Our laboratory is studying induction of non-classical apoptosis by the adenovirus E4orf4 protein. We found that E4orf4 induces malignant cell-specific apoptosis, which is p53-independent. Thus, E4orf4 may have a potential use in cancer therapy, and we are currently working to uncover the details of the E4orf4 apoptotic pathway.
We have shown that interaction between E4orf4 and a subpopulation of cellular protein phosphatase 2A (PP2A) molecules is required for induction of apoptosis by the viral protein. We have further shown that E4orf4 induces caspase-independent apoptosis in some cell lines, whereas in others it interacts with the death receptor pathway to induce caspase-dependent apoptosis. During the apoptotic process, E4orf4 translocates from the nucleus to the cytoplasm and to the cell membrane.
To further analyse the E4orf4 apoptotic pathway we are utilizing genetic models such as yeast and Drosophila. Although the yeast genome does not contain some of the genes encoding the core apoptotic machinery (such as Bcl-2 family members), it does encode members of an ancient cell death pathway, and was shown to be useful as a tool for apoptotic research. We found that E4orf4 induces PP2A-dependent growth arrest in yeast by targeting PP2A to the anaphase promoting complex / cyclosome, and inhibiting its activity. A genetic screen in yeast revealed another component of the E4orf4 toxic pathway, Ynd1, and we are currently studying the interactions between E4orf4, PP2A and human Ynd1 in mammalian cells and their contribution to E4orf4-induced apoptosis.
Studies in Drosophila, carried out in collaboration with Adi Salzberg's lab, show that E4orf4 induces apoptosis in this organism as well. We are currently generating fly strains that will allow us to test whether, similarly to the findings in tissue culture cells, cancer cells in vivo are also more sensitive to E4orf4-induced apoptosis. Future directions include a genetic screen in the fly to find additional components of the E4orf4 apoptotic network.
In addition, we found that E4orf4 associates with a chromatin remodeling factor and targets PP2A to the joint complex. The chromatin remodeling factor contributes to E4orf4 functions in apoptosis and control of virus infection. We continue to study the E4orf4-PP2A-chromatin remodeling factor complex and its contribution to E4orf4 functions.