Antitumor effects of a recombinant fowlpox virus expressing Apoptin in vivo and in vitro

Apoptin mediates mitophagy and endogenous apoptosis by regulating the level of ROS in hepatocellular carcinoma

Background

Apoptin, as a tumor-specific pro-apoptotic protein, plays an important anti-tumoral role, but its mechanism of autophagy activation and the interaction between autophagy and apoptosis have not been accurately elucidated. Here, we studied the mechanism of apoptin-induced apoptosis and autophagy and the interaction between two processes.

Methods

Using crystal violet staining and the CCK-8 assay, we analyzed the effect of apoptin in the inhibition of liver cancer cells in vitro and analyzed the effect of inhibiting liver cancer in vivo by establishing a nude mouse tumor model. Flow cytometry and fluorescence staining were used to analyze the main types of apoptin-induced apoptosis and autophagy. Subsequently, the relationship between the two events was also analyzed. Flow cytometry was used to analyze the effect of ROS on apoptin-mediated apoptosis and autophagy mediated by apoptin. The effect of ROS on two phenomena was analyzed. Finally, the role of key genes involved in autophagy was analyzed using gene silencing.

Results

The results showed that apoptin can significantly increase the apoptosis and autophagy of liver cancer cells, and that apoptin can cause mitophagy through the increase in the expression of NIX protein. Apoptin can also significantly increase the level of cellular ROS, involved in apoptin-mediated autophagy and apoptosis of liver cancer cells. The change of ROS may be a key factor causing apoptosis and autophagy.

Conclusion

The above results indicate that the increase in ROS levels after apoptin treatment of liver cancer cells leads to the loss of mitochondrial transmembrane potential, resulting in endogenous apoptosis and mitophagy through the recruitment of NIX. Therefore, ROS may be a key factor connecting endogenous apoptosis and autophagy induced by apoptin in liver cancer cells.

Graphical abstract

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-tumour effects of a dual cancer-specific oncolytic adenovirus on Breast Cancer Stem cells

Apoptin can specifically kill cancer cells but has no toxicity to normal cells. Human telomerase reverse transcriptase (hTERT) can act as a tumour‐specific promoter by triggering the expression of certain genes in tumour cells. This study aims to investigate the inhibitory effects and to explore the inhibitory pathway of a dual cancer‐specific recombinant adenovirus (Ad‐apoptin‐hTERTp‐E1a, Ad‐VT) on breast cancer stem cells. Breast cancer cell spheres were obtained from MCF‐7 cells through serum‐free suspension culture. The cell spheres were detected by flow cytometry for CD44⁺ CD24⁻ cell subsets. The stemness of MCF‐7‐CSC cells was confirmed by in vivo tumorigenesis experiments. The inhibitory effect of the recombinant adenoviruses on MCF‐7‐CSC cells was evaluated by CCK‐8 assay. In addition, the stemness of adenovirus‐infected MCF‐7‐CSC cells was analysed by testing the presence of CD44⁺ CD24⁻ cell subsets. The ability of the recombinant adenovirus to induce MCF‐7‐CSC cell apoptosis was detected by staining JC‐1, TMRM and Annexin V. Our results showed that a significantly higher proportion of the CD44⁺ CD24⁻ cell subsets was present in MCF‐7‐CSC cells with a significantly increased expression of stem cell marker proteins. The MCF‐7‐CSC cells, whlist exhibited a strong tumorigenic ability with a certain degree of stemness in mice, were shown to be strongly inhibited by recombinant adenovirus Ad‐VT through cell apoptosis. In addition, Ad‐VT was shown to exert a killing effect on BCSCs. These results provide a new theoretical basis for the future treatment of breast cancer.

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.

Apoptin Regulates Apoptosis and Autophagy by Modulating Reactive Oxygen Species (ROS) Levels in Human Liver Cancer Cells

Apoptin is a protein that specifically induces apoptosis in tumor cells. The anti-tumorigenic functions of Apoptin, including autophagy activation and its interaction with apoptosis, have not been precisely elucidated. Here we investigate the main pathways of apoptin-mediated killing of human liver cancer cells, as well as its putative role in autophagy and apoptosis. The anti-proliferative effect of apoptin in liver cancer cells was analyzed in vitro by crystal violet staining and MTS detection, and also in vivo using a tumor-based model. The main pathway related to apoptin-induced growth inhibition in vitro was evaluated by flow cytometry and fluorescence staining. The relationship between apoptosis and autophagy on apoptin-treating cells was analyzed using apoptosis and autophagy inhibitors, mitochondrial staining, Annexin V-FITC/PI flow detection, LC3 staining, and western blotting. The effect of ROS toward the apoptosis and autophagy of apoptin-treating cells was also evaluated by ROS detection, Annexin V-FITC/PI flow detection, LC3 staining, and western blotting. Inhibition of apoptosis in apoptin-treating liver cancer cells significantly reduced the autophagy levels in vitro. The overall inhibition increased from 12 h and the effect was most obvious at 48 h. Inhibition of autophagy could increase apoptin-induced apoptosis of cells in a time-dependent manner, reaching its peak at 24 h. Apoptin significantly alters ROS levels in liver cancer cells, and this effect is directly related to apoptosis and autophagy. ROS appears to be the key factor linking apoptin-induced autophagy and apoptosis through the mitochondria in liver cancer cells. Therefore, evaluating the interaction between apoptin-induced apoptosis and autophagy is a promising step for the development of alternate tumor therapies.

