The effect of P2Y-mediated platelet activation on the release of VEGF and endostatin from platelets

Vascular endothelial growth factor (VEGF) and endostatin are key protein modulators of angiogenesis found within platelets. The platelet activation pathways that control angiogenic protein release are incompletely elucidated. The differential release of pro-angiogenic and anti-angiogenic proteins from the platelet has been demonstrated for proteinase activated receptors (PARs). Given the ability of tumors to secrete ADP and the availability of ADP receptor antagonists clinically, we determined the influence of adenosine diphosphate (ADP) and the ADP receptors, P2Y(1) and P2Y(12), on platelet release of the angiogenic stimulator protein, VEGF, and the angiogenic inhibitor protein, endostatin. Minimally altered whole blood (WB) and platelet rich plasma (PRP) from healthy volunteers was stimulated with ADP alone (12.5 uM), in combination with a P2Y(1) antagonist (MRS2179) or a P2Y(12) antagonist (cangrelor). VEGF and endostatin protein concentrations were assessed by an ELISA assay. We report that maximally stimulating concentrations of ADP significantly increased VEGF release from platelets in both PRP and WB by 36+/-12% 36+/-12% 54+/-18% 36 +/- 12% (p < 0.05) respectively as compared to control. Both P2Y(1) and P2Y(12) receptor antagonism inhibited this release. Conversely, endostatin levels did not change following ADP stimulation in PRP, while a 4.7% (p = 0.03) increase was observed in WB. As compared to thrombin receptor activation, ADP activation was a weaker stimulus for VEGF release. We found that activation of platelets by ADP results in an increase in soluble VEGF concentrations with minimal effects on endostatin concentrations, suggesting ADP release in the tumor microenvironment may be, on balance, proangiogenic. P2Y receptor antagonism abrogates ADP mediated proangiogenic protein release and thus may represent a potential pharmacologic strategy for regulating platelet mediated angiogenesis.