Postprandial lipemia is modified by the presence of the polymorphism present in the exon 1 variant at the SR-BI gene locus

SR-B1, a Key Receptor Involved in the Progression of Cardiovascular Disease: A Perspective from Mice and Human Genetic Studies

High plasma level of low-density lipoprotein (LDL) is the main driver of the initiation and progression of cardiovascular disease (CVD). Nevertheless, high-density lipoprotein (HDL) is considered an anti-atherogenic lipoprotein due to its role in reverse cholesterol transport and its ability to receive cholesterol that effluxes from macrophages in the artery wall. The scavenger receptor B class type 1 (SR-B1) was identified as the high-affinity HDL receptor, which facilitates the selective uptake of cholesterol ester (CE) into the liver via HDL and is also implicated in the plasma clearance of LDL, very low-density lipoprotein (VLDL) and lipoprotein(a) (Lp(a)). Thus, SR-B1 is a multifunctional receptor that plays a main role in the metabolism of different lipoproteins. The aim of this review is to highlight the association between SR-B1 and CVD risk through mice and human genetic studies.

Quantile-dependent expressivity of postprandial lipemia

Purpose

“Quantile-dependent expressivity” describes an effect of the genotype that depends upon the level of the phenotype (e.g., whether a subject’s triglycerides are high or low relative to its population distribution). Prior analyses suggest that the effect of a genetic risk score (GRS) on fasting plasma triglyceride levels increases with the percentile of the triglyceride distribution. Postprandial lipemia is well suited for testing quantile-dependent expressivity because it exposes each individual’s genotype to substantial increases in their plasma triglyceride concentrations. Ninety-seven published papers were identified that plotted mean triglyceride response vs. time and genotype, which were converted into quantitative data. Separately, for each published graph, standard least-squares regression analysis was used to compare the genotype differences at time t (dependent variable) to average triglyceride concentrations at time t (independent variable) to assess whether the genetic effect size increased in association with higher triglyceride concentrations and whether the phenomenon could explain purported genetic interactions with sex, diet, disease, BMI, and drugs.

Results

Consistent with the phenomenon, genetic effect sizes increased (P≤0.05) with increasing triglyceride concentrations for polymorphisms associated with ABCA1, ANGPTL4, APOA1, APOA2, APOA4, APOA5, APOB, APOC3, APOE, CETP, FABP2, FATP6, GALNT2, GCKR, HL, IL1b, LEPR, LOX-1, LPL, MC4R, MTTP, NPY, SORT1, SULF2, TNFA, TCF7L2, and TM6SF2. The effect size for these polymorphisms showed a progressively increasing dose-response, with intermediate effect sizes at intermediate triglyceride concentrations. Quantile-dependent expressivity provided an alternative interpretation to their interactions with sex, drugs, disease, diet, and age, which have been traditionally ascribed to gene-environment interactions and genetic predictors of drug efficacy (i.e., personalized medicine).

Conclusion

Quantile-dependent expressivity applies to the majority of genetic variants affecting postprandial triglycerides, which may arise because the impaired functionalities of these variants increase at higher triglyceride concentrations. Purported gene-drug interactions may be the manifestations of quantile-dependent expressivity, rather than genetic predictors of drug efficacy.

Assessment of postprandial triglycerides in clinical practice: Validation in a general population and coronary heart disease patients

BACKGROUND

Previous studies have suggested that for clinical purposes, subjects with fasting triglycerides (TGs) between 89-180 mg/dl (1-2 mmol/l) would benefit from postprandial TGs testing.

OBJECTIVE

To determine the postprandial TG response in 2 independent studies and validate who should benefit diagnostically from an oral-fat tolerance test (OFTT) in clinical practice.

METHODS

A population of 1002 patients with coronary heart disease (CHD) from the CORDIOPREV clinical trial and 1115 white US subjects from the GOLDN study underwent OFTTs. Subjects were classified into 3 groups according to fasting cut points of TGs to predict the usefulness of OFTT: (1) TG < 89 mg/dl (<1 mmol/l); (2) TG, 89-180 mg/dl (1-2 mmol/l); and (3) TG > 180 mg/dl (>2 mmol/l). Postprandial TG concentration at any point > 220 mg/dl (>2.5 mmol/l) has been pre-established as an undesirable postprandial response.

