Platelet factor 4 binds to low-density lipoprotein receptors and disrupts the endocytic machinery, resulting in retention of low-density lipoprotein on the cell surface.
The influence of platelets on the cellular metabolism of atherogenic lipoproteins has not been characterized in detail. Therefore, we investigated the effect of platelet factor 4 (PF4), a cationic protein released in high concentration by activated platelets, on the uptake and degradation of low-density lipoprotein (LDL) via the LDL receptor (LDL-R). LDL-R-dependent binding, internalization, and degradation of LDL by cultured cells were inhibited 50%, 80%, and 80%, respectively, on addition of PF4. PF4 bound specifically to the ligand-binding domain of recombinant soluble LDL-R (half-maximal binding 0.5 microg/mL PF4) and partially (approximately 50%) inhibited the binding of LDL. Inhibition of internalization and degradation by PF4 required the presence of cell-associated proteoglycans, primarily those rich in chondroitin sulfate. PF4 variants with impaired heparin binding lacked the capacity to inhibit LDL. PF4, soluble LDL-R, and LDL formed ternary complexes with cell-surface proteoglycans. PF4 induced the retention of LDL/LDL-R complexes on the surface of human fibroblasts in multimolecular clusters unassociated with coated pits, as assessed by immuno-electron microscopy. These studies demonstrate that PF4 inhibits the catabolism of LDL in vitro in part by competing for binding to LDL-R, by promoting interactions with cell-associated chondroitin sulfate proteoglycans, and by disrupting the normal endocytic trafficking of LDL/LDL-R complexes. Retention of LDL on cell surfaces may facilitate proatherogenic modifications and support an expanded role for platelets in the pathogenesis of atherosclerosis.
Sachais
Bruce S
BS
Department of Pathology and Laboratory Medicine, University of Pennsylvania, Philadelphia 19104, USA. Sachais@mail.med.upenn.edu
Kuo
Alice
A
Nassar
Taher
T
Morgan
Jeanelle
J
Kariko
Katalin
K
Williams
Kevin Jon
KJ
Feldman
Michael
M
Aviram
Michael
M
Shah
Neelima
N
Jarett
Leonard
L
Poncz
Mortimer
M
Cines
Douglas B
DB
Higazi
Abd Al-Roof
AA
D000818:Animals; D001667:Binding, Competitive; D016466:CHO Cells; D002462:Cell Membrane / Q000378:metabolism / Q000648:ultrastructure; D002478:Cells, Cultured; D006224:Cricetinae; D004305:Dose-Response Relationship, Drug; D004705:Endocytosis; D005347:Fibroblasts / Q000378:metabolism / Q000648:ultrastructure; D006801:Humans; D008077:Lipoproteins, LDL / Q000378:metabolism* / Q000648:ultrastructure; D010978:Platelet Factor 4 / Q000378:metabolism* / Q000494:pharmacology*; D011509:Proteoglycans / Q000502:physiology; D011973:Receptors, LDL / Q000037:antagonists & inhibitors* / Q000378:metabolism / Q000648:ultrastructure
D008077:Lipoproteins, LDL; D011509:Proteoglycans; D011973:Receptors, LDL; D010978:Platelet Factor 4