Oncogenic YAP mediates changes in chromatin accessibility and activity that drive cell cycle gene expression and cell migration.

YAP, the key protein effector of the Hippo pathway, is a transcriptional co-activator that controls the expression of cell cycle genes, promotes cell growth and proliferation and regulates organ size. YAP modulates gene transcription by binding to distal enhancers, but the mechanisms of gene regulation by YAP-bound enhancers remain poorly understood. Here we show that constitutive active YAP5SA leads to widespread changes in chromatin accessibility in untransformed MCF10A cells. Newly accessible regions include YAP-bound enhancers that mediate activation of cycle genes regulated by the Myb-MuvB (MMB) complex. By CRISPR-interference we identify a role for YAP-bound enhancers in phosphorylation of Pol II at Ser5 at MMB-regulated promoters, extending previously published studies that suggested YAP primarily regulates the pause-release step and transcriptional elongation. YAP5SA also leads to less accessible 'closed' chromatin regions, which are not directly YAP-bound but which contain binding motifs for the p53 family of transcription factors. Diminished accessibility at these regions is, at least in part, a consequence of reduced expression and chromatin-binding of the p53 family member ΔNp63 resulting in downregulation of ΔNp63-target genes and promoting YAP-mediated cell migration. In summary, our studies uncover changes in chromatin accessibility and activity that contribute to the oncogenic activities of YAP.

Inhibition of the YAP-MMB interaction and targeting NEK2 as potential therapeutic strategies for YAP-driven cancers.

YAP activation in cancer is linked to poor outcomes, making it an attractive therapeutic target. Previous research focused on blocking the interaction of YAP with TEAD transcription factors. Here, we took a different approach by disrupting YAP's binding to the transcription factor B-MYB using MY-COMP, a fragment of B-MYB containing the YAP binding domain fused to a nuclear localization signal. MY-COMP induced cell cycle defects, nuclear abnormalities, and polyploidization. In an AKT and YAP-driven liver cancer model, MY-COMP significantly reduced liver tumorigenesis, highlighting the importance of the YAP-B-MYB interaction in tumor development. MY-COMP also perturbed the cell cycle progression of YAP-dependent uveal melanoma cells but not of YAP-independent cutaneous melanoma cell lines. It counteracted YAP-dependent expression of MMB-regulated cell cycle genes, explaining the observed effects. We also identified NIMA-related kinase (NEK2) as a downstream target of YAP and B-MYB, promoting YAP-driven transformation by facilitating centrosome clustering and inhibiting multipolar mitosis.

Expression of the cytokinesis regulator PRC1 results in p53-pathway activation in A549 cells but does not directly regulate gene expression in the nucleus.

Protein regulator of cytokinesis 1 (PRC1) is a microtubule-binding protein with essential roles in mitosis and cytokinesis. PRC1 is frequently overexpressed in cancer cells where it could contribute to chromosomal instability. Due to its nuclear localization in interphase, it has been speculated that PRC1 has additional functions that are involved in its pro-tumorigenic functions. In this study we investigated the potential nuclear functions of PRC1 in a lung cancer cell line. Genome wide expression profiling by RNA sequencing revealed that the expression of PRC1 results in activation of the p53 pathway and inhibition of the pro-proliferative E2F-dependent gene expression. A mutant of PRC1 that is unable to enter into the nucleus regulated the same gene sets as wildtype PRC1, suggesting that PRC1 has no nuclear-exclusive functions in A549 cells. Instead, induction of p53 by PRC1 correlates with multinucleation and depends on the localization of PRC1 to the midbody, suggesting that the induction of p53 is a consequence of overexpressed PRC1 to interfere with the normal function of PRC1 during cytokinesis. Activation of p53 by PRC1 results in cellular senescence but not in apoptosis. In conclusion, while PRC1 is frequently overexpressed in many cancers, the p53 pathways may initially protect cancer cells from the negative effects of PRC1 overexpression on cytokinesis. Because depletion of PRC1 also results in p53-pathway activation and senescence, levels of PRC1 need to be tightly regulated to allow unperturbed proliferation. Targeting the expression or function of PRC1 could create a therapeutic vulnerability for the treatment of cancer.