Beta 2 adrenergic receptors in asthma: a current perspective

Update on the Role of β2AR and TRPV1 in Respiratory Diseases

Respiratory diseases (RDs) constitute a common public health problem both in industrialized and developing countries. The comprehension of the pathophysiological mechanisms underlying these conditions and the development of new therapeutic strategies are critical for improving the quality of life of affected patients. β2-adrenergic receptor (β2AR) and transient receptor potential vanilloid 1 (TRPV1) are both involved in physiological responses in the airways. β2AR is implicated in bronchodilation, mucociliary clearance, and anti-inflammatory effects, while TRPV1 is involved in the mediation of pain and cough reflexes. In RDs, such as respiratory infections, asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis, the concentration and expression of these receptors can be altered, leading to significant consequences. In this review, we provided an update on the literature about the role of β2AR and TRPV1 in these conditions. We reported how the diminished or defective expression of β2AR during viral infections or prolonged therapy with β2-agonists can increase the severity of these pathologies and impact the prognosis. Conversely, the role of TRPV1 was pivotal in neuroinflammation, and its modulation could lead to innovative treatment strategies in specific patients. We indicate future perspectives and potential personalized treatments in RDs through a comprehensive analysis of the roles of these receptors in the physiological and pathological mechanisms of these pathologies.

Rhinovirus induces airway remodeling: what are the physiological consequences?

BACKGROUND

Rhinovirus infections commonly evoke asthma exacerbations in children and adults. Recurrent asthma exacerbations are associated with injury-repair responses in the airways that collectively contribute to airway remodeling. The physiological consequences of airway remodeling can manifest as irreversible airway obstruction and diminished responsiveness to bronchodilators. Structural cells of the airway, including epithelial cells, smooth muscle, fibroblasts, myofibroblasts, and adjacent lung vascular endothelial cells represent an understudied and emerging source of cellular and extracellular soluble mediators and matrix components that contribute to airway remodeling in a rhinovirus-evoked inflammatory environment.

MAIN BODY

While mechanistic pathways associated with rhinovirus-induced airway remodeling are still not fully characterized, infected airway epithelial cells robustly produce type 2 cytokines and chemokines, as well as pro-angiogenic and fibroblast activating factors that act in a paracrine manner on neighboring airway cells to stimulate remodeling responses. Morphological transformation of structural cells in response to rhinovirus promotes remodeling phenotypes including induction of mucus hypersecretion, epithelial-to-mesenchymal transition, and fibroblast-to-myofibroblast transdifferentiation. Rhinovirus exposure elicits airway hyperresponsiveness contributing to irreversible airway obstruction. This obstruction can occur as a consequence of sub-epithelial thickening mediated by smooth muscle migration and myofibroblast activity, or through independent mechanisms mediated by modulation of the β2 agonist receptor activation and its responsiveness to bronchodilators. Differential cellular responses emerge in response to rhinovirus infection that predispose asthmatic individuals to persistent signatures of airway remodeling, including exaggerated type 2 inflammation, enhanced extracellular matrix deposition, and robust production of pro-angiogenic mediators.

CONCLUSIONS

Few therapies address symptoms of rhinovirus-induced airway remodeling, though understanding the contribution of structural cells to these processes may elucidate future translational targets to alleviate symptoms of rhinovirus-induced exacerbations.

A cholesterol analog stabilizes the human β2-adrenergic receptor nonlinearly with temperature

In cell membranes, G protein-coupled receptors (GPCRs) interact with cholesterol, which modulates their assembly, stability, and conformation. Previous studies have shown how cholesterol modulates the structural properties of GPCRs at ambient temperature. Here, we characterized the mechanical, kinetic, and energetic properties of the human β2-adrenergic receptor (β2AR) in the presence and absence of the cholesterol analog cholesteryl hemisuccinate (CHS) at room temperature (25°C), at physiological temperature (37°C), and at high temperature (42°C). We found that CHS stabilized various structural regions of β2AR differentially, which changed nonlinearly with temperature. Thereby, the strongest effects were observed for structural regions that are important for receptor signaling. Moreover, at 37°C, but not at 25° or 42°C, CHS caused β2AR to increase and stabilize conformational substates to adopt to basal activity. These findings indicate that the nonlinear, temperature-dependent action of CHS in modulating the structural and functional properties of this GPCR is optimized for 37°C.

