Biomedical subjects

M F Linton

Publications and source records attributed to M F Linton.

At least 19 recordsLinked to original sources

PubMed2001

Isoform-specific effects of apolipoprotein E on atherogenesis: gene transduction studies in mice.

BACKGROUND: We recently used a bone marrow-based gene therapy approach to show that small amounts of retrovirus-derived human apolipoprotein E3 (apoE3) produced by macrophages are protective against early atherosclerosis in apoE-deficient mice. METHODS AND RESULTS: In the present study, we evaluated whether the effect produced by macrophage-derived apoE3 is related to its ability to bind cellular membranes. To this end, we used apoE2 and apoEcys142, dysfunctional human variants with reduced binding to the LDL receptor or to heparan sulfate proteoglycans, respectively. ApoE-deficient mice, 5 weeks of age, received transplants of apoE(-/-) bone marrow cells transduced with either parental retrovirus or apoE3, apoE2, or apoEcys142 retroviral vectors. Human apoE was detected by ELISA in the serum of apoE3, apoE2, and apoEcys142 mice as early as 4 weeks after bone marrow transplantation, and at 8 weeks, plasma apoE levels were 55.5+/-20.3, 50.5+/-8.7, and 15.3+/-7.3 microgram/dL, respectively. In all groups, cholesterol levels increased with age but were not affected by apoE expression. As previously demonstrated, the lesion area in male apoE3 mice (3808+/-2224 micrometer(2)/section) was 40% smaller than that in control mice (6503+/-3475 micrometer(2)/section). In apoE2 mice, however, the lesion area was similar to that of controls (5991+/-2771 micrometer(2)/section), and apoEcys142 mice showed an unexpected and significant increase in lesion size (10 320+/-6128 micrometer(2)/section). Thus, transplantation with marrow transfected with receptor binding-defective apoE variants did not replicate the antiatherogenic effect of apoE3. CONCLUSIONS: These data provide in vivo evidence suggesting that macrophage-derived apoE delays development of atherosclerosis through a receptor-dependent pathway.

Animals↗

PubMed2001

Retrovirus-mediated expression of apolipoprotein A-I in the macrophage protects against atherosclerosis in vivo.

We have previously reported that the lack of apolipoprotein (apo) E expression by macrophages promotes foam cell formation in vivo. Because transgenic mice overexpressing human apoA-I from the liver (h-apoA-I TgN) are protected from the atherogenesis induced by apoE deficiency, we hypothesized that the presence of apoA-I in the vessel wall could reduce the negative effect of apoE deficiency on lesion growth. To address this issue, we used both retroviral transduction and transgenic approaches to produce in vivo systems where apoA-I is expressed from macrophages. In the retroviral transduction study, apoA-I-deficient (apoA-I(-/-)) mice reconstituted with apoE-deficient (apoE(-/-)) bone marrow cells that were infected with a retroviral vector expressing human apoA-I (MFG-HAI) had 95% lower atherosclerotic lesion area than that of recipients of apoE(-/-) bone marrow cells infected with the parental virus (MFG). To determine whether the protective effect of locally produced apoA-I was due to the lack of systemic apoA-I, we conducted a different experiment using h-apoA-I TgN mice as recipients of apoE(-/-) bone marrow with or without human apoA-I (driven by a macrophage-specific transgene defined as mphi-AI). Aortic lesion area in apoE(-/-)/mphi-AI --> h-apoA-I TgN mice was decreased by 85% compared with apoE(-/-) --> h-apoA-I TgN mice. These data demonstrate that expression of apoA-I from macrophages protects against atherogenesis without affecting plasma apoA-I and high density lipoprotein cholesterol levels.

Animals↗

PubMed2001

The inflamed plaque: cytokine production and cellular cholesterol balance in the vessel wall.

