Beta-arrestins specifically constrain beta2-adrenergic receptor signaling and function in airway smooth muscle

Allosteric modulator potentiates β2AR agonist-promoted bronchoprotection in asthma models

Asthma is a chronic inflammatory disease associated with episodic airway narrowing. Inhaled β2-adrenergic receptor (β2AR) agonists (β2-agonists) promote - with limited efficacy - bronchodilation in asthma. All β2-agonists are canonical orthosteric ligands that bind the same site as endogenous epinephrine. We recently isolated a β2AR-selective positive allosteric modulator (PAM), compound-6 (Cmpd-6), which binds outside of the orthosteric site and modulates orthosteric ligand functions. With the emerging therapeutic potential of G-protein coupled receptor allosteric ligands, we investigated the impact of Cmpd-6 on β2AR-mediated bronchoprotection. Consistent with our findings using human β2ARs, Cmpd-6 allosterically potentiated β2-agonist binding to guinea pig β2ARs and downstream signaling of β2ARs. In contrast, Cmpd-6 had no such effect on murine β2ARs, which lack a crucial amino acid in the Cmpd-6 allosteric binding site. Importantly, Cmpd-6 enhanced β2 agonist-mediated bronchoprotection against methacholine-induced bronchoconstriction in guinea pig lung slices, but - in line with the binding studies - not in mice. Moreover, Cmpd-6 robustly potentiated β2 agonist-mediated bronchoprotection against allergen-induced airway constriction in lung slices obtained from a guinea pig model of allergic asthma. Cmpd-6 similarly enhanced β2 agonist-mediated bronchoprotection against methacholine-induced bronchoconstriction in human lung slices. Our results highlight the potential of β2AR-selective PAMs in the treatment of airway narrowing in asthma and other obstructive respiratory diseases.

The serotonin transporter sustains human brown adipose tissue thermogenesis

Activation of brown adipose tissue (BAT) in humans is a strategy to treat obesity and metabolic disease. Here we show that the serotonin transporter (SERT), encoded by SLC6A4, prevents serotonin-mediated suppression of human BAT function. RNA sequencing of human primary brown and white adipocytes shows that SLC6A4 is highly expressed in human, but not murine, brown adipocytes and BAT. Serotonin decreases uncoupled respiration and reduces uncoupling protein 1 via the 5-HT2B receptor. SERT inhibition by the selective serotonin reuptake inhibitor (SSRI) sertraline prevents uptake of extracellular serotonin, thereby potentiating serotonin's suppressive effect on brown adipocytes. Furthermore, we see that sertraline reduces BAT activation in healthy volunteers, and SSRI-treated patients demonstrate no 18F-fluorodeoxyglucose uptake by BAT at room temperature, unlike matched controls. Inhibition of BAT thermogenesis may contribute to SSRI-induced weight gain and metabolic dysfunction, and reducing peripheral serotonin action may be an approach to treat obesity and metabolic disease.

In silico identification of a β2-adrenoceptor allosteric site that selectively augments canonical β2AR-Gs signaling and function

Activation of β₂-adrenoceptors (β₂ARs) causes airway smooth muscle (ASM) relaxation and bronchodilation, and β₂AR agonists (β-agonists) are front-line treatments for asthma and other obstructive lung diseases. However, the therapeutic efficacy of β-agonists is limited by agonist-induced β₂AR desensitization and noncanonical β₂AR signaling involving β-arrestin that is shown to promote asthma pathophysiology. Accordingly, we undertook the identification of an allosteric site on β₂AR that could modulate the activity of β-agonists to overcome these limitations. We employed the site identification by ligand competitive saturation (SILCS) computational method to comprehensively map the entire 3D structure of in silico-generated β₂AR intermediate conformations and identified a putative allosteric binding site. Subsequent database screening using SILCS identified drug-like molecules with the potential to bind to the site. Experimental assays in HEK293 cells (expressing recombinant wild-type human β₂AR) and human ASM cells (expressing endogenous β₂AR) identified positive and negative allosteric modulators (PAMs and NAMs) of β₂AR as assessed by regulation of β-agonist-stimulation of cyclic AMP generation. PAMs/NAMs had no effect on β-agonist-induced recruitment of β-arrestin to β₂AR- or β-agonist-induced loss of cell surface expression in HEK293 cells expressing β₂AR. Mutagenesis analysis of β₂AR confirmed the SILCS identified site based on mutants of amino acids R131, Y219, and F282. Finally, functional studies revealed augmentation of β-agonist-induced relaxation of contracted human ASM cells and bronchodilation of contracted airways. These findings identify a allosteric binding site on the β₂AR, whose activation selectively augments β-agonist-induced Gs signaling, and increases relaxation of ASM cells, the principal therapeutic effect of β-agonists.

A cryptic mode of GPCR regulation revealed

Over three decades of research have provided thorough insights into G protein-coupled receptor (GPCR) regulation. In a recent issue of Molecular Cell, Fonseca et al. identified a previously overlooked desensitization mechanism. Agonist activation of the β2-adrenoceptor (β2AR) causes its S-nitrosylation that is required for the receptor to internalize and desensitize. Eliminating β2AR S-nitrosylation by mutation of C265 augments β2AR protein kinase A signaling, enables β2AR nitric oxide (NO) signaling, renders mice resistant to bronchoconstriction, and protects mice from allergen-induced asthma.

β2 -Adrenoceptor agonist profiling reveals biased signalling phenotypes for the β2 -adrenoceptor with possible implications for the treatment of asthma

BACKGROUND AND PURPOSE

β-Adrenoceptor agonists relieve airflow obstruction by activating β2 -adrenoceptors, which are G protein-coupled receptors (GPCRs) expressed on human airway smooth muscle (HASM) cells. The currently available β-adrenoceptor agonists are balanced agonists, however, and signal through both the stimulatory G protein (Gs )- and β-arrestin-mediated pathways. While Gs signalling is beneficial and promotes HASM relaxation, β-arrestin activation is associated with reduced Gs efficacy. In this context, biased ligands that selectively promote β2 -adrenoceptor coupling to Gs signalling represent a promising strategy to treat asthma. Here, we examined several β-adrenoceptor agonists to identify Gs -biased ligands devoid of β-arrestin-mediated effects.

EXPERIMENTAL APPROACH

Gs -biased ligands for the β2 -adrenoceptor were identified by high-throughput screening and then evaluated for Gs interaction, Gi interaction, cAMP production, β-arrestin interaction, GPCR kinase (GRK) phosphorylation of the receptor, receptor trafficking, ERK activation, and functional desensitization of the β2 -adrenoceptor.

KEY RESULTS

We identified ractopamine, dobutamine, and higenamine as Gs -biased agonists that activate the Gs /cAMP pathway upon β2 -adrenoceptor stimulation while showing minimal Gi or β-arrestin interaction. Furthermore, these compounds did not induce any receptor trafficking and had reduced GRK5-mediated phosphorylation of the β2 -adrenoceptor. Finally, we observed minimal physiological desensitization of the β2 -adrenoceptor in primary HASM cells upon treatment with biased agonists.

CONCLUSION AND IMPLICATIONS

Our work demonstrates that Gs -biased signalling through the β2 -adrenoceptor may prove to be an effective strategy to promote HASM relaxation in the treatment of asthma. Such biased compounds may also be useful in identifying the molecular mechanisms that determine biased signalling and in design of safer drugs.

β-arrestin-2 predicts the clinical response to β-blockers in cirrhotic portal hypertension patients: A prospective study

BACKGROUND

Portal hypertension, a common complication associated with liver cirrhosis, can result in variceal bleeding, which greatly impacts patient survival. Recently, β-arrestin-2 has been shown to predict the acute hemodynamic response to nonselective β-blocker therapy for cirrhotic portal hypertension. However, more data is needed on the long-term effects of and changes in β-arrestin-2 following nonselective β-blocker therapy.