Choline chloride attenuates the allergic airway disease by inhibiting the lysophosphatidylcholine induced response in mouse model

Aims

Allergic airway disease manifestation is induced by lysophosphatidylcholine (LPC) through CD1d-restricted Natural killer T (NKT) cells. Choline chloride (ChCl) and LPC both have the “choline” moiety in their structure and this may interplay the effect in allergic airway disease pathway.

Main methods

To test the hypothesis, mice were sensitized with cockroach extract (CE); challenged with CE or exposed to LPC and were given ChCl 1hr later.

Key findings

A significant increase in Airway hyperresponsiveness (AHR), total and differential cell count, Th2 cytokines, 8-isoprostanes level in bronchoalveolar lavage fluid (BALF) and inflammation score based on lung histology were observed on challenge with CE or exposure to LPC (p ​< ​0.05) indicating LPC induced airway disease manifestation in mice. These parameters were reduced significantly after administering mice with ChCl (p ​< ​0.05). The inflammatory parameters were significantly increased in LPC exposed mice, not sensitized with CE, which were significantly decreased when mice were administered with ChCl demonstrating its role in the inhibition of LPC induced allergic airway disease manifestation. Docking of CD1d with LPC and ChCl indicated the competitive inhibition of LPC induced effect by ChCl. This was validated in vivo in the form of decreased CD1d-restricted NKT cells in BALF and lung of the immunized mice on ChCl administration. There was no effect of ChCl administration on CD1d expression in BALF and lung cells.

Significance

This study shows that ChCl attenuates the allergic response by inhibiting the LPC induced- NKT cell mediated AHR, inflammation and oxidative stress by competitive inhibition to LPC in binding to CD1d.

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ChCl down regulates LPC (critical for allergic manifestation) induced response.

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Results were validated in cockroach extract immunized mice model.

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In silico studies indicate competitive inhibition to LPC by ChCl in binding to CD1d.

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In silico results were also validated in vivo in terms of CD1d-restricted NKT cells.

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Study explains the mechanism of ChCl action against allergic disease.

Simultaneous Identification of Multiple Binding Sites in Proteins: A Statistical Mechanics Approach

We present an extension of the Poisson-Boltzmann model in which the solute of interest is immersed in an assembly of self-orienting Langevin water dipoles, anions, cations, and hydrophobic molecules, all of variable densities. Interactions between charges are controlled by electrostatics, while hydrophobic interactions are modeled with a Yukawa potential. We impose steric constraints by assuming that the system is represented on a cubic lattice. We also assume incompressibility; i.e., all sites of the lattice are occupied. This model, which we refer to as the Hydrophobic Dipolar Poisson-Boltzmann Langevin (HDPBL) model, leads to a system of two equations whose solutions give the water dipole, salt, and hydrophobic molecule densities, all of them in the presence of the others in a self-consistent way. We use those to study the organization of the ions, cosolvent, and solvent molecules around proteins. In particular, peaks of densities are expected to reveal, simultaneously, the presence of compatible binding sites of different kinds on a protein. We have tested and validated the ability of HDPBL to detect pockets in proteins that bind to hydrophobic ligands, polar ligands, and charged small probes as well as to characterize the binding sites of lipids for membrane proteins.

β2 -Adrenoceptor agonist activity of higenamine

Higenamine was included in the World Anti-Doping Agency (WADA) Prohibited Substances and Methods List as a β2 -adrenoceptor agonist in 2017, thereby resulting in its prohibition both in and out of competition. The present mini review describes the physiology and pharmacology of adrenoceptors, summarizes the literature addressing the mechanism of action of higenamine and extends these findings with previously unpublished in silico and in vitro work. Studies conducted in isolated in vitro systems, whole-animal preparations and a small number of clinical studies suggest that higenamine acts in part as a β2 -adrenoceptor agonist. In silico predictive tools indicated that higenamine and possibly a metabolite have a high probability of interacting with the β2 -receptor as an agonist. Stable expression of human β2 -receptors in Chinese hamster ovary (CHO) cells to measure agonist activity not only confirmed the activity of higenamine at β2 but also closely agreed with the in silico prediction of potency for this compound. These data confirm and extend literature findings supporting the inclusion of higenamine in the Prohibited List.

