Truncation of the receptor carboxyl terminus impairs agonist-dependent phosphorylation and desensitization of the alpha 1B-adrenergic receptor

Protein phosphatase-protein kinase interplay modulates alpha 1b-adrenoceptor phosphorylation: effects of okadaic acid

In the present work we studied the effect of protein phosphatase inhibitors on the phosphorylation state and function of alpha(1b)-adrenoceptors. Okadaic acid increased receptor phosphorylation in a time- and concentration-dependent fashion (maximum at 30 min, EC(50) of 30 nM). Other inhibitors of protein phosphatases (calyculin A, tautomycin and cypermethrin) mimicked this effect. Staurosporine and Ro 31-8220, inhibitors of protein kinase C, blocked the effect of okadaic acid on receptor phosphorylation. Neither genistein nor wortmannin altered the effect of okadaic acid. The intense adrenoceptor phosphorylation induced by okadaic acid altered the adrenoceptor-G protein coupling, as evidenced by a small decreased noradrenaline-stimulated [(35)S]GTPgammaS binding. Okadaic acid did not alter the noradrenaline-stimulated increases in intracellular calcium or the production of inositol trisphosphate. Our data indicate that inhibition of protein phosphatases increases the phosphorylation state of alpha(1b)-adrenoceptors; this effect seems to involve protein kinase C. In spite of inducing an intense receptor phosphorylation, okadaic acid alters alpha(1b)-adrenergic actions to a much lesser extent than the direct activation of protein kinase C by phorbol myristate acetate.

Protein kinase C-mediated phosphorylation and desensitization of human alpha(1b)-adrenoceptors

Human alpha(1b)-adrenoceptors stably expressed (B(max) approximately 800 fmol/mg membrane protein) in mouse fibroblasts were able to increase intracellular Ca(2+) and inositol phosphate production in response to noradrenaline. Activation of protein kinase C desensitized the alpha(1b)-adrenergic-mediated actions but did not block the ability of the cells to respond to lysophosphatidic acid. Inhibition or downregulation of protein kinase C also blocked the action of the tumor promoter on the adrenergic effects. Photolabeling experiments indicated that the receptor has an apparent molecular weight of approximately 80 kDa. The receptors were phosphorylated in the basal state and such phosphorylation was increased when the cells were incubated with phorbol myristate acetate or noradrenaline. Incubation of the cells with phorbol myristate acetate or noradrenaline blocked noradrenaline-promoted [35S]GTP-gamma-S binding to membranes, suggesting receptor-G protein uncoupling. The results indicate that activation of protein kinase C blocked/desensitized human alpha(1b)-adrenoceptors and that such effect was associated to receptor phosphorylation.

Alpha 1-adrenoceptors: subtypes, signaling, and roles in health and disease

Alpha 1-adrenoceptors mediate some of the main actions of the natural catecholamines, adrenaline, and noradrenaline. They participate in many essential physiological processes, such as sympathetic neurotransmission, modulation of hepatic metabolism, control of vascular tone, cardiac contraction, and the regulation of smooth muscle activity in the genitourinary system. It is now clear that alpha 1-adrenoceptors mediate, in addition to immediate effects, longer term actions of catecholamines such as cell growth and proliferation. In fact, adrenoceptor genes can be considered as protooncogenes. Over the past years, considerable progress has been achieved in the molecular characterization of different alpha 1-adrenoceptor subtypes. Three main subtypes have been characterized pharmacologically and in molecular terms. Splice variants, truncated isoforms, and polymorphisms have also been detected. Similarly, it is now clear that these receptors are coupled to several classes of G proteins that, therefore, are capable of modulating different signaling pathways. In the present article, some of these aspects are reviewed, together with the distribution of the subtypes in different tissues and some of the known roles of these receptors in health and disease.

The proximal portion of the COOH terminus of the oxytocin receptor is required for coupling to g(q), but not g(i). Independent mechanisms for elevating intracellular calcium concentrations from intracellular stores

As the oxytocin receptor plays a key role in parturition and lactation, there is considerable interest in defining its structure/functional relationships. We previously showed that the rat oxytocin receptor transfected into Chinese hamster ovary cells was coupled to both G(q/11) and G(i/o), and that oxytocin stimulated ERK-2 phosphorylation and prostaglandin E(2) synthesis via protein kinase C activity. In this study, we show that deletion of 51 amino acid residues from the carboxyl terminus resulted in reduced affinity for oxytocin and a corresponding rightward shift in the dose-response curve for oxytocin-stimulated [Ca(2+)](i). However, oxytocin-stimulated ERK-2 phosphorylation and prostaglandin E(2) synthesis did not occur in cells expressing the truncated receptor. Oxytocin also failed to increase phospholipase A activity or activate protein kinase C, indicating that the mutant receptor is uncoupled from G(q)-mediated pathways. The Delta51 receptor is coupled to G(i), as oxytocin-stimulated Ca(2+) transients were inhibited by pertussis toxin, and a Gbetagamma sequestrant. Preincubation of Delta51 cells with the tyrosine kinase inhibitor, genistein, also blocked the oxytocin effect. A Delta39 mutant had all the activities of the wild type oxytocin receptor. These results show that the portion between 39 and 51 residues from the COOH terminus of the rat oxytocin receptor is required for interaction with G(q/11), but not G(i/o). Furthermore, an increase in intracellular calcium was generated via a G(i)betagamma-tyrosine kinase pathway from intracellular stores that are distinct from G(q)-mediated inositol trisphosphate-regulated stores.

Intracellular calcium and alpha 1b-adrenoceptor phosphorylation

BACKGROUND

Desensitization of G protein-coupled receptors is associated with receptor phosphorylation. Two groups of kinases seem to participate in such receptor phosphorylation, i.e., second messenger-activated protein kinases and G protein-coupled receptor kinases. Calcium seems to play a role in the phosphorylation of some G protein-coupled receptors. The role of calcium in alpha 1b-adrenoceptor phosphorylation has not been critically assessed.

METHODS

Rat-1 fibroblasts stably expressing the hamster alpha 1b-adrenergic receptor were used. To study receptor phosphorylation cells metabolically labeled with [32P]Pi were lysed and the receptor immunoprecipitated using a polyclonal antibody generated against the receptor carboxyl terminal decapeptide. Intracellular calcium was determined by using Fura-2 fluorescence.

RESULTS

Norepinephrine, endothelin-1, and lysophosphatidic acid increased intracellular calcium concentration. All these agents and phorbol myristate acetate (PMA) induce alpha 1b-adrenoceptor phosphorylation. The intracellular chelator, BAPTA, abolished the increase in intracellular calcium induced by the previously mentioned agents but did not affect the receptor phosphorylation induced by norepinephrine, PMA, or lysophosphatidic acid. Under these conditions, receptor phosphorylation induced by endothelin was slightly but consistently decreased. Thapsigargin increased intracellular calcium concentration but was unable to induce alpha 1b-adrenoceptor phosphorylation and decreased PMA-induced receptor phosphorylation. No increase in receptor phosphorylation was observed when calcium ionophores were used.

CONCLUSIONS

Our data indicate that an increase in [Ca2+]i is not sufficient to induce alpha 1b-adrenoceptor phosphorylation and that buffering of [Ca2+]i does not alter the receptor phosphorylation induced by norepinephrine, lysophosphatidic acid, and PMA. A marginal role of calcium in the alpha 1b-adrenoceptor phosphorylation induced by endothelin-1 cannot be discarded.

Interaction of the alpha(1B)-adrenergic receptor with gC1q-R, a multifunctional protein

gC1q-R, a multifunctional protein, was found to bind with the carboxyl-terminal cytoplasmic domain of the alpha(1B)-adrenergic receptor (173 amino acids, amino acids 344-516) in a yeast two-hybrid screen of a cDNA library prepared from the rat liver. In a series of studies with deletion mutants in this region, the ten arginine-rich amino acids (amino acids 369-378) were identified as the site of interaction. The interaction was confirmed by specific co-immunoprecipitation of gC1q-R with full-length alpha(1B)-adrenergic receptors expressed on transfected COS-7 cells, as well as by fluorescence confocal laser scanning microscopy, which showed co-localization of these proteins in intact cells. Interestingly, the alpha(1B)-adrenergic receptors were exclusively localized to the region of the plasma membrane in COS-7 cells that expressed the alpha(1B)-adrenergic receptor alone, whereas gC1q-R was localized in the cytoplasm in COS-7 cells that expressed gC1q-R alone; however, in cells that co-expressed alpha(1B)-adrenergic receptors and gC1q-R, most of the alpha(1B)-adrenergic receptors were co-localized with gC1q-R in the intracellular region, and a remarkable down-regulation of receptor expression was observed. These observations suggest a new role for the previously identified complement regulatory molecule, gC1q-R, in regulating the cellular localization and expression of the alpha(1B)-adrenergic receptors.

