Formation and disappearance of DNA interstrand cross-links in human colon tumor cell lines with different levels of resistance to chlorozotocin.

Three human colon tumor (HCT) cell lines, designated C, Moser and 116, exhibiting a gradation of resistance to chlorozotocin, a glucose-linked chloroethylnitrosourea (1-, 2.9-, and 5.8-fold respectively) were examined to assess the determinants of drug sensitivity. Although the O6-alkylguanine-DNA transferase content was relatively higher in the most resistant 116 cells than in the sensitive cell line C, its level in Moser cells did not correlate with the intermediate chlorozotocin sensitivity. Glutathione content in these tumor cell lines did not show a parallelism with drug resistance. The ethidium bromide fluorescence assay was used to quantitate the kinetics of DNA interstrand cross-link formation and its removal after drug exposure. The peak levels of DNA interstrand cross-links induced in HCT cells correlated with their resistance to chlorozotocin with cross-link indices of 0.03, 0.10 and 0.20, respectively, for 116, Moser and C cell lines. All three cell lines demonstrated DNA cross-link repair to different extents. While the smaller number of cross-links formed in resistant 116 and Moser cells were eliminated in a rapid phase of repair, the lesions formed at a much greater frequency in C cells remained largely unrepaired. These results draw attention to the role of increased DNA cross-link repair as a mechanism of nitrosourea resistance in the HCT cells studied.