Determinants Present in the Receptor Carboxy Tail Are Responsible for Differences in Subtype-Specific Coupling ofβ-Adrenergic Receptors to Phosphoinositide 3-Kinase

An agonist-occupied β 2 -adrenergic receptor ( β 2 -AR) recruits G protein receptor kinase-2 (GRK2) which is recruited to the membrane. Thus, the physical proximity of activated β 2 -AR and PI-3K allows the activation of the latter. In contrast, it has been observed that the β 1 -AR is unable to activate the PI-3K/Akt pathway. We hypothesized that the difference might be due to molecular determinants present in the carboxy termini of the two β -AR subtypes. Using transiently transfected HEK 293 cells expressing either β 1 - or β 2 -AR, we also observed that in presence of an agonist, β 2 -AR, but not β 1 -AR, is able to activate the PI-3K/Akt pathway. Switching the seventh transmembrane domain and the carboxy tail between the two receptors reverses this phenotype; that is, β 1 × β 2 -AR can activate the PI-3K/Akt pathway whereas β 2 × β 1 -AR cannot. Pretreatment with pertussis toxin abolished the activation of PI-3K by β 2 - or β 1 × β 2 -AR stimulation. Ligand-mediated internalization of the β 2 -AR induced by a 15-minute stimulation with agonist was abolished in the presence of a dominant negative of PI-3K or following pertussis toxin pretreatment. These results indicate that the subtype-specific differences in the coupling to PI-3K/Akt pathway are due to molecular determinants present in the carboxy tail of the receptor and further that β 2 -AR activates PI-3K via a pertussis toxin-sensitive mechanism.