The tail of the gonadotrophin-releasing hormone receptor: desensitization at, and distal to, G protein-coupled receptors

In recent years a general scheme for the rapid desensitization and cycling of G protein-coupled receptors (GPCRs) has emerged. In this scheme agonist-induced phosphorylation (most often in the receptors' C-terminal tail) causes association with beta-arrestin which not only reduces the efficiency of G-protein activation, but also targets these desensitized receptors for internalization, after which they may be either proteolytically degraded or resensitized and recycled back to the cell surface. Although sustained stimulation of pituitary gonadotrophs with gonadotrophin-releasing hormone (GnRH) is known to cause a pronounced desensitization of GnRH-stimulated gonadotrophin secretion, the discovery that mammalian GnRH receptors do not possess C-terminal tails raised the question of whether receptor desensitization is involved. This review outlines data demonstrating that tail-less mammalian GnRH receptors can be considered as natural desensitization and internalization deficient 'mutants'. This is in stark contrast to non-mammalian GnRH receptors which do possess tails and conform to the general scheme. In the absence of receptor desensitization, post receptor mechanisms take on increasing importance for desensitization of GnRH action via mammalian GnRH receptors. The down regulation of Ins(1,4,5)P3 receptors and consequent desensitization of GnRH effects on cytosolic Ca2+ are discussed as a novel mechanism for such desensitization.

G-protein coupled receptor kinases as modulators of G-protein signalling

G-protein coupled receptors (GPCRs) comprise one of the largest classes of signalling molecules. A wide diversity of activating ligands induce the active conformation of GPCRs and lead to signalling via heterotrimeric G-proteins and downstream effectors. In addition, a complex series of reactions participate in the 'turn-off' of GPCRs in both physiological and pharmacological settings. Some key players in the inactivation or 'desensitization' of GPCRs have been identified, whereas others remain the target of ongoing studies. G-protein coupled receptor kinases (GRKs) specifically phosphorylate activated GPCRs and initiate homologous desensitization. Uncoupling proteins, such as members of the arrestin family, bind to the phosphorylated and activated GPCRs and cause desensitization by precluding further interactions of the GPCRs and G-proteins. Adaptor proteins, including arrestins, and endocytic machinery participate in the internalization of GPCRs away from their normal signalling milieu. In this review we discuss the roles of these regulatory molecules as modulators of GPCR signalling.

Desensitization of P2Y2 receptor-activated transepithelial anion secretion

Desensitization of P2Y2 receptor-activated anion secretion may limit the usefulness of extracellular nucleotides in secretagogue therapy of epithelial diseases, e.g., cystic fibrosis (CF). To investigate the desensitization process for endogenous P2Y2 receptors, freshly excised or cultured murine gallbladder epithelia (MGEP) were mounted in Ussing chambers to measure short-circuit current (Isc), an index of electrogenic anion secretion. Luminal treatment with nucleotide receptor agonists increased the Isc with a potency profile of ATP = UTP > 2-methylthioATP >> alpha,beta-methylene-ATP. RT-PCR revealed the expression of P2Y2 receptor mRNA in the MGEP cells. The desensitization of anion secretion required a 10-min preincubation with the P2Y2 receptor agonist UTP and increased in a concentration-dependent manner (IC50 approximately 10(-6) M). Approximately 40% of the anion secretory response was unaffected by maximal desensitizing concentrations of UTP. Recovery from UTP-induced desensitization was rapid (<10 min) at preincubation concentrations less than the EC50 (1.9 x 10(-6) M) but required progressively longer time periods at greater concentrations. UTP-induced total inositol phosphate production and intracellular Ca2+ mobilization desensitized with a concentration dependence similar to that of anion secretion. In contrast, maximal anion secretion induced by Ca2+ ionophore ionomycin was unaffected by preincubation with a desensitizing concentration of UTP. It was concluded that 1) desensitization of transepithelial anion secretion stimulated by the P2Y2 receptor agonist UTP is time and concentration dependent; 2) recovery from desensitization is prolonged (>90 min) at UTP concentrations >10(-5) M; and 3) UTP-induced desensitization occurs before the operation of the anion secretory mechanism.

Constitutively active alpha-1b adrenergic receptor mutants display different phosphorylation and internalization features

We compared the phosphorylation and internalization properties of constitutively active alpha-1b adrenergic receptor (AR) mutants carrying mutations in two distant receptor domains, i.e., at A293 in the distal part of the third intracellular loop and at D142 of the DRY motif lying at the end of the third transmembrane domain. For the A293E and A293I mutants the levels of agonist-independent phosphorylation were 150% and 50% higher than those of the wild-type alpha-1b AR, respectively. On the other hand, for the constitutively active D142A and D142T mutants, the basal levels of phosphorylation were similar to those of the wild-type alpha-1b AR and did not appear to be further stimulated by epinephrine. Overexpression of the guanyl nucleotide binding regulatory protein-coupled receptor kinase GRK2 further increases the basal phosphorylation of the A293E mutant, but not that of D142A mutant. Both the wild-type alpha-1b AR and the A293E mutant could undergo beta-arrestin-mediated internalization. The epinephrine-induced internalization of the constitutively active A293E mutant was significantly higher than that of the wild-type alpha-1b AR. In contrast, the D142A mutant was impaired in its ability to interact with beta-arrestin and to undergo agonist-induced internalization. Interestingly, a double mutant A293E/D142A retained very high constitutive activity and regulatory properties of both the A293E and D142A receptors. These findings demonstrate that two constitutively activating mutations occurring in distant receptor domains of the alpha-1b AR have divergent effects on the regulatory properties of the receptor.

Modulation of basal intracellular calcium by inverse agonists and phorbol myristate acetate in rat-1 fibroblasts stably expressing alpha1d-adrenoceptors

In rat-1 fibroblasts stably expressing alpha1d-adrenoceptors BMY 7378, phentolamine, chloroethylclonidine and 5-methyl urapidil decreased basal [Ca2+]i. WB 4101 induced a very small effect on this parameter but when added before the other antagonists it blocked their effect. All these agents inhibited the action of norepinephrine. Phorbol myristate acetate also blocked the effect of norepinephrine and decreased basal [Ca2+]i. Staurosporine inhibited these effects of the phorbol ester. Our results suggest that: (1) alpha1d-adrenoceptors exhibit spontaneous ligand-independent activity, (2) BMY 7378, phentolamine, chloroethylclonidine and 5-methyl urapidil act as inverse agonists and (3) protein kinase C activation blocks spontaneous and agonist-stimulated alpha1d-adrenoceptor activity.

Molecular basis of receptor/G-protein-coupling selectivity

Molecular cloning studies have shown that G-protein-coupled receptors form one of the largest protein families found in nature, and it is estimated that approximately 1000 different such receptors exist in mammals. Characteristically, when activated by the appropriate ligand, an individual receptor can recognize and activate only a limited set of the many structurally closely related heterotrimeric G-proteins expressed within a cell. To understand how this selectivity is achieved at a molecular level has become the focus of an ever increasing number of laboratories. This review provides an overview of recent structural, molecular genetic, biochemical, and biophysical studies that have led to novel insights into the molecular mechanisms governing receptor-mediated G-protein activation and receptor/G-protein coupling selectivity.

