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Caspase-4 Interacts with TNF Receptor-Associated Factor 6 and Mediates Lipopolysaccharide-Induced NF-B-Dependent Production of IL-8 and CC Chemokine Ligand 4 (Macrophage-Inflammatory Protein-1β)¹

Cell Death and Human Diseases, Genomics and Genetics Division, Institute of Molecular and Cell Biology, Proteos, Singapore, Republic of Singapore

Human caspase-4 does not have a corresponding mouse ortholog. Caspase-4 falls within the class of "inflammatory caspases," being homologous with human caspases 1 and 5 and mouse caspases 1, 11, and 12. To address the function of caspase-4, we generated caspase-4-deficient human THP1 monocytic cell lines which exhibited substantially reduced LPS-induced secretion of several chemokines and cytokines, including IL-8 (CXCL8), CCL4 (macrophage-inflammatory protein-1β), CCL20 (macrophage-inflammatory protein-3), and IL-1β. The LPS-induced expression of the mRNAs encoding these cytokines was correspondingly reduced in the caspase-4-deficient clones. Because a specific NF-B inhibitor blocked LPS-induced IL-8 and CCL4 mRNA expression as well as IL-8 and CCL4 secretion in THP1 cells, we investigated the role of caspase-4 in NF-B signaling. LPS-induced NF-B nuclear translocation and activation were inhibited in all caspase-4-deficient clones. LPS stimulation led to the interaction of endogenous caspase-4 and TNFR-associated factor 6 (TRAF6) via a TRAF6-binding motif (PPESGE), which we identified in caspase-4. Mutation of this site in caspase-4 resulted in the loss of the TRAF6-caspase-4 interaction. Similar TRAF6-binding motifs are known to be functionally important for TRAF6 interactions with other molecules including caspase-8, and for mediating NF-B activation in various immune and nonimmune cell types. Our data suggest that the TRAF6-caspase-4 interaction, triggered by LPS, leads to NF-B-dependent transcriptional up-regulation and secretion of important cytokines and chemokines in innate immune signaling in human monocytic cells.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ This work was supported by funds from A*STAR to the Institute of Molecular and Cell Biology, Singapore.

² Address correspondence and reprint requests to Dr. Alan G. Porter, Cell Death and Human Diseases, Genomics and Genetics Division, Institute of Molecular and Cell Biology, Proteos, 61, Biopolis Drive, Singapore 138673, Republic of Singapore. E-mail address: mcbagp{at}imcb.a-star.edu.sg

³ Abbreviations used in this paper: ER, endoplasmic reticulum; PAMP, pathogen-associated molecular pattern; TIR, Toll/IL-1R; IRAK, IL-1R-associated protein; TAK, transforming growth factor β-activated kinase-1; TRAF6, TNFR-associated factor 6; TBS, TRAF6-binding site; VC, vector control; WT, wild type; IKK, IB kinase; shRNA, small hairpin RNA.

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