Objective-Tissue factor (TF), the initiator of the coagulation cascade, is expressed by cells in atherosclerotic lesions.Lysophosphatidic acid (LPA) is a component of oxidized lipoproteins and an agent released by activated platelets. The present study investigated whether and how TF expression is regulated by LPA. Methods and Results-Northern blotting, Western blotting, and TF activity assays demonstrated that LPA markedly induced TF mRNA, protein, and activity in vascular smooth muscle cells. LPA-induced TF expression is primarily controlled at the transcriptional level. Phosphorylation of mitogen-activated protein kinase kinase (MEK) and extracellular signaling-regulated kinases (ERK1/2) was rapidly and markedly induced by LPA. MEK inhibitors U0126 and PD98059 blocked both ERK activation and the increase in TF mRNA. In contrast, the specific p38 MAP kinase inhibitor SB203580 had no effect on LPA-induced TF mRNA increase. The G␣i protein inhibitor, pertussis toxin, abolished LPA-induced phosphorylation of MEKs and ERKs, as well as the induction of TF mRNA. Conclusions-Our

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