Pleiotropic beta-agonist-promoted receptor conformations and signals independent of intrinsic activity

Biased TAS2R Bronchodilators Inhibit Airway Smooth Muscle Growth by Downregulating Phosphorylated Extracellular Signal-regulated Kinase 1/2

Bitter taste receptor (TAS2R) agonists dilate airways by receptor-dependent smooth muscle relaxation. Besides their coupling to relaxation, we have found that human airway smooth muscle (HASM) cell TAS2Rs activate (phosphorylate) extracellular signal-related kinase 1/2 (ERK1/2), but the cellular effects are not known. In the present study, we show in HASM cells that TAS2R agonists initially stimulate phosphorylated ERK1/2 (pERK1/2) but by 24 hours cause a marked (50-70%) downregulation of pERK1/2 without a change in total ERK1/2. It was hypothesized that TAS2R agonists suppress cell growth through this pERK1/2 downregulation. Agonist-dependent inhibition of cell proliferation was indeed found in HASM cells derived from normal and asthmatic human lungs, as well as in an immortalized HASM cell line. pERK1/2 downregulation was linked to downregulation of the upstream kinase MEK1/2 (mitogen-activated protein kinase/extracellular signal-regulated kinase). Various structurally diverse TAS2R agonists evoked a range of inhibition of HASM proliferation, the magnitude of which directly correlated with the downregulation of pERK1/2 (R² = 0.86). Some TAS2R agonists were as effective as pharmacological inhibitors of Raf1 and MEK1/2 in suppressing growth. siRNA silencing of TAS2Rs (subtypes 10, 14, and 31) ablated the pERK1/2 and growth-inhibitory effects of TAS2R agonists. These phenotypes were attenuated by inhibiting the TAS2R G protein G_αi and by knocking down β-arrestin 1/2, indicating a dual pathway, although there may be additional mechanisms involved in this HASM TAS2R multidimensional signaling. Thus, TAS2R agonist structure can be manipulated to maintain the relaxation response and can be biased toward suppression of HASM growth. The latter response is of potential therapeutic benefit in asthma, in which an increase in smooth muscle mass contributes to airway obstruction.

Differential long-term regulation of TAS2R14 by structurally distinct agonists

Bitter taste receptor-14 (TAS2R14) is a GPCR also expressed on human airway smooth muscle cells, which signals to intracellular [Ca²⁺], resulting in relaxation of the airway, and is a novel target for bronchodilators. Here, we examine long-term, agonist-promoted down-regulation of TAS2R14 expression because tachyphylaxis would be an undesirable therapeutic characteristic. Five TAS2R structurally distinct full agonists were studied to ascertain biasing away from down-regulation. Agonist exposure for 18 h caused minimal desensitization by diphenhydramine (DPD) compared with ∼50% desensitization with all other agonists. Agonists evoked β-arrestin recruitment to TAS2R14, which was not seen with a phosphoacceptor-deficient mutant, TAS2R14-10A. All agonists except for DPD also caused subsequent TAS2R14 internalization and trafficking via early and late endosomes to down-regulation. TAS2R14-10A failed to undergo these events with any agonist. Molecular docking showed that DPD has specific interactions deep within a binding pocket that are not observed with the other agonists, which may lock the receptor in a conformation that does not internalize and therefore does not undergo down-regulation. Thus, TAS2R14 is subject to β-arrestin-mediated internalization and subsequent down-regulation with chronic exposure to most agonists. However, by manipulating the agonist structure, biasing toward G-protein coupling but away from long-term down-regulation can be achieved.-Woo, J. A., Castaño, M., Goss, A., Kim, D., Lewandowski, E. M., Chen, Y., Liggett, S. B. Differential long-term regulation of TAS2R14 by structurally distinct agonists.

A CREB-mediated increase in miRNA let-7f during prolonged β-agonist exposure: a novel mechanism of β2-adrenergic receptor down-regulation in airway smooth muscle

