Activation of human O-6-methylguanine-DNA methyltransferase gene by glucocorticoid hormone

O-6-methylguanine-DNA methyltransferase (MGMT), a ubiquitous DNA repair pro tein, removes the mutagenic DNA adduct O-6-alkylguanine, which is synthesiz ed both endogenously and after exposure to alkylnitrosamines and alkylating antitumor drugs such as 2-chloroethyl-N-nitrosourea (CNU), The MGMT gene i s highly regulated in mammalian cells and its overexpression, observed in m any types of tumor cells, is often associated with cellular resistance to C NU, Dexamethasone, a synthetic glucocorticoid hormone, was found to increas e MGMT expression in HeLa S3 cells, concomitant with their increased resist ance to CNU, Two putative glucocorticoid responsive elements (GREs) were id entified in the human MGMT (hMGMT) promoter. Transient expression of the lu ciferase reporter gene driven by an hMGMT promoter fragment containing thes e GREs was activated by dexamethasone. DNase I footprinting assays demonstr ated the binding of glucocorticoid receptor to these sequences. In vitro tr anscription experiment showed that these DNA sequences are functional in gl ucocorticoid receptor signal-mediated activation of transcription. These re sults suggest glucocorticoid-mediated induction of the MGMT gene contribute s to high level expression of MGMT.