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Increased selective uptake in vivo and in vitro of oxidized cholesteryl esters from high-density lipoprotein by rat liver parenchymal cells.

ABSTRACT: Oxidation of low-density lipoprotein (LDL) leads initially to the formation of LDL-associated cholesteryl ester hydroperoxides (CEOOH). LDL-associated CEOOH can be transferred to high-density lipoprotein (HDL), and HDL-associated CEOOH are rapidly reduced to the corresponding hydroxides (CEOH) by an intrinsic peroxidase-like activity. We have now performed in vivo experiments to quantify the clearance rates and to identify the uptake sites of HDL-associated [3H]Ch18:2-OH in rats. Upon injection into rats, HDL-associated [3H]Ch18:2-OH is removed more rapidly from the circulation than HDL-associated [3H]Ch18:2. Two minutes after administration of [3H]Ch18:2-OH-HDL, 19.6 +/- 2.6% (S.E.M.; n = 4) of the label was taken up by the liver as compared with 2.4 +/- 0.25% (S.E.M.; n = 4) for [3H]Ch18:2-HDL. Organ distribution studies indicated that only the liver and adrenals exhibited preferential uptake of [3H]Ch18:2-OH as compared with [3H]Ch18:2, with the liver as the major site of uptake. A cell-separation procedure, employed 10 min after injection of [3H]Ch18:2-OH-HDL or [3H]Ch18:2-HDL, demonstrated that within the liver only parenchymal cells take up HDL-CE by the selective uptake pathway. Selective uptake by parenchymal cells of [3H]Ch18:2-OH was 3-fold higher than that of [3H]Ch18:2, while Kupffer and endothelial cell uptake of the lipid tracers reflected HDL holoparticle uptake (as analysed with iodinated versus cholesteryl ester-labelled HDL). The efficient uptake of [3H]Ch18:2-OH by parenchymal cells was coupled to a 3-fold increase in rate of radioactive bile acid secretion from [3H]Ch18:2-OH-HDL as compared with [3H]Ch18:2-HDL. In vitro studies with freshly isolated parenchymal cells showed that the association of [3H]Ch18:2-OH-HDL at 37 degrees C exceeded [3H]Ch18:2-HDL uptake almost 4-fold. Our results indicate that HDL-associated CEOH are efficiently and selectively removed from the blood circulation by the liver in vivo. The selective liver uptake is specifically exerted by parenchymal cells and coupled to a rapid biliary secretion pathway. The liver uptake and biliary secretion route may allow HDL to function as an efficient protection system against potentially atherogenic CEOOH.

SUBMITTER: Fluiter K

PROVIDER: S-EPMC1217792 | biostudies-other | 1996 Oct

REPOSITORIES: biostudies-other

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Project description:We have observed recently that high-density lipoproteins (HDL) are the predominant carriers of cholesteryl ester hydroperoxides (CEOOH), the major class of lipid hydroperoxides detectable at nanomolar concentrations in the plasma of healthy fasting humans. The present study investigates the effect of such very low levels of CEOOH in apolipoprotein E-free HDL3 on lipoprotein particle metabolism and 'selective uptake' of its CE by human Hep G2 cells. Minimal oxidation with aqueous peroxyl radicals had a negligible effect on the binding, internalization and degradation of 125I-labelled HDL3. In contrast, with an increasing degree of radical-mediated oxidation of labelled HDL3, [3H]cholesteryl linoleate ([3H]Ch18:2) was taken up at an increasingly greater rate than were 125I-apoproteins. When [3H]cholesteryl linoleate hydroperoxide ([3H]Ch18:2-OOH was incorporated into unoxidized HDL3 by exchange from donor liposomes, it was taken up at a more than 8-fold higher rate than was incorporated [3H]Ch18:2. The same degree of preferential uptake of oxidized CE was observed when HDL3 was used that was doubly labelled with [3H]Ch18:2-OOH and cholesteryl [14C]oleate ([14C]Ch18:1). In both situations, uptake of [3H]Ch18:2-OOH exceeded that of 125I-apolipoprotein A-I some 40-fold. This increased selective uptake of [3H]Ch18:2-OOH from very mildly oxidized HDL3 was accompanied by a parallel increase in the intracellular levels of labelled free cholesterol. In contrast, lipid hydroperoxides were not detectable within Hep G2 cells, suggesting efficient detoxification of CEOOH by these cells. Neither the increased selective uptake of Ch18:2-OOHs nor the levels of intracellular free cholesterol were influenced by the presence of 50 microM chloroquine, suggesting extralysosomal hydrolysis of oxidized CEs. These results show that the selective uptake of HDL CEOOH by Hep G2 cells is more efficient than that of unoxidized CE, and support a protective role for rapid selective uptake in the removal of circulating HDL CEOOH.

