Hippo signaling pathway
The Hippo signaling pathway, also known as the Salvador/Warts/Hippo (SWH) pathway, controls organ size in animals through the regulation of cell proliferation and apoptosis. The pathway takes its name from one of its key signaling components—the protein kinase Hippo (Hpo). Mutations in this gene lead to tissue overgrowth, or a “hippopotamus”-like phenotype.
A fundamental question in developmental biology is how an organ knows to stop growing after reaching a particular size. Organ growth relies on several processes occurring at the cellular level, including cell division and programmed cell death (or apoptosis). The Hippo signaling pathway is involved in restraining cell proliferation and promoting apoptosis. As many cancers are marked by unchecked cell division, this signaling pathway has become increasingly significant in the study of human cancer.
The Hippo signaling pathway appears to be highly conserved. While most of the Hippo pathway components were identified in the fruit fly (Drosophila melanogaster) using mosaic genetic screens, orthologs to these components (genes that function analogously in different species) have subsequently been found in mammals. Thus, the delineation of the pathway in Drosophila has helped to identify many genes that function as oncogenes or tumor suppressors in mammals.
The Hippo pathway consists of a core kinase cascade in which Hpo phosphorylates the protein kinase Warts (Wts). Hpo (MST1/2 in mammals) is a member of the Ste-20 family of protein kinases. This highly conserved group of serine/threonine kinases regulates several cellular processes, including cell proliferation, apoptosis, and various stress responses. Once phosphorylated, Wts (LATS1/2 in mammals) becomes active. Wts is a nuclear DBF-2-related kinase. These kinases are known regulators of cell cycle progression, growth, and development. Two proteins are known to facilitate the activation of Wts: Salvador (Sav) and Mob as tumor suppressor (Mats). Sav (WW45 in mammals) is a WW domain-containing protein, meaning that this protein contains a sequence of amino acids in which a tryptophan and an invariant proline are highly conserved. Hpo can bind to and phosphorylate Sav, which may function as a scaffold protein because this Hpo-Sav interaction promotes phosphorylation of Wts. Hpo can also phosphorylate and activate Mats (MOBKL1A/B in mammals), which allows Mats to associate with and strengthen the kinase activity of Wts.
Activated Wts can then go on to phosphorylate and inactivate the transcriptional coactivator Yorkie (Yki). Yki is unable to bind DNA by itself. In its active state, Yki binds to the transcription factor Scalloped (Sd), and the Yki-Sd complex becomes localized to the nucleus. This allows for the expression of several genes that promote organ growth, such as cyclin E, which promotes cell cycle progression, and diap1 (Drosophila inhibitor of apopotosis protein-1), which, as its name suggests, prevents apoptosis. Yki also activates expression of the bantam microRNA, a positive growth regulator that specifically affects cell number. Thus, the inactivation of Yki by Wts inhibits growth through the transcriptional repression of these pro-growth regulators. By phosphorylating Yki at serine 168, Wts promotes the association of Yki with 14-3-3 proteins, which help to anchor Yki in the cytoplasm and prevent its transport to the nucleus. In mammals, the two Yki orthologs are Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ). When activated, YAP and TAZ can bind to several transcription factors including p73, Runx2 and several TEADs.
The upstream regulators of the core Hpo/Wts kinase cascade include the transmembrane protein Fat and several membrane-associated proteins. As an atypical cadherin, Fat (FAT1-4 in mammals) may function as a receptor, though an extracellular ligand has not been positively identified. While Fat is known to bind to another atypical cadherin, Dachsous (Ds), during tissue patterning, it is unclear what role Ds has in regulating tissue growth. Nevertheless, Fat is recognized as an upstream regulator of the Hpo pathway. Fat activates Hpo through the apical protein Expanded (Ex; FRMD6/Willin in mammals). Ex interacts with two other apically-localized proteins, Kibra (KIBRA in mammals) and Merlin (Mer; NF2 in mammals), to form the Kibra-Ex-Mer (KEM) complex. Both Ex and Mer are FERM domain-containing proteins, while Kibra, like Sav, is a WW domain-containing protein. The KEM complex physically interacts with the Hpo kinase cascade, thereby localizing the core kinase cacade to the plasma membrane for activation. Fat may also regulate Wts independently of Ex/Hpo, through the inhibition of the unconventional myosin Dachs. Normally, Dachs can bind to and promote the degradation of Wts.
