Anti-mitogenic effects of bitter taste receptor (BTR) agonists on human airway smooth muscle cells

Abstract Rationale: Obstructive diseases of airways such as asthma and COPD are characterized by airway remodeling. Clinical studies and animal models have demonstrated that ASM mass is increased in asthma, and ASM thickness is correlated with severity of the disease. Current asthma medications control inflammation and reverse airway obstruction effectively, yet have very limited effects in deterring airway remodeling. Recently we identified the expression of BTRs on human ASM cells. Activation with known BTR agonists resulted in elevation of intracellular calcium, membrane hyperpolarization and ASM relaxation. Aerosol challenge in normal and allergen sensitized- and challenged- mice resulted in a robust bronchodilation. Another recent study demonstrated that BTR expression, signaling and bronchodilatory effects are preserved during human asthma. These studies suggest that BTRs can be used as new therapeutic targets in the clinical management of obstructive lung diseases. The current study aimed at determining the effect of BTR agonists on ASM growth. Methods: Primary human ASM cells maintained in culture were pretreated with different concentrations of BTR agonists, chloroquine and quinine or vehicle, then stimulated with ASM mitogens fetal bovine serum (FBS), platelet-derived growth factor (PDGF) or epithelial growth factor (EGF). Regulation of ASM growth was subsequently assessed by cell counts, CyQuant assay and 3H-thymidine-incorporation assays. Parallel studies assessed the effects of BTR agonists on key mitogenic signaling pathways in the ASM by Western blotting. Results: In CyQuant assays, chloroquine and quinine significantly inhibited growth of normal and astmatic human ASM cells induced by each mitogen in a concentration-dependent manner. BTR agonists also inhibited increases in ASM cell number suggesting their anti-mitogenic effect is mediated via inhibition of hyperplasia. BTR agonists did not induce apoptosis or cell death in human ASM. Growth inhibitory effects of BTR agonists in ASM cells were not dependent on protein kinase A (PKA) as demonstrated for other Gs coupled G protein coupled receptor agonists (e.g. β-agonists and PGE2). Western blot analyses of key mitogenic signaling demonstrated that BTR agonists inhibit mitogen-induced activation of p42/p44, p38 and Akt pathways. Conclusion: Collectively, these data suggest that BTR agonists inhibit ASM cell growth by inhibiting key mitogenic signaling pathways in ASM via PKA-independent mechanism, suggesting a novel and unexploited mode of inhibiting ASM growth. Future studies are needed to establish in vivo effectiveness of BTR agonists on airway remodeling