Use of an anti-low density lipoprotein receptor antibody to quantify the role of the LDL receptor in the removal of chylomicron remnants in the mouse in vivo.

We have reported that transgenic mice overexpressing rat apo E shows marked reduction of plasma cholesterol and triglyceride levels due to the disappearance of VLDL and LDL. In this study, we investigated the metabolism of plasma lipoproteins in transgenic mice. After intravenous injection, the rates of clearance of '25I-VLDL and '25I-LDL were 3.0-and 2.4-fold greater in transgenic mice than in controls, respectively. Furthermore, clearance of chylomicron remnants estimated by oral retinyl palmitate-loading test was markedly enhanced in transgenic mice. The hepatic expression of LDL receptors by immunoblot analysis was similar in both groups. These data suggest that elimination of lipoproteins containing apo B was due to enhanced clearance of these lipoproteins enriched with apo E through hepatic LDL receptors. When fed a high cholesterol diet, controls showed twofold elevation of plasma cholesterol levels with marked increases in VLDL and LDL cholesterol on gel filtration chromatography. In contrast, cholesterol-fed transgenic mice showed resistance against these increases. High cholesterol feeding decreased the activity of hepatic LDL receptors and had no effect on enhancement of chylomicron remnant clearance in transgenic mice. Thus, overexpression of apo E facilitates metabolism of lipoproteins containing apo B presumably primarily via the LDL receptor pathway and possibly through an interaction with the chylomicron remnant receptor. (J. Clin.

Section: Discussionmentioning

confidence: 84%

Plasma lipoprotein metabolism in transgenic mice overexpressing apolipoprotein E. Accelerated clearance of lipoproteins containing apolipoprotein B.

Shimano¹,

Yamada²,

Katsuki³

et al. 1992

“…In transgenic animals, nearly all of this enhanced uptake occurred in the liver, consistent with earlier investigations that demonstrated chylomicrons with excess apo E are cleared preferentially by the liver (41, 43, 50). The major site for remnant removal is the space of Disse in the liver, where apo E directs binding to at least three different entities: the LDL receptor (53)(54)(55), the LRP (56), and HSPG (8,52,57). The LDL receptor accounts for more than half of remnant uptake (53), and it has a preference for particles smaller than 50 nm in diameter (58).…”

Section: Discussionmentioning

confidence: 99%

Increased expression of apolipoprotein E in transgenic rabbits results in reduced levels of very low density lipoproteins and an accumulation of low density lipoproteins in plasma.

Fan¹,

Ji²,

Huang³

et al. 1998

Transgenic rabbits expressing human apo E3 were generated to investigate mechanisms by which apo E modulates plasma lipoprotein metabolism. Compared with nontransgenic littermates expressing ‫ف‬ 3 mg/dl of endogenous rabbit apo E, male transgenic rabbits expressing ‫ف‬ 13 mg/dl of human apo E had a 35% decrease in total plasma triglycerides that was due to a reduction in VLDL levels and an absence of large VLDL. With its greater content of apo E, transgenic VLDL had an increased binding affinity for the LDL receptor in vitro, and injected chylomicrons were cleared more rapidly by the liver in transgenic rabbits. In contrast to triglyceride changes, transgenic rabbits had a 70% increase in plasma cholesterol levels due to an accumulation of LDL and apo E-rich HDL. Transgenic and control LDL had the same binding affinity for the LDL receptor. Both transgenic and control rabbits had similar LDL receptor levels, but intravenously injected human LDL were cleared more slowly in transgenic rabbits than in controls. Changes in lipoprotein lipolysis did not contribute to the accumulation of LDL or the reduction in VLDL levels. These observations suggest that the increased content of apo E3 on triglyceride-rich remnant lipoproteins in transgenic rabbits confers a greater affinity for cell surface receptors, thereby increasing remnant clearance from plasma. The apo E-rich large remnants appear to compete more effectively than LDL for receptormediated binding and clearance, resulting in delayed clearance and the accumulation of LDL in plasma. ( J. Clin. Invest. 1998.

“…The existence of a separate chylomicron remnant receptor was suggested by observations that humans (5), rabbits (6), and mice (7) lacking the LDLR do not significantly accumulate chylomicron remnants in plasma. On the other hand, antibodies to the LDLR partially inhibit chylomicron remnant removal in mice (8), indicating an important physiologic role for the LDLR in remnant clearance. The LRP binds specifically to apo E, thus suggesting a role for the LRP in chylomicron remnant clearance (9).…”

Section: Introductionmentioning

confidence: 98%

Hepatic apo E expression is required for remnant lipoprotein clearance in the absence of the low density lipoprotein receptor.

Linton¹,

Hasty²,

Babaev³

et al. 1998

According to the secretion-capture model of remnant lipoprotein clearance, apo E secreted by hepatocytes into the space of Disse serves to enrich the remnants with a ligand for receptor-mediated lipoprotein endocytosis. Current evidence supports a two-receptor model of lipoprotein removal, in which apo E-containing remnants bind either the low density lipoprotein receptor (LDLR) or the LDLR-related protein (LRP). Recently, we demonstrated that reconstitution of apo E( Ϫ / Ϫ ) mice with apo E( ϩ / ϩ ) marrow results in normalization of plasma lipoprotein levels, indicating that hepatic expression of apo E is not required for remnant clearance and calling into question the relevance of the secretion-capture mechanism. To dissect the relative contributions of LDLR and LRP to the cellular catabolism of remnant lipoproteins by the hepatocyte, bone marrow transplantation (BMT) was used to reconstitute macrophage expression of apo E in mice that were null for expression of both apo E and the LDLR. Reconstitution of macrophage apo E in apo E( Ϫ / Ϫ )/LDLR( Ϫ / Ϫ ) mice had no effect on serum lipid and lipoprotein concentrations, although it produced plasma apo E levels up to 16-fold higher than in C57BL/6 controls. Immunocytochemistry of hepatic sections revealed abundant staining for apo E in the space of Disse, but no evidence of receptor-mediated endocytosis of remnant lipoproteins. Transient expression of human LDLR in the livers of apo E( ϩ / ϩ ) → apo E( Ϫ / Ϫ )/LDLR( Ϫ / Ϫ ) mice by adenoviral gene transfer resulted in normalization of serum lipid levels and in the clearance of apo E-containing lipoproteins from the space of Disse. We conclude that whereas the LDLR efficiently clears remnant lipoproteins irrespective of the site of origin of apo E, endocytosis by the chylomicron remnant receptor (LRP) is absolutely dependent on hepatic expression of apo E. These data demonstrate in vivo the physiologic relevance of the apo E secretion-capture mechanism in the liver. ( J. Clin. Invest. 1998. 101:1726-1736.)