Interleukin-33 is biologically active independently of caspase-1 cleavage

J Biol Chem. 2009 Jul 17;284(29):19420-6. doi: 10.1074/jbc.M901744200. Epub 2009 May 22.


The new interleukin (IL)-1 family cytokine IL-33 is synthesized as a 30-kDa precursor. Like pro-IL-1beta, human pro-IL-33 was reported to be cleaved by caspase-1 to generate an 18-kDa fragment, which is sufficient to activate signaling by the IL-33 receptor T1/ST2. However, the proposed caspase-1 cleavage site is poorly conserved between species. In addition, it is not clear whether caspase-1 cleavage of pro-IL-33 occurs in vivo and whether, as for IL-1beta, this cleavage is a prerequisite for IL-33 secretion and bioactivity. In this study, we further investigated caspase-1 cleavage of mouse and human pro-IL-33 and assessed the potential bioactivity of the IL-33 precursor. We observed the generation of a 20-kDa IL-33 fragment in cell lysates, which was enhanced by incubation with caspase-1. However, in vitro assays of mouse and human pro-IL-33 indicated that IL-33 is not a direct substrate for this enzyme. Consistently, caspase-1 activation in THP-1 cells induced cleavage of pro-IL-1beta but not of pro-IL-33, and activated THP-1 cells released full-length pro-IL-33 into culture supernatants. Finally, addition of full-length pro-IL-33 induced T1/ST2-dependent IL-6 secretion in mast cells. However, we observed in situ processing of pro-IL-33 in mast cell cultures, and it remains to be determined whether full-length pro-IL-33 itself indeed represents the bioactive species. In conclusion, our data indicate that pro-IL-33 is not a direct substrate for caspase-1. In addition, our results clearly show that caspase-1 cleavage is not required for pro-IL-33 secretion and bioactivity, highlighting major differences between IL-1beta and IL-33.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Blotting, Western
  • Caspase 1 / metabolism*
  • Cell Line
  • Cells, Cultured
  • Enzyme Activation / drug effects
  • Green Fluorescent Proteins / genetics
  • Green Fluorescent Proteins / metabolism
  • Humans
  • Interleukin-1beta / metabolism
  • Interleukin-33
  • Interleukins / chemistry
  • Interleukins / genetics
  • Interleukins / metabolism*
  • Lipopolysaccharides / pharmacology
  • Mice
  • Microscopy, Confocal
  • Monocytes / cytology
  • Monocytes / drug effects
  • Monocytes / metabolism
  • Peptide Fragments / metabolism
  • Protein Precursors / genetics
  • Protein Precursors / metabolism
  • Tetradecanoylphorbol Acetate / pharmacology
  • Transfection


  • IL33 protein, human
  • Il33 protein, mouse
  • Interleukin-1beta
  • Interleukin-33
  • Interleukins
  • Lipopolysaccharides
  • Peptide Fragments
  • Protein Precursors
  • Green Fluorescent Proteins
  • Caspase 1
  • Tetradecanoylphorbol Acetate