Functional Receptor Coupling to GiIs A Mechanism of Agonist-Promoted Desensitization of the β2-Adrenergic Receptor

Publisher Website
Google Scholar
Abstract
The β2-adrenergic receptor (β2AR) couples to Gs, activating adenylyl cyclase (AC) and increasing cAMP. Such signaling undergoes desensitization with continued agonist exposure. β2AR also couple to Gi after receptor phosphorylation by the cAMP dependent protein kinase A, but the efficiency of such coupling is not known. Given the PKA dependence of β2AR-Gi coupling, we explored whether this may be a mechanism of agonist-promoted desensitization. HEK293 cells were transfected to express β2AR or β2AR and Giα2, and then treated with vehicle or the agonist isoproterenol to evoke agonist-promoted β2AR desensitization. Membrane AC activities showed that Giα2 overexpression decreased basal levels, but the fold-stimulation of the AC over basal by agonist was not altered. However, with treatment of the cells with isoproterenol prior to membrane preparation, a marked decrease in agonist-stimulated AC was observed with the cells overexpressing Giα2. in the absence of such overexpression, β2AR desensitization was 23 ± 7%, while with 5-fold Giα2 overexpression desensitization was 58 ± 5% (pi on desensitization was receptor-specific, in that forskolin responses were not altered by Giα2 overexpression. Thus, acquired β2AR coupling to Gi is an important mechanism of agonist-promoted desensitization, and pathologic conditions that increase Gi levels contribute to β2AR dysfunction.