Biology of the adenovirus E4orf4 protein: from virus infection to cancer cell death

Prospects for the use of viral proteins for the construction of chimeric toxins

One of the actively developing areas of drug development is the creation of chimeric toxins, recombinant bifunctional molecules designed to affect target cells selectively. The prevalent approach involves fusing bacterial and plant toxins with molecules that facilitate targeted delivery. However, the therapeutic use of such toxins often encounters challenges associated with negative side effects. Concurrently, viruses encode proteins possessing toxin-like properties, exerting multiple effects on the vital activity of cells. In contrast to bacterial and plant toxins, the impact of viral proteins is typically milder, presenting a significant advantage by potentially reducing the likelihood of side effects. This review delineates the characteristics of extensively studied viral proteins with toxic and immunomodulatory properties and explores the prospects of incorporating them into chimeric toxins.

Identification of the Potential Role of the E4orf4 Protein in Adenovirus A, B, C, and D Groups in Cancer Therapy: Computational Approaches

The human adenovirus (HADV) early region 4 open reading frame 4 (E4orf4) protein plays a regulatory role in promoting viral infection by interacting with various cellular proteins. E4orf4 can induce death in cancer cells. One of the death pathways that is induced by this protein is related to the formation of membrane blebbing following the phosphorylation of tyrosine amino acids. The activation of this pathway requires the interaction of E4orf4 with Src family kinases (SFKs). The modulation mechanism of Src-dependent signaling via E4orf4 is not yet fully understood. However, evidence suggests that a physical association between the Src kinase domain and the arginine-rich motif of E4orf4 is crucial. Physically connecting E4orf4 to Src kinase leads to the deregulation of the Src-related signaling pathway, thereby inducing cytoplasmic death. In this study, we mapped the E4orf4 interaction site in Src to investigate the interaction between E4orf4 and Src in detail. We also compared the binding strength of E4orf4 proteins from different HADV groups. To this end, we performed bioinformatics structural analysis of the Src kinase domain and E4orf4 to identify E4orf4 interaction sites. The group with the lowest binding energy was predicted to be the most likely candidate for the highest cytoplasmic death in tumor cells based on the energy of the E4orf4-Src complex in various HADV groups. These results show that HADV-A and HADV-C have minimal binding energies to the E4orf4-Src complex, while the dissociation constant (Kd) of HADV-A was less than that of HADV-C. According to the obtained results, E4orf4 of the HADV-A group is more effective at triggering cytoplasmic death based on its most robust interaction with the Src kinase domain.

Open Reading Frame-3a gene of the 2019 novel coronavirus inhibits the occurrence and development of colorectal cancer

Intestinal microecology is composed of bacteria, fungi and viruses. As a part of intestinal microecology, viruses participate in the occurrence and development of colorectal cancer. The 2019-nCoV was detected in stool samples from patients during COVID-19, suggesting that the 2019-nCoV may be associated with intestinal microecology. However, the relationship of the 2019-nCoV and CRC is unclear. The aim of this study is to explore the role of Open Reading Frame-3a (ORF3a) of the 2019-nCoV in CRC. After the pCDH-CMV-MCS-EF1-Puro vector that provides high expression of ORF3a was transfected into the SW480 CRC cell line, immunofluorescence was used to determine the localization of ORF3a in SW480 cells. The proliferation, migration, invasion, apoptosis, and cell cycle progression of SW480 cells were measured using the Cell Counting Kit-8 (CCK-8), wound healing, Transwell assay, flow cytometry, the TUNEL assay, and propidium iodide single staining. The results showed that ORF3a inhibited the proliferation, invasion, and migration of SW480 cells and induced their apoptosis after 24, 48, 72 h. Meanwhile, ORF3a inhibited the cell cycle and blocked SW480 CRC cells in the G1 phase. In in vivo experiments, high ORF3a expression was associated with decreased tumor volume, tumor weight, relative tumor volume, and tumor activity. ORF3a inhibited the growth and induced apoptosis and necrosis of tumor tissues. Based on this, we demonstrated that ORF3a might play a role in CRC, providing a new direction for the prevention and treatment of CRC.

