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
([Ca
]
) 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
mRNA or protein in HASM. We hypothesized that another G protein in HASM transmits TAS2R function. TAS2R signaling to [Ca
]
, extracellular signal-regulated kinase (ERK) 1/2, and physiologic relaxation was sensitive to pertussis toxin, confirming a role for a member of the G
family. α subunit expression in HASM was G
> G
= G
> G
≈ G
, with G
and G
at the limits of detection (>100-fold lower than G
). Small interfering RNA knockdowns in HASM showed losses of [Ca
]
and ERK1/2 signaling when G
, G
, or G
were reduced. G
and G
knockdowns had no effect on [Ca
]
and a minimal, transient effect on ERK1/2 phosphorylation. Furthermore, G
and G
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
. ASM cells from transgenic mice expressing a peptide inhibitor of G
had attenuated relaxation to TAS2R agonist. These data indicate that, unlike in taste cells, TAS2Rs couple to the prevalent G proteins, G
, G
, and G
, with no evidence for functional coupling to G
. This absence of function for the "canonical" TAS2R G protein in HASM may be due to the very low expression of G
, indicating that TAS2Rs can optionally couple to several G proteins in a cell type-dependent manner contingent upon G protein expression.