Dosch J, Kaina B
Division of Applied Toxicology, Institute of Toxicology, University of Mainz, Germany.
Oncogene. 1996 Nov 7;13(9):1927-35.
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.
细胞在受到紫外线照射及其他各种形式的基因毒性应激后，早期的即时反应是原癌基因c-fos和c-jun的诱导。为了探讨以下问题：（a）作为强力致癌物的甲基化剂是否能有效诱导fos和jun mRNA；（b）诱导是否受细胞修复能力的影响；（c）诱导是否伴随着基因毒性效应，在用N-甲基-N'-硝基-N-亚硝基胍（MNNG）和甲基磺酸甲酯（MMS）处理后，对DNA修复蛋白O6-甲基鸟嘌呤-DNA甲基转移酶（MGMT）缺陷（表型为Mex-）和 proficient（Mex+）的同基因中国仓鼠细胞系中c-fos、c-jun、junB和junD mRNA的水平进行了分析。两种甲基化剂在诱导fos和jun mRNA方面都非常有效，尽管它们在诱导DNA中O6-甲基鸟嘌呤的能力上有显著差异。反应最强烈的是c-fos和c-jun（mRNA水平增加高达80倍），而junB（高达9倍）和junD（高达2倍）分别表现出中等和较弱的反应。在Mex-和Mex+细胞之间未观察到这些mRNA诱导的剂量依赖性差异，这表明MNNG诱导的关键基因毒性和诱变损伤，即O6-甲基鸟嘌呤，可被MGMT迅速修复，并不是这种反应的触发因素。MNNG和MMS诱导fos和jun mRNA伴随着转录因子AP-1活性的剂量依赖性增加。为了诱导fos和jun mRNA以及AP-1，所需的MNNG剂量比诱导基因突变、重组事件（SCEs）和生殖细胞死亡的剂量高50倍以上，比在Mex细胞中诱导染色体畸变的剂量高5倍以上。因此，Mex-细胞暴露于MNNG后立即诱导fos和jun mRNA以及AP-1似乎对于MNNG诱导的诱变和基因毒性效应的产生并非必不可少，这可能是由于未修复的O6-甲基鸟嘌呤具有很高的基因毒性潜力。然而，对于本研究中作为比较纳入的MMS和紫外线，c-fos、c-jun、junB和junD mRNA以及AP-1的诱导与细胞杀伤效应、重组和染色体断裂诱导的剂量反应平行，表明Fos和Jun表达增加与MMS和紫外线诱导的基因变化的产生有关。这些数据符合一个模型，根据该模型，诱导的c-Fos和Jun蛋白参与了对紫外线辐射和其他DNA损伤剂的防御。
