Airway smooth muscle as a target in asthma : new insights into bronchorelaxation and hyperreactivity

Abstract

Patients with asthma suffer from reversible airway obstruction accompanied by airway hyperresponsiveness (AHR). Airway smooth muscle (ASM) is central to bronchoconstriction and AHR and is therefore the target of existing and potential new therapies for asthma.

Activation of bitter taste sensing type 2 receptors (TAS2Rs) on ASM was recently suggested to induce bronchorelaxation by a novel mechanism. Therefore, the first aim of this thesis was to determine the potential of TAS2Rs (and their respective agonists) as possible therapeutic targets on ASM. A secondary aim of this thesis was to assess whether inflammation induced by IL-13 can alter ASM function in intact human airways.

The methodology used in this thesis mainly involved the study of isometric contractions and relaxations in smooth muscle preparations from guinea-pigs and humans. The measurement of intracellular Ca2+ and analyses of mRNA expression following interventions (cytokines/siRNA) was performed in cultured ASM cells.

Defined TAS2R agonists including chloroquine, denatonium, dextromethorphan, noscapine and quinine were found to strongly relax guinea pig trachea, pre-contracted by muscarinic agonists. Although the TAS2R agonists were less potent bronchorelaxants than β2-adrenoceptor agonists, they were equally (or more) efficacious following stronger pre-contractions. Chloroquine relaxed antigen-induced contractions, whereas denatonium failed to relax other types of pre-contractions. TAS2R agonists were shown to induce relaxation via distinct mechanisms that do not involve conventional pathways.

Human pulmonary arteries, expressing TAS2R mRNA, were relaxed by chloroquine, dextromethorphan and noscapine, indicating that TAS2R agonists also affect vascular smooth muscle function. In guinea pig aorta, TAS2R agonists elicited relaxations following noradrenergic pre-contractions, independently of the endothelium, BK or L-type Ca2+ channels. Again, responses towards TAS2R agonists were shown to be dependent on the agents used for pre-contraction. Studies in guinea pig taenia coli demonstrated antagonism by denatonium and quinine at α-adrenoceptors. In human ASM cells, TAS2R mRNA expression was decreased by TNFα and glucocorticoids, and increased following 24-48 hours of serum-deprivation. Chloroquine, dextromethorphan, noscapine and quercetin induced relaxation of intact human airways, but differentially inhibited histamine-induced intracellular Ca2+ mobilisation. The siRNA-induced decrease of TAS2R14 mRNA could not prevent quercetin-mediated inhibitions in human ASM cells.

In human small airways, IL-13 induced a 2.5-3.1-fold increase in the potency of carbachol and histamine contractions, without affecting their amplitude. In addition, IL-13 impaired relaxant responses towards salbutamol, but not those towards formoterol, chloroquine and noscapine. IL-13 increased both the potency and the amplitude of histamine-induced intracellular Ca2+ mobilisation in human ASM cells. The IL-13-induced hyperreactivity of human bronchi and ASM cells was not prevented by the glucocorticoid dexamethasone.

In summary, although TAS2R agonists exert powerful relaxations in smooth muscle, their effects in ASM are not mediated by common pathways. As a consequence, the TAS2Rs may be questioned as a target for bronchorelaxation in asthma. On the other hand, IL-13 induces functional remodelling of the ASM in human airways that can contribute to AHR.