RESULTS

Of the total, 49% patients with CHD and 42% from the general population showed an undesirable response after the OFTT. The prevalence of undesirable postprandial TG in the CORDIOPREV clinical trial was 12.8, 50.3, and 89.7%, in group 1, 2, and 3, respectively (P < .001) and 11.2, 58.1, and 97.5% in group 1, 2, and 3, respectively (P < .001) in the GOLDN study.

CONCLUSIONS

These two studies validate the predictive values reported in a previous consensus. Moreover, the findings of the CORDIOPREV and GOLDN studies show that an OFTT is useful to identify postprandial hyperlipidemia in subjects with fasting TG between 1-2 mmol/l (89-180 mg/dL), because approximately half of them have hidden postprandial hyperlipidemia, which may influence treatment. An OFTT does not provide additional information regarding postprandial hyperlipidemia in subjects with low TG (<1 mmol/l, <89 mg/dL) or increased TG (>2 mmol/l, >180 mg/dl).

The Human Obesity Gene Map: The 2004 Update

This paper presents the eleventh update of the human obesity gene map, which incorporates published results up to the end of October 2004. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, transgenic and knockout murine models relevant to obesity, quantitative trait loci (QTLs) from animal cross-breeding experiments, association studies with candidate genes, and linkages from genome scans is reviewed. As of October 2004, 173 human obesity cases due to single-gene mutations in 10 different genes have been reported, and 49 loci related to Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and causal genes or strong candidates have been identified for most of these syndromes. There are 166 genes which, when mutated or expressed as transgenes in the mouse, result in phenotypes that affect body weight and adiposity. The number of QTLs reported from animal models currently reaches 221. The number of human obesity QTLs derived from genome scans continues to grow, and we have now 204 QTLs for obesity-related phenotypes from 50 genome-wide scans. A total of 38 genomic regions harbor QTLs replicated among two to four studies. The number of studies reporting associations between DNA sequence variation in specific genes and obesity phenotypes has also increased considerably with 358 findings of positive associations with 113 candidate genes. Among them, 18 genes are supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. Overall, >600 genes, markers, and chromosomal regions have been associated or linked with human obesity phenotypes. The electronic version of the map with links to useful publications and genomic and other relevant sites can be found at http://obesitygene.pbrc.edu.

Influence of polymorphisms and cholesterol-lowering treatment on SCARB1 mRNA expression

AIM

This study evaluated the influence of polymorphisms and cholesterol-lowering treatments on SCARB1 mRNA expression in peripheral blood mononuclear cells and in HepG2 and Caco-2 cells.

METHODS

Blood samples were drawn from normolipidemic (NL, n = 166) and hypercholesterolemic (HC, n = 123) individuals to extract DNA and total RNA and to analyze the lipid profile. After a 4-week washout period, 98 HC individuals were treated with atorvastatin (10 mg/day/4 weeks) whereas 25 were treated with ezetimibe (10 mg/day/4 weeks), followed by simvastatin (10 mg/day/8 weeks) and simvastatin plus ezetimibe (10 mg each/day/4 weeks). HepG2 and Caco-2 cells were treated with atorvastatin, simvastatin and ezetimibe at various concentrations for 12 and 24 h and collected for RNA extraction. SCARB1 mRNA expression was measured by TaqMan® assay and SCARB1 c.4G> A, c.726 + 54C> T and c.1080C> T polymorphisms were detected by PCR-RFLP.

RESULTS

High LDL cholesterol (> 160 mg/dL) values were associated with low baseline SCARB1 mRNA expression in PBMC. Allele T carriers for SCARB1 c.726+54C> T had lower basal SCARB1 transcription in PBMC (p < 0.05). Simvastatin, atorvastatin and ezetimibe treatments did not modify the SCARB1 mRNA level in PBMC from HC patients. Similarly, these cholesterol-lowering drugs did not modulate the SCARB1 expression in HepG2 and Caco-2 cells in spite of the concentration and time of exposure (p > 0.05).

