-Arrestin Mediates Desensitization and Internalization but Does Not Affect Dephosphorylation of the Thyrotropin-releasing Hormone Receptor

Department of Pharmacology and Physiology, University of Rochester, Rochester, New York, United States
Journal of Biological Chemistry (Impact Factor: 4.57). 12/2005; 280(46):38346-54. DOI: 10.1074/jbc.M502918200
Source: PubMed


The G protein-coupled thyrotropin-releasing hormone (TRH) receptor is phosphorylated and binds to β-arrestin after agonist
exposure. To define the importance of receptor phosphorylation and β-arrestin binding in desensitization, and to determine
whether β-arrestin binding and receptor endocytosis are required for receptor dephosphorylation, we expressed TRH receptors
in fibroblasts from mice lacking β-arrestin-1 and/or β-arrestin-2. Apparent affinity for [3H]MeTRH was increased 8-fold in cells expressing β-arrestins, including a β-arrestin mutant that did not permit receptor internalization.
TRH caused extensive receptor endocytosis in the presence of β-arrestins, but receptors remained primarily on the plasma membrane
without β-arrestin. β-Arrestins strongly inhibited inositol 1,4,5-trisphosphate production within 10 s. At 30 min, endogenous
β-arrestins reduced TRH-stimulated inositol phosphate production by 48% (β-arrestin-1), 71% (β-arrestin-2), and 84% (β-arrestins-1
and -2). In contrast, receptor phosphorylation, detected by the mobility shift of deglycosylated receptor, was unaffected
by β-arrestins. Receptors were fully phosphorylated within 15 s of TRH addition. Receptor dephosphorylation was identical
with or without β-arrestins and almost complete 20 min after TRH withdrawal. Blocking endocytosis with hypertonic sucrose
did not alter the rate of receptor phosphorylation or dephosphorylation. Expressing receptors in cells lacking Gαq and Gα11 or inhibiting protein kinase C pharmacologically did not prevent receptor phosphorylation or dephosphorylation. Overexpression
of dominant negative G protein-coupled receptor kinase-2 (GRK2), however, retarded receptor phosphorylation. Receptor activation
caused translocation of endogenous GRK2 to the plasma membrane. The results show conclusively that receptor dephosphorylation
can take place on the plasma membrane and that β-arrestin binding is critical for desensitization and internalization.

