Genetic heterogeneity of lipoproteins in inbred strains of mice: analysis by gel-permeation chromatography

Zebrafish ApoB-Containing Lipoprotein Metabolism: A Closer Look

Zebrafish have become a powerful model of mammalian lipoprotein metabolism and lipid cell biology. Most key proteins involved in lipid metabolism, including cholesteryl ester transfer protein, are conserved in zebrafish. Consequently, zebrafish exhibit a human-like lipoprotein profile. Zebrafish with mutations in genes linked to human metabolic diseases often mimic the human phenotype. Zebrafish larvae develop rapidly and externally around the maternally deposited yolk. Recent work revealed that any disturbance of lipoprotein formation leads to the accumulation of cytoplasmic lipid droplets and an opaque yolk, providing a visible phenotype to investigate disturbances of the lipoprotein pathway, already leading to discoveries in MTTP (microsomal triglyceride transfer protein) and ApoB (apolipoprotein B). By 5 days of development, the digestive system is functional, making it possible to study fluorescently labeled lipid uptake in the transparent larvae. These and other approaches enabled the first in vivo description of the STAB (stabilin) receptors, showing lipoprotein uptake in endothelial cells. Various zebrafish models have been developed to mimic human diseases by mutating genes known to influence lipoproteins (eg, ldlra, apoC2). This review aims to discuss the most recent research in the zebrafish ApoB-containing lipoprotein and lipid metabolism field. We also summarize new insights into lipid processing within the yolk cell and how changes in lipid flux alter yolk opacity. This curious new finding, coupled with the development of several techniques, can be deployed to identify new players in lipoprotein research directly relevant to human disease.

Quantitative characterizations of the cholesterol-related pathways in the retina and brain of hamsters

The retina and brain are separated from the systemic circulation by the anatomical barriers, which are permeable (the outer blood-retinal barrier) and impermeable (the blood-brain and inner blood-retina barriers) to cholesterol. Herein we investigated whether the whole-body cholesterol maintenance affects cholesterol homeostasis in the retina and brain. We used hamsters, whose whole-body cholesterol handling is more similar to those in humans than in mice and conducted separate administrations of deuterated water and deuterated cholesterol. We assessed the quantitative significance of the retinal and brain pathways of cholesterol input and compared the results with those from our previous studies in mice. The utility of the measurements in the plasma of deuterated 24-hydroxycholesterol, the major cholesterol elimination product from the brain, was investigated as well. We established that despite a 7-fold higher serum LDL to HDL ratio and other cholesterol-related differences, in situ biosynthesis remained the major source of cholesterol for hamster retina, although its quantitative significance was reduced to 53% as compared to 72-78% in mouse retina. In the brain, the principal pathway of cholesterol input was also the same, in situ biosynthesis, accounting for 94% of the total brain cholesterol input (96% in mice); the interspecies differences pertained to the absolute rates of the total cholesterol input and turnover. We documented the correlations between deuterium enrichments of the brain 24-hydroxycholesterol, brain cholesterol, and plasma 24-hydroxycholesterol, which suggested that deuterium enrichment of plasma 24-hydroxycholesteol could be an in vivo marker of cholesterol elimination and turnover in the brain.

Cyp2c-deficiency depletes muricholic acids and protects against high-fat diet-induced obesity in male mice but promotes liver damage

Objective

Murine-specific muricholic acids (MCAs) are reported to protect against obesity and associated metabolic disorders. However, the response of mice with genetic depletion of MCA to an obesogenic diet has not been evaluated. We used Cyp2c-deficient (Cyp2c^−/−) mice, which lack MCAs and thus have a human-like bile acid (BA) profile, to directly investigate the potential role of MCAs in diet-induced obesity.

Methods

Male and female Cyp2c^−/− mice and wild-type (WT) littermate controls were fed a standard chow diet or a high-fat diet (HFD) for 18 weeks. We measured BA composition from a pool of liver, gallbladder, and intestine, as well as weekly body weight, food intake, lean and fat mass, systemic glucose homeostasis, energy expenditure, intestinal lipid absorption, fecal lipid, and energy content.

Results

Cyp2c-deficiency depleted MCAs and caused other changes in BA composition, namely a decrease in the ratio of 12α-hydroxylated (12α-OH) BAs to non-12α-OH BAs, without altering the total BA levels. While WT male mice became obese after HFD feeding, Cyp2c^−/− male mice were protected from obesity and associated metabolic dysfunctions. Cyp2c^−/− male mice also showed reduced intestinal lipid absorption and increased lipid excretion, which was reversed by oral gavage with the 12α-OH BA and taurocholic acid (TCA). Cyp2c^−/− mice also showed increased liver damage, which appeared stronger in females.

Conclusions

MCA does not protect against diet-induced obesity but may protect against liver injury. Reduced lipid absorption in Cyp2c-deficient male mice is potentially due to a reduced ratio of 12α-OH/non-12α-OH BAs.

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Presence of MCA does not necessarily protect against diet-induced obesity.

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Cyp2c deficiency promotes resistance to diet-induced obesity in males.

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Cyp2c-knockout mice have decreased the ratio of 12α-OH/non-12α-OH BAs that promotes decreased intestinal lipid absorption.

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Cyp2c-knockout mice have improved glucose homeostasis.

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Cyp2c-deficiency promotes mild and severe liver injury in male and female mice, respectively.

Mild Mitochondrial Uncoupling Decreases Experimental Atherosclerosis, A Proof of Concept

Aim: Atherosclerosis is responsible for high morbidity and mortality rates around the world. Local arterial oxidative stress is involved in all phases of atherosclerosis development. Mitochondria is a relevant source of the oxidants, particularly under certain risky conditions, such as hypercholesterolemia. The aim of this study was to test whether lowering the production of mitochondrial oxidants by induction of a mild uncoupling can reduce atherosclerosis in hypercholesterolemic LDL receptor knockout mice.

Methods: The mice were chronically treated with very low doses of DNP (2,4-dinitrophenol) and metabolic, inflammatory and redox state markers and atherosclerotic lesion sizes were determined.

Results: The DNP treatment did not change the classical atherosclerotic risk markers, such as plasma lipids, glucose homeostasis, and fat mass, as well as systemic inflammatory markers. However, the DNP treatment diminished the production of mitochondrial oxidants, systemic and tissue oxidative damage markers, peritoneal macrophages and aortic rings oxidants generation. Most importantly, development of spontaneous and diet-induced atherosclerosis (lipid and macrophage content) were significantly decreased in the DNP-treated mice. In vitro, DNP treated peritoneal macrophages showed decreased H₂O₂ production, increased anti-inflammatory cytokines gene expression and secretion, increased phagocytic activity, and decreased LDL-cholesterol uptake.

Conclusions: These findings are a proof of concept that activation of mild mitochondrial uncoupling is sufficient to delay the development of atherosclerosis under the conditions of hypercholesterolemia and oxidative stress. These results promote future approaches targeting mitochondria for the prevention or treatment of atherosclerosis.

Physico-chemical and physiological determinants of lipo-nanoparticle stability

Liposome-based nanoparticles (NPs) comprised mostly of phospholipids (PLs) have been developed to deliver diagnostic and therapeutic agents. Whereas reassembled plasma lipoproteins have been tested as NP carriers of hydrophobic molecules, they are unstable because the components can spontaneously transfer to other PL surfaces-cell membranes and lipoproteins-and can be degraded by plasma lipases. Here we review two strategies for NP stabilization. One is to use PLs that contain long acyl-chains: according to a quantitative thermodynamic model and in vivo tests, increasing the chain length of a PL reduces the spontaneous transfer rate and increases plasma lifetime. A second strategy is to substitute ether for ester bonds which makes the PLs lipase resistant. We conclude with recommendations of simple ex vivo and in vitro tests of NP stability that should be conducted before in vivo tests are begun.

Insulin resistance dysregulates CYP7B1 leading to oxysterol accumulation: a pathway for NAFL to NASH transition

NAFLD is an important public health issue closely associated with the pervasive epidemics of diabetes and obesity. Yet, despite NAFLD being among the most common of chronic liver diseases, the biological factors responsible for its transition from benign nonalcoholic fatty liver (NAFL) to NASH remain unclear. This lack of knowledge leads to a decreased ability to find relevant animal models, predict disease progression, or develop clinical treatments. In the current study, we used multiple mouse models of NAFLD, human correlation data, and selective gene overexpression of steroidogenic acute regulatory protein (StarD1) in mice to elucidate a plausible mechanistic pathway for promoting the transition from NAFL to NASH. We show that oxysterol 7α-hydroxylase (CYP7B1) controls the levels of intracellular regulatory oxysterols generated by the "acidic/alternative" pathway of cholesterol metabolism. Specifically, we report data showing that an inability to upregulate CYP7B1, in the setting of insulin resistance, results in the accumulation of toxic intracellular cholesterol metabolites that promote inflammation and hepatocyte injury. This metabolic pathway, initiated and exacerbated by insulin resistance, offers insight into approaches for the treatment of NAFLD.

