The secondary fungal metabolite gliotoxin targets proteolytic activities of the proteasome

Background: The fungal epipolythiodioxopiperazine metabolite gliotoxin has a variety of toxic effects such as suppression of antigen processing, induc tion of macrophagocytic apoptosis and inhibition of transcription factor NF -kappa B activation. How gliotoxin acts remains poorly understood except th at the molecule's characteristic disulfide bridge is important for immunomo dulation, As this fungal metabolite stabilizes the NF-kappa B inhibitor I k appa B alpha in the cytoplasm, we decided to investigate its molecular mech anism of action. Results: We show that gliotoxin is an efficient, noncompetitive inhibitor o f the chymotrypsin-like activity of the 20S proteasome in vitro, Proteasome inhibition can be reversed by dithiothreitol, which reduces gliotoxin to t he dithiol compound. In intact cells, gliotoxin inhibits NF-kappa B inducti on through inhibition of proteasome-mediated degradation of I kappa B alpha . Conclusions: Gliotoxin targets catalytic activities of the proteasome effic iently. Inhibition by gliotoxin may be countered by reducing agents, which are able to inactivate the disulfide bridge responsible for the inhibitory capacity of gliotoxin,