Induction of fatal inflammation in LDL receptor and ApoA-I double-knockout mice fed dietary fat and cholesterol

Fully automated immunoassay for cholesterol uptake capacity to assess high-density lipoprotein function and cardiovascular disease risk

High-density lipoprotein (HDL) cholesterol efflux capacity (CEC), which is a conventional metric of HDL function, has been associated with coronary heart disease risk. However, the CEC assay requires cultured cells and takes several days to perform. We previously established a cell-free assay to evaluate cholesterol uptake capacity (CUC) as a novel measure of HDL functionality and demonstrated its utility in coronary risk stratification. To apply this concept clinically, we developed a rapid and sensitive assay system based on a chemiluminescent magnetic particle immunoassay. The system is fully automated, providing high reproducibility. Measurement of CUC in serum is completed within 20 min per sample without HDL isolation, a notably higher throughput than that of the conventional CEC assay. CUC decreased with myeloperoxidase-mediated oxidation of HDL or in the presence of N-ethylmaleimide, an inhibitor of lecithin: cholesterol acyltransferase (LCAT), whereas CUC was enhanced by the addition of recombinant LCAT. Furthermore, CUC correlated with CEC even after being normalized by ApoA1 concentration and was significantly associated with the requirement for revascularization due to the recurrence of coronary lesions. Therefore, our new assay system shows potential for the accurate measurement of CUC in serum and permits assessing cardiovascular health.

Apolipoprotein A1 Protects Against Necrotic Core Development in Atherosclerotic Plaques: PDZK1-Dependent High-Density Lipoprotein Suppression of Necroptosis in Macrophages

BACKGROUND

Atherosclerosis is a chronic disease affecting artery wall and a major contributor to cardiovascular diseases. Large necrotic cores increase risk of plaque rupture leading to thrombus formation. Necrotic cores are rich in debris from dead macrophages. Programmed necrosis (necroptosis) contributes to necrotic core formation. HDL (high-density lipoprotein) exerts direct atheroprotective effects on different cells within atherosclerotic plaques. Some of these depend on the SR-B1 (scavenger receptor class B type I) and the adapter protein PDZK1 (postsynaptic density protein/Drosophila disc-large protein/Zonula occludens protein containing 1). However, a role for HDL in protecting against necroptosis and necrotic core formation in atherosclerosis is not completely understood.

METHODS

Low-density lipoprotein receptor-deficient mice engineered to express different amounts of ApoA1 (apolipoprotein A1), or to lack PDZK1 were fed a high fat diet for 10 weeks. Atherosclerotic plaque areas, necrotic cores, and key necroptosis mediators, RIPK3 (receptor interacting protein kinase 3), and MLKL (mixed lineage kinase domain-like protein) were characterized. Cultured macrophages were treated with HDL to determine its effects, as well as the roles of SR-B1, PDZK1, and the PI3K (phosphoinositide 3-kinase) signaling pathway on necroptotic cell death.

RESULTS

Genetic overexpression reduced, and ApoA1 knockout increased necrotic core formation and RIPK3 and MLKL within atherosclerotic plaques. Macrophages were protected against necroptosis by HDL and this protection required SR-B1, PDZK1, and PI3K/Akt pathway. PDZK1 knockout increased atherosclerosis in LDLR^KO mice, increasing necrotic cores and phospho-MLKL; both of which were reversed by restoring PDZK1 in BM-derived cells.

CONCLUSIONS

Our findings demonstrate that HDL in vitro and ApoA1, in vivo, protect against necroptosis in macrophages and necrotic core formation in atherosclerosis, suggesting a pathway that could be a target for the treatment of atherosclerosis.

The Association between Blood Lipids and Systemic Lupus Erythematosus: A Two-Sample Mendelian Randomization Research

We evaluated the causal effects of blood lipid levels on systemic lupus erythematosus with a two-sample Mendelian randomization analysis. Independent single-nucleotide polymorphisms related to blood lipids levels (p < 5 × 10−8) were selected as instrumental variables (IVs) from a published genome-wide association study (GWAS). SLE GWAS analysis that included 4036 cases and 6959 controls of European ancestry provided the related roles between instrumental variables and result (SLE). The causal effects were evaluated with two-sample Mendelian randomization (MR) analyses. According to the inverse-variance weighted approaches, genes predictive of increased LDL cholesterol (OR: 1.131; 95% CI: 0.838, 1.528; p = 0.420), HDL cholesterol (OR: 1.093; 95% CI: 0.884, 1.352; p = 0.412), triglycerides (OR: 0.903; 95% CI: 0.716, 1.137; p = 0.384), Apolipoprotein A-I (OR: 0.854; 95% CI: 0.680, 1.074; p = 0.177), and Apolipoprotein B (OR: 0.933; 95% CI: 0.719, 1.211; p = 0.605) were not causally related to the risk of SLE, consistent with multivariate Mendelian randomization analysis. The reverse-MR analyses showed no massive causal roles between SLE and LDL cholesterol (OR: 0.998; 95% CI: 0.994, 1.001; p = 0.166) as well as Apolipoprotein B (OR: 0.998; 95% CI: 0.994, 1.001; p = 0.229). Nevertheless, a causal role of SLE in decreasing HDL cholesterol (OR: 0.993; 95% CI: 0.988, 0.997; p = 0.002), triglycerides (OR: 0.996; 95% CI: 0.993, 0.999; p = 0.010), and Apolipoprotein A-I (OR: 0.995; 95% CI: 0.990, 0.999; p = 0.026) was validated to some extent. Our study found no causal association between abnormal blood lipids and SLE nor a causal effect between SLE and LDL cholesterol as well as Apolipoprotein B. Nevertheless, some evidence showed that SLE exerted a causal effect on lowering HDL cholesterol, Apolipoprotein A-I, and triglyceride levels.

Kidney and lipids: novel potential therapeutic targets for dyslipidemia in kidney disease?

INTRODUCTION

Altered lipid distribution and metabolism may lead to the development and/or progression of chronic kidney disease (CKD). Dyslipidemia is a major risk factor for CKD and increases the risk of cardiovascular events and mortality. Therefore, lipid-lowering treatments may decrease cardiovascular risk and prevent death.

AREAS COVERED

Key players involved in regulating lipid accumulation in the kidney; contribution of lipids to CKD progression, lipotoxicity, and mitochondrial dysfunction in kidney disease; recent therapeutic approaches for dyslipidemia.

EXPERT OPINION

The precise mechanisms for regulating lipid metabolism, particularly in kidney disease, are poorly understood. Guidelines for lipid-lowering therapy for CKD are controversial. Several hypolipemic therapies are available, but compared to others, statin therapy is the most common. No clinical trial has evaluated the efficacy of proprotein convertase subtilisin/kexin type 9 inhibitors (PCSK9i) in preventing cardiovascular events or improving kidney function among patients with CKD or kidney transplant recipients. Attractive alternatives, such as PCSK9-small interfering RNA (siRNA) molecules or evinacumab are available. Additionally, several promising agents, such as cyclodextrins and the FXR/TGR5 dual agonist, INT-767, can improve renal lipid metabolism disorders and delay CKD progression. Drugs targeting mitochondrial dysfunction could be an option for the treatment of dyslipidemia and lipotoxicity, particularly in renal diseases.

Lack of ApoA-I in ApoEKO Mice Causes Skin Xanthomas, Worsening of Inflammation, and Increased Coronary Atherosclerosis in the Absence of Hyperlipidemia

Background:

HDL (high-density lipoprotein) and its major protein component, apoA-I (apolipoprotein A-I), play a unique role in cholesterol homeostasis and immunity. ApoA-I deficiency in hyperlipidemic, atheroprone mice was shown to drive cholesterol accumulation and inflammatory cell activation/proliferation. The present study was aimed at investigating the impact of apoA-I deficiency on lipid deposition and local/systemic inflammation in normolipidemic conditions.

Methods:

ApoE deficient mice, apoE/apoA-I double deficient (DKO) mice, DKO mice overexpressing human apoA-I, and C57Bl/6J control mice were fed normal laboratory diet until 30 weeks of age. Plasma lipids were quantified, atherosclerosis development at the aortic sinus and coronary arteries was measured, skin ultrastructure was evaluated by electron microscopy. Blood and lymphoid organs were characterized through histological, immunocytofluorimetric, and whole transcriptome analyses.

Results:

DKO were characterized by almost complete HDL deficiency and by plasma total cholesterol levels comparable to control mice. Only DKO showed xanthoma formation and severe inflammation in the skin-draining lymph nodes, whose transcriptome analysis revealed a dramatic impairment in energy metabolism and fatty acid oxidation pathways. An increased presence of CD4⁺ T effector memory cells was detected in blood, spleen, and skin-draining lymph nodes of DKO. A worsening of atherosclerosis at the aortic sinus and coronary arteries was also observed in DKO versus apoE deficient. Human apoA-I overexpression in the DKO background was able to rescue the skin phenotype and halt atherosclerosis development.

