Antitumor effects of apoptin expressed by the dual cancer-specific oncolytic adenovirus - a review.

Apoptin is a small molecular weight protein derived from chicken anemia virus, which can induce the apoptosis of transformed cells and tumor cells and leave primary and nontransformed cells unharmed. Apoptin's cell localization depends on its own phosphorylation state and cell type. In tumor cells, phosphorylated apoptin enters the nucleus and induces apoptosis. While, in normal cells apoptin mainly exists in the cytoplasm. Apoptin, as a disordered protein in cells, interacts with many proteins in cell signal pathways to induce apoptosis of tumor cells. The specific mechanism of apoptosis induced by apoptin has not been completely elucidated. Therefore, apoptin has become a potential anticancer agent. This review summarizes the research results of apoptin in our laboratory and reveals the specific antitumor mechanism of apoptin expressed by oncolytic virus vector on a variety of tumor cells and mouse models.

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.

Phylogenetic analysis and genetic characterization of chicken anemia virus isolates from Cambodia.

Three chicken anemia viruses (CAV) were detected by PCR during screening of field samples from village chickens collected in Cambodia in 2011/2012. Nearly full-length VP1 viral structural protein genes (nt 1-1,293) from the 3 CAV were sequenced and characterized. Phylogenetic analysis revealed that all 3 of the Cambodian CAV were clustered with CAV strains belonging to genotype II and were most closely related to CAV strains from Guangdong province, China. On the amino acid level, major substitutions were observed at 12 residues in the VP1 protein (positions 22, 75, 97, 125, 139, 144, 254, 287, 290, 370, 376, and 413) when compared with published reference CAV strains. In motifs associated with virulence, all Cambodian CAV had virulence-associated motifs composed of 75I, 89T, 125I, 139Q, 141Q, 144Q, and 394Q, which are commonly found in highly virulent genotype II viruses and some genotype III viruses. This is the first report of CAV isolated from village chickens in Southeast Asia as well as Cambodia.

Molecular characterization of contaminating infectious anemia virus of chickens in live commercial vaccines produced in the 1990s.

The presence of infectious chicken anemia virus (CAV) was detected in a previous study by nested-PCR as a contaminant in seven commercial vaccines, produced in the 1990s by three different manufacturers, prepared against the most relevant virus etiologies. In order to phylogenetically characterize the genome and compare it to CAV isolates from Brazil and other parts of the world, sequences of approximately 675 bp of the gene encoding the hypervariable region of VP1 protein of three CAV vaccine contaminant strains were studied. The CAV genome in contaminated vaccines showed high similarity (> 98.9%) with the Brazilian BR91/99 and Argentinian ArgA001028 (> 99%) strains. However, the comparison with the Cuxhaven-1 vaccine strain showed a lower identity of between 96.8% and 97.7%, and comparing it with the CAV26P4 vaccine strain showed an identity between 97.2% and 98.2%; both are available in Brazil. Such differences might be relevant for the highly conserved CAV genome. CAV contaminants were positioned in the same genetic group (clusters) with the Brazilian strain BR91/99 and Argentinian strain ArgA001028. Results indicated that the contamination of live vaccines by CAV may have influenced CAV epidemiology in the Brazilian and Argentinian poultry industry.

An optimized polymerase chain reaction assay to identify avian virus vaccine contamination with Chicken anemia virus.

The use of embryonating chicken eggs in preparation of avian virus vaccines is the principle cause for contamination with Chicken anemia virus (CAV). Identification of CAV in contaminated vaccines relies on the expensive, tedious, and time-consuming practice of virus isolation in lymphoblastoid cell lines. The experience of the last 2 decades indicates that polymerase chain reaction is extending to replace most of the classic methods for detection of infectious agents. In the present report, a simple, rapid, and accurate polymerase chain reaction method for detection of CAV in poultry vaccines is described. Oligonucleotide primers homologous to highly conserved sequences of the VP1 gene were used to amplify a fragment of 676 bp. The developed assay was specific for detecting CAV from different sources, with no cross reactivity with many avian viruses. No inter- and intra-assay variations were observed. The analytical sensitivity of the test was high enough to detect 5 TCID(50) (50% tissue culture infective dose) of the virus per reaction; however, different factors related to the vaccine matrix showed considerable effects on the detection limit. In conclusion, this method may represent a suitable alternative to virus isolation for identification of CAV contamination of poultry virus vaccines.

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.

Cloning and expression of chicken anemia virus VP3 protein in Escherichia coli.

Purification of chicken anemia virus (CAV) VP3 protein, expressed in a prokaryotic expression system as histidine-tagged fusion protein is demonstrated in the present study. CAV particle was obtained from infected liver of chicken and DNA was extracted. The VP3 protein gene was amplified from the extracted DNA by polymerase chain reaction (PCR) and cloned. The recombinant expression construct (pTrc-VP3) was identified by PCR and sequencing analysis. Expression of VP3 protein with a molecular mass of approximately 21kDa was confirmed by Western blotting analysis with CAV-specific antibodies. The in vitro expressed VP3 protein was purified to near homogeneity by elution from the gel, as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis. The purified VP3 protein was recognized by CAV antibodies in a Western blotting assay. This finding indicates that recombinant VP3 expressed in the pTrcHis2 vector system can be used as antigen to detect anti-CAV antibodies.

