Airway Relaxation Mechanisms and Structural Basis of Osthole to Improve Lung Function in Asthma

Abstract

Background and objective Overuse of β2‐adrenoceptor agonist bronchodilators causes receptor desensitization, resulting in decreased efficacy and an increased risk of death in asthma patients. This is a critical unmet problem in asthma management, and novel drugs are urgently needed. Here, we report that osthole, a coumarin derived from a traditional Chinese medicine, induces complete relaxation of preconstricted airways in mouse precision‐cut lung slices, irrespective of β2‐adrenoceptor desensitization. Methods Mouse precision‐cut lung slices as an in vitro model were used to analyze airway relaxation. The cAMP level in human airway smooth muscle (ASM) or HEK293 cells was determined by phosphorylation of vasodilator‐stimulated phosphoprotein using Western blot or cAMP dependent reporter luciferase assay. The secretion of prostaglandin E2 (PGE2) in ASM cells or lung slices was measured by ELISA assay. The effect of osthole on airway hyperresponsiveness (AHR) was measured in both house dust mite (HDM)‐sensitized mice and naïve regulator of G‐protein signaling 2 (RGS2) KO mice. The binding sites of osthole with PDE4D5 catalytic domain were analyzed by crystallization and X‐ray diffraction. Results Osthole administered in vivo ameliorates AHR, a hallmark of asthma, in murine asthma models. Mechanistic studies revealed that osthole inhibits phosphodiesterase 4D (PDE4D) activity to amplify endogenous PGE2 signaling in ASM cells, which eventually triggers cAMP/protein kinase A‐dependent relaxation of airways. The crystal structure of the PDE4D complexed with osthole reveals that osthole binds to the catalytic site to prevent cAMP binding and hydrolysis. Together, our studies elucidate a specific molecular target and mechanism by which osthole induces airway relaxation. Conclusion Identification of osthole binding sites on PDE4D will guide further development of novel therapeutics for β2‐adrenoceptor agonist‐resistant asthma. Support or Funding Information This work was supported in part by the National Institutes of Health (R01HL116849) and Nebraska State LB595 Research Program, USA; the National Laboratory of Biomacromolecules and the Center for Excellence in Biomacromolecules, Institute of Biophysics, Chinese Academy of Sciences (CAS); the CAS/SAFEA International Partnership Program for Creative Research Teams; the National Key R&D Program of China (2017YFA0205501); the National Natural Science Foundation of China (21777192 and 31500623)