title: Effect of O6-methylguanine on DNA interstrand cross-link formation by chloroethylnitrosoureas and 2-chloroethyl(methylsulfonyl)methanesulfonate.
abstract: Exposure of HT29 cells in culture to O6-methylguanine is known to result in a reduction in O6-alkylguanine-DNA alkyltransferase (AGT) activity and an enhancement of sensitivity to the cytotoxic effects of chloroethylating agents. Since cytotoxicity of these agents may be mediated by the formation of interstrand cross-links, alkaline elution analysis was performed on HT29 cells treated with 1-(2-chloroethyl)-1-nitrosourea, 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea, and Clomesone [2-chloroethyl(methylsulfonyl)methanesulfonate] in the presence or absence of O6-methylguanine pretreatment to determine if the enhanced toxicity was due to an increase in the number of cross-links formed. Interstrand cross-linking by 1-(2-chloroethyl)-1-nitrosourea or 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea was increased by pretreatment with 0.4 mM O6-methylguanine for 24 h. Cross-linking by Clomesone was observed only in cells exposed to 0.4 mM O6-methylguanine for 24 h prior to administration of the drug and for 12 h after administration, suggesting that the resynthesis of the AGT may prevent the cross-linking by Clomesone. Complete recovery of AGT activity after reduction to 20 to 30% of the basal level upon treatment with 0.4 mM O6-methylguanine required between 8 h and 15 h in both HT29 cells and in Raji cells which were also sensitized to 1-(2-chloro-ethyl)-3-cyclohexyl-1-nitrosourea by exposure to O6-methylguanine. These data suggest that the enhancement of chloroethylnitrosourea toxicity after treatment with O6-methylguanine may be related to an increase in the number of DNA cross-links and that the relatively rapid rate of AGT recovery plays a role in prevention of cross-links resulting from Clomesone.
authors.affiliation: Department of Physiology, Milton S. Hershey Medical Center, Pennsylvania State University, College of Medicine, Hershey 17033.
authors.forename: M E
authors.initials: ME
authors.lastname: Dolan
authors.forename: A E
authors.initials: AE
authors.lastname: Pegg
authors.forename: N K
authors.initials: NK
authors.lastname: Hora
authors.forename: L C
authors.initials: LC
authors.lastname: Erickson
mesh_terms: D002455:Cell Division / Q000187:drug effects; D002470:Cell Survival / Q000187:drug effects; D003432:Cross-Linking Reagents; D004249:DNA Damage; D004260:DNA Repair; D005038:Ethylnitrosourea / Q000031:analogs & derivatives* / Q000378:metabolism / Q000633:toxicity; D006147:Guanine / Q000031:analogs & derivatives* / Q000378:metabolism; D006801:Humans; D008130:Lomustine / Q000378:metabolism / Q000633:toxicity; D008698:Mesylates / Q000378:metabolism* / Q000633:toxicity; D008780:Methyltransferases / Q000378:metabolism*; D019853:O(6)-Methylguanine-DNA Methyltransferase; D014407:Tumor Cells, Cultured
chemical_list: D003432:Cross-Linking Reagents; D008698:Mesylates; C017139:1-(2-chloroethyl)-1-nitrosourea; C041189:clomesone; D006147:Guanine; D008130:Lomustine; C008449:O-(6)-methylguanine; D008780:Methyltransferases; D019853:O(6)-Methylguanine-DNA Methyltransferase; D005038:Ethylnitrosourea