Mediation of Electronegative Low-Density Lipoprotein Signaling by LOX-1

Abstract

Abstract-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, Ͼ160 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-␣.

Human electronegative LDL induces mitochondrial dysfunction and premature senescence of vascular cells in vivo

“…In addition, L5 induced LOX‐1 expression in the thoracic aortic tissues of C57B6/J mice (Figure 1b; Supporting Information Figure S3D) but failed to induce endothelial senescence in LOX‐1 −/− mice. We previously showed that LOX‐1 serves as a receptor for L5 and mediates its entry into endothelial cells, subsequently inducing apoptosis (Li, Cao, Berndt, Funder, & Liu, 1999; Lu et al., 2008, 2009). Therefore, our results support that LOX‐1 mediates L5‐dependent endothelial senescence.…”

“…Caffeine (1 mM) or NAC (5 mM) was added to cells incubated with L5 to evaluate their effects on L5‐induced senescence (Lu et al., 2009). …”

MicroRNA let-7c inhibits Bcl-xl expression and regulates ox-LDL-induced endothelial apoptosis

“…Early studies revealed that the morphological changes of cultured ECs exposed to ox-LDL are similar to those observed in the endothelium covering atherosclerotic lesions (8). It has been further shown that ox-LDL induced apoptosis in ECs has been associated with the down-regulation of the anti-apoptotic protein Bcl-xl (9,10), which decreases the ratio of Bcl-xl:Bax and thereby initiating the mitochondria-dependent apoptotic pathway (11). The Caspases, especially Caspase-3, are known to act downstream of Bcl-xl:Bax and play a key role in the execution of apoptosis (12).…”

Modified Forms of LDL in Plasma

“…The increment of electronegativity in LDL(-) is not sufficient to allow its binding to SRA in macrophages (Benitez, 2004b). However, it has been reported that LDL(-) binds to another SR, LOX-1, in endothelial cells (Lu, 2009). This binding does not promote foam cell formation, but could mediate the signaling of endothelial apoptosis.…”