Deletion of multiple genes induces virulence reduction of vaccinia virus Tiantan strain

The purpose of this study was to knock out two non-essential gene fragments (TC7L-TK2L and TJ2R) related to virulence, immunomodulation, and host range in the vaccinia virus Tian Tan strain (VTT), and combining with double-label screening and exogenous screening marker knockout techniques to construct attenuated strains with multiple gene deletions(rVTT-TC-TJ). The shuttle plasmids pSK-TC and pSK-TJ were constructed by designing 2 pairs of recombinant arms, combined with poxvirus early and late complex strong promoter pE/L and exogenous screening marker enhanced green fluorescent protein(EGFP). The results showed that knocking out the two gene fragments does not affect the replication ability of the virus and displays a good genetic stability. Furthermore, a series of in vivo and in vitro experiments demonstrate that although virulence of rVTT-TC-TJ is attenuated significantly, high immunogenicity was maintained. These results support the potential development of rVTT-TC-TJ as a safe viral vector or vaccine.

Human Gyrovirus Apoptin as a Potential Selective Anticancer Agent: An In Vitro Study

Background: Selective therapy has always been the main challenge in cancer treatments. Recently, it has been shown that Human Gyrovirus-derived protein apoptin (HGV-Apoptin) has selective cytotoxic effects on cancer cells similar to its homologue, Chicken Anemia Virus-derived Apoptin (CAV-Apoptin). However, apoptotic effects of Human Gyrovirus apoptin have been only evaluated on a few cancerous cell lines and need to be further investigated. In this study, we have evaluated the apoptotic effects of HGV-Apoptin and CAV-Apoptin expression on lung cancer (A549) and normal (HEK-293) cell lines, in order to provide more information about the specificity of these proteins on cancerous cells. Methods: Target cells were transfected by the calcium-phosphate precipitation method with constructed plasmids expressing HGV-Apoptin and CAV-Apoptin proteins as well as the control plasmid. Transfection efficiency was followed and imaged by fluorescence microscopy. Quantification of apoptosis was performed by flow cytometry. Measurements were compared by paired Student t-test. Results: Cells were successfully transfected with control and constructed plasmids. Flowcytometry analysis showed that A549 cells transfected with HGV-Apoptin and CAV-Apoptin expressing plasmids, undergone the apoptosis compared to A549 cells transfected with control plasmid (P<0.001). None of the plasmids could induce apoptosis in HEK-293 cells. Conclusion: Human Gyrovirus-derived apoptin (HGV-Apoptin) similar to its homologue, chicken anemia virus derived Apoptin (CAV-Apoptin) can induce apoptosis in Non-small-cell lung carcinoma cell line A549, but not in normal human embryonic kidney cell line HEK-293, which can be introduced as a promising novel specific antitumor agent.

Antitumor effects of a recombinant baculovirus displaying anti-HER2 scFv expressing Apoptin in HER2 positive SK-BR-3 breast cancer cells

Aim: Since HER2 targeted therapies have shown clinical benefit in breast cancer, in the present study recombinant baculovirus (BV) displaying anti-HER2 single-chain variable domain fragment (scFv) expressing Apoptin was generated. Methods: The binding specificity and surface display of anti-HER2 scFv were evaluated using enzyme-linked immunosorbent assay (ELISA) and electron microscopy, respectively. The targeting properties and cytotoxic effect on breast cancer cells determined by fluorescence microscopy and MTT assays. Results: The results demonstrated that recombinant BV could specifically bind to HER2-overexpressing SK-BR-3 cells but not to the HER2 negative MCF-7 cells and reduced the viability of SK-BR-3 cells by expressing Apoptin. Conclusion: These results suggest that the antitumor effect of Apoptin in combination with HER2 targeting of this recombinant BV makes it a promising vector in targeted cancer therapy.

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.

Construction of an attenuated Tian Tan vaccinia virus strain by deletion of TA35R and TJ2R genes

rVTT-TA35-TJ, an attenuated vaccinia virus Tian Tan strain (VTT), was constructed by knocking out two non-essential gene fragments (TA35R and TJ2R) related to virulence, immunomodulation, and host range; and by combining double marker screening with exogenous and endogenous selectable marker knock-out techniques. Here, the shuttle plasmids pSK-TA35 and pSK-TJ were constructed, containing two pairs of recombinant arms: early and late strong promoter pE/L and EGFP as an exogenous selectable marker. The recombinant vaccinia virus rVTT-TA35-TJ without exogenous selection markers was then obtained through homologous recombination technology and the Cre/loxP system. Knocking out the two gene fragments does not affect the replication ability of the virus and displays a good genetic stability. Furthermore, a series of in vivo and in vitro experiments demonstrate that although virulence of rVTT-TA35-TJ is attenuated significantly, high immunogenicity was maintained. These results support the potential development of rVTT-TA35-TJ as a safe viral vector or vaccine.

