We have been investigating differential gene expression associated with apoptosis in AK-5 cells (a spontaneously regressing rat histiocytoma) and have observed catalytic subunits beta 7 and alpha 5 of the 26S proteasome and ubiquitin to be upregulated during apoptosis induced by a variety of agents. The observed elevation in gene expression was parallel to a comparable increase in the cytosolic protein expression of the proteasome and ubiquitin and a markedly amplified increase in the proteasome activity. Inhibition of the increase in gene expression resulted in the inhibition of the rise in the proteasome activity subsequently leading to an inhibition of apoptosis. Similarly, pretreatment with proteasome inhibitors, MG132 and lactacystin, resulted in a significant inhibition of apoptosis pointing to the requirement of a highly active protein degradation machinery during apoptosis. The apoptosis inhibitory effect of the proteasome inhibitors involved an inhibition of the activation of various initiator and effector caspases but was independent of any changes in the mitochondrial membrane depolarization and cytochrome c release associated with apoptosis. Inhibition of proteasome activity or its upstream PI3 kinase activity inhibited NFkappaB translocation thereby suppressing apoptosis, which highlights the requirement of NFkappaB activation for completion of apoptosis in AK-5 cells. Hence, the apoptosis associated induction of the Ub-proteasome pathway components and the proteasome activity suggests that the proteasome, in its capacity as an efficient protein degradation complex, plays an important role in the successful execution of apoptosis.