As a first step to study the functional crosstalk between YAP and mTORC1 in HB, we analyzed the protein levels of β-catenin, YAP, TAZ (a paralog of YAP), and members of the mTOR pathway using two human HB cell lines, namely Hep293TT and HepG2 cells [18, 19]. The SNU449 human HCC cell line that harbors a mutation in the Pten tumor suppressor gene (thus displaying a constitutively high mTOR activity) was used as positive control. Both Hep293TT and HepG2 HB cell lines have N-terminal truncated forms of β-catenin, which could be readily detected as a lower band by Western blotting (Figure 1A). Total and nuclear YAP and TAZ were found to be expressed in the three cell lines, supporting the activation of the two protooncogenes in these cells (Figure 1A and 1C). Activation of mTORC1 in Hep293TT and HepG2 cells was evidenced by the elevated levels of phosphorylated/activated (p)-mTOR, phosphorylated/inactivated (p)-4EBP1, phosphorylated/activated (p)-S6K and phosphorylated/activated (p)-RPS6 (Figure 1A). Furthermore, levels of mTORC2 targets, including phosphorylated (p)-NDRG1 and phosphorylated/activated (p)-AKT (S473) were elevated, thus indicating the activation of mTORC2 in Hep293TT and HepG2 cells. In contrast, levels of PDK1 substrate phosphorylated/activated (p)-AKT (T308) were very low/undetectable by Western blotting in Hep293TT and HepG2 cell lines, whereas they were induced in SNU449 HCC cells (Figure 1A). Next, we analyzed the expression of these pathways in YAP/β-catenin-induced mouse HB tumor tissues (Figure 1B-1D). Consistently, truncated β-catenin and nuclear YAP and TAZ could be found in mouse HB tissues. In addition, levels of p-mTOR, p-S6K, p-RPS6 and p-4EBP1 were all higher in HB tumor tissues than normal liver tissues, supporting the activation of mTORC1 in YAP/β-catenin-induced HB tumors.