Hepatocyte (“hyperplastic”) nodules induced in the liver by initiation with diethylnitrosamine and selected by dietary 2-acetylaminofluorene plus partial hepatectomy (“resistant hepatocyte model”) have a special pattern of biochemical behavior and metabolic activity different than that seen acutely with many xenobiotics including many promoting agents and carcinogens. The nodule cells show a very low uptake of 2-acetylaminofluorene, relative to surrounding and normal liver, low levels of activity in the cytochromes P-450 and aryl hydrocarbon hydroxylase, high levels of activity in γ-glutamyltransferase, microsomal epoxide hydrolase, soluble glutathione-S-transferase and soluble UDP-glucuronyltransferase (UDP-GT1) and elevated levels of glutathione. This metabolic pattern appears to maximize the resistance of the nodules to xenobiotics generally, such as 2-acetylaminofluorene, and thereby may account for the resistant behavior of nodule hepatocytes to the inhibition of cell proliferation and the cytotoxicity by 2-acetylaminofluorene and other carcinogens. The possible importance of this seemingly new metabolic program in carcinogenesis is discussed briefly.
This study was undertaken to observe whether the administration of reduced glutathione intragastrically to male Fischer 344 rats during the precancerous steps of liver carcinogenesis has any protective effect on the development of hepatocellular carcinoma. Hepatocyte nodules were induced in the liver with a single initiating dose of diethylnitrosamine followed by selection of resistant hepatocytes to generate nodules by a two week exposure to dietary 2-acetylamino-fluorene coupled with partial hepatectomy. Animals had hepatocyte ('hyperplastic') nodules when examined by iaparotomy at three months. At that time, the animals were divided into two groups. One received daily intragastric giuta-thione for 8 months while the other received no further treatment. An additional control group received only the selecting (promoting) regimen with no initiator or glutathione. At 12 months, the animals receiving the initiator and promoter regimen had a 65% incidence of hepatocellular carcinoma and those receiving glutathione in addition bad a 71% incidence. Under these experimental conditions, the long term administration of glutathione appears to have no observable influence on liver cancer development in this model.
The appearance of resistance to a number of hepatotoxins in primary cultures of hepatocytes prepared at various time intervals up to 2 weeks after partial hepatectomy is the major focus in this study. Resistance to the cytocidal effect of aflatoxin B1, 2-acetylaminofluorene, N-hydroxy-2-acetylaminofluorene, methotrexate, or methyl methanesulfonate shows a progressive increase until 48 hr and then returns to the resting level of susceptibility by 2 weeks. The genesis of mutagens from 2-acetylaminofluorene and aflatoxin B1 by S-9 liver fractions shows a decrease from and return to control values after partial hepatectomy that parallels the resistance. The levels of total cellular cytochromes P-450 also decrease following partial hepatectomy and remain from 28 to 36% less than those of controls for at least 1 week. The glutathione and total soluble sulfhydryl ("glutathione") content increase following partial hepatectomy, and the pattern is consistent with a partial role for glutathione in the resistance phenomenon as it relates to 2-acetylaminofluorene. The possible relationship between resistance to the cytocidal effects in vitro and the resistance to inhibition of cell proliferation in vivo during liver carcinogenesis is discussed.
