Domains of apolipoprotein E contributing to triglyceride and cholesterol homeostasis in vivo. Carboxyl-terminal region 203-299 promotes hepatic very low density lipoprotein-triglyceride secretion

Reconstituted HDL-apoE3 promotes endothelial cell migration through ID1 and its downstream kinases ERK1/2, AKT and p38 MAPK

INTRODUCTION

Atherosclerotic Coronary Artery Disease (ASCAD) is the leading cause of mortality worldwide. Novel therapeutic approaches aiming to improve the atheroprotective functions of High Density Lipoprotein (HDL) include the use of reconstituted HDL forms containing human apolipoprotein A-I (rHDL-apoA-I). Given the strong atheroprotective properties of apolipoprotein E3 (apoE3), rHDL-apoE3 may represent an attractive yet largely unexplored therapeutic agent.

OBJECTIVE

To evaluate the atheroprotective potential of rHDL-apoE3 starting with the unbiased assessment of global transcriptome effects and focusing on endothelial cell (EC) migration as a critical process in re-endothelialization and atherosclerosis prevention. The cellular, molecular and functional effects of rHDL-apoE3 on EC migration-associated pathways were assessed, as well as the potential translatability of these findings in vivo.

METHODS

Human Aortic ECs (HAEC) were treated with rHDL-apoE3 and total RNA was analyzed by whole genome microarrays. Expression and phosphorylation changes of key EC migration-associated molecules were validated by qRT-PCR and Western blot analysis in primary HAEC, Human Coronary Artery ECs (HCAEC) and the human EA.hy926 EC line. The capacity of rHDL-apoE3 to stimulate EC migration was assessed by wound healing and transwell migration assays. The contribution of MEK1/2, PI3K and the transcription factor ID1 in rHDL-apoE3-induced EC migration and activation of EC migration-related effectors was assessed using specific inhibitors (PD98059: MEK1/2, LY294002: PI3K) and siRNA-mediated gene silencing, respectively. The capacity of rHDL-apoE3 to improve vascular permeability and hypercholesterolemia in vivo was tested in a mouse model of hypercholesterolemia (apoE KO mice) using Evans Blue assays and lipid/lipoprotein analysis in the serum, respectively.

RESULTS

rHDL-apoE3 induced significant expression changes in 198 genes of HAEC mainly involved in re-endothelialization and atherosclerosis-associated functions. The most pronounced effect was observed for EC migration, with 42/198 genes being involved in the following EC migration-related pathways: 1) MEK/ERK, 2) PI3K/AKT/eNOS-MMP2/9, 3) RHO-GTPases, 4) integrin. rHDL-apoE3 induced changes in 24 representative transcripts of these pathways in HAEC, increasing the expression of their key proteins PIK3CG, EFNB2, ID1 and FLT1 in HCAEC and EA.hy926 cells. In addition, rHDL-apoE3 stimulated migration of HCAEC and EA.hy926 cells, and the migration was markedly attenuated in the presence of PD98059 or LY294002. rHDL-apoE3 also increased the phosphorylation of ERK1/2, AKT, eNOS and p38 MAPK in these cells, while PD98059 and LY294002 inhibited rHDL-apoE3-induced phosphorylation of ERK1/2, AKT and p38 MAPK, respectively. LY had no effect on rHDL-apoE3-mediated eNOS phosphorylation. ID1 siRNA markedly decreased EA.hy926 cell migration by inhibiting rHDL-apoE3-triggered ERK1/2 and AKT phosphorylation. Finally, administration of a single dose of rHDL-apoE3 in apoE KO mice markedly improved vascular permeability as demonstrated by the reduced concentration of Evans Blue dye in tissues such as the stomach, the tongue and the urinary bladder and ameliorated hypercholesterolemia.

CONCLUSIONS

rHDL-apoE3 significantly enhanced EC migration in vitro, predominantly via overexpression of ID1 and subsequent activation of MEK1/2 and PI3K, and their downstream targets ERK1/2, AKT and p38 MAPK, respectively, and improved vascular permeability in vivo. These novel insights into the rHDL-apoE3 functions suggest a potential clinical use to promote re-endothelialization and retard development of atherosclerosis.

Genome-Wide Association Study of NAFLD Using Electronic Health Records

Genome-wide association studies (GWAS) have identified several risk loci for nonalcoholic fatty liver disease (NAFLD). Previous studies have largely relied on small sample sizes and have assessed quantitative traits. We performed a case-control GWAS in the UK Biobank using recorded diagnosis of NAFLD based on diagnostic codes recommended in recent consensus guidelines. We performed a GWAS of 4,761 cases of NAFLD and 373,227 healthy controls without evidence of NAFLD. Sensitivity analyses were performed excluding other co-existing hepatic pathology, adjusting for body mass index (BMI) and adjusting for alcohol intake. A total of 9,723,654 variants were assessed by logistic regression adjusted for age, sex, genetic principal components, and genotyping batch. We performed a GWAS meta-analysis using available summary association statistics. Six risk loci were identified (P < 5*10-8 ) (apolipoprotein E [APOE], patatin-like phospholipase domain containing 3 [PNPLA3, transmembrane 6 superfamily member 2 [TM6SF2], glucokinase regulator [GCKR], mitochondrial amidoxime reducing component 1 [MARC1], and tribbles pseudokinase 1 [TRIB1]). All loci retained significance in sensitivity analyses without co-existent hepatic pathology and after adjustment for BMI. PNPLA3 and TM6SF2 remained significant after adjustment for alcohol (alcohol intake was known in only 158,388 individuals), with others demonstrating consistent direction and magnitude of effect. All six loci were significant on meta-analysis. Rs429358 (P = 2.17*10-11 ) is a missense variant within the APOE gene determining ϵ4 versus ϵ2/ϵ3 alleles. The ϵ4 allele of APOE offered protection against NAFLD (odds ratio for heterozygotes 0.84 [95% confidence interval 0.78-0.90] and homozygotes 0.64 [0.50-0.79]). Conclusion: This GWAS replicates six known NAFLD-susceptibility loci and confirms that the ϵ4 allele of APOE is associated with protection against NAFLD. The results are consistent with published GWAS using histological and radiological measures of NAFLD, confirming that NAFLD identified through diagnostic codes from consensus guidelines is a valid alternative to more invasive and costly approaches.

Association between apolipoprotein E gene polymorphism and nonalcoholic fatty liver disease in Southern China: A case-control study

BACKGROUND

Apolipoprotein E (ApoE) polymorphisms have been reported to be associated with nonalcoholic fatty liver disease (NAFLD), but the conclusions of studies are inconsistent in different regions. The present study aims to investigate the role of ApoE genotypes on NAFLD in southern China.

METHODS

A total of 1064 subjects including 372 NAFLD patients and 692 controls who attended Meizhou People's Hospital located in southern China from March 1, 2016 to April 30, 2020 were enrolled in this study. The ApoE genotypes were detected and the laboratory parameters were examined.

RESULTS

Significant differences were observed between NAFLD patients and controls in the prevalence of ε3/ε3 (p < 0.001) and ε3/ε4 (p = 0.004). NAFLD patients presented higher frequency of ε4 allele than controls (p = 0.013). Logistic regression analysis suggested that ε3/ε3 was an independent risk factor (OR: 1.435, 95% CI: 1.084-1.891, p = 0.010), while ε3/ε4 was an independent protective factor (OR: 0.578, 95% CI: 0.404-0.828, p = 0.003) for development of NAFLD. In addition, allele ε4 showed a protective effect on NAFLD with an adjusted OR of 0.588 (95% CI: 0.420-0.824, p = 0.002).

CONCLUSION

Our results suggested that ApoE genotype was associated with the development of NAFLD in the population of southern China. Individuals carrying ε3/ε3 were at higher risk of NAFLD, while those carrying ε3/ε4 were at lower risk of NAFLD.

Isoform and tissue dependent impact of apolipoprotein E on adipose tissue metabolic activation: The role of apolipoprotein A1

Adipose organ is made of white (WAT) and brown (BAT) adipose tissue which are primarily responsible for lipid storage and energy production (heat and ATP) respectively. Metabolic activation of WAT may ascribe to this tissue characteristics of BAT, namely non-shivering thermogenesis and ATP production. Recent data indicate that apolipoproteins E (APOE) and A1 (APOA1) regulate WAT mitochondrial metabolic activation. Here, we investigated the functional cross-talk between natural human APOE2 and APOE4 isoforms with APOA1 in this process, using Apoe2^knock-in and Apoe4^knock-in mice. At baseline when Apoe2^knock-in and Apoe4^knock-in mice express both APOE and Apoa1, the Apoe2^knock-in strain appears to have higher mitochondrial oxidative phosphorylation levels and non-shivering thermogenesis in WAT compared to Apoe4^knock-in mice. When mice were switched to a high-fat diet for 18 weeks, circulating levels of endogenous Apoa1 in Apoe2^knock-in mice became barely detectable though significant levels of APOE2 were still present. This change was accompanied by a significant reduction in WAT mitochondrial Ucp1 expression while BAT Ucp1 was unaffected. Ectopic APOA1 expression in Apoe2^knock-in animals potently stimulated WAT but not BAT mitochondrial Ucp1 expression providing further evidence that APOA1 potently stimulates WAT non-shivering thermogenesis in the presence of APOE2. Ectopic expression of APOA1 in Apoe4^knock-in mice stimulated BAT but no WAT mitochondrial Ucp1 levels, suggesting that in the presence of APOE4, APOA1 is a trigger of BAT non-shivering thermogenesis. Overall, our data identified a tissue-specific role of the natural human APOE2 and APOE4 isoforms in WAT- and BAT-metabolic activation respectively, that appears dependent on circulating APOA1 levels.

