Apolipoprotein E deficiency in mice: gene replacement and prevention of atherosclerosis using adenovirus vectors

Hyperlipidemia impairs bone regeneration of closed bone defects and tooth extraction wounds in mice

OBJECTIVES

To evaluate the effect of hyperlipidemia on the healing of bone defects.

MATERIALS AND METHODS

Apolipoprotein E (ApoE)-deficient mice and wild-type (WT) C57BL/6J mice were fed with an atherogenic high-fat diet (HFD) or a standard chow diet (as control) for 6 weeks. Blood samples were collected to evaluate serum lipid levels. Closed bone defects and open tooth extraction wounds were then created in the mandibles of these animals. One or two weeks after surgery, animals were euthanized. Micro-CT analysis and histomorphometric analysis were conducted to evaluate the healing of bone defects and the alveolar ridge resorption.

RESULTS

Bone regeneration of closed bone defects was considerably delayed in the hyperlipidemic Apoe-/- mice and WT mice. No obvious difference was detected in the new bone formation of the tooth extraction wounds. The HFD-fed mice showed more prominent reduction in the lingual alveolar ridge height of the tooth extraction wounds when compared with the control group.

CONCLUSIONS

Hyperlipidemia results in delayed bone regeneration in large closed bone defects. Although tooth extraction wounds are small and normally regenerated in a hyperlipidemic microenvironment, the prominent reduction in the alveolar ridge height is also a challenge for future restoration of the dentition.

Adenoviral vectors for cardiovascular gene therapy applications: a clinical and industry perspective

Abstract

Despite the development of novel pharmacological treatments, cardiovascular disease morbidity and mortality remain high indicating an unmet clinical need. Viral gene therapy enables targeted delivery of therapeutic transgenes and represents an attractive platform for tackling acquired and inherited cardiovascular diseases in the future. Current cardiovascular gene therapy trials in humans mainly focus on improving cardiac angiogenesis and function. Encouragingly, local delivery of therapeutic transgenes utilising first-generation human adenovirus serotype (HAd)-5 is safe in the short term and has shown some efficacy in drug refractory angina pectoris and heart failure with reduced ejection fraction. Despite this success, systemic delivery of therapeutic HAd-5 vectors targeting cardiovascular tissues and internal organs is limited by negligible gene transfer to target cells, elimination by the immune system, liver sequestration, off-target effects, and episomal degradation. To circumvent these barriers, cardiovascular gene therapy research has focused on determining the safety and efficacy of rare alternative serotypes and/or genetically engineered adenoviral capsid protein-modified vectors following local or systemic delivery. Pre-clinical studies have identified several vectors including HAd-11, HAd-35, and HAd-20–42-42 as promising platforms for local and systemic targeting of vascular endothelial and smooth muscle cells. In the past, clinical gene therapy trials were often restricted by limited scale-up capabilities of gene therapy medicinal products (GTMPs) and lack of regulatory guidance. However, significant improvement of industrial GTMP scale-up and purification, development of novel producer cell lines, and issuing of GTMP regulatory guidance by national regulatory health agencies have addressed many of these challenges, creating a more robust framework for future adenoviral-based cardiovascular gene therapy. In addition, this has enabled the mass roll out of adenovirus vector-based COVID-19 vaccines.

Key messages

First-generation HAd-5 vectors are widely used in cardiovascular gene therapy.

HAd-5-based gene therapy was shown to lead to cardiac angiogenesis and improved function.

Novel HAd vectors may represent promising transgene carriers for systemic delivery.

Novel methods allow industrial scale-up of rare/genetically altered Ad serotypes.

National regulatory health agencies have issued guidance on GMP for GTMPs.

Pig and Mouse Models of Hyperlipidemia and Atherosclerosis

Atherosclerosis is a chronic inflammatory disorder that is the underlying cause of most cardiovascular disease. Resident cells of the artery wall and cells of the immune system participate in atherogenesis. This process is influenced by plasma lipoproteins, genetics, and the hemodynamics of the blood flow in the artery. A variety of animal models have been used to study the pathophysiology and mechanisms that contribute to atherosclerotic lesion formation. No model is ideal as each has its own advantages and limitations with respect to manipulation of the atherogenic process and modeling human atherosclerosis and lipoprotein profile. In this chapter we will discuss pig and mouse models of experimental atherosclerosis. The similarity of pig lipoprotein metabolism and the pathophysiology of the lesions in these animals with that of humans is a major advantage. While a few genetically engineered pig models have been generated, the ease of genetic manipulation in mice and the relatively short time frame for the development of atherosclerosis has made them the most extensively used model. Newer approaches to induce hypercholesterolemia in mice have been developed that do not require germline modifications. These approaches will facilitate studies on atherogenic mechanisms.

Oxidized LDL-dependent pathway as new pathogenic trigger in arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is hallmarked by ventricular fibro-adipogenic alterations, contributing to cardiac dysfunctions and arrhythmias. Although genetically determined (e.g., PKP2 mutations), ACM phenotypes are highly variable. More data on phenotype modulators, clinical prognosticators, and etiological therapies are awaited. We hypothesized that oxidized low-density lipoprotein (oxLDL)-dependent activation of PPARγ, a recognized effector of ACM adipogenesis, contributes to disease pathogenesis. ACM patients showing high plasma concentration of oxLDL display severe clinical phenotypes in terms of fat infiltration, ventricular dysfunction, and major arrhythmic event risk. In ACM patient-derived cardiac cells, we demonstrated that oxLDLs are major cofactors of adipogenesis. Mechanistically, the increased lipid accumulation is mediated by oxLDL cell internalization through CD36, ultimately resulting in PPARγ upregulation. By boosting oxLDL in a Pkp2 heterozygous knock-out mice through high-fat diet feeding, we confirmed in vivo the oxidized lipid dependency of cardiac adipogenesis and right ventricle systolic impairment, which are counteracted by atorvastatin treatment. The modulatory role of oxidized lipids on ACM adipogenesis, demonstrated at cellular, mouse, and patient levels, represents a novel risk stratification tool and a target for ACM pharmacological strategies.

Plasma proteomics reveals gestational age-specific responses to mechanical ventilation and identifies the mechanistic pathways that initiate preterm lung injury

The preterm lung is particularly vulnerable to ventilator-induced lung injury (VILI) as a result of mechanical ventilation. However the developmental and pathological cellular mechanisms influencing the changing patterns of VILI have not been comprehensively delineated, preventing the advancement of targeted lung protective therapies. This study aimed to use SWATH-MS to comprehensively map the plasma proteome alterations associated with the initiation of VILI following 60 minutes of standardized mechanical ventilation from birth in three distinctly different developmental lung states; the extremely preterm, preterm and term lung using the ventilated lamb model. Across these gestations, 34 proteins were differentially altered in matched plasma samples taken at birth and 60 minutes. Multivariate analysis of the plasma proteomes confirmed a gestation-specific response to mechanical ventilation with 79% of differentially-expressed proteins altered in a single gestation group only. Six cellular and molecular functions and two physiological functions were uniquely enriched in either the extremely preterm or preterm group. Correlation analysis supported gestation-specific protein-function associations within each group. In identifying the gestation-specific proteome and functional responses to ventilation we provide the founding evidence required for the potential development of individualized respiratory support approaches tailored to both the developmental and pathological state of the lung.

Promoting macrophage survival delays progression of pre-existing atherosclerotic lesions through macrophage-derived apoE

AIMS

Macrophage apoptosis is a prominent feature of atherosclerosis, yet whether cell death-protected macrophages would favour the resolution of already established atherosclerotic lesions, and thus hold therapeutic potential, remains unknown.

METHODS AND RESULTS

We irradiated then transplanted into Apoe(-/-) or LDLr(-/-) recipient mice harbouring established atherosclerotic lesions, bone marrow cells from mice displaying enhanced macrophage survival through overexpression of the antiapoptotic gene hBcl-2 (Mø-hBcl2 Apoe(-/-) or Mø-hBcl2 Apoe(+/+) LDLr(-/-)). Both recipient mice exhibited decreased lesional apoptotic cell content and reduced necrotic areas when repopulated with Mø-hBcl2 mouse-derived bone marrow cells. In contrast, only LDLr(-/-) recipients showed a reduction in plasma cholesterol levels and in atherosclerotic lesions. The absence of significant reduction of plasma cholesterol levels in the context of apoE deficiency highlighted macrophage-derived apoE as key in both the regulation of plasma and tissue cholesterol levels and the progression of pre-existing lesion. Accordingly, hBcl2 expression in macrophages was associated with larger pools of Kupffer cells and Ly-6C(low) monocytes, both high producers of apoE. Additionally, increased Kupffer cells population was associated with improved clearance of apoptotic cells and modified lipoproteins.

