Divergent Effects of Peroxisome Proliferator-Activated Receptor γ Agonists and Tumor Necrosis Factor α on Adipocyte ApoE Expression

ApoE is expressed in multiple mammalian cell types in which it supports cellular differentiated function. In this report we demonstrate that apoE expression in adipocytes is regulated by factors involved in modulating systemic insulin sensitivity. Systemic treatment with pioglitazone increased systemic insulin sensitivity and increased apoE mRNA levels in adipose tissue by 2-3-fold. Treatment of cultured 3T3-L1 adipocytes with ciglitazone increased apoE mRNA levels by 2-4-fold in a dose-dependent manner and increased apoE secretion from cells. Conversely, treatment of adipocytes with tumor necrosis factor (TNF) α reduced apoE mRNA levels and apoE secretion by 60%. Neither insulin nor a peroxisome proliferator-activated receptor (PPAR) α agonist regulated adipocyte apoE gene expression. In addition, treatment of human monocyte-derived macrophages with ciglitazone did not regulate expression of apoE. Additional analyses using reporter genes indicated that the effect of TNFα and PPARγ agonists on the apoE gene was mediated via distinct gene control elements. The TNFα effect was mediated by elements within the proximal promoter, whereas the PPARγ effect was mediated by elements within a downstream enhancer. However, the addition of TNFα substantially reduced the absolute levels of apoE reporter gene response even in the presence of ciglitazone. These results indicate for the first time that adipose tissue expression of apoE is modulated by physiologic regulators of insulin sensitivity

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

Expression of apoE in adipocytes has been shown to have an important role in modulating adipocyte triglyceride (TG) metabolism and gene expression that is independent of circulating and extracellular apoE. The impact of adipocyte expression of common human apoE isoforms was evaluated using adipocytes harvested from human apoE2, -3, and -4 knock-in mice. Expression of the apoE2 isoform was associated with an increase in adipocyte apoE gene expression and apoE synthesis. Newly synthesized apoE2 was unstable in adipocytes and demonstrated increased degradation and decreased secretion. ApoE2-expressing mice were hyperlipidemic, and had increased size of gonadal fat pads and of adipocytes, compared with apoE3 mice. In isolated cells, however, expression of the apoE2 isoform produced defective lipogenesis and increased TG hydrolysis. Incubation of adipose tissue with apoE3-containing TG-rich lipoproteins resulted in a significant increase in TG in adipose tissue from apoE3 and -E4 mice, but not apoE2 mice. Reduced capacity to internalize FFA as lipogenic substrate contributed to defective lipogenesis. Newly synthesized apoE2 is unstable in adipocytes and results in decreased adipocyte TG synthesis and defective FA uptake. These changes recapitulate those observed in apoE knockout adipocytes and have implications for understanding metabolic disturbances in humans expressing the E2 isoform

Selective suppression of adipose tissue apoE expression impacts systemic metabolic phenotype and adipose tissue inflammation

apoE is a multi-functional protein expressed in several cell types and in several organs. It is highly expressed in adipose tissue, where it is important for modulating adipocyte lipid flux and gene expression in isolated adipocytes. In order to investigate a potential systemic role for apoE that is produced in adipose tissue, mice were generated with selective suppression of adipose tissue apoE expression and normal circulating apoE levels. These mice had less adipose tissue with smaller adipocytes containing fewer lipids, but no change in adipocyte number compared with control mice. Adipocyte TG synthesis in the presence of apoE-containing VLDL was markedly impaired. Adipocyte caveolin and leptin gene expression were reduced, but adiponectin, PGC-1, and CPT-1 gene expression were increased. Mice with selective suppression of adipose tissue apoE had lower fasting lipid, insulin, and glucose levels, and glucose and insulin tolerance tests were consistent with increased insulin sensitivity. Lipid storage in muscle, heart, and liver was significantly reduced. Adipose tissue macrophage inflammatory activation was markedly diminished with suppression of adipose tissue apoE expression. Our results establish a novel effect of adipose tissue apoE expression, distinct from circulating apoE, on systemic substrate metabolism and adipose tissue inflammatory state

Regulation of macrophage apoE secretion and sterol efflux by the LDL receptor

Factors that regulate apolipoprotein E (apoE) secretion by macrophages will have important effects on vessel wall lipid flux and atherosclerosis. Macrophages express the LDL receptor, which binds apoE with high affinity and could thereby affect the net secretion of apoE from macrophages. In these studies, we demonstrate that treatment of J774 macrophages transfected to constitutively express a human apoE3 cDNA with simvastatin, to increase LDL receptor activity, reduces the secretion of apoE. To further examine the relationship between LDL receptor expression and apoE secretion from macrophages, mouse peritoneal macrophages (MPMs) were isolated from mice with constitutively high expression of human LDL receptor to increase overall LDL receptor expression by 2- to 3-fold. Cells with increased LDL receptor expression also showed reduced apoE secretion compared with MPMs with basal LDL receptor expression. The effect of changes in LDL receptor expression on apoE secretion was isoform-specific, with greater reduction of apoE4 compared with apoE3 secretion and no reduction of apoE2 secretion, paralleling the known affinity of each isoform for LDL receptor binding. The effect of the LDL receptor on apoE secretion for each isoform was further reflected in LDL receptor-dependent changes in apoE-mediated cholesterol efflux. These results establish a regulatory interaction between two branches of macrophage sterol homeostatic pathways that could facilitate a rapid

Intensive glucose lowering and cardiovascular disease prevention in diabetes: reconciling the recent clinical trial data

Intensive glucose lowering and cardiovascular disease prevention in diabetes: reconciling the recent clinical trial dat

Endogenous Adipocyte ApoE is Co-localized with Caveolin at the Adipocyte Plasma Membrane

Apolipoprotein E is well-established as a secreted protein that plays an important role in systemic lipoprotein metabolism and vascular wall homeostasis. Recently, endogenous expression of apoE in adipocytes has been shown to play an important role in adipocyte lipoprotein metabolism and gene expression consistent with a non-secreted cellular itinerary for apoE. We designed studies to evaluate if adipocyte apoE was retained as a constituent protein in adipocytes, and to identify a cellular retention compartment. Using confocal microscopy, coimmunoprecipitation, and sucrose density cellular fractionation, we establish that endogenous apoE shares a cellular itinerary with the constituent protein caveolin-1. Altering adipocyte caveolar number by modulating cellular cholesterol flux or altering caveolin expression regulates the distribution of cellular apoE between cytoplasmic and plasma membrane compartments. A mechanism for co-localization of apoE with caveolin was established by demonstrating a noncovalent interaction between an aromatic amino acid-enriched apoE N-terminal domain with the caveolin scaffolding domain. Absent apoE expression in adipocytes alters caveolar lipid composition. These observations provide evidence for an interaction between two proteins involved in cellular lipid metabolism in a cell specialized for lipid storage and flux, and rationalize a biological basis for the impact of adipocyte apoE expression on adipocyte lipoprotein metabolism