E4orf6 variants with separate abilities to augment adenovirus replication and direct nuclear localization of the E1B 55-kilodalton protein

Conserved E1B-55K SUMOylation in different human adenovirus species is a potent regulator of intracellular localization

Over the past decades, studies on the biology of human adenoviruses (HAdVs) mainly focused on the HAdV prototype species C type 5 (HAdV-C5) and revealed fundamental molecular insights into mechanisms of viral replication and viral cell transformation. Recently, other HAdV species are gaining more and more attention in the field. Reports on large E1B proteins (E1B-55K) from different HAdV species showed that these multifactorial proteins possess strikingly different features along with highly conserved functions. In this work, we identified potential SUMO-conjugation motifs (SCMs) in E1B-55K proteins from HAdV species A to F. Mutational inactivation of these SCMs demonstrated that HAdV E1B-55K proteins are SUMOylated at a single lysine residue that is highly conserved among HAdV species B to E. Moreover, we provide evidence that E1B-55K SUMOylation is a potent regulator of intracellular localization and p53-mediated transcription in most HAdV species. We also identified a lysine residue at position 101 (K101), which is unique to HAdV-C5 E1B-55K and specifically regulates its SUMOylation and nucleo-cytoplasmic shuttling. Our findings reveal important new aspects on HAdV E1B-55K proteins and suggest that different E1B-55K species possess conserved SCMs while their SUMOylation has divergent cellular effects during infection. Importance E1B-55K is a multifunctional adenoviral protein and its functions are highly regulated by SUMOylation. Although functional consequences of SUMOylated HAdV-C5 E1B-55K are well studied, we lack information on the effects of SUMOylation on homologous E1B-55K proteins from other HAdV species. Here, we show that SUMOylation is a conserved post-translational modification in most of the E1B-55K proteins, similar to what we know about HAdV-C5 E1B-55K. Moreover, we identify subcellular localization and regulation of p53-dependent transcription as highly conserved SUMOylation-regulated E1B-55K functions. Thus, our results highlight how HAdV proteins might have evolved in different HAdV species with conserved domains involved in virus replication and differing alternative functions and interactions with the host cell machinery. Future research will link these differences and similarities to the diverse pathogenicity and organ tropism of the different HAdV species.

Double-edged role of PML nuclear bodies during human adenovirus infection

PML nuclear bodies are matrix-bound nuclear structures with a variety of functions in human cells. These nuclear domains are interferon regulated and play an essential role during virus infections involving accumulation of SUMO-dependent host and viral factors. PML-NBs are targeted and subsequently manipulated by adenoviral regulatory proteins, illustrating their crucial role during productive infection and virus-mediated oncogenic transformation. PML-NBs have a longstanding antiviral reputation; however, the genomes of Human Adenoviruses and initial sites of viral transcription/replication are found juxtaposed to these domains, resulting in a double-edged capacity of these nuclear multiprotein/multifunctional complexes. This enigma provides evidence that Human Adenoviruses selectively counteract antiviral responses, and simultaneously benefit from or even depend on proviral PML-NB associated components by active recruitment to PML track-like structures, that are induced during infection. Thereby, a positive microenvironment for adenoviral transcription and replication is created at these nuclear subdomains. Based on the available data, this review aims to provide a detailed overview of the current knowledge of Human Adenovirus crosstalk with nuclear PML body compartments as sites of SUMOylation processes in the host cells, evaluating the currently known principles and molecular mechanisms.

