Effect ofp53 loss,p53 mutation and aflatoxin exposure on the control of hepatocyte proliferation in transgenic mice

Hepatocellular carcinoma (HCC, hepatoma) is one of the most lethal cancers in humans. The incidence of HCC is as high as 30 new cases per 100,000 men per year in high risk regions such as Qi Dong of China and Mozambique. The incidence of HCC is rising in the United States. Epidemiological studies suggest that chronic hepatitis related liver injury and aflatoxin contamination in food and drinking water are the major risk factors in the development of liver cancer. HCC from areas with high hepatitis virus carrier rate and high aflatoxin exposure often has high p53 mutation rate at codon 249. Previous experiments demonstrate that p53 loss, p53 mutation and aflatoxin exposure each contribute to the development of liver cancer in a transgenic mouse model. Further studies using proliferating cell nuclear antigen (PCNA) immunostaining indicate that loss of p53 increases hepatocyte proliferation in adult mouse livers as compared to wild-type controls, and mutation of p53 at codon 246 (equivalent to p53 mutation at codon 249 in humans) leads to accumulation of hepatocytes in the G1 phase of the cell cycle as compared to the p53 null mice. Here, experiments were undertaken to: (1) examine the possible mechanisms underlying the increased hepatocyte proliferation in p53 deficient mice. (2) investigate G1 phase hepatocyte accumulation in p53 mutant mice. (3) study the effect of neonatal aflatoxin exposure on hepatocyte proliferation in p53 wild-type mice. Mice of different p53 genotypes at three months of age were produced and used for the first two aims. For the third aim, mice were injected peritoneally with AFB1 at 7 days of age and sacrificed at 6 weeks of age. Our results suggest that p27 downregulation may be the major factor contributing to the hepatocyte proliferation in p53 deficient mice at 3 months of age. Although p21 is a direct downstream mediator of p53, its expression is low in p53 wild type mouse livers and is not significantly decreased in p53 deficient mouse livers. Introduction of a p53 mutant transgene increases expression of p21 and p27 as compared to their null counterparts. However, only a small percentage of hepatocytes express mutant p53 protein. Cells with increased expression of p21 and p27 could be blocked in G1 and account for the hepatocyte accumulation in G1 phase in p53 mutant mice. Our results also demonstrate that neonatal aflatoxin exposure can lead to prolonged hepatocyte and ductual cell proliferation in adult mice. The potential molecular interplay of p53 loss, p53 mutation, chronic hepatitis related liver injury and aflatoxin exposure in eventually causing dysregulated cell cycle control and the development of liver cancer is discussed in the context.