Involvement of the macrophage low density lipoprotein receptor-binding domains in the uptake of oxidized low density lipoprotein

Scavenger receptor BI promotes cytoplasmic accumulation of lipoproteins in clear-cell renal cell carcinoma

Clear-cell renal cell carcinomas (ccRCCs) are characterized by inactivation of the von Hippel-Lindau (VHL) gene and intracellular lipid accumulation by unknown pathomechanisms. The immunochemical analysis of 356 RCCs revealed high abundance of apoA-I and apoB, as well as scavenger receptor BI (SR-BI) in the ccRCC subtype. Given the characteristic loss of VHL function in ccRCC, we used VHL-defective and VHL-proficient cells to study the potential influence of VHL on lipoprotein uptake. VHL-defective patient-derived ccRCC cells and cell lines (786O and RCC4) showed enhanced uptake as well as less resecretion and degradation of radio-iodinated HDL and LDL (¹²⁵I-HDL and ¹²⁵I-LDL, respectively) compared with the VHL-proficient cells. The ccRCC cells showed enhanced vascular endothelial growth factor (VEGF) and SR-BI expression compared with normal kidney epithelial cells. Uptake of ¹²⁵I-HDL and ¹²⁵I-LDL by patient-derived normal kidney epithelial cells as well as the VHL-reexpressing ccRCC cell lines, 786-O-VHL and RCC4-O-VHL cells, was strongly enhanced by VEGF treatment. The knockdown of the VEGF coreceptor, neuropilin-1 (NRP1), as well as blocking of SR-BI significantly reduced the uptake of lipoproteins into ccRCC cells in vitro. LDL stimulated proliferation of 786-O cells more potently than 786-O-VHL cells in a NRP1- and SR-BI-dependent manner. In conclusion, enhanced lipoprotein uptake due to increased activities of VEGF/NRP1 and SR-BI promotes lipid accumulation and proliferation of VHL-defective ccRCC cells.

Expression profiles of microRNAs in oxidized low-density lipoprotein-stimulated RAW 264.7 cells

Macrophage-derived foam cells were one of the hallmarks of atherosclerosis, and microRNAs played an important role in the formation of foam cells. In order to explore the roles of miRNA in the formation of foam cells, we investigated miRNA expression profiles in foam cells through high-throughput sequencing technology. A total of 84 miRNAs were differentially expressed between RAW 264.7 macrophages and foam cells induced by ox-LDL. Thirty miRNAs were upregulated and 54 miRNAs were downregulated. GO terms and KEGG pathways analysis revealed that the target genes of most of DE miRNAs were mainly enriched in "cell differentiation," "endocytosis," "MAPK signaling pathway," and "FoxO signaling pathway." The target genes of some DE miRNAs were enriched in "Insulin signaling pathway," "Hippo signaling pathway," "TNF signaling pathway," "NF-kappa B signaling pathway," and "cell death." Using bioinformatics analyses and dual-luciferase reporter assays, we found that miR-28a-5p and miR-30c-1-3p directly inhibited LRAD3 and LOX-1 mRNA expression through targeting the 3'UTR of LRAD3 and LOX-1 mRNA, respectively. Our study indicates that miRNAs are extensively involved in the formation of foam cells, and provides a valuable resource for further study the role of miRNAs in atherosclerosis.

Minimally oxidized LDL inhibits macrophage selective cholesteryl ester uptake and native LDL-induced foam cell formation

