Platelets play a critical role in maintaining hemostasis and minimizing blood loss following vascular injury.¹ However, they also have roles in pathological conditions including cerebral thrombosis, myocardial infarction, cancer, inflammation, and atherosclerosis.^2,3 For example, activated platelets aggregate at ruptured atherosclerotic plaques and trigger thrombus formation, leading to atherothrombotic disease, a leading cause of mortality and disability.^1,3 Several antiplatelet therapies have been developed and used in the treatment of thrombotic diseases.¹ One of the most prescribed antiplatelet agents is aspirin, which inhibits COX-dependent synthesis of prostaglandins and thromboxane A₂ (TXA₂), a potent activator of platelets. Human platelets express numerous G protein-coupled receptors (GPCRs) through which TXA₂ and several prostaglandins regulate platelet activation and aggregation, including the TXA₂ receptor (TP), prostaglandin I₂ (PGI₂) receptor (IP), prostaglandin D₂ (PGD₂) receptor subtype DP₁, and the prostaglandin E₂ (PGE₂) receptor subtypes EP₂, EP₃, and EP₄.^2,4,5 In this article, we will discuss the roles of thromboxanes, prostaglandins, and their receptors in the regulation of platelet activation and aggregation.

Thromboxane A₂ (TXA₂)

TXA₂ is produced by activated platelets and endothelial cells and is well known for its role as a platelet activator with prothrombotic properties and vasoconstrictive activity.^1,2,4 Platelets synthesize TXA₂ following activation by collagen, adenosine diphosphate (ADP), epinephrine, and thrombin, as well as by TXA₂ itself, which creates a positive feedback mechanism and allows TXA₂ to activate platelets in an autocrine and paracrine fashion.^1,2 TXA₂ binds to the TXA₂ receptor (TP), which signals through G_q to activate phospholipase C (PLC), resulting in an increase in intracellular calcium (Ca²⁺) levels (Figure 1).² Activation of the TP receptor by TXA₂ also leads to activation of Rho-associated kinase (ROCK), which regulates platelet shape change and spreading.¹

TP receptor agonists, such as the prostaglandin H₂ (PGH₂) analogs U-46619 and U-44069, induce platelet aggregation, whereas the TP receptor antagonists SQ 29,548 and seratrodast inhibit U-46619- and U44069-induced platelet aggregation, respectively.^6-10

Figure 1. TXA₂ and several prostaglandins signal through GPCRs on the surface of human platelets to regulate platelet activation and aggregation.

Prostaglandin I₂ (PGI₂)

PGI₂, also known as prostacyclin, is a potent inhibitor of platelet activation, aggregation, and thrombosis produced by endothelial and smooth muscle cells and is one of several factors that inhibit platelet activation in the circulation under physiological conditions.^1,2,4,5 It binds to the PGI₂ receptor (IP), which is coupled to G_s, leading to activation of adenylate cyclase and an increase in intracellular cyclic AMP (cAMP) levels (Figure 1).^2,4

Stable PGI₂ analogs, such as carbaprostacyclin, iloprost, beraprost, and ciprostene, inhibit platelet aggregation induced by ADP, collagen, and other aggregating agents with varying degrees of efficacy.^11-14 Iloprost also inhibits aggregation induced by platelet-activating factor (PAF), which can in turn be inhibited by the IP receptor antagonist CAY10441.¹⁵ PGI₂ (as epoprostenol) and iloprost also have vasodilatory effects and have been used in the treatment of pulmonary arterial hypertension (PAH).^1,4,16

Prostaglandin D₂ (PGD₂)

PGD₂ is also an inhibitor of platelet aggregation, albeit less potent than PGI₂.² It is produced by mast cells and platelets and binds to the PGD₂ receptor subtype DP₁ on human platelets, but not murine platelets, as murine platelets lack this receptor.^2,4 The DP₁ receptor is similar to the IP receptor in that it is coupled with G_s and its activation therefore increases adenylate cyclase activity and intracellular cAMP concentrations (Figure 1).

The DP₁ receptor agonist BW 245C inhibits ADP-induced platelet aggregation.^17,18

Prostaglandin E₂ (PGE₂)

PGE₂ is the primary prostaglandin produced by endothelial cells in human microvasculature and binds to the PGE₂ receptor subtypes EP₂, EP₃, and EP₄ on platelets (Figure 1).⁴ Binding to G_i-linked EP₃ receptors inhibits adenylate cyclase and the production of cAMP and potentiates platelet activation, whereas binding to the G_s-linked EP₂ and EP₄ receptors increases intracellular cAMP levels and inhibits platelet activation and aggregation.^2,5,15 PGE₂ is unique among prostaglandins for its biphasic effect on platelet aggregation, with potentiating effects at lower concentrations and inhibitory effects at higher concentrations.²

Subtype-selective EP receptor agonists and antagonists have helped to unravel the differential effects of PGE₂ receptors on platelet aggregation. The EP₃ receptor agonist sulprostone increases platelet aggregation induced by PAF, and this effect can be counteracted by the EP₃ receptor antagonist DG-041.¹⁵ The EP₂ receptor agonist ONO-AE1-259 and EP₄ receptor agonist ONO-AE1-329 both inhibit PAF-induced platelet aggregation, and the effects of ONO-AE1-329 can be inhibited by the EP₄ receptor antagonist ONO-AE3-208.