β₂-Adrenergic receptors (β₂ARs) desensitize during continuous agonist activation, which manifests clinically as tachyphylaxis. β-Agonist desensitization of β₂ARs in human airway smooth muscle (HASM) cells is recognized in the treatment of asthma and may be related to poor outcomes. Rapid events in desensitization include receptor phosphorylation and internalization, but mechanisms responsible for the decrease in receptor protein after prolonged agonist exposure (down-regulation) are ill defined. The microRNA (miRNA) let-7f regulates β₂AR expression by translational repression. In cultured HASM cells from nonasthmatic and asthmatic lungs, 18 h of β-agonist exposure increased let-7f by 2-3-fold, concomitant with a ∼90% decrease in β₂ARs. Inhibition of let-7f attenuated this down-regulation response by ∼50%. The let-7f increase was found to be cAMP/PKA-dependent. The mechanism of the let-7f increase was found by chromatin immunoprecipitation to be from activated cAMP response element-binding protein (CREB) binding to the let-7f promoter, thereby increasing let-7f expression. Knockdown of CREB attenuated agonist-promoted β₂AR down-regulation by ∼50%. Thus, β₂AR down-regulation occurs as a result of not only internalized receptor degradation but also a novel cAMP/PKA/CREB-mediated increase in let-7f, which causes enhanced repression of the β₂AR gene, adrenoreceptor β₂ ( ADRB2) translation and represents ∼50% of the net loss of receptors observed after prolonged agonist exposure. This mechanism is apparent in asthmatic HASM cells, indicating relevance in a disease model.-Kim, D., Cho, S., Woo, J. A., Liggett, S. B. A CREB-mediated increase in miRNA let-7f during prolonged β-agonist exposure: a novel mechanism of β₂-adrenergic receptor down-regulation in airway smooth muscle.

β2-Adrenergic Receptors Chaperone Trapped Bitter Taste Receptor 14 to the Cell Surface as a Heterodimer and Exert Unidirectional Desensitization of Taste Receptor Function

Bitter taste receptors (TAS2Rs) are G-protein-coupled receptors now recognized to be expressed on extraoral cells, including airway smooth muscle (ASM) where they evoke relaxation. TAS2Rs are difficult to express in heterologous systems, with most receptors being trapped intracellularly. We find, however, that co-expression of β₂-adrenergic receptors (β₂AR) in HEK-293T routes TAS2R14 to the cell surface by forming receptor heterodimers. Cell surface TAS2R14 expression was increased by ∼5-fold when β₂AR was co-expressed. Heterodimer formation was shown by co-immunoprecipitation with tagged receptors, biomolecular fluorescence complementation, and merged confocal images. The dynamic nature of this interaction was shown by: a gene-dose relationship between transfected β₂AR and TAS2R14 expression, enhanced (up to 3-fold) TAS2R14 agonist stimulation of [Ca²⁺]_i with β₂AR co-transfection, ∼53% decrease in [Ca²⁺]_i signaling with shRNA knockdown of β₂AR in H292 cells, and ∼60% loss of [Ca²⁺]_i responsiveness in βAR knock-out mouse ASM. Once expressed on the surface, we detected unidirectional, conformation-dependent, interaction within the heterodimer, with β₂AR activation rapidly uncoupling TAS2R14 function (∼65% desensitization). Cross-talk was independent of β₂AR internalization and cAMP/PKA, and not accompanied by TAS2R14 internalization. With prolonged β-agonist exposure, TAS2R14 internalized, consistent with slow recycling of naked TAS2R14 in the absence of the heterodimeric milieu. In studies of ASM mechanics, rapid cross-talk was confirmed at the physiologic level, where relaxation from TAS2R14 agonist was decreased by ∼50% with β-agonist co-treatment. Thus the β₂AR acts as a double-edged sword: increasing TAS2R14 cell surface expression, but when activated by β-agonist, partially offsetting the expression phenotype by direct receptor:receptor desensitization of TAS2R14 function.

Drug-like actions of autoantibodies against receptors of the autonomous nervous system and their impact on human heart function

Antibodies against cholinergic and adrenergic receptors (adrenoceptors) are frequent in serum of patients with chronic heart failure. Their prevalence is associated with Chagas' disease, idiopathic dilated cardiomyopathy (DCM), and ischaemic heart disease. Among the epitopes targeted are first and second extracellular loops of the β-adrenergic (β-adrenoceptor) and M2 muscarinic receptor. β₁-adrenoceptor autoantibodies affect radioligand binding and cardiomyocyte function similar to agonists. Corresponding rodent immunizations induce symptoms compatible with chronic heart failure that are reversible upon removal of the antibodies, transferable via the serum and abrogated by adrenergic antagonists. In DCM patients, prevalence and stimulatory efficacy of β₁-adrenoceptor autoantibodies are correlated to the decline in cardiac function, ventricular arrhythmia and higher incidence of cardiac death. In conclusion, such autoantibodies seem to cause or promote chronic human left ventricular dysfunction by acting on their receptor targets in a drug-like fashion. However, the pharmacology of this interaction is poorly understood. It is unclear how the autoantibodies trigger changes in receptor activity and second messenger coupling and how that is related to the pathogenesis and severity of the associated diseases. Here, we summarize the available evidence regarding these issues and discuss these findings in the light of recent knowledge about the conformational activation of the human β₂-adrenoceptor and the properties of bona fide cardiopathogenic autoantibodies derived from immune-adsorption therapy of DCM patients. These considerations might contribute to the conception of therapy regimen aimed at counteracting or neutralizing cardiopathogenic receptor autoantibodies.

