Adenosine A1 receptors heterodimerize with β1- and β2-adrenergic receptors creating novel receptor complexes with altered G protein coupling and signaling

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 beta-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(1)ARs heterodimerize with beta(1)- and/or beta(2)-adrenergic receptors (beta R-1 and beta R-2), and whether such interactions lead to functional consequences. Co-immunoprecipitation and co-localization studies with differentially epitope-tagged A(1), beta(1), and beta(2) receptors transiently co-expressed in HEK-293 cells indicate that A(1)AR forms constitutive heterodimers with both beta R-1 and beta R-2. This heterodimerization significantly influenced orthosteric ligand binding affinity of both beta R-1 and beta R-2 without altering ligand binding properties of A(1)AR. Receptor-mediated ERK1/2 phosphorylation significantly increased in cells expressing A(1)AR/beta R-1 and A(1)AR/beta R-2 heteromers. beta-Receptor-mediated cAMP production was not altered in A(1)AR/beta R-1 expressing cells, but was significantly reduced in the A(1)AR/beta R-2 cells. The inhibitory effect of the A(1)AR on cAMP production was abrogated in both A(1)AR/beta R-1 and A(1)AR/beta R-2 expressing cells in response to the A(1)AR agonist CCPA. Co-immunoprecipitation studies conducted with human heart tissue lysates indicate that endogenous A(1)AR, beta R-1, and beta R-2 also form heterodimers. Taken together, our data suggest that heterodimerization between A(1) and beta 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. (C) 2012 Published by Elsevier Inc.