The Hippo Signaling Pathway in Cancer
In fruitfly, the Hippo signaling pathway involves a kinase cascade involving the Salvador (Sav), Warts (Wts) and Hippo (Hpo) protein kinases. Many of the genes involved in the Hippo signaling pathway are recognized as tumor suppressors, while Yki/YAP/TAZ is identified as an oncogene. In fact, YAP has been found to be elevated in some human cancers, including breast cancer, colorectal cancer, and liver cancer. This may be explained by YAP’s recently defined role in overcoming contact inhibition, a fundamental growth control property of normal cells in culture in which proliferation stops after cells reach confluence. This property is typically lost in cancerous cells, allowing them to proliferate in an uncontrolled manner. In fact, YAP overexpression antagonizes contact inhibition.
Many of the pathway components recognized as tumor suppressor genes are mutated in human cancers. For example, mutations in Fat4 have been found in breast cancer, while NF2 is mutated in familial and sporadic schwannomas. Additionally, several human cancer cell lines invoke mutations of the WW45 and MOBK1B proteins.
|Drosophila melanogaster||Human ortholog(s)||Protein Description & Role in Hippo Signaling Pathway|
|Dachsous (Ds)||DCHS1, DCHS2||Atypical cadherin that may act as a ligand for the Fat receptor|
|Fat (Ft)||FAT1, FAT2, FAT3, FAT4 (FATJ)||Atypical cadherin that may act as a receptor for the Hippo pathway|
|Expanded (Ex)||FRMD6/Willin||FERM domain-containing apical protein that associates with Kibra and Mer as an upstream regulator of the core kinase cascade|
|Dachs (Dachs)||Unconventional myosin that can bind Wts, promoting its degradation|
|Kibra (Kibra)||WWC1||WW domain-containing apical protein that associates with Ex and Mer as an upstream regulator of the core kinase cascade|
|Merlin (Mer)||NF2||FERM domain-containing apical protein that associates with Ex and Kibra as an upstream regulator of the core kinase cascade|
|Hippo (Hpo)||MST1, MST2||Sterile-20-type kinase that phosphorylates and activates Wts|
|Salvador (Sav)||WW45 (SAV1)||WW domain-containing protein that may act as a scaffold protein, facilitating Warts phosphorylation by Hippo|
|Warts (Wts)||LATS1, LATS2||Nuclear DBF-2-related kinase that phosphorylates and inactivates Yki|
|Mob as tumor suppressor (Mats)||MOBKL1A, MOBKL1B||Kinase that associates with Wts to potentiate its catalytic activity|
|Yorkie (Yki)||YAP, TAZ||Transcriptional coactivator that binds to Sd in its active, unphosphorylated form to activate expression of transcriptional targets that promote cell growth, cell proliferation, and prevent apoptosis|
|Scalloped (Sd)||TEAD1, TEAD2, TEAD3, TEAD4||Transcription factor that binds Yki to regulate target gene expression|
- Saucedo, Leslie J.; Edgar, Bruce A. (2007). "Filling out the Hippo pathway". Nature Reviews Molecular Cell Biology 8 (8): 613–21. doi:10.1038/nrm2221. PMID 17622252.
- Dan, Ippeita; Watanabe, Norinobu M.; Kusumi, Akihiro (2001). "The Ste20 group kinases as regulators of MAP kinase cascades". Trends in Cell Biology 11 (5): 220–30. doi:10.1016/S0962-8924(01)01980-8. PMID 11316611.
