Abstract 988: Leptin Induces Activation of Human Platelets: Insights Into the Intracellular Signaling Mechanisms Underlying the Prothrombotic Effects of the Adipokine
Leptin promotes platelet aggregation and thrombosis in vivo, and elevated plasma leptin levels have been associated with an increased cardiovascular risk. We sought to dissect the mechanisms underlying the effects of leptin on platelet function. As determined by fluorescence microscopy, stimulation of resting human platelets with leptin induced cytoskeletal reorganization and a significant increase in platelet size (1.9-fold; P<0.01 vs. control). Moroever, leptin enhanced adherence of platelets to fibrinogen-coated glass slides (3.4-fold; P<0.01 vs. unstimulated platelets). After co-stimulation with ADP, leptin enhanced binding of fibrinogen to washed platelets as determined by flow cytometry (1.3-fold; P<0.01; n=10 experiments), promoted platelet aggregation (1.6-fold; P<0.0001; n=37), and induced thromboxane generation (1.4-fold; P=0.019; n=11). RT-PCR and Western blotting experiments revealed that platelets express both the short (95 kDa) and the long isoform (200 kDa) of the leptin receptor. The long isoform was phosphorylated (Tyr1141) in response to leptin. In further experiments, leptin time- and dose-dependently activated the signaling molecules JAK2 and STAT3 with a maximal increase in phosphorylation (2.3- and 1.7-fold, respectively) after stimulation with 10 ng/mL for 2 min (P<0.01). Similarly, leptin induced phosphorylation of phosphatidyl-inositol-3 kinase (PI3K), AKT (1.4-fold; P=0.016) and the MAP kinases p44/42 (2.5-fold; P=0.006) and p38 (3.2-fold; P=0.04). Importantly, the effect of leptin on protein phosphorylation as well as platelet aggregation was prevented by specific inhibitors of JAK2 (AG490), PI3K (Ly294002) and p38 (SB203580), underlining the role of these pathways in leptin-mediated platelet activation. Thus, our results indicate that the adipokine leptin induces platelet activation. Moreover, they suggest that the effects of leptin on platelet aggregation are mediated by the long isoform of the leptin receptor and involve activation of JAK2, and of the PI3K and p38 MAP kinase pathways. These findings support the link between hyperleptinemia and the increased thrombosis risk in human obesity.