Apolipoprotein C1: Its Pleiotropic Effects in Lipid Metabolism and Beyond

Apolipoprotein C1 (apoC1), the smallest of all apolipoproteins, participates in lipid transport and metabolism. In humans, APOC1 gene is in linkage disequilibrium with APOE gene on chromosome 19, a proximity that spurred its investigation. Apolipoprotein C1 associates with triglyceride-rich lipoproteins and HDL and exchanges between lipoprotein classes. These interactions occur via amphipathic helix motifs, as demonstrated by biophysical studies on the wild-type polypeptide and representative mutants. Apolipoprotein C1 acts on lipoprotein receptors by inhibiting binding mediated by apolipoprotein E, and modulating the activities of several enzymes. Thus, apoC1 downregulates lipoprotein lipase, hepatic lipase, phospholipase A2, cholesterylester transfer protein, and activates lecithin-cholesterol acyl transferase. By controlling the plasma levels of lipids, apoC1 relates directly to cardiovascular physiology, but its activity extends beyond, to inflammation and immunity, sepsis, diabetes, cancer, viral infectivity, and-not last-to cognition. Such correlations were established based on studies using transgenic mice, associated in the recent years with GWAS, transcriptomic and proteomic analyses. The presence of a duplicate gene, pseudogene APOC1P, stimulated evolutionary studies and more recently, the regulatory properties of the corresponding non-coding RNA are steadily emerging. Nonetheless, this prototypical apolipoprotein is still underexplored and deserves further research for understanding its physiology and exploiting its therapeutic potential.

FoxO transcription factors are required for hepatic HDL cholesterol clearance

Insulin resistance and type 2 diabetes are associated with low levels of high-density lipoprotein cholesterol (HDL-C). The insulin-repressible FoxO transcription factors are potential mediators of the effect of insulin on HDL-C. FoxOs mediate a substantial portion of insulin-regulated transcription, and poor FoxO repression is thought to contribute to the excessive glucose production in diabetes. In this work, we show that mice with liver-specific triple FoxO knockout (L-FoxO1,3,4), which are known to have reduced hepatic glucose production, also have increased HDL-C. This was associated with decreased expression of the HDL-C clearance factors scavenger receptor class B type I (SR-BI) and hepatic lipase and defective selective uptake of HDL cholesteryl ester by the liver. The phenotype could be rescued by re-expression of SR-BI. These findings demonstrate that hepatic FoxOs are required for cholesterol homeostasis and HDL-mediated reverse cholesterol transport to the liver.

PCSK9 deficiency reduces atherosclerosis, apolipoprotein B secretion, and endothelial dysfunction

Proprotein convertase subtilisin/kexin type 9 (PCSK9) interacts directly with cytoplasmic apoB and prevents its degradation via the autophagosome/lysosome pathway. This process affects VLDL and LDL production and influences atherogenesis. Here, we investigated the molecular machinery by which PCSK9 modulates autophagy and affects atherogenesis. We backcrossed Pcsk9-/- mice with atherosclerosis-prone Ldlr-/-Apobec1-/- (LDb) mice to generate Ldlr-/-Apobec1-/-Pcsk9-/- (LTp) mice. Deletion of PCSK9 resulted in decreased hepatic apoB secretion, increased autophagic flux, and decreased plasma levels of IDL and LDL particles. The LDLs from LTp mice (LTp-LDLs) were less atherogenic and contained less cholesteryl ester and phospholipids than LDb-LDLs. Moreover LTp-LDLs induced lower endothelial expression of the genes encoding TLR2, Lox-1, ICAM-1, CCL2, CCL7, IL-6, IL-1β, Beclin-1, p62, and TRAF6 Collectively, these effects were associated with substantially less atherosclerosis development (>4-fold) in LTp mice. The absence of PCSK9 in LDb mice results in decreased lipid and apoB levels, fewer atherogenic LDLs, and marked reduction of atherosclerosis. The effect on atherogenesis may be mediated in part by the effects of modified LDLs on endothelial cell receptors and proinflammatory and autophagy molecules. These findings suggest that there may be clinical benefits of PCSK9 inhibition due to mechanisms unrelated to increased LDL receptor activity.

Impact of genetic deletion of platform apolipoproteins on the size distribution of the murine lipoproteome

Given their association with cardiovascular disease protection, there has been intense interest in understanding the biology of high density lipoproteins (HDL). HDL is actually a family of diverse particle types, each made up of discrete - but as yet undetermined - combinations of proteins drawn from up to 95 lipophilic plasma proteins. The abundant apolipoproteins (apo) of the A class (apoA-I, apoA-II and apoA-IV) have been proposed to act as organizing platforms for auxiliary proteins, but this concept has not been systematically evaluated. We assessed the impact of genetic knock down of each platform protein on the particle size distribution of auxiliary HDL proteins. Loss of apoA-I or apoA-II massively reduced HDL lipids and changed the plasma size pattern and/or abundance of several plasma proteins. Surprisingly though, many HDL proteins were not affected, suggesting they assemble on lipid particles in the absence of apoA-I or apoA-II. In contrast, apoA-IV ablation had minor effects on plasma lipids and proteins, suggesting that it forms particles that largely exclude other apolipoproteins. Overall, the data indicate that distinct HDL subpopulations exist that do not contain, nor depend on, apoA-I, apoA-II or apoA-IV and these contribute substantially to the proteomic diversity of HDL.

BIOLOGICAL SIGNIFICANCE

Plasma levels of high density lipoproteins (HDL) are inversely correlated with cardiovascular disease. These particles are becoming known as highly heterogeneous entities that have diverse compositions and functions that may impact disease. Unfortunately, we know little about the forces that maintain the composition of each particle in plasma. It has been suggested that certain 'scaffold' proteins, such as apolipoprotein (apo) A-I, apoA-II and apoA-IV, may act as organizing centers for the docking of myriad accessory proteins. To test this hypothesis, we took advantage of the genetic tractability of the mouse model and ablated these three proteins individually. We then tracked the abundance and size profile of the remaining HDL proteins by gel filtration chromatography combined with mass spectrometry. The results clearly show that certain cohorts of proteins depend on each scaffold molecule to assemble normal sized HDL particles under wild-type conditions. This work forms the basis for more detailed studies that will define the specific compositions of HDL subspecies with the possibility of connecting them to specific functions or roles in disease.

Increased high-density lipoprotein cholesterol levels in mice with XX versus XY sex chromosomes

OBJECTIVE

The molecular mechanisms underlying sex differences in dyslipidemia are poorly understood. We aimed to distinguish genetic and hormonal regulators of sex differences in plasma lipid levels.

APPROACH AND RESULTS

We assessed the role of gonadal hormones and sex chromosome complement on lipid levels using the four core genotypes mouse model (XX females, XX males, XY females, and XY males). In gonadally intact mice fed a chow diet, lipid levels were influenced by both male-female gonadal sex and XX-XY chromosome complement. Gonadectomy of adult mice revealed that the male-female differences are dependent on acute effects of gonadal hormones. In both intact and gonadectomized animals, XX mice had higher HDL cholesterol (HDL-C) levels than XY mice, regardless of male-female sex. Feeding a cholesterol-enriched diet produced distinct patterns of sex differences in lipid levels compared with a chow diet, revealing the interaction of gonadal and chromosomal sex with diet. Notably, under all dietary and gonadal conditions, HDL-C levels were higher in mice with 2 X chromosomes compared with mice with an X and Y chromosome. By generating mice with XX, XY, and XXY chromosome complements, we determined that the presence of 2 X chromosomes, and not the absence of the Y chromosome, influences HDL-C concentration.

CONCLUSIONS

We demonstrate that having 2 X chromosomes versus an X and Y chromosome complement drives sex differences in HDL-C. It is conceivable that increased expression of genes escaping X-inactivation in XX mice regulates downstream processes to establish sexual dimorphism in plasma lipid levels.

A comparison of the mouse and human lipoproteome: suitability of the mouse model for studies of human lipoproteins

Plasma levels of low density lipoproteins (LDL) and high density lipoproteins (HDL) exhibit opposing associations with cardiovascular disease in human populations and mouse models have been heavily used to derive a mechanistic understanding of these relationships. In humans, recent mass spectrometry studies have revealed that the plasma lipoproteome is significantly more complex than originally appreciated. This is particularly true for HDL which contains some 90 distinct proteins, a majority of which play functional roles that go beyond those expected for simple lipid transport. Unfortunately, the mouse lipoproteome remains largely uncharacterized-a significant gap given the heavy reliance on the model. Using a gel filtration chromatography and mass spectrometry analysis that targets phospholipid-bound plasma proteins, we compared the mouse lipoproteome and its size distribution to a previous, identical human analysis. We identified 113 lipid associated proteins in the mouse. In general, the protein diversity in the LDL and HDL size ranges was similar in mice versus humans, though some distinct differences were noted. For the majority of proteins, the size distributions, that is, whether a given protein was associated with large versus small HDL particles, for example, were also similar between species. Again, however, there were clear differences exhibited by a minority of proteins that may reflect metabolic differences between species. Finally, by correlating the lipid and protein size profiles, we identified five proteins that closely track with the major HDL protein, apolipoprotein A-I across both species. Thus, mice have most of the minor proteins identified in human lipoproteins that play key roles in inflammation, innate immunity, proteolysis and its inhibition, and vitamin transport. This provides support for the continued use of the mouse as a model for many aspects of human lipoprotein metabolism.

