Ionizing radiation affects 26s proteasome function and associated molecular responses, even at low doses.
OBJECTIVE
Ionizing radiation is known to activate certain signal transduction pathways, the regulation of which could involve post-transcriptional as well as transcriptional mechanisms. One of the most important post-transcriptional pathways in eukaryotic cells is the ATP- and ubiquitin-dependent degradation of proteins by the 26s proteasome. This process controls initiation of many cellular stress responses, as well as inflammatory responses under control of the transcription factor NF-kappaB. The literature on the relationship between radiation and inflammation seems somewhat paradoxical. At high doses, radiation is generally pro-inflammatory. On the other hand, low dose radiation has a long history of use in the treatment of inflammatory disease. This suggests the involvement of multiple mechanisms that may operate differentially at different dose levels.


METHODS
In this paper, the ability of different doses of ionizing radiation to directly affect 26s proteasome activity was tested in ECV 304 cells. Proteasome activity, IkappaBalpha protein levels, and NF-kappaB activation were monitored.


RESULTS
Inhibition of chymotrypsin-like 20s and 26s proteasome activity was observed immediately after low- and high-dose irradiation either of cells or purified proteasomes. The inhibitory effect was independent of the availability of the known endogenous proteasome inhibitor heat shock protein 90 (hsp90). Levels of IkappaBalpha, a physiological 26s proteasome substrate, 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 molecular framework within which low dose anti-inflammatory effects of radiation, and radiation-induced molecular responses in general, should be considered.
Pajonk
F
F
Department of Radiation Therapy, Radiological University Clinic, Hugstetter Strasse 55, 79106 i. Brsg., Freiburg, Germany.
McBride
W H
WH
D002478:Cells, Cultured; D004268:DNA-Binding Proteins / Q000378:metabolism / Q000528:radiation effects; D006801:Humans; D020672:I-kappa B Proteins; D007249:Inflammation / Q000378:metabolism*; D000072000:NF-KappaB Inhibitor alpha; D016328:NF-kappa B / Q000378:metabolism / Q000528:radiation effects; D010447:Peptide Hydrolases / Q000528:radiation effects*; D046988:Proteasome Endopeptidase Complex; D011839:Radiation, Ionizing
D004268:DNA-Binding Proteins; D020672:I-kappa B Proteins; D016328:NF-kappa B; C000606204:NFKBIA protein, human; D000072000:NF-KappaB Inhibitor alpha; D010447:Peptide Hydrolases; D046988:Proteasome Endopeptidase Complex; C061553:ATP dependent 26S protease