Involvement of Lgl and Mahjong/VprBP in cell competition.

During the initial stages of carcinogenesis, transformation events occur in a single cell within an epithelial monolayer. However, it remains unknown what happens at the interface between normal and transformed epithelial cells during this process. In Drosophila, it has been recently shown that normal and transformed cells compete with each other for survival in an epithelial tissue; however the molecular mechanisms whereby "loser cells" undergo apoptosis are not clearly understood. Lgl (lethal giant larvae) is a tumor suppressor protein and plays a crucial role in oncogenesis in flies and mammals. Here we have examined the involvement of Lgl in cell competition and shown that a novel Lgl-binding protein is involved in Lgl-mediated cell competition. Using biochemical immunoprecipitation methods, we first identified Mahjong as a novel binding partner of Lgl in both flies and mammals. In Drosophila, Mahjong is an essential gene, but zygotic mahjong mutants (mahj(-/-)) do not have obvious patterning defects during embryonic or larval development. However, mahj(-/-) cells undergo apoptosis when surrounded by wild-type cells in the wing disc epithelium. Importantly, comparable phenomena also occur in Mahjong-knockdown mammalian cells; Mahjong-knockdown Madin-Darby canine kidney epithelial cells undergo apoptosis, only when surrounded by non-transformed cells. Similarly, apoptosis of lgl(-/-) cells is induced when they are surrounded by wild-type cells in Drosophila wing discs. Phosphorylation of the c-Jun N-terminal kinase (JNK) is increased in mahj(-/-) or lgl(-/-) mutant cells, and expression of Puckered (Puc), an inhibitor of the JNK pathway, suppresses apoptosis of these mutant cells surrounded by wild-type cells, suggesting that the JNK pathway is involved in mahj- or lgl-mediated cell competition. Finally, we have shown that overexpression of Mahj in lgl(-/-) cells strongly suppresses JNK activation and blocks apoptosis of lgl(-/-) cells in the wild-type wing disc epithelium. These data indicate that Mahjong interacts with Lgl biochemically and genetically and that Mahjong and Lgl function in the same pathway to regulate cellular competitiveness. As far as we are aware, this is the first report that cell competition can occur in a mammalian cell culture system.

Novel roles of hakai in cell proliferation and oncogenesis.

During tumor development, cells acquire multiple phenotypic changes upon misregulation of oncoproteins and tumor suppressor proteins. Hakai was originally identified as an E3 ubiquitin-ligase for the E-cadherin complex that regulates cell-cell contacts. Here, we present evidence that Hakai plays a crucial role in various cellular processes and tumorigenesis. Overexpression of Hakai affects not only cell-cell contacts but also proliferation in both epithelial and fibroblast cells. Furthermore, the knockdown of Hakai significantly suppresses proliferation of transformed epithelial cells. Expression of Hakai is correlated to the proliferation rate in human tissues and is highly up-regulated in human colon and gastric adenocarcinomas. Moreover, we identify PTB-associated splicing factor (PSF), an RNA-binding protein, as a novel Hakai-interacting protein. By using cDNA arrays, we have determined various specific PSF-associated mRNAs encoding proteins that are involved in several cancer-related processes. Hakai affects the ability of PSF to bind these mRNAs, and expression of PSF short hairpin RNA or a dominant-negative PSF mutant significantly suppresses proliferation of Hakai-overexpressing cells. Collectively, these results suggest that Hakai is an important regulator of cell proliferation and that Hakai may be an oncoprotein and a potential molecular target for cancer treatment.