Evidence that hepatic lipase deficiency in humans is not associated with proatherogenic changes in HDL composition and metabolism

Phenotype in Individuals with Heterozygous Rare Variants in LIPC Encoding Hepatic Lipase

Hepatic lipase deficiency is a rare genetic condition caused by biallelic loss-of-function variants in the LIPC gene encoding hepatic lipase. These variants reduce or abolish the protein's lipolytic activity, resulting in elevated plasma lipids. The condition is classified as autosomal recessive, since dyslipidemia is inconsistently observed in heterozygous patients with only one LIPC variant. However, this has been concluded from historical studies encompassing a few families and having very small sample sizes. Here, we conduct a retrospective chart review of 46 heterozygous subjects, each harboring one rare pathogenic LIPC variant. We compare plasma levels of total cholesterol, low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), triglycerides, and apolipoprotein B to those of matched controls without LIPC variants. Variant pathogenicity is classified according to the guidelines of the American College of Medical Genetics and Genomics. We observe that levels of total cholesterol, LDL-C, and triglycerides are significantly elevated in the LIPC variant heterozygotes, but HDL-C and apolipoprotein B are not. When filtering solely with respect to pathogenic or likely pathogenic variants, all lipid variables emerge as significantly elevated compared to controls. Thus, hepatic lipase deficiency may not necessarily be a recessive condition, but perhaps semi-dominant since individuals with one variant are phenotypically distinct from the controls. These hypothesis-generating findings regarding LIPC genetic variations observed in a clinical cohort should be evaluated in larger populations and databases.

Deciphering structural aspects of reverse cholesterol transport: mapping the knowns and unknowns

Atherosclerosis is a major contributor to the onset and progression of cardiovascular disease (CVD). Cholesterol-loaded foam cells play a pivotal role in forming atherosclerotic plaques. Induction of cholesterol efflux from these cells may be a promising approach in treating CVD. The reverse cholesterol transport (RCT) pathway delivers cholesteryl ester (CE) packaged in high-density lipoproteins (HDL) from non-hepatic cells to the liver, thereby minimising cholesterol load of peripheral cells. RCT takes place via a well-organised interplay amongst apolipoprotein A1 (ApoA1), lecithin cholesterol acyltransferase (LCAT), ATP binding cassette transporter A1 (ABCA1), scavenger receptor-B1 (SR-B1), and the amount of free cholesterol. Unfortunately, modulation of RCT for treating atherosclerosis has failed in clinical trials owing to our lack of understanding of the relationship between HDL function and RCT. The fate of non-hepatic CEs in HDL is dependent on their access to proteins involved in remodelling and can be regulated at the structural level. An inadequate understanding of this inhibits the design of rational strategies for therapeutic interventions. Herein we extensively review the structure-function relationships that are essential for RCT. We also focus on genetic mutations that disturb the structural stability of proteins involved in RCT, rendering them partially or completely non-functional. Further studies are necessary for understanding the structural aspects of RCT pathway completely, and this review highlights alternative theories and unanswered questions.

Hypertriglyceridemia Induced Acute Pancreatitis Caused by a Novel LIPC Gene Variant in a Pediatric Patient

Hypertriglyceridemia induced acute pancreatitis is a rare cause of pancreatitis in children. Hepatic lipase deficiency is an extremely rare cause of hypertriglyceridemia, reported in only a few families to date. Hepatic lipase is the enzyme involved in the hydrolysis of triglycerides and phospholipids in remnants of triglyceride-rich lipoproteins that have a role in the conversion of very low density lipoprotein remnants to low density lipoproteins. Hepatic lipase deficiency is inherited in an autosomal recessive pattern. Detection of heterozygous carriers of hepatic lipase mutations remains accidental at the population level, as affected persons with a heterozygous state of hepatic lipase mutation do not display specific lipoprotein abnormalities and also patients with complete hepatic lipase deficiency have inconstant phenotype. The proximal promoter of the LIPC gene consists of four polymorphic sites in complete linkage disequilibrium. Five missense mutations in encoding exons have been described and proved to be responsible for hepatic lipase deficiency to date: S267F, T383M, L334F, A174T, and R186H, affecting the activity and secretion of hepatic lipase. We identified a primary disorder of the lipid metabolism as the cause of the acute episode of pancreatitis in a four years old patient, consisting of hepatic lipase deficiency caused by a novel genetic variant of the LIPC gene, a gross deletion of the genomic region encompassing exon 1. This variant was not previously described in the literature in persons with LIPC-related disorders and its significance is currently uncertain, but in the presented clinical and paraclinical context, it has the characteristics of a pathological variant inducing a hepatic lipase deficiency phenotype.

Lipid and metabolic syndrome traits in coronary artery disease: a Mendelian randomization study

Mendelian randomization (MR) of lipid traits in CAD has provided evidence for causal associations of LDL-C and TGs in CAD, but many lipid trait genetic variants have pleiotropic effects on other cardiovascular risk factors that may bias MR associations. The goal of this study was to evaluate pleiotropic effects of lipid trait genetic variants and to account for these effects in MR of lipid traits in CAD. We performed multivariable MR using inverse variance-weighted and MR-Egger methods in large (n ≥ 300,000) GWAS datasets. We found that 30% of lipid trait genetic variants have effects on metabolic syndrome traits, including BMI, T2D, and systolic blood pressure (SBP). Nonetheless, in multivariable MR analysis, LDL-C, HDL-C, TGs, BMI, T2D, and SBP are independently associated with CAD, and each of these associations is robust to adjustment for directional pleiotropy. MR at loci linked to direct effects on HDL-C and TGs suggests locus- and mechanism-specific causal effects of these factors on CAD.

