Role of Endoplasmic Reticulum and Mitochondrion in Proton Microbeam Radiation-Induced Bystander Effect

It is well known that mitochondria and endoplasmic reticulum (ER) plays important roles in radiation responses, but their functions in radiation-induced bystander effect (RIBE) are largely unknown. This study found that, when a small portion of cells in the population of human lung fibroblast MRC-5 cells were precisely irradiated through either nuclear or cytoplasm with counted microbeam protons, the yield of micronucleus (MN) in the nonirradiated cells neighboring to irradiated cells was increased and the levels of intracellular reactive oxygen species (ROS) in the whole population were also significantly increased after irradiation. The mito/ER-tracker assay illustrates that the mitochondria was obviously activated after nuclear irradiation and the mass of ER approached to a higher level after cytoplasmic irradiation. Moreover, the above radiation induced ROS was diminished by rotenone, an inhibitor of mitochondria activation, but it was not influenced by siRNA interference of BiP, an ER regulation protein. For nuclear irradiation, rotenone enhanced radiation-induced ER expression, and BiP siRNA eliminated radiation-induced activation of mitochondria. However, these phenomena were not observed for cytoplasmic irradiation. Bystander MN was reduced by rotenone but enhanced by BiP siRNA. When the cells were treated with both rotenone and BiP siRNA, the MN yield was reduced for nuclear irradiation but was enhanced for cytoplasmic irradiation. Our results suggest that the cell organelles of mitochondria and ER have different roles in RIBE with respect to nuclear and cytoplasmic irradiations, and the function of ER is a prerequisite for mitochondrial activation