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

Combination of VP3 and CD147-knockdown enhance apoptosis and tumor growth delay index in colorectal tumor allograft

BACKGROUND

Cancer therapies that kill cancer cells without affecting normal cells is the ultimate mode of treating cancers. The VP3, an avian virus-derived protein, can specifically initiate cell death through several signal transduction pathways leading to apoptosis. In cancer, chemoresistance and cell survivability implicate the cell surface protein, CD147.

METHODS

In this study, transfection of VP3 and silencing of CD147 genes was achieved through the treatment of tumors with pVIVO1-GFP/VP3 (VP3), psiRNA-CD147/2 (shCD147/2), and their combination of CT26 colon cancer cell-induced in mice. The effectiveness of tumor-treatment was ascertained by electrophoresis, TUNEL assay, and flow cytometry analysis. While histopathological and biochemical analysis were used as toxic side effect identification.

RESULTS

The tumor growth delay index (TGDI) after treatment with VP3, shCD147/2, and their combination treatments increased by 1.3-, 1.2-, 2.0- and 2.3-fold respectively, over untreated control. The VP3-shCD147/2 combination treatment was more efficacious then either VP3 or shCD147/2 alone in the retardation of mouse CT26 colorectal cell tumor allograft.

CONCLUSION

The antitumor effect of the combination treatment is the result of synergistic effects of VP3 and shCD147/2 on the tumor cells resulting in apoptosis. Thus, the study shows that combination of VP3 and shCD147/2 treatment can be developed into a potential approach for anticolorectal cancer treatment regimen.

Anticancer genes: inducers of tumour-specific cell death signalling

Recent studies have revealed a new class of genes encoding proteins with specific anticancer activity. Upon ectopic expression, these factors cause cell death specifically in tumour cells by apoptosis, autophagy or mitotic catastrophe, yet normal cells are spared. Some of these genes or their encoded proteins are in clinical development and show promising results, and their signalling pathways are currently under intense investigation. Defining these genes as anticancer genes, we review what is known about their functions, the specific cell death signals they induce and the status of cancer therapy approaches that emulate their function. Systematic screening for such anticancer genes might lead to the identification of a repertoire of signalling pathways directed against cellular alterations that are specific for tumour cells.

Proteins selectively killing tumor cells

All human cells have a genetic program that upon activation will cause cell death, named apoptosis. Cancer cells can grow due to unbalances in proliferation, cell cycle regulation and their apoptosis machinery: genomic mutations resulting in non-functional pro-apoptosis proteins or over-expression of anti-apoptosis proteins form the basis of tumor formation. Surprisingly, lessons learned from viruses show that cancer cannot be regarded simply as the opposite of apoptosis. For instance, adenovirus can only transform cells when both its anti- and pro-apoptotic proteins are produced. Oncolytic viruses are known to replicate selectively in tumor cells resulting in cell death. Proteins derived from viruses, i.e. chicken anemia virus (CAV)-derived apoptosis-inducing protein (apoptin), adenovirus early region 4 open reading frame (E4orf4) and parvovirus-H1 derived non-structural protein 1 (NS1), the human alpha-lactalbumin made lethal to tumor cells (HAMLET), which is present in human milk or the human cytokines melanoma differentiation-associated gene-7 (mda-7) and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) have all the ability to induce tumor-selective apoptosis. The tumor-selective apoptosis-inducing proteins seem to interact with transforming survival processes, which can become redirected by these proteins into cell death. Transformation-related processes have been identified, which seem to be crucial for the tumor-selectively killing activity of these proteins. For instance, the transformation-related protein phosphatase 2A (PP2A) plays a role in the induction of tumor-selective apoptosis. The proteins mda-7, TRAIL and HAMLET are already successfully tested in first clinical trials. Proteins harboring tumor-selective apoptosis characteristics represent, therefore, a therapeutic potential and a tool for unraveling tumor-related processes. Fundamental molecular and (pre)clinical therapeutic studies of the various tumor-selective apoptosis-inducing proteins apoptin, E4orf4, HAMLET, mda-7, NS1, TRAIL and related proteins will be discussed.

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.