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Open Access Highly Accessed Research article

A Smac-mimetic sensitizes prostate cancer cells to TRAIL-induced apoptosis via modulating both IAPs and NF-kappaB

Yao Dai14, Meilan Liu2, Wenhua Tang1, Yongming Li1, Jiqin Lian1, Theodore S Lawrence13 and Liang Xu13*

Author Affiliations

1 Department of Radiation Oncology, University of Michigan, Ann Arbor, MI 48109, USA

2 Department of Pathology, University of Michigan, Ann Arbor, MI 48109, USA

3 Comprehensive Cancer Center, University of Michigan, Ann Arbor, MI 48109, USA

4 Current address : Department of Radiation Oncology, University of Florida Health Science Center, Gainesville, FL 32610

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BMC Cancer 2009, 9:392  doi:10.1186/1471-2407-9-392

Published: 6 November 2009

Abstract

Background

Although tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising agent for human cancer therapy, prostate cancer still remains resistant to TRAIL. Both X-linked inhibitor of apoptosis (XIAP) and nuclear factor-kappaB function as key negative regulators of TRAIL signaling. In this study, we evaluated the effect of SH122, a small molecule mimetic of the second mitochondria-derived activator of caspases (Smac), on TRAIL-induced apoptosis in prostate cancer cells.

Methods

The potential of Smac-mimetics to bind XIAP or cIAP-1 was examined by pull-down assay. Cytotoxicity of TRAIL and/or Smac-mimetics was determined by a standard cell growth assay. Silencing of XIAP or cIAP-1 was achieved by transient transfection of short hairpin RNA. Apoptosis was detected by Annexin V-PI staining followed by flow cytometry and by Western Blot analysis of caspases, PARP and Bid. NF-kappaB activation was determined by subcellular fractionation, real time RT-PCR and reporter assay.

Results

SH122, but not its inactive analog, binds to XIAP and cIAP-1. SH122 significantly sensitized prostate cancer cells to TRAIL-mediated cell death. Moreover, SH122 enhanced TRAIL-induced apoptosis via both the death receptor and the mitochondrial pathway. Knockdown of both XIAP and cIAP-1 sensitized cellular response to TRAIL. XIAP-knockdown attenuated sensitivity of SH122 to TRAIL-induced cytotoxicity, confirming that XIAP is an important target for IAP-inhibitor-mediated TRAIL sensitization. SH122 also suppressed TRAIL-induced NF-kappaB activation by preventing cytosolic IkappaB-alpha degradation and RelA nuclear translocation, as well as by suppressing NF-kappaB target gene expression.

Conclusion

These results demonstrate that SH122 sensitizes human prostate cancer cells to TRAIL-induced apoptosis by mimicking Smac and blocking both IAPs and NF-kappaB. Modulating IAPs may represent a promising approach to overcoming TRAIL-resistance in human prostate cancer with constitutively active NF-kappaB signaling.