Antitumor effect of the Newcastle disease viral hemagglutinin-neuraminidase gene is expressed through an oncolytic adenovirus effect in osteosarcoma cells

Newcastle disease virus (NDV) can specifically kill cancer cells and has less toxicity to normal cells. The hemagglutinin-neuraminidase (HN) protein is an important structural protein in NDV pathogenesis and has been postulated as a promising candidate for antitumor therapy. The aim of this study was to investigate the anticancer potential of recombinant adenovirus Ad-HN-PEG3p-E1a. An MTS assay was performed to determine viral proliferation after viral infection, the data showed that the proliferation ability of osteosarcoma cells decreased, whereas there was no significant change in normal hepatic cells. DAPI and Annexin V experiments showed that osteosarcoma cells were killed because of apoptosis, active oxygen content, and augmented mitochondrial membrane potential loss. Caspase Activity Assay Kits were used to detect the caspase-3 activities of the treated OS-732 for increased expression. Western blot analysis showed that cytochrome C increased significantly and apoptosis of the virus was confirmed in tumor cells. In-vivo experiments show that NDV has an inhibitory effect on tumor growth. The recombinant adenovirus, which is composed of a HN protein and progressive increment promoter PEG3p, could inhibit the growth of OS-732 and promote the apoptosis of tumor cells. However, there was no clear relationship with normal cell (L02) apoptosis.

Engineering of double recombinant vaccinia virus with enhanced oncolytic potential for solid tumor virotherapy

Vaccinia virus (VACV) oncolytic therapy has been successful in a number of tumor models. In this study our goal was to generate a double recombinant vaccinia virus (VV-GMCSF-Lact) with enhanced antitumor activity that expresses exogenous proteins: the antitumor protein lactaptin and human granulocyte-macrophage colony-stimulating factor (GM-CSF). Lactaptin has previously been demonstrated to act as a tumor suppressor in mouse hepatoma as well as MDA-MB-231 human adenocarcinoma cells grafted into SCID mice. VV-GMCSF-Lact was engineered from Lister strain (L-IVP) vaccinia virus and has deletions of the viral thymidine kinase and vaccinia growth factor genes. Cell culture experiments revealed that engineered VV-GMCSF-Lact induced the death of cultured cancer cells more efficiently than recombinant VACV coding only GM-CSF (VV-GMCSF-dGF). Normal human MCF-10A cells were resistant to both recombinants up to 10 PFU/cell. The selectivity index for breast cancer cells measured in pair cultures MCF-7/MCF-10A was 200 for recombinant VV-GMCSF-Lact coding lactaptin and 100 for VV-GMCSF-dGF. Using flow cytometry we demonstrated that both recombinants induced apoptosis in treated cells but that the rate in the cells with active caspase −3 and −7 was higher after treatment with VV-GMCSF-Lact than with VV-GMCSF-dGF. Tumor growth inhibition and survival outcomes after VV-GMCSF-Lact treatment were estimated using immunodeficient and immunocompetent mice models. We observed that VV-GMCSF-Lact efficiently delays the growth of sensitive and chemoresistant tumors. These results demonstrate that recombinant VACVs coding an apoptosis-inducing protein have good therapeutic potential against chemoresistant tumors. Our data will also stimulate further investigation of coding lactaptin double recombinant VACV in clinical settings.

Generation of an Attenuated Tiantan Vaccinia Virus Strain by Deletion of Multiple Genes

An attenuated vaccinia virus-MVTT_EAB-was constructed by deletion of non-essential gene segments related to the immunomodulatory and virulence functions of the vaccinia virus Tiantan strain (VVTT). The shuttle plasmids pTC-EGFP, pTE-EGFP, pTA35-EGFP, pTB-EGFP, and pTA66-EGFP were constructed and combined with the early and late strong promoter pE/L and EGFP as an exogenous selectable marker. Then, through the homologous recombination technology and Cre/loxP system, the following gene fragments were gradually knocked out one by one: TC7L-TK2L, TE3L, TA35R, TB13R, and TA66R. Ultimately, the five-segment-deleted attenuated strain MVTT_EAB was obtained. Knockout of these segments and genetic stability of MVTT_EAB were confirmed, and it was also shown that knockout of these segments did not affect the replication ability of the virus. Further, a series of in vivo and in vitro experiments demonstrated that the virulence of MVTT_EAB was attenuated significantly, but at same time, high immunogenicity was maintained. These results indicate that MVTT_EAB has potential for clinical use as a safe viral vector or vaccine with good attenuation and immunogenicity.

