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Enzyme profile of rat bile ductular epithelial cells in reference to the resistance phenotype in hepatocarcinogenesis

An extensive bile ductular cell hyperplasia with the formation of well‐differentiated bile ductules is the most prominent feature of rat liver at 6 to 15 weeks after bile duct ligation. We have improved our previous cell isolation procedure and are now routinely able to obtain from such livers high yields of viable bile ductular epithelial cells. These cells were characterized with respect to their specific activities of γ‐glutamyl transpeptidase and β‐glucuronidase and of select Phase I and Phase II enzymes of biotransformation. At the time of their isolation, only a very small number of the bile ductular epithelial cells were observed to be in DNA synthesis. In addition, in histological sections prepared from intact hyperplastic bile ductular tissue isolates, only the bile ductular epithelial cells exhibited histochemical staining for γ‐glutamyl transpeptidase activity. Typically, greater than 95% of the cells isolated from this tissue were also found to be histochemically positive for γ‐glutamyl transpeptidase activity, and no hepatocytes were seen contaminating this cell population. Biochemically, the isolated bile ductular cells exhibited a γ‐glutamyl transpeptidase specific activity that was 100 times higher than that of hepatocytes isolated at the same time from the bile duct‐ligated rats and more than 300 times higher than the specific activity of the enzyme of freshly isolated normal rat hepatocytes. The bile ductular cells also expressed a β‐glucuronidase specific activity that was higher than that measured in normal rat hepatocytes, but in contrast to normal liver, they did not contain a detectable amidopyrine demethylation (Phase I) activity. On the other hand, they exhibited specific activities for the Phase II enzymes UDP‐glucuronosyl‐transferase and glutathione S‐transferase that were relatively close in amounts to those obtained with hepatocytes from the bile duct‐ligated rats. Substrate and/or inhibitor studies further suggested that the forms of the Phase II enzymes expressed by the bile ductular cells were similar to those shown by immunohistochemistry to be present in the interlobular bile ducts of normal untreated rats. Our findings represent the first direct biochemical measurements of Phase II enzyme activities of isolated bile ductular epithelial cells and indicate possible mechanisms which may favor their resistance to the cytotoxic and mito‐inhibitory effects of at least some types of hepatocarcinogenic and hepatotoxic chemicals.