Plasma levels of heme in the 20 to 600 μM range are found in clinical conditions associated with intravascular hemolysis including paroxysmal nocturnal hemoglobinuria and sickle cell disease, conditions also associated with a thrombotic tendency.

Objectives: To investigate whether heme, an inflammatory mediator and a product of intravascular hemolysis in patients with hemolytic anemia including sickle cell disease (SCD), could modulate hemostasis by an effect on endothelial tissue factor (TF) expression. Additionally, in SCD patient-related studies, we assessed whether any association existed between whole blood TF activity (WBTF) and levels of surrogate markers of intra-vascular hemolysis including lactate dehydrogenase (LDH) and reticulocyte counts.

Methods: Following incubation of human endothelial cells (from umbilical vein and/or lung microvasculature) with heme (1 to 100 μM) for various times (30 minutes to 8 hours), levels of TF protein were assessed using ELISA, flow cytometry and/or Western blotting; and TF mRNA by a semi-quantitative RT-PCR. An assay for TF functional activity was performed using a chromogenic tenase activity kit where specificity of TF activity was tested in antibody-blocking experiments. Three TF-specific antibodies including a rabbit polyclonal and two mouse monoclonal (clones hTF-1 and TF9-10H10) antibodies were used in assays involving TF protein analysis. All experiments were performed in media containing polymyxin B to neutralize any potential endotoxin contamination. In patient-related studies, 81 subjects with SCD (1 to 21 years) were evaluated for levels of WBTF, LDH, and reticulocyte counts and data analyzed for potential relationships.

Results: Heme induced TF protein expression on the surface of both macro- and micro-vascular endothelial cells in a concentration-dependent manner with 12- to 50-fold induction noted (ELISA assays) between 1 and 100 μM heme (P<0.05, n=3 to 6). Complementary flow cytometry studies showed that the heme-mediated endothelial TF expression was quantitatively similar to that induced by the cytokine TNF-α. Heme also up-regulated endothelial expression of TF mRNA (8- to 26-fold, peak expression at 2 hours postagonist treatment), protein (20- to 39-fold, peak expression at 4 hours) and procoagulant activity (5- to 13-fold, peak activity at 4 hours post-agonist treatment) in a time-dependent manner. Time-course of heme-mediated TF antigen expression paralleled induction of procoagulant activity with antibody blocking studies demonstrating specificity for TF protein. Potential involvement of endogenously released cytokines including IL-1α and TNF-α in mediating the heme effect was next explored. We found that the latter cytokines are not involved, since antibodies against IL-1α and TNF-α, and an IL-1- receptor antagonist failed to block heme-induced endothelial TF expression. Inhibition of heme-induced TF mRNA expression by sulfasalazine and curcumin suggested that the transcription factor NFκB was involved in mediating heme-induced effect. In patient-related studies, whole blood TF levels in SCD correlated positively with both LDH (r=0.72, p<0.000001), and reticulocyte count (r=0.60, p<0.000001).

Conclusions: Our findings demonstrate that heme induces TF expression in endothelial cells, and that the observed effects occurred at patho-physiologically relevant heme concentrations. Our results suggest 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.

Disclosures: No relevant conflicts of interest to declare.

Supported by grants U54 HL70585 from the National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, MD.

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