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.