Changes in the fine structure of rat hepatocytes during the early phases of chronic 2-acetylaminofluorene intoxication

Aromatic Amines in Experimental Cancer Research: Tissue-Specific Effects, an Old Problem and New Solutions

Carcinogenic aromatic amines usually produce tumors in specific target tissue, such as 2-acetylaminofluorene (AAF) producing liver tumors in rats, in contrast to some other structurally related arylamines. A hypothesis is presented that explains the mode of action in this rat liver model. Genotoxic and nongenotoxic effects work together and make AAF a complete rat liver carcinogen. The cytotoxic, promoting effects are particularly important. N-Hydroxy-2-aminofluorene and 2-nitrosofluorene, two metabolites of AAF, are able to uncouple the mitochondrial respiratory chain. They entertain a redox cycle that removes electrons from the respiratory chain and impairs ATP production. The dose-dependent opening of the mitochondrial permeability transition pore signals the viability of the cell. If the pore is opened to a certain extent, the cell is eliminated by apoptosis. As a consequence, oval cells proliferate, and as this process is overloaded, the liver transforms into a cirrhosis-like situation and thus provides the conditions under which initiated liver cells develop tumors. Such an interpretation is based on assumptions that have been debated for a long time. Some of these often forgotten developments are reviewed in support of the hypothesis, which allows a more comprehensive view of the complex in vivo situation at a time when in vitro models prevail.

Induction by paracetamol of bladder and liver tumours in the rat. Effects on hepatocyte fine structure

Groups of male and female inbred Leeds strain rats were fed diets containing either 0.5% or 1.0% paracetamol by weight for up to 18 months. At the 1.0% dosage level, 20% of rats of both sexes developed neoplastic nodules of the liver, a statistically significant incidence. These rats also showed gross enlargement of their livers and an increase in foci of cellular alteration, the latter also being observed in the low dosage male rats. Papillomas of the transitional epithelium of the bladder developed in all paracetamol-treated groups, and three rats bore bladder carcinomas. However, significant yields of bladder tumours were only obtained from low dosage females and high dosage males. Additionally, 20 to 25% of paracetamol-treated rats developed hyperplasia of the bladder epithelium, which was not coincident with the presence of bladder calculi. A low yield of tumours at various other sites also arose following paracetamol feeding. An electron microscope study of the livers of paracetamol-treated rats revealed ultrastructural changes in the hepatocytes that resemble those that result from exposure to a variety of known hepatocarcinogens.

Interactions of asbestos fibers with hepatocytes in vitro: an ultrastructural study

Cultures of rat hepatocytes have been used to study the interaction of chrysotile asbestos fibers with epithelial cells. The hepatocytes were incubated for 20 h with chrysotile asbestos (UICC B and a preparation of short fibers: 90% less than 2 micron) at concentrations of 1 or 10 micrograms/ml. Morphologic studies were then performed by means of transmission electron microscopy. Fibers were incorporated into plasma membrane invaginations, cytoplasmic vacuoles and phagolysosomes. These observations demonstrate that rat hepatocytes can engage in phagocytosis of chrysotile fibers. The results are compared to those previously obtained with mesothelial cells and tracheobronchial epithelium, exposed to asbestos in vitro. The advantages of the hepatocyte model to investigate the effects on cells of the association of asbestos fibers with genotoxic agents in relation to carcinogenesis are discussed.

Alterations in the enzyme activity and polypeptide composition of rat hepatic endoplasmic reticulum during acute exposure to 2-acetylaminofluorene

Studies have been made of the morphology, enzyme activity and protein composition of liver endoplasmic reticulum in rats exposed to acute doses of the carcinogen, 2-acetylaminofluorene (2-AAF). Electron microscopic examination revealed numerous ultrastructural changes in the hepatocyte; most consistent alterations were the disorganisation of endoplasmic reticulum system with apparent increase of smooth endoplasmic reticulum. Administration of 2-AAF to rats immediately depressed microsomal glucose-6-phosphatase activity and eventually induced epoxide hydratase activity 6--7-fold over control activity. The induction was time-dependent and maximal rates of induction were observed at dosages greater than 40 mg/kg body wt. The treatment also induced cytochrome b5 content, NADH and NADPH cytochrome c reductase activities (1.0--1.5-fold). Only very small changes in the total content of cytochrome P-45- were noted. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) of microsomal proteins from 2-AAF pretreated animals showed time-dependent induction of two polypeptides which differed slightly in migration, in the region of Mr = 48000; the fast-migrating induced polypeptide has been identified as epoxide hydratase. Two-dimensional PAGE analysis of microsomal proteins from 2-AAF exposed rats showed a reproducible deletion of a protein with molecular weight in the region of 67000. The basis for the alterations in the protein composition of endoplasmic reticulum in response to 2-AAF treatment is discussed.