The Effect of Natural LCAT Mutations on the Biogenesis of HDL

We have investigated how the natural LCAT[T147I] and LCAT[P274S] mutations affect the pathway of biogenesis of HDL. Gene transfer of WT LCAT in LCAT(-/-) mice increased 11.8-fold the plasma cholesterol, whereas the LCAT[T147I] and LCAT[P274S] mutants caused a 5.2- and 2.9-fold increase, respectively. The LCAT[P274S] and the WT LCAT caused a monophasic distribution of cholesterol in the HDL region, whereas the LCAT[T147I] caused a biphasic distribution of cholesterol in the LDL and HDL region. Fractionation of plasma showed that the expression of WT LCAT increased plasma apoE and apoA-IV levels and shifted the distribution of apoA-I to lower densities. The LCAT[T147I] and LCAT[P274S] mutants restored partially apoA-I in the HDL3 fraction and LCAT[T147I] increased apoE in the VLD/IDL/LDL fractions. The in vivo functionality of LCAT was further assessed based on is its ability to correct the aberrant HDL phenotype that was caused by the apoA-I[L159R]FIN mutation. Co-infection of apoA-I(-/-) mice with this apoA-I mutant and either of the two mutant LCAT forms restored only partially the HDL biogenesis defect that was caused by the apoA-I[L159R]FIN and generated a distinct aberrant HDL phenotype.

Effect of open-label infusion of an apoA-I-containing particle (CER-001) on RCT and artery wall thickness in patients with FHA

Reverse cholesterol transport (RCT) contributes to the anti-atherogenic effects of HDL. Patients with the orphan disease, familial hypoalphalipoproteinemia (FHA), are characterized by decreased tissue cholesterol removal and an increased atherogenic burden. We performed an open-label uncontrolled proof-of-concept study to evaluate the effect of infusions with a human apoA-I-containing HDL-mimetic particle (CER-001) on RCT and the arterial vessel wall in FHA. Subjects received 20 infusions of CER-001 (8 mg/kg) during 6 months. Efficacy was assessed by measuring (apo)lipoproteins, plasma-mediated cellular cholesterol efflux, fecal sterol excretion (FSE), and carotid artery wall dimension by MRI and artery wall inflammation by (18)F-fluorodeoxyglucose-positron emission tomography/computed tomography scans. We included seven FHA patients: HDL-cholesterol (HDL-c), 13.8 [1.8-29.1] mg/dl; apoA-I, 28.7 [7.9-59.1] mg/dl. Following nine infusions in 1 month, apoA-I and HDL-c increased directly after infusion by 27.0 and 16.1 mg/dl (P = 0.018). CER-001 induced a 44% relative increase (P = 0.018) in in vitro cellular cholesterol efflux with a trend toward increased FSE (P = 0.068). After nine infusions of CER-001, carotid mean vessel wall area decreased compared with baseline from 25.0 to 22.8 mm(2) (P = 0.043) and target-to-background ratio from 2.04 to 1.81 (P = 0.046). In FHA-subjects, CER-001 stimulates cholesterol mobilization and reduces artery wall dimension and inflammation, supporting further evaluation of CER-001 in FHA patients.

A computational model for the analysis of lipoprotein distributions in the mouse: translating FPLC profiles to lipoprotein metabolism

Disturbances of lipoprotein metabolism are recognized as indicators of cardiometabolic disease risk. Lipoprotein size and composition, measured in a lipoprotein profile, are considered to be disease risk markers. However, the measured profile is a collective result of complex metabolic interactions, which complicates the identification of changes in metabolism. In this study we aim to develop a method which quantitatively relates murine lipoprotein size, composition and concentration to the molecular mechanisms underlying lipoprotein metabolism. We introduce a computational framework which incorporates a novel kinetic model of murine lipoprotein metabolism. The model is applied to compute a distribution of plasma lipoproteins, which is then related to experimental lipoprotein profiles through the generation of an in silico lipoprotein profile. The model was first applied to profiles obtained from wild-type C57Bl/6J mice. The results provided insight into the interplay of lipoprotein production, remodelling and catabolism. Moreover, the concentration and metabolism of unmeasured lipoprotein components could be determined. The model was validated through the prediction of lipoprotein profiles of several transgenic mouse models commonly used in cardiovascular research. Finally, the framework was employed for longitudinal analysis of the profiles of C57Bl/6J mice following a pharmaceutical intervention with a liver X receptor (LXR) agonist. The multifaceted regulatory response to the administration of the compound is incompletely understood. The results explain the characteristic changes of the observed lipoprotein profile in terms of the underlying metabolic perturbation and resultant modifications of lipid fluxes in the body. The Murine Lipoprotein Profiler (MuLiP) presented here is thus a valuable tool to assess the metabolic origin of altered murine lipoprotein profiles and can be applied in preclinical research performed in mice for analysis of lipid fluxes and lipoprotein composition.

Hepatic ACAT2 knock down increases ABCA1 and modifies HDL metabolism in mice

OBJECTIVES

ACAT2 is the exclusive cholesterol-esterifying enzyme in hepatocytes and enterocytes. Hepatic ABCA1 transfers unesterified cholesterol (UC) to apoAI, thus generating HDL. By changing the hepatic UC pool available for ABCA1, ACAT2 may affect HDL metabolism. The aim of this study was to reveal whether hepatic ACAT2 influences HDL metabolism.

DESIGN

WT and LXRα/β double knockout (DOKO) mice were fed a western-type diet for 8 weeks. Animals were i.p. injected with an antisense oligonucleotide targeted to hepatic ACAT2 (ASO6), or with an ASO control. Injections started 4 weeks after, or concomitantly with, the beginning of the diet.

RESULTS

ASO6 reduced liver cholesteryl esters, while not inducing UC accumulation. ASO6 increased hepatic ABCA1 protein independently of the diet conditions. ASO6 affected HDL lipids (increased UC) only in DOKO, while it increased apoE-containing HDL in both genotypes. In WT mice ASO6 led to the appearance of large HDL enriched in apoAI and apoE.

CONCLUSIONS

The use of ASO6 revealed a new pathway by which the liver may contribute to HDL metabolism in mice. ACAT2 seems to be a hepatic player affecting the cholesterol fluxes fated to VLDL or to HDL, the latter via up-regulation of ABCA1.

Mycophenolate mofetil prevents high-fat diet-induced hypertension and renal glomerular injury in Dahl SS rats

We designed experiments to test the hypothesis that Dahl salt-sensitive (SS) rats are sensitive to high-fat diet (HFD)–induced hypertension and renal injury via an inflammatory mechanism. Twelve-week-old Dahl SS rats were maintained on a normal diet (ND; 14% fat), HFD (59% fat), or HFD supplemented with the lymphocyte immunosuppressive agent, mycophenolate mofetil (HFD + MMF; 30 mg/kg/day orally in diet), for a period of 4 weeks. Mean arterial pressure (MAP), metabolic parameters, T lymphocyte (CD3⁺) localization, and renal structural damage were assessed during the studies. Four weeks of HFD significantly elevated MAP and visceral adiposity without changing circulating levels of lipids or adipokines. Immunohistochemical analysis demonstrated that SS rats on HFD had significantly greater numbers of CD3⁺ cells in renal glomerular and medullary areas compared to ND SS rats. Additionally, HFD led to increased glomerular injury, but did not alter renal medullary injury. Chronic MMF treatment in HFD-fed Dahl SS rats reduced MAP, visceral adiposity, infiltration of CD3⁺ cells in the glomerulus, as well as glomerular injury. However, MMF treatment did not alter HFD-induced infiltration of CD3⁺ cells in the renal medulla. In conclusion, Dahl SS rats are sensitized to HFD-induced hypertension and renal glomerular injury via infiltration of T lymphocytes.