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    • "A similar pattern emerges when internalization is examined. Expression of dominant negative arrestin in normal cells or expression of receptors in arrestin-null cells severely restricts the rate and extent of endocytosis (Hanyaloglu et al., 2002; Jones and Hinkle, 2005, 2008). Endogenous arrestin levels control the desensitization and internalization of heterologously expressed TRH receptors; for example, HEK293 and even more so COS-7 cells have lower concentrations of arrestin and correspondingly less desensitization and internalization than pituitary cells (Falck-Pedersen et al., 1994; Vrecl et al., 1998). "
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    ABSTRACT: The pituitary receptor for thyrotropin-releasing hormone (TRH) is a calcium-mobilizing G protein-coupled receptor (GPCR) that signals through Gq/11, elevating calcium, and activating protein kinase C. TRH receptor signaling is quickly desensitized as a consequence of receptor phosphorylation, arrestin binding, and internalization. Following activation, TRH receptors are phosphorylated at multiple Ser/Thr residues in the cytoplasmic tail. Phosphorylation catalyzed by GPCR kinase 2 (GRK2) takes place rapidly, reaching a maximum within seconds. Arrestins bind to two phosphorylated regions, but only arrestin bound to the proximal region causes desensitization and internalization. Phosphorylation at Thr365 is critical for these responses. TRH receptors internalize in clathrin-coated vesicles with bound arrestin. Following endocytosis, vesicles containing phosphorylated TRH receptors soon merge with rab5-positive vesicles. Over approximately 20 min these form larger endosomes rich in rab4 and rab5, early sorting endosomes. After TRH is removed from the medium, dephosphorylated receptors start to accumulate in rab4-positive, rab5-negative recycling endosomes. The mechanisms responsible for sorting dephosphorylated receptors to recycling endosomes are unknown. TRH receptors from internal pools help repopulate the plasma membrane. Dephosphorylation of TRH receptors begins when TRH is removed from the medium regardless of receptor localization, although dephosphorylation is fastest when the receptor is on the plasma membrane. Protein phosphatase 1 is involved in dephosphorylation but the details of how the enzyme is targeted to the receptor remain obscure. It is likely that future studies will identify biased ligands for the TRH receptor, novel arrestin-dependent signaling pathways, mechanisms responsible for targeting kinases and phosphatases to the receptor, and principles governing receptor trafficking.
    Full-text · Article · Dec 2012 · Frontiers in Neuroscience
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    • "Critically, data from our studies ascertain that desensitization and internalization are two independent processes. In a recent study, dephosphorylation of TRH receptors was observed despite inhibition of receptor internalization (Jones and Hinkle, 2005) consistent with our view that receptors can be dephosphorylated at the plasma membrane. This phenomenon may allow for more rapid resensitization at plasma membrane following agonist-induced desensitization than for an internalized receptor. "
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    ABSTRACT: Phosphoinositide 3-kinase γ (PI3Kγ) is activated by G protein-coupled receptors (GPCRs). We show here that PI3Kγ inhibits protein phosphatase 2A (PP2A) at the β-adrenergic receptor (βAR, a GPCR) complex altering G protein coupling. PI3Kγ inhibition results in significant increase of βAR-associated phosphatase activity leading to receptor dephosphorylation and resensitization preserving cardiac function. Mechanistically, PI3Kγ inhibits PP2A activity at the βAR complex by phosphorylating an intracellular inhibitor of PP2A (I2PP2A) on serine residues 9 and 93, resulting in enhanced binding to PP2A. Indeed, enhanced phosphorylation of β2ARs is observed with a phosphomimetic I2PP2A mutant that was completely reversed with a mutant mimicking dephosphorylated state. siRNA depletion of endogenous I2PP2A augments PP2A activity despite active PI3K resulting in β2AR dephosphorylation and sustained signaling. Our study provides the underpinnings of a PI3Kγ-mediated regulation of PP2A activity that has significant consequences on receptor function with broad implications in cellular signaling.
    Full-text · Article · Mar 2011 · Molecular cell
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    • "The results are reminiscent of previous studies showing that the TRH receptor retains some ability to form an acid-resistant complex and undergo endocytosis in arrestin-null cells (Jones and Hinkle, 2005), pointing to the existence of an uncharacterized arrestin-independent pathway. The arrestin-TRH receptor complex has much higher affinity for TRH than the nonphosphorylated receptor (Jones and Hinkle, 2005, 2008), just as other GPCR-arrestin complexes bind agonists with higher affinity than the GPCRs alone (Gurevich et al., 1997). When equilibrium [ 3 H]MeTRH binding was measured in live HEK293 cells, the 6Q-TRH receptor had significantly lower apparent agonist affinity than the WT receptor (Table 1). "
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    ABSTRACT: The thyrotropin-releasing hormone (TRH) receptor undergoes rapid and extensive agonist-dependent phosphorylation attributable to G protein-coupled receptor (GPCR) kinases (GRKs), particularly GRK2. Like many GPCRs, the TRH receptor is predicted to form an amphipathic helix, helix 8, between the NPXXY motif at the cytoplasmic end of the seventh transmembrane domain and palmitoylation sites at Cys335 and Cys337. Mutation of all six lysine and arginine residues between the NPXXY and residue 340 to glutamine (6Q receptor) did not prevent the receptor from stimulating inositol phosphate turnover but almost completely prevented receptor phosphorylation in response to TRH. Phosphorylation at all sites in the cytoplasmic tail was inhibited. The phosphorylation defect was not reversed by long incubation times or high TRH concentrations. As expected for a phosphorylation-defective receptor, the 6Q-TRH receptor did not recruit arrestin, undergo the typical arrestin-dependent increase in agonist affinity, or internalize well. Lys326, directly before phenylalanine in the common GPCR motif NPXXY(X)(5-6)F(R/K), was critical for phosphorylation. The 6Q-TRH receptor was not phosphorylated effectively in cells overexpressing GRK2 or in in vitro kinase assays containing purified GRK2. Phosphorylation of the 6Q receptor was partially restored by coexpression of a receptor with an intact helix 8 but without phosphorylation sites. Phosphorylation was inhibited but not completely prevented by alanine substitution for cysteine palmitoylation sites. Positively charged amino acids in the proximal tail of the beta2-adrenergic receptor were also important for GRK-dependent phosphorylation. The results indicate that positive residues in helix 8 of GPCRs are important for GRK-dependent phosphorylation.
    Full-text · Article · Nov 2009 · Molecular pharmacology
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