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

The purposes of the present study were to evaluate the hepatocarcinogenicity of concurrent treatment of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) and diethylnitrosamine (DEN) in rats and to determine whether no effect levels of combinations of these two different structural categories of genotoxic hepatocarcinogens exist. Two 16-week rat hepatocarcinogenesis assays were performed using a total of 790 male F344 rats. In experiment 1, we evaluated the effects of concurrent treatment of a subcarcinogenic dose of DEN on rat hepatocarcinogenesis induced by various doses of MeIQx. In experiment 2, we determined hepatocarcinogenicities of combinations of MeIQx and DEN at subcarcinogenic doses, low carcinogenic doses and high carcinogenic doses. Quantitative analyses of glutathione S-transferase placental form (GST-P)-positive foci, a preneoplastic lesion of the liver in rats, revealed that concurrent treatment with subcarcinogenic doses of DEN did not enhance MeIQx-induced rat hepatocarcinogenicity. We also found that concurrent treatment with combinations of subcarcinogenic doses of DEN and MeIQx was not hepatocarcinogenic, indicating that the combined effects of subcarcinogenic doses of DEN and MeIQx were neither additive nor synergistic. Moreover, concurrent treatment with low carcinogenic doses of these 2 carcinogens did not show additive or synergistic effects. Synergetic effects were observed only in rats coadministered high carcinogenic doses of the 2 carcinogens. These results demonstrate the existence of no effect levels of combinations of these 2 genotoxic hepatocarcinogens, and provide new evidence supporting our idea that there is a threshold, at least a practical threshold, that should be considered when evaluating the risk of genotoxic carcinogens.     

Ethanol Does Not Promote MeIQx-initiated Rat Colon Carcinogenesis Based on Evidence from Analysis of a Colon Cancer Surrogate Marker

Epidemiological studies suggest that alcohol consumption increases the risk of developing colorectal cancer. However, the data are confounded by numerous cosegregating variables. To cast further light on the relationships between alcohol intake and colon cancer development, 21-day-old male F344/DuCrj rats were fed 200 ppm 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) in their diet for 8 weeks and doses of 0, 0.1, 0.3, 1, 3, 10 and 20% of ethanol in their drinking water ad libitum for 16 weeks thereafter. The rats were sacrificed after 24 weeks of experiment, and aberrant crypt foci (ACF), surrogate lesions for colon cancer, were examined under a light microscope at low magnification. Ethanol was found not to affect the ACF formation at any dose compared with the initiated-controls. Furthermore, ethanol did not alter colon epithelial cell proliferation. These data, obtained by analysis of a colon cancer surrogate marker lesion, indicate that ethanol lacks promotion activity for MeIQx-initiated rat colon carcinogenesis.