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 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.