Decreased susceptibility to diet-induced atherosclerosis in human apolipoprotein A-II transgenic mice

Interaction between Apo A-II -265T>C polymorphism and dietary total antioxidant capacity on some anthropometric indices and serum lipid profile in patients with type 2 diabetes mellitus

The present study aimed to investigate the interaction of Apo A-II polymorphism and dietary total antioxidant capacity (DTAC) with lipid profile and anthropometric markers in patients with type 2 diabetes (T2DM) that are at risk for atherosclerosis. This cross-sectional study was conducted on 778 patients with T2DM (35–65 years). Dietary intakes were assessed by a 147-item food frequency questionnaire. DTAC was computed using international databases. Participants were categorised into two groups based on rs5082 genotypes. The gene–diet interaction was analysed by an ANCOVA multivariate interaction model. Total cholesterol, TC; triacylglycerol, TG; high- and low-density lipoprotein, HDL and LDL; TC–HDL ratio; waist circumference, WC and body mass index, BMI were obtained according to standard protocols. Overall, the frequency of CC homozygous was 12⋅1 % among study participants. We found that a significant interaction between rs5082 variants and DTAC on mean WC (P_TEAC = 0⋅044), TC concentration (P_FRAP = 0⋅049 and P_TEAC = 0⋅031) and TC/HDL (P_FRAP = 0⋅031 and P_TRAP = 0⋅040). Among patients whose DTAC was higher than the median intake, the mean of weight, WC and TC/HDL were significantly higher only in individuals with CC genotype. Also, the high DTAC was associated with a lower TC concentration only in T-allele carriers (P_FRAP = 0⋅042). We found that adherence to a diet with high total antioxidant capacity can improve the complications of diabetes and atherosclerosis in the T carrier genotype more effectively than the CC genotype. These results could indicate the anti-atherogenic properties of Apo A-II. However, further studies are needed to shed light on this issue.

Human apolipoprotein A-II reduces atherosclerosis in knock-in rabbits

BACKGROUND AND AIMS

Apolipoprotein A-II (apoAII) is the second major apolipoprotein of the high-density lipoprotein (HDL) particle, after apoAI. Unlike apoAI, the biological and physiological functions of apoAII are unclear. We aimed to gain insight into the specific roles of apoAII in lipoprotein metabolism and atherosclerosis using a novel rabbit model.

METHODS

Wild-type (WT) rabbits are naturally deficient in apoAII, thus their HDL contains only apoAI. Using TALEN technology, we replaced the endogenous apoAI in rabbits through knock-in (KI) of human apoAII. The newly generated apoAII KI rabbits were used to study the specific function of apoAII, independent of apoAI.

RESULTS

ApoAII KI rabbits expressed exclusively apoAII without apoAI, as confirmed by RT-PCR and Western blotting. On a standard diet, the KI rabbits exhibited lower plasma triglycerides (TG, 52%, p < 0.01) due to accelerated clearance of TG-rich particles and higher lipoprotein lipase activity than the WT littermates. ApoAII KI rabbits also had higher plasma HDL-C (28%, p < 0.05) and their HDL was rich in apoE, apoAIV, and apoAV. When fed a cholesterol-rich diet for 16 weeks, apoAII KI rabbits were resistant to diet-induced hypertriglyceridemia and developed significantly less aortic atherosclerosis compared to WT rabbits. HDL isolated from rabbits with apoAII KI had similar cholesterol efflux capacity and anti-inflammatory effects as HDL isolated from the WT rabbits.

CONCLUSIONS

ApoAII KI rabbits developed less atherosclerosis than WT rabbits, possibly through increased plasma HDL-C, reduced TG and atherogenic lipoproteins. These results suggest that apoAII may serve as a potential target for the treatment of atherosclerosis.

Proteins Involved in the Induction of Procoagulant Activity and Autoimmune Response in Patients With Primary Antiphospholipid Syndrome

The aim of this study was to determine the plasma protein profile of patients with primary antiphospholipid syndrome (PAPS) compared to healthy controls and identify proteins that might be used in the evaluation, diagnosis, and prognosis of this condition. The sample consisted of 14 patients with PAPS and 17 sex- and age-matched controls. Plasma samples were submitted to proteomic analysis (albumin and immunoglobulin G depletion, concentration, digestion, and label-free data-independent mass spectrometry). The software ExpressionE was used to quantify intergroup differences in protein expression. The analysis yielded 65 plasma proteins of which 11 were differentially expressed (9 upregulated and 2 downregulated) in relation to controls. Four of these are known to play a role in pathophysiological mechanisms of thrombosis: fibrinogen α chain, fibrinogen α chain, apolipoprotein C-III, and α-1-glycoprotein-1. Our analysis revealed autoimmune response and the presence of proteins believed to be functionally involved in the induction of procoagulant activity in patients with PAPS. Further studies are necessary to confirm our findings and may eventually lead to the development of significantly more accurate diagnostic tools.

