Overexpression of human apolipoprotein A-II in mice induces hypertriglyceridemia due to defective very low density lipoprotein hydrolysis

Dietary acid load modifies the effects of ApoA2-265 T > C polymorphism on lipid profile and serum leptin and ghrelin levels among type 2 diabetic patients

This investigation with aimed the effect of APOA2-265 T > C polymorphism and dietary acid load (DAL) as either potential renal acid load (PRAL) and net endogenous acid production (NEAP) intake interaction on metabolic markers in type 2 diabetes mellitus (T2DM). In present cross-sectional study, 737 patients with T2DM (290 men and 447 women) were enlisted from diabetes centers in Tehran. The dietary intakes of all participants during the last year was acquired by a validated semi-quantitative food frequency (FFQ) questionnaire. Polymerase chain reaction (PCR) was used for genotyping the APOA2-265 T > C. Biochemical indises containing leptin, ghrelin, total cholesterol (Bailey et al., J Clin Invest 97:1147-1453, 1996), low-density lipoprotein cholestrol (LDL-C), high-density lipoprotein cholestrol (HDL-C), triglyceride (TG), superoxide dismutase (SOD), high sensitivy C-reactive protein (hs-CRP), total antioxidant capacity (TAC), pentraxin-3 (PTX3), prostaglandin F2α (PGF2α) and interleukin 18 (IL18) were measured by standard method. Atherogenic indices (AIP, AC, CR-I, CR-II) were calculated. The gene-diet interactions were evaluated using an GLM. The frequency overall prevalence of rs5082 genotypes was 63.82 and 36.17% for T-allele and C-allele respectively. TG, Ghrelin, and hs-CRP concentrations were significantly higher among carriers with C allele than TT homozygotes. However, TC/CC genotypes have lower PTX3 than TT homozygotes (P < 0.05). C-allele carriers had highest mean of BMI (P_NEAP=0.04, P_PRAL = 0.006), WC (P_NEAP=0.04, P_PRAL = 0.04), TC (P_NEAP=0.03, P_PRAL = 0.01), ghrelin (P_NEAP=0.01, P_PRAL = 0.04), and leptin (P_NEAP=0.04, P_PRAL = 0.03) when placed in top tertiles of NEAP and PRAL.BMI, WC, TC, ghrelin, and leptin levels may be modified in C carriers by decreasing DAL, though, further investigations are required to confirm these findings.

Apolipoprotein A-II, a Player in Multiple Processes and Diseases

Apolipoprotein A-II (apoA-II) is the second most abundant apolipoprotein in high-density lipoprotein (HDL) particles, playing an important role in lipid metabolism. Human and murine apoA-II proteins have dissimilar properties, partially because human apoA-II is dimeric whereas the murine homolog is a monomer, suggesting that the role of apoA-II may be quite different in humans and mice. As a component of HDL, apoA-II influences lipid metabolism, being directly or indirectly involved in vascular diseases. Clinical and epidemiological studies resulted in conflicting findings regarding the proatherogenic or atheroprotective role of apoA-II. Human apoA-II deficiency has little influence on lipoprotein levels with no obvious clinical consequences, while murine apoA-II deficiency causes HDL deficit in mice. In humans, an increased plasma apoA-II concentration causes hypertriglyceridemia and lowers HDL levels. This dyslipidemia leads to glucose intolerance, and the ensuing high blood glucose enhances apoA-II transcription, generating a vicious circle that may cause type 2 diabetes (T2D). ApoA-II is also used as a biomarker in various diseases, such as pancreatic cancer. Herein, we provide a review of the most recent findings regarding the roles of apoA-II and its functions in various physiological processes and disease states, such as cardiovascular disease, cancer, amyloidosis, hepatitis, insulin resistance, obesity, and T2D.

Anorexia Nervosa Is Associated with a Shift to Pro-Atherogenic Low-Density Lipoprotein Subclasses

Anorexia nervosa (AN) is a severe eating disorder affecting primarily female adolescents and younger adults. The energy deprivation associated with AN has been shown to alter lipoprotein metabolism, which may affect cardiovascular risk. However, the mechanisms leading to alterations in the composition, structure, and function of lipoproteins in AN patients are not well-understood yet. Here, we investigated the lipid abnormalities associated with AN, particularly changes in the distribution, composition, metabolism, and function of lipoprotein subclasses. In this exploratory study, we analyzed serum samples of 18 women diagnosed with AN (BMI < 17.5 kg/m2) and 24 normal-weight women (BMI from 18.5−24.9 kg/m2). Using the Quantimetrix Lipoprint® system, we determined low-density lipoprotein (LDL) subclass distribution, including quantitative measurements of very low-density lipoprotein (VLDL), intermediate density lipoprotein (IDL) and high-density lipoprotein (HDL) subclass distribution. We quantified the most abundant apolipoproteins of HDL and assessed lecithin-cholesterol acyltransferase (LCAT) and cholesteryl-ester transfer protein (CETP) activities. In addition, anti-oxidative capacity of apoB-depleted serum and functional metrics of HDL, including cholesterol efflux capacity and paraoxonase activity were assessed. The atherogenic lipoprotein subclasses VLDL and small LDL particles were increased in AN. Levels of VLDL correlated significantly with CETP activity (rs = 0.432, p = 0.005). AN was accompanied by changes in the content of HDL-associated apolipoproteins involved in triglyceride catabolism, such as apolipoprotein C-II (+24%) and apoA-II (−27%), whereas HDL-associated cholesterol, phospholipids, and triglycerides were not altered. Moreover, AN did not affect HDL subclass distribution, cholesterol efflux capacity, and paraoxonase activity. We observed a shift to more atherogenic lipoprotein subclasses in AN patients, whereas HDL functionality and subclass distribution were not altered. This finding underpins potential detrimental effects of AN on lipid metabolism and the cardiovascular system by increasing atherosclerotic risk factors.

A transgenic mouse model reproduces human hereditary systemic amyloidosis

Amyloidoses are rare life-threatening diseases caused by protein misfolding of normally soluble proteins. The fatal outcome is predominantly due to renal failure and/or cardiac dysfunction. Because amyloid fibrils formed by all amyloidogenic proteins share structural similarity, amyloidoses may be studied in transgenic models expressing any amyloidogenic protein. Here we generated transgenic mice expressing an amyloidogenic variant of human apolipoprotein AII, a major protein of high density lipoprotein. According to amyloid nomenclature this variant was termed STOP78SERApoAII. STOP78SER-APOA2 expression at the physiological level spontaneously induced systemic amyloidosis in all mice with full-length mature STOP78SER-ApoAII identified as the amyloidogenic protein. Amyloid deposits stained with Congo red were extracellular, and consisted of fibrils of approximately 10 nm diameter. Renal glomerular amyloidosis was a major feature with onset of renal insufficiency occurring in mice older than six months of age. The liver, heart and spleen were also greatly affected. Expression of STOP78SER-APOA2 in the liver and intestine in mice of the K line but not in other amyloid-laden organs showed they present systemic amyloidosis. The amyloid burden was a function of STOP78SER-APOA2 expression and age of the mice with amyloid deposition starting in two-month-old high-expressing mice that died from six months onwards. Because STOP78SER-ApoAII conserved adequate lipid binding capacity as shown by high STOP78SER-ApoAII amounts in high density lipoprotein of young mice, its decrease in circulation with age suggests preferential deposition into preformed fibrils. Thus, our mouse model faithfully reproduces early-onset hereditary systemic amyloidosis and is ideally suited to devise and test novel therapies.