MicroRNA let-7 establishes expression of beta2-adrenergic receptors and dynamically down-regulates agonist-promoted down-regulation

Although β(2)-adrenergic receptors (β(2)AR) are expressed on most cell types, mechanisms that establish expression levels and regulate expression by chronic agonist remain unclear. The 3' UTR of ADRB2 has a conserved 8-nucleotide seed region that we hypothesized is targeted by the let-7 family of miRNAs leading to translational repression. In luciferase assays with transfected cells, luc-β(2)WT3'UTR had decreased expression when cotransfected with let-7f, but a mutated luc-β(2)3'UTR lacking the seed was unaffected by let-7f; a mutated let-7f also had no effect on luc-β(2)WT3'UTR expression. ADRB2 mRNA was in greater abundance in immunoprecipitates of Ago2, a core component of the miRNA-induced silencing complex, when cells were transfected with let-7f, but not with a mutated let-7f, indicating a direct interaction with the silencing mechanism. H292 cells transfected with let-7f caused ∼60% decrease in native β(2)AR expression, but transfection with let-7f-specific locked nucleic acid anti-miRNA increased β(2)AR expression by ∼twofold. We considered that an increase in let-7f leading to greater repression of translation contributes to agonist-promoted down-regulation. Paradoxically, in cells and in lungs from mice treated in vivo, an ∼50% decrease in let-7f occurs during long-term agonist exposure, indicating a counterregulatory event. Consistent with this notion, let-7f locked nucleic acid transfection caused depressed agonist-promoted down-regulation. Thus, let-7f miRNA regulates baseline β(2)AR expression and decreases in let-7f evoked by agonist attenuate down-regulation. This positive feedback loop has not previously been described for a G protein-coupled receptor and its miRNA. Methods to decrease let-7f expression in targeted cells may increase therapeutic responses to β-agonist by increasing β(2)AR expression or minimizing tachyphylaxis.

A homogeneous single-label time-resolved fluorescence cAMP assay

G-protein-coupled receptors (GPCRs) are an important class of pharmaceutical drug targets. Functional high-throughput GPCR assays are needed to test an increasing number of synthesized novel drug compounds and their function in signal transduction processes. Measurement of changes in the cyclic adenosine monophosphate (cAMP) concentration is a widely used method to verify GPCR activation in the adenylyl cyclase pathway. Here, a single-label time-resolved fluorescence and high-throughput screening (HTS)-feasible method was developed to measure changes in cAMP levels in HEK293(i) cells overexpressing either β(2)-adrenergic or δ-opioid receptors. In the quenching resonance energy transfer (QRET) technique, soluble quenchers reduce the signal of unbound europium(III)-labeled cAMP in solution, whereas the antibody-bound fraction is fluorescent. The feasibility of this homogeneous competitive assay was proven by agonist-mediated stimulation of receptors coupled to either the stimulatory G(s) or inhibitory G(i) proteins. The reproducibility of the assays was excellent, and Z' values exceeded 0.7. The dynamic range, signal-to-background ratio, and detection limit were compared with a commercial time-resolved fluorescence resonance energy transfer (TR-FRET) assay. In both homogeneous assays, similar assay parameters were obtained when adenylyl cyclase was stimulated directly by forskolin or via agonist-mediated activation of the G(s)-coupled β(2)AR. The advantage of using the single-label approach relates to the cost-effectiveness of the QRET system compared with the two-label TR-FRET assay as there is no need for labeling of two binding partners leading to reduced requirements for assay optimization.

The Effects of β (2) -Adrenoreceptor Activation on the Contractility, Ca-Signals and Nitric Oxide Production in the Mouse Atria

The effects of the selective β(2)-adrenoreceptor agonist (fenoterol) on the functioning of mouse atrial were studied using both tensometry and fluorescent methods. It has been demonstrated that with the use of a high concentration of fenoterol (in the range of 1-50 µM), there is a more significant positive inotropic effect observed within a shorter period of time. In the case of relatively low doses of fenoterol (1 and 5 µM), its contractility effects are observed 20 min after the application of agonist, whereby in the case of high concentrations (25, 50 and 300 µM), the effects appear within the first minutes. During the first 10-15 min, 5 µM fenoterol causes an increase in the amplitude of Ca-signals in cardiomyocytes (this indicates an increase in the concentration of Ca ions during systole) and the activation of NO synthesis. However, after 20 min, the production of NO decreases; while the amplitude of Ca-signals remains high. The application of 50 µM fenoterol leads to a rapid increase in the amplitude of Ca-signals: at the same time, it causes a decrease in the production of NO, which we found to begin to increase after 10 min of agonist application. It is suggested that the dynamics of the positive inotropic effect occurring under pharmacological stimulation of β(2)-adrenoreceptors depend on the rate of increase in the amplitude of Ca-signals and on the degree of NO synthesis.