Seven major genomic deletions of vaccinia virus Tiantan strain are sufficient to decrease pathogenicity

Attenuated strain TTVAC7, as a multi-gene-deleted vaccinia virus Tiantan strain (VTT), was constructed by knocking out parts of non-essential genes related to virulence, host range and immunomodulation of VTT, and by combining double marker screening with exogenous selectable marker knockout techniques. In this study, shuttle vector plasmids pTC-EGFP, pTA35-EGFP, pTA66-EGFP, pTE-EGFP, pTB-EGFP, pTI-EGFP and pTJ-EGFP were constructed, which contained seven pairs of recombinant arms linked to the early and late strong promoter pE/L, as well as to enhanced green fluorescent protein (EGFP) as an exogenous selectable marker. BHK cells were co-transfected/infected successively with the above plasmids and VTT or gene-deleted VTT, and homologous recombination and fluorescence plaque screening methods were used to knock out the gene fragments (TC: TC7L ∼ TK2L; TA35: TA35L; TA66: TA66R; TE: TE3L ∼ TE4L; TB: TB13R; TI: TI4L; TJ: TJ2R). The Cre/LoxP system was then applied to knock out the exogenous selectable marker, and ultimately the gene-deleted attenuated strain TTVAC7 was obtained. A series of in vivo and in vitro experiments demonstrated that not only the host range of TTVAC7 could be narrowed and its toxicity weakened significantly, but its high immunogenicity was maintained at the same time. These results support the potential of TTVAC7 to be developed as a safe viral vector or vaccine.

Chemical and biological insights into uranium-induced apoptosis of rat hepatic cell line

Uranium release into the environment is a threat to human health, and the mechanisms of cytotoxicity caused by uranium are not well-understood. To improve our understanding in this respect, we herein evaluated the effects of uranium exposure on normal rat hepatic BRL cells. As revealed by scanning electron microscopy and transmission electron microscope analysis, uranyl nitrate was found to be transformed into uranyl phosphate particles in the medium and taken up by BRL cells in an endocytotic uptake manner, which presumably initiates apoptosis of the cell, although soluble uranyl ion may also be toxic. The apoptosis of BRL cells upon uranium exposure was also confirmed by both the acridine orange and ethidium bromide double staining assay and the Annexin V/propidium iodide double staining assay. Further studies revealed that uranium induced the loss of mitochondrial membrane potential in a dose-dependent manner. Moreover, the uranium-induced apoptosis was found to be associated with the activation of caspase-3, caspase-8 and caspase-9, indicating both a mitochondria-dependent signaling pathway and a death receptor pathway by a crosstalk. This study provides new chemical and biological insights into the mechanism of uranium toxicity toward hepatic cells, which will help seek approaches for biological remediation of uranium.

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.

Construction and characterization of novel fowlpox virus shuttle vectors

Viral vectors are important vehicles in vaccine research. Avipoxviruses including fowlpox virus (FPV) play major roles in viral vaccine vector development for the prevention and therapy of human and other veterinary diseases due to their immunomodulatory effects and safety profile. Recently, we analyzed the genomic and proteomic backgrounds of the Chinese FPV282E4 strain. Based on analysis of the whole genome of FPV282E4, the FPV150 and FPV193 loci were chosen as insertion sites for foreign genes, and two shuttle vectors with a triple-gene expression cassette were designed and constructed. Homologous recombination between the FPV virus genome and sequences within the shuttle plasmids in infected cells was confirmed. The recombinants were obtained through several rounds of plaque purification using enhanced green fluorescent protein as a reporter and evaluated for the correct expression of foreign genes in vitro using RT-PCR, real-time PCR and Western blotting. Morphogenesis and growth kinetics were assayed via transmission electron microscopy and viral titering, respectively. Results showed that recombinant viruses were generated and correctly expressed foreign genes in CEF, BHK-21 and 293T cells. At least three different exogenous genes could be expressed simultaneously and stably over multiple passages. Additionally, the FPV150 mutation, FPV193 deletion and insertion of foreign genes did not affect the morphogenesis, replication and proliferation of recombinant viruses in cells. Our study contributes to the improvement of FPV vectors for multivalent vaccines.

Apoptin enhances the oncolytic properties of vaccinia virus and modifies mechanisms of tumor regression

A recombinant vaccinia virus VVdGF-ApoS24/2 expressing apoptin selectively kills human cancer cells in vitro [Kochneva et al., 2013]. We compared the oncolytic activity of this recombinant with that of the parental strain L-IVP using a model of human A431 carcinoma xenografts in nude mice. Single intratumoral injections (2×10^7 PFU/mouse) of the viruses produced a dramatic decrease in tumor volumes, which was higher after injection of apoptin-producing virus. The tumor dried out after the injection of recombinant while injection of L-IVP strain resulted in formation of cavities filled with cell debris and liquid. Both viruses rapidly spread in xenografts and replicate exclusively in tumor cells causing their destruction within 8 days. Both viruses induced insignificant level of apoptosis in tumors. Unlike the previously described nuclear localization of apoptin in cancer cells the apoptin produced by recombinant virus was localized to the cytoplasm. The apoptin did not induce a typical apoptosis, but it rather influenced pathway of cell death and thereby caused tumor shrinkage. The replacement of destroyed cells by filamentous material is the main feature of tumor regression caused by the VVdGF-ApoS24/2 virus. The study points the presence of complicated mechanisms of apoptin effects at the background of vaccinia virus replication.