Posttranslational modifications of apolipoprotein A-II proteoforms in type 2 diabetes

BACKGROUND

Apolipoprotein A-II (apoA-II) is the second most abundant protein in high-density lipoprotein particles. However, it exists in plasma in multiple forms. The effect of diabetes on apoA-II proteoforms is not known.

OBJECTIVE

Our objective was to characterize plasma apoA-II proteoforms in participants with and without type 2 diabetes.

METHODS

Using a novel mass spectrometric immunoassay, the relative abundance of apoA-II proteoforms was examined in plasma of 30 participants with type 2 diabetes and 25 participants without diabetes.

RESULTS

Six apoA-II proteoforms (monomer, truncated TQ monomer, truncated Q monomer, dimer, truncated Q dimer, and truncated 2Qs dimer) and their oxidized proteoforms were identified. The ratios of oxidized monomer and all oxidized proteoforms to the native apoA-II were significantly greater in the diabetic group (P = .004 and P = .005, respectively) compared with the nondiabetic group.

CONCLUSION

The relative abundance of oxidized apoA-II is significantly increased in type 2 diabetes.

ApoA-II HDL Catabolism and Its Relationships With the Kinetics of ApoA-I HDL and of VLDL1, in Abdominal Obesity

We study the associations between apoA-II fractional catabolic rate (FCR) and the kinetics of VLDL subspecies and apoA-I and show that, in abdominally obese individuals, apoA-II FCR is positively and independently associated with both apoA-I FCR and VLDL1-TG indirect FCR.

Chylomicrons: Advances in biology, pathology, laboratory testing, and therapeutics

The adequate absorption of lipids is essential for all mammalian species due to their inability to synthesize some essential fatty acids and fat-soluble vitamins. Chylomicrons (CMs) are large, triglyceride-rich lipoproteins that are produced in intestinal enterocytes in response to fat ingestion, which function to transport the ingested lipids to different tissues. In addition to the contribution of CMs to postprandial lipemia, their remnants, the degradation products following lipolysis by lipoprotein lipase, are linked to cardiovascular disease. In this review, we will focus on the structure-function and metabolism of CMs. Second, we will analyze the impact of gene defects reported to affect CM metabolism and, also, the role of CMs in other pathologies, such as atherothrombotic cardiovascular disease and diabetes mellitus. Third, we will provide an overview of the laboratory tests currently used to study CM disorders, and, finally, we will highlight current treatments in diseases affecting CMs.

D25V apolipoprotein C-III variant causes dominant hereditary systemic amyloidosis and confers cardiovascular protective lipoprotein profile

Apolipoprotein C-III deficiency provides cardiovascular protection, but apolipoprotein C-III is not known to be associated with human amyloidosis. Here we report a form of amyloidosis characterized by renal insufficiency caused by a new apolipoprotein C-III variant, D25V. Despite their uremic state, the D25V-carriers exhibit low triglyceride (TG) and apolipoprotein C-III levels, and low very-low-density lipoprotein (VLDL)/high high-density lipoprotein (HDL) profile. Amyloid fibrils comprise the D25V-variant only, showing that wild-type apolipoprotein C-III does not contribute to amyloid deposition in vivo. The mutation profoundly impacts helical structure stability of D25V-variant, which is remarkably fibrillogenic under physiological conditions in vitro producing typical amyloid fibrils in its lipid-free form. D25V apolipoprotein C-III is a new human amyloidogenic protein and the first conferring cardioprotection even in the unfavourable context of renal failure, extending the evidence for an important cardiovascular protective role of apolipoprotein C-III deficiency. Thus, fibrate therapy, which reduces hepatic APOC3 transcription, may delay amyloid deposition in affected patients.

Creation of Apolipoprotein C-II (ApoC-II) Mutant Mice and Correction of Their Hypertriglyceridemia with an ApoC-II Mimetic Peptide

Apolipoprotein C-II (apoC-II) is a cofactor for lipoprotein lipase, a plasma enzyme that hydrolyzes triglycerides (TGs). ApoC-II deficiency in humans results in hypertriglyceridemia. We used zinc finger nucleases to create Apoc2 mutant mice to investigate the use of C-II-a, a short apoC-II mimetic peptide, as a therapy for apoC-II deficiency. Mutant mice produced a form of apoC-II with an uncleaved signal peptide that preferentially binds high-density lipoproteins (HDLs) due to a 3-amino acid deletion at the signal peptide cleavage site. Homozygous Apoc2 mutant mice had increased plasma TG (757.5 ± 281.2 mg/dl) and low HDL cholesterol (31.4 ± 14.7 mg/dl) compared with wild-type mice (TG, 55.9 ± 13.3 mg/dl; HDL cholesterol, 55.9 ± 14.3 mg/dl). TGs were found in light (density < 1.063 g/ml) lipoproteins in the size range of very-low-density lipoprotein and chylomicron remnants (40-200 nm). Intravenous injection of C-II-a (0.2, 1, and 5 μmol/kg) reduced plasma TG in a dose-dependent manner, with a maximum decrease of 90% occurring 30 minutes after the high dose. Plasma TG did not return to baseline until 48 hours later. Similar results were found with subcutaneous or intramuscular injections. Plasma half-life of C-II-a is 1.33 ± 0.72 hours, indicating that C-II-a only acutely activates lipolysis, and the sustained TG reduction is due to the relatively slow rate of new TG-rich lipoprotein synthesis. In summary, we describe a novel mouse model of apoC-II deficiency and show that an apoC-II mimetic peptide can reverse the hypertriglyceridemia in these mice, and thus could be a potential new therapy for apoC-II deficiency.

Metabolism of apolipoprotein A-II containing triglyceride rich ApoB lipoproteins in humans

OBJECTIVE

To characterize human triglyceride-rich lipoproteins (TRL) with and without apoA-II and to study their metabolism in vivo.

METHODS

Plasma from 11 participants on a controlled diet given a bolus infusion of [D5]l-phenylalanine to label apoB was combined into four pools and applied to anti-apoA-II immunoaffinity columns. Fractions with and without apoA-II were separated into VLDL and IDL by ultracentrifugation; lipids and apolipoproteins were measured. For kinetic measurements, apoB was isolated and hydrolyzed to the constituent amino acids. Tracer enrichment was measured by GCMS. Metabolic rates were determined by SAAM-II.

