An adenovirus mRNA which encodes a 14,700-Mr protein that maps to the last open reading frame of region E3 is expressed during infection

Immune evasion by adenoviruses: a window into host-virus adaptation

Human adenoviruses (HAdVs) are widespread pathogens that cause a number of partially overlapping, species-specific infections associated with respiratory, urinary, gastrointestinal, and ocular diseases. The early 3 (E3) region of adenoviruses is highly divergent between different species, and it encodes a multitude of proteins with immunomodulatory functions. The study of genetic diversity in the E3 region offers a unique opportunity to gain insight into how the various HAdVs have evolutionarily adapted in response to the selection pressures exerted by host immune defenses. The objective of this review was to discuss subversion of host antiviral immune responses by HAdVs, with a focus on suppression of MHC class I antigen presentation, as a window into host-HAdV adaptation.

Functions and mechanisms of action of the adenovirus E3 proteins

In the evolutionary battle between viruses and their hosts, viruses have armed themselves with weapons to defeat the host's attacks on infected cells. Various proteins encoded in the adenovirus (Ad) E3 transcription unit protect cells from killing mediated by cytotoxic T cells and death-inducing cytokines such as tumor necrosis factor (TNF), Fas ligand, and TNF-related apoptosis-inducing ligand (TRAIL). The viral protein E3-gp19 K blocks MHC class-I-restricted antigen presentation, which diminishes killing by cytotoxic T cells. The receptor internalization and degradation (RID) complex (formerly E3-10.4 K/14.5 K) stimulates the clearance from the cell surface and subsequent degradation of the receptors for Fas ligand and TRAIL, thereby preventing the action of these important immune mediators. RID also downmodulates the epidermal growth factor receptor (EGFR), although what role, if any, this function has in immune regulation is uncertain. In addition, RID antagonizes TNF-mediated apoptosis and inflammation through a mechanism that does not primarily involve receptor downregulation. E3-6.7 K functions together with RID in downregulating some TRAIL receptors and may block apoptosis independently of other E3 proteins. Furthermore, E3-14.7 K functions as a general inhibitor of TNF-mediated apoptosis and blocks TRAIL-induced apoptosis. Finally, after expending great effort to maintain cell viability during the early part of the virus replication cycle, Ads lyse the cell to allow efficient virus release and dissemination. To perform this task subgroup C Ads synthesize a protein late in infection named ADP (formerly E3-11.6 K) that is required for efficient virus release. This review focuses on recent experiments aimed at discovering the mechanism of action of these critically important viral proteins.

Distinct domains in the adenovirus E3 RIDalpha protein are required for degradation of Fas and the epidermal growth factor receptor

Adenovirus (Ad) types 2 and 5 encode at least five proteins within the E3 transcription unit that help the virus evade the immune system. Two such proteins, RIDalpha (formerly E3-10.4K) and RIDbeta (formerly E3-14.5K), form the RID (receptor internalization and degradation) complex (formerly E3-10.4K/14.5K). RID mediates clearance from the cell surface and lysosomal degradation of a number of important members in the tumor necrosis factor receptor (TNFR) superfamily and the receptor tyrosine kinase receptor family. Affected receptors include Fas, TRAIL (TNF-related apoptosis-inducing ligand) receptor 1 (TR1), TR2, and epidermal growth factor receptor (EGFR). Degradation of Fas and TRAIL receptors protects Ad-infected cells from apoptosis. To investigate the mechanism of action of RIDalpha, 14 mutant RIDalpha proteins, each containing a three- to five-amino-acid deletion, were constructed and then expressed from the E3 region of a replication-competent recombinant Ad in the same context as wild-type RIDalpha. Each mutant protein was characterized with regard to five physical properties associated with wild-type RIDalpha, namely, protein stability, proteolytic cleavage, insertion into the membrane, complex formation with RIDbeta, and transport to the cell surface. Additionally, the mutant proteins were tested for their ability to mediate internalization and degradation of EGFR and Fas and to protect cells from Fas-mediated apoptosis. The majority of mutant RIDalpha proteins (8 out of 14) were physically similar to wild-type RIDalpha. With regard to functional characteristics, the cytoplasmic domain of RIDalpha is largely unimportant for receptor internalization and degradation and the extracellular domain of RIDalpha is important for down-regulation of EGFR but not Fas.

