title: XIAP is not required for human tumor cell survival in the absence of an exogenous death signal.
abstract: BACKGROUND
  The X-linked Inhibitor of Apoptosis (XIAP) has attracted much attention as a cancer drug target. It is the only member of the IAP family that can directly inhibit caspase activity in vitro, and it can regulate apoptosis and other biological processes through its C-terminal E3 ubiquitin ligase RING domain. However, there is controversy regarding XIAP's role in regulating tumor cell proliferation and survival under normal growth conditions in vitro.
  
  
  METHODS
  We utilized siRNA to systematically knock down XIAP in ten human tumor cell lines and then monitored both XIAP protein levels and cell viability over time. To examine the role of XIAP in the intrinsic versus extrinsic cell death pathways, we compared the viability of XIAP depleted cells treated either with a variety of mechanistically distinct, intrinsic pathway inducing agents, or the canonical inducer of the extrinsic pathway, TNF-related apoptosis-inducing ligand (TRAIL).
  
  
  RESULTS
  XIAP knockdown had no effect on the viability of six cell lines, whereas the effect in the other four was modest and transient. XIAP knockdown only sensitized tumor cells to TRAIL and not the mitochondrial pathway inducing agents.
  
  
  CONCLUSIONS
  These data indicate that XIAP has a more central role in regulating death receptor mediated apoptosis than it does the intrinsic pathway mediated cell death.
authors.affiliation: Aragon Pharmaceuticals, 4215 Sorrento Valley Blvd, Suite 215 San Diego CA 92121, USA. jhager2@san.rr.com
authors.forename: John
authors.initials: J
authors.lastname: Sensintaffar
authors.forename: Fiona L
authors.initials: FL
authors.lastname: Scott
authors.forename: Robert
authors.initials: R
authors.lastname: Peach
authors.forename: Jeffrey H
authors.initials: JH
authors.lastname: Hager
mesh_terms: D000970:Antineoplastic Agents / Q000494:pharmacology; D017209:Apoptosis; D016923:Cell Death; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D002470:Cell Survival; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D008928:Mitochondria / Q000378:metabolism; D008954:Models, Biological; D017434:Protein Structure, Tertiary; D034741:RNA, Small Interfering / Q000378:metabolism; D015398:Signal Transduction; D053221:TNF-Related Apoptosis-Inducing Ligand / Q000737:chemistry; D051636:X-Linked Inhibitor of Apoptosis Protein / Q000378:metabolism*
chemical_list: D000970:Antineoplastic Agents; D034741:RNA, Small Interfering; D053221:TNF-Related Apoptosis-Inducing Ligand; D051636:X-Linked Inhibitor of Apoptosis Protein