Structural basis of agonist-induced desensitization and sequestration of the P2Y2 nucleotide receptor. Consequences of truncation of the C terminus

Molecular determinants of P2Y2 receptor desensitization and sequestration have been investigated. Wild-type P2Y2 receptors and a series of five C-terminal truncation mutants of the receptor were epitope-tagged and stably expressed in 1321N1 cells. These constructs were used to assess the importance of the intracellular C terminus on 1) UTP-stimulated increases in intracellular calcium concentration, 2) homologous desensitization of the receptor, and 3) agonist-induced decreases in cell-surface density (receptor sequestration) of epitope-tagged receptors using fluorescence-activated cell sorting. The potency and efficacy of UTP were similar for the wild-type and all mutant P2Y2 receptors. Truncation of 18 or more amino acids from the C terminus increased by approximately 30-fold the concentration of UTP necessary to desensitize the receptor. Both the rate and magnitude of UTP-induced receptor sequestration were decreased with progressively larger truncations of the C terminus. Furthermore, the recovery from sequestration was slower for the most extensively truncated receptor. Complete desensitization was obtained with >50% of the original receptor complement remaining on the cell surface. Protein kinase C activation, which desensitizes the P2Y2 receptor, had no effect on sequestration, consistent with the ideas that desensitization and sequestration are discrete events and that agonist occupancy is required for receptor sequestration.

G protein-coupled receptor kinases

G protein-coupled receptor kinases (GRKs) constitute a family of six mammalian serine/threonine protein kinases that phosphorylate agonist-bound, or activated, G protein-coupled receptors (GPCRs) as their primary substrates. GRK-mediated receptor phosphorylation rapidly initiates profound impairment of receptor signaling, or desensitization. This review focuses on the regulation of GRK activity by a variety of allosteric and other factors: agonist-stimulated GPCRs, beta gamma subunits of heterotrimeric GTP-binding proteins, phospholipid cofactors, the calcium-binding proteins calmodulin and recoverin, posttranslational isoprenylation and palmitoylation, autophosphorylation, and protein kinase C-mediated GRK phosphorylation. Studies employing recombinant, purified proteins, cell culture, and transgenic animal models attest to the general importance of GRKs in regulating a vast array of GPCRs both in vitro and in vivo.

Real-time optical monitoring of ligand-mediated internalization of alpha1b-adrenoceptor with green fluorescent protein

The study of G protein-coupled receptor signal transduction and behavior in living cells is technically difficult because of a lack of useful biological reagents. We show here that a fully functional alphalb-adrenoceptor tagged with the green fluorescent protein (alphalbAR/GFP) can be used to determine the molecular mechanism of intemalization of alphalbAR/ GFP in living cells. In mouse alphaT3 cells, alpha1bAR/GFP demonstrates strong, diffuse fluorescence along the plasma membrane when observed by confocal laser scanning microscope. The fluorescent receptor binds agonist and antagonist and stimulates phosphatidylinositol/Ca2+ signaling in a similar fashion to the wild receptor. In addition, alpha1bAR/ GFP can be internalized within minutes when exposed to agonist, and the subcellular redistribution of this receptor can be determined by measurement of endogenous fluorescence. The phospholipase C inhibitor U73,122, the protein kinase C activator PMA, and inhibitor staurosporine, and the Ca2+-ATPase inhibitor thapsigargin were used to examine the mechanism of agonist-promoted alphalbAR/GFP redistribution. Agonist-promoted internalization of alphalbAR/GFP was closely linked to phospholipase C activation and was dependent on protein kinase C activation, but was independent of the increase in intracellular free Ca2+ concentration. This study demonstrated that real-time optical monitoring of the subcellular localization of alphalbAR (as well as other G protein-coupled receptors) in living cells is feasible, and that this may provide a valuable system for further study of the biochemical mechanism(s) of agonist-induced receptor endocytosis.

The role of receptor kinases and arrestins in G protein-coupled receptor regulation

G protein-coupled receptors (GPRs) play a key role in controlling hormonal regulation of numerous second-messenger pathways. However, following agonist activation, most GPRs rapidly lose their ability to respond to hormone. For many GPRs, this process, commonly referred to as desensitization, appears to be primarily mediated by two protein families: G protein-coupled receptor kinases (GRKs) and arrestins. GRKs specifically bind to the agonist-occupied receptor, thereby promoting receptor phosphorylation, which in turn leads to arrestin binding. Arrestin binding precludes receptor/G protein interaction leading to functional desensitization. Many GPRs are then removed from the plasma membrane via clathrin-mediated endocytosis. Recent studies have implicated endocytosis in the resensitization of GPRs and have linked both GRKs and arrestins to this process. In this review, we discuss the role of GRKs and arrestins in regulating agonist-specific signaling and trafficking of GPRs.

Molecular mechanisms involved in the activation and regulation of the alpha 1-adrenergic receptor subtypes

The adrenergic receptors (ARs) belong to the superfamily of membrane-bound G protein coupled receptors (GPCRs). Our investigation has focused on the structure-function relationship of the alpha 1b-AR subtype used as the model system for other GPCRs. Site-directed mutagenesis studies have elucidated the structural domains of the alpha 1b-AR involved in ligand binding, G protein coupling or desensitization. In addition, a combined approach using site-directed mutagenesis and molecular dynamics analysis of the alpha 1b-AR has provided information about the potential mechanisms underlying the activation process of the receptor, i.e. its transition from the 'inactive' to the 'active' conformation.

Role of cytoplasmic tail of the type 1A angiotensin II receptor in agonist- and phorbol ester-induced desensitization

To investigate mechanisms underlying the agonist-induced desensitization of the type 1A angiotensin II receptor (AT1A-R), we have stably expressed in Chinese hamster ovary (CHO) cells the wild-type receptor and truncated mutants lacking varying lengths of the cytoplasmic tail. Assay of inositol 1,4,5-trisphosphate (IP3) formation in response to agonist demonstrated that the truncated mutants T318, T328, and T348 lacking the last 42, 32, or 12 amino acid residues, respectively, couple with Gq protein with an efficiency similar to that of full-length receptors, whereas coupling of Gq protein was abolished in the T310 truncated mutant devoid of the carboxyl-terminal 50 amino acids. Exposure of CHO/AT1A-R cells expressing the wild-type AT1A-R to angiotensin II resulted in rapid and dose-dependent homologous desensitization of receptor-mediated IP3 formation, which was independent of the receptor internalization. Mastoparan, an activator of G protein-coupled receptor kinase (GRK), induced desensitization of the AT1A-R. The agonist-induced desensitization of the receptor was largely prevented by heparin, a potent inhibitor of GRK, whereas it was only partially attenuated by a protein kinase C (PKC)-specific inhibitor. The homologous or heterologous desensitization of the receptor was greatly impaired in the truncated mutants T318 and T328, lacking the Ser/Thr-rich (13 or 12 Ser/Thr residues) cytoplasmic tail of the AT1A-R. Deletion of the last two Ser residues, including one PKC consensus site in the receptor tail, prevented only phorbol 12-myristate 13-acetate-induced desensitization by 30%. Moreover, we found an agonist-induced translocation of a heparin-sensitive kinase activity. The angiotensin II-stimulated heparin-sensitive kinase could phosphorylate a thioredoxin fusion protein containing the entire AT1A-R cytoplasmic tail (N295 to E359), which lacks consensus phosphorylation sites for GRK1, GRK2, and GRK3. The heparin-sensitive kinase may not be GRK2, GRK3, or GRK6 expressed in CHO/AT1A-R cells, since angiotensin II did not induce translocation of these receptor kinases. Potential Ser/Thr phosphorylation sites located between S328 and S347 in the cytoplasmic tail of AT1A-R seem to play a critical role in the heterologous and homologous desensitization of the receptor. A heparin-sensitive kinase other than GRK2, GRK3, or GRK6 may be involved in the agonist-induced homologous desensitization of the AT1A-R.

