Functions Of Bmi1 In Hepatic Carcinogenesis And Hepatic Stem Cell Expansion

Objectives:Bmil is a polycomb family proto-oncogene that has been implicated in multiple tumor types. However its role in HCC development and liver stem cell proliferation has not been well-studied. In this study, we are trying to explore the functions of Bmi1 in hepactic carcinogenesis and hepatic stem cell expansion.Methods:1. We applied qRT-PCR and western blotting to detect the expression of Bmi1 in human HCC tissue. Then we used the shRNA to knock down the expression of Bmi1 in HCC cell lines and to evaluate the impact of loss of Bmi1 on HCC cell growth. To investigate the role of Bmil in promoting liver cancer development in vivo, we stably expressed Bmi1 and/or the activated form of Ras (RasV12) in mouse liver. Mechanism analyis was finished by checking the expression of Ink4A/Arf in human and mice HCC tissues, Bmi1 knock down cell lines and primary mouse hepatocytes.2. To illustrate the role of Bmi1 in maintaining the growth of oval cell, we set up a mouse model. In this mouse model, both Bmi1 WT and KO mice were fed with DDC, a chemical to induce oval cell expansion. By comparing the phenotype of oval cell expansion between Bmi1 WT and Bmi1 KO mice, we can determine the role of Bmi1 in liver stem cell proliferation.Results:1. We found that Bmi1 is over-expressed in human HCC samples as well as HCC cell lines. When Bmi1 expression is knocked down in human HCC cells lines, it significantly inhibits HCC cell proliferation and perturbs cell cycle regulation. In mouse model, while Bmi1 or RasV12 alone is not sufficient to promote liver cancer development, co-expression of Bmi1 plus RasV12 induces HCC formation in mice. Intriguingly, we found no evidence that Bmi1 inhibits Ink4A/Arf expression in both in vitro and in vivo systems of liver tumor development.2. We found that in Bmi1 null mice, the oval cell, a potential hepatic stem cell, showed decreased proliferation capability. In wild type mice, liver injury resulted from DDC treatment is able to induce marked proliferation of oval cells. In Bmil knock-out mice, however, the cell numbers of oval cell induced by liver injury are decreased due to loss of Bmil.Conclusions:1. Bmi1 is required for HCC cell proliferation in vitro and it cooperates with other oncogenes to promote hepatic carcinogenesis in vivo. Bmi1 functions as an oncogene independent of Ink4A/Arf repression in liver cancer development; 2. Bmi1 is required for the proliferation of oval cells and is a potential mediator of oval cell expansion in mouse livers.