Effect of heparin-induced lipolysis on the distribution of apolipoprotein e among lipoprotein subclasses. Studies with patients deficient in hepatic triglyceride lipase and lipoprotein lipase

Closing the gaps in patient management of dyslipidemia: stepping into cardiovascular precision diagnostics with apolipoprotein profiling

In persons with dyslipidemia, a high residual risk of cardiovascular disease remains despite lipid lowering therapy. Current cardiovascular risk prediction mainly focuses on low-density lipoprotein cholesterol (LDL-c) levels, neglecting other contributing risk factors. Moreover, the efficacy of LDL-c lowering by statins resulting in reduced cardiovascular risk is only partially effective. Secondly, from a metrological viewpoint LDL-c falls short as a reliable measurand. Both direct and calculated LDL-c tests produce inaccurate test results at the low end under aggressive lipid lowering therapy. As LDL-c tests underperform both clinically and metrologically, there is an urging need for molecularly defined biomarkers. Over the years, apolipoproteins have emerged as promising biomarkers in the context of cardiovascular disease as they are the functional workhorses in lipid metabolism. Among these, apolipoprotein B (ApoB), present on all atherogenic lipoprotein particles, has demonstrated to clinically outperform LDL-c. Other apolipoproteins, such as Apo(a) - the characteristic apolipoprotein of the emerging risk factor lipoprotein(a) -, and ApoC-III - an inhibitor of triglyceride-rich lipoprotein clearance -, have attracted attention as well. To support personalized medicine, we need to move to molecularly defined risk markers, like the apolipoproteins. Molecularly defined diagnosis and molecularly targeted therapy require molecularly measured biomarkers. This review provides a summary of the scientific validity and (patho)physiological role of nine serum apolipoproteins, Apo(a), ApoB, ApoC-I, ApoC-II, ApoC-III, ApoE and its phenotypes, ApoA-I, ApoA-II, and ApoA-IV, in lipid metabolism, their association with cardiovascular disease, and their potential as cardiovascular risk markers when measured in a multiplex apolipoprotein panel.

Efficacy and Safety of Low-Dose Pemafibrate Therapy for Hypertriglyceridemia in Patients with Type 2 Diabetes

Introduction:

Pemafibrate is a potent selective peroxisome proliferator-activated receptor α (PPARα) modulator that may be safer than conventional PPARα agonists in the treatment of dyslipidemia. This study was designed to investigate the efficacy of low-dose pemafibrate (0.1 mg/day) therapy for hypertriglyceridemia in 31 patients with type 2 diabetes and high triglyceride (TG) levels at the Manda Memorial Hospital.

Methods:

TG, remnant lipoprotein cholesterol (RLP-C), high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), apolipoprotein (Apo) AI, Apo AII, Apo B, Apo CII, Apo CIII, and Apo E levels were evaluated. Liver, kidney, and muscle toxicity tests were also performed. Pemafibrate (0.1 mg) was administered once daily.

Results:

This treatment significantly decreased TG, RLP-C, Apo CII, Apo CIII, and Apo E levels while significantly increasing HDL-C, Apo AI, and Apo AII levels. No significant changes were observed in LDL-C and Apo B levels. There were no significant liver-, kidney-, or muscle-related adverse events.

Conclusions:

The results of this study show that low-dose pemafibrate administration improves the lipid profile in Japanese patients with hypertriglyceridemia and type 2 diabetes.

Old and new applications of non-anticoagulant heparin

The aim of this chapter is to provide an overview of non-anticoagulant effects of heparins and their potential use in new therapeutic applications. Heparin and heparin derivatives have been tested in inflammatory, pulmonary and reproductive diseases, in cardiovascular, nephro- and neuro-tissue protection and repair, but also as agents against angiogenesis, atheroschlerosis, metastasis, protozoa and viruses. Targeting and inhibition of specific mediators involved in the inflammatory process, promoting some of the above mentioned pathologies, are reported along with recent studies of heparin conjugates and oral delivery systems. Some reports from the institute of the authors, such as those devoted to glycol-split heparins are also included. Among the members and derivatives of this class, several are undergoing clinical trials as antimetastatic and antimalarial agents and for the treatment of labour pain and severe hereditary anaemia. Other heparins, whose therapeutic targets are non-anticoagulant such as nephropathies, retinopathies and cystic fibrosis are also under investigation.

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.

Antisense inhibition of apoB synthesis with mipomersen reduces plasma apoC-III and apoC-III-containing lipoproteins

Mipomersen, an antisense oligonucleotide that reduces hepatic production of apoB, has been shown in phase 2 studies to decrease plasma apoB, LDL cholesterol (LDL-C), and triglycerides. ApoC-III inhibits VLDL and LDL clearance, and it stimulates inflammatory responses in vascular cells. Concentrations of VLDL or LDL with apoC-III independently predict cardiovascular disease. We performed an exploratory posthoc analysis on a subset of hypercholesterolemic subjects obtained from a randomized controlled dose-ranging phase 2 study of mipomersen receiving 100, 200, or 300 mg/wk, or placebo for 13 wk (n = 8 each). ApoC-III-containing lipoproteins were isolated by immuno-affinity chromatography and ultracentrifugation. Mipomersen 200 and 300 mg/wk reduced total apoC-III from baseline by 6 mg/dl (38-42%) compared with placebo group (P < 0.01), and it reduced apoC-III in both apoB lipoproteins and HDL. Mipomersen 100, 200, and 300 mg doses reduced apoB concentration of LDL with apoC-III (27%, 38%, and 46%; P < 0.05). Mipomersen reduced apoC-III concentration in HDL. The drug had no effect on apoE concentration in total plasma and in apoB lipoproteins. In summary, antisense inhibition of apoB synthesis reduced plasma concentrations of apoC-III and apoC-III-containing lipoproteins. Lower concentrations of apoC-III and LDL with apoC-III are associated with reduced risk of coronary heart disease (CHD) in epidemiologic studies independent of traditional risk factors.