AIM

To investigate the expression and role of β-Arrestin-2 in predicting the long-term response to nonselective β-blockers in cirrhotic portal hypertensive patients.

METHODS

We prospectively enrolled 91 treatment-naïve patients with cirrhotic portal hypertension. Baseline clinical and laboratory data were obtained. Gastroscopy was performed for grading and treating varices and obtaining gastric antral biopsies. We measured the serum and antral expression of β-arrestin-2 and obtained Doppler measurement of the portal vein congestion index. Treatment with nonselective β-blockers was then started. The patients were followed up for 18 mo, after which they have undergone a repeat antral biopsy and re-evaluation of the portal vein congestion index.

RESULTS

A higher serum level and antral expression of β-arrestin-2 was associated with longer bleeding-free intervals, greater reduction in the portal vein congestion index, and improved grade of varices. Among patients with a low β-arrestin-2 expression, 17.6% were nonselective β-blocker responders, whereas, among those with high expression, 95.1% were responders (P < 0.001). A serum β-arrestin-2 value ≥ 2.23 ng/mL was associated with a lower likelihood of variceal bleeding (90% sensitivity and 71% specificity). β-arrestin-2 expression significantly decreased after nonselective β-blocker therapy.

CONCLUSION

β-arrestin-2 expression in cirrhotic portal hypertension predicts the clinical response to long-term nonselective β-blocker treatment. Serum β-arrestin-2 is a potential noninvasive biomarker for selecting the candidate patients for nonselective β-blockers.

Phenylalanine 193 in Extracellular Loop 2 of the β 2-Adrenergic Receptor Coordinates β-Arrestin Interaction

G protein-coupled receptors (GPCRs) transduce a diverse variety of extracellular stimuli into intracellular signaling. These receptors are the most clinically productive drug targets at present. Despite decades of research on the signaling consequences of molecule-receptor interactions, conformational components of receptor-effector interactions remain incompletely described. The β ₂-adrenergic receptor (β ₂AR) is a prototypical and extensively studied GPCR that can provide insight into this aspect of GPCR signaling thanks to robust structural data and rich pharmacopeia. Using bioluminescence resonance energy transfer -based biosensors, second messenger assays, and biochemical techniques, we characterize the properties of β ₂AR-F193A. This single point mutation in extracellular loop 2 of the β ₂AR is sufficient to intrinsically bias the β ₂AR away from β-arrestin interaction and demonstrates altered regulatory outcomes downstream of this functional selectivity. This study highlights the importance of extracellular control of intracellular response to stimuli and suggests a previously undescribed role for the extracellular loops of the receptor and the extracellular pocket formed by transmembrane domains 2, 3, and 7 in GPCR regulation that may contribute to biased signaling at GPCRs. SIGNIFICANCE STATEMENT: The role of extracellular G protein-coupled receptor (GPCR) domains in mediating intracellular interactions is poorly understood. We characterized the effects of extracellular loop mutations on agonist-promoted interactions of GPCRs with G protein and β-arrestin. Our studies reveal that F193 in extracellular loop 2 in the β₂-adrenergic receptor mediates interactions with G protein and β-arrestin with a biased loss of β-arrestin binding. These results provide new insights on the role of the extracellular domain in differentially modulating intracellular interactions with GPCRs.

Ivy leaves dry extract EA 575® mediates biased β2-adrenergic receptor signaling

BACKGROUND

β₂-adrenergic receptor (β₂-AR) stimulation activates the G protein/cAMP pathway, which is opposed by the GRK2/β-arrestin 2 pathway. The latter is undesirable in the treatment of respiratory diseases.

HYPOTHESIS/PURPOSE

EA 575® is capable of mediating a biased β₂-adrenergic signaling pathway.

METHODS

The impact of the ivy leaves dry extract EA 575® on β₂-adrenergic signaling was tested in a dynamic mass redistribution assay in HEK wild-type and in HEK β-arrestin knock-out cells. cAMP formation and recruitment of β-arrestin 2 were investigated using GloSensor™ and PathHunter® assays, respectively. NFκB transcriptional activity was determined in both HEK wild-type as well as HEK β-arrestin knock-out cells.

RESULTS

EA 575® inhibits the recruitment of β-arrestin 2 and thereby enhances G protein/cAMP signaling under β₂-stimulating conditions, as evidenced by a corresponding increase in cAMP formation. While β₂-AR-mediated inhibition of NFκB transcriptional activity is β-arrestin-dependent, EA 575® leads to significant inhibition of NFκB transcriptional activity in β-arrestin knock-out cells and thus via a β-arrestin-independent signaling pathway.

CONCLUSION

EA 575® is the first active phytopharmaceutical ingredient for which biased β₂-adrenergic activation has been described. This shift towards G protein/cAMP signaling provides the molecular basis for the clinically proven efficacy of EA 575® in the treatment of lower respiratory tract diseases. In this light, EA 575® could potentially reduce β-arrestin-mediated adverse effects in new combinatorial therapeutic approaches.

Relaxation Effect of Schisandra Chinensis Lignans on the Isolated Tracheal Smooth Muscle in Rats and Its Mechanism

Schisandra chinensis (S. chinensis) is one of the core drugs used for relieving cough and asthma in traditional Chinese medicine. However, there are few basic studies on the treatment of respiratory diseases with S. chinensis in modern pharmacology, and the material basis and mechanism of its antiasthmatic effect are still unclear. Lignans are the main active components of S. chinensis. The aim of this study was to observe the relaxation effect of S. chinensis lignans (SCL) on the tracheal smooth muscle of rats by in vitro tracheal perfusion experiments, and to explore the mechanism by preincubation with L-type calcium channel blocker verapamil, four potassium channel blockers glibenclamide, tetraethylamine, 4-aminopyridine and barium chloride (BaCl₂), β-adrenoceptor blocker propranolol, nitric oxide synthase inhibitor Nω-nitro-L-arginine methyl ester (L-NAME), and the cyclooxygenase inhibitor indomethacin, respectively. The results showed that SCL (0.25-1.75 mg/mL) reduced the contraction of isolated tracheal smooth muscle induced by acetylcholine, the preincubation with verapamil and glibenclamide could attenuate the relaxation effect, whereas propranolol, 4-aminopyridine, BaCl₂, tetraethylamine, L-NAME, and indomethacin had no such effect. These results suggest that SCL has a significant relaxation effect on the isolated tracheal smooth muscle of rats, and the mechanism may be related to the inhibition of extracellular calcium influx and intracellular calcium release from the sarcoplasmic reticulum, as well as the activation of ATP-sensitive potassium channels. These findings may provide a pharmacological basis for the traditional use of S. chinensis to treat asthma.

RGS proteins, GRKs, and beta-arrestins modulate G protein-mediated signaling pathways in asthma

Asthma is a highly prevalent disorder characterized by chronic lung inflammation and reversible airways obstruction. Pathophysiological features of asthma include episodic and reversible airway narrowing due to increased bronchial smooth muscle shortening in response to external and host-derived mediators, excessive mucus secretion into the airway lumen, and airway remodeling. The aberrant airway smooth muscle (ASM) phenotype observed in asthma manifests as increased sensitivity to contractile mediators (EC₅₀) and an increase in the magnitude of contraction (E_max); collectively these attributes have been termed "airways hyper-responsiveness" (AHR). This defining feature of asthma can be promoted by environmental factors including airborne allergens, viruses, and air pollution and other irritants. AHR reduces airway caliber and obstructs airflow, evoking clinical symptoms such as cough, wheezing and shortness of breath. G-protein-coupled receptors (GPCRs) have a central function in asthma through their impact on ASM and airway inflammation. Many but not all treatments for asthma target GPCRs mediating ASM contraction or relaxation. Here we discuss the roles of specific GPCRs, G proteins, and their associated signaling pathways, in asthma, with an emphasis on endogenous mechanisms of GPCR regulation of ASM tone and lung inflammation including regulators of G-protein signaling (RGS) proteins, G-protein coupled receptor kinases (GRKs), and β-arrestin.