RESULTS

VLDL and IDL with apoA-II comprised 7% and 9% of total VLDL and IDL apoB respectively. VLDL with apoA-II was enriched in apoC-III, apoE, and cholesterol compared to VLDL without apoA-II. Mean apoB FCR of VLDL with apoA-II was significantly lower than for VLDL without apoA-II (2.80 ± 0.96 pools/day v.s. 5.09 ± 1.69 pools/day, P = 0.009). A higher percentage of VLDL with apoA-II was converted to IDL than was cleared from circulation, compared to VLDL without apoA-II (96 ± 8% vs. 45 ± 22%; P = 0.007). The rate constants for conversion of VLDL to IDL were similar for VLDL with and without apoA-II. Thus, a very low rate constant for clearance accounted for the lower FCR of VLDL with apoA-II.

CONCLUSION

VLDL with apoA-II represents a small pool of VLDL particles that has a slow FCR and is predominantly converted to IDL rather than cleared from the circulation.

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.

Apolipoprotein A-II is a key regulatory factor of HDL metabolism as appears from studies with transgenic animals and clinical outcomes

The structure and metabolism of HDL are linked to their major apolipoproteins (apo) A-I and A-II. HDL metabolism is very dynamic and depends on the constant remodeling by lipases, lipid transfer proteins and receptors. HDL exert several cardioprotective effects, through their antioxidant and antiinflammatory capacities and through the stimulation of reverse cholesterol transport from extrahepatic tissues to the liver for excretion into bile. HDL also serve as plasma reservoir for C and E apolipoproteins, as transport vehicles for a great variety of proteins, and may have more physiological functions than previously recognized. In this review we will develop several aspects of HDL metabolism with emphasis on the structure/function of apo A-I and apo A-II. An important contribution to our understanding of the respective roles of apo A-I and apo A-II comes from studies using transgenic animal models that highlighted the stabilizatory role of apo A-II on HDL through inhibition of their remodeling by lipases. Clinical studies coupled with proteomic analyses revealed the presence of dysfunctional HDL in patients with cardiovascular disease. Beyond HDL cholesterol, a new notion is the functionality of HDL particles. In spite of abundant literature on HDL metabolic properties, a major question remains unanswered: which HDL particle(s) confer(s) protection against cardiovascular risk?

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.

Apolipoprotein A-II and adiponectin as determinants of very low-density lipoprotein apolipoprotein B-100 metabolism in nonobese men

Data from cellular systems and transgenic animal models suggest a role of apolipoprotein (apo) A-II in the regulation of very low-density lipoprotein (VLDL) metabolism. However, the precise mechanism whereby apoA-II regulates VLDL metabolism remains to be elucidated in humans. In this study, we examined the associations between the kinetics of high-density lipoprotein (HDL)-apoA-II and VLDL-apoB-100 kinetics, and plasma adiponectin concentrations. The kinetics of HDL-apoA-II and VLDL-apoB-100 were measured in 37 nonobese men using stable isotope techniques. Plasma adiponectin concentration was measured using immunoassays. Total plasma apoA-II concentration was positively associated with HDL-apoA-II production rate (PR) (r = 0.734, P < .01); both were positively associated with plasma triglyceride concentration (r = 0.360 and 0.369, respectively) and VLDL-apoB-100 PR (r = 0.406 and 0.427, respectively), and inversely associated with plasma adiponectin concentration (r = -0.449 and -0.375, respectively). Plasma adiponectin was inversely associated with plasma triglyceride concentration (r = -0.327), VLDL-apoB-100 concentration (r = -0.337), and VLDL-apoB-100 PR (r = -0.373). In multiple regression models including waist circumference and plasma insulin, plasma adiponectin concentration was an independent determinant of total plasma apoA-II concentration (β-coefficient = -0.508, P = .001) and HDL-apoA-II PR (β-coefficient = -0.374, P = .03). Conversely, total plasma apoA-II concentration (β-coefficient = 0.348, P = .047) and HDL-apoA-II PR (β-coefficient = -0.350, P = .035) were both independent determinants of VLDL-apoB-100 PR. However, these associations were not independent of plasma adiponectin. Variation in HDL apoA-II production, and hence total plasma apoA-II concentration, may exert a major effect on VLDL-apoB-100 production. Plasma adiponectin may also contribute to the variation in VLDL-apoB-100 production partly by regulating apoA-II transport.

Alteration of negatively charged residues in the 89 to 99 domain of apoA-I affects lipid homeostasis and maturation of HDL

In this study, we investigated the role of positively and negatively charged amino acids within the 89-99 region of apolipoprotein A-I (apoA-I), which are highly conserved in mammals, on plasma lipid homeostasis and the biogenesis of HDL. We previously showed that deletion of the 89-99 region of apoA-I increased plasma cholesterol and phospholipids, but it did not affect plasma triglycerides. Functional studies using adenovirus-mediated gene transfer of two apoA-I mutants in apoA-I-deficient mice showed that apoA-I[D89A/E91A/E92A] increased plasma cholesterol and caused severe hypertriglyceridemia. HDL levels were reduced, and approximately 40% of the apoA-I was distributed in VLDL/IDL. The HDL consisted of mostly spherical and a few discoidal particles and contained preβ1 and α4-HDL subpopulations. The lipid, lipoprotein, and HDL profiles generated by the apoA-I[K94A/K96A] mutant were similar to those of wild-type (WT) apoA-I. Coexpression of apoA-I[D89A/E91A/E92A] and human lipoprotein lipase abolished hypertriglyceridemia, restored in part the α1,2,3,4 HDL subpopulations, and redistributed apoA-I in the HDL2/HDL3 regions, but it did not prevent the formation of discoidal HDL particles. Physicochemical studies showed that the apoA-I[D89A/E91A/E92A] mutant had reduced α-helical content and effective enthalpy of thermal denaturation, increased exposure of hydrophobic surfaces, and increased affinity for triglyceride-rich emulsions. We conclude that residues D89, E91, and E92 of apoA-I are important for plasma cholesterol and triglyceride homeostasis as well as for the maturation of HDL.

Cholesterol metabolism and transport in the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder, affecting millions of people worldwide. Apart from age, the major risk factor identified so far for the sporadic form of AD is possession of the epsilon4 allele of apolipoprotein E (APOE), which is also a risk factor for coronary artery disease (CAD). Other apolipoproteins known to play an important role in CAD such as apolipoprotein B are now gaining attention for their role in AD as well. AD and CAD share other risk factors, such as altered cholesterol levels, particularly high levels of low density lipoproteins together with low levels of high density lipoproteins. Statins--drugs that have been used to lower cholesterol levels in CAD, have been shown to protect against AD, although the protective mechanism(s) involved are still under debate. Enzymatic production of the beta amyloid peptide, the peptide thought to play a major role in AD pathogenesis, is affected by membrane cholesterol levels. In addition, polymorphisms in several proteins and enzymes involved in cholesterol and lipoprotein transport and metabolism have been linked to risk of AD. Taken together, these findings provide strong evidence that changes in cholesterol metabolism are intimately involved in AD pathogenic processes. This paper reviews cholesterol metabolism and transport, as well as those aspects of cholesterol metabolism that have been linked with AD.

