Mediation of electronegative low-density lipoprotein signaling by LOX-1: a possible mechanism of endothelial apoptosis.
The lectin-like oxidized LDL receptor LOX-1 mediates endothelial cell (EC) uptake of experimentally prepared copper-oxidized LDL (oxLDL). To confirm the atherogenic role of this receptor cloned against copper-oxLDL, we examined whether it mediates EC uptake of L5, an electronegative LDL abundant in dyslipidemic but not normolipidemic human plasma. Hypercholesterolemic (LDL-cholesterol, >160 mg/dL) human LDL was fractionated into L1-L5, increasingly electronegative, by ion-exchange chromatography. In cultured bovine aortic ECs (BAECs), L5 upregulated LOX-1 and induced apoptosis. Transfection of BAECs with LOX-1-specific small interfering RNAs (siLOX-1) minimized baseline LOX-1 production and restrained L5-induced LOX-1 upregulation. Internalization of labeled L1-L5 was monitored in BAECs and human umbilical venous ECs by fluorescence microscopy. LOX-1 knockdown with siLOX-1 impeded the endocytosis of L5 but not L1-L4. In contrast, blocking LDL receptor with RAP (LDL receptor-associated protein) stopped the internalization of L1-L4 but not L5. Although chemically different, L5 and oxLDL competed for EC entry through LOX-1. Via LOX-1, L5 signaling hampered Akt phosphorylation and suppressed EC expression of fibroblast growth factor-2 and Bcl-2. L5 also selectively inhibited Bcl-xL expression and endothelial nitric oxide synthase phosphorylation but increased synthesis of Bax, Bad, and tumor necrosis factor-alpha. Blocking Akt phosphorylation with wortmannin increased LOX-1 expression, suggesting a modulatory role of Akt in LOX-1 synthesis; L5 upregulated LOX-1 by dephosphorylating Akt. Because endothelial nitric oxide synthase and Bcl-2 activities are Akt-dependent, L5 impairs Akt-mediated growth and survival signals in vascular ECs by way of LOX-1. Thus, the L5/LOX-1 complex may play a critical role in atherogenesis and illuminate important targets for disease intervention.
Lu
Jonathan
J
Department of Medicine, Baylor College of Medicine, Houston, TX 77030, USA.
Yang
Jun-Hai
JH
Burns
Alan R
AR
Chen
Hsin-Hung
HH
Tang
Daming
D
Walterscheid
Jeffrey P
JP
Suzuki
Shinichi
S
Yang
Chao-Yuh
CY
Sawamura
Tatsuya
T
Chen
Chu-Huang
CH
D000818:Animals; D017209:Apoptosis; D050197:Atherosclerosis / Q000378:metabolism* / Q000473:pathology; D002417:Cattle; D002478:Cells, Cultured; D002852:Chromatography, Ion Exchange; D004705:Endocytosis; D042783:Endothelial Cells / Q000201:enzymology / Q000378:metabolism* / Q000473:pathology; D016222:Fibroblast Growth Factor 2 / Q000378:metabolism; D006801:Humans; D026563:LDL-Receptor Related Protein-Associated Protein / Q000378:metabolism; D008077:Lipoproteins, LDL / Q000097:blood / Q000378:metabolism*; D008856:Microscopy, Fluorescence; D052250:Nitric Oxide Synthase Type III / Q000378:metabolism; D010766:Phosphorylation; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism; D034622:RNA Interference; D034741:RNA, Small Interfering / Q000378:metabolism; D051127:Scavenger Receptors, Class E / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction; D014162:Transfection; D014409:Tumor Necrosis Factor-alpha / Q000378:metabolism
D026563:LDL-Receptor Related Protein-Associated Protein; D008077:Lipoproteins, LDL; C495137:OLR1 protein, human; D019253:Proto-Oncogene Proteins c-bcl-2; D034741:RNA, Small Interfering; D051127:Scavenger Receptors, Class E; D014409:Tumor Necrosis Factor-alpha; C099252:oxidized low density lipoprotein; D016222:Fibroblast Growth Factor 2; D052250:Nitric Oxide Synthase Type III; D051057:Proto-Oncogene Proteins c-akt