Evidence for intracellular down-regulation of the epidermal growth factor (EGF) receptor during adenovirus infection by an EGF-independent mechanism

Role of EGF Receptor Regulatory Networks in the Host Response to Viral Infections

In this review article, we will first provide a brief overview of EGF receptor (EGFR) structure and function, and its importance as a therapeutic target in epithelial carcinomas. We will then compare what is currently known about canonical EGFR trafficking pathways that are triggered by ligand binding, versus ligand-independent pathways activated by a variety of intrinsic and environmentally induced cellular stresses. Next, we will review the literature regarding the role of EGFR as a host factor with critical roles facilitating viral cell entry and replication. Here we will focus on pathogens exploiting virus-encoded and endogenous EGFR ligands, as well as EGFR-mediated trafficking and signaling pathways that have been co-opted by wild-type viruses and recombinant gene therapy vectors. We will also provide an overview of a recently discovered pathway regulating non-canonical EGFR trafficking and signaling that may be a common feature of viruses like human adenoviruses which signal through p38-mitogen activated protein kinase. We will conclude by discussing the emerging role of EGFR signaling in innate immunity to viral infections, and how viral evasion mechanisms are contributing to our understanding of fundamental EGFR biology.

Adenovirus early region 3 RIDα protein limits NFκB signaling through stress-activated EGF receptors

The host limits adenovirus infections by mobilizing immune systems directed against infected cells that also represent major barriers to clinical use of adenoviral vectors. Adenovirus early transcription units encode a number of products capable of thwarting antiviral immune responses by co-opting host cell pathways. Although the EGF receptor (EGFR) was a known target for the early region 3 (E3) RIDα protein encoded by nonpathogenic group C adenoviruses, the functional role of this host-pathogen interaction was unknown. Here we report that incoming viral particles triggered a robust, stress-induced pathway of EGFR trafficking and signaling prior to viral gene expression in epithelial target cells. EGFRs activated by stress of adenoviral infection regulated signaling by the NFκB family of transcription factors, which is known to have a critical role in the host innate immune response to infectious adenoviruses and adenovirus vectors. We found that the NFκB p65 subunit was phosphorylated at Thr254, shown previously by other investigators to be associated with enhanced nuclear stability and gene transcription, by a mechanism that was attributable to ligand-independent EGFR tyrosine kinase activity. Our results indicated that the adenoviral RIDα protein terminated this pathway by co-opting the host adaptor protein Alix required for sorting stress-exposed EGFRs in multivesicular endosomes, and promoting endosome-lysosome fusion independent of the small GTPase Rab7, in infected cells. Furthermore RIDα expression was sufficient to down-regulate the same EGFR/NFκB signaling axis in a previously characterized stress-activated EGFR trafficking pathway induced by treatment with the pro-inflammatory cytokine TNF-α. We also found that cell stress activated additional EGFR signaling cascades through the Gab1 adaptor protein that may have unappreciated roles in the adenoviral life cycle. Similar to other E3 proteins, RIDα is not conserved in adenovirus serotypes associated with potentially severe disease, suggesting stress-activated EGFR signaling may contribute to adenovirus virulence.

Although most adenovirus infections produce a mild and self-limiting disease, they can be life threatening for immunocompromised individuals. Some serotypes also cause epidemic outbreaks that pose a significant health risk in people with no known predisposing conditions. Although the early region 3 (E3) of the adenovirus genome is known to play a critical role in viral pathogenesis, experimental evidence regarding the molecular mechanisms effecting damage in the host is still limited. Here we provide the first studies showing that adenovirus infection induced stress-activated EGF receptor (EGFR) pro-inflammatory signaling prior to nuclear translocation and transcription of viral DNA in non-immune epithelial target cells. We have also identified host molecular mechanisms co-opted by the E3 RIDα protein that potentially limit immune-mediated tissue injury caused by stress-activated EGFR. There is increasing evidence that many viruses exploit EGFR function to facilitate their replication and antagonize host antiviral responses. Until now, it was generally assumed that viruses co-opted mechanisms induced by conventional ligand-regulated pathways. Recognition that stress-activated EGFR signaling may play a critical role in viral pathogenesis is significant because unique host proteins regulating this pathway represent novel drug targets for therapeutic development.

