Vascular-Directed Tissue Factor Pathway Inhibitor Overexpression Regulates Plasma Cholesterol and Reduces Atherosclerotic Plaque Development

Abstract

Rationale: Tissue factor pathway inhibitor (TFPI) is a potent regulator of the tissue factor pathway and is found in plasma in association with lipoproteins. Objective: To determine the role of TFPI in the development of atherosclerosis, we bred mice which overexpress TFPI into the apolipoprotein E-deficient (apoE ؊/؊ ) background. Methods and Results: On a high-fat diet, smooth muscle 22␣ (SM22␣)-TFPI/apoE ؊/؊ mice were shown to have less aortic plaque burden compared to apoE ؊/؊ mice. Unexpectedly, SM22␣-TFPI/apoE ؊/؊ had lower plasma cholesterol levels compared to apoE ؊/؊ mice. Furthermore, SM22␣-TFPI mice fed a high-fat diet had lower cholesterol levels than did wild-type mice. Because TFPI is associated with lipoproteins and its carboxyl terminus (TFPIct) has been shown to be a ligand for the very-low-density lipoprotein (VLDL) receptor, we hypothesized that TFPI overexpression may regulate lipoprotein distribution. We quantified VLDL binding and uptake in vitro in mouse aortic smooth muscle cells from SM22␣-TFPI and wild-type mice. Mouse aortic smooth muscle cells from SM22␣-TFPI mice demonstrated higher VLDL binding and internalization compared to those from wild-type mice. Because SM22␣-TFPI mice have increased circulating levels of TFPI antigen, we examined whether TFPIct may act to alter lipoprotein distribution. In vitro, TFPIct increased VLDL binding, uptake, and degradation in murine embryonic fibroblasts. Furthermore, this effect was blocked by heparinase treatment. In vivo, systemic administration of TFPIct reduced plasma cholesterol levels in apoE ؊/؊ mice. Conclusions: These studies suggest that overexpression of TFPI lowers plasma cholesterol through the interaction of its carboxyl terminus with lipoproteins and heparan sulfate proteoglycans. (Circ Res. 2009;105:713-720.)Key Words: atherosclerosis Ⅲ coagulation Ⅲ murine models Ⅲ tissue factor Ⅲ lipoproteins T issue factor (TF) pathway inhibitor (TFPI), a Kunitz-type serine protease inhibitor, is an endogenous inhibitor of TF-mediated coagulation. TFPI suppresses factor Xa generation by binding via its Kunitz 1 domain to the TF:FVIIa catalytic complex and via its Kunitz 2 domain to factor Xa. 1 The formation of a quaternary TF:VIIa:TFPI:Xa complex dampens ongoing FXa generation. Additionally, TFPI also has been shown to regulate coagulation by inducing the internalization and degradation of TF:VIIa complex on cell surfaces. [2][3][4] TFPI is expressed in many cells relevant to atherosclerosis including platelets, endothelial cells, vascular smooth muscle cells, and monocyte/macrophages. [5][6][7][8][9] In human carotid plaques, the level of TFPI expression is inversely associated with TF activity, suggesting a local regulatory role. 10 In plasma, TFPI exists in small quantities (Ͻ5%) as a free full-length protein but is predominantly associated with lipoproteins. 11-13 Lipoprotein-associated TFPI has been shown to have less anticoagulant activity than free TFPI. 13 TFPI is cleared from the plasma by binding the low-density lipoprotein recepto...

Expression of Human Tissue Factor Pathway Inhibitor on Vascular Smooth Muscle Cells Inhibits Secretion of Macrophage Migration Inhibitory Factor and Attenuates Atherosclerosis in ApoE −/− Mice

“…For instance, mice deficient in TFPI show enhanced atherosclerosis. 18 However, there are 2 potential mechanisms operating in these mice, the first involving loss of the direct effects of TFPI on circulating lipids 19,20 and the second involving loss of regulation of TF activity and resulting enhanced thrombin generation. To illustrate the first mechanism, transgenic mice overexpressing soluble TFPI were shown to be resistant to atherosclerosis as a result of a direct effect of soluble TFPI on very-low-density lipoprotein metabolism.…”

“…To illustrate the first mechanism, transgenic mice overexpressing soluble TFPI were shown to be resistant to atherosclerosis as a result of a direct effect of soluble TFPI on very-low-density lipoprotein metabolism. 20 The importance of the second mechanism is illustrated when ApoE −/− mice are made deficient in HCII, a natural thrombin inhibitor, in that they develop severe atheroma. 21 Additionally, the direct thrombin inhibitor melagatran reduces lesion progression and maintains plaque stability in ApoE −/− mice.…”

Tissue factor pathway inhibitor as a multifunctional mediator of vascular structure

“…To determine the role of TFPI in the development of atherosclerosis, we bred SM22αTFPI into the ApoE −/− background (48). On a high fat diet, SM22αTFPI/ApoE −/− mice were shown to have less aortic plaque burden compared to ApoE −/− mice (Figure 4).…”

Interdependent biological systems, multi-functional molecules: The evolving role of tissue factor pathway inhibitor beyond anti-coagulation

“…Conversely, TFPI haploinsufficiency increases the thrombotic tendency and atherosclerotic burden in pro-atherosclerotic Apo E deficient mice [40, 41]. Interestingly, recent studies again involving mice overexpressing vascular TFPI bred into an Apo E deficient, atherosclerotic, background not only revealed reduced plaque burden but also a reduction in total plasma cholesterol [42]. This suggested a role for TFPI in systemic lipid metabolism.…”