Lipoxin A4 has been described as a major signal for the resolution of inflammation and is abnormally produced in the lungs of patients with cystic fibrosis (CF). In CF, the loss of chloride transport caused by the mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) Cl− channel gene results in dehydration, mucus plugging, and reduction of the airway surface liquid layer (ASL) height which favour chronic lung infection and neutrophil based inflammation leading to progressive lung destruction and early death of people with CF. This review highlights the unique ability of LXA4 to restore airway surface hydration, to stimulate airway epithelial repair, and to antagonise the proinflammatory program of the CF airway, circumventing some of the most difficult aspects of CF pathophysiology. The report points out novel aspects of the cellular mechanism involved in the physiological response to LXA4, including release of ATP from airway epithelial cell via pannexin channel and subsequent activation of and P2Y11 purinoreceptor. Therefore, inadequate endogenous LXA4 biosynthesis reported in CF exacerbates the ion transport abnormality and defective mucociliary clearance, in addition to impairing the resolution of inflammation, thus amplifying the vicious circle of airway dehydration, chronic infection, and inflammation.
Lipoxin A4 (LXA4) belongs to a class of newly identified specialised proresolution lipid mediators playing a central role in the resolution of inflammation which results from the sequential production of characteristic eicosanoids in a process termed “class switching” [
LXA4 is produced by multistep enzymatic process resulting from lipoxygenase (LO) activities in different cell types [
Two main pathways will result in LXA4 synthesis. One involves lipoxygenation of arachidonic acid by 15-LO in macrophages and epithelial cells. The 5-LO expressed by neutrophils can then utilise the 15(S)-hydroxyeicosatetranoic acid (15S-HETE) released as a substrate to synthesize LXA4 [
Lipoxin A4 biosynthesis by trans-cellular cooperation in the airways. The neutrophil donates LTA4 intermediate formed by the action of 5 lipoxygenase (5-LO) on arachidonic acid (AA) to the acceptor airway epithelial cell or alveolar macrophage whereby 15 lipoxygenase (15-LO) catalyses LXA4 formation (brown arrows). Airway epithelial cell or alveolar macrophage 15-LO activity catalyses the conversion of AA to 15S-HETE which is donated to the acceptor neutrophil and converted to LXA4 by 5-LO catalysis (blue arrows).
The anti-inflammatory action of LXA4 is mainly mediated by the formyl-peptide receptor 2 (FPR2) which is one member of a subgroup of receptors linked to inhibitory G-proteins, also called ALX [
LXA4 inhibits neutrophil effector functions [
Cystic fibrosis (CF) is the most common lethal genetic disorder in Caucasians caused by a mutation in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR). The disease was first characterised in 1938 by Andersen who described the cystic fibrosis of the pancreas and correlated it with the lung and intestinal disease that occurs in CF [
The CFTR protein is principally expressed in the apical membranes of epithelia where it acts as an anion channel providing a pathway for Cl− and bicarbonate
In addition to CFTR dysfunction, other abnormalities have been described in chronically inflamed and infected CF airways, including intrinsic proinflammatory properties, amplified inflammatory responses to infections, and reduced bacterial clearance. More specifically, the levels of LXA4 have been reported to be decreased in CF, like in other chronic airway inflammatory diseases such as asthma [
The lung must continually defend itself against bacteria that deposit on the airway surfaces during normal tidal breathing. Mucus clearance is a primary form of pulmonary defence and the efficiency of mucociliary clearance in large part depends upon the volume of the airway surface liquid layer (ASL). The ASL allows for mucus containing foreign bodies to be transported away from the lung to the oropharynx where it is either expelled from the body or swallowed and destroyed by the gut. The ASL provides a low viscosity solution allowing free ciliary beat and mucus transport [
In normal airways the airway surface liquid layer (ASL) provides an adequate mucociliary clearance which is maintained by a combination of Cl− secretion through the cystic fibrosis transmembrane conductance regulator (CFTR), Na+ absorption via the epithelial sodium channel (ENaC), and water transport through a paracellular pathway and membrane bound aquaporins (Aq). In CF, a defective CFTR leads to loss of Cl− secretion and Na+ hyperabsorption. The concomitant dehydration of the airway lumen favours bacterial infection and inflammation (mainly neutrophilic). LXA4 mediates an increase in ASL height and restores it to normal levels in CF bronchial epithelium. LXA4 also increase tight junction formation, reestablishing the epithelial barrier function. Taken together this work provides evidence for LXA4 as potentially a new therapy for CF patients.
