Apolipoprotein E in macrophages and hepatocytes is eegraded via the proteasomal pathway

Unraveling APOE4's Role in Alzheimer's Disease: Pathologies and Therapeutic Strategies

Alzheimer's disease (AD), the most common kind of dementia worldwide, is characterized by elevated levels of the amyloid-β (Aβ) peptide and hyperphosphorylated tau protein in the neurons. The complexity of AD makes the development of treatments infamously challenging. Apolipoprotein E (APOE) genes's ε4 allele is one of the main genetic risk factors for AD. While the APOE gene's ε4 allele considerably increases the chance of developing AD, the ε2 allele is protective compared to the prevalent ε3 variant. It is fiercely discussed how APOE affects the development and course of disease since it has a variety of activities that influence both neuronal and non-neuronal cells. ApoE4 contributes to the formation of tau tangles, deposition of Aβ, neuroinflammation, and other processes. Four decades of research have provided a significant understanding of the structure of APOE and how this may affect the neuropathology and pathogenesis of AD. APOE is a crucial lipid transporter essential for the growth of the central nervous system (CNS), upkeep, and repair. The mechanisms by which APOE contributes to the pathophysiology of AD are still up for discussion, though. Evidence suggests that APOE affects the brain's clearance and deposition of Aβ. Additionally, APOE has Aβ-independent pathways in AD, which has led to the identification of new functions for APOE, including mitochondrial dysfunction. This study summarizes important studies that describe how APOE4 affects well-known AD pathologies, including tau pathology, Aβ, neuroinflammation, and dysfunction of neural networks. This study also envisions some of the therapeutic approaches being used to target APOE4 in the hopes of preventing or treating AD.

Isoform- and cell-state-specific APOE homeostasis and function

Apolipoprotein E is the major lipid transporter in the brain and an important player in neuron-astrocyte metabolic coupling. It ensures the survival of neurons under stressful conditions and hyperactivity by nourishing and detoxifying them. Apolipoprotein E polymorphism, combined with environmental stresses and/or age-related alterations, influences the risk of developing late-onset Alzheimer's disease. In this review, we discuss our current knowledge of how apolipoprotein E homeostasis, i.e. its synthesis, secretion, degradation, and lipidation, is affected in Alzheimer's disease.

The Patatin-Like Phospholipase Domain Containing Protein 7 Facilitates VLDL Secretion by Modulating ApoE Stability

BACKGROUND AND AIMS

The regulation of hepatic very-low-density lipoprotein (VLDL) secretion is vital for lipid metabolism whose pathogenetic status is involved in fatty liver disease and dyslipidemia seen in hepatic steatosis. Accumulated evidence suggest that apolipoprotein E (ApoE) is closely related to hepatic VLDL secretion. Here, we report that the expression of patatin-like phospholipase domain containing protein 7 (PNPLA7) is strongly induced by hepatic steatosis and positively correlates with plasma triacylglycerol (TAG) levels in the human subjects, whereas the role of PNPLA7 in hepatic VLDL secretion is unknown.

APPROACH AND RESULTS

Herein, with genetic manipulation in the mice, the deficiency of hepatic PNPLA7 expression resulted in reduced VLDL secretion accompanied by enhanced hepatic lipid accumulation and decreased hepatic ApoE expression. Furthermore, knockdown of PNPLA7 in the livers of the db/db mice also resulted in significant reduction in plasma TAG level but aggravated hepatic steatosis. Importantly, we observed that PNPLA7 interacted with ApoE and presumably at the site of endoplasmic reticulum. Mechanistically, we have shown that PNPLA7 could modulate polyubiquitination and proteasomal-mediated degradation of ApoE. Overexpressed ApoE restored the impaired VLDL-TAG metabolism in PNPLA7-knockdown primary hepatocytes.

CONCLUSION

PNPLA7 plays a critical role in regulating hepatic VLDL secretion by modulating ApoE stability through its interaction with ApoE.

