Abstract
The pro-inflammatory cytokine interleukin-1β (IL-1β) plays important roles in immunity but is also implicated in autoimmune disease. The most well-established mechanism of IL-1β secretion is via activation of the NOD-like receptor family pyrin domain containing-3 (NLRP3) inflammasome which requires an initial priming signal followed by an activating signal. However, the precise mechanism by which the inflammasome is activated remains unclear. The role of reactive oxygen species (ROS) in this process is contradictory, with some studies suggesting that ROS are crucial while others describe opposite effects. In this study, we evaluated the effects of oxidative stress on IL-1β secretion. Gout is a disease driven solely by IL-1β secretion in response to monosodium urate (MSU) crystals which form during periods of hyperuricemia and thus presents an opportunity to study factors contributing to IL-1β secretion. Sera and monocytes were isolated from patients with gout to determine whether differences in antioxidant status could explain the susceptibility of these individuals to gout attacks. In addition, sera and monocytes were collected from patients with chronic kidney disease (CKD) for comparison as this condition is associated with high levels of oxidative stress and disturbances in serum uric acid levels. There were differences in some aspects of antioxidant defenses in gout patients and these were mainly due to higher serum uric acid. Monocytes from gout patients were more responsive to priming, but not activation, of the NLRP3 inflammasome. However, expression of the components of the NLRP3 inflammasome were unaffected by priming or activation of the inflammasome, nor were these expression levels differentially regulated in gout patients. Inhibition of ROS by N-Acetyl Cysteine inhibited TLR2-induced priming of the NLRP3 inflammasome, but had no effect on MSU-induced activation. Together these findings demonstrate that oxidative stress only affects priming of the NLRP3 inflammasome but does not influence activation.
Highlights
Pro-inflammatory cytokines play important roles in protecting against infection but are implicated in autoimmune and metabolic disease [1, 2]
Primary human monocytes secreted IL1β when treated with the TLR2 ligand, Pam3, and these levels increased substantially when cells were treated with Pam3 + monosodium urate (MSU) (Figure 1A) via activation of the NOD-like receptor family pyrin domain containing-3 (NLRP3) inflammasome, as demonstrated by inhibition of IL-1β secretion using NLRP3 inhibitor MCC950 [41]
To investigate whether reactive oxygen species (ROS) are involved in IL-1β secretion, cells were treated with Pam3 –/+ crystallized uric acid and ROS levels measured by ROS-GloTM assay
Summary
Pro-inflammatory cytokines play important roles in protecting against infection but are implicated in autoimmune and metabolic disease [1, 2]. Identification of the NOD-like receptor family pyrin domain containing-3 (NLRP3) inflammasome as an essential component of IL-1β processing as first described by Martinon et al [6] was a critical step in elucidating the mechanism by which intracellular pro-IL-1β is processed for subsequent secretion where it contributes to inflammatory responses. The NLR family pyrin domain containing 3 (NLRP3) inflammasome is a tripartite intracellular receptor consisting of three proteins, NLRP3, ASC, and pro-caspase-1 which oligomerize to form a cytosolic complex. Canonical NLRP3 inflammasome activation involves two distinct signals, an initial “priming” signal that increases transcription of pro-IL-1β and NLRP3 followed by a second signal to induce oligomerization. NLRP3 is activated by a wide array of stimuli, such as extracellular ATP [7], asbestos [8], and the influenza virus [9], and is involved in the pathogenesis of many diseases [10]
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