Strong, sustained hepatocellular proliferation precedes hepatocarcinogenesis in hepatitis B surface antigen transgenic mice

Multiple Roles for Hepatitis B and C Viruses and the Host in the Development of Hepatocellular Carcinoma

Chronic hepatitis B and C viral infections are major risk factors for hepatocellular carcinoma (HCC) in the United States and worldwide. Direct and indirect mechanisms of viral infection lead to the development of HCC. Chronic viral infection leads to inflammation and liver damage, culminating in cirrhosis, the penultimate step in the progression toward HCC. Host, viral, and environmental factors likely interact to promote oncogenesis. Clinical considerations include recommendations for screening for HCC in persons at risk, treatment with antivirals, and an emerging role for immunotherapy in HCC. We pose unanswered questions regarding HCC susceptibility and pathogenesis in the setting of chronic hepatitis B and C.

Transgenic zebrafish for modeling hepatocellular carcinoma

Liver cancer is the third leading cause of cancer‐related deaths throughout the world, and more than 0.6 million people die from liver cancer annually. Therefore, novel therapeutic strategies to eliminate malignant cells from liver cancer patients are urgently needed. Recent advances in high‐throughput genomic technologies have identified de novo candidates for oncogenes and pharmacological targets. However, testing and understanding the mechanism of oncogenic transformation as well as probing the kinetics and therapeutic responses of spontaneous tumors in an intact microenvironment require in vivo examination using genetically modified animal models. The zebrafish (Danio rerio) has attracted increasing attention as a new model for studying cancer biology since the organs in the model are strikingly similar to human organs and the model can be genetically modified in a short time and at a low cost. This review summarizes the current knowledge of epidemiological data and genetic alterations in hepatocellular carcinoma (HCC), zebrafish models of HCC, and potential therapeutic strategies for targeting HCC based on knowledge from the models.

Participation of liver stem cells in cholangiocarcinogenesis after aflatoxin B1 exposure of glutathione S-transferase A3 knockout mice

Aflatoxin B1, arguably the most potent human carcinogen, induces liver cancer in humans, rats, trout, ducks, and so on, but adult mice are totally resistant. This resistance is because of a detoxifying enzyme, mouse glutathione S-transferase A3, which binds to and inactivates aflatoxin B1 epoxide, preventing the epoxide from binding to DNA and causing mutations. Glutathione S-transferase A3 or its analog has not been detected in any of the sensitive species, including humans. The generation of a glutathione S-transferase A3 knockout (represented as KO or -/-) mice has allowed us to study the induction of liver cancer in mice by aflatoxin B1. In contrast to the induction of hepatocellular carcinomas in other species, aflatoxin B1 induces cholangiocarcinomas in GSTA3-/- mice. In other species and in knockout mice, the induction of liver cancer is preceded by extensive proliferation of small oval cells, providing additional evidence that oval cells are bipolar stem cells and may give rise to either hepatocellular carcinoma or cholangiocarcinoma depending on the nature of the hepatocarcinogen and the species of animal. The recent development of mouse oval cell lines in our laboratory from aflatoxin B1-treated GSTA3-/- mice should provide a new venue for study of the properties and potential of putative mouse liver stem cells.

Modeling human liver cancer heterogeneity: virally induced transgenic models and mouse genetic models of chronic liver inflammation

In addition to being the most common primary liver cancer, hepatocellular carcinoma (HCC) is the second most common cause of cancer-related death in humans. Treatment options are limited for this chemoresistant cancer, with liver transplantation and surgical intervention in early stages being the most successful treatments. Drug development over the past 15 years has focused on generating mouse models that mimic the human pathology for HCC. This has enabled the laboratory testing of potentially new human therapeutics. Described in this unit are the classification of HCC and an overview of hepatitis virus-related transgenic and genetically engineered mouse models (GEMMs) that are employed for elucidating the mechanism(s) responsible for the development of HCC, with particular emphasis on genetic, dietary, and environmental factors.

Biological Basis of Differential Susceptibility to Hepatocarcinogenesis among Mouse Strains

There is a vast amount of literature related to mouse liver tumorigenesis generated over the past 60 years, not all of which has been captured here. The studies reported in this literature have generally been state of the art at the time they were carried out. A PubMed search on the topic "mouse liver tumors" covering the past 10 years yields over 7000 scientific papers. This review address several important topics related to the unresolved controversy regarding the relevance of mouse liver tumor responses observed in cancer bioassays. The inherent mouse strain differential sensitivities to hepatocarcinogenesis largely parallel the strain susceptibility to chemically induced liver neoplasia. The effects of phenobarbital and halogenated hydrocarbons in mouse hepatocarcinogenesis have been summarized because of recurring interest and numerous publications on these topics. No single simple paradigm fully explains differential mouse strain responses, which can vary more than 50-fold among inbred strains. In addition to inherent genetics, modifying factors including cell cycle balance, enzyme induction, DNA methylation, oncogenes and suppressor genes, diet, and intercellular communication influence susceptibility to spontaneous and induced mouse hepatocarcinogenesis. Comments are offered on the evaluation, interpretation, and relevance of mouse liver tumor responses in the context of cancer bioassays.

