The Role of 12-Lipoxygenase in the Regulation of Platelet Function

[摘要] Platelets are small, anucleated cellular fragments derived from the megakaryocytes of the bone marrow. These small cellular fragments have a specialized role in maintaining hemostasis, a process that prevents blood loss, through the initiation of blood coagulation. However, excessive platelet reactivity can lead to a pathophysiological condition known as thrombosis. Platelet-mediated thrombosis is the primary underlying mechanism leading to cardiovascular life-threatening clinical events, such as myocardial infarction and stroke. Regulating excessive platelet reactivity is an essential aspect of antithrombotic therapy. A number of anti-platelet drugs have been developed to target specific signaling pathways or endpoints involved in platelet activation. Despite the effectiveness of current anti-platelet therapies, uncontrolled thrombosis or bleeding complications still persist. Therefore, elucidating the mechanisms involved in platelet activation is crucial for identifying the development of novel or alternative anti-thrombotic strategies.Platelet 12-lipoxygenase (12-LOX), an oxygenase principally found to convert freed polyunsaturated fatty acids (PUFA) substrates from the membrane phospholipids following cellular stimulation, was identified to be a potential target for regulating platelet reactivity. Thus, the studies described in this thesis seek to assess whether 12-LOX activity modulated platelet responses or reactivity by using newly developed selective 12-LOX inhibitors, NCTT-956 and ML355. Although both NCTT-956 and ML355 blocked 12-LOX activity in platelets, ML355 was shown to be much more potent in inhibiting platelet activation. In support of the pharmacological ex vivo studies, mice lacking 12-LOX (12-LOX-/-) in platelets were protected from platelet-induced activation as well as thrombosis in vivo. Additionally, the role of 12-LOX in immune-mediated platelet activation was also investigated in this thesis work. One form of immune-mediated platelet activation or thrombosis is heparin-induced thrombocytopenia and thrombosis (HITT), a life-threatening cardiovascular disorder. A key component of this disorder is the activation of FcRIIa, a transmembrane receptor, by immune-complexes. I had shown that 12-LOX potentiated immune-mediated platelet activation and 12-LOX activity regulated early signaling effectors in the immune signaling pathways of platelet activation. 12-LOX predominantly generates 12(S)-HETE, an oxylipin or metabolite, from arachidonic acid (AA), since this is the most abundant -6 PUFA found in the lipid bilayer of cells. Dietary supplementation of plant- or fish-based oils can alter and enrich the contents of the lipid bilayer by which they shift 12-LOX substrate from AA to other PUFAs. Consumption of borage or primrose oil, which is enriched in -6 PUFA dihomo--linolenic acid (DGLA), has been shown to be cardioprotective; however, its mechanism of action was unclear. In this thesis, I had also sought to elucidate the underlying mechanism by which DGLA could provide cardioprotection via the prevention of thrombosis. I had demonstrated that the oxylipin of DGLA generated by 12-LOX, 12(S)-HETrE, inhibited platelet activation and thrombosis in vivo in a G-signaling dependent manner. Altogether, my thesis work highlighted the importance of targeting 12-LOX in platelets or using its endogenous mechanism to generate oxylipins that have anti-platelet effects as potential therapeutic avenues for preventing unwanted platelet activation.

[发布机构] University of Michigan

[效力级别] 12-Lipoxygenase

[关键词] Platelet biology;12-Lipoxygenase;Immune-mediated thrombocytopenia and thrombosis;Molecular;Cellular and Developmental Biology;Science;Pharmacology