Hepatic ribosomal protein S6 (Rps6) insufficiency results in failed bile duct development and loss of hepatocyte viability; a ribosomopathy-like phenotype that is partially p53-dependent.

Defective ribosome biogenesis (RiBi) underlies a group of clinically diverse human diseases collectively known as the ribosomopathies, core manifestations of which include cytopenias and developmental abnormalities that are believed to stem primarily from an inability to synthesize adequate numbers of ribosomes and concomitant activation of p53. The importance of a correctly functioning RiBi machinery for maintaining tissue homeostasis is illustrated by the observation that, despite having a paucity of certain cell types in early life, ribosomopathy patients have an increased risk for developing cancer later in life. This suggests that hypoproliferative states trigger adaptive responses that can, over time, become maladaptive and inadvertently drive unchecked hyperproliferation and predispose to cancer. Here we describe an experimentally induced ribosomopathy in the mouse and show that a normal level of hepatic ribosomal protein S6 (Rps6) is required for proper bile duct development and preservation of hepatocyte viability and that its insufficiency later promotes overgrowth and predisposes to liver cancer which is accelerated in the absence of the tumor-suppressor PTEN. We also show that the overexpression of c-Myc in the liver ameliorates, while expression of a mutant hyperstable form of p53 partially recapitulates specific aspects of the hepatopathies induced by Rps6 deletion. Surprisingly, co-deletion of p53 in the Rps6-deficient background fails to restore biliary development or significantly improve hepatic function. This study not only reveals a previously unappreciated dependence of the developing liver on adequate levels of Rps6 and exquisitely controlled p53 signaling, but suggests that the increased cancer risk in ribosomopathy patients may, in part, stem from an inability to preserve normal tissue homeostasis in the face of chronic injury and regeneration.

Hepatoblastoma modeling in mice places Nrf2 within a cancer field established by mutant ß-catenin.

Aberrant wnt/ß-catenin signaling and amplification/overexpression of Myc are associated with hepatoblastoma (HB), the most prevalent type of childhood liver cancer. To address their roles in the pathogenesis of HB, we generated mice in which Myc and mutant ß-catenin were targeted to immature cells of the developing mouse liver. Perinatal coexpression of both genes promoted the preferential development of HBs over other tumor types in neonatal mice, all of which bore striking resemblance to their human counterparts. Integrated analysis indicated that tumors emerged as a consequence of Myc-driven alterations in hepatoblast fate in a background of pan-hepatic injury, inflammation, and nuclear factor (erythroid-derived 2)-like 2/Nrf2-dependent antioxidant signaling, which was specifically associated with expression of mutant ß-catenin but not Myc. Immunoprofiling of human HBs confirmed that approximately 50% of tumors demonstrated aberrant activation of either Myc or Nfe2l2/Nrf2, while knockdown of Nrf2 in a cell line-derived from a human HB with NFE2L2 gene amplification reduced tumor cell growth and viability. Taken together, these data indicate that ß-catenin creates a protumorigenic hepatic environment in part by indirectly activating Nrf2 and implicate oxidative stress as a possible driving force for a subset of ß-catenin-driven liver tumors in children.

Induction of hepatocyte proliferation and death by modulation of T-Antigen expression.

Mice expressing SV40 T-Antigen in liver under control of the phosphoenolpyruvate carboxykinase promoter were generated. By altering the carbohydrate content of the diet, TAg expression, the rate of hepatocyte proliferation and apoptosis, and hence hepatocarcinogenesis, could be regulated. Carbohydrate-mediated suppression of TAg resulted in slow hepatic growth that progressed to focal hepatocellular carcinoma (HCC) after a long latency period. In contrast, induction of TAg by feeding mice a low carbohydrate diet resulted in massive hepatomegaly that progressed rapidly to diffuse multifocal HCC. Hepatic TAg expression could be efficiently repressed by switching mice from the low to the high-carbohydrate diet, which if instigated prior to the development of HCC, resulted in rapid regression through a p53-independent reduction in hepatocyte proliferation and an increase in hepatocyte apoptosis. Although liver growth was accompanied by compensatory hepatocyte apoptosis, an apoptotic deficit developed following chronic exposure to high levels of TAg. This was associated with Akt phosphorylation and increased expression of the antiapoptotic molecules bfl-1/A1, TIAP, and A20. Mice were resistant to Fas-induced hepatocellular apoptosis due to severely impaired caspase activation and failed activation of the mitochondrial amplification loop. This model will be useful to investigate oncogene-mediated disruption of the cell cycle and apoptosis, and to determine which processes constitute fixed, or reversible aspects of the tumorigenic process.