High-density lipoprotein subpopulation profiles in lipoprotein lipase and hepatic lipase deficiency

BACKGROUND AND AIMS

Our aim was to gain insight into the role that lipoprotein lipase (LPL) and hepatic lipase (HL) plays in HDL metabolism and to better understand LPL- and HL-deficiency states.

METHODS

We examined the apolipoprotein (apo) A-I-, A-II-, A-IV-, C-I-, C-III-, and E-containing HDL subpopulation profiles, assessed by native 2-dimensional gel-electrophoresis and immunoblotting, in 6 homozygous and 11 heterozygous LPL-deficient, 6 homozygous and 4 heterozygous HL-deficient, and 50 control subjects.

RESULTS

LPL-deficient homozygotes had marked hypertriglyceridemia and significant decreases in LDL-C, HDL-C, and apoA-I. Their apoA-I-containing HDL subpopulation profile was shifted toward small HDL particles compared to controls. HL-deficient homozygotes had moderate hypertriglyceridemia, modest increases in LDL-C and HDL-C level, but normal apoA-I concentration. HL-deficient homozygotes had a unique distribution of apoA-I-containing HDL particles. The normally apoA-I:A-II, intermediate-size (α-2 and α-3) particles were significantly decreased, while the normally apoA-I only (very large α-1, small α-4, and very small preβ-1) particles were significantly elevated. In contrast to control subjects, the very large α-1 particles of HL-deficient homozygotes were enriched in apoA-II. Homozygous LPL- and HL-deficient subjects also had abnormal distributions of apo C-I, C-III, and E in HDL particles. Values for all measured parameters in LPL- and HL-deficient heterozygotes were closer to values measured in controls than in homozygotes.

CONCLUSIONS

Our data are consistent with the concept that LPL is important for the maturation of small discoidal HDL particles into large spherical HDL particles, while HL is important for HDL remodeling of very large HDL particles into intermediate-size HDL particles.

Advances in the Study of the Antiatherogenic Function and Novel Therapies for HDL

The hypothesis that raising high-density lipoprotein cholesterol (HDL-C) levels could improve the risk for cardiovascular disease (CVD) is facing challenges. There is multitudinous clear clinical evidence that the latest failures of HDL-C-raising drugs show no clear association with risks for CVD. At the genetic level, recent research indicates that steady-state HDL-C concentrations may provide limited information regarding the potential antiatherogenic functions of HDL. It is evident that the newer strategies may replace therapeutic approaches to simply raise plasma HDL-C levels. There is an urgent need to identify an efficient biomarker that accurately predicts the increased risk of atherosclerosis (AS) in patients and that may be used for exploring newer therapeutic targets. Studies from recent decades show that the composition, structure and function of circulating HDL are closely associated with high cardiovascular risk. A vast amount of data demonstrates that the most important mechanism through which HDL antagonizes AS involves the reverse cholesterol transport (RCT) process. Clinical trials of drugs that specifically target HDL have so far proven disappointing, so it is necessary to carry out review on the HDL therapeutics.

HDL biogenesis, remodeling, and catabolism

In this chapter, we review how HDL is generated, remodeled, and catabolized in plasma. We describe key features of the proteins that participate in these processes, emphasizing how mutations in apolipoprotein A-I (apoA-I) and the other proteins affect HDL metabolism. The biogenesis of HDL initially requires functional interaction of apoA-I with the ATP-binding cassette transporter A1 (ABCA1) and subsequently interactions of the lipidated apoA-I forms with lecithin/cholesterol acyltransferase (LCAT). Mutations in these proteins either prevent or impair the formation and possibly the functionality of HDL. Remodeling and catabolism of HDL is the result of interactions of HDL with cell receptors and other membrane and plasma proteins including hepatic lipase (HL), endothelial lipase (EL), phospholipid transfer protein (PLTP), cholesteryl ester transfer protein (CETP), apolipoprotein M (apoM), scavenger receptor class B type I (SR-BI), ATP-binding cassette transporter G1 (ABCG1), the F1 subunit of ATPase (Ecto F1-ATPase), and the cubulin/megalin receptor. Similarly to apoA-I, apolipoprotein E and apolipoprotein A-IV were shown to form discrete HDL particles containing these apolipoproteins which may have important but still unexplored functions. Furthermore, several plasma proteins were found associated with HDL and may modulate its biological functions. The effect of these proteins on the functionality of HDL is the topic of ongoing research.

Reactive Oxygen Species, SUMOylation, and Endothelial Inflammation

Although the exact mechanism through which NADPH oxidases (Nox's) generate reactive oxygen species (ROS) is still not completely understood, it is widely considered that ROS accumulation is the cause of oxidative stress in endothelial cells. Increasing pieces of evidence strongly indicate the role for ROS in endothelial inflammation and dysfunction and subsequent development of atherosclerotic plaques, which are causes of various pathological cardiac events. An overview for a causative relationship between ROS and endothelial inflammation will be provided in this review. Particularly, a crucial role for specific protein SUMOylation in endothelial inflammation will be presented. Given that SUMOylation of specific proteins leads to increased endothelial inflammation, targeting specific SUMOylated proteins may be an elegant, effective strategy to control inflammation. In addition, the involvement of ROS production in increasing the risk of recurrent coronary events in a sub-group of non-diabetic, post-infarction patients with elevated levels of HDL-cholesterol will be presented with the emphasis that elevated HDL-cholesterol under certain inflammatory conditions can lead to increased incidence of cardiovascular events.

