Dear Sir:

We haveread with great interest the paper by Ozata et al. (1).Their results are in contrast with recent evidence suggesting thatleptin may promote human platelet aggregation (2). Weperformed this study to investigate the role of leptin in theregulation of platelet aggregation in humans.

Our study consisted of 14 healthy men (age 25 to 35 years; bodymass index < 25 kg/m²) who gave informedconsent. All subjects underwent a complete clinical and laboratoryassessment to exclude major health problems. To participate in thestudy, all subjects were requested not to take drugs for 4 weeks beforesampling. After fasting overnight, a venous blood sample was obtained,without stasis, from the cubital vein of the arm of all subjects. Forthe platelet study, blood samples (25 mL) were collected in a conicalpolypropylene tube containing 3.33 mL/liter of ACD (consistingof 64 mM citric acid, 85 mM sodium citrate, and 111 mM dextrose).Platelet-rich plasma (PRP) was obtained by centrifugation at800g for 20 minutes at room temperature. The PRP wascentrifugated at 2000g to form a soft platelet pellet,resuspended in HEPES Tyrode buffer (HBS) at pH = 7.4, and washedonce. Platelets were counted and resuspended in HBS buffer at aconcentration of 0.5 to 1 10⁸platelets/mL. Platelet suspension (1 mL) was stimulated by the additionof leptin (5 to 100 ng/mL; Linco Research Inc., St. Charles, MO) at 37°C for 5 minutes. Furthermore, to investigate whether the effects ofleptin on platelet aggregation were mediated by the leptin receptor,platelets were preincubated with leptin alone (5 to 100 ng/mL) orleptin (100 ng/mL) and rabbit anti-human leptin receptor long form(Ob-Rb) antibody (5 to 10 L; Linco Research Inc.). Platelets wereaggregated in PRP with 2 M adenosine diphosphate (ADP).Platelet-poor plasma (PPP) was obtained by centrifugation at2000g for 15 minutes. PPP was used as a reference toindicate 100% in aggregation experiments. Aggregation was opticallymonitored with a Platelet Aggregation Chromogenic Kinetic System(PACKS4; Helena Laboratories, Beaumont, TE). Platelet aggregation wasexpressed as a percentage of maximal aggregation. Statistical analysiswas performed using ANOVA one-way test with Scheffè post hoctest. Data were expressed as mean SEM.

Neither leptin alone (100 ng/mL; percent aggregation, 15.9 2.9) nor ADP (2 M; percent aggregation, 15.4 3.1)affected platelet aggregation. Platelet aggregation was significantlyenhanced by the preincubation with leptin (leptin 5 ng/mL, 40.2 3.7%, p < 0.05; leptin 10 ng/mL, 51.3 3.8%,p < 0.01; leptin 50 ng/mL, 55.9 4.7%, p> 0.01; leptin 100 ng/mL, 69.2 4.1, p < 0.001).This effect was dose-dependent, and it was completely abated bypreincubation with leptin and anti-ObRb antibody (5–10L).

In agreement with the findings reported by Nakata et al.(2), we found that leptin is able to potentiateADP-induced platelet aggregation. Furthermore, the anti-ObRb antibodycompletely abated the effect of leptin on the platelet aggregationresponse to ADP, thus suggesting that the enhancement of plateletresponse to the agonist induced by leptin is mediated by the longform of its receptor on platelet membrane. This problem has beenstudied by Nakata et al. (2), and they found that leptinwas able to induce a significant dose- and time-dependentincrease in tyrosine phosphorylation in human platelet. In light ofthese findings, they suggested that platelet Ob-Rb is a member of theclass I cytokine receptor family that is linked to the Januskinase-signal transducer and activator of transcriptionsignaling pathway (2).

We agree with Ozata et al. (1) that the reason forthe conflicting results is unclear, and we think that much more work isneeded to clarify the role of leptin in platelet aggregation and toidentify the mediators of this effect. We also agree that factors otherthan leptin (i.e., insulin and catecholamines) are establishedmodulators of platelet function. However, we think that studies in thisfield are still in their stages of infancy, and the putative roleof leptin in the regulation of platelet aggregation need to beinvestigated further before claiming that "leptin does not play amajor role."

References

1. Ozata, M., Avcu, F., Durmus, O., Yilmaz, I., Ozdemir, I. C., Yalcin, A. (2001) Leptin does not play a major role in platelet aggregation in obesity and leptin deficiency. Obes Res. 9: 627–630.
2. Nakata, M., Yada, T., Soejima, N., Maruyama, I. (1999) Leptin promotes platelet aggregation of human platelets via the long form of its receptor. Diabetes. 48: 426–429. | Article | PubMed | ISI | ChemPort |