Treatment of human umbilical vein endothelial cells with
flavone acetic acid (FAA) at 800 micrograms/ml for 4 h resulted in a 3-11-fold increase in procoagulant activity. This increase was due to enhanced
tissue factor expression on the endothelial cell surface, as evidenced by the blocking of the enhanced clotting with antibody to
tissue factor, by substitution of normal with
factor VII deficient plasma, or by simultaneous treatment of the endothelial cells with
cycloheximide or
actinomycin D. FAA was not toxic to endothelial cell at concentrations up to 1.6 mg/ml over 4 h. Combined treatment with FAA and tumour
necrosis factor alpha (
TNF-alpha) (100 pg/ml) produced a 675-fold (range 160-1980) increase in
tissue factor activity, compared to 5-fold and 50-fold increases for the individual agents respectively. Northern blotting of total
RNA from cells treated with the combination of agents or either agent alone, followed by probing with a
cDNA to human
tissue factor demonstrated a synergistic increase in
tissue factor mRNA after combination treatment. In vivo, the combination of FAA and
TNF-alpha could be shown to induce greater growth delay in two murine tumours than would be predicted on the basis of the activity of either agent alone.