New Insights to Adenovirus-Directed Innate Immunity in Respiratory Epithelial Cells

The nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB) family of transcription factors is a key component of the host innate immune response to infectious adenoviruses and adenovirus vectors. In this review, we will discuss a regulatory adenoviral protein encoded by early region 3 (E3) called E3-RIDα, which targets NFκB through subversion of novel host cell pathways. E3-RIDα down-regulates an EGF receptor signaling pathway, which overrides NFκB negative feedback control in the nucleus, and is induced by cell stress associated with viral infection and exposure to the pro-inflammatory cytokine TNF-α. E3-RIDα also modulates NFκB signaling downstream of the lipopolysaccharide receptor, Toll-like receptor 4, through formation of membrane contact sites controlling cholesterol levels in endosomes. These innate immune evasion tactics have yielded unique perspectives regarding the potential physiological functions of host cell pathways with important roles in infectious disease.

Human Adenoviruses, Cholesterol Trafficking, and NF-κB Signaling

The interplay between viruses and host factors regulating inflammatory or cytotoxic responses directed against infected cells is well documented. Viruses have evolved a wide array of mechanisms that strike a balance between the elimination of virus and immune-mediated tissue injury by antiviral immune responses. The topic of this mini-review is a series of recent studies demonstrating a link between cholesterol trafficking and innate immune responses in cells infected with human adenoviruses that provide the backbone of commonly used vectors in gene medicine. Besides revealing an unexpected role for lipid metabolism in immune evasion, these studies have important implications for understanding the molecular basis of cholesterol trafficking in normal cells and various disease states. They also describe a previously unappreciated host-virus interaction that may be employed by other pathogens to interfere with the host innate immune system.

Adenovirus Modulates Toll-Like Receptor 4 Signaling by Reprogramming ORP1L-VAP Protein Contacts for Cholesterol Transport from Endosomes to the Endoplasmic Reticulum

Human adenoviruses (Ads) generally cause mild self-limiting infections but can lead to serious disease and even be fatal in high-risk individuals, underscoring the importance of understanding how the virus counteracts host defense mechanisms. This study had two goals. First, we wished to determine the molecular basis of cholesterol homeostatic responses induced by the early region 3 membrane protein RIDα via its direct interaction with the sterol-binding protein ORP1L, a member of the evolutionarily conserved family of oxysterol-binding protein (OSBP)-related proteins (ORPs). Second, we wished to determine how this interaction regulates innate immunity to adenovirus. ORP1L is known to form highly dynamic contacts with endoplasmic reticulum-resident VAP proteins that regulate late endosome function under regulation of Rab7-GTP. Our studies have demonstrated that ORP1L-VAP complexes also support transport of LDL-derived cholesterol from endosomes to the endoplasmic reticulum, where it was converted to cholesteryl esters stored in lipid droplets when ORP1L was bound to RIDα. The virally induced mechanism counteracted defects in the predominant cholesterol transport pathway regulated by the late endosomal membrane protein Niemann-Pick disease type C protein 1 (NPC1) arising during early stages of viral infection. However, unlike NPC1, RIDα did not reconstitute transport to endoplasmic reticulum pools that regulate SREBP transcription factors. RIDα-induced lipid trafficking also attenuated proinflammatory signaling by Toll-like receptor 4, which has a central role in Ad pathogenesis and is known to be tightly regulated by cholesterol-rich "lipid rafts." Collectively, these data show that RIDα utilizes ORP1L in a way that is distinct from its normal function in uninfected cells to fine-tune lipid raft cholesterol that regulates innate immunity to adenovirus in endosomes.IMPORTANCE Early region 3 proteins encoded by human adenoviruses that attenuate immune-mediated pathology have been a particularly rich source of information regarding intracellular protein trafficking. Our studies with the early region 3-encoded RIDα protein also provided fundamental new information regarding mechanisms of nonvesicular lipid transport and the flow of molecular information at membrane contacts between different organelles. We describe a new pathway that delivers cholesterol from endosomes to the endoplasmic reticulum, where it is esterified and stored in lipid droplets. Although lipid droplets are attracting renewed interest from the standpoint of normal physiology and human diseases, including those resulting from viral infections, experimental model systems for evaluating how and why they accumulate are still limited. Our studies also revealed an intriguing relationship between lipid droplets and innate immunity that may represent a new paradigm for viruses utilizing these organelles.

Open reading frame E3-10.9K of subspecies B1 human adenoviruses encodes a family of late orthologous proteins that vary in their predicted structural features and subcellular localization

