Impaired fibrinolytic system in ApoE gene-deleted mice with hyperlipidemia augments deep vein thrombosis

Background: Hyperlipidemia increases the level of blood plasminogen activator inhibitor-1 (PAI-1) that is responsible for regulating fibrinolysis by inhibiting both urokinase-type plasminogen activator (u-PA) and tissue-type plasminogen activator (t-PA). While this fibrinolytic pathway is well known, the role of PAI-1 in venous thrombosis (VT) under hyperlipidemic conditions has not been fully established. We sought to determine the effects of PAI-1 in an in vivo hyperlipidemic model of VT. Methods: C57BL/6 wild-type (WT) mice, apolipoprotein E gene-deleted mice (ApoE-/-) having hyperlipidemia, and PAI-1 gene-deleted (PAI-1-/-) mice were used in this study. Inferior vena cava (IVC) ligation below the level of the renal veins was performed to create a stasis VT. Endpoints included measuring acute thrombosis (day 2) and chronic thrombosis (days 6 and 14). At euthanasia, blood samples were collected for plasmin and PAI-1 activity. In addition, the IVC and its thrombus were evaluated for thrombus weight (TW), u-PA activity, and differential leukocyte count while the vein wall only was analyzed for monocyte chemoattractant protein-1 (MCP-1), matrix metalloproteinase (MMP) 2, and MMP-9. Results: Compared to WT at day 2, ApoE-/-mice demonstrated a statistically significant 14% increase in TW(P<. 05) and a significant 41% increase in circulating PAI-1 activity (P <.05), while showing a trend of decreased plasmin activity. In addition, TW in ApoE-/-mice was 45% higher than PAI-1-/-mice at day 2 (P <.05), 33% at day 6 (P <.01), and 41% at day 14 (P <.01). ApoE-/-mice exhibited undetectable levels of u-PA in both vein wall and thrombus, compared to WT, at all time points. Also, vein wall MMP-2 was significantly decreased by 64% at day 6 (P <.01) and 58% at day 14 (P <.05). MMP-9 was significantly decreased by 71% at day 2 (P <.01) and 48% at day 6 (P <.01), in ApoE-/-mice compared to WT mice. In addition, in ApoE-/-mice, MCP-1 was significantly decreased by 38% at day 2 (P <.01) and 67% at day 6 (P <.01) vs WT mice. As expected in ApoE mice, following a decrease in MCP-1, monocyte recruitment was significantly decreased at days 6 (P <.01) and 14 (P <.05). Conclusions: A significant increase of circulating PAI-1 levels in hyperlipidemic mice correlated with an early increase in TW due to impaired fibrinolysis. The undetectable levels of u-PA in ApoE-/-mice correlated to a decrease in vein wall MMP-2, MMP-9, MCP-1, and a decrease in monocyte recruitment diminishing thrombus resolution. (J Vasc Surg 2012; 55: 815-22.) Clinical Relevance: Recent studies have presented evidence of a connection between hyperlipidemia and deep vein thrombosis (DVT). Here we present our findings on characterizing a mouse model of DVT in hyperlipidemia. We found a dysfunction in the fibrinolytic system in hyperlipidemic mice undergoing venous thrombosis. This information may contribute to a better understanding of the mechanisms linking hyperlipidemia and DVT. In addition, we believe that our findings represent a solid characterization of a mouse model for future therapeutic studies involving DVT and hyperlipidemia.