TAS2R activation promotes airway smooth muscle relaxation despite
?2-adrenergic receptor tachyphylaxis
Steven S. An,1* Wayne C. H. Wang,2* Cynthia J. Koziol-White,3Kwangmi Ahn,4Danielle Y. Lee,1
Richard C. Kurten,5Reynold A. Panettieri, Jr.,3and Stephen B. Liggett6
1Program in Respiratory Biology and Lung Disease, Johns Hopkins University, Bloomberg School of Public Health,
Baltimore, Maryland;2Department of Medicine, University of Maryland School of Medicine, Baltimore, Maryland;
3Division of Pulmonary, Allergy and Critical Care, Airways Biology Initiative, University of Pennsylvania Medical Center,
Philadelphia, Pennsylvania;4National Institutes of Health, Bethesda, Maryland;5Department of Physiology and Biophysics,
University of Arkansas for Medical Sciences, Little Rock, Arkansas; and6Personalized Medicine Institute, University of South
Florida Morsani College of Medicine, Tampa, Florida
Submitted 13 April 2012; accepted in final form 7 June 2012
An SS, Wang WC, Koziol-White CJ, Ahn K, Lee DY, Kurten RC,
Panettieri RA Jr, Liggett SB. TAS2R activation promotes airway
smooth muscle relaxation despite ?2-adrenergic receptor tachyphylaxis.
Am J Physiol Lung Cell Mol Physiol 303: L304–L311, 2012. First
published June 8, 2012; doi:10.1152/ajplung.00126.2012.—Recently,
bitter taste receptors (TAS2Rs) were found in the lung and act to relax
airway smooth muscle (ASM) via intracellular Ca2?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 ?-ago-
nists. The ?2-adrenergic receptor (?2AR) of ASM undergoes exten-
sive 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 stud-
ied intracellular signaling and cell mechanics using isolated human
ASM, mouse tracheal responses, and human bronchial responses to
characterize TAS2R relaxation in the context of ?2AR 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 ?2AR desen-
sitization, the TAS2R agonist chloroquine relaxation response was
unaffected. TAS2R-mediated stimulation of intracellular Ca2?con-
centration in human ASM was unaltered by albuterol pretreatment, in
contrast to cAMP signaling, which was desensitized by ?90%. In
mouse trachea, ?2AR desensitization by ?-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 ?-agonist exposure culminated in 64 ? 5.7% (P ? 0.001)
desensitization of ?2AR-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 ?-agonist treatment.
These findings portend a favorable therapeutic profile for TAS2R
agonists for the treatment of bronchospasm in asthma or chronic
obstructive lung disease.
asthma; airway smooth muscle; ?2-adrenergic receptor desensitiza-
tion; chloroquine; bitter taste receptor
?-AGONISTS ARE ROUTINELY UTILIZED for the treatment of asthma,
for acute therapy for bronchospasm, and for preventative or
maintenance therapy when chronically administered. These
agents relieve airflow obstruction by activating ?2-adrenergic
receptors (?2ARs), which are G protein-coupled receptors
(GPCRs) expressed on airway epithelial and airway smooth
muscle (ASM) cells (6, 44). ?2ARs signal to G?s, which
activates adenylyl cyclase, thereby increasing intracellular
cAMP and activating protein kinase A (PKA) (7, 16). PKA
phosphorylation of myosin light chain kinase and related pro-
teins results in ASM relaxation and bronchodilation (2, 24, 42).
Human studies have shown that chronic administration of
standard doses of ?-agonist to subjects decreases ?2AR ex-
pression and agonist-stimulated generation of cAMP in circu-
lating mononuclear cells (9) and in airway epithelial cells and
macrophages obtained by bronchoalveolar lavage (43). In the
treatment of obstructive lung disease, chronic ?-agonist use
has been associated with tachyphylaxis (loss of clinical effi-
cacy) (15, 29). In addition, increased bronchial hyperreactivity
(11, 25) and adverse effects, including death (8, 20, 39, 40),
have been observed in various clinical trials of chronic ?-ago-
nists. It should be noted, though, that none of these effects have
been definitively linked to ?-agonist-evoked regulation in
ASM, although ?2AR desensitization seems to be the most
likely of these events to be mediated by such tachyphylaxis
For GPCRs, agonist-promoted desensitization is defined as a
loss of function during persistent agonist exposure. When
function is defined as coupling of ?2AR to Gswith activation
of adenylyl cyclase, ?2AR desensitization is often quantified in
terms of generation of the second-messenger cAMP. However,
in a broader context relevant to bronchodilation, desensitiza-
tion can be defined as a loss of physiological function, which
is a failure to relax ASM. In this regard, when pathways from
different classes of receptors ultimately converge on the same
physiological function, desensitization can occur at points
distant from the initial agonist-receptor coupling event. Fur-
thermore, desensitization can occur via cross talk between two
pharmacologically distinct GPCRs (28, 31). Such interactions
can take place via a number of interactions, including second-
messenger-promoted kinase activation, such as PKA and pro-
tein kinase C (27).
Using RNA microarrays, the complement of GPCRs on
human ASM has been shown to be much greater than previ-
ously recognized (22) and has led to consideration of other
receptors whose activation might lead to relaxation. One such
class of receptors is the bitter taste receptors (TAS2Rs), some
of which are expressed at levels greater than ?2AR (18).
TAS2Rs promote relaxation via specialized intracellular Ca2?
* S. S. An and W. C. H. Wang contributed equally to this work.
Address for reprint requests and other correspondence: S. S. An, Johns
Hopkins Univ., Bloomberg School of Public Health, 615 N Wolfe St., Rm.
E-7616, Baltimore, MD 21205 (e-mail: email@example.com).
Am J Physiol Lung Cell Mol Physiol 303: L304–L311, 2012.
First published June 8, 2012; doi:10.1152/ajplung.00126.2012.
1040-0605/12 Copyright © 2012 the American Physiological Societyhttp://www.ajplung.orgL304
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EFFICACY OF BITTER TASTANT CHLOROQUINE IN ?2AR TACHYPHYLAXIS
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