PDK1/Akt/PDE4D axis identified as a target for asthma remedy synergistic with β2AR agonists by a natural agent arctigenin

Background: Asthma is a heterogenetic disorder characterized by chronic inflammation with variable airflow obstruction and airway hyper-responsiveness. As the most potent and popular bronchodilators, beta(2) adrenergic receptor (beta(2)AR) agonists bind to the beta(2)ARs that are coupled via a stimulatory G protein to adenylyl cyclase, thereby improving cAMP accumulation and resulting in airway smooth muscle relaxation. We previously demonstrated arctigenin had a synergistic function with the beta(2)AR agonist, but the target for this remained elusive. Method: Chemical proteomics capturing was used to enrich and uncover the target of arctigenin in human bronchial smooth muscle cells, and reverse docking and molecular dynamic stimulation were performed to evaluate the binding of arctigenin and its target. In vitro enzyme activities and protein levels were demonstrated with special kits and Western blotting. Finally, guinea pig tracheal muscle segregation and ex vivo function were analysed. Results: Arctigenin bound to PDK1 with an ideal binding free energy-25.45 kcal/mol and inhibited PDK1 kinase activity without changing its protein level. Additionally, arctigenin reduced PKB/Akt-induced phosphorylation of PDE4D, which was first identified in this study. Attenuation of PDE4D resulted in cAMP accumulation in human bronchial smooth muscle. The inhibition of PDK1 showed a synergistic function with beta(2)AR agonists and relaxed the constriction of segregated guinea pig tracheal muscle. Conclusions: The PDK1/Akt/PDE4D axis serves as a novel asthma target, which may benefit airflow obstruction.