Up-stream events in the nuclear factor jB activation cascade in response to sparsely ionizing radiation

Radiation is a potentially limiting factor for manned long-term space missions. Prolonged exposure to galactic cosmic rays may shorten the healthy life-span after return to Earth due to cancer induction. During the mission, a solar flare can be life threatening. For better risk estimation and development of appropriate countermeasures, the study of the cellular radiation response is necessary. Since apoptosis may be a mechanism the body uses to eliminate damaged cells, the induction by cosmic radiation of the nuclear antiapoptotic transcription factor nuclear factor jB (NF-jB) could influence the cancer risk of astronauts exposed to cosmic radiation by improving the survival of radiation-damaged cells. In previous studies using a screening assay for the detection of NF-jB-dependent gene induction (HEK-pNF-jB-d2EGFP/Neo cells), the activation of this transcription factor by heavy ions was shown [Baumstark-Khan, C., Hellweg, C.E., Arenz, A., Meier, M.M. Cellular monitoring of the nuclear factor kappa B pathway for assessment of space environmental radiation. Radiat. Res. 164, 527–530, 2005]. Studies with NF-jB inhibitors can map functional details of the NF-jB pathway and the influence of radiation-induced NF-jB activation on various cellular outcomes such as survival or cell cycle arrest. In this work, the efficacy and cytotoxicity of four different NF-jB inhibitors, caffeic acid phenethyl ester (CAPE), capsaicin, the proteasome inhibitor MG-132, and the cell permeable peptide NF-jB SN50 were analyzed using HEK-pNF-jB-d2EGFP/Neo cells. In the recommended concentration range, only CAPE displayed considerable cytotoxicity. CAPE and capsaicin partially inhibited NF-jB activation by the cytokine tumor necrosis factor a. MG-132 completely abolished the activation and was therefore used for experiments with X-rays. NF-jB SN-50 could not reduce NF-jB dependent expression of the reporter destabilized Enhanced Green Fluorescent Protein (d2EGFP). MG-132 entirely suppressed the X-ray induced NF-jB activation in HEK-pNF-jB-d2EGFP/Neo cells. In conclusion, the degradation of the inhibitor of NF-jB (IjB) in the proteasome is essential for X-ray induced NF-jB activation, and MG-132 will be useful in studies of the NF-jB pathway involvement in the cellular response to heavy ion exposure and other space-relevant radiation qualities