Apoptin induces apoptosis in human transformed and malignant cells but not in normal cells

The interactions between PML nuclear bodies and small and medium size DNA viruses

Promyelocytic leukemia nuclear bodies (PM NBs), often referred to as membraneless organelles, are dynamic macromolecular protein complexes composed of a PML protein core and other transient or permanent components. PML NBs have been shown to play a role in a wide variety of cellular processes. This review describes in detail the diverse and complex interactions between small and medium size DNA viruses and PML NBs that have been described to date. The PML NB components that interact with small and medium size DNA viruses include PML protein isoforms, ATRX/Daxx, Sp100, Sp110, HP1, and p53, among others. Interaction between viruses and components of these NBs can result in different outcomes, such as influencing viral genome expression and/or replication or impacting IFN-mediated or apoptotic cell responses to viral infection. We discuss how PML NB components abrogate the ability of adenoviruses or Hepatitis B virus to transcribe and/or replicate their genomes and how papillomaviruses use PML NBs and their components to promote their propagation. Interactions between polyomaviruses and PML NBs that are poorly understood but nevertheless suggest that the NBs can serve as scaffolds for viral replication or assembly are also presented. Furthermore, complex interactions between the HBx protein of hepadnaviruses and several PML NBs-associated proteins are also described. Finally, current but scarce information regarding the interactions of VP3/apoptin of the avian anellovirus with PML NBs is provided. Despite the considerable number of studies that have investigated the functions of the PML NBs in the context of viral infection, gaps in our understanding of the fine interactions between viruses and the very dynamic PML NBs remain. The complexity of the bodies is undoubtedly a great challenge that needs to be further addressed.

Semisynthesis reveals apoptin as a tumour-selective protein prodrug that causes cytoskeletal collapse

Apoptin is a small viral protein capable of inducing cell death selectively in cancer cells. Despite its potential as an anticancer agent, relatively little is known about its mechanism of toxicity and cancer-selectivity. Previous experiments suggest that cancer-selective phosphorylation modulates apoptin toxicity, although a lack of chemical tools has hampered the dissection of underlying mechanisms. Here, we describe structure-function studies with site-specifically phosphorylated apoptin (apoptin-T108ph) in living cells which revealed that Thr108 phosphorylation is the selectivity switch for apoptin toxicity. Mechanistic investigations link T108ph to actin binding, cytoskeletal disruption and downstream inhibition of anoikis-resistance as well as cancer cell invasion. These results establish apoptin as a protein pro-drug, selectively activated in cancer cells by phosphorylation, which disrupts the cytoskeleton and promotes cell death. We anticipate that this mechanism provides a framework for the design of next generation anticancer proteins with enhanced selectivity and potency.

Microbial cancer therapeutics: A promising approach

The success of conventional cancer therapeutics is hindered by associated dreadful side-effects of antibiotic resistance and the dearth of antitumor drugs' selectivity and specificity. Hence, the conceptual evolution of anti-cancerous therapeutic agents that selectively target cancer cells without impacting the healthy cells or tissues, has led to a new wave of scientific interest in microbial-derived bioactive molecules. Such strategic solutions may pave the way to surmount the shortcomings of conventional therapies and raise the potential and hope for the cure of wide range of cancer in a selective manner. This review aims to provide a comprehensive summary of anti-carcinogenic properties and underlying mechanisms of bioactive molecules of microbial origin, and discuss the current challenges and effective therapeutic application of combinatorial strategies to attain minimal systemic side-effects.

Construction of chicken infectious anemia virus infectious clone and study on its pathogenicity

Chicken infectious anemia virus (CIAV) can be transmitted through contaminated live poultry vaccine. However, the pathogenicity of contaminated CIAV strains is rarely reported. Previously, the chickens showed the typical symptoms of anemia after using the attenuated live fowl pox virus (FPV) vaccine. Therefore, exogenous CIAV contamination was suspected. We detected anti-CIAV antibodies in SPF chicks vaccinated with the FPV vaccine. CIAV contamination was confirmed in the FPV vaccine, and the CIAV strain was named JS2020-FPV. This study aims to rescue JS2020-FPV by reverse genetic assays and investigate its pathogenicity. Firstly, double-copies infectious clone of JS2020-FPV was constructed. For the pathogenicity study, infectious clone of JS2020-FPV was used to inoculate 1-day-old SPF chicks. The typical symptoms of anemia were observed in the JS2020-PFV group 14  days post inoculation. The hematocrit and body weight of chicks in the JS2020-PFV group were significantly lower than those in the mock group. Notably, the thymus development index and antibody levels of NDV were lower in chicks in the JS2020-PFV group than those in the mock group. Different degrees of apoptosis of MSB1 and DF-1 were observed after inoculated with the JS2020-FPV VP3 recombinant fusion protein expressed by E. coli system, indicating that VP3 induced apoptosis in the transformed cells. Overall, the pathogenicity of the CIAV detected in the contaminated vaccine was confirmed by inoculating SPF chicks with the double-copies infectious DNA clone in this study. Our findings indicate that the dangers of vaccine contamination cannot be ignored.

All-In-One Biomimetic Nanoplatform Based on Hollow Polydopamine Nanoparticles for Synergistically Enhanced Radiotherapy of Colon Cancer

Even though radiotherapy is the most important therapeutic strategy for colon cancer treatment, there is an enormous demand to improve radiosensitivity in solid tumor destruction. For this purpose, a biomimetic nanoplatform based on hollow polydopamine nanoparticles (HP) with homologous targeting and pH-responsive drug release properties is designed. In this work, HP is constructed by using a chelation competition-induced polymerization strategy and then modified with the cancer cell membrane. Hollow polydopamine integrated with Pt nanoparticles (Pt@HP) has a catalase-like activity, which can be used to trigger endogenous H₂ O₂ into O₂ , relieving hypoxia of the tumor microenvironment (TME). With mesoporous shells and large cavities, Pt@HP shows efficient apoptin^100-109 (AP) and verteporfin (VP) loading to form AVPt@HP@M. Under X-ray irradiation, AVPt@HP@M exerts a radiosensitization effect via multiple strategies, including relieving hypoxia (Pt NPs), enhancing tumor apoptosis (AP), and X-ray-induced photodynamic therapy (X-PDT) (VP). Further metabonomics analysis shows that the specific mechanism of the AVPt@HP@M is through influencing purine metabolism. Without appreciable systemic toxicity, this nanoplatform highlights a new strategy for effective radiosensitization and provides a reference for treating malignant tumors.

Innovative Approaches of Engineering Tumor-Targeting Bacteria with Different Therapeutic Payloads to Fight Cancer: A Smart Strategy of Disease Management

Conventional therapies for cancer eradication like surgery, radiotherapy, and chemotherapy, even though most widely used, still suffer from some disappointing outcomes. The limitations of these therapies during cancer recurrence and metastasis demonstrate the need for better alternatives. Some bacteria preferentially colonize and proliferate inside tumor mass; thus these bacteria can be used as ideal candidates to deliver antitumor therapeutic agents. The bacteria like Bacillus spp., Clostridium spp., E. coli, Listeria spp., and Salmonella spp. can be reprogrammed to produce, transport, and deliver anticancer agents, eg, cytotoxic agents, prodrug converting enzymes, immunomodulators, tumor stroma targeting agents, siRNA, and drug-loaded nanoformulations based on clinical requirements. In addition, these bacteria can be genetically modified to express various functional proteins and targeting ligands that can enhance the targeting approach and controlled drug-delivery. Low tumor-targeting and weak penetration power deep inside the tumor mass limits the use of anticancer drug-nanoformulations. By using anticancer drug nanoformulations and other therapeutic payloads in combination with antitumor bacteria, it makes a synergistic effect against cancer by overcoming the individual limitations. The tumor-targeting bacteria can be either used as a monotherapy or in addition with other anticancer therapies like photothermal therapy, photodynamic therapy, and magnetic field therapy to accomplish better clinical outcomes. The toxicity issues on normal tissues is the main concern regarding the use of engineered antitumor bacteria, which requires deeper research. In this article, the mechanism by which bacteria sense tumor microenvironment, role of some anticancer agents, and the recent advancement of engineering bacteria with different therapeutic payloads to combat cancers has been reviewed. In addition, future prospective and some clinical trials are also discussed.