Beyond the CNS: The many peripheral roles of APOE

Apolipoprotein E (APOE) is a multifunctional protein synthesized and secreted by multiple mammalian tissues. Although hepatocytes contribute about 75% of the peripheral pool, APOE can also be expressed in adipose tissue, the kidney, and the adrenal glands, among other tissues. High levels of APOE production also occur in the brain, where it is primarily synthesized by glia, and peripheral and brain APOE pools are thought to be distinct. In humans, APOE is polymorphic, with three major alleles (ε2, ε3, and ε4). These allelic forms dramatically alter APOE structure and function. Historically, the vast majority of research on APOE has centered on the important role it plays in modulating risk for cardiovascular disease and Alzheimer's disease. However, the established effects of this pleiotropic protein extend well beyond these two critical health challenges, with demonstrated roles across a wide spectrum of biological conditions, including adipose tissue function and obesity, metabolic syndrome and diabetes, fertility and longevity, and immune function. While the spectrum of biological systems in which APOE plays a role seems implausibly wide at first glance, there are some potential unifying mechanisms that could tie these seemingly disparate disorders together. In the current review, we aim to concisely summarize a wide breadth of APOE-associated pathologies and to analyze the influence of APOE in the development of several distinct disorders in order to provide insight into potential shared mechanisms implied in these various pathophysiological processes.

Diacylglycerol acyltransferase-2 and monoacylglycerol acyltransferase-2 are ubiquitinated proteins that are degraded by the 26S proteasome

Acyl-CoA:1,2-diacylglycerol acyltransferase (DGAT)-2 is one of the two DGAT enzymes that catalyzes the synthesis of triacylglycerol, which is an important form of stored energy for eukaryotic organisms. There is currently limited information available regarding how DGAT2 and triacylglycerol synthesis are regulated. Recent studies have indicated that DGAT2 can be regulated by changes in gene expression. How DGAT2 is regulated post-transcriptionally remains less clear. In this study, we demonstrated that DGAT2 is a very unstable protein and is rapidly degraded in an ubiquitin-dependent manner via the proteasome. Many of the 25 lysines present in DGAT2 appeared to be involved in promoting its degradation. However, the six C-terminal lysines were the most important in regulating stability. We also demonstrated that acyl-CoA:monoacylglycerol acyltransferase (MGAT)-2, an enzyme with extensive sequence homology to DGAT2 that catalyzes the synthesis of diacylglycerol, was also ubiquitinated. However, MGAT2 was found to be much more stable than DGAT2. Interestingly, when co-expressed, MGAT2 appeared to stabilize DGAT2. Finally, we found that both DGAT2 and MGAT2 are substrates of the endoplasmic reticulum-associated degradation pathway.

Amyloid Precursor Protein (APP) May Act as a Substrate and a Recognition Unit for CRL4CRBN and Stub1 E3 Ligases Facilitating Ubiquitination of Proteins Involved in Presynaptic Functions and Neurodegeneration

The amyloid precursor protein (APP), whose mutations cause Alzheimer disease, plays an important in vivo role and facilitates transmitter release. Because the APP cytosolic region (ACR) is essential for these functions, we have characterized its brain interactome. We found that the ACR interacts with proteins that regulate the ubiquitin-proteasome system, predominantly with the E3 ubiquitin-protein ligases Stub1, which binds the NH2 terminus of the ACR, and CRL4(CRBN), which is formed by Cul4a/b, Ddb1, and Crbn, and interacts with the COOH terminus of the ACR via Crbn. APP shares essential functions with APP-like protein-2 (APLP2) but not APP-like protein-1 (APLP1). Noteworthy, APLP2, but not APLP1, interacts with Stub1 and CRL4(CRBN), pointing to a functional pathway shared only by APP and APLP2. In vitro ubiquitination/ubiquitome analysis indicates that these E3 ligases are enzymatically active and ubiquitinate the ACR residues Lys(649/650/651/676/688) Deletion of Crbn reduces ubiquitination of Lys(676) suggesting that Lys(676) is physiologically ubiquitinated by CRL4(CRBN) The ACR facilitated in vitro ubiquitination of presynaptic proteins that regulate exocytosis, suggesting a mechanism by which APP tunes transmitter release. Other dementia-related proteins, namely Tau and apoE, interact with and are ubiquitinated via the ACR in vitro This, and the evidence that CRBN and CUL4B are linked to intellectual disability, prompts us to hypothesize a pathogenic mechanism, in which APP acts as a modulator of E3 ubiquitin-protein ligase(s), shared by distinct neuronal disorders. The well described accumulation of ubiquitinated protein inclusions in neurodegenerative diseases and the link between the ubiquitin-proteasome system and neurodegeneration make this concept plausible.

