Mechanisms of Apoptin-induced cell death

Targeting autophagy and beyond: Deconvoluting the complexity of Beclin-1 from biological function to cancer therapy

Beclin-1 is the firstly-identified mammalian protein of the autophagy machinery, which functions as a molecular scaffold for the assembly of PI3KC3 (class III phosphatidylinositol 3 kinase) complex, thus controlling autophagy induction and other cellular trafficking events. Notably, there is mounting evidence establishing the implications of Beclin-1 in diverse tumorigenesis processes, including tumor suppression and progression as well as resistance to cancer therapeutics and CSC (cancer stem-like cell) maintenance. More importantly, Beclin-1 has been confirmed as a potential target for the treatment of multiple cancers. In this review, we provide a comprehensive survey of the structure, functions, and regulations of Beclin-1, and we discuss recent advances in understanding the controversial roles of Beclin-1 in oncology. Moreover, we focus on summarizing the targeted Beclin-1-regulating strategies in cancer therapy, providing novel insights into a promising strategy for regulating Beclin-1 to improve cancer therapeutics in the future.

Oncolytic Viruses in the Therapy of Lymphoproliferative Diseases

Cancer is a leading causes of death. Despite significant success in the treatment of lymphatic system tumors, the problems of relapse, drug resistance and effectiveness of therapy remain relevant. Oncolytic viruses are able to replicate in tumor cells and destroy them without affecting normal, healthy tissues. By activating antitumor immunity, viruses are effective against malignant neoplasms of various nature. In lymphoproliferative diseases with a drug-resistant phenotype, many cases of remissions have been described after viral therapy. The current level of understanding of viral biology and the discovery of host cell interaction mechanisms made it possible to create unique strains with high oncoselectivity widely used in clinical practice in recent years.

Apoptin Overexpression Efficiently Amplified Cytotoxic Effects of PI3K Inhibition Using BKM120 in Lymphoblastic Leukemia Cell Lines

Purpose: Although the complex structure of acute lymphoblastic leukemia (ALL) and involvement of diverse pathways in its pathogenesis have put an obstacle in the way of efficient treatments, identification of strategies to manipulate the genome of neoplastic cells has made the treatment prospective more optimistic.

Methods: To evaluate whether the transduction of apoptin __a gene encoding a protein that participates in the induction of apoptosis__ could reduce the survival of leukemic cells, we generated recombinant lentivirus expressing apoptin, and then, MTT assay, flow cytometric analysis of DNA content, western blotting, and quantitative reverse transcription polymerase chain reaction (qRT-PCR) were applied.

Results: Transduction of apoptin into different leukemic cells was coupled with the reduction in the viability and proliferative capacity of the cells. Among all tested cell lines, Nalm-6 and C8166 were more sensitive to the anti-leukemic property of apoptin. Moreover, we found that the transduction of apoptin in the indicated cell lines not only induced G2/M cell cycle arrest but also induced apoptotic cell death by altering the balance between pro- and anti-apoptotic target genes. The efficacy of apoptin transduction was not limited to these findings, as we reported for the first time that the overexpression of this gene could potentiate the anti-leukemic property of pan PI3K inhibitor BKM120.

Conclusion: The results of this study showed that the transduction of apoptin into lymphoblastic leukemia cell lines induced cytotoxic effects and enhanced therapeutic value of PI3K inhibition; however, further investigations are demanded to ascertain the safety and the efficacy of apoptin transduction in patients with ALL.

Effects of chicken anaemia virus on experimental leukosis, induced by avian myelocytomatosis virus Mc29

The effects of concomitant infection with chicken anaemia virus (CAV) on the incidence, clinical manifestation and mortality from leukosis, induced by the avian myelocytomatosis virus strain Mc29 were studied. Experimental one-day-old 15 I line White Leghorn chickens were inoculated simultaneously with Mc29 and CAV or with Mc29 alone and observed daily for clinical signs and mortality. Both groups of chickens inoculated with Mc29 virus strain alone or in combination with CAV deve­loped tumours and died within 57 days. Necropsy has been performed on all dead birds following the standard protocol. Organ samples from thymuses, spleens, bone marrow, and livers were collected and histopathologically investigated. Neoplasms detected included myelocytomas, nephroblastomas and hepatocellular carcinomas. In addition, 50% of the CAV/Mc29-inoculated chickens developed epithelial type thymomas. However, no such lesions were found in chickens infected with Mc29 alone. No significant differences in the clinical course of leukosis between the two experimental groups of chickens were observed. The results indicated that CAV infection did not affect substantially the incidence and mortality from avian leukosis, induced by myelocytomatosis virus strain Mc29, but contributed to greater variety of the induced tumours.

