Salmeterol-induced desensitization, internalization and phosphorylation of the human beta2-adrenoceptor

Asthma and COPD: A Focus on β-Agonists - Past, Present and Future

Asthma has been recognised as a respiratory disorder for millennia and the focus of targeted drug development for the last 120 years. Asthma is one of the most common chronic non-communicable diseases worldwide. Chronic obstructive pulmonary disease (COPD), a leading cause of morbidity and mortality worldwide, is caused by exposure to tobacco smoke and other noxious particles and exerts a substantial economic and social burden. This chapter reviews the development of the treatments of asthma and COPD particularly focussing on the β-agonists, from the isolation of adrenaline, through the development of generations of short- and long-acting β-agonists. It reviews asthma death epidemics, considers the intrinsic efficacy of clinical compounds, and charts the improvement in selectivity and duration of action that has led to our current medications. Important β2-agonist compounds no longer used are considered, including some with additional properties, and how the different pharmacological properties of current β2-agonists underpin their different places in treatment guidelines. Finally, it concludes with a look forward to future developments that could improve the β-agonists still further, including extending their availability to areas of the world with less readily accessible healthcare.

Kinetic analysis of endogenous β2 -adrenoceptor-mediated cAMP GloSensor™ responses in HEK293 cells

BACKGROUND AND AIM

Standard pharmacological analysis of agonist activity utilises measurements of receptor-mediated responses at a set time-point, or at the peak response level, to characterise ligands. However, the occurrence of non-equilibrium conditions may dramatically impact the properties of the response being measured. Here we have analysed the initial kinetic phases of cAMP responses to β2 -adrenoceptor agonists in HEK293 cells expressing the endogenous β2 -adrenoceptor at extremely low levels.

EXPERIMENTAL APPROACH

The kinetics of β2 -adrenoceptor agonist-stimulated cAMP responses were monitored in real-time, in the presence and absence of antagonists, in HEK293 cells expressing the cAMP GloSensor™ biosensor. Potency (EC50 ) and efficacy (Emax ) values were determined at the peak of the agonist GloSensor™ response and compared to kinetic parameters L50 and IRmax values derived from initial response rates.

KEY RESULTS

The partial agonists salbutamol and salmeterol displayed reduced relative IRmax values (with respect to isoprenaline) when compared with their Emax values. Except for the fast dissociating bisoprolol, preincubation with β2 -adrenoceptor antagonists produced a large reduction in the isoprenaline peak response due to a state of hemi-equilibrium in this low receptor reserve system. This effect was exacerbated when IRmax parameters were measured. Furthermore, bisoprolol produced a large reduction in isoprenaline IRmax consistent with its short residence time.

CONCLUSIONS AND IMPLICATIONS

Kinetic analysis of real-time signalling data can provide valuable insights into the hemi-equilibria that can occur in low receptor reserve systems with agonist-antagonist interactions, due to incomplete dissociation of antagonist whilst the peak agonist response is developing.

G Protein-Coupled Receptors in Asthma Therapy: Pharmacology and Drug Action

Asthma is a heterogeneous inflammatory disease of the airways that is associated with airway hyperresponsiveness and airflow limitation. Although asthma was once simply categorized as atopic or nonatopic, emerging analyses over the last few decades have revealed a variety of asthma endotypes that are attributed to numerous pathophysiological mechanisms. The classification of asthma by endotype is primarily routed in different profiles of airway inflammation that contribute to bronchoconstriction. Many asthma therapeutics target G protein-coupled receptors (GPCRs), which either enhance bronchodilation or prevent bronchoconstriction. Short-acting and long-acting β 2-agonists are widely used bronchodilators that signal through the activation of the β 2-adrenergic receptor. Short-acting and long-acting antagonists of muscarinic acetylcholine receptors are used to reduce bronchoconstriction by blocking the action of acetylcholine. Leukotriene antagonists that block the signaling of cysteinyl leukotriene receptor 1 are used as an add-on therapy to reduce bronchoconstriction and inflammation induced by cysteinyl leukotrienes. A number of GPCR-targeting asthma drug candidates are also in different stages of development. Among them, antagonists of prostaglandin D2 receptor 2 have advanced into phase III clinical trials. Others, including antagonists of the adenosine A2B receptor and the histamine H4 receptor, are in early stages of clinical investigation. In the past decade, significant research advancements in pharmacology, cell biology, structural biology, and molecular physiology have greatly deepened our understanding of the therapeutic roles of GPCRs in asthma and drug action on these GPCRs. This review summarizes our current understanding of GPCR signaling and pharmacology in the context of asthma treatment. SIGNIFICANCE STATEMENT: Although current treatment methods for asthma are effective for a majority of asthma patients, there are still a large number of patients with poorly controlled asthma who may experience asthma exacerbations. This review summarizes current asthma treatment methods and our understanding of signaling and pharmacology of G protein-coupled receptors (GPCRs) in asthma therapy, and discusses controversies regarding the use of GPCR drugs and new opportunities in developing GPCR-targeting therapeutics for the treatment of asthma.

G Protein-Coupled Receptor Endocytosis Confers Uniformity in Responses to Chemically Distinct Ligands

The ability of chemically distinct ligands to produce different effects on the same G protein-coupled receptor (GPCR) has interesting therapeutic implications, but, if excessively propagated downstream, would introduce biologic noise compromising cognate ligand detection. We asked whether cells have the ability to limit the degree to which chemical diversity imposed at the ligand-GPCR interface is propagated to the downstream signal. We carried out an unbiased analysis of the integrated cellular response elicited by two chemically and pharmacodynamically diverse β-adrenoceptor agonists, isoproterenol and salmeterol. We show that both ligands generate an identical integrated response, and that this stereotyped output requires endocytosis. We further demonstrate that the endosomal β2-adrenergic receptor signal confers uniformity on the downstream response because it is highly sensitive and saturable. Based on these findings, we propose that GPCR signaling from endosomes functions as a biologic noise filter to enhance reliability of cognate ligand detection.

A luminescent assay for real-time measurements of receptor endocytosis in living cells

Ligand-mediated endocytosis is a key autoregulatory mechanism governing the duration and intensity of signals emanating from cell surface receptors. Due to the mechanistic complexity of endocytosis and its emerging relevance in disease, simple methods capable of tracking this dynamic process in cells have become increasingly desirable. We have developed a bioluminescent reporter technology for real-time analysis of ligand-mediated receptor endocytosis using genetic fusions of NanoLuc luciferase with various G-protein-coupled receptors (GPCRs). This method is compatible with standard microplate formats, which should decrease work flows for high-throughput screens. This article also describes the application of this technology to endocytosis of epidermal growth factor receptor (EGFR), demonstrating potential applicability of the method beyond GPCRs.

Salmeterol Efficacy and Bias in the Activation and Kinase-Mediated Desensitization of β2-Adrenergic Receptors

Salmeterol is a long-acting β2-adrenergic receptor (β2AR) agonist that is widely used as a bronchodilator for the treatment of persistent asthma and chronic obstructive pulmonary disease in conjunction with steroids. Previous studies demonstrated that salmeterol showed weak efficacy for activation of adenylyl cyclase; however, its efficacy in the complex desensitization of the β2AR remains poorly understood. In this work, we provide insights into the roles played by the G protein-coupled receptor kinase/arrestin and protein kinase A in salmeterol-mediated desensitization through bioluminescence resonance energy transfer (BRET) studies of liganded-β2AR binding to arrestin and through kinetic studies of cAMP turnover. First, BRET demonstrated a much reduced efficacy for salmeterol recruitment of arrestin to β2AR relative to isoproterenol. The ratio of BRETISO/BRETSALM after 5-minute stimulation was 20 and decreased to 5 after 35 minutes, reflecting a progressive decline in BRETISO and a stable BRETSALM. Second, to assess salmeterol efficacy for functional desensitization, we examined the kinetics of salmeterol-induced cAMP accumulation (0-30 minutes) in human airway smooth muscle cells in the presence and absence of phosphodiesterase inhibition. Analysis of shaping of cAMP turnover for both agonists demonstrated significant salmeterol desensitization, although it was reduced relative to isoproterenol. Using an isoproterenol rescue protocol after either short-term (10 minutes) or long-term (2 and 14 hours) salmeterol pretreatments, we found that salmeterol progressively depressed isoproterenol stimulation but did not prevent subsequent rescue by isoproterenol and additional isoproterenol-mediated desensitization. Our findings reveal a complex efficacy for functional desensitization, demonstrating that although salmeterol shows weak efficacy for adenylyl cyclase activation and G protein-coupled receptor kinase/arrestin-mediated desensitization, it acts as a strong agonist in highly amplified protein kinase A-mediated events.

