Different expression of modified low density lipoprotein receptors in rabbit peritoneal macrophages and Kupffer cells

123I-Labeled oxLDL Is Widely Distributed Throughout the Whole Body in Mice

Purpose

Oxidized low-density lipoprotein (oxLDL) plays a key role in endothelial dysfunction, vascular inflammation, and atherogenesis. The aim of this study was to assess blood clearance and in vivo kinetics of radiolabeled oxLDL in mice.

Methods

We synthesized ¹²³I-oxLDL by the iodine monochloride method, and performed an uptake study in CHO cells transfected with lectin-like oxLDL receptor-1 (LOX-1). In addition, we evaluated the consistency between the ¹²³I-oxLDL autoradiogram and the fluorescence image of DiI-oxLDL after intravenous injection for both spleen and liver. Whole-body dynamic planar images were acquired 10 min post injection of ¹²³I-oxLDL to generate regional time-activity curves (TACs) of the liver, heart, lungs, kidney, head, and abdomen. Regional radioactivity for those excised tissues as well as the bladder, stomach, gut, and thyroid were assessed using a gamma counter, yielding percent injected dose (%ID) and dose uptake ratio (DUR). The presence of ¹²³I-oxLDL in serum was assessed by radio-HPLC.

Results

The cellular uptakes of ¹²³I-oxLDL were identical to those of DiI-oxLDL, and autoradiograms and fluorescence images also exhibited consistent distributions. TACs after injection of ¹²³I-oxLDL demonstrated extremely fast kinetics. The radioactivity uptake at 10 min post-injection was highest in the liver (40.8 ± 2.4% ID). Notably, radioactivity uptake was equivalent throughout the rest of the body (39.4 ± 2.7% ID). HPLC analysis revealed no remaining ¹²³I-oxLDL or its metabolites in the blood.

Conclusion

¹²³I-OxLDL was widely distributed not only in the liver, but also throughout the whole body, providing insight into the pathophysiological effects of oxLDL.

Metabolism of modified LDL and foam cell formation in murine macrophage-like RAW 264 cells

The uptake of modified low density lipoprotein (LDL) by arterial macrophages is a key event in the atherogenesis. We studied 1) the uptake and degradation of modified LDL, 2) LDL recognition by specific receptors, and 3) the foam cell formation with murine macrophage-like RAW 264 cells in vitro. The cells took up and degraded effectively 125I-labeled acetylated LDL (Ac-LDL) and aggregated LDL (Aggr-LDL). Also oxidized LDL (Ox-LDL) was taken up but it was degraded poorly. The degradation of 125I-Ac-LDL was efficiently competed by both unlabeled Ac-LDL and Ox-LDL, whereas the degradation of 125I-Ox-LDL was partially competed by unlabeled Ox-LDL and Aggr-LDL but not at all by unlabeled Ac-LDL. The incubation with increasing concentrations of Ac-LDL, Aggr-LDL or Ox-LDL resulted in marked foam cell formation in the RAW 264 cells. Ox-LDL was cytotoxic at 500 to 1000 microg/ml concentrations. The results show that RAW 264 cells have at least two classes of receptors for modified lipoproteins: one that recognizes both Ox-LDL and Ac-LDL, and is similar to the scavenger receptors, and another that recognizes Ox-LDL but not Ac-LDL. RAW 264 cells are a convenient model cell line for examining the metabolism of modified lipoproteins, not only that of Ac-LDL but also that of Ox-LDL and Aggr-LDL, and cellular accumulation of lipids derived from modified LDL.

Receptors for oxidized low density lipoprotein

An increasing body of evidence indicates that oxidized low density lipoprotein (LDL) is involved in the pathogenesis of atherosclerosis. One of the first biologic actions of oxidized LDL to be identified in vitro was its ability to interact with the 'acetyl LDL receptor' discovered by Goldstein and Brown. Over the past decade, considerable progress has been made in identifying and characterizing cell-surface receptors for oxidized LDL. Most of these receptors are thought to be multifunctional because they interact with several structurally different ligands, and accordingly have been termed 'scavenger receptors'. The objective of this article is to review the most important publications dealing with structure, ligand specificity, regulation, and function of scavenger receptors.

Oxidized LDLs but not native LDLs augment Ba2+ currents through L-type Ca2+ channels of the A7r5 smooth muscle-derived cell line

The whole-cell patch-clamp method was used on A7r5 smooth muscle-derived cell line, and Ba2+ currents through Ca2+ channels were recorded. The A7r5 cells showed voltage-dependent, long-lasting Ba2+ currents which were markedly inhibited by nifedipine (10 microM). The magnitude of the maximum Ba2+ current (IBa(max)) was augmented by an application of dbcAMP (1 mM), but not affected by TPA (80 nM). Noradrenaline (NA) at 100 microM caused an increase in the IBa(max) by 19.7% in the presence of phentolamine (10 microM). This effect was cancelled by Rp-cAMPs (10 microM). In the presence of propranolol (10 microM), NA tended to reduce the IBa(max). Application of Ox-LDLs at 100 microg protein/ml caused an increase in the IBa(max) by 15.7%, whereas native LDLs did not change the IBa(max). Rp-cAMPs was ineffective to the Ox-LDL action on the IBa(max). In the presence of Ox-LDLs, NA augmented the IBa(max) by 21.4% in the presence of phentolamine. These results suggest that Ox-LDLs activate L-type Ca2+ channels of A7r5 cells by a mechanism independent of cAMP/PKA signalling.

