Beta2-adrenoceptor signaling is required for the development of an asthma phenotype in a murine model. Proc Natl Acad Sci U S A. 2009;106:2435-2440. [PMID: 19171883 DOI: 10.1073/pnas.0810902106]

Inhibition of TRPA1, Endoplasmic Reticulum Stress, Human Airway Epithelial Cell Damage, and Ectopic MUC5AC Expression by Vasaka (Adhatoda vasica; Malabar Nut) Tea

This study tested whether a medicinal plant, Vasaka, typically consumed as a tea to treat respiratory malaise, could protect airway epithelial cells (AECs) from wood smoke particle-induced damage and prevent pathological mucus production. Wood/biomass smoke is a pneumotoxic air pollutant. Mucus normally protects the airways, but excessive production can obstruct airflow and cause respiratory distress. Vasaka tea pre- and co-treatment dose-dependently inhibited mucin 5AC (MUC5AC) mRNA induction by AECs treated with wood smoke particles. This correlated with transient receptor potential ankyrin-1 (TRPA1) inhibition, an attenuation of endoplasmic reticulum (ER) stress, and AEC damage/death. Induction of mRNA for anterior gradient 2, an ER chaperone/disulfide isomerase required for MUC5AC production, and TRP vanilloid-3, a gene that suppresses ER stress and wood smoke particle-induced cell death, was also attenuated. Variable inhibition of TRPA1, ER stress, and MUC5AC mRNA induction was observed using selected chemicals identified in Vasaka tea including vasicine, vasicinone, apigenin, vitexin, isovitexin, isoorientin, 9-oxoODE, and 9,10-EpOME. Apigenin and 9,10-EpOME were the most cytoprotective and mucosuppressive. Cytochrome P450 1A1 (CYP1A1) mRNA was also induced by Vasaka tea and wood smoke particles. Inhibition of CYP1A1 enhanced ER stress and MUC5AC mRNA expression, suggesting a possible role in producing protective oxylipins in stressed cells. The results provide mechanistic insights and support for the purported benefits of Vasaka tea in treating lung inflammatory conditions, raising the possibility of further development as a preventative and/or restorative therapy.

Fusion of the β2-adrenergic receptor with either Gαs or βarrestin-2 produces constitutive signaling by each pathway and induces gain-of-function in BEAS-2B cells

The β2AR is a prototypical G protein-coupled receptor (GPCR) known to orchestrate different cellular responses by the stimulation of specific signaling pathways. The best-established signaling pathways for the β2AR are the canonical Gs pathway and the alternative β arrestin 2 (βarr2) pathway. Exploring each pathway separately remains a challenging task due to the dynamic nature of the receptor. Here, we fused the β2AR with its cognate transducers, Gαs and βarr2, using short linkers as a novel approach for restricting the conformation of the receptor and preferentially activating one of its two signaling pathways. We characterized the behavior of our fusion proteins β2AR-Gαs and β2AR-βarr2 in HEK293 cells by measuring their constitutive activity, transducer recruitment, and pharmacological modulation. Our fusion proteins show (a) steric hindrance from the reciprocal endogenous transducers, (b) constitutive activity of the β2AR for the signaling pathway activated by the tethered transducer, and (c) pharmacologic modulation by β2AR ligands. Based on these characteristics, we further explored the possibility of a gain-of-function mechanism in the human lung non-tumorigenic epithelial cell line, BEAS-2B cells. This immortalized human bronchial epithelial cell line has immunomodulatory properties through cytokine release mediated by β2AR stimulation. Our findings suggest that each signaling pathway of the β2AR is biased toward either the Th1 or Th2 inflammatory response suggesting a role in regulating the immune phenotype of respiratory diseases. Our data imply that our fusion proteins can be used as tools to isolate the function elicited by a single signaling pathway in physiologically relevant cell types.

Oral antibiotics relieve allergic asthma in post-weaning mice via reducing iNKT cells and function of ADRB2

The role of normal gut microbiota in asthma or ovalbumin (OVA)-induced asthma tolerance (OT) remains unclear. Here, we established mouse models of asthma and OT followed by 2 weeks of antibiotic treatment, to clear the gut microbiota. Antibiotic treatment was found to alleviate allergic asthma accompanied with a reduction of invariant natural killer (iNKT) cells. By RNA-seq analysis, we found that β-adrenergic receptor (ADRB) genes, including Adrb1, Adrb2, and Adrb3, were downregulated in asthmatic lungs, but these changes were reversed in OT lungs. Moreover, Adrb2 and Adrb3 were significantly upregulated in asthmatic lungs after antibiotic treatment. Surprisingly, blocking ADRB with propranolol relieved allergic asthma while reducing T helper 2 (Th2) and Treg cell numbers. Further analyses using flow cytometry and immunofluorescence showed that the protein expression level of ADRB2 was higher in asthmatic lungs than that in the control and OT lungs. Notably, dendritic cells (DCs), especially the ADRB2⁺ DCs, were increased in asthmatic lungs compared to that in the control and OT lungs. In addition, ADRB2⁺ DCs were significantly reduced following the administration of the ADRB2-specific antagonist ICI118551. Our findings suggest that antibiotic treatment can alleviate OVA-induced allergic asthma via reducing the frequency of iNKT cells and function of ADRB2.

S-nitrosylation is required for β2AR desensitization and experimental asthma

The β2-adrenergic receptor (β2AR), a prototypic G-protein-coupled receptor (GPCR), is a powerful driver of bronchorelaxation, but the effectiveness of β-agonist drugs in asthma is limited by desensitization and tachyphylaxis. We find that during activation, the β2AR is modified by S-nitrosylation, which is essential for both classic desensitization by PKA as well as desensitization of NO-based signaling that mediates bronchorelaxation. Strikingly, S-nitrosylation alone can drive β2AR internalization in the absence of traditional agonist. Mutant β2AR refractory to S-nitrosylation (Cys265Ser) exhibits reduced desensitization and internalization, thereby amplifying NO-based signaling, and mice with Cys265Ser mutation are resistant to bronchoconstriction, inflammation, and the development of asthma. S-nitrosylation is thus a central mechanism in β2AR signaling that may be operative widely among GPCRs and targeted for therapeutic gain.

