Coupling of Airway Smooth Muscle Bitter Taste Receptors to Intracellular Signaling and Relaxation Is via G_αi1,2,3.

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 G_αgust mRNA 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 G_i family. α subunit expression in HASM was G_αi2 > G_αi1 = G_αi3 > G_αtrans1 ≈ G_αtrans2, with G_αgust and G_αo at the limits of detection (>100-fold lower than G_αi2). Small interfering RNA knockdowns in HASM showed losses of [Ca²⁺]_i and ERK1/2 signaling when G_αi1, G_αi2, or G_αi3 were reduced. G_αtrans1 and G_αtrans2 knockdowns had no effect on [Ca²⁺]_i and a minimal, transient effect on ERK1/2 phosphorylation. Furthermore, G_αgust and G_αo knockdowns 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 G_αi2. ASM cells from transgenic mice expressing a peptide inhibitor of G_αi2 had attenuated relaxation to TAS2R agonist. These data indicate that, unlike in taste cells, TAS2Rs couple to the prevalent G proteins, G_αi1, G_αi2, and G_αi3, with no evidence for functional coupling to G_αgust. This absence of function for the "canonical" TAS2R G protein in HASM may be due to the very low expression of G_αgust, indicating that TAS2Rs can optionally couple to several G proteins in a cell type-dependent manner contingent upon G protein expression.

KEYWORDS: {{ getKeywords(articleDetailText.words) }}