doi: 10.1161/01.ATV.0000201968.72967.f9
© 2006 American Heart Association, Inc.
Letters to the Editor |
Physiological Plasma Gas6 Levels Do Not Influence Platelet Aggregation
Institut National de la Santé et de la Recherche, Médicale (INSERM), Faculté des Sciences Pharmaceutiques et Biologique, Université Paris V, Paris, France
Division of Angiology and Haemostasis, Department of Internal Medicine, Faculty of Medicine and University Hospitals of Geneva, Geneva, Switzerland
INSERM, Faculté des Sciences Pharmaceutiques et Biologiques, Université Paris V, Paris, France
To the Editor:
Gas6 protein is a vitamin K–dependent protein originally identified as an apoptosis regulator.1 Gas6 binds to Axl, Mer, and Sky, 3 receptors of the tyrosine kinase subfamily, and was recently implicated in platelet aggregation.2 Gas6 knockout mice showed impaired aggregation in response to weak activation with agonists such as 5 µmol/L ADP, 10 µmol/L U46619, and 2 µg/mL collagen. Similarly, loss of any Gas6 receptor impaired the stabilization of platelet aggregates,3 and both Gas6 and Gas6 receptor knockout mice were protected from venous and arterial thrombosis. Gas6 was found to be located in 
-granules of murine platelets and to be released during platelet activation.2,4 Data on human platelets are less clear cut. Indeed, preincubation of human platelet-rich plasma (PRP) with an anti-Gas6 antibody inhibited platelet aggregation and secretion responses to both 5 µmol/L ADP and the protease-activated receptor-1 (PAR-1)–activating peptide SFLLRN (20 µmol/L) by >80%.5 Using several approaches, Balogh et al found that Gas6 was contained in human blood plasma but not in human platelets, suggesting that in humans, the putative role of Gas6 in platelet aggregation may be played by Gas6 originating from the circulation.6
Here, we examined whether human platelet aggregation was influenced by physiological Gas6 plasma levels. Samples from 100 healthy male volunteers 18 to 35 years of age (mean 24.3±3.7 years) were used for platelet aggregation studies and plasma Gas6 assay. Gas6 levels were determined by ELISA as described previously7 and were expressed as a percentage of the Gas6 level in a pool of normal plasma. PRP aggregation studies were performed as described previously8 with the following agonists: ADP, collagen, arachidonic acid (AA), U46619, and SFLLRN peptide. The results were expressed as the maximum percentage increase in light transmission over that of the initial platelet suspension, relative to that of autologous platelet-poor plasma (arbitrarily 100%). The collagen lag time was also collected. Each subject was studied twice, at a 1-week interval, and the platelet aggregation results were averaged as described previously.8 There was a good concordance between plasma Gas6 levels in each subject’s paired samples (r=94%; P<0.001), and the mean values between the 2 visits were therefore used. Plasma Gas6 levels ranged from 50.5% to 197.5% (median 81%).
Because the previously reported effects of Gas6 were observed only in conditions of weak activation, we performed platelet aggregation tests with similar or even lower agonist doses.2,5 The Spearman correlation test showed no relationship between the plasma Gas6 level and the platelet response to any of the agonists tested. Plasma Gas6 levels did not correlate with maximal aggregation to 2 µmol/L ADP, 1 µg/mL collagen, 1 mmol/L AA, 1 µmol/L U46619, 10 or 15 µmol/L SFLLRN peptide 7, or the collagen lag time.
These results show that plasma Gas6 levels in healthy subjects do not influence platelet aggregation ex vivo and suggest that they do not reflect platelet susceptibility to activation. The mechanism by which the Gas6 pathway modulates platelet aggregation remains elusive.
Acknowledgments
S.C. received a grant from Association Claude Bernard.
References
1. Manfioletti G, Brancolini C, Avanzi G, Schneider C. The protein encoded by a growth arrest-specific gene (gas6) is a new member of the vitamin K-dependent proteins related to protein S, a negative coregulator in the blood coagulation cascade. Mol Cell Biol. 1993; 13: 4976–4985.
2. Angelillo-Scherrer A, de Frutos P, Aparicio C, Melis E, Savi P, Lupu F, Arnout J, Dewerchin M, Hoylaerts M, Herbert J, Collen D, Dahlback B, Carmeliet P. Deficiency or inhibition of Gas6 causes platelet dysfunction and protects mice against thrombosis. Nat Med. 2001; 7: 215–221.[CrossRef][Medline] [Order article via Infotrieve]
3. Angelillo-Scherrer A, Brunier L, Flores N, Savi P, DeMol M, Schaeffer P, Herbert J, Lemke G, Goff SP, Matsushima GK, Earp HS, Vesin C, Hoylaerts M, Plaisance S, Collen D, Conway EM, Arnout J, Wehrle-Haller B, Carmeliet P. Role of Gas6 receptors in platelet signaling during thrombus stabilization and implications for antithrombotic therapy. J Clin Invest. 2005; 115: 237–246.[CrossRef][Medline] [Order article via Infotrieve]
4. Ishimoto Y, Nakano T. Release of a product of growth arrest-specific gene 6 from rat platelets. FEBS Lett. 2000; 466: 197–199.[CrossRef][Medline] [Order article via Infotrieve]
5. Gould WR, Baxi SM, Schroeder R, Peng YW, Leadley RJ, Peterson JT, Perrin LA. Gas6 receptors Axl, Sky and Mer enhance platelet activation and regulate thrombotic responses. J Thromb Haemost. 2005; 3: 733–741.[CrossRef][Medline] [Order article via Infotrieve]
6. Balogh I, Hafizi S, Stenhoff J, Hansson K, Dahlbach B. Analysis of Gas6 in human platelets and plasma. Arterioscler Thromb Vasc Biol. 2005; 25: 1280–1286.
7. Borgel D, Clauser S, Bornstain C, Bièche I, Bissery A, Remones V, Fagon JY, Martine Aiach M, Diehl JL. Elevated Gas6 plasma levels in patients with severe sepsis. Crit Care Med. 2006; 34: 219–222.[CrossRef][Medline] [Order article via Infotrieve]
8. Dupont A, Fontana P, Bachelot-Loza C, Reny JL, Bieche I, Desvard F, Aiach M, Gaussem P. An intronic polymorphism in the PAR-1 gene is associated with platelet receptor density and the response to SFLLRN. Blood. 2003; 101: 1833–1840.
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