Platelet function and Isoprostane biology. Should isoprostanes be the newest member of the orphan-ligand family?

Abstract

While there have been many reports investigating the biological activity and signaling mechanisms of isoprostanes, their role in biology, particularly in platelets, appears to still be underestimated. Moreover, whether these lipids have their own receptors is still debated, despite multiple reports that discrete receptors for isoprostane do exist on platelets, vascular tissues, amongst others. This paper provides a review of the important literature of isoprostanes and provides reasoning that isoprostanes should be classified as orphan ligands until their receptor(s) is/are identified.

Figure 1

Structure of arachidonic acid (the precursor for all prostaglandins), various TPR ligands, PGF_2α, and the most abundant isoprostane 8-iso-PGF_2α.

Figure 2

A schematic representation of the arachidonic acid metabolism pathway. After its liberation by phospholipases, ((i.e., phospholipase A₂ (PLA₂) or phospholipase C (PLC)), the free arachidonic acid may undergo enzymatic metabolism by the lipoxygenases which produce HPETEs and leukotrienes, and the cyclooxygenases (COX-1, COX-2) which generate prostaglandins and thromboxanes. The specific repertoire of the arachidonic acid metabolites produced may vary according to the expression profile of these enzymes in different cell types. In platelets, for example, arachidonic acid is metabolized by COX-1 into the prostaglandin endoperoxides, PGG₂ and PGH₂. Next, thromboxane synthetase further metabolizes PGH₂ into TXA₂, which is a potent activator of platelet aggregation, with a half-life of 20-30 seconds. Thromboxane A₂ is then hydrolyzed to the inactive form TXB₂ (not shown). On the other hand, if PGH₂ is metabolized by prostacyclin synthetase, then PGI₂ would be produced (e.g., in endothelial cells). Furthermore, if PGH₂ is acted upon by PGD or PGE isomerase, then PGD₂, and PGE₂ are produced, respectively (e.g., in renal cells). Finally, if the PG reductase metabolizes PGH₂, then PGF_2α is produced (e.g., pulmonary vessels). Thus, the biological functions of arachidonic acid are exerted indirectly after its metabolism into prostaglandin and thromboxane metabolites.

Figure 3

A schematic representation of the metabolic cascade for the non-enzymatic generation of isoprostanes. This is a proposed scheme in which four series of regioisomers of PGG₂ are formed, before they are reduced to PGF_2α isomers. As shown, isoprostanes can be formed from arachidonic acid in situ in phospholipids, from which they are presumably cleaved by phospholipases A₂. PGG₂ spontaneously rearranges to PGD₂ and PGE₂ thereby generating isoprostanes of the D and E series. The initial step in the formation of an isoprostane from arachidonic acid (I) is the generation of a lipid free radical by the abstraction of a hydrogen atom from one of the three methylene-interrupted carbon atoms, C7, C10, or C13, as shown here, by a free radical (FR•) which may be a hydroxyl radical (HO•), a superoxide radical (O₂^-•) or other free radical, and results in (II). Radical attack at C-10 is shown, abstraction at the other positions determines the relative proportion of the isomers formed. The lipid free radical is converted to a peroxy radical by reaction with molecular oxygen. The peroxy radical cyclizes in an intramolecular reaction that yields an endoperoxide (III). The free radical chain reaction will continue to propagate until quenched by an antioxidant.

Figure 4

Schematic representation of a model describing the inhibitory and stimulatory signaling pathways for TPR-dependent modulation of platelet activation by 8-iso-PGF_2α.