β-Arrestin-Dependent Spontaneous α1a-Adrenoceptor Endocytosis Causes Intracellular Transportation of α-Blockers via Recycling Compartments

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Abstract
The antagonist ligand BODIPY-FL-prazosin (QAPB) fluoresces when bound to bovine α1a-adrenoceptors (ARs). Data indicate that the receptor-ligand complex is spontaneously internalized by β-arrestin-dependent endocytosis. Internalization of the ligand did not occur in β-arrestin-deficient cells; was blocked or reversed by another α1 ligand, phentolamine, indicating it to reflect binding to the orthosteric recognition site; and was prevented by blocking clathrin-mediated endocytosis. The ligand showed rapid, diffuse, low-intensity, surface binding, superseded by punctate intracellular binding that developed to equilibrium in 50 to 60 min and was reversible on ligand removal, indicating a dynamic equilibrium. In cells expressing a human α1a-AR-enhanced green fluorescent protein (EGFP) 2 fusion protein, BODIPY-R-558/568-prazosin (RQAPB) colocalized with the fusion, indicating that the ligand gained access to all compartments containing the receptor, and, conversely, that the receptor has affinity for the ligand at all of these sites. The distribution of QAPB binding sites was similar for receptors with or without EGFP2, validating the fusion protein as an indicator of receptor location. The ligand partially colocalized with β-arrestin in recycling and late endosomes, indicating receptor transit without destruction. Organelles containing receptors showed considerable movement consistent with a transportation function. This was absent in β-arrestin-deficient cells, indicating that both constitutive receptor internalization and subsequent intracellular transportation are β-arrestin-dependent. Calculations of relative receptor number indicate that at steady state, less than 30% of receptors reside on the cell surface and that recycling is rapid. We conclude that α1a-ARs recycle rapidly by an agonist-independent, constitutive, β-arrestin-dependent process and that this can transport “α-blockers” into cells carrying these receptors.