Low-density lipoprotein (LDL) binds to a G-protein coupled receptor in human platelets - Evidence that the proaggregatory effect induced by LDL is modulated by down-regulation of binding sites and desensitization of its mediated signaling

We present evidence of a link between low-density lipoprotein (LDL) recepto r binding and activation of a platelet G-coupled protein. LDL stimulation i nduced cytosolic [Ca2+](i) mobilization, increase in inositol 1,4,5-triphos phate (IP3) formation and a rapid cytosol-to-membrane translocation of prot ein kinase C (PKC) enzymatic activity. Pertussis toxin inhibited all the st imulatory effects, whereas cholera toxin had no effect. Using ligand-bindin g assays, we demonstrated that exposing platelet LDL receptors to high conc entrations of LDL (1.5 g/l) caused a rapid down-regulation and desensitizat ion, as shown by the reduction in the B-max, intracellular [Ca2+](i) mobili zation and IP3 formation to 65, 73 and 63%, respectively. The inhibitory ef fects were reversible and dose and time dependent. Furthermore, VLDL (0.2 g /l) and IDL (0.07 g/l) induced similar desensitization effects. However, HD L3 (up to 1.5 g/l), chylomicrons (up to 0.5 g/l) and cyclohexandione-modifi ed LDL (which does not bind to platelets) had no significant effects. Prote in kinase C inhibitors (150 nmol/l staurosporine, 100 mu mol/l H-7, and 10 nmol/l bisindolylmaleimide inhibited desensitization to 71%, on average. Se questration blocking agents (0.30 g/l, concanavalin A) had no significant e ffect if phosphorylation was operative. However, there was a complete block ade with the concurrent inhibition of both pathways. In contrast, cAMP-depe ndent protein kinase inhibitors (PKI, 1 mu mol/l) or beta -adrenergic recep tor kinase inhibitors (100 nmol/l, heparin), had no effect. Overall results indicate that LDL binds to a pertussis sensitive G-protein coupled recepto r and that high levels of lipoproteins down-regulate the number of receptor s and desensitize its mediated response by a mechanism that involves PKC-ph osphorylation and sequestration of binding sites. This new regulatory mecha nism may have implications for the thrombogenicity in hyperlipidemia and fo r effects of lipid lowering therapy. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.