Conclusions:

HDL deficiency, in the absence of hyperlipidemia, is associated with severe alterations of skin morphology, aortic and coronary atherosclerosis, local and systemic inflammation.

Apolipoprotein A1 deficiency in mice primes bone marrow stem cells for T cell lymphopoiesis

The bone marrow has emerged as a potentially important target in cardiovascular disease as it generates all leukocytes involved in atherogenesis. In the current study, we evaluated whether a change in bone marrow functionality underlies the increased atherosclerosis susceptibility associated with high-density lipoprotein (HDL) deficiency. We found that HDL deficiency in mice due to the genetic lack of hepatocyte-derived apolipoprotein A1 (APOA1) was associated with an increase in the Lin⁻Sca-1⁺Kit⁺ (LSK) bone marrow stem cell population and lymphoid-primed multipotent progenitor numbers, which translated into a higher production and systemic flux of T cell subsets. In accordance with APOA1 deficiency-associated priming of stem cells to increase T lymphocyte production, atherogenic diet-fed low-density lipoprotein receptor knockout mice transplanted with bone marrow from APOA1-knockout mice displayed marked lymphocytosis as compared to wild-type bone marrow recipients. However, atherosclerotic lesion sizes and collagen contents were similar in the two groups of bone marrow recipients. In conclusion, systemic lack of APOA1 primes bone marrow stem cells for T cell lymphopoiesis. Our data provide novel evidence for a regulatory role of HDL in bone marrow functioning in normolipidemic mice.

Summary: Changes in cholesterol metabolism, that is, in high-density lipoprotein levels, can significantly impact leukocyte numbers via modulating bone marrow functionality.

Atherosclerosis: Pathogenesis and Key Cellular Processes, Current and Emerging Therapies, Key Challenges, and Future Research Directions

Atherosclerosis is the principal cause of cardiovascular disease that continues to be a substantial drain on healthcare systems, being responsible for about 31% of all global deaths. Atherogenesis is influenced by a range of factors, including oxidative stress, inflammation, hypertension, and hyperlipidemia, and is ultimately driven by the accumulation of low-density lipoprotein cholesterol within the arterial wall of medium and large arteries. Lipoprotein accumulation stimulates the infiltration of immune cells (such as monocytes/macrophages and T-lymphocytes), some of which take up the lipoprotein, leading to the formation of lipid-laden foam cells. Foam cell death results in increased accumulation of dead cells, cellular debris and extracellular cholesterol, forming a lipid-rich necrotic core. Vascular smooth muscle cells from the arterial media also migrate into the intima layer and proliferate, taking up the available lipids to become foam cells and producing extracellular matrix proteins such as collagen and elastin. Plaque progression is characterized by the formation of a fibrous cap composed of extracellular matrix proteins and smooth muscle cells, which acts to stabilize the atherosclerotic plaque. Degradation, thinning, and subsequent rupture of the fibrous cap leads to lumen-occlusive atherothrombosis, most commonly resulting in heart attack or stroke. This chapter describes the pathogenesis of atherosclerosis, current and emerging therapies, key challenges, and future directions of research.

Myeloperoxidase Inhibition Ameliorates Plaque Psoriasis in Mice

Plaque psoriasis is a common inflammatory condition of the skin characterized by red, flaking lesions. Current therapies for plaque psoriasis target many facets of the autoimmune response, but there is an incomplete understanding of how oxidative damage produced by enzymes such as myeloperoxidase contributes to skin pathology. In this study, we used the Aldara (Imiquimod) cream model of plaque psoriasis in mice to assess myeloperoxidase inhibition for treating psoriatic skin lesions. To assess skin inflammation severity, an innovative mouse psoriasis scoring system was developed. We found that myeloperoxidase inhibition ameliorated psoriasis severity when administered either systemically or topically. The findings of this study support the role of oxidative damage in plaque psoriasis pathology and present potential new therapeutic avenues for further exploration.

Gelsolin: a new biomarker of disease activity in SLE patients associated with HDL-c

OBJECTIVES

To identify potential biomarkers of disease activity analysing the proteome of high-density lipoprotein (HDL) particles from SLE patients in clinical remission and when they develop a flare compared with a healthy control group.

METHODS

Quantitative proteomic analyses of purified HDL were performed using Tandem Mass Tag isobaric tag-labelling and nanoLC-Orbitrap (nLC-MS/MS) from nine SLE patients in clinical remission when they developed a flare and from nine healthy controls (9-9-9). We verified the identified proteins by Western blot and ELISA in a cohort of 104 SLE women patients, 46 healthy women and 14 SLE patients when a flare developed.

RESULTS

We found 17 proteins with a significant fold-change (>1.1) compared with the control group. In lupus patients experiencing a flare compared with those in remission, we identified four proteins with a significant fold-change (C4, Indian Hedgehog protein, S100A8 and gelsolin). Plasma gelsolin (pGSN) levels were decreased in the 104 SLE patients (176.02(74.9) mcg/l) compared with the control group (217.13(86.7) mcg/l); P=0.005 and when they developed a clinical flare (104.84(41.7) mcg/l); P=0.002). pGSN levels were associated with HDL cholesterol levels (r = 0.316, P<0.001). Antimalarial treated patients showed significant higher levels of pGSN (214.56(88.94) mcg/l regarding 170.35(66.36) mcg/l); P = 0.017.

CONCLUSION

Decreased pGSN are associated with clinical disease activity in SLE patients. Antimalarial treatment and HDL cholesterol are associated with higher levels of pGSN.

Myeloid cells regulate plasma LDL-cholesterol levels

PURPOSE OF REVIEW

Leukocytosis, elevated blood leukocyte levels, is associated with enhanced cardiovascular risk in humans. Hematopoietic stem and progenitor cells (HSPCs) drive leukocyte production in a process called hematopoiesis, which mainly occurs in the bone marrow, and under certain conditions also in other organs such as the spleen. Cholesterol accumulation in HSPCs enhances hematopoiesis, increasing levels of blood monocytes that infiltrate into atherosclerotic plaques. Although HSPC proliferation and monocytosis enhance atherogenesis in several studies, concomitant decreases in LDL-cholesterol levels have also been reported, associated with anti-atherogenic effects. This review focuses on the link between HSPC proliferation, leukocytosis, plasma LDL-cholesterol levels, and atherogenesis.

RECENT FINDINGS

Recent studies have shown that an acute infection enhances cholesterol accumulation in HSPCs, driving HSPC proliferation, and leading to the expansion of myeloid cells (monocytes, neutrophils, and macrophages). Enhanced hematopoiesis is associated with low plasma LDL-cholesterol levels in animal models and humans, probably because of the increased number of myeloid cells that take up LDL-cholesterol. Despite low-plasma LDL-cholesterol levels, specific patient populations with enhanced hematopoiesis show increased cardiovascular risk.

SUMMARY

Enhanced hematopoiesis and monocytosis may accelerate atherogenesis. Studies on these processes may lead to the identification of new therapeutic targets for cardiovascular diseases.

Lipid-Free Apolipoprotein A-I Reduces Progression of Atherosclerosis by Mobilizing Microdomain Cholesterol and Attenuating the Number of CD131 Expressing Cells: Monitoring Cholesterol Homeostasis Using the Cellular Ester to Total Cholesterol Ratio

BACKGROUND

Atherosclerosis is a chronic inflammatory disorder whose development is inversely correlated with high-density lipoprotein concentration. Current therapies involve pharmaceuticals that significantly elevate plasma high-density lipoprotein cholesterol concentrations. Our studies were conducted to investigate the effects of low-dose lipid-free apolipoprotein A-I (apoA-I) on chronic inflammation. The aims of these studies were to determine how subcutaneously injected lipid-free apoA-I reduces accumulation of lipid and immune cells within the aortic root of hypercholesterolemic mice without sustained elevations in plasma high-density lipoprotein cholesterol concentrations.

METHODS AND RESULTS

Ldlr^-/- and Ldlr^-/- apoA-I^-/- mice were fed a Western diet for a total of 12 weeks. After 6 weeks, a subset of mice from each group received subcutaneous injections of 200 μg of lipid-free human apoA-I 3 times a week, while the other subset received 200 μg of albumin, as a control. Mice treated with lipid-free apoA-I showed a decrease in cholesterol deposition and immune cell retention in the aortic root compared with albumin-treated mice, regardless of genotype. This reduction in atherosclerosis appeared to be directly related to a decrease in the number of CD131 expressing cells and the esterified cholesterol to total cholesterol content in several immune cell compartments. In addition, apoA-I treatment altered microdomain cholesterol composition that shifted CD131, the common β subunit of the interleukin 3 receptor, from lipid raft to nonraft fractions of the plasma membrane.

