PLATELETS AND THROMBOPOIESIS Platelet 12-LOX is essential for Fc g RIIa-mediated platelet activation

• Platelet 12-LOX modulates FcgRIIa signaling and presents a viable therapeutic target in the prevention of immune-mediated thrombosis. • This novel therapeutic approach is supported by pharmacologic inhibition and genetic ablation of 12-LOX in human and mouse platelets. Platelets are essential in maintaining hemostasis following inflammation or injury to the vasculature. Dysregulated platelet activity often results in thrombotic complications leading to myocardial infarction and stroke. Activation of the FcgRIIa receptor leads to immune-mediated thrombosis, which is often life threatening in patients undergoing heparin-induced thrombocytopenia or sepsis. Inhibiting FcgRIIa-mediated activation in platelets has been shown to limit thrombosis and is the principal target for prevention of immune-mediated platelet activation. In this study, we show for the first time that platelet 12(S)-lipoxygenase (12-LOX), a highly expressed oxylipin-producing enzyme in the human platelet, is an essential component of FcgRIIa-mediated thrombosis. Pharmacologic inhibition of 12-LOX in human platelets resulted in significant attenuation of FcgRIIa-mediated aggregation. Platelet 12-LOX was shown to be essential for FcgRIIainduced phospholipase Cg2 activity leading to activation of calcium mobilization, Rap1 and protein kinase C activation, and subsequent activation of the integrin aIIbb3. Additionally, platelets from transgenic mice expressing human FcgRIIa but deficient in platelet 12-LOX, failed to form normal platelet aggregates and exhibited deficiencies in Rap1 and aIIbb3 activation. These results support an essential role for 12-LOX in regulating FcgRIIa-mediated platelet function and identifies 12-LOX as a potential therapeutic target to limit immune-mediated thrombosis. (Blood. 2014;124(14):2271-2279)