The hippo tumor suppressor network: from organ size control to stem cells and cancer.

The third meeting on the Hippo pathway was held as a Keystone Symposia Conference last May in Monterrey, California. Over the past several years, theHippo tumor suppressor pathway has emerged as a complex signaling network that has significant implications for our understanding of themolecular mechanisms of cancer, development, and stem cell biology (reviewed in refs. 1–3). The main effectors of this pathway, Yes-associated protein (YAP) and Transcriptional coactivator with PDZ-binding motif (TAZ), are transcriptional coactivators that act as regulators of stem cell homeostasis and potent oncogenes. Abnormal expression of Hippo pathway components, in particular YAP and TAZ, is often observed in human cancer, and mice with mutations that activate YAP and TAZ or that directly overexpress YAP develop cancer in a variety of tissues. Therefore, the Hippo pathway has become an attractive target for the development of anticancer drugs. However, many questions about the Hippo pathway are still unanswered and several paradoxes about the function of the pathway have emerged in recent years. For example, does YAP act as an oncogene or a tumor suppressor, and what are the mechanisms that regulate the activity of the pathway? A flurry of new discoveries was presented at this year's Hippo meeting, which reflected on all aspects of the Hippo pathway, from signal transductionmechanisms to cancer research and regeneration. Thus, the meeting witnessed the fast progress in this field, which is due, in large part, to an active dialog between Drosophila geneticists, mammalian signalers, and cancer and developmental biologists. Although many upstream components of the Hippo pathway have been identified in recent years, mechanisms of signal transduction and how different inputs are integrated into the pathway have been poorly understood. Substantial progress has been made in this area in the past 2 years and several researchers presented new findings of signal transduction mechanisms. Kun-Liang Guan (University of California, San Diego, La Jolla, CA) presented the first keynote lecture and showed that the Hippo pathway is acutely regulated by Gprotein–coupled receptor signaling. Activation of G12/13-coupled receptors, such as stimulation by lysophosphatidic acid, results in LATS kinase inactivation and YAP/TAZ activation in a manner independent of the MST1/2 kinases. In contrast, stimulation of Gs-coupled receptors, such as by epinephrine, activates LATS kinase activity and inhibits YAP (4). He showed that Gs-coupled receptors act through cyclic AMP (cAMP) via protein kinase A and Rho GTPases to stimulate LATS kinase activity and YAP phosphorylation (5). Joseph Avruch (Massachusetts General Hospital, Boston, MA) presented the second keynote lecture. Using gene-targeted mouse models, he showed that MST1/2 act as robust tumor suppressors in the liver and colon (6, 7). He also showed that theMST1/2 substrate and cofactor for LATS/nuclear Dbf-2-related (NDR) kinases, MOB1, regulates the DOCK-C family of Rac1/Cdc42 guanine nucleotide exchange factors (GEF; and perhaps other targets), independently of and in parallel to the NDR-family kinases. Interestingly, although MST1 is reversibly activated in T cells, once activated in liver, it is converted to a constitutively active form by proteolytic cleavage. In addition, he showed that YAP protein abundance is controlled at the level of transcription, in part by the ETS transcription factor GA-binding protein (GABP; ref. 8), as well as by phosphorylation-directed ubiquitination and degradation. The presented work was an ongoing collaboration between his group, Dawang Zhou (Xiamen University, Xiamen, Fujian, China) and Nabeel Bardeesy (Massachusetts General Hospital, Boston, MA) for studies in the liver. Nic Tapon (Cancer Research UK, London, United Kingdom) presented his laboratory's efforts to identify new regulators of Hippo signaling using cell-based RNA interference screens in Drosophila. Using aWarts/LATS activity reporter based on the Split-TEV protein–protein interaction detection system, they identified the salt-inducible kinases Sik2/3 as potential nutritional/hormonal inputs in Hippo signaling (9). More recent work was aimed at identifying ubiquitin ligases involved in regulating stability of the Hippo scaffold partner Salvador. The Hippo pathway is known to be regulated by the neurofibromatosis type 2 (NF2) tumor suppressor protein (also known as Merlin), although the mechanisms underlying this have remained unclear. Filippo Giancotti (Memorial SloanKettering Cancer Center, New York, NY) and colleagues previously reported that mammalian Merlin suppresses tumorigenesis by inhibiting the E3 ubiquitin ligase CRL4-DCAF1 in the nucleus (10). At the meeting, he presented evidence that CRL4-DCAF1 inhibits the output of the Hippo pathway by promoting ubiquitylation of LATS1 and 2. Genetic epistasis experiments indicated that this signaling connection sustains the oncogenicity of Merlin-deficient tumor cells. Jeffrey Schindler (Massachusetts Institute of Technology, Cambridge, MA) showed that the two major Merlin isoforms are able to Authors' Affiliations: VIB Center for the Biology of Disease, KU Leuven Center for Human Genetics, University of Leuven, Leuven, Belgium; and Stem Cell Program, Department of Stem Cell and Regenerative Biology, Boston Children's Hospital, Harvard University, Cambridge, Massachusetts