Received for publication August 12, 2004.
Revised September 27, 2004.
Accepted for publication September 28, 2004.

Cellular GRK Levels Regulate Sensitivity of the _2B-Adrenergic Receptor to Undergo Agonist-induced Down-regulation

Abstract

Chronic co-activation of _2B- and ₂-ARs was recently reported to down-regulate the _2B-AR at a lower threshold EPI concentration compared to when the _2B-AR alone is activated. This is the result of a modest ₂-AR dependent up-regulation of GRK3. In the present study we determined that increasing GRK2 or GRK3 levels, independent of ₂-AR activation, decreases the EC₅₀ concentration for agonist-induced down-regulation of the _2B-AR using NG108 cells with or without over-expression (2-10 fold) of GRK2 or GRK3. In parental NG108 cells, the EC₅₀ concentration of EPI required for down-regulation of the _2B-AR is 30 µM. A 2-3-fold over-expression of GRK3 in NG108 cells however reduces the EC₅₀ to 0.2 µM (a 150-fold decrease) while a comparable over-expression of GRK2 reduces it to 1 µM (a 30-fold decrease). However, when GRK3 or GRK2 in NG108 cells are overexpressed 8-10-fold, the EC₅₀ concentration (0.02 µM EPI) for _2B-AR down-regulation is reduced 1000-fold. These data clearly suggest that a modest (2-3 fold) up-regulation of GRK3 is more effective at enhancing the sensitivity of _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 _2B-AR down-regulation when upregulated 8-10 fold. To our knowledge, this is the first report to systematically demonstrate that GRKs, particularly GRK3, play a pivotal role in modulating the agonist EC₅₀ concentration that down-regulates the _2B-AR and thus adds a new dimension to an already intricate signaling network.

Key words: GRK, alpha-adrenoceptor, down-regulation, epinephrine, norepinephrine, signaling

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