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-inhibitormediated
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.
Dai,Y., Liu, M., Tang, W., Li, Y., Lian, J., Lawrence, T.S., Xu, L. A Smac-mimetic sensitizes prostrate cancer cells to TRAIL-induced apoptosis via modulating both IAPs and NF-kappaB. BMC Cancer 2009, 9:392. http://dx.doi.org/10.1186/1471-2407-9-392