Functional desensitization of the β 2 adrenoceptor is not dependent on agonist efficacy

Chronic treatment with β 2 adrenoceptor agonists is recommended as a first-line maintenance therapy for chronic obstructive pulmonary disease (COPD). However, a potential consequence of long-term treatment may be the loss of functional response (tachyphylaxis) over time. In this study, we have investigated the tendency of such agonists, with a range of efficacies, to develop functional desensitization to cAMP responses in primary human bronchial smooth muscle cells following prolonged agonist exposure. The data show that upon repeat exposure, all agonists produced functional desensitization to the same degree and rate. In addition, β 2 adrenoceptor internalization and β-arrestin-2 recruitment were monitored using β 2·eGFP visualization and the PathHunter™ β-arrestin-2 assay, respectively. All agonists were capable of causing robust receptor internalization and β-arrestin-2 recruitment, the rate of which was influenced by agonist efficacy, as measured in those assays. In summary, although a relationship exists between agonist efficacy and the rate of both receptor internalization and β-arrestin-2 recruitment, there is no correlation between agonist efficacy and the rate or extent of functional desensitization.

Development and characterization of pepducins as Gs-biased allosteric agonists

The β2-adrenergic receptor (β2AR) is a prototypical G protein-coupled receptor that mediates many hormonal responses, including cardiovascular and pulmonary function. β-Agonists used to combat hypercontractility in airway smooth muscle stimulate β2AR-dependent cAMP production that ultimately promotes airway relaxation. Chronic stimulation of the β2AR by long acting β-agonists used in the treatment of asthma can promote attenuated responsiveness to agonists and an increased frequency of fatal asthmatic attacks. β2AR desensitization to β-agonists is primarily mediated by G protein-coupled receptor kinases and β-arrestins that attenuate receptor-Gs coupling and promote β2AR internalization and degradation. A biased agonist that can selectively stimulate Gs signaling without promoting receptor interaction with G protein-coupled receptor kinases and β-arrestins should serve as an advantageous asthma therapeutic. To identify such molecules, we screened ∼50 lipidated peptides derived from the intracellular loops of the β2AR, known as pepducins. This screen revealed two classes of Gs-biased pepducins, receptor-independent and receptor-dependent, as well as several β-arrestin-biased pepducins. The receptor-independent Gs-biased pepducins operate by directly stimulating G protein activation. In contrast, receptor-dependent Gs-biased pepducins appear to stabilize a Gs-biased conformation of the β2AR that couples to Gs but does not undergo G protein-coupled receptor kinase-mediated phosphorylation or β-arrestin-mediated internalization. Functional studies in primary human airway smooth muscle cells demonstrate that Gs-biased pepducins are not subject to conventional desensitization and thus may be good candidates for the development of next generation asthma therapeutics. Our study reports the first Gs-biased activator of the β2AR and provides valuable tools for the study of β2AR function.

Pharmacologic characterization of GSK-961081 (TD-5959), a first-in-class inhaled bifunctional bronchodilator possessing muscarinic receptor antagonist and β2-adrenoceptor agonist properties

The objective of the present studies was to characterize the pharmacologic properties of GSK-961081 [TD-5959; (R)-1-(3-((2-chloro-4-(((2-hydroxy-2-(8-hydroxy-2-oxo-1,2-dihydroquinolin-5-yl)ethyl)amino)methyl)-5-methoxyphenyl)amino)-3-oxopropyl) piperidin-4-yl [1,1'-biphenyl]-2-ylcarbamate], a novel first-in-class inhaled bifunctional compound possessing both muscarinic antagonist (MA) and β2-adrenoceptor agonist (BA) properties (MABA). In competition radioligand binding studies at human recombinant receptors, GSK-961081 displayed high affinity for hM2 (Ki = 1.4 nM), hM3 muscarinic receptors (Ki = 1.3 nM) and hβ2-adrenoceptors (Ki = 3.7 nM). GSK-961081 behaved as a potent hβ2-adrenoceptor agonist (EC50 = 0.29 nM for stimulation of cAMP levels) with 440- and 320-fold functional selectivity over hβ1- and hβ3-adrenoceptors, respectively. In guinea pig isolated tracheal tissues, GSK-961081 produced smooth muscle relaxation through MA (EC50 = 50.2 nM), BA (EC50=24.6 nM), and MABA (EC50 = 11 nM) mechanisms. In the guinea pig bronchoprotection assay, inhaled GSK-961081 produced potent, dose-dependent inhibition of bronchoconstrictor responses via MA (ED50 = 33.9 µg/ml), BA (ED50 = 14.1 µg/ml), and MABA (ED50 = 6.4 µg/ml) mechanisms. Significant bronchoprotective effects of GSK-961081 were evident in guinea pigs via MA, BA, and MABA mechanisms for up to 7 days after dosing. The lung selectivity index of GSK-961081 in guinea pigs was 55- to 110-fold greater than that of tiotropium with respect to systemic antimuscarinic antisialagogue effects and was 10-fold greater than that of salmeterol with respect to systemic β2-adrenoceptor hypotensive effects. These preclinical findings studies suggest that GSK-961081 has the potential to be a promising next-generation inhaled lung-selective bronchodilator for the treatment of airway diseases, including chronic obstructive pulmonary disease.

cAMP regulation of airway smooth muscle function

Agonists activating β(2)-adrenoceptors (β(2)ARs) on airway smooth muscle (ASM) are the drug of choice for rescue from acute bronchoconstriction in patients with both asthma and chronic obstructive pulmonary disease (COPD). Moreover, the use of long-acting β-agonists combined with inhaled corticosteroids constitutes an important maintenance therapy for these diseases. β-Agonists are effective bronchodilators due primarily to their ability to antagonize ASM contraction. The presumed cellular mechanism of action involves the generation of intracellular cAMP, which in turn can activate the effector molecules cAMP-dependent protein kinase (PKA) and Epac. Other agents such as prostaglandin E(2) and phosphodiesterase inhibitors that also increase intracellular cAMP levels in ASM, can also antagonize ASM contraction, and inhibit other ASM functions including proliferation and migration. Therefore, β(2)ARs and cAMP are key players in combating the pathophysiology of airway narrowing and remodeling. However, limitations of β-agonist therapy due to drug tachyphylaxis related to β(2)AR desensitization, and recent findings regarding the manner in which β(2)ARs and cAMP signal, have raised new and interesting questions about these well-studied molecules. In this review we discuss current concepts regarding β(2)ARs and cAMP in the regulation of ASM cell functions and their therapeutic roles in asthma and COPD.

Discovery of regulators of receptor internalization with high-throughput flow cytometry

We developed a platform combining fluorogen-activating protein (FAP) technology with high-throughput flow cytometry to detect real-time protein trafficking to and from the plasma membrane in living cells. The hybrid platform facilitates drug discovery for trafficking receptors such as G protein-coupled receptors and was validated with the β₂-adrenergic receptor (β₂AR) system. When a chemical library containing ∼1200 off-patent drugs was screened against cells expressing FAP-tagged β₂ARs, all 33 known β₂AR-active ligands in the library were successfully identified, together with a number of compounds that might regulate receptor internalization in a nontraditional manner. Results indicated that the platform identified ligands of target proteins regardless of the associated signaling pathway; therefore, this approach presents opportunities to search for biased receptor modulators and is suitable for screening of multiplexed targets for improved efficiency. The results revealed that ligands may be biased with respect to the rate or duration of receptor internalization and that receptor internalization may be independent of activation of the mitogen-activated protein kinase pathway.

Formoterol and salmeterol induce a similar degree of β2-adrenoceptor tolerance in human small airways but via different mechanisms

BACKGROUND AND PURPOSE

Steroids prevent and reverse salbutamol-induced β(2)-adrenoceptor tolerance in human small airways. This study examines the effects of the long-acting β(2) agonists (LABAs) formoterol and salmeterol, and the ability of budesonide to prevent desensitization.

EXPERIMENTAL APPROACH

Long-acting β(2) agonists in the presence and absence of budesonide were incubated with human precision-cut lung slices containing small airways. Tolerance was deduced from measurements of reduced bronchodilator responses to isoprenaline and correlated with β(2)-adrenoceptor trafficking using a virally transduced, fluorescent-tagged receptor. The ability of the LABAs to protect airways against muscarinic-induced contraction was also assessed.

KEY RESULTS

Following a 12 h incubation, both formoterol and salmeterol attenuated isoprenaline-induced bronchodilatation to a similar degree and these effects were not reversible by washing. Pre-incubation with budesonide prevented the desensitization induced by formoterol, but not that induced by salmeterol. Formoterol also protected the airways from carbachol-induced bronchoconstriction to a greater extent than salmeterol. In the epithelial cells of small airways, incubation with formoterol promoted receptor internalization but this did not appear to occur following incubation with salmeterol. Budesonide inhibited the formoterol-induced reduction in plasma membrane β(2)-adrenoceptor fluorescence.

