Elevated Levels of Low Density Lipoprotein (LDL) Trigger VWF Self-Association into Fibers and Bundles and Potentiate Microvascular Thrombosis

Self-association of VWF on the surface of activated endothelium provides an efficient surface for the capture of platelets and is at least partially responsible for the microvascular dysfunction seen in several diseases, including diabetes, coronary heart disease, malaria, sepsis, ischemia/reperfusion injury, and the thrombotic microangiopathies, including thrombotic thrombocytopenic purpura. We showed previously that high density lipoprotein (HDL) can prevent VWF self-association (PMID: 26552698) and decided to explore whether other lipoproteins affected the phenomenon. Of all the lower density lipoproteins (chylomicrons, VLDL, and LDL) only LDL affected VWF self-association, having the opposite effect as HDL. Vortexing a solution of recombinant VWF (5 μg/ml) for 90 min resulted in complete loss of VWF from the solution by deposition on the wall of the tube. Addition of HDL at increasing concentrations progressively protected VWF in solution, with no VWF lost at 1.3 mg/ml of HDL. At an HDL concentration of 1.2 mg/ml, the addition of LDL at increasing concentrations progressively increased VWF loss, the effect being related to the ratio of LDL to HDL and not the absolute concentration of the lipoprotein particles. Similarly, HDL diminished deposition of VWF in a post-in-channel microfluidic device, whereas LDL markedly potentiated deposition, increasing both the rate and extent of VWF strand formation, demonstrated with a solution of purified VWF and with plasma. The effects of the lipoproteins appeared to be from direct interactions with VWF, as shown by the association of fluorescently labeled HDL and LDL with VWF strands.

Plasma from hypercholesterolemic patients with elevated LDL but equivalent levels of HDL also displayed accelerated VWF accumulation on the microfluidic device, the initial rate of deposition correlating linearly with the LDL level in the plasma.

We evaluated the physiological consequences of the LDL/VWF interaction in mice with molecular imaging ultrasound and contrast enhanced ultrasound. In mice deficient in ADAMTS13 and the LDL receptor fed a high-fat diet for two weeks, the high LDL levels enhanced VWF and platelet adhesion to the myocardial microvasculature, reducing myocardial microvascular perfusion, impairing systolic function, and producing early signs of cardiomyopathy.

Finally, by intravital microscopy of the mesenteric microvasculature of wild-type C57Bl6 stimulated by calcium ionophore to activate endothelial secretion of VWF, preinjection of LDL markedly enhanced the formation of microvascular thrombosis. The intensity of thrombosis exceeded even that seen in similarly treated ADAMTS13-deficient mice that did not receive an LDL infusion, with a greater number of large thrombi being formed and a prolonged time to thrombus resolution (> 20 min in LDL-treated WT mice vs 10 min in ADAMTS13-deficient mice).

In summary, we show that LDL, in addition to its role in cholesterol delivery and atherosclerosis, interacts with the hemostatic system, increasing the efficiency of VWF self-association. Regulation of VWF self-association and therefore platelet adhesion by lipoproteins has some obvious clinical implications. Long term, one would expect that the counterbalancing effects of HDL and LDL on VWF self-association would contribute to the well-known effects of these lipoproteins on the genesis and progression of atherosclerosis, largely a disease of large blood vessels. But the balance between the two lipoproteins is also likely to influence the course of acute and chronic diseases affecting the microcirculation, including the thrombotic microangiopathies.