Platelet factor 4 enhances generation of activated protein C in vitro and in vivo

• Published in: Blood 2003-07, Vol.102 (1), p.146-151
• Main Authors: Slungaard, Arne, Fernandez, Jose A., Griffin, John H., Key, Nigel S., Long, Janel R., Piegors, Donald J., Lentz, Steven R.
• Format: Article
• Language: English
• Subjects: Animals; Biological and medical sciences; Blood Coagulation - drug effects; Blood coagulation. Blood cells; Dose-Response Relationship, Drug; Endothelium, Vascular - cytology; Endothelium, Vascular - drug effects; Endothelium, Vascular - metabolism; Fundamental and applied biological sciences. Psychology; General aspects, investigation methods, hemostasis, fibrinolysis; Humans; Macaca fascicularis; Microcirculation - cytology; Models, Animal; Molecular and cellular biology; Partial Thromboplastin Time; Platelet Factor 4 - pharmacology; Platelet Factor 4 - physiology; Protein C - biosynthesis; Thrombin - administration & dosage; Thrombomodulin
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood-2002-11-3529

Platelet factor 4 (PF4), an abundant platelet α-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 μg 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 μg/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 control’s from 10 through 120 minutes (P ≤ .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. (Blood. 2003;102:146-151)