CONCLUSIONS AND IMPLICATIONS

Although both formoterol and salmeterol attenuate isoprenaline-induced bronchodilatation, they appear to induce β(2)-adrenoceptor tolerance via different mechanisms; formoterol, but not salmeterol, enhances receptor internalization. Budesonide protection against β(2)-adrenoceptor tolerance was correlated with the retention of receptor fluorescence on the plasma membrane, thereby suggesting a mechanism by which steroids alter β(2)-adrenoceptor function.

Pharmacogenetics of β2 adrenergic receptor gene polymorphisms, long-acting β-agonists and asthma

Adrenergic β2 receptor (ADRβ2) agonists are widely used in asthma. Approximately 10% of patients have severe, poorly controlled disease despite extensive use of ADRβ2 agonists. Variations in responses to ADRβ2 agonists can, in part, be attributed to genetic variation, with 49 different polymorphisms having been identified for the ADRβ2 gene. Although clear associations exist between ADRβ2 gene polymorphisms, such as +46G>A, and patient response, the importance of these polymorphisms remains controversial. Patient selection, the number of polymorphisms analysed, differences in the type/dose of ADRβ2 agonist, use of inhaled corticosteroids and population sizes have all varied. Most studies were limited to mild or moderate asthmatics using ADRβ2 agonists sparingly. It is difficult to extrapolate from these studies to individual patients who have severe asthma, use a variety of ADRβ2 agonists and do so frequently. The extent to which ADRβ2 gene polymorphisms are relevant to asthma management needs further review, both clinically and at the molecular level. In vitro studies have helped to define the functional changes induced by specific ADRβ2 gene polymorphisms, including 3'-untranslated region poly-C repeat. The resulting ADRβ2 gene haplotypes (rather than genotypes), the interactions among ADRβ2 gene haplotypes and variations in the chemistry of different agonists deserve more detailed assessment. Responses to ADRβ2 agonists depend on effective downstream signalling following ADRβ2 activation and also on receptor regulation. Studies on other regulators of ADRβ2 receptor signalling and trafficking may be equally important in understanding the functional role of ADRβ2 gene polymorphisms. The role of ADRβ2 gene polymorphisms in the pathogenesis and management of severe asthma cannot be clearly defined until more specific and targeted research studies are performed.

Anti-mitogenic effects of β-agonists and PGE2 on airway smooth muscle are PKA dependent

Inhaled β-agonists are effective airway smooth muscle (ASM)-relaxing agents that help reverse bronchoconstriction in asthma, but their ability to affect the aberrant ASM growth that also occurs with asthma is poorly understood. β-Agonists exhibit PKA-dependent antimitogenic effects in several cell types. However, recent studies suggest that Epac, and not PKA, mediates the antimitogenic effect of cAMP in both ASM and fibroblasts. This study aims to clarify the role of PKA in mediating the effect of G(s)-coupled receptors on human ASM growth. Pretreatment of ASM cultures with β-agonists albuterol, isoproterenol, or salmeterol (100 nM to 10 μM) caused a significant (∼ 25-30%) inhibition of EGF-stimulated ASM thymidine incorporation and cell proliferation, whereas a much greater inhibition was observed from pretreatment with PGE(2) (75-80%). However, all agents were ineffective in cells expressing GFP chimeras of either PKI (a PKA inhibitor) or a mutant PKA regulatory subunit relative to the control cells expressing GFP. The antimitogenic efficacy of PGE(2) in inhibiting control cultures was associated with greater ability to stimulate sustained PKA activation and greater inhibition of late-phase promitogenic p42/p44 and PI3K activities. These findings suggest that therapeutic approaches enabling superior PKA activation in ASM will be most efficacious in deterring ASM growth.

Agonist efficacy and receptor desensitization: from partial truths to a fuller picture

It has been demonstrated that the degree of agonist-induced desensitization of the beta(2)-adrenoceptor is related to agonist efficacy (strength of signalling), whereby high-efficacy agonists (e.g. formoterol) cause more phosphorylation and internalization of the receptor than low-efficacy agonists (e.g. salmeterol). These early studies, however, used a protocol where agonists were matched for receptor occupancy rather than functional effect. In this issue of the BJP, Duringer and colleagues have extended these studies to compare the ability of agonists to cause desensitization at equi-effective (cAMP signalling) concentrations rather than equal occupancy. Their data and conclusions are quite different from those previously described. After prolonged exposure, all the agonists caused a similar degree of desensitization, whereas a pulse protocol uncovered a greater loss of responsiveness with the low-efficacy ligands. This is consistent with the notion that high-efficacy agonists have 'spare receptors', and are therefore less sensitive to loss of receptors through desensitization. It also reflects experience in the clinic, where both formoterol and salmeterol show a similar early decline in bronchoprotection, after which their effects remain stable. These findings challenge the notion that high-efficacy ligands always cause more functional desensitization.

Agonist-specific patterns of beta 2-adrenoceptor responses in human airway cells during prolonged exposure

BACKGROUND AND PURPOSE

Beta(2)-adrenoceptor agonists (beta(2)-agonists) are important bronchodilators used in the treatment of asthma and chronic obstructive pulmonary disease. At the molecular level, beta(2)-adrenergic agonist stimulation induces desensitization of the beta(2)-adrenoceptor. In this study, we have examined the relationships between initial effect and subsequent reduction of responsiveness to restimulation for a panel of beta(2)-agonists in cellular and in vitro tissue models.

EXPERIMENTAL APPROACH

Beta(2)-adrenoceptor-induced responses and subsequent loss of receptor responsiveness were studied in primary human airway smooth muscle cells and bronchial epithelial cells by measuring cAMP production. Receptor responsiveness was compared at equi-effective concentrations, either after continuous incubation for 24 h or after a 1 h pulse exposure followed by a 23 h washout. Key findings were confirmed in guinea pig tracheal preparations in vitro.

KEY RESULTS

There were differences in the reduction of receptor responsiveness in human airway cells and in vitro guinea pig trachea by a panel of beta(2)-agonists. When restimulation occurred immediately after continuous incubation, loss of responsiveness correlated with initial effect for all agonists. After the 1 h pulse exposure, differences between agonists emerged, for example isoprenaline and formoterol induced the least reduction of responsiveness. High lipophilicity was, to some extent, predictive of loss of responsiveness, but other factors appeared to be involved in determining the relationships between effect and subsequent loss of responsiveness for individual agonists.

CONCLUSIONS AND IMPLICATIONS

There were clear differences in the ability of different beta(2) agonists to induce loss of receptor responsiveness at equi-effective concentrations.

Salmeterol restores secretory functions in cystic fibrosis airway submucosal gland serous cells

The activity of the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) can be mediated by surface G protein-coupled receptors such as the beta(2)-adrenergic receptor. In this study, we explored the effect of a long-acting beta(2)-adrenergic agonist, salmeterol, on the CFTR-dependent secretory capacity of a human CF tracheal gland serous cell line (CF-KM4), homozygous for the delF508 mutation. We showed that, compared with the untreated CF serous cells, a 24-hour pre-incubation period with 200 nM salmeterol induced an 83% increase in delF508-CFTR-mediated chloride efflux. The restoration of the bioelectric properties is associated with increased apical surface pool of delF508-CFTR. Salmeterol induced a decrease in ion concentration and an increase in the level of hydration of the mucus packaged inside the CF secretory granules. The effects of salmeterol are not associated with a persistent production of cAMP. Western blotting on isolated secretory granules demonstrated immunoreactivity for CFTR and lysozyme. In parallel, we measured by atomic force microscopy an increased size of secretory granules isolated from CF serous cells compared with non-CF serous cells (MM39 cell line) and showed that salmeterol was able to restore a CF cell granule size similar to that of non-CF cells. To demonstrate that the salmeterol effect was a CFTR-dependent mechanism, we showed that the incubation of salmeterol-treated CF serous cells with CFTR-inh172 suppressed the restoration of normal secretory functions. The capacity of salmeterol to restore the secretory capacity of glandular serous cells suggests that it could also improve the airway mucociliary clearance in patients with CF.

