Tumor Suppression by Cell Competition Through Regulation of the Hippo Pathway

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

Shar-pei Mediates Cell Proliferation Arrest During Imaginal Disc Growth in Drosophila

During animal development, organ size is determined primarily by the amount of cell proliferation, which must be tightly regulated to ensure the generation of properly proportioned organs. However, little is known about the molecular pathways that direct cells to stop proliferating when an organ has attained its proper size. We have identified mutations in a novel gene, shar-pei, that is required for proper termination of cell proliferation during Drosophila imaginal disc development. Clones of shar-pei mutant cells in imaginal discs produce enlarged tissues containing more cells of normal size. We show that this phenotype is the result of both increased cell proliferation and reduced apoptosis. Hence,shar-pei restricts cell proliferation and promotes apoptosis. By contrast, shar-pei is not required for cell differentiation and pattern formation of adult tissue. Shar-pei is also not required for cell cycle exit during terminal differentiation, indicating that the mechanisms directing cell proliferation arrest during organ growth are distinct from those directing cell cycle exit during terminal differentiation. shar-pei encodes a WW-domain-containing protein that has homologs in worms, mice and humans, suggesting that mechanisms of organ growth control are evolutionarily conserved

The Fat Cadherin Acts through the Hippo Tumor-Suppressor Pathway to Regulate Tissue Size

Background: The Hippo tumor-suppressor pathway has emerged as a key signaling pathway that controls tissue size in Drosophila. Merlin, the Drosophila homolog of the human Neurofibromatosis type-2 (NF2) tumor-suppressor gene, and the related protein Expanded are the most upstream components of the Hippo pathway identified so far. However, components acting upstream of Expanded and Merlin, such as transmembrane receptors, have not yet been identified. Results: Here, we report that the protocadherin Fat acts as an upstream component in the Hippo pathway. Fat is a known tumor-suppressor gene in Drosophila, and fat mutants have severely overgrown imaginal discs. We found that the overgrowth phenotypes of fatmutants are similar to those of mutants in Hippo pathway components: fat mutant cells continued to proliferate after wild-type cells stopped proliferating, and fat mutant cells deregulated Hippo target genes such as cyclin E and diap1. Fat acts genetically and biochemically upstream of other Hippo pathway components such as Expanded, the Hippo and Warts kinases, and the transcriptional coactivator Yorkie. Fat is required for the stability of Expanded and its localization to the plasma membrane. In contrast, Fat is not required for Merlin localization, and Fat and Merlin act in parallel in growth regulation. Conclusions: Taken together, our data identify a cell-surface molecule that may act as a receptor of the Hippo signaling pathway

DisAsymNet: Disentanglement of Asymmetrical Abnormality on Bilateral Mammograms using Self-adversarial Learning

Asymmetry is a crucial characteristic of bilateral mammograms (Bi-MG) when abnormalities are developing. It is widely utilized by radiologists for diagnosis. The question of 'what the symmetrical Bi-MG would look like when the asymmetrical abnormalities have been removed ?' has not yet received strong attention in the development of algorithms on mammograms. Addressing this question could provide valuable insights into mammographic anatomy and aid in diagnostic interpretation. Hence, we propose a novel framework, DisAsymNet, which utilizes asymmetrical abnormality transformer guided self-adversarial learning for disentangling abnormalities and symmetric Bi-MG. At the same time, our proposed method is partially guided by randomly synthesized abnormalities. We conduct experiments on three public and one in-house dataset, and demonstrate that our method outperforms existing methods in abnormality classification, segmentation, and localization tasks. Additionally, reconstructed normal mammograms can provide insights toward better interpretable visual cues for clinical diagnosis. The code will be accessible to the public

Lethal giant discs, a novel C2-domain protein, restricts notch activation during endocytosis

The Notch signaling pathway plays a central role in animal growth and patterning, and its deregulation leads to many human diseases, including cancer. Mutations in the tumor suppressor lethal giant discs (lgd) induce strong Notch activation and hyperplastic overgrowth of Drosophila imaginal discs. However, the gene that encodes Lgd and its function in the Notch pathway have not yet been identified. Here, we report that Lgd is a novel, conserved C2-domain protein that regulates Notch receptor trafficking. Notch accumulates on early endosomes in lgd mutant cells and signals in a ligand-independent manner. This phenotype is similar to that seen when cells lose endosomal-pathway components such as Erupted and Vps25. Interestingly, Notch activation in lgd mutant cells requires the early endosomal component Hrs, indicating that Hrs is epistatic to Lgd. These data suggest that Lgd affects Notch trafficking between the actions of Hrs and the late endosomal component Vps25. Taken together, our data identify Lgd as a novel tumor-suppressor protein that regulates Notch signaling by targeting Notch for degradation or recycling.status: publishe

The bantam microRNA is a target of the hippo tumor-suppressor pathway

The Hippo tumor-suppressor pathway has emerged as a key signaling pathway that controls tissue size in Drosophila. Hippo signaling restricts tissue size by promoting apoptosis and cell-cycle arrest, and animals carrying clones of cells mutant for hippo develop severely overgrown adult structures. The Hippo pathway is thought to exert its effects by modulating gene expression through the phosphorylation of the transcriptional coactivator Yorkie. However, how Yorkie regulates growth, and thus the identities of downstream target genes that mediate the effects of Hippo signaling, are largely unknown.status: publishe

Hippo promotes proliferation arrest and apoptosis in the Salvador/Warts pathway

Proliferation and apoptosis must be precisely regulated to form organs with appropriate cell numbers and to avoid tumour growth. Here we show that Hippo (Hpo), the Drosophila homologue of the mammalian Ste20-like kinases, MST1/2, promotes proper termination of cell proliferation and stimulates apoptosis during development. hpo mutant tissues are larger than normal because mutant cells continue to proliferate beyond normal tissue size and are resistant to apoptotic stimuli that usually eliminate extra cells. Hpo negatively regulates expression of Cyclin E to restrict cell proliferation, downregulates the Drosophila inhibitor of apoptosis protein DIAP1, and induces the proapoptotic gene head involution defective (hid) to promote apoptosis. The mutant phenotypes of hpo are similar to those of warts (wts), which encodes a serine/threonine kinase of the myotonic dystrophy protein kinase family, and salvador (sav), which encodes a WW domain protein that binds to Wts. We find that Sav binds to a regulatory domain of Hpo that is essential for its function, indicating that Hpo acts together with Sav and Wts in a signalling module that coordinately regulates cell proliferation and apoptosis.status: publishe

Tumor suppression by cell competition through regulation of the Hippo pathway

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.status: publishe