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Advances in Brief |
1 The Sidney Kimmel Comprehensive Cancer Center and 2 Howard Hughes Medical Institute, and 3 Department of Radiation Oncology and Molecular Radiation Sciences, The Johns Hopkins University School of Medicine, Baltimore, Maryland
| ABSTRACT |
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| Introduction |
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During both naturally occurring neoplasia and subsequent chemotherapy, cancer cells develop defects that make them less likely to undergo apoptosis and thereby provide a selective growth advantage. One mechanism of resistance to apoptotic stimuli involves the overexpression of the inhibitor of apoptosis (IAP) family of proteins, which include closely related X-linked IAP (XIAP), cIAP-1, and cIAP-2 (3 , 4) . These proteins have been shown to prevent apoptosis by binding to initiator and effector caspases and thereby protecting them from cleavage activation (3 , 5) . The importance of XIAP relative to the other IAP family members remains conjectural (3) . XIAP-null mice are completely normal and have no detectable defect in apoptosis. In such mice, there is a compensatory up-regulation of cIAP-1 and -2, implying that XIAP may be redundant with other IAP family members (6) . Transgenic animals that globally overexpress XIAP have shown no increase in cancer incidence (7) . In contrast, the Drosophila homologue of XIAP seems to play an important role in cell proliferation, with loss of function resulting in embryonic cell death (8) . Although contributing to our understanding of how XIAP may function, these diverse studies fail to provide a clear prediction as to how XIAP inhibition might affect human cancer cells (9) .
An antiapoptotic effect of XIAP expression on the TRAIL effector pathway has been inferred from experiments that have relied on the forced expression of either XIAP or of endogenous inhibitors of IAPs (5) , Smac/DIABLO and XIAP-associated factor 1 (XAF1). Efforts to enhance TRAIL-mediated apoptosis with Smac peptides (10 , 11) and small molecules designed to inhibit IAPs (12) have yielded promising results, but the precise targets of these molecules are not fully defined (9) . In this report, we describe a genetic approach to examine the consequences of XIAP loss of function in human cancer cells.
| Materials and Methods |
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Targeted Deletion of the Human XIAP Locus.
The strategy used for creating knockouts with AAV vectors was as described in Hirata et al. (13)
. The targeting construct pAAV-Neo-XIAP was made by PCR, using bacterial artificial chromosome clone RP1-315G1 (Invitrogen, Carlsbad, CA) as the template for the homology arms. Details of the vector design and sequences of all PCR primers are available from the authors on request. Stable G418-resistant clones were initially selected in the presence of 0.4 mg/ml Geneticin (Invitrogen, Carlsbad, CA) and then were routinely propagated in the absence of selective agents.
Microscopic Analysis.
Cells were collected by incubation in trypsin/EDTA followed by centrifugation and were fixed in a solution containing 3.7% formaldehyde, 0.5% NP40, and 10 µg/ml Hoechst 33258 in PBS. Apoptotic indices were determined by visual scoring of at least 300 nuclei.
Clonogenic Survival.
For each drug concentration, 103 and 104 cells were plated into duplicate 100-mm dishes and allowed to attach. Cells were then treated with the indicated final concentrations of TRAIL for 24 h. After the treatment period, cells were washed and incubated in medium without TRAIL for an additional 14 days. Colonies with more than 50 cells were counted after staining with crystal violet.
Immunoblotting.
Protein extracts were resolved via SDS-PAGE and transferred to polyvinylidene difluoride membranes. Filters were blocked in Tris-buffered saline containing 0.1% Tween 20 and 5% BSA, and then were incubated at room temperature with primary antibody in blocking buffer for 16 h. All of the antibodies used in this study were purchased from Cell Signaling Technology (Beverly, MA).
| Results and Discussion |
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XIAP-Dependent Alteration of Caspase Processing.
Next, we explored the biochemical changes in caspase processing that followed treatment with TRAIL. XIAP has been shown to bind in vitro to effector caspases-3 and -7 as well as to caspase-9, a protease that plays an important role in the processing of both caspases-3 and -7 (3)
. Lysates from TRAIL-treated XIAP-WT and XIAP-KO cells were immunoblotted with anti-caspase antibodies. Quantitative and qualitative differences in the cleavage products of caspase-3 were observed (Fig. 3)
. Overall, TRAIL-treated XIAP-WT cells, which underwent relatively little apoptosis, exhibited low amounts of caspase-3-derived cleavage products, whereas TRAIL-treated XIAP-KO cells had markedly higher levels of cleaved caspase-3. The time-dependent appearance of low-molecular weight caspase-3 protein also differed between XIAP-WT and XIAP-KO cells (Fig. 3)
, suggesting that XIAP protected the cleaved intermediate from further processing. This phenotype is consistent with biochemical studies that have demonstrated that the binding of XIAP to caspase-3 requires initial procaspase-3 cleavage and that XIAP can target activated caspase-3 for ubiquitin-mediated degradation (3)
. In contrast, despite previous reports that XIAP can associate with caspase-7 and caspase-9, we found that the proteolytic activation of these caspases was similar in XIAP-WT and XIAP-KO cells (Fig. 3)
. Likewise, no appreciable differences in the processing of caspase-8 were observed.
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TRAIL has been shown to selectively affect neoplastic cells (1) . This selectivity has generated considerable enthusiasm for TRAIL as a potential therapy with a favorable therapeutic index (1 , 2) . Our results demonstrate that the sensitivity of cancer cells to TRAIL can be increased by inhibiting the function of XIAP. Additional studies are clearly warranted to determine whether such a strategy would be effective in vivo.
| FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Requests for reprints: Fred Bunz, Department of Radiation Oncology and Molecular Radiation Sciences, The Johns Hopkins University School of Medicine, 1650 Orleans Street, Baltimore, MD 21231. Phone: (410) 502-7941; Fax: (410) 502-7234; E-mail: bunzfre{at}jhmi.edu
Received 1/ 7/04. Revised 2/26/04. Accepted 3/ 4/04.
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