Synergistic Effects of Low‐dose Hepatocarcinogens in Induction of Glutathione S‐Transferase P‐positive Foci in the Rat Liver

The effects of combined administration of hepatocarcinogens at low doses on the development of glutathione S‐transferase P‐form (GST‐P)‐positive foci of rat liver were examined utilizing a bioassay model which consists of a single injection of diethylnitrosamine (DEN, 200 mg/kg, ip), two‐thirds partial hepatectomy at week 3 and a 6‐week administration of test compounds. The chemicals used, 2‐acetylaminofluorene (2‐AAF), 3′‐methyl‐4‐dimethylaminoazobenzene (3′‐Me‐DAB), phenobarbital (PB), thioacetamide (TAA), N‐ethyl‐N‐hydroxyethylnitrosamine (EHEN), benzo[a]pyrene (B[a]P), carbazole, and α‐hexachlorocyclohexane (α‐HCH) were incorporated in the diet, except for EHEN which was dissolved in the drinking water, at levels of 1/6 of the doses usually used. The combinations were: I) 2‐AAF, 3′‐Me‐DAB, PB, TAA, EHEN and B[a]P, II) 2‐AAF, 3′‐Me‐DAB and PB, III) TAA, EHEN and B[a]P, IV) 2‐AAF, 3′‐Me‐DAB, carbazole, TAA, EHEN and a‐HCH, V) 2‐AAF, 3′‐Me‐DAB and carbazole, and VI) TAA, EHEN and α‐HCH. All combinations, except for II, caused an increase in the area of the foci as evaluated by the ratios of areas in the combined administration groups to the sum totals of 3 or 6 individual data: I) 1.75, II) 0.81, III) 2.01, IV) 3.62, V) 1.34 and VI) 2.91. The non‐synergistic effect in combination II might be related to PB induction of hepatic microsomal enzymes leading to enhanced enzymatical detoxification of 2‐AAF and 3′‐Me‐DAB. The present results indicate that exposure to several chemicals of similar organotropism, even at doses lower than the apparent carcinogenic levels, might be critical to the carcinogenic process.