Assessing the role of E1A in the differential oncogenicity of group A and group C human adenoviruses

Structural Determinants within the Adenovirus Early Region 1A Protein Spacer Region Necessary for Tumorigenesis

It has long been established that group A human adenoviruses (HAdV-A12, -A18, and -A31) can cause tumors in newborn rodents, with tumorigenicity related to the presence of a unique spacer region located between conserved regions 2 and 3 within the HAdV-A12 early region 1A (E1A) protein. Group B adenoviruses are weakly oncogenic, whereas most of the remaining human adenoviruses are nononcogenic. In an attempt to understand better the relationship between the structure of the AdE1A spacer region and oncogenicity of HAdVs, the structures of synthetic peptides identical or very similar to the adenovirus 12 E1A spacer region were determined and found to be α-helical using nuclear magnetic resonance (NMR) spectroscopy. This contrasts significantly with some previous suggestions that this region is unstructured. Using available predictive algorithms, the structures of spacer regions from other E1As were also examined, and the extent of the predicted α-helix was found to correlate reasonably well with the tumorigenicity of the respective virus. We suggest that this may represent an as-yet-unknown binding site for a partner protein required for tumorigenesis.IMPORTANCE This research analyzed small peptides equivalent to a region within the human adenovirus early region 1A protein that confers, in part, tumor-inducing properties to various degrees on several viral strains in rats and mice. The oncogenic spacer region is α-helical, which contrasts with previous suggestions that this region is unstructured. The helix is characterized by a stretch of amino acids rich in alanine residues that are organized into a hydrophobic, or "water-hating," surface that is considered to form a major site of interaction with cellular protein targets that mediate tumor formation. The extent of α-helix in E1A from other adenovirus species can be correlated to a limited degree to the tumorigenicity of that virus. Some serotypes show significant differences in predicted structural propensity, suggesting that the amino acid type and physicochemical properties are also of importance.

Mapping the interactions of adenoviral E1A proteins with the p160 nuclear receptor coactivator binding domain of CBP

Many viruses deregulate the cell and force transcription of viral genes by competing with cellular proteins for binding to the transcriptional co-activators CREB-binding protein (CBP) and p300. Through its interactions with CBP/p300 and the retinoblastoma protein, the adenovirus (AdV) early region 1A (E1A) oncoprotein hijacks the cell cycle and, in rodents, transforms the cell; the mechanistic and structural basis for these effects remain unclear. In this study we compare the affinity of protein constructs from the E1A proteins from two adenovirus serotypes, non-oncogenic AdV5 and highly oncogenic AdV12, for binding to the nuclear receptor coactivator binding domain (NCBD) of CBP. NMR spectra show that the E1A constructs from both serotypes are intrinsically disordered in the free state and that each contains three homologous binding sites for the NCBD, one in the N-terminal region and two within conserved region 1 (CR1) of E1A. The binding sites in CR1 correspond to the motifs that bind the retinoblastoma protein and the TAZ2 domain of CBP/p300. The E1A and NCBD peptides fold synergistically upon complex formation. Binding affinities determined from NMR titrations show that, although the overall affinities for AdV5 and AdV12 E1A are comparable, there are significant differences between the two E1A serotypes in the relative strength with which their constituent interaction motifs bind to the NCBD. The individual E1A interaction motifs were unable to compete effectively with p53 for binding to the NCBD and both the N-terminal region and CR1 region of E1A are required for efficient competition with p53.

Control of human adenovirus type 5 gene expression by cellular Daxx/ATRX chromatin-associated complexes