A serine cluster prevents recycling of the V2 vasopressin receptor

Receptor recycling plays a critical role in the regulation of cellular responsiveness to environmental stimuli. Agonist-promoted phosphorylation of G protein-coupled receptors has been related to their desensitization, internalization, and sequestration. Dephosphorylation of internalized G protein-coupled receptors by cytoplasmic phosphatases has been shown to be pH-dependent, and it has been postulated to be necessary for receptors to recycle to the cell surface. The internalized V2 vasopressin receptor (V2R) expressed in HEK 293 cells is an exception to this hypothesis because it does not recycle to the plasma membrane for hours after removal of the ligand. Because this receptor is phosphorylated only by G protein-coupled receptor kinases (GRKs), the relationship between recycling and GRK-mediated phosphorylation was examined. A nonphosphorylated V2R, truncated upstream of the GRK phosphorylation sites, rapidly returned to the cell surface after removal of vasopressin. Less-drastic truncations of V2R revealed the presence of multiple phosphorylation sites and suggested a key role for a serine cluster present at the C terminus. Replacement of any one of Ser-362, Ser-363, or Ser-364 with Ala allowed quantitative recycling of full-length V2R without affecting the extent of internalization. Examination of the stability of phosphate groups incorporated into the recycling S363A mutant V2Rs revealed that the recycling receptor was dephosphorylated after hormone withdrawal, whereas the wild-type V2R was not, providing molecular evidence for the hypothesis that GRK sites must be dephosphorylated prior to receptor recycling. These experiments uncovered a role for GRK phosphorylation in intracellular sorting and revealed a GRK-dependent anchoring domain that blocks V2R recycling.

Molecular cloning, genomic characterization and expression of novel human alpha1A-adrenoceptor isoforms

We have isolated and characterized from human prostate novel splice variants of the human alpha1A-adrenoceptor, several of which generate truncated products and one isoform, alpha(1A-4), which has the identical splice site as the three previously described isoforms. Long-PCR on human genomic DNA showed that the alpha(1A-4) exon is located between those encoding the alpha(1A-1) and alpha(1A-3) variants. CHO-K1 cells stably expressing alpha(1A-4) showed ligand binding properties similar to those of the other functional isoforms as well as agonist-stimulated inositol phosphate accumulation. Quantitative PCR analyses revealed that alpha(1A-4) is the most abundant isoform expressed in the prostate with high levels also detected in liver and heart.

Crosstalk: phosphorylation of alpha1b-adrenoceptors induced through activation of bradykinin B2 receptors

The action of bradykinin was studied in rat-1 fibroblasts stably expressing alpha1b-adrenoceptors. It was observed that bradykinin and kallidin markedly increase cytosol calcium concentration, but that the B1 agonist, des-Arg9-bradykinin, only mimicked this effect to a minimal extent. Antagonists, selective for the B2 subtype, such as Hoe 140, blocked this effect of bradykinin and kallidin. Similarly, bradykinin and kallidin stimulated the production of inositol phosphates and B2 antagonists blocked their actions. The possibility that bradykinin could modulate alpha1b-adrenoceptors was studied. It was observed that bradykinin and kallidin increased alpha1b-adrenoceptor phosphorylation and that such effect was also blocked by Hoe 140. Interestingly, the ability of norepinephrine to increase intracellular calcium concentration was not altered by pretreatment of the cells with bradykinin, i.e. bradykinin induced alpha1b-adrenoceptor phosphorylation but this did not lead to receptor desensitization.

Chloroquine inhibits alpha1B-adrenergic action in hepatocytes

Noradrenaline increased phosphorylase a activity through activation of alpha1B-adrenoceptors in rat hepatocytes. Such effect was inhibited by chloroquine (Ki approximately 55 nM) and only slightly reduced by high concentrations of primaquine. Chloroquine did not inhibit the activation of phosphorylase a induced by vasopressin or angiotensin II. Binding competition experiments using [3H]prazosin showed that both chloroquine and primaquine interact with alpha1B-adrenoceptors, but only at very high concentrations. This indicates that the ability of chloroquine to block the alpha1B-adrenergic action was not due to antagonism at the receptor level. Noradrenaline increased phosphatidylinositol resynthesis and inositol trisphosphate production; these effects were inhibited by chloroquine and phorbol 12-myristate 13-acetate. Staurosporine and Ro 31-8220 (3-[1-[3-(amidinothio)propyl-1H-indol-3-yl]-3-(1-methyl-1H-indol-3 -yl)maleimide), reduced the inhibitions induced by the active phorbol ester and the antimalarial drug on adrenergic-stimulated phosphatidylinositol resynthesis. Similarly, staurosporine blocked the inhibitory actions of chloroquine and phorbol 12-myristate 13-acetate on noradrenaline-stimulated inositol trisphosphate production. These data suggest the possibility that protein kinases, such as protein kinase C, could be involved in the actions of chloroquine.

Chloroethylclonidine is a partial alpha1A-adrenoceptor agonist in cells expressing recombinant alpha1-adrenoceptor subtypes

Chloroethylclonidine increased cytosol [Ca2+] in rat-1 fibroblasts stably expressing alpha1a-adrenoceptors. The effect of the imidazoline was dose-dependent with a maximal effect (approximately 3-fold increase in [Ca2+]i) at 10 microM and it was blocked by phentolamine and 5-methyl urapidil, indicating that it was mediated through alpha1-adrenoceptors. Noradrenaline (1 microM) induced a much bigger effect (approximately 6-8-fold) in the same cells. When chloroethylclonidine was added before noradrenaline a dose-dependent inhibition of the effect of the natural catecholamine was observed. Chloroethylclonidine did not modified cytosol [Ca2+] in rat-1 fibroblast expressing alpha1b- or alpha1d-adrenoceptors. However, the imidazoline acutely inhibited the effect of noradrenaline in these cells. It is concluded that chloroethylclonidine interacts with alpha1a-adrenoceptors as a partial agonist inducing Ca2+ mobilization in a very short time frame and that it is able to inhibit the action of noradrenaline when co-incubated with the catecholamine in cells expressing any of the three alpha1-adrenoceptor subtypes.

Evidence of an important and direct role for protein kinase C in agonist-induced phosphorylation leading to desensitization of the angiotensin AT1A receptor

1. The role of protein kinase C (PKC) in the mechanism underlying rapid agonist-induced desensitization of angiotensin AT1 receptors remains unresolved. A major problem has been to isolate these receptors in a sufficiently purified form to allow study of their phosphorylation state. 2. A cleavable (His)6 affinity tag was introduced into the N-terminus of the recombinant AT1A receptor and stably expressed in human embryonic kidney cells. This affinity tag allowed rapid isolation, purification and determination of the phosphorylation state of the AT1A receptor. Using these cells, we determined the role of PKC in both agonist-induced receptor phosphorylation and desensitization under identical conditions. 3. Agonist-induced phosphorylation of the AT1A receptor was observed at both low and high concentrations of angiotensin II (AII). Preincubation of cells with Ro-31-8220 (a PKC specific inhibitor) revealed that at low concentrations of AII (1 nM), PKC appeared to be the main kinase involved in receptor phosphorylation. In contrast, at high concentrations of AII (100 nM), although PKC-mediated phosphorylation of the receptor was observed, this was overshadowed by a second kinase. 4. In preliminary desensitization studies we observed that at a low concentration of AII, preincubation with Ro-31-8220 attenuated desensitization, whilst at high concentrations of AII (100 nM) it had little or no effect on the level of desensitization observed. 5. These data directly demonstrate an association between PKC-induced receptor phosphorylation and desensitization at low concentrations of AII. Since circulating concentrations of AII are in the picomolar range, we propose that PKC is the physiologically relevant mediator of AT1 receptor desensitization.

Characterization of the phosphorylation sites involved in G protein-coupled receptor kinase- and protein kinase C-mediated desensitization of the alpha1B-adrenergic receptor

Catecholamines as well as phorbol esters can induce the phosphorylation and desensitization of the alpha1B-adrenergic receptor (alpha1BAR). In this study, phosphoamino acid analysis of the phosphorylated alpha1BAR revealed that both epinephrine- and phorbol ester-induced phosphorylation predominantly occurs at serine residues of the receptor. The findings obtained with receptor mutants in which portions of the C-tail were truncated or deleted indicated that a region of 21 amino acids (393-413) of the carboxyl terminus including seven serines contains the main phosphorylation sites involved in agonist- as well as phorbol ester-induced phosphorylation and desensitization of the alpha1BAR. To identify the serines invoved in agonist- versus phorbol ester-dependent regulation of the receptor, two different strategies were adopted, the seven serines were either substituted with alanine or reintroduced into a mutant lacking all of them. Our findings indicate that Ser394 and Ser400 were phosphorylated following phorbol ester-induced activation of protein kinase C, whereas Ser404, Ser408, and Ser410 were phosphorylated upon stimulation of the alpha1BAR with epinephrine. The observation that overexpression of G protein-coupled kinase 2 (GRK2) could increase agonist-induced phosphorylation of Ser404, Ser408, and Ser410, strongly suggests that these serines are the phosphorylation sites of the alpha1BAR for kinases of the GRK family. Phorbol ester-induced phosphorylation of the Ser394 and Ser400 as well as GRK2-mediated phosphorylation of the Ser404, Ser408, and Ser410, resulted in the desensitization of alpha1BAR-mediated inositol phosphate response. This study provides generalities about the biochemical mechanisms underlying homologous and heterologous desensitization of G protein-coupled receptors linked to the activation of phospholipase C.