CONCLUSIONS

ApoA-I treatment reduced lipid and immune cell accumulation within the aortic root by systemically reducing microdomain cholesterol content in immune cells. These data suggest that lipid-free apoA-I mediates beneficial effects through attenuation of immune cell lipid raft cholesterol content, which affects numerous types of signal transduction pathways that rely on microdomain integrity for assembly and activation.

Using Mendelian Randomization studies to Assess Causality and Identify New Therapeutic Targets in Cardiovascular Medicine

Integration of knowledge generated from genetic studies on intermediate biomarkers and CHD can provide a reliable approach to help assess causal pathways in coronary heart disease. Mendelian Randomization (MR) studies are a powerful tool to assess causal relevance of a range of pathways. These analyses use genetic variants as proxies for soluble biomarkers in association studies of disease risk. MR studies can provide unbiased estimates of causal effects and avoid distortions due to confounding factors arising later in life, because genetic variants are fixed at conception. MR studies have provided evidence pointing towards the likelihood of a causal relevance of a range of pathways in CHD, including LDL-C, triglycerides, lipoprotein (a), and interleukin-6 receptor. On the other hand, MR studies have refuted the causal relevance of a number of biomarkers, including C-reactive protein (CRP), fibrinogen, uric acid, LpPLA2 activity, and homocysteine. Carefully conducted MR studies should overcome the limitations that are inherent to other observational studies (e.g., residual confounding and reverse causality) to help assess causal relevance of a range of pathways in CHD.

Haptoglobin is a serological biomarker for adenocarcinoma lung cancer by using the ProteomeLab PF2D combined with mass spectrometry

Identification of serological biomarker is urgently needed for cancer screening, monitoring cancer progression, treatment response, and surveillance for recurrence in lung cancer. Therefore, we try to find new serological biomarker that has more specificity and sensitivity for lung cancer diagnostics. In this study, the 2-D liquid phase fractionation system (PF2D) and mass spectrometry approach has been used for comparison the serum profiles between lung cancer patients and healthy individuals. Eight proteins were identified form PF2D and subsequently by mass spectrometry. Among these proteins, haptoglobin (HP) and apolipoprotein AI (APOA1) were chosen and validated with turbidimetric assay. We found that HP levels were significantly higher and APOA1 levels were significantly lower in lung cancer patients. However, after the participants were stratified by gender, the expression trends of HP and APOA1 in lung cancer patients existed only in men, which is gender specific phenomenon. HP, APOA1 and carcinoembryonic antigen (CEA), used for distinguishing lung adenocarcinoma, had a sensitivity of 64%, 64% and 79%, respectively. Area under the ROC curve (AUC) of HP, APOA1 and CEA were 0.768, 0.761 and 0.884, respectively. When restricted to male subjects, HP, APOA1 and CEA showed sensitivity of 89%, 73% and 100%, respectively. AUC of HP, APOA1 and CEA were 0.929, 0.840 and 0.877, respectively. Therefore, our results showed that combined with PF2D system and mass spectrometry, this is a promising novel approach to identify new serological biomarkers for lung cancer research. In addition, HP may be a potential serological biomarker for lung adenocarcinoma diagnostics, especially in male subjects.

Microdomains, Inflammation, and Atherosclerosis

Elevated levels of cholesteryl ester (CE)-enriched apoB containing plasma lipoproteins lead to increased foam cell formation, the first step in the development of atherosclerosis. Unregulated uptake of low-density lipoprotein cholesterol by circulating monocytes and other peripheral blood cells takes place through scavenger receptors and over time causes disruption in cellular cholesterol homeostasis. As lipoproteins are taken up, their CE core is hydrolyzed by liposomal lipases to generate free cholesterol (FC). FC can be either re-esterified and stored as CE droplets or shuttled to the plasma membrane for ATP-binding cassette transporter A1-mediated efflux. Because cholesterol is an essential component of all cellular membranes, some FC may be incorporated into microdomains or lipid rafts. These platforms are essential for receptor signaling and transduction, requiring rapid assembly and disassembly. ATP-binding cassette transporter A1 plays a major role in regulating microdomain cholesterol and is most efficient when lipid-poor apolipoprotein AI (apoAI) packages raft cholesterol into soluble particles that are eventually catabolized by the liver. If FC is not effluxed from the cell, it becomes esterified, CE droplets accumulate and microdomain cholesterol content becomes poorly regulated. This dysregulation leads to prolonged activation of immune cell signaling pathways, resulting in receptor oversensitization. The availability of apoAI or other amphipathic α-helix-rich apoproteins relieves the burden of excess microdomain cholesterol in immune cells allowing a reduction in immune cell proliferation and infiltration, thereby stimulating regression of foam cells in the artery. Therefore, cellular balance between FC and CE is essential for proper immune cell function and prevents chronic immune cell overstimulation and proliferation.

HDL is redundant for adrenal steroidogenesis in LDLR knockout mice with a human-like lipoprotein profile

The contribution of HDL to adrenal steroidogenesis appears to be different between mice and humans. In the current study, we tested the hypothesis that a difference in lipoprotein profile may be the underlying cause. Hereto, we determined the impact of HDL deficiency on the adrenal glucocorticoid output in genetically modified mice with a human-like lipoprotein profile. Genetic deletion of APOA1 in LDL receptor (LDLR) knockout mice was associated with HDL deficiency and a parallel increase in the level of cholesterol associated with nonHDL fractions. Despite a compensatory increase in the adrenal relative mRNA expression levels of the cholesterol synthesis gene, HMG-CoA reductase, adrenals from APOA1/LDLR double knockout mice were severely depleted of neutral lipids, as compared with those of control LDLR knockout mice. However, basal corticosterone levels and the adrenal glucocorticoid response to stress were not different between the two types of mice. In conclusion, we have shown that HDL is not critical for proper adrenal glucocorticoid function when mice are provided with a human-like lipoprotein profile. Our findings provide the first experimental evidence that APOB-containing lipoproteins may facilitate adrenal steroidogenesis, in an LDLR-independent manner, in vivo in mice.

Comparative transcriptomic profiling of hydrogen peroxide signaling networks in zebrafish and human keratinocytes: Implications toward conservation, migration and wound healing

Skin wounds need to be repaired rapidly after injury to restore proper skin barrier function. Hydrogen peroxide (H₂O₂) is a conserved signaling factor that has been shown to promote a variety of skin wound repair processes, including immune cell migration, angiogenesis and sensory axon repair. Despite growing research on H₂O₂ functions in wound repair, the downstream signaling pathways activated by this reactive oxygen species in the context of injury remain largely unknown. The goal of this study was to provide a comprehensive analysis of gene expression changes in the epidermis upon exposure to H₂O₂ concentrations known to promote wound repair. Comparative transcriptome analysis using RNA-seq data from larval zebrafish and previously reported microarray data from a human epidermal keratinocyte line shows that H₂O₂ activates conserved cell migration, adhesion, cytoprotective and anti-apoptotic programs in both zebrafish and human keratinocytes. Further assessment of expression characteristics and signaling pathways revealed the activation of three major H₂O₂–dependent pathways, EGF, FOXO1, and IKKα. This study expands on our current understanding of the clinical potential of low-level H₂O₂ for the promotion of epidermal wound repair and provides potential candidates in the treatment of wound healing deficits.

High-density lipoprotein deficiency in genetically modified mice deeply affects skin morphology: A structural and ultrastructural study

Cutaneous lipids, endogenously synthetized and transported by lipoproteins, play a pivotal role in maintaining skin barrier. An impairment of extracutaneous lipid trafficking leads to the development of xanthomas, mostly arising in hyperlipidemic patients, but also in subjects with high-density lipoprotein (HDL) deficiency. The aim of this work was to evaluate, in a genetically modified mouse model, lacking two protein components of HDL particles, apolipoprotein(apo)E and apoA-I, the effect of HDL deficiency on skin morphology. Control mice (C57BL/6), apoE deficient mice (EKO), apoA-I deficient mice (A-IKO) and apoA-I/apoE double knockout mice (A-IKO/EKO) were maintained on a low-fat/low-cholesterol diet up to 30 weeks of age. At sacrifice, skin biopsies were processed for light (LM) and transmission electron microscopy (TEM). Whereas the skin of EKO, A-IKO, and C57BL/6 mice was comparable, LM analysis in A-IKO/EKO mice showed an increase in dermal thickness and the presence of foam cells and T lymphocytes in reticular dermis. TEM analysis revealed the accumulation of cholesterol clefts in the papillary dermis and of cholesterol crystals within foam cells. In conclusion, A-IKO/EKO mice represent an experimental model for investigating the cutaneous phenotype of human HDL deficiency, thus mimicking a condition in which human xanthomatous lesions can develop.

