Quantitative Approaches to Assess Key Carcinogenic Events of Genotoxic Carcinogens

Abstract

Chemical carcinogenesis is a multistep process. Genotoxic carcinogens, which are DNA-reactive, induce DNA adduct formation and genetic alterations in target cells, thereby generating mutated cells (initiation). Subsequently, preneoplastic lesions appear through clonal proliferation of the mutated cells and transform into tumors (promotion and progression). Many factors may influence these processes in a dose-dependent manner. Therefore, quantitative analysis plays an important role in studies on the carcinogenic threshold of genotoxic carcinogens. Herein, we present data on the relationship between key carcinogenic events and their deriving point of departure (PoD). Their PoDs were also compared to those of the carcinogenesis pathway. In an experiment, the liver of rats exposed to 2-amino-3,8-dimethylimidazo-(4,5-f)quinoxaline (MeIQx) was examined to determine the formation of MeIQx-DNA adducts, generation of mutations at LacI transgene, and induction of preneoplastic glutathione S-transferase placental form (GST-P)-positive foci and tumors (benign and malignant). The PoDs of the above key events in the carcinogenicity of MeIQx were increased as the carcinogenesis advanced; however, these PoDs were lower than those of tumor induction. Thus, the order of key events during tumor induction in the liver was as follows: formation of DNA adducts << Mutations << GST-positive foci (preneoplasia) << Tumor (adenoma and carcinoma). We also obtained similar data on the genotoxic and carcinogenic PoDs of other hepatocarcinogens, such as 2-amino-3,8-dimethylimidazo(4,5-f)quinoline. These results contribute to elucidating the existence of a genotoxic and carcinogenic threshold.

Keywords: Cancer risk assessment; Genotoxic carcinogen; IQ; Linear no-threshold carcinogenicity; MeIQx; Point of departure.

Fig. 1

MeIQx-DNA adduct level in the liver of F344 rats treated with MeIQx at various doses for 4 weeks (7). MeIQx, 2-amino-3,8-dimethylimidazo-(4,5-f)quinoxaline.

Fig. 2

8-OHdG level in the liver of F344 rats treated with MeIQx at various doses for 4 weeks. *p< 0.01 versus 0 ppm group (7). 8-OHdG, 8-hydroxy-2′-deoxyguanosine; MeIQx, 2-amino-3,8-dimethylimidazo-(4,5-f)quinoxaline.

Fig. 3

GST-P positive foci in the liver of F344 rats treated with MeIQx at various doses for 16 and 32 weeks. *p<0.01 versus respective 0 ppm group (7). GST-P, glutathione S-transferase placental form; MeIQx, 2-amino-3,8-dimethylimidazo-(4,5-f)quinoxaline.

Fig. 4

lacI gene mutation frequencies (A) and GST-P positive foci (B) in the liver of Big Blue^® rats treated with MeIQx at various doses for 16 weeks. *p< 0.001 versus 0 ppm group (8). GST-P, glutathione S-transferase placental form; MeIQx, 2-amino-3,8-dimethylimidazo-(4,5-f)quinoxaline.

Fig. 5

Dose-response plots and derived BMDL₁₀ values for DNA adducts, mutations, GST-P positive foci, and hepatocellular adenoma and carcinoma in the liver of F344 rats treated with MeIQx at various doses (12). BMDL₁₀, lower 95% confidence limit of the benchmark dose corresponding to a 10% extra risk; GST-P, glutathione S-transferase placental form; MeIQx, 2-amino-3,8-dimethylimidazo-(4,5-f)quinoxaline.

Fig. 6

IQ-DNA adduct formation in the liver of F344 rats treated with IQ at various doses for 16 weeks (17). IQ, 2-amino-3-methylimidazo(4,5-f)quinoline.

Fig. 7

GST-P positive foci in the liver of F344 rats treated with IQ at various doses for 16 weeks. *p< 0.01 versus 0 ppm group. **p< 0.001 versus 0 ppm group (17). GST-P, glutathione S-transferase placental form; IQ, 2-amino-3-methylimidazo(4,5-f)quinoline.

Fig. 8

Dose-response curves of genotoxic carcinogenesis events derived from BMDL₁₀, lower 95% confidence limit of the benchmark dose corresponding to a 10% extra risk.