Regulation of human chemokine receptors CXCR4. Role of phosphorylation in desensitization and internalization

Members of the chemokine receptor family CCR5 and CXCR4 have recently been shown to be involved in the entry of human immunodeficiency virus (HIV) into target cells. Here, we investigated the regulation of CXCR4 in rat basophilic leukemia cells (RBL-2H3) stably transfected with wild type (Wt CXCR4) or a cytoplasmic tail deletion mutant (DeltaCyto CXCR4) of CXCR4. The ligand, stromal cell derived factor-1 (SDF-1) stimulated higher G-protein activation, inositol phosphate generation, and a more sustained calcium elevation in cells expressing DeltaCyto CXCR4 relative to Wt CXCR4. SDF-1 and phorbol 12-myristate 13-acetate (PMA), but not a membrane permeable cAMP analog induced rapid phosphorylation as well as desensitization of Wt CXCR4. Phosphorylation of DeltaCyto CXCR4 was not detected under any of these conditions. Despite lack of receptor phosphorylation, calcium mobilization by SDF-1 in DeltaCyto CXCR4 cells was partially desensitized by prior treatment with SDF-1. Of interest, the rapid release of calcium was inhibited without affecting the sustained calcium elevation, indicating independent regulatory pathways for these processes. PMA completely inhibited phosphoinositide hydrolysis and calcium mobilization in Wt CXCR4 but only partially inhibited these responses in DeltaCyto CXCR4. cAMP also partially inhibited these responses in both Wt CXCR4 and DeltaCyto CXCR4. SDF-1, PMA, and cAMP caused phosphorylation of phospholipase Cbeta3 in Wt and DeltaCyto CXCR4 cells. Both SDF-1 as well as PMA induced rapid internalization of Wt CXCR4. SDF-1 but not PMA induced internalization of DeltaCyto CXCR4 albeit at reduced levels relative to Wt CXCR4. These results indicate that signaling and internalization of CXCR4 are regulated by receptor phosphorylation dependent and independent mechanisms. Desensitization of CXCR4 signaling, independent of receptor phosphorylation, appears to be a consequence of the phosphorylation of phospholipase Cbeta3.

Subtype-specific differences in subcellular localization of alpha1-adrenoceptors: chlorethylclonidine preferentially alkylates the accessible cell surface alpha1-adrenoceptors irrespective of the subtype

Selective inactivation of alpha1B-adrenoceptor (AR) by the site-directed alkylating agent chlorethylclonidine (CEC) has been used as one of major pharmacological criteria to subclassify alpha1-AR; however, the mechanism for the differential CEC sensitivity of the two subtypes is uncertain, and the extent of CEC inactivation varies depending on the treatment employed. In this study, we examined the correlation between the subcellular localization of alpha1-AR subtypes (alpha1A and alpha1B) and CEC sensitivity. Constructing alpha1-AR tagged with the FLAG epitope at the amino terminus and/or green fluorescent protein (GFP) at the carboxyl terminus, we examined the subcellular distribution of alpha1-ARs expressed in COS-7 cells. Flow cytometry analysis showed that most populations of GFP-expressing alpha1B-AR cells, but very few GFP-expressing alpha1A-AR cells, were detected by the anti-amino terminus antibodies. The immunocytochemical and GFP-fluorescence confocal micrographs showed that alpha1A-ARs predominantly localize intracellularly, whereas alpha1B-ARs localize on the cell surface. Furthermore, CEC (10 microM) treatment of intact cells resulted in an inactivation of approximately 42% of alpha1A-ARs and 93% of alpha1B-ARs, whereas treatment of the membrane preparations resulted in an inactivation of approximately 83% of alpha1A-ARs and 88% of alpha1B-ARs, respectively. Together, the results showed that a hydrophilic alkylating agent CEC preferentially inactivates alpha1-AR on the cell surface irrespective of its subtype, and that the subtype-specific subcellular localization rather than the receptor structure is a major determinant for CEC inactivation of alpha1-AR. Subtype-specific subcellular localization suggests an additional class of functional properties that provide new insight into drug action.

Activation of endothelin ETA receptors induces phosphorylation of alpha1b-adrenoreceptors in Rat-1 fibroblasts

The effect of endothelin-1 on the phosphorylation of alpha1b-adrenoreceptors, transfected into rat-1 fibroblasts, was studied. Basal alpha1b-adrenoreceptor phosphorylation was markedly increased by endothelin-1, norepinephrine, and phorbol esters. The effect of endothelin-1 was dose dependent (EC50 approximately 1 nM), reached its maximum 5 min after stimulation, and was inhibited by BQ-123, an antagonist selective for ETA receptors. Endothelin-1-induced alpha1b-adrenoreceptor phosphorylation was attenuated by staurosporine or genistein and essentially abolished when both inhibitors were used together. The effect of norepinephrine was not modified by either staurosporine or genistein alone, and it was only partially inhibited when both were used together. These data suggest the participation of protein kinase C and tyrosine kinase(s) in endothelin-1-induced receptor phosphorylation. However, phosphoaminoacid analysis revealed the presence of phosphoserine and traces of phosphothreonine, but not of phosphotyrosine, suggesting that the putative tyrosine kinase(s), activated by endothelin, could act in a step previous to receptor phosphorylation. The effect of endothelin-1 on alpha1b-adrenoreceptor phosphorylation was not mediated through pertussis toxin-sensitive G proteins. Calcium mobilization induced by norepinephrine was diminished by endothelin-1. Norepinephrine and endothelin-1 increased [35S]GTPgammaS binding to control membranes. The effect of norepinephrine was abolished in membranes obtained from cells pretreated with endothelin-1. Interestingly, genistein plus staurosporine inhibited this effect of the endothelial peptide. Endothelin-1 did not induce alpha1b-adrenoreceptor internalization. Our data indicate that activation of ETA receptors by endothelin-1 induces alpha1b-adrenoreceptor phosphorylation and alters G protein coupling.

Carboxyl terminus of delta opioid receptor is required for agonist-dependent receptor phosphorylation

The wild-type delta opioid receptor (DOR) and a carboxyl terminus-truncated mutant DOR lacking the last 31 amino acids (DOR-T) were expressed in neuroblastoma x glioma hybrid NG108-15 cells to investigate the role of the carboxyl terminus of DOR in agonist-dependent receptor phosphorylation. Stimulation of the cells with delta specific agonists significantly induced DOR phosphorylation whereas no phosphorylation of DOR-T was detected under the same conditions. Neither overexpression of G protein-coupled receptor kinases (GRK2 or GRK5) nor activation of protein kinase C promoted agonist-induced phosphorylation of DOR-T, in contrast to their strong stimulatory effect on the agonist-dependent phosphorylation of DOR. Furthermore, DOR-T failed to be internalized after agonist stimulation, probably due to its inability to be phosphorylated. Our results indicate that the carboxyl terminus of DOR is required for agonist-dependent receptor phosphorylation and the phosphorylation site(s) of DOR is likely located at its carboxyl terminus.

Role of protein kinase C and carboxyl-terminal region in acute desensitization of vasopressin V1a receptor

The role of protein kinase C activation and carboxyl-terminal region in rapid desensitization of the vasopressin V1a receptor was investigated in Xenopus oocytes. Preincubation of the oocytes with vasopressin or with the diacylglycerol analog 1-oleoyl-2-acetyl-sn-glycerol (OAG), or direct injection of active protein kinase C, all blunted the calcium response of the V1a receptor. Truncation of the 51 terminal amino acids (S374STOP) modified neither the intracellular calcium response to vasopressin nor its desensitization by vasopressin or OAG. These data suggest that desensitization of the V1a receptor is mediated by PKC activation and that its carboxyl-terminal domain is not required for signal transduction and rapid desensitization.

