Platelet Tissue Factor Activity In Patients with Hypercholesterolemia: Modulation of Procoagulant Activity by Statins

Abstract 156

Human platelets store TF that expresses procoagulant activity (PCA) associated with platelet activation (Panes O. et al, Blood 2007;109:5242-50). We have also observed that platelet TF-induced PCA is specifically and rapidly induced by activation of GPIb complex by VWF-Ristocetin (Panes O. et al. Blood 2008;112:A113). Hypercholesterolemia has been associated with increased platelet function and hypercoagulability, but the mechanisms are still unknown. The cholesterol content in cell membranes is directly related with the plasma concentration of the sterol. Inhibition of HMG-CoA reductase by statins reduces cardiovascular risk by decreases in plasma cholesterol and by other independent (“pleiotropic”) effects.

We studied the relationship between total and LDL plasma cholesterol with platelet TF content and TF-dependent PCA of washed platelets activated for 5 min with VWF-Ristocetin (VWF-R). We also examined these variables after atorvastatin or rosuvastatin intake (80 and 20 mg/day × 1 month, respectively) in 25 subjects with hypercholesterolemia. TF was measured by ELISA and PCA by FXa generation (chromogenic assay) in washed platelet suspensions before and after activation. We found no significant differences between both statins regarding the decrease in plasma LDL-cholesterol levels and platelet TF-dependent PCA; both statins had any significant effects on the levels of blood inflammatory markers (usCRP, fibrinogen and VWF). Thus, we analyzed the pooled data of patients receiving either one or the other statin. Total and LDL-Chol dropped from 271±41 and 185±40 to 167±30 and 87±23 mg dL^-1, respectively. This was associated with decrease in basal PCA (non-stimulated platelets) from 34±27 to 22±14 nmol of FXa/2*10⁷platelets (p=0.02). After VWF-R stimulation, a mean 3.48-fold increase in platelet PCA was observed in hypercholesterolemic patients before taking statins. Surprisingly, a mean 20-fold increase in FXa generation in activated platelets was observed after 1 month on statin therapy, and this difference was statistically significant (p=0.018). These changes in PCA were not associated with significant changes in TF content of platelet membranes (621±481 to 555±300 pg/mg prot). We also found that plasma LDL-Chol was negatively correlated with platelet PCA induced by VWF-R activation (r = −0.239, p = 0.026) and with the ratio of PCA between activated and non-activated platelets (r = −0.27, p = 0.011). The striking post-statin increase of platelet-PCA without change in the total platelet TF protein suggests that PCA depends more on the fraction of platelet TF available for activation than on the total mass of platelet TF. The decrease in basal PCA of non-stimulated platelets after statin therapy suggests that lower concentration of cholesterol in platelet membranes makes the platelets more stable during the isolation procedure. The higher increase after stimulation would reflect a better hemostatic response to a specific stimulus. Thus, changes in plasma cholesterol, possibly through decrease in platelet membrane cholesterol, modulates the PCA of platelets, either by making platelets more stable under resting conditions and more responsive after specific activation.