APOA II genotypes frequency and their interaction with saturated fatty acids consumption on lipid profile of patients with type 2 diabetes

BACKGROUND & AIM

Several studies have suggested that APOA II-265T/C polymorphism affect lipid profile. The aim of this study was to investigate the effect of -265T/C APOA II polymorphism and saturated fatty acids (SFA) intake interaction on lipid profile in diabetic population who are at risk for lipid disorders.

METHODS

In this cross sectional study, 697 type 2 diabetic patients participated. Food consumption data were collected using validated semi-quantitative FFQ during the last year. Realtime-PCR was used to determine APOA II-265T/C genotypes. The interaction between the genotypes and SFA intake with lipid profile was tested using analysis of covariance (ANCOVA).

RESULTS

According to APOA II-265T/C (rs5082) genotype distribution results, CC genotype with a frequency of 12.9% and TC with that of 47.7% showed the lowest and highest frequency in our population, respectively. CC genotype subjects had significantly lower total cholesterol, triglyceride, Cholesterol/HDL-c ratio and non-HDL cholesterol than T allele carriers (p = 0.009, p = 0.02, p = 0.02 and p = 0.002, respectively). The interaction between genotype and SFA intake contributed to significant higher levels of LDL-c and LDL/HDL in CCs (p = 0.05 and p = 0.01), suggesting vulnerability of these individuals to high intake of SFA in the diet.

CONCLUSION

APOA II polymorphism may influence the saturated fatty acid intake required to prevent dyslipidemia in the type 2 diabetic population.

Setting the course for apoAII: a port in sight?

ApoAII, the second most abundant protein of the human plasma HDLs, was discovered nearly 50 years ago. Over the subsequent years, nearly 2000 studies - epidemiological, cell-based, biochemical, mouse and human - have attempted to unravel its role in human lipid metabolism. On the basis of these studies, apoAII has been described as an activator and inhibitor of various plasma activities, and as both pro- and anti-atherogenic. Here, we summarize the studies of apoAII, use the preponderance of evidence to propose that the apoAII compass can be reset towards an antiatherogenic course, and suggest ways to stay the course.

Expression and purification of recombinant human apolipoprotein A-II in Pichia pastoris

Apolipoprotein A-II (ApoA-II) is the second most abundant protein constituent of high-density lipoprotein (HDL). The physiologic role of ApoA-II is poorly defined. ApoA-II may inhibit lecithin:cholesterol acyltransferase and cholesteryl-ester-transfer protein activities, but may increase the hepatic lipase activity. ApoA-II may also inhibit the hepatic cholesteryl uptake from HDL probably through the scavenger receptor class B type I depending pathway. Interpretation of data from transgenic and knockout mice of genes involved in lipoprotein metabolism has been often complicated as clinical implications because of species difference. So it is important to obtain human ApoA-II for further studies about its functions. In our studies, Pichia pastoris expression system was first used to express a high-level secreted recombinant human ApoA-II (rhApoA-II). We have cloned the cDNA encoding human ApoA-II and achieved its high-level secreting expression with a yield of 65 mg/L of yeast culture and the purification process was effective and easy to handle. The purified rhApoA-II can be used to further study its biological activities.

Human apolipoprotein A-II protects against diet-induced atherosclerosis in transgenic rabbits

OBJECTIVE

Apolipoprotein (apo) A-II is the second major apo of high-density lipoproteins, yet its pathophysiological roles in the development of atherosclerosis remain unknown. We aimed to examine whether apo A-II plays any role in atherogenesis and, if so, to elucidate the mechanism involved.

METHODS AND RESULTS

We compared the susceptibility of human apo A-II transgenic (Tg) rabbits to cholesterol diet-induced atherosclerosis with non-Tg littermate rabbits. Tg rabbits developed significantly less aortic and coronary atherosclerosis than their non-Tg littermates, while total plasma cholesterol levels were similar. Atherosclerotic lesions of Tg rabbits were characterized by reduced macrophages and smooth muscle cells, and apo A-II immunoreactive proteins were frequently detected in the lesions. Tg rabbits exhibited low levels of plasma C-reactive protein and blood leukocytes compared with non-Tg rabbits, and high-density lipoproteins of Tg rabbit plasma exerted stronger cholesterol efflux activity and inhibitory effects on the inflammatory cytokine expression by macrophages in vitro than high-density lipoproteins isolated from non-Tg rabbits. In addition, β-very-low-density lipoproteins of Tg rabbits were less sensitive to copper-induced oxidation than β-very-low-density lipoproteins of non-Tg rabbits.

CONCLUSIONS

These results suggest that enrichment of apo A-II in high-density lipoprotein particles has atheroprotective effects and apo A-II may become a target for the treatment of atherosclerosis.

