Inhibition of O6-alkylguanine-DNA-alkyltransferase activity potentiates cytotoxicity and induction of SCEs in human glioma cells resistant to 1,3-bis(2-chloroethyl)-1-nitrosourea

Abstract

SF-188 is a human glioma-derived cell line resistant to the cytotoxic effects of and the induction of sister chromatid exchanges (SCEs) by 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). Pretreatment of SF-188 cells with N-methyl-N-nitrosourea (MNU) for 1 h increased the cytotoxicity of a 1-h treatment with BCNU 2-to 10-fold and doubled the number of SCEs; the magnitude of these effects was dependent on the dose of both agents. Treatment of SF-188 cells with MNU resulted in a dose-dependent inhibition of O⁶-alkylguanine-DNA-alkyltransferase (O⁶-AT) activity. Low doses of MNU, which did not significantly inhibit O⁶-AT, did not potentiate SCE induction. Higher doses of MNU inhibited O⁶-AT and potentiated cytotoxicity and the induction of SCEs. These results are consistent with the hypothesis that in resistant cells treated with BCNU, O⁶-AT repairs O⁶-chloroethylguanine before it can form a DNA interstrand cross-link. Inhibition of this enzyme allows for the formation of BCNU-induced DNA interstrand cross-links resulting in increases in cytotoxicity and induction of SCEs. The correlation between cytotoxicity and the induction of SCEs suggests that measurement of SCEs may be useful for determining the cellular response of normal and tumor cells to in vivo treatment with combinations of chemotherapeutic agents.