In vitro progression-associated c-H-ras activation in neoplastic hepatocyte lines established from SV40-T antigen gene-harboring transgenic mice

Conditional transformation of mouse liver epithelial cells. An in vitro model for analysis of genetic events in hepatocarcinogenesis

Primary rodent and human hepatocytes have a very limited lifespan in culture and are not readily applicable to transformation studies in vitro. To facilitate the investigation of early genetic events involved in hepatocarcinogenesis, we examined a transformation assay system utilizing conditionally immortalized mouse liver epithelial cells as an alternative to primary hepatocytes. By infecting primary mouse hepatocytes with a recombinant retrovirus carrying a temperature-sensitive simian virus 40 large T antigen gene, two mouse liver epithelial cell lines, CHST8 and CHST10-2.1, were established. Because of the heat-labile nature of the large T antigen, the cell lines proliferated rapidly at 33 degrees C, but were growth-arrested at 39 degrees C. Because activated c-H-ras and c-myc oncogenes are frequently found to be involved in mouse hepatocarcinogenesis in vivo, we assessed whether those oncogenes can complement the immortalizing function of the large T antigen at the nonpermissive temperature. When CHST8 cells were doubly transfected with activated c-H-ras and c-myc at 33 degrees C, they exhibited clonal growth ability even after shifting the temperature to 39 degrees C. However, neither c-H-ras nor c-myc alone allowed growth at 39 degrees C. On the other hand, c-H-ras alone was sufficient for overcoming the growth defect of CHST10-2.1 cells at 39 degrees C, whereas c-myc alone was again ineffective. Northern blot studies revealed that endogenous c-myc expression was significantly downregulated in the parental CHST8 cells after a temperature shift from 33 to 39 degrees C. In contrast, in the parental CHST10-2.1 cells, appreciable c-myc expression was observed at both temperatures. These results indicate that c-H-ras and c-myc can cooperate in complementing the ability of the temperature-sensitive large T antigen to immortalize mouse liver cells at the nonpermissive temperature. In addition, the mutant c-H-ras, but not c-myc, cooperated with the functional T antigen at 33 degrees C to allow growth in soft agarose of the CHST8 and CHST10-2.1 cell lines. However, cell lines carrying mutant c-H-ras and overexpressing c-myc were unable to grow in soft agarose at 39 degrees C. Thus, the two cellular oncogenes were insufficient for full transformation of the liver epithelial cells. The present in vitro model should be useful for investigating molecular events involved in both early and late stages of hepatocarcinogenesis.

Frequent loss of heterozygosity on chromosome 4 in diethylnitrosamine-induced C3H/MSM mouse hepatocellular carcinomas in culture

Genetic changes, in particular the loss of heterozygosity (LOH) and the presence of c-Ha-ras codon 61 point mutations, were investigated in diethylnitrosamine-induced hepatocellular carcinomas (HCCs) in C3H/MSM F1 mice. (MSM are wild mice.) LOH analysis of 48 primary tumors with microsatellite probes covering at least one proximal and one distal site of each autosome revealed no obvious positive results for LOH. Analysis of 23 cell lines established from seven of these HCCs, however, showed LOH on chromosome 4 in all (seven of seven), even in early passages (G2-G3). With regard to other chromosomes, LOH was observed only rarely on chromosomes 16 and 19. These allelotype features were maintained in later passages (G11-G14), with only a few additional occurrences of LOH appearing on chromosomes 1, 6, and 8. Extensive analyses with multiple microsatellite probes from chromosome 4 and with 52 cell lines established from 24 HCCs of 18 mice revealed LOH in 22 of the tumors (92%), with the shortest region about 10 cM distal to the alpha-interferon gene. No c-Ha-ras oncogene activation in codon 61 was observed. These data indicate that loss of tumor suppressor genes on chromosome 4 may play an important role in mouse hepatocarcinogenesis in progression in vivo or in immortalization in vitro or both.

The structure and function of the H-ras proto-oncogene are not altered in rat liver tumors initiated by 2-acetylaminofluorene, 2-acetylaminophenanthrene and trans-4-acetylaminostilbene

Liver tumors were generated in Wistar rats in an initiation-promotion experiment. 2-Acetylaminofluorene (AAF), 2-acetylaminophenanthrene (AAP), and trans-4-acetylaminostilbene (AAS) were administered to newborn animals as initiators, and phenobarbital as a promoter was added to the drinking water after weaning. Livers were examined after 26, 52, 78, and 104 weeks. Tumors were present in all groups except for at the first time point. The potency of the initiators decreased in the order AAS > AAP > AAF. DNA from tumors of all groups and of control livers was analyzed for mutations in the H-ras gene, but no mutations could be found. The sequence of almost the entire H-ras gene was determined and was compared to other H-ras genes. There are some differences with the sequence in other rat strains, particularly in intron D containing the alternative splicing site. The expression of the H-ras gene has also been studied by various methods in enzyme altered foci and tumors, but no alterations could be found. It is, therefore, concluded that structural of functional alterations of this proto-oncogene are not involved in the generation of liver tumors in Wistar rats by the three genotoxic arylamines.

