The Roles of p21^Waf1/CIP1 and Hus1 in Generation and Transmission of Damage Signals Stimulated by Low-Dose Alpha-Particle Irradiation

Previously reported studies have demonstrated the involvement of p21^Waf1/CIP1 in radiation-induced bystander effects (RIBE). Mouse embryonic fibroblasts (MEFs) lacking Hus1 fail to proliferate in vitro, but inactivation of p21 allows for the continued growth of Hus1-deficient cells, indicating the close connection between p21 and Hus1 cells. In this study, wild-type MEFs, Hus1^+/+p21^–/– MEFs and p21^–/–Hus1^–/– MEFs were used in a series of radiation-induced bystander effect experiments, the roles of p21 and Hus1 in the induction and transmission of radiation-induced damage signals were investigated. Our results showed that after 5 cGy α particle irradiation, wild-type MEFs induced significant increases in γ-H2AX foci and micronuclei formation in bystander cells, whereas the bystander effects were not detectable in p21^–/–Hus1^+/+ MEFs and were restored again in p21^–/–Hus1^–/– MEFs. Media transfer experiments showed that p21^–/–Hus1^+/+ MEFs were deficient in the production bystander signals, but could respond to bystander signals. We further investigated the mitogen-activated protein kinases (MAPKs) that might be involved in the bystander effects. It was found that although knocking out p21 did not affect the expression of connexin43 and its phosphorylation, it did result in inactivation of some MAPK signal pathway kinases, including JNK1/2, ERK1/2 and p38, as well as a decrease in reactive oxygen species (ROS) levels in irradiated cells. However, the activation of MAPK kinases and the ROS levels in irradiated cells were restored in the cell line by knocking out Hus1. These results suggest that p21^Waf1/CIP1 and Hus1 play crucial roles in the generation and transmission of bystander damage signals after low-dose α-particle irradiation.