Procollagen C-endopeptidase Enhancer Protein 2 (PCPE2) Reduces Atherosclerosis in Mice by Enhancing Scavenger Receptor Class B1 (SR-BI)-mediated High-density Lipoprotein (HDL)-Cholesteryl Ester Uptake

Studies in human populations have shown a significant correlation between procollagen C-endopeptidase enhancer protein 2 (PCPE2) single nucleotide polymorphisms and plasma HDL cholesterol concentrations. PCPE2, a 52-kDa glycoprotein located in the extracellular matrix, enhances the cleavage of C-terminal procollagen by bone morphogenetic protein 1 (BMP1). Our studies here focused on investigating the basis for the elevated concentration of enlarged plasma HDL in PCPE2-deficient mice to determine whether they protected against diet-induced atherosclerosis. PCPE2-deficient mice were crossed with LDL receptor-deficient mice to obtain LDLr(-/-), PCPE2(-/-) mice, which had elevated HDL levels compared with LDLr(-/-) mice with similar LDL concentrations. We found that LDLr(-/-), PCPE2(-/-) mice had significantly more neutral lipid and CD68+ infiltration in the aortic root than LDLr(-/-) mice. Surprisingly, in light of their elevated HDL levels, the extent of aortic lipid deposition in LDLr(-/-), PCPE2(-/-) mice was similar to that reported for LDLr(-/-), apoA-I(-/-) mice, which lack any apoA-I/HDL. Furthermore, LDLr(-/-), PCPE2(-/-) mice had reduced HDL apoA-I fractional clearance and macrophage to fecal reverse cholesterol transport rates compared with LDLr(-/-) mice, despite a 2-fold increase in liver SR-BI expression. PCPE2 was shown to enhance SR-BI function by increasing the rate of HDL-associated cholesteryl ester uptake, possibly by optimizing SR-BI localization and/or conformation. We conclude that PCPE2 is atheroprotective and an important component of the reverse cholesterol transport HDL system.

Macrophage apoAI protects against dyslipidemia-induced dermatitis and atherosclerosis without affecting HDL

Tissue cholesterol accumulation, macrophage infiltration, and inflammation are features of atherosclerosis and some forms of dermatitis. HDL and its main protein, apoAI, are acceptors of excess cholesterol from macrophages; this process inhibits tissue inflammation. Recent epidemiologic and clinical trial evidence questions the role of HDL and its manipulation in cardiovascular disease. We investigated the effect of ectopic macrophage apoAI expression on atherosclerosis and dermatitis induced by the combination of hypercholesterolemia and absence of HDL in mice. Hematopoietic progenitor cells were transduced to express human apoAI and transplanted into lethally irradiated LDL receptor(-/-)/apoAI(-/-) mice, which were then placed on a high-fat diet for 16 weeks. Macrophage apoAI expression reduced aortic CD4(+) T-cell levels (-39.8%), lesion size (-25%), and necrotic core area (-31.6%), without affecting serum HDL or aortic macrophage levels. Macrophage apoAI reduced skin cholesterol by 39.8%, restored skin morphology, and reduced skin CD4(+) T-cell levels. Macrophage apoAI also reduced CD4(+) T-cell levels (-32.9%) in skin-draining lymph nodes but had no effect on other T cells, B cells, dendritic cells, or macrophages compared with control transplanted mice. Thus, macrophage apoAI expression protects against atherosclerosis and dermatitis by reducing cholesterol accumulation and regulating CD4(+) T-cell levels, without affecting serum HDL or tissue macrophage levels.

Mouse models of disturbed HDL metabolism

High-density lipoprotein (HDL) is considered to be an anti-atherogenic lipoprotein moiety. Generation of genetically modified (total body and tissue-specific knockout) mouse models has significantly contributed to our understanding of HDL function. Here we will review data from knockout mouse studies on the importance of HDL's major alipoprotein apoA-I, the ABC transporters A1 and G1, lecithin:cholesterol acyltransferase, phospholipid transfer protein, and scavenger receptor BI for HDL's metabolism and its protection against atherosclerosis in mice. The initial generation and maturation of HDL particles as well as the selective delivery of its cholesterol to the liver are essential parameters in the life cycle of HDL. Detrimental atherosclerosis effects observed in response to HDL deficiency in mice cannot be solely attributed to the low HDL levels per se, as the low HDL levels are in most models paralleled by changes in non-HDL-cholesterol levels. However, the cholesterol efflux function of HDL is of critical importance to overcome foam cell formation and the development of atherosclerotic lesions in mice. Although HDL is predominantly studied for its atheroprotective action, the mouse data also suggest an essential role for HDL as cholesterol donor for steroidogenic tissues, including the adrenals and ovaries. Furthermore, it appears that a relevant interaction exists between HDL-mediated cellular cholesterol efflux and the susceptibility to inflammation, which (1) provides strong support for the novel concept that inflammation and metabolism are intertwining biological processes and (2) identifies the efflux function of HDL as putative therapeutic target also in other inflammatory diseases than atherosclerosis.

The pleiotropic role of HDL in autoimmune diseases

As is widely known, the classic function of HDL is reverse cholesterol transport (RCT), thus removing cholesterol from peripheral tissues. Early epidemiological studies, such as Framingham's, stated that increased HDL levels were associated with a significant decrease in relative risk for cardiovascular disease (CVD) mortality. However, those with heightened expectations in recent years for the development of therapeutic targets to increase HDL levels have been disappointed, because efforts have demonstrated the opposite effect on cardiovascular and global mortality. However, in contrast, studies have highlighted the complexity and the intriguing role of HDL in different pathological conditions, such as infections, neoplasms, and autoimmune diseases. In this review an attempt is made to summarize some biological pathways that link HDL function with the immune system, and its possible clinical repercussions in autoimmune diseases.

Mitochondrial dysfunction in obesity: potential benefit and mechanism of Co-enzyme Q10 supplementation in metabolic syndrome

Co-enzyme Q10 (Co-Q10) is an essential component of the mitochondrial electron transport chain. Most cells are sensitive to co-enzyme Q10 (Co-Q10) deficiency. This deficiency has been implicated in several clinical disorders such as heart failure, hypertension, Parkinson's disease and obesity. The lipid lowering drug statin inhibits conversion of HMG-CoA to mevalonate and lowers plasma Co-Q10 concentrations. However, supplementation with Co-Q10 improves the pathophysiological condition of statin therapy. Recent evidence suggests that Co-Q10 supplementation may be useful for the treatment of obesity, oxidative stress and the inflammatory process in metabolic syndrome. The anti-inflammatory response and lipid metabolizing effect of Co-Q10 is probably mediated by transcriptional regulation of inflammation and lipid metabolism. This paper reviews the evidence showing beneficial role of Co-Q10 supplementation and its potential mechanism of action on contributing factors of metabolic and cardiovascular complications.

Why targeting HDL should work as a therapeutic tool, but has not

Atherosclerosis is one of the most common causes of death and disability in the United States today despite the availability of statins, which reduce hyperlipidemia, a risk factor that predisposes individuals to this disease. Epidemiology of human populations has overwhelmingly demonstrated an inverse correlation between the concentration of plasma high-density lipoprotein (HDL) cholesterol (HDL-C) and the likelihood of developing cardiovascular disease (CVD). Decades of observations and mechanistic studies suggest that one protective function of HDL is its central role in reverse cholesterol transport. In this pathway, the ATP-binding cassette transporter A1 releases intracellular cholesterol, which is packaged with apolipoprotein A-I (apoA-I) into nascent HDL particles and released from the plasma membrane. Further lipidation and maturation of HDL occur in plasma with the eventual uptake by the liver where cholesterol is removed. It is generally accepted that CVD risk can be reduced if plasma HDL-C levels are elevated. Several different pharmacological approaches have been tried; the most popular approach targets the movement of cholesteryl ester from HDL to triglyceride-rich particles by cholesteryl ester transfer protein. Inhibition of cholesteryl ester transfer protein increases plasma HDL-C concentration; however, beneficial effects have yet to be demonstrated, likely the result of off-target effects. These revelations have led to a reevaluation of how elevating HDL concentration could decrease risk. A recent, landmark study showed that the inherent cholesterol efflux capacity of an individual's plasma was a better predictor of CVD status than overall HDL-C concentration. Even more provocative are recent studies showing that apoA-I, the principle protein component of HDL modulates cellular inflammation and oxidation. The following will review all these potential routes explaining how HDL apoA-I can reduce the risk of CVD.