Apoptotic induction of lung adenocarcinoma A549 cells infected by recombinant RVG Newcastle disease virus (rL-RVG) in vitro

Newcastle disease virus (NDV) is a member of the genus Avulavirus in the Paramyxoviridae family and its antitumor properties depend on its ability to kill malignant cells while not affecting normal cells. The present study investigated a recombinant avirulent NDV LaSota strain (wild-type NDV strain) expressing the rabies virus glycoprotein (rL-RVG), examined its oncolytic effect on the lung adenocarcinoma A549 cell line and evaluated its potential to serve as a vaccine against lung cancer. A549 cells were infected with the rL-RVG virus and analyzed by MTT, western blot, polymerase chain reaction (PCR), immunofluorescence, terminal deoxynucleotidyl transferase dUTP nick end labeling and flow-cytometric analyses. PCR, western blot and immunofluorescence showed that the RVG gene and protein were stably expressed in A549 cells following infection with rL-RVG. The growth of A549 cells in the rL-RVG group was inhibited more effectively compared to those infected with the wild-type NDV strain. MTT results showed that cell growth inhibition rates in the rL-RVG group were significantly higher than those in the NDV group (P<0.05). Early apoptosis in the rL-RVG group was also more evident, with the apoptotic index being increased in rL-RVG group. The expression of the pro-apoptotic proteins caspase-3, -8 and -9 increased. The expression of caspase-3 decreased following application of the broad-specificity caspase inhibitor Z-VAD-FMK. However, the expression of the inhibitory apoptosis protein B-cell lymphoma 2 (bcl-2) did not change, but bcl-2-associated X/bcl-2 ratio was higher in the rL-RVG group than that in the NDV group. The rL-RVG strain was able to suppress lung cancer cell growth and promote lung cancer cell apoptosis to a greater extent than the wild-type NDV strain. Therefore, the rL-RVG strain is a potent antitumor agent.

Apoptins: selective anticancer agents

Therapies that selectively target cancer cells for death have been the center of intense research recently. One potential therapy may involve apoptin proteins, which are able to induce apoptosis in cancer cells leaving normal cells unharmed. Apoptin was originally discovered in the Chicken anemia virus (CAV); however, human gyroviruses (HGyV) have recently been found that also harbor apoptin-like proteins. Although the cancer cell specific activity of these apoptins appears to be well conserved, the precise functions and mechanisms of action are yet to be fully elucidated. Strategies for both delivering apoptin to treat tumors and disseminating the protein inside the tumor body are now being developed, and have shown promise in preclinical animal studies.

Anti-tumor effects of an oncolytic adenovirus expressing hemagglutinin-neuraminidase of Newcastle disease virus in vitro and in vivo

Oncolytic virotherapy has been an attractive drug platform for targeted therapy of cancer over the past few years. Viral vectors can be used to target and lyse cancer cells, but achieving good efficacy and specificity with this treatment approach is a major challenge. Here, we assessed the ability of a novel dual-specific anti-tumor oncolytic adenovirus, expressing the hemagglutinin-neuraminidase (HN) gene from the Newcastle disease virus under the human telomerase reverse transcriptase (hTERT) promoter (Ad-hTERTp-E1a-HN), to inhibit esophageal cancer EC-109 cells in culture and to reduce tumor burden in xenografted BALB/c nude mice. In vitro, infection with Ad-hTERT-E1a-HN could inhibit the growth of EC-109 cells significantly and also protect normal human liver cell line L02 from growth suppression in 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays. Ad-hTERT-E1a-HN also effectively and selectively decreased the sialic acid level on EC-109 cells, but not on L02 cells. Furthermore, Ad-hTERT-E1a-HN was shown to induce the apoptosis pathway via acridine orange and ethidium bromide staining (AO/EB staining), increase reactive oxygen species (ROS), reduce mitochondrial membrane potential and release cytochrome c. In vivo, xenografted BALB/c nude mice were treated via intratumoral or intravenous injections of Ad-hTERT-E1a-HN. Although both treatments showed an obvious suppression in tumor volume, only Ad-hTERT-E1a-HN delivered via intratumoral injection elicited a complete response to treatment. These results reinforced previous findings and highlighted the potential therapeutic application of Ad-hTERT-E1a-HN for treatment of esophageal cancer in clinical trials.

