Mechanisms of allergy and clinical immunology
Role of nicotinic receptors and acetylcholine in mucous cell metaplasia, hyperplasia, and airway mucus formation in vitro and in vivo

https://doi.org/10.1016/j.jaci.2012.04.002Get rights and content

Background

Airway mucus hypersecretion is a key pathophysiologic feature in a number of lung diseases. Cigarette smoke/nicotine and allergens are strong stimulators of airway mucus; however, the mechanism of mucus modulation is unclear.

Objectives

We sought to characterize the pathway by which cigarette smoke/nicotine regulates airway mucus and identify agents that decrease airway mucus.

Methods

IL-13 and γ-aminobutyric acid type A receptors (GABAARs) are implicated in airway mucus. We examined the role of IL-13 and GABAARs in nicotine-induced mucus formation in normal human bronchial epithelial (NHBE) and A549 cells and secondhand cigarette smoke–induced, ovalbumin-induced, or both mucus formation in vivo.

Results

Nicotine promotes mucus formation in NHBE cells; however, the nicotine-induced mucus formation is independent of IL-13 but sensitive to the GABAAR antagonist picrotoxin. Airway epithelial cells express α7-, α9-, and α10-nicotinic acetylcholine receptors (nAChRs), and specific inhibition or knockdown of α7- but not α9/α10-nAChRs abrogates mucus formation in response to nicotine and IL-13. Moreover, addition of acetylcholine or inhibition of its degradation increases mucus in NHBE cells. Nicotinic but not muscarinic receptor antagonists block allergen- or nicotine/cigarette smoke–induced airway mucus formation in NHBE cells, murine airways, or both.

Conclusions

Nicotine-induced airway mucus formation is independent of IL-13, and α7-nAChRs are critical in airway mucous cell metaplasia/hyperplasia and mucus production in response to various promucoid agents, including IL-13. In the absence of nicotine, acetylcholine might be the biological ligand for α7-nAChRs to trigger airway mucus formation. α7-nAChRs are downstream of IL-13 but upstream of GABAARα2 in the MUC5AC pathway. Acetylcholine and α7-nAChRs might serve as therapeutic targets to control airway mucus.

Section snippets

Methods

NHBE and A549 cells were cultured by using standard procedures.15 DO11.10 ovalbumin (OVA)–T-cell receptor (TCR) transgenic mice on a BALB/c background were exposed to air, secondhand smoke (SS) or nicotine (1.5 mg total particulate material/m3), heat-aggregated OVA aerosol (5 mg/m3), or OVA plus SS or nicotine for 2 weeks (6 h/d for 5 d/wk). In some experiments normal (wild-type) BALB/c mice were sensitized to Aspergillus fumigatus extract, as described previously.16 Where indicated, mice were

Nicotinic receptors are critical in mucus formation

We examined the effects of nicotine (100 nmol/L) on mucus formation in NHBE cells grown at the air-liquid interface (ALI). This is a realistic concentration of nicotine and is several-fold lower than the median effective concentration (10-100 μmol/L) of nicotine/nicotine agonists required to activate the ligand-gated cationic channel in neurons.22 As seen with AB/PAS mucus staining (Fig 1, A), control NHBE cells have a low baseline level of mucus; however, when the cells were treated with

Discussion

NAChRs are seen in tissues from both vertebrates and invertebrates, such as nematodes and insects.40 In the central nervous system nAChRs are ligand-gated ion channels that mediate fast synaptic cholinergic transmission, and these properties are strongly conserved across species. nAChRs are present in neuronal32 and many nonneuronal13, 29, 30 cell types. At least in some nonneuronal cell types nAChRs are not ligand-gated ion channels but signal through second messengers.29 Thus far, the

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    Supported in part by grants from the US Army Medical Research and Material Command (GW093005), the National Institutes of Health (R01-DA017003), and the Lovelace Respiratory Research Institute (IMMSPT).

    Disclosure of potential conflict of interest: K. S. Harrod is a member of the Avisa Pharma Board of Directors. The rest of the authors declare that they have no relevant conflicts of interest.

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