The adenovirus E3/10.4K-14.5K proteins down-modulate the apoptosis receptor Fas/Apo-1 by inducing its internalization

Adenoviruses (Ads) have evolved multiple mechanisms to evade the host immune response. Several of the immunomodulatory Ad proteins are encoded in early transcription unit 3 (E3). The E3/19K protein interferes with antigen presentation and T cell recognition, whereas the E3/10.4K, 14.5K, and 14.7K proteins can protect cells from tumor necrosis factor alpha-mediated lysis. Here, we describe an additional activity of E3 proteins. Transfectants expressing all E3 proteins of Ad2 exhibit a profound reduction of the apoptosis receptor CD95 (Fas, APO-1) on the cell surface. In contrast, cells expressing only the E3A region have normal Fas levels. Thus, one of the E3B proteins (10.4K, 14.5K, or 14.7K) seems to be responsible for this effect. To identify the E3B products involved, each individual E3B ORF was selectively disrupted. Examination of stable cell lines containing the mutated E3 regions showed that Fas expression is restored when either the 10.4K or the 14.5K ORF is disrupted, whereas mutation of the 14.7K ORF does not rescue Fas expression. Loss of Fas on the cell surface is accompanied by a similar decrease of total Fas levels. However, in the presence of lysosomotropic agents Fas accumulates in endosomal/lysosomal vesicles, indicating that 10.4K-14.5K induce internalization and degradation of Fas. Down-regulation of Fas but not CD40 is also observed during infection and as a consequence, Ad-infected cells are protected from Fas-mediated apoptosis. Thus, the Fas system is implicated in Ad pathogenesis.

Adenovirus death protein, a transmembrane protein encoded in the E3 region, is palmitoylated at the cytoplasmic tail

The 11.6-K protein of human adenovirus 2 (Ad2), which was recently renamed as adenovirus death protein (ADP), is a type III membrane glycoprotein that ultimately localizes to the nuclear membrane. ADP is encoded in the E3 transcription unit of Ad2 and migrates as a set of multiple bands in SDS-PAGE with three major forms. The corresponding gene product of adenovirus 5 (Ad5) has a slightly lower molecular weight and shows the same pattern in SDS-PAGE. We report here the covalent attachment of fatty acids to cysteine residues of ADP. In the case of Ad5-ADP all three major forms of this protein can be labeled by [3H]palmitic acid, but not by [3H]myristic acid, whereas only two [3H]palmitic acid-labeled Ad2-ADP species could be detected. The label is sensitive to treatment with 1 M hydroxylamine at pH 7 and with 20% beta-mercaptoethanol indicating that the fatty acids are linked via a thioester bond. By thin layer chromatography, the vast majority of the incorporated label was identified as palmitic acid. Two cysteine residues at the boundary between transmembrane domain and cytoplasmic tail which could serve as acceptor sites were mutated to alanine residues by site-directed mutagenesis of the cloned Ad5-ADP gene. Expression of wild-type Ad5-ADP and the resulting mutants was performed in HeLa cells using the vaccinia virus T7 expression system. As demonstrated by labeling with [3H]palmitic acid, only the mutants with one remaining cysteine residue in the cytoplasmic tail were able to incorporate [3H]palmitic acid, indicating that either could serve as acceptor site. In contrast the double cysteine mutant could not be labeled by [3H]palmitic acid, clearly demonstrating that cysteines 53 and 54 are required for palmitoylation and probably represent the palmitoylation sites in Ad5-ADP.

