In vivo DNA damage and resistance to 1-methyl-1-nitrosourea and 1,3-bis(2-chloroethyl)-1-nitrosourea in L1210 leukemia cells.

Damages in secondary DNA structure and inactivation or activation of some repair enzymes such as DNA polymerases alpha and beta and poly(ADP-riboso)polymerase induced by 1-methyl-1-nitrosourea (MNU) and 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) after administration of the drugs at therapeutic single doses to mice bearing parent L1210 leukemia cells (L1210/0) and MNU- and BCNU-resistant L1210 leukemia cells (L1210/MNU and L1210/BCNU) were studied. Damages in DNA structure of all three leukemia strains were investigated using centrifugation on alkaline or neutral sucrose gradients. More MNU-induced single-strand breaks (SSB) and alkali-labile lesions in L1210/0 and L1210/MNU cells were revealed in newly replicated DNA as compared with those in preexisting DNA. BCNU induced fewer SSB in newly replicated DNA of L1210/0 cells than MNU. The fastest repair of the damages in newly replicated DNA was detected in L1210/BCNU and especially in L1210/MNU cells as compared with L1210/0 cells. These results suggest that there are prone errors in the repair of DNA template as many SSB were revealed in the newly replicated DNA synthesized on the repaired DNA. Repair of DNA damages in L1210/BCNU and especially in L1210/MNU cells was accompanied by activation of DNA polymerases alpha and beta and poly(ADP-riboso)polymerase. It was shown that both DNA polymerases alpha and DNA polymerase beta were involved in the repair of damages induced by MNU and only DNA polymerase beta was involved in repair of damages induced by BCNU.