The Hippo-YAP Pathway: New Connection between Cancer and Stem Cells.
Laboratories studying Drosophila development first discovered the Hippo-YAP signaling pathway (also known as the Salvador-Warts-Hippo Pathway) 18 years ago1-3. This novel pathway plays a role in organism development, stem cell biology, and cancer biology4. While researchers still have much to learn about the Hippo-YAP pathway, this signaling mechanism could lead to a promising new paradigm in regenerative medicine and cancer treatment.
How the Pathway Works
While we’re far from fully understanding the Hippo-YAP signaling pathway, researchers have uncovered some of its components. In mammalian cells, the first signal modulator stimulated is mammalian STE-20 protein kinase 1 & 2 (Mst1/2). This stimulation triggers autophosphorylation, which starts a kinase cascade that phosphorylates the proteins Salvador homolog 1 (Sav1), MOB kinase activator 1 (Mob1), and large tumor suppressor 1 & 2 (Lats1/2). Once activated, Lats1/2 phosphorylates YAP (Yes-associated protein)4. This phosphorylation sequesters YAP outside the cell and leads to its proteosomal degradation, blocking its ability to form a complex with the protumor TEAD transcription factors — which in turn inhibits proliferation and blocks apoptosis inhibition4. Researchers have also demonstrated other alternative mechanisms, such as directly targeting YAP via the WNT pathway, or activating YAP/TAZ via the SMAD signaling pathway through TGFβ and BMP5.
While researchers are still uncovering how the Hippo pathway gets stimulated, they’ve discovered two mechanisms so far: cell-cell contact and activation of G-protein coupled receptors4,5. Stimulating G-protein Coupled Receptors (GPCRs) — Go with the ligands LPA or S1P, and Gs with glucagon and epinephrine — activates the Hippo-YAP signaling pathway, causing phosphorylation of Mst1/26. The cell-cell contact method of Hippo activation, on the other hand, most likely phosphorylates Mst1/2 through the upstream component Merlin6. However, while both stimulation methods phosphorylate Mst1/2, researchers don’t yet know what lies upstream of Mst1/2 in either pathway, aside from one or two components. This signaling pathway is clearly complex, and its upstream signals may be redundant6.
A Role in Embryonic Development
The Hippo-YAP signaling pathway plays a crucial role in embryological development. At the center of this pathway is the transcriptional co-activator with PDZ-binding motif (TAZ, also known as WWTR1). TAZ can regulate the signaling mechanisms of the SMAD2/3-4 pathway4 — a pathway that regulates the TGF-beta signaling cascade important in early embryogenesis7. Researchers have also shown that functional loss of the TAZ protein — but not YAP — leads to uncontrolled differentiation of human embryonic stem cells (hESCs), as well as loss of hESC self-renewal4. Surprisingly, even though YAP matters less than TAZ for blocking differentiation, YAP becomes inactivated during normal hESC differentiation4. Beyond stem cell differentiation, the Hippo-YAP signaling pathway also plays an important role in tissue polarization8 (both planar and apicobasal cell polarity)5, tissue shape and patterning9, and overall tissue homeostasis9.
When the Pathway Goes Wrong
While activating the Hippo-YAP pathway seems important for embryogenesis, dysregulating this pathway appears to play a striking role in tumorigenesis9. Deleting the upstream Mst1/2 component of the Hippo-YAP pathway causes uncontrolled liver growth. Microscopic analysis of liver biopsies revealed that these tissues were full of hepatocellular carcinoma and cholangiocarcinoma4. Similarly, overactivating the YAP protein caused uncontrolled, extreme thickening of the epidermal layer4. However, researchers believe a different pathway drives this YAP activation — signaling through alpha catenin, rather than the canonical Hippo-YAP pathway. The catenin family and the Hippo-YAP signaling pathway further showed their connection when YAP overexpression indirectly boosted expression of the Notch/Wnt signaling pathway through YAP-driven overexpression of beta catenin4. Since the Notch/Wnt pathways matter for cancer stem cell phenotype10 and cancer metastasis11, further investigation into the Hippo-YAP signaling pathway’s role could carry significant clinical value.
Looking Toward Treatment
As of this writing, no proposed drug directly targets the Hippo signaling pathway. However, researchers are investigating many possible therapy targets that would also affect the Hippo signaling pathway5. One target is homeodomain-interacting protein kinase 2 (HIPK2), which has been shown to activate YAP5. Researchers have also shown that GPCR antagonists, such as Dobutamine, can decrease YAP activation levels5. Promisingly, researchers have also solved many domains of the YAP structure, which may lead to specific inhibitors that target this oncogene12. Given the role the Hippo signaling pathway may play in inhibiting tumor growth, candidate drugs targeting this pathway may not be far from entering the FDA drug pipeline.
Further Reading:
1 Justice, R. W., Zilian, O., Woods, D. F., Noll, M. & Bryant, P. J. The Drosophila tumor suppressor gene warts encodes a homolog of human myotonic dystrophy kinase and is required for the control of cell shape and proliferation. Genes & development 9, 534-546 (1995).
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).
4 Ramos, A. & Camargo, F. D. The Hippo signaling pathway and stem cell biology. Trends in cell biology 22, 339-346, doi:10.1016/j.tcb.2012.04.006 (2012).
5 Harvey, K. F., Zhang, X. & Thomas, D. M. The Hippo pathway and human cancer. Nature reviews. Cancer 13, 246-257, doi:10.1038/nrc3458 (2013).
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).
7 Massague, J. TGFbeta signalling in context. Nature reviews. Molecular cell biology 13, 616-630, doi:10.1038/nrm3434 (2012).
8 Yu, F. X. & Guan, K. L. The Hippo pathway: regulators and regulations. Genes & development 27, 355-371, doi:10.1101/gad.210773.112 (2013).
9 Pan, D. The hippo signaling pathway in development and cancer. Developmental cell 19, 491-505, doi:10.1016/j.devcel.2010.09.011 (2010).
10 Takebe, N., Harris, P. J., Warren, R. Q. & Ivy, S. P. Targeting cancer stem cells by inhibiting Wnt, Notch, and Hedgehog pathways. Nature reviews. Clinical oncology 8, 97-106, doi:10.1038/nrclinonc.2010.196 (2011).
11 Fodde, R. & Brabletz, T. Wnt/beta-catenin signaling in cancer stemness and malignant behavior. Current opinion in cell biology 19, 150-158, doi:10.1016/j.ceb.2007.02.007 (2007).
12 Sudol, M., Shields, D. C. & Farooq, A. Structures of YAP protein domains reveal promising targets for development of new cancer drugs. Seminars in cell & developmental biology 23, 827-833, doi:10.1016/j.semcdb.2012.05.002 (2012).
