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Synergy is achieved by complementation with Apo2L/TRAIL and actinomycin D
in Apo2L/TRAIL-mediated apoptosis of prostate cancer cells: role of XIAP in
resistance
Synergy is achieved by complementation with Apo2L/TRAIL and
actinomycin D in Apo2L/TRAIL-mediated apoptosis of prostate cancer cells: role
of XIAP in resistance.
Prostate
2002 Dec 1;53(4):286-99
Ng CP, Zisman A, Bonavida B.
Department of Microbiology, Immunology, and Molecular Genetics, UCLA School of
Medicine and Jonsson Comprehensive Cancer Center, University of California,
Los Angeles, California 90095-1747, USA.
BACKGROUND: Tumors have an inherent immunogenicity that can be exploited by
immunotherapy. However, often tumors develop mechanisms that render them
resistant to most immunologic cytotoxic effector mechanisms. This study
examines the underlying mechanism of resistance to Apo2L/TRAIL (tumor necrosis
factor-related apoptosis-inducing ligand)-mediated apoptosis. METHODS: We
studied prostate tumor cell lines for their sensitivity to
Apo2L/TRAIL-mediated apoptosis in the presence and absence of the sensitizing
agent actinomycin D (Act D). Apoptosis was determined by flow cytometry and
signaling for apoptosis by Western blot. RESULTS: Treatment with subtoxic
concentrations of Act D significantly sensitizes the tumor cells (CL-1,
DU-145, and PC-3 prostate tumor cells) to Apo2L/TRAIL-mediated apoptosis. The
cytotoxicity of Act D-sensitized prostate tumor cells was a result of
synergistic activation of caspases (caspase-3, -9, and -8), detectable after 6
hr of treatment. Treatment with Apo2L/TRAIL alone, although it was
insufficient to induce apoptosis, resulted in the loss of mitochondrial
membrane potential and release of cytochrome c from the mitochondria into the
cytoplasm in the absence of significant caspases activation. These findings
suggested that a major apoptosis resistance factor blocking the Apo2L/TRAIL
apoptotic signaling events is present downstream of the mitochondrial
activation. The expression of receptors and anti-apoptotic proteins were
examined in Act D-sensitized CL-1 cells. The earliest and the most pronounced
change induced by Act D was down-regulation of X-linked inhibitor of apoptosis
(XIAP) and up-regulation of Bcl-xL/-xS proteins. The role of XIAP in
resistance was demonstrated by overexpression of Smac/DIABLO, which inhibited
inhibitors of apoptosis (IAPs) and sensitized the cells to Apo2L/TRAIL.
Apo2L/TRAIL receptors (DR4, DR5, DcR1, and DcR2), c-FLIP, Bcl-2, and other IAP
members (c-IAP1 and c-IAP2) were marginally affected at later times in the
cells sensitized by Act D. CONCLUSION: This study suggests that the
combination of Act D-induced down-regulation of XIAP (Signal I) and
Apo2L/TRAIL-induced release of cytochrome c (Signal II) leads to the reversal
of resistance to Apo2L/TRAIL-mediated apoptosis in the tumor cells. The
sensitization of tumor cells to Apo2L/TRAIL by Act D is of potential clinical
application in the immunotherapy of drug/Apo2L/TRAIL refractory tumors.
Copyright 2002 Wiley-Liss, Inc.