α and β Catalytic Subunits of cAMP-dependent Protein Kinase Regulate Formoterol-induced Inflammatory Gene Expression Changes in Human Bronchial Epithelial Cells

BACKGROUND & PURPOSE

It has been proposed that genomic mechanisms contribute to the adverse-effects that are often experienced by asthmatic subjects who take regular, inhaled β₂ -adrenoceptor agonists as a monotherapy. Moreover, data from preclinical models of asthma suggest that these gene expression changes are mediated by β-arrestin-2 rather than PKA. Herein, we tested this hypothesis by comparing the genomic effects of formoterol, a β₂ -adrenoceptor agonist, with forskolin in human primary bronchial epithelial cells (HBEC).

EXPERIMENTAL APPROACH

Gene expression changes were determined by RNA-sequencing. Gene silencing and genome editing were employed to explore the roles of β-arrestin-2 and PKA.

KEY RESULTS

The formoterol-regulated transcriptome in HBEC treated concurrently with TNFα, was defined by 1480 unique gene expression changes. TNFα-induced transcripts modulated by formoterol were annotated with enriched gene ontology terms related to inflammation and proliferation, notably "GO:0070374~positive regulation of ERK1 and ERK2 cascade", which is an established β-arrestin-2 target. However, expression of the formoterol- and forskolin-regulated transcriptomes were highly rank-order correlated and the effects of formoterol on TNFα-induced inflammatory genes were abolished by an inhibitor of PKA. Furthermore, formoterol-induced gene expression changes in BEAS-2B bronchial epithelial cell clones deficient in β-arrestin-2 were comparable to those expressed by their parental counterparts. Contrariwise, gene expression was partially inhibited in clones lacking the α-catalytic subunit (Cα) of PKA and abolished following the additional knockdown of the β-catalytic subunit (Cβ) paralogue.

CONCLUSIONS

The effects of formoterol on inflammatory gene expression in airway epithelia are mediated by PKA and involve the cooperation of PKA-Cα and PKA-Cβ.

Cooperativity between β-agonists and c-Abl inhibitors in regulating airway smooth muscle relaxation

Current therapeutic approaches to avoid or reverse bronchoconstriction rely primarily on β2 adrenoceptor agonists (β-agonists) that regulate pharmacomechanical coupling/cross bridge cycling in airway smooth muscle (ASM). Targeting actin cytoskeleton polymerization in ASM represents an alternative means to regulate ASM contraction. Herein we report the cooperative effects of targeting these distinct pathways with β-agonists and inhibitors of the mammalian Abelson tyrosine kinase (Abl1 or c-Abl). The cooperative effect of β-agonists (isoproterenol) and c-Abl inhibitors (GNF-5, or imatinib) on contractile agonist (methacholine, or histamine) -induced ASM contraction was assessed in cultured human ASM cells (using Fourier Transfer Traction Microscopy), in murine precision cut lung slices, and in vivo (flexiVent in mice). Regulation of intracellular signaling that regulates contraction (pMLC20, pMYPT1, pHSP20), and actin polymerization state (F:G actin ratio) were assessed in cultured primary human ASM cells. In each (cell, tissue, in vivo) model, c-Abl inhibitors and β-agonist exhibited additive effects in either preventing or reversing ASM contraction. Treatment of contracted ASM cells with c-Abl inhibitors and β-agonist cooperatively increased actin disassembly as evidenced by a significant reduction in the F:G actin ratio. Mechanistic studies indicated that the inhibition of pharmacomechanical coupling by β-agonists is near optimal and is not increased by c-Abl inhibitors, and the cooperative effect on ASM relaxation resides in further relaxation of ASM tension development caused by actin cytoskeleton depolymerization, which is regulated by both β-agonists and c-Abl inhibitors. Thus, targeting actin cytoskeleton polymerization represents an untapped therapeutic reserve for managing airway resistance.

Can GPCRs Be Targeted to Control Inflammation in Asthma?

Historically, the drugs used to manage obstructive lung diseases (OLDs), asthma, and chronic obstructive pulmonary disease (COPD) either (1) directly regulate airway contraction by blocking or relaxing airway smooth muscle (ASM) contraction or (2) indirectly regulate ASM contraction by inhibiting the principal cause of ASM contraction/bronchoconstriction and airway inflammation. To date, these tasks have been respectively assigned to two diverse drug types: agonists/antagonists of G protein-coupled receptors (GPCRs) and inhaled or systemic steroids. These two types of drugs "stay in their lane" with respect to their actions and consequently require the addition of the other drug to effectively manage both inflammation and bronchoconstriction in OLDs. Indeed, it has been speculated that safety issues historically associated with beta-agonist use (beta-agonists activate the beta-2-adrenoceptor (β₂AR) on airway smooth muscle (ASM) to provide bronchoprotection/bronchorelaxation) are a function of pro-inflammatory actions of β₂AR agonism. Recently, however, previously unappreciated roles of various GPCRs on ASM contractility and on airway inflammation have been elucidated, raising the possibility that novel GPCR ligands targeting these GPCRs can be developed as anti-inflammatory therapeutics. Moreover, we now know that many GPCRs can be "tuned" and not just turned "off" or "on" to specifically activate the beneficial therapeutic signaling a receptor can transduce while avoiding detrimental signaling. Thus, the fledging field of biased agonism pharmacology has the potential to turn the β₂AR into an anti-inflammatory facilitator in asthma, possibly reducing or eliminating the need for steroids.

A Systematic Review of Inverse Agonism at Adrenoceptor Subtypes

As many, if not most, ligands at G protein-coupled receptor antagonists are inverse agonists, we systematically reviewed inverse agonism at the nine adrenoceptor subtypes. Except for β3-adrenoceptors, inverse agonism has been reported for each of the adrenoceptor subtypes, most often for β2-adrenoceptors, including endogenously expressed receptors in human tissues. As with other receptors, the detection and degree of inverse agonism depend on the cells and tissues under investigation, i.e., they are greatest when the model has a high intrinsic tone/constitutive activity for the response being studied. Accordingly, they may differ between parts of a tissue, for instance, atria vs. ventricles of the heart, and within a cell type, between cellular responses. The basal tone of endogenously expressed receptors is often low, leading to less consistent detection and a lesser extent of observed inverse agonism. Extent inverse agonism depends on specific molecular properties of a compound, but inverse agonism appears to be more common in certain chemical classes. While inverse agonism is a fascinating facet in attempts to mechanistically understand observed drug effects, we are skeptical whether an a priori definition of the extent of inverse agonism in the target product profile of a developmental candidate is a meaningful option in drug discovery and development.

