Abstract
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 Ca(2+) ([Ca(2+)](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 [Ca(2+)](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 [Ca(2+)](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 [Ca(2+)](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.