The adenovirus E1B 55K (E1B) protein plays major roles in productive adenoviral infection and cellular transformation. Interest in E1B increased because of the potential of adenoviruses as therapeutic vectors, and the E1B gene is commonly deleted from adenovirus vectors for anticancer therapy. E1B activities are spatiotemporally regulated through SUMOylation and phosphorylation, and through interactions with multiple partners that occur presumably at different intracellular sites and times postinfection. E1B is implicated in the formation of viral replication compartments and regulates viral genome replication and transcription, transcriptional repression, degradation of cellular proteins, and several intranuclear steps of viral late mRNA biogenesis. Here, we review advances in our understanding of E1B during productive adenovirus replication and discuss fundamental aspects that remain unresolved.

Human adenovirus (HAdV) infections cause a wide variety of clinical symptoms, ranging from mild upper respiratory tract disease to lethal outcomes, particularly in immunocompromised individuals. To date, neither widely available vaccines nor approved antiadenoviral compounds are available to efficiently deal with HAdV infections. Thus, there is a need to thoroughly understand HAdV-induced disease, and for the development and preclinical evaluation of HAdV therapeutics and/or vaccines, and consequently for suitable standardizable in vitro systems and animal models. Current animal models to study HAdV pathogenesis, persistence, and tumorigenesis include rodents such as Syrian hamsters, mice, and cotton rats, as well as rabbits. In addition, a few recent studies on other species, such as pigs and tree shrews, reported promising data. These models mimic (aspects of) HAdV-induced pathological changes in humans and, although they are relevant, an ideal HAdV animal model has yet to be developed. This review summarizes the available animal models of HAdV infection with comprehensive descriptions of virus-induced pathogenesis in different animal species. We also elaborate on rodent HAdV animal models and how they contributed to insights into adenovirus-induced cell transformation and cancer.

Extensive studies on viral‐mediated oncogenic transformation by human adenoviruses have revealed much of our current understanding on the molecular mechanisms that are involved in the process. To date, these studies have shown that cell transformation is a multistep process regulated by the cooperation of several adenoviral gene products encoded in the early regions 1 (E1) and 4 (E4). E1A immortalizes primary rodent cells, whereas coexpression of E1B induces full manifestation of the transformed phenotype. Beside E1 proteins, also some E4 proteins have partial transforming activities through regulating many cellular pathways. Here, we summarize recent data of how adenoviral oncoproteins may contribute to viral transformation and discuss the challenge of pinpointing the underlying mechanisms.

Adenoviruses are a group of double-stranded DNA viruses that can mainly cause respiratory, gastrointestinal, and eye infections in humans. In addition, adenoviruses are employed as vector vaccines for combatting viral infections, including SARS-CoV-2, and serve as excellent gene therapy vectors. These viruses have the ability to modulate the host cell machinery to their advantage and trigger significant restructuring of the nuclei of infected cells through the activity of viral proteins. One of those, the adenovirus DNA-binding protein (DBP), is a multifunctional non-structural protein that is integral to the reorganization processes. DBP is encoded in the E2A transcriptional unit and is highly abundant in infected cells. Its activity is unequivocally linked to the formation, structure, and integrity of virus-induced replication compartments, molecular hubs for the regulation of viral processes, and control of the infected cell. DBP also plays key roles in viral DNA replication, transcription, viral gene expression, and even host range specificity. Notably, post-translational modifications of DBP, such as SUMOylation and extensive phosphorylation, regulate its biological functions. DBP was first investigated in the 1970s, pioneering research on viral DNA-binding proteins. In this literature review, we provide an overview of DBP and specifically summarize key findings related to its complex structure, diverse functions, and significant role in the context of viral replication. Finally, we address novel insights and perspectives for future research.

The human adenovirus species C type 5 (HAdV-C5) early region 1B 55 kDa (E1B-55K) protein is a multifunctional protein that promotes viral replication and adenovirus-mediated cell transformation through various mechanisms that primarily counteract host intrinsic and innate immunity. These include post-translational activities that exploit the host cell ubiquitin and small ubiquitin-like modifier (SUMO) conjugation machineries to regulate antiviral cellular restriction factors. However, despite significant advancements in this field, several underlying mechanisms governing these processes remain unidentified to date. Here, we performed stable isotope labeling by amino acids in cell culture (SILAC)-based quantitative SUMO proteomics to better understand cellular consequences of E1B-55K-mediated host cell modulation and adenovirus infection in general. We assessed cellular proteins for abundance changes and SUMO2 conjugate proteome changes during infection with wild-type HAdV-C5 or an E1B-55K deletion mutant. We provide evidence that changes in the SUMOylated proteome have the potential to regulate the DNA damage response, cell cycle control, chromatin assembly, and gene transcription and present these data as a resource for the research community. Strikingly, we identified a SUMO-dependent, ubiquitin-mediated degradation mechanism for some SUMO substrates, suggesting that E1B-55K may use multiple mechanisms to alter the activity of restrictive cellular pathways. IMPORTANCE: Human adenoviruses (HAdVs) generally cause mild and self-limiting diseases of the upper respiratory and gastrointestinal tracts but pose a serious risk to immunocompromised patients and children. Moreover, they are widely used as vectors for vaccines and vector-based gene therapy approaches. It is therefore vital to thoroughly characterize HAdV gene products and especially HAdV virulence factors. Early region 1B 55 kDa protein (E1B-55K) is a multifunctional HAdV-encoded oncoprotein involved in various viral and cellular pathways that promote viral replication and cell transformation. We analyzed the E1B-55K dependency of SUMOylation, a post-translational protein modification, in infected cells using quantitative proteomics. We found that HAdV increases overall cellular SUMOylation and that this increased SUMOylation can target antiviral cellular pathways that impact HAdV replication. Moreover, we showed that E1B-55K orchestrates the SUMO-dependent degradation of certain cellular antiviral factors. These results once more emphasize the key role of E1B-55K in the regulation of viral and cellular proteins in productive HAdV infections.

It is well established that human adenoviruses such as species C, types 2 and 5 (HAdV-C2 and HAdV-C5), induce a nearly complete shutoff of host-cell protein synthesis in the infected cell, simultaneously directing very efficient production of viral proteins. Such preferential expression of viral over cellular genes is thought to be controlled by selective nucleocytoplasmic export and translation of viral mRNA. While detailed knowledge of the regulatory mechanisms responsible for the translation of viral mRNA is available, the viral or cellular mechanisms of mRNA biogenesis are not completely understood. To identify parameters that control the differential export of viral and cellular mRNAs, we performed global transcriptome analyses (RNAseq) and monitored temporal nucleocytoplasmic partitioning of viral and cellular mRNAs during HAdV-C5 infection of A549 cells. Our analyses confirmed previously reported features of the viral mRNA expression program, as a clear shift in viral early to late mRNA accumulation was observed upon transition from the early to the late phase of viral replication. The progression into the late phase of infection, however, did not result in abrogation of cellular mRNA export; rather, viral late mRNAs outnumbered viral early and most cellular mRNAs by several orders of magnitude during the late phase, revealing that viral late mRNAs are not selectively exported but outcompete cellular mRNA biogenesis.