Presence and function of β-adrenergic receptors in primary equine bronchial epithelia cells

The β-adrenergic receptor (β-AR) plays an important role in regulating a variety of cell and organ functions in different animal species and is an important target in asthma pathogenesis and therapy. The β-AR expression and function in equine bronchial epithelial cells (EBEC) were not known but innervation and significant decrease in receptor level were reported in the equine bronchial tissues from asthmatic horses. ¹²⁵I-iodocyanopindolol (ICYP) binding studies were undertaken in primary freshly isolated and cultured EBEC to identify the presence of the β-ARs. The receptor distribution was assessed using subtype-selective β-AR antagonists (ICI 118 551 (β₂) and CGP 20712A (β₁). The β-AR function was confirmed by measuring the agonist-induced intracellular cAMP accumulation in freshly isolated and cultured EBEC. In both freshly isolated and cultured EBEC, the specific ICYP binding was saturable and of high affinity. The maximal receptor density (B_max) was 9763 ± 140 binding sites/cell (mean ± SEM, n = 7) and 10575 ± 194 binding sites/cell (mean ± SEM, n = 5) in freshly isolated and cultured EBEC, respectively. The receptor affinity to the ligand (K_D) was also not different between the two cell conditions. ICI 118.551 displaced ICYP with 25 000-fold higher affinity than CGP 20712A. Moreover, in both fresh isolated and cultured EBEC, cAMP-accumulation was stimulated with a rank-order of potency of isoproterenol > adrenaline > noradrenaline. These results highlight the β₂-AR to be a key subtype in both freshly isolated and cultured primary EBEC.

Co-spraying of carriers (mannitol-lactose) as a method to improve aerosolization performance of salbutamol sulfate dry powder inhaler

Although in dry powder inhaler (DPI) formulations a single carrier is used, a single carrier is not able to provide an excellent aerosolization performance when it is used in DPI formulations. Thereby, the aim of this study was to engineer a suitable ternary mixture of mannitol-lactose-leucine to be used in a DPI formulation with enhanced aerosolization performance. To this end, binary mixtures of mannitol:lactose containing a constant amount of leucine (5% w/w of carriers) were spray-dried as a single solution. Spray-dried samples were blended with salbutamol sulfate to determine the efficiency of their aerosolization performance. Interestingly, note that lactose was in its amorphous state stabilized by the presence of mannitol in the samples. Spray-dried mannitol without lactose showed a combination of the α- and β-polymorphic forms which was the case in all other ratios of mannitol:lactose. It was shown that the highest fine particle fraction (FPF) was 62.42 ± 4.21% which was obtained for the distinct binary mixtures (1:3 mannitol:lactose) compared to a single carrier. This study opens a new window to investigate further the implementation of binary mixtures of sugar carriers containing leucine in DPI formulations to overcome poor aerosolization performance the mentioned DPI formulations.

Biological Obstacles for Identifying In Vitro-In Vivo Correlations of Orally Inhaled Formulations

Oral inhalation of drugs is the classic therapy of obstructive lung diseases. In contrast to the oral route, the link between in vitro and in vivo findings is less well defined and predictive models and parameters for in vitro-in vivo correlations are missing. Frequently used in vitro models and problems in obtaining in vivo values to establish such models and to identify the action of formulations in vivo are discussed. It may be concluded that major obstacles to link in vitro parameters on in vivo action include lack of treatment adherence and incorrect use of inhalers by patients, variation in inhaler performance, changes by humidity, uncertainties about lung deposition, and difficulties to measure drug levels in epithelial lining fluid and tissue. Physiologically more relevant in vitro models, improvement in inhaler performance, and better techniques for in vivo measurements may help to better understand importance and interactions between individual in vitro parameters in pulmonary delivery.

Targeting sphingosine-1-phosphate signaling in lung diseases

Sphingosine-1-phosphate (S1P), a simple, bioactive sphingolipid metabolite, plays a key role, both intracellularly and extracellularly, in various cellular processes such as proliferation, survival, migration, inflammation, angiogenesis, and endothelial barrier integrity. The cellular S1P level is low and is tightly regulated by its synthesis and degradation. Sphingosine Kinases (SphKs) 1 and 2, catalyze the ATP-dependent phosphorylation of sphingosine to S1P, while the degradation is mediated by the reversible dephosphorylation catalyzed by the S1P phosphatases and lipid phosphate phosphatases and the irreversible degradation to hexadecenal and ethanolamine phosphate by sphingosine-1-phosphate lyase (S1PL). As a ligand for specific G-protein-coupled receptors, S1P1-5, which are differentially expressed in different cell types, S1P generates downstream signals that play crucial role in developmental and disease related pathologies. In addition to acting extracellularly on receptors located on the plasma membrane, S1P can also act intracellularly, independently of S1P1-5, affecting calcium homeostasis and cell proliferation. The SphKs /S1P /S1PL metabolic pathway is implicated in numerous human pathologies including respiratory disorders, thereby raising the possibility that manipulating intracellular S1P levels could offer therapeutic potential in ameliorating lung diseases. This review focuses on the prospects of targeting S1P signaling and S1P metabolizing enzymes using small molecule inhibitors, receptor agonists, and antagonists in the treatment of lung diseases.