Apolipoprotein E predicts incident cardiovascular disease risk in women but not in men with concurrently high levels of high-density lipoprotein cholesterol and C-reactive protein

Although there is great interest in the notion that dysfunctional transformation of high-density lipoprotein (HDL) facilitates development of atherosclerosis and cardiovascular disease (CVD), few studies in human populations directly address this issue. As apolipoprotein E (apoE) is a constituent of HDL thought to be important for HDL antiatherogenic function, we sought to assess the role of apoE in CVD risk in subjects likely to display dysfunctional transformation of HDL. Association of apoE levels with incident CVD risk was investigated using Cox multivariable proportional hazards modeling. Analyses were performed in subgroups of women and men likely to display dysfunctional transformation of HDL deriving from previous subgroup identification based upon defining characteristics of concurrently high levels of HDL cholesterol and systemic inflammation as reflected by high C-reactive protein levels. Results revealed apoE levels (dichotomized as highest quartile vs combined 3 lowest quartiles) as predicting subgroup risk in women (hazard ratio, 4.52; 95% confidence interval, 1.07-19.12; P = .040) but not in men. Further sex differences were manifested in terms of the relationship of apoE levels with age. Analysis revealed positive correlation of apoE levels with age in women (r = 0.47, P < .0001) but not in men (r = 0.04, P = .43). Apolipoprotein E levels predict incident CVD risk in women with high levels of HDL cholesterol and C-reactive protein but not in men. Future studies should be oriented toward investigations of apoE as related to multiplicity of HDL functionality and toward assessment of potential roles for apoE in dysfunctional transformation of HDL.

Apolipoprotein A-II influences apolipoprotein E-linked cardiovascular disease risk in women with high levels of HDL cholesterol and C-reactive protein

Background

In a previous report by our group, high levels of apolipoprotein E (apoE) were demonstrated to be associated with risk of incident cardiovascular disease in women with high levels of C-reactive protein (CRP) in the setting of both low (designated as HR1 subjects) and high (designated as HR2 subjects) levels of high-density lipoprotein cholesterol (HDL-C).

To assess whether apolipoprotein A-II (apoA-II) plays a role in apoE-associated risk in the two female groups.

Methodology/Principal

Outcome event mapping, a graphical data exploratory tool; Cox proportional hazards multivariable regression; and curve-fitting modeling were used to examine apoA-II influence on apoE-associated risk focusing on HDL particles with apolipoprotein A-I (apoA-I) without apoA-II (LpA-I) and HDL particles with both apoA-I and apoA-II (LpA-I:A-II). Results of outcome mappings as a function of apoE levels and the ratio of apoA-II to apoA-I revealed within each of the two populations, a high-risk subgroup characterized in each situation by high levels of apoE and additionally: in HR1, by a low value of the apoA-II/apoA-I ratio; and in HR2, by a moderate value of the apoA-II/apoA-I ratio. Furthermore, derived estimates of LpA-I and LpA-I:A-II levels revealed for high-risk versus remaining subjects: in HR1, higher levels of LpA-I and lower levels of LpA-I:A-II; and in HR2 the reverse, lower levels of LpA-I and higher levels of LpA-I:A-II. Results of multivariable risk modeling as a function of LpA-I and LpA-I:A-II (dichotomized as highest quartile versus combined three lower quartiles) revealed association of risk only for high levels of LpA-I:A-II in the HR2 subgroup (hazard ratio 5.31, 95% CI 1.12–25.17, p = 0.036). Furthermore, high LpA-I:A-II levels interacted with high apoE levels in establishing subgroup risk.

Conclusions/Significance

We conclude that apoA-II plays a significant role in apoE-associated risk of incident CVD in women with high levels of HDL-C and CRP.

Positive association of the hepatic lipase gene polymorphism c.514C > T with estrogen replacement therapy response

Background

Hepatic lipase (HL), an enzyme present in the hepatic sinusoids, is responsible for the lipolysis of lipoproteins. Human HL contains four polymorphic sites: G-250A, T-710C, A-763G, and C-514T single-nucleotide polymorphism (SNPs). The last polymorphism is the focus of the current study. The genotypes associated with the C-514T polymorphism are CC (normal homozygous - W), CT (heterozygous - H), and TT (minor-allele homozygous - M). HL activity is significantly impaired in individuals of the TT and CT genotypes. A total of 58 post-menopausal women were studied. The subjects were hysterectomized women receiving hormone replacement therapy consisting of 0.625 mg of conjugated equine estrogen once a day. The inclusion criteria were menopause of up to three years and normal blood tests, radiographs, cervical-vaginal cytology, and densitometry. DNA was extracted from the buccal and blood cells of all 58 patients using a commercially available kit (GFX^® - Amersham-Pharmacia, USA).

