IRRADIATION-INDUCED EXPRESSION OF O6-METHYLGUANINE-DNA METHYLTRANSFERASE IN MAMMALIAN-CELLS

O6-Methylguanine-DNA methyltransferase (MGMT) is a DNA repair protein which plays an important role in chemotherapy, mutagenesis, and carcin ogenesis. The specific activity of MGMT in female rat liver can be ind uced by approximately 20-fold by treatment of the rats with lambda-irr adiation. Maximum response occurred 48 h after 15 Gy irradiation. MGMT levels in male rats were induced by only 3-fold. MGMT activity was al so induced by irradiation of rat hepatoma H4IIE cells with a 3-fold in crease noted after treatment with 3 Gy. Northern analysis and nuclear run-on assays indicated that the induction of MGMT was regulated at th e transcriptional level. The radiation-mediated increase in MGMT was b locked by H7, a protein kinase inhibitor, but not by H89, an inhibitor of protein kinase A. Hydroxyl radicals may play a role in the inducti on mechanism since dimethyl sulfoxide, a radical scavenger, blocked th e radiation-mediated increase in MGMT. MGMT activity was also increase d by treatment of the cells with H2O2, in accordance with the involvem ent of activated oxygen species in the induction of MGMT. Finally, the addition of cycloheximide, an inhibitor of protein synthesis, prior t o but not after irradiation, abolished the increase in MGMT activity.