Mechanisms of cancer cell killing by the adenovirus E4orf4 protein

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.

Study on Propagation and Adaptation of EDS-76 Avian Adenovirus in Duck and SPF Primary Embryonic Chicken Cell Culture Comparison to Duck and SPF Embryonated Chicken Eggs

Egg drop syndrome (EDS) is a major viral infectious poultry disease with severe economic losses in laying hens. The disease is caused by an adenovirus and can be transmitted horizontally and vertically. This study investigated the EDS virus (EDSV) infection in duck embryo fibroblasts (DEF), specific pathogen-free (SPF) embryo fibroblasts, and SPF egg embryos using different methods. The results were compared to the virus culture in duck and SPF chicken eggs. Duck and chicken fibroblast cells were used as the primary cell culture in Dulbecco's Modified Eagle Medium, and the low-pathogenic duck adenovirus was used to infect the ducks and SPF fibroblasts primary cell cultures, as well as the duck and SPF eggs. The titer of the virus was measured by hemagglutination assay, ECID50, plaque-forming unit, and TCID50 methods. The results revealed that EDSV could proliferate in the chorioallantoic membrane of DEF cells and duck eggs, compared to the chorioallantoic membrane of chicken embryo fibroblasts (CEF) and SPF chicken eggs. The findings showed that duck egg embryos and primary DEF cell lines are more appropriate for EDSV replication, compared to CEF and SPF chicken eggs. This suggests that the use of DEF culture for producing avian adenovirus EDS-76 is a suitable alternative for the embryonic egg culture.

Generation of a soluble and stable apoptin-EGF fusion protein, a targeted viral protein applicable for tumor therapy

A promising candidate for tumor targeted toxins is the chicken anemia-derived protein apoptin that induces tumor-specific apoptosis. It was aimed to design a novel apoptin-based targeted toxin by genetic fusion of apoptin with the tumor-directed ligand epidermal growth factor (EGF) using Escherichia coli as expression host. However, apoptin is highly hydrophobic and tends to form insoluble aggregates. Therefore, three different apoptin-EGF variants were generated. The fusion protein hexa-histidine (His)-apoptin-EGF (HAE) was expressed in E. coli and purified under denaturing conditions due to inclusion bodies. The protein solubility was improved by maltose-binding protein (MBP) or glutathione S-transferase. The protein MBP-apoptin-EGF^His (MAEH) was found favorable as a targeted toxin regarding final yield (4-6 mg/L) and stability. MBP was enzymatically removed using clotting factor Xa, which resulted in low yield and poor separation. MAEH was tested on target and non-target cell lines. The targeted tumor cell line A431 showed significant toxicity with an IC₅₀ of 69.55 nM upon incubation with MAEH while fibroblasts and target receptor-free cells remained unaffected. Here we designed a novel EGF receptor targeting drug with high yield, purity and stability.

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.

An Interaction with PARP-1 and Inhibition of Parylation Contribute to Attenuation of DNA Damage Signaling by the Adenovirus E4orf4 Protein

The adenovirus (Ad) E4orf4 protein was reported to contribute to inhibition of ATM- and ATR-regulated DNA damage signaling during Ad infection and following treatment with DNA-damaging drugs. Inhibition of these pathways improved Ad replication, and when expressed alone, E4orf4 sensitized transformed cells to drug-induced toxicity. However, the mechanisms utilized were not identified. Here, we show that E4orf4 associates with the DNA damage sensor poly(ADP-ribose) polymerase 1 (PARP-1) and that the association requires PARP activity. During Ad infection, PARP is activated, but its activity is not required for recruitment of either E4orf4 or PARP-1 to virus replication centers, suggesting that their association occurs following recruitment. Inhibition of PARP-1 assists E4orf4 in reducing DNA damage signaling during infection, and E4orf4 attenuates virus- and DNA damage-induced parylation. Furthermore, E4orf4 reduces PARP-1 phosphorylation on serine residues, which likely contributes to PARP-1 inhibition as phosphorylation of this enzyme was reported to enhance its activity. PARP-1 inhibition is important to Ad infection since treatment with a PARP inhibitor enhances replication efficiency. When E4orf4 is expressed alone, it associates with poly(ADP-ribose) (PAR) chains and is recruited to DNA damage sites in a PARP-1-dependent manner. This recruitment is required for inhibition of drug-induced ATR signaling by E4orf4 and for E4orf4-induced cancer cell death. Thus, the results presented here demonstrate a novel mechanism by which E4orf4 targets and inhibits DNA damage signaling through an association with PARP-1 for the benefit of the virus and impacting E4orf4-induced cancer cell death.IMPORTANCE Replication intermediates and ends of viral DNA genomes can be recognized by the cellular DNA damage response (DDR) network as DNA damage whose repair may lead to inhibition of virus replication. Therefore, many viruses evolved mechanisms to inhibit the DDR network. We have previously shown that the adenovirus (Ad) E4orf4 protein inhibits DDR signaling, but the mechanisms were not identified. Here, we describe an association of E4orf4 with the DNA damage sensor poly(ADP-ribose) polymerase 1 (PARP-1). E4orf4 reduces phosphorylation of this enzyme and inhibits its activity. PARP-1 inhibition assists E4orf4 in reducing Ad-induced DDR signaling and improves the efficiency of virus replication. Furthermore, the ability of E4orf4, when expressed alone, to accumulate at DNA damage sites and to kill cancer cells is attenuated by chemical inhibition of PARP-1. Our results indicate that the E4orf4-PARP-1 interaction has an important role in Ad replication and in promotion of E4orf4-induced cancer-selective cell death.