Subspecies B1 human adenoviruses (HAdV-B1s) are important causative agents of acute respiratory disease, but the molecular bases of their distinct pathobiology are still poorly understood. Marked differences in genetic content between HAdV-B1s and the well-characterized HAdV-Cs that may contribute to distinct pathogenic properties map to the E3 region. Between the highly conserved E3-19K and E3-10.4K/RIDα open reading frames (ORFs), and in the same location as the HAdV-C ADP/E3-11.6K ORF, HAdV-B1s carry ORFs E3-20.1K and E3-20.5K and a polymorphic third ORF, designated E3-10.9K, that varies in the size of its predicted product among HAdV-B1 serotypes and genomic variants. As an initial effort to define the function of the E3-10.9K ORF, we carried out a biochemical characterization of E3-10.9K-encoded orthologous proteins and investigated their expression in infected cells. Sequence-based predictions suggested that E3-10.9K orthologs with a hydrophobic domain are integral membrane proteins. Ectopically expressed, C-terminally tagged (with enhanced green fluorescent protein [EGFP]) E3-10.9K and E3-9K localized primarily to the plasma membrane, while E3-7.7K localized primarily to a juxtanuclear compartment that could not be identified. EGFP fusion proteins with a hydrophobic domain were N and O glycosylated. EGFP-tagged E3-4.8K, which lacked the hydrophobic domain, displayed diffuse cellular localization similar to that of the EGFP control. E3-10.9K transcripts from the major late promoter were detected at late time points postinfection. A C-terminally hemagglutinin-tagged version of E3-9K was detected by immunoprecipitation at late times postinfection in the membrane fraction of mutant virus-infected cells. These data suggest a role for ORF E3-10.9K-encoded proteins at late stages of HAdV-B1 replication, with potentially important functional implications for the documented ORF polymorphism.

Adenovirus RID-alpha activates an autonomous cholesterol regulatory mechanism that rescues defects linked to Niemann-Pick disease type C

Host-pathogen interactions are important model systems for understanding fundamental cell biological processes. In this study, we describe a cholesterol-trafficking pathway induced by the adenovirus membrane protein RID-alpha that also subverts the cellular autophagy pathway during early stages of an acute infection. A palmitoylation-defective RID-alpha mutant deregulates cholesterol homeostasis and elicits lysosomal storage abnormalities similar to mutations associated with Niemann-Pick type C (NPC) disease. Wild-type RID-alpha rescues lipid-sorting defects in cells from patients with this disease by a mechanism involving a class III phosphatidylinositol-3-kinase. In contrast to NPC disease gene products that are localized to late endosomes/lysosomes, RID-alpha induces the accumulation of autophagy-like vesicles with a unique molecular composition. Ectopic RID-alpha regulates intracellular cholesterol trafficking at two distinct levels: the egress from endosomes and transport to the endoplasmic reticulum necessary for homeostatic gene regulation. However, RID-alpha also induces a novel cellular phenotype, suggesting that it activates an autonomous cholesterol regulatory mechanism distinct from NPC disease gene products.

Pregnancy-upregulated nonubiquitous calmodulin kinase induces ligand-independent EGFR degradation

We describe here an important function of the novel calmodulin kinase I isoform, pregnancy-upregulated nonubiquitous calmodulin kinase (Pnck). Pnck (also known as CaM kinase Ibeta(2)) was previously shown to be differentially overexpressed in a subset of human primary breast cancers, compared with benign mammary epithelial tissue. In addition, during late pregnancy, Pnck mRNA was shown to be strongly upregulated in epithelial cells of the mouse mammary gland exhibiting decreased proliferation and terminal differentiation. Pnck mRNA is also significantly upregulated in confluent and serum-starved cells, compared with actively growing proliferating cells (Gardner HP, Seung HI, Reynolds C, Chodosh LA. Cancer Res 60: 5571-5577, 2000). Despite these suggestive data, the true physiological role(s) of, or the signaling mechanism(s) regulated by Pnck, remain unknown. We now report that epidermal growth factor receptor (EGFR) levels are significantly downregulated in a ligand-independent manner in human embryonic kidney-293 (HEK-293) cells overexpressing Pnck. MAP kinase activation was strongly inhibited by EGFR downregulation in the Pnck-overexpressing cells. The EGFR downregulation was not the result of reduced transcription of the EGFR gene but from protea-lysosomal degradation of EGFR protein. Knockdown of endogenous Pnck mRNA levels by small interfering RNA transfection in human breast cancer cells resulted in upregulation of unliganded EGFR, consistent with the effects observed in the overexpression model of Pnck-mediated ligand-independent EGFR downregulation. Pnck thus emerges as a new component of the poorly understood mechanism of ligand-independent EGFR degradation, and it may represent an attractive therapeutic target in EGFR-regulated oncogenesis.

