Induced Synthesis of O⁶‐Methylguanine‐DNA Methyltransferase in Rat Hepatoma Cells Exposed to DNA‐damaging Agents

When the rat hepatoma cell line H4IIE was treated with DNA‐damaging agents such as N‐methyl‐N′‐nitro‐N‐nitrosoguanidine (MNNG), ultraviolet light and γ‐rays, the O⁶‐methylguanine‐DNA methyltransferase activity increased 2 to 3 times over the level seen in non‐treated cells. SDS/polyacrylamide gel electrophoresis followed by fluorography revealed that a single species of methyltransferase protein with a molecular weight of 25,500 was present in both non‐treated and treated cells. Northern blot analysis using a cloned rat cDNA as a probe revealed that the enzyme activity increased because transcription of the gene was enhanced. The level of enzyme activity increased within 48 h after UV irradiation and remained at a higher level for 150 h. Following UV irradiation, the cells become more resistant than the normal cells to MNNG.