br Results br Discussion Our current study demonstrates that

Results
Discussion Our current study demonstrates that the sustained activation of Yap overrides the p27-mediated checkpoint, at least in part, through the degradation of FoxO family proteins, allowing polyploid cells to proliferate inappropriately with mitotic defects and resulting in centrosome amplification, genomic instability, and cell oncogenesis. In fact, p27 plays a dual role in the regulation of the Thonzonium Bromide australia and genomic stability in the mouse liver (Carrano et al., 1999, Kossatz et al., 2004, Nakayama et al., 2004, Serres et al., 2012). For cell-cycle control, elevated p27 arrests the cell cycle via the inhibition of CDKs to prevent cell division. In contrast, increased p27 results in mitotic defects and promotes endoreduplication cycles to induce cell polyploidy and genomic instability in mouse livers, while a loss of p27 reduces cell polyploidy, thereby maintaining genomic stability. In addition, p27 has long been known to be an assembly factor for cyclin D/Cdk4 complexes (LaBaer et al., 1997). Thus, elevated p27 in polyploid cells might increase Cdk4 activity and promote cell division, aneuploidy, and genomic instability, thus contributing to the development of cancer. In the context of the much higher fraction of polyploid cells in Mst1/2 DKO liver tissues, it is not surprising to observe that the loss of p27 in Hippo-deficient livers resulted in decreased cell polyploidy and a significantly reduced number and volume of tumor size. Thus, these data support a key role of p27 expression in the tumorigenesis of polyploid organs, such as livers with deregulated Hippo signaling. Previous studies have shown that tetraploid cells arrest their cell cycle in a p53-dependent manner. Hippo tumor suppressor pathway Lats1/2 kinases were shown to induce tetraploid cell-cycle arrest by preventing Mdm2-mediated p53 degradation (Aylon et al., 2006, Iida et al., 2004, McPherson et al., 2004). However, we found that Yap overexpression or the deletion of Yap inhibitory components such as WW45, Mst1/2, and Lats1/2 in mouse livers results in highly increased p53 expression and activity. Moreover, the combined losses of Hippo signaling and p53 lead to greatly increased polyploidy with multiple nuclei and results in a higher incidence and earlier onset of liver tumors. Thus, the increased p53-mediated response might be a potent negative feedback loop in response to increased cell polyploidy upon the disruption of Hippo signals. Alternatively, other effectors downstream of Yap that positively augment the p53 response possibly exist. These results suggested that Hippo signaling might play a dual role in p53 regulation, positively as a blockage of Mdm2-mediated p53 degradation by Lats1/2 but negatively as an inhibitor of the Yap-induced p53 response. It will be of particular interest to determine how Yap regulates p53 activity in future studies. In addition, a previous study showed that tetraploid cells have lower Rho activity, which is mainly due to increased Rac activation in the presence of excess microtubules nucleated by extra centrosomes (Ganem et al., 2014). Restoring Rho activity enables the cell to bypass G1 arrest. Consistently, enhanced cell-matrix adhesion, which activates Rho, is reported to reduce the G1 arrest of tetraploid cells. Interestingly, our previous work demonstrated that the Hippo kinases Mst1 and Mst2 are required for Rac activation (Geng et al., 2015). Thus, whether the reintroduction of active Rac induces the G1 arrest of tetraploid cells is an interesting open question. Moreover, a previous study reported that Mst1 limits the kinase activity of aurora B to promote stable kinetochore-microtubule attachment (Oh et al., 2010). Thus, there might be other effectors downstream of Hippo signaling that mediate polyploidy. In conclusion, our results reveal that the modulation of the Hippo signaling pathway orients hepatocytes into a specific cell-cycle program, leading to the generation of diploid or polyploid cells. It is of interest to determine whether Hippo signaling is also involved in the regulation of other polyploid cell types such as megakaryocyte polyploidization.