Reduced level of DNA cross-links and sister chromatid exchanges in 1,3-bis(2-chloroethyl)-1-nitrosourea-resistant rat brain tumor cells.

We found that 9L-2 cells, a cell line derived from the in vivo 9L rat brain tumor model, are approximately 8-fold more resistant to the cytotoxic effect of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) than are sensitive 9L cells. Treatment with BCNU induces sister chromatid exchanges in both lines, but to produce similar levels of exchanges, 9L-2 cells must be treated with a 14-fold higher concentration of BCNU. The extent of DNA methylation was the same in both cell lines after a 1-hr treatment with 100 microM methylnitrosourea. While the levels of the alkylation products N-7-methylguanine and N-3-methyladenine were similar in both lines, the level of O6-methylguanine was 20% lower in 9L-2 than in 9L cells, which implies that 9L-2 cells repair O6-alkylguanine derivatives more efficiently than do 9L cells. The number of DNA interstrand cross-links formed in 9L-2 cells after treatment with BCNU was approximately 50% of the number formed in 9L cells. These results suggest that the repair of O6-alkylguanine derivatives formed in BCNU-treated 9L-2 cells may be related to the reduced number of DNA interstrand cross-links formed and may have a role in the mechanism of cellular resistance of 9L-2 cells to BCNU. However, our results indicate that, in itself, the reduction in the number of DNA cross-links may not be sufficient to account entirely for the cellular resistance of 9L-2 cells to BCNU and suggest that additional mechanisms may be involved in cellular resistance of 9L-2 cells to BCNU treatment.