title: O6-methylguanine-DNA methyltransferase activity and induction of novel immunogenicity in murine tumor cells treated with methylating agents.
abstract: To investigate the mechanism of the generation of immunogenic tumor variants by mutagenic drugs, murine leukemia cells exhibiting different sensitivity to killing by the alkylator 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and different ability to repair O6-methyl-guanine in their DNA were treated in vitro with a series of methylating agents, including triazene derivatives, temozolomide, and streptozotocin. At the population level, we found that BCNU-resistant cells (L1210/BCNU) that appeared to be cross-resistant to killing by a dimethyltriazene and expressed high levels of O6-methylguanine-DNA methyltransferase activity (mer+ phenotype) failed to generate highly immunogenic variant sublines on repeated exposure to the methylating agents. In contrast, all cells (L1210) that were susceptible to DNA alkylation damage and deficient in O6-methylguanine repair (mer-) developed immunogenic variant sublines. A noticeable exception was represented by streptozotocin treatment, which was equally effective in mer+ and mer- cells. At the clonal level, a single exposure to streptozotocin or a triazene derivative resulted in a high incidence (33% and 50%, respectively) of immunogenic cell generation in mer- cells only. In mer+ cells, streptozotocin treatment led to a 33% incidence of immunogenic clones only when the cells were concurrently exposed to O6-methylguanine as a free base. The activity of O6-methylguanine-DNA methyltransferase in mer+ cells was greatly reduced by treatment with O6-methylguanine or streptozotocin, and the combination of the two drugs led to enzyme levels similar to those observed in mer- cells. Taken together, these data suggest that the mechanism of O6-alkylation may be operative in the induction of novel tumor-cell antigenicity by methylating agents.
authors.affiliation: Dip. Medicina Sperimentale-Sez. Farmacologia, Perugia, Italy.
authors.forename: R
authors.initials: R
authors.lastname: Bianchi
authors.forename: L
authors.initials: L
authors.lastname: Citti
authors.forename: R
authors.initials: R
authors.lastname: Beghetti
authors.forename: L
authors.initials: L
authors.lastname: Romani
authors.forename: M
authors.initials: M
authors.lastname: D'Incalci
authors.forename: P
authors.initials: P
authors.lastname: Puccetti
authors.forename: M C
authors.initials: MC
authors.lastname: Fioretti
mesh_terms: D000477:Alkylating Agents / Q000494:pharmacology*; D000478:Alkylation / Q000187:drug effects; D000818:Animals; D004249:DNA Damage; D004260:DNA Repair / Q000187:drug effects; D004273:DNA, Neoplasm / Q000187:drug effects; D005260:Female; D007939:Leukemia L1210 / Q000201:enzymology* / Q000276:immunology; D008297:Male; D008745:Methylation / Q000187:drug effects; D008780:Methyltransferases / Q000187:drug effects* / Q000378:metabolism; D051379:Mice; D008807:Mice, Inbred BALB C; D008811:Mice, Inbred DBA; D009368:Neoplasm Transplantation; D019853:O(6)-Methylguanine-DNA Methyltransferase; D014407:Tumor Cells, Cultured / Q000187:drug effects
chemical_list: D000477:Alkylating Agents; D004273:DNA, Neoplasm; D008780:Methyltransferases; D019853:O(6)-Methylguanine-DNA Methyltransferase