Cholesterol increases kinetic, energetic, and mechanical stability of the human β2-adrenergic receptor

The steroid cholesterol is an essential component of eukaryotic membranes, and it functionally modulates membrane proteins, including G protein-coupled receptors. To reveal insight into how cholesterol modulates G protein-coupled receptors, we have used dynamic single-molecule force spectroscopy to quantify the mechanical strength and flexibility, conformational variability, and kinetic and energetic stability of structural segments stabilizing the human β(2)-adrenergic receptor (β(2)AR) in the absence and presence of the cholesterol analog cholesteryl hemisuccinate (CHS). CHS considerably increased the kinetic, energetic, and mechanical stability of almost every structural segment at sufficient magnitude to alter the structure and functional relationship of β(2)AR. One exception was the structural core segment of β(2)AR, which establishes multiple ligand binding sites, and its properties were not significantly influenced by CHS.

An environmental epigenetic study of ADRB2 5'-UTR methylation and childhood asthma severity

BACKGROUND

Beta-2 adrenergic receptor (ADRB2) is the primary target of both short- and long-acting beta-agonist asthma medications. ADRB2 5'-UTR methylation changes in blood have the potential to act as a surrogate biomarker of responsiveness to beta-agonist treatment and childhood asthma severity.

OBJECTIVE

To study the association between ADRB2 5'-UTR methylation, NO (2) exposure and childhood asthma severity.

METHODS

We compared ADRB2 5'-UTR methylation levels in blood between 60 children with mild asthma and 122 children with severe asthma using methylation-specific PCR. We also investigated potential joint effects between NO (2) exposure and ADRB2 5'-UTR methylation.

RESULTS

We found a significant association between intermediate (OR: 4.11, 95% CI: 1.58-10.73) and high levels (OR: 7.63, 95% CI: 3.02-19.26) of ADRB2 methylation and severe childhood asthma. In addition, we found a significant association between indoor exposure to NO (2) , an air pollutant and known asthmogen, and severe asthma among children exhibiting high ADRB2 methylation (OR: 4.59, 95% CI: 1.03-20.55) but no association among children exhibiting low levels of ADRB2 methylation (OR: 0.35, 95% CI: 0.01-14.13).

CONCLUSIONS AND CLINICAL RELEVANCE

These findings support the potential use of ADRB2 5'-UTR methylation as a biomarker of both asthma severity and risk for NO (2) -associated asthma exacerbations in children, and present the first evidence of an epigenetic link between an important environmental exposure and childhood asthma severity.

Ligand-specific interactions modulate kinetic, energetic, and mechanical properties of the human β2 adrenergic receptor

G protein-coupled receptors (GPCRs) are a class of versatile proteins that transduce signals across membranes. Extracellular stimuli induce inter- and intramolecular interactions that change the functional state of GPCRs and activate intracellular messenger molecules. How these interactions are established and how they modulate the functional state of GPCRs remain to be understood. We used dynamic single-molecule force spectroscopy to investigate how ligand binding modulates the energy landscape of the human β2 adrenergic receptor (β2 AR). Five different ligands representing either agonists, inverse agonists or neutral antagonists established a complex network of interactions that tuned the kinetic, energetic, and mechanical properties of functionally important structural regions of β2 AR. These interactions were specific to the efficacy profile of the ligands investigated and suggest that the functional modulation of GPCRs follows structurally well-defined interaction patterns.