Adrenoceptor-related decrease in serum triglycerides is independent of PPARα activation

Adrenoceptor (AR)-linked pathways belong to the major components of the stress response system and are associated with the pathophysiology of diseases within the spectrum of metabolic syndrome. In this study, the role of adrenoceptor stimulation in serum triglyceride (TG) regulation in mice was investigated. For this purpose, α₁ -ARs were activated with phenylephrine (PH) and β_1/2 -ARs with isoprenaline (ISOP). Both AR agonists markedly reduced serum TG levels independently of PPARα activation. These drugs also significantly activated the hormone-sensitive lipase in the white adipose tissue indicating increased mobilization of TGs in this tissue. In addition, PH and ISOP up-regulated Lpl, Nr4A, Dgat1, Mttp, Aadac and Cd36 genes, critical in TG regulation, whereas the observed decrease in serum TG levels was independent of the hepatic very low-density lipoprotein (VLDL)-TG secretion. Interestingly, PH and ISOP also inactivated the hepatic insulin/PI3k/AKT/FoxO1 signaling pathway, holding a critical role in the regulation of genes involved in TG synthesis. Taken together, the findings of the present study indicate that stimulation of α₁ - and β_1/2 -ARs markedly reduced serum TG steady-state levels as a result of alterations in TG synthesis, uptake, transport, hydrolysis, metabolism and clearance, an effect induced by PPARα independent mechanisms.

Karavia E, Constantinou C, Hatziri A, Kalogeropoulou C, Xepapadaki E, Zvintzou E. Apolipoprotein E in diet-induced obesity: a paradigm shift from conventional perception

Apolipoprotein E (APOE) is a major protein component of peripheral and brain lipoprotein transport systems. APOE in peripheral circulation does not cross blood brain barrier or blood cerebrospinal fluid barrier. As a result, peripheral APOE expression does not affect brain APOE levels and vice versa. Numerous epidemiological studies suggest a key role of peripherally expressed APOE in the development and progression of coronary heart disease while brain APOE has been associated with dementia and Alzheimer's disease. More recent studies, mainly in experimental mice, suggested a link between Apoe and morbid obesity. According to the latest findings, expression of human apolipoprotein E3 (APOE3) isoform in the brain of mice is associated with a potent inhibition of visceral white adipose tissue (WAT) mitochondrial oxidative phosphorylation leading to significantly reduced substrate oxidation, increased fat accumulation and obesity. In contrast, hepatically expressed APOE3 is associated with a notable shift of substrate oxidation towards non-shivering thermogenesis in visceral WAT mitochondria, leading to resistance to obesity. These novel findings constitute a major paradigm shift from the widely accepted perception that APOE promotes obesity via receptor-mediated postprandial lipid delivery to WAT. Here, we provide a critical review of the latest facts on the role of APOE in morbid obesity.

Pleiotropic effects of apolipoprotein C3 on HDL functionality and adipose tissue metabolic activity

APOC3 is produced mainly by the liver and intestine and approximately half of plasma APOC3 associates with HDL. Though it was believed that APOC3 associates with HDL by simple binding to preexisting particles, recent data support that biogenesis of APOC3-containing HDL (APOC3-HDL) requires Abca1. Moreover, APOC3-HDL contributes to plasma triglyceride homeostasis by preventing APOC3 association with triglyceride-rich lipoproteins. Interestingly, APOC3-HDL also shows positive correlation with the morbidly obese phenotype. However, the roles of APOC3 in HDL functionality and adipose tissue metabolic activity remain unknown. Therefore, here we investigated the direct effects of APOC3 expression on HDL structure and function, as well as white adipose tissue (WAT) and brown adipose tissue (BAT) metabolic activity. C57BL/6 mice were infected with an adenovirus expressing human APOC3 or a recombinant attenuated control adenovirus expressing green fluorescent protein and blood and tissue samples were collected at 5 days postinfection. HDL was then analyzed for its apolipoprotein and lipid composition and particle functionality. Additionally, purified mitochondria from BAT and WAT were analyzed for uncoupling protein 1, cytochrome c (Cytc), and Cytc oxidase subunit 4 protein levels as an indirect measure of their metabolic activity. Serum metabolomic analysis was performed by NMR. Combined, our data show that APOC3 modulates HDL structure and function, while it selectively promotes BAT metabolic activation.

Apolipoprotein E - A Multifunctional Protein with Implications in Various Pathologies as a Result of Its Structural Features

Apolipoprotein E (apoE), a 34 kDa glycoprotein, mediates hepatic and extrahepatic uptake of plasma lipoproteins and cholesterol efflux from lipid-laden macrophages. In humans, three structural different apoE isoforms occur, with subsequent functional changes and pathological consequences. Here, we review data supporting the involvement of apoE structural domains and isoforms in normal and altered lipid metabolism, cardiovascular and neurodegenerative diseases, as well as stress-related pathological states. Studies using truncated apoE forms provided valuable information regarding the regions and residues responsible for its properties. ApoE3 renders protection against cardiovascular diseases by maintaining lipid homeostasis, while apoE2 is associated with dysbetalipoproteinemia. ApoE4 is a recognized risk factor for Alzheimer's disease, although the exact mechanism of the disease initiation and progression is not entirely elucidated. ApoE is also implicated in infections with herpes simplex type-1, hepatitis C and human immunodeficiency viruses. Interacting with both viral and host molecules, apoE isoforms differently interfere with the viral life cycle. ApoE exerts anti-inflammatory effects, switching macrophage phenotype from the proinflammatory M1 to the anti-inflammatory M2, suppressing CD4+ and CD8+ lymphocytes, and reducing IL-2 production. The anti-oxidative properties of apoE are isoform-dependent, modulating the levels of various molecules (Nrf2 target genes, metallothioneins, paraoxonase). Mimetic peptides were designed to exploit apoE beneficial properties. The "structure correctors" which convert apoE4 into apoE3-like molecules have pharmacological potential. Despite no successful strategy is yet available for apoE-related disorders, several promising candidates deserve further improvement and exploitation.

Distinct Roles of Apolipoproteins A1 and E in the Modulation of High-Density Lipoprotein Composition and Function

In addition to high-density lipoprotein cholesterol (HDL-C) levels, HDL quality also appears to be very important for atheroprotection. Analysis of various clinical paradigms suggests that the lipid and apolipoprotein composition of HDL defines its size, shape, and functions and may determine its beneficial effects on human health. Previously, we reported that like apolipoprotein A-I (Apoa1), apolipoprotein E (Apoe) is also capable of promoting the de novo biogenesis of HDL with the participation of ATP binding cassette A lipid transporter member 1 (Abca1) and plasma enzyme lecithin:cholesterol acyltransferase (Lcat), in a manner independent of a functional Apoa1. Here, we performed a comparative analysis of the functions of these HDL subpopulations. Specifically, Apoe and Apoa1 double-deficient (Apoe(-/-) × Apoa1(-/-)) mice were infected with APOA1- or APOE3-expressing adenoviruses, and APOA1-containing HDL (APOA1-HDL) and APOE3-containing HDL (APOE3-HDL), respectively, were isolated and analyzed by biochemical and physicochemical methods. Western blot and lipidomic analyses indicated significant differences in the apolipoprotein and lipid composition of the two HDL species. Moreover APOE3-HDL presented a markedly reduced antioxidant potential and Abcg1-mediated cholesterol efflux capacity. Surprisingly, APOE3-HDL but not APOA1-HDL attenuated LPS-induced production of TNFα in RAW264.7 cells, suggesting that the anti-inflammatory effects of APOA1 are dependent on APOE expression. Taken together, our data indicate that APOA1 and APOE3 recruit different apolipoproteins and lipids on the HDL particle, leading to structurally and functionally distinct HDL subpopulations. The distinct role of these two apolipoproteins in the modulation of HDL functionality may pave the way toward the development of novel pharmaceuticals that aim to improve HDL functionality.