Although the concept that inflammation plays a role in the biology of atherosclerosis is now well accepted, the basic feature of the arterial lesion remains the accumulation of clusters of foam cells. These clusters are the consequence of the enhanced recruitment of monocytes in the vessel wall induced by the hyperlipidemia and of the disproportionate accumulation of lipids in the cytoplasm of macrophages deriving from monocytes. Ultimately, every molecular force and pathway with modulating activity over the developing lesion will have to act on a convergence point with factors regulating cholesterol balance in the macrophage. Consistent with this view is the recent report that cytokines, such as tumor necrosis factor-alpha, can influence the expression of the scavenger receptor, whereas interferon-gamma can inhibit adenosine triphosphate-binding cassette transporter-1, the main effector of cholesterol efflux in the peripheral cell. Conversely, recent data have shown that primary alterations in macrophage cholesterol balance, such as those produced by the total absence of acylcoenzyme A:cholesterol acyltransferase-1, may determine local changes compatible with the activation of inflammatory pathways. In this brief review, we discuss some of the convergence points between inflammation and cholesterol balance, and we highlight the additional therapeutic targets suggested by these new developments in vascular biology.

Animals↗

PubMed2001

A recycling pathway for resecretion of internalized apolipoprotein E in liver cells.

We have investigated the recycling of apoE in livers of apoE(-)/- mice transplanted with wild type bone marrow (apoE(+/+) --> apoE(-)/-), a model in which circulating apoE is derived exclusively from macrophages. Nascent Golgi lipoproteins were recovered from livers of apoE(+/+) --> apoE(-)/- mice 8 weeks after transplantation. ApoE was identified with nascent d < 1.006 and with d 1.006-1.210 g/ml lipoproteins at a level approximately 6% that of nascent lipoproteins from C57BL/6 mice. Hepatocytes from apoE(+/+) --> apoE(-)/- mice were isolated and cultured in media free of exogenous apoE. ApoE was found in the media primarily on the d < 1.006 g/ml fraction, indicating a resecretion of internalized apoprotein. Secretion of apoE from C57BL/6 hepatocytes was consistent with constitutive production, whereas the majority of apoE secreted from apoE(+/+) --> apoE(-)/- hepatocytes was recovered in the last 24 h of culture. This suggests that release may be triggered by accumulation of an acceptor, such as very low density lipoproteins, in the media. In agreement with the in vivo data, total recovery of apoE from apoE(+/+) --> apoE(-)/- hepatocytes was approximately 6% that of the apoE recovered from C57BL/6 hepatocytes. Since plasma apoE levels in the transplanted mice are approximately 10% of control levels, the findings indicate that up to 60% of the internalized apoE may be reutilized under physiologic conditions. These studies provide definitive evidence for the sparing of apoE and its routing through the secretory pathway and demonstrate that internalized apoE can be resecreted in a quantitatively significant fashion.

Animals↗

PubMed2001

Mouse models of hyperlipidemia and atherosclerosis.

The mouse is the most utilized model to study lipids and atherosclerosis. Before the advent of the techniques of genetic manipulation it was well known that different inbred strains had varying degrees of susceptibility to diet-induced atherosclerosis. The C57BL/6 mouse was adopted as the standard even though the arterial lesions induced by even extreme diets were limited in size, complexity, and distribution. This changed with the production of several gene knockout and transgenic mice, which in many cases produced remarkable effects on plasma lipoproteins and arterial lesions even without dietary manipulations. The most typical example remains the apoE deficient model, in which a massive hyperlipidemia is accompanied with the development of severe atherosclerotic plaques at the aortic root and throughout the aortic tree. With the creation of the LDL receptor knockout, and the different knockout and transgenic mice with changes in apoB, apoE, and the HDL system, a solid body of new information has emerged on the mechanisms regulating plasma lipoprotein levels and controlling the initial stages of atherogenesis. This paper presents an overview of the most utilized mouse models and a summary of the results obtained with the technique of bone marrow transplantation, an approach to study macrophages in atherosclerosis.

Animals↗

PubMed2001

Peroxisome proliferator-activated receptor-gamma agonist troglitazone protects against nondiabetic glomerulosclerosis in rats.