A novel interaction between adrenergic receptors and the alpha-subunit of eukaryotic initiation factor 2B

The alpha-subunit of eukaryotic initiation factor 2B (eIF-2B), a guanine nucleotide exchange protein that functions in regulation of translation, was observed to associate with the carboxyl-terminal cytoplasmic domains of the alpha2A- and alpha2B-adrenergic receptors in a yeast two-hybrid screen of a cDNA library prepared from 293 cells. This protein association was confirmed in vitro by affinity chromatography and was shown to be specific for a subset of G protein-coupled receptors, including the alpha2A-, alpha2B-, alpha2C-, and beta2-adrenergic receptors, but not the vasopressin (V2) receptor. Association of these proteins in vivo was confirmed by specific co-immunoprecipitation of eIF-2Balpha with full-length beta2-adrenergic receptors expressed in transfected 293 cells and by fluorescence microscopy showing co-localization of these proteins in intact cells. Remarkably, eIF-2Balpha co-localized with receptors exclusively in regions of the plasma membrane that are in contact with the extracellular medium, but failed to associate with membranes making cell-cell contacts. Overexpression of eIF-2Balpha in 293 cells caused a small (approximately 15%) but significant enhancement of beta2-adrenergic receptor-mediated activation of adenylyl cyclase, without affecting forskolin or V2 receptor-mediated activation. These observations suggest a new role for a previously identified guanine nucleotide exchange protein in membrane biology and cell signaling.

Phosphorylation of phospholipase C-coupled receptors

Early work on G-protein-coupled receptor (GPCR) phosphorylation focused on the adenylyl cyclase-linked beta-adrenoceptor, where phosphorylation at sites on the C-terminal tail and within the third intracellular loop results in receptor desensitisation. In recent years, intense research activity has revealed that a large number of GPCR subtypes exist as phosphoproteins, where the level of phosphorylation is dramatically increased subsequent to receptor stimulation. Among these receptor subtypes are those receptors coupled to phospholipase C (PLC). It appears, therefore, that regulation via receptor phosphorylation is a mechanism employed by all but a few GPCRs, including those coupled to PLC. Because the majority of GPCRs are coupled to the phosphoinositide signalling pathway, receptor phosphorylation of PLC-coupled receptors is a regulatory process with profound physiological significance for a huge array of biological responses. This review discusses the properties of homologous and heterologous phosphorylation of PLC-coupled receptors, together with the receptor kinases involved and the functional significance of receptor phosphorylation.

Role of the C terminus in histamine H2 receptor signaling, desensitization, and agonist-induced internalization

To evaluate the role of the histamine H2 receptor C terminus in signaling, desensitization, and agonist-induced internalization, canine H2 receptors with truncated C termini were generated. Wild-type (WT) and truncated receptors were tagged at their N termini with a hemagglutinin (HA) epitope and expressed in COS7 cells. Most of the C-terminal intracellular tail could be truncated (51 of 70 residues, termed T308 mutant) without loss of functions: cAMP production, tiotidine binding, and plasma membrane targeting. In fact, the T308 mutant produced more cAMP than the WT when cell-surface expression per cell was equivalent. Pretreatment of cells with 10(-5) M histamine desensitized cAMP productions via WT and T308 receptors to similar extents. Incubation of cells expressing WT receptors with 10(-5) M histamine reduced cell-surface anti-HA antibody binding by approximately 30% (by 30 min, t1/2 approximately 15 min), but did not affect the Bmax of tiotidine in membrane fractions, which represents total receptor amounts, suggesting that WT receptors were internalized from the cell surface. In contrast, no internalization was observed with T308 receptors following histamine treatment. A mutant with a deletion of the 30 C-terminal amino acids, termed T329, was functional but was as potent as the WT in terms of cAMP production. Apart from being desensitized by histamine, the internalization of the receptor was indistinguishable from that of the WT. Internalization was observed in the T320 but not in T313 mutant, narrowing the region involved in internalization to that between Glu314 and Asn320 (ETSLRSN). Of these seven residues, either Thr315, Ser316, or both, were replaced with Ala. Thr315 and Ser316 are conserved among species. The mutation at Thr315 (but not that at Ser316) abolished internalization. Taken together, these results demonstrate that Thr315 is involved in agonist-induced internalization. Furthermore, the finding that T308 receptors were desensitized in the absence of internalization suggests that internalization and desensitization are meditated by independent mechanisms.

An X-linked NDI mutation reveals a requirement for cell surface V2R expression

Function and biochemical properties of the V2 vasopressin receptor (V2R) mutant R337ter, identified in patients suffering from X-linked recessive nephrogenic diabetes insipidus, were investigated by expression in COS.M6 or HEK293 cells. Binding assays and measurements of adenylyl cyclase activity failed to detect function for the truncated receptor, although metabolic labeling demonstrated normal levels of protein synthesis. ELISA assays performed on cells expressing the receptors tagged at the amino terminus with the HA epitope failed to detect V2R R337ter on the plasma membrane. Treatment with endoglycosidase H revealed that the receptor was present only as a precursor form because the mature R337ter V2R, resistant to endoglycosidase H treatment, was not detected. The precursor of V2R-R337ter had a longer half-life than that of the wild type V2R, suggesting that arrested maturation may slow the degradation of the precursor. Unrelated experiments had demonstrated that V2R-G345ter, containing eight additional amino acids, was expressed on the plasma membrane and functioned normally. Receptor truncations longer than 337ter revealed that four of the eight amino acids identified initially provided the minimum length required for the protein to acquire cell surface expression. This was shown by the production of mature receptor (V2R-341ter) detectable in SDS-PAGE, which mediated arginine vasopressin stimulation of adenylyl cyclase activity and bound ligand. In addition, the identity of amino acid 340 was found to play a role in this phenomenon. In conclusion, these data demonstrate that the V2R R337ter is nonfunctional because it does not reach the plasma membrane and that the minimal protein length required for translocation of the V2R to the cell surface is sufficient to confer function to the receptor protein. They also suggest the existence of a protein quality control in the endoplasmic reticulum independent of glycosylation.

Ligand-induced desensitization of the human CXC chemokine receptor-2 is modulated by multiple serine residues in the carboxyl-terminal domain of the receptor

We have characterized the ligand-enhanced phosphorylation of the CXC chemokine receptor-2 (CXCR2) in a series of clonal 3ASubE cell lines expressing receptors truncated or mutated in the carboxyl-terminal domain. Truncation of CXCR2 by substitution of a stop codon for Ser-342 (342T) or Ser-331 (331T) results in total loss of melanoma growth stimulatory activity/growth-related protein (MGSA/GRO)-enhanced receptor phosphorylation, which cannot be explained based upon altered ligand binding affinity or receptor number. 3ASubE cells expressing 342T or CXCR2 with mutation of Ser-342, -346, -347, and -348 to alanine (4A) exhibit strong mobilization of Ca2+ in response to ligand (interleukin-8 or MGSA/GRO), with a recovery phase significantly slower than that of cells expressing wild type (WT) CXCR2. In contrast to the WT CXCR2, which is 93% desensitized by 20 nM ligand, the 331T, 342T, and 4A CXCR2 mutants do not undergo significant ligand-induced desensitization, and respond to a second ligand challenge by mobilizing Ca2+. The 3ASubE cells expressing CXCR2 with mutation of Ser-346, -347, and -348 to alanine, or with mutation of only one serine in this domain, continue to be phosphorylated in response to ligand and are 60-70% desensitized following the initial ligand challenge. WT CXCR2 phosphorylation and desensitization occur in <1 min, while receptor sequestration is a much later event (30-60 min). However, mutant receptors that are neither phosphorylated nor desensitized in response to ligand are <10% sequestered 60 min following ligand challenge. These data demonstrate for the first time that ligand binding to CXCR2 results in receptor phosphorylation, desensitization, and sequestration and that serine residues 342 and 346-348 participate in the desensitization and sequestration processes.

