JMolMed DOI 10.1007/s00109-017-1542-4 REVIEW The expansive role of oxylipins on platelet biology Jennifer Yeung 1 & Megan Hawley1 & Michael Holinstat1,2 Received: 11 February 2017 /Revised: 29 April 2017 /Accepted: 4 May 2017 # Springer-Verlag Berlin Heidelberg 2017 Abstract In mammals, three major oxygenases, responses in the blood following physiological and patho- cyclooxygenases (COXs), lipoxygenases (LOXs), and cyto- physiological disturbance of the endothelium lining the vessel chrome P450 (CYP450), generate an assortment of unique wall [2]. The inability to properly regulate platelet reactivity lipid mediators (oxylipins) from polyunsaturated fatty acids often leads to atherothrombotic events, including myocardial (PUFAs) which exhibit pro- or anti-thrombotic activity. Over infarction and stroke. Recent work in the field has uncovered a the years, novel oxylipins generated from the interplay of number of lipid products, eicosanoids, derived from ω-3 or -6 theoxygenase activity in various cells, such as the specialized polyunsaturated fatty acids (PUFAs) that significantly regulate pro-resolving mediators (SPMs), have been identified and in- and alter platelet function. The PUFAs include arachidonic vestigated in inflammatory disease models. Although platelets acid (AA), linoleic acid (LA), eicospentaenoic acid (EPA), have been implicated in inflammation, the role and mecha- docosapentaenoic acid (DPA), docosahexaenoic acid (DHA), nism of these SPMs produced from immune cells on platelet and dihomo-γ-linolenic acid (DGLA). Understanding how function are still unclear. This review highlights the these newly identified lipids fit into the overall regulation of burgeoning classes of oxylipins that have been found to reg- platelets in the vessel will aid in our understanding of lipid- ulate platelet function; however, their mechanism of action platelet interactions, often resulting from altered diet or fatty still remains to be elucidated. acid supplementation, that play key roles in the ability of the platelet to form a hemostatic Bplug^ following vascular injury or alternatively form an occlusive thrombus following patho- Keywords Lipoxygenase . Cyclooxygenase . Oxygenases . physiologic insult to the vessel. Finally, understanding how Eicosanoids . Prostaglandins . Thrombosis these lipids and lipid products are generated and regulate platelet reactivity should reveal novel targets for therapeutic intervention to prevent thrombosis while limiting the risk for Introduction bleeding following vessel injury. Thus, this review will be limited to describing the various lipids and bioactive lipid Cardiovascular disease remains the leading the cause of mor- products shown to regulate platelet function and modulate tality globally accounting for nearly 1 in 3 deaths annually [1]. hemostasis and thrombosis in the vessel. Platelet activation leading to clot formation and thrombosis is PUFAs are generally inert and depend on oxygenase activ- an essential component of both the hemostatic and thrombotic ity to generate a wide array of structurally distinct bioactive fatty acids metabolites. The formation of lipid products is typically initiated by stimulation of the cell that results in an * Michael Holinstat [email protected] increase in intracellular calcium. This calcium flux results in translocation of cytosolic phospholipase A2 (cPLA2)tothe 1 Department of Pharmacology, University of Michigan, 1150 W. lipid membrane where it cleaves the fatty acid from the sn-2 Medical Center Dr., Room 2220D, Ann Arbor, MI 48109-5632, USA position of the phospholipids to release free fatty acids for 2 Department of Internal Medicine, Division of Cardiovascular oxidation in the cell. Once cleaved from the lipid membrane, Medicine, University of Michigan, Ann Arbor, MI, USA the freed fatty acids can be metabolized by cyclooxygenase J Mol Med (COX), lipoxygenase (LOX) or cytochrome P450 (CYP450) inflammation and tumorigenesis [4]. This section describes to form oxidized lipids (oxylipins). Oxylipins have been the select prostanoid lipids generated from the PUFAs through thought to predominantly function by regulating cellular prop- the COX pathway that regulate platelet function. erties and signaling through one of three pathways. The first involves binding to G protein-coupled receptors (GPCRs) to COX-derived metabolites and their regulatory roles further propagate intercellular signaling. Secondly, fatty acids on platelet function or their oxylipins can directly interact with peroxisome proliferator-activated receptors (PPARs) within the cell. COX transforms AA to series 2 PGs (PGE2,PGD2,PGI2)and While fatty acids are thought to be weak activators of thromboxanes (TX) A2 that can exhibit either pro-thrombotic PPARs, when they accumulate