Pretreatment of human colon tumor cells with DNA methylating agents inhibits their ability to repair chloroethyl monoadducts

Publisher Website
Google Scholar
Abstract
A variety of human tumor cell lines are capable of preventing chloroethylnitrosourea (CNU)-induced DNA crosslinks, presumably by removing guanine O6-chloroethyl DNA monoadducts before crosslinks can form. Those cells capable of preventing crosslinking were of the Mer+ (Methylation repair) phenotype, and have been shown to be proficient at repair of O6-methyl-guanine adducts by the repair enzyme guanine O6-methyl-transferase. Mer- tumor cell lines are deficient at O6-methyl-guanine repair, incapable of preventing CNU interstrand crosslinking, and have recently been shown to lack the repair enzyme O6-methyl-transferase. Pretreatment of Mer+ cells (HT-29 human colon carcinoma cells and IMR-90 normal human fibroblasts) with the DNA methylating agent MNNG [N-methyl-N''-nitro-N-nitrosoguanidine], under conditions which should inactivate O6-methyl-transferase, apparently saturates the monoadduct repair system, and allows CNU to form interstrand crosslinks in these cells. This effect was also seen when MNU [N-methyl-N-nitrosourea] pretreatment was used, but not with MMS [methyl methanesulfonate] or streptozotocin. The formation of CNU-induced interstrand crosslinks following MNNG or MNU pretreatment was coincident with a dramatic increase in cytotoxicity as measured by colony formation assays. In contrast, cytotoxicity was only slightly increased when MMS or streptozotocin pretreatment was used.