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Proceedings of the National Academy of Sciences of the United States of America


Homeostatic mechanisms can eliminate abnormal cells to prevent diseases such as cancer. However, the underlying mechanisms of this surveillance are poorly understood. Here we investigated how clones of cells mutant for the neoplastic tumor suppressor gene scribble (scrib) are eliminated from Drosophila imaginal discs. When all cells in imaginal discs are mutant for scrib, they hyperactivate the Hippo pathway effector Yorkie (Yki), which drives growth of the discs into large neoplastic masses. Strikingly, when discs also contain normal cells, the scrib− cells do not overproliferate and eventually undergo apoptosis through JNK-dependent mechanisms. However, induction of apoptosis does not explain how scrib− cells are prevented from overproliferating. We report that cell competition between scrib− and wild-type cells prevents hyperproliferation by suppressing Yki activity in scrib− cells. Suppressing Yki activation is critical for scrib− clone elimination by cell competition, and experimental elevation of Yki activity in scrib−cells is sufficient to fuel their neoplastic growth. Thus, cell competition acts as a tumor-suppressing mechanism by regulating the Hippo pathway in scrib− cells.

Animals have evolved homeostatic mechanisms to eliminate abnormal and cancerous cells, protecting the animal from harm (1). A prominent example of an organism removing abnormal cells that have the potential to form tumors is the elimination of scribble mutant (scrib−) cells from Drosophila imaginal discs (2–8). scrib is a conserved tumor-suppressor gene that is essential for the establishment of apical–basal cell polarity (8–10). Scrib is a scaffold protein that localizes to basolateral cell junctions and functions together with the Discs large (Dlg) and Lethal giant larvae (Lgl) adaptor proteins to govern apical–basal cell polarity in epithelial cells (8, 10). Imaginal discs from Drosophila larvae that are homozygous mutant for scrib, dlg, or lgl grow into large tumorous masses of neoplastic cells that display several hallmarks of carcinomas: They lose apical–basal cell polarity, hyperproliferate, and have defects in differentiation (10). Interestingly, the neoplastic phenotype of scrib− cells depends on their cellular environment. When scrib− cells are produced in patches (clones) of mutant cells that are surrounded by normal cells, they do not hyperproliferate, remain small, and eventually are eliminated (2–7, 11–13). Similar effects are observed for lgl− and dlg− clones, although they may not be eliminated very efficiently (11, 14, 15). Thus, the presence of wild-type cells prevents scrib−, lgl−, and dlg−cells from manifesting their tumorigenic potential (2–7, 11–15). Several groups have shown that the JNK stress–response pathway is activated in scrib− clones, leading to engulfment and death or extrusion of mutant cells from the epithelium (2–4, 6, 11, 16). Activation of JNK is required for the elimination of scrib− cells because blocking JNK activity in scrib−cells results in massive overgrowth of clones that is reminiscent of the tumorous overgrowth of entirely mutant discs (2–4, 6, 12, 13). However, blocking apoptosis does not cause overproliferation of scrib− clones (2, 3). Therefore, in addition to inducing apoptosis, JNK suppresses the potential of scrib− cells to hyperproliferate (2, 3). However, how scrib−cells are prevented from hyperproliferating is not known.

The presence of normal cells is required for the elimination of tumorigenic scrib− clones because genetically ablating the normal tissue surrounding scrib− cells results in hyperproliferation of the scrib− cells (2, 3). It has been suggested that cell competition, a process by which viable cells of lower fitness are removed from a tissue and replaced through extra proliferation of fitter neighbors (17), is responsible for the elimination of scrib−and lgl− cell clones (2, 14). However, the hypothesis that scrib− and lgl− clones are eliminated by cell competition is in conflict with other reports and thus is controversial.

It has been reported that cells with compromised Scrib or Lgl function exhibit elevated activity of Yorkie (Yki), a transcriptional coactivator and downstream effector of the Hippo growth-control pathway (13, 14, 18–20). The Hippo pathway is a conserved tumor-suppressor pathway that suppresses growth by antagonizing the activity of Yki (21). Thus, loss of Hippo pathway activity or elevated levels of Yki activity result in hyperproliferation of imaginal disc cells and resistance to apoptosis that normally would eliminate extra cells (21). Notably, an increase in Yki activity can rescue weak cells, such as cells heterozygous for Minute (M) mutations, from being eliminated by cell competition (22). M mutations occur in ribosomal protein-encoding genes and were the first class of genes identified as having cell-competition phenotypes (23). Homozygous M mutations are lethal, but heterozygous Manimals are viable, although their cells have reduced growth rates (23). In genetic mosaics, however, interaction between wild-type and M+/− cells leads to the elimination of the M+/−cells and expansion of the wild-type population, a phenomenon termed “cell competition” (17). Thus, M+/− cells are less competitive than wild-type cells. Importantly, elevated levels of Yki can rescue M+/− cells from being eliminated by cell competition and also can transform normal cells into supercompetitors that induce apoptosis in their neighbors and proliferate at their neighbors’ expense (22, 24, 25). Yki may increase the competitiveness of cells by inducing the expression of Myc, a known regulator of cell competition (24–27). However, the reports that scrib− cells have high levels of Yki activity and the hypothesis that scrib− cells are eliminated by cell competition present a paradox. If scrib− cells indeed have elevated levels of Yki activity, why does that elevated Yki activity not protect scrib−cells from cell competition?

Here we investigated this paradox further. We show that scrib− cells are indeed eliminated by cell competition. We found that for this elimination to occur, scrib− cells undergo a JNK-dependent suppression of Yki activity; this suppression of Yki activity prevents scrib− cells from hyperproliferating and enables their removal. The modulation of Yki activity in scrib−cells thus is a critical effect of the JNK-dependent cell-competition process that removes such tumorigenic cells from imaginal discs. Finally we show that the Myc and Ras oncogenes, which can rescue scrib− clones from elimination (2, 4, 15), do so by conferring competitive fitness to scrib− cells and thereby prevent the down-regulation of Yki activity in scrib− cells. Our results thus further characterize the effects of cell-competition pathways in removing tumorigenic scrib− cells from imaginal discs.

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