Results

Statistically significant reductions in triglycerides (t = 2.16; n = 58; p = 0.03) but not in total cholesterol (t = 0.14; n = 58; p = 0.89) were found after treatment. This group of good responders were carriers of the T allele; the CT and TT genotypes were present significantly more frequently than in the group of non-responders (p = 0.02 or p = 0.07, respectively). However, no significant difference in HDL-C (t = 0.94; n = 58; p = 0.35) or LDL-C (t = -0.83; n = 58; p = 0.41) was found in these patients.

Conclusions

The variation in lipid profile associated with the C-514T polymorphism is significant, and the T allele is associated with the best response to ERT.

Role of hepatic lipase and endothelial lipase in high-density lipoprotein-mediated reverse cholesterol transport

Reverse cholesterol transport (RCT) constitutes a key part of the atheroprotective properties of high-density lipoproteins (HDL). Hepatic lipase (HL) and endothelial lipase (EL) are negative regulators of plasma HDL cholesterol levels. Although overexpression of EL decreases overall macrophage-to-feces RCT, knockout of both HL and EL leaves RCT essentially unaffected. With respect to important individual steps of RCT, current data on the role of EL and HL in cholesterol efflux are not conclusive. Both enzymes increase hepatic selective cholesterol uptake; however, this does not translate into altered biliary cholesterol secretion, which is regarded the final step of RCT. Also, the impact of HL and EL on atherosclerosis is not clear cut; rather it depends on respective experimental conditions and chosen models. More mechanistic insights into the diverse biological properties of these enzymes are therefore required to firmly establish EL and HL as targets for the treatment of atherosclerotic cardiovascular disease.

Secretion of triacylglycerol-poor VLDL particles from McA-RH7777 cells expressing human hepatic lipase

Hepatic lipase (HL) plays a role in the catabolism of apolipoprotein (apo)B-containing lipoproteins through its lipolytic and ligand-binding properties. We describe a potential intracellular role of HL in the assembly and secretion of VLDL. Transient or stable expression of HL in McA-RH7777 cells resulted in decreased (by 40%) incorporation of [(3)H]glycerol into cell-associated and secreted triacylglycerol (TAG) relative to control cells. However, incorporation of [(35)S]methionine/cysteine into cell and medium apoB-100 was not decreased by HL expression. The decreased (3)H-TAG synthesis/secretion in HL expressing cells was not attributable to decreased expression of genes involved in lipogenesis. Fractionation of medium revealed that the decreased [(3)H]TAG from HL expressing cells was mainly attributable to decreased VLDL. Expression of catalytically-inactive HL (HL(SG)) (Ser-145 at the catalytic site was substituted with Gly) in the cells also resulted in decreased secretion of VLDL-[(3)H]TAG. Examination of lumenal contents of microsomes showed a 40% decrease in [(3)H]TAG associated with lumenal lipid droplets in HL or HL(SG) expressing cells as compared with control. The microsomal membrane-associated [(3)H]TAG was decreased by 50% in HL expressing cells but not in HL(SG) expressing cells. Thus, expression of HL, irrespective of its lipolytic function, impairs formation of VLDL precursor [(3)H]TAG in the form of lumenal lipid droplets. These results suggest that HL expression in McA-RH7777 cells result in secretion of [(3)H]TAG-poor VLDL.

Clinical and laboratory assessment of cardiovascular risk in children: Guidelines for screening, evaluation, and treatment

The early lesions of atherosclerosis begin in childhood and are related to antecedent cardiovascular disease (CVD) risk factors. Environmental and genetic factors (eg, diet, obesity, exercise, and certain inherited dyslipidemias) influence progression of such lesions. Identification of youth at risk for atherosclerosis includes an integrated assessment of these predisposing factors. Treatment starts with a diet low in total and saturated fat and cholesterol, use of water-soluble fiber, plant stanols and plant sterols, weight control, and exercise. Drug therapy, for example, with inhibitors of hydroxymethylglutaryl-CoA reductase, bile acid sequestrants, and cholesterol absorption inhibitors, can be considered in those with a positive family history of premature CVD and low-density lipoprotein cholesterol >160 mg/dL after dietary and hygienic measures. Candidates for drug therapy often include those with familial hypercholesterolemia, familial combined hyperlipidemia, the metabolic syndrome, polycystic ovarian syndrome, type 1 diabetes, and the nephrotic syndrome. Such dietary and drug therapy appears safe and efficacious. Early identification and treatment of youth with CVD risk factors and dyslipidemia are likely to retard the atherosclerotic process. Optimal detection and treatment of high-risk children either from the general population or from families with premature CVD will require a comprehensive universal screening and evaluation program.

