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Bioactivation of asymmetric N-dialkylnitrosamines in rat tissues derived from the ventral entoderm
B Ludeke1, T Meier, P Kleihues
1Institute of Pathology, University of Zurich, Switzerland.
Abstract:
Aliphatic methylalkylnitrosamines with a chain length of three to six carbon atoms are powerful oesophageal carcinogens in rats and have been shown to methylate target organ DNA preferentially. This class of carcinogens is efficiently metabolized not only in the oesophageal mucosa but also in the mucosa of the nasal and oral cavity, trachea and bronchioli, i.e., tissues derived from the rat ventral entoderm. In order to determine whether more than one cytochrome P450 isozyme is involved in the bioactivation of asymmetrical aliphatic dialkylnitrosamines in these tissues, we have studied the effects of various modulators of nitrosamine metabolism, including dietary zinc deficiency, ethanol and disulfiram, on DNA alkylation by N-nitrosomethyl-n-butylamine (NMBA) and its ethyl analogue N-nitrosoethyl-n-butylamine (NEBA). Formation of O6-methyl- and O6-ethyldeoxyguanosine by a single dose of NMBA and NEBA, respectively, was quantified after a survival time of 6 h by immuno-slot-blot assay. In control rats, methylation of DNA by NMBA was highest in oesophagus, followed by nasal mucosa, liver and lung. Formation of O6-ethyldeoxyguanosine from NEBA, however, was twice as high in liver as in nasal mucosa and lung and four times as high in liver as in oesophagus. In oesophagus, trachea and bronchioli, both nitrosamines were selectively metabolized in mucosal cells. Bioactivation of NMBA and NEBA was almost completely inhibited in nasal mucosa by ethanol. In contrast, a striking interorgan shift in DNA methylation by NMBA from liver (-50%) to lung (+100%), oesophagus (+300%) and nasal mucosa (+400%) was obtained with dietary disulfiram, whereas only 20-50% increases in extrahepatic DNA ethylation were determined for NEBA.(ABSTRACT TRUNCATED AT 250 WORDS)
Insights
This study reveals that different modulators affect nitrosamine metabolism differently. Ethanol inhibits bioactivation in nasal tissue, while disulfiram shifts DNA methylation from the liver to other organs.
Area of Science:
- Toxicology
- Carcinogenesis
- Biochemistry
Background:
- Aliphatic methylalkylnitrosamines are potent esophageal carcinogens in rats.
- These compounds preferentially methylate DNA in target organs, including the esophagus, nasal cavity, trachea, and bronchioli.
- The metabolism and bioactivation of these carcinogens involve cytochrome P450 isozymes.
Purpose of the Study:
- To investigate the involvement of multiple cytochrome P450 isozymes in the bioactivation of asymmetrical aliphatic dialkylnitrosamines.
- To examine the effects of dietary zinc deficiency, ethanol, and disulfiram on DNA alkylation by N-nitrosomethyl-n-butylamine (NMBA) and N-nitrosoethyl-n-butylamine (NEBA).
Main Methods:
- Quantification of O6-methyl- and O6-ethyldeoxyguanosine formation using immuno-slot-blot assay after a single dose of NMBA and NEBA.
- Assessment of DNA methylation levels in various rat tissues (esophagus, nasal mucosa, liver, lung, trachea, bronchioli).
- Evaluation of the impact of ethanol and disulfiram on nitrosamine-induced DNA alkylation.
Main Results:
- In control rats, NMBA-induced DNA methylation was highest in the esophagus, followed by nasal mucosa, liver, and lung.
- NEBA-induced DNA ethylation was highest in the liver, followed by nasal mucosa, lung, and esophagus.
- Ethanol significantly inhibited the bioactivation of both NMBA and NEBA in the nasal mucosa.
- Disulfiram caused a substantial interorgan shift in NMBA-induced DNA methylation from the liver to the lung, esophagus, and nasal mucosa.
Conclusions:
- The bioactivation of NMBA and NEBA is selectively mediated by mucosal cells in the esophagus, trachea, and bronchioli.
- Ethanol and disulfiram differentially modulate the metabolism and DNA alkylation patterns of aliphatic dialkylnitrosamines.
- These findings suggest the involvement of multiple cytochrome P450 isozymes in nitrosamine bioactivation and highlight the potential for therapeutic intervention through modulator agents.