The transcriptional co-repressor CtBP is a negative regulator of growth that antagonizes the Yorkie and JNK/AP-1 pathways

Multicellular organisms require strict growth control mechanisms to ensure that an organ reaches, but does not grossly exceed, its appropriate size and shape. In an unbiased mosaic screen for genes involved in growth regulation, we identified a loss-of-function allele of the gene CtBP that conferred a growth advantage to homozygous mutant tissue. CtBP encodes a transcriptional co-repressor found in diverse organisms, yet its role in regulating tissue growth is not known. We found that CtBP functions as a negative regulator of growth by restricting the expression of the growth-promoting microRNA bantam (ban). ban is a known target of the Hippo pathway effector Yorkie (Yki). We show that loss of CtBP function leads to the activation of a minimal enhancer of ban via both Yki-dependent and AP-1 transcription factor-dependent mechanisms. AP-1 is downstream of the Jun N-terminal Kinase (JNK) pathway and thus JNK could regulate growth during development via ban. Furthermore, we show that distinct isoforms of the AP-1 component Fos differ in their ability to activate this enhancer. Since the orthologous pathways in mammalian cells (YAP/TEAD and AP-1) converge on enhancers implicated in tumor progression, a role for mammalian CtBP proteins at those enhancers merits attention.