Treatment of COS-7 Cells with Proteasome Inhibitors or γ-Interferon Reduces the Increase in Caspase 3 Activity Associated with Staurosporine-Induced Apoptosis

Proteasomes play a major role in intracellular protein degradation and have been implicated in apoptosis. In this study we have investigated proteasome activity and the effects of inhibition of proteasomes or modulation of proteasome complexes on staurosporine-induced apoptosis in COS-7 cells. Staurosporine treatment of COS-7 cells had little direct effect on proteasome activity and did not cause dissociation of 26S proteasomes. There was also no major redistribution of proteasomes accompanying apoptosis in COS-7 cells. However, when the cells were pretreated with proteasome inhibitors, both the caspase 3 activity of the cells and the percentage of apoptotic cells measured by the TUNEL assay were reduced compared to staurosporine-treated cells, which had no inhibitor added. Proteasome inhibitors were also found to reduce the activation of caspase 3 in living cells which was assayed using a FRET-based method. However, proteasome inhibitors did not prevent some of the morphological changes associated with staurosporine-induced apoptosis. Pretreatment of cells with γ-interferon, which increases immunoproteasomes and PA28 complexes and reduces 26S proteasome levels, had an antiapoptotic effect. These results are consistent with a role for 26S proteasomes in regulating the activation of caspase 3 through the degradation of key regulatory proteins.