Project description:Cholesteryl esters (CEs) are the water-insoluble transport and storage form of cholesterol. For both transport and storage, phospholipids and proteins embrace the CEs to form an amphipathic monolayer that surrounds the CEs. CEs are transported extracellularly in lipoproteins and are stored intracellularly as cytoplasmic lipid droplets. To clarify the molecular phenomena related to the above structures, we conducted atomistic molecular-dynamics simulations for a spherical, approximately high density lipoprotein sized lipid droplet comprised of palmitoyl-oleoyl-phosphatidylcholine (POPC) and cholesteryl oleate (CO) molecules. An additional simulation was conducted for a lamellar lipid trilayer consisting of the same lipid constituents. The density profiles showed that COs were located in the core of the spherical droplet. In trilayer simulations, CO molecules were also in the core and formed two denser strata. This is remarkable because the intra- and intermolecular behaviors of the COs were similar to previous findings from bulk COs in the fluid phase. In accordance with previous experimental studies, the solubility of COs in the POPC monolayers was found to be low. The orientation distribution of the sterol moiety with respect to the normal of the system was found to be broad, with mainly isotropic or slightly parallel orientations observed deep in the core of the lipid droplet or the trilayer, respectively. In both systems, the orientation of the sterol moiety changed to perpendicular with respect to the normal close to the phopsholipid monolayers. Of interest, within the POPC monolayers, the intramolecular conformation of the COs varied from the previously proposed horseshoe-like conformation to a more extended one. From a metabolic point of view, the observed solubilization of CEs into the phospholipid monolayers, and the conformation of CEs in the phospholipid monolayers are likely to be important regulatory factors of CE transport and hydrolysis.

Project description:BACKGROUND:Macrophage accumulation in the vascular wall is a hallmark of atherosclerosis. Studies showed that shifting of oxidized lipids-induced inflammatory macrophages towards an anti-inflammatory phenotype by promoting oxidative metabolism attenuated atherosclerosis progression. Therefore, this study aimed to investigate whether metformin, which has ameliorated atherosclerosis in animal models and clinical trials, modulated oxidized low-density lipoprotein (Ox-LDL) induced inflammatory status in macrophages by regulating cellular oxidative metabolism. METHODS:Murine raw264.7 macrophages were incubated with Ox-LDL (50 μg/mL) in the presence or absence of metformin (15 μmol/L) for 24 h. Real-time polymerase chain reaction was used to quantify the transcription of classically activated (M1) pro-inflammatory and alternatively activated (M2) anti-inflammatory markers and mitochondrial DNA copy numbers. Cellular reactive oxygen species (ROS) production and mitochondrial membrane potential were detected by immunofluorescence. Cellular adenosine triphosphate (ATP) synthesis, glucose uptake, and lactic acid production were measured by commercial kit and normalized to cellular lysates. Western blotting analysis was performed to detect the expression of mitochondrial fusion/fission related proteins, enzymes mediating lipid metabolism and signaling pathway of glucose transport. Differences between groups were analyzed using one-way analysis of variance. RESULTS:Metformin improved Ox-LDL-impaired anti-inflammatory phenotype in raw264.7 macrophages as shown by up-regulated transcription of anti-inflammatory markers including interleukin 10 (0.76 ± 0.04 vs. 0.94 ± 0.01, P = 0.003) and Resistin-like molecule alpha (0.67 ± 0.08 vs. 1.78 ± 0.34, P = 0.030). Conversely, Ox-LDL-diminished phosphorylation of Akt was up-regulated by metformin treatment (0.47 ± 0.05 vs. 1.02 ± 0.08, P = 0.040), associated with an improvement of mitochondrial function, characterized by decreased ROS generation (2.50 ± 0.07 vs. 2.15 ± 0.04, P = 0.040), increased lipid oxidation, and elevated cellular ATP production (0.026 ± 0.001 vs. 0.035 ± 0.003, P = 0.020). Moreover, metformin-mediated Akt activation increased Akt substrate of 160 kDa (AS160) phosphorylation (0.51 ± 0.04 vs. 1.03 ± 0.03, P = 0.0041), promoted membrane translocation of glucose transporter 1, and increased glucose influx into the cells (4.78 ± 0.04 vs. 5.47 ± 0.01, P < 0.001). CONCLUSION:This study suggested that targeting macrophage metabolism with new or existing drugs had therapeutic potential for the prevention and treatment of diabetes-accelerated atherosclerosis.

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Increased selective uptake in vivo and in vitro of oxidized cholesteryl esters from high-density lipoprotein by rat liver parenchymal cells.

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