Characterization and phylogenetic analysis of Brazilian chicken anaemia virus.

Chicken anaemia virus (CAV) was detected by a Nested-PCR assay in field samples from different regions of Brazil. The 539 bp amplified fragments of vp1 gene from 44 field samples were sequenced and 10 new nucleotide sequences of CAV were observed. These sequences were phylogenetically analysed by Mega2 using neighbour joining distance methods with 1000 bootstrap replications. Phylogenetic analysis did not show correlation between CAV pathology pattern and genetic groups. The 10 nucleotide sequences of the Brazilian samples were also analysed together with 30 sequences of CAV strains previously described from other countries. The genetic variability observed was not related to the geographical distribution. Amino acid substitutions were detected at 9 positions of the Brazilian sequences and two of them had not been observed before, (65)R replacing the Q residue and (98)F replacing Y residue.

Mutation of chicken anemia virus VP2 differentially affects serine/threonine and tyrosine protein phosphatase activities.

Novel dual-specificity protein phosphatases (DSPs), which catalyse the removal of phosphate from both phosphotyrosine and phosphoserine/phosphothreonine substrates, have recently been identified in two viruses within the family Circoviridae. Viral protein 2 (VP2) of chicken anemia virus (CAV) and ORF2 of TT virus have been shown to possess DSP activity in vitro. CAV VP2 is unusual in possessing two vicinal cysteines within the protein phosphatase signature motif. The first cysteine residue (C95) within the motif has been identified by mutagenesis as the essential catalytic cysteine. In this study, it was shown that virus mutated at this residue displayed a marked inhibition of growth, with titres reduced 10(4)-fold, and reduced cytopathogenic effect in cell culture, indicating that viral DSP activity may be significant during infection. As with virus mutated at the first cysteine residue, mutation of the second cysteine (C97) within the motif resulted in a marked reduction in viral growth and attenuation of cytopathogenicity in infected cell cultures. However, mutagenesis of this second cysteine only reduced phosphotyrosine phosphatase activity to 70 % of that of wild-type VP2, but increased phosphoserine/phosphothreonine phosphatase activity by as much as 700 %. The differential effect of the C97S mutation on VP2 activity does not appear to have parallels in other DSPs and suggests a unique role for the second cysteine in the function of these viral proteins, particularly in vivo.

The tumour specific pro-apoptotic factor apoptin (Vp3) from chicken anaemia virus.

Cancer is a growing problem for human health world-wide. Dramatic breakthroughs have increased our understanding of the molecular mechanisms involved in the process of tumorigenesis, allowing us to develop more refined anti-cancer treatments, expanding the repertoire of available anti-cancer drugs, and increasing the efficiency of their delivery to malignant cells. Nevertheless, even with improved understanding of the complex origins of cancer cells, there is a dearth of efficient and above all specific anti-cancer treatments. Apoptin (viral protein 3 - VP3), a gene product derived from the Chicken Anaemia Virus (CAV) represents a novel anti-cancer tool. It appears to have innate tumour-specific p53-independent, Bcl-2-enhanced pro-apoptotic activity, and hence may be of great utility in the endeavour to achieve specific and efficient elimination of cancer cells, particularly in cases of drug resistance through Bcl-2 overexpression/loss of p53 function etc. This review will examine the unique aspects of apoptin's properties, and in particular, its ability to localise specifically in the nucleus of transformed but not normal cells. The latter ability, importantly, appears to be integrally related to its tumour-specific pro-apoptotic action.

Identification of a 24 kDa protein expressed by chicken anaemia virus.

Antisera raised against oriented peptide conjugates were used to identify and partially characterize a 24 kDa protein product expressed by chicken anaemia virus (CAV). The peptides derived from the N and C termini of the protein were shown to react against the native protein, expressed within virus-infected cells, by immunofluorescence, immunoperoxidase and immunogold thin section electron microscopy techniques. The protein product was located by immunogold single labelling in intranuclear inclusions similar to those described previously for the 13 kDa CAV protein, which causes apoptosis. The 24 kDa protein was co-localized to the nuclear inclusions with the CAV 13 kDa protein by simultaneous dual labelling immunogold electron microscopy. Following isolation of the CAV proteins by nuclei isolation and SDS-PAGE, the antisera were used to probe for the protein by immunoblotting. The antisera recognized an expressed protein product of apparent molecular mass 30 kDa. An immunofluorescence time course study of CAV protein expression was carried out and the peptide antisera reacted against the protein at 12 h post-infection. Antisera against the 13 kDa protein reacted at similar times post-infection. This was in contrast to antisera raised against the 52 kDa capsid protein which is detectable by immunofluorescence only after 24 h. The 13 kDa and 24 kDa proteins thus appear to be early antigens produced by CAV during infection.