Apolipoprotein A-II: evaluating its significance in dyslipidaemia, insulin resistance, and atherosclerosis

Reduced HDL cholesterol, commonly found in subjects with obesity and type 2 diabetes, is associated with increased risk of cardiovascular disease (CVD). ApoA-II, a constituent apolipoprotein of certain HDL particles, plays an important role in the regulation of cholesterol efflux, HDL remodelling, and cholesteryl ester uptake via its interactions with lipid transfer proteins, lipases, and cellular HDL receptors. Recent studies have linked apoA-II directly with triglyceride and glucose metabolism. Most of the data are, however, derived from cellular systems and transgenic animal models. Direct evidence from human studies is scarce. Clinical studies demonstrate that apoA-II is a strong predictor of risk for CVD. There is no evidence, however, that selective therapeutic modification of apoA-II impacts on atherosclerosis and clinical outcomes. More research is required to investigate further the significance of apoA-II in clinical medicine.

Gene-diet interactions in childhood obesity

Childhood overweight and obesity have reached epidemic proportions worldwide, and the increase in weight-associated co-morbidities including premature type 2 diabetes mellitus (T2DM) and atherosclerotic cardiovascular disease will soon become major healthcare and economic problems. A number of studies now indicate that the childhood obesity epidemic which has emerged during the past 30 years is a complex multi-factorial disease resulting from interaction of susceptibility genes with an obesogenic environment. This review will focus on gene-diet interactions suspected of having a prominent role in promoting childhood obesity. In particular, the specific genes that will be presented (FTO, MC4R, and NPC1) have recently been associated with childhood obesity through a genome-wide association study (GWAS) and were shown to interact with nutritional components to increase weight gain. Although a fourth gene (APOA2) has not yet been associated with childhood obesity, this review will also present information on what now represents the best characterized gene-diet interaction in promoting weight gain.

Prospective studies on the relationship between high-density lipoprotein cholesterol and cardiovascular risk: a systematic review

Epidemiological studies have extensively evaluated the association between high-density lipoprotein cholesterol (HDL-C) and cardiovascular disease (CVD) risk. The objective of this systematic review was to enumerate the number of original prospective studies that showed a significant association between HDL-C and CVD risk and provided evidence of the consistency of this association across other lipid risk factors. A systematic MEDLINE literature search identified 53 prospective cohort and five nested case-control studies that provided multivariate assessments of the association between HDL-C and CVD risk. Among these 58 prospective studies, 31 studies found a significant inverse association between HDL-C and CVD risk for all CVD outcomes and subpopulations studied, whereas 17 studies found a significant association for some CVD outcomes and/or subpopulations assessed. The ratio of studies that found a significant association out of the total studies identified was similar across all CVD outcomes, although there was less evidence for stroke and atherosclerotic outcomes. Only seven studies tested for the consistency of this association across other lipid risk factors, of which six studies suggested that the association was consistent across other lipid levels. In conclusion, the association between HDL-C and CVD risk is significant and strong, although further evidence may be needed to establish whether this association is consistent across other lipid risk factors. Furthermore, uncertainties remain regarding the mechanism in which HDL-C exerts its effects, suggesting a need for further research focused on new methods for reliable measurement.

Rosuvastatin selectively stimulates apolipoprotein A-I but not apolipoprotein A-II synthesis in Hep G2 cells

Hydroxymethylglutaryl-coenzyme A reductase inhibitors (statins) are extensively used to regulate dyslipidemia and to reduce atherosclerotic cardiovascular disease. In addition to effectively lowering cholesterol and low-density lipoprotein levels, rosuvastatin and certain other statins can also increase plasma high-density lipoprotein (HDL) cholesterol modestly. However, the mechanism of action of rosuvastatin on HDL metabolic processes is not understood. Using cultured human hepatoblastoma cells (Hep G2) as an in vitro model system, we assessed the effect of rosuvastatin on apolipoprotein (apo) A-I and apo A-II (the major proteins of HDL) synthesis and HDL catabolic processes. Rosuvastatin dose-dependently increased messenger RNA expression and de novo synthesis of apo A-I but not apo A-II. Rosuvastatin selectively increased the synthesis of HDL particles containing only apo A-I (LP A-I) but not particles containing both apo A-I and A-II (LP A-I + A-II). The HDL(3)-protein or HDL(3)-cholesterol ester uptake by Hep G2 cells was not affected by rosuvastatin. The apo A-I-containing particles secreted by rosuvastatin-treated Hep G2 significantly increased cholesterol efflux from fibroblasts. The data indicate that rosuvastatin increases hepatic apo A-I but not apo A-II messenger RNA transcription, thereby selectively increasing the synthesis of functionally active apo A-I-containing HDL particles, which mediate cholesterol efflux from peripheral tissues. We suggest that this mechanism of action of rosuvastatin to increase apo A-I production without apo A-I/HDL removal may result in increased apo A-I turnover that results in accelerated reverse cholesterol transport.