Transcription factor Nrf1 negatively regulates the cystine/glutamate transporter and lipid-metabolizing enzymes

Liver-specific Nrf1 (NF-E2-p45-related factor 1) knockout mice develop nonalcoholic steatohepatitis. To identify postnatal mechanisms responsible for this phenotype, we generated an inducible liver-specific Nrf1 knockout mouse line using animals harboring an Nrf1(flox) allele and a rat CYP1A1-Cre transgene (Nrf1(flox/flox)::CYP1A1-Cre mice). Administration of 3-methylcholanthrene (3-MC) to these mice (Nrf1(flox/flox)::CYP1A1-Cre+3MC mice) resulted in loss of hepatic Nrf1 expression. The livers of mice lacking Nrf1 accumulated lipid, and the hepatic fatty acid (FA) composition in such animals differed significantly from that in the Nrf1(flox/flox)::CYP1A1-Cre control. This change was provoked by upregulation of several FA metabolism genes. Unexpectedly, we also found that the level of glutathione was increased dramatically in livers of Nrf1(flox/flox)::CYP1A1-Cre+3MC mice. While expression of glutathione biosynthetic enzymes was unchanged, xCT, a component of the cystine/glutamate antiporter system x(c)(-), was significantly upregulated in livers of Nrf1(flox/flox)::CYP1A1-Cre+3MC mice, suggesting that Nrf1 normally suppresses xCT. Thus, stress-inducible expression of xCT is a two-step process: under homeostatic conditions, Nrf1 effectively suppresses nonspecific transactivation of xCT, but when cells encounter severe oxidative/electrophilic stress, Nrf1 is displaced from an antioxidant response element (ARE) in the gene promoter while Nrf2 is recruited to the ARE. Thus, Nrf1 controls both the FA and the cystine/cysteine content of hepatocytes by participating in an elaborate regulatory network.

apoE3[K146N/R147W] acts as a dominant negative apoE form that prevents remnant clearance and inhibits the biogenesis of HDL

The K146N/R147W substitutions in apoE3 were described in patients with a dominant form of type III hyperlipoproteinemia. The effects of these mutations on the in vivo functions of apoE were studied by adenovirus-mediated gene transfer in different mouse models. Expression of the apoE3[K146N/R147W] mutant in apoE-deficient (apoE(-/-)) or apoA-I-deficient (apoA-I(-/-))×apoE(-/-) mice exacerbated the hypercholesterolemia and increased plasma apoE and triglyceride levels. In apoE(-/-) mice, the apoE3[K146N/R147W] mutant displaced apoA-I from the VLDL/LDL/HDL region and caused the accumulation of discoidal apoE-containing HDL. The WT apoE3 cleared the cholesterol of apoE(-/-) mice without induction of hypertriglyceridemia and promoted formation of spherical HDL. A unique property of the truncated apoE3[K146N/R147W]202 mutant, compared with similarly truncated apoE forms, is that it did not correct the hypercholesterolemia. The contribution of LPL and LCAT in the induction of the dyslipidemia was studied. Treatment of apoE(-/-) mice with apoE3[K146N/R147W] and LPL corrected the hypertriglyceridemia, but did not prevent the formation of discoidal HDL. Treatment with LCAT corrected hypertriglyceridemia and generated spherical HDL. The combined data indicate that the K146N/R147W substitutions convert the full-length and the truncated apoE3[K146N/R147W] mutant into a dominant negative ligand that prevents receptor-mediated remnant clearance, exacerbates the dyslipidemia, and inhibits the biogenesis of HDL.

Allele-dependent thermodynamic and structural perturbations in ApoE variants associated with the correction of dyslipidemia and formation of spherical ApoE-containing HDL particles

Overexpression of ApoE4[Leu261Ala/Trp264Ala/Phe265Ala] mutant (ApoE4mutC) prevents hypertriglyceridemia and promotes formation of spherical ApoE-containing HDL in ApoE(-/-) or ApoA-I(-/-) mice. Although, a similar phenotype was observed with ApoE2[Leu261Ala/Trp264Ala/Phe265Ala] (ApoE2mutC), small differences in cholesterol distribution to IDL/LDL, HDL2 and HDL3 fractions and ApoE distribution to HDL2 and HDL3 fractions suggested that ApoE allelic background can influence mutant ApoE properties. To understand the structural basis behind the properties of ApoE2mutC and ApoE4mutC variants we analyzed their structural and thermodynamic integrity in comparison to their wild-type counterparts. Circular dichroism spectroscopy revealed a significantly reduced helical content for both mutants compared to wild-type. The presence of mutation only marginally affected the thermal stability of ApoE4 but greatly affected the thermal stability profile of ApoE2 leading to a previously uncharacterized intermediate stage. Both ApoE4mutC and ApoE2mutC were slightly stabilized against chemical denaturation compared to their wild-type counterparts. ApoE2mutC, in contrast to ApoE4mutC, exposed a larger hydrophobic surface to the solvent as determined by a fluorescent probe. Both mutants remodeled 1,2-dimyristoyl-sn-glycero-3-phosphocholine vesicles with identical kinetics to the wild-type proteins. Given the known conformational differences between ApoE2 and ApoE4, our findings suggest that the 261-265 region may be involved in inter-domain interactions within the ApoE molecule. Overall, we show that substitution of Leu261, Trp264 and Phe265 with Ala in ApoE2 leads to more pronounced perturbations of thermodynamic stability and structure than in ApoE4. The minimal perturbations in ApoE4mutC may make it a more suitable candidate for therapeutic applications for the correction of remnant removal disorders and atheroprotection.

Pharmacodynamic and pharmacokinetic analysis of apoE4 [L261A, W264A, F265A, L268A, V269A], a recombinant apolipoprotein E variant with improved biological properties

Physiological levels of wild-type (wt) apolipoprotein E (apoE) in plasma mediate the clearance of cholesterol-rich atherogenic lipoprotein remnants while higher than normal plasma apoE concentrations fail to do so and trigger hypertriglyceridemia. This property of wt apoE reduces significantly its therapeutic value as a lead biological for the treatment of dyslipidemia. Recently, we reported the generation of a recombinant apoE variant, apoE4 [L261A, W264A, F265A, L268A, V269A] (apoE4mut1) with improved biological functions. Specifically, in apoE-deficient (apoE(-/-)) mice this variant can normalize high plasma cholesterol levels without triggering hypertriglyceridemia, even at supraphysiological levels of expression. In the present study we performed pharmacodynamic and pharmacokinetic analysis of apoE4mut1 in experimental mice. Using adenovirus-mediated gene transfer in LDL receptor deficient (LDLr(-/-)) mice, we show that the cholesterol lowering potential of apoE4mut1 is dependent on the expression of a functional classical LDLr. Bolus infusion of apoE4mut1-containing proteoliposomes in apoE(-/-) mice fed western-type diet for 6 weeks indicated that exogenously synthesized apoE4mut1 maintains intact its ability to normalize the high cholesterol levels of these mice with a maximum pharmacological effect obtained at 10h post-treatment. Interestingly, plasma cholesterol levels remained significantly reduced up to 24h following intravenous administration of apoE4mut1 proteoliposomes. Measurements of plasma apoE levels indicated that apoE4mut1 in the form of proteoliposomes used in the study has a half-life of 15.8h. Our data suggest that purified apoE4mut1 may be an attractive new candidate for the acute correction of hypercholesterolemia in subjects expressing functional LDL receptor.

Apolipoprotein A-I modulates processes associated with diet-induced nonalcoholic fatty liver disease in mice

Apolipoprotein A-I (apoA-I) is the main protein of high-density lipoprotein (HDL). We investigated the involvement of apoA-I in diet-induced accumulation of triglycerides in hepatocytes and its potential role in the treatment of nonalcoholic fatty liver disease (NAFLD). ApoA-I-deficient (apoA-I(-/-)) mice showed increased diet-induced hepatic triglyceride deposition and disturbed hepatic histology while they exhibited reduced glucose tolerance and insulin sensitivity. Quantification of FASN (fatty acid synthase) [corrected], DGAT-1 (diacylglycerol O-acyltransferase 1), and PPARγ (peroxisome proliferator-activated receptor γ) mRNA expression suggested that the increased hepatic triglyceride content of the apoA-I(-/-) mice was not due to de novo synthesis of triglycerides. Similarly, metabolic profiling did not reveal differences in the energy expenditure between the two mouse groups. However, apoA-I(-/-) mice exhibited enhanced intestinal absorption of dietary triglycerides (3.6 ± 0.5 mg/dL/min for apoA-I(-/-) versus 2.0 ± 0.7 mg/dL/min for C57BL/6 mice, P < 0.05), accelerated clearance of postprandial triglycerides and a reduced rate of hepatic very low density lipoprotein (VLDL) triglyceride secretion (9.8 ± 1.1 mg/dL/min for apoA-I(-/-) versus 12.5 ± 1.3 mg/dL/min for C57BL/6 mice, P < 0.05). In agreement with these findings, adenovirus-mediated gene transfer of apoA-I(Milano) in apoA-I(-/-) mice fed a Western-type diet for 12 wks resulted in a significant reduction in hepatic triglyceride content and an improvement of hepatic histology and architecture. Our data extend the current knowledge on the functions of apoA-I, indicating that in addition to its well-established properties in atheroprotection, it is also an important modulator of processes associated with diet-induced hepatic lipid deposition and NAFLD development in mice. Our findings raise the interesting possibility that expression of therapeutic forms of apoA-I by gene therapy approaches may have a beneficial effect on NAFLD.

A case-control study on the effects of the apolipoprotein E genotypes in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) has reached epidemic proportions being the most common cause of chronic liver disease in Western countries. The Apolipoprotein E (ApoE) gene has three major isoforms encoded by the ε2, ε3, and ε4 alleles, with the ε4 allele associated with hypercholesterolemia and the ε2 allele with the opposite effect. The role of apoE genotypes on NAFLD has been previously investigated with conflicting results. Our hospital-based case-control study conducted in Italy aims to explore the effect of the apoE genotypes on NAFLD risk and their effect on the clinical features of NAFLD patients. 310 NAFLD cases and 422 controls were genotyped for apoE. Adjusted odds ratios (ORs) and 95% confidence intervals (CI) from logistic regression were used to explore the relationship between NAFLD and apoE genotypes, as well as their interaction with selected demographic and lifestyle factors. ApoE ε4 allele carriers showed a statistically significant two-fold reduction of NAFLD risk (OR = 0.51, 95% CI: 0.28-0.93) compared with ε3 homozygotes. A statistically significant lower HDL cholesterol level was observed for ApoE ε4 carriers if compared with ε3/ε3 genotype or ApoE ε2 carriers with a nearly linear decreasing trend from ApoE ε2 to ApoE ε4 carriers. Our study reports for the first time a protective effect of the ε4 allele towards NAFLD that might be attributable to its role in the regulation of hepatic triglycerides rich very low-density lipoproteins secretion.

