Radiation-induced Activation of Nuclear Factor-κB Involves Selective Degradation of Plasma Membrane-associated IκBα

In contrast to nuclear factor-κB (NF-κB) activation by tumor necrosis factor-α (TNF-α), the specific processes involved in the activation of this transcription factor by ionizing radiation (IR) have not been completely defined. According to the classical paradigm, a critical event in NF-κB activation is the degradation of IκBα. Data presented herein show that, in contrast to treatment with TNF-α, IR-induced NF-κB activation was not accompanied by degradation of IκBα in the U251 glioblastoma cell line as determined in whole cell lysates. However, treatment with the proteosome inhibitor MG-132 inhibited NF-κB activation induced by IR, suggesting that IκBα degradation was a critical event in this process. To reconcile these results, U251 cell lysates were separated into soluble and insoluble fractions and IκBα levels evaluated. Although IκBα was found in both subcellular fractions, treatment with IR resulted in the degradation of IκBα only in the insoluble fraction. Further subcellular fractionation suggested that the IR-sensitive, insoluble pool of IκBα was associated with the plasma membrane. These data suggest that the subcellular location of IκBα is a critical determinant in IR-induced NF-κB activation.