Impaired Secretion of Apolipoprotein E2 from Macrophages

Abstract

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 receptorrelated 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.ApoE, 2 a 299-residue exchangeable plasma apolipoprotein, is a major player in lipoprotein metabolism and cardiovascular disease (1). It also plays critical roles in many other important biological processes, including Alzheimer disease and cognitive function, immunoregulation and response to infectious agents, intracellular cholesterol trafficking and control of oxidation, and apoptosis (2). ApoE is synthesized and secreted primarily by the liver but also by brain, skin, and tissue macrophages throughout the body (1, 3). Although most lipoprotein-associated apoE is of hepatic origin, macrophage apoE is a major regulator of cholesterol entry and exit within the atherosclerotic plaque. The atheroprotective role of macrophage apoE has been well documented. Macrophage-derived apoE has been shown to protect against atherosclerosis, both early (4 -6) and late during plaque development (7,8), and even when expressed in amounts too low to affect plasma lipid levels (9). Moreover, C57BL/6 mice reconstituted with apoE Ϫ/Ϫ bone marrow develop 10-fold more atherosclerosis than control mice fed a butter fat diet without any differences ...

Expression of the human apoE2 isoform in adipocytes: altered cellular processing and impaired adipocyte lipogenesis

“…The defect in adipocyte apoE2 secretion and the instability of the newly synthesized apoE2 isoform have also been reported in macrophages ( 21 ). We were surprised, however, by the increase in apoE gene expression and synthesis that accompanied expression of the apoE2 gene under control of its endogenous control elements.…”

“…Human expression of the apoE2 and E4 isoforms has been associated with important human diseases (15)(16)(17). Isoform-specifi c effects on cellular-differentiated function have been established for neurons and macrophages (20)(21)(22)(23)(24)(25)(26), and . TG turnover in apoE isoform knock-in adipocytes.…”

“…TG hydrolysis was estimated by measuring the release of glycerol into incubation medium over 2 h ( 6 ). cell function, have been described in macrophages (20)(21)(22)(23) and neuronal cells (24)(25)(26). For example, in macrophages, the secretion of apoE2 is impaired, and there is an associated alteration in macrophage sterol effl ux mediated by apoE2.…”

Macrophage-derived apoESendai suppresses atherosclerosis while causing lipoprotein glomerulopathy in hyperlipidemic mice

“…Figure 1C shows that macrophages retain the apoE2 isoform, but not apoE3 and apoE Sendai . We have previously reported intracellular retention of apoE2 in macrophages due to its interaction with LRP1 ( 9 ). Here, we show that apoE Sendai is weakly associated with LRP1 and is not retained in macrophages.…”

No association between ApoE polymorphism and febrile seizures

“…In Europeans free of any neurological disorders, the distribution of different alleles is stable, with an ApoE2 frequency of 0.068, an ApoE3 frequency of 0.813 and an ApoE4 frequency of 0.119 [22]. ApoE2 is more highly retained in astrocytes than ApoE3 and ApoE4 (Fan and Fazio, personal communication; see [23] for similar results in macrophages) and might thus facilitate the prostaglandin-induced, calcium-dependent exocytosis of glutamate by astrocytes [16,17]. In turn, increased glutamate concentration in the synaptic cleft might induce prolonged depolarization with repetitive spiking in neurons leading to seizures.…”