The mammalian Hippo signaling pathway, short for MST1/2-WW45-LATS1/2 signaling, is a critical pathway that determines cell growth rate and organ size[8-9]. MST1/2 (short for mammalian Ste20-like kinase 1 and 2) phosphorylates LATS1/2 (large tumor suppressor 1 and 2) and Mob1 (Mobkl1a/b), leading to their activation[10,11]. LATS1/2 phosphorylates YAP1 (Yes-associated protein 1)[9] and TAZ (WW domain-containing transcription regulator 1)[12] and promotes 14-3-3 binding to phosphorylated YAP1/TAZ, causing YAP1/TAZ cytoplasmic accumulation and sequestering its oncogenic function. The unphosphorylated YAP1 and TAZ are translocated to the nucleus and bind with TEAD1-4 (TEA domain DNA-binding transcription factors 1-4), inducing transcriptional activity for cell proliferation and differentiation[13-15].

As the most important negative regulator of YAP1/TAZ, much studies have provided new findings into the Hippo signaling pathway[9], elucidating novel phosphorylation-dependent[16] and independent mechanisms of YAP1/TAZ inhibition by the Hippo pathway. The key components of Hippo pathways form a phosphokinase cascade and play a crucial role to inhibit the downstream effectors, YAP1 and TAZ. Meanwhile, the Hippo pathway is the same important for homeostasis control and dysregulation of Hippo pathway contributes to carcinogenesis[17].

The tumor suppressor function of Hippo pathway is enhanced by E-cadherin/catenin complex[18,19], AMOT family proteins[20] and LKB1-MARK signaling[21], but is negatively regulated by GPCR (G-protein-coupled receptor) signaling[22]. The protease activated receptors[23] inhibits the LATS1/2 kinase activity via Rho GTPase and G12/13-coupled receptors[24].

The tight junction (TJ)[25] and adherens junction (AJ) components contact with YAP1 and quench its oncogenic function. α-catenin, a component of AJ, binds with 14-3-3 to form complex and phosphorylates YAP1, promoting YAP1 inhibition[19]. AMOT proteins restrict YAP1 activity in a LAST1/2 dependent and LATS1/2 independent manners[20,26]. As YAP1/TAZ is also a mechanotransduction sensor[27], the cell spreading, tension, attachment and detachment modulate YAP1/TAZ activity which is associated with Rho GTPase activity and reorganization of actin cytoskeleton[28]. YAP1/TAZ is also positively or negatively regulated by GPCR signaling pathway through RhoGTPases/actin/LATS1/2 cascade, which is determined by different GPCR ligands. Gα12/13-, Gαq/11-, and Gαi/o-coupled ligands activate YAP1/TAZ but Gαs-coupled ligands suppress YAP1/TAZ activity[22,29].

The deregulated Hippo pathway shares crosstalks with Wnt/β-catenin[30], Notch[31] and TGF-β (transforming growth factor beta)[32] signaling pathways to promote tumorigenesis coordinately. Many studies have highlighted the regulation of Hippo pathway on Wnt/β-catenin pathway. The cytoplasmic accumulation of YAP1/TAZ directly interacts with β-catenin and inhibits β-catenin nuclear translocation[33], whereas in the nuclei YAP1 binds with β-catenin to enhance the tumorigenicity[34]. The subcellular localization and phosphorylation status of YAP1/TAZ also determine the effect of Hippo signaling on TGF-β signaling. In the cytoplasm, YAP1/TAZ binds with Smad protein to suppress the TGF-β induced transcription, but in the nuclei, YAP1/TAZ retains Smad protein and enhances transcription under the TGF-β stimulation[35,36]. The interplay of Hippo pathway and Notch pathway was also demonstrated in mouse intestine. YAP1 induces the expression Notch ligand Jagged-1 and activates Notch signaling[37]. Meanwhile in MST1/2-deficient intestinal epithelium, β-catenin and Notch signaling are strongly activated[38]. In addition, the EGFR ligand amphiregulin (AREG) is another transcriptional target of YAP1, which functions as a contributor for YAP1-mediated proliferation and invasion[39], suggesting that the Hippo pathway also regulates the growth factor receptor tyrosine kinase signaling.

