Enhancement of O6-methylguanine-DNA-methyltransferase activity induced by various treatments in mammalian cells.

The O6-methylguanine-DNA-methyltransferase (methyltransferase) activity was determined in a rat hepatoma cell line after treatment with ultraviolet or gamma-irradiation, heat treatment, or incubation with cis-diamminedichloroplatinum(II),2-methyl-9-hydroxyellipticinium, or bleomycin. The assay measured the removal of O6-methylguanine from 3H-alkylated DNA by cellular extracts. The results show that 48 h after the various treatments, the methyltransferase activity is increased by 2- to 5-fold. This increase is due to de novo specific protein synthesis. It is not related to a modification of the cell cycle parameters, as a similar enhancement is observed in plateau-phase cells treated with ionizing radiations or cis-dichlorodiammineplatinum(II). The increase of the methyltransferase activity measured using an alkylated substrate represents an actual increase of the active molecules in the cells, as the mutation frequency is much lower in cells treated with N-methyl-N'-nitro-N-nitrosoguanidine 48 h after an irradiation (3 Gy) than in nonirradiated cells. This induction of the methyltransferase was not observed in Chinese hamster ovary cells after gamma-irradiation, and therefore it does not seem to occur in cells which have a low constitutive level of O6-methylguanine repair.