CONCLUSION

LDL cholesterol levels and SCARB1 c.726 + 54C> T are associated with low mRNA expression in mononuclear cells. Cholesterol-lowering drugs do not modulate SCARB1 expression in PBMC from HC subjects or in HepG2 and Caco-2 cells.

Identification of novel inhibitors of dietary lipid absorption using zebrafish

Pharmacological inhibition of dietary lipid absorption induces favorable changes in serum lipoprotein levels in patients that are at risk for cardiovascular disease and is considered an adjuvant or alternative treatment with HMG-CoA reductase inhibitors (statins). Here we demonstrate the feasibility of identifying novel inhibitors of intestinal lipid absorption using the zebrafish system. A pilot screen of an unbiased chemical library identified novel compounds that inhibited processing of fluorescent lipid analogues in live zebrafish larvae. Secondary assays identified those compounds suitable for testing in mammals and provided insight into mechanism of action, which for several compounds could be distinguished from ezetimibe, a drug used to inhibit cholesterol absorption in humans that broadly inhibited lipid absorption in zebrafish larvae. These findings support the utility of zebrafish screening assays to identify novel compounds that target complex physiological processes.

Effects of variations in the APOA1/C3/A4/A5 gene cluster on different parameters of postprandial lipid metabolism in healthy young men

The APOA1/C3/A4/A5 gene cluster encodes important regulators of fasting lipids, but the majority of lipid metabolism takes place in the postprandial state and knowledge about gene regulation in this state is scarce. With the aim of characterizing possible regulators of lipid metabolism, we studied the effects of nine single nucleotide polymorphisms (SNPs) during postprandial lipid metabolism. Eighty-eight healthy young men were genotyped for APOA1 -2630 (rs613808), APOA1 -2803 (rs2727784), APOA1 -3012 (rs11216158), APOC3 -640 (rs2542052), APOC3 -2886 (rs2542051), APOC3 G34G (rs4520), APOA4 N147S (rs5104), APOA4 T29T (rs5092), and A4A5_inter (rs1263177) and were fed a saturated fatty acid-rich meal (1g fat/kg of weight with 60% fat, 15% protein and 25% carbohydrate). Serial blood samples were extracted for 11 h after the meal. Total cholesterol and fractions [HDL-cholesterol, LDL-cholesterol, trifacylglycerols (TGs) in plasma, TG-rich lipoproteins (TRLs) (large TRLs and small TRLs), apolipoprotein A-I and apolipoprotein B] were determined. APOA1 -2803 homozygotes for the minor allele and A4A5_inter carriers showed a limited degree of postprandial lipemia. Carriers of the rare alleles of APOA4 N147S and APOA4 T29T had lower APOA1 plasma concentration during this state. APOC3 -640 was associated with altered TG kinetics but not its magnitude. We have identified new associations between SNPs in the APOA1/C3/A4/A5 gene cluster and altered postprandial lipid metabolism.

ABCA1 gene variants regulate postprandial lipid metabolism in healthy men

OBJECTIVE

Genetic variants of ABCA1, an ATP-binding cassette (ABC) transporter, have been linked to altered atherosclerosis progression and fasting lipid concentration, mainly high-density lipoproteins and apolipoprotein A1; however, results from different studies have been inconsistent.

METHODS AND RESULTS

To further characterize the effects of ABCA1 variants in human postprandial lipid metabolism, we studied the influence of 3 single nucleotide polymorphisms (i27943 [rs2575875]; i48168 [rs4149272]; R219K [rs2230806]) in the postprandial lipemia of 88 normolipidemic young men who were given a fatty meal. For i27943 and i48168 single nucleotide polymorphisms, fasting and postprandial values of apolipoprotein A1 were higher and postprandial lipemia was much lower in homozygotes for the major alleles, total triglycerides in plasma, and large triglyceride-rich lipoprotein triglycerides. These persons also showed a higher apolipoprotein A1/apolipoprotein B ratio. Major allele homozygotes for i48168 and i27943 showed additionally higher high-density lipoproteins and lower postprandial apolipoprotein B.

