title: Beta-arrestin-mediated activation of MAPK by inverse agonists reveals distinct active conformations for G protein-coupled receptors.
abstract: It is becoming increasingly clear that signaling via G protein-coupled receptors is a diverse phenomenon involving receptor interaction with a variety of signaling partners. Despite this diversity, receptor ligands are commonly classified only according to their ability to modify G protein-dependent signaling. Here we show that beta2AR ligands like ICI118551 and propranolol, which are inverse agonists for Gs-stimulated adenylyl cyclase, induce partial agonist responses for the mitogen-activated protein kinases extracellular signal-regulated kinase (ERK) 1/2 thus behaving as dual efficacy ligands. ERK1/2 activation by dual efficacy ligands was not affected by ADP-ribosylation of Galphai and could be observed in S49-cyc- cells lacking Galphas indicating that, unlike the conventional agonist isoproterenol, these drugs induce ERK1/2 activation in a Gs/i-independent manner. In contrast, this activation was inhibited by a dominant negative mutant of beta-arrestin and was abolished in mouse embryonic fibroblasts lacking beta-arrestin 1 and 2. The role of beta-arrestin was further confirmed by showing that transfection of beta-arrestin 2 in these knockout cells restored ICI118551 promoted ERK1/2 activation. ICI118551 and propranolol also promoted beta-arrestin recruitment to the receptor. Taken together, these observations suggest that beta-arrestin recruitment is not an exclusive property of agonists, and that ligands classically classified as inverse agonists rely exclusively on beta-arrestin for their positive signaling activity. This phenomenon is not unique to beta2-adrenergic ligands because SR121463B, an inverse agonist on the V2 vasopressin receptor-stimulated adenylyl cyclase, recruited beta-arrestin and stimulated ERK1/2. These results point to a multistate model of receptor activation in which ligand-specific conformations are capable of differentially activating distinct signaling partners.
mesh_terms: D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000818:Animals; D019390:Arrestins / Q000378:metabolism*; D002460:Cell Line; D019204:GTP-Binding Proteins / Q000378:metabolism*; D006801:Humans; D051379:Mice; D008856:Microscopy, Fluorescence; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D011412:Propanolamines / Q000494:pharmacology; D011487:Protein Conformation; D011956:Receptors, Cell Surface / Q000737:chemistry / Q000378:metabolism*; D000071559:beta-Arrestin 1; D000071560:beta-Arrestin 2; D000071557:beta-Arrestins
chemical_list: C000605253:ARRB1 protein, human; C000605254:ARRB2 protein, human; D000318:Adrenergic beta-Agonists; C000605255:Arrb1 protein, mouse; C000605258:Arrb2 protein, mouse; D019390:Arrestins; D011412:Propanolamines; D011956:Receptors, Cell Surface; D000071559:beta-Arrestin 1; D000071560:beta-Arrestin 2; D000071557:beta-Arrestins; C026777:ICI 118551; D020928:Mitogen-Activated Protein Kinases; D019204:GTP-Binding Proteins