Influence of insulin sensitivity and the TaqIB cholesteryl ester transfer protein gene polymorphism on plasma lecithin:cholesterol acyltransferase and lipid transfer protein activities and their response to hyperinsulinemia in non-diabetic men

Dyslipidemia in Type 1 Diabetes: AMaskedDanger

Type 1 diabetes (T1D) patients show lipid disorders which are likely to play a role in their increased cardiovascular (CV) disease risk. Quantitative abnormalities of lipoproteins are noted in T1D with poor glycemic control. In T1D with optimal glycemic control, triglycerides and LDL-cholesterol are normal or slightly decreased whereas HDL-cholesterol is normal or slightly increased. T1D patients, even with good glycemic control, show several qualitative and functional abnormalities of lipoproteins that are potentially atherogenic. An association between these abnormalities and CV disease risk has been reported in recent studies. Although the mechanisms underlying T1D dyslipidemia remain unclear, the subcutaneous route of insulin administration, that is responsible for peripheral hyperinsulinemia, is likely to be an important factor.

Impact of Phospholipid Transfer Protein in Lipid Metabolism and Cardiovascular Diseases

PLTP plays an important role in lipoprotein metabolism and cardiovascular disease development in humans; however, the mechanisms are still not completely understood. In mouse models, PLTP deficiency reduces cardiovascular disease, while its overexpression induces it. Therefore, we used mouse models to investigate the involved mechanisms. In this chapter, the recent main progresses in the field of PLTP research are summarized, and our focus is on the relationship between PLTP and lipoprotein metabolism, as well as PLTP and cardiovascular diseases.

Antibodies Against the C-Terminus of ApoA-1 Are Inversely Associated with Cholesterol Efflux Capacity and HDL Metabolism in Subjects with and without Type 2 Diabetes Mellitus

Background: We determined relationships of cholesterol efflux capacity (CEC), plasma cholesterol esterification (EST) and cholesteryl ester transfer (CET) with anti-c-terminus apoA-1 (Ac-terAA1) and anti-apolipoprotein (apo)-1 (AAA1) autoantibodies in subjects with and without Type 2 diabetes mellitus (T2D). Methods: In 75 T2D subjects and 75 nondiabetic subjects, Ac-terAA1 and AAA1 plasma levels were measured by enzyme-linked immunosorbent assay. CEC was measured as [³H]-cholesterol efflux from human cultured fibroblasts to diluted individual subject plasma. Plasma EST and CET were assayed by isotope methods. Results: Ac-terAA1 and AAA1 levels and were similar between T2D and control subjects. Univariate regression analysis (n = 150) demonstrated that Ac-terAA1 levels were inversely correlated with CEC, EST, CET, total cholesterol, non-HDL cholesterol, triglycerides and apolipoprotein B, (p < 0.05 to p < 0.01), but not with glucose and HbA1c. In separate multivariable linear regression models, CEC, EST and CET were inversely associated with Ac-terAA1 levels independently of age, sex, T2D and drug use (β = −0.186, p = 0.026; β = −0.261, p < 0.001; and β = −0.321, p < 0.001; respectively). These associations were lost after additional adjustment for non-HDL cholesterol and triglycerides. No associations were observed for AAA1. Conclusions: CEC, plasma EST and CET are inversely associated with Ac-terAA1 autoantibodies, conceivably attributable to an inverse relationship of these autoantibodies with apolipoprotein B-containing lipoproteins.

Plasma lecithin:cholesterol acyltransferase and phospholipid transfer protein activity independently associate with nonalcoholic fatty liver disease

BACKGROUND

Nonalcoholic fatty liver disease (NAFLD) is a highly prevalent condition which contributes to atherogenic apolipoprotein B dyslipoproteinemias. Lecithin:cholesterol acyltransferase (LCAT) and phospholipid transfer protein (PLTP) are both synthesized by the liver and are important in lipid metabolism. Here, we interrogated the impact of NAFLD on plasma LCAT and PLTP activities.

METHODS

Plasma LCAT activity (exogenous substrate assay) and PLTP activity (phospholipid vesicles-HDL assay) were determined in 348 subjects (279 men; 81 subjects with type 2 diabetes (T2DM); 123 with metabolic syndrome (MetS)). A Fatty Liver Index (FLI) ≥60 was used as a proxy of NAFLD. Insulin resistance was determined by homoeostasis model assessment (HOMA-IR).

RESULTS

A total of 147 participants had an FLI ≥60 coinciding with T2DM and MetS (P < 0.001 for each). Plasma LCAT activity and PLTP activity were on average 12% and 5% higher, respectively, in subjects with an FLI ≥ 60 (P < 0.001 for each). In age- and sex-adjusted partial linear regression analysis, LCAT activity and PLTP activity were positively related to various obesity measures and HOMA-IR (P < 0.001 for each). In multivariable linear regression analyses adjusted for age and sex, LCAT activity was associated with an FLI ≥ 60 independent of T2DM and MetS, the waist/hip ratio, or HOMA-IR (β = 0.307 to 0.366, P < 0001 for all models). PLTP activity was also associated with an FLI ≥ 60 independent of these variables (β = 0.151 to 0223, P = 0.013 to 0.001).

CONCLUSION

NAFLD, as inferred from an FLI≥60, confers higher plasma LCAT and to a lesser extent PLTP activity, even when taking account of T2DM, MetS, central obesity and insulin resistance.

Longitudinal association between egg consumption and the risk of cardiovascular disease: interaction with type 2 diabetes mellitus

Background/objectives

It remains unclear if high egg consumption has beneficial or adverse effects on cardiometabolic health. The present study prospectively evaluated the longitudinal association between egg-consumption levels and incident cardiovascular disease (CVD) among Korean adults.

Subjects/methods

We conducted a prospective cohort study of 9248 Korean adults aged 40–69 years without CVD or cancer at the baseline from the Korean Genome and Epidemiology Study, Ansung–Ansan cohort, South Korea. The egg intake of the participants was estimated using a validated semiquantitative food frequency questionnaire at the baseline and the second follow-up examination and categorized into quartiles. CVD cases were identified using biennial questionnaires and confirmed through repeated in-depth personal interviews. Hazard ratios (HR) and 95% confidence interval (CI) were analyzed using Cox proportional hazard regression.

Results

During the average follow-up of 7.3 years, 570 cases of CVD were newly diagnosed. After adjusting for multiple confounding variables, egg-intake levels were not associated with CVD incidence (HR: 1.14, 95% CI: 0.87–1.49, P for trend: 0.7). However, the association was modified by type 2 diabetes mellitus (T2DM) status. Egg consumption was significantly associated with an increased risk for incident CVD among participants with T2DM; individuals with the highest egg intake (4.2 ± 0.04 eggs/week) had a 2.8 times higher incidence of CVD (HR: 2.81, 95% CI: 1.25–6.30, P for trend: 0.02) than those with the lowest egg intake (0.1 ± 0.02 eggs/week). However, no association was observed among individuals without T2DM (HR: 1.03, 95% CI: 0.77–1.38, P for trend: 0.8).

Conclusions

Higher egg consumption may increase the risk for CVD in Korean patients with T2DM. Our findings provide a basis for the development of an optimal dietary cholesterol intake guideline for the Korean population.

Phospholipid transfer protein: its impact on lipoprotein homeostasis and atherosclerosis

Phospholipid transfer protein (PLTP) is one of the major modulators of lipoprotein metabolism and atherosclerosis development in humans; however, we still do not quite understand the mechanisms. In mouse models, PLTP overexpression induces atherosclerosis, while its deficiency reduces it. Thus, mouse models were used to explore the mechanisms. In this review, I summarize the major progress made in the PLTP research field and emphasize its impact on lipoprotein metabolism and atherosclerosis, as well as its regulation.

Pathophysiology of diabetic dyslipidaemia: where are we?

Cardiovascular disease is a major cause of morbidity and mortality in patients with type 2 diabetes mellitus, with a two- to fourfold increase in cardiovascular disease risk compared with non-diabetic individuals. Abnormalities in lipid metabolism that are observed in the context of type 2 diabetes are among the major factors contributing to an increased cardiovascular risk. Diabetic dyslipidaemia includes not only quantitative lipoprotein abnormalities, but also qualitative and kinetic abnormalities that, together, result in a shift towards a more atherogenic lipid profile. The primary quantitative lipoprotein abnormalities are increased triacylglycerol (triglyceride) levels and decreased HDL-cholesterol levels. Qualitative lipoprotein abnormalities include an increase in large, very low-density lipoprotein subfraction 1 (VLDL1) and small, dense LDLs, as well as increased triacylglycerol content of LDL and HDL, glycation of apolipoproteins and increased susceptibility of LDL to oxidation. The main kinetic abnormalities are increased VLDL1 production, decreased VLDL catabolism and increased HDL catabolism. In addition, even though LDL-cholesterol levels are typically normal in patients with type 2 diabetes, LDL particles show reduced turnover, which is potentially atherogenic. Although the pathophysiology of diabetic dyslipidaemia is not fully understood, the insulin resistance and relative insulin deficiency observed in patients with type 2 diabetes are likely to contribute to these lipid changes, as insulin plays an important role in regulating lipid metabolism. In addition, some adipocytokines, such as adiponectin or retinol-binding protein 4, may also contribute to the development of dyslipidaemia in patients with type 2 diabetes.

Grape polyphenols increase the activity of HDL enzymes in old and obese rats

HDL particles are protein-rich particles that act as a vehicle for reverse cholesterol transport from tissues to the liver. The purpose of this study was to investigate age-dependent changes in the functional activity of HDL and the effect of high-energy diet on this index, as well as to correct it under the influence of grape polyphenols from “Enoant” obtained from Vitis vinifera grapes. We observed the age-dependent composition changes in HDL particle. It was shown that total lipids and triacylglycerol (TG) levels were higher in 24-month-old animals. In obese rats, HDL total lipids and TG levels were higher in 24-month-old than in the 3-month-old and 12-month-old groups but did not differ from 24-month-old group. The plasma HDL paraoxonase (PON) and lecithin:cholesterol acyltransferase (LCAT) activity levels were decreased in old-aged rats, and cholesteryl ester transfer protein (CETP) activity was higher in old rats. Keeping 12-month-old animals on high-fructose diet completely leveled the age differences in the data that have been measured between 12-month-old and 24-month-old rats. After “Enoant” administration, an increase of HDL PON and LCAT activity levels and a reduction of CETP activity were found in 24-month-old and obese rats.

Lecithin:cholesterol acyltransferase: old friend or foe in atherosclerosis?

Lecithin:cholesterol acyltransferase (LCAT) is a key enzyme that catalyzes the esterification of free cholesterol in plasma lipoproteins and plays a critical role in high-density lipoprotein (HDL) metabolism. Deficiency leads to accumulation of nascent preβ-HDL due to impaired maturation of HDL particles, whereas enhanced expression is associated with the formation of large, apoE-rich HDL(1) particles. In addition to its function in HDL metabolism, LCAT was believed to be an important driving force behind macrophage reverse cholesterol transport (RCT) and, therefore, has been a subject of great interest in cardiovascular research since its discovery in 1962. Although half a century has passed, the importance of LCAT for atheroprotection is still under intense debate. This review provides a comprehensive overview of the insights that have been gained in the past 50 years on the biochemistry of LCAT, the role of LCAT in lipoprotein metabolism and the pathogenesis of atherosclerosis in animal models, and its impact on cardiovascular disease in humans.

Cholesterol efflux from macrophages is influenced differentially by plasmas from overweight insulin-sensitive and -resistant subjects

OBJECTIVE

In vitro measurements of cholesterol efflux from macrophages have recently been shown to associate with cardiovascular risk. We investigated whether cholesterol efflux from macrophages incubated with plasmas from overweight/obese subjects with metabolic syndrome was influenced by the presence of insulin resistance.

METHODS

Plasmas were obtained from 47 men and women with metabolic syndrome, of whom 25 were found to be insulin resistant (IR) and 22 insulin sensitive (IS) (Matsuda, De Fronzo equation based on oral glucose tolerance test). Activated human macrophage THP-1 cells in which cholesterol had been radiolabelled were incubated with the subjects' plasmas to allow calculation of % cholesterol efflux.

RESULTS

Body mass index and waist measurements, as well as plasma lipid levels, did not differ between the two groups. Homeostatic model assessment-insulin resistance value as well as plasma insulin and leptin concentrations were higher in IR subjects. Cholesterol efflux was found to be significantly greater with plasmas from IR subjects (9.1%) than from IS subjects (6.7%) (P=0.005). Further, cholesterol efflux was significantly inversely associated with insulin sensitivity index (P<0.001), directly with arterial insulin concentration (P<0.001) and directly with cholesteryl ester transfer protein (CETP) mass (P=0.044).

CONCLUSION

Plasmas from overweight subjects with insulin resistance induced greater in vitro cholesterol efflux compared with IS subjects. Efflux inversely correlated with insulin sensitivity suggesting an increase in reverse cholesterol transport in the IR state that may lead to greater transfer of cholesterol to apoB lipoproteins from high-density lipoproteins via CETP as a factor in the association between IR and atherosclerosis.

Linkage and association of phospholipid transfer protein activity to LASS4

Phospholipid transfer protein activity (PLTPa) is associated with insulin levels and has been implicated in atherosclerotic disease in both mice and humans. Variation at the PLTP structural locus on chromosome 20 explains some, but not all, heritable variation in PLTPa. In order to detect quantitative trait loci (QTLs) elsewhere in the genome that affect PLTPa, we performed both oligogenic and single QTL linkage analysis on four large families (n = 227 with phenotype, n = 330 with genotype, n = 462 total), ascertained for familial combined hyperlipidemia. We detected evidence of linkage between PLTPa and chromosome 19p (lod = 3.2) for a single family and chromosome 2q (lod = 2.8) for all families. Inclusion of additional marker and exome sequence data in the analysis refined the linkage signal on chromosome 19 and implicated coding variation in LASS4, a gene regulated by leptin that is involved in ceramide synthesis. Association between PLTPa and LASS4 variation was replicated in the other three families (P = 0.02), adjusting for pedigree structure. To our knowledge, this is the first example for which exome data was used in families to identify a complex QTL that is not the structural locus.

Plasma lecithin: cholesterol acyltransferase activity is elevated in metabolic syndrome and is an independent marker of increased carotid artery intima media thickness

CONTEXT

Lecithin:cholesterol acyltransferase (LCAT), which esterifies free cholesterol to cholesteryl esters, is required for normal plasma lipoprotein structure and is instrumental in high density lipoprotein (HDL) remodeling, but the relationship of variation in plasma LCAT activity with subclinical atherosclerosis is unclear.

OBJECTIVES

The aim of the study was to determine the effect of the metabolic syndrome (MetS) on plasma LCAT activity and its relationship with carotid artery intima media thickness (IMT).

SETTING

The study was conducted at the vascular laboratory of a university medical center.

METHODS

In 74 subjects with MetS and 90 subjects without MetS (National Cholesterol Education Program Adult Treatment Panel III criteria), mean carotid artery IMT, plasma lipids, LCAT activity (exogenous substrate method), high-sensitive C-reactive protein, and homeostasis model assessment insulin resistance (HOMA(ir)) were documented.

RESULTS

IMT was greater (P = 0.01) and plasma LCAT activity was higher (P < 0.001) in subjects with MetS compared to subjects without MetS. Similar increases in IMT and LCAT were found in MetS subjects without type 2 diabetes mellitus. Multiple linear regression analysis demonstrated that plasma LCAT activity was independently and positively related to HOMA(ir), plasma triglycerides, non-HDL cholesterol, and HDL cholesterol (all P < 0.001). After adjustment for age and sex, IMT was positively associated with LCAT activity (P < 0.01), independently of the presence of MetS (or alternatively of plasma lipids), HOMA(ir), and high-sensitive C-reactive protein.

CONCLUSIONS

Plasma LCAT activity is elevated in MetS and may be a marker of subclinical atherosclerosis. Our findings do not support the contention that strategies to elevate LCAT are necessarily beneficial for cardioprotection.

Increased phospholipid transfer protein activity associated with the impaired cellular cholesterol efflux in type 2 diabetic subjects with coronary artery disease

Reverse cholesterol transport (RCT) is the pathway, by which the excess of cholesterol is removed from peripheral cells to the liver. An early step of RCT is the efflux of free cholesterol from cell membranes that is mediated by high-density lipoproteins (HDL). Phospholipid transfer protein (PLTP) transfers phospholipids between apolipoprotein-B-containing lipoproteins (i.e., chylomicrons and very low-density lipoproteins) and HDL. PLTP contributes to the HDL maturation and increases the ability of HDL to extract the cellular cholesterol. It is known that RCT is impaired in type 2 diabetic patients, especially when cardiovascular complication is associated with. In this study, we measured the serum capacity that promotes cellular cholesterol efflux and the plasma PLTP activity in type 2 diabetic patients with coronary artery disease (CAD) (n = 35), those without CAD (n = 24), and 35 healthy subjects as a sex- and age-matched control. In patients with CAD, plasma triglyceride level was higher compared to controls (p < 0.01) and HDL-cholesterol was lower (p < 0.01 vs control and the patients without CAD). In diabetic patients with or without CAD, PLTP activity was consistently increased, compared to controls, while cellular cholesterol efflux activity was decreased by 20% (p < 0.001) or 13.5% (p < 0.01), respectively. In conclusion, plasma PLTP activity was increased in type 2 diabetic patients with or without CAD, which could impair cellular cholesterol removal and might accelerate atherosclerosis in diabetic patients.

Relationship of endogenous hyperleptinemia to serum paraoxonase 1, cholesteryl ester transfer protein, and lecithin cholesterol acyltransferase in obese individuals

Altered activities of high-density lipoprotein (HDL)-associated antioxidant enzyme paraoxonase 1 (PON1) and lipid transfer proteins, for example, cholesteryl ester transfer protein (CETP) and lecithin cholesterol acyltransferase (LCAT), participating in lipoprotein remodeling seem to play important roles in obesity-related accelerated atherosclerosis. Inverse associations of PON1 with obesity and serum leptin levels have been demonstrated. However, the relationship of leptin with CETP and LCAT in humans is less clear. Our aims were to investigate whether the elevated leptin level is (a) an independent predictor of low PON1 and (b) associated with alterations of CETP and LCAT activities. Seventy-four white subjects forming 3 age- and sex-matched groups were included into the study (groups 1 and 2: nondiabetic obese patients, n = 25 with body mass index [BMI] 28-39.9 kg/m2 and n = 25 with BMI >or=40 kg/m2, respectively; and group 3: 24 healthy, normal-weight control subjects). Paraoxonase 1 correlated inversely with BMI (r = -0.39, P < .01), waist circumferences (r = -0.42, P < .001), and leptin concentrations (r = -0.38, P < .001). However, in a multiple regression model, neither these variables nor others, for example, age, sex, blood pressure, insulin resistance (in homeostasis model assessment of insulin resistance [HOMA-IR]), HDL cholesterol, low-density lipoprotein cholesterol, or lipid peroxidation (measured as thiobarbituric acid reactive substances), proved to be independent predictors of PON1. Lecithin cholesterol acyltransferase correlated negatively with BMI (r = -0.40, P < .01), waist circumferences (r = -0.42, P < .001), and leptin levels (r = -0.40, P < .01). During multiple regression analyses, BMI was an independent predictor of LCAT after adjustments for age, sex, HOMA-IR, and HDL cholesterol. However, this was replaced by leptin and HOMA-IR when leptin was also included into the model. The CETP activities correlated with HOMA-IR (r = 0.33, P < .01), thiobarbituric acid reactive substances (r = 0.45, P < .001), and leptin (r = 0.36, P < .01) levels in univariate but not in multivariate models. Elevated leptin level is an independent predictor of low LCAT, but not PON1, activity. In a population with a wide range of BMI, LCAT correlates inversely with obesity and CETP directly with insulin resistance.

Alterations in lipoprotein defense against oxidative stress in metabolic syndrome

Metabolic syndrome (MetS) is a high-risk condition for premature atherosclerotic vascular disease. Patients with MetS display a lipoprotein profile in which dense low-density lipoproteins (LDL), which are more susceptible to oxidation, predominate. Oxidation of lipoproteins can be attenuated in vivo by enzymatic and nonenzymatic antioxidant defenses, but high-density lipoproteins (HDL) play a key role in the protection of LDL from oxidation. Such activity depends on the presence of apolipoproteins (apoA-I, apoA-II, apoA-IV, apoE) and enzymes (paraoxonase 1, platelet activating factor-acetylhydrolase, lecithin:cholesterol acyltransferase, glutathione peroxidase). The impairment of HDL antioxidative activity in MetS is partly related to an enrichment of small HDL in triglycerides and their depletion in cholesteryl esters, to the replacement of apoA-I by serum amyloid A, and to glycation and oxidation of apoA-I. Therapeutic normalization of the quantity and the quality of HDL particles may constitute a novel approach to attenuate atherosclerosis and cardiovascular risk in MetS.

Serum cholesteryl ester transfer protein concentrations are associated with serum levels of total cholesterol, beta-lipoprotein and apoproteins in patients with type 2 diabetes mellitus

OBJECTIVE

To investigate the role of serum cholesterol ester transfer protein (CETP) and the metabolism of various lipids including apoproteins in patients with type 2 diabetes.

MATERIALS AND METHODS

The relationships between serum concentrations of CETP and various lipids and apoproteins were investigated in 193 patients with type 2 diabetes mellitus and 68 age-matched healthy subjects. Serum CETP concentrations were measured by an enzyme-linked immunosorbent assay.

RESULTS

Serum CETP values were lower in diabetic patients than in healthy controls (p < 0.01). Female diabetic patients had significantly higher CETP concentrations than male patients. Serum CETP concentrations exhibited a significant positive correlation with serum concentrations of cholesterol (TC) and beta-lipoproteins in diabetic patients (r = 0.485, p = 0.013). Patients with relatively high serum concentrations of high-density lipoprotein cholesterol (HDL-C) tended to have much lower CETP concentrations than patients with lower HDL-C concentrations. Serum CETP concentrations showed significant positive correlations with those of apoproteins B (Apo B; r = 0.384, p = 0.024) and E (Apo E; r = 0.341, p = 0.035).

CONCLUSION

The data indicate that serum CETP is closely involved in the metabolism of TC, beta-lipoprotein, Apo B and Apo E in type 2 diabetic patients.

Phospholipid transfer protein activity is associated with inflammatory markers in patients with cardiovascular disease

Plasma phospholipid lipid transfer protein (PLTP) has several known key functions in lipoprotein metabolism. Recent studies suggest that it also may play a role in the inflammatory response. Inflammatory cell activity contributes to the development of atherosclerosis. To seek further evidence for the association of PLTP with inflammation, we studied the relationship between PLTP activity and five inflammatory markers [C-reactive protein (CRP), serum amyloid A (SAA), interleukin 6 (IL-6), white blood cells (WBC), and fibrinogen] in 93 patients with low HDL and cardiovascular disease (CVD). Plasma PLTP activity had the strongest correlation with CRP (r=0.332, P<0.001) followed by SAA (r=0.239, P=0.021). PLTP, CRP, and SAA were significantly associated with body mass index (BMI), insulin or glucose, apolipoprotein (apo) B, and/or apo E level (r=0.264-0.393, P<0.01). PLTP, SAA, and IL-6 also were associated with the concentration of HDL particles without apo A-II [Lp(A-I)](r=0.373-0.472, P<0.005, n=56), but not particles with apo A-II. Smoking was associated with increased PLTP activity, CRP, and WBC, and hypertension with increased PLTP activity. In linear models, CRP remained significantly associated with PLTP after adjustment of CVD risk factors and insulin resistance. Also, much of the variability of plasma PLTP activity was explained by CRP, BMI, Lp(A-I), smoking, glucose, and blood pressure. These findings show for the first time that plasma PLTP activity is associated positively with CRP in CVD, a state of chronic inflammation.

Interactions of dietary fat intake and the hepatic lipase -480C-->T polymorphism in determining hepatic lipase activity: the Hoorn Study

BACKGROUND

Gene-nutrient interactions affecting hepatic lipase (HL) activity may contribute to the interindividual variability of the cardiovascular disease risk associated with dietary fat intake.

OBJECTIVE

We determined the associations of dietary fat intake with postheparin HL activity and the possible modifying effect of the HL -480C-->T polymorphism on these associations.

DESIGN

Subjects were recruited from participants in the 2000-2001 follow-up examination of the Hoorn Study. HL activity was determined in postheparin plasma in a sample of 211 men and 218 women aged 60-87 y. Information about dietary intake of the participants was obtained with a validated food-frequency questionnaire. Linear regression was performed, adjusted for age.

RESULTS

Total dietary fat was positively associated with HL activity (standardized beta: 0.11; 95% CI: 0.02, 0.21), and this association was also seen for saturated fat (0.10; 0.01, 0.20) and monounsaturated fatty acid (0.10; 0.01, 0.19). We observed a significant interaction of the HL polymorphism with the relation between total fat intake and HL activity. The association of total fat with HL activity was stronger in subjects with CT (0.27; 0.11, 0.43) and TT (0.39; -0.22, 1.00) genotypes than in subjects with the CC genotype (0.06; -0.06, 0.18; P for interaction < 0.10). The interaction remained statistically significant in models that included age, sex, carbohydrate and protein intakes, and insulin or body mass index.

CONCLUSIONS

Higher intakes of total and saturated fat were positively associated with higher HL activity. In addition, the observed association of total fat with HL activity was modified by the HL-480C-->T polymorphism, after adjustment for age, sex, carbohydrate and protein intakes, and insulin or body mass index.

Lipoprotein lipase (LPL) mass in preheparin serum reflects insulin sensitivity

Lipoprotein lipase (LPL) is one of the enzymes regulated by insulin and its plasma activity reflects insulin sensitivity. Although intravenous heparin injection is required to measure LPL activity, we can detect LPL mass in preheparin serum (Pr-LPL mass) by immunoassay. In this study, we examined whether Pr-LPL mass reflects insulin sensitivity. We measured Pr-LPL mass, insulin sensitivity (Si), and acute insulin release in response to a glucose bolus (AIRg) in subjects with normal glucose tolerance (NGT; n = 23), impaired glucose tolerance (IGT; n = 10), and Type II diabetes mellitus (DM; n = 48). Si and AIRg were determined by minimal model analysis. We also compared Pr-LPL mass with the homeostasis model assessment of insulin resistance (HOMA-R) and the urinary excretion of C-peptide (urine CPR). We found that Pr-LPL mass correlated significantly with Si ( r = 0.354, P < 0.01) in all the subjects. This correlation was still significant in the NGT group (P < 0.472, P < 0.05), DM group (r = 0.311, P < 0.01), and DM group with a fasting plasma glucose >150 mg/dl ( n = 20, r = 0.459. P < 0.05). Moreover, Pr-LPL mass correlated negatively with HOMA-R (r = -0.272. P < 0.05) and fasting IRI (r = -0.256, P < 0.05). By contrast, Pr-LPL mass was not correlated with either urine CPR or logAIRg that reflect the ability to secrete insulin. In conclusion, Pr-LPL mass reflects insulin sensitivity. We speculate that Pr-LPL mass might be used to assess insulin sensitivity not only in the general population but also in advanced diabetic patients.

Alterations in high-density lipoprotein metabolism and reverse cholesterol transport in insulin resistance and type 2 diabetes mellitus: role of lipolytic enzymes, lecithin:cholesterol acyltransferase and lipid transfer proteins

Insulin resistance and type 2 diabetes mellitus are generally accompanied by low HDL cholesterol and high plasma triglycerides, which are major cardiovascular risk factors. This review describes abnormalities in HDL metabolism and reverse cholesterol transport, i.e. the transport of cholesterol from peripheral cells back to the liver for metabolism and biliary excretion, in insulin resistance and type 2 diabetes mellitus. Several enzymes including lipoprotein lipase (LPL), hepatic lipase (HL) and lecithin: cholesterol acyltransferase (LCAT), as well as cholesteryl ester transfer protein (CETP) and phospholipid transfer protein (PLTP), participate in HDL metabolism and remodelling. Lipoprotein lipase hydrolyses lipoprotein triglycerides, thus providing lipids for HDL formation. Hepatic lipase reduces HDL particle size by hydrolysing its triglycerides and phospholipids. A decreased postheparin plasma LPL/HL ratio is a determinant of low HDL2 cholesterol in insulin resistance. The esterification of free cholesterol by LCAT increases HDL particle size. Plasma cholesterol esterification is unaltered or increased in type 2 diabetes mellitus, probably depending on the extent of triglyceride elevation. Subsequent CETP action results in transfer of cholesteryl esters from HDL towards triglyceride-rich lipoproteins, and is involved in decreasing HDL size. An increased plasma cholesteryl ester transfer is frequently observed in insulin-resistant conditions, and is considered to be a determinant of low HDL cholesterol. Phospholipid transfer protein generates small pre beta-HDL particles that are initial acceptors of cell-derived cholesterol. Its activity in plasma is elevated in insulin resistance and type 2 diabetes mellitus in association with high plasma triglycerides and obesity. In insulin resistance, the ability of plasma to promote cellular cholesterol efflux may be maintained consequent to increases in PLTP activity and pre beta-HDL. However, cellular cholesterol efflux to diabetic plasma is probably impaired. Besides, cellular abnormalities that are in part related to impaired actions of ATP binding cassette transporter 1 and scavenger receptor class B type I are likely to result in diminished cellular cholesterol efflux in the diabetic state. Whether hepatic metabolism of HDL-derived cholesterol and subsequent hepatobiliary transport is altered in insulin resistance and type 2 diabetes mellitus is unknown. Specific CETP inhibitors have been developed that exert major HDL cholesterol-raising effects in humans and retard atherosclerosis in animals. As an increased CETP-mediated cholesteryl ester transfer represents a plausible metabolic intermediate between high triglycerides and low HDL cholesterol, studies are warranted to evaluate the effects of these agents in insulin resistance- and diabetes-associated dyslipidaemia.

High plasma phospholipid transfer protein levels as a risk factor for coronary artery disease

OBJECTIVE

Plasma phospholipid transfer protein (PLTP) mediates both net transfer and exchange of phospholipids between different lipoproteins. Animal studies have shown that it is closely related to the development of atherosclerosis. PLTP-deficient mice have demonstrated increased antioxidation potential as well as a decrease in apolipoprotein B secretion and atherosclerotic lesions. In humans, high PLTP is associated with type II diabetes and obesity.

METHODS AND RESULTS

To assess the relationship between PLTP activity and coronary artery disease (CAD), a novel, high-throughput method to measure plasma PLTP activity was used, relating it to CAD in 1102 cases and 444 controls. This demonstrated that PLTP activity in patients with CAD was significantly higher than in controls (25.5 versus 22.4 pmol/microL per h; P<0.0001). Using multivariate logistic regression analysis, PLTP activity was found to have independent predictive value for CAD. Patients within the highest quintile of PLTP activity revealed a 1.9-fold increase in risk for CAD compared with patients within the lowest quintile.

CONCLUSIONS

These findings indicate that PLTP activity is positively and independently related to CAD and suggest that (1) prospective studies to evaluate this relationship are warranted and (2) PLTP should be considered a therapeutic target.

PLTP activity decreases with weight loss: changes in PLTP are associated with changes in subcutaneous fat and FFA but not IAF or insulin sensitivity

Phospholipid transfer protein (PLTP) activity is elevated in obese and diabetic subjects. No prospective studies have examined the effect of weight loss on PLTP activity and assessed whether the resultant changes in activity are related to changes in body weight, insulin resistance, or both. PLTP activity was measured at baseline in 46 subjects (body mass index = 19-64 kg/m2) and after diet-induced weight loss in 19 of the obese subjects. Total body fat mass (FM) by dual-energy X-ray absorptiometry, intraabdominal fat (IAF), and abdominal subcutaneous fat (SQF) by CT scan, insulin sensitivity (SI) by frequently sampled intravenous glucose tolerance test, leptin, and lipids were determined. At baseline, PLTP activity correlated with FM (r = 0.36, P = 0.02) and SQF (r = 0.31, P = 0.045), but not with IAF (r = 0.16, P = 0.32) or SI (r = 0.10, P = 0.52). With diet-induced weight loss (16 +/- 7.3 kg), PLTP activity significantly decreased 9.1% (P = 0.002). The change in PLTP activity correlated with the change in SQF (r = 0.55, P = 0.014) (33.6% decrease), but not with IAF (r = 0.09, P = 0.73) or SI (r = 0.18, P = 0.44), and was highly correlated with the change in nonesterified fatty acid (NEFA) (r = 0.71, P < 0.001). In conclusion, elevated PLTP activity in obese subjects is likely a result of increased body fat, reflected by SQF, and is influenced by NEFAs but is not directly related to insulin resistance.

Natural genetic variation as a tool in understanding the role of CETP in lipid levels and disease

Since the identification of cholesteryl ester transfer protein (CETP), its role in the modulation of HDL levels and cardiovascular disease has been debated. With the early detection of genetic variants followed by the finding of families deficient in CETP, genetic studies have played a large role in the attempts to understand the association of CETP with lipids and disease; however, results of these studies have often led to disparate conclusions. With the availability of a greater variety of genetic polymorphisms and larger studies in which disease has been examined, it is now possible to compare the breadth of CETP genetic studies and draw better conclusions. The most broadly studied polymorphism is TaqIB for which over 10,000 individuals have been genotyped and had HDL levels determined. When these studies are subjected to a meta-analysis, the B2B2 homozygotes are found to have higher HDL levels than B1B1 homozygotes (0.12 mmol/l, 95% CI = 0.11-0.13, P < 0.0001). A similar analysis of the I405V polymorphism yields 0.05 mmol/l higher HDL levels in 405VV homozygotes than in 405II homozygotes (95% CI = 0.03-0.07, P < 0.0001). The implications of these studies for cardiovascular disease will be addressed.

Effects of gender, hepatic lipase gene polymorphism and type 2 diabetes mellitus on hepatic lipase activity in Chinese

Genetic variation in the hepatic lipase (HL) gene (LIPC) promoter is an important determinant of HL activity in Caucasians. As HL activity is increased in patients with type 2 diabetes mellitus, we have investigated whether the -514 C-to-T polymorphism acted independently of type 2 diabetes to regulate HL activity. The frequency of this polymorphism and its effect on plasma HL activity and lipids were examined in 203 Chinese patients with type 2 diabetes and 205 controls. The frequency of the T allele was 0.343 and 0.376 in male and female diabetic patients, respectively, compared with 0.371 and 0.372 in male and female controls. The effect of LIPC genotype on HL activity was similar between men and women, and between diabetic patients and non-diabetic controls, with the lowest HL activity being found in those subjects with the TT genotype. On multivariate analysis, gender, LIPC genotype, the presence of type 2 diabetes and body mass index were independent predictors of HL activity, accounting for 22, 9, 5 and 3%, respectively, of the variance in HL activity (whole model adjusted R(2)=0.39, P<0.0001). The T allele was associated with higher high-density lipoprotein in the controls but not in the diabetic patients, and no associations were found between LIPC genotype and low-density lipoprotein subfractions in either groups. In conclusion, despite the higher frequency of the T allele in Chinese than in Caucasians, gender was the best predictor for HL activity, with LIPC gene polymorphism and type 2 diabetes making relatively smaller contributions to the variation in HL activity.

Short-term Acipimox decreases the ability of plasma from Type 2 diabetic patients and healthy subjects to stimulate cellular cholesterol efflux: a potentially adverse effect on reverse cholesterol transport

AIMS

To evaluate the effect of short-term administration of the anti-lipolytic agent, Acipimox, on the ability of plasma to stimulate cellular cholesterol removal, which represents one of the first steps in the anti-atherogenic process of reverse cholesterol transport.

METHODS

Eight male Type 2 diabetic patients and eight healthy subjects were studied after a 12-h fast at baseline, after 24 h of Acipimox administration, 250 mg every 4 h, and again after 1 week (recovery). Plasma lipids, apolipoprotein AI, phospholipid transfer protein (PLTP) activity, pre-beta high-density lipoproteins (HDL) in incubated plasma and efflux of radiolabelled cholesterol from Fu5AH rat hepatoma cells to plasma were measured at each time point.

RESULTS

Acipimox lowered plasma triglycerides in diabetic patients (P = 0.001) and healthy subjects (P = 0.002), whereas plasma non-esterified fatty acids were decreased in diabetic patients (P = 0.001) compared with the averaged values at baseline and recovery. Acipimox decreased HDL cholesterol in healthy subjects (P = 0.007) and plasma apolipoprotein AI in both groups (P = 0.001 for diabetic patients; P = 0.008 for healthy subjects). Not only plasma PLTP activity (P = 0.001 for diabetic patients; P = 0.01 for healthy subjects), but also pre-beta HDL in incubated plasma (P = 0.001 for diabetic patients; P = 0.03 for healthy subjects) and cellular cholesterol efflux to plasma (P = 0.04 for diabetic patients; P = 0.005 for healthy subjects) were lowered by Acipimox in both groups.

CONCLUSIONS

Short-term Acipimox administration impairs the ability of plasma from Type 2 diabetic patients and healthy subjects to stimulate cellular cholesterol efflux, in conjunction with alterations in HDL parameters and in PLTP activity. If the impairment of cellular cholesterol efflux to plasma is sustained with long-term treatment, this potentially adverse effect should be considered when treating diabetic dyslipidaemia with Acipimox. Diabet. Med. 18, 509-513 (2001)

The impact of phospholipid transfer protein (PLTP) on HDL metabolism

High-density lipoproteins (HDL) play a major protective role against the development of coronary artery disease. Phospholipid transfer protein (PLTP) is a main factor regulating the size and composition of HDL in the circulation and plays an important role in controlling plasma HDL levels. This is achieved via both the phospholipid transfer activity of PLTP and its capability to cause HDL conversion. The present review focuses on the impact of PLTP on HDL metabolism. The basic characteristics and structure of the PLTP protein are described. The two main functions of PLTP, PLTP-mediated phospholipid transfer and HDL conversion are reviewed, and the mechanisms and control, as well as the physiological significance of these processes are discussed. The relationship between PLTP and the related cholesteryl ester transfer protein (CETP) is reviewed. Thereafter other functions of PLTP are recapitulated: the ability of PLTP to transfer cholesterol, alpha-tocopherol and lipopolysaccharide (LPS), and the suggested involvement of PLTP in cellular cholesterol traffic. The discussion on PLTP activity and mass in (patho)physiological settings includes new data on the presence of two forms of PLTP in the circulation, one catalytically active and the other inactive. Finally, future directions for PLTP research are outlined.

Plasma cholesteryl ester transfer and hepatic lipase activity are related to high-density lipoprotein cholesterol in association with insulin resistance in type 2 diabetic and non-diabetic subjects

We evaluated the hypothesis that plasma cholesteryl ester transfer (CET) and lipase activities are influenced by insulin sensitivity and contribute to the low high-density lipoprotein (HDL) cholesterol observed in type 2 diabetic patients and insulin-resistant non-diabetic subjects. Sixteen type 2 diabetic and 16 non-diabetic subjects participated. Diabetic and non-diabetic subjects were divided in equal groups of eight subjects with low or high insulin sensitivity, which was documented as the glucose infusion rate (M-value) during the last hour of a 3-h euglycaemic hyperinsulinaemic clamp (150 mU kg(-1) h(-1), blood glucose target 4.6 mmol L(-1)). Post-heparin plasma lipoprotein lipase (LPL) and hepatic lipase (HL) activities were measured in samples obtained 1-2 weeks before the clamp. Plasma CET was measured by a radioisotope method. Compared to non-diabetic men with high insulin sensitivity (n = 8) HDL cholesterol was lower in type 2 diabetic men (n=8, p<0.01) and non-diabetic men (n=8, p <0.05) with low insulin sensitivity, and the HDL cholesterylester content was lower in type 2 diabetic men with high insulin sensitivity (n=8, p<0.05). In non-diabetic subjects with high insulin sensitivity, plasma CET was lower than in the other groups (p<0.05 for all). Multiple regression analysis showed that plasma CET (p=0.001) and HL activity (p=0.02) were independently and negatively associated with the M-value. No association between the M-value and LPL activity was observed. Independent negative relationships of HDL cholesterol with plasma CET (p = 0.04) and HL activity (p=0.03) were observed. This study supports the hypothesis that a low HDL cholesterol associated with insulin resistance in type 2 diabetic and non-diabetic subjects is related to a high plasma CET and a high HL activity.

Role of phospholipid transfer protein and prebeta-high density lipoproteins in maintaining cholesterol efflux from Fu5AH cells to plasma from insulin-resistant subjects

Plasma phospholipid transfer protein (PLTP) enhances the generation of prebeta-high density lipoproteins (HDL) that may act as initial acceptors of cellular cholesterol, and are likely to play an important role in the antiatherogenic process of reverse cholesterol transport. We examined the interrelationships between insulin resistance, the ability of plasma to stimulate cellular cholesterol efflux, HDL cholesterol, plasma PLTP activity and prebeta-HDL in 12 non-diabetic, non-smoking, normotriglyceridaemic men. Cholesterol efflux from Fu5AH cells to plasma, plasma lipoproteins, PLTP activity and prebeta-HDL formation as measured in incubated plasma were determined after a 12-h fast. Insulin sensitivity was assessed by a euglycaemic, hyperinsulinaemic clamp (M-value). HDL cholesterol was positively correlated with the M-value (r=0.65, p< 0.05), whereas plasma PLTP activity (r= -0.59, p <0.05) and prebeta-HDL in incubated plasma (r= -0.66, p<0.05) were negatively correlated with the M-value. Thus, the lower the insulin sensitivity, the lower was HDL cholesterol and the higher were plasma PLTP activity and prebeta-HDL. Cellular cholesterol efflux tended to be correlated with HDL cholesterol (r=0.55, p < 0.10) as well as with plasma PLTP activity (r=0.56, p<0.10) and was positively correlated with prebeta-HDL in incubated plasma (r=0.74, p<0.01). No positive correlation between the M-value and cellular cholesterol efflux was found (r= -0.34, ns). These preliminary results support the hypothesis that, despite a lower HDL cholesterol, the ability of plasma from insulin-resistant subjects to promote cellular cholesterol efflux is not impaired, as a consequence of a higher plasma PLTP activity and enhanced prebeta-HDL formation.

Cholesteryl ester transfer protein and atherosclerosis

Plasma cholesteryl ester transfer protein facilitates the transfer of cholesteryl ester from HDL to apolipoprotein B-containing lipoproteins. Its significance in atherosclerosis has been debated in studies of human population genetics and transgenic mice. The current review will focus on human plasma cholesteryl ester transfer protein research, including TaqIB, 1405V, and D442G polymorphisms. Plasma cholesteryl ester transfer protein has a dual effect on atherosclerosis, depending on the metabolic background. In hypercholesterolaemia or combined hyperlipidaemia, plasma cholesteryl ester transfer protein may be pro-atherogenic and could be a therapeutic target.

Phospholipid transfer protein in lipid metabolism

Phospholipid transfer protein (PLTP) is one of the main modulators of plasma HDL size and composition. The publications discussed in the present review have substantially increased our knowledge on the physiological importance of PLTP-mediated phospholipid transfer, especially between triglyceride-rich lipoproteins and HDL. Furthermore, novel data have provided clues about the transfer mechanism, and evidence for the direct involvement of PLTP in atheroprotection has recently been presented. The development of assays for PLTP mass determination has offered new tools for the elucidation of the physiological role of PLTP.

Structure and function of lecithin cholesterol acyl transferase: new insights from structural predictions and animal models

The enzyme lecithin cholesterol acyl transferase is responsible for the synthesis of most of the cholesteryl esters in plasma, and therefore plays a key role in lipoprotein metabolism. The relationship between the structure and function of lecithin cholesterol acyl transferase has been extensively studied in the past years, and new data appeared in 1999 documenting the substrate specificity and physiological role of lecithin cholesterol acyl transferase. The discovery of natural mutants, together with the proposal of a three-dimensional model for the enzyme, has provided new tools to unravel the function of specific residues of lecithin cholesterol acyl transferase. The use of transgenic animals and the production of knock-out lecithin cholesterol acyl transferase mice has further contributed to the understanding of the lecithin cholesterol acyl transferase 'in vivo' function. Evidence for a protective role of lecithin cholesterol acyl transferase against the development of atherosclerosis through the hydrolysis of oxidized lipids was recently proposed. Lecithin cholesterol acyl transferase patterns in several pathologies were further clarified. These newer developments are reviewed here.