The effect of vascular smooth muscle cell-targeted expression of tissue factor pathway inhibitor in a murine model of arterial thrombosis.

Tissue factor pathway inhibitor (TFPI) is a Kunitz-type protease inhibitor that regulates the extrinsic pathway of coagulation by inhibiting the factor VIIa/tissue factor (TF) catalytic complex. TFPI is expressed by both endothelial and smooth muscle cells in the vasculature and circulates at low levels. The role of local vascular TFPI in thrombosis and the development of vascular disease is unknown. To establish an experimental animal model to directly modulate smooth muscle cell-derived TFPI on the development of arterial thrombosis, transgenic mice in which a cDNA encoding murine TFPI is expressed from the murine SM22alpha promoter were generated. Expression of transgenic mRNA was 4-fold higher than the level of endogenous TFPI mRNA in arteries from transgenic mice. In situ hybridization confirmed that expression of the transgene was limited to medial vascular smooth muscle cells. Vascular TFPI activity was increased to 2 to 3-fold in carotid homogenates. There was no difference in plasma TFPI levels or hemostatic measures (PT, aPTT and tail vein bleeding times) between these mice and their wildtype littermates. In a ferric chloride-induced model of carotid thrombosis, homozygotic transgenic mice demonstrated resistance to thrombotic occlusion compared to wildtype littermates. In transgenic mice 22% occluded within 30 minutes of application while 84% of wild type mice occluded within the same time frame (p<0.01). Heterozygotic transgenic mice had an intermediate thrombotic phenotype. Taken together, these data indicated that local VSMC-specific TFPI overexpression attenuated ferric chloride-induced thrombosis without systemic or hemostatic effects. Furthermore, this transgenic mouse model should prove useful for studying the role of TFPI in the development and progression of vascular disease.