Endogenous Gs-coupled receptors in smooth muscle exhibit differential susceptibility to GRK2/3-mediated desensitization

Although G protein-coupled receptor (GPCR) kinases (GRKs) have been shown to mediate desensitization of numerous GPCRs in studies using cellular expression systems, their function under physiological conditions is less well understood. In the current study, we employed various strategies to assess the effect of inhibiting endogenous GRK2/3 on signaling and function of endogenously expressed G s-coupled receptors in human airway smooth muscle (ASM) cells. GRK2/3 inhibition by expression of a Gbetagamma sequestrant, a GRK2/3 dominant-negative mutant, or siRNA-mediated knockdown increased intracellular cAMP accumulation mediated via beta-agonist stimulation of the beta-2-adrenergic receptor (beta 2AR). Conversely, neither 5'-( N-ethylcarboxamido)-adenosine (NECA; activating the A2b adenosine receptor) nor prostaglandin E2 (PGE 2; activating EP2 or EP4 receptors)-stimulated cAMP was significantly increased by GRK2/3 inhibition. Selective knockdown using siRNA suggested the majority of PGE 2-stimulated cAMP in ASM was mediated by the EP2 receptor. Although a minor role for EP3 receptors in influencing PGE 2-mediated cAMP was determined, the GRK2/3-resistant nature of EP2 receptor signaling in ASM was confirmed using the EP2-selective agonist butaprost. Somewhat surprisingly, GRK2/3 inhibition did not augment the inhibitory effect of the beta-agonist on mitogen-stimulated increases in ASM growth. These findings demonstrate that with respect to G s-coupled receptors in ASM, GRK2/3 selectively attenuates beta 2AR signaling, yet relief of GRK2/3-dependent beta 2AR desensitization does not influence at least one important physiological function of the receptor.

Similarities and differences in the autonomic control of airway and urinary bladder smooth muscle

The airways and the urinary bladder are both hollow organs serving very different functions, i.e. air flow and urine storage, respectively. While the autonomic nervous system seems to play only a minor if any role in the physiological regulation of airway tone during normal breathing, it is important in the physiological regulation of bladder smooth muscle contraction and relaxation. While both tissues share a greater expression of M2 than of M3 muscarinic receptors, smooth muscle contraction in both is largely mediated by the smaller M3 population apparently involving phospholipase C activation to only a minor if any extent. While smooth muscle in both tissues can be relaxed by beta-adrenoceptor stimulation, this primarily involves beta2-adrenoceptors in human airways and beta3-adrenoceptors in human bladder. Despite activation of adenylyl cyclase by either subtype, cyclic adenosine monophosphate plays only a minor role in bladder relaxation by beta-agonists; an important but not exclusive function is known in airway relaxation. While airway beta2-adrenoceptors are sensitive to agonist-induced desensitization, beta3-adrenoceptors are generally considered to exhibit much less if any sensitivity to desensitization. Gene polymorphisms exist in the genes of both beta2- and beta3-adrenoceptors. Despite being not fully conclusive, the available data suggest some role of beta2-adrenoceptor polymorphisms in airway function and its treatment by receptor agonists, whereas the available data on beta3-adrenoceptor polymorphisms and bladder function are too limited to allow robust interpretation. We conclude that the distinct functions of airways and urinary bladder are reflected in a differential regulation by the autonomic nervous system. Studying these differences may be informative for a better understanding of each tissue.

Embracing emerging paradigms of G protein-coupled receptor agonism and signaling to address airway smooth muscle pathobiology in asthma

G protein-coupled receptors (GPCRs) regulate numerous airway cell functions, and signaling events transduced by GPCRs are important in both asthma pathogenesis and therapy. Indeed, most asthma therapies target GPCRs either directly or indirectly. Within recent years, our understating of how GPCRs signal and are regulated has changed significantly as new concepts have emerged and traditional ideas have evolved. In this review, we discuss current concepts regarding constitutive GPCR activity and receptor agonism, functional selectivity, compartmentalized signaling, and GPCR desensitization. We further discuss the relevance of these ideas to asthma and asthma therapy, while emphasizing their potential application to the GPCR signaling in airway smooth muscle that regulates airway patency and thus disease severity.

A study of antagonist affinities for the human histamine H2 receptor

BACKGROUND AND PURPOSE

Ligand affinity has been a fundamental concept in the field of pharmacology and has traditionally been considered to be constant for a given receptor-ligand interaction. Recent studies have demonstrated that this is not true for all three members of the G(s)-coupled beta-adrenoceptor family. This study evaluated antagonist affinity measurements at a different G(s)-coupled receptor, the histamine H(2) receptor, to determine whether antagonist affinity measurements made at a different family of GPCRs were constant.

EXPERIMENTAL APPROACH

CHO cells stably expressing the human histamine H(2) receptor and a CRE-SPAP reporter were used and antagonist affinity was assessed in short-term cAMP assays and longer term CRE gene transcription assays.

KEY RESULTS

Nine agonists and seven antagonists, of sufficient potency at the H(2) receptor to examine in detail, were identified. Measurements of antagonist affinity were the same regardless of the efficacy of the competing agonist, time of agonist incubation, cellular response measured or presence of a PDE inhibitor.

CONCLUSIONS AND IMPLICATIONS

Antagonist affinity at the G(s)-coupled histamine H(2) receptor obeys the accepted dogma for antagonism at GPCRs. This study further confirms that something unusual is indeed happening with the beta-adrenoceptors and is not an artefact related to the transfected cell system used. As the human histamine H(2) receptor does not behave in a similar manner to any of the human beta-adrenoceptors, it is clear that information gathered from one GPCR cannot be simply extrapolated to predict the behaviour of another GPCR. Each GPCR therefore requires careful and detailed evaluation on its own.

Phosphorylation of the beta2-adrenergic receptor in plasma membranes by intrinsic GRK5

Characterization of the GRKs participating in the phosphorylation of the beta2-adrenergic receptor (beta2AR) have in part been limited by the lack of a simple cell-free assay with membrane-bound beta2AR and GRKs. We describe here a cell-free assay for GRK phosphorylation of the beta2AR in a postnuclear 600g fraction and washed membranes by intrinsic GRK activity using the GRK phosphosite-specific antibody that recognizes pS(355,356). Treatment of these cell-free preparations with 1.0 microM isoproterenol (ISO) caused a rapid maximal 10-15-fold increase in GRK site phosphorylation of the beta2AR (t1/2 = 1 min) with an EC50 for ISO stimulation of approximately 80 nM. Extensively washed plasma membrane fractions retained the 10-15-fold ISO stimulation of GRK site phosphorylation and GRK5 levels while being depleted of GRK2 and GRK6. Stimulation of GRK site phosphorylation by a range of partial agonists correlated well with their intrinsic efficacy for stimulation of adenylyl cyclase. GRK phosphorylation of the beta2AR in the washed membrane fraction caused minimal desensitization of ISO stimulation of adenylyl cyclase activity. Association of GRK5 with the beta2AR in intact cells was demonstrated by a high level of basal BRET2 using beta2AR-Rluc and GRK5-GFP2 that was not diminished by agonist stimulation. BRET2 between the beta2AR-Rluc and GFP2-betaarrestin 2 was increased by agonist, whereas BRET2 between the beta2AR and GRK2-GFP2 was not significant. On the basis of the level of GRK5-mediated phosphorylation we observe in isolated membrane fractions and co-localization of the beta2AR and GRK5, we conclude that GRK5 plays a distinctive role in the phosphorylation of the beta2AR.

Bioluminescence resonance energy transfer as a screening assay: Focus on partial and inverse agonism

The reported data for compound screening with the bioluminescence resonance energy transfer (BRET2) assay is very limited, and several questions remain unaddressed, such as the behavior of agonists. Eleven beta2 adrenergic receptor (beta2-AR) agonists were tested for full or partial agonism in an improved version of the receptor/beta-arrestin2 BRET2 assay and in 2 cyclic adenosine monophosphate (cAMP) assays (column cAMP assay and ALPHAscreen cAMP assay). Tested in the highly sensitive ALPHAscreen cAMP assay, all selected agonists behaved as full agonists, using isoproterenol as a reference compound. In the less sensitive column cAMP assay, ephedrine and dopamine had a clear partial response. For the BRET2 assay, a highly graded picture was obtained. Moreover, beta2-AR antagonists were tested for inverse agonism. Pronounced inverse agonism was detected in the ALPHAscreen cAMP assay. Only marginal inverse agonistic responses were seen for alprenolol and pindolol in the column cAMP assay, and no inverse agonism was seen in the BRET2 assay. For the beta2-AR, the BRET2 assay may be superior for secondary screening of agonists where a separation of full and partial agonists is needed and the ALPHAscreen cAMP assay may be preferred for primary screening of agonists where all receptor activating compounds are desired.

