Extracellular Adenosine 5’-Triphosphate in Pulmonary Disorders

Abstract

Adenosine 5’-triphosphate (ATP) is present in every cell of the human body, where it plays an essential role in cellular energetics and metabolism. ATP is released from cells under physiological and pathophysiological conditions; extracellular ATP is rapidly degraded to adenosine 5’-diphosphate (ADP) and adenosine by ecto-enzymes, primarily CD39 and CD73. Before its degradation, ATP acts as an autocrine and paracrine agent, exerting its effects on targeted cells by activating cell surface receptors called P2 purinergic receptors. These receptors are expressed by various cell types in the lungs, the activation of which is implicated in multiple pulmonary disorders. This review summarizes the role of ATP in inflammation processes associated with these disorders, including bronchoconstriction, cough, mechanical ventilation-induced lung injury, and idiopathic pulmonary fibrosis. These remain significant unmet clinical needs. Therefore, the diverse ATP-signaling pathways in pulmonary inflammation are attractive targets for novel drug candidates to improve the management of patients with multiple pulmonary diseases.

Introduction

Adenosine 5’-triphosphate (ATP) exists in every cell of the human body, playing a vital role in cellular metabolism and energetics. ATP is released from cells under various physiological and pathological conditions and acts as an autocrine and paracrine agent affecting target cells by activating P2 purinergic receptors (P2R). The P2 receptors are divided into two families: P2YR, which are seven-transmembrane domain G protein-coupled receptors, and P2XR, which are transmembrane cationic channels. Eight P2YR and seven P2XR subtypes have been cloned. Two decades after the purinoceptor concept was introduced by Burnstock, it was demonstrated that ATP activates P2XR located on vagal sensory nerve terminals in canine lungs. At the same time, studies showed that aerosolized ATP is a potent bronchoconstrictor in both healthy human subjects and, to a greater extent, in asthmatic patients. Later observations confirmed that ATP’s activation of pulmonary vagal sensory nerve terminals and ATP-induced bronchoconstriction are cause-and-effect phenomena. This understanding led to the proposal of the “Adenosine 5’-triphosphate Axis in Obstructive Airway Diseases,” which aligns with the established purinergic signaling in the lungs. Since that time, many studies have confirmed this hypothesis and contributed significant data with major clinical implications. In light of the expanding research in this area, particularly in airway immune responses, this review presents a succinct summary of the state of the art. Given the volume of literature, some topics and references could not be included.

ATP-Induced Bronchoconstriction

Initial studies revealed that extracellular ATP generates afferent neural traffic in pulmonary slow-conducting C fibers by activating P2XR located on these fibers’ terminals. These fibers are bimodal—they are stimulated by both mechanical stretch and chemical compounds such as capsaicin and ATP. Further research found that ATP can also stimulate pulmonary fast-conducting Aδ fibers, and that P2X3R and/or P2X2/3R mediate this activity. Activation of vagal sensory nerve terminals in the lungs results in a central pulmonary-pulmonary vagal reflex, which leads to bronchoconstriction, cough, and likely the localized release of pro-inflammatory neuropeptides via an axon reflex.

Experimental data demonstrated that intravenous and aerosolized ATP cause bronchoconstriction in animals and humans. Patients with chronic obstructive pulmonary diseases such as asthma and COPD exhibit hypersensitivity to aerosolized ATP. Lung inflammation results in increased ATP release into the extracellular space, leading to abnormally high ATP levels in the lungs of COPD patients. Moreover, gene expression of ATP-degrading enzymes is decreased in these individuals. These findings support a mechanistic role of extracellular ATP in bronchoconstriction in asthma and COPD. Targeting this pathway could provide new bronchodilator therapies different from current long-acting adrenergic or muscarinic antagonists, or corticosteroids. Recent data show that aerosolized selective P2X3R antagonists effectively reduce ATP-induced bronchoconstriction in animal models, suggesting their potential as new therapeutic agents.

ATP-Induced Cough

Extracellular ATP lowers the threshold for cough induction, as shown in guinea pig models. This is unsurprising, given ATP stimulates pulmonary vagal C and Aδ fibers, both of which are key to the cough mechanism. Notably, studies indicate that airway sensory nerve density is increased in people with chronic cough, suggesting neuroplasticity that contributes to hypersensitivity. These nerve fibers are likely to be ATP-sensitive vagal C fibers. The tussigenic action of ATP is mediated via P2X3R. Selective antagonists, such as aerosolized DT-0111 and orally administered BLU-5937, have been shown to suppress ATP-induced cough in animals and reduce cough frequency in patients. Non-selective P2X3R/P2X2/3R antagonists like gefapixant also achieve efficacy in clinical trials, with moderate side effects reported. Such drug candidates may soon represent a novel class of therapeutics for chronic cough management.

ATP in Pulmonary Embolism

Platelets contain millimolar concentrations of ATP, much of which is released upon activation. Platelet activation also leads to the release of mediators such as serotonin and histamine. Both ATP and serotonin induce vagal-dependent bronchoconstriction in animal models. Therefore, the massive activation of platelets during pulmonary embolism could result in the localized release of ATP, stimulating sensory nerve endings and increasing vagal input to the lung and heart, potentially causing bradycardia and atrioventricular conduction block.

