G Protein-coupled Receptor Kinase and β-Arrestin-mediated Desensitization of the Angiotensin II Type 1A Receptor Elucidated by Diacylglycerol Dynamics

Receptor desensitization progressively limits responsiveness of cells to chronically applied stimuli. Desensitization in the continuous presence of agonist has been difficult to study with available assay methods. Here, we used a fluorescence resonance energy transfer-based live cell assay for the second messenger diacylglycerol to measure desensitization of a model seven-transmembrane receptor, the Gq-coupled angiotensin II type 1(A) receptor, expressed in human embryonic kidney 293 cells. In response to angiotensin II, we observed a transient diacylglycerol response reflecting activation and complete desensitization of the receptor within 2-5 min. By utilizing a variety of approaches including graded tetracycline-inducible receptor expression, mutated receptors, and overexpression or short interfering RNA-mediated silencing of putative components of the cellular desensitization machinery, we conclude that the rate and extent of receptor desensitization are critically determined by the following: receptor concentration in the plasma membrane; the presence of phosphorylation sites on the carboxyl terminus of the receptor; kinase activity of G protein-coupled receptor kinase 2, but not of G protein-coupled receptor kinases 3, 5, or 6; and stoichiometric expression of beta-arrestin. The findings introduce the use of the biosensor diacylglycerol reporter as a powerful means for studying Gq-coupled receptor desensitization and document that, at the levels of receptor overexpression commonly used in such studies, the properties of the desensitization process are markedly perturbed and do not reflect normal cellular physiology.