Therapeutic efficacy of an hTERT promoter-driven oncolytic adenovirus that expresses apoptin in gastric carcinoma

The efficacy and specificity of treatment are the major challenges for cancer gene therapy. Oncolytic virotherapy is an attractive drug delivery platform of cancer gene therapy. Previous studies have determined that apoptin is a p53-independent, Bcl-2-insensitive apoptotic protein that has the ability to induce apoptosis specifically in tumor cells. In this study, we show that the administration of a dual cancer-specific oncolytic adenovirus construct, Ad-hTERT-E1a-apoptin [in which the adenovirus early region 1a (E1a) gene is driven by the cancer-specific promoter of human telomerase reverse transcriptase (hTERT) and that expresses apoptin simultaneously], suppresses tumor growth in gastric carcinoma cells in vitro and reduces the tumor burden in vivo in xenografted nude mice. The observation that infection with the Ad-hTERT-E1a-apoptin construct significantly inhibited the growth of gastric cancer cells and protected normal human gastric epithelium from growth inhibition confirmed the induction of cancer cell-selective adenovirus replication, growth inhibition and apoptosis by this therapeutic approach. In vivo assays were performed using BALB/c nude mice that had established primary tumors. Subcutaneous primary tumor volume was reduced not only in the intratumoral injection group but also in the systemic delivery mice following treatment with Ad-hTERT-E1a-apoptin. Furthermore, treatment of primary models with Ad-hTERT-E1a-apoptin increased the mouse survival time. These data reinforce previous research and highlight the potential therapeutic application of Ad-hTERT-E1a-apoptin for the treatment of neoplastic diseases in clinical trials.

Efficient production of an engineered apoptin from chicken anemia virus in a recombinant E. coli for tumor therapeutic applications

Background

Apoptin, a nonstructural protein encoded by the VP3 gene of chicken anemia virus (CAV), has been shown to not only induce apoptosis when introduced into the precursors of chicken thymocytes, but has been found to specifically kill human cancer cells, tumor cell and transformed cells without affecting the proliferation of normal cells. This tumor-specific apoptotic characteristic of the protein potentially may allow the development of a protein drug that has applications in tumor therapy. However, several major problems, which include poor expression and poor protein solubility, have hampered the production of apoptin in bacteria.

Results

Significantly increased expression of recombinant full-length apoptin that originated from chicken anemia virus was demonstrated using an E. coli expression system. The CAV VP3 gene was fused with a synthetic sequence containing a trans-acting activator of transcription (TAT) protein transduction domain (PTD). The resulting construct was cloned into various different expression vectors and these were then expressed in various E. coli strains. The expression of the TAT-Apoptin in E. coli was significantly increased when TAT-Apoptin was fused with GST-tag rather than a His-tag. When the various rare amino acid codons of apoptin were optimized, the expression level of the GST-TAT-Apoptin_opt in E. coli BL21(DE3) was significantly further increased. The highest protein expression level obtained was 8.33 g/L per liter of bacterial culture after induction with 0.1 mM IPTG for 4 h at 25 °C. Moreover, approximately 90% of the expressed GST-TAT-Apoptin_opt under these conditions was soluble. After purification by GST affinity chromatography, the purified recombinant TAT-Apoptin_opt protein was used to evaluate the recombinant protein’s apoptotic activity on tumor cells. The results demonstrated that the E. coli-expressed GST-TAT-apoptin_opt showed apoptotic activity and was able to induce human premyelocytic leukemia HL-60 cells to enter apoptosis.

Conclusions

On expression in E. coli, purified recombinant TAT-Apoptin_opt that has been fused to a GST tag and had its codons optimized, was found to have great potential. This protein may in the future allow the development of a therapeutic protein that is able to specifically kill tumor cells.

Protection against SHIV-KB9 infection by combining rDNA and rFPV vaccines based on HIV multiepitope and p24 protein in Chinese rhesus macaques

Developing an effective vaccine against HIV infection remains an urgent goal. We used a DNA prime/fowlpox virus boost regimen to immunize Chinese rhesus macaques. The animals were challenged intramuscularly with pathogenic molecularly cloned SHIV-KB9. Immunogenicity and protective efficacy of vaccines were investigated by measuring IFN-γ levels, monitoring HIV-specific binding antibodies, examining viral load, and analyzing CD4/CD8 ratio. Results show that, upon challenge, the vaccine group can induce a strong immune response in the body, represented by increased expression of IFN-γ, slow and steady elevated antibody production, reduced peak value of acute viral load, and increase in the average CD4/CD8 ratio. The current research suggests that rapid reaction speed, appropriate response strength, and long-lasting immune response time may be key protection factors for AIDS vaccine. The present study contributes significantly to AIDS vaccine and preclinical research.

Targeting Gene-Viro-Therapy with AFP driving Apoptin gene shows potent antitumor effect in hepatocarcinoma

Background

Gene therapy and viral therapy are used for cancer therapy for many years, but the results are less than satisfactory. Our aim was to construct a new recombinant adenovirus which is more efficient to kill hepatocarcinoma cells but more safe to normal cells.

