Mutagenesis induced by xenobiotics : factors of importance and modification, with special emphasis on glutathione and glutathione transferases

Detta är en avhandling från Stockholm : Stockholm University

Sammanfattning: Genetic changes induced by chemical agents can result in cancer. Such tumor formation can be linked to activationof oncogenes, often through specific, definable mutagenic events, e.g., single base-pair mutation in specific codons.Most mutagenic and carcinogenic compounds must be activated in order to obtain mutagenic and carcinogenicproperties. This activation can be spontaneous through chemical reactions with other compounds, but is mostfrequently mediated through the action of enzymes. Bioactivated compounds can subsequently be detoxicated throughenzymatic conjugation with hydrophilic biomolecules or, occasionally in spontaneous reactions.This thesis deals with certain factors influencing the chain of event resulting in mutagenicity detected in vitro or invivo. Mutagenicity was detected as HPRT mutants in V79 Chinese hamster cells or by the use of the Somatic Mutationand Recombination Test in Drosophila melanogasler. V79 cells, which lack the capacity to bioactivate the modelcompounds studied, were also used as target cells for the detection of mutagenicity in co-cultivation with other cellscompetent in such bioactivation. By the use of this technique, it was possible to investigate factors of importance forbioactivation and detoxication in both primary non-dividing cells, as well as in various established cell lines.During this study several model compounds have been used. Compounds that either require metabolic activation toobtain mutagenic properties or that are spontaneously activated. Compounds belonging to the former group werebenzo(a)pyrene (B(a)P), its 7,8-diol and 1,2-dichloroethane (DCE). The latter group was represented byA'-methyl-A'1.nitro-W-nitrososoguanidine (MNNG), V-ethyl-W -nitro-W-nitrososoguanidine (ENNG) and nitrosocimetidine (NC).The results obtained are presented in six separate publications. In paper I co-cultivation experiments with alveolarmacrophages (PAMs) show that B(a)P-induced mutagenicity was enhanced 5-10 fold as a consequence of thephagocytic process triggered by the addition of particles. In the study presented in paper II the principal aim was tocompare human PAMs from smokers and non-smokers with respect to their capacity to induce mutations in cocultivatedV79 cells when treated with B(a)P-7,8-diol. No statistical difference between the two groups was detected.Papers IV and V deals with the importance of glutathione transferases (GSTs) for the mutagenicity. The resultspresented in paper IV show the protective role of certain GSTs in mutagenesis induced by polycyclic aromatichydrocarbons. However, the results presented in paper V indicate that GSTs can alto participate in bioactivation ofcertain compounds, e.g., DCE.In papers III and VI the influence of glutathione and other thiols on the mutagenicity of MNNG, ENNG and NC isinvestigated. These compounds are spontaneously activated by nucleophiles such as thiols. If this activation occursextracellularly, the products also react extracellularly with molecules in the surroundings. Therefore, due to their highreactivity these products are unable to reach the DNA and cause mutations. On the other hand, if this activation occursintracellularly, these compounds are highly mutagenic and the mutagenicity is also dependent on the intracellular levelof thiols.