The Hippo signaling pathway is an important regulator of cellular proliferation and organ size. However, little is known about the role of this cascade in the control of cell fate. Employing a combination of lineage tracing, clonal analysis, and organoid culture approaches, we demonstrate that Hippo-pathway activity is essential for the maintenance of the differentiated hepatocyte state. Remarkably, acute inactivation of Hippo-pathway signaling in vivo is sufficient to de-differentiate, at very high efficiencies, adult hepatocytes into cells bearing progenitor characteristics. These hepatocyte-derived progenitor cells demonstrate self-renewal and engraftment capacity at the single cell level. We also identify the NOTCH signaling pathway as a functional important effector downstream of the Hippo transducer YAP. Our findings uncover a potent role for Hippo/YAP signaling in controlling liver cell fate, and reveal an unprecedented level of phenotypic plasticity in mature hepatocytes, which has implications for the understanding and manipulation of liver regeneration.

A remarkable feature of regenerative processes is their ability to halt proliferation once an organ’s structure has been restored. The Wnt signaling pathway is the major driving force for homeostatic self-renewal and regeneration in the mammalian intestine. The mechanisms that counterbalance Wnt-driven proliferation are poorly understood. We demonstrate here that YAP, a protein known for its powerful growth-inducing and oncogenic properties1-2, has an unexpected growth-suppressive function restricting Wnt signals during intestinal regeneration. Transgenic expression of YAP reduces Wnt target gene expression and results in the rapid loss of intestinal crypts. In addition, loss of YAP results in Wnt hypersensitivity during regeneration, leading to hyperplasia, expansion of intestinal stem cells (ISCs) and niche cells, and formation of ectopic crypts and microadenomas. We find that cytoplasmic YAP restricts elevated Wnt signaling independently of the APC/Axin/GSK3β complex partly by limiting the activity of Dishevelled (DVL). DVL signals in the nucleus of ISCs and its forced expression leads to enhanced Wnt signaling in crypts. YAP dampens Wnt signals by restricting DVL nuclear translocation during regenerative growth. Finally, we provide evidence that YAP is silenced in a subset of highly aggressive and undifferentiated human colorectal carcinomas (CRC) and its expression can restrict the growth of CRC xenografts. Collectively, our work describes a novel mechanistic paradigm for how proliferative signals are counterbalanced in regenerating tissues. Additionally, our findings have important implications for the targeting of YAP in human malignancies.

SUMMARY Global downregulation of microRNAs (miRNAs) is commonly observed in human cancers and can have a causative role in tumorigenesis. The mechanisms responsible for this phenomenon remain poorly understood. Here we show that YAP, the downstream target of the tumor-suppressive Hippo signaling pathway regulates miRNA biogenesis in a cell density-dependent manner. At low cell density, nuclear YAP binds and sequesters p72 (DDX17), a regulatory component of the miRNA processing machinery. At high cell density, Hippo-mediated cytoplasmic retention of YAP facilitates p72 association with Microprocessor and binding to a specific sequence motif in pri-miRNAs. Inactivation of the Hippo pathway or expression of constitutively active YAP causes widespread miRNA suppression in cells and tumors and a corresponding post-transcriptional induction of MYC expression. Thus, the Hippo pathway links contact-inhibition regulation to miRNA biogenesis and may be responsible for the widespread miRNA repression observed in cancer.