Modulation of human endothelial cell procoagulant activity in tumour models in vitro

Several tumour-derived factors have recently been identified which induce tissue factor (TF) expression in endothelial cells in vitro. However, there is only limited evidence that endothelial cells lining tumour blood vessels express elevated procoagulant activity (PCA) in vivo. We have investigated the effects of human breast and small cell lung cancer cell lines on the PCA of human micro- and macrovessel endothelial cell monolayers using a one-stage clotting assay, as well as detection of TF mRNA by RT-PCR. Only conditioned medium from the MDA-MB-231 breast adenocarcinoma cell line produced a consistent although transient increase in endothelial cell surface PCA, which was maximal by 6-9 hr. TF mRNA was detectable in the endothelial cells after 1 hr incubation with MDA-MB-231-conditioned medium and subsequently fell below detectable levels. Following 24 hr stimulation, nearly half the endothelial cell PCA was due to the presence of TF-containing membrane vesicles shed by the MDA-MB-231 cells. Consistent with these findings, the MDA-MB-231 cell line expressed high levels of cell surface-associated TF activity. Co-culture of MDA-MB-231 and endothelial cells for up to 5 days increased (approx. 118-fold) PCA associated with endothelial cell monolayers, due mainly to sequestration of shed tumour cell vesicles. Our results suggest that induction of TF de novo is not a common feature in the supporting endothelium of these tumour types.