Interactive effects of APOE haplotype, sex, and exercise on postheparin plasma lipase activities

Hepatic lipase (HL) and lipoprotein lipase (LPL) activities (HLA, LPLA) modify lipoproteins and facilitate their binding to hepatic receptors. Apolipoprotein E (APOE) physically interacts with the lipases, and the three common haplotypes of the APOE gene (ε2, ε3, and ε4) yield protein isoforms (E2, E3, and E4, respectively) that are functionally different. Lipase activities themselves differ by sex and exercise training status. The interaction of APOE genotype, exercise training, and sex effects on lipase activities has not been studied. We measured postheparin plasma lipase activities in normolipidemic men and women with the three most common APOE genotypes, which are the haplotype combinations ε2/ε3 (n = 53 ), ε3/ε3 (n = 62), and ε4/ε3 (n = 52), enrolled in 6 mo of aerobic exercise training. These haplotype combinations comprise an estimated 11.6, 62.3, and 21.3% of the population, respectively. Baseline HLA was 35% lower in women than in men (P < 0.0001). In men but not women, HLA was higher in ε2/ε3 group compared with ε4/ε3 (P = 0.01) and ε3/ε3 (P = 0.05). Neither sex nor APOE genotype affected baseline LPLA. Training decreased HLA by 5.2% (P = 0.018) with no APOE effect. The apparent increase in LPLA following exercise was significant and APOE dependent only when corrected for baseline insulin (P < 0.05). Exercise decreased LPLA by 0.8 μmol free fatty acid (FFA)·ml⁻¹·h⁻¹ (-6%) in ε3/ε3 compared with the combined increases of 6.6% in ε2/ε3 and 12% in ε4/ε3 (P = 0.018 vs. ε3/ε3). However, these differences were statistically significant only after correcting for baseline insulin. We conclude that common APOE genotypes interact with 1) sex to modulate HLA regardless of training status, with ε2/ε3 men demonstrating higher HLA than ε3/ε3 or ε4/ε3 men, and 2) aerobic training to modulate LPLA, regardless of sex, with ε3/ε3 subjects showing a significant decrease compared with an increase in ε2/ε3 and ε3/ε4 after controlling for baseline insulin.

Lipid metabolism in diet-induced obese rabbits is similar to that of obese humans

Whereas diet-induced obese rabbits have been used to study various aspects of obesity, alterations of lipid metabolism in this model have not been clarified. This study aimed to compare plasma nonesterified fatty acid (NEFA) and triglyceride (TG) kinetics in obese and lean rabbits by means of U-(13)C16-palmitate infusion. Young female rabbits consumed either a high-fat diet (49% energy from fat) ad libitum to develop obesity (n = 6) or a normal diet (7.9% energy from fat) as lean control (n = 5). After 10 wk of feeding, the body weight of obese rabbits (5.33 +/- 0.05 kg) was greater (P < 0.001) than that of lean rabbits (3.89 +/- 0.07 kg). The obese rabbits had higher concentrations of plasma NEFA and TG and a greater rate of fatty acid (FA) turnover. Whereas the fractional secretion rates of hepatic TG did not differ, 100% of hepatic secretory TG was synthesized from plasma NEFA in the lean rabbits compared to 59% in the obese rabbits (P < 0.001). In the lean rabbits, hepatic lipase-mediated hydrolysis of lipoprotein TG did not contribute to the FA pool for synthesis of secretory TG, consistent with the naturally occurring deficit in hepatic lipase in this species. We conclude that lipid metabolism in diet-induced obese rabbits is similar to that in obese humans. The deficiency in hepatic lipase in rabbits simplifies the quantitation of hepatic lipid kinetics.

Effects of dietary phospholipid level in cobia (Rachycentron canadum) larvae: growth, survival, plasma lipids and enzymes of lipid metabolism

A study was conducted to determine the effects of dietary phospholipid (PL) levels in cobia (Rachycentron canadum) larvae with regard to growth, survival, plasma lipids and enzymes of lipid metabolism. Fish with an average weight of 0.4 g were fed diets containing four levels of PL (0, 20, 40 and 80 g kg(-1)dry matter: purity 97%) for 42 days. Final body weight (FBW), weight gain (WG) and survival ratio were highest in the 8% PL diet group and mortality was highest in PL-free diet group. We examined the activities of lipoprotein lipase (LPL) and hepatic lipase (HL) in liver, lecithin-cholesterolacyltransferase (LCAT) in plasma as well as plasma lipids and lipoprotein. LCAT activity showed a decrease of more than two-fold in PL-supplemented diet groups compared with the PL-free diet group. HL activity was highest in the 8% PL diet group and the other three groups showed no difference. LPL activity was significantly higher in the PL-supplemented diet groups than in the PL-free diet group. The dietary intervention significantly increased plasma phospholipids and total cholesterol (TC) levels, and the higher free cholesterol (FC) level contributed to the TC level. However, the fish fed PL exhibited a significantly decreased plasma triglyceride (TG) level. The lipoprotein fractions were also affected significantly by the PL. The PL-supplemented diet groups had significantly higher high-density lipoprotein (HDL) compared with the PL-free diet group, but showed a marked decrease in very low-density lipoprotein (VLDL). The results suggested that PL could modify plasma lipoprotein metabolism and lipid profile, and that the optimal dietary PL level may well exceed 80 g kg(-1) for cobia larvae according to growth and survival.

ACTH reduces the rise in ApoB-48 levels after fat intake

It has been repeatedly demonstrated that ACTH administration lowers plasma lipid concentrations in man. The present study was designed to test the hypothesis, based on observations of decreased apolipoprotein B (ApoB) synthesis and secretion in vitro, that ACTH administration inhibits the postprandial output of ApoB in man. Therefore, we studied the response to a fat-rich meal supplemented with Vitamin A in eight healthy volunteers, who underwent this test without premedication, after 4 days administration of ACTH, and after 4 days administration of a glucocorticoid (betamethasone). As expected, fasting plasma levels of low-density lipoproteins (LDL)-cholesterol (-25%) and ApoB (-17%) decreased after ACTH, but not after betamethasone administration. Also, the elevation of plasma ApoB-48 in response to fat intake (to twice the basal levels) was markedly reduced after ACTH administration. However, the postprandial rise in plasma triglycerides and retinyl palmitate was unimpaired, suggesting that ACTH administration induced the secretion of fewer but larger chylomicrons. The effect of betamethasone on the postprandial response was similar but less pronounced. This study confirms earlier reports on the lipid-lowering effects of ACTH and supports our theory, based on in vitro studies, that the lipid-lowering effects of ACTH administration in man involves an inhibition of ApoB production.

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.

Hepatic steatosis: a mediator of the metabolic syndrome. Lessons from animal models

Epidemiological studies in humans, as well as experimental studies in animal models, have shown an association between visceral obesity and dyslipidemia, insulin resistance, and type 2 diabetes mellitus. Recently, attention has been focused on the excessive accumulation of triglycerides (TG) in the liver as part of this syndrome. In this review, important principles of the pathophysiological involvement of the liver in the metabolic syndrome obtained in rodent models are summarized. We focus on non-alcoholic causes of steatosis, because the animal experiments we refer to did not include alcohol as an experimental condition. In general, there is continuous cycling and redistribution of non-oxidized fatty acids between different organs. The amount of TG in an intrinsically normal liver is not fixed but can readily be increased by nutritional, metabolic, and endocrine interactions involving TG/free fatty acid (FFA) partitioning and TG/FFA metabolism. Several lines of evidence indicate that hepatic TG accumulation is also a causative factor involved in hepatic insulin resistance. Complex interactions between endocrine, metabolic, and transcriptional pathways are involved in TG-induced hepatic insulin resistance. Therefore, the liver participates passively and actively in the metabolic derangements of the metabolic syndrome. We speculate that similar mechanisms may also be involved in human pathophysiology.