Expanding the toolbox of exosome-based modulators of cell functions

Exosomes are cell-derived extracellular vesicles and play important roles in mediating intercellular communications. Due to their unique advantages in transporting a variety of biomolecules, exosomes have been emerging as a new class of nanocarriers with great potential for therapeutic applications. Despite advancements in loading chemotherapeutics and interfering RNAs into exosomes, active incorporation of protein molecules into exosomes remains challenging owing to their distinctive physicochemical properties and/or a lack of knowledge of cargo sorting during exosome biogenesis. Here we report the generation of a novel type of engineered exosomes with actively incorporated membrane proteins or soluble protein cargos, named genetically infused functionally tailored exosomes (GIFTed-Exos). Through genetic fusion with exosome-associated tetraspanin CD9, transmembrane protein CD70 and glucocorticoid-induced tumor necrosis factor receptor family-related ligand (GITRL) could be displayed on exosome surface, resulting in GIFTed-Exos with excellent T-cell co-stimulatory activities. By genetically linking to a CD9-photocleavable protein fusion, fluorescent protein mCherry, apoptosis-inducing protein apoptin, and antioxidant enzyme catalase could be effectively packed into exosomes for light-controlled release. The generated GIFTed-Exos display notable in vitro and in vivo activities for delivering distinct types of protein cargos to target cells. As a possibly general approach, GIFTed-Exos provide new opportunities to create exosomes with new functions and properties for biomedical research.

Origin, Evolution and Stability of Overlapping Genes in Viruses: A Systematic Review

During their long evolutionary history viruses generated many proteins de novo by a mechanism called “overprinting”. Overprinting is a process in which critical nucleotide substitutions in a pre-existing gene can induce the expression of a novel protein by translation of an alternative open reading frame (ORF). Overlapping genes represent an intriguing example of adaptive conflict, because they simultaneously encode two proteins whose freedom to change is constrained by each other. However, overlapping genes are also a source of genetic novelties, as the constraints under which alternative ORFs evolve can give rise to proteins with unusual sequence properties, most importantly the potential for novel functions. Starting with the discovery of overlapping genes in phages infecting Escherichia coli, this review covers a range of studies dealing with detection of overlapping genes in small eukaryotic viruses (genomic length below 30 kb) and recognition of their critical role in the evolution of pathogenicity. Origin of overlapping genes, what factors favor their birth and retention, and how they manage their inherent adaptive conflict are extensively reviewed. Special attention is paid to the assembly of overlapping genes into ad hoc databases, suitable for future studies, and to the development of statistical methods for exploring viral genome sequences in search of undiscovered overlaps.

Apoptosis Triggered by ORF3 Proteins of the Circoviridae Family

Apoptosis, a form of the programmed cell death, is an indispensable defense mechanism regulating cellular homeostasis and is triggered by multiple stimuli. Because of the regulation of apoptosis in cellular homeostasis, viral proteins with apoptotic activity are particular foci of on antitumor therapy. One representative viral protein is the open reading frame 3 (ORF3) protein, also named as apoptin in the Circoviridae chicken anemia virus (CAV), and has the ability to induce tumor-specific apoptosis. Proteins encoded by ORF3 in other circovirus species, such as porcine circovirus (PCV) and duck circovirus (DuCV), have also been reported to induce apoptosis, with subtle differences in apoptotic activity based on cell types. This article is aimed at reviewing the latest research advancements in understanding ORF3 protein-mediated apoptosis mechanisms of Circoviridae from three perspectives: subcellular localization, interactions with host proteins, and participation in multiple apoptotic signaling pathways, providing a scientific basis for circovirus pathogenesis and a reference on its potential anticancer function.

A dual cancer-specific recombinant adenovirus suppresses the growth of liver cancer cells in vivo and in vitro

Oncolytic virus therapy is emerging as important means in cancer treatment. In a previous study, we constructed a dual cancer-specific antitumor recombinant adenovirus, designating it Ad-apoptin-hTERTp-E1a (Ad-VT). This study aimed to investigate the anticancer potential of recombinant adenovirus Ad-apoptin-hTERTp-E1a (Ad-VT) in liver cancer. Crystal Violet staining and CCK-8 assays were used to analyse the inhibitory effect of recombinant adenovirus on human hepatoma cell line QGY-7703 and SMMC-7721. Ad-VT had a significant tumour killing inhibitory effect on QGY-7703 and SMMC-7721 cells that was both dose and a time dependent. Ad-VT-induced apoptosis of QGY-7703 cells was detected using Hoechst, Annexin V, and JC-1 staining, as well as western blotting. Recombinant adenovirus had a strong apoptosis-inducing effect on QGY-7703 cells, and killed QGY-7703 cells mainly through the mitochondrial apoptotic pathway. QGY-7703 cells invasion were detected using cell-scratch and Transwell assays. Recombinant adenovirus could significantly inhibit the invasion of QGY-7703 cells over a short period of time. The pGL4.51 plasmid was used to transfect QGY-7703 cells to construct tumour cells stably expressing luciferase (QGY-7703-LUC). The tumour inhibition effect of Ad-VT in vivo was subsequently confirmed by establishing a tumour-bearing nude mouse model. Ad-VT could effectively inhibit tumour growth and prolong survival of the mice. Recombinant adenovirus Ad-VT has the characteristics of tumour-specific replication and specific tumour killing, and could inhibit the growth of liver cancer QGY-7703 cells and promote their apoptosis.

Anti-tumor effect of a dual cancer-specific recombinant adenovirus on ovarian cancer cells

BACKGROUND

Apoptin can specifically kill cancer cells but has no toxicity to normal cells. Human telomerase reverse transcriptase (hTERT) acts as a tumor-specific promoter, triggering certain genes to replicate or express only in tumor cells, conferring specific replication and killing abilities. This study aimed at investigating the anticancer potential of the recombinant adenovirus Ad-apoptin-hTERTp-E1a (Ad-VT) in ovarian cancer treatment.

METHODS

Crystal Violet staining and WST-1 assays were used to analyze the inhibitory effect of Ad-VT on ovarian cancer SKOV3 and OVCAR-3 cells. Ad-VT-induced apoptosis of ovarian cancer cells, was detected using Hoechst, Annexin V-FITC/PI, JC-1 staining. Cell migration and invasion of ovarian cancer cells were detected using cell-scratch and Transwell assays. The pGL4.51 plasmid was used to transfect and to generate SKOV3-LUC cells, that stably express luciferase. The in vivo tumor inhibition effect of Ad-VT was subsequently confirmed using a tumor-bearing nude mouse model.

RESULTS

Ad-VT had a strong apoptosis-inducing effect on SKOV3 and OVCAR-3 cells, that was mainly mediated through the mitochondrial apoptotic pathway. The Ad-VT could significantly increase the inhibition of ovarian cancer cell migration and invasion. The Ad-VT also can inhibit tumor growth and reduce toxicity in vivo.

CONCLUSIONS

The recombinant adenovirus, comprising the apoptin protein and the hTERTp promoter, was able to inhibit the growth of ovarian cancer cells and promote their apoptosis.

p28 Bacterial Peptide, as an Anticancer Agent

Cancer remains a major cause of morbidity and mortality irrespective of the type of conventional chemotherapy. Therefore, there is an urgent need for new and effective anticancer therapeutic agents. Bacterial proteins and their derivative peptides appear as a promising approach for cancer treatment. Several, including an amphipathic, α-helical, 28-amino acid peptide derived from azurin, a 128-amino acid copper-containing redox protein secreted from Pseudomonas aeruginosa, show clinical promise in the treatment of adult and pediatric solid tumors. The peptide, p28, is a post-translational, multi-target anticancer agent that preferentially enters a wide variety of solid tumor cells. Mechanistically, after entry, p28 has two major avenues of action. It binds to both wild-type and mutant p53 protein, inhibiting constitutional morphogenic protein 1 (Cop1)-mediated ubiquitination and proteasomal degradation of p53. This results in increased levels of p53, which induce cell-cycle arrest at G2/M and an eventual apoptosis that results in tumor cell shrinkage and death. In addition, p28 also preferentially enters nascent endothelial cells and decreases the phosphorylation of FAK and Akt inhibiting endothelial cell motility and migration. Here, we review the current basic and clinical evidence suggesting the potential of p28 as a cancer therapeutic peptide.

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.

