Involvement of G protein-coupled receptor kinase (GRK) 3 and GRK2 in down-regulation of the α2B-adrenoceptor

Increasing the cellular levels of G protein-coupled receptor kinase (GRK) 2 or GRK3 renders the alpha(2B)-adrenoceptor (AR) more sensitive to agonist-induced down-regulation (J Pharmacol Exp Ther 312: 767 - 773, 2005). However, an absolute requirement of GRK3 and GRK2 for alpha(2B)-AR down-regulation is controversial. In this study, using NG108 cells (endogenous alpha(2B)-AR), we provide strong evidence for a critical role of both GRK3 and GRK2 in down-regulation of the alpha(2B)-AR. Pretreatment of NG108 cells with 20 mu M epinephrine (EPI) begins down-regulating the alpha(2B)-AR by 2 h. The translocation of GRK3 and GRK2 to the membrane peaks at 30 min, decreasing by 1 h. Although these results may implicate GRK3 and GRK2 in alpha(2B)-AR down-regulation, significant receptor down-regulation is not observed until 2 h, after GRK3 and GRK2 translocation has peaked and is declining. To more directly establish a role for GRK3 and GRK2 in alpha(2B)-AR down-regulation, NG108 cells were transfected to express GRK3ct, which binds to liberated G(beta gamma) subunits, preventing GRK3 and GRK2 translocation to the membrane. Overexpression of GRK3ct prevented not only the translocation of GRK3 and GRK2 but also the down-regulation of the alpha(2B)-AR caused by 24-h pretreatment with 20 mu M EPI. Taken together, these data provide direct evidence for a role of GRK3 and GRK2 in the down- regulation of the alpha(2B)-AR and contribute significantly to the increasing evidence in the literature for a pivotal role of GRKs in modulating the agonistinduced down- regulation of the alpha(2)-AR.