The mechanism of body and organ size control is an unsolved puzzle. Recent Drosophila genetics studies established the key role of the Hippo pathway and its downstream target Yki in organ size control. Yki is the homolog of the mammalian Yes-associated protein (YAP), a transcription co-activator. However, the regulation of YAP activity was not well understood. My study elucidated the mechanism of YAP regulation by the Hippo pathway in mammalian cells in response to cell density. At high cell density, phosphorylation of S127 by the Lats tumor suppressor kinase leads to cytoplasmic retention and inactivation of YAP. Attenuation of this phosphorylation of YAP or Yki potentiates their oncogenic transformation activity in vitro and growth-promoting function in vivo. YAP overexpression regulates gene expression in a manner opposite to cell density, and overcomes cell contact inhibition. Inhibition of YAP function restores contact inhibition in the ACHN human cancer cell line. This evidence supports the involvement of Hippo-YAP pathway in cell contact inhibition.

As a transcription co-activator, YAP has to interact with transcription factors to activate gene expression. A critical transcription factor mediating YAP function was unknown. By screening a transcription factor library, I identified TEAD family transcription factors as the most potent YAP targets. Experiments further demonstrated that TEADs are required for YAP induced gene expression, cell growth, and oncogenic transformation. In addition, I identified CTGF (connective tissue growth factor) as a direct target gene of YAP-TEAD mediating their biological functions.

However, evidence suggests that YAP function also requires other transcription factors. WW domains of YAP, a structure mediating protein-protein interactions, are implicated in mediating interactions with other transcription factors. Consistently, I showed that the WW domains of YAP have a critical role in inducing a subset of YAP target genes independent of or in cooperation with TEAD. Mutation of the WW domains diminishes the ability of YAP to stimulate cell proliferation and oncogenic transformation.

The above data suggest a model that YAP plays a key role in the Hippo pathway to regulate cell proliferation, organ size, and oncogenic transformation by inducing expression of genes including CTGF through interaction with TEAD family and WW domain-binding transcription factors.