title: Simultaneous alpha2B- and beta2-adrenoceptor activation sensitizes the alpha2B-adrenoceptor for agonist-induced down-regulation.
abstract: We recently reported that alpha(2A)-adrenoceptor (AR) desensitization and down-regulation occurs after 24-h treatment with epinephrine (EPI) (0.3 microM) in BE(2)-C cells that express both alpha(2)- and beta(2)-ARs. The same concentration of norepinephrine (NE) has no effect. The effect of EPI is prevented by beta(2)-AR blockade and is associated with an increase in G protein-coupled receptor kinase 3 (GRK3) expression. Because differences in agonist-induced down-regulation of the alpha(2A)-versus alpha(2B)-ARs have been reported, the present study examines the effects of simultaneous activation of alpha(2B)- and beta(2)-ARs on alpha(2B)-AR number and signaling. We studied NG108 cells that naturally express alpha(2B)-ARs, and BN17 cells, NG108 cells transfected to express the human beta(2)-AR. In NG108 cells, alpha(2B)-AR desensitization and down-regulation require treatment with 20 microM EPI or NE; GRK expression was not changed. In BN17 cells expressing beta(2)-ARs, the threshold EPI concentration for alpha(2B)-AR desensitization and down-regulation was reduced to 0.3 microM; 10 microM NE was required for the same effect. Furthermore, 24-h EPI or NE treatments that produced desensitization also resulted in a selective 2-fold up-regulation of GRK3; GRK2 was unchanged. The beta-AR antagonist alprenolol (1 microM) and GRK3 antisense (but not sense) DNA blocked 0.3 microM EPI- and 10 microM NE-induced desensitization and down-regulation of the alpha(2B)-AR as well as GRK3 up-regulation. In conclusion, simultaneous activation of alpha(2B)- and beta(2)-ARs results in a 67-fold decrease in the threshold concentration of EPI required for alpha(2B)-AR down-regulation. This lower threshold for down-regulation is associated with alpha(2B)- and beta(2)-AR dependent up-regulation of GRK3 expression.
authors.affiliation: Department of Pharmacological and Pharmaceutical Sciences, College of Pharmacy, University of Houston, Houston, TX 77204-5037, USA.
authors.forename: Aarti N
authors.initials: AN
authors.lastname: Desai
authors.forename: Kelly M
authors.initials: KM
authors.lastname: Standifer
authors.forename: Douglas C
authors.initials: DC
authors.lastname: Eikenburg
mesh_terms: D000322:Adrenergic Agonists / Q000494:pharmacology; D000818:Animals; D015536:Down-Regulation; D054770:G-Protein-Coupled Receptor Kinase 3; D006801:Humans; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D018341:Receptors, Adrenergic, alpha-2 / Q000378:metabolism*; D018343:Receptors, Adrenergic, beta-2 / Q000378:metabolism*; D014407:Tumor Cells, Cultured
chemical_list: C547771:ADRA2B protein, human; D000322:Adrenergic Agonists; D018341:Receptors, Adrenergic, alpha-2; D018343:Receptors, Adrenergic, beta-2; D017346:Protein Serine-Threonine Kinases; D054770:G-Protein-Coupled Receptor Kinase 3; C517848:GRK3 protein, human