The selectivity of beta-adrenoceptor antagonists at the human beta1, beta2 and beta3 adrenoceptors

α- and β-Adrenergic receptors differentially modulate the emission of spontaneous and amphetamine-induced 50-kHz ultrasonic vocalizations in adult rats

Amphetamine (AMPH) increases adult rat 50-kHz ultrasonic vocalizations, preferentially promoting frequency-modulated (FM) calls that have been proposed to reflect positive affect. The main objective of this study was to investigate a possible noradrenergic contribution to AMPH-induced calling. Adult male Long-Evans rats were tested with AMPH (1 mg/kg intraperitoneal) or saline combined with various systemic pretreatments: clonidine (α2 adrenergic agonist), prazosin (α1 antagonist), atipamezole (α2 antagonist), propranolol, betaxolol, and/or ICI 118,551 (β1/β2, β1, and β2 antagonists, respectively), nadolol (β1/β2 antagonist, peripheral only), or NAD-299 (5HT(1A) antagonist). In addition, effects of cirazoline (α1 adrenergic agonist) and cocaine (0.25-1.5 mg/kg intravenous) were studied alone. AMPH-induced calling was suppressed by low-dose clonidine and prazosin. Cirazoline and atipamezole did not significantly affect calling rate. Propranolol, without affecting the call rate, dose dependently promoted 'flat' calls under AMPH while suppressing 'trills,' thus reversing the effects of AMPH on the 'call subtype profile.' This effect of propranolol seemed to be mediated by simultaneous inhibition of CNS β1 and β2 rather than by 5HT(1A) receptors. Finally, cocaine elicited fewer calls than did AMPH, but produced the same shift in the call subtype profile. Taken together, these results reveal differential drug effects on flat vs trill vs other FM 50-kHz calls. These findings highlight the value of detailed call subtype analyses, and show that 50-kHz calls are associated with adrenergic α1- and β-receptor mechanisms. These preclinical findings suggest that noradrenergic contributions to psychostimulant subjective effects may warrant further investigation.

High-throughput analysis of ligand-induced internalization of β2-adrenoceptors using the coiled-coil tag-probe method

Receptor internalization is a useful indicator of the activity of ligands. The N-terminus of the β(2)-adrenergic receptor expressed on the cell surface was labeled with fluorophores using a novel coiled-coil labeling system. Endocytosis of the receptors was automatically detected using a fluorescence image analyzer by evaluating (1) translocation of the receptor from cell-surface to intracellular regions and (2) acidification in endosomes. Both parameters increased upon agonist stimulation in a dose-dependent manner. The extent of endocytosis was significantly dependent on the agonist used, indicating the presence of a biased signaling for endocytosis. The receptor antagonists can also be screened by competitive inhibition of agonist-induced endocytosis. The image analysis approach has proven to be useful for high-throughput characterization and screening of GPCR ligands.

Cardio-selective beta-blocker: pharmacological evidence and their influence on exercise capacity

For the past 40 years, beta-blockers have been widely used in cardiovascular medicine, reducing morbidity as well as mortality. Beta-blockers are currently used in a number of cardiovascular conditions such as systolic heart failure, postmyocardial infarction, and in prevention and treatment of arrhythmias. They are not recommended as the first line antihypertensive therapy, particularly in the elderly, unless there are specific indications. Despite the benefits of beta-blockers, tolerability concerns in patients with co-morbidities have limited their use. Some of these problems were overcome with the discovery of cardioselective beta-blockers. The third generation beta-blockers have additional properties of vasodilatation and advantages in terms of minimizing the adverse effects of beta-blockers. Some of the advantages include improvement of insulin resistance, decrease in cholesterol as well as alleviation of erectile dysfunction. Acute treatment with beta-blockers modifies local muscular metabolic properties and impairs endurance exercise capacity whereas the influence of chronic is debated controversially.

Biased signaling pathways in β2-adrenergic receptor characterized by 19F-NMR

Extracellular ligand binding to G protein-coupled receptors (GPCRs) modulates G protein and β-arrestin signaling by changing the conformational states of the cytoplasmic region of the receptor. Using site-specific (19)F-NMR (fluorine-19 nuclear magnetic resonance) labels in the β(2)-adrenergic receptor (β(2)AR) in complexes with various ligands, we observed that the cytoplasmic ends of helices VI and VII adopt two major conformational states. Changes in the NMR signals reveal that agonist binding primarily shifts the equilibrium toward the G protein-specific active state of helix VI. In contrast, β-arrestin-biased ligands predominantly impact the conformational states of helix VII. The selective effects of different ligands on the conformational equilibria involving helices VI and VII provide insights into the long-range structural plasticity of β(2)AR in partial and biased agonist signaling.

Cell-based and in-silico studies on the high intrinsic activity of two boron-containing salbutamol derivatives at the human β₂-adrenoceptor

Salbutamol is a well-known β(2) adrenoceptor (β(2)AR) partial agonist. We synthesized two boron-containing salbutamol derivatives (BCSDs) with greater potency and efficacy, compared to salbutamol, for inducing β(2)AR-mediated smooth-muscle relaxation in guinea-pig tracheal rings. However, the mechanism involved in this pharmacological effect remains unclear. In order to gain insight, we carried out binding and functional assays for BCSDs in HEK-293T cells transfected with the human β(2)AR (hβ(2)AR). The transfected hβ(2)AR showed similar affinity for BCSDs and salbutamol, but adenosine 3',5'-cyclic phosphate (cAMP) accumulation induced by both BCSDs was similar to that elicited by isoproterenol and greater than that induced by salbutamol. The boron-containing precursors (boric and phenylboronic acids, 100 μM) had no significant effect on salbutamol binding or salbutamol-induced cAMP accumulation. These experimental results are in agreement with theoretical docking simulations on lipid bilayer membrane-embedded hβ(2)AR structures. These receptors showed slightly higher affinity for BCSDs than for salbutamol. An essential change between putative active and inactive conformational states depended on the interaction of the tested ligands with the fifth, sixth and seventh transmembrane domains. Overall, these data suggest that BCSDs induce and stabilize conformational states of the hβ(2)AR that are highly capable of stimulating cAMP production.

Impedance responses reveal β₂-adrenergic receptor signaling pluridimensionality and allow classification of ligands with distinct signaling profiles

The discovery that drugs targeting a single G protein-coupled receptor (GPCR) can differentially modulate distinct subsets of the receptor signaling repertoire has created a challenge for drug discovery at these important therapeutic targets. Here, we demonstrate that a single label-free assay based on cellular impedance provides a real-time integration of multiple signaling events engaged upon GPCR activation. Stimulation of the β₂-adrenergic receptor (β₂AR) in living cells with the prototypical agonist isoproterenol generated a complex, multi-featured impedance response over time. Selective pharmacological inhibition of specific arms of the β₂AR signaling network revealed the differential contribution of G_s-, G_i- and Gβγ-dependent signaling events, including activation of the canonical cAMP and ERK1/2 pathways, to specific components of the impedance response. Further dissection revealed the essential role of intracellular Ca²⁺ in the impedance response and led to the discovery of a novel β₂AR-promoted Ca²⁺ mobilization event. Recognizing that impedance responses provide an integrative assessment of ligand activity, we screened a collection of β-adrenergic ligands to determine if differences in the signaling repertoire engaged by compounds would lead to distinct impedance signatures. An unsupervised clustering analysis of the impedance responses revealed the existence of 5 distinct compound classes, revealing a richer signaling texture than previously recognized for this receptor. Taken together, these data indicate that the pluridimensionality of GPCR signaling can be captured using integrative approaches to provide a comprehensive readout of drug activity.

