A possible mechanism of mutagenesis : oxidative stress and nuclear translocation of superoxide dismutase.

细胞外超氧化物歧化酶(EC-SOD)是目前已知的唯一一种分泌型超氧化物歧化酶同工酶。EC-SOD在C端有一个带正电荷的肝素结合域，被认为可以在分泌后的组织中进行酶的特性分布。组织化学检查显示细胞外超氧化物歧化酶(EC-SOD)的细胞核染色，免疫组化和Western印迹分析表明EC-SOD在培养细胞中也有核转位。分泌到培养液中的EC-SOD被掺入3T3-LI细胞，其中很大一部分被摄取的物质定位于细胞核。定点突变表明EC-SOD的肝素结合区作为核定位信号发挥作用。这些结果表明，EC-SOD核转运机制涉及一系列N端信号肽和C端He…更多依赖于Parin结合域的分泌、重新摄取和随后的核移位过程。这一发现支持EC-SOD的作用是保护基因组DNA免受活性氧的损伤和/或氧化还原敏感基因表达的转录调控。此外，还研究了氧化应激对EC-SOD细胞摄取和核转位的影响。用H_2O_2(5 mM，5min)处理3T3-LI细胞，然后用CHO-EK培养液孵育，可降低EC-SOD的摄取。与CHO-EK培养液孵育后，H_2O_2处理可明显促进EC-SOD的核转位。EC-SOD是目前已知的唯一一种主动转运到细胞核内的抗氧化酶。这些结果提示，氧化应激上调EC-SOD的核转位可能在其保护细胞核免受基因组DNA氧化损伤的机制中起作用。较少

Extracellular superoxide dismutase(EC-SOD) is the only known secretory SOD isoenzyme. EC-SOD has a positively-charged heparin-binding domain at the C-terminus and it has been thought that this domain could allow the enzyme characteristic distribution in tissues after the secretion.Histochemical examination of mouse tissues showed nuclear staining of extracellular superoxide dismutase(EC-SOD),and the nuclear translocation of EC-SOD was also confirmed in cultured cells that had been transfected with its gene, as shown by immunohistochemistry and Western blot analysis. The EC-SOD which was secreted into the medium was incorporated into 3T3-LI cells and a significant fraction of the material taken up was localized in the nucleus. Site-directed mutagenesis indicated that the heparin-binding domain of EC-SOD functions as the nuclear localization signal. These results suggest that the mechanism of the nuclear transport of EC-SOD involves a series of N-terminal signal peptide-and C-terminal he … More parin-binding domain-dependent processes of secretion, re-uptake and the subsequent nuclear translocation. The findings herein provide support for the view that the role of EC-SOD is to protect of genome DNA from damage by reactive oxygen species and/or the transcriptional regulation of redox-sensitive gene expression. Moreover, the effect of oxidative stress on the cellular uptake and nuclear translocation of EC-SOD was investigated. Treatment of the 3T3-LI cells with H_2O_2,(5mM for 5min) followed by incubation with CHO-EK medium downregulated the uptake of EC-SOD. Nuclear translocation of the incorporated EC-SOD was clearly enhanced by H_2O_2 treatment following incubation with the CHO-EK medium. EC-SOD is the only anti-oxidant enzyme which is known at this time to be actively transported into nuclei. The results suggest that the upregulation of the nuclear translocation of EC-SOD by oxidative stress might play a role in the mechanism by which the nucleus is protected against oxidative damage of genomic DNA. Less