| As the layer of tissue exposed directly to the microbe rich air, the airway epithelium is largely responsible for preventing infection. It is believed that the epithelium achieves this by mediating mucociliary clearance, secreting antibiotic factors, and regulating airway surface liquid pH; all of which are dependent on salt and therefore ion transport. Thus, when ion transport becomes compromised, infection results. The best example of this is Cystic Fibrosis (CF), a lethal disease characterized by progressive lung infection, caused by mutations in the CFTR Cl− channel.; This study has focused on the regulation of airway epithelial ion transport by the endogenous autocrine/paracrine mediator—adenosine, in three model cell systems: the A549 cell line, the Calu-3 cell line, and primary cultures of normal human bronchial epithelium (NHBE). The regulatory actions of adenosine are mediated by four distinct membrane receptors. We have used complementary molecular and functional techniques to show that A1 receptors are responsible for ion channel regulation in A549 cells, while A2A and A 2B receptors are involved in Calu-3 and NHBE cells.; Further study revealed that the concentration of adenosine in the vicinity of these receptors is controlled by at least three factors: equilibrative nucleoside transporters (ENTs), adenosine kinase, and 5 ′-nucleotidase. ENT1, mediates the rate-limiting step in adenosine signal termination, such that inhibition of this transporter stimulates adenosine-dependent ion transport. Adenosine kinase and 5′-nucleotidase were found to be important in the metabolism and production of adenosine, respectively.; The ion channel targets of adenosine signaling were also investigated. In A549 cells, adenosine regulates the intermediate conductance Ca 2+-dependent K+ channel. In contrast, in Calu-3 and NHBE cells, cAMP-dependent channels are targeted, in particular CFTR and an Outwardly Rectifying Cl− Channel. The basolateral localization of the latter was further characterized, and a novel role for this channel was proposed in anion secretion that may explain how preferential HCO 3− secretion occurs.; Thus, we have characterized the factors involved in adenosine-dependent ion transport. Furthermore, we have proposed pharmacological agents that target these factors, some of which are already approved for clinical use, which may be of benefit in the treatment of CF. |
