Emerging roles of the RB/E2F pathway in fatty liver disease and cancer

Abstract

Liver cancer in humans is ranked number five concerning cancer related deaths accounted worldwide. Many risk factors related to liver cancer have been identified including hepatitis virus infection, exposure to mycotoxins, and fatty liver disease. These risk factors predispose livers to develop cancer in part by modifying cell’s metabolism and tumor suppression mechanism. The studies contained in this thesis focused on understanding the role of RB/E2F pathways in postnatal development, fatty liver disease and liver cancer by utilizing conditional mice models. The first study covers investigations on the role of atypical E2Fs in the development of fatty liver disease. Deletion of E2f7 and E2f8 genes in the mouse with liver specific deficiency of PTEN, a model of nonalcoholic fatty liver disease limited the ability of hepatocytes to store lipids and prevented the development of fatty liver disease. Moreover, an extended study to explore possible cooperation between atypical E2Fs and the tumor suppressor PTEN, a member of an important pathway that is frequently altered in liver cancer was conducted. Interestingly, combined deletion of atypical E2Fs and PTEN accelerate the formation of hepatocellular carcinoma (HCC). Furthermore, liver specific inactivation of Pten was accompanied by increased expression of p53-dependent stress response genes, including P21. Remarkably this stress response was partially rescued through additional deletion of E2f7/8, indicating that atypical E2Fs contribute to a proper p53 stress response in hepatocytes. Together, the study showed that atypical E2Fs and PTEN cooperate in suppressing liver tumor formation, potentially through antagonistic regulation of the p53-stress response. Atypical E2Fs are known to be indispensable for mouse embryonic development, but it was unknown whether atypical E2F function is also critical for the postnatal survival of mice. Therefore, another study focused in understanding the role of atypical E2Fs in postnatal development by simultaneously deleting E2f7 and E2f8 in newborn pups utilizing a tamoxifen inducible knockout approach. Surprisingly these mice lived healthy until old age. Notably, the exocrine and endocrine pancreas cells displayed less polyploidization without major impact on the production of pancreatic hormones and enzymes. Reduction in polyploid was extended to hepatocyte, confirming previous findings in liver deficient of atypical E2Fs. This study demonstrate that atypical E2Fs are essential for polyploidization in the pancreas and liver, while other polyploid cells such as megakaryocytes were not affected. Finally, the last study in this thesis discovered that therapeutic approaches aimed to remove tumor tissue in the liver, can actually initiate the formation of undifferentiated malignant liver tumors at the site of surgical intervention. The study could demonstrate that liver cancer develops at the site of surgical or radiofrequency ablation (RFA). Furthermore, post-intervention follow up and lineage tracing studies, could demonstrate that, cancer initiation occurred as a result of specific migration, expansion and transformation of cytokeratin-19+-liver (CK-19+) cells at the necrotic and hypoxic surgical intervention site. The discoveries in this study suggest that patients with non-functional mutations for p53 and Rb in the liver at the time of surgical or thermal intervention are at risk of developing tumors at the intervention site.