The Mechanism And Function Of The Hippo Pathway Effector YAP In Regulation Of Gene Transcription

The Hippo pathway is a newly identified evolutionarily conserved signaling cascade that plays a key role in regulation of organ size, tissue homeostasis and tumorigenesis. The Hippo pathway responds to upstream signals such as GPCR signal, mechanical stress, cytoskeleton remodeling and cell polarity complexes. Upon stimulation, the Hippo pathway core kinase cascade phosphorylates and inhibits the effectors YAP and TAZ transcription co-activators, thus regulates gene transcription and cell physiology. Therefore, the mechanism by which YAP and TAZ regulates transcription is a key question in the Hippo pathway field. As transcription co-activators, YAP/TAZ do not bind DNA directly. Instead, they bind to gene promoters through other transcription factors. Works from our lab and others have previously identified TEAD family transcription factors as key mediators of YAP-induced gene transcription and organ size control. However, we have also noticed that the WW domains of YAP, which is a protein-protein interaction domain, are also required for the induction of a fraction of YAP target genes. This suggests that PPXY-motif-containing transcription factors, such as Smads, TBX5, Runx2, and p73 may mediate YAP target gene expression and organ size control. Nevertheless, such possibility has not been demonstrated. In this study, we found that integrin ITGB2 is a new target gene of YAP/TAZ requiring both TEAD and a WW domain-interacting transcription factor for its induction. By screening a transcription factor library and by affinity purification, we found PRDM4, a histone methyltransferase family member, as a new target transcription factor of YAP, TEAD and PRDM4 bind to distinct regions of ITGB2 promoter, and both interact with YAP, thus cooperatively induce ITGB2 expression. Functionally, ITGB2 expression induced by YAP may promote cell adhesion and cell migration while PRDM4 may further promote cellular transformation and tumorigenesis through other target genes. Furthermore, we identified a previously unknown negative feedback mechanism of the Hippo pathway. We noticed that expression of a constitutively active YAP mutant strongly induces phosphorylation and inhibition of endogenous YAP, which suggests a negative feedback mechanism. Further analysis indicated that through TEAD, YAP directly induces transcription and activation of its upstream inhibitory kinase Lats2. Interestingly, this feedback mechanism not only responses to physiological signals of the Hippo pathway such as serum and cell adhesion, but is also activated during YAP-induced liver tumorigenesis in vivo. In conclusion, we identified a novel YAP target transcription factor PRDM4 and a new target gene ITGB2, and partially elucidated the transcriptional mechanism of YAP-induced cell adhesion, migration, and Hippo signaling homeostasis.