Biased agonism at β-adrenergic receptors

The β-adrenergic receptors (βARs) include three subtypes, β₁, β₂ and β₃. These receptors are widely expressed and regulate numerous physiological processes including cardiovascular and metabolic functions and airway tone. The βARs are also important targets in the treatment of many diseases including hypertension, heart failure and asthma. In some cases, the use of current βAR ligands to treat a disease is suboptimal and can lead to severe side effects. One strategy to potentially improve such treatments is the development of biased agonists that selectively regulate a subset of βAR signaling pathways and responses. Here we discuss the compounds identified to date that preferentially activate a G_s- or β-arrestin-mediated signaling pathway through βARs. Mechanistic insight on how these compounds bias signaling sheds light on the potential development of even more selective compounds that should have increased utility in treating disease.

Phosphodiesterase isoforms and cAMP compartments in the development of new therapies for obstructive pulmonary diseases

The second messenger molecule 3'5'-cyclic adenosine monophosphate (cAMP) imparts several beneficial effects in lung diseases such as asthma, chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF). While cAMP is bronchodilatory in asthma and COPD, it also displays anti-fibrotic properties that limit fibrosis. Phosphodiesterases (PDEs) metabolize cAMP and thus regulate cAMP signaling. While some existing therapies inhibit PDEs, there are only broad family specific inhibitors. The understanding of cAMP signaling compartments, some centered around lipid rafts/caveolae, has led to interest in defining how specific PDE isoforms maintain these signaling microdomains. The possible altered expression of PDEs, and thus abnormal cAMP signaling, in obstructive lung diseases has been poorly explored. We propose that inhibition of specific PDE isoforms can improve therapy of obstructive lung diseases by amplifying specific cAMP signals in discreet microdomains.

Androgen Effects on the Adrenergic System of the Vascular, Airway, and Cardiac Myocytes and Their Relevance in Pathological Processes

INTRODUCTION

Androgen signaling comprises nongenomic and genomic pathways. Nongenomic actions are not related to the binding of the androgen receptor (AR) and occur rapidly. The genomic effects implicate the binding to a cytosolic AR, leading to protein synthesis. Both events are independent of each other. Genomic effects have been associated with different pathologies such as vascular ischemia, hypertension, asthma, and cardiovascular diseases. Catecholamines play a crucial role in regulating vascular smooth muscle (VSM), airway smooth muscle (ASM), and cardiac muscle (CM) function and tone.

OBJECTIVE

The aim of this review is an updated analysis of the role of androgens in the adrenergic system of vascular, airway, and cardiac myocytes. Body. Testosterone (T) favors vasoconstriction, and its concentration fluctuation during life stages can affect the vascular tone and might contribute to the development of hypertension. In the VSM, T increases α1-adrenergic receptors (α ₁-ARs) and decreases adenylyl cyclase expression, favoring high blood pressure and hypertension. Androgens have also been associated with asthma. During puberty, girls are more susceptible to present asthma symptoms than boys because of the increment in the plasmatic concentrations of T in young men. In the ASM, β ₂-ARs are responsible for the bronchodilator effect, and T augments the expression of β ₂-ARs evoking an increase in the relaxing response to salbutamol. The levels of T are also associated with an increment in atherosclerosis and cardiovascular risk. In the CM, activation of α _1A-ARs and β ₂-ARs increases the ionotropic activity, leading to the development of contraction, and T upregulates the expression of both receptors and improves the myocardial performance.

CONCLUSIONS

Androgens play an essential role in the adrenergic system of vascular, airway, and cardiac myocytes, favoring either a state of health or disease. While the use of androgens as a therapeutic tool for treating asthma symptoms or heart disease is proposed, the vascular system is warmly affected.

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.

Agonist-specific desensitization of PGE2-stimulated cAMP signaling due to upregulated phosphodiesterase expression in human lung fibroblasts

Pulmonary fibrosis is characterized by fibroblasts persisting in an activated form, producing excessive fibrous material that destroys alveolar structure. The second messenger molecule cyclic 3',5'-adenosine monophosphate (cAMP) has antifibrotic properties, and prostaglandin E₂ (PGE₂) can stimulate cAMP production through prostaglandin E (EP)₂ and EP₄ receptors. Although EP receptors are attractive therapeutic targets, the effects of long-term exposure to PGE₂ have not been characterized. To determine the effects of long-term exposure of lung fibroblasts to PGE₂, human fetal lung (HFL)-1 cells were treated for 24 h with 100 nM PGE₂ or other cAMP-elevating agents. cAMP levels stimulated by acute exposure to PGE₂ were measured using a fluorescent biosensor. Pretreatment for 24 h with PGE₂ shifted the concentration-response curve to PGE₂ rightward by approximately 22-fold but did not affect responses to the beta-adrenoceptor agonist isoproterenol. Neither isoproterenol nor forskolin pretreatment altered PGE₂ responses, implying that other cAMP-elevating agents do not induce desensitization. Use of EP₂- and EP₄-selective agonists and antagonists suggested that PGE₂-stimulated cAMP responses in HFL-1 cells are mediated by EP₂ receptors. EP₂ receptors are resistant to classical mechanisms of agonist-specific receptor desensitization, so we hypothesized that increased PDE activity mediates the loss of signaling after PGE₂ pretreatment. PGE₂ treatment upregulated messenger RNA for PDE3A, PDE3B, PDE4B, and PDE4D and increased overall PDE activity. The PDE4 inhibitor rolipram partially reversed PGE₂-mediated desensitization and PDE4 activity was increased, but rolipram did not alter responses to isoproterenol. The PDE3 inhibitor cilostazol had minimal effect. These results show that long-term exposure to PGE₂ causes agonist-specific desensitization of EP₂ receptor-stimulated cAMP signaling through the increased expression of PDE isozymes, most likely of the PDE4 family.

Diacylglycerol kinase ζ promotes allergic airway inflammation and airway hyperresponsiveness through distinct mechanisms

Asthma is a chronic allergic inflammatory airway disease caused by aberrant immune responses to inhaled allergens, which leads to airway hyperresponsiveness (AHR) to contractile stimuli and airway obstruction. Blocking T helper 2 (T_H2) differentiation represents a viable therapeutic strategy for allergic asthma, and strong TCR-mediated ERK activation blocks T_H2 differentiation. Here, we report that targeting diacylglycerol (DAG) kinase zeta (DGKζ), a negative regulator of DAG-mediated cell signaling, protected against allergic asthma by simultaneously reducing airway inflammation and AHR though independent mechanisms. Targeted deletion of DGKζ in T cells decreased type 2 inflammation without reducing AHR. In contrast, loss of DGKζ in airway smooth muscle cells decreased AHR but not airway inflammation. T cell-specific enhancement of ERK signaling was only sufficient to limit type 2 airway inflammation, not AHR. Pharmacological inhibition of DGK diminished both airway inflammation and AHR in mice and also reduced bronchoconstriction of human airway samples in vitro. These data suggest that DGK is a previously unrecognized therapeutic target for asthma and reveal that the inflammatory and AHR components of asthma are not as interdependent as generally believed.