- Ma, J.; Benz, C.; Grimaldi, R.; Stockdale, C.; Wyatt, P.; Frearson, J.; Hammarton, T. C. (2010). "Nuclear DBF-2-related Kinases Are Essential Regulators of Cytokinesis in Bloodstream Stage Trypanosoma brucei". Journal of Biological Chemistry 285 (20): 15356–68. doi:10.1074/jbc.M109.074591. PMC 2865264. PMID 20231285.
- Andre, B.; Springael, J.Y. (1994). "WWP, a New Amino Acid Motif Present in Single or Multiple Copies in Various Proteins Including Dystrophin and the SH3-Binding Yes-Associated Protein YAP65". Biochemical and Biophysical Research Communications 205 (2): 1201–5. doi:10.1006/bbrc.1994.2793. PMID 7802651.
- Wu, Shian; Huang, Jianbin; Dong, Jixin; Pan, Duojia (2003). "Hippo Encodes a Ste-20 Family Protein Kinase that Restricts Cell Proliferation and Promotes Apoptosis in Conjunction with salvador and warts". Cell 114 (4): 445–56. doi:10.1016/S0092-8674(03)00549-X. PMID 12941273.
- Wei, Xiaomu; Shimizu, Takeshi; Lai, Zhi-Chun (2007). "Mob as tumor suppressor is activated by Hippo kinase for growth inhibition in Drosophila". The EMBO Journal 26 (7): 1772–81. doi:10.1038/sj.emboj.7601630. PMC 1847660. PMID 17347649.
- Huang, Jianbin; Wu, Shian; Barrera, Jose; Matthews, Krista; Pan, Duojia (2005). "The Hippo Signaling Pathway Coordinately Regulates Cell Proliferation and Apoptosis by Inactivating Yorkie, the Drosophila Homolog of YAP". Cell 122 (3): 421–34. doi:10.1016/j.cell.2005.06.007. PMID 16096061.
- Thompson, Barry J.; Cohen, Stephen M. (2006). "The Hippo Pathway Regulates the bantam microRNA to Control Cell Proliferation and Apoptosis in Drosophila". Cell 126 (4): 767–74. doi:10.1016/j.cell.2006.07.013. PMID 16923395.
- Nolo, Riitta; Morrison, Clayton M.; Tao, Chunyao; Zhang, Xinwei; Halder, Georg (2006). "The bantam MicroRNA is a Target of the Hippo Tumor-Suppressor Pathway". Current Biology 16 (19): 1895–904. doi:10.1016/j.cub.2006.08.057. PMID 16949821.
- Wang, Kainan; Degerny, Cindy; Xu, Minghong; Yang, Xiang-Jiao (2009). "YAP, TAZ, and Yorkie: A conserved family of signal-responsive transcriptional coregulators in animal development and human diseaseThis paper is one of a selection of papers published in this Special Issue, entitled CSBMCB's 51st Annual Meeting – Epigenetics and Chromatin Dynamics, and has undergone the Journal's usual peer review process". Biochemistry and Cell Biology 87 (1): 77–91. doi:10.1139/O08-114. PMID 19234525.
- Badouel, Caroline; Garg, Ankush; McNeill, Helen (2009). "Herding Hippos: Regulating growth in flies and man". Current Opinion in Cell Biology 21 (6): 837–43. doi:10.1016/j.ceb.2009.09.010. PMID 19846288.
- Cho, E.; Irvine, KD (2004). "Action of fat, four-jointed, dachsous and dachs in distal-to-proximal wing signaling". Development 131 (18): 4489–500. doi:10.1242/dev.01315. PMID 15342474.
- Baumgartner, Roland; Poernbacher, Ingrid; Buser, Nathalie; Hafen, Ernst; Stocker, Hugo (2010). "The WW Domain Protein Kibra Acts Upstream of Hippo in Drosophila". Developmental Cell 18 (2): 309–16. doi:10.1016/j.devcel.2009.12.013. PMID 20159600.