CONCLUSION

Our work shows that major allele homozygotes for ABCA1 single nucleotide polymorphisms i27943 and i48168 have a lower postprandial response as compared to minor allele carriers. This finding may further characterize the role of ABCA1 in lipid metabolism.

Adiponectin gene variants are associated with insulin sensitivity in response to dietary fat consumption in Caucasian men

Adiponectin (adipoQ) gene variants have been associated with type 2 diabetes mellitus and insulin resistance. Our aim was to examine whether the presence of several polymorphisms at the adipoQ gene locus (-11391 G > A, -11377 C > G, 45 T > G, and 276 G > T) influences the insulin sensitivity to dietary fat. Healthy volunteers (30 men and 29 women) consumed 3 diets for 4 wk each: an initial period during which all subjects consumed a SFA-rich diet (38% total fat, 20% SFA), followed by a carbohydrate-rich diet (CHO) (30% total fat, 55% carbohydrate) or a monounsaturated fatty acid (MUFA)-rich diet (38% total fat, 22% MUFA) following a randomized, crossover design. After participants consumed each diet, we tested peripheral insulin sensitivity with the insulin suppression test and measured plasma adiponectin concentrations. C/C homozygous men for the -11377 C > G single nucleotide polymorphism (SNP) had a significantly greater decrease in the steady-state plasma glucose concentrations when changing from the SFA-rich (8.95 +/- 0.6 mmol/L) to the MUFA-rich (6.04 +/- 0.31 mmol/L) and CHO-rich (6.35 +/- 0.38 mmol/L) diets than did those carrying the minor G allele (SFA, 6.65 +/- 0.4 mmol/L; MUFA, 6.45 +/- 0.4 mmol/L; CHO, 5.83 +/- 0.3 mmol/L) (P sex x gene x diet interaction = 0.016). These differences did not occur in female participants. Furthermore, C/C men had lower plasma adiponectin concentrations than did C/C women (P sex x gene interaction = 0.015), independently of the dietary fat consumed. None of the variables examined were significantly associated with -11426 A > G, 45T > G, or 276 G > T SNP. In conclusion, C/C homozygous men for the -11377 C > G SNP at adipoQ gene were significantly less insulin resistant after consumption of the MUFA- and CHO-rich diets compared with the SFA-rich diet. This information should help in the identification of vulnerable populations or persons who will benefit from more personalized and mechanism-based dietary recommendations.

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.

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.

Peroxisome proliferator-activated receptor α polymorphisms and postprandial lipemia in healthy men

Peroxisome proliferator-activated receptor alpha (PPARalpha) is a ligand-dependent transcription factor that plays a key role in lipid and glucose homeostasis. This study evaluated whether variants of PPARalpha are associated with postprandial lipemia. Subjects were given a single fat load composed of 60% calories as fat, 15% as protein, and 25% as carbohydrate. Blood was drawn every hour from baseline to 6 h, then every 2.5 h to 11 h to determine triglyceride (TG) levels. The minor allele of the nonsynonymous p.Leu162Val variant was associated with higher fasting total cholesterol, LDL-cholesterol, and apolipoprotein B. There were no significant associations with all of the postprandial parameters examined. Conversely, the noncoding variant c.140+5435T>C was not associated with fasting lipid concentrations but was significantly associated with decreased postprandial TG and cholesterol in the small TG-rich lipoprotein particle. Although the minor allele carriers displayed lower mean concentrations of TG and cholesterol throughout the postprandial period, the differences were most pronounced in the latter period. These data suggest that PPARalpha variants may modulate the risk of cardiovascular disease by influencing both fasting and postprandial lipid concentrations.

