Dose-response relationship and low dose extrapolatioin in chemical carcinogenesis

Data supporting various dose-response relationships in chemical carcinogenesis are summarized. General principles are derived to explain the relationships between exposure dose, DNA adduct level, induction of genetic changes, and tumor incidence. Some mechanistic aspects of epigenetic carcinogens (stimulation of cell division and maldifferentiation) are analyzed in a similar way. In a homogeneous population, non-linearities are frequent. They are due to phenomena of induction or saturation of enzymatic activities and to the multi-step nature of carcinogenesis: if a carcinogen accelerates more than one step, the superposition of the dose-response curves for the individual steps can result in an exponential relationship. A fourth power of the dose was the maximum seen in animals (formaldehyde). At the lowest dose levels, a proportionality between dose and tumor induction is postulated independent of the mechanism of action if the carcinogen accelerates the endogenous process responsible for spontaneous tumor formation. Low-dose thresholds are expected only for situations where the carcinogen acts in a way that has no endogenous counterpart. Epidemlological studies in humans show linear dose-response curves in all but two investigations. The difference from the strongly non-linear slopes seen in animal studies could be due to the heterogeneity of the human population: if the individual sensitivity to a carcinogen is governed by a large number of genetic and life-style factors, the non-linearities will tend to cancel each other out and the dose-response curve becomes ‘quasi-linear

In vivo covalent binding of organic chemicals to DNA as a quantitative indicator in the process of chemical carcinogenesis

The covalent binding of chemical carcinogens to DNA of mammalian organs is expressed per unit dose, and a 'Covalent-Binding Index', CBI, is defined. CBI for various carcinogens span over 6 orders of magnitude. A similar range is observed for the carcinogenic potency in long-term bioassays on carcinogenicity. For the assessment of a risk from exposure to a carcinogen, the total DN A darnage can be estimated if the actual dose is also accounted for. A detailed description is given for planning and performing a DNA-binding assay. A complete literature survey on DNA binding in vivo (83 compounds) is given with a calculation of CBI, where possible, 153 compounds are listed where a covalent binding to any biological macromolecule has been shown in vivo or in vitro. Recent, so far unpublished findings with aflatoxin Mh macromolecule- bound aflatoxin Bh ·diethylstilbestrol, and 1,2-epithiobutyronitrile are included. A comparison of CBI for rat-liver DNA with hepatocarcinogenic potency reveals a surprisingly good quantitative correlation. Refinements for a DN A-binding assay are proposed. Possibilities and Iimitations in the use of D NA binding in chemical carcinogenesis are discussed extensively

Covalent binding of styrene to DNA in rat and mouse

Covalent binding of (7-3H)styrene (S) to DNA in vivo was measured and evaluated in a quantitative manner in order to investigate whether DNA adduct formation could form a mechanistic basis for tumor induction in a carcinogenicity bioassay. [7-3H]S was administered by inhalation in a closed chamber to male and female CD rats and B6C3F1 mice. After 4.5-6 h (rats) and 6-9 h (pools of four mice), S doses of 23-39 and 85-110 mg/kg respectively had been metabolized. DNA was purified to constant specific radioactivity which was measurable in all samples. DNA was enzymatically degraded to the 3'-nucleotides which were separated by HPLC for the detection of radiolabeled nucleotide-S adducts. The fractions with the normal nucleotides contained most of the radioactivity. In mouse liver DNA, a minute but significant level of adduct radioactivity was also detected. In the units of the Covalent Binding Index CBI = (μmol adduct/mol DNA nucleotide)/(mmol chemical/kg body wt), values of 0.05-0.09 and 0.07-0.18 were calculated for males and females respectively. In the rat, no DNA adducts were detectable in the liver at a limit of detection of 0.1 CBI units. Two of the four lung samples of the female rats showed adduct-related radioactivity corresponding to 0.07 CBI units. The CBI values are compatible with styrene 7,8-oxide as the reactive intermediate. The data are compared with CBI values and carcinogenic potencies of established genotoxic carcinogens. It is concluded that the DNA-binding potency of S is so low that significant tumor induction in a standard bioassay for carcinogenicity is unlikely to be due to DNA adduct formation alone. Consequences for a human risk estimation are discusse