The adenovirus death protein (E3-11.6K) is required at very late stages of infection for efficient cell lysis and release of adenovirus from infected cells

Adenovirus (Ad) infection is concluded by assembly of virions in the cell nucleus followed by lysis of cells by an unknown mechanism. We have described an Ad nuclear membrane glycoprotein of 11,600 kDa (E3-11.6K) which is encoded by the E3 transcription unit and which is synthesized in small amounts from the E3 promoter at early stages of infection but in large amounts from the major late promoter at very late stages of infection. We now report that E3-11.6K is required for the efficient lysis (death) of Ad-infected cells, and we propose that the function of E3-11.6K is to mediate the release of Ad progeny from infected cells. We have renamed E3-11.6K the Ad death protein (ADP). Virus mutants that lack ADP replicated as well as adp+ Ad, but the cells lysed more slowly, virus release from the cell was retarded, and the plaques were small and developed slowly. Cells infected with adp+ viruses began to lyse at 2 or 3 days postinfection (p.i.) and were completely lysed by 5 or 6 days p.i. In contrast, cells infected with adp mutants did not begin significant lysis until 5 or 6 days p.i. Cell lysis and viability were determined by plaque size, extracellular virus, cell morphology, release of lactate dehydrogenase, trypan blue exclusion, the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay for mitochondrial activity, RNA degradation, and DNA degradation as determined by agarose gel electrophoresis and the terminal deoxynucleotidyltransferase end labeling assay. Protein synthesis was almost nonexistent at 3 days p.i. in cells infected with adp+ Ads, but it was still increasing in cells infected with adp mutants. Host cell protein synthesis was undetectable at 1 day p.i. in cells infected with adp+ Ads or adp mutants. Cells infected with adp mutants showed Ad cytopathic effect at 1 or 2 days p.i. in that they rounded up and detached, but the cells remained metabolically active and intact for >5 days p.i. When examined by electron microscopy, the nuclei were extremely swollen and full of virus, and the nuclear membrane appeared to be intact. ADP is unrelated in sequence to other known cell death-promoting proteins.

The nucleotide sequence of adenovirus type 11 early 3 region: comparison of genome type Ad11p and Ad11a

The early 3 region (E3) of two strains (genome type Ad11p and Ad11a) of human adenovirus serotype 11, causing persistent urinary and acute respiratory illnesses, respectively, has been identified and partially sequenced. The sequenced E3 regions of Ad11p and Ad11a were 1980 and 1966 bp long and encoded three complete ORFs, 18.5, 20.3, 20.6k within the Ad11p genome and 18.5, 20.3, 20.2k within the Ad11a genome. The sequence analysis of the 18.5k gene product demonstrated that a transmembrane domain and a cytoplasmic domain of Ad11p, Ad11a, and Ad35 was identical. Ad11p and Ad35 were homologous in the signal sequence. There was one amino acid mismatch between Ad11p and Ad11a, represented by an alanine instead of a proline. The endoplasmic reticulum lumenal domain, which binds to class I MHC, was relatively conserved between Ad11p and Ad11a with the exception of Glu80 and Glu104 in Ad11p, which were replaced by Gln80 and Lys104 in Ad11a. Within the 20.2k protein of Ad11a, the amino acid sequence Thr-Thr-Ser-His was deleted from a position immediately upstream the transmembrane region of the Ad11p 20.6k protein. The 9.0k E3 open reading frame (ORF) of Ad3 was deleted in the genomes of Ad11p and Ad11a. It is noteworthy that Ad11p and Ad35 which both cause persistent infection of the urinary tract display a remarkable similarity in several ORFs of the E3 region.

A 12,500 MW protein is coded by region E3 of adenovirus

There is an open reading frame between ATG291 and TGA612 in the early region E3 transcription unit of adenovirus 2 (Ad2) that could encode a protein of 12,500 MW (12.5K). To address whether this protein is synthesized, we generated an antiserum against a TrpE-12.5K fusion protein which was expressed in Escherichia coli. This antiserum immunoprecipitated a doublet of about 12.5K apparent MW from [35S]Cys-labeled cells infected with Ad2, Ad5, and various mutants in other E3 genes. Mutants in the 12.5K gene did not produce this protein, and an in-frame deletion mutant showed a protein with a corresponding decrease in size. Cell-free translation of hybridization-purified RNA indicated that 12.5K is coded by E3 mRNA i. mRNA i is relatively scarce, and 12.5K is synthesized in correspondingly small amounts. The 12.5K protein was synthesized at early and late stages of infection in comparable amounts. Pulse-chase experiments indicated that 12.5K has a half-life of about 10 hr. The function of 12.5K is unknown, and the 12.5K gene can be deleted without affecting virus growth in cell culture. However, 12.5K is likely to be important in vivo because the gene is highly conserved in both Ad2 and Ad5 (group C adenoviruses), and also in Ad3 (group B).

