Ligand-induced phosphorylation, clustering, and desensitization of A1 adenosine receptors

Through immunocytochemistry with the use of antibodies against A1 adenosine receptors (A1Rs) and confocal microscopy, we show that stimulation of A1Rs by the agonist (R)-phenylisopropyladenosine [(R)-PIA] caused a rapid (5-15 min) aggregation (clustering) of receptor molecules on the surface of DDT1MF-2 cells. Internalization of the chronically stimulated receptor was slower and occurred concomitantly, with a time-dependent decrease (50%) in the number of cell surface [3H](R)-PIA binding sites. The reduction of binding sites was due partly (30%) to internalization and partly (20%) to the presence of desensitized cell surface receptor molecules that were unable to bind the ligand. Chronic exposure of DDT1MF-2 cells to 50 nM (R)-PIA produced functional desensitization, as deduced from second messenger production assays. Quantification of the content of A1Rs by immunoblotting and flow cytometry in cells pretreated with 50 nM (R)-PIA indicates a time-dependent slow down-regulation of the receptor. Receptor clustering and agonist-induced receptor phosphorylation, which occurred in serine and tyrosine, were simultaneous. The finding that activators of protein kinase A or C were able to induce functional desensitization of A1Rs, phosphorylate A1Rs in serine and threonine, and trigger clustering of the receptor suggests that phosphorylation of A1Rs in serine/threonine is involved in desensitization-related events.