Dietary fat increases high density lipoprotein (HDL) levels both by increasing the transport rates and decreasing the fractional catabolic rates of HDL cholesterol ester and apolipoprotein (Apo) A-I. Presentation of a new animal model and mechanistic studies in human Apo A-I transgenic and control mice

Effects of Therapeutic Lifestyle Change diets high and low in dietary fish-derived FAs on lipoprotein metabolism in middle-aged and elderly subjects

The effects of Therapeutic Lifestyle Change (TLC) diets, low and high in dietary fish, on apolipoprotein metabolism were examined. Subjects were provided with a Western diet for 6 weeks, followed by 24 weeks of either of two TLC diets (10/group). Apolipoprotein kinetics were determined in the fed state using stable isotope methods and compartmental modeling at the end of each phase. Only the high-fish diet decreased median triglyceride-rich lipoprotein (TRL) apoB-100 concentration (-23%), production rate (PR, -9%), and direct catabolism (-53%), and increased TRL-to-LDL apoB-100 conversion (+39%) as compared with the baseline diet (all P < 0.05). This diet also decreased TRL apoB-48 concentration (-24%), fractional catabolic rate (FCR, -20%), and PR (-50%) as compared with the baseline diet (all P < 0.05). The high-fish and low-fish diets decreased LDL apoB-100 concentration (-9%, -23%), increased LDL apoB-100 FCR (+44%, +48%), and decreased HDL apoA-I concentration (-15%, -14%) and PR (-11%, -12%) as compared with the baseline diet (all P < 0.05). On the high-fish diet, changes in TRL apoB-100 PR were negatively correlated with changes in plasma eicosapentaenoic and docosahexaenoic acids. In conclusion, the high-fish diet decreased TRL apoB-100 and TRL apoB-48 concentrations chiefly by decreasing their PR. Both diets decreased LDL apoB-100 concentration by increasing LDL apoB-100 FCR and decreased HDL apoA-I concentration by decreasing HDL apoA-I PR.

Non-cholesterol sterols in different forms of primary hyperlipemias

BACKGROUND AND AIMS

We investigated the behaviour of non-cholesterol sterols, surrogate markers of cholesterol absorption (campesterol and sitosterol) and synthesis (lathosterol), in primary hyperlipemias.

METHODS AND RESULTS

We studied 53 patients with polygenic hypercholesterolemia (PH), 38 patients with familial combined hyperlipemia (FCH), and 19 age- and sex-matched healthy control subjects. In all participants, plasma sitosterol, campesterol and lathosterol were determined by gas chromatography coupled to mass spectrometry. To correct for the effect of plasma lipid levels, non-cholesterol sterol concentrations were adjusted for plasma cholesterol (10² μmol/mmol cholesterol). Patients with FCH were more frequently men, and had higher body mass index (BMI), fasting glucose, insulin and HOMA-IR. Lathosterol was higher in FCH than in pH or controls (p < 0.05). Campesterol was significantly lower in FCH (p < 0.05), while no differences were found between pH and controls. Sitosterol displayed higher values in pH compared to FCH (p < 0.001) and controls (p < 0.05). Spearman's rank correlations showed positive correlations of lathosterol with BMI, waist circumference, HOMA-IR, triglycerides, apoprotein B, and a negative one with HDL-cholesterol. Sitosterol had a negative correlation with BMI, waist circumference, HOMA-IR, triglycerides, and a positive one with HDL-cholesterol and apoprotein AI. Multivariate regression analyses showed that cholesterol absorption markers predicted higher HDL-cholesterol levels, while HOMA-IR was a negative predictor of sitosterol and BMI a positive predictor of lathosterol.

CONCLUSIONS

Our findings suggest the occurrence of an increased cholesterol synthesis in FCH, and an increased cholesterol absorption in pH. Markers of cholesterol synthesis cluster with clinical and laboratory markers of obesity and insulin resistance.

Rosuvastatin does not affect human apolipoprotein A-I expression in genetically modified mice: a clue to the disputed effect of statins on HDL

BACKGROUND AND PURPOSE

Besides a significant reduction of low-density lipoprotein (LDL) cholesterol, statins moderately increase high-density lipoprotein (HDL) levels. In vitro studies have indicated that this effect may be the result of an increased expression of apolipoprotein (apo)A-I, the main protein component of HDL. The aim of the present study was to investigate in vivo the effect of rosuvastatin on apoA-I expression and secretion in a transgenic mouse model for human apoA-I.

EXPERIMENTAL APPROACH

Human apoA-I transgenic mice were treated for 28 days with 5, 10 or 20 mg·kg(-1) ·day(-1) of rosuvastatin, the most effective statin in raising HDL levels. Possible changes of apoA-I expression by treatment were investigated by quantitative real-time RT-PCR on RNA extracted from mouse livers. The human apoA-I secretion rate was determined in primary hepatocytes isolated from transgenic mice from each group after treatment.

KEY RESULTS

Rosuvastatin treatment with 5 and 10 mg·kg(-1) ·day(-1) did not affect apoA-I plasma levels, whereas a significant decrease was observed in mice treated with 20 mg·kg(-1) ·day(-1) of rosuvastatin (-16%, P < 0.01). Neither relative hepatic mRNA concentrations of apoA-I nor apoA-I secretion rates from primary hepatocytes were influenced by rosuvastatin treatment at each tested dose.

CONCLUSIONS AND IMPLICATIONS

In human apoA-I transgenic mice, rosuvastatin treatment does not increase either apoA-I transcription and hepatic secretion, or apoA-I plasma levels. These results support the hypothesis that other mechanisms may account for the observed HDL increase induced by statin therapy in humans.

Visceral fat positively correlates with cholesterol synthesis in dyslipidaemic patients

BACKGROUND

Quantification of plasma noncholesterol sterols allows the study of cholesterol absorption and synthesis. A pattern of low cholesterol absorption and high synthesis has been demonstrated in patients with obesity and insulin resistance. To understand the relationship between cholesterol absorption/synthesis and visceral obesity, we investigated surrogate markers of cholesterol absorption (campesterol and sitosterol) and synthesis (lathosterol) in dyslipidaemic patients with different representation of abdominal fat, estimated by ultrasonographic measurement of visceral fat area (VFA).

METHODS

In 126 patients with primary hyperlipaemias, plasma sitosterol, campesterol and lathosterol were determined by gas chromatography coupled with mass spectrometry. Visceral and subcutaneous fats were evaluated by ultrasonography. The study population was divided into two groups on the basis of VFA median values, below/equal and above 154 cm(2) . RESULTS Patients with higher VFA had significantly higher lathosterol levels (median 109 vs. 76 × 10(2) μmol/mmol cholesterol P < 0·004), body mass index, waist circumference, blood pressure, triglycerides, insulin, homoeostatic model assessment (HOMA)-IR and lower high-density lipoprotein (HDL)-C. VFA was positively correlated with lathosterol (ρ = 0·35, P < 0·001) and negatively with HDL-C (ρ = -0·43, P < 0·001), campesterol (ρ = -0·23, P = 0.01) and sitosterol (ρ = -0·35, P < 0·001). VFA was an independent predictor of lathosterol values (β = 0·389, P < 0·0001, P of the model < 0·0001);age, systolic blood pressure, BMI, waist circumference, triglycerides, HDL-C and HOMA failed to enter the final equation.

CONCLUSIONS

  In hyperlipidaemic patients, the amount of visceral fat correlates with cholesterol synthesis; the use of ultrasonographic detection of abdominal adiposity allows a better characterization of cholesterol pathway, potentially useful for a tailored therapeutic approach.

The Cholesterol Content of Western Diets Plays a Major Role in the Paradoxical Increase in High-Density Lipoprotein Cholesterol and Upregulates the Macrophage Reverse Cholesterol Transport Pathway

Objective—

A high–saturated fatty acid– and cholesterol-containing (HFHC) diet is considered to be a major risk factor for cardiovascular disease. The present study aimed to determine the effects of this Western-type diet on high-density lipoprotein (HDL) metabolism and reverse cholesterol transport (RCT) from macrophages to feces.

Methods and Results—

Experiments were carried out in mice fed a low-fat, low-cholesterol diet, an HFHC diet, or an HFHC diet without added cholesterol (high–saturated fatty acid and low-cholesterol [HFLC]). The HFHC diet caused a significant increase in plasma cholesterol, HDL cholesterol, and liver cholesterol and enhanced macrophage-derived [ 3 H]cholesterol flux to feces by 3- to 4-fold. These effects were greatly reduced in mice fed the HFLC diet. This HFHC diet–mediated induction of RCT was sex independent and was not associated with obesity or insulin resistance. The HFHC diet caused 1.4- and 3-fold increases in [ 3 H]cholesterol efflux to plasma and HDL-derived [ 3 H]tracer fecal excretion, respectively. Unlike a low-fat, low-cholesterol and HFLC diets, the HFHC diet increased liver ABCG5/G8 expression. The effect of the HFHC diet on fecal macrophage-derived [ 3 H]cholesterol excretion was totally blunted in ABCG5/G8-deficient mice.