The APOB -516C/T polymorphism is associated with differences in insulin sensitivity in healthy males during the consumption of diets with different fat content

Several apo B polymorphic sites have been studied for their potential use as markers for CHD in the population and for potential gene-diet interactions. Our aim was to determine whether the presence of the -516C/T polymorphism in the APOB gene promoter modifies insulin sensitivity to dietary fat. We studied fifty-nine healthy volunteers (thirty men and twenty-nine women, thirty-six homozygotes for the -516C allele (C/C) (nineteen males and seventeen females) and twenty-three heterozygotes for the -516T allele (C/T) (eleven males and twelve females)). Subjects consumed three diets during the feeding study, 4 weeks each: an SFA-rich diet (38 % fat, 20 % SFA), followed by a carbohydrate (CHO)-rich diet (30 % fat, 55 % CHO) or a MUFA-rich diet (38 % fat, 22 % MUFA) following a randomised cross-over design. For each diet, we investigated peripheral insulin sensitivity with the insulin suppression test. Male carriers of the -516T allele showed a significantly greater decrease in steady-state plasma glucose concentrations when changing from the SFA-rich diet (9.18 (sd 1.35) mmol/l) to the MUFA (6.55 (sd 0.74) mmol/l) or the CHO (6.31 (sd 0.93) mmol/l) diets than did those who were homozygous for the C allele (P = 0.040). Furthermore, C/T subjects presented higher plasma NEFA values after consumption of the SFA diet compared with the MUFA and CHO diets (P = 0.001). This effect was not observed in females (P = 0.908). Our findings show that male carriers of the -516T allele, C/T, have a significant increase in insulin resistance after consumption of all diets, but the difference is more exaggerated after the SFA diet compared with the MUFA- and CHO-rich diets.

Effects of amino acid substitutions at glycine 420 on SR-BI cholesterol transport function

Scavenger receptor class B type I (SR-BI) facilitates the uptake of HDL cholesteryl esters (CEs) in a two-step process involving binding of HDL to its extracellular domain and transfer of HDL core CEs to a metabolically active membrane pool, where they are subsequently hydrolyzed by a neutral CE hydrolase. Recently, we characterized a mutant, G420H, which replaced glycine 420 in the extracellular domain of SR-BI with a histidine residue and had a profound effect on SR-BI function. The G420H mutant receptor exhibited a reduced ability to mediate selective HDL CE uptake and was unable to deliver HDL CE for hydrolysis, despite the fact that it retained the ability to bind HDL. This did not hold true if glycine 420 was replaced with an alanine residue; G420A maintained wild-type HDL binding and cholesterol transport activity. To further understand the role that glycine 420 plays in SR-BI function and why there was a disparity between replacing glycine 420 with a histidine versus an alanine, we generated a battery of point mutants by substituting glycine 420 with amino acids possessing side chains that were charged, hydrophobic, polar, or bulky and tested the resulting mutants for their ability to support HDL binding, HDL cholesterol transport, and delivery for hydrolysis. The results indicated that substitution with a negatively charged residue or a proline impaired cell surface expression of SR-BI or its interaction with HDL, respectively. Furthermore, substitution of glycine 420 with a positively charged residue reduced HDL CE uptake as well as its subsequent hydrolysis.

Scavenger receptor class B type I (SCARB1) c.1119C>T polymorphism affects postprandial triglyceride metabolism in men

The scavenger receptor class B type I (SCARB1) is a cell surface glycoprotein that plays a key role in reverse cholesterol transport. A polymorphism in exon 8 (c.1119C>T) has been associated with fasting HDL- and LDL- cholesterol concentrations in Caucasian populations. This study evaluated whether this variant is associated with postprandial lipemia in 59 normolipidemic males. After an overnight fast, the subjects were given a single fat load comprising 60% of energy as fat, 15% as protein, and 25% as carbohydrate. Blood was drawn every hour from baseline to 6 h, then every 2.5 h until h 11. We measured plasma lipid concentrations including triglycerides (TG) in large and small triglyceride rich lipoprotein particles (TRL). Changes in postprandial small TRL TG differed among groups over time (Pgeno x time = 0.034) whereby TT and CT subjects maintained lower concentrations throughout most of the postprandial period compared with CC subjects. Significant differences occurred at h 1, 2, 4, and 5 (P < 0.05). Postprandial changes in large TRL TG (Pgeno x time = 0.923) or total TG (Pgeno x time = 0.529) did not differ by genotype. These results suggest that the c.1119C>T polymorphism is associated with a lower postprandial TG response in the smaller, partially catabolized lipoprotein fraction.

