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.

A variety of phenotypic cellular changes emerge in the liver of different species prior to the appearance of hepatocellular adenomas and carcinomas induced by carcinogenic agents (chemicals, radiation, hepadna viruses) or develop "spontaneously." Foci of altered hepatocytes have been studied most extensively in rats treated with chemical carcinogens; they are considered preneoplastic lesions and have been used in several laboratories as endpoints in carcinogenicity testing. The principles and problems of the morphological classification of foci of altered hepatocytes are presented. In addition to the 4 types of foci generally accepted (clear, acidophilic, basophilic and mixed cell foci), further subtypes (intermediate cell foci) or other types of foci, namely tigroid cell foci and amphophilic cell foci, have more recently been separated as distinct pathomorphological entities. Whereas the amphophilic foci might result from a modulation of clear and acidophilic cell foci, the tigroid cell foci apparently represent a stage in a separate cell lineage leading to hepatocellular adenomas. It remains open whether the tigroid cell foci may also progress to carcinomas. Extrafocal phenotypic changes of hepatocytes might also be involved in hepatocarcinogenesis. The cellular phenotypes within foci also depend strongly, among many other factors, on the dose and duration of the carcinogenic treatment. Cytomorphological, cytochemical, microbiochemical and stereological studies suggest that the predominant sequence of cellular changes during hepatocarcinogenesis leads from the clear and acidophilic cell foci storing glycogen in excess through mixed cell foci and nodules to basophilic cell populations prevailing in hepatocellular carcinomas. A multitude of metabolic aberrations is associated with the sequential cellular changes. Aberrations in carbohydrate metabolism are particularly prominent and might be causally related to the neoplastic transformation of the hepatocytes.

Using an antibody raised against the mitochondrial enzyme cytochrome c oxidase (EC 1.9.3.1), it was possible to visualize microoncocytomas, oncocytic tubules and even single oncocytes in the renal cortex of rats treated with N-nitrosomorpholine, and to localize the lesions to the collecting duct of the nephron. In human specimens, the antibody also stained oncocytomas and oncocytic tubules in the surrounding macroscopically unaffected cortex. Thus, this antibody may be used for detection as well as for investigation of the development of oncocytic lesions in various species.

Treatment of male Sprague-Dawley rats with N-nitrosomorpholine (NNM) in the drinking water at a dose of 120 mg/l for 7 weeks resulted in a subsequent enhanced development of focal and nodular lesions in the adrenal cortex. Sequential observation revealed that focal lesions in the zona reticularis/fasciculata or the zona glomerulosa developed both earlier and at a significantly higher incidence in animals treated with carcinogen than in untreated controls. Foci observed within or adjacent to the zona glomerulosa were all of pale cell appearance and contained large numbers of electron-dense cytoplasmic granules similar to those observed in normal granulosa cells. The foci and nodules which arose in the zona reticularis/fasciculata were, in contrast, characterized by a reduction or loss of the dense osmiophilic droplets normally seen in the cells of this region of the adrenal cortex, a pronounced increase in pleomorphic mitochondria of atypical appearance and the development of vacuoles.

The renal tubular segment from which clear cell tumors originate was investigated in the kidneys of rats treated with N-nitrosomorpholine. This tumor type, which in the rat closely resembles that in man, is made up of clear and granular acidophilic cells and arises from tubules lined by clear cells. The tubular origin of the tumors was established in serial sections by demonstrating connections between both clear cell tumors and tubules lined by clear cells, and renal tubules of normal appearance. In 45 clear cell lesions (17 tumors and 28 tubules) one or more such connections were identified which belonged to the collecting system. In accordance with their localisation in the kidney, the clear cell lesions were connected predominantly to tubules of the cortical collecting system and occasionally to outer medullary collecting ducts. As previously reported, oncocytic tubules and microoncocytomas were observed to originate from the same portions of the collecting system. Rarely, microadenomas and tubules consisting of both oncocytes and clear or granular acidophilic cells were also observed in the kidneys studied.

Long-term dietary administration of the adrenal hormone dehydroepiandrosterone (DHEA) to male Sprague-Dawley rats induced significant alterations in the activities of enzymes involved in liver carbohydrate metabolism. Although glycogen synthase activity was increased and phosphorylase decreased, glycogen stores were reduced. This was presumably related to lysosomal glycogen degradation, since alpha-glucosidase was increased. All rate-limiting enzymes of glucose metabolism which were studied (glucose-6-phosphate dehydrogenase, total hexokinases, pyruvate kinase, fructose-1,6-bisphosphatase) revealed markedly reduced activity, only glucose-6-phosphatase activity was increased. These enzymatic changes point to a far-reaching metabolic shift towards energy loss via decreased glucose consumption and increased glucose output. The enzyme pattern induced by DHEA is in many respects opposite to that induced in preneoplastic and neoplastic liver lesions by chemical hepatocarcinogens.