Conclusion—

Despite its deleterious effects on atherosclerosis, the HFHC diet promoted a sustained compensatory macrophage-to-feces RCT. Our data provide direct evidence of the crucial role of dietary cholesterol signaling through liver ABCG5/G8 upregulation in the HFHC diet–mediated induction of macrophage-specific RCT.

New insights into the mechanism of low high-density lipoprotein cholesterol in obesity

Obesity, a significant risk factor for various chronic diseases, is universally related to dyslipidemia mainly represented by decreasing high-density lipoprotein cholesterol (HDL-C), which plays an indispensible role in development of cardiovascular disease (CVD). However, the mechanisms underlying obesity and low HDL-C have not been fully elucidated. Previous studies have focused on the alteration of HDL catabolism in circulation following elevated triglyceride (TG). But recent findings suggested that liver and fat tissue played pivotal role in obesity related low HDL-C. Some new molecular pathways like microRNA have also been proposed in the regulation of HDL metabolism in obesity. This article will review recent advances in understanding of the potential mechanism of low HDL-C in obesity.

Group 1B phospholipase A₂ deficiency protects against diet-induced hyperlipidemia in mice

Excessive absorption of products of dietary fat digestion leads to type 2 diabetes and other obesity-related disorders. Mice deficient in the group 1B phospholipase A₂ (Pla2g1b), a gut digestive enzyme, are protected against diet-induced obesity and type 2 diabetes without displaying dietary lipid malabsorption. This study tested the hypothesis that inhibition of Pla2g1b protects against diet-induced hyperlipidemia. Results showed that the Pla2g1b(-/-) mice had decreased plasma triglyceride and cholesterol levels compared with Pla2g1b(+/+) mice subsequent to feeding a high-fat, high-carbohydrate (hypercaloric) diet. These differences were evident before differences in body weight gains were observed. Injection of Poloxamer 407 to inhibit lipolysis revealed decreased VLDL production in Pla2g1b(-/-) mice. Supplementation with lysophosphatidylcholine, the product of Pla2g1b hydrolysis, restored VLDL production rates in Pla2g1b(-/-) mice and further elevated VLDL production in Pla2g1b(+/+) mice. The Pla2g1b(-/-) mice also displayed decreased postprandial lipidemia compared with Pla2g1b(+/+) mice. These results show that, in addition to dietary fatty acids, gut-derived lysophospholipids derived from Pla2g1b hydrolysis of dietary and biliary phospholipids also promote hepatic VLDL production. Thus, the inhibition of lysophospholipid absorption via Pla2g1b inactivation may prove beneficial against diet-induced hyperlipidemia in addition to the protection against obesity and diabetes.

Effects of antisense-mediated inhibition of 11β-hydroxysteroid dehydrogenase type 1 on hepatic lipid metabolism

11β-hydroxysteroid dehydrogenase 1 (11β-HSD1) converts inactive 11-keto derivatives to active glucocorticoids within tissues and may play a role in the metabolic syndrome (MS). We used an antisense oligonucleotide (ASO) to knock down 11β-HSD1 in livers of C57BL/6J mice consuming a Western-type diet (WTD). 11β-HSD1 ASO-treated mice consumed less food, so we compared them to ad libitum-fed mice and to food-matched mice receiving control ASO. Knockdown of 11β-HSD1 directly protected mice from WTD-induced steatosis and dyslipidemia by reducing synthesis and secretion of triglyceride (TG) and increasing hepatic fatty acid oxidation. These changes in hepatic and plasma lipids were not associated with reductions in genes involved in de novo lipogenesis. However, protein levels of both sterol regulatory element-binding protein (SREBP) 1 and fatty acid synthase were significantly reduced in mice treated with 11β-HSD1 ASO. There was no change in hepatic secretion of apolipoprotein (apo)B, indicating assembly and secretion of smaller apoB-containing lipoproteins by the liver in the 11β-HSD1-treated mice. Our results indicate that inhibition of 11β-HSD1 by ASO treatment of WTD-fed mice resulted in improved plasma and hepatic lipid levels, reduced lipogenesis by posttranslational regulation, and secretion of similar numbers of apoB-containing lipoproteins containing less TG per particle.

Reversal of obesity and insulin resistance by a non-peptidic glucagon-like peptide-1 receptor agonist in diet-induced obese mice

BACKGROUND

Glucagon-like peptide-1 (GLP-1) is recognized as an important regulator of glucose homeostasis. Efforts to utilize GLP-1 mimetics in the treatment of diabetes have yielded clinical benefits. A major hurdle for an effective oral therapy has been the difficulty of finding a non-peptidic GLP-1 receptor (GLP-1R) agonist. While its oral bioavailability still poses significant challenges, Boc5, one of the first such compounds, has demonstrated the attainment of GLP-1R agonism in diabetic mice. The present work was to investigate whether subchronic Boc5 treatment can restore glycemic control and induce sustainable weight loss in diet-induced obese (DIO) mice, an animal model of human obesity and insulin resistance.

METHODOLOGY/PRINCIPAL FINDINGS

DIO mice were treated three times a week with Boc5 (0.3, 1 and 3 mg) for 12 weeks. Body weight, body mass index (BMI), food intake, fasting glucose, intraperitoneal glucose tolerance and insulin induced glucose clearance were monitored regularly throughout the treatment. Glucose-stimulated insulin secretion, β-cell mass, islet size, body composition, serum metabolic profiles, lipogenesis, lipolysis, adipose hypertrophy and lipid deposition in the liver and muscle were also measured after 12 weeks of dosing. Boc5 dose-dependently reduced body weight, BMI and food intake in DIO mice. These changes were associated with significant decreases in fat mass, adipocyte hypertrophy and peripheral tissue lipid accumulation. Boc5 treatment also restored glycemic control through marked improvement of insulin sensitivity and normalization of β-cell mass. Administration of Boc5 (3 mg) reduced basal but enhanced insulin-mediated glucose incorporation and noradrenaline-stimulated lipolysis in isolated adipocytes from obese mice. Furthermore, circulating leptin, adiponectin, triglyceride, total cholesterol, nonesterified fatty acid and high-density lipoprotein/low-density lipoprotein ratio were normalized to various extents by Boc5 treatment.

CONCLUSIONS/SIGNIFICANCE

Boc5 may produce metabolic benefits via multiple synergistic mechanisms and may represent an attractive tool for therapeutic intervention of obesity and diabetes, by means of non-peptidic GLP-1R agonism.

Differential regulation of human apolipoprotein AI and high-density lipoprotein by fenofibrate in hapoAI and hapoAI-CIII-AIV transgenic mice

Fenofibrate, a PPAR-α agonist, lowers triglycerides (TG) and raises high-density lipoproteins (HDL-C) in humans. While fenofibrate is very effective in lowering TG, it does not raise HDL-C in humans to the same extent as seen in human apoAI transgenic (hAI-Tg) mice. We studied the mechanism of this discordance using the following compounds as tools: cholic acid that down-regulates human apoAI, and fenofibrate, that elevates hapoAI and HDL-C in hAI-Tg mice. We hypothesized that additional sequences, including apoCIII and AIV genes on chromosome 11, not present in the hapoAI transgene may be responsible for the dampened effect of fibrates on HDL-C seen in humans. For this, hAI-Tg mice with 11kb DNA segment and hapoAI-CIII-AIV-Tg mice with 33kb DNA segment harboring apoCIII and AIV genes were employed. These mice were treated with fenofibrate and cholic acid. Fenofibrate increased apoAI and HDL-C levels, and HDL size in the apoAI-Tg mice via up-regulation of the hapoAI mRNA and increased activity and mRNA of PLTP, respectively. Consistent with earlier findings, cholic acid showed similar effects of lowering HDL-C, and elevating LDL-C in hAI-Tg mice as well as in the hAI-CIII-AIV-Tg mice. Fenofibrate decreased TG and increased HDL size in hAI-CIII-AIV-Tg mice as well, but surprisingly, did not elevate serum levels of hapoAI or hepatic AI mRNA, suggesting that additional sequences not present in the hapoAI transgene (11kb) may be partly responsible for the dampened effect on HDL-C seen in hAI-CIII-AIV-Tg mice. Since hAI-CIII-AIV-Tg mouse mimics fenofibrate effects seen in humans, this transgenic mouse could serve as a better predictive model for screening HDL-C raising compounds.