Apolipoprotein A-I protection against atherosclerosis is dependent on genetic background

OBJECTIVE

Inbred mouse strains have different susceptibilities to experimental atherosclerosis. The C57BL/6 strain is among the most sensitive and has, therefore, been the most widely used in atherosclerosis studies, whereas many strains are resistant. The FVB/N strain is highly resistant to atherosclerosis on the apolipoprotein E (apoE)- and low-density lipoprotein (LDL) receptor-deficient backgrounds. High-density lipoprotein and its major apoprotein, apoA-I, have been shown to be protective against atherogenesis on the C57BL/6 background. We here examine the influence of genetic background on the atheroprotective nature of apoA-I.

APPROACH AND RESULTS

ApoE-deficient/apoA-I-deficient mice were generated in the C57BL/6 and FVB/N strains from apoE-deficient mice. After 6 to 10 weeks on a Western-type diet, plasma lipids and atherosclerotic lesion size were assessed. Macrophage recruitment, cholesterol regulation, and blood monocyte levels were examined as potential mechanisms driving lesion size differences. FVB/N knockout mice had higher plasma very-LDL/LDL cholesterol than their C57BL/6 counterparts. ApoA-I deficiency decreased very-LDL/LDL cholesterol in C57BL/6 mice but not in FVB/N mice. FVB/N single and double knockout mice had less lesion than C57BL/6 6 to 10 weeks on diet. ApoA-I deficiency augmented lesion development only in C57BL/6 mice. Macrophage recruitment to thioglycollate-treated peritoneum and diet-induced blood monocyte levels reflected the pattern of lesion development among the 4 genotypes. ApoA-I deficiency increased macrophage cholesterol content only in C57BL/6. FVB/N plasma was a better acceptor for macrophage cholesterol efflux than C57BL/6.

CONCLUSIONS

ApoA-I is atheroprotective only in certain genetic contexts. In the C57BL/6 context, but not FVB/N, apoA-I decreases inflammatory macrophage recruitment and monocytosis, contributors to lesion formation.

Hypercholesterolemia and reduced HDL-C promote hematopoietic stem cell proliferation and monocytosis: studies in mice and FH children

Previous studies have shown that mice with defects in cellular cholesterol efflux show hematopoietic stem cell (HSPC) and myeloid proliferation, contributing to atherogenesis. We hypothesized that the combination of hypercholesterolemia and defective cholesterol efflux would promote HSPC expansion and leukocytosis more prominently than either alone. We crossed Ldlr(-/-) with Apoa1(-/-) mice and found that compared to Ldlr(-/-) mice, Ldlr(-/-)/Apoa1(+/-) mice, with similar LDL-cholesterol levels but reduced HDL-cholesterol (HDL-C) levels, had expansion of HSPCs, monocytosis and neutrophilia. Ex vivo studies showed that HSPCs expressed high levels of Ldlr, Scarb1 (Srb1), and Lrp1 and were able to take up both native and oxidized LDL. Native LDL directly stimulated HSPC proliferation, while co-incubation with reconstituted HDL attenuated this effect. We also assessed the impact of HDL-C levels on monocytes in children with familial hypercholesterolemia (FH) (n = 49) and found that subjects with the lowest level of HDL-C, had increased monocyte counts compared to the mid and higher HDL-C levels. Overall, HDL-C was inversely correlated with the monocyte count. These data suggest that in mice, a balance of cholesterol uptake and efflux mechanisms may be one factor in driving HSPC proliferation and monocytosis. Higher monocyte counts in children with FH and low HDL-cholesterol suggest a similar pattern in humans.

High density lipoprotein biogenesis, cholesterol efflux, and immune cell function

This review provides a summary of recent research on the role of high-density lipoprotein (HDL)/apolipoprotein A-I cholesterol efflux and immune cell function. Plasma concentrations of HDL have been known to inversely correlate with risk for coronary vascular disease. Bulk transport of HDL cholesterol from the peripheral tissues to the liver is a major pathway, termed reverse cholesterol transport, responsible for maintaining whole body cholesterol homeostasis. In addition to participating in this pathway, HDL and apolipoprotein A-I exert anti-inflammatory effects through different pathways. One pathway that seems to be important in atherosclerosis and autoimmunity is its role in modulation of T cell activation. HDL/apolipoprotein A-I helps regulate cell signaling by accepting membrane cholesterol from ATP binding cassette transporter A1 on immune cells and, thereby, fine tuning the amount of cholesterol present in plasma membrane lipid rafts.

Genetic basis of atherosclerosis: insights from mice and humans

Atherosclerosis is a complex and heritable disease involving multiple cell types and the interactions of many different molecular pathways. The genetic and molecular mechanisms of atherosclerosis have, in part, been elucidated by mouse models; at least 100 different genes have been shown to influence atherosclerosis in mice. Importantly, unbiased genome-wide association studies have recently identified a number of novel loci robustly associated with atherosclerotic coronary artery disease. Here, we review the genetic data elucidated from mouse models of atherosclerosis, as well as significant associations for human coronary artery disease. Furthermore, we discuss in greater detail some of these novel human coronary artery disease loci. The combination of mouse and human genetics has the potential to identify and validate novel genes that influence atherosclerosis, some of which may be candidates for new therapeutic approaches.

Site-specific oxidation of apolipoprotein A-I impairs cholesterol export by ABCA1, a key cardioprotective function of HDL

The mechanisms that deprive HDL of its cardioprotective properties are poorly understood. One potential pathway involves oxidative damage of HDL proteins by myeloperoxidase (MPO) a heme enzyme secreted by human artery wall macrophages. Mass spectrometric analysis demonstrated that levels of 3-chlorotyrosine and 3-nitrotyrosine - two characteristic products of MPO - are elevated in HDL isolated from patients with established cardiovascular disease. When apolipoprotein A-I (apoA-I), the major HDL protein, is oxidized by MPO, its ability to promote cellular cholesterol efflux by the membrane-associated ATP-binding cassette transporter A1 (ABCA1) pathway is diminished. Biochemical studies revealed that oxidation of specific tyrosine and methionine residues in apoA-I contributes to this loss of ABCA1 activity. Another potential mechanism for generating dysfunctional HDL involves covalent modification of apoA-I by reactive carbonyls, which have been implicated in atherogenesis and diabetic vascular disease. Indeed, modification of apoA-I by malondialdehyde (MDA) or acrolein also markedly impaired the lipoprotein's ability to promote cellular cholesterol efflux by the ABCA1 pathway. Tandem mass spectrometric analyses revealed that these reactive carbonyls target specific Lys residues in the C-terminus of apoA-I. Importantly, immunochemical analyses showed that levels of MDA-protein adducts are elevated in HDL isolated from human atherosclerotic lesions. Also, apoA-I co-localized with acrolein adducts in such lesions. Thus, lipid peroxidation products might specifically modify HDL in vivo. Our observations support the hypotheses that MPO and reactive carbonyls might generate dysfunctional HDL in humans. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Dysfunctional HDL containing L159R ApoA-I leads to exacerbation of atherosclerosis in hyperlipidemic mice

The mutation L159R apoA-I or apoA-I(L159R) (FIN) is a single amino acid substitution within the sixth helical repeat of apoA-I. It is associated with a dominant negative phenotype, displaying hypoalphaproteinemia and an increased risk for atherosclerosis in humans. Mice lacking both mouse apoA-I and LDL receptor (LDL(-/-), apoA-I(-/-)) (double knockout or DKO) were crossed>9 generations with mice transgenic for human FIN to obtain L159R apoA-I, LDLr(-/-), ApoA-I(-/-) (FIN-DKO) mice. A similar cross was also performed with human wild-type (WT) apoA-I (WT-DKO). In addition, FIN-DKO and WT-DKO were crossed to obtain WT/FIN-DKO mice. To determine the effects of the apoA-I mutations on atherosclerosis, groups of each genotype were fed either chow or an atherogenic diet for 12weeks. Interestingly, the production of dysfunctional HDL-like particles occurred in DKO and FIN-DKO mice. These particles were distinct with respect to size, and their enrichment in apoE and cholesterol esters. Two-dimensional gel electrophoresis indicated that particles found in the plasma of FIN-DKO mice migrated as large α(3)-HDL. Atherosclerosis analysis showed that FIN-DKO mice developed the greatest extent of aortic cholesterol accumulation compared to all other genotypes, including DKO mice which lack any apoA-I. Taken together these data suggest that the presence of large apoE enriched HDL particles containing apoA-I L159R lack the normal cholesterol efflux promoting properties of HDL, rendering them dysfunctional and pro-atherogenic. In conclusion, large HDL-like particles containing apoE and apoA-I(L159R) contribute rather than protect against atherosclerosis, possibly through defective efflux properties and their potential for aggregation at their site of interaction in the aorta. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Suppressed monocyte recruitment drives macrophage removal from atherosclerotic plaques of Apoe-/- mice during disease regression

