[Annexin A1 activates the G protein-coupled formyl peptide receptor type 2-dependent endothelial nitric oxide synthase pathway to alleviate sepsis associated acute lung injury].

Objective: To investigate whether annexin A1 (ANXA1) improves sepsis-induced lung injury by activating G protein-coupled formyl peptide receptor type 2 (FPR2)-dependent endothelial nitric oxide synthase (eNOS) pathway.

Methods: Twenty-four male SD rats were randomly divided into normal group (Control group), lipopolysaccharide (LPS) induced lung injury model group (LPS group), LPS+ANXA1 mimetic peptide group (LPS+Ac2-26 group) and LPS+ANXA1 mimetic peptide+FPR2 inhibitor group (LPS+Ac2-26+WRW4 group), with 6 rats in each group. On the third day before modeling, rats of the LPS+Ac2-26 group were injected with 1 mg/kg Ac2-26 by the tail vein and rats of LPS+Ac2-26+WRW4 group were injected with 1 mg/kg Ac2-26 and 2.2 mg/kg WRW4 by the tail vein. The rats of control group and LPS group were injected same volume of physiological saline. After 48 hours of modeling, the rats were anesthetized and the carotid blood was taken to detect the oxygenation index (OI). Lung tissue was taken from the euthanized rats. The wet/dry (W/D) ratio was determined. The pathological changes of lung tissue were observed under light microscope and pathological score was performed. The levels of tumor necrosis factor-α (TNF-α), interleukins (IL-1β, IL-6, IL-10), malondialdehyde (MDA) and myeloperoxidase (MPO) were detected by enzyme-linked immunosorbent assay (ELISA). The protein expressions of eNOS, inducible nitric oxide synthase (iNOS) and nuclear factor-κB (NF-κB) were detected by Western blotting.

Results: Under light microscope, compared with LPS group, the infiltration degree of inflammatory cells in the lung tissue of LPS+Ac2-26 group was reduced, and the thickness of the alveolar septum was improved. The degree of inflammatory cell infiltration in the lung tissue of LPS+Ac2-26+WRW4 group was more severe than that of LPS+Ac2-26 group, and the thickness of the alveolar septum increased. These findings suggested that ANXA1 significantly inhibited inflammatory cell infiltration and improved alveolar septal thickness, WRW4 reversed the lung improvement effects of ANXA1. Compared with control group, OI in LPS group was significantly decreased, and W/D ratio, pathological score and TNF-α, IL-1β, IL-6, MDA and MPO levels in lung tissue were significantly increased. Compared with LPS group, OI and IL-10 levels in lung tissue were significantly increased in LPS+Ac2-26 group, while W/D ratio, pathological score, TNF-α, IL-1β, IL-6, MDA and MPO levels in lung tissue were significantly decreased. These results indicated that ANXA1 can improve the oxygenation capacity, improve lung tissue leakage, reduce edema, and inhibit lung tissue inflammation in rats with lung injury. Compared with LPS+Ac2-26 group, the LPS+Ac2-26+WRW4 group showed significant decreases in OI and lung tissue IL-10 level [OI (mmHg, 1 mmHg ≈ 0.133 kPa): 132.16±24.00 vs. 248.67±18.70, IL-10 (ng/L): 27.30±3.04 vs. 36.10±3.92, both P < 0.05], the lung tissue W/D ratio, pathological score and levels of TNF-α, IL-1β, IL-6, MDA and MPO were significantly increased [W/D ratio: 5.29±0.02 vs. 4.83±0.02, pathological score: 5.00±0.28 vs. 2.67±0.52, TNF-α (ng/L): 39.80±4.36 vs. 32.10±2.15, IL-1β (ng/L): 200.00±15.68 vs. 152.60±9.74, IL-6 (ng/L): 181.50±18.02 vs. 148.50±7.34, MDA (mmol/mg): 82.01±8.22 vs. 70.43±5.69, MPO (pg/mg): 6.50±0.32 vs. 4.60±0.56, all P < 0.05]. These results suggested that WRW4 could block the above improvement of ANXA1. Western blotting results showed that compared with control group, the expression of eNOS, iNOS and NF-κB in LPS group was significantly up-regulated. Compared with LPS group, the protein expression of eNOS in LPS+Ac2-26 group was significantly up-regulated (eNOS/β-actin: 0.25±0.01 vs. 0.14±0.01, P < 0.05), and the protein expression of iNOS and NF-κB was significantly down-regulated (iNOS/β-actin: 0.09±0.02 vs. 0.12±0.02, NF-κB/β-actin: 0.35±0.06 vs. 0.59±0.13, both P < 0.05). These findings suggested that ANXA1 might activate the eNOS pathway and down-regulate the expression of NF-κB. Compared with LPS+Ac2-26 group, the protein expression of eNOS in LPS+Ac2-26+WRW4 group was significantly down-regulated (eNOS/β-actin: 0.17±0.02 vs. 0.25±0.01, P < 0.05), while the protein expression of iNOS and NF-κB was significantly up-regulated (iNOS/β-actin: 0.12±0.02 vs. 0.09±0.02, NF-κB/β-actin: 0.52±0.10 vs. 0.35±0.06, both P < 0.05). These results suggested that WRW4 blocked the activation of the eNOS pathway by ANXA1.

Conclusions: ANXA1 can improve lung injury associated with sepsis by activating FPR2-dependent eNOS pathway.