Hepatocellular injury, hyperplasia and putative premalignancy in rats fed 2-acetylaminofluorene.

Hepatocellular growth inhibition and restorative hyperplasia were studied in 80 young male Buffalo rats that were fed .02% 2-AAF for up to four weeks. Between 30 and 58 days of age, the control rats tripled their body weights and more than doubled their liver weights. But these increases were respectively inhibited by 25 and 22% in 2-AAF fed rats. Because the rats were in a phase of rapid growth, it was necessary to evaluate the age-related hepatic changes in control animals prior to analyzing the experimental data. Thirty-seven-day-old control rats had 3H-thymidine labeling indices of hepatocytes that were two to five times higher than in 58-day-old rats. A replicative gradient was also noted in the hepatic acini. At three ages of sacrifice (37, 44 and 58 days), the control hepatocytes in the periportal regions (Zone 1) had labeling indices that were at least three times higher than those surrounding the terminal hepatic veins (Zone 3). Histochemical GGTase activity also showed age-related zonal changes. Particularly prominent was the decrease with age in the enzyme activity of the most actively replicating Zone 1 hepatocytes. GGTase activity was observed in hepatocytes occupying 7.6% of tissue section areas in rats sacrificed at 30 days of age, but this decreased to 0.3% by the fourth week of the study. After four weeks of feeding the carcinogen toxic growth inhibition was most impressive in midzonal (Zone 2) hepatocytes, which also showed decreased glycogen, and decreased cytoplasmic RNA. The localization of injury to acinar midzones is particularly noteworthy in view of the extreme rarity of the finding in the hepatic toxicology literature. Growth inhibition was already apparent after only one week of 2-AAF feeding, when the 3H-thymidine labeling indices of Zone 2 hepatocytes were 90% lower (0.9%) than in control rats (8.6%). Hepatocellular hyperplasia, presumably a restorative response to the midzonal growth inhibition, was prominent in Zone 1, but also noted in Zone 3, after four weeks of the study. However, most of the Zone 1 hyperplastic cells showed canalicular GGTase activity, while none of the Zone 3 hyperplastic cells showed this phenotype. Occasional discrete foci of hyperplastic hepatocytes that were considered putatively premalignant because of their very high GGTase activity and low G6Pase and ATPase activities were also noted after four weeks of 2-AAF feeding. The foci were usually in close physical association with hyperplastic Zone 1 hepatocytes, but could be distinguished by their higher GGTase and lower G6Pase activities.(ABSTRACT TRUNCATED AT 400 WORDS)