Experimental models of atherosclerosis suggest that recruitment of monocytes into plaques drives the progression of this chronic inflammatory condition. Cholesterol-lowering therapy leads to plaque stabilization or regression in human atherosclerosis, characterized by reduced macrophage content, but the mechanisms that underlie this reduction are incompletely understood. Mice lacking the gene Apoe (Apoe-/- mice) have high levels of cholesterol and spontaneously develop atherosclerotic lesions. Here, we treated Apoe-/- mice with apoE-encoding adenoviral vectors that induce plaque regression, and investigated whether macrophage removal from plaques during this regression resulted from quantitative alterations in the ability of monocytes to either enter or exit plaques. Within 2 days after apoE complementation, plasma cholesterol was normalized to wild-type levels, and HDL levels were increased 4-fold. Oil red O staining and quantitative mass spectroscopy revealed that esterified cholesterol content was markedly reduced. Plaque macrophage content decreased gradually and was 72% lower than baseline 4 weeks after apoE complementation. Importantly, this reduction in macrophages did not involve migratory egress from plaques or CCR7, a mediator of leukocyte emigration. Instead, marked suppression of monocyte recruitment coupled with a stable rate of apoptosis accounted for loss of plaque macrophages. These data suggest that therapies to inhibit monocyte recruitment to plaques may constitute a more viable strategy to reduce plaque macrophage burden than attempts to promote migratory egress.

Apolipoprotein A-I modulates regulatory T cells in autoimmune LDLr-/-, ApoA-I-/- mice

The immune system is complex, with multiple layers of regulation that serve to prevent the production of self-antigens. One layer of regulation involves regulatory T cells (Tregs) that play an essential role in maintaining peripheral self-tolerance. Patients with autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis have decreased levels of HDL, suggesting that apoA-I concentrations may be important in preventing autoimmunity and the loss of self-tolerance. In published studies, hypercholesterolemic mice lacking HDL apoA-I or LDLr(-/-), apoA-I(-/-) (DKO), exhibit characteristics of autoimmunity in response to an atherogenic diet. This phenotype is characterized by enlarged cholesterol-enriched lymph nodes (LNs), as well as increased T cell activation, proliferation, and the production of autoantibodies in plasma. In this study, we investigated whether treatment of mice with lipid-free apoA-I could attenuate the autoimmune phenotype. To do this, DKO mice were first fed an atherogenic diet containing 0.1% cholesterol, 10% fat for 6 weeks, after which treatment with apoA-I was begun. Subcutaneous injections of 500 μg of lipid-free apoA-I was administered every 48 h during the treatment phase. These and control mice were maintained for an additional 6 weeks on the diet. At the end of the 12-week study, DKO mice showed decreased numbers of LN immune cells, whereas Tregs were proportionately increased. Accompanying this increase in Tregs was a decrease in the percentage of effector/effector memory T cells. Furthermore, lipid accumulation in LN and skin was reduced. These results suggest that treatment with apoA-I reduces inflammation in DKO mice by augmenting the effectiveness of the LN Treg response.

AAV gene therapy as a means to increase apolipoprotein (Apo) A-I and high-density lipoprotein-cholesterol levels: correction of murine ApoA-I deficiency

BACKGROUND

Inherited apolipoprotein (Apo) A-I deficiency is an orphan disorder characterized by high-density lipoprotein (HDL)-cholesterol deficiency and premature atherosclerosis. Constitutive over-expression of ApoA-I might provide a means to treat this disease. The present study provides a comprehensive evaluation of adeno-associated virus (AAV)-mediated ApoA-I gene delivery to express human (h)ApoA-I and correct the low HDL-cholesterol phenotype associated with ApoA-I deficiency.

METHODS

In an effort to maximize AAV-mediated gene expression, we performed head-to-head comparisons of recombinant AAVs with pseudotype capsids 1, 2, 6 and 8 administered by different routes with the use of five different liver-specific promoters in addition to cytomegalovirus as single-stranded or as self-complementary (sc) AAV vectors.

RESULTS

Intravenous administration of 1 x 10(13) gc/kg scAAV8, in combination with the liver-specific promoter LP1, in female ApoA-I(-/-) mice resulted in hApoA-I expression levels of 634 +/- 69 mg/l, which persisted for the duration of the study (15 weeks). This treatment resulted in full recovery of HDL-cholesterol levels with correction of HDL particle size and apolipoprotein composition. In addition, we observed increased adrenal cholesterol content and a significant increase in bodyweight in treated mice.

CONCLUSIONS

The present study demonstrates that systemic delivery of a scAAV8 vector provides a means for efficient liver expression of hApoA-I, thereby correcting the lipid abnormalities associated with murine ApoA-I deficiency. Importantly, the study demonstrates that AAV-based gene therapy can be used to express therapeutic proteins at a high level for a prolonged period of time and, as such, provides a basis for further development of this strategy to treat hApoA-I deficiency.

Group III secreted phospholipase A2 transgenic mice spontaneously develop inflammation

PLA2 (phospholipase A2) group III is an atypical sPLA2 (secretory PLA2) that is homologous with bee venom PLA2 rather than with other mammalian sPLA2s. In the present paper, we show that endogenous group III sPLA2 (PLA2G3) is expressed in mouse skin and that Tg (transgenic) mice overexpressing human PLA2G3 spontaneously develop skin inflammation. Pla2g3-Tg mice over 9 months of age frequently developed dermatitis with hyperkeratosis, acanthosis, parakeratosis, erosion, ulcer and sebaceous gland hyperplasia. The dermatitis was accompanied by infiltration of neutrophils and macrophages and by elevated levels of pro-inflammatory cytokines, chemokines and prostaglandin E2. In addition, Pla2g3-Tg mice had increased lymph aggregates and mucus in the airway, lymphocytic sialadenitis, hepatic extramedullary haemopoiesis, splenomegaly with increased populations of granulocytes and monocytes/macrophages, and increased serum IgG1. Collectively, these observations provide the first demonstration of spontaneous development of inflammation in mice with Tg overexpression of mammalian sPLA2.

Apolipoprotein A-I and its role in lymphocyte cholesterol homeostasis and autoimmunity

OBJECTIVE

The purpose of this study was to determine the effects of an atherogenic diet on immune function in LDLr(-/-), ApoA-I(-/-) mice.

METHODS AND RESULTS

When LDLr(-/-), ApoA-I(-/-) (DKO), and LDLr(-/-) (SKO) mice were fed an atherogenic diet, DKO had larger peripheral lymph nodes (LNs) and spleens compared to SKO mice. LNs were enriched in cholesterol and contain expanded populations of T, B, dendritic cells, and macrophages. Expansion of all classes of LN cells was accompanied by a approximately 1.5-fold increase in T cell proliferation and activation. Plasma antibodies to dsDNA, beta2-glycoprotein I, and oxidized LDL were increased in DKO, similar to levels in diet-fed Fas(lpr/lpr) mice, suggesting the development of an autoimmune phenotype. Both LN enlargement and cellular cholesterol expansion were "prevented" when diet-fed DKO mice were treated with helper dependent adenovirus expressing apoA-I. Independent of the amount of dietary cholesterol, DKO mice consistently showed lower plasma cholesterol than SKO mice, yet greater aortic cholesterol deposition and inflammation.

CONCLUSIONS

ApoA-I prevented cholesterol-associated lymphocyte activation and proliferation in peripheral LN of diet-fed DKO mice. A approximately 1.5-fold increase in T cell activation and proliferation was associated with a approximately 3-fold increase in concentrations of circulating autoantibodies and approximately 2-fold increase in the severity of atherosclerosis suggesting a common link between plasma apoA-I, inflammation, and atherosclerosis.

