Induction of c-fos, c-jun, junB and junD mRNA and AP-1 by alkylating mutagens in cells deficient and proficient for the DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) and its relationship to cell death, mutation induction and chromosomal instability.

An early and immediate response of cells upon irradiation with UV light and various other forms of genotoxic stress is the induction of the proto-oncogenes c-fos and c-jun. To address the questions of whether (a) methylating agents that are powerful carcinogens are effective in induction of fos and jun mRNAs, (b) induction is affected by the repair capacity of the cells, and (c) induction is accompanied by genotoxic effects, the levels of c-fos, c-jun, junB and junD mRNA were analysed in isogenic Chinese hamster cell lines deficient (phenotypically Mex-) and proficient (Mex+) for the DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) after treatment with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and methyl methanesulfonate (MMS). Both methylating agents were very effective in inducing fos and jun mRNAs, although they differ markedly in their potency to induce O6-methylguanine in DNA. Most responsive were c-fos and c-jun (up to 80-fold increases in mRNA level) whereas junB (up to ninefold) and junD (up to twofold) displayed an intermediate and weak response, respectively. No difference in the dose-dependence of induction of these mRNAs was observed between Mex- and Mex+ cells indicating that the critical genotoxic and mutagenic lesion induced by MNNG, i.e. O6-methylguanine, which is rapidly repaired by MGMT, does not act as a trigger for this response. Induction of fos and jun mRNAs by MNNG and MMS was accompanied by a dose-dependent increase in the activity of the transcription factor AP-1. To induce fos and jun mRNAs as well as AP-1, doses of MNNG were required which were more than 50-fold higher than those inducing gene mutations, recombination events (SCEs) and reproductive cell death, and fivefold higher than those inducing chromosomal aberrations in Mex cells. Therefore, the immediate induction of fos and jun mRNAs and AP-1 in Mex- cells upon their exposure to MNNG appears not to be essential for the generation of MNNG-induced mutagenic and genotoxic effects, which is possibly due to the high genotoxic potential of non-repaired O6-methylguanine. However, for MMS and UV light, which was included in this study for comparison, c-fos, c-jun, junB and junD mRNA as well as AP-1 induction paralleled the dose-response for induction of cell killing effects, recombination and chromosomal breakage indicating that increased expression of Fos and Jun is related to the generation of MMS and UV-induced genetic changes. These data are in line with a model according to which the induced c-Fos and Jun proteins are involved in defense against UV radiation and other DNA damaging agents.