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.