Pretreatment of normal human fibroblasts and human colon carcinoma cells with MNNG allows chloroethylnitrosourea to produce DNA interstrand crosslinks not observed in cells treated with chloroethylnitrosourea alone

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Abstract
Chloroethylnitrosoureas (CNU) are antitumor agents which produce DNA interstrand crosslinks. We have proposed that crosslinks are produced in DNA via monoadduct formation at the guanine-O 6 position, followed by a delayed reaction with the opposite DNA strand. Human cells are known to differ in their capacity to repair the O 6 -methylguanine lesion. One example of this repair capacity is the ability of cells to reactivate adenovirus which has been damaged by in vitro treatment with N-methyl-N′-nitro-N-nitrosoguanidine (MNNG). Cells that repair the virus are designated Mer + and deficient cells Mer − . In a recent report, we showed a clear correlation between CNU-induced DNA interstrand cross-linking and the Mer phenotype. Mer − cells produced consistently higher levels of interstrand crosslinks than did Mer + cells. In the present study we have measured the CNU-induced DNA interstrand crosslinking in IMR-90 normal human fibroblasts (Mer + ), HT-29 human colon carcinoma cells (Mer + ), and VA-13 SV-40 transformed human cells (Mer − ) following pretreatment with MNNG. Cells were treated for 1 h with MNNG, then for an additional 1 h with CNU. Comparable levels of CNU-induced DNA interstrand crosslinking were observed in all cell lines. This crosslinking has been previously undetected in the IMR-90 and HT-29 cells. Cytotoxicity studies showed that MNNG pretreatment greatly enhanced the killing of IMR-90 and HT-29 cells by CNU, however, in VA-13 cells the increase in cell kill was smaller. These data suggest that in Mer + cells a DNA repair system may remove chloroethyl monoadducts before the lethal DNA interstrand crosslinks can form. However, pretreatment of cells with MNNG may saturate this repair system rendering it inoperable.