YAP Promotes Breast Cell Proliferation and Survival Partially through Stabilizing the KLF5 Transcription Factor

The Yes-associated protein (YAP), an oncoprotein in the Hippo tumor suppressor pathway, regulates tumorigenesis and has been found in a variety of tumors, including breast, ovarian, and hepatocellular cancers. Although YAP functions through its WW domains, the YAP WW domain-binding partners have not yet been completely determined. With this study, we demonstrate that YAP functions partially through its binding to KLF5, a transcription factor that promotes breast cell proliferation and survival. YAP interacted with the KLF5 PY motif through its WW domains, preventing the E3 ubiquitin ligase WWP1 from ubiquitinating KLF5. Overexpression of the wild-type YAP but not the WW domain-mutated YAP up-regulated KLF5 protein levels and mRNA expression levels of KLF5 downstream target genes, including FGFBP1 (alias FGF-BP) and ITGB2. In addition, knockdown of YAP decreased expression levels of KLF5, FGF-BP, and ITGB2. Depletion of either YAP or KLF5 decreased breast cell proliferation and survival in MCF10A and SW527 breast cell lines, and stable knockdown of either YAP or KLF5 suppressed SW527 xenograft growth in mice. The YAP upstream kinase LATS1 suppressed the KLF5-FGF-BP axis, as well as cell growth through YAP signaling. Both YAP and KLF5 are coexpressed in estrogen receptor ERα-negative breast cell lines. These findings suggest that KLF5 could be an important transcription factor partner for YAP and may contribute to the Hippo pathway.