Post-prandial lipid levels for assessing target goal achievement in type 2 diabetic patients taking statin

It is inconvenient to perform serum lipid analysis in fasting state in diabetic patients with drug treatment. In patients with statin treatment and Asian diet, it has not been clearly known whether non-fasting values could be used for the clinical decision making in diabetic patients. In this study, fasting and post-prandial plasma lipid profiles of hospitalized type 2 diabetic patients taking statin, were measured in whom standard diabetic breakfast in traditional Korean style were provided. In repeated-measures ANOVA, there were no significant differences among fasting, post-prandial 2 and 4 hr low-density lipoprotein (LDL) and high density lipoprotein (HDL) cholesterol values. When compared to fasting levels, both post-prandial 2 hr and 4 hr LDL cholesterol levels were misclassified as not achieved target goal only in 4% of patients. Post-prandial HDL cholesterol matched with fasting values in women, without exception. In conclusion, the fasting and post-prandial LDL and HDL cholesterol levels are not significantly different each other and can be used in the assessment of achieving target goal in type 2 diabetes taking statin after Korean diet.

Very low density lipoprotein metabolism and plasma adiponectin as predictors of high-density lipoprotein apolipoprotein A-I kinetics in obese and nonobese men

CONTEXT

Hypercatabolism of high-density lipoprotein (HDL) apolipoprotein (apo) A-I results in low plasma apoA-I concentration. The mechanisms regulating apoA-I catabolism may relate to alterations in very low density lipoprotein (VLDL) metabolism and plasma adiponectin and serum amyloid A protein (SAA) concentrations.

OBJECTIVE

We examined the associations between the fractional catabolic rate (FCR) of HDL-apoA-I and VLDL kinetics, plasma adiponectin, and SAA concentrations.

STUDY DESIGN

The kinetics of HDL-apoA-I and VLDL-apoB were measured in 50 obese and 37 nonobese men using stable isotopic techniques.

RESULTS

In the obese group, HDL-apoA-I FCR was positively correlated with insulin, homeostasis model of assessment for insulin resistance (HOMA-IR) score, triglycerides, VLDL-apoB, and VLDL-apoB production rate (PR). In the nonobese group, HDL-apoA-I FCR was positively correlated with triglycerides, apoC-III, VLDL-apoB, and VLDL-apoB PR and negatively correlated with plasma adiponectin. Plasma SAA was not associated with HDL-apoA-I FCR in either group. In multiple regression analyses, VLDL-apoB PR and HOMA-IR score, and VLDL-apoB PR and adiponectin were independently predictive of HDL-apoA-I FCR in the obese and nonobese groups, respectively. HDL-apoA-I FCR was positively and strongly associated with HDL-apoA-I PR in both groups.

CONCLUSIONS

Variation in VLDL-apoB production, and hence plasma triglyceride concentrations, exerts a major effect on the catabolism of HDL-apoA-I. Insulin resistance and adiponectin may also contribute to the variation in HDL-apoA-I catabolism in obese and nonobese subjects, respectively. We also hypothesize that apoA-I PR determines a steady-state, lowered plasma of apoA-I, which may reflect a compensatory response to a primary defect in the catabolism of HDL-apoA-I due to altered VLDL metabolism.

Effect of obesity on high-density lipoprotein metabolism

Reduced levels of high-density lipoproteins (HDL) in non-obese and obese states are associated with increased risk for the development of coronary artery disease. Therefore, it is imperative to determine the mechanisms responsible for reduced HDL in obese states and, conversely, to examine therapies aimed at increasing HDL levels in these individuals. This paper examines the multiple causes for reduced HDL in obese states and the effect of exercise and diet--two non-pharmacologic therapies--on HDL metabolism in humans. In general, the concentration of HDL-cholesterol is adversely altered in obesity, with HDL-cholesterol levels associated with both the degree and distribution of obesity. More specifically, intra-abdominal visceral fat deposition is an important negative correlate of HDL-cholesterol. The specific subfractions of HDL that are altered in obese states include the HDL2, apolipoprotein A-I, and pre-beta1 subfractions. Decreased HDL levels in obesity have been attributed to both an enhancement in the uptake of HDL2 by adipocytes and an increase in the catabolism of apolipoprotein A-I on HDL particles. In addition, there is a decrease in the conversion of the pre-beta1 subfraction, the initial acceptor of cholesterol from peripheral cells, to pre-beta2 particles. Conversely, as a means of reversing the decrease in HDL levels in obesity, sustained weight loss is an effective method. More specifically, weight loss achieved through exercise is more effective at raising HDL levels than dieting. Exercise mediates positive effects on HDL levels at least partly through changes in enzymes of HDL metabolism. Increased lipid transfer to HDL by lipoprotein lipase and reduced HDL clearance by hepatic triglyceride lipase as a result of endurance training are two important mechanisms for increases in HDL observed from exercise.

Composition, structure and absorption of milk lipids: a source of energy, fat-soluble nutrients and bioactive molecules

Milkfat is a remarkable source of energy, fat-soluble nutrients and bioactive lipids for mammals. The composition and content of lipids in milkfat vary widely among mammalian species. Milkfat is not only a source of bioactive lipid components, it also serves as an important delivery medium for nutrients, including the fat-soluble vitamins. Bioactive lipids in milk include triacylglycerides, diacylglycerides, saturated and polyunsaturated fatty acids, and phospholipids. Beneficial activities of milk lipids include anticancer, antimicrobial, anti-inflammatory, and immunosuppression properties. The major mammalian milk that is consumed by humans as a food commodity is that from bovine whose milkfat composition is distinct due to their diet and the presence of a rumen. As a result of these factors bovine milkfat is lower in polyunsaturated fatty acids and higher in saturated fatty acids than human milk, and the consequences of these differences are still being researched. The physical properties of bovine milkfat that result from its composition including its plasticity, make it a highly desirable commodity (butter) and food ingredient. Among the 12 major milk fatty acids, only three (lauric, myristic, and palmitic) have been associated with raising total cholesterol levels in plasma, but their individual effects are variable-both towards raising low-density lipoproteins and raising the level of beneficial high-density lipoproteins. The cholesterol-modifying response of individuals to consuming saturated fats is also variable, and therefore the composition, functions and biological properties of milkfat will need to be re-evaluated as the food marketplace moves increasingly towards more personalized diets.

The effect of select nutrients on serum high-density lipoprotein cholesterol and apolipoprotein A-I levels

One of the factors contributing to the increased risk of developing premature atherosclerosis is low plasma concentrations of high-density lipoprotein (HDL) cholesterol (HDLc). Multiple potential mechanisms account for the cardioprotective effects of HDL and its main protein apolipoprotein A-I (apo A-I). The low plasma concentrations of HDL could be the result of increased fractional clearance and reduced expression of apo A-I. To this end, nutrients play an important role in modulating the fractional clearance rate, as well as the rate of apo A-I gene expression. Because medical nutrition therapy constitutes the cornerstone of management of dyslipidemias, it is essential to understand the mechanisms underlying the changes in HDL level in response to alterations in dietary intake. In this review, we will discuss the effect of select nutrients on serum HDLc and apo A-I levels. Specifically, we will review the literature on the effect of carbohydrates, fatty acids, and ketones, as well as some of the nutrient-related metabolites, such as glucosamine and the prostanoids, on apo A-I gene expression. Because there are multiple mechanisms involved in the regulation of serum HDLc levels, changes in gene transcription do not necessarily correlate with clinical observations on serum levels of HDLc.

High-density lipoprotein apolipoprotein A-I kinetics in obesity

OBJECTIVE

Low plasma concentrations of high-density lipoprotein (HDL)-cholesterol and apolipoprotein A-I (apoA-I) are independent predictors of coronary artery disease and are often associated with obesity and the metabolic syndrome. However, the underlying kinetic determinants of HDL metabolism are not well understood.

RESEARCH METHODS AND PROCEDURES

We pooled data from 13 stable isotope studies to investigate the kinetic determinants of apoA-I concentrations in lean and overweight-obese individuals. We also examined the associations of HDL kinetics with age, sex, BMI, fasting plasma glucose, fasting insulin, Homeostasis Model Assessment score, and concentrations of apoA-I, triglycerides, HDL-cholesterol and low-density lipoprotein-cholesterol.