Enhancing therapeutic efficacy of oncolytic vaccinia virus armed with Beclin-1, an autophagic Gene in leukemia and myeloma

Different strategies were taken to make virotherapy more effective at killing cancer cells. Among them, oncolytic virus which arms the therapeutic gene to enhance antitumor activity is a prevalent approach. In this study, a newly developed oncolytic vaccinia virus (OVV) that expresses Beclin-1 (OVV-BECN1) was tested for its in vitro and in vivo oncolytic activity in blood cancer. Results showed that the OVV exhibited higher infectivity for leukemia cells. OVV-BECN1 induced significant apoptosis-independent cell death either in wild-type leukemia and multiple myeloma (MM) cell lines or caspase-3 shRNA leukemia cell lines, and had a superior antitumor activity compared to the parent OVV. Autophagic cell death induced by OVV-BECN1 was demonstrated in vitro and in vivo experiments. Finally, upregulation of SIRT-1, a member of class III histone deacetylases, by OVV-BECN1 resulted in the deacetylation of LC3 and its distribution from the nucleus toward the cytoplasm, which might contribute to induction of autophagy. Overall, our data showed a favorable therapeutic effect of the oncolytic vaccinia virus on blood cancers through oncolytic and autophagic mechanisms, and may therefore constitute a promising and effective therapeutic strategy for treating human leukemia and MM. However, further studies are warranted for its reliable clinical translation.

LdrB Toxin with In Vitro and In Vivo Antitumor Activity as a Potential Tool for Cancer Gene Therapy

Due to the high prevalence of cancer in recent years, it is necessary to develop new and more effective therapies that produce fewer side effects. Development of gene therapy for cancer based on the use of suicide genes that can damage the tumor cell, without requiring a prodrug for its lethal effect, is one of the recent foci of gene therapy strategies. We evaluated the cytotoxic impact of the LdrB toxin from Escherichia coli k12 as a possible tool for cancer gene therapy. For that, colorectal and breast cancer cells were transfected under the control of a TRE3G promoter inducible by doxycycline. Our results showed that ldrB gene expression induced a drastic inhibition of proliferation in vitro, in both 2D and 3D experimental models. Moreover, unlike conventional chemotherapy, the ldrB gene induced a severe loss of proliferation in vivo without any side effects in our animal model. This antitumor outcome was modulated by cell cycle arrest in the G0/G1 phase and apoptotic death. Scanning electronic microscopy demonstrates that the LdrB toxin conserves its pore-forming ability in HCT-116 cells as in E. coli k12. Taken together, our results provide, for the first time, a proof of concept of the antitumor capacity of the ldrB gene in colorectal and breast cancer.

Deciphering the Mechanism of Action Involved in Enhanced Suicide Gene Colon Cancer Cell Killer Effect Mediated by Gef and Apoptin

Despite the great advances in cancer treatment, colorectal cancer has emerged as the second highest cause of death from cancer worldwide. For this type of tumor, the use of suicide gene therapy could represent a novel therapy. We recently demonstrated that co-expression of gef and apoptin dramatically inhibits proliferation of the DLD-1 colon cell line. In the present manuscript, we try to establish the mechanism underlying the enhanced induction of apoptosis by triggering both gef and apoptin expression in colon tumor cells. Scanning microscopy reveals that simultaneous expression of gef and apoptin induces the apparition of many "pores" in the cytoplasmic membrane not detected in control cell lines. The formation of pores induced by the gef gene and accentuated by apoptin results in cell death by necrosis. Moreover, we observed the presence of apoptotic cells. Performing protein expression analysis using western blot, we revealed an activation of mitochondrial apoptosis (increased expression of Pp53, cytochrome c, Bax, and caspase 9) and also the involvement of the extrinsic pathway through caspase 8activation. In conclusion, in this manuscript we demonstrate for the first time that the extrinsic pathway of apoptosis and pore formation is also involved in the cell death caused by the co-expression of the gef and apoptin genes. Our results suggest that co-expression of gef and apoptin genes induces an increase in post-apoptotic necrotic cell death and could be a valuable tool in the design of new antitumor strategies focused on the enhancement of the immune response against cancer cell death.

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.

