LDL receptor binds newly synthesized apoE in macrophages: a precursor pool for apoE secretion

Oxysterols and retinal degeneration

Retinal degeneration, characterised by the progressive death of retinal neurons, is the most common cause of visual impairment. Oxysterols are the cholesterol derivatives produced via enzymatic and/or free radical oxidation that regulate cholesterol homeostasis in the retina. Preclinical and clinical studies have suggested a connection between oxysterols and retinal degeneration. Here, we summarise early and recent work related to retina oxysterol-producing enzymes and the distribution of oxysterols in the retina. We examine the impact of loss of oxysterol-producing enzymes on retinal pathology and explore the molecular mechanisms associated with the toxic or protective roles of individual oxysterols in different types of retinal degeneration. We conclude that increased efforts to better understand the oxysterol-associated pathophysiology will help in the development of effective retinal degeneration therapies. LINKED ARTICLES: This article is part of a themed issue on Oxysterols, Lifelong Health and Therapeutics. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.16/issuetoc.

Apoproteins E, A-I, and SAA in Macrophage Pathobiology Related to Atherogenesis

Macrophages are core cellular elements of both early and advanced atherosclerosis. They take up modified lipoproteins and become lipid-loaded foam cells and secrete factors that influence other cell types in the artery wall involved in atherogenesis. Apoproteins E, AI, and SAA are all found on HDL which can enter the artery wall. In addition, apoE is synthesized by macrophages. These three apoproteins can promote cholesterol efflux from lipid-loaded macrophages and have other functions that modulate macrophage biology. Mimetic peptides based on the sequence or structure of these apoproteins replicate some of these properties and are potential therapeutic agents for the treatment of atherosclerosis to reduce cardiovascular diseases.

Apoprotein E and Reverse Cholesterol Transport

Apoprotein E (apoE) is a multifunctional protein. Its best-characterized function is as a ligand for low-density lipoprotein (LDL) receptor family members to mediate the clearance of apoB-containing atherogenic lipoproteins. Among its other functions, apoE is involved in cholesterol efflux, especially from cholesterol-loaded macrophage foam cells and other atherosclerosis-relevant cells, and in reverse cholesterol transport. Reverse cholesterol transport is a mechanism by which excess cellular cholesterol is transported via lipoproteins in the plasma to the liver where it can be excreted from the body in the feces. This process is thought to have a role in the attenuation of atherosclerosis. This review summarizes studies on the role of apoE in cellular cholesterol efflux and reverse cholesterol transport and discusses the identification of apoE mimetic peptides that may promote these pathways.

Regulation of endogenous apolipoprotein E secretion by macrophages

Apolipoprotein E has critical roles in the protection against atherosclerosis and is understood to follow the classical constitutive secretion pathway. Recent studies have indicated that the secretion of apoE from macrophages is a regulated process of unexpected complexity. Cholesterol acceptors such as apolipoprotein A-I, high density lipoprotein, and phospholipid vesicles can stimulate apoE secretion. The ATP binding cassette transporter ABCA1 is involved in basal apoE secretion and in lipidating apoE-containing particles secreted by macrophages. However, the stimulation of apoE secretion by apoA-I is ABCA1-independent, indicating the existence of both ABCA1-dependent and -independent pathways of apoE secretion. The release of apoE under basal conditions is also regulated, requiring intact protein kinase A activity, intracellular calcium, and an intact microtubular network. Mathematical modeling of apoE turnover indicates that whereas some pools of apoE are committed to either secretion or degradation, other pools can be diverted from degradation toward secretion. Targeted inhibition or stimulation of specific apoE trafficking pathways will provide unique opportunities to regulate the biology of this important molecule.

