Osmanthus fragrans Flower Ameliorates Compound 48/80-Induced Pseudo-Allergic Reactions in vitro and in vivo

Osmanthus fragrans flower is a highly popular edible flower due to its long-lasting aroma. Although it has been reported to possess multiple activities, no study has focused on its effects on mas-related G protein-coupled receptor (Mrgpr)-mediated mast cell activation.

The ethanol extract of O. fragrans flower (OFE) was prepared and analyzed by the NaNO₂-Al(NO₃)₃-NaOH colorimetric method and HPLC. Compound 48/80 (C48/80) was used for activating Mrgpr signaling. Supernatant β-hexosaminidase level was assayed by using the fluorescence probe 4-methylumbelliferyl N-acetyl-β-D-glucosaminide. The levels of cytoplasmic Ca2+ (Cac2+) and intracellular mitochondrial Ca2+ (Cam2+) were determined by fluorescence probes Fluo-3 AM and Rhod-2/AM, respectively. The Ca2+ level outside the mitochondria was indicated by the fluorescence probe Calcium Green-5N. C48/80-induced vascular leakage, scratching behavior, and hypothermia models were used for evaluating the in vivo effects of OFE.

The highly water-soluble flavonoid-rich OFE significantly inhibited C48/80-induced degranulation in LAD2 cells and primary rat peritoneal mast cells. It could rapidly and reversibly decrease Cac2+ level independent of the activation of Mrgpr signaling. Among the four classical pathways responsible for Cac2+ removal, OFE failed to affect plasma membrane Ca2+-ATPase, sarco/endoplasmic Ca2+-ATPase, and Na+/Ca2+ exchanger, but promoted the uptake of Cam2+. In vivo, OFE alleviated C48/80-induced vascular leakage, scratching, as well as hypothermia.

This study indicates, for the first time, that OFE is a mast cell stabilizer. It decreases Cac2+ to suppress mast cell degranulation by directly enhancing Cam2+ uptake. These findings may unveil a potential medicinal value of O. fragrans flower as a promising candidate for relieving mast cell activation-related diseases.