Characterization of an epithelial, nearly diploid liver cell strain, from Chinese hamster, able to activate promutagens

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Abstract
Epithelial liver cells of the Chinese hamster (CHEL cells) were propagated in culture for 35 passages. At favourable cell densities, the population doubling time in normal medium, was 20 h. L-Tyrosine amino transferase activity was retained at a measurable level, but its enhancement by dexamethasone was detected solely in cells of early passages. Pyruvate kinase was strongly activated by fructose-1,6-biphosphate at low substrate concentrations. These enzymatic properties suggest that the CHEL cells are derived from a sub-population of parenchymal hepatocytes or from cells closely related to parenchymal hepatocytes. With a lag period of a few hours, CHEL cultures metabolized benzo[a]pyrene. In cell homogenates the various monooxygenase activities investigated were below the detection limits. However, other xenobiotic—metabolizing activities, such as cytochrome P-450 reductase, glutathione transferase and UDP-glucuronosyl-transferase were high, with levels comparable to those observed in freshly isolated rat parenchymal cells. Epoxide hydrolase activity was also detected, but was lower than in the liver. The CHEL cells were able to activate benzo[a]pyrene, 7,12-dimethylbenz[a]anthracene and aflatoxin B1 to mutagens, as shown in a co-culture assay with V79 cells, in which acquisition of resistance to 6-thioguanine was studied. At early passages, the CHEL cells had a near diploid set of chromosomes. Then, gradually the frequency of cells with slight changes in the number of chromosomes and the frequency of tetraploids were increased. During the observation period (up to passage 20) the modal number of chromosomes shifted from 22 to 23. No gross morphological changes in the cultures were noticed during the 20 passages. However, the cloning efficiency in normal medium was increased from ∼1% initially to 9%. In conditioned medium, CHEL cells, even those of early passages, showed enhanced growth rates and cloning efficiencies of up to 60%. The former, but not the latter, effect was partially mimicked by the addition of insulin and epidermal growth factor to normal medium. CHEL cells may be useful as indicator cells in mutagenicity experiments as they (i) have a relatively stable karyotype consisting of ∼ 22 chromosomes; (ii) grow well in culture, even at clonal densities (in conditioned medium) and (iii) have retained various xenobiotic—metabolizing activities. They may also be potentially useful in the study of regulatory phenomena in the hepatic metabolism of endogenous compounds, as well as acting as a model system for malignant transformation of epithelial cells.