Kleczkowska H E, Althaus F R
Institute of Pharmacology and Toxicology, University of Zürich-Tierspital, Zürich, Switzerland.
Mutat Res. 1996 Mar 1;367(3):151-9. doi: 10.1016/0165-1218(95)00090-9.
Since alkylating agents are widely present in the environment and constitute a continuous challenge to genome integrity, cells and organisms have developed defense mechanisms to remove such lesions. We monitored the response of human keratinocytes to a very low concentration of a methylating agent, namely 2.5 nM N-methyl-N'-nitro- N-nitrosoguanidine (MNNG). The effect of a 60-min exposure of quiescent cells to 2.5 nM MNNG was studied in terms of DNA integrity, poly(ADP-ribose) metabolism, clonogenic survival and DNA synthesis. We observed two waves of DNA strand break formation and resealing. Interestingly, the amount of DNA strand breaks in exposed cells was lower than in unexposed control cells. This phenomenon was also observed when cells were exposed to MNNG in the presence of a protein synthesis inhibitor, or when they were maintained on ice during the treatment. A dose of 2.5 nM MNNG stimulated poly(ADP-ribose) turnover, reduced the intracellular NAD+ content, stimulated DNA synthesis and caused a remarkable increase in clonogenic survival. Thus, the cellular responses to extremely low concentrations of MNNG differ sharply from those observed at higher doses of this carcinogen. We conclude that the very low dose response cannot be extrapolated from usual dose-response analyses.
由于烷化剂广泛存在于环境中，对基因组完整性构成持续挑战，细胞和生物体已进化出防御机制来清除此类损伤。我们监测了人类角质形成细胞对极低浓度甲基化剂（即2.5 nM N-甲基-N'-硝基-N-亚硝基胍，MNNG）的反应。研究了静止细胞暴露于2.5 nM MNNG 60分钟后在DNA完整性、聚（ADP-核糖）代谢、克隆形成存活率和DNA合成方面的影响。我们观察到两波DNA链断裂形成和重新封闭的过程。有趣的是，暴露细胞中的DNA链断裂数量低于未暴露的对照细胞。当细胞在蛋白质合成抑制剂存在下暴露于MNNG时，或者在处理过程中保持在冰上时，也观察到了这种现象。2.5 nM的MNNG剂量刺激了聚（ADP-核糖）周转，降低了细胞内NAD+含量，刺激了DNA合成，并导致克隆形成存活率显著增加。因此，细胞对极低浓度MNNG的反应与在该致癌物较高剂量下观察到的反应截然不同。我们得出结论，极低剂量反应不能从常规剂量反应分析中推断出来。