Selective elimination of cancer cells by the adenovirus E4orf4 protein in a Drosophila cancer model: a new paradigm for cancer therapy

The adenovirus (Ad) E4orf4 protein contributes to efficient progression of virus infection. When expressed alone E4orf4 induces p53- and caspase-independent cell-death, which is more effective in cancer cells than in normal cells in tissue culture. Cancer selectivity of E4orf4-induced cell-death may result from interference with various regulatory pathways that cancer cells are more dependent on, including DNA damage signaling and proliferation control. E4orf4 signaling is conserved in several organisms, including yeast, Drosophila, and mammalian cells, indicating that E4orf4-induced cell-death can be investigated in these model organisms. The Drosophila genetic model system has contributed significantly to the study of cancer and to identification of novel cancer therapeutics. Here, we used the fly model to investigate the ability of E4orf4 to eliminate cancer tissues in a whole organism with minimal damage to normal tissues. We show that E4orf4 dramatically inhibited tumorigenesis and rescued survival of flies carrying a variety of tumors, including highly aggressive and metastatic tumors in the fly brain and eye discs. Moreover, E4orf4 rescued the morphology of adult eyes containing scrib^- cancer clones even when expressed at a much later stage than scrib elimination. The E4orf4 partner protein phosphatase 2A (PP2A) was required for inhibition of tumorigenesis by E4orf4 in the system described here, whereas another E4orf4 partner, Src kinase, provided only minimal contribution to this process. Our results suggest that E4orf4 is an effective anticancer agent and reveal a promising potential for E4orf4-based cancer treatments.

Biphasic Functional Interaction between the Adenovirus E4orf4 Protein and DNA-PK

The adenovirus (Ad) E4orf4 protein contributes to virus-induced inhibition of the DNA damage response (DDR) by reducing ATM and ATR signaling. Consequently, E4orf4 inhibits DNA repair and sensitizes transformed cells to killing by DNA-damaging drugs. Inhibition of ATM and ATR signaling contributes to the efficiency of virus replication and may provide one explanation for the cancer selectivity of cell death induced by the expression of E4orf4 alone. In this report, we investigate a direct interaction of E4orf4 with the DDR. We show that E4orf4 physically associates with the DNA-dependent protein kinase (DNA-PK), and we demonstrate a biphasic functional interaction between these proteins, wherein DNA-PK is required for ATM and ATR inhibition by E4orf4 earlier during infection but is inhibited by E4orf4 as infection progresses. This biphasic process is accompanied by initial augmentation and a later inhibition of DNA-PK autophosphorylation as well as by colocalization of DNA-PK with early Ad replication centers and distancing of DNA-PK from late replication centers. Moreover, inhibition of DNA-PK improves Ad replication more effectively when a DNA-PK inhibitor is added later rather than earlier during infection. When expressed alone, E4orf4 is recruited to DNA damage sites in a DNA-PK-dependent manner. DNA-PK inhibition reduces the ability of E4orf4 to induce cancer cell death, likely because E4orf4 is prevented from arriving at the damage sites and from inhibiting the DDR. Our results support an important role for the E4orf4-DNA-PK interaction in Ad replication and in facilitation of E4orf4-induced cancer-selective cell death.IMPORTANCE Several DNA viruses evolved mechanisms to inhibit the cellular DNA damage response (DDR), which acts as an antiviral defense system. We present a novel mechanism by which the adenovirus (Ad) E4orf4 protein inhibits the DDR. E4orf4 interacts with the DNA damage sensor DNA-PK in a biphasic manner. Early during infection, E4orf4 requires DNA-PK activity to inhibit various branches of the DDR, whereas it later inhibits DNA-PK itself. Furthermore, although both E4orf4 and DNA-PK are recruited to virus replication centers (RCs), DNA-PK is later distanced from late-phase RCs. Delayed DNA-PK inhibition greatly contributes to Ad replication efficiency. When E4orf4 is expressed alone, it is recruited to DNA damage sites. Inhibition of DNA-PK prevents both recruitment and the previously reported ability of E4orf4 to kill cancer cells. Our results support an important role for the E4orf4-DNA-PK interaction in Ad replication and in facilitation of E4orf4-induced cancer-selective cell death.