The long cytoplasmic carboxyl terminus of the prostaglandin E2 receptor EP4 subtype is essential for agonist-induced desensitization

The 488-amino acid human prostaglandin E2 receptor EP4 subtype, which couples to stimulation of adenylyl cyclase, shares the major structural features of G protein-coupled receptors, having seven putative transmembrane domains, an extracellular amino terminus, and a cytoplasmic carboxyl terminus. The latter is composed of 156 amino acids and contains 38 serine and threonine residues, which are potential phosphorylation sites. The carboxyl terminus may be important in receptor function; in some receptors, truncation of the cytoplasmic tail abolishes desensitization. In others, truncation leads to constitutive activity, and in other receptors, truncation has no effect on receptor function. To investigate the role of the long cytoplasmic tail of the EP4 receptor, we constructed a mutant EP4 that lacks the last 138 amino acids at the carboxyl terminus, including 36 serine and threonine residues. The truncated EP4 receptor was stably expressed in Chinese hamster ovary cells at levels comparable to that of the wild-type receptor and exhibited a Kd value for [3H]PGE2 binding similar to that of the wild-type receptor. PGE2-mediated adenylyl cyclase activity as a function of PGE2 concentration was similar in cells expressing the wild-type and truncated EP4 receptors. Neither the wild-type receptor nor the truncated form showed any constitutive activity. However, the wild-type EP4 receptor underwent PGE2-induced desensitization fully within 15-20 min, whereas the truncated EP4 receptor, lacking 36 of the 38 carboxyl-terminal serines and threonines, displayed a sustained activation. Despite the continuous presence of PGE2, the rate of cAMP synthesis via stimulation of the truncated receptor remained constant over > or = 20 min. These findings suggest that the cytoplasmic tail of EP4 plays an important role in agonist-induced desensitization.

Functional consequences of the phosphorylation of the gonadotropin receptors

When target cells are exposed to a hormone, their responsiveness wanes with time, in spite of the continuous presence of the hormone. This phenomenon, referred to as desensitization, is due to regulatory steps that occur at the level of the hormone receptor as well as at post-receptor steps. While post-receptor events may be specific for the metabolic pathways activated in a given target cell, receptor events are more general in nature and conserved within a given family of receptors. There are at least two categories of regulatory events that occur at the level of the receptor and contribute to the process of desensitization. One of them, referred to as receptor uncoupling, will be used here to denote a change in the functional properties of a constant number of receptors resulting in a reduction in the ability of the receptor to activate its most proximal effector system. The other, referred to as receptor down-regulation, will be used here to denote a reduction in the density of cell surface receptors. Uncoupling is generally faster than down-regulation, and is believed to be due to post-translational modifications of the receptor. Down-regulation, on the other hand, is slower, and could be due to an increase in the rate of receptor internalization, sequestration or degradation, and/or to a decrease in the rate of receptor externalization, processing or synthesis. In this paper, recent studies from my laboratory designed to directly address the potential involvement of gonadotropin receptor phosphorylation in the process of uncoupling have been reviewed.

Amino acids of the alpha1B-adrenergic receptor involved in agonist binding: differences in docking catecholamines to receptor subtypes

Site-directed mutagenesis and molecular dynamics analysis of the 3-D model of the alpha1B-adrenergic receptor (AR) were combined to identify the molecular determinants of the receptor involved in catecholamine binding. Our results indicate that the three conserved serines in the fifth transmembrane domain (TMD) of the alpha1B-AR play a distinct role in catecholamine binding versus receptor activation. In addition to the amino acids D125 in TMDIII and S207 in TMDV directly involved in ligand binding, our findings identify a large number of polar residues playing an important role in the activation process of the alpha1B-AR thus providing new insights into the structure/function relationship of G protein-coupled receptors.

Cross-desensitization among receptors for platelet activating factor and peptide chemoattractants. Evidence for independent regulatory pathways

Cross-desensitization among receptors for peptide chemoattractants have been shown to involve two independent processes, receptor phosphorylation and inhibition of phospholipase C (PLC) activation. Receptors for lipid chemoattractants, i.e. platelet activating factor (PAF) and leukotriene B4, did not inhibit the responses of peptide chemoattractant receptors, suggesting distinct signaling pathways. To examine cross-desensitization between receptors for lipid and peptide chemoattractants, cDNA encoding the PAF receptor (PAFR) was co-expressed into RBL-2H3 cells with cDNAs encoding receptors for either formylated peptides (FR), a product of the fifth component of complement (C5aR) or interleukin-8 A (IL-8RA). PAFR was homologously phosphorylated and desensitized by PAF, and cross-phosphorylated and cross-desensitized by fMet-Leu-Phe, C5a, and IL-8. In contrast, the receptors for peptide chemoattractants were neither cross-phosphorylated nor cross-desensitized by PAF. Staurosporine blocked cross-phosphorylation and cross-desensitization of the PAFR by peptide chemoattractants. Truncation of the cytoplasmic tail of PAFR (mPAFR) abolished its homologous and cross-phosphorylation. mPAFR was also resistant to cross-desensitization by peptide chemoattractants at the level of PLC activation. Interestingly, mPAFR mediated a sustained Ca2+ mobilization in response to PAF and was more active in inducing GTPase activity, phosphoinositide hydrolysis, secretion, and phospholipase D activation than the wild type PAFR. In contrast to PAFR, stimulation of the mPAFR cross-phosphorylated and cross-desensitized responses to IL-8RA. As expected, FR, which is resistant to cross-phosphorylation by C5aR and IL-8RA, was not phosphorylated by mPAFR. However, unlike C5aR and IL-8RA, mPAFR did not inhibit the ability of FR to activate PLC. Blocking Ca2+ influx inhibited mPAFR-mediated sustained Ca2+ response, phospholipase D activation and secretion, but not phosphoinositide hydrolysis and cross-phosphorylation and cross-desensitization of IL-8RA. The data herein suggest that cross-desensitization of PAFR by peptide chemoattractants is solely due to receptor phosphorylation. The PAFR and the peptide chemoattractant receptors do not cross-regulate each other at the level of PLC, suggesting distinct regulatory pathways.

Role of receptor phosphorylation in desensitization and internalization of the secretin receptor

The secretin receptor is prototypic of a recently described family of G protein-coupled receptors. We recently demonstrated its phosphorylation in response to agonist stimulation and elimination of this covalent modification by C-terminal truncation (F. Ozcelebi et al. (1995) Mol. Pharmacol. 48, 818-824). Here, we explore the functional impact of receptor phosphorylation and structural determinants for desensitization by comparing receptor behavior after agonist exposure in cell lines expressing wild-type and truncated receptor. To characterize receptor internalization, a novel fluorescent full agonist, [rat secretin-27]-Gly-rhodamine, was developed, which bound specifically and with high affinity. Both receptor constructs bound secretin normally, leading to normal G protein coupling and cAMP accumulation and prompt receptor internalization. Exposure to 10 nM secretin for 5 min or 12 h prior to washing and restimulation with a full range of concentrations demonstrated absent cAMP responses in wild-type receptor-bearing cells and responses 25 to 30% of control and shifted 1 order of magnitude to the right in the truncated receptor-bearing cells. Thus, the major mechanism of desensitization was phosphorylation-independent receptor internalization. Phosphorylation was associated with a distinct process that likely represents interference with G protein coupling, manifest as a reduced rate of cAMP stimulation. Thus, dual distinct mechanisms of desensitization exist in the secretin receptor family that should help protect receptor-bearing cells from overstimulation.

Acute regulation of the receptor-mediated phosphoinositide signal transduction pathway

It is apparent that the phosphoinositide signalling pathway is subject to a variety of regulatory features which will ultimately dictate the magnitude and profile of cellular responses to agonist occupation of PIC-linked receptors. Our understanding of these mechanisms is far from complete but will be crucial in revealing both the specificity of receptor signalling and the integration of signals arising from the potentially wide variety of metabotropic and ionotropic receptors on individual cells.

