Activation of the murine EP3 receptor for PGE2 inhibits cAMP production and promotes platelet aggregation
Jean-Etienne Fabre, … , Thomas M. Coffman, Beverly H. Koller
Jean-Etienne Fabre, … , Thomas M. Coffman, Beverly H. Koller
Published March 1, 2001
Citation Information: J Clin Invest. 2001;107(5):603-610. https://doi.org/10.1172/JCI10881.
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Article

Activation of the murine EP3 receptor for PGE2 inhibits cAMP production and promotes platelet aggregation

Abstract

The importance of arachidonic acid metabolites (termed eicosanoids), particularly those derived from the COX-1 and COX-2 pathways (termed prostanoids), in platelet homeostasis has long been recognized. Thromboxane is a potent agonist, whereas prostacyclin is an inhibitor of platelet aggregation. In contrast, the effect of prostaglandin E2 (PGE2) on platelet aggregation varies significantly depending on its concentration. Low concentrations of PGE2 enhance platelet aggregation, whereas high PGE2 levels inhibit aggregation. The mechanism for this dual action of PGE2 is not clear. This study shows that among the four PGE2 receptors (EP1–EP4), activation of EP3 is sufficient to mediate the proaggregatory actions of low PGE2 concentration. In contrast, the prostacyclin receptor (IP) mediates the inhibitory effect of higher PGE2 concentrations. Furthermore, the relative activation of these two receptors, EP3 and IP, regulates the intracellular level of cAMP and in this way conditions the response of the platelet to aggregating agents. Consistent with these findings, loss of the EP3 receptor in a model of venous inflammation protects against formation of intravascular clots. Our results suggest that local production of PGE2 during an inflammatory process can modulate ensuing platelet responses.

Authors

Jean-Etienne Fabre, MyTrang Nguyen, Krairek Athirakul, Kenneth Coggins, John D. McNeish, Sandra Austin, Leslie K. Parise, Garret A. FitzGerald, Thomas M. Coffman, Beverly H. Koller

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Effects of PGE2 on cytosolic calcium and cAMP levels. (a) Absence of int...
Effects of PGE2 on cytosolic calcium and cAMP levels. (a) Absence of internal calcium mobilization upon treatment of wild-type platelets with 10–7 and 10–5 M PGE2. Change in fluorescence upon subsequent addition of 10 μM U46619 demonstrated that these platelets were able to mobilize their internal calcium stores. After exposure to 10–5 M PGE2, platelets did not respond strongly to 30 μM ADP because of the inhibitory effect of high PGE2 concentration on calcium mobilization. PGE2 (10–4 M) induced calcium mobilization in IP-deficient, but not in IP- and TP-deficient platelets. Experiments examining calcium mobilization in wild-type platelets were repeated five times. Experiments examining calcium mobilization in Ip–/– and Ip–/– × Tp –/– platelets were repeated four times, and similar results were observed in all experiments. A representative trace is shown. (b) Effects of 5 × 10–5 M PGE2 and 7 × 10–9 M PGI2 on accumulated cAMP production in each of the receptor-deficient mouse lines compared with their age- and strain-matched controls. Values were normalized to the cAMP level obtained in appropriate control animals (DBA for Ep1–/–, 129 for Ep2–/–, Ep3–/–, and Ip–/–, and mixed background for Ep4–/– mice), and the bars represent the mean of the percent change observed in four experiments (each bar graph represents the values obtained from 16 mice). Error bars = SEM. ASignificant difference: P < 0.01 (ANOVA test, and Dunnett as post test). Inset: effect of increasing concentration of PGE2 on cAMP level elevated by treatment of platelets of IP-deficient platelets with 10–5 M adenosine. Bar A (inset), the cAMP level in platelets treated with 10–5 M adenosine was set at 100%; Bar B (inset), cAMP level in untreated platelets. The remaining bars indicate the percentage of maximal cAMP observed in platelets treated with 10–5 M adenosine and the indicated amount of PGE2 (10–9, 10–7, and 10–5 M). Three experiments were carried out. Error bars = SEM. ASignificant difference: P < 0.01 (ANOVA test, and Dunnett as post test).