Emerging evidence suggests that gut microbiota-derived short-chain fatty acids (SCFAs; acetate, propionate, and butyrate) are important modulators of the inflammatory state in diseases such as asthma. However, the functional expression of the Gi protein-coupled free fatty acid receptors (FFAR2/GPR43 and FFAR3/GPR41) has not been identified on airway smooth muscle (ASM). Classically, acute activation of Gi-coupled receptors inhibits cyclic AMP (cAMP) synthesis, which impairs ASM relaxation and can also induce crosstalk between Gi- and Gq-signaling pathways, potentiating increases in intracellular Ca2+ concentration ([Ca2+]i), favoring ASM contraction. In contrast, chronic activation of Gi-coupled receptors can sensitize adenylyl cyclase resulting in increased cAMP synthesis favoring relaxation. We questioned whether the Gi-coupled FFAR2 or FFAR3 is expressed in human ASM, whether they modulate cAMP and [Ca2+]i, and whether SCFAs modulate human ASM tone. We detected the protein expression of FFAR3 but not FFAR2 in native human ASM and primary cultured human airway smooth muscle (HASM) cells. In HASM cells, acute activation of FFAR3 with SCFAs inhibited forskolin-stimulated cAMP accumulation, but chronic activation did not sensitize cAMP synthesis. SCFAs induced [Ca2+]i increases that were attenuated by pertussis toxin, gallein, U73122, or xestospongin C. Acute treatment with SCFAs potentiated acetylcholine-stimulated [Ca2+]i increases and stress fiber formation in cells and contraction of ex vivo human airway tissues. In contrast, chronic pretreatment of human ASM with propionate did not potentiate airway relaxation. Together, these findings demonstrate that FFAR3 is expressed in human ASM and contributes to ASM contraction via reduced cAMP and increased [Ca2+]i.
Keywords: actin; cyclic AMP; immunohistochemistry; intracellular calcium; siRNA.