BACKGROUND: Peroxisome proliferator-activated receptor-gamma (PPAR gamma) is a member of the nuclear receptor superfamily of ligand-dependent transcriptional factors with beneficial effects in diabetes mediated by improved insulin sensitivity and lipid metabolism, but potential adverse effects in atherosclerosis by promoting in vitro foam cell formation. We explored whether a PPAR gamma agonist, troglitazone (TGL), affects sclerosis by mechanisms unrelated to insulin and lipid effects in a model of nondiabetic glomerulosclerosis. METHODS: Adult male Sprague Dawley rats underwent 5/6 nephrectomy and were treated for 12 weeks as follows: control (CONT), no further treatment; triple antihypertensive therapy (TRX); and TGL or TGL + TRX. Functional, morphological, and molecular analyses were performed. RESULTS: Systolic blood pressure (SBP) was increased in CONT and TGL groups (161 +/- 1 and 160 +/- 3 mm Hg), but not in TGL + TRX and TRX (120 +/- 3 vs. 126 +/- 1 mm Hg, P < 0.0001 vs. non-TRX). Serum triglyceride and cholesterol levels in all groups remained normal except for slightly higher serum cholesterol levels in TRX group. TGL groups had reduced proteinuria, serum creatinine, and glomerulosclerosis versus CONT, in contrast to no significant effect with TRX alone (sclerosis index, 0 to 4+ scale: CONT 1.99 +/- 0.42, TGL 0.85 +/- 0.12, TGL + TRX 0.56 +/- 0.14, TRX 1.30 +/- 0.21; TGL, P < 0.05; TGL + TRX, P = 0.01 vs. CONT). Glomerular cell proliferation, assessed by proliferating cell nuclear antigen (PCNA), was decreased after treatment with TGL or TGL + TRX, in parallel with decreases in glomerular p21 mRNA and p27 protein compared with CONT and TRX (PCNA + cells/glomerulus: CONT 2.04 +/- 0.64, TGL 0.84 +/- 0.21, TGL + TRX 0.30 +/- 0.07, TRX 1.38 +/- 0.37; TGL, P < 0.05, TGL + TRX, P < 0.01 vs. CONT). Glomerular plasminogen activator inhibitor-1 (PAI-1) immunostaining was decreased in TGL or TGL + TRX groups (0 to 4+ scale, CONT 2.42 +/- 0.32, TGL 1.40 +/- 0.24, TGL + TRX 1.24 +/- 0.17, TRX 2.53 +/- 0.24; TGL or TGL + TRX vs. CONT, P < 0.05), with a parallel decrease in PAI-1 mRNA by in situ hybridization. Glomerular and tubular transforming growth factor-beta (TGF-beta) mRNA expression was decreased with TGL treatment. Glomerular macrophages, present in CONT and TRX rats, did not express PPAR gamma, in contrast to PPAR gamma + macrophages in control carotid artery plaque. PPAR gamma was expressed in resident cells. CONCLUSIONS: Our results demonstrate in vivo that the PPAR gamma ligand TGL ameliorates the progression of glomerulosclerosis in a nondiabetic model. Macrophages show phenotypic diversity in glomerular versus vascular sclerosis, with macrophage PPAR gamma expression in only the latter. PPAR gamma beneficial effects are independent of insulin/glucose effects and are associated with regulation of glomerular cell proliferation, hypertrophy, and decreased PAI-1 and TGF-beta expression.

Animals↗

PubMed2001

Class A scavenger receptors, macrophages, and atherosclerosis.

The scope of this review is to discuss the new advances in our understanding of the role of scavenger receptor class A in the initiation and modulation of the atherosclerotic process. Through the approaches of gene manipulation in the mouse model, a substantial body of literature has accumulated that depicts scavenger receptor class A as a central player in atherogenesis. In studies of scavenger receptor class A overexpression in macrophages through bone marrow transplantation using transgenic donor material, recipient mice with hyperlipidemia caused either by apolipoprotein E or LDL receptor deficiency did not show convincing changes in the degree of atherosclerosis development compared with controls. Conversely, the deletion of the scavenger receptor class A gene in the mouse has shown, in a consistent and significant fashion, that this receptor serves a pro-atherogenic function under hyperlipidemic conditions, as both apolipoprotein E and LDL receptor-deficient mice had reduced atherosclerosis in the absence of scavenger receptor class A. In addition, we have recently shown that C57BL/6 mice are protected from diet-induced atherosclerosis when they lack scavenger receptor class A, and that the macrophage is the cell type responsible for the effect of scavenger receptor class A deficiency in reducing lesion formation in C57BL/6 and LDL receptor null mice. Together, these results demonstrate that macrophage scavenger receptor class A contributes significantly to atherosclerotic lesion formation, and suggest that the uptake of oxidized or modified lipoproteins by vessel wall macrophages is a central process in atherogenesis.