Absence of stearoyl-CoA desaturase-1 ameliorates features of the metabolic syndrome in LDLR-deficient mice

A combination of the interrelated metabolic risk factors obesity, insulin resistance, dyslipidemia, and hypertension, often described as the "metabolic syndrome," is known to increase the risk of developing cardiovascular disease and diabetes. Stearoyl-coenzyme A desaturase (SCD) activity has been implicated in the metabolic syndrome, but detailed studies of the beneficial metabolic effects of SCD deficiency have been limited. Here, we show that absence of the Scd1 gene product reduces plasma triglycerides and reduces weight gain in severely hyperlipidemic low density lipoprotein receptor (LDLR)-deficient mice challenged with a Western diet. Absence of SCD1 also increases insulin sensitivity, as measured by intraperitoneal glucose and insulin tolerance testing. SCD1 deficiency dramatically reduces hepatic lipid accumulation while causing more modest reductions in plasma apolipoproteins, suggesting that in conditions of sustained hyperlipidemia, SCD1 functions primarily to mediate lipid stores. In addition, absence of SCD1 partially ameliorates the undesirable hypertriglyceridemic effect of antiatherogenic liver X receptor agonists. Our results demonstrate that constitutive reduction of SCD activity improves the metabolic phenotype of LDLR-deficient mice on a Western diet.

The -256T>C polymorphism in the apolipoprotein A-II gene promoter is associated with body mass index and food intake in the genetics of lipid lowering drugs and diet network study

BACKGROUND

Apolipoprotein A-II (APOA2) plays an ambiguous role in lipid metabolism, obesity, and atherosclerosis.

METHODS

We studied the association between a functional APOA2 promoter polymorphism (-265T>C) and plasma lipids (fasting and postprandial), anthropometric variables, and food intake in 514 men and 564 women who participated in the Genetics of Lipid Lowering Drugs and Diet Network (GOLDN) study. We obtained fasting and postprandial (after consuming a high-fat meal) measures. We measured lipoprotein particle concentrations by proton nuclear magnetic resonance spectroscopy and estimated dietary intake by use of a validated questionnaire.

RESULTS

We observed recessive effects for this polymorphism that were homogeneous by sex. Individuals homozygous for the -265C allele had statistically higher body mass index (BMI) than did carriers of the T allele. Consistently, after multivariate adjustment, the odds ratio for obesity in CC individuals compared with T allele carriers was 1.70 (95% CI 1.02-2.80, P = 0.039). Interestingly, total energy intake in CC individuals was statistically higher [mean (SE) 9371 (497) vs 8456 (413) kJ/d, P = 0.005] than in T allele carriers. Likewise, total fat and protein intakes (expressed in grams per day) were statistically higher in CC individuals (P = 0.002 and P = 0.005, respectively). After adjustment for energy, percentage of carbohydrate intake was statistically lower in CC individuals. These associations remained statistically significant even after adjustment for BMI. We found no associations with fasting lipids and only some associations with HDL subfraction distribution in the postprandial state.

CONCLUSIONS

The -265T>C polymorphism is consistently associated with food consumption and obesity, suggesting a new role for APOA2 in regulating dietary intake.

Paradoxical exacerbation of combined hyperlipidemia in human apolipoprotein A-II transgenic mice treated with fenofibrate

Apolipoprotein (apo) A-II has been biochemically and genetically linked to familial combined hyperlipidemia. Human ApoA-II transgenic mice and peroxisome proliferator-activated receptor alpha (PPARalpha)-deficient mice share some similar phenotypic characteristics. The aim of this study was to determine whether a fibrate-induced PPARalpha activation corrects the combined hyperlipidemia present in human apoA-II transgenic mice. ApoA-II transgenic mice were treated with fenofibrate (250 mg/kg) for 13 days. After this period, they presented a remarkable 8-fold increase in plasma triglycerides. This was concomitant with a 4-fold increase in non-high-density lipoprotein (non-HDL) cholesterol, a quantitatively similar decrease in HDL cholesterol and a severe reduction in mouse plasma apoA-I and apoA-II. Fenofibrate stimulated liver fatty acid beta-oxidation, increased the transcriptional expression of carnitine palmitoyltransferase 1 and phospholipid transfer protein, and decreased expression of apoA-I and apoC-III. However, very-low-density lipoprotein (VLDL)-triglyceride production and lipoprotein lipase (LPL) activities and the expression of other PPARalpha target genes were similar in mice treated with vehicle and fenofibrate. Further, fenofibrate-treated mice presented decreased in vivo [3H]VLDL catabolism and decreased VLDL-triglyceride hydrolysis by exogenous LPL. Therefore, the paradoxical enhancement of hyperlipidemia in fenofibrate-treated apoA-II transgenic mice is mainly due to decreased VLDL catabolism and, also, to a partial impairment in PPARalpha-signaling.

Fibrates

Atherosclerosis of the large arteries is the main origin of cerebro- and cardiovascular diseases, the leading causes of mortality and morbidity in industrialized countries. The pathophysiology of coronary and cerebrovascular atherosclerosis is multifactorial and complex. Fibrates are hypolipidemic drugs that lower progression of atherosclerotic lesions mainly through activation of the nuclear receptor peroxisome-proliferator activated receptor-alpha. In addition, fibrates exert pleiotropic and anti-inflammatory actions. In this chapter, we will focus on the different effects of fibrates impacting on the development of atherosclerosis.

