Elimination of Tumor Suppressor Proteins during Liver Carcinogenesis

Abstract

Liver cancer is one of the most lethal cancers. Quiescent liver expresses up to 20 tumor suppressor proteins including Rb, p53, CCAAT-Enhancer-Binding Protein (C/EBP)α, Hepatocyte Nuclear Factor (HNF4)α and p16 and it is well protected from development of liver cancer. However, the negative control of liver proliferation by these factors and other tumor suppressor genes is eliminated in liver cancer. Studies of liver regeneration after surgery and injury have provided fundamental mechanisms on how liver neutralizes tumor suppressor proteins for the time of regeneration; however, studies of liver cancer in animal models and in human samples showed several additional pathways of this neutralization. One of these addi- tional pathways includes activation of a small subunit of the proteasome, Gankyrin. Gankyrin is dramatically increased in human hepatocellular carcinoma (HCC) and in animal models of car- cinogenesis. Once activated Gankyrin triggers degradation of main tumor suppressor proteins during development of liver cancer using slightly different mechanisms. Recent studies identi- fied mechanisms which repress Gankyrin in quiescent livers and mechanisms of activation of Gankyrin in liver cancer. These mechanisms involve a communication between Farnesoid X Receptor (FXR) signaling and chromatin remodelling proteins mediated by members of C/EBP family. It has been recently shown that C/EBPα plays a critical role in this network and that the activation of C/EBPα in cirrhotic livers with HCC inhibits cancer progression. This C/EBPα- dependent inhibition of liver cancer involves activation of a majority of tumor suppressor genes and repression of tumor initiating pathways such as β-catenin and c-myc. These recent findings provide a background for FXR-based and C/EBPα-based approaches to treat liver cancer.