(The) role of Yes Associated Protein (YAP) in promoting stemness in mammary epithelial cell and breast cancer

Abstract

The switch between stem/progenitor cell expansion and differentiation is critical for organ homeostasis and may be a viable target for cancer therapeutics. The mammalian Hippo pathway effector oncoprotein YAP (Yes-Associated Protein) is a transcription coactivator known to expand undifferentiated stem/progenitor cells in various tissues. While several upstream cues, collectively constituting the Hippo pathway, have been shown to regulate YAP activity, the YAP associating transcription factors and YAP downstream targets underlying the stemness-promoting activity are poorly understood. In my doctoral research, I searched for a novel transcription target of YAP by employing tamoxifen inducing form of YAP. I found that the immediate target genes of YAP included many mammary stem cell (MaSC) signature genes, and YAP can indeed induce MaSC-like property in mammary epithelial cell line, MCF-10A. I also demonstrate that SRF (serum response factor) associates with YAP and recruits it to the promoters of multiple mammary stem cell (MaSC) signature genes, including IL6 (interleukin 6), DLL1 (delta-like 1), ETS1 (v-ets avian erythroblastosis virus E26 oncogene homolog 1). Among these, IL6 was identified as a prime target of SRF-YAP that is essential for endowment of MaSC-like properties. Specifically, I found that inactivation of either SRF or IL6 decreased the competence of cells to form self-renewable mammospheres, whereas inactivation of YAP’s previously known TEAD and CTGF-mediated oncogenic pathway failed to reverse the promotion of stem-like properties by YAP. Extending my findings to breast cancer, I showed that SRF and YAP/TAZ expression are correlated with stem cell-like, basal-like breast cancer (BLBC) with high IL6 expression. Finally, I show that high SRF expression in BLBC enables YAP to more efficiently induce IL6 and stemness compared to luminal type breast cancer. Collectively, my results establish the importance of SRF-YAP-IL6 signaling in promoting mammary stem cell-like properties in mammary epithelial cells and breast cancer.

I was also intrigued by the fact that TEAD-YAP and SRF are both regulated by similar subset of extracellular stimuli including serum treatment and actomyosin tension. Analogous to YAP for TEAD, MRTF (Myocardin Related Transcription Factor) shuttles from cytoplasm to nucleus to translate extracellular physical and chemi-cal stimuli to activity of its partner transcription factor SRF. Therefore, I examined for possible crosstalk be-tween TEAD-YAP and MRTF-SRF pathway. Surprisingly, I found that MRTF is a strong activator of TEAD transcription factor and it is required for full TEAD-YAP activity. Further, I demonstrate that MRTF directly binds YAP via WW domain-PPXY motif interaction. This interaction between YAP and MRTF is critical for YAP target gene expression.

In conclusion, I found a novel signaling pathway downstream of YAP involving SRF. SRF and YAP bind to induce specific subset of target genes involved in MaSC-like property, among which IL6 is critical. I also deline-ate the functional crosstalk between TEAD-YAP and SRF-MRTF pathway by showing that MRTF is a critical mediator of functional TEAD-YAP complex. Considering that the both transcription machinery is critical medi-ator translating extracellular physical and chemical cue to cellular physiology, I suggest that the two mecha-nisms may work cooperatively to induce stemness under supportive niche.