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.

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.

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.