Mechanistic Basis for Nonlinearities and Thresholds in Rat Liver Carcinogenesis by the DNA-Reactive Carcinogens 2-Acetylaminofluorene and Diethylnitrosamine

To explore differences in mechanisms of carcinogenicity at low and high exposures, we have conducted a series of exposure-response studies of hepatocarcinogenesis in rats using 2 well-studied DNA-reactive carcinogens, 2-acetylaminofluorene and diethylnitrosamine. In these studies, we have used intraperitoneal injection or intragastric instillation to deliver exact doses during an initiation segment followed by phenobarbital as a liver tumor promoter to enhance manifestation of initiation. This protocol results in carcinogenicity comparable to that produced by lifetime exposure to the carcinogens. Our findings in these experiments provide evidence for the following: (a) formation of DNA adducts can be nonlinear, with a plateau at higher exposures; (b) cytotoxicity shows no-effect levels and is related to exposure; (c) compensatory hepatocyte proliferation shows no-effect levels and can be supralinear at high exposures; (d) formation of preneoplastic hepatocellular altered foci can show no-effect levels and appears supralinear at high exposures; (e) no-effect levels can exist for tumor development, and the exposure response can be supralinear. We interpret these findings to reflect thresholds for hepatocellular initiating effects of these carcinogens and exaggerated responses at high exposures attributable to cytotoxicity and compensatory hepatocyte proliferation. Such enhanced proliferation of hepatocytes harboring DNA damage likely results in an exaggerated yield of mutations in critical genes, leading to supralinear initiation of carcinogenesis. Thus, mechanisms differ between low and high exposures. Based on these observations, we suggest that linear extrapolation from high toxic exposures to postulated low-exposure effects of DNA-reactive carcinogens can yield overestimates. Such extrapolation must be supported by mechanistic information. The finding of no-effect levels provides a basis for understanding why low-level environmental exposures of humans to even DNA-reactive carcinogens may convey no cancer risk.

[1]  J A Swenberg,et al.  Dose-response relationships for carcinogens. , 1995, Toxicology letters.

[2]  R. Gebhardt,et al.  Glutamine synthetase and hepatocarcinogenesis. , 1995, Carcinogenesis.

[3]  A. Levine,et al.  Several hydrophobic amino acids in the p53 amino-terminal domain are required for transcriptional activation, binding to mdm-2 and the adenovirus 5 E1B 55-kD protein. , 1994, Genes & development.

[4]  G. Williams,et al.  N-nitrosodiethylamine mechanistic data and risk assessment: bioactivation, DNA-adduct formation, mutagenicity, and tumor initiation. , 1996, Pharmacology & therapeutics.

[5]  G. Williams,et al.  Cell cycle-specific mutagenesis at the hypoxanthine phosphoribosyltransferase locus in adult rat liver epithelial cells. , 1980, Proceedings of the National Academy of Sciences of the United States of America.

[6]  D. Jerina,et al.  Dose-dependent differences in the mutational profiles of (-)-(1R,2S,3S,4R)-3,4-dihydroxy-1, 2-epoxy-1,2,3,4-tetrahydrobenzo[c]phenanthrene and its less carcinogenic enantiomer. , 1996, Cancer research.

[7]  G. Williams,et al.  2-Acetylaminofluorene mechanistic data and risk assessment: DNA reactivity, enhanced cell proliferation and tumor initiation. , 1996, Pharmacology & therapeutics.

[8]  G. Williams.  Mechanistic considerations in cancer risk assessment. , 1999, Inhalation toxicology.

[9]  H J Clewell,et al.  Implementation of EPA Revised Cancer Assessment Guidelines: Incorporation of Mechanistic and Pharmacokinetic Data. , 1997, Fundamental and applied toxicology : official journal of the Society of Toxicology.

[10]  L. Valcovic,et al.  Hazard evaluation of chemicals that cause accumulation of alpha 2u-globulin, hyaline droplet nephropathy, and tubule neoplasia in the kidneys of male rats. , 1993, Environmental health perspectives.

[11]  P. Vineis,et al.  Aromatic Amines and Cancer , 1998 .

[12]  G. Williams,et al.  The sensitivity and heterogeneity of histochemical markers for altered foci involved in liver carcinogenesis. , 1979, The American journal of pathology.

[13]  G. Williams,et al.  d-limonene mechanistic data and risk assessment: absolute species-specific cytotoxicity, enhanced cell proliferation, and tumor promotion. , 1996, Pharmacology & therapeutics.

[14]  J. Miller.  Sulfonation in chemical carcinogenesis--history and present status. , 1994, Chemico-biological interactions.

[15]  A. Kopp-Schneider,et al.  Effects of low doses of N-nitrosomorpholine on the development of early stages of hepatocarcinogenesis. , 1995, Carcinogenesis.

[16]  Harris Cc,et al.  Molecular epidemiology of human cancer: contribution of mutation spectra studies of tumor suppressor genes. , 1998 .

[17]  L. Loeb,et al.  Cancer cells exhibit a mutator phenotype. , 1998, Advances in cancer research.

[18]  R. Kodell,et al.  Dose and time responses models for the incidence of bladder and liver neoplasms in mice fed 2-acetylaminofluorene continuously. , 1980, Journal of environmental pathology and toxicology.

[19]  C. Harris,et al.  Molecular epidemiology of human cancer. , 1998, Recent results in cancer research. Fortschritte der Krebsforschung. Progres dans les recherches sur le cancer.

[20]  R. Gebhardt,et al.  Diethylnitrosamine exposure-responses for DNA damage, centrilobular cytotoxicity, cell proliferation and carcinogenesis in rat liver exhibit some non-linearities. , 1996, Carcinogenesis.