Frequent spontaneous sister chromatid exchange in hepatocytes of transgenic mice harboring the SV40-T antigen gene

In order to shed light on the causal mechanisms of hepatocarcinogenesis in the transgenic mouse into which the albumin-promotor-regulated SV40-T antigen gene has been introduced (T+ mouse), and especially on the frequent chromosomal aberrations seen in cultured hepatocytes and hepatocellular neoplasms derived from such animals, the frequency of sister chromatid exchange (SCE) and karyotype abnormalities were investigated in a hepatocyte primary culture system. Cells were obtained through collagenase perfusion from T+ mice at 16-18 days of age, when no morphological changes are apparent, and from nontransgenic littermates, and cultured in the presence of bromodeoxyuridine. SCE was seen in transgenic hepatocytes twice as frequently as in their normal counterparts. No karyotype abnormalities in terms of numerical change or gross aberration were detected at this phase. The results thus suggest mutagenic properties for the T antigen, which may play an important role in hepatocarcinogenesis in this transgenic mouse.

p53 mutations are absent from carcinogen-induced mouse liver tumors but occur in cell lines established from these tumors

Mutations in the p53 gene are frequent genetic alterations in human hepatocellular carcinomas. We have examined, by single-strand conformation polymorphism analysis of polymerase chain reaction products, a total of 93 carcinogen-induced liver tumors from mice of three different strains (C3H/He, C57BL/6J, and B6C3F1) for the presence of p53 aberrations. Hepatoma lines, established from 12 liver tumors, were also included in the analysis. While structural aberrations of the p53 gene were not detected in any of the primary mouse liver tumors analyzed, single-base substitutions occurred at different locations within the p53 gene in three of the cell lines during in vitro propagation. One hepatoma line carried two point mutations on separate alleles. All four mutations were either G:C----C:G or C:G----G:C transversions. Mutations at codon 61 of the c-Ha-ras gene, which were frequent in primary liver tumors from C3H/He and B6C3F1 mice, were not detected in the hepatoma lines. Our data indicate (i) that c-Ha-ras but not p53 mutations play an important role during the early stages of mouse hepatocarcinogenesis and (ii) that p53 mutations confer a selective growth advantage to the mutated hepatoma cells in vitro.

Transgenic models of tumor development

Numerous cancer-prone strains of mice have been created by the introduction of candidate tumor-promoting genes into fertilized eggs. Each transgenic strain is predisposed to develop specific types of tumors, but they usually arise stochastically because of the need for spontaneous mutation of genes that collaborate with the introduced oncogene. These mice are providing insights into the effects of individual oncogenes on cellular proliferation, differentiation, and viability, as well as on oncogene cooperativity. Their predisposed state imposes sensitivity to viral and chemical carcinogenesis, and the mice should prove valuable in tests of potential carcinogens, therapies, and preventive measures.

Hepatocarcinogenesis in transgenic mice carrying albumin-promoted SV40 T antigen gene

We have developed transgenic mice that inherit albumin promoter-regulated simian virus 40 (SV40) large T antigen gene, expressed specifically in hepatocytes. These mice all develop multifocal hepatocellular carcinomas at around 5 months and die of liver insufficiency by 7 months. Sequential morphological observation of hepatocarcinogenesis revealed 5 distinct stages: (I) newborn to 2 weeks of age, neither recognizable histological changes nor cellular replication in spite of T antigen expression; (II) between 3 and 7 weeks, diffuse cytomegalic change of hepatocytes with numerous abnormal mitoses, usually resulting in cell death; (III) from 7 weeks onwards, quasi-regenerative small hepatocyte foci with a decreased tendency for cytomegaly in spite of T antigen expression, rapidly replacing the hepatic tissue; (IV) 11 weeks of age and thereafter, neoplastic foci and nodules with enzymatic alteration; (V) 20 weeks of age and thereafter, gross hepatocellular carcinomas with occasional pulmonary metastases. Considerable variation existed both in morphological and enzymatic features and T antigen expression among neoplastic lesions, including carcinomas. Thus, these transgenic mice clearly show a multistep process in hepatocarcinogenesis with remarkable synchrony and provide a promising model for analyzing the essential events of carcinogenesis at different stages.

Cytogenetic analysis of hepatic cell lines derived from SV40-T antigen gene-harboring transgenic mice

A total of five hepatic cell lines were established from SV40-T transgenic mice at different stages of development. Karyotype analysis performed on these cell lines revealed 1) chromosomal instability manifested by numerous karyotypic alterations, 2) non-random numerical changes of chromosome number 18 and 3) frequent occurrence of marker chromosomes in four of the five lines. These findings indicate that genetic abnormalities occur very frequently in hepatocytes of the transgenic mice, most probably caused by the mutagenic action of T-antigen, raising the possibility that they play essential roles in initiation and progression in this model of murine hepatocarcinogenesis.