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.

Role of the mucins in pathogenesis of COPD: implications for therapy

Introduction: Evidence accumulated in the last decade has started to reveal the enormous complexity in the expression, interactions and functions of the large number of different mucins present in the different compartments of the human lower airways. This occurs both in normal subjects and in COPD patients in different clinical phases and stages of severity.Areas covered: We review the known physiological mechanisms that regulate mucin production in human lower airways of normal subjects, the changes in mucin synthesis/secretion in COPD patients and the clinical efficacy of drugs that modulate mucin synthesis/secretion.Expert opinion: It is evident that the old simplistic concept that mucus hypersecretion in COPD patients is associated with negative clinical outcomes is not valid and that the therapeutic potential of 'mucolytic drugs' is under-appreciated due to the complexity of the associated molecular network(s). Likewise, our current knowledge of the effects of the drugs already available on the market that target mucin synthesis/secretion/structure in the lower airways is extremely limited and often indirect and more well-controlled clinical trials are needed in this area.

Proinflammatory Cytokines Mediate GPCR Dysfunction

Proinflammatory reaction by the body occurs acutely in response to injury that is considered primarily beneficial. However, sustained proinflammatory cytokines observed with chronic pathologies such as metabolic syndrome, cancer, and arthritis are detrimental and in many cases is a major cardiovascular risk factor. Proinflammatory cytokines such as interleukin-1, interleukin-6, and tumor necrosis factor α (TNFα) have long been implicated in cardiovascular risk and considered to be a major underlying cause for heart failure (HF). The failure of the anti-TNFα therapy for HF indicates our elusive understanding on the dichotomous role of proinflammatory cytokines on acutely beneficial effects versus long-term deleterious effects. Despite these well-described observations, less is known about the mechanistic underpinnings of proinflammatory cytokines especially TNFα in pathogenesis of HF. Increasing evidence suggests the existence of an active cross-talk between the TNFα receptor signaling and G-protein-coupled receptors such as β-adrenergic receptor (βAR). Given that βARs are the key regulators of cardiac function, the review will discuss the current state of understanding on the role of proinflammatory cytokine TNFα in regulating βAR function.

Chronic β2AR stimulation limits CFTR activation in human airway epithelia

Traditional pulmonary therapies for cystic fibrosis (CF) target the downstream effects of CF transmembrane conductance regulator (CFTR) dysfunction (the cause of CF). Use of one such therapy, β-adrenergic bronchodilators (such as albuterol), is nearly universal for airway clearance. Conversely, novel modulator therapies restore function to select mutant CFTR proteins, offering a disease-modifying treatment. Recent trials of modulators targeting F508del-CFTR, the most common CFTR mutation, suggest that chronic β-agonist use may undermine clinical modulator benefits. We therefore sought to understand the impact of chronic or excess β-agonist exposure on CFTR activation in human airway epithelium. The present studies demonstrate a greater than 60% reduction in both wild-type and modulator-corrected F508del-CFTR activation following chronic exposure to short- and long-acting β-agonists. This reduction was due to reduced cellular generation of cAMP downstream of the β-2 adrenergic receptor-G protein complex. Our results point towards a posttranscriptional reduction in adenylyl cyclase function as the mechanism of impaired CFTR activation produced by prolonged β-agonist exposure. β-Agonist-induced CFTR dysfunction was sufficient to abrogate VX809/VX770 modulation of F508del-CFTR in vitro. Understanding the clinical relevance of our observations is critical for CF patients using these drugs, and for investigators to inform future CFTR modulator drug trials.

β2-Adrenoceptor signaling in airway epithelial cells promotes eosinophilic inflammation, mucous metaplasia, and airway contractility

The mostly widely used bronchodilators in asthma therapy are β₂-adrenoreceptor (β₂AR) agonists, but their chronic use causes paradoxical adverse effects. We have previously determined that β₂AR activation is required for expression of the asthma phenotype in mice, but the cell types involved are unknown. We now demonstrate that β₂AR signaling in the airway epithelium is sufficient to mediate key features of the asthmatic responses to IL-13 in murine models. Our data show that inhibition of β₂AR signaling with an aerosolized antagonist attenuates airway hyperresponsiveness (AHR), eosinophilic inflammation, and mucus-production responses to IL-13, whereas treatment with an aerosolized agonist worsens these phenotypes, suggesting that β₂AR signaling on resident lung cells modulates the asthma phenotype. Labeling with a fluorescent β₂AR ligand shows the receptors are highly expressed in airway epithelium. In β₂AR^-/- mice, transgenic expression of β₂ARs only in airway epithelium is sufficient to rescue IL-13-induced AHR, inflammation, and mucus production, and transgenic overexpression in WT mice exacerbates these phenotypes. Knockout of β-arrestin-2 (βarr-2^-/-) attenuates the asthma phenotype as in β₂AR^-/- mice. In contrast to eosinophilic inflammation, neutrophilic inflammation was not promoted by β₂AR signaling. Together, these results suggest β₂ARs on airway epithelial cells promote the asthma phenotype and that the proinflammatory pathway downstream of the β₂AR involves βarr-2. These results identify β₂AR signaling in the airway epithelium as capable of controlling integrated responses to IL-13 and affecting the function of other cell types such as airway smooth muscle cells.

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.

Effects of β-blockers on house dust mite-driven murine models pre- and post-development of an asthma phenotype

BACKGROUND

Our previous studies suggested certain β-adrenoceptor blockers (β-blockers) attenuate the asthma phenotype in ovalbumin driven murine models of asthma. However, the ovalbumin model has been criticized for lack of clinical relevance.

METHODS

We tested the non-selective β-blockers, carvedilol and nadolol, in house dust mite (HDM) driven murine asthma models where drugs were administered both pre- and post-development of the asthma phenotype. We measured inflammation, mucous metaplasia, and airway hyper-responsiveness (AHR). We also measured the effects of the β-blockers on extracellular-signal regulated kinase (ERK 1/2) phosphorylation in lung homogenates.

RESULTS

We show that nadolol, but not carvedilol, attenuated inflammation and mucous metaplasia, and had a moderate effect attenuating AHR. Following HDM exposure, ERK1/2 phosphorylation was elevated, but the level of phosphorylation was unaffected by β-blockers, suggesting ERK1/2 phosphorylation becomes dissociated from the asthma phenotype.