RESULTS

Compared with lean individuals, overweight-obese individuals had significantly higher HDL apoA-I fractional catabolic rate (0.21+/-0.01 vs. 0.33+/-0.01 pools/d; p<0.001) and production rate (PR; 11.3+/-4.4 vs. 15.8+/-2.77 mg/kg per day; p=0.001). In the lean group, HDL apoA-I PR was significantly associated with apoA-I concentration (r=0.455, p=0.004), whereas in the overweight-obese group, both HDL apoA-I fractional catabolic rate (r=-0.396, p=0.050) and HDL apoA-I PR (r=0.399, p=0.048) were significantly associated with apoA-I concentration. After adjustment for fasting insulin or Homeostasis Model Assessment score, HDL apoA-I PR was an independent predictor of apoA-I concentration.

DISCUSSION

In overweight-obese subjects, hypercatabolism of apoA-I is paralleled by an increased production of apoA-I, with HDL apoA-I PR being the stronger determinant of apoA-I concentration. This could have therapeutic implications for the management of dyslipidemia in individuals with low plasma HDL-cholesterol.

High-Density Lipoprotein Cholesterol and Coronary Heart Disease

There is a large body of evidence demonstrating an inverse correlation between circulating levels of high-density lipoprotein (HDL) cholesterol and cardiovascular disease risk. For every 1-mg/dL increase in HDL, it is estimated that the risk of cardiovascular events decreases by 2% to 3%. HDL is one of many factors that contribute to the regulation of the atherosclerotic process. HDL mediates reverse cholesterol transport and exhibits numerous beneficial properties, including antioxidant, antiinflammatory, and antithrombotic effects on the vasculature. Recent studies have expanded our understanding of the vasoprotective mechanisms of HDL to include enhanced nitric oxide production and improved endothelium-dependent relaxation. Progress has also been made in determining the molecular mechanisms that mediate reverse cholesterol transport. Recently published National Cholesterol Education Program Adult Treatment Panel guidelines have broadened the definition of low levels of HDL and encourage more aggressive screening and treatment of lipid abnormalities. Several therapeutic interventions can augment HDL concentrations, and there is increasing evidence that these interventions improve cardiovascular outcomes. Research focusing on defining the molecular roles of HDL will likely identify potential therapeutic targets for decreasing the incidence and progression of coronary heart disease. This review highlights the role of HDL in coronary heart disease, from basic mechanisms of action to recent clinical trial results.

Endothelial lipase is a major genetic determinant for high-density lipoprotein concentration, structure, and metabolism

High-density lipoprotein (HDL) protects against atherosclerosis. Endothelial lipase (EL) has been postulated to be involved in lipoprotein, and possibly HDL, metabolism, yet the evidence has been scarce and conflicting. We have inactivated EL in mice by gene targeting. EL(-/-) mice have elevated plasma and HDL cholesterol, and increased apolipoproteins A-I and E. NMR analysis reveals an abundance of large HDL particles. There is down-regulation of the transcripts for phospholipid transfer protein, but up-regulation of those for hepatic lipase and lipoprotein lipase. Plasma lecithin:cholesterol acyltransferase is unchanged despite an increase in hepatic mRNA; lecithin:cholesterol acyltransferase activity toward endogenous EL(-/-) substrate is, however, reduced by 50%. HDL clearance is decreased in EL(-/-) mice; both the structure of HDL and the presence of EL are factors that determine the rate of clearance. To determine EL's role in humans, we find a significant association between a single-nucleotide polymorphism 584C/T in the EL (LIPG) gene and HDL cholesterol in a well characterized population of 372 individuals. We conclude that EL is a major determinant of HDL concentration, structure, and metabolism in mice, and a major determinant of HDL concentration in humans.

Targeted replacement of mouse apolipoprotein A-I with human ApoA-I or the mutant ApoA-IMilano. Evidence of APOA-IM impaired hepatic secretion

Despite a pro-atherogenic profile, individuals carrying the molecular variant (R173C) of apolipoprotein (apo)A-I, named apoA-I(Milano) (apoA-I(M)), appear to be at reduced risk for cardiovascular disease. To develop an in vivo system to explore, in a controlled manner, the effects of apoA-I(M) on lipid metabolism, we have used the gene targeting technology, or "gene knock-in" (gene k-in), to replace the murine apoA-I gene with either human apoA-I or apoA-I(M) genes in embryonic stem cells. As in human carriers, mice expressing apoA-I(M) (A-I(M) k-in) are characterized by low concentrations of the human apolipoprotein and reduced high density lipoprotein cholesterol levels, compared with A-I k-in animals. The aim of the present study was to investigate the basic mechanisms of hypoalphalipoproteinemia associated with the apoA-I(M) mutation. ApoA-I and apoA-I(M) mRNA expression, as assessed by Northern blot analysis and quantitative real time reverse transcription-PCR, did not exhibit significant differences in either liver or intestine. Moreover, human apolipoprotein synthesis rates were similar in the k-in lines. When the secretion rate of the human apolipoproteins was assessed in cultured hepatocytes from the mouse lines, secretion from apoA-I(M)-expressing cells was markedly reduced (42% for A-I(M) k-in and 36% for A-I/A-I(M) k-in mice) as compared with that of A-I k-in hepatocytes. These results provide the first evidence that the hypoalphalipoproteinemia in apoA-I(M) human carriers may be partially explained by impaired apoA-I(M) secretion.

Mechanisms of HDL deficiency in mice overexpressing human apoA-II

To ascertain the mechanisms underlying the hypoalphalipoproteinemia present in mice overexpressing human apolipoprotein A-II (apoA-II) (line 11.1), radiolabeled HDL or apoA-I were injected into mice. Fractional catabolic rate of [(3)H]cholesteryl oleoyl ether HDL ([(3)H]HDL) was 2-fold increased in 11.1 transgenic mice compared with control mice and this was concomitant with increased radioactivity in liver, gonads, and adrenals. However, scavenger receptor class B, type I (SR-BI) was increased only in adrenals. [(3)H]HDL of 11.1 transgenic mice presented greater binding but decreased uptake compared with control mice when Chinese hamster ovary cells transfected with SR-BI were used, thereby pointing to unknown but SR-BI-independent mechanisms as being responsible for the increased (3)H-radioactivity seen in liver and gonads. Synthesis rate (SR) of plasma [(3)H]HDL was 2-fold decreased in 11.1 transgenic mice. Mouse (125)I-apoA-I was 2-fold more rapidly catabolized (mainly by the kidney) in transgenic mice. Mouse apoA-I displacement from HDL by the addition of isolated human apoA-II was reproduced ex vivo; thus, this mechanism may be involved in the increased renal catabolism of apoA-I. ApoA-I SR was 2-fold decreased in 11.1 transgenic mice and this was concomitant with a 2.3-fold decrease in hepatic apoA-I mRNA abundance. Our findings show that multiple mechanisms are involved in the HDL deficiency presented by mice overexpressing human apoA-II.

Arg123-Tyr166 domain of human ApoA-I is critical for HDL-mediated inhibition of macrophage homing and early atherosclerosis in mice

Atherosclerosis was studied in apolipoprotein E (apoE) knockout mice expressing human apolipoprotein A-I (apoA-I) or an apoA-I/apolipoprotein A-II (apoA-II) chimera in which the Arg123-Tyr166 central domain of apoA-I was substituted with the Ser12-Ala75 segment of apoA-II. High density lipoprotein (HDL) cholesterol levels were identical in apoA-I and apoA-I/apoA-II mice, but at 4 months, plaques were 2.7-fold larger in the aortic root of the apoA-I/apoA-II mice (P<0.01). The macrophage-to-smooth muscle cell ratio of lesions was 2.1-fold higher in apo-I/apoA-II mice than in apoA-I mice (P<0.01). This was due to a 2.7-fold higher (P<0.001) in vivo macrophage homing in the aortic root of apoA-I/apoA-II mice. Plasma platelet-activating factor acetyl hydrolase activity was lower (P<0.01) in apoA-I/apoA-II mice, resulting in increased oxidative stress, as evidenced by the higher titer of antibodies against oxidized low density lipoprotein (P<0.01). Increased oxidative stress resulted in increased stimulation of ex vivo macrophage adhesion by apoA-I/apoA-II beta-very low density lipoprotein and decreased inhibition of beta-very low density lipoprotein-induced adhesion by HDL from apoA-I/apoA-II mice. The cellular cholesterol efflux capacity of HDL from apoA-I/apoA-II mice was very similar to that of apoA-I mice. Thus, the Arg123-Tyr166 central domain of apoA-I is critical for reducing oxidative stress, macrophage homing, and early atherosclerosis in apoE knockout mice independent of its role in HDL production and cholesterol efflux.

