Adenosine and its nucleotides activate wild-type and R117H CFTR through an A2B receptor-coupled pathway

ATP and itsmetabolites stimulate Clsecretion in human epithelium in vitro and in vivo. The specificpurinergic receptor subtypes that govern these effects have beendifficult to separate, in part due to multiple parallel pathways forCl secretion in respiratoryand intestinal epithelia. In a simplified model using COS-7 cells, wedemonstrate acquisition of an ATP-, ADP-, AMP-, and adenosine(ADO)-regulated halide permeability specifically following expressionof wild-type (wt) cystic fibrosis transmembrane conductance regulator(CFTR). This halide permeability is blocked by theP₁ purinergic receptor antagonist8-phenyl theophylline, sensitive to the protein kinase A inhibitorH-89, and associated with a modest, dose-dependent increase in cellularcAMP concentration. Phorbol esters poorly activate halide permeabilitycompared with ADO, and ADO-stimulated efflux was not affected bytreatment with the protein kinase C inhibitor bisindolylmaleimide I. The A₂ ADO receptor (AR) agonists5'-N-ethylcarboxamide adenosineand ADO were strong activators, whereas theA₁ AR agonistR-phenylisopropyladenosine failed toactivate halide permeability. Metabolic conversion of ADO nucleotidesby surface ecto-5'-nucleotidase to more active (lessphosphorylated) forms contributes to anion transport activation inthese cells. Immunoprecipitation withanti-A_2B AR antibody identified a31-kDa protein in both COS-7 and human bronchial epithelial cells.Together, these findings indicate that ADO and its nucleotides arecapable of activating wtCFTR-dependent halide permeability throughA_2B AR and that this AR subtype ispresent in human bronchial epithelium. We also present data showingthat this pathway can activate clinically significant mutant CFTRmolecules such as R117H.