Coupling of Airway Smooth Muscle Bitter Taste Receptors to Intracellular Signaling and Relaxation Is via Gαi1,2,3.
Bitter taste receptors (TAS2Rs) are expressed on human airway smooth muscle (HASM) and evoke marked relaxation. Agonist interaction with TAS2Rs activates phospholipase C and increases compartmentalized intracellular Ca2+([Ca2+]i) via inositol 1,4,5 triphosphate. In taste cells, the G protein gustducin couples TAS2R to phospholipase C; however, we find very low levels of GαgustmRNA or protein in HASM. We hypothesized that another G protein in HASM transmits TAS2R function. TAS2R signaling to [Ca2+]i, extracellular signal-regulated kinase (ERK) 1/2, and physiologic relaxation was sensitive to pertussis toxin, confirming a role for a member of the Gifamily. α subunit expression in HASM was Gαi2> Gαi1= Gαi3> Gαtrans1≈ Gαtrans2, with Gαgustand Gαoat the limits of detection (>100-fold lower than Gαi2). Small interfering RNA knockdowns in HASM showed losses of [Ca2+]iand ERK1/2 signaling when Gαi1, Gαi2, or Gαi3were reduced. Gαtrans1and Gαtrans2knockdowns had no effect on [Ca2+]iand a minimal, transient effect on ERK1/2 phosphorylation. Furthermore, Gαgustand Gαoknockdowns did not affect any TAS2R signaling. In overexpression experiments in human embryonic kidney-293T cells, we confirmed an agonist-dependent physical interaction between TAS2R14 and Gαi2. ASM cells from transgenic mice expressing a peptide inhibitor of Gαi2had attenuated relaxation to TAS2R agonist. These data indicate that, unlike in taste cells, TAS2Rs couple to the prevalent G proteins, Gαi1, Gαi2, and Gαi3, with no evidence for functional coupling to Gαgust. This absence of function for the "canonical" TAS2R G protein in HASM may be due to the very low expression of Gαgust, indicating that TAS2Rs can optionally couple to several G proteins in a cell type-dependent manner contingent upon G protein expression.
1 Department of Medicine and the Center for Personalized Medicine and Genomics, University of South Florida Morsani College of Medicine, Tampa, Florida.
Donghwa
D
Kim
1 Department of Medicine and the Center for Personalized Medicine and Genomics, University of South Florida Morsani College of Medicine, Tampa, Florida.
Jung A
JA
Woo
2 Department of Environmental Health and Engineering, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland; and.
Ezekiel
E
Geffken
2 Department of Environmental Health and Engineering, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland; and.
Steven S
SS
An
3 Departments of Internal Medicine and Molecular Pharmacology and Physiology, and the Center for Personalized Medicine and Genomics, University of South Florida Morsani College of Medicine, Tampa, Florida.
Stephen B
SB
Liggett
D000595:Amino Acid Sequence; D000818:Animals; D019206:GTP-Binding Protein alpha Subunits, Gi-Go / Q000737:chemistry / Q000378:metabolism*; D055785:Gene Knockdown Techniques; D057809:HEK293 Cells; D006801:Humans; D042541:Intracellular Space / Q000187:drug effects / Q000378:metabolism; D008168:Lung / Q000378:metabolism*; D008822:Mice, Transgenic; D009126:Muscle Relaxation / Q000187:drug effects; D009130:Muscle, Smooth / Q000378:metabolism*; D010455:Peptides / Q000494:pharmacology; D037342:Pertussis Toxin / Q000633:toxicity; D021122:Protein Subunits / Q000378:metabolism; D034741:RNA, Small Interfering / Q000378:metabolism; D043562:Receptors, G-Protein-Coupled / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects; D000098706:Taste Receptors, Type 2
D010455:Peptides; D021122:Protein Subunits; D034741:RNA, Small Interfering; D043562:Receptors, G-Protein-Coupled; D000098706:Taste Receptors, Type 2; D037342:Pertussis Toxin; D019206:GTP-Binding Protein alpha Subunits, Gi-Go
G proteins; airway smooth muscle; asthma; bitter taste receptors; chronic obstructive pulmonary disease