Characterization of a novel mutation causing hepatic lipase deficiency among French Canadians

Individuals with hepatic lipase (HL) deficiency are often characterized by elevated levels of triglycerides (TGs) and cholesterol. The aim of the present study was to characterize the molecular defect leading to severe HL deficiency in a Québec-based kindred. In the proband and two of her brothers, the very low to undetectable HL activity resulted from compound heterozygosity for two rare HL gene mutations, a previously unknown missense mutation in exon 5 designated A174T and the previously reported T383M mutation in exon 8 of the HL gene. The mutation at codon 174 resulted in the substitution of alanine for threonine, a polar amino acid, in a highly conserved nonpolar region of the protein involved in the catalytic activity of the enzyme. The severe HL deficiency among the three related compound heterozygotes was associated with a marked TG enrichment of LDL and HDL particles. The two men with severe HL deficiency also presented with abdominal obesity, which appeared to amplify the impact of HL deficiency on plasma TG-rich lipoprotein levels. Our results demonstrated that HL deficiency in this Québec kindred is associated with an abnormal lipoprotein-lipid profile, which may vary considerably in the presence of secondary factors such as abdominal obesity.

Detection, quantification, and characterization of potentially atherogenic triglyceride-rich remnant lipoproteins

Triglyceride-rich lipoprotein (TRL) remnants are formed in the circulation when apolipoprotein (apo) B-48-containing chylomicrons of intestinal origin or apoB-100-containing VLDL of hepatic origin are converted by lipoprotein lipase, and to a lesser extent by hepatic lipase, into smaller and more dense particles. Compared with their nascent precursors, TRL remnants are depleted of triglyceride, phospholipid, and C apolipoproteins and are enriched in cholesteryl esters and apoE. They can thus be identified, separated, and/or quantified in plasma according to their density, charge, size, specific lipid components, apolipoprotein composition, and/or apolipoprotein immunospecificity. Each of these approaches has contributed to our current understanding of the compositional characteristics of TRL remnants and their potential to promote atherosclerosis. An ongoing search is nevertheless under way for more accurate and clinically applicable remnant lipoprotein assays that will be able to better define coronary artery disease risk in patients with hypertriglyceridemia.

The role of hepatic lipase in lipoprotein metabolism

Hepatic lipase (HL) is one of two major lipases released from the vascular bed by intravenous injection of heparin. HL hydrolyzes phospholipids and triglycerides of plasma lipoproteins and is a member of a lipase superfamily that includes lipoprotein lipase and pancreatic lipase. The enzyme can be divided into an NH2-terminal domain containing the catalytic site joined by a short spanning region to a smaller COOH-terminal domain. The NH2-terminal portion contains an active site serine in a pentapeptide consensus sequence, Gly-Xaa-Ser-Xaa-Gly, as part of a classic Ser-Asp-His catalytic triad, and a putative hinged loop structure covering the active site. The COOH-terminal domain contains a putative lipoprotein-binding site. The heparin-binding sites may be distributed throughout the molecule, with the characteristic elution pattern from heparin-sepharose determined by the COOH-terminal domain. Of the three N-linked glycosylation sites, Asn-56 is required for efficient secretion and enzymatic activity. HL is hypothesized to directly couple HDL lipid metabolism to tissue/cellular lipid metabolism. The potential significance of the HL pathway is that it provides the hepatocyte with a mechanism for the uptake of a subset of phospholipids enriched in unsaturated fatty acids and may allow the uptake of cholesteryl ester, free cholesterol and phospholipid without catabolism of HDL apolipoproteins. HL can hydrolyze triglyceride and phospholipid in all lipoproteins, but is predominant in the conversion of intermediate density lipoproteins to LDL and the conversion of post-prandial triglyceride-rich HDL into the post-absorptive triglyceride-poor HDL. It has been suggested that enzymatically inactive HL can play a role in hepatic lipoprotein uptake forming a 'bridge' by binding to the lipoprotein and to the cell surface. This raises the interesting possibility that production and secretion of mutant inactive HL could promote clearance of VLDL remnants. We have described a rare family with HL deficiency. Affected patients are compound heterozygotes for a mutation of Ser267Phe that causes an inactive enzyme and a mutation of Thr383Met that results in impaired secretion of HL and reduced specific activity. Human HL deficiency in the context of a second factor causing hyperlipidemia is strongly associated with premature coronary artery disease.

Hepatic lipase deficiency

Hepatic lipase (HL) is an enzyme that is made primarily by hepatocytes (and also found in adrenal gland and ovary) and hydrolyzes phospholipids and triglycerides of plasma lipoproteins. It is secreted and bound to the hepatocyte surface and readily released by heparin. It is a member of the lipase superfamily and is homologous to lipoprotein lipase and pancreatic lipase. The enzyme can be divided into an NH2-terminal domain containing the catalytic site joined by a short spanning region to a smaller COOH-terminal domain. The NH2-terminal portion contains an active site serine in a pentapeptide consensus sequence, Gly-Xaa-Ser-Xaa-Gly, as part of a classic Ser-Asp-His catalytic triad, and a putative hinged loop structure covering the active site. The COOH-terminal domain contains a putative lipoprotein-binding site. The heparin-binding sites may be distributed throughout the molecule, with the characteristic elution pattern from heparin-sepharose determined by the COOH-terminal domain. Of the three N-linked glycosylation sites, Asn-56 is required for efficient secretion and enzymatic activity. HL is hypothesized to directly couple HDL lipid metabolism to tissue/cellular lipid metabolism. The potential significance of the HL pathway is that it provides the hepatocyte with a mechanism for the uptake of a subset of phospholipids enriched in unsaturated fatty acids and may allow the uptake of cholesteryl ester, free cholesterol, and phospholipid without catabolism of HDL apolipoproteins. HL can hydrolyze triglyceride and phospholipid in all lipoproteins, but is predominant in the conversion of intermediate density lipoproteins to LDL and the conversion of post-prandial triglyceride-rich HDL into the postabsorptive triglyceride-poor HDL. HL plays a secondary role in the clearance of chylomicron remnants by the liver. Human post-heparin HL activity is inversely correlated with intermediate density lipoprotein cholesterol concentration only in subjects with a hyperlipidemia involving VLDL. This is consistent with intermediate-density lipoproteins being a substrate for HL. HDL cholesterol has been reported to be inversely correlated to HL activity, and on this basis it has been suggested that lowering HL would increase HDL cholesterol. However, the correlation could also be due to a common hormonal factor such as estrogen, which has been shown to up-regulate apoAI and HDL cholesterol and lower HL. A striking feature of severe deficiency of HL is the increase in HDL cholesterol and apolipoprotein AI and an approximately 10-fold increase in HDL triglyceride. Hyper-alpha-triglyceridemia is not a feature of antiatherogenic HDL. HL binds not only to heparan, but also to the LDL receptor-related protein. It has been suggested that enzymatically inactive HL can play a role in hepatic lipoprotein uptake, forming a "bridge" by binding to the lipoprotein and to the cell surface. This raises the interesting possibility that production and secretion of mutant inactive HL could promote clearance of VLDL remnants. We have described a rare family with HL deficiency. Affected patients are compound heterozygotes for a mutation of Ser267 to Phe that results in an inactive enzyme and a mutation of Thr383 to Met that results in impaired secretion and reduced specific activity. Human HL deficiency in the context of a second factor causing hyperlipidemia is strongly associated with premature coronary artery disease. Recently, it has been reported that mutations affecting the structure of HL (e.g., T383M) are relatively frequent in the Finnish population. A C-to-T polymorphism in the promotor region of the HL gene is associated with lowered HL activity and less strongly with increased HDL cholesterol. In summary, there is a good understanding of what HL does in lipoprotein metabolism; however, there is little understanding of its physiological importance, that is, why HL does what it does. (ABSTRACT TRUNCATED)

