The inducibility of the mammalian O6-methylguanine-DNA
methyltransferase (MGMT) gene encoding the MGMT
protein (EC 2.1.1.63) responsible for removal of the procarcinogenic and promutagenic lesion O6-alkylguanine from
DNA was examined by an analysis of transcription of the MGMT gene following exposure of repair-competent (Mex+) and repair-deficient (Mex-) cells to
N-methyl-N'-nitro-N-nitrosoguanidine (
MNNG). While human and rodent Mex- cells (CHO-9, V79, HeLa MR) showed no detectable MGMT
mRNA despite the presence of the gene in their genome, the amount of it in several Mex+ lines (NIH 3T3, HeLa S3, HepG2) paralleled their MGMT activity. However, none of these cell lines showed an increase in the MGMT
mRNA level
after treatment with various concentrations of
MNNG. In contrast,
MNNG-treated rat
hepatoma cells, H4IIE and FTO-2B, both Mex+, had three- to fivefold more MGMT
mRNA than the corresponding untreated controls as measured 12 to 72 h after alkylation.
N-Methyl-N-nitrosourea,
methyl methanesulfonate, N-hydroxyethyl-N-chloroethylnitrosourea, UV light, and X rays caused a similar accumulation of MGMT
mRNA in rat
hepatoma cells. Studies with inhibitors of
RNA and
protein synthesis indicate that the induced increase in the amount of MGMT
mRNA was due to enhanced transcription of the gene. Furthermore, they revealed the turnover of the MGMT
mRNA to be relatively low (half-life, greater than 7 h).
Mutagen-induced increase of transcription of MGMT
mRNA in H4IIE cells was accompanied by elevation of MGMT repair activity and resulted in reduction of mutation frequency after a challenge dose of
MNNG. Although induction of MGMT
mRNA transcription has been observed in two rodent
hepatoma cell lines so far, this appears to be the first demonstration of inducibility of a mammalian gene encoding a clearly define DNA repair function. The transcription activation of the MGMT gene protects cells from the mutagenic effects of methylating agents.