TAS2R activation promotes airway smooth muscle relaxation despite 2-adrenergic receptor tachyphylaxis

Program in Respiratory Biology and Lung Disease, Johns Hopkins University, Bloomberg School of Public Health, 615 N Wolfe St., Rm. E-7616, Baltimore, MD 21205, USA.
AJP Lung Cellular and Molecular Physiology (Impact Factor: 4.08). 06/2012; 303(4):L304-11. DOI: 10.1152/ajplung.00126.2012
Source: PubMed


Recently, bitter taste receptors (TAS2Rs) were found in the lung and act to relax airway smooth muscle (ASM) via intracellular Ca(2+) concentration signaling generated from restricted phospholipase C activation. As potential therapy, TAS2R agonists could be add-on treatment when patients fail to achieve adequate bronchodilation with chronic β-agonists. The β(2)-adrenergic receptor (β(2)AR) of ASM undergoes extensive functional desensitization. It remains unknown whether this desensitization affects TAS2R function, by cross talk at the receptors or distal common components in the relaxation machinery. We studied intracellular signaling and cell mechanics using isolated human ASM, mouse tracheal responses, and human bronchial responses to characterize TAS2R relaxation in the context of β(2)AR desensitization. In isolated human ASM, magnetic twisting cytometry revealed >90% loss of isoproterenol-promoted decrease in cell stiffness after 18-h exposure to albuterol. Under these same conditions of β(2)AR desensitization, the TAS2R agonist chloroquine relaxation response was unaffected. TAS2R-mediated stimulation of intracellular Ca(2+) concentration in human ASM was unaltered by albuterol pretreatment, in contrast to cAMP signaling, which was desensitized by >90%. In mouse trachea, β(2)AR desensitization by β-agonist amounted to 92 ± 6.0% (P < 0.001), while, under these same conditions, TAS2R desensitization was not significant (11 ± 3.5%). In human lung slices, chronic β-agonist exposure culminated in 64 ± 5.7% (P < 0.001) desensitization of β(2)AR-mediated dilation of carbachol-constricted airways that was reversed by chloroquine. We conclude that there is no evidence for physiologically relevant cross-desensitization of TAS2R-mediated ASM relaxation from chronic β-agonist treatment. These findings portend a favorable therapeutic profile for TAS2R agonists for the treatment of bronchospasm in asthma or chronic obstructive lung disease.