Adenovirus RIDalpha regulates endosome maturation by mimicking GTP-Rab7

The small guanosine triphosphatase Rab7 regulates late endocytic trafficking. Rab7-interacting lysosomal protein (RILP) and oxysterol-binding protein-related protein 1L (ORP1L) are guanosine triphosphate (GTP)-Rab7 effectors that instigate minus end-directed microtubule transport. We demonstrate that RILP and ORP1L both interact with the group C adenovirus protein known as receptor internalization and degradation alpha (RIDalpha), which was previously shown to clear the cell surface of several membrane proteins, including the epidermal growth factor receptor and Fas (Carlin, C.R., A.E. Tollefson, H.A. Brady, B.L. Hoffman, and W.S. Wold. 1989. Cell. 57:135-144; Shisler, J., C. Yang, B. Walter, C.F. Ware, and L.R. Gooding. 1997. J. Virol. 71:8299-8306). RIDalpha localizes to endocytic vesicles but is not homologous to Rab7 and is not catalytically active. We show that RIDalpha compensates for reduced Rab7 or dominant-negative (DN) Rab7(T22N) expression. In vitro, Cu(2+) binding to RIDalpha residues His75 and His76 facilitates the RILP interaction. Site-directed mutagenesis of these His residues results in the loss of RIDalpha-RILP interaction and RIDalpha activity in cells. Additionally, expression of the RILP DN C-terminal region hinders RIDalpha activity during an acute adenovirus infection. We conclude that RIDalpha coordinates recruitment of these GTP-Rab7 effectors to compartments that would ordinarily be perceived as early endosomes, thereby promoting the degradation of selected cargo.

A tyrosine-based signal plays a critical role in the targeting and function of adenovirus RIDalpha protein

Early region 3 genes of human adenoviruses contribute to the virus life cycle by altering the trafficking of cellular proteins involved in adaptive immunity and inflammatory responses. The ability of early region 3 genes to target specific molecules suggests that they could be used to curtail pathological processes associated with these molecules and treat human disease. However, this approach requires genetic dissection of the multiple functions attributed to early region 3 genes. The purpose of this study was to determine the role of targeting on the ability of the early region 3-encoded protein RIDalpha to downregulate the EGF receptor. A fusion protein between the RIDalpha cytoplasmic tail and glutathione S-transferase was used to isolate clathrin-associated adaptor 1 and adaptor 2 protein complexes from mammalian cells. Deletion and site-directed mutagenesis studies showed that residues 71-AYLRH of RIDalpha are necessary for in vitro binding to both adaptor complexes and that Tyr72 has an important role in these interactions. In addition, RIDalpha containing a Y72A point mutation accumulates in the trans-Golgi network and fails to downregulate the EGF receptor when it is introduced into mammalian cells as a transgene. Altogether, our data suggest a model where RIDalpha is trafficked directly from the trans-Golgi network to an endosomal compartment, where it intercepts EGF receptors undergoing constitutive recycling to the plasma membrane and redirects them to lysosomes.

A novel dileucine lysosomal-sorting-signal mediates intracellular EGF-receptor retention independently of protein ubiquitylation

One of the main goals of this study was to understand the relationship between an epidermal growth factor (EGF) receptor dileucine (LL)-motif (679-LL) required for lysosomal sorting and the protein ubiquitin ligase CBL. We show that receptors containing 679-AA (di-alanine) substitutions that are defective for ligand-induced degradation nevertheless bind CBL and undergo reversible protein ubiquitylation similar to wild-type receptors. We also demonstrate that 679-LL but not CBL is required for EGF receptor downregulation by an endosomal membrane protein encoded by human adenoviruses that uncouples internalization from post-endocytic sorting to lysosomes. 679-LL is necessary for endosomal retention as well as degradation by the adenovirus protein, and is also transferable to reporter molecules. Using NMR spectroscopy, we show that peptides with wild-type 679-LL or mutant 679-AA sequences both exhibit alpha-helical structural propensities but that this structure is not stable in water. A similar analysis carried out in hydrophobic media showed that the alpha-helical structure of the wild-type peptide is stabilized by specific interactions mediated by side-chains in both leucine residues. This structure distinguishes 679-LL from other dileucine-based sorting-signals with obligatory amino-terminal acidic residues that are recognized in the form of an extended beta or beta-like conformation. Taken together, these data show that 679-LL is an alpha-helical stabilizing motif that regulates a predominant step during lysosomal sorting, involving intracellular retention under both sub-saturating and saturating conditions.

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.

