Lipoprotein accumulation in macrophages via toll-like receptor-4-dependent fluid phase uptake.
Toll-like receptor (TLR)4 recognizes microbial pathogens, such as lipopolysaccharide, and mediates lipopolysaccharide-induced proinflammatory cytokine secretion, as well as microbial uptake by macrophages. In addition to exogenous pathogens, TLR4 recognizes modified self, such as minimally oxidized low-density lipoprotein (mmLDL). Here we report that mmLDL and its active components, cholesteryl ester hydroperoxides, induce TLR4-dependent fluid phase uptake typical of macropinocytosis. We show that mmLDL induced recruitment of spleen tyrosine kinase (Syk) to a TLR4 signaling complex, TLR4 phosphorylation, activation of a Vav1-Ras-Raf-MEK-ERK1/2 signaling cascade, phosphorylation of paxillin, and activation of Rac, Cdc42, and Rho. These mmLDL-induced and TLR4- and Syk-dependent signaling events and cytoskeletal rearrangements lead to enhanced uptake of small molecules, dextran, and, most importantly, both native and oxidized LDL, resulting in intracellular lipid accumulation. An intravenous injection of fluorescently labeled mmLDL in wild-type mice resulted in its rapid accumulation in circulating monocytes, which was significantly attenuated in TLR4-deficient mice. These data describe a novel mechanism leading to enhanced lipoprotein uptake in macrophages that would contribute to foam cell formation and atherosclerosis. These data also suggest that cholesteryl ester hydroperoxides are an endogenous ligand for TLR4. Because TLR4 is highly expressed on the surface of circulating monocytes in patients with chronic inflammatory conditions, and cholesteryl ester hydroperoxides are present in plasma, lipid uptake by monocytes in circulation may contribute to the pathological roles of monocytes in chronic inflammatory diseases.
Choi
Soo-Ho
SH
Department of Medicine, University of California at San Diego, 9500 Gilman Dr, La Jolla, CA 92093, USA.
Harkewicz
Richard
R
Lee
Jee Hyun
JH
Boullier
Agnès
A
Almazan
Felicidad
F
Li
Andrew C
AC
Witztum
Joseph L
JL
Bae
Yun Soo
YS
Miller
Yury I
YI
D000818:Animals; D019556:COS Cells; D002522:Chlorocebus aethiops; D002788:Cholesterol Esters / Q000235:genetics / Q000378:metabolism*; D002908:Chronic Disease; D003599:Cytoskeleton / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D005487:Foam Cells / Q000378:metabolism* / Q000473:pathology; D006801:Humans; D007249:Inflammation / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism; D008024:Ligands; D008077:Lipoproteins, LDL / Q000235:genetics / Q000378:metabolism*; D020935:MAP Kinase Signaling System; D051379:Mice; D008817:Mice, Mutant Strains; D019950:Mitogen-Activated Protein Kinase 1 / Q000235:genetics / Q000378:metabolism; D048052:Mitogen-Activated Protein Kinase 3 / Q000235:genetics / Q000378:metabolism; D010766:Phosphorylation / Q000235:genetics; D010873:Pinocytosis; D011505:Protein-Tyrosine Kinases / Q000235:genetics / Q000378:metabolism; D000072377:Syk Kinase; D051197:Toll-Like Receptor 4 / Q000235:genetics / Q000378:metabolism*; D020764:cdc42 GTP-Binding Protein / Q000235:genetics / Q000378:metabolism; D020744:rac GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism; D020741:rho GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism
D002788:Cholesterol Esters; D047908:Intracellular Signaling Peptides and Proteins; D008024:Ligands; D008077:Lipoproteins, LDL; C493487:Tlr4 protein, mouse; D051197:Toll-Like Receptor 4; C099252:oxidized low density lipoprotein; D011505:Protein-Tyrosine Kinases; C000606896:SYK protein, human; D000072377:Syk Kinase; C000606897:Syk protein, mouse; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D020764:cdc42 GTP-Binding Protein; D020744:rac GTP-Binding Proteins; D020741:rho GTP-Binding Proteins