Prostaglandin E₁ (PGE₁)

PGE₁ is an inhibitor of platelet aggregation produced by many cell types, including activated platelets.^4,19 While it can interact with IP and EP receptors, it has been determined that its inhibitory effects on platelets are mediated primarily through IP receptors (Figure 1).^2,19 The IP receptor antagonist CAY10441 reduces PGE₁-mediated inhibition of platelet aggregation induced by U-46619, whereas the EP₄ receptor antagonist ONO-AE3-208 has no effect.¹⁹

Available from Cayman

As leading experts in lipid biology and chemistry, Cayman offers a variety of tools and resources to study the roles of thromboxanes, prostaglandins, and their receptors in platelet activation and aggregation, including high-quality lipids, receptor agonists and antagonists, and assay kits to measure intracellular Ca²⁺ and cAMP levels.

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References

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2. Kashiwagi, H., Yuhki, K., Imamichi, Y., et al. Roles of prostanoids in the regulation of platelet function. Thromb. Haemost. Res. 2(2), 1014 (2018).

3. Chaudhary, P.K., Kim, S., and Kim, S. An insight into recent advances on platelet function in health and disease. Int. J. Mol. Sci. 23(11), 6022 (2022).

4. Braune, S., Küpper, J.-H., and Jung, F. Effect of prostanoids on human platelet function: An overview. Int. J. Mol. Sci. 21(23), 9020 (2020).

5. Zhu, L., Zhang, Y., Guo, Z., et al. Cardiovascular biology of prostanoids and drug discovery. Arterioscler. Thromb. Vasc. Biol. 40(6), 1454–1463 (2020).

6. Abramovitz, M., Adam, M., Boie, Y., et al. The utilization of recombinant prostanoid receptors to determine the affinities and selectivities of prostaglandins and related analogs. Biochim. Biophys. Acta 1483(2), 285-293 (2000).

7. Tymkewycz, P.M., Jones, R.L., Wilson, N.H., et al. Heterogeneity of thromboxane A₂ (TP-) receptors: Evidence from antagonist but not agonist potency measurements. Br. J. Pharmacol. 102(3), 607-614 (1991).

8. Pollock, W.K., Armstrong, R.A., Brydon, L.J., et al. Thromboxane-induced phosphatidate formation in human platelets. Relationship to receptor occupancy and to changes in cytosolic free calcium. Biochem. J. 219(3), 833-842 (1984).

9. Ogletree, M.L., Harris, D.N., Greenberg, R., et al. Pharmacological actions of SQ 29,548, a novel selective thromboxane antagonist. J. Pharmacol. Exp. Ther. 234(2), 435-441 (1985).

10. Imura, Y., Terashita, Z., Shibouta, Y., et al. Antagonistic action of AA-2414 on thromboxane A₂/prostaglandin endoperoxide receptor in platelets and blood vessels. Jpn. J. Pharmacol. 52(1), 35-43 (1990).

11. Whittle, B.J.R., Moncada, S., Whiting, F., et al. Carbacyclin – A potent stable prostacyclin analogue for the inhibition of platelet aggregation. Prostaglandins 19(4), 605-627 (1980).

12. Schrör, K., Darius, H., Matzky, R., et al. The antiplatelet and cardiovascular actions of a new carbacyclin derivative (ZK 36 374) – Equipotent to PGI₂ in vitro. Naunyn Schmiedebergs Arch. Pharmacol. 316(3), 252-255 (1981).

13. Yang, L., Yatomi, Y., Satoh, K., et al. Inhibitory effects of beraprost on platelet aggregation: Comparative study utilizing two methods of aggregometry. Thromb. Res. 94(1), 25–32 (1999). 

14. Whittle, B.J. and Moncada, S. Platelet actions of stable carbocyclic analogues of prostacyclin. Circulation 72(6), 1219-1225 (1985).

15. Iyú, D., Glenn, J.R., White, A.E., et al. The role of prostanoid receptors in mediating the effects of PGE₂ on human platelet function. Platelets 21(5), 329-342 (2010).

16. Hill, N.S., Preston, I.R., and Roberts, K.E. Inhaled therapies for pulmonary hypertension. Respir. Care 60(6), 794-805 (2015).

17. Boie, Y., Sawyer, N., Slipetz, D.M., et al. Molecular cloning and characterization of the human prostanoid DP receptor. J. Biol. Chem. 270(32), 18910-18916 (1995).

18. Town, M.H., Casals-Stenzel, J., and Schillinger, E. Pharmacological and cardiovascular properties of a hydantoin derivative, BW 245 C, with high affinity and selectivity for PGD₂ receptors. Prostaglandins 25(1), 13-28 (1983).

19. Iyú, D., Jüttner, M., Glenn, J.R., et al. PGE₁ and PGE₂ modify platelet function through different prostanoid receptors. Prostaglandins Other Lipid Mediat. 94(1-2), 9-16 (2011).