It has been reported that in cells exposed to low-dose radiation, radio-adaptive responses can be induced which make irradiated cells refractory to subsequent high-dose irradiation. However, whether adaptive responses are possible when only the cytoplasm, not the nucleus, of the cell is exposed to radiation is still unclear. In this study, using the proton microbeam facility at the National Institute of Radiological Sciences (Japan), we found that cytoplasmic irradiation activates radio-adaptive responses in normal human lung fibroblast WI-38 cells. Our results showed that when cells received cytoplasmic irradiation with 500 protons prior to 2 Gy or 6 Gy X-ray broad-beam irradiation, the DNA double-strand break levels were significantly reduced. In contrast, at cytoplasmic irradiation with less than 100 protons, the radio-adaptive response was not detected. Moreover, the time interval between cytoplasmic irradiation and whole-cell X-ray irradiation should be longer than 6 h for the induction of adaptive responses. In addition, cytoplasmic irradiation elevated the level of cellular mitochondrial superoxide, which enhanced the phosphorylation of extracellular signal-regulated kinases 1/2 (ERK 1/2) and its mediated nuclear accumulation of nuclear factor (erythroid-derived 2)-like 2 (NRF2). This signaling pathway contributed to cytoplasmic irradiation-induced adaptive response as supported by the observations that treatment with the mitochondrial superoxide scavenger mito-tempol, ERK 1/2 inhibitor U0126 or NRF2 inhibitor ML385 could repress the adaptive response. Overall, we showed that cytoplasmic irradiation induces radio-adaptive responses and that mitochondrial superoxide/ERK 1/2/NRF2 signaling is a mechanism. Our results provide new information on the biological effects induced by cytoplasm-targeted irradiation.

据报道，在暴露于低剂量辐射的细胞中，可以诱导放射适应性反应，这使得被辐照的细胞对随后的高剂量辐照是难治的。然而，当仅将细胞的细胞质而不是细胞核暴露于辐射时，是否可能做出适应性反应尚不清楚。在这项研究中，使用日本国立放射科学研究所的质子微束设备，我们发现细胞质辐射激活了正常人肺成纤维细胞WI-38细胞中的放射适应性反应。我们的结果表明，当细胞在2 Gy或6 Gy X射线宽束辐照之前接受500质子的细胞质辐照时，DNA双链断裂水平显着降低。相反，在质子少于100个质子的情况下，未检测到无线电自适应响应。此外，胞质照射和全细胞X射线照射之间的时间间隔应大于6小时，以诱导适应性反应。此外，细胞质辐射提高了细胞线粒体超氧化物的水平，从而增强了细胞外信号调节激酶1/2（ERK 1/2）的磷酸化及其介导的核因子（类胡萝卜素2）样2的核积累（ NRF2）。该信号通路有助于细胞质辐射诱导的适应性反应，这一点得到以下观察结果的支持：用线粒体超氧化物清除剂米托普尔，ERK 1/2抑制剂U0126或NRF2抑制剂ML385可以抑制适应性反应。总体，我们表明，细胞质辐射诱导放射适应性反应，线粒体超氧化物/ ERK 1/2 / NRF2信号传导是一种机制。我们的结果提供了有关细胞质靶向照射诱导的生物学效应的新信息。