Enhanced cellular uptake of remnant high-density lipoprotein particles: a mechanism for high-density lipoprotein lowering in insulin resistance and hypertriglyceridemia

A low level of high-density lipoprotein (HDL) cholesterol is characteristic of insulin resistance and hypertriglyceridemia and likely contributes to the increased risk of cardiovascular disease associated with these conditions. One pathway involves enhanced clearance of lipolytically modified HDL particles, but the underlying mechanisms remain poorly understood. Here, we examine the effect of triglyceride enrichment and hepatic lipase hydrolysis on HDL binding, internalization, and degradation in cultured liver and kidney cells. Maximal binding of remnant HDL (HDL enriched with triglycerides followed by hepatic lipase hydrolysis), but not binding affinity, was markedly higher than native and triglyceride-rich HDL in both HepG2 cells and HEK293 cells. Compared with native and triglyceride-rich HDL, remnant HDL was internalized to a greater extent in both cell types and was more readily degraded in HEK293 cells. The increased binding of remnant HDL was not mediated by the low-density lipoprotein receptor or scavenger receptor class B type I (SR-BI), because enhanced remnant HDL binding was observed in low-density lipoprotein receptor-deficient cells with or without SR-BI overexpression. Disruption of cell surface heparan sulfate proteoglycans or blockage of apolipoprotein E-mediated lipoprotein binding also did not abolish the enhanced remnant HDL binding. Our observations indicate that remodeling of triglyceride-enriched HDL by hepatic lipase may result in enhanced binding, internalization, and degradation in tissues involved in HDL catabolism, contributing to rapid clearance and overall lowering of plasma HDL cholesterol in insulin resistance and hypertriglyceridemia.

No change in apolipoprotein AI metabolism when subcutaneous insulin infusion is replaced by intraperitoneal insulin infusion in type 1 diabetic patients

In type 1 diabetic patients, the replacement of subcutaneous insulin infusion by intraperitoneal insulin infusion restores the normal physiological gradient between the portal vein and the peripheral circulation, which is likely to modify HDL metabolism. This stable isotope kinetic study was designed to compare HDL apolipoprotein (apo) AI metabolism in seven type 1 diabetic patients first treated by continuous subcutaneous insulin infusion by an external pump and then 3 months after the beginning of intraperitoneal insulin infusion by an implantable pump. Glycaemic control was comparable under subcutaneous and intraperitoneal insulin infusion (HbA1c=7.34+/-0.94% versus 7.24+/-1.00%, NS). HDL composition was similar under both insulin regimens (esterified cholesterol=20.1+/-2.5% versus 24.0+/-3.0% (NS), free cholesterol=3.4+/-1.1% versus 3.3+/-0.9% (NS), triglycerides=2.4+/-0.9% versus 2.1+/-0.9% (NS), phospholipids=22.7+/-5.3% versus 25.2+/-6.5% (NS) and proteins=51.2+/-6.3% versus 45.5+/-4.7% (NS)). The replacement of subcutaneous insulin infusion by intraperitoneal insulin infusion induced significant changes neither in apoAI fractional catabolic rate, nor in apoAI production rate, nor in apoAI pool size (respectively, 0.199+/-0.051 pool d(-1) versus 0.211+/-0.017 pool d(-1), 12.0+/-3.2 mg kg(-1)d(-1) versus 12.1+/-1.8 mg kg(-1)d(-1), 60.4+/-5.0 mg kg(-1) versus 57.5+/-7.5 mg kg(-1)). In conclusion, HDL metabolism is not modified by the replacement of subcutaneous insulin infusion by intraperitoneal insulin infusion when glycaemia is well controlled under both insulin regimens. As far as HDL metabolism is concerned there is no advantage in favour of one way of insulin administration or another.

Kinetics of plasma apolipoprotein C-III as a determinant of diet-induced changes in plasma triglyceride levels

OBJECTIVE

To compare the effect of a high monounsaturated fatty acid (MUFA) diet and of a control low-fat diet consumed under ad libitum conditions on plasma apolipoprotein (apo) C-III metabolism.

DESIGN

Randomized, two-arm parallel dietary trial.

SETTING

Diets were prepared and consumed at the metabolic kitchen of the Department of Food Sciences and Nutrition, and laboratory analyses were performed at the Institute of Nutraceuticals and Functional Foods at Laval University.

SUBJECTS AND INTERVENTIONS

Eighteen men were randomly assigned to either the high MUFA diet or the low-fat control diet, which they consumed for 6-7 weeks. Before and after the dietary intervention, subjects received a primed-constant infusion of [5,5,5-D(3)]-L-leucine for 12 h under constant feeding conditions for the determination of plasma apoC-III kinetics.

RESULTS

The high-MUFA diet and the low-fat control diet had no significant impact on plasma apoC-III production rate (PR) or fractional catabolic rate. However, diet-induced variations in plasma apoCIII PR predicted the reduction in plasma triglycerides and apoC-III levels (r=0.85, P<0.01 and r=0.73, P<0.05, respectively) in the high MUFA group only.

CONCLUSIONS

These results suggest that the hypotriglyceridemic effect of a high-MUFA diet may be attributable in part to a reduced hepatic production of apoC-III.

SPONSORSHIP

This study was supported in part by an operating grant from the Canadian Institutes of Health Research (CIHR), and the Canada Research Chair in Nutrition and Cardiovascular Health (B Lamarche).

