Inhibitory Effects of Flavonoids Extracted from Licorice on Lipopolysaccharide-induced Acute Pulmonary Inflammation in Mice

Airway inflammation plays important roles in the pathogenesis of acute respiratory distress syndrome (ARDS), asthma and chronic obstructive pulmonary disease (COPD), and anti-inflammatory treatment effectively improves the symptoms of these diseases. To develop the potentially therapeutic compounds for the treatment of pulmonary inflammation, we investigated the effects of licorice flavonoids (LF) extracted from the roots of Glycyrrhiza uralensis (licorice) on lipopolysaccharide (LPS)-induced acute pulmonary inflammation in mice. Acute pulmonary inflammation was induced by intracheal instillation with LPS, treatment with LF at dosages of 3, 10 and 30 mg/kg significantly reduced the LPS-induced inflammatory cells, including neutrophils, macrophages and lymphocytes accumulation in bronchoalveolar lavage fluids (BALF), among these inflammatory cells, LF predominately inhibited neutrophil infiltration, and the maximal effect (30 mg/kg) was as comparable as dexamethasone treatment at 1 mg/kg. Consistent with its effects on neutrophil infiltration, LF treatment significantly increased LPS-induced BALF superoxide dismutase activity, and significantly decreased lung myeloperoxidase activity as well. Furthermore, treatment with LF at 30 mg/kg significantly reduced LPS-induced lung TNFalpha and IL-1beta mRNA expression at 6 h and 24 h after LPS instillation, respectively. Finally, LF at different dosages not only significantly decreased the elevation of lung water content, but also markedly attenuated LPS-induced histological alteration. Therefore, we suggest that LF effectively attenuates LPS-induced pulmonary inflammation through inhibition of inflammatory cells infiltration and inflammatory mediator release which subsequently reduces neutrophil recruitment into lung and neutrophil-mediated oxidative injury, and this study provides with the potential rationale for development of anti-inflammatory compounds from flavonoid extracts of licorice.