CONCLUSION

Our findings in HDM models administering drugs both pre- and post-development of the asthma phenotype are consistent with previous results using ovalbumin models and show differential effects for nadolol and carvedilol on the asthma phenotype. Lastly, our data suggest that ERK1/2 phosphorylation may be involved in development of the asthma phenotype, but may have a limited role in maintaining the phenotype.

Phosphodiesterase 4 Inhibitors Attenuate the Asthma Phenotype Produced by β2-Adrenoceptor Agonists in Phenylethanolamine N-Methyltransferase-Knockout Mice

Mice lacking the endogenous β2-adrenoceptor (β2AR) agonist epinephrine (phenylethanolamine N-methyltransferase [PNMT]-knockout mice) are resistant to developing an "asthma-like" phenotype in an ovalbumin sensitization and challenge (Ova S/C) model, and chronic administration of β2AR agonists to PNMT-KO mice restores the phenotype. Based on these and other studies showing differential effects of various β2AR ligands on the asthma phenotype, we have speculated that the permissive effect of endogenous epinephrine and exogenous β2AR agonists on allergic lung inflammation can be explained by qualitative β2AR signaling. The β2AR can signal through at least two pathways: the canonical Gαs-cAMP pathway and a β-arrestin-dependent pathway. Previous studies suggest that β-arrestin-2 is required for allergic lung inflammation. On the other hand, cell-based assays suggest antiinflammatory effects of Gαs-cAMP signaling. This study was designed to test whether the in vitro antiinflammatory effects of phosphodiesterase 4 inhibitors, known to increase intracellular cAMP in multiple airway cell types, attenuate the asthma-like phenotype produced by the β2AR agonists formoterol and salmeterol in vivo in PNMT-KO mice, based on the hypothesis that skewing β2AR signaling toward Gαs-cAMP pathway is beneficial. Airway inflammatory cells, epithelial mucus production, and airway hyperresponsiveness were quantified. In Ova S/C PNMT-KO mice, formoterol and salmeterol restored the asthma-like phenotype comparable to Ova S/C wild-type mice. However, coadministration of either roflumilast or rolipram attenuated this formoterol- or salmeterol-driven phenotype in Ova S/C PNMT-KO. These findings suggest that amplification of β2AR-mediated cAMP by phosphodiesterase 4 inhibitors attenuates the asthma-like phenotype promoted by β-agonists.

β2-adrenoreceptor Inverse Agonist Down-regulates Muscarine Cholinergic Subtype-3 Receptor and Its Downstream Signal Pathways in Airway Smooth Muscle Cells in vitro

Mechanisms underlying β₂-adrenoreceptor (β₂AR) inverse agonist mediated bronchoprotectiveness remain unknown. We incubated ICI118,551, formoterol, budesonide, and formoterol plus budesonide, as well as ICI118,551 or pindolol plus formoterol, ICI118,551 plus forskolin, SQ22,536 or H89 plus formoterol in ASMCs to detect expressions of M₃R, PLCβ₁ and IP₃. The level of M₃R in the presence of 10⁻⁵ mmol/L ICI118,551 were significantly decreased at 12 h, 24 h and 48 h (P < 0.05), and at 24 h were significantly reduced in ICI118,551 with concentration of 10⁻⁵mmol/L, 10⁻⁶mmol/L, 10⁻⁷mmol/L, and 10⁻⁸mmol/L (P < 0.05). The level of IP₃ in 10⁻⁵mmol/L ICI118,551 was significantly diminished at 24 h (P < 0.01), except for that at 1 h, neither was in the level of PLCβ₁. A concentration of 10⁻⁵mmol/L ICI118,551 at 24 h showed a significant reduction of M₃R level compared to formoterol (P < 0.01), budesonide (P < 0.01), and formoterol + budesonide (P < 0.05), but significant reduction of PLCβ₁ and IP₃ was only found between 10⁻⁵mmol/L ICI118,551 and formoterol at 24 h, but not in the comparison of budesonide or formoterol + budesonide. Pindolol and H89 could not inhibit the formoterol-induced expression of M₃R (P > 0.05), but SQ22,536 significantly antagonized the formoterol-induced M₃R expression (P < 0.05). In conclusions, β₂AR inverse agonist, ICI118,551, exerts similar bronchoprotective effects to corticosteroids via decreasing the expression of M₃R and inhibiting the production of IP₃.

Biphasic effects of propranolol on tumour growth in B16F10 melanoma-bearing mice

BACKGROUND AND PURPOSE

Propranolol is a vasoactive drug that shows antiangiogenic and antitumour activities in melanoma. However, it is unknown whether these activities are dose-dependent and whether there is a relationship between systemic vascular effects of propranolol and anti-melanoma activity.

EXPERIMENTAL APPROACH

Effects of increasing doses of propranolol (10, 20, 30 and 40 mg·kg^-1 ·day^-1 ) on tumour growth were studied in B16F10 melanoma-bearing mice. Histological and biochemical analyses were used to assess propranolol effects on angiogenesis and cancer cell proliferation. Systemic vascular resistance (SVR) was evaluated by measuring cardiac output and arterial BP.

KEY RESULTS

In vitro analyses revealed that B16F10 cells expressed β-adrenoceptors, but neither isoprenaline, a β-adrenoceptor agonist, nor the β-blocker propranolol affected cancer cell proliferation. In vivo studies showed that the antitumour efficacy of propranolol follows a U-shaped biphasic dose-response curve. Low doses (10 and 20 mg·kg^-1 ·day^-1 ) significantly inhibit tumour growth, whereas higher doses are progressively less effective. We also found that high-dose propranolol stimulates tumour arteriogenesis whereas no effect on angiogenesis was observed at any dose. Based on these data and considering that propranolol is a vasoactive drug, we hypothesized that it causes systemic vasoconstriction or vasodilation depending on the dose and thus alters tumour perfusion and growth. Consistent with this hypothesis, we found that propranolol has a biphasic effect on SVR with low and high doses producing vasoconstriction and vasodilation respectively.

CONCLUSIONS AND IMPLICATIONS

Propranolol inhibits melanoma growth in a U-shaped biphasic manner. A direct relationship exists between SVR and anti-melanoma activity.