Animals↗

PubMed2001

Increased cholesterol efflux in apolipoprotein AI (ApoAI)-producing macrophages as a mechanism for reduced atherosclerosis in ApoAI((-/-)) mice.

The concentration of apolipoprotein (apo) AI in the artery wall is thought to enhance cellular cholesterol efflux and protect against atherosclerosis. It has been shown that although macrophages do not make apoAI, they respond to it by increased cholesterol efflux. We hypothesized that macrophage production of apoAI would increase cholesterol efflux and reduce atherogenesis. In this study, we produced mice expressing human apoAI under the control of the macrophage-specific scavenger receptor-A promoter (mphi-AI). Human apoAI was detectable in the serum HDL fraction of mphi-AI transgenic mice at concentrations too low to affect serum cholesterol or HDL levels. Immunoblotting showed the presence of human apoAI in transgenic macrophage culture supernatants, mostly as lipoprotein-free protein, with a small component associated with HDL-like particles. Atherosclerosis studies using apoAI((-/-)) mice transplanted with mphi-AI bone marrow showed that in the absence of macrophage-derived apoE, local expression of apoAI reduced diet-induced lesions in the proximal aorta. Additionally, mphi-AI macrophages showed a 40% increase in cholesterol efflux compared with control macrophages. These data support the hypothesis that apoAI production by macrophages in the artery wall is protective against atherosclerosis. This protection is likely mediated by increased cholesterol efflux and decreased foam cell formation in vivo.

Animals↗

PubMed2001

Increased atherosclerosis in LDL receptor-null mice lacking ACAT1 in macrophages.

During atherogenesis, circulating macrophages migrate into the subendothelial space, internalize cholesterol-rich lipoproteins, and become foam cells by progressively accumulating cholesterol esters. The inhibition of macrophage acyl coenzyme A:cholesterol acyltransferase (ACAT), which catalyzes the formation of cholesterol esters, has been proposed as a strategy to reduce foam cell formation and to treat atherosclerosis. We show here, however, that hypercholesterolemic LDL receptor-deficient (LDLR(-/-)) mice reconstituted with ACAT1-deficient macrophages unexpectedly develop larger atherosclerotic lesions than control LDLR(-/-) mice. The ACAT1-deficient lesions have reduced macrophage immunostaining and more free cholesterol than control lesions. Our findings suggest that selective inhibition of ACAT1 in lesion macrophages in the setting of hyperlipidemia can lead to the accumulation of free cholesterol in the artery wall, and that this promotes, rather than inhibits, lesion development.

Animals↗

PubMed2000

Macrophage lipoprotein lipase promotes foam cell formation and atherosclerosis in low density lipoprotein receptor-deficient mice.

The role of macrophage lipoprotein lipase (LPL) expression in atherosclerotic lesion formation was examined in low density lipoprotein receptor (LDLR(-/-)) mice using dietary conditions designed to induce either fatty streak lesions or complex atherosclerotic lesions. First, LDLR(-/-) mice chimeric for macrophage LPL expression were created by transplantation of lethally irradiated female LDLR(-/-) mice with LPL(-/-) (n = 12) or LPL(+/+) (n = 14) fetal liver cells as a source of hematopoietic cells. To induce fatty streak lesions, these mice were fed a Western diet for 8 weeks, resulting in severe hypercholesterolemia. There were no differences in plasma post-heparin LPL activity, serum lipid levels, or lipoprotein distribution between these two groups. The mean lesion area in the proximal aorta in LPL(-/-) --> LDLR(-/-) mice was significantly reduced by 33% compared with LPL(+/+) --> LDLR(-/-) mice, and a similar reduction (38%) in lesion area was found by en face analysis of the aortae. To induce complex atherosclerotic lesions, female LDLR(-/-) mice were lethally irradiated, transplanted with LPL(-/-) (n = 14), LPL(+/-) (n = 13), or LPL(+/+) (n = 14) fetal liver cells, and fed the Western diet for 19 weeks. Serum cholesterol and triglyceride levels did not differ between the three groups. After 19 weeks of diet, the lesions in the proximal aorta were complex with relatively few macrophages expressing LPL protein and mRNA in LPL(+/+) --> LDLR(-/-) mice. Analysis of cross-sections of the proximal aorta demonstrated no differences in the extent of lesion area between the groups, whereas en face analysis of the aortae revealed a dose-dependent effect of macrophage LPL on mean aortic lesion area in LPL(-/-) --> LDLR(-/-), LPL(-/+) --> LDLR(-/-), and LPL(+/+) --> LDLR(-/-) mice (1.8 +/- 0. 2%, 3.5 +/- 0.5% and 5.9 +/- 0.8%, respectively). Taken together, these data indicate that macrophage LPL expression in the artery wall promotes atherogenesis during foam cell lesion formation, but this impact may be limited to macrophage-rich lesions.