High density lipoprotein cholesteryl ester metabolism in the pony, an animal species without plasma cholesteryl ester transfer protein activity: transfer of high density lipoprotein cholesteryl esters to lower density lipoproteins and the effect of the amount of fat in the diet

The metabolism of high density lipoprotein cholesteryl esters (HDL CE) was studied in the pony, an animal species without plasma cholesteryl ester transfer protein (CETP) activity. Studies were done in ponies fed a low- (1.5% fat, w/w) and a high-fat diet (11.5%, w/w fat). The ponies fed the high-fat diet had higher plasma HDL CE concentrations (1.08+/-0.15 vs. 0.84+/-0.11 mmol/l, mean+/-S.D., n=6, P<0.01) and plasma lipoprotein lipase (LPL) activities (14.3+/-4.0 vs. 5.7+/-3.4 micromol free fatty acids (FFA)/ml per h, P<0.05) than those on the low-fat diet. Plasma triacylglycerol (TAG) concentrations were lower on the high-fat diets (0.129+/-0.043 vs. 0.180+/-0.050 mmol/l), but these differences were not statistically significant. There was a negative correlation between the levels of plasma TAG (r=0.598, P<0.05) and VLDL CE (r=0.658, P<0.05) on the one hand and the HDL CE concentrations on the other hand. The transport rates of HDL CE were not significantly different between ponies fed high-fat (0.029+/-0.008 mmol HDL CE/h per l plasma) and those fed low-fat diets (0.024+/-0.004). HDL CE were transferred to low density lipoproteins (LDL) and we calculated that the percentage of LDL CE derived from HDL was 0.69+/-0.13 in the ponies fed the low-fat diet and 0.53+/-0.05 in the ponies fed the high-fat diet (P<0.05). The results of these in vivo studies suggest that in ponies, similarly as reported in rats and pigs, HDL CE can be transferred to LDL despite the absence of plasma CETP activity, and that the magnitude of this transfer is related to the levels of HDL CE as induced by the amount of fat in the diet.

Effects of walnut consumption on plasma fatty acids and lipoproteins in combined hyperlipidemia

BACKGROUND

Epidemiologic studies show an inverse relation between nut consumption and coronary heart disease.

OBJECTIVE

We determined the effects of walnut intake on plasma fatty acids, lipoproteins, and lipoprotein subclasses in patients with combined hyperlipidemia.

DESIGN

Participants sequentially adhered to the following diets: 1) a habitual diet (HD), 2) a habitual diet plus walnuts (HD+W), 3) a low-fat diet (LFD), and 4) a low-fat diet plus walnuts (LFD+W).

RESULTS

In 13 postmenopausal women and 5 men ( +/- SD age 60 +/- 8 y), walnut supplementation did not increase body weight despite increased energy intake and the LFD caused weight loss (1.3 +/- 0.5 kg; P < 0.01). When comparing the HD with the HD+W, linoleic acid concentrations increased from 29.94 +/- 1.14% to 36.85 +/- 1.13% and alpha-linolenic acid concentrations increased from 0.78 +/- 0.04% to 1.56 +/- 0.11%. During the LFD+W, plasma total cholesterol concentrations decreased by 0.58 +/- 0.16 mmol/L when compared with the HD and by 0.46 +/- 0.14 mmol/L when compared with the LFD. LDL-cholesterol concentrations decreased by 0.46 +/- 0.15 mmol/L when compared with the LFD. Measurements of lipoprotein subclasses and particle size suggested that walnut supplementation lowered cholesterol preferentially in small LDL (46.1 +/- 1.9% compared with 33.4 +/- 4.3%, HD compared with HD+W, respectively; P < 0.01). HDL-cholesterol concentrations decreased from 1.27 +/- 0.07 mmol/L during the HD to 1.14 +/- 0.07 mmol/L during the HD+W and to 1.11 +/- 0.08 mmol/L during the LFD. The decrease was seen primarily in the large HDL particles.

CONCLUSIONS

Walnut supplementation may beneficially alter lipid distribution among various lipoprotein subclasses even when total plasma lipids do not change. This may be an additional mechanism underlying the antiatherogenic properties of nut intake.

Dietary cholic acid lowers plasma levels of mouse and human apolipoprotein A-I primarily via a transcriptional mechanism

To induce dietary atherosclerosis in mice, high-fat/high-cholesterol (HF) diets are frequently supplemented with cholic acid (CA). This diet produces low plasma levels of high-density lipoprotein (HDL) and high levels of low-density lipoprotein (LDL). However, HF diets without any added CA, which more closely resemble human diets, increase levels of both HDL and LDL, suggesting that CA may be responsible for the lowering of HDL. Our aim was to examine the potential mechanism responsible for the lowering of HDL. Nontransgenic (NTg) C57BL mice and apoA-I-transgenic (apoAI-Tg) mice, with greatly increased basal apoA-I and HDL levels, were used. Mice were fed the following four diets: control (C), high-fat/high-cholesterol (HF), control and 1% cholate (CA) and HF + CA. Dietary CA reduced plasma HDL levels by 35% in NTg and 250% in apoAI-Tg mice, independent of the fat or cholesterol content of the diet. Hepatic apoA-I mRNA decreased 30% in NTg and 180% in apoAI-Tg mice. Hepatic apoA-I synthesis and apoA-I mRNA transcription rates also decreased in parallel with apoA-I mRNA levels, suggesting that the CA-induced decreases in plasma apoA-I levels occurred primarily via decreasing apoA-I mRNA transcription rates. An HF diet increased HDL levels 1.8-fold in NTg and 1.5-fold in apoAI-Tg mice. Addition of CA to the HF diet lowered HDL levels by 1.6-fold in NTg and 2. 5-fold in apoAI-Tg mice. Transfection studies with the apoA-I promoter suggested the presence of a putative cis-acting element responsible for the CA-mediated down-regulation of the apoA-I promoter activity. Measurements of apoA-I regulatory protein-1 (ARP-1) mRNA, a negative regulator of the apoA-I gene in the mouse liver showed that CA increased the ARP-1 mRNA levels. Because apoA-I gene transcription alone was not sufficient to account for the lowering of plasma HDL levels, scavenger receptor-B1 (SR-B1) and hepatic lipase (HL) mRNAs levels were quantitated. The levels of SR-B1 and HL mRNA were not changed by dietary CA. These studies suggest that dietary cholate regulates plasma levels of apoA-I primarily by a transcriptional mechanism via a putative bile acid response element involving a negative regulator of apoA-I, and partly by an unidentified post-transcriptional mechanism.

High density lipoprotein, apolipoprotein A-I, and coronary artery disease

High density lipoproteins (HDL), one of the main lipoprotein particles circulating in plasma, is involved in the reverse cholesterol transport. Several lines of evidence suggest that elevated levels of HDL is protective against coronary heart disease. The role of HDL in the removal of body cholesterol and in the regression of atherosclerosis add to the importance of understanding the molecular-cellular processes that determine plasma levels of HDL. Factors modulating plasma levels of HDL may have influence on the predisposition of an individual to premature coronary artery disease. Apolipoprotein (apo) A-I is the main apolipoprotein component of HDL and, to a large extent, sets the plasma levels of HDL. Thus, understanding the regulation of apoA-I gene expression may provide clues to raise plasma levels of HDL. This review discusses the various pathways that alter plasma levels of HDL. Since apoA-I is the main protein component of HDL and determines the plasma levels of HDL, this review also covers the regulation of apoA-I gene expression.

Changes in plasma lipoproteins during low-fat, high-carbohydrate diets: effects of energy intake

BACKGROUND

Low-fat diets can increase plasma triacylglycerol and reduce HDL cholesterol. Changes in energy intake and body weight can influence the lipoprotein response.

OBJECTIVE

We sought to prospectively examine the effects of euenergetic and ad libitum dietary fat restriction on plasma lipoproteins in healthy postmenopausal women.

DESIGN

Participants first received a controlled euenergetic diet in which dietary fat was reduced stepwise from 35% to 25% to 15% over 4 mo. Thereafter, participants followed an ad libitum 15%-fat diet for 8 mo; 54 women completed the intervention.

