Clinical observations of individuals with a

Clinical observations of individuals with a decrease in 12-LOX a short text are consistent with 12-LOX potentiating platelet activation and thrombus formation [40]. However, the relative contribution of 12-LOX to these complex disorders cannot be fully elucidated in the absence of a biochemical assessment of how 12-LOX and its oxylipins are mechanistically regulating platelet reactivity, thrombosis, and subsequent MI and stroke. In the absence of these data, these clinical observations remain associative as opposed to causative with regard to the extent of regulatory control 12-LOX and its oxylipins play in these clinicopathological conditions. For example, ∼40% of individuals with myeloproliferative disorders, hematological neoplasms in the bone marrow, have platelets with a decrease in 12-LOX expression and activity [41]. The reduction in 12-LOX expression in these patients is associated with a decreased risk of thrombosis, and an increased risk of bleeding relative to patients with myeloproliferative disorders that express similar levels of 12-LOX as healthy donors 42, 43, 44. Additionally, individuals with haploinsufficiency of Runt-related transcription factor (RUNX)1, a hematopoietic-specific transcription factor, have thrombocytopenia, hypofunctional platelets, and bleeding diathesis that is associated with a decrease in expression of 12-LOX [45]. The effect of RUNX1 haploinsuficiency on the expression of other platelet proteins is not well understood and therefore the relative contribution of 12-LOX to the decrease in platelet function remains unclear. Mice deficient in 12-LOX (12-LOX−/−) are viable and have no spontaneous bleeding; however, upon stress or challenge, 12-LOX−/− mice exhibit prolonged tail vein bleeding compared to wild-type (WT) mice [12]. Even with the generation of 12-LOX−/− mice, the extent of regulation imparted by 12-LOX in platelet activation is laboratory- and challenge-dependent. An early study of platelets from 12-LOX−/− mice has suggested that they were hyper-responsive to one agonist, ADP, but had similar levels of responsiveness to other agonists tested compared to platelets from WT mice [34]. More recent studies, however, have demonstrated that platelets from 12-LOX−/− mice exhibit attenuated aggregation compared to platelets from WT mice in response to multiple agonists tested including collagen, PAR4-activating peptide (AP), and ADP 12, 32, 46. In response to laser-induced vascular injury, 12-LOX−/− mice exhibit a decrease in thrombus formation compared to WT mice, suggesting that 12-LOX is essential for thrombus formation in this mouse model [46]. It remains to be determined if a significant component of the thrombosis observed in vivo is due to enhanced platelet signaling shown in human and mouse platelets ex vivo, thrombin generation, or a combination of the two 12-LOX-mediated effects. It is important to acknowledge that while essential for determining potential in vivo involvement in hemostasis and thrombosis, mouse models of thrombosis in the background of 12-LOX−/− mice need to be interpreted with caution because mice express a distinct set of LOX isoforms from humans, including other LOX enzymes in the cardiovascular system that have the potential to generate 12-HETE [9].
Early 12-LOX Inhibitors LOX inhibitors are divided into categories based on their mechanism of action including redox inhibitors and fatty acid analogs [9]. Studies have consistently demonstrated that different categories of 12-LOX inhibitors reduce platelet activation and aggregation in response to a variety of platelet agonists including collagen, thrombin, ADP, and U46619 12, 31, 47, 48, 49, 50. Similar to other secondary feedback drugs, 12-LOX inhibitors do not ablate platelet aggregation, but rather they shift the dose response to agonist. This attenuated activity, whereby the platelet becomes less sensitive to the endogenous agonists exposed to the platelet under prothrombotic conditions, is viewed as a favorable pharmacological and physiological trait because full inhibition would result in a significant bleeding risk and not be viewed as a viable path forward toward prevention of thrombosis, without significant increases in bleeding.