Understanding how long-acting β2 -adrenoceptor agonists enhance the clinical efficacy of inhaled corticosteroids in asthma - an update

In moderate-to-severe asthma, adding an inhaled long-acting β₂ -adenoceptor agonist (LABA) to an inhaled corticosteroid (ICS) provides better disease control than simply increasing the dose of ICS. Acting on the glucocorticoid receptor (GR, gene NR3C1), ICSs promote anti-inflammatory/anti-asthma gene expression. In vitro, LABAs synergistically enhance the maximal expression of many glucocorticoid-induced genes. Other genes, including dual-specificity phosphatase 1(DUSP1) in human airways smooth muscle (ASM) and epithelial cells, are up-regulated additively by both drug classes. Synergy may also occur for LABA-induced genes, as illustrated by the bronchoprotective gene, regulator of G-protein signalling 2 (RGS2) in ASM. Such effects cannot be produced by either drug alone and may explain the therapeutic efficacy of ICS/LABA combination therapies. While the molecular basis of synergy remains unclear, mechanistic interpretations must accommodate gene-specific regulation. We explore the concept that each glucocorticoid-induced gene is an independent signal transducer optimally activated by a specific, ligand-directed, GR conformation. In addition to explaining partial agonism, this realization provides opportunities to identify novel GR ligands that exhibit gene expression bias. Translating this into improved therapeutic ratios requires consideration of GR density in target tissues and further understanding of gene function. Similarly, the ability of a LABA to interact with a glucocorticoid may be suboptimal due to low β₂ -adrenoceptor density or biased β₂ -adrenoceptor signalling. Strategies to overcome these limitations include adding-on a phosphodiesterase inhibitor and using agonists of other Gs-coupled receptors. In all cases, the rational design of ICS/LABA, and derivative, combination therapies requires functional knowledge of induced (and repressed) genes for therapeutic benefit to be maximized.

Inundation of asthma target research: Untangling asthma riddles

Asthma is an inveterate inflammatory disorder, delineated by the airway inflammation, bronchial hyperresponsiveness (BHR) and airway wall remodeling. Although, asthma is a vague term, and is recognized as heterogenous entity encompassing different phenotypes. Targeting single mediator or receptor did not prove much clinical significant, as asthma is complex disease involving myriad inflammatory mediators. Asthma may probably involve a large number of different types of molecular and cellular components interacting through complex pathophysiological pathways. This review covers the past, present, and future therapeutic approaches and pathophysiological mechanisms of asthma. Furthermore, review describe importance of targeting several mediators/modulators and receptor antagonists involved in the physiopathology of asthma. Novel targets for asthma research include Galectins, Immunological targets, K ⁺ Channels, Kinases and Transcription Factors, Toll-like receptors, Selectins and Transient receptor potential channels. But recent developments in asthma research are very promising, these include Bitter taste receptors (TAS2R) abated airway obstruction in mouse model of asthma and Calcium-sensing receptor obliterate inflammation and in bronchial hyperresponsiveness allergic asthma. All these progresses in asthma targets, and asthma phenotypes exploration are auspicious in untangling of asthma riddles.

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.

Development of GABAA Receptor Subtype-Selective Imidazobenzodiazepines as Novel Asthma Treatments

Recent studies have demonstrated that subtype-selective GABAA receptor modulators are able to relax precontracted human airway smooth muscle ex vivo and reduce airway hyper-responsiveness in mice upon aerosol administration. Our goal in this study was to investigate systemic administration of subtype-selective GABAA receptor modulators to alleviate bronchoconstriction in a mouse model of asthma. Expression of GABAA receptor subunits was identified in mouse lungs, and the effects of α4-subunit-selective GABAAR modulators, XHE-III-74EE and its metabolite XHE-III-74A, were investigated in a murine model of asthma (ovalbumin sensitized and challenged BALB/c mice). We observed that chronic treatment with XHE-III-74EE significantly reduced airway hyper-responsiveness. In addition, acute treatment with XHE-III-74A but not XHE-III-74EE decreased airway eosinophilia. Immune suppressive activity was also shown in activated human T-cells with a reduction in IL-2 expression and intracellular calcium concentrations [Ca(2+)]i in the presence of GABA or XHE-III-74A, whereas XHE-III-74EE showed only partial reduction of [Ca(2+)]i and no inhibition of IL-2 secretion. However, both compounds significantly relaxed precontracted tracheal rings ex vivo. Overall, we conclude that the systemic delivery of a α4-subunit-selective GABAAR modulator shows good potential for a novel asthma therapy; however, the pharmacokinetic properties of this class of drug candidates have to be improved to enable better beneficial systemic pharmacodynamic effects.

β2-adrenoceptor in obstructive airway diseases: Agonism, antagonism or both?

Obstructive airway disease is a complex disease entity including several maladies characterized by bronchoconstriction and abnormal airway inflammation. Reversing bronchoconstriction leads to symptomatic relief and improvement in quality of life, both in reversible (bronchial asthma) and partially reversible (chronic obstructive airway disease) obstructive airway diseases. β₂-adrenoceptor expressed in human airway is the main β-receptor subtype, and its activation in airway smooth muscle cells leads to bronchodilatation. Drugs targeting β-adrenoceptors have been around for many years, for which agonists of the receptors are used in bronchodilation while antagonists are used in cardiovascular diseases. This review article summarizes the effect and usage of β₂-agonist in obstructive airway disease, addressing the benefits and potential risks of β₂-agonist. The article also looks at the safety of β-blocker usage for cardiovascular disease in patients with obstructive airway disease. There is also emerging evidence that non-selective β-blockers with inverse agonism ironically can have long-term beneficial effects in obstructive airway disease that is beyond cardiovascular protection. Further trials are urgently needed in this area as it might lead to a dramatic turnaround in clinical practice for obstructive airway diseases as has already been seen in the usage of β-blockers for heart failure.

Long-Acting Beta Agonists Enhance Allergic Airway Disease

Asthma is one of the most common of medical illnesses and is treated in part by drugs that activate the beta-2-adrenoceptor (β₂-AR) to dilate obstructed airways. Such drugs include long acting beta agonists (LABAs) that are paradoxically linked to excess asthma-related mortality. Here we show that LABAs such as salmeterol and structurally related β₂-AR drugs such as formoterol and carvedilol, but not short-acting agonists (SABAs) such as albuterol, promote exaggerated asthma-like allergic airway disease and enhanced airway constriction in mice. We demonstrate that salmeterol aberrantly promotes activation of the allergic disease-related transcription factor signal transducer and activator of transcription 6 (STAT6) in multiple mouse and human cells. A novel inhibitor of STAT6, PM-242H, inhibited initiation of allergic disease induced by airway fungal challenge, reversed established allergic airway disease in mice, and blocked salmeterol-dependent enhanced allergic airway disease. Thus, structurally related β₂-AR ligands aberrantly activate STAT6 and promote allergic airway disease. This untoward pharmacological property likely explains adverse outcomes observed with LABAs, which may be overcome by agents that antagonize STAT6.