Scavenger receptor-mediated uptake of oxidized LDL (oxLDL) is thought to be the major mechanism of foam cell generation in atherosclerotic lesions. Recent data has indicated that native LDL is also capable of contributing to foam cell formation via low-affinity receptor-independent LDL particle pinocytosis and selective cholesteryl ester (CE) uptake. In the current investigation, Cu(2+)-induced LDL oxidation was found to inhibit macrophage selective CE uptake. Impairment of selective CE uptake was significant with LDL oxidized for as little as 30 min and correlated with oxidative fragmentation of apoB. In contrast, LDL aggregation, LDL CE oxidation, and the enhancement of scavenger receptor-mediated LDL particle uptake required at least 3 h of oxidation. Selective CE uptake did not require expression of the LDL receptor (LDL-R) and was inhibited similarly by LDL oxidation in LDL-R(-/-) versus WT macrophages. Inhibition of selective uptake was also observed when cells were pretreated or cotreated with minimally oxidized LDL, indicating a direct inhibitory effect of this oxLDL on macrophages. Consistent with the effect on LDL CE uptake, minimal LDL oxidation almost completely prevented LDL-induced foam cell formation. These data demonstrate a novel inhibitory effect of mildly oxidized LDL that may reduce foam cell formation in atherosclerosis.

The Relationship Between Lipid Peroxidation and LDL Desialylation in Experimental Atherosclerosis

ABSTRACT High serum total cholesterol concentration has been strongly connected with atherosclerosis in numerous studies. Being the main carrier of cholesterol in blood, low-density lipoprotein (LDL) is also the principal lipoprotein causing atherosclerosis. Sialic acids are a family of amino sugars that are commonly found as terminal oligosaccharide residues on glycoproteins and are sialylated on their apolipoprotein and glycolipid constituents. In several studies, it was demonstrated that LDL has a 2.5- to 5-fold lower content of sialic acid in patients with coronary artery disease compared with healthy subjects. The role of oxidatively modified LDL in the pathogenesis has been well documented. These studies have focused on modifications in the lipid and protein parts of LDL. But recently, desialylated LDL and its relation with the oxidation mechanisms have received attention in the pathogenesis of atherosclerosis and coronary artery disease (CAD). From these points, we have performed atheroma plaques in an experimental atherosclerosis model with rabbits and examined the LDL and plasma sialic acid and thiobarbituric acid reactive substance (TBARS) levels in the same model. We also have determined serum sialidase enzyme activities relevant with these parameters. LDL sialic acid levels were significantly decreased in the progression of the atherosclerosis (by the 30th, 60th, and 90th days). LDL and plasma TBARS levels and plasma sialidase enzyme activities were significantly elevated by the same time periods. In conclusion, serum sialidase enzyme may play an important role in the desialylation mechanism, and reactive oxygen substance (ROS) may affect this reaction.

CD44, alpha(4) integrin, and fucoidin receptor-mediated phagocytosis of apoptotic leukocytes

Various types of phagocytes mediate the clearance of apoptotic cells. We previously reported that human and murine high endothelial venule (HEV) cells ingest apoptotic cells. In this report, we examined endothelial cell fucoidin receptor-mediated phagocytosis using a murine endothelial cell model mHEV. mHEV cell recognition of apoptotic leukocytes was blocked by fucoidin but not by other phagocytic receptor inhibitors such as mannose, fucose, N-acetylglucosamine, phosphatidylserine (PS), or blocking anti-PS receptor antibodies. Thus, the mHEV cells used fucoidin receptors for recognition and phagocytosis of apoptotic leukocytes. The fucoidin receptor-mediated endothelial cell phagocytosis was specific for apoptotic leukocytes, as necrotic cells were not ingested. This is in contrast to macrophages, which ingest apoptotic and necrotic cells. Endothelial cell phagocytosis of apoptotic cells did not alter viable lymphocyte migration across these endothelial cells. Antibody blocking of CD44 and alpha4 integrin on the apoptotic leukocyte inhibited this endothelial cell phagocytosis, suggesting a novel function for these adhesion molecules in the removal of apoptotic targets. The removal of apoptotic leukocytes by endothelial cells may protect the microvasculature, thus ensuring that viable lymphocytes can successfully migrate into tissues.

Lipid peroxidation in membranes and low-density lipoproteins: similarities and differences

Lipid peroxidation has been a central aspect of studies of the nature of free radical species and their origin in biological systems. Moreover, there has been a growing interest in lipid peroxidation based on evidence that biologically active products are formed that influence cell function and the course of major human diseases. A review of the work in this area is contributed by Lars Ernster is presented with an emphasis on the mechanisms by which lipid peroxidation is initiated in biological lipid systems. Based on what was described for metal catalyzed oxidation of cell membranes, and the seminal studies on cytochrome P-450-mediated lipid peroxidation, several parallel and distinct aspects of lipid peroxidation are described. A key distinction between lipid peroxidation in cell membranes and lipoproteins reveals aspects of free radical initiated reactions involving proteins and lipids that determine pro- vs. anti-oxidant outcomes, and the role of lipid structure and order in delineating the progress of oxidation.

