Purinergic Signaling in Pulmonary Inflammation

Abstract

Purine nucleotides and nucleosides are at the center of biologic reactions. In particular, adenosine triphosphate (ATP) is the fundamental energy currency of cellular activity and adenosine has been demonstrated to play essential roles in human physiology and pathophysiology. In this review, we examine the role of purinergic signaling in acute and chronic pulmonary inflammation, with emphasis on ATP and adenosine. ATP is released into extracellular space in response to cellular injury and necrosis. It is then metabolized to adenosine monophosphate (AMP) via ectonucleoside triphosphate diphosphohydrolase-1 (CD39) and further hydrolyzed to adenosine via ecto-5'-nucleotidase (CD73). Adenosine signals via one of four adenosine receptors to exert pro- or anti-inflammatory effects. Adenosine signaling is terminated by intracellular transport by concentrative or equilibrative nucleoside transporters (CNTs and ENTs), deamination to inosine by adenosine deaminase (ADA), or phosphorylation back into AMP via adenosine kinase (AK). Pulmonary inflammatory and hypoxic conditions lead to increased extracellular ATP, adenosine diphosphate (ADP) and adenosine levels, which translates to increased adenosine signaling. Adenosine signaling is central to the pulmonary injury response, leading to various effects on inflammation, repair and remodeling processes that are either tissue-protective or tissue destructive. In the acute setting, particularly through activation of adenosine 2A and 2B receptors, adenosine signaling serves an anti-inflammatory, tissue-protective role. However, excessive adenosine signaling in the chronic setting promotes pro-inflammatory, tissue destructive effects in chronic pulmonary inflammation.

Keywords: acute pulmonary inflammation; adenosine; chronic pulmonary inflammation; ectonucleotidase; nucleotides; purinergic signaling.

Figure 1

Release of ATP during inflammatory conditions. During conditions of inflammation such as during ischemia-reperfusion injury, hypoxia, inflammatory bowel disease, acute lung injury, and vascular thrombosis, ATP and ADP are released into the extracellular space via several mechanisms. ATP and ADP are released from apoptotic cells through pannexin-hemichannels and from connexin-hemichannels located on activated immune cells. Additionally, ATP and ADP can be released after cell lysis occurs in necrotic cells and though vesicular release by activated platelets. Once released, ATP and ADP act as potent signaling molecules by binding to and activating P2X receptors (ligand-gated ion channels) and P2Y receptors (G-protein-coupled receptors). Pictured are several examples of how ATP and ADP activate P2x and P2Y receptors during inflammatory states. P2Y6 and P2X7 receptors on vascular endothelium promote inflammation whereas activation of the P2X1, P2Y1, and P2Y12 receptors mediate platelet activation. In the setting of chronic lung diseases such as asthma, P2X7 and P2Y2 receptors promote activation of dendritic cells. Components of the figure were modified from SMART Servier Medical Art Library.

Figure 2

Extracellular adenosine signaling and its termination. Adenosine partakes in a number of signaling events during inflammatory conditions. ATP and ADP that is released serve as the source of extracellular adenosine. CD39 (Ecto-nucleotide triphosphate diphosphohydrolase 1, E-NTPDase1) dephosphorylates extracellular ATP and ADP on the cell surface to generate AMP, which is further dephosphorylated by CD73 (ecto-5'-nucleotidase, Ecto5'NTase) into adenosine. Once generated by enzymatic dephosphorylation, adenosine plays several important roles in regulating inflammation and immunity. Binding with A2AAR and A2BAR on immune cells promotes the inhibition of inflammation mediated by a number of innate immune cells including dendritic cells, monocytes, macrophages, and neutrophils. Adenosine interaction with A2AAR on T cells has demonstrated the suppression of effector functions and promotes the transition to T-regulatory status. On vascular endothelial cells, A2AAR and A2BAR activation decreases cellular inflammatory responses and promotes the integrity of barrier functions, respectively. Epithelial cells from several tissues including lung, gastrointestinal, myocardial, and renal contain A2BAR that, when activated by adenosine, are shown to play critical roles in decreasing inflammation and promoting barrier integrity during inflammation and injury. Several mechanisms are involved in regulating adenosine in order to allow for appropriate termination of signaling. Equilibrative nucleoside transporters (ENT)-1 and−2 deplete the extracellular accumulation of adenosine by transporting it into the nucleus. Adenosine kinase and adenosine deaminase are enzymes that both act to “inactivate” adenosine and inhibit its ability to bind to its receptors. Adenosine deaminase converts adenosine back to AMP and adenosine deaminase converts adenosine to inosine, which is an important step in the metabolism of nucleotides. Components of the figure were modified from SMART Servier Medical Art Library.

Figure 3

Adenosine signaling in acute pulmonary inflammation. ATP and ADP (not shown) are released into the extracellular space from various cells during pulmonary injury. Extracellular ATP is rapidly metabolized to AMP via CD39 and then to adenosine via CD73. In acute lung injury/ARDS, adenosine can signal through one of four adenosine receptors: the A1AR, A2AAR, A2BAR, and A3AR with A2AAR and A2BAR receptor expression being induced during hypoxic and inflammatory conditions on several cell types including alveolar epithelial cells, neutrophil, and macrophages. Studies have shown an anti-inflammatory, tissue protective role with attenuation of pulmonary inflammation and edema with A2AAR and A2BAR adenosine signaling.

Figure 4

Purinergic signaling in chronic pulmonary inflammation. In chronic lung injury, elevated extracellular ATP activates P2 receptors on neutrophils to release CXCL3/elastase and fibroblasts to release IL-6. Elevated extracellular adenosine signals through the A2BAR and A3AR on various pulmonary cell types to induce aberrant cell differentiation and production of pro-inflammatory, pro-fibrotic mediators, including IL-4, IL-6, IL-8, fibronectin, and TGF-beta. A2BAR engagement on mast cells induces production of IL-4, IL-8, IL-13, and VEGF. A2BAR activation promotes fibroblast and myofibroblast proliferation and differentiation. Signaling through A2BAR led to the production of IL-6 and osteopontin from macrophages, MCP-1, and IL-6 from bronchial smooth muscle cells, IL-6 release from fibroblasts, fibronectin expression in type 2 airway epithelial cells (AECs), and hyaluronan synthetases from macrophages and vascular smooth muscle cells. A2BAR signaling is responsible for the maintenance of vascular barrier integrity in endothelial cells.