No abstract

3254 Platelet activation plays a pivotal role in thrombosis and hemostasis. Understanding the signaling events mediating this process is essential in preventing unwanted clot formation, which can lead to the development of heart attack, stroke, and venous thromboembolism. Fatty acids may play an important role in determining the level of platelet reactivity, however the mechanism(s) by which this occurs are not entirely clear. 12-lipoxygenase (12-LOX) has been shown to oxidize the fatty acid, arachidonic acid (AA), in order to produce the bioactive eicosanoid 12-HETE, which has recently been shown to play a role in tissue factor activation and subsequent thrombin generation in the platelet (Thomas et al., 2010, J Biol Chem; 285:6891–903). 12-HETE has also been shown to signal to cells, in part, through the G protein-coupled receptor GPR31 (Guo et al, 2011, J Biol Chem; epub). While research on 12-LOX-mediated eicosanoid regulation of platelets has primarily focused on 12-HETE, the potential for regulation of platelets by eicosanoids derived from other fatty acids has been overlooked. As the fatty acid content on the platelet membrane is extremely dynamic and fatty acid supplementation is correlated with a reduced risk for cardiovascular disease, we hypothesized that 12-LOX oxidation of another fatty acid may play a direct role in regulating platelet function. To test this hypothesis, the Ω-6 fatty acid dihomo-γ-linolenic acid (DGLA) as well as its eicosanoid derived from 12-LOX oxidation, 12-hydroxyeicosatrienoic acid (12-HETrE), were applied to washed platelets followed by stimulation with thrombin, PAR1-AP, or PAR4-AP. Platelets treated with either DGLA or 12-HETrE showed significant attenuation in platelet aggregation following stimulation with thrombin or PAR-AP. A number of biochemical intermediates were also tested in the presence of 12-HETrE or DGLA including Rap1 activation, αIIbβ3 integrin activation, α-granule secretion, and dense granule secretion. All endpoints tested were attenuated in the presence of DGLA or 12-HETrE relative to control. To confirm the regulation was unique to DGLA and its metabolite, the same endpoints were measured in the presence of AA or 12-HETE. Pre-treatment with either AA or 12-HETE did not attenuate any of the agonist-mediated platelet activation endpoints. Thus, our data supports a unique role for 12-HETE and 12-HETrE and that fatty acid regulation of platelet function may be highly dependent on the lipid content of the platelet. Shifting the ratios of fatty acids in the platelet through dietary supplementation or pharmacological intervention may be sufficient to induce a cardio-protective state, in part, through increasing 12-HETrE formation and subsequently inhibiting platelet activation. Disclosures: No relevant conflicts of interest to declare.

Excessive platelet activation often results in unwanted clot formation leading to vessel occlusion and stroke. One approach to preventing clot formation in the vessel may be through regulation of signaling events in the platelet. 12‐lipoxygenase (12‐LOX), an enzyme regulating bioactive metabolite formation in human platelets, oxidizes a number of free fatty acids such as arachidonic acid (AA) and dihomo‐γ‐linolenic acid (DGLA) following platelet activation and fatty acid liberation from the phospholipid membrane. We hypothesized the metabolites formed by each fatty acid may have opposing regulatory effects in the platelet. To investigate this possibility, a number of biochemical endpoints were assessed in the absence or presence of AA, DGLA, or their metabolites (12‐HETE and 12‐HETrE, respectively). Washed human platelets treated with DGLA or 12‐HETrE significantly attenuated agonist‐induced platelet aggregation, αIIbβ3 activation, and Rap1 activation; whereas AA and 12‐HETE had no inhibitory effect on platelet‐mediated activation. Our data supports an inhibitory role for 12‐HETrE and a pro‐thrombotic role for 12‐HETE in the platelet. Hence regulation of fatty acid content or metabolite formation may represent alternative mechanisms for regulating platelet reactivity in vivo. This work was supported by Sigma Xi Grants‐in‐Aid and NIH grant HL089457 (to MH).

2169 Platelets play a pivotal role in thrombotic events leading to clot formation and clot stability in vivo. Uncontrolled signaling events in the platelet can result in unwanted thrombosis, which may eventually lead to the development of myocardial infarction or stroke. Previously, we have shown that ex vivo treatment of human platelets with the ω-6 fatty acid dihomo-γ-linolenic acid (DGLA) or its eicosanoid derived from 12-LOX oxidation, 12-hydroxyeicosatrienoic acid (12(S)-HETrE), inhibits PAR and collagen-induced platelet aggregation, clot retraction, and GPIIbIIIa activation. Since early studies have shown dietary supplementation of fatty acids increase fatty acid incorporation into the platelet lipid membrane (Barre, DE Lipids 1992; 27(5): 315–320; Marry, MJ Prostaglandins Leukot Essent Fatty Acids 1997; 56(3):223–223), we postulated that altering fatty acid composition in the platelet through dietary supplementation in vivo may be a viable approach to inhibiting platelet function. Therefore, a longitudinal study of wild-type mice on normal chow compared to mice supplemented with high (.5 g/kg) or low (.13 g/kg) DGLA diets was conducted. Each set of mice (7–8 mice) was given the designated diet for a period of 1, 2, or 3 months. At each time point, tail bleeding times and ex vivo platelet function in PRP were performed. Tail bleeding times from mice on the high DGLA diet were significantly prolonged by more than 15 minutes. Further, a smaller but statistically significant delay in clotting time was observed in mice on the low DGLA diet compared with control mice. Additionally, ex vivo aggregation response to collagen (1 μg/mL to 20 μg/mL) and PAR4-AP (50 μM to 500 μM) in platelets from mice on the high DGLA diet showed significant shifts to the right in their ability to induce platelet aggregation compared with control mice suggesting these mice were protected against thrombosis. JON/A and P-selectin binding to the PRP of high and low DGLA were also significantly attenuated in response to PAR4-AP. This study, which evaluated the in vivo and ex vivo effects of DGLA on regulation of platelet reactivity, supports DGLA as a potent, endogenous anti-thrombotic agent. Understanding the mechanistic details by which DGLA protects against thrombosis and maintains hemostasis through its COX-1 and 12-LOX-dependent bioactive metabolites will help to identify the potential viability of this target for anti-platelet intervention. Disclosures: No relevant conflicts of interest to declare.