Deficiency in apolipoprotein E has a protective effect on diet-induced nonalcoholic fatty liver disease in mice

Apolipoprotein E (apoE) mediates the efficient catabolism of the chylomicron remnants very low-density lipoprotein and low-density lipoprotein from the circulation, and the de novo biogenesis of high-density lipoprotein. Lipid-bound apoE is the natural ligand for the low-density lipoprotein receptor (LDLr), LDLr-related protein 1 and other scavenger receptors. Recently, we have established that deficiency in apoE renders mice resistant to diet-induced obesity. In the light of these well-documented properties of apoE, we sought to investigate its role in the development of diet-induced nonalcoholic fatty liver disease (NAFLD). apoE-deficient, LDLr-deficient and control C57BL/6 mice were fed a western-type diet (17.3% protein, 48.5% carbohydrate, 21.2% fat, 0.2% cholesterol, 4.5 kcal·g(-)) for 24 weeks and their sensitivity to NAFLD was assessed by histological and biochemical methods. apoE-deficient mice were less sensitive than control C57BL/6 mice to diet-induced NAFLD. In an attempt to identify the molecular basis for this phenomenon, biochemical and kinetic analyses revealed that apoE-deficient mice displayed a significantly delayed post-prandial triglyceride clearance from their plasma. In contrast with apoE-deficient mice, LDLr-deficient mice fed a western-type diet for 24 weeks developed significant accumulation of hepatic triglycerides and NAFLD, suggesting that apoE-mediated hepatic triglyceride accumulation in mice is independent of LDLr. Our findings suggest a new role of apoE as a key peripheral contributor to hepatic lipid homeostasis and the development of diet-induced NAFLD.

The aminoterminal 1-185 domain of human apolipoprotein E suffices for the de novo biogenesis of apoE-containing HDL-like particles in apoA-I deficient mice

AIMS

Recently we showed that apolipoprotein E promotes the de novo biogenesis of apoE-containing HDL particles in a process that requires the function of the lipid transporter ABCA1. Here, we sought to identify the domain of apoE that is responsible for its functional interactions with ABCA1 and the formation of apoE-rich HDL-like particles.

METHODS AND RESULTS

Recombinant attenuated adenoviruses expressing carboxy-terminal truncated forms of apoE4 (apoE4[1-259], apoE4[1-229], apoE4[1-202], and apoE4[1-185]) were administered to apoA-I-deficient mice at a low dose of 8×10(8) pfu and five days post-infection plasma samples were isolated and analyzed for HDL formation. Fractionation of plasma lipoproteins of the infected mice by density gradient ultracentrifugation and FPLC revealed that all forms were capable of promoting HDL formation. Negative staining electron microscopy analysis of the HDL density fractions confirmed that all C-terminal truncated forms of apoE4 promoted the formation of particles with diameters in the HDL region. Interestingly, apoE4[1-259], apoE4[1-229], and apoE4[1-202] led to the formation of spherical particles while plasma from apoE4[1-185] expressing mice contained a mixture of spherical and discoidal particles.

CONCLUSIONS

Taken together, our data establish that the aminoterminal 1-185 region of apoE suffices for the formation of HDL particles in vivo. Our findings may have important ramifications in the design of new biological drugs for the treatment of dyslipidemia, atherosclerosis and coronary heart disease.

Domains of apoE4 required for the biogenesis of apoE-containing HDL

INTRODUCTION. We have studied the functions of truncated apoE4 forms in vitro and in vivo in order to identify the domains of apoE4 required for the biogenesis of apoE-containing high-density lipoprotein (HDL). RESULTS. We have found that apoE4-185, -202, -229, or -259 could promote ATP-binding cassette transporter A1 (ABCA1)-dependent cholesterol efflux in vitro, although less efficiently than Full-length apoE4, and had diminished capacity to activate lecithin cholesterol acyltransferase (LCAT). Formation of HDL in vivo was assessed by various methods following gene transfer in apolipoprotein A-I(-/-) × apoE(-/-) mice. Fast protein liquid chromatography of plasma showed that the truncated apoE forms, except apoE4-185, generated an apoE-containing HDL peak. Two-dimensional gel electrophoresis of plasma and electron microscopy showed that truncated apoE forms generated distinct HDL subpopulations and formed discoidal HDL particles which could be converted to spherical by co-administration of truncated apoE4-202 and LCAT. CONCLUSION. Overall, the in-vivo and in-vitro data are consistent and indicate that apoE4-185 is the shortest truncated form that supports formation of discoidal apoE4-containing HDL particles.

Alteration of negatively charged residues in the 89 to 99 domain of apoA-I affects lipid homeostasis and maturation of HDL

In this study, we investigated the role of positively and negatively charged amino acids within the 89-99 region of apolipoprotein A-I (apoA-I), which are highly conserved in mammals, on plasma lipid homeostasis and the biogenesis of HDL. We previously showed that deletion of the 89-99 region of apoA-I increased plasma cholesterol and phospholipids, but it did not affect plasma triglycerides. Functional studies using adenovirus-mediated gene transfer of two apoA-I mutants in apoA-I-deficient mice showed that apoA-I[D89A/E91A/E92A] increased plasma cholesterol and caused severe hypertriglyceridemia. HDL levels were reduced, and approximately 40% of the apoA-I was distributed in VLDL/IDL. The HDL consisted of mostly spherical and a few discoidal particles and contained preβ1 and α4-HDL subpopulations. The lipid, lipoprotein, and HDL profiles generated by the apoA-I[K94A/K96A] mutant were similar to those of wild-type (WT) apoA-I. Coexpression of apoA-I[D89A/E91A/E92A] and human lipoprotein lipase abolished hypertriglyceridemia, restored in part the α1,2,3,4 HDL subpopulations, and redistributed apoA-I in the HDL2/HDL3 regions, but it did not prevent the formation of discoidal HDL particles. Physicochemical studies showed that the apoA-I[D89A/E91A/E92A] mutant had reduced α-helical content and effective enthalpy of thermal denaturation, increased exposure of hydrophobic surfaces, and increased affinity for triglyceride-rich emulsions. We conclude that residues D89, E91, and E92 of apoA-I are important for plasma cholesterol and triglyceride homeostasis as well as for the maturation of HDL.

Enhanced binding of apolipoprotein A-I variants associated with hypertriglyceridemia to triglyceride-rich particles

Hypertriglyceridemia (HTG) is a common lipid abnormality in humans. However, its etiology remains largely unknown. It was shown that severe HTG can be induced in mice by overexpression of wild-type (WT) apolipoprotein E (apoE) or specific apoA-I mutants. Certain mutations in apoE4 were found to affect plasma triglyceride (TG) levels in mice overexpressing the protein. HTG appeared to positively correlate with the ability of the apoE4 variants to bind to TG-rich particles, protein destabilization, and the exposure of protein hydrophobic surface in solution. Here, we propose that the apoA-I mutations that cause HTG may also lead to changes in the conformation and stability that promote binding of apoA-I to TG-rich lipoproteins. To test this hypothesis, we studied binding to TG-rich emulsion and biophysical properties of the apoA-I mutants that induce HTG, apoA-I[E110A/E111A] and apoA-I[Δ(61-78)], and compared them to those of WT apoA-I and another apoA-I mutant, apoA-I[Δ(89-99)], that does not induce HTG but causes hypercholesterolemia in mice. We found that the apoA-I[E110A/E111A] and apoA-I[Δ(61-78)] mutations lead to enhanced binding of apoA-I to TG-rich particles, destabilization, and greater exposure of the hydrophobic surface of the protein. The apoA-I[Δ(89-99)] mutant did not show enhanced binding to the emulsion or a more exposed hydrophobic surface. Thus, like apoE4, the apoA-I variants that cause HTG in mice have the altered conformation and stability that facilitate their binding to TG-rich lipoproteins and thereby may lead to the reduced level of lipolysis of these lipoproteins. While many factors may be involved in induction of HTG, we suggest that an increased level of association of destabilized loosely folded apolipoproteins with TG-rich lipoproteins may contribute to some cases of HTG in humans.

Molecular etiology of a dominant form of type III hyperlipoproteinemia caused by R142C substitution in apoE4

We have used adenovirus-mediated gene transfer in apolipoprotein (apo)E(-/-) mice to elucidate the molecular etiology of a dominant form of type III hyperlipoproteinemia (HLP) caused by the R142C substitution in apoE4. It was found that low doses of adenovirus expressing apoE4 cleared cholesterol, whereas comparable doses of apoE4[R142C] greatly increased plasma cholesterol, triglyceride, and apoE levels, caused accumulation of apoE in VLDL/IDL/LDL region, and promoted the formation of discoidal HDL. Co-expression of apoE4[R142C] with lecithin cholesterol acyltransferase (LCAT) or lipoprotein lipase (LPL) in apoE(-/-) mice partially corrected the apoE4[R142C]-induced dyslipidemia. High doses of C-terminally truncated apoE4[R142C]-202 partially cleared cholesterol in apoE(-/-) mice and promoted formation of discoidal HDL. The findings establish that apoE4[R142C] causes accumulation of apoE in VLDL/IDL/LDL region and affects in vivo the activity of LCAT and LPL, the maturation of HDL, and the clearance of triglyceride-rich lipoproteins. The prevention of apoE4[R142C]-induced dyslipidemia by deletion of the 203-299 residues suggests that, in the full-length protein, the R142C substitution may have altered the conformation of apoE bound to VLDL/IDL/LDL in ways that prevent triglyceride hydrolysis, cholesterol esterification, and receptor-mediated clearance in vivo.

