Quercitrin induces the downregulation of GPVI signaling events and modulates the contribution of TxA2 signaling in human platelets
Since quercitrin significantly reduced platelet aggregation and ATP secretion following CRP stimulation, we investigated the mechanism by which quercitrin regulates CRP-induced GPVI signaling, which activates numerous kinases including Syk, PLCγ2, PI3K, and AKT. To match the experimental condition with platelet aggregation assays, human platelets were stimulated with CRP (0.1 µg/ml) under stirring conditions in an aggregometer and lysed immediately after 5-minute progress. We found that Syk, PLCγ2, PI3K, and AKT were phosphorylated after CRP stimulation in vehicle–pretreated platelets, whereas quercitrin significantly reduced the phosphorylation of Syk, PLCγ2, PI3K, and AKT following CRP stimulation in a concentration-dependent manner (Figure 6A and 6B). Since quercitrin impaired TxA2-induced platelet aggregation and TxA2 reflects the response to collagen-mediated human platelet aggregation,(Cho et al., 2003) we also studied whether quercitrin modulates platelet activation following TxA2 stimulation. The phosphorylation levels of PLCβ3, PI3K, and AKT were elevated in U46619-activated control platelets with stirring, but the phosphorylation patterns were markedly diminished by quercitrin in a concentration-dependent manner (Figure 6C and 6D). These results suggest that quercitrin regulates both GPVI-and TxA2-mediated platelet signaling, thereby regulating platelet activation.