Summary
Tissue factor (TF) is a transmembrane glycoprotein that complexes with factor VIIa
to initiate blood coagulation. It was reported in an earlier study that expression
of high levels of TF in a human melanoma cell line promotes metastasis, and that the
cytoplasmic domain of TF is required for this metastatic effect. To analyze the functions
of the cytoplasmic and extracellular domains of TF in metastasis, two TF mutants were
constructed; in one mutant alanine was substituted for each of the three serine residues
in the cytoplasmic domain, preventing phosphorylation; in the other mutant alanine
was substituted for four key residues in the extracellular domain, preventing binding
of factor VIIa and consequently eliminating the initiation of blood coagulation by
the TF-VIIa complex. Melanoma lines expressing high levels of either mutant form of
TF were weakly metastatic in SCID mice, indicating that phosphorylation of the cytoplasmic
domain and formation of a complex with VIIa by the extracellular domain are required
for the full metastatic effect of TF. It was also found that increasing TF expression
in human melanoma cells does not increase expression of vascular endothelial growth
factor or promote growth and vascularization of tumors derived from the melanoma cells,
suggesting that TF acts by a mechanism other than angiogenesis to promote metastasis.
