LDL increases inactive tissue factor on vascular smooth muscle cell surfaces: hydrogen peroxide activates latent cell surface tissue factor.

BACKGROUND Tissue factor, which is required for the initiation of the extrinsic coagulation cascade, is known to be upregulated in cells within atherosclerotic lesions, including smooth muscle cells. Tissue factor expression on the smooth muscle cell surface could be of pathological significance as a contributor to plaque growth, thrombus formation, and the acute coronary syndrome after plaque rupture. METHODS AND RESULTS In this study, we show that LDL increased tissue factor mRNA and cell surface protein in smooth muscle cells without a marked increase in surface tissue factor activity. Hydrogen peroxide activated tissue factor on the cell surface but did not increase tissue factor mRNA or cell surface protein. Sequentially added LDL and hydrogen peroxide increased mRNA, cell surface protein, and activity; surface activity was greater than that observed with hydrogen peroxide alone. The action of hydrogen peroxide did not involve a regulatory mechanism associated with the cytoplasmic tail of tissue factor because a truncated tissue factor lacking the cytoplasmic tail was activated by hydrogen peroxide. CONCLUSIONS These results suggest a novel 2-step pathway for increased tissue factor activity on smooth muscle cell surfaces in which lipoproteins regulate synthesis of a latent tissue factor and oxidants activate the protein complex.