Ionizing radiation and short wavelength UV activate NF-κB through two distinct mechanisms

We examined the mechanisms by which two different types of photonic radiation, short wavelength UV (UV-C) and gamma radiation, activate transcription factor NF-kappa B, Exposure of mammalian cells to either form of radiation resulted in induction with similar kinetics of NF-kappa B DNA binding activity, nuclear translocation of its p65(RelA) subunit, and degradation of the major NF-kappa B inhibitor I kappa B alpha. In both cases, induction of NF-kappa B activity was attenuated by proteasome inhibitors and a mutation in ubiquitin-activating enzyme, suggesting that both UV-C and gamma radiation induce degradation of I kappa Bs by means of the ubiquitin/proteasome pathway. However, although the induction of I kappa B alpha degradation by gamma rays was dependent on its phosphorylation at Ser-32 and Ser-36, UV-C-induced I kappa B alpha degradation was not dependent on phosphorylation of these residues. Even the "super repressor" I kappa B alpha mutant, which contains alanines at positions 32 and 36, was still susceptible to UV-C-induced degradation. Correspondingly, we found that gamma radiation led to activation of IKK, the protein kinase that phosphorylates I kappa B alpha at Ser-32 and Ser-36, whereas UV-C radiation did not. Furthermore, expression of a catalytically inactive IKK beta mutant prevented NF-kappa B activation by gamma radiation, but not by UV-C. These results indicate that gamma radiation and UV-C activate NF-kappa B through two distinct mechanisms.