Recent research indicates that the
proteasome is one of the non-
caspase proteases involved in apoptotic signaling pathways.
Nuclear factor-kappaB (
NF-kappaB) activation, one of the key factors in apoptosis, can be prevented through abrogation of
IkappaBalpha degradation by
proteasome inhibition. We have investigated the effects of the
proteasome inhibitors carbobenzoxyl-L-leucyl-L-leucyl-
L-leucinal (
MG132) and N-acetyl-L-leucinyl-L-leucinyl-L-norleucinal (LLnL) on apoptosis and
NF-kappaB activation induced by
etoposide, using a human
leukemia cell line (U937) and
leukemia blasts freshly isolated from patients with acute
leukemia. Pretreatment of U937 cells with
MG132 or LLnL inhibited
etoposide-induced morphological apoptosis and
caspase-3 activation. Furthermore,
MG132 or LLnL prevented
NF-kappaB activation and
IkappaBalpha degradation, but not
IkappaBalpha phosphorylation at Ser32. Other inhibitors of
NF-kappaB activation, including pyrrrolidine dithiocarbamate (an
antioxidant) and the
peptide SN50 (an inhibitor of translocation of activated
NF-kappaB into the nucleus), also attenuated
etoposide-induced apoptosis. In
leukemia blasts, although
proteasome inhibitors suppressed
NF-kappaB activation induced by
etoposide, they were unable to prevent morphological apoptosis. Moreover,
proteasome inhibitors by themselves caused apoptosis in
leukemia blasts at the concentrations employed in this study. These results suggest that the role that
NF-kappaB plays in apoptosis induced by
etoposide in a human
leukemia cell line may be different from the role it plays in freshly isolated
leukemia blasts.