CONCLUSION

Collectively, these data show that promoting macrophage survival provides a supplemental source of apoE, which hinders pre-existing plaque progression.

The crucial roles of apolipoproteins E and C-III in apoB lipoprotein metabolism in normolipidemia and hypertriglyceridemia

PURPOSE OF REVIEW

To describe the roles of apolipoprotein C-III (apoC-III) and apoE in VLDL and LDL metabolism

RECENT FINDINGS

ApoC-III can block clearance from the circulation of apolipoprotein B (apoB) lipoproteins, whereas apoE mediates their clearance. Normolipidemia is sustained by hepatic secretion of VLDL and IDL subspecies that contain both apoE and apoC-III (VLDL E+C-III+). Most of this VLDL E+C-III+ is speedily lipolyzed, reduced in apoC-III content, and cleared from the circulation as apoE containing dense VLDL, IDL, and light LDL. In contrast, in hypertriglyceridemia, most VLDL is secreted with apoC-III but without apoE, and so it is not cleared until it loses apoC-III during lipolysis to dense LDL. In normolipidemia, the liver also secretes IDL and large and medium-size LDL, whereas in hypertriglyceridemia, the liver secretes more dense LDL with and without apoC-III. These pathways establish the hypertriglyceridemic phenotype and link it metabolically to dense LDL. Dietary carbohydrate compared with unsaturated fat suppresses metabolic pathways mediated by apoE that are qualitatively similar to those suppressed in hypertriglyceridemia.

SUMMARY

The opposing actions of apoC-III and apoE on subspecies of VLDL and LDL, and the direct secretion of LDL in several sizes, establish much of the basic structure of human apoB lipoprotein metabolism in normal and hypertriglyceridemic humans.

Hyperlipidemia compromises homing efficiency of systemically transplanted BMSCs and inhibits bone regeneration

PURPOSE

Mesenchymal stem cells (MSCs) can selectively home to bone defects and play an essential role in promoting bone regeneration. As an adverse effect factor for bone metabolism, hyperlipidemia significantly impairs bone regeneration. In this study, bone marrow stromal cells (BMSCs) were systemically transplanted into a hyperlipidemic mouse model to explore the effect of hyperlipidemia on stem cell recruitment and bone regeneration.

METHODS

Hyperlipidemia was established in ApoE-/- mice (on C57BL/6J background) fed with a high fat diet (HFD) for five weeks. C57BL/6 mice fed with the same diet served as controls. BMSCs labeled with the green fluorescent protein (GFP) were then injected via the tail vein and bone defects were created in the mandibles. The animals were sacrificed at weeks 1, 2 and 4 after surgery, and the fate of the transplanted BMSCs was monitored with a fluorescence microscope and immunohistochemical analysis. After hematoxylin and eosin (HE) staining and Masson's Trichrome (MT) staining, histomorphometric analysis was performed to evaluate bone regeneration.

RESULTS

In both groups transplanted with BMSCs, the number of GFP-positive BMSCs detected in the bone defects reached its peak at 1 week after surgery and was decreased thereafter. However, at all time points, less GFP+ cells were detected in the ApoE-/- mice than in the corresponding control mice. BMSCs transplantation significantly enhanced new bone formation, but to a lesser degree in the ApoE-/- mice when compared with the control mice.

CONCLUSIONS

Hyperlipidemia compromises homing efficiency of systemically transplanted BMSCs and inhibits bone regeneration.

Acrolein modification impairs key functional features of rat apolipoprotein E: identification of modified sites by mass spectrometry

Apolipoprotein E (apoE), an antiatherogenic apolipoprotein, plays a significant role in the metabolism of lipoproteins. It lowers plasma lipid levels by acting as a ligand for the low-density lipoprotein receptor (LDLr) family of proteins, in addition to playing a role in promoting macrophage cholesterol efflux in atherosclerotic lesions. The objective of this study is to examine the effect of acrolein modification on the structure and function of rat apoE and to determine the sites and nature of modification by mass spectrometry. Acrolein is a highly reactive aldehyde, which is generated endogenously as one of the products of lipid peroxidation and is present in the environment in pollutants such as tobacco smoke and heated oils. In initial studies, acrolein-modified apoE was identified by immunoprecipitation using an acrolein-lysine specific antibody in the plasma of 10-week old male rats that were exposed to filtered air (FA) or low doses of environmental tobacco smoke (ETS). While both groups displayed acrolein-modified apoE in the lipoprotein fraction, the ETS group had higher levels in the lipid-free fraction compared with the FA group. This observation provided the rationale to further investigate the effect of acrolein modification on rat apoE at a molecular level. Treatment of recombinant rat apoE with a 10-fold molar excess of acrolein resulted in (i) a significant decrease in lipid-binding and cholesterol efflux abilities, (ii) impairment in the LDLr- and heparin-binding capabilities, and (iii) significant alterations in the overall stability of the protein. The disruption in the functional abilities is attributed directly or indirectly to acrolein modification yielding an aldimine adduct at K149 and K155 (+38); a propanal adduct at K135 and K138 (+56); an N(ε)-(3-methylpyridinium)lysine (MP-lysine) at K64, K67, and K254 (+76), and an N(ε)-(3-formyl-3,4-dehydropiperidino)lysine (FDP-lysine) derivative at position K68 (+94), as determined by matrix-assisted laser desorption/ionization-time of flight/time of flight mass spectrometry (MALDI-TOF/TOF MS). The loss of function may also be attributed to alterations in the overall fold of the protein as noted by changes in the guanidine HCl-induced unfolding pattern and to protein cross-linking. Overall, disruption of the structural and functional integrity of apoE by oxidative modification of essential lysine residues by acrolein is expected to affect its role in maintaining plasma cholesterol homeostasis and lead to dysregulation in lipid metabolism.

Regression of atherosclerosis: insights from animal and clinical studies

BACKGROUND

Based on studies that date back to the 1920s, regression and stabilization of atherosclerosis in humans has gone from just a dream to one that is achievable. Review of the literature indicates that the successful attempts at regression generally applied robust measures to improve plasma lipoprotein profiles. Examples include extensive lowering of plasma concentrations of atherogenic apolipoprotein B and enhancement of reverse cholesterol transport from atheromata to the liver.

FINDINGS

Possible mechanisms responsible for lesion shrinkage include decreased retention of atherogenic apolipoprotein B within the arterial wall, efflux of cholesterol and other toxic lipids from plaques, emigration of lesional foam cells out of the arterial wall, and influx of healthy phagocytes that remove necrotic debris as well as other components of the plaque. This review will highlight the role key players such as LXR, HDL and CCR7 have in mediating regression.

CONCLUSION

Although much progress has been made, there are many unanswered questions. There is, therefore, a clear need for preclinical and clinical testing of new agents expected to facilitate atherosclerosis regression with the hope that additional mechanistic insights will allow further progress.

Leptin promotes neointima formation and smooth muscle cell proliferation via NADPH oxidase activation and signalling in caveolin-rich microdomains

AIMS

Apolipoprotein E (apoE) may act as a vasculoprotective factor by promoting plasma lipid clearance and cholesterol efflux. Moreover, apoE accumulates at sites of vascular injury and modulates the effect of growth factors on smooth muscle cells (SMCs). Experimental data suggested that hypothalamic apoE expression is reduced in obesity and associated with leptin resistance. In this study, we examined the role of apoE in mediating the effects of leptin on vascular lesion formation.

