3.1 TNF-α induced NF-κB/p65 activation and translocation to nuclear in A549 cells
As NF-κB acts as an essential role in the progression of ALI, we evaluated its expression and activity in A549 during TNF-α induced ALI. We first detected NF-κB/p65 mRNA expression by PCR. The results showed that compared with the control group, the NF-κB/p65 mRNA expression was notably upregulated in A549 treated with TNF-α for 24 hours (Fig. 1A). Furthermore, we used western blot to analyze the lever of NF-κB/p65 activation and translocation to nuclear. It was found that TNF-α induced the activation and translocation of NF-κB/p65 (Fig. 1B and C). Besides, the data of RNA interference experiments (the p65 siRNA Group) demonstrated the effective down-regulation of NF-κB/p65 after NF-κB/p65 siRNA transfection.
3.2 Effects of NF-κB/p65 on TNF-α-induced intracellular ROS production in A549 cells
Excessive production of ROS induces the expression of inflammatory cytokine, leading to lung injury[5]. Thus, we attempted to explore whether NF-κB/p65 regulates TNF-α-induced ROS generation in Type Ⅱ alveolar epithelial cells (A549). DCFH-DA assay was used to assess the activation of ROS generation in A549 cells after 24 h’ treatment with TNF-α. As demonstrated in Fig. 2, compared with exposure to control conditions, intracellular ROS production was significantly increased after the treatment of TNF-α. Moreover, we pre-transfected NF-κB/p65 siRNA in A549 cells and the data showed that NF-κB/p65 siRNA notably reduced the intracellular ROS level, which indicates that NF-κB may attenuate ROS generation in TNF-a induced ALI.
3.3 Effects of NF-κB/p65 on the regulation of NOX1, NOX2 and NOX4 mRNA and protein expression in TNF-α-stimulated A549 cells
The NADPH oxidases are considered as important originators of reactive oxygen species (ROS) in ALI. Recent studies showed that NF-κB modulates the NOX family gene activity in various cell types. Therefore, we explored whether NF-κB regulates TNF-α-induced NADPH oxidases activation in A549 cells. The mRNA level of NADPH oxidases was analyzed by real-time PCR. Compared with the control group, TNF-α treatment induced NOX1, NOX2 and NOX4 mRNA expression. Moreover, transfection with NF-κB/p65 siRNA markedly diminished the NOX1, NOX2 and NOX4 activation in TNF-α-stimulated A549 (Fig. 3A). However, there was no obvious difference in NOX3, NOX5, DUOX1 and DUOX2 (Fig. 3B).
To certify the role of NF-κB on TNF-α-induced NOX1, NOX2 and NOX4 activation, we detected the protein expression by western blot. As presented in Fig. 3C, Fig. 3D and Fig. 3E, TNF-α treatment induced NOX1, NOX2 and NOX4 protein activation, and this overexpression in A549 could be inhibited by NF-κB siRNA significantly. In conclusion, these results reflected that TNF-a stimulated NOX1, NOX2 and NOX4 mRNA and protein activation through NF-κB in A549 cells.
3.4 Identification of NF-κB binding sites on the predicted elements of NOX1 promoter
Previous research found that NOX1, but not NOX2 and NOX4, plays an important role in ROS production and cell death in hyperoxia-induced acute alveolar epithelial cells injury[7–9]. Therefore, in this study, we took NOX1 as the target gene of the ALI prevention and control strategy. To further test the regulation of NOX1 expression by NF-κB, bioinformatic analysis was performed on the NOX1 promoter. According to the online prediction website, there might be a strong targeting relationship between NF-κB and NOX1. Two potential NF-κB binding sites in the human NOX1 proximal promoter gene (1439 bp) were assessed using Alibaba 2.1 software (TRANSFACR) (Additional file 1): -1095/-1086 bp (NOX1/κB1), -261/-252 bp (NOX1/κB2). The sequences of two transcription factor binding sites are as below: Nox1/κB1: 5’-CAGGAAAAAC- 3’; Nox1/κB2: 5’-TAAAATCCCC- 3’ (Fig. 4A). To further investigate the functional NF-κB binding sites in NOX1 promoter in TNF-α-stimulated A549, we performed electromobility shift assays (EMSA) (Fig. 4B). The results revealed that a Shift band was detected in lane 2 which containing probe 2 designed based on Nox1/κB 2. To confirm whether NF-κB was able to recognize and bind to the putative NOX1/κB2 region of NOX1 promoter, supershift assay was performed. As showed in Fig. 4C, the anti-NF-κB p65 antibody was an addition in Lane 5, resulting in the formation of a bigger complex known as the supershift band, by comparison to the Shift band. Collectively, these findings suggested there is a physical interaction between NF-κB and the putative proximal elements of NOX1 promoter.
3.5 Regulatory effects of NF-𝜅B on human NOX1 promoter constructs
To further confirm the activation of NF-κB in the NOX1 promoter, a wt-NOX1 promoter-luciferase reporter vector (called Vector1 for short) was constructed and transfected into A549 cells in the absence or presence of NF-κB/p65 siRNA, then treated with TNF-α for 24 hours. The results illustrated that TNF-α treatment up-regulated the activity level of NOX1 promoter compared, which could be reduced by NF-κB/p65 siRNA (Fig. 5A). These findings further indicated the involvement of NF-κB in TNF-α-induced activation of the NOX1 promoter.
Next, to investigate whether the putative NF-κB binding sites mediated transcriptional activation of NOX1 genes, we construct a deletion mutant of the human NOX1 promoter-luciferase reporter vector (called Vector2 for short), without the fragment of the NOX1/κB2. Empty vector, Vector1 and Vector2 human NOX1 promoter constructs were transiently transfected into A549 cells, respectively, and the transcription activities were detected in the presence or the absence of TNF-α. As presented in Fig. 5B, TNF-α up-regulated the luciferase activity of Vector1 and Vector2 NOX1 promoters, respectively. Moreover, when the deletion mutant was made in the − 261/-252 bp (NOX1/κB2, TAAAAATCCC) region of the NOX1 promoter, the effect of TNF-α on luciferase activity was lower than the integrity of the NOX1 proximal promoters. Collectively, our results suggested that NF-κB could specifically bind to the − 261/-252 bp (NOX1/κB2, TAAAAATCCC) region of NOX1 promoter and active NOX1 expression in in TNF-α-stimulated A549 cells.