Animals↗

PubMed2000

Current perspectives on statins.

Statins (HMG-CoA reductase inhibitors) are used widely for the treatment of hypercholesterolemia. They inhibit HMG-CoA reductase competitively, reduce LDL levels more than other cholesterol-lowering drugs, and lower triglyceride levels in hypertriglyceridemic patients. Statins are well tolerated and have an excellent safety record. Clinical trials in patients with and without coronary heart disease and with and without high cholesterol have demonstrated consistently that statins reduce the relative risk of major coronary events by approximately 30% and produce a greater absolute benefit in patients with higher baseline risk. Proposed mechanisms include favorable effects on plasma lipoproteins, endothelial function, plaque architecture and stability, thrombosis, and inflammation. Mechanisms independent of LDL lowering may play an important role in the clinical benefits conferred by these drugs and may ultimately broaden their indication from lipid-lowering to antiatherogenic agents.

Cardiology↗

PubMed2000

Re-emergence of fibrates in the management of dyslipidemia and cardiovascular risk.

It is now widely accepted that low-density lipoprotein (LDL) is not the only atherogenic component of the lipid profile and that abnormalities in the metabolism and plasma levels of triglycerides and high-density lipoprotein (HDL) may lead to accelerated growth of atherosclerotic lesions. Fibrates are the drugs of first choice in the management of hypertriglyceridemia, and are also able to substantially raise HDL. The recently published Veterans Administration-High-density Lipoprotein Intervention Trial (VA-HIT) trial showed that fibrate treatment in patients with coronary heart disease (CHD), low HDL, modestly elevated triglycerides, and normal LDL reduces the risk of a recurrent coronary event by 25%. A reasonable approach to the dyslipidemic patient with high CHD risk is to tailor the intervention to the specific lipoprotein abnormality. Under these assumptions fibrate therapy should become widespread, considering that the most common lipid alteration in CHD and patients with diabetes is low HDL and high triglycerides.

Angiography↗

PubMed2000

The cell biology and physiologic relevance of ApoE recycling.

Apolipoprotein E (apoE) is unique among the different apoproteins because of its many functions in the assembly, processing, and removal of plasma lipoproteins. Some of these functions require the presence of apoE inside the cell or in close association with its outer membrane. For example, the presence of apoE increases triglyceride content in newly formed very low-density lipoprotein (VLDL), whereas the fast removal of chylomicrons requires the presence of a cellular pool of apoE ready to surface and bind to membrane proteoglycans and possibly to lipoprotein receptors such as the LDL receptor-related protein (LRP). Our recent discovery that VLDL-apoE internalized by hepatocytes is partially protected from lysosomal degradation and recycles through the Golgi apparatus suggests that the biologic cycle of apoE may not be concluded when the lipoprotein is taken up by the cell, and that recycling apoE may have a physiologic role in lipoprotein assembly, remnant removal, and cholesterol efflux.

Animals↗

PubMed2000

Reduced atherosclerotic lesions in mice deficient for total or macrophage-specific expression of scavenger receptor-A.

