Failure of a second x-ray dose to activate nuclear factor κB in normal rat astrocytes

Induced gene expression and subsequent cytokine production have been implicated in the normal tissue injury response to radiotherapy. However, studies of radiation-induced gene expression have used single radiation doses rather than the fractionated exposures typical of the clinical situation. To study the effects of multiple radiation doses on gene expression, we investigated nuclear factor κB (NFκB) DNA binding activity in primary astrocyte cultures after one and two exposures to x-rays. After a single dose of x-rays (3.8-15 gray (Gy)), NFκB binding activity in astrocytes increased in a dose-dependent manner, reaching a maximum by 2-4 h and returning to control levels by 8 h after irradiation. In split-dose experiments, when an interval of 24 h was used between two doses of 7.5 Gy, the second 7.5-Gy exposure failed to induce NFκB activation. The period of desensitization induced by the first radiation exposure was dose-dependent, persisting approximately 72 h after 7.5 Gy compared with 24 h after 1.5 Gy. No changes in IκBα protein levels were detected. However, the presence of a transcription inhibitor prevented the desensitizing effect of the initial irradiation. Irradiation also prevented NFκB activation in astrocytes by a subsequent exposure to H₂O₂, but it had no effect on the activation induced by tumor necrosis factor-α. These data indicate that an initial x-ray exposure can desensitize astrocytes to the NFκB-activating effects of a subsequent radiation exposure. Furthermore, they suggest that this desensitization depends on gene transcription and may have some specificity for NFκB activation mediated by reactive oxygen species.