Cellular G Protein-Coupled Receptor Kinase Levels Regulate Sensitivity of the alpha_(2B)-Adrenergic Receptor to Undergo Agonist-Induced Down-Regulation

摘要

Chronic coactivation of alpha_(2B)-and beta_2-adrenoceptors(AR)was recently reported to down-regulate the alpha_(2B)-AR at a lower threshold epinephrine(EPI)concentration compared with the activation of alpha_(2B)-AR alone.This is the result of a modest beta_2-AR-dependent up-regulation of G protein-coupled receptor kinase 3(GRK3).In the present study,we determined that increasing GRK2 or GRK3 levels,independent of beta_2-AR activation,decreases the EC_50 concentration for agonist-induced down-regulation of the alpha_(2B)-AR using NG108 cells with or without overexpression(2-to 10-fold)of GRK2 or GRK3.In parental NG108 cells,the EC_50 concentration of EPI required for down-regulation of the alpha_(2B)-AR is 30 muM.A 2-to 3-fold overexpression of GRK3 in NG108 cells,however,reduces the EC_50 to 0.2 muM(a 150-fold decrease),whereas a comparable overexpression of GRK2 reduces it to 1 muM(a 30-fold decrease).However,when GRK3 or GRK2 in NG108 cells are overexpressed 8-to 10-fold,the EC_50 concentration(0.02 muM EPI)for alpha_(2B)-AR down-regulation is reduced 1000-fold.These data clearly suggest that a modest(2-to 3-fold)up-regulation of GRK3 is more effective at enhancing the sensitivity of alpha_(2B)-AR to down-regulation after exposure to EPI than a modest up-regulation of GRK2,but that both GRK2 and GRK3 are equally effective at inducing alpha_(2B)-AR down-regulation when up-regulated 8-to 10-fold.To our knowledge,this is the first report to systematically demonstrate that GRKs,particularly GRK3,play a pivotal role in modulating the agonist EC_50 concentration that down-regulates the alpha_(2B)-AR and thus adds a new dimension to an already intricate signaling network.