RESULTS

During the controlled euenergetic diet, plasma triacylglycerol increased from 1.70 +/- 0.10 to 2.30 +/- 0.16 mmol/L, total cholesterol decreased from 5.87 +/- 0.13 to 5.53 +/- 0. 13 mmol/L, LDL cholesterol decreased from 3.41 +/- 0.10 to 2.87 +/- 0.10 mmol/L, HDL cholesterol decreased from 1.76 +/- 0.08 to 1.50 +/- 0.08 mmol/L, and apolipoprotein (apo) A-I decreased from 5.11 +/- 0.14 to 4.78 +/- 0.14 mmol/L (P < 0.0001 for all changes). Hormone replacement therapy did not affect the relative change in HDL cholesterol. Plasma glucose, insulin, hemoglobin A(1C,) free fatty acid, and apo B concentrations did not change significantly. During the ad libitum 15%-fat diet, participants lost 4.6 +/- 0.4 kg. Plasma triacylglycerol and LDL cholesterol returned to baseline values (1.77 +/- 0.12 and 3.31 +/- 0.08 mmol/L, respectively), whereas HDL cholesterol and apo A-I remained low (1.40 +/- 0.08 and 4.82 +/- 0.18 mmol/L, respectively). HDL cholesterol and apo A-I concentrations stabilized in subjects who were not receiving hormone replacement therapy but continued to decline in women who were receiving hormone therapy.

CONCLUSIONS

The ad libitum 15%-fat diet resulted in significant weight loss. The euenergetic but not the ad libitum diet caused hypertriacylglycerolemia. HDL cholesterol decreased during both low-fat diets.

Role of the kidney in regulating the metabolism of HDL in rabbits: evidence that iodination alters the catabolism of apolipoprotein A-I by the kidney

To evaluate the factors that regulate HDL catabolism in vivo, we have measured the clearance of human apoA-I from rabbit plasma by following the isotopic decay of (125)I-apoA-I and the clearance of unlabeled apoA-I using a radioimmunometric assay (RIA). We show that the clearance of unlabeled apoA-I is 3-fold slower than that of (125)I-apoA-I. The mass clearance of iodinated apoA-I, as determined by RIA, is superimposable with the isotopic clearance of (125)I-apoA-I. The data demonstrate that iodination of tyrosine residues alters the apoA-I molecule in a manner that promotes an accelerated catabolism. The clearance from rabbit plasma of unmodified apoA-I on HDL(3) and a reconstituted HDL particle (LpA-I) were very similar and about 3-4-fold slower than that for (125)I-apoA-I on the lipoproteins. Therefore, HDL turnover in the rabbit is much slower than that estimated from tracer kinetic studies. To determine the role of the kidney in HDL metabolism, the kinetics of unmodified apoA-I and LpA-I were reevaluated in animals after a unilateral nephrectomy. Removal of one kidney was associated with a 40-50% reduction in creatinine clearance rates and a 34% decrease in the clearance rate of unlabeled apoA-I and LpA-I particles. In contrast, the clearance of (125)I-labeled molecules was much less affected by the removal of a kidney; FCR for (125)I-LpA-I was reduced by <10%. The data show that the kidneys are responsible for most (70%) of the catabolism of apoA-I and HDL in vivo, while (125)I-labeled apoA-I and HDL are rapidly catabolized by different tissues. Thus, the kidney is the major site for HDL catabolism in vivo. Modification of tyrosine residues on apoA-I may increase its plasma clearance rate by enhancing extra-renal degradation pathways.

Impaired HDL response to fat in men with coronary artery disease

A low HDL cholesterol is found frequently in subjects with premature coronary artery disease. We speculated that individuals with a normal total cholesterol and coronary artery disease have an impaired HDL response to dietary fat. Twenty-one men with recently diagnosed coronary artery disease and total plasma cholesterol of <6 mmol/l were matched by age, weight and cholesterol with 26 men with no personal or family history of coronary artery disease. They were placed sequentially on a 25% fat diet for 2 weeks, a high carbohydrate supplement which reduced fat to 16% of energy for 3 weeks and a high monounsaturated fat supplement which increased fat to 35% for a final 3-week period. Half of the subjects underwent an intravenous glucose tolerance test at the end of the intervention periods. The high fat supplement increased HDL cholesterol from 0.79 to 0.89 mmol/l in the men with coronary artery disease while HDL increased from 0.88 to 1.05 mmol/l in the control group (P<0.05 for group difference). Plasma triglyceride fell by 0.79 and 0.45 mmol/l in cases and controls respectively (P<0.05 for group difference). LDL cholesterol fell by 0.2 mmol/l in both groups. Men with coronary artery disease had an enhanced insulin response during the intravenous glucose tolerance test (P<0.03) particularly in the low fat phase. Thus men with premature coronary artery disease and a low HDL cholesterol appear to have an impaired elevation of HDL cholesterol in response to dietary fat, and insulin resistance may underlie this response.

Dietary restriction of saturated fat and cholesterol decreases HDL ApoA-I secretion

We examined the mechanisms responsible for the decrease in HDL cholesterol (HDL-C) levels after the consumption of a diet low in total fat, saturated fat, and cholesterol. Twenty-one subjects with a mean age of 58+/-12 years were placed on a baseline isocaloric diet (15% protein, 49% carbohydrate, 36% fat, and 150 mg/1000 kcals of cholesterol) and then switched to an NCEP Step 2 diet (15% protein, 60% carbohydrate, 25% fat, and 45 mg/1000 kcals of cholesterol). After 6 or 24 weeks on each diet, subjects received a 15-hour primed-constant infusion of [5,5,5-2H3]-L-leucine. HDL apoA-I and apoA-II tracer curves were determined by gas chromatography-mass spectrometry and fitted to a monoexponential equation. Compared with the baseline diet, consumption of the Step 2 diet lowered HDL-C mean levels by 15% (1.03+/-0.23 to 0.88+/-0.16 mmol/L, P<0.001), apoA-I by 12% (1.25+/-0.15 to 1.10+/-0.13 g/L, P<0. 001) and the TC/HDL-C ratio by 5% (0.145+/-0.04 to 0.137+/-0.03). No significant changes were observed in apoA-II levels and HDL particle size with diet. HDL apoA-I fractional catabolic rate did not change (0.219+/-0.052 to 0.220+/-0.043 pools/day, P=0.91) but HDL apoA-I secretion rate decreased by 8% (12.26+/-3.07 to 10.84+/-2.11 mg. kg-1. day-1, P=0.03) during consumption of the Step 2 diet. There was no effect of diet on apoA-II fractional catabolic rate or secretion rate. Our results indicate that the decrease in HDL-C and apoA-I levels during the isocaloric consumption of a Step 2 diet paralleled the reductions in apoA-I secretion rate.

Pleiotropy and genotype by diet interaction in a baboon model for atherosclerosis: a multivariate quantitative genetic analysis of HDL subfractions in two dietary environments

We investigated dietary effects on pleiotropic relationships among 3 HDL cholesterol (C) subfractions (HDL1-C, HDL2-C, and HDL3-C; levels quantified by gradient gel electrophoresis) for 942 pedigreed baboons (Papio hamadryas) who were fed a basal (Chow) diet and a high cholesterol, saturated fat (HCSF) challenge diet. Using multivariate maximum likelihood methods we estimated heritabilities for all 6 traits, genetic and environmental correlations (rhoG and rhoE) between them, and the additive genetic variance of each subfraction's response to the diets. On the Chow diet, genetic correlations between the 3 subfractions were significant, and we observed complete pleiotropy between HDL1-C and HDL3-C (rhoG=-0.81). On the HCSF diet, only the genetic correlation between HDL1-C and HDL3-C (rhoG=-0.61) was significant. Genetic correlations between individual subfractions on the Chow and HCSF diets did not differ significantly from 1.0, indicating that the same additive genes influenced each subfraction's levels regardless of diet. However, the additive genetic variance of response to the diets was highly significant for HDL1-C and HDL2-C, but not for HDL3-C. Similar sets of genes influence variation in the 3 HDL subfractions on the Chow diet, and the same set influences variation in each subfraction on the HCSF diet. However, the expression of genes influencing HDL1-C and HDL2-C is altered by the HCSF diet, disrupting the pleiotropy observed between the 3 subfractions on the Chow diet.

Immunolocalization of high-density lipoproteins in arterial walls of rats

The inverse correlation between serum high-density lipoprotein (HDL) levels and coronary heart disease in humans suggests that HDL has a protective effect against the development of atherosclerosis. However, there is a lack of data concerning its distribution across the arterial wall. In order to detect this lipoprotein, we performed immunogold labeling on ultrathin sections of L.R. White embedded rat arterial tissue. Electron microscopic examination revealed that HDL was localized in the cytoplasm of the endothelial cells and the smooth muscle cells, but not in the nucleus or other organelles. The HDL was also present in the subendothelial space, the extracellular matrix as well as the intercellular clefts between the endothelial cells. Quantitative study revealed that rats on a high cholesterol diet for one month have more immunogold labeling (P < 0.05) in the subendothelial space, the smooth muscle cells and the extracellular matrix as compared to rats on a normal diet. After 12 months of normal diet, the intracellular labeling was significantly increased (P < 0.05) in the endothelial cells and the smooth muscle cells as compared to 1 month on the normal diet. The increase was greater (P < 0.05) for the high-cholesterol diet than for the normal diet treatment.