Chemical carcinogens and overnutrition in diet-related cancer

The intake of known dietary carcinogens was compiled and the cancer risk was estimated on the basis of carcinogenic potencies in animals as derived from the Carcinogenic Potency Database by Gold and co-workers. The total cancer risk was compared with the number of cancer cases attributed by epidemiologists to dietary factors (one-third of all cancer cases, i.e. ∼ 80 000 per one million lives). Except for alcohol, the known dietary carcinogens could not account for more than a few hundred cancer cases. This was seen both with the DNA-reactive carcinogens (heterocyclic aromatic amines, polycyclic aromatic hydrocarbons, N-nitroso compounds, estragole, aflatoxin B1, ethyl carbamate, to name the most important factors) as well as with those carcinogens which have not been shown to react with DNA (e.g. caffeic acid and the carcinogenic metals arsenic and cadmium). Residues and contaminants turned out to be negligible. Among the various possibilities to explain the discrepancy we investigated the role of overnutrition. Dietary restriction in animals is well known for its strong reducing effect on spontaneous tumor formation. These data can be used to derive a carcinogenic potency for excess macronutrients: the tumor incidence seen with the restricted animals is taken as a control value and the increased tumor incidence in the animals fed ad libitum is attributed to the additional feed intake. For excess standard diet in rats, a carcinogenic potency TD50 of 16 g/kg/day was deduced from a recent study. Overnutrition in Switzerland, estimated to be 5.5 kcal/kg/day, was converted to excess food (1.9 g/kg/day) and the cancer incidence was calculated. The result, 60 000 cancer cases per one million lives, is provocatively close to the number of cases not explained by the known dietary chemical carcinogens. Mechanistic studies will be required to test our hypothesis and investigate the role of different types of macronutrients in overnutritio

Dose-Incidence Relationships Derived from Superposition of Distributions of Individual Susceptibility on Mechanism-Based Dose Responses for Biological Effects

Dose-response relationships for incidence are based on quantal response measures. A defined effect is either present or not present in an individual. The dose-incidence curve therefore reflects differences in individual susceptibility (the "tolerance distribution”). At low dose, only the more susceptible individuals manifest the effect, while higher doses are required for more resistant individuals to be recruited into the affected fraction of the group. Here, we analyze how such dose-incidence relationships are related to mechanism-based dose-response relationships for biological effects described on a continuous scale. As an example, we use the quantal effect "cell division” triggered by occupancy of growth factor receptors (R) by a hormone or mitogenic ligand (L). The biologically effective dose (BED) is receptor occupancy (RL). The dose-BED relationship is described by the hyperbolic Michaelis-Menten function, RL/Rtot = L / (L + KD). For the conversion of the dose-BED relationship to a dose-cell division relationship, the dose-BED curve has to be combined with a function that describes the distribution of susceptibilities among the cells to be triggered into mitosis. We assumed a symmetrical sigmoid curve for this function, approximated by a truncated normal distribution. Because of the supralinear dose-BED relationship due to the asymptotic saturation of the Michaelis-Menten function, the composite curve that describes cell division (incidence) as a function of dose becomes skewed to the right. Logarithmic transformation of the dose axis reverses this skewing and provides a nearly perfect fit to a normal distribution in the central 95% incidence range. This observation may explain why dose-incidence relationships can often be described by a cumulative normal curve using the logarithm of the administered dose. The dominant role of the tolerance distribution for dose-incidence relationships is also illustrated with the example of a linear dose-BED relationship, using adducts to protein or DNA as the BED. Superimposed by a sigmoid distribution of individual susceptibilities, a sigmoid dose-incidence curve results. Linearity is no longer observed. We conclude that differences in susceptibility should always be considered for toxicological risk assessment and extrapolation to low dos

Methylation of DNA by incubation with methylamine and nitrite

DNA was incubated in septum-closed reaction vials with [14C]methylamine and nitrite. The DNA was purified, hydrolysed with hydrochloric acid, and the purines were analysed by h.p.I.c. 7-Methylguanine was detectable as a result of DNA methylation in experiments performed in 100 mM acetate at pH 4. Using different concentrations of amine and nitrite a first order reaction for total amine and a second order for total nitrite could be shown. A study on the pH dependence using 100 mM malonate buffer, pH 2.0-6.0, revealed a maximum rate at pH 3.5, with steep slopes above and below this pH value, in agreement with a mathematical analysis of the reaction equations. The data show that the alkylating agent formed spontaneously by nitrosation and deamination of a primary amine has a long enough lifetime to react with DNA in vitro. Using the reaction orders established here, an extrapolation to lower concentrations found in the stomach can now be performed. Future in vivo experiments on the methylation of gastro-intestinal DNA then would show to what extent DNA in a cell is protected from alkylatio

Short communication : Mouse skin papilloma formation by chronic dermal application of 7,12-dimethylbenz[a]anthracene is not reduced by diet restriction