[21]  J F Dicello,et al.  The genotype of the human cancer cell: implications for risk analysis. , 1996, Mutation research.

[22]  R. Gebhardt,et al.  Dose response effects of 2-acetylaminofluorene on DNA damage, cytotoxicity, cell proliferation and neoplastic conversion in rat liver. , 1993, Cancer letters.

[23]  P. Thomas,et al.  Carcinoembryonic antigen induces signal transduction in Kupffer cells. , 1997, Cancer letters.

[24]  G. Williams,et al.  Modulation by butylated hydroxytoluene of liver and bladder carcinogenesis induced by chronic low-level exposure to 2-acetylaminofluorene. , 1991, Cancer research.

[25]  T. Lange,et al.  Dissimilarity in aflatoxin dose-response relationships between DNA adduct formation and development of preneoplastic foci in rat liver. , 1997, Chemico-biological interactions.

[26]  J I Goodman,et al.  Principles underlying dose selection for, and extrapolation from, the carcinogen bioassay: dose influences mechanism. , 1995, Regulatory toxicology and pharmacology : RTP.

[27]  P Grasso,et al.  Effects on 4080 rats of chronic ingestion of N-nitrosodiethylamine or N-nitrosodimethylamine: a detailed dose-response study. , 1991, Cancer research.

[28]  O. Boxma,et al.  Oncogenesis by mutations in anti-oncogenes: a view. , 1990, Anticancer research.

[29]  G. Williams,et al.  Epigenetic carcinogens: evaluation and risk assessment. , 1996, Experimental and toxicologic pathology : official journal of the Gesellschaft fur Toxikologische Pathologie.

[30]  R. Gebhardt,et al.  Nonlinearities in 2-acetylaminofluorene exposure responses for genotoxic and epigenetic effects leading to initiation of carcinogenesis in rat liver. , 1998, Toxicological sciences : an official journal of the Society of Toxicology.

[31]  P Grasso,et al.  Dose and time relationships for tumor induction in the liver and esophagus of 4080 inbred rats by chronic ingestion of N-nitrosodiethylamine or N-nitrosodimethylamine. , 1991, Cancer research.

[32]  G. Williams.  Chemicals with carcinogenic activity in the rodent liver; mechanistic evaluation of human risk. , 1997, Cancer letters.

[33]  V. Dellarco,et al.  U.S. Environmental Protection Agency guidelines for carcinogen risk assessment: past and future. , 1996, Mutation research.

[34]  Sen Np.  Recent studies in Canada on the occurrence and formation of N-nitroso compounds in foods and food contact materials. , 1991 .

[35]  N. P. Sen.  Recent studies in Canada on the occurrence and formation of N-nitroso compounds in foods and food contact materials. , 1991, IARC scientific publications.

[36]  G. Williams,et al.  Butylated hydroxyanisole mechanistic data and risk assessment: conditional species-specific cytotoxicity, enhanced cell proliferation, and tumor promotion. , 1996, Pharmacology & therapeutics.

[37]  S. Cohen.  Cell Proliferation in the Evaluation of Carcinogenic Risk and the Inadequacies of the Initiation-Promotion Model , 1998 .

[38]  D. Jerina,et al.  Dose-dependent differences in the profile of mutations induced by (+)-7R,8S-dihydroxy-9S,10R-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene in the coding region of the hypoxanthine (guanine) phosphoribosyltransferase gene in Chinese hamster V-79 cells. , 1993, Cancer research.

[39]  M. You,et al.  Dose-dependent ras mutation spectra in N-nitrosodiethylamine induced mouse liver tumors and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone induced mouse lung tumors. , 1993, Carcinogenesis.

[40]  G. Williams,et al.  Enhancement of rat hepatocellular-altered foci by the liver tumor promoter phenobarbital: evidence that foci are precursors of neoplasms and that the promoter acts on carcinogen-induced lesions. , 1978, Journal of the National Cancer Institute.

[41]  R. Gebhardt,et al.  Glutamine synthetase heterogeneous expression as a marker for the cellular lineage of preneoplastic and neoplastic liver populations. , 1989, Carcinogenesis.

[42]  G. Williams,et al.  Quantitative kinetics of development of N-2-fluorenylacetamide-induced, altered (hyperplastic) hepatocellular foci resistant to iron accumulation and of their reversion or persistence following removal of carcinogen. , 1978, Journal of the National Cancer Institute.

[43]  R. Gebhardt,et al.  Non-linearity of neoplastic conversion induced in rat liver by low exposures to diethylnitrosamine. , 1993, Carcinogenesis.

[44]  T. Sugimura,et al.  Human exposure to mutagenic/carcinogenic heterocyclic amines and comutagenic beta-carbolines. , 1997, Mutation research.

[45]  C. Frith,et al.  Biologic and morphologic characteristics of urinary bladder neoplasms induced in BALB/c female mice with 2-acetylaminofluorene. , 1980, Journal of Environmental Pathology And Toxicology.

[46]  G. Williams,et al.  The pathogenesis of rat liver cancer caused by chemical carcinogens. , 1980, Biochimica et biophysica acta.

[47]  J. Hotchkiss.  Preformed N-nitroso compounds in foods and beverages. , 1989, Cancer surveys.

[48]  Chung-Xiou Wang,et al.  Diethylnitrosamine exposure-responses for DNA ethylation, hepatocellular proliferation, and initiation of carcinogenesis in rat liver display non-linearities and thresholds , 1999, Archives of Toxicology.

[49]  K. Husgafvel‐Pursiainen,et al.  Interaction between dose and susceptibility to environmental cancer: a short review. , 1997, Environmental health perspectives.