Mechanisms mediating insulin resistance in transgenic mice overexpressing mouse apolipoprotein A-II

We previously demonstrated that transgenic mice overexpressing mouse apolipoprotein A-II (apoA-II) exhibit several traits associated with the insulin resistance (IR) syndrome, including increased atherosclerosis, hypertriglyceridemia, obesity, and IR. The skeletal muscle appeared to be the insulin-resistant tissue in the apoA-II transgenic mice. We now demonstrate a decrease in FA oxidation in skeletal muscle of apoA-II transgenic mice, consistent with reports that decreased skeletal muscle FA oxidation is associated with increased skeletal muscle triglyceride accumulation, skeletal muscle IR, and obesity. The decrease in FA oxidation is not due to decreased carnitine palmitoyltransferase 1 activity, because oxidation of palmitate and octanoate were similarly decreased. Quantitative RT-PCR analysis of gene expression demonstrated that the decrease in FA oxidation may be explained by a decrease in medium chain acyl-CoA dehydrogenase. We previously demonstrated that HDLs from apoA-II transgenic mice exhibit reduced binding to CD36, a scavenger receptor involved in FA metabolism. However, studies of combined apoA-II transgenic and CD36 knockout mice suggest that the major effects of apoA-II are independent of CD36. Rosiglitazone treatment significantly ameliorated IR in the apoA-II transgenic mice, suggesting that the underlying mechanisms of IR in this animal model may share common features with certain types of human IR.

ApoA-II modulates the association of HDL with class B scavenger receptors SR-BI and CD36

The class B scavenger receptors SR-BI and CD36 exhibit a broad ligand binding specificity. SR-BI is well characterized as a HDL receptor that mediates selective cholesteryl ester uptake from HDL. CD36, a receptor for oxidized LDL, also binds HDL and mediates selective cholesteryl ester uptake, although much less efficiently than SR-BI. Apolipoprotein A-II (apoA-II), the second most abundant HDL protein, is considered to be proatherogenic, but the underlying mechanisms are unclear. We previously showed that apoA-II modulates SR-BI-dependent binding and selective uptake of cholesteryl ester from reconstituted HDL. To investigate the effect of apoA-II in naturally occurring HDL on these processes, we compared HDL without apoA-II (from apoA-II null mice) with HDLs containing differing amounts of apoA-II (from C57BL/6 mice and transgenic mice expressing a mouse apoA-II transgene). The level of apoA-II in HDL was inversely correlated with HDL binding and selective cholesteryl ester uptake by both scavenger receptors, particularly CD36. Interestingly, for HDL lacking apoA-II, the efficiency with which CD36 mediated selective uptake reached a level similar to that of SR-BI. These results demonstrate that apoA-II exerts a marked effect on HDL binding and selective lipid uptake by the class B scavenger receptors and establishes a potentially important relationship between apoA-II and CD36.

Low high-density lipoprotein cholesterol: physiological background, clinical importance and drug treatment

Low high-density lipoprotein (HDL) cholesterol is an important risk factor for coronary heart disease (CHD). In vitro, HDL exerts several potentially anti-atherogenic activities. HDLs mediate the reverse cholesterol transport (RCT) from peripheral cells to the liver, inhibit oxidation of low-density lipoprotein (LDL), adhesion of monocytes to the endothelium, apoptosis of vascular endothelial and smooth muscle cells and platelet activation, and stimulate the endothelial secretion of vasoactive substances as well as smooth muscle cell proliferation. Hence, raising HDL-cholesterol levels has become an interesting target for anti-atherosclerotic drug therapy. Levels of HDL cholesterol and the composition of HDL subclasses in plasma are regulated by apolipoproteins, lipolytic enzymes, lipid transfer proteins, receptors and cellular transporters. The interplay of these factors leads to RCT and determines the composition and, thereby, the anti-atherogenic properties of HDL. Several inborn errors of metabolism, as well as genetic animal models, are characterised by both elevated HDL cholesterol and increased rather than decreased cardiovascular risk. These findings suggest that the mechanism of HDL modification rather than simply increasing HDL cholesterol determine the efficacy of anti-atherosclerotic drug therapy. In several controlled and prospective intervention studies, patients with low HDL cholesterol and additional risk factors benefited from treatment with fibric acid derivatives (fibrates) or HMG-CoA reductase inhibitors (statins). However, only in some trials was prevention of coronary events in patients with low HDL cholesterol and hypertriglyceridaemia related to an increase in HDL cholesterol. We discuss the clinical and metabolic effects of fibrates, statins, nicotinic acid and sex steroids, and present novel therapeutic strategies that show promise in modifying HDL metabolism. In conclusion, HDL-cholesterol levels increase only moderately after treatment with currently available drugs and do not necessarily correlate with the functionality of HDL. Therefore, the anti-atherosclerotic therapy of high-risk cardiovascular patients should currently be focused on the correction of other risk factors present besides low HDL cholesterol. However, modification of HDL metabolism and improvement of RCT remain an attractive target for the development of new regimens of anti-atherogenic drug therapy.

