K+ CHANNEL BLOCKERS INHIBIT TISSUE FACTOR EXPRESSION BY HUMAN MONOCYTIC CELLS

Human monocytes express the important procoagulant protein, tissue factor (TF), after stimulation by a variety of agents, including bacterial lipopolysaccharide (LPS). Monocyte TF expression may contribute to intravascular coagulation in a number of disease states. The present studies show that monocytic cell TF expression can be inhibited by several agents known to block cellular K+ channels. Exposure of human peripheral blood to 100 ng/mL LPS for 2 hours led to pronounced TF procoagulant activity associated with the mononuclear cell fraction. This was inhibited by 4-aminopyridine (2 mmol/L), tetraethylammonium chloride (10 mmol/L), and apamin (1 mu mol/L). In contrast, charybdotoxin (100 nmol/L) was inactive. More detailed studies were carried out in cultured human monocytic tumor THP-1 cells. These cells exhibited low but detectable levels of TF mRNA, measured by reverse transcription and polymerase chain reaction; cell surface procoagulant activity, measured by a plasma clotting assay; and cell homogenate TF antigen, measured by immunoassay. Exposure of THP-1 cells to 1 mu g/mL LPS led to threefold to fivefold increases in all three parameters. Basal and LPS-induced levels of all three parameters were reduced in a dose-dependent manner by 4-aminopyridine (I-50, 1 mmol/L) and tetraethylammonium chloride (I-50, 20 mmol/L) but not by apamin or charybdotoxin. Expression of TF activity was also inhibited by glibenclamide, an inhibitor of ATP-dependent K+ channels (I-50, 25 mu mol/L). These results suggest that facilitation of TF synthesis may be an important role for K+ channels in monocytes.