S-Adenosylmethionine increases circulating very-low density lipoprotein clearance in non-alcoholic fatty liver disease

BACKGROUND & AIMS

Very-low-density lipoproteins (VLDLs) export lipids from the liver to peripheral tissues and are the precursors of low-density-lipoproteins. Low levels of hepatic S-adenosylmethionine (SAMe) decrease triglyceride (TG) secretion in VLDLs, contributing to hepatosteatosis in methionine adenosyltransferase 1A knockout mice but nothing is known about the effect of SAMe on the circulating VLDL metabolism. We wanted to investigate whether excess SAMe could disrupt VLDL plasma metabolism and unravel the mechanisms involved.

METHODS

Glycine N-methyltransferase (GNMT) knockout (KO) mice, GNMT and perilipin-2 (PLIN2) double KO (GNMT-PLIN2-KO) and their respective wild type (WT) controls were used. A high fat diet (HFD) or a methionine deficient diet (MDD) was administrated to exacerbate or recover VLDL metabolism, respectively. Finally, 33 patients with non-alcoholic fatty-liver disease (NAFLD); 11 with hypertriglyceridemia and 22 with normal lipidemia were used in this study.

RESULTS

We found that excess SAMe increases the turnover of hepatic TG stores for secretion in VLDL in GNMT-KO mice, a model of NAFLD with high SAMe levels. The disrupted VLDL assembly resulted in the secretion of enlarged, phosphatidylethanolamine-poor, TG- and apoE-enriched VLDL-particles; special features that lead to increased VLDL clearance and decreased serum TG levels. Re-establishing normal SAMe levels restored VLDL secretion, features and metabolism. In NAFLD patients, serum TG levels were lower when hepatic GNMT-protein expression was decreased.

CONCLUSIONS

Excess hepatic SAMe levels disrupt VLDL assembly and features and increase circulating VLDL clearance, which will cause increased VLDL-lipid supply to tissues and might contribute to the extrahepatic complications of NAFLD.

Protein Arginylation in Rat Brain Cytosol: A Proteomic Analysis

Arginine can be post-translationally incorporated from arginyl-tRNA into the N-terminus of soluble acceptor proteins in a reaction catalyzed by arginyl-tRNA protein transferase. In the present study, several soluble rat brain proteins that accepted arginine were identified after arginine incorporation by two dimensional electrophoresis and mass spectrometry. They were identified as: contrapsin-like protease inhibitor-3, alpha-1-antitrypsin, apolipoprotein E, hemopexin, calreticulin and apolipoprotein A-I. All of these proteins shared a signal sequence for the translocation of proteins across endoplasmic reticulum membranes. After losing the signal peptide, these proteins expose amino acids described as compatible for post-translational arginylation. Although the enzymatic system involved in arginylation is confined mainly in cytosol and nucleus, all the substrates described herein enter to the exocytic pathway co-translationally. Therefore, we postulate that the substrates for arginylation could reach the cytosol by retro-translocation and be then arginylated.

Degradation of perilipin is mediated through ubiquitination-proteasome pathway

Perilipin protein coats the surface of intracellular lipid droplets and plays fundamental roles in lipid droplet formation and triacylglycerol hydrolysis. Perilipin is transcriptionally regulated through peroxisome proliferator-activated receptor and post-translationally stabilized by stored intracellular neutral lipids. In this study, we show that perilipin protein accumulates in transfected Chinese hamster ovary cells cultured in the presence of fatty acids but in turn is destabilized when lipid precursors for triacylglycerol synthesis are removed from culture serum. Adding fatty acids in the culture medium prevents the degradation of perilipin. Moreover, specific proteasome inhibitors, MG132, lactacystin, and ALLN, block the degradation, whereas inhibitors of other proteases are ineffective. Pulse-chase experiments confirm that perilipin is degraded through proteasome, a process that is inhibited by MG132 or ALLN and blunted by the addition of oleic acid. We have detected the co-immunoprecipitation of perilipin and ubiquitin, thus confirming that perilipin is conjugated to poly-ubiquitin and targeted for proteasomal degradation. Treatment with MG132 increases the expression of perilipin associated with lipid droplets as well as modestly throughout the cytosol. We conclude that the degradation of perilipin is mediated through an ubiquitination-proteasome pathway, which suggests another mode for the post-translational regulation of perilipin.

