Polyphosphate elicits pro-inflammatory responses that are counteracted by activated protein C in both cellular and animal models

Background: Recent results have indicated that polyphosphate, released by activated platelets, can function as a procoagulant to modulate the proteolytic activity of serine proteases of the blood clotting cascade. Objective: To determine whether polyphosphate is involved in inducing signal transduction in cellular and animal models. Methods: The effect of polyphosphate on human umbilical vein endothelial cells was examined by monitoring cell permeability, apoptosis and activation of NF-κB after treating cells with different concentrations of polyphosphate. Moreover, the expression of cell surface adhesion molecules (VCAM-1, ICAM-1 and E-selectin) and the adhesion of THP-1 cells to polyphosphate-treated cells were monitored using established methods. In the in vivo model, the pro-inflammatory effect of polyphosphate was assessed by monitoring vascular permeability and migration of leukocytes to the peritoneal cavity of mice injected with polyphosphate. Results: Polyphosphate, comprised of 45, 65 and 70 phosphate units, enhanced the barrier permeability and apoptosis in cultured endothelial cells and up-regulated the expression of cell adhesion molecules, thereby mediating the adhesion of THP-1 cells to polyphosphate-treated endothelial cells. These effects of polyphosphate were mediated through the activation of NF-κB and could not be recapitulated by another anionic polymer, heparin. Polyphosphate also increased the extravasation of the bovine serum albumin (BSA)-bound Evans blue dye and the migration of leukocytes to the mouse peritoneal cavity, which was prevented when activated protein C (APC) was intravenously (i.v.) injected 2h before the challenge. Conclusion: Polyphosphate, in addition to up-regulation of coagulation, can elicit potent pro-inflammatory responses through the activation of NF-κB, possibly contributing to the pro-inflammatory effect of activated platelets.