Vol. 294, Issue 2, 664-671, August 2000

Attenuation of O⁶-Methylguanine-DNA Methyltransferase Activity and mRNA Levels by Cisplatin and Temozolomide in Jurkat Cells¹

Stefania D'Atri, Grazia Graziani, Pedro Miguel Lacal, Vittoria Nisticò, Sara Gilberti, Isabella Faraoni, Amanda J. Watson, Enzo Bonmassar and Geoffrey P. Margison

Istituto Dermopatico Dell'Immacolata, Rome, Italy (S.D., P.M.L., E.B.); Department of Neurosciences, Pharmacology and Medical Oncology Section, University of Rome "Tor Vergata", Rome, Italy (G.G., V.N., S.G., I.F.); and CRC Carcinogenesis Group, Paterson Institute for Cancer Research, Christie Hospital NHS Trust, Manchester, United Kingdom (A.J.W., G.P.M.)

The DNA repair protein O⁶-methylguanine-DNA methyltransferase (MGMT) is important in cellular resistance to certain alkylating antitumor agents such as the methylating drug temozolomide (TMZ). To provide a more rational basis for clinical combinations with another commonly used drug, cisplatin, we assessed the modulation of MGMT protein and mRNA levels in the human leukemic cell line Jurkat after treatment with these agents. Cisplatin decreased MGMT activity in a time- and dose-dependent manner, with maximal suppression (50%) observed 24 h after treatment with 25 µM cisplatin. This was probably the result of decreased transcription of the MGMT gene, because there was an earlier nadir of MGMT mRNA levels after cisplatin treatment and neither cisplatin nor DNA reacted with cisplatin in vitro was able to inhibit MGMT activity in an in vitro assay. TMZ alone depleted MGMT activity in a time- and dose-dependent manner with almost complete loss of activity occurring immediately after treatment with 500 µM TMZ. Combinations of cisplatin (12.5 µM) and TMZ (250 µM) caused substantial and prolonged MGMT depletion with recovery to only 30% of pretreatment levels by 48 h. These results suggest that the clinical efficacy of TMZ and cisplatin may be improved by appropriate schedules of combinations of these agents.