Inhibition of proteasome activity by tyropeptin A in PC12 cells.

Tyropeptin A, a potent proteasome inhibitor not reported before, was produced by Kitasatospora sp. MK993-dF2. In this study, we investigated the effects of tyropeptin A on proteasome activity in PC12 cells. Tyropeptin A inhibited the intracellular proteasome activity in a dose-dependent way and seemed to cause neurite outgrowth. As expected, ubiquitinated proteins that should be substrates for the proteasome accumulated in cells treated with tyropeptin A. Hence, it appears that tyropeptin A can permeate into cells and there inhibit the intracellular proteasome activity.