The hierarchy of events accompanying induction of apoptosis by the
proteasome inhibitor Bortezomib was investigated in Jurkat lymphoblastic and U937 myelomonocytic
leukemia cells. Treatment of Jurkat or U937 cells with
Bortezomib resulted in activation of
c-Jun-N-terminal kinase (JNK) and
p38 MAPK (
mitogen-activated protein kinase), inactivation of extracellular signal-regulating
kinase 1/2 (ERK1/2),
cytochrome c release,
caspase-9, -3, and -8 activation, and apoptosis.
Bortezomib-mediated
cytochrome c release and
caspase activation were blocked by the pharmacologic JNK inhibitor
SP600125, but lethality was not diminished by the
p38 MAPK inhibitor
SB203580. Inducible expression of a constitutively active MEK1 construct blocked
Bortezomib-mediated ERK1/2 inactivation, significantly attenuated
Bortezomib lethality, and unexpectedly prevented JNK activation. Conversely, pharmacologic
MEK/ERK1/2 inhibition promoted
Bortezomib-mediated JNK activation and apoptosis. Lastly, the
antioxidant N-acetyl-l-cysteine (LNAC) attenuated
Bortezomib-mediated
reactive oxygen species (ROS) generation, ERK inactivation, JNK activation,
mitochondrial dysfunction, and apoptosis. In contrast, enforced MEK1 and ERK1/2 activation or JNK inhibition did not modify
Bortezomib-induced ROS production. Together, these findings suggest that in human
leukemia cells,
Bortezomib-induced oxidative injury operates at a proximal point in the cell death cascade to antagonize cytoprotective ERK1/2 signaling, promote activation of the stress-related JNK pathway, and to trigger
mitochondrial dysfunction,
caspase activation, and apoptosis. They also suggest the presence of a feedback loop wherein
Bortezomib-mediated ERK1/2 inactivation contributes to JNK activation, thereby amplifying the cell death process.