Apoptin as a Tumor-Specific Therapeutic Agent: Current Perspective on Mechanism of Action and Delivery Systems

Cancer remains one of the leading causes of death worldwide in humans and animals. Conventional treatment regimens often fail to produce the desired outcome due to disturbances in cell physiology that arise during the process of transformation. Additionally, development of treatment regimens with no or minimum side-effects is one of the thrust areas of modern cancer research. Oncolytic viral gene therapy employs certain viral genes which on ectopic expression find and selectively destroy malignant cells, thereby achieving tumor cell death without harming the normal cells in the neighborhood. Apoptin, encoded by Chicken Infectious Anemia Virus' VP3 gene, is a proline-rich protein capable of inducing apoptosis in cancer cells in a selective manner. In normal cells, the filamentous Apoptin becomes aggregated toward the cell margins, but is eventually degraded by proteasomes without harming the cells. In malignant cells, after activation by phosphorylation by a cancer cell-specific kinase whose identity is disputed, Apoptin accumulates in the nucleus, undergoes aggregation to form multimers, and prevents the dividing cancer cells from repairing their DNA lesions, thereby forcing them to undergo apoptosis. In this review, we discuss the present knowledge about the structure of Apoptin protein, elaborate on its mechanism of action, and summarize various strategies that have been used to deliver it as an anticancer drug in various cancer models.

The Role of Apoptin in Chicken Anemia Virus Replication

Apoptin is the Vp3 protein of chicken anemia virus (CAV), which infects the thymocytes and erythroblasts in young chickens, causing chicken infectious anemia and immunosuppression. Apoptin is highly studied for its ability to selectively induce apoptosis in human tumor cells and, thus, is a protein of interest in anti-tumor therapy. CAV apoptin is known to localize to different subcellular compartments in transformed and non-transformed cells, depending on the DNA damage response, and the phosphorylation of several identified threonine residues. In addition, apoptin interacts with molecular machinery such as the anaphase promoting complex/cyclosome (APC/C) to inhibit the cell cycle and induce arrest in G2/M phase. While these functions of apoptin contribute to the tumor-selective effect of the protein, they also provide an important fundamental framework to apoptin’s role in viral infection, pathogenesis, and propagation. Here, we reviewed how the regulation, localization, and functions of apoptin contribute to the viral life cycle and postulated its importance in efficient replication of CAV. A model of the molecular biology of infection is critical to informing our understanding of CAV and other related animal viruses that threaten the agricultural industry.

New insights into the evolutionary features of viral overlapping genes by discriminant analysis

Overlapping genes originate by a mechanism of overprinting, in which nucleotide substitutions in a pre-existing frame induce the expression of a de novo protein from an alternative frame. In this study, I assembled a dataset of 319 viral overlapping genes, which included 82 overlaps whose expression is experimentally known and the respective 237 homologs. Principal component analysis revealed that overlapping genes have a common pattern of nucleotide and amino acid composition. Discriminant analysis separated overlapping from non-overlapping genes with an accuracy of 97%. When applied to overlapping genes with known genealogy, it separated ancestral from de novo frames with an accuracy close to 100%. This high discriminant power was crucial to computationally design variants of de novo viral proteins known to possess selective anticancer toxicity (apoptin) or protection against neurodegeneration (X protein), as well as to detect two new potential overlapping genes in the genome of the new coronavirus SARS-CoV-2.

Anti-tumor Synergistic Effect of a Dual Cancer-Specific Recombinant Adenovirus and Paclitaxel on Breast Cancer

This study aimed at investigating the anticancer potential of the recombinant adenovirus Ad-apoptin-hTERTp-E1a (Ad-VT) and its synergistic combination with paclitaxel (PTX) in breast cancer treatment. First, we used the Calcusyn software to analyze the synergy between the Ad-VT and paclitaxel, and to determine the final drug concentration. Second, we used crystal violet staining and WST-1 assays to analyze the inhibitory effect of Ad-VT and paclitaxel combination treatment on MCF-7, MDA-MB-231, and MCF-10A cells. Subsequently, we used Hoechst, Annexin V, JC-1 staining to analyze the inhibition pathway of drugs on breast cancer cells. We also used Transwell assays to analyze the cell migration and invasion of MCF-7 and MDA-MB-231 cells. The pGL4.51 plasmid was used to transfect and to generate MDA-MB-231 cells, that stably express luciferase (MDA-MB-231-LUC). The in vivo tumor inhibition effect of Ad-VT and paclitaxel combination treatment was subsequently confirmed using a tumor-bearing nude mouse model. This combination treatment can increase the inhibition of breast cancer cells and reduce paclitaxel toxicity. Ad-VT had a strong apoptosis-inducing effect on MCF-7 and MDA-MB-231 cells, that was mainly mediated through the mitochondrial apoptotic pathway. The combination of Ad-VT and paclitaxel could significantly increase the inhibition of breast cancer cell migration and invasion. Combination of Ad-VT and paclitaxel can inhibit tumor growth and reduce toxicity in vivo. Ad-VT can also inhibit the growth of breast cancer cells and promote their apoptosis. Meanwhile, when it is combined with paclitaxel, Ad-VT could play a significant role in a synergistic tumor inhibition.

Bacteria-cancer interactions: bacteria-based cancer therapy

Recent advances in cancer therapeutics, such as targeted therapy and immunotherapy, have raised the hope for cures for many cancer types. However, there are still ongoing challenges to the pursuit of novel therapeutic approaches, including high toxicity to normal tissue and cells, difficulties in treating deep tumor tissue, and the possibility of drug resistance in tumor cells. The use of live tumor-targeting bacteria provides a unique therapeutic option that meets these challenges. Compared with most other therapeutics, tumor-targeting bacteria have versatile capabilities for suppressing cancer. Bacteria preferentially accumulate and proliferate within tumors, where they can initiate antitumor immune responses. Bacteria can be further programmed via simple genetic manipulation or sophisticated synthetic bioengineering to produce and deliver anticancer agents based on clinical needs. Therapeutic approaches using live tumor-targeting bacteria can be applied either as a monotherapy or in combination with other anticancer therapies to achieve better clinical outcomes. In this review, we introduce and summarize the potential benefits and challenges of this anticancer approach. We further discuss how live bacteria interact with tumor microenvironments to induce tumor regression. We also provide examples of different methods for engineering bacteria to improve efficacy and safety. Finally, we introduce past and ongoing clinical trials involving tumor-targeting bacteria.

Live tumor-targeting bacteria can selectively induce cancer regression and, with the help of genetic engineering, be made safe and effective vehicles for delivering drugs to tumor cells. In a review article, Jung-Joon Min and colleagues from Chonnam National University Medical School in Hwasun, South Korea, discuss the clinical history of using natural or engineered bacterial strains to suppress cancer growth. Because bacteria such as Salmonella and Listeria preferentially home in on tumors or their surrounding microenvironments, researchers have harnessed these microbial agents to attack cancer cells without causing collateral damage to normal tissues. Bioengineers have also armed bacteria with stronger tumor-sensing and more targeted drug delivery capabilities, and improved control of off-target toxicities. An increasing number of therapeutic bacterial strains are now entering clinical testing, promising to enhance the efficacy of more conventional anticancer treatments.

Cancer Treatment Goes Viral: Using Viral Proteins to Induce Tumour-Specific Cell Death

Cell death is a tightly regulated process which can be exploited in cancer treatment to drive the killing of the tumour. Several conventional cancer therapies including chemotherapeutic agents target pathways involved in cell death, yet they often fail due to the lack of selectivity they have for tumour cells over healthy cells. Over the past decade, research has demonstrated the existence of numerous proteins which have an intrinsic tumour-specific toxicity, several of which originate from viruses. These tumour-selective viral proteins, although from distinct backgrounds, have several similar and interesting properties. Though the mechanism(s) of action of these proteins are not fully understood, it is possible that they can manipulate several cell death modes in cancer exemplifying the intricate interplay between these pathways. This review will discuss our current knowledge on the topic and outstanding questions, as well as deliberate the potential for viral proteins to progress into the clinic as successful cancer therapeutics.

Targeted Inhibitory Effect of Nasopharyngeal Carcinoma Cells by Hre2.Grp78 Chimeric Promoter Regulating Fusion Gene TK/VP3

Objective:

To construct plasmids with Hre₂.Grp78 chimeric promoter regulating fusion gene TK/VP3 and elaborate the effects of overexpressed TK/VP3 on nasopharyngeal carcinoma cells.

Methods:

Four plasmids were constructed, including pcDNA3.1-CMV-TK/VP3, pcDNA3.1-Hre₂.TK/VP3, pcDNA3.1-Grp78.TK/VP3, and pcDNA3.1-Hre₂.Grp78.TK/VP3. The human nasopharyngeal carcinoma cell line HNE1 cells were transfected with the 4 plasmids, respectively. Cell viabilities were evaluated using 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, and apoptosis was conducted using flow cytometry analysis. The expression of TK, VP3, Grp78, and hypoxia-inducible factor 1α and apoptosis-related proteins was determined by real-time quantitative polymerase chain reaction and Western blotting.