Characterization of beta2-adrenergic receptor dephosphorylation: Comparison with the rate of resensitization

Dephosphorylation of the cyclic AMP-dependent protein kinase (PKA) site phosphoserine 262 and the G protein-coupled receptor kinase (GRK) site phosphoserines 355 and 356 of the beta2-adrenergic receptor (beta2AR) were characterized in both intact human embryonic kidney 293 cells and subcellular fractions and were correlated with the rate of resensitization of isoproterenol stimulation of adenylyl cyclase after treatment with isoproterenol and blockade by antagonist. Dephosphorylation of the PKA site after stimulation with 300 pM isoproterenol occurred with a t(1/2) of 9 min (k = 0.08 +/- 0.016/min) in intact cells in the absence of internalization. Dephosphorylation of the GRK sites in intact cells after treatment with 1.0 microM isoproterenol for 5 min exhibited a lag phase of approximately 5 min, after which dephosphorylation proceeded slowly with a t(1/2) of 18 min (k = 0.039 +/- 0.006/min). Consistent with the slow rate of GRK site dephosphorylation, the phosphatase inhibitors calyculin A and okadaic acid failed to augment phosphorylation in intact cells during continuous agonist stimulation indicating that GRK site dephosphorylation was minimal. However, both inhibited dephosphorylation of the GRK sites after the addition of antagonist. Slow GRK site dephosphorylation after antagonist treatment was also demonstrated by the relative stability of internalized phosphorylated beta2AR in cells as observed both by immunofluorescence microscopy using a phospho-site-specific antibody and by studies of the subcellular localization of the GRK-phosphorylated beta2AR on sucrose gradients that revealed nearly equivalent levels of GRK site phosphorylation in the plasma membrane and vesicular fractions. In addition, dephosphorylation of the GRK sites by intrinsic phosphatase activity occurred only in the heavy vesicle fractions. In contrast to the slow rates of dephosphorylation, the rate of resensitization of isoproterenol stimulation of adenylyl cyclase was 5- and 10-fold faster (k = 0.43 +/- 0.009/min; t(1/2) = 1.6 min), than PKA and GRK site dephosphorylation, respectively, clearly dissociating the rapid phase of resensitization (0-5 min) from dephosphorylation.

Salmeterol stimulation dissociates beta2-adrenergic receptor phosphorylation and internalization

Salmeterol is a long-acting beta(2)-adrenergic receptor (beta(2)AR) agonist commonly used in the treatment of asthma and chronic obstructive pulmonary disease. It differs from other beta-agonists in that it has a very low intrinisic efficacy, especially when compared with the other available long-acting beta-agonist, formoterol. Receptor desensitization and down-regulation has been described with the chronic use of beta-agonists. This effect may not be the same with all beta-agonists and may be related to their stabilization of altered receptor states. The extreme hydrophobicity and high-affinity quasi-irreversible binding of salmeterol have rendered studies examining the mechanisms by which it mediates receptor desensitization, down-regulation, and internalization difficult. We determined the capacity of salmeterol to induce beta(2)AR endocytosis, G protein-coupled receptor kinase (GRK)-site phosphorylation, degradation, and beta-arrestin2 translocation in HEK293 cells as compared with other agonists of varying intrinsic efficacies. Despite stimulating GRK-mediated phosphorylation of Ser355,356 after 30 min and 18 h to an extent similar to that observed with agonists of high intrinsic efficacy, such as epinephrine and formoterol, salmeterol did not induce significant beta(2)AR internalization or degradation and was incapable of stimulating the translocation of enhanced green fluorescent protein-beta-arrestin2 chimera (EGFP-beta-arrestin2) to the cell surface. Salmeterol-induced receptor endocytosis was rescued, at least in part, by the overexpression of EGFP-beta-arrestin2. Our data indicate that salmeterol binding induces an active receptor state that is unable to recruit beta-arrestin or undergo significant endocytosis or degradation despite stimulating considerable GRK-site phosphorylation. Defects in these components of salmeterol-induced receptor desensitization may be important determinants of its sustained bronchodilation with chronic use.

Targeting G protein-coupled receptor signaling in asthma

The complex disease asthma, an obstructive lung disease in which excessive airway smooth muscle (ASM) contraction as well as increased ASM mass reduces airway lumen size and limits airflow, can be viewed as a consequence of aberrant airway G protein-coupled receptor (GPCR) function. The central role of GPCRs in determining airway resistance is underscored by the fact that almost every drug used in the treatment of asthma directly or indirectly targets either GPCR-ligand interaction, GPCR signaling, or processes that produce GPCR agonists. Although many airway cells contribute to the regulation of airway resistance and architecture, ASM properties and functions have the greatest impact on airway homeostasis. The theme of this review is that GPCR-mediated regulation of ASM tone and ASM growth is a major determinant of the acute and chronic features of asthma, and multiple strategies targeting GPCR signaling may be employed to prevent or manage these features.

Role of the G protein-coupled receptor kinase site serine cluster in beta2-adrenergic receptor internalization, desensitization, and beta-arrestin translocation

There is considerable evidence for the role of carboxyl-terminal serines 355, 356, and 364 in G protein-coupled receptor kinase (GRK)-mediated phosphorylation and desensitization of beta(2)-adrenergic receptors (beta(2)ARs). In this study we used receptors in which these serines were changed to alanines (SA3) or to aspartic acids (SD3) to determine the role of these sites in beta-arrestin-dependent beta(2)AR internalization and desensitization. Coupling efficiencies for epinephrine activation of adenylyl cyclase were similar in wild-type and mutant receptors, demonstrating that the SD3 mutant did not drive constitutive GRK desensitization. Treatment of wild-type and mutant receptors with 0.3 nm isoproterenol for 5 min induced approximately 2-fold increases in the EC(50) for agonist activation of adenylyl cyclase, consistent with protein kinase A (PKA) site-mediated desensitization. When exposed to 1 mum isoproterenol to trigger GRK site-mediated desensitization, only wild-type receptors showed significant further desensitization. Using a phospho site-specific antibody, we determined that there is no requirement for these GRK sites in PKA-mediated phosphorylation at high agonist concentration. The rates of agonist-induced internalization of the SD3 and SA3 mutants were 44 and 13%, respectively, relative to that of wild-type receptors, but the SD3 mutant recruited enhanced green fluorescent protein (EGFP)-beta-arrestin 2 to the plasma membrane, whereas the SA3 mutant did not. EGFP-beta-Arrestin2 overexpression triggered a significant increase in the extent of SD3 mutant desensitization but had no effect on the desensitization of wild-type receptors or the SA3 mutant. Expression of a phosphorylation-independent beta-arrestin 1 mutant (R169E) significantly rescued the internalization defect of the SA3 mutant but inhibited the phosphorylation of serines 355 and 356 in wild-type receptors. Our data demonstrate that (i) the lack of GRK sites does not impair PKA site phosphorylation, (ii) the SD3 mutation inhibits GRK-mediated desensitization although it supports some agonist-induced beta-arrestin binding and receptor internalization, and (iii) serines 355, 356, and 364 play a pivotal role in the GRK-mediated desensitization, beta-arrestin binding, and internalization of beta(2)ARs.

G Protein‐Coupled Receptor Internalization Assays in the High‐Content Screening Format

High-content screening (HCS), a combination of fluorescence microscopic imaging and automated image analysis, has become a frequently applied tool to study test compound effects in cellular disease-modeling systems. This chapter describes the measurement of G protein-coupled receptor (GPCR) internalization in the HCS format using a high-throughput, confocal cellular imaging device. GPCRs are the most successful group of therapeutic targets on the pharmaceutical market. Accordingly, the search for compounds that interfere with GPCR function in a specific and selective way is a major focus of the pharmaceutical industry today. This chapter describes methods for the ligand-induced internalization of GPCRs labeled previously with either a fluorophore-conjugated ligand or an antibody directed against an N-terminal tag of the GPCR. Both labeling techniques produce robust assay formats. Complementary to other functional GPCR drug discovery assays, internalization assays enable a pharmacological analysis of test compounds. We conclude that GPCR internalization assays represent a valuable medium/high-throughput screening format to determine the cellular activity of GPCR ligands.

Multiple signaling states of G-protein-coupled receptors

Studies have been amassed in the past several years indicating that an agonist can conform a receptor into an activation state that is dependent upon an intrinsic property of the agonist usually based upon its chemical composition. Theoretically, each different agonist could impart its own unique activation state. Evidence for multiple signaling states for the G-protein-coupled receptors will be reviewed and is derived from many different pharmacological behaviors: efficacy, kinetics, protean agonism, differential desensitization and internalization, inverse agonism, and fusion chimeras. A recent extension of the ternary complex model is suggested by evidence that the different processes that govern deactivation, such as desensitization and internalization, is also regulated by conformers specific to the agonist. Rhodopsin may serve as a primer for the study of multiple activation states. Therapeutic implications that utilize multiple signaling states hold vast promise in the rationale design of drugs.

