Attenuation of O-6-methylguanine-DNA methyltransferase activity and mRNA levels by cisplatin and temozolomide in Jurkat cells

The DNA repair protein O-6-methylguanine-DNA methyltransferase (MGMT) is im portant in cellular resistance to certain alkylating antitumor agents such as the methylating drug temozolomide (TMZ). To provide a more rational basi s for clinical combinations with another commonly used drug, cisplatin, we assessed the modulation of MGMT protein and mRNA levels in the human leukem ic cell line Jurkat after treatment with these agents. Cisplatin decreased MGMT activity in a time- and dose-dependent manner, with maximal suppressio n (50%) observed 24 h after treatment with 25 mu M cisplatin. This was prob ably the result of decreased transcription of the MGMT gene, because there was an earlier nadir of MGMT mRNA levels after cisplatin treatment and neit her cisplatin nor DNA reacted with cisplatin in vitro was able to inhibit M GMT activity in an in vitro assay. TMZ alone depleted MGMT activity in a ti me- and dose-dependent manner with almost complete loss of activity occurri ng immediately after treatment with 500 mu M TMZ. Combinations of cisplatin (12.5 mu M) and TMZ (250 mu M) caused substantial and prolonged MGMT deple tion with recovery to only 30% of pretreatment levels by 48 h. These result s suggest that the clinical efficacy of TMZ and cisplatin may be improved b y appropriate schedules of combinations of these agents.