Role of the Ha-ras gene in the malignant transformation of rat liver oval cells

Proteins related to early changes in carcinogenesis of hepatic oval cells after treatment with methylnitronitrosoguanidine

Hepatic oval cells are considered as facultative progenitor/stem cells of liver and able to differentiate into either hepatocytes or biliary epithelial cells. The transformed oval cells by carcinogen possess potential to develop carcinomas in animal models. In order to better understand the molecular mechanism in carcinogenetic process, we used a proteomic approach to assess the early changes in protein expression of oval cells (OC3W3-15) initiated by methylnitronitrosoguanidine (MNNG). Meanwhile, we compared cell biologic characteristics of the MNNG treated OC3W3-15 cells and control oval cells by electron microscopy, flow cytometry, karyotype and soft agar assay. The mRNA levels of GGT and GSTP1 determined by real-time PCR were also detected in both cell lines. Our results showed that MNNG-treated OC3W3-15 cells exhibited characteristics of malignant transformation, including growth rate, chromosomal aberrations, abnormal DNA content, and the ability to form colonies. The cells expressed higher levels of the tumor marker AFP, GGT and GSTP1 mRNA than that of control cells. Significant changes of several proteins involved in the malignant transformation process, including cell cycle related proteins, proteins involved in organism development and cell differentiation, are found in OC3W3-15 cells. The proteins may provide early affection in malignant transformation of hepatic oval cells, and yield further insight into mechanism of carcinogenesis of hepatocellular carcinoma.

Generation and analysis of genetically defined liver carcinomas derived from bipotential liver progenitors

Hepatocellular carcinoma is a chemoresistant cancer and a leading cause of cancer mortality; however, the molecular mechanisms responsible for the aggressive nature of this disease are poorly understood. In this study, we developed a new liver cancer mouse model that is based on the ex vivo genetic manipulation of embryonic liver progenitor cells (hepatoblasts). After retroviral gene transfer of oncogenes or short hairpin RNAs targeting tumor suppressor genes, genetically altered liver progenitor cells are seeded into the liver of otherwise normal recipient mice. We show that histopathology of the engineered liver carcinomas reveals features of the human disease. Furthermore, representational oligonucleotide microarray analysis (ROMA) of murine liver tumors initiated by two defined genetic hits revealed spontaneously acquired genetic alterations that are characteristic for human hepatocellular carcinoma. This model provides a powerful platform for applications like cancer gene discovery or high-throughput preclinical drug testing.

The role of wild-type and mutated N-ras in the malignant transformation of liver cells

In order to determine the role of N-ras overexpression and mutation in malignant liver cell transformation, wild-type and mutated N-ras were transfected into the rat liver epithelial cell line OC/CDE 22, and N-ras expression, growth kinetics, growth in soft agar, and tumorigenicity in vivo as well as the involvement of the mitogen-activated protein kinase (MAPK) signal transduction pathway in the expression of the malignant phenotype were analyzed. Although OC/CDE 22 cells transfected with wild-type N-ras showed a high expression of N-ras at the mRNA and protein levels, the cells did not grow in soft agar and were not tumorigenic in vivo. In contrast, OC/CDE 22 cells transfected with mutated N-ras showed anchorage-independent growth and were tumorigenic. When cultured in fetal bovine serum-supplemented medium, OC/CDE 22 cells expressing mutant N-ras showed a higher proliferation rate than nontransfected OC/CDE 22 cells or OC/CDE 22 cells transfected with wild-type N-ras. When held in serum-free medium, untreated OC/CDE 22 cells did not grow at all, while OC/CDE 22 cells transfected with wild-type or mutant N-ras proliferated at a similar rate, which can be explained by the high MAPK activity in these cells. Selective inhibition of the MAPK cascade abolished the growth of OC/CDE 22 cells carrying mutant N-ras in soft agar; furthermore, these cells ceased pile up and formed monolayers on Petri dishes. Thus, activation of the MAPK signaling pathway, though alone not sufficient to malignantly transform liver cells (as shown in liver cells overexpressing wild-type N-ras), is not only essential for growth control but also for the expression of the malignant phenotype (as demonstrated in liver cells transformed by mutated N-ras).