Mass spectrometric analysis of agonist effects on posttranslational modifications of the beta-2 adrenoceptor in mammalian cells

Posttranslational modifications (PTMs) of the beta-2 adrenoceptor (B2AR) play a fundamental role in receptor regulation by agonists. We have examined the effects of several agonists on net levels of B2AR palmitoylation and phosphorylation using epitope tagging in stably transfected human embryonal kidney (HEK) 293 cells, immunoaffinity purification, and mass spectrometry combined with the method of stable isotope labeling by amino acids in cell culture (SILAC). Palmitoylation of Cys341 was confirmed and did not change detectably after 30 min exposure of cells to saturating concentrations of dopamine, epinephrine, or isoproterenol. However, all of these agonists produced a marked increase in net phosphorylation. Phosphorylation of the third cytoplasmic loop was increased to a similar degree by all three agonists, whereas differences between agonists were observed in net phosphorylation of the carboxyl-terminal cytoplasmic domain (isoproterenol approximately epinephrine >> dopamine). Interestingly, agonist-induced phosphorylation of the carboxyl-terminal cytoplasmic domain was observed exclusively in a proximal portion (between residues 339-369). None of the agonists produced detectable phosphorylation in a distal portion of the cytoplasmic tail, which contains all sites of agonist-induced phosphorylation identified previously by in vitro reconstitution. These results provide insight to agonist-dependent regulation of the B2AR in intact cells, suggest the existence of significant differences in regulatory phosphorylation events occurring between in vitro and in vivo conditions, and outline a general analytical approach to investigate regulated PTM of receptors in mammalian cells.

Functional adrenergic receptor polymorphisms and idiopathic orthostatic intolerance

OBJECTIVES

Idiopathic orthostatic intolerance (IOI) is a common disorder that is characterized by chronic orthostatic symptoms and substantial increases in heart rate and plasma norepinephrine concentrations that are disproportionately high while standing. Several features of the syndrome, including the tachycardia, tremulousness, and exaggerated norepinephrine have been considered potentially due to hypoactive or hyperactive states of adrenergic receptors of the sympathetic nervous system. The aim of this study was therefore to ascertain whether genotypes at eight polymorphic loci within five relevant adrenergic receptor genes (alpha2A, alpha2B, alpha2C, beta1 and beta2) influence the risk for IOI.

METHODS

We studied 80 young men in military service (20 patients with IOI and 60 age-matched controls). All participants underwent a tilt table test including monitoring of blood pressure, heart rate and plasma catecholamines, in the supine position and during 30 min of standing. Genotyping at the eight loci (alpha2ALys251, alpha2BDel301-303, alpha2CDel322-325, beta1Gly49, beta1Arg389, beta2Arg16, beta2Glu27, beta2Ile164) was performed in all participants. Chi-square tests of independence were used to test for associations between IOI and genotype. In addition, an association of the polymorphisms with haemodynamic variables (heart rate, supine and upright blood pressure) was ascertained using one-way variance analysis.

RESULTS

For the beta1Gly49 polymorphism we found a decrease in the risk of IOI among persons who were homozygous (odds ratio, 0.88; 95% confidence interval, 0.81-0.97). In addition, we found an association between beta1Gly49 and decreased heart rate in the upright position, regardless of IOI diagnosis. There were no associations with the other studied polymorphisms and IOI.

CONCLUSIONS

Our current results suggest that the beta1Gly49 polymorphism is protective for IOI. This is likely one of several common genetic loci that may represent modifiers of IOI phenotypes.

Site of action of beta-ligands at the human beta1-adrenoceptor

Antagonist affinity measurements have traditionally been considered important in defining the receptor or receptor subtypes present within cells or tissues. Any change in this value has normally been taken as evidence for the presence of a second receptor. However, highly efficacious ligands induce a time and phosphorylation-dependent change in the beta2-adrenoceptor resulting in 10-fold lower affinity for antagonists. Also the beta1-adrenoceptor is now considered to exist in two different active conformations which are distinguished by their pharmacological properties. In this study, the site of action of a range of beta-agonists and beta-antagonists was determined using the human beta1-adrenoceptor stably expressed in Chinese hamster ovary cells with cyclic AMP response element reporter genes. Adrenaline and noradrenaline were confirmed as having agonist actions via the catecholamine site, whereas all antagonists had higher affinity for the catecholamine rather than secondary site. However, the rank order of affinity for the two sites was different suggesting that they are indeed separate entities. The measurements of antagonist affinity at the catecholamine site, however, were found to depend upon the agonist present. For example, xamoterol, cimaterol, terbutaline, and formoterol agonist responses were more readily antagonized by CGP 20712A[2-hydroxy-5-(2-[{hydroxy-3-(4-[1-methyl-4-trifluoromethyl-2-imidazolyl]phenoxy)propyl}amino]ethoxy)benzamide] than the catecholamine responses themselves. This, however, was not related to agonist efficacy as has previously been reported for the human beta2-adrenoceptor. Therefore, it may be that some agonists (e.g., cimaterol) purely activate the catecholamine site and others purely activate the secondary site (e.g., CGP 12177 [(-)-4-(3-tert-butylamino-2-hydroxypropoxy)-benzimidazol-2-one]), whereas the others (e.g., catecholamines) activate both sites to differing degrees.

Homologous desensitization of calcitonin gene-related peptide-induced relaxation in rat intramural coronary arteries

We investigated the type of desensitization of calcitonin gene-related peptide (CGRP)-induced responses in rat isolated intramural coronary arteries using isometric myograph and FURA-2 technique. In coronary arteries precontracted with 9,11-dideoxy-11alpha,9alpha-epoxymethanoprostaglandin F2alpha (U46619), development of tachyphylaxis to CGRP is characterized by significant attenuation of CGRP-induced maximal reduction in the tension and [Ca2+](i) during the second CGRP concentration-response curve; however, there was no further reduction in the CGRP-induced maximum relaxation during the third CGRP concentration-response curve. There was no sign of tachyphylaxis to CGRP when CGRP concentration-response curves were recorded in 36 mM K+-depolarized coronary arteries contrary to the results obtained in 300 nM U46619-precontracted coronary arteries. Preincubation with colchicine did not prevent the development of tachyphylaxis to CGRP in U46619-precontracted coronary arteries, indicating no role for endocytosis. Development of tachyphylaxis to CGRP was completely abolished by preincubating the coronary arteries with 1 microM RO 31-8220, indicating a role for protein kinases. Pre-exposure of the coronary arteries to isoprenaline or forskolin did not attenuate the CGRP-induced relaxation in these vessels, indicating that the cAMP-protein kinase A (PKA) pathway is not involved. Like CGRP, the coronary arteries developed tachyphylaxis toward isoprenaline during the second exposure. However, there was no sign of tachyphylaxis to either forskolin or dibutyryl cAMP (dbcAMP) during the second exposure. In conclusion, these results suggest that development of tachyphylaxis to CGRP in U46619-precontracted coronary is related to CGRP receptor-mediated activation of protein kinase.

Influence of agonist intrinsic activity on the desensitisation of beta2-adrenoceptor-mediated responses in mast cells

1. The aim of the present study was to determine whether the intrinsic activity of an agonist influences the extent of desensitisation of beta(2)-adrenoceptor-mediated responses in human lung mast cells. 2. The effects of a wide range of beta-adrenoceptor agonists (10(-10)-10(-5) m) on the IgE-mediated release of histamine from mast cells were determined. The intrinsic activity of agonists was established by comparing the maximal inhibitory response (E(max)) of an agonist relative to the maximal response obtained with the full agonist, isoprenaline. The intrinsic activity order for the inhibition of histamine release was isoprenaline (1.0)>formoterol (0.94)>fenoterol (0.89)>terbutaline (0.84)>salbutamol (0.69)>clenbuterol (0.65)>salmeterol (0.30)>dobutamine (0.20). 3. There was a significant (P<0.05) positive correlation (r=0.81) between the extent to which beta-adrenoceptor agonists inhibited histamine release and the degree to which the agonists caused elevations in cAMP in mast cells. 4. Further studies investigated the effects of long-term (24 h) incubation of mast cells with beta-adrenoceptor agonists on the subsequent ability of isoprenaline to inhibit histamine release. At concentrations of agonists selected to occupy a large percentage (88%) of beta(2)-adrenoceptors, there was a significant (P<0.05) correlation (r=0.73) between the relative intrinsic activity of agonists as inhibitors of histamine release and the extent of functional desensitisation induced by the agonists. At lower receptor occupancies, however, there was no correlation between the relative intrinsic activity of agonists and the extent of agonist-induced desensitisation. 5. These data indicate that, under experimental conditions where high receptor occupancies prevail, agonist intrinsic activity influences the extent of desensitisation of beta(2)-adrenoceptor-mediated responses in mast cells.

