Differential signaling of the endogenous agonists at the beta2-adrenergic receptor.
The concept of "functional selectivity" or "biased signaling" suggests that a ligand can have distinct efficacies with regard to different signaling pathways. We have investigated the question of whether biased signaling may be related to distinct agonist-induced conformational changes in receptors using the β(2)-adrenergic receptor (β(2)AR) and its two endogenous ligands epinephrine and norepinephrine as a model system. Agonist-induced conformational changes were determined in a fluorescently tagged β(2)AR FRET sensor. In this β(2)AR sensor, norepinephrine caused signals that amounted to only ≈50% of those induced by epinephrine and the standard "full" agonist isoproterenol. Furthermore, norepinephrine-induced changes in the β(2)AR FRET sensor were slower than those induced by epinephrine (rate constants, 47 versus 128 ms). A similar partial β(2)AR activation signal was revealed for the synthetic agonists fenoterol and terbutaline. However, norepinephrine was almost as efficient as epinephrine (and isoproterenol) in causing activation of G(s) and adenylyl cyclase. In contrast, fenoterol was quite efficient in triggering β-arrestin2 recruitment to the cell surface and its interaction with β(2)AR, as well as internalization of the receptors, whereas norepinephrine caused partial and slow changes in these assays. We conclude that partial agonism of norepinephrine at the β(2)AR is related to the induction of a different active conformation and that this conformation is efficient in signaling to G(s) and less efficient in signaling to β-arrestin2. These observations extend the concept of biased signaling to the endogenous agonists of the β(2)AR and link it to distinct conformational changes in the receptor.
Reiner
Susanne
S
Institute of Pharmacology and Toxicology, Versbacher Strasse 9, D-97078 Würzburg, Germany.
Ambrosio
Manuela
M
Hoffmann
Carsten
C
Lohse
Martin J
MJ
D000262:Adenylyl Cyclases / Q000378:metabolism; D000322:Adrenergic Agonists / Q000494:pharmacology*; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000818:Animals; D016466:CHO Cells; D002462:Cell Membrane / Q000187:drug effects* / Q000378:metabolism; D006224:Cricetinae; D003412:Cricetulus; D004837:Epinephrine / Q000494:pharmacology; D005280:Fenoterol / Q000494:pharmacology; D057809:HEK293 Cells; D006801:Humans; D007545:Isoproterenol / Q000494:pharmacology; D008164:Luminescent Proteins / Q000235:genetics / Q000378:metabolism; D018613:Microscopy, Confocal; D008856:Microscopy, Fluorescence; D009638:Norepinephrine / Q000494:pharmacology; D010766:Phosphorylation / Q000187:drug effects; D011487:Protein Conformation / Q000187:drug effects; D018343:Receptors, Adrenergic, beta-2 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects*; D014162:Transfection
D000322:Adrenergic Agonists; D000316:Adrenergic alpha-Agonists; D000318:Adrenergic beta-Agonists; D008164:Luminescent Proteins; D018343:Receptors, Adrenergic, beta-2; D011993:Recombinant Fusion Proteins; D005280:Fenoterol; D000262:Adenylyl Cyclases; D007545:Isoproterenol; D009638:Norepinephrine; D004837:Epinephrine