Effects of high dietary fat and cholesterol on expression of PPAR alpha, LXR alpha, and their responsive genes in the liver of apoE and LDLR double deficient mice

The significance of transcription factors PPAR alpha, LXR alpha, and their responsive/target genes for the pathogenesis of atherosclerosis in apolipoprotein E and low-density lipoprotein receptor double deficient (AL) mice fed with high fat and cholesterol (HF) diet were studied. C57BL/6J wild-type (WT) mice were used as control to the AL mice. Plasma lipid metabolites and morphological atherosclerotic lesions in aortic wall were determined. Semi- and real-time quantitative RT-PCR were used to measure gene expression patterns between AL mice and the controls, which were fed with HF or normal chow diet. The results showed that in AL mice fed with HF diet, plasma lipid levels, hepatic lipid accumulation, and atherogenesis together with upregulated PPAR alpha, LXR alpha, and their target genes, i.e., FAT, SCD1, FAS, Angptl3, and apoB100 significantly increased in a 12-week long feeding period. In contrast, apoAI, apoAIV, apoF, LPL, and SR-BI were decreased compared to chow-fed group. In WT mice, PPAR alpha, LXR alpha, FAS, Angpt13, CPT1, apoF, ACOX1, LPL, and SR-BI were increased with HF treatment, while apoAI and apoAIV were decreased markedly. The different changes of lipid metabolism-related genes between AL and WT mice, fed with HF diet or chow diet indicated that the mechanisms of dietary effects on gene mutant mice are different from those of intact WT mice. Since lipid metabolic system defected genetically in AL mice, we suggest that the changes of PPAR alpha, LXR alpha, and their target genes aggravated lipid metabolic disorder in the liver and further accelerated the development of atherosclerosis on a stress of HF diet feeding in AL mice.

VLDL best predicts aortic root atherosclerosis in LDL receptor deficient mice

Hyperlipidemia is a major risk factor for developing atherosclerosis in humans, and epidemiological studies have correlated specific lipoprotein levels with cardiovascular disease risk. Murine models of atherosclerosis rely on the induction of hyperlipidemia for vascular lesions to form, but the pathogenic contributions attributed to different lipoprotein populations are not well defined. To address this issue, we analyzed over 300 LDL receptor (LDLR) deficient mice that have been fed a high-fat diet and for which a full lipoprotein profile and aortic root atherosclerosis values were assessed. Overall, aortic root atherosclerosis is best predicted by plasma VLDL cholesterol levels with less predictive value derived from either LDL or HDL cholesterol. Triglyceride levels are more atherogenic in female mice, especially immune competent females, and depletion of the adaptive immune system leads to a global reduction in plasma lipid levels and aortic root lesion size yet does not appear to alter the atherogenic potential of individual lipoprotein subspecies. In contrast, HDL-cholesterol is a better predictor of aortic root atherosclerosis in apoE-deficient mice. In summary, this large scale analysis of high-fat diet fed LDLR deficient mice highlight the relationship between different plasma lipid components, especially VLDL-cholesterol, and aortic root atherosclerosis.

Atherosclerosis and liver inflammation induced by increased dietary cholesterol intake: a combined transcriptomics and metabolomics analysis

BACKGROUND

Increased dietary cholesterol intake is associated with atherosclerosis. Atherosclerosis development requires a lipid and an inflammatory component. It is unclear where and how the inflammatory component develops. To assess the role of the liver in the evolution of inflammation, we treated ApoE*3Leiden mice with cholesterol-free (Con), low (LC; 0.25%) and high (HC; 1%) cholesterol diets, scored early atherosclerosis and profiled the (patho)physiological state of the liver using novel whole-genome and metabolome technologies.

RESULTS

Whereas the Con diet did not induce early atherosclerosis, the LC diet did so but only mildly, and the HC diet induced it very strongly. With increasing dietary cholesterol intake, the liver switches from a resilient, adaptive state to an inflammatory, pro-atherosclerotic state. The liver absorbs moderate cholesterol stress (LC) mainly by adjusting metabolic and transport processes. This hepatic resilience is predominantly controlled by SREBP-1/-2, SP-1, RXR and PPARalpha. A further increase of dietary cholesterol stress (HC) additionally induces pro-inflammatory gene expression, including pro-atherosclerotic candidate genes. These HC-evoked changes occur via specific pro-inflammatory pathways involving specific transcriptional master regulators, some of which are established, others newly identified. Notably, several of these regulators control both lipid metabolism and inflammation, and thereby link the two processes.

CONCLUSION

With increasing dietary cholesterol intake the liver switches from a mainly resilient (LC) to a predominantly inflammatory (HC) state, which is associated with early lesion formation. Newly developed, functional systems biology tools allowed the identification of novel regulatory pathways and transcriptional regulators controlling both lipid metabolism and inflammatory responses, thereby providing a rationale for an interrelationship between the two processes.

Myeloperoxidase and inflammatory proteins: pathways for generating dysfunctional high-density lipoprotein in humans

High-density lipoprotein (HDL) inhibits atherosclerosis by removing cholesterol from artery wall macrophages. Additionally, HDL is anti-inflammatory in animal studies, suggesting that this property might also be important for its cardioprotective effects. Recent studies in subjects with established cardiovascular disease (CVD) demonstrate that myeloperoxidase targets HDL for oxidation and blocks the lipoprotein's ability to remove excess cholesterol from cells, raising the possibility that the enzyme provides a specific mechanism for generating dysfunctional HDL in humans. Shotgun proteomic analysis of HDL identified multiple complement regulatory proteins, protease inhibitors, and acute-phase response proteins, supporting a central role for HDL in inflammation. Mass spectrometry and biochemical analyses demonstrated that HDL(3) from CVD subjects was selectively enriched in apolipoprotein E, suggesting that it carries a unique cargo of proteins in humans with clinically significant CVD. Thus, oxidative modifications to HDL and changes in its protein composition might be useful biomarkers-and perhaps mediators-of CVD.

Myeloperoxidase: an inflammatory enzyme for generating dysfunctional high density lipoprotein

PURPOSE OF REVIEW

Evidence indicates that high density lipoprotein (HDL) is cardioprotective and that several mechanisms are involved. One important pathway is a membrane-associated ATP-binding cassette transporter, ABCA1, that clears cholesterol from macrophage foam cells. Anti-inflammatory and antioxidant properties also might contribute to HDL's ability to inhibit atherosclerosis.

RECENT FINDINGS

Myeloperoxidase targets HDL for oxidation, raising the possibility that the enzyme provides a specific mechanism for generating dysfunctional HDL in humans. Myeloperoxidase-dependent oxidation of apolipoprotein A-I, the major protein in HDL, blocks HDL's ability to remove excess cholesterol from cells by the ABCA1 pathway. Analysis of mutated forms of apoA-I and oxidized apoA-I treated with methionine sulfoxide reductase implicate oxidation of specific tyrosine and methionine residues in impairing the ABCA1 transport activity of apoA-I. The crystal structure of lipid-free apoA-I suggests that such oxidative damage might disrupt negatively charged regions on the protein's surface or alter its remodeling, resulting in conformations that fail to interact with ABCA1.

SUMMARY

Oxidation of HDL by myeloperoxidase may represent a specific molecular mechanism for converting the cardioprotective lipoprotein into a dysfunctional form, raising the possibility that the enzyme represents a potential therapeutic target for preventing vascular disease in humans. Moreover, oxidized HDL might prove useful as a blood marker for clinically significant cardiovascular disease in humans.

Ratio determination of plasma wild-type and L159R apoA-I using mass spectrometry: tools for studying apoA-IFin

In this report, methods are described to isolate milligram quantities of a mutant apolipoprotein A-I (apoA-I) protein for use in structure-function studies. Expression of the L159R apoA-I mutation in humans reduces the concentration of plasma wild-type apoA-I, thus displaying a dominant negative phenotype in vivo. Earlier attempts to express and isolate this mutant protein resulted in extensive degradation and protein misfolding. Using an Escherichia coli expression system used predominantly for the isolation of soluble apoA-I mutant proteins, we describe the expression and purification of L159R apoA-I (apoA-I(Fin)) from inclusion bodies. In addition, we describe a mass spectrometric method for measuring the L159R-to-wild-type apoA-I ratio in a 1 microl plasma sample. These new methods will facilitate further studies into the mechanism behind the dominant negative phenotype associated with the expression of the L159R apoA-I protein in humans.

Overexpression of human ApoAI transgene provides long-term atheroprotection in LDL receptor-deficient mice

The long-term effect of elevated levels of human apolipoprotein AI (apoAI) on atherosclerosis was assessed using human apoAI transgenic mice on a double mutant LDL receptor-deficient (LDLr-/-) and mouse apoAI-deficient (apoAI-/-) background. When they were fed a high fat diet, atherosclerosis in transgenic human apoAI, LDLr-/-, apoAI-/- mice (huapoAITg) was compared with LDLr-/- mice that expressed normal amounts of apoAI (msapoAI) or LDLr-/- mice that lacked mouse apoAI (noapoAI). The atheroprotective effect of human apoAI was demonstrated by a greater than six-fold inhibition in lesion areas in the aortic wall and heart valves compared to the two control strains after 27 or 36 weeks. Plasma apoAI concentrations in huapoAITg mice were considerably higher than in msapoAI mice (600 and 37 mg/dL, respectively). The human apoAI transgene led to several plasma HDL subpopulations, with high levels of prebeta-HDL and a significant decrease in total plasma cholesterol. This was observed without a change in total HDL cholesterol levels. Thus, elevated levels of human apoAI in LDL receptor-deficient mice lacking mouse apoAI conferred profound protection against diet-induced over extended periods of time.