Methods

By using the Cancer Targeting Gene-Viro-Therapy strategy, Apoptin, a promising cancer therapeutic gene was inserted into the double-regulated oncolytic adenovirus AD55 in which E1A gene was driven by alpha fetoprotein promoter along with a 55 kDa deletion in E1B gene to form AD55-Apoptin. The anti-tumor effects and safety were examined by western blotting, virus yield assay, real time polymerase chain reaction, 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide assay, Hoechst33342 staining, Fluorescence-activated cell sorting, xenograft tumor model, Immunohistochemical assay, liver function analysis and Terminal deoxynucleotidyl transferase mediated dUTP Nick End Labeling assay.

Results

The recombinant virus AD55-Apoptin has more significant antitumor effect for hepatocelluar carcinoma cell lines (in vitro) than that of AD55 and even ONYX-015 but no or little impair on normal cell lines. Furthermore, it also shows an obvious in vivo antitumor effect on the Huh-7 liver carcinoma xenograft in nude mice with bigger beginning tumor volume till about 425 mm3 but has no any damage on the function of liver. The induction of apoptosis is involved in AD55-Apoptin induced antitumor effects.

Conclusion

The AD55-Apoptin can be a potential anti-hepatoma agent with remarkable antitumor efficacy as well as higher safety in cancer targeting gene-viro-therapy system.

Apoptin enhances the oncolytic properties of Newcastle disease virus

OBJECTIVE

Naturally occurring strains of Newcastle disease virus (NDV) have demonstrated the potential to kill cancer cells in both preclinical and clinical studies. Previous studies showed that apoptin, the VP3 protein of chicken infectious anemia virus, is a p53-independent, Bcl-2-insensitive apoptotic protein with the ability to specifically induce apoptosis in transformed cells. In this study, we tested the hypothesis that apoptin enhances NDV-mediated tumor cell death.

METHODS

Reverse genetics was used to engineer an oncolytic NDV strain, FMW, to express apoptin. The antitumor effects of the recombinant virus (rFMW/AP) were also evaluated in the tumor cell lines and tumor-bearing mice.

RESULTS

Compared to the parental strain FMW, rFMW/AP was more potent in killing A459 and SMMC7721 tumor cells. Recombinant NDV also exhibited higher efficacy in suppressing tumor growth in mice bearing A549-induced tumors. Furthermore, rFMW/AP did not display apparent toxic effects in either normal cells or control mice.

CONCLUSION

Our results suggest that the recombinant NDV expressing apoptin is a promising novel antitumor agent.

Antitumor effects of a recombinant pseudotype baculovirus expressing Apoptin in vitro and in vivo

Apoptin, a chicken anemia virus-derived, p53-independent, bcl-2-insenstive apoptotic protein with the ability to specifically induce apoptosis in tumor or transformed cells, is a promising tool for cancer gene therapy. In this study, pseudotype baculovirus, a recently developed alternative gene delivery system, was used as a vector to express Apoptin. The resultant recombinant baculovirus (BV-Apoptin) efficiently expressed the Apoptin protein and induced apoptosis in HepG2 and H22 cells. Studies in vivo showed that intratumoral injection of BV-Apoptin into a xenogeneic tumor (derived from H22 murine hepatoma cells in C57BL/6 mice) significantly suppressed tumor growth, and significantly prolonged the survival of tumor-bearing mice compared to a control pseudotype baculovirus that expressed EGFP. Taken together, these results suggest that Apoptin, expressed from the pseudotype baculovirus vector, has the potential to become a therapeutic agent for the treatment of solid tumors.

Potent anti-tumor effects of a dual specific oncolytic adenovirus expressing apoptin in vitro and in vivo

Background

Oncolytic virotherapy is an attractive drug platform of cancer gene therapy, but efficacy and specificity are important prerequisites for success of such strategies. Previous studies determined that Apoptin is a p53 independent, bcl-2 insensitive apoptotic protein with the ability to specifically induce apoptosis in tumor cells. Here, we generated a conditional replication-competent adenovirus (CRCA), designated Ad-hTERT-E1a-Apoptin, and investigated the effectiveness of the CRCA a gene therapy agent for further clinical trials.

Results

The observation that infection with Ad-hTERT-E1a-Apoptin significantly inhibited growth of the melanoma cells, protecting normal human epidermal melanocytes from growth inhibition confirmed cancer cell selective adenoviral replication, growth inhibition, and apoptosis induction of this therapeutic approach. The in vivo assays performed by using C57BL/6 mice containing established primary or metastatic tumors expanded the in vitro studies. When treated with Ad-hTERT-E1a-Apoptin, the subcutaneous primary tumor volume reduction was not only observed in intratumoral injection group but in systemic delivery mice. In the lung metastasis model, Ad-hTERT-E1a-Apoptin effectively suppressed pulmonary metastatic lesions. Furthermore, treatment of primary and metastatic models with Ad-hTERT-E1a-Apoptin increased mice survival.

Conclusions

These data further reinforce the previously research showing that an adenovirus expressing Apoptin is more effective and advocate the potential applications of Ad-hTERT-E1a-Apoptin in the treatment of neoplastic diseases in future clinical trials.

