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Differential sensitivity to Ad5 E1B-21kD and Bcl-2 proteins of apoptin-induced versus p53-induced apoptosis
S M Zhuang1, A Shvarts, A G Jochemsen
1Laboratory for Molecular Carcinogenesis, Sylvius Laboratories, Leiden University, The Netherlands.
Abstract:
Apoptin, a small protein derived from chicken anemia virus (CAV), induces apoptosis in human tumor cell lines regardless of whether these express p53 or not. We examined whether the small adenovirus 5 E1B protein of 21 kDa (E1B-21kD), also called E1B-19kD) and Bcl-2 could inhibit apoptin-induced apoptosis in human tumor cell lines and compared this with p53-induced apoptosis. E1B-21kD, but not Bcl-2 was found to inhibit apoptin-induced apoptosis in the osteosarcoma cell lines U2OS and Saos-2. However, neither expression of E1B-21kD nor of Bcl-2 resulted in inhibition of apoptin-induced apoptosis in Hep3B hepatoma cells and kidney rhabdoid tumor G401 cells. Both Bcl-2 and Ad5 E1B-21kD were able to inhibit p53-induced apoptosis in the human tumor cell lines Saos-2 and Hep3B. In Saos-2 and U2OS, but not in Hep3B and G401, expression of E1B-21kD leads to retention of apoptin in the cytoplasm, in that way preventing its nuclear function. These results indicate that proteins inhibiting the p53-induced apoptotic pathway do not block apoptin-induced apoptosis or do so only in a cell type-specific manner. The apoptin-induced apoptotic pathway is distinct from that induced by p53 and, therefore, apoptin is a potential antitumor agent.
Insights
Apoptin induces tumor cell death independently of p53. Viral proteins like E1B-21kD inhibit this process in some cells, suggesting distinct apoptotic pathways and apoptin
Area of Science:
- Oncology
- Virology
- Molecular Biology
Background:
- Apoptin, derived from chicken anemia virus (CAV), triggers apoptosis in human tumor cells, irrespective of p53 expression.
- The p53 tumor suppressor protein also induces apoptosis, a critical cellular process.
- Viral proteins, such as adenovirus 5 E1B-21kD (E1B-21kD) and Bcl-2, are known to interfere with apoptosis.
Purpose of the Study:
- To investigate whether E1B-21kD and Bcl-2 can inhibit apoptin-induced apoptosis in human tumor cell lines.
- To compare the inhibitory effects of E1B-21kD and Bcl-2 on apoptin-induced apoptosis with their effects on p53-induced apoptosis.
Main Methods:
- Transfection of human tumor cell lines (U2OS, Saos-2, Hep3B, G401) with vectors expressing E1B-21kD or Bcl-2.
- Induction of apoptosis using apoptin or by activating the p53 pathway.
- Assessment of apoptosis levels and localization of apoptin within cells using microscopy.
Main Results:
- E1B-21kD inhibited apoptin-induced apoptosis in osteosarcoma cell lines (U2OS, Saos-2) but not in hepatoma (Hep3B) or kidney rhabdoid tumor (G401) cells.
- Neither E1B-21kD nor Bcl-2 inhibited apoptin-induced apoptosis in Hep3B and G401 cells.
- Both E1B-21kD and Bcl-2 effectively inhibited p53-induced apoptosis in Saos-2 and Hep3B cells.
- E1B-21kD caused cytoplasmic retention of apoptin in Saos-2 and U2OS cells, preventing nuclear translocation and function.
Conclusions:
- Apoptin-induced apoptosis operates via a pathway distinct from p53-mediated apoptosis.
- Inhibition of p53-related apoptosis does not necessarily block apoptin's effects, indicating cell-type specificity.
- Apoptin demonstrates potential as an anticancer therapeutic agent due to its unique mechanism of action.