Regulation of Airway Smooth Muscle Contraction in Health and Disease

Airway smooth muscle (ASM) extends from the trachea throughout the bronchial tree to the terminal bronchioles. In utero, spontaneous phasic contraction of fetal ASM is critical for normal lung development by regulating intraluminal fluid movement, ASM differentiation, and release of key growth factors. In contrast, phasic contraction appears to be absent in the adult lung, and regulation of tonic contraction and airflow is under neuronal and humoral control. Accumulating evidence suggests that changes in ASM responsiveness contribute to the pathophysiology of lung diseases with lifelong health impacts.Functional assessments of fetal and adult ASM and airways have defined pharmacological responses and signaling pathways that drive airway contraction and relaxation. Studies using precision-cut lung slices, in which contraction of intrapulmonary airways and ASM calcium signaling can be assessed simultaneously in situ, have been particularly informative. These combined approaches have defined the relative importance of calcium entry into ASM and calcium release from intracellular stores as drivers of spontaneous phasic contraction in utero and excitation-contraction coupling.Increased contractility of ASM in asthma contributes to airway hyperresponsiveness. Studies using animal models and human ASM and airways have characterized inflammatory and other mechanisms underlying increased reactivity to contractile agonists and reduced bronchodilator efficacy of β₂-adrenoceptor agonists in severe diseases. Novel bronchodilators and the application of bronchial thermoplasty to ablate increased ASM within asthmatic airways have the potential to overcome limitations of current therapies. These approaches may directly limit excessive airway contraction to improve outcomes for difficult-to-control asthma and other chronic lung diseases.

Specificity of NHERF1 regulation of GPCR signaling and function in human airway smooth muscle

Na+/H+ exchanger regulatory factor 1 (NHERF1; also known as ezrin-radixin-moesin-binding phosphoprotein 50) is a PSD-95, disc large, zona occludens-1 adapter that acts as a scaffold for signaling complexes and cytoskeletal-plasma membrane interactions. NHERF1 is crucial to β-2-adrenoceptor (β2AR)-mediated activation of cystic fibrosis transmembrane conductance regulator (CFTR) in epithelial cells, and NHERF1 has been proposed to mediate the recycling of internalized β2AR back to the cell membrane. In the current study, we assessed the role of NHERF1 in regulating cAMP-mediated signaling and immunomodulatory functions in airway smooth muscle (ASM). NHERF1 knockdown attenuated the induction of (protein kinase A) phospho-vasodilator-stimulated phosphoprotein (p-VASP) by isoproterenol (ISO), prostaglandin E2 (PGE2), or forskolin (FSK) as well as the induction of p-heat shock protein 20 after 4 h of stimulation with ISO and FSK. NHERF1 knockdown fully abrogated the ISO-, PGE2-, and FSK-induced IL-6 gene expression and cytokine production without affecting cAMP-mediated phosphodiesterase 4D (PDE4D) gene expression, phospho-cAMP response element-binding protein (p-CREB), and cAMP response element (CRE)-Luc, or PDGF-induced cyclin D1 expression. Interestingly, NHERF1 knockdown prevented ISO-induced chromatin-binding of the transcription factor CCAAT-enhancer-binding protein-β (c/EBPβ). c/EBPβ knockdown almost completely abrogated the cAMP-mediated IL-6 but not PDE4D gene expression. The differential regulation of cAMP-induced signaling and gene expression in our study indicates a role for NHERF1 in the compartmentalization of cAMP signaling in ASM.-Pera, T., Tompkins, E., Katz, M., Wang, B., Deshpande, D. A., Weinman, E. J., Penn, R. B. Specificity of NHERF1 regulation of GPCR signaling and function in human airway smooth muscle.

Detailed insight on β-adrenoceptors as therapeutic targets

Human G protein-coupled receptors (GPCRs), especially adrenoceptors, play a crucial role in maintaining important physiological activities including cardiovascular and pulmonary functions. Among all adrenoceptors, β-adrenoceptors are the best characterized GPCRs and possess distinctive features as drug targets. Similarly, ligands that activate/deactivate β-adrenoceptors also hold a significant position in the field of biomarker identification and drug discovery. Several studies regarding molecular characterization of the β-adrenoceptor ligands have revealed that ligands with abilities to inhibit basal or intrinsic receptor activity or prevent receptor desensitization are particularly important to efficiently manage detrimental health conditions, including chronic heart failure, asthma, chronic obstructive pulmonary disease, obesity, and diabetes. Given the importance of β-adrenoceptors as molecular targets for many pathological conditions, this review aims to provide a detailed insight on the structural and functional aspects of β-adrenoceptors, with a particular emphasis on their importance as biomarkers and therapeutic targets.

Regulation of ovarian cancer G protein-coupled receptor-1 expression and signaling

Ovarian cancer G protein-coupled receptor 1 (OGR1) is a recently deorphanized G protein-coupled receptor shown to signal in response to low extracellular pH (↓pH_o) or certain benzodiazepines. The pleiotropic nature of OGR1 signaling in human airway smooth muscle (HASM) cells suggests that OGR1 is a potential therapeutic target for the management of obstructive lung diseases. However, the basic pharmacological and regulatory features of OGR1 remain poorly understood. We employed model systems of heterologously expressed [human embryonic kidney 293 (HEK293) cells] or endogenous (HASM) OGR1 to assess changes in expression, subcellular localization, and signaling capabilities following acute or chronic treatment with ↓pH_o or the benzodiazepines lorazepam and sulazepam. In HEK293 cells expressing OGR1, treatment with ↓pH_o and/or lorazepam, but not sulazepam, caused rapid OGR1 internalization. In HASM cells, acute treatment with ↓pH_o or benzodiazepines did not alter abundance of OGR1 mRNA; however, significant downregulation was observed following chronic treatment. Acute and chronic pretreatment of HASM cells with sulazepam or lorazepam resulted in receptor desensitization as demonstrated by reduced phosphorylation of vasodilator-stimulated phosphoprotein (VASP) or p42/p44 upon rechallenge. Acid (acute but not chronic) pretreatment of HASM cells induced desensitization of OGR1-mediated VASP (but not p42/p44) phosphorylation. In contrast to a recent study reporting OGR1 upregulation and sensitization in cardiac tissue subject to ischemic/acidic insult, chronic OGR1 activation in multiple model systems did not increase OGR1 expression or signaling capacity. The ability to induce OGR1 internalization and desensitization was activator dependent, reflecting the ability of different activators to induce specific receptor confirmations and engagement of specific heterotrimeric G proteins.

Cooperativity of E-prostanoid receptor subtypes in regulating signaling and growth inhibition in human airway smooth muscle

Prostaglandin E2 (PGE₂) is produced in the airway during allergic lung inflammation and both promotes and inhibits features of asthma pathology. These mixed effects relate to 4 E-prostanoid (EP) receptor subtypes (EP1, 2, 3 and 4) expressed at different levels on different resident and infiltrating airway cells. Although studies have asserted both EP2 and EP4 expression in human airway smooth muscle (HASM), a recent study asserted EP4 to be the functionally dominant EP subtype in HASM. Herein, we employ recently-developed subtype-selective ligands to investigate singular or combined EP2 and EP4 receptor activation in regulating HASM signaling and proliferation. The subtype specificity of ONO-AE1-259-01 (EP2 agonist) and ONO-AE1-329 (EP4 agonist) was first demonstrated in human embryonic kidney 293 cells stably expressing different EP receptor subtypes. EP receptor knockdown and subtype-selective antagonists demonstrated EP2 and EP4 receptor responsiveness in HASM cells to the specific ONO compounds, whereas PGE₂ appeared to preferentially signal via the EP4 receptor. Both singular EP2 and EP4 receptor agonists inhibited HASM proliferation, and combined EP2 and EP4 receptor agonism exhibited positive cooperativity in both chronic G_s-mediated signaling and inhibiting HASM proliferation. These findings suggest both EP2 and EP4 are functionally important in HASM, and their combined targeting optimally inhibits airway smooth muscle proliferation.-Michael, J. V. Gavrila, A., Nayak, A. P., Pera, T., Liberato, J. R., Polischak, S. R., Shah, S. D., Deshpande, D. A., Penn, R. B. Cooperativity of E-prostanoid receptor subtypes in regulating signaling and growth inhibition in human airway smooth muscle.