Carvedilol binding to β2-adrenergic receptors inhibits CFTR-dependent anion secretion in airway epithelial cells

Carvedilol functions as a nonselective β-adrenergic receptor (AR)/α1-AR antagonist that is used for treatment of hypertension and heart failure. Carvedilol has been shown to function as an inverse agonist, inhibiting G protein activation while stimulating β-arrestin-dependent signaling and inducing receptor desensitization. In the present study, short-circuit current (Isc) measurements using human airway epithelial cells revealed that, unlike β-AR agonists, which increase Isc, carvedilol decreases basal and 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate-stimulated current. The decrease in Isc resulted from inhibition of the cystic fibrosis transmembrane conductance regulator (CFTR). The carvedilol effect was abolished by pretreatment with the β2-AR antagonist ICI-118551, but not the β1-AR antagonist atenolol or the α1-AR antagonist prazosin, indicating that its inhibitory effect on Isc was mediated through interactions with apical β2-ARs. However, the carvedilol effect was blocked by pretreatment with the microtubule-disrupting compound nocodazole. Furthermore, immunocytochemistry experiments and measurements of apical CFTR expression by Western blot analysis of biotinylated membranes revealed a decrease in the level of CFTR protein in monolayers treated with carvedilol but no significant change in monolayers treated with epinephrine. These results demonstrate that carvedilol binding to apical β2-ARs inhibited CFTR current and transepithelial anion secretion by a mechanism involving a decrease in channel expression in the apical membrane.

β-Blockers have differential effects on the murine asthma phenotype

BACKGROUND AND PURPOSE

Our previous studies have shown the β2 -adrenoceptor and its endogenous ligand, adrenaline, are required for development of the asthma phenotype in murine asthma models. Chronic administration of some, but not other, β-blockers attenuated the asthma phenotype and led us to hypothesize that biased signalling was the basis of their differential effects, experimentally and clinically.

EXPERIMENTAL APPROACH

We used mice with no detectable systemic adrenaline (PNMT(-/-) ) and wild-type (WT) mice to study the effects of four β-blockers, alprenolol, carvedilol, propranolol and nadolol, in an ovalbumin sensitization and challenge (Ova S/C) murine model of asthma. The parameters measured were inflammatory cell infiltration, mucous metaplasia and airway hyperresponsiveness. To interpret the pharmacological action of these ligands quantitatively, we conducted computer simulations of three-state models of receptor activation.

KEY RESULTS

Ova S/C PNMT(-/-) mice do not develop an asthma phenotype. Here, we showed that administration of alprenolol, carvedilol or propranolol in the absence of interference from adrenaline using Ova S/C PNMT(-/-) mice resulted in the development of an asthma phenotype, whereas nadolol had no effect. Ova S/C WT mice did develop an asthma phenotype, and administration of alprenolol, propranolol and carvedilol had no effect on the asthma phenotype. However, nadolol prevented development of the asthma phenotype in Ova S/C WT mice. Computer simulations of these four ligands were consistent with the isolated three-state receptor model.

CONCLUSION AND IMPLICATIONS

β-Blockers have different effects on the murine asthma phenotype that correlate with reported differences in activation or inhibition of downstream β2 -adrenoceptor signalling pathways.

Bronchial lesions of mouse model of asthma are preceded by immune complex vasculitis and induced bronchial associated lymphoid tissue (iBALT)

We systematically examined by immune histology the lungs of some widely used mouse models of asthma. These models include sensitization by multiple intraperitoneal injections of soluble ovalbumin (OVA) or of OVA with alum, followed by three intranasal or aerosol challenges 3 days apart. Within 24 h after a single challenge there is fibrinoid necrosis of arterial walls with deposition of immunoglobulin (Ig) and OVA and infiltration of eosinophilic polymorphonuclear cells that lasts for about 3 days followed by peribronchial B-cell infiltration and slight reversible goblet cell hypertrophy (GCHT). After two challenges, severe eosinophilic vasculitis is present at 6 h, increases by 72 h, and then declines; B-cell proliferation and significant GCHT and hyperplasia (GCHTH) and bronchial smooth muscle hypertrophy recur more prominently. After three challenges, there is significantly increased induced bronchus-associated lymphoid tissue (iBALT) formation, GCHTH, and smooth muscle hypertrophy. Elevated levels of Th2 cytokines, IL-4, IL-5, and IL-13, are present in bronchial lavage fluids. Sensitized mice have precipitating antibody and positive Arthus skin reactions but also develop significant levels of IgE antibody to OVA but only 1 week after challenge. We conclude that the asthma like lung lesions induced in these models is preceded by immune complex-mediated eosinophilic vasculitis and iBALT formation. There are elevations of Th2 cytokines that most likely produce bronchial lesions that resemble human asthma. However, it is unlikely that mast cell-activated atopic mechanisms are responsible as we found only a few presumed mast cells by toluidine blue and metachromatic staining limited to the most proximal part of the main stem bronchus, and none in the remaining main stem bronchus or in the lung periphery.

Epinephrine Activation of the β2-Adrenoceptor Is Required for IL-13-Induced Mucin Production in Human Bronchial Epithelial Cells

Mucus hypersecretion by airway epithelium is a hallmark of inflammation in allergic asthma and results in airway narrowing and obstruction. Others have shown that administration a T_H2 cytokine, IL-13 is sufficient to cause mucus hypersecretion in vivo and in vitro. Asthma therapy often utilizes β₂-adrenoceptor (β₂AR) agonists, which are effective acutely as bronchodilators, however chronic use may lead to a worsening of asthma symptoms. In this study, we asked whether β₂AR signaling in normal human airway epithelial (NHBE) cells affected mucin production in response to IL-13. This cytokine markedly increased mucin production, but only in the presence of epinephrine. Mucin production was blocked by ICI-118,551, a preferential β₂AR antagonist, but not by CGP-20712A, a preferential β₁AR antagonist. Constitutive β₂AR activity was not sufficient for IL-13 induced mucin production and β-agonist-induced signaling is required. A clinically important long-acting β-agonist, formoterol, was as effective as epinephrine in potentiating IL-13 induced MUC5AC transcription. IL-13 induced mucin production in the presence of epinephrine was significantly reduced by treatment with selective inhibitors of ERK1/2 (FR180204), p38 (SB203580) and JNK (SP600125). Replacement of epinephrine with forskolin + IBMX resulted in a marked increase in mucin production in NHBE cells in response to IL-13, and treatment with the inhibitory cAMP analogue Rp-cAMPS decreased mucin levels induced by epinephrine + IL-13. Our findings suggest that β₂AR signaling is required for mucin production in response to IL-13, and that mitogen activated protein kinases and cAMP are necessary for this effect. These data lend support to the notion that β₂AR-agonists may contribute to asthma exacerbations by increasing mucin production via activation of β₂ARs on epithelial cells.

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.

Effects of one month treatment with propranolol and metoprolol on the relaxant and contractile function of isolated trachea from rats exposed to cigarette smoke for four months

BACKGROUND AND OBJECTIVE

The effects of one-month treatment of β-adrenergic receptor (AR) blockers on the relaxant function of airway in chronic obstructive pulmonary disease (COPD) are unknown. A passive cigarette smoking rat model was used to investigate the effect of β-AR blockers.

METHODS

Thirty-six rats were randomly divided into six groups: control (C); propranolol (P); metoprolol (M); smoking (S); smoking + propranolol (SP); and smoking + metoprolol (SM). Histological analysis of the lungs was done and isolated tracheal segments were studied in vitro to assess the contractile and relaxant function of the smooth muscle. The level of norepinephrine (NE) was measured.

RESULTS

Airway inflammation was more severe in group S than group C (p < 0.001), and it was alleviated in group SP (p < 0.001) while had no change in group SM. After propranolol or metoprolol administration, the level of NE was significantly lower in both SP and SM groups (p < 0.01). The maximal contractile response to acetylcholine (Ach) in groups S and SM was increased compared to group C (p < 0.01). Propranolol significantly inhibited contractile response in group SP (p < 0.05), while metoprolol did not show the same effect. The curve of maximum relaxant rate to isoprenaline (Iso) shifted to the left in group SP (p < 0.01), while group SM showed no changes.

CONCLUSION

One-month treatment with propranolol increased the relaxant effect mediated by the β₂-AR and decreased the contractile response to Ach by reducing the level of NE.

Current and novel bronchodilators in respiratory disease

PURPOSE OF REVIEW

β2-Agonists and muscarinic antagonists are widely used to treat asthma and chronic obstructive pulmonary disease (COPD), and a number of novel drug targets are being investigated for potential clinical utility. This review will summarize current developments in the field.

RECENT FINDINGS

The clinical effectiveness of a number of once a day inhaled β2-agonists and muscarinic antagonists is a major advance providing sustained bronchodilation in asthma and COPD. The identification of novel targets (e.g. bitter taste receptor TASR2), the demonstration of clinical effectiveness of others [e.g. phosphodiesterase (PDE)3/4] and exploring the potential of inverse agonists/biased agonists are evidence of continuing interest in the development of novel bronchodilators.

SUMMARY

Novel long-acting β2-agonists (e.g. indacaterol, vilanterol, olodaterol and carmoterol) and muscarinic antagonists (e.g. tiotropium, aclidinium, glycopyrronium and umeclidinium bromide) document sustained bronchodilation and their combination provides additional benefits over monotherapy. Not surprisingly, inhaled long-acting β2-agonist and long-acting muscarinic antagonists remain the drugs of choice for maintenance bronchodilation. However, there is a continued interest in developing novel bronchodilators illustrated by the clinical effectiveness of long acting mixed PDE3/4 inhibitors, vasointestinal peptide adenylyl cyclase agonists and inverse agonists/biased agonists for the β2-adrenoceptor, and the identification of intracellular (e.g. Rho kinase, exchange proteins activated by cyclic AMP) and cell surface (e.g. TAS2R, natriuretic peptide receptor) targets.

Agonist-induced β2-adrenoceptor desensitization and downregulation enhance pro-inflammatory cytokine release in human bronchial epithelial cells

It is not clear whether increased asthma severity associated with long-term use of β2-adrenoceptor (β2-AR) agonists can be attributed to receptor degradation and increased inflammation. We investigated the cross-talk between β-AR agonist-mediated effects on β2-AR function and expression and cytokine release in human bronchial epithelial cells. In 16HBE14o(-) cells grown in the presence and absence of β-AR agonists and/or antagonists, the β2-AR density was assessed by radioligand binding; the receptor protein and mRNA was determined using laser scanning cytometer and RT-PCR; cAMP generation, the cytokines IL-6 and IL-8 release were determined using AlphaScreen Assay and ELISA, respectively. Isoprenaline (ISO) and salbutamol (Salbu) induced a concentration- and time-dependent significant decrease in β2-AR density. Both Salbu and ISO reduced cAMP generation in a concentration-dependent manner while in same cell culture the IL-6 and IL-8 release was significantly enhanced. These effects were antagonized to a greater extent by ICI 118.551 than by propranolol, but CGP 20712A had no effect. Reduction of the β2-AR protein and mRNA could be seen when cells were treated with ISO for 24 h. Our findings indicate a direct link between cytokine release and altered β2-AR expression and function in airway epithelial cells. β2-AR desensitization and downregulation induced by long-term treatment with β2-AR agonists during asthma may account for adverse reactions also due to enhanced release of pro-inflammatory mediators and should, thus, be considered in asthma therapy.