Essential involvement of 12-lipoxygenase in regiospecific andstereospecific oxidation of low density lipoprotein by macrophages

To establish a role of the 12-lipoxygenase on the generation of oxidized low density lipoprotein (LDL) in macrophages that leads to foam cell formation in atherosclerosis, we overexpressed 12-lipoxygenases in a macrophage-like cell line, J774A.1, that does not show intrinsic enzyme activity. When the 12-lipoxygenase-expressing cells were incubated with 400 microg.mL-1 LDL in Dulbecco's modified Eagle's medium at 37 degrees C for 12 h, LDL oxidation, as determined by thiobarbituric acid reactive substance, was markedly increased compared with the mock-transfected cells. Oxygenated products in the modified LDL were examined by HPLC before and after alkaline hydrolysis. Most of the oxygenated derivatives were of an esterified form, and the major product was identified as 13S-hydroxyoctadeca-9Z,11E-dienoic acid. These results clearly demonstrate that esterified fatty acids in LDL are oxygenated by the 12-lipoxygenases expressed in the J774A.1 cells. Furthermore, the oxidized LDL generated by intracellular 12-lipoxygenases was recognized by a scavenger receptor as assessed by macrophage degradation assay.

The antioxidant and antiatherogenic effects of MAK-4 in WHHL rabbits

Oxidized low-density lipoprotein (ox-LDL) plays an important role in atherogenesis. Atheroma formation is reduced significantly in Watanabe heritable hyperlipidemic (WHHL) rabbits by antioxidants such as probucol and vitamin E. The herbal mixture Maharishi Amrit Kalash-4 (MAK-4) inhibits Cu+2 -induced LDL oxidation, and enzymatic- and nonenzymatic-induced microsomal lipid peroxidation. We tested the effect of MAK-4 on the development of atheroma in WHHL rabbits. Eleven rabbits were divided into two groups: controls (n = 5) and a group fed 6% (w/w) MAK-4 (n = 6). Blood was drawn for biochemical analysis every two months and at necropsy, six months after the special diet was started. The aortas were preserved in formalin. The percentage area of aortic arch covered with visible plaque in the MAK-4 group (22.5 +/- 4.2%, mean +/- SE) was significantly reduced (p < 0.01) compared to the control group (47.6 +/- 6.8%, mean +/- SE). The MAK-4 group showed a significant decrease (p < 0.05) in lipid peroxide, and a significant increase (p < 0.05) in glutathione peroxidase and resistance of LDL to endothelial cell-induced and cupric ion-catalyzed oxidation (4.5 h and 5 h lag phase, respectively, for the MAK-4 group; 0 h lag phase for both for the controls). These findings suggest MAK-4 reduces atheroma formation through its antioxidant activity.

Inhibition of low-density lipoprotein oxidation by oral herbal mixtures Maharishi Amrit Kalash-4 and Maharishi Amrit Kalash-5 in hyperlipidemic patients

