Quantitative electron microscopy of carcinogen-induced alterations in hepatocyte rough endoplasmic reticulum. I. Chronic effect of 3'MeDAB and short-term effects of azo dyes of different carcinogenic potentials.

Abstract

A new method has been developed for the quantitative analysis of an ultrastructural change in hepatocyte rough endoplasmic reticulum (ER) that is induced by liver carcinogens. Hitherto only subjective observations of this alteration had been made. Male inbred Leeds rats were fed a diet containing 0.06% of the carcinogenic azo dye 3'-methyl-4-dimethyl-aminoazobenzene (3'MeDAB). Groups of treated rats, together with untreated controls, were sacrificed after 10 days, 4 weeks, 10 weeks and 17 weeks. Samples of liver tissue from each animal and from 10 3'MeDAB-induced hepatocellular carcinomas (HCC), were examined by electron microscopy. A quantitative study was carried out to investigate the effect of chronic exposure to 3'MeDAB upon the state of aggregation of the hepatocyte rough ER into parallel arrays. As early as 10 days after the start of treatment, the rough ER showed a highly significant degree of disaggregation: the mean number of ER cisternae per array fell from the control value of 6.20 to 3.73. This change was sustained throughout the experiment. At 17 weeks, comparison of the mean array size in the HCC cells with that in the surrounding hepatocytes revealed a further significant decline, from 3.46 to 2.12. Further groups of rats were fed other azo dyes for 4 weeks and subjected to the same assay. The carcinogens 4'-methyl-4-dimethylaminoazobenzene (4'MeDAB) and N,N-dimethyl-4-amino-N-acetyl-N-monomethyl-4-aminoazobenzene (DAAMAB) resembled 3'MeDAB with respect to the degree of rough ER disaggregation they induced. In contrast, the non-carcinogen 3'-trifluoromethyl-4-dimethylaminoazobenzene (3'TFMeDAB) had no such effect, while the weak initiator 2-methyl-4-dimethylaminoazobenzene (2MeDAB) induced disaggregation to a lesser degree than the strong carcinogens. At least with the azo dyes used in this study, the extent of rough ER disaggregation appears to be related to hepatocarcinogenesis.