Current understanding of the role of high-density lipoproteins in atherosclerosis and senescence

Numerous epidemiologic and interventional studies revealed that high-density lipoprotein (HDL) is an important risk factor for coronary heart disease. There are several well documented HDL functions that may account for the antiatherogenic effects of this lipoprotein. The best recognized of these is the capacity of HDL to transport cholesterol from the periphery to the liver, and thereby prevent cholesterol deposition in the arterial wall. Further properties of HDL that may also be antiatherogenic include its potent anti oxidative and anti-inflammatory action. In addition, HDL seems to be involved in processes related to senescence at both the cellular and whole-organism level. Both protein components of HDL (such as apolipoprotein A-I) and its lipid components (such as, lysosphingolipids) appear to mediate the antiatherogenic and anti-aging effects of HDL. The purpose of this review is to summarize the novel functions of HDL that may protect from atherosclerosis and senescence.

Diet and murine atherosclerosis

Lipid-enriched diets are often used to induce or accelerate the rate of atherosclerotic lesion development in murine models of atherosclerosis. It appears that the induction of persistent hypercholesterolemia to levels > or approximately to 300 mg/dL is required for the development of experimental atherosclerosis in the mouse. A variety of different diets have been used that vary in the level of cholesterol, the level and type of fatty acid, and the absence or presence of cholate. Each of these components as well as the protein source has been shown to influence lipoprotein level and/or atherosclerosis, with dietary cholesterol being the major proatherogenic component. In some instances the effects of these components on the expression of hepatic genes relevant to lipid homeostasis has been observed. An appreciation of the effect of the differences in diet composition on these processes is important to compare results from different atherosclerosis studies, so the composition of the diets used should always be reported or referenced. Cholate should not be used unless its effects are being specifically investigated.

What is so special about apolipoprotein AI in reverse cholesterol transport?

An initial step in reverse cholesterol transport is the movement of unesterified cholesterol from peripheral cells to high-density lipoproteins (HDLs). This transfer usually occurs in extracellular spaces, such as the subendothelial space of a vessel wall, and is promoted by the interaction of lipid-free or lipid-poor apolipoprotein (apo)AI with ATP binding cassette A1 cellular transporters on macrophages (MPhi). Because HDL does not interact with MPhi ATP binding cassette A1 and apoAI is not synthesized by macrophages, this apoAI must be generated from spherical HDL. In this brief review, we propose that spherical apoAI is derived from HDL by remodeling events that are accomplished by proteins secreted by cholesteryl ester-loaded foam cells, including the lipid transfer proteins, phospholipid transfer protein, and cholesteryl ester transfer protein, and the triglyceride hydrolases hepatic lipase and lipoprotein lipase.

The PPARdelta agonist GW0742X reduces atherosclerosis in LDLR(-/-) mice

Several lines of evidence suggest a biological role for peroxisome proliferator-activated receptor (PPARdelta) in the pathogenesis of atherosclerosis. Administration of synthetic PPARdelta agonists to obese rhesus monkeys elevates serum high-density lipoprotein (HDL) cholesterol as a result of increased reverse cholesterol transport whilst in vitro studies have suggested a role for PPARdelta in lipid uptake into macrophages. Recent studies have found that PPARdelta depletion from macrophages in LDL receptor (LDLR(-/-)) mice decreases lesion area via modulation of the inflammatory status of the macrophage, an effect also seen on pharmacological activation of PPARdelta in vitro. We demonstrate here that the PPARdelta agonist, GW0742X has potent anti-atherogenic activity in the LDLR(-/-) mouse, decreasing lesion area by up to 50%. Administration of GW0742X had no effect on total cholesterol, HDL or LDL cholesterol and modest effects on very low-density lipoprotein (VLDL). Treatment with GW0742X resulted in decreased expression of monocyte chemoattractant protein 1 (MCP-1) and intracellular adhesion moleculae 1 (ICAM-1) in the aortae of treated mice. In addition, GW0742X decreased tumour necrosis factor-alpha (TNFalpha) expression in peritoneal macrophages, aortae and adipose tissue in comparison with control animals. Changes in gene expression were reflected in decreased plasma levels of MCP-1. These observations support an atheroprotective effect of PPARdelta agonists in vivo.

Effects of D-4F on vasodilation and vessel wall thickness in hypercholesterolemic LDL receptor-null and LDL receptor/apolipoprotein A-I double-knockout mice on Western diet

Previously we showed L-4F, a novel apolipoprotein A-I (apoA-I) mimetic, improved vasodilation in 2 dissimilar models of vascular disease: hypercholesterolemic LDL receptor-null (Ldlr(-/-)) mice and transgenic sickle cell disease mice. Here we determine the mechanisms by which D-4F improves vasodilation and arterial wall thickness in hypercholesterolemic Ldlr(-/-) mice and Ldlr(-/-)/apoA-I null (apoA-I(-/-)), double-knockout mice. Ldlr(-/-) and Ldlr(-/-)/apoA-I(-/-) mice were fed Western diet (WD) with and without D-4F. Oral D-4F restored endothelium- and endothelial NO synthase (eNOS)-dependent vasodilation in direct relationship to duration of treatments and reduced wall thickness in as little as 2 weeks in vessels with preexisting disease in Ldlr(-/-) mice. D-4F had no effect on total or HDL cholesterol concentrations but reduced proinflammatory HDL levels. D-4F had no effect on plasma myeloperoxidase concentrations but reduced myeloperoxidase association with apoA-I as well as 3-nitrotyrosine in apoA-I. D-4F increased endothelium- and eNOS-dependent vasodilation in Ldlr(-/-)/apoA-I(-/-) mice but did not reduce wall thickness as it had in Ldlr(-/-) mice. Vascular endothelial cells were treated with 22(R)-hydroxycholesterol with and without L-4F. 22(R)-Hydroxycholesterol decreased NO (*NO) and increased superoxide anion (O2*-) production and increased ATP-binding cassette transporter-1 and collagen expression. L-4F restored *NO and O2*- balance, had little effect on ATP-binding cassette transporter-1 expression, but reduced collagen expression. These data demonstrate that although D-4F restores vascular endothelial cell and eNOS function to increase vasodilation, HDL containing apoA-I, or at least some critical concentration of the antiatherogenic lipoprotein, is required for D-4F to decrease vessel wall thickness.

Increased atherosclerosis in mice lacking apolipoprotein A-I attributable to both impaired reverse cholesterol transport and increased inflammation

To test the hypothesis that apolipoprotein A-I (apoA-I) functions specifically to inhibit atherosclerosis independent of the level of high-density lipoprotein cholesterol (HDL-C) by promoting both reverse cholesterol transport and HDL antiinflammatory function in vivo, we established a murine atherosclerosis model of apoA-I deficiency in which the level of HDL-C is well maintained. ApoA-I-/- mice were crossed with atherosclerosis susceptible low-density lipoprotein receptor-/-/apobec-/- (LA) mice to generate LA mice with apoA-I+/+, apoA-I+/-, and apoA-I-/- genotypes. There were no major differences in the amounts of non-HDL-C and HDL-C in the plasma between different apoA-I genotypes. A significant inverse relationship was observed, however, between apoA-I gene dose and atherosclerosis in both female and male mice. Compared with LA-apoA-I+/+ mice, serum from LA-apoA-I-/- mice had a significantly reduced capacity to function as an acceptor of ABCA1- and SR-BI-mediated cellular cholesterol efflux, and also had markedly reduced lecithin cholesterol acyltransferase activity. In addition, LA-apoA-I-/- mice had significantly reduced macrophage-derived cholesterol esterification and reverse cholesterol transport in vivo. There was significantly reduced plasma paraoxonase (PON-1) activity, impaired HDL vascular antiinflammatory function, and increased basal levels of monocyte chemotactic protein-1 in the plasma of LA-apoA-I-/- mice compared with LA-apoA-I+/+ mice. In LA-apoA-I-/- mice, there was also greater induction of some, but not all, inflammatory cytokines and chemokines in response to intraperitoneal injection of lipopolysaccharide than in LA-apoA-I+/+ mice. We conclude that apoA-I inhibits atherosclerosis by promoting both macrophage reverse cholesterol transport and HDL antiinflammatory function, and that these anti-atherogenic functions of apoA-I are largely independent of the plasma level of HDL-C in this mouse model.