Results:

The recombinant plasmids that could steadily overexpress TK and VP3 were successfully constructed. Expression of TK and VP3 in cells transfected with pcDNA3.1-Hre₂.TK/VP3 and pcDNA3.1-Grp78.TK/VP3 was significantly higher than pcDNA3.1-CMV-TK/VP3, and expression in cells transfected with pcDNA3.1-Hre₂.Grp78.TK/VP3 was the highest. Under glucose deprivation or hypoxia condition, Grp78 or hypoxia-inducible factor 1α was overexpressed so that expression of TK and VP3 was significantly upregulated, which could further inhibit cell proliferation and enhance cell apoptosis.

Conclusion:

We successfully constructed 4 plasmids with Hre₂.Grp78 chimeric promoter regulating fusion gene TK/VP3, which could significantly inhibit the proliferation as well as enhance the apoptosis of nasopharyngeal carcinoma cells under glucose deprivation or hypoxia condition.

Recombinant adenoviruses expressing apoptin suppress the growth of MCF‑7 breast cancer cells and affect cell autophagy

Autophagy and apoptosis both promote cell death; however, the relationship between them is subtle, and they mutually promote and antagonize each other. Apoptin can induce apoptosis of various tumor cells; however, tumor cell death is not only caused by apoptosis. Whether apoptin affects tumor cell autophagy is poorly understood. Therefore, the present study aimed to explore the potential mechanisms underlying the effects of apoptin using recombinant adenoviruses expressing apoptin. Reverse transcription-quantitative polymerase chain reaction, immunoblotting, flow cytometry, fluorescence microscopy and proteomics analyses revealed that apoptin could induce autophagy in MCF-7 breast cancer cells. The results also suggested that apoptin affected autophagy in a time- and dose-dependent manner. During the early stage of apoptin stimulation (6 and 12 h), the expression levels of autophagy pathway-associated proteins, including Beclin-1, microtubule-associated protein 1A/1B-light chain 3, autophagy-related 4B cysteine peptidase and autophagy-related 5, were significantly increased, suggesting that apoptin promoted the upregulation of autophagy in MCF-7 cells. Conversely, after 12 h of apoptin stimulation, the expression levels of apoptosis-associated proteins were decreased, thus suggesting that apoptosis may be inhibited. Therefore, it was hypothesized that apoptin may enhance autophagy and inhibit apoptosis in MCF-7 cells at the early stage. In conclusion, apoptin-induced cell death may involve both autophagy and apoptosis. The induction of autophagy may inhibit apoptosis, whereas apoptosis may inhibit autophagy; however, occasionally both pathways operate at the same time and involve apoptin. This apoptin-associated selection between tumor cell survival and death may provide a potential therapeutic strategy for breast cancer.

Efficacy of CpG-ODN and Freund's immune adjuvants on antibody responses induced by chicken infectious anemia virus VP1, VP2, and VP3 subunit proteins

Chicken infectious anemia virus is an important pathogen that causes severe anemia and immunosuppression in chickens, leading to serious economic losses worldwide in the poultry industry. However, no commercialized inactivated vaccine, subunit vaccine, or genetically engineered vaccine that is effective for controlling this virus is available. In this study, 3 recombinant plasmids were constructed to produce corresponding viral proteins in an Escherichia coli system. The immune effects of the subunit proteins accompanied by CpG-ODN or Freund's immune adjuvants were evaluated and analyzed in systemic animal experiments. The results showed that VP1 induced the highest antibody titers with the participation of VP2 protein, indicating better protection under combined treatment, and the CpG-ODN adjuvant induced higher antibody titers and smaller dispersion of antibody titers than Freund's adjuvants. This is the first study to demonstrate that VP1 protein formulated with VP2 and CpG-ODN adjuvant can induce highest antibody titers and markedly enhance the immune response, indicating its promise as a vaccine candidate.

Antitumor effect of a dual cancer-specific oncolytic adenovirus on prostate cancer PC-3 cells

PURPOSE

Apoptin can specifically kill cancer cells but has no toxicity to normal cells. Human telomerase reverse transcriptase (hTERT) acts as a tumor-specific promoter, triggering certain genes to replicate or express only in tumor cells, conferring specific replication and killing abilities. This study aimed to investigate the anticancer potential of the recombinant adenovirus Ad-apoptin-hTERTp-E1a (Ad-VT) in prostate cancer.

METHODS

The pGL4.51 plasmid was used to transfect PC-3 cells to construct tumor cells stably expressing luciferase (PC-3-luc). Crystal violet staining and MTS assays determined the ability of Ad-VT to inhibit cell proliferation. Ad-VT-induced apoptosis of PC-3-luc cells was detected using Hoechst, Annexin V, JC-1 staining, and caspases activity analysis. PC-3-luc cells invasion and migration were detected using cell-scratch and Transwell assays. In vivo tumor inhibition was detected using imaging techniques.

RESULTS

Crystal violet staining and MTS results showed that the proliferation ability of PC-3-luc cells decreased significantly. Hoechst, JC-1, and Annexin V experiments demonstrated that Ad-VT mainly induced apoptosis to inhibit PC-3-luc cell proliferation. Ad-VT could significantly inhibit the migration and invasion of PC-3-luc cells over a short period of time. In vivo, Ad-VT could effectively inhibit tumor growth and prolong survival of the mice.

CONCLUSIONS

The recombinant adenovirus, comprising the apoptin protein and the hTERTp promoter, was able to inhibit the growth of prostate cancer PC-3 cells and promote their apoptosis.

Porcine circovirus type 2 ORF3 protein induces apoptosis in melanoma cells

BACKGROUND

The current treatment of malignant melanoma is limited by the lack of effective therapeutic approaches, and alternative treatments are needed. Proliferative diseases such as melanoma and other cancers may be treatable by virally-encoded apoptotic proteins that are targeted to rapidly multiplying cells. Caspase-dependent apoptosis, that is frequently used in chemotherapy, can boost the cell proliferation that caspase-independent cell death does not.

METHODS

In the current study, the porcine circovirus type 2 (PCV2), proapoptotic protein ORF3 was expressed in mouse and human cancer cell lines, and its apoptotic activity was assessed.

RESULTS

Quantitative assessment of the apoptotic cells by flow cytometry showed that apoptotic cell death was significantly increased in ORF3-expressing malignant cells, compared to ORF3 non-expressing cells. Our data show that PCV2 ORF3 induces apoptosis in a caspase-3 and -8 independent manner. ORF3 expression seems to cause an increase in abnormal mitosis in B16F10 melanoma cells by interacting with centrosomes and thereby disrupting the formation of the mitotic spindle. In addition, we show that ORF3 of PCV2 also exhibits significant anti-tumor effects in vivo. Although the expression of Regulator of G protein Signaling (RGS)-16 by recipient mice inhibited the development of grafted melanoma in vivo, it was not required for the antitumoral activity of ORF3.

CONCLUSION

PCV2 ORF3 causes abnormal mitosis in rapidly dividing cells and increases the apoptosis of cancer cells. Apoptin might, therefore, be considered to develop future antitumoral strategies.

Marek's disease virus oncoprotein Meq physically interacts with the chicken infectious anemia virus-encoded apoptotic protein apoptin

Marek's disease (MD) is a neoplastic disease of poultry caused by Marek's disease virus (MDV), a highly contagious alphaherpesvirus. Meq, the major MDV oncoprotein, induces neoplastic transformation of T-cells through several mechanisms, including inhibition of apoptosis. In contrast, the chicken anemia virus (CAV)-encoded protein apoptin (VP3) is a powerful inducer of apoptosis of tumor cells, a property that is exploited for anticancer therapeutics. Although the molecular mechanisms of selective induction of tumor cell apoptosis by apoptin are not fully understood, its tumor cell–restricted nuclear translocation is thought to be important. Co-infection with MDV and CAV is common in many countries, CAV antigens are readily detectable in MD lymphomas, and the MDV-transformed T-lymphoblastoid cell lines such as MSB-1 is widely used for propagating CAV for vaccine production. As MDV-transformed cell lines express high levels of Meq, we examined here whether CAV-encoded apoptin interacts with Meq in these cells. Using immunofluorescence microscopy, we found that apoptin and Meq co-localize to the nucleus, and biochemical analysis indicated that the two proteins do physically interact. Using a combination of Meq mutagenesis and co-immunoprecipitation, we demonstrate that apoptin interacts with Meq within a region between amino acids 130 and 140. Results from the IncuCyte assay suggested that Meq inhibits apoptin-induced apoptosis activity. In summary, our findings indicate that Meq interacts with and inhibits apoptin. Insights into this novel interaction between Meq and apoptin will relevance for pathogenesis of coinfections of the two viruses and in CAV vaccine production using MDV-transformed cell lines.

