title: P-selectin expression on platelets determines size and stability of platelet aggregates.
abstract: BACKGROUND
  P-selectin mediates rolling of platelets and leukocytes on activated endothelial cells. After platelet activation, P-selectin is translocated from intracellular granules to the external membrane, whereas fibrinogen aggregates platelets by bridging glycoprotein (GP) IIb/IIIa between adjacent platelets.
  
  
  RESULTS
  In this study, we define a novel role for P-selectin in platelet aggregation. Expression of P-selectin on the platelet surface correlated strongly with the mean platelet aggregate size. Inhibition of P-selectin binding to its ligand by either monoclonal anti-P-selectin antibodies directed against the lectin domain or soluble human P-selectin reversed platelet aggregation even when added up to 5 minutes after activation; however, fibrinogen binding to platelets was not affected. This deaggregating effect significantly reduced the maximal size and number of platelet aggregates. When added 1 minute after platelet activation, anti-P-selectin antibody achieved 95% to 100% of the deaggregating effect of EDTA, whereas the anti-GP IIb/IIIa antibody abciximab had no effect. Monoclonal antibodies against known P-selectin ligands, such as P-selectin GP ligand-1 (PSGL-1) or GP Ib, had no effect on platelet aggregation, suggesting a different ligand for P-selectin in platelet aggregate stabilization. In kinetic studies, P-selectin was maximally expressed 10 minutes after platelet activation, whereas maximal activation of GP IIb/IIIa occurred within the first 10 seconds, suggesting that P-selectin operates after fibrinogen binding to activated GP IIb/IIIa.
  
  
  CONCLUSIONS
  These results indicate that P-selectin interaction with a ligand, different from PSGL-1 or GP Ib, stabilizes initial GP IIb/IIIa-fibrinogen interactions, allowing the formation of large stable platelet aggregates.
authors.affiliation: Division of Cardiology, Department of Internal Medicine, University of Texas Houston Medical School 77030, USA. michael_merten@yahoo.com
authors.forename: M
authors.initials: M
authors.lastname: Merten
authors.forename: P
authors.initials: P
authors.lastname: Thiagarajan
mesh_terms: D000077284:Abciximab; D000244:Adenosine Diphosphate / Q000378:metabolism / Q000494:pharmacology; D000911:Antibodies, Monoclonal / Q000494:pharmacology; D001792:Blood Platelets / Q000166:cytology / Q000378:metabolism*; D004492:Edetic Acid / Q000494:pharmacology; D005455:Fluorescent Antibody Technique; D006801:Humans; D007140:Immunoglobulin Fab Fragments / Q000494:pharmacology; D008024:Ligands; D008562:Membrane Glycoproteins / Q000037:antagonists & inhibitors / Q000378:metabolism; D019007:P-Selectin / Q000096:biosynthesis* / Q000276:immunology / Q000494:pharmacology; D010446:Peptide Fragments / Q000494:pharmacology; D010974:Platelet Aggregation / Q000187:drug effects / Q000502:physiology*; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000037:antagonists & inhibitors / Q000378:metabolism; D010980:Platelet Membrane Glycoproteins / Q000037:antagonists & inhibitors / Q000378:metabolism; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000494:pharmacology
chemical_list: D000911:Antibodies, Monoclonal; D007140:Immunoglobulin Fab Fragments; D008024:Ligands; D008562:Membrane Glycoproteins; D019007:P-Selectin; C084882:P-selectin ligand protein; D010446:Peptide Fragments; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; D010980:Platelet Membrane Glycoproteins; D011994:Recombinant Proteins; C076901:thrombin receptor-activating peptide (P508-530); D000244:Adenosine Diphosphate; D004492:Edetic Acid; D000077284:Abciximab