Functional cooperation between human adenovirus type 5 early region 4, open reading frame 6 protein, and cellular homeobox protein HoxB7

Human adenovirus type 5 (HAdV5) E4orf6 (early region 4 open reading frame 6 protein) is a multifunctional early viral protein promoting efficient replication and progeny production. E4orf6 complexes with E1B-55K to assemble cellular proteins into a functional E3 ubiquitin ligase complex that not only mediates proteasomal degradation of host cell substrates but also facilitates export of viral late mRNA to promote efficient viral protein expression and host cell shutoff. Recent findings defined the role of E4orf6 in RNA splicing independent of E1B-55K binding. To reveal further functions of the early viral protein in infected cells, we used a yeast two-hybrid system and identified the homeobox transcription factor HoxB7 as a novel E4orf6-associated protein. Using a HoxB7 knockdown cell line, we observed a positive role of HoxB7 in adenoviral replication. Our experiments demonstrate that the absence of HoxB7 leads to inefficient viral progeny production, as HAdV5 gene expression is highly regulated by HoxB7-mediated activation of various adenoviral promoters. We have thus identified a novel role of E4orf6 in HAdV5 gene transcription via regulation of homeobox protein-dependent modulation of viral promoter activity.

The E3 ubiquitin ligase activity associated with the adenoviral E1B-55K-E4orf6 complex does not require CRM1-dependent export

The adenovirus type 5 (Ad5) E1B-55K and E4orf6 (E1B-55K/E4orf6) proteins are multifunctional regulators of Ad5 replication, participating in many processes required for virus growth. A complex containing the two proteins mediates the degradation of cellular proteins through assembly of an E3 ubiquitin ligase and induces shutoff of host cell protein synthesis through selective nucleocytoplasmic viral late mRNA export. Both proteins shuttle between the nuclear and cytoplasmic compartments via leucine-rich nuclear export signals (NES). However, the role of their NES-dependent export in viral replication has not been established. It was initially shown that mutations in the E4orf6 NES negatively affect viral late gene expression in transfection/infection complementation assays, suggesting that E1B-55K/E4orf6-dependent viral late mRNA export involves a CRM1 export pathway. However, a different conclusion was drawn from similar studies showing that E1B-55K/E4orf6 promote late gene expression without active CRM1 or functional NES. To evaluate the role of the E1B-55K/E4orf6 NES in viral replication in the context of Ad-infected cells and in the presence of functional CRM1, we generated virus mutants carrying amino acid exchanges in the NES of either or both proteins. Phenotypic analyses revealed that mutations in the NES of E1B-55K and/or E4orf6 had no or only moderate effects on viral DNA replication, viral late protein synthesis, or viral late mRNA export. Significantly, such mutations also did not interfere with the degradation of cellular substrates, indicating that the NES of E1B-55K or E4orf6 is dispensable both for late gene expression and for the activity associated with the E3 ubiquitin ligase.

Viral-mediated stabilization of AU-rich element containing mRNA contributes to cell transformation

E4orf6 is one of the oncogene products of adenovirus, and it also has an important role for transportation of cellular and viral messenger RNA (mRNA) during the late phase of virus infection. We previously revealed that E4orf6 controls the fate of AU-rich element (ARE) containing mRNA by perturbing the chromosome maintenance region 1-dependent export mechanism. Here, we show that E4orf6 stabilizes ARE-mRNA through the region required for its oncogenic activity and ubiquitin E3 ligase assembly. Cells that failed to stabilize ARE-mRNA after HuR knockdown were unable to produce colonies in soft agar, even when E4orf6 was expressed. Furthermore, the stabilized ARE-mRNA induced the transformation of rodent immortalized cells. These findings indicate that stabilized ARE-mRNA is necessary, if not all, for the oncogenic activity of E4orf6 and has the potential to transform cells, at least under a certain condition.

Obstacles to the successful development of an efficacious T cell-inducing HIV-1 vaccine

An efficacious vaccine to HIV-1 is direly needed to stem the global pandemic. Immunogens that elicit broadly cross-neutralizing antibodies to HIV-1 remain elusive, and thus, most HIV-1 vaccine efforts are focusing on induction of T cells. The notion that T cells can mediate protection against HIV-1 has been called into question by the failure of the STEP trial, which was designed to test this concept by the use of an E1-deleted Ad vaccine carrier. Lack of efficacy of the STEP trial vaccine underscores our limited knowledge about correlates of immune protection against HIV-1 and stresses the need for an enhanced commitment to basic research, including preclinical and clinical vaccine studies. In this review, we discuss known correlates of protection against HIV-1 and different vaccine strategies that have been or are being explored to induce such correlates, focusing on T cell-inducing vaccines and particularly on Ad vectors.

