title: Interleukin-33 is biologically active independently of caspase-1 cleavage.
abstract: 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.
authors.affiliation: Division of Rheumatology, University Hospital, and Department of Pathology and Immunology, University of Geneva School of Medicine, 1211 Geneva 4, Switzerland.
authors.forename: Dominique
authors.initials: D
authors.lastname: Talabot-Ayer
authors.forename: Céline
authors.initials: C
authors.lastname: Lamacchia
authors.forename: Cem
authors.initials: C
authors.lastname: Gabay
authors.forename: Gaby
authors.initials: G
authors.lastname: Palmer
mesh_terms: D000818:Animals; D015153:Blotting, Western; D020170:Caspase 1 / Q000378:metabolism*; D002460:Cell Line; D002478:Cells, Cultured; D004789:Enzyme Activation / Q000187:drug effects; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D006801:Humans; D053583:Interleukin-1beta / Q000378:metabolism; D000067596:Interleukin-33; D007378:Interleukins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008070:Lipopolysaccharides / Q000494:pharmacology; D051379:Mice; D018613:Microscopy, Confocal; D009000:Monocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D010446:Peptide Fragments / Q000378:metabolism; D011498:Protein Precursors / Q000235:genetics / Q000378:metabolism; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014162:Transfection
chemical_list: C506536:IL33 protein, human; C506535:Il33 protein, mouse; D053583:Interleukin-1beta; D000067596:Interleukin-33; D007378:Interleukins; D008070:Lipopolysaccharides; D010446:Peptide Fragments; D011498:Protein Precursors; D049452:Green Fluorescent Proteins; D020170:Caspase 1; D013755:Tetradecanoylphorbol Acetate