Familial dyslipidaemias: an overview of genetics, pathophysiology and management

Plasma lipid disorders can occur either as a primary event or secondary to an underlying disease or use of medications. Familial dyslipidaemias are traditionally classified according to the electrophoretic profile of lipoproteins. In more recent texts, this phenotypic classification has been replaced with an aetiological classification. Familial dyslipidaemias are generally grouped into disorders leading to hypercholesterolaemia, hypertriglyceridaemia, a combination of hyper-cholesterolaemia and hypertriglyceridaemia, or abnormal high-density lipoprotein-cholesterol (HDL-C) levels. The management of these disorders requires an understanding of plasma lipid and lipoprotein metabolism. Lipid transport and metabolism involves three general pathways: (i) the exogenous pathway, whereby chylomicrons are synthesised by the small intestine, and dietary triglycerides (TGs) and cholesterol are transported to various cells of the body; (ii) the endogenous pathway, whereby very low-density lipoprotein-cholesterol (VLDL-C) and TGs are synthesised by the liver for transport to various tissues; and (iii) the reverse cholesterol transport, whereby HDL cholesteryl ester is exchanged for TGs in low-density lipoptrotein (LDL) and VLDL particles through cholesteryl ester transfer protein in a series of steps to remove cholesterol from the peripheral tissues for delivery to the liver and steroidogenic organs. The plasma lipid profile can provide a framework to guide the selection of appropriate diet and drug treatment. Many patients with hyperlipoproteinaemia can be treated effectively with diet. However, dietary regimens are often insufficient to bring lipoprotein levels to within acceptable limits. In this article, we review lipid transport and metabolism, discuss the more common lipid disorders and suggest some management guidelines. The choice of a particular agent depends on the baseline lipid profile achieved after 6-12 weeks of intense lifestyle changes and possible use of dietry supplements such as stanols and plant sterols. If the predominant lipid abnormality is hypertriglyceridaemia, omega-3 fatty acids, a fibric acid derivative (fibrate) or nicotinic acid would be considered as the first choice of therapy. In subsequent follow-up, when LDL-C is >130 mg/dL (3.36 mmol/L) then an HMG-CoA reductase inhibitor (statin) should be added as a combination therapy. If the serum TG levels are <500 mg/dL (2.26 mmol/L) and the LDL-C values are over 130 mg/dL (3.36 mmol/L) then a statin would be the first drug of choice. The statin dose can be titrated up to achieve the therapeutic goal or, alternatively, ezetimibe can be added. A bile acid binding agent is an option if the serum TG levels do not exceed 200 mg/dL (5.65 mmol/L), otherwise a fibrate or nicotinic acid should be considered. The decision to treat a particular person has to be individualised.

Effects of fenofibrate on apolipoprotein kinetics in patients with coexisting dysbetalipoproteinemia and heterozygous familial hypercholesterolemia

Dysbetalipoproteinemia (dysb) and familial hypercholesterolemia (FH) are two genetic disorders giving rise to severe disturbances of lipid homeostasis and premature atherosclerosis. The co-occurrence of both metabolic abnormalities is very rare and is estimated to affect 1 individual per 2,500,000 in the general population. However, the relative contribution of these two dyslipidemias to the combined lipoprotein phenotype is unknown. The two objectives of this study were (1) to compare the in vivo kinetics of triglyceride-rich lipoprotein (TRL) apolipoprotein (apo) B48, VLDL, IDL and LDL apo B100 as well as plasma apo A-l labelled with a stable isotope (l-(5,5,5-D3) leucine) in two subjects presenting both heterozygous FH and dysbetalipoproteinemia (FH+/dysb+), in six FH heterozygotes and in five normolipidemic controls, and (2) to examine the impact of a 6-week treatment with micronized fenofibrate 200 mg/d on apolipoprotein kinetics in FH+/dysb+. As compared with FH heterozygotes and controls, the two FH+/dysb+ subjects showed elevated TRL apo B48 and VLDL, IDL apo B100 pool sizes (PS) mainly due to lower fractional catabolic rates (FCR). Moreover, as compared with FH heterozygotes, FH+/dysb+ subjects presented lower LDL apo B100 PS due to a higher FCR. Pool size, FCR and production rate (PR) of apo A-l were higher in FH+/dysb+ subjects than in FH heterozygotes. In FH+/dysb+ subjects, fenofibrate treatment was associated with a decreased TRL apo B48 PS (-52 and -61%), VLDL apo B100 (-61 and -63%) and IDL apo B100 (-37 and -16%) and an increased FCR of TRL apo B48 (10 and 67%), VLDL apo B100 (123 and 57%) and IDL apo B100 (29 and 10%). Fenofibrate also increased LDL apo B100 PS (3 and 57%) due to an increase in PR (80 and 26%) but had divergent effects on LDL apo B100 FCR. These results indicate that the coexistence of dysbetalipoproteinemia and heterozygous FH results in a mixed lipoprotein phenotype that is intermediate between the two pure phenotypes and that fenofibrate treatment partially reverses lipoprotein abnormalities, mostly through changes in PR and FCR of apo B48- and B100-containing lipoproteins.

Studying apolipoprotein turnover with stable isotope tracers: correct analysis is by modeling enrichments

Lipoprotein kinetic parameters are determined from mass spectrometry data after administering mass isotopes of amino acids, which label proteins endogenously. The standard procedure is to model the isotopic content of the labeled precursor amino acid and of proteins of interest as tracer-to-tracee ratio (TTR). It is shown here that even though the administered tracer alters amino acid mass and turnover, apolipoprotein synthesis is unaltered and hence the apolipoprotein system is in a steady state, with the total (labeled plus unlabeled) masses and fluxes remaining constant. The correct model formulation for apolipoprotein kinetics is shown to be in terms of tracer enrichment, not of TTR. The needed mathematical equations are derived. A theoretical error analysis is carried out to calculate the magnitude of error in published results using TTR modeling. It is shown that TTR modeling leads to a consistent underestimation of the fractional synthetic rate. In constant-infusion studies, the bias error percent is shown to equal approximately the plateau enrichment, generally <10%. It is shown that, in bolus studies, the underestimation error can be larger. Thus, for mass isotope studies with endogenous tracers, apolipoproteins are in a steady state and the data should be fitted by modeling enrichments.

