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Upstream regulatory role for XIAP in receptor-mediated apoptosis

John C Wilkinson1, Enrique Cepero, Lawrence H Boise

  • 1Departments of Pathology, University of Michigan, Ann Arbor, Michigan 48109, USA.

Molecular and Cellular Biology
|July 30, 2004
PubMed

Insights

X-linked inhibitor of apoptosis (XIAP) prevents cell death by inhibiting caspases. Full-length XIAP, but not a caspase 9-only mutant, supports cell survival and proliferation after CD95 activation.

Area of Science:

  • Cell biology
  • Apoptosis regulation
  • Immunology

Background:

  • X-linked inhibitor of apoptosis (XIAP) is a key endogenous inhibitor of programmed cell death.
  • XIAP functions by directly suppressing the activity of effector caspases (caspase 3 and 7) and initiator caspases (caspase 9).
  • Understanding XIAP's precise role in cell death pathways is crucial for developing targeted therapies.

Purpose of the Study:

  • To investigate the functional role of full-length XIAP and a caspase 9-inhibiting XIAP mutant in regulating CD95-mediated apoptosis.
  • To elucidate the mechanism by which XIAP influences cell survival, proliferation, and mitochondrial signaling.
  • To determine if XIAP acts upstream or downstream of mitochondrial outer membrane permeabilization.

Main Methods:

  • Generation of Jurkat cell lines stably overexpressing full-length XIAP or a XIAP truncation mutant.
  • Stimulation of CD95 signaling pathway to induce apoptosis.
  • Assessment of cell survival, proliferation, caspase activation, and mitochondrial events (cytochrome c and Smac/DIABLO release, mitochondrial membrane potential).
  • Utilized RNA interference to suppress XIAP levels in Bcl-x(L)-overexpressing cells.

Main Results:

  • Full-length XIAP expression supported short- and long-term cell survival and proliferation following CD95 activation, similar to Bcl-x(L).
  • XIAP full-length inhibited CD95-induced caspase 3 processing, cytochrome c and Smac/DIABLO release, and loss of mitochondrial membrane potential.
  • The XIAP truncation mutant failed to prevent these cell death events, indicating the importance of caspase 3 and 7 inhibition.
  • XIAP suppression sensitized Bcl-x(L)-overexpressing cells to death receptor-induced apoptosis.

Conclusions:

  • Full-length XIAP actively inhibits caspase activation essential for the mitochondrial amplification of death receptor signals.
  • XIAP acts upstream of mitochondrial activation, thereby preserving mitochondrial integrity and function.
  • XIAP plays a critical role in supporting long-term cell proliferative capacity following death receptor stimulation.

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