The adenovirus E3 14.5-kilodalton protein, which is required for down-regulation of the epidermal growth factor receptor and prevention of tumor necrosis factor cytolysis, is an integral membrane protein oriented with its C terminus in the cytoplasm

We previously reported that the adenovirus type 5 E3 14.5-kilodalton protein (14.5K) forms a complex with E3 10.4K and that both proteins are required to down-regulate the epidermal growth factor receptor in adenovirus-infected human cells. Both proteins are also required to prevent cytolysis by tumor necrosis factor of most mouse cell lines infected by adenovirus mutants that lack E3 14.7K. The E3 14.5K amino acid sequence suggests that 14.5K is an integral membrane protein with an N-terminal signal sequence for membrane insertion. Here we show that 14.5K was found exclusively in cytoplasmic membrane fractions. Radiochemical sequencing of 14.5K indicated that the N-terminal signal sequence is cleaved predominantly between Cys-18 and Ser-19. With a mutant that does not express 10.4K, cleavage occurs predominantly between Phe-17 and Cys-18, indicating that the presence or absence of 10.4K affects the signal cleavage site. 14.5K was extracted into the detergent phase with Triton X-114, it remained associated with membranes after extraction with Na2CO3 at pH 11.5, and it was partially protected by membranes from proteinase K digestion; these observations indicate that 14.5K is an integral membrane protein. Proteinase K digestion followed by immunoprecipitation with antipeptide antisera directed against the N or C terminus of mature 14.5K indicated that 14.5K is oriented in the membrane with its N terminus in the lumen and its C terminus in the cytoplasm. Thus, 14.5K is a type I bitopic membrane protein. Previous studies indicated that 10.4K is also an integral membrane protein oriented with its C terminus in the cytoplasm. Altogether, these findings suggest that cytoplasmic membranes are the site of action when 10.4K and 14.5K down-regulate the epidermal growth factor receptor and prevent tumor necrosis factor cytolysis.

The 10,400- and 14,500-dalton proteins encoded by region E3 of adenovirus function together to protect many but not all mouse cell lines against lysis by tumor necrosis factor

We have reported that the E3 14,700-dalton protein (E3 14.7K protein) protects adenovirus-infected mouse C3HA fibroblasts against lysis by tumor necrosis factor (TNF) (L. R. Gooding, L. W. Elmore, A. E. Tollefson, H. A. Brady, and W. S. M. Wold, Cell 53:341-346, 1988). We have also observed that the E1B 19K protein protects adenovirus-infected human but not mouse cells against TNF lysis (L. R. Gooding, L. Aquino, P. J. Duerksen-Hughes, D. Day, T. M. Horton, S. Yei, and W. S. M. Wold, J. Virol. 65:3083-3094, 1991). We now report that, in the absence of E3 14.7K, the E3 10.4K and E3 14.5K proteins are both required to protect C127 as well as several other mouse cell lines against TNF lysis. The 14.7K protein can also protect these cells from TNF in the absence of the 10.4K and 14.5K proteins. This protection by the 10.4K and 14.5K proteins was not observed in the C3HA cell line. These conclusions are based on 51Cr release assays of cells infected with virus E3 mutants that express the 14.7K protein alone, that express both the 10.4K and 14.5K proteins, and that delete the 14.7K in combination with either the 10.4K or 14.5K protein. The 10.4K protein was efficiently coimmunoprecipitated together with the 14.5K protein by using an antiserum to the 14.5K protein, suggesting that the 10.4K and 14.5K proteins exist as a complex in the infected mouse cells and consistent with the notion that they function in concert. Considering that three sets of proteins (E3 14.7K, E1B 19K, and E3 10.4K/14.5K proteins) exist in adenovirus to prevent TNF cytolysis of different cell types, it would appear that TNF is a major antiadenovirus defense of the host.

