Characterization of the transcriptional complexes enrolled by Yes-associated protein (YAP) in hepatoma cells and their regulation by ERK5/MAPK

The transcriptional cofactor Yes-Associated Protein (YAP) is known to be a master regulator of gene expression programs involved in several cell functions. It cooperates with several transcriptional factors, mainly with those belonging to the TEAD family. There is increasing evidence that the composition and genomic occupancy of YAP-recruiting transcriptional complexes may depend on tissue and cellular context, thus dynamically driving specific gene expression and different functional outcomes. The formation and specific composition of these complexes may be regulated by the phosphorylation status of the components, which eventually depends on the activity of enzymes belonging to different signaling pathways. Based on our recent work identifying the transcriptional factor STAT3 as a new interactor of YAP in liver cells and ERK5 as a new regulator of YAP activity, and on recent literature data showing lncRNAs as functional components of transcriptional molecular platforms, this PhD project aimed to the structural and functional characterization of YAP-enrolled transcriptional complexes in transformed liver cells also focusing on their upstream regulation. Specifically, I have i) investigated the role of STAT3 in the YAP-dependent gene expression, ii) analyzed the mechanistic and functional role of ERK5/MAPK in the control of the assembly/activity of YAP/STAT3 and YAP/TEAD complexes and of their recruitment on DNA, and iii) evaluated lncRNA MALAT1 as potential functional member of YAP-dependent transcriptional platforms. Provided data demonstrated that, in hepatoma cells, i) STAT3 regulates YAP transcriptional activity by cooperating with TEAD on previously characterized YAP/TEAD target genes (i.e. Ctgf and Cyr61); ii) YAP/STAT3 DNA binding is dependent on ERK5 kinase activity; iii) YAP is a new direct target of ERK5 kinase activity. Furthermore, we gathered evidence that lncRNA MALAT1 physically interacts with YAP in an ERK5-dependent manner and that it is required for YAP-target gene expression. On the basis of these results, a model for the dynamic formation and activation of YAP/STAT3/TEAD transcriptional complex has been proposed for CTGF promoter, extendable to other YAP target genes and possibly involving other components, including MALAT1. Overall, our findings provide a possible paradigm of how specific YAP-including transcriptional complexes can be dynamically assembled and drive specific YAP-dependent gene expression and cellular outcomes. In perspective, these studies may pave the way to new therapeutic approaches, aimed at interfering with YAP activity in pathologies where it has been found deregulated.