In the present study, we investigated the anti-inflammatory effects of bergenin using TNF-α-stimulated human bronchial epithelial cells model. Our results showed that bergenin significantly decreased TNF-α-mediated production of IL-6 and IL-8 at mRNA and protein in 16-HBE cells, which was found to as a result of its NF-κB inhibition property. Further study demonstrated that the NF-κB inhibition property of bergenin was associated with increased SIRT1 activity. Moreover, the combination of bergenin and dexamethasone treatment could exert additive reduction effects on TNF-α-mediated expression of IL-6 and IL-8.
Bronchial epithelium cells served as a barrier to maintain lung homeostasis and protected against external harmful stimulus. However, persistent damages to the bronchial epithelium driven dysregulation of pro-inflammatory response, which was supposed to be involved in the context of airway remodeling and progression to asthma.
Diverse research suggested that the early immune response in the airway is regulated mainly by the pro-inflammatory cytokine TNF-α [25, 29]. Both of abnormal inflammation and airway remodeling are key histopathologic features in asthma. Upon TNF-α stimulation, the neutrophils and eosinophils airway recruitment, and the increased release of cytokines have been demonstrated to contribute to early onset asthma and asthma exacerbation [21, 31]. In addition, TNF-α could upregulate the expression of transforming growth factor (TGF) -β1, which has been shown to be implicated in airway remodeling in severe asthma . These evidences all indicated that the pathogenesis role of TNF-α during the progression of asthma. For these reasons, TNF-α has been suggested as an important target for asthma control. Therefore, in this study, we employed 16-HBE cells with TNF-α stimulation to investigate whether bergenin could exert inhibitory effects on TNF-α-induced pro-inflammatory response.
Our study showed that bergenin has an inhibitory effect on TNF-α-mediated elevation of IL-6 and IL-8 in 16-HBE cells. Accumulated evidences have provided a correlation between these pro-inflammatory mediators and asthma in both mouse models of allergic asthma and patients with asthma. The amounts of IL-6 has been found to be increased in the sputum and BAL fluid of patients with asthma [33, 34]. IL-6 levels significantly correlated with disease severity and lower forced expiratory volume in 1 second (FEV1) in children and adults with asthma [35, 36]. Blockade of IL-6 attenuated airway recruitment of eosinophil and neutrophil in allergen-challenged mice . In addition, IL-8, a powerful chemoattractant of neutrophil, has also been thought to play an indispensable role in initiating and/or exacerbating of asthma. Kinetics of IL-8 showed that level of IL-8 was significantly increased in acute exacerbation of asthma, but was decreased at resolution . It is postulated that the increased IL-8 was responsible for virus-driven asthma exacerbations and correlated with more neutrophils in the sputum of asthma patients . IL-8-recruited neutrophil was believed to lead to development of severe clinical forms of neutrophilic asthma, which was intractable for asthma treatment due to frequent steroid resistance . Thus, IL-8 could be developed as attractive target for neutrophilic asthma. Based on these evidences, down-regulation of IL-6 and IL-8 by bergenin treatment may sever as effective strategy for asthma management
The transcription factor NF-κB was well-known for pro-inflammatory cytokines initiation, and has been demonstrated to be closely associated with the development of asthma . Aberrant activation of NF-κB caused the proliferation of airway smooth muscle cells, resulting in airway narrowing and hyperresponsiveness in asthmatic . Increased NF-κB nuclear translocation has been observed in the airway recruited inflammatory cells of asthmatics . Besides, NF-κB subunit (including p50 or c-Rel) deficiency in mice abrogated the development of allergic asthma, which directly supported that functional role of NF-κB in the pathogenesis of allergic airways disease [45, 46]. Upon allergens airway exposure, released of many pro-inflammatory mediators including IL-6 and IL-8 were regulated, at least in part, by NF-κB [47, 48]. The results of the present study demonstrated that bergenin suppressed degradation of IkBα, phosphorylation of p65 NF-κB and NF-κB nuclear translocation. Therefore, we can suppose that the decreased of IL-6 and IL-8 secretion by bergenin was due to inhibition of NF-κB activation.
Interestingly, our results showed that expression levels of SIRT1 was not affected by bergenin, but the decreased SIRT1 activity mediated by TNF-α stimulation was significantly reversed by bergenin. SIRT1, belonging to a member of NAD+-dependent class III histone/protein deacetylases (HDACs), has been reported to be implicated in deacetylate a variety of substrates, and thus affected a series of cellular pathophysiology process. Research showed that the suppression of SIRT1 activity has been shown to be involved in promoting the progression of asthma . This implied us that increased the SIRT1 activity by novel agents might appears to be a very promising strategy for the treatment of severe uncontrolled asthma. In addition, previous study indicated that activated SIRT1 could attenuate NF-κB-driven inflammatory response upon LPS or TNF-α stimulation [27, 51]. Our above-mentioned results showed that bergenin inhibited NF-κB activation. Therefore, we investigated the effect of increased SIRT1 activity by bergenin on the post-translational modification of NF-κB-p65 in TNF-α-stimulated 16-HBE cells. As expected, acetylation status of NF-κB-p65 was decreased by bergenin treatment. Moreover, the SIRT1 inhibitor salermide reversed the anti-inflammatory effects of bergenin in TNF-α-stimulated 16-HBE cells. Therefore, we can suppose that the increased SIRT1 activity by bergenin could deacetylate NF-κB-p65; in turn the inhibition of NF-κB reduced TNF-α-induced production of IL-6 and IL-8.
Inhaled or oral glucocorticoids has been extensively applied to control excess airway inflammation of asthmatic in clinic. However, it has been shown that increased number of patients with asthma gradually fail to respond to glucocorticoids. The molecular mechanism underlying of glucocorticoid resistance or insensitivity was due to excessive activation of MAPK and NF-κB signaling pathway . Furthermore, increased NF-κB activation is closely correlated with the severity of asthma . Therefore, it was proposed that inactivation of NF-κB could provide beneficial effects in the treatment of asthma. In accordance with this, our data indicated that combination of bergenin and dexamethasone exerted additive effects on TNF-α-mediated production of IL-6 and IL-8. Thus, we can suppose that the additive anti-inflammatory effect may be a result of the inhibition property on NF-κB by bergenin. Therefore, we believed that bergenin can contribute as an adjunct to the treatment of glucocorticoid resistance in asthma.