Regulation of postprandial lipemia: an update on current trends

People spend a large percentage of their waking hours in the postprandial state. Postprandial lipemia is associated with disruptions in lipoprotein metabolism and inflammatory factors, cardiovascular disease, MetS, and diabetes. Commonly, the dietary sources of fat exceed the actual needs and the tissues are faced with the excess, with accumulation of chylomicrons and remnant particles. This review will summarize recent findings in postprandial lipemia research with a focus on human studies. The effects of dietary factors and other meal components on postprandial lipemia leads to the following question: do we need a standardized oral lipid tolerance test (OLTT)? An overview of recent findings on FABP2, MTP, LPL, apoAV, and ASP and the effects of body habitus (sex influence and body size), as well as exercise and weight loss, on postprandial lipemia will be summarized.

Dietary fat, genes and insulin sensitivity

Both insulin resistance and dyslipidaemia are determined by genetic and environmental factors. Depending on their expression and their function, gene variants may influence either insulin action or dyslipidaemia. The purpose of this review was to give some examples from recent studies of gene variants that influence insulin signalling and the interaction between gene and diet to predispose insulin resistance. Recent findings indicate a major role for genetic susceptibility to the insulin resistance syndrome. Nutrition also plays an important role in the development and progression of the condition. Genetic background may interact with habitual dietary fat composition, affecting predisposition to the insulin resistance syndrome and individual responsiveness to changes in dietary fat intake. Due to the complex nature of gene-environment interactions, therefore, therapeutic dietary therapy may require a 'personalized' nutrition approach in the future. Although results have not always been consistent, gene variants that affect primary insulin action or dyslipidaemia, and particularly their interaction with the environment, are important modulators of glucose metabolism and insulin resistance syndrome.

Genetic and nutrient determinants of the metabolic syndrome

PURPOSE OF REVIEW

The metabolic syndrome is a very common condition that is associated with an increased risk of type 2 diabetes mellitus and cardiovascular disease. The diverse clinical characteristics illustrate the complexity of the disease, involving several dysregulated metabolic pathways and multiple genetic targets. The increasing prevalence of obesity heightens the requirement to reduce the risk of the metabolic syndrome. In order to understand the aetiology, it is critical to appreciate the nature of multiple gene-gene and gene-nutrient interactions relevant to the metabolic syndrome.

RECENT FINDINGS

Research indicates a major role for genetic susceptibility to the metabolic syndrome. Nutrition clearly plays an important role in the development and progression of the condition. Genetic background can interact with habitual dietary fat composition, thereby affecting predisposition to the metabolic syndrome, and may also determine an individual's responsiveness to altered dietary fat intake. These studies indicate that therapeutic dietary therapy may require a 'personalized nutrition' approach, wherein a particular genetic profile may determine responsiveness of patients to specific dietary fatty acid interventions.

SUMMARY

Understanding the biological impact of gene-nutrient interactions will provide a key insight into the pathogenesis and progression of diet-related polygenic disorders. This review explores the hypothesis that genetic components of the metabolic syndrome may be modified by dietary fatty acid composition.

Postprandial lipoprotein metabolism, genes and risk of cardiovascular disease

PURPOSE OF REVIEW

Several lines of evidence suggest that postprandial lipemia increases the risk of atherogenesis, and in each of the systems involved in postprandial metabolism the roles of many genes have been explored in order to establish the possible implications of their variability in coronary heart disease risk.