Acute and persistent ultrastructural changes in rat liver after a single dose of 2-acetylaminofluorene: effect of co-administration of cycloheximide

Electron microscope studies have been made of rat hepatic cell morphology, 24 hr, 48 hr and 24 weeks after a single intragastric dose of 2-acetylaminofluorene (2-AFF), alone or together with a single s.c. dose of cycloheximide (CHM). CHM prevents the acute perinuclear glycogen pooling, peripheral displacement of cytoplasmic organelles and nuclear shrinkage induced by 2-AAF. However, the induction by 2-AAF of changes in the granular endoplasmic reticulum, mitochondria and bile canaliculi is enhanced by co-administration of CHM. These changes persist for at least 24 weeks and are similar to those induced during 2-AAF hepatocarcinogenesis and observed in hepatocellular tumours. In view of the similar results obtained in previous studies with the azocarcinogen, 3'-methyl-4-dimethylaminoazobenzene, the possibility is discussed that these persistent changes might be associated with the initiation phase of neoplastic induction.

Acute fine structural changes in rat hepatocytes induced by a single large dose of 2-acetylaminofluorene

Male rats were given a single intragastric dose of 2-acetylaminofluorene, 600 mg/kg body weight, killed at intervals up to 14 days after treatment, and their hepatic tissue examined by electron microscopy. The early cytoplasmic lesion produced in hepatocytes by lower doses, consisting of perinuclear glycogen pooling, peripheral displacement of organelles and pyknosis, was delayed for several days. Among the changes which appeared to be independent of this lesion were disorganization and decrease of the granular endoplasmic reticulum and abnormalities of the bile canaliculi. These changes were similar to those which are seen during chronic exposure to 2-AAF and other hepatocarcinogens and in hepatic cell tumours.

Scanning electron microscopic surface morphology of isolated hepatocytes from normal and premalignant rat liver

Scanning electron microscopy was performed on hepatocytes isolated after collagenase perfusion of livers from rats fed 2-acetylaminofluorene (AAF) in the diet for 6--8 weeks and on hepatocytes from control rats. The premalignant AAF-cells showed a greater variation in cell size and shape. They also exhibited a reduced number of microvilli and more blebs and other protrusions on the surface than hepatocytes from control animals.

Fine structural changes induced in rat hepatocytes by single doses of 3'-methyl-4-dimethylaminoazobenzene

Male Leeds rats were given single oral doses of 150 or 300 mg/kg body weight of 3'-methyl-4-dimethylaminoazobenzene (3'-MeDAB). They were sacrificed 24 or 48 h after treatment and their hepatic tissues examined by electron microscopy. The development was observed of an unusual cytoplasmic change, which consisted of large perinuclear areas of decreased hyaloplasmic density, devoid of glycogen or organized structures, which displaced the organelles to the cell periphery. This arose by the formation of large glycogen lakes, which coalesced and then lost their glycogen content, and this was accompanied by nuclear irregularity and shrinkage. Other changes, affecting the endoplasmic reticulum and the cell surface, appeared to be similar to those induced by chronic azo dye feeding. It was concluded that the acute lesion observed may limit any specific responses of the hepatocytes to the presence of 3'-MeDAB.

The effects of aflatoxin B 1 in vivo on membrane--ribosome association

The effects of aflatoxin B(1) on the endoplasmic reticulum of rat liver has been examined in vivo. Electron microscopy has shown a disorganization and degranulation of rough surfaced membrane under these conditions and evidence is presented that this is a primary effect of the toxin, and results from the direct attack of the aflatoxin on the steroid-dependent ribosome binding sites on the membrane. A technique is described by which the presence of degranulated rough membrane may be detected in microsomal preparations.