Lack of genetic and in vitro metabolic activity of potently carcinogenic azoxyalkanes

Aeons of distress: an evolutionary perspective on the bacterial SOS response

The SOS response of bacteria is a global regulatory network targeted at addressing DNA damage. Governed by the products of the lexA and recA genes, it co-ordinates a comprehensive response against DNA lesions and its description in Escherichia coli has stood for years as a textbook paradigm of stress-response systems in bacteria. In this paper we review the current state of research on the SOS response outside E. coli. By retracing research on the identification of multiple diverging LexA-binding motifs across the Bacteria Domain, we show how this work has led to the description of a minimum regulon core, but also of a heterogeneous collection of SOS regulatory networks that challenges many tenets of the E. coli model. We also review recent attempts at reconstructing the evolutionary history of the SOS network that have cast new light on the SOS response. Exploiting the newly gained knowledge on LexA-binding motifs and the tight association of LexA with a recently described mutagenesis cassette, these works put forward likely evolutionary scenarios for the SOS response, and we discuss their relevance on the ultimate nature of this stress-response system and the evolutionary pressures driving its evolution.

A stable and sensitive genotoxic testing system based on DNA damage induced gene expression in Saccharomyces cerevisiae

A sensitive and stable genotoxic testing system has been developed based on the induction of a Saccharomyces cerevisiae RNR3-lacZ reporter gene expression in response to a broad range of DNA-damaging agents and agents that interfere with DNA synthesis. All 11 tested known carcinogenic and genotoxic agents, ranging from DNA alkylating agents, oxidative chemicals and radiations, were able to induce RNR3-lacZ expression at a sublethal dose. In particular, a potent colon carcinogen, 1,2-dimethyl hydrazine (SDMH), was not detected as a mutagen by a standard Ames test, but was able to induce RNR3-lacZ expression. In contrast, both non-mutagenic and non-genotoxic chemicals tested were unable to induce RNR3-lacZ expression. We have compared three yeast DNA damage-inducible genes for their sensitivity and inducibility, and found that RNR3 is more sensitive than RNR2 and MAG1. The effects of damage dose, post-treatment incubation time and cell growth stage on RNR3-lacZ expression have also been determined and optimized. In order to create a stable and user-friendly testing system, we integrated the RNR3-lacZ cassette into the yeast genome and demonstrated that its inducibility is indistinguishable from that of plasmid-based studies.

Effects of vitamins A and E on methylazoxymethanol-induced mutagenesis in Salmonella typhimurium strain TA100

The aim of this study is to report the antimutagenic effect of vitamin A and vitamin E towards methylazoxymethanol (MAM)-induced mutagenesis in Salmonella typhimurium strain TA100 sensitive to alkylating agents. In order to characterize different levels of action of these two fat-soluble vitamins towards the mutagenicity of MAM, several assays have been considered to show the antimutagenic effect and the possible interactions of vitamins with MAM or with the bacteria. Thus, for each vitamin, three different assays with three different incubations have been conducted: (i) MAM, bacteria and vitamins together, (ii) MAM and vitamins, (iii) bacteria and vitamins. The results showed that both vitamins A and E present an antimutagenic effect towards MAM induced mutagenesis. alpha-Tocopherol seems to have an action directly on to the mutagenic agent, whereas the action of retinol is likely due to a protection of the bacterial genoma against MAM. These in vitro results could help to interpret results of colon carcinogenesis studies using animals induced by 1,2-dimethylhydrazine and fed vitamins supplemented diet.

Mutational specificities of environmental carcinogens in the lacl gene of Escherichia coli H. V: DNA sequence analysis of mutations in bacteria recovered from the liver of Swiss mice exposed to 1,2-dimethylhydrazine, azoxymethane, and methylazoxymethanolacetate

The host-mediated assay (HMA) was used to determine the spectra of mutations induced in the lacl gene of Escherichia coli cells recovered from the livers of Swiss mice exposed to the carcinogens 1,2-dimethylhydrazine (SDMH), azoxymethane (AOM), and methylazoxymethanolacetate (MAMA). These spectra were further compared with changes induced by dimethylnitrosamine (DMNA) in the HMA methodology. A total of 177 independent lacl mutations arising in the HMA following exposure to SDMH, AOM, and MAMA were analyzed. Single-base substitutions accounted for 97% of all mutations analyzed. The vast majority of the single-base substitutions consisted of G:C----A:T transitions (94% of all mutations). The remaining mutations consisted of A:T----G:C transitions (3% of all mutations) while non-base substitutions accounted for only 3% of the total mutagenesis. The latter mutations consisted of one frameshift mutation and four lacO deletions. The distribution of G:C----A:T transitions induced by the three chemicals in the first 200 bp of the lacl gene was not random, but rather clustered at sites where a target guanine was flanked at the 5' site by a purine residue.

Deuterium isotope effects in carcinogenesis by N-nitroso compounds

A number of N-nitroso compounds and an azoxyalkane have been labeled with deuterium in various positions and have been administered to rats, hamsters, or mice in parallel with the unlabeled compounds. The treatments with the labeled and analogous unlabeled compounds were equimolar and for the same time. Mortality rates from tumors and tumor incidences were compared between deuterium-labeled and the unlabeled analogs. In many cases more than one dose level was used for the comparisons. An increased rate of mortality from tumors or an increased incidence of induced tumors was considered an index of increased potency of one treatment compared with the other. Using these criteria deuterium in the alpha positions of nitrosodimethylamine, nitrosomorpholine, nitrosoheptamethyleneimine, and nitrosoazetidine reduced carcinogenic potency compared with the unlabeled compounds. This indicated that cleavage of a carbon-hydrogen bond in the alpha position was a rate-limiting step in carcinogenesis by these nitrosamines. In both nitrosomethylethylamine and nitroso-2,6-dimethylmorpholine, the presence of deuterium at different positions increased or decreased carcinogenic potency, suggesting that competition for oxidation between these sites might be the determining factor in activation of the molecule. This also applied to nitrosomethyl-n-butylamine and nitrosomethyl-phenylethylamine with deuterium at the methyl group or at the alpha carbon of the butyl or phenylethyl groups, and to azoxymethane with deuterium in the 1-methyl or 4-methyl group. In nitrosomethylcyclohexylamine, nitrosomethyl-n-dodecylamine, and dinitroso-2,6-dimethylpiperazine there was no detectable effect of deuterium on carcinogenic potency, suggesting that the conditions did not provide sufficient sensitivity for detection of an isotope effect, or that oxidation at the alpha carbon was not a rate-limiting step in carcinogenesis by these molecules.