Effect of pretreatment with beta-naphthoflavone on tumorigenesis by N-nitrosoethylurea in five mouse strains

Methods of in vitro toxicology

In vitro methods are common and widely used for screening and ranking chemicals, and have also been taken into account sporadically for risk assessment purposes in the case of food additives. However, the range of food-associated compounds amenable to in vitro toxicology is considered much broader, comprising not only natural ingredients, including those from food preparation, but also compounds formed endogenously after exposure, permissible/authorised chemicals including additives, residues, supplements, chemicals from processing and packaging and contaminants. A major promise of in vitro systems is to obtain mechanism-derived information that is considered pivotal for adequate risk assessment. This paper critically reviews the entire process of risk assessment by in vitro toxicology, encompassing ongoing and future developments, with major emphasis on cytotoxicity, cellular responses, toxicokinetics, modelling, metabolism, cancer-related endpoints, developmental toxicity, prediction of allergenicity, and finally, development and application of biomarkers. It describes in depth the use of in vitro methods in strategies for characterising and predicting hazards to the human. Major weaknesses and strengths of these assay systems are addressed, together with some key issues concerning major research priorities to improve hazard identification and characterisation of food-associated chemicals.

Differences in susceptibility to colonic stem cell somatic mutation in three strains of mice

Different species and different strains of animals commonly show very different sensitivities to carcinogenic regimes, which are often unexplained. A major possible contributory factor is variation in susceptibility to mutation, but this has not been directly demonstrated. This study therefore quantified the colonic stem cell mutation frequency in three strains of mice using two carcinogens. Stem cell mutations were identified using loss of function of glucose 6-phosphate dehydrogenase (G6PD) in individual crypts, a technique validated by several previous studies. The carcinogens dimethylhydrazine (DMH) and ethyl nitrosurea (ENU) were given to Balb/C, C57BL/6J, and C3H mice. In response to DMH, Balb/C mice were most susceptible, with approximately double the stem cell mutation frequency found in C3H and more than ten-fold that found in C57BL/6J (3.3+/-0.71 vs. 1.5+/-0.52 vs. 0.28+/-0.8x10(-4)). In response to ENU, Balb/C mice and C3H mice were equally susceptible, showing a stem cell mutation frequency approximately twice that of C57BL/6J (3.1+/-0.4 vs. 3.1+/-0.65 vs. 1.63+/-0.28x10(-4)). The observed differences among the strains with respect to somatic mutation following DMH treatment are likely to be due to the previously documented differences in metabolic conversion to the active metabolite. However, as ENU is a directly acting, rapidly inactivated mutagen, strain differences in response to ENU are unlikely to be due to strain-dependent metabolism of the mutagen and are likely to reflect differences in DNA repair efficiency, or possibly in stem cell kinetics among the strains studied. Susceptibility to the induction of colonic stem cell mutation is an important factor in susceptibility to carcinogens, whether due to differences in DNA repair or to other factors. Direct quantification of stem cell mutation frequency allows the separate identification of this component of the carcinogenic cascade and shows that it can make a major contribution to the differing susceptibility of different mouse strains.