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

An agonist-occupied beta(2)-adrenergic receptor (beta(2)-AR) recruits G protein receptor kinase-2 (GRK2) which is recruited to the membrane. Thus, the physical proximity of activated beta(2)-AR and PI-3K allows the activation of the latter. In contrast, it has been observed that the beta(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 beta-AR subtypes. Using transiently transfected HEK 293 cells expressing either beta(1)- or beta(2)-AR, we also observed that in presence of an agonist, beta(2)-AR, but not beta(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, beta(1) x beta(2)-AR can activate the PI-3K/Akt pathway whereas beta(2) x beta(1)-AR cannot. Pretreatment with pertussis toxin abolished the activation of PI-3K by beta(2)- or beta(1) x beta(2)-AR stimulation. Ligand-mediated internalization of the beta(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 beta(2)-AR activates PI-3K via a pertussis toxin-sensitive mechanism.