RUNX1 permits E4orf6-directed nuclear localization of the adenovirus E1B-55K protein and associates with centers of viral DNA and RNA synthesis

The localization of the adenovirus E1B-55K-E4orf6 protein complex is critical for its function. Prior studies demonstrated that E4orf6 directs the nuclear localization of E1B-55K in human cells and in rodent cells that contain part of human chromosome 21. We show here that the relevant activity on chromosome 21 maps to RUNX1. RUNX1 proteins are transcription factors that serve as scaffolds for the assembly of proteins that regulate transcription and RNA processing. After transfection, the RUNX1a, RUNX1b, and RUNX1-DeltaN variants allowed E4orf6-directed E1B-55K nuclear localization. The failure of RUNX1c to allow nuclear colocalization was relieved by the deletion of amino-terminal residues of this protein. In the adenovirus-infected mouse cell, RUNX1 proteins were localized to discrete structures about the periphery of viral replication centers. These sites are enriched in viral RNA and RNA-processing factors. RUNX1b and RUNX1a proteins displaced E4orf6 from these sites. The association of E1B-55K at viral replication centers was enhanced by the RUNX1a and RUNX1b proteins, but only in the absence of E4orf6. In the presence of E4orf6, E1B-55K occurred in a perinuclear cytoplasmic body resembling the aggresome and was excluded from the nucleus of the infected mouse cell. We interpret these findings to mean that a dynamic relationship exists between the E4orf6, E1B-55K, and RUNX1 proteins. In cooperation with E4orf6, RUNX1 proteins are able to modulate the localization of E1B-55K and even remodel virus-specific structures that form at late times of infection. Subsequent studies will need to determine a functional consequence of the interaction between E4orf6, E1B-55K, and RUNX1.

Production and formulation of adenovirus vectors

Adenovirus vectors have attracted considerable interest over the past decade, with ongoing clinical development programs for applications ranging from replacement therapy for protein deficiencies to cancer therapeutics to prophylactic vaccines. Consequently, considerable product, process, analytical, and formulation development has been undertaken to support these programs. For example, "gutless" vectors have been developed in order to improve gene transfer capacity and durability of expression; new cell lines have been developed to minimize recombination events; production conditions have been optimized to improve volumetric productivities; analytical techniques and scaleable purification processes have advanced towards the goal of purified adenovirus becoming a "well-characterized biological"; and liquid formulations have been developed which maintain virus infectivity at 2-8 degrees C for over 18 months. These and other advances in the production of adenovirus vectors are discussed in detail in this review. In addition, the needs for the next decade are highlighted.

Adenovirus type 5 E4orf3 protein targets the Mre11 complex to cytoplasmic aggresomes

Virus infections have dramatic effects on structural and morphological characteristics of the host cell. The gene product of open reading frame 3 in the early region 4 (E4orf3) of adenovirus serotype 5 (Ad5) is involved in efficient replication and late protein synthesis. During infection with adenovirus mutants lacking the E4 region, the viral genomic DNA is joined into concatemers by cellular DNA repair factors, and this requires the Mre11/Rad50/Nbs1 complex. Concatemer formation can be prevented by the E4orf3 protein, which causes the cellular redistribution of the Mre11 complex. Here we show that E4orf3 colocalizes with components of the Mre11 complex in nuclear tracks and also in large cytoplasmic accumulations. Rearrangement of Mre11 and Rad50 by Ad5 E4orf3 is not dependent on interactions with Nbs1 or promyelocytic leukemia protein nuclear bodies. Late in infection the cytoplasmic inclusions appear as a distinct juxtanuclear accumulation at the centrosome and this requires an intact microtubule cytoskeleton. The large cytoplasmic accumulations meet the criteria defined for aggresomes, including gamma-tubulin colocalization and formation of a surrounding vimentin cage. E4orf3 also appears to alter the solubility of the cellular Mre11 complex. These data suggest that E4orf3 can target the Mre11 complex to an aggresome and may explain how the cellular repair complex is inactivated during adenovirus infection.