Three mechanisms have been suggested to explain syncope in pulmonary embolism: massive embolism reducing cardiac output and cerebral perfusion, complete heart block in certain conditions, and central vagal reflex triggered by vagal sensory nerve stimulation. Considering the role of ATP release during platelet activation and its capacity to trigger pulmonary and cardiac vagal reflexes, ATP may be a key mechanistic contributor to bradycardia and syncope in pulmonary embolism.

ATP in Pulmonary Inflammation

It is well established that ATP is released from inflammatory cells and plays a significant mechanistic role in inflammation, especially in the lungs under various disease conditions.

ATP in Mechanical Ventilation-Induced Lung Injury

Mechanical ventilation applies physical stresses to lung tissue, which may result in ventilator-induced lung injury (VILI), characterized by edema, increased permeability, tissue damage, and elevated inflammatory markers. ATP release in response to mechanical stress is well documented. Recent studies implicate ATP in the pathogenesis of VILI, though the full mechanisms remain to be clarified. One theory postulates that ATP activates P2X3R on vagal sensory nerve endings, leading to the localized release of neuropeptides such as substance P by axon reflex, contributing to inflammation and injury. Blockade of P2X3R may thus ameliorate VILI.

ATP in Asthma

ATP and adenosine, the product of its enzymatic breakdown, exert potent effects on multiple lung cell types involved in asthma. Asthma is a chronic inflammatory disease marked by Th2 and Th17 cytokine expression, with evidence indicating a role for ATP in its pathogenesis. Allergen challenge rapidly raises ATP in bronchoalveolar fluid in patients and animal models. In experimental asthma, ATP mediates inflammation and bronchospasm, effects that are attenuated by ATP-hydrolyzing enzymes or P2 receptor antagonists. Exogenous ATP enhances Th2-type sensitization. ATP’s chemotactic action on eosinophils, mast cell degranulation, and subsequent mediator release further underscore its role in asthma.

ATP in Pulmonary Ischemia-Reperfusion Injury

Lung ischemia-reperfusion injury is a significant complication after lung transplantation, featuring inflammation, increased permeability, and edema. In murine models, ischemia–reperfusion leads to increased expression of pannexin-1 channels in lung endothelial cells, facilitating ATP release. Extracellular ATP in this context mediates vascular permeability, inflammation, and leukocyte infiltration. Pharmacological antagonism of pannexin-1 may serve as a therapeutic strategy. Additionally, ATP’s degradation product, adenosine, is protective via A2a receptor activation in this setting.

ATP in Cystic Fibrosis

The cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the ATP-binding cassette (ABC) transporter family but functions as an ATP-gated chloride channel in epithelial cells. ATP binding is critical for CFTR channel opening, and mutations impairing ATP binding contribute to cystic fibrosis. Nearly 2000 mutations in CFTR have been identified, with a broad range of impacts on channel function. Airway dehydration is a pathological feature of cystic fibrosis; notably, inhibition of ATP hydrolysis can restore airway liquid production in affected tissues.

ATP in Idiopathic Lung Fibrosis

Pulmonary fibrosis is a feature of numerous chronic and often fatal lung diseases, including idiopathic pulmonary fibrosis (IPF). Overgrowth of fibroblasts and extracellular matrix deposition occur, disrupting lung function. Increased ATP concentrations have been found in bronchoalveolar fluid in IPF, as well as in animal models of lung injury. The P2X7 receptor has been implicated, as P2X7 knockout mice exhibit reduced markers of tissue fibrosis. Thus, ATP and its signaling through P2X7 may drive lung inflammation and fibrosis.

ATP in Pulmonary Hypertension

Pulmonary hypertension is a progressive disease with various causes, all leading to increased pulmonary artery pressure and right heart failure. Reduced expression of CD39 and increased P2X1 receptor expression have been observed in affected patients and animal models. This suggests that either the enhancement of ATP/ADP breakdown or the blockade of P2X1R may be viable treatment targets. Endothelial P2X1, P2X4, P2Y6, and P2Y11 receptor expression is pronounced, and their activation induces robust intracellular Ca2+ responses. Animal studies indicate that ATP infusion can decrease pulmonary artery pressure, and ATP-MgCl2 is effective and safe in acute pediatric pulmonary hypertension and in postoperative evaluation.

ATP in Lung Cancer

Extracellular ATP and nucleotides may have therapeutic potential in cancer through P2R signaling, but can also foster mitogenic effects in lung epithelial cells via P2Y2 and P2Y6 receptors. Elevated extracellular ATP is found at tumor sites and may be internalized by tumor cells, promoting growth. ATP increases cell survival, proliferation, and motility, in part via autocrine signaling through P2X7R. Both pro- and anti-tumor activities have been noted, depending on ATP levels and receptor subtypes involved. Some in vitro and in vivo studies show that ATP and its nucleotides can inhibit tumor growth or enhance the effects of chemotherapeutics, while others highlight their role in cancer cell survival. Clinical trials of intravenous ATP in advanced lung cancer have not demonstrated disease-modifying benefit, but suggest potential benefit in ameliorating cachexia and improving liver ATP status.