METHODS AND RESULTS

Leptin was administered to apoE knockout (apoE-/-) mice via osmotic pumps to increase its circulating levels. Morphometric analysis revealed that leptin did not alter neointima formation and failed to increase α-actin- or PCNA-immunopositive SMCs after vascular injury. Similar findings were obtained after analysis of atherosclerotic lesions. Comparison of apoE-/-, wild-type, or LDL receptor-/- mice and functional analyses in aortic SMCs from WT or apoE-/- mice or human arterial SMCs after treatment with small interfering (si)RNA or heparinase revealed that leptin requires the presence of apoE, expressed, secreted and bound to the cell surface, to fully activate leptin receptor signalling and to promote SMC proliferation and neointima formation. Mechanistically, leptin induced the phosphorylation and membrane translocation of caveolin (cav)-1, and apoE down-regulation or caveolae disruption inhibited the leptin-induced p47phox activation, ROS formation and SMC proliferation. Finally, leptin failed to increase neointima formation in mice lacking cav-1.

CONCLUSION

Our findings suggest that apoE mediates the effects of leptin on vascular lesion formation by stabilizing cav-1-enriched cell membrane microdomains in SMCs, thus allowing NADPH oxidase assembly and ROS-mediated mitogenic signalling.

Reduction of connexin36 content by ICER-1 contributes to insulin-secreting cells apoptosis induced by oxidized LDL particles

Connexin36 (Cx36), a trans-membrane protein that forms gap junctions between insulin-secreting beta-cells in the Langerhans islets, contributes to the proper control of insulin secretion and beta-cell survival. Hypercholesterolemia and pro-atherogenic low density lipoproteins (LDL) contribute to beta-cell dysfunction and apoptosis in the context of Type 2 diabetes. We investigated the impact of LDL-cholesterol on Cx36 levels in beta-cells. As compared to WT mice, the Cx36 content was reduced in islets from hypercholesterolemic ApoE-/- mice. Prolonged exposure to human native (nLDL) or oxidized LDL (oxLDL) particles decreased the expression of Cx36 in insulin secreting cell-lines and isolated rodent islets. Cx36 down-regulation was associated with overexpression of the inducible cAMP early repressor (ICER-1) and the selective disruption of ICER-1 prevented the effects of oxLDL on Cx36 expression. Oil red O staining and Plin1 expression levels suggested that oxLDL were less stored as neutral lipid droplets than nLDL in INS-1E cells. The lipid beta-oxidation inhibitor etomoxir enhanced oxLDL-induced apoptosis whereas the ceramide synthesis inhibitor myriocin partially protected INS-1E cells, suggesting that oxLDL toxicity was due to impaired metabolism of the lipids. ICER-1 and Cx36 expressions were closely correlated with oxLDL toxicity. Cx36 knock-down in INS-1E cells or knock-out in primary islets sensitized beta-cells to oxLDL-induced apoptosis. In contrast, overexpression of Cx36 partially protected INS-1E cells against apoptosis. These data demonstrate that the reduction of Cx36 content in beta-cells by oxLDL particles is mediated by ICER-1 and contributes to oxLDL-induced beta-cell apoptosis.

Targeted In Situ Gene Correction of Dysfunctional APOE Alleles to Produce Atheroprotective Plasma ApoE3 Protein

Cardiovascular disease is the leading worldwide cause of death. Apolipoprotein E (ApoE) is a 34-kDa circulating glycoprotein, secreted by the liver and macrophages with pleiotropic antiatherogenic functions and hence a candidate to treat hypercholesterolaemia and atherosclerosis. Here, we describe atheroprotective properties of ApoE, though also potential proatherogenic actions, and the prevalence of dysfunctional isoforms, outline conventional gene transfer strategies, and then focus on gene correction therapeutics that can repair defective APOE alleles. In particular, we discuss the possibility and potential benefit of applying in combination two technical advances to repair aberrant APOE genes: (i) an engineered endonuclease to introduce a double-strand break (DSB) in exon 4, which contains the common, but dysfunctional, ε2 and ε4 alleles; (ii) an efficient and selectable template for homologous recombination (HR) repair, namely, an adeno-associated viral (AAV) vector, which harbours wild-type APOE sequence. This technology is applicable ex vivo, for example to target haematopoietic or induced pluripotent stem cells, and also for in vivo hepatic gene targeting. It is to be hoped that such emerging technology will eventually translate to patient therapy to reduce CVD risk.

ApoE promotes hepatic selective uptake but not RCT due to increased ABCA1-mediated cholesterol efflux to plasma

ApoE plays an important role in lipoprotein metabolism. This study investigated the effects of adenovirus-mediated human apoE overexpression (AdhApoE3) on sterol metabolism and in vivo reverse cholesterol transport (RCT). In wild-type mice, AdhApoE3 resulted in decreased HDL cholesterol levels and a shift toward larger HDL in plasma, whereas hepatic cholesterol content increased (P < 0.05). These effects were dependent on scavenger receptor class B type I (SR-BI) as confirmed using SR-BI-deficient mice. Kinetic studies demonstrated increased plasma HDL cholesteryl ester catabolic rates (P < 0.05) and higher hepatic selective uptake of HDL cholesteryl esters in AdhApoE3-injected wild-type mice (P < 0.01). However, biliary and fecal sterol output as well as in vivo macrophage-to-feces RCT studied with (3)H-cholesterol-loaded mouse macrophage foam cells remained unchanged upon human apoE overexpression. Similar results were obtained using hApoE3 overexpression in human CETP transgenic mice. However, blocking ABCA1-mediated cholesterol efflux from hepatocytes in AdhApoE3-injected mice using probucol increased biliary cholesterol secretion (P < 0.05), fecal neutral sterol excretion (P < 0.05), and in vivo RCT (P < 0.01), specifically within neutral sterols. These combined data demonstrate that systemic apoE overexpression increases i) SR-BI-mediated selective uptake into the liver and ii) ABCA1-mediated efflux of RCT-relevant cholesterol from hepatocytes back to the plasma compartment, thereby resulting in unchanged fecal mass sterol excretion and overall in vivo RCT.

A2b adenosine receptor regulates hyperlipidemia and atherosclerosis

BACKGROUND

The cAMP-elevating A(2b) adenosine receptor (A(2b)AR) controls inflammation via its expression in bone marrow cells.

METHODS AND RESULTS

Atherosclerosis induced by a high-fat diet in apolipoprotein E-deficient mice was more pronounced in the absence of the A(2b)AR. Bone marrow transplantation experiments indicated that A(2b)AR bone marrow cell signals alone were not sufficient to elicit this effect. Intriguingly, liver expression of the A(2b)AR in wild-type mice was vastly augmented by a high-fat diet, raising the possibility that this upregulation is of functional significance. A(2b)AR genetic ablation led to elevated levels of liver and plasma cholesterol and triglycerides and to fatty liver pathology typical of steatosis, assessed by enzymatic assays and analysis of liver sections. Western blotting and quantitative polymerase chain reaction revealed elevated expression of the following molecules in the liver of A(2b)AR-null mice: the transcription factor sterol regulatory element binding protein-1 (SREBP-1) and its 2 downstream targets and regulators of lipogenesis, acetyl CoA carboxylase and fatty acid synthase. Pharmacological activation or inhibition of A(2b)AR in primary hepatocytes confirmed the regulation of SREBP-1 by this receptor. A(2b)AR-mediated changes in cAMP were found to regulate levels of the transcriptionally active form of SREBP-1. Finally, adenovirally mediated restoration of the A(2b)AR in the liver of A(2b)AR-null mice reduced the lipid profile and atherosclerosis. Similarly, in vivo administration of the A(2b)AR ligand BAY 60-6853 in control mice on a high-fat diet reduced the lipid profile and atherosclerosis.

CONCLUSION

This study provides the first evidence that the A(2b)AR regulates liver SREBP-1, hyperlipidemia, and atherosclerosis, suggesting that this receptor may be an effective therapeutic target.

Xanthine oxidoreductase is involved in macrophage foam cell formation and atherosclerosis development

OBJECTIVE

Hyperuricemia is common in patients with metabolic syndrome. We investigated the role of xanthine oxidoreductase (XOR) in atherosclerosis development, and the effects of the XOR inhibitor allopurinol on this process.