Biophysical properties of apolipoprotein E4 variants: implications in molecular mechanisms of correction of hypertriglyceridemia

In humans and animal models, high plasma concentrations of apolipoprotein (apo) E are associated with hypertriglyceridemia. It has been shown that overexpression of human wild-type (WT) apoE4 in apoE-deficient mice induces hypertriglyceridemia. In contrast, overexpression of an apoE4 variant, apoE4-mut1 (apoE4(L261A, W264A, F265A, L268A, V269A)), does not induce hypertriglyceridemia and corrects hypercholesterolemia. Furthermore, overexpression of another variant, apoE4-mut2 (apoE4(W276A, L279A, V280A, V283A)), induces mild hypertriglyceridemia and does not correct hypercholesterolemia. To better understand how these mutations improve the function of apoE4, we investigated the conformation and stability of apoE4-mut1 and apoE4-mut2 and their binding to dimyristoyl phosphatidylcholine (DMPC) vesicles and to triglyceride (TG)-rich emulsion particles. We found that the mutations introduced in apoE4-mut1 lead to a more stable and compactly folded conformation of apoE4. These structural changes are associated with a slower rate of solubilization of DMPC vesicles by apoE4-mut1 and reduced binding of the protein to emulsion particles compared with WT apoE4. Under conditions of apoE4 overexpression, the reduced binding of apoE4-mut1 to TG-rich lipoprotein particles may facilitate the lipolysis of these particles and may alter the conformation of the lipoprotein-bound apoE in a way that favors the efficient clearance of the lipoprotein remnants. Mutations introduced in apoE4-mut2 result in smaller structural alterations compared with those observed in apoE4-mut1. The slightly altered structural properties of apoE4-mut2 are associated with slightly reduced binding of this protein to TG-rich lipoprotein particles and milder hypertriglyceridemia as compared with WT apoE4.

Adenovirus mediated expression "in vivo" of the chemokine receptor CXCR1

A major hurdle in the structural analysis of membrane proteins is the expression of a functional and homogeneous form of the protein. Except for rhodopsin, most G protein-coupled receptors (GPCRs) are endogenously expressed at very low levels. Heterologous expression of GPCRs in bacteria, yeast, insect cells or mammalian cell lines often yields proteins with large amounts of misfolded proteins and heterogeneous posttranslational modifications. Here, we report a novel mammalian "in vivo" system for the expression of the chemokine receptor CXCR1. This receptor was expressed in liver of mice infected with adenovirus encoding CXCR1. Liver plasma membranes from infected mice displayed high-levels of (125)I-labeled human interleukin-8 (IL-8) binding. The pharmacological profile of the recombinant CXCR1 expressed "in vivo" was similar to those expressed in neutrophils. We found that the incorporation of the detergent solubilized CXCR1 into phospholipid vesicles in the presence of Gi/Go proteins is required for the reconstitution of (125)I-IL-8 binding. On the basis of the presence of the several endogenous His residues and glycosylation moieties in CXCR1 we fractionated the detergent-solubilized plasma membranes by employing Ni- and Concanavalin A-based chromatography. Fractions enriched with CXCR1 were monitored by (125)I-IL-8-bound to the receptor and Western blots with anti-CXCR1 antibodies. This robust expression system could be readily applied for the expression of GPCRs and other eukaryotic membrane proteins.

Apolipoprotein E predisposes to obesity and related metabolic dysfunctions in mice

Obesity is a central feature of the metabolic syndrome and is associated with increased risk for insulin resistance and typeII diabetes. Here, we investigated the contribution of human apoliproteinE3 and mouse apoliproteinE to the development of diet-induced obesity in response to western-type diet. Our data show that apolipoproteinE contributes to the development of obesity and other related metabolic disorders, and that human apolipoproteinE3 is more potent than mouse apolipoproteinE in promoting obesity in response to western-type diet. Specifically, we found that apolipoproteinE3 knock-in mice fed western-type diet for 24 weeks became obese and developed hyperglycemia, hyperinsulinemia, hyperleptinemia, glucose intolerance and insulin resistance that were more severe than in C57BL/6 mice. In contrast, apolipoproteinE-deficient mice fed western-type diet for the same period were resistant to diet-induced obesity, had normal plasma glucose, leptin and insulin levels, and exhibited normal responses to glucose tolerance and insulin resistance tests. Furthermore, low-density lipoprotein receptor-deficient mice were more sensitive to the development of diet-induced obesity and insulin resistance than apolipoprotein E-deficient mice, but were still more resistant than C57BL/6 mice, raising the possibility that low-density lipoprotein receptor mediates, at least in part, the effects of apolipoproteinE on obesity. Taken together, our findings suggest that, in addition to other previously identified mechanisms of obesity, apolipoproteinE and possibly the chylomicron pathway are also important contributors to the development of obesity and related metabolic dysfunctions in mice.

Biophysical analysis of progressive C-terminal truncations of human apolipoprotein E4: insights into secondary structure and unfolding properties

Apolipoprotein E4 (apoE4) is a risk factor for Alzheimer's disease and has been associated with a variety of neuropathological processes. ApoE4 C-terminally truncated forms have been found in brains of Alzheimer's disease patients. Structural rearrangements in apoE4 are known to be key to its physiological functions. To understand the effect of C-terminal truncations on apoE4 lipid-free structure, we produced a series of recombinant apoE4 forms with progressive C-terminal deletions between residues 166 and 299. Circular dichroism measurements show a dramatic loss in helicity upon removal of the last 40 C-terminal residues, whereas further truncations of residues 203-259 lead to recovery of helical content. Further deletion of residues 186-202 leads to a small increase in helical content. Thermal denaturation indicated that removal of residues 260-299 leads to an increase in melting temperature but truncations down to residue 186 did not further affect the melting temperature. The progressive C-terminal truncations, however, gradually increased the cooperativity of thermal unfolding. Chemical denaturation of the apoE4 forms revealed a two-step process with a clear intermediate stage that is progressively lost as the C-terminus is truncated down to residue 230. Hydrophobic fluorescent probe binding suggested that regions 260-299 and 186-202 contain hydrophobic sites, the former being solvent accessible in the wild-type molecule and the latter being accessible only upon truncation. Taken together, our results show an important but complex role of apoE4 C-terminal segments in secondary structure stability and unfolding and suggest that interactions mediated by the C-terminal segments are important for the structural integrity and conformational changes of apoE4.

Residues Leu261, Trp264, and Phe265 account for apolipoprotein E-induced dyslipidemia and affect the formation of apolipoprotein E-containing high-density lipoprotein

Overexpression of apolipoprotein E (apoE) induces hypertriglyceridemia in apoE-deficient mice, which is abrogated by deletion of the carboxy-terminal segment of residues 260-299. We have used adenovirus-mediated gene transfer in apoE-/- and apoA-I-/- mice to test the effect of three sets of apoE mutations within the region of residues 261-265 on the induction of hypertriglyceridemia, the esterification of cholesterol of very low-density lipoprotein (VLDL) and high-density lipoprotein (HDL), and the formation of spherical or discoidal apoE-containing HDL. A single-amino acid substitution (apoE4[Phe265Ala]) induced hypertriglyceridemia in apoE-/- or apoA-I-/- mice, promoted the accumulation of free cholesterol in the very low-density lipoprotein (VLDL) and HDL region, and decreased HDL cholesterol levels. A double substitution (apoE4[Leu261Ala/Trp264Ala]) induced milder hypertriglyceridemia and increased HDL cholesterol levels. A triple substitution (apoE4[Leu261Ala/Trp264Ala/Phe265Ala] or apoE2[Leu261Ala/Trp264Ala/Phe265Ala]) did not induce hypertriglyceridemia and increased greatly the HDL cholesterol levels. Electron microscopy (EM) analysis of the HDL fractions showed that apoE4[Leu261Ala/Trp264Ala/Phe265Ala] and apoE2[Leu261Ala/Trp264Ala/Phe265Ala] contained spherical HDL, apoE4[Leu261Ala/Trp264Ala] contained mostly spherical and few discoidal HDL particles, and apoE4[Phe265Ala] contained discoidal HDL. We conclude that residues Leu261, Trp264, and Phe265 play an important role in apoE-induced hypertriglyceridemia, the accumulation of free cholesterol in VLDL and HDL, and the formation of discoidal HDL. Substitution of these residues with Ala improves the apoE functions by preventing hypertriglyceridemia and promoting formation of spherical apoE-containing HDL.

The physiological and molecular regulation of lipoprotein assembly and secretion

Triglycerides are insoluble in water and yet are transported at milligram per millilitre concentrations in the bloodstream. This is made possible by the ability of the liver and intestine to assemble lipid-protein emulsions (i.e. lipoproteins), which transport hydrophobic molecules. The assembly of triglyceride-rich lipoproteins requires the coordination of protein and lipid synthesis, which occurs on the cytoplasmic surface of the endoplasmic reticulum (ER), and their concerted assembly and translocation into the luminal ER secretory pathway as nascent lipoprotein particles. The availability of lipid substrate for triglyceride production and the machinery for lipoprotein assembly are highly sensitive to nutritional, hormonal, and genetic modulation. Disorders in lipid metabolism or an imbalance between lipogenesis and lipoprotein assembly can lead to hyperlipidemia and/or hepatic steatosis. We selectively review recently-identified machinery, such as transcription factors and nuclear hormone receptors, which provide new clues to the regulation of lipoprotein secretion.