Elevated LDL triglyceride concentrations in subjects heterozygous for the hepatic lipase S267F variant

Although naturally occurring loss-of-function mutations in human hepatic lipase (HL) have been described, the biochemical phenotype of heterozygous HL deficiency remains ill defined. This may be due to the relatively small numbers of heterozygous adult carriers of HL mutations in index kindreds. We have identified several new heterozygotes for the catalytically inactive, nonsecreted HL variant S267F in the kindred that was originally ascertained because of hypertriglyceridemia due to the mutant, secreted, circulating apolipoprotein (apo) CII variant apo CII-T. Pairwise comparisons with family controls showed that only the plasma low density lipoprotein triglycerides (LDL TGs) were higher in 11 simple heterozygotes for HL S267F (P=0.002). In contrast, both plasma total TGs and LDL TGs were significantly higher in 12 simple heterozygotes for apo CII-T than in family-matched control subjects (P=0.005 and 0.009, respectively). These findings suggest that the TG content of LDL is increased by heterozygosity for 2 different mutations that affect different proteins involved in lipolysis. However, the mechanisms underlying this compositional change in LDL appear to be different for the 2 mutations, because the total TGs are also elevated in subjects heterozygous for apo CII-T but not in subjects heterozygous for HL S267F.

Proposal of a multicompartmental model for use in the study of apolipoprotein E metabolism

Apolipoprotein (apo) E is a 299-amino acid glycoprotein that serves a number of functions in lipoprotein metabolism. Apo E binds to the triglyceride-rich lipoproteins (TRL), very-low-density lipoprotein (VLDL), and chylomicrons, as they are lipolyzed, mediating their removal from plasma via lipoprotein receptors. Apo E is also found associated with high-density lipoprotein (HDL) and has been suggested to play a role in reverse cholesterol transport. Studies on the kinetic behavior of apo E from the TRL and HDL fractions provide insights into the metabolic relationships between TRL and HDL in vivo. We sought to develop a compartmental model that can be used for analysis of kinetic data in studies on the metabolism of TRL and HDL apo E. Using radioactive tracers, it has been previously observed that, in some instances, a portion of VLDL apo E that is removed from plasma subsequently reappears in VLDL. Four multicompartmental models were considered that could account for this type of behavior: model A, in which there is transfer of apo E from HDL to VLDL; model B, in which there is a bidirectional extravascular exchange; model C, in which there is removal and subsequent reintroduction of TRL apo E into plasma; and model D, in which there is secretion of TRL apo E into plasma directly and via an extravascular pathway. Models C and D provided the best fit to the experimental data. While no physiologically plausible analog to model C could be found, an extravascular delay, analogous to newly secreted apo E that enters the lymphatic system before appearing in plasma, was postulated for model D. It was this model that was used to analyze kinetic data from metabolic studies of apo E. The model was able to provide a satisfactory fit to kinetic data in studies in which subjects were given a primed-constant infusion of 2H3-leucine. It was determined that TRL apo E from the six subjects studied had a mean residence time of 0.11 +/- 0.05 days and a mean production rate of 10.6 +/- 7.2 mg/kg/d, while HDL apo E had a mean residence time of 2.96 +/- 0.99 days and a mean production rate of 0.07 +/- 0.07 mg/kg/d. We conclude that this model describes a potential pathway for the metabolism of a portion of apo E in plasma and can be used to calculate the residence time and production rate of TRL and HDL apo E under a variety of conditions.

Effects of short-term stanozolol administration on serum lipoproteins in hepatic lipase deficiency

We have identified a kindred in Providence, RI, deficient in hepatic triglyceride lipase (HL). The two affected brothers have coronary heart disease and elevated levels of triglycerides, total cholesterol, high-density lipoprotein (HDL) cholesterol, and apolipoprotein [apo] A-I. The lipoprotein lipase (LPL) activity is normal. We and others have postulated that the effects of oral anabolic steroids on HDL metabolism are mediated by HL. To test this hypothesis, we treated these two men and two controls with the oral androgen stanozolol (6 mg/d) for 2 weeks. Consistent with other reports, HL activity increased a mean of 277% in controls with a concomitant decrease in HDL cholesterol (49%), HDL2 cholesterol (90%), HDL3 cholesterol (16%), and apo A-I (41%) and no change in apo A-II. Although stanozolol failed to induce HL activity in the HL-deficient man, HDL cholesterol, HDL2 cholesterol, and apo A-I were reduced a mean of 20%, 48%, and 32%, respectively. In contrast to controls, HDL3 cholesterol (46%) and apo A-II (14%) increased in HL-deficient subjects. Stanozolol treatment also increased LPL activity (124% +/- 86%, n = 4) and decreased lipoprotein(a) ([Lp(a)] 66% +/- 3%, n = 3) in the three men with detectable levels. The data indicate that in addition to stimulation of HL activity, stanozolol treatment changes HDL cholesterol concentration and subfraction distribution by other mechanisms.