E3-13.7 integral membrane proteins encoded by human adenoviruses alter epidermal growth factor receptor trafficking by interacting directly with receptors in early endosomes

Animal cell viruses provide valuable model systems for studying many normal cellular processes, including membrane protein sorting. The focus of this study is an integral membrane protein encoded by the E3 transcription region of human adenoviruses called E3-13.7, which diverts recycling EGF receptors to lysosomes without increasing the rate of receptor internalization or intrinsic receptor tyrosine kinase activity. Although E3-13.7 can be found on the plasma membrane when it is overexpressed, its effect on EGF receptor trafficking suggests that the plasma membrane is not its primary site of action. Using cell fractionation and immunocytochemical experimental approaches, we now report that the viral protein is located predominantly in early endosomes and limiting membranes of endosome-to-lysosome transport intermediates called multivesicular endosomes. We also demonstrate that E3-13.7 physically associates with EGF receptors undergoing E3-13.7-mediated down-regulation in early endosomes. Receptor-viral protein complexes then dissociate, and EGF receptors proceed to lysosomes, where they are degraded, while E3-13.7 is retained in endosomes. We conclude that E3-13.7 is a resident early endocytic protein independent of EGF receptor expression, because it has identical intracellular localization in mouse cells lacking endogenous receptors and cells expressing a human cytomegalovirus-driven receptor cDNA. Finally, we demonstrate that EGF receptor residues 675-697 are required for E3-13.7-mediated down-regulation. Interestingly, this sequence includes a known EGF receptor leucine-based lysosomal sorting signal used during ligand-induced trafficking, which is also conserved in the viral protein. E3-13.7, therefore, provides a novel model system for determining the molecular basis of selective membrane protein transport in the endocytic pathway. Our studies also suggest new paradigms for understanding EGF receptor sorting in endosomes and adenovirus pathogenesis.

The adenovirus E3-10.4K/14.5K complex mediates loss of cell surface Fas (CD95) and resistance to Fas-induced apoptosis

Cytotoxic T cells use Fas (CD95), a member of the tumor necrosis factor (TNF) receptor superfamily, to eliminate virus-infected cells by activation of the apoptotic pathway for cell death. The adenovirus E3 region encodes several proteins that modify immune defenses, including TNF-dependent cell death, which may allow this virus to establish a persistent infection. Here we show that, as an early event during infection, the adenovirus E3-10.4K/14.5K complex selectively induces loss of Fas surface expression and blocks Fas-induced apoptosis of virus-infected cells. Loss of surface Fas occurs within the first 4 h postinfection and is not due to decreased production of Fas protein. The decrease in surface Fas is distinct from the 10.4K/14.5K-mediated loss of the epidermal growth factor receptor on the same cells, because intracellular stores of Fas are not affected. Further, 10.4K/14.5K, which was previously shown to protect against TNF cytolysis, does not induce a loss of TNF receptor, indicating that this complex mediates more than one function to block host defense mechanisms. These results suggest yet another mechanism by which adenovirus modulates host cytotoxic responses that may contribute to persistent infection by human adenoviruses.

Signalling by Src family kinases: lessons learnt from DNA tumour viruses

Virus replication and spreading in a host population depends on highly specific interactions of viral proteins with infected cells, resulting in subversion of multiple cellular signal transduction pathways. For instance, viral proteins cause cell cycle progression of the infected host cell in order to establish a cellular environment favourable for virus replication. Of equal importance for successful virus propagation is virus-mediated attenuation of a host's immune response. Many of the pathways controlling these aspects of cell behaviour are regulated by cellular tyrosine kinases. One particular family of these enzymes, Src family kinases, are involved in processing signals emanating from the plasma membrane upon stimulation by growth factors, by cell-substratum or by cell-cell contact. Two families of DNA viruses, polyoma- and herpesviruses, encode proteins targeted at tyrosine kinases. The middle-T antigens expressed by mouse and hamster polyomavirus associate with and activate Src family tyrosine kinases. Two members of the herpes family of DNA viruses, Epstein-Barr virus (EBV) and herpesvirus saimiri (HVS), encode proteins, LMP2A and Tip, respectively, that associate with cellular tyrosine kinases of the Src and Syk/Zap family. Upon association with these viral proteins, the activity of these tyrosine kinases is changed resulting in altered signal output. Middle-T, LMP2A and Tip are therefore excellent tools to study the regulation of Src family kinases.

The HIV-1 Nef protein acts as a connector with sorting pathways in the Golgi and at the plasma membrane

The HIV Nef protein down-regulates the cell surface expression of CD4 and of MHC I at least in part through accelerated endocytosis. To investigate further the mechanism of this effect, we created chimeric integral membrane proteins comprising the extracellular and transmembrane regions of CD4 or CD8 and Nef as the cytoplasmic domain. These fusion molecules could down-modulate CD4 in trans in a dileucine-dependent manner. Furthermore, in spite of lacking receptor-derived internalization signals, the Nef-containing chimeras underwent both Golgi retention and rapid endocytosis via clathrin-coated pits. Taken together, these data suggest that Nef down-regulates CD4 and probably MHC I by physically connecting these receptors with sorting pathways in the Golgi and at the plasma membrane.

