EFFECTS OF FIBRIN ON THE SECRETION OF PLASMINOGEN-ACTIVATOR INHIBITOR-1 FROM ENDOTHELIAL-CELLS AND ON PROTEIN-KINASE-C

We previously demonstrated that cultured human umbilical vein endothel ial cells (HUVECs) overlaid with a fibrin clot induced a slight increa se in tissue-type plasminogen activator (t-PA) secretion and marked re duction in plasminogen activator inhibitor-1 (PAI-1) secretion. In thi s study, the intracellular signal transduction after fibrin stimulatio n was further investigated by analyzing cyclic AMP (cAMP) and protein kinase C (PK-C). When HUVECs were stimulated by fibrin clots, t-PA mRN A increased to 130% but PAI-1 mRNA decreased to 42%. These changes con curred with the data on the protein levels of t-PA and PAI-1 as previo usly reported. The effect of fibrin on t-PA production in HUVECs was n ot significantly altered after the elevation of cAMP by either forskol in or dibutyryl cAMP. Furthermore, an effect of fibrin on t-PA product ion did not appear when the cells were treated by phorbol 12-myristate 13-acetate (PMA) or 1-(5-isoquinolinesulfonyl)-2-methylpiper (H-7) Th e suppressive effect of fibrin on PAI-1 secretion from HUVECs was not altered by elevation of cAMP. Regarding the activation of PK-C by PMA, PAI-1 secretion was enhanced, but was suppressed by fibrin stimulatio n. H-7 suppressed PAI-1 secretion and further stimulation by fibrin al most completely abolished PAI-I secretion. These changes were well ass ociated with mRNA levels of t-PA and PAI-1. These results suggested th at fibrin on HUVECs preferably down-regulates PK-C resulting in a decr ease of PAI-1 in both the protein and mRNA levels and that effect of f ibrin on t-PA secretion is neither involved in PK-C nor cAMP pathway.