Oxidized cholesterol compounds or oxysterols are thought to be potent membrane-destabilizing agents. Anionic phospholipids, chiefly phosphatidylserine, have a procoagulant potential due to their ability to favour the membrane assembly of the characteristic clotting enzyme complexes including the tissue factor-dependent initiating complex. However, in resting cells, phosphatidylserine is sequestered in the inner leaflet of the plasma membrane. When THP-1 monocytic cells were cultured in the presence of 7β-hydroxycholesterol (7β-OH) or 25-hydroxycholesterol (25-OH), prothrombinase, which reflects anionic phospholipid exposure and tissue factor (TF) procoagulant activities, increased in a time- and dose-dependent manner. 7β-OH appeared 1.5- to 2-fold more potent than 25-OH. Interestingly, no effect of cholesterol itself could be detected on procoagulant activities. Nevertheless, no difference in TF activity could be detected between oxysterol-treated and control cells after disruption. TF antigen expression was the same in oxysterol-treated and control cells as shown by flow cytometry. In contrast, the use of labelled annexin V, a protein probe of anionic phospholipids, revealed an elevated number of cells with exposed phosphatidylserine. Because the latter also constitutes a signal for phagocyte recognition of apoptotic cells and fragments, and a proportion of cells displayed altered morphology with condensed chromatin and membrane blebs, analysis of DNA was performed and indicated apoptosis in oxysterol-treated cells. Hence, oxysterol-induced phosphatidylserine exposure and enhanced TF activity may result from apoptosis. These results suggest relationships between oxysterol and the amplification of coagulation reactions by monocytic cells resulting from induced phosphatidylserine exposure.

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