Heme induces endothelial tissue factor expression: potential role in hemostatic activation in patients with hemolytic anemia.

OBJECTIVES We explored the possibility that heme, an inflammatory mediator and a product of intravascular hemolysis in patients with hemolytic anemia including sickle cell disease, could modulate hemostasis by an effect on endothelial tissue factor (TF) expression. METHODS Levels of TF mRNA, protein and procoagulant activity were measured in heme-treated endothelial cells. RESULTS Heme induces TF expression on the surface of both macrovascular and microvascular endothelial cells in a concentration-dependent manner, with 12-fold to 50-fold induction being noted (enzyme-linked immunosorbent assay) between 1 and 100 microm heme (P < 0.05). Complementary flow cytometry studies showed that the heme-mediated endothelial TF expression was quantitatively similar to that of tumor necrosis factor-alpha (TNF-alpha). Heme also upregulated the expression of TF mRNA (8-fold to 26-fold), protein (20-fold to 39-fold) and procoagulant activity (5-fold to 13-fold) in endothelial cells in a time-dependent manner. The time-course of heme-mediated TF antigen expression paralleled the induction of procoagulant activity, with antibody blocking studies demonstrating specificity for TF protein. Interleukin (IL)-1alpha, and TNF-alpha are not involved in mediating the heme effect, as antibodies against these cytokines and IL-1-receptor antagonist failed to block heme-induced TF expression. Inhibition of heme-induced TF mRNA expression by sulfasalazine and curcumin suggested that the transcription factor nuclear factor kappaB is involved in mediating heme-induced TF expression in endothelial cells. CONCLUSIONS Our results demonstrate that heme induces TF expression by directly activating endothelial cells, and that heme-induced endothelial TF expression may provide a pathophysiologic link between the intravascular hemolytic milieu and the hemostatic perturbations previously noted in patients with hemolytic anemia including sickle cell disease.