Physiological and pathological roles of the Hippo-YAP/TAZ signaling pathway in liver formation, homeostasis, and tumorigenesis.

The liver plays central homeostatic roles in metabolism and detoxification, and has a remarkable capacity to fully recover from injuries caused by the various insults to which it is constantly exposed. To fulfill these functions, the liver must maintain a specific size and so must regulate its cell numbers. It must also remove senescent, transformed, and/or injured cells that impair liver function and can lead to diseases such as cirrhosis and liver cancer. Despite their importance, however, the mechanisms governing liver size control and homeostasis have resisted delineation. The discovery of the Hippo intracellular signaling pathway and its downstream effectors, the transcriptional coactivators Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ), has provided partial elucidation of these mechanisms. The Hippo-YAP/TAZ pathway is considered to be a cell's sensor of its immediate microenvironment and the cells that surround it, in that this pathway responds to changes in elements such as the ECM, cell-cell tension, and cell adhesion. Once triggered, Hippo signaling negatively regulates the binding of YAP/TAZ to transcription factors such as TEAD and Smad, controlling their ability to drive gene expression needed for cellular responses such as proliferation, survival, and stemness. Numerous KO mouse strains lacking YAP/TAZ, as well as transgenic mice showing YAP/TAZ hyperactivation, have been generated, and the effects of these mutations on liver development, size, regeneration, homeostasis, and tumorigenesis have been reported. In this review, I summarize the components and regulation of Hippo-YAP/TAZ signaling, and discuss this pathway in the context of liver physiology and pathology.