title: Minimally modified low-density lipoprotein induces macrophage endoplasmic reticulum stress via toll-like receptor 4.
abstract: Minimally modified low-density lipoprotein (mm-LDL) induces intimal foam cell formation, which is promoted by endoplasmic reticulum stress (ERS), a cross-point to link cellular processes with multiple risk factors that exist in all stages of atherosclerosis. However, it remains unclear whether mm-LDL-induced lipid accumulation in macrophages involves ERS and its underlying mechanisms. We showed that mm-LDL induced the accumulation of lipid droplets in RAW264.7 macrophages with increased free cholesterol in the endoplasmic reticulum, which was markedly attenuated by pretreatment with an antibody against toll-like receptor 4 (TLR4). Additionally, mm-LDL stimulated the transport of Cy3-labeled activating transcription factor 6 (ATF6), a key sensor to the unfolded protein response (UPR), from cytoplasm into nucleus. The expression of phosphorylated inositol-requiring enzyme 1 (p-IRE1), another sensor to the UPR, and its two downstream molecules, X box binding protein 1 and glucose-regulated protein 78 (GRP78), were significantly upregulated by mm-LDL. The alterations induced by mm-LDL were all significantly inhibited by antibodies against TLR4 or CD36. In addition, the upregulation of p-IRE1 and GRP78 and the nuclear translocation of ATF6 induced by mm-LDL were significantly attenuated by TLR4 siRNA. These results suggest that mm-LDL may induce free cholesterol accumulation in the endoplasmic reticulum and subsequently stimulate ERS and activate the UPR signaling pathway mediated by ATF6 and IRE1 in macrophages, a process that is potentially mediated by TLR4.
authors.affiliation: Institute of Atherosclerosis, Key Laboratory of Atherosclerosis in Universities of Shandong, Taishan Medical University, Taian 271000, China. yst228@126.com
authors.forename: Shutong
authors.initials: S
authors.lastname: Yao
authors.forename: Nana
authors.initials: N
authors.lastname: Yang
authors.forename: Guohua
authors.initials: G
authors.lastname: Song
authors.forename: Hui
authors.initials: H
authors.lastname: Sang
authors.forename: Hua
authors.initials: H
authors.lastname: Tian
authors.forename: Cheng
authors.initials: C
authors.lastname: Miao
authors.forename: Ying
authors.initials: Y
authors.lastname: Zhang
authors.forename: Shucun
authors.initials: S
authors.lastname: Qin
mesh_terms: D051702:Activating Transcription Factor 6 / Q000819:agonists / Q000235:genetics; D000818:Animals; D000906:Antibodies / Q000494:pharmacology; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism; D002784:Cholesterol / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D004705:Endocytosis; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D000091342:Endoplasmic Reticulum Chaperone BiP; D059865:Endoplasmic Reticulum Stress / Q000502:physiology*; D005786:Gene Expression Regulation; D006360:Heat-Shock Proteins / Q000235:genetics / Q000378:metabolism; D008077:Lipoproteins, LDL / Q000494:pharmacology / Q000502:physiology*; D008264:Macrophages / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D051379:Mice; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism; D021381:Protein Transport; D000071701:Regulatory Factor X Transcription Factors; D015398:Signal Transduction; D051197:Toll-Like Receptor 4 / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D056811:Unfolded Protein Response / Q000235:genetics
chemical_list: D051702:Activating Transcription Factor 6; D000906:Antibodies; C497112:Atf6 protein, mouse; D004268:DNA-Binding Proteins; D000091342:Endoplasmic Reticulum Chaperone BiP; D006360:Heat-Shock Proteins; C000717248:Hspa5 protein, mouse; D008077:Lipoproteins, LDL; D008565:Membrane Proteins; D000071701:Regulatory Factor X Transcription Factors; C493487:Tlr4 protein, mouse; D051197:Toll-Like Receptor 4; D014157:Transcription Factors; C099252:oxidized low density lipoprotein; D002784:Cholesterol; C486556:Ern2 protein, mouse; D017346:Protein Serine-Threonine Kinases