β2-adrenergic receptor gene polymorphisms in normal and asthmatic individuals in the Eastern Province of Saudi Arabia

BACKGROUND AND OBJECTIVES

Several polymorphisms of the β2-adrenergic receptor (β2-AR) gene have been identified, including the amino acid substitution from arginine (Arg) to glycine (Gly) at codon 16 and from glutamine (Gln) to glutamic acid (Glu) at codon 27. These substitutions affect receptor function and show significantly more agonist-promoted receptor down-regulation than cells expressing the Arg 16/Gln 27 variants. Although the ethnic dependency of this polymorphism has been described in other populations, no studies investigating its relationship to asthma have been conducted in the Saudi population . Therefore, our main objective was to determine the prevalence of these two mutations among patients with asthma in the Eastern Province and in matched healthy controls.

DESIGN AND SETTING

A case-control study conducted at a university hospital among Saudi patients.

PATIENTS AND METHODS

Blood samples were collected from 73 asthmatic patients and from 85 controls, and the β2-AR gene polymorphisms at codon 16 and codon 27 were assessed by restriction fragment length polymorphism.

RESULTS

Although a significant difference was observed in genotype frequencies at codon 16 (Arg/Gly) between the asthmatic and normal control subjects (P<.05), no statistically significant difference was observed in allele frequencies between the two groups. In addition, no statistically significant differences were observed in genotype and allele frequencies at codon 27 (Gln/Glu) between the normal (control) and asthmatic groups (β2=0.75, P>.68). Using the THESIAS statistical program, no significant association of any haplotype with asthma was found.

CONCLUSIONS

Our findings indicate a poor association of individual single-nucleotide polymorphisms with asthma. However, further study is required to ascertain the interactions of different haplotypes and the response of patients with different haplotypes to various treatments.

Structure-based discovery of beta2-adrenergic receptor ligands

Aminergic G protein-coupled receptors (GPCRs) have been a major focus of pharmaceutical research for many years. Due partly to the lack of reliable receptor structures, drug discovery efforts have been largely ligand-based. The recently determined X-ray structure of the beta(2)-adrenergic receptor offers an opportunity to investigate the advantages and limitations inherent in a structure-based approach to ligand discovery against this and related GPCR targets. Approximately 1 million commercially available, "lead-like" molecules were docked against the beta(2)-adrenergic receptor structure. On testing of 25 high-ranking molecules, 6 were active with binding affinities <4 microM, with the best molecule binding with a K(i) of 9 nM (95% confidence interval 7-10 nM). Five of these molecules were inverse agonists. The high hit rate, the high affinity of the most potent molecule, the discovery of unprecedented chemotypes among the new inhibitors, and the apparent bias toward inverse agonists among the docking hits, have implications for structure-based approaches against GPCRs that recognize small organic molecules.

High-resolution crystal structure of an engineered human beta2-adrenergic G protein-coupled receptor

Heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors constitute the largest family of eukaryotic signal transduction proteins that communicate across the membrane. We report the crystal structure of a human beta2-adrenergic receptor-T4 lysozyme fusion protein bound to the partial inverse agonist carazolol at 2.4 angstrom resolution. The structure provides a high-resolution view of a human G protein-coupled receptor bound to a diffusible ligand. Ligand-binding site accessibility is enabled by the second extracellular loop, which is held out of the binding cavity by a pair of closely spaced disulfide bridges and a short helical segment within the loop. Cholesterol, a necessary component for crystallization, mediates an intriguing parallel association of receptor molecules in the crystal lattice. Although the location of carazolol in the beta2-adrenergic receptor is very similar to that of retinal in rhodopsin, structural differences in the ligand-binding site and other regions highlight the challenges in using rhodopsin as a template model for this large receptor family.

beta2-Adrenergic receptor polymorphisms and asthma in the North Indian population

INTRODUCTION

The beta(2)-adrenergic receptor (ADRB2) polymorphisms are known to be functionally relevant and disease modifying in subjects with asthma. However, the association of these polymorphisms with asthma remains to be established. Our objective is to investigate the association of the ADRB2 polymorphisms and haplotypes with asthma in North Indian subjects.

METHODS

A subset of 101 unrelated cases and 55 unrelated unaffected individuals were used for a case-control disease-association test.

RESULTS

Ten variable single nucleotide polymorphism (SNP) sites within a span of 2.193 kb were identified in the ADRB2 gene by the sequencing and genotyping of 351 bronchial asthma patients and healthy individuals. The distributions of genotype and allele frequencies for individual SNPs in the ADRB2 gene and ADRB2 haplotype frequencies were estimated in unrelated asthmatics and healthy individuals. No significant association was observed between ADRB2 genotypes and alleles with disease status after Bonferroni correction for multiple testing (reference p value = 0.0083). However, haplotype GGCTTTGCAA was found to be significantly associated with asthma (p = 0.021) in the studied population.

CONCLUSION

Our results suggest that there is likely to be a functional significance of the ADRB2 gene with asthma.

Localization of beta1-adrenergic receptors in the cochlea and the vestibular labyrinth

Sympathetic activation in a "fight or flight reaction" may put the sensory systems for hearing and balance into a state of heightened alert via beta1-adrenergic receptors (beta1-AR). The aim of the present study was to localize beta1-AR in the gerbil inner ear by confocal immunocytochemistry, to characterize beta1-AR by Western immunoblots, and to identify beta1-AR pharmacologically by measurements of cAMP production. Staining for beta1-AR was found in strial marginal cells, inner and outer hair cells, outer sulcus, and spiral ganglia cells of the cochlea, as well as in dark, transitional and supporting cells of the vestibular labyrinth. Receptors were characterized in microdissected inner ear tissue fractions as 55 kDa non-glycosylated species and as 160 kDa high-mannose-glycosylated complexes. Pharmacological studies using isoproterenol, ICI-118551 and CGP-20712A demonstrated beta1-AR as the predominant adrenergic receptor in stria vascularis and organ of Corti. In conclusion, beta1-AR are present and functional in inner ear epithelial cells that are involved in K+ cycling and auditory transduction, as well as in neuronal cells that are involved in auditory transmission.

G protein-coupled receptor kinase 5 regulates airway responses induced by muscarinic receptor activation

G protein-coupled receptors (GPCRs) transduce extracellular signals into intracellular events. The waning responsiveness of GPCRs in the face of persistent agonist stimulation, or desensitization, is a necessary event that ensures physiological homeostasis. GPCR kinases (GRKs) are important regulators of GPCR desensitization. GRK5, one member of the GRK family, desensitizes central M(2) muscarinic receptors in mice. We questioned whether GRK5 might also be an important regulator of peripheral muscarinic receptor responsiveness in the cardiopulmonary system. Specifically, we wanted to determine the role of GRK5 in regulating muscarinic receptor-mediated control of airway smooth muscle tone or regulation of cholinergic-induced bradycardia. Tracheal pressure, heart rate, and tracheal smooth muscle tension were measured in mice having a targeted deletion of the GRK5 gene (GRK5(-/-)) and littermate wild-type (WT) control mice. Both in vivo and in vitro results showed that the airway contractile response to a muscarinic receptor agonist was not different between GRK5(-/-) and WT mice. However, the relaxation component of bilateral vagal stimulation and the airway smooth muscle relaxation resulting from beta(2)-adrenergic receptor activation were diminished in GRK5(-/-) mice. These data suggest that M(2) muscarinic receptor-mediated opposition of airway smooth muscle relaxation is regulated by GRK5 and is, therefore, excessive in GRK5(-/-) mice. In addition, this study shows that GRK5 regulates pulmonary responses in a tissue- and receptor-specific manner but does not regulate peripheral cardiac muscarinic receptors. GRK5 regulation of airway responses may have implications in obstructive airway diseases such as asthma or chronic obstructive pulmonary disease.

Association of asthma with beta(2)-adrenergic receptor gene polymorphism and cigarette smoking

Recent studies have suggested that two polymorphisms of the beta(2)-adrenergic receptor (beta(2)AR) gene at codons 16 (arginine to glycine) and 27 (glutamine to glutamate) affect an individual's airway responsiveness, or response to acute or chronic beta(2)-agonist therapy but are not risk factors for asthma. We hypothesize that there is an interaction effect on asthma between the beta(2)AR gene polymorphisms and cigarette smoking. A case-control study was conducted in 128 asthma cases and 136 control individuals identified from 10,014 studied subjects in rural Anqing, China. Allele-specific polymerase chain reaction (PCR) was used to genotype beta(2)AR gene polymorphisms. Multiple logistic regression was used to adjust for potential confounding factors. We found a marginally significant interaction between cigarette smoking and beta(2)AR-16 genotype after adjusting for important confounding factors (p = 0.06). Specifically, we found that compared with never-smoking Gly-16 homozygotes, those ever-smokers who are Arg-16 homozygotes had a significantly increased risk of asthma (odds ratio [OR] = 7.81; 95% confidence interval [CI]: 2.07 to 29.5). This association showed a clear dose-response relationship with the number of cigarettes smoked. However, there was no significant association of asthma with polymorphisms of the beta(2)AR at position 27 (OR = 1.38; 95% CI: 0.69 to 2.73). Our study suggests a gene-environment interaction between the Arg-16 genotype and ever cigarette smoking with respect to the susceptibility of an individual to asthma.

