Dose and time dependence of the cellular phenotype in rat hepatic preneoplasia and neoplasia induced by continuous oral exposure to N-nitrosomorpholine

The dose and time dependence of the cellular phenotype in preneoplastic and neoplastic liver lesions was evaluated quantitatively in groups of male Sprague-Dawley rats continuously exposed to 0, 6, 12 and 24 mg/kg body wt of N-nitrosomorpholine (NNM) and studied at different time points up to 80, 50, 37 and 20 weeks respectively. Continuous oral administration of NNM resulted in a dose- and time-dependent increase in the total number and volume of preneoplastic foci of altered hepatocytes (FAH) and in the incidence of hepatocellular adenomas (HCA) and carcinomas (HCC) at all dose levels studied. In contrast to earlier stop experiments with 24 mg NNM/kg body wt, there was no reversion-linked phenotypic instability but a rapid progression of FAH of the mixed cell type to a high incidence of hepatocyte nodules and HCC after continuous treatment with NNM at this dose level. At the two lower dose levels of NNM (12 and 6 mg/kg) the well-known sequence of cellular changes leading from glycogenotic (clear, combined clear/acidophilic and acidophilic) to mixed and diffusely basophilic cell populations, in which HCC prevailed. A considerable part of the glycogenotic foci contained exclusively acidophilic cells, and HCA consisting of a mixture of acidophilic and basophilic cells were the most common benign tumour type in these groups. At the end of the observation period there was also a high incidence (> 50%) of HCC at both dose levels, indicating the potential of persistent nodules (HCA) containing acidophilic cells to progress to HCC. FAH and HCA exhibiting a tigroid cell pattern appeared only at the two lower dose levels, but for this type of HCA it remained open whether it may progress to HCC. From a comparison of the different dosing regimens of NNM studied in this and previous experiments we conclude that the rapid, potentially reversible shift from glycogenotic to mixed cell populations at the highest dose level of continuously applied NNM (24 mg/kg) and the high proportion of pure acidophilic glycogen storage foci observed after continuous administration of NNM at the two lower dose levels (6 and 12 mg/kg) represent different phenotypic expressions of promoting effects exerted by the ongoing influence of the carcinogen on FAH initiated by the same compound. The metabolic and molecular changes underlying these 'initiating' and 'promoting' effects of NNM seem to differ in terms of quantity rather than quality.