Adenosine A1 receptors heterodimerize with β_1- and β_2-adrenergic receptors creating novel receptor complexes with altered G protein coupling and signaling

摘要

G protein coupled receptors play crucial roles in mediating cellular responses to external stimuli, and increasing evidence suggests that they function as multiple units comprising homo/heterodimers and hetero-oligomers. Adenosine and β-adrenergic receptors are co-expressed in numerous tissues and mediate important cellular responses to the autocoid adenosine and sympathetic stimulation, respectively. The present study was undertaken to examine whether adenosine A_1ARs heterodimerize with β_1- and/or β_2-adrenergic receptors (β_1R and β_2R), and whether such interactions lead to functional consequences. Co-immunoprecipitation and co-localization studies with differentially epitope-tagged A_1, β_1, and β_2 receptors transiently co-expressed in HEK-293 cells indicate that A_1AR forms constitutive heterodimers with both β_1R and β_2R. This heterodimerization significantly influenced orthosteric ligand binding affinity of both β_1R and β_2R without altering ligand binding properties of A_1AR. Receptor-mediated ERK1/2 phosphorylation significantly increased in cells expressing A_1AR/β_1R and A_1AR/β_2R heteromers. β-Receptor-mediated cAMP production was not altered in A_1AR/β_1R expressing cells, but was significantly reduced in the A_1AR/β_2R cells. The inhibitory effect of the A_1AR on cAMP production was abrogated in both A_1AR/β_1R and A_1AR/β_2R expressing cells in response to the A_1AR agonist CCPA. Co-immunoprecipitation studies conducted with human heart tissue lysates indicate that endogenous A_1AR, β_1R, and β_2R also form heterodimers. Taken together, our data suggest that heterodimerization between A_1 and β receptors leads to altered receptor pharmacology, functional coupling, and intracellular signaling pathways. Unique and differential receptor cross-talk between these two important receptor families may offer the opportunity to fine-tune crucial signaling responses and development of more specific therapeutic interventions.