The
proteasome is a multiprotein complex that is involved in the intracellular protein degradation in eukaryotes. Here, we show that human
malignant glioma cells are susceptible to apoptotic cell death induced by the
proteasome inhibitors,
MG132 and
lactacystin. The execution of the apoptotic death program involves the processing of
caspases 2, 3, 7, 8, and 9. Apoptosis is inhibited by ectopic expression of X-linked inhibitor of apoptosis (XIAP) and by coexposure to the broad-spectrum
caspase inhibitor, benzoyl-VAD-fluoromethyl
ketone (
zVAD-fmk), but not by the preferential
caspase 8 inhibitor, crm-A. It is interesting that specific morphological alterations induced by
proteasome inhibition, such as dilated rough endoplasmic reticulum and the formation of cytoplasmic vacuoles and dense mitochondrial deposits, are unaffected by
zVAD-fmk. Apoptosis is also inhibited by ectopic expression of Bcl-2 or by an inhibitor of
protein synthesis,
cycloheximide. Further,
cytochrome c release and disruption of mitochondrial membrane potential are prominent features of apoptosis triggered by
proteasome inhibition. Bcl-2 is a stronger inhibitor of
cytochrome c release than
zVAD-fmk. XIAP and crm-A fail to modulate
cytochrome c release. These data place
cytochrome c release downstream of Bcl-2 activity but upstream of XIAP- and crm-A-sensitive
caspases. The partial inhibition of
cytochrome c release by
zVAD-fmk indicates a positive feedback loop that may involve
cytochrome c release and
zVAD-fmk-sensitive
caspases. Finally, death
ligand/receptor interactions, including the CD95/
CD95 ligand system, do not mediate apoptosis induced by
proteasome inhibition in human
malignant glioma cells.