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

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 mug 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 mug/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 less than or equal to .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. (C) 2003 by The American Society of Hematology.