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Updated: Jul 14, 2026
08:47
Lighting Up the Pathways to Caspase Activation Using Bimolecular Fluorescence Complementation
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
Summary
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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