Modulation of genotoxicity of azathioprine by intracellular glutathione in hepatocytes.

The role of intracellular glutathione (GSH) against the genotoxicity of azathioprine (AZA) was examined by the use of rat hepatocytes and alkaline and neutral elution techniques. Treatment of hepatocytes with AZA for 3 h induced DNA fragmentation in alkaline conditions but not in neutral conditions. A dose-dependent increase in DNA single-strand breaks was observed with the treatment of AZA ranging from 0.3 mM to 1.0 mM with concomitant cytotoxicity. However, neither 6-mercaptopurine, which is a major metabolite of AZA, nor 6-mercaptopurine riboside, an active form of the former, induced the DNA damage at the same concentrations. Moreover, the elution rate of DNA fragmentation, even at low dose of AZA that is not cytotoxic, significantly increased in the presence of buthionine sulfoximine, which is a selective inhibitor of gamma-glutamylcysteine synthetase; i.e., depletion of GSH in hepatocytes enhanced the DNA damage by AZA. Thus, AZA has been proved to be genotoxic, and the genotoxicity is likely to be protected by GSH present in hepatocytes, suggesting that GSH depletion potentiates the carcinogenic effect of AZA.