Acute lung injury in association with gastric aspiration carries a mortality of up to 30% (15
). NF-κB plays a key role in the molecular and cellular events in acid-induced acute lung injury (16
). Depletion of alveolar macrophages results in decreased production of inflammatory mediators in acid aspiration (23–80%) (15
). In addition, the neutrophil is an important mediator of acid-induced acute lung injury (17
). Thus, with an animal model of acid-induced lung injury, it is possible to test the hypothesis that activation of α7 nAChR may regulate pulmonary edema and proinflammatory responses in acid-induced acute lung injury.
The importance of the cholinergic anti-inflammatory pathway has recently been recognized (1
). Acetylcholine released by efferent vagus nerves inhibits macrophage activation (18
). α7 nAChR is required for acetylcholine inhibition of macrophage TNF release (2
). Nicotine, as the prototype agonist of nAChR, can protect mice from lethal sepsis (4
) by inhibiting HMGB1 production. A selective α7 nAChR agonist (2.4-dimethoxybenzylidene anabaseine dihydrochloride, GTS-21) has been used to attenuate experimental pancreatitis (19
). In this study, to the best of our knowledge, we first demonstrated that activation of α7 nAChR with its agonists (nicotine, choline, and PNU-282987) protected against acid-induced acute lung injury as reflected by a marked reduction in excess lung water, lung vascular permeability, proinflammatory cytokines and protein in the BAL, neutrophil infiltration, and epithelial cell injury (decreased RAGE level in the BAL). Conversely, blockade of α7 nAChR with MLA (an antagonist) counteracted the protective effect of nicotine and deficiency of α7 nAChR worsened acid-induced acute lung injury.
Activation of α7 nAChR by its endogenous agonists (acetylcholine, or choline, a metabolite of acetylcholine) is a key pathway to limit lung inflammation. This notion was confirmed by deficiency of α7 nAChR resulted in a 2-fold increase in excess lung water and lung vascular permeability in the acid-induced acute lung injury. Alveolar macrophages and neutrophils expressed α7 nAChR, which possessed acting sites for endogenous and exogenous α7 nAChR agonists. In the injured lung, α7 nAChR expression in alveolar macrophages and neutrophils was increased. Up-regulation of α7 nAChR expression in these proinflammatory cells may probably compensate for the inflammation and also may provide more binding sites for exogenous α7 nAChR agonists. Activation of α7 nAChR inhibited NF-κB activity, which was demonstrated by reduction of NF-κB p65 translocation. As a result of reduced NF-κB activation, activation of α7 nAChR reduced proinflammatory cytokine (TNF-α, MIP-2) levels in the airspaces of the lung. Decreased MIP-2 production from alveolar macrophages may also reduce neutrophil chemotaxis to the airspaces and further reduce the severity of lung inflammation and injury.
Lung airway epithelia are innervated by the vagus nerve, where alveolar epithelial cells have access to endogenous α7 nAChR agonists (acetylcholine and choline). Also, alveolar epithelial II type cells express α7 nAChR. The lung epithelium is an important locus of NF-κB activation during inflammation (20
). It is still unclear whether stimulation of α7 nAChR can suppress NF-κB activity and reduce proinflammatory cytokine production. Thus, we cannot rule out an inhibitory effect on inflammation of activation of α7 nAChR by lung epithelial cells in this study.
In addition to human bronchial cells, the endothelial cells express α7 nAChR (21
). It is possible that activation of α7 nAChR in the endothelial cells reduces neutrophil adhesion and infiltration. As expected, neutrophils were the main cell population in the airspaces of the lung 4 h after acid instillation. Administration of nicotine, choline, and PNU-282987 reduced neutrophil accumulation in the airspaces. This result can be explained by reduction of MIP-2 and neutrophil chemotaxis. Another explanation is that activation of α7 nAChR in the endothelial cells may reduce endothelial cell activation and leukocyte binding (22
In the endotoxemia mouse model, activation of α7 nAChR by vagus nerve stimulation reduced the levels of TNF-α in the plasma, liver, and spleen, but not in the lung (23
). There may be several reasons to explain why their findings were different from ours: (1
) The endotoxemia model is different from our acid-induced acute lung injury model. (2
) The TNF-α was measured only in the lung homogenate. Thus, we cannot rule out that there was a difference in the TNF-α level in the BAL because alveolar macrophages are the major sources of TNF-α. (3
) LPS was given by intravenous injection rather than intratracheal instillation. Even with intratracheal instillation of LPS, 12–24 h are required to develop peak pulmonary edema and inflammation. Thus, we believe that the lungs were probably not injured sufficiently at 90 and 180 min after intravenous injection of LPS to demonstrate the beneficial effect of vagus nerve stimulation.
Taken together, activation of α7 nAChR can inhibit NF-κB activation in alveolar macrophages and reduces the production of proinflammatory cytokines (MIP-2 and TNF-α). Thus, administration of α7 nAChR agonist can reduce neutrophil accumulation in the airspaces of the lung as well as decrease pulmonary edema and lung inflammation. These results suggest that activation of α7 nAChR may be useful as a novel anti-inflammatory therapy to treat acute lung injury.