Apoptin, a tumor-selective killer

Apoptin, a small protein from chicken anemia virus, has attracted great attention, because it specifically kills tumor cells while leaving normal cells unharmed. The subcellular localization of apoptin appears to be crucial for this tumor-selective activity. In normal cells, apoptin resides in the cytoplasm, whereas in cancerous cells it translocates into the nucleus. The nuclear translocation of apoptin is largely controlled by its phosphorylation. In tumor cells, apoptin causes the nuclear accumulation of survival kinases including Akt and is phosphorylated by CDK2. Thereby, apoptin redirects survival signals into cell death responses. Apoptin also binds as a multimeric complex to DNA and interacts with several nuclear targets, such as the anaphase-promoting complex, resulting in a G2/M phase arrest. The proapoptotic signal of apoptin is then transduced from the nucleus to cytoplasm by Nur77, which triggers a p53-independent mitochondrial death pathway. In this review, we summarize recent discoveries of apoptin's mechanism of action that might provide intriguing insights for the development of novel tumor-selective anticancer drugs.

Apoptosis-inducing proteins in chicken anemia virus and TT virus

Torque teno viruses (TTVs) share several genomic similarities with the chicken anemia virus (CAV). CAV encodes the protein apoptin that specifically induces apoptosis in (human) tumor cells. Functional studies reveal that apoptin induces apoptosis in a very broad range of (human) tumor cells. A putative TTV open reading frame (ORF) in TTV genotype 1, named TTV apoptosis inducing protein (TAIP), it induces, like apoptin, p53-independent apoptosis in various human hepatocarcinoma cell lines to a similar level as apoptin. In comparison to apoptin, TAIP action is less pronounced in several analyzed human non-hepatocarcinoma-derived cell lines. Detailed sequence analysis has revealed that the TAIP ORF is conserved within a limited group of the heterogeneous TTV population. However, its N-terminal half, N-TAIP, is rather well conserved in a much broader set of TTV isolates. The similarities between apoptin and TAIP, and their relevance for the development and treatment of diseases is discussed.

Construction and identification of the recombinant adenovirus expressing Apoptin gene of chicken anemia virus

AIM: To construct a recombinant adenovirus carrying Apoptin gene so as to provide a basis for further studying the molecular mechanism of Apoptin gene in inducing tumor cell apoptosis.

METHODS: The plasmid pVAX1-Apoptin was digested by endonuclease BamHⅠ and SpeⅠ, and the obtained Apoptin segment was inserted into vector pacAd5 CMV K-N pA to construct a shuttle plasmid pacAd5-Apoptin. After PacⅠ digestion and linearized process, the plasmid pacAd5-Apoptin and pAD (genome plasmid) were co-transfected into AAV-293 cells by liposome mediation. The DNA containing Apoptin gene of the recombinant adenovirus was identified by plaque purification, reverse transcription-polymerase chain reaction (RT-PCR) and Western blot. The titer of the obtained adenovirus was also examined.

RESULTS: The recombinant adenovirus expressed Apoptin gene and the molecular weight of the protein was about 13 kDa, which was consistent with the CVA-positive control. The protein of Apoptin could be effectively expressed in the recombinant adenovirus, and this protein had response to the CAV-positive serum. The titer of the recombinant virus was 10¹¹ PFU/L.

CONCLUSION: The adenovirus containing Apoptin gene is successfully constructed, and the virus titer is able to meet the requirements of in vitro and in vivo experiments.

Apoptin: therapeutic potential of an early sensor of carcinogenic transformation

The avian virus-derived protein apoptin induces p53-independent apoptosis in a tumor-specific way. Apoptin acts as a multimeric complex and forms superstructures upon binding to DNA. In tumor cells, apoptin is phosphorylated and mainly nuclear, whereas in normal cells it is unphosphorylated, cytoplasmic, and becomes readily neutralized. Interestingly, apoptin phosphorylation, nuclear translocation, and apoptosis can transiently be induced in normal cells by cotransfecting SV40 large T oncogene, indicating that apoptin recognizes early stages of oncogenic transformation. In cancer cells, apoptin appears to recognize survival signals, which it is able to redirect into cell death impulses. Apoptin targets include DEDAF, Nur77, Nmi, Hippi, and the potential drug target APC1. Apoptin-transgenic mice and animal tumor models have revealed apoptin as a safe and efficient antitumor agent, resulting in significant tumor regression. Future antitumor therapies could use apoptin either as a therapeutic bullet or as an early sensor of druggable tumor-specific processes.

Viral protein apoptin as a molecular tool and therapeutic bullet: implications for cancer control

The chicken anemia virus-derived protein apoptin induces apoptosis in human tumor cells via a p53-independent pathway, while leaving normal cells intact. Moreover, apoptin treatment in preclinical animal studies leads to reduced tumor growth or remission without a detectable effect on healthy tissues. Apoptin is activated by a still unknown tumor-specific kinase activity. The mode of action of apoptin is under intense investigation, as certain features make it a promising tool for discovering early events in tumorigenesis, identifying druggable targets for antitumor treatment and possibly serving as an antitumor therapy in itself.