Tissue Factor Pathway Inhibitor Is Expressed by Human Monocyte–Derived Macrophages

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Abstract
—Lipid-laden macrophages express tissue factor (TF), which may activate the extrinsic coagulation pathway on rupture of the atherosclerotic plaque. Tissue factor pathway inhibitor (TFPI) is a major regulator of TF-induced coagulation. We evaluated the possibility that monocyte-derived macrophages express this protein, thereby contributing to regulation of TF activity (TFact). Equally, we investigated the effect of cholesterol and of oxidized LDL (Ox-LDL) on the expression of TFPI and TF by human monocyte–derived macrophages (HMDMs). Northern blot analysis of TFPI mRNA from cultured HMDMs revealed a single band at 4.2 kb with weak intensity; this finding was confirmed by reverse transcription–polymerase chain reaction. Gel filtration of HMDM supernatants showed the presence of an active 100-kDa form of TFPI, which was confirmed by sodium dodecyl sulfate–polyacrylamide gel electrophoresis under nonreducing conditions; under reducing conditions, however, the immunoblot revealed a 40-kDa form of TFPI. The TFPI in HMDM supernatants possessed heparin-binding affinity, suggesting potential interaction of TFPI with heparan sulfate proteoglycans. Stimulation of foam cell formation by incubation of macrophages for 48 hours with exogenous free cholesterol indicated that neither the biological activity nor the de novo synthesis of TFPI protein was affected. In contrast, cholesterol loading with exogenous free cholesterol induced significant upregulation of total TFact (2.6-fold: 25.0 versus 9.4 mU/mg cell protein, cholesterol-treated versus control cells; PP<0.05); by contrast, the amount of TF antigen remained unchanged (7.1 versus 7.9 ng/mg cell protein, Ox-LDL–treated versus control cells). Our data indicate that enhancement of the procoagulant activity of TF in macrophage-derived foam cells is not counterbalanced by upregulation of TFPI activity, suggesting that lesion foam cells are in a procoagulant state; they may therefore contribute to thrombus generation on plaque rupture.