Clinical chemistry of common apolipoprotein E isoforms

Apolipoprotein E plays a central role in clearance of lipoprotein remnants by serving as a ligand for low-density lipoprotein and apolipoprotein E receptors. Three common alleles (apolipoprotein E(2), E(3) and E(4)) give rise to six phenotypes. Apolipoprotein E(3) is the ancestral form. Common apolipoprotein E isoforms derive from nucleotide substitutions in codons 112 and 158. Resulting cysteine-arginine substitutions cause differences in: affinities for low-density lipoprotein and apolipoprotein E receptors, low-density lipoprotein receptor activities, distribution of apolipoprotein E among lipoproteins, low-density lipoprotein formation rate, and cholesterol absorption. Accompanying changes in triglycerides, cholesterol and low-density lipoprotein may promote atherosclerosis development. Over 90% of patients with familial dysbetalipoproteinaemia have apolipoprotein E(2)/E(2). Apolipoprotein E(4) may promote atherosclerosis by its low-density lipoprotein raising effect. Establishment of apolipoprotein E isoforms may be important for patients with diabetes mellitus and several non-atherosclerotic diseases. Apolipoprotein E phenotyping exploits differences in isoelectric points. Isoelectric focusing uses gels that contain pH 4-7 ampholytes and urea. Serum is directly applied, or prepurified by delipidation, lipoprotein precipitation or dialysation. Isoelectric focusing is followed by immunofixation/protein staining. Another approach is electro- or diffusion blotting, followed by protein staining or immunological detection with anti-apolipoprotein E antibodies and an enzyme-conjugated second antibody. Apolipoprotein E genotyping demonstrates underlying point mutations. Analyses of polymerase chain reaction products are done by allele-specific oligonucleotide probes, restriction fragment length polymorphism, single-stranded conformational polymorphism, the primer-guided nucleotide incorporation assay, or denaturating gradient gel electrophoresis. Detection with primers that either or not initiate amplification is performed with the amplification refractory mutation system. Disparities between phenotyping and genotyping may derive from isoelectric focusing methods that do not adequately separate apolipoprotein E posttranslational variants, storage artifacts or faint isoelectric focusing bands.

Plasma concentration of apolipoprotein E in intermediate-sized remnant-like lipoproteins in normolipidemic and hyperlipidemic subjects

Triglyceride-rich lipoprotein (TRL) remnants have been strongly implicated in the pathogenesis of atherosclerosis. To further investigate plasma remnant lipoprotein metabolism, we have determined the plasma concentration of apolipoprotein (apo) E (by polyclonal enzyme-linked immunoassay) in remnant-like lipoproteins, isolated by automated gel filtration chromatography as a fraction intermediate in size between VLDL and HDL. In normolipidemic subjects (n = 12), 1.2 +/- 0.11 mg/dL (33 +/- 2%, mean +/- SE) of total plasma apoE was associated with this fraction (termed ISL apoE). In hypercholesterolemic (type IIa, n = 12), hypertriglyceridemic (type IV, n = 12), and mixed hyperlipidemic (type IIb, n = 12) subjects, mean ISL apoE concentrations were 2.1 +/- 0.2, 2.5 +/- 0.2, and 3.8 +/- 0.4 mg/dL, respectively (P < .001 versus normal values) (45 +/- 2%, 32 +/- 2%, and 44 +/- 2% of total). ISL apoE was 8.7 +/- 1.4 mg/dL (42 +/- 3%) in type III dyslipidemic subjects (apoE2/2, n = 8). ISL apoE was positively correlated with plasma triglyceride (r = .41, P < .01), and at any given level of plasma triglyceride, subjects with an apoE2/2 or apoE3/2 phenotype tended to have a higher concentration of ISL apoE (P < .01) than apoE3/3 or E4/3 individuals. ISL apoE was also correlated (P < .001) with total plasma cholesterol (r = .66), TRL cholesterol (r = .49), TRL apoE (r = .47), LDL apoB (r = .42), and LDL+HDL triglyceride (r = .74). These results suggest that (1) a significant proportion of plasma apoE resides within an intermediate-sized remnant-like lipoprotein fraction in both normolipidemic and hyperlipidemic subjects; (2) plasma remnant lipoprotein accumulation is associated with an elevation in ISL apoE concentration; and (3) ISL apoE concentration is significantly correlated with various proatherogenic lipid parameters and may itself be a potentially important atherogenic index.

Influence of lipoprotein lipase and hepatic lipase on the transformation of VLDL and HDL during lipolysis of VLDL

In order to study the relative effects of lipolytic enzymes on the removal of lipids and apolipoproteins, in particular apolipoprotein (apo) E and cholesteryl ester, from human very low density lipoprotein (VLDL) during its conversion to product lipoproteins, the action of lipoprotein lipase (LPL) and the combined action of lipoprotein lipase and hepatic lipase (HL) were studied in the presence of physiological proportions of high density lipoprotein (HDL) (10 mg protein), VLDL (2 mg protein) and albumin in an amount sufficient for the binding of all released fatty acids. The HDL used in the incubation was free of apo E in order to facilitate assessment of apo E transfer from VLDL to HDL. The redistribution of lipid and apolipoprotein mass and the movement of labeled cholesteryl ester from VLDL to other lipoprotein fractions was assessed by density gradient ultracentrifugation. Following 90%-95% lipolysis of VLDL triglycerides by rat heart LPL in 2 h, there was an almost complete transfer of apo C-II and apo C-III to HDL but only 20% of VLDL apo E was transferred to HDL. There was significant augmentation of HDL unesterified cholesterol and phospholipid mass during LPL action despite a substantial overall phospholipid hydrolysis (30%). The transfer of cholesteryl ester mass to HDL was variable (0%-13%) with a mean transfer of 7% of VLDL cholesteryl ester. Transfer of labeled VLDL cholesteryl ester to HDL was 3%-6%. A considerable amount of the VLDL lipid mass appeared in the light fraction of the low density lipoprotein (LDL) region, but a substantial amount remained in the VLDL/intermediate density lipoprotein (IDL) region. The post-lipolysis particles that were isolated in the VLDL-LDL density range were larger than LDL and contained a high ratio of surface lipids relative to core lipids as compared to plasma LDL. The inclusion of human HL with LPL did not alter the redistribution of apolipoproteins proteins or lipids from VLDL to LDL or to HDL. The major effect of HL, relative to that observed with LPL alone, was a marked hydrolysis of HDL triglycerides (68%). Despite the combined action of LPL and HL on VLDL in the presence of HDL and over 90% lipolysis of triglycerides, a major portion of residual VLDL mass remained in fractions lighter than normal LDL density and retained apo E. It is concluded that lipoprotein lipase of LPL in combination with HL are ineffective in bringing about the complete conversion of plasma VLDL to LDL. Lipoprotein lipase was effective in substantially augmenting the HDL mass including cholesteryl while the major effect of HL was the selective hydrolysis of HDL triglycerides.

Methods for the analysis of triacylglycerols

This article discusses the methods most commonly employed in the analysis of the triacylglycerols (TAGs) in natural fats and considers the main advantages and disadvantages of each and the techniques for optimising analytical conditions. Complete analysis of the composition of a natural fat calls for a method of extracting and purifying the triglyceride fraction, normally by preparatory thin-layer and column chromatography. Determination of the individual components of triglyceride mixtures still entails certain difficulties, namely, the separation and identification of the TAGs in natural fats. High-performance liquid chromatography (HPLC) offers significant advantages over gas and thin-layer chromatography. Many workers have developed non-aqueous, reversed-phase HPLC systems capable of successfully resolving complex mixtures of TAGs, and combining reversed-phase (RP) HPLC and argentation chromatography may improve the results. Identification of the TAGs separated by HPLC becomes an extremely complex task if many different fatty acids are involved and if the sn-stereoscopic positions on the glycerol are to be determined. Enzymatic analysis and chiral-phase chromatography are capable of localising fatty acids on the TAG molecule. In closing, some of the most interesting biomedical applications of TAG analysis are summarised.

