Abstract 1940: Syk as a Selective Target to Reduce Shear-Induced Platelet Thrombosis
Syk is a 72 kDa non-receptor tyrosine kinase that contributes significantly to platelet activation induced by exposure of subendothelial collagen or thrombin generation during arterial thrombosis. Platelets exposed to pathophysological shear stress that occurs at sites of arterial injury, particularly upon atherosclerotic plaque rupture, also undergo a substantial increase in Syk tyrosine (Y) phosphorylation. In contrast, conditions that mimic physiologic flow have minimal effects on Syk phosphorylation making Syk an attractive target for pharmacological intervention. Shear-induced phosphorylation occurs at Syk Y525/526 and Y352, which are critical for Syk kinase and adapter functions, respectively. Phosphorylation at both sites is mediated through the activity of Src-family kinase(s). Phosphorylation of Y525/526 is dependent upon ADP signaling, but not contingent upon outside-in signaling through GPIIb-IIIa or aspirin-sensitive TxA2 signaling. GPIIb-IIIa antagonists or aspirin have no significant effects in blocking Syk phosphorylation due to increased shear stress. These results provide a molecular basis that platelet P2Y1/12 receptor(s) blockade or ADP scavengers may reduce ischemic events triggered by shear-dependent platelet activation. Syk inhibitors are exceedingly effective in inhibiting shear-induced phosphorylation of Y525/526 and Y352 and shear-induced aggregation. These inhibitors are apparently Syk selective as they do not target upstream Src kinase. Using an in vitro perfusion system to examine thrombus formation on collagen substrates under flow, our studies support that Syk activity is fundamental for thrombus formation under pathophysiological flow conditions. These studies suggest that selective pharmacological modulation of Syk offers a viable pathway for the treatment of pro-thrombotic lesions or arterial thrombosis while minimizing effects on hemostasis and consequent increased incidences of bleeding.