Activation of Wnt/β-
catenin signaling is important for human and rodent hepatocarcinogenesis. In mice, the
tumor promoter phenobarbital (PB) selects for hepatocellular
tumors with activating β-
catenin mutations via
constitutive androstane receptor activation. PB-dependent
tumor promotion was studied in mice with genetic inactivation of Apc, a negative regulator of β-
catenin, to circumvent the problem of randomly induced mutations by chemical initiators and to allow monitoring of PB- and Wnt/β-
catenin-dependent
tumorigenesis in the absence of unknown genomic alterations. Moreover, the study was designed to investigate PB-induced proliferation of liver cells with activated β-
catenin. PB treatment provided Apc-deficient hepatocytes with only a minor proliferative advantage, and additional
connexin 32 deficiency did not affect the proliferative response. PB significantly promoted the outgrowth of Apc-deficient
hepatocellular adenoma (HCA), but simultaneously inhibited the formation of Apc-deficient
hepatocellular carcinoma (HCC). The probability of
tumor promotion by PB was calculated to be much lower for hepatocytes with loss of Apc, as compared to mutational β-
catenin activation. Comprehensive transcriptomic and phosphoproteomic characterization of HCA and HCC revealed molecular details of the two
tumor types. HCC were characterized by a loss of differentiated hepatocellular gene expression, enhanced proliferative signaling, and massive over-activation of Wnt/β-
catenin signaling. In conclusion, PB exerts a dual role in liver
tumor formation by promoting the growth of HCA but inhibiting the growth of HCC. Data demonstrate that one and the same compound can produce opposite effects on hepatocarcinogenesis, depending on context, highlighting the necessity to develop a more differentiated view on the tumorigenicity of this model compound.