Interplay between Tumor Cell-Associated and Circulating Coagulation Factors in Establishing Metastatic Potential.

Tumor cell-associated tissue factor (TF) is an established determinant of metastatic potential. In order to explore the interplay between tumor cell-associated TF and circulating hemostatic system components in malignancy, we established a line of hRas-transformed tumor cells from C57Bl/6-derived TF^−/− embryonic fibroblasts. These cells were subsequently transfected with expression vectors encoding wildtype murine TF (TF^WT), a mutant form of murine TF lacking the cytoplasmic tail (TF^ΔTail), or no transgene product (TF^O). Stable transfectants expressing TF^WT and TF^ΔTail were selected by fluorescence-activated cell sorting using ICON protein (a chimeric derivative of murine fVIIa linked to the Fc portion of human IgG). All three tumor cell lines supported similar and robust tumor growth following subcutaneous transplantation into immunocompetent C57Bl/6-inbred mice. However, consistent with previous reports, TF^WT cells were dramatically more metastatic than TF^O cells. Loss of the cytoplasmic tail significantly reduced metastases relative to TF^WT cells, but TF^ΔTail cells were far more metastatic than TF^O cells, indicating that the TF cytoplasmic tail is important, but not strictly-required to support metastasis. Imposing genetic deficits in either platelet function (i.e., Gαq deficiency) or coagulation function (i.e., fibrinogen deficiency) significantly diminished the metastatic potential of TF-expressing tumor cells. The low metastatic potential of TF^ΔTail cells was further diminished in mice with impaired platelet activation, indicating that loss of the cytoplasmic tail does not uncouple metastatic potential from circulating hemostatic system components. Studies of the early fate/survival of [¹²⁵I]-iododeoxyuridine radiolabeled TF^WT and TF^O cells revealed that neither fibrinogen nor tumor-associated tissue factor were required for initial tumor cell adhesion within the pulmonary vasculature. However, the combination of tumor cell TF and circulating fibrinogen was shown to enhance the longer-term survival of tumor cells within the lungs. Based on the earlier finding that both platelets and fibrinogen support metastasis in part by impeding natural killer cell mediated clearance of embolic tumor cells, a favored working hypothesis is that circulating hemostatic factors couple tumor cell-associated TF to evasion of innate anti-tumor immune surveillance mechanisms.