Baicalein, an active component of Scutellaria baicalensis, protects against lipopolysaccharide-induced acute lung injury in rats

Abstract

Ethnopharmacological relevance

Baicalein (BE), a phenolic flavonoid extracted mainly from the root of Scutellaria baicalensis Georgi, a Chinese herb, is traditionally used in oriental medicine. Several studies have demonstrated that BE exerts many beneficial effects including anti-inflammatory and antioxidant activities. However, its effect on acute lung injury (ALI) and the molecular mechanisms involved remain unclear and warrant further investigation. The aim of the study is to investigate whether BE improves lipopolysaccharide (LPS, intratracheally, i.t.)-induced ALI in rats, and further study the underlying mechanisms of its activity.

Material and methods

Rats were administrated with LPS (5 mg/kg/body weight, i.t.) through a 24-gauge catheter to establish the ALI model. The effects of BE on the levels of pro-inflammatory cytokines, nitrite/nitrate in bronchoalveolar lavage fluid, and the expression of nuclear factor-erythroid 2-related factor 2 (Nrf2), heme oxygenase-1 (HO-1) and nuclear factor-kappa B (NF-κB) activation as well as the histopathological changes were evaluated.

Results

Results showed that BE (20 mg/kg, i.p.) treatment markedly attenuated LPS-induced lung edema, elevation of the levels of IL-1β, TNF-α, IL-6, CINC-3, and nitrite/nitrate in bronchoalveolar lavage fluid accompanied by a remarkable improvement of lung histopathological symptoms. The LPS-enhanced inflammatory cell infiltration and myeloperoxidase activity, O₂⁻ formation and the expression of inducible nitric oxide synthase and nitrotyrosin in lungs were all attenuated by BE. Notably, BE could augment Nrf2/HO-1 cascade, but inhibited NF-κB activation in LPS-instilled lungs that was strongly reversed by blocking HO-1 activity.

Conclusion

This study is the first to demonstrate that BE protects against LPS-induced ALI in rats. The underlying mechanisms may include inhibition of NF-κB-mediated inflammatory responses and upregulation of Nrf2/HO-1 pathway, which ultimately alleviates the pathological symptoms of ALI.

Introduction

Acute lung injury (ALI) and its most severe form, acute respiratory distress syndrome (ARDS), remain the leading diseases causing the high mortality in critically ill patients (Frutos-Vivar et al., 2006). The ALI is characterized by a sustained and uncontrolled inflammatory process in lungs, thereby resulting in increased pulmonary vascular permeability, pulmonary edema and polymorphonuclear neutrophils (PMNs) infiltration into the alveolar space, which in turn impairs respiratory function (Piantadosi and Schwartz, 2004). Clinical observations have indicated that the gram-negative bacterial infection plays an important role in the development of ALI. Exposure of lung with lipopolysaccharide (LPS) or endotoxin, a major constituent of the outer membrane of gram-negative bacteria, may lead to ALI by increasing inflammatory and chemotactic cytokine production and subsequent inflammatory cell sequestration in lung tissues (Togbe et al., 2007). Moreover, in response to LPS, the lung endothelial cells, alveolar cells, airway epithelial cells, and alveolar immune cells can produce a large amount of reactive oxygen species (ROS), a critical mediator amplifying the inflammatory responses and lung injury (Blackwell et al., 1996).

The transcription factor nuclear factor-erythroid 2–related factor 2 (Nrf2)-mediated induction of heme oxygenase-1 (HO-1) is responsible for the degradation of heme into three products: biliverdin, free Fe²⁺ irons and carbon monoxide (CO) (Kikuchi et al., 2005). HO-1 reportedly possesses a protective effect against oxidative stress, inflammation or apoptosis-induced cytotxicity and tissue damage (Cho et al., 2006, Kirkby and Adin, 2006, An et al., 2012). Among HO isoforms, the HO-1 gene is inducible, whereas the genes encoding HO-2 and HO-3 are constitutively expressed. The lung protection due to up-regulation of HO-1 has been confirmed in many experimental models of ALI and sepsis (Sarady-Andrews et al., 2005, Ryter and Choi, 2010, An et al., 2012). Thus, suppression of the inflammatory responses and/or induction of HO-1 may be a potential strategy to alleviate the progression of ALI caused by LPS.

Scutellaria baicalensis Georgi (Lamiaceae family), also called Huang Qin in Chinese, is a Chinese herbal medicine. Scutellaria baicalensis Georgi is used traditionally for treating various diseases including bacterial and viral infections of the respiratory and the gastrointestinal tract, cardiovascular disease, and inflammation (Huang et al., 2005, Shang et al., 2010). The baicalein (BE), (5,6,7-trioxyflavone-7-O-beta-d-glucuronide), is isolated mainly from the root of Scutellaria baicalensis Georgi. BE is known to exhibit a therapeutic effect in endotoxin-related multiple tissue injury including myocardial dysfunction (Lee et al., 2011), acute liver failure (Wu et al., 2010), glomerulonephritis (Yeh et al., 2010), and endotoxic shock in rats (Cheng et al., 2007). Furthermore, it has been reported that the inhibitory effects of BE on LPS-induced pro-inflammatory inducible nitric oxide synthase (iNOS), cyclooxygenase-2 expression and tumor necrosis factor-α (TNF-α) formation in vitro and in vivo are mediated by blocking nuclear factor-kappa B (NF-κB) activation that is critical for modulating several inflammatory gene expression (Cheng et al., 2007, Wu et al., 2010). Recent evidence has indicated that the anti-inflammatory activity of BE in endotoxaemic rats is associated with up-regulation of HO-1 expression (Lee et al., 2011). These findings suggest that BE is a potential therapeutic drug for LPS-related inflammatory diseases. To date, whether BE improves intratracheal LPS-induced ALI is still unknown. Therefore, the present study was designed to evaluate the action of BE on LPS-induced ALI in rats, and further investigate the underlying mechanisms involved.