Altered airway and cardiac responses in mice lacking G protein-coupled receptor kinase 3

Contraction and relaxation of airway smooth muscles is mediated, in part, by G protein-coupled receptors (GPCRs) and dysfunction of these receptors has been implicated in asthma. Phosphorylation of GPCRs, by G protein-coupled receptor kinase (GRK), is an important mechanism involved in the dampening of GPCR signaling. To determine whether this mechanism might play a role in airway smooth muscle physiology, we examined the airway pressure time index and heart rate (HR) responses to intravenous administration of the cholinergic agonist methacholine (MCh) in genetically altered mice lacking one copy of GRK2 (GRK2 +/-), homozygous GRK3 knockout (GRK3 -/-), and wild-type littermates. (GRK2 -/- mice die in utero.) GRK3 -/- mice demonstrated a significant enhancement in the airway response to 100 and 250 microgram/kg doses of MCh compared with wild-type and GRK2 +/- mice. GRK3 -/- mice also displayed an enhanced sensitivity of the airway smooth muscle response to MCh. In addition, GRK3 -/- mice displayed an altered HR recovery from MCh-induced bradycardia. Although direct stimulation of cardiac muscarinic receptors measured as vagal stimulation-induced bradycardia was similar in GRK3 -/- and wild-type mice, the baroreflex increase in HR associated with sodium nitroprusside-induced hypotension was significantly greater in GRK3 -/- than wild-type mice. Therefore, these data demonstrate that in the mouse, GRK3 may be involved in modulating the cholinergic response of airway smooth muscle and in regulating the chronotropic component of the baroreceptor reflex.

Differences between asthma exacerbations and poor asthma control

BACKGROUND

Increased variation in peak expiratory flow (PEF) is characteristic of poorly controlled asthma, and measurement of diurnal variability of PEF has been recommended for assessment of asthma severity, including during exacerbations. We aimed to test whether asthma exacerbations had the same PEF characteristics as poor asthma control.

METHODS

Electronic PEF records from 43 patients with initially poorly controlled asthma were examined for all exacerbations that occurred after PEF reached a plateau with inhaled corticosteroid treatment. Diurnal variability of PEF was compared during exacerbations, run-in (poor asthma control), and the period of stable asthma before each exacerbation.

FINDINGS

Diurnal variability was 21.3% during poor asthma control and improved to 5.3% (stable asthma) with inhaled corticosteroid treatment. 40 exacerbations occurred in 26 patients over 2-16 months; 38 (95%) of exacerbations were associated with symptoms of clinical respiratory infection. During exacerbations, consecutive PEF values fell linearly over several days then improved linearly. However, diurnal variability during exacerbations (7.7%) was not significantly higher than during stable asthma (5.4%, p=0.1). PEF data were consistent with impaired response to inhaled beta2-agonist during exacerbations but not during poorly controlled asthma.

INTERPRETATION

Asthmatics remain vulnerable to exacerbations during clinical respiratory infections, even after asthma is brought under control. Calculation of diurnal variability may fail to detect important changes in lung function. PEF variation is strikingly different during exacerbations compared with poor asthma control, suggesting differences in beta2-adrenoceptor function between these conditions.

Effects of enantiomers of beta 2-agonists on ACh release and smooth muscle contraction in the trachea