Temporal characteristics of cAMP response element-mediated gene transcription: requirement for sustained cAMP production

Many clinically used drugs are G-protein-coupled receptor (GPCR) antagonists and are given long-term to prevent receptor activation by endogenous agonists. Most GPCR antagonists are considered to have little agonist efficacy of their own. However, many beta antagonists do stimulate very small beta(2) adrenoceptor-mediated cAMP responses, but these responses become substantial at the level of cAMP response element (CRE)-gene transcription. Here, we compared the temporal characteristics of these beta(2) adrenoceptor-mediated cAMP and CRE-gene transcription responses with ligands of differing agonist efficacy. Within a minute, full agonists (e.g., isoprenaline) stimulated large increases in intracellular and exported cAMP. Very weak partial agonists (e.g., alprenolol) did not increase intracellular cAMP (only stimulating a small export). However, all agonists (regardless of efficacy) stimulated an increase in CRE-gene transcription after a 2-h incubation. An initial 30-min continual stimulation was required to initiate the process of CRE-gene transcription for all ligands. Longer agonist incubations resulted in larger gene transcription responses in a proportional manner for both weak and full agonists alike, and this was despite the lack of intracellular cAMP detection for the weaker ligands. Thus, the major initiator for CRE-gene transcription was not cAMP concentration or total quantity generated but a sustained turnover of intracellular cAMP and hence sustained stimulation of CREB phosphorylation. Thus, long-acting agonists and long-term treatments with very weak partial agonists (including many drugs classified previously as antagonists based on traditional second-messenger assays, e.g., several clinically used "beta-blockers") may cause more substantial gene transcription than previously believed.

Characterization of agonist stimulation of cAMP-dependent protein kinase and G protein-coupled receptor kinase phosphorylation of the beta2-adrenergic receptor using phosphoserine-specific antibodies

Agonist-stimulated desensitization of the beta2-adrenergic receptor (beta2AR) is caused by both a potent cAMP-dependent protein kinase (PKA)-mediated phosphorylation and a less potent, occupancy-dependent, G protein-coupled receptor kinase (GRK)-mediated phosphorylation that leads to beta-arrestin binding and internalization. In this study the kinetics of phosphorylation of the third intracellular loop PKA site Ser262 and the putative C-tail GRK sites Ser355, Ser356 of the human beta2AR overexpressed in human embryonic kidney (HEK) 293 cells were characterized using phosphoserine-specific antibodies. Specificity of the antibodies was shown by their lack of reactivity with mutant beta2ARs lacking the respective sites. In addition, overexpression of GRK2 and GRK5 increased basal levels of phosphorylation of the GRK sites Ser355, Ser356 in both COS-7 and HEK 293 cells. Epinephrine, prostaglandin E1, and forskolin at maximum concentrations stimulated phosphorylation of the beta2AR PKA site (Ser262) by 4-fold, whereas PMA stimulated it by 2-fold. Epinephrine stimulated PKA site phosphorylation with an EC50 of 20 to 40 pM. In contrast, epinephrine stimulated GRK site phosphorylation (Ser355,Ser356) with an EC50 of 200 nM (1-min treatments), which is more than 4000-fold higher relative to PKA site phosphorylation, consistent with an occupancy-driven process. After 10 to 30 min, the EC50 for epinephrine stimulation of GRK site phosphorylation was reduced to 10 to 20 nM but was still approximately 200-fold greater than for the PKA site. The EC50 for internalization correlated with GRK site phosphorylation and showed a similar shift with time of epinephrine stimulation. The kinetics of epinephrine-stimulated GRK site phosphorylation were not altered in a mutant of the beta2AR lacking the PKA consensus sites. The initial levels (2 min) of a range of agonist-stimulated GRK site phosphorylations were correlated with their efficacy for activation of adenylyl cyclase, namely epinephrine > or = formoterol = fenoterol > terbutaline = zinterol = albuterol > salmeterol > dobutamine > or = ephedrine. However, after 20 to 30 min of treatment, agonists with intermediate strengths, such as albuterol and salmeterol, stimulate GRK site phosphorylations that are approximately equal to that produced by epinephrine, and the correlation breaks down. The GRK and PKA site antibodies were also effective in detecting phosphorylation of the endogenous beta2AR expressed in A431 human epidermoid carcinoma cells. To summarize, our results show a remarkable amplification of PKA site phosphorylation relative to the putative GRK site phosphorylation, heterologous stimulation of the PKA site phosphorylation, no dependence of GRK site phosphorylation on PKA sites, and a reasonable correlation of initial levels of GRK site phosphorylation with the strength of a range of agonists.

Desensitisation of mast cell beta2-adrenoceptor-mediated responses by salmeterol and formoterol

1. The long-acting beta(2)-adrenoceptor agonist formoterol (10(-10)-10(-6) m) inhibited the IgE-dependent release of histamine from human lung mast cells in a concentration-dependent manner. Formoterol was more potent and a full agonist relative to the nonselective beta-adrenoceptor agonist isoprenaline. By contrast, the long-acting beta(2)-adrenoceptor agonist salmeterol (10(-10)-10(-6) m) was about two-thirds less efficacious than either formoterol or isoprenaline as an inhibitor of histamine release. 2. Isoprenaline, formoterol and salmeterol (all at 10(-5) m) increased total cell cAMP levels in mast cells over basal by 361+/-90 (P<0.05), 321+/-89 (P<0.05) and 64+/-24% (P>0.05), respectively. 3. Long-term (24 h) incubation of mast cells with formoterol (10(-6) m) or salmeterol (10(-6) m) essentially abolished the subsequent ability of isoprenaline to inhibit histamine release. Both formoterol and salmeterol were more effective at inducing the functional desensitisation than isoprenaline (10(-6) m) or the short-acting beta(2)-adrenoceptor agonist salbutamol (10(-6) m). 4. The desensitisation induced by long-term treatments with salmeterol and formoterol was specific for beta(2)-adrenoceptor-mediated inhibition of histamine release as the inhibitory effects of alternative cAMP-elevating compounds, prostaglandin E(2), a receptor-mediated activator of adenylate cyclase, and forskolin, a direct activator of adenylate cyclase, were unaffected by desensitising treatments. 5. Radioligand binding studies were performed to determine beta(2)-adrenoceptor density in cell membranes after pretreatment (24 h) of cells with agonists. Isoprenaline, formoterol and salmeterol (all at 10(-6) m) reduced beta(2)-adrenoceptor density by 13+/-5 (P>0.05), 49+/-13 (P<0.05) and 35+/-17% (P>0.05), respectively. 6. These data indicate that long-term exposure of mast cells to both salmeterol and formoterol can cause substantial levels of desensitisation to beta(2)-adrenoceptor-mediated responses in mast cells.

Influence of agonist efficacy and receptor phosphorylation on antagonist affinity measurements: differences between second messenger and reporter gene responses

The ability of an antagonist to bind to a receptor is an innate property of that ligand-receptor chemical interaction. Provided no change in the antagonist or receptor chemical nature occurs, this affinity should remain constant for a given antagonist-receptor interaction, regardless of the agonists used. This fundamental assumption underpins the classification of receptors. Here, measurements of beta2-adrenoceptor-mediated cAMP accumulation and cAMP response-element (CRE)-mediated reporter-gene transcription revealed differences in antagonist affinity that depended upon agonist incubation time and the efficacy of the competing agonist. In cAMP accumulation studies (10-min agonist incubation), antagonist affinities were the same regardless of the agonist used. The CRE-reporter gene assay (5 h of incubation) antagonist affinities were 10-fold lower in the presence of isoprenaline and adrenaline than when salbutamol or terbutaline were present (e.g., log KD propranolol -8.65 +/- 0.08, n = 22, and -9.68 +/- 0.07, n = 17, for isoprenaline and salbutamol-induced responses, respectively). Isoprenaline and adrenaline were more efficacious in functional studies, and their ability to internalize GFP-tagged human beta2-adrenoceptors. Longer-term cAMP studies also showed significant differences in KD values moving toward that seen with gene transcription. Agonist-dependent differences in antagonist affinity were reduced for reporter-gene responses when a phosphorylation-deficient mutant of the beta2-adrenoceptor was used. This study suggests that high-efficacy agonists induce a chemical modification in beta2-adrenoceptors (via phosphorylation) that reduces antagonist affinities. Because reporter-gene assays are used for high-throughput screening in drug discovery, less efficacious or partial agonists may be more reliable than highly efficacious agonists when reporter-gene techniques are used to estimate antagonist affinity.

