Introduction
Caspase-1 is a pro-inflammatory enzyme which gets activated by autoprocessing following the assembly of multiprotein complexes called inflammasomes. Mature caspase-1 is responsible for the activation of the pro-inflammatory cytokines interleukin (IL)-1β and IL-18. Luksch and colleagues reported naturally occurring CASP1 genetic variants in patients suffering from unexplained recurrent febrile episodes. Paradoxically, in vitro and in vivo analyses revealed decreased enzymatic activity of these caspase-1 variants leading to impaired cytokine production. A study of Lamkanfi and colleagues provides a possible explanation by indicating a link between enzymatically inactive procaspase-1 and activation of NF-κB, a pro-inflammatory transcription factor.
Objectives
We tried to solve the indicated paradox by analyzing NF-κB activation in the presence of the procaspase-1 variants found.
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Methods
NF-κB activity was determined using a luciferase reporter assay system in transfected HEK 293T cells. RIP2 cleavage and ubiquitination studies were also performed in these cells. Protein/protein interactions of RIP2 and procaspase-1 were investigated in THP-1 cells by co-immunoprecipitation and in human monocyte derived macrophages by confocal fluorescence microscopy.
Results
Procaspase-1 variants with reduced enzymatic activity increased NF-κB activation by interacting with RIP2 (receptor interacting protein kinase 2). In contrast, wildtype (wt) procaspase-1 reduced NF-κB activity by cleaving RIP2 and decreasing RIP2 ubiquitination which is essential for NF-κB activation. In addition to transfection experiments, we showed RIP2/procaspase-1 interaction in the human monocyte cell line THP-1 and in human monocyte derived macrophages after stimulation with LPS in a time dependent manner.
Conclusion
Our results support the hypothesis that procaspase-1 variants with reduced enzymatic activity bind to RIP2 and thereby increase NF-κB activitation. This may contribute to pro-inflammatory signalling and thereby contribute to unexplained recurrent febrile episodes in the patients.
Disclosure of interest
None declared
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This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.