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1.  Agonist-Promoted Homologous Desensitization of Human Airway Smooth Muscle Bitter Taste Receptors 
Bitter taste receptors (TAS2Rs) were shown to be expressed in human airway smooth muscle (ASM). They couple to specialized [Ca2+]i release, leading to membrane hyperpolarization, the relaxation of ASM, and marked bronchodilation. TAS2Rs are G-protein–coupled receptors, known to undergo rapid agonist-promoted desensitization that can limit therapeutic efficacy. Because TAS2Rs represent a new drug target for treating obstructive lung disease, we investigated their capacity for rapid desensitization, and assessed their potential mechanisms. The pretreatment of human ASM cells with the prototypic TAS2R agonist quinine resulted in a 31% ± 5.1% desensitization of the [Ca2+]i response from a subsequent exposure to quinine. No significant change in the endothelin-stimulated [Ca2+]i response was attributed to the short-term use of quinine, indicating a homologous form of desensitization. The TAS2R agonist saccharin also evoked desensitization, and cross-compound desensitization with quinine was evident. Desensitization of the [Ca2+]i response was attenuated by a dynamin inhibitor, suggesting that receptor internalization (a G-protein coupled receptor kinase [GRK]-mediated, β-arrestin–mediated process) plays an integral role in the desensitization of TAS2R. Desensitization was insensitive to antagonists of the second messenger kinases protein kinase A and protein kinase C. Using intact airways, short-term, agonist-promoted TAS2R desensitization of the relaxation response was also observed. Thus these receptors, which represent a potential novel target for direct bronchodilators, undergo a modest degree of agonist-promoted desensitization that may affect clinical efficacy. Collectively, the results of these mechanistic studies, along with the multiple serines and threonines in intracellular loop 3 and the cytoplasmic tail of TAS2Rs, suggest a GRK-mediated mode of desensitization.
doi:10.1165/rcmb.2011-0061OC
PMCID: PMC3361362  PMID: 21642585
airway smooth muscle relaxation; taste receptors; tachyphylaxis; phosphorylation; G-protein–coupled receptor kinases
2.  Common ADRB2 Haplotypes Derived from 26 Polymorphic Sites Direct β2-Adrenergic Receptor Expression and Regulation Phenotypes 
PLoS ONE  2010;5(7):e11819.
Background
The β2-adrenergic receptor (β2AR) is expressed on numerous cell-types including airway smooth muscle cells and cardiomyocytes. Drugs (agonists or antagonists) acting at these receptors for treatment of asthma, chronic obstructive pulmonary disease, and heart failure show substantial interindividual variability in response. The ADRB2 gene is polymorphic in noncoding and coding regions, but virtually all ADRB2 association studies have utilized the two common nonsynonymous coding SNPs, often reaching discrepant conclusions.
Methodology/Principal Findings
We constructed the 8 common ADRB2 haplotypes derived from 26 polymorphisms in the promoter, 5′UTR, coding, and 3′UTR of the intronless ADRB2 gene. These were cloned into an expression construct lacking a vector-based promoter, so that β2AR expression was driven by its promoter, and steady state expression could be modified by polymorphisms throughout ADRB2 within a haplotype. “Whole-gene” transfections were performed with COS-7 cells and revealed 4 haplotypes with increased cell surface β2AR protein expression compared to the others. Agonist-promoted downregulation of β2AR protein expression was also haplotype-dependent, and was found to be increased for 2 haplotypes. A phylogenetic tree of the haplotypes was derived and annotated by cellular phenotypes, revealing a pattern potentially driven by expression.
Conclusions/Significance
Thus for obstructive lung disease, the initial bronchodilator response from intermittent administration of β-agonist may be influenced by certain β2AR haplotypes (expression phenotypes), while other haplotypes may influence tachyphylaxis during the response to chronic therapy (downregulation phenotypes). An ideal clinical outcome of high expression and less downregulation was found for two haplotypes. Haplotypes may also affect heart failure antagonist therapy, where β2AR increase inotropy and are anti-apoptotic. The haplotype-specific expression and regulation phenotypes found in this transfection-based system suggest that the density of genetic information in the form of these haplotypes, or haplotype-clusters with similar phenotypes can potentially provide greater discrimination of phenotype in human disease and pharmacogenomic association studies.
doi:10.1371/journal.pone.0011819
PMCID: PMC2912278  PMID: 20686604

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