The Hippo-YAP pathway in organ size control and tumorigenesis: an updated version

Abstract

The Hippo signaling pathway is gaining recognition as an important player in both organ size control and tumorigenesis, which are physiological and pathological processes that share common cellular signaling mechanisms. Upon activation by stimuli such as high cell density in cell culture, the Hippo pathway kinase cascade phosphorylates and inhibits the Yes-associated protein (YAP)/TAZ transcription coactivators representing the major signaling output of the pathway. Altered gene expression resulting from YAP/TAZ inhibition affects cell number by repressing cell proliferation and promoting apoptosis, thereby limiting organ size. Recent studies have provided new insights into the Hippo signaling pathway, elucidating novel phosphorylation-dependent and independent mechanisms of YAP/Yki inhibition by the Hippo pathway, new Hippo pathway components, novel YAP target transcription factors and target genes, and the three-dimensional structure of the YAP-TEAD complex, and providing further evidence for the involvement of YAP and the Hippo pathway in tumorigenesis.

Figure 1.

Domain organization and key modifications of the Hippo pathway components. The illustrations are drawn in scale unless indicated otherwise. Human sequences are drawn unless indicated by D.m., which stands for Drosophila sequences. (CA) Cadeherin repeats; (EGF) EGF-like domain; (LamG) Laminin G domain; (TM) transmembrane region; (WW) WW domain; (C2) C2 domain; (FERM) FERM domain; (C1) C1 domain; (RA) Ras association domain; (SARAH) SARAH domain; (UBA) ubiquitin-associated domain; (PPXY) PPXY motif; (HM) hydrophobic motif; (P-rich) proline-rich domain; (C-C) coiled-coil domain; (TEA) TEA DNA-binding domain. Drosophila Fat is processed into two fragments (Feng and Irvine 2009; Sopko et al. 2009). The approximate cleavage site is indicated. Fat cytoplasmic domain is phosphorylated by Dco (Feng and Irvine 2009; Sopko et al. 2009). Mer is phosphorylated by PAK1/2 on S518, which affects Mer conformation and inactivates Mer as a tumor suppressor (Rong et al. 2004). Mst1 activation loop autophosphorylation (T183) is essential for its kinase activity. Caspase cleavage, as indicated, activates Mst1 (Graves et al. 1998). Lats1 is activated by autophosphorylation on the activation loop (S909), and phosphorylation by Mst1/2 on the hydrophobic motif (T1079) (Chan et al. 2005). Mob1 is phosphorylated by Mst1/2 on T12 and T35, and this phosphorylation stimulates its interaction with Lats1/2 (Praskova et al. 2008). Sav1 is also phosphorylated by Mst1/2 on an unidentified site (Callus et al. 2006). YAP/TAZ/Yki is phosphorylated by Lats1/2 on S61, S109, S127, S164, and S381 (TAZ S66, S89, S117, S311, Yki S111, S168, and S250) in the HXRXXS motifs (Zhao et al. 2007; Lei et al. 2008; Oh and Irvine 2008). S127 phosphorylation induces 14–3–3 binding and cytoplasmic retention (Zhao et al. 2007). S381 phosphorylation primes CK1δ/ɛ phosphorylation of S384, and S387 finally leads to SCF^β-TRCP-mediated ubiquitination and degradation (Zhao et al. 2010). YAP is also phosphorylated by c-Abl on Y391 (Levy et al. 2008).

Figure 2.

Models of the Hippo pathway in Drosophila and mammals. In Drosophila, Fat protocadherin may initiate the Hippo pathway signal in response to Ds binding, and is modulated by binding of Lft and phosphorylation by Dco (Feng and Irvine 2009; Mao et al. 2009; Sopko et al. 2009). Fat may inhibit a nonconventional myosin Dachs, which represses Wts protein levels (Cho et al. 2006). Fat may also activate Ex with an unknown mechanism (Bennett and Harvey 2006; Silva et al. 2006; Willecke et al. 2006; Tyler and Baker 2007). Mer and Ex also activate the Hippo pathway (Hamaratoglu et al. 2006). They may form a complex with Hpo and Sav (Yu et al. 2010). Kibra interacts with both Mer and Ex, and may also be in the complex (Yu et al. 2010). Hpo kinase interacts with and phosphorylates a scaffold protein, Sav (Wu et al. 2003). Together, they phosphorylate and activate Wts kinase and its associated protein, Mats (Lai et al. 2005). Wts phosphorylates a transcription coactivator, Yki, on three sites (Oh and Irvine 2009). Phosphorylation of Yki S168 induces 14–3–3 binding and cytoplasmic retention (Dong et al. 2007). Yki may also be retained in the cytoplasm by physical interaction with Ex, Wts, and Hpo (Badouel et al. 2009; H Oh et al. 2009). When Yki is relieved from inhibition and gets into the nucleus, it binds and activates a transcription factor, Sd, to induce cycE, diap1, and ex expression (Goulev et al. 2008; Wu et al. 2008; L Zhang et al. 2008). Yki induces bantam microRNA through Hth and Tsh (Peng et al. 2009). In mammals, functional significance of Fat and Ex homologs are not clear. However, Mer may still activate the Hippo pathway (Yokoyama et al. 2008). RASSF, a subgroup of Ras effector proteins, may also activate Mst1/2 (Hpo homolog) (Oh et al. 2006). Relationships between Hpo, Sav, Wts, and Mats are basically conserved in mammalian Mst1/2, Sav1 (Sav homolog), Lats1/2 (Wts homolog), and Mob (Mats homolog). Lats1/2 phosphorylates YAP on five conserved HXRXXS motifs (four on TAZ) (Zhao et al. 2007). Dependent on cell context, there may exist another YAP kinase in response to Mst1/2 and another Lats1/2 kinase (Zhou et al. 2009). S127 (S89 in TAZ) phosphorylation-dependent 14–3–3 binding and cytoplasmic retention are conserved in YAP/TAZ (Zhao et al. 2007; Lei et al. 2008). YAP is also inhibited by S381 phosphorylation, which primes CK1δ/ɛ phosphorylation of S384, and S387 finally leads to SCF^β-TRCP-mediated ubiquitination and degradation (Zhao et al. 2010). Sd homologs, TEADs, are major YAP target transcription factors. They mediate expression of CTGF, Gli2, and many other target genes (Zhao et al. 2008b). AREG is induced by YAP through an unidentified transcription factor (J Zhang et al. 2009). YAP and TAZ also bind Smad1 and Smad2/3 to activate expression of TGF-β and BMP target genes, respectively, to maintain stem cell pluripotency (Varelas et al. 2008; Alarcon et al. 2009).