Adenovirus prevents dsRNA formation by promoting efficient splicing of viral RNA

Eukaryotic cells recognize intracellular pathogens through pattern recognition receptors, including sensors of aberrant nucleic acid structures. Sensors of double-stranded RNA (dsRNA) are known to detect replication intermediates of RNA viruses. It has long been suggested that annealing of mRNA from symmetrical transcription of both top and bottom strands of DNA virus genomes can produce dsRNA during infection. Supporting this hypothesis, nearly all DNA viruses encode inhibitors of dsRNA-recognition pathways. However, direct evidence that DNA viruses produce dsRNA is lacking. Contrary to dogma, we show that the nuclear-replicating DNA virus adenovirus (AdV) does not produce detectable levels of dsRNA during infection. In contrast, abundant dsRNA is detected within the nucleus of cells infected with AdV mutants defective for viral RNA processing. In the presence of nuclear dsRNA, the cytoplasmic dsRNA sensor PKR is relocalized and activated within the nucleus. Accumulation of viral dsRNA occurs in the late phase of infection, when unspliced viral transcripts form intron/exon base pairs between top and bottom strand transcripts. We propose that DNA viruses actively limit dsRNA formation by promoting efficient splicing and mRNA processing, thus avoiding detection and restriction by host innate immune sensors of pathogenic nucleic acids.

E4orf1 Suppresses E1B-Deleted Adenovirus Vaccine-Induced Immune Responses

As demonstrated by the recent COVID pandemic, vaccines can reduce the burden arising from infectious agents. Adenoviruses (Ads) with deletion of the early region 1B55K (ΔE1B Ad) are currently being explored for use in vaccine delivery. ΔE1B Ads are different from Ads with deletions in early region 1 and early region 3 (ΔE1/E3) used in most Ad vaccine vectors in that they contain the Ad early region 1A (E1A), and therefore the ability to replicate. Common to almost all Ads that are being explored for clinical use is the Ad early region 4 (E4). Among the E4 genes is open reading frame 1 (E4orf1), which mediates signals through the PI3-kinase/Akt pathway that is known to modulate immune responses. This suggests that E4orf1 might also modulate immune responses, although it has remained unexplored in ΔE1B Ad. Here, we show that cells infected with an E1B55K and E4orf1-deleted (ΔE41) Ad exhibited reduced levels of phosphorylated Akt (Ser473 and Thr308)) and expressed different intrinsic innate immune cytokines from those induced in cells infected with an E4orf1-containing, ΔE1B parental Ad that exhibited elevated levels of phosphorylated Akt. Rhesus macaques immunized with a ΔE41 Ad that expressed rhFLSC (HIV-1BaL gp120 linked to rhesus CD4 D1 and D2), exhibited higher levels of rhFLSC-specific interferon γ-producing memory T-cells, higher titers of rhFLSC-specific IgG1 binding antibody in serum, and antibodies able to mediate antibody-dependent cellular cytotoxicity (ADCC) with greater killing capacity than the ΔE1B Ad. Therefore, E4orf1, perhaps by acting through the PI3-kinase/Akt pathway, limits intrinsic innate and system-wide adaptive immune responses that are important for improved ΔE1B Ad-based vaccines.

PP2A Phosphatase as an Emerging Viral Host Factor

Protein phosphatase 2A (PP2A) is one of the most ubiquitous cellular proteins and is responsible for the vast majority of Ser/Thr phosphatase activity in eukaryotes. PP2A is a heterotrimer, and its assembly, intracellular localization, enzymatic activity, and substrate specificity are subject to dynamic regulation. Each of its subunits can be targeted by viral proteins to hijack and modulate its activity and downstream signaling to the advantage of the virus. Binding to PP2A is known to be essential to the life cycle of many viruses and seems to play a particularly crucial role for oncogenic viruses, which utilize PP2A to transform infected cells through controlling the cell cycle and apoptosis. Here we summarise the latest developments in the field of PP2A viral targeting; in particular recent discoveries of PP2A hijacking through molecular mimicry of a B56-specific motif by several different viruses. We also discuss the potential as well as shortcomings for therapeutic intervention in the face of our current understanding of viral PP2A targeting.

The adenoviral protein E4orf4: a probing tool to decipher mechanical stress-induced nuclear envelope remodeling in tumor cells

The human adenovirus (Ad) type 2/5 early region 4 (E4) ORF4 protein (E4orf4) exerts a remarkable tumor cell-selective killing activity in mammalian cells. This indicates that E4orf4 can target tumor cell-defining features and is a unique tool to probe cancer cell vulnerabilities. Recently, we found that E4orf4, through an interaction with the polarity protein PAR3, subverts nuclear envelope (NE) remodeling processes in a tumor cell-selective manner. In this Perspective, we outline mechanical signals that modify nuclear dynamics and tumor cell behavior to highlight potential mechanisms for E4orf4's tumoricidal activity. Through an analysis of E4orf4's cellular targets, we define a protein subnetwork that comprises phosphatase systems interconnected to polarity protein hubs, which could contribute to enhanced NE plasticity. We infer that elucidating E4orf4's protein network at a functional level could uncover key mechanisms of NE remodeling that define the tumor cell phenotype.