METHODS AND RESULTS

Oral administration of allopurinol to ApoE knockout mice markedly ameliorated lipid accumulation and calcification in the aorta and aortic root. In addition, allopurinol treatment or siRNA-mediated gene knockdown of XOR suppressed transformation of J774.1 murine macrophage cells, treated with acetylated LDL or very low density lipoprotein (VLDL) into foam cells. This inhibitory effect of allopurinol was also observed in primary cultured human macrophages. In contrast, overexpression of XOR promoted transformation of J774.1 cells into foam cells. Interestingly, SR-A1, SR-B1, SR-B II, and VLDL receptors in J774.1 cells were reduced by XOR knockdown, and increased by XOR overexpression. Conversely, expressions of ABCA1 and ABCG1 were increased by XOR knockdown and suppressed by XOR overexpression. Finally, productions of inflammatory cytokines accompanied by foam cell formation were also reduced by allopurinol administration.

CONCLUSIONS

These results strongly suggest XOR activity and/or its expression level to contribute to macrophage foam cell formation. Thus, XOR inhibitors may be useful for preventing atherosclerosis.

Apolipoprotein E mediates enhanced plasma high-density lipoprotein cholesterol clearance by low-dose streptococcal serum opacity factor via hepatic low-density lipoprotein receptors in vivo

OBJECTIVE

Recombinant streptococcal serum opacity factor (rSOF) mediates the in vitro disassembly of human plasma high-density lipoprotein (HDL) into lipid-free apolipoprotein (apo) A-I, a neo-HDL that is cholesterol poor, and a cholesteryl ester-rich microemulsion (CERM) containing apoE. Given the occurrence of apoE on the CERM, we tested the hypothesis that rSOF injection into mice would reduce total plasma cholesterol clearance via apoE-dependent hepatic low-density lipoprotein receptors (LDLR).

METHODS AND RESULTS

rSOF (4 μg) injection into wild-type C57BL/6J mice formed neo-HDL, CERM, and lipid-free apoA-I, as observed in vitro, and reduced plasma total cholesterol (-43%, t(1/2)=44±18 minutes) whereas control saline injections had a negligible effect. Similar experiments with apoE(-/-) and LDLR(-/-) mice reduced plasma total cholesterol ≈0% and 20%, respectively. rSOF was potent; injection of 0.18 μg of rSOF produced 50% of maximum reduction of plasma cholesterol 3 hours postinjection, corresponding to a ≈0.5-mg human dose. Most cholesterol was cleared hepatically (>99%), with rSOF treatment increasing clearance by 65%.

CONCLUSIONS

rSOF injection into mice formed a CERM that was cleared via hepatic LDLR that recognize apoE. This reaction could provide an alternative mechanism for reverse cholesterol transport.

Ethnicity differences in plasma apoC-III levels between African American and Caucasian youths

BACKGROUND

Little is known about the association between apoC-III and lipoprotein-lipids in African American (AA) and Caucasian (CA) youths. The aim of this study was to investigate if plasma apoC-III level is associated with ethnicity differences in atherogenic lipoprotein-lipids between AA and CA youths.

METHODS

A total of 202 youths (mean age 16.1±1.3 y, range 13.8-18.9 y) consisting of 122 AA (boys/girls, 52/70) and 80 CA (boys/girls, 40/40) youths were recruited via flyers sent to local high schools. For AA youths, body mass index (BMI) values were 22.5±5.0 kg/m(2) and 25.0±6.8 kg/m(2) for boys and girls, respectively. For CA youths, BMI values were 22.0±4.8 kg/m(2) and 22.1±5.0 kg/m(2) for boys and girls, respectively. Anthropometric variables were measured using standard procedures. Body fat was measured by dual-energy X-ray absorptiometry. Fasting glucose and insulin, lipoprotein-lipids, and apolipoproteins were measured in fasting plasma samples.

RESULTS

AA youths had significantly lower values in apoC-III (P<0.001), triglyceride (P<0.001), and total cholesterol/high-density lipoprotein cholesterol (P=0.011) and higher values in HDLC (P=0.004), apoE (P=0.016), insulin (P=0.027), and homoeostasis model of assessment insulin resistance (HOMA-IR) (P=0.025) than CA youths. Body composition and insulin resistance parameters were significantly associated with apoC-III levels in CA youths, but not in AA youths. Regression analyses showed that waist circumference and HOMA-IR were significant predictors for apoC-III in CA, not AA, youths.

CONCLUSIONS

The findings of the current study suggest that ethnicity differences in atherogenic lipids between AA and CA youths may be associated with differences in apoC-III and apoE levels.

Atherosclerosis regression and high-density lipoproteins

Atherosclerosis regression has been demonstrated clearly in animal experimental models and, to a lesser extent, in human clinical studies. Imaging techniques for study of the arterial wall are playing a key role in promoting our appreciation of regression. LDL lowering remains the mainstay of current lipid treatment, but given the multiple antiatherosclerotic functions of HDL, including reverse cholesterol transport, agents that target HDL may represent the next generation of treatment for atherosclerotic disease. Currently available agents, including nicotinic acid, have documented antiatherosclerotic effects and trials examining clinical outcomes in the context of contemporary LDL treatment are now underway. Future approaches to HDL treatment may include cholesteryl ester transfer protein inhibitors and apolipoprotein A-I mimetics.

Dietary interventions that lower lipoproteins containing apolipoprotein C-III are more effective in whites than in blacks: results of the OmniHeart trial

BACKGROUND

Blacks have lower average triglyceride and LDL cholesterol concentrations than do whites but higher rates of coronary heart disease. Apolipoprotein (apo) C-III in VLDL and LDL stimulates atherogenic processes in vascular cells. In blacks, the concentration of lipoproteins with apo C-III is unknown, and the response to dietary strategies that lower triglyceride and apo C-III has not been investigated

OBJECTIVE

We compared the concentration of and dietary effects on apo C-III-containing lipoproteins in blacks and whites.

DESIGN

In a randomized, 3-period feeding study [OmniHeart (Optimal Macronutrient Intake Trial to Prevent Heart Disease)], we measured lipoprotein concentrations in 89 blacks and 73 whites who consumed self-selected diets (baseline) and after 3 healthful diets emphasizing carbohydrate, unsaturated fat, or protein. Participants had prehypertension or hypertension, and 79% were overweight or obese.

RESULTS

While consuming self-selected diets, blacks had lower apo C-III in total plasma, VLDL, and LDL than did whites. Unsaturated fat and protein diets lowered plasma apo C-III (16% and 18%, respectively) and triglyceride (12% and 21%, respectively) in whites but not in blacks, reducing racial differences. Most important, blacks had a lower concentration of atherogenic LDL with apo C-III at baseline and after study diets (34-41% lower, P < 0.02). The molar ratio of apo E to apo B was higher in blacks than in whites in total plasma and LDL at baseline and after the study diets.

CONCLUSIONS

Blacks have lower concentrations of atherogenic lipoproteins that contain apo C-III than do whites when consuming diverse diets and an attenuated dietary response of triglyceride and apo C-III. Dietary efforts to lower triglyceride and apo C-III may be more effective in whites than in blacks. The OmniHeart Trial was registered at www.clinicaltrials.gov as NCT00051350.

Gene therapy for dyslipidemia: a review of gene replacement and gene inhibition strategies

Despite numerous technological and pharmacological advances and more detailed knowledge of molecular etiologies, cardiovascular diseases remain the leading cause of morbidity and mortality worldwide claiming over 17 million lives a year. Abnormalities in the synthesis, processing and catabolism of lipoprotein particles can result in severe hypercholesterolemia, hypertriglyceridemia or low HDL-C. Although a plethora of antidyslipidemic pharmacological agents are available, these drugs are relatively ineffective in many patients with Mendelian lipid disorders, indicating the need for new and more effective interventions. In vivo somatic gene therapy is one such intervention. This article summarizes current strategies being pursued for the development of clinical gene therapy for dyslipidemias that cannot effectively be treated with existing drugs.

Role of triglyceride-rich lipoproteins in diabetic nephropathy

Diabetic nephropathy is an increasingly important cause of morbidity and mortality worldwide. A large body of evidence suggests that dyslipidemia has an important role in the progression of kidney disease in patients with diabetes. Lipids may induce renal injury by stimulating TGF-beta, thereby inducing the production of reactive oxygen species and causing damage to the glomeruli and glomerular glycocalyx. Findings from basic and clinical studies strongly suggest that excess amounts of a variety of lipoproteins and lipids worsens diabetes-associated microvascular and macrovascular disease, increases glomerular injury, increases tubulointerstitial fibrosis, and accelerates the progression of diabetic nephropathy. The increasing prevalence of obesity, type 2 diabetes mellitus, and diabetic nephropathy means that interventions that can interrupt the pathophysiological cascade of events induced by lipoproteins and lipids could enable major life and cost savings. This Review discusses the structural, cellular, and microscopic findings associated with diabetic nephropathy and the influence of lipoproteins, specifically triglyceride-rich lipoproteins (TGRLs), on the development and perpetuation of diabetic nephropathy. Some of the accepted and hypothesized mechanisms of renal injury relating to TGRLs are also described.

