Modulation of Platelet Aggregation by the Ep3 Receptor for Prostaglandin E2
Prostaglandins (PGs) are vasoactive eicosanoids that regulate vascular tone and platelet functions. PGE2 is produced by activated platelets and is known to have dual effects on platelet function. At high concentrations, PGE2 inhibits platelet aggregation while at lower concentrations, PGE2 may potentiate aggregation. The physiologic functions of PGE2 are mediated through binding to specific PGE2 receptors (EPs). Four EP receptor isoforms (EP1-4) have been identified, each with distinctive tissue distributions and signaling mechanisms. However, roles for the individual EP receptor isoforms in regulating platelet functions are not clearly defined. In some cell types, the EP3 receptor couples to the inhibitory protein Gi, resulting in reduced intra-cellular cAMP levels. As reduced cAMP levels enhance platelet aggreation, we hypothesized that the EP3 receptor might mediate the pro-aggregatory actions of PGE2. To address this issue, we studied mice with targeted disruption of EP3 receptor gene (EP3-/-). Bleeding times in EP3-/- mice were similar to wild type controls (70±18 sec vs. 84±18 sec, p=0.57). To study the effects of EP3 receptor on platelet function in vitro, platelet aggregation was determined by light absorbance. PGE2 at a dose of 10 nM had no significant effect on platelet aggregation in either EP3-/- or wild type mice. However, in platelets from wild-type mice, the combination of 10 nM PGE2 and 500 nM of thromboxane receptor (TP) agonist U46619 caused vigorous aggregation (75±3.5%). In contrast, 10 nM of PGE2 with 500 nM TP agonist had no effect on EP3-deficient platelets (4.3±2.8%, p<0.01 vs wild-type). These studies identify actions of the EP3 receptor to promote platelet aggregation.