Major Reservoir for Heparin-Releasable TFPIα (Tissue Factor Pathway Inhibitor α) Is Extracellular Matrix

Objective: Human endothelial cells produce 2 alternatively spliced TFPI (tissue factor pathway inhibitor) isoforms that maintain anticoagulant properties of the vasculature. TFPIβ is glycosylphosphatidylinositol anchored on the cell surface. TFPIα has a basic C terminus sharing homology with VEGF (vascular endothelial growth factor) and is a heparin-releasable protein, suggesting it binds glycosaminoglycans on the endothelium surface. However, this is unclear because TFPIα is not on the surface of cultured endothelial cells. This study identifies the source of heparin-releasable TFPIα. Approach and Results: ELISA assays localized heparin-releasable TFPIα to the extracellular matrix (ECM) of Ea.hy926 cells and human umbilical vein endothelial cells. Immunofluorescence microscopy for TFPIα showed punctate intracytoplasmic staining and ECM staining beneath individual cells. Flow cytometry identified TFPIβ but not TFPIα on the cell surface. TFPIα localization to ECM was confirmed with ELISA and immunohistochemistry studies of umbilical cord veins. The TFPIα C terminus interacted with Ea.hy926 ECM glycosaminoglycans, and a homologous VEGF peptide competed for this binding, suggesting these interactions modulate VEGF responses. Immobilized TFPIα C-terminal peptide bound to several ECM proteoglycans in Ea.hy926 conditioned media. Immunofluorescence studies of human kidney colocalized TFPIα with 4 of these proteoglycans surrounding the microvasculature: glypican-1, syndecan-4, thrombospondin, and laminin-5. The absence of TFPIα on the surface of endothelial cells and its co-localization with specific ECM proteins suggests TFPIα binds to unique proteoglycan structures. Conclusions: ECM contained the primary vascular pool of heparin-releasable TFPIα. By localizing to ECM, TFPIα is positioned to inhibit the procoagulant activity of tissue factor surrounding the vasculature.