The absence of the scavenger receptor A (SR-A)-I/II has produced variable effects on atherosclerosis in different murine models. Therefore, we examined whether SR-AI/II deficiency affected atherogenesis in C57BL/6 mice, an inbred strain known to be susceptible to diet-induced atherosclerotic lesion formation, and whether the deletion of macrophage SR-AI/II expression would modulate lesion growth in C57BL/6 mice and LDL receptor (LDLR)(-/-) mice. SR-AI/II-deficient (SR-AI/II(-/-)) female and male mice on the C57BL/6 background were challenged with a butterfat diet for 30 weeks. No differences were detected in plasma lipids between SR-AI/II(-/-) and SR-AI/II(+/+) mice, whereas both female and male SR-AI/II(-/-) mice had a tremendous reduction (81% to 86%) in lesion area of the proximal aorta compared with SR-AI/II(+/+) mice. Next, to analyze the effect of macrophage-specific SR-AI/II deficiency in atherogenesis, female C57BL/6 mice were lethally irradiated, transplanted with SR-AI/II(-/-) or SR-AI/II(+/+) fetal liver cells, and challenged with the butterfat diet for 16 weeks. In a separate experiment, male LDLR(-/-) mice were reconstituted with SR-AI/II(-/-) or SR-AI/II(+/+) fetal liver cells and challenged with a Western diet for 10 weeks. No significant differences in plasma lipids and lipoprotein profiles were noted between the control and experimental groups in either experiment. SR-AI/II(-/-)-->C57BL/6 mice, however, had a 60% reduction in lesion area of the proximal aorta compared with SR-AI/II(+/+)-->C57BL/6 mice. A similar level of reduction (60%) in lesion area was noted in the proximal aorta and the entire aorta en face of SR-AI/II(-/-)-->LDLR(-/-) mice compared with SR-AI/II(+/+)-->LDLR(-/-) mice. These results demonstrate in vivo that SR-AI/II expression has no impact on plasma lipid levels and that macrophage SR-AI/II contributes significantly to atherosclerotic lesion formation.

Animals↗

PubMed2000

Massive xanthomatosis and altered composition of atherosclerotic lesions in hyperlipidemic mice lacking acyl CoA:cholesterol acyltransferase 1.

Inhibitors of acyl CoA:cholesterol acyltransferase (ACAT) have attracted considerable interest as a potential treatment for atherosclerosis. Currently available inhibitors probably act nonselectively against the two known ACATs. One of these enzymes, ACAT1, is highly expressed in macrophages in atherosclerotic lesions, where it contributes to foam-cell formation. In this study, we examined the effects of selective ACAT1 deficiency in two mouse models of atherosclerosis. In the setting of severe hypercholesterolemia caused by deficiency in apoE or the LDL receptor (LDLR), total ACAT1 deficiency led to marked alterations in cholesterol homeostasis and extensive deposition of unesterified cholesterol in the skin and brain. Bone marrow transplantation experiments demonstrated that ACAT1 deficiency in macrophages was sufficient to cause dermal xanthomas in hyperlipidemic LDLR-deficient mice. ACAT1 deficiency did not prevent the development of atherosclerotic lesions in either apoE-deficient or LDLR-deficient mice, despite causing relatively lower serum cholesterol levels. However, the lesions in ACAT1-deficient mice were atypical in composition, with reduced amounts of neutral lipids and a paucity of macrophages in advanced lesions. Although the latter findings may be associated with increased lesion stability, the marked alterations in cholesterol homeostasis indicate that selectively inhibiting ACAT1 in the setting of severe hyperlipidemia may have detrimental consequences.

Animals↗

PubMed1999

A direct role for the macrophage low density lipoprotein receptor in atherosclerotic lesion formation.

To evaluate the contribution of the macrophage low density lipoprotein receptor (LDLR) to atherosclerotic lesion formation, we performed bone marrow transplantation studies in different mouse strains. First, LDLR(-/-) mice were transplanted with either LDLR(+/+) marrow or LDLR(-/-) marrow and were challenged with an atherogenic Western type diet. The diet caused severe hypercholesterolemia of a similar degree in the two groups, and no differences in the aortic lesion area were detected. Thus, macrophage LDLR expression does not influence foam cell lesion formation in the setting of extreme LDL accumulation. To determine whether macrophage LDLR expression affects foam cell formation under conditions of moderate, non-LDL hyperlipidemia, we transplanted C57BL/6 mice with either LDLR(-/-) marrow (experimental group) or LDLR(+/+) marrow (controls). Cholesterol levels were not significantly different between the two groups at baseline or after 6 weeks on a butterfat diet, but were 40% higher in the experimental mice after 13 weeks, mostly due to accumulation of beta-very low density lipoprotein (beta-VLDL). Despite the increase in cholesterol levels, mice receiving LDLR(-/-) marrow developed 63% smaller lesions than controls, demonstrating that macrophage LDLR affects the rate of foam cell formation when the atherogenic stimulus is beta-VLDL. We conclude that the macrophage LDLR is responsible for a significant portion of lipid accumulation in foam cells under conditions of dietary stress.

