The secondary fungal metabolite gliotoxin targets proteolytic activities of the proteasome.

The fungal epipolythiodioxopiperazine metabolite gliotoxin has a variety of toxic effects such as suppression of antigen processing, induction of macrophagocytic apoptosis and inhibition of transcription factor NF-kappaB activation. How gliotoxin acts remains poorly understood except that the molecule's characteristic disulfide bridge is important for immunomodulation. As this fungal metabolite stabilizes the NF-kappaB inhibitor IkappaBalpha in the cytoplasm, we decided to investigate its molecular mechanism of action. We show that gliotoxin is an efficient, noncompetitive inhibitor of the chymotrypsin-like activity of the 20S proteasome in vitro. Proteasome inhibition can be reversed by dithiothreitol, which reduces gliotoxin to the dithiol compound. In intact cells, gliotoxin inhibits NF-kappaB induction through inhibition of proteasome-mediated degradation of IkappaBalpha. Gliotoxin targets catalytic activities of the proteasome efficiently. Inhibition by gliotoxin may be countered by reducing agents, which are able to inactivate the disulfide bridge responsible for the inhibitory capacity of gliotoxin.