Functional specificity of two hormone response elements present on the human apoA-II promoter that bind retinoid X receptor alpha/thyroid receptor beta heterodimers for retinoids and thyroids: synergistic interactions between thyroid receptor beta and upstream stimulatory factor 2a

DNA binding and mutagenesis in vitro established that the -67/-55 region of the apoA-II (apolipoprotein A-II) promoter contains a thyroid HRE (hormone response element), which strongly binds RXRalpha (retinoid X receptor alpha)/T(3)Rbeta (thyroid receptor beta) heterodimers and weakly T(3)Rbeta homodimers, but does not bind other homo- or heterodimers of RXRalpha or orphan nuclear receptors. Transactivation was abolished by point mutations in the thyroid HRE. In co-transfection experiments of HEK-293 (human embryonic kidney 293) cells, the -911/+29 human apoA-II promoter was transactivated strongly by RXRalpha/T(3)Rbeta heterodimers in the presence of RA (9- cis retinoic acid) or T(3) (tri-iodothyronine). Homopolymeric promoters containing either three copies of the -73/-40 (element AIIAB) or four copies of the -738/-712 (element AIIJ) apoA-II promoter could be transactivated by RXRalpha/T(3)Rbeta heterodimers in COS-7 cells only in the presence of T(3) or RA respectively. RXRalpha/T(3)Rbeta heterodimers and USF2a (upstream stimulatory factor 2a) synergistically transactivated the -911/+29 apoA-II promoter in the presence of T(3). USF2a also enhanced the activity of a GAL4-T(3)Rbeta fusion protein in the presence of T(3) and suppressed the activity of a GAL4-RXRalpha fusion protein in the presence of RA. These findings suggest a functional specificity of the two HREs of the apoA-II promoter for retinoids and thyroids, which is modulated by synergistic or antagonistic interactions between RXRalpha/T(3)Rbeta heterodimers and the ubiquitous transcription factor USF2a.

High density lipoproteins (HDLs) and atherosclerosis; the unanswered questions

The concentration of high density lipoprotein-cholesterol (HDL-C) has been found consistently to be a powerful negative predictor of premature coronary heart disease (CHD) in human prospective population studies. There is also circumstantial evidence from human intervention studies and direct evidence from animal intervention studies that HDLs protect against the development of atherosclerosis. HDLs have several documented functions, although the precise mechanism by which they prevent atherosclerosis remains uncertain. Nor is it known whether the cardioprotective properties of HDL are specific to one or more of the many HDL subpopulations that comprise the HDL fraction in human plasma. Several lifestyle and pharmacological interventions have the capacity to raise the level of HDL-C, although it is not known whether all are equally protective. Indeed, despite the large body of information identifying HDLs as potential therapeutic targets for the prevention of atherosclerosis, there remain many unanswered questions that must be addressed as a matter of urgency before embarking wholesale on HDL-C-raising therapies as strategies to prevent CHD. This review summarises what is known and highlights what we still need to know.

Apolipoprotein A-II: beyond genetic associations with lipid disorders and insulin resistance

PURPOSE OF REVIEW

Apolipoprotein A-II, the second major HDL apolipoprotein, was often considered of minor importance relatively to apolipoprotein A-I and its role was controversial. This picture is now rapidly changing, due to novel polymorphisms and mutations, to the outcome of clinical trials, and to studies with transgenic mice.

RECENT FINDINGS

The -265 T/C polymorphism supports a role for apolipoprotein A-II in postprandial very-low-density lipoprotein metabolism. Fibrates, which increase apolipoprotein A-II synthesis, significantly decrease the incidence of major coronary artery disease events, particularly in subjects with low HDL cholesterol, high plasma triglyceride, and high body weight. The comparison of transgenic mice overexpressing human or murine apolipoprotein A-II has highlighted major structural differences between the two proteins; they have opposite effects on HDL size, apolipoprotein A-I content, plasma concentration, and protection from oxidation. Human apolipoprotein A-II is more hydrophobic, displaces apolipoprotein A-I from HDL, accelerates apolipoprotein A-I catabolism, and its plasma concentration is decreased by fasting. Apolipoprotein A-II stimulates ATP binding cassette transporter 1-mediated cholesterol efflux. Human and murine apolipoprotein A-II differently affect glucose metabolism and insulin resistance. A novel beneficial role for apolipoprotein A-II in the pathogenesis of hepatitis C virus has been shown.

SUMMARY

The hydrophobicity of human apolipoprotein A-II is a key regulatory factor of HDL metabolism. Due to the lower plasma apolipoprotein A-II concentration during fasting, measurements of apolipoprotein A-II in fed subjects are more relevant. More clinical studies are necessary to clarify the role of apolipoprotein A-II in well-characterized subsets of patients and in the insulin resistance syndrome.