Food restriction by intermittent fasting induces diabetes and obesity and aggravates spontaneous atherosclerosis development in hypercholesterolaemic mice

Different regimens of food restriction have been associated with protection against obesity, diabetes and CVD. In the present study, we hypothesised that food restriction would bring benefits to atherosclerosis- and diabetes-prone hypercholesterolaemic LDL-receptor knockout mice. For this purpose, 2-month-old mice were submitted to an intermittent fasting (IF) regimen (fasting every other day) over a 3-month period, which resulted in an overall 20 % reduction in food intake. Contrary to our expectation, epididymal and carcass fat depots and adipocyte size were significantly enlarged by 15, 72 and 68 %, respectively, in the IF mice compared with the ad libitum-fed mice. Accordingly, plasma levels of leptin were 50 % higher in the IF mice than in the ad libitum-fed mice. In addition, the IF mice showed increased plasma levels of total cholesterol (37 %), VLDL-cholesterol (195 %) and LDL-cholesterol (50 %). As expected, in wild-type mice, the IF regimen decreased plasma cholesterol levels and epididymal fat mass. Glucose homeostasis was also disturbed by the IF regimen in LDL-receptor knockout mice. Elevated levels of glycaemia (40 %), insulinaemia (50 %), glucose intolerance and insulin resistance were observed in the IF mice. Systemic inflammatory markers, TNF-α and C-reactive protein, were significantly increased and spontaneous atherosclerosis development were markedly increased (3-fold) in the IF mice. In conclusion, the IF regimen induced obesity and diabetes and worsened the development of spontaneous atherosclerosis in LDL-receptor knockout mice. Although being efficient in a wild-type background, this type of food restriction is not beneficial in the context of genetic hypercholesterolaemia.

Dietary restriction in rats and mice: a meta-analysis and review of the evidence for genotype-dependent effects on lifespan

Laboratory survival experiments have shown that dietary restriction (DR) can increase median and maximum lifespan. This paper provides a meta-analysis of laboratory experiments that have evaluated the effects of DR on lifespan in rats and mice (1934-present). In rats, DR increased median lifespan by 14-45% in half of all experiments, but in mice the effects of DR have been much weaker (4-27%). The least favorable effects of DR on lifespan have been observed among inbred rather than non-inbred mouse strains. In fact, some inbred mouse strains do not necessarily live longer with DR, including DBA/2 male mice and several strains from the ILSXISS recombinant inbred panel. Shortening of lifespan with DR has also been observed and confirmed for ILSXISS strain 114. Importantly, all rodent studies may be biased by the effects of laboratory breeding, since one study has shown that median lifespan is not improved by DR in wild-derived mice. These findings suggest that the set of genetic backgrounds studied in rodent DR experiments should be diversified. This will broaden the scope of genotypes studied in aging research, but may also be critical for translation of findings from rodents to historically outbred and genetically heterogeneous primate species.

The apolipoprotein C-I content of very-low-density lipoproteins is associated with fasting triglycerides, postprandial lipemia, and carotid atherosclerosis

Background. Experimental studies in animals suggest that apolipoprotein (apo) C-I is an important regulator of triglycerides in fasting and postprandial conditions and associated with carotid atherosclerosis. Methods. A cross-sectional study was conducted with 81 subjects, aged 56–80 years recruited from a population health survey. The participants underwent a fat tolerance test (1 g fat per Kg body weight) and carotid atherosclerosis was determined by ultrasound examination. VLDL particles, Sf 20–400, were isolated and their lipid composition and apoC-I content determined. Results. The carotid plaque area increased linearly with the number of apoC-I molecules per VLDL particles (P = 0.048) under fasting conditions. Fasting triglycerides increased across tertiles of apoC-I per VLDL particle in analyses adjusted for apoC-II and -C-III, apoE genotype and traditional cardiovascular risk factors (P = 0.011). The relation between apoC-I in VLDL and serum triglycerides was conveyed by triglyceride enrichment of VLDL particles (P for trend <0.001. The amount of apoC-I molecules per VLDL was correlated with the total (r = 0.41, P < 0.0001) and incremental (r = 0.35, P < 0.001) area under the postprandial triglyceride curve. Conclusions. Our findings support the concept that the content of apoC-I per VLDL particle is an important regulator of triglyceride metabolism in the fasting and postprandial state and associated with carotid athrosclerosis.

CETP expression reverses the reconstituted HDL-induced increase in VLDL

Human data suggest that reconstituted HDL (rHDL) infusion can induce atherosclerosis regression. Studies in mice indicated that rHDL infusion adversely affects VLDL levels, but this effect is less apparent in humans. This discrepancy may be explained by the fact that humans, in contrast to mice, express cholesteryl ester transfer protein (CETP). The aim of this study was to investigate the role of CETP in the effects of rHDL on VLDL metabolism by using APOE*3-Leiden (E3L) mice, a well-established model for human-like lipoprotein metabolism. At 1 h after injection, rHDL increased plasma VLDL-C and TG in E3L mice, but not in E3L mice cross-bred onto a human CETP background (E3L.CETP mice). This initial raise in VLDL, caused by competition between rHDL and VLDL for LPL-mediated TG hydrolysis, was thus prevented by CETP. At 24 h after injection, rHDL caused a second increase in VLDL-C and TG in E3L mice, whereas rHDL had even decreased VLDL in E3L.CETP mice. This secondary raise in VLDL was due to increased hepatic VLDL-TG production. Collectively, we conclude that CETP protects against the rHDL-induced increase in VLDL. We anticipate that studies evaluating the anti-atherosclerotic efficacy of rHDL in mice that are naturally deficient for CETP should be interpreted with caution, and that treatment of atherogenic dyslipidemia by rHDL should not be combined with agents that aggressively reduce CETP activity.

Apolipoprotein B knockdown by AAV-delivered shRNA lowers plasma cholesterol in mice

Serum low-density lipoprotein cholesterol (LDL-C) levels are proportionate to the risk of atherosclerotic cardiovascular disease. In order to reduce serum total cholesterol and LDL-C levels in mice, RNA interference (RNAi) was used to inhibit expression of the structural protein of LDL-C, apolipoprotein B100 (ApoB). We developed and screened 19 short hairpin RNAs (shRNAs) targeting conserved sequences in human, mouse, and macaque ApoB mRNAs (shApoB) and subsequently narrowed our focus to one candidate for in vivo testing. Self-complementary adeno-associated virus serotype 8 (scAAV8) was used for long-term transduction of murine liver with shApoB. A strong dose-dependent knockdown of ApoB mRNA and protein was observed, which correlated with a reduction in total cholesterol levels, without obvious signs of toxicity. Furthermore, shApoB was found to specifically reduce LDL-C in diet-induced dyslipidemic mice, whereas high-density lipoprotein cholesterol (HDL-C) remained unaffected. Finally, elevated lipid accumulation was shown in murine liver transduced with shApoB, a known phenotypic side effect of lowering ApoB levels. These results demonstrate a robust dose-dependent knockdown of ApoB by AAV-delivered shRNA in murine liver, thus providing an excellent candidate for development of RNAi-based gene therapy for the treatment of hypercholesterolemia.

Ciprofibrate increases cholesteryl ester transfer protein gene expression and the indirect reverse cholesterol transport to the liver

Background

CETP is a plasma protein that modulates atherosclerosis risk through its HDL-cholesterol reducing action. The aim of this work was to examine the effect of the PPARα agonist, ciprofibrate, on the CETP gene expression, in the presence and absence of apolipoprotein (apo) CIII induced hypertriglyceridemia, and its impact on the HDL metabolism.

Results

Mice expressing apo CIII and/or CETP and non-transgenic littermates (CIII, CIII/CETP, CETP, non-Tg) were treated with ciprofibrate during 3 weeks. Drug treatment reduced plasma triglycerides (30-43%) and non-esterified fatty acids (19-47%) levels. Cholesterol (chol) distribution in plasma lipoprotein responses to ciprofibrate treatment was dependent on the genotypes. Treated CIII expressing mice presented elevation in VLDL-chol and reduction in HDL-chol. Treated CETP expressing mice responded with reduction in LDL-chol whereas in non-Tg mice the LDL-chol increased. In addition, ciprofibrate increased plasma post heparin lipoprotein lipase activity (1.3-2.1 fold) in all groups but hepatic lipase activity decreased in treated CETP and non-Tg mice. Plasma CETP activity and liver CETP mRNA levels were significantly increased in treated CIII/CETP and CETP mice (30-100%). Kinetic studies with ³H-cholesteryl ether (CEt) labelled HDL showed a 50% reduction in the ³H-CEt found in the LDL fraction in ciprofibrate treated compared to non-treated CETP mice. This means that ³H-CEt transferred from HDL to LDL was more efficiently removed from the plasma in the fibrate treated mice. Accordingly, the amount of ³H-CEt recovered in the liver 6 hours after HDL injection was increased by 35%.

Conclusion

Together these data showed that the PPARα agonist ciprofibrate stimulates CETP gene expression and changes the cholesterol flow through the reverse cholesterol transport, increasing plasma cholesterol removal through LDL.