Biased signaling of the proton-sensing receptor OGR1 by benzodiazepines

GPCRs have diverse signaling capabilities, based on their ability to assume various conformations. Moreover, it is now appreciated that certain ligands can promote distinct receptor conformations and thereby bias signaling toward a specific pathway to differentially affect cell function. The recently deorphanized G protein-coupled receptor OGR1 [ovarian cancer G protein-coupled receptor 1 ( GPR68)] exhibits diverse signaling events when stimulated by reductions in extracellular pH. We recently demonstrated airway smooth muscle cells transduce multiple signaling events, reflecting a diverse capacity to couple to multiple G proteins. Moreover, we recently discovered that the benzodiazepine lorazepam, more commonly recognized as an agonist of the γ-aminobutyric acid A (GABAA) receptor, can function as an allosteric modulator of OGR1 and, similarly, can promote multiple signaling events. In this study, we demonstrated that different benzodiazepines exhibit a range of biases for OGR1, with sulazepam selectively activating the canonical Gs of the G protein signaling pathway, in heterologous expression systems, as well as in several primary cell types. These findings highlight the potential power of biased ligand pharmacology for manipulating receptor signaling qualitatively, to preferentially activate pathways that are therapeutically beneficial.-Pera, T., Deshpande, D. A., Ippolito, M., Wang, B., Gavrila, A., Michael, J. V., Nayak, A. P., Tompkins, E., Farrell, E., Kroeze, W. K., Roth, B. L., Panettieri, R. A. Jr Benovic, J. L., An, S. S., Dulin, N. O., Penn, R. B. Biased signaling of the proton-sensing receptor OGR1 by benzodiazepines.

Crosstalk between mAChRM3 and β2AR, via acetylcholine PI3/PKC/PBEP1/Raf-1 MEK1/2/ERK1/2 pathway activation, in human bronchial epithelial cells after long-term cigarette smoke exposure

BACKGROUND

Cigarette smoke extract (CSE) affects the expression of non-neuronal components of cholinergic system in bronchial epithelial cells and, as PEBP1/Raf-mediated MAPK1/2 and ERK1/2 pathway, promotes inflammation and oxidative stress.

AIMS

We studied whether Acetylcholine (ACh) is involved in the mechanism of crosstalk between mAChRM3 and β2Adrenergic receptors (β2AR) promoting, via PI3/PKC/PBEP1/Raf/MEK1/2/ERK1/2 activation, β2AR desensitization, inflammation and, oxidative stress in a bronchial epithelial cell line (16HBE) after long-term exposure to cigarette smoke extract (LECSE).

METHODS

We evaluated mAChRM3 and Choline Acetyltransferase (ChAT) expression, ACh production, PEBP1, ERk1/2, and β2AR phosphorylation, as well as NOX-4, ROS production and IL-8 release in 16HBE after LECSE. The inhibitory activity of Hemicholinium (HCh-3) (a potent choline uptake blocker), LY294002 (a highly selective inhibitor of PI3 kinase), Tiotropium (Spiriva®) (anticholinergic drug) and Olodaterol (β2AR agonist), were tested in 16HBE after LECSE.

RESULTS

mAChRM3, ChAT, ACh activity, pPEBP1, pβ2AR, pERK1/2, ROS, NOX-4 and IL-8 increased after LECSE in 16HBE LECSE compared to untreated cells. HCh-3 and LY294002 (alone or in combination) as well as Tiotropium (Spiriva®) or Olodaterol (alone or in combination) all reduced the levels of pPEBP1, pβ2AR, pERK1/2, ROS, NOX-4, and IL-8 in 16HBE LECSE compared to untreated cells.

CONCLUSIONS

LECSE promotes ACh production which enhances PI3/PKC/PEBP1/Raf-ERK1/2 pathway activation, heterologous β2AR desensitization, as well as release of inflammatory and oxidative mediators in bronchial epithelial cells. The use of anticholinergic drugs and long-acting β2-agonists, alone or in combination may be dampen these inflammatory mechanisms when used in combination in some epithelial cell types.

β2 Agonists

History suggests β agonists, the cognate ligand of the β₂ adrenoceptor, have been used as bronchodilators for around 5,000 years, and β agonists remain today the frontline treatment for asthma and chronic obstructive pulmonary disease (COPD). The β agonists used clinically today are the products of significant expenditure and over 100 year's intensive research aimed at minimizing side effects and enhancing therapeutic usefulness. The respiratory physician now has a therapeutic toolbox of long acting β agonists to prophylactically manage bronchoconstriction, and short acting β agonists to relieve acute exacerbations. Despite constituting the cornerstone of asthma and COPD therapy, these drugs are not perfect; significant safety issues have led to a black box warning advising that long acting β agonists should not be used alone in patients with asthma. In addition there are a significant proportion of patients whose asthma remains uncontrolled. In this chapter we discuss the evolution of β agonist use and how the understanding of β agonist actions on their principal target tissue, airway smooth muscle, has led to greater understanding of how these drugs can be further modified and improved in the future. Research into the genetics of the β₂ adrenoceptor will also be discussed, as will the implications of individual DNA profiles on the clinical outcomes of β agonist use (pharmacogenetics). Finally we comment on what the future may hold for the use of β agonists in respiratory disease.

Heat Shock-Related Protein 20 Peptide Decreases Human Airway Constriction Downstream of β2-Adrenergic Receptor

Severe bronchospasm refractory to β-agonists is a challenging aspect of asthma therapy, and novel therapeutics are needed. β-agonist-induced airway smooth muscle (ASM) relaxation is associated with increases in the phosphorylation of the small heat shock-related protein (HSP) 20. We hypothesized that a transducible phosphopeptide mimetic of HSP20 (P20 peptide) causes relaxation of human ASM (HASM) by interacting with target(s) downstream of the β2-adrenergic receptor (β2AR) pathway. The effect of the P20 peptide on ASM contractility was determined in human and porcine ASM using a muscle bath. The effect of the P20 peptide on filamentous actin dynamics and migration was examined in intact porcine ASM and cultured primary HASM cells. The efficacy of the P20 peptide in vivo on airway hyperresponsiveness (AHR) was determined in an ovalbumin (OVA) sensitization and challenge murine model of allergic airway inflammation. P20 peptide caused dose-dependent relaxation of carbachol-precontracted ASM and blocked carbachol-induced contraction. The β2AR inhibitor, (±)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol hydrochloride (ICI 118,551), abrogated isoproterenol but not P20 peptide-mediated relaxation. The P20 peptide decreased filamentous actin levels in intact ASM, disrupted stress fibers, and inhibited platelet-derived growth factor-induced migration of HASM cells. The P20 peptide treatment reduced methacholine-induced AHR in OVA mice without affecting the inflammatory response. These results suggest that the P20 peptide decreased airway constriction and disrupted stress fibers through regulation of the actin cytoskeleton downstream of β2AR. Thus, the P20 peptide may be a potential therapeutic for asthma refractory to β-agonists.

Bitter taste receptor agonists alter mitochondrial function and induce autophagy in airway smooth muscle cells

Airway remodeling, including increased airway smooth muscle (ASM) mass, is a hallmark feature of asthma and COPD. We previously identified the expression of bitter taste receptors (TAS2Rs) on human ASM cells and demonstrated that known TAS2R agonists could promote ASM relaxation and bronchodilation and inhibit mitogen-induced ASM growth. In this study, we explored cellular mechanisms mediating the antimitogenic effect of TAS2R agonists on human ASM cells. Pretreatment of ASM cells with TAS2R agonists chloroquine and quinine resulted in inhibition of cell survival, which was largely reversed by bafilomycin A1, an autophagy inhibitor. Transmission electron microscope studies demonstrated the presence of double-membrane autophagosomes and deformed mitochondria. In ASM cells, TAS2R agonists decreased mitochondrial membrane potential and increased mitochondrial ROS and mitochondrial fragmentation. Inhibiting dynamin-like protein 1 (DLP1) reversed TAS2R agonist-induced mitochondrial membrane potential change and attenuated mitochondrial fragmentation and cell death. Furthermore, the expression of mitochondrial protein BCL2/adenovirus E1B 19-kDa protein-interacting protein 3 (Bnip3) and mitochondrial localization of DLP1 were significantly upregulated by TAS2R agonists. More importantly, inhibiting Bnip3 mitochondrial localization by dominant-negative Bnip3 significantly attenuated cell death induced by TAS2R agonist. Collectively the TAS2R agonists chloroquine and quinine modulate mitochondrial structure and function, resulting in ASM cell death. Furthermore, Bnip3 plays a central role in TAS2R agonist-induced ASM functional changes via a mitochondrial pathway. These findings further establish the cellular mechanisms of antimitogenic effects of TAS2R agonists and identify a novel class of receptors and pathways that can be targeted to mitigate airway remodeling as well as bronchoconstriction in obstructive airway diseases.

