Enhancement of GST-P positive liver cell foci development by combined treatment of rats with five heterocyclic amines at low doses

Abstract

Potential synergism between five heterocyclic amines at low doses was evaluated in a medium-term liver bioassay system for carcinogens. F344 male rats were given a single i.p. injection of diethylnitrosamine (DEN, 200 mg/kg) and then received test compound(s) in their diet for 6 weeks beginning 2 weeks later. Control groups received DEN or test compound(s) alone. All rats were subjected to two-thirds partial hepatectomy at week 3 and killed at week 8. Compounds tested and reported positive were 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1, 150 p.p.m.), 2-aminodipyrido[1,2-a:3',2'-d]imidazole (Glu-P-2, 500 p.p.m), 2-amino-3-methylimidazo[4,5-f]quinoline (MeIQ, 300 p.p.m.), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx, 400 p.p.m.). Groups were given each chemical at the carcinogenic dose, or 1/5 or 1/25 of this. Other groups received the five chemicals in combination, each at the 1/5 or 1/25 levels. Enhancing activity was assessed by quantitative analysis of glutathione S-transferase placental form (GST-P) positive foci, the numbers being significantly increased with all chemicals at the highest dose. Trp-P-1, IQ and MeIQ also exerted positive influence even at the 1/5 dose level. Similar results were obtained regarding areas of foci at the highest dose levels, with the exception of Glu-P-2. An increase was also observed for MeIQ at the 1/5 dose. Additive or synergistic effects between the chemicals were evident in the groups given the five chemicals together at both the 1/5 and 1/25 dose levels, development of GST-P positive foic being increased over the sum totals of individual data for the 1/5 or 1/25 dose groups. Thus, carcinogenicity was predicted for all five heterocyclic amines tested in dose-dependent manner in the present system of 8 weeks duration, synergistic effects being apparent especially at the low dose level.

Carcinogenic Effects of Mixtures of Chemicals

“…As well demonstrated by researchers in Ito's group 20,21,[23][24][25][26] , the medium-term liver bioassay and the medium-term multiorgan model are plausible alternatives to long-term studies. Particularly, in the medium-term liver bioassay, the endpoints, the numbers and areas of GST-P positive liver foci, allow very sensitive evaluation of the tumorigenicity of test compounds in the liver, and this model is particularly appropriate for investigating summation, synergism, or inhibition of carcinogenic responses with mixtures of multiple chemicals.…”

“…Genotoxic carcinogens like nitrosamines and HCAs induce DNA damage through formation of DNA adduct(s) and/or oxygen radical species produced as bi-products and alteration of the genome is thought to be irreversible once it has take place, leading to the assumption that risks may be additive even at low doses. The data demonstrated by Burger 10 and Ito's group [23][24][25] , however, provide evidence that combination treatment with genotoxic carcinogens at low doses does not necessarily entail additive risk for carcinogenicity. This phenomenon could be partly explained by detoxification of carcinogens administered and/or successful repair of DNA damage caused by low doses of carcinogens.…”

“…The data clearly show that exposure to multiple nongenotoxic chemicals at ADI levels entails practically very low risk in humans. Another significant finding is that the combination of 5 or 10 carcinogenic HCAs did not exert summation or synergistic tumor induction when given at 1/ 25 or 1/100 carcinogenic doses [23][24][25] . These findings suggest that at very low doses, exposure to multiple chemicals may not be risk factors for human populations at least under the conditions conducted in animal experiments.…”

“…In the first experiment 23,25 , potential synergism between five HCAs at low doses was evaluated. Other groups received the five chemicals in combination, each at the 1/5 or 1/25 levels.…”

Lack of Hepatocarcinogenicity of Combinations of Low Doses of 2-amino-3, 8-dimethylimidazo[4,5-<i>f</i>]quinoxaline and Diethylnitrosamine in Rats: Indication for the Existence of a Threshold for Genotoxic Carcinogens

“…For example, concurrent treatment with low doses of 3 N -nitroso compounds (genotoxic carcinogens) did not show additive effects on liver tumor development; only treatment with high doses exerted an additive effect 8 . Furthermore, concurrent treatment with low doses of 5 or 10 heterocyclic amines did not enhance development of preneoplastic lesions of the liver either additively or synergistically when given at the post-initiation stage in medium-term live bioassays (Ito test) 9 , 10 , 11 . These findings suggested that concurrent treatment with genotoxic carcinogens at low doses did not necessarily entail additive risk for carcinogenicity 7 .…”