Apolipoprotein A-II, HDL metabolism and atherosclerosis

Apolipoprotein (Apo) A-I and apo A-II are the major apolipoproteins of HDL. It is clearly demonstrated that there are inverse relationships between HDL-cholesterol and apo A-I plasma levels and the risk of coronary heart disease (CHD) in the general population. On the other hand, it is still not clearly demonstrated whether apo A-II plasma levels are associated with CHD risk. A recent prospective epidemiological (PRIME) study suggests that Lp A-I (HDL containing apo A-I but not apo A-II) and Lp A-I:A-II (HDL containing apo A-I and apo A-II) were both reduced in survivors of myocardial infarction, suggesting that both particles are risk markers of CHD. Apo A-II and Lp A-I:A-II plasma levels should be rather related to apo A-II production rate than to apo A-II catabolism. Mice transgenic for both human apo A-I and apo A-II are less protected against atherosclerosis development than mice transgenic for human apo A-I only, but the results of the effects of trangenesis of human apo A-II (in the absence of a co-transgenesis of human apo A-I) are controversial. It is highly suggested that HDL reduce CHD risk by promoting the transfer of peripherical free cholesterol to the liver through the so-called 'reverse cholesterol transfer'. Apo A-II modulates different steps of HDL metabolism and therefore probably alters reverse cholesterol transport. Nevertheless, some effects of apo A-II on intermediate HDL metabolism might improve reverse cholesterol transport and might reduce atherosclerosis development while some other effects might be deleterious. In different in vitro models of cell cultures, Lp A-I:A-II induce either a lower or a similar cellular cholesterol efflux (the first step of reverse cholesterol transport) than Lp A-I. Results depend on numerous factors such as cultured cell types and experimental conditions. Furthermore, the effects of apo A-II on HDL metabolism, beyond cellular cholesterol efflux, are also complex and controversial: apo A-II may inhibit lecithin-cholesterol acyltransferase (LCAT) (potential deleterious effect) and cholesteryl-ester-transfer protein (CETP) (potential beneficial effect) activities, but may increase the hepatic lipase (HL) activity (potential beneficial effect). Apo A-II may also inhibit the hepatic cholesteryl uptake from HDL (potential deleterious effect) probably through the SR-BI depending pathway. Therefore, in terms of atherogenesis, apo A-II alters the intermediate HDL metabolism in opposing ways by increasing (LCAT, SR-BI) or decreasing (HL, CETP) the atherogenicity of lipid metabolism. Effects of apo A-II on atherogenesis are controversial in humans and in transgenic animals and probably depend on the complex effects of apo A-II on these different intermediate metabolic steps which are in weak equilibrium with each other and which can be modified by both endogenous and environmental factors. It can be suggested that apo A-II is not a strong determinant of lipid metabolism, but is rather a modulator of reverse cholesterol transport.

Increased ABCA1 activity protects against atherosclerosis

The ABC transporter ABCA1 plays a key role in the first steps of the reverse cholesterol transport pathway by mediating lipid efflux from macrophages. Previously, it was demonstrated that human ABCA1 overexpression in vivo in transgenic mice results in a mild elevation of plasma HDL levels and increased efflux of cholesterol from macrophages. In this study, we determined the effect of overexpression of ABCA1 on atherosclerosis development. Human ABCA1 transgenic mice (BAC(+)) were crossed with ApoE(-/-) mice, a strain that spontaneously develop atherosclerotic lesions. BAC(+)ApoE(-/-) mice developed dramatically smaller, less-complex lesions as compared with their ApoE(-/-) counterparts. In addition, there was increased efflux of cholesterol from macrophages isolated from the BAC(+)ApoE(-/-) mice. Although the increase in plasma HDL cholesterol levels was small, HDL particles from BAC(+)ApoE(-/-) mice were significantly better acceptors of cholesterol. Lipid analysis of HDL particles from BAC(+)ApoE(-/-) mice revealed an increase in phospholipid levels, which was correlated significantly with their ability to enhance cholesterol efflux.

Improved lipid and lipoprotein profile, hepatic insulin sensitivity, and glucose tolerance in 11beta-hydroxysteroid dehydrogenase type 1 null mice