Cimicifugamide from Cimicifuga rhizomes functions as a nonselective β-AR agonist for cardiac and sudorific effects

Cimicifuga rhizomes (CR) are used in the treatment of respiratory and cardiovascular diseases in traditional Chinese medicine, but their key effective components and mechanism of action have not yet been reported. In this study, the cardiac, antipyretic and sudorific effects of CR were evaluated using the toad heart failure in vitro model and mice fever and sweating in vivo models. Moreover, the UPLC/Q-TOF-MS-integrated β2-AR luciferase reporter gene assay system was used to screen the bioactive ingredients from CR extract, and the activity of this ingredient were verified using the above-mentioned in vitro and vivo models. Our results showed that CR had anti-heart failure, antipyretic and sweating effects, which could be antagonized by propranolol. On the other hand, cimicifugamide was screened as β2-AR agonist from CR and cimicifugamide could activate β1, 2-ARs more significantly than β3-AR in β-ARs selectivity assessment. The results not only revealed the key effective components and mechanism of CR in traditional use but also supplied a characteristic complementary ingredient for quality control of CR.

Angiotensin type 1A receptor regulates β-arrestin binding of the β2-adrenergic receptor via heterodimerization

Heterodimerization between angiotensin type 1A receptor (AT₁R) and β₂-adrenergic receptor (β₂AR) has been shown to modulate G protein-mediated effects of these receptors. Activation of G protein-coupled receptors (GPCRs) leads to β-arrestin binding, desensitization, internalization and G protein-independent signaling of GPCRs. Our aim was to study the effect of heterodimerization on β-arrestin coupling. We found that β-arrestin binding of β₂AR is affected by activation of AT₁Rs. Costimulation with angiotensin II and isoproterenol markedly enhanced the interaction between β₂AR and β-arrestins, by prolonging the lifespan of β₂AR-induced β-arrestin2 clusters at the plasma membrane. While candesartan, a conventional AT₁R antagonist, had no effect on the β-arrestin2 binding to β₂AR, TRV120023, a β-arrestin biased agonist, enhanced the interaction. These findings reveal a new crosstalk mechanism between AT₁R and β₂AR, and suggest that enhanced β-arrestin2 binding to β₂AR can contribute to the pharmacological effects of biased AT₁R agonists.

Bronchoprotection and bronchorelaxation in asthma: New targets, and new ways to target the old ones

Despite over 50years of inhaled beta-agonists and corticosteroids as the default management or rescue drugs for asthma, recent research suggests that new therapeutic options are likely to emerge. This belief stems from both an improved understanding of what causes and regulates airway smooth muscle (ASM) contraction, and the identification of new targets whose inhibition or activation can relax ASM. In this review we discuss the recent findings that provide new insight into ASM contractile regulation, a revolution in pharmacology that identifies new ways to "tune" G protein-coupled receptors to improve therapeutic efficacy, and the discovery of several novel targets/approaches capable of effecting bronchoprotection or bronchodilation.

Antimitogenic effect of bitter taste receptor agonists on airway smooth muscle cells

Airway remodeling is a hallmark feature of asthma and chronic obstructive pulmonary disease. Clinical studies and animal models have demonstrated increased airway smooth muscle (ASM) mass, and ASM thickness is correlated with severity of the disease. Current medications control inflammation and reverse airway obstruction effectively but have limited effect on remodeling. Recently we identified the expression of bitter taste receptors (TAS2R) on ASM cells, and activation with known TAS2R agonists resulted in ASM relaxation and bronchodilation. These studies suggest that TAS2R can be used as new therapeutic targets in the treatment of obstructive lung diseases. To further establish their effectiveness, in this study we aimed to determine the effects of TAS2R agonists on ASM growth and promitogenic signaling. Pretreatment of healthy and asthmatic human ASM cells with TAS2R agonists resulted in a dose-dependent inhibition of ASM proliferation. The antimitogenic effect of TAS2R ligands was not dependent on activation of protein kinase A, protein kinase C, or high/intermediate-conductance calcium-activated K(+) channels. Immunoblot analyses revealed that TAS2R agonists inhibit growth factor-activated protein kinase B phosphorylation without affecting the availability of phosphatidylinositol 3,4,5-trisphosphate, suggesting TAS2R agonists block signaling downstream of phosphatidylinositol 3-kinase. Furthermore, the antimitogenic effect of TAS2R agonists involved inhibition of induced transcription factors (activator protein-1, signal transducer and activator of transcription-3, E2 factor, nuclear factor of activated T cells) and inhibition of expression of multiple cell cycle regulatory genes, suggesting a direct inhibition of cell cycle progression. Collectively, these findings establish the antimitogenic effect of TAS2R agonists and identify a novel class of receptors and signaling pathways that can be targeted to reduce or prevent airway remodeling as well as bronchoconstriction in obstructive airway disease.

Genetic Deletion of β-Arrestin-2 and the Mitigation of Established Airway Hyperresponsiveness in a Murine Asthma Model

β-Arrestin-2 (βarr2) is a ubiquitously expressed cytosolic protein that terminates G protein-coupled receptor signaling and transduces G protein-independent signaling. We previously showed that mice lacking βarr2 do not develop an asthma phenotype when sensitized to, and challenged with, allergens. The current study evaluates if an established asthma phenotype can be mitigated by deletion of βarr2 using an inducible Cre recombinase. We sensitized and challenged mice to ovalbumin (OVA) and demonstrated that on Day (d) 24 the allergic asthma phenotype was apparent in uninduced βarr2 and wild-type (WT) mice. In a second group of OVA-treated mice, tamoxifen was injected on d24 to d28 to activate Cre recombinase, and OVA aerosol challenge was continued through d44. The asthma phenotype was assessed using lung mechanics measurements, bronchoalveolar lavage cell analysis, and histological assessment of mucin and airway inflammation. Compared with their respective saline-treated controls, OVA-treated WT mice and mice expressing the inducible Cre recombinase displayed a significant asthma phenotype at d45. Whereas tamoxifen treatment had no significant effect on the asthma phenotype in WT mice, it inhibited βarr2 expression and caused a significant reduction in airway hyper-responsiveness (AHR) in Cre-inducible mice. These findings suggest that βarr2 is actively required for perpetuation of the AHR component of the allergic asthma phenotype. Our finding that βarr2 participates in the perpetuation of AHR in an asthma model means that targeting βarr2 may provide immediate and potentially long-term relief from daily asthma symptoms due to AHR irrespective of inflammation.

Akt kinase C-terminal modifications control activation loop dephosphorylation and enhance insulin response

The Akt protein kinase, also known as protein kinase B, plays key roles in insulin receptor signalling and regulates cell growth, survival and metabolism. Recently, we described a mechanism to enhance Akt phosphorylation that restricts access of cellular phosphatases to the Akt activation loop (Thr(308) in Akt1 or protein kinase B isoform alpha) in an ATP-dependent manner. In the present paper, we describe a distinct mechanism to control Thr(308) dephosphorylation and thus Akt deactivation that depends on intramolecular interactions of Akt C-terminal sequences with its kinase domain. Modifications of amino acids surrounding the Akt1 C-terminal mTORC2 (mammalian target of rapamycin complex 2) phosphorylation site (Ser(473)) increased phosphatase resistance of the phosphorylated activation loop (pThr(308)) and amplified Akt phosphorylation. Furthermore, the phosphatase-resistant Akt was refractory to ceramide-dependent dephosphorylation and amplified insulin-dependent Thr(308) phosphorylation in a regulated fashion. Collectively, these results suggest that the Akt C-terminal hydrophobic groove is a target for the development of agents that enhance Akt phosphorylation by insulin.