Genetic organization, size, and complete sequence of early region 3 genes of human adenovirus type 41

The complete nucleotide and predicted amino acid sequences for open reading frames (ORFs) of the human adenovirus type 41 (Ad41) early region 3 (E3) gene have been determined. The sequence of the Ad41 E3 gene (map units 74 to 83.9) consists of 3,373 nucleotides and has one TATA box and two polyadenylation signals (AATAAA). Analysis of the nucleotide sequence reveals that the E3 gene can encode six ORFs, designated RL1 to RL6. These are all expressed at the mRNA level, as determined by reverse transcription-PCR analysis of AD41-infected cell RNA. When compared with known E3 sequences of most other human adenoviruses deposited in GenBank, the sequences of RL1 to RL3 were found to be unique to subgroup F adenoviruses (Ad40 and Ad41). They encode putative proteins of 173 amino acids (19.4 kDa) and 276 amino acids (31.6 kDa) in one reading frame as well as a 59- amino-acid (6.7 kDa) protein in an overlapping reading frame. RL4 encodes a 90-amino-acid protein (10.1 kDa) with 40% homology to the Ad2 E3 10.4-kDa protein, which induces degradation of the epidermal growth factor receptor and functions together with the Ad2 E3 14.5-kDa protein to protect mouse cell lines against lysis. RL5 encodes a protein of 107 amino acid residues (12.3 kDa) and is analogous to the Ad E3 14.5-kDa protein. RL6 codes for a protein of 122 amino acids (14.7 kDa) that is analogous to the Ad2 14.7-kDa protein, which functions to protect Ad-infected cells from tumor necrosis factor-induced cytolysis. This finding of three unique (RL1 to RL3) E3 gene ORFs may explain why subgroup F adenoviruses differ substantially from other human adenoviruses in their host range; i.e., they replicate predominantly in the host's gastrointestinal rather than respiratory tract. A recent phylogenetic study that compared subgroup F Ad40 DNA sequences with representatives of subgroups B (Ad3), C (Ad2), and E (Ad4) reached a similar conclusion about the uniqueness of RL1 and RL2.

Region E3 of subgroup B human adenoviruses encodes a 16-kilodalton membrane protein that may be a distant analog of the E3-6.7K protein of subgroup C adenoviruses

There is an open reading frame in the E3 transcription unit of adenovirus type 3 (Ad3) and Ad7 that could encode a protein of 16 kDa (16K protein). Ad3 and Ad7 are members of subgroup B of human adenoviruses. Using a rabbit antipeptide antiserum, we show that the 16K protein is expressed in Ad3- and Ad7-infected cells at early and late stages of infection; it is not expressed in cells infected with an Ad7 mutant that deletes the 16K gene. The 16K protein was also transcribed and translated in vitro from DNA containing the open reading frame for the 16K protein. The 16K protein has two hydrophobic domains typical of integral membrane proteins; consistent with this, we detected 16K in the crude membrane but not the cytosol cellular fractions. Although 16K has two potential sites for Asn-linked glycosylation, the protein is not glycosylated. The 16K gene is located in the same position in region E3 as the gene for the 6.7K protein of subgroup C adenoviruses (Ad2 and Ad5). E3-6.7K is an Asn-linked integral membrane glycoprotein, localized in the endoplasmic reticulum, whose function is unknown. The 16K protein has a putative transmembrane domain located in the same place in 16K as is the transmembrane domain in 6.7K, and the C-terminal portion of 16K is partially homologous to the C-terminal cytoplasmic domain of 6.7K; we suggest that these domains in 16K and 6.7K may have a similar function. The N-terminal 102 residues in 16K are not found in 6.7K; these residues may have a function that is unique to the 16K protein. In common with all known E3 proteins, the 16K protein is dispensable for virus replication in cultured cells; this suggests that the 16K protein may function in virus-host interactions.

The adenovirus E3 10.4K and 14.5K proteins, which function to prevent cytolysis by tumor necrosis factor and to down-regulate the epidermal growth factor receptor, are localized in the plasma membrane

The adenovirus type 2 and 5 E3 10,400- and 14,500-molecular-weight (10.4K and 14.5K) proteins are both required to protect some cell lines from lysis by tumor necrosis factor and to down-regulate the epidermal growth factor receptor. We have shown previously that both 10.4K and 14.5K are integral membrane proteins and that 14.5K is phosphorylated and O glycosylated. The 10.4K protein coimmunoprecipitates with 14.5K, indicating that the two proteins function as a complex. Here we show, using immunofluorescence and two different cell surface-labeling techniques, that both proteins are localized in the plasma membrane. In addition, we show that trafficking of each protein to the plasma membrane depends on concomitant expression of the other protein. Finally, neither protein could be immunoprecipitated from conditioned media, indicating that neither is secreted. Taken together, these results suggest that the plasma membrane is the site at which 10.4K and 14.5K function to inhibit cytolysis by tumor necrosis factor and to down-regulate the epidermal growth factor receptor.

