Induction of genes involved in dna alkylation repair in rat hepatoma cells

Base alltyl adducts in DNA are repaired by 06-methylguanine-DNA methyltransferase (MGMT) and a set of enzymes of the base excision repair (BER) pathway including N-methylpurine-DNA glycosylase (MPG), apurinic endonuclease (APE), DNA polymerase beta (Pol beta) and DNA ligase. To address the question of whether alky la t ion damage repair can be stimulated in mammalian cells by DNA damaging agents, we studied the expression of various BER genes in rat hepatoma cells (H4IIE) treated with N-methyl-N!-nitro-N-nitrosoguanidine (MNNG) and Xrays- Furthermore we exposed these cells to dexamethasone in order to test whether DNA repair genes are also activated by glucocorticoid hormones. MGMT mRNA was induced by MNNG, X-rays and dexamethasone. A weak induction by X-rays and dexamethasone treatment was also observed for APE. Transfection experiments with the cloned human MGMT promoter and H4IIE cells revealed that the MGMT promoter can be stimulated by mutagen treatments and dexamethasone indicating that the observed increases in MGMT mRNA level are due to transcriptional activation of the gene. We have recently cloned the rat MPG promoter and tested also its transcriptional activity in H4I1E cells. The promoter had a high basal activity that was not significantly increased further by alkylating agents and X-rays. Taken together, these data strongly suggest that MGMT is a DNA damage and glucocorticoid hormone-inducible repair gene in mammalian cells. It may not be co-regulated with MPG and other alkylated base excision repair genes in mutagen-treated rat hepatoma cells (DFGKa724/4-2,CA31721).