Excess tissue glucocorticoid action may underlie the dyslipidemia, insulin resistance, and impaired glucose tolerance of the metabolic syndrome. 11beta-Hydroxysteroid dehydrogenase type 1 (11beta-HSD-1) catalyzes conversion of circulating inert 11-dehydrocorticosterone into active corticosterone, thus amplifying local intracellular glucocorticoid action, particularly in liver. The importance of 11beta-HSD-1 in glucose homeostasis is suggested by the resistance of 11beta-HSD-1(-/-) mice to hyperglycemia upon stress or obesity, due to attenuated gluconeogenic responses. The present study further investigates the metabolic consequences of 11beta-HSD-1 deficiency, focusing on the lipid and lipoprotein profile. Ad lib fed 11beta-HSD-1(-/-) mice have markedly lower plasma triglyceride levels. This appears to be driven by increased hepatic expression of enzymes of fat catabolism (carnitine palmitoyltransferase-I, acyl-CoA oxidase, and uncoupling protein-2) and their coordinating transcription factor, peroxisome proliferator-activated receptor-alpha (PPARalpha). 11beta-HSD-1(-/-) mice also have increased HDL cholesterol, with elevated liver mRNA and serum levels of apolipoprotein AI. Conversely, liver Aalpha-fibrinogen mRNA levels are decreased. Upon fasting, the normal elevation of peroxisome proliferator-activated receptor-alpha mRNA is lost in 11beta-HSD-1(-/-) mice, consistent with attenuated glucocorticoid induction. Despite this, crucial oxidative responses to fasting are maintained; carnitine palmitoyltransferase-I induction and glucose levels are similar to wild type. Refeeding shows exaggerated induction of genes encoding lipogenic enzymes and a more marked suppression of genes for fat catabolism in 11beta-HSD-1(-/-) mice, implying increased liver insulin sensitivity. Concordant with this, 24-h refed 11beta-HSD-1(-/-) mice have higher triglyceride but lower glucose levels. Further, 11beta-HSD-1(-/-) mice have improved glucose tolerance. These data suggest that 11beta-HSD-1 deficiency produces an improved lipid profile, hepatic insulin sensitization, and a potentially atheroprotective phenotype.

Cholesteryl ester transfer protein, high density lipoprotein and arterial disease

The reported relationships between cholesteryl ester transfer protein, high density lipoproteins and arterial disease are confusing and conflicting. Several papers published during the review period add substantially to the evidence base regarding the atherogenicity (or anti-atherogenicity) of cholesteryl ester transfer protein, although none clearly resolves the continuing conflict. These new papers are presented against the backdrop of the previous state of knowledge.

Apolipoprotein A-II modulates the binding and selective lipid uptake of reconstituted high density lipoprotein by scavenger receptor BI

High density lipoprotein (HDL) represents a mixture of particles containing either apoA-I and apoA-II (LpA-I/A-II) or apoA-I without apoA-II (LpA-I). Differences in the function and metabolism of LpA-I and LpA-I/A-II have been reported, and studies in transgenic mice have suggested that apoA-II is pro-atherogenic in contrast to anti-atherogenic apoA-I. The molecular basis for these observations is unclear. The scavenger receptor BI (SR-BI) is an HDL receptor that plays a key role in HDL metabolism. In this study we investigated the abilities of apoA-I and apoA-II to mediate SR-BI-specific binding and selective uptake of cholesterol ester using reconstituted HDLs (rHDLs) that were homogeneous in size and apolipoprotein content. Particles were labeled in the protein (with (125)I) and in the lipid (with [(3)H]cholesterol ether) components and SR-BI-specific events were analyzed in SR-BI-transfected Chinese hamster ovary cells. At 1 microg/ml apolipoprotein, SR-BI-mediated cell association of palmitoyloleoylphosphatidylcholine-containing AI-rHDL was significantly greater (3-fold) than that of AI/AII-rHDL, with a lower K(d) and a higher B(max) for AI-rHDL as compared with AI/AII-rHDL. Unexpectedly, selective cholesterol ester uptake from AI/AII-rHDL was not compromised compared with AI-rHDL, despite decreased binding. The efficiency of selective cholesterol ester uptake in terms of SR-BI-associated rHDL was 4-5-fold greater for AI/AII-rHDL than AI-rHDL. These results are consistent with a two-step mechanism in which SR-BI binds ligand and then mediates selective cholesterol ester uptake with an efficiency dependent on the composition of the ligand. ApoA-II decreases binding but increases selective uptake. These findings show that apoA-II can exert a significant influence on selective cholesterol ester uptake by SR-BI and may consequently influence the metabolism and function of HDL, as well as the pathway of reverse cholesterol transport.

Prevention of coronary heart disease by raising high-density lipoprotein cholesterol?

Consistent with several potentially anti-atherogenic activities of high-density lipoproteins in vitro, low plasma levels of high-density lipoprotein cholesterol are associated with an increased risk of coronary heart disease. In addition to genes, lifestyle factors (e.g. smoking, being overweight and physical inactivity) strongly affect plasma high-density lipoprotein cholesterol levels. Moreover, a low level of high-density lipoprotein cholesterol interacts with other risk factors. Raising of high-density lipoprotein cholesterol by either adjustments of lifestyle or drug intervention as well as elimination of additional risk factors are thus thought to affect coronary risk. Here, we summarize the outcomes of observational and interventional studies as well as genetic and experimental research which have recently much advanced our understanding of the function and regulation of high-density lipoprotein metabolism. We conclude from the data that changes in the kinetics and functionality of high-density lipoprotein rather than changes in plasma high-density lipoprotein cholesterol levels per se will affect the anti-atherogenicity of therapeutic interference with high-density lipoprotein metabolism.