in the vicinity of the PPAR, or anti-thrombotic modulation of platelet function (Table 1) reports have shown their affinity for activating PPAR signal- [5, 6]. In terms of thrombosis, TXA2, when formed, is released ing is significantly increased [3]. The third regulatory mecha- and acts through the thromboxanereceptor (TPα), which is nism utilized by fatty acids and oxylipins in the platelet is coupled to Gαq and Gα13 and functions to amplify platelet direct inhibition of oxylipin-producing enzymatic pathways activation leading to enhanced aggregation and thrombosis or further metabolic transformation of lipids within the cell [7]. In contrast, PGD2 derived mainly from mast cells, leuko- (Fig. 1). The review will cover the oxygenase pathways, clas- cytes, and some platelets [8], had been shown to dampen ses of structurally distinct oxylipins, and their biological ef- platelet activation [9–11] through its binding to the DP1 recep- fects on the platelets. tor and subsequent elevation of cAMP [12–14]; however, there are evidence that PGD2 can also directly activate 12 PPARs [15]. PGD2 can be further dehydrated to PGJ2, Δ - 12,14 PGJ2, and 15-deoxy-Δ -PGJ2, and inhibit platelet activa- Cyclooxygenase tion through a number of signaling pathways including acti- vation of PPARs [16, 17]. While PGD2 derivatives are known Cyclooxygenase (COX) exists in two isoforms, COX-1 and PPAR ligands, the role of PPARs in platelet activation has not COX-2 in the body; however, the platelet expresses primarily been fully elucidated. Similar to PGD2,PGI2 (prostacyclin), a COX-1, and its inhibition is thought to be a primary target for well-characterized vasodilator [18], has been shown to acti- reduction of platelet reactivity in the patients with cardiovas- vate adenylate cyclase in the platelet via the prostacyclin (IP) cular risk. COX activation primarily results in the generation receptor and in turn antagonizes platelet aggregation at sites of of prostanoids (prostaglandins (PGs) and thromboxanes injury [6, 19]. Although PGI2 has been shown to exert anti- (TXs)) derived from PUFAs that are responsible for maintain- platelet effects in vivo and is also clinically available to treat ing either physiological or pathophysiologic states, such as cardiovascular related diseases, a major concern of the use is the increased occurrence of hypotension. Moreover, PGE2 exhibits pleiotropic effects by which it can induce both pro- and anti-platelet responses depending on the concentration [20, 21]through the binding of one of more of its prostaglan- din receptors: EP1,EP2,EP3 and EP4 [22]. DGLA is an ω-6 PUFA that can be acquired through sup- plementation of γ-linolenic acid (GLA) in the diet. COX con- verts DGLA to series 1 prostaglandins (PGD1, PGE1) and TXA1 [23, 24], which inhibit platelet function in vitro and in vivo [25]. These DGLA-derived COX prostanoids exerted their anti-platelet action by activating the Gαs-coupled GPCRs, prostaglandin (EP2 and EP4)orIPreceptors. Similar to DGLA, COX acts on EPA, an ω-3 PUFA, to generate anti-inflammatory [26] lipid mediators, series 3 PGs and TXs [27, 28]. EPA-derived metabolites of COX (PGE3, PGD3,PGI3) inhibit platelet aggregation [29–32]andP- Fig. 1 Polyunsaturated fatty acids (PUFAs) are released from the selectin expression induced by platelet activating factor (PAF) embedded phospholipid bilayer membrane, which are then converted as well as inhibiting platelet-rich plasma (PRP) [7]. Evidence by intracellular oxygenases (COX, LOX, or CYP450) to generate wide for the series 3 PGs receptors is scant. PGE3 hadbeensuggested array of oxylipins that can diffuse across the cellular membrane to be to be a partial agonist of the EP receptors in human kidney cells further converted by oxygenases, act on intracellular signaling component, peroxisome proliferator-activated receptor (PPAR), or act with varying degree of affinity potencies [29], inducing second- on receptor to regulate platelet function ary messenger actions. For instance, PGE3 mediates Gαq JMolMed Table 1 Oxylipin regulation of platelets Oxygenase PUFA Oxylipin Actions COX AA PGD2 Inhibits platelet activation via DP receptor and possibly PPAR AA PGE2 Exhibits both anti-platelet and pro-platelet activation, depending on concentrations and binding to receptors EP1-EP4 AA PGI2 Inhibits platelet function via IP receptor and PPAR AA TXA2 Activates platelet function via TPα receptor DGLA PGD1 Inhibits platelet function via EP2,EP4 and IP receptors DGLA PGE1 Inhibits platelet function via EP2,EP4 and IP receptors DGLA TXA1 Inhibits platelet function via EP2,EP4 and IP receptors EPA PGD3 Inhibits platelet function via DP receptor and PPAR EPA PGE3 Inhibits platelet function via EP2 and P4 receptors EPA PGI3 Inhibits platelet function via IP