The adenovirus E4-6/7 protein directs nuclear localization of E2F-4 via an arginine-rich motif

E2F transcription factors are key participants in the regulation of proliferation, apoptosis, and differentiation in mammalian cells. E2Fs are negatively regulated by members of the retinoblastoma protein (pRb) family. During adenovirus (Ad) infection, viral proteins that displace pRb family members from E2Fs and recruit E2F complexes to viral and cellular promoter regions are expressed. This recruitment of E2F involves the induction of stable E2F binding to inverted E2F binding sites in the Ad E2a and cellular E2F-1 promoters and induces both viral and cellular gene expression. E2F-4 has abundant E2F activity within cells, and the majority of E2F-4 in asynchronous cells is found in the cytoplasm. Upon expression of the adenovirus E4-6/7 protein, a significant portion of E2F-4 is translocated to the nucleus, and its activity constitutes the majority of Ad-induced nuclear E2F DNA binding activity. This redirection of E2F-4 from cytoplasm to the nucleus requires an N-terminal arginine-rich nuclear localization sequence within E4-6/7. The directed targeting of E4-6/7 to the nucleus is important for the function of this protein in the context of viral infection. This function of E4-6/7 has a redundant component as well as nonredundant components in cooperation with the adenovirus E1A oncoproteins to deregulate and usurp host cell E2F function.

The Adenovirus E4orf6 Protein Inhibits DNA Double Strand Break Repair and Radiosensitizes Human Tumor Cells in an E1B-55K-independent Manner

The adenoviral protein E4orf6 has been shown to inhibit both in vitro V(D)J recombination and adenoviral DNA concatenation, two processes that rely on cellular DNA double strand break repair (DSBR) proteins. Most of the known activities of E4orf6 during adenoviral infection require its interaction with another adenoviral protein, E1B-55K. Here we report that E4orf6, stably expressed in RKO human colorectal carcinoma cells or transiently expressed by adenoviral vector in U251 human glioblastoma cells, inhibits DSBR and induces significant radiosensitization in the absence of E1B-55K. Expression of a mutant form of E4orf6 (L245P) failed to radiosensitize RKO cells. E4orf6 reduced DSBR capacity in transfected and infected cells, as measured by sublethal DNA damage repair assay and phosphorylated H2AX (gamma-H2AX) levels, respectively. Consistent with the inhibitory effect of E4orf6 on DSBR, expression of wild-type but not mutant E4orf6 reduced recovery of a transfected, replicating reporter plasmid (pSP189) in 293 cells but did not increase the mutation frequency measured in the reporter plasmid. The kinase activity of DNA-PKcs (the DNA-dependent protein kinase catalytic subunit) toward heterologous substrates was not affected by expression of E4orf6; however, autophosphorylation of DNA-PKcs at Thr-2609 following ionizing radiation was prolonged in the presence of E4orf6 when compared with control-infected cells. Our results demonstrate for the first time that E4orf6 expression hinders the cellular DNA repair process in mammalian cells in the absence of E1B-55K or other adenoviral genes and suggest that viral-mediated delivery of E4orf6, combined with localized external beam radiation, could be a useful approach for the treatment of radioresistant solid tumors such as glioblastomas.

