The role of nuclear factor κB in the radiation-induced bystander response

Kurzfassung

Radiation-induced bystander effects play a special role in the cellular response to ionizing radiation. Besides direct consequences of radiation exposure such as cell death, cell cycle arrest and deoxyribonucleic acid (DNA) repair, signaling pathways are activated that result in the secretion of different factors for intercellular communication (cytokines, radicals, damage markers and extracellular vesicles). These factors incite multiple effects in nearby non-irradiated target cells (bystander cells), among those are induction of DNA damage as well as further signal transduction. DNA damage in bystander cells can lead to cell death or activation of repair, similar to direct irradiation responses. Continuing activation of signaling pathways leads to further secretion of signaling factors thereby amplifying and promoting the damage signal originating from the irradiated cell. A key molecule in intra- and intercellular signal transduction is the transcription factor nuclear factor κB (NF-κB). Target genes of the transcription factor encode proteins that affect intracellular processes like repair and cell cycle progression as well as cytokines that are secreted for intercellular communication. In this work the role of NF-κB in the radiation-induced bystander response was investigated. To this end, embryonic fibroblasts from wildtype (wt) and NF-κB essential modulator (NEMO) knock-out (ko) mice were used. In these NEMO ko murine embryonic fibroblasts (MEF), the NF-κB response is dysfunctional. Direct X-ray exposure of MEF wt cells resulted in reduced survival, induction of premature senescence at high doses, cell cycle arrest in G2/M phase and DNA double strand breaks (DSB) that were partially repaired with time. Furthermore, X-irradiation of MEF wt cells led to nuclear translocation of the NF-κB subunit p65, indicating activation of NF-κB. MEF NEMO ko cells show a similarly reduced survival upon X-irradiation, a more sensitive response regarding senescence induction, cell cycle arrest in G2/M phase and DNA DSB induction compared to MEF wt. Bystander MEF cells were incubated with culture medium conditioned by irradiated cells. MEF wt bystander cells show NF-κB activation, a dose threshold-dependent reduction of cellular survival, induction of premature senescence and induction of DNA DSB, but no changes in cell cycle progression. MEF NEMO ko bystander cells show an increased survival fraction after treatment with conditioned medium and a more sensitive response regarding senescence induction, but no changes in cell cycle progression similar to MEF wt cells. The amount of DNA DSB in MEF NEMO ko bystander cells depends on incubation time and conditioning dose. The survival response of bystander cells has been found to depend on the NF-κB status of the recipient cells, indicating involvement of NF-κB in the amplification and transmission of the bystander signal.