Cyclic AMP-mobilizing agents and glucocorticoids modulate human smooth muscle cell migration

Hyperplasia and cell migration of smooth muscle are features of both airway and pulmonary vascular diseases. The precise cellular and molecular mechanisms that regulate smooth muscle migration in the lungs remain unknown. In this study, we examined the effect of cAMP-mobilizing agents and steroids on smooth muscle cell migration. Platelet-derived growth factor (PDGF), transforming growth factor-alpha, vascular endothelial growth factor, and basic fibroblast growth factor significantly stimulated cell migration in pulmonary vascular smooth muscle (PVSM) cells. Airway smooth muscle (ASM) migration was also stimulated by PDGF, transforming growth factor-alpha, and basic fibroblast growth factor, but vascular endothelial growth factor was without effect. Interestingly, the smooth muscle mitogen thrombin did not stimulate migration of either cell type. Agents capable of elevating intracellular cAMP inhibited basal (unstimulated) cell migration in both cell types, whereas their effects on PDGF-stimulated migration were more variable. Prostaglandin E2, salmeterol, and the phosphodiesterase type 4 inhibitor cilomolast inhibited basal ASM and PVSM migration by 30-60%. Prostaglandin E2 and cilomolast also inhibited PDGF-stimulated migration of ASM and PVSM cells, but salmeterol was without effect. Preincubation of ASM cells with dexamethasone or fluticasone inhibited basal and PDGF-stimulated migration, and enabled an inhibitory effect of salmeterol on PDGF-induced cell migration. Steroids alone did not stimulate cAMP production or cAMP/PKA-dependent gene transcription (CRE-Luc activity), but slightly augmented salmeterol-stimulated CRE-Luc activity. Collectively, these findings demonstrate that cAMP-mobilizing agents and steroids modulate human smooth muscle cell migration, likely by distinct mechanisms.

Signaling and regulation of G protein-coupled receptors in airway smooth muscle

Signaling through G protein-coupled receptors (GPCRs) mediates numerous airway smooth muscle (ASM) functions including contraction, growth, and "synthetic" functions that orchestrate airway inflammation and promote remodeling of airway architecture. In this review we provide a comprehensive overview of the GPCRs that have been identified in ASM cells, and discuss the extent to which signaling via these GPCRs has been characterized and linked to distinct ASM functions. In addition, we examine the role of GPCR signaling and its regulation in asthma and asthma treatment, and suggest an integrative model whereby an imbalance of GPCR-derived signals in ASM cells contributes to the asthmatic state.

Beta(2)-adrenergic receptor lacking the cyclic AMP-dependent protein kinase consensus sites fully activates extracellular signal-regulated kinase 1/2 in human embryonic kidney 293 cells: lack of evidence for G(s)/G(i) switching

Stimulation of the beta(2)-adrenergic receptor (beta(2)AR) in human embryonic kidney (HEK) 293 cells causes a transient activation of Extracellular Signal-Regulated Kinase (ERK) 1/2. One of the mechanisms proposed for this activation is a PKA-mediated phosphorylation of the beta(2)AR that switches receptor coupling from G(s) to G(i) and triggers internalization of the receptor. To examine these phenomena, we characterized agonist activation of ERK1/2 in HEK293 cells by the endogenous beta(2)AR and in HEK293 cells stably overexpressing either the wild-type beta(2)AR or a substitution mutant beta(2)AR (PKA(-)) that lacks the cyclic AMP-dependent protein kinase (PKA) consensus phosphorylation sites (S261A, S262A and S345A, S346A). As the baseline, we established that epinephrine stimulation of the endogenous beta(2)AR in HEK293 cells (20-30 fmol/mg) caused a rapid and transient activation of ERK1/2 with an EC(50) of 5 to 6 nM. In contrast, the potency of epinephrine stimulation of ERK1/2 in cells stably overexpressing WTbeta(2)AR and PKA(-) (2-4 pmol of beta(2)AR/mg) was increased by over 100-fold relative to HEK293 cells, the EC(50) values being 20 to 60 pM. The nearly identical 100-fold shift in EC(50) for ERK1/2 activation in the PKA(-) and WTbeta(2)AR relative to that in the HEK293 showed that the PKA(-) are fully capable of activating ERK1/2. We also found maximal activation of ERK1/2 in the overexpressing cell lines at concentrations of epinephrine that cause no internalization (i.e., the EC(50) for internalization was 75 nM). Pertussis toxin pretreatment caused only a weak inhibition of epinephrine activation of ERK1/2 in the HEK293 (7-16%) and no inhibition in the PKA(-) cells. Finally we found that the Src family kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (10 microM) caused a >90% inhibition of epinephrine or forskolin activation of ERK1/2 in both cell lines. Our results indicate that the dominant mechanism of beta(2)AR activation of ERK1/2 does not require PKA phosphorylation of the beta(2)AR, receptor internalization or switching from activation of G(s) to G(i) but clearly requires activation of a Src family member that may be downstream of PKA.

Pharmacological characterization of CGP 12177 at the human beta(2)-adrenoceptor

1 It has recently been reported that CGP 12177 can act as an agonist at a novel secondary site within the human beta(1)-adrenoceptor. The aim of this study was to undertake a detailed pharmacological study of the effects of CGP 12177 on the human beta(2)-adrenoceptor. 2 CGP 12177 acted as a potent partial agonist of (3)H-cyclic AMP accumulation (log EC(50)-8.90+/-0.06) and CRE-mediated reporter gene transcription (log EC(50)-9.66+/-0.04) in CHO-K1 cells expressing the human beta(2)-adrenoceptor. These CGP-induced responses were antagonized by the beta(2)-selective antagonist ICI 118551 (apparent log K(D) values of -8.84+/-0.15 and -9.51+/-0.02 for the cyclic AMP and reporter gene responses respectively). 3 CGP 12177 was also able to antagonize both cyclic AMP and reporter gene responses to more efficacious beta(2)-agonists with similar log K(D) values (e.g. -9.57+/-0.15 and -10.04+/-0.096 respectively with salbutamol as agonist). 4 (3)H-CGP 12177 binding to beta(2)-adrenoceptors in intact CHO-beta(2) cells yielded a log K(D) value of -9.84+/-0.06, but indicated that the ligand dissociates very slowly from the receptor (t(1/2) for dissociation=65 min). However, studies with a Green Fluorescent Protein (GFP)-tagged beta(2)-adrenoceptor indicated that CGP 12177 does not stimulate beta(2)-adrenoceptor internalization. 5 This study demonstrates that CGP 12177 is a high affinity partial agonist of both cAMP accumulation and CRE-mediated gene transcription at the human beta(2)-adrenoceptor. It provides no evidence that CGP 12177 can discriminate a secondary site on the beta(2)-adrenoceptor analogous to that observed for the human beta(1)-adrenoceptor. However, despite its very weak actions on cAMP accumulation, the potent agonist effects of CGP 12177 on CRE-mediated gene transcription at the human beta(2)-adrenoceptor, coupled with its long duration of action, offers a potential lead for drug development for the treatment of chronic inflammatory airway diseases.

Prolonged isoproterenol infusion impairs the ability of beta(2)-agonists to increase alveolar liquid clearance

We determined if prolonged isoproterenol (Iso) infusion in rats impaired the ability of the beta(2)-adrenergic agonist terbutaline to increase alveolar liquid clearance (ALC). We infused rats with Iso (at rates of 4, 40, or 400 microg.kg(-1).h(-1)) or vehicle (0.001 N HCl) for 48 h using subcutaneously implanted miniosmotic pumps. After this time, the rats were anesthetized, and ALC was determined (by mass-balance after instillation of Ringer lactate containing albumin into the lungs) under baseline conditions and after terbutaline administration. Baseline and terbutaline-stimulated ALC in vehicle-infused rats averaged, respectively, 19.6 +/- 1.2% (SE) and 44.7 +/- 1.5%/h. The ability of terbutaline to increase ALC was eliminated at 400 microg.kg(-1).h(-1)Iso, inhibited by 26% at 40 microg.kg(-1).h(-1) Iso, and was not affected by 4 microg.kg(-1).h(-1) Iso. beta-adrenergic receptor (betaAR) density of freshly isolated alveolar epithelial type II (ATII) cells from Iso-infused rats was reduced by the 40 and 400 microg.kg(-1).h(-1) infusion rates. These data demonstrate that prolonged exposure to beta-agonists can impair the ability of beta(2)-agonists to stimulate ALC and produce ATII cell betaAR downregulation.