Multiple roles of Ring 1 and YY1 binding protein in physiology and disease

Ring 1 and YY1 binding protein (RYBP) was first identified in 1999, and its structure includes a conserved Npl4 Zinc finger motif at the N‐terminus, a central region that is characteristically enriched with arginine and lysine residues and a C‐terminal region enriched with serine and threonine amino acids. Over nearly 20 years, multiple studies have found that RYBP functions as an organ developmental adaptor. There is also evidence that RYBP regulates the expression of different genes involved in various aspects of biological processes, via a mechanism that is dependent on interactions with components of PcG complexes and/or through binding to different transcriptional factors. In addition, RYBP interacts directly or indirectly with apoptosis‐associated proteins to mediate anti‐apoptotic or pro‐apoptotic activity in both the cytoplasm and nucleus of various cell types. Furthermore, RYBP has also been shown to act as tumour suppressor gene in different solid tumours, but as an oncogene in lymphoma and melanoma. In this review, we summarize our current understanding of the functions of this multifaceted RYBP in physiological and pathological conditions, including embryonic development, apoptosis and cancer, as well as its role as a component of polycomb repressive complex 1.

Attenuated Salmonella Typhimurium Carrying TRAIL and VP3 Genes Inhibits the Growth of Gastric Cancer Cells in Vitro and in Vivo

Background

Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand (TRAIL) and apoptin (VP3) of chicken anemia virus can selectively induce apoptosis in human tumor cell lines by two different pathways. Salmonella not only delivers functional genes to mammalian cells but also possesses antitumor activity and therefore could be adopted as a novel vector for anticancer therapy.

Materials and methods

TRAIL and VP3 genes were cloned into a pBudCE4.1 vector and delivered by attenuated Salmonella typhimurium into gastric cancer cells, and their expression and antitumor effects in nude mice were monitored by Western blot, fluorescence microscopy, MTT assay, TUNEL staining, and immunohistochemistry.

Results

pBud-VP3 and pBud-TRAIL-VP3 plasmids were constructed to express TRAIL and apoptin in gastric cancer cells, leading to inhibition of cancer cell proliferation after 48 hours (P <0.05). TRAIL and VP3 genes in pBudCE4.1 vector were also successfully delivered by attenuated S. typhimurium into gastric cancer cells in vivo, in which both TRAIL and apoptin were expressed. In vivo data indicated that S. typhimurium carring pBud-TRAIL-VP3 induced significant cell growth inhibition and tumor regression (P <0.05). Moreover, expression of TRAIL and apoptin increased the expression of caspase-3 and caspase-9, resulting in enhanced apoptosis.

Conclusion

Delivery of TRAIL and VP3 genes by attenuated S. typhimurium can significantly inhibit the growth of gastric cancer cells in vitro and in vivo.

A novel anti-CD22 scFv-apoptin fusion protein induces apoptosis in malignant B-cells

CD22 marker is a highly internalizing antigen which is located on the surface of B-cells and is being used as a promising target for treatment of B cell malignancies. Monoclonal antibodies targeting CD22 have been introduced and some are currently under investigation in clinical trials. Building on the success of antibody drug conjugates, we developed a fusion protein consisting of a novel anti-CD22 scFv and apoptin and tested binding and therapeutic effects in lymphoma cells. The recombinant protein was expressed in E. coli and successfully purified and refolded. In vitro binding analysis by immunofluorescence and flow cytometry demonstrated that the recombinant protein specifically binds to CD22 positive Raji cells but not to CD22 negative Jurkat cells. The cytotoxic properties of scFv–apoptin were assessed by an MTT assay and Annexin V/PI flow cytometry analysis and showed that the recombinant protein induced apoptosis preferentially in Raji cells with no detectable effects in Jurkat cells. Our findings indicated that the recombinant anti-CD22 scFv–apoptin fusion protein could successfully cross the cell membrane and induce apoptosis with high specificity, make it as a promising molecule for immunotherapy of B-cell malignancies.

Unexpected heat shock element binding ability and tumor-killing activity of the combinatorial function domain of apoptin

Apoptin, derived from the chicken anemia virus, has been found to exert tumor-preferential apoptotic activity. It is a potential anticancer agent with direct clinical applications. However, if this viral protein were to be used as a new drug, it might also induce a strong immune response, causing toxic side effects. In a previous study, our group showed that TAT-apoptin downregulates the stress expression of heat shock protein 70 by competing with heat shock factor protein 1 in binding to the heat shock element (HSE) of the promoter region of heat shock protein 70, thus inducing specific apoptosis in HepG2 cells. In this study, we investigated the HSE-binding properties of the minimal functional region of apoptin. We showed that apoptin's nuclear localization signals 1 and nuclear localization signals 2 represented functional regions that could bind with HSE and that this binding capacity was increased by polymers formed through the introduction of a leucine-rich stretch. Our data also showed that truncated combinatorial apoptin peptide has greater tumor-specific cell-killing activity and could be a potential antitumor agent.

Identification of the interaction and interaction domains of chicken anemia virus VP2 and VP3 proteins

Chicken anemia virus (CAV) is a small, single-stranded DNA virus of Anelloviridae family. Its genome segments encode three proteins, VP1, VP2, and VP3. This study identified an interaction between VP2 and VP3 and mapped the interaction domains. Through the yeast two-hybrid (Y2H) system, VP2 was found to interact with VP3. The presence of the VP2-VP3 complex in CAV-infected chicken cells was confirmed by co-immunoprecipitation. Confocal microscopy showed that VP2 and VP3 were expressed in the cytoplasm in cotransfected Vero cells. In the Y2H system, the interaction domains were identified as being within the N-terminal aa 1-30 and C-terminal aa 17-60 for VP2 and the N-terminal aa 46-60 and C-terminal aa 1-7 for VP3. This study showed the interaction between VP2 and VP3 of CAV and identified multiple independent interactive domains within the two proteins. This provides novel information for investigating the biological functions of these proteins.

PTD4-apoptin induces Bcl-2-insensitive apoptosis in human cervical carcinoma in vitro and in vivo

Worldwide, cervix carcinoma is among the most dangerous cancer types, and novel therapies are under development. Cancer treatments are often hampered because of lack of specificity. The chicken anemia virus-derived apoptin induces apoptosis selectively in tumor cells and leaves normal cells unharmed. Here, we have carried out in-vitro and in-vivo studies on the cytotoxic effect of apoptin in a cervix carcinoma model. Apoptin was fused to the protein transduction domain 4 (PTD4), enabling delivery of the fusion protein across cellular membranes. PTD4-apoptin protein is located in the nuclei of human cervical carcinoma HeLa cells and in the cytoplasm of normal cells L02. By MTT and flow cytometry analysis, we have proven that PTD4-apoptin protein induced apoptosis in the cervical carcinoma cells. PTD4-apoptin enhanced the level of active executioner caspase-3. Neither caspase-3 activation nor apoptin-induced accumulation of the mitochondrial outer-membrane protein Mfn-2 was affected by ectopic Bcl-2 expression. In contrast, apoptin-mediated AKT activation was inhibited by Bcl-2. In vivo, cervix carcinoma xenografts were treated for 7 days with PTD4-apoptin protein. The PTD4-apoptin treatment induced a decrease in the cervix carcinoma, whereas the PTD4-GFP protein-treated controls expanded significantly. TUNEL analysis showed that PTD4-apoptin protein induced apoptosis in cervix carcinoma cells, in contrast to the control PTD-GFP-treated ones. Our results indicate that apoptin is a potential anticancer agent for treating cervix carcinoma.

Viruses with Circular Single-Stranded DNA Genomes Are Everywhere!

Circular single-stranded DNA viruses infect archaea, bacteria, and eukaryotic organisms. The relatively recent emergence of single-stranded DNA viruses, such as chicken anemia virus (CAV) and porcine circovirus 2 (PCV2), as serious pathogens of eukaryotes is due more to growing awareness than to the appearance of new pathogens or alteration of existing pathogens. In the case of the ubiquitous human circular single-stranded DNA virus family Anelloviridae, there is still no convincing direct causal relation to any specific disease. However, infections may play a role in autoimmunity by changing the homeostatic balance of proinflammatory cytokines and the human immune system, indirectly affecting the severity of diseases caused by other pathogens. Infections with CAV (family Anelloviridae, genus Gyrovirus) and PCV2 (family Circoviridae, genus Circovirus) are presented here because they are immunosuppressive and affect health in domesticated animals. CAV shares genomic organization, genomic orientation, and common features of major proteins with human anelloviruses, and PCV2 DNA may be present in human food and vaccines.