Death domain-associated protein (Daxx) cooperates with X-linked α-thalassaemia retardation syndrome protein (ATRX), a putative member of the sucrose non-fermentable 2 family of ATP-dependent chromatin-remodelling proteins, acting as the core ATPase subunit in this complex, whereas Daxx is the targeting factor, leading to histone deacetylase recruitment, H3.3 deposition and transcriptional repression of cellular promoters. Despite recent findings on the fundamental importance of chromatin modification in host-cell gene regulation, it remains unclear whether adenovirus type 5 (Ad5) transcription is regulated by cellular chromatin remodelling to allow efficient virus gene expression. Here, we focus on the repressive role of the Daxx/ATRX complex during Ad5 replication, which depends on intact protein-protein interaction, as negative regulation could be relieved with a Daxx mutant that is unable to interact with ATRX. To ensure efficient viral replication, Ad5 E1B-55K protein inhibits Daxx and targets ATRX for proteasomal degradation in cooperation with early region 4 open reading frame protein 6 and cellular components of a cullin-dependent E3-ubiquitin ligase. Our studies illustrate the importance and diversity of viral factors antagonizing Daxx/ATRX-mediated repression of viral gene expression and shed new light on the modulation of cellular chromatin remodelling factors by Ad5. We show for the first time that cellular Daxx/ATRX chromatin remodelling complexes play essential roles in Ad gene expression and illustrate the importance of early viral proteins to counteract cellular chromatin remodelling.

Adenovirus type 5 early region 1B 55K oncoprotein-dependent degradation of cellular factor Daxx is required for efficient transformation of primary rodent cells

Early region 1B 55K (E1B-55K) from adenovirus type 5 (Ad5) is a multifunctional regulator of lytic infection and contributes in vitro to complete cell transformation of primary rodent cells in combination with Ad5 E1A. Inhibition of p53 activated transcription plays a key role in processes by which E1B-55K executes its oncogenic potential. Nevertheless, additional functions of E1B-55K or further protein interactions with cellular factors of DNA repair, transcription, and apoptosis, including Mre11, PML, and Daxx, may also contribute to the transformation process. In line with previous results, we performed mutational analysis to define a Daxx interaction motif within the E1B-55K polypeptide. The results from these studies showed that E1B-55K/Daxx binding is not required for inhibition of p53-mediated transactivation or binding and degradation of cellular factors (p53/Mre11). Surprisingly, these mutants lost the ability to degrade Daxx and showed reduced transforming potential in primary rodent cells. In addition, we observed that E1B-55K lacking the SUMO-1 conjugation site (SCS/K104R) was sufficient for Daxx interaction but no longer capable of E1B-55K-dependent proteasomal degradation of the cellular factor Daxx. These results, together with the observation that E1B-55K SUMOylation is required for efficient transformation, provides evidence for the idea that SUMO-1-conjugated E1B-55K-mediated degradation of Daxx plays a key role in adenoviral oncogenic transformation. We assume that the viral protein contributes to cell transformation through the modulation of Daxx-dependent pathways. This further substantiates the assumption that further mechanisms for efficient transformation of primary cells can be separated from functions required for the inhibition of p53-stimulated transcription.

Sprouty is a cytoplasmic target of adenoviral E1A oncoproteins to regulate the receptor tyrosine kinase signalling pathway

Background

Oncoproteins encoded by the early region of adenoviruses have been shown to be powerful tools to study gene regulatory mechanisms, which affect major cellular events such as proliferation, differentiation, apoptosis and oncogenic transformation. They are possesing a key role to favor viral replication via their interaction with multiple cellular proteins. In a yeast two-hybrid screen we have identified Sprouty1 (Spry1) as a target of adenoviral E1A Oncoproteins. Spry proteins are central and complex regulators of the receptor tyrosine kinase (RTK) signalling pathway. The deregulation of Spry family members is often associated with alterations of the RTK signalling and its downstream effectors, leading to the ERK pathway.

Results

Here, we confirm our yeast two-hybrid data, showing the interaction between Spry1 and E1A in GST pull-down and immunoprecipitation assays. We also demonstrated the interaction of E1A with two further Spry isoforms. Using deletion mutants we identified the N-terminus and the CR conserved region (CR) 3 of E1A- and the C-terminal half of Spry1, which contains the highly conserved Spry domain, as the essential sites for direct interaction between Spry and E1A. Immunofluorescent microscopy data revealed a co-localization of E1A_13S with Spry1 in the cytoplasm. SRE and TRE reporter assays demonstrated that co-expression of Spry1 with E1A_13S abolishes the inhibitory function of Spry1 in RTK signalling, which is consequently accompanied with a decrease of E1A_13S-induced gene expression.