Low-density lipoprotein (LDL) oxidation is central to the pathogenesis of atherosclerosis. We have shown previously that the herbal mixtures Maharishi Amrit Kalash-4 (MAK-4) and Maharishi Amrit Kalash-5 (MAK-5) inhibit LDL oxidation induced by cupric ions (Cu+2) and endothelial cells in vitro and that MAK-4 reduces atherosclerosis in Watanabe heritable hyperlipidemic rabbits that were fed this herbal mixture. This study evaluates the antioxidant activity of MAK-4 and MAK-5 in vivo. Ten hyperlipidemic patients prescribed stable hypolipidemic therapy were treated with MAK-4 and MAK-5 for 18 weeks. Plasma lipoprotein, plasma lipid peroxide, and LDL oxidation studies were performed every 6 weeks. Apolipoprotein A, apolipoprotein B, and lipoprotein (a) levels were measured at baseline and 18 weeks. After 12 weeks of treatment with MAK-4 and MAK-5, a time-dependent increase in the lag phase and delay in the propagation phase of oxidation of LDL by Cu+2 and endothelial cells was seen. Lag phases at baseline and after 6, 12, and 18 weeks of MAK-4 and MAK-5 ingestion were 6.66 hours +/- 0.19 (mean +/- standard error of mean), 6.77 hours +/- 0.31, 7.22 hours +/- 0.24, and 18.00 hours +/- 0.73, respectively, for Cu(+2)-catalyzed LDL oxidation. Lag phases were 14.89 hours +/- 0.77, 13.33 hours +/- 0.50, 20.22 hours +/- 0.76, and 20.00 hours +/- 0.79, respectively, for endothelial cell-induced LDL oxidation. The levels of plasma lipid peroxide did not change significantly. No significant changes were seen in the plasma lipoproteins and the levels of apolipoprotein A, apolipoprotein B, and lipoprotein (a). The results show that MAK-4 and MAK-5 inhibit LDL oxidation in patients with hyperlipidemia. Therefore, MAK-4 and MAK-5 may be useful in the prevention and treatment of atherosclerosis.

Cellular effects of hypercholesterolemia in modulation of cancer growth and metastasis: a review of the evidence

Hypercholesterolemia and increased cancer risk have been associated, particularly with the high fat diets characteristic of Western societies. We were interested in the possible association between preexisting hypercholesterolemia and the rapidity and extent of tumor metastases in these patients. To date there has been only a few studies that have suggested and explored this determinant of cancer metastases although it may play a role in a subset of patients who develop cancers. This article will review the literature on the effects of LDL-cholesterol on cell proliferation and differentiation and speculate on mechanisms of involvement of a hypercholesterolemic milieu on cancer progression and enhancement of metastatic potential.

Increased uptake of LDL by oxidized macrophages is the result of an initial enhanced LDL receptor activity and of a further progressive oxidation of LDL

Iron ions were recently shown to induce cellular lipid peroxidation in macrophages, and these oxidized cells can convert native low-density lipoprotein (LDL) to oxidized LDL (Ox-LDL). The present study demonstrates that deoxycholic acid (DCA) and angiotensin II (ANG-II) can also induce oxidative modification of macrophages via metal ions independent mechanisms. Furthermore, incubation of LDL (200 micrograms of protein/ml) for 24 h at 37 degrees C with DCA, ANG-II, as well as FeSO4-induced oxidized macrophages, resulted in oxidative modification of the lipoprotein as evidenced by increased TBARS formation in LDL (by 50, 105, and 258%, respectively), decreased TNBS reactivity (by 45, 56, and 42%, respectively), and increased cellular uptake (by 60, 166, and 230%, respectively). A positive correlation (n = .88) was found between the extent of the cellular lipid peroxidation and the increment in the cellular uptake of the LDL. The oxidative modification of LDL by oxidized macrophages was found to be a progressive process. Incubation of LDL with oxidized macrophages for increasing periods of time up to 24 h resulted in progressive increment in: (1) the electrophoretic mobility of the LDL; (2) the TBARS formation in LDL; (3) the cellular uptake of LDL by the oxidized macrophages via the Ox-LDL receptor. Upon fractionation on a heparin-sepharose column of LDL that was incubated for different periods of time with oxidized macrophages, a gradual increment in the unbound LDL fraction was obtained, up to 72% after 24 h of incubation. During the first hour of LDL incubation with the oxidized macrophages a twofold increase in the cellular uptake of LDL by these cells was detected, although no significant oxidation of the lipoprotein occurred during this short time period. This effect could be attributed to an increased number of LDL receptors on the cell surface of the oxidized macrophages. In conclusion, increased uptake of LDL by oxidized macrophages results from two routes: (1) enhanced uptake via the LDL receptor due to increased LDL receptor activity; (2) lipoprotein uptake via the Ox-LDL receptors due to cellular modification of LDL. Both of these processes lead to macrophage cholesterol accumulation and foam cell formation, and thus contribute to accelerated atherosclerosis under oxidative stress.

