For a long period, it has been generally considered that carcinogens, particularly genotoxic ones, have no threshold in exerting their potential for cancer induction. However, the non-threshold theory can be challenged with regard to assessment of cancer risk to humans. Here we show that a food-derived, genotoxic hepatocarcinogen, 2-amino-3,8-dimethylimidazo[4,5-f ]quinoxaline, forms DNA adducts at low doses, but does not induce glutathione S-transferase placental form (GST-P)-positive foci (considered to be preneoplastic lesions) or 8-hydroxy-2′ ′ ′ ′-deoxyguanosine in rat liver.Moreover a N-nitroso compound, N-nitrosodiethylamine, at low doses was also found not to induce GST-P-positive foci in rat liver. These results imply that there is a no-observed effect level for hepatocarcinogenesis by these genotoxic carcinogens.

Inhibitory effects of naturally occurring antioxidants on the initiation stage of hepatocarcinogenesis were studied. Group 1 rats were given a diet containing β‐carotene (β8‐CT, 0.02%), α‐tocopherol (α‐TP, 1.5%), glutathione (GLT, 5%), vanillin (VNL, 1%), quercetin (QCT, 1%) or ellagic acid (ELA, 1%), or 3 doses of diallyl sulfide (DAS, 200 mg/kg, i.g) over an 8‐day period. On day 7, the animals received a single dose of 2‐amino‐3‐methylimidazo[4,5‐f]jumoline (IQ, 100 mg/kg, i.g.), 12 h after two‐thirds partial hepatectomy for initiation and 2 weeks thereafter, were placed on promotion regimen comprising phenobarbital (0.05% in diet) and a single dose of D‐galactosamine (100 mg/kg, i.p.). Groups 2 and 3 were treated as described for Group 1, but without test material or IQ, respectively. Survivors were killed at week 11 and antioxidant influence was assessed by comparing values for preneoplastic glutathione S‐transferase placental form‐positive (GST‐P+) foci between Groups 1 and 2. All lesions larger than 70 βm in diameter consisting of approximately 5 cells in cross section were counted. Numbers of GST‐P+ foci/cm2 in Group 1 were: β ‐CT, 7.99; α‐TP, 8.21; GLT, 9.71; DAS, 10.37; VNL, 10.57; QCT, 11.1; ELA, 12.5 (n = 11‐15). All, except ELA, showed a significant decrease as compared with the Group 2 value of 14.54 (n=15). Only β ‐CT showed a significant decrease for the area value. This is the first report to show that β ‐CT, α‐TP, GLT, DAS, VNL, QCT exert inhibitory effects on initiation of hepatocarcinogenesis by the food carcinogen IQ, suggesting that these antioxidants might find application as chemopreventive agents. Furthermore, the current protocol proved practical for the assessment of chemopreventive agents within 11 weeks, a relatively Short period.

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Until recently it has been generally considered that genotoxic carcinogens have no threshold in exerting their potential for cancer induction. However, the nonthreshold theory can be challenged with regard to assessment of cancer risk to humans. In the present study we show that a food derived, genotoxic hepatocarcinogen, 2-amino-1-methyl-6-phenolimidazo[4,5-b]pyridine (PhIP), does not induce aberrant crypt foci (ACF) as preneoplastic lesions at low dose (below 50 ppm) or 8-hydroxy-2'-deoxyguanosine (below 400 ppm) in the rat colon. Moreover PhIP-DNA adducts were not formed at the lowest dose (below 0.01 ppm). Thus, the dose required to initiate ACF is approximately 5000 times higher than that needed for adduct formation. The results imply a no-observed effect level (existence of a threshold) for colon carcinogenesis by a genotoxic carcinogen.

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