Oxidized or acetylated low density lipoproteins are rapidly cleared by the liver in mice with disruption of the scavenger receptor class A type I/II gene

Oxidized low density lipoprotein (LDL) and acetyl LDL are recognized by the scavenger receptor class A type I/II (SR-AI/II) on macrophages and liver endothelial cells. Several investigators have suggested that there are additional receptors specific for oxidized LDL, but characterization of these alternate receptors for oxidized LDL and evaluation of their quantitative importance in uptake of oxidized LDL has been difficult because of overlapping ligand specificity with SR-AI/II. The purpose of this study was to determine the importance of SR-AI/II in the removal of modified LDL from the bloodstream in vivo. The clearance rate of oxidized LDL from plasma in normal mice was very rapid, and > 90% of injected dose was removed from the blood within 5 min. Clearance rates of oxidized LDL were equally high in SR-AI/II knockout mice, indicating that this receptor is not required for removal of oxidized LDL from plasma. Surprisingly, there was no difference in the clearance rate of acetyl LDL in wild-type and SR-AI/II knockout animals. The plasma clearance of radioiodinated acetyl LDL was almost fully blocked by a 50-fold excess of unlabeled acetyl LDL, but the latter only inhibited oxidized LDL clearance by approximately 5%. Both modified LDLs were cleared mostly by the liver, and there was no difference in the tissue distribution of modified LDL in control and knockout mice. Studies in isolated nonparenchymal liver cells showed that Kupffer cells accounted for most of the uptake of oxidized LDL. Extensively oxidized LDL and LDL modified by exposure to fatty acid peroxidation products were efficient competitors for the uptake of labeled oxidized LDL by SR-AI/II-deficient Kupffer cells, while acetyl LDL and malondialdehyde-modified LDL were relatively poor competitors.

Acetylated LDL endocytosis by the human monocytic Mono Mac 6sr cells is not mediated by the macrophage type I and II scavenger receptors

We recently reported that the human monocytic Mono Mac 6sr cell line constitutively takes up and degrades acetylated (acLDL) and oxidized LDL through receptor-specific pathways. The present studies were undertaken to further characterize the acLDL binding site on a functional and molecular basis. The degradation of acLDL increased during differentiation of Mono Mac 6sr cells with lipopolysaccharide (10 ng/mL, 72 hours) and low concentrations of phorbol 12-myristate 13-acetate (PMA; 0.1 to 1.0 ng/mL, 72 hours). Higher doses of PMA (5 or 10 ng/mL), however, decreased acLDL degradation. Scatchard plots of acLDL binding in untreated and LPS-differentiated Mono Mac 6sr cells were nonlinear and suggested the presence of more than one binding site. Although the ligand specificity of the acLDL receptor in Mono Mac 6sr cells resembles that of the macrophage type I and type II scavenger receptors, we did not detect mRNA of either receptor type in untreated or differentiated Mono Mac 6sr cells by means of Northern blotting and reverse transcription polymerase chain reaction. Furthermore, ligand blotting with 125I-acLDL failed to detect the 220-kD types I and II scavenger receptor protein. Thus, Mono Mac 6sr cells express an acLDL receptor that is distinct from the type I and type II scavenger receptor found in human monocyte-derived macrophages but that, like the latter, is induced during monocytic differentiation.

High affinity saturable uptake of oxidized low density lipoprotein by macrophages from mice lacking the scavenger receptor class A type I/II

