Lorie Leo

Prothrombotic Effects of Hyperhomocysteinemia and Hypercholesterolemia in ApoE-Deficient Mice

Objective—We tested the hypothesis that hyperhomocysteinemia and hypercholesterolemia promote arterial thrombosis in mice. Methods and Results—Male apolipoprotein E (Apoe)-deficient mice were fed one of four diets: control, hyperhomocysteinemic (HH), high fat (HF), or high fat/hyperhomocysteinemic (HF/HH). Total cholesterol was elevated 2-fold with the HF or HF/HH diets compared with the control or HH diets (P<0.001). Plasma total homocysteine (tHcy) was elevated (12 to 15 &mgr;mol/L) with the HH or HF/HH diets compared with the control or HF diets (4 to 6 &mgr;mol/L; P<0.001). Aortic sinus lesion area correlated strongly with total cholesterol (P<0.001) but was independent of tHcy. At 12 weeks of age, the time to thrombotic occlusion of the carotid artery after photochemical injury was >50% shorter in mice fed the HF diets, with or without hyperhomocysteinemia, compared with the control diet (P<0.05). At 24 weeks of age, carotid artery thrombosis was also accelerated in mice fed the HH diet (P<0.05). Endothelium-dependent nitric oxide–mediated relaxation of carotid artery rings was impaired in mice fed the HF, HH, or HF/HH diets compared with the control diet (P<0.05). Conclusions—Hyperhomocysteinemia and hypercholesterolemia, alone or in combination, produce endothelial dysfunction and increased susceptibility to thrombosis in Apoe-deficient mice.

Paradoxical absence of a prothrombotic phenotype in a mouse model of severe hyperhomocysteinemia

Hyperhomocysteinemia confers a high risk for thrombotic vascular events, but homocysteine-lowering therapies have been ineffective in reducing the incidence of secondary vascular outcomes, raising questions regarding the role of homocysteine as a mediator of cardiovascular disease. Therefore, to determine the contribution of elevated homocysteine to thrombosis susceptibility, we studied Cbs(-/-) mice conditionally expressing a zinc-inducible mutated human CBS (I278T) transgene. Tg-I278T Cbs(-/-) mice exhibited severe hyperhomocysteinemia and endothelial dysfunction in cerebral arterioles. Surprisingly, however, these mice did not display increased susceptibility to arterial or venous thrombosis as measured by photochemical injury in the carotid artery, chemical injury in the carotid artery or mesenteric arterioles, or ligation of the inferior vena cava. A survey of hemostatic and hemodynamic parameters revealed no detectible differences between control and Tg-I278T Cbs(-/-) mice. Our data demonstrate that severe elevation in homocysteine leads to the development of vascular endothelial dysfunction but is not sufficient to promote thrombosis. These findings may provide insights into the failure of homocysteine-lowering trials in secondary prevention from thrombotic vascular events.

Human Thrombomodulin Knock-In Mice Reveal Differential Effects of Human Thrombomodulin on Thrombosis and Atherosclerosis

Objective—We sought to develop a murine model to examine the antithrombotic and antiinflammatory functions of human thrombomodulin in vivo. Methods and Results—Knock-in mice that express human thrombomodulin from the murine thrombomodulin gene locus were generated. Compared with wild-type mice, human thrombomodulin knock-in mice exhibited decreased protein C activation in the aorta (P<0.01) and lung (P<0.001). Activation of endogenous protein C following infusion of thrombin was decreased by 90% in knock-in mice compared with wild-type mice (P<0.05). Carotid artery thrombosis induced by photochemical injury occurred more rapidly in knock-in mice (12±3 minutes) than in wild-type mice (31±6 minutes; P<0.05). No differences in serum cytokine levels were detected between knock-in and wild-type mice after injection of endotoxin. When crossed with apolipoprotein E–deficient mice and fed a Western diet, knock-in mice had a further decrease in protein C activation but did not exhibit increased atherosclerosis. Conclusion—Expression of human thrombomodulin in place of murine thrombomodulin produces viable mice with a prothrombotic phenotype but unaltered responses to systemic inflammatory or atherogenic stimuli. This humanized animal model will be useful for investigating the function of human thrombomodulin under pathophysiological conditions in vivo.