β2-Adrenoceptor involved in smoking-induced airway mucus hypersecretion through β-arrestin-dependent signaling

Progression of chronic obstructive pulmonary disease is associated with small airway obstruction by accumulation of inflammatory mucous exudates. However, the mechanism of mucin hypersecretion after exposure to cigarette smoke (CS) is still not clear. In this study, we explored the contribution of β2-adrenoceptor (β2-AR) signaling to CS extract (CSE)-induced mucus hypersecretion in vitro and examined the effect of a β-blocker on airway mucin hypersecretion in vivo. NCI-H292 epithelial cell line was used to determine the contribution of β2-AR signaling to CSE-induced MUC5AC production by treatment with β2-AR antagonists propranolol and ICI118551 and β2-AR-targeted small interfering RNA. The effect of propranolol on airway mucus hypersecretion was examined in a rat model exposed to CS. MUC5AC expression was assayed by real-time PCR, immunohistochemistry and ELISA. β2-AR and its downstream signaling were detected by western blot analysis. We found that pretreating NCI-H292 cells with propranolol, ICI118551 for 30 min or β2AR-targeted siRNA for 48 h reduced MUC5AC mRNA and protein levels stimulated by CSE. However,inhibiting the classical β2AR-cAMP-PKA pathway didn't attenuate CSE-induced MUC5AC production, while silencing β-arretin2 expression significantly decreased ERK and p38MAPK phosphorylation, thus reduced the CSE-stimulated MUC5AC production. In vivo, we found that administration of propranolol (25 mg kg(-1) d(-1)) for 28 days significantly attenuated the airway goblet cell metaplasia, mucus hypersecretion and MUC5AC expression of rats exposed to CS. From the study, β2-AR-β-arrestin2-ERK1/2 signaling was required for CS-induced airway MUC5AC expression. Chronic propranolol administration ameliorated airway mucus hypersecretion and MUC5AC expression in smoking rats. The exploration of these mechanisms may contribute to the optimization of β2-AR target therapy in chronic obstructive pulmonary disease.

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 ligands: pathway validation for novel GPCR therapeutics

G protein-coupled receptors (GPCRs), in recent years, have been shown to signal via multiple distinct pathways. Furthermore, biased ligands for some receptors can differentially stimulate or inhibit these pathways versus unbiased endogenous ligands or drugs. Biased ligands can be used to gain a deeper understanding of the molecular targets and cellular responses associated with a GPCR, and may be developed into therapeutics with improved efficacy, safety and/or tolerability. Here we review examples and approaches to pathway validation that establish the relevance and therapeutic potential of distinct pathways that can be selectively activated or blocked by biased ligands.

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.

Ligand bias prevents class equality among beta-blockers

β-Blockers are used for a wide range of diseases from hypertension to glaucoma. In some diseases/conditions all β-blockers are effective, while in others only certain subgroups are therapeutically beneficial. The best-documented example for only a subset of β-blockers showing clinical efficacy is in heart failure, where members of the class have ranged from completely ineffective, to drugs of choice for treating the disease. Similarly, β-blockers were tested in murine asthma models and two pilot clinical studies. A different subset was found to be effective for this clinical indication. These findings call into question the current system of classifying these drugs. To consider 'β-blockers', as a single class is misleading when considering their rigorous pharmacological definition and their appropriate clinical application.

Clinical issues of mucus accumulation in COPD

Airway mucus is part of the lung's native immune function that traps particulates and microorganisms, enabling their clearance from the lung by ciliary transport and cough. Mucus hypersecretion and chronic productive cough are the features of the chronic bronchitis and chronic obstructive pulmonary disease (COPD). Overproduction and hypersecretion by goblet cells and the decreased elimination of mucus are the primary mechanisms responsible for excessive mucus in chronic bronchitis. Mucus accumulation in COPD patients affects several important outcomes such as lung function, health-related quality of life, COPD exacerbations, hospitalizations, and mortality. Nonpharmacologic options for the treatment of mucus accumulation in COPD are smoking cessation and physical measures used to promote mucus clearance. Pharmacologic therapies include expectorants, mucolytics, methylxanthines, beta-adrenergic receptor agonists, anticholinergics, glucocorticoids, phosphodiesterase-4 inhibitors, antioxidants, and antibiotics.

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.

Prenatal and postnatal polycyclic aromatic hydrocarbon exposure, airway hyperreactivity, and Beta-2 adrenergic receptor function in sensitized mouse offspring

Despite data associating exposure to traffic-related polycyclic aromatic hydrocarbons (PAH) in asthma, mechanistic support has been limited. We hypothesized that both prenatal and early postnatal exposure to PAH would increase airway hyperreactivity (AHR) and that the resulting AHR may be insensitive to treatment with a β 2AR agonist drug, procaterol. Further, we hypothesized that these exposures would be associated with altered β 2AR gene expression and DNA methylation in mouse lungs. Mice were exposed prenatally or postnatally to a nebulized PAH mixture versus negative control aerosol 5 days a week. Double knockout β 2AR mice were exposed postnatally only. Prenatal exposure to PAH was associated with reduced β 2AR gene expression among nonsensitized mice offspring, but not increases in DNA methylation or AHR. Postnatal exposure to PAH was borderline associated with increased AHR among sensitized wildtype, but not knockout mice. In the first study that delivers PAH aerosols to mice in a relatively physiological manner, small effects on AHR and β 2AR gene expression, but not β 2AR agonist drug activity, were observed. If confirmed, the results may suggest that exposure to PAH, common ambient urban pollutants, affects β 2AR function, although the impact on the efficacy of β 2AR agonist drugs used in treating asthma remains uncertain.

Functional selectivity of G-protein-coupled receptors: from recombinant systems to native human cells

In the mid 1990s, it was assumed that a two-state model, postulating an inactive (R) state and an active (R*) state provides the molecular basis for GPCR activation. However, it became clear that this model could not accommodate many experimental observations. Accordingly, the two-state model was superseded by a multi-state model according to which any given ligand stabilizes a unique receptor conformation with distinct capabilities of activating down-stream G-proteins and β-arrestin. Much of this research was conducted with the β2-adrenoceptor in recombinant systems. At the molecular level, there is now no doubt anymore that ligand-specific receptor conformations, also referred to as functional selectivity, exist. This concept holds great potential for drug discovery in terms of developing drugs with higher selectivity for specific cells and/or cell functions and fewer side effects. A major challenge is the analysis for functional selectivity in native cells. Here, I discuss our current knowledge on functional selectivity of three representative GPCRs, the β2-adrenoceptor and the histamine H2- and H4-receptors, in recombinant systems and native human cells. Studies with human neutrophils and eosinophils support the concept of functional selectivity. A major strategy for the analysis of functional selectivity in native cells is to generate complete concentration/response curves with a large set of structurally diverse ligands for multiple parameters. Next, correlations of potencies and efficacies are analyzed, and deviations of the correlations from linearity are indicative for functional selectivity. Additionally, pharmacological inhibitors are used to dissect cell functions from each other.