The effect of 4 weeks dietary administration of the hormone dehydroepiandrosterone (DHEA) on enzyme and morphological phenotype of focal lesions previously induced by dimethylaminoazobenzene (DAB) treatment was investigated in Sprague-Dawley rats. In contrast to the DAB-alone livers where large numbers of glycogen-storing, mixed cell nodules homogeneously positive for glutathione S-transferase P form (GST-P) were apparent, DHEA treated animals were characterized by significantly fewer, more heterogeneous lesions, in some cases demonstrating increased amphophilia and structured basophilia. The enhanced heterogeneity, in some ways reminiscent of that reported earlier for ‘reversibility’ or ‘remodelling’ of rapidly induced nodular lesions, was associated with increased catalase (CAT), acid phosphatase (AP) and glucose-6-phosphatase (G6Pase) and decreased glycogen content and phosphorylase (PHO) activity in both nodules and background parenchyma. Glucose-6-phosphate dehydrogenase (G6PD) activity was elevated irregularly, focal lesions also demonstrating a heterogeneous reaction. The experimental data suggest two separate effects of the hormone treatment, the first involving modulation of the usual altered phenotype of preneoplastic lesions with a shift towards ‘tigroid’ cell character and the second, similar to that reported earlier for rapidly induced nodules, involving enhanced phenotypic instability and leading to reduction in numbers.

Human and experimental liver tumors are classified according to the similarity in morphology and histological arrangement of their cells to specific normal counterparts. Thus, hepatocellular, cholangiocellular and diverse mesenchymal tumors are distinguished. This histogenetic classification was established by light microscopy. It has been supported by many electron microscopical results and more recently also by immunohistochemical investigations of certain cytoskeletal components, especially the so-called intermediate filaments.

Male Sprague-Dawley rats were investigated after N-nitrosomorpholine (NNM) treatment with concomitant and subsequent administration of dehydroepiandrosterone (DHEA) for development of pre-neoplastic and neoplastic liver lesions. In addition to clear, acidophilic, mixed cell and basophilic foci, a hitherto undescribed lesion type demonstrating a unique morphological and histochemical phenotype was observed in animals receiving both NNM and DHEA. The cells of the majority of these lesions for which we propose the designation amphophilic foci were characterized by increased granular acidophilia and randomly scattered cytoplasmic basophilia. Histochemically, reduced glycogen content and elevated activity of glucose-6-phosphate dehydrogenase (G6PDH), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), acid phosphatase (AP), succinate dehydrogenase (SDH) and catalase (CAT) were evident. The lack of gamma-glutamyl transpeptidase (GGT) or glutathione S-transferase placental form (GST-P) in foci of this type allowed clear differentiation from other NNM-induced focal lesions while suggesting certain similarities to pre-neoplastic cells induced by hypolipidemic agents. Similar enzyme histochemical patterns were characteristic for foci and later appearing nodules (adenomas) composed of amphophilic/tigroid cells the basophilic material of which was increased and frequently arranged in long striped bands. DHEA treatment, while not itself inducing any preneoplastic foci, was thus associated with altered phenotypic expression of foci and adenomas generated by NNM.

Administration of the hormone dehydroepiandrosterone (DHEA) (0.6% in the diet) during or subsequent to injections of the carcinogen dihydroxy-di-n-propylnitrosamine (DHPN) (2 X 1000 mg/kg body weight, i.p.) brought about alteration in the yield of preneoplastic lesions in liver and lung of both male and female F344 rats. Concomitant treatment with DHEA was associated with decrease in the numbers and size of glutathione S-transferase (GST-P)-positive hepatocellular foci while effecting a significant increase in development of lung lesions, especially in females. Long-term treatment with the hormone subsequent to carcinogen exposure brought about a reduction in numbers of liver foci in both sexes but in males was also associated with the development of large GST-P-negative foci and nodules of amphophilic/tigroid cell character. DHEA itself did not induce any focal lesions in the lungs or livers of either sex. Thus the hormone increased sensitivity to 'initiation' in the lung while decreasing that in the liver and exerted a sex-dependent pronounced modulation of the phenotype of a proportion of hepatocellular lesions.

Dietary administration of 0.25% dehydroepiandrosterone (DHEA) during and subsequent to 7 weeks treatment with N-nitrosomorpholine (NNM) resulted in significantly reduced development of adrenal cortical lesions and hemangiosarcomas in the liver. In addition, distinct phenotypic modulation of hepatocellular tumours was observed after combined hormone and carcinogen treatment. Thus the neoplasms were characterized by a higher degree of differentiation, lower mitotic rate and reduced potential for metastasis as compared to tumours observed after NNM alone. The data clearly indicate that DHEA exerts an inhibitory effect on both adrenal and hepatocarcinogenesis similar to that earlier reported for neoplasia in lung, thyroid, colon and skin.