Animals↗

PubMed1999

Retroviral gene therapy in ApoE-deficient mice: ApoE expression in the artery wall reduces early foam cell lesion formation.

BACKGROUND: Apolipoprotein E (apoE) has long been known to play an important role in the clearance of plasma lipoproteins. More recently, a direct role for apoE in delaying atherogenesis has been proposed. Macrophage production of apoE in the artery wall has been demonstrated to provide protection against atherosclerotic lesion development independently from its role in lipoprotein clearance. However, whether macrophage apoE can affect lesion growth at all stages of atherogenesis remains to be established. METHODS AND RESULTS: To evaluate the role of macrophage apoE in different stages of atherogenesis, as well as to establish a novel gene therapy approach to atherosclerotic vascular disease, we used an apoE-expressing retrovirus to transduce apoE-deficient (-/-) bone marrow for transplantation into apoE(-/-) recipient mice. Three weeks after bone marrow transplantation, apoE was expressed from arterial macrophages and was detectable in plasma associated with lipoproteins at 0.5% to 1% of normal levels but did not affect plasma cholesterol levels. We used 2 groups of recipient mice: younger mice with lesions consisting primarily of foam cells and older mice with more advanced lesions. When either the mouse or human apoE transgenes were expressed in mice from 5 to 13 weeks of age, there was a significant reduction in lesion area, whereas no effects were detected in mice that expressed apoE from 10 to 26 weeks of age. CONCLUSIONS: We demonstrate that arterial macrophage apoE secretion can delay atherogenesis if expressed during foam cell formation but is not beneficial during the later stages of atherogenesis. These data also provide evidence that apoE transgene expression from arterial macrophages may have therapeutic applications.

Animals↗

PubMed1999

Recycling of apolipoprotein E in mouse liver.

Following the internalization of low density lipoprotein (LDL) by the LDL receptor within cells, both the lipid and the protein components of LDL are completely degraded within the lysosomes. Remnant lipoproteins are also internalized by cells via the LDL receptor as well as other receptors, but the events following the internalization of these complexes, which use apolipoprotein E (apoE) as their ligand for receptor capture, have not been defined. There is evidence that apoE-containing beta-very low density lipoproteins follow differential intracellular routing depending on their size and apoE content and that apoE internalized with lipoproteins can be resecreted by cultured hepatocytes and fibroblasts. In the present studies, we addressed the question of apoE sparing or recycling as a physiologic phenomenon. Remnant lipoproteins (d < 1.019 g/ml) from normal mouse plasma were iodinated and injected into normal C57BL/6 mice. Livers were collected at 10, 30, 60, and 120 min after injection, and hepatic Golgi fractions were prepared for gel electrophoresis analysis. Golgi preparations were analyzed for galactosyltransferase enrichment (>40-fold above cell homogenate) and by appearance of the Golgi stacks and vesicles on electron microscopy. Iodinated apoE was consistently found in the Golgi fractions peaking at 10 min and disappearing by 2 h after injection. Although traces of apoB48 were present in the Golgi fractions, the apoE/apoB ratio in the Golgi was 50-fold higher compared with serum. Quantitatively similar results were obtained when the very low density lipoprotein remnants were injected into mice deficient in either apoE or the LDL receptor, indicating that the phenomenon of apoE recycling is not influenced by the production of endogenous apoE and is not dependent on the presence of LDL receptors. In addition, radioactive apoE in the Golgi fractions was part of d = 1.019-1.21 g/ml complexes, indicating an association of recycled apoE with either newly formed lipoproteins or the internalized complexes. These studies show that apoE recycling is a physiologic phenomenon in vivo and establish the presence of a unique pathway of intracellular processing of apoE-containing remnant lipoproteins.

Animals↗