A dominant negative form of the transcription factor c-Jun affects genes that have opposing effects on lipid homeostasis in mice

c-Jun is a transcription factor activated by phosphorylation by the stress-activated protein kinase/c-Jun N-terminal kinase pathway in response to extracellular signals and cytokines. We show that adenovirus-mediated gene transfer of the dominant negative form of c-Jun (dn-c-Jun) in C57BL/6 mice increased greatly apoE hepatic mRNA and plasma levels, increased plasma cholesterol, triglyceride, and very low density lipoprotein levels, and resulted in the accumulation of discoidal high density lipoprotein particles. A similar but more severe phenotype was generated by overexpression of the mouse apoE in C57BL/6 mice, suggesting that dyslipidemia induced by dn-c-Jun was the result of apoE overexpression. Unexpectedly, infection of apoE(-/-) mice with adenovirus expressing dn-c-Jun reduced plasma cholesterol by 70%, suggesting that dn-c-Jun affected other genes that control plasma cholesterol levels. To identify these genes, we performed whole genome expression analysis (34,000 genes) of isolated livers from two groups of five apoE(-/-) mice, infected with adenoviruses expressing either the dn-c-Jun or the green fluorescence protein. Bioinformatic analysis and Northern blotting validation revealed that dn-c-Jun increased 40-fold the apoE mRNA and reduced by 70% the Scd-1 (stearoyl-CoA-desaturase 1) mRNA. The involvement of Scd-1 in lowering plasma cholesterol was confirmed by restoration of high cholesterol levels of apoE(-/-) mice following coinfection with adenoviruses expressing dn-c-Jun and Scd-1. In conclusion, dn-c-Jun appears to trigger two opposing events in mice that affect plasma cholesterol and triglyceride levels as follows: one results in apoE overexpression and triggers dyslipidemia and the other results in inhibition of Scd-1 and offsets dyslipidemia.

Pathway of biogenesis of apolipoprotein E-containing HDL in vivo with the participation of ABCA1 and LCAT

We have investigated the ability of apoE (apolipoprotein E) to participate in the biogenesis of HDL (high-density lipoprotein) particles in vivo using adenovirus-mediated gene transfer in apoA-I-/- (apolipoprotein A-I) or ABCA1-/- (ATP-binding cassette A1) mice. Infection of apoA-I-/- mice with 2x10(9) pfu (plaque-forming units) of an apoE4-expressing adenovirus increased both HDL and the triacylglycerol-rich VLDL (very-low-density lipoprotein)/IDL (intermediate-density lipoprotein)/LDL (low-density lipoprotein) fraction and generated discoidal HDL particles. ABCA1-/- mice treated similarly failed to form HDL particles, suggesting that ABCA1 is essential for the generation of apoE-containing HDL. Combined infection of apoA-I-/- mice with a mixture of adenoviruses expressing both apoE4 (2x10(9) pfu) and human LCAT (lecithin:cholesterol acyltransferase) (5x10(8) pfu) cleared the triacylglycerol-rich lipoproteins, increased HDL and converted the discoidal HDL into spherical HDL. Similarly, co-infection of apoE-/- mice with apoE4 and human LCAT corrected the hypercholesterolaemia and generated spherical particles, suggesting that LCAT is essential for the maturation of apoE-containing HDL. Overall, the findings indicate that apoE has a dual functionality. In addition to its documented functions in the clearance of triacylglycerol-rich lipoproteins, it participates in the biogenesis of HDL-sized apoE-containing particles. HDL particles generated by this pathway may account at least for some of the atheroprotective functions of apoE.

Alterations in lipid homeostasis of mouse dorsal root ganglia induced by apolipoprotein E deficiency: a shotgun lipidomics study

One of the fundamental goals of lipidomics research is to identify the linkage of an individual gene with a given lipidome, thereby revealing the role of that gene in lipid metabolism, transport, and homeostasis. In this study, we have identified four apolipoprotein E (apoE)-induced alterations in the lipidome of mouse dorsal root ganglia (DRG) through utilizing the technology of shotgun lipidomics. First, apoE mediates sulfatide mass content in mouse DRG, which is comparable to its role in the CNS. Second, apoE contributes to galactosylceramide and ceramide homeostasis in mouse DRG. Third, apoE significantly modulates cholesterol levels in mouse DRG. The latter two functions of apoE are distinct from those in the CNS. Finally, mice null for apoE have dramatically less triacylglycerol mass content in DRG which are opposite to the effects observed in the peripheral organs and vascular system. Collectively, this study identifies the specific alterations in the DRG lipidome induced by apoE knockout and suggests the potential roles of apoE in lipid transport and homeostasis in a tissue specific manner, thereby providing insights into the biochemical mechanisms underlying the functions of apoE in the PNS.

Specific and efficient targeting of adenovirus vectors to macrophages: application of a fusion protein between an adenovirus-binding fragment and avidin, linked to a biotinylated oligonucleotide

BACKGROUND

The application of serotype 5 adenoviruses (Ad5) in macrophages is hampered by the absence of the endogenous coxsackie adenovirus receptor (CAR).

METHODS

To overcome this limitation, we first generated a linker protein consisting of the virus-binding domain of CAR and the C-terminus of avidin. Second, to target macrophages, this linker protein was equipped with the biotinylated (bio) oligonucleotide dA6G10, which was previously shown to display a high affinity for the scavenger receptor A (SR-A).

RESULTS

As compared to nontargeted virus, the linker protein equipped with bio-dA6G10 showed a 500-fold increased reporter gene expression in mouse macrophage RAW264.7 cells. A linker protein equipped with a bio-dA16 control oligonucleotide was inactive. Moreover, the bio-dA6G10-equipped linker showed a 390-fold increased luciferase expression in the macrophage cell line J774 and 276- and 150-fold increased reporter gene expression in primary peritoneal and bone marrow (BM)-derived macrophages, respectively. Using BM-derived macrophages from SR-A knockout mice, it was shown that the dA6G10-mediated uptake is predominantly SR-A-mediated.

CONCLUSIONS

Thus, we have developed a novel tool to link biotinylated ligands to a virus-binding fragment of CAR and have exploited this linker protein to extend the applicability of Ad5 to infect transformed and primary macrophages.

LDL receptor deficiency or apoE mutations prevent remnant clearance and induce hypertriglyceridemia in mice

We have used adenovirus-mediated gene transfer and bolus injection of purified apolipoprotein E (apoE) in mice to determine the contribution of LDL receptor family members in the clearance of apoE-containing lipoproteins in vivo and the factors that trigger hypertriglyceridemia. A low dose [5 x 10(8) plaque-forming units (pfu)] of an adenovirus expressing apoE4 did not normalize plasma cholesterol levels of apolipoprotein E-deficient (apoE(-/-)) x low density lipoprotein receptor-deficient (LDLr(-/-)) mice and induced hypertriglyceridemia. A similar phenotype of combined dyslipidemia was induced in apoE(-/-) or apoE(-/-) x LDLr(-/-) mice after infection with a low dose (4 x 10(8) pfu) of an adenovirus expressing the apoE4[R142V/R145V] mutant previously shown to be defective in receptor binding. In contrast, a low dose of 5 x 10(8) pfu of the apoE4-expressing adenovirus corrected hypercholesterolemia in apoE(-/-) mice and did not trigger hypertriglyceridemia. Bolus injection of purified apoE in apoE(-/-) x LDLr(-/-) mice did not clear plasma cholesterol levels and induced mild hypertriglyceridemia. In contrast, similar injection of apoE in apoE(-/-) mice cleared plasma cholesterol and caused transiently mild hypertriglyceridemia. These findings suggest that a) the LDL receptor alone can account for the clearance of apoE-containing lipoproteins in mice, and the contribution of other receptors is minimal, and b) defects in either the LDL receptor or in apoE that affect its interactions with the LDL receptor, increase the sensitivity to apoE-induced hypertriglyceridemia in mice.

Efficient targeting of adenoviral vectors to integrin positive vascular cells utilizing a CAR-cyclic RGD linker protein

Vascular smooth muscle (VSMC) and endothelial cells (EC) are particularly resistant to infection by type 5 adenovirus (Ad) vectors. To overcome this limitation and target Ad vectors to ubiquitously expressed alpha(V)beta(3/5) integrins, we have generated a linker protein consisting of the extracellular domain of the coxsackie adenovirus receptor (CAR) connected via avidin to a biotinylated cyclic (c) RGD peptide. After optimization of CAR to cRGD and to Ad coupling, infection of mouse heart endothelial cells (H5V) could be augmented significantly, as demonstrated by 600-fold increased transgene expression levels. In EOMAs, a hemangioendothelioma-derived cell line, the fraction of infected cells was enhanced 4- to 6-fold. Furthermore, the fraction of infected primary mouse VSMC was increased from virtually 0% to 25%. Finally, in human umbilical vein endothelial cells, the number of GFP positive cells was enhanced from 2% to 75%. In conclusion, CAR-cRGD is a versatile and highly efficient construct to target Ad vectors to both transformed and primary VSMC and EC.