RECENT FINDINGS

This report focuses on recent results pertaining to postprandial lipoprotein metabolism and genes, their variability and their relationship with intermediate phenotypes and coronary heart disease. The postprandial lipid response was modified by polymorphisms within the genes for apolipoprotein AI, apolipoprotein E, apolipoprotein B, apolipoprotein CI, apolipoprotein CIII, apolipoprotein AIV, apolipoprotein AV, lipoprotein lipase, hepatic lipase, fatty acid-binding protein-2, the fatty acid transport proteins, microsomal triglyceride transfer protein and scavenger receptor class B type I. We also discuss recent advances in the effects of gene regulation using knockdown animal models on postprandial lipoprotein metabolism.

SUMMARY

The review discusses several of these factors as well as the potential impact of gene polymorphism on the variability of postprandial lipoprotein metabolism as intermediate phenotypes for coronary heart disease. The variability in postprandial lipid response is highly complex. Future studies will need to be large if they are to assess the effects of multiple polymorphisms.

Accelerated lipid absorption in mice overexpressing intestinal SR-BI

Dietary cholesterol absorption contributes to a large part of the circulating cholesterol. However, the mechanism of sterol intestinal uptake is not clearly elucidated. Scavenger receptor class B type I (SR-BI), major component in the control of cholesterol homeostasis, is expressed in the intestine, but its role in this organ remains unclear. We have generated transgenic mice overexpressing SR-BI primarily in the intestine by using the mouse SR-BI gene under the control of intestinal specific "apoC-III enhancer coupled with apoA-IV promoter." We found SR-BI overexpression with respect to the natural protein along the intestine and at the top of the villosities. After a meal containing [(14)C]cholesterol and [(3)H]triolein, SR-BI transgenic mice presented a rise in intestinal absorption of both lipids that was not due to a defect in chylomicron clearance nor to a change in the bile flow or the bile acid content. Nevertheless, SR-BI transgenic mice showed a decrease of total cholesterol but an increase of triglyceride content in plasma without any change in the high density lipoprotein apoA-I level. Thus, we described for the first time a functional role in vivo for SR-BI in cholesterol but also in triglyceride intestinal absorption.

Genetic variation and lipid metabolism: modulation by dietary factors

Cardiovascular diseases (CVD) result from complex interactions between genetic and environmental factors. The evidence supports that gene-environment interactions modulate plasma lipid concentrations and potentially CVD risk. The findings from studies examining gene-diet interactions and lipid metabolism have been promising. Several loci (eg, APOA1, APOE, LIPC) are providing proof of concept for the application of genetics in the context of personalized nutrition for CVD prevention. The spectrum of candidate genes has been expanding to incorporate those involved in intracellular lipid metabolism (eg, iPPARs, CYP7A1). However, the practical application of these findings is not ready for prime time. There is a compelling need for replication using a higher level of scientific evidence. Moreover, we need to evolve from the simple scenarios examined nowadays (ie, one single dietary component, SNP, and risk factor) to more realistic situations involving multiple interactions. In summary, there is need for both large population studies and well-standardized intervention studies.

Hepatic high-density lipoprotein receptors: roles in lipoprotein metabolism and potential for therapeutic modulation

High-density lipoprotein (HDL) plays an important role in protection against atherosclerosis. A major part of HDL's antiatherogenic role is through mediating reverse cholesterol transport from peripheral cells, such as macrophages and other cells in the artery wall, to the liver. Hepatic HDL receptors should, therefore, play an important role in either mediating or modulating HDL-dependent reverse cholesterol transport. The scavenger receptor class B type I (SR-BI) was first identified as a hepatic HDL receptor almost 10 years ago and is well characterized at the molecular level. This review highlights recent studies that provide insight into the cellular pathways involved in SR-BI-mediated lipid transfer between bound lipoproteins and cells, supports a role for this receptor in reverse cholesterol transport and protection against experimental atherosclerosis in mice, and explores the consequences of sequence variations in the gene encoding SR-BI in humans.

