NLRP3 Deletion Protects from Hyperoxia-induced Acute Lung Injury

Inspiration of a high concentration of oxygen, a therapy for acute lung injury (ALI), could unexpectedly lead to reactive oxygen species (ROS) production and hyperoxia-induced acute lung injury (HALI). Nucleotide-binding domain and leucine-rich repeat PYD-containing protein 3 (NLRP3) senses the ROS, triggering inflammasome activation and interleukin-1β (IL-1β) production and secretion. However, the role of NLRP3 inflammasome in HALI is unclear. The main aim of this study is to determine the effect of NLRP3 gene deletion on inflammatory response and lung epithelial cell death. Wild-type (WT) and NLRP3−/− mice were exposed to 100% O2 for 48–72 h. Bronchoalveolar lavage fluid and lung tissues were examined for proinflammatory cytokine production and lung inflammation. Hyperoxia-induced lung pathological score was suppressed in NLRP3−/− mice compared with WT mice. Hyperoxia-induced recruitment of inflammatory cells and elevation of IL-1β, TNFα, macrophage inflammatory protein-2, and monocyte chemoattractant protein-1 were attenuated in NLRP3−/− mice. NLRP3 deletion decreased lung epithelial cell death and caspase-3 levels and a suppressed NF-κB levels compared with WT controls. Taken together, this research demonstrates for the first time that NLRP3-deficient mice have suppressed inflammatory response and blunted lung epithelial cell apoptosis to HALI. acute lung injury (ALI) is characterized by severe alveolar damage resulting from an acute inflammatory response that leads to proinflammatory cytokine production, neutrophil, macrophage infiltration, and edema. The most severe form of ALI is acute respiratory distress syndrome (ARDS), which is a major cause for admission to critical care units. Hyperoxia therapy is a necessary part of treatment for patients with acute and chronic cardiovascular and pulmonary diseases. However, prolonged exposure to hyperoxia could deteriorate ALI (19, 47). Currently, there are several animal models available to study the mechanism of ALI. The hyperoxia-induced acute lung injury (HALI) animal model became widely used to study human ALI after Cochrane et al. (7) revealed the increase of oxidants in the lungs of patients with ARDS. It is now well established that there are clinically relevant similarities between the animal model of HALI and human lung injury (26). However, the molecular mechanisms that initiate and amplify the lung inflammation in response to inhaled oxygen are not well understood. IL-1β is one of the most potent early cytokines found in ALI patients, and it induces the production of other cytokines (12). The proinflammatory cytokine IL-1β is also known to be one of the most biologically important inflammatory mediators in the air space of patients with early ALI (35). Interestingly, IL-1β can also act as an important activator and prosurvival cytokine for neutrophils (37). However, mechanisms that initiate IL-1β processing in ALI are not clearly defined. Martinon et al. (25) first reported in 2002 that caspase-1-mediated processing of IL-1β is mediated by the nucleotide-binding domain and leucine-rich repeat PYD-containing protein 3 (NLRP3) inflammasome. The NLRP3 inflammasome is a multiprotein complex, which contains NLRP3, the caspase recruitment domain containing protein Cardinal, apoptosis-associated speck-like protein (ASC), and caspase-1 (33). NLRP3 inflammasome is implicated in sensing stress caused by reactive oxygen species (9, 32). Recently we showed that hyperoxia induces inflammasome activation (21, 22). However, whether inhibition or deletion of NLRP3 inflammasome is critical to confer protection against HALI has not been studied yet. Since the HALI model is thoroughly characterized in terms of reactive oxygen species involvement, assessing the effect of NLRP3 deletion on HALI will provide important information about how NLRP3 plays a role in HALI and might result in novel therapeutic strategies to treat ALI. In this study for the first time we used NLRP3-deficient mice to identify the role of inflammasomes in hyperoxia-induced lung injury