The FDA-Approved Drug Nelfinavir Inhibits Lytic Cell-Free but Not Cell-Associated Nonlytic Transmission of Human Adenovirus

Adenoviruses (AdVs) are prevalent and give rise to chronic and recurrent disease. Human AdV (HAdV) species B and C, such as HAdV-C2, -C5, and -B14, cause respiratory disease and constitute a health threat for immunocompromised individuals. HAdV-Cs are well known for lysing cells owing to the E3 CR1-β-encoded adenovirus death protein (ADP). We previously reported a high-throughput image-based screening framework and identified an inhibitor of HAdV-C2 multiround infection, nelfinavir mesylate. Nelfinavir is the active ingredient of Viracept, an FDA-approved inhibitor of human immunodeficiency virus (HIV) aspartyl protease that is used to treat AIDS. It is not effective against single-round HAdV infections. Here, we show that nelfinavir inhibits lytic cell-free transmission of HAdV, indicated by the suppression of comet-shaped infection foci in cell culture. Comet-shaped foci occur upon convection-based transmission of cell-free viral particles from an infected cell to neighboring uninfected cells. HAdV lacking ADP was insensitive to nelfinavir but gave rise to comet-shaped foci, indicating that ADP enhances but is not required for cell lysis. This was supported by the notion that HAdV-B14 and -B14p1 lacking ADP were highly sensitive to nelfinavir, although HAdV-A31, -B3, -B7, -B11, -B16, -B21, -D8, -D30, and -D37 were less sensitive. Conspicuously, nelfinavir uncovered slow-growing round HAdV-C2 foci, independent of neutralizing antibodies in the medium, indicative of nonlytic cell-to-cell transmission. Our study demonstrates the repurposing potential of nelfinavir with postexposure efficacy against different HAdVs and describes an alternative nonlytic cell-to-cell transmission mode of HAdV.

The Adenovirus Death Protein - a small membrane protein controls cell lysis and disease

Human adenoviruses (HAdVs) cause widespread acute and persistent infections. Infections are usually mild and controlled by humoral and cell-based immunity. Reactivation of persistently infected immune cells can lead to a life-threatening disease in immunocompromised individuals, especially children and transplant recipients. To date, no effective therapy or vaccine against HAdV disease is available to the public. HAdV-C2 and C5 are the best-studied of more than 100 HAdV types. They persist in infected cells and release their progeny by host cell lysis to neighbouring cells and fluids, a process facilitated by the adenovirus death protein (ADP). ADP consists of about 100 amino acids and harbours a single membrane-spanning domain. It undergoes post-translational processing in endoplasmic reticulum and Golgi compartments, before localizing to the inner nuclear membrane. Here, we discuss the current knowledge on how ADP induces membrane rupture. Membrane rupture is essential for both progression of disease and efficacy of therapeutic viruses in clinical applications, in particular oncolytic therapy.

Adenoviruses - Infection, pathogenesis and therapy

Both well-known and emerging viruses increasingly affect humans and cause disease, sometimes with devastating impact on society. The viruses present in the biosphere are the top predators in the life chain, virtually without enemies, except perhaps the immune system, and harsh environmental physicochemical conditions restricting their dissemination. We know a lot about viruses, but do we know enough? This series of reviews is dedicated to adenoviruses (AdVs), a family of nonenveloped DNA viruses occurring in vertebrates, including humans. AdVs have been the focus of intense research for more than 67 years. Besides causing disease, they have immensely contributed to the advance of life sciences and medicine over the past decades. Recently, AdVs have been widely used as vehicles in gene therapy and vaccination. They continue to provide fundamental insights into virus-host interactions in cells, tissues and organisms, as well as systems and metabolic networks. This special issue of FEBS Letters presents a unique collection of 23 state-of-the-art review articles by leading adenovirologists. In this prelude, I present the chapters, which provide a solid basis for further exploring the rich heritage in adenovirus molecular cell biology, structural biology, genetics, immunology, gene therapy and epidemiology. I conclude with an essential discussion of six blind spots in adenovirology.