Hemoglobin enhances tissue factor expression on human malignant cells

Tissue Factor (TF) is a transmembrane glycoprotein that complexes with fact or VII/activated factor VII to initiate blood coagulation. TF may be expres sed on the surface of various cells including monocytes and endothelial cel ls. Over-expression of TF in human tumor cell lines promotes metastasis. We recently showed that hemoglobin (Hb) forms a specific complex with TF puri fied from human malignant melanoma cells and enhances its procoagulant acti vity (DCA). To further study this interaction, we examined the effect of Hb on the expression of TF on human malignant (TF+) cells and KG1 myeloid leu kemia (TF-) cells. Human melanoma A375 and J82 bladder carcinoma cells, whi ch express TF at moderate and relatively high levels, respectively, were in cubated with varying concentrations (0-1.5 mg/ml) of Hb. After washing, cel ls were analyzed for Hb binding and TF expression using flow cytometry and confocal microscopy. Hb bound to the cells in a concentration-dependent man ner, and increased both TF expression and PCA. The human A375 malignant mel anoma cells incubated with Hb (1 mg/ml) expressed up to six times more TF a ntigen than cells without Hb. This increase in TF expression and PCA of int act cells incubated with Hb was significantly inhibited by cycloheximide at a concentration of 10 mug/ml (P < 0.01). An increase in total cellular TF antigen content was demonstrated by specific immunoassay. In contrast, Hb ( 5 mg/ml) did not induce TF expression and PCA on KG1 cells as determined by flow cytometry and TF (FXAA) activity. We conclude that Hb specifically bi nds to TF-bearing malignant cells and increases their PCA. This effect seem s to be at least partly due to de novo synthesis of TF and increased surfac e expression. However, the exact mechanism by which Hb binds and upregulate s TF expression remains to be determined. Blood Coagul Fibrinolysis 12:171- 177 (C) 2001 Lippincott Williams & Wilkins.