An apoA-I mimetic peptide containing a proline residue has greater in vivo HDL binding and anti-inflammatory ability than the 4F peptide

Modifying apolipoprotein (apo) A-I mimetic peptides to include a proline-punctuated alpha-helical repeat increases their anti-inflammatory properties as well as allows better mimicry of full-length apoA-I function. This study compares the following mimetics, either acetylated or biotinylated (b): 4F (18mer) and 4F-proline-4F (37mer, Pro). b4F interacts with both mouse HDL (moHDL) and LDL in vitro. b4F in vivo plasma clearance kinetics are not affected by mouse HDL level. Administration of biotinylated peptides to mice demonstrates that b4F does not associate with lipoproteins smaller than LDL in vivo, though it does associate with fractions containing free hemoglobin (Hb). In contrast, bPro specifically interacts with HDL. b4F and bPro show opposite binding responses to HDL by surface plasmon resonance. Administration of acetylated Pro to apoE(-/-) mice significantly decreases plasma serum amyloid A levels, while acetylated 4F does not have this ability. In contrast to previous reports that inferred that 4F associates with HDL in vivo, we systematically examined this potential interaction and demonstrated that b4F does not interact with HDL in vivo but rather elutes with Hb-containing plasma fractions. bPro, however, specifically binds to moHDL in vivo. In addition, the number of amphipathic alpha-helices and their linker influences the anti-inflammatory effects of apoA-I mimetic peptides in vivo.

Human cholesteryl ester transfer protein expression enhances the mouse survival rate in an experimental systemic inflammation model: a novel role for CETP

Mice expressing human cholesteryl ester transfer protein (huCETP) are more resistant to Escherichia coli bacterial wall LPS because death rates 5 days after intraperitoneal inoculation of LPS were higher in wild-type than in huCETP+/+ mice, whereas all huCETP+/+ mice remained alive. After LPS inoculation, plasma concentrations of TNF-alpha and IL-6 increased less in huCETP+/+ than in wild-type mice. LPS in vitro elicited lower TNF-alpha production by CETP expressing than by wild-type macrophages. In addition, TNF-alpha production by RAW 264.7 murine macrophages increased on incubation with LPS but decreased in a dose-dependent manner when human CETP was added to the medium. Human CETP in vitro enhanced the LPS binding to plasma high-density lipoprotein/low-density lipoprotein. The liver uptake of intravenous infused 14C-LPS from Salmonella typhimurium was greater in huCETP+/+ than in wild-type mice. Present data indicate for the first time that CETP is an endogenous component involved in the first line of defense against an exacerbated production of proinflammatory mediators.

Overactive endocannabinoid signaling impairs apolipoprotein E-mediated clearance of triglyceride-rich lipoproteins

The endocannabinoid (EC) system regulates food intake and energy metabolism. Cannabinoid receptor type 1 (CB1) antagonists show promise in the treatment of obesity and its metabolic consequences. Although the reduction in adiposity resulting from therapy with CB1 antagonists may not account fully for the concomitant improvements in dyslipidemia, direct effects of overactive EC signaling on plasma lipoprotein metabolism have not been documented. The present study used a chemical approach to evaluate the direct effects of increased EC signaling in mice by inducing acute elevations of endogenously produced cannabinoids through pharmacological inhibition of their enzymatic hydrolysis by isopropyl dodecylfluorophosphonate (IDFP). Acute IDFP treatment increased plasma levels of triglyceride (TG) (2.0- to 3.1-fold) and cholesterol (1.3- to 1.4-fold) in conjunction with an accumulation in plasma of apolipoprotein (apo)E-depleted TG-rich lipoproteins. These changes did not occur in either CB1-null or apoE-null mice, were prevented by pretreatment with CB1 antagonists, and were not associated with reduced hepatic apoE gene expression. Although IDFP treatment increased hepatic mRNA levels of lipogenic genes (Srebp1 and Fas), there was no effect on TG secretion into plasma. Instead, IDFP treatment impaired clearance of an intravenously administered TG emulsion, despite increased postheparin lipoprotein lipase activity. Therefore, overactive EC signaling elicits an increase in plasma triglyceride levels associated with reduced plasma TG clearance and an accumulation in plasma of apoE-depleted TG-rich lipoproteins. These findings suggest a role of CB1 activation in the pathogenesis of obesity-related hypertriglyceridemia and underscore the potential efficacy of CB1 antagonists in treating metabolic disease.

Opposite lipemic response of Wistar rats and C57BL/6 mice to dietary glucose or fructose supplementation

The metabolic effects of carbohydrate supplementation in mice have not been extensively studied. In rats, glucose- and fructose-rich diets induce hypertriacylglycerolemia. In the present study, we compared the metabolic responses to two monosaccharide supplementations in two murine models. Adult male Wistar rats (N = 80) and C57BL/6 mice (N = 60), after 3 weeks on a standardized diet, were submitted to dietary supplementation by gavage with glucose (G) or fructose (F) solutions (500 g/L), 8 g/kg body weight for 21 days. Glycemia was significantly higher in rats after fructose treatment (F: 7.9 vs 9.3 mM) and in mice (G: 6.5 vs 10 and F: 6.6 vs 8.9 mM) after both carbohydrate treatments. Triacylglycerolemia increased significantly 1.5 times in rats after G or F supplementation. Total cholesterol did not change with G treatment in rats, but did decrease after F supplementation (1.5 vs 1.4 mM, P < 0.05). Both supplementations in rats induced insulin resistance, as suggested by the higher Homeostasis Model Assessment Index. In contrast, mice showed significant decreases in triacylglycerol (G: 1.8 vs 1.4 and F: 1.9 vs 1.4 mM, P < 0.01) and total cholesterol levels (G and F: 2.7 vs 2.5 mM, P < 0.05) after both monosaccharide supplementations. Wistar rats and C57BL/6 mice, although belonging to the same family (Muridae), presented opposite responses to glucose and fructose supplementation regarding serum triacylglycerol, free fatty acids, and insulin levels after monosaccharide treatment. Thus, while Wistar rats developed features of plurimetabolic syndrome, C57BL/6 mice presented changes in serum biochemical profile considered to be healthier for the cardiovascular system.

Fenofibrate increases HDL-cholesterol by reducing cholesteryl ester transfer protein expression

In addition to efficiently decreasing VLDL-triglycerides (TGs), fenofibrate increases HDL-cholesterol levels in humans. We investigated whether the fenofibrate-induced increase in HDL-cholesterol is dependent on the expression of the cholesteryl ester transfer protein (CETP). To this end, APOE*3-Leiden (E3L) transgenic mice without and with the human CETP transgene, under the control of its natural regulatory flanking regions, were fed a Western-type diet with or without fenofibrate. Fenofibrate (0.04% in the diet) decreased plasma TG in E3L and E3L.CETP mice (-59% and -60%; P < 0.001), caused by a strong reduction in VLDL. Whereas fenofibrate did not affect HDL-cholesterol in E3L mice, fenofibrate dose-dependently increased HDL-cholesterol in E3L.CETP mice (up to +91%). Fenofibrate did not affect the turnover of HDL-cholesteryl ester (CE), indicating that fenofibrate causes a higher steady-state HDL-cholesterol level without altering the HDL-cholesterol flux through plasma. Analysis of the hepatic gene expression profile showed that fenofibrate did not differentially affect the main players in HDL metabolism in E3L.CETP mice compared with E3L mice. However, in E3L.CETP mice, fenofibrate reduced hepatic CETP mRNA (-72%; P < 0.01) as well as the CE transfer activity in plasma (-73%; P < 0.01). We conclude that fenofibrate increases HDL-cholesterol by reducing the CETP-dependent transfer of cholesterol from HDL to (V)LDL, as related to lower hepatic CETP expression and a reduced plasma (V)LDL pool.

Cholesteryl Ester Transfer Protein Decreases High-Density Lipoprotein and Severely Aggravates Atherosclerosis in APOE*3-Leiden Mice

Objective— The role of cholesteryl ester transfer protein (CETP) in the development of atherosclerosis is still undergoing debate. Therefore, we evaluated the effect of human CETP expression on atherosclerosis in APOE*3-Leiden ( E3L ) mice with a humanized lipoprotein profile.

Methods and Results— E3L mice were crossbred with human CETP transgenic mice. On a chow diet, CETP expression increased plasma total cholesterol (TC) (+43%; P <0.05). To evaluate the effects of CETP on the development of atherosclerosis, mice were fed a Western-type diet containing 0.25% cholesterol, leading to 4.3-fold elevated TC levels in both E3L and CETP.E3L mice ( P <0.01). On both diets, CETP expression shifted the distribution of cholesterol from high-density lipoprotein (HDL) toward very-low-density lipoprotein (VLDL)/low-density lipoprotein (LDL). Moreover, plasma of CETP.E3L mice had reduced capacity (−39%; P <0.05) to induce SR-BI–mediated cholesterol efflux from Fu5AH cells than plasma of E3L mice. After 19 weeks on the Western-type diet, CETP.E3L mice showed a 7.0-fold increased atherosclerotic lesion area in the aortic root compared with E3L mice ( P <0.0001).

Conclusions— CETP expression in E3L mice shifts the distribution of cholesterol from HDL to VLDL/LDL, reduces plasma-mediated SR-BI–dependent cholesterol efflux, and represents a clear pro-atherogenic factor in E3L mice. We anticipate that the CETP.E3L mouse will be a valuable model for the preclinical evaluation of HDL-raising interventions on atherosclerosis development.

