NLRP3 Inflammasome Cutting Edge: Nitric Oxide Inhibits the

Although the NLRP3 inflammasome plays a pivotal role in host defense, its uncontrolled activation is associated with inflammatory disorders, suggesting that regulation of the inflammasome is important to prevent detrimental effects. Type I IFNs and long-term LPS stimulation were shown to negatively regulate NLRP3 activation. In this study, we found that endogenous NO is involved in the regulation of NLRP3 inflammasome activation by either IFN-b pretreatment or long-term LPS stimulation. Furthermore, S-nitroso-N-acetylpenicillamine (SNAP), an NO donor, markedly inhibited NLRP3 inflammasome activation, whereas the AIM2 and NLRC4 inflammasomes were only partially inhibited by SNAP. An increase in mitochondrial reactive oxygen species induced by ATP was only modestly affected by SNAP treatment. Interestingly, S-nitrosylation of NLRP3 was detected in macrophages treated with SNAP, and this modification may account for the NO-mediated mechanism controlling inflammasome activation. Taken together, these results revealed a novel role for NO in regulating the NLRP3 inflammasome. T he inflammasome is a multiprotein complex that mediates the activation of caspase-1, which then processes pro–IL-1b and pro–IL-18 into mature IL-1b and IL-18. Among a number of sensor proteins reported to be involved in inflammasome activation, NLRP3, NLRP1b, NLRC4, and AIM2, have been established as the major sensors for the recognition of various pathogens or damage-associated molecular patterns. NLRP3 activation is triggered by different types of stimuli (e.g., whole pathogens, uric acid crystals, nigericin, ATP) (1). The precise mechanism of NLRP3 inflammasome activation has not been determined; however, NLRP3 agonist-induced generation of mitochondrial reactive oxygen species (ROS) is likely to play a major role in NLRP3 activation (2–5). Despite the fact that NLRP3 inflammasome contributes to host defense against microbial pathogens, excessive activation due to mutations in the NLRP3 gene has been associated with a spectrum of autoin-flammatory disorders collectively known as " cryopirin-associated periodic syndromes " (1). Additionally, NLRP3 has been implicated in obesity-induced inflammation and insulin resistance (6). Thus, appropriate regulation of inflammasome activation appears to be important to avoid detrimental effects. It was demonstrated that activation of the inflammasome is regulated by several mechanisms. For instance, the protein level of NLRP3 is relatively low in resting macrophages, so that NLRP3 inflammasome formation is hardly induced until the expression level is increased by exogenous and endogenous factors, including TLR agonists and proinflammatory cyto-kines. This suggests that activation of the NLRP3 inflam-masome is limited under normal conditions (7). In contrast, long-term priming with the TLR4 agonist LPS results in the …