Tissue Factor Pathway Inhibitor Overexpression Inhibits Hypoxia-Induced Pulmonary Hypertension

Pulmonary hypertension (PH) is a commonly recognized complication of chronic respiratory disease. Enhanced vasoconstriction, pulmonary vascular remodeling, and in situ thrombosis contribute to the increased pulmonary vascular resistance observed in PH associated with hypoxic lung disease. The tissue factor pathway regulates fibrin deposition in response to acute and chronic vascular injury. We hypothesized that inhibition of the tissue factor pathway would result in attenuation of pathophysiologic parameters typically associated with hypoxia-induced PH. We tested this hypothesis using a chronic hypoxia-induced murine model of PH using mice that overexpress tissue factor pathway inhibitor (TFPI) via the smooth muscle-specific promoter SM22 (TFPI(SM22)). TFPI(SM22) mice have increased pulmonary TFPI expression compared with wild-type (WT) mice. In WT mice, exposure to chronic hypoxia (28 d at 10% O(2)) resulted in increased systolic right ventricular and mean pulmonary arterial pressures, changes that were significantly reduced in TFPI(SM22) mice. Chronic hypoxia also resulted in significant pulmonary vascular muscularization in WT mice, which was significantly reduced in TFPI(SM22) mice. Given the pleiotropic effects of TFPI, autocrine and paracrine mechanisms for these hemodynamic effects were considered. TFPI(SM22) mice had less pulmonary fibrin deposition than WT mice at 3 days after exposure to hypoxia, which is consistent with the antithrombotic effects of TFPI. Additionally, TFPI(SM22) mice had a significant reduction in the number of proliferating (proliferating cell nuclear antigen positive) pulmonary vascular smooth muscle cells compared with WT mice, which is consistent with in vitro findings. These findings demonstrate that overexpression of TFPI results in improved hemodynamic performance and reduced pulmonary vascular remodeling in a murine model of hypoxia-induced PH. This improvement is in part due to the autocrine and paracrine effects of TFPI overexpression.