title: Alpha2A- and alpha2C-adrenergic receptors form homo- and heterodimers: the heterodimeric state impairs agonist-promoted GRK phosphorylation and beta-arrestin recruitment.
abstract: 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.
authors.affiliation: Department of Medicine, University of Cincinnati College of Medicine, Cincinnati, Ohio 45267, USA.
authors.forename: Kersten M
authors.initials: KM
authors.lastname: Small
authors.forename: Mary Rose
authors.initials: MR
authors.lastname: Schwarb
authors.forename: Clare
authors.initials: C
authors.lastname: Glinka
authors.forename: Cheryl T
authors.initials: CT
authors.lastname: Theiss
authors.forename: Kari M
authors.initials: KM
authors.lastname: Brown
authors.forename: Carrie A
authors.initials: CA
authors.lastname: Seman
authors.forename: Stephen B
authors.initials: SB
authors.lastname: Liggett
mesh_terms: D019390:Arrestins / Q000378:metabolism*; D015153:Blotting, Western; D002478:Cells, Cultured; D019281:Dimerization; D054769:G-Protein-Coupled Receptor Kinase 2; D006801:Humans; D047468:Immunoprecipitation; D007668:Kidney; D018613:Microscopy, Confocal; D010766:Phosphorylation; D018341:Receptors, Adrenergic, alpha-2 / Q000378:metabolism*; D015398:Signal Transduction; D014162:Transfection; D051552:beta-Adrenergic Receptor Kinases / Q000378:metabolism*; D000071557:beta-Arrestins
chemical_list: C547768:ADRA2A protein, human; C547774:ADRA2C protein, human; D019390:Arrestins; D018341:Receptors, Adrenergic, alpha-2; D000071557:beta-Arrestins; C496567:GRK2 protein, human; D051552:beta-Adrenergic Receptor Kinases; D054769:G-Protein-Coupled Receptor Kinase 2