Non-Structural protein 1 (NS1) gene of Canine Parvovirus-2 regresses chemically induced skin tumors in Wistar rats

The Non-Structural protein 1 of Canine Parvovirus-2 (CPV2.NS1) plays a major role in viral cytotoxicity and pathogenicity. CPV2.NS1 has been proven to cause apoptosis in HeLa cells in vitro in our laboratory. Here we report that CPV2.NS1 has no toxic side effects on healthy cells but regresses skin tumors in Wistar rats. Histopathological examination of tumor tissue from CPV2.NS1 treated group revealed infiltration of mononuclear and polymorphonuclear cells with increased extra cellular matrix, indicating signs of regression. Tumor regression was also evidenced by significant decrease in mitotic index, AgNOR count and PCNA index, and increase in TUNEL positive apoptotic cells in CPV2.NS1 treated group. Further, CPV2.NS1 induced anti-tumor immune response through significant increase in CD8(+) and NK cell population in CPV2.NS1 treated group. These findings suggest that CPV2.NS1 can be a possible therapeutic candidate as an alternative to chemotherapy for the treatment of cancer.

Selective apoptosis induction in MCF-7 cell line by truncated minimal functional region of Apoptin

BACKGROUND

Chicken Anemia Virus (CAV) VP3 protein (also known as Apoptin), a basic and proline-rich protein has a unique capability in inducing apoptosis in cancer cells but not in normal cells. Five truncated Apoptin proteins were analyzed to determine their selective ability to migrate into the nucleus of human breast adenocarcinoma MCF-7 cells for inducing apoptosis.

METHODS

For identification of the minimal selective domain for apoptosis, the wild-type Apoptin gene had been reconstructed by PCR to generate segmental deletions at the N' terminal and linked with nuclear localization sites (NLS1 and NLS2). All the constructs were fused with maltose-binding protein gene and individually expressed by in vitro Rapid Translation System. Standardized dose of proteins were delivered into human breast adenocarcinoma MCF-7 cells and control human liver Chang cells by cytoplasmic microinjection, and subsequently observed for selective apoptosis effect.

RESULTS

Three of the truncated Apoptin proteins with N-terminal deletions spanning amino acid 32-83 retained the cancer selective nature of wild-type Apoptin. The proteins were successfully translocated to the nucleus of MCF-7 cells initiating apoptosis, whereas non-toxic cytoplasmic retention was observed in normal Chang cells. Whilst these truncated proteins retained the tumour-specific death effector ability, the specificity for MCF-7 cells was lost in two other truncated proteins that harbor deletions at amino acid 1-31. The detection of apoptosing normal Chang cells and MCF-7 cells upon cytoplasmic microinjection of these proteins implicated a loss in Apoptin's signature targeting activity.

CONCLUSIONS

Therefore, the critical stretch spanning amino acid 1-31 at the upstream of a known hydrophobic leucine-rich stretch (LRS) was strongly suggested as one of the prerequisite region in Apoptin for cancer targeting. Identification of this selective domain provides a platform for developing small targets to facilitating carrier-mediated-transport across cellular membrane, simultaneously promoting protein delivery for selective and effective breast cancer therapy.

Therapeutic anti-tumor effect of exogenous apoptin driven by human survivin gene promoter in a lentiviral construct

INTRODUCTION

The aim of this study was to construct a lentivirus vector with survivin promoter (pSur)-driven apoptin and test its efficiency in suppressing the growth of tumor cells.

MATERIAL AND METHODS

Expression cassettes with different fragments of survivin gene promoter (pSur, 161 bp, 272 bp, 990 bp) driving 6XHis-tagged apoptin were constructed to generate recombinant lentivirus, of which the inhibitory effect on tumor cells was compared. The activity of different pSur in 293FT, and 272 bp pSur in primary bone marrow mesenchymal stem cells (BMSCs), SW480, Hela and MCF-7 was examined by Western blot. Their ability to induce apoptosis in SW480 cells was determined by annexin-V staining. The inhibitory effect of letivirus containing different pSur-driven apoptin on nude mice-xenografted SW480 cells was assessed by tumor size and pathological observation.

RESULTS

The 272 bp and 990 bp pSur displayed comparable effects in terms of promoter activity, cell apoptosis/necrosis and G1 phase arrest in vitro, and growth of xenograft tumor in vivo. When lentivirus containing 272 bp pSur was tested, it drove high apoptin expression in tumor cells (SW480, Hela and MCF-7) and weak expression in primary bone marrow mesenchymal stem cells. Xenograft to nude mice using infected Sw480 cells showed that lentiviruses possessing 272 bp and 990 bp pSur were able to significantly induce tumor cell death, focal necrosis, and tumor growth lag.

CONCLUSIONS

The data indicated that pSur-apoptin expression cassette in lentivirus vector ensures specific suppression of tumor cells, and may be applicable to monitor malignant transformation of transplanted cells.