Pharmaceutical induction of ApoE secretion by multipotent mesenchymal stromal cells (MSCs)

Background

Apolipoprotein E (ApoE) is a molecular scavenger in the blood and brain. Aberrant function of the molecule causes formation of protein and lipid deposits or "plaques" that characterize Alzheimer's disease (AD) and atherosclerosis. There are three human isoforms of ApoE designated ε2, ε3, and ε4. Each isoform differentially affects the structure and function of the protein and thus the development of disease. Homozygosity for ApoE ε4 is associated with atherosclerosis and Alzheimer's disease whereas ApoE ε2 and ε3 tend to be protective. Furthermore, the ε2 form may cause forms of hyperlipoproteinemia. Therefore, introduction of ApoE ε3 may be beneficial to patients that are susceptible to or suffering from these diseases. Mesenchymal stem cells or multipotent mesenchymal stromal cells (MSCs) are adult progenitor cells found in numerous tissues. They are easily expanded in culture and engraft into host tissues when administered appropriately. Furthermore, MSCs are immunosuppressive and have been reported to engraft as allogeneic transplants. In our previous study, mouse MSCs (mMSCs) were implanted into the brains of ApoE null mice, resulting in production of small amounts of ApoE in the brain and attenuation of cognitive deficits. Therefore human MSCs (hMSCs) are a promising vector for the administration of ApoE ε3 in humans.

Results

Unlike mMSCs, hMSCs were found not to express ApoE in culture; therefore a molecular screen was performed for compounds that induce expression. PPARγ agonists, neural stem cell conditioned medium, osteo-inductive media, dexamethasone, and adipo-inductive media (AIM) were tested. Of the conditions tested, only AIM or dexamethasone induced sustained secretion of ApoE in MSCs and the duration of secretion was only limited by the length of time MSCs could be sustained in culture. Upon withdrawal of the inductive stimuli, the ApoE secretion persisted for a further 14 days.

Conclusion

The data demonstrated that pre-treatment and perhaps co-administration of MSCs homozygous for ApoE ε3 and dexamethasone may represent a novel therapy for severe instances of AD, atherosclerosis and other ApoE-related diseases.

Human apolipoprotein E expression from mouse skeletal muscle by electrotransfer of nonviral DNA (plasmid) and pseudotyped recombinant adeno-associated virus (AAV2/7)

Plasma apolipoprotein E (apoE) has multiple atheroprotective actions. However, although liver-directed adenoviral gene transfer of apoE reverses hypercholesterolemia and inhibits atherogenesis in apoE-deficient (apoE(-/-)) mice, safety considerations have revived interest in nonviral DNA (plasmid) and nonpathogenic adeno-associated viral (AAV) vectors. Here, we assess the effectiveness of these two delivery vehicles by minimally invasive intramuscular injection. First, we constructed AAV2-based expression plasmids harboring human apoE3 cDNA, driven by two muscle-specific promoters (CK6 and C5-12) and one ubiquitous promoter (CAG); each efficiently expressed apoE3 in transfected cultured C2C12 mouse myoblasts, although muscle-specific promoters were active only in differentiated multinucleate myotubes. Second, a pilot study verified that electrotransfer of the CAG-driven plasmid (p.CAG.apoE3) into tibialis anterior muscles, pretreated with hyaluronidase, of apoE(-/-) mice significantly enhanced (p < 0.001) local intramuscular expression of apoE3. However, in a 7-day experiment, the CK6- and C5-12-driven plasmids produced less apoE3 in muscle than did p.CAG.apoE3 (0.61 +/- 0.38 and 0.45 +/- 0.38 vs. 13.38 +/- 7.46 microg of apoE3 per muscle, respectively), but plasma apoE3 levels were below our detection limit (<15 ng/ml) in all mice and did not reverse the hyperlipidemia. Finally, we showed that intramuscular injection of a cross-packaged AAV serotype 7 viral vector, expressing human apoE3 from the CAG promoter, resulted in increasing levels of apoE3 in plasma over 4 weeks, although the concentration reached (1.40 +/- 0.35 microg/ml) was just below the threshold level needed to reduce the hypercholesterolemia. We conclude that skeletal muscle can serve as an effective secretory platform to express the apoE3 transgene, but that improved gene transfer vectors are needed to achieve full therapeutic levels of plasma apoE3 protein.

Gene delivery to the liver

Viral gene transfer to the liver has proven extremely effective in animal models and is currently being evaluated in clinical trials for a variety of metabolic disorders. In rodents, a single tail vein injection of an adenoviral vector can transduce most hepatocytes in vivo. This provides a convenient model for assessing vector design as well as for evaluating the effects of specific transgenes in genetic mouse models of human disease. Protocols for optimized in vivo hepatic gene transfer are described.

Rapid regression of atherosclerosis: insights from the clinical and experimental literature

Looking back at animal and clinical studies published since the 1920s, the notion of rapid regression and stabilization of atherosclerosis in humans has evolved from a fanciful goal to one that might be achievable pharmacologically, even for advanced plaques. Our review of this literature indicates that successful regression of atherosclerosis generally requires robust measures to improve plasma lipoprotein profiles. Examples of such measures include extensive lowering of plasma concentrations of atherogenic apolipoprotein B (apoB)-lipoproteins and enhancement of 'reverse' lipid transport from atheromata into the liver, either alone or in combination. Possible mechanisms responsible for lesion shrinkage include decreased retention of apoB-lipoproteins within the arterial wall, efflux of cholesterol and other toxic lipids from plaques, emigration of foam cells out of the arterial wall, and influx of healthy phagocytes that remove necrotic debris and other components of the plaque. Unfortunately, the clinical agents currently available cause less dramatic changes in plasma lipoprotein levels, and, thereby, fail to stop most cardiovascular events. Hence, there is a clear need for testing of new agents expected to facilitate atherosclerosis regression. Additional mechanistic insights will allow further progress.

Prospects for atherosclerosis regression through increase in high-density lipoprotein and other emerging therapeutic targets

In a process often seen as progressive and irreversible, deposition and retention of lipoproteins and the consequent inflammatory reaction result in the accumulation of atherosclerotic plaques from an early age. However, striking effects observed in experimental models support the concept that atherosclerosis can regress. This is often accompanied by changes in plaque composition favouring stability and decreased likelihood of rupture. Large clinical trials have established the value of low-density lipoprotein cholesterol reduction with statin treatment, although this may prevent no more than 30% of all cardiovascular events, and the magnitude of effect on plaque regression seems relatively modest. High-density lipoprotein cholesterol (HDL-C) is well recognised as an important and independent protective factor, although treatment options to increase HDL-C have until now been limited. The recent emergence of new treatments will probably establish increased HDL-C as another important strategy in antiatherosclerosis treatment. Beyond HDL-C increases, further appreciation of mechanisms of cellular lipid homoeostasis and regulation of gene transcription have revealed new targets for atherosclerosis treatment. This review considers emerging approaches to plaque regression together with some of the parallel developments in imaging technology that will improve our appreciation of response to treatment.