Role of the C terminus of the interleukin 8 receptor in signal transduction and internalization

Interleukin 8 (IL-8) is a potent neutrophil chemoattractant and activator. Two IL-8 receptor subtypes, A and B, are expressed in neutrophils. In this work, we analyzed the role of the C terminus domain of the IL-8 receptor on the signal transduction and receptor internalization mechanisms. The IL-8 receptor A was tagged with an epitope corresponding to the monoclonal antibody 1D4 to monitor the localization of the IL-8 receptor. We demonstrated IL-8-dependent receptor internalization by monitoring the density of surface 125I-labeled IL-8 binding sites and by immunofluorescence microscopy. Truncation of the last 27 amino acids of the IL-8 receptor A severely impaired the IL-8-induced internalization of the receptor. Of importance was the observation that binding of IL-8 to receptors A and B triggered a dramatically faster rate of internalization of receptor B than receptor A, suggesting that the heterologous C termini among receptor subtypes modulate the rate of internalization of IL-8 receptors. However, substitution of the C terminus of the receptor subtype A for the C terminus of receptor B reduced the internalization rate of receptor A. Furthermore, we found that the rate of internalization of IL-8 receptor B triggered by IL-8 was faster than the one induced by the IL-8-related peptide, melanoma growth stimulatory activity. Studies with human neutrophils pretreated with 100 nM IL-8 for 5 min revealed a positive and a negative calcium response mediated by receptors A and B, respectively. In contrast, neutrophils pretreated with melanoma growth stimulatory activity showed positive calcium responses to both receptors A and B. These data suggest that the neutrophil responses mediated by IL-8 are modulated by the rate of internalization of receptors.

Effect of different G protein-coupled receptor kinases on phosphorylation and desensitization of the alpha1B-adrenergic receptor

The alpha1B-adrenergic receptor (alpha1BAR), its truncated mutant T368, different G protein-coupled receptor kinases (GRK) and arrestin proteins were transiently expressed in COS-7 or HEK293 cells alone and/or in various combinations. Coexpression of beta-adrenergic receptor kinase (betaARK) 1 (GRK2) or 2 (GRK3) could increase epinephrine-induced phosphorylation of the wild type alpha1BAR above basal as compared to that of the receptor expressed alone. On the other hand, overexpression of the dominant negative betaARK (K220R) mutant impaired agonist-induced phosphorylation of the receptor. Overexpression of GRK6 could also increase epinephrine-induced phosphorylation of the receptor, whereas GRK5 enhanced basal but not agonist-induced phosphorylation of the alpha1BAR. Increasing coexpression of betaARK1 or betaARK2 resulted in the progressive attenuation of the alpha1BAR-mediated response on polyphosphoinositide (PI) hydrolysis. However, coexpression of betaARK1 or 2 at low levels did not significantly impair the PI response mediated by the truncated alpha1BAR mutant T368, lacking the C terminus, which is involved in agonist-induced desensitization and phosphorylation of the receptor. Similar attenuation of the receptor-mediated PI response was also observed for the wild type alpha1BAR, but not for its truncated mutant, when the receptor was coexpressed with beta-arrestin 1 or beta-arrestin 2. Despite their pronounced effect on phosphorylation of the alpha1BAR, overexpression of GRK5 or GRK6 did not affect the receptor-mediated response. In conclusion, our results provide the first evidence that betaARK1 and 2 as well as arrestin proteins might be involved in agonist-induced regulation of the alpha1BAR. They also identify the alpha1BAR as a potential phosphorylation substrate of GRK5 and GRK6. However, the physiological implications of GRK5- and GRK6-mediated phosphorylation of the alpha1BAR remain to be elucidated.

Identification of a novel receptor kinase that phosphorylates a phospholipase C-linked muscarinic receptor

Phosphorylation of G-protein-linked receptors is thought to play a central role in receptor regulation and desensitization. Unlike the case of the extensively studied beta-adrenergic receptor/adenylate cyclase pathway, in which receptor-specific phosphorylation is known to be mediated by beta-adrenergic receptor kinase ( beta-ARK), the kinases responsible for phosphorylation of phospholipase C-linked receptors have yet to be identified, although a role for beta-ARK has been implicated. This study describes the purification of a novel 40-kDa receptor kinase from porcine cerebellum that is able to phosphorylate the phospholipase C-linked m3-muscarinic receptor in an agonist-dependent manner. The assay for kinase activity was based on the ability of the kinase to phosphorylate a bacterial fusion protein, Ex-m3, containing amino acids Ser345-Leu463 of the third intracellular loop of the m3-muscarinic receptor. Purification of the muscarinic receptor kinase from a high speed supernatant fraction of porcine cerebellum was achieved using the following steps: (i) 30-60% ammonium sulfate cut and successive chromatography on (ii) butyl-Sepharose (iii) Resource Q, (iv) Resource S, and (v) heparin-Sepharose. The purified protein kinase represented an approximately 18,600-fold purification and was a single polypeptide with a molecular weight of approximately 40 kDa. Based on the chromatographic mobility, molecular weight, and kinase inhibitor studies, the kinase, designated MRK, was shown to be distinct from previously characterized second messenger regulated protein kinases, beta-ARK, and other members of the G-protein-linked receptor kinase family. It therefore represents a new class of receptor kinase.

Desensitization and internalization of the m2 muscarinic acetylcholine receptor are directed by independent mechanisms

The phenomenon of acute desensitization of G-protein-coupled receptors has been associated with several events, including receptor phosphorylation, loss of high affinity agonist binding, receptor:G-protein uncoupling, and receptor internalization. However, the biochemical events underlying these processes are not fully understood, and their contributions to the loss of signaling remain correlative. In addition, the nature of the kinases and the receptor domains which are involved in modulation of activity have only begun to be investigated. In order to directly measure the role of G-protein-coupled receptor kinases (GRKs) in the desensitization of the m2 muscarinic acetylcholine receptor (m2 mAChR), a dominant-negative allele of GRK2 was used to inhibit receptor phosphorylation by endogenous GRK activity in a human embryonic kidney cell line. The dominant-negative GRK2K220R reduced agonist-dependent phosphorylation of the m2 mAChR by approximately 50% and prevented acute desensitization of the receptor as measured by the ability of the m2 mAChR to attenuate adenylyl cyclase activity. In contrast, the agonist-induced internalization of the m2 mAChR was unaffected by the GRK2K220R construct. Further evidence linking receptor phosphorylation to acute receptor desensitization was obtained when two deletions of the third intracellular loop were made which created m2 mAChRs that did not become phosphorylated in an agonist-dependent manner and did not desensitize. However, the mutant mAChRs retained the ability to internalize. These data provide the first direct evidence that GRK-mediated receptor phosphorylation is necessary for m2 mAChR desensitization; the likely sites of in vivo phosphorylation are in the central portion of the third intracellular loop (amino acids 282-323). These results also indicate that internalization of the m2 receptor is not a key event in desensitization and is mediated by mechanisms distinct from GRK phosphorylation of the receptor.

A single mutation of the neurokinin-2 (NK2) receptor prevents agonist-induced desensitization. Divergent conformational requirements for NK2 receptor signaling and agonist-induced desensitization in Xenopus oocytes

Receptor activation and agonist-induced desensitization of the human neurokinin-2 (NK2) receptor expressed in Xenopus oocytes have been investigated. When neurokinin A (NKA) was applied repeatedly at 5-min intervals, the second and subsequent applications gave no responses. This desensitization was not observed with the specific agonists (Lys3, Gly8-R-gamma-lactam-Leu9)NKA(3-10) (GR64349) or (Nle10)-NKA(4-10). However, in the presence of the protein kinase inhibitor staurosporine, stimulation with GR64349 or (Nle10)-NKA(4-10) induced receptor desensitization. In contrast, the protein kinase C inhibitor Ro-31-8220 was not able to enhance GR64349-mediated desensitization. We created a mutation (F248S) in the third cytoplasmic loop of NK2 that impairs NKA-induced desensitization. In the presence of either staurosporine or Ro-31-8220, the mutant receptor was desensitized in response to NKA application but not to GR64349. Also, truncation mutants delta 62 and delta 87, lacking serine and threonine residues in the cytoplasmic COOH-terminal tail, were functionally active and were partially resistant to desensitization. These observations indicate that 1) there are different conformational requirements for NK2 receptor signalling and agonist-induced desensitization, 2) the third intracellular loop and the cytoplasmic tail of NK2 are functional domains important for agonist-induced desensitization, and 3) some agonists at the NK2 receptor cause much more desensitization than others and suggest that this might result from phosphorylation by receptor-specific kinases and other non-identified protein kinases.