Some other studies focus on findings of new Hippo pathway components and crosstalks, novel YAP1/TAZ target genes and the three-dimensional structure of the YAP1-TEAD complex[40,41]. This provides further evidence and insight for the involvement of the deregulated Hippo pathway and YAP1/TAZ activation in tumorigenesis[42].

YAP1 is located at Chromosome 11q22, a recurrent amplicon region in esophageal squamous cell carcinoma and liver cancer[43,44]. The amplification within this region is also identified in a subset of cervical and lung cancers[45,46]. The modular structure of YAP1 contains a WW domain, TEAD-binding domain, SH3-binding motif and PDZ-binding motif[47]. YAP1 is a modular adaptor protein with multiple protein interaction domains and it was first identified by its affinity to bind the SH3 domain of nonreceptor tyrosine kinase c-Yes, a Src protein kinase[48]. The majority of the interactions are mediated by the WW domain of YAP1. Through WW domain, YAP1, PPxY motif-containing LATS1 kinase, and AMOTL1 protein form functional complex[49,50]. YAP1 binds to the ERBB4 and functions as a transcription co-activator for the cytoplasmic fragment of ERBB4 which is translocated to the nucleus[51]. TEAD family is one of the most important binding partners of YAP1[40] through TEAD-binding domain which is responsible for promoting oncogenic transformation[52] and metastasis[53]. PDZ-binding motif of YAP1 is a critical region for its cytoplasmic-nuclear translocation[54] and accumulation thus to exert its oncogenic function in tumorigenesis.

YAP1 has been concordantly identified as a candidate oncogene that promotes tumorigenesis in many different types of solid tumor[55-59]. In non-transformed mammary epithelial cells, the upregulation and activation of YAP1 promotes epithelial-mesenchymal transition (EMT) and induces growth factor independent growth and anchorage independent proliferation in soft agar[60]. In non-small cell lung cancer, YAP1 binds with Oct4 through its WW domain and transcriptionally induces Sox2 activation thus to endow the stem-like properties[61]. In medulloblastoma, Shh signaling induces YAP1 expression and promotes YAP1 nuclear localization to promote tumorigenesis[62]. In uveal melanoma, Gαq promotes the YAP-dependent growth via a Trio-Rho/Rac signaling circuitry which promotes actin re-polymerization. This process is independent of phospholipase Cβ and the canonical Hippo pathway[63]. In ovarian cancer cell lines, YAP1 can enhance the transformed phenotype and confers drug resistance to chemotherapeutic agents, which are commonly used in clinical ovarian cancer[64].

On the contrary, other studies identify that YAP1 stabilizes p73 and prevents its ubiquitination[65] thus selectively activating the transcription of proapoptotic genes as a consequence of damage to the DNA[66]. Therefore the function of YAP1 in favoring tumor suppression is thought to through apoptosis induction. Promyelocytic leukemia, a tumor suppressor gene and direct transcriptional target of p73/YAP1, directly binds with YAP1 physically via the PVPVY domain and WW domain thus to stabilize YAP1 from degradation[67]. The binding affinity of protein-protein interaction domains is thought to be critical in directing the biological function of YAP1[68]. O’Neill and his colleagues pointed out the molecular background such as loss of RASFF1A expression switches YAP1 from a tumor suppressor to an oncogene[69].

In hepatic oval cells, the mammalian Hippo pathway controls the proliferation rate and thereby restricts liver size and prevents the initiation of hepatocellular carcinoma (HCC)[94]. Overexpression of MST1/2 inhibits cell proliferation[95], promotes YAP1 phosphorylation[96] and downregulates the mRNA expression of CTGF, AREG and Survivin[97]. YAP1 plays oncogenic even cancer-driven role in HCC development[98,99]. YAP1 knockdown by siRNA-lipid nanoparticles (siRNA-LNPs) dramatically restores hepatocyte differentiation in advanced HCC and leads to tumor regression[100]. The activation of YAP1 is an early event[101] in HCC and serves as a an independent prognostic factor[102,103]. The PDZ binding motif in YAP1 is crucial for its activation of CTGF, a cell proliferation gene and TEAD-dependent transcription target[104]. CREB (cyclic adenosine monophosphate response element-binding protein) promotes YAP1 transcriptional output through binding to -608/-439, a novel region from the YAP promoter[105,106].

