Induction of O6-methylguanine-DNA-methyltransferase and N3-methyladenine-DNA-glycosylase in human cells exposed to DNA-damaging agents.

The inducibility of two DNA repair proteins, the O6-methylguanine-DNA-methyltransferase (MGMT) and the N3-methyladenine-DNA-glycosylase (ANPG), was studied by measuring the protein activities and the transcription of the MGMT and ANPG genes in a human hepatoma cell line (LICH cells). The two protein activities are enhanced after treatment with a variety of DNA-damaging agents. They are maximum 72 hr after the inducing treatments and remain elevated for about 120 hr. This induction is abolished when the cells are grown in the presence of protein or RNA synthesis inhibitors. Northern blot analysis shows that the DNA-damaging agents increase to different extents the transcription of the MGMT or ANPG genes. The transferase activity is also increased by DNA damage in a human glioblastoma cell line (T98G cells), but is not significantly modified in human normal fibroblasts, suggesting that this repair activity enhancement might occur preferentially in transformed cells, as we have previously shown for cells of rat origin. Therefore, these increased repair activities may play an important role in removing the lethal N3-methyladenine residues, the promutagenic O6-methylguanine lesions, and the potentially lethal chloroethyl adducts formed by the nitrosoureas used in cancer chemotherapy more efficiently from the cellular DNA.