Effects of omega-3 fatty acids on intravascular lipolysis of very-low-density lipoproteins in humans

Very-low-density lipoproteins (VLDLs) are the major carriers of fasting plasma triglyceride (TG). TG-enriched VLDLs become cholesterol (C)-enriched low-density lipoproteins (LDLs) through hydrolysis facilitated by lipoprotein lipase (LPL). Omega-3 fatty acid (n-3 FA) supplementation may increase LDL-C while decreasing plasma TG in hypertriglyceridemic patients. It has been proposed that n-3 FAs increase LDL-C by promoting production of TG-poor VLDL and accelerating conversion of VLDL to LDL. To study the effects of n-3 FA supplementation on in vivo lipolysis of VLDL directly, we treated 11 hypertriglyceridemic subjects with n-3 FA (3.3 g/d). Each participant was studied three times: at baseline, after a 1-month period of run-in olive oil placebo, and after 1 more month of n-3 FA supplementation. Lipolysis was induced by intravenous infusion of heparin for 2 hours. Plasma samples were obtained every 30 minutes for determination of lipids and apoproteins (apos), separation of individual lipoproteins by fast protein liquid chromatography (FPLC), and measurement of LPL and hepatic TG lipase (HTGL) levels. n-3 FA supplementation decreased fasting plasma TG (2.51 +/- 0.23 v 3.97 +/- 0.46 mmol/L), VLDL-TG (1.08 +/- 0.18 v 2.35 +/- 0.35 mmol/L), and VLDL-C (0.39 +/- 0.05 v 0.72 +/- 0.13 mmol/L) while increasing LDL-C (3.59 +/- 0.21 v 3.00 +/- 0.23 mmol/L) and plasma apo B (3.31 +/- 0.19 v 2.90 +/- 0.17 mmol/L). The absolute rate of TG lipolysis correlated with fasting TG (r = .74, P < .005) and was lower after n-3 FA supplementation (0.11 +/- 0.01 mmol/mL/min) as compared with placebo (0.19 +/- 0.01, P < .01), whereas percent decreases from baseline TG levels were similar at entry onto the study (57.4% +/- 2.5%), after placebo (58.8% +/- 2.7%), and after n-3 FA (52% +/- 3.6%).(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular characterization of human hepatic lipase deficiency. In vitro expression of two naturally occurring mutations

Individuals with hepatic lipase (HL) deficiency are often characterized by elevated levels of triglycerides and cholesterol and may be subject to premature atherosclerosis. Missense mutations in the HL gene have been identified in two affected families: substitutions of serine for phenylalanine at amino acid 267 and threonine for methionine at amino acid 383 (S267F and T383M, respectively). To confirm the role of S267F and T383M, respectively). To confirm the role of mutations separately into human HL cDNA by site-directed mutagenesis, and the resulting constructs were independently expressed in COS cells. HL activity and mass were measured and compared with wild-type HL transfectants to determine the effect of these mutations on lipase activity and secretion. Although similar amounts of HL protein were detected intracellularly after transfection with the wild-type and mutant constructs, S267F and T383M HL activity levels were markedly decreased: in S267F, no HL activity was detected, and activity levels in T383M were 38% of wild-type HL. Heparin-induced secretion of the two HL mutants was also severely affected: no detectable activity could be measured in the media of S267F, although some inactive mass (12% of wild-type HL) was secreted; mutant T383M secreted 4% and 20% of wild-type activity and mass, respectively. These results indicate that the single amino acid substitution present in HL S267F is sufficient to render the enzyme completely nonfunctional; in contrast, the T383M mutant retains partial activity but is poorly secreted. Thus, these defects appear capable of accounting for the HL-deficient phenotypes exhibited by individuals carrying the T383M and S267F mutations.

Determination of fatty acid and triacylglycerol composition of human very-low-density lipoproteins

The fatty acid composition of human very-low-density lipoproteins (VLDL) was studied in a population from western Andalusia with a diet in which the fat content came mainly from olive oil. The lipid composition of VLDL, including the fatty acid composition of the phospholipids and triacylglycerols, was examined by capillary gas chromatography. Twenty-five peaks were resolved, ranging in chain length from 14 to 24 carbon atoms, including geometric and positional isomers. The major fatty acids present in phospholipids were 16:0, 18:0, 18:1(n - 9) and 18:2(n - 6), and in triacylglycerols were 18:1(n - 9), 16:0 and 18:2(n - 6). The major triacylglycerol was POO, followed by PLO and OOO. MLP, PPS and LLL were absent. The presence of a large amount of OOO in this fraction demonstrates that the triacylglycerol composition of the VLDL depends on the type of diet consumed.

Beta-VLDL in hepatic lipase deficiency induces apoE-mediated cholesterol ester accumulation in macrophages

Hepatic lipase-deficient subjects in the Ontario kindred are compound heterozygotes for hepatic lipase mutations (Ser267-->Phe and Thr383-->Met). Cholesteryl ester-rich beta-very-low-density lipoprotein (beta-VLDL) accumulates in plasma and such subjects have premature atherosclerosis. To determine a possible mechanism, we hypothesized that hepatic lipase-deficient beta-VLDL, homozygous for apolipoprotein (apo) E3, would cause cholesteryl ester accumulation and foam cell formation in macrophages. beta-VLDL and pre-beta-VLDL were isolated by Pevikon electrophoresis and incubated with J774 macrophages, cells that do not secrete apoE. beta-VLDL increased cellular cholesteryl ester content 13-fold, whereas pre-beta-VLDL increased cholesteryl ester sevenfold. beta-VLDL increased acyl CoA:cholesterol acyltransferase activity fourfold (measured as [14C]oleate incorporation into cholesteryl ester). Preincubation of hepatic lipase-deficient beta-VLDL with the anti-apoE monoclonal antibody 1D7, which inhibits binding of apoE to low-density lipoprotein receptors, inhibited cellular cholesteryl ester accumulation by 75%, whereas the anti-apoB blocking monoclonal antibody 5E11 failed to inhibit cellular cholesteryl ester accumulation. In contrast to hepatic lipase deficiency, beta-VLDL from type III subjects (E2/E2) failed to increase cellular cholesteryl ester or acyl CoA:cholesterol acyltransferase more than 1.5-fold. Thus, hepatic lipase-deficient beta-VLDL readily induces cholesteryl ester accumulation in J774 macrophages, a process mediated by functional apoE3. This may explain the premature atherosclerosis observed in this kindred.

