Apolipoprotein E (apoE) is an important determinant for the uptake of triglyceride-rich lipoproteins and emulsions by the liver, but the intracellular pathway of apoE following particle internalization is poorly defined. In the present study, we investigated whether retroendocytosis is a unique feature of apoE as compared with apoB by studying the intracellular fate of very low density lipoprotein-sized apoE-containing triglyceride-rich emulsion particles and LDL after LDLrmediated uptake. Incubation of HepG2 cells with

Lipoprotein lipase (LPL) stimulates the uptake of low-density lipoprotein (LDL) and very-low-density lipoprotein (VLDL) in different cell types, including macrophages, through bridging of LPL between lipoproteins and extracellular heparan sulphate proteoglycans (HSPG). Because macrophages produce LPL and because modified lipoproteins are present in the arterial wall in vivo, we wondered whether LPL also enhances the uptake of oxidized LDL by J774 macrophages. LDL samples with different degrees of oxidation, as evaluated by relative electrophoretic mobility (REM) as compared with native LDL are used as well as native and acetylated LDL. Addition of 5 microg/ml LPL to the J774 cell culture medium stimulated the binding of both native LDL and moderately oxidized LDL (REM < 3.5) 50-100-fold, and their uptake was stimulated approx. 20-fold. The LPL-mediated binding of native LDL and moderately oxidized LDL was dose-dependent. Preincubation of the cells with heparinase (2.4 units/ml) inhibited the stimulatory effect of LPL, indicating that this LPL-mediated stimulation was due to bridging between the lipoproteins and HSPG. The binding to J774 macrophages of severely oxidized LDL (REM=4.3) was stimulated less than 3-fold by LPL, whereas its uptake was not stimulated significantly. The binding and uptake of acetylated LDL (AcLDL) were not stimulated by LPL, although the LPL-molecule itself does bind to AcLDL. Measurements of the cellular lipid content showed that addition of LPL also stimulated the accumulation in the cells of cholesteryl ester derived from both native LDL and moderately oxidized LDL in a dose-dependent manner. We conclude that our results present experimental evidence for the hypothesis that LPL serves as an atherogenic component in the vessel wall.

Apolipoprotein (apo) E-deficient mice display marked accumulation in the plasma of VLDL deficient in both apoE and apoB100 but containing apoB48, apoA-I, apoCs, and apoA-IV. Since apoE-deficient mice develop severe atherosclerotic lesions with lipid-laden macrophages, we reasoned that the uptake of lipoproteins by intimal macrophages can take place in the absence of both apoE and apoB100. To get more insight into the mechanism of foam cell formation in apoE-deficient mice, we measured the interaction of VLDL from apoE-deficient mice (apoEnull VLDL) with the murine macrophage cell line J774. Scatchard analysis revealed that apoEnull VLDL is bound to J774 cells with a Kd value comparable to that of control VLDL (8.1 versus 4.7 micrograms/mL) and with a Bmax value about half that of control VLDL (40 versus 70 ng/mg cell protein, respectively). ApoEnull VLDL is also taken up and degraded by J774 macrophages via a high-affinity process less efficiently than control mouse VLDL (6-fold and 50-fold less efficiently, respectively). In line with this observation, incubation of J774 cells with 50 micrograms/mL apoEnull VLDL for 24 hours resulted in an increase in intracellular cholesteryl ester (CE) content, although 5-fold less pronounced than after incubation with 50 micrograms/mL control mouse VLDL. Under the conditions applied, simultaneous addition of 5 micrograms/mL lipoprotein lipase (LPL) stimulated the cellular uptake and degradation of apoEnull VLDL about 10-fold and resulted in a 5-fold stimulation of the intracellular CE accumulation, from 9 +/- 2 to 46 +/- 5 micrograms CE per milligram cell protein. In contrast to control mouse VLDL, apoEnull VLDL could not compete with 125I-labeled LDL for binding to the LDL receptor of J774 cells. Furthermore, neither LDL nor acetylated LDL could compete with 125I-labeled apoEnull VLDL for binding to these cells, whereas control mouse VLDL, VLDL from a hypertriglyceridemic patient, and apoEnull VLDL itself were efficient competitors. Thus, VLDL from apoE-deficient mice is taken up by J774 macrophages through recognition by a distinct receptor, which could be the triglyceride-rich lipoprotein receptor. We conclude that in apoE-deficient mice, foam cell formation occurs via a receptor-mediated uptake of apoEnull VLDL, which can be stimulated by the presence of LPL.

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Abstract-The present study examined the relative contributions of the different pathways by which oxidatively modified VLDL (oxVLDL) promotes the uptake and intracellular accumulation of lipids in J774 macrophages. VLDL was oxidized for a maximum of 4 hours, resulting in an increase in thiobarbituric acid-reactive substances and an increased electrophoretic mobility on agarose gel. The lipid composition of the relatively moderately oxidized VLDL samples did not differ significantly from that of nonoxidized VLDL samples. The uptake of 125 I-labeled VLDL by the J774 cells increased with oxidation time and was completely blocked on coincubation with polyinosinic acid (PolyI), indicating that oxVLDL is taken up by the cells via the scavenger receptor only. Despite the 2-fold increased uptake of oxVLDL protein, the cell association of triglyceride (TG)-derived fatty acids by the J774 macrophages after incubation with oxVLDL was only 50% of that with native VLDL. In line with these observations, the induction of de novo synthesis of TG by J774 cells was Ϸ3-fold less efficient after incubation with oxVLDL than after incubation with native VLDL. The induction of de novo synthesis of TG with oxVLDL was even further decreased on simultaneous incubation with PolyI, whereas PolyI did not affect the native VLDL-induced TG synthesis. These results indicate that oxVLDL induces endogenous TG synthesis predominantly through particle uptake via the scavenger receptor and much less via the extracellular lipoprotein lipase (LPL)-mediated hydrolysis of TG, as is the case for native VLDL. In line with these observations, we showed that the suitability of VLDL as a substrate for LPL decreases with oxidation time. Addition of oxVLDL to the LPL assay did not interfere with the lipolysis of native VLDL. However, enrichment of the oxidized lipoprotein particle with native apoC2 was able to fully restore the impaired lipolysis. Thus, from these studies it can be concluded that on oxidation, VLDL becomes less efficient in inducing TG accumulation in J774 cells as a consequence of a defect in apoC2 as an activator for the LPL-mediated extracellular lipolysis. (1) the receptor-mediated uptake of intact VLDL particles and (2) the direct uptake of free fatty acids (FFAs) as generated by the extracellular lipoprotein lipase (LPL)-mediated hydrolysis of VLDL-TG, followed by intracellular reesterification into lipids. The resulting cholesterol-enriched remnant particles are thereafter taken up via a receptor-mediated process.In analogy with LDL, 5,6 it has been shown that in vitro exposure of ␤-VLDL to endothelial cells causes oxidation of this lipoprotein. 7,8 This results in a 2-to 3-fold increased degradation by mouse peritoneal macrophages 7 and rabbit smooth muscle cells 8 compared with unoxidized ␤-VLDL and in a 2-to 3-fold increased intracellular cholesterol esterification rate. Isolated human VLDL, like human LDL, was shown to be effectively oxidized in vitro on incubation with free radicals. 9 The degradation of oxidized human VLDL (oxVLD...