Late viral RNA export, rather than p53 inactivation, determines ONYX-015 tumor selectivity

ONYX-015 is an adenovirus that lacks the E1B-55K gene product for p53 degradation. Thus, ONYX-015 was conceived as an oncolytic virus that would selectively replicate in p53-defective tumor cells. Here we show that loss of E1B-55K leads to the induction, but not the activation, of p53 in ONYX-015-infected primary cells. We use a novel adenovirus mutant, ONYX-053, to demonstrate that loss of E1B-55K-mediated late viral RNA export, rather than p53 degradation, restricts ONYX-015 replication in primary cells. In contrast, we show that tumor cells that support ONYX-015 replication provide the RNA export function of E1B-55K. These data reveal that tumor cells have altered mechanisms for RNA export and resolve the controversial role of p53 in governing ONYX-015 oncolytic selectivity.

Both BC-box motifs of adenovirus protein E4orf6 are required to efficiently assemble an E3 ligase complex that degrades p53

Small DNA tumor viruses typically encode proteins that either inactivate or degrade p53. Human adenoviruses encode products, including E4orf6 and E1B55K, that do both. Each independently binds to p53 and inhibits its ability to activate gene expression; however, in combination they induce p53 degradation by the ubiquitin pathway. We have shown previously that p53 degradation relies on interactions of E4orf6 with the cellular proteins Cul5, Rbx1, and elongins B and C to form an E3 ligase similar to the SCF and VBC complexes. Here we show that, like other elongin BC-interacting proteins, including elongin A, von Hippel-Lindau protein, and Muf1, the interaction of E4orf6 is mediated by the BC-box motif; however, E4orf6 uniquely utilizes two BC-box motifs for degradation of p53 and another target, Mre11. In addition, our data suggest that the interaction of E1B55K with E4orf6 depends on the ability of E4orf6 to form the E3 ligase complex and that such complex formation may be required for all E4orf6-E1B55K functions.

Adenovirus E1B 55-kilodalton oncoprotein binds to Daxx and eliminates enhancement of p53-dependent transcription by Daxx

The adenovirus E1B 55-kDa protein impairs the p53 pathway and enhances transformation, although the underlying mechanisms remain to be defined. We found that Daxx binds to the E1B 55-kDa protein in a yeast two-hybrid screen. The two proteins interact through their C termini. Mutation of three potential phosphorylation sites (S489/490 and T494 to alanine) within the E1B 55-kDa protein did not affect its interaction with Daxx, although such mutations were previously shown to inhibit E1B's ability to repress p53-dependent transcription and to enhance transformation. In addition to their coimmunoprecipitation in 293 extracts, purified Daxx interacted with the E1B 55-kDa protein in vitro, indicating their direct interaction. In 293 cells, Daxx colocalized with the E1B 55-kDa protein within discrete nuclear dots, where p53 was also found. Such structures were distinct from PML (promyelocytic leukemia protein) bodies, and it appeared that Daxx was displaced from PML bodies. Thus, the Daxx concentration was diminished in dots with a prominent presence of PML and vice versa. Indeed, PML overexpression led to dramatic redistribution of Daxx from p53-E1B 55-kDa protein complexes to PML bodies. Additionally, expression of the E1B 55-kDa protein in Saos2 osteosarcoma cells reduced the number of PML bodies. Our data suggest that E1B and PML compete for available Daxx in the cell. Surprisingly, Daxx significantly augmented p53-mediated transcription and the E1B 55-kDa protein eliminated this effect. Thus, it is likely that the E1B 55-kDa protein sequesters Daxx and p53 in specific nuclear locations, where p53 cannot activate transcription. One consequence of the Daxx-E1B interaction might be an alteration of normal interactions of Daxx, PML, and p53, which may contribute to cell transformation.