Induction of intracellular calcium concentration by environmental benzo(a)pyrene involves a β2-adrenergic receptor/adenylyl cyclase/Epac-1/inositol 1,4,5-trisphosphate pathway in endothelial cells

Polycyclic aromatic hydrocarbons (PAHs) such as benzo(a)pyrene (B(a)P) are widely distributed environmental contaminants, known as potent ligands of the aryl hydrocarbon receptor (AhR). These chemicals trigger an early and transient increase of intracellular calcium concentration ([Ca(2+)](i)), required for AhR-related effects of PAHs. The mechanisms involved in this calcium mobilization were investigated in the present study. We demonstrated that B(a)P-mediated [Ca(2+)](i) induction was prevented in endothelial HMEC-1 cells by counteracting β2-adrenoreceptor (β2ADR) activity using pharmacological antagonists, anti-β2ADR antibodies, or siRNA-mediated knockdown of β2ADR expression; by contrast, it was strongly potentiated by β2ADR overexpression in human kidney HEK293 cells. B(a)P was shown, moreover, to directly bind to β2ADR, as assessed by in vitro binding assays and molecular modeling. Pharmacological inhibition and/or siRNA-mediated silencing of various signaling actors acting downstream of β2ADR in a sequential manner, such as G protein, adenylyl cyclase, Epac-1 protein, and inositol 1,4,5-trisphosphate (IP(3))/IP(3) receptor, were next demonstrated to prevent B(a)P-induced calcium signal. Inhibition or knockdown of these signaling elements, as well as the use of chemical β-blockers, were finally shown to counteract B(a)P-mediated induction of cytochrome P-450 1B1, a prototypical AhR target gene. Taken together, our results show that B(a)P binds directly to β2ADR and consequently utilizes β2ADR machinery to mobilize [Ca(2+)](i), through activation of a G protein/adenylyl cyclase/cAMP/Epac-1/IP(3) pathway. This β2ADR-dependent signaling pathway activated by PAHs may likely be crucial for PAH-mediated up-regulation of AhR target genes, thus suggesting a contribution of β2ADR to the health-threatening effects of these environmental pollutants.

Predicting in vivo cardiovascular properties of β-blockers from cellular assays: a quantitative comparison of cellular and cardiovascular pharmacological responses

β-Adrenoceptor antagonists differ in their degree of partial agonism. In vitro assays have provided information on ligand affinity, selectivity, and intrinsic efficacy. However, the extent to which these properties are manifest in vivo is less clear. Conscious freely moving rats, instrumented for measurement of heart rate (β1; HR) and hindquarters vascular conductance (β2; HVC) were used to measure receptor selectivity and ligand efficacy in vivo. CGP 20712A caused a dose-dependent decrease in basal HR (P<0.05, ANOVA) at 5 doses between 6.7 and 670 μg/kg (i.v.) and shifted the dose-response curve for isoprenaline to higher agonist concentrations without altering HVC responses. In contrast, at doses of 67 μg/kg (i.v.) and above, ICI 118551 substantially reduced the HVC response to isoprenaline without affecting HR responses. ZD 7114, xamoterol, and bucindolol significantly increased basal HR (ΔHR: +122 ± 12, + 129 ± 11, and + 59 ± 11 beats/min, respectively; n=6), whereas other β-blockers caused significant reductions (all at 2 mg/kg i.v.). The agonist effects of xamoterol and ZD 7114 were equivalent to that of the highest dose of isoprenaline. Bucindolol, however, significantly antagonized the response to the highest doses isoprenaline. An excellent correlation was obtained between in vivo and in vitro measures of β1-adrenoceptor efficacy (R(2)=0.93; P<0.0001).

The development of label-free cellular assays for drug discovery

INTRODUCTION

The need to improve drug research and development productivity continues to drive innovation in pharmacological assays. Technologies that can leverage the advantages of both molecular and phenotypic assays would hold great promise for discovery of new medicines.

AREAS COVERED

This article briefly reviews current label-free platforms for cell-based assays and is primarily focused on fundamental aspects of these assays using dynamic mass redistribution technology as an example. The article also presents strategies for relating label-free profiles to molecular modes of actions of drugs.

EXPERT OPINION

Emerging evidence suggests that label-free cellular assays are phenotypic in nature, yet permit molecular mechanistic deconvolution. Together with unique competency in throughput, sensitivity and pathway coverages, label-free cellular assays allow users to screen drugs against endogenous receptors in native cells (including disease relevant primary cells) and determine the molecular modes of action of drug molecules. However, there are challenges for label-free in both basic research and drug discovery: the deconvolution of the cellular and molecular mechanisms for the biosensor signatures of receptor-drug interactions, new methodologies for data analysis and the development of new biosensor technologies. These challenges will need to be met for the wide adoption of these assays in drug discovery.

GPCRs regulate the assembly of a multienzyme complex for purine biosynthesis

G protein-coupled receptors (GPCRs) transmit exogenous signals to the nucleus, promoting a myriad of biological responses via multiple signaling pathways in both healthy and cancerous cells. However, little is known about the response of cytosolic metabolic pathways to GPCR-mediated signaling. Here we applied fluorescent live-cell imaging and label-free dynamic mass redistribution assays to study whether purine metabolism is associated with GPCR signaling. Through a library screen of GPCR ligands in conjunction with live-cell imaging of a metabolic multienzyme complex for de novo purine biosynthesis, the purinosome, we demonstrated that the activation of endogenous Gα(i)-coupled receptors correlates with purinosome assembly and disassembly in native HeLa cells. Given the implications of GPCRs in mitogenic signaling and of the purinosome in controlling metabolic flux via de novo purine biosynthesis, we hypothesize that regulation of purinosome assembly and disassembly may be one of the downstream events of mitogenic GPCR signaling in human cancer cells.

Homology model and docking studies on porcine β₂ adrenoceptor: description of two binding sites

The affinity of the classical β(2) adrenoceptor-selective inverse agonist ICI118,551 is notoriously lower for porcine β(2) adrenoceptors (p(2)βAR) than for human β(2) adrenoceptors (hβ(2)AR) but molecular mechanisms for this difference are still unclear. Homology 3-D models of pβ(2)AR can be useful in predicting similarities and differences, which might in turn increase the comparative understanding of ligand interactions with the hβ(2)AR. In this work, the pβ(2)AR amino acid sequence was used to carry out homology modeling. The selected pβ(2)AR 3-D structure was structurally and energetically optimized and used as a model for further theoretical study. The homology model of pβ(2)AR has a 3-D structure very similar to the crystal structures of recently studied hβ(2)AR. This was also corroborated by sequence identity, RMSD, Ramachandran map, TM-score and docking results. Upon performing molecular docking simulations with the AutoDock4.0.1 program on pβ(2)AR, it was found that a set of well-known β(2)AR ligands reach two distinct binding sites on pβ(2)AR. Whereas one of these sites is similar to that reported on the hβ(2)AR crystal structure, the other can explain some important experimental observations. Additionally, the theoretical affinity estimated for ICI118,551 closely agrees with affinities estimated from experimental in vitro data. The experimental differences between the human/porcine β(2)ARs in relation to ligand affinity can in part be elucidated by observations in this molecular modeling study.

Synthesis and characterization of high-affinity 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene-labeled fluorescent ligands for human β-adrenoceptors

The growing practice of exploiting noninvasive fluorescence-based techniques to study G protein-coupled receptor pharmacology at the single cell and single molecule level demands the availability of high-quality fluorescent ligands. To this end, this study evaluated a new series of red-emitting ligands for the human β-adrenoceptor family. Upon the basis of the orthosteric ligands propranolol, alprenolol, and pindolol, the synthesized linker-modified congeners were coupled to the commercially available fluorophore BODIPY 630/650-X. This yielded high-affinity β-adrenoceptor fluorescent ligands for both the propranolol and alprenolol derivatives; however, the pindolol-based products displayed lower affinity. A fluorescent diethylene glycol linked propranolol derivative (18a) had the highest affinity (log K_D of −9.53 and −8.46 as an antagonist of functional β2- and β1-mediated responses, respectively). Imaging studies with this compound further confirmed that it can be employed to selectively label the human β2-adrenoceptor in single living cells, with receptor-associated binding prevented by preincubation with the nonfluorescent β2-selective antagonist 3-(isopropylamino)-1-[(7-methyl-4-indanyl)oxy]butan-2-ol (ICI 118551) (J. Cardiovasc. Pharmacol.1983, 5, 430–437.)

