Finally there are poxvirus antiapoptosis proteins containing ankyrin repeat regions that are thought to interact with cellular proteins.
Sindbis virus causes acute encephalitis in mice and serves as a useful model for encephalitic alphaviruses that infect humans. The outcome of infection is determined by whether infected neurons are resistant to virus-induced programmed cell death or activate their apoptotic pathway. The host immune response may also cause death of infected neurons. Determinants of neuronal apoptosis include the maturity of the neuron, the virulence of the infecting virus and the cellular immune response to infection. In many situations viral and cellular factors that decrease virus replication also decrease apoptosis. Antiviral antibody can downregulate virus replication in surviving neurons without affecting cell viability. Other innate and induced host immune responses can alter the outcome of infection without a change in virus production. Failure to induce apoptosis in infected neurons leads to long-term persistence of small amounts of viral RNA in the nervous system of infected mice despite the clearance of infectious virus. The molecular mechanisms that govern these pathogenesis factors are beginning to be elucidated.
Chemokines play a key role in orchestrating leukocytic recruitment during inflammatory responses, including those to viral infections. Chemokines are soluble cytokines which mediate their effects through specific G protein-coupled, seven-transmembrane receptors which are expressed on a wide range of cells, including monocytes, T-cells, dendritic cells, and NK cells. Analyses of herpesvirus genomes have revealed that these viral pathogens encode their own versions of both chemokines and chemokine receptors. Viral genes encoding chemokine elements were likely to have been acquired from the host genome and have been remodeled during virus evolution to presumably optimize function or acquire new properties not displayed by their cellular homologues. Virus-encoded chemokines and chemokine receptors are important players in the continuing confrontation between viruses and their mammalian hosts. Detailed characterization of these elements will provide a better understanding of how the immune system responds to viral infection and may suggest new antiviral drug targets and new avenues for the development of antiviral therapies. We will review here the chemokine elements encoded by herpesviruses and how they may aid viral infection and propagation.
It has been postulated that virus-induced apoptosis is an important factor in pathogenesis caused by cytopathogenic viruses. Apoptosis is induced by both influenza A and influenza B viruses, which are cytopathogenic negative-strand RNA viruses. The importance of influenza viruses as worldwide pathogens in humans and domestic animals is well recognized. In this report, we review the current state of knowledge of the mechanism(s) of influenza virus-induced apoptosis. Several cellular factors and at least one virus-specific protein, the nonstructural protein NS1, have been implicated in influenza virus-induced apoptosis. However, the mechanism(s) of influenza virus-induced apoptosis are not well understood at the present time.
