Uptake by J774 Macrophages of Very-Low-Density Lipoproteins Isolated From ApoE-Deficient Mice Is Mediated by a Distinct Receptor and Stimulated by Lipoprotein Lipase

Abstract

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.

Macrophage Phenotype in Mice Deficient in Both Macrophage-Colony–Stimulating Factor (Op) and Apolipoprotein E

“…Recently, a specific and saturable receptor activity for ␤-VLDL derived from apoE-deficient mice was characterized in a murine M cell line. 55 Binding of ␤-VLDL to this receptor is not competed for by LDL or by AcLDL but is competed for by normal VLDL. 55 Recently, hepatic SR-A was overexpressed in transgenic mice to investigate the functional role of these site-specific receptors in clearing potentially atherogenic lipoproteins.…”

“…55 Binding of ␤-VLDL to this receptor is not competed for by LDL or by AcLDL but is competed for by normal VLDL. 55 Recently, hepatic SR-A was overexpressed in transgenic mice to investigate the functional role of these site-specific receptors in clearing potentially atherogenic lipoproteins. 56 The mouse transferrin promoter targeted expression of the bovine SR-A type I to murine liver.…”

Different cellular traffic of LDL-cholesterol and acetylated LDL-cholesterol leads to distinct reverse cholesterol transport pathways

“…AcLDL was prepared as described by Goldstein et al (29). bVLDL was isolated from cholesterol-fed apoE-null mice (30).…”

Sphingomyelinase, an Enzyme Implicated in Atherogenesis, Is Present in Atherosclerotic Lesions and Binds to Specific Components of the Subendothelial Extracellular Matrix

“…9,11,30 Arterial-wall macrophages loaded with large amounts of cholesteryl ester (macrophage foam cells) are thought to play a major role throughout atherogenesis. [31][32][33][34][35][36][37][38] Studies with cultured macrophages have revealed that massive cholesteryl ester accumulation cannot simply be induced by many types of monomeric lipoproteins that are thought to be atherogenic, such as native LDL, 39 certain forms of oxidized LDL, 40 -43 and lipoproteins from atherosclerotic apolipoprotein E-knockout (E0) mice 44 (S.M. et al, unpublished data, 1999).…”