The E1B 19,000-molecular-weight protein of group C adenoviruses prevents tumor necrosis factor cytolysis of human cells but not of mouse cells

Tumor necrosis factor (TNF) is a multifunctional immunoregulatory protein that is secreted by activated macrophages and is believed to have antiviral activities. We reported earlier that when mouse C3HA fibroblasts are infected with human adenoviruses, the 289R and 243R proteins encoded by region E1A render the cells susceptible to lysis by TNF, and a 14,700-molecular-weight protein (14.7K protein) encoded by region E3 protects the cells against lysis by TNF. We now report that the 19,000-molecular-weight (19K) (176R) protein encoded by the E1B transcription unit can protect human HEL-299 fibroblasts and human ME-180 cervical carcinoma cells against lysis by TNF. This was determined by infecting cells with adenovirus double mutants that lack region E3 and do or do not express the E1B-19K protein and by measuring cytolysis by using a short-term (18-h) 51Cr-release assay. Under these assay conditions, the 51Cr release was specific to TNF and was not a consequence of the cyt phenotype associated with E1B-19K protein-negative mutants. Also, by using virus double mutants that lack E3 in combination with other early regions, we found that E1A, the E1B-55K protein-encoding gene, E3, and E4 are not required to protect HEL-299 cells against TNF cytolysis. Three additional human cancer cell lines (HeLa, HCT8, and RC29) and a simian virus 40-transformed WI38 cell line (VA-13) also required E1B for protection against TNF cytolysis, indicating that the E1B-19K protein is required to protect many if not all human cell types against lysis by TNF when infected by adenovirus. The E1B-19K protein was not able to protect six different adenovirus-infected mouse cell lines against TNF lysis, even though the protein was shown to be efficiently expressed in one of the cell lines. HEL-299 or ME-180 cells infected by a mutant that lacks the E1B-19K protein but retains region E3 were not lysed by TNF, indicating that one or more of the E3 proteins can protect these cells against TNF lysis in the absence of the E1B-19K protein. Thus, the E3-14.7K but not the E1B-19K protein can protect adenovirus-infected mouse cells against TNF cytolysis, whereas the E1B-19K protein as well as one or more of the E3 proteins can protect adenovirus-infected human cells against TNF cytolysis.

A 6700 MW membrane protein is encoded by region E3 of adenovirus type 2

There is an open reading frame between ATG1022 and TGA1205 in the E3 transcription unit of adenovirus 2 that could encode a protein of MW 6700 (6.7K) (61 amino acids). To address whether this protein is expressed, we prepared an antiserum against a synthetic peptide corresponding to residues 47-61 in the 6.7K protein. This antiserum immunoprecipitated two series of protein bands, a 7K-8K doublet and a 15K-16K doublet or triplet, as observed by electrophoresis on 10-18% gradient SDS-polyacrylamide gels. These bands were not obtained from cells infected with mutants that lack the 6.7K gene. Most, if not all, of the 7K-8K and 15K-16K bands were detected by immunoblot, indicating that they are modified versions of the 6.7K protein. Only an 8K band was observed after cell-free translation of hybridization-purified mRNA, suggesting that this may be the primary translation product. As judged by DNA sequence, the 6.7K protein has a hydrophobic domain of at least 22 residues (residues 16-37), suggesting that 6.7K may be a membrane protein. Consistent with this, the 7K-8K and 15K-16K bands were observed in the crude membrane but not the cytosol or nuclear fractions of biochemically fractionated cells. The 6.7K protein was underproduced by mutants which underproduce E3 mRNAs a and c, indicating that 6.7K is translated from these mRNAs. Since the E3-gp 19K protein is also translated from mRNAs a and c, these mRNAs are bicistronic. The 6.7K protein is well-conserved in Ad5 (Ad2 and Ad5 are group C adenoviruses), and appears to be marginally conserved in Ad3 (group B).

