LDL BINDING-SITES ON PLATELETS DIFFER FROM THE CLASSICAL RECEPTOR OF NUCLEATED CELLS

Washed human platelets bound radioiodinated low density lipoprotein (I-125-LDL) to a class of saturable binding sites; they numbered 1,348+/-126 per platelet, and the dissociation constant (K(D)) was 50.7+/-9 nM. I-125-LDL binding to platelets was reversible, and apparent equilibrium was attained within 25 minutes at 22-degrees-C and was characterized by forward and reverse rate constants of 1.47 X 10(4) X SeC-1 X M-1 and 8 X 10(4) X sec-1 X M-1, respectively. Such binding was largely unaltered by temperature, divalent ions, and chelating agents. In addition, neither did receptor regulation (up or down) occur when platelets were loaded with cholesterol, nor did prostaglandin E1 (PGE1) increase the binding of I-125-LDL to platelets. On the other hand, the specificity of LDL binding was not typical of the LDL receptor of nucleated cells. Lipoproteins competed for the occupancy of LDL binding sites in platelets with the following order of potency: very low density much greater than intermediate density>high density subfraction 2. High density lipoprotein subfraction 3, heparin, and PGE1 had no effect on this binding. I-125-LDL binding to lymphocytes and fibroblasts and proteolytic degradation of I-125-LDL by lymphocytes was inhibited by the monoclonal antibody IgG-C7 directed against the LDL receptor to 88%, 85%, and 85% (p<0.001), respectively. However, with this monoclonal antibody, a blocking effect on neither I-125-LDL binding to platelets nor on LDL-enhanced platelet aggregation induced by ADP and collagen was found. Moreover, we confirmed the existence of LDL binding in platelets from patients with familial hypercholesterolemia. Our results indicate that human platelets bind LDL by saturable sites, which clearly differ from the "classical" LDL receptor in their binding properties, absence of receptor regulation, presence in platelets of familial hypercholesterolemia patients, and the lack of a blocking effect of IgG-C7 on LDL binding and LDL biological activity.