Molecular basis of ligand dissociation in β-adrenergic receptors

The important and diverse biological functions of β-adrenergic receptors (βARs) have promoted the search for compounds to stimulate or inhibit their activity. In this regard, unraveling the molecular basis of ligand binding/unbinding events is essential to understand the pharmacological properties of these G protein-coupled receptors. In this study, we use the steered molecular dynamics simulation method to describe, in atomic detail, the unbinding process of two inverse agonists, which have been recently co-crystallized with β₁ and β₂ARs subtypes, along four different channels. Our results indicate that this type of compounds likely accesses the orthosteric binding site of βARs from the extracellular water environment. Importantly, reconstruction of forces and energies from the simulations of the dissociation process suggests, for the first time, the presence of secondary binding sites located in the extracellular loops 2 and 3 and transmembrane helix 7, where ligands are transiently retained by electrostatic and Van der Waals interactions. Comparison of the residues that form these new transient allosteric binding sites in both βARs subtypes reveals the importance of non-conserved electrostatic interactions as well as conserved aromatic contacts in the early steps of the binding process.

Ligand-dependent perturbation of the conformational ensemble for the GPCR β2 adrenergic receptor revealed by HDX

Mechanism of G protein-coupled receptor (GPCR) activation and their modulation by functionally distinct ligands remains elusive. Using the technique of amide hydrogen/deuterium exchange coupled with mass spectrometry, we examined the ligand-induced changes in conformational states and stability within the beta-2-adrenergic receptor (β(2)AR). Differential HDX reveals ligand-specific alterations in the energy landscape of the receptor's conformational ensemble. The inverse agonists timolol and carazolol were found to be most stabilizing even compared with the antagonist alprenolol, notably in intracellular regions where G proteins are proposed to bind, while the agonist isoproterenol induced the largest degree of conformational mobility. The partial agonist clenbuterol displayed conformational effects found in both the inverse agonists and the agonist. This study highlights the regional plasticity of the receptor and characterizes unique conformations spanning the entire receptor sequence stabilized by functionally selective ligands, all of which differ from the profile for the apo receptor.

A stress response pathway regulates DNA damage through β2-adrenoreceptors and β-arrestin-1

The human mind and body respond to stress, a state of perceived threat to homeostasis, by activating the sympathetic nervous system and secreting the catecholamines adrenaline and noradrenaline in the 'fight-or-flight' response. The stress response is generally transient because its accompanying effects (for example, immunosuppression, growth inhibition and enhanced catabolism) can be harmful in the long term. When chronic, the stress response can be associated with disease symptoms such as peptic ulcers or cardiovascular disorders, and epidemiological studies strongly indicate that chronic stress leads to DNA damage. This stress-induced DNA damage may promote ageing, tumorigenesis, neuropsychiatric conditions and miscarriages. However, the mechanisms by which these DNA-damage events occur in response to stress are unknown. The stress hormone adrenaline stimulates β(2)-adrenoreceptors that are expressed throughout the body, including in germline cells and zygotic embryos. Activated β(2)-adrenoreceptors promote Gs-protein-dependent activation of protein kinase A (PKA), followed by the recruitment of β-arrestins, which desensitize G-protein signalling and function as signal transducers in their own right. Here we elucidate a molecular mechanism by which β-adrenergic catecholamines, acting through both Gs-PKA and β-arrestin-mediated signalling pathways, trigger DNA damage and suppress p53 levels respectively, thus synergistically leading to the accumulation of DNA damage. In mice and in human cell lines, β-arrestin-1 (ARRB1), activated via β(2)-adrenoreceptors, facilitates AKT-mediated activation of MDM2 and also promotes MDM2 binding to, and degradation of, p53, by acting as a molecular scaffold. Catecholamine-induced DNA damage is abrogated in Arrb1-knockout (Arrb1(-/-)) mice, which show preserved p53 levels in both the thymus, an organ that responds prominently to acute or chronic stress, and in the testes, in which paternal stress may affect the offspring's genome. Our results highlight the emerging role of ARRB1 as an E3-ligase adaptor in the nucleus, and reveal how DNA damage may accumulate in response to chronic stress.

The pharmacological effects of the thermostabilising (m23) mutations and intra and extracellular (β36) deletions essential for crystallisation of the turkey β-adrenoceptor

The X-ray crystal structure of the turkey β-adrenoceptor has recently been determined. However, mutations were introduced into the native receptor that was essential for structure determination. These may cause alterations to the receptor pharmacology. It is therefore essential to understand the effects of these mutations on the pharmacological characteristics of the receptor. This study examined the pharmacological effects of both the m23 mutations and the β36 deletions, both alone and then in combination in the β36-m23 mutant used in the crystallisation and structure determination of the turkey β-adrenoceptor. Stable CHO-K1 cell lines were made of each of the receptor mutants and the affinity and efficacy of ligands assessed by (3)H-CGP 12177 whole cell ligand binding, (3)H-cAMP accumulation, and CRE-SPAP gene transcription assays. The m23 mutations reduced affinity for agonists, partial agonists and neutral antagonists by about tenfold whilst the β36 deletions alone had no effect on ligand affinity. Both sets of changes appeared to reduce the agonist activation of the receptor. Both the m23 and the β36 receptors retained two active agonist-induced receptor conformations similar to that of the original tβtrunc receptor. The combined β36-m23 receptor bound ligands with similar affinity to the m23 receptor; however, agonist activation was only observed with a few agonists including the catecholamines. Although the combination of mutations severely reduced the activation ability, the final crystallised receptor (β36-m23) was still a fully functional receptor capable of binding agonist and antagonist ligands and activating intracellular agonist responses.

The β2-subtype of adrenoceptors mediates inhibition of pro-fibrotic events in human lung fibroblasts

Fibrosis is part of airway remodelling observed in bronchial asthma and COPD. Pro-fibrotic activity of lung fibroblasts may be suppressed by β-adrenoceptor activation. We aimed, first, to characterise the expression pattern of β-adrenoceptor subtypes in human lung fibroblasts and, second, to probe β-adrenoceptor signalling with an emphasis on anti-fibrotic actions. Using reverse transcription PCR, messenger RNA (mRNA) encoding β(2)-adrenoceptors was detected in MRC-5, HEL-299 and primary human lung fibroblasts, whereas transcripts for β(1)- and β(3)-adrenoceptors were not found. Real-time measurement of dynamic mass redistribution in MRC-5 cells revealed β-agonist-induced G(s)-signalling. Proliferation of MRC-5 cells (determined by [(3)H]-thymidine incorporation) was significantly inhibited by β-agonists including the β(2)-selective agonist formoterol (-logIC(50), 10.2) and olodaterol (-logIC(50), 10.6). Formoterol's effect was insensitive to β(1)-antagonism (GCP 20712, 3 μM), but sensitive to β(2)-antagonism (ICI 118,551; apparent, pA (2), 9.6). Collagen synthesis in MRC-5 cells (determined by [(3)H]-proline incorporation) was inhibited by β-agonists including formoterol (-logIC(50), 10.0) and olodaterol (-logIC(50), 10.3) in a β(2)-blocker-sensitive manner. α-Smooth muscle actin, a marker of myo-fibroblast differentiation, was down-regulated at the mRNA and the protein level by about 50% following 24 and 48 h exposure to 1 nM formoterol, a maximally active concentration. In conclusion, human lung fibroblasts exclusively express β(2)-adrenoceptors and these mediate inhibition of various markers of pro-fibrotic cellular activity. Under clinical conditions, anti-fibrotic actions may accompany the therapeutic effect of long-term β(2)-agonist treatment of bronchial asthma and COPD.

Human atrial β(1L)-adrenoceptor but not β₃-adrenoceptor activation increases force and Ca(2+) current at physiological temperature

BACKGROUND AND PURPOSE

It has been proposed that BRL37344, SR58611 and CGP12177 activate β₃-adrenoceptors in human atrium to increase contractility and L-type Ca(2+) current (I(Ca-L)). β₃-adrenoceptor agonists are potentially beneficial for the treatment of a variety of diseases but concomitant cardiostimulation would be potentially harmful. It has also been proposed that (-)-CGP12177 activates the low affinity binding site of the β₁-adrenoceptor in human atrium. We therefore used BRL37344, SR58611 and (-)-CGP12177 with selective β-adrenoceptor subtype antagonists to clarify cardiostimulant β-adrenoceptor subtypes in human atrium.

