title: A Smac-mimetic sensitizes prostate cancer cells to TRAIL-induced apoptosis via modulating both IAPs and NF-kappaB.
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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Radiation Oncology, University of Michigan, Ann Arbor, MI 48109, USA. daiyao@umich.edu
authors.forename: Yao
authors.initials: Y
authors.lastname: Dai
authors.forename: Meilan
authors.initials: M
authors.lastname: Liu
authors.forename: Wenhua
authors.initials: W
authors.lastname: Tang
authors.forename: Yongming
authors.initials: Y
authors.lastname: Li
authors.forename: Jiqin
authors.initials: J
authors.lastname: Lian
authors.forename: Theodore S
authors.initials: TS
authors.lastname: Lawrence
authors.forename: Liang
authors.initials: L
authors.lastname: Xu
mesh_terms: D017209:Apoptosis / Q000187:drug effects*; D051017:Apoptosis Regulatory Proteins; D045744:Cell Line, Tumor; D006801:Humans; D051034:Inhibitor of Apoptosis Proteins / Q000235:genetics / Q000378:metabolism*; D047908:Intracellular Signaling Peptides and Proteins / Q000819:agonists* / Q000494:pharmacology; D008297:Male; D024101:Mitochondrial Proteins / Q000819:agonists* / Q000494:pharmacology; D016328:NF-kappa B / Q000235:genetics / Q000378:metabolism*; D011471:Prostatic Neoplasms / Q000188:drug therapy / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding; D053221:TNF-Related Apoptosis-Inducing Ligand / Q000494:pharmacology*; D051636:X-Linked Inhibitor of Apoptosis Protein / Q000235:genetics / Q000378:metabolism
chemical_list: D051017:Apoptosis Regulatory Proteins; C412403:DIABLO protein, human; D051034:Inhibitor of Apoptosis Proteins; D047908:Intracellular Signaling Peptides and Proteins; D024101:Mitochondrial Proteins; D016328:NF-kappa B; D053221:TNF-Related Apoptosis-Inducing Ligand; D051636:X-Linked Inhibitor of Apoptosis Protein