SINGLE NUCLEOTIDE POLYMORPHISMS THAT INFLUENCE LIPID METABOLISM: Interaction with Dietary Factors

Cardiovascular disease (CVD) risk is the result of complex interactions between genetic and environmental factors. During the past few decades, much attention has focused on plasma lipoproteins as CVD risk factors. The current evidence supports the concept that gene-environment interactions modulate plasma lipid concentrations and potentially CVD risk. The findings from studies examining gene-diet interactions and lipid metabolism have been highly promising. Several loci (i.e., APOA1, APOA4, APOE, and LIPC) are providing proof-of-concept for the potential application of genetics in the context of personalized nutritional recommendations for CVD prevention. However, the incorporation of these findings to the clinical environment is not ready for prime time. There is a compelling need for replication using a higher level of scientific evidence. Moreover, we need to evolve from the simple scenarios examined nowadays (i.e., one single dietary component, single nucleotide polymorphism, and risk factor) to more realistic situations involving interactions between multiple genes, dietary components, and risk factors. In summary, there is need for both large population studies and well-standardized intervention studies.

Scavenger receptor BI and ATP-binding cassette transporter A1 in reverse cholesterol transport and atherosclerosis

PURPOSE OF REVIEW

The appearance of scavenger receptor class B type I (SR-BI) and ATP-binding cassette transporter A1 (ABCA1) in macrophages and liver implicates these transporters in different stages of reverse cholesterol transport. This review focuses on the role of SR-BI and ABCA1 in reverse cholesterol transport in the context of atherosclerotic lesion development.

RECENT FINDINGS

Recent studies indicate that hepatic expression of ABCA1 and SR-BI is important for the generation of nascent HDL and the delivery of HDL cholesteryl esters to the liver, respectively. Although macrophage SR-BI and ABCA1 do not contribute significantly to circulating HDL levels, the perpetual cycle of HDL lipidation and delipidation by the liver ensures the availability of acceptors for cholesterol efflux that maintain cholesterol homeostasis in arterial macrophages, thereby reducing atherogenesis. In addition to its established role in the selective uptake of HDL cholesteryl esters, there is now evidence that hepatic SR-BI facilitates postprandial lipid metabolism, and that hepatic secretion of VLDL is dependent on ABCA1-mediated nascent HDL formation. Thus, remnant and HDL metabolism are more intimately intertwined in hepatic lipid metabolism than has previously been appreciated.

SUMMARY

Recent advances in the understanding of the role of ABCA1 and SR-BI in HDL metabolism and their atheroprotective properties indicate the significant potential of modulating ABCA1 and SR-BI expression in both arterial wall macrophages and the liver for the treatment of atherosclerotic coronary artery disease.

Adenovirus-mediated hepatic overexpression of scavenger receptor class B type I accelerates chylomicron metabolism in C57BL/6J mice

The function of scavenger receptor class B type I (SR-BI) in mediating the selective uptake of HDL cholesteryl esters is well established. In SR-BI-deficient mice, we recently observed a delayed postprandial triglyceride (TG) response, suggesting an additional role for SR-BI in facilitating chylomicron (CM) metabolism. Here, we assessed the effect of adenovirus-mediated hepatic overexpression of SR-BI (Ad.SR-BI) in C57BL/6J mice on serum lipids and CM metabolism. Infection of 5 x 10(8) plaque-forming units per mouse of Ad.SR-BI significantly decreases serum cholesterol (>90%), phospholipids (>90%), and TG levels (50%), accompanied by a 41.4% reduction (P < 0.01) in apolipoprotein B-100 levels. The postprandial TG response is 2-fold lower in mice treated with Ad.SR-BI compared with control mice (area under the curve = 31.4 +/- 2.4 versus 17.7 +/- 3.2; P < 0.05). Hepatic mRNA expression levels of genes known to be involved in serum cholesterol and TG clearance are unchanged and thus could not account for the decreased plasma TG levels and the change in postprandial response. We conclude that overexpression of SR-BI accelerates CM metabolism, possibly by mediating the initial capture of CM remnants by the liver, whereby the subsequent internalization can be exerted by additional receptor systems such as the LDL receptor (LDLr) and LDLr-related protein 1.