Thyronines and probucol inhibition of human capillary endothelial cell-induced low density lipoprotein oxidation

Oxidized lipoproteins have been implicated as important factors in the pathogenicity of atherosclerosis. Thus, antioxidants play a significant role in inhibiting a critical step in atheroma progression. Previously, we demonstrated that thyronine analogs inhibit Cu(2+)-induced low density lipoprotein (LDL) oxidation. In the present study, we examined the effect of thyronine analogs on endothelial cell (EC)-induced LDL oxidation. LDL was incubated with or without EC in the presence or absence of various concentrations of thyronine, vitamin C, or probucol at 37 degrees in a humidified atmosphere (95% air, 5% CO2). Thyronine analogs, probucol, and vitamin C inhibited EC-induced LDL oxidation in a concentration-dependent manner. The concentration of each agent (microM) producing 50% inhibition (IC50) of EC-induced LDL oxidation for thiobarbituric acid reactive substances (TBARS) and electrophoretic mobility, respectively, was as follows: 0.294 and 0.417 for levothyroxine (L-T4); 0.200 and 0.299 for L-triiodothyronine (L-T3); 0.125 and 0.264 for dextro-thyroxine (D-T4); 0.203 and 0.304 for reversed triiodothyronine (rT3); 1.02 and 1.44 for probucol; and 13.6 and 14.9 for vitamin C. Thyroid binding globulin (TBG) inhibited EC-induced LDL oxidation; further, thyronines bound to TBG exhibited more antioxidant activity than unbound thyronines. Pretreatment of EC with any of the thyronines decreased the ability of EC to oxidize LDL. Also, our results showed that a synergistic interaction exists between vitamin C and T4 in the inhibition of EC-induced LDL oxidation. The T4 and TBG concentrations that inhibited LDL oxidation were in the physiological range. We conclude that T4, like the pharmacological agent probucol, reduces oxidative modification of LDL and thus may act as a natural inhibitor of atherogenesis.

Low density lipoprotein isolated from patients with essential hypertension exhibits increased propensity for oxidation and enhanced uptake by macrophages: a possible role for angiotensin II

In patients with essential hypertension, the increased risk for atherosclerosis is related not only to the blood pressure levels per se, but also to other, unknown, factors. Recent observations have indicated that oxidation of low density lipoprotein (LDL) and macrophage uptake of oxidized LDL are implicated in human atherosclerosis. We tested both the susceptibility of LDL, derived from hypertensive patients, to lipid peroxidation as well as its uptake by macrophages, in comparison with control LDL obtained from healthy subjects. The LDL that was derived from 25 patients with essential hypertension demonstrated increased propensity for lipid peroxidation with a 63%, 91% and 69% elevation in the content of the lipoprotein malondialdehyde, peroxides and conjugated dienes, respectively, in comparison with control LDL. Minimally modified LDL (MM-LDL) (prepared by 6 months' storage of the LDL at 4 degrees C) derived from the hypertensive patients also demonstrated increased lipid peroxidation with a 94%, 130% and 96% elevation in lipoprotein malondialdehyde, peroxides and conjugated dienes, respectively, compared with the control LDL. The susceptibility of the patients' LDL to lipid peroxidation decreased by 32% and 44% (measured as malondialdehyde) after 3 weeks of therapy with the angiotensin converting enzyme inhibitors captopril and enalapril, respectively, with no parallel reduction in the patients' blood pressure. The patients' LDL was shown to contain increased content of lipid peroxides and unsaturated fatty acids, which may explain its increased susceptibility to lipid peroxidation. In vitro experiments revealed that LDL can bind angiotensin II, and that angiotensin II has a stimulatory effect on copper-mediated oxidation of LDL, as well as on LDL degradation by macrophages. These results were secondary to cell-mediated oxidation of the LDL and to its cellular uptake via the scavenger receptor. We conclude that LDL derived from patients with essential hypertension is more susceptible to lipid peroxidation than control LDL, and this may be secondary to angiotensin II stimulation of LDL lipid peroxidation in these patients. Furthermore, this LDL demonstrates enhanced cellular uptake by macrophages in comparison with normal LDL which can also be related to angiotensin II-mediated LDL oxidation. Both these phenomena have been shown to be associated with accelerated atherosclerosis, and thus suggest a new mechanism for increased atherogenecity in hypertensive patients.