EXPERIMENTAL APPROACH

Human right atrium was obtained from patients without heart failure undergoing coronary artery bypass or valve surgery. Cardiomyocytes were prepared to test BRL37344, SR58611 and CGP12177 effects on I(Ca-L). Contractile effects were determined on right atrial trabeculae.

KEY RESULTS

BRL37344 increased force which was antagonized by blockade of β₁- and β₂-adrenoceptors but not by blockade of β₃-adrenoceptors with β₃-adrenoceptor-selective L-748,337 (1 µM). The β₃-adrenoceptor agonist SR58611 (1 nM-10 µM) did not affect atrial force. BRL37344 and SR58611 did not increase I(Ca-L) at 37°C, but did at 24°C which was prevented by L-748,337. (-)-CGP12177 increased force and I(Ca-L) at both 24°C and 37°C which was prevented by (-)-bupranolol (1-10 µM), but not L-748,337.

CONCLUSIONS AND IMPLICATIONS

We conclude that the inotropic responses to BRL37344 are mediated through β₁- and β₂-adrenoceptors. The inotropic and I(Ca-L) responses to (-)-CGP12177 are mediated through the low affinity site β(1L)-adrenoceptor of the β₁-adrenoceptor. β₃-adrenoceptor-mediated increases in I(Ca-L) are restricted to low temperatures. Human atrial β₃-adrenoceptors do not change contractility and I(Ca-L) at physiological temperature.

Are there functional β₃-adrenoceptors in the human heart?

β₃-Adrenoceptor mRNA is expressed in the human heart, but corresponding receptor protein has not yet consistently been demonstrated. Furthermore, their physiological role remains highly controversial. For example, in human atria these receptors apparently do not promote cAMP formation. Evidence presented in this issue of the BJP suggests that a previously reported β₃-adrenoceptor-mediated stimulation of Ca(2+) channels at room temperature is absent at physiological temperatures, and that β₃-adrenoceptors have no effect on atrial contraction. Drugs classified as β₃-adrenoceptor agonists cause contraction in human atria but in most cases this involves β₁- and/or β₂-adrenoceptors. In contrast, in human ventricles β₃-adrenoceptor agonists can exhibit negative inotropic effects, potentially involving Pertussis toxin-sensitive G-proteins and activation of a NO synthase. However, firmer pharmacological evidence is required that these effects indeed occur via β₃-adrenoceptors. Whether the expected future use of β₃-adrenoceptor agonists in the treatment of urinary bladder dysfunction is associated with adverse events related to cardiac function remains to be determined from clinical studies.

The increase in rat ventricular automaticity induced by salbutamol is mediated through β(1)- but not β(2)-adrenoceptors: role of phosphodiesterases

AIMS

While β(2)-adrenoceptor (AR) agonists are useful bronchodilators, they also produce cardiac arrhythmias. These agents are not fully selective and also activate β(1)-AR, but the involvement of β(1)-AR and β(2)-AR in the observed pro-arrhythmic effect has not been established. We studied the effect of β(1)-AR and β(2)-AR activation on ventricular automaticity and the role of phosphodiesterases (PDE) in regulating this effect.

MAIN METHODS

Experiments were performed in the spontaneously beating isolated right ventricle of the rat heart. We also measured cAMP production in this tissue.

KEY FINDINGS

The β(2)-AR agonist salbutamol (1-100 μM) produced a concentration-dependent increase in ventricular automaticity that was not affected by 50nM of the β(2)-AR antagonist ICI 118551. This effect was enhanced by the non-selective PDE inhibitor theophylline (100 μM) and by the selective PDE4 inhibitors rolipram (1 μM) and Ro 201724 (2 μM), but not modified by the selective PDE3 inhibitors cilostamide (0.3 μM) or milrinone (0.2 μM). The effects of salbutamol alone and in the presence of either theophylline or rolipram were virtually abolished by 0.1 μM β(1)-AR antagonist CGP 20712A. Salbutamol (10 μM) increased the cAMP concentration, and this effect was abolished by CGP 20712A (0.1 μM) but enhanced by theophylline (100 μM) or rolipram (1 μM). Cilostamide (0.3 μM) failed to modify the effect of salbutamol on cAMP concentration.

SIGNIFICANCE

These results indicate that the increase of ventricular automaticity elicited by salbutamol was exclusively mediated through β(1)-AR and enhanced by non-selective PDE inhibition with theophylline or selective PDE4 inhibition. However, PDE3 did not appear to regulate this effect.

Implications of nonshivering thermogenesis for energy balance regulation in humans

The incidence of the metabolic syndrome has reached epidemic levels in the Western world. With respect to the energy balance, most attention has been given to reducing energy (food) intake. Increasing energy expenditure is an important alternative strategy. Facultative thermogenesis, which is the increase in energy expenditure in response to cold or diet, may be an effective way to affect the energy balance. The recent identification of functional brown adipose tissue (BAT) in adult humans promoted a renewed interest in nonshivering thermogenesis (NST). The purpose of this review is to highlight the recent insight in NST, general aspects of its regulation, the major tissues involved, and its metabolic consequences. Sustainable NST in adult humans amounts to 15% of the average daily energy expenditure. Calculations based on the limited available literature show that BAT thermogenesis can amount to 5% of the basal metabolic rate. It is likely that at least a substantial part of NST can be attributed to BAT, but it is possible that other tissues contribute to NST. Several studies on mitochondrial uncoupling indicate that skeletal muscle is another potential contributor to facultative thermogenesis in humans. The general and synergistic role of the sympathetic nervous system and the thyroid axis in relation to NST is discussed. Finally, perspectives on BAT and skeletal muscle NST are given.

β-adrenoceptor agonist effects in experimental models of bladder dysfunction

β-adrenoceptor stimulation can enhance the storage function of the urinary bladder by acting on detrusor smooth muscle tone, mediator release from the urothelium and/or afferent nerve activity. In humans this may occur predominantly if not exclusively via the β₃-subtype. The effects of β-adrenoceptor agonists including several β₃-selective agonists have been studied in vitro and in vivo, in healthy animals of both genders and various age groups and in a wide range of animal (mostly rat) models of genetic or acquired bladder dysfunction. Such models included bladder irritation by intravesical instillation of acetic acid or prostaglandin E₂, bladder outlet obstruction, stroke, diabetes, spontaneously hypertensive rats, and NO synthase inhibition. Across all of these models β-adrenoceptor agonists had effects consistent with improved bladder storage function. β₃-adrenoceptor effects are resistant to agonist-induced desensitization in many cell types, but whether this also applies to the human bladder is unknown. The efficacy of β-adrenoceptor agonists appears to be largely unaffected by common polymorphisms of the β₃-adrenoceptor gene. Taken together these findings suggest that β₃-adrenoceptor agonists may become useful drugs for the treatment of bladder storage dysfunction, a view supported by recent phase III clinical studies for one such agent, mirabegron.

Evolution of β-blockers: from anti-anginal drugs to ligand-directed signalling

Sir James Black developed β-blockers, one of the most useful groups of drugs in use today. Not only are they being used for their original purpose to treat angina and cardiac arrhythmias, but they are also effective therapeutics for hypertension, cardiac failure, glaucoma, migraine and anxiety. Recent studies suggest that they might also prove useful in diseases as diverse as osteoporosis, cancer and malaria. They have also provided some of the most useful tools for pharmacological research that have underpinned the development of concepts such as receptor subtype selectivity, agonism and inverse agonism, and ligand-directed signalling bias. This article examines how β-blockers have evolved and indicates how they might be used in the future.

Functional and biochemical rationales for the 24-hour-long duration of action of olodaterol

β(2)-Adrenoceptor (β(2)-AR) agonists are powerful bronchodilators and play a pivotal role in the management of pulmonary obstructive diseases, such as asthma and chronic obstructive pulmonary disease. Although these agents first were used many years ago, progress in drug development has resulted in better tolerated, long-acting β(2)-AR agonists (LABAs), such as formoterol and salmeterol. Although LABAs have been on the market for several years, relatively little is known on the rationale(s) behind their long duration of action. In this study, we focused on olodaterol (previously known as BI1744CL), a novel inhaled LABA, which provides a bronchodilating effect lasting 24 h and is currently in Phase III clinical trials. To understand the rationale behind its long duration of action, different aspects of olodaterol were analyzed (i.e., its lipophilicity and propensity to accumulate in the lipid bilayer as well as its tight binding to the β(2)-AR). In line with its physicochemical properties, olodaterol associated moderately with lipid bilayers. Instead, kinetic as well as equilibrium binding studies indicated the presence of a stable [(3)H]olodaterol/β(2)-AR complex with a dissociation half-life of 17.8 h due to ternary complex formation. The tight binding of olodaterol to the human β(2)-AR and stabilization of the ternary complex were confirmed in functional experiments monitoring adenylyl cyclase activity after extensive washout. Taken together, binding, kinetic, and functional data support the existence of a stable complex with the β(2)-AR that, with a dissociation half-life >17 h, might indeed be a rationale for the 24-h duration of action of olodaterol.