Apoptin Gene Delivery by the Functionalized Polyamidoamine (PAMAM) Dendrimer Modified with Ornithine Induces Cell Death of HepG2 Cells

The use of tumor-specific therapeutic agents is a promising option for efficient and safe nonviral gene transfer in gene therapy. In this study, we describe the efficacy of polyamidoamine (PAMAM)-based nonviral gene delivery carriers, namely, an ornithine conjugated PAMAM (PAMAM-O) dendrimer in delivering apoptin, a tumor-specific killer gene, into human hepatocellular carcinoma (HepG2 cells) and dermal fibroblasts. We analyzed the transfection efficiency by the luciferase assay and assessed cell viability in both cell types. The transfection efficiency of the PAMAM-O dendrimer was found to be higher than that of the PAMAM dendrimer. Moreover, the cytotoxicity of the PAMAM-O dendrimer was very low. We treated both cell types with a polyplex of PAMAM-O dendrimer with apoptin, and analyzed its cellular uptake and localization by confocal microscopy. Cell cycle distribution, tetramethylrhodamine, ethyl ester (TMRE) analysis, and transmission electron microscopy imaging showed that apoptin induced cell death in HepG2 cells. We therefore demonstrated that a PAMAM-O/apoptin polyplex can be used as an effective therapeutic strategy in cancer owing to its effectiveness as a suitable nonviral gene vector for gene therapy.

C-terminal region of apoptin affects chicken anemia virus replication and virulence

Background

Chicken anemia virus (CAV) causes anemia and immune suppression, which are important diseases in the poultry industry. CAV VP3, also referred as ‘apoptin’, has been shown to selectively kill tumor cells, raising great hopes for its utilization as an anticancer therapy. The ability of apoptin to induce apoptosis is closely related to its nuclear localization. The C-terminal region of apoptin contains a bipartite nuclear localization signals (NLS), and a nuclear export signal (NES) is located between the arms of the NLS. Most previous studies have expressed apoptin of different lengths in vitro to understand the relationship between its localization and its induction of apoptosis.

Methods

In this study, we investigated the replication of CAV and its induction of apoptosis in vitro and in vivo with VP3-truncated infectious virus. Quantitative PCR was used to detect viral replication in MDCC-MSB1 cells, and the viral localization was observed by confocal microscopy. Flow cytometry was uesed to analyze virus-induced apoptosis in MDCC-MSB1 cells. Additionally, chickens infected with the rescued viruses compared with the parental virus rM9905 to evaluate the viral replication in vivo and virulence.

Results

Based on the infectious clone, we rescued two viruses in which were deleted NES–NLS2 (rCAV-VP3N88) or NLS1–NES–NLS2 (rCAV-VP3N80) in the C-terminal region of apoptin. The viral load of rCAV-VP3N88 decreased significantly between 60 and 108 hpi, and was always 10–100-fold lower than that of the parental virus rM9905. The levels of rCAV-VP3N80 were also 10–100-fold lower than that of rM9905 and declined significantly at three time points. There was almost no difference in the viral loads of rCAV-VP3N88 and rCAV-VP3N80. Additionally, rM9905 induced 85.39 ± 2.18% apoptosis at 96 hpi, whereas rCAV-VP3N88 and rCAV-VP3N80 induced 63.08 ± 4.78% and 62.56 ± 7.35% apoptosis, respectively, which were significantly (about 20%) lower than that induced by the parental virus. The rescued viruses altered the nuclear localization in MDCC-MSB1 cells. Moreover, deletion of C-terminal region of apoptin impaired viral replication in vivo and reduced the virulence of CAV in chickens.

Conclusions

In summary, we have demonstrated that the C-terminal deletion of apoptin in infectious CAV affected the replication of the virus. The deletion of the C-terminal region of apoptin not only significantly reduced viral replication in vitro but also reduced its induction of apoptosis, which correlated with the loss of its nuclear localization. The deletion of the C-terminal region of apoptin also impaired the replication of CAV and attenuated its virulence in chickens.

Apoptin Gene Delivery by the Functionalized Polyamidoamine Dendrimer Derivatives Induces Cell Death of U87-MG Glioblastoma Cells

Malignant glioma is the most common and aggressive form of primary brain tumor in adults. In this study, we describe the efficacy of nonviral gene delivery carriers, histidine- and arginine- or histidine- and lysine-grafted polyamidoamine (PAMAM) dendrimers (PAMAM-H-R and PAMAM-H-K), in delivering a therapeutic and a tumor-selective killer gene, apoptin, using human glioma cells (U87-MG) and newborn human dermal fibroblast cells. We analyzed transfection efficiency using luciferase and a plasmid DNA encoding for enhanced green fluorescent protein and assessed cell viability in both cells. The results show that transfection efficiency of PAMAM-H-R and PAMAM-H-K was greatly increased compared with that of native PAMAM. Moreover, among PAMAM derivatives, cytotoxicity of PAMAM-H-K was very low. We treated both cells with complexes of PAMAM-H-R or PAMAM-H-K and apoptin and analyzed their cellular uptake by flow cytometry and localization by confocal microscopy. Furthermore, cell cycle distribution, caspase 3 activity assay, and JC-1 analysis showed cell death induced by apoptin in U87-MG cells. The present study demonstrates that a PAMAM-H-R/apoptin complex is an effective gene carrier system in glioma cell culture.

Insights into the mechanism of Apoptin's exquisitely selective anti-tumor action from atomic level characterization of its conformation and dynamics

Apoptin is a 121 residue protein which forms large, soluble aggregates and possesses an exceptionally selectively cytotoxic action on cancer cells. In the accompanying paper, we described the design, production and initial characterization of an Apoptin truncated variant called H₆-ApopΔProΔLeu. Whereas both the variant and wild type protein possess similar selective cytotoxicity against cancer cells following transfection, only the variant is cytotoxic when added externally. Remarkably, as observed by gel filtration chromatography and dynamic light scattering, H₆-ApopΔProΔLeu lacks the tendency of wild type Apoptin to form large aggregates, which greatly facilitated the study of its biological properties. Here, we characterize the conformation and dynamics of H₆-ApopΔProΔLeu. Using a battery of 2D, 3D and (4,2)D NMR spectra, the essentially complete ¹H, ¹³C and ¹⁵N resonance assignments of H₆-ApopΔProΔLeu were obtained. The analysis of these data shows that the variant is an intrinsically disordered protein, which lacks a preferred conformation. This conclusion is corroborated by a lack of protection against proteolytic cleavage and hydrogen/deuterium exchange. Moreover, the CD spectra are dominated by random coil contributions. Finally, ¹H-¹⁵N NOE ratios are low, which indicates flexibility on the ps-ns time scale. Interestingly, H₆-ApopΔProΔLeu's intrinsically disordered ensemble is not significantly altered by the redox state of its Cys residues or by Thr phosphorylation, which has been proposed to play a key role in Apoptin's selective cytotoxicity. These results serve to better comprehend Apoptin's remarkably selective anticancer action and provide a framework for the future design of improved Apoptin variants.

Genetically engineered and self-assembled oncolytic protein nanoparticles for targeted cancer therapy

The integration of a targeted delivery with a tumour-selective agent has been considered an ideal platform for achieving high therapeutic efficacy and negligible side effects in cancer therapy. Here, we present engineered protein nanoparticles comprising a tumour-selective oncolytic protein and a targeting moiety as a new format for the targeted cancer therapy. Apoptin from chicken anaemia virus (CAV) was used as a tumour-selective apoptotic protein. An EGFR-specific repebody, which is composed of LRR (Leucine-rich repeat) modules, was employed to play a dual role as a tumour-targeting moiety and a fusion partner for producing apoptin nanoparticles in E. coli, respectively. The repebody was genetically fused to apoptin, and the resulting fusion protein was shown to self-assemble into supramolecular repebody-apoptin nanoparticles with high homogeneity and stability as a soluble form when expressed in E. coli. The repebody-apoptin nanoparticles showed a remarkable anti-tumour activity with negligible side effects in xenograft mice through a cooperative action of the two protein components with distinct functional roles. The repebody-apoptin nanoparticles can be developed as a systemic injectable and tumour-selective therapeutic protein for targeted cancer treatment.