Glycosylated low density lipoprotein is more sensitive to oxidation: implications for the diabetic patient?

Oxidised low density lipoprotein (LDL) is considered to be atherogenic. This study examined the relationship between glycosylation and oxidation of LDL from 10 normocholesterolaemic Type 2 diabetic patients, 10 hypercholesterolaemic Type 2 diabetic patients, and 10 normocholesterolaemic non-diabetic subjects. LDL was isolated by sequential ultracentrifugation and susceptibility to oxidation assessed by measuring thiobarbituric reactive substances (TBARS) during a 4-h oxidation period. LDL glycosylation was measured by aminophenylborate gel chromatography. Results demonstrated an increased susceptibility to oxidation in LDL from both diabetic groups, the mean 3-h TBARS values being 35.2 +/- 2.1 and 36.4 +/- 2.6 nmol MDA/mg LDL protein for normocholesterolaemic and hypercholesterolaemic diabetic patients compared with 24.5 +/- 2.5 nmol MDA/mg LDL protein for control subjects. LDL glycosylation of 2.20% +/- 0.11% and 2.89% +/- 0.46% for normocholesterolaemic and hypercholesterolaemic diabetic LDL was significantly higher than that for the non-diabetic control subjects of 1.60% +/- 0.12% (P < 0.02). There was a significant positive correlation (P < 0.005) between LDL glycosylation and LDL oxidation. The esterified/free cholesterol ratio which correlated positively with oxidation (P < 0.01) was significantly higher in LDL from both diabetic groups compared with LDL from control subjects (P < 0.01). Thus the increased incidence of atherosclerosis in diabetes may be related to glycosylation of LDL through its increased susceptibility to oxidation.

Apolipoprotein E and lipoprotein lipase reduce macrophage degradation of oxidized very-low-density lipoprotein (VLDL), but increase cellular degradation of native VLDL

Oxidized low-density lipoprotein (Ox-LDL) has been shown to be taken up by the macrophage-scavenger receptor at an enhanced rate in comparison to native LDL, with consequent cellular cholesterol accumulation. In the present study, we analyzed macrophage interaction with very-low-density lipoprotein (VLDL) from normolipidemic subjects (N-VLDL) that was oxidized in the presence of 10 mumol/L copper ions. Oxidized VLDL (Ox-VLDL) contained increased conjugated dienes and malondialdehyde (MDA) equivalents and showed increased electrophoretic mobility. Gradual fragmentation of VLDL apolipoproteins (apo) was noted, with apo B-100 being the first to be fragmented, followed by apo E and apo C. Degradation of Ox-VLDL by mouse peritoneal macrophages (MPM) was increased almost twofold in comparison to N-VLDL. Upon incubation of VLDL with lipoprotein lipase (LPL), the LPL-treated lipoprotein demonstrated up to 50% increased degradation by macrophages in comparison to control N-VLDL. However, the degradation of LPL-treated Ox-VLDL was decreased by up to 20% in comparison to control Ox-VLDL. Similarly, the addition of apo E to VLDL enhanced its cellular degradation by 56%, whereas a 20% reduction in the degradation of apo E-treated Ox-VLDL was demonstrated in comparison to nontreated Ox-VLDL. These results showed that LPL and apo E, two important regulatory substances in cellular metabolism of plasma lipoproteins, increased macrophage degradation of native VLDL, but reduced the degradation of Ox-VLDL. These inhibitory effects on macrophage uptake of Ox-VLDL suggest that apo E and LPL may possess antiatherogenic potential.