Platelet HDL3 binding sites are not related to integrin αIIbβ3 (GPIIb–IIIa)

Early studies considered that fibrinogen receptor (glycoprotein [GP] IIb–IIIa or platelet integrin αIIbβ3) is the binding site for low-density lipoprotein (LDL) and high-density lipoprotein type 3 (HDL3). Recent data, however, do not support the hypothesis that the binding of LDL to human intact resting platelets is related to integrin αIIbβ3. In this study we present evidence that platelet integrin αIIbβ3 is also not involved in the interaction of HDL3 and human intact resting platelets. Firstly, specific ligands for platelet integrin αIIbβ3, such as fibrinogen, vitronectin, von Willebrand factor and fibronectin, were unable to inhibit the binding of HDL3 to intact resting platelets. Secondly, the HDL3 binding characteristics (Kd and Bmax values), the activation of protein kinase C (PKC) and the inhibition of thrombin-induced inositoltriphosphate (IP3) formation and calcium (Ca2+) mobilization mediated by HDL3 particles were similar in platelets from control subjects and patients with type I and type II Glanzmannʹs thrombasthenia, which are characterized by total and partial lack of GPIIb–IIIa and fibrinogen, respectively. In contrast, nitrosylation of tyrosine residues of HDL3 by tetranitromethane fully abolished both the ability of particles to interact with its specific binding sites and the functional effects. Thirdly, polyclonal antibodies against the GPIIb–IIIa complex (edu-3 and 5B12), human antiserums against platelet alloantigens (anti-Baka/B and anti-PLA1/2), anti-integrin subunits (anti-αV and anti-β3), and a wide panel of monoclonal antibodies (mAbs) against well-known epitopes of GPIIb (M3, M4, M5, M6, M8 and M95-2b) and GPIIIa (P23-7, P33, P37, P40, and P97) did not affect the binding of HDL3 particles to human intact resting platelets. Overall results show that neither the GPIIb–IIIa complex nor GPIIb or GPIIIa individually are the membrane binding proteins for HDL3on intact resting platelets.