Effect of ethanol treatment on metabolic activation and detoxification of esophagus carcinogenic N-nitrosamines in rat liver

Abstract

In order to elucidate the mechanism underlying enhancement by ethanol of N-nitrosodiethylamine (DEN)- and N-nitrosomethylbenzylamine (NMBA)-induced esophageal tumorigenesis in rats, hepatic levels of cytochrome P-450 (CYP) enzymes, mutagenic activation of several N-nitrosamines and three kinds of UDP-glucuronyltransferase (UDPGT) activities were assayed in F344 rats. Immunoblot analyses of microsomal CYP proteins revealed induction of CYP2E1 (approximately 2-fold), but not CYP2B1/2, 1A1/2 or 3A2, by treatment with 10% ethanol in the drinking water for 2 weeks. In contrast, s.c. treatment with 0.5 mg/kg NMBA three times per week for 2 weeks produced no significant alterations in the levels of these CYP species. Ethanol treatment also elevated the mutagenic activities of N-nitrosodimethylamine (DMN), DEN and N-nitrosopyrrolidine (NPYR) in strain TA100 up to 2.1-, 1.6- and 2.3-fold above each control, respectively. However, this was not the cases for four N-nitrosamines, including NMBA, in strain TA100 and two heterocyclic amines and aflatoxin B(1) in strain TA98. In addition, ethanol did not affect UDPGT activities towards 4-nitrophenol, bilirubin and testosterone. Hepatic CYP species responsible for mutagenic activation of selected N-nitrosodialkylamines were confirmed by use of specific CYP inducers and inhibitors with the liver from F344 and Wistar rats, indicating that DMN, DEN and NMBA are selectively activated by CYP2E1, predominantly by CYP2E1 with a slight contribution by CYP2B2 and selectively by CYP2B1/2, respectively. These results demonstrate that ethanol exerts an enhancing effect on mutagenic activation by CYP2E1 of DMN, DEN and NPYR, but does not affect that of NMBA and the other carcinogens by CYP2B1/2, 1A1/2 and 3A2 and UDPGT1A1, 1A6 and 2B1 activities. Consequently, this suggests that enhancement by ethanol of DEN-induced esophageal carcinogenesis in F344 rats can be attributed to an increase in hepatic activation during the initiation phase, but that of NMBA-induced tumorigenesis is not attributable to metabolic activation and inactivation via glucuronidation in liver.

DNA DAMAGE MEASURED BY COMET ASSAY AND 8-OH-dG FORMATION RELATED TO BLOOD CHEMICAL ANALYSES IN AGED RATS

“…As another alkylating agent, DEN was used in this study, and DEN is metabolically activated mainly by CYP2E1 to alkylate DNA (Mori et al, 2002). In this study, DEN increased DNA migration at pH 12.1 and pH 13, and the largest portion of the DNA migration was considered to be SSBs, because the DNA migration at pH 13 at 3 hr after DEN-treatments was comparable to or slightly more than at pH 12.1.…”

Cytochrome P450 and GlutathioneS-Transferase mRNA Expression in Human Fetal Liver Hematopoietic Stem Cells

“…Alternatively, it is possible that the lack of detectable CYP1A protein in fetal HSC could be explained by alternative CYP1A1 gene splicing during translation that occurs in rat and human brain (Chinta et al, 2005). Similar to CYP1A2, CYP2E1 is expressed at low levels in fetal liver with up-regulation of expression occurring at birth (Mori et al, 2002). CYP2E1 is the highaffinity liver P450 isoform involved in the metabolism and activation of several aliphatic and aromatic halogenated hydrocarbons (Hines and McCarver, 2002;Gonzalez, 2005).…”

Interindividual variability in hepatic drug glucuronidation: studies into the role of age, sex, enzyme inducers, and genetic polymorphism using the human liver bank as a model system

“…Although the mechanism for this effect is unknown, one study suggested that it was dependent on the presence of hepatic Kupfer cells (Kardon et al, 2002). On the other hand another study that involved adding 10% alcohol to the drinking water of rats showed no effect on hepatic bilirubin, nitrophenol or testosterone glucuronidation (Mori et al, 2002). …”