Desirable properties of β3-adrenoceptor agonists: implications for the selection of drug development candidates

β3-adrenoceptor agonists are currently in clinical development for the treatment of overactive bladder and considered for several other indications. This Perspective discusses desirable properties of such drugs mainly based on the example of overactive bladder, but at least partly they should also be applicable to other indications of β(3)-adrenoceptor agonists or other drug classes and therapeutic areas. These include degree of selectivity for the molecular target in terms of affinity, intrinsic efficacy and ligand-directed signaling. The ability to cause agonist-induced desensitization and the potential impact of gene polymorphisms also need to be considered. Depending on intended indication, specific pharmacokinetic considerations may also apply. These findings challenge the usefulness of high-throughput screening assays based upon a single molecular response in an artificial system and emphasize the need for early use of in vivo testing in species considered to be predictive for the human situation.

The effects of acute and chronic nadolol treatment on β2AR signaling in HEK293 cells

Nadolol (NAD) is a β-adrenergic receptor blocker with inverse agonist activity at βARs. Previous studies in our laboratory showed that chronic treatment with NAD decreased airway resistance response (R (aw)) to the muscarinic agonist methacholine in a murine model of asthma while acute treatment with NAD increased R (aw) (Callaerts-Vegh et al., Proc Natl Acad Sci U S A 101:4948-4953, 2004). Chronic treatment with NAD also caused decreased airway inflammation and mucin content in a murine asthma model (Nguyen et al., Am J Respir Cell Mol Biol 38:256-262, 2008). In this study, we examined the effects of nadolol on β(2)AR levels and signaling components downstream of the β(2)AR using a line of HEK293 cells expressing human β(2)ARs. Chronic treatment with NAD increased β(2)AR protein levels and decreased receptor degradation, consistent with receptor stabilization by the inverse agonist. Basal cAMP levels decreased after 5 min of treatment with NAD but increased after a 24-h treatment. A 5-min treatment with NAD decreased forskolin-stimulated phosphorylation at the β(2)AR PKA site Ser 262 while a 24-h treatment with NAD increased it. In contrast, chronic treatment with NAD had no effect on phosphorylation of the β(2)AR GRK site at Ser 355, 356. Chronic treatment with NAD upregulated cellular levels of G(α)s but had no effect on G(α)i. Chronic NAD treatment therefore increases cellular cAMP levels by mechanisms that include the upregulation of β(2)AR and G(α)s. This effect may explain in part the beneficial effects of chronic nadolol treatment on airway contractility.

A full pharmacological analysis of the three turkey β-adrenoceptors and comparison with the human β-adrenoceptors

BACKGROUND

There are three turkey β-adrenoceptors: the original turkey β-adrenoceptor from erythrocytes (tβtrunc, for which the X-ray crystal structure has recently been determined), tβ3C and tβ4C-receptors. This study examined the similarities and differences between these avian receptors and mammalian receptors with regards to binding characteristics and functional high and low affinity agonist conformations.

METHODOLOGY/PRINCIPAL FINDINGS

Stable cell lines were constructed with each of the turkey β-adrenoceptors and 3H-CGP12177 whole cell binding, CRE-SPAP production and (3)H-cAMP accumulation assays performed. It was confirmed that the three turkey β-adrenoceptors are distinct from each other in terms of amino acid sequence and binding characteristics. The greatest similarity of any of the turkey β-adrenoceptors to human β-adrenoceptors is between the turkey β3C-receptor and the human β2-adrenoceptor. There are pharmacologically distinct differences between the binding of ligands for the tβtrunc and tβ4C and the human β-adrenoceptors (e.g. with CGP20712A and ICI118551). The tβtrunc and tβ4C-adrenoceptors appear to exist in at least two different agonist conformations in a similar manner to that seen at both the human and rat β1-adrenoceptor and human β3-adrenoceptors. The tβ3C-receptor, similar to the human β2-adrenoceptor, does not, at least so far, appear to exist in more than one agonist conformation.

CONCLUSIONS/SIGNIFICANCE

There are several similarities, but also several important differences, between the recently crystallised turkey β-adrenoceptor and the human β-adrenoceptors. These findings are important for those the field of drug discovery using the recently structural information from crystallised receptors to aid drug design. Furthermore, comparison of the amino-acid sequence for the turkey and human adrenoceptors may therefore shed more light on the residues involved in the existence of the secondary β-adrenoceptor conformation.

Can [I]-Iodocyanopindolol Label β(3)-Adrenoceptors in Rat Urinary Bladder?

β₃-Adrenoceptors have been demonstrated to mediate urinary bladder smooth muscle relaxation but proof of their expression at the protein level has been missing because of lack of suitable antibodies or radioligands. As among various available radioligands [¹²⁵I]-iodocyanopindolol ([¹²⁵I]-ICYP) exhibited the smallest problems in labeling cloned human β₃-adrenoceptors in previous studies, we have explored its suitability to label β₃-adrenoceptors in rat urinary bladder in saturation and competition radioligand binding experiments. Rat lung was used as an internal control and exhibited all characteristics expected from this tissue with regard to β1/β2-adrenoceptor labeling. Saturation and competition binding studies with [¹²⁵I]-ICYP in rat bladder yielded saturable binding sites with an affinity compatible with β₃-adrenoceptors. In competition experiments various agonists and antagonists largely exhibited a profile compatible with a population consisting largely of β₃-adrenoceptors. However, the binding competition properties of ICI 118,551 and SR 59,230A were not easily explained by the idea of labeling a homogeneous β₃-adrenoceptor population but interpretation of the data was limited by a high degree of non-specific binding in [¹²⁵I]-ICYP concentrations required to label the receptors. We conclude that [¹²⁵I]-ICYP can be used to label tissue β₃-adrenoceptors but results obtained with this ligand have to be interpreted with caution.

A fluorescent ligand-binding alternative using Tag-lite® technology

G-protein-coupled receptors (GPCRs) are crucial cell surface receptors that transmit signals from a wide range of extracellular ligands. Indeed, 40% to 50% of all marketed drugs are thought to modulate GPCR activity, making them the major class of targets in the drug discovery process. Binding assays are widely used to identify high-affinity, selective, and potent GPCR drugs. In this field, the use of radiolabeled ligands has remained so far the gold-standard method. Here the authors report a less hazardous alternative for high-throughput screening (HTS) applications by the setup of a nonradioactive fluorescence-based technology named Tag-lite(®). Selective binding of various fluorescent ligands, either peptidic or not, covering a large panel of GPCRs from different classes is illustrated, particularly for chemokine (CXCR4), opioid (δ, µ, and κ), and cholecystokinin (CCK1 and CCK2) receptors. Affinity constants of well-known pharmacological agents of numerous GPCRs are in line with values published in the literature. The authors clearly demonstrate that the Tag-lite binding assay format can be successfully and reproducibly applied by using different cellular materials such as transient or stable recombinant cells lines expressing SNAP-tagged GPCR. Such fluorescent-based binding assays can be performed with adherent cells or cells in suspension, in 96- or 384-well plates. Altogether, this new technology offers great advantages in terms of flexibility, rapidity, and user-friendliness; allows easy miniaturization; and makes it completely suitable for HTS applications.