Post-translational regulation of adipose differentiation-related protein by the ubiquitin/proteasome pathway

Adipose differentiation-related protein (ADRP) is localized to lipid droplets in most mammalian cells. ADRP, proposed to regulate fatty acid mobilization and lipid droplet formation, is linked to lipid accumulation in foam cells of human atherosclerotic lesions. In this report, we show that ADRP protein accumulates in Chinese hamster ovary fibroblastic cells cultured in the presence of oleic acid but is destabilized when fatty acid sources are removed from culture serum. The latter effect was blocked by the proteasome inhibitor MG132, whereas inhibitors of other proteolytic processes were ineffective. Pulse-chase experiments confirmed that ADRP degradation is inhibited by MG132. Conditions that stimulate ADRP degradation also promoted the covalent modification of ADRP by ubiquitin, whereas the addition of oleic acid to culture media, which promotes triacylglycerol deposition, blunted the appearance of ubiquitinated-ADRP. Treatment with MG132 increased the levels of ADRP associated with lipid droplets, as well as throughout the cytosol. Finally, we demonstrate that the disappearance of ADRP protein after the onset of perilipin expression during adipocyte differentiation is due to degradation by proteasomes Thus, proteolytic degradation of ADRP mediated through the ubiquitin/proteasome pathway appears to be a major mode for the post-translational regulation of ADRP.

The key role of apolipoprotein E in atherosclerosis

Apolipoprotein E is a multifunctional protein that is synthesized by the liver and several peripheral tissues and cell types, including macrophages. The protein is involved in the efficient hepatic uptake of lipoprotein particles, stimulation of cholesterol efflux from macrophage foam cells in the atherosclerotic lesion, and the regulation of immune and inflammatory responses. Apolipoprotein E deficiency in mice leads to the development of atherosclerosis and re-expression of the protein reduces the extent of the disease. This review presents evidence for the potent anti-atherogenic action of apolipoprotein E and describes our current understanding of its multiple functions and regulation by factors implicated in the pathogenesis of cardiovascular disease.

Post-transcriptional regulation of apoC-I synthesis and secretion in human HepG2 cells

ApoC-I plays an important role in controlling plasma lipid metabolism, however little is known about factors regulating the hepatic synthesis and secretion of this apolipoprotein. In the present study, we have carried out experiments with human hepatoma (HepG2) cells, in order to determine the effect of different tissue culture conditions on cellular lipid levels and on the production of apoC-I (and apoE) at the protein and mRNA level. Cells incubated for 48 h with 10% human serum had significantly higher cellular triglyceride (22%, P<0.05) and cholesterol levels (19%, P<0.01), higher medium apoC-I and apoE levels (2.6- and 2.9-fold, respectively), but similar levels of apoC-I and apoE mRNA, compared to cells incubated with 10% human lipoprotein-deficient serum (LPDS). Serum containing only HDL, or containing HDL with LDL, also increased cellular lipids and increased secreted apoC-I and apoE levels without altering apoC-I and apoE mRNA levels. Incubation of cells with Intralipid triglyceride (625 microM), increased cellular triglyceride (2.8-fold, P<0.001), decreased cellular cholesterol (32%, P<0.01), decreased cellular and medium apoC-I (24 and 26%, P<0.01) and had no effect on apoC-I mRNA levels. Additional experiments in which cells were loaded with cholesterol (incubation with 10 microg/ml cholesterol plus 1 microg/ml 25-hydroxycholesterol) or depleted of cholesterol (statin treatment) confirmed that secretion of apoC-I by HepG2 cells was dependent on cellular cholesterol levels and independent of changes in apoC-I mRNA levels. These results demonstrate that cellular cholesterol rather than triglyceride levels play a role in controlling apoC-I production by HepG2 cells and that this regulation occurs at a post-transcriptional level.

Synthesis and secretion of apoC-I and apoE during maturation of human SW872 liposarcoma cells

Little is known about the regulation of apolipoprotein (apo) C-I production by human adipocytes. The aim of the present study, therefore, was to investigate the effect of different tissue culture conditions on the synthesis and secretion of apoC-I and apoE in human SW872 liposarcoma cells. After 3-4 d in culture (0.5 x 10(6) cells/well, DMEM/F-12 medium with 10% fetal calf serum), cells reached confluence and became growth arrested. The molar ratio of apoE:apoC-I in the cell was 8.9 +/- 0.6 and in the medium was 6.6 +/- 0.5. After 17 d in culture, SW872 cells contained significantly more cholesterol (100%) and triglyceride (3-fold) and secreted more apoC-I [4 vs. 17 d: 0.11 +/- 0.01 vs. 0.23 +/- 0.01 pmol/(10(6) cells . 24 h), P < 0.001] and apoE [0.7 +/- 0.1 vs. 3.1 +/- 0.3 pmol/(10(6) cells . 24 h), P < 0.001]. Cellular apoC-I increased 7-fold and apoE increased 16-fold. Cell maturation was associated with significantly higher levels of apoE mRNA but not apoC-I mRNA. Increases in cell lipids, apoC-I, and apoE were not dependent on the presence of extracellular lipids because similar changes occurred in cells incubated with lipoprotein-deficient serum or in cells incubated without serum. Treatment (7 d) of cells during maturation with insulin (10 or 1000 nmol/L) significantly reduced the secretion of apoC-I and apoE. These results demonstrate that in maturing SW872 cells, cholesterol and triglyceride accumulation in the presence or absence of extracellular lipids, is associated with increased apoC-I and apoE production. Furthermore, apoC-I and apoE production are differentially regulated at the transcriptional level, and long-term treatment with insulin has an inhibitory rather than stimulatory effect on apoC-I and apoE production.

