title: Platelet factor 4 enhances generation of activated protein C in vitro and in vivo.
abstract: Platelet factor 4 (PF4), an abundant platelet alpha-granule protein, accelerates in vitro generation of activated protein C (APC) by soluble thrombin/thrombomodulin (TM) complexes up to 25-fold. To test the hypothesis that PF4 similarly stimulates endothelium-associated TM, we assessed the influence of human PF4 on thrombin-dependent APC generation by cultured endothelial monolayers. APC generated in the presence of 1 to 100 microg PF4 was up to 5-fold higher than baseline for human umbilical vein endothelial cells, 10-fold higher for microvascular endothelial cells, and unaltered for blood outgrowth endothelial cells. In an in vivo model, cynomolgus monkeys (n = 6, each serving as its own control) were infused with either PF4 (7.5 mg/kg) or vehicle buffer, then with human thrombin (1.0 microg/kg/min) for 10 minutes. Circulating APC levels (baseline 3 ng/mL) peaked at 10 minutes, when PF4-treated and vehicle-treated animals had APC levels of 67 +/- 5 ng/mL and 39 +/- 2 ng/mL, respectively (P <.001). The activated partial thromboplastin time (APTT; baseline, 28 seconds) increased maximally by 27 +/- 6 seconds in PF4-treated animals and by 9 +/- 1 seconds in control animals at 30 minutes (P <.001). PF4-dependent increases in circulating APC and APTT persisted more than 2-fold greater than that of controls from 10 through 120 minutes (P < or =.04). All APTT prolongations were essentially reversed by monoclonal antibody C3, which blocks APC activity. Thus, physiologically relevant concentrations of PF4 stimulate thrombin-dependent APC generation both in vitro by cultured endothelial cells and in vivo in a primate thrombin infusion model. These findings suggest that PF4 may play a previously unsuspected physiologic role in enhancing APC generation.
authors.affiliation: Department of Medicine, Section of Hematology, Oncology and Transplantation, University of Minnesota, Mayo Mail Code 480, 420 Delaware St SE, Minneapolis, MN 55455, USA. slung001@tc.umn.edu
authors.forename: Arne
authors.initials: A
authors.lastname: Slungaard
authors.forename: Jose A
authors.initials: JA
authors.lastname: Fernandez
authors.forename: John H
authors.initials: JH
authors.lastname: Griffin
authors.forename: Nigel S
authors.initials: NS
authors.lastname: Key
authors.forename: Janel R
authors.initials: JR
authors.lastname: Long
authors.forename: Donald J
authors.initials: DJ
authors.lastname: Piegors
authors.forename: Steven R
authors.initials: SR
authors.lastname: Lentz
mesh_terms: D000818:Animals; D001777:Blood Coagulation / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D004730:Endothelium, Vascular / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D006801:Humans; D008252:Macaca fascicularis; D008833:Microcirculation / Q000166:cytology; D023421:Models, Animal; D010314:Partial Thromboplastin Time; D010978:Platelet Factor 4 / Q000494:pharmacology* / Q000502:physiology; D011486:Protein C / Q000096:biosynthesis*; D013917:Thrombin / Q000008:administration & dosage; D018180:Thrombomodulin
chemical_list: D011486:Protein C; D018180:Thrombomodulin; D010978:Platelet Factor 4; D013917:Thrombin