Transcription mapping of mouse adenovirus type 1 early region 3

Early region 3 (E3) of mouse adenovirus type 1 was analyzed using S1 nuclease protection and primer extension assays, cDNA sequencing, and genomic sequencing. We present the genomic sequence from 79 to 83 map units of the viral genome, the precise ends and splice sites of the E3 mRNAs, and the predicted protein sequence encoded by the mRNAs. Three major classes of early mRNAs were identified; all were approximately 1 kb long, consisted of three exons, and shared 5' and 3' ends. The three classes had alternative splicing at the junction between the second and third exon. The three proteins predicted by the three mRNAs were slightly similar to the E3 19K glycoprotein of human adenovirus type 3; the longest of the three was the most similar. Open reading frames corresponding to late proteins were also identified in the translated mouse adenovirus type 1 DNA sequence. In mouse adenovirus, as in the human adenoviruses, L4 overlaps E3, and L5 starts just downstream of the E3 region.

A protein serologically and functionally related to the group C E3 14,700-kilodalton protein is found in multiple adenovirus serotypes

A 14.7-kilodalton protein (14.7K protein) encoded by the E3 region of group C adenoviruses has been shown to protect virus-infected fibroblasts from lysis by tumor necrosis factor (TNF) (L.R. Gooding, L.W. Elmore, A.E. Tollefson, H.A. Brady, and W.S.M. Wold, Cell 53:341-346, 1988). In this study we show that adenoviruses of other groups are also protected from TNF-induced cytolysis. Representative serotypes of groups A, B, D, and E produce a protein analogous to the 14.7K protein found in human group C adenoviruses. Deletion of this protein in group C viruses permits virus infection to induce cellular susceptibility to TNF killing. As with group C adenoviruses, cells infected with wild-type adenoviruses of other serotypes are not killed by TNF and are protected from lysis induced by TNF plus cycloheximide. However, cells are susceptible to TNF-induced lysis when infected with adenovirus type 4 mutants from which the 14.7K gene has been deleted. Although all known adenovirus serotypes infect epithelial cells, adenoviruses cause several diseases with various degrees of pathogenesis. Our findings suggest that the 14.7K protein provides a function required for the in vivo cytotoxicity of many adenoviruses independent of the site of infection or degree of pathogenesis.

A 14,500 MW protein is coded by region E3 of group C human adenoviruses

There is an ORF in the early region E3 transcription unit of human adenovirus 5 (Ad5) which could encode a protein of 14,500 MW (14.5K). This ORF is conserved in Ad5 and Ad2, both group C adenoviruses, and also in Ad3 and Ad7, both group B adenoviruses. To address whether the 14.5K protein is synthesized, we prepared antisera against synthetic peptides corresponding to residues 19-34 or 118-132 in the Ad5 version of 14.5K, and also against a TrpE-14.5K fusion protein expressed in Escherichia coli. These antisera immunoprecipitated the [35S]Met-labeled 14.5K protein from KB cells infected with rec700 (an Ad5-Ad2-Ad5 recombinant), Ad2, and a variety of E3 mutants. Mutants in the 14.5K ORF did not produce the 14.5K protein. The 14.5K is coded in large part, although probably not exclusively, by E3 mRNA f, as indicated by immunoprecipitation of 14.5K from cells infected with mutants that overproduce or underproduce mRNA f. The 14.5K migrated as five to six bands on SDS-PAGE after immunoprecipitation or Western blot, suggesting that it undergoes post-translational modification. Two bands of 14.5K were obtained by cell-free translation of 14.5K from mRNA purified by hybridization from infected cells.

A 10,400-molecular-weight membrane protein is coded by region E3 of adenovirus

Previous studies with adenovirus mutants have indicated that a 10,400-molecular-weight (10.4K) protein predicted to be coded by an open reading frame in region E3 of adenovirus functions to down regulate the epidermal growth factor receptor (C. R. Carlin, A. E. Tollefson, H. A. Brady, B. L. Hoffman, and W. S. M. Wold, Cell 57:135-144, 1989). We now demonstrate that the 10.4K protein is in fact synthesized in cells infected by group C adenoviruses. This was done by immunoprecipitation of 10.4K from cells infected by a variety of E3 mutants, using antisera against three different synthetic peptides corresponding to the predicted 10.4K sequence. The 10.4K protein was translated primarily from E3 mRNA f, as indicated by cell-free translation of mRNA purified by hybridization from cells infected with an RNA processing mutant that synthesizes predominantly mRNA f. The 10.4K protein was overproduced or underproduced in vivo, respectively, by mutants that overproduce or underproduce E3 mRNA f, also indicating that the 10.4K protein is translated primarily from mRNA f. The 10.4K protein migrated as two bands with apparent molecular weights of 16,000 and 11,000 (10 to 18% gradient gels); both bands contained 10.4K epitopes, as shown by Western blot (immunoblot). Only the 16K band was obtained by cell-free translation, suggesting that the 16K protein is the precursor to the 11K protein. The 10.4K protein is a membrane protein, as shown by cell fractionation experiments and as predicted from its sequence. The predicted 10.4K sequence as well as a putative N-terminal signal sequence and 30-residue transmembrane domain are conserved in adenovirus types 2 and 5 (group C) and in types 3, 7, and 35 (group B).