Effects of β-adrenoceptor antagonists on anaphylactic hypotension in conscious rats

Anaphylactic shock is sometimes fatal or resistant to therapy in patients treated with propranolol, a nonselective β-adrenoceptor antagonist, against cardiovascular diseases. However, it remains unknown which subtype of β-adrenoceptors, β(1)- or β(2)-adrenoceptor, is primarily responsible for the detrimental effects of propranolol on anaphylactic hypotension. Effects of β(1)- and β(2)-adrenoceptor antagonists were therefore determined on the survival rate and systemic hypotension in conscious Sprague-Dawley rats that suffered from anaphylactic shock. Mean arterial pressure and portal venous pressure were simultaneously measured. The control rats showed a decrease in mean arterial pressure and an increase in portal venous pressure, but did not die within 48h after an injection of ovalbumin antigen. The survival rate of the rats pretreated with propranolol (1mg/kg; n=7), the selective β(2)-adrenoceptor antagonist ICI 118,551 (0.5mg/kg; n=7), or adrenalectomy (n=7) was significantly smaller than that with the selective β(1)-adrenoceptor antagonist atenolol (2mg/kg; n=7). However, the changes in mean arterial pressure and portal venous pressure were similar for 10min after antigen among any groups, although propranolol and atenolol attenuated the antigen-induced increase in heart rate. Furthermore, bolus injections of epinephrine (3μg/kg) at 3 and 5min after antigen prevented the death of the atenolol-pretreated rats, but only marginally prolonged the survival rates for the ICI 118,551- or propranolol-pretreated and adrenalectomized rats. In conclusion, in rat anaphylactic shock, inhibition of β(2)-adrenoceptor causes more detrimental effects than that of the β(1)-adrenoceptor. These β-adrenoceptor antagonists may exert detrimental effects on rat systemic anaphylaxis via inhibiting beneficial actions of catecholamines endogenously released from the adrenal gland.

The selectivity of beta-adrenoceptor agonists at human beta1-, beta2- and beta3-adrenoceptors

BACKGROUND AND PURPOSE

There are two important properties of receptor-ligand interactions: affinity (the ability of the ligand to bind to the receptor) and efficacy (the ability of the receptor-ligand complex to induce a response). Ligands are classified as agonists or antagonists depending on whether or not they have efficacy. In theory, it is possible to develop selective agonists based on selective affinity, selective intrinsic efficacy or both. This study examined the affinity and intrinsic efficacy of 31 beta-adrenoceptor agonists at the three human beta-adrenoceptors to determine whether the current agonists are subtype selective because of affinity or intrinsic efficacy.

EXPERIMENTAL APPROACH

Stable clonal CHO-K1 cell lines, transfected with either the human beta(1), beta(2) or beta(3)-adrenoceptor, were used, and whole-cell [(3)H]-CGP 12177 radioligand binding and [(3)H]-cAMP accumulation were measured.

KEY RESULTS

Several agonists were found to be highly subtype selective because of selective affinity (e.g. salmeterol and formoterol, for the beta(2)-adrenoceptor over the beta(1) or beta(3)), while others (e.g. isoprenaline) had little affinity-selectivity. However, the intrinsic efficacy of salmeterol, formoterol and isoprenaline was similar across all three receptor subtypes. Other ligands (e.g. denopamine for beta(1); clenbuterol, AZ 40140d, salbutamol for beta(2)) were found to have subtype-selective intrinsic efficacy. Several ligands appeared to activate two agonist conformations of the beta(1)- and beta(3)-adrenoceptors.

CONCLUSIONS AND IMPLICATIONS

There are agonists with subtype selectivity based upon both selective affinity and selective intrinsic efficacy. Therefore, there is scope to develop better selective agonists based upon both selective affinity and selective intrinsic efficacy.

Ghrelin secretion stimulated by {beta}1-adrenergic receptors in cultured ghrelinoma cells and in fasted mice

Ghrelin, an octanoylated peptide hormone produced in the stomach, rises dramatically in mouse plasma during chronic severe calorie deprivation, an event that is essential to maintain life. The mechanism for this increase is not understood. Here, we study the control of ghrelin secretion in tissue culture cells derived from mice bearing ghrelinomas induced by a tissue-specific SV40 T-antigen transgene. We found that the ghrelin-secreting cells express high levels of mRNA encoding beta(1)-adrenergic receptors. Addition of norepinephrine or epinephrine to the culture medium stimulated ghrelin secretion, and this effect was blocked by atenolol, a selective beta(1)-adrenergic antagonist. When WT mice were treated with reserpine to deplete adrenergic neurotransmitters from sympathetic neurons, the fasting-induced increase in plasma ghrelin was blocked. Inhibition was also seen following atenolol administration. We conclude that ghrelin secretion during fasting is induced by adrenergic agents released by sympathetic neurons and acting directly on beta(1) receptors on the ghrelin-secreting cells of the stomach.

Lateral paracapsular GABAergic synapses in the basolateral amygdala contribute to the anxiolytic effects of beta 3 adrenoceptor activation

Norepinephrine (NE) is known to play an integral role in the neurobiological response to stress. Exposure to stressful stimuli increases NE levels in brain regions that regulate stress and anxiety, like the basolateral amygdala (BLA). NE is thought to increase excitability in these areas through alpha- and beta-adrenoceptors (ARs), leading to increased anxiety. Surprisingly, recent studies have shown that systemic beta 3-AR agonist administration decreases anxiety-like behaviors, suggesting that beta 3-ARs may inhibit excitability in anxiety-related brain regions. Therefore, in this study we integrated electrophysiological and behavioral approaches to test the hypothesis that the anxiolytic effects of beta 3-AR agonists may be mediated by an increase in BLA GABAergic inhibition. We examined the effect of a selective beta 3-AR agonist, BRL37344 (BRL), on GABAergic synapses arising from local circuit interneurons and inhibitory synapses originating from a recently described population of cells called lateral paracapsular (LPCS) interneurons. Surprisingly, BRL selectively enhanced LPCS-evoked inhibitory postsynaptic currents (eIPSCs) with no effect on local GABAergic inhibition. BRL also had no effect on glutamatergic synaptic excitation within the BLA. BRL potentiation of LPCS eIPSCs was blocked by the selective beta 3-AR antagonist, SR59230A, or by intracellular dialysis of Rp-CAMPS (cAMP-dependent protein kinase inhibitor), and this enhancement was not associated with any changes in spontaneous IPSCs or LPCS paired-pulse ratio. BRL also increased the amplitude of unitary LPCS IPSCs (uIPSCs) with no effect on uIPSC failure rate. Finally, bilateral BLA microinjection of BRL reduced anxiety-like behaviors in an open-field assay and the elevated plus-maze. Collectively, these data suggest that beta 3-AR activation selectively enhances LPCS, but not local, BLA GABAergic synapses, and that increases in LPCS-mediated inhibition may contribute to the anxiolytic profile of beta 3-AR agonists.

Mapping the druggable allosteric space of G-protein coupled receptors: a fragment-based molecular dynamics approach

To address the problem of specificity in G-protein coupled receptor (GPCR) drug discovery, there has been tremendous recent interest in allosteric drugs that bind at sites topographically distinct from the orthosteric site. Unfortunately, structure-based drug design of allosteric GPCR ligands has been frustrated by the paucity of structural data for allosteric binding sites, making a strong case for predictive computational methods. In this work, we map the surfaces of the β₁ (β₁AR) and β₂ (β₂AR) adrenergic receptor structures to detect a series of five potentially druggable allosteric sites. We employ the FTMAP algorithm to identify ‘hot spots’ with affinity for a variety of organic probe molecules corresponding to drug fragments. Our work is distinguished by an ensemble-based approach, whereby we map diverse receptor conformations taken from molecular dynamics (MD) simulations totaling approximately 0.5 μs. Our results reveal distinct pockets formed at both solvent-exposed and lipid-exposed cavities, which we interpret in light of experimental data and which may constitute novel targets for GPCR drug discovery. This mapping data can now serve to drive a combination of fragment-based and virtual screening approaches for the discovery of small molecules that bind at these sites and which may offer highly selective therapies.

beta-Adrenergic activation of electrogenic K+ and Cl- secretion in guinea pig distal colonic epithelium proceeds via separate cAMP signaling pathways

