Abstract 5786: Prostaglandin E2 and EP3 Activation Potentiate Human Platelet Responsiveness
Activated human platelets synthesize prostaglandin (PG) E2, although at much lower rates than thromboxane A2. PGE2 binds different receptors (EP1–4). EP3-deleted mice displayed lower platelet response to sub-threshold concentrations of agonists and less susceptibility to experimental thrombosis than wild type mice. However, the role of PGE2 in human platelet function remains poorly characterized. We investigated the effect of PGE2 and EP-agonists on human platelet responsiveness to other agonists. Platelets were incubated with increasing concentrations of PGE2 or different EP agonists, and stimulated by aggregating doses of adenosine diphosphate (ADP), collagen or arachidonic acid (AA). Aggregation was measured by the optical method. EP expression was studied by immunohistochemistry in platelets and bone marrow megakaryocytes. PGE2 in the microM range (1–200 microM) dose-dependently inhibited 10microM ADP-induced aggregation. However nanoM PGE2 concentrations (2–200 nanoM), dose-dependently increased the slope (velocity) of the secondary wave of 8 microM ADP-induced platelet aggregation, with an EC50 of 25±6 nanoM (n=15), without affecting maximal aggregation. At lower (4 microM) ADP concentration, PGE2 reverted the reversible aggregation, with an EC50 of 29±10 nanoM (n=6). In in vitro aspirinated platelets, 200 nanoM PGE2 in part counteracted the effect of aspirin in abolishing the secondary wave of 8 microM ADP-induced aggregation. In addition, PGE2 dose-dependently abolished shape change caused by 10 microg/ml collagen and 50 microg/ml AA, with an IC50 of 157±4 nanoM. The EP3 agonist, 11-deoxy-16,16-dimetyl PGE2, mimicked the PGE2 effect on the secondary wave of ADP-induced aggregation, with an EC50 58±1 nanoM (n=10), while the EP2 agonist butaprost was ineffective. The EP3 agonist at microM concentrations, dose-dependently induced spontaneous platelet aggregation with an EC50 of 5.5±1 microM. Immunostains revealed EP3 expression in human platelets and megakaryocytes. We conclude that PGE2-EP3 interaction potentiates human platelet response to physiological concentrations of common agonists. EP3 might represent a novel pharmacological target for inhibiting platelet responsiveness in atherothrombotic diseases.