Conclusions

Extracellular ATP and its cell surface receptors play central mechanistic roles in a range of pulmonary disorders, including asthma, COPD, chronic cough, ventilator-induced lung injury, idiopathic pulmonary fibrosis, pulmonary hypertension, and lung cancer. These conditions remain critical unmet clinical needs. Accordingly, pathways of ATP signal transduction represent attractive targets for novel drug development and innovative therapies to improve patient outcomes.

Epilogue: A Personal Note on Collaboration and Friendship with Geoffrey Burnstock

Five years after the purinergic neurotransmission hypothesis was proposed in 1972, I joined the Department of Physiology at the University of Virginia, becoming immersed in an environment known for its focus on adenosine. Despite initial skepticism by prominent researchers, over the years, mounting evidence supported Geoffrey Burnstock’s theory about the existence of extracellular ATP and its signaling roles, eventually leading to widespread recognition and honors.

My journey led from curiosity about ATP’s effects on cardiac physiology to close collaborations with clinical and basic researchers. Observations in clinical electrophysiology deepened my interest in the mechanism of ATP’s cardiovascular actions, confirming the role of both adenosine and vagal pathways. Interactions with Burnstock deepened both scientific collaboration and friendship, influencing research directions and resulting in joint publications and novel drug development efforts. As recognition for Burnstock’s work grew, his commitment to collaboration and his generosity as a mentor and consultant became evident. His death marks a great loss; his contributions to the understanding of ATP’s roles in physiology and disease remain deeply valued.

EPILOGUE: A Personal Note on Collaboration and Friendship with Geoffrey Burnstock

Five years after Geoffrey Burnstock proposed the hypothesis of purinergic neurotransmission in 1972, I joined the Department of Physiology at the University of Virginia in Charlottesville as an NIH postdoctoral fellow. The department, led by Robert M. Berne, M.D., was already a prominent center for adenosine research, earning the moniker “Adenosine Mecca.” Many future leaders in adenosine studies passed through this institution as graduate students, postdoctorates, or visitors. Although most in the department focused on aspects of adenosine, my initial path lay in basic cardiac electrophysiology at Nick Sperelakis’ lab. However, my close proximity to adenosine research inevitably influenced my focus on ATP.

There was skepticism about Burnstock’s hypothesis among Berne and other leading figures, as they believed intact ATP could not persist extracellularly due to its rapid enzymatic degradation. For many years, this novel idea was met with resistance and even ridicule. However, as supportive evidence emerged worldwide, Burnstock’s contributions gained recognition, culminating in the Royal Society Gold Medal in 2000. Ironically, while Berne’s adenosine hypothesis regarding coronary blood flow has not stood the test of time, subsequent studies have illustrated adenosine’s negative feedback role in various tissues during hypoxic or ischemic conditions.

After leaving Virginia, I spent four years at Ichilov Hospital in Tel Aviv, focusing on basic cardiac electrophysiology and collaborating with Dr. Bernard Belhassen. It was during this time that a clinical encounter—where ATP was used to terminate a case of paroxysmal supraventricular tachycardia—sparked my keen interest in ATP’s clinical and physiological actions. With little knowledge available in the literature about extracellular ATP at the time, I proposed a joint research project with Dr. Belhassen, including systematic comparisons of ATP and adenosine on heart rate and conduction. Our first major study revealed that ATP’s negative chronotropic and dromotropic effects in canine hearts were mediated not only by adenosine but also by increased vagal tone triggered by ATP. This experience solidified my position in what Burnstock lightheartedly referred to as the “ATP camp” versus the “adenosine camp.”

My growing involvement in ATP and adenosine research led to significant roles in the development of ATP- and adenosine-based drugs, including Adenocard®, Adenoscan®, and the selective A2a adenosine agonist Lexiscan®. These advancements benefited from collaborative, multidisciplinary research and were grounded in a growing understanding of the extracellular actions of these nucleotides.

A pivotal moment in my professional journey occurred during the 3rd International Symposium on Adenosine in Munich in 1986, where I met Burnstock. Our initial interactions fostered a lasting personal and professional relationship, culminating in my stint as a visiting professor in Burnstock’s lab at University College London—supported by the Burroughs Wellcome Fund. Geoff was an exceptional host and mentor, whose dedication and insight greatly benefited my work and that of many others.

During my time in Burnstock’s lab, I observed his unwavering support for scientific collaboration and innovation. He was always willing to share his wealth of knowledge and was instrumental in supporting our efforts—even years later, when he provided scientific consultation for Duska Therapeutics, a company I founded to develop ATP-related drugs. Our final co-authored publication centered on a novel P2X3R antagonist for the treatment of COPD and chronic cough, demonstrating Geoff’s enduring interest in translating basic scientific discoveries into clinical applications.

Geoffrey Burnstock’s passing is a tremendous loss to the scientific community and to ATP research in particular. His wisdom, generosity, and collaborative spirit left a lasting mark on all who worked with him.Adenosine 5′-diphosphate As a friend and colleague, his memory and impact will be deeply cherished.