The Anaphase Promoting Complex/Cyclosome (APC/C): A Versatile E3 Ubiquitin Ligase

In the present chapter we discuss the essential roles of the human E3 ubiquitin ligase Anaphase Promoting Complex/Cyclosome (APC/C) in mitosis as well as the emerging evidence of important APC/C roles in cellular processes beyond cell division control such as regulation of genomic integrity and cell differentiation of the nervous system. We consider the potential incipient role of APC/C dysregulation in the pathophysiology of the neurological disorder Alzheimer's disease (AD). We also discuss how certain Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA) viruses take control of the host's cell division regulatory system through harnessing APC/C ubiquitin ligase activity and hypothesise the plausible molecular mechanisms underpinning virus manipulation of the APC/C. We also examine how defects in the function of this multisubunit protein assembly drive abnormal cell proliferation and lastly argue the potential of APC/C as a promising therapeutic target for the development of innovative therapies for the treatment of chronic malignancies such as cancer.

RIPK3 promotes adenovirus type 5 activity

Oncolytic adenoviral mutants infect human malignant cells and replicate selectively within them. This induces direct cytotoxicity that can also trigger profound innate and adaptive immune responses. However, the mechanism by which adenoviruses produce cell death remains uncertain. We previously suggested that type 5 adenoviruses, including the E1A CR2 deletion mutant dl922-947, might induce a novel form of programmed death resembling necroptosis. Here we have investigated the roles of core necrosis proteins RIPK1, RIPK3 and MLKL in the cytotoxicity of dl922-947 and other adenovirus serotypes. By electron microscopy, we show that dl922-947 induces similar necrotic morphology as TSZ treatment (TNF-α, Smac mimetic, zVAD.fmk). However, dl922-947-mediated death is independent of TNF-α signalling, does not require RIPK1 and does not rely upon the presence of MLKL. However, inhibition of caspases, specifically caspase-8, induces necroptosis that is RIPK3 dependent and significantly enhances dl922-947 cytotoxicity. Moreover, using CRISPR/Cas9 gene editing, we demonstrate that the increase in cytotoxicity seen upon caspase inhibition is also MLKL dependent. Even in the absence of caspase inhibition, RIPK3 expression promotes dl922-947 and wild-type adenovirus type 5 efficacy both in vitro and in vivo. Together, these results suggest that adenovirus induces a form of programmed necrosis that differs from classical TSZ necroptosis.

The Adenovirus E4orf4 Protein Provides a Novel Mechanism for Inhibition of the DNA Damage Response

The DNA damage response (DDR) is a conglomerate of pathways designed to detect DNA damage and signal its presence to cell cycle checkpoints and to the repair machinery, allowing the cell to pause and mend the damage, or if the damage is too severe, to trigger apoptosis or senescence. Various DDR branches are regulated by kinases of the phosphatidylinositol 3-kinase-like protein kinase family, including ataxia-telangiectasia mutated (ATM) and ATM- and Rad3-related (ATR). Replication intermediates and linear double-stranded genomes of DNA viruses are perceived by the cell as DNA damage and activate the DDR. If allowed to operate, the DDR will stimulate ligation of viral genomes and will inhibit virus replication. To prevent this outcome, many DNA viruses evolved ways to limit the DDR. As part of its attack on the DDR, adenovirus utilizes various viral proteins to cause degradation of DDR proteins and to sequester the MRN damage sensor outside virus replication centers. Here we show that adenovirus evolved yet another novel mechanism to inhibit the DDR. The E4orf4 protein, together with its cellular partner PP2A, reduces phosphorylation of ATM and ATR substrates in virus-infected cells and in cells treated with DNA damaging drugs, and causes accumulation of damaged DNA in the drug-treated cells. ATM and ATR are not mutually required for inhibition of their signaling pathways by E4orf4. ATM and ATR deficiency as well as E4orf4 expression enhance infection efficiency. Furthermore, E4orf4, previously reported to induce cancer-specific cell death when expressed alone, sensitizes cells to killing by sub-lethal concentrations of DNA damaging drugs, likely because it inhibits DNA damage repair. These findings provide one explanation for the cancer-specificity of E4orf4-induced cell death as many cancers have DDR deficiencies leading to increased reliance on the remaining intact DDR pathways and to enhanced susceptibility to DDR inhibitors such as E4orf4. Thus DDR inhibition by E4orf4 contributes both to the efficiency of adenovirus replication and to the ability of E4orf4 to kill cancer cells.