PROSTACYCLIN ANALOGS INHIBIT TISSUE FACTOR EXPRESSION IN THE HUMAN MONOCYTIC CELL-LINE THP-1 VIA A CYCLIC AMP-DEPENDENT MECHANISM

Increased expression of tissue factor procoagulant by peripheral blood monocytes has been implicated in a number of thrombotic disorders. The present studies were undertaken to determine whether stable analogues of prostacyclin, a potent endothelium-derived platelet inhibitor and vasodilator, could inhibit tissue factor expression by human monocytic cells. Exposure of monocytic tumor THP-1 cells to 100 ng/ml endotoxin, 2 units/ml interleukin-1-beta, or 5 ng/ml tumor necrosis factor-alpha for 4 hours led to increased tissue factor procoagulant activity. Preincubation for 30 minutes with iloprost, ciprostene, and carbacyclin led to a dose-dependent inhibition of tissue factor expression induced by all three challenging agents. Iloprost was the most potent: 50% inhibition occurred at 5 nM, a concentration close to the reported dissociation constant for iloprost binding to the platelet prostacyclin receptor. An orally active analogue, cicaprost, was equally effective against endotoxin-induced tissue factor expression. Carbacyclin and ciprostene were 100 times less potent. Iloprost prevented the endotoxin-induced expression of tissue factor antigen on the surface of THP-1 cells, as determined by flow cytometry. Iloprost (500 pM-50 nM) increased intracellular levels of cyclic AMP. This effect was potentiated by isobutylmethylxanthine, an inhibitor of phosphodiesterase. The inhibitory effects of iloprost on tissue factor expression were also potentiated by isobutylmethylxanthine and mimicked by forskolin and dibutyryl cyclic AMP but not dibutyryl cyclic GMP. These results suggest that prostacyclin may play a role in downregulating tissue factor expression in monocytes, at least in part via elevation of intracellular levels of cyclic AMP.