Oxidation of low density lipoproteins (LDL) has been implicated as a causal factor in the pathogenesis of atherosclerosis. Oxidized LDL has been found to exhibit numerous potentially atherogenic properties in vitro, including receptor-mediated uptake by macrophages. Oxidized LDL is a ligand for the class A scavenger receptor type I/II (SR-AI/II), but cross-competition studies with cultured macrophages suggested that there is an additional receptor(s) that is specific for oxidized LDL and that does not interact with acetyl LDL or other chemically modified LDL. A number of macrophage membrane proteins, including CD36, FcgammaRII-B2, scavenger receptor BI, and macrosialin/CD68, have been found to bind to oxidized LDL in vitro and have been proposed as candidate oxidized LDL receptors. However, because of overlapping ligand specificity with the SR-AI/II, it has been difficult to evaluate the relative importance of these proteins in the uptake of oxidized LDL by macrophages. In the present report, we have studied the uptake and degradation of oxidized LDL by macrophages from mice in which the SR-AI/II gene had been disrupted. The uptake of acetyl LDL was reduced by more than 80% in macrophages from scavenger receptor knockout mice, confirming that most of the uptake of acetyl LDL by macrophages can be attributed to this receptor. In contrast, the uptake of extensively oxidized LDL was reduced by only 30% and showed high affinity, saturable uptake with apparent Km of about 5 microg/ml, similar to that of the SR-AI/II. This indicates that about 70% of the uptake of oxidized LDL in macrophages is attributable to an alternate oxidized LDL receptor(s). In contrast to findings reported with CD36, mildly oxidized LDL was internalized much more slowly than extensively oxidized LDL. Unlabeled oxidized LDL, polyinosinic acid, phosphatidylserine-rich liposomes, and LDL or bovine albumin modified by fatty acid oxidation products were effective competitors for the uptake of radioiodinated oxidized LDL by macrophages from knockout mice, whereas acetyl LDL and malondialdehyde-modified LDL were relatively poor competitors. This ligand specificity differs from that of CD36-related (class B) scavenger receptors but is similar to the reported specificity of macrosialin/CD68 in ligand blots. However, the rate of uptake of oxidized LDL by knockout macrophages was not increased by phorbol ester or in thioglycollate-elicited macrophages, both of which are expected to increase the amount of macrosialin on the cell surface. In macrophages from SR-AI/II knockout mice, ligand blots of membrane proteins with iodinated, oxidized, or acetylated LDL revealed several bands, with apparent molecular size on SDS-polyacrylamide gel electrophoresis of 60, 94, 124, and 210 kDa, but none of the bands were specific for oxidized LDL. These results provide direct evidence that a receptor other than SR-AI/II is responsible for most of the uptake of oxidized LDL in murine macrophages, but further studies are needed to identify the receptor(s) involved.

Mechanism of uptake of copper-oxidized low density lipoprotein in macrophages is dependent on its extent of oxidation

Several investigators have reported nonreciprocal cross-competition between unlabeled acetyl low density lipoprotein (LDL) and oxidized LDL for the degradation of the corresponding labeled LDLs. The failure of acetyl LDL to compete fully for the degradation of oxidized LDL has been interpreted as evidence for additional receptor(s) specific for oxidized LDL. In the present study, it is demonstrated that the ability of oxidized LDL to compete for the degradation of acetyl LDL is determined largely by its extent of oxidation. Extensively oxidized LDL competed for 90% of acetyl LDL degradation in murine macrophages, and hence there appears to be no pathway in these cells that is specific for acetyl LDL but not oxidized LDL. The reciprocal situation (competition by acetyl LDL for uptake and degradation of oxidized LDL) proved to be more complicated. Oxidized LDL is known to be susceptible to aggregation, and less than half of the aggregates found in the present experiments were large enough to be removed by filtration or centrifugation at 10,000 x g. When oxidized LDL was prepared under conditions that resulted in minimal aggregation, acetyl LDL competed for greater than 80% of oxidized LDL degradation. With more extensive oxidation and aggregation of LDL, acetyl LDL only competed for about 45% of oxidized LDL degradation, while polyinosinic acid remained an effective competitor. Individual preparations of oxidized LDL that differed in degree of oxidation were separated into aggregated and nonaggregated fractions, and it was shown that both fractions were competed to a similar degree by acetyl LDL in mouse peritoneal macrophages and in Chinese hamster ovary cells transfected with human scavenger receptor type I cDNA. Hence, aggregation by itself did not alter the apparent rate of uptake by the scavenger receptor pathway. These results indicate that the extent of oxidation of LDL affects its mechanism of uptake and that about half of the uptake of very extensively oxidized LDL appears to be via a pathway distinct from the scavenger receptor type I/II. The uptake of very extensively oxidized LDL was not affected by cytochalasin D, an inhibitor of phagocytosis. As well, it was not affected by an antibody to CD36 in human monocyte-derived macrophages or in THP-1 cells, suggesting that this alternate pathway does not involve CD36.

Decrease in scavenger receptor expression in human monocyte-derived macrophages treated with granulocyte macrophage colony-stimulating factor

To determine whether scavenger receptors are susceptible to regulation by granulocyte macrophage colony-stimulating factor (GM-CSF), a macrophage-specific cytokine, human monocytes were differentiated into macrophages in the absence or presence of 20 U/mL GM-CSF. Binding, uptake, and degradation of acetylated LDL (Ac-LDL) and oxidized LDL (Ox-LDL) were measured. Treatment with GM-CSF resulted in a significant twofold to threefold decrease in the number of binding sites for Ac-LDL and Ox-LDL on the surface of macrophages without affecting the affinity of the receptor for these ligands. Competition experiments revealed that two binding sites were responsible for the recognition and uptake of Ac-LDL; one specific for Ac-LDL and one that recognized both Ac-LDL and Ox-LDL. No binding site specific for Ox-LDL could be detected in either control or GM-CSF-treated macrophages. Treatment of human monocyte-derived macrophages with GM-CSF resulted in a decrease of the Ac-LDL/Ox-LDL receptor but did not affect the binding site specific for Ac-LDL. Northern blot analysis showed that mRNA levels of both types I and II scavenger receptor were reduced in macrophages differentiated in the presence of GM-CSF. Human macrophages that were differentiated in the presence of GM-CSF accumulated approximately 50% fewer cholesteryl esters. Taken together, these results indicate that GM-CSF can downregulate both types I and II scavenger receptor in human monocyte-derived macrophages, which might have implications for foam cell formation.