Inflammatory stimuli regulate caspase substrate profiles

Area of Science:

• Immunology
• Molecular Biology
• Proteomics

Background:

• Inflammatory caspases (caspase-1, -4, -5) regulate physiological processes in response to inflammatory signals.
• Known substrates for these caspases are limited, primarily including other caspases and interleukin-1 family cytokines.
• A comprehensive understanding of inflammatory caspase substrates is crucial for elucidating their roles in inflammation.

Purpose of the Study:

• To comprehensively identify and characterize substrates cleaved by inflammatory caspases (caspase-1, -4, -5).
• To compare substrate profiles induced by different inflammatory stimuli in vivo versus in vitro conditions.
• To investigate the role of substrate localization in regulating inflammatory caspase activity.

Main Methods:

• Utilized an enzymatic N-terminal enrichment method combined with mass spectrometry-based proteomics.
• Analyzed THP-1 monocytic cell lysates treated with recombinant purified caspases (in vitro).
• Investigated inflammatory caspases activated by gout mimics, bacterial infection, and viral infection mimics (in vivo), employing quantitative SILAC labeling.

Main Results:

• Identified 82 putative caspase-1 substrates, 3 for caspase-4, and none for caspase-5 in vitro.
• In vivo, activated inflammatory caspases cleaved 27 (gout mimic), 16 (bacterial mimic), and 22 (viral mimic) substrates.
• In vivo stimuli induced overlapping but distinct substrate profiles, with only half of in vivo cleavages matching in vitro findings.

Conclusions:

• This study provides the most extensive list of caspase-1-cleaved products to date, with caspases-4 and -5 having significantly fewer substrates.
• Discrepancies between in vitro and in vivo data underscore the critical role of substrate localization in regulating inflammatory caspase activity.
• Inflammation inducers may subtly modulate caspase-1 substrate profiles, impacting cellular responses.