A non-canonical repressor function of JUN restrains YAP activity and suppresses YAP-dependent liver cancer growth [SLAM_Seq]

The transcriptional coactivators YAP/TAZ are the critical downstream regulators of the Hippo pathway that regulate gene expression in response to changes in pathway activity, mainly by binding to TEAD transcription factors. Uncontrolled transcriptional output of YAP/TAZ can lead to rapid induction of aggressive tumor growth. AP-1 is a dimeric basic leucine zipper (bZIP) transcription factor complex with JUN and FOS proteins being the most abundant members of this family. Unlike FOS, JUN can also form homodimers, but in the cell, JUN presumably preferentially forms heterodimers with members of the FOS family, which act as potent transcriptional activators. Previous studies identified substantial co-occupancy of YAP/TAZ and AP-1 at genomic sites, and they demonstrated that JUN/FOS heterodimers cooperate with the YAP/TAZ and TEAD transcription factors to drive YAP/TAZ target gene expression. Here, we now elucidate a negative feedback mechanism in which high YAP activity is restrained by the recruitment of homodimeric JUN::JUN/NCOR1 repressor complexes and show that this noncanonical JUN function is part of a tumor suppressor mechanism in the liver. Overall design: To analyze the impact of JUN overexpression on gene expression, 10A cells expressing the necessary SAM expression constructs (dCas9-VP64 and MCP-p65-HSF1) were infected with a non-targeting control or a sgJUN#1 targeting the activating dCas9 complex to the JUN promoter.