[Anthropometric and dietary determinants of blood levels of HDL cholesterol in a population-based study. The REGICOR study. Researchers of the REGICOR study]

OBJECTIVES

The aim of the present study was to identify dietary and anthropometric factors influencing HDL cholesterol levels in the region of Girona. POBLATION AND METHODS: A cross-sectional study was designed with random recruitment and 798 men and 862 women were included. Anthropometric variables were collected, the energy expenditure in physical activity was calculated and a dietary questionnaire was supplied in order to obtain nutritional data. Furthermore, lipid levels and lipoprotein concentrations were determined.

RESULTS

Significant differences were found in serum triglycerides, body mass index, glucose levels and alcohol intake between the upper and the lower tertils of HDL cholesterol in both men and women. In men, energy expenditure in physical activity was significantly associated with HDL cholesterol levels, as well as total fat and monounsaturated fat. In women, together with the waist-to-hip ratio and fasted glycemia, vitamin C was the dietary factor positively associated with HDL cholesterol levels.

CONCLUSIONS

Moderate alcohol intake, physical activity, vitamin C consumption and optimizing body weight strongly contribute to increased HDL cholesterol levels in our region.

Substitution of the carboxyl-terminal domain of apo AI with apo AII sequences restores the potential of HDL to reduce the progression of atherosclerosis in apo E knockout mice

HDL metabolism and atherosclerosis were studied in apo E knockout (KO) mice overexpressing human apo AI, a des- (190-243)-apo AI carboxyl-terminal deletion mutant of human apo AI or an apo AI-(1-189)-apo AII-(12-77) chimera in which the carboxyl-terminal domain of apo AI was substituted with the pair of helices of apo AII. HDL cholesterol levels ranked: apo AI/apo E KO approximately apo AI-(1-189)-apo AII- (12-77)/apo E KO > > des-(190-243)-apo AI/apo E KO > apo E KO mice. Progression of atherosclerosis ranked: apo E KO > des-(190-243)-apo AI/apo E KO > > apo AI-(1-189)- apo AII-(12-77)/apo E KO approximately apo AI/apo E KO mice. Whereas the total capacity to induce cholesterol efflux from lipid-loaded THP-1 macrophages was higher for HDL of mice overexpressing human apo AI or the apo AI/apo AII chimera, the fractional cholesterol efflux rate, expressed in percent cholesterol efflux/microg apolipoprotein/h, for HDL of these mice was similar to that for HDL of mice overexpressing the deletion mutant and for HDL of apo E KO mice. This study demonstrates that the tertiary structure of apo AI, e.g., the number and organization of its helices, and not its amino sequence is essential for protection against atherosclerosis because it determines HDL cholesterol levels and not cholesterol efflux. Amino acid sequences of apo AII, which is considered to be less antiatherogenic, can be used to restore the structure of apo AI and thereby its antiatherogenicity.

Negative regulation of Apo A-I gene expression by retinoic acid in rat hepatocytes maintained in a coculture system

Rat hepatocytes cocultured with rat liver epithelial cells (RLEC) were used to investigate the influence of all-trans retinoic acid (RA) on the regulation of apolipoproteins (Apo) A-I and A-II gene expression, the major protein constituent of high-density lipoproteins. In contrast to rat hepatocytes in conventional primary culture, Apo A-I and Apo A-II gene expression remained high and stable for several days in parenchymal cells in coculture. Treatment of cocultured rat hepatocytes with RA resulted in a specific decrease in Apo A-I mRNA levels whereas no marked difference in Apo A-II mRNA levels was observed. Such a negative effect of RA was already detected as early as 2 days of treatment and was effective for the entire experimental period (6 days). As controls, RARbeta mRNA levels increased whereas those of GAPDH mRNA were not affected by the RA treatment. The decrease in Apo A-I mRNA levels was associated with lower amounts of Apo A-I secreted in the culture medium within day 1 of treatment. This effect required active transcription and protein synthesis. These results show that, contrary to primary pure hepatocyte cultures and hepatoma cell lines, cocultures of rat hepatocytes reproduce the in vivo results suggesting that only well differentiated hepatocytes may correctly respond to RA. Furthermore, they demonstrate that RA can directly act on hepatocytes and differently affect Apo A-I and Apo A-II gene expression.

The full induction of human apoprotein A-I gene expression by the experimental nephrotic syndrome in transgenic mice depends on cis-acting elements in the proximal 256 base-pair promoter region and the trans-acting factor early growth response factor 1

To identify molecular factors regulating apo A-I production in vivo, we induced in transgenic mice the experimental nephrotic syndrome, which results in elevated levels of HDL cholesterol (HDL-C), plasma apo A-I, and hepatic apo A-I mRNA. Human (h) apo A-I transgenic mice with different length 5' flanking sequences (5.5 or 0.256 kb, the core promoter for hepatic-specific basal expression) were injected with nephrotoxic (NTS) or control serum. With nephrosis, there were comparable (greater than twofold) increases in both lines of HDL-C, h-apo A-I, and hepatic h-apo A-I mRNA, suggesting that cis-acting elements regulating induced apo A-I gene expression were within its core promoter. Hepatic nuclear extracts from control and nephrotic mice footprinted the core promoter similarly, implying that the same elements regulated basal and induced expression. Hepatic mRNA levels for hepatocyte nuclear factor (HNF) 4 and early growth response factor (EGR) 1, trans-acting factors that bind to the core promoter, were measured: HNF4 mRNA was not affected, but that of EGR-1 was elevated approximately fivefold in the nephrotic group. EGR-1 knockout (EGR1-KO) mice or mice expressing EGR-1 were injected with either NTS or control serum. Levels of HDL-C, apo A-I, and hepatic apo A-I mRNA were lowest in nonnephrotic EGR1-KO mice and highest in nephrotic mice expressing EGR-1. Although in EGR1-KO mice HDL-C, apo A-I, and apo A-I mRNA levels also increased after NTS injection, they were approximately half of those in the nephrotic EGR-1-expressing mice. We conclude that in this model, basal and induced apo A-I gene expression in vivo are regulated by the trans-acting factor EGR-1 and require the same cis-acting elements in the core promoter.

C57BL/6 mice fed high fat diets as models for diabetes-accelerated atherosclerosis

Non-insulin-dependent diabetes mellitus (NIDDM) is a major risk factor for the development of atherosclerosis in humans. The development of an animal model that displays accelerated atherosclerosis associated with NIDDM will aid in elucidating the mechanisms that associate these disorders. C57BL/6 mice may provide such a model system. This strain becomes obese, hyperglycemic and insulin resistant when fed a high fat diet (diabetogenic diet) and is susceptible to atherosclerotic lesion development when fed a separate high fat diet containing cholesterol and bile acids (atherogenic diet). This report tests whether a diet commonly used to induce atherosclerosis also provokes a diabetic phenotype and whether a diet used to induce diabetes provokes the development of aortic fatty streak lesions. Mice of strains C57BL/6, C3H/He, BALB/c and seven recombinant inbred (RI) strains were fed an atherogenic diet for 14 weeks and glucose parameters were measured. No correlation was observed between atherosclerosis susceptibility and fasting insulin or glucose levels, or glucose clearance following short-term insulin or glucose treatment. Analysis of the RI strains suggested that multiple genes control these glucose metabolic parameters. Feeding the diabetogenic diet for 14 weeks to C57BL/6 mice induced obesity and diabetes and 2-fold increases in plasma lipoprotein concentrations. Also, small aortic sinus lipid deposits were observed in 40% of the mice. Thus, analysis of the diabetogenic diet fed C57BL/6 mouse may provide an important tool for further studies of diabetes accelerated vascular disease.