Several binding partners were identified to enhance the oncogenic property exerted by YAP1. YAP1 and PI3K exerts oncogenic cooperation in HCC[107]. The interaction of MEK1-YAP1 is critical for cell proliferation and transformed phenotype maintenance of HCC cells[108]. Amot-p130 was associated with the YAP1-TEAD transcriptional complex and contributed to the regulation of a bunch of YAP1 targeting genes. These targeted genes are associated with liver tumorigenesis[109]. SIRT1 was reported to deacetylate YAP1 protein in HCC cells and SIRT1-mediated deacetylation strengthened the YAP1-TEAD4 interaction, leading to the activation of YAP1/TEAD4 transcriptional ability and stimulating tumor growth in HCC cells[110]. YAP1 up-regulates Jag-1 to activate Notch signaling in HCC cells[37] and it also activates PI3K-mTOR signaling pathway[111], indicating the crosstalks of Hippo pathway and oncogenic pathways. As the same with GC, AXL is a target for YAP1-dependent oncogenic transcription in HCC and has been reported to be a potential therapeutic target[112].

In colorectal cancer (CRC), decreased expression of LATS1 in CRC was associated with promoter hypermethylation and such reduced expression promotes progression of CRC[113]. The Hippo pathway is suppressed by a critical regulator named integrin-linked kinase (ILK) via phosphorylation of MYPT1-PP1, leading to the Merlin inactivation[114]. MST1 and MST2 significantly repress the abundance and activation of YAP1 in normal intestinal epithelium. As tumor suppressors, MST1 and MST2 have an anti-proliferative function, however this function is often overcome by the over-abundance of YAP1 in CRC[38]. YAP1 overexpression in CRC cells is partly due the activation of β-catenin pathway. The β-catenin/TCF4 complexes bind to a DNA enhancer element within the first intron of the YAP1 gene to promote the transcription of YAP1[115]. The interplay between overexpressed YAP1 and β-catenin drives proliferation of colon cancer cells[116]. In KRAS-dependent cells when KRAS was suppressed, overexpression of YAP1 could rescue cell viability, suggesting that YAP1 was required for KRAS-induced cell transformation like EMT in CRC[117]. YAP1 transcription levels positively correlates with 5-fluorouracil resistance[118], relapse and shorter patient survival[119]. TAZ, the paralog of YAP1, regulates AXL in CRC and also plays an critical role in clonogenicity and non-adherent growth[120]. However, YAP1 is negatively regulated by E2A, which encodes two bHLH (basic helix-loop-helix) transcription factors E12 and E47[121].

In pancreatic ductal adenocarcinoma (PDAC), YAP1 was identified as a critical and functional downstream which plays a role in the oncogenic switch between KRAS pathway and MAPK (mitogen-activated protein kinase) pathway. YAP1 promotes the expression of genes which encode secretory factors. These factors accumulatively sustained the neoplastic proliferation, a stromal tumorigenic response from the tumor microenvironment[122]. The recent findings reveal that YAP1 and TEAD2, a TEAD transcriptional factor, interact and function cooperatively with E2F transcription factors to drive KRAS(G12D)-independent tumor initiation. In PDAC, this complex activates cell cycle progression and DNA synthesis program and finally escapes from the oncogenic KRAS addiction[123]. AGR2 up-regulates the expression of AREG, a EGFR ligand with growth-promoting potential, which is mediated by activation of YAP1 in PDAC[124]. YAP1 is negatively regulated by miR-141[125] and miR-375[126], which serves an independent prognostic factor for PDAC patients and exerts tumor suppressor functions.