The receptor binding domain of apolipoprotein E is responsible for its antioxidant activity

Apolipoprotein E (apoE) is a 34-kDa lipid-associated protein present in plasma and in the central nervous system. Previous studies have demonstrated that apoE has multiple functions, including the ability to transport lipids, regulate cell homeostasis, and inhibit lipid oxidation. The lipid binding domain of apoE has been localized to the carboxyl-terminal domain, whereas a cluster of basic amino acid residues within the N-terminal domain is responsible for its receptor binding activity. This study was undertaken to identify the domain in apoE responsible for its antioxidant activity. Results showed that apoE inhibits Cu(2+)-induced LDL oxidation by delaying conjugated diene formation in a concentration-dependent manner. Reductive methylation of lysine residues or cyclohexanedione modification of arginine residues in apoE abolished its ability to inhibit LDL oxidation. Additional studies showed that a 22-kDa peptide containing the N-terminal domain of apoE3 was more effective than a similar peptide with the apoE4 sequence in inhibiting Cu(2+)-induced LDL oxidation. In contrast, the 10-kDa peptide that contains the C-terminal domain of apoE was ineffective. Inhibition of Cu(2+)-induced LDL oxidation can also be accomplished with a peptide containing either a single sequence or a tandem repeat sequence of the receptor binding domain (residues 141-155) of apoE. Taken together, these results localized the antioxidant domain of apoE to its receptor binding domain and the basic amino acids in this domain are important for its antioxidant activity.

Deletions of helices 2 and 3 of human apoA-I are associated with severe dyslipidemia following adenovirus-mediated gene transfer in apoA-I-deficient mice

The objective of this study was to determine the effect of two amino-terminal apolipoprotein A-I (apoA-I) deletions on high-density lipoprotein (HDL) biosynthesis and lipid homeostasis. Adenovirus-mediated gene transfer showed that the apoA-I[Delta(89-99)] deletion mutant caused hypercholesterolemia, characterized by increased plasma cholesterol and phospholipids, that were distributed in the very low density/intermediate density/low-density lipoprotein (VLDL/IDL/LDL) region, and normal triglycerides. The capacity of the mutant protein to promote ATP-binding cassette transporter A1- (ABCA1-) mediated cholesterol efflux and to activate lecithin:cholesterol acyltranserase (LCAT) was approximately 70-80% of the wild-type (WT) control. The phospholipid transfer protein (PLTP) activity of plasma containing the apoA-I[Delta(89-99)] mutant was decreased to 32% of the WT control. Similar analysis showed that the apoA-I[Delta(62-78)] deletion mutant in apoA-I-deficient mice caused combined hyperlipidemia characterized by increased triglycerides, cholesterol, and phospholipids in the VLDL/IDL region. There was enrichment of the VLDL/IDL with mutant apoA-I that resulted in reduction of in vitro lipolysis. The capacity of this mutant to promote ABCA1-mediated cholesterol efflux was normal, and the capacity to activate LCAT in vitro was reduced by 53%. The WT apoA-I and the apoA-I[Delta(62-78)] mutant formed spherical HDL particles, whereas the apoA-I[Delta(89-99)] mutant formed discoidal HDL particles. We conclude that alterations in apoA-I not only may have adverse effects on HDL biosynthesis but also may promote dyslipidemia due to interference of the apoA-I mutants on the overall cholesterol and triglycerides homeostasis.

ApoC-III deficiency prevents hyperlipidemia induced by apoE overexpression

Adenovirus-mediated overexpression of human apolipoprotein E (apoE) induces hyperlipidemia by stimulating the VLDL-triglyceride (TG) production rate and inhibiting the LPL-mediated VLDL-TG hydrolysis rate. Because apoC-III is a strong inhibitor of TG hydrolysis, we questioned whether Apoc3 deficiency might prevent the hyperlipidemia induced by apoE overexpression in vivo. Injection of 2 x 10(9) plaque-forming units of AdAPOE4 caused severe combined hyperlipidemia in Apoe-/- mice [TG from 0.7 +/- 0.2 to 57.2 +/- 6.7 mM; total cholesterol (TC) from 17.4 +/- 3.7 to 29.0 +/- 4.1 mM] that was confined to VLDL/intermediate density lipoprotein-sized lipoproteins. In contrast, Apoc3 deficiency resulted in a gene dose-dependent reduction of the apoE4-associated hyperlipidemia (TG from 57.2 +/- 6.7 mM to 21.2 +/- 18.5 and 1.5 +/- 1.4 mM; TC from 29.0 +/- 4.1 to 16.4 +/- 9.8 and 2.3 +/- 1.8 mM in Apoe-/-, Apoe-/-.Apoc3+/-, and Apoe-/-.Apoc3-/- mice, respectively). In both Apoe-/- mice and Apoe-/-.Apoc3-/- mice, injection of increasing doses of AdAPOE4 resulted in up to a 10-fold increased VLDL-TG production rate. However, Apoc3 deficiency resulted in a significant increase in the uptake of TG-derived fatty acids from VLDL-like emulsion particles by white adipose tissue, indicating enhanced LPL activity. In vitro experiments showed that apoC-III is a more specific inhibitor of LPL activity than is apoE. Thus, Apoc3 deficiency can prevent apoE-induced hyperlipidemia associated with a 10-fold increased hepatic VLDL-TG production rate, most likely by alleviating the apoE-induced inhibition of VLDL-TG hydrolysis.

Generation of a recombinant apolipoprotein E variant with improved biological functions: hydrophobic residues (LEU-261, TRP-264, PHE-265, LEU-268, VAL-269) of apoE can account for the apoE-induced hypertriglyceridemia

To identify the residues in the carboxyl-terminal region 260-299 of human apolipoprotein E (apoE) that contribute to hypertriglyceridemia, two sets of conserved, hydrophobic amino acids between residues 261 and 283 were mutated to alanines, and recombinant adenoviruses expressing these apoE mutants were generated. Adenovirus-mediated gene transfer of apoE4-mut1 (apoE4 (L261A, W264A, F265A, L268A, V269A)) in apoE-deficient mice (apoE(-/-)) corrected plasma cholesterol levels and did not cause hypertriglyceridemia. In contrast, gene transfer of apoE4-mut2 (apoE4 (W276A, L279A, V280A, V283A)) did not correct hypercholesterolemia and induced mild hypertriglyceridemia. ApoE-induced hyperlipidemia was corrected by co-infection with a recombinant adenovirus expressing human lipoprotein lipase. Both apoE4 mutants caused only a small increase in hepatic very low density lipoprotein-triglyceride secretion. Density gradient ultracentrifugation analysis of plasma and electron microscopy showed that wild-type apoE4 and apoE4-mut2 displaced apoA-I from the high density lipoprotein (HDL) region and promoted the formation of discoidal HDL, whereas the apoE4-mut1 did not displace apoA-I from HDL and promoted the formation of spherical HDL. The findings indicate that residues Leu-261, Trp-264, Phe-265, Leu-268, and Val-269 of apoE are responsible for hypertriglyceridemia and also interfere with the formation of HDL. Substitutions of these residues by alanine provide a recombinant apoE form with improved biological functions.

Substitutions of glutamate 110 and 111 in the middle helix 4 of human apolipoprotein A-I (apoA-I) by alanine affect the structure and in vitro functions of apoA-I and induce severe hypertriglyceridemia in apoA-I-deficient mice

Hypertriglyceridemia is a common pathological condition in humans of mostly unknown etiology. Here we report induction of dyslipidemia characterized by severe hypertriglyceridemia as a result of point mutations in human apolipoprotein A-I (apoA-I). Adenovirus-mediated gene transfer in apoA-I-deficient (apoA-I(-)(/)(-)) mice showed that mice expressing an apoA-I[E110A/E111A] mutant had comparable hepatic mRNA levels with WT controls but greatly increased plasma triglyceride and elevated plasma cholesterol levels. In addition, they had decreased apoE and apoCII levels and increased apoB48 levels in very low-density lipoprotein (VLDL)/intermediate-density lipoprotein (IDL). Fast protein liquid chromatography (FPLC) analysis of plasma showed that most of cholesterol and approximately 15% of the mutant apoA-I were distributed in the VLDL and IDL regions and all the triglycerides in the VLDL region. Hypertriglyceridemia was corrected by coinfection of mice with recombinant adenoviruses expressing the mutant apoA-I and human lipoprotein lipase. Physicochemical studies indicated that the apoA-I mutation decreased the alpha-helical content, the stability, and the unfolding cooperativity of both lipid-free and lipid-bound apoA-I. In vitro functional analyses showed that reconstituted HDL (rHDL) particles containing the mutant apoA-I had 53% of scavenger receptor class B type I (SR-BI)-mediated cholesterol efflux capacity and 37% capacity to activate lecithin:cholesterol acyltransferase (LCAT) as compared to the WT control. The mutant lipid-free apoA-I had normal capacity to promote ATP-binding cassette transporter A1 (ABCA1)-dependent cholesterol efflux. The findings indicate that subtle structural alterations in apoA-I may alter the stability and functions of apoA-I and high-density lipoprotein (HDL) and may cause hypertriglyceridemia.