One of the greatest challenges of fundamental research into reversing the CF defect in the lung has been to design a strategy to overcome the absence of functional CFTR by stimulating chloride secretion via alternative pathways, thus restoring airway hydration and mucociliary clearance. This can be achieved via the stimulation of calcium activated Cl− by agents that raise the intracellular concentration of calcium. Yet, this strategy has been plagued by the side effects of the amplification of the calcium-dependent proinflammatory response resulting in undesirable activation of NF
Mason et al. first proposed that extracellular ATP regulates ion transport rates when added to either the apical or basolateral surface of human airway epithelium and found that these effects appear to be mediated by cell surface receptors that respond to ATP by regulating ion transport rates through the release of Ca2+ from internal stores and extracellular Ca2+ influx [
Pharmacological data has shown that the P2Y11 receptor is preferentially activated by ATP and is uniquely coupled to both the phosphoinositide and the cAMP pathways [
The complex cellular composition of the airways that is ciliated cells and mucin-secretory goblet cells suggests that several mechanisms and pathways are involved in the release of nucleotides into the airways. Two general mechanisms for the release of ATP from cells have been proposed as vesicular release and channel-mediated release. While vesicular release of ATP is well documented, ATP release can also occur in the absence of vesicules. For example, human erythrocyte which is devoid of cytoplasmic vesicle can release ATP in low oxygen content or in response to shear stress [
The increase in Ca2+ can activate Panx1 channels and subsequent release of ATP provides a new source for extracellular ATP to reach more distant cells [
The mechanism by which LXA4 stimulates Ca2+-activated Cl− secretion and ASL height increase has been elucidated. Higgins et al. reported that LXA4 induces an apical ATP release from non-CF and CF airway epithelial cell lines and CF primary cultures. This ATP release induced by LXA4 is completely inhibited by antagonists of the FPR2 receptor and Panx1 channels suggesting a major role of Panx1 in this effect. Furthermore, LXA4 induces an increase in intracellular cAMP and calcium, which are abolished by the selective inhibition of the P2RY11 purinoreceptor. Panx1 and ATP hydrolysis inhibition and P2RY11 purinoreceptor knockdown all abolish the increase of ASL height induced by LXA4. Inhibition of the A2b adenosine receptor does not affect the ASL height increase induced by LXA4, whereas the PKA inhibitor partially inhibits this response. Taken together this report provides evidence for a novel role of LXA4 in stimulating apical ATP secretion via Panx1 channel and subsequent P2RY11 purinoreceptor activation in airway epithelial cells leading to an ASL height increase (Figure
Lipoxin A4 enhances epithelial barrier integrity by stimulating an increase in airway surface liquid (ASL) layer height, epithelial repair, and tight junction formation. Stimulation of the FPR2 receptor by LXA4 induces an apical ATP release through the pannexin (Panx1) channel activating a purinoreceptor pathway. Activation of P2Y11 receptors stimulates chloride secretion out of the cell by calcium activated chloride channels (CaCC) and inhibition of sodium absorption by amiloride sensitive epithelial sodium channels (ENaC) which result in a restored ASL height in CF bronchial epithelial cells. The calcium signal induced by P2Y11 activation also stimulates epithelial repair and tight junction formation. Taken together, the physiological effects induced by LXA4 have the potential to delay the invasion of bronchial epithelial cells by bacteria (green and orange structures).
In CF, recurrent infections and inflammatory insults result in damage to the airways and trigger the repair process [
The lipid mediator LXA4 triggers epithelial cell migration and proliferation and thus plays a role in repair of epithelia including bronchial epithelium from patients with CF [
There is currently no treatment available that fully corrects the biochemical abnormality in CF and leads to a cessation of the typical pathobiology seen in the condition. Therapies to date have been centred on slowing the decline in pulmonary function over time to prolong survival. Medication is predominantly used to optimise nutrition (pancreatic enzymes, fat soluble vitamin supplementation), treat infection (oral, inhaled, and intravenous antibiotics), and facilitate effective mucociliary clearance (DNAse, hypertonic saline). Several anti-inflammatory approaches have been examined in CF; however, the ideal anti-inflammatory drug is not yet available [
Two promising avenues of therapy have recently emerged: small molecule correctors and gene therapy. The flagship small molecule corrector has been Ivacaftor (VX-770). This compound facilitates gating of defective CFTR where the cause of CFTR dysfunction is a gating mutation—predominantly G551D. This has been remarkably clinically successful but can be taken by only approximately 5% of patients worldwide [
For now, we continue to search for new and effective therapies to slow or prevent the decline in pulmonary function in CF.
A variety of airway clearance therapies have been developed for patients with CF [
In conclusion, the discovery of the multiple impacts of LXA4 in restoring bronchial epithelium ion transport, in enhancing ASL height, in restoring epithelial barrier function, and in reducing inflammation might provide significant advance in treatment of the CF airway disease (Figure
The authors declare that there is no conflict of interests regarding the publication of this paper.