Abstract
The adenovirus E4orf4 protein regulates the progression of viral infection, and when expressed alone in mammalian tissue culture cells it induces protein phosphatase 2A (PP2A)-B55- and Src-dependent cell death, which is more efficient in oncogene-transformed cells than in normal cells. This form of cell death is caspase-independent, although it interacts with classic caspase-dependent apoptosis. PP2A-B55-dependent E4orf4-induced toxicity is highly conserved in evolution from yeast to mammalian cells. In this work we investigated E4orf4-induced cell death in a whole multicellular organism, Drosophila melanogaster. We show that E4orf4 induced low levels of cell killing, caused by both caspase-dependent and -independent mechanisms. Drosophila PP2A-B55 (twins/abnormal anaphase resolution) and Src64B contributed additively to this form of cell death. Our results provide insight into E4orf4-induced cell death, demonstrating that in parallel to activating caspase-dependent apoptosis, E4orf4 also inhibited this form of cell death induced by the proapoptotic genes reaper, head involution defective, and grim. The combination of both induction and inhibition of caspase-dependent cell death resulted in low levels of tissue damage that may explain the inefficient cell killing induced by E4orf4 in normal cells in tissue culture. Furthermore, E4orf4 inhibited JNK-dependent cell killing as well. However, JNK inhibition did not impede E4orf4-induced toxicity and even enhanced it, indicating that E4orf4-induced cell killing is a distinctive form of cell death that differs from both JNK- and Rpr/Hid/Grim-induced forms of cell death.
