Caspase-9 activation by the apoptosome is not required for fas-mediated apoptosis in type II Jurkat cells

Area of Science:

• Cellular biology
• Molecular biology
• Immunology

Background:

• Receptor-mediated apoptosis in type II cells involves the mitochondrial pathway.
• Bid cleavage to truncated Bid (tBid) is crucial for mitochondrial recruitment.
• The exact signaling for executioner caspase activation post-mitochondria is debated.

Purpose of the Study:

• Investigate molecular requirements for Fas-induced apoptosis in type II cells.
• Examine the role of the mitochondrial pathway and apoptosome in this process.
• Clarify the signaling cascade leading to executioner caspase activation.

Main Methods:

• Utilized distinct Jurkat T-lymphocyte clones with inhibited mitochondrial apoptotic pathways.
• Overexpressed Bcl-2 or Bcl-x(L) to assess protection.
• Generated Apaf-1-deficient Jurkat cells.
• Employed pharmacological inhibitors (Z-DEVD-fmk) and protein domains (XIAP BIR1/BIR2).
• Used Smac mimetics to sensitize cells.

Main Results:

• Bcl-2 and Bcl-x(L) overexpression protected cells from anti-Fas-induced apoptosis.
• Apaf-1-deficient cells remained sensitive to anti-Fas, showing Bid cleavage, Bak activation, cytochrome c/Smac release, and caspase-3 activation.
• Inhibition of downstream caspases or XIAP expression reduced anti-Fas-induced apoptosis.
• Smac mimetic sensitized Bcl-x(L)-overexpressing cells to Fas-induced apoptosis.

Conclusions:

• Fas-mediated executioner caspase activation and apoptosis in type II cells are independent of apoptosome-mediated caspase-9 activation.
• Post-mitochondrial signaling pathways are critical for initiating apoptosis.
• These findings challenge the conventional model of type II apoptosis signaling.