Impaired secretion of apolipoprotein E2 from macrophages

Human apoE is a multifunctional and polymorphic protein synthesized and secreted by liver, brain, and tissue macrophages. Here we show that apoE isoforms and mutants expressed through lentiviral transduction display cell-specific differences in secretion efficiency. Whereas apoE3, apoE4, and a natural mutant of apoE4 (apoE-Cys(142)) were efficiently secreted from macrophages, apoE2 and a non-natural apoE mutant (apoE-Cys(112)/Cys(142)) were retained in the perinuclear region and only minimally secreted. The secretory block for apoE2 in macrophages was not affected by the ablation of LDLR (low density lipoprotein receptor), ABCA-1, or SR-BI (scavenger receptor class B type I) but was released in the absence of low density lipoprotein receptor related protein (LRP). In co-immunoprecipitation experiments, an anti-apoE antibody pulled down two times more LRP in apoE2-transduced macrophages than in apoE3-expressing macrophages. Non-reducing SDS-PAGE/Western blot analyses showed that macrophage apoE2 is mostly dimeric and multimeric, whereas apoE3 is predominantly monomeric. ApoE2 retention and multimer formation also occurred in human macrophages derived from the monocyte cell line THP-1. These results were specific for macrophages, as in transduced mouse primary hepatocytes: 1) ApoE2 was secreted as efficiently as apoE3 and apoE4; 2) all isoforms were exclusively in monomeric form; 3) there was no co-immunoprecipitation of apoE and LRP. A microsomal triglyceride transfer protein (MTP) inhibitor nearly deleted apoB100 secretion from hepatocytes without affecting apoE secretion. These data show that macrophages retain apoE2, a highly expressed protein carried by about 8% of the human population. Given the role of locally produced apoE in regulating cholesterol efflux, modulating inflammation, and controlling oxidative stress, this unique property of apoE2 may have important impacts on atherogenesis.

Harmful effects of increased LDLR expression in mice with human APOE*4 but not APOE*3

OBJECTIVE

Increased expression of the low-density lipoprotein receptor (LDLR) is generally considered beneficial for reducing plasma cholesterol and atherosclerosis, and its downregulation has been thought to explain the association between apolipoprotein (apo) E4 and increased risk of coronary heart disease in humans.

METHODS AND RESULTS

Contrary to this hypothesis, doubling Ldlr expression caused severe atherosclerosis with marked accumulation of cholesterol-rich, apoE-poor remnants in mice with human apoE4, but not apoE3, when the animals were fed a Western-type diet. The increased Ldlr expression enhanced in vivo clearance of exogenously introduced remnants in mice with apoE4 only when the remnants were already enriched with apoE4. The rates of nascent lipoprotein production were the same. The adverse effects of increased LDLR suggest a possibility that the receptor can trap apoE4, reducing its availability for the transfer to nascent lipoproteins needed for their rapid clearance, thereby increasing the production of apoE-poor remnants that are slowly cleared. The lower affinity for the LDLR of apoE3 compared with apoE4 could then explain why increased receptor expression had no adverse effects with apoE3.

CONCLUSIONS

Our results emphasize the occurrence of important and unexpected interactions between APOE genotype, LDLR expression, and diet.

Isoform-dependent cholesterol efflux from macrophages by apolipoprotein E is modulated by cell surface proteoglycans

OBJECTIVE

Apolipoprotein E (apoE) mediates cellular cholesterol efflux and plays a crucial role in the inhibition of atherogenesis. We investigated whether there is an isoform-specific difference in its function for cholesterol efflux from cholesterol-loaded RAW264.7 cells, a murine macrophage cell line that lacks endogenous apoE expression.

METHODS AND RESULTS

When human apoE was expressed in RAW264.7 cells, apoE2 reduced cellular total cholesterol (TC) and esterified cholesterol (EC) levels significantly, whereas apoE3 and apoE4 had no effect. However, treatment of cells with 4-methylumbelliferyl-7-beta-D-xyloside (beta-DX) resulted in all 3 isoforms' reducing cellular TC and EC contents significantly. We also investigated the effect of exogenously derived apoE on cholesterol efflux by utilizing the medium harvested from HeLa cells expressing apoE. ApoE2 and E3 reduced both cellular TC and EC contents significantly, whereas apoE4 did not. However, treatment of the cells with beta-DX resulted in all 3 exogenously derived apoE isoforms' reducing TC and EC contents significantly. The binding ability of apoE to heparan sulfate proteoglycans examined by heparinase I treatment revealed less binding ability of apoE2 compared with that of apoE3 or apoE4.