Posttranscriptional control of the expression and function of diacylglycerol acyltransferase-1 in mouse adipocytes

Acyl-CoA:diacylglycerol acyltransferase-1 (DGAT1) catalyzes the final step of triglyceride synthesis in mammalian cells. Data obtained from DGAT1-knockout mice have indicated that this enzyme plays an important role in energy homeostasis. We investigated the regulation of the expression and function of DGAT1 in mouse 3T3-L1 cell as a model for mammalian adipocytes. We demonstrated that the DGAT1 protein level increased by approximately 90-fold following differentiation of 3T3-L1 into mature adipocytes, a change that was accompanied by approximately 7-fold increase in DGAT1 mRNA. On the other hand, forced overexpression of DGAT1 mRNA by >20-fold via a recombinant adenovirus only resulted in approximately 2-fold increase in DGAT1 protein in mature adipocytes and little increase in preadipocytes. These results indicated that gene expression of DGAT1 in adipocytes is subjected to rigorous posttranscriptional regulation, which is modulated significantly by the differentiation status of 3T3-L1 cells. Protein stability is not a significant factor in the control of DGAT1 expression. The steady-state levels of DGAT1 were unaffected by blockage of proteolytic pathways by ALLN. However, translational control was suggested by sequence analysis of the 5'-untranslated region of human DGAT1 (hDGAT1) mRNA. We found that the level of DGAT1 activity was predominantly a function of the steady-state level of DGAT1 protein. No significant functional changes were observed when the conserved tyrosine phosphorylation site in hDGAT1 was mutated by a single base pair substitution. Despite only a approximately 2-fold increase in DGAT1 protein caused by recombinant viral transduction, a proportionate increase in cellular triglyceride synthesis resulted without affecting the triglyceride lipolysis rate, leading to >2-fold increase in intracellular triglyceride accumulation. No change in adipocyte morphology or in the expression levels of lipoprotein lipase, proxisomal proliferation-activating receptor-gamma, and aP2 was evident secondary to DGAT1 overexpression at different stages in 3T3-L1 differentiation. These data suggest that dysregulation of DGAT1 may play a role in the development of obesity, and manipulation of the steady-state level of DGAT1 protein may offer a potential means to treat or prevent obesity.

ATP binding cassette transporter ABCA1 modulates the secretion of apolipoprotein E from human monocyte-derived macrophages

Apolipoprotein E (apoE) produced by macrophages in the arterial wall protects against atherosclerosis, but the regulation of its secretion by these cells is poorly understood. Here we investigated the contribution of the adenosine triphosphate binding cassette transporters ABCA1 and ABC8 to the secretion of apoE from either primary human monocyte-derived macrophages (HMDM) or human THP1 macrophages. During incubations of up to 6 h, apoE secretion from both THP1 macrophages and HMDM was stimulated by 8-Br-cAMP, which activates ABCA1 expression. The putative ABCA1 inhibitor glyburide and antisense oligonucleotides directed against ABCA1 mRNA significantly reduced apoE secretion from THP1 macrophages and HMDM. Antisense oligonucleotides directed against ABC8 mRNA also inhibited apoE secretion, although this inhibition was less pronounced and consistent than in the case of ABCA1. ApoE secretion from HMDM of ABCA1-deficient patients with Tangier disease was also decreased. ApoE mRNA expression was not affected by inhibition of ABCA1 or ABC8 in normal HMDM or the lack of functional ABCA1 in HMDM from Tangier disease patients. Inhibition of ABCA1 in HMDM prevented the occurrence of anti-apoE-immunoreactive granular structures in the plasma membrane. We conclude that ABCA1 and, to a lesser extent, ABC8 both promote secretion of apoE from human macrophages.