Conclusions

These results establish Spry1 as a cytoplasmic localized cellular target for E1A oncoproteins to regulate the RTK signalling pathway, and consequently cellular events downstream of RTK that are essential for viral replication and transformation.

Evasion of the immune system by adenoviruses

Human adenoviruses (Ads) have the ability to transform primary cells, and certain Ads, the subgenus A adenoviruses such as Ad12, induce tumours in immunocompetent rodents. The oncogenic phenotype of the subgenus A adenoviruses is determined by the viral E1A oncogene. In order to generate tumours, Ad12-transformed cells must evade the cellular immune system of the host. Ad12 E1A gene products mediate transcriptional repression of several genes in the major histocompatibility complex (MHC) involved in antigen processing and presentation, resulting in evasion of cytotoxic T lymphocyte (CTL) killing of transformed cells. In this review, the molecular mechanisms of E1A-mediated transcriptional repression of MHC gene expression are described. In addition, evasion of natural killer (NK) cell killing by Ad-transformed cells is also considered.

Cell transformation by human adenoviruses

The last 40 years of molecular biological investigations into human adenoviruses have contributed enormously to our understanding of the basic principles of normal and malignant cell growth. Much of this knowledge stems from analyses of their productive infection cycle in permissive host cells. Also, initial observations concerning the carcinogenic potential of human adenoviruses subsequently revealed decisive insights into the molecular mechanisms of the origins of cancer, and established adenoviruses as a model system for explaining virus-mediated transformation processes. Today it is well established that cell transformation by human adenoviruses is a multistep process involving several gene products encoded in early transcription units 1A (E1A) and 1B (E1B). Moreover, a large body of evidence now indicates that alternative or additional mechanisms are engaged in adenovirus-mediated oncogenic transformation involving gene products encoded in early region 4 (E4) as well as epigenetic changes resulting from viral DNA integration. In particular, detailed studies on the tumorigenic potential of subgroup D adenovirus type 9 (Ad9) E4 have now revealed a new pathway that points to a novel, general mechanism of virus-mediated oncogenesis. In this chapter, we summarize the current state of knowledge about the oncogenes and oncogene products of human adenoviruses, focusing particularly on recent findings concerning the transforming and oncogenic properties of viral proteins encoded in the E1B and E4 transcription units.

Adenovirus early region 1A protein binds to mammalian SUG1-a regulatory component of the proteasome

Adenovirus early region 1A (Ad E1A) is a multifunctional protein which is essential for adenovirus-mediated transformation and oncogenesis. Whilst E1A is generally considered to exert its influence on recipient cells through regulation of transcription it also increases the level of cellular p53 by increasing the protein half-life. With this in view, we have investigated the relationship of Ad E1A to the proteasome, which is normally responsible for degradation of p53. Here we have shown that both Ad5 and Ad12 E1A 12S and 13S proteins can be co-immunoprecipitated with proteasomes and that the larger Ad12 E1A protein binds strongly to at least three components of the 26S but not 20S proteasome. One of these interacting species has been identified as mammalian SUGI, a proteasome regulatory component which also plays a role in the cell as a mediator of transcription. In vitro assays have demonstrated a direct interaction between Ad12 E1A 13S protein and mouse SUGI. Following infection of human cells with Ad5 wt and Ad5 mutants with lesions in the E1A gene it has been shown that human SUG1 can be co-immunoprecipitated with full-length E1A and with E1A carrying a deletion in conserved region 1 which is the region considered to be responsible for increased expression of p53. We have concluded therefore that Ad EIA binds strongly to SUGI but that this interaction is not responsible for inhibition of proteasome activity. This is consistent with the observation that purified Ad12 E1A inhibits the activity of the purified 20S but not 26S proteasomes. We have also demonstrated that SUGI can be co-immunoprecipitated with SV40 T and therefore we suggest that this may represent a common interaction of transforming proteins of DNA tumour viruses.