Thesis title: Extracellular Signal-Regulated Kinase 5 (ERK5) is required for the Yes-associated protein (YAP) co-transcriptional activity
Yes-associated protein (YAP) is a transcriptional cofactor involved in the regulation of various physio-pathological cellular processes. YAP is the main downstream effector of the tumor-suppressive Hippo pathway. When Hippo signaling is activated, it induces YAP phosphorylation and cytoplasmic sequestration, thus inhibiting its co-transcriptional activity. On the contrary, when Hippo signaling is inhibited, YAP translocates into the nucleus where it drives several transcriptional programs in a cell- and context-dependent manner. However, recent observations indicate that YAP activity can be also modulated by Hippo independent/integrating pathways, still largely unexplored. In this study, we demonstrated the role of the extracellular signal-regulated kinase 5 (ERK5)/mitogen-activated protein kinase in the regulation of YAP activity. By means of ERK5 inhibition/silencing and overexpression experiments, and by using as model liver stem cells, hepatocytes, and hepatocellular carcinoma (HCC) cell lines, we provided evidence that ERK5 is required for YAP-dependent gene expression. Mechanistically, ERK5 is required for the recruitment of YAP on the promoters of target genes and for its physical interaction with the transcriptional partner TEAD4. Moreover, we observed that ERK5 modulates YAP activation in cell adhesion dynamics, TGFβ-induced EMT of liver cells and cell migration. Furthermore, we demonstrated that ERK5 modulates the activity also of a YAP mutant non-phosphorylatable by LATS1/2, thus providing evidence of its, at least in part, independence from the Hippo pathway. Finally, preliminary results obtained in mice overexpressing YAP in the liver confirmed the relevance of ERK5/YAP axis in vivo and suggested the ERK5 involvement in the YAP-induced liver fibrosis. Therefore, our observations identify ERK5 as a novel upstream regulator of YAP activity, thus unveiling a new target for therapeutic approaches aimed at interfering with its function.