Transfected lymphocyte extracts of patients with urological tumours: complement temperature-sensitive adenovirus mutants in vitro

Patients with renal or bladder cancers exhibit a unique association with adenovirus (Ad) infections. About 60% of them contain antibodies to Ad early antigens. Both in their tumour cells and peripheral blood lymphocytes (PBL) they have detectable early Ad antigens known to be involved in malignant cell transformation. Transfection of tumour cell extracts resulted in complementing temperature-sensitive (ts) Ad mutants at nonpermissive temperatures (39 degrees C) indicating that some cells of the tumour mass possess active functions for Ad. Only 4 to 18% of control subjects were positive in these tests. Here we studied whether lymphocytes might be involved in tumourigenesis by Ad. PBL extracts of patients were transfected into HEp-2 culture cells, which were subsequently superinfected with Ad-5 ts18 and ts19 mutants at 39 degrees C. Titration of virus yields indicated complementation in 76% of patients with renal and bladder cancers in contrast to 20% of control individuals. Complementing ability of lymphocytes which had been prestimulated with phytohaemagglutinin (PHA) approached that of tumour extracts. It means that both specimens contain advanced functions in contrast to resting lymphocytes. Lymphocytes are nonpermissive for latently carried Ad infections. Expression, possible transfer of early Ad gene products via frequent contacts with tissue cells can result in removal of tumour suppressor gene products from complexes regulating cell cycle negatively. Further interaction with hormone-sensitive protooncogenes explains tissue, age and gender specificity of urological malignancies. These phenomena suggest an important cofactorial role for Ad in kidney and bladder tumours.

Adenovirus E3 protein causes constitutively internalized epidermal growth factor receptors to accumulate in a prelysosomal compartment, resulting in enhanced degradation

We have previously identified and characterized an integral membrane protein coded for by the early transcription region 3 (E3) of human group C adenoviruses that down-regulates the epidermal growth factor receptor (EGFR). The goal of this study was to characterize the early receptor trafficking events leading to enhanced EGFR degradation in adenovirus-infected cells. Specifically, we wished to determine whether adenovirus increases the rate of EGFR internalization or alters the subcellular compartmentalization of internalized EGFRs. Once the optimal time for measuring early trafficking events was determined, surface EGFRs were labeled with a cleavable biotin reagent to measure internalization rates and with a receptor-specific monoclonal antibody (MAb) conjugated to colloidal gold for intracellular localization studies. We first showed that the rate of EGFR internalization in adenovirus-infected cells is indistinguishable from the constitutive internalization rate for unoccupied EGFRs. The possibility that the E3 protein can affect trafficking of EGFRs internalized at a low constitutive rate was further supported by studies showing that adenovirus-mediated down-regulation occurs independently of EGFR oligomerization and intrinsic EGFR tyrosine kinase activity, which are required for efficient ligand-induced internalization. Other tyrosine kinases inhibited by genistein are also not required for adenovirus-induced down-regulation. When the intracellular localization of EGFRs during adenovirus-mediated down-regulation was examined by electron microscopy, there was a threefold increase in the number of EGFRs localized to multivesicular bodies. The multivesicular body has been proposed to be important for regulating intracellular membrane protein sorting, since trafficking patterns for receptors that recycle and receptors that are degraded diverge in this organelle. These data therefore suggest that adenovirus may enhance EGFR degradation by causing constitutively internalized EGFRs to accumulate in a prelysosomal compartment. This is the first example of a mechanism that efficiently down-regulates EGFR without significantly increasing the rate of internalization or that does not require EGFR tyrosine kinase activity. Since viral proteins often mimic or modify a host counterpart, this suggests that there are normal physiological conditions when receptor destruction without tyrosine signalling is beneficial.