Regulation of human apolipoprotein A-I gene expression by gramoxone

To induce oxidative stress, HepG2 cells were exposed to a compound known as gramoxone. This compound undergoes a one-electron reduction to form a stable free radical which is capable of generating reactive oxygen species. We demonstrated that exposure of HepG2 cells to gramoxone (0.1 microM) resulted in a 2-fold decrease in apoA-I mRNA with no significant change in apoB and apoE mRNA levels. To examine if increased rates of mRNA degradation were responsible for the reduction in apoA-I mRNA levels, mRNA half-lives were measured in the presence of actinomycin D with and without gramoxone treatment. These studies revealed a 4-fold increase in the rate of apoA-I mRNA degradation in cells exposed to gramoxone. In similarly treated cells, nuclear run-off assays indicated that the transcription rate of the apoA-I gene was also increased 2-fold. Consistent with nuclear run-off assays, transient transfection experiments using a series of pGL2-derived luciferase reporter plasmids containing the human apoAI proximal promoter demonstrated that gramoxone treatment increased apoA-I promoter activity 2-fold. We have identified a potential "antioxidant response element" (ARE) in the apoA-I promoter region that may be responsible for the increase in apoA-I transcriptional activity by gramoxone. Gel mobility shift assays with an ARE oligonucleotide revealed increased levels of a specific protein-DNA complex that formed with nuclear extracts from gramoxone-treated cells. UV cross-linking experiments with the ARE and nuclear extracts from either untreated or gramoxone-treated cells detected proteins of approximately 100 and 115 kDa. When a single copy of the ARE was inserted upstream of the SV40 promoter in a luciferase reporter plasmid, a significant 2-fold induction in luciferase activity was observed in HepG2 cells in the presence of gramoxone. In contrast, a plasmid containing a mutant apoAI-ARE did not confer responsiveness to gramoxone. Furthermore, pGL2 (apoAI-250 mutant ARE), in which point mutations eliminated the ARE in the apoAI promoter, showed no increase in luciferase activity in response to gramoxone. Taken together, the data suggest that gramoxone affects apoA-I mRNA levels by both transcriptional and post-transcriptional mechanisms.

Role of the carboxy-terminal domain of human apolipoprotein AI in high-density-lipoprotein metabolism--a study based on deletion and substitution variants in transgenic mice

Cholesterol levels in high-density lipoprotein (HDL) of transgenic mice overexpressing human apolipoprotein AI (apoAI), a des-(190-243)-apoAI deletion mutant or an apoAI-(1-189)-apoAII-(12-77) chimera were 2.8-fold (P<0.001), 1.3-fold (P<0.05) and 2.2-fold (P<0.001) higher than in control mice, respectively. Human apolipoprotein levels in apoAI and in apoAI-(1-189)-apoAII-(12-77) transgenic mice were 5.2-fold and 3.5-fold higher than in des-(190-243)-apoAI transgenic mice, whereas their HDL cholesterol levels were 2.1-fold and 1.6-fold higher. PAGE of HDL isolated by ultracentrifugation revealed that murine HDL migrated as 9.6-nm and 7.2-nm particles. Overexpression of human apoAI and apoAI-(1-189)-apoAII-(12-77) resulted in the production of polydisperse HDL (9.6, 9.2, 8.4 and 7.2 nm) particles, whereas overexpression of des-(190-243)-apoAI primarily resulted in an increase of 7.2-nm particles. The fractional catabolic rates of human apoAI and apoAI-(1-189)-apoAII-(12-77) were very similar, whereas that of des-(190-243)-apoAI was 4.9-fold higher. The endogenous production rates of human apoAI, des-(190-243)-apoAI and apoAI-(1-189)-apoAII-(12-77) in transgenic mice were very similar. It is concluded that deletion of the carboxy-terminal domain of apoAI reduces its lipoprotein association, resulting in the production of small, phospholipid-rich HDL particles that are cleared more rapidly. Substitution of the carboxy-terminal helices of apoAI with helices of apoAII restores lipoprotein association, resulting in the production of HDL, which migrates as human HDL3 and HDL2. Although the carboxy-terminal domain of the chimera contained more than 80% of the amino acid sequence of apoAII, its HDL-distribution profile in transgenic mice was very similar to that of human apoAI. This study demonstrates the importance of the helical structure of apoAI of the carboxy-terminal domain of apoAI, rather than of its exclusive amino acid sequence, in HDL metabolism.

Efficacy of simvastatin and pumpkin-seed oil in the management of dietary-induced hypercholesterolemia

Pumpkin-seed oil (PSO), a natural supplement rich with antioxidants and polyunsaturated fatty acids (PUFAs), was given in combination with simvastatin, as antihypercholesterolemic drug, to high cholesterol-fed rabbits, for three weeks. In comparison with normal rabbits, a significant increase of the aortic contractile response to norepinephrine was observed which could be attributed to endothelium dysfunction. In addition, serum levels of total lipids, triacylglycerols, total cholesterol and low density lipoprotein-cholesterol (LDL-C) were increased while phospholipids and high density lipoprotein-cholesterol (HDL-C) were decreased in hypercholesterolemic rabbits. These changes could be related to the predominance of LDL and oxidized-LDL particles caused by high levels of reactive oxygen species during hypercholesterolemia (HC). Treatment with simvastatin modulated most of the altered parameters affected during HC that might be, in part, due to inhibition of cholesterol biosynthesis. While concomitant administration of simvastatin and PSO succeeded to cause marked reduction of the aortic contractile response to norepinephrine and to normalize the most adverse effects observed during HC. These effects were explained by the potentiating effects of simvastatin with antioxidants and essential fatty acids in PSO. On the contrary, serum activities of aminotransferases and creatine phosphokinase were increased with simvastatin treatment but not with the combination therapy in hypercholesterolemic rabbits.

Intestinal transcription and synthesis of apolipoprotein AI is regulated by five natural polymorphisms upstream of the apolipoprotein CIII gene

To understand the factors contributing to the synthesis of human apolipoprotein AI (apoAI), relative apoAI synthesis was measured from endoscopic biopsy samples obtained from 18 healthy volunteers. The relative amount of apoAI synthesis was directly correlated with steady state intestinal apoAI mRNA levels and a 10-fold within-group variability was observed. Analysis of genomic DNA from the subjects revealed five polymorphic sites which defined two haplotypes in the intestinal enhancer region of the apoAI gene located upstream of the apolipoprotein CIII gene transcriptional start site (+ 1): (-641 C to A, -630 G to A, -625 T to deletion, -482 C to T, and -455 T to C). The population frequencies of the wild-type and mutant alleles were 0.53 and 0.44, respectively. Mean steady state apoAI mRNA levels and mean relative apoAI synthesis were 49 and 37% lower, respectively, in homozygotes for the mutant allele and 28 and 41% lower, respectively, in heterozygotes than in homozygotes for the wild-type allele (P < 0.05 for both). Site-directed mutants of apoAI gene promoter/reporter constructs containing the above mutations were transfected into Caco-2 cells and showed a 46% decrease in transcriptional activity compared with the wild type (P < 0.001); however, no significant differences were observed in HepG2 cells. Electrophoretic mobility shift assays showed that the mutated sequences from -655 to -610 bound Caco-2 cell nuclear protein(s) while the wild type did not. These results indicate that intestinal apoAI gene transcription and protein synthesis are genetically determined and are reduced in the presence of common mutations which induced binding of nuclear protein(s), possibly a transcriptional repressor.

Effect of a high or a low fat diet on cardiovascular risk factors in male and female runners

Dietary fat may be associated with coronary heart disease (CHD). Studies suggest that restricting fat intake may compromise endurance performance and that increasing fat intake may improve endurance performance. We studied the effects of varying dietary fat intake on CHD risk factors in runners. Twelve male and 13 female runners increased fat from 16% to 30% of daily calories (4 wk each). Of this group, six males and six females increased fat to 42% of daily calories (4 wk). Physiological and lipoprotein risk factors were measured after each diet. Results were analyzed by repeated measures ANOVA. Increasing dietary fat from 16% to 42% of daily calories did not change adiposity, weight, heart rate, blood pressure, serum triglycerides, total cholesterol, LDL cholesterol. Apolipoprotein B, or the Apo A1/Apo B ratio. Compared with those eating higher fat, subjects eating 16% fat had lower HDL cholesterol (50 +/- 3 vs 62 +/- 3 mg.dl-1, P < 0.0001) and Apolipoprotein A1 (111 +/- 6 v. 134 +/- 6 mg/dl, P < 0.0005) and a higher TC/HDL-C ratio (4.05 +/- 0.27 vs 3.42 +/- 0.24, P < 0.0005). Runners who increased fat intake to 42% further raised HDL cholesterol (64 +/- 6 to 69 +/- 5 mg.dl-1, P < 0.04) without adversely affecting other lipoproteins. In conclusion, a 42% fat diet maintained favorable CHD risk factors in female and male runners whereas a 16% fat diet lowered Apo A1 and HDL-C and raised the TC/HDL-C ratio.