α2A- and α2C-Adrenergic Receptors Form Homo- and Heterodimers: The Heterodimeric State Impairs Agonist-Promoted GRK Phosphorylation and β-Arrestin Recruitment

Dimerization of seven transmembrane-spanning receptors diversifies their pharmacologic and physiologic properties. The alpha(2)-adrenergic receptor (alpha(2)AR) subtypes A and C are both expressed on presynaptic nerves and act to inhibit norepinephrine release via negative feedback. However, in vivo and in vitro studies examining the roles of the two individual alpha(2A)- and alpha(2C)AR subtypes are not readily reconciled. We tested the hypothesis that the receptors form homo- and heterodimers and that the alpha(2A)-alpha(2C) heterodimer has unique properties. SDS-PAGE of epitope-tagged receptors revealed potential oligomers including dimers. BRET of live HEK-293 cells transfected with the subtypes fused to Rluc or YFP revealed that both subtypes form dimers and the heterodimer. A lower BRET(50) for the alpha(2A)-alpha(2C) heterodimer (0.79 +/- 0.20) compared to that of the alpha(2A) or alpha(2C) homodimer (2.331 +/- 0.44 or 3.67 +/- 0.69, respectively) suggests that when both subtypes are expressed, there is a greater likelihood that the two receptors will form the heterodimer than homodimers. Co-immunoprecipitation studies confirmed homo- and heterodimer formation. The presence of the alpha(2C)AR within the heterodimer resulted in a marked reduction in the level of GRK2-mediated alpha(2A)AR phosphorylation, which was accompanied by a qualitative attenuation of beta-arrestin recruitment. Signaling of the alpha(2A)-alpha(2C) heterodimer to the beta-arrestin-dependent activation of Akt was decreased compared to that of the alpha(2A)AR homodimer, while p44/p42 MAP kinase activation was unaffected. Thus, the alpha(2C)AR alters alpha(2A)AR signaling by forming oligomers, and these complexes, which appear to be preferred over the homodimers, should be considered a functional signaling unit in cells in which both subtypes are expressed.