Abstract
Bitter taste receptors (TAS2Rs) in airway smooth muscle (ASM) mediate potent bronchodilation, yet the specific G protein coupling underlying this response remains incompletely defined. Here, we investigated whether GNAT3, the α-subunit of the gustducin G protein and a hallmark of the Gi/o family, is required for phenanthroline (Phen)-induced airway relaxation. Using smooth muscle–specific Gnat3−/− mice and pharmacological approaches, we demonstrate that Phen elicits robust, dose-dependent relaxation of methacholine-contracted murine airways, with maximal efficacy comparable to the β2-agonist R-albuterol, albeit at higher concentrations. This relaxant effect was significantly attenuated in Gnat3−/− air-ways, indicating partial dependence on GNAT3. At the cellular level, Phen sup-pressed methacholine-induced global intracellular calcium elevation ([Ca2+]i) in ASM cells—a key mechanism driving relaxation. Critically, this inhibitory effect on [Ca2+]i was abolished by pertussis toxin (PTX), a selective Gi/o inhibitor, and markedly reduced in Gnat3−/− cells. Together, these findings provide convergent genetic and pharmacological evidence that Phen induces bronchodilation via a GNAT3-dependent, PTX-sensitive Gi/o signaling pathway that dampens [Ca2+]i mobilization. The incomplete blockade by Gnat3 deletion or PTX suggests contributions from additional G proteins or parallel mechanisms. Our study establishes GNAT3 as a functional component of the TAS2R–gustducin axis in ASM and advances the mechanistic understanding of bitter tastant–mediated airway relaxation with therapeutic potential for obstructive lung diseases.
References
[1] Yoshida R, Ninomiya Y. Mechanisms and Functions of Sweet Reception in Oral and Extraoral Organs. Int J Mol Sci. 2024;25(13):7398.
[2] Fang Y, Wang Q, Wu S, et al. Defining the Critical Role of α-Gustducin for NF-κB Inhibition and Anti-Inflammatory Signal Transduction by Bitter Agonists in Lung Epithelium. Int J Mol Sci. 2026;27(2):997.
[3] Liu LL, Li F, Zhu MM, et al. Contribution of the bitter taste signaling pathway to lung inflammation during Staphylococcus aureus-induced pneumonia. Front Immunol. 2025;16:1647780.
[4] Zhang Z, Zhou Z, Liu J, et al. Salicin alleviates periodontitis via Tas2r143/gustducin signaling in fibroblasts. Front Immunol. 2024;15:1374900.
[5] Le Gléau L, Rouault C, Osinski C, et al. Intestinal alteration of α-gustducin and sweet taste signaling pathway in metabolic diseases is partly rescued after weight loss and diabetes remission. Am J Physiol Endocrinol Metab. 2021;321(3):E417-E432.
[6] Liu L, Wess J. Adipocyte G Protein-Coupled Receptors as Potential Targets for Novel Antidiabetic Drugs. Diabetes. 2023;72(7):825-834.
[7] Wu M, Deng L, Zhu G, Li YP. G Protein and its signaling pathway in bone development and disease. Front Biosci. 2010;15(3):957-985.
[8] Hoffman MT, Kemp SB, Salas-Escabillas DJ, et al. The Gustatory Sensory G-Protein GNAT3 Suppresses Pancreatic Cancer Progression in Mice. Cell Mol Gastroenterol Hepatol. 2021;11(2):349-369.
[9] Kim D, An SS, Lam H, Leahy JW, Liggett SB. Identification and Characterization of Novel Bronchodilator Agonists Acting at Human Airway Smooth Muscle Cell TAS2R5. ACS Pharmacol Transl Sci. 2020;3(6):1069-1075.
[10] Deshpande DA, Wang WCH, McIlmoyle EL, et al. Bitter taste receptors on airway smooth muscle bronchodilate by a localized calcium flux and reverse obstruction. Nat Med. 2010;16(11):1299-1304.
[11] Camoretti-Mercado B, Pauer SH, Yong HM, et al. Pleiotropic Effects of Bitter Taste Receptors on [Ca2+]i Mobilization, Hyperpolarization, and Relaxation of Human Airway Smooth Muscle Cells. PloS One. 2015;10(6):e0131582.
[12] Sharma P, Yi R, Nayak AP, et al. Bitter Taste Receptor Agonists Mitigate Features of Allergic Asthma in Mice. Sci Rep. 2017;7:46166.
[13] Grassin-Delyle S, Abrial C, Fayad-Kobeissi S, et al. The expression and relaxant effect of bitter taste receptors in human bronchi. Respir Res. 2013;14(1):134.
[14] Tan X, Sanderson MJ. Bitter tasting compounds dilate airways by inhibiting airway smooth muscle calcium oscillations and calcium sensitivity. Br J Pharmacol. 2014;171(3):646-662.
[15] Zhang CH, Chen C, Lifshitz LM, Fogarty KE, Zhu MS, ZhuGe R. Activation of BK channels may not be required for bitter tastant-induced bronchodilation. Nat Med. 2012;18(5):648-650; author reply 650-651.
[16] Qu M, Lu P, Lifshitz LM, Moore Simas TA, Delpapa E, ZhuGe R. Phenanthroline relaxes uterine contractions induced by diverse contractile agents by decreasing cytosolic calcium concentration. Eur J Pharmacol. 2024;968:176343.
[17] Sywanycz S, Hill BL, Miller ZA, et al. T2R5 agonist phendione decreases cell viability and induces apoptosis in head and neck squamous cell carcinoma. Mol Cancer Ther. Published online November 7, 2025.