Comprehensive QTL analysis of serum cholesterol levels before and after a high-cholesterol diet in SHRSP

The stroke-prone spontaneously hypertensive rat (SHRSP) showed an exaggerated response to a high-fat, high-cholesterol (HFC) diet, and the resulting reactive hypercholesterolemia was suggested to exacerbate the atherogenic process in this rat. We thus performed a quantitative trait locus (QTL) analysis on the serum cholesterol level of SHRSP before and after the HFC diet, with the final goal being the identification of the genetic mechanisms of its reactive hypercholesterolemia. Three hundred fifty-eight F2 rats between SHRSP and Wistar-Kyoto rat were employed in the study. The serum cholesterol and apoprotein E were measured before and after 2 wk of feeding with the HFC diet. Multiple QTLs for the basal cholesterol level were identified on chromosomes 1 and 5, whereas those for the postdietary cholesterol level were on chromosomes 7, 15, and 16. The cholesterol QTLs before and after HFC diet did not overlap with one another, implying that the involved metabolic processes were considerably different between the two conditions. Supporting this, VLDL and LDL cholesterol were the major components of the postdietary serum cholesterol, whereas the basal cholesterol level consisted mainly of HDL cholesterol. A substantial difference of the QTLs between males and females was observed, especially after the HFC diet. The QTL on chromosome 15 had an inverse effect on the cholesterol level, suggesting that the congenic substitution of the SHRSP fragment with that of Wistar-Kyoto rats could induce a greater cholesterol level in SHRSP. This observation is significant in establishing a new model for atherosclerosis with hypertension in rats.

Structural and functional variations in human apolipoprotein E3 and E4

There are three major apolipoprotein E (apoE) isoforms. Although APOE-epsilon3 is considered a longevity gene, APOE-epsilon4 is a dual risk factor to atherosclerosis and Alzheimer disease. We have expressed full-length and N- and C-terminal truncated apoE3 and apoE4 tailored to eliminate helix and domain interactions to unveil structural and functional disturbances. The N-terminal truncated apoE4-(72-299) and C-terminal truncated apoE4-(1-231) showed more complicated or aggregated species than those of the corresponding apoE3 counterparts. This isoformic structural variation did not exist in the presence of dihexanoylphosphatidylcholine. The C-terminal truncated apoE-(1-191) and apoE-(1-231) proteins greatly lost lipid binding ability as illustrated by the dimyristoylphosphatidylcholine turbidity clearance. The low density lipoprotein (LDL) receptor binding ability, determined by a competition binding assay of 3H-LDL to the LDL receptor of HepG2 cells, showed that apoE4 proteins with N-terminal (apoE4-(72-299)), C-terminal (apoE4-(1-231)), or complete C-terminal truncation (apoE4-(1-191)) maintained greater receptor binding abilities than their apoE3 counterparts. The cholesterol-lowering abilities of apoE3-(72-299) and apoE3-(1-231) in apoE-deficient mice were decreased significantly. The structural preference of apoE4 to remain functional in solution may explain the enhanced opportunity of apoE4 isoform to display its pathophysiologic functions in atherosclerosis and Alzheimer disease.

Apolipoprotein E recycling: implications for dyslipidemia and atherosclerosis

After receptor-mediated endocytosis, the intracellular fate of triglyceride-rich lipoproteins (TRLs) is far more complex than the classical degradation pathway of low-density lipoproteins. Once internalized, TRLs disintegrate in peripheral endosomes, followed by a differential sorting of TRL components. Although core lipids and apolipoprotein B are targeted to lysosomes, the majority of TRL-derived apolipoprotein E (apoE) remains in peripheral recycling endosomes. This pool of TRL-derived apoE is then mobilized by high-density lipoproteins (HDLs) or HDL-derived apoA-I to be recycled back to the plasma membrane, followed by apoE resecretion and the subsequent formation of apoE-containing HDL. The HDL-induced recycling of apoE is accompanied by cholesterol efflux and involves the internalization and targeting of HDL-derived apoA-I to endosomes containing both apoE and cholesterol. These findings point to a yet unknown intracellular link between TRL-derived apoE, cellular cholesterol transport, and HDL metabolism. Recent studies provide first evidence that impaired recycling of TRL-derived apoE4, but not apoE3, is associated with intracellular cholesterol accumulation, which might explain some well-documented effects of apoE4 on HDL metabolism. This review summarizes the current understanding of apoE recycling and its potential role in the regulation of plasma apoE levels in the postprandial state.

Gene therapy for lipoprotein disorders

Existing approaches to the treatment of refractory hypercholesterolaemia, severe hypertriglyceridaemia, low levels of high-density lipoprotein cholesterol and certain inherited disorders of intracellular lipid metabolism are ineffective in a substantial number of patients. Somatic gene therapy is considered to be a potential approach to the therapy of several of these lipid disorders. In many cases preclinical proof-of-principle studies have already been performed, and in one (homozygous familial hypercholesterolaemia) a clinical trial has been conducted. Other clinical gene therapy trials for dyslipidaemia are likely to be initiated within the next several years.

Are Remnant-Like Particles Independent Predictors of Coronary Heart Disease Incidence?

Background— Remnant-like particles have been proposed as a new risk factor for coronary heart disease (CHD). This is the first long-term prospective investigation of the relationship between remnant-like particles and a cardiovascular disease outcome in healthy men.

Methods and Results— A cohort of 1156 Japanese-American men aged 60 to 82 from the Honolulu Heart Program was followed for 17 years. During that period 164 incident cases of CHD were identified. In multivariate Cox regression analyses, baseline remnant-like particle cholesterol (RLP-C) and triglyceride (RLP-TG) levels were significantly related to CHD incidence independently of nonlipid cardiovascular risk factors and of total cholesterol or high-density and low-density lipoprotein cholesterol levels. Total triglyceride levels were an independent predictor of CHD incidence. However, in models including RLP and triglyceride level simultaneously, neither variable was significant when adjusted for the other. This finding can be attributed to the strong correlation between RLP-C and RLP-TG levels and total triglycerides. When individuals with normal triglyceride levels (n=894) were separated from those with elevated triglycerides (n=260), the association between RLPs and CHD relative risk was only significant for the group with elevated triglyceride levels.

Conclusions— RLP levels predicted CHD incidence independently of nonlipid risk factors and of total cholesterol or high-density and low-density lipoprotein cholesterol levels. However, RLP levels did not provide additional information about CHD incidence over and above total triglyceride levels. Therefore, this study does not support the need for testing of remnants in men if measures of fasting triglycerides are available.

Distinct signaling mechanisms for apoE inhibition of cell migration and proliferation

Over the years, the vascular protective role of apolipoprotein (apo) E has been attributed to the ability of apoE to induce cholesterol efflux from macrophage foam cells and its transport of extrahepatic cholesterol to the liver for excretion out of the body. Recently, apoE has been shown to protect against vascular disease by additional mechanisms that are independent of its cholesterol transport functions. This review summarizes data demonstrating apoE binding to specific cell surface receptors and proteoglycans in smooth muscle cells triggers distinct signalling pathways that result in inhibition of cell migration, proliferation, and excessive extracellular matrix deposition. apoE binding to the low density lipoprotein receptor-related protein is responsible for inhibition of cell migration, due to the induction of cyclic AMP accumulation and protein kinase A activation. apoE inhibition of cell proliferation is mediated by its binding to proteoglycans and the resulting activation of inducible nitric oxide synthase. apoE also inhibits excessive extracellular matrix protein synthesis. The receptor responsible for this latter apoE function remains to be identified.

Apolipoprotein E gene polymorphisms and thrombosis and restenosis after coronary artery stenting

Experimental data support a protective function of apolipoprotein E (apoE) against restenosis, the main factor limiting the long-term benefit of percutaneous coronary interventions. We investigated the possibility that the single nucleotide polymorphisms (SNPs)--219G/T, 113G/C, 334T/C, and 472C/T of the gene encoding apoE (APOE) are associated with the incidence of death and myocardial infarction or restenosis after stenting in coronary arteries. In addition, we asked whether the apoE isotype-related epsilon2/epsilon3/epsilon4 polymorphism, defined by specific allele combinations (haplotypes) of the 334T/C and 472C/T polymorphism, and other APOE haplotypes, derived from all four SNPs investigated, are associated with adverse clinical and angiographic outcomes after stenting. Our study included 1,850 consecutive patients with symptomatic coronary artery disease (CAD) who underwent stent implantation. Follow-up angiography was performed in 1,556 patients (84.1%) at 6 months after the intervention. We found that none of the APOE SNPs is associated with death and myocardial infarction or restenosis after stenting. In addition, we observed no relationship between APOE haplotypes and adverse outcomes. In conclusion, the APOE -219G/T, 113G/C, 334T/C, and 472C/T polymorphisms, either alone or in combination, do not represent genetic markers of the risk of thrombotic and restenotic complications in patients with CAD treated with coronary stenting.