Specificity of arrestin subtypes in regulating airway smooth muscle G protein-coupled receptor signaling and function

Arrestins have been shown to regulate numerous G protein-coupled receptors (GPCRs) in studies employing receptor/arrestin overexpression in artificial cell systems. Which arrestin isoforms regulate which GPCRs in primary cell types is poorly understood. We sought to determine the effect of β-arrestin-1 or β-arrestin-2 inhibition or gene ablation on signaling and function of multiple GPCRs endogenously expressed in airway smooth muscle (ASM). In vitro [second messenger (calcium, cAMP generation)], ex vivo (ASM tension generation in suspended airway), and in vivo (invasive airway resistance) analyses were performed on human ASM cells and murine airways/whole animal subject to β-arrestin-1 or -2 knockdown or knockout (KO). In both human and murine model systems, knockdown or KO of β-arrestin-2 relative to control missense small interfering RNA or wild-type mice selectively increased (40-60%) β2-adrenoceptor signaling and function. β-arrestin-1 knockdown or KO had no effect on signaling and function of β2-adrenoceptor or numerous procontractile GPCRs, but selectively inhibited M3 muscarinic acetylcholine receptor signaling (∼50%) and function (∼25% ex vivo, >50% in vivo) without affecting EC50 values. Arrestin subtypes differentially regulate ASM GPCRs and β-arrestin-1 inhibition represents a novel approach to managing bronchospasm in obstructive lung diseases.

Defective Resensitization in Human Airway Smooth Muscle Cells Evokes β-Adrenergic Receptor Dysfunction in Severe Asthma

β₂-adrenergic receptor (β₂AR) agonists (β₂-agonist) are the most commonly used therapy for acute relief in asthma, but chronic use of these bronchodilators paradoxically exacerbates airway hyper-responsiveness. Activation of βARs by β-agonist leads to desensitization (inactivation) by phosphorylation through G-protein coupled receptor kinases (GRKs) which mediate β-arrestin binding and βAR internalization. Resensitization occurs by dephosphorylation of the endosomal βARs which recycle back to the plasma membrane as agonist-ready receptors. To determine whether the loss in β-agonist response in asthma is due to altered βAR desensitization and/or resensitization, we used primary human airway smooth muscle cells (HASMCs) isolated from the lungs of non-asthmatic and fatal-asthmatic subjects. Asthmatic HASMCs have diminished adenylyl cyclase activity and cAMP response to β-agonist as compared to non-asthmatic HASMCs. Confocal microscopy showed significant accumulation of phosphorylated β₂ARs in asthmatic HASMCs. Systematic analysis of desensitization components including GRKs and β-arrestin showed no appreciable differences between asthmatic and non-asthmatic HASMCs. However, asthmatic HASMC showed significant increase in PI3Kγ activity and was associated with reduction in PP2A activity. Since reduction in PP2A activity could alter receptor resensitization, endosomal fractions were isolated to assess the agonist ready β₂ARs as a measure of resensitization. Despite significant accumulation of β₂ARs in the endosomes of asthmatic HASMCs, endosomal β₂ARs cannot robustly activate adenylyl cyclase. Furthermore, endosomes from asthmatic HASMCs are associated with significant increase in PI3Kγ and reduced PP2A activity that inhibits β₂AR resensitization. Our study shows that resensitization, a process considered to be a homeostasis maintaining passive process is inhibited in asthmatic HASMCs contributing to β₂AR dysfunction which may underlie asthma pathophysiology and loss in asthma control.

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.

Quantification of beta adrenergic receptor subtypes in beta-arrestin knockout mouse airways

In allergic asthma Beta 2 adrenergic receptors (β₂ARs) are important mediators of bronchorelaxation and, paradoxically, asthma development. This contradiction is likely due to the activation of dual signaling pathways that are downstream of G proteins or β-arrestins. Our group has recently shown that β-arrestin-2 acts in its classical role to desensitize and constrain β₂AR-induced relaxation of both human and murine airway smooth muscle. To assess the role of β-arrestins in regulating β₂AR function in asthma, we and others have utilized β-arrestin-1 and -2 knockout mice. However, it is unknown if genetic deletion of β-arrestins in these mice influences β₂AR expression in the airways. Furthermore, there is lack of data on compensatory expression of βAR subtypes when either of the β-arrestins is genetically deleted, thus necessitating a detailed βAR subtype expression study in these β-arrestin knockout mice. Here we standardized a radioligand binding methodology to characterize and quantitate βAR subtype distribution in the airway smooth muscle of wild-type C57BL/6J and β-arrestin-1 and β-arrestin-2 knockout mice. Using complementary competition and single-point saturation binding assays we found that β₂ARs predominate over β₁ARs in the whole lung and epithelium-denuded tracheobronchial smooth muscle of C57BL/6J mice. Quantification of βAR subtypes in β-arrestin-1 and β-arrestin-2 knockout mouse lung and epithelium-denuded tracheobronchial tissue showed that, similar to the C57BL/6J mice, both knockouts display a predominance of β₂AR expression. These data provide further evidence that β₂ARs are expressed in greater abundance than β₁ARs in the tracheobronchial smooth muscle and that loss of either β-arrestin does not significantly affect the expression or relative proportions of βAR subtypes. As β-arrestins are known to modulate β₂AR function, our analysis of βAR subtype expression in β-arrestin knockout mice airways sets a reference point for future studies exploiting these knockout mice in various disease models including asthma.

Bias analyses of preclinical and clinical D2 dopamine ligands: studies with immediate and complex signaling pathways

G protein-coupled receptors (GPCRs) often activate multiple signaling pathways, and ligands may evoke functional responses through individual pathways. These unique responses provide opportunities for biased or functionally selective ligands to preferentially modulate one signaling pathway over another. Studies with several GPCRs have suggested that selective activation of signaling pathways downstream of a GPCR may lead to safer and more effective drug therapies. The dopamine D2 receptor (D2R) is one of the main drug targets in the therapies for Parkinson's disease and schizophrenia. Recent studies suggest that selective modulation of individual signaling pathways downstream of the D2R may lead to safer antipsychotic drugs. In the present study, immediate effectors of the D2R (i.e., Gαi/o, Gβγ, β-arrestin recruitment) and more complex signaling pathways (i.e., extracellular signal-regulated kinase phosphorylation, heterologous sensitization, and dynamic mass redistribution) were examined in response to a series of D2R ligands. This was accomplished using Chinese hamster ovary cells stably expressing the human D2L dopamine receptor in the PathHunter β-Arrestin GPCR Assay Platform. The use of a uniform cellular background was designed to eliminate potential confounds associated with cell-to-cell variability, including expression levels of receptor as well as other components of signal transduction, including G protein subunits. Several well characterized and clinically relevant D2R ligands were evaluated across each signaling pathway in this cellular model. The most commonly used methods to measure ligand bias were compared. Functional selectivity analyses were also used as tools to explore the relative contribution of immediate D2R effectors for the activation of more complex signaling pathways.

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.

