Abstract
IntroductionMonosodium urate crystals (MSU), the etiological agent of gout, are one of the most potent proinflammatory stimuli for neutrophils. The modulation of MSU-induced neutrophil activation by inhibitory receptors remains poorly characterized. The expression of the myeloid inhibitory C-type lectin-like receptor (MICL) in neutrophils is downregulated by several proinflammatory stimuli, suggestive of a role for this receptor in neutrophil function. We thus investigated the potential role of MICL in MSU-induced neutrophil activation.MethodsThe expression of MICL was monitored in human neutrophils by flow cytometry and Western blot analysis after stimulation with MSU. Protein tyrosine phosphorylation was also assessed by Western blot analysis and the production of IL-1 and IL-8 by enzyme-linked immunosorbent assay. Changes in the concentration of cytoplasmic free calcium were monitored with the Fura-2-acetoxymethyl ester calcium indicator. MICL expression was modulated with an anti-MICL antibody in neutrophils and siRNA in the PLB-985 neutrophil-like cell line.ResultsMSU induced the downregulation of MICL expression in neutrophils. A diminution in the expression of MICL induced by antibody cross-linking or siRNA enhanced the MSU-dependent increase in cytoplasmic calcium levels, protein tyrosine phosphorylation and IL-8 but not IL-1 production. Pretreatment of neutrophils with colchicine inhibited the MSU-induced downregulation of MICL expression.ConclusionsOur findings strongly suggest that MICL acts as an inhibitory receptor in human neutrophils since the downregulation of MICL expression enhances MSU-induced neutrophil activation. Since MSU downregulates the expression of MICL, MICL may play a pathogenic role in gout by enhancing neutrophil effector functions. In support of this notion, colchicine counteracts the MSU-induced loss of MICL expression. Our findings thus also provide further insight into the potential molecular mechanisms behind the anti-inflammatory properties of this drug.
Highlights
Monosodium urate crystals (MSU), the etiological agent of gout, are one of the most potent proinflammatory stimuli for neutrophils
A diminution in the expression of myeloid inhibitory C-type lectin-like receptor (MICL) induced by antibody cross-linking or small interfering RNA (siRNA) enhanced the MSU-dependent increase in cytoplasmic calcium levels, protein tyrosine phosphorylation and IL-8 but not IL-1 production
Our findings strongly suggest that MICL acts as an inhibitory receptor in human neutrophils since the downregulation of MICL expression enhances MSU-induced neutrophil activation
Summary
Monosodium urate crystals (MSU), the etiological agent of gout, are one of the most potent proinflammatory stimuli for neutrophils. According to the current understanding of the pathogenesis of acute gout, MSU activate resident articular cells (for example, macrophages) during the initiation phase, most commonly in the metatarsophalangeal joint [3]. The activation of resident cells by MSU induces the synthesis of several inflammatory mediators, including active interleukin 1b (IL-1b), a cytokine that plays a pivotal role in the pathogenesis of gout, implicating nucleotide-binding oligomerization domain-like receptor containing pyrin domain 3 (NLRP3) inflammasome in this inflammatory disorder. IL-1b contributes to the initiation and perpetuation of the effector phase by virtue of its ability to stimulate endothelial cells of the vasculature to express potent chemokines (for example, IL-8) and adhesion molecules responsible for the massive recruitment of neutrophils to the joint
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