Different hydrolytic efficiencies of adipose tissue lipoprotein lipase on very-low-density lipoprotein subfractions separated by heparin-Sepharose chromatography

Human very-low-density lipoproteins (VLDL) were subfractionated by heparin-Sepharose chromatography into an unbound (A) and three bound (B, C and D) populations at increasing ionic strengths. Subfractions were characterized regarding their chemical composition and efficiency of triacylglycerol hydrolysis by rat adipose tissue LPL. The triacylglycerol content decreased, whereas the cholesterol and protein contents increased from subfractions A and B to subfraction D. VLDL-D showed the highest apo E/apo C ratio, though all the subfractions contained appreciable apo E. Appearance of VLDL-A resulted from exceeding the binding capacity of the column, since practically all its particles eluted at positions of bound VLDL under re-chromatography. Subfractions B, C and D stimulated LPL activity on emulsified tri[14C]oleoylglycerol to a similar extent, indicating that their apo C-II content was equally effective activating LPL. Incubation of tri[14C]oleoylglycerol labeled VLDL subfractions with fat pad pieces in the presence or absence of heparin resulted in greater hydrolysis and fatty acid uptake for VLDL-B and -C than for VLDL-D, a pattern observed over a wide range of LPL activities in the media. We conclude: (1) any VLDL particle can interact with heparin, which is consistent with the presence of apo E in all the subfractions, and (2) triacylglycerols in apo E-rich VLDL are less efficiently hydrolyzed by LPL than those in apo E-poor particles. We propose that richness in apo E impairs LPL action upon VLDL and decreases the rate of delivery of fatty acids to peripheral tissues.

LDL particle size distribution. Results from the Framingham Offspring Study

Using 2-16% gradient gel electrophoresis, we examined low density lipoprotein (LDL) particle size in relation to plasma lipoproteins in 1,168 women and 1,172 men from the Framingham Offspring Study. In addition, we studied the effect of dietary intake on LDL size in a subset of the population. Seven LDL size peaks were identified, with the largest, LDL 1, being found in the density range 1.019-1.033 g/ml; LDL 2 and LDL 3 in d = 1.033-1.038 g/ml; LDL 4 and LDL5 in d = 1.038-1.050 g/ml; and the smallest, LDL 6 and 7, in d = 1.050-1.063 g/ml. Seventy-seven percent of the population had one major and at least one minor LDL peak. Secondary LDL peaks accounted for 23% of the total LDL relative area, based on laser scanning densitometry. LDL size distribution was skewed toward larger LDL particles in women (prevalence of LDL 1, 30% and of LDL 2, 31%), whereas men exhibited a more symmetric distribution (prevalence of LDL 3, 42%). The prevalence of small (< 255 A), dense (d > 1.038 g/ml) LDL particles 4-7 was 33% in men, 5% in premenopausal women, and 14% in postmenopausal women. In agreement with previous reports, small, dense LDL particles were significantly (p < 0.0001) associated with increased triglyceride and apolipoprotein (apo) B levels and decreased HDL cholesterol and apo A-I levels. In addition, we found a significant (p < 0.0001) association between LDL cholesterol and LDL size. The highest LDL cholesterol levels were found among women with LDL 4 (148 mg/dl) and men with LDL 3-5 (138 mg/dl). In addition, the presence of LDL 3 or 4 as secondary peaks was significantly associated with higher LDL cholesterol levels, while smaller secondary LDL peaks were associated with higher triglyceride levels. We also found that compared with subjects with optimal LDL cholesterol levels (< 130 mg/dl), individuals with high-risk LDL cholesterol levels (> or = 160 mg/dl) had 1) a higher prevalence of LDL 3 and 4 (women only) and a lower prevalence of LDL 1 and 2 (women only) and 2) 11% higher LDL cholesterol to apo B ratios, even when matched for LDL particle size. Furthermore, low saturated fat and cholesterol intakes were significantly associated (p < 0.01) with smaller LDL particles. Therefore, the identification of small, dense LDL particles per se may not be a good indicator of coronary artery disease risk in population studies.(ABSTRACT TRUNCATED AT 400 WORDS)

Down-regulation of hepatic lipase gene expression and activity by fenofibrate

The influence of the hypolipidemic drug, fenofibrate, on hepatic lipase (HL) gene expression and activity was investigated in the rat. Fenofibrate treatment provoked a dose-dependent decrease in HL mRNA levels. At a dose of 0.5% (w/w), HL mRNA levels were reduced to nearly 50% the levels in untreated controls. This decrease was parallelled by a comparable reduction in liver HL activity. The decrease in HL mRNA levels was already observed after 1 day of fenofibrate treatment. Whole liver perfusion experiments showed that the heparin-releasable HL activity in fenofibrate-treated livers dropped to 10% the activity in control livers. In conclusion, treatment with fenofibrate decreases HL gene expression, leading to a lowered activity of endothelium bound HL in fenofibrate-treated livers.

Independent regulation of chylomicron lipolysis and particle removal rates: effects of insulin and thyroid hormones on the metabolism of artificial chylomicrons

The processes of chylomicron lipolysis and removal from plasma were investigated by the intra-arterial infusion of doubly labeled artificial chylomicrons in rats. The rate of lipolysis was measured as a delipidation index (DI), which is the glyceryl-tri-9,10(N)-3H oleate (3H-TO) fraction removed from the particle as fatty acids, whereas the cholesteryl(1-14C) oleate (14C-CO) plasma disappearance rate measures the splanchnic organ particle uptake. In the alloxan-diabetic rats, despite a normal DI, the 14C-CO plasma residence time (RT) was longer than in control animals and remained longer after stimulation of the lipoprotein lipase by heparin. DI and 14C-CO removal rate were not significantly altered by insulin administration to glucose-supplemented control rats. Lipolysis was remarkable in propylthiouracil (PTU)-induced hypothyroidism, and yet the 14C-CO removal rate was retarded. In hypothyroidism, heparin enhanced the 14C-CO removal more than in the control group; however, after heparin, the 14C-CO RT still remained higher in the hypothyroid animals as compared with the control group. Hyperthyroidism lowered the DI; nevertheless, the 14C-CO disappearance rate was faster than in controls. In summary, lack or excess of thyroid hormone influences both the chylomicron lipolysis and removal systems, whereas lack of insulin impairs mostly the particle removal from plasma, and excess of insulin has no effect on the chylomicron metabolism.