Thematic review series: patient-oriented research. What have we learned about HDL metabolism from kinetics studies in humans?

Plasma measurements of lipids, lipoproteins, and apolipoproteins provide information on the static levels of these fractions without providing key information on the dynamic fluxes of lipoproteins in the circulation. Kinetics studies, in contrast, provide additional information on the production and clearance rates of lipoproteins and the flow of lipids and apolipoproteins through lipoprotein fractions. This information is crucial in accurately delineating the metabolism of HDL in plasma, because plasma concentrations of HDL are the net result of the de novo production and catabolism of HDL as well as the recycling of HDL particles and the contribution to HDL from components of other lipoproteins. Studies aimed at measuring the metabolism of HDL particles have shown that HDL metabolism in vivo is complex and consists of multiple components. Kinetics studies provide a window into the metabolism of HDL, allowing us to better understand the mechanisms of HDL decrease in human conditions and the functionality of HDL particles. Here, we review the progress in our understanding of HDL metabolism derived from in vivo kinetics studies, focusing primarily on studies in humans but also reviewing key studies in animal models.

Endothelial lipase concentrations are increased in metabolic syndrome and associated with coronary atherosclerosis

BACKGROUND

Endothelial lipase (EL), a new member of the lipase family, has been shown to modulate high-density lipoprotein (HDL-C) metabolism and atherosclerosis in mouse models. We hypothesized that EL concentrations would be associated with decreased HDL-C and increased atherosclerosis in humans.

METHODS AND FINDINGS

Healthy individuals with a family history of premature coronary heart disease (n = 858) were recruited as part of the Study of the Inherited Risk of Atherosclerosis. Blood was drawn in the fasting state before and, in a subgroup (n = 510), after administration of a single dose of intravenous heparin. Plasma lipids were measured enzymatically, lipoprotein subclasses were assessed by nuclear magnetic resonance, and coronary artery calcification (CAC) was quantified by electron beam computed tomography. Plasma EL mass was measured using a newly developed enzyme-linked immunosorbent assay. Median EL mass in pre-heparin plasma was 442 (interquartile range = 324-617) ng/ml. Median post-heparin mass was approximately 3-fold higher, 1,313 (888-1,927) ng/ml. The correlation between pre-heparin EL mass and post-heparin EL mass was 0.46 (p < 0.001). EL mass concentrations in both pre- and post-heparin plasma significantly correlated with all NCEP ATPIII-defined metabolic syndrome factors: waist circumference (r = 0.28 and 0.22, respectively, p < 0.001 for each), blood pressure (r = 0.18 and 0.24, p < 0.001 for each), triglycerides (r = 0.22, p < 0.001; and 0.13, p = 0.004), HDL cholesterol (r = -0.11, p = 0.002; and -0.18, p < 0.001), and fasting glucose (r = 0.11 and 0.16, p = 0.001 for both). EL mass in both routine (odds ratio [OR] = 1.67, p = 0.01) and post-heparin (OR = 2.42, p = 0.003) plasma was associated with CAC as determined by ordinal regression after adjustment for age, gender, waist circumference, vasoactive medications, hormone replacement therapy (women), and established cardiovascular risk factors.

CONCLUSIONS

We report, to our knowledge for the first time, that human plasma EL concentrations, in both post-heparin and routine pre-heparin plasma, are significantly associated with metabolic syndrome features and with subclinical atherosclerosis. EL may be a pro-atherogenic factor in humans, especially in overweight individuals and those with metabolic syndrome.

Effect of Fenofibrate on Plasma Lipoprotein Composition and Kinetics in Patients With Complete Hepatic Lipase Deficiency

Objective— The goal of this study was to characterize the effect of microcoated fenofibrate (160 mg/day for 6 months) on plasma lipoprotein composition and kinetics in 2 patients with complete hepatic lipase (HL) deficiency.

Methods and Results— Fenofibrate treatment normalized the plasma lipoprotein profile of patients with complete HL deficiency, as evidenced by significant reductions in the plasma concentration of cholesterol (−49%) and triglycerides (−82%) and a significant increase in low-density lipoprotein (LDL) size (251.5±1.8 versus 263.5±0.7 Å). The in vivo kinetics of very low–density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), and LDL apolipoprotein (apo)B-100 and plasma apoA-I and apoA-II were studied using a primed-constant infusion of L-[5,5,5-D 3 ]-leucine for 12 hours in the fasted state. Fenofibrate treatment in complete HL-deficient patients substantially decreased plasma concentrations of VLDL, IDL, and LDL apoB-100 attributable to important increases in VLDL (+325%), IDL (+129%), and LDL (+218%) apoB-100 fractional catabolic rates (FCR). IDL apoB-100 FCR nevertheless remained 60% lower after treatment compared with values obtained in controls (n=5). The kinetics of plasma apoA-I and apoA-II as well as the capacity of total plasma and of high-density lipoprotein particles to efflux cellular cholesterol from normal human skin fibroblasts was not altered by fenofibrate.

