Objective: To investigate the effects of soluble uric acid (UA) on expression and activation of the NOD-like receptor (NLR) pyrin domain containing protein 3 (NLRP3) inflammasome in human monocytes to elucidate the role of hyperuricemia in the pathogenesis of gout. Methods: Primary human monocytes and the THP-1 human monocyte cell line were used to determine the effects of short- and longterm exposure to UA on activation of the NLRP3 inflammasome and subsequent interleukin 1β (IL-1β) secretion by ELISA and cell-based assays. Expression of key NLRP3 components in monocytes from patients with a history of gout were analyzed by quantitative PCR. Results: Precipitation of UA was required for activation of the NLRP3 inflammasome and subsequent release of IL-1β in human monocytes. Neither monosodium urate (MSU) crystals nor soluble UA had any effect on activation of the transcription factor, nuclear factor-κB. Prolonged exposure of monocytes to soluble UA did not alter these responses. However, both MSU crystals and soluble UA did result in a 2-fold increase in reactive oxygen species. Patients with gout (n = 15) had significantly elevated serum UA concentrations compared to healthy individuals (n = 16), yet secretion of IL-1β and expression of NLRP3 inflammasome components in monocytes isolated from these patients were not different from those of healthy controls. Conclusion: Despite reports indicating that soluble UA can prime and activate the NLRP3 inflammasome in human peripheral blood mononuclear cells, precipitation of soluble UA into MSU crystals is essential for in vitro NLRP3 signaling in primary human monocytes.
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