Expression of Human ApoAII in Transgenic Rabbits Leads to Dyslipidemia

Objective— Apolipoprotein AII (apoAII) is the second major apolipoprotein in high-density lipoprotein (HDL). However, the physiological functions of apoAII in lipoprotein metabolism have not been fully elucidated.

Methods and Results— We generated human apoAII transgenic (Tg) rabbits, a species that normally does not have an endogenous apoAII gene. Plasma levels of human apoAII in Tg rabbits were ≈30 mg/dL, similar to the plasma levels in healthy humans. The expression of human apoAII in Tg rabbits resulted in increased levels of plasma triglycerides, total cholesterol, and phospholipids accompanied by a marked reduction in HDL-cholesterol levels compared with non-Tg littermates. Analysis of lipoprotein fractions showed that hyperlipidemia exhibited by Tg rabbits was caused by elevated levels of very-low-density lipoproteins (VLDL) and intermediate-density lipoproteins. Furthermore, postheparin lipoprotein lipase activity significantly decreased in Tg rabbits compared with non-Tg rabbits.

Conclusions— These results indicate that apoAII plays an important role in both VLDL and HDL metabolism, possibly through the inhibition of lipoprotein lipase activity. ApoAII Tg rabbits may become a new model for the study of human familial combined hyperlipidemia.

Human apolipoprotein A-II determines plasma triglycerides by regulating lipoprotein lipase activity and high-density lipoprotein proteome

INTRODUCTION

Apolipoprotein (apo) A-II is the second most abundant high-density lipoprotein (HDL) apolipoprotein. We assessed the mechanism involved in the altered postprandial triglyceride-rich lipoprotein metabolism of female human apoA-II-transgenic mice (hapoA-II-Tg mice), which results in up to an 11-fold increase in plasma triglyceride concentration. The relationships between apoA-II, HDL composition, and lipoprotein lipase (LPL) activity were also analyzed in a group of normolipidemic women.

METHODS AND RESULTS

Triglyceride-rich lipoprotein catabolism was decreased in hapoA-II-Tg mice compared to control mice. This suggests that hapoA-II, which was mainly associated with HDL during fasting and postprandially, impairs triglyceride-rich lipoprotein lipolysis. HDL isolated from hapoA-II-Tg mice impaired bovine LPL activity. Two-dimensional gel electrophoresis, mass spectrometry, and immunonephelometry identified a marked deficiency in the HDL content of apoA-I, apoC-III, and apoE in these mice. In normolipidemic women, apoA-II concentration was directly correlated with plasma triglyceride and inversely correlated with the HDL-apoC-II+apoE/apoC-III ratio [corrected]. HDL-mediated induction of LPL activity was inversely correlated with apoA-II and directly correlated with the HDL-apoC-II+apoE/apoC-III ratio [corrected]. Purified hapoA-II displaced apoC-II, apoC-III, and apoE from human HDL2. Human HDL3 was, compared to HDL2, enriched in apoA-II but poorer in apoC-II, apoC-III, and apoE.

CONCLUSIONS

ApoA-II plays a crucial role in triglyceride catabolism by regulating LPL activity, at least in part, through HDL proteome modulation.

Evaluating the association of common APOA2 variants with type 2 diabetes

Background

APOA2 is a positional and biological candidate gene for type 2 diabetes at the chromosome 1q21-q24 susceptibility locus. The aim of this study was to examine if HapMap phase II tag SNPs in APOA2 are associated with type 2 diabetes and quantitative traits in French Caucasian subjects.

Methods

We genotyped the three HapMap phase II tagging SNPs (rs6413453, rs5085 and rs5082) required to capture the common variation spanning the APOA2 locus in our type 2 diabetes case-control cohort comprising 3,093 French Caucasian subjects. The association between these variants and quantitative traits was also examined in the normoglycaemic adults of the control cohort. In addition, meta-analysis of publicly available whole genome association data was performed.

Results

None of the APOA2 tag SNPs were associated with type 2 diabetes in the French Caucasian case-control cohort (rs6413453, P = 0.619; rs5085, P = 0.245; rs5082, P = 0.591). However, rs5082 was marginally associated with total cholesterol levels (P = 0.026) and waist-to-hip ratio (P = 0.029). The meta-analysis of data from 12,387 subjects confirmed our finding that common variation at the APOA2 locus is not associated with type 2 diabetes.

Conclusion

The available data does not support a role for common variants in APOA2 on type 2 diabetes susceptibility or related quantitative traits in Northern Europeans.

Sensing of dietary lipids by enterocytes: a new role for SR-BI/CLA-1

Background

The intestine is responsible for absorbing dietary lipids and delivering them to the organism as triglyceride-rich lipoproteins (TRL). It is important to determine how this process is regulated in enterocytes, the absorptive cells of the intestine, as prolonged postprandial hypertriglyceridemia is a known risk factor for atherosclerosis. During the postprandial period, dietary lipids, mostly triglycerides (TG) hydrolyzed by pancreatic enzymes, are combined with bile products and reach the apical membrane of enterocytes as postprandial micelles (PPM). Our aim was to determine whether these micelles induce, in enterocytes, specific early cell signaling events that could control the processes leading to TRL secretion.

Methodology/Principal Findings

The effects of supplying PPM to the apex of Caco-2/TC7 enterocytes were analyzed. Micelles devoid of TG hydrolysis products, like those present in the intestinal lumen in the interprandial period, were used as controls. The apical delivery of PPM specifically induced a number of cellular events that are not induced by interprandial micelles. These early events included the trafficking of apolipoprotein B, a structural component of TRL, from apical towards secretory domains, and the rapid, dose-dependent activation of ERK and p38MAPK. PPM supply induced the scavenger receptor SR-BI/CLA-1 to cluster at the apical brush border membrane and to move from non-raft to raft domains. Competition, inhibition or knockdown of SR-BI/CLA-1 impaired the PPM-dependent apoB trafficking and ERK activation.

Conclusions/Significance

These results are the first evidence that enterocytes specifically sense postprandial dietary lipid-containing micelles. SR-BI/CLA-1 is involved in this process and could be a target for further study with a view to modifying intestinal TRL secretion early in the control pathway.

Influence of genetic factors in the modulation of postprandial lipemia

Postprandial lipemia is traditionally defined by the extent and duration of the increase in plasma triglycerides in response to a fat-enriched meal. The relationship between alimentary lipemia and coronary disease is of great interest in view of the epidemiological and experimental evidence that underlies it. The rate of synthesis of triglyceride-rich lipoproteins, lipoprotein lipase-mediated triglyceride hydrolysis, and the hepatic capture of chylomicron remnants via the interaction of the lipoprotein receptor with APOE and LPL, are the fundamental pillars of the metabolism and modification of these lipoproteins. The modulation of such phenomena is influenced by both genetic and environmental factors, thus explaining their extraordinary individual variance. This review presents the current evidence linking a number of candidate genes to the modulation of postprandial lipid metabolism.