Intrabody-based strategies for inhibition of vascular endothelial growth factor receptor-2: effects on apoptosis, cell growth, and angiogenesis

VEGF, an endothelial-specific mitogen, is an important tumor angiogenesis growth factor. The major receptor for VEGF on endothelial cells is KDR. We hypothesized that an intrabody could bind newly synthesized KDR and block receptor transport to the cell surface, thereby inhibiting important VEGF effects. We expressed a single chain antibody (p3S5) to KDR with or without the endoplasmic reticulum (ER) retention signal (KDEL), using either a plasmid (p3S5-HAK) or a tet-off adenoviral system (Ad-HAK). Plasmid-mediated expression of the tethered intrabody significantly reduced KDR expression (from 82.5+/-12.5% to 27.9+/-13.6% of cells; P<0.01) and thymidine incorporation in successfully transfected cells. Ad-HAK infection resulted in intrabody expression in >90% of human umbilical vein endothelial cells (HUVECs), producing marked (80%) apoptosis at 48 h postinfection. The intrabody was essential for these effects, as confirmed by inhibiting its expression with doxycycline or by expressing irrelevant genes (lacZ, GFP). Cell death was dependent on KDR, because Ad-HAK infection of cell lines with minimal or no KDR had little effect on cell viability. Infected HUVECs were unable to form tubes on Engelbreth Holm-Swarm (EHS) tumor gel matrix. These results demonstrate the potential for development of an intrabody-based strategy to block angiogenesis and prevent tumor growth.

An activity associated with human chromosome 21 permits nuclear colocalization of the adenovirus E1B-55K and E4orf6 proteins and promotes viral late gene expression

The adenovirus E1B-55K and E4orf6 proteins cooperate during virus infection while performing several tasks that contribute to a productive infection, including the selective nucleocytoplasmic transport of late viral mRNA. Previous studies have shown that the E4orf6 protein retains the E1B-55K protein in the nucleus of human and monkey cells, but not in those of rodents, suggesting that primate-specific cellular factors contribute to the E4orf6-mediated retention of the E1B-55K protein in the nucleus. In an effort to identify these proposed primate-specific cellular factors, the interaction of the E1B-55K and E4orf6 proteins was studied in a panel of stable human-rodent monochromosomal somatic cell hybrids. Analysis of this panel of cell lines has demonstrated the existence of an activity associated with human chromosome 21 that permits the E1B-55K and E4orf6 proteins to colocalize in the nucleus of a rodent cell. Additional hybrid cells bearing portions of human chromosome 21 were used to map this activity to a 10-megabase-pair segment of the chromosome, extending from 21q22.12 to a region near the q terminus. Strikingly, this region also facilitates the expression of adenovirus late genes in a rodent cell background while having little impact on the expression of early viral genes.

Analysis of the adenovirus E1B-55K-anchored proteome reveals its link to ubiquitination machinery

During the early phase of infection, the E1B-55K protein of adenovirus type 5 (Ad5) counters the E1A-induced stabilization of p53, whereas in the late phase, E1B-55K modulates the preferential nucleocytoplasmic transport and translation of the late viral mRNAs. The mechanism(s) by which E1B-55K performs these functions has not yet been clearly elucidated. In this study, we have taken a proteomics-based approach to identify and characterize novel E1B-55K-associated proteins. A multiprotein E1B-55K-containing complex was immunopurified from Ad5-infected HeLa cells and found to contain E4-orf6, as well as several cellular factors previously implicated in the ubiquitin-proteasome-mediated destruction of proteins, including Cullin-5, Rbx1/ROC1/Hrt1, and Elongins B and C. We further demonstrate that a complex containing these as well as other proteins is capable of directing the polyubiquitination of p53 in vitro. These ubiquitin ligase components were found in a high-molecular-mass complex of 800 to 900 kDa. We propose that these newly identified binding partners (Cullin-5, Elongins B and C, and Rbx1) complex with E1B-55K and E4-orf6 during Ad infection to form part of an E3 ubiquitin ligase that targets specific protein substrates for degradation. We further suggest that E1B-55K functions as the principal substrate recognition component of this SCF-type ubiquitin ligase, whereas E4-orf6 may serve to nucleate the assembly of the complex. Lastly, we describe the identification and characterization of two novel E1B-55K interacting factors, importin-alpha 1 and pp32, that may also participate in the functions previously ascribed to E1B-55K and E4-orf6.