Summary
The role of tissue factor (TF)-containing microparticles in clot propagation has been
established, but the ability of circulating microparticles to initiate coagulation
has been disputed. However, TF-bearing microparticles, particularly endothelial-microparticles
generated during disease, may interact with extracellular matrices which in turn can
localise circulating TF to sites of injury. In order to examine this hypothesis in vitro, microparticles were isolated from human coronary artery endothelial cells transfected
to overexpress TF, tumour-necrosis factor (TNF)α-treated cells or non-transfected
cells lacking TF. The ability of microparticles to bind collagen-IV, fibronectin and
fibrin was examined under static conditions and arterial shear rates (650 s-1), and also in the presence of inhibitory antibodies against β1-, β3-, α3-and αv-integrins
or an anti-TF antibody. TF-microparticles showed increases of up to 43% and 24% in
adherence to collagen-IV and fibronectin, respectively, compared to control microparticles
under shear flow. Furthermore, TF-containing microparticles, but not the transfected
parent cells had increased levels of β1-integrin compared to TF-deficient microparticles.
Pre-incubation of microparticles with a β1-integrin-blocking antibody counteracted
the additional adhesion of TFmicroparticles compared to control microparticles. Finally,
adherence of TF microparticles to collagen-IV or fibronectin resulted in increased
TF activity by concentrating TF onto the surface. In conclusion, the presence of TF
within microparticles enhances the interactions of endothelial cell-derived microparticles
with extracellular matrices in an integrin-dependent manner. Accumulation and localisation
of these microparticles in turn results in the enhancement of TF activity. This may
be an innate mechanism by which TF-bearing microparticles induce coagulation upon
vascular injury.
Keywords
Tissue factor - endothelial-derived microparticles - extracellular matrices - integrins
- procoagulant activity