Increased removal of beta-very low density lipoproteins after ethinyl estradiol is associated with increased mRNA levels for hepatic lipase, lipoprotein lipase, and the low density lipoprotein receptor in Watanabe heritable hyperlipidemic rabbits

The mechanism by which ethinyl estradiol (EE) decreases the concentration of lipids in the d less than 1.019 g/ml fraction (beta-very low density lipoprotein [beta-VLDL]) of homozygous Watanabe heritable hyperlipidemic (WHHL) rabbits was studied. Treatment with EE increased the activity of hepatic lipase (HL) twofold to threefold in postheparin plasma and in liver biopsies. Postheparin plasma and adipose tissue lipoprotein lipase (LPL) activities were also increased twofold to fourfold after EE. The effects of EE on HL and LPL activities were associated with a threefold to sixfold elevation in liver HL mRNA and a fourfold elevation in adipose tissue LPL mRNA steady-state levels, pointing to an effect of EE on HL and LPL gene transcription. EE also increased liver low density lipoprotein (LDL) receptor mRNA levels threefold to fivefold. These results suggest a concerted action of LPL, HL, and the LDL receptor in the removal of beta-VLDL in homozygous WHHL rabbits with a defective LDL receptor. In addition, the content of apolipoprotein E in the d less than 1.019 g/ml fraction changed toward normal after EE. Because the remaining particles contained apolipoprotein B-100 almost exclusively, it is likely that apolipoprotein E-containing beta-VLDLs are preferentially removed. This may be the result of the increased activity of LPL and HL influencing the conformation of apolipoprotein E on the beta-VLDL particle in such a way that it is directly removed from the circulation, possibly by the induced LDL receptor.

The association of lipoprotein and hepatic lipase activities with high density lipoprotein subclass levels in men with myocardial infarction at a young age

The relations between postheparin plasma lipase activities and concentrations of lipoproteins, in particular plasma high density lipoprotein (HDL) subclasses determined by gradient gel electrophoresis, were examined in 39 men who had survived a first myocardial infarction before the age of 45 years and in 20 age-matched control men. Reduced lipoprotein lipase (LPL) and hepatic lipase (HL) activities were found in the patients due to low LPL activity in patients with hypertriglyceridaemia, and low HL activity in those with a normal lipoprotein pattern or hypercholesterolaemia. Considerably lower plasma HDL2b and HDL2a protein concentrations and higher plasma HDL3b and HDL3c protein levels were found in the patients compared with the healthy control subjects. The subgroup of patients with hypertriglyceridaemia accounted for the major proportion of the case control differences for the HDL subspecies. However, significantly lower HDL2b and HDL2a concentrations were seen also among the normotriglyceridaemic patients. Analysis of the correlations between concentrations of HDL subclasses and lipase activities revealed positive associations between LPL and HDL2b and negative associations between HL and HDL2b. For LPL, this relationship was confined to hypertriglyceridaemic and for HL to normotriglyceridaemic subjects. HL was indicated to be positively connected with HDL3b levels, irrespective of lipoprotein pattern, whereas LPL seemed to be unassociated with HDL3b. It is concluded that low LPL and HL activities partly account for the change in HDL subclass distribution observed in patients with myocardial infarction at a young age.

Coexistence of abnormalities of hepatic lipase and lipoprotein lipase in a large family

A large family is reported with familial hepatic triglyceride lipase (HTGL) deficiency and with the coexistence of reduced lipoprotein lipase (LPL) similar to the heterozygote state of LPL deficiency. The proband was initially detected because of hypertriglyceridemia and chylomicronemia. He was later demonstrated to have beta-VLDL despite an apo E3/E3 phenotype and the lack of stigmata of type III hyperlipoproteinemia. The proband had no HTGL activity in postheparin plasma. Two of his half-sisters had very low HTGL activity (39 and 31 nmol free fatty acids/min/ml; normal adult female greater than 44). His son and daughters had decreased HTGL activity (normal male and preadolescent female greater than 102), which would be expected in obligate heterozygotes for HTGL deficiency. Low HTGL activity was associated with LDL particles which were larger and more buoyant. Several family members, including the proband, had reduced LPL activity and mass less than that circumscribed by the 95% confidence-interval ellipse for normal subjects and had hyperlipidemia similar to that described in heterozygote relatives of patients with LPL deficiency. All the sibs with hyperlipidemia had a reduced LPL activity and mass, while subjects with isolated reduced HTGL (with normal LPL activity) had normal lipid phenotypes. Analysis of genomic DNA from these subjects by restriction-enzyme digestion revealed no major abnormalities in the structure of either the HTGL or the LPL gene. Compound heterozygotes for HTGL and LPL deficiency show lipoprotein physiological characteristics typical for HTGL deficiency, while their variable lipid phenotype is typical for LPL deficiency.

Plasma lipoproteins in familial hepatic lipase deficiency

We have studied the lipoproteins, apolipoproteins, and postheparin lipase activities in an extended pedigree with familial hepatic lipase deficiency. A deficiency of hepatic lipase was found in three of five brothers and in one of their children. Triglyceride enrichment of low density and high density lipoproteins was identified as the constitutive phenotype. beta-very low density lipoprotein was observed in hepatic lipase-deficient subjects, but it was absent when the plasma triglyceride concentration was less than 1 mM/l. The hepatic lipase-deficient subjects had normal or elevated low density lipoprotein cholesterol and high density lipoprotein cholesterol concentrations. Hyperprebetalipoproteinemia, hyperbetalipoproteinemia, and hyperalphalipoproteinemia were observed in both affected and unaffected family members. Compared with the unaffected family members, the hepatic lipase-deficient subjects had no significant differences in very low density lipoprotein cholesterol, very low density lipoprotein triglyceride, or low density lipoprotein cholesterol. These observations are consistent with the presence of additional genes causing hyperlipidemia in this family, independent of the deficiency of hepatic lipase.

Serum apolipoprotein E in children and adolescents: the Bogalusa Heart Study

Serum apolipoprotein (apo) E levels and its relationship to lipids and lipoprotein cholesterol fractions were examined in a random subsample (n = 561) of children and adolescents (7 to 17 years of age) from a total biracial community. Mean (+/- SD) levels of apo E were higher in blacks (males 4.8 +/- 1.8 mg/dL; females 5.2 +/- 1.8 mg/dL) than in whites (males 3.9 +/- 1.2 mg/dL; females 4.3 +/- 1.0 mg/dL) irrespective of sex (P less than .001). The black-white difference in apo E persisted after controlling for the covariates: sexual maturation, age, adiposity, cigarette smoking, alcohol use, and oral contraceptive use (P less than .001). A sex differential (females greater than males, P less than .01) for apo E was seen in both racial groups. Apo E levels were inversely associated with age (P less than .01) and sexual maturation (P less than .05) only in white males. Apo E related positively and significantly to total cholesterol, low-density lipoprotein cholesterol, and high-density lipoprotein cholesterol fractions (HDL2-C and HDL3-C) in certain race-sex groups. Race, HDL2-C, triglycerides (very-low density lipoprotein cholesterol), HDL3-C, and sex were identified as predictor variables for apo E, in that order, and accounted for 21% of its variability in serum. It is conceivable that the observed race-sex differences in apo E may be related to apo E-HDL subfraction, which is thought to participate in the reverse cholesterol transport.