Combination effects of forty carcinogens administered at low doses to male rats

Sensitivity of the immature rat uterotrophic assay to mixtures of estrogens

We have evaluated whether mixtures of estrogens, present in the mix at doses that are individually inactive in the immature rat uterotrophic assay, can give a uterotrophic response. Seven chemicals were evaluated: nonylphenol, bisphenol A (BPA), methoxychlor, genistein (GEN), estradiol, diethylstilbestrol, and ethinyl estradiol. Dose responses in the uterotrophic assay were constructed for each chemical. The first series of experiments involved evaluating binary mixtures of BPA and GEN at dose levels that gave moderate uterotrophic responses when tested individually. The mixtures generally showed an intermediate or reduced uterotrophic effect compared with when the components of the mixture were tested alone at the dose used in the mixture. The next series of experiments used a multicomponent (complex) mixture of all seven chemicals evaluated at doses that gave either weakly positive or inactive uterotrophic responses when tested individually in the assay. Doses that were nominally equi-uterotrophic ranged over approximately six orders of magnitude for the seven chemicals. Doses of agents that gave a weak uterotrophic response when tested individually gave a marginally enhanced positive response in the assay when tested combined as a mixture. Doses of agents that gave a negative uterotrophic response when tested individually gave a positive response when tested as a mixture. These data indicate that a variety of different estrogen receptor (ER) agonists, present individually at subeffective doses, can act simultaneously to evoke an ER-regulated response. However, translating these findings into the process of environmental hazard assessment will be difficult. The simple addition of the observed, or predicted, activities for the components of a mixture is confirmed here to be inappropriate and to overestimate the actual effect induced by the mixture. Equally, isobole analysis is only suitable for two- or three-component mixtures, and concentration addition requires access to dose-response data and EC50 values (concentration giving 50% of the maximum response) for the individual components of the mixture--requirements that will rarely be fulfilled for complex environmental samples. Given these uncertainties, we conclude that it may be most expedient to select and bioassay whole environmental mixtures of potential concern.

Food-borne heterocyclic amines. Chemistry, formation, occurrence and biological activities. A literature review

This review summarizes the abundant literature on food-borne heterocyclic amines, their chemistry and formation, their occurrence in food, their biological activities including mutagenicity, induction of DNA damage and carcinogenicity. Pharmacokinetics and biotransformation are also discussed. Factors that influence these effects are given consideration, with special emphasis on dietary factors that might counteract detrimental biological effects. The annual per capita intake of heterocyclic amines via food is estimated. Risk extrapolations that have been published suggest that food-borne heterocyclic amines are relevant for human cancer etiology.

[The combination effects of quercetin with the herbicides atrazine, cyanazine and gesamprim in mutagenicity tests]

The plant flavonol quercetin and the triazine herbicides atrazine, cyanazine, and gesamprim were examined individually and in combination for the induction of genotoxic effects. The sister chromatid exchange (SCE) assay and the gene mutation assay for 6-thioguanine resistance (HPRT) were carried out with Chinese hamster ovary (CHO) cells. Whereas no evidence of an increased SCE rate was found, the test substances caused a slightly increased mutation rate in the HPRT assay after metabolic activation with a subcellular liver enzyme preparation. Combination studies with two or three of the test substances did not result in higher mutation rates than those observed for the individual compounds tested singly.

Synergism of environmental carcinogens and promoters on bladder cancer development initiated by N-butyl-N-(4-hydroxybutyl)nitrosamine in F344 rats

Synergistic or additive effects of combined treatments with carcinogens or promoters on N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN)-initiated rat bladder carcinogenesis were examined. Male F344 rats were given BBN as an initiator followed by low doses of 3 sodium salts (sodium bicarbonate, sodium L-ascorbate and sodium citrate) and/or 3 antioxidants (butylated hydroxyanisole, butylated hydroxytoluene and tertiary butylhydroxyquinone). Combined treatments with 3 sodium salts or 3 antioxidants, and especially all 6 chemicals together promoted bladder carcinogenesis. In addition, these combined treatments were associated with increased DNA synthesis of the bladder epithelium. Combined administration of the carcinogens, o-anisidine, p-cresidine, and 4-chloro-o-phenylenediamine at low doses also enhanced BBN-initiated bladder carcinogenesis. These results indicate that environmental carcinogens or promoters can exert synergistic or additive actions on bladder cancer induction.

