The Hippo pathway controls organ size in diverse species, whereas pathway deregulation can induce tumours in model organisms and occurs in a broad range of human carcinomas, including lung, colorectal, ovarian and liver cancer. Despite this, somatic or germline mutations in Hippo pathway genes are uncommon, with only the upstream pathway gene neurofibromin 2 (NF2) recognized as a bona fide tumour suppressor gene. In this Review, we appraise the evidence for the Hippo pathway as a cancer signalling network, and discuss cancer-relevant biological functions, potential mechanisms by which Hippo pathway activity is altered in cancer and emerging therapeutic strategies.

Establishing and maintaining homeostasis is critical to the well-being of an organism and is determined by the balance of cell proliferation and death. Two genes that function together to regulate growth, proliferation, and apoptosis in Drosophila are warts (wts), encoding a serine/threonine kinase, and salvador (sav), encoding a WW domain containing Wts-interacting protein. However, the mechanisms by which sav and wts regulate growth and apoptosis are not well understood. Here, we describe mutations in hippo (hpo), which encodes a protein kinase most related to mammalian Mst1 and Mst2. Like wts and sav, hpo mutations result in increased tissue growth and impaired apoptosis characterized by elevated levels of the cell cycle regulator cyclin E and apoptosis inhibitor DIAP1. Hpo, Sav, and Wts interact physically and functionally, and regulate DIAP1 levels, likely by Hpo-mediated phosphorylation and subsequent degradation. Thus, Hpo links Sav and Wts to a key regulator of apoptosis.

The number of cells in an organism is determined by regulating both cell proliferation and cell death. Relatively few mechanisms have been identified that can modulate both of these processes. In a screen for Drosophila mutations that result in tissue overgrowth, we identified salvador (sav), a gene that promotes both cell cycle exit and cell death. Elevated Cyclin E and DIAP1 levels are found in mutant cells, resulting in delayed cell cycle exit and impaired apoptosis. Salvador contains two WW domains and binds to the Warts (or LATS) protein kinase. The human ortholog of salvador (hWW45) is mutated in three cancer cell lines. Thus, salvador restricts cell numbers in vivo by functioning as a dual regulator of cell proliferation and apoptosis.

Intense research over the past four years has led to the discovery and characterization of a novel signalling network, known as the Salvador-Warts-Hippo (SWH) pathway, involved in tissue growth control in Drosophila melanogaster. At present, eleven proteins have been implicated as members of this pathway, and several downstream effector genes have been characterized. The importance of this pathway is emphasized by its evolutionary conservation, and by increasing evidence that its deregulation occurs in human tumours. Here, we review the main findings from Drosophila and the implications that these have for tumorigenesis in mammals.

Collectively, our data reveal that the cell polarity regulators Lgl, aPKC, and Crumbs regulate the SWH pathway by two distinct pathways: Lgl acts antagonistically to aPKC to regulate Hippo and RASSF localization, whereas Crumbs regulates Expanded localization. Thus, our study implicates Lgl, aPKC, and Crumbs as regulators of tissue growth via the SWH pathway.

Fat restricts organ size during Drosophila development via the Salvador/Warts/Hippo pathway. These studies aid our understanding of developmental organ size control and have implications for human hyperproliferative disorders, such as cancers.

The Salvador-Warts-Hippo (SWH) pathway was first discovered in Drosophila melanogaster as a potent inhibitor of tissue growth. The SWH pathway is highly conserved between D. melanogaster and mammals, both in function and in the mechanism of signal transduction. The mammalian SWH pathway limits tissue growth by inhibiting the nuclear access and expression of the transcriptional co-activator, Yes-associated protein (YAP). Mutation and altered expression of SWH pathway proteins has been observed in several types of human cancer, but the contribution of these events to tumorigenesis has been unclear. Here we show that YAP can enhance the transformed phenotype of ovarian cancer cell lines and that YAP confers resistance to chemotherapeutic agents that are commonly used to treat ovarian cancer. We find that high nuclear YAP expression correlates with poor patient prognosis in a cohort of 268 invasive epithelial ovarian cancer samples. Segregation by histotype shows that the correlation between nuclear YAP and poor survival is predominantly associated with clear cell tumors, independent of stage. Collectively our findings suggest that YAP derepression contributes to the genesis of ovarian clear cell carcinoma and that the SWH pathway is an attractive therapeutic target.

A prevailing paradigm posits that Polycomb Group (PcG) proteins maintain stem cell identity by repressing differentiation genes, and abundant evidence points to an oncogenic role for PcG in human cancer 1,2. Here we demonstrate using Drosophila that a conventional PcG complex can also have a potent tumor suppressive activity. Mutations in all core PRC1 components cause dramatic hyperproliferation of eye imaginal tissue, accompanied by deregulation of epithelial architecture. The mitogenic JAK/STAT pathway is strongly and specifically activated in mutant tissue; activation is driven by transcriptional upregulation of Unpaired (Upd) family ligands. We show that upd genes are direct targets of PcG-mediated repression in imaginal discs. Ectopic JAK/STAT activity is sufficient to induce overproliferation, while reduction of JAK/STAT activity suppresses the PRC1 mutant tumor phenotype. These findings show that PcG proteins can restrict growth directly by silencing mitogenic signaling pathways, shedding light onto an epigenetic mechanism underlying tumor suppression.