Activation of beta-catenin, the central effector of the canonical Wnt pathway and a recognized oncogene, has been implicated in hepatocellular carcinoma. We examined N-nitrosodiethylamine (DEN)-induced tumorigenesis in hepatic beta-catenin conditional knockout mice (beta-cat KO). Male beta-cat KO and age- and sex-matched littermate controls were given a single intraperitoneal DEN injection and followed for 6-12 months for hepatic tumors. Hepatic tumors were characterized for histology, proliferation, apoptosis, oxidative stress, and specific proteins by way of western blot, immunohistochemistry, and coprecipitation studies. For in vivo tumor intervention studies, specific inhibitors were administered intraperitoneally or through drinking water. Intriguingly, beta-cat KO mice showed a paradoxical increase in susceptibility to DEN-induced tumorigenesis. This accelerated tumorigenesis is due to increased injury and inflammation, unrestricted oxidative stress, fibrosis, and compensatory increase in hepatocyte proliferation secondary to platelet-derived growth factor receptor alpha (PDGFRalpha)/phosphoinositide 3-kinase (PIK3CA)/Akt activation and c-Myc overexpression. In vitro suppression of beta-catenin expression in hepatoma cells led to enhanced PDGFRalpha expression, which was abrogated in the presence of nuclear factor kappaB (NF-kappaB) inhibitor. Daily treatment of 6-month-old DEN-exposed beta-cat KO with PDGFRalpha inhibitor dramatically reduced tumor numbers and size. Inclusion of N-acetyl-L-cysteine, a known antioxidant and NF-kappaB inhibitor, in the drinking water led to complete abolition of tumorigenesis in DEN-exposed beta-cat KO.
Conclusion: Loss of beta-catenin impairs the liver's ability to counteract DEN-induced oxidative stress and enhances tumorigenesis through PDGFRalpha/PIK3CA/Akt signaling. Blockade of PDGFRalpha or oxidative stress dramatically affects beta-catenin-deficient tumorigenesis. Also, hepatoma cells use PDGFRalpha/PIK3CA signaling as an escape mechanism following beta-catenin suppression, and their sequential suppression profoundly impedes tumor proliferation.