Native high-density lipoproteins inhibit platelet activation via scavenger receptor BI. Role of negatively charged phospholipids

Objectives: HIGH-density lipoproteins (HDL) are a negative predictor of platelet-dependent thrombus formation and reduced platelet activation has been observed in vitro in the presence of HDL3, a major HDL fraction. However, mechanisms underlying the anti-thrombotic effects of HDL3 are poorly understood. Scavenger receptors class B represent possible HDL3 binding partners on platelets. We here investigated the role of scavenger receptor class B type I (SR-BI) and CD36 in mediating inhibitory effects of native HDL3 on thrombin-induced platelet activation. Methods and results: Rhodamine isothiocyanate-labeled HDL3 bound specifically to platelets and HDL3 binding was inhibited by scavenger receptor class B ligands such as phosphatidylserine (PS)- or phosphatidylinositol (PI)-containing liposomes or maleylated albumin (mBSA). By contrast, scavenger receptor class A ligands failed to displace HDL3 from platelets. HDL3, PS- and PI-liposomes, and mBSA inhibited thrombin-induced platelet aggregation, fibrinogen binding, P-selectin expression and mobilization of intracellular Ca²⁺. In addition, PS- and PI-liposomes emulated HDL3-induced intracellular signaling cascades including diacylglycerol production and protein kinase C activation. The reduction of platelet activation by liposomes was related to their PS or PI content. Moreover, inhibitory effects of native HDL3 were enhanced after enriching lipoproteins with PS, while PS- and PI-poor HDL2 failed to inhibit platelet aggregation and Ca²⁺ mobilization. Both, HDL3 and PS-containing liposomes failed to inhibit thrombin-induced activation of platelets obtained from SR-BI-deficient mice but not CD36-deficient mice. Conclusion: We suggest that SR-BI is a functional receptor for native HDL3 on platelets that generates an inhibitory signal for platelet activation. The content of negatively charged phospholipids (PS, PI) in HDL may be an important determinant of their anti-thrombotic potential.