CONCLUSIONS

The present study clarified the differential cellular cholesterol-modulating effect of apoE isoforms in macrophages, which would be due to the difference in their binding to proteoglycans.

Apolipoprotein E and atherosclerosis

Apolipoprotein E plays a key protective role in atherosclerosis. Its capacity to safeguard against this disease can be attributed to at least three distinct functions. First, plasma apolipoprotein E maintains overall plasma cholesterol homeostasis by facilitating efficient hepatic uptake of lipoprotein remnants. Second, lesion apolipoprotein E in concert with apolipoprotein A-I facilitates cellular cholesterol efflux from macrophage foam cells within the intima of the lesion. Third, lesion apolipoprotein E directly modifies both macrophage- and T lymphocyte-mediated immune responses that contribute to this chronic inflammatory disease.

Transport and processing of endogenously synthesized ApoE on the macrophage cell surface

We have previously established the presence of a pool of apoE sequestered on the macrophage cell surface by demonstrating its displacement from a cell monolayer at 4 degrees C. In this series of experiments, we use a cell surface biotinylation protocol to directly quantitate apoE on the macrophage cell surface and evaluate its transport to and from this cell surface pool. In human monocyte-derived macrophages labeled to equilibrium and in a mouse macrophage cell line transfected to constitutively express human apoE3, approximately 8% of total cellular apoE was present on the surface, but only a portion of this surface pool served as a direct precursor to secreted apoE. The half-life of apoE on the macrophage cell surface was calculated to be approximately 12 min. On SDS-polyacrylamide gel electrophoresis, the apoE isolated from the surface fraction of cells labeled to equilibrium migrated in an isoform pattern distinct from that observed from the intracellular fraction, with the surface fraction migrating predominantly in a higher molecular weight isoform. Pulse labeling experiments demonstrated that newly synthesized apoE reached the cell surface by 10 min but was predominantly in a low molecular weight isoform. There was also a lag between appearance of apoE on the cell surface and its appearance in the medium. Biotinylated apoE, which accumulated in the medium, even from pulse labeled cells, was predominantly in the high molecular weight isoform. Additional experiments demonstrated that low molecular weight apoE present on the cell surface was modified to higher molecular weight apoE by the addition of sialic acid residues prior to secretion and that this conversion was inhibited by brefeldin A. These results demonstrate an unexpected complexity in the transport and cellular processing of macrophage cell surface apoE. Factors that modulate the size and turnover of the cell surface pool of apoE in the macrophage remain to be identified and investigated.

The role of the LDL receptor in apolipoprotein B secretion

Familial hypercholesterolemia is caused by mutations in the LDL receptor gene (Ldlr). Elevated plasma LDL levels result from slower LDL catabolism and a paradoxical lipoprotein overproduction. We explored the relationship between the presence of the LDL receptor and lipoprotein secretion in hepatocytes from both wild-type and LDL receptor-deficient mice. Ldlr(-/-) hepatocytes secreted apoB100 at a 3.5-fold higher rate than did wild-type hepatocytes. ApoB mRNA abundance, initial apoB synthetic rate, and abundance of the microsomal triglyceride transfer protein 97-kDa subunit did not differ between wild-type and Ldlr(-/-) cells. Pulse-chase analysis and multicompartmental modeling revealed that in wild-type hepatocytes, approximately 55% of newly synthesized apoB100 was degraded. However, in Ldlr(-/-) cells, less than 20% of apoB was degraded. In wild-type hepatocytes, approximately equal amounts of LDL receptor-dependent apoB100 degradation occured via reuptake and presecretory mechanisms. Adenovirus-mediated overexpression of the LDL receptor in Ldlr(-/-) cells resulted in degradation of approximately 90% of newly synthesized apoB100. These studies show that the LDL receptor alters the proportion of apoB that escapes co- or post-translational presecretory degradation and mediates the reuptake of newly secreted apoB-containing lipoprotein particles.