Inhibition of Early‐phase Exogenous and Endogenous Liver Carcinogenesis in Transgenic Rats Harboring a Rat Glutathione S‐Transferase Placental Form Gene

Abstract

Hepatocarcinogenesis initiated with N-nitrosodiethylamine (DEN) and that initiated by feeding of a choline-deficient, L-amino acid-defined (CDAA) diet were compared in transgenic male Wistar rats harboring a rat glutathione S-transferase placental form (GST-P) gene (GST-P-Tg rats) and non-transgenic (N-Tg) rats. Eight-week-old GST-P-Tg and N-Tg rats were administered DEN intraperitoneally at 100 mg/kg body weight, subjected to a selection procedure with 2-acetylaminofluorene and CCl 4 , and killed at the end of weeks 5 and 12. Other groups were fed the CDAA diet for 12 weeks and killed. Five weeks after the DEN treatment, numbers and sizes of γ γ γ γ-glutamyltransferase (GGT)-or GST-P-positive lesions and 8-hydroxyguanine (8-OHG) levels in the livers were significantly less in GST-P-Tg rats than in N-Tg rats. The lesion numbers were unchanged between the ends of weeks 5 and 12 in GST-P-Tg rats, but decreased in N-Tg rats. The lesion sizes were increased in GST-P-Tg rats, but unchanged in N-Tg rats. While the proliferating cell nuclear antigen labeling indices (PCNA L.I.) in and surrounding the lesions were decreased, more prominently in GST-P-Tg rats than in N-Tg rats, the 8-OHG levels were also decreased but similarly in both cases. After 12 weeks on the CDAA diet, the lesion incidences, numbers and sizes, 8-OHG levels, PCNA L.I. in and surrounding the lesions, and liver injury were significantly less in GST-P-Tg rats than in N-Tg rats. These results indicate that insertion of a rat GST-P transgene alters the early phase of exogenous and endogenous rat hepatocarcinogenesis, presumably due to enhanced detoxification by GST-P expressed both transiently during the initiation and chronically in the altered hepatocyte populations.Key words: Glutathione S-transferase placental form -N-Nitrosodiethylamine -Choline-deficient, L-amino acid-defined diet -Transgenic rat -Rat liver carcinogenesisThe glutathione S-transferases (GSTs; EC 2.5.1.18) are a family of multifunctional, phase II detoxification enzymes catalyzing the conjugation of the reduced form of glutathione with electrophilic metabolites of xenobiotics.1, 2) Among the isozymes of the GSTs, GST placental form (GST-P) is present in small quantities in various rat tissues, but only in trace amounts in normal liver.3-5) It is lacking in fetal liver and not increased in regenerating liver, 3) but has attracted particular attention since it is highly expressed in putative preneoplastic, focal lesions as well as in hepatocellular carcinomas induced by most chemical carcinogens.3, 4, 6, 7) GST-P has been shown to be a very useful marker for quantitation of lesions in assessment of both initiation and promotion. 8,9) However, the biological roles and the mechanisms underlying GST-P expression within the preneoplastic focal lesions are still largely obscure.To cast light on this problem, we have cloned the rat GST-P gene 10,11) and identified a strong enhancer, GST-P enhancer I, having 2 activator protein 1 binding site-like sequences at −2.5 kb of the 5′ flanking regio...

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