No abstract availabl

Stimulation of DNA synthesis in rat and mouse liver by various tumor promoters

In order to investigate whether the stimulation of liver DNA synthesis might be used to detect one class of hepatic tumor promoters, the incorporation of orally administered radio-labelled thymidine into liver DNA was determined in rats and mice 24 h after a single oral gavage of test compounds at various dose levels. Three DNA-binding hepatocarcinogens, aflatoxin B1,- benzidine and carbon tetrachloride, did not stimulate but rather inhibited DNA synthesis (not for CCl4 Four hepatic tumor promoters, clofibrate, DDT, phenobarbital and thioacetainide, gave rise to a stimulation in a dose-dependent manner. Single oral doses between 0.02 and 0.3 mmol/kg were required to double the level of thymidine incorporation into liver DNA (= doubling dose, DD). Differences between species or sex as obsserved in long-term carcinogenicity studies were reflected by a different stimulation of liver DNA synthesis. In agreement with the bioassay data, aldrin was positive only in male mice (DD = 0.007 mmol/kg) but not in male rats or female mice. 2,3,7,8-TCDD was positive in male mice (DD = 10−6 mmol/kg) and in female rats (DD = 2 × 10−6 mmol/kg) but not in male rats. The assay was also able to distinguish between structural isomers with different carcinogenicities. [alpha]Hexchlorocyclohexane stimulated liver DNA synthesis with a doubling dose of about 0.2 mmol/kg in male rats whereas the [gamma]-isomer was ineffective even at 1 mmol/kg. So far, only one result was inconsistent with carcinogenicity bioassay data. The different carcinogenicity of di(2-ethylhexyl)adipate (negative in rats) and di(2-ethylhexyl)phthalate (positive) was not detectable. Both plasticizers were positive in this short-term system with DD's of 0.7 mmol/kg for DEHA and 0.5 mmol/kg for DEHP. The proposed assay is discussed as an attempt to devise short-term assays for carcinogens not detected by the routine genotoxicity test system

Mouse skin papilloma formation by chronic dermal application of 7, 12-dimethylbenz[a] anthracene is not reduced by diet restriction

Diet restriction has repeatedly been shown to reduce the incidence of spontaneous and chemically induced tumors in rodents. However, no conclusive data are available to show whether carcinogenesis by chronic exposure to a genotoxic agent can also be retarded. In this study, diet restriction to 70% was investigated for a protective effect on the formation of skin papilloma in male NMRI mice treated twice weekly with 20 nmol 7,12-dimethylbenz(a)anthracene (DMBA). Rather surprisingly, no protection was seen. Both time of onset of papilloma formation (13 weeks in both groups) and time of 50% cumulative incidence (t50; 17.5 and 18 weeks) were similar in the unrestricted and the restricted group. In contrast, a clearly protective elTect was found in mice initiated with 100 nmol DMBA and promoted twice weekly with 2.5 nmol 12-O-tetradecanoylphorbol-13-acetate: the onset of papilloma formation increased from 7 to 11.5 weeks, the t50 was shifted from 8.5 to 19 weeks. Diet restriction, therefore, was not protective under conditions of chronic exposure to a genotoxic carcinogen. It cannot be considered a universal measure of cancer preventio

Different Types of Combination Effects for the Induction of Micronuclei in Mouse Lymphoma Cells by Binary Mixtures of the Genotoxic Agents MMS, MNU, and Genistein

Distinction between dose addition and response addition for the analysis of the toxicity of mixtures may allow differentiation of the components regarding similar versus independent mode of action. For nonlinear dose responses for the components, curves of dose addition and response addition differ and embrace an "envelope of additivity.” Synergistic or antagonistic interaction may then be postulated only if the mixture effect is outside this surface. This situation was analyzed for the induction of micronuclei in L5178Y mouse lymphoma cells by the two methylating agents methyl methanesulfonate (MMS) and N-methyl-N-nitrosourea (MNU) and the topoisomerase-II inhibitor genistein (GEN). All three chemicals reproducibly generated sublinear (upward convex) dose-response relationships. For the analysis of mixture effects, these genotoxic agents were investigated in the three binary combinations. Statistical testing for dose addition along parallel exponential dose responses was performed by linear regression with interaction based on the logarithm of the number of cells that contain micronuclei. For MMS+MNU, the mixture effect was compatible with dose addition (i.e., significantly larger than calculated for the addition of net responses). For MMS+GEN, the measured effect was larger than for response addition but smaller than for dose addition. For MNU+GEN, the measured effect was below response addition, indicative of true antagonism. In the absence of knowledge on the sublinear dose-response relationships for the individual components, a synergistic effect of MMS on both MNU and GEN would have been postulated erroneously. The observed difference between MMS and MNU when combined with GEN would not have been predicted on the basis of a simplistic interpretation of DNA methylation as the mode of action and may be due to differences in the profile of DNA methylations and/or epigenetic effects. We conclude that knowledge of nonlinearities of the dose-response curves of individual components of a mixture can be crucial to analyze for synergism or antagonism and that an in-depth mechanistic knowledge is useful for a prediction of similarity or independence of actio