Role of early region 3 (E3) in pathogenesis of adenovirus disease

The cotton rat Sigmodon hispidus has provided an animal model of adenovirus pneumonia that permits investigation of the viral gene products required to produce the disease and the molecular mechanisms effecting the damage. This study was carried out to test the hypothesis that early region 3 (E3) of the adenovirus genome plays a critical role in pathogenesis of the virus's disease process even though none of its gene products are essential for its replication. Mutants whose E3 region is largely deleted (i.e., H2dl801 and H5dl327) replicated like wild-type virus in the cotton rats' lungs, but the lymphocyte and macrophage/monocyte inflammatory response was markedly increased. Viruses containing mutations that ablated production of the 19-kDa glycoprotein had the same effect as H2dl801 and H5dl327. However, mutants with deletions in the other E3 open reading frames, some of which encode known proteins, did not differ from wild-type virus in their pathogenic properties. The 19-kDa glycoprotein markedly reduces expression of the class I major histocompatibility complex antigens on the surface of infected cells. A complete correlation was found between those mutants that had increased pathogenic effects and those that lost the ability to reduce transport of the class I major histocompatibility complex antigens to surface of infected cells (i.e., all mutants unable to express the 19-kDa glycoprotein). H5sub304, which has a deletion between 83.2 and 85.1 map units in the E3B region and expresses the 19-kDa glycoprotein, did not increase the extent of pneumonia but qualitatively changed the inflammatory response in that increased numbers of polymorphonuclear leukocytes accumulated, often in small foci.

Epidermal growth factor receptor is down-regulated by a 10,400 MW protein encoded by the E3 region of adenovirus

Epidermal growth factor (EGF) binds to specific high affinity receptors (EGF-Rs) and induces endosome-specific internalization and degradation of ligand-receptor complexes in lysosomes. We report here that EGF-R is down-regulated in an analogous manner during early infection of a variety of cell types by group C human adenoviruses. This effect is not a function of viral entry, nor is it due to a nonspecific increase in turnover of membrane proteins. Using a series of virus deletion mutants, the gene responsible for EGF-R down-regulation was mapped to the E3 transcription unit. The E3 gene product, a protein of MW 10,400 (10.4K), induces internalization and degradation of EGF-R, but does not affect synthesis of the EGF-R precursor. The 10.4K protein is not an EGF-like autocrine growth factor, but is similar in sequence to a region in EGF-R at the cytoplasmic face of the transmembrane domain. This suggests that down-regulation of EGF-R during adenovirus infection may occur by a novel mechanism that involves the formation of hetero-oligomers composed of 10.4K and EGF-R.

A 14,700 MW protein from the E3 region of adenovirus inhibits cytolysis by tumor necrosis factor

We find that cells infected with wild-type group C human adenoviruses are not killed by exposure to tumor necrosis factor (TNF), but cells infected with adenoviruses that delete the E3 transcription unit are highly sensitive to TNF lysis. Mock-infected cells are resistant to TNF. Thus, adenovirus infection induces cellular susceptibility to lysis by TNF, and a product of E3 protects against lysis by TNF. The E3-dependent resistance to TNF was investigated using virus mutants that delete different segments of E3. Resistance was found to depend on the presence of a 14,700 MW protein, which has only recently been identified and for which there was no known function. Our results support the hypothesis that one of the functions of TNF in vivo is to combat virus infections, and that the 14,700 MW protein evolved in adenovirus to counteract the antiviral effects of TNF.