Adrenergic stimulation of isolated guinea pig distal colonic mucosa produced transient Cl(-) and sustained K(+) secretion. Transient short-circuit current (I(sc)) depended on beta(2)-adrenergic receptors (beta(2)-AdrR), and sustained I(sc) relies on a beta(1)-AdrR/beta(2)-AdrR complex. Epinephrine (epi) increased cAMP content with a biphasic time course similar to changes in epi-activated I(sc) ((epi)I(sc)). Inhibition of transmembrane adenylyl cyclases (tmACs) reduced peak (epi)I(sc) and cAMP to near zero without decreasing sustained (epi)I(sc), consistent with cAMP from tmAC signaling for only Cl(-) secretion. Inhibition of soluble adenylyl cyclase (sAC) reduced sustained (epi)I(sc) and cAMP to near zero without decreasing peak (epi)I(sc) or cAMP, consistent with cAMP from sAC signaling for K(+) secretion. Sensitivity to phosphodiesterase (PDE) inhibitors and peptide YY (PYY) stimulation further supported separate signaling for the two components. PDE3 or PDE4 inhibitors enhanced peak (epi)I(sc) but not sustained (epi)I(sc), consistent with these PDEs as part of the beta(2)-AdrR signaling domain. PYY suppressed peak (epi)I(sc) in a pertussis toxin (PTx)-sensitive manner, supporting Galpha(i)-dependent inhibition of tmACs producing cAMP for Cl(-) secretion. Since PYY or PTx did not alter sustained (epi)I(sc), signaling for K(+) secretion occurred via a Galpha(i)-independent mechanism. Presence of multiple sAC variants in colonic epithelial cells was supported by domain-specific antibodies. Responses to specific activators and inhibitors suggested that protein kinase A was not involved in activating peak or sustained components of (epi)I(sc), but the cAMP-dependent guanine nucleotide exchange factor, Epac, may contribute. Thus beta-adrenergic activation of electrogenic Cl(-) and K(+) secretion, respectively, required tmAC- and sAC-dependent signaling pathways.

A novel chemogenomics analysis of G protein-coupled receptors (GPCRs) and their ligands: a potential strategy for receptor de-orphanization

Background

G protein-coupled receptors (GPCRs) represent a family of well-characterized drug targets with significant therapeutic value. Phylogenetic classifications may help to understand the characteristics of individual GPCRs and their subtypes. Previous phylogenetic classifications were all based on the sequences of receptors, adding only minor information about the ligand binding properties of the receptors. In this work, we compare a sequence-based classification of receptors to a ligand-based classification of the same group of receptors, and evaluate the potential to use sequence relatedness as a predictor for ligand interactions thus aiding the quest for ligands of orphan receptors.

Results

We present a classification of GPCRs that is purely based on their ligands, complementing sequence-based phylogenetic classifications of these receptors. Targets were hierarchically classified into phylogenetic trees, for both sequence space and ligand (substructure) space. The overall organization of the sequence-based tree and substructure-based tree was similar; in particular, the adenosine receptors cluster together as well as most peptide receptor subtypes (e.g. opioid, somatostatin) and adrenoceptor subtypes. In ligand space, the prostanoid and cannabinoid receptors are more distant from the other targets, whereas the tachykinin receptors, the oxytocin receptor, and serotonin receptors are closer to the other targets, which is indicative for ligand promiscuity. In 93% of the receptors studied, de-orphanization of a simulated orphan receptor using the ligands of related receptors performed better than random (AUC > 0.5) and for 35% of receptors de-orphanization performance was good (AUC > 0.7).

Conclusions

We constructed a phylogenetic classification of GPCRs that is solely based on the ligands of these receptors. The similarities and differences with traditional sequence-based classifications were investigated: our ligand-based classification uncovers relationships among GPCRs that are not apparent from the sequence-based classification. This will shed light on potential cross-reactivity of GPCR ligands and will aid the design of new ligands with the desired activity profiles. In addition, we linked the ligand-based classification with a ligand-focused sequence-based classification described in literature and proved the potential of this method for de-orphanization of GPCRs.

A vHTS approach for the identification of beta-adrenoceptor ligands

Using a vHTS based on a pharmacophore alignment on known beta3-adrenoceptor ligands, a set of intriguing beta-adrenoceptor ligands was identified, optimization of which resulted in a selective and potent human beta2-AR antagonist.

An efficient asymmetric synthesis of the potent beta-blocker ICI-118,551 allows the determination of enantiomer dependency on biological activity

A highly efficient, practical and flexible two-step asymmetric synthesis of the beta(2)-selective beta-blocker ICI 118,551 is reported, allowing an unambiguous determination of the dependency of biological activity with optical activity, revealing the S,S-enantiomer to be the most potent.

Function of cardiac beta1- and beta2-adrenoceptors of newborn piglets: role of phosphodiesterases PDE3 and PDE4

The structures of porcine and human beta(2)-adrenoceptors differ but the repercussions for porcine cardiac function are unknown. We investigated the function of porcine beta(2)-adrenoceptors in 3 cardiac regions, sinoatrial node, left atrium and right ventricle of newborn piglets. Both (-)-noradrenaline and (-)-adrenaline caused sinoatrial tachycardia: 60+/-10% and 62+/-7% of the maximum response (E(max)) to (-)-noradrenaline (-logEC(50)=9.0) and (-)-adrenaline (-logEC(50)=7.5) respectively, were resistant to antagonism by the beta(1)-selective CGP20712A (2-hydroxy-5-[2-[[2-hydroxy-3-[4-[1-methyl-4-(trifluoromethyl)-1H-imidazol-2-yl]phenoxy]propyl]amino]ethoxy]-benzamide) (300 nM) but antagonized by beta(2)-selective ICI118551 (erythro(+/-)-[1-(2,3-dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol) (50 nM), consistent with mediation through beta(2)-adrenoceptors. The phosphodiesterase3-selective inhibitor cilostamide and phosphodiesterase4-selective inhibitor rolipram did not affect catecholamine chronotropic potencies. Only small CGP20712A-resistant positive inotropic effects of (-)-adrenaline were detected in the left atria (13+/-2% of E(max)) and ventricular trabeculae (14+/-5% of E(max)). The atrial inotropic responses to (-)-noradrenaline and (-)-adrenaline faded; fades were prevented by rolipram but not cilostamide or concurrent cilostamide+rolipram respectively. (-)-Noradrenaline (ICI118551 present) increased left atrial cAMP levels through beta(1)-adrenoceptors that were markedly enhanced by rolipram but unaffected by cilostamide. Concurrent cilostamide+rolipram uncovered inotropic and cAMP responses to (-)-adrenaline (CGP20712A present). We conclude that sinoatrial beta(2)-adrenoceptors are more important than beta(1)-adrenoceptors in the mediation of tachycardia caused by both (-)-noradrenaline and (-)-adrenaline in the newborn piglet. beta(2)-adrenoceptors have only a minor role in the mediation of left atrial and ventricular inotropic effects of (-)-adrenaline. Catecholamine-evoked tachycardia is not controlled by PDE3 or PDE4. PDE4, but not PDE3, controls the atrial inotropic and cAMP beta(1)-adrenoceptor-mediated responses to (-)-noradrenaline. Both PDE3 and PDE4 blunt left atrial inotropic and cAMP responses to (-)-adrenaline through beta(2)-adrenoceptors.

Docking studies on a refined human beta(2) adrenoceptor model yield theoretical affinity values in function with experimental values for R-ligands, but not for S-antagonists

G-protein coupled receptors (GPCR) belong to the largest group of membrane proteins involved in signal transduction. These receptors are implicated in diverse physiological and pathological events. The human beta(2) adrenergic receptor (hbeta(2)AR) is one of the few GPCRs whose 3-D structures are available on the Protein Data Bank. Because there is great interest by drug developers for hbeta(2)AR as a target, it is necessary to study its ligand-recognition process at the atomic level. The hbeta(2)AR can recognize both R/S enantiomeric ligands, R-agonists result in a greater activation than do S-agonists (eutomers and distomers for activation, respectively), according to experimental results. In this work is reported the ligand recognition on a refined hbeta(2)AR-structure of a set of well-known R/S-ligands by means of docking studies. Data obtained in silico were analyzed and compared with those reported in vitro. The theoretical affinity values were reproduced for agonists, but not for antagonist (or inverse agonists). However, theoretical data for R-antagonists are in function to experimental data. The theoretical results confirm the role of amino acids previously reported by mutagenesis studies due to their important roles in drug affinity and stereoselectivity.