Hepatitis B virus pre-S2 mutant surface antigen induces degradation of cyclin-dependent kinase inhibitor p27Kip1 through c-Jun activation domain-binding protein 1

The hepatitis B virus (HBV) large surface antigen (LHBS) mutant with deletion at the pre-S(2) region accumulates in endoplasmic reticulum (ER) and is associated with HBV-induced hepatocellular carcinogenesis. In this study, we found that the pre-S(2) LHBS mutant directly interacts with the Jun activation domain-binding protein 1 (JAB1). Association of pre-S(2) LHBS with JAB1 dissociated JAB1 from the JAB1/IRE1 complex in ER. The free (active) JAB1 then translocated into cell nuclei and rendered the Cdk inhibitor p27(Kip1) to cytosolic proteasome for degradation. The pre-S(2) LHBS mutant induced hyperphosphorylation of tumor suppressor retinoblastoma (RB) via cyclin-dependent kinase 2 (Cdk2), a downstream molecule regulated by p27(Kip1). This effect is independent of the ER stress signaling pathway. The transgenic mice carrying the pre-S(2) mutant LHBS gene also exhibited Cdk2 activation, p27(Kip1) degradation, as well as RB hyperphosphorylation. The mouse hepatocytes exhibited morphologic abnormalities such as chromatin condensation, multinucleation, and dysplasia of hepatocytes. In summary, the pre-S(2) LHBS mutant causes p27(Kip1) degradation through direct interaction with JAB1. The pre-S(2) mutant LHBS is suggested to be a potential oncoprotein for HBV-related hepatocellular carcinoma.

Sustained hepatic expression of FoxM1B in transgenic mice has minimal effects on hepatocellular carcinoma development but increases cell proliferation rates in preneoplastic and early neoplastic lesions

Increased hepatic expression of the Forkhead transcription factor FoxM1B in adult mice accelerates hepatocyte proliferation after partial hepatectomy, while in hepatocytes in intact liver the transgenic (Tg) protein is inactive and has no effect on proliferation. To investigate the influence of FoxM1B on liver tumor formation, we examined the effect of sustained enrichment of FoxM1B in the hepatocytes of mice treated with a diethylnitrosamine (DEN)/phenobarbital tumor induction protocol. Tg enrichment of FoxM1B in hepatocytes did not increase the proliferation rate in normal liver tissue even when the protein was localized to the nucleus. However, it did cause an increase in the proliferation rate and size of preneoplastic and early neoplastic lesions, although having no effects on the total numbers of these lesions. As tumors progressed to hepatocellular carcinomas, the additional Tg FoxM1B protein had no effect on cell proliferation, and there was no increase in tumor burden compared to wild-type animals. This suggests that the artificial enrichment of FoxM1B in the liver, which has been suggested as a gene therapy protocol for liver dysfunction with aging, may not be tumorigenic in that organ.

Synergy between truncated c-Met (cyto-Met) and c-Myc in liver oncogenesis: importance of TGF-beta signalling in the control of liver homeostasis and transformation

The c-Met tyrosine kinase receptor and its ligand, Hepatocyte Growth Factor/ Scatter Factor, have been implicated in human cancer. We have previously described that the transgenic expression of a truncated form of human c-Met (cyto-Met) in the liver confers resistance to several apoptotic stimuli. Here we show the impact of cyto-Met expression on liver proliferation and transformation. Despite a sixfold increase of hepatocyte proliferation, adult transgenic livers displayed normal size and architecture. We present evidence showing that activation of TGF-beta1 signalling controls the liver mass in cyto-Met mice. The oncogenic potential of cyto-Met was further assessed in the context of c-Myc-induced hepatocarcinogenesis, using WHV/c-Myc transgenic mice. Co-expression of cyto-Met and c-Myc further enhanced hepatocyte proliferation and caused a dramatic acceleration of the Myc-induced tumorigenesis, leading to the emergence of hepatocarcinomas in 3-4-month-old animals. Importantly, the TGF-beta receptor type II expression was strongly downregulated in most tumours, indicating that impairment of TGF-beta1-mediated growth inhibition plays a major role in accelerated neoplastic development. The strong potential of cyto-Met for oncogenic cooperation without direct transforming activity designates cyto-Met mice as an ideal tool for studying the early steps of multistage hepatocarcinogenesis and for identification of prognostic markers of transformation.