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Published on: March 5, 2018

XIAP activity dictates Apaf-1 dependency for caspase 9 activation

Andrew T Ho1, Qin H Li, Hitoshi Okada

  • 1Division of Cell & Molecular Biology, Toronto General Research Institute-University Health Network, Toronto, Ontario, Canada M5G 2M1.

Molecular and Cellular Biology
|June 15, 2007
PubMed

Insights

The intrinsic apoptosis pathway can activate caspase 9 independently of Apaf-1, particularly in myoblasts where Smac/Diablo overcomes XIAP inhibition. This finding redefines caspase 9 activation mechanisms in apoptosis.

Area of Science:

  • Cell Biology
  • Molecular Biology
  • Apoptosis Research

Background:

  • The intrinsic apoptotic pathway traditionally requires Apaf-1 and Smac/Diablo for caspase 9 activation.
  • Apoptotic stimuli trigger mitochondrial caspase activation, involving procaspase 9 dimerization and inhibition of apoptosis proteins (IAPs).
  • The interplay between Apaf-1, Smac/Diablo, and IAPs is crucial for regulating apoptosis.

Purpose of the Study:

  • To investigate whether caspase 9 activation can occur independently of Apaf-1 during intrinsic apoptosis.
  • To determine the role of Smac/Diablo and X-linked IAP (XIAP) in Apaf-1-independent caspase 9 activation.
  • To explore the differential regulation of apoptosis in various cell types, such as myoblasts and fibroblasts.

Main Methods:

  • Utilized Apaf-1 knockout (Apaf-1(-/-)) primary myoblasts and fibroblasts.
  • Administered cytotoxic drugs to induce apoptotic stimuli.
  • Assessed caspase 9 activation, Smac/Diablo accumulation, and XIAP levels.
  • Employed Smac/Diablo or pharmacological XIAP inhibitors in experiments.

Main Results:

  • Caspase 9 activation was observed in Apaf-1(-/-) myoblasts treated with drugs, independent of Apaf-1.
  • Smac/Diablo accumulation and subsequent sequestration of XIAP facilitated caspase 9 activation in myoblasts due to lower XIAP expression.
  • In Apaf-1(-/-) fibroblasts, higher XIAP to Smac/Diablo ratios prevented caspase 9 activation, but this could be overcome by XIAP antagonists.

Conclusions:

  • Apaf-1 is not essential for caspase 9 activation in response to intrinsic apoptotic stimuli.
  • Smac/Diablo-mediated inhibition of XIAP can independently trigger caspase 9 activation in specific cellular contexts.
  • These findings decouple caspase 9 activation from Apaf-1 in vivo, highlighting the significance of IAP regulation.

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