Human serum albumin-mediated apoptin delivery suppresses breast cancer cell growth in vitro and in vivo

Gene therapy is one of the most promising potential therapeutic strategies for many types of cancer. Cell apoptosis is an active, programmed physiological process of the body, and its disruption has been closely associated with the occurrence of tumor development. Apoptin is known to induce tumor cell apoptosis. In the present study, the MCF-7 breast cancer cell line was transfected with a human serum albumin (HSA) and apoptin expressing plasmid [HSA-polyethylenimine (PEI)-pcDNA-Apoptin]. Reverse transcription-quantitative polymerase chain reaction and western blotting were performed to detect the expression of apoptin in the transfected MCF-7 cells, while MTT assays and flow cytometry were conducted to detect cell viability and apoptosis. Furthermore, hematoxylin and eosin staining was used to observe the morphology of xenografts from mice injected with MCF-7 cells. It was demonstrated that the HSA-PEI-pcDNA-Apoptin expression plasmid resulted in the upregulation of apoptin in MCF-7 cells, and efficiently suppressed breast tumor growth in vivo. These findings indicated that the use of HSA as an apoptin expression vector has potential therapeutic benefits for cancer and confirms the requirement for the further evaluation of apoptin in clinical trials.

The chimeric multi-domain proteins mediating specific DNA transfer for hepatocellular carcinoma treatment

AIM

This study was aimed to evaluate the therapeutic efficiency of a non-virus based specific chimeric multi-domain DNA transferred with apoptin in human hepatocellular carcinoma (HCC) HepG-2 cells in vitro and in mice H22 cells in vivo.

METHODS

We firstly constructed the multi-domain recombinant chimeric proteins based on recombinant proteins [G (yeast GAL4), NG (none GAL4), TG (GAL4 + Tat protein) and TNG (Tat protein)] and pUAS-Apoptin plasmid, and transfected them into human HepG-2 cells. The antitumor effect of this multi-domain recombinant chimeric proteins to HCC cells were detected by MTT assay, AO/EB staining, DAPI staining and Annexin V assay. In order to find the pathway of cell apoptosis, the Caspase (1, 3, 6 and 8) activity was detected. We then constructed the H22 liver cancer mice model and analyzed the anti-tumor rate and mice survival rate after treated with G/pUAS-Apoptin NG/pUAS-Apoptin TG/pUAS-Apoptin, and TNG/pUAS-Apoptin.

RESULTS

MTT results showed that the Tat protein (TG and TNG) significantly induced cell death in a time dependent manner. AO/EB, DAPI, Annexin V and Caspases assay results indicated that the Caspase 1, 3, 6 and 8 were highly expressed in TG/pUAS-Apoptin, and TNG/pUAS-Apoptin treated mouse groups. The antitumor rate and survival rate in TG/pUAS-Apoptin, and TNG/pUAS-Apoptin treated mouse groups were higher than in the other groups.

CONCLUSION

The Tat-apoptin is a potential anti-tumor agent for HCC treatment with remarkable anti-tumor efficacy and high safety based on non-virus gene transfer system. The anti-tumor function may be associated with high expression of Caspase 1, 3, 6 and 8.

Dipeptide-functionalized polyamidoamine dendrimer-mediated apoptin gene delivery facilitates apoptosis of human primary glioma cells

Glioblastoma multiform (GBM) is the most frequent and aggressive form of brain tumors in adults. However, the development of more efficient and safe nonviral vector gene therapy represents a promising therapeutic approach, using a tumor-specific killer gene, named apoptin. In this study, we describe the efficacy of non-viral gene delivery vectors, the amino acid-conjugated PAMAM derivatives (PAMAM-H-R and PAMAM-H-K) in delivering a therapeutic gene, displaying affinity toward human primary glioma cells (GBL-14 cells) and dermal fibroblasts. We analyzed transfection efficiency, using luciferase (Luci) and a pDNA encoding for enhanced fluorescent protein (EGFP), and cytotoxicity in both cells. The results show that transfection efficiency of PAMAM-H-R improved compared to native PAMAM dendrimer, but cytotoxicity of PAMAM-H-R and PAMAM-H-K were very low. We treated both cells with a polyplex formation of PAMAM-H-R or PAMAM-H-K/apoptin, and analyzed their cellular uptake and localization by flow cytometry and confocal microscopy. Furthermore, we analyzed the endosomal escape effect using TEM images, and found that PAMAM-H-R showed very fast escape from endosome to the cytosol. Caspase 3 activity assay, cell cycle distribution, and JC-1 analysis showed apoptosis induced by apoptin in GBL-14 cells. This indicates that PAMAM-H-R can be a potential nonviral vector gene delivery carrier for brain tumor therapy. The present study demonstrates that PAMAM-H-R/apoptin gene polyplex can be used as an effective therapeutic candidate for GBM due to its selective induction of apoptosis in primary glioma cells as a potential nonviral gene delivery carrier for brain tumor therapy.

Activation of the Chicken Anemia Virus Apoptin Protein by Chk1/2 Phosphorylation Is Required for Apoptotic Activity and Efficient Viral Replication

Chicken anemia virus (CAV) is a single-stranded circular DNA virus that carries 3 genes, the most studied of which is the gene encoding VP3, also known as apoptin. This protein has been demonstrated to specifically kill transformed cells while leaving normal cells unharmed in a manner that is independent of p53 status. Although the mechanistic basis for this differential activity is unclear, it is evident that the subcellular localization of the protein is important for the difference. In normal cells, apoptin exists in filamentous networks in the cytoplasm, whereas in transformed cells, apoptin is present in the nucleus and appears as distinct foci. We have previously demonstrated that DNA damage signaling through the ataxia telangiectasia mutated (ATM) pathway induces the translocation of apoptin from the cytoplasm to the nucleus, where it induces apoptosis. We found that apoptin contains four checkpoint kinase consensus sites and that mutation of either threonine 56 or 61 to alanine restricts apoptin to the cytoplasm. Furthermore, treatment of tumor cells expressing apoptin with inhibitors of checkpoint kinase 1 (Chk1) and Chk2 causes apoptin to localize to the cytoplasm. Importantly, silencing of Chk2 rescues cancer cells from the cytotoxic effects of apoptin. Finally, treatment of virus-producing cells with Chk inhibitor protects them from virus-mediated toxicity and reduces the titer of progeny virus. Taken together, our results indicate that apoptin is a sensor of DNA damage signaling through the ATM-Chk2 pathway, which induces it to migrate to the nucleus during viral replication.

IMPORTANCE

The chicken anemia virus (CAV) protein apoptin is known to induce tumor cell-specific death when expressed. Therefore, understanding its regulation and mechanism of action could provide new insights into tumor cell biology. We have determined that checkpoint kinase 1 and 2 signaling is important for apoptin regulation and is a likely feature of both tumor cells and host cells producing virus progeny. Inhibition of checkpoint signaling prevents apoptin toxicity in tumor cells and attenuates CAV replication, suggesting it may be a future target for antiviral therapy.

Combined administration of the apoptin gene and poly (I:C) induces potent anti-tumor immune response and inhibits growth of mouse mammary tumors

BACKGROUND

Many viral proteins exhibit selective cytotoxicity for tumor cells without affecting the normal diploid cells. The apoptin protein of chicken infectious anemia virus is one of such proteins, which has been shown to kill tumor cells specifically. However, an effective cancer treatment strategy also requires assistance from the immune system. Recently, poly (I:C) has been shown to be an effective cancer vaccine adjuvant.

AIM

In this study, we assessed the anti-tumor potential of apoptin gene transfer alone and in combination with poly (I:C) in a 4T1 mouse mammary tumor model.

METHODS

4T1 cells were used to induce mammary tumor in Balb/c mice. Mice bearing tumors were divided into 6 groups, and each group received six intratumoral injections during a period of one month. After the last immunization, the animals were sacrificed, and peripheral blood, spleen, lungs, liver, heart, kidney and tumor tissues were collected for immunological, molecular and pathological analysis.

RESULTS

We report that intratumoral administration of apoptin plasmid along with poly (I:C) not only significantly inhibited the growth of mammary tumor, but also induced a potent anti-tumor immune response as indicated by the increase in blood CD4+, CD8+ cells and infiltration of immune cells in the tumor tissue. Further, blood serum analysis of the cytokines revealed increased secretion of Th1 cytokines (IFN-γ and IL-2).

CONCLUSIONS

The results of our study demonstrate that the inclusion of poly (I:C) significantly enhanced the anti-tumor activity of apoptin mainly by inducing a potent anti-tumor immune response. Therefore, we report the use of apoptin and poly (I:C) combination as a novel and powerful strategy for cancer immunotherapy.

