Medizinische Universität Graz Austria/Österreich - Forschungsportal - Medical University of Graz
Gewählte Publikation:
Philipose, S.
NOVEL ROLES OF PROSTANOID AGONISTS AND ANTAGONISTS IN PLATELET FUNCTION
[ Dissertation ] Medical University of Graz; 2011. pp. 108
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- Autor*innen der Med Uni Graz:
- Sonia Philipose Sonia Philipose
- Betreuer*innen:
- Frank Sasa
- Schuligoi Rufina
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- Abstract:
- Platelets are small anucleate cells which are derived from fragmentation of megakaryocytes and are extruded into the circulation at a rate of approximately 85 million per day. The major role of platelets is in maintaining haemostasis. Certain features of platelets make them amenable to drug targeting, and antiplatelet therapy is among the most commonly prescribed and used class of medications. Under inflammatory conditions, prostanoids are synthesized by phospholipase A2 and cyclooxygenase which are involved in mediating their effects on thrombosis. Prostanoids like thromboxane (TX)A2, prostaglandin (PG)I2, PGD2 and PGE2 are released. While TXA2 via activation of the TP receptor causes platelet aggregation, PGI2 via the IP receptor, and PGD2 via the DP receptor inhibit aggregation, whereas PGER2R is known to play a biphasic role in platelet aggregation. At lower concentrations it is primarily pro-aggregatory, mediating its effects via the EP3 receptor, and at higher concentrations it is known to exhibit an anti-aggregatory effect. First, I showed the presence of the EP4 receptor on the surface of human platelets. The EP4 agonist, ONO AE1-329, inhibited adenosindiphosphate (ADP)- and collagen-induced aggregation in a concentrationdependent manner. The inhibitory effect of EP4 activation was reversed only by the EP4 antagonists leading to the conclusion that this inhibitory effect is specifically conferred by the EP4 receptor. Next I investigated the cellular mechanisms underlying this process. The EP4 agonist inhibited CaP2+ P flux and the PGER2R-induced inhibition of CaP 2+ P flux was reversed by using EP4 antagonists. P-selectin is important for plateletleukocyte-endothelial interaction and heterodimerization of the glycoprotein (GP)IIb/IIIa forms the fibrinogen receptor. PGER2R and the EP4 agonist ONO AE1-329 inhibited P-selectin expression levels on the surface of platelets as well as GPIIb/IIIa heterodimerization, and both the effects were revoked by the EP4 antagonist. Whole blood assays revealed that both PGER2R and ONO AE1-329 inhibited thrombogenesis and the EP4 antagonist reversed these effects. It was also observed that acetylsalicylic acid and the EP4 agonist act synergistically to inhibit thrombogenesis.In the second part of my study I investigated the effects of laropiprant (at a pharmacologically relevant concentration of 1 ¿M) with regard to the cellular mechanisms of PGDR2R/DP receptor based inhibition of platelet aggregation. In addition, laropiprant also reversed the inhibition of thrombogenesis caused by DP receptor activation. I also identified that laropiprant acts as a TP- as well as EP3-receptor antagonist (at a higher concentration of 10 ¿M). It was reported previously, that laropiprant had some residual effect on the TP receptor, but nothing has been reported so far about EP3 antagonistic effects of laropiprant. Activation of the EP3 and TP receptors enhances platelet aggregation and this could be reversed by laropiprant. P-selectin expression and GPIIb/IIIa heterodimerization which was enhanced by EP3 and TP receptor activation could also be revoked by laropiprant. As reported previously, I found that niacin is antithrombogenic, however, the mechanism is not elucidated so far. Laropiprant at 10 ¿M, also inhibited thrombus formation but did not affect the inhibitory actions of niacin or acetylsalicylic acid. Interestingly, pretreatment of whole blood samples with acetylsalicylic acid did not reverse the inhibitory effect of niacin on thrombus formation, suggesting, that - in contrast to the ...