Apolipoprotein AII is a regulator of very low density lipoprotein metabolism and insulin resistance

Apolipoprotein AII (apoAII) transgenic (apoAIItg) mice exhibit several traits associated with the insulin resistance (IR) syndrome, including IR, obesity, and a marked hypertriglyceridemia. Because treatment of the apoAIItg mice with rosiglitazone ameliorated the IR and hypertriglyceridemia, we hypothesized that the hypertriglyceridemia was due largely to overproduction of very low density lipoprotein (VLDL) by the liver, a normal response to chronically elevated insulin and glucose. We now report in vivo and in vitro studies that indicate that hepatic fatty acid oxidation was reduced and lipogenesis increased, resulting in a 25% increase in triglyceride secretion in the apoAIItg mice. In addition, we observed that hydrolysis of triglycerides from both chylomicrons and VLDL was significantly reduced in the apoAIItg mice, further contributing to the hypertriglyceridemia. This is a direct, acute effect, because when mouse apoAII was injected into mice, plasma triglyceride concentrations were significantly increased within 4 h. VLDL from both control and apoAIItg mice contained significant amounts of apoAII, with approximately 4 times more apoAII on apoAIItg VLDL. ApoAII was shown to transfer spontaneously from high density lipoprotein (HDL) to VLDL in vitro, resulting in VLDL that was a poorer substrate for hydrolysis by lipoprotein lipase. These results indicate that one function of apoAII is to regulate the metabolism of triglyceride-rich lipoproteins, with HDL serving as a plasma reservoir of apoAII that is transferred to the triglyceride-rich lipoproteins in much the same way as VLDL and chylomicrons acquire most of their apoCs from HDL.

Apolipoprotein-AII concentrations are associated with liver steatosis in patients with chronic hepatitis C

It has been shown that the hepatitis C virus (HCV) core protein reduces the activity of the microsomal triglyceride transfer protein (MTP) and could lead to steatosis in HCV-infected patients. Experimentally, apolipoprotein-AII (apoAII), which restores triglyceride secretion altered by the HCV core protein, could be protective against HCV steatosis. On the other hand, increasing plasma concentrations of mouse apoAII in transgenic mice produced several aspects of insulin-resistance syndrome, which also is implicated in the pathogenesis of HCV steatosis. This study was designed to investigate the role of apoAII in HCV-related steatosis in humans. Sixty-five hospitalized patients with chronic HCV were included in this study to assess the effects of apoAII, body mass index (BMI), age, insulin sensibility (HOMA), and leptin level on steatosis. Steatosis was observed in 55.3% of patients. Apo-AII was significantly associated with HOMA and with leptin concentrations. In univariate analyses, age, BMI, increased leptin level, increased HOMA, and increased apoAII concentration were associated with steatosis. In multivariate analysis, steatosis was associated with apoAII concentration, age, gender, and BMI. Contrary to previous hypotheses, apoAII is not a protective factor against HCV steatosis but is significantly associated with the development of liver steatosis. The fact that the plasma levels of apoAII correlate with HOMA and leptin levels in HCV-infected patients suggests that apoAII may contribute to hepatic steatosis progression in relationship to visceral obesity, insulin resistance, and metabolism of triglyceride-rich lipoproteins.

Dietary, physiological, genetic and pathological influences on postprandial lipid metabolism

Most of diurnal time is spent in a postprandial state due to successive meal intakes during the day. As long as the meals contain enough fat, a transient increase in triacylglycerolaemia and a change in lipoprotein pattern occurs. The extent and kinetics of such postprandial changes are highly variable and are modulated by numerous factors. This review focuses on factors affecting postprandial lipoprotein metabolism and genes, their variability and their relationship with intermediate phenotypes and risk of CHD. Postprandial lipoprotein metabolism is modulated by background dietary pattern as well as meal composition (fat amount and type, carbohydrate, protein, fibre, alcohol) and several lifestyle conditions (physical activity, tobacco use), physiological factors (age, gender, menopausal status) and pathological conditions (obesity, insulin resistance, diabetes mellitus). The roles of many genes have been explored in order to establish the possible implications of their variability in lipid metabolism and CHD risk. The postprandial lipid response has been shown to be modified by polymorphisms within the genes for apo A-I, A-IV, A-V, E, B, C-I and C-III, lipoprotein lipase, hepatic lipase, fatty acid binding and transport proteins, microsomal triglyceride transfer protein and scavenger receptor class B type I. Overall, the variability in postprandial response is important and complex, and the interactions between nutrients or dietary or meal compositions and gene variants need further investigation. The extent of present knowledge and needs for future studies are discussed in light of ongoing developments in nutrigenetics.

An apolipoprotein A-II polymorphism (-265T/C, rs5082) regulates postprandial response to a saturated fat overload in healthy men

Apolipoprotein (Apo) A-II is an apolipoprotein with an unknown role in lipid metabolism. It has been suggested that the presence of the less frequent allele of a single nucleotide polymorphism (Apo A-II -265T/C, rs5082) reduces the transcription rate of Apo A-II and enhances VLDL postprandial clearance in middle-aged men. To further investigate the role of Apo A-II -265T/C on lipid metabolism, we studied 88 normolipidemic young men. The participants were given a fatty meal containing 1 g fat and 7 mg cholesterol/kg weight and capsules containing 60,000 IU vitamin A (retinyl palmitate, 15.15 mg RE) per square meter body surface area. Postprandial lipemia was assessed during the 11 h following the meal. Total cholesterol (Chol) and triacylglycerols (TG) in plasma and TG-rich lipoproteins (TRL) (large TRL and small TRL) were measured, as well as HDL, Apo A-I, Apo B, Apo B-48, and Apo B-100. Postprandial responses were higher in the TT group than in carriers of the minor allele (CC/TC) for total TG in plasma (21.37% of change of area under curve, P = 0.014), large TRL-TG (24.75% change, P = 0.017) and small TRL-Chol (26.63% change, P = 0.003). Our work shows that carriers of the minor allele for Apo A-II -265T/C (CC/TC) have a lower postprandial response compared with TT homozygotes. This finding may partially explain the role of Apo A-II in lipid metabolism and can identify a population with a decreased risk of cardiovascular disease, as corresponds to the lower level of postprandial hypertriglyceridemia.

