First discovered by laboratories studying Drosophila development 18 years ago1-3, the Hippo-YAP signaling pathway (also known as the Salvador-Warts-Hippo Pathway) is a novel pathway implicated in organism development, stem cell biology, and cancer biology4. While not much is known about the Hippo-YAP pathway, this signaling mechanism could lead to a promising paradigm in regenerative medicine and treating cancer.
While we are far from fully discovering every aspect Hippo-YAP signaling pathway, some components of this novel pathway have been uncovered. In mammalian cells, the first signal modulator to be stimulated is mammalian STE-20 protein kinase 1 & 2(Mst1/2). This stimulation causes autophosphorylation, which in turn starts a kinase cascade; phosphorylating the proteins Salvador homolog 1 (Sav1), MOB kinase activator 1 (Mob1), and large tumor suppressor 1 & 2 (Lats1/2). Once Lats1/2 is activated, it phosphorylates YAP (Yes-associated protein)4. This phosphorylation of YAP sequesters it outside of the cell and leads to its proteosomal degradation and thus blocking its ability to complex with the protumor TEAD transcription factors, which in turn inhibits proliferation and blocks inhibition of apoptosis4. Interestingly, other alternative mechanisms, such as directly targeting YAP via the WNT pathway, or activation of YAP/TAZ via the SMAD signaling pathway by TGFβ and BMP, have been demonstrated5. While the stimulation of the Hippo pathway is still being revealed, researchers have discovered two mechanisms hippo stimulation: cell-cell contact and activation of G-protein coupled receptors4,5. Stimulation of G-protein Coupled Receptors (GPCRs), Go with the ligands LPA or S1P and Gs with glucagon and epinephrine, have been shown to activate the Hippo-YAP signaling pathway, causing phosphorylation of Mst1/26. On the other hand, the cell-cell contact method of hippo activation most likely phosphorylates Mst1/2 through the upstream component: Merlin6. However, while there is phosphorylation of Mst1/2 in both stimulation methods, neither stimulation pathway is known, save one or two components, upstream of Mst1/2. Furthermore, the complexity of this signaling pathway is certain and upstream signals may be redundant6.
The Hippo-YAP signaling pathway plays a crucial role in embryological development. At the middle of this is the Hippo signaling component is transcriptional co-activator with PDZ-binding motif (TAZ, also known as WWTR1). TAZ is able to regulate the signaling mechanisms of the SMAD2/3-4 signaling pathway4; a pathway that regulates the TGF-beta signaling cascade that is important in early embryogenesis7. Furthermore, it has been demonstrated that functional loss of the TAZ protein, and not YAP, will lead to uncontrolled differentiation of human embryonic stem cells (hESCs) as well as loss of self-renewal of hESCs4. Surprisingly, although YAP is not as important as TAZ to block differentiation, YAP is inactivated during normal hESC differentiation4. In addition to stem cell differentiation, the Hippo-YAP signaling pathway has been shown to be important for polarization of tissues8 in both planar and apicobasal cell polarity5, tissue shape and patterning9, and overall tissue homeostasis9.
While the activation of the Hippo-YAP pathway seems to be important for embryogenesis, the dysregulation of the Hippo-YAP pathway seems to play a striking role in tumorigenesis9. Deletion of the upstream Mst1/2 component of the Hippo-YAP pathway has been shown to cause uncontrolled liver growth. Microscopic analysis of liver biopsies revealed that these tissues were full of hepatocellular carcinoma and cholangiocarcinoma4. Likewise, overactivation of the YAP protein caused uncontrolled, extreme thickening of epidermal layer4. However, the pathway that is thought to be involved in this process of YAP activation is not the canonical Hippo-YAP pathway, but a signaling through alpha catenin. The catenin family and the Hippo-YAP signaling pathway were further shown to interact when overexpression of YAP facilitated the expression of Notch/Wnt signaling pathway indirectly by YAP-driven overexpression of beta catenin4. Because the Notch/Wnt pathways are important for cancer stem cell phenotype10 and cancer metastasis11, further investigation into the roles of the Hippo-YAP signaling pathway could bring a lot of clinical significance.
At the writing of this blog, no proposed drug has been proposed that directly targets the Hippo signaling pathway. However, many possible targets for therapy are being investigated that would also affect the Hippo signaling pathway5. One of these is targeting the homeodomain-interacting protein kinase 2 (HIPK2) which has been demonstrated to activate YAP 5.In addition, using GPCR antagonists, such as Dobutamine, have been shown to decrease activation levels of YAP5. Also promising, researchers have solved many domains of the YAP structure, which may lead to specific inhibition of this oncogene by potential inhibitors12. Because of the role that the Hippo signaling pathway may play on tumor growth inhibition, it may not be long before candidate drugs targeting this pathway will start to enter the FDA drug pipeline.
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2 Xu, T., Wang, W., Zhang, S., Stewart, R. A. & Yu, W. Identifying tumor suppressors in genetic mosaics: the Drosophila lats gene encodes a putative protein kinase. Development 121, 1053-1063 (1995).
3 Wu, S., Huang, J., Dong, J. & Pan, D. hippo encodes a Ste-20 family protein kinase that restricts cell proliferation and promotes apoptosis in conjunction with salvador and warts. Cell 114, 445-456 (2003).
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6 Yu, F. X. et al. Regulation of the Hippo-YAP pathway by G-protein-coupled receptor signaling. Cell 150, 780-791, doi:10.1016/j.cell.2012.06.037 (2012).
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