Structure-bias relationships for fenoterol stereoisomers in six molecular and cellular assays at the β2-adrenoceptor

Functional selectivity is well established as an underlying concept of ligand-specific signaling via G protein-coupled receptors (GPCRs). Functionally, selective drugs could show greater therapeutic efficacy and fewer adverse effects. Dual coupling of the β2-adrenoceptor (β2AR) triggers a signal transduction via Gsα and Giα proteins. Here, we examined 12 fenoterol stereoisomers in six molecular and cellular assays. Using β2AR-Gsα and β2AR-Giα fusion proteins, (R,S')- and (S,S')-isomers of 4'-methoxy-1-naphthyl-fenoterol were identified as biased ligands with preference for Gs. G protein-independent signaling via β-arrestin-2 was disfavored by these ligands. Isolated human neutrophils constituted an ex vivo model of β2AR signaling and demonstrated functional selectivity through the dissociation of cAMP accumulation and the inhibition of formyl peptide-stimulated production of reactive oxygen species. Ligand bias was calculated using an operational model of agonism and revealed that the fenoterol scaffold constitutes a promising lead structure for the development of Gs-biased β2AR agonists.

Role for β-arrestin in mediating paradoxical β2AR and PAR2 signaling in asthma

G protein-coupled receptors (GPCRs) utilize (at least) two signal transduction pathways to elicit cellular responses including the classic G protein-dependent, and the more recently discovered β-arrestin-dependent, signaling pathways. In human and murine models of asthma, agonist-activation of β2-adrenergic receptor (β2AR) or Protease-activated-receptor-2 (PAR2) results in relief from bronchospasm via airway smooth muscle relaxation. However, chronic activation of these receptors, leads to pro-inflammatory responses. One plausible explanation underlying the paradoxical effects of β2AR and PAR2 agonism in asthma is that the beneficial and harmful effects are associated with distinct signaling pathways. Specifically, G protein-dependent signaling mediates relaxation of airway smooth muscle, whereas β-arrestin-dependent signaling promotes inflammation. This review explores the evidence supporting the hypothesis that β-arrestin-dependent signaling downstream of β2AR and PAR2 is detrimental in asthma and examines the therapeutic opportunities for selectively targeting this pathway.

Biased ligand modulation of seven transmembrane receptors (7TMRs): functional implications for drug discovery

Seven transmembrane receptors (7TMRs), also known as G-protein-coupled receptors (GPCRs), have proven to be valuable targets for the development of therapeutics. The expansion of our understanding of 7TMR downstream signaling pathways beyond G-proteins has broadened our appreciation of the versatility of these cell surface receptors. In particular, the increased awareness of 7TMR engagement of β-arrestin signaling has opened up additional avenues for drug discovery. 7TMRs can adopt different conformations and in response to various ligands can lead to a bias in downstream signaling mechanisms when comparing the overall efficacy between G-protein and β-arrestin dependent pathways. In 2012, we organized a session at the Spring National Meeting of the American Chemical Society on biased signaling in 7TMRs.1-4 Building on that experience, we provide in this Miniperspective some examples that exemplify developments in the area of biased 7TMR signaling and highlight some cautionary notes as well as some of the exciting opportunities for drug discovery.

Crosstalk between beta-2-adrenoceptor and muscarinic acetylcholine receptors in the airway

The M3 and M2 muscarinic acetylcholine receptors (mAChRs) and beta-2-adrenoceptors (β2ARs) are important regulators of airway cell function, and drugs targeting these receptors are among the first line drugs in the treatment of the obstructive lung diseases asthma and chronic obstructive lung disease (COPD). Cross-regulation or crosstalk between mAChRs and β2ARs in airway smooth muscle (ASM) helps determine the contractile state of the muscle, thus airway diameter and resistance to airflow. In this review we will detail mAChR and β2AR-signaling and crosstalk, focusing on events in the ASM cell but also addressing the function of these receptors in other cell types that impact airway physiology. We conclude by discussing how recent advances in GPCR pharmacology offer a unique opportunity to fine tune mAChR and β2AR signaling and their crosstalk, and thereby produce superior therapeutics for obstructive lung and other diseases.

GPCRs and arrestins in airways: implications for asthma

The obstructive lung disease asthma is treated by drugs that target, either directly or indirectly, G protein-coupled receptors (GPCRs). GPCRs coupled to Gq are the primary mediators of airway smooth muscle (ASM) contraction and increased airway resistance, whereas the Gs-coupled beta-2-adrenoceptor (β2AR) promotes pro-relaxant signaling in and relaxation of ASM resulting in greater airway patency and reversal of life-threatening bronchoconstriction. In addition, GPCR-mediated functions in other cell types, including airway epithelium and hematopoietic cells, are involved in the control of lung inflammation that causes most asthma. The capacity of arrestins to regulate GPCR signaling, via either control of GPCR desensitization/resensitization or G protein-independent signaling, renders arrestins an intriguing therapeutic target for asthma and other obstructive lung diseases. This review will focus on the potential role of arrestins in those GPCR-mediated airway cell functions that are dysregulated in asthma.

Airway smooth muscle in airway reactivity and remodeling: what have we learned?

It is now established that airway smooth muscle (ASM) has roles in determining airway structure and function, well beyond that as the major contractile element. Indeed, changes in ASM function are central to the manifestation of allergic, inflammatory, and fibrotic airway diseases in both children and adults, as well as to airway responses to local and environmental exposures. Emerging evidence points to novel signaling mechanisms within ASM cells of different species that serve to control diverse features, including 1) [Ca(2+)]i contractility and relaxation, 2) cell proliferation and apoptosis, 3) production and modulation of extracellular components, and 4) release of pro- vs. anti-inflammatory mediators and factors that regulate immunity as well as the function of other airway cell types, such as epithelium, fibroblasts, and nerves. These diverse effects of ASM "activity" result in modulation of bronchoconstriction vs. bronchodilation relevant to airway hyperresponsiveness, airway thickening, and fibrosis that influence compliance. This perspective highlights recent discoveries that reveal the central role of ASM in this regard and helps set the stage for future research toward understanding the pathways regulating ASM and, in turn, the influence of ASM on airway structure and function. Such exploration is key to development of novel therapeutic strategies that influence the pathophysiology of diseases such as asthma, chronic obstructive pulmonary disease, and pulmonary fibrosis.

β-Arrestins 1 and 2 are critical regulators of inflammation

β-Arrestins 1 and 2 couple to seven trans-membrane receptors and regulate G protein-dependent signaling, receptor endocytosis and ubiquitylation. Recent studies have uncovered several unanticipated functions of β-arrestins, suggesting that the role of β-arrestins in cell signaling is much broader than originally thought. It is now recognized that β-arrestins can transduce receptor signaling independent of G proteins. The expression of β-arrestins is differentially regulated in immune cells and tissues in response to specific inflammatory stimuli, and β-arrestins are critical regulators of the inflammatory response. This review will focus on β-arrestins in immune cells and the impact of altered expression on the pathogenesis of specific inflammatory diseases. Understanding the role of β-arrestins in inflammation may lead to new strategies to treat inflammatory diseases, such as sepsis, rheumatoid arthritis, asthma, multiple sclerosis, inflammatory bowel disease and atherosclerosis.

Targeting G protein coupled receptor-related pathways as emerging molecular therapies

G protein coupled receptors (GPCRs) represent the most important targets in modern pharmacology because of the different functions they mediate, especially within brain and peripheral nervous system, and also because of their functional and stereochemical properties. In this paper, we illustrate, via a variety of examples, novel advances about the GPCR-related molecules that have been shown to play diverse roles in GPCR pathways and in pathophysiological phenomena. We have exemplified how those GPCRs’ pathways are, or might constitute, potential targets for different drugs either to stimulate, modify, regulate or inhibit the cellular mechanisms that are hypothesized to govern some pathologic, physiologic, biologic and cellular or molecular aspects both in vivo and in vitro. Therefore, influencing such pathways will, undoubtedly, lead to different therapeutical applications based on the related pharmacological implications. Furthermore, such new properties can be applied in different fields. In addition to offering fruitful directions for future researches, we hope the reviewed data, together with the elements found within the cited references, will inspire clinicians and researchers devoted to the studies on GPCR’s properties.