Beta 3-adrenoreceptor regulation of nitric oxide in the cardiovascular system

The presence of a third beta-adrenergic receptor (beta 3-AR) in the cardiovascular system has challenged the classical paradigm of sympathetic regulation by beta1- and beta2-adrenergic receptors. While beta 3-AR's role in the cardiovascular system remains controversial, increasing evidence suggests that it serves as a "brake" in sympathetic overstimulation - it is activated at high catecholamine concentrations, producing a negative inotropic effect that antagonizes beta1- and beta2-AR activity. The anti-adrenergic effects induced by beta 3-AR were initially linked to nitric oxide (NO) release via endothelial NO synthase (eNOS), although more recently it has been shown under some conditions to increase NO production in the cardiovascular system via the other two NOS isoforms, namely inducible NOS (iNOS) and neuronal NOS (nNOS). We summarize recent findings regarding beta 3-AR effects on the cardiovascular system and explore its prospective as a therapeutic target, particularly focusing on its emerging role as an important mediator of NO signaling in the pathogenesis of cardiovascular disorders.

Ligand-specific regulation of the extracellular surface of a G-protein-coupled receptor

G-protein-coupled receptors (GPCRs) are seven-transmembrane proteins that mediate most cellular responses to hormones and neurotransmitters. They are the largest group of therapeutic targets for a broad spectrum of diseases. Recent crystal structures of GPCRs have revealed structural conservation extending from the orthosteric ligand-binding site in the transmembrane core to the cytoplasmic G-protein-coupling domains. In contrast, the extracellular surface (ECS) of GPCRs is remarkably diverse and is therefore an ideal target for the discovery of subtype-selective drugs. However, little is known about the functional role of the ECS in receptor activation, or about conformational coupling of this surface to the native ligand-binding pocket. Here we use NMR spectroscopy to investigate ligand-specific conformational changes around a central structural feature in the ECS of the beta(2) adrenergic receptor: a salt bridge linking extracellular loops 2 and 3. Small-molecule drugs that bind within the transmembrane core and exhibit different efficacies towards G-protein activation (agonist, neutral antagonist and inverse agonist) also stabilize distinct conformations of the ECS. We thereby demonstrate conformational coupling between the ECS and the orthosteric binding site, showing that drugs targeting this diverse surface could function as allosteric modulators with high subtype selectivity. Moreover, these studies provide a new insight into the dynamic behaviour of GPCRs not addressable by static, inactive-state crystal structures.

Norepinephrine enhances the LPS-induced expression of COX-2 and secretion of PGE2 in primary rat microglia

Background

Recent studies suggest an important role for neurotransmitters as modulators of inflammation. Neuroinflammatory mediators such as cytokines and molecules of the arachidonic acid pathway are generated and released by microglia. The monoamine norepinephrine reduces the production of cytokines by activated microglia in vitro. However, little is known about the effects of norepinephrine on prostanoid synthesis. In the present study, we investigate the role of norepinephrine on cyclooxygenase- (COX-)2 expression/synthesis and prostaglandin (PG)E₂ production in rat primary microglia.

Results

Interestingly, norepinephrine increased COX-2 mRNA, but not protein expression. Norepinephrine strongly enhanced COX-2 expression and PGE₂ production induced by lipopolysaccharide (LPS). This effect is likely to be mediated by β-adrenoreceptors, since β-, but not α-adrenoreceptor agonists produced similar results. Furthermore, β-adrenoreceptor antagonists blocked the enhancement of COX-2 levels induced by norepinephrine and β-adrenoreceptor agonists.

Conclusions

Considering that PGE₂ displays different roles in neuroinflammatory and neurodegenerative disorders, norepinephrine may play an important function in the modulation of these processes in pathophysiological conditions.

Analysis of hydrophobic interactions of antagonists with the beta2-adrenergic receptor

The adrenergic receptors mediate a wide variety of physiological responses, including vasodilatation and vasoconstriction, heart rate modulation, and others. Beta-adrenergic antagonists ('beta-blockers') thus constitute a widely used class of drugs in cardiovascular medicine as well as in management of anxiety, migraine, and glaucoma. The importance of the hydrophobic effect has been evidenced for a wide range of beta-blocker properties. To better understand the role of the hydrophobic effect in recognition of beta-blockers by their receptor, we carried out a molecular docking study combined with an original approach to estimate receptor-ligand hydrophobic interactions. The proposed method is based on automatic detection of molecular fragments in ligands and the analysis of their interactions with receptors separately. A series of beta-blockers, based on phenylethanolamines and phenoxypropanolamines, were docked to the beta2-adrenoceptor binding site in the crystal structure. Hydrophobic complementarity between the ligand and the receptor was calculated using the PLATINUM web-server (http://model.nmr.ru/platinum). Based on the analysis of the hydrophobic match for molecular fragments of beta-blockers, we have developed a new scoring function which efficiently predicts dissociation constant (pKd) with strong correlations (r(2) approximately 0.8) with experimental data.

Comparative 3D QSAR study on β(1)-, β(2)-, and β(3)-adrenoceptor agonists

A quantitative structure–activity relationship study of tryptamine-based derivatives of β₁-, β₂-, and β₃-adrenoceptor agonists was conducted using comparative molecular field analysis (CoMFA). Correlation coefficients (cross-validated r²) of 0.578, 0.595, and 0.558 were obtained for the three subtypes, respectively, in three different CoMFA models. All three CoMFA models have different steric and electrostatic contributions, implying different requirements inside the binding cavity. The CoMFA coefficient contour plots of the three models and comparisons among these plots provide clues regarding the main chemical features responsible for the biological activity variations and also result in predictions which correlate very well with the observed biological activity. Based on the analysis, a summary regeospecific description of the requirements for improving β-adrenoceptor subtype selectivity is given.

Think the impossible: beta-blockers for treating asthma

Asthma was originally thought to be associated with an intrinsic defect in beta2ADR (beta2-adrenoceptor) function, tipping the balance towards parasympathetic bronchoconstriction. Hence beta-blocking drugs (such as beta2ADR antagonists and inverse agonists) may cause acute bronchoconstriction which, in turn, may be attenuated by anti-cholinergic agents. Although beta2-agonists are highly effective for the acute relief of bronchoconstriction, their chronic use is accompanied by an adaptive reduction in beta2ADR numbers and associated desensitization of response, resulting in increased exacerbations and rare cases of death. The hypothesis examined in the present article is that, while single dosing with a beta-blocker may cause acute bronchoconstriction, chronic dosing may afford putative beneficial effects including attenuated airway hyperresponsiveness.

Analysis of full and partial agonists binding to beta2-adrenergic receptor suggests a role of transmembrane helix V in agonist-specific conformational changes

The 2.4 A crystal structure of the beta(2)-adrenergic receptor (beta(2)AR) in complex with the high-affinity inverse agonist (-)-carazolol provides a detailed structural framework for the analysis of ligand recognition by adrenergic receptors. Insights into agonist binding and the corresponding conformational changes triggering G-protein coupled receptor (GPCR) activation mechanism are of special interest. Here we show that while the carazolol pocket captured in the beta(2)AR crystal structure accommodates (-)-isoproterenol and other agonists without steric clashes, a finite movement of the flexible extracellular part of TM-V helix (TM-Ve) obtained by receptor optimization in the presence of docked ligand can further improve the calculated binding affinities for agonist compounds. Tilting of TM-Ve towards the receptor axis provides a more complete description of polar receptor-ligand interactions for full and partial agonists, by enabling optimal engagement of agonists with two experimentally identified anchor sites, formed by Asp113/Asn312 and Ser203/Ser204/Ser207 side chains. Further, receptor models incorporating a flexible TM-V backbone allow reliable prediction of binding affinities for a set of diverse ligands, suggesting potential utility of this approach to design of effective and subtype-specific agonists for adrenergic receptors. Systematic differences in capacity of partial, full and inverse agonists to induce TM-V helix tilt in the beta(2)AR model suggest potential role of TM-V as a conformational "rheostat" involved in the whole spectrum of beta(2)AR responses to small molecule signals.