FAILURE OF A 2ND X-RAY DOSE TO ACTIVATE NUCLEAR FACTOR KAPPA-B IN NORMAL RAT ASTROCYTES

Induced gene expression and subsequent cytokine production have been i mplicated in the normal tissue injury response to radiotherapy. Howeve r, studies of radiation-induced gene expression have used single radia tion doses rather than the fractionated exposures typical of the clini cal situation. To study the effects of multiple radiation doses oil ge ne expression, we investigated nuclear factor kappa B (NF kappa B) DNA binding activity in primary astrocyte cultures after one and two expo sures to x-rays. After a single dose of x-rays (3.8-15 gray (Gy)), NF kappa B binding activity in astrocytes increased in a dose-dependent m anner, 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 NF kappa B 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 c hanges in I kappa B alpha protein levels were detected, However, the p resence of a transcription inhibitor prevented the desensitizing effec t of the initial irradiation. Irradiation also prevented NF kappa B ac tivation in astrocytes by a subsequent exposure to H2O2, but it had mo effect on the activation induced by tumor necrosis factor-alpha. Thes e data indicate that an initial x-ray exposure can desensitize astrocy tes to the NF kappa B-activating effects of a subsequent radiation exp osure, Furthermore, they suggest that this desensitization depends on gene transcription and may have some specificity for NF kappa B activa tion mediated by reactive oxygen species.