Atherosclerosis is enhanced by testosterone deficiency and attenuated by CETP expression in transgenic mice

In this work, we investigated the impact of testosterone deficiency and cholesteryl ester transfer protein (CETP) expression on lipoprotein metabolism and diet-induced atherosclerosis. CETP transgenic mice and nontransgenic (nTg) littermates were studied 4 weeks after bilateral orchidectomy or sham operation. Castrated mice had an increase in the LDL fraction (+36% for CETP and +79% for nTg mice), whereas the HDL fraction was reduced (-30% for CETP and -11% for nTg mice). Castrated mice presented 1.7-fold higher titers of anti-oxidized LDL (Ox-LDL) antibodies than sham-operated controls. Plasma levels of CETP, lipoprotein lipase, and hepatic lipase were not changed by castration. Kinetic studies showed no differences in VLDL secretion rate, VLDL-LDL conversion rate, or number of LDL and HDL receptors. Competition experiments showed lower affinity of LDL from castrated mice for tissue receptors. Diet-induced atherosclerosis studies showed that testosterone deficiency increased by 100%, and CETP expression reduced by 44%, the size of aortic lesion area in castrated mice. In summary, testosterone deficiency increased plasma levels of apolipoprotein B-containing lipoproteins (apoB-LPs) and anti-OxLDL antibodies, decreased LDL receptor affinity, and doubled the size of diet-induced atherosclerotic lesions. The expression of CETP led to a milder increase of apoB-LPs and reduced atherosclerotic lesion size in testosterone-deficient mice.

Antiatherogenic effects of S-nitroso-N-acetylcysteine in hypercholesterolemic LDL receptor knockout mice

BACKGROUND

The pathophysiology of the NO/NO synthase system and dysfunctional changes in the endothelium in the early phases of the atherogenic process are incompletely understood. In this study, we investigated the effects of the nitrosothiol NO donor S-nitroso-N-acetylcysteine (SNAC) in the early prevention of plaque development in the hypercholesterolemic LDLr-/- mice as well as the changes in endothelium-dependent relaxation and NO synthase expression.

METHODS AND RESULTS

LDLr-/- mice were fed a 1.25% cholesterol-enriched diet for 15 days. Plasma cholesterol/triglyceride levels increased and this increase was accompanied by the development of aortic root lesions. Aortic vasorelaxation to acetylcholine was increased, although endothelium-independent relaxation in response to sodium nitroprusside did not change, which suggest stimulated NO release enhanced. This dysfunction was associated with enhanced aortic superoxide production and with increased levels of constitutive NOS isoform expression, particularly neuronal NOS. SNAC (S-nitroso-N-acetylcysteine) administration (0.51 micromol/kg/day i.p. for 15 days) decreased the extent of the plaque by 55% in hypercholesterolemic mice, but had no effects on vasomotor changes. It did, however, lead to a decrease in constitutive NOS expression. The SNAC induced only minor changes in plasma lipid profile.

CONCLUSION

The present study has shown that, in early stages of plaque development in LDLr-/- mice, specific changes in NO/NO synthase system develop, that are characterized by increased endothelium-dependent vasorelaxation and increased constitutive NOS expression. Since the development of plaque and the indicator of endothelial cell dysfunction were prevented by SNAC, such treatment may constitute a novel strategy for the halting of progression of early plaque.

Severe hypertriglyceridemia in human APOC1 transgenic mice is caused by apoC-I-induced inhibition of LPL

Studies in humans and mice have shown that increased expression of apolipoprotein C-I (apoC-I) results in combined hyperlipidemia with a more pronounced effect on triglycerides (TGs) compared with total cholesterol (TC). The aim of this study was to elucidate the main reason for this effect using human apoC-I-expressing (APOC1) mice. Moderate plasma human apoC-I levels (i.e., 4-fold higher than human levels) caused a 12-fold increase in TG, along with a 2-fold increase in TC, mainly confined to VLDL. Cross-breeding of APOC1 mice on an apoE-deficient background resulted in a marked 55-fold increase in TG, confirming that the apoC-I-induced hyperlipidemia cannot merely be attributed to blockade of apoE-recognizing hepatic lipoprotein receptors. The plasma half-life of [3H]TG-VLDL-mimicking particles was 2-fold increased in APOC1 mice, suggesting that apoC-I reduces the lipolytic conversion of VLDL. Although total postheparin plasma LPL activity was not lower in APOC1 mice compared with controls, apoC-I was able to dose-dependently inhibit the LPL-mediated lipolysis of [3H]TG-VLDL-mimicking particles in vitro with a 60% efficiency compared with the main endogenous LPL inhibitor apoC-III. Finally, purified apoC-I impaired the clearance of [3H]TG-VLDL-mimicking particles independent of apoE-mediated hepatic uptake in lactoferrin-treated mice. Therefore, we conclude that apoC-I is a potent inhibitor of LPL-mediated TG-lipolysis.

Effects of diabetes and CETP expression on diet-induced atherosclerosis in LDL receptor-deficient mice

The role of CETP expression and diabetes in atherogenesis was investigated in mice with heterozygous disruption of the LDL receptor gene (LDLR1). LDLR1 mice with and without CETP expression were treated with streptozotocin (STZ) and maintained on a standard diet for one month before switching to an atherogenic diet for an additional month. STZ-sensitive mice had approximately 2.5-fold higher glycemia and 7.5- to 8.0-fold higher cholesterolemia. Factorial analysis of variance showed no significant effect of diabetes, CETP or diabetes-CETP interaction on the size of the atherosclerotic lesions. CETP expression in non-diabetic mice resulted in a 50% reduction in the area of the atherosclerotic lesions. Multiple regression analysis showed a positive and independent atherogenic effect of triglyceridemia in LDLR1 mice and of cholesterolemia in diabetic mice. Logistic analysis showed that elevated plasma cholesterol level significantly increased the risk of developing large lesion size (>75th percentile). In conclusion, CETP expression did not alter the lesion formation in response to diabetes, although it may be protective in the euglycemic state; the triglyceride level was an independent risk factor for LDL receptor-deficient mice but not for CETP-expressing mice; and elevated plasma cholesterol levels increased the risk of developing large atherosclerotic lesions, independently of CETP and diabetes.

Cholesteryl ester transfer protein expression is down-regulated in hyperinsulinemic transgenic mice

Cholesteryl ester transfer protein (CETP) mediates cholesteryl ester (CE) and triglyceride redistribution among plasma lipoproteins. In this work, we investigated whether varying levels of insulin regulate the CETP expression in vivo. Insulin deficiency [streptozotocin (STZ) injection], and hyperinsulinemia (insulin injections, 14 days) were induced in transgenic mice expressing a human CETP minigene flanked by its natural regulatory sequences. Glucose supplementation was provided to the hyperinsulinemic group (INS+GLUC) and to an extra group of mice (GLUC). In the STZ group, endogenous CE transfer rate, plasma CETP, and hepatic CETP mRNA levels were enhanced 3.0-, 1.5-, and 2.5-fold, respectively, as compared with controls. Insulin replacement in STZ mice normalized their glycemia and liver mRNA levels. Higher plasma CETP levels were observed in GLUC mice, which were decreased in INS+GLUC mice. Hepatic CETP mRNA was not altered in GLUC mice and was reduced by one-third in INS+GLUC mice. These results show that: 1) STZ treatment increases CETP plasma levels and liver mRNA expression; 2) diet glucose supplementation increases plasma CETP levels but does not change liver mRNA abundance; and 3) daily insulin injections blunt the glucose-stimulated CETP expression by reducing its liver mRNA levels. These data suggest that insulin down-regulates CETP gene expression.

Lipoprotein subclass profiles of hyperlipidemic diabetic mice measured by nuclear magnetic resonance spectroscopy

Dyslipidemia accelerates vascular complications of diabetes. Nuclear magnetic resonance (NMR) analysis of lipoprotein subclasses is used to evaluate a mouse model of human familial hypercholesterolemia +/- streptozotocin (STZ)-induced diabetes. A double knockout (DKO) mouse (low-density lipoprotein receptor [LDLr] -/-; apolipoprotein B [apoB] mRNA editing catalytic polypeptide-1 [Apobec1] -/-) was studied. Wild-type (WT) and DKO mice received sham or STZ injections at age 7 weeks, yielding control (WT-C, DKO-C) and diabetic (WT-D, DKO-D) groups. Fasting serum was collected when the mice were killed (age 40 weeks) for Cholestech analysis (Cholestech Corp, Hayward, CA) and NMR lipoprotein subclass profile. By Cholestech, fasting triglyceride and total cholesterol increased in DKO-C versus WT-C. Diabetes further increased total cholesterol in DKO. High-density lipoprotein cholesterol (HDL-C) was similar among all groups. NMR revealed that LDL in all groups was present in a subclass the size of large human LDL and was increased 48-fold in DKO-C versus WT-C animals, but was unaffected by diabetes. HDL was found in a subclass equivalent to large human HDL, and was similar among groups. In conclusion, NMR analysis reveals lipoprotein subclass distributions and the effects of genetic modification and diabetes in mice, but lack of particles the size of human small LDL and small HDL may limit the relevance of the present animal model to human disease.

