Activated Protein C Resistance: Effect of Platelet Activation, Platelet-Derived Microparticles, and Atherogenic Lipoproteins

A novel role for CD36 in VLDL-enhanced platelet activation

Type 2 diabetes is characterized by increased plasma triglyceride levels and a fourfold increase in ischemic heart disease, but the mechanism is unclear. CD36 is a receptor/transporter that binds fatty acids of lipoproteins. CD36 deficiency has been linked with insulin resistance. There is strong evidence of in vivo interaction between platelets and atherogenic lipoproteins suggesting that atherogenic triglyceride-rich lipoproteins, such as VLDL, that are increased in diabetic dyslipidemia are important in this process. This study demonstrates that VLDL binds to the platelet receptor CD36, enhances platelet thromboxane A2 production, and causes increased collagen-mediated platelet aggregation. VLDL enhanced collagen-induced platelet aggregation by 1) shortening the time taken for aggregation to begin (lag time) to 70% of control (P = 0.001); 2) increasing maximum aggregation to 170% of control (P = 0.008); and 3) increasing thromboxane production to 3,318% of control (P = 0.004), where control represents platelets stimulated with collagen (100%). A monoclonal antibody against CD36 attenuated VLDL-enhanced collagen-induced platelet aggregation by 1) inhibiting binding of VLDL to platelets by 75% (P = 0.041); 2) lengthening lag time to 190% (P < 0.001); and 3) decreasing thromboxane production to 8% of control (P < 0.001). In support of this finding, platelets from Cd36-deficient rats showed no increase in aggregation, thromboxane production, and VLDL binding in contrast to platelets from rats expressing CD36. These data suggest that platelet Cd36 has a key role in VLDL-induced collagen-mediated platelet aggregation, possibly contributing to atherothrombosis associated with increased VLDL levels.

Triglyceride-rich lipoproteins: are links with atherosclerosis mediated by a procoagulant and proinflammatory phenotype?

Specific treatment that primarily reduces low density lipoprotein cholesterol (LDLc) levels improves survival of patients with pre-existing vascular disease by 20-30%. Failure to produce a more marked improvement in outcome is most likely explained by: (1) the observation from angiographic studies that established atherosclerotic vascular disease (AVD) is largely irreversible with current therapy and (2) other important factors cause AVD besides LDLc. One such risk factor predicting development of AVD is the atherogenic lipoprotein phenotype (ALP), comprising abnormalities of triglyceride enriched lipoproteins, high density lipoprotein cholesterol (HDLc) and small dense LDL particles. Despite strong links between the ALP and AVD, the mechanism(s) linking these relatively subtle lipoprotein abnormalities to vascular disease is poorly understood. Recent evidence suggests that a procoagulant and proinflammatory state develops within the vasculature, perhaps mediating a link between the ALP and AVD. The purpose of this review is to discuss mechanisms by which the ALP, and specifically, certain triglyceride-rich lipoproteins, may cause AVD by adverse affects on platelet function, coagulation and vascular inflammation. All rights reserved.