The anti-inflammatory effect of 2-(4-hydroxy-3-prop-2-enyl-phenyl)-4-prop-2-enyl-phenol by targeting Lyn kinase in human neutrophils

The undesirable respiratory burst in neutrophils can lead to inflammation and tissue damage. This study investigates the effect and the underlying mechanism of 2-(4-hydroxy-3-prop-2-enyl-phenyl)-4-prop-2-enyl-phenol (honokiol), a lignan extracted from the stem bark of Magnolia officinalis Rehd. et Wils (Magnoliaceae), on N-formyl-l-methionyl-l-leucyl-l-phenylalanine (fMLP)-induced respiratory burst in human neutrophils. Signaling pathways regulated by honokiol which modulate fMLP-induced respiratory burst and cathepsin G release were evaluated by phosphorylation of Src family kinase induced by fMLP, Src family kinases activities and by immunoblotting analysis of the downstream targets of Src kinase. Briefly, honokiol inhibited fMLP-induced superoxide anion production (IC₅₀ = 9.80 ± 0.21 μM, n = 4), cathepsin G release (IC₅₀ = 14.23 ± 1.43 μM, n = 4) and migration (IC₅₀ = 5.69 ± 1.51 μM, n = 4) in a concentration dependent manner. Further, honokiol specifically suppresses fMLP-induced Lyn (a member of the Src kinase family) phosphorylation, by inhibiting Lyn kinase activity. Consequently, honokiol attenuated the downstream targets of Lyn kinase, such as Tec translocation from the cytosol to the inner leaflet of the plasma membrane, phosphorylation of AKT, P38, PLCγ2, protein kinase C and membrane localization of p47^phox. On the other hand, fMLP-induced phosphorylation of Hck, Fgr kinase activity (other members of Src kinase), downstream phosphorylation of Vav1 and extracellular signal-regulated kinase remained unaffected. In addition, honokiol neither inhibited NADPH oxidase activity nor increased cyclic AMP levels. Honokiol is not a competitive or allosteric antagonist of fMLP. In conclusion, honokiol specifically modulates fMLP-mediated neutrophil activation by inhibiting Lyn activation which subsequently interferes with the activation of PLCγ2, AKT, p38, protein kinase C, and p47^phox.