Radiation-induced activation of nuclear factor-κB involves selective degradation of plasma membrane-associated IκBa

In contrast to nuclear factor-kappaB (NF-kappaB) activation by tumor necrosis factor-alpha (TNF-alpha), 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-kappaB activation is the degradation Of IkappaBalpha. Data presented herein show that, in contrast to treatment with TNF-alpha, IR-induced NF-kappaB activation was not accompanied by degradation Of IkappaBalpha in the U251 glioblastoma cell line as determined in whole cell lysates. However, treatment with the proteosome inhibitor MG-132 inhibited NF-kappaB activation induced by IR, suggesting that IkappaBalpha degradation was a critical event in this process. To reconcile these results, U251 cell lysates were separated into soluble and insoluble fractions and IkappaBalpha levels evaluated. Although IkappaBalpha was found in both subcellular fractions, treatment with IR resulted in the degradation Of IkappaBalpha only in the insoluble fraction. Further subcellular fractionation suggested that the IR-sensitive, insoluble Pool Of IkappaBalpha was associated with the plasma membrane. These data suggest that the subcellular location of IkappaBalpha is a critical determinant in IR-induced NF-kappaB activation.