Platelet Factor 4 Enhances the Binding of Oxidized Low-density Lipoprotein to Vascular Wall Cells

Taher Nassar,Bruce S Sachais,Sa'Ed Akkawi,Maria Anna Kowalska,Khalil Bdeir,Eran Leitersdorf,Edna Hiss,Leah Ziporen,Michael Aviram,Douglas Cines,Mortimer Poncz,Abd Al-Roof Higazi

doi:10.1074/jbc.m208894200

Abstract

Accumulation of low-density lipoprotein (LDL)-derived cholesterol by macrophages in vessel walls is a pathogenomic feature of atherosclerotic lesions. Platelets contribute to lipid uptake by macrophages through mechanisms that are only partially understood. We have previously shown that platelet factor 4 (PF4) inhibits the binding and degradation of LDL through its receptor, a process that could promote the formation of oxidized LDL (ox-LDL). We have now characterized the effect of PF4 on the binding of ox-LDL to vascular cells and macrophages and on the accumulation of cholesterol esters. PF4 bound to ox-LDL directly and also increased ox-LDL binding to vascular cells and macrophages. PF4 did not stimulate ox-LDL binding to cells that do not synthesize glycosaminoglycans or after enzymatic cleavage of cell surface heparan and chondroitin sulfates. The effect of PF4 on binding ox-LDL was dependent on specific lysine residues in its C terminus. Addition of PF4 also caused an approximately 10-fold increase in the amount of ox-LDL esterified by macrophages. Furthermore, PF4 and ox-LDL co-localize in atherosclerotic lesion, especially in macrophage-derived foam cells. These observations offer a potential mechanism by which platelet activation at sites of vascular injury may promote the accumulation of deleterious lipoproteins and offer a new focus for pharmacological intervention in the development of atherosclerosis.

Highlights

Accumulation of low-density lipoprotein (LDL)-derived cholesterol by macrophages in vessel walls is a pathogenomic feature of atherosclerotic lesions
One potential candidate that might be involved in the accumulation of LDL is platelet factor 4 (PF4), a cationic protein that is released in large amounts when platelets are activated and that is found in atherosclerotic lesions [15, 16]
Platelet activation promotes the accumulation of LDL by macrophages, PF4 inhibits the uptake of LDL by fibroblasts [31]

Summary

Parameters analyzed

TBARS (nmol MDA/mg LDL protein) Lipid peroxides (nmol/mg LDL protein) CLOOH (nmol/mg LDL protein) Electrophoretic mobility (cm) may facilitate the incorporation of LDL into cells, we previously reported that PF4 diverts the uptake of LDL from the LDL receptor to the relatively inefficient PG-dependent endocytic pathway, likely increasing its residency time in tissue and its propensity to undergo oxidation [47]. LDL that has not been internalized and degraded is subject to oxidation in the vascular microenvironment [32]. Unlike LDL, which is internalized primarily through the LDL receptor, oxidized LDL is internalized by the scavenger receptor and several other, more recently described receptors [33,34,35]. In the present study we extend these studies by showing that, in contrast to LDL, PF4 stimulates the binding of ox-LDL by vascular cells and promotes its esterification. We propose a novel mechanism by which platelet activation may contribute to the development of atherosclerosis through the release of PF4

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION