Lipases and HDL metabolism

Walking the VLDL tightrope in cardiometabolic diseases

Very-low-density lipoprotein (VLDL), a triglyceride-rich lipoprotein secreted by hepatocytes, is pivotal for supplying peripheral tissues with fatty acids for energy production. As if walking on a tightrope, perturbations in the balance of VLDL metabolism contribute to cardiometabolic dysfunction, promoting pathologies such as cardiovascular disease (CVD) or metabolic dysfunction-associated steatotic liver disease (MASLD). Despite the advent of lipid-lowering therapies, including statins and proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors, risks for cardiovascular events persist. With limitations to currently available CVD therapeutics and no US Food and Drug Administration (FDA)-approved treatment for MASLD, this review summarizes the current understanding of VLDL metabolism that sheds light on novel therapeutic avenues to pursue for cardiometabolic disorders.

Effects of portable pedal machines at work on lipoprotein subfraction profile in sedentary workers - the REMOVE study

BACKGROUND

Sedentary behaviour at work is a major cause of atherosclerosis, particularly in tertiary workers. However, no studies have ever assessed the effect of active workstation on lipoprotein subfraction profile. This study aimed to evaluate the effect of 12-week portable pedal machines (PPMs) on lipoprotein subfraction profile among healthy sedentary workers.

METHODS

Healthy administrative workers were randomized into an intervention group using PPMs for 12 weeks or a control group using normal-desk. Lipoprotein subfractions were assessed using Lipoprint® electrophoresis. Main outcomes were explored using mixed models with sensitivity analyses (four models).

RESULTS

We included 40 participants (43.7 ± 8.6 years old, 100% women, BMI 23.8 ± 3.4 kg/m2; sedentary time at work 7.7 ± 1.8 h/day). Groups did not differ at baseline in any outcomes. 32 participants finished the trial. Changes in lipoprotein subfractions were especially marked for LDL profile. There was an interaction time x group for all parameters related to LDL and their subfractions: total LDL-cholesterol (p = 0.012), LDL particle size (p = 0.027), large LDL subfractions 1 and 2 (p = 0.001), and small dense LDL subfractions 3 to 7 (p = 0.046), using the crude model. The interaction reflects difference in the direction of changes between groups. The LDL particle size significantly increased in the intervention group (from 271.9 ± 2.5 at t0 to 272.8 ± 1.9 Ångström at t1, p = 0.037) while it did not change in the control group (272.5 ± 1.7 at t0 to 271.8 ± 1.5Å at t1, p = 0.52). All interactions were constantly significant whatever the models. Influencing variables were mainly stress at work that was associated with an increase in total LDL-cholesterol (coefficient 3.15, 95CI 0.20 to 6.11 mg/dl, p = 0.038), and BMI that was associated with Large-LDL, Large-HDL, IDL-C and triglycerides.

CONCLUSIONS

Lipoprotein profile was improved after a 12-week PPMs intervention at work in healthy administrative workers. Changes were mainly showed for LDL and LDL subfractions. Lipoprotein profile was worsened by stress at work, BMI and age.

TRIAL REGISTRATION

NCT04153214.

Using Synthetic ApoC-II Peptides and nAngptl4 Fragments to Measure Lipoprotein Lipase Activity in Radiometric and Fluorescent Assays

Lipoprotein lipase (LPL) plays a crucial role in preventing dyslipidemia by hydrolyzing triglycerides (TGs) in packaged lipoproteins. Since hypertriglyceridemia (HTG) is a major risk factor for cardiovascular disease (CVD), the leading cause of death worldwide, methods that accurately quantify the hydrolytic activity of LPL in clinical and pre-clinical samples are much needed. To date, the methods used to determine LPL activity vary considerably in their approach, in the LPL substrates used, and in the source of LPL activators and inhibitors used to quantify LPL-specific activity, rather than other lipases, e.g., hepatic lipase (HL) or endothelial lipase (EL) activity. Here, we describe methods recently optimized in our laboratory, using a synthetic ApoC-II peptide to activate LPL, and an n-terminal Angiopoietin-Like 4 fragment (nAngptl4) to inhibit LPL, presenting a cost-effective and reproducible method to measure LPL activity in human post-heparin plasma (PHP) and in LPL-enriched heparin released (HR) fractions from LPL secreting cells. We also describe a modified version of the triolein-based assay using human serum as a source of endogenous activators and inhibitors and to determine the relative abundance of circulating factors that regulate LPL activity. Finally, we describe how an ApoC-II peptide and nAngptl4 can be applied to high-throughput measurements of LPL activity using the EnzChek™ fluorescent TG analog substrate with PHP, bovine LPL, and HR LPL enriched fractions. In summary, this manuscript assesses the current methods of measuring LPL activity and makes new recommendations for measuring LPL-mediated hydrolysis in pre-clinical and clinical samples.

Effect of Selective Androgen Receptor Modulator on Cholesterol Efflux Capacity, Size and Subspecies of HDL Particles

Context

Selective androgen receptor modulators (SARMs), because of their preferential muscle vs prostate selectivity, are being developed for muscle-wasting conditions. Oral SARMs suppress high-density lipoprotein cholesterol (HDL-C) but their effects on functional capacity and atherogenic potential of HDL particles are unknown.

Objective

To determine the effects of an oral SARM (OPK-88004) on cholesterol efflux capacity, HDL particle number and size, apolipoprotein particle number and size and HDL subspecies

Methods

We measured cholesterol efflux capacity (CEC); HDL particle number and size; APOB; APOA1; and protein-defined HDL subspecies associated with coronary heart disease (CHD) risk in men, who had undergone prostatectomy for low-grade prostate cancer during 12-week treatment with placebo or 1, 5, or 15 mg of an oral SARM (OPK-88004).

Results

SARM significantly suppressed HDL-C (P < .001) but HDL particle size did not change significantly. SARM had minimal effect on CEC of HDL particles (change + 0.016, –0.036, +0.070, and –0.048%/µmol-HDL/L^–1 at 0, 1, 5, and 15 mg SARM, P = .045). SARM treatment suppressed APOAI (P < .001) but not APOB (P = .077), and reduced APOA1 in HDL subspecies associated with increased (subspecies containing α2-macroglobulin, complement C3, or plasminogen) as well as decreased (subspecies containing APOC1 or APOE) CHD risk; relative proportions of APOA1 in these HDL subspecies did not change. SARM increased hepatic triacylglycerol lipase (HTGL) (P < .001).

Conclusion

SARM treatment suppressed HDL-C but had minimal effect on its size or cholesterol efflux function. SARM reduced APOA1 in HDL subspecies associated with increased as well as decreased CHD risk. SARM-induced increase in HTGL could contribute to HDL-C suppression. These data do not support the simplistic notion that SARM-associated suppression of HDL-C is necessarily proatherogenic; randomized trials are needed to determine SARM’s effects on cardiovascular events.

Effect of thyroid function on pre-β1 high-density lipoprotein levels in patients with Graves' disease undergoing radioiodine treatment

CONTEXT

The metabolism of HDL is altered in thyroid dysfunctions. Preβ-1 HDL is a very small discoidal precursor HDL and promotes cholesterol efflux via ABCA₁. The effects of thyroid dysfunctions on pre-β1 HDL are unknown. Thyroid hormone regulates ANGPTL3 expression, which may participate in HDL metabolism in thyroid dysfunctions.

OBJECTIVE

To determine the variation of HDL subfractions, especially preβ-1 HDL in thyroid dysfunctions, and whether ANGPTL3 mediates the effect of thyroid function on HDL metabolism.

METHODS

We recruited 26 patients with Graves' disease undergoing radioiodine treatment. They were evaluated at three time points: at baseline, when they were hypothyroid after radioiodine treatment, and when they were on stable levothyroxine replacement and euthyroid.

RESULTS

The concentrations of smaller HDL particles Preβ-1 HDL and HDL3 were highest at the hyperthyroid state, and lowest at the hypothyroid state. While the larger HDL particles HDL2 and HDL1 changed just the opposite. Preβ1-HDL and HDL3 were positively correlated to fT₃ and fT₄, while were negatively correlated to TSH. In contrast, HDL1 was negatively associated with fT₃ and fT₄, while was positively associated with TSH. The correlations between thyroid hormones and HDL subfractions remained significant after adjusting for ANGPTL3.

CONCLUSIONS

There is a shift form smaller HDL particles pre-β1 HDL and HDL3 to larger HDL particles HDL2 and HDL1 in hypothyroidism, while the change is just the opposite in hyperthyroidism. In future, cholesterol efflux capacity should be measured to determine if the function of HDL particles also changes with the shifting of HDL subfractions.

Polypeptide N-acetylgalactosaminyltransferase-Associated Phenotypes in Mammals

Mucin-type O-glycosylation involves the attachment of glycans to an initial O-linked N-acetylgalactosamine (GalNAc) on serine and threonine residues on proteins. This process in mammals is initiated and regulated by a large family of 20 UDP-GalNAc: polypeptide N-acetylgalactosaminyltransferases (GalNAc-Ts) (EC 2.4.1.41). The enzymes are encoded by a large gene family (GALNTs). Two of these genes, GALNT2 and GALNT3, are known as monogenic autosomal recessive inherited disease genes with well characterized phenotypes, whereas a broad spectrum of phenotypes is associated with the remaining 18 genes. Until recently, the overlapping functionality of the 20 members of the enzyme family has hindered characterizing the specific biological roles of individual enzymes. However, recent evidence suggests that these enzymes do not have full functional redundancy and may serve specific purposes that are found in the different phenotypes described. Here, we summarize the current knowledge of GALNT and associated phenotypes.

Assessment of HDL Cholesterol Removal Capacity: Toward Clinical Application

While there is a controversy regarding the causal relationship between high-density lipoprotein cholesterol (HDL-C) and cardiovascular disease (CVD), recent studies have demonstrated that the cholesterol efflux capacity (CEC) of HDL is associated with the incidence of CVD. However, there are several limitations to current assays of CEC. First, CEC measurements are not instantly applicable in clinical settings, because CEC assay methods require radiolabeled cholesterol and cultured cells, and these procedures are time consuming. Second, techniques to measure CEC are not standardized. Third, the condition of endogenous cholesterol donors would not be accounted for in the CEC assays. Recently, we established a simple, high-throughput, cell-free assay system to evaluate the capacity of HDL to accept additional cholesterol, which is herein referred to as "cholesterol uptake capacity (CUC)". We demonstrated that CUC represents a residual cardiovascular risk in patients with optimal low-density lipoprotein cholesterol control independently of traditional risk factors, including HDL-C. Establishing reproducible approaches for the cholesterol removal capacity of HDL is required to validate the impact of dysfunctional HDL on cardiovascular risk stratification in the "real world".

Protective Effect of Chickpea Protein Hydrolysates on Colon Carcinogenesis Associated With a Hypercaloric Diet

OBJECTIVE

Colon cancer occupies the third place in incidence worldwide; eating habits, in particular, consumption of hypercaloric diets, are relevant in its etiopathogenesis. On the other hand, foods can also modulate carcinogenesis: for example, proteins, which when hydrolyzed release peptides with biological activities, and legumes, especially, chickpea, represent a good source of hydrolysates. The objective of this work was to verify the inhibitory effect of chickpea hydrolyzed protein on azoxymethane (AOM)-induced carcinogenesis in mice fed a hypercaloric diet.

METHODS

We hydrolyzed chickpea protein by pepsin, pancreatin, and a combined pepsin-pancreatin system, to test its anticarcinogenic and hypercaloric activity in mice that had consumed a hypercaloric diet or a normal diet but were injected with azoxymethane (AOM).

RESULTS

A concentrate (70% proteins) was obtained from chickpea seeds (18.5% proteins), and extensive hydrolysates were obtained at 15 minutes, in all tested enzyme systems. The greatest activity was evidenced in the hydrolysates obtained with pepsin-pancreatin at 90 minutes. Animals that consumed the hypercaloric diet had a higher concentration of cholesterol and a higher atherogenic index, which were significantly reduced with the administration of chickpea protein hydrolysates with a dose-response effect (10, 20, or 30 mg/kg), whereas no effect was observed in animals that consumed the normal diet. In animals given AOM, aberrant crypts were observed, at a higher rate in animals that consumed the hypercaloric diet; with the consumption of hydrolysates by the animals that consumed either diet, the number of aberrant crypts was reduced with the 3 doses tested, and the effect was better in those animals fed the hypercaloric diet. The best effect in all tests was with 30 mg/kg body weight.

CONCLUSION

The consumption of chickpea protein hydrolysates might confer a protective effect against colon carcinogenesis.

Possible association between ABCC8 C49620T polymorphism and type 2 diabetes in a Nigerian population

BACKGROUND

The association between ABCC8 gene C49620T polymorphism and type 2 diabetes (T2D) in populations of diverse ethnic backgrounds has been reported. However, such occurrence in an African population is yet to be established. This case-control study involving 73 T2D and 75 non-diabetic (ND) patients investigated the occurrence of this polymorphism among T2D patients in Nigeria and assessed its relationship with body lipids of patients.

METHODS

Demographic and clinical characteristics of patients were collected and lipid profile indices including total cholesterol (TC), triglyceride (TG), low density lipoprotein (LDL) and high density lipoprotein (HDL) were assayed. Restriction fragment length polymorphism-PCR (RFLP-PCR) was employed to genotype the ABCC8-C49620T polymorphism using PstI restriction enzyme.

RESULTS

This study revealed significantly (p < 0.05) higher prevalence of the T allele of the ABCC8 gene in T2D patients (33.1%) compared to ND patients (28.0%). The mutant TT genotype was also higher than the CC and CT genotypes in T2D patients compared to ND patients but did not show any significant risk (p>0.05) of T2D for the unadjusted codominant, dominant and recessive models. Following age adjustment, the mutant genotypes (CT and TT) showed significant (p<0.05) risk of T2D for all the models with the recessive model presenting the greatest risk of T2D (OR: 2.39, 95% CI: 1.16-4.91, p<0.018). The TT genotype significantly (p<0.05) associated with high level of HDL and reduced levels of TC, TG and LDL in non-diabetic patients but was not associated with any of the demographic and clinical characteristics among T2D patients.

CONCLUSIONS

ABCC8 C49620T polymorphism showed possible association with T2D marked by predominance of the mutant TT genotype in T2D patients. However, the relationship between TT genotype and lipid abnormalities for possible beneficial effect on people suffering from T2D is unclear.

Neo High-Density Lipoprotein Produced by the Streptococcal Serum Opacity Factor Activity against Human High-Density Lipoproteins Is Hepatically Removed via Dual Mechanisms

Injection of streptococcal serum opacity factor (SOF) into mice reduces the plasma cholesterol level by ∼40%. In vitro, SOF converts high-density lipoproteins (HDLs) into multiple products, including a small HDL, neo HDL. In vitro, neo HDL accounts for ∼60% of the protein mass of the SOF reaction products; in vivo, the accumulated mass of neo HDL is <1% of that observed in vitro. To identify the underlying cause of this difference, we determined the fate of neo HDL in plasma in vitro and in vivo. Following incubation with HDL, neo HDL-PC rapidly transfers to HDL, giving a small remnant, which fuses with HDL. An increased level of SR-B1 expression in Huh7 hepatoma cells and a reduced level of LDLR expression in CHO cells had little effect on neo HDL-[³H]CE uptake. Thus, the dominant receptors for neo HDL uptake are not LDLR or SR-B1. The in vivo metabolic fates of neo HDL-[³H]CE and HDL-[³H]CE were different. Thirty minutes after the injection of neo HDL-[³H]CE and HDL-[³H]CE into mice, plasma [³H]CE counts were 40 and 53%, respectively, of injected counts, with 10 times more [³H]CE appearing in the livers of neo HDL-[³H]CE-injected than in those of HDL-[³H]CE-injected mice. These data support a model of neo HDL-[³H]CE clearance by two parallel pathways. At early post-neo HDL-[³H]CE injection times, some neo HDL is directly removed by the liver; the remainder transfers its PC to HDL, leaving a remnant that fuses with HDL, which is also hepatically removed more slowly. Given that SR-B1 and SOF both remove CE from HDL, this novel mechanism may also underlie the metabolism of remnants released by hepatocytes following selective SR-B1-mediated uptake of HDL-CE.

Impact of gastrectomy on high-density lipoprotein cholesterol elevation in nonobese patients during a 10-year follow-up

AIM

The role of gastrectomy in glycemic control has been established in the current era of bariatric surgery for obesity. Gastrectomy in obese patients is associated with increased levels of high-density lipoprotein cholesterol (HDL-C). However, limited data on the effects of gastrectomy in nonobese patients are available. We herein investigated the long-term plasma lipid changes in nonobese patients who had undergone gastrectomy.

METHODS

Patients were enrolled as part of routine healthcare examinations from 1984 to 2003. Preoperative and postoperative data from patients who had undergone curative gastrectomy were analyzed for up to 10 years postoperatively. Three age- and sex-matched controls were assigned to each case.

RESULTS

Sixty-four nonobese patients without diabetes mellitus or a history of having taken lipid-lowering drugs who underwent curative gastrectomy during the study period were enrolled (60 subtotal gastrectomies, four total gastrectomies). The median follow-up period was 7.6 years. The mean body mass index was 9.6% lower one year after gastrectomy (p < 0.01), then plateaued with a slight recovery. Intriguingly, the preoperative HDL-C level was 21% higher one year after gastrectomy (p < 0.01), increased by another 30% six years after gastrectomy and remained at this level for the rest of the follow-up period. No significant changes in the HDL-C level were observed in the controls. The degree of HDL-C elevation was consistently significant, irrespective of the baseline triglyceride level, HDL-C level or body weight.

CONCLUSIONS

Gastrectomy in nonobese patients was associated with consistent and distinct long-term HDL-C elevations and body mass index reductions.

Down-regulation of lipoprotein lipase increases ABCA1-mediated cholesterol efflux in THP-1 macrophages

The ATP-binding cassette transporter A1 (ABCA1) mediates the efflux of excess cholesterol from foam cells to lipid-poor apolipoprotein A-I, in a process called reverse cholesterol transport. Lipoprotein lipase (LPL) is a lipolytic enzyme expressed by macrophages within atherosclerotic lesions. Lentivirus-mediated RNA interference was used to genetically knock-down (KD) the expression of LPL in THP-1 macrophages. Silencing of the LPL gene was confirmed by end-point PCR, real time PCR, and protein analysis. Suppression of LPL expression correlated with a 1.6-fold up-regulation of ABCA1 mRNA levels, and resulted in a 4.5-fold increase in ABCA1-dependent cholesterol efflux. Replenishing LPL by addition of purified bovine LPL to the cell culture media resulted in down-regulation of ABCA1-mediated cholesterol efflux in both wild-type and LPL knockdown cells. These findings suggest an inverse correlation between macrophage LPL levels and ABCA1 cholesterol transport activity.

Effects of cigarette smoking on HDL quantity and function: implications for atherosclerosis

Cigarette smoking has been identified as an independent and preventable risk factor for atherosclerosis and cardiovascular disease. Population studies have shown that plasma high density lipoprotein (HDL) cholesterol levels are inversely related to the risk of developing cardiovascular disease. Cigarette smoking is associated with reduced HDL cholesterol levels. Cigarette smoking can alter the critical enzymes of lipid transport, lowering lecithin: cholesterol acyltransferase (LCAT) activity and altering cholesterol ester transfer protein (CETP) and hepatic lipase activity, which attributes to its impact on HDL metabolism and HDL subfractions distribution. In addition, HDL is susceptible to oxidative modifications by cigarette smoking, which makes HDL become dysfunctional and lose its atheroprotective properties in smokers. Therefore, cigarette smoking has a negative impact on both HDL quantity and function, which can explain, in part, the increased risk of cardiovascular disease in smokers.

Role of SN1 lipases on plasma lipids in metabolic syndrome and obesity

OBJECTIVE

To assess the phospholipase activity of endothelial (EL) and hepatic lipase (HL) in postheparin plasma of subjects with metabolic syndrome (MS)/obesity and their relationship with atherogenic and antiatherogenic lipoproteins. Additionally, to evaluate lipoprotein lipase (LPL) and HL activity as triglyceride (TG)-hydrolyses to complete the analyses of SN1 lipolytic enzymes in the same patient.

APPROACH AND RESULTS

Plasma EL, HL, and LPL activities were evaluated in 59 patients with MS and 36 controls. A trend toward higher EL activity was observed in MS. EL activity was increased in obese compared with normal weight group (P=0.009) and was negatively associated with high-density lipoprotein-cholesterol (P=0.014 and P=0.005) and apolipoprotein A-I (P=0.045 and P=0.001) in control and MS group, respectively. HL activity, as TG-hydrolase, was increased in MS (P=0.025) as well as in obese group (P=0.017); directly correlated with low-density lipoprotein-cholesterol (P=0.005) and apolipoprotein B (P=0.003) and negatively with high-density lipoprotein-cholesterol (P=0.021) in control group. LPL was decreased in MS (P<0.001) as well as in overweight and obese compared with normal weight group (P=0.015 and P=0.004, respectively); inversely correlated %TG-very low-density lipoproteins (P=0.04) and TG/apolipoprotein B index (P=0.013) in control group. These associations were not found in MS.

CONCLUSIONS

We describe for the first time EL and HL activity as phospholipases in MS/obesity, being both responsible for high-density lipoprotein catabolism. Our results elucidate part of the remaining controversies about SN1 lipases activity in MS and different grades of obesity. The impact of insulin resistance on the activity of the 3 enzymes determines the lipoprotein alterations observed in these states.

Endothelial Lipase Activity Predicts High-Density Lipoprotein Catabolism in Hemodialysis

Objective—

A novel phospholipase assay was used to measure for the first time the behavior of endothelial and hepatic phospholipase activities in postheparin human plasma of hemodialyzed patients and its relationship with atherogenic and antiatherogenic lipoprotein levels.

Methods and Results—

Endothelial and hepatic phospholipase activity was assessed in a total SN1-specific phospholipase assay, using (1-decanoylthio-1-deoxy-2-decanoyl-sn-glycero-3-phosphoryl) ethylene glycol as the substrate. Hemodialyzed patients presented lower values of total and hepatic phospholipase activity than controls: 4.4 (1.9–9.0) versus 7.5 (3.6–18.0) and 2.6 (0.7–6.2) versus 6.6 (1.3–15.2) μmol of fatty acid released per milliliter of postheparin plasma per hour, respectively ( P <0.001); however, endothelial lipase (EL) phospholipase activity was increased in patients: 1.7 (0.8–3.0) versus 1.1 (0.1–2.7) μmol of fatty acid released per milliliter of postheparin plasma per hour ( P =0.008). EL was negatively associated with high-density lipoprotein (HDL)-cholesterol ( r =–0.427; P =0.001), and apolipoprotein A-I levels, total phospholipase, and hepatic lipase activity were directly associated with low-density lipoprotein-cholesterol and apolipoprotein B. The association of EL and HDL-cholesterol remained significant when adjusting for waist circumference (β=–0.26; P =0.05), and the effect of hepatic lipase on low-density lipoprotein-cholesterol continued after adjusting for age (β=0.46; P = 0.001).

Conclusion—

Our results support the hypothesis that EL is the predominant enzyme responsible for lipolytic catabolism of HDLs in hemodialyzed patients and resolve the apparent paradox observed between low hepatic lipase activity and decreased HDL-cholesterol levels observed in these patients. In addition, the ability to assess total hepatic lipase and EL phospholipase activity in plasma will increase our knowledge of the mechanisms involved in controlling HDL levels and cardiovascular risk in hemodialyzed patients, as well as other populations with low levels of HDL-cholesterol.

A review of the role of apolipoprotein C-II in lipoprotein metabolism and cardiovascular disease

The focus of this review is on the role of apolipoprotein C-II (apoC-II) in lipoprotein metabolism and the potential effects on the risk of cardiovascular disease (CVD). We searched PubMed/Scopus for articles regarding apoC-II and its role in lipoprotein metabolism and the risk of CVD. Apolipoprotein C-II is a constituent of chylomicrons, very low-density lipoprotein, low-density lipoprotein, and high-density lipoprotein (HDL). Apolipoprotein C-II contains 3 amphipathic α-helices. The lipid-binding domain of apoC-II is located in the N-terminal, whereas the C-terminal helix of apoC-II is responsible for the interaction with lipoprotein lipase (LPL). At intermediate concentrations (approximately 4 mg/dL) and in normolipidemic subjects, apoC-II activates LPL. In contrast, both an excess and a deficiency of apoC-II are associated with reduced LPL activity and hypertriglyceridemia. Furthermore, excess apoC-II has been associated with increased triglyceride-rich particles and alterations in HDL particle distribution, factors that may increase the risk of CVD. However, there is not enough current evidence to clarify whether increased apoC-II causes hypertriglyceridemia or is an epiphenomenon reflecting hypertriglyceridemia. A number of pharmaceutical interventions, including statins, fibrates, ezetimibe, nicotinic acid, and orlistat, have been shown to reduce the increased apoC-II concentrations. An excess of apoC-II is associated with increased triglyceride-rich particles and alterations in HDL particle distribution. However, prospective trials are needed to assess if apoC-II is a CVD marker or a risk factor in high-risk patients.

Removal from the plasma of the free and esterified forms of cholesterol and transfer of lipids to HDL in type 2 diabetes mellitus patients

BACKGROUND

The aim was to investigate new markers for type 2 diabetes (T2DM) dyslipidemia related with LDL and HDL metabolism. Removal from plasma of free and esterified cholesterol transported in LDL and the transfer of lipids to HDL are important aspects of the lipoprotein intravascular metabolism. The plasma kinetics (fractional clearance rate, FCR) and transfers of lipids to HDL were explored in T2DM patients and controls, using as tool a nanoemulsion that mimics LDL lipid structure (LDE).

RESULTS

14C- cholesteryl ester FCR of the nanoemulsion was greater in T2DM than in controls (0.07 ± 0.02 vs. 0.05 ± 0.01 h-1, p = 0.02) indicating that LDE was removed faster, but FCR 3 H- cholesterol was equal in both groups. Esterification rates of LDE free-cholesterol were equal. Cholesteryl ester and triglyceride transfer from LDE to HDL was greater in T2DM (4.2 ± 0.8 vs. 3.5 ± 0.7%, p = 0.03 and 6.8 ± 1.6% vs. 5.0 ± 1.1, p = 0.03, respectively). Phospholipid and free cholesterol transfers were not different.

CONCLUSIONS

The kinetics of free and esterified cholesterol tended to be independent in T2DM patients and the lipid transfers to HDL were also disturbed. These novel findings may be related with pathophysiological mechanisms of diabetic macrovascular disease.

Ajuga iva aqueous extract improves reverse cholesterol transport in streptozotocin-induced diabetic rat

Objectives

The aim of this study was to determine the effects of Ajuga iva aqueous extract on lecithin : cholesterol acyltransferase (LCAT) activity and amount and composition of high-density lipoprotein (HDL)2 and (HDL)3, in streptozotocin (STZ)-induced diabetic rats.

Methods

Diabetes was induced in male Wistar rats by intraperitoneal injection of STZ (60 mg/kg body weight). Diabetic rats (n = 12) were divided into two groups. The diabetic control group (D) received a 20% casein diet and the diabetic treated group received the same diet supplemented with A. iva aqueous extract (0.5 g/100 g diet) (DAi), for 4 weeks.

Key findings

Total cholesterol and HDL3-C were respectively decreased by 32% and 55% in the DAi group compared with the D group, whereas HDL2-C was increased by 30%. The amounts of HDL2 and HDL3, which were the sum of apolipoproteins, unesterified cholesterol (UC), cholesteryl esters (CEs), triacylglycerols (TGs) and phospholipids (PLs), showed no significant difference. A. iva treatment increased LCAT by 33% and its cofactor-activator, apolipoprotein A-I, by 58%. HDL3-PL (enzyme substrate) and HDL3-UC (acyl group acceptor) were respectively decreased by 70% and 57%, whereas HDL2-CE (product of LCAT reaction) was enhanced by 30%.

Conclusions

In STZ-induced diabetic rats, A. iva improves reverse cholesterol transport by enhancing LCAT activity, leading to anti-atherogenic effects.

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.

Lipase maturation factor 1 is required for endothelial lipase activity

Lipase maturation factor 1 (Lmf1) is an endoplasmic reticulum (ER) membrane protein involved in the posttranslational folding and/or assembly of lipoprotein lipase (LPL) and hepatic lipase (HL) into active enzymes. Mutations in Lmf1 are associated with diminished LPL and HL activities ("combined lipase deficiency") and result in severe hypertriglyceridemia in mice as well as in human subjects. Here, we investigate whether endothelial lipase (EL) also requires Lmf1 to attain enzymatic activity. We demonstrate that cells harboring a (cld) loss-of-function mutation in the Lmf1 gene are unable to generate active EL, but they regain this capacity after reconstitution with the Lmf1 wild type. Furthermore, we show that cellular EL copurifies with Lmf1, indicating their physical interaction in the ER. Finally, we determined that post-heparin phospholipase activity in a patient with the LMF1(W464X) mutation is reduced by more than 95% compared with that in controls. Thus, our study indicates that EL is critically dependent on Lmf1 for its maturation in the ER and demonstrates that Lmf1 is a required factor for all three vascular lipases, LPL, HL, and EL.

Determination of lipoprotein lipase activity using a novel fluorescent lipase assay

A novel, real-time, homogeneous fluorogenic lipoprotein lipase (LPL) assay was developed using a commercially available substrate, the EnzChek lipase substrate, which is solubilized in Zwittergent. The triglyceride analog substrate does not fluoresce, owing to apposition of fluorescent and fluorescent quenching groups at the sn-1 and sn-2 positions, respectively, fluorescence becoming unquenched upon release of the sn-1 BODIPY FA derivative following hydrolysis. Increase in fluorescence intensity at 37°C was proportional to LPL concentration. The assay was more sensitive than a similar assay using 1,2-O-dilauryl-rac-glycero-3-glutaric acid-(6-methylresorufin ester) and was validated in biological samples, including determination of LPL-specific activity in postheparin mouse plasma. The simplicity and reproducibility of the assay make it ideal for in vitro, high-throughput screening for inhibitors and activators of LPL, thus expediting discovery of drugs of potential clinical value.

Biliary cholesterol secretion: More than a simple ABC

Biliary cholesterol secretion is a process important for 2 major disease complexes, atherosclerotic cardiovascular disease and cholesterol gallstone disease. With respect to cardiovascular disease, biliary cholesterol secretion is regarded as the final step for the elimination of cholesterol originating from cholesterol-laden macrophage foam cells in the vessel wall in a pathway named reverse cholesterol transport. On the other hand, cholesterol hypersecretion into the bile is considered the main pathophysiological determinant of cholesterol gallstone formation. This review summarizes current knowledge on the origins of cholesterol secreted into the bile as well as the relevant processes and transporters involved. Next to the established ATP-binding cassette (ABC) transporters mediating the biliary secretion of bile acids (ABCB11), phospholipids (ABCB4) and cholesterol (ABCG5/G8), special attention is given to emerging proteins that modulate or mediate biliary cholesterol secretion. In this regard, the potential impact of the phosphatidylserine flippase ATPase class I type 8B member 1, the Niemann Pick C1-like protein 1 that mediates cholesterol absorption and the high density lipoprotein cholesterol uptake receptor, scavenger receptor class B type I, is discussed.

Reverse cholesterol transport in type 2 diabetes mellitus

High-density lipoprotein (HDL) plays an important protective role against atherosclerosis, and the anti-atherogenic properties of HDL include the promotion of cellular cholesterol efflux and reverse cholesterol transport (RCT), as well as antioxidant, anti-inflammatory and anticoagulant effects. RCT is a complex pathway, which transports cholesterol from peripheral cells and tissues to the liver for its metabolism and biliary excretion. The major steps in the RCT pathway include the efflux of free cholesterol mediated by cholesterol transporters from cells to the main extracellular acceptor HDL, the conversion of free cholesterol to cholesteryl esters and the subsequent removal of cholesteryl ester in HDL by the liver. The efficiency of RCT is influenced by the mobilization of cellular lipids for efflux and the intravascular remodelling and kinetics of HDL metabolism. Despite the increased cardiovascular risk in people with type 2 diabetes, current knowledge on RCT in diabetes is limited. In this article, abnormalities in RCT in type 2 diabetes mellitus and therapeutic strategies targeting HDL and RCT will be reviewed.

Hepatic uptake and metabolism of phosphatidylcholine associated with high density lipoproteins

BACKGROUND

Phosphatidylcholine (PC) is the predominant phospholipid associated with high density lipoproteins (HDL). Although the hepatic uptake of cholesteryl esters from HDL is well characterized, much less is known about the fate of PC associated with HDL. Thus, we investigated the uptake and subsequent metabolism of HDL-PC in primary mouse hepatocytes.

METHODS AND RESULTS

The absence of scavenger receptor-BI resulted in a 30% decrease in cellular incorporation of [(3)H]PC whereas [(3)H]cholesteryl ether uptake was almost completely abolished. Although endocytosis is not involved in the uptake of cholesteryl esters from HDL, we demonstrate that HDL internalization accounts for 40% of HDL-PC uptake. Extracellular remodeling of HDL by secretory phospholipase A(2) significantly enhances HDL lipid uptake. HDL-PC taken up by hepatocytes is partially converted to triacylglycerols via PC-phospholipase C-mediated hydrolysis of PC and incorporation of diacylglycerol into triacylglycerol. The formation of triacylglycerol is independent of scavenger receptor-BI and occurs in extralysosomal compartments.

CONCLUSIONS AND GENERAL SIGNIFICANCE

These findings indicate that HDL-associated PC is incorporated into primary hepatocytes via a pathway that differs significantly from that of HDL-cholesteryl ester, and shows that HDL-PC is more than a framework molecule, as evidenced by its partial conversion to hepatic triacylglycerol.

Citicoline (CDP-choline): What role in the treatment of complications of infectious diseases

A dysregulated host immune response, as opposed to the intrinsic virulence of a microbial pathogen induces a large part of the pathology seen in infectious diseases. However, current therapies are designed to target the pathogen rather than the underlying pathogenic mechanisms responsible for the manifestation of the pathology. Recent studies have highlighted the role of endothelial cell alteration in the pathology induced in sepsis and cerebral malaria. The endothelial onslaught described, is similar to that seen during ischemia reperfusion in stroke. Protecting endothelial cell membranes during sepsis and cerebral malaria, using citicoline in the same way as in stroke, has thus emerged as a new strategy that needs to be evaluated urgently. Citicoline is a natural compound that is registered for use in ischemic stroke, head trauma and neurological disorders. It enters the phosphatidylcholine synthesis pathway as a rate-limiting step and is involved in the modulation of a large number of metabolic pathways and neurotransmitter levels, and also in the biosynthesis of phospholipids in neuronal membranes. This short review highlights the potential role of citicoline as part of adjunct therapy in the treatment of infectious diseases.

PPARs Mediate Lipid Signaling in Inflammation and Cancer

Lipid mediators can trigger physiological responses by activating nuclear hormone receptors, such as the peroxisome proliferator-activated receptors (PPARs). PPARs, in turn, control the expression of networks of genes encoding proteins involved in all aspects of lipid metabolism. In addition, PPARs are tumor growth modifiers, via the regulation of cancer cell apoptosis, proliferation, and differentiation, and through their action on the tumor cell environment, namely, angiogenesis, inflammation, and immune cell functions. Epidemiological studies have established that tumor progression may be exacerbated by chronic inflammation. Here, we describe the production of the lipids that act as activators of PPARs, and we review the roles of these receptors in inflammation and cancer. Finally, we consider emerging strategies for therapeutic intervention.

HbA1c negatively correlates with LCAT activity in type 2 diabetes

AIMS

Abnormal high-density lipoproteins (HDL) metabolism is a major cardiovascular risk factor in type 2 diabetes mellitus (DM2). Lecithin:cholesterol acyltransferase (LCAT) increases HDL size by transferring 2-acyl groups from lecithin or phosphatidylethanolamine to unesterified cholesterol. The purpose of this study was to determine the independent correlates of LCAT activity in DM2 patients.

METHODS

A total of 45 (male: 20) consecutive adult DM2 patients aging 50.0+/-7.0 years (range: 40-64 years) with a median diabetes duration of 4 years (range: 2-18) were studied. Exclusion criteria were: smoking, positive history of cardiovascular, thyroid, renal or liver disease, pregnancy, treatment with metformin, insulin, lipid lowering drugs, angiotensin-converting enzyme inhibitors, aspirin or antioxidant supplements. Univariate and multivariate analyses were performed.

RESULTS

From a comprehensive list of variables studied, only HbA1c (rho=-0.951) and oxidized LDL (rho=-0.779) had statistically significant correlation with LCAT activity (p<0.001). These two variables were themselves strongly correlated to each other (rho=0.809, p<0.001). To eliminate potential confounding effects, we performed multivariate analysis, where HbA1c emerged as a strong independent predictor of LCAT activity (adjusted OR=-0.928, p<0.001).

CONCLUSIONS

Glycemia-induced glycation of HDL decreases LCAT activity. The fact that HbA1c is an accurate measure of glycation and can therefore reflect glycated HDL levels explains the association found in the present study. In conclusion, HbA1c provides an easy-to-assess, accurate measure of LCAT activity in DM2.

Increasing apoA-I production as a target for CHD risk reduction

Dyslipidemia leading to coronary heart diseases (CHD) enables venues to prevent or treat CHD by other strategies than only lowering serum LDL cholesterol (LDL-C) concentrations, which is currently the most frequently targeted change. Unlike LDL-C, elevated high-density lipoprotein cholesterol (HDL-C) concentrations may protect against the development of CHD as demonstrated in numerous large-scale epidemiological studies. In this review we describe that besides elevating serum HDL-C concentrations by increasing alpha-HDL particles, approaches to elevate HDL-C concentrations by increasing pre-beta HDL particle concentrations seems more attractive. Besides infusion of apoA-I(Milano), using apoA-I mimetics, or delipidation of alpha-HDL particles, elevating de novo apoA-I production may be a suitable target to functionally increase pre-beta HDL particle concentrations. Therefore, a detailed description of the molecular pathways underlying apoA-I synthesis and secretion, completed with an overview of known effects of pharmacological and nutritional compounds on apoA-I synthesis will be presented. This knowledge may ultimately be applied in developing dietary intervention strategies to elevate apoA-I production and serum HDL-C concentrations and consequently lower CHD risk.

Inactivity, exercise training and detraining, and plasma lipoproteins. STRRIDE: a randomized, controlled study of exercise intensity and amount

Exercise has beneficial effects on lipoproteins. Little is known about how long the effects persist with detraining or whether the duration of benefit is effected by training intensity or amount. Sedentary, overweight subjects ( n = 240) were randomized to 6-mo control or one of three exercise groups: 1) high-amount/vigorous-intensity exercise; 2) low-amount/vigorous-intensity exercise; or 3) low-amount/moderate-intensity exercise. Training consisted of a gradual increase in amount of exercise followed by 6 mo of exercise at the prescribed level. Exercise included treadmill, elliptical trainer, and stationary bicycle. The number of minutes necessary to expend the prescribed kilocalories per week (14 kcal·kg body wt−1·wk−1for both low-amount groups; 23 kcal·kg body wt−1·wk−1for high-amount group) was calculated for each subject. Average adherence was 83–92% for the three groups; minutes per week were 207, 125, and 203 and sessions per week were 3.6, 2.9, and 3.5 for high-amount/vigorous-intensity, low-amount/vigorous intensity, and low-amount/moderate-intensity groups, respectively. Plasma was obtained at baseline, 24 h, 5 days, and 15 days after exercise cessation. Continued inactivity resulted in significant increases in low-density lipoprotein (LDL) particle number, small dense LDL, and LDL-cholesterol. A modest amount of exercise training prevented this deterioration. Moderate-intensity but not vigorous-intensity exercise resulted in a sustained reduction in very-low-density lipoprotein (VLDL)-triglycerides over 15 days of detraining ( P < 0.05). The high-amount group had significant improvements in high-density lipoprotein (HDL)-cholesterol, HDL particle size, and large HDL levels that were sustained for 15 days after exercise stopped. In conclusion, physical inactivity has profound negative effects on lipoprotein metabolism. Modest exercise prevented this. Moderate-intensity but not vigorous-intensity exercise resulted in sustained VLDL-triglyceride lowering. Thirty minutes per day of vigorous exercise, like jogging, has sustained beneficial effects on HDL metabolism.

PPARs and their metabolic modulation: new mechanisms for transcriptional regulation?

Peroxisome proliferator-activated receptors (PPARs) as ligand-activated nuclear receptors involved in the transcriptional regulation of lipid metabolism, energy balance, inflammation, and atherosclerosis are at the intersection of key pathways involved in the pathogenesis of diabetes and cardiovascular disease. Synthetic PPAR agonists like fibrates (PPAR-alpha) and thiazolidinediones (PPAR-gamma) are in therapeutic use to treat dyslipidaemia and diabetes. Despite strong encouraging in vitro, animal model, and human surrogate marker studies with these agents, recent prospective clinical cardiovascular trials have yielded mixed results, perhaps explained by concomitant drug use, study design, or a lack of efficacy of these agents on cardiovascular disease (independent of their current metabolic indications). The use of PPAR agents has also been limited by untoward effects. An alternative strategy to PPAR therapeutics is better understanding PPAR biology, the nature of natural PPAR agonists, and how these molecules are generated. Such insight might also provide valuable information about pathways that protect against the metabolic problems for which PPAR agents are currently indicated. This approach underscores the important distinction between the effects of synthetic PPAR agonists and the unequivocal biologic role of PPARs as key transcriptional regulators of metabolic and inflammatory pathways relevant to diabetes and atherosclerosis.

Disorders in high-density metabolism with insulin resistance and chronic kidney disease

Cardiovascular risk increases with each decrement in renal function. Low-density lipoprotein (LDL) cholesterol levels are not associated with increased mortality, but high-density lipoprotein (HDL) levels are inversely associated with cardiovascular risk. Lipoprotein composition with increased abundance of small dense LDL and HDL and reduced levels of more buoyant isoforms is similar to what is found in states of insulin resistance and in the metabolic syndrome (MS). In both cases, high triglyceride levels are associated with reduced HDL levels. Chronic kidney disease (CKD) is itself associated with increasing insulin resistance as renal function fails. In both instances, decreased levels of apo A-I and apo A-II are a consequence of increased fractional catabolic rate (FCR), resulting from a predominance of small HDL particles. HDL maturation is impaired in CKD through decreased activity of lecithin:cholesterol acyltransferase (LCAT), and increased cholesterol ester transfer protein (CETP) activity in MS shuttles triglycerides back into HDL, thereby destabilizing it. Whether insulin resistance is entirely responsible for disorders of HDL metabolism in CKD, or whether the process is a result of unrelated pathophysiology, is currently unknown.

Determinants of plasma HDL concentrations and reverse cholesterol transport

PURPOSE OF REVIEW

One of the major mechanisms whereby HDL particles are felt to protect against atherosclerosis is that of reverse cholesterol transport from atherosclerotic lesion macrophages to the liver, with subsequent excretion of cholesterol in the bile. This review focuses on recent progress in our understanding of reverse cholesterol transport and the factors that determine plasma HDL cholesterol concentrations.

RECENT FINDINGS

The liver and intestine are the major sites of apolipoprotein A-I synthesis and nascent HDL particle secretion. The liver has recently been shown to be a major contributor to the plasma HDL-cholesterol concentration, but the precise site or mechanism whereby hepatically-synthesized HDL acquire the bulk of their lipid content remains to be determined. Contrastingly, macrophages contribute little to the plasma HDL cholesterol pool, whereas the quantitatively small macrophage-specific reverse cholesterol transport contributes disproportionately to protection against atherosclerosis. Studies have highlighted the coordinate action of cell surface lipid transporters, cholesterol esterification enzymes and lipid transfer factors in the early steps of reverse cholesterol transport and the recycling of pre-beta HDL particles to create a ready supply of cholesterol acceptor HDL particles. Most of the variation in plasma HDL-cholesterol levels in human populations is accounted for by variations in HDL clearance rather than production.

SUMMARY

Our understanding of the in-vivo metabolism of HDL particles and their role in reverse cholesterol transport is rapidly evolving, with long-standing concepts being constantly challenged by emerging evidence. An in-depth understanding of HDL metabolism will guide the rational design of novel pharmacological therapies that effectively protect against atherosclerosis.

The beneficial effects of high-density lipoprotein on adipocytes may relate to its anti-atherogenic properties

High-density lipoprotein (HDL) has significant anti-atherogenic properties, whereas the underlying mechanisms are complex and have not been completely elucidated. Adipocytes produce a variety of adipokines with cardiovascular effects. The dysregulated secretion of adipokines by adipocytes may contribute to the increased risk of atherosclerosis associated with obesity. Clinical evidences indicate that higher plasma HDL-C levels are associated with a favourable adipokines secretion profile, suggesting that HDL might improve the dysregulated adipokines secretion. HDL may diminish lipid accumulation in adipocytes through phosphorylation of PPARgamma and inhibition of aP2 expression, which possibly account for the favourable effects of HDL on adipokines secretion. Therefore, we hypothesize that HDL might exert several beneficial effects on adipocytes, which may relate to its anti-atherogenic properties.

Apolipoprotein A-V Deficiency Results in Marked Hypertriglyceridemia Attributable to Decreased Lipolysis of Triglyceride-Rich Lipoproteins and Removal of Their Remnants

OBJECTIVE

ApoAV, a newly discovered apoprotein, affects plasma triglyceride level. To determine how this occurs, we studied triglyceride-rich lipoprotein (TRL) metabolism in mice deficient in apoAV.

METHODS AND RESULTS

No significant difference in triglyceride production rate was found between apoa5(-/-) mice and controls. The presence or absence of apoAV affected TRL catabolism. After the injection of 14C-palmitate and 3H-cholesterol labeled chylomicrons and (125)I-labeled chylomicron remnants, the disappearance of 14C, 3H, and (125)I was significantly slower in apoa5(-/-) mice relative to controls. This was because of diminished lipolysis of TRL and the reduced rate of uptake of their remnants in apoa5(-/-) mice. Observed elevated cholesterol level was caused by increased high-density lipoprotein (HDL) cholesterol in apoa5(-/-) mice. VLDL from apoa5(-/-) mice were poor substrate for lipoprotein lipase, and did not bind to the low-density lipoprotein (LDL) receptor as well as normal very-low-density lipoprotein (VLDL). LDL receptor levels were slightly elevated in apoa5(-/-) mice consistent with lower remnant uptake rates. These alterations may be the result of the lower apoE-to-apoC ratio found in VLDL isolated from apoa5(-/-) mice.

CONCLUSIONS

These results support the hypothesis that the absence of apoAV slows lipolysis of TRL and the removal of their remnants by regulating their apoproteins content after secretion.

Higher order lipase gene association with plasma triglycerides

Lipoprotein lipase, HL, and endothelial lipase (EL) are proteoglycan-bound enzymes that regulate plasma lipoprotein levels through coordinated triglyceride (TG) lipase and phospholipase activity. We hypothesized that single nucleotide polymorphisms (SNPs) in lipase genes would have higher order impact on plasma lipoproteins beyond the influence of individual SNPs. In a sample of asymptomatic Caucasian subjects (n = 738), we used a two-stage approach, first identifying groups of subjects with similar multilocus lipase genotypes and then characterizing the relationships between genotype groups and plasma lipids. Using complementary methods, including a permutation test procedure and a mixed-effects modeling approach, we found a higher order interaction between four SNPs in three lipase genes (EL 2,237 3' untranslated region, EL Thr111Ile, HL -514C/T, and LPL HindIII) and plasma TG levels. Subjects who were heterozygous for all four lipase SNPs had significantly higher plasma TG levels beyond the effect of individual lipase SNPs and environmental factors, even after correcting for multiple comparisons. In conclusion, lipase genes had synergistic association with plasma TG beyond individual gene effects. Higher order multilocus genotype contributions to dyslipidemia and atherosclerotic cardiovascular disease need to be considered a priori because they may have an important effect even in the absence of significant main effects of the individual genes.

Association of endothelial lipase gene (LIPG) haplotypes with high-density lipoprotein cholesterol subfractions and apolipoprotein AI plasma levels in Japanese Americans

The LIPG gene on chromosome 18 encodes for endothelial lipase, a member of the triglyceride lipase family. Mouse models suggest that variation in LIPG influences high-density lipoprotein (HDL) metabolism, but only limited data are available in humans. This study examined associations of LIPG haplotypes, comprising a single nucleotide polymorphism (SNP) in the promoter region (-384A>C), and a nonsynonymous SNP in exon 3 (Thr111Ile or 584C>T), with lipoprotein risk factors in 541 adult Japanese Americans. A marginal association was found between LIPG diplotypes and HDL cholesterol (p=0.045). Stronger associations were seen for HDL3 cholesterol (p=0.005) and Apolipoprotein AI plasma levels (p=0.002). After adjustment for age, gender, smoking and medications, individuals homozygous for the minor allele at both SNPs (*4 haplotype) had a more favorable risk factor profile, compared to other haplotype combinations. No relationship was seen for plasma triglyceride levels or low-density lipoprotein (LDL) size, but the homozygous *4 diplotype was also associated with lower Apolipoprotein B and LDL cholesterol levels (p=0.001 and 0.015, respectively). In conclusion, this community-based family study of Japanese Americans demonstrates that LIPG variants are associated with HDL related risk factors, and may play a role in susceptibility to cardiovascular disease in this population.

New Insights Into the Regulation of HDL Metabolism and Reverse Cholesterol Transport

The metabolism of high-density lipoproteins (HDL), which are inversely related to risk of atherosclerotic cardiovascular disease, involves a complex interplay of factors regulating HDL synthesis, intravascular remodeling, and catabolism. The individual lipid and apolipoprotein components of HDL are mostly assembled after secretion, are frequently exchanged with or transferred to other lipoproteins, are actively remodeled within the plasma compartment, and are often cleared separately from one another. HDL is believed to play a key role in the process of reverse cholesterol transport (RCT), in which it promotes the efflux of excess cholesterol from peripheral tissues and returns it to the liver for biliary excretion. This review will emphasize 3 major evolving themes regarding HDL metabolism and RCT. The first theme is that HDL is a universal plasma acceptor lipoprotein for cholesterol efflux from not only peripheral tissues but also hepatocytes, which are a major source of cholesterol efflux to HDL. Furthermore, although efflux of cholesterol from macrophages represents only a tiny fraction of overall cellular cholesterol efflux, it is the most important with regard to atherosclerosis, suggesting that it be specifically termed macrophage RCT. The second theme is the critical role that intravascular remodeling of HDL by lipid transfer factors, lipases, cell surface receptors, and non-HDL lipoproteins play in determining the ultimate metabolic fate of HDL and plasma HDL-c concentrations. The third theme is the growing appreciation that insulin resistance underlies the majority of cases of low HDL-c in humans and the mechanisms by which insulin resistance influences HDL metabolism. Progress in our understanding of HDL metabolism and macrophage reverse cholesterol transport will increase the likelihood of developing novel therapies to raise plasma HDL concentrations and promote macrophage RCT and in proving that these new therapeutic interventions prevent or cause regression of atherosclerosis in humans.

Mixed modelling to characterize genotype-phenotype associations

We propose using mixed effects models to characterize the association between multiple gene polymorphisms, environmental factors and measures of disease progression. Characterizing high-order gene-gene and gene-environment interactions presents an analytic challenge due to the large number of candidate genes and the complex, undescribed interactions among them. Several approaches have been proposed recently to reduce the number of candidate genes and post hoc approaches to identify gene-gene interactions are described. However, these approaches may be inadequate for identifying high-order interactions in the absence of main effects and generally do not permit us to control for potential confounders. We describe how mixed effects models and related testing procedures overcome these limitations and apply this approach to data from a cohort of subjects at risk for cardiovascular disease. Four (4) genetic polymorphisms in three genes of the same gene family are considered. The proposed modelling approach allows us first to test whether there is a significant genetic contribution to the variability observed in our disease outcome. This contribution may be through main effects of multi-locus genotypes or through an interaction between genotype and environmental factors. This approach also enables us to identify specific multi-locus genotypes that interact with environmental factors in predicting the outcome. Mixed effects models provide a flexible statistical framework for controlling for potential confounders and identifying interactions among multiple genes and environmental factors that explain the variability in measures of disease progression.

Lipoproteins, macrophage function, and atherosclerosis: beyond the foam cell?

Atherogenesis requires and is highly influenced by the interaction between lipoproteins and macrophages. Most of the focus to date has been on the ability of atherogenic lipoproteins (such as low-density lipoproteins, LDL) to promote and of anti-atherogenic lipoproteins (such as high-density lipoproteins, HDL) to prevent the development of the cholesteryl ester-enriched macrophage-derived foam cell. However, lipoprotein-macrophage interactions have the potential to modulate macrophage function in a variety of additional ways that may impact on atherosclerosis. These include modulating cellular cholesterol and oxysterol content, providing fatty acids as ligands for PPARs, and acting as ligands for macrophage scavenger and Toll-like receptors. We suggest that atherogenic lipoproteins promote and anti-atherogenic lipoproteins inhibit atherogenesis by modulating macrophage function in a variety of ways beyond cholesteryl ester accumulation and foam cell formation.

A biphasic response of hepatobiliary cholesterol metabolism to dietary fat at the onset of obesity in the mouse

Human obesity is associated with abnormal hepatic cholesterol homeostasis and resistance to leptin action. Because leptin administration to rodents promotes the biliary elimination of plasma cholesterol, this study was designed to elucidate a pathophysiological role for leptin during the development of obesity. We fed mice diets containing high or low saturated fat contents. Before and after the onset of obesity, we measured downstream targets of leptin action and evaluated plasma, hepatic, and biliary cholesterol metabolism. Although not obese at 28 days, mice fed a high fat diet became hyperleptinemic. Sensitivity to leptin was evidenced by downregulation of both hepatic stearoyl CoA desaturase-1 and fatty acid synthase. Due principally to upregulation of adenosine triphosphate-binding cassette proteins A1 and G5, plasma high density lipoprotein (HDL) cholesterol concentrations increased, as did relative secretion rates of biliary cholesterol. A smaller, more hydrophilic bile salt pool decreased intestinal cholesterol absorption. In this setting, hepatic cholesterol synthesis was downregulated, indicative of increased uptake of plasma cholesterol. After 56 days of high fat feeding, obesity was associated with leptin resistance, as evidenced by marked hyperleptinemia without downregulation of stearoyl CoA desaturase-1 or fatty acid synthase and by upregulation of hepatic cholesterol and bile salt synthesis. Hypercholesterolemia was attributable to overproduction and decreased clearance of large HDL(1) particles. In conclusion, before the onset of obesity, preserved leptin sensitivity promotes biliary elimination of endogenous cholesterol in response to dietary fat. Leptin resistance due to obesity leads to a maladaptive response whereby newly synthesized cholesterol in the liver is eliminated via bile.

Endothelial Lipase Modulates Susceptibility to Atherosclerosis in Apolipoprotein-E-deficient Mice

Endothelial lipase (EL) expression correlates inversely with circulating high density lipoprotein (HDL) cholesterol levels in genetic mouse models, and human genetic variation in this locus has been linked to differences in HDL cholesterol levels. These data suggest a role for EL in the development of atherosclerotic vascular disease. To investigate this possibility, LIPG-null alleles were bred onto the apoE knockout background, and the homozygous double knockout animals were characterized. Both apoE knockout and double knockout mice had low HDL cholesterol levels when compared with wild-type mice, but the HDL cholesterol levels of the double knockout mice were higher than those of apoE knockout mice. Atherogenic very low density lipoprotein and intermediate density lipoprotein/low density lipoprotein cholesterol levels of the double knockout mice were also greater than those of the apoE knockout animals. Despite this lipid profile, there was a significant approximately 70% decrease in atherosclerotic disease area in double knockout mice on a regular diet. Immunohistochemistry and protein blot studies revealed increased EL expression in the atherosclerotic aortas of the apoE knockout animals. An observed decrease in macrophage content in vessels lacking EL correlated with ex vivo vascular monocyte adhesion assays, suggesting that this protein can modulate monocyte adhesion and infiltration into diseased tissues. These data suggest that EL may have indirect atherogenic actions in vivo through its effect on circulating HDL cholesterol and direct atherogenic actions through vascular wall processes such as monocyte recruitment and cholesterol uptake.