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    • "This is the first study to show that RGZ and CQ can maintain potency and complete small airway relaxation following β-adrenoceptor desensitization in mouse lung slices. The finding is consistent with previous results obtained with CQ in mouse trachea and human precision cut lung slices following 18 h incubation with ISO or the long acting β-adrenoceptor agonist salmeterol respectively [8]. The ability of both RGZ and CQ to cause relaxation under conditions of β-adrenoceptor desensitization provides further evidence that these dilators are working via a different mechanism to β-adrenoceptor agonists. "
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    ABSTRACT: Current therapy for relieving bronchoconstriction may be ineffective in severe asthma, particularly in the small airways. The aim of this study was to further characterise responses to the recently identified novel bronchodilators rosiglitazone (RGZ) and chloroquine (CQ) under conditions where beta-adrenoceptor agonist efficacy was limited or impaired in mouse small airways within lung slices. Relaxation to RGZ and CQ was assessed following submaximal methacholine (MCh) pre-contraction, in slices treated overnight with either RGZ, CQ or albuterol (ALB) (to induce beta-adrenoceptor desensitization), and in slices treated with caffeine/ryanodine in which contraction is associated with increases in Ca2+ sensitivity in the absence of contractile agonist-induced Ca2+ oscillations. Furthermore, the effects of RGZ, CQ, ALB and isoproterenol (ISO) on the initiation and development of methacholine-induced contraction were also compared. RGZ and CQ, but not ALB or ISO, elicited complete relaxation with increasing MCh pre-contraction and maintained their potency and efficacy following beta-adrenoceptor desensitization. RGZ, CQ and ALB maintained efficacy following overnight incubation with RGZ or CQ. Relaxation responses to all dilators were generally maintained but delayed after caffeine/ryanodine. Pre-treatment with RGZ, but not CQ, ALB or ISO, reduced MCh potency. This study demonstrates the superior effectiveness of RGZ in comparison to CQ and beta-adrenoceptor agonists as a dilator of mouse small airways. Further investigation of the mechanisms underlying the relatively greater efficacy of RGZ under these conditions are warranted and should be extended to include studies in human asthmatic airways.
    Full-text · Article · Mar 2014 · Respiratory research
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    • "The importance of taste signalling in asthma was recently suggested in an analysis of TAS2R expression in peripheral blood leukocytes from asthmatic children [3]. Furthermore, the potential value of TAS2R as a drug target is enhanced by the fact that (at least in vitro) TAS2R agonists were effective in relaxing airway smooth muscle even when β2-adrenergic receptors (the current cornerstone target of bronchodilators) were subject to tachyphylaxis [40]. The development of selective TAS2R antagonists and more potent, selective TAS2R agonists is nevertheless a prerequisite for better characterizing the TAS2Rs' involvement in relaxation and understanding the corresponding molecular signalling pathways. "
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    ABSTRACT: Bitter-taste receptors (TAS2Rs) have recently been involved in the relaxation of mouse and guinea pig airways, and increased expression of TAS2Rs was shown in blood leucocytes from asthmatic children. We sought to identify and characterize the TAS2Rs expressed in isolated human bronchi and the subtypes involved in relaxation. Human bronchi were isolated from resected lungs and TAS2R transcripts were assessed with RT-qPCR. Relaxation to TAS2R agonists was tested in organ bath in the presence or absence of pharmacological modulators of the signalling pathways involved in bronchial relaxation. We detected the expression of TAS2R transcripts in human bronchi. The non-selective agonists chloroquine, quinine, caffeine, strychnine and diphenidol produced a bronchial relaxation as effective and potent as theophylline but much less potent than formoterol and isoproterenol. Denatonium, saccharin and colchicine did not produce relaxation. Receptor expression analysis together with the use of selective agonists suggest a predominant role for TAS2R5, 10 and 14 in bitter taste agonist-induced relaxation. The mechanism of relaxation was independent of the signalling pathways modulated by conventional bronchodilators and may be partly explained by the inhibition of phosphatidylinositol-3-kinases. The TAS2Rs may constitute a new therapeutic target in chronic obstructive lung diseases such as asthma.
    Full-text · Article · Nov 2013 · Respiratory research
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    • "Interestingly, the magnitude of bronchodilation achieved by high-dose TAS2R agonists in many of these studies rivaled maximal β-agonist treatments and mechanistically was found to be cAMP- and PKC-independent. This group has recently extended this observation to show that in relevant models of β2-adrenoceptor desensitization, chloroquine-mediated TAS2R activation in ASM retains its pro-bronchodilatory effects, a finding of considerable clinical relevance given the well-described concern of β-agonist tachyphylaxis that occurs with repetitive β-agonism [29]. Yet, it should be noted that TAS2R activation in ASM can lead to desensitization via a GRK-mediated, β-arrestin pathway, which may limit its therapeutic usefulness as it is seen currently with β-adrenoceptor agonists [30]. "
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    ABSTRACT: Bronchodilators are the first line therapy during acute asthmatic exacerbations to reverse airway obstruction primarily by relaxing airway smooth muscle. Only three categories of bronchodilators exist in clinical practice: β-adrenergic agonists, anticholinergics, and methylxanthines. Each of these categories have specific drugs dating back to the early 20th century, raising the question of whether or not we can find better bronchodilators. While caffeine, theophylline, atropine, and epinephrine were the first generations of therapeutics in each of these drug classes, there is no question that improvements have been made in the bronchodilators in each of these classes. In the following editorial, we will briefly describe new classes of potential bronchodilators including: novel PDE inhibitors, natural phytotherapeutics, bitter taste receptor ligands, and chloride channel modulators, which have the potential to be used alone or in combination with existing bronchodilators to reverse acute airway obstruction in the future.
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