Conclusion— Fenofibrate therapy exerts a pronounced antiatherogenic effect on triglyceride-rich lipoproteins even in the complete absence of HL.

Treatment of dyslipidemia in children and adolescents

The early lesions of atherosclerosis begin in childhood, and are related to antecedent cardiovascular disease risk factors. Environmental and genetic factors such as diet, obesity, exercise, and certain inherited dyslipidemias influence the progression of such lesions. The identification of youth at risk for atherosclerosis includes an integrated assessment of these predisposing factors. Treatment starts with a diet low in total and saturated fat and cholesterol, the use of water-soluble fiber and plant sterols, weight control, and exercise. Drug therapy, for example, with inhibitors of hydroxymethylglutaryl CoA reductase, bile acid sequestrants, and cholesterol absorption inhibitors, can be considered in those with a positive family history of premature coronary artery disease and a low-density lipoprotein cholesterol above 160 mg/dL, after dietary and hygienic measures. Candidates for drug therapy often include those with familial hypercholesterolemia, familial combined hyperlipidemia, the metabolic syndrome, polycystic ovarian syndrome, type I diabetes, and the nephrotic syndrome. We review the safety and efficacy of dietary and drug therapy, and propose an updated diagnostic and therapeutic algorithm that includes the metabolic syndrome. The early identification and treatment of youth with dyslipidemias is likely to retard the atherosclerotic process.

Elevated HDL is a risk factor for recurrent coronary events in a subgroup of non-diabetic postinfarction patients with hypercholesterolemia and inflammation

Recent studies demonstrate important roles for inflammation in development of atherosclerosis with current attention focusing on interactions of inflammation with traditional lipoprotein risk factors. Since the nature of such relationships is largely unknown, we sought to investigate interactions of inflammation with hyperlipidemia in generating cardiovascular risk in a way that would allow recognition of such interactions whether anticipated or not. Thus, we searched for subgroups at high risk for recurrent coronary events in 767 non-diabetic postinfarction patients using an exploratory three-dimensional graphical screening technique with previously established factor analysis-derived inflammatory and lipoprotein-related factors. Results indicated a high-risk patient subgroup defined by factor interaction that was best characterized clinically by high levels of C-reactive protein (CRP) and total cholesterol. Kaplan-Meier and Cox multivariate analysis confirmed high-risk. Additionally, within-subgroup risk related to metabolic, inflammatory, and thrombogenic blood markers was assessed using Cox analysis with results showing only elevated HDL as a significant and independent predictor of risk with hazard ratio, 2.24 (95% CI; 1.12, 4.49; p = 0.023). We conclude that in non-diabetic postinfarction patients, elevated HDL is predictive of risk of recurrent coronary events within a subgroup of patients characterized by simultaneous elevations in serum CRP and total cholesterol.

Plasma metabolism of apoB-containing lipoproteins in patients with hepatic lipase deficiency

The metabolism of apoB-containing lipoproteins was investigated in the fasted state in three complete and three partial hepatic lipase (HL)-deficient subjects as well as in seven normotriglyceridemic (NTG) and two hypertriglyceridemic (HTG) controls using a 12 h primed-constant infusion of L-[5,5,5-D(3)]-leucine. Two males with complete HL deficiency had increased plasma pool sizes of VLDL and IDL apoB-100 due to substantial reductions in fractional catabolic rate (FCR) of VLDL and IDL apoB-100 compared with both NTG and HTG controls. Reductions in LDL apoB-100 production rate (PR) were also observed in these two patients compared with NTG and HTG controls. Complete HL deficiency in the female proband was associated with normal VLDL apoB-100 kinetics, while plasma IDL apoB-100 pool size was increased by 124% due to an 82% decrease in the FCR of IDL apoB-100. The FCR and PR of LDL apoB-100 were reduced by 64 and 51%, respectively, in the proband compared with sex-matched controls. Partial HL-deficient patients were characterized by apoB-containing lipoprotein metabolism similar to that of controls. These results indicate that complete HL deficiency is associated with a potentially atherogenic apoB-containing lipoprotein metabolism that can be modulated considerably by secondary factors such as gender and abdominal obesity.

Genetics of Variation in HDL Cholesterol in Humans and Mice

Plasma high-density lipoprotein cholesterol (HDL-C) concentrations are genetically determined to a great extent, and quantitative trait locus (QTL) analysis has been used to identify chromosomal regions containing genes regulating HDL-C levels. We discuss new genes found to participate in HDL metabolism. We also summarize 37 mouse and 30 human QTLs for plasma HDL-C levels, finding that all but three of the mouse QTLs have been confirmed by a second cross or a homologous human QTL, that the mouse QTL map is almost saturated because 92% of recently reported QTLs are repeats of those already found, and that 28 of the 30 human QTLs are located in regions homologous to mouse QTLs. This high degree of concordance between mouse and human QTLs suggests that the underlying genes may be the same. Strategies to more rapidly identify genes underlying mouse and human QTLs for HDL-C include focusing on the mouse and using mouse–human homologies, combining crosses, and haplotyping to narrow the region. Sequence analysis and expression studies can distinguish candidate genes consistent across multiple mouse crosses, and testing the candidate genes in human association studies can provide additional evidence for the candidacy of a gene. Together these strategies can accelerate the pace of finding genes that regulate HDL.