Interactive effects of c-myc and transforming growth factor alpha transgenes on liver tumor development in simian virus 40 T antigen transgenic mice

Quantifying growth and transformation frequency of oncogene-expressing mouse hepatocytes in vivo

Gene changes can affect cancer cells in many ways, but changes that increase disease severity--by allowing cells to proliferate when they should be quiescent, by enhancing their rate of growth under growth permissive conditions, or by increasing the risk that they will accumulate additional carcinogenic alterations--must be identified so that strategies to counter their effects can be developed. We describe a novel in vivo assay system based on hepatocyte transplantation that permits us to accomplish this objective for genetically modified hepatocytes. We find that the oncogenes c-myc and transforming growth factor alpha, but not simian virus 40 T-antigen, increase the rate of hepatocyte growth under growth permissive conditions. However, no single oncogene can induce hepatocyte growth in quiescent liver. In contrast, at least one oncogene combination, transforming growth factor alpha/T-antigen, was sufficient to direct cell autonomous growth even in this nonpermissive environment. Furthermore, we could quantify risk for progression to neoplasia associated with oncogene expression; increased transformation frequency was the principal carcinogenic effect of T-antigen.

CONCLUSION

This system identifies biological mechanistic role(s) in carcinogenesis for candidate genetic changes implicated in development of human liver cancer. The quantitative and comparative evaluation of gene effects on liver cancer allows us to prioritize targets for therapeutic intervention.

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.

Altered differentiation of hepatocytes in a transgenic mouse model of hepatocarcinogenesis

Transgenic mice carrying the SV40 T antigen (TAg) gene, which develop hepatocellular and biliary cell tumors by 4 mo of age, show ductular structures in the neonatal liver. Coexpression of c-myc with TAg increases the extent and persistence of ductular lesions and also accelerates tumor development. To analyze possible links between altered gene expression and cell differentiation and to determine the relationship between the ductular structures and tumor development in these mice, ductular cells in single (TAg) and bitransgenic (TAg x c-myc) mice were characterized for biliary and hepatocellular differentiation, transgene expression, and proliferation activity. The results show that the ductular cells in these transgenic mice have characteristics of biliary cells, including basement membrane formation, positive laminin staining, and bile duct-specific lectin (Dolichos biflorus agglutinin and peanut agglutinin) binding, and characteristics of hepatocytes, including albumin expression and ultrastructural features such as round nuclei with 1 or 2 nucleoli and well-developed cytoplasmic organelles. However, differences in transgene expression and cell proliferation between the ductular cells and nonductular hepatocytes were not apparent. Thus, the ductular cells could not be defined as tumor progenitor cells in these mouse livers. However, this model suggests that manipulation of gene expression can alter differentiation of hepatic parenchymal cells.