Bortezomib is a highly selective inhibitor of the
26S proteasome and has been approved for clinical use in the treatment of relapsing and refractory
multiple myeloma and
mantle cell lymphoma. Clinical trials are also underway to assess the role of
bortezomib in several other human
malignancies, including
leukemia. However, the mechanism(s) by which
bortezomib acts remain to be fully understood. Here, we studied the molecular requirements of
bortezomib-induced apoptosis using the human T-cell leukemic Jurkat cells stably transfected with or without
shRNA against apoptotic
protease-activating factor-1 (Apaf-1). The Apaf-1-deficient Jurkat T cells were resistant to
bortezomib-induced apoptosis, as assessed by
caspase-3 activity,
poly(ADP-ribose) polymerase cleavage,
phosphatidylserine externalization, and hypodiploid
DNA content. In contrast, Apaf-1-deficient cells were sensitive to Fas-induced apoptosis.
Bortezomib induced an upregulation of the
pro-apoptotic protein Noxa, loss of mitochondrial transmembrane potential, and release of
cytochrome c in cells expressing or not expressing Apaf-1. Transient silencing of Apaf-1 expression in RPMI 8402 T-cell leukemic cells also diminished
bortezomib-induced apoptosis. Fas-associated death domain (FADD)-deficient Jurkat cells were resistant to Fas-mediated apoptosis yet remained sensitive to
bortezomib. Our results show that
bortezomib induces apoptosis by regulating pathways that are mechanistically different from those activated upon
death receptor ligation. Furthermore, in silico analyses of public transcriptomics databases indicated elevated Apaf-1 expression in several
hematologic malignancies, including acute lymphoblastic and
myeloid leukemia. We also noted variable Apaf-1 expression in a panel of samples from patients with
acute lymphoblastic leukemia. Our results suggest that the expression of Apaf-1 may be predictive of the response to
proteasome inhibition.