Summary
Tissue factor pathway inhibitor (TFPI) abrogates coagulation initiated by the factor
VIIa/tissue factor catalytic complex. While the gene structure of TFPI suggests that
it is a secreted protein, a large pool of TFPI is associated with the vascular endothelium
through its affinity for a glycosylphosphatidylinositol (GPI)-linked membrane protein.
Inhibition of tissue factor by TFPI coincides with the translocation of quaternary
complexes containing tissue factor, factor VIIa, factor Xa, and TFPI to detergent-insoluble
plasma membrane domains rich in cholesterol, sphingomyelin, and GPI-linked proteins
known as lipid rafts and caveolae. It is not known if localization of TFPI to these
membrane domains is required for its inhibition of tissue factor procoagulant activity.
We generated chimeric TFPI molecules linked directly to the plasma membrane via a
GPI anchor or hydrophobic transmembrane domain and expressed these in HEK293 cells
that produce tissue factor but not endogenous TFPI. The GPI-anchored chimera was exclusively
enriched in detergent-insoluble membrane fractions while the transmembrane molecule
was not. Transfectants expressing equal levels of the GPI-linked or transmembrane
TFPI displayed equal anticoagulant potency as assessed by tissue factor-mediated conversion
of factor X to factor Xa. Disruption of lipid rafts with cyclodextrin likewise had
no effect on the inhibitory activity of the transmembrane or GPI-linked TFPI chimeras
in HEK293 cells, nor on endogenous TFPI expressed by ECV304 cells. Thus, we conclude
that the GPI anchor and membrane localization to lipid rafts does not enhance inhibition
of factor VIIa/ tissue factor by cell-surface associated TFPI.
Keywords
Tissue factor pathway inhibitor - glycosyl phosphatidylinositol - lipid rafts - coagulation
- endothelikum