Adenovirus E3 protein causes constitutively internalized epidermal growth factor receptors to accumulate in a prelysosomal compartment, resulting in enhanced degradation

We have previously identified and characterized an integral membrane protein coded for by the early transcription region 3 (E3) of human group C adenoviruses that down-regulates the epidermal growth factor receptor (EGFR). The goal of this study was to characterize the early receptor trafficking events leading to enhanced EGFR degradation in adenovirus-infected cells. Specifically, we wished to determine whether adenovirus increases the rate of EGFR internalization or alters the subcellular compartmentalization of internalized EGFRs. Once the optimal time for measuring early trafficking events was determined, surface EGFRs were labeled with a cleavable biotin reagent to measure internalization rates and with a receptor-specific monoclonal antibody (MAb) conjugated to colloidal gold for intracellular localization studies. We first showed that the rate of EGFR internalization in adenovirus-infected cells is indistinguishable from the constitutive internalization rate for unoccupied EGFRs. The possibility that the E3 protein can affect trafficking of EGFRs internalized at a low constitutive rate was further supported by studies showing that adenovirus-mediated down-regulation occurs independently of EGFR oligomerization and intrinsic EGFR tyrosine kinase activity, which are required for efficient ligand-induced internalization. Other tyrosine kinases inhibited by genistein are also not required for adenovirus-induced down-regulation. When the intracellular localization of EGFRs during adenovirus-mediated down-regulation was examined by electron microscopy, there was a threefold increase in the number of EGFRs localized to multivesicular bodies. The multivesicular body has been proposed to be important for regulating intracellular membrane protein sorting, since trafficking patterns for receptors that recycle and receptors that are degraded diverge in this organelle. These data therefore suggest that adenovirus may enhance EGFR degradation by causing constitutively internalized EGFRs to accumulate in a prelysosomal compartment. This is the first example of a mechanism that efficiently down-regulates EGFR without significantly increasing the rate of internalization or that does not require EGFR tyrosine kinase activity. Since viral proteins often mimic or modify a host counterpart, this suggests that there are normal physiological conditions when receptor destruction without tyrosine signalling is beneficial.

Adenovirus and protein kinase C have distinct molecular requirements for regulating epidermal growth factor receptor trafficking

The ligand-activated tyrosine kinase receptor for epidermal growth factor (EGF) is down-regulated by an integral membrane protein coded for by the E3 early transcription unit of group C adenoviruses. The E3 protein appears to block recycling of constitutively internalized receptors, causing them instead to traffic to lysosomes where they are degraded. Expression of functional EGF receptors is also regulated by protein kinase C (PKC), which directly phosphorylates the EGF receptor at Thr-654. The goal of this study was to determine potential interactions between PKC and the E3 protein, since membrane-bound PKC activity is elevated by the adenovirus E1A protein. Our results show that although tumor promoters which activate PKC cause a coordinate induction of E3 protein synthesis and EGF receptor degradation, the E3 protein-induced pathway for receptor down-regulation functions independently of PKC and other kinases that are inhibited by staurosporine. This suggests that in contrast to other mechanisms that modulate receptor expression (i.e., ligand and PKC), the E3 protein is not regulated by phosphorylation but is constitutively active. We also report that adenovirus-mediated degradation is the preferred pathway in infected cells stimulated with 12-O-tetradecanoylphorbol-13-acetate (TPA) to induce receptor recycling.

Structurally related class I and class II receptor protein tyrosine kinases are down-regulated by the same E3 protein coded for by human group C adenoviruses

Receptor tyrosine kinases (RTKs) are grouped into subcategories based on shared sequence and structural features. Human group C adenoviruses down-regulate EGF receptors, which are members of the class I family of RTKs, during the early stages of infection. Adenovirus appears to utilize a nonsaturable intracellular pathway since it causes EGF-R down-regulation even in cells that significantly overexpress EGF-R. Adenovirus-induced down-regulation is mediated by a small hydrophobic molecule coded for by the E3 early transcription region that has recently been localized to plasma membrane. Here we examine intracellular trafficking of other RTKs in adenovirus-infected cells, to better understand the molecular basis for the action of the E3 protein. Although p185c-neu, which is a class I RTK closely related to the EGF receptor, is down-regulated in cells expressing physiological concentrations of this molecule, it is not down-regulated in tumor cell lines that significantly overexpress p185c-neu. Cell surface receptors for insulin and IGF1, which are class II RTKs, are also reduced in cells expressing the E3 protein, although to a slightly lesser extent than the EGF receptor. Moreover, whereas EGF receptors are degraded between 3- and 9-h postinfection, insulin and IGF1 receptors are degraded between 6- and 12-h postinfection under identical conditions. In contrast to the class I and class II RTKs, there is no difference in the expression of the class III receptors for PDGF and aFGF in cells infected with a virus with an intact E3 region versus a virus mutant with an internal deletion in the relevant E3 gene. These results suggest that the E3 protein provides an internalization and degradative sorting signal for some class I and class II RTKs, although down-regulation of class II RTKs is somewhat less efficient. Molecular recognition of class I and class II RTKs during adenovirus infection may not be due strictly to amino acid structure, however, since EGF-R but not p185c-neu is down-regulated in cells where it is significantly overexpressed.

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.