Storage of human plasma samples leads to alterations in the lipoprotein distribution of apoC-III and apoE

The effect of frozen storage on lipoprotein distribution of apolipoprotein C-III (apoC-III) and apoE was investigated by measuring apoC-III and apoE by ELISA in HDL and apoB-containing lipoproteins of human plasma samples (n = 16) before and after 2 weeks of frozen storage (-20 degrees C). HDLs were separated by heparin-manganese precipitation (HMP) or by fast-protein liquid chromatography (FPLC). Total plasma apoC-III and apoE levels were not affected by frozen storage. HDL-HMP apoC-III and apoE levels were significantly higher in frozen versus fresh samples: 7.7 +/- 0.7 versus 6.7 +/- 0.7 mg/dl (P < 0.05) and 2.0 +/- 0.1 versus 1.2 +/- 0.1 mg/dl (P < 0.001), respectively. HDL-FPLC apoC-III and apoE, but not triglyceride (TG) or cholesterol, levels were also higher in frozen samples: 12.0 +/- 1.2 versus 7.5 +/- 0.6 mg/dl (P < 0.001) and 2.7 +/- 0.2 versus 1.6 +/- 0.2 mg/dl (P < 0.001), respectively. Frozen storage led to a decrease in apoC-III (-17 +/- 9%) and apoE (-19 +/- 9%) in triglyceride-rich lipoprotein. Redistribution of apoC-III and apoE was most evident in samples with high TG levels. HDL apoC-III and apoE levels were also significantly higher when measured in plasma stored at -80 degrees C. Our results demonstrate that lipoprotein distribution of apoC-III and apoE is affected by storage of human plasma, suggesting that analysis of frozen plasma should be avoided in studies relating lipoprotein levels of apoC-III and/or apoE to the incidence of coronary artery disease.

Apolipoprotein E binding to low density lipoprotein receptor-related protein-1 inhibits cell migration via activation of cAMP-dependent protein kinase A

Smooth muscle cell migration and proliferation contribute to neointimal hyperplasia and vascular stenosis after endothelial denudation. Previous studies revealed that apolipoprotein E (apoE) is an effective inhibitor of platelet-derived growth factor-directed smooth muscle cell migration and proliferation and that the anti-migratory function is mediated via apoE binding to low density lipoprotein receptor-related protein-1 (LRP-1). This study was undertaken to identify the intracellular pathway by which apoE binding to LRP-1 results in inhibition of smooth muscle cell migration. The results showed that apoE increased intracellular cAMP levels 3-fold after 5 min, and the increase was sustained for more than 1 h. As a consequence, apoE also increased protein kinase A (PKA) activity in smooth muscle cells. Importantly, suppression of PKA activity with a cell-permeable peptide inhibitor of PKA abolished the inhibitory effect of apoE on smooth muscle cell migration. These results indicated that apoE inhibition of smooth muscle cell migration is mediated via the activation of cAMP-dependent PKA. Additional experiments revealed that apoE also inhibited fibroblasts migration toward platelet-derived growth factor by a similar mechanism of cAMP-dependent PKA activation. It is noteworthy that apoE failed to increase cAMP levels or inhibit migration of LRP-1-negative mouse embryonic fibroblasts and LRP-1-deficient smooth muscle cells. Taken together, these findings established the mechanism by which apoE inhibits cell migration, i.e. via cAMP-dependent protein kinase A activation as a consequence of its binding to LRP-1.

Severe hypercholesterolemia, impaired fat tolerance, and advanced atherosclerosis in mice lacking both low density lipoprotein receptor-related protein 5 and apolipoprotein E

LDL receptor-related protein 5 (LRP5) plays multiple roles, including embryonic development and bone accrual development. Recently, we demonstrated that LRP5 is also required for normal cholesterol metabolism and glucose-induced insulin secretion. To further define the role of LRP5 in the lipoprotein metabolism, we compared plasma lipoproteins in mice lacking LRP5, apolipoprotein E (apoE), or both (apoE;LRP5 double knockout). On a normal chow diet, the apoE;LRP5 double knockout mice (older than 4 months of age) had approximately 60% higher plasma cholesterol levels compared with the age-matched apoE knockout mice. In contrast, LRP5 deficiency alone had no significant effects on the plasma cholesterol levels. High performance liquid chromatography analysis of plasma lipoproteins revealed that cholesterol levels in the very low density lipoprotein and low density lipoprotein fractions were markedly increased in the apoE;LRP5 double knockout mice. There were no apparent differences in the pattern of apoproteins between the apoE knockout mice and the apoE;LRP5 double knockout mice. The plasma clearance of intragastrically loaded triglyceride was markedly impaired by LRP5 deficiency. The atherosclerotic lesions of the apoE;LRP5 double knockout mice aged 6 months were approximately 3-fold greater than those in the age-matched apoE-knockout mice. Furthermore, histological examination revealed highly advanced atherosclerosis, with remarkable accumulation of foam cells and destruction of the internal elastic lamina in the apoE;LRP5 double knockout mice. These data suggest that LRP5 mediates both apoE-dependent and apoE-independent catabolism of plasma lipoproteins.

Role of the hepatic ABCA1 transporter in modulating intrahepatic cholesterol and plasma HDL cholesterol concentrations

The current model for reverse cholesterol transport proposes that HDL transports excess cholesterol derived primarily from peripheral cells to the liver for removal. However, recent studies in ABCA1 transgenic mice suggest that the liver itself may be a major source of HDL cholesterol (HDL-C). To directly investigate the hepatic contribution to plasma HDL-C levels, we generated an adenovirus (rABCA1-GFP-AdV) that targets expression of mouse ABCA1-GFP in vivo to the liver. Compared with mice injected with control AdV, infusion of rABCA1-GFP-AdV into C57Bl/6 mice resulted in increased expression of mouse ABCA1 mRNA and protein in the liver. ApoA-I-dependent cholesterol efflux was increased 2.6-fold in primary hepatocytes isolated 1 day after rABCA1-GFP-AdV infusion. Hepatic ABCA1 expression in C57Bl/6 mice (n = 15) raised baseline levels of TC, PL, FC, HDL-C, apoE, and apoA-I by 150-300% (P < 0.05 all). ABCA1 expression led to significant compensatory changes in expression of genes that increase hepatic cholesterol, including HMG-CoA reductase (3.5-fold), LDLr (2.1-fold), and LRP (5-fold) in the liver. These combined results demonstrate that ABCA1 plays a key role in hepatic cholesterol efflux, inducing pathways that modulate cholesterol homeostasis in the liver, and establish the liver as a major source of plasma HDL-C.

Effect of atorvastatin on plasma apoE metabolism in patients with combined hyperlipidemia

Atorvastatin, a synthetic HMG-CoA reductase inhibitor used for the treatment of hyperlipidemia and the prevention of coronary artery disease, significantly lowers plasma cholesterol and low-density lipoprotein cholesterol (LDL-C) levels. It also reduces total plasma triglyceride and apoE concentrations. In view of the direct involvement of apoE in the pathogenesis of atherosclerosis, we have investigated the effect of atorvastatin treatment (40 mg/day) on in vivo rates of plasma apoE production and catabolism in six patients with combined hyperlipidemia using a primed constant infusion of deuterated leucine. Atorvastatin treatment resulted in a significant decrease (i.e., 30-37%) in levels of total triglyceride, cholesterol, LDL-C, and apoB in all six patients. Total plasma apoE concentration was reduced from 7.4 +/- 0.9 to 4.3 +/- 0.2 mg/dl (-38 +/- 8%, P < 0.05), predominantly due to a decrease in VLDL apoE (3.4 +/- 0.8 vs. 1.7 +/- 0.2 mg/dl; -42 +/- 11%) and IDL/LDL apoE (1.9 +/- 0.3 vs. 0.8 +/- 0.1 mg/dl; -57 +/- 6%). Total plasma lipoprotein apoE transport (i.e., production) was significantly reduced from 4.67 +/- 0.39 to 3.04 +/- 0.51 mg/kg/day (-34 +/- 10%, P < 0.05) and VLDL apoE transport was reduced from 3.82 +/- 0.67 to 2.26 +/- 0.42 mg/kg/day (-36 +/- 10%, P = 0.057). Plasma and VLDL apoE residence times and HDL apoE kinetic parameters were not significantly affected by drug treatment. Percentage decreases in VLDL apoE concentration and VLDL apoE production were significantly correlated with drug-induced reductions in VLDL triglyceride concentration (r = 0.99, P < 0.001; r = 0.88, P < 0.05, respectively, n = 6). Our results demonstrate that atorvastatin causes a pronounced decrease in total plasma and VLDL apoE concentrations and a significant decrease in plasma and VLDL apoE rates of production in patients with combined hyperlipidemia.