Endothelial lipase is a major determinant of HDL level

A new member of the lipase gene family, initially termed endothelial lipase (gene nomenclature, LIPG; protein, EL), is expressed in a variety of different tissues, suggesting a general role in lipid metabolism. To assess the hypothesis that EL plays a physiological role in lipoprotein metabolism in vivo, we have used gene targeting of the native murine locus and transgenic introduction of the human LIPG locus in mice to modulate the level of EL expression. Evaluation of these alleles in a C57Bl/6 background revealed an inverse relationship between HDL cholesterol level and EL expression. Fasting plasma HDL cholesterol was increased by 57% in LIPG(-/-) mice and 25% in LIPG(+/-) mice and was decreased by 19% in LIPG transgenic mice as compared with syngeneic controls. Detailed analysis of lipoprotein particle composition indicated that this increase was due primarily to an increased number of HDL particles. Phospholipase assays indicated that EL is a primary contributor to phospholipase activity in mouse. These data indicate that expression levels of this novel lipase have a significant effect on lipoprotein metabolism.

Critical review of acylation-stimulating protein physiology in humans and rodents

In the last few years, there has been increasing interest in the physiological role of acylation-stimulating protein (ASP). Recent studies in rats and mice, in particular in C3 (-/-) mice that are ASP deficient, have advanced our understanding of the role of ASP. Of note, the background strain of the mice influences the phenotype of delayed postprandial triglyceride clearance in ASP-deficient mice. Administration of ASP in all types of lean and obese mice studied to date, however, enhances postprandial triglyceride clearance. On the other hand, regardless of the background strain, ASP-deficient mice demonstrate reduced body weight, reduced leptin and reduced adipose tissue mass, suggesting that ASP deficiency results in protection against development of obesity. In humans, a number of studies have examined the relationship between ASP, obesity, diabetes and dyslipidemia as well as the influence of diet, exercise and pharmacological therapy. While many of these studies have small subject numbers, interesting observations may help us to better understand the parameters that may influence ASP production and ASP action. The aim of the present review is to provide a comprehensive overview of the recent literature on ASP, with particular emphasis on those studies carried out in rodents and humans.

Cholesteryl ester transfer protein expression attenuates atherosclerosis in ovariectomized mice

Reduced estrogen levels result in loss of protection from coronary heart disease in postmenopausal women. Enhanced and diminished atherosclerosis have been associated with plasma levels of cholesteryl ester transfer protein (CETP); however, little is known about the role of CETP-ovarian hormone interactions in atherogenesis. We assessed the severity of diet-induced atherosclerosis in ovariectomized (OV) CETP transgenic mice crossbred with LDL receptor knockout mice. Compared with OV CETP expressing ((+)), OV CETP non-expressing ((-)) mice had higher plasma levels of total, VLDL-, LDL-, and HDL-cholesterol, as well as higher antibodies titers against oxidized LDL. The mean aortic lesion area was 2-fold larger in OV CETP(-) than in OV CETP(+) mice (147 +/- 90 vs. 73 +/- 42 x 10(3) micro m(2), respectively). Estrogen therapy in OV mice blunted the CETP dependent differences in plasma lipoproteins, oxLDL antibodies, and atherosclerosis severity. Macrophages from OV CETP(+) mice took up less labeled cholesteryl ether (CEt) from acetyl-LDL than macrophages from OV CETP(-) mice. Estrogen replacement induced a further reduction in CEt uptake and an elevation in HDL mediated cholesterol efflux from pre-loaded OV CETP(+) as compared with OV CETP(-) macrophages. These findings support the proposed anti-atherogenic role of CETP in specific metabolic settings.

Expression of simian CETP in normolipidemic Fisher rats has a profound effect on large sized apoE-containing HDL

In order to investigate the direct effect of cholesteryl ester transfer protein (CETP) on the structure and composition of HDL in vivo, simian CETP was expressed in Fisher rat that spontaneously displays high plasma levels of HDL1. In the new CETPTg rat line, the production of active CETP by the liver induced a significant 48% decrease in plasma HDL cholesterol, resulting in a 34% decrease in total cholesterol level (P < 0.01 in both cases). Among the various plasma HDL subpopulations, the largest HDL were those mostly affected by CETP, with a 74% decrease in HDL1 versus a significantly weaker 38% decrease in smaller HDL2 (P < 0.0001). Apolipoprotein E (apoE)-containing HDL1 were selectively affected by CETP expression, whereas apoA content of HDL remained unmodified. The reduction in the apoE content of serum HDL observed in CETPTg rats compared to controls (53%, P < 0.02) suggests that apoE in HDL may constitute in vivo a major determinant of their ability to interact with CETP. These results bring new insight into the lack of HDL1 in plasma from CETP-deficient heterozygotes despite their substantial 50% decrease in CETP activity. In addition, they indicate that HDL1 constitute reliable and practicable sensors of very low plasma CETP activity in vivo.

Improved insulin sensitivity is associated with restricted intake of dietary glycoxidation products in the db/db mouse

Advanced glycation end products (AGEs), known promoters of diabetic complications, form abundantly in heated foods and are ingested in bioreactive forms. To test whether dietary AGEs play a role in the progression of insulin resistance, C57/BL/KsJ db/db mice were randomly placed for 20 weeks on a diet with either a low AGE content (LAD) or a 3.4-fold higher content of AGE (high AGE diet [HAD]), including (epsilon)N-carboxymethyllysine (CML) and methylglyoxal (MG). LAD-fed mice showed lower fasting plasma insulin levels throughout the study (P = 0.01). Body weight was reduced by approximately 13% compared with HAD-fed mice (P = 0.04) despite equal food intake. LAD-fed mice exhibited significantly improved responses to both glucose (at 40 min, P = 0.003) and insulin (at 60 min, P = 0.007) tolerance tests, which correlated with a twofold higher glucose uptake by adipose tissue (P = 0.02). Compared with the severe hypertrophy and morphological disorganization of islets from HAD-fed mice, LAD-fed mice presented a better-preserved structure of the islets. LAD-fed mice demonstrated significantly increased plasma HDL concentrations (P < 0.0001). Consistent with these observations, LAD-fed mice exhibited twofold lower serum CML and MG concentrations compared with HAD-fed mice (P = 0.02). These results demonstrate that reduced AGE intake leads to lower levels of circulating AGE and to improved insulin sensitivity in db/db mice.

Disruption of the sterol carrier protein 2 gene in mice impairs biliary lipid and hepatic cholesterol metabolism

Hepatic up-regulation of sterol carrier protein 2 (Scp2) in mice promotes hypersecretion of cholesterol into bile and gallstone formation in response to a lithogenic diet. We hypothesized that Scp2 deficiency may alter biliary lipid secretion and hepatic cholesterol metabolism. Male gallstone-susceptible C57BL/6 and C57BL/6(Scp2(-/-)) knockout mice were fed a standard chow or lithogenic diet. Hepatic biles were collected to determine biliary lipid secretion rates, bile flow, and bile salt pool size. Plasma lipoprotein distribution was investigated, and gene expression of cytosolic lipid-binding proteins, lipoprotein receptors, hepatic regulatory enzymes, and intestinal cholesterol absorption was measured. Compared with chow-fed wild-type animals, C57BL/6(Scp2(-/-)) mice had higher bile flow and lower bile salt secretion rates, decreased hepatic apolipoprotein expression, increased hepatic cholesterol synthesis, and up-regulation of liver fatty acid-binding protein. In addition, the bile salt pool size was reduced and intestinal cholesterol absorption was unaltered in C57BL/6(Scp2(-/-)) mice. When C57BL/6(Scp2(-/-)) mice were challenged with a lithogenic diet, a smaller increase of hepatic free cholesterol failed to suppress cholesterol synthesis and biliary cholesterol secretion increased to a much smaller extent than phospholipid and bile salt secretion. Scp2 deficiency did not prevent gallstone formation and may be compensated in part by hepatic up-regulation of liver fatty acid-binding protein. These results support a role of Scp2 in hepatic cholesterol metabolism, biliary lipid secretion, and intracellular cholesterol distribution.