Apolipoprotein A-II is catabolized in the kidney as a function of its plasma concentration

We investigated in vivo catabolism of apolipoprotein A-II (apo A-II), a major determinant of plasma HDL levels. Like apoA-I, murine apoA-II (mapoA-II) and human apoA-II (hapoA-II) were reabsorbed in the first segment of kidney proximal tubules of control and hapoA-II-transgenic mice, respectively. ApoA-II colocalized in brush border membranes with cubilin and megalin (the apoA-I receptor and coreceptor, respectively), with mapoA-I in intracellular vesicles of tubular epithelial cells, and was targeted to lysosomes, suggestive of degradation. By use of three transgenic lines with plasma hapoA-II concentrations ranging from normal to three times higher, we established an association between plasma concentration and renal catabolism of hapoA-II. HapoA-II was rapidly internalized in yolk sac epithelial cells expressing high levels of cubilin and megalin, colocalized with cubilin and megalin on the cell surface, and effectively competed with apoA-I for uptake, which was inhibitable by anti-cubilin antibodies. Kidney cortical cells that only express megalin internalized LDL but not apoA-II, apoA-I, or HDL, suggesting that megalin is not an apoA-II receptor. We show that apoA-II is efficiently reabsorbed in kidney proximal tubules in relation to its plasma concentration.

Human apolipoprotein A-II associates with triglyceride-rich lipoproteins in plasma and impairs their catabolism

Postprandial hypertriglyceridemia and low plasma HDL levels, which are principal features of the metabolic syndrome, are displayed by transgenic mice expressing human apolipoprotein A-II (hapoA-II). In these mice, hypertriglyceridemia results from the inhibition of lipoprotein lipase and hepatic lipase activities by hapoA-II carried on VLDL. This study aimed to determine whether the association of hapoA-II with triglyceride-rich lipoproteins (TRLs) is sufficient to impair their catabolism. To measure plasma TRL residence time, intestinal TRL production was induced by a radioactive oral lipid bolus. Radioactive and total triglyceride (TG) were rapidly cleared in control mice but accumulated in plasma of transgenic mice, in relation to hapoA-II concentration. Similar plasma TG accumulations were measured in transgenic mice with or without endogenous apoA-II expression. HapoA-II (synthesized in liver) was detected in chylomicrons (produced by intestine). The association of hapoA-II with TRL in plasma was further confirmed by the absence of hapoA-II in chylomicrons and VLDL of transgenic mice injected with Triton WR 1339, which prevents apolipoprotein exchanges. We show that the association of hapoA-II with TRL occurs in the circulation and induces postprandial hypertriglyceridemia.

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.

Formation of high density lipoproteins containing both apolipoprotein A-I and A-II in the rabbit

Human plasma HDLs are classified on the basis of apolipoprotein composition into those that contain apolipoprotein A-I (apoA-I) without apoA-II [(A-I)HDL] and those containing apoA-I and apoA-II [(A-I/A-II)HDL]. ApoA-I enters the plasma as a component of discoidal particles, which are remodeled into spherical (A-I)HDL by LCAT. ApoA-II is secreted into the plasma either in the lipid-free form or as a component of discoidal high density lipoproteins containing apoA-II without apoA-I [(A-II)HDL]. As discoidal (A-II)HDL are poor substrates for LCAT, they are not converted into spherical (A-II)HDL. This study investigates the fate of apoA-II when it enters the plasma. Lipid-free apoA-II and apoA-II-containing discoidal reconstituted HDL [(A-II)rHDL] were injected intravenously into New Zealand White rabbits, a species that is deficient in apoA-II. In both cases, the apoA-II was rapidly and quantitatively incorporated into spherical (A-I)HDL to form spherical (A-I/A-II)HDL. These particles were comparable in size and composition to the (A-I/A-II)HDL in human plasma. Injection of lipid-free apoA-II and discoidal (A-II)rHDL was also accompanied by triglyceride enrichment of the endogenous (A-I)HDL and VLDL as well as the newly formed (A-I/A-II)HDL. We conclude that, irrespective of the form in which apoA-II enters the plasma, it is rapidly incorporated into spherical HDLs that also contain apoA-I to form (A-I/A-II)HDL.

Wild-type PCSK9 inhibits LDL clearance but does not affect apoB-containing lipoprotein production in mouse and cultured cells

Mutations in Proprotein Convertase Subtilisin Kexin 9 (PCSK9) have been associated with autosomal dominant hypercholesterolemia. In vivo kinetic studies indicate that LDL catabolism was impaired and apolipoprotein B (apoB)-containing lipoprotein synthesis was enhanced in two patients presenting with the S127R mutation on PCSK9. To understand the physiological role of PCSK9, we overexpressed human PCSK9 in mouse and cellular models as well as attenuated the endogenous expression of PCSK9 in HuH7 hepatoma cells using RNA interference. Here, we show that PCSK9 dramatically impairs the expression of the low density lipoprotein receptor (LDLr) and, in turn, LDL cellular binding as well as LDL clearance from the plasma compartment in C57BL6/J mice but not in LDLr-deficient mice, establishing a definitive role for PCSK9 in the modulation of the LDLr metabolic pathway. In contrast to data obtained in S127R-PCSK9 patients presenting with increased apoB production, our study indicates that wild-type PCSK9 does not significantly alter the production and/or secretion of VLDL apoB in either cultured cells or mice. Finally, we show that unlike PCSK9 overexpression in mice, the S127R mutation in patients led to increased VLDL apoB levels, suggesting a potential gain of function for S127R-PCSK9 in humans.

Grape seed procyanidins improve atherosclerotic risk index and induce liver CYP7A1 and SHP expression in healthy rats

Moderate consumption of red wine reduces risk of death from cardiovascular disease. The polyphenols in red wine are ultimately responsible for this effect, exerting antiatherogenic actions through their antioxidant capacities and modulating intracellular signaling pathways and transcriptional activities. Lipoprotein metabolism is crucial in atherogenesis, and liver is the principal organ controlling lipoprotein homeostasis. This study was intended to identify the primary effects of procyanidins, the most abundant polyphenols in red wine, on both plasma lipoprotein profile and the expression of genes controlling lipoprotein homeostasis in the liver. We show that procyanidins lowered plasma triglyceride, free fatty acids, apolipoprotein B (apoB), LDL-cholesterol and nonHDL:nonLDL-cholesterol levels and slightly increased HDL-cholesterol. Liver mRNA levels of small heterodimer partner (SHP), cholesterol 7alpha-hydroxylase (CYP7A1), and cholesterol biosynthetic enzymes increased, whereas those of apoAII, apoCI, and apoCIII decreased. Lipoprotein lipase (LPL) mRNA levels increased in muscle and decreased in adipose tissue. In conclusion, procyanidins improve the atherosclerotic risk index in the postprandial state, inducing in the liver the overexpression of CYP7A1 (suggesting an increase of cholesterol elimination via bile acids) and SHP, a nuclear receptor emerging as a key regulator of lipid homeostasis at the transcriptional level. These results could explain, at least in part, the beneficial long-term effects associated with moderate red wine consumption.

