Platelet HDL(3) binding sites are not related to integrin alpha(IIb)beta(3) (GPIIb-IIIa)

Abstract

Early studies considered that fibrinogen receptor (glycoprotein [GP] IIb-IIIa or platelet integrin alpha(IIb)beta(3)) is the binding site for low-density lipoprotein (LDL) and high-density lipoprotein type 3 (HDL(3)). Recent data, however, do not support the hypothesis that the binding of LDL to human intact resting platelets is related to integrin alpha(IIb)beta(3). In this study we present evidence that platelet integrin alpha(IIb)beta(3) is also not involved in the interaction of HDL(3) and human intact resting platelets. Firstly, specific ligands for platelet integrin alpha(IIb)beta(3), such as fibrinogen, vitronectin, von Willebrand factor and fibronectin, were unable to inhibit the binding of HDL(3) to intact resting platelets. Secondly, the HDL(3) binding characteristics (K(d) and B(max) values), the activation of protein kinase C (PKC) and the inhibition of thrombin-induced inositoltriphosphate (IP(3)) formation and calcium (Ca(2+)) mobilization mediated by HDL(3) particles were similar in platelets from control subjects and patients with type I and type II Glanzmann's thrombasthenia, which are characterized by total and partial lack of GPIIb-IIIa and fibrinogen, respectively. In contrast, nitrosylation of tyrosine residues of HDL(3) 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-Bak(a/B) and anti-PL(A1/2)), anti-integrin subunits (anti-alpha(V) and anti-beta(3)), 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 HDL(3) 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 HDL(3)on intact resting platelets.