DNA modification and repair in vivo: towards a biochemical basis of organ-specific carcinogenesis by methylating agents

Alterations in global DNA methylation and metabolism-related genes caused by zearalenone in MCF7 and MCF10F cells

Zearalenone (ZEN) is a non-steroidal estrogenic mycotoxin produced by Fusarium fungi. ZEN has endocrine disruptor effects and could impair the hormonal balance. Here, we aimed at investigating possible effects of ZEN on metabolism-related pathways and its relation to epigenetic mechanisms in breast adenocarcinoma (MCF7) and breast epithelial (MCF10F) cells. Using the MTT and neutral red uptake (NRU) cell viability tests, IC₅₀ values of ZEN after 24 h were found to be 191 μmol/L and 92.6 μmol/L in MCF7 cells and 67.4 μmol/L and 79.5 μmol/L in MCF10F cells. A significant increase on global levels of 5-methylcytosine (5-mC%) was observed for MCF7 cells, correlating with the increased expression of DNA methyltransferases. No alterations were observed on levels of 5-mC% and expression of DNA methyltransferases for MCF10F cells. Further, at least threefold upregulation compared to control was observed for several genes related to nuclear receptors and metabolism in MCF7 cells, while some of these genes were downregulated in MCF10F cells. The most notably altered genes were IGF1, HK2, PXR, and PPARγ. We suggested that ZEN could alter levels of global DNA methylation and impair metabolism-related pathways.

Effects of inhibition of O6-alkylguanine-DNA alkyltransferase in rats on carcinogenesis by methylnitrosourea and ethylnitrosourea

Many alkylating agents are potent carcinogens and there is considerable evidence that the formation of O6-alkylguanine in DNA can lead to mutations and the initiation of neoplastic growth. The repair of O6-methyl- or O6-ethylguanine in DNA is known to be brought about by the action of a protein termed O6-alkylguanine-DNA alkyltransferase. In order to investigate the role of this activity in the carcinogenic action of methylnitrosourea and ethylnitrosourea, O6-benzylguanine, a potent inhibitor of the alkyltransferase, was used. Groups of 20 female F344 rats were treated with the nitrosourea (0.2 mmol) by gavage in 10 weekly doses and a parallel group was also treated with 4 mg of O6-benzylguanine, 2 h prior to each dose of the nitrosourea. This dose of O6-benzylguanine was sufficient to reduce the alkyltransferase activity to zero in the liver for at least 8 h but activity had returned to about 60% of normal within 24 h. Animals were maintained until they became moribund, when they were killed, or until death related to tumors. The median week of death in the animals receiving methylnitrosourea was reduced from 60 wk to 52 wk by co-treatment with O6-benzylguanine. There was a smaller reduction from 55 to 50 wk in the rats receiving ethylnitrosourea. The treatment with O6-benzylguanine caused no significant change in the incidence of the principal tumors induced by the alkylnitrosoureas and there were no liver tumors produced by the combined treatments. These results show that the level of inactivation of alkyltransferase produced by this dose of O6-benzylguanine was not sufficient to greatly alter the potent carcinogenic effect of these doses of alkylnitrosoureas in this system.

Structure-activity relations in carcinogenesis by N-nitroso compounds

For a large number of N-nitroso compounds a comparison of their carcinogenic effects in rats and Syrian golden hamsters has been made. Nitrosamines, which require metabolic activation, and nitrosoalkylamides, which do not, produce quite different tumor responses. There are also large differences in the types of tumor induced in rats and in hamsters. In all the studies doses of the various compounds, equimolar to the extent that was possible, are administered orally. Continuous doses (in drinking water or food) often produce a response different from that after administration of the same compound in pulsed doses (by gavage), even though the same total dose is delivered. Continuous doses of nitrosamines are usually more effective than pulsed doses, but with the nitrosoalkylureas, the reverse is more generally the case. Rat and hamster liver is a common target of many nitrosamines, but rarely of nitrosamides. The most common site of tumor induction in rats by N-nitroso compounds is the esophagus, but the hamster esophagus never responds. The pancreas duct of the hamster is a common target of nitrosamines containing a beta-oxygenated propyl group, but pancreas duct tumors are never seen in rats. Nitrosomethyl-n-alkylamines (with an even numbered carbon chain) induce bladder tumors in rats and hamsters. Many nitrosoalkylureas induce tumors of the nervous system in rats, as well as a great variety of other tumors. In hamsters, nitrosoalkylureas give rise only to tumors of the forestomach and spleen, but no tumors of the nervous system. The similar carcinogenic actions of certain groups of N-nitroso compounds can be related to their generation, directly or by metabolism, of similar simple moieties having certain organs as their target.

Carcinogenesis by oxygenated nitrosomethylpropylamines in Syrian hamsters

Three oxygenated propylnitrosomethylamines were administered to female Syrian hamsters at doses similar to those which had induced high incidences of esophageal neoplasms in rats. Nitrosomethyl-2-oxopropylamine (NMOP) given at the rate of 2 mg/animal/week, whether as one application of 2 mg or two applications of 1 mg, led to early death of the animals, mostly with liver neoplasms; administration of 1 mg/animal/week led to longer survival, but most animals died with both liver neoplasms and neoplasms of the nasal mucosa. Only one hamster treated with NMOP had a neoplasm of the pancreatic duct. Of the 14 hamsters treated with the higher dose of nitrosomethyl-2-hydroxypropylamine (NMHP) and surviving beyond 6 weeks, most had liver neoplasms and nine had neoplasms of the pancreatic ducts. At the lower dose of NMHP, most hamsters developed neoplasms of the nasal mucosa, as did those receiving the same dose of NMOP, and seven animals had hemangioendothelial tumors of the liver, but only one animal had a carcinoma of the pancreatic duct. Nitrosomethyldihydroxypropylamine (NMDHP) was a much weaker carcinogen than the other two compounds and induced mainly neoplasms of the nasal mucosa, with little shortening of life.

Species differences in nitrosamine carcinogenesis

The carcinogenic action of approximately 50 N-nitroso compounds, nitrosamines, and nitrosoalkylamides has been compared in rats and in Syrian golden hamsters. The compounds were administered PO, as far as possible at comparable dose rates. The relative potencies of the treatments were assessed mainly by the time to death of the animals with tumors. The esophagus and other parts of the upper gastrointestinal tract were the most common sites for tumor induction in rats, but the esophagus was hardly ever affected in hamsters, although several compounds induced tumors of the forestomach in both rats and hamsters. No conclusion could be drawn about the relative susceptibility of the rat and hamster to these N-nitroso compounds, which varied with different compounds. Few generalizations can be made about these results, although it appeared that the 2-hydroxypropyl group was usually necessary for the induction of pancreas tumors in hamsters.

Induction of tumors of the esophagus in rats by nitrosomethylalkylamines

The carcinogenic effectiveness of nitrosomethyl-n-butylamine,-n-hexylamine and -n-heptylamine was compared by administration to F344 rats at various concentrations in drinking water and by gavage in oil. Nitrosomethylbutylamine induced only tumors of the upper gastrointestinal tract, and was very toxic. Nitrosomethylhexylamine was less toxic and induced tumors of the liver and lung, as well as those the upper GI tract, when given by gavage, but only tumors of the GI tract when given in drinking water. In contrast, nitrosomethylheptylamine induced tumors of the liver and lung when given in drinking water or by gavage, but there were tumors of the esophagus only in those rats treated via the drinking water. As measured by the dose needed to produce a given carcinogenic effect, nitrosomethylheptylamine was a weaker carcinogen than the other two nitrosamines, but was not inactive as has been reported previously.