Apple procyanidins decrease cholesterol esterification and lipoprotein secretion in Caco-2/TC7 enterocytes

Decrease of plasma lipid levels by polyphenols was linked to impairment of hepatic lipoprotein secretion. However, the intestine is the first epithelium that faces dietary compounds, and it contributes to lipid homeostasis by secreting triglyceride-rich lipoproteins during the postprandial state. The purpose of this study was to examine the effect of apple and wine polyphenol extracts on lipoprotein synthesis and secretion in human Caco-2/TC7 enterocytes apically supplied with complex lipid micelles. Our results clearly demonstrate that apple, but not wine, polyphenol extract dose-dependently decreases the esterification of cholesterol and the enterocyte secretion of lipoproteins. Apple polyphenols decrease apolipoprotein B (apoB) secretion by inhibiting apoB synthesis without increasing the degradation of the newly synthesized protein. Under our conditions, cholesterol uptake, apoB mRNA, and microsomal triglyceride protein activity were not modified by apple polyphenols. The main monomers present in our mixture did not interfere with the intestinal lipid metabolism. By contrast, apple procyanidins reproduced the inhibition of both cholesteryl ester synthesis and lipoprotein secretion. Overall, our results are compatible with a mechanism of action of polyphenols resulting in impaired lipid availability that could induce the inhibition of intestinal lipoprotein secretion and contribute to the hypolipidemic effect of these compounds in vivo.

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.

Substitutions of glutamate 110 and 111 in the middle helix 4 of human apolipoprotein A-I (apoA-I) by alanine affect the structure and in vitro functions of apoA-I and induce severe hypertriglyceridemia in apoA-I-deficient mice

Hypertriglyceridemia is a common pathological condition in humans of mostly unknown etiology. Here we report induction of dyslipidemia characterized by severe hypertriglyceridemia as a result of point mutations in human apolipoprotein A-I (apoA-I). Adenovirus-mediated gene transfer in apoA-I-deficient (apoA-I(-)(/)(-)) mice showed that mice expressing an apoA-I[E110A/E111A] mutant had comparable hepatic mRNA levels with WT controls but greatly increased plasma triglyceride and elevated plasma cholesterol levels. In addition, they had decreased apoE and apoCII levels and increased apoB48 levels in very low-density lipoprotein (VLDL)/intermediate-density lipoprotein (IDL). Fast protein liquid chromatography (FPLC) analysis of plasma showed that most of cholesterol and approximately 15% of the mutant apoA-I were distributed in the VLDL and IDL regions and all the triglycerides in the VLDL region. Hypertriglyceridemia was corrected by coinfection of mice with recombinant adenoviruses expressing the mutant apoA-I and human lipoprotein lipase. Physicochemical studies indicated that the apoA-I mutation decreased the alpha-helical content, the stability, and the unfolding cooperativity of both lipid-free and lipid-bound apoA-I. In vitro functional analyses showed that reconstituted HDL (rHDL) particles containing the mutant apoA-I had 53% of scavenger receptor class B type I (SR-BI)-mediated cholesterol efflux capacity and 37% capacity to activate lecithin:cholesterol acyltransferase (LCAT) as compared to the WT control. The mutant lipid-free apoA-I had normal capacity to promote ATP-binding cassette transporter A1 (ABCA1)-dependent cholesterol efflux. The findings indicate that subtle structural alterations in apoA-I may alter the stability and functions of apoA-I and high-density lipoprotein (HDL) and may cause hypertriglyceridemia.

Bile acids lower triglyceride levels via a pathway involving FXR, SHP, and SREBP-1c

We explored the effects of bile acids on triglyceride (TG) homeostasis using a combination of molecular, cellular, and animal models. Cholic acid (CA) prevents hepatic TG accumulation, VLDL secretion, and elevated serum TG in mouse models of hypertriglyceridemia. At the molecular level, CA decreases hepatic expression of SREBP-1c and its lipogenic target genes. Through the use of mouse mutants for the short heterodimer partner (SHP) and liver X receptor (LXR) alpha and beta, we demonstrate the critical dependence of the reduction of SREBP-1c expression by either natural or synthetic farnesoid X receptor (FXR) agonists on both SHP and LXR alpha and LXR beta. These results suggest that strategies aimed at increasing FXR activity and the repressive effects of SHP should be explored to correct hypertriglyceridemia.

In vitro transcriptional induction of the human apolipoprotein A-II gene by glucose

Type 2 diabetic patients present high triglyceride and low HDL levels, significant determinants for the risk of atherosclerosis. Transgenic mice overproducing human apolipoprotein (apo)A-II, one of the two major apos of HDLs, display the same lipid disorders. Here, we investigated the possible regulation of apoA-II gene expression by glucose. In primary rat hepatocytes and in HepG2 cells, the transcription of the human apoA-II gene was upregulated by glucose. This response was mediated by a hormone-responsive element within the enhancer of the apoA-II promoter and was dependent on hepatocyte nuclear factor-4alpha. Accordingly, in transgenic mice, the human apoA-II gene is stimulated by a high-carbohydrate diet after fasting and at weaning. By contrast, the apoA-II mRNA level is not modified in streptozotocin-induced diabetic rats. In transgenic mice overexpressing the human apoA-II gene, plasma human apoA-II concentration was positively correlated with blood glucose levels. These mice displayed a marked delay in plasma glucose tolerance as compared with control mice. We hypothesize that the following pathogenic pathway might occur in the course of type 2 diabetes: increased apoA-II level causes a rise in plasma triglyceride level and glucose intolerance, resulting in hyperglycemia, which in turn might further increase apoA-II gene transcription.

Lipid micelles stimulate the secretion of triglyceride-enriched apolipoprotein B48-containing lipoproteins by Caco-2 cells

Intestinal triglyceride-rich lipoproteins (TRL) are synthesized from dietary lipids. This study was designed to evaluate the effects of lipid micelles, mimicking post-digestive duodenal micelles, on the fate of apolipoprotein B (apoB)48-containing lipoproteins by Caco-2 cells. Such micelles, consisting of oleic acid (OA), taurocholate, 2-monooleoylglycerol (2-MO), cholesterol (Chol), and L-alpha-lysophospatidylcholine, were the most efficient inducers of OA uptake and esterification. The efficiency of TG and apoB48 secretion increased specifically as a function of cell differentiation. PAGE analysis of secreted lipoproteins separated by sequential ultracentrifugation after [35S] labeling revealed differences in the secretion of apoB100- and apoB48-containing lipoproteins. In absence of micelles, apoB48 was secreted mostly in "HDL-like" particles, as observed in enterocytes in vivo. Micelle application increased 2.7-fold the secretion of apoB, resulting in 53 times more apoB48 being recovered as TG-enriched lipoproteins at d < 1.006 g/ml. Electron microscopy revealed the presence of lipid droplets in the secretory pathway and the accumulation of newly synthesized TG in cytoplasmic lipid droplets, as in enterocytes in vivo. We showed that these droplets could be used for secretion. However, apoB48 preferentially bound to newly synthesized TG in the presence of micelles, accounting in part for the functional advantage of apoB editing in the intestine. While Caco-2 cells express both apoB isoforms, our results show that the apical supply of complex lipid micelles favors the physiological route of apoB48-containing TG-enriched lipoproteins.