Organ-specific modification of tumor development by low-dose combinations of agents in a rat wide-spectrum carcinogenesis model

The combined effects of low doses of various carcinogens and carcinogenesis modifiers on tumor development were investigated by using a wide-spectrum organ carcinogenesis model in F344 rats. These agents were administered as three groups: (1) a group of known hepatocarcinogens; (2) a group of nitroso compounds having various target organ specificities; and (3) a group of antioxidants having various inhibiting or enhancing activities depending on the target organ. Doses were used which were generally below the known effective level for the individual chemical. These groups of chemicals were administered with or without prior administration of N-diethylnitrosamine (100 mg/kg body wt., i.p.), N-methylnitrosourea (4 x 20 mg/kg body wt., i.p.) and dihydroxy-di-N-propylnitrosamine (0.1% in drinking water for 2 weeks). The hepatocarcinogen group in combination with various nitroso compounds increased the incidences of liver hyperplastic nodules and hepatocellular carcinomas. In contrast, incidences were clearly reduced when the hepatocarcinogens and/or the nitroso compounds were administered in combination with the antioxidants. For the urinary bladder, the combination with nitroso compounds and antioxidants enhanced cancer development, and the addition of hepatocarcinogens further increased tumorigenesis. For the glandular stomach, additive effects on the numbers of pepsinogen isozyme 1-altered pyloric glands, a putative preneoplastic lesion, were produced by the combination treatment of antioxidants and the nitroso compounds. No synergistic effects on tumor development were seen in other organs. The results of the present study demonstrated that combinations of various compounds at low doses can additively or synergistically exert either enhancing or inhibitory effects on the development of preneoplastic and neoplastic lesions in different organs in a single model having a wide spectrum of organ effects.

Implications of the carcinogenic hazard of low doses of three hepatocarcinogenic N-nitrosamines

The data of a large-scale experiment on single and combination effects of very low doses of the hepatocarcinogenic N-nitrosamines N-nitrosodiethylamine (NDEA), N-nitrosopyrrolidine (NPYR), and N-nitrosodiethanolamine (NDElA) were modeled by new statistical methods to derive implications of the carcinogenic hazard of dose ranges so low as to result in long-term toxic effects only slightly different from the background. According to this model a linear relationship was found to exist between daily exposure to low N-nitrosamine levels and time to death with liver tumor. Extrapolation of these data to zero exposure suggested the occurrence of one to ten percent of spontaneous liver tumors in animals surviving more than 1000 days. At this advanced age a further reduction of carcinogen-induced liver tumor incidence does not contribute to a longer overall survival due to competitive, probably independent, causes of death. A quasi-threshold in terms of a "no-observed-effect level" can thus be derived from the data. The observed combination effect indicates a mere additivity in liver tumor occurrence, even at very low doses which alone cause no significant carcinogenic effect during the animal's lifetime. Within the combination of NDEA, NPYR and NDElA, N-nitrosodiethylamine was found to be the most carcinogenic agent: it contributes by at least 17 orders of magnitude more to the relative risk of dying with liver tumor than the other two compounds, if the daily dose is increased by one unit (0.1 mg/kg). Likewise, NDEA shows the steepest slope when assessing the relationship between daily carcinogen doses and time to liver tumor occurrence; relative to NDElA--the least potent carcinogen on a weight basis--and NPYR a 40-fold and 9-fold quicker appearance of liver tumors has to be expected, if the daily doses are increased by an equivalent amount.