Estrogen increases hepatic lipase levels in inbred strains of mice: a possible mechanism for estrogen-dependent lowering of high density lipoprotein

We have shown mouse to be an useful animal model for studies on the estrogen-mediated synthesis and secretion of lipoproteins since, unlike in rats, low density lipoprotein receptors are not upregulated in mice. This results into the elevation of plasma levels of apolipoprotein (apo) B and apoE, and lowering of apoA-1-containing particles. The mechanisms of apoB and apoE elevation by estrogen have been elucidated, but the mechanism of lowering of plasma levels of HDL is still not known. Among other factors, apoA-I, cholesterol ester transfer protein (CETP), scavenger receptor B1 (SR-B1), and hepatic lipase are potential candidates that modulate plasma levels of HDL. Since estrogen treatment increased hepatic apoA-I mRNA and apoA-I synthesis, and mouse express undetectable levels of CETP, we tested the hypothesis that estradiol-mediated lowering of HDL in mice may occur through modulation of hepatic lipase (HL). Four mouse strains (C57L, C57BL, BALB, C3H) were administered supraphysiological doses of estradiol, and plasma levels of HDL as well as HL mRNA were quantitated. In all 4 strains estradiol decreased plasma levels of HDL by 30%, and increased HL mRNA 2-3 fold. In a separate experiment groups of male C57BL mouse were castrated or sham-operated, and low and high doses of estradiol administered. We found 1.4-2.5 fold elevation of HL mRNA with concomitant lowering of HDL levels. Ten other mouse strains examined also showed estradiol-induced elevation of HL mRNA, but the extent of elevation was found to be strain-specific. Based on these studies, we conclude that hepatic lipase is an important determinant of plasma levels of HDL and that HL mRNA is modulated by estrogen which in turn may participate in the lowering of plasma levels of HDL.

7-Ketocholesterol delivered to mice in chylomicron remnant-like particles is rapidly metabolised, excreted and does not accumulate in aorta

Cholesterol oxidation products (oxysterols) have been implicated in atherogenesis due to their presence in atherosclerotic tissue and their potent effects in vitro. One of the major oxysterols currently of interest is 7-ketocholesterol (7K) and it has been suggested that the diet is an important source of this oxysterol. This investigation tested the hypothesis that 7K, delivered in a physiologically relevant vehicle, chylomicron remnant-like emulsion (CMR), would be metabolised and excreted by mice in a similar manner and to a similar extent as previously observed in rats when delivered in a chemically modified lipoprotein, acetylated low-density lipoprotein (acLDL). Indeed, the metabolism of 14C-7K delivered in CMR mirrored that of acLDL and was much more rapid than (3)H-cholesterol delivered simultaneously. The 7K-derived (14)C was cleared from the liver, appeared in the intestine and was excreted in the faeces. A substantial proportion of the 7K-derived (14)C in the intestine and faeces was aqueous-soluble, indicating metabolism to polar products, presumably bile acids. Moreover, while cholesterol-derived (3)H increased in the aorta, (14)C appeared transiently and there was no observable accumulation within 24 h. The data confirm our previous findings of rapid hepatic metabolism of 7K when delivered in acLDL and demonstrate that 7K delivered in a vehicle of dietary significance is similarly metabolised and excreted. Indeed, the data encourage further investigation into the contribution that dietary oxysterols may or may not make to atherogenesis.

Plasma lipases and lipid transfer proteins increase phospholipid but not free cholesterol transfer from lipid emulsion to high density lipoproteins

BACKGROUND

Plasma lipases and lipid transfer proteins are involved in the generation and speciation of high density lipoproteins. In this study we have examined the influence of plasma lipases and lipid transfer protein activities on the transfer of free cholesterol (FC) and phospholipids (PL) from lipid emulsion to human, rat and mouse lipoproteins. The effect of the lipases was verified by incubation of labeled (3H-FC,14C-PL) triglyceride rich emulsion with human plasma (control, post-heparin and post-heparin plus lipase inhibitor), rat plasma (control and post-heparin) and by the injection of the labeled lipid emulsion into control and heparinized functionally hepatectomized rats.

RESULTS

In vitro, the lipase enriched plasma stimulated significantly the transfer of 14C-PL from emulsion to high density lipoprotein (p<0.001) but did not modify the transfer of 3H-FC. In hepatectomized rats, heparin stimulation of intravascular lipolysis increased the plasma removal of 14C-PL and the amount of 14C-PL found in the low density lipoprotein density fraction but not in the high density lipoprotein density fraction. The in vitro and in vivo experiments showed that free cholesterol and phospholipids were transferred from lipid emulsion to plasma lipoproteins independently from each other. The incubation of human plasma, control and control plus monoclonal antibody anti-cholesteryl ester transfer protein (CETP), with 14C-PL emulsion showed that CETP increases 14C-PL transfer to human HDL, since its partial inhibition by the anti-CETP antibody reduced significantly the 14C-PL transfer (p<0.05). However, comparing the nontransgenic (no CETP activity) with the CETP transgenic mouse plasma, no effect of CETP on the 14C-PL distribution in mice lipoproteins was observed.

CONCLUSIONS

It is concluded that: 1-intravascular lipases stimulate phospholipid transfer protein mediated phospholipid transfer, but not free cholesterol, from triglyceride rich particles to human high density lipoproteins and rat low density lipoproteins and high density lipoproteins; 2-free cholesterol and phospholipids are transferred from triglyceride rich particles to plasma lipoproteins by distinct mechanisms, and 3 - CETP also contributes to phospholipid transfer activity in human plasma but not in transgenic mice plasma, a species which has high levels of the specific phospholipid transfer protein activity.

LCAT modulates atherogenic plasma lipoproteins and the extent of atherosclerosis only in the presence of normal LDL receptors in transgenic rabbits

Elevated low density lipoprotein cholesterol (LDL-C) and reduced high density lipoprotein cholesterol (HDL-C) concentrations are independent risk factors for coronary heart disease. We have previously demonstrated that overexpression of an enzyme with a well established role in HDL metabolism, lecithin:cholesterol acyltransferase (LCAT), in New Zealand White rabbits not only raises HDL-C concentrations but reduces those of LDL-C as well, ultimately preventing diet-induced atherosclerosis. In the present study, the human LCAT gene (hLCAT) was introduced into LDL receptor (LDLr)-deficient (Watanabe heritable hyperlipidemic) rabbits to (1) investigate the role of the LDLr pathway in the hLCAT-mediated reductions of LDL-C and (2) determine the influence of hLCAT overexpression on atherosclerosis susceptibility in an animal model of familial hypercholesterolemia. Heterozygosity or homozygosity for the LDLr defect was determined by polymerase chain reaction, and 3 groups of hLCAT-transgenic (hLCAT+) rabbits that differed in LDLr status were established: (1) LDLr wild-type (LDLr+/+), (2) LDLr heterozygotes (LDLr+/-), and (3) LDLr homozygotes (LDLr-/-). Data for hLCAT+ rabbits were compared with those of nontransgenic (hLCAT-) rabbits of the same LDLr status. Plasma HDL-C concentrations were significantly elevated in the hLCAT+ animals of each LDLr status. However, LDL-C levels were significantly reduced only in hLCAT+/LDLr+/+ and hLCAT+/LDLr+/- rabbits but not in hLCAT+/LDLr-/- rabbits (405+/-14 versus 392+/-31 mg/dL). Metabolic studies revealed that the fractional catabolic rate (FCR, d(-1)) of LDL apolipoprotein (apo) B-100 was increased in hLCAT+/LDLr+/+ (26+/-4 versus 5+/-0) and hLCAT+/LDLr+/- (4+/-1 versus 1+/-0) rabbits, whereas the FCR of LDL apoB-100 in both groups of LDLr-/- rabbits was nearly identical (0.16+/-0.02 versus 0.15+/-0.02). Consistently, neither aortic lipid concentrations nor the extent of aortic atherosclerosis was significantly different between hLCAT+/LDLr-/- and hLCAT-/LDLr-/- rabbits. Significant correlations were observed between the percent of aortic atherosclerosis and both LDL-C (r=0.985) and LDL apoB-100 FCR (-0.745), as well as between LDL-C and LDL apoB-100 FCR (-0.866). These data are the first to establish that LCAT modulates LDL metabolism via the LDLr pathway, ultimately influencing atherosclerosis susceptibility. Moreover, LCAT's antiatherogenic effect requires only a single functional LDLr allele, identifying LCAT as an attractive gene therapy candidate for the majority of dyslipoproteinemic patients.

Acylation stimulating protein (ASP) deficiency alters postprandial and adipose tissue metabolism in male mice

Acylation stimulating protein (ASP) is a potent stimulator of triglyceride synthesis in adipocytes. In the present study, we have examined the effect of an ASP functional knockout (ASP(-/-)) on lipid metabolism in male mice. In both young (14 weeks) and older (26 weeks) mice there were marked delays in postprandial triglyceride clearance (80% increase at 14 weeks and 120% increase at 26 weeks versus wild type (+/+)). Postprandial nonesterified fatty acids were also increased in ASP(-/-) mice versus ASP(+/+) mice by 37% (low fat 10% Kcal) and by 73% (high fat 40% Kcal) diets, although there were no differences in fasting lipid levels. The ASP(-/-) mice had moderately increased energy intake (16% +/- 2% p < 0.0001) and reduced feed efficiency (33% increase in calories/g of body weight gained on low fat diet) versus wild type. The ASP(-/-) mice also had modest changes in insulin/glucose metabolism (30% to 40% decrease in insulin.glucose product), implying increased insulin sensitivity. As well, there were decreases in leptin (29% shift in leptin to body weight ratio) and up to a 26% decrease in specific adipose tissue depots versus the wild type mice on both low fat and high fat diets. These results demonstrate that ASP plays an important role in adipose tissue metabolism and fat partitioning.