Roles of N-Myc and STAT interactor in cancer: From initiation to dissemination

N‐myc & STAT Interactor, NMI, is a protein that has mostly been studied for its physical interactions with transcription factors that play critical roles in tumor growth, progression and metastasis. NMI is an inducible protein, thus its intracellular levels and location can vary dramatically, influencing a diverse array of cellular functions in a context‐dependent manner. The physical interactions of NMI with its binding partners have been linked to many aspects of tumor biology including DNA damage response, cell death, epithelial‐to‐mesenchymal transition and stemness. Thus, discovering more details about the function(s) of NMI could reveal key insights into how transcription factors like c‐Myc, STATs and BRCA1 are contextually regulated. Although a normal, physiological function of NMI has not yet been discovered, it has potential roles in pathologies ranging from viral infection to cancer. This review provides a timely perspective of the unfolding roles of NMI with specific focus on cancer progression and metastasis.

Apoptin interacts with and regulates the activity of protein kinase C beta in cancer cells

Apoptin, the VP3 protein from chicken anaemia virus (CAV), induces tumour cell-specific cell death and represents a potential future anti-cancer therapeutic. In tumour but not in normal cells, Apoptin is phosphorylated and translocates to the nucleus, enabling its cytotoxic activity. Recently, the β isozyme of protein kinase C (PKCβ) was shown to phosphorylate Apoptin in multiple myeloma cell lines. However, the exact mechanism and nature of interaction between PKCβ and Apoptin remain unclear. Here we investigated the physical and functional link between PKCβ and CAV-Apoptin as well as with the recently identified Apoptin homologue derived from human Gyrovirus (HGyV). In contrast to HCT116 colorectal cancer cells the normal colon mucosa cell lines expressed low levels of PKCβI and showed reduced Apoptin activation, as evident by cytoplasmic localisation, decreased phosphorylation and lack of cytotoxic activity. Co-immunoprecipitation and proximity ligation assay studies identified binding of both CAV- and HGyV-Apoptin to PKCβI in HCT116 cells. Using Apoptin deletion constructs the N-terminal domain of Apoptin was found to be required for interacting with PKCβI. FRET-based PKC activity reporter assays by fluorescence lifetime imaging microscopy showed that expression of Apoptin in cancer cells but not in normal cells triggers a significant increase in PKC activity. Collectively, the results demonstrate a novel cancer specific interplay between Apoptin and PKCβI. Direct interaction between the two proteins leads to Apoptin-induced activation of PKC and consequently activated PKCβI mediates phosphorylation of Apoptin to promote its tumour-specific nuclear translocation and cytotoxic function.

Transcriptomic Profiling of Virus-Host Cell Interactions following Chicken Anaemia Virus (CAV) Infection in an In Vivo Model

Chicken Anaemia Virus (CAV) is an economically important virus that targets lymphoid and erythroblastoid progenitor cells leading to immunosuppression. This study aimed to investigate the interplay between viral infection and the host's immune response to better understand the pathways that lead to CAV-induced immunosuppression. To mimic vertical transmission of CAV in the absence of maternally-derived antibody, day-old chicks were infected and their responses measured at various time-points post-infection by qRT-PCR and gene expression microarrays. The kinetics of mRNA expression levels of signature cytokines of innate and adaptive immune responses were determined by qRT-PCR. The global gene expression profiles of mock-infected (control) and CAV-infected chickens at 14 dpi were also compared using a chicken immune-related 5K microarray. Although in the thymus there was evidence of induction of an innate immune response following CAV infection, this was limited in magnitude. There was little evidence of a Th1 adaptive immune response in any lymphoid tissue, as would normally be expected in response to viral infection. Most cytokines associated with Th1, Th2 or Treg subsets were down-regulated, except IL-2, IL-13, IL-10 and IFNγ, which were all up-regulated in thymus and bone marrow. From the microarray studies, genes that exhibited significant (greater than 1.5-fold, false discovery rate <0.05) changes in expression in thymus and bone marrow on CAV infection were mainly associated with T-cell receptor signalling, immune response, transcriptional regulation, intracellular signalling and regulation of apoptosis. Expression levels of a number of adaptor proteins, such as src-like adaptor protein (SLA), a negative regulator of T-cell receptor signalling and the transcription factor Special AT-rich Binding Protein 1 (SATB1), were significantly down-regulated by CAV infection, suggesting potential roles for these genes as regulators of viral infection or cell defence. These results extend our understanding of CAV-induced immunosuppression and suggest a global immune dysregulation following CAV infection.

The therapeutic effect of death: Newcastle disease virus and its antitumor potential

Programmed cell death is essential to survival of multicellular organisms. Previously restricted to apoptosis, the concept of programmed cell death is now extended to other mechanisms, as programmed necrosis or necroptosis, autophagic cell death, pyroptosis and parthanatos, among others. Viruses have evolved to manipulate and take control over the programmed cell death response, and the infected cell attempts to neutralize viral infections displaying different stress signals and defensive pathways before taking the critical decision of self-destruction. Learning from viruses and their interplay with the host may help us to better understand the complexity of the self-defense death response that when altered might cause disorders as important as cancer. In addition, as the fields of immunotherapy and oncolytic viruses advance as promising novel cancer therapies, the programmed cell death response reemerges as a key point for the success of both therapeutic approaches. In this review we summarize the research of the multimodal cell death response induced by Newcastle disease viruses (NDV), considered nowadays a promising viral oncolytic therapeutic, and how the manipulation of the host programmed cell death response can enhance the NDV antitumor capacity.

Human-gyrovirus-Apoptin triggers mitochondrial death pathway--Nur77 is required for apoptosis triggering

The human gyrovirus derived protein Apoptin (HGV-Apoptin) a homologue of the chicken anemia virus Apoptin (CAV-Apoptin), a protein with high cancer cells selective toxicity, triggers apoptosis selectively in cancer cells. In this paper, we show that HGV-Apoptin acts independently from the death receptor pathway as it induces apoptosis in similar rates in Jurkat cells deficient in either FADD (fas-associated death domain) function or caspase-8 (key players of the extrinsic pathway) and their parental clones. HGV-Apoptin induces apoptosis via the activation of the mitochondrial intrinsic pathway. It induces both mitochondrial inner and outer membrane permebilization, characterized by the loss of the mitochondrial potential and the release into cytoplasm of the pro-apoptotic molecules including apoptosis inducing factor and cytochrome c. HGV-Apoptin acts via the apoptosome, as lack of expression of apoptotic protease-activating factor 1 in murine embryonic fibroblast strongly protected the cells from HGV-Apoptin–induced apoptosis. Moreover, QVD-oph a broad-spectrum caspase inhibitor delayed HGV-Apoptin–induced death. On the other hand, overexpression of the anti-apoptotic BCL-XL confers resistance to HGV-Apoptin–induced cell death. In contrast, cells that lack the expression of the pro-apoptotic BAX and BAK are protected from HGV-Apoptin induced apoptosis. Furthermore, HGV-Apoptin acts independently from p53 signal but triggers the cytoplasmic translocation of Nur77. Taking together these data indicate that HGV-Apoptin acts through the mitochondrial pathway, in a caspase-dependent manner but independently from the death receptor pathway.

The anticancer gene ORCTL3 targets stearoyl-CoA desaturase-1 for tumour-specific apoptosis

ORCTL3 is a member of a group of genes, the so-called anticancer genes, that cause tumour-specific cell death. We show that this activity is triggered in isogenic renal cells upon their transformation independently of the cells' proliferation status. For its cell death effect ORCTL3 targets the enzyme stearoyl-CoA desaturase-1 (SCD1) in fatty acid metabolism. This is caused by transmembrane domains 3 and 4, which are more efficacious in vitro than a low molecular weight drug against SCD1, and critically depend on their expression level. SCD1 is found upregulated upon renal cell transformation indicating that its activity, while not impacting proliferation, represents a critical bottleneck for tumourigenesis. An adenovirus expressing ORCTL3 leads to growth inhibition of renal tumours in vivo and to substantial destruction of patients' kidney tumour cells ex vivo. Our results indicate fatty acid metabolism as a target for tumour-specific apoptosis in renal tumours and suggest ORCTL3 as a means to accomplish this.

Viral genes as oncolytic agents for cancer therapy

Many viruses have the ability to modulate the apoptosis, and to accomplish it; viruses encode proteins which specifically interact with the cellular signaling pathways. While some viruses encode proteins, which inhibit the apoptosis or death of the infected cells, there are viruses whose encoded proteins can kill the infected cells by multiple mechanisms, including apoptosis. A particular class of these viruses has specific gene(s) in their genomes which, upon ectopic expression, can kill the tumor cells selectively without affecting the normal cells. These genes and their encoded products have demonstrated great potential to be developed as novel anticancer therapeutic agents which can specifically target and kill the cancer cells leaving the normal cells unharmed. In this review, we will discuss about the viral genes having specific cancer cell killing properties, what is known about their functioning, signaling pathways and their therapeutic applications as anticancer agents.