title: Agonist-dependent phosphorylation and desensitization of the rat A3 adenosine receptor. Evidence for a G-protein-coupled receptor kinase-mediated mechanism.
abstract: A3 adenosine receptor (A3AR) activation contributes to both the cardioprotective and antihypertensive effects of adenosine. To date, no studies have examined the mechanisms by which this receptor undergoes rapid homologous desensitization. Therefore, a functional hemagglutinin epitope-tagged A3AR has been stably expressed in Chinese hamster ovary cells, and its regulation by the AR agonist 5'-N-ethylcarboxamidoadenosine (NECA) has been studied. Cellular exposure to NECA induces rapid (t1/2 = approximately 1 min) A3AR phosphorylation on serine and threonine residues. This is associated with a functional desensitization and a 30-40% reduction in the number of high affinity agonist binding sites as determined by radioligand binding assays. Activation of second messenger-regulated kinases could not mimic the effect of NECA, suggesting a role for G-protein-coupled receptor kinases (GRKs). In vitro phosphorylation assays demonstrate that phosphorylation of agonist-occupied A3ARs is enhanced by GRK2 and that cellular pretreatment with NECA dramatically inhibits subsequent GRK2-mediated phosphorylation in vitro. Therefore, the A3AR is phosphorylated in situ by a kinase similar or identical to GRK2, and this may be involved in rapid functional desensitization of the A3AR.
mesh_terms: D000241:Adenosine / Q000031:analogs & derivatives / Q000494:pharmacology; D019830:Adenosine-5'-(N-ethylcarboxamide); D000595:Amino Acid Sequence; D000818:Animals; D016466:CHO Cells; D006224:Cricetinae; D054771:G-Protein-Coupled Receptor Kinase 4; D019204:GTP-Binding Proteins / Q000502:physiology*; D006031:Glycosylation; D008969:Molecular Sequence Data; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases; D058906:Purinergic P1 Receptor Agonists; D051381:Rats; D020794:Receptor Protein-Tyrosine Kinases / Q000502:physiology*; D018047:Receptors, Purinergic P1 / Q000378:metabolism*
chemical_list: D058906:Purinergic P1 Receptor Agonists; D018047:Receptors, Purinergic P1; D019830:Adenosine-5'-(N-ethylcarboxamide); D020794:Receptor Protein-Tyrosine Kinases; D017346:Protein Serine-Threonine Kinases; D054771:G-Protein-Coupled Receptor Kinase 4; C517854:Grk4 protein, rat; D019204:GTP-Binding Proteins; D000241:Adenosine