Carbon monoxide releasing molecule-2 protects against particulate matter-induced lung inflammation by inhibiting TLR2 and 4/ROS/NLRP3 inflammasome activation.

Exposure to airborne particulate matter (PM) not only causes lung inflammation and chronic respiratory diseases, but also increases the incidence and mortality of cardiopulmonary diseases. The nucleotide-binding domain and leucine-rich repeat protein 3 (NLRP3) inflammasome activation has been shown to play a critical role in the formation of many chronic disorders. On the other hand, carbon monoxide (CO) has been shown to possess anti-inflammatory and antioxidant effects in many tissues and organs. Here, we investigated the effects and mechanisms of carbon monoxide releasing molecule-2 (CORM-2) on PM-induced inflammatory responses in human pulmonary alveolar epithelial cells (HPAEpiCs). We found that PM induced C-reactive protein (CRP) expression, NLRP3 inflammasome activation, IL-1β secretion, and caspase-1 activation, which were inhibited by pretreatment with CORM-2. In addition, transfection with siRNA of Toll-like receptor 2 (TLR2) or TLR4 and pretreatment with an antioxidant (N-acetyl-cysteine, NAC), the inhibitor of NADPH oxidase (diphenyleneiodonium, DPI), or a mitochondria-specific superoxide scavenger (MitoTEMPO) reduced PM-induced inflammatory responses. CORM-2 also inhibited PM-induced NADPH oxidase activity and NADPH oxidase- and mitochondria-derived ROS generation. However, pretreatment with inactivate CORM-2 (iCORM-2) had no effects on PM-induced inflammatory responses. Finally, we showed that CORM-2 inhibited PM-induced CRP, NLRP3 inflammasome, and ASC protein expression in the lung tissues of mice and IL-1β levels in the serum of mice. PM-enhanced leukocyte count in bronchoalveolar lavage fluid in mice was reduced by CORM-2. The results of this study suggested a protective role of CORM-2 in PM-induced lung inflammation by inhibiting the TLR2 and TLR4/ROS-NLRP3 inflammasome-CRP axial.