Ionizing radiation affects 26s proteasome function and associated molecular responses, even at low doses

Background and purpose: Ionizing radiation is known to activate certain sig nal transduction pathways, the regulation of which could involve post-trans criptional as well as transcriptional mechanisms. One of the most important post-transcriptional pathways in eukaryotic cells is the ATP- and ubiquiti n-dependent degradation of proteins by the 26s proteasome. This process con trols initiation of many cellular stress responses, as well as inflammatory responses under control of the transcription factor NF-kappaB. The literat ure on the relationship between radiation and inflammation seems somewhat p aradoxical. At high doses, radiation is generally pro-inflammatory. On the other hand, low dose radiation has a long history of use in the treatment o f inflammatory disease. This suggests the involvement of multiple mechanism s that may operate differentially at different dose levels. Materials and methods: In this paper, the ability of different doses of ion izing radiation to directly affect 26s proteasome activity was tested in EC V 304 cells. Proteasome activity, I kappaB alpha protein levels, and NF-kap paB activation were monitored. Results: Inhibition of chymotrypsin-like 20s and 26s proteasome activity wa s observed immediately after low- and high-dose irradiation either of cells or purified proteasomes. The inhibitory effect was independent of the avai lability of the known endogenous proteasome inhibitor heat shock protein 90 (hsp90). Levels of I kappaB alpha, a physiological 26s proteasome substrat e, were increased only at low doses (0.25 Gy) and unaltered at higher doses whereas only the highest doses (8 and 20 Gy) activated NF-kappaB. Conclusions: We conclude that the proteasome is a direct target of ionizing radiation and suggest that inhibition of proteasome function provides a mo lecular framework within which low dose anti-inflammatory effects of radiat ion, and radiation-induced molecular responses in general, should be consid ered. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.