JNK and Yorkie drive tumor progression by generating polyploid giant cells in Drosophila

Epithelial cancer tissues often possess polyploid giant cells, which are thought to be highly oncogenic. However, the mechanisms by which polyploid giant cells are generated in tumor tissues and how such cells contribute to tumor progression remain elusive. We previously noticed in Drosophila imaginal epithelium that cells mutant for the endocytic gene rab5 exhibit enlarged nuclei. Here we find that mutations in endocytic ‘neoplastic tumor-suppressor’ genes, such as rab5, vps25, erupted, or avalanche result in generation of polyploid giant cells. Genetic analyses on rab5-defective cells reveal that cooperative activation of JNK and Yorkie generates polyploid giant cells via endoreplication. Mechanistically, Yorkie-mediated upregulation of Diap1 cooperates with JNK to downregulate the G2/M cyclin CycB, thereby inducing endoreplication. Interestingly, malignant tumors induced by Ras activation and cell polarity defect also consist of polyploid giant cells, which are generated by JNK and Yorkie-mediated downregulation of CycB. Strikingly, elimination of polyploid giant cells from such malignant tumors by blocking endoreplication strongly suppressed tumor growth and metastatic behavior. Our observations suggest that JNK and Yorkie, two oncogenic proteins activated in many types of human cancers, cooperatively drive tumor progression by generating oncogenic polyploid giant cells.