- Pan, Duojia (2010). "The Hippo Signaling Pathway in Development and Cancer". Developmental Cell 19 (4): 491–505. doi:10.1016/j.devcel.2010.09.011. PMC 3124840. PMID 20951342.
- Cho, Eunjoo; Feng, Yongqiang; Rauskolb, Cordelia; Maitra, Sushmita; Fehon, Rick; Irvine, Kenneth D (2006). "Delineation of a Fat tumor suppressor pathway". Nature Genetics 38 (10): 1142–50. doi:10.1038/ng1887. PMID 16980976.
- Kango-Singh, Madhuri; Singh, Amit (2009). "Regulation of organ size: Insights from theDrosophilaHippo signaling pathway". Developmental Dynamics 238 (7): 1627–37. doi:10.1002/dvdy.21996. PMID 19517570.
- Zender, Lars; Spector, Mona S.; Xue, Wen; Flemming, Peer; Cordon-Cardo, Carlos; Silke, John; Fan, Sheung-Tat; Luk, John M. et al. (2006). "Identification and Validation of Oncogenes in Liver Cancer Using an Integrative Oncogenomic Approach". Cell 125 (7): 1253–67. doi:10.1016/j.cell.2006.05.030. PMC 3026384. PMID 16814713.
- Steinhardt, Angela A.; Gayyed, Mariana F.; Klein, Alison P.; Dong, Jixin; Maitra, Anirban; Pan, Duojia; Montgomery, Elizabeth A.; Anders, Robert A. (2008). "Expression of Yes-associated protein in common solid tumors". Human Pathology 39 (11): 1582–9. doi:10.1016/j.humpath.2008.04.012. PMC 2720436. PMID 18703216.
- Eagle, Harry; Levine, Elliot M. (1967). "Growth Regulatory Effects of Cellular Interaction". Nature 213 (5081): 1102–6. doi:10.1038/2131102a0. PMID 6029791.
- Hanahan, Douglas; Weinberg, Robert A (2000). "The Hallmarks of Cancer". Cell 100 (1): 57–70. doi:10.1016/S0092-8674(00)81683-9. PMID 10647931.
- Zhao, B.; Wei, X.; Li, W.; Udan, R. S.; Yang, Q.; Kim, J.; Xie, J.; Ikenoue, T. et al. (2007). "Inactivation of YAP oncoprotein by the Hippo pathway is involved in cell contact inhibition and tissue growth control". Genes & Development 21 (21): 2747. doi:10.1101/gad.1602907.
- Qi, Chao; Zhu, Yiwei Tony; Hu, Liping; Zhu, Yi-Jun (2009). "Identification of Fat4 as a candidate tumor suppressor gene in breast cancers". International Journal of Cancer 124 (4): 793–8. doi:10.1002/ijc.23775. PMC 2667156. PMID 19048595.
- Evans, D G. R; Sainio, M; Baser, ME (2000). "Neurofibromatosis type 2". Journal of Medical Genetics 37 (12): 897–904. doi:10.1136/jmg.37.12.897. PMC 1734496. PMID 11106352.
- Tapon, Nicolas; Harvey, Kieran F.; Bell, Daphne W.; Wahrer, Doke C.R.; Schiripo, Taryn A.; Haber, Daniel A.; Hariharan, Iswar K. (2002). "Salvador Promotes Both Cell Cycle Exit and Apoptosis in Drosophila and is Mutated in Human Cancer Cell Lines". Cell 110 (4): 467–78. doi:10.1016/S0092-8674(02)00824-3. PMID 12202036.
- Lai, Zhi-Chun; Wei, Xiaomu; Shimizu, Takeshi; Ramos, Edward; Rohrbaugh, Margaret; Nikolaidis, Nikolas; Ho, Li-Lun; Li, Ying (2005). "Control of Cell Proliferation and Apoptosis by Mob as Tumor Suppressor, Mats". Cell 120 (5): 675–85. doi:10.1016/j.cell.2004.12.036. PMID 15766530.