Prevalence of beta-2 adrenergic receptor (beta 2 AR) polymorphisms and its influence on a model used to assess endothelial function using pulse wave analysis (PWA)

BACKGROUND

Pulse wave analysis (PWA) combined with beta(2)-agonist challenge has recently been used to assess endothelial function. beta-2 adrenergic receptor (beta(2)AR) polymorphisms may affect response to beta(2)-agonist. We determined whether beta(2)AR polymorphisms influence endothelial response in our model using PWA and salbutamol.

METHODS

388 healthy Malay subjects (177 males, 211 females) were genotyped for 5 functionally important single nucleotide polymorphisms (SNPs) of beta(2)AR; 298 subjects proceeded with assessment of endothelial function. The parameter augmentation index (AIx) was recorded non-invasively using SphygmoCor. Recording of AIx at baseline was followed by administration of 500 microg sublingual glyceryl trinitrate (GTN). AIx recordings were repeated at 3, 5, 10, 15 and 20 min post-GTN. Subjects then inhaled 400 microg of salbutamol before AIx recordings at 5 min intervals up to 20 min. Maximum changes in AIx after GTN and salbutamol represented endothelium independent and endothelium dependent vasodilatation (EDV) respectively.

RESULTS

Allele frequencies of mutated Gly16, Glu27, Ile164, -20C and -47C were 47%, 6.8%, 0%, 30% and 9.3% respectively. No significant differences in EDV were noted between genotype groups of each studied SNPs.

CONCLUSIONS

Assessment of endothelial function using PWA and salbutamol was not influenced by beta(2)AR polymorphisms.

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

BACKGROUND AND PURPOSE

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

EXPERIMENTAL APPROACH

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

KEY RESULTS

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

CONCLUSIONS AND IMPLICATIONS

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

Mechanisms of pharmacogenomic effects of genetic variation within the cardiac adrenergic network in heart failure

One of the goals of pharmacogenomics is the use of genetic variants to predict an individual's response to treatment. Although numerous candidate and genome-wide associations have been made for cardiovascular response-outcomes, little is known about how a given polymorphism imposes the phenotype. Such mechanisms are important, because they tie the observed human response to specific signaling alterations and thus provide cause-and-effect relationships, aid in the design of hypothesis-based clinical studies, can help to devise workaround drugs, and can reveal new aspects of the pathophysiology of the disease. Here we discuss polymorphisms within the adrenergic receptor network in the context of heart failure and beta-adrenergic receptor blocker therapy, where multiple approaches to understand the mechanism have been undertaken. We propose a comprehensive series of studies, ranging from transfected cells, transgenic mice, and ex vivo and in vitro human studies as a model approach to explore mechanisms of action of pharmacogenomic effects and extend the field beyond observational associations.

Three generations of ongoing controversies concerning the use of short acting beta-agonist therapy in asthma: a review

An increase in asthma mortality in 1960s noted by British authors stirred a debate about the use of beta-adrenergic therapy that has persisted in the medical literature. The cause appears to be isoproterenol and fenoterol overuse. A second debate evolved around the possible deleterious, pro-inflammatory effects, of the albuterol distomer. Most clinical studies showed improved bronchodilatation, but limited benefits from using levalbuterol. Recently, genotyping has uncovered a single nucleotide polymorphism at codon 16 that appears to affect the long term response to both regular and as needed use of albuterol, calling for a new genotype based therapeutic approach in asthma.

Alveolar epithelial beta2-adrenergic receptors

beta(2)-adrenergic receptors are present throughout the lung, including the alveolar airspace, where they play an important role for regulation of the active Na(+) transport needed for clearance of excess fluid out of alveolar airspace. beta(2)-adrenergic receptor signaling is required for up-regulation of alveolar epithelial active ion transport in the setting of excess alveolar edema. The positive, protective effects of beta(2)-adrenergic receptor signaling on alveolar active Na(+) transport in normal and injured lungs provide substantial support for the use of beta-adrenergic agonists to accelerate alveolar fluid clearance in patients with cardiogenic and noncardiogenic pulmonary edema. In this review, we summarize the role of beta(2)-adrenergic receptors in the alveolar epithelium with emphasis on their role in the regulation of alveolar active Na(+) transport in normal and injured lungs.