Recent advances in liver-directed gene therapy for dyslipidemia

As currently available preventive and therapeutic interventions for hypercholesterolemia are ineffective in a substantial proportion of patients, severe dyslipidemias associated with atherosclerotic vascular disease remain an important target for the development of novel gene therapies. The development of a safe and efficient gene transfer vector has been a major challenge in liver-directed gene therapy, but recently significant progress has been made in this area. Proof-of-principle experiments indicate that the transfer of lipid-modifying genes to the liver is an effective method to restore normal plasma lipids and protect against atherosclerosis. This article summarizes recent developments in liver-directed gene delivery and reviews data on the treatment of dyslipidemias and prevention of atherosclerosis in animals. The evidence presented suggests that some of the approaches taken in animals may be ready for clinical trials in the near future.

Inhibition of atherosclerosis in apolipoprotein-E-deficient mice following muscle transduction with adeno-associated virus vectors encoding human apolipoprotein-E

Apolipoprotein E (apoE) is a multifunctional plasma glycoprotein involved in lipoprotein metabolism and a range of cell signalling phenomena. ApoE-deficient (apoE(-/-)) mice exhibit severe hypercholesterolaemia and are an excellent model of human atherosclerosis. ApoE somatic gene transfer and bone marrow transplantation in apoE(-/-) mice results in reversal of hypercholesterolaemia, inhibition of atherogenesis and regression of atherosclerotic plaque density. Replication defective adeno-associated virus vectors (rAAVs) are an attractive system currently in clinical trial for muscle-based heterologous gene therapy to express secreted recombinant plasma proteins. Here we have applied rAAV transduction of skeletal muscle to express wild-type (epsilon3) and a defective receptor-binding mutant (epsilon2) human apoE transgene in apoE(-/-) mice. In treated animals, apoE mRNA was present in transduced muscles and, although plasma levels of recombinant apoE fell below the detection levels of our ELISA (ie <10 ng/ml), circulating antibodies to human apoE and rAAV were induced. Up to 3 months after a single administration of rAAV/apoE3, a significant reduction in atherosclerotic plaque density in aortas of treated animals was observed (approximately 30%), indicating that low-level rAAV-mediated apoE3 expression from skeletal muscle can retard atherosclerotic progression in this well-defined genetic model.

Cell-derived apolipoprotein E (ApoE) particles inhibit vascular cell adhesion molecule-1 (VCAM-1) expression in human endothelial cells

Sub-endothelial infiltration of monocytes occurs early in atherogenesis and is facilitated by cell adhesion molecules that are up-regulated on activated endothelium. Apolipoprotein E (apoE) helps protect against atherosclerosis, in part, because apoE particles secreted by macrophages have local beneficial effects at lesion sites. Here, we hypothesize that such protection includes anti-inflammatory actions and investigate whether cell-derived apoE can inhibit tumor necrosis factor-alpha-mediated up-regulation of vascular cell adhesion molecule-1 (VCAM-1) in human umbilical vein endothelial cells (HUVECs). Two models were used to mimic endothelial exposure to macrophage-derived apoE. In the first, HUVECs were transiently transfected to secrete apoE; VCAM-1 induction inversely correlated with secretion of apoE into the media (r = -0.76, p < 0.001). In the second, incubation of HUVECs with media from recombinant Chinese hamster ovary (CHO) cells expressing apoE (CHO(apoE)) also reduced VCAM-1 in a dose-dependent manner (r = -0.70, p < 0.001). Characterization of CHO(apoE) cell-derived apoE revealed several similarities to apoE particles secreted by human blood monocyte-derived macrophages. The suppression of endothelial activation by apoE most likely occurs via stimulation of endothelial nitric oxide synthase; apoE increased levels of intracellular nitric oxide and its surrogate marker, cyclic guanosine monophosphate, while the nitric oxide synthase inhibitor, ethyl-isothiourea, blocked its effect. We propose that apoE secreted locally at lesion sites by macrophages may be anti-inflammatory by stimulating endothelium to release NO and suppress VCAM-1 expression.

Gene therapy for cardiovascular disease: a case for cautious optimism

There is currently intense interest in the development of gene therapy for cardiovascular disease. The stimulation of therapeutic angiogenesis for ischemic heart disease has been one of the areas of greatest promise. Encouraging results have been obtained with the angiogenic cytokines vascular endothelial growth factor (VEGF) and basic fibroblast growth factor in animal models, leading to clinical trials in ischemic heart disease. VEGF also has therapeutic potential in a second area of cardiovascular gene therapy, the enhancement of arterioprotective endothelial functions to prevent postangioplasty restenosis and bypass graft arteriopathy. The endothelial cell growth and survival functions of VEGF promote endothelial regeneration, whereas VEGF-induced endothelial production of NO and prostacyclin inhibits vascular smooth muscle cell proliferation. Inhibition of neointimal hyperplasia may also be achieved by gene transfer of endothelial NO synthase (eNOS), PGI synthase, or cell cycle regulators (retinoblastoma, cyclin or cyclin-dependent kinase inhibitors, p53, growth arrest homeobox gene, fas ligand) or antisense oligonucleotides to c-myb, c-myc, proliferating cell nuclear antigen, and transcription factors such as nuclear factor kappaB and E2F. An improved understanding of etiologically complex pathologies involving the interplay of genes and the environment, such as atherosclerosis and systemic hypertension, has led to the identification of new targets for gene therapy, with the potential to alleviate inherited genetic defects such as familial hypercholesterolemia. The use of vasodilator gene overexpression and antisense knockdown of vasoconstrictors to reduce blood pressure in animal models of systemic and pulmonary hypertension offers the prospect of gene therapy for human hypertensive disease. The renin-angiotensin system has been the target of choice for antihypertensive strategies because of its wide distribution and additional effects on fibrinolytic and oxidative stress pathways. Gene therapy in cardiovascular disease has an exciting future but remains at an early stage. Further developments in gene transfer vector technology and the identification of additional target genes will be required before its full therapeutic potential can be realized.

Dramatic remodeling of advanced atherosclerotic plaques of the apolipoprotein E-deficient mouse in a novel transplantation model

OBJECTIVE

Regression of atherosclerotic lesions is an important goal. No extensive experimental evidence shows that it can be achieved for advanced lesions. To study this, we developed a model to maintain a long-term change in the plasma lipoprotein environment of advanced arterial lesions of hyperlipidemic (apolipoprotein E [apoE]-deficient) mice.

METHODS

The apoE-deficient mice (plasma total cholesterol of 1334 +/- 219 [+/- SEM] mg/dL) on a typical Western diet for 38 weeks had advanced atherosclerotic lesions (ie, beyond the macrophage foam cell stage) throughout the arterial tree. Lesion-containing thoracic aortas were transplanted (replacing a segment of abdominal aorta) into either apoE-deficient or wild-type (WT) (total cholesterol of 86 +/- 10 mg/dL) recipients. Grafts were harvested after 9 weeks.

RESULTS

Compared with pretransplant lesions (area = 0.0892 +/- 0.0179 mm(2)), lesion size tended to increase in apoE-deficient to apoE-deficient grafts (0.2411 +/- 0.0636 mm(2); P =.06), whereas a significant reduction was seen in apoE-deficient to WT grafts (0.0214 +/- 0.0049 mm(2); P <.001). Also, foam cells were absent in apoE-deficient to WT grafts, but abundant in pretransplant lesions and apoE-deficient to apoE-deficient grafts. Grafts were evaluated noninvasively in vivo with magnetic resonance imaging, and wall thickening was detected in the apoE-deficient to apoE-deficient group.

CONCLUSIONS

Nearly complete regression of advanced atherosclerotic lesions can be achieved with sustained normalization of the plasma lipoprotein profile. Syngeneic arterial transplantation in mice is a novel and valuable model system for atherosclerosis research; and magnetic resonance imaging can detect differences in characteristics in lesions undergoing regression.