Modification of toxic liver injury in the rat II. Protective effect of cycloheximide on ethionine-induced damage and autoprotective effects of high doses of ethionine, 3'-methyl-4-dimethylaminoazobenzene, and 2-acetylaminofluorene

Detection of genotoxic carcinogens in the in vivo-in vitro hepatocyte DNA repair assay

The in vivo-in vitro hepatocyte DNA repair assay has been shown to be useful in the evaluation of the carcinogenic potential of chemicals. The purpose of this study was to apply this assay to determining the genotoxicity of the compounds from a wide variety of structural classes. Male Fischer-334 rats were treated by gavage or ip injection with compounds dissolved in either corn oil, water, or dimethyl sulfoxide (DMSO). At selected times after treatment, hepatocytes were isolated by liver perfusion and cultured with 3H-thymidine, Unscheduled DNA synthesis (UDS) was measured by quantitative autoradiography as net grains/nucleus (NG); greater than or equal to 5 NG was considered positive. Water, corn oil, or DMSO controls produced -3 to -6 NG with less than or equal to 6% of the cells in repair. All genotoxic hepatocarcinogens tested produced strong positive responses of greater than 15 NG including dimethylnitrosamine, 2-acetylaminofluorene (2-AAF), azoxymethane, 1,2-dimethylhydrazine, benzidine, aflatoxin B1, 2,6-dinitrotoluene, and 2,4-diaminotoluene. The noncarcinogen, 2,6-diaminotoluene, was negative. The mutagen and rat brain carcinogen methyl methanesulfonate (MMS) and the rat pancreatic carcinogen azaserine were also positive. The carcinogens benzo(a)pyrene and 7,12-dimethylbenz(a)anthracene yielded from -2 to -4 NG. This negative response is consistent with their lack of carcinogenic activity in rat liver. MMS produced the greatest amount of UDS 2 hr after treatment whereas 2-AAF did not induce its maximum response until 12 hr post-treatment. The potent hepatotoxin carbon tetrachloride induced a 40-fold elevation in DNA replication 48 hr after a 400 mg/kg dose, but no UDS was observed at 2, 12, 24, or 48 hr post-treatment. The weak hepatocarcinogen safrole induced no UDS suggesting that it is either nongenotoxic or is metabolized to an active form at an extremely slow rate following a single administration. These results demonstrate that this assay is valuable for the detection and study of a variety of genotoxic carcinogens.

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.

Histopathology of the rat liver following a single autoprotective dose of 3'-methyl-4-dimethylaminoazobenzene

Male inbred Leeds strain rats were given single intragastric doses of 1200 mg/kg of 3'-methyl-4-dimethylaminoazobenzene, autoprotective against liver necrosis and then sacrificed at intervals up to 14 months. About 40 per cent of the animals died, apparently as a result of pancreatic necrosis, between 9 and 13 days after treatment while the remainder survived until they were killed. The liver changes induced included an early intense ductular cell reaction, presistent hepatocellular abnormalities and at later stages, cholangiofibrosis and increasing numbers of parenchymal clear cells. Although no tumours arose as a result of treatment, the observed histopathological changes are similar to those seen during liver carcinogenesis induced by a variety of carcinogens and may be due specifically to the carcinogenic action of the azo dye.