The beta 2-agonists currently used as bronchodilators are racemic mixtures of R- and S-enantiomers. In the present study, we examined the effects of enantiomers of the beta 2-agonists albuterol and formoterol on acetylcholine (ACh) release from equine trachealis parasympathetic nerves. ACh release was evoked by electrical field stimulation (20 V, 0.5 ms, 0.5 Hz) and measured by high-performance liquid chromatography coupled with electrochemical detection. We also tested the effects of enantiomers of albuterol and formoterol on equine tracheal smooth muscle (TSM) contraction in response to exogenous ACh. R- and RS-albuterol (10(-8) to 10(-5) M) and RR- and RR/SS-formoterol (10(-8) to 10(-5) M) augmented ACh release in a concentration-dependent manner. Beginning at 10(-6) M, SS-formoterol significantly increased ACh release, and at 10(-5) M, release increased by 71.9 +/- 8.7% over baseline. This effect was only observed, however, when the prejunctional muscarinic autoinhibitory effect of ACh was prevented with atropine. Both the RR- and SS-formoterol-induced increases in ACh release were abolished by the beta 2-antagonist ICI-118551 (3 x 10(-7) M). The effect of S-albuterol on ACh release was variable, and the mean increase induced by 10(-5) M was 30.8 +/- 16.1% in the presence of atropine. In the muscle tension study, R- and RS-albuterol and RR- and RR/SS-formoterol (10(-8) to 10(-5) M) but not the S-enantiomers inhibited TSM contraction. Even though R-enantiomers augment ACh release, they potently inhibit TSM contraction. Because racemic beta 2-agonists are bronchodilators on acute administration, the postjunctional spasmolytic effects of R-enantiomers predominate over the spasmogenic effect evoked via increased ACh release. The S-enantiomers, in contrast, do not inhibit TSM contraction and therefore would not contribute to the observed bronchodilation of the racemate. The S-enantiomers do prejunctionally facilitate ACh release when prejunctional muscarinic autoreceptors are dysfunctional, suggesting a potentially deleterious effect.

Dissociation between bronchial hyperreactivity in vivo and reduced beta-adrenoceptor sensitivity in vitro in allergen-challenged guinea pigs

In a recently developed guinea pig model of allergic asthma, we investigated the relationships between allergen-induced bronchial hyperreactivity in vivo, tracheal smooth muscle function in vitro, and the number of inflammatory cells in the bronchoalveolar lavage. At 6 h after allergen provocation (after the early asthmatic reaction) bronchial hyperreactivity to histamine aerosol was observed, which was still present, but reduced, at 24 h after the challenge (after the late asthmatic reaction). The severity of bronchial hyperreactivity at 6 h and at 24 h after each of four daily allergen provocations was progressively reduced. The contractile properties of tracheal smooth muscle preparations in response to methacholine or histamine were not changed at 6 h and 24 h after a single allergen provocation, as well as at 24 h after the fourth of the repeated provocations. However, the sensitivity to isoprenaline-induced relaxation of a half-maximal contraction obtained with methacholine or histamine was significantly reduced at 24 h after either a single or the fourth of the repeated provocations. The time course of the reduced beta-adrenoceptor sensitivity in vitro did not correlate with that of bronchial hyperreactivity in vivo. However, it was parallelled by a progressive infiltration of inflammatory cells in the airways, suggesting that mediators from these cells may decrease airway smooth muscle beta-adrenoceptor sensitivity.

Expression of beta 2-adrenergic receptor mRNA in peripheral lung in asthma and chronic obstructive pulmonary disease

Previous studies have indicated an increased number of beta 2-adrenergic receptors (beta 2AR) on bronchial smooth muscle in fatal asthma. This study evaluates the utility of autopsy lung for studies of gene expression and examines the hypothesis that increased expression of beta 2 AR mRNA in peripheral lung underlies the increased receptor number reported in central airways in fatal asthma. beta 2AR mRNA levels have been quantitated using the ribonuclease protection assay on RNA from peripheral lung obtained both at autopsy and thoracotomy from subjects with normal lungs as well as subjects with asthma or chronic obstructive pulmonary disease (COPD). Glucocorticosteroid and serum induction of beta 2AR mRNA in human epidermoid carcinoma A431 cells, which display a high abundance of beta 2AR receptors, was also examined to provide aliquots of RNA containing relatively high levels of beta 2AR mRNA for use as positive controls and internal standards. In A431 cells maintained after confluence in serum-free media for 72 h, maximal beta 2AR mRNA levels in response to 10% fetal bovine serum were 85% of maximal levels following serum plus 10 microM dexamethasone. Both autopsy and resected lung yielded undegraded RNA with a similar relative abundance of beta 2AR mRNA. Although geometric mean beta 2AR mRNA levels were similar in all three patient groups, relatively high levels were observed in resected lung in a subpopulation of subjects with mild or moderate asthma but not in autopsy lung from subjects with severe asthma. High levels of beta 2AR mRNA, presumably reflecting lung growth or asthma, were demonstrated in peripheral lung of a 4-yr-old child with asthma.(ABSTRACT TRUNCATED AT 250 WORDS)