Aesculetin Suppresses Airway Epithelial Barrier Disruption Induced by Inhalation of Urban Coarse Particulate: Involvement of Neutrophil Elastase and Protease-Activated Receptor-2

Particulate matter (PM) is a mixture of solid and liquid air pollutants floating in the air, and it is the most harmful form of air pollutant because it can penetrate deep into the lungs and bloodstream and cause various breathing problems. Neutrophil elastase (NE) is known to be a major constituent of lung proteolytic activity and potently stimulates mucus secretion during airway inflammation. Aesculetin is a coumarin derivative that has anti-inflammatory properties in blood vessels and the immune system. This study investigated whether aesculetin inhibited bronchial barrier destruction caused by urban PM10 (uPM10, particles less than 10 μm). Balb/c mice were orally administrated with 10 mg/kg aesculetin while inhaling 6 μg/mL of uPM10 for 8 weeks. In addition, human bronchial epithelial BEAS-2B cells were exposed to 2 μg/mL uPM10 or 0.5 μg/mL NE in the presence of 1-20 μM aesculetin. Oral administration of aesculetin attenuated neutrophil infiltration and accumulation in the small airways inflamed by uPM10 and suppressed NE-mediated neutrophil inflammation in the airways. The supplementation of aesculetin boosted bronchial levels of junction proteins of occludin-1 and ZO-1, depleted due to uPM10 inhalation, indicating that this compound blocked airway epithelial barrier disruption caused by uPM10. Consistently, aesculetin enhanced the induction of occludin-1 and ZO-1 in BEAS-2B cells exposed to either uPM10 or NE. On the other hand, aesculetin suppressed bronchial matrix metalloproteinase (MMP)-2 in uPM10-loaded mice. Moreover, aesculetin inhibited the bronchial induction of protease-activated receptor (PAR)-2. Collectively, aesculetin improved the upper airways damaged by the inhalation of urban coarse PM through inhibiting NE-driven neutrophil inflammation and MMP-2 activation involving PAR-2. Therefore, anti-inflammatory aesculetin may be a promising natural agent to strengthen the airway epithelial barrier disrupted by activation of PAR-2-NE and PAR-2-MMP-2 in dusty urban environments.