Regulation of alpha-1B adrenergic receptor localization, trafficking, function, and stability
Introduction
The alpha-1B adrenergic receptors (α1BARs), like the prototypical beta-2 adrenergic receptors (β2ARs) and most other G protein-coupled receptors (GPCRs), are not only activated by agonist binding but also undergo a complex series of adaptive changes generally referred to as desensitization. These include rapid decreases in receptor function (uncoupling), changes in subcellular localization (internalization), as well as slower decreases in the level of expression of receptors detectable by radioligand binding (down-regulation). This article highlights recent studies from our laboratory and others focused on the localization and trafficking of the α1BARs and on changes in receptor stability that are likely to be involved in regulating receptor expression. Our understanding of regulation for the α1BAR subtype provides a model for future studies of the differential regulation of the other α1AR subtypes and may lead to identification of new molecular targets for therapeutic intervention in a variety of disease states.
Section snippets
α1BAR function and agonist-induced uncoupling
The α1BARs are one of three subtypes of α1ARs, the other two being α1AARs and α1DARs; these together constitute one of three families of adrenergic receptors, the other two being the α2ARs and the βARs. The α1BARs, as well as the α1AARs and α1DARs, couple to the Gq/11 family of G proteins, and agonist binding leads to activation of phospholipase C (PLC) and stimulation of phosphoinositide (PI) hydrolysis, with increases in intracellular free calcium and activation of protein kinase C (PKC) as
α1BAR internalization: sequestration, endocytosis, and the role of PKC
In unstimulated cells, α1BARs are localized primarily in the plasma membrane, but agonist exposure leads to rapid changes in cell surface accessibility and membrane localization. Localization and trafficking of α1BARs can be detected by their accessibility to hydrophilic ligands that do not readily cross cell membranes, by changes in their distribution on sucrose density gradient centrifugation, and more recently by confocal fluorescence microscopy (CFM) localization with either receptor
α1BAR C-tail domains involved in internalization and down-regulation
Several recent studies clearly demonstrate critical roles of specific C-tail domains in all aspects of α1BAR desensitization. Truncation of the hamster α1BAR after Arg-368 (T368) did not greatly alter its binding or functional properties, but receptor phosphorylation and desensitization were largely eliminated (Lattion et al., 1994). Further studies identified Ser-404, -408, and -410 as the major sites for GRK phosphorylation and desensitization, and Ser-394 and -400 as the sites for PKC
α1BAR signaling to extracellular signal-regulated kinases (ERKs)
Similar to many other GPCRs, α1BAR activation can lead to phosphorylation and activation of multiple members of the mitogen-activated protein kinase (MAPK) family, critical mediators of transcription factor regulation controlling cell growth, apoptosis, and differentiation Auer et al., 1998, Zhong and Minneman, 1999a, Keffel et al., 2000, Chalothorn et al., 2002. Because of multiple studies suggesting a requirement of receptor endocytosis for GPCR activation of the extracellular
α1BAR localization in caveolae or membrane lipid rafts
The ability of α1BARs to sequester within the plasma membrane without endocytosing into intracellular vesicles, together with their apparent ability to down-regulate their binding capacity without being internalized and delivered to classical degradative compartments, led us to explore the possible localization or trafficking of α1BARs into alternate plasma membrane compartments. Caveolae or membrane lipid rafts are specialized membrane domains that are known to regulate receptor signaling
Regulation of α1BAR structural stability and α1BAR up- and down-regulation
Studies of the down-regulation properties of the G protein coupling-defective IC3-mutated Δ12 and Δ5 α1BARs revealed the unexpected result that agonist exposure led to an up-regulation of [3H]prazosin-binding activity rather than down-regulation (Wang et al., 2002). The mechanism(s) involved in this up-regulation have been further investigated (Prinster et al., 2003). Multiple studies eliminated various transcriptional up-regulation mechanisms, and the fact that up-regulation was not blocked by
Concluding remarks
The studies summarized here delineate many of the important mechanisms regulating localization, trafficking, and stability for α1BARs, the best characterized of the α1AR subtypes. Though not discussed here, several recent studies indicate that the functions, localization and trafficking, and other aspects of regulation can be quite different for the three α1AR subtypes. For example, the α1DAR is reported to be primarily intracellular, and it also appears to exhibit considerable constitutive
Acknowledgements
These studies were supported by NIH Research Grant GM34500.
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2024, Science and SportsPKC-isoform specific regulation of receptor desensitization and KCNQ1/KCNE1 K<sup>+</sup> channel activity by mutant α<inf>1B</inf>-adrenergic receptors
2022, Cellular SignallingCitation Excerpt :Within this time frame, several mechanism are proposed to regulate receptor internalization, e.g. the interaction of α1B-AR with β-arrestin and the clathrin adaptor complex AP2 [17] or with Rab4 and Rab5 proteins [39]. Mutational studies on the hamster α1B-AR identified the aa 403–425 domain of the C-tail as critical for internalization [41]. This domain contains the GRK phosphorylation sites (for the human ortholog S406,S410,S412) that seem to be essential for receptor internalization since coexpression of a dominant-negative GRK2 markedly decreased receptor internalization [42].
Sites phosphorylated in human α<inf>1B</inf>-adrenoceptors in response to noradrenaline and phorbol myristate acetate
2019, Biochimica et Biophysica Acta - Molecular Cell ResearchCitation Excerpt :Pioneer contributions by Cotecchia and coworkers, using receptor mutants with deletions and mutagenesis of specific sites, together with functional studies, identified that relevant phosphorylation sites for hamster α1B-AR are located in a cluster at the Ctail, which seem to be targeted by G protein-coupled receptor kinases (GRK) and PKC [30,32]. In addition, the group of Toews [35,36] has observed that, in the hamster receptor, two domains play key roles in receptor internalization/down regulation. Using deletions and site-directed mutagenesis, these authors observed that some mutated receptors internalize but do not down-regulate, other mutants that down-regulate without internalization, and that there are also receptors defective in both internalization and down-regulation [35].
Receptor species-dependent desensitization controls KCNQ1/KCNE1 K<sup>+</sup> channels as downstream effectors of Gq protein-coupled receptors
2016, Journal of Biological ChemistryCitation Excerpt :Although truncation studies demonstrated that homologous desensitization of the α1B-AR depends unequivocally on PKC-induced phosphorylation of serine residues in the C terminus (35, 40), conflicting results were reported upon the contribution of GRK to α1B-AR phosphorylation and desensitization (41, 42), depending on the cell expression system. Mutational studies on hamster α1B-AR indicate that PKC activation and phosphorylation of residues within the C-tail promote rapid receptor internalization (43). More recent studies propose that interactions of α1B-AR with β-arrestin and the clathrin adaptor complex AP2 (36) or with Rab4 and Rab5 proteins that are associated with early endosomes (25) regulate receptor internalization.
Characterization of the α<inf>1</inf>-adrenoceptor subtype activating extracellular signal-regulated kinase in submandibular gland acinar cells
2008, European Journal of PharmacologySignal transduction and regulation: Are all α<inf>1</inf>-adrenergic receptor subtypes created equal?
2007, Biochemical PharmacologyCitation Excerpt :Moreover, the regulation of receptor expression can also be induced by molecules which do not directly act on the receptor, i.e. heterologous regulation. As the regulation of α1-AR expression in general has been reviewed previously [90,93,94], we will focus on studies comparing the regulation of two or more α1-AR subtypes. A subtype-selective regulation of α1-ARs has been demonstrated in many tissues, e.g. related to physiological factors such as sex [95], ageing [96] or β-AR stimulation [97].