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.

Association of liver steatosis with lipid oversecretion and hypotriglyceridaemia in C57BL/6j mice fed trans-10,cis-12-linoleic acid

Conjugated linoleic acids (CLA) have recently been recognized to reduce body fat and plasma lipids in some animals. This study demonstrated that the steatosis accompanying the fat loss induced by trans-10,cis-12-C(18:2) (CLA2) and not cis-9,trans-11-C(18:2) (CLA1) isomer in C57BL/6j mice was not due to an alteration of the liver lipoprotein production that was even increased. The 3-fold decrease in plasma triacylglycerol contents and the induction of mRNA expression of low-density lipoprotein receptors concomitantly observed in CLA2-fed mice suggested an increase in the lipoprotein clearance at the level of the liver itself. CLA1 feeding produced similar but attenuated effects on triglyceridaemia only.

Apolipoprotein A5, a crucial determinant of plasma triglyceride levels, is highly responsive to peroxisome proliferator-activated receptor alpha activators

The recently discovered APOA5 gene has been shown in humans and mice to be important in determining plasma triglyceride levels, a major cardiovascular disease risk factor. apoAV represents the first described apolipoprotein where overexpression lowers triglyceride levels. Since fibrates represent a commonly used therapy for lowering plasma triglycerides in humans, we investigated their ability to modulate APOA5 gene expression and consequently influence plasma triglyceride levels. Human primary hepatocytes treated with Wy 14,643 or fenofibrate displayed a strong induction of APOA5 mRNA. Deletion and mutagenesis analyses of the proximal APOA5 promoter firmly demonstrate the presence of a functional peroxisome proliferator-activated receptor response element. These findings demonstrate that APOA5 is a highly responsive peroxisome proliferator-activated receptor alpha target gene and support its role as a major mediator for how fibrates reduce plasma triglycerides in humans.

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.

Mouse models as tools for dissecting disorders of lipoprotein metabolism

The overexpression of proteins as transgenes or by adenovirus-mediated gene transfer as well as the disruption of genes by homologous DNA recombination in the mouse provide powerful tools to dissect the role of individual proteins in complex biological pathways. These and similar techniques have been widely used to characterize the function of most of the players involved in lipoprotein metabolism. These models are expected to greatly advance the finding of new therapeutic strategies for the treatment of disorders of lipoprotein metabolism.

The farnesoid X-receptor is an essential regulator of cholesterol homeostasis

To address the importance of the farnesoid X-receptor (FXR; NR1H4) for normal cholesterol homeostasis, we evaluated the major pathways of cholesterol metabolism in the FXR-deficient (-/-) mouse model. Compared with wild-type, FXR(-/-) mice have increased plasma high density lipoprotein (HDL) cholesterol and a markedly reduced rate of plasma HDL cholesterol ester clearance. Concomitantly, FXR(-/-) mice exhibit reduced expression of hepatic genes involved in reverse cholesterol transport, most notably, that for scavenger receptor BI. FXR(-/-) mice also have increased: (i) plasma non-HDL cholesterol and triglyceride levels, (ii) apolipoprotein B-containing lipoprotein synthesis, and (iii) intestinal cholesterol absorption. Surprisingly, biliary cholesterol elimination was increased in FXR(-/-) mice, despite decreased expression of hepatic genes thought to be involved in this process. These data demonstrate that FXR is a critical regulator of normal cholesterol metabolism and that genetic changes affecting FXR function have the potential to be pro-atherogenic.

Overexpression of apoC-I in apoE-null mice: severe hypertriglyceridemia due to inhibition of hepatic lipase

Apolipoprotein C-I (apoC-I) has been proposed to act primarily via interference with apoE-mediated lipoprotein uptake. To define actions of apoC-I that are independent of apoE, we crossed a moderately overexpressing human apoC-I transgenic, which possesses a minimal phenotype in the WT background, with the apoE-null mouse. Surprisingly, apoE-null/C-I mice showed much more severe hyperlipidemia than apoE-null littermates in both the fasting and non-fasting states, with an almost doubling of cholesterol, primarily in IDL+LDL, and a marked increase in triglycerides; 3-fold in females to 260 +/- 80 mg/dl and 14-fold in males to 1409 +/- 594 mg/dl. HDL lipids were not significantly altered but HDL were apoC-I-enriched and apoA-II-depleted. Production rates of VLDL triglyceride were unchanged as was the clearance of post-lipolysis remnant particles. Plasma post-heparin hepatic lipase and lipoprotein lipase levels were undiminished as was the in vitro hydrolysis of apoC-I transgenic VLDL. However, HDL from apoC-I transgenic mice had a marked inhibitory effect on hepatic lipase activity, as did purified apoC-I. LPL activity was minimally affected. Atherosclerosis assay revealed significantly increased atherosclerosis in apoE-null/C-I mice assessed via the en face assay. Inhibition of hepatic lipase may be an important mechanism of the decrease in lipoprotein clearance mediated by apoC-I.

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.

Quantitative trait loci and candidate genes regulating HDL cholesterol: a murine chromosome map

OBJECTIVE

Summarizing the many discovered mouse and human quantitative trait loci (QTL) for high density lipoprotein (HDL) cholesterol (HDL-C) levels is important for guiding future research on the genetic regulation of HDL concentrations and for finding gene targets for upregulating HDL levels in mice and humans.

METHODS AND RESULTS

We summarized the 27 QTL and candidate genes associated with HDL-C concentrations in mice and plotted them on a mouse chromosome map. We also summarized the 22 human QTL for HDL-C levels and compared them with those of the mouse by comparative genomics. At least part of the mouse homologies for 18 of the 22 human HDL-C QTL were within the murine HDL-C QTL.

CONCLUSIONS

Murine QTL for HDL-C levels may predict their homologous location in humans, and their underlying genes may be appropriate genes to test in humans.

Lipases and HDL metabolism

Plasma levels of high-density lipoprotein (HDL) cholesterol are strongly inversely associated with atherosclerotic cardiovascular disease, and overexpression of HDL proteins, such as apolipoprotein A-I in animals, reduces progression and even induces regression of atherosclerosis. Therefore, HDL metabolism is recognized as a potential target for therapeutic intervention of atherosclerotic vascular diseases. The antiatherogenic properties of HDL include promotion of cellular cholesterol efflux and reverse cholesterol transport, as well as antioxidant, anti-inflammatory and anticoagulant properties. The molecular regulation of HDL metabolism is not fully understood, but it is influenced by several extracellular lipases. Here, we focus on new developments and insights into the role of secreted lipases on HDL metabolism and their relationship to atherosclerosis.