A non-canonical Hippo pathway represses the expression of ΔNp63.

The p63 transcription factor, a member of the p53 family, plays an oncogenic role in squamous cancers, while in breast cancers its expression is often repressed. In the canonical conserved Hippo pathway, known to play a complex role in regulating growth of cancer cells, the protein kinases MST1/2 and LATS1/2 act sequentially to phosphorylate and inhibit the YAP/TAZ transcription factors. We found that in the MCF10A mammary epithelial cell line as well as in squamous and breast cancer cell lines, expression of ΔNp63 RNA and protein is strongly repressed by inhibition of the Hippo pathway protein kinases in a manner that is independent of p53. While MST1/2 and LATS1 are required for p63 expression, the next step of the pathway, namely phosphorylation and degradation of the YAP/TAZ transcriptional activators is not required for repression of p63. This suggests that regulation of p63 expression occurs by a non-canonical version of the Hippo pathway. We additionally identified additional genes that were similarly regulated suggesting the broader importance of this pathway. Interestingly, we observed that experimentally lowering p63 expression leads to increased YAP protein levels, thereby constituting a feedback loop. These results, which reveal the intersection of the Hippo and p63 pathways, may prove useful for the control of their activities in cancer cells. One Sentence Summary: Regulation of p63 expression occurs by a non-canonical version of the Hippo pathway in mammary epithelial, breast carcinoma and head and neck squamous carcinoma cells.