Abstract
Mutations in the Wnt/β-catenin pathway occur in most colorectal cancers (CRCs), and these mutations lead to increased nuclear accumulation of the β-catenin transcriptional co-activator. In the nucleus, β-catenin associates with TCF/LEF sequence specific transcription factors to activate target gene expression. The Hippo pathway restricts cellular growth by preventing nuclear accumulation of the Yes-associated protein (YAP) transcriptional co-activator. YAP expression is elevated in CRCs suggesting that, like Wnt/β-catenin signaling, the Hippo pathway may contribute to colorectal carcinogenesis. Regulation of YAP at the post-translational level has been well studied but the transcription factors that control YAP gene expression are unknown. Here we demonstrate that β-catenin/TCF4 complexes bind a DNA enhancer element within the first intron of the YAP gene to drive YAP expression in CRC cells. As such, reducing β-catenin expression in CRC cells using shRNAs leads to decreased YAP mRNA and protein levels. YAP is abundantly expressed in the cytoplasm and nuclei of several established human colon cancer cell lines and this localization pattern is insensitive to plating density. Finally, we show that YAP expression is elevated in the majority of a panel of primary human colorectal tumors compared with its expression in uninvolved colonic mucosa, and that YAP and β-catenin localize to the nuclear compartment of tumor cells. Together, these results implicate YAP as an oncogene whose expression is driven by aberrant Wnt/β-catenin signaling in human CRC cells.