ABSTRACT
The ß2-adrenergic receptor (ß2AR) is relevant for surfactant formation in alveolar type 2 cells and reduction of intracellular calcium concentration in bronchial muscle cells and thus for secretolytic and bronchospasmolytic effects. Herbal medicinal products that affect the ß2AR system are used to treat common cold and bronchitis accompanied with mucus covered and narrowed airways. The present work compares the influence of an ivy preparation and an ivy/thyme combination on the ß2-adrenergic signal transduction. For receptor binding studies and characterization of the lateral mobility of ß2AR we have used single molecule detection by fluorescence correlation spectroscopy and single particle tracking. For the determination of both the second messenger cAMP and the internalization of ß2AR we have generated luciferase based reporter cell lines, which produce a cAMP-dependent luciferase in the cytosol and express ß2AR with extracellular luciferase moiety in the plasma membrane. While both preparations increased the ß2AR binding, a significant increase of the cAMP level was observed only for the ivy preparation, which can be explained by the inhibited internalization of HiBiT-tagged ß2AR under stimulating conditions. In contrast, isoprenaline-mediated internalization of HiBiT-tagged ß2AR of ivy/thyme combination pre-treated cells was not inhibited. Cells comparatively pre-treated with a thyme preparation did not show inhibition of ß2AR internalization either. Furthermore, SNAP-tagged ß2AR of ivy preparation pre-treated cells, which were not internalized after isoprenaline stimulation, showed a redistribution from fast-to-slowly diffusing ß2AR. A corresponding redistribution of these receptors was not observed after pre-treatment with both the ivy/thyme combination and the thyme preparation. Comparable to the ivy/thyme combination, no decrease in the intratrack transitioning probability ratio (p23/p32) for fast and slow diffusing ß2AR was found for the thyme preparation, which, however, significantly decreased for control cells and for pre-treatment with the ivy preparation under stimulating conditions. It can therefore be concluded that the thyme fluid extract fraction in the ivy/thyme combination may have in part a negative effect on the ß2-adrenergic signal transduction.
ABSTRACT
BACKGROUND: Recently is has been shown that α- and ß-hederin increase the ß2-adrenergic responsiveness of alveolar type II cells (A549) and human airway smooth muscle cells (HASM), respectively, by inhibiting the internalization of ß2-adrenergic receptors (ß2AR) under stimulating conditions. Internalization of ß2AR is initiated by phosphorylations of certain serines and threonines by cAMP dependent protein kinase A (PKA) and G protein-coupled receptor kinases (GRK). PURPOSE: To evaluate the effect of α-hederin on PKA and GRK2 mediated phosphorylation of GFP-tagged ß2AR. STUDY DESIGN: To study this process we performed In-Cell Western using isoprenaline stimulated HEK293 cells overexpressing ß2AR as GFP fusion protein and specific antibodies against PKA (Ser345/346) and GRK2 (Ser355/356) phosphorylation sites. RESULTS: There was no effect found on the PKA mediated phosphorylation (n = 14) but we could show that α-hederin (1 µM, 12 h) significantly inhibits GRK2 mediated phosphorylation at Ser355/356 by 11 ± 5% (n ≥ 29, p ≤ 0.01) under stimulating conditions compared to the positive control. In Förster resonance energy transfer (FRET) experiments using the isolated kinases in solution α-hederin did not show any influence neither to GRK2 nor to PKA. CONCLUSION: Taken together, these results indicate that α-hederin acts as an indirect GRK2 inhibitor leading to a reduced homologous desensitization of ß2AR-GFP in HEK293 cells.
