Effects of Non-Genotoxic Hepatocarcinogens Phenobarbital and Nafenopin on Phenotype and Growth of Different Populations of Altered Foci in Rat Liver

Abstract

Non-genotoxic hepatocarcinogens share the ability to induce liver growth in rodents. Phenobarbital (PB), as one prototype compound, promotes the development of liver tumors; altered cell foci of the clear-eosinophilic phenotype, also identified by gamma-glutamyltransferase expression, appear to be precursor lesions. These foci seem to over-respond to the growth-inducing effect of PB. In contrast, the question as to whether peroxisome inducers are also tumor promoters is still unsettled. We will present evidence which strongly suggests that the peroxisome inducer, nafenopin (Naf), promotes tumor development in rat livers by stimulating selective growth of a hitherto undescribed subtype of altered foci. This subtype is characterized by weak diffuse cytoplasmic basophilia of its hepatocytes. Initiation in rats by aflatoxin B1 followed by promotion with Naf produced numerous adenomas and carcinomas; their morphology resembled that of the weakly basophilic foci. Both clear-eosinophilic and weakly basophilic foci appear "spontaneously" in the liver of aging rats. Promotion of such lesions by PB-type compounds or peroxisome inducers may explain cancer formation by these non-genotoxic agents.

Promotion of Hepatocarcinogenesis by Phenobarbital in c-myc/TGF-? Transgenic Mice

“…Because the results were very similar at 24 and 26 wk, the groups were combined (16 animals in each group). PB appeared to cause a small, but not statistically signi®cant, shift from clear cell and basophilic toward eosinophilic lesions, as has been found in rat and mouse models [3,7,8,17,19]. The signi®cance of the morphology of preneoplastic lesions is still unresolved, but PB seems to be acting in a manner consistent with that seen in other systems.…”

Comparison of the Hepatic Effects of Nafenopin and WY-14, 643 on Peroxisome Proliferation and Cell Replication in the Rat and Syrian Hamster

“…Several hypotheses have been proposed to account for the formation of liver tumors by peroxisome proliferators in rats and mice (2,(6)(7)(8)(9)(13)(14). If these hypotheses are combined, a role for increased cell replication in peroxisome proliferator-induced hepatocarcinogenicity may be identified.…”

“…If these hypotheses are combined, a role for increased cell replication in peroxisome proliferator-induced hepatocarcinogenicity may be identified. For example, if hepatocytes are transformed either by oxidative stress-induced damage (2,(6)(7)(8)(9) or spontaneously (14), such initiated cells could be promoted into liver tumors by enhanced cell replication (13). Certainly the present rat study with NAF and WY supports the concept that liver tumors are produced more rapidly by doses of peroxisome proliferators that produce a sustained stimulation of cell replication.…”

“…This imbalance is due to the fact that peroxisome proliferators markedly stimulate enzymes of the peroxisomal fatty acid f-oxidation cycle (which generate hydrogen peroxide), whereas only a small increase is observed in catalase activity, and selenium-dependent glutathione peroxidase activity is normally inhibited by these chemicals (7,8,11,12). Although some evidence has been obtained for the oxidative stress hypothesis (7,8), other workers have suggested that peroxisome proliferator-induced hepatocarcinogenicity may be due to alternative mechanisms including the sustained stimulation of replicative DNA synthesis and the promotion of spontaneously formed preneoplastic liver lesions (13,14).…”

Tumor-Promoting Effects of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin and Phenobarbital in Initiated Weanling Sprague-Dawley Rats: A Quantitative, Phenotypic, and ras p21 Protein Study

“…In addition, by evaluating H&E staining characteristics, the tumor-promoting ability of PB was similar to TCDD as early as day 170. These findings would indicate that various populations of initiated cells were responsive to the promoting stimulus of PB and that some cells continued through subsequent stages of the carcinogenesis process without regression (28). Because there was a positive tumor-promoting response to PB based on hepatocellular neoplasms and AHF characterized by H&E staining, but not by the histochemical marker ATPase, these data suggest that subpopulations of initiated cells negative for ATPase may not be required for PB tumor promotion under the experimental protocol utilized.…”