ApoAV Reduces Plasma Triglycerides by Inhibiting Very Low Density Lipoprotein-Triglyceride (VLDL-TG) Production and Stimulating Lipoprotein Lipase-mediated VLDL-TG Hydrolysis

ApoAV has been discovered recently as a novel modifier of triglyceride (TG) metabolism, but the pathways involved are currently unknown. To gain insight into the function of apoAV, adenovirus-mediated gene transfer of murine apoa5 to C57Bl/6 mice was employed. The injection of low doses of Ad-apoa5 (1-5 x 10(8) plaqueforming units/mouse) dose-dependently reduced plasma very low density lipoprotein (VLDL)-TG levels. First, we evaluated whether a reduced hepatic VLDL production contributed to the TG-lowering effect. Ad-apoa5 treatment dose-dependently diminished (29-37%) the VLDL-TG production rate without affecting VLDL particle production, suggesting that apoAV impairs the lipidation of apoB. Second, Ad-apoa5 treatment dose-dependently reduced (68-88%) the postprandial hypertriglyceridemia following an intragastric fat load, suggesting that apoAV also stimulates the lipoprotein lipase (LPL)-dependent clearance of TG-rich lipoproteins. Indeed, recombinant apoAV was found to dose-dependently stimulate LPL activity up to 2.3-fold in vitro. Accordingly, intravenously injected VLDL-like TG-rich emulsions were cleared at an accelerated rate concomitant with the increased uptake of emulsion TG-derived fatty acids by skeletal muscle and white adipose tissue in Ad-apoa5-treated mice. From these data, we conclude that apoAV is a potent stimulator of LPL activity. Thus, apoAV lowers plasma TG by both reducing the hepatic VLDL-TG production rate and by enhancing the lipolytic conversion of TG-rich lipoproteins.

Contribution of the Hormone-Response Elements of the Proximal ApoA-I Promoter, ApoCIII Enhancer, and C/EBP Binding Site of the Proximal ApoA-I Promoter to the Hepatic and Intestinal Expression of the ApoA-I and ApoCIII Genes in Transgenic Mice

We have generated and studied the pattern of expression of transgenic mouse lines carrying the human apoA-I and apoCIII gene cluster mutated at different sites. In two lines, we have either mutated the hormone-response element (HRE) of element G of the apoCIII enhancer or the C/EBP binding site of the proximal apoA-I promoter. In a third line, we have mutated the two HREs of the apoA-I promoter and the HRE of the apoCIII enhancer. Mutations in the HRE of element G reduced the hepatic and intestinal expressions of the reporter chloramphenicol acetyltransferase (CAT) gene (which substituted the apoCIII gene) to 4 and 13% of the wild-type (WT) control, whereas the hepatic and intestinal expressions of the apoA-I gene were reduced to 92 and 25% of the WT control, respectively. A mutation in the C/EBP site increased the hepatic and intestinal expressions of the apoA-I gene approximately 1.25- and 1.6-fold, respectively, and did not affect the expression of the CAT gene. The mutation in the three HNF-4 binding sites of the apoA-I promoter/apoCIII enhancer nearly abolished the expression of apoA-I and the reporter CAT gene in all tissues. These findings establish the importance of the HREs for the hepatic and intestinal expressions of the apoA-I and apoCIII genes and suggest that C/EBP does not play a central role in the expression of the apoA-I gene.

Probing the pathways of chylomicron and HDL metabolism using adenovirus-mediated gene transfer

PURPOSE OF THE REVIEW

This review clarifies the functions of key proteins of the chylomicron and the HDL pathways.

RECENT FINDINGS

Adenovirus-mediated gene transfer of several apolipoprotein (apo)E forms in mice showed that the amino-terminal 1-185 domain of apoE can direct receptor-mediated lipoprotein clearance in vivo. Clearance is mediated mainly by the LDL receptor. The carboxyl-terminal 261-299 domain of apoE induces hypertriglyceridemia, because of increased VLDL secretion, diminished lipolysis and inefficient VLDL clearance. Truncated apoE forms, including apoE2-202, have a dominant effect in remnant clearance and may have future therapeutic applications for the correction of remnant removal disorders. Permanent expression of apoE and apoA-I following adenoviral gene transfer protected mice from atherosclerosis. Functional assays, protein cross-linking, and adenovirus-mediated gene transfer of apoA-I mutants in apoA-I deficient mice showed that residues 220-231, as well as the central helices of apoA-I, participate in ATP-binding cassette transporter A1-mediated lipid efflux and HDL biogenesis. Following apoA-I gene transfer, an amino-terminal deletion mutant formed spherical alpha-HDL, a double amino- and carboxyl-terminal deletion mutant formed discoidal HDL, and a carboxyl-terminal deletion mutant formed only pre-beta-HDL. The findings support a model of cholesterol efflux that requires direct physical interactions between apoA-I and ATP-binding cassette transporter A1, and can explain Tangier disease and other HDL deficiencies.

SUMMARY

New insights are provided into the role of apoE in cholesterol and triglyceride homeostasis, and of apoA-I in the biogenesis of HDL. Clearance of the lipoprotein remnants and increase in HDL synthesis are obvious targets for therapeutic interventions.

Regulatory gene mutations affecting apolipoprotein gene expression: functions and regulatory behavior of known genes may guide future pharmacogenomic approaches to therapy

A pharmacogenomic approach to therapy requires systematic knowledge of the regulatory regions of the genes, as well as basic understanding of transcriptional regulatory mechanisms of genes. Using the apolipoprotein (apo) A-I/CIII gene cluster as a model system, we have identified by in vitro and in vivo studies the regulatory elements and the factors which control its transcription. Studies in transgenic mice established that the hepatocyte nuclear factor (HNF-4) binding site of the apoCIII enhancer, which controls transcription of both genes, is required for the intestinal expression of apoA-I and apoCIII genes, and enhances synergistically their hepatic transcription in vivo. The three Sp1 sites of the enhancer are also required for the intestinal expression of apoA-land apoCIII genes in vivo, and for the enhancement of the hepatic transcription. The regulation of the apoE/apoCI/apoCIV/apoCII cluster is also cited. It is expected that identification of the regulatory regions of genes will be soon accelerated by the sequencing of several mammalian genomes. The functional analyses of the regulatory domains of genes involved in lipid homeostasis, combined with cross-species sequence comparisons in the near future, may identify natural regulatory gene polymorphisms in the general population that will permit rational pharmacogenomic approaches for treatment of dyslipidemias.

Hyperlipidemia in APOE2 transgenic mice is ameliorated by a truncated apoE variant lacking the C-terminal domain

Familial dysbetalipoproteinemia associated with the apolipoprotein E2 (APOE2) genotype is a recessive disorder with low penetrance. We have investigated whether additional expression of full-length APOE3, APOE4, or a truncated variant of APOE4 (APOE4-202) can reduce APOE2- associated hyperlipidemia. This was achieved using adenovirus-mediated gene transfer to mice transgenic for human APOE2 and deficient for endogenous Apoe (APOE2.Apoe-/- mice). The hyperlipidemia of APOE2.Apoe-/- mice was readily aggravated by APOE3 and APOE4 overexpression. Only a very low dose of APOE4 adenovirus was capable of reducing the serum cholesterol and triglyceride (TG) levels. Expression of higher doses of APOE4 was associated with an increased VLDL-TG production rate and the accumulation of TG-rich VLDL in the circulation. In contrast, a high dose of adenovirus carrying APOE4-202 reduced both the cholesterol and TG levels in APOE2.Apoe-/- mice. Despite the absence of the C-terminal lipid-binding domain, APOE4-202 is apparently capable of binding to lipoproteins and mediating hepatic uptake. Moreover, overexpression of APOE4-202 in APOE2.Apoe-/- mice does not aggravate their hypertriglyceridemia. These results extend our previous analyses of APOE4-202 expression in Apoe-/- mice and demonstrate that apoE4-202 functions even in the presence of clearance-defective apoE2. Thus, apoE4-202 is a safe and efficient candidate for future therapeutic applications.

Reconstituted discoidal ApoE-phospholipid particles are ligands for the scavenger receptor BI. The amino-terminal 1-165 domain of ApoE suffices for receptor binding

The high density lipoprotein receptor, scavenger receptor class B type I (SR-BI), recognizes lipid-bound apolipoprotein A-I (apoA-I) and other apolipoproteins. Here, we have used large scale cultures of apoE-expressing cells to purify apoE and prepare apoE containing reconstituted discoidal 1-palmitoyl-2-oleoyl-l-phosphatidylcholine (POPC)-apoE particles. These particles have been used to examine their binding to wild-type and mutant forms of SR-BI expressed in transfected ldlA-7 cells. Specific binding to SR-BI was determined by subtracting from the total binding, nonspecific values measured using either control untransfected ldlA-7 cells or by inhibiting SR-BI-mediated binding with a high titer antireceptor-blocking antibody. POPC-apoE particles generated using apoE2, apoE3, apoE4, or the carboxyl-terminally truncated forms apoE165, apoE202, apoE229, and apoE259 all bound tightly to wild-type SR-BI with similar affinities (K(d) = 35-45 microg/ml). Binding was nearly abolished in a cell line expressing the ldlA (Q402R/Q418R) double mutant form of SR-BI that is unable to bind native high density lipoprotein but binds low density lipoprotein normally. The findings establish that apoE is a ligand for SR-BI and that the receptor binding domain is located in the amino-terminal 1-165-region of the protein. SR-BI-apoE interactions may contribute to cholesterol homeostasis in tissues and cells expressing SR-BI that are accessible to apoE-containing lipoproteins.